OVERSIZE 

(200) 

I B 

no. jst;O 

c . ~ 

v. f 

-1~ ASSESSING 

EARTHQUAKE HAZARDS 

AND REDUCING RISK 

1l 1\.CIFIC NORTHWEST 
Cover. Insert. ground-shaking damage from the 1949 Pu get ound earthquake to unrcinforccdma. on ry in cattle. Wah. Photograph by George an koncn. eaulc Times. Background. lands lide damage to the railbed between Olympia and Tumwater. Wash .. in the 

1965 Pugct Sound earthquake. Photograph by Greg Gilbert . Daily Olympian. 

Cover design by Carol A. Quesenbe rry. 
Assessing Earthquake Hazards and 

Reducing Risk in the Pacific Northwest 

Albert M. Roger , Timothy J. Walsh, Willi am J. Kockelman, and 

George R. Pri e t, Editors 

Volume 1 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1560 

An investigation of th e earthquake potential in th e 

Pa ·ific Northwest and examination of the measures 

necessary to reduce seismic hazards 

ov 2 6 996 

C' 

1 C r A R Y 

UNITED ST TE GOVER ME T PRI TING OFHCE, WAS HI GTO : 1996 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For sale by U.S. Geologica l Sur . In formation Service 

Box 25286, Federal Center 

D nver, CO 80225 

A ny use of trade. product, or fim1 names in thi s publi cati on i for descripti ve purposes onl y and 

cl es not impl y endorsement by the . ovcrnment 

Library of Congress Cataloging-in-Publication Data 

Assessing earthquake haLard; and red uci ng risk in the Pacific 1 nh"e"t I Alben M. 

Rogcr, ... [et a i. J. editor>. 

p. em.-( .S. Geological Survey profes,ional pape r : I 560) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.16: I 560 

I. Eart hquake haurd ana lysi>- onhwest. Pacific. ~- Earthquake!>- onhwc!>l. 

Pac ific. I. Rogers. A.M. II. Series. 

QE535 .2. 6A 25 1997 

363.3'-l95-dc20 9 -37559 

CIP 
PREFACE 

To increase knowledge and understanding of geologic hazard. and ri k in the Pacifi 

orthwe t, the .S. Geologica l Sur ey initiated a research program fo used on the. e is ue. 

in the late 1970' s (volcanic hazards have traditionall y been studied separately). Thi s stud 

area wa cho en for everal rea on . Large, damaging earthqu ake arc part of the regional 

documentary record , yet for variou reasons we do not have confidence in ex i. ting es timate. of earthquake hazards. ew in formation ha increa ed our concern about the potential for great ubduction-zone earthquake and large, hallow earthqu ake . . Furthermore. the 

rapid increa e in population den ity and urban dcvclopm nt in thi s area during the Ia. t 

decade ha increased the eismi c ri sk. For the. e rea. on , funding for the e . tudies > as 

increased in 1987 to upport a 5-year accelerated re carch program under the . pon orship 

of the ati onal Earthquake Hazard Reduction Program. The . tudic ' ere primaril conducted under an element of the national program termed "'Urban and Regional Hazards 

A se · ment," whi ch previously supported intensive . tudie. in Salt Lake Ci ty. tah, and 

Lo Angeles and San Franci co, Ca lif. Typi call y. scienti st.. engineers, and soc iologist: 

from Federal agencies, State geological organi za tions, univer ities, and private in lu. try 

have parti cipated in thi s re earch program. Some of the cicntific . tudic from the first 3 

year of the program arc pre ented in thi s profess ional paper. The o cr ic' chapter summari zes the state of knowledge concerning geologic hazard and ri. k in thi. r gi n through 

199 I , including the tudi es herein and other relevant . tudic . 

The broade t goal of thi s profe ional paper are to prom te the recognition, a . cs:-

ment. and reducti on of earthquake hazards in the Pacific onh wes t. Engineers. pl ann r . . 

dcci ion makers, and land and building owner . hould, in the long term , usc thi. inf rmati on 

to reduce the effects of expec ted future earthquake in Wa hington, Oregon. northern aliforni a, and Briti sh Columbi a, Canada. 

Becau e per ons in many parts of society are re p n ibl for understanding earthquake hazards and mak ing deci ion to reduce earthquake effect . we hope thi. prof . -

sional paper will have a wide audience. To some ex tent , the diversity in the content of the 

paper renect. the di ver ity of the audience. A lthough many of th resea rch report. add res. 

pecialist ha ing ome familiarity wi th the geologic and geophy. ical . cicnces and earthquake-hazard miti gation. the overview chapter and report s on implementation of hazardreduction t chniquc (volume 2) are intended to inform both technical and nomechnical 

readers. T fu rther aid the lay reader. we have defined technical term u eel throughout the 

prof ssional pap r in the glo ary. 

A.M. R ger 

T.J . Wal h 

W .J . Kockel man 

G.R. Priest 

Ill 

CONTENTS 

VOLUME 1 

INTROD UCTION 

Earthquake Hazards in the Pacific onhwest- An Overview 

Bv Albert M . Roger , Timothy J. Wal h, William J. Kockelman, and 

George R. Prie t. ... ..... ... .................... ... ... ................. ........ ... ......... .... ....... I 

GLOSSARY .......... ..... ....... .......... ................................. ......................... ............ 55 

CONVERS ION TABLE...................................... ........ ......................... ... ......... ........... 67 

TECTONIC SETTING 

An Introduction to Earthquake Source of the Pacific orthwe t 

By Timothy J. Walsh .... .................................... ......................... ................. . 7 1 

PALEOSEISMICITY 

Coastal Evidence for Great Earthquake in We tern Washingt n 

By Brian F. Atwater.. ....................... ............................................................ 77 

Great-Earthquake Potential in Ore0 on and Washington-An Overview of 

Recent Coastal Geologic Studie and Their Bearing on Segmentati on of 

Holocene Ruptures, Central Ca cadia Subducti on Zone 

By Ian R. elson and Stephen F. Personius .......... ... ........... .... .... ........... . 9 1 

Discrimination of limatic. Oceanic, and Tec tonic M echani ms of clic 

M ar h Burial. I ea Bay, Oregon 

By urt D. Peterson and M ark E. Darienzo ............................................ ... 115 

Great Earthquake Recorded by Turbidites Off the Oregon-Wa hington oast 

By John Adam ... ...... .. .... .. ....... ........ .... ............ ........ ... ............. ....... ... ........ 14 7 

TECTO lCS A 1D GEOPHYSIC 

Cen zoic Evolution of the Continental M argin of Oregon and Washington 

By Parke D. Snavely, Jr. , and Ray E. Well s. ... ..................... .. ........... ......... 16 1 

Tectoni c. of the Will amette Valley. Oregon 

By Robert S. Yeat , Erik P. Graven , Kenneth S. Werner, 

Chri Goldfinger, and Thomas A . P pow ki ....................... ................. 183 

ti ve Strike-Slip Faulting and Folding of the a cadia Subduction-Zone Pl ate 

Boundary and Forearc in Central and 1onhern Oregon 

By hri s Gold fi nger. LaVern D. Kuhn. RobertS. Yeats. 

Bruce Appel gate. M ary E. M acKay, and Guy R. ochrane .................. 223 

Wes tern Washington anhquake Focal Mechani sm and Their Relati onship to 

R gi nal Te t nic tre 

By Li t! a. Rob n ro. son, and Ruth Ludwin. ........................................... r7 

Es timate. of ei. mic ource Regi n · from on ideration s of the anhquake 

Di tributi on and Regional Tectonic in the Pa ific Northwest 

Br raig . Weaver and Kaye M . hedlock .............. ............ ..................... 2 5 

INTRODUC 
Preceding page. IIISel"l, debris left by the Apri l 29, 1965, Seattle. Wash .. earthquake. 

Photograph courtesy of OAA/EDIS. Background, des truction caused by the fall of an 

unbraced masonry parapet in downtown Klamath Fal l , Oreg., during the ept. 20, 1993. 

M 5.9 and M 6.0 earthquake . Photograph by Lou Sennick of the Herald and ews. 

Kl amath Falls, Oreg. (from Dewey. J.W. , 1993, Damage from the 20 September earthquakes near Klamath Fal l , Oregon: Earthquakes & Volcanoe , v. 24. no. 3. p. 12 1- 128). 
EARTHQUAKE HAZARDS IN THE 

PACIFIC NORTHWEST-AN OVERVIEW 

B) Albert M. Rogers, 1 Timothy J. Wal h,

2 Willi am J. Kockelman ,3 and George R. Pri . r~ 

ABSTRACT 

S ientific re earch on earthquake hazard in the Pa ific 

orthwest through 199 1 suggests that the potential for large 

earthquakes and losses i greater than has been previously 

recogni zed. A great earthquake of magnitude 8 or more on 

the thru t fault that separates the orth Ameri ca and Juan de 

Fuca pl ates (commonl y termed the a cadia thru t or megath ru t fault) or a large earthquake of about magnitude 8 or 

less on shallow fault in the continental cru t would produce 

much greater damage than previou ly experienced by the 

modern inhabitants of thi region. Furthermore. damaging 

events similar to the 1949 and 1965 Puget Sound earth -

quake , whi ch occurred in the subducted Juan de Fuca plate, 

are expected to recur. In omc circumstance , marginall y di fferent locations or larger magnitudes for the c earthquake 

could sub tantiall y increa e the level. of damage compared 

to levels ob erved histori ca ll y. 

A variety of geologic and g ophy. ical data sugge t 

that great earthquakes (those with magnitudes between 8 

and 9.5) have occurred in the Ia t 7,000 year on the a cadia thrust fault. First. marsh deposit along parts of the 

Washington and Oregon coa t record epi sodic sudden ubmergcncc. The uddcn . ubmcrgence event , which have 

been in ferred from peaty oil s o crlain b tidal mud depo -

it in coas tal e tuarie . ha c been a cri bcd t tectonic subid nee a sociated with pa t great earthquake along the 

a cadia thru t fault. In some pl aces. the buri ed peaty soil 

arc capped by thin . and layer. that . ug0 e t and-bearing 

tsunami. caused by some earthqu ake coincided with the 

drowning of tidal wetl ands. c ond. gcodeti measurement. . uggcst that the ontincntal crust in " c. tern Wa hington i undergoing comprc . . ion. \ here the direction of 

1 .S. cological Survey, 13 ox 25046. M 966. Federal en1er. 

Denver. 0 8022-. 

ashing10n Department of at ura l Re>ourccs. PY - 12. Olympia. 

WA 98504. 

·

1 Deccm.cd. 

"Oregon Dc t art ment of Geosc ience. 9 10 State Office 13 ui lding. 1400 

.W. 5th Ave .. Portland. OR 9720 1. 

maximum compres. ional increa. c i. parallel to th dire -

tion of on ergencc between the Juan de Fuca and onh 

America plates. Third, Plcisto en and Holoccn folding 

and faulting of off horc and coas tal sediment. ar cv id n c 

of deformation that may ha c a com panied subducti on. 

Fourth, the Ca cadi a subduction zone has . c era! chant teri. tic similar to those in other ubduction zon s that have 

produced great earthquake. hi tori ca ll . And fi fth. ground 

shaking during prehi tori c earthquake. ma have tri oocrcd 

13 sean oor turbidite depos its at widely scatter d lo ations 

along the Cascadia subduction-zone margin . M ost . cicnti sts 

believe that the e and other data are consi. tent with continued ubduction, and . ome belie e the data arc con i. tent 

with sei mic ubducti on and gr at prchi. tori c earthquakes. 

High-prcci ion radiocarbon age. suggc t th at the Ia t 

·udden coa. tal ubmerocncc and tsunami occurred about 

00 years ago. The time between great carthquak s. however, may range from a few ccnturic. to more than a millennium . At pre ent, a reliable forecast of the time until the nex t 

great earth 1uake i. n t pos ible. oncthelc s, bcca u. c the 

effec ts of a great earthquake arc likely t be sub. tantial and 

wide pread, uch an event hould be consider cl in earthquake ri k a ses mcnt and eli sa. tcr pl anning for the region. 

The consequences of a great earthquake n the ascadia thru t fault are signi ficant b cau. e of the large area 

ex posed to mo t type of ea rthquake hazard . Damag ing 

baking may reach aero much of " e. tern Oregon. we tern 

Washington. and northern Californi a. Thi s type of earthquake may a! o produce widely cauered ground failure in 

su ccptible area . T. unami and seiche that may a ·company 

a great earthquake can further co mpound the le I. of cl amage from ground failure and baking in coa tal areas. 

Some scienti ts. howe er. think that subduction of the 

Juan de Fuca plate is a. ci mi c. For example, no gr at earthquake have occurred ince the arri va l of Europ an settlers 

in the area. nor have sensiti ve seismographs detected . mall 

arthquake on the a cadia thru. t fault. Theca c for asci -

mic ubduction is a! o ba eel on inference from theoreti cal 

modeling about fault-zone properti c and slab temperature 

that sugge t the downdip width of the locked cc tion of the 

thru st fault i limited. The max imum magnitude of gr at 
2 SSESSI E RTHQ AKE HAZ RDS 'D RED CI 1

G Rl K I THE PACIFI ORTHWEST 

a. cadia thru t-fault earthquake. may be limited and. if 

. omc model prcdi ti ons prevail. sei. mic subducti on may 

even be precluded. It i al. o argued that some e !Uary ubmcrgcnce events may have cau c. oth r than great earthquakes. For example, the effec t: f eu tatic sea-level ri se, 

elating procedure ·. and local tectonics loud the interpretation of . ome of the ubmcrgcnce e ems. Y ct, the cv id nee 

for great eanhquakes appears stronger than the evidence 

again. 1 them. 

The potenti al for . hall w contincn tal-cru. t earthqu akes 

of about magnitude 8 or lc .. is also incrca ingly apparent. 

cw geologic evidence indica tes that faults capable of generating large continental-crust earthquakes may ex ist in 

we. tern Washington. wes tern Oreg n. and nonhwe. tern 

Californi a. Th sc earthquakes have the potential to produce 

locall y higher lc el of damage than great subduction earth -

quakes where they underli e or pa s near urban areas . The 

long-term hazard from earthquakes on these faults ar 

unknown bccau e many . hallow faul t. may be unmapped, 

and earthquake r currcncc rate. and max imum magnitudes 

on these faults arc unknown. 

Intermed iate-depth earthquake. (40- 80 km deep in the 

subductcd Ju an de Fuca plate), imilar to those in 1949 and 

1965. arc expected to recur. These earthquakes may occur 

on a shallower pan of the subductcd plate. may strike 

clo. er to urban areas , or may have greater magni!Udes than 

those in the past. A max imum magnitude of about 8 i. possible for thi . type of earthquake. Hence', thee earthquakes 

have the potential to produce greater damage and life to 

th an ha been observed hi. tori call y. Earthquake ri sk asses -

mcnt and eli . aster-planning efforts . hould al. o include these 

hypothesized earthquake . . 

Much new re carch, presented in thi s profc. sional 

paper and other recent studic . . timul ates increased awareness of earthquake haza rds in the Pac ific orthwc t and 

encourage. tep to reduce the ri . k. ' uch tep should 

in vol ve hazard prediction. loss estim ation, citizen educa ti on, 

elisa. tcr management, urban planni ng, and continued evaluation and enforcement of earthquake building code . 

INTRODUCTION 

The Pacific onhwc. t i. a very large region that 

inc lu lc. all of Oregon. Washington. northern California, and 

western Briti sh olum bia. anada. In thi s professional 

paper, Pacific Nonhwe t refers primaril y to the area west of 

the a. cadc Range in this region, incl uding Vancouver 

I land (figs. I and 2). The study area is large enough t fac ilitate under tandi ng of earthquake hazard with a tec tonicprovi nce per. pcctivc. A lthough our discussion of the Pac ific 

onhwcs t in ol ve. the entire region. much of the research 

reported in th is profcs ional paper nece. saril y focuses on 

topica l studi es within . mailer areas. primaril y in Wa. hington 

and Oregon. Unfortunately, the topical and geographic limits 

of thi re earch currentl y pre lucie a fu ll de cription and 

understanding of canhquak hazar I . Because som i ue 

are complex and an not be simplified without losing significant understanding. ' e rc mmencl reading other di scu -

ion to further larify some aspec t of the issue or to 

prov ide additional detail and a different perspective (for 

cxam tlc, Riddih ugh, 197 ; Heaton and Hartzell , 1987; 

o. on and others, 1988; Rogers, 1988a, 1988b; Shedlock 

and Weaver, 199 1 ). 

Recent re carch has al exposed several new and significant unre olvcd i sues that continue to be studied and 

discussed. The e i uc. arc to some ex tent a con equence of 

the remarkable eli ersity and complex ity of past and present 

tectoni c 1 rocesse, a fact that may permi t multi ple inter-pretation. of the same data. Bu t the va riety of inter-pretations 

aLo result from . ant data and the infancy of scientific 

research on some tot ics. Future data co ll ec ti on, new technique , and continued rc earch will ultimately result in 

greater under tanding f earthquake hazards th an is currentl y poss ibl . The pre ·cnt . tate of knowledge, however, is 

uffic ient to continue planning and dcci. ion making to 

reduce the ri k. cau. ed by futur earthquakes. Both increasing understanding of the hazard. and efforts to reduce them 

should be viewed as long-term goals panning decades or 

more, tcp that will ultimately increase the level of protecti on for urban areas in thi. region. 

ACKNOWLEDGMENTS 

The author. are gratefu l f r the excepti nall y thorough 

re iew of thi s chapter pro idcd by Brian Atwater. Craig 

Weaver, and Robert ro. son. Their omment. helped to significantly improve the clarity. rganization, and accuracy of 

the manu cript. Wriucn com ments and el i cus ions provided 

by William Spence. James avagc, Dav id Perkins, and .T . 

A lgermi. en helped clari fy sc era! i. ue for the author . 

We at. o benefited from eli. cu sion. , preprint , faul t mapping. and parti al rev iew. by Parke navcly, Jr. , amuel 

Clark e, Ian M aclin . Garry ar cr. A lan el on. V rnc Kulm , 

R.D. Hyndman. and Jame. You nt. We thank Arthur TarT for 

helping to complete the seismicity figure u ing Gl . We 

wish to thank Barbara Hill ier and John ynncfaki , our .S. 

Geologica l Survey technica l ed it rs, for their tremcndou 

efforts in prov iding each auth r with final ed iting copy and 

for their managing of the detail s of the c mplction of thi 

publica tion. Without their help, thi publi cation would not 

have been poss ible. 

We deeply regret the untimely death of our co-edi tor, 

Willi am J. Kockclman. Bill was a trcmend u help in the 

completi on of thi paper. The Implementation s ction of the 

prof e. sional paper could n t have been produced without hi s 

parti cipation. But beyond thi. paper. he also contributed . ignificantl y to earthquake hazard awareness in the Pacifi c 

orthwes t, tah, Californi a, and el. ewherc through hi s 
126° 

48° 

42° 

EARTHQ AKE HAZARDS I THE PACIFIC 1 RTHWE T- A t 0 ER lEW 

124° m· 120° 118° 

0 

Port Albemi BRITISH COLUMBIA 

Cape 

Blanco 

Cape 

Mendocino 

-------------------------- A Mt. Baker 

Skagit Ri ver Delta 

Glacier 

Peak A SNOHOMI H 

COUNTY 

See inset 

KING 

COUNTY 

Olympi.i' o Fort isqually 

THURSTON PIERCE 

COUNTY COUNTY A o Mt. Rainier Centralia 

~ 

t Q 

~ 

% Mt. Jefferson A "{' 

° Corvallis 

Eugene 0 A Three Sisters 

(/) 

z 

.... Newberry Volcano 

OREGON 

(; 

Crater Lake 

0

A Mt. Mazama 

Mt. Mclaughlin A 

0 Klamath Falls 

------------ Crescent City -----------1-

Medictne 

A Lake Volcano 

A Mt. Shasta 

0 Eureka CALIFORNIA 

A Lassen Peak 

Spokane 0 

WASHINGTON 

\ 

I 

0 

:r: 

<( 

9 

--0--------( 

Freewaler ' 

Lake 

Wa shington 

NEVADA 

I i l ii:( 

"' I 

c.§' / 

I 

I ~ <( 

I 0 '-----

I 

I 

_ _j __ 

50 100 ILOMETEAS 

Figure I. lnde · map of1h c Pa ·ifi e nhwest swdy are::t. Triangle. show maj or Quaterna ry canoc~. 
4 SSE .. I G E RTHQ KE HAZA RDS A D REDU I G RISK I T HE PA IFIC ORTHWEST 

48° 

46° 

44° 

42° 

40° 

PACIFIC 

OCEAN 

PACIFIC 

PLATE 

100 200 KILOMETERS 

GORDA 

SOUTH 

PLATE 

MENDOCINO 

FRACTURE ZONE 

124° 120° 

BRITISH COLUMBIA 

NORTH AMERICA PLATE 

OREGON 

I 

I 

116° 

\ ~ 9 

--------------r----------

NEVADA 

CALIFORNIA 

Figure 2. M aj or geologic feature. in the Pac ific orth west. Barbs show relati ve . lip directi ons acros. fracture zone . 

The arrows show the directi on of relati ve convergence between the orth A meri ca and Juan de Fuca pl ates. Ridges 

are major fracture. in the litho phere where plates separate and new crustal materi al is emiued. The fracwre zones are 

trike-slip tran. form faults th at accommodate differemial motion between plates. The d ued l ine mark ing the continelllalmargin is the we ·tern boundary of the main thru t fault where the oceanic plate begi n to underthrust or subduct 

beneath the c nti nental plate. 

ex tensive parti cipati on and leader ·hip in workshops on thi s 

topic on a number of occasions and hi s publicati on contributi on . . His dedica tion to increas ing public awareness of the 

earthquake hazard in many region i widely recogni zed and 

appreciated. A a personal friend to each of the co-editors of 

thi paper. he wil l be mi ssed. 

RECOGNIZING POTENTIAL 

EARTHQUAKE HAZARDS AND RISK 

IN THE PACIFIC NORTHWEST 

The earlie t inhabitants of the Pacifi c orth wes t suffered the consequences of a wide range of geologic hazard s 

(Heaton and Snavely. 19 5; Grant and Minor, 199 1 ). Even 

though generati ons of retelling have left some aspe t 

unclear. Storie are till told among nati ve people concerning ome of these event . Stori es told by the M akah and other 

tribes uggest the occurrence of a large tsunami along the 

coas t of Wa hington. Yurok mythology relates the ex i. tence 

of a powerful earthqu ake god (Heaton and navely. 1985). 

and the Chinook have tradition de cribing arthquakes that 

shook their hou es and raised the ground (Gibbs, 1955); 

Gibb ( 1955 ) noted ev idence for recent raising and lowering 

of the land at Shoal water (now Will apa) Bay, W ash .. a locale 

of the Chinook tribe. Re ent findin g about the geology of 

the r gion . ugge t th at at leas t some of the e tradition · have 

bas i in fact (for example, Atwater. thi s volume). 

Some earl y event. are unequ i vocal in the documentary 

record . A large ash fall between 1770 and the ea rly pan of 

the 19th century contri bu ted to sickne s and death in the 
EARTHQ AKE HAZARD I THE PACIFIC ORTHWEST- AN OV · R IE\ 

Table 1. Large, damaging earthquakes of the Pacific orthwcst ince I 33. 

!Modified Mercalli intensi ty (MM I) of VII or greater. Intensi ty value~ adopted from Algermi;scn (19 3) unlc; foo tnoted. Leaders(---), no data available[ 

Date Location 

Dec. 14, 1872 ear Lake Chelan, Wash. 

ov. 22, 1873 Southwestern Oregon 

Oct. 12, 1877 Ca cade Range, Oregon 

M ar. 7, 1893 matilla, Oreg. 

Apr. 14, 1898 Mendocino County, Calif. 

M ar. I 7, I 904 About 60 km northwe t of Seattle. Wash. 

Jan. II , 1909 orth of Seattle, near the Bri ti sh Columbia border 

Dec. 6, 19 18 Vancouver Island, British Columbia 

Jan. 24, 1920 Strait of Georgia. Briti sh Columbia 

Jan. 22. 1923 Cape Mendocino (off hore), Calif. 

July 15, 1936 ear Frecwater, Oreg. 

ov. 13, 1939 orthwest of Olympia, Wash. 

Apr. 29. 1945 About 50 km outhea t of Seattle. Wash. 

Feb. 15, 1946 About 35 km north-northeast of Tacoma. Wash. 

June 23, 1946 Vancouver Island, Briti h Columbia 

Apr. l 3. 1949 Between Olympia and Tacoma, Wash. 

Apr. 29, 1965 Between Tacoma and Scanle, Wa h. 

Nov. 8, 1980 North of Cape Mendoci no (offshore), Calif. 

Aug. 17, 199 1 ear the coa t of northern California 

Apr. 25 , 1992 ear the coast of northern Cal ifornia 

M ar. 25, 1993 car the Wa hington-Orcgon border 

Sept. 2 1, 1993 car Klamath Fall , Oreg. 

1

M.G. Hopper. wriHen commun., 1990. 

Maximum 

MMI 

IX 1 

VIII 

VIII 

VII 

Vlii- IX 

VII 

VII 

Vlll2 

Vll2 

VII- VIII 

VII 

VII 

VII 

VII 

VIII2 

VIII 

VIII 

VII 

Vll 9 

IX 11 

VlJ I2 

VII 13 

urface wave 

magnitude (M,) 

7 

6.3 

57.2- 6·77.3 

6.77.1 

6.5 

97.0- 7.2 

106.2 

11 7.1 

6.125.5 

135.7 - 5.9 

2Estimmed maximum MM I or magniwdc estimated from MM I data. The magnitude of the I 72 eanhquake is e;ti rmtcd "M 7.4 (Malone and Bor. 197?), 

a'suming a depth of 60 krn ; M 7 is estimated from the maximum MMI of IX. iL,elf estimated a;suming a shallow cnrstal depth (M.G Hopper. wriHen 

cornmun ., 1990) 

l-roppozada and others ( 1981 ). 4

Coffman and Hake ( 1973 ). 5Rogers and Hasegawa ( 1978). 

{'Body-wave magnitude. m b. 

7Abe ( 19 I). 8

mb or M ,. Algermissen and Harding ( 1965). 9Person ( 1981 ) 10U.S. Geological Survey ( 199 1 ). Several other large canhquake occurred wrthin a month to 'everal hours of this event; they were farther offshore and 

did not produce onshore MMI:, above V These earthquake o urred on July I (M, 6.9, MMI V), August 16 (M , 6.3, MMI V), and August 17 (M, 7.1. 

MMIV) 11 0ppenheimer and others (1993). Thi earthquake is significant not on!} be"ruse it produced con ider:tblc damage (estimated between $48 and $66 

million. but because 11 may have occurred on the Cascadia thrust fault or a subpamllel fault. In either case. it likel y indicate.' active subduction of the Gorda 

plate. The earthquake w;" followed by two large aftershock. on April26 that also caused damage (both M , 6.6, MMI VIII). 12U.S. Geological Survey (1993). 

"u.S Geological Survey (1993). An aftershock with about the same magnitude and location occurred about 2 hours after the main shock. Another 

earthquake in thi. 'cries that produced MMI VII occu rred on December 12. 

5 

anpoi l tribe (po .. ibl y several eruptions from Mt. Hood. 

Glac ier Peak, or M ount t. Helens: sec fig. I ) (Coombs and 

other . . 1977). The carli c. t documented earthqu ake ' a. associated with the I 20 erupti on of Mt. Rainier (then ailed Mt. 

Tacoma) ( oombs and others, 1977). Settlers at Fort 

isquall y. Wash. (fi g. I). fel t another earthquake in I 33 

(Bradford. 1935). In 1872. uropcan . euler and Indians 

alike reported the effec ts of the magnitude (M ) 7.4 Lake 

hclan ea rthqu ake, wh ich was felt from Eug n , Oreg., to 

Briti sh olumbia. The direct and indirec t cffe ts of thi s 

earthquake cau eel many death. among the Indi an . E en 

though no mortality figures cxi t. mu ltiple accounts mention 

deaths from r ck fall . em i. sion. of poi onou. gasc . and 

famine following the earthquake (Coombs and others. 1977). 

Al togt:thcr. at lea. t 57 earthquakes in the Pacific orthwest 

have been large enough to produce . omc damage (M odifi d 

M crcalli inten ity. or MM I, V I or greater: Coffman and 

Hake, 1973). A few of these earthquake cau. eel or had 

potential to cause sub. tantial damage (MM I V II or greater) 

(table I : figs. 3-5). 
6 SSESSI G EARTHQ AKE HAZ RD A DR DU I G Rl K I TH E PA IFI RTHWEST 

Figure 3. Ground-shaking damage from the 1949 earthquake to unrein foreed masonry in Seattle. Photograph by 

George Cankonen, Seattle Times. 

R ccntl y collected geologic and geodetic data 

(Darienzo and Peterson, 1990: avagc and other. , 199 1; 

Ad ams, thi s vo lume: Atwater, thi volume; el on and Personius, thi s vo lume: Peter on and Darienzo. thi s volume) 

uggc. t that the region coul I have maj or earthquake along 

the ascadia subduction zone in the future. Future rupture 

of a 300- to 1.200-km egmcnt of the ascadia thrust fault 

would pr duce a great earthq uake of M 8- 9.5 (Heaton and 

Hartzell. 19 7). which i much larger than any hi tori cal 

earthquake in the region. !though hi tori cal earthquakes 

have had a sign ificant impac t on the Pacific orthwest, no 

great earthquakes (M 8 or greater) have occurred in the 

Pacifi c 1onhwe. t in the 200-year record of European ettlcmcnt, and none ha c had the intcn. ity or the geographic 

ex tent of damage that is likely in a great Cascadia thrustfault event. 

nfonunately. little information currentl y ex i t conerning great-earthquake charactcri . tic such as frequency, 

izc. expected locati on, and expected inten. ity and geograJ hie di tri bution of ground haking and ground failure. 

Furthermore. the frequency of large, damaging earthqu akes 

is low compared with many ubduction zones around the 

world . The apparent di. crcpancy between the hi tori cal and 

geologi records, di ffering intcq relati ons of these and other 

relevant data, and cant data on omc of the a pects ju t 

noted have produced uncertainty and debate regarding the 

degree of earthquake hazard from great earthquake . 

It i di fficult to cstabli h the full con cquence of a 

great earthquake in thi. region for several rca ons. First. 

ome properti es of the earthqu ake ource area. including the 

lateral and downdip extent of the expec ted rupture area, are 

not cstab li hed; the dimcn ions of the slipping ecti n of the 

fault strongly control the magnitude of the exp cted earthquake and the prox imity of the fault ec tion that produce 

ground shaking in urban areas. If the lipping ecti on of the 

Cascadia thru. t fault is close to urban area . the hazard wi ll 

be greater than if it i. di. tant. ccond. the attenu ati on of 

trong ground haking in thi region is not adequately known 

for any earthquake ourcc type. Third, the characteri stic of 

ground- haking effects related to geologic site condi ti on in 

urban area are j u t beginning to be tudied (Wong and ther . . 1990; Carver and other . vo lume 2; M adin. volu me 2), 

yet the e effec t could strongly innuence the levels of haking in urban area . Until many of the c is ues are better 
EARTHQ KE HAZARDS I ORTHWEST- 0 ER IE 

Figure 4. Ground-shaking damage from the 1949 earthquake to the bri ck veneer of a cntralia. v ash .. bui lding. The 

parapet c ll ap. e resulted in one death . Photograph from the A. L. Miller ollecti on. ni v r. ity of 'vVashington archi ves . 

Figure 5. Landslide damage to the rail bed between Olympia and Tumwater. Wash .. in the 1965 earthqu ake. Ph otograph 

by Gr g Gilbert . Dail Olympi an. 

7 
ASSES. I G EARTHQUAKE HAZA RD D REDU I G Rl K I TH - PACIFIC 1

0RTHWEST 

resolved. our understanding of the level of hazard from an 

earthquake of thi. type' ill remain uncertain . 

Should a great earthquake occur. the resulting shaking 

and ground failure li kely wi ll affect a wid region bccau. c of 

the great length and shallow dip of the a. cadia thrust fault : 

other hazards such as tsunami and seiche are also likel y. 

Thu . . in a inglc great earthquake, thi s combinati on of 

effect. could produce concu rren t damage in many urban centers along the wcstcm region of Oregon . Wa hington, 

northern al iforn ia, and Briti sh Columbia. 

As ide from the great-earthquake issue, howe cr, the 

hi storical and geologic records tell us that large earthquakes 

can be expected on faults other than the Cascadia thrust fault. 

For example. large continental- ru t earthquake have 

occurred on Vancouver Island and possibl y in the northern 

Ca. cade Range. Geologic evidence suggests that young. 

shallow fault. may cx i. t close to major population center in 

both the Pugct Sound and Will amctte ba. in region. (for 

exampl e. sec Wil son and other . 1979; Harding and oth rs, 

198 ; Buck nam and Barnhard, 1989 : Yeat., thi :, volume). 

B cause of the proximity of shallow fault. to developed 

areas. moderate to large earthquakes on the. c fault. might be 

as damagi ng in urban area. as more distant earthquake · on 

the Ca. cadia thrust fault. 

on tinued subduction of the sccti n of the lithosphere 

below the ascad ia thrust fault is likel y to produce 1949- and 

1965-sty lc earthquakes wi th no further convergence bet ween 

the 1orth America and Ju an de Fuca plates. The occurrence 

of earthquakes similar to tho c in the past, but with . lightl y 

different magnitude or locati ons. cou ld lead to greater lo s 

of life and property than has been ob crvcd hi tori ally. 

Together, the 1949 and 1965 earthquakes in the Pugct Sound 

region cau eel well over $200 million (in 19 4 dollars) in 

property damage and 15 deaths ( ay and oson. 19 6). The 

. Geological Survey ( 1975) e. tim atcd that an earthquake 

similar to the 1949 event could result in 2,200 deaths, 8,700 

seri ous injuric . . and as many as 23.500 homeless in the 

Wa hington counti es of King, Kit sap, Mason, Pierce, Snohomish. and Thur. ton (fi g. I). The hypotheti cal Benioffzone earthquake (. ce the glos ary) assumed in that tudy 

produced effects more severe than the 1949 and 1965 event , 

for several reasons. The assumed magnitude (M 7.5) was 

larger. the a sumcd hypocenter was at shallower depth (50 

km). and the assumed epicenter was clo. er to urban area. 

th an the earlier earthquakes (of the scenario con idcrcd in 

thi s study. the grca tc: t life lo. s and injury was produced by 

an earthquake hypocen ter beneath southern Seattle). onethcless. the assumption arc rea li ti c for an earthqu ake of thi s 

type. whi ch wa. a recognized earthquake . ource at the time 

of the study. 

Infrastructure and population have ignificantly 

incrca. eel in Washington and Oregon . incc the 1975 esti -

mates were made. add ing to the number of lives and dollar. 

at risk. M ay and No. on ( 1986) estimated that si nce the U.S. 

Geological Survey ( 1975) study, the populati on in the Pugct 

ound region alone has increased by 25 percent, and the total 

a sc sed aluc has increased by 240 percent. At pre cnt. 

however. we do not know the effec ts of the e change. on 

anticipated los. c . 

!though the documented record of moderate earthquakes in the region has prompted some application of building code to the dcvclopm nt of modern infrastructure, it i 

pos iblc that the hazard s have been undcrc timated and that 

ex isting action. to reduce ri . k arc inadc jUatc. con idering 

1mich new data. The most populous parts of W a hington and 

Oregon lie within cismic zones 2 and 3 of the Uniform 

Building ode (Internati onal Conference of Building Officials. 1991 ). cia sification. that have special earthquakerc i tant de ign req uirement f r certain types of structures. 

However. absence of th large. t hypothes ized earthquakes 

from the documentary record of the region may weaken the 

resolve among contemporary citi zens to adapt and enforce 

hazard-reduction techniques. We know, for example, that 

omc municipalities have not stri ctl y enforced earthquake 

element of the building codes (May and oson. 1986). Lack 

of experi ence. debate about the level of hazard. and our . cant 

knowledge regarding fundamental aspects of earthquake 

occurrence in thi s region hamper efforts to cck hazardred uction mea ures. 

THE NATURE OF 

EARTHQUAKE HAZARDS 

large earthquake can cause wide. prcad regional damage resulting from sc era! different geologic and sci mic 

effects. n earthquake occurs when rocks on opposite . ides 

of a fault move or lip abruptl y in respon c to strcs that 

exceed the strength of the fault. In map view. the fault and 

associated fractures arc com monl y confined to a narrow 

zone a few meter to a few kilom tcr. wide and , everal 

meters to a thousand or more kilometers long. The downdip 

ex tent of the fault. refc1Tcd to as the fault width. can range 

from a few meter for a very small earthquake (magn itude 

le than 2) to I 00 km or more for a very large earthqu ake (M 

7.5 or greater). ot all earthquake. occur on fault that produce . lip at the Earth's surface. a fact that damaging earthquakes of the Pac ific orthwc t commonly demonstrate (for 

example. see Weaver and Smith, 1983). oncthelcss, faults 

with surface ex pres ion do cxi. tin thi region (pl. I). particularl y offshore between the Cascadia thru st fault and the 

coas t. and some of these fault (both on hore and off hore) 

have the potential to produce earthquakes. 

In other areas, fault lip commonly innict. damage on 

buildings, roadways, pipel ines. or other tructurcs lying 

within a fault zone that has surface exprc. sion. Di sruption of 

lifelines(. cc the glossary) by faulting can have significant 

con equences in urban area (Earthquake Engineering 

Rc. ea rch Institute, 1986). Young, mappable faults inter.cct 
EARTHQ AKE HAZARDS I THE PACIFIC ORTHWEST- 0 ER lEW 9 

the land surface in thi s region in . everal place , and geophysical data suggest that many such faults may exi st that are 

obscured by sediments (for exampl . Harding and others, 

1988; Finn. 1990; Yount and Gower, 1991 ). 

The destruction from an earthquake, how ver, commonl y extend much beyond the rupture zone. Damaging 

energy in the form of seismic wave propagates away from 

the fault to distances a great as s era! hundred kilometers 

in a large earrhquake. The strong shaking that occur when 

the energy reaches the Earth ' surface is re pon sible for the 

largest prop n y and life loss in most earthqu akes (figs . 3-5). 

Although shaking levels generall y decrease with di tance 

from the rupture becau. e of the natural allenu ati on of the 

Earth and geometri c spreading f seismic waves, level of 

ground shaking can increase at sites underl ain by oft oil s or 

allu vium and (or) deep edimentary basins. Thi effect is 

comm only termed ... ite amplifi cati on'' or simpl y the "s ite 

effect. '' The phenomenon. whi ch may be en han ed at di tant 

site. by seismic-wave reflecti on. from deep rock layer , wa 

clearl y seen in several recent earthquakes uch a the 1985 

Michoacan, M exi co; 1985 Chile; and 1989 L oma Prieta. 

Calif. , events (Ai germi . sen and thers, 1985; Singh and others, 1988 ; Theil , 1990). In the 1985 Michoacan earthquake, 

collap ed or damaged buildings in M ex ico City were nearl y 

400 km from the . ource. oil amplificati on was a factor 

affec ting shaking in the 1949 and 1965 Puget Sound earthquakes (A igermi . sen and Harding, 1965; Mullineaux and 

others. 1967; U.S. Geological Survey. 1975) and it i 

expec ted to be a factor during future earthquake in the 

Pacific Northwest ( arver and others, vo lume 2; M ad in. volume 2; Sil va, volume 2). 

Strong or prolonged shaking also produce other clamaging geologic effecL such as landslide (fi g. 5). liquefacti on, and lateral spreading. In ome earthquake . . land. !ide. 

and so il failure may prod uc greater damage and life lo . 

than ground shaking, parti cul arl y for urban area in moun -

tainou terrain . oson and others ( 19 8) noted that as many 

a 14 earthquake have cau. ed landslides in Washington 

between 1872 and 1980. The 1949 and 1965 earthquakes 

triggered at lea l I 05 separate land I ides ( hleborad and 

Schuster, 1989). Potenti all y di a trou prehi stori c landslides 

are known to have o urred. such as the Osceola muclnow 

that ex tended from th slopes f M ount R·tinier down the 

White and Puya llup Ri ers as far a. the pre. cnt-day location 

of Au burn and Tacoma ( randcll. 197 1 ). Earthquake shaking may have induced . omc prehi stori c landslide For 

example, about 1.100 cars ago. large-sca le land I iding submerged bl ocks along the . horc of ake Wa. hingt n (Jacoby 

and Willi am . . 1990; Jacob and others, 1992); the timing of 

these landslide. approx imately coincides with other regional 

geologic events that may be earthquake related (for exam pi . 

see Karlin and Abell a, 1992; Schu ster and others, 1992). 

Liquefaction i. a phenomenon wherein oil s of certain 

types lose strength owing to the shaking-induced now of 

water from depth toward s the . urfacc : lateral spreading is a 

related chara teri stic of soil s th at results in permanent 

hori zontal di splacement of th ground. Both phenom na can 

be de tructi vc to faciliti c. that lie in gr und-failure zon s. 

The e effect may ha c au . eel '1S much as 25 perc nt of th 

damage in the 1949 and 1965 earthquakes in the Pac ifi 

ortll\ est (Grant, 1989). 

Ground failure ill occur in the future in th Pacific 

orthwes t, induced by both rainfall and ea rthquake . haking. 

Schu l r and hi borad ( 19 9), hlcbora I and Schu. ter 

( olume 2), and Grant and oth rs ( olu mc 2), among oth rs, 

ha e eli cu sed the lo ation. of pas t land. lide. and th areas 

susceptible to future land. liding. 

Earthquake and landslide. offsh r can produce large 

. ea-wave trains, or t: unami. ( e M ulloch. 1985. for a 

rev iew). Tsunami · trav I away from the sour c ar a at 

speed. of a much as 800 km/hour (M ulloch. 1985). Their 

height is low in the open ocean but can rca h t ns of mel r. 

on approaching shore, causing fl ooding of low-ly ing area. as 

much a thousands of ki lometcrs from th source. Th ut ' r 

coasts of Washington. Oregon, and Californi a ar not onl 

u ceptibl e to tsun ami: generated b earthquakes rupturing 

the ascadia . ubduction zon (Hebenstreit. 198 ), but they 

are also su ceJLiblc to . mailer waves from di stant ea rthquakes along the Aleutian and Japan . e trenches (M e ullo h, 1985). The t. un ami from the 1964 Al a ka ea rthquake 

damaged the A las ka. Briti sh olumbia, Washington. Oregon, and northern Ca li forni a coa. L. (includino 13 killed and 

II million in damage, prim aril at rc. cent ity. ali f. ; 

M e ulloch. 1985). Sa nd clepo. it. in CO<SLal Wa. hington and 

Oregon suggc. t that t. unami s a com1 anicd se veral larg 

earthquakes on the ascadia subducti on zone in th Ia l 

5.000 years (A twater. 1987; B urgcoi. and Reinhart. 1988; 

Darienzo and Peterson. 1990; At\ ater, thi s vo lume; Peter-

. on and Darienzo, thi. vo lume). Bucknam and Bam hard 

( 1989) and Bucknam and other ( 1992) found ev idence for 

recent uplift of 7 m at Re torati on Point in Puget Sound (fi g. 

1). I f deformati on of thi s type ccurs suddenl y, it is likely 

that a L unami will be generated. In fac t. e idence doe exi t 

that a L unami occurred in Pugct Sound in a ociation with 

thi s uplift (Atwater and Moore. 1992). Permanent modification of the h reline can also be exp cted. 

Finall y, damaging eiches can occur in the Pac ific 

Northwest. A seiche i imil ar to the resonant . lo. hing of 

water in a bucket that has been eli . turbed. The e wave , 

whi ch are obs rv d in lake and other bocli e of mostl y 

nclosecl water. can produce consid rable damage and 

fl ooding along shorelines. Seiches arc commonl y induced by 

the long-peri od ground shaking that accompani es earthquakes but ca n also be induced by submarine or shoreline 

ground failure. sudden uplift or subsidence, or sudden tilt 

(M e ulloch, 1985). Pas t earthquake have caus d sei he. in 

L ake Washington and Lake nion in Sea ttle. as well as at 

other lakes in Washington and Oregon (see Thor en, 198 , 

for a summary). It i rea onabl e to question whether the 

seismic- or nonscismi c-inducecl co llap. c of a large delta such 

as the Skagit Ri ver delta (fi g. I) could produce a damaging 

water wave (M cCulloch. 1985 : Finn and others. 19 9). 
10 -ARTHQ KE HAZ RDS D RED Cl RISK I THE PA IFIC ORTHWEST 

PACIFIC NORTHWEST 

EARTHQUAKES: ORIGIN, 

LOCATION, SIZE, AND TIMING 

M o. t earthquake. in the Pac ific orth\ est arc either 

direc tl y r indirec tl y related to the interac ti on of the ru stal 

plates . tcm of the r gion, wh ich includes the Pacific, Juan 

de Fuca. Gorda 1 orth , Gorda South, Explorer, and North 

America plates (fi g. 2) (Atwater. 1970; Fox and Engebretson, 1983; Spence, 1989). Much evidence sugge. ts that the e 

brittle plates converge obliquely along the I ,200-km-long 

Cascad ia subducti on zon , causing the oceani c pl ate. to 

de. cend, or subdu t, beneath the continent (fig. 6). Thi. proc .. can lead to severa l types of earthqu akes at widely separated locati on: not onl y off: hore but throughout much of the 

we. t rn regions of Wa hington , Oregon, northern alifornia. and British olumbi a (fig . 7 and 8). W e class ify the 

earthquakes mos t likely to affect populati on center in thi s 

subducti on zone into four principal type. based on fault locati on (fi g. 6C): ( I ) shallow cru. tal earthquake. in the orth 

Ameri ca I late; (2) earthquak es that rupture fault: within the 

. ubductcd oceanic litho. phere below the thrust fault (these 

earthquakes partl y define the dipping Benioff zone and commonly ex tend from the decpe t section (20- 30 km) of the 

locked thrust fault to as much a. 0 km deep): (3) earth -

quakes on the main boundary thrust fault between the oceanic and continental plates (these earthquake also help 

define the Benioff zone in some cases but n t at the Cascadia 

subducti on zone; and (4) shallow earthquake. w ithin the 

oceani c plate or along plate margins. Earthquake types that 

occur n fa ul t. wi thin plates are termed " intrapl ate events.'· 

and those along pl ate boundari es arc termed "imcrplate 

e ent. : · All except type 3 earthquakes have been ob:erved 

hi. tori ca ll y in the Pacific orthwe t; evidence for thi type, 

however, may ex i t in the geologic record (for example. 

Atwaicr and others. 199 1; Atwater. thi velum : Peterson 

and Darienzo, thi s olume). 

In thi report. we limit di. cuss ion of ome earthquake 

sources because they are of little importance in . ci michazard eva luation or are outside the . tudy area . For exampl e. 

significa nt earthquakes are likely cast of the Cascade Rang , 

w hi ch could affec t urban areas on both side of the a:cadcs. 

The largest hi stori cal earthquake in W ashington and Oregon 

(M 7- 7.4: see table I) (Milne, 1956; M alone and Bor. 1979; 

M .G. Hopper, .S. Geologica l Survey , written commun .. 

1989) wa. in 1872 and probabl y occurred outside our tudy 

region, ncar L ake Chelan. Thi s was probably a cru stal earth -

quake (Algenni sscn, 1983), but no surface ex pre sion of the 

fault has yet been found ( hannon & Wil son, In .. 1977). A s 

recentl y noted in aliforni a, however, shallow earthquakes 

on some thrust faults (termed blind thru. t ) do not alway 

rupture to the surface. Many young cru stal faults exist east of 

the ascadc (pl. I ), and some of these fault may be capable 

of earthquake that would affec t urban area on both sides of 

the ascades (for example, . cc Hawkins, Foley . and 

LaForge, 1989; Pcity and ther. , 1990). A !though such 

fau lts arc not the focu s of thi s report and we do not eli cu 

them furth r, we shou ld be aware th at they do contribute to 

the potential overall hazard in the study area. 

Figure 6 (facing page). The Cascadi a . ubduction zone from 

three different perspecti ves. emphasi7.ing one or more a pects di scussed in the tex t. A. the relationship between the principal geographic and geologic feature in the Pacific onhwest. The shaded 

region is the contact zone between 1hc onh Ameri ca and Juan de 

Fuca plalcs, termed the ascaclia thrust fault. The sloping lines on 

thi s fault are drawn primari ly 10 aiel in visuali zation of a threedimensional effect, but the subcluctcd plate may indeed be broken 

into one or more sections. M FZ. Mendocino fracture Lone. Verti cal 

scale is exaggerated. B. roughly east-west cross cction through the 

Earth' s crust and upper mantle near the latilllcle of Puget Sound. 

showing continental-plate compress ion and warpi ng that occurs 

during the peri ods between great earthquakes anclthe resulting variabilit in uplift and tilt th at i. expected as a functi on of geographic 

pos ition. The warping of the upper plate and the arch in the lower 

plate are greatl y exaggerated. The. ubductecl plate has a lower clip 

along the top of the arch, ex tending the thrust fault farther inl and 

ncar Pugct ounclthanto the north or south. Potential segmentation 

of the slab may ex tend into the thru. t fault or even into the oceanic 

I itho. phcre we 1 of the subduction zone but. at present, no e iclence 

ex ists to connect potential segmentation offshore with that below 

the thrust fault. Barbs show relati ve motion across the segmel)lation 

fault, and arrows show relati ve movement of vari ous parts of the 

pl ates. , cross section ncar the latitude of Puget Sound showing 

the positions of the most significamtype. of earthquakes in the Pacific rthwcst. The large t historical earthquakes in 1his region 

were in the Benioff zone and in the continental cru s!. Earthquakes 

in the Benioff zone occur within both th . ubductcd oceanic crust 

ancl unclcrl ying lithosphere. In most places along the ascadia subduction zone, the accretionary wedge is nearly aseismic; howe cr. 

a few hi storical earthquakes occurred in the younger section. of the 

accretionary wedge (age increases from we t to easl). and nsiclcrablc sci. micity occurs in the accretionary wedge wc. t of Vancouver Island and west of the northern ali fornia coast (sec fi g. 7). 

M any earthquakes also initi ate in the oceanic plate in these two area. (west of the region clepictecl in thi . drawing). omc sc icnti l. 

. uggc. tthat a thin section of . oft sediments. not shown here. is subcluctccl along the thrust fault. The briule-cluctile boundary (see the 

glos. ary). here arbi traril y drawn at a depth of25 km. i. the probable 

depth limit for seismic slip on the thrust fault anclthc depth limit for 

earthquake occurrence within the continental rocks. In thi s drawing, the backstop (see the glossary) is approx imately coincident 

with the outer arc ridge (not hown here: . ce the glossary) (P.D. 

Snavely. Jr., . . Geol gical Survey, oral commun .. 199 1; Snavely, 1992). Barbs . how relative motion on fau lt. 
EARTHQ A K HAZA RD I THE PA !FIC ORTHWEST - A 1 0 ER lEv 

Subducted Juan 

de Fuca plate and 

oceanic lithosphere 

A 

Spreading 

ridge 

JUAN DE FUCA PLATE 

Mantle upwelling 

B 

(f) 

a: 

UJ 

fUJ 

:2 

0 

_J 

;;;: 

~ 

:r:' 

f-

(l_ 

UJ 

0 

c 

20 

40 

60 

80 

100 

EXPLA ATIO 

D Area of continental-crust earthquakes 

[/.LYU.d Area of Benioff-zone earthquakes 

---- Potentially sei mogenic thrust fault 

Zone of 

coastal uplift Zone of inland 

NORTH AMERICA PLATE 

Potential segmentation 

of the subducted plate 

below the thrust fault 

CASCADE 

RANGE 

II 
12 A SESS I G EARTHQ AKE HAZ RDS A D REDU I G RISK I T HE PA IFI ORTHWEST 

13 2' 130' 128' 124' 122' 120' 11 8° 11 6° 

so• 

BRITISH COL.UMBIA ·( . 

I ~ \ . __:,_ _____ .,- . ;- ~-1---- \ . J\:~ : ... . •, • . • I ( I ~ ~---~·~~ ·' ·· · •· . . · \ ' . I , if 48' 

~ 

0 

~ 

u 

•a• 0 

u 

':5 ·-

u 

cf: 

'§ 

., 44' ~ 

~ ~ 

• ~ 

• • .. 

•• Pacific plate 42' 

• 

40' 

100 200 

.. ·,: ·.·•. . . \ 

,... . ~-· ~ .t, . : ' . . ..... <!;. •• .•. ,.. . . 1 

·:r. ~: WASHINGTON I 

. ·,. . . ··~ \ ... ~~ ·. , : .. ,' • .. · • . t 

... . . ·. ~---\ 0 ' ·., . ~ .. ,:r: \:: ~'--. 1<1: :-\:......;. .. -.... . . Cl ..• """"" ~ I ··i· . .. ' . I 

(' ·. ~ 

North America plate . A 

OREGC?N 

300 KILOMETERS 

~--, 

I 

I 

\ 

. ·~ . D :; . 

.. ... 

Figure 7. Epicenter map showing the largest eart hquake. in the Pacific onhwest for the period I 33 through December 1993 based on 

the data of Ludwin and others ( 1991 ) and U.S. Geological Survey ( 1991, 1992. 1993). The size of the epicenter symbol i. proportional to 

the earthquake magnitude as follows: M?.7, largest circle: 65fi4<7, nex t smaller: 55fi4<6, penci I point: M<S. smallest dot. Double I i ncs show 

ocea nic . preading ridges: heavy solid lines show the approximate po. itions of major transform fau lts (modified from Riddihough, 1983): 

the thin solid l ine indicates the inferred position of the ontinental margin and, therefore. the we tern mo. t ex tent of the Cascadia thrust fault 

(modified from Connard and others. 1984). Tri angles are volcanoes. 

We also largely ignore the earthquake potential along 

the . preading ridges in the Pa ific Ocean because eanhquakes in those areas are too di stant from urban areas to cause 

widespread damage. arthqu akes along the fracture zones. 

such as the Bl anco and M endocino fracture zones (fig. 2), or 

wi thin the continental and oceanic plates off hore have 

produced significant damage in the past. The 1980 earthquake off California (tab le I). for exampl e, injured ix people, caused a bridge co ll apse. and produced a max imum MM I 

V II near Eureka ( .S. Geologica l Su rvey and ati onal Oceanic and Atmospheri c Adm inistration. 1982). Three ea rthquakes, ranging in magnitude from 6.0 to 7. 1. occurred off 
E RTHQ AKE HAZARDS It THE PACIFIC ORTHWEST- 0 ER lEW 

126° 124° 122° 120° 

48° 

46° 

SEA 

LEVEL 

(f) 25 a: 

w 

Iw 

~ 

0 

_J 

:;;: 

!:; 

I . 

I-

"-w 

0 

100 

BRITISH COLUMBIA 

WASHINGTO 

.. , 

.. .. .. . . ... ' 

.. ·.· 

.:~ 

u:.. .· 

,. . . ~ .. 

.. PACIFIC • ·.1, . 

·. ··~ OCEAN St. Helens 

• seismic zone 

Columbia• 

,. 

. . . . .. .. ORE GO 

A 0 50 100 KILOMETERS 

NORTHWEST SOUTHEAST 

A A ' 

a tx:laoo • ao o 

~ o oa aocJJ ~if\ c Dao 

- D ~DO DC D 

Lower b - - ~ a 0 " Top of the Juan oundary of 9 -:- - _ de Fuca plate 

a -rni/Jion- - - _:_ Year-old I' h - tt osphere - - -

0 25 50 KILOMETERS 

B 

Figure 8. Locations or anhquake epi centers with well- onstrained depths in the Pacifi c onhw st. A. epi centers (dots) ba. ed on the data or Ludwin and others ( 199 1) and the .S. Geological 

urve ( 199 1 ). Box shows area or epicenters proj ected to the plane of eros: section A-A· in pan B. 

13. en ss . ecti on A- A· showing earthqu ake hypocemers (squares) relati ve to the position or the top 

of the subductin g oceanic (Juan de Fuca) plate as infetTed by M ooney and Wea er ( 1989). 

13 
14 SSESSI G EA RTHQ A KE HAZARD A D RED CI RI. K I THE PAC! IC 1

0RTHWEST 

northern alifornia in 1991 ( .S. Geological ur ey , 199 1). 

The smallc. t of the. e e cnt. , whi ch was near the coa t. produced maximum MMI VII in coas tal north rn aliforni a. If 

the M 7 . I , 199 1 event had occurred clo c to . horc. damage 

alon0 coastal Californi a and Oregon would have been ubstantial. I though the. c off. h rc structures are important in 

a .. essing the earthquake hazards of thi s region. \ e do not 

fo us on them in thi. report. In the following sections of thi. 

chapter. we emphas ize the potential for damaging earthquakes within the continental crust. within the subducted 

pl ate and lithosphere. and on the a. cad i a thrust fault. 

CONTINENTAL CRUSTAL EARTHQUAKES 

Some pans of the continental cru st in th Pacifi c orthwcst arc clearl y . cismicall y active be au. c many historica l 

low-magnitude anhquakcs and several hi stori ca l large 

earthquakes (fi g. 7) occurred in the continental (1 orth Ameri a) plate (Lud\ in, 19 9). Low-magnitude seismicity in the 

continental plate has defined the north -northwest-s triking 

right-lateral t. Helens :ei mic zone (fig. 8A) (Weaver and 

mith, 19 3). The 191 body-wave magnitude (mb) 7.2 and 

1946 111 LJ 7.3 Vancouver I. land earthquake. re also cru stal 

c cnts (Rogers and Hasegawa. 197 : Cas idy and other. . 

19 ). These earthquake may have a second-order relation 

to . ubduction . If the thrust fault is locked. tre ses produced 

by . ubducti on and relati ve pl ate motions may be tran ferr d 

to the continental pl ate (Fitch , 1972). 

Young fault. recentl y recogni zed by geologists must 

have proclu eel . hallow earthquake in recent prehistoric 

time. Pl ate I . hows faults wi th known or USJ ec tcd Quaternary di. placements and gcophysica ll y inferred lineaments in 

the Pa ific onhwcs t (note that the decrease in densi ty of 

you.thful faults in Oregon at the Oregon- alifornia border, 

apparent on plate I . is not a tec tonic feature but rather an anifact of the greater cmpha. i on fault mapping in California). 

car Pugct ound. the Saddle ountain Ea. t. Saddle M ountai n Wes t. and Hood anal faults arc of Holocene age; the 

Dow M ountain and ushman Valley fault. are of late Quatern ary age (Wil. on. 19 3). Faults within and bounding the 

Portland basin in Oregon appear to be cismi call y acti ve 

(Yelin and Patton. 199 1 ). Ev idence cx i t. for as many as 

three H locenc slip events on the Liulc Salmon fault zone in 

n nhwc. tcrn Californi a and for le s frequent and smaller 

c cnts on the ncarb M ad Ri er fault zone (Carver. 19 7; 

Carver and Burke. 19 7b; Kelsey and Carver, 1988; arver 

and others. 19 9: Iarke and Car er. 19 9: Clarke and 

Carver. 1992). Bucknam and Barnhard ( 1989) observed 

Holoccn uplift at Rc. torati on P int on Bainbridge Island 

and at Lynch Cove ncar Belfair. Wa. h. (fi g. I). which may 

be related to sli p n a shallow fault that cuts through po ·tglacial deposit. on the noor of Pugct Sound (Yount and Gower, 

199 1 ). Deformation on local shallow fau lt. may have 

produ eel subsidence at oo. Bay. Oreg. ( clson and Person ius. thi. volume). 

M apping of offshore depo. it shows many offshore 

fault s and folds in the continental pl ate. navely ( 1987) and 

navcly and other. ( 1980) di scussed Holocene fau lts both 

on horc and on the continental shelf that have the potential 

to produce dcstructi vc earthquake. . Iarke ( 1990, 1992), and 

Clarke and Carver ( 1992) mapped many ffshore fold and 

thrust . tructurcs. The age of the c : tructures is not preci sely 

known. Bccau e blind thrust fau lt may not rupture through 

ro k. as young a. the age of faulting, . ome faults may be 

younger than the youngest rock unit penetrated. Where these 

faults come ashore. in som places they have been dated as 

Holocene and Plcistoc n ( Iarke and a rver, 1989). Clarke 

( 1990) stated that the. c offshore"* * * structure originated 

or w re reactivated as a consequence of deformation accompanyi ng plate con ergencc during Plio cne to Holocene 

time." Off. h rc fau lt. and fold. cutting Holocene sediment 

arc also abundant on the continental maroin off Wa hington 

(Wagner, 19 5; Wagner and others, 19 6) and in the Strait 

of Ju·m de Fuca (Wagner and Tomson, 1987). Finally, Harding and Barnhard ( 19 7) and Harding and thers ( 19 ) 

di scussed features, identified in se ismic-refl ection profiles, 

that offset or disturb th s climent on the noor of Puget 

ound and Lake Washington. In som place , submarine 

. lumps may produce these offset.. but in other places the 

depth penetrati on and di1 of th feature. are evidence of 

faulti ng of Holocene age. Some of these fault. may represent 

. econdary ruptur in response to slip on the Ca cadia thrust 

fault ( Iarke and arver, 1992). The return time for earthquakes on such sccondar faults could be linked with the 

return time for great earthquakes on th ascadia thru t fau lt. 

lip on some continental fau lt. , h we er, may be independent of the ascadia thrust fault. 

Whether th sc faults represent prim ary or econdary 

rupture, many arc long enough to be considered potenti al 

source. f large. damaging earthqu ake .. On the basis of histori ca l sci mic data. a max imum magnitude of 7.4 is pos iblc. if even t such as the I 72 Lake Chelan earthquake arc 

po. ible at other loca le we. t of the Ca. cadc Range. A . imilar magnitude is suggested by the Vancouver earthquakes 

(tab le I). The St. Helens scismi zone (Weaver and mith. 

1983) produced an earthquake of local magnitude (ML) 5.5 

in 198 1. Weaver and Shedlock ( 19 9) suggested that thi s 

fault i. capable of an earthquake as large a M 6.8. omc 

mapped faults in the region may be capable of maximum 

magnitudes greater than 7.4. For example, based n fau lt offsets in H loccnc . ccl imcnts on the Little Salmon, M cKin -

leyville. and M ad Ri ver fault zon . Clarke and Carver 

( 19 9. 1992) and Dengler and thcr. ( 199 1) in ferred prehistori c ea rthquakes as. ociatcd with these fault hav ing mao nitudcs ranging between 7.5 and +. 

The lack of data hampers es tabli shment of a reli able 

upper limit for the magnitude of shallow con tinental-cru t 

earthquakes, whi ch. in any case, should be detcrminccl . cparatcly for each fault. De. pitc the ev idence cited , I ittlc is presentl y known abou t the locati on, size, or recurrence interva l 
EARTHQU KE HAZA RD I 1 THE PACIFIC ORTH\ EST- A 0 ER I \V 

of earthquake on . haiiO\ crustal fault. . and it is likely that 

many hallow faults with the potential to produce large 

earthquakes have not been rccoo ni zed. It is not yet po iblc 

to state the locati on of all . uch earthqu akes. Even where the 

location or surface faulting is known , few data are yet available about the timing of past movement. evertheles , exi ling fault data uggest that large. shallow earthquake. are 

pos ible at ome location wes t of the Cascade Range, and 

the size and locati on of the e faults uggest that the hazard 

from cominental-cru t earthquake may be as large a or 

greater than that from other source type . 

EARTHQUAKES WITHIN AND BELOW 

THESUBDUCTEDPLATE 

The Benioff zone i the dipping zone of earthqu ake , 

ob. erved in ac ti ve subducti on zones worldwide. that parti all y define. the subducting oceanic lithosphere. Commonly. thi . . eismi zone includes earthquakes on the thrust 

fault (interpl atc) and earthquakes within the subducting oceanic lithosphere below the thru t fault (intraplate). In the 

Pac ific North we. l. the ascadia thru. t fault ha. not produced 

detec table earthquakes (Lud win an I other. , 199 1 ), therefore, 

in thi s region, the Benioff zone is defined onl y by intraplate 

earthqu ake .. Large. damaging intraplate Benioff-zone earthquakes have occurred in the pas t in thi region. and simil ar 

events will likely occur in the future. Tw of the large thi stori cal earthqu akes, whi ch occurred in 1949 (111 b 7. I) and 

1965 (111 b 6.5 ) (table I) at depths between 50 and 60 km . were 

intraplate Benioff-zone event . Ludw in and others ( 199 1) 

uggcstcd that the I 73 earth 1uakc near the OregonCaliforni a border (table I) may have been an intrapl ate 

earthquake because it lacked aftershocks. a fcaturt.: that is 

comm on to many such ea rthqu ake. (A. ti z and others, 1988). 

Although sci. mi c evidence for a well-defined Benioff zone 

in thi. reg ion was lacking in the pa. t ( ro son. 1972), recent 

. eismic-network upgrade . . improved earthquake locati on 

techniques. and an ex tended period of monitoring have 

hown that a Bcniofl zone doc xist beneath we. t rn Washington and ' c. tcm Briti sh Columbia and that it is clear! 

defined by small- to moderate-magnitude earthquakes (Ludwin andothcrs. l 99 1). idcnce is. tilll a kin ofor a . ei ·micall y defined Benioff zone b neath Oregon (di. cussed 

below). Str ng c idcncc, howe er, docs ex ist for the pre -

cnce of a subducting pl ate beneath Oregon, Wa hington, 

northern ali forni a. and wes tern Briti. h olumbia from 

tudic of sci. mi -wave trav I times (M cKenzie and Julian. 

197 1; in. 1974 ; Lang. ton. 19 I a; ichacl on and Weaver. 

19 6; Taber and Lcw i . . 19 6: Zcrva. and rosson. 19 6: 

Cro . on an I o,. en. . 19 7; Owen. and others. 19 8: 

Rasmus. en and Humphreys. 19 ). 

The downward gravitational pull of the oceanic cru ·t 

into the Earth 's mantle i likely the fund amental procc. s that 

cau. cs Beni IT-zone earthquake (Spence, 19 7; A. ti z and 

other. . 19 ). The force of grav it not onl y ausc. earthquakes in the subductcd . lab b ext n. i nal faulting. but it is 

also the predominant dri ing for c that produces strc:s on 

the thru st fault. which can lead to gr 'at . ubduction-zon 

earthquake . . The subductcd slab can also d form in ways 

other than by normal faulting. For xamplc, differential 

xtcn ion between faulted . cgments of the dm ngoing slab 

may cause . trike-s lip faulting (fi g. 68 ) (Tab r and Smith. 

1985: Baker and Lang. ton. 19 7). 

Intrapl ate Beniof f-zone carthquak s do n t ha c a wcllc. tabli shedupp r-b undmagnitud .. ti zanl oth rs( l 98<) 

noted that M 8 earthquake. of thi. t pc ha e been ob. crved 

worldw ide, and thi s magnitud ha. been ugg steel by . ome 

sc ienti st. a. a max imum ( oppcrsmith and Youn os. 1990). 

The probabl e thi kncss of th subduct d slab at th as adia 

ubduction zone ma impl y that the slab an sustain intrapl ate earthqu ake. no larg r than M 7.5 (Wa. hinoton Public 

Power upply stem. 19 8). To compute the probabili stic 

seismic hazard of the region. in ~ and others ( 19 0) 

assumed an upper l imit of M 7.9 for thi s type of carthquak ' . 

THE POTENTIAL FOR GREAT 

THR ST-FA LT EARTHQUAKE IN THE 

CASCA DIA SU BD CTION ZONE 

Both prehi storic and future grea t earthquake. > ith magnitudes b tween 8 and 9.5 ha c been hyp thcsiz d for the 

a adia thrust faul t on the ba. is of a wide vari ct of evidence (Savage and other· . 19 I ; H aton and Kanamori . 

1984: Heaton and Hartzell. 19 6. 1987). cicnt ific controver. y exist. concernin g thi s hypothes is. however. because 

no inform ati on uneq ui voca ll y indica tes either prehi stori c 

great earthquakes r the pre onditions for such an even t in 

the future. Furthermore. the Cascadi a thru st fault has not 

produ eel Jete tab! earthquake at any magnitude lev I during the 90-ycar in trumental record or the 200-year hi stori cal 

re ord of the r gion (LucJ , in and other. . 199 1 ). The . cienti fic controvcr y center on se cral related que. tion . . Fir. !. 

is the Ju an d Fuca plate continuing to subduct beneath 

orth mcri a'l ccond. is ubducti on. if ongoing. cismic 

or ascismi ·) Th ird. if . cismi c . ubdu ti on is 1 ossible. how 

large ca n the earthqu ake be and how likely i. ir) 

In the fo llowing sc ·ti ons of thi s chapter, we di.'cuss the 

potential for great ea rthquake on the ascadia pl ate boundary and some aspec ts of the debate on thi s topic. Th di:cussion is an outline intended to review and cite th mos t 

signi ficant argument. for and aga in. t ac ti c se ismic subdu -

ti on. Tab les 2 and 3 . ummarizc the tex t discu, sion that follows in th nex t several ·cctions. The eli. cu .. ion and 

citati ns arc not intended to be all inc lusive; reade r can fj ncJ 

a greater level of detail and other arguments about thi s topic 

in th references ·itcd. 
16 A ES l GE RTHQ A KE H Z RD A DR - D CI GR I Kl T HEP CIFI ORTHWEST 

Table 2. Principal arguments for and against contemporary acti ve and inacli ve subduction in the Cascadia subduction zone. 

[Argument references and additional discussion are found in the text! 

Suppon ing argument and evidence Counterarguments 

Active subduction 

Seafloor magnetic lineaments indicate spreading of the Juan de 

Fuca ridge, and global plate- motion recon !ruction indicates 

convergence between the Juan de Fuca and orth America 

plates 

The direction of greatest compres ivc stress inferred from 

strain-rate data in Washington is subparallel to the plateconvergence direction, and compre sive strain i increasing 

The observed hori zontal-strain and coastal-uplift rates are 

successfull y modeled by a locked thrust fault converging at the 

independently estimated plate-convergence rate 

A Benioff zone ex ists beneath Washington and western British 

Columbia, and seismicity is present along the Mendocino, 

Blanco, and ootka fracture zones 

The presence of norm al-fault earthquak es in the Benioff zone 

suggests acti ve slab pull against a locked thrust fault. Slab pull 

is the predominant force producing subduction 

Geophysical data indicate the presence of a subducted slab 

downdip from the thrust fault, which supports subduction 

Offshore and onshore late Pleistocene and probable Holocene 

folding and faulting are consistent with ubduction 

The inferred rate of seafl oor spreading decreased between 6 M a 

and 700 ka, the time of the last magnetic reversal. If seafloor 

spreading continued to decrea e after th at, subduction may 

have ended. Plate-motion reconstructions are presently only 

loosely constrained 

The strain rates are low and, hence, the measurement errors may 

be comparable to the measured strain-rate values 

Ob crved strain rates could be produced by permanent 

deformation of the continental plate without subduction. The 

strain measurements may be influenced by processes near the 

Earth 's surface, uch as topography or loca l deformation 

A Benioff zone is lacking beneath Oregon and northern 

Cali forni a 

ormal faulting in the Benioff zone could continue in the 

subducted slab arter the thrust fault had permanently locked 

Thi s evidence support ancient subduction but is not evidence of 

current subducti on 

Tectonic processes other than ubduction could produce some of 

these structures 

Inactive subduction 

Seismicity is lacking on the Cascadia thrust fault and is low at 

the Juan de Fuca ridge 

The direction of greatest compress ive stress in ferred from 

earthqu ake foca l mechani sms and borehole breakouts is northnortheast to north-northwest in we tern Washington, Oregon, 

northern Californi a, Briti sh Columbia, and in the Juan de Fuca 

plate; thi direction i not parallel to the convergence direction 

(ea t-northeast) 

Low hori zontal-strain rates are observed near Puget Sound and 

on the Olympic Peninsula, which could indicate low rates of 

subduction or stopped subducti on 

A poorl y defined trench could be evidence that subduction has 

ended 

Seismic gaps ex ist on many faults that have produced large 

earthqu akes. The Juan de Fuca ridge continues to produce new 

cru st and has heat-flow values comparable to other acti ve 

ri dge . Subduction could continue without acti ve ridge 

spreading, owing to slab-pull fo rces and overriding by the 

continental plate 

Acti ve subductj on stresses may be in a low-s tress phase of the 

earthqu ake cycle. Landward counterclockwi se rotation of the 

stress ori entation may ex ist because of permanent 

deformation of the continental shelf and slope. Or, since the 

las t ancient earthquake on the Cascadia thru t fault, clastic 

rebound in the continental crust may have produced presentday extensional strain ubparallel to the maximum strain-rate 

direction 

Strain rates may change with time. Low strain rates, however, 

have been predicted on the bas is of a locked thrust fault and 

the con vergence of the Juan de Fuca and continental plates 

Buoyant oceanic crust that is well coupled but still undergoing 

acti ve subduction may produce a poorl y defined trench based 

on compari sons with other subduction zones worldwide. High 

sedimentation rates in the Pac ific Northwest obscure the 

trench by filling it with sediments 
EARTH Q A KE H ZA RD IN T HE P CIFI ORTH\ T- 1 OVER IE\ 17 

BD CTIO :ACTIVE OR !1 ACTIVE'? 

GEOPHYSICAL D TA 

Seafl oor magnetic lineaments arc the principal evidence for ac ti ve subduction during the last scv ral million 

years. These lin aments are the remnants of the magn tic 

field of the Earth and how ancient re er. a is in thi field 

recorded by ocean-bottom rocks a the rock. cooled after 

ej ec tion from the oceanic rid oes (Raff and M a. on, 196 1; 

Vine and M atthews. 1963). The. c feature. and other geophys ical and geologic data how that the Juan de Fuca ri dge 

(fi g. I) is spreading at a rate of about 3 em/year in an castsoutheas t direc ti on with respect to the Pacific pl ate, whereas 

the orth Ameri ca pl ate is moving at about 5.8 em/year in a 

southeas t direc tion with respec t to the Pacific pl ate (Atwater. 

1970; i himura and other . 19 4: Riddihough. 1984). 

These motions re. olvc into eas t-northeastward/wes t-southwes tward relative convergence (fi g. 2) between the Juan de 

Fuca and North Ameri ca pl ates at about 2.5--4.3 em/year. 

ompelling ev idence for ac ti c ubducti on al o come. 

from strain-rate (see the glossary) measurement in Washington. The directi on of maximum contraction inferred from 

geodeti mea urement of strain. . 59±6.6° E. in the Olympic Pen in ·ul a, N. 68° E. in Seattl e ( avage and other.. 199 1 ). 

and E. 3° S. in wes tern Oregon (Paul Vincent, ni vcr. ity of 

Oregon, written commun. , 1991 ), i subparallel with the convergence directi on (N. 68° E.) determined from pl ate motions 

and. thu., is consi tent wi th acti e subduction. Vari able measurements from the Vancouver I land area, . 2± 13° E. in 

Queen Charlotte Strait. . 18± I I o E. at Port A lberni (Dragert. 

199 1 ), suggest c mpl ex ity in the sub lucti on process in the 

Van ou cr I land region but arc con. istent in mao nitude with 

a locked-thru st-fault model (Dragert, 199 1 ). 

Ti lc-gaug and lcvclino data indi ca te gradual uplift of 

coa. tal areas in both Wa. hington and Oregon. avagc and 

others ( 199 1) showed th at thi. observation i con istent' ith 

ongoing . h rtening of the 1orth mcri a plate (fi g. 68 ). 

Several in v ti gators (Sil ver. 1972: navely. Wagner. 

and Lander. 1980: navel , 19, 7: Kelsey. 1990: M clncll y 

and Kelsey. 1990: and Iarke, 1992: among others) concluded. from a variety f offshore gcophy ical data. that the 

outer continental margin and coa. tal region arc undergoing 

acti ve youth ful (Holocene at . omc localiti ) folding and 

faulting consistent with ubdu ·tion of the Juan de Fuca plate. 

The north -south ex tent of the deformati on suggest. that 

ongoing subduction a!. o includes the - xpl orer (Riddihough. 

197 : Hyndman and others. 1979, 1990) and orth and 

South Gorda subplatcs (fi g. 2) (Kclsc and arvcr. 19 ). 

SEISi\IICITY 

One of the ob:cr ati on. leading to controvcrs is that 

no great arthquakcs have been detec ted during the a~ t 

200 year on the ascadia thru. t fault (Rogers. 198 a): 

today. the fault zone i. ismicall quiet ~ en at th ' Jo, est 

detectable magnitude cis (Crosson. 1972: rosson. 

19, 3: Lud' in and others. 199 1 )s I. o. no earthquake. 

ha e been observed in large reg ions of the offshore ontincntal 1latc in . pite of the youthful fault inn and foldin n of 

off hor sc liment s noted above. cismicity is al. o absent at 

the Juan de Fuca ridg and in large sections of the o cani · 

plate wes t of the . ubduction thrust fault (fi g. 7). alth ugh 

thi s observation i. common to man subdu ·ti on zones 

worldwide ( mcri can Associati on of Petroleum Geologists, 

198 1 ). Seismicit y do s occur. however. alonQ the offshore 

frac ture zones and within the Gorda South plate. We el i.-

cu. s these dilemmas in the following paragraphs. 

The lack f hi stori ca l se ismicity on the suhdu ti on 

thru. t fault ha. be n interpreted in sc era! ways: ( I ) the subducti on process has . topped. that is. there is no longer any 

clo. ing motion bet> ccn the North m ri a andth Juan etc 

Fuca pl ate. , or . lab pull (s~;c the glos ary) i. insuiTici nt to 

initiate slip on the thru. t fault ( ro. son. 1972; Farrar and 

Di xon, 1980): (2) th pl ates arc unloc ked and the subducti on 

proccs i proccc ling without generati ng earthquake (An do 

and Balaz . 1979): and C') the two pl ate arc stron ly locked 

but converging noncthele. s: thi s con lition would lead to 

comprcs ion and bending of the 1 orth Amcri a pl ate and 

storing of ei;'Isti c energy. to be rclca. cd in a future great 

earthquake ( avagc and others. 198 1: Heaton and Kanamori. 

1984). In intcq relati on 3. the hi. tori cal period is presumabl y 

part of the cismi c cle bet\ ccn great ~; rth c 

In imple. t term s. the sci. mi ·cycle ref rs to the buil dup 

and rclea e of stres on faults. whi ch result in alternating 

pcri o Is of quiescence and sci micity. Th relati onship 

bet ween thi. y le and earthquake occurrence i not yet 

completely clear (Kanamori. 19 I ). In one model of the eismi c cycle. howcv r. moti on of cru stal plates 1 roduces strain 

accumulation rc. ultino in . evcral sci micity phase . . At low 

. train levels. a few earthquake. may occur at a rate that 

defines a low. or background. level. A train accumulate . 

the rate of earthquake oc urrence common! increase 

throughout the stnincd olumc of cru tal rock. At larger 

strain I vel . some faults or fault segments lock and become 

5Two earthquake:- have occurred recentl y !hat may have been produced by slip o n or ,ubpara llcl 10 !he :- ubduc tion thrust fault. Low-angle 

nodal planes and !hru,H li p style computed fo r 1hesc ea rthqua kes ( .S. 

Geolog ical urve . 1991. 1992: Oppenhe imer and others. 1993) arc 1hc ba-

, i, for this conc lu, ion. The ea rthqua c' occurred o n August 17. 1991 (M 

6.2). and April 25. 1992 (AI 7. 1 ). The 1992 eart hquake ha; ab o been a iscu;,ed by Hubert and chwa111 ( 1993). Koch ( 1993 ). and ra ves ( 199 ). 
ASSESS I EART HQ A KE H ZARDS A 10 RED Cl G RISK I TH E PACIFIC ORTHWEST 

quiet again . although scismicit may continue on urrounding faui L. . . train increa. c. further, a sequence of fore-

. hoc k. may develop\ ithin the formerl y quiet zone. nea r the 

impending mainshock in time and locati on. When strain 

rcachc. a criti ca l level on a gi ven fault, the mainshock earth -

quake occurs followed by a seri of after. ho k . after which 

the yclc repeats. If the rate of long-term . train accumulati on 

is con: tant and the faul t-zone propertie. are uniform. the 

time between main sho ks depend on the amount of . lip in 

the last main. hock (Bufe and others, 1977). ndcr the conditions of th is. impli sti c model. an earthquake with large . lip 

will be preceded by a longer quiescent period than an earth -

quake with mall slip. 

The seismic cycle concept partl . tern s from th 

absence of ci ·micity on some young faults, a comm onl y 

observed charac teri sti c (Lay and other. . 1982); yet, some of 

these fault. have been sci. mica ll y ac ti ve hi stori ca ll y. Rogers ( 1988a) cited the subduction zone in cw Zea land near 

Wellington as an example of a fault that produced a large 

subducti on-zone earthqu ake in the last century bu t has no 

detectable present-day seismicity on the thrust fault. Scismt gaps along other . ubducti on zones may forecast the 

potential for fuwrc large earthquakes (Kelleher. 1970: 

M cCann and other , 1979). Sections of the San Andrea 

fault in Cali forn ia, although not a thru st fault. suggc. t 

another example of the . cismic cycle. As noted by Heaton 

and Kanamori ( 1984). the section of the fault that ruptured 

in 1906 is now eismica ll y quiet, whereas the creeping section that has not produced a large earthquake hi stori ca ll y is 

sci mica lly acti ve at low magnitude levels. Thu . . we can 

take neither the pre .. cnce of small earthquakes a con lusive ev idence of the potential for large e ems nor the 

absence of small arthquake: as e idcncc of a fault wi th no 

potent ial for large events (Lomnitz and Na a, 1983). everthele. s, ba. ed on these examples, the Cascadia thrust 

fault may be in a quiet period of its sci. mic cyc le, considering that other data . uggc t that plate motion. continue. 

For xample. seismicity occur on . omc boundaries of 

the regional plate sy tem that includes the boundaries 

between the 1onh Ameri ca, Juan de Fuca. onh and South 

Gorda, and Explorer plate. . Because the M endoc ino. 

Blanco, and Nootka fracture zone (compar fi gs. 2 and 7) 

1 roducc earthqu akes. it i po . iblc to det rmine if slip rates 

inferred from earthquake magni tude on these boundari es 

arc consistent with independentl y determined plate-motion 

rates . Hyndman and Weichert ( 19 3) showed that slip rare 

ca l ulatcd from the magnitudes of earthquakes on the plateboundary faults agree wi th ·li p rates ba. eel on global pl ate 

kin mati cs. Their result. however. did not reso lve whi ch 

plates arc in moti on but onl y showed that relati ve moti on 

occur. on these faults at rates consistent with plate-moti on 

recon. tructi ons. 

- xaminati n of lip direction and . tres cs derived 

from earthquake focal mechani. m is another means of 

ob. erv ing relati ve cru tal plate motion . . but these ob crvations are equi oca l regarding acti ve ubduction. Focalmechani. ms and sci. mi city alignmem · indicate right-lateral strike 

slip on the M endocino and Bl anco fracture zones (fig. 2) 

(M cE ill , 1968; Spence, 1989). Ri ght-lateral slip on thes 

faul t. is consistent w ith either transport of the Juan de Fuca 

pl ate into the trench, a fi xed Ju an de Fuca plate and northwest tran port of the Pacific plate. or moti on of both plate . 

Onl y the first and third alternati ve are wholl y consi tent 

with acti ve ubducti on. A s noted in the followi n:o paragraph . . the econd alternative may or may not be con i tent 

with acti ve subduction, depending on other a sumption 

The direc tion of greatc t compres i ve . tress inferred 

from fo a! mechani sms is principally north-south in the 

Pac i fi e 1orthwest (Spence, 1989). If con vcrgenc of the 

two plates is ongoing and the thrust fault is locked, a more 

east-northea ·tward compre si ve stre : directi n should be 

observed, parallel to the convergence direction. M ost of 

the foca l-mechani sm observati ons do not confirm thi s 

stre. s direc ti on. 

Spence ( 1989) computed tress directions and magnitude throughout the Juan de Fuca plate and part. of Wa hington and Oregon, ba eel on a model that onl y requires 

moti on of the Pacific and onh Ameri ca plate and a locked 

. ubducti on zone. Th i model as um · no motion of the Juan 

de Fuca plate. The ori entation of the greatest compre sive 

tre. s omputed u ing thi model roughl y fit the focalmec hani m-inferred direction of greates t compres. ion 

north-south . The agreement between modeling and data 

implies that the Juan de Fuca 1 late wes t of the trench is not 

presently under subducting compress ion, although the modcling docs not preclude subducti on in the future (see additional discuss ion of thi s modeling in the ection '·Strain rate 

measurements. cru. tal sh rtening, and focal mechani ms''). 

Seismi ev ict n e for moti on of th e subdu ted slab 

below the loc ked zone trengthen the argument for acti e 

. ubducti on. Taber and Smi th ( 1985), who swdied the focal 

mechani sm for mall earthqu akes in the Benioff zone (in 

location : imilar to earthquake wi thi n the ubducting lithosphere shown in fig. , B), found that these events were predomin antl y norm al-fault earthquakes with in ferred tension 

strc_. axes parallel to the downgoing lab. Both the 1965 and 

1949 earthquakes also had thi strc s orientation (Weaver 

and Baker, 1988). Ex ten ion in the Benioff zone is con is tent 

\ ith ac tive subducti on b cause the slab is being pul led into 

the mantle by gravitati onal force . Spence ( 1989) argued that 

the slab-pu l l force extends the downgoing litho phere, producing norm al-fault ea rthquake with downdip tension axes 

because the locked thru st fault pi ns the slab and re ists 

downdip slab force . . l f the thrust fault is unlocked, evidence 

from other ubducti on zones sugge. ts that an ongoing seri es 

of normal-fau lt earthquake. should oc ·ur out to the trench 

and farther wes t (Spence, 1989) as the slab-pull force cau es 
EARTHQ AKE HAZARD It TH E P IF! ORTHW EST- 0 ER JEW 19 

exten ion and bending of the oceanic lithosphere. If the 

thru t fault undergo s a seismic cycle. normal-fault earthquake. near the trench are expec ted onl y after a subductionzone great earthquake permit s an ex ten ional stress pulse to 

move updip pa t the momentaril y unlock d fault. At the ascadi a . ubduction zone, normal-fault earthquakes do not 

extend as far a the trench, although th 'Y doe tend beneath 

the Olympi c Peninsula (Taber and Sm ith, 1985), sugge. ting 

that a part of the thru t fault is locked. 

The 1949 earthquake focal mechani sm was strik slip 

rather than normal faulting. From ground-moti on modeling 

. tudie . Baker and Langston ( 1987) inferred left-lateral 

strike lip on an cast-west-s triking fault for thi. earthquake. 

Thi s slip ty le can be produced by differential moti on 

between different segment of the downgoing . lab (fi g. 68 ). 

Thus, an east-west tear in the slab could re. ult in trike-sli p 

movement on thi s fau lt (Baker and Langston , 19 7) if the 

southern segment of the slab moved d wndip relati ve to the 

northern egmcnt. In any case. thi s earthquake suggests 

acti ve subduction. 

OTHER JEWS A 0 COI\I PLEX IT IES 

The interpretati on of acti c subducti on is not unquesti ned. In carl ier studies. Crosson ( 1972). M i Inc and others 

( 1978), and Sbar ( 1983) noted the lack of a eli tinct Benioff 

zone and an earthqu ake-inferred compressi ve-. tress direction clif'fercnt from the convergence direc ti on. wherca. for 

many subducti on zones these two directi ons arc approx imately equal. From thi s inform ation, they argued that subducti on may have cea. cd. Since then. however. new data on 

the . ci. mi it y f the region ha. impro eel the definition of 

the Benioff zone beneath Wa hingt n (Cros. on, 1983 : Crosson and Owens, 19 7: Lud win and thers. 199 1 ). Yet. the 

lack of a di stinct Benioff zone beneath Oregon (Weaver and 

Mi chael. on, 19 5) has not been adequately explained. Bolt 

and others ( 1968) dis u . cd s ismi c and gcologi viden c 

that a part of the G rcl a and orth Ameri ca pl ate have 

formed a . ingl pl ate owing to the ex tension of the San 

Andrea fault to the northwc. t be ond the cnclocino fracture zone. Thi. h p the. is impli es that a . outhcrn sec ti n of 

the Ca. ad ia subducti on zone is perm anentl y lock d. 

Other argument. ha e been made to suggest that . ubducti on has ended. The pattcm f ocean-bottom magneti c 

rc cr. als indicates that the Ju an de Fuca oc an-ridge. prcacling rate dccrca eel during the las t 6 milli n years (Riddihough. 1977, 19 4: arl son. 19 I ; Duncan and M cElwee. 

1984). nfortunatcly. the last age for whi ch acti e spreading 

can be ca lculated i. about 700 ka: thus. the magn tic lineament data give no informati on on the ridg -. preacling rate to 

the present day. Furthermore. at present. the Juan de Fuca 

rid.,c i · nearl y a. eismic. whi ch could impl y th at spreading 

ha. cnclcd. If spreading ha. slowed and subdu ti on rates arc 

I w. th thrust fault could have pcm1 an ntl y loc ked during 

the la. t 700,000 year. (for example. sec twatcr. 1970). 

Oth r ev idence or arguments counter the ceasedsubduction h 'POLh es i . . For example. scismi n te. are commonly lo' al ong man ·tcti ve l spreadin g ri dges ' orld wid 

(Ameri can A. sociat ion of Petroleum Gcologi. ts. 19 I). ncl 

even though the Juan d Fuca riclg is as ismic. t n. of kilometer of new ru st have formed in som sec t ions of the Juan 

de Fuca rid ge (D laney and other .. 198 1; Henton and Hnrtzcll , 19 7) during the las t 700.000 cars. and hcat-fl w alucs over the 1 reading ridge arc omparabl to those ov r 

other ac tive spreading ridge. (Korgcn and others, 197 1 ). 

Som workers regard the Ia k of an offshore tr nch as 

c id nee that subducti on has . topped (for example. Ricldihough, 197 ). Howe cr. high . ed imcntation rat . from the 

Columbia and Fraser Ri ver .. incc the las t glac iation, a slo' 

. ubduction rate. and a buoyant o ca ni c pl ate ma bsc urc or 

inhibit d vclopment of a prominent tr nch. G ophy. i ·al lata 

. upport thi s infer nee (for exampl . Kulm and I ctcrson. 

1984). Finall . Heaton and Hart zell ( 19 6) sugge ted that 

the poorl y defined scclimcnt-fill cd a. cadi a subducti nzone tren h i. harac tcri stic of youthful. bu ) ant. strong! 

locked, acti ve obducti on zones ' orld wici '. 

In our vi w, the cvidcn c favors ongoin o a ti vc onvcrgcncc a ro .. the ascadia thru. t fault. Acti ve slab pull could 

produce sub lucti on and the c cntual o curTcn c of larg 

earthquake. on the thru st fault (. pence, 19 7). The mos t 

direct c idcncc for contcmporar subdu ti on is the offshor 

dcformati n of young . cdim nts in the ac rctionary wedge. 

strain data that indi ca te ·tcti ve com pres. ion or th continental 

plate. coa tal uplift infcrr d from leveling and sca-le cl data. 

and the ex istence of a Benioff zone hav ing earthqu akes '· ith 

downdip ex tensional . trc. sax s beneath Wa. hington. 

IS THE A CAD IA THR ST FA LT SEI Ml ? 

E n though a ti c subducti on is idcly acccptccl . on-

. id rabl c debate ex ist: about whether the ascadi a thrust 

fault i. capabl e of producing large earthquake (for example. 

A ndo and Balazs . 1979: Heaton and Kanamori. 19 4, 19 5; 

Achar a. 19 5: B rn and ykcs . 1987: S kes and Byrne, 

19 7: twatcr. 19 a: B rne and other. . 19 ; ammi and 

other . . 19 : Washington Public Power uppl y Sy. tcm. 

1988: We. t and M cCrumb. 19 a. 19 b: pence, 19 9: 

ykcs. 19 9: ppcr mith and Young .. 1990). In a rec Ill 

pollin g of 14 . cient i t. ( oppersmith and Young , 1990). 

estim ates were reque·ted about the potential for significant 

earthquakes (moment magnitude, Mw. gr ater than 5.0) on 

the thru. t fault. The rc, ponscs varied widely. from near certainty that uch earthquakes would not occur (probability 

ncar 0) to ncar certain! that they would occur (probability 

ncar I). The maj orit y of the respondent. . however. a!. igncd 

a probability between 0.5 and 0.9 for . uch an earth 1u akc. 

ignificant nc' data suggesting :cismic . ubduction. discus. eel in thi. paper. were not known to thes scicnti ts at the 

time of their va luati on in 198 . 
20 A SE EA RTHQ KE HAZ RD D RED RISK I TH E PA IFI ORTH'vVEST 

Table 3. Principal arguments for and against contemporary seismic and aseismic subdu tion in theCa cadia subduction zone. 

[Argument references and additional discussion arc found in the text. Because a lo ked thrust fault is a prcrc tuisite for seismi c subduction, some arguments center on whether the 

effects of a locked fault are present in Lhe vari ous obscrvaLions. Ncit.hcr arguments nor countcrargurnems are conclusive at present and. in some a~cs , further response to 

counter.rrguments is possible (see the original studies referenced in the text for this level of detail). In our view, however, the weight of the evi dence favors seismic 

subdu ction. In pa rti cula r, the evid ence for sudden subsidence, tsun ami deposits atop buried peats, and the modeling of strain daHl by a locked th r ust fault arc all highly 

significant. Additional work to improve the accuracy of radi ocarbon dating and connection of these events with shaki ng evidence. such as liquefaction and landslid ing, could 

make the case for great subduction eanhquakes unequivoca l! 

Supporting arguments and evidence Countcrarguments 

Seismic subduction 

Geologic evidence exi t for mu ltiple episode of sudden 

prehistoric coa tal ubsidence 

Sand deposits atop peats, buried by rapid coa tal subsidence, 

uggest the occurrence of tsunami 

Thirteen episodes of submarine turbidites arc observed in 

offshore sea-fan deposits less than 7,500 years old 

Observed horizontal-strain and coa ·tal-upli ft rates are 

successfu lly modeled by a locked thrust fau lt converging at 

the independently estimated plate-convergence rate 

Leveli ng data that show northward tilt in Oregon and eastward 

tilt of the Coast Ranges in Washi ngton are consistent wi th a 

locked thrust fault 

The Cascadia subduction zone is analogous to subduction zone 

off southern Chile, Mexico, southwestern Japan, and 

Colombia 

Exceptionally young, buoyant oceanic slab should couple and 

store stres more effectively with the continental plate. A 

locked thrust fault i upported by the downdip ten ion 

direction in the Benioff zone that results from slab pull 

against a locked thru t fault and by the presence of sei micity 

in the continental plate 

Off hore and onshore late Plei tocene and probable Holocene 

folding and faulting is con i tent with subduction 

The mostly consi tent compressive- tress direction (northnortheast to north-northwest), inferred from focal 

mechani ms and well breakouts in both the Juan de Fuca and 

orth America plates, suggests a locked thrust fault. See table 

2 for other stress-related argu ment that appl y 

Known Holocene landslides in the region yield 14C ages imilar 

to those for some inferred ubduction earthquakes 

Rapid eustatic changes in sea level could produce si milar effect 

at some locations. Coastal subsidence at some locales may be 

produced by deformati on on shallow or local faults. oastal 

submergence may also have occurred during intcrseismic 

periods at some locales. The ages of peat burial are not 

demonstrably equal at all e warics. Aseismic slip on the plate 

boundary could produce subsidence 

Over decades fo llowing rapid or gradual coastal subsidence, 

e w ary flooding by high tides, unu ual river discharge, or 

tsunamis from distant earthqu akes could also produce the e 

layers in some places 

Turbidites could be triggered naturally by cxce s sedimentation 

or by earthqu akes other than subduction events 

Ob crvcd strain rates could be produced by permanent 

deformation of the continental plate wi thout subduction. The 

train mea urcments may be influenced by processe ncar the 

Earth ' surface, such as topography or local deformation 

one 

The Cascadia subduction zone has unique fea!Ures. o other 

acti ve subducti on zone compare exactly with thi one 

If the fault locks too strongly or the subducted crust is too 

buoyant, the lab-pull force may be too small to overcome the 

frictional stress on the fault. ormal faulting in the Benioff 

zone could continue in the subducted lab after the thrust fau lt 

had permanent ly locked 

Folding and faulting could also be formed during a eismic 

subduction processes. parti cularly in the soft sediments of the 

accretionary wedge 

The lack of agreement between thi s tress direction and the 

convergence direction ugge ts that subduction-produced 

compressive stres is low compared to the compressive stres 

being produced by the Pacilic- onh America plate 

interaction. See table 2 for other tress-related arguments th at 

apply 

Rainfa ll or local earthqu akes could also have produced Holocene 

landslides 
EARTHQUAKE HAZARDS IN THE PACIFI C ORTHWE T - A 0 ER IE'Y 2 1 

Table 3. Principal arguments for and against contemporary seismic and aseismic subduction in the a cadia subduction zoneContinued 

Supporting arguments and evidence Counterargument 

Aseismic subduction 

Lack of earthquakes along the thrust fault suggests aseismic 

subduction 

Holocene coa tal terraces are moslly ab ent in the region 

Low hori zontal-strain rates are observed near Puget Sound and 

on the Olympic Peninsula, possibly indicating low or topped 

subduction, especially if the error in strain-rate measurements 

i. comparable to the measurements 

In one model, eastward tilt is ex pected for a zone undergoing 

aseismic subduction 

High heat now and high nuid pres ure in the accretionary wedge 

and subducted oft sediments along the thrust fault may 

prevent a cumulation of the required strcs c for a great 

subduction-zone earthquake 

A locked thru t fault and accumulating stre s are prerequi site to large ubduction-zone earthquake , although 

not all agree on thi s point (Chen and other . 1982 : Wa hington Public Power Suppl y Sy tern , 19 8). nethelcs , the 

ei. mic-a ei mic debate about the ascadi a thrust fault centers on data that are rele ant to whether the fault is locked 

and accumul ating com pre sional stre s. nfortun ately, these 

data may lead to equivoca l conclu ion . Though a se sment 

of the potential for large earthquakes i not completed. we 

review the principal a. pccts of the di cu .. ion in the following paragraphs. Table 3 al. o outl ines the vidence on b th 

. ide or the d bate. 

On question to be a. k d i . . what con. titute un quivo al id nee for the hypothc i that a great earthquake 

"ill ccur on the ascadia thrust fault? On the one hand. 

the only compl t I uncqui oca l idcnce is probabl a 

great-earthquake oc urrence. On the other hand. rca ·onab l 

con istcnt c idcncc f . v ral type. . om bincd ith a 

model that fi ts the. data well using tati. ti al measures. 

ould prod uce a high le cl of onfiden c in th h pothesis. 

If this lc cl of confid nc i. considcrl;d hi gh by a group of 

ob crver. . then mo. t ob cr cr would oncludc that . u h 

Other acti ve ubduction zones are commonly asci mic for orne 

period of the cismic cycle. If only the offshore ection of 

the thru t fault i. locked, it may be di ffi cult to detect small 

earthquakes in that region using exi sting onshore sci mic 

networks 

Terraces may be ab ent becau c of repeated cosei mic coastal 

subsidence and the position of the coast relative to the locked 

thrust fault. Continuing eustatic ea-level ri se or high rates of 

coa tal erosion may have obliterated terraces 

Strain rate may change with time. uccessful modeling of low 

strain rates with a locked fault model upport the ubduction 

earthquake hypothes is 

In other model . either eastward or westward tilt may be 

observed depending on po ition relati ve lO the locked thru l 

fault 

Inferences about the temperatures, nuid pressure , rock 

properti es, and presence of subducted oft cdiments along the 

thrust fault are difficult to te t and arc considered by some to 

be circumstanti al evidence 

earthquak will occur. For example, i f all the following 

were true. then in our view. confidence in the h pothc. i. 

woul d be high: ( I ) consistent time. of coa. tal ubmcrgencc 

at w idely eparatcd loca litie . so that onl y . lip on the ascad ia thru t fault cou ld ha e produced the effect; (2) evidence or ground bak ing in th form of tsunami s. 

land !ide . li quefacti on, and turbidite . each occutTing at 

time consistent w ith the submergence event ; (3) a model 

of the locked . ubducti on zone that explain the geodeti c 

observa tions. ca-lc cl change . . and ancient ubmergcncc 

events : and (4) developmen t of a model that permit · tick 

l ip 0 cr part. or the thru. t fault, Ll ing inference ab Lit 

tcmperatu r . and pore pre. sure along the thru. t fault. Conver. ely. the hypothesi. mi.,ht be rej ected b cau e of: ( I ) 

lack of s n hronou. occurrence of ubmergence event 

along the coa. t and (or) of ubmergence event. and haki ng 

ev idence : and (2) development f model that ex plain geodetic data. a-level data. and rapid coastal ubmergcnce in 

term s of a ei. mi c slip. A. i hown in the fo llowing ecti ons. onfidcnce in the hypothe. i is stronger than confidence in it. rcje tion. Arguab ly, the most igni ficant 

ev idence . ugge t. that at lea t some egment of the 
22 I G E RTHQ AKE H Z RD A D RED Cl G Rl K I THE PA IFI ORTHWE T 

a adia thrust fau lt arc locked , that . ubducti on continue . 

and that . cismic . lip on the thru. t fault can be expect d in 

the future (Heaton and Hart zell , 1986. 1987 ; Spence, 19 9; 

Savage and other . 199 1; At\ atcr. this volume). 

I FER EN ES FROJ"vl <IODELS OF 

PLATE AND FA LT-ZO E PROPERTIES 

In one model of the plate-boundary thru. t fault (Byrne 

and others. 1988), the fault is characteri zed by four zone 

(fi g. 6C, downdip from the urfacc trace of the fault): ( I) the 

dcf rmati onal front (accreti onary wedge). or lead ing edge of 

the continental pl ate. where clay-laden sed iment are relati vely . oft and affected by high pore pre urc; (2) a zone of 

igneou roc k and dewatercd , lithi ficd. con olidated edimcnt. , referred to as the ·'backstop,·· that is, strong enough to 

support tress accumulation; (3) continental crustal rocks; 

and (4) a thin zone of uncon olidated . ed iment. along the 

thrust fau lt that accumulated on the oceanic rust. before 

their . ubcluction. The. c unconsolidated sediments form a 

boundary of vari able thickness between the ubduct I lab 

and the vcrlying continental rock . High fluid pressure and 

. ubductcd uncon olidated . ediments could rcdu c the area 

of a loc ked thru. t fault in zones I and 2 (Byrne and Sykes, 

1987 ; Sykes and Byrne, 19 7; Byrne and other . 19 ; ros-

:on, 1989. 1990; Kulm, 1989; M agee and Zoback. 1989). 

Thi. model is a hypothe is, and many of its feature may be 

difficult to te t. For example, the pore pre sure and temperatures at the depth of the thru t fault can only be inferred 

from shallow measurements and modeling. and the a ti ons 

of thru t fault under cond itions characteri stic of subduction 

are not yet well under tood. Finall y, ome recent gcophy ical data suggest that ediments are mostl y . craped off rhe 

downgoing Juan de Fuca plate at ome pl aces and are not 

. ubducted (Hyndman and others, 1990). The long-term 

aseismicity of the thrust fault, however, could ultimately validate thi s hypothc is. 

In zone. 2 and 3. temperature on the fault may exceed 

300-450°C (Davi and others, 1990; Hyndman and others, 

1990), which wou ld prevent tick-slip motion on thi s secti on 

of the fault (Scheidegger, 1984; Byrne and other . 1988; 

Sammi and others, 19 ). Temperature measurements in 

shallow eafl oor borehole and modeling of thermal charactcri ti c. of the cru t form the ba i of thi s hypothe i . nder 

. ome conditions. which may apply to the Cascadia thrust 

fault, the. e effect could r duce the area of potential locked 

fault zonc tozero(Byrne andSyke , 19 7; ykesandByrne, 

19 7; Byrne and ther. . 1988). 

The age and temperature of the downgoing . lab form 

the bas is of other hypothese. about . ubducti on in thi region. 

young. relati vely hotter oceani plate may slow subduction because it i. more buoyant, reducing the slab-pull force 

and in rcasing interpl ate coupling (Spence. 1989). In thi 

model. large earthquakes may be pos ible even wi th a 

reduced rate of subduction (for example, sec Heaton and 

Kanamori, 19 4; Spence. 1989). Contn ry to thi · in t rpretation, however, others b lieve that evidence ex ist. that a very 

outhful . ubducting pl at and relati vely low rate f subduction could crea te a condition of aseismic subducti n (Kanamori and Asti z, 1985). 

0 'IPARI SO S WITII S LlD TION-ZO 'ES 

WORLDWIDE 

To variou degrees. each ubducti on zone i · unique, as 

noted by Heaton and Hart zell ( 1986) and Spen e ( 19 9). 

onctheless, one wa f assessing the lik lihood that the 

Cascadia ubduction zone can prod uce gr at earthquake is 

to compare its characteri sti cs to oth r acti ve subducti n 

zones worldwide. Heaton and Kanamori ( 19 4). Heaton and 

Hart zell ( 19 6, 1987), and Rogers ( 19 8a) found that the 

Ca cadia . ubduction zone . hares many charac t ri . tic wi th 

subduction zones off outhwestern Japan, south rn hile, 

and olombia. all zone. that ha e produ cl great earthquake hi toricall y. ome of these harac tcristi cs arc: ( I) an 

age and plate convergence rate that. combine I, group thi. 

zon with other that have produced great earthquake ; (2) 

Ia k of an active ba karc ba in ; ( ) abs nee of . eismicity in 

the Benioff zone below abou t 80 km : (4) lack of a welldefined offshore trench; and (5) a shallow-clipping Benioff 

zone. The e . tudie give detail comparing a cadia wi th 

other ubduction zone . We ite the Ri vera subduction z ne 

off the west coast of Mexico a an important exam pi ha ing 

propertie very similar to the acadia sub luction zone. The 

Ri vera and Juan de Fuca plate arc imilar in size and hare 

everal other ignificant characteri ti . (table 4). The most 

significant difference i that the Ri vera plat may have produced a urface-wa e magnitude (M s) 8.2 earthquake in 

1932 (Singh and other , 1985), yet the Ju an de Fuca thrust 

fault has produced no ignifieant earthqu ak . in the pa t 200 

years. Unfortunately, thi analog i ambiguou. becau e ome 

uggest that the I 932 earthquake occurred on the Coco 

thru t fau lt south of the Rivera pl ate (Ei : lcr and M e all y. 

19 4: Wa hington Public Power Suppl y Sy. tcm, 19 ). 

ELEVATIO CHA GE :SUDDEN S BMERGE ' E 

A DTILTOFTHE EA RTH'SS RFA CE 

In ac ti ve locked ubducti on zone . the compre . . ion of 

the continental margin lead to ecular change in elevati on 

and shortening of the crust (fig. 68 ). Some elevation change. 

may ccur abruptl y at the time of a great earthquake. ecular 

change in the til t of the Earth' urface at ome location 

may also accompany the elevation change. Becau e of the 

complex ity of ele at ion and tilt ac ti vity a a function of geographic position. the dip of the fault. the po ition of the 

locked part of the fault, and the rheologica l properties of the 

continental rocks and fau lt zone, the ob. crvations at different 
EARTHQ AKE HAZARD l THE PACiFIC ORTHWEST - A 0 ER lEW 23 

Table 4. Compari son of subduction-zone characteri stic of the Juan de Fuca and Rivera tectonic plate . 

I From Rogers ( 1988a) I 

Characteristics 

Age of subduction plate (millions of years) 

Convergence rate (em/year) 

Contemporary seismicity on the thrust fault 

Contemporary seismicity on the spreading ridge 

locations may appear contradictory, leading to differing 

interpretations. Progre s i being made, however, in developing models that expl ain the wide range of ob. ervations. 

The . tronge t ev idence for seismic subduction comes 

from three ourccs . Fir t, multiple equcnces of buried peats 

in coa tal interti dal lowl and appear to have been caused by 

sudden submergence of coas tal areas (Atwater, 1987; Grant 

an I M cL aren. 1987 ; Atwater, thi s volume; Peter on and 

Darienzo, thi volume). Second, thin sand layer overlie 

some peats, ugge ting that ra1 id submergence was followed 

by a t unami . Third, as many as 13 epi sode. of offshore turbidites occur in the Ca. cadi a Seachannel, Astori a Canyon , 

and at two ites off Cape Blanco, Oreg. (Adam , thi s volume). Turbidites are lumps of scan or sediments that liquefy and now over larg di Lance . Excessi ve sedimentation 

or earthquake shaking could trigger turbidite now (Adam , 

1990). The igni ficance of the. e 13 turbidite i that they all 

lie above the M ount M azama a h (a widespread volcanic 

depo. it about 7,-oo year old) (Adams, this volume), and 

they come from Cascadia Seachannel tributarie having 

widely separated sources, evidence that turbidite occurred at 

different locati ons at about the same time. Fourth, landslide 

or other ground failures occur in several places that may temporall y correlate with sub idence (Schu ter and others, 1992: 

Willi am and Jacoby, 1989; Jacoby and Willi am. , 1990). 

From . tudies f estuar depo it . ev idence in the form 

of buried peat dep sit. ha. been found at about 50 radio arbon-dated locati on. bowing that coas tal lowland have 

undergone epi odic. rapid subm rgen c ( twater. 1987 : 

~ ater, thi s volu me; Peters n and Darienzo. this vo lume). 

Some of the e locati on al o ha c 300-ycar-old tree tu mp 

and . nags now surrounded by . alt- and bracki. h-water tidal 

marshe . . Because the. e trc mu t have grown above the 

reach of sa lt water. it is li ke! that s a-le el ri or Janel ubidcn e killed th m. Tree-ring growth 1 att crn s and the pre -

cnce of intact gra .. in the buried peat deposit ugge t that 

sudden ubmcrocnce of a. tal lowlands mo t like! kill ed 

these organi sm. (Atwater and Yamagu hi. 199 1 ). 

If the ela ting of submergen c event , turbidite depo. it 

liquefacti on. and ancient Janel. lid . showed common age. at 

many place. in a evcral-hundrcd- kilometcr north- outh ccti on of the subdu tion zone. the evidence f r great 

Juan de Fuca pl ate 

7 

4.5 

A eismic 

early aseismic 

Ri vera plmc 

10 

2.5 

early asci mic 

early aseismic 

ubducti on-zone earthquakes of M 8 or larger might b conclusive. Ev idence for ground . baking (in the form of liqu -

faction. land. lides . and turbidit y depo. it ) that ' a 

synchronou with rapid coa. tal ele ati on hange would b 

parti cularl important data . UJ porting earthquake occurrence. A. . hown by cl. on ( 1992<) and t\· ater ( 1992), 

however, carbon- 14 age. may di ffer b centurie . making 

the temporal correlation of di fferent tyJ . f c ent. at many 

. ite hard to tc. t. The ombined diffi culty of obtaining . ampies representati ve of the age of a parti cular subsidcnc or 

ground-failure e ent and or ambiguity inherent in age-dating 

technique produce the ob er cd . cattcr in the age determination . . Of course. uch ca tter could, in part. also indicate 

inadequacy of the earthquake mod I a umcd. onetheles . . 

the data suggest a orrelati n in the time of occurrence of 

omc of the c c ent 

For example, Atwater and other ( 199 1 ), taking 

advantage of more accurate age estimates afforded b highpreci ion carbon- 14 dating technique and tree-ring data, 

concluded that sudden submergence in area. 55 km apart 

wa the re ult of coord inated . lip on one fault or one . y. -

tem of faults. Becau. c the . ubducti on-zonc thrust fault i. 

the onl y fault recogni zed a ex tending thr ugh the cntir 

55-km-long area, At\ atcr and other ( 199 1) a ociatcd the 

·ubmcrgence event ,. ith a ascadia thru t-fault earthquake. 

The high-preci ion rad iocarbon dating fi x d the time of thi s 

submergence in the inter al A.D. 1695- 17 10 (95-percent 

confidence interval: 296-2 I YBP) (Atwater and other. . 

1991). referred to below a time A . Le preci e age indicate that uclden submergence oc urred about thi time at 

other lo ation. along the Wa hington and Oregon coa t. 

sugge ting that the 300-year-old earthquake ruptured more 

than 55 km of the thru t fault. 

The record of probable tsunami depo it in es tu ary ediment i important ev id nee of prehi tori c large canhquakes. M odeling the geographic di tribution of sand atop 

the 300-y ar-old buried peat (time A) at Will apa Bay . Wa h., 

it landward deposition pattern , and amount of sedimentation, R in han and Bourgeoi ( 19 9) found that a I O-m-high 

t. un ami could have pr duced th is sand d posit. wherea 

nooding by ri cr di. charge wa an improbable cau e. Atwat r and Yamaguchi ( 199 1) argued that the landward thinning 
24 ASSESS ! G EARTHQ AKE HAZARD D RED I 'G RISK 11 THE PACIFI ORTHWE. T 

of the and de1 o its and their oar:en s. suggc. t .. an ex traordinary. landward-directed su rge that compri cd pul c. and 

approx imately coincid d with rapid . ubmcrg nee of coast 

above the Cascadia subduction zone .. (see also At water. 

19 7: Darienzo and Peter. on. 1990). 

Approx imately coin ident timing of geologic event 

older than time A may cx i. t at scauered locations (William and Jacoby, 1989). For example, approximate coincidence of . everal types of geologic event (su h a. udden 

ubmergencc, sudden uplift, land licling, and sand nting) 

ex ists at two separate age , about 600- 1,700 YBP (time B) 

and I ,400- 1,900 YBP (time C, po sibl y two events in thi 

age range; Atwater, 1992). Although Atwater ( 1992) quote 

an age range for each event. time B has been referred to a · 

the I , ISO Y B P event. and e ent ha been referred to as 

the 1,700 YBP event (Williams and Jacoby, 1989). Lack f 

coseismic . ubsidence at the Copali s Ri er. Wash., estuary 

(fi g. I) at time B raise a que tion concerning wh th r thi s 

event was produced by movement on the pl ate-boundary 

fault or another fault. Subsidence is b ervecl . however. at 

other coa. tal Washington and Oregon . ites at time B 

(Atwater. 1988b: Grant , 1989; Darienzo and Peter. on, 

1990; Peterson and Darienzo, thi s volume). The Copali s 

Ri ver site docs have ev idence for vented ·and deposits that 

are approx imately co incident with time B. but Atwater 

( 1992, p. 19 17) attributed thi s to folding and (or) faulting of 

deep aqui fers rather than shaking-induced liquefacti on. 

Land. I ides al. o occurred at times B and at Lake Washington (Willi ams and Jacoby, 1989: P.L. Willi am . 

Lawrence Berkeley L aboratory, written comm.. 1992). 

Landslide at about time B formed within a 30-km rad iu. of 

the Sadd le M ountain ea t fault in Wa. hington ( chu. ter 

and other , 1992). Thi s fault and other. nearby may have 

also slipped at time B (Wil son, 1983). plift at Restorati on 

Point and Lynch Cove occurred at time 8 (Bucknam and 

Barnhard. 19 9; Bucknam. 199 1 ). These geologic c cnt 

could have resulted from a subduction-zone earthquake, 

one or more continen tal-cru t earthquakes. or a temporal 

clu. tcring of both types of earthquakes. 

Geologic data at a few localities suggest that some vertica l oa tal mo ement may have interpretations other than 

great earthquakes. For example. buried e tuary peat. at some 

localiti . can be auributed to localized tectonic processc or 

sed imentati on ( clson. 1992b; elson and Per. oniu . . thi 

vo lume). Muhs and others ( 1990) al. o noted that local truelure can affect the amount and rate of uplift. Although 

explanation of these data doe not alway require great . ubduction-zone earthquakes, the data do impl y that active 

deformation of coas tal region is ongoing. Thi deformation 

is likely to be in re. pon c to acti ve ubduction. 

Much ev idence cx i. ts for tilting of the Earth ' surface 

in the Pacific orthwc t. Geologic evidence ex ists for longterm eastward tilting of coastal region (Adams. 1984), and 

geodetic (for example. Rcilinger and Adam s, 19 2) and ealevcl data indica te that ca tward tilting of these region 

continues at present ( dam . 19 4). In earlier studi s, Ando 

and Balazs ( 1979) interpreted the down-to-thc-easttilt of the 

outer coa tal zones in Washington and Oregon, the falling 

ca lc el at cah Bay, stori a, and Crc. ccnt City (fi g. I). 

and ri sino . ca le cl at caulc and Friday Harbor with a 

model of coa. tal uplift and inland sub. idcncc that was on-

. istent with a eismic subduction. Their model required 

clown-to-the-we ·t pre ei mi tilt and ubsiclencc of the leading edge of the continental pl ate proclu eel by aseismic . lip 

along the thrust fau lt. They concluclecl , based on comparison 

' ith data from Japan. that landward tilt i. incon istcnt with 

. ei mic ·ubcluction. 

In contras t to ndo and Balaz. ( 1979), other: interpreted the elevation and tilt data to indica te seismi c subducti n. For example. Adams' ( 1984) interpretati on of these 

data is for sci mic accumulati on of incla:tic deformation 

ncar the plate margin and ela ti c deformation farther inland. 

consi tent with a locked thru. t fault. avage ( 1983). Weaver 

and Smith ( 1983), and Darienzo and Peterson ( 1990) suggc ted that some regions of a coast may undergo prcscismi 

sub. idence while oth r coas tal z nes ri se (fig. 68 ). The 

mo ement in a parti cular region depend. , at leas t in part. on 

it location with rc. pec tto the crest of the uplift. th geometry of the thrust fault. and the position of the locked zone 

(Atwater, 1988a; We t and M e rumb. 19 8b). a age and 

other ' ( 199 1) modeling of . train and elevation data wa 

ba eel on acti ve subducti on and a locked thrust fault (further 

eli . cu ion below) and wa in accord wi th the data. Vincent 

and other ( 1989) and Paul Vincen t ( nivcrsi ty of Orcg n, 

writ! n com mun., 199 1 ). using a combi nation of north- outh 

and ca. t-wc t le eling data, al o concluded that these data 

upport subduction wi th trong intcrplatc c upling. 

On some coa t adjacent to other . ubduction zone . . 

ancient beach erosional feature . termed ··marine terrae .·· 

ha c been upl ifted when large thru. t-fau lt ea rthquakes 

occurred. The scarcity of uplifted Holocene marine terrace 

along the coas ts of Oregon and Washington (Laj oie and others. 19 2) ha been u. cd as an argument that the a. cadi a 

subducti on zone i. not seismic (West and M e rum b. 19 8a; 

Sykc . \ 989). Late Quatcrnar terraces do ex i t at omc 

Pa ific onhwe t locati ons, uch a. Cap Blanco. Oreg. 

( Kel ey. 1990). and the Cape Arago-Bandon region (fi g. I) 

(M clnell y and Kel. cy, 1990). A lthough Kel ey ( 1990) found 

ev idence of a Holocene uplifted beach berm at ape Bl an o. 

the five terraces at ape Blanco and Cape Arago arc of late 

Quaternary age and not Holocene. Th terraces were 

deformed and tilted. and the berm wa uplifted as a result f 

local folding duri ng the Holocene at the two locati ons. The e 

feature ma be co ei. mi . secondary deformation related to 

. ubducti on, a . uggc. ted by M clnell y and Kcl ey ( 1990) and 

Kelsey ( 1990). 

Terrace are not alway pre ent at locations havi ng hi -

tori ca l great earthquake . uch as southern Chile (Heaton 

and Hartze ll , 1986). Other factors such a high coas tal ereion rates in the Pacific 1 onh west could have ob cured 
EARTHQ AKE H ZARDS I 1 THE PA IFI ORTHWE T- N 0 ER lEW 25 

terrace (Heat nand Hart zell, 1987). Furthermore, the po. ition of the ere. t of uplift at the ascadia . ubduction zone 

may be such that rapid ubmcrgcncc of the coa t occur at 

some locati on at th time of the earthquake. wherea uplift 

i required to produce marine terrace 

Evidence for pa t and pre ent changes in ele ation and 

land tilt from geologic and gcod tic data strongly ugge t, 

but do not prove, that multiple prehi tori earthquake 

al ternate! graduall y upli fted some coa. tal areas between 

earthqu akes and abruptl y ubmergcd the same zones when 

each earthquake occurred. That measurable ongoing uplift 

and tilt can be modeled by convergence and a locked thru t 

fault . trongly suggest but doe. not prove that such events 

will be repeated. 

STRAIN-RATE MEAS REME 'T . CRU'TAL 

SI-IORTEN l t G, AND FOCAL MECI-IANIS I 

From strc s- ri cntation and strain-rate mea uremcnts, 

the state of interplatc coupling between the Juan de Fuca 

and orth America plate can be in ferred. High train rate. 

and the directi on of greatest compressive stre. s being parallel to the plate convergence wou ld impl y converging pl at • 

a locked thrust fault , and the potential for large earthquake . . On the other hand. I w train rate and (or) compre. si vc strc direc tion not parallel to con ergcnce may 

im1l y weak interpl ate coupling and fewer earthquakes or 

even a. cismic subduction. 

Data on trc ·. directions come primaril y from three 

source :earthquake f ca l mechani m , train measurements. 

and borehole breakout . Earthqu ake focal mechani sm provide an estimate of the principal stre. s orientati on by fining 

model. of earthquake ground-motion radiati on to observed 

first-motion directi ons and amplitudes. The average focalmechani sm stre s directions are com mon ly a umed to indicate the principal tre s ori entati ons at earthquake depths. 

"Borehole brea kout" i. a term that refer. to elliptica l elongati on and cracking of the wall of a borehole under stress. The 

. hort and lon11. axe. of the ellipse ali gn parallel to the grcatc~t and lcast-hori.wntal c mprcssivc . trcssc . . rcspccti cl . 

Like st rain measuremen ts, borehole breakout. rcfle t the 

. tress orientations n ar the surface. 

avagc and others ( 199 1) (. c also Sa age and other.. 

198 1: avag . 19 9: and avagcandLi owski.l991 ) found 

that alth ugh strain rate. arc IO\ (IO\ cr in fact than indicat d 

by earlier mea. urcments). the strain-rate data from both 

Pu get Sound and the Olympic Pen in. ula were pr dieted by a 

model having the thrust fault lock d off. h r and abou t 4 

em/year convergence bet\ ccn the Juan de Fu a and orth 

mcri ca plates. Thi. model predicted low . train rates and 

agreed with the obs rved uplift rates in western Washington 

d tcrmincd from tide gauges. The . uc css in fining thi s variety of data i. c. pcciall y significant b ·cau c parameter. u ed 

in the model were determined independentl y. rather than 

fitting the obscr ation. by parameter ariation. Still , the 

obs rvati on of low ·train rate .. the limited oeographi c ·tent 

of strain measurement.. and the pos ibilit that p nnancnt 

deformation of the lymJ ic Pen in. ula produce. th 

ob erved . train effects preclud s the uncqui ocal interpretation of a locked ubduction zone. 

Oth r interpretati ns of the train data als sugge. t that 

the two plate. are actively converging and lo ked . Mclo h 

( 1987, 1989). using finite-el mcnt technique , modeled 

uplift and . train data and con ludcd that a locked thrust fault 

and plate convergence fit the data best. Other modeling . tudie. su11 on thi s conclusion. For c ·ample, analo, trcs. modcling (Fox and Engcbret. on, 19 3) of a lo ked thrust fault 

predicted . omc aspect. of th young geologic d f rmation 

hi tory for the pa. t 7 million ca r. . 

On the other hand, ros. on ( 19 6. 1990) and M a and 

others (thi s volume) interpreted obs rvation of low h ri zontal strain rate. cpi odic strain rate. and north-. outh omprcssion in the Pu o t ound region a arguments for aseismic 

ubduction. Cros. on ( 1986) al. o argu dthat . train-rate rTor 

bar arc comparab le to the observati n. themselves and. 

therefore. cannot be u. eel to unequivocall y upp n a locked 

subduction zone. 

Other apparent conflict ari e in the ompari son of 

tre and train-rate principal . tre direction . . The direction of om pre . ive . tr s inf rrcd from f ca l mechani sms 

differ. substantiall y from that inferred from . train-rate meauremcnt. . The strain-i nferTed direction of great t compression approx imately parallel the direc ti on of convergence 

between the Juan de Fuca and rth America pl ates. In ontras t, the direction of greate l comprcs i c strc s inferred 

from earthquake focal mechani sms, oc urTino in either the 

continental plate or in the Juan de Fuca plate (> e, t of the 

thrust fault), i ommonl y perp ndi ular to the c nvergcn c 

direction. Focal mechani . m for earthquake in both pl ates 

general! , uppon a north- outh com pre .. i e tres. direc ti on 

that characteri ze the int racti on between the Pa ifi and 

North America 1 lates, a, obs r ed along the San Andrea 

fault in aliforni a. Thi s di crcpancy ha been u eel to argue 

again ·t ac ti ve subduction. !though the eli crepancy 

bet ween the orientation of focal-mechani. m tre c. and 

principal . train-rate direc tion. i. uncommon in locked subduction zones around the world ( da and Kanamori , 

1979). \ e not one xccpti on to thi. generality along the 

ankai trough off the coa l of Japan. Here, the c nvcrgence 

direction bet\ cen the Eura ia and Phili1 pine plate i. northwe ·t. butt he compr ss ive ·tre s direction inferred from focal 

m chani. m , : train rate . and geologic deformation is eastwest to ca 1-. outhea t/ w · t-nonhwest (Tabci. 19 9). Thi 

subducti on zone. however. ha a documented hi story of great 

anhquake Furthermore. the apparent difference in . tres 

direc tion · ma b b cau e we are comparing orientation. 

deri ved from ·tr s (strain) rate wi th orientati on r lated to 
26 ASSE I G E RTHQ AKE H Z RDS D RED I G RISK I TH E P IFIC ORTHWE T 

This difference was noted by M el h ( 1989). who 

attempted to reconcile the tre s-direc tion connict with a 

mod I in vol ing ela. tic rebound of the continental cru. t. In 

the earl y to middle pha e. of the earthquake cycle, the continental pl ate may be in a . tate of ex tension becau e of ela.-

tic rebound of the continental plate fo llowing the last 

earthquake ( ndo and Balaz , 1979; pence, 1987). In thi s 

phase of the . ubduction cycle, the stre orientati on would 

rever.c o that the directi on of ex tension i. parallel to the 

dire tion of convergence, but becau e con ergence continues, a compre . ional train rate is ob er ed al ng the direction of extension. As the ycle continues. the rebound i. 

ab. orbed a the continental plate recomprcs e . Late in the 

cyc le. earthquake- and train-inferred compre sive-. tres. 

direction would coincide, and a thru st-fault earthquake 

would ultimately occur. 

pence ( 1989) ugge ·ted that a focal-m chani. Illinferred com1 ress ive- trc . directi on not parallel to the conergcnce directi on i the re. ult of temporall y varying stres -

field uperpositi n. Spence ( 19 9) argued that sti·esse 

rencc ting the Pacifi c-Juan de Fuca pl ate interaction and slab 

pull are tran fcrred to the 1orth Ameri ca plate through a 

locked thru t faulr. In trc . -field modeling studie . Spen e 

( 19 9) assumed that the trc produced by the Paci fi - orth 

Ameri ca plate interacti on pre cntl y predominate over 

tre. e related to lab pull. pence' ( 1989) model that best 

fit ob erved north- outh pr s ure axes onl y require moti n 

of the orth Ameri ca and Paci fie pl ate., with a pas i ve Juan 

de Fuca pl ate as umecl . ubcluction is not required, although 

the mod I implicitly permit ubcluction when trc e 

induced by lab pull , by accumulation f ridge- preacling 

forces over time or by overriding of the continental pl ate. 

become ufficientl y great. Thi model qualitati vely account 

for the difference · between train- and focal-mechani sminferred pre . ure orientation. 

Becau e of the complex nature of deform ation in a . ubduction zone, ex ten ion commonl y occur in the continental 

cru t during ac tive ubduction. E idence exi st. that cas twe. t ex tension and north- outh compre sion may be a pattern that peri ted during a significant part of Cenozoic time 

(62- 12 M a) in Oregon and southern Washington. Well 

( 1990) interpreted clockwi se paleomagneti c rotation of 

forearc. arc, and backarc rock with a complex model of 

clockwi. e blo k rotati on in the arc and forearc region . 

Well ( 1990) inferred that, in Oregon, the rotati on produced 

equal amount of ex ten ion and ri ght-lateral hear. whereas 

in Wa. hington mailer rotation were proclucccl primaril y by 

right-lateral shear. The orientati on of the infeiTed shear is 

about north we t. Well s ( 1990) cited the M ount St. Helens 

. eismi c zone as an example of contemporary deformation 

fitting thi model. Observed temporal clockwi e rotati on of 

dike and normal-fault . tri ke direc tion into Quaternary time 

led Prie t ( 1990) to observe that arc extension may continue 

t the pre. cnt and that the direction of lea t com pre . ion is 

about eas t-we ·tin arc and forearc rocks. If thi s deform ati on 

per i t to the pre ent cl ay , one can infer that the orth 

meri ca-Pacific plate interaction dominate the cont mporar tre pattern , producing oblique subducti on and backar ex tension (Well , 1990). 

In our view, the evidence laroely favors a loc ked thru t 

fault with the potential for a great subcluction-z ne earthquake. We acknowledge, however, many counterarguments 

(beside the reference cited , sec also Washington Publi 

Power upply y tem. 1988). 

MAXIMUM MAGNITUDE EARTHQUAKE 

ON THE CASCADIA THRU T FAULT 

The question of maximum magnitude CMmax• a. sumed 

to be the same a Mw) on the ascadia thru. t fault i of vital 

concern. Earthquake size or magnitude i one of the most 

significant factors relating the level , duration , and lateral 

extent of trong haking. The length and downdip width of 

the rupture determine the earthquake ize and, thu. , the level. 

duration, and geographic ex tent of shaking. In a w rst-case 

ituation, the fault-rupture zone may ex tend the north-south 

length of the ascadia ubduction zone and eas tward to the 

Puget Sound-Willamette Valley lowland . Alternati ely, if 

the rupture area is limited in ize, th geographic area 

affected i proporti onately reduced and may lie farther from 

urban area than in the wor t case ituation. Bccau e th 

ex tent and po it ion of the fault ruplllre control many aspec ts 

of earthquake effects, it i. important that the mo. t reliable 

e timate pos ible be obtain d for thi s parameter. 

The di tributi on and amount of lip on the fault also control the magnitude. haking levels, and duration of shaking in 

an earthquake. These a pect of the earthquake source, however. arc difficult to clct rmine before the earthquake occurs. 

Thus, em( irical relations b tween max imum fault-rupture 

dimensions and magnitude or other empirical data are common I u. eel toe tabli h the max imum magnitude. 

Difference of Of inion ex i t about M max that ar fundamentall y related to the interplate coupling of the Juan de 

Fuca and North Ameri ca plate , the segmentation of the a -

cadia . ubclucti on zone, and the length and clowndip width of 

the fault area likely to rupture in a single earthquake. ing 

worldwide data, the max imum e pcc tccl magnitude ha been 

empiricall y related to the ubduction rat and age of the ubducting plate (Heaton and Kanamori , 19 4). High rates and 

youth ful ubducting pl ate arc as ociated with the largest 

magnitude . Heaton and Kanamori ( 19 4) u ed thi ob. ervati n to empiricall y predict a max imum magnitude of .3±0.4 

for the Ca cacli a subducti on zon . 1 oting the imilarities 

between the a cadia and outhern Chile ubcluction zone . 

Heaton and Hart zell ( 1986) concluded that rupture of I ,000 

km f the Ca cadi a subduction zone could produce an earthquake as large a M 9.5 . Both Adam ( 1990) and Rogers 

( 19 8a) computed a max imum magnitude of 9. 1 for the Juan 

de Fuca egment u ing different as. umptions. R ger ' 
EARTHQUAKE HAZARDS I THE PAC.IFI t ORTH EST- At OVERVI E 27 

( I988a) estimate i based on fault area and rupture of the 

Cascadia thru t fault on th 900-km-long Ju an de Fuca segment from the ootka fault (fig. 2) on the north to the Gorda 

South pl ate. a uming a I 00-km fault width. The magnitude 

is related to the fault area by an empirical relation (Wy s. 

1979). Inclu. ion of the Gorda South pl ate in thi earthquake 

rai. es the maximum magnitude to 9.2. Adams ( 1990) 

assumed a on vergence rate of 45 mm/year and an average 

recurrence interval of 590 year to compute an average slip 

of 26 m per arthquake, if all lip is seismic. He further 

assumed that the fault would rupture 750 km from the 

Blanco fracture zone to the ootka fault with a I 00-km 

width. The magnitude was computed from the definition of 

earthquake moment and an empirica l relation between magnitude and moment. In the following di cus ion , we review 

the evidence that could limit the length or downdip width of 

the fault and, thus. limit the maximum magnitude to value. 

Ie s than those ju ·t di cussed above. 

LIMITS 0 ' MAXIM 1 MAGNIT DE 

RELATED TO FA LT SEGMENTATIO 

egmentation refers to the po sibility that the downgoing lab wes t of the thru. t fault is composed of two or more 

pl ate eparatcd laterally by frac ture zone . Segmentation 

raise. the po ibility that the plates . ubduct independentl y of 

one another. Vari ations in interpl ate oupling along the fault 

trike may al. o lead to earthquake. that rupture limited sections of the fault. The zone of trong interpl ate coupling are 

termed '·a peri tie : · and earthquake can rupture one or more 

asperities. Thi s mode of rupture leads to the po sibility of a 

range of pos ible earthqu ake izcs, where very large event 

that rupture through several a peritie are the least likely. 

Commonl y. segmented thru t fault rupture onl y one segment in an earthquake, although ea rthquakes have occurred 

with probable multiple-. egment rupture in a single large 

event (Ando, 1975; Kanamori and M e all y. 1982). a in the 

I ,000- km-long 1960 Chile rupture. There are no examples 

worldwide. howe er. for the rupture of an entire subdu ti on 

zone in one earthquake (Spence, 19 9). a c of ""fault rupture contagi n·· (Perkin . 1987) also c ·i. tin which rupture of 

a . egmcnt leads to a equence of ru pture. of adjac nt egmcn ts at ti m inter als rangino from econd. to centurie. 

(Kelleher. 1970; ogi, 1974; ndo, 1975). The magnitude. 

of a sequence of carthqu ak s that oc ur cr an ex tended ti me 

arc likely to be . mailer than that of a . inglc . nchronou rupture of e era! segment. o cut-ring in . evcral minute . 

Alth ugh . egmcntati on of the a cad ia thru. t fault doc not 

ca tegori ca ll y limit the max imum magniwdc. it uggest the 

po. sibility of a range feanhquakc . ize on the fault and. perhap . less frequent multi segmcnt great earthqu ake . Great 

earthquake. with magnitude. ex cdin g M .0 are po . ible. 

h wever. with the rupture of indi vidual segments. 

Some data . ugge t prchi. tori c ruptur aero. egment 

boundaries on the Cascadia thru t fault. For example. 13 turbidite c cnt that occun·ed after dcpo ition of th M ount 

M azama ash on the Juan de Fuca and G rda pl ate (Adam., 

1990) may have be n . ynchronou., which suggest that 

wide pread baking triggered the:e c ent . Furthermore, the 

most recent e tuary- ub id n c cv nts in Wa hington are 

approximately . nchronou. with slip on maj r on. bore 

faults in northem Califomia ( arvcr and Burke. 1987a: 

ar er, 1989). All of the e event. ma be a re pon e to 

motion on the thru t fault (Cl arke and ar r, 19 9). If rupture of the. c and other onshore fault occurred at the amc 

time a abrupt oa tal sub idcnce. th large geographic pa -

ing between the e ent . uggest .. lip on more than on thrustfault egment. Of course, everal alt rnati ve inteq retation. 

are al. o po ible. including indcpend nt rupture of ev ral 

. egments at about the same time. 

The vidence for a . egmcntcd asca lia thru st fault a!. 

appear strong. It is likely. for e ·am1 lc. that the Gorda pl at 

on the outh and the Expl orer pl ate on the north cparated 

from the Juan de Fuca pl ate about 4 million YBP (Riddihough.l 9 4).AL o.the750-km-long Ju andeFuca cc ti n of 

the a cadia thrust fault, > hich ex tend from the 1 ootka 

fault on the north to the Blan o fracture zone, may be further 

egmented. M organ ( 196 ) how at least fi e fault. in th 

Juan de Fuca plate offsetting magnetic lineament . Like the 

Juan de Fuca ridge and the a cadia thrust fault, ho> e cr, 

these fault , if ei micall y acti ve. are acti ve at very I w rates 

and (or) low magnitude level . 

egment boundar ma cx i t at lat 46° . (M agee and 

Zoback. 1989: oppersmith and Young . . 1990). A change in 

·tre . direction and upli ft rat . has been bserved between 

!at 45° I and !at 46° . that could be related to co mentati on 

(Mitchell and other.. 199 1: Palmer and other. , 199 1 ). From 

borehole brea kout . focal mcchani. m ·. and other data, 

Weaver and mith ( 19 3). Zoback and Zoba k ( 198 ), and 

M agee and Zoback ( 1989) in ferred that the directi ons of 

greates t hori zontal stres and pl ate convcroencc arc similar 

in Washington north of I at 46° . and we:t of th a. cadc 

Range. indi ca tin g a lo ked thrust fault in that zone. Their 

data arc con i tent. how vcr. with a di fcrcnce between these 

directi ons from south of lat 46°N. to the M cndocin fracture 

zone (fig. 2) ( ee al. Wemcr and other . 199 1 ), which wa 

inferred to indicate weak interplate coupling between the 

subducting lab and the continental rust outh of lat 46° . 

in Oregon. In contra t to thi interpretati on. M a and others 

(thi s volume) oncluded that the data do not require a rotation in the comprc. sivc stress orientation outh of lat 46° ., 

although their anal i doe not prec lude a rotation. 

idcscan onar, eaBeam bathymetry. and single multichannel ci. mic record how three to even left-lateral 

trike-. lip fault. off the oa. t of central and northem Oregon 

that may be ac ti e egment boundari s in the Juan de Fuca 

plat (Gold finger and other . 1990: Kulm and others, 1991: 

Goldfinger and other . . 1992: Goldfinger and others. this 
28 ASSESS! G EARTHQ AKE HAZARDS D RED CI G RISK I 1 THE PACIFI ORTHWE T 

olume). The. e faults . trike . 62° W . to . 7 o W .. ubparallel to the Blanco fracture zone, and there i. ev iden e that 

lip on omc f the faults at some pl ace. occurred during the 

Holocene (Kulm and others. 199 1 ). Although Goldfinger 

and other (thi volume) have in ferred a block-rotation 

mechanism for the off horc left-lateral fault , they al. o note 

th at other interpretations arc poss ible. Much remain unclear 

about the kinematic sionificance of these faul t . Well and 

oc ( 19 5) interpreted older onshore fault in south we tern 

Wash ington of . imilar orientation and slip ty le (\ estnorthwest and left lateral) as a econdary response (Riedel 

shear ) to clockwi e block rotation within a maj or . ys tem of 

north-northwest-. triking ri ght-lateral faults. 

Pan of the Juan de Fuca plate downdip from the thru t 

fault may also be segmented (Michaelson and Wca er, 

1986), although the evidence on rhi point is not conclusive 

(Ra mu . en and Humphreys, 1988). From . cismi ity data, 

the dip directi on of the Juan de Fuca plate appear to vary 

from northeast under north we. tern Washington to about 

cas t-. ou theast under outhwestern Washington and n rthern 

Oregon (Crosson and Owens, 19 7; Weaver and Baker. 

1988) (fi g. 6A). These data . uggc t that the dip angle varies 

a. well , from about 15-20° north and south of an arch in the 

plate that extends beneath Puget Sound. to about I 0- 12° 

along the arch axi. . Recent eismic-rcncction tudi cs by 

Hyndman and others ( 1990) partl y confirm thi s interpretati on. In any case, Weaver and Michaelson ( 1985) interpreted 

the e data as two pl ate . egmcnts separated by a transition 

zone. In further analyses . Michaelson and Wea er ( 19 6) 

ugge ted that the Juan de Fuca pl ate may be cgmentcd in 

three sections in Wa hington and northern Oregon between 

about I at 43°N. and I at 49° Based on these data, the maximum egment length is about 250 km , and rupLUre of thi s 

. egment could produce an earthquake of M max 8.25±0.25 

(Washington Public Power Supply System, 1988). Rupture 

of the Gorda outh pl ate egmcnt could produce a M max of 

8.4 (Clarke and Carver, 199 1, 1992). nfortunatcly. we do 

not know whether segmentation of the lab below the thrust 

fault implic. segmentati on of the slab in the locked thrust 

zone and (or) we t of the thrust fault. At present. it is also 

unclear whether plate geometry, uch a the bend in the lab, 

could control the lateral extent of faulting. 

Seri al rupture of Cascadia thrust fault segmen t \-vould 

produce discordant sub idence that might be detected by 

high-preci ion dating if the eli cordance exceed a few 

decade. (Atwater and others. 1991 ). In fact. con iderablc 

variability in subsidence-e ent age is observed. but the 

vari ability among age at indi vidual localitie is commonl y 

as large a. the variability among localitic (A twater, 1992; 

elson, 1992a). 

Peters n and Darienzo (thi s volume) argue that the 

presence of probable tsun ami -generated . ands onl y on the 

tops of buried peats rather than within the intervening baymud layers is inconsistent with . egmcntcd ruptures becau. e 

rupLUre. of adjacent or even more di tant segment would 

likely produce t unami deposits interspersed in the peat horizons of the nonrupwring segment. We believe, however, 

that egmcntcd rupture with close temporal . pacing could 

also produce and only at the top of the peat , and the rupture of one egment might not produce t. unami large 

nough to breach bay- mouth bar in ·~ dja cc nt or di. tall! segments, inhibiting the depo it ion oft. unami sand in bay-mud 

deposit. of the adjacent egment . 

Th i. sue of egmen tation and it bearing on max imum magnitude remain unre ol d. !though Cascadia 

thru t-fault . egmentation i. likely, at pre. ent no argument 

r data preclude ruptur of multiple segment . For the 

present, a maximum rupture length of 800- 1.200 km 

hould be con idered pi au ible. but we ha e littl idea of 

the likelihood of uch an event. Conversion of the range of 

po iblc segment length. into expec ted max imum magnitudes require other as umption about the downdip wid th 

of th fault and the amount of lip. The max imum expected 

magnitude, even with . egmentation, ranges between and 

9+ ( opp r. mith and Young, 1990; U.S. uclcar Regu latory ommi sion, 199 1). 

VlHAT IS THE A TICIPATED DOW DIP 

EXTE T OF R PT RE'? 

The width of the fau lt (fig. 68). namely the downdip 

ex tent f rupture and the area over wh ich stick-slip di placement occurs, i. not well defined and i an important unrc-

. ol eel is ue bearing on the max imum magnitude of 

earthqu ake and the level of trong ground shak ing in inland 

urban area . The locked . ection of the faul t rad iates seismic 

energy at the time of . lip. Several competing views cx i t 

concern ing the . ize of the locked . ection f the thrust fault. 

The fault could rupture downdip f r I 00-200 km from the 

offshore deformati on front to the inter. ecti n f the thrust 

fault with the continental cru t-mantle boundary< t a depth of 

about 30-40 km (Wea er and Shedlock. 19 9). Others sugge t that the locked zone may ex tend from the trench eastward to the con tinental lope and outer hclf (Da is and 

others. 1990) or farther eastward to the outer coa t (Savage 

and other , 198 1; Weaver and Shedlock , thi s volume). Some 

of the e models propose the ex istence of a locked zone and 

tran. it ion zone (Clarke and Carver, 199 1: Sa vag and others. 

1991 ). In the tran it ion zone, which is a umed to ex i t 

downdip from the locked zone. both seismic and aseismic 

slip arc expected. In the ex treme view, no section of the fault 

lock (Byrne and Sykc . 1987). We review these hypothese 

in the following paragraph . . 

train data from near Seattle are consi tent with a 

locked thru t fault ex tending 120 km from the off hore continental margin to about the outer coast ( avage and other , 

1981 ; Weaver and Smith, 1983; Li sowski and others. 1987). 

M elo h ( 1987 . 1989) discu sed fini te-element modeling of 

slip and strain accumulation along the fault that al o fit s tilt 
EARTHQUAKE HAZARDS IN TH E PA IFI ORTHWEST- A OVER lEv 29 

ob. ervations and the record of sudden coastal ubmcrgence 

and gradual uplift. In thi s model. the ea tern limit of the 

locked fau lt i we t of the outer coast. 

In a tudy based on additional train-rate data from the 

Olympic Peninsula, Savage and Li sow ki ( 199 1) filled both 

the train-rate and the geologic data (inters i mic uplift 

rate at the coas t and cosei mi subsidence) well. Their 

model, wh ich a umed a on vergence rate of 4 em/year, 

required a locked fault ex tending from the offshore trace of 

the thru t fault about I 00 km ea twa rd. Farther eastward. 

the fault is a umed to be unlocked. but the continental 

cru t in thi region accumulates compre ional strain . The 

abrupt slip accompany ing the earthquake would occur in 

the locked zone but may propagate into the unlocked zone. 

In thi model, the total downdip ex tent over which ei. mic 

radiating sli p occurs i 175 km or le s. The zon of ei mic 

slip would be less than 175 km if . lip near the updip and 

downdip tips of the rupture occurred as rapid post. ci mic 

creep (J.C. Savage, .S. Geo logical Survey, oral commun., 

199 1 ). We infer from thi s model that the energy-radiating 

ection of the fault may extend ea t of the outer coa. t, well 

into the Olympic Peninsula. 

Argument. have been made suggesting that the Cascadia thrust fault i unabl e to ustain stre so er ecti on of the 

accreti onary wedge and at depth below about 20 km owing 

to properti e of the Juan de Fuca pl ate and condition along 

the fault zone. In the e interpretation , plasti call y deforming 

oft ediments and high pore pres. ure in the leading edge of 

the accretionary wedge limit the \ e tern ex tent of faulting 

(Syke and Byrne, 1987; Byrne and Sykes, 1987; Byrne and 

others, 1988), wherea . ubducti on of ignificant quantities 

of young sediment and unu uall y high temperature of the 

ubdu ting slab limit the eas tern ex tent of faulting. The 

youthfulness of the slab and the covering of sediments may 

produce a . lab temperature that prevents . ti ck . lip 

(Scheidegger, 1984: Dav i and other., 1990). The volume of 

ediment deposition in the trench, produced by the Columbia 

and Fraser Ri v rs, is rcater than that in some ac ti ve subduction z ne con idcrcd similar to thi s one. Th sc young ediment. not onl y thermall y in sulate the downgoing oceanic 

pl ate, but also insulate the accreti onar wedge. rai ing the 

w dgc temperature and. sub equentl y. the temperature at the 

thru t-fault boundar . Under su h conditions. the hallow 

sec tion of the thru t fault uld lip aseismi all y . In thi case. 

the locked fault\ idth. if a locked e tion cx i ·ts, i limited to 

the zon bel\· ccn the back. top (relati vely more competent 

continental rocks) and the depth at whi ch the fault eros e 

the 400°-450° i othcrm (the brittle-ductil e boundary. fi g. 

6 ). Thi boundary i. c. timated to b at a depth of between 

20 km (Da i and thcrs. 1990) and 35 km (Byrne and others, 19 ). Thi. limited sec tion of the fault zone probabl y 

lies beneath the continental lope and the utcr ontincntal 

he If. The locked secti on of the fault i narrO\ cr or noncxi tent, if soft cdimcnt. arc carried below the back top ( yke 

and Byrne, 19 7). The. c factors could ignifi antl y limit the 

iz of earthquake. on th thrust fault or. pcrhap . pre ludc 

th ir occurrence. 

Thi model rai cs e era! i. ue . Fir t. Sch idcggcr 

( 1984) showed a zone of .4 km!_econd- clocit y . cdiment. 

that thi cken eastward and o crli c the basalt at one lo alit ~0 

km we t of th ba e of the lope. He interpreted thi. highv locit y layer a thermall y dewatcrcd . dim nt. , hich presumabl y cou ld u tain . hear str·cs . Th s and oth r data 

argue again. t subduction of ·ignificant quanti ti c f . oft . ediment at ome localiti es . Sc ond, e tim ation f the depth 

and map po it ion of the 450° i. otherm on the top of the subducted slab i highl y d pendent on a number f as umptions, 

uch as accretion and cro ion rates. initial thermal oradient , 

and frictional heating on the thrust fault (Ba1T and Dahlen, 

1989). Finall y, the e. timated position of the 4. oo is th rm 

compu ted by Da i and other. ( 1990) applic on! to the 

region off Vancouver I. land . Because of th ar h in the . ubducting slab, the map positi n of thi s i. th rm on th top of 

the slab i pr bably well n. horc in the model of ros. on 

and Owens ( 1987) or n ar. horc in th model of Weaver and 

Baker ( 1988). 

Difference ex ist con rning the position of the lo ke I 

thru t fault relati ve to the back. top in ariou: models. Th 

possible cx i tence of multiple back tops and (or) differing 

d fin itions for thi term may be responsible forth se difference . For example, . omc sugge. t that th locked secti on of 

the fault lie updip from th back t p (Clarke and ar r, 

199 1; Savage and others, 199 1; Wano and Hyndman, 

199 1 ). I n the c model. . the accretionary wedge i. ascismi 

from it leading edge to roughl y 20 km downdip. In thi 

zone, the thrust fault als lip ascismicall y. A locked . ccti n lie further dO\ ndip that is of ariablc width as a functi on of geographic po ition. car the Ol ympi Pcnin ul a, 

the locked . ection may extend 95 km downdip (Wang and 

Hyndman , 199 1 ); in northern aliforni a, the locked secti on 

may extend 70- 80 km downdip (Clarke and arv r. 19 I). 

Do' ndip from the lo ked . cction. a tran itional zone may 

ex ist that can produce both aseismic and seismic slip. This 

zone also likely vari es in width as a function of latitude but 

may range between an 0- km length near the Olympic Penin ula to 65 km off Oregon (Wang and Hyndman, 199 1 ). or 

it may ex tend to the subducted pl ate knee bend (Clark and 

arver. 199 1) where the pl ate dip changes from I I o to 

about 25° (fig. 6C). 

Definition of the location and downdip width of the 

I eked zone i an important parameter required to model 

train and tre data and secular verti ca l motion of the 

coa tal region . Definit ion of the locked sec tion of the fault. 

however. may not fu ll y specify the fault secti on that radiate 

energy at the time of the earthquake (W ang and Hyndman. 

199 1 ). The locked ection is expec ted to radiate eismic 

energy wh n rupture ccurs: h we r, if the rupture propagate into unlocked fault sec tions either updip or downdip. 

the e section may al. o radiate energy. Thus, the locked fault 

secti on may onl y be a lower bound on the fault dimen. ions 

radiating ci mic energy. Very little is known about thi s 

a pcct of subduction-zone earthquakes. 
30 SSESSI G EARTHQ KE HAZARDS A DR DUCI G RISK IN THE PACIFI ORTHWE T 

M odel. that a . ume that the rupture zone extend to 

on horc region implici tl y . ugge t greater hazard to urban 

area . . Be ause the e model fit evcral kind of data, more 

weight . hould be accorded them compared to model that are 

ba. ed on indirect e idence and hypothesi. . verthele . . 

pecification of the fau lt width i. a matter of curren t debate 

not yet resolved. Continued monitoring of regional SLrain 

rates, broader train network coverage, and refined modeling 

technique arc likely to produce more certain es timate of the 

width and length of the locked fault in the future. Re earch 

is al o needed, however, to better define the relation hip 

between the izc of the locked zone and the ec tion of the 

fault that rad iate energy in subduction-zone earthquakes. 

EARTHQUAKE RECURRENCE 

Earthquake-recurrence estimate. are cx prcs ed in several way . . Specificati on of the prcci e time, location, and 

magnitude f the nex t earthquake i. termed ·'earthquake prediction ." nfortun ately. at pre. ent, method of earthquake 

prediction are . till being dev i ed and ha e not been adequately te ted. The nex t lower level of . pec ification i the 

ca lculation of probabi li ty of pecifi c earthquake magn itud s 

on particular fau lt for a given period. Thi s ca lculation 

depend on the hi. toric seismic record, th geologic paleoseismic record of times between events, and the time ince 

the las t event. Thi s level of pecificati on of earthquak 

occurrence has b en termed an "earthquake forecast. .. At 

present, offi cial earthquake forecasts are onl y available for 

elec ted fault. in Cali fornia (U.S. Geological Survey, 198 ). 

In the Pacific 1orthwest, studie are underway to determine 

paleosei. mic interval between event and the time incc the 

Ia. t e ent, data that are the basis of preliminary earthquake 

fo1'ccasts (. ee below). The lowest level of specification of 

earthquake recurrence is a mean rate of occurrence for a 

given magnitude level in a given region. When the logarithm 

of the m an rate of occurrence is plotted a a functi on of 

magnitude, the values form a traight line that i referred to 

a the Gutenberg-Richter recurrence relationship. In the 

Pacific rthwe t at pre ent, even thi. level of pecificati on 

i difficult because the hi tori cal record of earthquake in the 

region i. too short . 

It is worth noting that pec ification of a mean rate of 

earthqu ake occurrence, although u eful for . ome applicati ons. does not necessaril y de cribe temporal characteri stics 

of earthquake occurrence in spec ific cases. That is, 

although earthquake occu1Tcnce may fit the GutenbergRichter statisti cal law in a region that include multiple 

faults when averaged over sufficient time, earthqu akes in 

ubregion or on pecific fault commonl y recur with conidcrablc irregu larity. 

The di ver. c type of earthquake ources in the Pacific 

Northwe t c mplicatc the estimati on of earthquake recurrence in comparison with mos t other regions of the nited 

State . . Thru. t-fault earthquake , oceanic-plat earthquake . 

contincntal-cru t arthquakcs, and intra lab earthquake are 

each likely to have different mean recurrence intervals for a 

given magnitude. Even for a given ource type, subregional 

recurrence vari ation likely ex ist; for example, there are 

lower sei mi rates in wes tern Or gon relati e to western 

Wa hington and lower rates within the oceanic plates relati ve to those along their margins. 

Although we are very uncertain about the accuracy of 

cismic rate . w can compute e timate for each ource 

type. At pre ent, we mu t rely on scanty hi torical sei rni c 

data to es timate recurrence rates of continental-cru t earthquake . Ongoing paleoseismicity . tudie of individual urface fault may eventually yield recurrence-interval data for 

crustal prehistoric earthquake , at lea t for large events on 

selected individual fault . Sparse hi. torical earthquake in 

the subducted lithosphere pr vide a mall data ba e for estimation of recurrence rate of Benioff-zone earthqu ake 

Because no hi tori ca l ei mic data ex i t for thru t-type ubduction-zone earthquakes, estimati n of rccu1Tence interva ls mu t rel y entirely on plate-convergence-rate estimates 

and geologic data, . uch as the time intervals between rapid 

ub idence events (Atwater, thi. olume) or ubrnarinc 

. lope-failure event (Adams, thi volume). These cornplexiti e. add to our uncertainty about earthquake recurrence for 

each . ourcc type. 

Although th recurrence rate for crustal, Benioff zone, 

and great subduction earthquakes are not yet well e tabli hed, estimate have been calculated for several source 

type and . ource area (table 5). The earthquake magnitudes 

in table 5 are the magnitudes at which a given author specified the recurrence rate (in ome tudie , the auth r also 

inferred the e magni tude to be the maximum magnitude; 

notwith tanding these values, in our view, the di cu sion of 

max imum magnitude given above eems most plausible). 

These re U!Tence rate gi ve a rough basi for es timating the 

rec urrence of damaging earthquake for each ource type. 

Large cru. tal earthquakes (about M 6) have a mean retum 

period of about 200 years (Cro son, 1989) ba ed on seismic 

data from the Puget Sound region. ote. however, that three 

to four cru stal earthquakes wi th magnitudes between 7 and 8 

ha e occurred in the Pacific orthwe t ince the fir t documented earthquake in I 33 ( 1872 Washington. probably 

cru tal; 19 1 and 1946 Vancouver I land ; table 1). rudy of 

ancient earthquake on several indi vidual fau lts in northcm 

Cali fornia and southern Oregon indicate the average return 

peri od (M 7.6- 7.8) on the e fault is ab ut 500- 600 years 

( arver, 1989; Clarke and Carver, 1989). From the . mall 

ei ·micity data ba e, M 7.4 earthquakes in the Benioff zone 

have abou t a 200-year return period (Cros on. 1989). 

On the basi s of age dating of buried e tuary peat , ·ubduction-zone earthquakes occurred on average every 600 

years for the last 3,500 years but had irregular interseismic 

interval. (sec Grant and others, 19 9; Yeat , 1989; Atwater, 

thi. volume; Darienzo and Peter on, thi s volume). Turbidite 
EARTHQ AKE H ZA RDS I THE PACIFIC ORTHWE T- AN 0 ERV IE\J 3 1 

TableS. Recurrence interval for large earthqu ake in the Pacific orthwest. 

1 Values are in years. rounded to the nearest hundred; M is the magnitude at which the rate is cstimmed; leaders (---) indicme no estimate. This table shows the variability in returnperiods based on e ither historical seismi ci ty or geologic or strain data. Earthquakes may not recur at regular intervals; hen e, return period is a margi nally useful concept. Still. the 

variability in these va lues and the range about which these measurements cluster gives a sense of the time between events expected in this region based on these datal 

Intervals determined Mean intervaL determined 

by geologic studie by seismic studies 

Data source Subduction Benioff-zone Crustal earthquakes Benioff-zone and 

earthquakes eanhquakes (except ubduction) em. tal earthquakes 

Atwater (this vo lume), south coasta l Wa hington .... . . 300- 1,200 

Peterson and Darienzo (this volume), central coastal Oregon .. 300-700 

Clark and Carver ( 1991 ), Gorda South plate ....... . 150-550 (M 8.4) 

Heaton and Hartzell ( 1986) 1 . • . • • • . . . . • .•••••• I 00-250 (M 8.5) 

250-500 (M 9.5 ) 

Adams (this volume) .. . . ... . .... . . ...... . ... . 600 

Adams ( 1984) . . . . . . . . . . . . . . . . . . .... . 200-500 

Ba ed on Juan de Fuca plate convergence rates ...... .. ... . 400- 1,300 

(see tex t; and Rogers, I988a) 

Based on strain-rate measureme nts 

Savage and Lisowski ( 1991 ) ......... . .... ... ... .. . . 400- 1, 100 

50- tr in accumul ation s (see tex t) 550 

Cros on ( 1989), Puget Sound .............. . ... .. . ... . 200 (M 7.4) 200 (M 6. 1) 

Ra mu ssen and others ( 1974), Puget ound .. . .. . .. . . ... . IOO (M 7.0) 

Perkins and othe rs ( 1980), Puget Sound ...... .. . .. . .... . 100 (M 7.0) 

Perkins and others ( 1980), Will amclle basin ...... . .. ... . . 700 (M 7.0) 

1

Rates inferred from stra in measurements and (or) comparison with other subduction zones. 

deposition during about the last 7,500 years had an a erage 

occurrence rate remarkably similar to that deduced from buried e tuary peats. The uni form thicknes. of turbidite deposit. 

sugge t that ubduction-zone earthquakes happen at relatively uni form interva l of about 590± I 70 years (Adams. 

thi s volume). On the ba i of both data set , the most recent 

earthquake occun·ed about 300 years before I 990, although 

earlier dates do not always coincide for turbidite and . ubidence event . Approx imate or possible correspondence. 

however. ex ists for events at 300, I , I 00- 1,200, and about 

4.300 ear ago ( dam , thi olume). The lack of a cord 

between c cnt dates over the entire record for th two data 

ources may be r lated to their mea urement and interpretati on. For example, e era! processe . such a. ero ion or oxidation. could remove or ma. k me markers in the 

ub idcnce record. It i. al o piau. iblc that . ome large earth -

quake do not pr duce up! i ft at th co a t. a ha been 

ob erved for . ome e cnt in hil e. Furthermore, e timati on 

of the date of turbidite depo ition, ba ed on the thick ne s of 

layers that separate turb id ite depo. it and assumption about 

erosion of these layer . i onl y approx imate ( dam , this 

vo lume). oncthclcss. the close agreement between the 

long-term mean rate of e ent o currence and the imilarity 

in some of the age c timate. from two independent data sets 

is compelling but till cqui cal, evidence that the geologic 

record mark. the effec ts of paleo ei mic earthquake . 

Continued refinement of paleoseismic techniques and 

ex ten. ion of the data base to older earthqu ake will h lp te t 

the. e re ults. Further study wi ll al. o clari fy the dcgr e to 

whi ch err r in radi carbon dating or other errors bias the. c 

re. ult. and will help determine whether some earthquake. 

are mi sing in the geologic rcc rd . 

If we kn w the rate at which the Ju an de uca and orth 

Ameri ca plates con erge and if we know or as umc th total 

slip occurring in a great earthquake, it i. possible to ca lculat 

the return period between event. (return peri od eq uals total 

sli pd i idedbysliprate). R gcr. ( 19 a)cstimatcd th mean 

r peat time for great earthquakes on the a. cadi a th rust fau lt 

b combining con ergence-rate e timate with a sumptions 

about max imum slip during great earthquake and the ratio 

of seismic li p to total slip. U ing thi technique. returnperiod e timate. range from 400 to 1.300 years for the Juan 

de Fuca segment of the Cascadia thru t fa ult. The recurrence 

interval obtained u ing thi method for the Expl rer plate 

( I 00- 300 years) and the Gorda pl ate. (70-200 years) arc 

mall. lead ing Rogers ( 1988a) to suggest that the c egmen t 

have the highe t probability of rupture. 

Inference about the time between great earthquake 

from train-rate mea urements appears to roughl y agree with 

the geologic data from Washington and Oregon. It is estimated 40- 100 tr in accumulated before fault rupture in 

other great earthquakes. independent of magnitude (J.l . train 

is a dimcnsionles quantit y referring to deformati on of 

cru. tal ro ks of one part in one million: al o. ee the definiti on of train rate in the glos ary) (Sbar, 198 ; Adam. , 1984; 

Heaton and Hartzell, 19 6). Gi ven the compre ional strain 
32 ASSE G E RTHQ A KE HAZARDS A D RED l G RISK l THE PA IF! 

RTHWE T 

rate of 0.092 ~ train per ear ( a age and other . 199 1 ), and 

a imilar strain rate for we tern Oreg n. that i . 0.0 pstrain 

per year (Paul Yin ent. niver. ity of Oregon, written commun., 199 1), the period between earthquake i about 

400- 1, I 00 years. Repeating the ca lculation u ing the 50-

~ train max imum e tim ated for the great ubduction-zone 

earthquakes in Chile in 1960 and Al as ka in 1964 (Pi afker 

and Savage. 1970: Pl afker, 1972), the calculated return 

peri d i about 550 year , a value approx imating the longterm mean rates deduced from geologic data. 

. ing current c timate of the mean repeat time. the 

time since the Ia t earthquake, and the tandard en·or in 

the. c e timate , the probability of a large earthquake on the 

Ca. cadia thru t fault can be foreca t for pecified time periods. dams (thi volume) u ed the turbidite recurrence 

interva l to compute a 10-percent probability of a great 

earthquake in the nex t I 00 year. and a 25-pcrcent probability in the nex t 200 year .. He note that these estimate 

could be in error by a factor of 2 and doe not include the 

possibility of event clu tering in time (Adam . thi volume). We con icier the al ue quoted from thi s tudy to be 

preliminary estimate becau. e of pre ent uncertaintie in 

the underl ying as umptions and data. 

ote that the time ince the Ia t great earthquake, based 

on geologic studie , i about 300 year. , whereas the average 

repeat time is about 600 years. If the repeat time are regular, one could in fer that the nex t earthquake is 300 year 

hence. If the repeat time. vary between 300 and I ,200 years, 

however, it i difficult to infer the time of the next event with 

tati ti cal significance or practi cal utility. A lthough much is 

known , many questions remain unan. wered concerning the 

timing and recurrence of great earthquakes. 

LOWER SEISMIC RATES I OREGO 

Compared to we tern Washington, much of we tern 

Oregon ha lower seismic acti vity (for example, see Wea ver 

and Michaelson, 1985). A I though a subducted plate i 

present beneath western Oregon, seismicity defining the 

B nioff zone is ab ent, and few earthquakes ha e been 

detected in the continental cru st. The meager number of ei -

mograph tation in Oregon, however, probabl y reduce our 

ability to d teet earthquakes with magnitude below some 

thre hold. eve11heles . it i likely that a difference doe 

exist in the underl y ing activity rate at magnitude level 

where detection i complete. The cause of thi s difference and 

its implicati on for earthquake hazard in Oregon are importan t problem that have not been ati. factoril y addre sed. 

Two simplified hyp thee ·, similar to tho e used to explain 

the lack of ei micity on the thru t fault, may be in voked: ( I) 

the thru t fau lt is unlocked in the Oregon ecti on, and the 

continental cru st and ubducted lab are in a low-stre . tate 

that produce. few earthqu akes (for example, sec M agee and 

Zoback , 19 9); (2) the thru. t fault and continental fau lts are 

locked. but the cismic cycl i. such that few ea rth 1u akes arc 

pre ently produced in either the continental ·rust or the 

Benioff zone (for example. sec pence, 1989). A s With many 

te tonic i ue in thi s region, ex isting data do not allow an 

unequi vocal interpretation. The hi tori ca l aseismicity in 

we. tern Oregon implie lower . hort-t rm hazard compared 

with western Washington, but the long-t rrn hazard is not 

nece. arily lower. Thi s conclu. ion wou ld require definiti ve 

e idence that the ubduction 1 roce s is a. eismic in thi. section of theCa cadia thru t fau lt. At pre. ent , we con ider the 

long-term earthquake hazard in Oregon comparable to that 

of other . ections of the ascadia thrust fau It because th geologic evider.ce ugge t an acti e a. cadia thru. t fault ; furthermore, both off hore and onshore youthful faultin and 

folding ugge t acti ve continental earthquake process s. 

In detail , everal concepts related to these two hyp theses can or hav been consider d, but som of the. e idea 

raise further dilemmas. For exampl e. Riddihough ( 19 4) 

sugge ted that ubduction i lowed in the Oregon ction 

relative to the Wa. hington ccti on be au. c, inclu ling the 

Gorda orth plate, the ubducting . lab adjac nt to southern 

Oregon i young r !han that adjacent to Wa:hington. In thi s 

model, a young oceanic slab ha more buoyancy and locks 

more effecti vely with the continen tal ru l , red uci ng . ubdu -

tion rate (Kanamori, 197 1; yeda and Kanamori , 1979; 

Ruff and Kanamori. 1980, 19 3). Differences in buoyancy 

cause the Juan d Fuca plate to pivot about a pole located 

south of the plate, with lower ubducti on on the south and 

fa ter subduction on the north (M enard , 1978; Riddihough, 

1984). In thi model. slowed . ubduction on the outhern ection of the Juan de Fuca plate could reduce strain and ei mic 

rates in the continental cru t of Oregon. 

Several i sue ari e in the con id ration of Riddihough' ( 19 4) hypothesi. . From earthqu ake location. 

(Cros on and Owen , 1987) and travel-time studie ( recn 

and others, 1986; Michael on and Wea er, 19 6: Owens and 

others, 1988), an arched ubducted slab dip more hallowly 

benea th northern Washington and ha . teeper dip to the 

north and south of thi s . ection (fig. 6A). If the lab beneath 

Oregon is more youthful (and therefore more buo ant), one 

might e ·pect the plate dip there to be hal lower, not teeper, 

than that beneath Wa hington, although other factors uch a 

oblique conv rgence, varyi ng strike of the Ca cadia thru. t 

fault, and pl ate buoyancy cou ld combine to produce complex 

plate geometry (Cros on and Owens, 19 7). 

If the subducted plate i arched. interplate coupling i. 

expected to be greate t where the plate dips more . hal lowly 

and to be less in the teepl y dipping sec ti ons . uch a 

beneath Oregon (Weaver and Michaelson, 19 5). 

Decrea. eel interplate coupling of the thru t fault beneath 

Oregon differ from Riddihough's ( 1984) model but i al o 

con istent with lower sei mic rates in westem Oregon. An 

unlocked thrust fault benea th Oregon would not onl y p rmit a e1 m1c subduction but would reduce sei mic rates in 

the continental cru t and subducted litho. phere (R ger . . 
EARTHQ AKE HAZ RDS I TH E P CIFI 1

0RTHWEST- A 0 - R IE 3 

1985; Weaver and Michael on, 1985). For example. an 

unlocked condition would reduce th lab-pull ex ten, ional 

. tre s neccs ary to produce Benioff-zan - arthquake . 

If motion of the Juan de Fuca plate i independent of the 

Gorda ( orth and outh) and Explorer plate. , the buoyantpl ate hypothesis wou ld 1 reduce a result cl iam rrically 

oppo eel to Riclclihough's ( 1984) on lusion. Rogers ( 1985) 

argued th at buoyanc was greater on the northern ection of 

the Ju an de Fuca pl ate (con iclering the Juan de Fuca pl ate 

alone. age increase from north to south). Thi hypothe. is 

differs from Ridclihough' ( 1984) model, which treated Lhe 

Juan de Fuca and the Gorda orth plates a a single pl ate. ln 

Rogers' ( 1985) model, the age difference inc rea es the interplate coupling on the north compared with the outh. In thi. 

interpretation. the ec tion of the trench off the Oregon coa ·t 

is mov ing . eaward (referred to a. trench rollback) at about 

the amc rate as the we twarcl component of orth Ameri ca-

] uan de Fuca rclati ve plate motion . Roger. ( 1985) supported 

thi s interpretati on by noting that parti al compressional 

unloading of the continental cru t in Oregon may ha e permilled extensional- tyle volcani sm in western Oregon in th 

last 2.5 million years. Priest ( 1990) also argued that ca twe t ex ten i n in Oregon and ea tward migration of the volcanicarcduring the la t 4-7million ycar support compresional unloading. Thu , a difference in the style of tec toni sm 

between the northern and southern ections of the continental cru st adjacent to the Cascadia ubducti on zone could al o 

re ult in differential eismic rate . 

Other hypothe e. have also been uggc. ted. For example. a complete break in the downgoing lab at depth. 

between 40 and I 00 km could reduce lab-pull tres 

(Weaver and Michael on, 1985; Michaelson and Weaver. 

19 6; VanDecar and other , 199 1 ). In an extension of Riddihouoh'. ( 1984) model. the lack of a Benioff zone beneath 

Oregon might be expl ained by reduced . lab-pull force 

owing to the buoyancy of young subductecl crust. Spence 

( 19 9) argued that a slow ubduction rate under Oregon permits the subducted-plate temperature to increa e. Under thi. 

condition, the den ity of the downgoing lab beneath Oregon 

d crcas s relati ve to that beneath Washington, reducing the 

ab pull force. pence ( 1989) u cd ei. mic-velocity 

hangcs in th . lab noted b ichael on and Weaver ( 19 6) 

to in f r slab den ity. either the lack of lab ei. micit nor 

these models. howe cr, unequi ocall y indi cate that large 

Benioff-zone earthquake. (. imilar to th . e in Wa. hington in 

1949 and 1965) or thrust-fault earthquakes arc unlike! 

beneath Orcoon. 

GRO NO-SHAKING-HAZARD 

E TIMATION 

A curate c. timation of the level of ground shaking in an 

earthquake requires much infom1ation ab ut the earthquake 

rupture. including it depth. ruptur area, orientation. and the 

slip el i. tribution onth fault. Other factor are equall important . . uch a. the eli tance bet\ en the fa ult and the . ite under 

con ·idcrati on. the propert i s of the rock or . oil underl ying 

th sit (including ed iment oeometr ). and the attenuation 

of . baking that is expec ted betw en the fault and the site. 

Earthquake. occurring in the ontinental crust. the Beni ff 

zone. or on the thru t fault must ach be tr at d . eparat I in 

baking e timates becau c of fac tor. such a. differences in 

earthquake depth. e ·pc ted ma imum magnitude. and various source parameters. Much of th lata required to . timatc 

earthquake ground motion. may not be a ailable until after 

an earthquake occur. . ncertaint ab ut th parameters for 

each of the ource type. , in Judi no the likely po ition of rupture both within and bet\ cen the orth mcri a and Juan 

de Fuca pl ates. magnifies the diffi ulty of c mputing 

ground- bakin g estimate in the Pacific onhwcst. Thu . . 

predicti on of shaking levels require. e. tim ati on f these 

parameter. from scant geologic. geodetic. and seismi c informati on and earthquake ac ti vit in other localiti . . 

Ground-. bakin g estimate an take sc raJ forms. For 

example . . om map dcpi t the geographi variation in 

ground shaking with mea. urc .. u h as oclifi d M er alii 

intensity, peak acceleration. spectral ace leration. p tral 

velocity (sec the glo :ary for a definiti on of the ·e terms). 

mea. urcs of relati ve motion at it s underlain by . oil ompared to rock, or omc other parameter. Ground-motion 

maps arc useful in the de. ign of rdinary tructure. , urban 

pl ann ing. earthquake emergency response. zoning, c. timation of los es. and so forth . M ethods are also available to 

c timat the. c ground-moti on parameter f r pccific itcs. a 

prac ti ce that i used in the de ign of high-ri. e buildings. h . -

pitals, and other criti cal . tructurc. . itc- pecific baking 

es timates common I include m rc detailed informati on than 

is fca. ible or map esti mates and, thus. are usuall y more 

accurate than map va lues. 

Ground- ·baking time se ri es (for example, groun d 

accelerati on versu time) and related spec tra arc commonl y 

c timatcd from theor ti cal and hybrid (combination of 

empiricism and theory) methods. E. tim ati on of the m del 

parameters is difficult. and intcn. ivc computati on are 

req uired with th e method : thu . . thi type of predicti on i 

mo. t ommonl applied in site . tudie for criti cal structure . 

In the foliO\ ing . ec tion . we review variou ground-shaking 

estimates for hypotheti ca l and hi tori ca l Pacific orth\ e t 

earthquake to gi ve the reader a . en e of the expected le els 

f shaking for vari ou. earthquake type and to outline the 

unc rtain tie . urrounding thee e. timates. 

PROBABILISTIC GRO NO-MOTtO 1 ESTIMATES 

One t pc of empiricall y based ground-motion e timate 

take the form of statements about the level of ground shaking that will not be exceeded luring a specified cx p ure 

peri d with a specified probability level. M ap of uniform 
34 I G E RTHQ AKE H ZARDS A 0 REDUCI G Rl K I THE PA IFIC ORTHW ST 

Table 6. Probabilistic earthquake ground-shaking-hazard e timate for various urban area . 

1 Peak horizontal accelerati ons and velocities. with a I 0-percent probability of ex cedance in the number of years shown. on firm soil. 

(> ), greater than I 

Expo. ure time 

Location Peak acceleration (percent gt) Peak velocity (centimeters per second) 

50 years 250 years 50 year. 250 years 

Seattle, Wash .. . ....... . 2.330 341 - 260 317- 228 323 - 252 

Portland, Oreg. . ... . .. . . 

Salt Lake City, Utah . . 

San Francisco, Cali f. .. . . . 

2,3 16 330 - 236 310- 215 315 - 230 

229 - 435 260- 470 222 260 

28o 2>80 28o 2>80 

Memphi s, Tenn. . ...... . 230 240 220 240 

1

g, local gravitational acceleration, approximately eq ual to 980 cm/s2

. 

2

Aigermissen and others ( 1990). Includes uncertai nties for some parameters. 3Perk:i ns and others (1980). Does not include par:uneter uncertainties. 4Youngs and others (1987). Includes parameter unccnaintie .. 

probabili. tic ground-shaking hazard are computed for specified site conditi ons. commonl y firm oil or rock. or 

example, the mapped parameter may be the peak hori zontal 

acceleration. peak ve loci ty, or . pectral acceleration having 

a 10-percent probability of being exceeded in an exposure 

period of 50 year . The A pplied Technology Council 

( 1978) u ed such maps (updated every 3 years) for design 

ground-motion specifica tion included in their propo ed 

sei mic regulation for building:. The ati onal Earthquake 

Hazard Reduction Program·s ·' EHRP Recommended 

Prov isions for the Development of Sei mic Regulations for 

New Building,. (Bu ilding Seismic Safety Council , 1992) 

u cd similar specificati on. for ground motion in the 1988 

niform Buil ding Code (I nternational Conference of 

Building Official. , 199 1 ). 

Computation of probabi listie hazard maps require ( I ) 

the locati on, depth, orientation, and seismic rate on each 

acti ve fault for the range of earthquake capabl e of generating trong shaking. (2) an es tim ate of the maximum magnitude f r each . ourcc. (3) the ground-motion attenuation 

curve as a function of magnitude and di stance from the 

ource. and (4) an c timate of each parameter" s vari ability. 

In lieu of complete informati on about specific faults, ei mic 

rate arc com monl y attri buted to ourcc zone that may be 

composed of a few or no specific faults, "background faults'· 

(a uni forml y paced fault gri d of specified ori entati on and 

max imum magnitude), and uni forml y di tributed point 

s urcc for earthquake. of magnitude lc . than about 6-6.5. 

Seismi c rates may be di tributed on these elements uniforml y or may be apportioned based on ei mic observati on 

or geologic constraints. In principal, the max imum magnitude could be defined as the max imum magnitude observed 

in the source zone or on a . pecific fault. However, the maximum magnitude for some ource zones i unknown because 

the hi stori ca l period i too short to have observed the largest 

earthquake. In uch ca es, the maximum magnitude may be 

estim ated from the length of young faults in th ource zone 

or from compari son of the tectonic . etting with imilar circumstance. in other pl ace., or it is assumed to be a value 

arbitraril y equal to or larger than th at observed hi stori cally . 

Commonly. the attenuati on functi on is ba ed on historical 

strong-motion data, which i predominantl y composed of 

records of shallow earthquake in aliforni a. 

Past probabili ti c ground-shaking-hazard c tim ates for 

the Pacific onhwe t . ugge. ted that the hazard i ignificant. E timate. uch as those by Algcrmi en and others 

( 1982). Perkins and others ( 19 0), or Algermi sen and other 

( 1990) were ba ed on the assumption that damaging future 

events will recur as they have in the pas t, that i , as hallowand intermediate-depth earthquakes no larger than M 8.2. 

These probabili stic estimates did not incorporate great ubducti on-zone earthquakes becau e evidence for such event 

was not yet recogni zed. The e probabili tic ground-shakinghazard maps show that levels of haking that might be 

exceeded in a 50-year period with a I 0-percent probability of 

exceedance are comparable to tho e in the alt L ake ity, 

tah, area (table 6) or to those in the moderately . ei mic 

area. of alifornia, such as the entral Valley and sections 

of northern California (Aigennissen and other , 1990). 

Al germi s. en ( 198 ) al. o produced preliminary probabili tic hazard e timate that do account for potential subduction-zone earthquake . He a sumed. based n the 

paleo. eismic data. that a ubducti on-zone earthquake of M 

8.5 could have average recurrence intervals of 300, 600, or 

900 years. range that roughl y parallel tho e ugge ted by 

geologic studi e . The calculations also account for hallow 

eismicity and large Benioff-zone earthquakes (M w 7.5 or 

less). The. e ca lculation did not account for the time ince 

the Ia t earthquake but a umed a Poisson earthqu akeoccurrence process . Besides the three recurrence intervals 
EARTHQ AKE HAZARDS l THE PACiFIC NORTHW ST- 1 0 ER lEv 

as umed, two fault models were separately applied. In ne 

model, the eastern ex tent of lip was assumed to be the point 

at which the fault penetrated to a depth of 20 km , which limit its ext nt we t of the outer coa t. In the econd model, the 

eastern extent of lip was as umed to be the point at which 

the fault penetrated to a depth of 50 km , which permit th 

rupture to ex tend a far ea t a Puget Sound. For the rupture 

model to 20-km depth, Algermi en ( 1988) found that the 

50-year expo ure-time peak acceleration on rock with a I 0-

percent probability of exceedance wa not apprec iably different from the value given in tabl e 6 for Seattle. Thi. rc ult 

held for all three a sumed recurrence interval . For the 

deeper rupture extending to Puget Sound. the probabi li tic 

pea k acceleration and velocity were about equal to that in 

table 6 when the 900-year average recurrence inter al wa 

as umed, but the peak velocity was double the table 6 value 

when the 300-year recurrence interval wa as umed. 

M ore recent modeling (A lgermi ssen and Leyendecker. 

! 992) of the subducti on zone to compute spectral probabili -

tic ground motions is in general agreement with the peak 

accelerati on results just di cu ed. The e results . how that 

for an expo ure time of 50 years, probabili stic spec tral accelerations at 0.3 sand 1.0 (5-perccnt damping) on stiff soils 

(S2 cl as ifica tion, Building Seismic Safety Council , 1992) 

are about 85 cm/s2 and 35 em/ 2, re pec ti vely, at Seattle. 

Spectral accelerations f r a 250-year exposure time are about 

double the 50-year alue . . The pec tral value can be 

approx imately converted to peak values by dividing the 0. 3-

s-peri od values by 2.12 ( ewmark and Hall , 1982). Thus, 

the peak acceleration expected in Seattle on tiff oil. for a 

model that incorporate all three . ource types is about 0.4g 

(g=980 em/ 2. ee the glos ary). The spectral value are 

unchanged at Seattl e wh ther or not the great . ubducti on 

zone earthquake is included. 

The impon ant conclusion is that, if the rupture surface 

in the ubducti on zone i more than about 125 km from Seattle and if a erage recurrence rate of large M s 8.5 earthquakes equal about 500 years, large subducti on-zone . hock 

ha c littl effect on th max imum probabili stic acceleration 

( I 0-perccnt chance of cxce dance) at eattl c for peri od of 

interest ( xpo ure ti me. ) of a much a about 250 years. 

The result that the probabili. tic hazard with and without 

inclusion of large thru t-fault earthquake is mostl y 

unchanged for . ome ground-motion parameter · i due in part 

to the greater di tanc of the ubduction fault rupture from 

th urban areas of Wa. hington and Or 0 0n rclati e to faul t. 

in the contin ntal pl ate and Benioff zone and in part to the 

infrequency of great ubducti on-zone earthquakes relati ve to 

other earthquake ourccs. Thi rc. ult doe not impl y that 

ubducti n-zonc earthquake. are an inconsequential component of the earthquake-. baking hazard . Fir t, trong baking 

in a Iaroe subduction-zone earthquake will be wide pread. 

Al though mapped probabili stic ground-motion e timatcs 

would . how thi s fact clearl y. point c timate such a. those 

di cu sed ab vc (A lgermi sscn, 19 8) do not delineate the 

geographic ex tent of baking. Furthermore . . tatemcnt. ·1bout 

the probabili stic peak ac elcration and t a lesser tent 

peak-velocity hazard do not adequately repre. ent the hazard 

due to low-frequen ground moti ns of long durati on that 

would be of great importan e in a . . ing the effc t of thi s 

type of earthquake on tall building. and bridge . . Th fficiency of a great earthquake in generating low-frequen 

motions is al o of prime import ance in increasing the d'gree 

and di stributi on of land liding and liquefacti on. cond, 

note that probabili ti hazard ca l ulations pr . entl y do not 

full y account forth effect. f ite g ology, whi h can . igni ficantl y increa. e the I el of ground . baking at sites underlain by low-velocity . dimcnt or at site. affected b gr undmotion focusing du to . ediment gcom tr . Finall , peak 

acceleration or vclo ity computed by thi s method docs not 

nece. saril y represent the max imum ground moti on cx pectc I 

in each ource zone. The ma imum e pe t d moti n can be 

estim ated by determini stic means (sec bclo ) or by using 

probabilisti c ground motion for long xposure p riod . 

Other limitation of probabili. tic ground-motion maps 

are wide! recogniz d. Th ca lculation. arc no better than 

the accuracy of the in put parameter. used in the models. 

For example. the return peri od and ex pected size for each 

type of earthquake in the Pacific Northwes t is p orl y d termin ·d. parti cularl y for ontincntal-crust earthqu akes. Probabili. tic maps ould un lerstatc the hazard for continentalcrust earthquake:. A lthough pa t probabi li ti hazar I ca lculati on a umed that large continental-crust ea nhquak 's 

were po . iblc, it i increasingly apparent that the number of 

hallow faults capable of producing large earthquakes and 

the rate of earthquake. on these faul t. i. pre. entl y 

unknown. tatcd another way, gi en the complex ity of the 

dcf rm ati on in the Pacific orth wc. t, the hi. tori eal record 

probably does not provide an adequate ample of the s ismic rate or the modes of dcformati n. and there is prescmly 

no means to cntir' ly account f r thi s fact in ob·~ bili ti c 

hazard cal ulati on . On the other hand, M cGuire and Barn -

hard ( 19 I) found in a study of the hina earthquake 

record th at the best haza rd sti mate for the nex t 50-year 

inter al i. the mo t recent 50-year period. a re ult that supports the utility of probabili ti c foreca. t . 

The uti lity of probabi li tic hazard maps stems from the 

need of user. to compare the relati ve hazard aero geographi regions u. ing our CUITent tate of knowledge in a 

way that integrate. different hi stori ca l sci mic rate. , groundmotion attenuation. max imum magnitude. and tectoni . tyle 

( ati onal Res arch ouncil , 1988). The ab olute hazard 

level predi cted by the e methods i use ful only in ofar a the 

parameter incorp rated into uch map. are an accurate representation of phy ica l earthquake processe : howe er, the 

quality of thi knowledge it elfvari c geographica ll y. As we 

continue to improve ur knowledge of earthquake proce se . 

probabili sti c e tim ate. of the ground- haking hazard can 

serve a a guide in dec isions made by de igner . code wri ter . regulator , owners, and public official Probabili. tic 
36 SSE Sl G EARTHQU KE HAZ RDS A D RED Cl G Rl K I TH E P IFI ORTHWE T 

hazard calculations s rvc to e tablish the change in ri sk 

incurred for variou dec i ions within the c nstraints of the 

model and our pre ent . tate of knowledge. 

Probabili stic hazard calculations are commonly u ed 

in in frastructure de. ign dec ision and no form the ba. i 

for design ground motion in most building ode . Ideall y. 

de. ign level . hould vary for buildings of diff rent type . . 

The u. e of building-de ign motion corre ponding to 

longer cx p . urc periods. for exampl e. is a con er ati c 

approach that can be used for critical facilitie to mitigate 

the effec ts of imperfec t return-peri od and time-ofoccurrence e ti mate f r great subducti on-zone earthquake. . The d ign level may be higher for school and 

ho pita! , where the ri k for lo of li fe i higher. than for 

certain types of low-occupancy, low-ri e commercial truelUre . Thus, for a type of structure with low economic 

value or life-loss ri. k, a ground moti on a ociated with a 

50-y ar cx p . ure period (or honer) may be adequate. 

whereas for a . tructurc with high value. high life-to s ri sk, 

and (or) a lono proj ec ted lifetime, a ground motion a ociated with a 250-ycar expo. ure peri od (or longer) may be 

the design moti on required to achieve an acceptabl y low 

level of ri k. The point is that probabili sti c e. timate. of the 

ground- baking hazard permit one to as c s the relati ve ri. k 

incurred for variou de ign ground-motion decision . 

OTHER EMPIRICAL 

GRO 0-MOTION ESTIMATE 

A s an example of c timated baking values in map 

fom1 , the .S. Geological Survey ( 1975) produced map of 

expec ted M odified M ercalli inten ities for cvcral hypothc-

. izcd Benioff-zone earthquakes in Wa. hington. The. e M 

7.5 events were . eparately assumed to initiate at 50- km 

depth at the locati on of the 1965 earthquake and at a location near Seattl e. M odified M erca lli intensitie were e timated for a . ix-county area that included Seattle, Tacoma, 

and Ol ympia. The predicted M odified M crcalli inten itie 

in the Seattle area ranged between VI and IX. depending on 

ncar-surface geologic conditions. The c map were the 

ba. i of ca. ualty and damage estim ate in the six-county 

region (we summari zed the result. rn the "Recogni zing 

Potential Earthqu ake Hazard and Ri k in the Pacific 

orthwes t" . ec ti on). 

n important method of predi cting the ground moti on 

for subducti on-zone and Benioff-zone earthquakes is to fit 

earthquake ob. crvations recorded in other subduction zones 

with an equation that includes term. for magnitude. geometri c and anclastic attenuati on, and . ite condition . Thi s technique has been used in . evcral studie relevant to the Pacific 

onh we. t ( rou. e and others, 1988; Young. and other , 

1988 : Youngs and oppersmi th . 1989). A lthough the data 

eli. tri bution and lata pr pcrti cs are generall y inadequate to 

inclcpcndcntly determine all the coe fficients in an eq uati on 

ex pre. sing ea h of the e fa tor , one or more of the 1 arameter are commonl y fi xed by theory r oth r independent 

information. Furth rmore, ground motion generated by 

indi vidual ubduction zone may differ because of unmodeled parameter uch as dip of the fault, di stributi on of lip 

on the fault, and the type of earthquake (that is, a thru t-fault 

or Benioff-zone event). till the e are among the most reliable method currently available to predict cletermini tic 

ground-shaking value . The probabili tic computations al o 

require empiri cal equati ons of thi s type. 

Empirical ground-motion value predicted u ing the 

r lati ons developed by Young and others ( 1988) and 

Young and Coppersmith ( 1989) for both great thrust-fault 

and Benioff-zone earthquak s arc summari zed in table 7. F r 

a thru t-fault earthquake, " e considered two case . In the 

first case. we as umed the thru t-fault rupture extends eastward as far a the 40-km-depth c ntour on the fault. Some 

disagreement exi t concerning the gc graphic pos ition of 

the 40-km contour. Cro on and Owens ( 19 7) placed it 35 

km we t of Seattle, as uming that the thru t fault lies within 

the sei. mically quiet zone between the p . iti n of the crustal 

earthquake and the earthquake with in and below th subducted . lab (the seismicity cross section in fi g. 88 is representative of the data). Weaver and Baker ( 1988) inferred a 

po ition 90 km west of cattle based on the po it ion of the 

Benioff zone but as umed that the thru t fault lies along the 

top of the Benioff-zone sei micity. From refrac ti on profi le 

and other geophysical data. M ooney and Weaver ( 1989) 

placed the 40-km contour about 30 km we t of Seattl e. These 

contour locati on corre pond to a 50- 100- km range in closest di stance to the fault. Close t di stance to the fault (R, table 

7) is the equation parameter required in Young and others 

( 1988) and Youngs and oppersmith ( 1989) formul ation. To 

complete our example calculations, we used the contour of 

M ooney and Wea er ( 1989). The . hones t di stance to the 

fault is about 50 km to cattle and 40 km to Portland. In the 

second case, we assume the thru st fault ruptures to the 20-km 

depth contour on the fau lt. The honest eli. tancc to the rupture i about 135 km from Portl and and about 160 km from 

Seattle. We a sume that part of the rupturing segment f the 

Ca cadia thrust fault are adjacent to but wes t of each cit 

The magnitude we as ume i arbitrary and ind pend nt of 

the implicitl y assumed fault length. 

The computed results in table 7 illustrate se era! important point about the ground-shaking hazard at Seattl e and 

Portl and from Ca cadia thru t-fault earthquakes. Fir t, under 

any as umption about the pos ition of the rupture. an earthquake of thi ize is expec ted to produce damaging ground 

moti on at least a far ea t a. the urban area of the Puget 

Sound-Will amette Valley lowl and. Second, the groundmoti on value expec ted at Portl and are marginall y higher 

than those expected at Sea ttle. Third, the va lues at Seattle or 

Portl and under case 2 could be roughl y twice the values 

under case I . Four1h. based on the Youngs and others ( 1988) 

equati on . ground-moti on value at . itcs underl ain by soil 
Table 7. Empirically predicted and observed seismic ground motions for Seattle, Wash., and Portland, Oreg. 

[Based on relations developed by Youngs and others (1988) and Youngs and Coppersmith (1989). R, closest distance to the fault. Predicted acceleration is rounded to the nearest 0.05g, and predicted velocity is rounded to the nearest 

5% g (where g is approltimately 980 cm/s2

). Predicted spectral-acceleration values are computed by the appropriate transfom1ation of spectral velocity given by the relations developed by Youngs and others (1988) and Youngs and 

Coppersmith 19891 

M 8.5 thrust-fault rupture M 7.9 Benioff-zone M7.1, 1949 M 6.5, 1965 

rupture predicted earthquake, earthquake, 

ground motions, 1 observed observed 

R=50 km ground ground Rupture to the 40-km fault contour Rupture to the 20-km fault contour 

motions.' motions, 1 

Rock Soil Rock Soil R=49 km R=91 km 

R=50 km R=40km R=50 km R=40 km R=l65 km R=145 krn R= 165 km R=145 km Rock Soil Soil 2 Soi12 

(Seattle) (Portland) (Seattle) (Portland) (Seattle) (Port.land) (Seattle) (Portl and) 

Peak ground acceleration 20 20 30 35 10 10 15 15 25 40 328 420 

(percent g) 

Five-percent spectral 40-30 45-35 70-50 75- 55 15-10 20-15 30-20 35-25 50-35 90-60 558-25 542-20 

acceleration at 0.3 and 

0. 7 seconds (percent g) 

Five-percent spectral velocity 30 35 50 55 10 15 20 25 35 60 535 523 

at 1-s period (centimeters 

per second) 

-

1

0lympia Highway Test Laboratory. 2

Largest of two horizontal components. 3Langston ( 1981 b) 4

Baker and Langston ( 1987} 5

Trifunac (1977) 

rn 

)> 

;;v 

-l 

:r: 

.0 

c 

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rn 

:r: 

)> 

N 

)> 

;;v 

0 

en 

z 

-l 

:r: 

rn 

"C 

)> 

:::1 

0 

;;v 

-l 

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--.) 
38 S -SS I G EARTHQ KE HAZARD D RED Cl G Rl K l TH E PACIFIC ORTHWE T 

are as much as t\ icc the value. at site. underl ain by rock . 

Other data, howe cr. suggc t that ro k- oil ite- pectral diff renccs can be larger than hown in table 7 f r ome ite 

type. at ome period (for example. Algermissen and other . 

19 5; Theil. 1990; Rogers and other. , 199 1 ). 

alculated ground-motion alues using equations 

developed for Benioff-zone earthquakes by Young and 

others ( 19 ) and Youngs and Coppersmith ( 1989) arc al o 

hown in table 7. To illu. trate their equations, we po. tul ate 

a Benioff-zone earthquake of M 7.9 direc tl y beneath either 

Scaulc or P rtl and, almost a wor t-ea c hypothe. is for an 

e rthquakc f thi s type. Ground motion for thi s type of 

earthquake is ubstantiall y larger than those expected from 

the thrust-fault earthquakes. The gr und motion values 

observed in the 1949 and 1965 earthquake · at the Olympia 

Highway Te t Laborator are included in table 7. Although 

the observed and predicted values cannot be compared 

direc tl y because of differences in magnitude and eli tancc, 

a ca. ual compari . on hows that the empirical prediction 

are reasonabl e. 

nccrtainty about the po. ition of intraplate continental- ru st fault. and the potential max imum magnitude of 

earthquake on these faults preclude. estim ati on of the 

ground-. baking hazard a. ociated wi th them. If faults uch 

a the Portl and Hill. , Hood Canal, and Saddle Mountain, or 

the geophy ically inferred tructure beneath Puget Sound, 

are capable of gen rating earthquakes. the Scaule and Portland levels of ground shaking from earthquake on the e 

faults could exceed that from either thru t-fault or Benioffzone earthquakes. 

THEORETICAL GROU1 0-MOTION ESTIMATES FOR 

BENIOFF-ZO 'E EA RT HQ AKE 

Theoretical estimation of ground shaking for Pacific 

orthwe t earthquake at . pccific . itcs has been attempted 

for a few scenarios. Ground-motion time hi toric and pectra at a specific . itc can be produced u ing theoreti cal model s 

that incorporate fault rupture, propagation path , and site 

propenie . . For exampl e, Lang. ton ( 198 1 b) modeled the 

vel city and di splacement time hi torie that were recorded 

during the 1965 Benioff-zone earthquake at Seattle, Tacoma, 

and Ol ympia. ignificant di screpancies between the modeled and recorded moti ons were auributed to in ufficient 

knowledge about Earth . tructurc and the source. The inadequacy of thi s m del led to more reali sti c auempts, such as 

ray-tracing ca lculati ons in a two-dimen ional model of the 

Du wami. h Ri ver alley (Lang ton and Lee, 1983). This 

tudy predicted that order of magnitude change in groundmoti n di . J laccmcnt arc po ·iblc over eli tances of 200 m 

due to focusing of . ei. mi c energy by ba in geometry. Lang-

. ton and Lee ( 198 ) suggested that the geographic variabi lity 

in ground shaking during the 1965 earthquake could be 

expl ained on this basi . A three-dimen ional model was also 

c aluated wi th ray-tracing technique (Ihnen and Hadley, 

19 6) and compared with the records for the 1965 earthquake. In thi study, map of peak accelerati on for the Puget 

Sound region showed vari ability of a factor of 10 at a given 

distance from the fault, due primarily t focu ing by ba in 

geometry and secondarily to amplificati n by near-surface 

low-velocity layers. Their model predicted peak acceleration ob erved at eattle and Tacoma rea onably well but 

undcre. timatcd the peak acceleration at Olympia by a factor 

greater than 4. Ihnen and Hadley ( 1986) indicated that their 

re ults qualitatively agreed with the observed inten ity variation in the 1965 earthquake. By way of compari on with 

the prediction given in table 7, a large fraction of Ihnen and 

Had ley 's ground-moti n valu s grouped in the range 

between 0.03 and 0.3g at epiccntral eli Lance between 0 and 

70 km , though ome of their predicted va lue were as large 

as 0.6g near the mouth of the Du wami h River. 

ilva and other (volume 2) u ed a. tochasti c groundmotion model and random-vibration-theory technique to 

prcdi t the ground moti ons for the 1949 and 1965 earthquakes. Their computation a sumcd eismic-energy 

relea -e at a point on the fault ncare t the . ite, auenuating 

pr pagation between urce and ite, and an attenuating 

nat-layered near-surface veloci ty model that approx imates 

the site effect. In the e imulation , th known hypocenter 

for the. e earthquake i the as umed point of energy 

release. The peak a celeration values predicted by thi s 

technique compare favorably with the observed value 

(table 7) except at station OHT (Olympia Highway Test 

Laboratory) for the 1965 earthquake. For thi tation, the 

predicted value are too low by about a factor of 2 (table 8) 

(Silva and other . volume 2). Thi s di screpancy ha been 

attributed to exces ive model attenuati on in the nearsurface sediments. Sil va and other ' (volume 2) predicted 

spectral acceleration curves al o compare favorably during 

mo t periods \ ith the ob erved pcctral acceleration in the 

1965 and 1949 earthquakes at tations in the Olympi a 

Highway Te t Laboratory and the Federal offi ce building in 

Seattle. The. e com pari on how the value of u ing thi 

technique for pr eli tin ground motion!> in thi ~ region at 

other site or for other types of earthquake . 

T HEORETI CAL GRO NO-MOTION E Tl IATES FOR 

THRU T-FA LT EARTHQ AKES 

The results of three theoreti cal modeling studi c for 

hypotheti cal earthquakes on the a cadia thrust fault are 

hown in table 8. Silva and other (volume 2) applied the 

model described in the Ia t ection to predict the ground 

motion for a thru. t-fault ea rthquake. In their simulations, 

they a umed the point of energy release to be at 70-km 

hypocentral di tance from the Federal office building station 

in Seattl e. Thu , thi s model simulated rupture to the 40-kmdepth contour on the thru t fault (see the discu sion in the 

previous sec tion). 
EARTHQ AKE HAZARD I THE PAC.IFIC ORTHWEST - A 0 ER lEW . 9 

Table 8. Theoreti cal deterministic ground-motion parameters at Seattle for a Cascadia ubduction-zone earthquake. 

I Accelerations have been rounded 10 the ncares1 5-percenl g (where g ;, approxima1ely 9 0 cm/s2). Leaders (···) indi cmc no c;limmc l 

References 

ilva and others (volume 2) 

Cohee and others (volume 2) ..... . . . 

Washington model ( ee tex t) 

Heaton and Hartzell ( 1987. 1989) 

Magni tude 

.5 

9.5 

loses! distance 

to the fault 

(kilometer. ) 

146 

6 

Peak ground acceleration 

(percent g) 

Rock 

10- 15 

(deep model) 

Soil 

15 

25 

(deep model) 

M ax imum 5-pcr en! 

spectral a cclcration at 

0.3 and 0.7 . econds (percent g) 

Rock Soil 

35 

20-10 

1Avcragc of soil· and rock-site recordl\. The value~ arc for Pugct Sound rather than Seattle. 

2

Cohce and o1hers ( 199 1 ). 

Cohee and other (volume 2) applied a Green'sfunction technique ( ee the glo ary), with empirical source 

functions recorded during the 1985 hile thrust-fault earthquake at two ite , one underl ain by roc k and the other by 

soil. Thi s source function is modified by theoreti ca l functi ons that simulate the effec ts of gross cru tal structure and 

geometric attenuati on. Two M thru t-fault earthquake arc 

modeled in this manner, one in Wa hington and one in Oregon. Each rupture zone had a differ nt length, downdip 

width, and dip, largely in accord with the plate geometry 

determined by Weaver and Baker ( 19 8). Rupture extended 

to the 40-km-depth fault contour in both ca es. The model 

included the effects of cru tal tructure and implicitl y 

included near-surface ite effect . The calcu lati on eparately assumed three di stributi ons of energy release on the 

fault: hallow (upper one-third of the fault). middle (middle 

one-third), and deep (lower one-third). The large t acceleration at Seattle and Portland are for both deep-rupture zones 

because thi model place the greatest energy release nearc t 

the e urban area . Peak acceleration at Portl and for the deep 

earthquake (O. Ig for rock and 0.2g for soil) were ·lightly 

lower than tho e at Seattle (table 8). 

Heat n and Hartzell ( 19 7, 19 9) applied an empirical 

Green ' s-functi on technique whereby they added together the 

radiated ground-shak ing contribution of lower magnitude 

earthquakes di tributed on the fault to . imulate the radiation 

of a M 9.5 earthquake. In their wdy, an a ortment of 

record produced by thru. t-fault earthquake . ranging in 

magn itude from 7.4 to .2. from Japane. e and Ala kan ubduction zones ervcd a Green' functi on . Heaton and Hartzell ( 1987 , 1989) considered three faul t model . but the 

va lues we quote in table are for the fau lt model that ruptures from the trench axi. eas t to the coast. In their study and 

that of Cohee and other (volume 2), the 0 round-moti on a lues were substantially larger than tho e shown in table 8 for 

other case when the rupture ex tended ca l\ ard to Pugct 

Sound . Ex tension of the r11 Jture ea tward i inferred if the 

lab i arched into the thrust-fault region. a is ugg sted by 

orn e data ets. 

MMARY OF GRO 0-MOTIO ES 

Several important factor may con trol the level of 

ground baking in a great ubducti on-zone earthquake. First, 

the eastward limit of energ -releasing rupture wil l have a 

strong innuencc on the level of shaking in the urban area of 

the Puget Sound-Willamette Valley lowland. If the thrust 

fault rupture no farther cast than the outer oa ·t. . bak ing 

damage will be limited compared with a rupture cx tcndino 

beneath the Olympic Pcnin ula or farther as t\ ard . 1\ rupture limi ted to the outer coast wou ld reduce the shaki ng damage, parti cularl y for low-rise building , compared to the 

effec t shown in table 6- 8. Even in thi s ituati on. however. 

the hazard for some clas. c of tructure · orca tcr than t\ o to 

five . torie in height may remain high, particu larly for those 

tructures underl ain by geologi ba ·ins or thick sed imen ts. 

The amplificati on of long-period ground haki no in sedimen tar basin. and the lower rate of atten uat ion of longperi od motion with di stance could lead to damage of some 

tall structure . The M ichoacan earthquake off the c a: t of 

M ex i o damaged and col lap. ed some type. f building · in 

M exico Ci ty, about 360 krn from the epicenter. Building of 

se en to twelve storie. were particularl y affected at ome 

exico City locales because geologic condition ( ingh and 

others. 19 8) increa eel the level and duration of shak ing to 

damaging levels for building in thi s cia ( molka and Bcrz, 

1989). The 19 9 Loma Prieta M 7 .I earthquake al o produced damage a much a 80 km from the rupture zone 

owi ng to the effect of . ite geology (for example, ce Theil , 

1990). Al though the most vulnerable geologic condition. in 

M e ·ico City and an Franci co are ex treme ca e . similar 

deposits are found at orne site in the Pacific orthwest. 

Furthermore, a Cascadia thru t-fault rupture to the outer 

coas t result in a hypocentral di tance to the Puget SoundWill amette Valley lowland of less than about 160 km , conidcrably closer than the distance to M cxic ity from the 

M ichoacan earthquake. 
40 A SESSI G EART HQ A KE HAZA RD A D RED Cl G RISK I TH E PA IFI ORTHWE T 

Table 9. Examples of potential users of earthquake-hazard information in the Pacific Northwest. 

City, county, and multicounty government u cr. 

Local building, engineering, zoning, and afety departments 

City and county offices of emergency ervices or management 

County tax assessors 

M ayors and city council members 

Multicounty planning, development, and preparedness agencies 

Municipal engineers, planners, and administrators 

Planning and zoning officials, commi sions, and departments 

Police, fire, and sheriffs departmenL 

School districts, admini trator , and teacher 

State gove rnment users 

Building Codes Agency (Oregon) 

Building and Construction Safety Inspection Di vi ion 

(Washington) 

Department of ommunity Development (Washington) 

Department of Ecology, Dam Safety Section (Wa hington) 

Department of Energy (Oregon) 

Department of En vironmental Quality (Oregon) 

Department of Education (Washington and Oregon) 

Department of Geology and Minerallndustrie (Oregon) 

Department of Information Services (Washi ngton) 

Department of Land Conservation and Development (Oregon) 

Department of Transportation (Washington and Oregon) 

Department of Water Resources (Oregon) 

Di vision of Energy Management (Washington and Oregon) 

Di vision of Geology and Earth Resources (Washington) 

Di vision of Ri sk M anagement (Washington) 

Fire Mar hall (Oregon) 

Fire Protection Services Divi sion (Washington) 

Legislature 

Museum of Science and Industry (Oregon) 

ational Guard 

Offi e of the Governor 

Utilit y and Transportation Commi ss ion (Washington) 

Federal government users 

Army orps of Engineers 

Department of Energy 

Congress and congressional taffs 

Department of Housing and Urban Development 

Department of the Interior (Bureau of Land M anagement Bureau 

of Rec lamation, U.S. Geological Survey) 

Department of Tran portation 

Environmental Protection Agency 

Farmer Horne Adrnini ·tration 

Federal Emergency Management Agency 

Federal Housing Administration 

Federal Insurance Admini stration 

cdcral Power ornmi ssion 

Forest Service 

General Servi ce Adrnini tration 

National Insti tute of Standards and Technology 

ational Oceanic and Atmospheric Admini stration 

ational Security ouncil 

uclca r Regulatory Commi ss ion 

Small Business Admini stration 

------------------------------- Other national users 

American ational Red Cro s 

Applied Technology Counci l 

American Association of tate Highway and Tran portation 

Ameri can Public Work s As ociati on 

Association of Engineering Geologists 

American Association of tate Geologi 

Earthquake Engineering Re earch Institute 

International Conference of Building Official 

ational as ociations of cities, counties, and States 

ational As ociation of Insurance Commi sioners 

ational Center for Earthquake Engineering Rc carch 

ational Institute of Building Sciences 

ational Hazards Research and Appli cations enter 

Pri vate, corporate, and quasi-public users 

Ci vic, religious, and voluntary groups 

Concerned citi zens 

Construction companic 

Consulting planners, geologist , architects, and engineers 

Extracti ve, manu facturing, and processing industri es 

Lending and insurance institutions 

Landowners, developers, and real-estate salespersons 

ews media 

Professional and scienti fic ocictie (including geologic, 

engineering, architecture, and planning societies) 

Uni versity department (including geology, geophys ics, 

environmental, civil engineering, structural engineering, 

architecture, and urban and regional planning) 

tility di tricts 

Second, the length and downdip width of rupture not 

onl y affect the level of haking at a given locati on. but they 

also determine the size of the geographi c area affec ted. An 

earthquake of M 8- 9+ an be expected to produce damage 

from the outer coa tal areas to the urban areas of the Puget 

Sound-Willamcttc Vall y lowland. Simultanc ~ damage to 

a region of thi s ize po c seri ous problem for emergency 

re pon e and recovery. 

For Benioff-zone earthquakes. an important i ue is 

whether the e event. arc possible at any locati on in the subdueled litho 1 here for depth between about 40 and 80 km. 

If so, then the e earthquakes arc pos ible below Seattle. 

Portl and. and other urban area. of the Pugct SoundWillamctte Valley lowland (W ea er and. hecll ock . th i volume). Occurrence of the largest pos ible Benioff-zone eanhquake , Mrn ax 7.5- 8 ( uc lear Regul atory Commi. ion. 

199 1; W eaver and Shedl ock , thi s volume), at locati ons near 

or beneath these urban zone woul d produce . igni ficantly 

larger ground motion than were ob. erved at the same locati ons in the 1949 and 1965 earthquakes. 

A lmo t every aspect of continental-cru st earthquakes is 

an i sue at present. The potenti al locations, max imum magnitudes, and baking attenuati on are poorl y known for thi s 

type of earthquake in thi s region. onctheless. geologic 
EA RTHQUAKE HAZA RDS I THE PACIFI ORTHWE. T- A 0 ERVI · \ 4 1 

evidence exist for youthful contincntal-cru. t fault . and 

eanhquak have oc urred on uch faults in the region hi. -

tori call y. Significant damaging grou nd motion could result 

from moderate-to-large continental-cru t earthquake occurring near urban area . Notably. thi s type of earthquake has 

the potential to produce more intense damaoe to structures of 

all cl as. cs than thrust-fault and Benioff-zone earthquake . if 

the shallow earthquake occurs within or near an urban area. 

An earthquake of thi type, however, would likely affec t a 

smaller region than a great Cascadia subduction-zone earth -

quake. Few ground-motion estim ates have been produced 

for thi s type of earthquake in the Pacific orthwcs t to date, 

primaril y becau e of uncertainty abou t which fault shou ld 

be con ·idcrcd ource of future earthquake . 

USERS AND USES OF EARTHQUAKE 

HAZARD INFORMATION 

Earthquakes are a recognized hazard in the Pacific 

orthwe l. a fact that has rc ulted in . cientific and engineering studies to asse s the hazard . . Thi report contain. some 

of the signi fican t new information produced by thee studic . . 

Thi s inform ation. when tran lated for and tran fcrrcd to 

potcmial users (table 9). should result in the selecti on and 

adoption of appropri ate earthquake-hazard reduction techniques as, for example, those introduced by Kock !man (volume 2). Thi s report prov ides an opportun ity to build a bridge 

between the producer and the users of earthquake-hazard 

in formation in the Pacific orthwcst. 

The rcspon ibility for reducing earthquake hazards i 

. harcd by al l segments of society. Potential u. crs of earthquake-hazard informati on include many people at community. state, and national levels, both public and private (table 

9). These may be scientists and engineers who usc the information to rc. olve . cientific or technical que tions. pl an ner. 

and dcci sionmakers who must con. ider hazards in the context f other land u. e and development criteria. or just interested citi zen. who must onsidcr the likelihood of future 

earthquake. before maki ng per. onal decisions. 

Variou. levels of government. bu. incs .. industry. the 

scrv i cs sector. volu ntary organi za tions. profcs. ional . ocictic. , and special-interest group play important rol si n reducing earthquake hazard . . Kockclman (volume 2) pre cnts 

examples of . omc groups· utilization of earthqu ake-hazard 

information to r ducc casual tie . damage, and . ocioeconomic interrupti ons. 

CITY, COUNTY, AN D AREA WIDE 

GOVERNMENT SERS 

ity and coun ty governments arc empowered and 

obligated to provide for the publi c health. safety, and welfare of their citi zens. For example. the Wa. hington tate 

Legislature ( 1990a. b) rcquir rtain cou ntic. and ci ti s to 

adopt growth regulation pre luding land uses or developments that arc incompatibl with de. ignatecl "critical 

area .... The minimum guid lines for cia .. ifying ritical 

areas include ci. mic-hazard area. defined a. ··ar as . ubj 

to . evere ri k of damage a a result of earthquak -indu cd 

ground haking. lope fai lur , settlement,. oil liquefacti on. 

or urfacc faulting" (Washington tate Department of 

ommunity De elopmcnt, 1990). Public liabi li ty i: an 

added incenti ve. a di cu. sed by Perkin. and M oy ( olumc 

2). Lcgi. lature. com monly authorize and require the adoption and admi nistration of local zoning, subdi i:ion, building, grading, and safet ordinances. For example, 

recogni zing the likelihood and sc cri ty of a fu ture earthquake, one of the large. t iti cs in the nitccl tatcs- Los 

Angeles- inventoried and evaluated its un ~af'. unrcinforccd-masonry bearing-\ all buildings. lt th n enacted an 

ordinanc requiring owners to . tr ngthcn or d moli. h 8,000 

such buildings in a 14-ycar p ri d. 

The various plans, codes, and map overlays introduced 

by Booth and Bethel ( olumc 2) arc particularly good hazard 

reduct ion technique into which carthquakc-hat.ard in fonnati on can be incorporated. For example, King ount ·s" cnsiti vc Area. ,. ordinance can be amended t in lucie Ill rc 

spec ific earthquake-hazard in formati on. uch as ground-

. baking severity, land !ide potential. and liquefacti on potentia l, as it becomes a ailablc. 

The principal governmental resource at the si te of an 

emergency or disaster is usually local government agencies. 

They will necessaril y be heavil y invol eel in preparing for. 

coordinating on-the-scene response to, recovering from , and 

rccon tructing after damaging earthquakes. For example, 

school preparations ha c incl uded an earthquake safct program (M arten. , 19 ). an earthquake emergency planning 

booklet ( o. on and Marten. , 1987), and a manual for . ccuring n ns tructural building component. (Wa hington State 

Sup rintcndcnt of Publi c ln!>tructi on, 1989). 

STATE GOVERNMENT USERS 

State ha c the ultimate non-Federal public re pon. ibility for the health. afety. and welfare of their people. For 

example. the Oregon State Land Conservation and De clopment Commi ion ( 1990, p. ) adopted statewide mandatory 

planning goals and suggest d ouidcline . . One of the goals 

requir that developments ··. ubject to damage or that cou ld 

rc. ult in lo. s f life shall not be planned or located in known 

areas of natural * * * hazar Is without appropriate safeguard . . " tate agencic. incorporate their own sci. mic-sa fcty 

standard into their operation and also work with local governments to encourage com munity sci. mic-. afety effort s. 

Califomia mapped official zones of potentially acti ve fau ltrupture area . . It then mandated that all cities and countie. 

require a geologic report for these areas prior to development 

and a minimum setback from the trace of ac tive faults for 

m st human-occupancy structures. 
42 ASSESS! G EARTHQ AKE HAZA RD A D REDUCI G RISK l THE PACIFIC ORTHWE T 

Official. in almo t every tate-government agency 

(table 9) need to consider ei mic sa fety in carrying out their 

dutie. . For example, the W ashington State Sei mic Safety 

Counci l ( 19 6) recommended e entia! ac tions that both the 

legi. lature and eight State agencie . hould undertake for a 

long-range . ei. mic-risk-r duction program. In appendixes 

to its re ommendation. , the council identified the prim ary 

and econdary earthqu ake hazard., compared its earthquak 

acti vi ties to Cali fornia' and di ·cussed potentiallosse , vulnerability of school building , liability, and ri sk of chemical 

acc idents. The council also in ventoried the hazard-reduction 

acti vi ties of selected State agencies. 

The Wa hington State Legi lature ( 1990a) directed the 

Department of ommunity Development (which includes 

the Di vision of Emergency M anagement) to create a Seismic 

Safety Advi sory ommittee. Funding wa prov ided 

***solely for the depanmentlo de clop a seismic safety program 10 asses 

and make recommendations regarding the slate's canhquake prepared ness. 

The department shall create a sci. mic safety advisory board 10 develop a 

comprehensive plan and make recommendations 10 the legislature for 

improving the Slate's earthquake preparedness. The plan shall include an 

assessment of and recommendations on the adequacy of communications 

systems. structural integrity of public buildings, including hospital s and 

public schools. local government emergency response sy 1ems, and prioritization of measures 10 improve the state's earthquake readiness • • * 

Thi s advisory committee presentl y include participati on by 15 State agen ies, busine sand industry repre entati ves, and two local engineering consulting firms. The 

advi ory committee has completed its initial report that 

recommends prioriti e. for action, including es tablishment 

of a . eismic-safety oversight committee, improved emergency planning, . trengthening of buildings, and trengthening of lifeline (Wa hingt n State Sei. mic Safety Advi sory 

Committee, 199 1 ). 

Likewi ·e, in Oregon, Senate Bill 955 wa passed in 

1989 directing the Department of Geology and Mineral 

lndu trie (DOGAMI) to asse and mitigate earthquake 

hazards. In 1990 the Governor appointed the Seismic Safety 

Advisory Commi ion (SSPAC) to provide policy guidance 

on earthquake-hazard reducti on. Also, in 1990, DOGAMI 

created a new Earthquake Engineer po ition. In 199 1, the 

legislature e tabli hed the SSPAC in Senate Bill 96 and 

309. S nate Bill 96 also takes the followi ng actions: 

• Establi he earthquake drills in publi , pri vate, and 

parochi al chools with 50 or more pupi ls. 

• Amends the ·tate building code to: 

I . Evaluate new es:ential facilities, hazardou fac ilitie , major structures (greater than 6 Storie and 

greater th an 60,000 square feet), tructure greater 

than 10 stori s, and all parking structures . 

2. Install strong-motion accelerograph in and near 

selec ted bui ldings. 

3. Review geologic and engineering reports for seismic de:ign of new, large, or critical buildings. 

4. File for public use n ninterpreti ve data from eismic site evaluations. 

5. E tabli h a surcharge of I percent of building-permit fees to support the above ac ti vities. 

Senate Bill 309 authori zes the Oregon Building Codes 

Ag ncy to adopt regulations to require correc tion of un afe 

conditions caused by earthquake. in ex isting building . The 

bill also grants building codes inspector. authorit y to in pect 

for damage after an earthquake. 

Blair and Spangle ( 1979, p. 24) noted that the " location 

and construction of public fac ilitie , management of State 

lands, pro ision of ervices, and delegation of power and 

responsibilitie to loca l governments should all reflect an 

awareness that damaging earthquakes are inevitable." Publ ic 

awareness of the co t of damaging earthquakes, and technique to reduce such co ts, have been addressed in the 

Pacific orthwe tin various way .. The e include the Wa hington State S ismic Safety Counci l ' s recommendations 

(introduced above), bibliography and index to eismic hazard (M anson, 1988), serial publica tion such a Oregon 

Geo logy and Washington Geology (formerly Washington 

G ologic Newsletter), onferences and work hops (Walsh 

and others, 1990), outreach programs, and guidebook 

( oson and other , 198 ). 

FEDERAL GOVERNMENT USERS 

The Federal Government is crucial in stimulating State 

and local government. to improve their seismic-safety 

efforts. The development and di tribution of lo s and damage scenarios are a part of its rol . For example, the .S. 

Geo logical Survey ( 1975) prepared a study of potential 

earthquake losse in the Puget Sound area and is fu nding an 

earthquake-hazard a se sments program in the Pacific 

orthwest. Federal effort to reduce ri sks have evolved from 

a national commitment to prov ide di saster relief to State 

and local areas devastated by earthquakes. One of these 

efforts includes the preparation and wide di stributi on of a 

series of guidebooks for reducing earthquake hazards (Kockelman , volume 2). 

Federal agencie are also re ponsible for the . ei mic 

safety of their own faci lities. After the 197 1 San Fernando 

( outhern Californi a) earthquake, the U.S. Veteran Administration ( 197o, 1986) developed earthquake-res istant design 

requirements for Veteran Admini strati on hospi tal facilitie 

and e tabli hed seismic-protecti on provi ions for furniture, 

equipment. and supplies for these hospi tals. In addition, the 

Government, through the Federal Emergency M anagement 

Agency (which carri es out the National arthquake Hazards 

Reduction Program), formu late pl ans for nati onal action in 

response to ca ta trophi c earthquake . These pl ans include 

large-scale preparedne. s for, respon e to, and recovery from 

major damage or loss of li fe. Such pl ans may include search 

and rescue, evacuation, provision for medical care, food , 

shelter, poli ce protection, and other emergency help. 

The Federal Government, through the U.S. uclear 

Regulatory Commi sion, i also re pon ible for the sa fe iting, de ign, and operation of nuclear power pl ants. They 
EARTHQUAKE HAZA RDS I THE PA IFI ORTI-IWE T- OVERVI EW 43 

have commi ss ioned many tudies by pO\ er-plant applicants 

and ngineering firm and reviews on the geologic and ei -

mic hazards that mioht affect nuclear facilitie. in the Pacific 

orthwe t ( .S. uclear Regulatory om mi ion. 1991 ). 

The .S. uclear Regulatory Commi . ion recently 

increa ed the magnitude of the afe Shutdown arthquake 

(the largest earthquake for which major components of the 

power plant are expected to remain operati onal) for Ca cad i a 

thrust-fault earthquake to M 8.25 (U.S. ucl ar Regulatory 

Commi ion, 199 1),partl y inrecognition ofwork p n ored 

by the .S. Geological Survey. The .S. Nuclear Regulator 

Comm iss ion also funds an independen t research program on 

earthquake hazards. 

The Federal Govemmcnt i. al o respon sible for the 

sei mic afety of many dam . As part of thi s resp n. ibility. 

both the .S. Army Corp. of Engineers and the U .. 

Bureau of Reclamation conduct tudie con erning the tectonic etting of dam and earthquake and volcanic hazard. 

to these facilitie in the Pacifi c 1orthwc. t (for example, see 

Hawkin , Foley, and LaForge, 1989). The Bureau of Reclamation a! o recognize M 8.25 a the maximum-magnitude 

subduction earthquake on the Cascad ia thrust fa ult (Peity 

and others, 1990). 

OTHER NATIONAL USERS 

Voluntary organization (which pro ide aid to victim. 

of di as ter ), national in titute , and cientifi c and profe.-

sional organizati n make major contributions to eism ic 

afety. For exampl e, the American ational Red Cross 

( 1986) prepared a resource guidebook that include strategic on how to identify and u. e re ourcc , onduct research 

and evaluations, work with Red Cros and other r urces, 

and develop public relations. The earthqu ake prepar dnc 

program of Snohomi sh County. Wash .. wa included a. one 

of 43 examples of Red Cro chapter activitie . 

Re earchers from many di cipline . including . ei mology, geology. ea rthquake engineering, and the social . cicnce . have a major part in reducing earthquake hazards by 

helping local, tate, and Federal planner. and dcci ionmakcrs formu late and evaluate their plan and pr grams. 

Expert from the re earch commun ity pr vide technical 

adv ice to . pccial ad vi ory groups that prepare model building codes . uch as the Uniform Building ode by the International Conference of Building Offi ial s ( 199 1 ). S ientific 

and profcs ional organi za ti on arc major contributors to the 

worldwide exchange of inf rmati n through publica tions 

and technical meeting . 

PRIVATE, CORPORATE, AND 

QUASI-PUBLIC USERS 

The pri vate (including corporate) . ector has perhaps the 

strongest influence on engineering de. ign and land development in the United States. In choosing ' her to locate and 

how to on tll.lct facilitie . indu trial and ommercial d veloper ar s nsitivc to tax and oth r incentive off red b 

State and local government to fa orablc labor and r tai I 

markets, to transportation n twork. , and tot rrain and ' aterre ource ondition . Often, natural hazard ar not weighted 

heav il y in the e development con. id ration. unle. s they arc 

likely to affect the facilitic within their amoniz d !i f time 

or period of owner hip. Ace rding to the .. Executive 

Office of the President ( 1978, p. _9). 

Business. industry, and the services sector play the lead roles in o•Nru ·ting 

new buildings and in developing land. ei"ni de. ign provisions in local 

codes • • arc minimum standards. Thoughtful businessmen 111tcrested in 

providing a safe en ironment for their onsumers and employees. and in 

protecting their capital investment will want to gi\C careful consideration to 

ea11hquake hazards in planning. constructing and maintaining their fa illli~s 

• • * In some instances shon-tcrm profits may be redu ed to incrc;"c the 

long-term benefits of saving lives, rcdu ing property damage. and maJntainino the funct ioning of the economy in the face of a major eanhquah~ . 

The seismi c retrofiuing of the Heritao Building an I 

nion Station (Pcrbix , 1990a, b) in e·Htl e ar good example f busine s de i ions intended t conserve apital 

investment and protect the public. 

An important but le s di. cussed a pcct of noincering 

design include th ecuring of non tructural building component uch as parapets, bookca. cs, office equipment, li ght 

fixture , and partition . A parti ·u!arly good guid book on 

red ucing non tructural earthquake damage has b en prepared 

by Rei therman ( 1985). Th hazard-reducti on technique recommended in thi s guidebook are relatively incxpen ivc and 

are full y applicable to the Pacific N nh wes t. 

Kockelman (volume 2) cmpha izc. e aluati nand rcviion of hazard-reduction technique . A report on the 1989 

Loma Prieta earthquake by the aliforni a ssociation of 

Hospi tal and Heal th y. tern ( 1990) is similar in scope. 

Thi r port addr . e. structural , non. tructural. staffing, 

comm unications, co t-rcimbur. ement, media-relation . . and 

re. carch i ucs. M any re ommcndati ons w r made to 

improve building afety. preparednc . re ponse. and reco -

ery. uch tudi are needed to effecti ely update planning 

for earthqu ake di aster. 

PREREQUISITES FOR USE 

The effecti e u e of geologic. ei mologic. and engineering information to avoid damage or to reduce lo 

requires a considerable effort by both the producer and the 

u. crs of the information . For an integrated hazarda. se . ment method, ee Preus and Hebenstreit ( olume 2). 

scientific and engineering re ults arc specificall y 

tran. lated. the effecti ve u er commu nity is limited to other 

geologi ts, . ei mologi. t . and engineers. If u crs do not 

become proficient in interpreting and appl ying technica l 

informati on, the information is likely to be mi u. ed or even 

neglec t din the dcci. ionmaki ng proce . . 
44 ASSESS ! G EARTHQUAKE HAZA RD A D RED Cl G Rl K I THE P IFIC ORTHWEST 

A swd y by Kockelman ( 1990) on the u e of earthscience in formation by pl anners and decisionmakers in tah 

. howcd that the mo t effective u c of hazard information wa 

achieved when map that clearl y depicted the likelihood, 

locati on. and severity of the hazard were provided. Funhermorc, the use of thi s information for hazard reduction wa 

more likely hen the in forma ti on wa deli vered to potential 

users along with technical a sistance and encouragement. 

The effccti enc . of any earthquake-hazard-reduction program i dependent upon the awarenes , under tanding, and 

moti at ion of engineer. , pl anner., and deci ion makers, public and pri va te. M ay (volume 2) addresse the prospect for 

hazard reducti on in the Pacific orth wc. t. 

SUMMARY 

ln part, the sti mulu. for incrca ed earthquake-hazard. 

rc earch funding in the Pacific orthwe t wa. the recognition that thi s regi n may ha e had M 8-9.5 earthquake. along 

the Cascadia thru. t fault during the Ia t 7,000 year . Other 

factor also continue to fuel intere t in potential regional 

ea rthquake hazard . For example, recent re earch indica te 

th at large continental-crust earthquake are po ible in or 

ncar the principal urban areas of the Pac ific orthwe t. Gi ven 

our current level of understanding. it is al. o likely that 

Benioff-zone earthquake similar to the 1965 and 1949 

events will recur. Some of these earthquake could be larger 

(M 8 or less) or could oc ur closer to urbani zed area of the 

Pugct Sound-Will amette Valley lowland than in the pa t. 

Fu rthermore, this area has also undergone maj or increase in 

p pu lation and economic value at ri sk since the last large 

earthquake in 1965. Together, the e considerati ons provide 

incentive for continued evaluation of the tectonics and earthquake hazards of the region. The significance of this hazard 

al. o prov ide im petu for continued evaluati on of the suitability. level of implementati on. and cffec ti vene of ex i ting 

plan and regu lations to reduce fuwre earthquake effect. . 

The most important i ues regarding the level of 

earthquake hazard in the Pacific orth wc t arc summarized as fo llow. : 

• Large. hallow crustal earthquakes are likely in the 

future but , at present. little i known about the recurrence of these event or their potential locations. cw 

gcol gic data . uggc. t. however, th at uch earthquake. 

arc possible at locati ons clo. c to urban area and that 

events of thi type (not necessaril y on the fault near 

urban area. ) could be as large as about M 8. 

• Great earthquake. are pos ible on at least some segment of the ascadia th ru. t fault, and mos t scienti sts 

believe that these earthquakes cou ld ha e magniwdes 

at leas t as large as . although magnitudes as large as 

9- 9.5 have been suggested. 

• nfavorable ground condition in the Puget oundWillamettc Valley lowland are expected to . ub tantia lly incr a e the shaking hazard at ome ite , particularl y for high-ri e tructure underlain by deep 

edimcntary basins. 

• The ex tent of downdip rupture in a ubducti on earthquake on the Cascadia thrust fault wi ll strongly control . baking level in the principal urban areas . A 

model fitting both strain and uplift rate uggests that 

the fault could rupture downdip to points beneath the 

Olympic P nin ula, which wou ld substantially 

increase shaking level relati c to m del that limit 

rupture to the Pacific coa t or further west. 

• Future large Benioff-zone earthquakes arc likely, and 

me cienti. t believe that the e event. are possible 

within the ubducted litho. phere from we tern Briti h 

Columbia to northwc tern Cali forni a. The probable 

depth of the e earthquake ranges between 40 and 0 

km. Their max imum magnitud is likely to be 

between 7.5 and .0. Thu . earthqu ake of thi s type 

appear to be poss ible and have location and maximum magnitudes that would produce ubstantially 

grea ter damage than the hi storical Benioff-z ne 

earthquakes. 

Progre in under tanding the potential for great earthquake. or contincntal-cru. t earthquake wi ll come from continued paleo cismicity wdie. , in trumental ei micity 

tudic . and expanded geodetic mea urement . Much additi onal work i. also needed to produce us fulmap that depict 

the effects of geologic conditions on ground haking and the 

areas of vari ou types of ground failure. A lthough much 

remain to be done to further our under tanding of the earthquake hazard in the Pa i fie orth wcs t, progre . ha been 

made in several area , as evidenced by the results presented 

in this report. 

Al though thi s colic tion f re carch papers wil l be of 

interest to a wide audience, informing a diver c audience i 

hampered by the complex ity of the problem. The controversy surrounding . ome i ues in the Pac ific orthwe t further complica te communicati on. These factors pre cribe 

sub tantial techni al di cu. ion to convey both the hazard 

and the limitation of our present data and methods of analy. i. . M os t important, howe cr. the hi tori ca l and geologic 

le on in hand tell u. that earthquake hazard in thi region 

are ub tan ti al and that thi s fac t hould be made clear to 

those responsible for the afety of others and the preservation of property in future earthquake . A n important goal of 

thi s publica ti on is to provide in formation to the e users and 

other citi zen. concerning the scientific state of knowledge 

about Paci fi e North we t earthquake hazard s. I n addition, 

clarifi ca tion of the issues wi II improve the focus of cienti fie 

research on factors that strongly control hazard a se sment. 

Thi proce swill lead to increased understanding and recognition of the problem, to more informed decisions by engineers. pl anners. land and bu ilding owner , and public 

officials and. ultimately. to the adopti on of appropri ate ri skred uction techniques. 
EARTHQ AKE HAZARDS I TH E PA IFIC ORTHWE T- A 0 ER IE\ 4 

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GLO S RY 

GLOSSARY 

[Terms set in bold type arc defined e lsewhere in the glossary. Definitions arc modified from Zion). J.l. . cd. 

19 5. Evalua ting eart hquake haatrds in the Los Ange les r gion- An earth-s icnce pcr;.pcctivc: . . Geological 

Survey Professiona l Paper 1360. p. 15-22] 

Acceler ation. The rate of hange of velocity of a reference point. ommonl ex pre. sed 

as a fraction or percentage of the acceleration due t ora vity (g). whercg = 980 m/s2 

Accelerogram. The record from an acceler ograph . hawing ground acceleration as a 

function of time. 

Accelerograph. A compac t. rugged. and relatively incxpcn i e in. trumcnt that record. 

the signal from an accelerometer. Fi lm is the mos t common recording medium . 

Accelerometer. A sensor whose output i. almo t direc tl y proportional to ground ac el rat ion. The conventional strong-moti on accelerometer i a implc. nearl y criti ca ll 

damped oscillator hav ing a natural frequency of about _o Hz. 

Accretionary wedge. Sediment that accumulate and deform wh r oc anic and cominental pl ates collide. These sediments are scraped off the top of th do' ngoing o eanic cru . tal plate and are added to the lead ing edge of the continental plate. 

Active fault. A fault that is con ·idercd likely to undergo ren wed movement ' ithin a 

p ri od of concern to human Fault are commonl y considered to be a ·ti c if the 

have moved one or more times in the last I 0,000 year . . bu t they may also be considered acti ve when assc. sing the hazard for some applications even if movement has 

occurred in the last 500,000 years. 

After shocks. Secondary tremors th at may follow the large. t shock of an carthquak 

sequence. Such tremor can extend o er a period of week . . months. or year. . 

Alluvium. Loosely compac ted gravel, sand, ilt, or clay d posit d by . tream .. 

A mplification. An incrca e in . eismic-s ignal amplitude within omc range of frequency a. wave propagate through different Earth materi als. The. ional is both 

amplified and deamplified at the same ite in a manner that is dependent on the frequency band. The degree of amplification i also a complex function of the level of 

shaking uch that, as the level of shaking increases, the amount of amplifica ti on 

may decrea e. Shaking levels at a site may al o be increased b focusing of seismic 

energy caused by the geometry of the . ediment ve locity stru tu re. uch a. ba in . ubsurfacc topography. or by urface topography. 

A mplitude. Zero-to-r eak va lue of any wavelike di sturbance. 

Arc. ommonl y refers to the chain of volcanoes (volcanic ar ) that som times form 

inland and that arc produced by subduction. 

A rias intensit y. A ground-moti on parameter deri ved from an acceler ogram and pr parti ona! to the integral over time of the acceleration quared. xpre . ed in uni t. of 

vclocit (meter per . ccond or centimeter per . cond). 

Aspe1·ity. A region on a fault of high strength produ cd by one or more of the following 

condition. : incrca cd norm al strcs , high fri cti on. low pore pre sure. r geometric 

change. in the fault . uch a fault bends, offs t . or roughne. Thi. term i u. ed in 

two contc ·ts: it may refer to ections of a fault that radiate unc mm n ci mic 

energy or it may refer to locked secti on of the fault that cause fault egmentation. 

Aseismic. Refcring to a fault on whi ch no earthqu ake ha e been ob. ervcd. ci ·mic 

behav ior ma be due to lack of hear tre aero s the fault, a locked-fault condition with or without shear ·trc :. or rclea e of tre ·s by fault cr eep. 

A ttenuation. A dccrea ·c in eismi c- ignal amplitude a wave propagate from the ci. -

mic source. A ttenuation i aused by geometric preading of ei. mic-wavc cnerg 

and by the ab. orpti on and scattering of sei. mic energy in different Earth material. 

(termed anclastic attenuati on). Q and kappa are att nuation parameter used in modcling the attenuati on of ground moti on 

Backarc. The region landward of the chain of olcanoc. (volcani arc) in a ubducti on 

. ys tem. 

55 
56 ASSESSI NG EA RTH Q AKE H ZARDS A D REI UCI G RISK I TH E PA IFI ORTHWEST 

Backstop. ontincntal rocks in the backarc that arc landward from the trace of the subduction thru 1 fault and that arc strong enough to support strc s accumulation. These 

rock arc both igneous and dcwatcrcd, lithificd, con. olidatcd . ediments that probabl y were pan of the acc retionar y wedge. The softer accretionary-wedge rocks are 

strongly deformed as they accumulate against the backstop. The exact pos ition and 

dip direc tion of the back top i. not well determined, and more than one backstop 

may ex i. t. 

Basement. Igneous and metamorphi c rocks that underlie the main sedimentary-roc k 

sequences of a region and ex tend downward to th base of th crust. 

Bedrock. Relati vely hard. olicl rock that commonly underli s oftcr r ck, sediment. or 

soil. 

Benioff zone. A clipping pl anar zone of earthqu akes that i produced by the interaction of 

a downgoing oceanic crustal pl ate with a continental pl ate. The. e earthquakes can b 

produced by slip along the . ubduction thrust fault (sometimes referred to as the 

thrust interface fault because it is the interface between the continental plate and the 

oceani c pl ate) or by slip on faults within the downgoing plate as a re. ult of bending 

and extension as the pl ate i. pull ed into the mantle. Slip may also initiate bctwc n 

adjac nt segment. of downgoing pl ates. The Benioff zone in the Pac ific orthwcs t 

is not as well deve loped as it is in other subducti on zones . The earthquake. in thi. 

region do not appear to be produced by slip along the thrust fault. Al so known as the 

Wadati-Bcnioff zone. 

Body wave. A sci. mi c wave that propagates through the interi or of the Earth . as opposed 

to surface waves that propagate nca r the Earth' s surface. P and S waves arc examples. Each type of wave has distincti ve stra in characteri stic . . 

Bl'ittle-ductile boundar y. A depth in the crust acros which the thcrmomcchanica l properti es of the cru st change from bri!tlc above to ductile below. large percent age of 

the earthquake. in the crust initiate at or above thi. depth on high-angle faults; below 

thi s depth. fault slip may be aseismi c and may grade from high angle to low angle. 

Bulk density. The weight of a materi al di vided by its volume, including the volume of its 

pore paces. 

14C age date. A rel ati ve age obtained for geologic materi als containing bits or pieces of 

carbon u. ing mea. uremcnts of the proportion of radioacti ve ca rbon ( 14 ) to daughter carbon ( 12C). These dates arc independentl y calibrated with ca lendar dates. 

Coher ent slides. Landslides that con. ist of a few relatively int ac t bl ck of rock or soil 

th at m0vc wgc thcr. The lxtsn l fa ilure ur fuee o f IIIO:> t o f the;,e ;, !ide;, i:-. ;,cvcral 

meters or ten. of meters below the land surface. 

Cohcsionless. Rcfering to the condition of a cdiment who c . hear strength depends onl y 

on fri ction bccau. c there is no bonding between the grains. Thi s conditi n is typica l 

of clay-free sandy deposits. 

Colluvium. Loose . oil or roc k fragment. on or at the base of gentle . lopes or hil l. ides. 

Depos ited by or moving under the innuencc of rainwash or downhill creep. 

Compressional wave. ec P wave. 

Convolution. A mathemati ca l operati on that de cribcs the acti on of a linear sys tem on a 

signal. . uch as that of a filter on a cismi c . ignal. 

Corner frequency. The frequency at which the curve representing the Fourier amplitude spec trum of a recorded earthquake seismic signal abruptl y changes slope. 

Thi s frequency is a propert y of the source function related to fault size. 

Cr eep. Sl ow, more or less continuous movement occurring on fault. due t ongoing tectonic deform ation. Al so applied to slow movement of land. I ide masses down a . lope 

ec au ~c of gra itational forces . Fau lts that undergo significant and (or) ongoing 

creep arc like! to be aseismic or capable of onl y small or moderate earthquakes. 

Thi s fault condition is comm nl y referred to a. unlocked ( ce locked fault and 

intcrplate coupling). 
GLO ARY 

Critical facilities. tructurc who c ongoing 1 e1formance during an emergency i 

required or whose failure could threaten many li ves. M ay in Jude (I) structures such 

a nuclear power reactors or large dam who e failure might be ca ta. trophi : (2) 

maj or communication, utility, and tran pon ation ystcms; (3) involuntary- or highoccupancy buildings such as chool. or pri on ; and (4) em rgency facil iti s uch as 

hospital , police and fire stati ons. and di sa ter-re pon e center . 

Crust. The outermo t major layer of the arth, ranging from about I 0 to 65 km in thi kne worldwide. The continemal cru t i about 40 km thi kin the Pacific t orthwe t. 

The thickness of the oceanic cru st in thi s region vari c. between ab ut I 0 and 15 km . 

The crust is charac teri zed by P-wave velocities le than about km/s. The uppermos t 15- 35 km of cru t is brittl e enough to produce earthquakes. The s i mi · cru. t 

i. separated from the lower cru. t by the brittle-ductile boundary. 

Damping. The reduction in amplitude f a s ismic wa c or oscillator due to fri ti on 

and (or) the imernal ab orption of energy by matter. Sec Geometri al attenuation. 

Deformational front. See Accretionary wedge. 

Design earthquake. The po tul ated earthquake (commonl y including a pccification of 

the ground motion at a ite) that is u ed for eva luating the earthquake rc ·istance of 

a parti cular tructure. 

Dete rministic methods. Refers to methods of calculating ground motions for h pothcti· 

cal earthquakes based on earthquake-source models and wave-propagati on methods 

that exclude random effec ts. 

Dip. Inclination of a planar geologic . urfacc (for example. a fault or a bed) from the 

hori zontal. 

Dip slip. Se Fault. 

Dit·ecti vity. An effect of a propagating fault rupture whereby earthquake ground motion 

in the directi on of propagati n i more severe than that in other directi on from the 

earthquake ource. 

Displacement. The difference betwc n th initial po ition of a reference point and any 

later po ition. (I) In cismology. displacement is the ground moti on comm nly 

inferred from a seismogram. For example, it may be calculated by integrating an 

accele rogram twi ce with respect to time and is ex pre sed in unit of length . . u h a 

centimeter. . (2) In geolog . di pl acement is the permanent off ct of a gcologi or 

manmade reference point along a fault or a landslide. 

Disrupted slides and fa lls. Landslide that arc broken during movement into chaoti 

masses of mall block . rock fragments. or individual grains. The ba al fa ilure . urface f most such slides i within a few meter of the land surface. 

Earthquake. Ground baking and radiated ei mi energy caused mo t commonly by 

sudden lip on a fault, olcani c or magmatic a ti vity. or other udden strcs change 

in the Earth . An earthqu ake of magnitude 8 or larger is termed a great earthquake. 

Earthquake haza rd. Any physical phenomenon as ociated w ith an earthquake that may 

produce adver. c effec ts on human acti vities. Thi s incl udes urface faulting, 

ground shaking, landslides. liquefaction. tectoni c deformation, tsunami. and 

seiche and their effect on land usc, manmade . tructures . and ocio-economic y -

tcm . . A commonl y u. cd res tri cted definition of earthquake hazard is the probability 

of occurrence of a . pecified level of ground . baking in a specified period of time. 

Ea rthquake risk. The expected (or probable) life los., injury. r building damage that 

will happen. given the probability that s me earthquake hazard occur . Earthquake ri . k and earthquake hazard arc cca ional ly u ed interchangeabl y. 

Elastic di location theory. In ei. molog , a theoreti ca l de cription of how an elasti 

Earth respond to fault sli p. a. representc ! by a di tribution of displacement discontinuities . 

Epicente r. The point on the Earth·. surface crti ca ll y above the point (focus or hypocenter) in the cru. t where a . cismi c ruptur nucleate . 

57 
58 AS E Si t G EARTHQ AK H ZARDS A D REDU I G RISK I THE PA IFI ORTHWE T 

~"max · The frequency above which liule . eismic energy is ob cr ed at most . trong-moti on 

stations. Thi s frequency cutoff may be produced by attenuation of baking by 

uncon olidated s dimcnt underl ying the recording ·ire or may be a property of the 

source function. 

Fault. A frac ture along which there has been . ignificant displacement of the two sides 

relati ve to each other parallel to the fracwre. Strike-slip faults arc verti cal (or nearl y 

vertical) fracwrc along which rock ma e have mo. tl y shifted hori zontally. If the 

block opposite an ob cr cr I ok ing across the fault move to the right, the slip style 

is termed right lateral; if the bl ck moves to the left, the moti on i. termed left lateral. 

Dip- li p fau lts arc inclined fracwre along which roc k mas. cs have mostly shifted 

venicall . If the rock ma above an inclined fa ult i dcpres ed b slip, the fault i 

termed normal, whereas if the rock above the fault is elc atcd by slip. the fault i 

termed rever c (or thru st). Oblique- lip faults have significant components of both 

sli p style . . 

Fault-plane solution. An analy is using stcreographi c projecti on or its mathematical 

equi valent to determ ine the altitude of the ausati vc fault and it directi on f slip 

from the rad iation pattern of sci. mi c wave · u ing earthquake records at many tat ions. The analy:is mo. t commonl y uses the directi on of first motion of P waves 

and yields two po siblc orientations for the fault ru ptur and the direction of eismogenic slip. From the c data, inference: can be made concerning th princi1 al axes 

of stress in the region of the earthquake. The principal . tress axes determined in thi 

method arc the compress ional ax is (a lso called the P-ax i . . the ax i of greatest compresion. or 0 1) . the tension axi: (a lso known a the T-ax i , ax i of lea t c mpre. -

sion. or 0 3), and the intermediate su·es. ax i (0 2). 

Fault scarp. teplike linear landform coincidcilt with a fault trace and cau cd by geologica ll y recent . lip on the fault. 

Fault trace. lntcrsecti n of a fault wi th the ground urfacc; aL o, the line commonl y plotted on gc logic map. to represent a fault. 

Filter. In seismology, a physica l y tem or a mathemati ca l pcration that changes the 

waveform or amplitude fa signal. 

Filtering. A ttenuation of certain frequency component of a sci. mic ignal and the 

amplification of others. F r a recorded ignal. the process can be accompli hed 

electroni ca ll y or numeri ca lly in a digital computer. Filtering also occurs naturall y as 

. eismic energy pa cs through the Earth . 

Firs t motion. On a cismogram, the direction of gr ound motion as the P wave arri e at 

the seismometer. Upward ground motion indicate. an expansion in the source 

region; downward motion indicate. a contraction. 

Focal depth. A term that refer. to the depth of an earthqu ake focus. 

Focal-mechanism solution. See Fault-plane olution. 

Focus. ee H ypocenter. 

Focusing. Sec A mplification. 

Forear c. The region between the trace of the subduction thrust fault and the vo lcanic 

chain (volcanic arc). 

Foreshocks, mainshock , aftershocks. Forcshock are relati vely smaller earthquakes that 

1 recede the biggc t earthquake in a erie , which i. termed the mainshock. After-

. hock arc relati ve ly . mailer earthquake that follow the mainshock. 

l•ourier amplitude spectrum. The relati ve amplitude at different component frequencies that arc deri ved from a time histor y by Fourier analysis. 

Fourier transform. The mathemati cal operation that resol es a time eric (for xample, 

a recording of ground motion) into a scrie of number that characteri ze the relati ve 

amplitude and phase components of the signal as a functi on of frequency. 

Fr equency. umber of cycles occurring in unit time. Hertz (H z). the unit of freq uency, 

i. equal to the number of cycle. per second. 

Fundamental period. The longest period for which a . truclllrc bows a maximum 

response. The reciprocal of natural frequency. 
GLOSSA RY 

G or g. See Acceleration. 

Gaussian noise pectrum. The pcctrum of a time hi tor whose sampl value. arc ocnerated by random . election from a stati stical popul ation that has a spc iflcd mean 

and . tandard dev iati on. The alue (ordinate. ) have a bell-shaped distri bution ab ut 

the mean. In earthquake rudies. thi s type of spectrum i comm nly multipli d by a 

theoreti cal earthquake source pectrum to obtain predicted ground-motion . pcctra 

for hypotheti cal earthquakes. 

Geodeti c. Rcfcring to the determination of the ize and . hapc of the Earth and th precis 

locati on of points on its surface. 

Geometri cal attenuation. That component of attenuation of . ci mic-wa e amplitudes 

due to the radial spreading of ei mic energy with di tancc from a gi en . ur e. 

Geomorphology. The ·tudy of the charac ter and origin of landf rm 

Geotechnical. Refering to the usc of cientific mcth ds and engineerin g principle. to 

acquir . interpret. and appl y knowledge of Earth materi als for ol ing en inecring 

problems. 

Gt·avity. The attraction between two ma e , such a the Earth and an bjcct on it surface. ommonly referred to a the acceleration of gravit y. hangc. in the ora ity 

field can be u cd to infer in formation about the ·tructure of the Earth'. lithosphere 

and upper mantle. Interpretations of change in the gra ity fiell ar generall y 

applied to grav ity va lues correc ted for extraneous cffc t . The corrected va lue arc 

referred to by variou. term . such as free-air gravity. Bouguer ora ity. and i. o. tatic 

grav ity, depending on the number of correcti ons. 

Green's function. A mathemati cal repre entation that, in reference to earthquake . baking, is u. ed to represent the ground motion cau ed by instantancou. sli p on a mall 

pan of a fault. Green'. functions can be ummcd over a large fault : urfacc to compute the ground baking for a large earthquake rupturing a fa ult f finite size. The 

fracti onal fault-slip events that arc summed can be record from small earthquakes 

on the fault or they can be theoreti cally comput d mall-earthquake records. 

Ground fa ilu re. A general reference to landslides, liquefaction. and lateral spreads. 

Ground motion [shaking]. General term referring to the qualitati c or quantitative 

aspec t of movement of the Earth ' . urfacc from eanhquak . or expl osions. Ground 

moti on is produced by wave that arc generated by uddcn li p on a fault or . udden 

pre sure at the explo. ive our e and travel through th Earth and along its . urfacc. 

Halfspace. A mathemati cal model bounded by a pl anar surface but othcrwi: in fi nite. 

Properti es \ ith in the model are commonl y as umcd to be homog n ou. and i. otropic. unlike the Earth it. elf. which is heterogeneous and ani otropic. 

Hazard. See Ea rthquake hazat·d . 

Hertz (Hz). A unit of freq uency. Ex pres. ed in c cle per ·ccond. 

Holocene. Refers to a period of time bet\ een the present and I 0.000 year. before pre. ent. 

A pplied to rocks or faults. thi. term indi cate the peri od of rock formati on or the ti me 

of m tr cent fa ult . lip. Faultsof thi sagearecomm nl ycon idercd activc. based on 

the obscr ati n of hi tori al a ti ity on fault of thi age in other locale . . 

Hypocenter. The point within the Earth where an earthquake rupture initiates. lso 

ommonly termed the focus. 

Int ensity. subjecti ve numeri ca l index describing the severity of an earthquake in terms 

of it effect on the anh ' urfa c and on human and their tru ture . Several 

ca l . ex i t. but the ones mo t ommonl y u ed in the nited tate- are the M odified 

M erca lli calc and the Ros i-Forel scale. 

Intraplate and interplatc. Intrapl ate pertain. to processe within the Earth ·. cru. tal 

plate . . lnterpl ate pertain to proce. c between the pl ates. 

lnterplate coupling. The qualitati e ability of a subduction th rust fault to lock and 

accumulat . trc. s .. trong intcrplat coupling implies that the fa ult i locked and 

capable of accumulating strc . wh r a weak coupling impli es that the fault is 

unlocked or only capable of accumulating low . tr s. A fault with weak interpl atc 

coupling could b asei. mi c or could lip by creep. See Locked fault. 

59 
60 A ESSI G EARTHQUAKE HAZARD A D RED lNG RISK I THE PA IFIC 1

0RTHWEST 

Isoseismal. Refcring to a line on a map bounding points of equal intensity f r a particular 

earthquake. 

Kinematic. Refcring to the general movement patterns and direction of the Earth' rock 

that produce rock deformation. 

Late Quaternary. Refering to an age between the present and 500.000 years b fore the 

pre ent. au its of this age are ometimes consid red active ba ed on th observati on 

of hi tori ca l acti vity on faults of Lhi age in some locales . 

Landslide. The down lope movement of oi l and (or) rock. 

Lateral spreads and flows. Term refering to landslides that common ly form on gentle 

lope and that ha e rapid fluid-like flow movement. 

Least-squares fit. An approx imation of a set of data with a curve uch that the um of the 

quare of the difference between the ob ervcd point and th as umed curve is a 

minimum . 

Lifelines. Structures that arc important or criti ca l for urban functionality. Examples are 

roadway , pipelines, powerline , ewers, comm unications, and port fac ilities. 

Liquefaction. Process by which water- aturated sediment temporari ly lose strength and 

acts as a fluid . This effect can be cau ed by earthquake shaking. 

Lithology. The des ription of rock compo ition and tex tu re. 

Lithosphere. The outer solid part of the anh , including the crust and uppermo l mantle. The litho phere is about 100 km thick. although its th icknc . i age dCJ endcnt. 

The lithosphere below the crust is brittle enough at ome location to produce earthquake by fault ing, such a within a ubducted oceanic plate. 

Locked fault. A fault that i not slipping because fri ctional re i lance on the fault is 

greater than the hear tre s aero s the fault. Su h fau lt may store strain for 

ex tended peri ods that is eventuall y relca cd in an earthquake when fri ctional rc istance i overcome. locked fault condition contra t wi th faul t-creep condition. 

and an unlocked fault. See Interplate coupling. 

Love wave. A type of seismic surface wa e hav ing a hori zontal moti on that is tran ver e 

to the direction of propagation. 

Ma. An abbreviati on for one mil lion year ago. 

Magnetic polarity reversal. A change of the Earth ' magnetic field to the oppo ite 

polarity that has occurred at irregular interval during geologic time. Polarity reversal can be pre erved in sequences of magneti zed rocks and compared with standard 

polarity-change time . calcs to e timate geologic ages of the rock Rock created 

along the spreading oceanic ridges commonly pre erve this pattern of polarity 

rever als as they cool, and thi pattern can be u ·cd to determine the rate of oceanridge . preading. The reversal pattern. recorded in the rock arc termed sea-floor 

magnetic I i neaments. 

Magnitude. A number that characteri ze the relati ve ize of an earthquake. M agnitude i 

ba. ed on mea. urement of the max imum motion recorded by a seismograph ( omctimc for earthquake wave of a particular frequency), COITected for attenuati on to a 

standardi zed distance. c eral calc have been defi ned, but the mos t commonly 

u ed are (I) local magnitude (ML)· commonly referred to as " Richter magniwde,' ' (2) 

urfacc-wavc magnitude (M ), (3) body-wave magnitude (mb). and (4) moment 

magnitude (M.., ). calcs 1-3 have limited range and applicability and do not ati sfactoril y measure the size of the largest earthquakes. The moment magnitude (Mw) 

. ca le, based on the oncept of seismic moment, i uniform ly applicable to all sizes 

of earthquakes but is more difficult to compute th an the other types. In principal. all 

magnitude sca le. could be eros calibrated to yield the same va lue for any given 

earthquake, but thi s expectati on has prov n to be only approx imately true, thu the 

need to spec ify the magnitude type as well as it value. 

Mantle. That pan of the Earth ·s interior between the metallic core and the crust. 

Microzonation. The geographic delineation at local or site sca le of area. having different potentials for hazardous earthquake effec ts, such as ground shaking potential. 

Microzonation for any of the ea rthquake hazards can be produced. 
GLO S RY 

Moho. A di continuity in ci. mic velocity that mark the boundar b t\ een the Earth·. 

crust and mantle. Al so termed the Mohorovi ic' di continuit , after th · Croati an 

seismologist Andrija ohorovicic' ( I 57- 1936) wh di overed it. The boundary is 

between 25 and 60 km deep beneath the ontinent and bet\ c n 5 and km deep 

beneath the ocean floor. 

Moment magnitude. ee Magnitude. 

Natural frequency(ies). The di screte frequency(ies) at which a parti cular lastic . stem 

vibrate. when it is et in motion by a single impul. e and not influenced b other 

ex tern al force. or by damping. The reciprocal of fundamental period. 

Newmark analysi . A numeri cal technique that model a potential land lid a. a rioid 

bl ock re ting on a frictional lope, pres ribing d nami forces on th bloc k from 

a umed ground shaking record in order to calculate the cxpc ted di. placem nt of 

the block. 

Normal stress. That stress component perpendicular to a oiven plane. 

Oceanic spreading ridge. A fracture zone along the ocean bottom that a mmodatc. 

upwelling of mantle materi al to the surface, thus creating new crust. Thi. fra turc is 

topographicall y marked by a line of ridge. that form a molt n rock r ache. th 

ocean bottom and . olidifie . . 

Oceanic ti·ench. A I in car de pres ion of the sea fl oor cau cd by and approx imate I coincident with a subduction thrust fault. 

Oscillator. A mass that moves with oscillating motion under the influence of c ·t rna! 

forces and one or more force that re. tore the mass to it. stable at-rc t positi n. In 

earthquake engineering, an oscillator is an idea li zed damped ma. s-. pring system 

u cd a a model of the response of a . tructurc to earthquake ground motion. A 

seismog•·aph i al o an o ci llator of thi s type. 

Outer a1·c ridge. A zone landward from the trace of the subducti on thrust fault of lcva ted ea floor probably related to the compre ion of the rocks in the accretionary 

'"edge. Al so referred to a. the outer arc high. 

P wave. A seismic body wave that involves parti cle motion (alternating omprcs ion and 

ex ten ion) in the direction of propagation . 

Paleoseismic. Referring to the prchi. rori c seismic record a in ferred from oung geologic sediment . 

Peak acceleration. Sec Acceleration. 

Pedogenic. Pertaining to processes that add, transfer, transform . or remove soil con ti!llents. 

Period. The time inter al required for one full cycle of a wa c. 

Period band. That range of periods being considered in an analysi. of g•·ound motion. 

Phase. (I) A stage in periodic moti n. such as wave moti on r the motion of an oscillato•·, 

mea. ur d with rc. pe 1 to a given initial point and ex pre sed in angular measure. (2) 

A pul c of . cismic energy arri ving at ad finite time. (3) rage in the phy. ical properties of ro k. or minerals under differing cond ition of pre ure. temperature. and 

wa ter availability. 

Physiogr aphic. Rcfcring to the character and di tri bu ti on of land form . 

Plate tectonics. theory supported by a wide range of ev idence that con idcr the 

Earth ' crust and upper mantle to be compo cd of s vera I large. thin, relatively 

ri gid plate that mo c relative to one another. lip on faults that define the plate 

boundaries commonly re ults in earthquake . Several style of fau lt bound the 

plates, includi ng thrust fault. along wh ich plate materi al i ubductcd or con: umcd 

in the mantle. oceanic spreading ridges along wh ich ne• cru. tal material is produced. and transform faults that accommodate hori zontal slip (s trike slip) between 

adjoining pl ates. 

Plei tocene. The time period between about I 0.000 year before present and about 

1.650,000 cars before pre ent. A a descripti e term applied to rock or fau lt . it 

marks the period of rock formation or the time of mo t recent fault li p. respecti vely. Faults of Pleistocene age may be con idcred acti ve though their activity rate 

arc common! lower than younger faults. 

6 1 
62 SSE SI G EARTHQ AKE HAZARDS A D REDUCt G RISK I THE PA IFI ORTHW ST 

Poisson distribution. A probability di tribution that charac teri ze di crete events occurring independently of one another in time. 

Probabilistic earthquake hazard. See Earthquake hazard and Earthquake risk . 

Pseudorelative acceleration spectrum. ee Response spectrum. 

PSRV. Pseudorelati vc velocity rc pon e spectrum. Sec Response spectrum. 

Q. ce Attenuation. 

Quaternary. The geologic time period compri sing about the Ia t 1.65 million years. 

Radiometric. Pertaining to the mea uremcnt of geologi time by the analysis of certain 

radioi otope in rocks and their known rates of decay. 

Random vibration theot·y. A theoreti ca l probabili tic formul ation that lin k band-limited 

Gaussian noise spectra, repre. enting the pectra of earthquake ground motions, 

with corresponding time history peak values. 

Rayleigh wave. A seismic surface wave cau. ing an elliptical motion of a particle at the 

free urface. with no transverse motion. 

Ray-tracing method. A computational method of computing ground- baking estimate 

that a. sume that the ground moti n i composed of multiple arrival s that leave the 

source and are reflected or refracted at velocity boundarie according to Snell ' Law. 

The amplitude · of reflected and refracted wave at each boundary arc recalculated 

according to the law of con crvati on of energy. 

Recun-ence interval. The average time span between events ( uch as large earthqu akes, 

ground shak ing exceeding a particular value, or liquefac tion) at a particular ite. 

Also termed return period. 

Reflection. The energy or wave from a . ei mic ourcc that has been returned (reflected) 

from an interface between material of different clastic propertie within the Earth. 

Reflector. An interface between materi als of different ela tic propcrtie that refl ects ei -

mic waves. 

Refraction . (I) The deflection of the ray path of a . cismic wave caused by its passage 

from one mat rial to another having different cia tic properti c . (2) Bending of a 

tsunami wave front owing to variation in the water depth along a coa tline. 

Regression analysis. A statisti cal technique applied to data to determin , for predicti ve 

purpose., the degree of corre:ation of a dependent vari able with one or more independent vari able . ce Least-squares fit. 

Relaxation theory. oncept wherein rad iated sci mic wa c of an earthquake re ult 

when stored train with in the anh i released at the time of slip along a fault; adjacent faul t blocks reach new state of equilibrium . 

Rigidity. Sec Shear modulus. 

Residual. The difference between the mea urcd and predicted value of some quantity. 

Resonance. An increa e in the amplitude of vibration in an elas tic body or system when 

the frequency f the haking force is clo e to one or more of the natural frequencies of a shaking body. 

Response. The motion in a system resulting from shak ing under specified condition . 

Re ·ponse spectrum. A cur e hewing the mathematically computed max imum response 

of a et of imple damped harmonic oscillators of different natur al frequency(ies) 

to a parti cular record of earthqu ake ground acceleration. Response spec tra, commonl y plotted on tripartite logarithmi c graph paper. show the oscillator 's max imum 

acceler ation, velocity, and displacement a a functi on of oscillator frequency for 

va riou levels of oscillator damping. A computati onal approximation to the 

rc pon e . pectrum is referred to as the pseudorelati ve velocity respon e pectrum 

(PSRV). The e cur c are used by engineer to e tim ate the max imum re pon e of 

i mple structures to complex ground motions. For example, the 5-percent spectral 

accelerati on at I second is the max imum acceleration of the top of a tructure with 5-

percent damping whose natural period of vibration i I second when subje ted to a 

given input ground-acceleration record . 

Return period. See Recurrence interval. 

Risk. See Ea t·thquake ri k. 
GLOS . RY 

Rheological properties. The properti es of rock that de. cribe their abi lity to deform and 

now a a function of temperature. pr ssure. and chemical condition . 

Root mean quar e. quare root of the mean quare value of a random ariablc. 

Rupture front. The instantaneous boundary b tween the slippino and lo ked parts of a 

fault during an earthquake. Rupture in one direction on the fault is referred to as unilateral. Rupture may radiate outward in a cir ular manner or it ma radiate toward 

the two ends of the fault fr m an interior point, behavior ref rrcd to a. bilateral. 

Rupture velocity. The speed at ,. hich a rupture front propagate during an earthquake. 

S wave. A ei mic body wave that involves a hearing moti n in a direction perpendicular to the direction of propagation. When it i rc. olvcd into two onhog nal c mponcnts in the plane perpendicular to the direction of pro1 agation, H denotes the 

hori zontal component and SV denotes the orthogonal component. 

Sand boil. Sand and water ejected to the ground . urface as a resu lt of liqucfa ·ti on at shallow depth: the con ical sed iment deposit that remain. a evidence of liquefaction. 

ea-floor magnetic lineaments. ec M agnetic polarity rever sal. . 

Secular. Refering to long-term changes that take place slow! and imperc ptahly. ommanly used to de cribe change in elc ation. tilt, and strc . or . train rare. that ar 

related to long-term tectonic deformation. 

Segmentation. The braking of fault along their length by other fau lts that cross th m or 

their limitation in length by other factors such as topography or bend. in the trikes 

of the faults. egmentation can limit the length of faulting in a single earthquake to 

some fraction of the total fault length, thu also limiting the izc of the earthquake. 

eiche. 0 . cillation of the urface of an enclo ed body of water owing to earthquake 

shaking. 

Seismic hazard. See Earthquake hazard and Earthquake risk . 

Seismic impedance. ei. mic P-wave velocity multiplied by the bulk density of a 

medium. 

Seismic wave. An ela. tic wave generated by an impul. c such as an carthquak or an 

explosion . Seismic waves may propagate either along or near the Earth's su rface 

(for example. Rayleigh and L ove waves) or through the Earth's interior (P and S 

waves). 

Seismicity. The geographic and hi tori cal distribution of earthquakes. 

Seismic gap. A ection of a fault that has produced earthquakes in the past but is now 

quiet. For . ome . eismic gaps. no earthquake. have been ob. ervcd historically, but it 

is belie ed that the fault segment i capable of producing earthquakes on some other 

basis. such as plate-motion information or . train mea. urcments. This latter case 

may apply to the Ca. cad ia thru L fault. 

eismic moment. measure of the size of an earthquake based on the area of fault rupture. the average amount of slip. and the shea r modulus of the rocks offset b faulting. cismic moment can also be ca lcu lated from the amplitude . pe tra of ci. mic 

waves. 

Seismic r eflection or refraction line. A . ct of sci. mographs commonly di tributed along 

the Earth · .. urface to record ci m ic waves generated by an e · plosion for the purpo c of recording rene tion and refractions f the. e waves from veloci ty di con tinuitie within the Earth. The data co llec ted can be used to infer the internal structure 

of the Earth. 

Seismic risk. The probabi I ity of social or economi con. equence of an earthquake. Sec 

Probabilistic earthquake hazard . 

Seismi c zonation. Geographic delineation of area. having different potential for hazardau. effect from future earthqu ake . Seismic zonation can be done at any 

scale- nati onal. regional. local. or site. Sec M ict·ozonation. 

eismogenic. apablc of oencrating earthqu akes. 

Seismogr am. A record written by a ei. mograph in re pon c to ground motions produced by an earthquake, explo. ion, or other grou nd-moti on ourccs. 

63 
64 A SESS I 1G EARTHQ A KE HAZ RD A D RED Cl G Rl K I TH E PACIFIC ORTHWEST 

eismometer or seismogr aph. A sci. mometer i a damped o cillating ma. s. su h a. a 

damped ma s-. pring system. used to detect . ei mic-wa c energy. The m tion of the 

mas. is commonly transformed into an elec trica l voltage. The electri ca l voltage i. 

recorded on paper, magnetic tape. or another r cording medium. Thi s record i proporti onal to the motion of the seismometer rn a relative to the Earth. but it can be 

mathemati call y convened to a record f the absolute moti on of the ground . Seismograph is a t rm that refers to the ei momcter and its recording devi ce a. a single unit. 

Separation. The di tancc between any two pans of a reference 1 lane (for examp le, a . edimentary bed or a geomorphi c . urface) offset by a fault, measured in any pl ane. Separation i. the apparent amount of fault displacement and i. nearly always lc .. than 

the actual slip. 

Shea r modulus. The ratio of . hear stre s to hear strain of a materi al during simple hear. 

Shear stress. The stress component parallel to a given urfacc . . uch a a fault plane, that 

result from forces applied parallel to the surface or from remote force transmitted 

thr ugh the surrounding rock. 

Shea r wave. See S wave. 

Slab. In thi document, slab refers to the oceani c crustal plate that undcnhrusts the continental pl ate and is consumed by the Earth 's mantle. ee Plate tectonics. 

Slab pull. The force of grav ity cau ing the slab to . ink into the mantle. The downdip 

component of thi force leads to downdip ex tensional . tre s in the slab and may produce earthquakes within the subducted slab. lab pull may al o contribute to stress 

on the subducton thrust fault if the fault is locked. 

Slip. The relati ve displacement of fom1erly adjacent point on opposite side. of a fault, 

mea ured on the fault surface. 

Slip model. A kinematic model that de cribe the amount, di tributi on. and timing of slip 

associ ated with a real or postulated earthquake. 

Slip rate. The average rate of displacement at a point along a fault a. determined from 

geodetic mea. urement . from off et manmade structures, or from offset geologic features whose age can be es timated. It i measured parallel to the predominant slip 

direc ti on or estim ated from the vertica l or hori zontal separ ation of geologic markers. 

oil. (I) In engineering. all uncon. olidated material above bedrock. (2) In soil . cicncc. 

naturall y occurring layers of mineral and (or) organi c con titucnts that differ from 

the underl ying parent materi al in their phy ical, chemical. mineralogic. and morphologic charac ter bccau e of pedogenic procc scs. 

Soil pr·ofile. The crti cal arrangement of soil hori zons clown to the parent materi al or to 

bedrock. Commonl y subcliviclecl into A, B. and hori zon 

Source. (I) The geologic tructurc that generates a parti cular earthquake. (2) The explosion used to generate acou tic or seismic wa c . . 

Source function. The ground motion generated at the fault during rupture. u uall y a~ preel i ted by a theoreti cal model and repre. en ted by a time histor y or spectrum . The 

terms Brune pec trum, Aki . pec trum, and Ha. kel model refer to varying rcprc entation of the ourcc function, each based on different assumption . as de i cd by the 

sc ienti t for which the model i. named. 

Spectral acceleration. ommonl y refers to either the Fourier amplitude spectrum of 

ground accelerati on or the PSRV. 

Spectral amplification. A measure of the relati ve . haking r e ponse of different geologic 

materi als. The rati o of the Fourier amplitude spectrum of a seismogram recorded 

on one materi al to that computed from a seismogram recorded on another materi al 

for the . arn e earthquake or cxplo ion. 

pectral ratio. Sec Spectral amplification. 

pectrum. In seismology, a curve showing amplitude and phase a a function of frequency or pel'iod . 

Standard deviation. The quare root of the average of the square of deviati ons about 

the mean of a se t of data . Standard deviation is a stati sti ca l measure of spread or 

vari ability. 
GLO RY 

Standard penetration resistance. A measure of relati c den ·ity c ·pr sed b the number of bl ows (blow count) needed to pu h a pr be a. tandard distance into s dimcnt. 

The tand ard penetration te t determine the number of blows r quircd to drive a 

tandard ampling poon I ft into the . ediment by repeated! dropping a 140-pound 

weight from a height of 30 in. 

Station. A ground position at which a geo1 hy i al in. trument is lo a ted for an obser at ion. 

Stick slip. The rapid di placement that occur between t\ o ide. of a fault when the 

. hear strc s on the fault exceed the fri ctional tres . tick- lip di. pi a cment on a 

fault radiates energy in the form of ei mic wave , creating an earthquake. 

Stochastic. Applied to proce es that have random charact ri ti cs. 

Strain. Small changes is length and volume a. so iated wi th deformati on f the Earth b 

tectonic stre se. or by the pa . age of seismic waves. 

Strain mte. Strain measurements are computed from obser ed hangcs in length on th 

Earth·. surface. commonl y along multiple path . Because th hange. in length ar 

observed over varying time peri od and path length., they ar pre .. d a. the 

change in length di vided by the mea. urcment di lance divided by the measurement 

time peri od. Thi s number, which i expre ed a the hangc in length per unit 

length per unit time. is termed the strain rate. These mea urcmcnts arc used to infer 

the directions of prin ipal train and tre rates ncar the Earth' s surfa c. 

Stratigraphy. The SlUdy of the character, form , and cquencc of Ia crcd ro ks. 

Stress. Force per unit area acting on a plane with in a body. Six va lue. arc rcquir d to 

characteri ze completely the tre s at a point: three norm al components and three 

shear components. 

Stress drop. The difference between the stress across a fault before and after an ea rthquake. A parameter in many model of the earthquake source that has a b a ring on the 

level of high-frequency shaking that the fault radiates . Commonl y stated in units 

termed bar or mega pascal ( I bar equal I kg/cm2. and I mega pasca l eq ual I 0 bars). 

Strike. Trend or bearing, r lative to north, of the line defined by the intersection of a planar geologic surface (for example, a fault or a bed) and a hori zontal . urfac . 

Strike slip. See Fault. 

Strong motion. Ground motion of sufficient amplitude and durati on to be potential! 

damaging to a buil ding's structural components or architectural feature . . 

Subduction. A plate tectonics term for the proce s whereby the canic lithos phere 

co llides with and descends beneath the continental lithosphere. 

ubduction thrust fault. The fault that accommodates the differential motion between 

the downgoing oceanic crustal pl ate and the continental pl ate as subduction o ·cur. . 

Thi s fault is the contac t between the top of the oceanic pl ate and the bonom of the 

newly formed continental accretionary wedge. AI o alternately referred to as the 

plate-boundary thrust fault. the thrust intcrfac fault. or the mcgathru l fault. 

Surface faulting. Displacement that reache the Earth ' urfacc during slip along a fault. 

ommonl y ac ompani cs moderate and large earthquake hav ing focal depths les 

than 20 km . urface faulting al. o may accompany a ci mic tectonic rcep or natural or man-induced ubsidcnce. 

urface wave. ei mic wave that propagates along the Earth 's urface. Love and Rayleigh waves are the mos t common. 

Surface-wave magnitude. ec Magnitude. 

Tectonic. Refer. to ro k-dcforming proccs es and re ulting strucwr that occur over 

regional s cti on of the lithosphere. 

Teleseismic. Pertaining to earthquake at listances greater than I ,000 km from the mea-

. urement site. 

Tilt. An ob. crvcd rate of change in the . lope of the Earth's urface. 

Thrust interface fault. ee Benioff zone and subduction thrust fault. 

Time history. The sequence of values of any time-varying quantity ( uch a a groundmotion mea urcmcnt) measured at a set of fixed times . I. o termed time . eri c . 

65 
66 SE I G EARTHQ AKE HAZA RDS A D REDUCI G Rl K I TH E PA IFI ORTHWEST 

Transform fault. A pecial vari ety of trike- lip fault that accommodate relative horizontal . lip between oth r tectonic clement. . such a oceanic crustal pl ate . 

Traveltime curve. A graph of arri al times. commonl y P or S waves, recorded at different poi nt. a a function of di tance from the eismic source. Seismi c velocitie 

within the Earth can be computed from the lope of the resulting curves. 

Tsunami . An impul ·ivcly generated sea wave of I cal or di stant origin that result from 

large- ca lc ea noor di spla emcnt a sociated ' ith large earthquake. major . ubmarinc slides. or exploding vo lcanic i. lands. 

Tsunamigenic. Rcfcring to tho. e earthquake sources, commonly along maj or subduction-zone pl ate boundari es such as tho e bordering th Pacific Ocean, that can generate t. unami . 

Tsunami magnitude (Mt). number u. cd to compare ize. of tsun amis generated by different earthquakes and calculated from the logarithm of the maximum amplitude of 

the tsunami wave measured by a tide gauge distant from the tsunami ource. 

Turbidites. ca-bottom deposit formed by mas ive slope failures where ri ers have 

deposited large d ltas. These slope fail in respon e to earthquake baking or execs-

. ive sedimentation load. The temporal correlation of turbidite occurrence for ome 

delta of the Pacific Northwest suggests that these deposits ha e been formed by 

earthquake . . 

nlocked fault. ee Locked fault and Creep. 

Velocity. In reference to earthquake haking, veloc ity is the time rate of change of ground 

displacement of a reference point during the pa sage of earthquake seismic waves. 

ommonly expre sed in centimeters per econd. 

Velocity structure. A generali zed regional model of the Earth 's crust that represents 

cru stal structure u ing layers having different a sumcd sei mic velocitie . 

Water table. The upper surface of a body of unconfined ground water at which th water 

pres ure i equal to the atmospheric pre ure. 

Wavelength. The eli . lance between succcs ivc points of equal amplitude and phase on a 

wave (for example, crest to ere t or trough to trough). 

YBP. An abbreviation for year before present. 
CO VERSIO F CTORS 

FACTORS FOR CONVERTING METRIC UNITS 

TO INCH-POUND UNITS 

67 

Quantitati ve information in thi s profe ional paper is given in the International System f niL, or metric . ~m. The 

following table convert s the most common metri c unit to inch-pound uniL for the convenience f those \ ho rrefer or ar' 

more familiar with the U.S. Customary Sy tem of units. Also included arc definitions of geochronologic dating terms used to 

denote elapsed time from the pre ent to the pa t. 

Multiply metric unit 

Area 

square meter (m2) 

square kilometer (km2) 

Density 

gram per cubic centimeter (g/cm3) 

Length 

millimeter (mm) 

entimeter (em) 

meter (m) 

kil ometer (km) 

kilometer (km) 

Volume 

cubic centimeter (cm3) 

cubic meter (111 3) 

cubic kil ometer (km3) 

By 

10.76 

0.3 6 1 

62.43 

0.03937 

0.3937 

3.2 I 

0.62 14 

0.5400 

0.06 102 

35.3 1 

0.2399 

Abbreviations: ka, thousand year ago; Ma, million year. ago. 

To obtain inch-pound unit 

square foot (ft-) 

s ru are mile (mi 2) 

pound per cubic foot (lb/ft3) 

inch (in) 

inch (in) 

foot (ft) 

mil (mi) 

mile, nautica l (nmi) 

cubic inch (in3) 

cubic foot (ft 3) 

cubic mil (m i3) 

TECTONIC SETTING 
Preceding page. IIISerl, masonry damage cau ed by the April 29, 1965, Seattle, 

Wa h., earthquake. Photograph courtesy of NOAA/EDIS. Background, damage to unrein forced masonry wall and parapet in downtown Klamath Fall s, Oreg., during the Sept. 

20, 1993, M 5.9 and M 6.0 earthquakes (from Dewey, J.W. , 1993, Damage from the 20 

September earthquakes near Kl amath Falls, Oregon: Earthquake & Volcanoes, v. 24, 

no. 3, p. 12 1- 128). 
AN INTRODUCTION TO EARTHQUAKE SOURCES 

OF THE PACIFIC NORTHWEST 

By Timothy I. Wal hI 

INTRODUCTION 

The id nti fi cati on of earthquake ource , a eragc recurrence inter als, and likely magnitudes i, e. sential for accurate earthquake ri sk a es. ment. These arc most easil y 

characteri zed by historica l records, but the recordc I hi. tory 

of the Pacific orthwe. t is hort. The instrumental record of 

earthquake in Wa hington and Oregon is fairly complete at 

about the magnitude 4 level only since 1960 (Lud win and 

others, 199 1 ). Prior to 1960. onl y a few large earthquakes 

had been in. trumentall y located in Wa. hington and Oregon, 

and earthquake catal gues rely on felt reports. Felt report 

are, unfortunately, parse and difficult to interpret. Although 

the rate of seismicity in the Puget Sound area appears to be 

cyclica l (Palmer, 199 1 ), the data base is too , mall for stati sti cally valid conclu ions. In contra t, Hutton and Jones 

( 199 1) were able to document change in rates of , ei micity 

in southcm Californi a becau e they had a large-en ugh data 

ba e to draw stari. tical inference . For instance, Townley 

and All en ( 1939) reported I earthquake in Oregon 

between 184 1 and 1928 and 166 earthquakes in Washington 

between 1833 and 1928 but more than 3.000 earthqu akes in 

aliforni a for the same period. 

1 ot onl y are th earthquake ca talogue. for the Pacific 

1 orthwes t hort , but they still require ignificant ed iting of 

the prcinstrumental entries. Rasmussen ( 1967) deleted a 

number of entries in the Townley and Allen ( 1939) atalogue 

but , till reported more earthquake ( I 6) in Washington 

between I 4 1 and 1928 . Ludwin and others ( 199 1) . uggested that the ca talogue for magnitude 6 and larger carthjuake. is probably complete at eight event in the Pacifi c 

orthwc t. but Ludwin and Qamar ( 199 1 ), who rev iewed 

newspaper report s a, part of a continuing cff rt to edit the 

prc- 1928 cata logue, recti covered yet another event of that 

approx imate magnitude. 

1 Di i n of Geology and Ea rt h Re,ources, Washi ngton State Department of 1 antral Resource,. Ol ympia. WA 9 50 7007. 

The heer number of li sting docs not give a ompl 1c 

account of the true sei. mic nu x in th region. or 

in tance, Califomia does have sub lan tiall y mor arthquakc than the Pacifi c torth\ e. t, but bccau s~ 

earthquake. are more lik I t be . hallo' . th 

lower magnitude level , and the ca talogue includ a larg 

number of after hock as ' ell. Thi. account. for at lea. t 

some of the difference. 

Another conscquen c f the shallower , i. mtctty in 

Cali forni a i that fault arc more likely to rupLUn: to th . urface. It ha frequen tl y been po . iblc t lo at surfa faul ting after earthquake in aliforni a. \ hcreas thi s has ctt he 

accomplished in Washington or Oregon. onfidcnt ly relating earthquake. to a causati ve tructu rc is n ssary for sitespecific seismic ri sk a cssment.. Withoutthi. kind of in formati on, engineers in the Pacifi onhwcs t commonly ha e 

to rely on " noating earthquake .. (. hallo\ crustal earthquakes that arc a .. umed to be po .. ible anywh r wi thin a 

geologic or phy. iograi hie province) for dcs ion spec ificati on . . This make inference of rc urrencc interval. diffi ·ult. 

at lea t on a ite-. pccifi c bas is, becau. c . ci. micit is tied to 

a region rather than a geologic . tructurc. Therefore, land -usc 

planners. who in Wa hington are required to 7.0n f r . cismi c ri sk. are un able to relate cismic risk to causa ti structures and u. uall y zone only for liqu fac tion su ccptibility. 

Thu . the relati ve paucity of histori cal informati on 

means that earthquake-hazard a. es ment in the Paci fic 

onhwcst must rely more heav ily on geologic mapping. 

geophysical and tectonic modeling, and paleo cismology 

than on past experience. Thi volume of the pr fe sional 

1 aper contains r port. that de cribe earthquake source. 111 

the Pacifi orthwe tin their tectonic contex t. 

TECTONICS 

na ely and Well (thi volume) ummari zc the enozoic tectonic devel pment of the continental margin of th 

Pac i fie onhwc t. They show that. throughout Tertiary 

71 
72 ASSE I 'G ARTHQ AKE H ZARDS A D RED lNG RISK I THE PA IFI ORTHWEST 

time, oblique . ubduction produced epi odic uplift of the 

coa t range , folding and thru sting of accretionary-wedge 

ediment , and trike-slip faulting. M any of the e feature 

areal o pre ent in Quaternary sediments. They note e era! 

unconformitie in Quaternary depo it on the continental 

shelf that they infer to have been cau. ed by both epi odic 

downwarping and diapiric uplift. 1

oting cveral Quaternary 

faults on the shelf, along the coa t. and at the northern end of 

the Ol ympic Penin ula. th y ugge t that Holocene seismicity may have triggered large land tides that have been 

mapped in the Olympic M ountain . 

Yeat. and others (thi s volume) compile the geologic 

mapping, expl orati on and water-well-drilling data and 

sei mic-refl ection profile · of Oregon' Willamette Valley 

and analyze it enozoic (parti cularl y post-Miocene) tectonic hi tory. They show that there are at lea. t a dozen 

po !-Miocene crustal faults in the va lley, at least one of 

whi ch (the M ount Angel fault) i acti e. Becau. e offset in 

mid-Quaternary ediment is small and mappable length of 

all structures i short, the authors conclude that lip rate 

mu t be small and potential earthquake magnitudes are 

probabl y moderate. 

Goldfinger and others (thi volume) compile the available seismic-refl ecti on data, along with SeaMARC and 

GLOR IA sonar record , SeaBeam bathymetry, and observati ons from the ubmer. ible AL VI , to map late Cenozoic 

strucwres on the central and northern Oregon continental 

margin . In a controversial interpretation, they map a erie of 

left-lateral trike- lip fault. on the abys al pl ain and sugge t 

that the fault continue onto the continental lope. These 

fault trend at high angle to most of the tructures in and 

overl ying the Juan de Fuca and Gorda plates (although they 

are para II I to the maj or transform faults that offset the Gorda 

Ridge). At leas t one of the e faults, the Wecoma, shows significant late Quaternary off et and appears still to be active. 

Th authors offer several hypothese. for the formati on of 

the. e structure but, as yet. their . igni ficance i unclear. 

Still , very lillie is known about the . eismic capability of 

the corm st other tructure in the Pacific onhwest. The 

above author . Roger and others (thi. volume), and M adin 

(volume 2) show many suspected ac ti ve faults, but onl y the 

M ount Angel fault (Yeats and other. . thi s volume) has been 

confidentl y linked to hi stori ca l earthquakes. Paleosei mic 

studie. arc beginning to link some fauiL to prehi stori c earthquake (for instance, Wit on and others, 1979; Campbell and 

Bentley, 198 1; Bucknam and Barnhard , 1989). and tudies 

f . ci micity have delineated cvcral fault zones (Weaver 

and Smith. 19 3; Zollwcg and J hnson, 1989; Yelin and Patton, 199 1 ). However, much remain. to be done to a se s the 

ri . ks of shallow cru tal faulting. 

GEOPHYSICS 

Weaver and Shedlock (thi volume) summari ze the 

potential ei mic ource region as ociatcd with oblique subduction in the Paci fie orth wcsl. The. c regions are ( I) the 

cru t of the orth America plate, which has produced large 

earthquakes in Wa hington in I 72 and in Briti h olumbia 

in 1946; (2) the subducting labs of the Juan de Fuca and 

Gorda pl ate . which produced large earthquakes in Wa bington in 1949 and 1965 (and, they uggest, the Port Orford area 

of Oregon in 1873); and (3) the interface between the two, 

which ha been a ei. mic in historical time with the po sible 

exception of the 1992 ape M endoc ino earthquake 

(Michael, 1992). 

M a and others (th is volume) analyze a well-constrained 

data set recorded by the ni vcrsity of Washington sci mic 

net and conclude that th orth Ameri can cru t i. responding principall y to north-south compre . ion. impl ying that the 

contribution of pl ate con ergence to the state of stre s mu t 

be small in compari on. They al o suggest that, although 

focal mechani ms from the t. Helen ci. mic zone (Weaver 

and Smith, 19 3) impl y a rotation of the regional stre s field 

from north-south comprcs ion to compre: ion directed 

toward 30° E .. the data al. o permit an unrotated tress 

field that take advantage of a preex i. ting zone of weaknes . 

PALEOSEISMICITY 

!though no hi tori cal subduction-zone earthquakes 

have occurred in the region, and there i no recognized 

contemporary thrust seismicity on the ubduction interface, 

recent geodetic tudies ( avagc and oth rs, 198 1, 199 1) 

and compari son with other subducti on zones (Heaton and 

Kanamori , 1984; Heaton and Hartzell , 1987; Roger , 1988) 

suggested the possibility of large (M 8+) thru t earthquake. 

on the Ca. cadi a subduction zone. Because the potential 

rupture urface is inacce iblc, traditional paleoseismology 

tool uch as trenching fault trace are unavailable. In 

1985, during a Nati onal Earthquake Hazards Reduction 

Program workshop in Seattle. a meeting wa held to pl an 

trategie for te ting the hypothesi of the Cascadia subduction-zone megathrust earthquake (Schwartz, 1986). As 

a result of that m eting, B.F. Atwater of the U.S. Geological Survey ought to tes t the hypothesis of Ando and Balazs ( 1979) that aseismic subducti on . hould result in coa tal 

uplift. Instead, he found ev idence of udden submergence, 

dated at more than 250 yea r ago, along the southern 

Washington coa t (Atwater, 1987). Because it had been 

observed that great . ubduction-zone earthquakes cau e 

both sudden uplift and subsidence (Pi afker, 1969; Pl afkcr 
AN I TROD CTIO TO EARTHQUAKE 0 RCE OF THE P IFIC NORTH WE T 73 

and Savage, 1970), thi evidence of ubmergence uggc. ted that great subduction-zone earthquake may have 

occurred on the Cascadia subduction zone b yond the hi storical record of the Pacifi c orthwe r. 

Atwater (thi volume) pre ents a pictori al view of the 

evidence for inferred co eismic sub idence followed by depo ition of a sand heel by a t unami. At least ix peaty soil 

interpreted to be coseismically sub id d lowlands arc 

present in coa tal southern Wa hington. 

Peterson and Darienzo (this volume), as in a previous 

work (Darienzo and Peter on, 1990), also report ev idence 

of sudden ubmergence in A lsea Bay on the central Oregon coast. They investigate other models for rapid submergence, such as riverine nood , storm waves, and changes 

in tide range. They conclude that, because the oil are laterally persi tent, verti call y si milar in grain ize and 

organic-carbon content, and common ly overl ain by and 

inferred to be tsunami deposit , they are most likely due to 

coseismic subsidence. 

However, elson and Person ius (thi s vol ume) examine 

the record of mar h stratigraphy both in the Pacific Northwe t and on nontectonic coasts and conclude that udden 

oscillation in . ea level accompanied by changes in local 

sediment innux and irregular compaction can produce a similar stratigraphic record. They argue that on ly where there is 

evidence of substantial changes in water depth can cosei mic 

subsidence be confidently inferred. 

Adam (this volume) prov ide permi ss ive evidence for 

two more mt ss tng piece of the subduction-zoneearthquake puzzle. Examining the record of turbidite in 

the Cascadia Channel, documented by Grigg. ( 1969) and 

Griggs and Kulm ( 1970), he show that if the e turbidites 

were triggered by earthqu akes, a originall y in ferred by 

Grigg . they imply . ome degree of synchroneity over ignificant distance .. Adams (this volume) report that there 

arc more turbidite per tributary to the Ca cad ia hannel 

than there arc in the mai n channel. Thi implie that turbidity curren ts from the tributary channel coa le ced in the 

main channel, depositing one turbidite in tead of the mu ltiple overlapping one that would be expected if the turbidity 

currents arrived at different times. 

A lthough the papers in thi s volume indi vidually pre ent 

evidence that in general i. merely permissive, collectively 

they argue per uas i vcly that the Pacific Northwes t is at ri k 

from a type of earthquake not seen in hi torical time. 

Although much work remain to characteri ze uch an event, 

it would be prudent to incorporate a ubduction-zone earthquake in disas ter planning and engineering practi ce in the 

Pacific orth wcst. 

REFERENCES CITED 

Ando, M asataka, and Salaz . . E. I.. 1979. Geodeti idence for 

aseism ic. ubducti on of the Juan de Fu a plat<.!: Journal of Geophysical Research, v. 4, no. 86, p. 3023- 302 . 

Atwater, B.F., 19 7. E idence for great Holocene earthquakes 

along the outer coa. t of Wa. hington tate: cience, . 236, no. 

4 04, p. 942- 944. 

Bucknam, R.C .. and Barnh ard, T.P., 19 9. Eviden e f sudden late 

Holocene uplift in th central Puget Lowlan I, Washington: 

EO [American Geophysical nion Transaction.], v. 70, no. 

43, p. 1332. 

Campbell, .P, and Bentley. R.D .. 19 I. Late Quaternary deformation of the Toppenish Ridg uplift in south-central Washington: Geology, . 9, no. II , p. 519- 524. 

Darienzo, M .E .. and Peterson. .D., 1990, pisodic te ·tonic subsidence of late Holocene salt mar. he. , northern reg n central 

a. adia margi n: Tectonic . . v. 9, no. I, p. 1- 22. 

Gri gg. , G.B .. 1969, a cadi a hannel- The anatomy of a deep-. ea 

channel: Corvallis. Oregon State niversity. Ph.D. dissertation, 183 p. 

Griggs, G.B .. and Kulm, L.D .. 1970, edimentation in ascadia 

deep-sea channel: Geological Society of America Bull tin, v. 

I. no. 5, p. 136 1- 1 4. 

Heat n, T.H ., and Hanzel!, .H., 1987, Earthquake hazards on the 

Cascadia subduction zone: cience. v. 236. no. 4798. p. 

162- 168. 

Heaton, T. H., and Kanamori. Hiroo, 1984, eismic potential associated with subduction in the northwestern nited tates: Seismoloo ical ociety of America Bulletin. v. 74, no. 3, p. 

933- 941. 

Hutton, L. K., and Jones, L.M ., 199 1. hanges in the rate f. ei. mi -

it in sou thern aliforni a between 19 2 and 1989 [abs.j: Seismological Research Letter , v. 62, no. I . p. 47. 

Ludwin, R.S .. and Qamar, A. l.. 199 1, 1882 earthqu ake red is overed: Washington Geology. v. 19, no. 2, p. 12- 13. 

Ludwin. R.S .. Weaver, .S .. and rosson. R . . , 199 1, Sei smicity of 

Washington and Oregon. in Iemmons. D.B., Engdahl. .R., 

Zoback . M .D., and Blackwell, D. D .. ed .. , eotectonics of 

onh America: 8 uller, olo .. Geol gica l ociety of Ameri -

ca, Decade M ap Volume I, p. 77- 97. 

Michael, Andrew, 1992. Three's a crowd in Cali fornia: ature, v. 

357,no.6374.p. 111 - 11 2. 

Palmer. S.P., 199 1, Modified Mercalli intensity VI and greater 

em1hquakes in Washington State, 1928- 1990: Washington 

Geology, v. 19, no. 2, p. 3- 7. 

Plafker, George, I 969, Tectonics of the M arch 27, 1964, Iaska 

earthqu ake: U. Geological urvey Profess ional Paper 

543- I, 74 p. 

Plafker, George, and avage, J.C., 1970, Mechanism of the Chilean 

earthquakes of May 2 1 and 22, 1960: Geological Society of 

America Bulletin, . 81. no. 4, p. I 00 1- 103 1. 

Rasmussen, orman, 1967, Wash ington State earthquake· 1840 

through 1965: eismological Society of America Bulleti n, . 

57, no. 3, p.463 76. 

Rogers, G.C .. 1988, An assessment of the megathrust earthquake 

p tenti al of the Cascadia subduction zone: anadian Journal of 

Earth Sciences, v. 25 . no. 6, p. 844- 52. 

Savage, J . .. Lisowski, Michael, and Prescott. W.H ., 198 1. Geodetic strain measurements in Washington: Journal of Geophys ical 

Research, v. 6, no. 86. p. 4929-4940. 
74 ASSESS! 1

G EARTHQUAKE HAZARD A D REDUCI G RISK I THE PACIFIC ORTHWEST 

--- 199 1, Strain accumulation in western Washington: Journal 

of Geophy ical Research, v. 96, no. B9, p. 14493- 14507. 

Schwartz, D.P., 1986, Subduction zone earthquake in the Pacific 

orthwe !- Evaluating the potential- Thought for discussion at the Seattle workshop, in Hays, W.W. , and Gori, P.L., 

eds., Proceedings of Conference XXXIII, Work hop on earthquake hazards in the Puget Sound, Wa ·hington area: U.S. Ge -

logical Survey Open-File Report 86- 253, p. 58- 68. 

Town ley, S.D., and Allen, M .W., 1939, Descriptive catalog of earthquakes of the Pacific coast of the United States, 1769 to 1928: 

Seismological Society of America Bulletin , v. 29, no. I, p. 1- 297. 

Weaver, C.S., and Smith, S. W., 1983, Regional tectonic and earthquake hazards implications of a crustal fault zone in 

sou thwestern Washington: Journal of Geophysical Re earch

v. 88, no. Bl 2, p. 1037 1- 103 3. 

Wil on, J.R., Bartholomew, M .J. , and Carson, R.J ., 1979, Late 

Quaternary faults and their relationship to tectonism in the 

Olympi Penin ula, Washington: Geology, v. 7, no. 5. p

235- 239. 

Yelin, T .. , and Patton, H.J.. 199 1, eismotectonics of the Portland

Oregon, region: Sei mological Society of America Bulletin, v

81, no. I, p. 109- 130. 

Zollweg, J.E., and Johnson, P.A., 1989, The Dan·ington seismic 

zone in northwe tern Washington: Seismological ociety o

America Bulletin, v. 79, no. 6, p. I 33- 1845. 
Paleoseismicity 
Preceding page. lnse rL, damage from the 1949 Puget Sound earthquake. Photograph 

by William P. onser, by permi ss ion of the Olympia Heritage Commiss ion. Background, 

damage to unreinforced rn a onry wall and parapet in downtown Klamath Falls, Oreg., 

during the Sept. 20, 1993, M 5.9 and M 6.0 earthquakes (from Dewey, J.W. , 1993, Damages from the 20 September earthquakes near Klamath Fall , Oregon: Earthquakes & 

Volcanoes, v. 24, no. 3, p. 12 1- 128). 
COASTAL EVIDENCE FOR GREAT EARTHQUAKES 

IN WESTERN WASHINGTON 

By Brian F. Atwater 1 

ABSTRACT 

Buried marsh and fore t soil s in . outhern coastal Washington show that earthquak es of magnitude or larger (great 

ea11hquakes) probably occur at the Ca cadia subduction 

zone. Simi lar soil s in Alaska and Chile record lowering of 

coastal land that accompanied great ubduction-zonc earthquakes in tho e region . Fossil plants how that many of the 

oi l in Wa hington stood above most or all high tid until 

the onset of buri al, when the soils were frequently ubmerged by tidal water at least 'h m deep. In ome ca e , fossil 

plants further how th at thi ubmergence re ulted from 

abrupt lowering of the land, not from gradual ri e of the sea . 

Some of the buried soil s arc mantled with sand that was 

probably deposited by tsun ami . Sequences of buried oil 

give ev idence that hundreds of year commonly elapse 

between great earthquakes in southern coa tal Wa. hington. 

INTRODUCTION 

Coastal geology ha provided strong ev idence for the 

pa t occurrence of great earthquake at the Ca cad ia 

. ubduction zone (Roger and others, thi s olume). uch geologic evidence has been found at many c tuari es along the 

Pacific coast from outhcrn Briti h Columbia to northern 

California (A twater and other , 1995). ome of the evidence 

1U.S. Geo logica l Survey. De partment of Geological Scie nces, 

nivcrsit y of Was hington AJ-20. cattle. WA 9 195. 

from coa tal Oregon is described in el on and Per onius 

(thi s volume) and Peter on and Dari nzo (thi s volume). 

In thi chapter, I pre ent mostly pictorial evidence from 

outhern coa tal Wa hington, and I compare thi c idcnce 

with a few of the geologic effect of great hi torical earthquakes in Ala ka and Chile. The Alaskan and hi lean example . how that a great subduction-zone earthquake can leave 

geologic records of abrupt land-level change and an en uing 

tsunami. I infer that uch geologic records arc present in 

outhern coa. tal Wa hington and that the earthquakes 

re ponsible for them occurred hundred of year apart. 

The chapter doe not pre ent evidence that sei mic 

haking accompanied the inferred earthquake . trong evidence of thi s kind was fir t recognized in 1992, two year. 

after the chapter was mostl y completed. The evidence . how. 

that liquefaction accompanied the mo t recent of the abrupt 

land-level change in outhern coa tal Wa hington. Detail 

about thi evidence have been reported by Obermeier 

( 1994), who di. covered it, and Atwater ( 1994). 

ACKNOWLEDGMENTS 

I thank Mary Ann Reinhart for con tributing figure 228 . 

am al o grateful to Alan R. Nelson, H. Edward Clifton, 

David K. Yamaguchi , Barbara C. Hillier, John Synnefakis, 

and Timothy J. Wal h for improv ing the manu cript through 

review and di cu ions. 

77 
7 I G EARTHQ AK HAZ RD A D RED I G Rl K I THE PA IFI ORTHWEST 

GREAT EARTHQUAKES AT SUBDUCTION ZONES CAN CAUSE SUDDEN 

COASTAL SUBMERGENCE . . . 

Regional lowering of the land commonly accompanie 

great earthquakes at subduction zone . Thi ubsidence can 

be viewed a part of a cyc le of deformation in the continental 

plate (fig. 9). Before an earthquake. the continental plate 

bulge. over and behind a tuck patch on the pl ate boundary. 

During the earthquake. the bulge collap es and the leading 

edge of the continental plate stretche and thin. from lurching . eaward whi le mo t of the remai nder of the plate stand. 

sti ll. Such ela. tic thinn ing can produce a belt of ubsidence, 

a. illustrated by great subduction-zone earthquakes in 

A Ia. ka and hi le (fig. I 0). 

The ub idence in Al a ka and Chile dropped 

well-vegetated lowlands into the intertidal zone. This ubmergence led to the wide pread depositi on of tide flat mud on 

. uch lowlands a. an Alaskan pruce forest (fig. II A) and a 

Chilean pa ture (fig. II 8 ). The change from well-vegetated 

lowland to tidefl at began immediately after ea h earthquake. 

irst-year casualties in Ala ka included Si tka pruce 

(Piafker, 1969, fi gs. 12, 19, 22). T idal-marsh plants later coloni zed the post-earthquake tideflats (fig. II ). 

BEFORE 

EARTHQUAKE 

DURING 

EARTHQUAKE 

boundary 

Figure 9. Deform ation of a continental plate before and during a 

subduction-zone ea rthquake. Black arrows show directions of 

strc. s within the plates; colored arTows show exaggerated vertical 

di!>placemcnt of the land . urfacc: barbs indi cate direction of plate 

mo emcnt. From models of Plafkcr ( 1972, p. 9 13) and Thatcher 

and Rundle ( 1984). 

I• igurc I 0 (faci ng column). Land-level change accompanying 

two great subduction-1.onc earthquakes. A. Alaska, 1964, magnitude 9.2 (Pi afkcr, 1969). B. Chile. 1960, magnitude 9.5 (Piafkcr and 

:JVagc. 1970). M agnitudes arc from Kanamori ( 1977). 

62' 

56' 

38' 

44' 

156' 150' 

• 

B 

ALASKA 

78' 

EXPLANATION 

1' 2 Uplift-Showing maximum 

amount, in meters 

~ 1 Subsidence-Showing 

• 

maximum amount. in 

meters 

Limit of subsidenceDashed where inferred 

Plate boundarySawteeth on upper 

plate 

PACIFIC 

OCEA 

0 100 KILOMETERS 

L___,__j 

144' 

' ' ' 

PACIFIC 

OCEAN 

0 100 KILOMETERS 

L___,__j 

I 

I 

I 

I 

72' 
OASTAL EYIOE E FOR GREAT EARTHQ AKES I WESTER WASH!N TO 79 

... AND THE CONSEQUENT BURIAL OF COASTAL LOWLANDS 

--- m 

A 

Figure 1!. Geologic record. of coastal land . ubsid nee from great earthquakes in Alaska and Chile. Tidal-mar. h plams (p) locall y' 

coloni ze mud (m) of post-earthquake tiden ats. A. dead itka spruce forest near Portage. Ala ka (fi g. l OA). Mud l-2 m thi ck accumulated 

on the forest noor between 1964. when th forest became tiden at, and !973. when the picture was taken (Ovenshine and others, !976). 

Photograph by A.T. Ovenshine. B. form r pasture near M aull in, hile (fig. I 08). mostl y covered by mud since sub iding in !960. 

Still-exposed soil (s) of pre-earthqu ake pasture undergoes erosion along meandering tidal channel (c). Photograph taken at low tide in 19 . 
80 AS ESS I G ARTHQ AKE HAZA RDS 0 REOUCI G Rl K I THE PA !FIC ORTHWE T 

BURIED LOWLANDS ARE COMMON AND CONSPICUOUS IN 

SOUTHERN COASTAL WASHINGTON 

Buried lowlands border all four c tuarie of southern 

coa tal Wa hington- thc lower Copalis River. Grays 

Harbor, Willapa Bay. and the lower Columbia Ri ver (fig. 12). 

130" 

45" 

PACIFIC 

OCEAN 

A 

100 KILOMETERS ~ 

10 KILOMETERS ~ 

The large dot in figure 128 locate example shown in thi 

chapter. 

t!any of the buried lowlands retain recogni zab l lowland features . The oil of the former marsh in figure 13 has 

lateral extent comparable to that of the modem marsh. Rooted 

in the buried-mar h ·oi l arc ros il tcms and leaves oft he grass 

Deschampsia caespiwsa, which i al:o the dominant plant of 

the modern marsh (sec detailed photographs in fig. 19). 

The m st conspicuous buried lowland. arc marked by 

standing dead trun k of we. tern rcdc dar (fig. 14). Such 

nag impressed a naturali st on a nineteenth-century railroad 

su rvey : '"On . omc or the tide-meadows about Shoal water 

[Will apaJ ba dead tree or this spc ics onl y arc standing. 

sometime in groves * * *'' ( oopcr. I 60, p. 26). The redcedar snag. , mo tl y rooted below present high tide, arc . cattercel along nearly 100 km ofcoa. t from the Copali s Ri ver to 

the Columbia Ri ver. The trees died about A.D. 1700 

(Yamaguchi and others. 19 9: Atwat rand others, 1991 ). 

Figure 12 (fa cing column). The Cascadia subduction ;one and 

southern coa. tal Washington. A, Cascadia subd ucti on zone. Sawteeth arc on ovcrri ling on h America plate. Sis th almon River 

estuary. B. southern coastal Washington. B is Bay enter; . hchali. Ri ver and Blue Slough: E. Elliou Slough; J. Johns River; 1 

iawiakum Ri ver: and W, Willapa River. 

Figure 13. Soil of a former marsh (m), buried and later ex humed along the Johns Ri ver, Grays Harbor (fig. 12 B). Especially resistant to 

erosion , the soi l . uppon. the bench on which the people (p) arc standing. The water is at low tide: the highc t modern tides inundate the live 

mar h behind the people. Locality 13 of Atwater ( 1992). 
COASTAL EVIDE E FOR GREAT EARTHQ AKE I WESTERN WA HI GTO 81 

/ A 

I 

/ .. :( 

Figure 14. Dead forest of western red eclar (Thujo plicata) 1 rotruding through a brac ki sh-water tidal marsh along the Copalis Ri ver 

(Atwater. 199-. figs. 2. 5). The noor f the dead fore. t lie. buried below the marsh surface. The live forest in the background o cupies the 

upland above the highest tid . . 
2 

A 

B 

A 

MHHW 

MLLW 

SSESS I G EART HQ KE H Z RDS I RED I G RI K I 1 T ilE PAC IFI 'ORT HWEST 

BURIAL OF LOWLANDS IN COASTAL WASHINGTON 

RESULTED FROM SUBMERGENCE 

SUBMERGENCE 

DEPOSITION ONLY 

Sitka 

spruce, 

--~) DEAD FOREST 

PROTRUDING 

THROUGH THE 

TIDAL MARSH 

--~) MORE LIVE FOREST 

Western 

Diverse 

Tidal ubmergcncc (fig. I -A) led t the dcmi. c of the 

redccdar forest hov n in figure 14. Deposition alone- by 

storm , nood, or tsunami- would n t have promoted such 

. ubmcrgcnce (fi g. 158). 

Buried spruce forest in coastal Washington gi c further cviden e for submergence.. ud ju t above the oil s of 

the. c fore ts commonl y contains the below-ground stem 

(rhi zome ) of Trig loch in maritima or Ca rex lyngbyei, gras -

like plant that in coa tal Washington arc rooted mainl y Y2 

to I Y2 rn lower in elevation than the lowest "itka spruce 

(fig . 16, 17). The prucc tumJ . and T. maritima rhi zomes 

. hO\ n in figure 17 thu .. ugg t that the land was lowered 

about Y2 t I Y2 m relati ve to the . ca. In compari son, the 

max imum regional subsidence from great ubduction-zonc 

earthquake. in Al a ka and hilc ausccl about 2 m or 

. ubmergcn e (fig. I 0). 

Figure IS (facing column). Buri al of the flo r of a coastal lowland forest. /\, depos ition tri ggered by ti dal submergence. The 

result is a dead forest that 1 rotrudes throu0 h a tidal marsh, a' in 

fi gures IIA and 14. 13. deposition by storm , fl ood, or tsunami without las ting tidal submergence. The forest fl oor build. fa rther above 

high tides, contrary to the re. ult . hown in figures II A and 14. 

Figure I 6. Dominant plant> of coastal mar ·hes and swamps in . ou thern Washitl:,,tOn. A, distribution of plants re lati ve to tides and . ali nity. M H HW is mean higher high water; MLL W is mean lower low water. From descriptions by Franklin and Dyrness ( 1973, p. 294- 295). 

Kun;.c and orne! ius ( 19 2), and Weinmann and others ( 19 4 ). 8 , Triglocltin maririma on a muddy tideflat. Wi ll apa Bay. The shovel hanelk i' 0.5 m long. 
High 

tide 

B 

COASTAL EVIDENCE FOR GREAT EARTHQ AKE I 1 WESTER WA HI GTO 

SUBMERGENCE ) 

Spruce 

forest 

2 3 

Tideflat T. maritima 

marsh 

BURIAL ) 

Deschampsia 

caespicosa 

4 

D. caespitosa 

marsh 

EXHUMATION 

5 

Beach 

Figure 17. Former! buried spruce stumps exhumed on a bca h at Bay Center. Willapa Ba (fi g. 12/3). The shovel hand le (h) in A and 

3 

is 0.5 m long. Local it 16 of Atwater ( 1992). A. overview. showing SI rucc stumps (a-d) at low tide on the beach. Leaves of ccl gra s (e. 

Zostera mari11a ). a sa lt-water aquati c plant. mamle much of the bca h and mark th le cl of highest summ nimc tides (t). 13. inferred sequence of eve nt~ lead ing 10 burial and ex humation. C. detail of swmp (a) . howing peaty soi l (s) in which the slUmp is rooted and lowermost 

0.3 m of the ti deflat mud (m) that accumu lated on thi ~ soil. D. vertica l vic' of Triglocilill maritima rhi;omcs in mud at lower ri ght in 

Individual rhit.omc . . which include anachcdlcaf ba s'~. arc mostl y perpendicular 10 15- m scale. 
84 A SESSI G EARTHQUAKE HAZ RDS A DR DUCI G RISK I THE PACIFIC ORTHWE T 

SUBMERGENCE OF BURIED LOWLANDS IN COASTAL WASHINGTON 

HAPPENED ABRUPTLY 

Fossil pl ant and abrupt top of buried lowl and soil in 

. outhern coastal Washington show that ubmergence of the 

lowlands happened so quickly that it is better explained by 

abrupt lowering of the land than by gradual ri se of the ea. 

Some of the spruce tump rooted in the younge t buried 

lowland oil show little or no thinning of outer annual growth 

rings (fig. 18). Thi s lack of thinning suggests spruce death 

as udden a that from earthquake-induced ubmergence in 

Alaska (fi g. II A). Thinning of outer rings would be expected 

of trees that suffered slow death from gradual submergence. 

Herbaceous plant rooted in ome buried lowland oil s 

retain stem and lea es that are entombed in overl ying 

tidefl at mud. The most conspi cuous of the ·e fossi Is belong 

to Deschampsia caespitosa, the chief grass in high parts of 

sa lt- and brackish-water lowl ands in coa tal Wa hington 

(figs. 16A, 19A- C). Sudden subm rgen c followed by 

rapid buri al be t explains the preservation of the fossil 

gra (fig. 19D- F). 

The boundary between buried lowl and soil and overl ying tidefl at mud is typica lly sharp-evid nee th at the change 

from lowland to tidefl at happened abruptl y (fi gs. 17 , 22A, 

22C, 23 8). Erosion cannot have cau. ed thi s abruptness 

while paring fo sil grass that tand rooted in buried soil s 

(fi g. 19). 

Figure 18. Outermost ring of a Sitka spruce stump rooted in the you ngest buried lowland soil at Willapa Bay (fig. 17A, sru mp b). The 

outermost ring i stained from decay of the bark , which fell off duri ng sampling. 
OASTAL EVIDE CE FOR GREAT EARTHQUAKES I WESTER WASH I GTO 5 

D 

GRASS 

GRADUAL SUBMERGENCE--)~ DECOMPOSED 

WITH AGGRADATION IN SOIL 

E High marsh Slowly aggrading high marsh 

SUDDEN ---) GRASS 

SUBMERGENCE ENTOMBED 

Deschampsia 

caespitosa 

High marsh 

IN TIDEFLAT 

MUD 

Rapidly aggrading tideflat 

GRADUAL SUBMERGENCE 

WITHOUT AGGRADATION 

Deschampsia 

caespitosa 

\ 

GRASS 

-~) DECOMPOSED 

AT SURFACE 

Triglochin 

maritima 

\ 

F Nonaggrading, progressively lower marsh 

Figure I 9. M odern and fossil leave: and s1ems of !he IUfled gras. Deschampsia caespitosa a1 1he iawiakum River. Willapa Bay (AI\ mer 

and Yamaguchi, 199 1. fi g. 4). A, oblique closeup of lllfted s1em. and leave. growing low on a modern plan!. The scale is 15 em long. B. 

venical view of siC m~ and leaves of a fo. sil grass IUf! surrounded by 1ideOa1 mud (m). The scale is 15 em long. The dark soil (s) in which 

the tufl is rooled lie. 2 em b ·low mos1 of 1hc surface in vi w. The probable lime of burial was bel ween I ,400 and 1.900 year. ago (soilS. 

fi gs. 24, 25). . a modern plan!. S1ripcs on I he scale arc I 0 em long. D. likely se JU encc of c cn1. 1ha1 produced 1he fo. sillllfl in B. E and F. 

even1s unlikely to have produced I he lllfl sh wn in B. 
6 A I G E RTHQUAK - H Z RD. D RED Rl K It T H - PA IFI ORTHWEST 

TSUNAMIS FROM GREAT SUBDUCTION ZONE EARTHQUAKES 

CAN LEAVE SAND ON COASTAL LOWLANDS ... 

Becau e a great ubduction-zone earthquake usual! 

produce. a t. unami (Abe, 1977). coa tal lowlands in the 

region of the earthquake may receive edi ment from the train 

of r. unami wave that come a hore soon after the earthquake. Such onland depo ition followed the 1960 Chile 

earthquake (Wright and Mella. 1963, p. 1389; Bourgeois and 

Rei nhart , 1989). l ear M aullfn, hile (fig. 108). the 

t. unami-laid sand widely blanket the 1960 soi l (fi g. 20). 

The and. mostl y le than I 0 em thick, commonly contains 

the rooted tem and leave. of herbaceou. plant. around 

which the and accumulated. Locally, the sand contains. ilty 

layer. that may represent lull s between waves (fi g. 20C). 

TSUNAMI ) SAND 

SHEET 

~ Water with '"'";""' ""' 

,;,, igh rl \]) ~ r--:-1 

D Subsided marsh 

Figure 20. and depo. ited on lowlands near aullfn, Chile (fig. I 0 13 ). by the tsunami from the 1960 Chi le earthquake. Photographs taken 

in 19 9. Stripes on handle·in B and Care I 0 cm long. A. two . and bed. (lower right) on property of Juan Vera (left), who reported finding 

tsunami-laid ~and in thi s area in 1960. The upper layer accumul ated in 1960. the lower layer during an earlier t. unami. Rio Ballenar local ity 

of A twater and others ( 1992). B. 1960 sa nd bed (s) etched by high ti des along an eroding, otherwi se peaty bank I km west-southwest of 

loca lit y shown in/\. , lamination in the 1960 sa nd bed (s) near locality . hown in 13 . Resistant laminae are silty. the sa nd having been etched 

by tide,. Stringy, largely verti ca l feawres arc roots. Buried 1960 soil (b) is not as dark a. in A because tides have coated it \ ith mud and 

algae. D. sequence of event, th at produced sa nel y tsunami deposits on hi lea n lowlands in 1960. 
0 TAL EVLDE CE FOR GREAT ARTHQ KES I WESTER ' WA HI T01 7 

... AND PROBABLY DID SO IN WASHINGTON AND OREGON 

Some of the buried coastal soil in . outhern Washington and northern Oregon are mantled wi th andy deposit 

th at were probably left by t unam is. Such depo. its widely 

overlie the younge t buried soil (figs. 2 1. 22). The sand 

records an unu ual event; relati e to other sand above the 

soil. it is excepti onall y coarse or thi ck or both. During the 

event. the and moved landward. the direction in which the 

sand thin and disappears (fig. 228) (Atwater. 1992. fig. 2). 

Deposition of the sand wa interrupted by lull during which 

mud settled (fig. 22A, C). Depo ition approximat co incided ith udden sub id nee of th · land b cau e the andy 

interval surrounds rooted tuft of gra that li ved on the lowland at the time of ub idence (fig. 22C). Ex traord inar . 

landward dir cted, interrupted by lull.. and approximately 

coincident with coa tal ub idence in Wa hingt n, thee en t 

wa probably a r unami from a great subdu tion-zone earthquake rather than a nood or . Lorm, or a t unami of . omc 

other origin. 

Figure 21. Sand bed (s) etched by high tides along the lower almon Ri ver, nonhern Oregon (fig. 12A) ( rant and McLaren , 1987). Probabl 

analogous wi th the Chilean sand bed shown in fi gure 208. 

46' 

37' 

30' 

Figure 22. Sandy interval o erl ing the youngest buried lowland 

s il along the iawiakum Ri ver. Willapa Bay ( twater and 

Yamaguchi, 199 1. fi g. 5). A. outcrop showin, abrupt contact between buried I wland soil (dark) and laminated sandy interval (s). 

Blade of sho el rest. on modern tidal marsh: handle is 0.5 m long. 

B. detail of the left part of the sand inter a! (s) shown in A. The 

lamina of and and mud (compare with fig. 20 ) are interrupted at 

center left. where they accumulated around tuft d leaves and stems 

of Deschampsio caespi10sa (fi g. 19) rooted in the buried soil. The 

scale is centimeters. , di stribution and thickness of sandy interval 

' hown in A (Reinhart and Bourgeois, 1987). ompiled from core 

logs an I outcrop sketche. by M.A. Reinhart. K.A. Bevi s. W.C. 

rant , B.F. Atwater, R.T. Versical. and S. McMullen. 

t23'55' 

THICKNESS OF PROBABLE 

TSU AMI DEPOSIT 

• More than 5 em thick 

• Less than 5 em thick 

• Less than 1 em thick 

o o deposit 

c 0 0 5 KILOMETERS 
ASSE l G ARTHQ AKE HAZARDS A D RED Cl G Rl K l T HE PA IFf ORTHWEST 

CENTURIES COMMONLY ELAPSE BETWEEN GREAT EARTHQUAKES 

IN COASTAL WASHINGTON 

Sequence. of buried lowland soil gi e clue. about how 

much time may elap e between succe. sivc great earthquakes 

in coa tal Wa hington. The seq uence shown in figures 

23-25 . ugge t that the elap ed time com monly pan hundred of year . Earthquakes that incubated longe.t may be 

widely marked by peaty fore t oi ls (Y and S in the figures) 

beneath which the buried soi ls ( and N, re pcctively) contain littl e organic matter. Prolonged uplift between earthquakes- the upward bulging in figure 9- may have allowed 

tree to colonize bold oi l Y whi le admi tting oxygen into 

organic matter of soil , which thereby became faint ; the 

same may be true forb lei soil S and faint oil N (fig. 25). 

There are many certai nties, howe er, about th history 

of great earthquake at the a cadia subduction zone. One 

major unknown i the degree to which great earthquakes 

corre. pond one-to-one wi th buried soil s. orne buried oil 

might reprc cnt nons ismic changes in tide levels or earthquake. smaller than magnitude 8 ( clson and Person ius, this 

volume). Other buried soi ls produced by great earthquake 

may have di appeared through decompositi on, a .. hown in 

figure 25 for a hypotheti cal correlati ve of . oil W (fig. 238). 

A ·econd major unknown is the size range of the great 

earthquakes. Abrupt lowering of coastal land about 300 

years ago, during the be. t understood period of prehistori c 

plate-boundary seism icity at the ascadia subduction zone, 

may record I ,000 km of rupture between southern Briti h 

Columbia and northern ali fornia. However, radiocarbon 

dating lack the resolution t show whether thi s ex tensive 

rupture happened piecemeal, during seri al earthquakes of 

magnitude 8, or whether it au ndcd a single earthquake of 

magnilllde 9 (Atwater and other., 1995). 

Figure 23. Low-tide outcrop showing ·uccessive buried lowland soil. in southern coastal Washington. A, six buried . oils 

along the iawiaku m River, Willapa Bay. Soi ls U and N lack organic matter res i ·tant to erosion. Soils J and L de. cend away 

from Pleistocene bedrock (b). probably because of differential sett lement. The modern marsh (m) extends to the forest. which 

covers an upland . Locality 17 of Atwater ( 1992). 13. four buried soils at the mouth of Elliott Iough, Grays Harbor. Two of the 

. oi ls contai n spruce roots (r). The isible part of the shovel handle is 0.4 rn long. Locality 9 of Atwater ( 1992). 
f :::::N 

1(:\ Buried spruce 

~ stump 

_1._ MarshQueried where 

fossils aren't 

entombed in 

overlying mud 

tj Bold surficial 

soil horizon 

Faint surficial 

soil horizon 

t

O OE~H 05 IN 

METERS 

1 

COASTAL EYIDEN E FOR REAT EARTHQUAKES I W TER W SHI GTO 

Niawiakum River, 

Willapa Bay 

Johns River, 

Grays Harbor 

J 

0.0, 0.2, 0.2, 0.3, 0.4 

------

~l

1.6 1 7 

~ 

1a 

· 2.5, 2.5, 2.6, 2.6, 2.8 

0.3, 0.3, 0.3 

1.5, 1.5, 1.9 

Wi/lapa River, 

Willapa Bay 

Chehalis River and 

Blue Slough, 

Grays Harbor 

~ 

1.7, 1.7, 7.9 

2.5 

2.6, 3.0, 3.2 

3.0, 3.1' 3.3 

9 

0.1, 0.2 

1.5, 1.6, 1.7, 1.8 

2.5, 2.8, 3.2 

Figure 24. equences of buried lowland oils (bold letters) at Grays Harbor and Willapa Bay. The numbers arc ages in thousands of 

radiocarbon years rounded t the nearest hundred. The standard deviati on is probabl y 50- _00 radiocarbon years for most ages. The ages ' ere 

measured on woody roots. sti cks. cones. and surficial hori z n. of soil s. Lines between columns link oil. that may record a single earthqu ake 

or a brief seri es of earthquakes. Localities of Atwater ( 1992). from left to ri ght : 17; 14; I 5; I 0 and I I combined. 

J L 

Tideflat 

EXPLANATION 

- Land surface in relation 

to constant sea level 

Bold surficial horizon 

Faint surficial horizon 

Oxidation zone 

3,000 

N 

I 0.5-3 m for 

land height 

1m for soil 

depth 

Est imated uncertainty in time of 

submerg ence and burial: 

2,000 

s u w 

: 200 years 

Soils J- W 

1,000 

APPROXIMATE AGE, IN CALENDAR YEARS BEFORE A.D. 1990 

y 

: 10years 

- ;...J Soil Y 

0 

y 

u 

s 

N 

L 

J 

Figure 25. lnferTcd history of land:cape. soil s. and earthqu akes at the Willapa Ba locality . hown in fi gure 23 A. Bold letters ' ignify buried 

soi l hori m ns .. oil W is hypothcti al at thi s localit y (Atwater. 1992). 
90 A ESS! 1G EART HQ AKE H ZA RDS A 'D REDUCI G RISK I TH E P IFIC ORTHWEST 

REFERENCES CITED 

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inferred from tsunam i data: Journal of Geophysical Re ·earch. 

. 8-1 . no. B4. p. 156 1- 156 . 

Atwater, B.F., 1992. Geologic evidence for earthquake during the 

past 2000 years along the opali Ri ver, southern coastal 

Washington: Journal of Geophy. ical Re. em·ch. v. 97, no. B2. 

p. 190 1- 19 19. 

At water, B .F .. comp .. 1994, Geology of liquefacti on feature. about 

300 years old along the lower Columbia River at M ar h, Bru ·h. 

Price. Hunting. and Wallace Island , Oregon and Wa. hingwn: 

. Geological urvey Open-File Report 94-209. 64 p. 

Atwater, B.F .. Jim nez unez, Hector, and Yita-Finzi, !audio, 

1992. et late Holocene emergence de. pite earthquake-induced . ubmergence, . outh-central Chi le. i11 Ota. 

Yoko, el. on, A.R. , and BeiTyman, K.R., ed . . , Impact: of tectonics on Quaternary coa tal evolwion: Quaternary llllernati onal, v. 15116, p. 77- 85. 

Atwater, B.F., elson, A.R. , Clague, J.J., Carver, G.A., Yamaguchi, O.K .. Bobrowski. P.T., Bourgeois. Joanne, Dari enzo. 

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Jacoby, G.C., ishenko, S.P., Palmer, S.P., Peterson, C. D., and 

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Earthquake Spectra,v. JI , no. l , p. l- 18. 

Atwater. B.F., tui ver, Minze, and Yamaguchi, O.K., 1991 , Radiocarbon test of earthquake magnitude at the Cascadia subducti on zone: atu re. v. 353 . no. 6340, p. 156-158. 

Atwater, B.F., and Yamaguchi. O.K., 1991, Sudden, probabl y 

coseismic submergence f Holocene trees and grass in coastal 

Washington state: Geology, v. 19, no. 7, p. 706- 709. 

Bourgeois, Joanne. and Reinhart, M .A., 1989, Onshore eros ion and 

depo. it ion by the 1960 tsunami at the Rio Lingue estuary, 

. outh-celllral hile [abs.]: EOS [American Geophysical nion 

Transa ti onsi, v. 70, no. 43, p. 133 1. 

Cooper. J.G .. I 60, Repon n the botany of the route, i11 Reports of 

exploration. and urveys to ascertain the most practi cable and 

economical route for a rai lroad from the Miss is. ippi Riverto the 

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Franklin, J .F. , and Dyrne. s, C.T .. 1973, at ural vegetation of Oregon and WashingtOn: U.S. Forest Service. Pacific orth w st 

Forest and Range Experimelll Station. Forest Service Techn ica l Report P W- 8, 417 p. 

Gram, W . .. and McLaren, D.O .. 19 7, Evidence for Holocene 

. ubduction earthqu akes along the nonhern Oregon coast [ab .]: 

EO. [Ameri can Geophysical Union Transacti on. ], v. 6 . 

no. 44, p. 1239. 

Kanamori , Hiroo, 1977, The energy release in great eanhquakcs: 

Journal of Gcoph sica I Research. . 82. n . 20. p. 298 1- 29 7. 

Kun ze. L. t.. and Cornelius. L. .. 19 2. Baseline in ventory of rare, 

threatened and endangered plant species/communities along 

WashingtOn '. Pacific coast: Washington awral Herit aoe Program. oastal Zone Management Grant o. G82- 029, Wa. hington Department of Ecology (01 mJ ia, WA 98504-87 11), 

164 p. 

Obermeier, S.F .. 1994 , Preliminary limit: for the strength of shaking for the Columbia River valley and the southern half of 

coastal Wa. hington for the a. cad ia subduction zone earthquake of about 300 years ago: U.S. Geological Survey 

Open-File Report 94-589 . 

Ovenshine. A.T.. Lawson, D.E .. and Bartsch-Winkler, .R., 1976. 

The Pia er Ri ver Silt- An intertidal deposit caused by the 

1964 Iaska ea rthquake: . Ge logical urvey Journ-11 of 

Re. earch, v. 4, no. 2. p. 15 1- 162. 

Plafker. George, 1969, Tectoni c. of the March 27, 1964, Alaska 

earthquake: . . Geol gical urvey Profess ional Paper 543- 1, 

74 p. 

---1972, Alaskan earthquake of 1964 and hilean earthquake 

of 1960- lmplication. for arc teet nics: Journal of Geophy. ica l Research, v. 77. no. 5, p. 90 1- 925. 

Pl afk r, George. and Sa age, J ... 1970, Mechanism of the hi lean 

earthquake. of M ay 21 and 22, 1960: Geological Society of 

America Bulletin , v. I. no. 4, p. 100 1- 103 1. 

Reinhart. M .A .. and Bourgeois, Joanne, 1987. Distribution of 

anomalous sand at Will apa Bay, Washingt n- Evidence for 

large-scale landward-directed proces e · [a b . . ]: EO f America n Geophysi al nion Tran actions]. v. 68, no. 44, p. 1469. 

That her. Wayne, and Rundl e. J.B., 19 4. A vi. oela. ti c coupling 

model for the cyc lic deformation clue to peri ocli all y repeated 

earthquakes at ubduction zones: Journal of Geophysical 

Research, v. 9. no. B9. p. 7631 - 7640. 

Weinmann. F.C .. Bou le. M ark , Brunner, Kenneth, Malek. John, and 

Yo. hi no. Victor. 1984, Wetland plants of t he Pac ific orthwest: 

Seanle, Wa h., . . Army orps of Engineers, 5 1 . 

Wright, Charles, and Mella. Arnoldo, 1963, M difications to the 

oi l pall ern of south-central hile re. ulting from . eismic and 

associated phenomena during the peri od M ay to August 1960: 

S ismologicil l Soci Iy of mcri ca Bulletin, v . 53. no. 6. 

p. 1367- 1402. 

Yamaguchi. O.K., Wo dhou. e, C. .. and Reid, M . . , 1989. 

Tree-ring evidence for . ynchronous rapid submergence of the 

southwestern Washington coa. t 300 years B.P. f abs.]: EOS 

[American Geophysical Union Transactions], v. 70, no. 43. 

p. 1332. 
GREAT-EARTHQUAKE POTENTIAL IN OREGON AND 

WASHINGTON-AN OVERVIEW OF RECENT 

COASTAL GEOLOGIC STUDIES AND THEIR BEARING 

ON SEGMENTATION OF HOLOCENE RUPTURES, 

CENTRAL CASCADIA SUBDUCTION ZONE 

By A lan R. elson 1 and Stephen F. Per. oniu 1 

ABSTRACT INTRODUCTION 

Fundamental que ti ons in earthquake-hazards research 

in the Pacific 1orthwcs t are concerned with the magnitude 

and recurrence of great earthquake in the Cascadia ubduction zone (CSZ). Geologic work of the Ia t few year has 

produced convincing ev idence of co ei mic subsidence 

along the Wa hington and Oregon coast . Regional ubsidcncc recorded by estu arine deposit sugge ts that plateinterface earthqu akes of at lea l magnitude 8 with more than 

100-km-long ruptures occurred during the late Holocene in 

nonh we. tern Oregon and southern Washington. Differences 

in the types of coastal-marsh equence between northwestern and south-central Or gon, however, uggest that the Oregon coa tlinc did not subside due to regional deformation 

south of about I at 44° . orth of thi latitude, the coa. t may 

in tersec t the seaward edge of a zone of co eismic sub id nee 

that may con tinue. outhea tward on hore. Altern ati vely, the 

C Z may be segmented near lat 44°-45° 1 ; a segment 

boundary at thi location wou ld indica te that pl ate-interface 

earthquakes of abou t magniwde along the central CSZ arc 

more frequent than earthquakes of magnitude 9. South of thi. 

boundary. in the oo. Bay region. the tectoni c framework 

developed through mapping and dating of marine and fluvial 

terra c. indicates that at least some cpi odes of abrupt tidalmar. h burial in outh-ccntra l Oregon arc th produ t of 

deformation of loca l . tructurc . . Some f the local deformati n could be a ociatcd with moderate earthquake. (magnitude less than 6). t most . itc in outh-ccntral Oregon. 

however. it i. unclear whether the ob ·ervcd deformation is 

related to local fau lting and folding during mod ratemagnitud earthquakes, to regional deformation during great 

pl ate-interface earthquakes, or to both. 

1 .S. Geological Survey. Box 25046. MS 966. Federal Center. 

Denver. CO 80225. 

One of the fundamental que ti ns in earthquakehazard rc. arch in the Pacific rthwes t i what is the 

potential for great earthquake in th a cadia ubdu tion 

zone ( SZ), the region . hown in figure 26 (Heaton and Hartzell , 1987: Atwater, 19 7)? The pre cnt . ei mi c quie ccncc 

in the region. initiall y attributed to as i mic subduction 

(Ando and Balazs, 1979; Acharya, 198 1: Reiling r and 

Adams, 19 2), i increa ingly regarded a evidence for a 

locked pl ate interface where a cumulating trc will be 

rclca. ed in a great earthquake of m men! magnitude (Mw) 

greater than 8 ( avage and other , 19 I ; Heaton and Kanamori , 1984; Adams, 1984; Heaton and Hartzell , 19 6: Roger . 1988: S1 cnce. 1989). Re ent geologi field . Ludi c 

upport th locked plate-interface model (Atwater. 19 7; 

Kcl ey and Carver, 1988: Kulm , 19 9; lark and Carver, 

19 9; Adams. 1990: Darienzo and Pet r on, 1990: P ter on 

and Darienzo, thi s volume) but have thus far pr ide I onl y 

rough e timatc of great-earthquake recurrence and Iiulc 

informati on on earthquake magnitude. 

Resolution of seismic-safety is uc in the region 

require rea onablc es timate of the magnitude, ex tent. and 

re urrence of prehi toric plate-interface earthquakes during 

the Ia 1 5,000 year of the late Holocene (Shedlock and 

Wea er. 19 9). A probable locked pl at interface, a con ergence rate f 30 0 mm/ycar. and the young age (less than 

I 0 M a) of the ubducting cru t sugge t plate-interface earthquak . of Mw -9 are plausible (Heaton and Kanamori. 

19 4). But the recu1Tcnce and magnitude of the e earthquake probably differs from place to place along the Z 

(Spence. 198 ; Weaver and hcdlock. 1989). The temporal 

and spacial pauern f earthquake needs to be determined for 

the Ia. t f w seismic cyc le (Thatcher, 1989) to accurately 

assess the hazard from great earthquak s (Ni shenko. 1989). 

9 1 
92 ASSE GEARTHQUA KEHAZARDS 

Figure 26. M ajor feature of the 

Cascadia ubduction zone in the nonhwestem United State and . outhwestern Canada. Large open arrow , . orne 

with queries, mark generali zed area 

along the coa t that coincide with 

boundaries between tectonic subplates, 

projection of boundaries between volca nic segments in theCa cade Range 

(Hughes and others, 1980), or other area where eismicity and subductingplate parameter. may change, corresponding with boundari e between 

segments of the Pacific plate as outlined by Spence ( 1989). o query is 

hown at the Mendocino fracture zone 

becau. e it boundary i accurately 

known. Distance between boundaries 

(bold numbers, in ki lometers) are 

shown only a ranges of pos ible segment lengths. The range of di tances 

shown for segments ju t nonh and 

south of I at 44.5° . reflects several locations for a possible boundary along 

thi part of the coast. We don't know 

whether mo t Holocene ruptures along 

the CSZ have been influenced by these 

po tulated boundaries. Triangles show 

volcanoes in the Cascade Range; barbs 

and dark arrows show relative direction of plate movement on fault s. M odified from Rogers ( 1988), pence 

( 1989), and Wilson ( 1989). 

135° 

so· 

45° 

40° 

ORED 

EXPLORER 

SUBPLATE 

PACIFIC 

PLATE 

130° 

G Rl K I THE PACIFI 

JUAN DE 

FUCA PLATE 

MENDOCINO 

FRACTURE ZONE 

125° 

ORTHWE 'T 

120° 

BRITISH COLUMBIA 

NORTH 

AMERICA 

PLATE 

-----

WASHINGTON 

... 

... 

.& OREGON 

-----

CAUFORNIA 

0 100 200 KILOMETERS 

The degree to which changes in the characteri sti cs of 

the plate interface along the CSZ control the extent of 

Holocene rupture , that i , the extent to which the CSZ is 

segmented, i cri ti cal in determining whether earthquake of 

Mw 8 or Mw 9 have occurred more often. Subduction continues at a higher rate beneath Washington than beneath Oregon due to rotation of the Juan de Fuca plate (fig. 26) about 

a pole in northern California ( ishimura and other , 1984; 

Riddihough, 19 4; Wi I on, 1986). The propertie of the subducting plate and the width of the locked zone also differ 

from north to south ( ro son and Owens, 1987; Ra mussen 

and Humphreys, 1989; Weaver and Shedlock, 1989). Furthermore, the tre s regime in the crust throughout the CSZ 

is dominated by north-south compre ion due to the interaction of the Pacific, Gorda, Juan de Fuca, and orth America 

plate (M agee and Zoback , 1989; Spence, 1989). The complex interaction. of these plate and the limited seismicity 

data uggest that the CSZ is composed of four to seven segments (Hughe and others, 1980; Weaver and Michaelson, 

1985; Heaton and Hartzell , 1986; Rogers, 1988; Weaver and 

Baker, 1988; Spence, 1989). Although plate-interface propertie wi thin areas of high eismic-moment relea e near egment boundaries may be the primary control on rupture 

initiati on and ex tent (Thatcher, 1990), the boundarie 

between these segment. probably reflect the endpoints of 

many pl ate-interface ruptu res during the Quaternary. 
RE E T COA TAL T DIE OF SEGME TATIO 1 OF HOLO E E R PT RE ADl S BD CTIOt ZO E 9 

We wi ll rev iew ome of the preliminary conclusion · of 

geologi field tud ies that are underway in the oa tal areas 

of Oregon and Washington (a of December 19 9) and peculate on ome of their implications for the hazards po ed by 

plate-interface earthquake in the CSZ. Beeau e the record 

of paleo eismic events is fragmentary, a variety of methods 

is being used to piece together the late Quaternary tectoni c 

and paleo eismic history of the region. Work in progress i 

of two kind - paleo ei mology tudie of the late Holocene 

coas tal record , and tudie of cumulative middle and late 

Quatern ary tec tonic deformati on as ex pre ed by marine and 

nuvial terrace . 

ACKNOWLEDGMENTS 

Our work was supported by the ational Earthquake 

Hazard. Reducti on Program of the .S. Geological Survey 

and by the .S. Nuclear Regulatory Commission. We thank 

Jame arlton, Paul Rudy, and other taff of the Oregon 

Institute for M arine Biology, Charles ton, Oreg., for providing facilitie , logi ti cal . upport, di cu sion , and encouragement throughout thi s study. Michael Graybill trongly 

encouraged our tudies in the South Slough ati nal E tuarine Re carch Reser e, Coo Bay. Able field as istance wa 

provided by Peter Vetto, William Manley, and Stephen 

McDuffy. Joint field work and ex ten ive di cus ions with 

Cu rt Peter on and M ark Darienzo were in valuable, as were 

discuss ion with Brian Atwater, Gary Carver. Harvey 

Kel. ey, Wendy Grant, M ary Reinhart, William Spence, and 

Robert Bucknam. Peterson and Darienzo also provided core 

data from AI. ea Bay (di cu. ed ex ten ively in their paper 

in thi volume). Lee-Ann Brad ley and u an Rhea prepared 

the illustrati on. . omments by Brian Atwater, u an Bartsch-Winkler, and Daniel Muh. ignifi cantl y improved the 

manu cript. Thi a contribution to IGCP Project 274 

(Quaternary Coastal Evolution) and to the I Q A 1 eotectonic Commi s ion. 

HOLOCENEPALEOSEISMOLOGY 

RECOGNITION OF COSEISMIC 

COASTAL DEFORMATION IN 

W A HINGTON AND OREGON 

A linear pattern of cosei mic deformation has been 

observed along . ubduction-zone coa t during gr at plateinterface earthquakes (Pi afker, 1965; Ando, 1975 : reviewed 

in Lajoie. 1986). Pl afker ( 1969, 1972) de eloped a model 

of cosci mic deform ation. havi ng an elongated zone of 

uplift and a parallel zone of ubsiclence. ba. ed on mapping 

of the co. ei. mic deformation in the oas tal area of . outhern Alaska and south-central hile after the two large t 

earthquakes of thi entur . ncoupling of the lead ing d,e 

of the o erriding plate during megathru t earthquakes can 

produce a much a 2 m of regi nal co ei mic uplift in a 

zon 80- 160 km wide. Locally in thi s zone, as mu h a. 10 

m of co ei mic upl ift can o cur by thrusting on . mall, . teep 

fault in the toe of the overriclin o pl ate (Piafker. 1969; 

Pl afker and Rubin, 197 ; Yonekura and himazaki , 19 0). 

The zone of ub idence, which is landward of the zon of 

upl ift, is commonly wider than the zone f uplift and can be 

hundreds of kilom ter. long; max imum regional sub. idcn 

can reach 2-3 m during the large t earthquakes (Pi afker 

and Savage, 1970). 

T ITIAL EARCH FOR E IDE E 

OF HOLOCE E PLIFT 

Mo t previou palco ei mology studie along subduction-zone coa t have focused on horeline features 

inferred to have been rai ed by uddcn cosci. mic upli ft 

(reviewed in Lajoie, 1986; Berryman, 1987). Alth ugh 

large storm. can depo it terracclike beach berm , night. of 

Holocene marine terrace along active convergent margin. 

are commonl y interpreted to be of co ei mi c origin. xamples of probable Holocene coseismic terraces from ther 

subducti on zones include tho e in hile (Kaizuka and others, 1973), Japan (Mat uda and other , 197 ), ew 

Zea land (Berryman and others, 1989), the New Heb ride. 

I land (Tay lor and other , 1990), and A Ia ka (Piafker and 

Rubin, 1978; Win low, 19 8). 

Initial paleoseismic study of the C Z also focu. eel n a 

earch for feature produced by cosei mic upli ft, bu t no 

ex ten i Holocene tetTaces were found , even along the 

outhern pan of the Oregon coast that lie clo e t to the . ubcluction-zone trench (the eli tance from the coa. t to the edg 

of the trench at the ba e of the continental sl pe i. 60- 105 

km ; fig. 27) (Golder As ociate , Inc., 1986). We. t and 

M Crumb ( 1988a) interpreted the broad, m dern wa c-cut 

platforms and the lack of Holocene marin terrace along the 

Wa hington and Oregon coa t as e idence that repeated 

great plat -i nterface earthquake probably had not occurred 

(at lea t in central and outhern Oregon). They ugge ted 

that if plate-interface earthquake had occurred during the 

late Holocene. then sea-level ri e must have equaled or 

exceeded the rate of co ei mic plus inter ei mic upli ft, or 

that plate-interface earthqu ake were too mall to produce 

ignificant amount of coseismic uplift. 

In the outhernmo t part of the CSZ, Holocene marine 

terrace sugge ting uplift rate of 3-4 mm/year ha e been 

reported from the plate triple junction area where the M endocino frac ture zone joins the San Andreas fau lt (fig. 26) 

(Lajoie and other . 1982; arver and Burke, 1987; Carver 

and other, 19 9). In addition, Kel ey ( 19 9, 199 1) found a 

ingle example of a Holocene beach berm of probable 

co eismi c ori gin near the Elk River, 6 km south of ape 
94 ASSES I 'G EARTH Q KE HAZ RDS A D RED Cl G RISK I 1 TH • PAC IFIC ORTHWE T 

Blanco on the outhem Oregon oa t. Cape Blanco i. onl y 

about 60 km ea. t of the ubduction-zone trench and i. 

inferred to lie within a zone of co ·ci mic uplift during plateinterface arthquake (fig. 28). Rate of late Plei. tocenc 

uplift for Cape Blanco are higher than rates to the north and 

south (We t and McCrumb, 19 a; Kelsey, 1990; Muh and 

other , 1990). The proximity of Cape Blanco to the trench 

may thu explain the location of thi raised berm along a 

coa tline otherwi e devoid of e idence for Holocene uplift. 

EVIOE CE FOR COSEISMIC S BSIOE CE 

Heaton and Hartzell ( 1986). Atwater and Grant ( 1986), 

and Atwater ( 19 7) changed the focu. of pa leosci micity 

studie in the Pacific onhwest by propo ing that much of 

the coastline in Oregon and Wa hington is in a zone of co. eismic . ub idence rather than uplift during great earthquake . 

Atwater ( 1987) based hi interpretation of repeated 

co ci mic subsidence on the stratigraphy of es tuarine deposit. in outhwe tern Wa. hington . He inferred that. uddenjerk 

of subsidence had repeatedly submerged tidal wetland soils 

on the margin of e. tuarie during great earthquake . Plafker 

( 1972, p. 9 17) had prev iously uggested the paleo ei mic 

potential of thi. type of stratigraphi c record , and Oven hine 

and other ( 1976), Bartsch-Winkl er and other ( 1983), ombellick ( 1986), and Bartsch-Winkler ( 1988) had inve tigated 

record of thi s type in Alaska. However, Atwater ( 1987) wa 

the first to recognize evidence for co ei mic . ubsidencc in the 

Pacific Northwe t and to usc trati graphic methods to show 

th at uch sub idence had occurred repeatedl y over a large 

area. Darienzo ( 1987), Darienzo and Peter on ( 1987), Peterson and other ( 198 ), and Darienzo and Peter on ( 1990), 

working independently in the etart s Bay e tuary in northwe. tern Oregon, adopted the co ei mic- ubsidence model to 

explain similar sequences of interbedded tidal-marsh peat 

and mud. They and other inves tigators ub equently found 

. imilar evidence for co eismic . ubsidence in many other 

estuarie along the Oregon and Wa hington coa t (Grant 

and McLaren. 1987; Hull , 1987; elson, 1987; elson and 

other , 1987; Grant. 19 8; Peterson and Darienzo, 1988a. 

1989, thi volume; Mclncll y and other , 1989), a well a. in 

the Humboldt Bay area of Californi a (fig. 26) (Vick, 198 : 

Carver and other., 1989; Grant and others, 19 9). 

The repetiti ve . tratigraphy identified by Atwater ( 1987, 

1988d) in tidal-mar h cores and outcrops consists of peaty 

soil s reprc enting den. ely vegetated wetland abruptly overlai n by intertidal mud . Typically. the peats are thin 

(0.05- 0.2 m), have abrupt upper contact and gradati onal 

lower contact , are separated by thick (0.5- 1.0 m) interval. 

of intertidal mud, and commonly have pruce or we tern redcedar tump. rooted in them. Atwater' ( 1987) ev idence that 

the peaty . oil. were ubmergcd uddenl y due to co eismic 

sub. idencc. in some place accompanied by tsunami s, 

in ludcd: ( I) the abruptness of th upper contac ts of peat , 

48° 

46° 

44° 

42° 

\ 

\ 

126° 124° 

\ 

\ 

\ 

\ Q) 

%\ 

-.. , 

..., \ ~ ehalem Riuer 

~} Tillamook Bay o 1 Netarts Bay - 0 Sand Lake- c;I Nestucca Riuer 

I Salmon Riuer ~ Siletz Bay 

I 

Yaquina Riuer I Alsea Bay 

~107 km~ 

I Siuslaw River 

~104km~ 

I Smith Riuer 

jUmpqua Ri ver North Slough 

I Coos Bay 

So,uth Slough 

Coquille Ri ver I 

Si~es River 

Elk River 

\ 

122° 

WASHINGTON 

OREGON 

100 KILOMETERS 

Figure 27. Locati ons and names f e tu aries in Oregon and out hem Washington. The distances of the AI ea Bay and iuslaw Ri ver 

estuari es from the ba e of the continental slope (eastern edge of the 

a. cadi a . ubducti on-zone trench) are shown in bold . 

(2) the amount of ubmergcnce (0.5-2.0 m) uggestcd by 

plant macrof ssil above and below the abrupt contact , (3) 

the similar character of the peat-mud couplet both laterall y 

at different ite and in sequence, (4) the presence of continuous peat-mud couplet where these equences lap up on 

older, Pleistocene . ediments at valley ide , (5) the finingupward , landward-th inning sand bed capping some peats at 

ite where no sand i presently being depo ited, and (6) the 

. imilarity of these equences to those reported to have been 

depo ited following great earthqu akes in Ala ka and Chile 

(Wright and Mella, 1963 ; Ovenshine and other . 1976). In 

southwes tern Wa hington. Atwater ( 1987, 1988c) identi fied 

at lea t eight epa rate buried peats; the five mo t wide pread 

peaty soi Is are dated at les than 3 ka and all are les than 5 

ka. Atwater ( 1987) also u. cd the cosei. mi c- ubsidence 

model to reconci le the relatively low regional uplift of the 
RECE T COASTAL STUDIES Of' EGME TAT!O OF HOLO E E RUPTURES. SCADIA BD TIO ZO 95 

coast (0. 1- 0.4 mm/ycar) ex pre. sed by raised Pleistocene 

marine ten·acc in ou tiH c tern Washington with the much 

higher. regional rate of uplift obtained from tide-gauge 

records (2- 3 mm/ycar). Periods of jerky sub. idenc (0.5- 2 

m). superimpo ed on a low intcrscismic uplift rate throughout the late Plei stocene. were hypothesized to yield an even 

lower net uplift rate. 

Because interbedded peat-mud couplets can also be 

produced by non tectonic processc (di. cuss d below), the 

mo t convincing stu die of co. cismic sub. idcncc focu. cd on 

evidence suggesting sudden. significant rises in relative sea 

level (submergence). Rooted . tems of herbaceous plants arc 

found locall y entombed by overl ying c tuarinc mud at the 

upper contact of ome peaty soil. in southwestern Washington (Atwater. 1988c). These fo sil s provide compelling vidence for sudden . ubmergcnce because they arc generall y 

not pre. crvcd at the marsh surface for more than a few years. 

Tree-ring . tudi e by Yamaguchi and others ( 1989a) have 

al o hown that rcdccdar trees rooted in the youngest buried 

. oi l at four sites in southwestern Washington probably died 

suddenly within a few year. of each other. ln-progre .. 

microfo . il studic of some peat-mud couplets in sou thcen tral Oregon (A. R. clson and A.E. Jenning. , Uni ver. ity 

of Colorado; and K aoru Kashima, Kyushu Uni versi ty. 

unpub. data, 1989), northwe tern Oregon (Darienzo, 1987: 

Peterson. 19 9; Dari enzo and Peter on, 1990). and ou thwcstern Wa hington (Hemphill-Haley. 1989) upport A twater'. ( 1987) deduction that the trans itions across the upper 

contact_ of the peat-mud couplet. represent 0.5- 2.0 m of 

sudden submergence. 

Atwater ( 1987, 19 d) uggestcd that evidence of sediment dcpo ition by tsunami s, and edimcnt liquefacti on 

directly overl ying the abruptl y submerged wetland soil s 

would be compelling evidence of regional coseism ic subsidence. Initial mapping of the sand beds that cap some peaty 

. oi l in some part of the marshe bordering ti dal streams at 

Willapa Bay, Wash. (fi g. 27) (Atwater. 19 7). showed that 

the sand pinches out landward- the expected result of tsunami dcpo ition. The and bed were onl y found capping 

peaty ~oils and not within the thick estuarine muds. suggesting that al l . and bed. were deposited immediately after sudden submergence of the marsh. imilar . and bed. cappi ng 

buried peaty soi ls at Grays Harbor. Wa h. (Reinhart and 

Bourgeois. 1988). ctarts Bay. Oreg. (Peterson and others. 

19 : Darienzo and Peter. n, 1990). along the almon 

Ri ver, Oreg. (Gran t and M cLaren. 19 7). at Si lctz Bay. 

Oreg. (Peter. on. 19 9). and at AI. ea Bay. Oreg. (Darienzo. 

19 9; Peter on and Darienzo. thi volume), show . imilar 

relations and have also been attributed tot. unami . . Field and 

modeling studies of the sand bed at Will apa Bay (Reinhart 

and Bourgeois. 1987, 198 . 19 9; Bourgeois and R inhart. 

1988) indica te that large, landward-directed surges of sandy 

wat r that accompanied some of the subsid nee events '· ere 

locally generated t. unami s rather than . torm su rges, seiches. 

or distant tsu nam is. Microfos. il . tudi cs al. o lend upport to 

the tsunami hypothesi. , e. pecially wh re abrupt peat c ntact · lack capping sand b d . . In e. tuarine mud directly above 

an abrupt peat con tact in Will apa Ba . Ei l en H mphillHal y (U. Geological Surve , unpub. data. 1989) has 

found marine diatom. , which are n t characteristic of . tuarine en ironment. : po. ibly. these diatom were tran. ported 

from the continental . hclf by a tsunami. 

So far, onl y one e tuarinc si te in sou th\ es tern Wa. hington has yielded definitive e idcn of prehistoric . cdimcnt liquefacti on, but the erupti on of a . and blo' at thi. si te 

wa not concurrent with a subsidenc e nt ( twatcr, 

1988c). However, liquefaction feature common ly develop 

in well- ortcd fine and or si lt beds that arc . aturat d and 

confined by overlying finer grain d bed. (Obcrm icr and 

other , 1989). Furthermore, adequate ( 1- 2 m high) crti al 

expo ures where the water tabl is now b low the base of the 

expo ure are required to stud . u h fea ture. . o . yst matic 

search for liquefaction feature. ha. yet been made in . and 

area along the Oregon or Washington coas ts, but the continuing ri e of late Holocene sea le el in the r gion ensures 

that any exposures of liquefiable cdimcn ts will be difficult 

to find. Thus, the pr sent lack of well-documen ted palcoliqucfaction sites doe. not preclude great earthquakes along 

the CSZ during the late Holocene. 

ALTER ATIVE TOTHE 

COSEI IIC UBSIDEt CE HYPOTHESI 

Much of the initial debate about th appli cabi lity of the 

coseismic-subsidence m del to Oregon and Washinp_ton has 

centered on whether obscr eel changes in the sequences of 

estuarine cd imcnt are distinctive enough to be due to coseismic movements or whether they might have been produced 

by nontectonic pr ces cs. Earl invc. tioations (am ng other. . Darienzo. 19 7; Darienzo and Peterson. 1987; Rei nhart 

and Bourgeois, 19 7) questioned whether at least . omc of 

the peat-mud coupl · ts might not r ult from ( I) storm 

urges. eli. tan t tsunamis. or floods coupled with relati ely 

rapid rises in regional . ea level. (2) complex pattern , of tidal 

hannel cuts and fills, or (3) udclen changes in tidal range 

caused by migration of bar near the mouths of tidal inlet.. 

Atwater ( 1987) argued that storm urges. di tant tStlllami . 

and flood could only rai e ca level brief! (le s than a few 

day.) and that the c procc es could not permanently change 

woody wetlands into intertidal mud flat . a suggc ted b the 

litholog and pl ant macrofos il s in the sequences in outhwc. tern Wa hington. M ar. h vegetation hould r ad il y incorporate any . ed iment deposited on the mar h from floods or 

storm without permanent submergence of the mar h. In 

support of these argument , Peterson and Darienzo ( 19 8a, 

b, thi s vo lume) found no ev idence of significan t sedimen t 

dCJ osition inc tuarinc marsh cq ucnce during either major 

hi toric flood or large torm surge in fluvially dominated 

c tu ari cs in Oregon. M os t inve tigator (for example. 
96 RTHQUAK HAZA RD D RED Cl G RISK I THE P CIFI ORTHW ST 

WASHINGTON 

Grays Harbor 

Willa~ Bay 

I \ "' I ~ 

46' 46' I ..., 

'CI 

"' .'§. I Nehalem 

c River 

<: <: 

'31 Netarts / 

lfi lfi 'Q I Bay 

& u 

0 I I 

u u I 

c:: c:: Alsea Bay 

G G I 

if: I.J.J if: I I.J.J 

44' t:J 44' t:J 2 2 q: q: <r <r 

I.J.J I.J.J a a q: (} u 

(/) (/) 

q: (} u 

42' 42' 

126' 124' 122' 126' 124' 122' 

I 

I 

I WASHINGTON 

I I 

I I "' I "' I ~ ~ 

46' I 46' I ..., ..., 'CI 'CI 

"' .'§. I "' .'§. I c c 

<: 

'3 1 '3 I Netarts 

<: Bay d 'Q I lfi '0 1 

~I u 

0 0 I 

u I u I c:: !f G 

if: I.J.J if: I.J.J 

44' t:J 44' t:J 2 2 q: q: <r <r 

I.J.J I.J.J a a (} (} (/) (/) 

(} (} 

0 100 KILOMETERS 
RECE T COASTAL ST DI ES OF SEGM TATIO OF HOLO ENE R PT RES. CA DlA BD TlO Z t E 97 

Atwater, 19 7, 1988e; Darienzo, 19 9; Darienzo and Peterson, 1990; Peter on and Darienzo, thi olume) al o found 

that the lateral continuity ofmo t of the buried peaty oi l (a 

demonstrated by transects of cores or laterally ex tcn i c outcrop. ) eliminated ti dal chan nel cut-and-fill proce e as a 

pos ible cau e at mo t ites. The regular equence of lithologic change within peat-mud couplet and the changing 

environments indi cated by microfo sil a. semblages (A .R. 

telson, A.E. Jenning , and Kaoru Kashima, unpub. data, 

1989; Hem phi II-Haley. 1989) arc also incompatible with a 

channel origin. 

Sudden changes in tidal range are more difficult to dismiss as a pos ible cause of interbedded peat-mud sequence 

(Darienzo, 1987). In fact, thi s mechani m is a com mon 

explanation of sequences in northwestern Europe (Shennan, 

1986) that are similar to the peat-mud couplets in the Paci fie 

Northwest. On the Atl antic coas t of the Un ited States. Thomas and others ( 1989) also ugge ted thi s explanation for the 

udden changes in lithology and foraminiferal a emblage 

in c re from a Connecticut alt marsh. Sudden change in 

tida l range could explain the burial of some peat. in some 

inlets in the Pacific orth west, but correlation of imilar 

equence between sites in inlets of differenL morphology 

and tidal characteri sti cs show. that local change in ti dal 

range cannot have prod uced most peat-mud couplets. 

Figure 28 (facing page). Four models of the locati on. of zones 

of co. eismic subsidence duri ng plate-interface earthquakes in the 

central Ca. cadi a subduction zone. Heavy I i ncs are boundarie. of areas of coseismic sub. idence. A, a rupwre of more th an 700 km of 

the plate boundary from an earthquake of at least Mw 9 could produce a zone of co eismic upli ft extending eastward from the trench 

and a zone of co. eismic subsidence cast of the zone of uplift. Estuaries north of the Siuslaw Ri ver estuary contain strati graphi c sequence with evidence of aseismic subsidence, whereas the 

Siti.Iaw Ri ver e:tuary comain. e idence of a slow. uniform ri se in 

relati ve sea level. Thu . if Mw 9+ earthquake: ha ve occurred, the 

edge of the zone of co. ei mic subsidence during these earthquakes 

may have trended between the iuslaw Ri er and Alsea Bay e tuari es. The eastward ex tent of the zone of aseismic subsidence for 

such earthquake. i. unknown. /3 , ru ptun.:s of 300- 600 km of the 

plate boundary during M" Y2- 9 earthquakes would al. o produce 

ex tensive zones of coseismic subsidence. If these zo e~ were farther west than in A. they could not include the iu. law Ri ver estuary, and the rupwrcs cou ld not ex tend north or south of the Siu. law 

Ri ver. . rupture of single pl ate-boundary segment. or pan. of segments during earthqu ake. of les. th an Mw '/.1 could produce zone. 

of coseismic sub ·iden e lc ·s than 250 km long. These smaller 

earthqu akes might not produce any . ignificant (more than 0.3 m) 

permanent subsidence and could include the area f the iuslaw 

River e. w ary. Sli p along some pans of the plate boundary ould 

also be aseismic (Sa age, 19 9). D. a vari ati on of model A and C 

i. th at earthqu akes of Mw ?J,(i-8 1

/.> could be most c mmon, with rare 

earthquake. of M, 3;.1 that rupture two segments of the plate 

boundary. However. in thi s model. the zone of coseismic sub idence for the rare. largest earthqu akes would have to b cast of the 

ius law Ri ver e. w ary. 

The weake. t part of At\ atcr' s ( 19 7) ca c for co eismic ubsidence wa hi uggc tion that th equencc of 

interbedded peat and mud o t pica! f late Holoc ne estuarine sequences in the Pacific onhwe. t are characteristi of 

tectonicall y acti ve coa t . He argued that on nont · t ni all 

acti ve coast , th low, uniform ri e of a lc cl during th 

late Holocene has produ ed thick. uniform lay r. ofp at lik 

tho fou nd in Puget Sound, an Francis o Ba , and ape 

Cod marshe (fi g. 29, equencc A). ing thi line of rca oning, the widespread peat-mud coup! ts in s uthwe t rn 

Wa. hington were thought to b good e idencc f r th j rk 

ri e of Holocene ea le el. Each jerk wa inferred to rc. ult 

from an episode of udden relati v . ca-l vel ri c due LO 

co. eismic ub iden e (fi g. 29, scqu n c B). But many studie on nontectonic coa ts, an I particularl y tho from n nhwe tern Europe. how equcn ·e. that arc. at leas t 

superfi cia ll y, similar to tho c in the Pacific onhwc. t (for 

example, Devoy, 1987; Tooley and hennan, 1987). an 

of the e long-term studic. (for exampl . Jclgersma. 196 1; 

Tooley, 1978 : Pi a che, 19 2; Ter , 19 7) arc mu ·h more 

extensi ve than any work o far att mptcd in th Pacifi 

orthwe t and are upported by thou. and of 14 age · (for 

example, ee Berendsen. 19 4; hcnnan. 19 7). 

The trarigraphy at many individual site n nontectonic coa t sugge t. that local rate of sea-level ri have 

varied mark dl y (Shennan , 19 6); man procc. c have 

been in voked to explain the e inferred rate chang s. M ost 

upper and lower contact. of peaty units in cqucnces on nontectonic coa t are gradational (fi g. 29, . equcnce with 

overlying and underl ying int nidal mud . suggc ting transiti on between upper intertidal and lower intertidal en ironment that Ia ted at least ten of year . . Osc illations in late 

Holocene ea level arc the mos t often cited expl anation for 

the interbedded peat and muds on stable coast (for c ' <1111 -

ple, see Rampino and andcr . 198 1; Shennan, 19 2. 19 6; 

Fairbridge, 19 7; Ter , 1987). De pite the limits f resoluti on of al titudin al and 14 age data in . ca-level tudic . some 

regional o ·c illations f ea level have been identifi d ( hennan. 1987), but the co cillations arc not necessaril y synchronous from region to region (Kidson. 19 2). Other 

proce c. that may be in vol ved in sea-level change that produce peat-mud couplet on non tec tonic eoa t include ( I ) 

large torm (Tooley. 19 5: ullingford and ther . 19 9). 

(2) long peri od with many storm surge ( treif. 1980). ( ) 

changes in the hapcs and gradient of es tuarine y tems due 

to ri ver change and ri ing Holocene sea le el (Pias che. 

1980). and (4) gradual regional tec tonic warping ( ewman 

and others. 1980; Shennan. 1989). However. I ithologic 

changes in the peat-mud sequence may not be direct prox ie 

of mall ea-level change (Orson and other , 1985; Ste enon and others. 19 6). Such lithologi chang show. however, that mar h aggradation has not kept up with late 

Holocene . ea-level ri se along many non tec tonic coa ts 

(Brook and other . 1979; Kearney and Ward , 1986) and that 

the lack of thi k , equence. of peat is not neces ari ly evid nee for tectonic in tabi lity. 
98 A ES I G E RTHQ KE HAZARDS A D RED Cl 1GRI K l THEPA IF! ORTHWE T 

Sequence A Sequence 8 Sequence C 

REGIONAL (f) 6 6 6 a: 

w 4 4 

SEA-LEVEL fw 2 2 ~ mm/year 

CHANGE ~ 0 0 

4 2 0 2 0 4 2 0 

Coseismic lnterseismic 

0.3- 1.0 m/event 0.2 mm/year 

VERTICAL 

MOVEMENTS 

EARTH 

(f) 

a: 

fw 

w 

~ 

~ 0 

2 

1 

4 2 0 

0 

2 

4 2 0 4 

~ 2 0 

~ 6 :v4 6 

RELATIVE w 4 4 fw 2 2 SEA LEVEL ~ 

~ 0 

4 2 . 0 

0 

2 0 4 2 0 

LOCAL (f) 

3 3 3 a: I We: 

SEDIMENT t;:; ~ 2 2 ""'- 2 

;:;>- ~ -~ DEPOSITION -'a: 1 :::!w 1 to 2 mm/year 1 to 2 mm/year 

RATE ~"-

0 0 I 

~ 4 2 0 4 2 0 2 0 

THOUSANDS OF YEARS BEFORE PRESENT 

0 

RESULTING (f) 

a: 

w STRATIGRAPHIC t;:; 2 

SEQUENCE IN ~ 

~ 

ESTUARY INLET ~ 3 "-w 

0 

4 

5 

Abrupt contact 

Gradationa l contact 

2 

3 

4 

5 

EXPLANATION 

Peat 

Muddy peat 

0 

~ 

2 

3 

4 -

5 

B Peaty mud 

Mud 

Figure 29. M odel of regional sea-level ri e, land-level movements. re. ulting relati ve sea-level change , and sedimentdepo: ition rates in tidal inlets in the Pac ific onhwest during the late Holocene. The curves illustrate how different but 

pl ausible histories of sea-level ri se, coastal uplift. and edimentati on rate could produce different type of strati graphic 

sequences in these in lets. equen e A could onl y be produced in an inlet th at had not had major (more than 0.3 m) sudden 

change in relati ve . ca level. The abrupt upper contacts of the peat units in sequence 13 sugge. t . uclden ri ses in sea level 

or eros ion. Gradati onal contact. characteri ze sequence C. which could be produced by nuctuating regional ea level and 

rapidl y changing sedimentati on rates. Sequence 13 could easil y be confused with sequence C, whi ch was produ eel wi thout any sudden changes in relati e sea level. 
RE E TCOA TALST DIESOF EGM E TATIO OFHOLOCE E R PT RES, A CAD!AS 80 Tl ZONE 99 

There have been no detailed tudic of Holocene ea 

level in Oregon and Wa hington. o the rate and character 

( mooth v . o ci llatory) of sca-le el ri se in this region i 

poor! known. In general, the rate of ri c i thought to be 

similar to that for the nited States ca t coast, about 1-2 

111 m/year (Pet r on and others, 1984, fi g. 6; Phipps and 

Peter on, 1989). However, small (le s than 1.0 m), henterm ( 10- 500 year ), nontectonic o cillati on. in regional sea 

level. combined with changes in local . cdiment influx and 

irregular local sediment compaction (Kaye and Barghoorn , 

1964). cannot be ruled out a the cause of orne peat-mud 

couplets (fi g. 29, cquence C). Co eismic sub idencc can 

onl y be inferred where there i evidence of a sudden, subtantial change in water depth aero the upper contact of an 

uneroded peaty soil. 

The inability to ru le out mall regional osc il lati ons in the 

rate of sea-level ri se ca t. uncertainty on the suggestion that 

ome gradual con tact in marsh equenccs in nonhwe tern 

Oregon were produced by slow accumulati on of tec tonic 

train (Peter. on and Darienzo, 198 a; Peterson, 19 9; 

Darienzo and Peter. on, 1990). Current model of seismicstrain accumulation, release, and recovery (for example. Savage, 1983; Thatcher, 1984), make a tectoni c mechani sm for 

the ori gin of gradual contact pi au ible. However, it i. difficult to di stinguish the lithology in marsh core. or outcrop. 

re ulting from thee grad ual, tectonica ll y induced changes in 

regional uplift rate from the lith logy re. ulting from grad ual. 

nontectonic changes in the rate of ea-lcvel ri se. 

REGIONAL DIFFERENCES IN LATE 

HOLOCENE TIDAL-MARSH SEQUENCES 

IN OREGON 

Studies of late Holocene tidal-marsh tratigraphy in 

Oregon and Wa hington began with Dari enzo· ( 1987) initial field work in 1985 at ctan s Bay (fig. 27). However. 

most project are still in progre and onl y the Netarts Bay 

work has been publi shed a. a full-length paper (Darienzo 

and Peterson. 1990). Reconnai ance in ve Li gati on have 

been made in mo t of the 27 cswari es al ng the Oregon and 

Washington coa ts. bu t detailed work ha been ompletcd in 

fewer than half. De pi tc the 1 rcliminary nature of th interpretation. of sequence in many esw aric , there ar pear to be 

ignificant difference in the character of mar h stratigraphic 

. eq ucncc. along the Oregon coas t. 

ORTHWESTER OREGO 

Studies at ctans Bay (Peter. on and Darienzo, 1988a; 

Peterson and others. 1988; Dari enzo and Peterson, 1990) and 

at the Nehalem Ri v r and almon Ri ver e tuari e. (fig. 27) 

(Grant, 1988. 1989) how that late Holocene estu arin 

equenccs in nonhwc tern Oregon contain multiple (3- 5) 

wide pread buri d tidal-marsh and omc spruce-. wamp 

abruptly o erlain b intertidal muds and, in some pi a e. , 

and b~.;d . In a reconnai ancc . ur e at the c. tu ca Ri r 

e wary, P ter. on and Darienzo ( 19 8a) aL o found three to 

fi e buried peat . oi l . The peat-mud couplets inn rth c. tern Oregon are similar to those tudicd by B.F. Atwat r in 

outhwe ·tern Wa. hington. bu t the peaty horizon. are thi kcr 

(0.2- 1 m) and are eparat d b th inner int r aL of mud 

(0.3- 1.5 m). Stratigraphic relation are more diffi ult to d cument in Oregon because the height of tidal out rop. i 

lower and outcrops are le. ex ten. ivc than in . out in . tern 

Wa hington. Darienzo and Peter. on ( 1990) c. timated that 

the peat-mud couplets at etart Bay each represent 1- 1 .5 m 

of sudden co ei mic sub idenc . Thus. imilaritic. in mar. h 

tratigraph in northwestern Orcg n and outh\. est rn 

Washington sugge t re urrcnt co ci mi . ub. idcncc of 

regional (greater than 200 km) c tent (Grant. 19 9; Dari nzo 

and Peter on, 1990). 

CE TRAL OREGO 

Few detai I. of marsh-. tratigraphy stu die at e. tu arie. 

along the central Oregon co a. t have be n pub! ish d. Peterson 

and Darienzo ( 1988b) reported three to fi ve buried mar. h surfaces in the upper 5 m of core from the ilctz Ri v r estu ary 

(fi g. 27). M ore extensive work at the AI. ea Ri ver estu ary 

howcd3- 10pcaty mar. h . oil in c res4-7 mdccp(Pct rs n 

and Darienzo, 1988a, thi volume; Darienzo, 19 9). M o. t of 

the buried peaty oi I. at A I ea Bay ha c . harp upp r ontacts 

and gradational lower contact , but peaty units in the upper 

2.5 m of mo t cor are eparat d by thin (0.02- 0.4 m) interval of mud (fig. 30, core AB8). Buried peaty . oil in the 

lower part of the cores are separat d by greater thicknesses 

of mud (0.4- 1.8 m). similar to the peat-mud couplets in 

nonhwe. tern Oregon. Low outcrop and tran. ec t. f cores 

show that many tratigraphi c units can be correlated for hundreds of meter aero s the marsh. Sandy capping beds ar lcs. 

well developed on the buried p aty . oils in the fluvially in fluenced es tu arie of the Sil tz and !sea Ri ver. than they ar 

in the more tidall y influenced e tuarie uch a etarts Bay 

(Peter. on and Darienzo, 198 a). Howe er. di tincti e landward-thinning, sandy capping beds on the peat unit in the 

we tern Alsea Ri ver mm·she., combined with the repetitive, 

abrupt upper contact of the peat . led Peterson and Darienzo 

(thi olume) to conclude that the contact. were produced by 

co. ci mic subsidence accompanied by t unami . Based on 

change in sed iment lithology aero the abrupt peat-mud 

contacts, the amount of ubsidence during each event was 

e timated to be 0.5- 1.0 m. less than that e. timated fore cnt 

in nonhwe. tern Oregon (Darienzo. 1989; Peter on and 

Darienzo, thi s volume). 

We found a different type of marsh tratigraphic 

equcncc preserved in the Siuslaw River e tuary about 50 

km t the south of AI ea Bay (fig. 27). About ~ .5-4.0 m of 
100 ASSES I G EARTHQUA KE HAZARD A D RED IJ G Rl K I THE PACIFI ORTHWEST 

continuou., uni form tidal-marsh peat o erlie. tidal sand and 

mud at an inlet on the south ide of the ri er (fig. 30. core 

1- 09); 3 km to the nonh, at the mouth of the onh Fork of 

the Siuslaw Ri ver, about 4 m of tidal-marsh peat overlie. 3 

m of bracki h or fre hwater peat. The thi kne of the tidalmar h peats and se era! 14C age how that the upward 

growth of the mar h urface was able to keep up with ri sing 

ea level during a time span of 2,000 year (fig. 29, equence 

A). Reconnai sance coring upstream of the ite with thick 

peat showed interbedded peats (0.05- 0.3 m thick) and muds 

(0.0 1- 0.5 m thick), but contacts are mo tl y gradational; 

these contacts suggest gradual rather than . udden submergence. Thu, , there may have been gradual change in th rate 

of sea-level ri eat the iuslaw River estuary, but the thick 

peat indicate no ignifi cant (more than 0.3- 0.5 m), sudden 

change in . ea level. 

Although the type of mar h strati, raphic equences in 

the Siuslaw and AI ea Ri ver core differ significantly, the 

difference i le s di tinct than the di fference between 

sequences in north we ·tern Oregon and the Siuslaw River. 

Perhaps a few of the mo t gradual contacts in the upper part 

of the AI ea Bay core correlate with some of the ubtle, 

gradual lithologic change in the thick peat. in our cores 

from the Siuslaw Ri er. If so, we cannot ru le out a nontectonic ori gin for the e lithologic change because they repreent smaller water-level change. (probabl y les than 0.3 m) 

and are much less abrupt th an lithologic change ob erved in 

similar sequences on nontectoni call y active coa ts. Tectonic 

and nontectonic proces es may be producing identical small 

couplets along the Oregon coas t. Present methods of tratigraphi c analysi may not be capable of distinguishing 

bet ween mall tectonic components of sea-level change and 

small o cillati on in sea level and sediment input. 

SO THERN OREGO 

South of the Siu. law Ri ver, reconnai ssance coring at 

many ite in the mpqua Ri ver, Coo Bay, and Coquille 

Ri ver estu ari es (fig. 27) by elson ( 1987, 1988), Peter on 

and Dari enzo ( 1989), and Dari enzo ( 1989) showed no consistent regional pattern of multiple abruptly buried peaty 

oils like tho e found in north we tern Oregon. M any 

sequences have a ingle abruptl y buried oil 5- 50 em thick 

at 0. 8- 1.5 m depth ; the oil at many ite was apparentl y submerged suddenl y and at others more graduall y. An outcrop 

along the oquille Ri ver exposes such a buried soil, which 

ha the trunks of a few small spruce trees rooted in it and is 

very imilar to buried soils in outhwe tern Wa hington. 

Thi peaty soil i thin (0.05 m), but a po sibl y correlati ve oil 

at North Iough, in northern Coos Bay, is much thicker (0.5 

m) and resembl es the thick tidal-marsh peat found at the 

iu. law Ri ver. The thickne , depth, and organic materi al 

(f) 

cr: 

UJ 

1-

UJ 

~ 

~ 

0 

SHl9 

0 

2 1.5 ka 

::i 3 .....,_ __ 1- 2.3-2.5 ka 3 

h:: 

UJ 

0 

4 

South Slough, 

Coos Bay 

(lat 43.3'N.) 

Peat 

2.0 ka 

6 

South Inlet, 

Siuslaw River 

(lat 44' N.) 

EXPLANATION 

Muddy peat 

Peaty mud 

Mud 

0 

2 

3 

4 

5 

6 

7 

ABB 

Al sea Bay 

(lat 44.4'N.) 

Abrupt contact 

Gradational 

contact 

Figure 30. Com pari on of typical core from ti dal-marsh . equences at three ite. along the south-central Oregon coa. 1. Age estimates for di fferent levels in the cores are ba eel on unpublished 

14C ages from thee or nearby cores. Core WC- 12 contain. nine 

abruptl y buried peaty soil s, some of which may record coseismic 

ubsidence during movements on local stru lUres. The thick peat in 

core SI- 09 ugge ts that no udden change. in relati ve sea level 

have occurred since 2.0 ka. The abrupt contacts at the top of peaty 

soil in core AB8 have been interpreted by Peterson and Dari enzo 

(thi s volume) a. marking coseismic subsidence events of regional 

ex telll (0.5- 1 m subsidence). 
RECE T COASTAL STUDIES OF EGME1 TATIO OF HOLOCE E RUPT R . CA DIA BD CTIO ZO E 10 1 

content of uncommon. deeper (greater than 1.5 m d pth) 

peaty oil s in the oos Bay region is highly variable; some 

oil have abrupt upper contacts and other do not. However, 

multiple buried peat are rarely found more than 5-10 m 

from va lley ide slope , and few marshes appear to have 

been a ex tensive in the past 3,000 years a they are today. 

An exception to the lack of widespread, multiple peatmud couplets in the Coo Bay region is at South Slough (fig. 

27), which occupies the axis of an acti vely deforming northtrending yncline in western Coos Bay. oring in a small 

marsh at the south end of the slough re ea led as many a I 0 

buried tidal-mar. h ·urfaces in cores 5-8 m long (fi g. 30, core 

WC- 12). At lea t four of the buried . urface can be traced in 

cores aero s the inlet. Baed on lithology, unit thickness and 

preliminary foraminiferal and diatom data (A.R. 1elson, 

A.E. Jenning , and Kaoru Kashima, unpub. data, 1989), 

these couplets repres nt about 0.5 m of sudden submergence 

followed by deposition of 0.3- 0.5 m f intertidal ecl iment. 

A core from Day reek, near the middle of South Slough, 

penetrated a . imilar sequence of eight buried peaty soil 

(Peter on and Darienzo, 1989), sugge ting about 0.5 m of 

ubmergence during these event. . Reconnaissance coring by 

A.R. el on (unpub. data, 1988) at other sites in the we tern 

part of South Slough howed two to four abruptly buried 

peats, two of which are capped by thin bed of ilty and. 

elson ( 1987), Peter on and Darienzo ( 1989), Peterson 

( 1989), M clnelly and Kelsey ( 1990), and Mcinelly and others ( 19 9) suggested that at lea. t ome of the ubmergence 

events marked by the buried peaty soil s at South Slough arc 

the result of cosei mic displacement on local tructure . 

Mapping and dating of Plei tocene marine terrace and 

underl ying tructures in the Coos Bay region by Mclnell y 

and Kel ey ( 1990) showed that the South Slough . yilCline is 

being deformed but that the style of deformation in thi 

region is more complex than previou ly as umed by Adam 

( 1984). M clnell y and Kelsey ( 1990) have also dated tree 

stumps in the intertidal zone adjacent to nexurc-slip faults 

within the South Slough syncline, indicating Holocene displ acements of these tructures. Thu , some of the abruptly 

buried marsh surface in South Slough are probably the 

rc. ult of coseismic eli. placement. due to fo lding or faulting 

of the . yncli ne. The. e displacements arc mall (about 0.5 m) 

and can not easily be correlated with the few buried marsh 

surface found elsewhere in the Coo. Bay area. 

South of Cape Blanco, the onl y . izable e tuaries in 

outhemmo t Oregon arc at the mouths of the high-gradiem 

Rogu and Chctco Ri ver (fig. 27). Sediment in the e two 

e tuari es arc predomi nantly gravel and sand, and only small 

areas of marsh fringe the ri v -rs . The lack of quiet-water 

intertidal mud in these cstu ari c indicates little chance for a 

well-pre crvcd, unambiguou. stratigraphic record of late 

Holocene sea-level changes. 

GREAT -EARTHQUAKE RECURRE E AND 

THE LIMITATIONS OF 14C DATING 

ntil th ad cnt of recent coa tal field studies. estimates of the recurrence of great arthquak . in the CSZ w r 

limited to average aluc , large! ba. cd on anal gi • with 

other ubclu ti on zones. Heaton and Kanamori ( 19 4) u cd 

empirical and theoreti ca l relation. to cal ulate an a crag 

earthquake of Mw .3±0.5 for the Juan de Fuca plate; such an 

earthquake could recur every 126 t 420 year depending n 

the ratio of a eismic to ei mic lip (Kanamori and ti z. 

1985). Heaton and Hartzell ( 19 6) compared the Juan de 

Fuca pl ate to other ubduction zones in south-central hilc. 

outhwe tem Japan, and uthem Ala ka. sing the e three 

analogous . ubduction zon . and the con rgen e rate of the 

Juan de Fuca pl ate, they uggested great earthqu ake recurrences for the Juan de uca plat of 250- 500 year . . I 00- 250 

years, and po sibl y 1.000 year . re. pcctivel In a tud f 

turbidite deposition on the continental helf, Adam ( 19 9. 

1990) inferred a fairly uniform recurrence of 590± 170 years 

for maj or earthqu ake . However, thi. tud left open qu stion. about the size and location of earthquak ource and 

whether all turbidite. on the hclf were earthquake g ncraLec!. For example. Thatcher's ( 1990) r cent analy ·i of great 

earthquake in other circum-Pac ific ubduction zones . ugge ts recurrence of plate-interface earthquake. in the Z 

might be very nonuniform . 

A key a umption in initial palco. eismi c studie in the 

Pacific orthwes t was that con cntional radiocarbon elating 

could provide fairly accurate estimates of great-earthquake 

recurrence through analy i. of organi materials at or ju t 

abovc thc upper contact ofabruptl yburiecl pca ty . oil . lt al o 

was hoped that the age. of buried p at were separated by 

enough time to allow the soil to be correlated from on estuary to another on the ba is of conventional rad iocarb n elating. Howe er, tudi cs with more than a hundred 

conventional 14C age. in Washington (Atwater, 1988c; Hull , 

198 ) and ou r comparati ve ela ting pr gram in s uthern Oregon showed that con cntional 14 age fr m the upperm os t 

parts of buried wetl and soil. vary widely depending on the 

type of organic material analyzed and how it was prepared 

prior to analy is (Grant and others, 1989). For example, 

mean ages from identi ca l . plit of the same peat ample · 

from oos Bay sent to two different laboratori es that u e different pretreatmelll methods aried by I 00- 700 year.. Detrital wood picked from some of these arn e peat samples 

varied from I 00 year older to 1.000 year younger than age 

on the bulk ample . More limited data for older peats how 

imilar vari abilit y. 

These re. ults indicate that. with the excepti on of tree 

trunks well rooted in peaty soil , conven tional l-lC age on 

most materi als can onl y provide a general es timate 
102 A I G EART HQ KE HAZARDS A D RED Cl G Rl K I T HE P IFIC ORTHWEST 

(± 150- 300 year. ) of the time of peat burial (Grant and other.. 19 9): mo t age from the buried peaty oi I are max imum e timate of the time of buri al. The wide range in ages 

from the same peat layer al o uggest that many buried 

peaty . oil formed during time inter als of at lea t 

200- 1,000 years. Where time intervals between earthquak 

are more than I ,000 year , or where multiple age on different material above and below the upper contac t of peaty 

oil can be used to constrain the age of the urface of the 

soil , such a. in some of Atwater' s ( 19 8c) site., conventional 14C age may provide e timate of burial times that arc 

prcci e enough to allow buri d oil s to be correlated. How-

. ever, where the mean earthquake-recurrence interval i le s 

than 500 years and high-quality tree-trunk ample are lacking (central and outhern Oregon), conventional 14C age 

cannot be u. ed to demon. trate correlation . 

De. pite the e pr blems, am pies from mos t of the oil. 

inferred to have been buried during as many a 10 late 

Holocene co eismic event in the coas tal Pacifi c orthwe. t 

have been dated by conventional 14C method. (Grant and 

others, 1989). Atwater ( 1987, 1988c) collected a variety of 

materi al from above and below the upper contact of fi ve of 

the youngest of eight buried wetl and soil in outhwes tcrn 

Washington. M ean 14C age for the ubsidence event 

inferred from these sample were about 0. 3, 1.6, 1.7, 2.7 and 

3. 1 ka. An additional ub idence event about I .0 ka wa 

identified at the Waatch Ri ver estuary near the north we tern 

tip of Wa hingt n and at an inlet on the north side of the 

mouth of the olumbia Ri ver (Atwater, 198 c). Grant 

( 1989) obtained mean ages of about 0.4, 1.0, and 1.6 ka from 

three buried soil along the Nehalem River and about 0.4 ka 

from the single buried soil expo ed along the Salmon Ri ver. 

Darienzo and Peter on ( 1990) u ed peat age on arnplcs 

from six buried soi Is at etart Bay to uggest a synchr ny 

of events in northwe tern Oregon and Wa hington about 

0.3-0.5, 1.0- 1.3, 1.4- 1. , and 3.0-3.3 ka. At Al sea Bay in 

central Oregon, Peterson and Darienzo (this volume) also 

dated pea t from 6 of I 0 peaty soil th at were buried within 

the pa. t 5,000 years. Finall y. A.R. el on (unpub. data. 

1989) used 20 age on peat and detrital wood from 9 buried 

soil s in a core from the south end of South Iough in Coos 

Bay to in fer time of peat buri al at 0.3- 1.0 ka. 1.0- 1.3 ka. 

1.1- 1.5 ka, 1.7 ka, 2. 1- 2.5 ka, 2.3- 2.5 ka, 2.6-2.9 ka, 

2.7- 3.0 ka, and 3. 3.9 ka (fi g. 5, core WC- 12). C. D. Peter-

. on and M.E. Darienzo (Geology Dept.. Portl and St ate University, unpub. data, 19 9) obtained . imilar ages on six of 

eight buried oil in a core from central South Slough. But 

few f the ages from Washington and northwestern Oregon 

and none from central and outhern Oregon are preci e 

enough to allow correlati on of mo t buried oil . 

The conventional 14C age. show that recurrence of submergence c ent. at individual ites i nonuniform . Average 

recurrence interval are in the range of 400- 700 year. but 

the sites with many ages suggest recurrence inter als a. 

small a. I 00- 200 year and a. large as I ,000- 1 ,600 years. 

f-: 

I 

(.9(/) 

-a: 

ww 

Ifww 

U;2' 

~z a:-

w 

f60 

40 

20 

0 

9 

"Q) '00 o.t::: Q; Q; Q; ~ > >>> Q; Ole ~ u OlOl Q) 

Q) ·- Oc "'::J > > ~ "' "'"'"' > al alal al > 

Q~ 'tro ~ 0 a: a: a: a: "' Cll-"' E a: 

Oiii <{- N u "' Q)(f) "' ~ "' toO> ~~ tO> ::J Q) ~ u roo_ :0 '" (/) 

a. .I:: Cf "' oa. (/) 2 mE ro .,- a. Cii <{ Vi ~ z~~ E u "-"' u5 ::J ::J u (/) E Vi z i=Z 0 ::::> u 

DISTANCE ALONG OREGON COAST, IN KILOMETERS 

0 

f-Vl 

100 f-a: 

~~ 80 8 ~ :2g 

60 0 '>2 

[:E~ 

40 ~I. zU 

20 ~ ~ o,_ 

Figure 3 1. Relati on between height of the lowest marin terrace 

and di. tance from ihe Oregon coast to the eastern edge of the a -

cadi a trench (base of the continental slope). The upper cur c shows 

the distance from the coast to the trench (scale at ri ght), as measured 

from Peterson and others ( 1986). The lower curve. hows the pre. cnt 

height (. ca le at left) of the lowest ( 0- 125 ka) marine terrace along 

the Oregon coast between Port Orford and the olumbi a Ri ver. olid line ele ati ns in the Cape Blanco-Cape Arago region arc from 

the detailed mapping of Kelsey ( 1990) and M clncll y and Kcl. cy 

( 1990). Da hedlinc elevati ons between outh Slough and the Columbi a Ri ver are deri ved from We 1 and M cCrumb ( 198 a). Terraces are poorl y preserved along much of the Oregon coast, so ten·ace 

cuiTelati ons nonh of the South Slough area are tentati e. 

M o t age on the younge t buried oi l sugg st the late. t 

event occurred during the inter al 0.1 - 0.5 ka, but wide fluctuation in 14C flu x in the upper atmosphere during this 

peri od (Stuiver and Pear on. 1986) and the large error. on 

ages from peaty soil s make it impossible to determine 

whether or not the younge t soil wa. buried about the same 

time everywhere (Grant and others, 1989). 

Tree-ring tudie , such a tho e by Yamaguchi ( 1988), 

arc the mo l accurate method of dating ubsidcnce event . 

M ax imum-limiting calendar ages of A.D. 16 18- 16 7 constrain the timing of the mo t recent ub idence event at four 

ite that pan I 00 km of the outhwe tern Wa. hington coa t, 

but ome interpretive problem remain (Yamaguchi and other , 1989b). nfonunately, this method can onl y be applied 

to those few ite with well-pre. erved foss il redcedar tree . 

and uch ite have been found onl y in outhwestern Washington. High-preci ion con ventional 14C analysi (Pear. on, 

1979) of large am pies of les than I 0 rings from large buttress root (near the ba e of the trunk) may re ·ult in the dating 

of I or 2 earlier event to within a few decades in northwestern Oregon and southwe tern Wa. hington where pruce 

trunks rooted in buried peaty oil s are common (Atwater. 

1988c. table 2). The e are probably the be. t available method forte ting the synchrony of indi vidual ub idcnce event 

along ubstantial parts of the Paci fie onhwe t coast. 
RECE T COASTAL ST DIES OF SEGME TATIO OF HOLOCE E R PT RE , CAS ADI BD TIO ZO 10 

MIDDLE AND LATE 

QUATERNARY DEFORMATION 

Determination of rates and style of long-term Quaternary deformation i criti cal in interpreting ev idence of 

Holocene eismic or a ci mic deform ation in acti ve subduction zone (Bloom, 1980). Studie of rai ed marine terrae 

are the mo t traditional type of geolo0 ic in vestigati on of 

Quatern ary deformation along tec tonically ac tive coa tline . 

Other types of geologic in ve tigation include examination 

of fluvial-terrace . equ nces and analy is of regional pattern 

of river and drainage-bas in morphology (Well s and other , 

!988; Rhea, 1993; M erritt and Vincent, 1989). The c in ve.-

tigations can yield in formation about the cumulative effect. 

of Quaternary deformation, wh ich may help determine 

whether a coastal regi n has been affected by earthquake of 

regional or more local extent. Quaternary deformation is 

commonly 1uantified through analysis of uplift rates. identifi cation of acti ve structures. and geodetic and geologic analy. i. of regional tilting. The use of marine and fluvial terrace 

to de cribe the nature of these types of Qu aternary deformation along the C Z i di cussed below. 

UPLIFT RATES 

Uplifted marine terraces are found along many tectonicall y acti ve coastline . M ost Pleistocene marine terrace represent ancient shoreline that were formed during sea-level 

high stand a soc iated wi th interglacial periods and sub equcnt tectonic uplift (Lajoie, 19 6). F r thi reason, terrace 

tudie have been conducted along many tec tonically acti ve 

coast! in e. around the world (Berryman, 1987); of parti cular 

interc tare studie in region undergoing a ti ve ubduction 

such a Japan (Yonekura and Ota, 1986), the ew Hebrides 

Islands in the South Pacific (Tay lor and others. 19 5), and 

Chile (H u and others, 1989). Analys is of up I i ft rates in other 

acti ve area has helped to define the nature of tectoni c deformation associated wi th the C Z. 

Calculation of uplift rate from marine-terrace data 

req uires that the age and highest pre cn t elevation of the 

wave-cu t platform be knO\ n. a " ell a. the elevation of . ea 

level at th time the terrae was formed. The elevation of a 

form r sea level i determined by comparing the age of the 

platform to age-elevation cur e. of . ea level clcri ved from 

well- tudied area .. uch as w Guinea, aliforni a. and 

Japan (Lajoie, 19 6). The present cle ati on of the terra is 

then corrected for the elevation f . ea level at the time the 

terrace was formed and for any tilting that may have 

occurred since the terrace formed. The resul ting amount of 

tectonic uplift can th n be u. ed with the age of the terrace to 

ca lculate the net rate f uplift since the terrace wa formed. 

pli ft rates are commonly ompared from place to Jlace 

along the coast. and to other ac tive coas t . to identify patterns of regional deform ati n (Ota and Yoshikawa, 1978). 

Plei tocene marine terrace arc pre. rvcd discon tinuous! along the Oregon and Wa hington oa t (fig. 3 1) but 

are be. t de eloped and have been mapped and dated in the 

mo t detail near ape Blanco and ap rago in southern 

Oregon (Grigg , 1945; K nn d and th r.. 198_; Adams. 

19 4; Golder A .. ociate . 19 6; We. t and c rum b. 19 a; 

Kcl. ey. 1990: M clnell y and Kcl cy, 1990). recent d tailed 

study by Muhs and other ( 1990) used n w uranium-. ri 

ages and amino-acid and oxygen-isotope data to rrnlat 

and date the two lowe. t terraces at ape Blanco. Their 

rc ult. indica te uplift rate of about 0.85- 1.-5 111 p r th u-

. and years for the 0-ka ape Blan o terra c and 'tb ut 

0.8- 1.5 m per thou. and ear for the I o--ka Pionc r terrace. 

The c uplift rates are lower than tho. c reported b arlier 

tudie. (We t and M cCrumb, 19 a), but the arc ' till th 

highest rat s reported from marine tcn·accs a! ng the Oregon 

and Washington coa ts. 

Muhsand othcr ( 1990)al.ocomparcd th irnc\ uplift 

rate with rates fr 111 well-dated terra . JU Cnce 'H other 

ac ti ve subduction zone and concl uded that uplift rates at 

ape B Janco were not unusuall y high or 10\ . The a!. o 

found no clear relation bet> ccn the st le of phtc on ergence and uplift rate. of marine-t rrace cq u n cs in oth r 

subduction zones. The e con lu ion. onfli t with those of 

West and M e rumb ( 1988a), who used su h a compari son t 

infer that large pl ate-interface earthquake had probabl not 

occurred on the SZ in late Holocene time. 

In addition to marine-terra c tudie , we exam ined flu -

vial terrace becau e these fcatur scan be usefu l for measuring inci ion rates at con. idcrable distance. from the coast. In 

this approach, we consider in ·i ion rate: to be an indirect 

mea ure of uplift rate . Our studies of stream inci ion along 

the Siletz, Siu law, Smith, and mpqua Ri vers (fi g. 27) indiate rates of regional up! ift of 0.2- 0.8 m per thou:and y ars 

in the central Oregon oa. t Range (fig. 3_). The c rate. arc 

ba eel on terrace height. measured from the modern ri vcr 

le el to the tream-cut bedrock bench (s trath) underlying 

each terrace. Terrace age. arc derived from radiocarbon elating of Holocene and late. t Plei. tocenc terrace sed im nt s and 

from thermoluminescence (TL) dating of lder terrace 

clepo it . Th u e of TL dating is rclati cl new. and our 

dates are the fir t uch analy. e. f fluvial sediment in the 

Paci fie 1 orthwe t. 

DEFORMATION OF 

QUATERNARY STRUCTU RES 

The new marine-terrace age of M uh. and other ( 1990) 

were u. ed by !J cl nell and Kelsey ( 1990) and Kel ey ( 1990) 

to evaluate the amount and style of deformation of spec ific 

tructur s ncar ape Arago and ape BIanco (fig. 3 1 ). 

Kel. ey ( 1990) mapped a prominen t we !-trending fold (th 

ape Blanco anticline) that deform late Pleistocene marine 

terrace. at ape Blanco. The uplift rate determined by 
104 

(f) 

a: 

UJ 

I-

~ 200 

~ 

_j 

~ 150 UJ 

-' 

<( 

UJ 

(f) 

UJ 100 > 

0 

CD 

<( 

II 

S2 

UJ 

I 

(f) 

a: 

UJ 

IUJ 

~ 

~ 

_j 

UJ 

> 

UJ 

-' 

<( 

UJ 

(f) 

UJ 

> 0 

CD 

<( 

II 

<.9 

w 

I 

50 

160 

140 

120 

100 

80 

60 

40 

20 

0 

A I G EARTHQ AKE HAZ RDS A o ·RED CI G RISK I THE PACIFIC ORTHWE T 

0 

A 

EXPLANATION 

Terrace measurement 

.A. Radiocarbon age 

e TL age 

······· ········· · >200 ka 

(<0.4 m/kyr) 

20 40 

B 

MT 

-(80·125 ka ) 

0 

60 80 100 120 140 160 180 

DISTANCE FROM COAST, IN KILOMETERS 

EXPLANATION 

Terrace measurement 

& Radiocarbon age 

<}-{> Anticline 

I 

l······+·'·····l········· ···· · j..----"'1 

DISTANCE FROM COAST, IN KILOMETERS 

200 

50 

Figu re 32. Longitudinal profile of terrace. along the Umpqua and Siu law Rivers, central Oregon Coa:t Range. Heavier solid lines are 

elevation of the U mpqua and Siuslaw Ri ver . and lighter solid line connect terraces that can b phy icall y correlated; dashed lines connect 

teiTace that are probably correlative, and dotted lines are speculative correlations. Terrace height may have venica l errors of ±5 m for the 

higher ten·aces and ±2m for the lower terraces. Radiocarbon ages are located wi th tri angles and labeled '·yr B.P."' (radiocarbon years before 

1950); therm luminc cence (TL) ages are located with solid circle and labeled "ka" (thousand · of years ago). For comparison, 1he heights 

of the lowest marine terraces, labeled " MT,'' are marked near the mouths of the rivers (heights from Golder A. sociate , 1986). These marine 

terraces have been tentatively correlated with one of the oxygen-i otope stage-5 marine terraces ncar Cape Blanco, Oregon, whi h have been 

dated by M uhs and others ( 1990) and Kelsey ( 1990) at 0- 125 ka. Incision rate . labeled "m/ ky r'' (meters per thousand years). are calculated 

from the age determinations and the height of the bedrock benches underl ying the fluvi al terraces. A, longitudinal profiles of terraces along 

the mpqua Ri ver. Close examinati on of the nearly continuous earl y Holocene terrace revealed no ev idence of latest Plei. tocene folding or 

faul ting along the mpqua Ri ver. 8 , longi tudinal profil e of terraces along the lower part of the Siu. law River. Although no age data has 

been obtained from the pre-Holocene terraces. their relati on to the po ition of the lowest marine terrace and the pre. ence of very well developed soil s on the terrace deposit indicate that they were probably formed more than I 00 ka. The terrace r mnants are clearl y modified 

by an anti cl ine (double arrow . ymbol below graph ) that i present in the underl y ing Eocene bedrock. We ca nnot determine if terraces below 

an elevati on of about 50 m have been folded becau. c they are poorly preserved below thi s height. The nearl y continuous earl y Holocene 

terrace appears to be undeformed. The large area affected by the folding (more than 20 km wide) indi cates that these types of structure . 

. hould be easy to detect on terrace profi le from other oast Range rivers. 
RECE T COAST L ST DI · S OF SEGME TATI O OF HOLO E E R PT RES. ASCA DI S BD CTIO Z E 105 

Muh. and other ( 1990) in thi s area were mea ured along the 

ax i of thi s anticline. The location of a Holocene beach berm 

of possible co ei mi c ori gin on the outh fl ank of the amicline may indicate that th fold has been acti ve in th late 

Holocene. Kel ey ( 1990) also noted everal fault that off et 

Plei. tocene marine-terrace urfaces near ap Blanco, but 

the apparent a . ociati on of the tructures with major terrane-bounding suture zone may limit their u efulne s as 

indicator of modern . tre conditions. Similar studie in the 

reo ion between Cape Arago and South Slough by M lnelly b 

and Kelsey ( 1990) identified cveral north we !-trending 

fold and flexural-slip and high-angle fault . Stratigraphic 

evidence from marshe in the outh Sl ough . yn line ma 

indicate repeated late Holocene coseismi c folding and flexural-s lip faulting events in thi s region. The re ults of these 

studies indi cate that cru tal . hortening along the uthern 

Oregon coas t is th re ult at I ast in part , of the gr wth of 

wes t- and nonhwe Hrending fold and flexure-s lip and 

high-angle fault and i not impl y a result of r gional upli ft. 

In our study of flu vial-terrace eq uen e along the central Oregon coast, we examined a north-trending anticline 

(prcviou. ly de cribed by Adams. 19 4; and Per on ius, 1993) 

that deformed Quaternary deposits along the Siuslaw River 

(fig . 27 and 328 ), but we found no ev idence of fold or fault s 

with demonstrated Holocene or lates t Plci tocene movement. 

Although we may have mi ssed a few . tructure . e peciall y 

where terrace are poorl y preserved or where deform ati on 

rate. are I w, the general lack of deformati on of local stru -

tures indicates that the ei motectonic selling of the central 

Oregon coas t is di fferem from the . etting of the Cape BlancoCape Arago-Sourh Sl ough region in southern Oregon. 

The recent tudie of marine and fl uvial terraces ummari zed above indica! that in the southern pan of the CSZ. 

Quatemary deformation has been dominated by acti vity of 

local tructures rather than by regional uplift. As sugge ted 

by Muhs and others ( 1990), deformation of local tructure. 

may be the mo t important rea on that uplift rates can ary 

widely along acti ve subduction zone . Vari able uplift rates 

in Oregon (fig. 3 1) and elsewhere may refl ect deformation 

of local folds and fault rather than change in the style of 

plate convergence. 

The onl y other place where on hore Qu aternary truelure in the forear of the CSZ ha e been de cribed in detail 

arc inn rthern Cali forni a (Kcl y and ar er. 19 ar er 

and others, 19 9; lark and arvcr, 1992) ncar the outhern 

end of the C Z. Although other mall Quaternary tructurc. 

ha e been described at ca ttered location. along the Oregon 

and Wa ·hington coa ts (Adam . 1984: Me rory. 1992; Ticknor and others, 1992; Well and other.. 1992). the acti ve 

structure. in the Cape Blanco- ape A rago region mark a di -

tinct change in tylc of Quatern ary deformati on along the 

SZ. Thi change in . tructural style. n ar lat 43. · oN., is probably related in part to the central coa. t' . greater el i tance from 

the trench (fi g . 27 and 3 1 ). M ap. of lates t Quatern ary off-

. hore deformati on ( Iarke and others, 1985; Peter. on and 

other , 19 6: navely. 198 : Goldfinger and others. 1992) 

show a fold and thru t belt that appears to trend on. hor j u. t 

north of C os Bay (Dari enzo. 19 9; Peterson and Darienzo. 

19 9). The Quatern ar anticline along the iuslaw Ri erma 

mark the relati ve! inacti ve ea ternmo t part of thi s fold and 

thru t belt. Alt rnati vely, the increa eel tru tu ral omplex it 

near oo Bay may be r lated to a change in pl at -interfa c 

geometry or egm ntation of the Juan d Fuca pl ate. 

REGIONAL TILTING 

L andward-tilted ten·ace ha ommonl b n 

ob er ed along acti e ubduction zones (Ota, 19 6). so geodetic and geologic e idencc for regional tiltin o ha. a! o been 

used a e idence for ubduction at the SZ (Rei lingcr and 

Adams 19 2; Adam , 19 4; Vine nt and other. , 19 9; 

Mitchell and other., 199 1 ). Reilinger and Adam ( 19 -) 

noted that their geodeti call y mea urcd . hon -term rates of 

ea t-west landward tilt of the Oregon oa t were . imilar to 

long-term rates of landward tilt calculat d from rcc nnaissance in form ation on marine terraces along the Orcoon 

coas t. Th y interpr ted the agreement bctwe n long and 

. hort-term rate as evidence of a eismic subducti on. Adams 

( 1984) discu. sed thi agrcem nt in more d tai l and ca l ulated cru tal hort cning rate forth Or gon and 'V a. hington 

coast. u. ing geodeti c and limited (and in some pl ace. confli cting) geologic tilt data. He noted that the shortenin g rat · 

suggested coupling of the pl ate interface during the Pl eistocene but concluded that there wa in ufficient ev iden to 

determine whether pre ent-day . ubdu lion was ismic or 

asci mi c. M ore recent marine-terrace mapping by Kelsc 

( 1990) and M clnelly and Kelsey ( 1990) indica! d that landward ti I ti ng of marine terrace discu. eel by Adams ( 1984) in 

the Cape Blanco and Cape Arago re0 ion is allributable t 

d formation of local folds rather than to regional deformation. At ape Blanco. only the three lowe !terraces tilt ian !-

ward ; two higher terrace tilt caward (Kelsey. 1990). 

Similarl y. our work on flu ialterraces in the r on oa. t 

Range has uncovered no ev idence of a regional pauern of 

either landward or seaward tilting as ugge ted by Adam. 

( 1984) but instead indicates uniform eas t-we tupli ft of the 

oast Range (fig. "2A) com plicated by movement of local 

. tructurc such a the anticline on the iuslaw Ri er (fig. 

328 ). Therefore. shortening aero s ·tructur in the orth 

meri ca plate in outhern Oregon indicate continuing ubduction along the CSZ. but terrace tilting and the agreement 

between geologicall y and geodeti call y determined east-we. t 

tilt rate in Oregon are not evidence for either seismic or 

a eismic lip on the plate interface. M ore recentl y. Vincent 

and others ( 1989) and Mitchell and others ( 199 1) interpreted 

new geodetic e idence of long-wa elength north- outh 

deformation along the Oregon coas t. Repeated leveling urey between 194 1 and 1988 how a broad trough centered 

between the Yaquina and Till amook e tuarie , Oregon, and 

upward tilts southward to the Coquille River and northward 

to the olumbia Ri ver. Vincent and other ( 1989) interpreted 

these elevation hange a ev idence of inter ei mic deform ation due to trong interplatc coupling beneath the YaquinaTi llamook region. 
106 A E Sl IG E RTHQUAKE HAZARDS A D REDUCI G Rl K I THE P CIFI ORTHWE T 

POTENTIAL MAGNITUDE AND 

RECURRENCE OF 

PLATE-INTERFACE EARTHQUAKES 

IN THE CASCADIA SUBDUCTION 

ZONE 

Although the interpretation and implication of the 

emerging paleosei mic record in the CSZ continue to be 

debated, the ei mic historie of ubduction zone that have 

orne characteri tic in common with the CSZ limit the probable magnitude and recuiTence of future earthquakes in the 

zone. The re ult of recent geologic studie ugge t some 

additional con traint on po tulated earthquake rupture hi. -

tories, but everal different hypothe es of probable magnitude and rupture extent are p rmitted by the avai lable 

preliminary data. 

CONSTRAINTS BASED ON 

OTHER SUBD CTION ZONES 

Several recent geophysical . tudies (Heaton and Hartzell , 1987; Roger., 198 ; Spence. 1989) concluded that part 

of the plate interface in the CSZ are locked and that there i 

a ign ifican t potential for great earthquake. of Mw 7~ 9Y2. 

Because no plate-interface earthquakes have been recorded 

in trumentall y in the central CSZ, the magnitude of po tulated future great earthquakes seems to depend mo tl y on the 

modern analog ubduction zone most favored by any particular study. Some subduction zones imilar to the CSZ, uch 

a. in ·outhwes tern Mex ico, are characteri zed by earthquakes 

of Mw 71A- 81A that rupture 60- 250 km of the zone (Singh 

and others, 19 I ; i henko and Singh, 1987). Other zone , 

however, have had much larger earthquake , uch 3 the 

1960 south-central hilc earthquake of Mw 9.5 that ruptured 

900 km of the plate interface (Pi afker and Savage, 1970; 

i henko, 1985; Cifuentes, 1989). Studie that propose 

outh-central Chi le as a good analogy for the CSZ argue that 

an Mw 9+ earthquake . hould be seriously considered 

(Heaton and Hartzell , 1986; Rogers, 1988). Heaton and 

Hartzell ( 1986) al. o suggested that the largest earthquake in 

the ankai Trough, Japan, or in the subduction zone in western olombia (Mw 8.5- . ) may be the best analogies for 

SZ earthquake . In contrast. Kanamori ( 19 6), Roger 

( 198 ), and pence ( 1989) uggested that the 1932 Jali co, 

Mex ico, earthquake (Mw 8.2) on the Ri vera plate interface 

may be the clo c t analog for plate-interface earthquakes in 

the C Z. But Heaton and Hartzell ( 1986) stated that the low 

level of modern sei micity at the C Z ruled out an analogy 

with southwestern M ex ico and other imilar subduction 

zone . tudi cs of empirical correlation of ubduction-zone 

parameters and modern analogs are limited, however, 

because there arc . ignificant differences between the ubduction selling at the Z and all . uggested analog ource 

zones (Kanamori and A. ti z, 1985; Heaton and Hart zell , 

19 6; 'pen c, 1989). 

In fact. the histori cal record of ubducti on-zone earthquakes shows that. al though the maximum magnitude of 

thrust earthquake. dep~:::nds on plate parameters (Ruff, 1989), 

earthquakes of variable magnitude typify most subduction 

zone · (Thatcher. 1990). For example, in th olombian subduction zone, the Mw .8 earthquake of 1906 was followed 

by thre Mw 7.6- 8.2 earthquakes about 20 year apart (Kanamori and M e all , 1982). Other example of vari able 

magnitudes are compiled in ishenko ( 1989). That her 

( 1989. 1990) argued that great plate-interface earthquake 

along segments of ubduction zon s occur in cycle and that 

al though the magnitude of earthquakes difC rs from cycle to 

cycle, the duration of each seismi c cyc le is similar. Of more 

importance for evaluating earthquake potential, the earthquakes within each cycle eem clustered near the end of the 

cycle and tend to occur with incrca. ing rupture length and 

magnitude in the latter part of the cyc le. Reviews by Syke. 

and Quit meyer (l 9 I), Kanamori ( 1986), Ruff ( 1989), and 

other how that the majority of ubduction zone arc characterized b earthquake clo er to Mw than to Mw 9. If 

these general izati on developed from other ubducting circum-Pacific margin apply to the Z, earthquake near M, 

8 and Mw 9 might both be pos iblc, bu t Mw 8 earthquake 

would be much more common than Mw 9 earthquake and 

they would tend to cluster near the end of the seismic cycle. 

!though the magnitude of plate-interface earthquake 

may vary considerably, the location of rupture along a subduction zone i not random (Thatcher, 1990). Hi tori cal (for 

example. Ando, 1975) and geologic data (for example, Wells 

and others, 1988; Berryman and others. 1989: Taylor and 

other . 1990), as wel l a. instrumental records (Sykes and 

Quitmeyer, 198 1 ). ·how that subducti on zones are segmented by rupture that tend t fill seism ic gap (Ruff, 

1989). Several ei mic cycles may be required to fill some 

gaps, and thi s timing suggest that asperi tie (zone. of high 

shear strength on the plate interface) may control the size. 

timing, and tarring point of rupture (Kanamori , 1986; 

Ruff. 19 9; Thatcher. 1989). Where a perities innuence the 

ex tent of ruptures through . everal eismic cycle , the ubduction zone i egmented. Identification of poten tial egmcnt boundaries along a subduction zone i~ of fundamen tal 

importance becau e the length of segmen t , their relati on to 

the trucwral geometry of the convergence zone, and their 

relati on to the recent hi tory of ruptures along the zone. set 

limits on the po ible lateral and down-dip ex tent of the mo t 

probable plate-interface earthquakes. 

Although segment boundarie are commonl y as ociatcd 

with weak zone between asperitic , major zones of fau lting 

or other deformation that divide the subducting plate into 

ubplates al o tend to limit the extent of ruptures on the plate 

interface (Ruff. 1989). At the CSZ, faul t dividing the northern and southern end of the Juan de Fuca pl ate into subplates 

(fig. 26) are likely locati on. for segment boundarie · ( pence. 

19 9). hanges in I wer pl ate dip or the position of the bend 

within a ubducting plate mu t also have a major effect on the 
RECE T COASTAL ST DIES OF SEGME 'TATIO OF HOLOCE E R PT RE . CASC DIA BDUCTIOt ZO E 107 

lateral and dO\ n-dip ex ten t of plate-interface rupture (Kanamori, 1986; Spence. 19 7); uch change. in the CSZ ha e 

been identified at !at 47° and 49°N. (Riddihough. 19 4; 

Michael son and Weaver. 1986). Hughe and others ( 1980) 

noted variations in the alignment and type of olcanoe in the 

Cascade Range that can be projected to mo t of the pos ible 

segment boundaries (. hown by large open arrow in fig. 26). 

haracteristic sei micity patterns may also cr e to identify 

segments with differino states of . tre . . 

Finally, the instrumental record of earthquake in ubduction zone. shows that the rupture proce i very complex 

(Savage, 19 9; Thatcher, 1990) and that . implc model. of 

faul t segmentation and earthquake recurrence may be of limited u c in accurately forecasting grea t earthquake . Because 

the phy ical charac teristics of the plate interface and the 

mechanical pr perti e of asperitie. within fault egments 

differ. magnitude and recurrence characteri tic of earthquake during ea rthquake cycle differ from segment to segment ( ee Tay lor and others, 1990). However, cgment. may 

per ist on widely ranging time scales. A major fault zone 

between two subplatcs might alway halt propagating ruptures. whereas a diffuse zone of wcakne s between a. peri tie 

or an asperity of moderate izc might limit rupture ex tent 

onl y one quarter of the time (Schwartz and Sib on, 1989). 

For thi. rea on. even for subduction zone. that have been 

characteri zed by hi storical earthquakes of relatively moderate ma0 nitude, the rupture of two to five segments in a Mw 9 

earthquake cannot be precluded (Heaton and Hartzell , 19 6). 

CONSTRAINTS BASED ON RECENT 

GEOLOGIC STUDIES IN OREGON 

Although mo t coastal paleo ei mologi ts now agree 

that Mw 8+ pl ate-interface earthquake. have occurred during 

the late Holocene in the northern half of the Juan de Fuca 

pl ate (Atwater. 19 7; Grant, 1989; Peter. on, 1989; Darienzo 

and Peterson, 1990; Pet rson and Darienzo. thi s vo lume). 

discuss ion about future earthquake in the SZ tend to 

focu on ne of t wo hypotheses for the recurrence and magnitude of late Holocene plate-intetface earthquake . In the 

fir t hypoth si . Mw 9+ earthquake charact rize the C Z; 

those earthquakes were ac ompani cd by ruptur . that 

ex tended for more than 600 km along the Juan de Fuca plate 

interface and parts of adjacent subplates (fig. 28A). Based on 

analog great arthqu akes in other ubducti on zone and the 

rate of pl ate onvcrgcn e in the Z, th recurrence interval 

of the c Mw 9+ earthqu ake would be many hundred of 

years. In the ccond hypothc ·i , the Z i. segmented in a 

manner similar to that hown in figure 26. and ruptur of 

I 00- 250 km a. ociatcd with Mw 7:v4- 81

4 earthquake (fig. 

2 D) wou ld oc ur much more frequently than larger earthquakes. Only rarely, pcrha1 . near the end of a long earthquake cycle (Thatcher. 1989). would cveral egment 

rupture in an earthquake of about Mw 9 (fig. 28 ). If the 

second hypothc. i. is cotTcct. earthquake r urr nee in the 

C Z i apparently nonuniform bccau e there ha e been no 

great earthquakes on the plate interface in the pa t 150- 200 

year . . As di scus cd pr iously. there ar di. tin t differences 

in the tidal-mar h s qucnce attribut d to co ci mic def rmation along the Oregon coa t (P t r on, 1989; Dari nzo, 

19 9). Thee differences are be. t explained b as gmcnt d 

. ubduction zone (mo t earthquakes lcs. than Mw 'h) rath ·r 

than an un egm ntcd zone, but present data do not rule ut 

earthquake larger than M, Y2. 

The egmentcd-zon hypothc is require at lea t four 

egment boundarie . Likely egmcnt boundari for the Z 

(fig. 26) were suggested by Hughc. and other ( 19 0) using 

the distribution and alignment of different types of ol a noes 

in the Cascade Range and by pence ( 19 9) u ing the work 

of Riddihough ( 19 4), Michaelson and Wca cr ( 19 ), and 

Weaver and Baker ( 19 ). Lateral off. et. of som n rthsouth-trending tructur . in the upper plate on the ontinental shelf (Snavely, 198 ) mi oht aL o r fie t . cgment b undari cs. The independent motion. of the Expl r r and Gorda 

subplatcs show that their plate onta ts mark scgm nt 

boundarie near the n rthern and outhern nds of the Z 

(Spence. 19 8). Le ccnain are differences in scismi ity 

rate within the . ubducting plates and difference in pl at 

den. itie and dip in the north-central part of the zon ncar 

!at 49°, 4 .5°, and 47° . that may mark eomctll boundarie 

The very low cismi city rate in the . outh- cntral part of the 

zone, our primary area of intere t, makes identi fi at ion of th 

properties of the ubducting Juan de Fuca plate very difficult 

(for exam1 lc. ec Keach and others. 19 9), but Spence 

( 19 9) suggested that the low . ci micity rate b tween I at 

45° 1. and the Blanco frac ture zone (!at 43° .) might al. 

characteri ze a separate egmcnt of the Z. 

A egmcnt boundary in central Oregon between !at 44° 

and 45° . (fig. 26) that would tend to limit the propagation 

of plate-interface rupture i. one po ible explanation for 

change. in the type of mar. h . tratigraphi c ·eq uence that 

occur between northwc tern and south-central Oregon. For 

example, the thick peat equcncc. in the Siuslaw Ri ver e tuary may mark the outhern ex tent of late Holocene plateinterface rupture (fig . . 288 and 28D). The co eismic movement. in ferr d from mar h equenc decrea e from as much 

a. 2.7 m of ubsidence e timated for ome ite in outhwe tern Wa hington ( ileen Hemphill-Haley. .S. Geological 

urvey. written commun., 1989) to the 0.5- 1 m subsidence 

es timated by Peterson and Dari enzo (thi s volume) in Alsea 

Bay . Thus, the AI ea Bay sequence. may record the outhcrnmo t ex tent of plat -interface ruptures that are hundred 

of kil ometer long (fig. 288) (Darienzo, 1989). Alternati ely, the central Oregon coa t may have had only relatively 

hort rupture (most le than I 00-200 km) produced during 

moderate earthquake (fig. 28D), pcrhap with a cycleending earthquake of 250- 00 km (see Thatcher, 1990). In 

thi asc. the : ize ai1d recurrence of earthquakes in central 

Oregon wou ld differ from tho e in nonhwe tern Oregon, but 
108 A E SING EARTHQ KE HAZ RD A D RED I G Rl K I THE P IFIC 'ORTHWE T 

the Siu law River equence. \ ould till mark the outhern 

limit of the long. large- lip rupture (greater than 0.5 m 

co ei mic ub idence; fig. 2 D). 

An explanation for the variations in the type of mar h 

sequence that i con i tent with the Mw 9 earthquake 

hypothe i discus ed above can be related to the width and 

locati on of the zone of ubsidence inferred to have been produced duri ng repeated great (Mw 9+) eanhquake (Atwater, 

1988b). ff the we tern edge of the zone of ub idence for all 

these po tulated great earthquake was between Al sea Bay 

( I 07 km east of the trench) and the Siuslaw Ri ver e tuary 

( I 04 km from the trench; figs. 27 and 28A), only the AI. ea 

Bay equences wou ld be expected to show evidence of 

repeated cosei mic ubsidence. Of cour e, the length, width, 

and location of the zone of subsidence arie from earthquake to earthquake bu t it must alway have been east of th 

Siuslaw Ri ver estuary, wh ich how no evidence of major 

co eismic- ub idence event. . 

The difference in the type of mar h record from the 

Siu law Ri ver and AI ea Bay sites uggest that these ites are 

probably on separate late Holocene egment of the CSZ. 

Both sites are about the same di tance from the trench (fig. 

27), and the ages of ed iment in core from both estuari es 

probably span at least the last two to five ei mic cycles 

(2,000- 3,000 year ). Thu . mo t earthquakes recorded at 

AI ea Bay . hould have been recorded at the Siuslaw Ri ver if 

the ruptures on the pl ate interface during the e earthquakes 

extended throughout this area. Pos ibly some long ruptures 

ex tended far south of the Siu law Ri ver (fig. 28C) but, if so, 

the zone of co eism ic subsidence wa inland of the oast and 

most of these long-rupture earthquakes were not recorded at 

A I ea Bay. Unfortunately. the wid th of the zone of ub idence 

and its distance from the trench during hi tori c great earthquakes in other subduction zones are ufficientl y variable 

(Atwater, 1988b; We t and M cCrumb, 198 b) that none of 

the po sibl rupture modes in figure 3 can be ruled out u ing 

argument ba ed on analogies with other subduction zones. 

It al o is unfortunate that strati graphic studie of the 

marsh record may not be capab le of conclu ively di tinguishing between the two egmentation hypothese (fig . 

2 A and 28D). In central Oregon, es tuari es wi th mar .. hes of 

. uffi ient ize to allow the extensive cori ng needed to document co eismi c changes in ea level are paced 28- 50 km 

apart. Furthermore, in large parts of these es tuaries, 

equenccs con i t mo tl y of andy tidal-channel. eolian, and 

nuvial ediment in which the record of small sea-level 

changes is ob cure. The size of these ri ver-dominated e tu -

arie (Peter on and other , 19 4) and the di tributi on of their 

mar hcs also limit to le than 8 km the length of the eastwe t-trending zone over which evidence of ea-level 

hangcs can be identified. Thus, onl y four or five estu arie in 

central Oregon are likely to yield defin iti ve data on cosei -

mic changes in sea level. The di tance of potential core ite 

in these e. tuari c from the SZ tren h do not differ enough 

to di tingui h between the two egmentati on hypothc e 

di cu ed above, and there are not enough widely paced 

ite wi thin each e tuary to determine difference in the 

amount of co i mic . ub idence in an east-west directi on. 

Such difference might be used to better define the position 

of the zone of ubsidence during past eanhquake . 

In contra t to th northern and central Oregon coa t, the 

earthquake re ord of the Coos Bay region appears to be a 

compo ite of regional and local earthquakes. Here, the ac ti ve 

fold and thrust belt of the overriding pl ate is well documented by on hore (Kelsey, 1990; Mclnell y and Kelsey, 

1990) and off. hore tudies (Clarke and others, 1985). Within 

thi. tectonic framework , we ascribe some of the multi ple, 

abruptl y buried marsh peat in the South Iough yncline to 

coseism ic sub idence during local faulting and fo lding of the 

yncline or adjacent anticlines (M clnell y and others, 1989; 

Peter on, 19 9). Abrupt buri al of some other p aty oil 

el ewhere in the Coo Bay ar a may al o be due to local 

deformation of other structure . . Le . er amounts of local 

cose i mic deform ation could ex tend as far north as the iuslaw River (Peter on and Darienzo. 19 9), although we have 

as yet found no evidence of Holocene cosei mic deformation 

there. Although deform ation in the oo Bay region i probabl y control! d by local struct ure , plate-interface earthquake. are not precluded because mo ement on I cal 

structures commonl y occur during plate-interface earthquake (Pi afker, 1969; yke , 19 9), and local co ei mic 

deformation could be overprinting regional deformation 

( el on, 1987; Mclnell y and other , 1989; Peter on and 

Darienzo, 1989). 

A egment boundary north of the Siuslaw Ri ver in central Oregon would indicate that the 180- to 230-km-long pan 

of the central CSZ from that boundary outh to Cape Blanco 

con i t of one or possibly two segment (Peter on and 

Darienzo, 19 9). For example. between Coos Ba and the 

Siu law Ri ver, the submerged mouths of ri ver alleys and 

the relativel y low elevation of Plei tocenc marine terraces 

uggest that thi part of the coas t ha been up I if ted at a lower 

rate than areas to the north and outh (Adam. , 19 4). If thi s 

zone of relative ubmergence i renecting north- outh 

change in the properties of the subducting plate, then it 

could mark a egment boundary. However, because thi zone 

of submergence coincides with the eastern edge of the fold 

and thru t bel t, it i more likely to be related to an ea t-we. t 

structural tran ition in the overriding orth America plate 

(Peterson and Darienzo, 19 9). The change in tructural 

style hown by the marine- and nu vial-terrace tudie probably shows the ame transition. 

Determining the recurrence of prehistori c earthquakes 

in the fold and thru t belt region of southern Oregon is complicated by the probable com pos ite nature of the paleoseismic record . Analogie wi th similar areas (Berryman and 
RECE T COASTAL ST DIES OF SEGM E TATIOt OF HOLOCE R PTURES, ASCADIA S BD CTIO 1 ZO E 109 

other . 1989; Carver and other . 19 9) ugge t om local 

earthquakes in the Coos Bay region may have been of mall 

surface-wave magnitude (M 5 le than 7) and were not n ce aril triggered by larger pl ate- intcrfa e earthquake (for a 

different view. see Syke , 19 9). A argued by Year and 

other ( 198 1) in outhern Californi a. earthquake like tho e 

recorded in the South Slough yncline may be ery . hallow, 

quite pos ibl y le than Ms 6. and occurrino on fl ex ural-slip 

fault during co eismic folding. nfonunately . without a 

mean of preci ely dating or directl y correlating incli idual 

ubmergence event. , the possibility . eems remote that mar h 

rratigraphic method can be u eel con istentl y to di stingui sh 

local from regional earthquakes on a egment or segments of 

the CSZ b tween the Siuslaw Ri ver and Cape Blanco. 

CONCLUSIONS 

Field . tudies of the pa t few years have produced convincing evidence of cosei mic changes in land level along 

the Washington and Oregon coas ts. Studies of coa tal 

deposits suggest that regional plate-interfa e earthquakes 

of at least Mw 8 w ith more than I 00-km-long rupture have 

occurred in northwe tern Oregon and outhwes tern 

Wa hington. 

Difference in the type of tidal-marsh sequence 

between north and south-central Oregon are con i t nt w ith 

everal hypothe e for the recurrence and magnitude of 

plate-interface earthquake . We uspect that a segment 

boundary ex i t along the CSZ near lat 44°-45° .; such a 

boundary w uld sugge t that the SZ is egmented and, 

therefore, that plate-interface earthquake of abou t Mw 8 

along the central CSZ are more frequent than larger earth -

quakes. A ltern atively, the we tern edge of the zone of 

co eismic subsidence, due to larger (M w 9), more in freq uent 

plate-interface earthquake , may inter cctthe coast in central 

Oregon and could produce the ob erved . trati graphic 

change. in marsh equences. 

Gcophy ica l and . tructural studie and mapping of 

marine and fluvi al terrace indicate that many pi ode of 

abrupt marsh buri al in . outh-ccntral Oregon arc be t interpreted a the product of deform ati on of local tructures . 

Some of the local deform ation coul d be a ·soc iatcd with 

earthquake f moderate magn itude (M 5 lc · than 6). A t mos t 

ires in thi s area, however, it is still unclear w hether cosei -

mic event were re pon. es to local faulting or folding. to 

regional deform ation during great plate-interface earth -

quakes, or to both. Ad ances in dating techno! g and cx tenive mapping of Pleistocene tructure. w ill be need d to 

distingui h regional from local earthquake . 

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during the Flandrian stage: Oxford. Great Britain. larendon 

Press. 235 p. 

---19 5. limate, sea-level and coa~ al change:. in Toole . 

M.J ., and Sheail. G.M .. ed .. The climate scene: London. 

George Allen and Unwin. p. 206- 2 4. 

Tooley. M.J .. and Shennan, Tan. eds., 1987. Sea-level changes: 

London, Institute of British Geographers pecial Publicat ion 

20. Basi l Bla kwel l. 397 p. 

Vick, G .. , 1988, Late Holocene paleoseismicity and relative . ea 

level changes of the Mad Ri ver Sl ugh. northern Humboldt 

Bay. Cali forn ia: Arcata. Cali f., Humboldt State niversity, 

M .S. thesis, 7 p. 

Vincem, Paul. Ri rd ~. M .A .. and Weldon, R.J ., II. 1989. Vertica l deformation of the Oregon continental margin [ab . . ]: 

EOS [American Geophy ·ical nion Transactions]. v. 70, no. 

43, p. 1332. 

Weaver, C.S .. and Baker. G.E., 19 8. Geometry of the Juan de Fuca 

plate beneath Wa. hington and northern Oregon from seismicity: Sei smological ociety of America Bulletin. v. 7 , no. I. p. 

26 275. 

Weaver, C.S., and Michaelson, C.A., 1985. Seismicity and volcani. m in the Pacific orthwest- idence for the segmentation 

of the Juan de Fuca plate: Geophys ica l Research Letters. v. 12. 

no. 4, p. 2 15-2 18. 

Weaver, C.S., and Shedlock, K.M .. 1989, Potential . ubduction, 

probable intraplate. and known crustal earthquake . ource areas 

in the Ca. cadi a subducti on zone. in Hays. W . W .. eds .. Proceedings of Conference XLVII, 3d annual workshop on earthquake hazards in the Puget Sound. Portl and area: U.S. 

Geological Survey Open-File Report 89-465, p. I 0-26. 

Wells, R.E., Snavely. P.O., Jr .. and iem. A.R .. 1992, Quaternary 

hru~ t faulting at art~ Bay, northern Oregon coast: Geological Society of Ameri ca Abs tr ac ~ with Programs. v. 2-1 , no. -. 

p. 9. 

Wells. S.G .. Bu llard. T.F., ge~. C.M., Drake. P.G .. Karas. P.A .. 

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---1988b. Repl to B.F. twater's comment on "Coastline 

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I 0259- 1 0269. 

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DISCRIMINATION OF CLIMATIC, OCEANIC, AND 

TECTONIC MECHANISMS OF CYCLIC 

MARSH BURIAL, ALSEA BAY, OREGON 

By Curt D. Peterson 1 and Mark E. Darienzo 1 

ABSTRACT 

Late Holo enc sequences of 2- 10 buried peaty hori zon. 

( uch a wetland soil layers) from marsh cutbank and from 

J9marsh core site · in A lsea Bay, Oreg., have been analyzed 

for compo. ition, lateral correlation. and radiocarbon age. 

Thee analyses are used to di scri minate between different 

mechanisms of marsh submergence and buri al including 

river flooding. ocean set-up. barrier-spit breaching, and 

coseismic subsidence. System-wide mar h respon. cs to 

abrupt but persistent submergence events in Al sea Bay are 

indicated by ( I) lateral correlation of key peaty hori zons 

over di tance of a. much as 3 km . (2) con i tent verti cal 

trends in peaty burial units, generall y including increa ing 

organic material and dccrea ing sand content upunit, and (3) 

clo e a ociation of anomalou . and-ri ch t unami deposit. 

immediately overl ying the buried peaty hori zons. The tudy 

re ult rule out marsh buri al by cata trophic climatic or 

oceanic. proces es and argue for epi odic coastal submergence forced by abru pt co ei mic ubsidcnce in an acti ve 

subduction zone. 

n average recurrence inter al of 500 radi carbon 

year is estimated from eight repeat imcr als of cosei. mi c 

ubsidcncc between 4.5 10 and 4 0 Radiocarbon Year. 

Before Pre. cnt (R YBP). By compari . n. an average recurrene interval of340 rad iocarbon year i. c timatcd from the 

three most recent repeat intervals f co. eismic . ubsidence 

between 1.490 and 4 0 RCYBP. Recurrence inter al 

between succcs ivc burial c cnt. were found t range from 

250 to I ,370 years (uncalibrated radiocarbon year ). The 

average minimum range of crti ca l . ubsidence in AI ca Ba 

(0.5- 1 m) at lat 44.4° is . ub. tantiall y les. than that 

reported for ctarts Bay, Oreg., ( 1- 1.5 m) at lat 45.4° . but 

more than that for Siuslaw Bay. Oreg., (0- 0.5 m) at lat 44° 

1

Dcpanmcn1 of Geology, Portland Hllc nivcrsil . Portland. OR 

97207. 

Thi. trend in average co eismic . ub. idcnce implies a coa tline clo ure wi th a zero i. obas . that is, a zon of little or no 

upper litho pheri c plate clastic fl exure, outh of lat 4-l . If 

confirmed by future work. the po ition of a zero isobas 

located at least 90 km landward of the trench in the a: adia 

ubduction zone would define the land• ard limit r the 

locked zone in central Oregon. Finall y, a high c rrcspondence between mar h subsidence and t. un ami sand-layer 

deposition was ob erv d in lsca Bay, even though the c timatcd am unt of local coa. tal . ub idcncc (0.5- 1 m) i. small. 

Such a high COITespondencc between tsunami and and 

coastal subsidence c ents can be used to tc. t models of 

potential earthquake rupture length in the central as ad ia 

ubducti on zone margin . 

INTRODUCTION 

Recent report. f epi odic burial of mar. h and wetl and 

hori zon. by tidal-fl at muds in several tidal ba. in of Washington and Oregon have greatl y incrca. cd the controversy 

over the neote tonic hi tory of the ascadia . ubducti on zone 

(CSZ) (Atwater, 1987; Peter on and others. 198 ). For example. ev idence of alternating coa tal UJiift and ub idcnce 

might indicate acti e ubduction of the ceanic Juan de Fuca 

tectonic plate under the continental onh Ameri ca plate 

land\ ard of th ea-floor trench (fig. 33). uch peri odic 

coa tal eli . placement. are a sociatcd with similar cosei mic 

ub luction in the strongly coupled ubducti on zone of 

hile, southea tern Ala ka. and Japan (Pi afk cr. 1972: yeda 

and Kanamori. 1979: Heaton and Hartze ll. 1986). 

brupt change. in rclati e tide level mu t have produced the cata trophic burial of the upratidal vegetated 

surface b lower intertidal . ediments. a ob er ed in some 

SZ coastal tidal ba ins (Atwater. 1987, 1988 ; Darienzo and 

Pctcr. on. 1990). However. abrupt increa es in the height of 

relative tide level in . omc bay might hav been caused by 

115 
11 6 ASSESS! G EARTHQ AKE HAZ RDS A 10 RED Cl G RISK I THE PA IFI ORTHWEST 

130° 125° 120° 

6. 

so· 

BRITISH 

COLUMBIA 

\ -rr_----

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6. 

/ 

6. 

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z 

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:ll 

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n 

OREGON )> 

CALIFORNIA-~ I 

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PLATE 1 

I 

\ 

0 100 200 400 ILOMETERS 

Figure 33. Index map of the Pacific onhwest howing the 

AI ca Bay study area (large dot) and several other bay wetland areas (small dot ) reponed to record coa. tal ubmergence. The 

dashed line shows the estimated position of the sediment-filled 

trench between the . ubducting Juan de Fuca plate and the overriding orth Ameri ca plate. Arrows show approximate directions of 

relati ve plate movement (from Riddihough, 19 4). Triangles show 

volcanoes in the Ca. ade volcanic arc. 

mcchani m. unrelated to either local or regional tecton ic . 

Such non tectonic mechani ms might include ( I) river fl ooding, (2) ocean torm surges, (3) anomalou condition of 

ccanic circulation, and (4) change in tidal-inlet morphology and the a ociated basin tidal pri sm, that is, the vo lume 

or tidal flow in the bays. 

A many a. six coa tal- ubmergence event have been 

documented in mar h and wetland depo its from large e ·tuarie. in Washington such a Willapa Bay and Gray Harbor 

( twater, 1987, 1988) and from mailer bays, including a 

barrier lagoon ( etan Bay) in Oregon (Darienzo and Peterson, 1990; Darienzo, 199 1 ). Buried peaty deposit have also 

been found in two fluviall y dominated estuari e., ehalem 

and Salmon Bays, a few ten of kilometers to the north and 

outh of Nctart: Bay (fig. 33), re. pec ti ely (Grant and 

M cLaren. 19 7; Grant. 19 9). The buried peaty horizons 

(wetl and oil ) in these bays have been rad io arbon dated a. 

being of late H loccnc age. They range in age from at least 

3.300 to about 350 Radiocarbon Year Before Pr sent 

(RCYB P) in etarts Bay (Darienzo and Peter on. 1990), and 

from 5,000 t 300 R YBP in Will apa Bay (Atwater, 1988). 

In contrast, r-lati vely long . tratigraphic secti on. of continuou or nearl y continuou. peaty edim nt · from 0 to 4 m 

depth have been ob. erved in iuslaw Bay (fig. 33) on the 

central Oregon coast ( cis n, 1987). 

Di crimination between tectonic and nont ctonic mechani. ms of relative ea-1 vel change are criti ca l to the correct 

interpretation of the submergence events do umcnted in 

coa tal wetl and of the central csz margin . or particu lar 

concern to the con trover ial history of the Z i. the po sible 

mi taking of tectonic sub idcnce event for climati c. oceanographi c, or pit-breaching ri e in tide level. These me hani sm: need to be di riminated to confirm po sible ev idence 

of regional tecton ic ub idence and a ociat d t unami excitation. uch c idencc wou ld clearl y indicate the poten ti al for 

gr at subduction earthquake in the central SZ margin. 

In thi report , we describe the late Holoccn tratigraphy of the !sea Bay, Oreg., mar h, a documented from cutbank and shallow coring tran cct (2- 7m depth) that extend 

aero . the lower and upper reaches of thee tuary . Di stincti ve 

hori zon in elec ted mar h c r s were analyzed for ( I) relative abundance of organic material, (2) sed iment grain-size 

di tribution, (3) sand ource (marine or flu vial), and (4) 

radiocarbon age of buried peaty depo it . The re. ult of our 

tudy how that the observed marsh strati graphy in AI ea 

Bay permi ts the di. crimination of tectonic and non tectonic 

mechani m of marsh burial. Due to the wide range of mar h 

environment in AI ea Bay, we believe that the resu lts of thi s 

tudy ite apply to other coasta l mar h y tern of the central 

CSZ margin. 

ACKNOWLEDGMENTS 

We thank Henry Pinock for pr iding records of wind 

peed, atmospheric pre ure, and corresponding torm-surgc 

level from Yaquina Bay, Oreg. We appreciate the time Alan 

el on of the U.S. Geologica l Survey spent in hawi ng u the 

marsh tratigraphy in Siuslaw Bay . Charl es Iough assi ted 

with urvey and lateral correlations of buried peat in cutbanks f AI ea Bay . Larry Kauffman provided u with a boat, 

motor. and dock facility for the ex ten ive coring and urveying of the A lsea Bay marsh. M argaret Mumford and arolyn 

Peterson produced many of the original line drawing . We 

thank Wendy Grant of the .. Geological Survey and Jim 

Phi pp · of Gray Harbor College, Aberdeen, Wash , for 

reviewing thi manu cript and providing many ugge tion 

for its improvement. Thi re earch wa supported by the .S. 

Geo logical Survey under the ational Earthquake Hazard. 

Red uction Pr gram, grant number 14- 0 - 000 1- G 151 2, and 

by National Science Foundati on grant EA R- 8903903. 
CLIMATIC. 0 EAt IC. At D TECTO I IECHA I M OF CY LI C MAR H B RI L. L E BAY, OR EGO 11 7 

N 

r 

0 2 KILOMETERS 

Upper estuarine reaches 

Figu•·c 34. The AI. ea Bay e tuary, showing channel comours of mean lower low water (dashed lines) and the fringing marsh 

system (plant symbols). M ore than 90 percent of the sediment s nooring thi high-gradient. nuviall y dominated estuary are in the 

sa nd-size range. The bay is roughl y di ided on the ba. is of. and source into the marine-dominated lover reaches. the nu viall y 

dominated upper reaches, and the transitional middle reaches (labeled in fi gure). Line of secti on A- A· at the tide inlet shows 

cross-section traverses detailed in fi gure 54. 

STUDY -SITE SELECTION 

We selected AI ea Bay (fig. 34) a a . tudy area because 

its phy. iography was thought to be particul arl y well uited to 

the di crimination of potential climati c. oceanic. and tectonic mechani sms of mar h burial. or example. Al sea Bay 

ha. mar h deposits that ex tend continuou. ly aero s nu viall y 

and tidal I y dom i natcd estuarine en iron mcnt. (Peter. on and 

other , 19 - ). The different marsh environment. . hould 

how either ( I) · ffcct from river fl ooding in the c n. tricted. 

upper c. w ary rcachc. , (_)effect. from ocean torm urge in 

the ex po. eel. broad lower c tuary. (3) effect from spit 

br ach ing during 19 -- 19 9 (Peter on and other , 1990) in 

modern mar. h depo it . or (4) effect of co eismic coas tal 

ubsidcncc, " hich . hould extend acros. upper and I ' cr 

estuary mar. h environment. and whi ch . hould be ab. ent 

from hi stori cal mar h dcpo it 

The . ccond rca on for selecting thi tudy itc is that the 

hyd rography and sedimen tology of thi small tidal ba in 

have been studi ed in detail (Goodwin and others. 1970: 

Boley, 1973: cKcnzic. 1975; Peterson and others. 19 2: 

Pet r. on. and other , 19 4: Peterson and other . . I 90). Di. -

tincti vc mineral compositions of ri cr and marine our c 

sands in Al sea Bay (Pet rson and other . 1982) arc 

particu larl y important to the in c. tigations or marsh-burial 

mechani ms in the estuary. For example. the identifica ti on of 

sand source (ri ver r ocean beach . and) upplied to the 

marsh edi ments should firmly di. tingui h between seaward- and landward-directed . cdimcnt transport during 

mar h burial. "inall , bu ried peaty hori zon. topped by thin 

sand layers were known to cx i t widel y in expo ed tidalcreek banks of the I ea Ba mar h (K.F. Scheidegger and 

.D. Peterson. unpub. data. 1980). 

POTENTIAL MECHANISMS OF 

MARSH BURIAL 

Wcll-dcvc l ped salt mar hes are pre. ent within a 

re trictcd range on either ide of mean tide lc cl (MTL). 

roughl y 0.5 m below to 2 m ab ve MTL. Thi range is a 

functi on of plant tolerance to local condition or d iccation. 

ali nit . and ub tratc . tability ( icring and Warren, 19 0). 

Emergence of a sa lt marsh abo c the reach of tide re ult in 

a transition to upland vegetation (supratidal elevati on), 

whcrca . ubmcrgencc to lower intertidal or . ubtidal level 

results in burial by tidal-fl at mud. 
II ASSE I G EART HQ A KE H ZARD A 1D RED RISK I T HE PAC IFI RTHW ST 

Slump block 

Figure 35. A hypothetical mar ·h vertical sequence developed wi thin a tidal-creek meander bend. nits in the stratigr:-tphic column corre pond to depositional . urface. in the channel eros. section. The 

sequence is produced a. the channel migrates laterall y. cutting the left 

bank and building the right bank. 

Although meandering ti dal channel can produ e 

depo it that mimic emergent ti dal-fl at to mar. h . equences 

(fig. 35), these channel-fill dcpo it do not overli e undi sturbed mar h urface and are only locall y developed within 

the back-filled meander bends of tidal channel . In ontra t, 

broadl y defined emergence or ubmergence records that 

span intervening tidal creeks indicate change. in MTL relati ve to the bay marsh system. 

Gradual ubmcrgcncc or minor flu tuati ons in the submergence of late Holocene :alt-mar. h en ironmcnts arc gen. 

erally reported from pa si e continental margins of the 

ea. tern and gulf coasts of the nited States (Kraft. 197 1; 

Redfield, 1972: Or n and other.. 19 5). These trends of 

nearl y continuou: coa. tal ubmergcncc refl ect relati vely 

gradual condition of u. tatic sca-le cl ri . e and (or) local 

ub idcnce by tectonic warping. In contrast. the episod ic 

event of mar. h emcrg nee and ·ubmergencc ob. ervcd in 

ome Pacific orthwe t coas tal bay · (Atwater, 19 7; 

Darienzo and Peterson. 1990) must be produ eel by more 

dynamic, reversible proces e. of relati ve sea-level change. 

c era! po ible mechani sm of epi odi c mar h buri al as 

recorded in coa tal dcpo. its or the central CSZ margin arc 

discu eel below. 

RIVER FLOODING 

Tidal bas ins of the Pac ific orth wes t coa. t vary greatl y 

in the relati ve influ nee of tidal and ri ver flow. In the fluvially dominated e tu aries. the peak ri ver di scharge during a 

gi ven tidal cycle can exceed 50 percent of the ba ·in tidalpri m discharge (Percy and other . 1974). The upp r e tuarine reache . often constricted by steep-s ided ri ver valley , 

are the area mo. t su ceptible to fl ooding by ex treme ri ver 

Figure 36. The upper reaches of Al sea Bay showing the central and eastern mar. h area . . View is to the east. See fi gure 

34 for the map of the estuary. Photograph by P.D. Komar. College of Oceanography. Oregon State ni versity. 
CLIMATI C. 0 Ef\1 IC, A D TECTO IC MECHA I M OF CYCLI MAR H B Rl L. L BAY, OREG 119 

River tidal channel 

PROXIMAL 

STRATIGRAPHY 

Marsh levee Backmarsh 

DISTAL 

STRATIGRAPHY 

Figure 37. Hypotheti cal ri ver-flood deposi t with proximal (marsh levee) and distal (back mar h) stratigraphic olumns showi ng cx 1 cctc I 

fi ning-upward sequences . Ri ver tidal channels in most f the high-gradient estuari es of the Paciri North ~ e t arc floored b . and anti (or) 

gravel. providing prox imal s urce. of coarse-grained scdi mcms to channel-margin en vir nmcnts during catastrophi · floods . 

discharge event.. For example, ex treme river floods ( I 00-

year flood ) in AI ea Bay that occurred in 1890 and 1964 

produced di charges of more than I , 133 m3/ . The e river 

fl oods raised the ri ver level about 7 m above the average 

level ( 1.5 m at 47 m3/s) at a gauging tation located at the 

head of tide, a di stance of 26 km from the bay mouth ( .S. 

Geologica l urvey, 1988). Fi ne- and coar e-grained sediments are transported down tidal channel of the upper estuary during winter months of max imum rain fa ll (Peter. on and 

other .1 982).Con civably,suspended ri ver ediment. could 

bury all-marsh urfaces in the con tricted upper e tuary during the mos t ex treme events of ri ver flooding (fi g. 36). 

The resulting flood ediments, deposited above the 

peaty marsh horizon, might include fining-upward 

equcnce (fine sand grading upward to silt and clay) (fig. 

37) that are typica l of fl ood overbank depos ition (Reineck 

and Si1igh. 1980, p. 288-29 1 ). AI o, tJ1 e coarse t ediments 

in basa l layer above the buried mar h would be expected to 

dimi nish in relative abundance with increasing di stance from 

the tidal-channel ax is and wi th increas ing di tance downtream from the constricted ri ver va lley. Finall y and mo t 

import an t. mar h-burial depo its of . and should ref! ct the 

ri ver- ourcc min ral c mposition if they are derived from 

flood overbank d p . it ion. 

OCEAN SET-UP 

On. horc water-forcing prevail along the Pacifi 

orthwcs t coa l when off bore storm (c cloni c lowpre .. urc en ter ) approach land from the west or outhwe t 

(Smith. 1974). The resulting cean ct-up Cinerea e in sea 

level) from local . torm winds and the as ociated low atmo-

. phcric pre . urc temporaril y increa es tide levels within the 

bay s. For cxampl , measur d tide level of at lea t 1.2 m 

above predicted tide. in Yaquina Bay ( 15 km north of 

Alsea Bay) arc known to result from ex treme onshore 

wind (as high as 90 knot. ) that force on hore ma. s 

tran. port of ocean water (fig. 38) (H nry Pittock, oil ge 

of Ocean graphy, Or gon State ni versity. unpub. data. 

19 I). Although the effects of such torm set-up cond ition. 

on coa tal-marsh s stems in the Pacifi onhw t ha c not 

been reported. hurricanes arc kno' n to ha e produced 

torm ·ed iment hycrs 1-3 em thick in mar h sy tcm. f th 

nitcd States gulf coa. t (Rej manek and other. , 198 ). 

Longer period of cpi odic sea-level ri e ca n oc ur 

from anomalous conditions of climatic-oceanic cir ulati on. 

Small increa cs in monthly mean ea level are produc d 

from sea onally persistent condition of low atmospheri 

pre . urc (Huycr and others, 1979; Pittock and others. 19 2) 

and (or) the north ward coastal propagati on of El ino S uthcrn 0 cill ati on (EN 0) ea-level bu lge. (Enfield and !len. 

1980). TheE SO equatori al (ca tward) ma. transport that 

rc u!t fr m the relaxation of equatori al trade winds is 

diverted . outh and north at the eq uatori al continental margin. 

The sea-le el bulge then propagates along the maroin a a 

coa tal-trapped Kel in wave. Although the ri se in the 

monthly mean ea levels along the Or 0 0n and Washington 

coa ts have histori call y been small ( I 0- 0 em) ( nficld and 

A llen. 19 0: Huycr and others, 1983), the I ng durati n 

(month ) of elevated sea level (fig. 39) could conceivably 

have an impact on tidal marshe , as di cu ed below. 

ondition of elevated ea le el from torm surge and 

(or) anomalou oceanic ci rculation wou ld increa. e the depth 

and fetch of intraba in wind wave in the characteri. ti call y 

shallow ba of the Pacific orthwe t. The tidal mar hes of 

the broad. lower c tu ary that are exp sed to direct cean 

well s or intrabas in wind waves wou ld be the mo t susceptible to buri al by ed iment re u pended and depo ited during 

major wind tonn . The burial deposit. could conceivably 

contain layered sand and silt (Allen. 1984) resulting from 

time-vari able tidal currents and the torm re. u pen ion of 

e limcnts from hallow sand and mud fl ats (fig. 40). lay 

hould not c nstitute a major fraction of the torm-burial 

layers, a it is likely to be held in suspcn ion during catastrophic ' ind-storm e cnts. In addition, the immediate 
120 AS ESSI G EARTHQUAKE HAZ RDS A D RED Cit G RI K I THE PA IF! 'ORTHWE T 

PACIFIC STANDARD TIME ON NOVEMBER 13-14,1981, IN HOURS 

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0 

::;; 

735 >--

..: 

1600 2000 0000 0400 0800 1200 

PACIFIC STANDARD TIME ON NOVEMBER 13- 14,1981, IN HOURS 

Figu1·e 38. Historical ex treme event of ocean set-up from a storm 

surge in Yaquina Bay, Oreg .. 15 km north of Al sea Bay, during ovember 13- 1-l, 198 1. M easured tide level. wind velocity, and atmospheric pre~s ur were recorded simultaneously at the Hatfield 

Marine Science en ter at Yaquina Bay (Henry Pittock , allege of 

Oceanography, Oregon State Univer. it y, unpub. data. 19 I ). The 

mea: ured tide level ranges from 0.- to I m above the predicted tide 

level during the maximum period of northeasterl y to easterl y directed wind tr ess (0000- 0 00 hours. ov. 1-l). Yaqui na Bay is similar 

to AI. ca Bay in site and morphology, but a larger tidal entrance to 

Yaquina Bay probably enhances storm set-up there. 

sources of tidal-flat . ediments in the flu viall y dominated 

es tuaries such as Al sea Bay have a significant beach-sand 

c mponcnt (Peter. on and others. 19 2). The coarse-grained 

comp ncnt of the torm deposits shou ld reflect the sand-

. ourcc mineral ompo. it ion of the nearby tidal flats from 

which they would be derived. Fin all y. marsh environments 

mo t sensitive to repea ted storm -surge deposition should be 

dominated by hi gh-energy coarse-grained . cdiments ( and). 

_j 

w 

> 

w 

...J(/) 

..:a: 

ww (/)>-w z::;; 

..:- w>-

::;;z w wu 

:2:z 

>---

:5 

w 

a: 

1982 

/ 

/ 

/ 

~/ 

' ' ' ' I 

I 

I 

I 

I 

I 

I 

I 

' ' ' , ___ ,. 

1983 

Figure 39. M onthl y relative mean sea levels from the Yaquina 

Bay. Oreg .. tide gauge for the 198-- 83 El ino Southern Osc illation 

(E 0) period (da-.hcd line), and COITCsponding 10-ycar averages 

(solid line) for the 197 1- 8 1 decade (Huycr and others, 1983). The 

measured increase of monthly mean sea lcveb peaked at greater 

th an 30 em abo e normal for the duration of the 1983 winter. a historically ex treme event of intra-annual elevated rel ati ve sea k1cl. 

BREACHES IN BARRIER SPITS 

All of the mcchani. ms of . ubmergcncc di scussed ab vc 

in olvc temporary increase: in mean tide level. However. 

long-term changes in the tide- level range might also impact 

mar h plant growth and sediment deposition (Clark and 

Patterson. 1985). Such long-term changes in tide-level range 

can occur from vari ation in tidal-inlet morphology that control the tidal flow between the free ocean surface and the 

in. horc tidal ba in . earl y all of the tidal bas in. on the open 

coas ts of Washington, Oregon. and northern aliforni a arc 

fronted by sand-spit barrier (Dingler and Clifton, 1993). 

Tidal inlets to the c bays either cut through the barrier . pit or 

arc pinned b the spit against a resi tant ca cliff, a. in Alsea 

Bay (fig. 34 ). The eros. - ·ectional areas of the tidal in lets arc 

generall y proportional to the size and con·e. ponding tidal 

pri m of the tidal ba in co· Brien. 1969). The tidal pri sm. of 

the larger bays do not allow the spits to completely cut off 

these cstuaric from o can tide level. However, vari abl e wave 

climate (energy and direction of urf) and the resulting mobility of littoral sand can cause a net in-filling or erosion of the 

tidal inlet. thereby reducing or increasing ba in tidal flow. 

A reduction in tidal fl ow (choking) across an inlet 

result. in a diminished tidal pri sm within the tidal basin. For 

all but the mo. t fluvially dominated c tuari c . a decrca. c in 

the tidal pri. m translates into a decreased maximum tide 

level in the broad lower estuary. Convcr ely, opening of the 

inlet (increa. c of tidal flow) incrca es the tidal range. thereby 

increas ing the depth but not the period of salt-marsh . ubmcrgcncc during high tide. For example, a ubstantial 50 percent 

increase in tide-level range above the equi librium range (2 

m) from mean lower low water (MLLW) to mean higher 

high water (MH HW) (Perc y and others. 1974) could add an 
CLIMATIC.OCEA !C. A DTECTONIC ECHA ISMS OF Y LI MARSH B RIAL, L E B Y, OR GO 121 

Mean tide level ... , ..... __ ..... ...... 

STRATIGRAPHY 

Coarse sand 

Organic material 

Figure 40. Hypotheti cal torm deposits composed of cyclic vertica l sequences of sa nd and silt bur ing marsh hori zon . . Waning 

storm conditions might produce fining-upward sequences. whereas variable ocean set-up and tidal cycle. mi ght 1 r duce more 

complex layering of sand and . ilt. The burial of uccessi e supratidal marsh hori zons by . torm resuspension and deposition 

would require catastrophic storms superimposed on a longer term 

trend of coastal submergence. 

additional 0.5 m of submergence during high tides in AI ea 

Bay. The effect. of a changing tide-level ran ge on ex i ting 

mar h sy tem in the Paci fi c Northwe l bays have not previou ly been exam ined. However. . ignifi cant changes in tide 

range could pos ibly . tress mar h sy tem , resulting in 

dimini shed pl ant growth and po sible burial by sediment. 

SUBDUCTION-ZONE TECTONICS 

Fin all y, ubducti n-zone tectonic might produce epiodic mar. h emergence and submergence in the convergen tmargin setting of the Pacific orth we l. In the CSZ, coa tal 

sub idence or uplift might rc ult from the cycl ic strain accumul ation and release a . oc iatcd wi th fric tional coupling of 

the overriding continental pl ate and ubducting oceanic plate 

(Fitch and Scholtz. 197 1; Savage. 1983). The direc ti on of 

vertical eli placement generall y depend on the coas tline 

position rclati c t an clasti ncxu re line ca lled the zero i obase of verti ca l dcf rm ation (Pl afkcr and Kachadoori an. 

1966) (fig. 41 ). In the cas of a oa:tlinc po. it ion landward 

of the zero isoba. c, coa. tal uplift hould be gradual 

(i ntersicsm ic strain ac umul ati on) \ hcrca. coa tal sub idencc . hould be abrupt ( o. eismic . train rcl ea. e). n in ver. e 

relation should hold for coas tline po ition eaward of the 

zero is ba. c. However. lo al upper-plate faults and fold can 

complica te thi deform ation pauern (Plafker, 1972: Iarke 

and Car er, 1992). The tidal-marsh deposit. of the central 

Oreg n coas t could r ord abrupt sub idence even ts if interplate clasti c . train i. rei ascd pi sodica ll y and the coas tal 

po ition of the mar. h site i. landward of the zero isobase. 

M arsh-buri al sequ nee. produced by cos ismic 

region al . ub. idence and ett li ng (Plafker and Kachadoorian: 

1966) sh uld occur throughout the e. tu ary mar. h y tem and 

Oceanic plate 

A lnterseismic 

strain accumulation 

B Coseismic 

strain release 

.c 

(.) 

c: 

., 

.:: 

., 

(/) 

"' .D 

0 

.!!? 

Continental plate 

FigUJ·e 41. Verti ca l tectonic relati n as ociated wi th (A) coupled 

train accumulation and (B) co ei mi shea r dislocation between a 

subducting oceanic plate and an overriding ontincntal plate (Ando 

and Bal azs. 1979). The relati ve position of the coastline and the upper plate nexure line, or zero i obasc (Piafker and Kachadoori an, 

1966). determines the directi n f coa. tal t ctoni displa ement 

during alternatin g periods of ascismi strain accumulation and 

coseismic dislocation. Arrows sho\ direction of relati e vertical 

mo emem on either side of the zero isobase. Barb show dire ti ns 

of relati ve plate movement along the fault. 

STRATIGRAPHY 

M ean tide level 

--

Figure 42. Hypotheti ca l ubsidcncc depo. it including a finingupward verti al sequence of sand to mud and colonizing mar. h 

above the buried preex i. ting mar. h surfa e. Sand and mud rati os in 

basal lower-intertidal layers might vary between ·ite · as a function 

of local sediment suppl y and re. u. pens ion energy. However. the 

upper-intertidal to supratidal layer. hould increase in mud and organic material as the d pos iti nal surface approaches the max imum 

reach of hi gh-tide levels. 

hould con i. t of low-energy tidal-nat dcpo it of finely 

laminated or bi turbated mud overly ing the submerged 

mar h surface (fi g. 42). uch lam inated or bioturbated muds 

are mo. t commonly deposited in the protec ted tidal-n at environmen ts of the Pac ific orthwe t es tuari es (Clifton and 

Phillips, 19 0: Peter. on and other. , 1982). They indica te 

prolonged period of low rates of depo ition. 
122 AS ESS I G E RTHQ AKE H Z RD tO RED I G RISK I THE PAC IFI ORTH\ E T 

Of parti cular intere t to palcosei. mologi t is the great 

potential for t. unami excitati n a ociatcd with co cismic 

coa tal di pl acement (Heaton and Hartzell. 19 6). E idcnce 

oft unami cdimem depo it immediately above the buried 

mar. h urface would indicate mar. h burial by coseismic 

ub idence ( t\ ater. 19 7). Such depo it . hould reflect a 

marine- ource mineral compo ilion and hould dimini h in 

thickne wi th increa ing di stance upri ver (landward) if they 

arc indeed produced by landward-directed t. unami s. 

FIELD AND LABORATORY 

PROCEDURES 

Cutbank expo ure of the Al sea Bay mar. h in the northern ide of the south channel and in both side. of the north 

channel (fig. 34) were examined for evidence of distinct 

peat hori zon . The cutbank s ( 1- 1.5 m verti cal exposure) are 

a continuou , circumferential exposure of the hallow 

stratigraphy of the AI ca Bay mar h sy. tcm. Two buried 

peaty hori zon and associated sandy sedim nt capping layer. 

(SCL. ) were tratigraphically correlated around the centralmar h and north-channel cutbank . . The sub. urface depth of 

the upper contac ts of the two buri ed peaty hori zon were 

measured in representati ve stratigraphic ec tion . total of 

62 shallow . tratigraphic sec tion were mea. urcd at spaced 

interva ls around the central-marsh circumference (sec fi g. 43 

for cutbank locations). The mar h-interior tratigraphy wa 

in vc tigated by deeper coring. 

A total of 2 1 core ( 19 eli fferent ire ) were taken from 

2- 7 m below the urface along a we t-east traverse (4 km 

between site AB I and AB 13) and two north- outh 

traverses (about I km each between sites AB2 1 and AB 18 

and between ites AB 19 and AB 17) (fig. 44). The wes t-east 

traverse extends from the middle e w ary (exposed to windtorm proces es) to the upper e w ary (influenced by ri er 

proce. es) a defined by the hydrography (M cKenzie. 

1975) and sediment-di spersal patterns (Peterson and others. 

19 2). The two north- outh traver e are oriented p rpendi ular to the outh channel, the maj or conduit of ri ver-tidal 

flow in the upp r e. lllary. The Al sea Ri ver valley and fringing marshes narrow greatl y to the ea t (upri ver) of core ite 

AB 12. ore itc AB I (at the we tern end of the traverse) 

contains onl y two thin mar h hori zon (25 em thick) over 

barren tidal-fl at and, and it probably represents the most 

westward po. ition attained by the contiguou. marsh sy. tern 

in late Holocene time. 

Two type. of mar. h core were taken in the study 

area, including continuous core in ABS pl astic pipe (Sand 7.5-cm diameter) and gouge cores (2. 5-cm diameter) 

returned from uccc ively deeper 1-m intervals to depth 

of :1 much a 7 m. The uncontaminated continuous core 

orth channel N 

EXPLANATION t 62 

D Alsea Bay central marsh 

Continuous cutbank 

exposure 1 

\ \:-~ 55 t-+-~ t 

;(Jf;H 1Q I 

~ Stratigraphic station 

Sou1h channel 

0 Alsea River 

Figure 43. M ap of cutbank exposures cxamint:d in the middle 

and upper rcache of i ·ea Bay. The ·olid line represents continuous exposure of two buried peaty hori zons. trati graphi c sections 

were measured at 62 stations (e cry fifth stati on is numbered). 

were u eel for quantitati ve sediment analy e. and for radiocarbon dating of peat hori zons. ediment contamination 

can occur in gouge cores, but they ield n gligible 

sediment compac tion during coring. Gouge core were generally used for hori zon depth comr I at continuou. core 

ite and for stratigraphic c rrclation of key hori zon 

between cutbank site and cominuous- ore . ite . . 

The core site were surveyed to the local MTL based on 

a tidal bench mark (Oreg. o. 9434938) adjacent to core ite 

AB 12. Elevation urveys of mo t core itcs and of the modern mar h and tidal fl at (adjacem to core ite AB20 and 

A B 17) were made u ing an autolevel (accuracy, ±0.1 m 

MTL). The elevati ons of core site AB2 1, AB6. AB 12. and 

B 13 were estimated ba ed on both mar. h-. urface elevations from adjacent core ite and long-di stance . ighting 

with the sur ey ing level (accuracy, ±0.3 m MTL). 

M ar h ores were returned to the laboratory for detailed 

examination, ediment ubsampling, and archi val in the 

refri gerated core fac ility at Oregon State ni ver ity in Corva lli s. Core were logged to the neare t !-e m-depth interval 

in the laboratory. Vi ual e timate of sediment tex ture and 

organic content in core. were calibrat d again. t quantitati vely analyzed ample from core site AB 8, AB 12, and 

A B2 1 (see below). Selected sediment hori zons at the e three 

ites were analyzed for sediment dry bulk density aft r drying 

at 60°C for 24 h ur (Gardner, 1965): relati ve abundance f 

organi c materi al, that is, weight los on ignition at 380°C for 

at least I 0 hour (Andr jko and others, 19 3): and relati ve 
LIM TIC, 0 EA IC, A D TE TO IC M E HA I MS OF CYCLIC AR H B RIAL AL EA B Y, OREGO 123 

PACIFIC 

OCEAN 

0 

N 

r 

ALSEA BA Y 

2 ILOMETERS 

EXPLANATION 

AB2 1 • Core site-Showing number 

.>\!!< Marsh 

Alsea Riuer 

Figure 44. Core sit es used to stud y the Alsea Bay marsh-interi or strati graph y. 

abundance of sand (s ieve diameter greater than 0.062 mm). 

silL and clay after organic oxidation by H20 2 (Folk, 1980). 

Sand-source mineral composition wa e tabli shed for 

elec ted core intervals from . ites AB2, AB4, AB9, ABI 2, 

ABI 5, ABI , ABI 9, and AB2 1 by petrographic analy is of 

the heavy- mineral fraction. At lea t 200 diagno ti c mineral 

grain (pyroxene ) were counted per sample slide to estim ate 

relati e abundance of ri er and beach and. Ri ver pyroxene 

minerals are angular and con i t . olel y of augite, wherea 

beach pyroxene minerals arc rounded and ontain an average 

hypersthcn :augite rati o of 0.60 (Peter on and other , 1982 : 

Peter. nand other. , 19 4). M ea urement of grain roundnes and hyp rsthene:augite rati o were normali zed to yield 

percent b ach and and percent ri ver and. 

Approx imate age. of marsh-buri al events were e timated by rad iocarbon dating of the top 5- 10 em of . elec ted 

buried peats from core ite AB9. The peat were hand 

picked , oven dried. and . ent to a commercial firm (Beta Analyti c, Inc.. oral Gable , Fl a.) for radi carbon age determination. tandard pretreatment leaches were performed there 

to remove potential contaminant . Peat age. were adju. ted 

by 12 ; 13 rati o and'" ere calibrated from e tablished treering ca librati on curve. (Stui ver and Reimer. 19 6). All peat 

age arc a . umed to predate the con·esponding burial events. 

STUDY RESULTS 

TIDE-ELEVATION RANGES OF THE 

MODERN MARSH 

M odern mar h development in Al sea Bay i clearl y 

zoned with re. pec t to tide level (JetTer on, 1975). Our 

ob erva tions and mea urement of marsh-pl ant a . embl agcs 

in Al sea Bay (table I 0) indica te that the den ely vegetated 

urface of the modern high mar h range between 1.4 and I . 

m abo e MTL. High-mar h pl ant in A I ea Bay include Deschamp. ia caespitosa, }uncus sp .. Potentilla pa ·ifica , and 

Grinde lia integrifolia. Trees ( pruce and alder) are pre entl y 

coloni zino the highe t mar h upland surfaces (higher than 

I . m above MTL) on elevated dike and levees and in landward mar. h peri meter . 

The lowe t tidal ele ation of continuou marsh development (low mar h) range from 0.6 to 1. 1 m above MTL. 

The low-mar h plant include, among other , alicomia vir-

'inica and Tri lochin maritimum . Isolated patches of colonizing mar. h in protected settings are found between 0.3 and 

0.6 m above MTL. nvegctated ti dal-flat ediment in the 

central and upp r reache of AI ea Bay are typically 0.3-0. 6 

m higher than MTL. In ummary. the laterall y continuous 

devel opment of mo t of the modern salt mar h in Al sea Bay 
124 AS I tG EARTHQ AKE HAZA RDS A D REDUCI G RISK I THE PA IFI ORTHWEST 

Table 10. Modern mar h elevations and tide levels in Alsea Bay. 

Marsh type Tida l stage Elevation range re lative to 

mean tide level (meter ) 

High marsh .......... ................................. .. ....... ............................... .... . + 1.4 lO +1.8 

+1.1 tO +1.4 

+I. I 

Transitional mar h ........ .................................................................... . 

Low marsh (lop) ... ... .. ..... .................................................................. . 

Mean higher high water (MHHW) .... . + 1.0 1 

+0.80 

+0.6 

Mean high water (MHW) ........... ... ... . . 

Low marsh (bollom) .................................. .. ................................. ·· ···· 

Tidal flat ........ ......................................... ..................................... ..... . +0.4 tO +0.6 

+0.3 tO +0.6 

0.00 

-0.08 

Colonizing mar h ....... .. .................... .............................................. . . 

Mean tide level (MTL) .. .. ................ .. . 

Mean low water (MLW) ................... . 

Mean lower low water (MLLW) ....... . - 1.0 1 

ex tend. over a tidal range of I m (0.6- 1.6 m above MTL). 

However. at lea t 90 percen t of the modern mar h surface is 

re tricted to a much narrower elevation range, that i., high 

mar. h at 1.4- 1 .8 m above MTL. 

The modern marsh surface is a well-constrained tidelevel datum . Depths and ages of prehi storic ti dal-mar. h urfaces record pas t changes in relati e ea-level position. 

However, specific indica tor of high-marsh, low-marsh, and 

coloni zing-marsh settings arc needed to e tablish the elevati ons of the buried mar h surfaces relati ve to their corresponding paleotide levels. One such indicator is the relati ve 

abundance of peaty materi al in the marsh subsurface. The 

organic content of the m dern mar h substrate (rooted zone) 

decreases greatl y from the highe t marsh elevations to the 

un egctated tidal fl ats in A lsea Bay. as in other Oregon 

marsh systems (Darienzo and Peterson, 1990). 

Generall y peaking, as the mar h urface rcachc 

supratidal elevation , the tid al-ri ver uppl y of inorganic 

sediments i .. hut off and the percentage of organic material. (peat) increase . Sub equent ox idation can decrease the 

relati ve abundance of organi c materials but cannot increase 

it. Thus, mea uremellls of the percentage of organic materi al in marsh sediments provides u eful criteria for e tabli shing paleotide elevati on of 1 rehi stori c marsh . urfaces. 

Di atoms and plant macrofoss il s in mar h sediments can 

al provide information about paleosalinity and therefore 

paleotide level in orn e mar h depo. it (Atwater, 1988; 

Darienzo and Peterson. 1990). However. the abrupt vertical 

transition from a freshwater diatom fl ora to an overl y ing 

bracki . h water-marine fl ora, as seen in etarts Bay core 

(Darienzo and Peter on. 1990). wa not observed in A lsea 

Bay core . . Thu., diatom were not u ed as palcotide-level 

indicators in thi s tudy. Pl ant macrof s il , pecifica ll y Triglochin maritimum rhi zomes (a common tidal-fl at coloni zer), were iden ti fied in some cores at subsurface depth 

greater than 1.5 m. Their pre ence at the base of . everal 

developed peaty horizons sugge ts transition from co lonizing marsh to high-mar h sClling . 

For cutbank and core-log descripti ons. the relati ve peat 

development in the !sea Bay mar. he i. defined a. follows: 

peat (0 rcater than 50 percent organic materi al by dr 

weight). muddy or sandy peat (25- 50 weight percent organic. ). p aty mud or and ( I 5- 25 weio ht percent organic ). 

slightl y peaty mud or and ( 10- 15 weight percent organics). 

rooted or slightl y rooted mud or sand (5- 10 weight percent 

organics), and barren mud or sand (les. than 5 weight percent 

organ ics). Semiquantitative descriptions of the relati ve 

abundance of organic materi al. arc based on laboratory comparison. with core samples quantitatively analyzed by drysa m! le ' eight lo on ignttton. 

Quan titative mea urcment. of organic content from 

s lec ted marsh h ri zon in Alsea Bay were performed to 

( I) e tab li sh relative abundances of organic material in 

modern mar. h hori zon with known tide ele at ions, (2) calibrate isual e timates of peat development with mea ured 

percentages of organic materi als in marsh cores. and (3) 

document downcorc ari ations in percentage of organic 

materi al in mar h cores. M ea. ured organic matter in core 

ample from modern high-marsh hori zon (table I 0) contained great r than 25 percent organic material. By comparison, barren-mud deposits in Oregon bay generall y contai n 

le than 5 percem organic material (Powell , 1980; 

Dari enz and Peter on. 1990). 

The transition between colon izing-mar h and e tabli shed low-mar: h ele ation (table 10) i com monly complica ted by the patchy development of coloni zing-mar h plants. 

Therefore, assignment of a re1 re entati ve organic content to 

thi s transition zone is somewhat arbitrary. evenheles . the 

examinati on of marsh cores from A I ea Bay indi cate th at a 

minimum value of 10 percent organic material over a 5- 10-

cm-depth interval can be used to di scrimin ate between e tabli shed low-marsh hori zon and the coloni zing marsh or algal 

mud-fl at hori zon in A lsea Bay. 
Figure 45. Two buried peaty horizons in cutbank exposures in the Alsea Bay marsh. A, northwest cutbank of south channel; 8 , northeast cutbank of ~o uth 

channel; C, northeast bank of north channel: 0 , northwest bank of north channel. Sandy layers (generall y several centimeters thi ck) directl y above peaty 

hori zons arc differenti all y eroded, producing two shadowed horizontal indentati ons that correspond to the two peat tops in A. C. and 0 . Coin ~ mark the tops 

of the two buried peat hori w ns in B. 

n 

r 

;::, 

)> 

--l 

n 

0 

n 

rn 

)> 

0 

)> 

z 

0 

--l 

rn 

n 

--l 

0 

rn 

n 

:r: 

)> 

Vl 

s 

Vl 

0 

., 

-< 

n 

r 

n 

s 

)> 

;o 

Vl 

:r: 

co 

c 

;o 

! 

r 

Vl 

rn 

co 

)> 

-< 

0 

;o 

rn 

0 

0 

N 

V1 
126 ASSESS! G EARTHQ AKE HAZA RDS A D RED CI G RISK I THE PA IFI ORTHWEST 

0 

10 

20 

If) 

a: 

UJ 30 fUJ 

:::E 40 i= 

z 

UJ 

u 

~ 50 

u.j 

u 

<{ 60 Ua: 

::J 

If) 

I 

If) 

a: 

<{ 

:::E 

z 

a: 

UJ 

0 

0 

:::E 

s: 

0 

_J 

UJ 

(I) 

I EXPLANATION fa._ 

UJ 

0 ---0-- Upper buried peat 

- -e-- Lowe r buried peat 

STRATIGRAPHIC STATION 

Figure 46. Correlation of two buried peat horizon. (buried mar. h top ) in 62 measured sections from the central-marsh area in Alsea Bay. 

Depths of the two buried peat tops below the modern mar. h surface are plotted against strati graphic station number around the centralmar. h-island perimeter (a total distance of 4.5 km). The modern marsh surface is arbitrarily set at 0 em depth for thi s figure. See figur -1 3 

for stratigraphic station locations and table II for corresponding measuremems of horizon depth. 

CORRELATIONS OF SHALLOW 

PEATY HORIZONS IN CUTBANKS 

The central Alsea Bay marsh is cu t by two major tida l 

channel.. the outh channel and the north channel (fig. 34). 

Due to modification of the tidal fl ow in the e channels at 

their upstream confluence beginning in the earl y 1900' to 

divert water first into the north and later into the outh channel. the banks of the channel have eroded, expos ing the 

undi turbcd marsh stratigraphy. M ar h depo it are cry 

well exposed to 1- 1.5 m below the modern marsh surface 

along the channel cutbank . . Cutbank arc present but arc lc s 

continuou up. trcam of the confluence of the north and sou th 

channels. The !sea Bay mar h y tcm po sibly i. unique 

within coastal Oregon in containing nearl y continuous cutbank exposure. of abou t 13 km in length that ex tend 6 km 

ca. t-wcst, or about on -half the length of the bay. A total of 

6.4 km of cutbank exposures in the central-marsh area were 

mapped for shallow-mar h strati graphy (fig. 43). 

Cutbank expo. urc in the central AI ea Bay mar h 

reveal two buried peaty hori zon at average depths of about 

0.5 m and I rn below the modern root zone. A third buried 

peaty horizon was infrequently observed 1- 1.5 m below the 

modern marsh. its expo ure being limited by modern cdimentation along the channel banks . The two upper muddypeat horizon have gradational lower contact with peaty 

mud or slightly peaty mud and have sharp upper contacts 

with thin ( 1- 10 em thick) sandy layer.. The andy layer 

above each of the upper buried peat have been differentially 

eroded by cu rren t and wind-wave cour to produce horizontal indentations directly above each buried peaty horizon 

(fig. 45). An orange-brown ox idi zed zone ( 1- 2 em thick) 

was al o rarely ob ervcd at a depth of about 0.25 m below the 

modern marsh urface. o sandy layer were ob erved 

directly above this anomalous ox idi zed zone. 

The two upper buried peat hori zon were nearly con tinuously correlated for 4.5 km around the circumference of the 

central-marsh island (fi g. 34) and along 1.9 km of the 

southern bank of the north channel (fig . . 43. 46). The depth 
LIMA TIC, OCEA IC, A D TECTO I ME H ISMS OF CYCLI C MAR H B RIAL. L E BAY. OREGO 127 

Table 11. Depth of shallow peaty horizons below modern marsh 

in cutbanks at AI ea Bay. 

{Sec figure 43 for locati ons of strati graphic stati ons} 

Strati graphic Depth of upper buried Depth of lower buried 

sLarion peat (centimeters) peat (centimeters) 

AB I 40 76 

AB2 58 87 

AB3 90 120 

AB4 50 5 

Al35 eroded 87 

AB6 eroded 

Al37 eroded 75 

AB8 50 90 

Al39 67 100 

Al31 0 6 120 

Al311 82 11 3 

Al31 2 95 12 

AB I3 57 100 

Al31 4 64 110 

ABI S 66 97 

ABI 6 80 100 

ABI7 73 11 0 

Al31 8 77 104 

AB I 9 75 105 

Al320 59 7 

Al321 60 9 

Al322 5 95 

AB23 41 0 

AB24 76 114 

AB25 62 94 

AB26 74 102 

A B27 66 89 

AB28 73 107 

AB29 55 8 1 

AB30 54 94 

A l33 1 56 97 

AB32 36 79 

AB33 38 9 

AB34 4 1 

A B35 46 91 

AB36 5 1 9 

AB37 56 90 

AB38 62 99 

AB39 65 10 1 

AB40 5 1 93 

AB4 1 54 97 

AB42 5 96 

AB43 32 71 

AB44 6 99 

Al345 55 92 

Al346 69 9 

Al347 54 92 

AB48 50 85 

Al349 52 8 

AB50 49 3 

Al351 53 I 

Al352 6 

AB53 62 10 1 

Al3 4 56 89 

AB55 69 101 

AB56 65 97 

AB57 69 10 

AB58 53 90 

AB59 59 100 

AB60 6 10 

AB6 1 57 100 

AB62 6 104 

to the top of the t\. o peat hori zons var from 3_ to 95 em 

(upper horizon) and from 7 1 to 128 em (lower hori zon) below 

the modem mar h urfa c (table II ). The cparation di tance 

between the two buried peat layer ' a al o found t ary 

ignificantly, from 20 to 51 em. The vari ati on in stratigraphi 

depth of the buried peaty horizons ha re ultcd fr m top -

graphic relief of both the paleomar ·h and modern mar h urface . Such relief i typicall y on th order of 0 0 m for 

the modem high marsh in central AI ea Bay (table I 0). 

MARSH VERTICAL SEQUENCE 

The deeper tratigraphy of the AI ea 8a mar h wa 

determined on the ba. i of detailed log taken from a > e leas t core traverse (fi g. 47A, B) and a north-. outh or 

traver. e (fig. 47C) of the cen tral-mar. h . y. tcm. M aximum 

core length. ranged from 2 m ( ite A8_) to 7 m (. ite 8 8 

and A89). Our hand-coring dev ices w re un able t p n trate 

thick sandy hori zons (0- 1 m below MTL) in either the 

lower-bay mar h (site. 8 I , A 82. A 84, and B20) or th · 

. outh-channcl bank. ( ite A8 17 and B I ). B comparison, relati vely deep core. (-4 to - 5.5 m rclati e to MTL) 

were con istentl y obtained from the central mar. h (. it s 

AB7, AB8, A89. A8 I I , and A8 19), which ha. very fin 

grain deposit . 

All of the analyzed cores from the Al sea Bay central 

mar h how multiple hori zon of peat or peat mud (5- r 

em thick) that are c mmonl y sc1 aratcd by very low oroani 

(VLO) sediments (fig. 47). The tops of the black. peaty horizon. (buried mar h . urfa e ) generally consist of sharp c ntacts (less than I em ,.vide) with overl ying deposit In 

contra t, the contact between the bottoms f the peaty horizons and the underl ying VLO cdimcnt are generall y gradational, that i . organic content de rca cs down ore. An 

analy is of 95 indi vidual peaty hori zons (more than 5 em 

thick) from the 19 core site howed that at lea. t 80 p rccnt 

of the peat unit have sharp upper contacts and gradational 

lower contact . These relati vely con i tent change in peat 

development define the top and batt ms of di. tinct mar. h 

hori zons, even where inter ening layer of VLO mud or sand 

are not present. 

The layer. of VLO . cdiment between the peaty horizon are quite variable in thi knc ·s and compo ition. The 

barren or lightl y rooted mud layers are either finely laminated or homogeneou to the naked eye and range from 2 to 

125 em thick (fi g. 47). At lea t 50 lamination ( 1- 5 mm 

thick) were counted in on VLO mud layer (20 em total 

thickn ) x-rayed in a thin core slab from ite AB4. The 

VLO layers generall y decrea e in thickne s (50 down to 5 

em thick) from deep core interval to shallow core interval . 

ome of the youngc t peaty hori zons in the uppermo t core 

interval are separated by very th in rooted mud or sand layer. ( 1- 2 em thi ck), uch as that shown at about I m above 

MTL at . ite AB2 (fi g. 47A). In contra t, thick basal layers 

(more than 0.5 m thick) of barren and or muddy sand occur 

below the peaty marsh depo. it in the we tern core ites 

(AB2, AB4. A8 18, an I A820; ee fi gs . 44 and 47A). The e 

VLO basal depo it probabl y represent barren tidal-Oat environments on ly recentl y colon ized by a prograded mar h. 
128 ASSESS ! G EARTHQ AKE HAZARDS D RED CI G RISK I THE PA IFI ORTHWEST 

Depth WEST 

(meters) AB21 AB20 

2 

Burial 

unit 

+ 0 a 

a 

a 

2 g 

a 

3 

a 

5 

M ea n g 

tide 

level 

a 6 

g 

- 1 

-2 

- 3 

Modern peat a 

Buried peat b 

Mud d 

Sand g 

Wood 

h 

AB15 AB2 

pm 

bms 

g s/pm 

s/rsm 

pm 

rm 

s/rm 

bsm 

bms 

bsm 

bsm 

bms 

s/rms 

bms 

EXPLANATION 

Abrupt contact Laminae 

Barren m Mud or muddy 

Organic ox Oxidized 

detritus 

Gradual contact p Peat or peaty 

Shells r Rooted 

AB 18 

CENTRAL 

AB4 

rsm 

bms 

s Sand or sandy 

sf Slightly 

tri Trigloch in 

w Wood 

oxrm s 

s/pm 

rsm 

s/pm 

s/rmsml 

s/pm 

s/rsm 

s/pm 

s/rm 

rsm tri 

rms 

rsm 

bms 

A 

Figure 47 (above and following two pages). Core log and strati graphic correlations of key peaty hori zons 

in the Al sea Bay marsh sy tem. A, west-central marsh; 8 , east-centralmar h; C, central marsh (north to south). 

Solid lines are apparent correlations; dashed I ine are less cert ain con·eiations. Burial units are numbered 0- 1 0 

in core secti ons from sites AB2 1, AB 12, and AB8. Elevati ons of secti ons for cores AB6, AB 12, AB I , and 

AB2 1 are approx imate. Additional quantitative analyses of . elected hori zons are li sted in table 12. 
CLIMATIC,OCEA IC.A DTECTOt iCMECHA I M OFCYCLICM RSHB Rl L.AL EABAY.ORE Ot 129 

Depth 

(meters) 

2 

Mean 

tide 

level 

- 1 

- 2 

- 3 

- 4 

- 5 

CENTRAL 

AB16 

9 

a 

9 

9 

9 

9 

a 

9 

9 

a 

9 

AB5 ABll AB6 

Burial 

unit 

AB12 

bsm 

bms 

bsm 

rm 

bm 

9 

a 

a 

a 

a 

EAST 

AB13 

a ~=l:l =::::-'"'"' rn 

B 

Figure 47. ore logs and stratigraphi c c JTelations of key peaty h rizons in the Al sea Bay marsh 

system- ominued 
130 ASSESS I 1G EARTHQ AKE HAZARDS A D REDUCI G Rl K I T HE PACIFI 

Depth 

(meters) 

2 

Mean 

tide 

level 

- 1 

- 2 

- 3 

- 4 

- 5 

9 

9 

9 

NORTH 

AB19 

9 

a 

tri 

a 

AB9 

Burial 

unit 

9 

slpm 

ABB AB10 AB7 

pm 

tri 

rsm 

d 

d 

rm 

slrsm 

w 

pm 

ORTHWEST 

SOUTH 

AB17 

rml 

mp 

rsm 

mp 

rm 

pm 

rm 

pm 

s/pm 

slrm 

slpm 

slrm 

bm 

bms 

c 

Figure 47. ore log. and stratigraphi c con·elations of key peaty horizons in the AI. ea Bay marsh 

sys1em- on1inued 
LIM ATI . OCEAN IC. A D TE TO IC MECHA I M S OF CYCLIC MARSH B Rl L. AL B Y. OREGO 13 1 

The base of some VLO layer and . ome peat hori zons 

(O.S- 1.5 m above MTL) in the western-marsh ite. and in 

ite along the north channel of the central marsh are characteristicall y enriched in sand (fig. 47). The thin, tructureless 

and (SCL ) contras t with both the overl ying laminated 

mud. or peaty mud and with the underl ying peaty hori zon . 

The observed SCLs in the Al sea Bay marsh cores range from 

25 em to less than I em thick. The thicknesse of the S L 

oenerally decrea e with di Lance from the northwestern bay 0 

mar h to the eastern confluence of the north channel, or from 

siteA B2 1to iteABI6 (fig .44,47). Fin ally, thebase of the 

two to three uppermo t peaty hori zon are vari ably oxidized 

(orange brown) at many of the marsh core ites (AB4, AB7 , 

AB8, ABIO, ABII , ABI 3, ABI5 , and ABI6), indicating 

temporary des iccation and exp su re to ox idi zi ng condition . 

The verti cal sequence of an SCL and (or) VLO layer 

grading UJ ward into a full y developed peaty hori zon represents a complete burial unit. However . . orne preserved buri al 

unit locall y do not contain an SCL r th ick VLO mud layer. 

evenhele , the verti ca l repetition of multiple buri al unit 

throughout the Al sea Bay mar. h repre ellls a repeating 

equence of epi odic marsh emergence (peat development). 

then ubmergence (peat buri al by VLO mud). Final ly, the 

dominant colllac t relation of the buried peaty hori zon (gradational lower contact. and sharp upper colllact ) indicate 

that emergence event were generall y gradual , wherea ubmergence event were abrupt. 

MARSH STRATIGRAPHIC CORRELATION 

sing both the relati ve depth. and number of peaty 

vertical sequences. we correlated key . tratigraphic hori zons 

between adjacent core site . For example, three peat burial 

units in the uppermo t 1.5 m of the central mar h are correlated between site. AB8, AB9. AB IO. ABII. AB I 6. and 

AB 19 (fig. 478. C) A described in a preceding sec ti on, the 

upper two unit have been traced continuou. ly for several 

kilometers in expo. ed cutbank. in the cen tral-marsh area . At 

least I 0 peat buri al units " ere ob crvecl to depth of - 5.5 m 

relati ve to TL (max imum core pcnctrati n) at sites B 

and AB9 (fi g. 47C). 

Similar! . di. tinct peaty hori zon from the upper I .- m 

of the we. tern and ea. tern mar h area. are strati graphically 

correlated with k y horizon of the central mar h (fig. 47). 

Verti cal off. cts of I 0- 30 em between corresp nding peat 

horizons from nearby core sites are relatively common in the 

we tern marsh area. The. c vcnica l off. et. ar con istcnt 

with the cle ati on differences 0~0 em) of the modern 

marsh urfacc (tabl e I 0). and they probably reflect the vari -

ability of the older marsh clcvati n. 1 ri or to buri al. By compari son, corrc ponding peaty hori zons in the ca tern mar h 

area are relati vely consi tent in clevati n except for the furthe t upri ver location (s ite AB 13 in fi g. 47 B). 

It i. not pos ible to confident! c tend the deeper mar h 

tratigraph (0 to - 5 m rclativ to TL) b ond the centralmarsh ar a becau e of the hall w depth. of or penetration 

in the . and-rich western ite and the rclati ely poor preservati on of peat horizon or lack of mar. h dcvclopm nt 

below MTL in the we tem and ca. tern c r tra crse . Ho> -

e er, the broad correlation of the hallo' peat hori zon. in th 

top 2 m throughout mo t of the cntral. ' . t rn . and ca tern 

marsh itcs pro ides the nece ar ontr I to t . t e cral different mechani sms of mar h burial in I ca ~ . The fol lowQUANTITATIVE ANALYSES OF 

PEATY NITS 

Quantitative ana lyse. of organi content and dim nt 

grain size in the modern marsh and in burial unit s were performed on sample. from core itcs A 82 1 (\ estern . itc). 8 

(central itc). and AB 12 (ca tern si te) (fig. 44, table 12). 

A nalyse of buri al unit mar. h top (MT) and mar h b tt ms 

(M8) or underlying VLO mud arc u cd to crify isual 

interpretation of core-. ite . tratigraph a. hown in the figure 47 cor log . Due to the wide s paration of the. e thre 

core sites. th quantitati c rc ult. can b u ed to mparc 

marsh respon. e to burial pro s. cs between tidal- an I riverdominated . ettings. In addition, grai n-size analy. is was performed on the SCLs fr m ite A82 1 toe tab li. h the ariability of and- izc-frac tion abundances betwc n th se 

a noma lou andy layer. and the underl ying marsh top . . For 

purpo. cs of clarit y, we re. tatc the h rizon identifica tion 

cheme used here. Su ce. si vel deq er burial unit are numbered from Ito IO(fig.47A. sitcAB2 1;fig. 47B .. it ·ABI 2; 

fi g. 47C. . ite AB ). F r example, the top of burial unit 2 is 

the marsh top (2MT) wherca the bottom of the unit i. the 

corresponding 2S L . Th rcforc. 2S L overli es 3MT, and 

2MT under! ic. I SCL. 

As indica ted in the marsh core logs (fi o. 478, ), the 

organic content and percentages of . and cr u. si lt-clay 

(mud) in the inorganic fraction vary s temati ca lly clowncore 

in burial unit from the central and ea. tern mar. he (table 12). 

Organic content, e tabli. heel by los on ignition , increa e 

one- to threefold from the bouom to the top f mo t burial 

unit from ite AB8 and AB 12 (fi g. 48). In contra. l. the percentage of sand in the inorganic fraction. tend to decrease 

ov r the same interva l (fig. 49). Organ ic content and and 

percentage show the strongest inver. c correlations in the 

lower tratigraphi ection. (burial unit. 2. 3. ancl4). Thee 

trend demon tratc a climini hing uppl y of inorganic sediment s, particul arl y coar e size frac ti ons, relati ve to organic 

materi al in the developing peaty hori zons. The increa. e in 

relati ve abundance of organic materi al is due to a decrease 

in the suppl y f watcr-born c siliciclas tic sediments a the co lonizing marsh emerge · to supratidal elevations. 
132 SSE SI G EARTHQ AKE HAZARDS A D REDUCING RISK I THE PA IFIC ORTHWEST 

Table 12. Organic content and sedi ment grain sizes of buried layers in three cores from A lsea Bay mar shes. 

!The burial unit number is fo llowed by the marsh horizon de ignation: MT. pemy marsh top; MB. peaty marsh bonom: RMT. romcd mud 

top: RMB. rooted mud bonm; S. very low organic (ba rren) scdimenl: CL. scdimclll capping layer. The surface is 1.50 m above lhe mean 

tide level. (--). no dawl 

Horizon 

OMT 

OMB 

IMT 

1MB 

ISCL 

2MT 

2MB 

2SCL 

3RMT 

3RMB 

3SCL 

5MT 

5S 

5SCL 

6MT 

6S 

OMT 

OMB 

IMT 

1MB 

2MT 

2MB 

3MT 

3MB 

4MT 

4 

5MT 

5S 

6MT 

6S 

7MT 

7MB 

8MT 

8MB 

9MT 

9S 

IOMT 

OMT 

OMB 

IMT 

1MB 

2MT 

2S 

3MT 

3S 

4MT 

4S 

Depth relative to mean 

tide level (meters) 

1.46 

1.40 

1.34 

1.25 

1. 18 

1.06 

1.00 

0.75 

0.63 

0.41 

0.35 

0.32 

0.06 

-0.37 

-0.4 1 

-0.65 

1.49 

1.43 

1.35 

1.1 8 

1.10 

0.74 

0.55 

0.38 

0.19 

-0.12 

-0.23 

-0.97 

- 1.10 

- 1.96 

- 2.12 

- 2.62 

- 2. 2 

- 2.86 

- .00 

-4.94 

- 5.22 

1.46 

1.37 

1.28 

1.01 

0.94 

0.46 

0.38 

0.32 

0.27 

0.03 

Density 

(grams/cubic centimeter) 

Organic material 

(percent) 

Core sit e AB21 (western marsh)

1 

0.2 

0.42 

1.47 

0.65 

0.53 

0.52 

29.0 

34.4 

41.1 

41.8 

32.2 

10.0 

7.9 

5.7 

9.9 

6.0 

13.0 

5.3 

Core site ABS (central marsh) 

0.23 

0.26 

0.29 

0.47 

0.85 

0.53 

0.42 

0.33 

0.41 

43.7 

40.0 

37.6 

37.5 

48.2 

12.2 

17.1 

14.0 

13. 

6.9 

27.6 

4.3 

12.2 

6.9 

23.7 

10.3 

30.9 

16.4 

32.9 

7.7 

23.9 

Co re site AIH2 (eastern marsh) 

0.30 

0.3 1 

0.37 

0.90 

32.1 

26.6 

37 .2 

24.1 

30.2 

6.3 

15.4 

9.0 

12.4 

7.0 

1

Buri al unit4 could not be distinguished in this core. 

Sand 

(perce nt) 

10.8 

13.0 

13.0 

12.1 

76.9 

8.9 

40.0 

88.5 

10.8 

0.3 

58. 1 

0.7 

4.1 

8 1.8 

0.9 

1.1 

2.2 

6.7 

0.9 

1.8 

0.3 

20.8 

2.3 

1.0 

0.2 

1.9 

0.5 

58.4 

5.3 

6.8 

0.9 

0.8 

1.2 

1.1 

4.2 

12.4 

0.6 

2.0 

5.1 

5.1 

4.8 

0.8 

12.5 

7.6 

32.3 

5.7 

68.5 

Si lt and clay 

(perce nt) 

89.2 

87.0 

87.0 

87.9 

23.1 

9 1. 1 

60.0 

11 .5 

89.2 

99.7 

41.9 

99.3 

95.9 

18.2 

99.1 

98.9 

97.8 

93.3 

99.1 

98.2 

99.7 

79.2 

97.7 

99.0 

99.8 

98.1 

99.5 

41.6 

94.7 

93.2 

99.1 

99.2 

98.8 

98.9 

95.8 

87.6 

99.4 

98.0 

94.9 

94.9 

95.2 

99.2 

87.5 

92.4 

67.7 

94.3 

31.5 
CLIMATIC. 0 E 1J , A D TECTO IC MECHA I MS F CYCLIC MARSH 8 Rl L. L E 8 Y, OR G N 133 

OMT 

OMB 

1MT 

1MB 

z 2MT 0 

N 

a: 

0 2MB I 

3MT 

3MB 

4MT 

./ 

4MB 

0 

' 

· .. ' . , 

1 

. I 

.... . ·/ 

'/ 

I 

... 

... 

.. · ... ., 

I 

• 

I 

EXPLANATION 

---o- Core site AB21 

- - • - - Core site AB8 

· · · · • · · · · Core site AB12 

10 20 30 40 

ORGANIC MATERIAL, IN PERCENT 

50 

Figure 48. Do' ncore plots of relative organic-material abundance in peaty hori zons of the modern marsh (0 prefi x) and four uppermost burial units from three core ·ires in Alsea Bay. nit 4 was 

not found at . itc AB21. MT. marsh lop: M 8, marsh bouom. Core 

. i1e: arc shown on fi gure 44. Measured elevat ion and addi1ional 

quamitalive dma for these horiLons arc . hown in table 12. 

By compari son, cyclic vari ati ons in either organic content or and abundance are generall y absent fr m the upper 

burial units at we tern -mar h ite AB2 1 (fi gs . 48. 49). A ionificant increase in organic content above unit 2MB at . ite 

AB2 1 corTespond. to . imilar increase at central and ca tern 

sites AB 8 and AB 12, respecti vely. Whcrea. the two lower 

units (3 and 4) generall y show upward transition from colonizing marsh to low mar h (5- 15 percent organ ic material). 

th e upper unit (0 and I) appear to include predominant ly 

hi gh-marsh . ub trate (24-4 percent organic materi al). The 

change in organic content between the 2MT and 2MB h ri -

zons at all three core site. indica tes a . ys tcm-\ ide rc pon ·e 

to relative . ca- lc vel change and further . ubstanti ates our 

tratigraphic correlation. of shallO\· burial units in the A I. ca 

Bay mar. h (fig. 47). 

ediment grain -siz analy. cs of the SCL from site 

A B2 1 show substantial di fference. in the relati ve sand abundan e between the thinS L and the underlying peat hori -

zon. (table 12). Ex treme enrichment of the . and fracti on wa 

found at each of the harp contact. bet\ ccn the L and 

con·c. ponding buried mar. h surface. (fig. 50) For example. 

sand abu ndan ·c increased from 0.9 percent at 6MT to 8 1.9 

percent at5S L aero. s thi s relati vely deep c nta t and from 

.9 percent at 2MT to 76.9 percent at I CL acros thi s shallow contact. Relative . and abundance in mar h bottom r 

YLO muds from site AB2 1 arc al. o substanti all y less than 

th ose measured in the CL s. In . ummary . the sandy SCL s 

., 

OMB .,. 

/· 

1MT . I . 

1MB Ii 

1.' 

6 2MT . 

N 

a: 

~2MB 

3MT 

3MB 

EXPLA ATJO 

---o- Core site AB21 

- - • - - Core site AB8 

· · · · • · · · · Core site AB12 

.. . 

..... 

20 30 40 50 

SAND VOLUME, IN PERCE NT 

60 70 

Figure 49. Downcore plot of rcla1ive sand abundance in lop and 

bouom peaty hori zons of 1he mod rn marsh (0 prefix) and four uppermost burial un i1s from 1hrce core si1cs in AI. ca 8a nil -1 ' as 

not f und at si1 e A 821. MT. mar. h 10p: M 8, marsh bouom. ore 

sites are shown on fi gure 44. Measured eleva1ion. and addi tional 

1uanti1a1i e data f r these hori zon> are . hown in !able 12. 

mu l ha e an ori gin very di fferent from either the peaty horizon or YLO muds that they . cparatc in some interva ls of 

core AB2 1. 

Both th thi ckne. and relati c sand pcrccntaoc in S L 

from central-mar h . itc were found to g n rall y deer asc 

with increasing di tancc up the north channel. For exampl e, 

2SCL decrea e in thi kne s from grea ter than 20 m at ite 

AB2 1 to le. s than 0.5 em at site B 12. a distan c of evcral 

kilometers (fi g. 47A. B). The L s also dccrcas in thi cknc s 

w ith increasing di tance away from the north-channel ax is. 

For example, 2SCL i greater than 5 em th ick at A B 19 bu t is 

not found at A B I 0. a di tance of 200 m from the channel 

(fi g . 44. 47C). 

The CL are identi fied here by their relati e cn ri hmcnt of . and. However .. ome si lt-rich laminae (le s than I 

em thick) direc tl y overlie buried peaty hori zons in the central- and ea tern-marsh areas. such a. above 3MT at ite. 

AB and A89 (fig. 47C). The. c anomalou . silty laminae 

li kely correspond to sand-ri ch L. above 3MT at the 

we tem -mar h itc . inall y, there i no evidence of SCL. 

above either I MT or 4MT within the central-marsh area. 

The lack of an SCL above the I MT hori zon is po sibly 

con i tent \ ith the lack f definition r continuity of the 

I MT upper conta t w ith OMB. In contra t, the 4MT upper 

contac t i. well defined at the central-marsh sites. and the 

lack of a corre p nding _,SCL make. the 4MT hori zon parti cularl y di stincti c in I ca Bay. 
134 AS ES I G EARTHQ AKE H ZARD D REDU I G RISK I THE PACIFI NORTHWE T 

MINERALOGY OF SEDIMENT 

CAPPING LAYERS 

The heavy-mineral a emblage of elected S L s 

were inve. tigated t cs tab li. h the ource (river or marine) 

of these anomalou. and layer . The thicke t SCL ( I SCL 

above 2MT. 2 L above 3MT, and 4 CL above 5M T) are 

found along the northea tern bay margin ( ite AB2 1, 

AB20, and AB 15) and up the north channel (site AB 19) 

(fig. 47A, C). Heavy-mineral eparate from fi ne- and- ize 

fraction. of these SCL. were analyzed for river and beach 

and ·. The end-memb r ourccs were di scriminated on the 

ba i. of two independent method - hyper thenc:augite 

ratio. and relati ve grain rounding (Peter on and thcr , 

1982). Heavy-m ineral anal yse of the and fraction from 

underlying marsh top were at o performed to identi fy any 

differences in sand source between the anomalous SCLs 

and the buried peaty horizons. 

and- ource analy e. were performed on repre entati ve core from the north channel (site A82 1, AB 15, and 

AB 19) and from the outh channel ( ites AB I , AB4. and 

A 8 12). The re ults of the analy es show striking vari ations 

in sand-source campo ition between SCLs and the underl ying buried mar h top at northern marsh ite AB21, AB 15, 

and B 19 (the con tact at site A B 12 was al o observed 

under mi cro cope, but only river sand grains con tituting 

le s than I weight percent of the ed iment ma were 

z 

0 

!::! 

a: 

0 

I 

lMT 

1MB 

2MT 

2MB 

2SCL 

3MT 

3MB 

3SCL 

5MT 

55 

5SCL 

6MT 

20 40 60 80 100 

ORGANIC MATERIAL, IN PERCE NT 

Figure 50. Downcorc plot of relati ve sand abundance in peaty 

h ri zons and anomalous sand-rich ·ediment capping layers (S L.) 

th at directly overlie the buried peaty hori zon · at core site AB21 in 

Alsea Bay. nit 4 wa. not fou nd at si te A82 1. MT, marsh top; MB, 

marsh bottom: S. very low organ ic (barren) sediment. M easured elevati ons of the Ls arc provided in table 12. 

found) (table 13. fi 0 . 5 1). Although ri ver and predominate. in all of the analyzed and fracti on. (53- I 00 p rcent 

river sand). the S Ls of the northern bay site. are . ub tantiall y enriched in beach sand. For example, the S Ls ran oe 

b 

from 14 to 47 percent beach and at we. tern ite ABIand A82 1. re pe ti vely. In contrast. none of the underl ying 

mar. h-top hori zon · or L s from the outhcrn bay ite 

(A 8 18 and 84) ex cccl I 0 percent beach and. 

Pl ot. of relative abunclanc of beach sand in SCL 

along north chann I ite AB2 1, A B 15, and A B 19 how that 

th percentage of beach and in each of three analyzed SCLs 

decrea cs with incrca ing distance landward (fi g. 5 1). The 

beach- and pcrcentag s of 2SCL at sites A82 1, AB 15, 

AB 19, and A B 12 arc hown in figure 52 along with modern 

ancl-compo ition contours in the bay, a previously reponed 

in Peter. on and others ( 19 2). The high abundance of beach 

sand (3 1-47 per'ccnt) at marsh . ites A815 and A B2 1 contra ts harply with adja cnt tidal-fl at sands, averaging le 

than 20 percent beach . and (fig. 52). The nearc t possible 

and source (greater than 50 percent beach sand) for 2SCL 

(which has as much a 47 percent beach . and) is 1.5 km cl ue 

we. t (. caward) of ite AB2 1. The clecrea ing beach-sand 

per en rages up th north channel corre ·pond to the dccrea -

ing total -sand percentage and clecrea ing and-layer thicknc e of the SCLs (fig. 47). 

The three trend in S L deposition, that is, the layer 

thickne. s, and per enrage, and sand mineralogy independentl y confirm a climini hing transport energy with increasing di ranee lanclwarcl in AI ca Bay. The SCL were 

depo ited by marine urge that entered the I. ea Bay mouth 

and propagated (mo tl y) up the north channel. The landward 

attenuati on of surge-SCL depo it ion occur within a di stance 

of about 7 km from the mouth of Al sea Bay (fi g. 52). However. thi eli . tance repre ent. more than 75 per ent of the 

length of th is mall ba . 

RADIOCARBON AGES OF 

PEATY HORIZONS 

Selected interval of buri ed mar h tops at core site A89 

(figs. 44, 47C) and tratigraphi tation 4 1 (fig. 43) from the 

central mar h were anal zed for radiocarbon age. The radiocarbon-clatecl marsh tops (intervals 5- 10 em thick) correspond to burial-unit hori zons I MT to I OMT. A total of eight 

bulk-peat rad iocarbon ages and one radiocarbon age from a 

detrital wood fragment (from interval 2MT) are shown on 

figure 53. A should be expec ted, the analy es how increa -

ing age (4 0 R YBP to 4.5 10 RCYBP) wi th increa ing 

depth ( 1.1 m to -5 .1 m relati ve to MTL). 

The uppermo. t buried mar h interval yielded a rel ati vely young rad iocarbon age ( 160±50 RCYBP; fi g. 53) 

below the anomalous I MT upper contac t, a noncontinuou , 

weak ly-ox idi zed horizon. To date the second buried marsh 

interval, a detrital wood fragment (2 em diameter) was 
CLIMATI C, CEA !C. A D TECTO IC M ECHA lJ MS OF Y LIC M AR H B RIA . AL E B Y, ORE 0 I 5 

z 

0 

N 

a: 

0 

I 

2MT 

2SCL 

3MT 

• 

' ' ' ' 

-o- Core site AB21 

--• -- Core site ABlS 

Core site AB19 

4SCL • • 

./ 

0 

/ 

/ 

/ 

/ 

/ 

10 20 30 

BEACH SAND, IN PERCENT 

40 50 

Figure 51. Downcore pl ot of relati ve abundance of beach-source 

component in analyzed sa nd fractions (0.06-0.25 !Jill) from sediment capping layers (S Ls) and marsh-top (MT) horizon. at core 

si tes AB2 1. ABI 5, and A BI 9 in Al sea Bay (sand fractions were 

also analyzed from . ite AB 12 and found to contain no beach sand). 

Beach- and ri ver-source omponents total 100 percent by volume 

abundance. The measured elevations of SCLs and mar h-top hori -

zons are shown in table 12 and figure 47. 

collected from buri ed marsh top 2MT at tratigraphi c station 

41 (fig. 43, table I I ). The hoice of the detrital wood fragment circum vented the problem of modern root contamin ation in thi hallow , buried mar h hori zon. The age of the 

wood fragment and the dated peaty intervals arc a umed to 

predate the time of the corre pending events o f mar h ubmergence and burial by sediment . The fourth buried mar h 

top (4MT) was not radiocarbon dated due to potenti al contamin ati on v ia des ending roots from the overl y ing peat 

hori zon. The deep . t peat hori zon ored ( I OMT, at - 5. 1_ m 

relati ve to MTL) repre ents th oldc. t buried mar h 

(5,3 19-4, 69 cal r BP) to be reported in the central Oreg n 

coa. t. It date ex tend. the kno n peri od of epi s di mar. h 

buri al in thi . region ba k to middle Holocen time. The total 

of at lea t nine mar. h . ub id n e e cnt that oc urred during 

the peri od from 4,- 10 to 4 0 R YBP indicate. an average 

re urrence interval of about 500 year. . 

DISCUSSION 

YSTEM-WIDE RESPONSE TO 

MARSH-BURIAL EVENTS 

Multiple hori zon of buried peat or p aty mud arc 

widely pre. ervcd in cutbank and interi or site. through ut 

the central-mar h . y tcm of I ea Bay (fi gs. 43, 4 --47). Th 

total number of peaty hori zon (3- 10) at ea h core . ite i. 

generall y proportional to the corresponding ma imum depth 

of core penetration (-0.5 to - 5.5 m relati ve to MTL). 

!though the ucce . i e development and 1 rc. er at ion f 

peaty hori zons must indi ate an o erall condition of rclati v 

ea-lc el ri e. the di tinct mar h buri al unit refl ect r crsing 

cpi ode of relati ve sea-le I change. At lea. t 80 per cnt of 

the indi vidual peaty hori zon analyzed contain both gradational 10\ cr conta t (gradual mar. h emergence) and sharp 

up1 cr contac ts (rapid mar h ubmergence and buri al). The 

verti ca l sequences of mar h buri al are largely define I by thi s 

a ymmctry of mar h development, parti cul ar! in the younge t buri al unit that Ia k . ub tanti al VLO layer. . 

The ma h buri al . equencc , comm nl including distincti ve S L s, prov ide key strati graphic hori zon. that arc 

correlated throughout the central-mar h area (fi gs. 45-47). 

For exampl e, two buried mar. h tops (2MT and MT) and 

thei r overl y ing S L s ( I SCL and 2 L ) are traced around 6.4 

km of ex1 o ed cutbanks in the central-mar h area (fi g. 43). 

Table 13. Relati ve abundance of beach and in analyzed sand fractions from buried marsh 

sediment capping layers along the north channel of AI ea Bay. 

IS L. scdimcnl cappi ng layer: 11'. peaty marsh top! 

Amount of beach 

Hori zon Site AB21 Site ABI S 

I CL 2 1 16 

2MT 7 10 

2SCL 47 31 

3MT 10 7 

4SCL 27 14 

5MT 5 0 

and (percent) 

itc ABI 9 

12 

0 

9 

0 

4 

0 

1

Site ABI 2 

0 

0 

0 

0 

0 

0 

1

Thc bases of burial units at site i\ 131 2 arc used for mineral analysis to rcprcsem equivalentS L horizons sampled 

at core sites AB21. i\131 5, and Al31 9. 
1"6 S ESSI G EARTHQ AKE HAZARDS A D REDU 1 G RISK 1 THE P !FIC ORTHW T 

N 

1 

EXPLANATION 

- 20 -- Contour-Showing amount of 

beach sand. in percent 

• • • Direction of sand transport 

by marine surge 

e AB12 Core site-Showing number 

and percentage of beach 

sand component 

• 47 percent 

AB21 

~ oa . 31 percent 

AB15 

2 KILOMETERS 

Figure 52. Percentage of beach-sand component in sa nd fractions from modern estu ary sediments (contours) and in the 2 L h ri zon at 

core ites AB21 , AB 15, AB I9, and ABI2 at AI ea Bay. Arrows sho' inferred landward direction of marine surge. Gray shading indicates 

the area of mean lower low water. The sand mineralogy ( I 0 percent beach sa nd) for the 2 CL horizon at core site AB I (south channel) 

indicate· a prox imal channel ource. so it could not be used to discriminate transport direction. 

The lateral con tinuity of key burial unit in cutbanks ubstanti ate the tratigraphic correlation of the 4-5 uppermost buried marsh surface identified in the upper 2m of marsh cores. 

Several factor indicate a sy tem-wide re ·pon e to catatrophic events of mar h burial. including ( I ) continuous lateral correlati on of two peaty hori zons (2MT and 3MT) in 

cutbank of the central-mar h y. tem, (2) a imilar number 

of down ore buri al equences at equivalent depth at mo. t 

adjacent core sites, and (3) the similarity of generall y harp 

upper contact. and gradati onal lower contact of the peaty 

hori zon. at mo t mar h c re si tes. 

Quantitati ve analy es of edimenl compos ition in verti ca l pr files of representati ve cores from ire AB21. AB . 

and ABI 2 (fig. 44) from the central marsh confirm the 

stratigraphic interpretation above. For example. in verse 

relati ns between the percentages of organic materi al and 

river . and from ires B8 and AB 12 indicate succes ive 

cycles of I w-marsh to high-marsh transition (fig . 48, 49). 

The. e vertical depos iti nal sequence uggest epi ode. of 

marsh . ubmergence relati ve to MTL, a reponed for other 

tidal basins of s uthwestern Washington (Atwater, 1987; 

198 ) and northwes tern Oregon (Peter on and other , 19 

Dari enzo and Peter on, 1990). Below, we evaluate the evidence of several potential mechani sm of epi sodic marsh 

buri al re ord d in AI. ea Bay. 

RIVER-FLOOD PROCESSES 

Ri ver flooding is an important means of sed iment transport down the Al sea Ri ver channel in winter month. of hi gh 

ri er di charge (Peterson and other , 19 2). However, predicted tratigraph ic features of ca tas trophic-fl ood overbank 

depo it ion (fi g. 37) were not ob. erved in the upper e lUary 

core it s (fig. 47). What was ob erved was a gradual 

decreasing percentage of . and upward within each of the 

buri al uni t. . As the mar h system emerged above lower 

intertidal level , it r e above the reach of the higher veloc ity 

current that are capable of su pending sand (tab! 12, fi g . 

48 and 49). These re ults indica te long peri ods of marsh 

accretion from tidal-cycle depo it ion rather than ca tastrophic 

marsh burial from great ri ver fl oods. Furthermore. the SCL 
LIMA T IC. OCEA IC. A DTECTO IC M ECHA ISMS OF y Ll MAR H BURl L. AL E BAY. ORE 0 

Hori zon Sampl e Horizon Age Depth elevatio n no. (meters) (meters) RCYBP C13 adj cal. BP 

Slightly 1.27 27182 MT1 160:t50 120:t 50 0- 290 oxidized 

horizon 

1.14 39181 MT2 480:t60 500:t60 474- 645 

0.64 27184 MT3 800:t80 760:t80 561 - 903 

Thin break 

in peat 0.25 MT4 not dated not dated not dat ed content 

Mean 

tid e level 

- 0.18 26791 MT5 1 ,490:t80 1 ,450:t80 1,260- 1,530 

- 0.87 27 185 MT6 2,210:t80 2,200:t80 1,999- 2,3 49 

- 1 

EXPLANATION 

= - 1.68 26792 MT7 2,620:t60 2,570:t60 2,479- 2,783 

Modern pea t 

Buried pea t -2 

Mud 

- 2.57 27186 MT8 2,890:t 70 2,830:t 70 2, 779- 3,205 

- 3.05 38861 MT9 3, 140:t50 3,070:t80 3,168- 3,389 

- 5.12 26790 MT10 4,510:t80 4,480:t80 4,869- 5,319 

Figure 53. ore log and radiocarbon ages fr m core site AB9 and one detrit al wood fragment from . trati graph ic station 

41 in Al sea Bay. Radiocarbon el ating " as performed by Beta Anal ytic. Inc .. of oral Gables. Fla .. and corresponding lab 

number. arc . hown for each sample. Peat ages arc shown in radiocarbon years before present (RCYB P. one . tanclarcl deviati on); tJ adjusted ages (C 13 aclj) are ba. eel on measured sample t

2c;t C ratios: and ca li brated age. before present 

(cal. BP) using 2 units of standard dev iati on about the ~ calibrati on curves are establi. heel from tree-ring elating (Stui er 

and Reimer, 19 6). The poor se parat ion of 4MT from 3MB did not allo\ dating by bulk -carbon meth ods at the time the 

testing was performed. The peat top . . used for radiocarbon clatin o. predate each associated marsh submergence and burial 

and poss ibl y arc older than the ages of the subsiclen e events. 

I 7 
13 A SE I G E RTHQ K HAZA RD A D REDUCI G Rl K I THE PA IFIC ORTHW - T 

of the we. tern and central core . itc cou ld not have been supplied by ri er flood because they contain ubstantial 

amount of beach and (fig. 5 I ). 

The lack of cata trophic ri ver-fl ood deposition in the 

high tidal marshc of AI ea Bay is explained. in pan, by 

reduced ri ver-flood level in thee tuary. Relative to the contricted ri ver valley of the tributaries, the ti dall y dominated 

c. tuary i. characterized by large areas of intertidal and lower 

. upratidal . urfacc. (Percy and others, 1974). Con equentl y. 

high river tagc have much-reduced effect in the areas 

where river-flood water are prcad out o er the broad urfacc . Al so, high-mar h flooding i limited to relati vely brief 

in tervals of high tide level, wh ich might not coincide with 

peak river discharge. However, the mo t important ob taclc 

to high-mar h buri al by ri ver flood is the confinement of 

coarse-grained ediments to the channel.. 

Sediment carried down the channel. of the bay tributari es are li kely to settle out of su. pen ion during the period 

f decelerating river flow that leads to flood-tide slack water. 

As a re ult, the coar er river ed iment ·, parti cularly and, are 

unlik ly to be depo ited over the high-marsh surface . which 

undergo the greatest ubmergencc only during max imum 

high-tide level. Sediment re u pension in channel i likely 

to lag the on. et f ebb tidal fl ow, therefore reaching maximum concentrations during falling tidal level. Whereas 

coar c-grained ri ver sediments move down th upper es tuary 

along hannel bottom , the fine. t grained sediment remain 

in . uspcnsion. Clay and fine ilt arc either deposited on the 

high marshes and protected tidal fl at or are effecti vely 

tran ported down the es tuary to be di persed offshore (Peteron and others, 1984). 

In ummary. ca ta trophic river flooding a the mechani. m of alt-mar h burial in A lsea Bay i contrad icted by the 

following ob. ervati on : ( I) the lack of thick overbank flood 

deposits at the marsh channel levees, (2) the upward 

decrease in 1 ercentage of river . and within burial units, indica ting gradual transitions from low intertidal to supratidal 

elevati ons. (3) the general thickening of SCL , including 

marine-sand component , with increasing di tance from the 

river ourcc. and (4) dimini. hcd effects of salt-marsh submergence and burial by high ri ver di charge due to hyd rogral hie con. traints in ti dal basin . 

OCEAN SET-UP PROCESSES 

An alternati ve means of mar h submergence and buri al 

might I e ocean set-up resulting from ex treme oc an storm 

. urges and (or) anomalous oceanic circulation. Ocean torm 

urge. are generall y hon li ved in the Pacific onhwes t (fig. 

38), but they c incide wi th period. of max imum wind-wave 

generation o cr ome tidal flats. The combination of cleated tide levels and wave o. cillatory current have undoubtdly added and to the we ternmo t mar h core si te (AB I . 

AB2, and AB2 1 in fig. 44). or example, the top intervals of 

the f ur you ngc t mar ·h hori zon (OMT, I MT, 2MT. and 

3MT) at core ite AB2 1 generall y ex eed I 0 percent sand 

(fig. 50 and table 12). wherea corrc pending mar. h top 

from protected interior ites B8 and 8 12 average only 2.5 

percent and in II analyzed hori zon . . Mineraloo ica l analyc of the sand. from the upper four mar. h tops (peaty deposits) in we. tern site A82 1 indicate a prox imal source from the 

nearby tidal fl ats (fig . 5 I and 52), confirming the gradual 

add ition of and by storm wind-wave resuspen ion . 

In contrast, the coarse-. ediment frac tion in the peaty 

hori zons and muds, e eluding the SCL. , from the centraland ea tern -mar h core ites (A B 19 and AB 12) are almo. t 

cxclu ively com1 o eel of ri er-supplied and (tab! II and 

12). Ocean storm urge have been in apabl of adding any 

ignificant amounts of tidal-flat and, identified by beachsand minerals, to high-marsh peaty hori zon of the centralor ea. tern-marsh core site . A lso, the sediments of the YLO 

horizons in the central-marsh itcs con ·istmo tl y of ilt and 

clay (mud) bu t not of the coar cr izc frac ti ons (sand) that are 

as. umcd to be re uspended from sand tidal fl at during 

maj or torm (fig. 40). 

In ummary, no apparent torm- urge depo. it of bay 

tidal-fla t sands were found in peaty hori zons from the interior core sites of the central- r eastern-marsh area . . The 

lack of . torm- urge depo. it ion within the Ia. t everal thouand years rul es out cata trophic torm-surge events a 

mechani sm · of system-wide marsh submergence and buri al 

in !sea Bay. Pacific onhwc t storm ar frequent but f 

short duration, with relati ely low wind elocitics that arc 

generall y well under I 00 knot (fi g. 38). Furthermore. the 

narro c ntinental helf and mall tidal bas in do not 

amplify effect of storm-induced ocean et-up al ng thi. 

coa t. We conclude that midlatitudc ocean . torms of the 

central SZ margin are not of ufficient magnitude to 

transport and deposit ub tantial edimcnt layers over broad 

high-marsh urfacc ar ites di tant from the tidal inlet or 

open-bay reaches. 

By compari son, longer periods (months) of elevated 

m -an . ca I vel might have a more gn ca nt impact on 

marsh development and (or) burial due to the additi ve effect 

of ubmergence during pring-tide ex tremes and to ex tended 

. ubm rgcnce during winter torm and high river di charge. 

The large. t measured increa. e of monthl y mean a le cl (30 

em above a I 0-year mean) occurred very recentl y (winter 

and . pring of 1983) a a result of the 1982-83 El ino Southern Oscillation (E SO) event (fig. 39) (Huyer and others. 

1983). Thi s wa al o a year of above-average precipitat ion 

and high ri ver di scharge. However, close examinati on of the 

modern marsh horizon (OMT) at all of the A I sea Bay core 

. ites showed no anomalou edimentation or decrea e in the 

percentage of organic materi al re ulting from thi s most 

recent E SO event. The 0.3 m ri . c in mean sea level did not 

rc:ult in mar h deteriorati on in AI. ea Bay. Thu , thi s value 

provid sa lower limit of the amount of submergence that is 

required to drown and bury a high mar h in thi s tidal bas in. 
CLI ATI .OCEA IC, A D TECTO IC MECHA ISMS OF YCLI MAR H B RI L. L E BA . OR GOt I "9 

6 

4 

(/) 2 

SOUTH 

A 

NORTH 

A' 

cr: ow 

1-1- ----------------------------------- ww 0 >~ ·· /''c 

' ~- . i=Z <{- - 2 ..J_j 

ww 

\ . . ·. ,' ................ --.....::· .. - .. - .. ~ __ .. _ .. _ .. _ .. _ .. __ 'i · ...... ·: 1 ~ _...... .. ............ . CC> -4 zw o..J -W -6 1-0 <{- >Iwz -8 ..J<{ Ww 

~ - 10 

0 100 METERS 

~ \ ~· I / .· 

\~ ;-. t .· 

. / \ ·)/ 50 

- 12 '·J 

EXPLANATION 

AREA 

Dec. 1985 2,075 m2 

May 1986 1,527 m2 

June 1988 

Apr. 1989 

AREA 

663m2 

592 m2 

Figure 54. hangcs in the shape and eros -. ctional area of Alsea Bay inlet between 1985 and 19 9 as a result of a spit breach 

in 19 5 and recovery by 1989 (Peterson and others, 1990). Vertical exaggeration is x 12. Location of the line of . ection is shown 

in figure 34. 

That i to say, more than 0.3 m of abrupt and persistelll ealevcl ri se i needed to kill and bury a high all mar h in th 

central C Z margin. 

Whereas E SO event arc p ss ibly capable of producing sca-le el ri e. in exec s of30 em, their durati ons are limited to relatively brief interval. of relaxed trade-wind tres 

and corre ponding Kelvin-wave tran port (Enfield and 

Allen. 1980). Furthermore, the E SO evelll are unlikely to 

produce the trong asymmetry in abrupt . ubmergence and 

gradual emergence found in the marsh records. AI o, it se m 

very unlikely that major ENSO c cnts would coincide with 

. inglc ca tastrophic torm urge. to yield the nearl y one-toone correspondence between Ls and mar h burial unit in 

Alsea Bay. However, the stronge. t evidence again t regional 

E 0 forcing of salt-ma. h burial in AI ea Bay is the lack of 

uch burial c em. in the we tern mar. he. of Siu. law Ba 

( clson, 1987). located j ust 45 km south of I ea Ba . 

BARRIER-SPIT BREACHES 

M ar. h growth in AI. ea Bay could potentially be 

impacted by vari ati ons in tide-level range due to changes in 

th bay barrier-. pit morpho! gy. Sandy tidal inl ts do not 

completely block es tuari es that have tidal-pri sm and rivereli charge volume. a large a those in A lsea Bay (O' Bri en. 

1969: Percy and others. 1974). However, adjustment in barrier-spit morphology can result in ignifi cant con tri cti on or 

widening of the tidal inlet , producing corresp nding 

changes in the tidal pri m. or example, major chanoe in the 

cro .. -. ectional area of the A lsea Bay tidal inlet occurred in 

association with an anomalous br aching and recovery of the 

Al sea Bay spit from 1985 to 1987 (Jackson and Ro cnfeld , 

1987: Peter on 'tnd others. 1990). The total cr s. -sectional 

area of the inlet increa ed from about 600 m2 (pri or to 19 5) 

to at least 2.000 m2 (December 19 5). The tidal inlet and spit 

fin all y returned to the pre- 1985 size and hape by 1989 (fi g. 

54). This unu. ual event was initiated by beach-sand eli.-

placement during the 198" E S period (Komar, 19 6). It 

provide an excellent example of the effect of a spit breach 

an I associated inlet widening on mean tide level within the 

bay, particul arl y in term. of the impact of increa ed tidelevel range on the bay marsh system. 

The 400-percent increa c in tidal-inlet eros - ectional 

area in 19 5 i. e timated to ha c produced a greater than 60-

perccnt incrca. c in peak tidal flow through th AI. ea Bay 

inlet (J ack on and Ro enfeld , 1987). The greater tidal fl ow 

increa eel the c tim ated bay:ocean surface- level ratio from 

0.71 (pre-19 5) to 1.00 ( 1985). The nearl y 30-percent 

incrca e in peak tid I vel in !sea Bay could yield an additional 0.7 m of marsh ubmcrgence during max imum high 

tides (Jackson and Ro enfcld, 1987). Thi is a very sub tanti al increase of peak tidal ubmergence con idering that the 

eq uilibrium MHHW- MLLW tidal range in Alsea Bay is 

only 2 m (table I 0). 
140 ASSESSI G EARTHQ KE HAZ RDS 0 RED Cl 1G Rl K I THE PACIFI ORTHWEST 

However. close examination of the modern mar. h horizons (OMT) at all of the core ites revealed no observab le 

effects of thi. major change in tide-level rang on plant 

growth or on the fine- calc tratigraphy of the modern marsh 

top. For example, there was no evidence of S L depo. ition 

or any increa. e in and depo it ion from . torm wind-wave 

re u pension over the marsh surface at the we. tern coresite . Thi. major pit breach (fourfold increa e in tidal inlet 

cro s-sectional area) did not ignificantl y influence the tidal 

marshes in Al sea Bay. The e result. show that prehi. tori 

events of inlet breaching or widening could not have produced the effect of catas trophic mar. h submergence and 

burial that are recorded throughout the bay mar h sys tem. 

COSEISMIC SUBSIDENCE 

Perhap. the strongest evidence again. t the burial of the 

AI. ca Bay marsh by relatively brief periods of ocean setup, anomalou. oceanic circu lation, or changing ti cle-lc cl 

range is the observed asymmetry f p at d elopment. 

M rc th an 80 percent of the peaty horizons from the A I a 

Bay core site have both sharp upper contact (mar h tops) 

and gradati onal lower contact (lower-mar. h or coloni zingmarsh sub trates). These vertical trends in peat development indicate conditions of abrupt marsh submergence 

(upper contact.) followed by gradual marsh emergence 

(lower contac ts). This strong a ymmctry of peat development is remarkably consi ten t throughout the entire marsh 

system in lsea Bay (fig. 47). Such a ymmetry is consistent with coseismic tec tonic sub. iclcncc altern ating with 

sedimentati on and (or) gradual tectonic uplift (figs. 41. 42) 

landward of the upper-plate zero isobasc. 

Specifically, the gradual accumulati on of clay and silt, 

a shown by the finely laminated or biowrbated muds. indicates long-term deposition in low-energy es tuarine environments. These burial dcpo its demon. tratc many low-energy 

depositional cycles as oppo eel to one high-energy depositional event. Rather than succumbing to cata. trophic burial 

by storm or river flood . . the mar h plant growth wa. terminated or dimini . heel by abrupt and persi. tent mar h submergence relative to MTL. A lthough not present in lsca Bay. 

high-mar. h emergence to fore. ted wetl ands is rccorclccl in 

buried-mar. h strati graphy at northern Oregon bays including 

clans Bay (Darienzo and Peter. on. 1990). Wetl and emergence above tide level requires an clement of tec tonic uplift, 

because tidal . cclimcnt. can not be tran ported above the 

reach of tides. Episodic c ems of coseismic su b iclencc and 

intcrsci mic uplift that arc superimposed on long-terrn 

cu tatic . ca-lc cl rise arc the unique mcchani ·ms that 

account for the buried-mar. h stratigraphy in Alsea Bay. 

MAGNITUDE OF VERTICAL 

DISPLACEMENT 

The tec tonic displacements needed to IO\ cr the highmarsh surfaces to intertidal levels of barren tidal flats or coloni zing-marsh substrates arc on the order of I m in AI ea 

Bay (table I 0). However. the ap1 a ren t mar. h transitions over 

burial units 2-4 (low-mar h to co loni zi ng-mar. h tidal flat) 

and the transition o cr burial unit 1-2 (predom inan tl y high 

marsh to upper low marsh) . uggc. t smaller vcrti al eli . placement. , that is. a little as 0.5 m (fig. 48). I though horizon 

I MT might indicate mar h burial. it lacks ev idence of abrupt 

burial by co eismi c . ubsidcncc. so it i not included in average displacement e. timatc. for AI. ca Bay. 

Vertical displacement due to sed iment ompacti on i. 

a sumcd to be small as compared to the coseismic ubsidcncc eli . placement in I sea Bay. This as. umption is partly 

base I on the relatively smal l changes in bulk density downcore. which arc further dccrca eel when normalized against 

percent inorganic content (table 12). However. stronger 

ev idence lie in the comparison of marsh tratigraphic elevati ons that overlie a hallow bench of consolidated Pl eistocene deposits and thi ck, unconsolidated Holocene 

deposits in the ax ial river va lley. Specificall y, elevation. of 

burial-unit hori zons ( I MT - 5MT) ary by a total of about 

0.6 m between core site AB21 and sites AB8 . AB9. and 

AB I 0 (figs. 44, 47). The higher buried-mar. h elevati on. 

occur at site AB21, which bottoms out only a few meters 

above con ol ida ted Pleistocene deposit. (Peter ·on and other , 1984 ). The deeper buried-marsh horizons occur at . itcs 

A B8, AB9. and AB I 0, which o erli c 30-50 m of unconsolidated Holocene fill. The total off et of 0.6 m average onl y 

15 em of differential ub idcnce settling for each of the last 

four subsidence events between the very hallow and very 

deep fi II area . 

Table 14. Minimum subsidence esLimated for the four mo t recent subsiden e events in three Oregon bays along the Cascadia subduction 

z.one margin. 

!Seq .. Lransiti on sequence in marsh environmenl due to subsidence: Dbpl.. cosci•mic verti ca l disp lacement. in meters; VHM. very high marsh ; Tf', ti da l nat; HM, high marsh; LM , 

low marsh; MM, undefined marsh elevation; ( --). no sub,idence reported. Netarts Bay data from Darienzo and Peterson ( 1990). Siuslaw Bay data from elson ( 1987)1 

Tidal Distance to subduction funU ~ ~ ~ A vg. displacement 

basin zone trench (k ilometers) Seq. Dis pl. Seq. Dis pl. Seq. Di pl. Seq. Displ. range (melers) 

etart s Bay 110 VHM- TF 1.5 HM- TF 1- 1.5 HM- TF 1- 1.5 VHM- TF 1.5 1- 1.5 

Alsea Bay 100 HM- HM 0.5 HM- LM 0.5- 1 LM- TF 0.5- 1 LM- TF 0.5- 1 0.5- 1 

iuslaw Bay 90 MM- MM 0-0.5 0-0.5 
LIMATI , 0 EA IC A D TE TO 1

1 ME H ISMS OF YCLIC MARSH BURIA L AL E B Y, OREGO 141 

The verti ca l di spl acement of t clOnic sub idence that 

are es tim ated for A I ca Bay (0.5- LO m sub idence) are 

ubstantially smaller than those reported for 1 etart s Bay 

( J- 1.5 m . ub idence) in north we tern Oregon (fig. J3. table 

14) (Darienzo and Peter on. 1990). By compari on. the 

0.5- 1.0 m erti ca l di pl acement in Al sea Bay are apparently greater than those 45 km to the outh in iuslaw Bay 

(0- 0.5 m ubsidence), where there are minor or no significant breaks in p at de elopmcnt at the western end of the 

bay ( elson, 1987). 

The 90- 120-km di tance of the central Oregon coa t 

from the ubduction-zone trench (Peter on and others. 1986) 

(fi g. 33) indica tes the coastline's prox imity to the upperplate zero i obase (fi g. 4 1 ), ba ed on o cismic di splacements from other subduction zones. For example, co eismi c 

coastal subsidence occurred along the coa t of Japan ( 1944 

and 1946). outhern Chile ( 1960), Alaska ( 1964). and 

Colombia ( 1979); each subsidence zone was to atcd 

I 00- 130 km landward of a subduction-zone trench (Heaton 

and Hartzell . 1986; We t and M cC rumb. 1988). 

The coast-to-trench di tances of mar. h ys tems in 

'etart. , A lsea. and Siuslaw Bays (fi g. 33), and the corresponding mar h co eismic verti cal di pl acements (table 14), 

are compared in fi gure 55 . The plots show a positi ve correlation b tween decreas ing distance from the trench and 

decrea ing magnitudes of verti cal coastal di placement. The 

upper-plate nexu re line. or i oba. e of zero di spl acement 

(Piafker and Kachadoorian. 1966), for the SZ margin is 

estimated to be about 90 km ea. t of the trench. in tercepting 

the coa tline at tat 43.S0-44° I (fig. 56). 

An additional indi cation of the relati ve landfall position 

of the zero isobasc i the trend of intcrseismic uplift. which 

reache a max imum ome di. tancc landward of the zero isobase (fi g. 41 ). The buried wetl and in southern Wa hington, 

which are most di tant from the trench (about 140 km ). commonl y reach and exceed upratidal elevation . that i. lO say, 

fore. t oil arc common! d vclopcd prior to cosei mic . ubidence (A twater. 1988). By compari son. onl y two of the Ia t 

four burial uni t in ctart s Ba ( II 0 km from the trench) 

emerged above the reach of ti de. (table 14) (Dari nzo and 

Peter on, 1990). Al sea Bay. at a distance of I 00 km from the 

trench, doc n t contain any wetl and burial . eq uencc. that 

reached fu ll y upratidal elevations . . uch as shrub-forest 

oils. Thu . th re was in uffi cient inter i mi c tectOnic uplift 

to rai. e tidal-mar h surfaces above the r ach of tides in AI. ea 

Bay. further supJ orting the bay's relati vely lo. c prox imity 

to the zero isoba. c. 

The positi n of the zero isobase is important in constraining the landward termination of the scismogcni c 

locked zone. The width of the locked zone pant y determine 

the magnitude of the mcgathrust ea rthqu ake and the di stance 

between coa. tal population center. and the earthquake epicenter. Additi onal studies are underway by the c author. to 

confirm the location of the landfall of the upt cr pl ate zero 

isobasc along the central Oregon oa:t. 

2.0 r----.-----.-~---.----,-1---, 

1.5 - • -

1.0 f- • • -

(/) 

a: 

llJ 

IllJ 

:2 0.5 f- • • -

~ 

~ 

llJ 

z A 

:2 0.0 I I ' llJ 

u 43.5 - 44 .5 45.5 

< ...J COASTAL LATITUDE, IN DEGREES "-(/) 

0 

llJ 2.0 u I I z 

llJ 

0 

iii 

ro 

;::) 

(/) 1.5 f- • - ...J 

< I-

(/) 

< 

0 1.0 r- • • u -

0.5 f- • • -

o.o L__B ____ _JI _____ __~.I ____ ____J 

90 100 110 120 

DISTANCE BETWEEN COAST AND TRENCH, IN KILOMETERS 

Figure 55. Estimated range of minimum CO<L tal subsidence of 

marsh systems in iu. law, Alsea, and etarts Bays, Oreg. A. $ubsidence in relati on to coastal latitude. B. subsidence related tO the 

distance between coast and . ubdu ti on-tone tren ·h. Sec table 14 for 

basis of sub idence magn itude e~ tim e .. 

TSUNAMI DEPOSITS 

The clearest evidenc of abrupt tectOnic ub idence in 

the AI ca Bay mar. h i the ca ta trophic burial of the westcentral marsh peat. by the anomalous S L (fi g. 47A, C). 

Landward-dire ted marine surge. carried beach and (or) 

tidal-nat sands at lea t 1.5 km east' ard to the we. ternmo t 

marsh ore ite. AB21. and depos ited thin. and-ri ch SCLs 

over marsh urface. of the northern bay margin and along the 

north channel (fi gs. 5 I . 52). The SCL show no cv idcn e of 

bioturbati on or internal cro. stratifica ti on, but . omc faint 

layering i ob. erved at . ome sites . SCL sands ap1 ear to have 

been deposited directl y out of turbu lent suspension. These 

characteri tic . . taken altogether, indica te sediment u. pension. landward transport, and cata. trophic deposi tion by tsunami' ave. (Atwater. 19 7; Reinhart and Bourgcoi . 1989: 

Darienzo and Peterson. 1990). The direct correspondence 
142 A SE I G E RTHQUAKE HAZA RDS A D RED Cl G RISK I T HE PA IF! ORTHWEST 

between mar h ub. idence and S L depo it ion in I. ea Ba 

indicate that the e t unami s were general d by tec tonic di -

pl acement in the CSZ margi n. 

Although the Al sea Bay t unami deposits thin with 

increa ing distance landward, the th icker t unam i and of 

the we tern mar h were generally deposited on low marsh 

surface (fig. 47A). In addition, the t unami urge that 

fl owed up the north channel were in ufficient to cross o er 

the narrow mar h at core ite AB4 (figs. 44, 52). We interpret 

the e pauerns of SCL depo ition to indicate th at t unami 

urge fl owed through the bay mouth and then due ea t to the 

northea tern bay margin . t thi point. the surges depo ited 

relati vely thi ck layer of . and and muddy and over the 

mar !Hop horizons (2MT, 3MT, and 5MT). The tsunami 

surge continued up the north channel bu t rapidl y dimini hed 

in fl ow energy, depositing onl y thin andy mud or ilty mud 

layers in the central-marsh ite . The inundation of broad 

tidal fl ats and marsh surface between core ites AB I and 

AB5 (fig . . 34, 36, and 44) wou ld have greatl y reduced the 

mass and associated momentum of the land ward-directed 

urge . . The landward attenuati on of tsunami depo ition in 

AI ea Bay i. apparentl y due to the flooding of br ad intertidal area in the middle e tuary. 

The record of l un ami sed imentation in AI ea Bay provide ome imponam constraints on local tsunami depo ilion in the CSZ margin . or example, a imilarity in t unamidepo ited layer thickness ( 1-25 em) occurs between Al.ea 

Bay and etart Bay even though their verti cal subsidence 

value. differ by a factor of two. The small sub idence di -

placements in Al sea Bay (0.5- 1 m in fig. 55) did not preclude the sedimentation and preservation of a. ociated 

tsunami deposits. A lthough ome SCLs do thicken over 

lower marsh . etting in AI ea Bay, the greate t variation in 

l unami-depo it thi ckne i controlled by the landward 

attenuati on of the urge (fi gs . 47, 52). The apparent independence of tsunami depo ition and the magnitude of local, 

co. eismic sub idence mean that large CSZ-generated t unami. should be capable f leaving di tinctt unami deposit on 

mar h surface that have experienced liule or no corre pending coseismic ub. idence. 

The ob. erved S L depo it in I ea Bay are onl y a ociated with ven t f mar h subsidence apparentl y cau ed by 

c s ismic pl ate deformation. A similar observati on in etart 

Bay wa u ed to in fer that the pre erved t unami depo it. 

there were produced by locall y generated t unami and not 

by di stantl y produced t unami from other Pac ific-rim margin. (Peter on and other , 19 8). Taking thi approach a . tep 

further, it might be po sible to use the correspondence 

between t unami depo it and coseismic . ub idence in a 

marsh record toe tab! ish whether a t. unami wa generated 

within a parti cular egment of the SZ margin . 

A si mple te l of regional versu local segment rupture 

is provided by high-re olution records of coastal ub idence 

and tsunami depo ition. If tsunami depo it. are recor led in 

marsh sequence. that do not how orresponding coas tal 

\ 

46" 

44" 

42" 

\ 

\ 

1 PACIFIC 

I OCEA 

I 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

I 

I 

I 

.c:l 

g\ 

~I 

-I 

~I 

Ol 

~I 

.gl 

\51 

1l 1 

I 

I -~I 

~I 

~I 

U l 

I 

I 

I 

I 

I 

WASHINGTON 

OREGON 

Figure 56. Predicted upper-plate flexure line, or zero isobase, in 

the ascad ia subducti on zone. based on an assumed trenchparallel orientati on and a coastal intercepti on point between lat 

43.5° 1

• and lat 44° . The trench po it ion in the south -central continental margin is approx imated from the mapped ba. e f the continental slope (Peter. nand others, 19 6). Five bay marsh . ystcm 

that have recorded epi. odi c, abrupt coastal subsiden e (Atwater, 

19 7: Grant and M cLaren, 1987; Dari nzo and Peter on. 1990) 

arc shown nonh of Siu>la\ Bay. All are ea t of the trend of the 

predicted zero i obase. 

subsidence, then it can be as. umed that the t unami were 

generated outside of the local egmenr. For example, such 

records of independent t unami deposits wou ld be expected 

in Alsea Bay if mall egment of the C Z margin were 

relea ed at time other than tho e in the AI ea Bay area (fig. 

57). In contras t, if the marsh tratigraphy onl y shows tsunami depo ition with local coas tal subsidence, then the local 

sub. idence must be part of a much larger (regional) tectonic 

di spl acement. Indeed, the observed mar h records in AI ea 

Bay do not show any evidenc of t unam i depo its independent of coa tal subsidence. A nearl y one-to-one correlati on 

of SCL development and local mar h . ubsidence i recorded 

for the Ia t half-dozen subsidence events (fi g. 47). Thi s high 

correlation could impl y ynchronous co eismic di splacement over large area of the SZ margin. We are currentl y 
CLIM ATIC, 0 EA I , A D TECTO IC M ECHA I MS OF YCLI M RSH B Rl L. LS BAY. OR GOt 143 

44° 

\ 

I 

I 

\ 

\ 

I 

\ 

I 

I 

I 

Hypothetical 

northern segm ent 

124° 

Tsunami front -

r-------------------

Hypotheti cal 

central segment 

Modell 

Segmented 

·-

c: 

::> 

·"' .:: 

::> 

a:l 

Mode12 

Unseg mented 

c: 

::> 

·"' .:: 

::> 

a:l 

c: 

::> 

·~ ::> 

a:l 

Hypotheti ca l 

southern seg ment 

EXPLANATION 

Figure 57. Models of segmented and 

un. egmented interpl ate di. lo at ion in th 

north -centra l a. cadi a subdu tion-w ne 

margin (n nhern Oregon). In the segmented margin (model I). tsunami generation o urs in a ruptured segment and 

the tsunami waves propagate into aseismic . egment. . The tsunami surges deposit di stinct sandy layers within marsh 

buri al units in the aseismic segment s. 

These Ia ers would not directl y o erl ic 

buried marsh tops. In the unsegmented 

margin (model 2), t ·unami generation 

and coa tal subsidence are simultaneous 

throughout all adjoining or nearby coastal area .. Resulting tsunami deposits ar' 

only associated ' ith the tops of buried 

marsh or wetland horiwns. indi atinu 

contemporaneous coastal sub. idcncc. 

Model 2 is favored for the !sea Bay r' -

gion due to a strong corre. pondence between the anomal us sandy Ia ers and 

buried marsh tops. 

50 1 00 KILOMETERS 

Modern peat 

Buried peat 

Mud or peaty mud 

Tsunami sand 

Hypothetical platesegment boundary 

---- Tsunami propagation 

front 

inve Li gating whether simi lar one-to-one correspondences of 

tsunami deposition and coa tal subsidence are found in other 

tsun ami- en iti ve bays along the northern Oregon coast. 

CHRONOLOGY OF BURIAL EVENTS 

Signifi cantl y. the younge t peat top co ei mi call y . ub-

. idcd in A I ca Bay (2MT interval) has a radiocarbon age 

(4 0 R YBP, fig. 53) that i on i tent w ith the radiocarbon 

age · of other youn gest buried peat. and forest soi I repon ed 

for northern Oregon (Darienzo and Peterson, 1990: 

Darienzo, 199 1) and W ashington (At water, 19 ). A more 

accurate age of the most r cent vent of co ei mic sub. idcnce in . outh wc tern W a hington (about A .D. 1700) is provided b tree-ring dates of cedar snags killed by their 

submergence to intertid al levels (Y amaguchi and others, 

19 9). nfortu natcly. tree trunk s. ui table for tree-ring dating 

have yet to be identified in buried peat. of the AI. ca Bay 

marsh. 

The nex t younge. L buried hori zon in AI. ea Bay (3MT) 

has an intriguing date of 800 R YBP (56 1- 903 Before 

Present in calibrated age). Th is was the first indica tion of 

coas tal sub idence in the central and northern Orcg n coast 

hav ing occurred durino thi time period . Add itional . tratigraJ hi e work in northern Oregon bay (Peterson and thers, 

1993) ha hown that a l unami pr ducecl from thi . event 

affected widespread area north of Al sea Bay, but sub idcn c from the ame event ha not been ob erved north of 

Yaquina Bay. 

Recurrence in terval between succes ive subsidence 

event in I. ea Bay were found to range from a little a 250 

years (hori zon 9MT- 8MT. fi g. 53) to a much a 1.370 

years (hori zons I OMT - 9MT). based on corresponding, 

uncalibrated peat radi ocarbon date . Dating errors of as 

much as cveral hundred year have r . ulted from radi ocarbon el ating of indi idual C Z coa tal-sub idencc event 

(Atwater. 1988; Peterson and Dari nzo, 1990). However, the 

accura y of c timating average recurrence intervals is 

increa e I o cr longer time peri od a random errors arc canceled by the larger number of ubsidence event.. or example. the average recurrence interval of the three younge t 

subsiden e event (5MT-2MT hori zon ) from I ,490 to 480 

R YBP is es timated to be about 340 year . By compari son. 

the average recurrence interval of the nex t three youngest 

subsidence event (8 MT - 5MT horizon ) from 2, 90 to 
144 A ES I G EARTHQ AK HAZA RD A D RED CI G Rl K I THE P IFIC 1 RTHWEST 

I ,490 RCYBP i e. timated to be abou t 470 year.. The average recurrence interval for the longest peri d of record 

(4,51 0 to 480 RCYBP) i estimated to be about500 c, r. for 

eight ub idence events. 

Generally, the older stratigraphic interval how larger 

recurrence interval , probably resulting from the dec lining 

record-pre crvation potential with greater age. For example. 

burial units become increa ingly difficult to orrelate at 

Jcpth. below MTL in Al..ea Bay due to mi. ing marsh horizons at adjacent core itc . At the central-mar h ites, burial 

unit 6 is mis ing at itc B I 0 and AB7. wherea burial unit 

7 i apparently mi s. ing at AB 19 (fig. 47 ). Finall y, a po. sible burial unit at a depth of 4 m below MTL at core ite AB7 

i. absent from A B8 and AB9. The ab. ence of thi s unit at 

radiocarbon-dated ite AB9 might account for the anomalou ly long peri od (rouohl y 1.500 years) between 9MT and 

I OMT (fig. -3). In A lsea Bay. the most reliable strati graphic 

interval for e timating an average recurrence interval is 

thought to be between 5MT and 2MT, the shallow buriedmarsh hori zons near or above MTL. sing our calibrated 

radiocarbon date of I ,530- 1 ,260 year Before Pre ent for 

5MT and the reported 00 Calendar Years Before Pre ent 

date for 2MT (Yamaguchi and other , 1989; twater and 

other , 199 1 ), we e. tim ate an average recurrence interval of 

320-4 10 years for the Ia t three co eism ic sub. idence event 

in A l ·ea Bay . 

The long-term average recurrence interval of coa tal 

subsidence events in AI ea Bay, about 500 years, i. not igni ficantly different from that rep rted for etart Bay 

(Dari enz and Peterson, 1990; Darienzo, 199 1 ). The similarity of these long-term in terval. implies regional linking of 

coas tal subsiden e e ent in the north-central SZ margin . 

However, the relati ve inaccuracie of radiocarbon dating 

prohibit regional correlation of indi vidual sub. idence 

event . olely on the ba i of radiocarbon age. The long-term 

average recurrence interva l of coastal subsidence in Al sea 

Bay is somewhat I s than the average recurrence interva l of 

po sible co. eismic turbidity fl ow (590 years) reponed for 

the SZ continental slope (Adam., 1990). Howe er, we 

tress that average recurrence in ter al measured over 

shorter and younger time pan in A lsea Bay imply substanti all y shorter average re urrence inter als, uch as 400 years 

or lc , between co. ei. mic subsidence events. 

CONCLUSIONS 

The broad correlation of key peaty hori zon. in the top 2 

m of the A I ca Bay marsh. and the similar nature of marshburial equences throughout the AI ea Bay e. tuary, how 

epi. odic events of coastal ubmergence in late Holocene 

time. P at-burial cqucnccs in A lsea Bay do not reflect cata-

. tro1 hie burial by brief climatic or oceanic events: rather, 

they demon tratc rapid and per istcnt ub. idence fol lowed 

by grad Lnl deposition of suspend d materi al and as ociated 

mar h em rgence. Fu rthermore, hi. tori cal ex treme even ts of 

ri ver fl od . . torm urge:, and tidal-inlet change. in !sea 

Bay have not produ ed hanges in mean tide level or tide 

range that were sufficient to have : ignificantl y impacted 

modern mar. h de clopment. 

aseismi c coastal . ubsidcnce appear to b the unique 

mechani m apable of prod ucing the observed record s of 

episodi c marsh burial in I ea Bay. Spec ifi all y, the cyclic 

epi sode f gradual coastal uplift and sedimentation followed by abrupt coas tal : ubsidence in several bays of the 

central SZ margin refl ec t alternating peri d. of cru. tal 

train accumulation (gradual) and . train rei a. e (abrupt) in 

an acti ve tectonic etting. om pari on of the erti cal movements in A I ca Bay with other Oregon marsh sy tcms . how 

similar trend in the a. ymmctr of verti ca l moti ns. However, along margin trend of decreas ing t ctonic sub i lence 

and uplift "ith decreasing di tance from the trench imply 

regional rather than local te toni c control of the observed 

coa tal sub iden e. The e r gional trend of tectonic coas tal 

di placements are con. istent with upper-pl ate fl ex ure in a 

strongly coupled subducti on zone. urthem10re. they possibl y indica te the rclati e pos ition of an upper-pl ate fl ex ure 

line or zero i. oba e intersecting the c ntral Oregon coa t 

near I at 44° I 

Co ci mic ·ubsi dence of the preex isting mar h urfacc 

in A lsea Bay is indica ted by . e ral independent lines of evidence for contemporaneous t un ami deposition. T unami 

deposits (S L ) above buried marsh urfaces are anomalously sand ri ch. l:o. the contain ele atcd abundance of 

marine- ourcc and relati ve to adjacent ti dal-fl at or riverchannel depos it . Both the relative thicknes and beach-sand 

content of the SCLs indicate landward-directed attenuation 

of marine- urge tran port . A lthough the S L s lie directl y 

above subsided peats, the mall magnitude of e timated subsidence in AI ea Bay doc not prec lude their depo ition or 

preservation. A nearl y one to one correlati on between tsunami dcpo ition and coa tal subs iden e recorded in the 

upper four burial unit from AI ea Bay might indica te ynchronou co eismi c di spl acements over substantial lengths 

of the central Oregon margin. 

We find that the age of the most recent co ei mi ca ll y 

subsided mar h top at Al sea Bay (480 RCYBP) i consi tent 

wi th reported age of the mo t recent co eismic ub idence 

(300 Calendar years Before Pre ent) at other SZ mar h sy -

tem in Oregon and Wa hington. Average recurrence intervals for the younge t three cosei mic-subsidence event in 

A I ea Bay are e timated to be between 320 and 4 10 years. 

ba ed partl y on calibrated radiocarbon dates. By compari-

. on, an average recurrence interval of 500 yea rs i. es tim ated 

for the longest peri od of record (4,5 1 0-4 0 R YBP) in 

AI ea Bay . Thi interva l i similar to long-term recurrence 

intervals reported from other CSZ mar h ystems, impl ying 

regional linking of coa tal subsidence in the north-central 

Ca cad ia subduction-zone margin of northern Oregon and 

southern Washington. 
CLIM T IC. 0 EA IJC, D TECTO IC ECI-IA ISMS OF Y Ll M R H B RIAL, AL EA BAY. RE 0 1-+5 

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Allen. J.R.L.. 19 4. Sedimentary structure., their character and 

physical basis: ew York. Elsevier, 663 p. 

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146 ASSESS! G EARTHQUAKE HAZARD A D RED C!NG Rl K I TH E PA !FIC ORTHWEST 

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435-442. 

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Summarie of technica l report. , Volume XXVI II : .S. Geologica l Sur ey Open-File Report 89-453, p. 162- 169. 
GREAT EARTHQUAKES RECORDED BY TURBIDITES 

OFF THE OREGON-WASHINGTON COAST 

By John Adams 1 

ABSTRACT 

Pi ston cores from the Ca. cadia Seachannel contain 

.equences of wrbidite that can be correlat d and dated by 

u. ing gla .. shards from the M ount M azama, Oregon, eruption of about 6,850 year ago as a common dawm. Turbidity 

curren t in the chan nel' s tributary canyon. appear to have 

occurred simultancou ly and then merged, deposi ting single 

turbidite units in the main channel. In the Ca. cad ia Seachannel and at other places along the ba e of the OregonWa hington continental slope, there arc 13 lllrbidites that 

postdate the M ount M azama erupti on. Thin layers of pelagic 

depo ·its on each succc ivc turbidite ugge t that in each 

place. the 13 lllrbidity cu rrent occurred every 590± 170 

year . on average. The be t explanation of the spatial and 

temporal di. tri bution of the lllrbidite. is that the turbidit y currents were triggered by 13 great Ca cadia ubducti n zone 

earthqu akes. The thicknc . of the topmost pelagic layer. sugge t that the ounge t turbidite wa deposited 300±60 years 

ago, a ti me consi tent with the mo ·t recent sudden- ubsidence event on the Oregon-Wa hington coast. The turbidite 

evidence implie appreciable near-term ri k of a great earthquake (magnitud or , reatcr) that might ruplllre the ascadia subduction zone from Yancou cr Island. Briti h 

Columbia. t Cape Blanco. Or gon. or even farther outh. 

INTRODUCTION 

bout I 5 years ago, p rc ption of the nature of the 

·ubduction zone beneath . outhern Briti. h Columbi a. Wa hington, and Oregon began to change (for example, cc Riddihough and Hyndman. 1976). Earlier work had es tabli shed 

that the Juan de Fuca 1 late wa con ergi ng on 1onh America. However, the lack of sci micity on the interface between 

the Juan de Fuca and the orth America pl ates led omc 

workers to con ider that the ubducti on was occurring 

ex tremely slowly or had stopped. 

1

Gcophysics Division. .:ological urvcy of Canada. I Obscrva10ry 

Crcsccm. Onawa KIA OY3. an ada. 

M ore recent studie. of the dcf rmation front at the ba e 

of the continental lope (for cxampl , Barnard , 197 ). of 

geodeti c deformation rate. on land (Ando and Balazs, 1979; 

Reilinger and Adams. 19 2: H !dah l and ther , 19 9), and 

of on hore tectonic deformation such a warp d ten·a cs 

( dams, 1984) ha c confirmed that the Oregon-Washington 

cominental margin is being deformed at rate. as rapid as 

tho e at other subducti on zon s. Heaton and Kanamori 

( 1984) and Rogers ( 1988) as c .. d . imilariti s b tween the 

ascadia ubducti on z nc and other zone w rldwid and 

concluded that the a cadia subducti on zon 

earthquake. of magniwdc 8.3+. 

The chief hypothc is of my work is that great earthquake , should they occur n the a. cadi a ubduction zone, 

would be ex tremely ca tas trophi events and would leave 

their mark , not onl y in th on. horc geologica l record but 

a! o in the off horc record. In 1984 and 1985. I uggc ·t d 

that the simp! st way to reconcile the discrcpancie 

between long-term and hon-tcrm ev idence for deformation, train ac umulation, and . tress directi on. was with a 

great-earthquake cyc le of long duration (Adams. 19 4. 

1985). I di cussed how an analy is of landslides, land. I idedammed lake ·, drowned tree bclo' sea level, and uplifted 

beache cou ld be used to d termi ne ev idence for past great 

earthquakes and I ·tated that lllrbidite units deposited by 

lllrbidity urrent in the Ca cadia Seachannel. a. de cribed 

by Grigg and Kulm ( 1970). could indicate a minimum 

recurrence inter al for uch anhquakcs. 

Earthquake ha e long been known to cau e submarine 

turbidity currents (Heezen and Ewing. 1952), but becau e 

the very frequent turbidity current off some deltas (every 

few year off the M agdalena and Congo Ri er ) sugge ted 

that many current. were cau eel by . edimcnt in lability due 

to rapid sedimentation, the role of infrequent earthquakes 

was le ea y to a ess . The present chapter wa wri tten in 

parallel with a full paper (Adam , 1990) and updates a ummary work (Adam , 1989) that howed that the turbidite 

record provides strong e idcncc for great earthquakes occurring about every 600 year.. The turbidite evidence complements the large body of on hore re carch into 

paleoseismicity that ha been conducted since 1986 (for 

147 
148 ASSESS! G E/\RTHQ AKE HAZ RD A D RED I G RISK I THE P IF! ORTHWE T 

example. twatcr. 19 7a. b) and i thu important for the 

current debate (for example. sec Heaton and Hartzell , 19 7) 

about the le el of sei mic hazard in the Pacific orthwc t. 

ACKNOWLEDGMENTS 

Thi . tudy wou ld not ha e been po ible wi thout the 

meticulou work of G.B. Grigg , hi fellow Ph.D. tudent . 

and their superv isor. La Verne Kulm . 1 thank Mitch L yle and 

the College of Oceanography. Oregon tate ni cr ity, for 

their cooperation and for making the core log available. 

Da id Piper assi ted with technical adv ice regarding the 

behav ior of turbidity currents, and B.F. Atwater pointed out 

the importance of the bias in the unca librated radiocarbon 

ages. Permis. ion of the American Geophysical Union to 

reuse materi al from Adam ( 1990) is gratefull y acknowledged. I thank B.F. Atwater, P.W. Ba ham. M.J. Berry, L.D. 

Kulm. D.J .W . Piper. and G.C. Roger for aluable criti al 

comments n earl y report s of thi study, and Hans clson 

and M ark Holme. for thoughtful reviews. This paper is Geologica l Survey of Canada Contribution 45389. 

CAN THE CASCADIA SEACHANNEL 

TURBIDITES BE CORRELATED? 

For the pa t 7.000 years, suspended sed iment from the 

olumbia Ri ver has been carried north along the co n~in

tal hclf and depo ited on the. hclf and the edge of the continental lope (fig. 58) (Barnard . 197 ; Sternberg, 1986). 

The ed imcnt accumulate as a thin sheet of mud until 

slope fai lure occurs, and then it flows down the variou 

submarine canyon. and channels a a muddy turbidity current (fig. 59). Griggs and Kulm ( 1970) calculated that each 

major turbidity current takes about 2 days to travel the 735-

km length of the a. adia cachanncl and carrie about 

525x I 06 m3 of sedimen t in a flow as large as 17 km wide 

and I 00 m high. The predominantly muddy sed imen tati on 

in the channel and the similarity of Cascadia Seachannel 

pist n core . uggc. t that many of the cores repre ent a 

complete record of turbidity currents in the Holocene. By 

contrast. on ly two turbidite depo. itcd after the M ount 

M azama crupti n in outhwestcrn Oregon ex tend the length 

of the A. tori a eachanncl (fig. 58) ( clson. 196 , 1976; 

I on and others. 19 ), pcrhap because of the northward 

drift of . ediment away from Astori a Canyon. 

Be ·a usc large turbidity current travel the length of the 

ascacl ia cachanncl. turbidite along th length of the channel should corr late. unique event in the last 10,000 year 

was th eruption of M ount Mazama at 6.845±50 radiocarbon 

car before presen t (Bacon, 1983). Though little if any of 

th air-fall tephra fell a. far wes t as the coas tal ranges (fi g. 

5 ). much fell in the drainage ba in of the olumbia Ri ver 

and was washed into the ocean and cl posited on the ontinental shelf. apparentl y mo tl y n rth of the river mouth. 

When this helf-edgc dcpo. it slumped, the turbidit y current 

carried the tephra-rich sediment clown into the scachannel . 

Because of the presence of thi s tephra, the oldes t turbidite wi th abundant gla can be traced cl own the a. cadia 

Scachannel, and the mean period between turbidity current 

can be ca lculated. The position of the gla. s in three piston 

cores from the middle and lower ascadia eachan nel upports rigg. ' ( 1969) a . en ion that not onl y mu t the lowe t 

turbidite containing unt M azama tephra correlate 

between core , but o must each of the 12 overl ying turbidite. (that is. the 13 turbidites in th cores repre ent the same 

13 turbidit y current ). The correlati on is further upportecl by 

the . imilar pattern of bioturbati on in the top eight layers of 

pelagic sediment in cores 6509-27. 6609- 24, and 6509- 26 

(fig. 58) from a 65-km-long sec ti on of the lower as adia 

eachanncl (fi g. 60) (Grigg and others, 1969). Although the 

pattern of bioturbati n is di tincti vc in these three cores, it is 

unlikely that the correlation could be ex tended to the ore 

from the tributary channel. di cus. eel below because th ose 

core. are uniforml y more bioturbated. AI o. until the reason 

for the degrees of bioturbation are fully under tood, Ca. cadi a Scachannel core. cannot be correlated on thi s ba. i alone 

with core from off ou thern Oregon. 

DO SINGLE TURBIDITES RESULT 

FROM SIMULTANEOUS TURBIDITY 

CURRENTS IN SEPARATE 

TRIBUTARIES? 

Cor 6508- K I (fig. 58) i. from the Ca cad i a Seachannel down tream f the confluence of two main tributUJ") 

channels, the northern from the Ju an de Fuca and nearby 

canyon and the sou thern from Will a! a, Gray , and 

Quinault Canyons. In the northern tributary, core from site 

6705-6 ha 14 turbidites depo. ited incc the M ount 

M azama erupti on, the amc number a wa fou nd in core 

from below the con fluence. Farther upchannel from core 

6705- 6. core 6705-2 contain 16 turbidite depo ited after 

the M ount Mazama erupti on (according to Griggs and 

Kulm. 1970); by inference, 2 small turbidity currents did 

not flow down the channel as far as core 6705-6. In 

Quinault Canyon, core 53- 18 contains ·'at least 14" turbidite dcpo ited after the M ount M azama eruption (Barnard. 

1978, p. Ill ). Further downchannel, in the main Will apa 

anyon, core 6705-5 has 15 turbidite ince the M ount 

M azama eruption; one expl anation for thi s di screpancy is 

that one small turbidity current did not fl ow downchannel 

as far a the confluence and core 6508-K I . 
GREAT EARTHQUAKES RECORDED BY T RBIDITE OFF T HE OR EGO -W SH it GTOt 

126" 

\ 

50 

48" 

46" 

6609-27 

'"' 

·c ~ 

13 ~ 6509-15 ~ 0 

~ 

. 6509-27 ~ 

~ 

t 

JUAN DE 0> 

I ; FUCA PLATE l_g 

44" 

I "' 

I 6609-24 

I 

I 

. 6509-26 I 

J I 

I 

~'"1?~~-ttc I 

I c.,.v~P I l:<o~ ~ I 

PACIFIC I 660~ PLATE 13 I 

l! I 

I <c GORDA 66~ o"J PLATE 13 

8' / 01 I 

42" I 

124" 

<;::,~ 

~ 

MOUNT 

ST. HELENS 

:QOREGON 

Figure 58. M ap f 1he Oregon-Washington eominental margin sh wi ng the ex tent of the airfa ll 

M ount M azama tephra (Fryxell. 1965), the panern of . ediment dispersal non h from the olumbia 

River (. tippled area with heavy arTows: numbers ho\. amount of sedi ment as a percentage of the 

ri ver outpu t. from ternberg. 19 6). ca nyons n the Wa hington continental slope (J. Juan de Fuca: 

Q, Quinault: , Gray. : W. Wi llapa: and A. A. Iori a). submarine channels leading 1 the deep-sea 

fl o r (lines with arrow. ), the sites of pi ston cores described in the tex t (circles and d ts: all from 

regan State University except core 53- 18. which is from the Uni ve rsity of Wa. hington). and the 

number of wrbidit s deposited after the M Lilli M azama erupti on (numbers within the circles) as discussed in the tex t. From dams ( 1990, fig. 1). 

A T 1-19 
ISO ASSESSI G E RTHQUAKE HAZ RD A D RED lNG RISK I THE PA IFI ORTHWEST 

Figure 59. Schematic diagram showing the pr posed gene. is of 

turbidity current. in the Ca cacl ia Seachannel. ecli ment is carried 

clown the Columbia Ri ver ( I), drifts n rth along the continental shelf 

(2). and accumulate. at the top of the con tinental slope in the heads 

of deep-sea canyon. (3). According 10 the hypothe ·is presented in 

thi s paper, a greatthru t earthquake on the a:cadia subdu ti on zone 

(4) trongly shakes the helf and slop (barbs show direction of relati ve mo ement across fault plane). The shaking cau es sediment 

liquefaction and slumping simultaneously at many place along the 

slope. There. ultantmassive undersea debris flows mix with the water to become a seri e. of turbidity current. traveling imultaneously 

down separate channels (5). At j uncti ons, the tributary turbidity currents coalesce to travel down the ascadia eachannel as one large 

lllrbidity current (6). From Adams ( 1990, fi g. 2). 

The cores from the tributaries . uggc t that a few (2 or 3 

of a max imum of 16) small turbidit y current. may have been 

generated that re ulted in lcs ex t nsiv deposits than the 13 

turbidit y currents in th lower chan nel. Therefore, the 

remarkable inference can be made that pair. of turbidity currents were generated in the two tributaries at the . ame time. 

Had the 15 turbidites deposited in the Willapa Seachannel 

and the 14 in the northern channel occurred independentl y, 

there would be 29 wrbiditc below the connuence in tead of 

the 14 that have been found. Therefore. I conclude that turbidi ty cu rrent. were generated simultaneously in two independent channel. and that each pair of small current merged 

to form one large turbidity current. 

That only 14 turbidi tes were found in the main channel 

implies th at every single turbidity current in the two tributaric oc urred at the . ame time, that is, imultaneou occurrence wa. the rule rather than the exception. Simi lar merging 

of turbidity current. from mult tplc sources to form a ingle 

turbidity current ha. been documented from the M ed iterran an Sea and from the Puerto Rico Trench (Pilkey. 1988) 

and ha. al o been interpreted to rcpre ent the effects of a 

simultaneous regional trigger. 

6509-27 6609-24 6509-26 

2 

3 

" " (J) ' 0:: ' w 4 ' 0:: 

1- w w " ' C) 

~ " ' ~ 

i= " ' ::::> 

z " z w w u ' 1-

~ 0 

:x:· C) 

6 0:: 1- -. \ ::::> t:J 1- z \ w \ ...J \ 

\ \ 

\ 7 

\ \ \ \ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

Figure 60. Three cores from the ascad ia 

Seachannel showing how different degrees ofbiolllrbation in the pelagic sediments confirms the stratigraphi c correlation of the turbidites. ore site arc 

shown in fi gure 58. The pelagic layers arc shown in 

olid black. Note the uniform thi ckness of the turbidites . In core 6609-24. the earli est M ount M azama 

gla sis in the 13th lllrbidite (not shown) from the top 

oft he core. Redrawn (with changes) wi th penni ion 

from Deep-Sea Research. v. 16. Griggs. G.B .. Carey. 

A.G., and Kulm, L.D .. "Deep-sea sedimentation and 

ed iment-fauna interaction in Cascadia Channel and 

on ascacl ia Abyssal Plain ," copy right 1969. Pergamon Pre. s PLC. 

HOW FAR ALONG THE 

CONTINENTAL MARGIN CAN THE 

13 DEPOSITIONAL EVENTS SINCE 

THE MOUNT MAZAMA ERUPTION 

BE TRACED? 

Beside the cores dis u. cd above, there arc many others from site off Oregon and Washington that contain the 

M ount M azama glass and varying numbers of overl ying turbidites . Figure 5 show. three core . ites outside the Cascadia 

Seachannel, one at Astoria Canyon and two off Cape Blanco. 

A tori a Canyon, off the olumbia Ri ver mouth (fi g. 

58), was a major conduit for . cclimcnt until 7.000 years 
GREAT EARTHQ KE RECORDED BY T RB IDITE OFFTHEOREGO -WA HI GT 

Table 15. Sediment-layer thicknesse in selected Cascadia subduction zone piston cores. 

)Numbers are mean values per layer. Summari zed from Adams (1990. ~ab e I). (>).greater than) 

OAT 

Turbidites Pelagic deposit 

Core umber of Mean thi ckness Variability2 

tu rbidi tes 1 (millimeters) (percent) 

6705- 2 ..... . . . . . 16 11 5 50 

6705- 6 14 155 69 

6705- 5 . . . . ' . . . . . 15 355 73 

36508-KI . . . . . . . . . 14 360 86 

6509- 15 . . . . . . . . . 13 65 44 

6509- 27 >8 1,060 33 

36609- 24 . . ... .... 13 450 28 

6509-26 ......... >10 660 17 

6502- PC I ....... . 13 130 47 

36604- 12 .. ' ... . . . 13 420 44 

1 ince en•plion of Mount Mazama (6,845±50 radiocarbon years before the present). 

2

Standard deviati on expressed as a percentage of the mean thicknes . 

Mean thi ckne .. Variabilit y2 

(millimeters) (percent) 

62 6 

55 44 

31 49 

33 26 

15 36 

28 44 

55 35 

41 32 

148 24 

105 61 

3Mean pe lagic-layer thi ckness excludes two abnormally thin layers immediately above the first turbi dite contai ning Mount Mazama glass. 

13 L_ __ _L __ ~~---L----L----" 

0 50 100 150 200 250 0 50 100 150 0 200 400 600 800 

TU RBIDITE UNIT THICKNESS, IN MILLIMETERS 

Figure 61. Thi cknes. es of wrbidite unit · in three cores fr m off the Oregon-Washington coast ( ore . ite. 

sho' n in fi g. 58). Large d t denote the oldest turbidite containing M ount M azama gl<L<;S; arrows show th at pl otted va lue. arc minimum thi cknesses. A lthough . omc thi ckness vari ati ons may con·elatc (such as near the base of 

each graph), such correlati on are not obvious. 
152 A ESSI IG EARTHQ AKE H ZARDS A D REDUCI G Rl K I THE PA IFI ORTHWEST 

ago. Although the lower Astoria Seachannel has been less 

acti e ince two 200-500-mm-th ick turbidite were depo -

ited oon after the M ount azama eruption ( el on and 

other.. 1988). core 6502- P I how that I thin (average 

thickne of 130 mm) turbidite were depo ited in toria 

Canyon after the eruption ( arl on, 196 ). Apparently, 

these small turbidity current did not travel all the way 

down the A toria Scachannel. 

Both the ite off Cape Blanco (core 6604- 12) and the 

one off the Rogue River (6609- 1) are remote from the 

olumbia Ri ver mouth, and heavy-mineral analy i of the 

core show that, at lea t since the M ount M azama ruption. 

their sed iment ha come from the Klamath M ountain (Duncan and Kulm, 1970), which are inland and outh of Cape 

Blanco. De. pite their remotenes. from the Ca cadia 

Scachannel and A tori a Canyon ystem , each core contain 

13 turbidite deposited after the eruption. 

The distance from Juan de Fuca anyon to Quinault 

Canyon (fig. 58) is at leas t 50 km; the distance aero the 

Quinault-Will apa anyon ystcm is 100 km: on to Astoria 

Canyon i 30 km; from there to Cape Blanco is 350 km; and 

to the Rogue Ri ver i another 50 km. AilS pans of the margin 

have had 13 turbidity current ince the M ount M azama erupti on. The simple t explanation for the simultaneous current 

in neighboring tributaries and the ame numbers of cun·ent. 

at . ite. 580 km apart i a seri es of great earthquakes. 

As noted by Griggs ( 1969) and Grigg and Kulm 

( 1970), th turbidite. in the ascadia eachannel y tcm 

have generall y . imilar thicknc se and are separated by 

inter als of pelagic deposit , also of generall y constant 

thickness. Pelagic and turbidite thicknesse for I 0 ·ores are 

given in Adam ( 1990), from which the summary statistic 

in table 15 are deri ved. The rather similar turbidite thicknesses within each core (fig. 6 1) suggest that the volume of 

each turbid ity current wa similar. A plot of cumulati ve 

pel agic thick ness ver ·us turbi dite sequence number (Adam , 

1990, fig. 5) was prcv i u. ly used to demonstrate that the 13 

even ts affecting the continen tal margin were fairl y regular in 

time and thu. that the mean thi ck ne interval between 

event. is a rea li sti c measure of the event timing. 

WHAT IS THE MEAN INTERVAL 

BETWEEN THE 

DEPOSITIONAL EVENTS? 

The M ount M azama eruption is well elated by multiple 

carbon- 14 dates at 6, 45±50 radiocarbon year before 

1 resent (Bacon. 1983), though Brown an I others ( 1989) ugocs ted 6,480±60 rad iocarbon year before present might be 

a better c. timatc. The rad iocarbon age is eq ui valent to 

7,620 +~;( cal ibratcd years before 1950 (at the 2o confidence le el u ing er ion 2.0 of the computer program of 

Stui ver and Reimer, 1986). The difference bet ween the+ 159 

year and - 51 year error is . mall relati ve to the to hastic 

rror di cussed below . . o adju ting for the 40 yea r incc the 

1950rcferencec!ategive 7.660± 100 ca libratccl year b fore 

1990. Like the St. Helens tephra (Sternberg, 1986), the 

M ount M azama tephra was probabl y carried n11 idl y to the 

continen tal helfedge and wa available for slumping almo t 

immediately aft r crupti n. When did the nex t turbidity current occur? For 13 turbidit y CUITCnt s, the mean return period 

i about600 years, so the nex t one would probabl y have been 

300±300 year. later. Therefore. 7 ,3 60±400 year before 

1990 is adopted as. the age of the oldes t turbidite wi th M ount 

M azama tephra. 

Since the olde. t turbidite. there ha e been 12 turbidite , 

12 between-turbidite interval. , and a peri od of time since the 

youngc t turbidite. The length of the las t peri od is not 

known. o it i. taken a · a 0.5±0.5 interval. for a total of 

12.5±0.5 interval:. The mean interval between turbidite i 

then 7,360±400 years divided by 12.5±0.5 interval., or 

590±50 year (for com pari. on. the Brown and others ( 1989) 

age forth M ount Mazama crupti n gi c a mean inter al of 

570±60 year.). 

WHAT IS THE VARIABILITY 

OF THE MEAN? 

The above error on the mean return period doe · not 

measure the variability of turbidit y-current timing, which 

can be estimated by exam ining the thi cknc variation of 

the depo ited layers. Tab le 15 show the mean accumulation 

per e ent and the tanclarcl deviation ex pres. eel as a percentage of th mean f r turbidite and pelagic dcpo its in some of 

theCa cadia Seachanncl core . The ex tremely low variability for the turbidite thickne. se in core 6509-26 (± 17 percent; table 15, fig. 60) suggests a very regular seri es of events 

including constant sediment accumulation on the continental 

helf, rhythmic tri ggering, and con. i tent turbidity-fl ow 

characteri sti cs that include the amount of ero ion. For the 

pelagic layers, accumulation rates and their vari ability are 

generall y large t near horc. whereas the deeper core show 

more even thick nesse , ±26 percent in 6508-K I and ±32 

percent in 6509-26. If the cores record the same 13 event . 

those with the more variable pelagic thickn . se must 

include a larger amount of vari ance clue to mea urement 

errors and erosion and so prov ide poor estim ates of the variability of event timing. Therefore, the average variabili ty for 

the three lowe t vari abi I ity cores ( I o =28 percent) i cho en 

a the representative vari ability of event timing. For a 590-

year recurrence interval, thi s translates to a variab ility of 

about 170 year and an implied standard error on the mean 
GREAT EARTHQUAKE RECORDED BY T RBJDITE OFF THE OREGO -\ ASHI1 GTO OA T 153 

of 50 year . Thus, if the intervals are normally distributed, 

there i one chance in two that I of the 12 inter al. would lie 

out ide J..l± 1.70 , or out ide the range of 300- 900 year.. 

WHAT PHENOMENA COULD 

TRIGGER THE TURBIDITES? 

Although Gary Griggs (then at Oregon State ni erity) original ly believed that earthquakes were the cau. e of 

the turbidity curren t (L.D. Kulm, Oregon State niver ity, 

oral commun ., 1988), the lack of large earthquake nearby 

made thi s po ilion difficult to support in the middle to late 

1960's. Griggs and Kulm ( 1970) noted that i f sediment is 

upplied by the Columbia Ri ver at a constant rate, it wou ld 

accumulate in the off hore canyon head until there was 

enough to tri gger co l lap e, whereupon the cycle would start 

again. uch a self-triggering ystem would tend to repeat 

itself and would generate similar- ize turbidite in the offhare channel system. Of cour e, triggering by external 

event of a cyclic nature (such as great earthquake. ) wou ld 

al o tend to displace similar mas e of ediment and produce 

similar-size turbidite . Perhaps the stronge t argument 

agai n !the elf- tri ggering hyp the ·i i that imilar numb r 

of even ts are noted at widely separated place along the continental margin where ediment accumulation amount are 

variable; it wou ld be highl y improbable that every canyon 

would have the ame temp raJ re ponse to the varying rates 

of edimentation that occur along the margin. Therefore, I 

conclude that the tri ggering events have occurred more frequentl y than the time needed to accumulate the critical mas 

required for collap e in most canyon and o the events 

would preempt the endogenic . lumping proce . 

For a similar rea on, event local to one canyon head 

(such a a magnitude 6.5 earthquake within 50 km) do not 

sufficientl y expl ain . imultaneou turbidity currents in separate tributarie and the same number of event along the continental margin . Therefore. any cau c of the imultane u 

event mu. t be ex ternal t the canyon head and mu t affect 

the Oregon-Washington margin a a whole. Three uch 

ex tern al tri gger with the required patial ex tent are tsunami . wave-i nduced lumping during large storm . and great 

earthquakes on the Ca cad ia . ubduction zone. 

TSUNAMIS 

The mo. t recent damaging t unami on the OregonWashington coas t wa gcn rated by the 1964 Ala ka earthquake, the second largest earthqu ake of thi century. It did 

not tri gger one of the 13 great turbidity current in the Ca -

cadia cachannel bccau ·e both the thicknes of pelagic edimcnt found on top of the cores and the amount of animal life 

seen on the channel floor during the 1965- 67 research 

crui e (for example, ec Grigg and other . 1969) ar too 

large for uch a recentturbidit current. T. unami-ocncrat ing 

earthquake arc relati vely ommon around the Pacifi 

Ocean. and if the large t unami in 1964 did not tri gger a turbidity current. it eem unlikely that a c upling of multiple 

independent t unami ource. with the timc-dep ndcnt tability change. in the canyon head would gi e both I ng recurrence interval. and the am number of e cnts along the 

con tinental margin. 

WAVE-INDUCED SLUMPING 

ll1e a c for wave-induced slumping n u h a large 

calc i. poor! documented. and amputation. ugoe. 1 that 

for all but the teepe t slop . . wa e I ad ing doc not influence lope stability in water depth. greater than 120 m 

(Moran and Hurlbut, 19 6), whi hi hallow r than m . 1 of 

the offshore canyon heads. Physica l parameters con. train th 

maximum size of torms and torm -g ncrat · d wa cs . o that 

an ex tremely rare torm i. not much larg r than a mor ommon storm . Therefore, the I 00- and I ,000-ycar storms would 

have rather imilar effects, the s diment probabl y w ulcl 

need to be close to failure anyway, and . o, c !lapse would 

happen at different times for different place . Like tsunami s, 

the proce s would be unlikely to gi ve both long rccurren e 

interval and the . amc number of event for the whole continental margin . 

EARTHQUAKES 

Earthquake arc unu ual nawral phenomena in that the 

trength and duration of gr und shaking continue to 

increase dramati ca ll y as the probability of occurrence drops. 

Thus, a grea t earthquake is so overwhelmingly large that it 

wou ld tri gger col lap e of both maroinally and otherwi se normall y table canyon-head dcp it. . Great earthqu akes, 

. hould they occur on the a cacl ia subduction tone. \ ould 

probably have a long return period and, indeed. return period of . everal hundred to a thou and years have been e timated by Adam ( 1984) and Heaton and Hartzel l ( 1986) 

based on the rate of geodeti c strain accumulation. As a fir t 

approx imation, pl ate-boundary earthqu akes occur fairly regularl y and tend to have a charac t ri stic size. For uch characteri sti c earthquake . there may be a ca le-indepenclent 

ari ability in timing amounting to about one-fifth of the 

mean recurrence interval ( 1i hcnko and Buland, 1987). 

hence 120 years for a 590-ycar recurrence interval. Thi variability i close to the 20- 30 percent ariability in p lagic 

deposit thicknes es found in the cores (which includes not 

onl y variability clue to the eanhquake cycle but al o that due 

to variable parti al ero ion and measurement error in determining the thickness of the pelagic layer ). 
154 A. ESS I G E RTHQ AKE HAZA RDS A D RED I G Rl K I THE P IF! ORTHWE T 

HOW DOES THE 

TURBIDITE EVIDENCE COMPARE 

WITH ONLAND EVIDENCE 

OF GREAT EARTHQUAKES? 

Evidence that the 13 turbidity current indeed repre ent 

earthquakes i circum tantial. and cstabli hing proof i. 

unlikely in the near fulllre. evenhclcss, because a great 

thru. t earthquake is an ex tremely large and relati vely rare 

event, it . hould cause imultaneous effects throughout the 

coastal Pacific orthwe. r. uch confim1atory pa t evidence 

would include . udden coa tal ubsidence or uplift, landslide., cd imcnt liquefaction and and volcanoes, ediment 

lumping in large lake , and . ubmarine dcbri flow . Other 

les. direct effect. wou ld be abnormal edimentation events, 

deformed tree growth , abandonment of prehisroric Indian 

settlement. , and secondary faulting on cru stal fa ult . 

Although the turbidite record may well be complete because 

of the muddy nature of the ediment , the inferred earthquake record might be incomplete becau e sub. equent 

nearby earthquake. (a econd main hock or large aftershock) soon after a great earthquake wou ld probably not 

generate their own turbidity current(. ), all of the unstable 

sediment. hav ing already slumped. 

The first onshore evidence in ferred to indicate past 

grea t a. cadi a ubduction zone earthquakes was reported by 

Atwater ( 1987a), who ascribed buried Holocene marshes in 

coa. tal Washington to udden co cismic subsidences of the 

c ast. Hi s mo. t recent work (Atwater, 19 ) suggest five 

ubsidence event and one baking event in the Ia t 3, I 00 

years. Corre. pending evidence comes al o from eight buried 

oil s in about 5,000 years at another coastal Wa hington ite 

(Hull. 1987), even buried mar he in about 3,550 year in 

northern Oregon (Peterson and others, 1988), and eight buried marshe in abou t 5,000 years in southern Oregon ( clson 

and other , 1989). At the southern end of the Cascadia subduction zone, a rver and Burke ( 1987) deduced recurrence 

intervals of ab ut 600 year for earthquake on thrust faul t 

in the accreti onary pri m just north of Cape M endocino, 

al i f. qui valent ev idence is described in the other chapter 

of thi s volume. In each plac , the impli ed recurrence inter al 

is about 500- 600 years. in clo c agreement with the mean 

recurrenc interva l and . patial extent cstabli heel for the lllrbidity currents (fig. 62A). 

B.F. Atwater (U.S. Geological Survey, written commun., 19 8) has also drawn my attention to the coincidence 

between the age of 4.290±80 rad iocarbon years before 

pre nt for the eighth peat at Will apa Bay (sample H 1- B of 

Atwater, 1988) and the age of 4,645± 190 radiocarbon years 

b fore pre cnt for pl ant fibers in the eighth turbidite of core 

6509- 27 (G ri gg: and other , 1969). The fibers had accumulated in the offshore canyon head. for about 600 year prior 

to the dated turbidity current. . ubtracting 300 year. (half 

the inferred accumulation time) from the core date bring. the 

two ages into remarkable agreemen t. Thi s ao rcement i all 

the more . ignificalll becau e each date is on the eighth event. 

. ugge ting a one-to-one correspondence b tween turbidites 

and subsidence event. and reinforcing the argumen t made 

from the mean rccutTence interval. eli cus ed above. 

WHAT ARE THE IMPLICATIONS 

FOR GREAT THRUST EARTHQUAKES 

ON THE CASCADIA 

SUBDUCTION ZONE? 

The inferred occurrence of 13 great thru. t earthquake . 

their mean return period of 590 years, a ari ability of 170 

year , and the 580-km ex tent f triggered turbidity current 

(and. hence. infetTed strong ground motion) cons train the 

slip, rupture dimension , magnitude, and tyle of rupture of 

the mar0 in . . ubduction of the Juan de Fuca plate at 45 

mm/ ear and the mean return period of 590 years re ults in 

26 m forth average . lip per earthquake if all the lip i 

cismic. A 26-m slip for the 750-km-long Juan de Fuca 

plate north of the Blanco fracture zone and a fault wid th of 

I 00 km could produce an earthqu ake of moment magnitude 

(Mw) 9.1 , which is in accord with the maximum size propo eel by Rogers ( 1988). A ari abi I ity of 2 percent in 

earthquake timing would lead to lip di pl acements of 

between 19 and 33 m but a variability in magnitude of onl 

±0.1 magnitude unit. 

Would a rupture of the a cadia ubducti on zone 

extend uth of the shoreward ex tension of the B Janco fracture zone? Core 6604-12, which came from the ex tension of 

the fracture zone, and core 6609- 1 from the ubdu ting 

Gorda plate (fig. 58) contain 13 turbidite deposited after the 

Mount M azama eruption, which arc presumed to repre em 

great earthquake on the Juan de Fu a part of the ascadia 

subducti on zone. If the Gorda pl ate had a hi tory of independent ubduction (such a Mw 8.3 earthquake ; . ee Roger . 

1988), core 6604- 12 i close enough to have hown additi onal turbidity currents that might have been generated by 

earth 1uake either to the north or outh. Therefore, the indica ti on of only 13 turbidite. in the e 2 cores sugge t that 

either every Cascadia subducti on zone rupture extend. pa t 

the Blanco fracture zone (indicating at least Mw 9.2 earthquake ) or the earthquakes on the Gorda plate segment are 

synchroni zed with those on the Ju an de Fuca pl ate segment 

(a zipper effect in which an earthquake on one segment triggers earthquake on adjacent segment in succes. ion a few 

hour. to year later; hence, no . ed imcnt would be left to be 

triggered by the second earthquake). 
GRE T EARTHQ AKE RECORDED BY T RBIDITES OFF THE OREGO -WA HI GTO OAT ISS 

48' 

46' 

44' 

42' 

126' 

Vancouver 

Island 

124' 

WASHINGTON 

' ' 

OREGON 

CALIFORNIA 

48' 

46' 

44' 

42' 

126' 124' 

OREGON 

Figure 62. patial di. tribution of 

age c idencc for gr at ca11hquakcs in 

the ascadia subduction 1.0ne, coastal Oregon and Washington. A. mc·m 

re unence inter als for turbidites and 

onshore sub. iden e events, in years. 

B. age of the youngest wrbidite. submarine slumping, or onshore subsidence event, in years before 1990. 

Hca y lines ' ith arrow. arc submarine canyons and hanncls leading to 

th deep-. ea Ooor. From Adams 

( 1990. fi g. 6). 

50 1 00 KILOMETERS 600 50 100 KILOMETERS 

A 

' I 

WHEN WAS THE 

LAST EARTHQUAKE? 

B 

The age of the last earthquake can b e timated roughly 

fr m the thick ne of pelag ic sed iment on top of the younge tturbidit relati ve to the mean sediment accumulation rate. 

From six ore. (table 16), an age of ab ut 300±60 year · 

(before 1995) is e tim atcd. Thi age i con istent with the 

lack of a great earthquake in the ! 50-year histori cal record 

but may be a bia ed undcrc tim atc if some sediment ha been 

washed out from the top of each core. It i noteworthy that 

both the Astori a anyon core and the core from ff ape 

Blanc give similar ages (to within the poor resolution) as 

the four ascadia Scachannel core . thereby howing that 

the youn0 es t turbidite in each place did not occur at greatl y 

different times . 

A date for the you ngest turbidite can also be derived 

from the amount of sediment th at has accumulated in the 

submarine anyon . ince the ed iment las t slumped away 

(for example, se Barn ard , 1978, p. I I I), but e timates range 

from I 00 to 400 year becau e the sedimen t accumulation 

rate arc uncertain. The c timatc weakly upport the inference that about 300 year have elapsed since the las t earthquake. Tree-ring dates on drowned tree at ix sites in coas tal 

Wa hington sugge t that the youngc t rapid ubmergence 

even t wa about 00 years ago (Yamaguchi and other , 

1989). omparable el ate. for the younge. t buried marsh soil 

uggest that the younge t ub iclencc event occurred about 

300 year ago in southwe tem Wa hington (Atwater and 

other., 19 7), les than 400±200 year ago in northern Oregon (Peter on and others. 1988), less than 380±60 year ago 

in southern Oregon ( el on and other , 1989), and 270±30 

years ago in northern Californi a (Gary Carver, Humboldt 

State Univer ity. oral commun. , 1989); these dates are con-

. i. tent with the preceding analy is (fig. 628 ) and sugge t a 

regional earthquake (or closely 1 aced erie of earthquakes) 

that affected almo t the whole Cascadia subduction zone. 

Working from the pelagic thicknesses in core 

6508- K I (fig. 58) (cho en because it pelagic thickne e 

are the mo t regular), a. suming that no sediment ha been 

eroded by the overlying turbidite, and adopting 300 year 
156 A SESS I G EART HQUAKE HAZA RDS A D RED Cl1 G RISK I TH E PA IFIC ORTHWEST 

for the age of the last earthquake the following date for 

the last fi ve earthquakes are deduced: A.D . I ,690±60. A .D. 

I ,300± 130, A.D. 00±200. 180±450 B.C., and 790±520 

B.C. (Adam . 1990, table 3). The low accumulation rate. 

of the pelagic . ediment and the disturbance by the coring 

equipment make these dates rather impreci. e, with the 

error. accumulating for the older date However, these 

dates should prove u. eful to other researcher eeking to 

match their di covered onshore events to the record of great 

earthquakes indicated by the turbidites. 

WHAT IS THE PROBABILITY 

OF THE NEXT EARTHQUAKE? 

From the mean interva l between event (590 year:), the 

tandard dev iati on of the mean (about 170 years), the time 

since the youngest event (about 300 years), and a norm aleli tributi on model for simple recurrent ruptures without clustering, the likelihood of the next great Cascadi a ubducti on 

zone earthquake can be es timated (fi g. 63). At present, there 

is about a 5 percent chance that the next earthquake should 

have already happened. For the future, the conditional probabilities are (crudely) 0.1 percent in the next year, 5 percent 

in the nex t 50 year , I 0 percent in the next I 00 years. and 25 

percent in the nex t 200 years. These values are probably correc t to only a fac tor of 2 and need to be refined using a revi sed 

date for the Ia t earthquake and perhaps a lognormal di tributi on model (following i ·henko and Buland, 1987). 

Some of the uncertainty ari ses from the po sibility that 

the earthquakes cluster in time ( ay, three events in one 

millennium, followed by long quie cence) and from the 

still-unknown mode of failure of the subducti on zone. 

Although the same number of turbidites in the Cascadia 

Seachanncl, Astoria Canyon, and at the sites off Cape 

Blanco trongly suggests imultaneou turbidit y current . a 

zipper effect whereby mailer earthquakes rupture the pl ate 

::::; 

Ci'i 

80 

;;3foZ 60 a:W 

a.U 

wa: 

>~ 

~ ~ 40 ..J 

::::J 

2 

::::J 

u 

20 

200 

El ·:: I 

I ::l l 

cL I 

I 

I 

I 

400 

M ean = 590 years 

Standard 

deviation = 170 years 

600 800 1000 

TIME SINCE PREVIOUS GREAT EARTHQUAKE, IN YEARS 

Figure 63. Cumul ati ve normal probability distribution with a 

mean age of 590 years and a standard deviati on of 170 years, as infen·ed for great ea rthquakes on theCa cadia subduction zone. The 

thi ck bar on the abscissa at 300±60 years is an estimate of where I he 

present time is relati ve to the last earth 1u ake, and the range of conditional probabilities of the nex t earthquake for the nex t 50, 100, 

and 200 years arc illustrated by the values at the top. From Adarns 

( 1990, fi g. 7). 

boundary in a short-term sequence cannot yet be ruled out. 

Such multiple m de of rupture-sometime single great 

earthqu akes, sometimes sequence of mailer earthquakes- would compli ca te the paleoseismic history and arc 

probabl y beyond the resolving power of the turbidite 

record. However, the mode of rupture could be re olvcd 

through onsho·re in v stigati on using dendrochronology or 

other precise dating of earthquake effec ts. 

Even if the rupture mode is complex, the net effect on 

hazard estimates may not be great because circumstantial 

evidence . ugge ts th at the (hypothetical) rupture segments 

would need to" tay in step" and the time since the last earthquake is already clo e t half the c tim ated mean recurrence 

interva l. In addition, the damage impli cations for any pl ace 

Table 16. Age of the youngest turbidite in selected Cascadia ubduction zone pi ston cores. 

!Age estimated from the th ickness of the overl ying pelagic sediment. From Adams (1990, table 2)1 

Core 

Pe lagic-layer thieknes t 

(millimete rs) 

6705-2 .. . . .. .. . 

6705- 6 .. .. .... . . 

6509- 26 .. .. . . .. . 

6509- 27 .. . ..... . 

6502- PCI . .. ... . . 

6604-12 .. ..... . . 

23±6 

25±3 

17±5 

16±3 

70±20 

47±10 

1

(±). possible error in 1965- 67. 

Accumulation rate2 

(millimeters/event) 

62 

55 

41 

28 

148 

105 

Mean age 

2

From tublc 15; mean event spacing is 590 years. 3Re lative to 1965-67; add 30 years for years before 1995. 

Age of youngest turbidite3 

(years) 

220±60 

270±30 

245±70 

335±60 

280±80 

260±60 

4270±60 

4

0r about 300 years before 1995. May be biased towards bei ng too you ng because of washouts uttop of core. 
GREAT EARTHQUAKE RECORDED BY T RBIDITES OFF TH E OR EGO -WASH! GTO 0 T 157 

along the continental margin are not greatl y different 

between the . ingle magnitude 9 or multiple magnitude 8 

earthquake scenarios, though it is clear that hould one magnitude 8 earthquake happen omewhere n the ubducti on 

zone, the likelihood of others in the near future would be 

greatl y increa. ed becau e of the temporal clu tering implied 

by the alternati ve hypothesis. 

CONCLUSIONS 

Turbidite in the Ca cad ia Seachannel and at other 

places along the Oregon-Wa hington continental margin 

were depo ited from turbidity curren t probably tri ggered by 

13 great ascad ia ubduction zone earthquake since the 

Mount M azama erupti on. Earthqu ake of the order of magnitude 9 have occurred every 590 years on average, an interval that agree well with the mean interval betw en known 

coa tal sub. idence events. Analysis of the pelagic-sediment 

intervals between the turbidite sugge ts that the earthquakes 

occurred fairly regularl y, with a standard de iation of 170 

years or less for the earthquake timing. a vari ability imilar 

to that for great earthquake cyc le elsewhere. 

The thickness of the topmo t pelagic layer sugge t that 

the last earthquake was 300±60 years ago. Thi age i consistent with the younge t subsidence e ent di covered on the 

Washington coa. t and implie appreciable near-te1111 ri k of 

a great earthquake in the Pacifi c orthwes t. 

Rhythmi c triggering of turbidity currents by great 

earthquake may be a much more comm n phenomenon 

th an hitherto reali zed and might be expected at other con tinental margin such as Al a ka, Ja1 an. ew Zealand. and 

Chile, where great thru t earthquakes with a long return 

period are combined wi th a moderate uppl y of . edimentto 

the edge of the con tinental shelf. If ampled correctl y, the 

turbidite record can provide a qui ck estimate of the paleoeismicit y of a ntinentalmargin and s guide confirmati on 

by nshorc palcosei mology tudi c . 

REFERENCES CITED 

Adams. J hn, 1984, Acti e deformati on of the Pacific 1 orth west 

continental margin: Tectoni cs , v. 1

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107 1. 

---1989. Turbidite. off the Oregon-W ashington margin re ore! 

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---1990. Palcoscismicity of the Cas adi a subdu tion 

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---1987b. ubduction-earthquake telltale. beneath coastal 

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of Conference XXX IX. Dir ctions in paleoseismol g : 

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79- 114. 

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Duncan. J.R., and Kuhn . L.D .. 1970. Mineralogy, provenance, and 

eli per. a! hi tory of late Quatern ary deep-seas, nds of ascadi a 

bas in and Blanco fracture zone off Oregon: J urnal of cd imentary Petrology, . 40, p. 74- 87. 

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channel: Cor alli s, Oregon State University, Ph .D. di ssert ati on, I 3 p. 

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and on Cascadia aby. sa l plain: Deep-Sea Research. v. 16. p. 

157- 170. 

Grigg . . G.B .. and Kulm. L.D .. 1970. edimentati on in Cascadia 

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I. no. 5. p. 136 1- 1384. 

Heaton, T.H .. and Hanzel!. .H .. 19 6. Source characteristi cs of 

hypotheti ca l . ubducti on eanhquakcs in the orth western United tate. : Seismologi a! ociety of A meri ca Bu lletin. v. 76. 

no. 3. p. 6r - 70 . 

---1987 , Earthquake hazards on the ascadia subducti on z ne: 

cience. v. 236, no. 4 798 . p. 162- 16 . 

Heaton. T .H., and Kanamori. Hi roo, 1984. cismic potential a ·sociatecl 1 ith subduction in the orthwestern nited States : Seismologica l ociety of America Bulletin. v. 74, no. 3. p. 

933- 94 1. 
158 ASSESS! G EARTHQ AKE HAZ RDS A D RED Cl Rl K l TH E PACIFIC ORTHWEST 

Hee;:en, B. .. and E\ ing, auricc, 1952. Turbidity ClllTent · and 

. ubmarine . lumps, and the 1929 Grand Banks earthqu ake: 

American Journal of Scien e, v. 250, p. 849- 873 . 

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vertical cru stal motion in the Pacific orth we t, in Cohen, 

S.C., and Yanicek, P., eds., Slow deformation ancltran ·mission 

of . tress in the Earth : American Geophysical nion Geophysical M onograph 49, p. 17-29. 

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earthqu akes during the la. t 5000 years: EOS [American Gcophy ical nion Transaction. ], v. 6 . no. 44, p. 1468- 1469. 

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ational Earthquake Hazard Reducti on Program, ummaries 

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---1976, Late Pleistocene and Holocene depositional trends. 

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229, p. 93-99. 

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1022- 1030. 

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Geophysical nion Tran action. ]. v. 70, n . 43, p. 1332. 
Tectonics and Geophysics 
Preceding page. Insert, re cue of a motori t trapped by fallen bricks moments after the 

April 29, 1965, Seattle, Wash., earthquake. Photograph by Ken Harri s of the Seattl e Postlntelligencer. Background, damage to unreinforced masonry wall and parapet in downtown Kl amath Fall , Oreg., during the Sept. 20, 1993, M 5.9 and M 6.0 earthquakes (from 

Dewey, J. W., 1993, Damages from the 20 September earthquakes near Kl amath Fall s, 

Oregon: Earthquake & Volcanoes, v. 24, no. 3, p. 12 1- 128). 
CENOZOIC EVOLUTION OF THE CONTINENTAL 

MARGIN OF OREGON AND WASHINGTON 

By Parke D. Snavely, Jr., 1 and Ray E. W lls2 

ABSTRACT 

The Cenozoic tectonic and depo itional hi tory of the 

Oregon and Wa hington continental margin was marked by 

underthrusting, tran CUITent faulting, and ex tension during 

oblique convergence of the orth Ameri ca plate and oceanic 

plates to the west. Sedimentation, punctuated by epi ode of 

volcanism, was e entially continuou. in the forearc bas in 

who e ax is lay along the present inner continental shelf. The 

oldest rock in the Oregon-Washington Coa t Range are 

Paleocene and lower Eocene oceanic basalt capped in pl ace 

by i lands and seamounts. They most likely formed by inplace erupti ons (62-56 M a) during oblique rifting and ex tension of the continental margin. These basa lts were erupted 

adjacent to the continental margin, as evidenced by locally 

interbedded teiTestri al sediment . A thick lower Eocene siltstone and middle Eocene turbidite equence buried the 

i land while the basin wa · subsiding. The middle Eocene 

turbidite strata overlap both oceanic ba alt and pre-Tertiary 

rocks of the Kl amath Mountains in southwes t Oreg n, e. tabli hing that uturing of the Coast Range-Olympi c M ountains 

terrane to North Ameri ca occurred at about 50 M a. Continued 

high rates of moderately oblique convergence may have 

cau ed trike-s lip fau lting and tectonic rotati ons along the 

continental margin . The north-trending Fulmar fault on the 

Oregon continental shelf juxtaposes lower Eocene 

graywacke on the west against lower ocene oceanic cru t of 

the Coa. t Range and is interpreted a a right-lateral fault with 

a minimum offset of 200 km . Eruption of upper middle and 

upper Eocene tholeiiti c and alkalic ba alt uggests a transten ionaltectonic environment toward the coas t. Red uction 

of the rate of convergence between the Farallon and orth 

America plate after 49 M a was followed by Ca cade arc olcanism (about44 M a) and. ultimately, the demi e of the rightlateral Fulmar fault, a recorded by the onlap of uppennos t 

1

U.S . Geological Su rvey. MS 999. 345 Middlefield Rd., Me nlo Park, 

CA 94025- 359 1. 

2 .S. Geolog ical Survey, MS 975. 345 Middlefie ld Rd .. Menlo Park. 

CA 94025-359 1 . 

Eocene strata. Depo ition of Oli gocene tu ffac ou marin 

trata in the forearc signaled the beginnin f oluminou. 

Cascade arc vol an ism at about 35 M a and cont mporane u. 

subsidence of the forearc. !though ri ft olcanism chara t rized the forearc during the late -o enc and carl ligoccne, 

there i evidence for late Eocene underthru tin"' in the 0 1 mpic Mountains accrcti nary wedge. Major upli ft of the Oregon and Washington continental margin oc urr d in th lat ' 

middle Miocene, as indica ted by a rcgi nal un onfonnity on 

the continental shelf and by strongly deformed r cks of the 

Miocene Astoria Formation and the olumbia Ri ver Ba. all 

Group in the Coast Range. Offshore, Oligocene and Mio cnc 

melange wedges are overlain by a much a. 2,000 m of upper 

Miocene and Pliocene . trata that arc roldcd and thrust 

faulted. Oblique convergence continues today and has caused 

Quaternary trata to be episodically fold d and thrust alon 

the deformation front on the continental slop . A long the 

coas t, marine terraces have be n tilted during uplift of the 

central and outhern Oregon Coa t Range. At the strand line, 

Holocene marsh depo it record several epi ode. of rapid 

coa tal ub. idence, pos ibly related to earthquake on the 

thru. t interface with the downgoing plate. Late enozoic 

fault along the coast are adjacent to subsiding region . . and 

some of these fault how Quat rn ary di. placement. 

INTRODUCTION 

Sedimentary and volcanic rock s in the Coa t Range of 

Oregon and Wa hington, in the Olympic M ou ntains, and on 

the adj acent continental margin comain a nearl y continuou. 

record of Cenozoic ed imentation and tectoni m along th 

oblique convergent margin between the oceanic and orth 

Ameri ca plates. Although ubducti on has pl ayed a major 

pan in the evolution of the convergent margin, the geologic 

record indica! s a complex hi story that include stri ke-. li p 

faulting. bl ock rotati on, ex tension. and magmati sm, along 

with typica l convergent-margin processes of thru ting, 

underpl ating, and development of a major accreti onary 

wedge. In thi s arti cle, we summari ze the on. hore and 

16 1 
162 ASSESS! G EARTHQ AKE HAZA RDS D RED Cl G RISK l T HE PAC!Fl ORTHWEST 

off hore enozoic geologic framework of the continental 

margin, building on pre ious ynthese by navely ( 1987) 

and Wells and other ( 19 4). We empha ize the geologic 

hi story, regional structure, and off hore evidence for neotectoni c to set the tagc for evaluation of earthquake 

potential along thi. active margin. 

ACKNOWLEDGMENTS 

We thank Exxon Company, .S.A .. for generously 

making available their sci mic-rcflection profile off the 

mouth of the Columbia Ri ver, a thi profile clearl y images 

the structure of thi . egment of the Oregon continental margin. Weldon W. Rau and David Bukry pro ided the bio tratigraphic framework for the Tertiary equencc that is the ba is 

for ti ming of mos t geologic event . John G. Vedder, A lan R. 

icm, and Timothy Wal h rev iewed the manuscript and 

offered ugge tion that improved its quality. The writers are 

parti cularl y grateful to Diane L. Minas ian, who drafted the 

illustrati ons and criti ca ll y re iewed the manu. cript. 

CENOZOIC GEOLOGIC HISTORY 

PALEOCENE TO MIDDLE EOCENE 

ORIGI OF THE COAST RA GE OCEA IC BASEME T 

The oldest rock of the Oregon and Wa hington oast 

Range are tholeiitic pi llow basalt and interbedded breccia 

and marine sed imentary rocks (fig. 64, x pattern) (Duncan, 

1982; Bukry and Snavely. 1988). These oceanic ba alt , 

capped in places by i land and seamount , include the Si letz 

Ri ver Volcanics in Oregon (Snavely and other , 196 ), the 

Crescent Formation in Wa hington (Arnold, 1906; Brown 

and other , 1960; Cady, 1975), and the Metcho in Volcanic 

on southern Vancouver Island, British olumbia (Clapp, 

19 17; Muller, 1977). A lthough several models have been 

propo ed for the origin of the. c ba. alt (for example, Duncan, 1982; Wel l and other , 1984), we prefer the model of 

ri fting and volca ni sm along the leading edge of the continental margin of western orth America during a peri od of 

rapid, highly obl ique northeast moti on (with re pcct to the 

orth America pl ate) of the Kul a and Farallon pl ate that 

was initiated in the Paleocene (fig. 65) (Well s and other , 

19 4; Snavely, 1987). 

The we tern boundary of the oast Range oceanic 

basement is a north-south-trending fault on the Oregon continental sh If that has been delineated by aeromagnetic 

data, seismic refl ec tion, and deep tes t well (table 17). Thi 

fault was named the Fulmar fault by Snavely, Wagner, and 

Lander ( 1980) and is interpreted to have at lea t 200 km of 

dex tral-s lip moti on, based on the j uxtapo. ition of lower 

ocenc turbidite sandstone we. t of the Coast Range oceani c ba cment. 

The Fu lmar fau lt is thought to be the ' es tern boundary of the oblique marginal rift ba ·in that i floored by the 

Paleocene to lower Eocene tholeiitic pil low ba alt. Along 

with other dex tral faults farther to the east, it formed a trantcnsional zone of right-lateral shear along the ocenc continental margin of Oregon and western Wa hington. The 

major movement along the Fulmar fault most likely ended 

by late Eocene time becau eon the Oregon shelf, thi fault 

i overl app d by . trata of lat st Eocene age (about 37 Ma) 

( navely, Wagner, and Lander, 19 0). However, minor 

mo ement ha. occurred subsequentl y, as overl ying strata a 

young a. Pleistocene how small verti ca l offsets on sci -

mic-reflection profiles. 

BASI FILLI G A 0 OEFORMATIO 

Within th rift basin, a 3,000-m-thi ck seq uence of silt-

. tone, turbidite andstonc, and conglomerate of earl y Eocene 

age was depo ited adjacent to the vo lcanic high area . The 

configuration of the ba in wa likely inherited from the relief 

on the Si letz Ri ver and rescent vo lcani c pil e because nearhare deposits a old as early Eocene unconformably onl ap 

these volcanic highs (Bukry and Snavely, 19 8; navely. 

199 1 ). Convergence between the Kul a-Farallon and orth 

America plates in late early Eocene time (about 52 Ma) 

deformed the island chain and overl ying marine edimentary 

rocks again t the orth A meri ca buttrc . In the southern 

part of the Oregon oast Range. a et of southeast-dipping 

imbricate thrust heets and fault-propagati on fold deformed 

the Paleocene and lower Eocene oceanic basalt. and overlying lower Eocene edimcntary rock (Baldwin, 1974; Heller 

and Ryberg, 1983; M olenaar, 1985; Well. and Heller, 1988; 

iem and iem. 1990). In the central part of the oa t 

Range, the Si letz Ri ver Volcani cs were faulted and folded 

prior to rapid down warping and the clepo ition of the overl ying mildl y deformed Tyee Formation of earl y middle Eocene 

age (Sn avely and other . 1976a; Bu kry and navely, 19 8). 

Thi compres. ional event also deformed and uplifted 

continental terrane adjacent to the rift bas in and accelerated 

erosion of pre-Terti ary rock in the Kl amath M ountain to 

the south and on Vancouver I land to the north . Debri flows 

and conglomerate derived from the Kl amath M ountain 

occur along the southern margin of the Coa t Range basin . In 

the southern Oregon part of the forearc basin , 2,000 m of 

Figure 64 (facing page). Generalized onshore geologic map of 

western Oregon. we tern Washington, and . outhern Vancouver Island, British olumbia. with inferred locations of major faults on 

the adj acent continental . helf. Offshore faults concealed by younger strata are indicated by sh rt da hes. Outcrops of middle Miocene 

basalt are not shown in the Willamette Valley; upper middle 

Eo ene volcanic rocks arc not shown in the eastern part of Pugct 

Sound. M odified from Snavely ( 1987). 
44 ° 

0 

CE OZOI EYOL TIO OF THE CO Tl E TAL MARGI 1 OF OR EGO A D V ASH I GTO 

15 

a 

('") 

CT1 

)> 

z \ 

\ 

50 KILOM ETERS 

I 

I 

t 

I 

J. 

I 

I 

\ 

t 

I 

't 

\ 

~ 

I 

EXPLANATION 

Cenozotc sedtmentary rocks 

Cenozotc Cascade arc volcanic rocks 

Mrddle Mrocene basalT flows and sills 

Late Oligocene to mrddle Miocene melange 

and broken formauon 

Eocene melange and broken formation 

Middle 10 upper Eocene alkalic basalT 

Middle Eocene turbrdrte sandstone 

Paleocene 10 lower Eocene oceanic basalts 

Pre-Tertiary rocks 

Inferred western limit of Oligocene and 

Miocene melange 

Fault- Dashed where concealed; arrows show 

strike-slip displacement 

Thrust fault- Dashed where concealed; 

sawteeth on upper plate 

Syncline 

Oflshore exploration well-showrng number 

(table 1) 

163 
164 ASSE SI G EARTH QUA KE HAZA RD D RED CI G Rl K I TH E PA IFI NORTHWEST 

arko ic, lithic, and feld pathic turbidit sand tone and ittstone of the lower middle Eocene Tyee Formation (Snavely 

and other , 1964; Lovell 1969; Baldwin, 1974; Chan and 

Dott, 19 3) wa deri ved in part from an uplifted Kl amath terrane and from the Idaho batholith and adjacent terrane 

(Heller and others, 1985), buried preex i. ting olcanic high., 

and deformed lower Eocene sedimentary rock . Thi edimentary equence laps aero the tectonic boundary between 

the Klamath pre-Tertiary rocks and the Eocene oceanic 

ba aiL to the north and indicates that the oa t Range oceanic ba ement wa accreted to the orth Ameri ca plate in 

earl y middle Eocene time (about 50 M a). Thick-bedded 

lithic quartzo e andstone and conglomerate of earl y middle(?) Eocene age in the Olympic M ountains core rocks 

(Tabor and Cady, 1978a: Snavely and others, 1986) repreent materi al eroded from an uplifted pre-Tertiary terrane of 

Vancouver Island and the northern Ca cade Range. In the 

Puget Sound area, stream tran ported large quantities of 

arkosic and lithic and into the northern part of the ba. in 

across broad low-lying flood pl ains. The coal-bearing continental flu vial depo it. , repre ented by the lower part of the 

Puget Group (Wolfe and others, 196 1 ), the Chuckanut Formation (McLellan, 1927; Johnson, 1984), and the Swauk 

Formation (Ru sell, 1900; Tabor and other , 1984), formed 

in lowl and area . Part of thi s coar e cia tic material may 

have been tran ported we tward through submarine channel 

into the deeper parts of the ba in to form turbidite fan . Some 

highl y deformed lower and middle Eocene thick-bedded carbonaceou , lithic, and arkosic sandstone units within the 

Olympic M ountain core rocks (Tabor and other , 1972; 

Tabor and Cady, 1978b) and in the ape Fl attery, Wa h. , 

coa. tal area (fig. 64) (Snavel y and other , 1986) may be 

marine counterparts of these continental deposits. 

LATE MIDDLE EOCENE TO LATE EOCENE 

CONTI ENTAL-MARGIN MAGMA TIS 1 

Tholeiitic volcani m continued along the continental 

margin between 44 and 37 M a, although the maj or volcanic 

center became re tricted to outhwes t Washington and the 

northern and central Oregon Coast Range (M acLeod and 

Snavely, 1973). The lavas were erupted from regional dike 

warm. that trended northea t before rotation. Thi orientation ugge ts a transtensional relation hip to the ri ght-lateral 

Fulmar fault (Well and others, 1984). The ba. alt are 

chieny subaerial; however, pill ow lava and brecc ia in the 

lower part of these units imertongue laterall y with deepwater siltstone of the upper middle Eocene Yamhill and the 

upper Eocene Ne tucca Formation (Snavely and others, 

1990b). The. e ba alt nows and breccia include the Grays 

Ri ver volcanic rocks of southwe t Washington (Li vingston, 

1966; Wells, 198 1; Wal h and others, 1987) and the 

Till amook Volcani cs in north wes t Oregon (Warren and othrs, 1945: Snavely and other , 1969; M agill and others. 

198 1; Well. and others, 19 3; iem and 1iem, 1985). They 

N 

r 

KULA PLATE 

I 

\ 

\ 

\ ~ 

\ PLATE 

RA~L:~ \ 

100 200 KILOMETERS 

I I 

Fulmar Fault 

Figure 65. Diagram showing inferred cont inental-margin-ri fting 

model for the origin of the Paleocene to lower middle Eocene oceanic basalt (x pattern), oceanic i land (circul ar area. with black 

ubaeri al cap. ), and continental crust (diagonal pattern) in western 

Oregon and Wa hington. Large black arrow denotes directi on of 

movement of the Kula plate relati ve to the orth America plate (Engebretson and others, 1985); large open arrows show local . preading 

direction. Heavy lines are faults, with . awteeth on upper plate and 

tick. on downthrown block: aiTows show lateral movement. Ri vers 

and marine ediment. (stipple pattern ) are shown schematically. 

were followed cl o ely by the Yachats Basalt ( navely and 

M acLeod, 1974) and the basalts of Cascade Head (Snavely 

and Voke , 1949; Snavely and other , 1990a) in central 

coa tal Oregon. The volcanic rocks are commonly porphyriti c and range widely in compo ition , from tholeiitic to alkalic basa lt, ba altic andesite. and dacite. Thi s compositional 

range suggests that the ba altic magma that produced these 

volcanic rocks wa differentiated in high-level magma 

chamber before ex trusion. 
CE OZOIC EVOLUTIO OF THE CO T I E TAL MARGI OF OREGO NO WASH! GTO 165 

Table 17. Locations, depth , and oldest rock penetrated in exploratory well drilled on the Oregon and Washington continental hel f 

and selected wells drilled on the shelf off outhern Vancouver I sland, British Columbia. 

Location 

Well no. 1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

II 

12 

13 

14 

15 

Company and 

name of well 

Pan American Oil Co. 

o. I , P-0 11 2 

Union Oil Co. 

Fulmar P-0 130 

Shell Oil Co. 

P-087 I ET - 2ET 

Union Oil Co. 

Grebe P- 093 

Standard Oil Co. 

autilus No. I 

P0-0 103 

Shell Oil Co. 

P-072 lET 

Shell Oil Co. P- 075 I ET 

Shell and Pan American 

Oil Co. P-0 150 

Shell Oil Co. 

P- 0 155 l ET 

Union Oil Co. 

Tidelands State o. 2 

unshine Mining Co. 

Medina o. I 

Pan American Oil Co. 

P- 0 141 

hell Canada Ltd. 

Anglo Cygnet J- 100 

Shell anada Ltd. 

Anglo Prometheus 

H- 68 

hell anada Ltd. 

Anglo Zeus I- 65 

cc figure 64 for mapped locations of wells. 

Year 

drilled 

1967 

1966 

1965 

1966 

1965 

1966 

1966 

1966 

1967 

1962 

1962 

1967 

1969 

1967 

196 

BA I FORMATION A D SEDIMENTATIO 

Latitude 

( orth) 

43°04.8' 

44°03.6' 

44°13.3' 

44°29.8' 

44°5 I .5' 

46°02.8' 

46°09.1 ' 

46°43.5' 

46°51 .2' 

47°03.22' 

47°03 .03' 

47°39.7' 

48°1 9.67' 

48°48.9' 

48°54.6' 

A lthough differen ti al upli ft occurred in the areas of ubaeri al volcanism . uch a the Heceta Head-Cape Perpetua 

area on the central Oregon coa t (Snavely and M acL eod, 

1974), regional downwarping occurred in mo t places in the 

forearc ba in . Bath yal thin -bedded siltstone and andstone 

Longitude 

(West) 

124°35.6' 

124°38.8' 

124°28.2' 

124°24.9' 

124°16.7' 

124°29.9' 

124°24.5' 

124°21.3' 

124°24.5' 

I 24°12.8 I ' 

124°10.02' 

124°47.5' 

125°43.96' 

125°39.6' 

126°09.15' 

Total depth 

(meters) 

I ,873 

3,744 

2,546 

3,05 1 

3,849 

2,505 

3,097 

4,017 

3,402 

1,546 

1,262 

3,160 

2,460 

2,335 

3,042 

Olde. t rock 

penetrated 

Lower Eocene arkosic wacke 

(Snavely and others , 1982) 

Lower Eocene arko ic wacke 

(Snavely and other , 1982) 

Lower(?) Eocene basalt 

(Snavely and other , 1982) 

Lower(?) Eocene basalt 

(Snavely and other , 1982) 

Middle to upper Eocene 

basalt(?) (Snavely and 

others, 1982) 

pper Oligocene to middle 

Miocene melange(?) 

Middle(?) Eocene diabase si ll 

Upper Oligocene to lower 

Miocene melange and 

broken formation 

Upper Oligocene melange 

(Snavely and Wagner, I 982) 

Lower to middle M iocene 

melange and broken 

form ation 

Lower Miocene melange 

(onshore well produced 

12,000 barrel s of oi I) 

Middle to upper Eocene 

melange (Snavely and 

Kvenvolden, 1989) 

Lower to middle Miocene 

(Snavely and Wagner, 198 1; 

Should ice, 197 1) 

Lower(?) Eocene part of 

Crescent Formation 

(Shouldice, 197 1) 

Lower(?) Eocene arkosic 

wacke (Shouldice, 197 I) 

were unconform ably depo ited on lower Eocene oceanic 

ba alt and sedim ntary rocks. The geometry of po t-middle 

Eocene depo its wa controlled in part by the distribution of 

thick upper Eocene olcan ic . equence For example, a 

> e t-trending ri dge of the Yachat Ba alt separate the oo 

Bay ba in from the Newport embay ment to the north . Other 

coas tal ba in. appear to b fault con trolled. The Tillamook 
166 ASSES I G EARTHQ AKE HAZ RD A D REDUCI G RI SK I TH E PACIFI ORTHWEST 

embayment in north\ e. t Oregon i bordered by a major 

northwc !-trending en-echelon fault zone that tra erscs the 

entire Coa t Range (Well and others, 1983). 

In the northern part of the Olympic Pen in. ul a, the middle and upper Eocene ilt tone of the Aldwell Formation 

(Brown and other , I 960) overl ap Jder sedimentary rock to 

rest unconformably upon basalt of the lower Eocene re cent 

Formation. The ba al contact is marked in place. by beds 

I 0-30 m thick of obble and boulder onglomerate derived 

from the underlying Crescent Formation. In the north-central 

part of the Oregon Coa t Range, bathyal ilt. tone of the upper 

middle Eocene Yamhill Formation (Baldwin and others. 

I 955; Bukry and Snavely. I 9 8) unconformably overl ap. 

folded trata of the Tyee Formation to rc. t upon ba alt of the 

iletz River Volcanics (Well s and other , I 983; navely, 

1987). In the northern part of the Oregon oast Range, the 

Yamhill Formation intertongue with pillow lava. and breccia of the middle Eocene Tillamook Volcanic (Wells and 

others, I 9 3; Sna ely and others, I 990b). 

A water-laid dacitic tuff bed as much as 30 m thi ck 

occurs at or near the ba e of the Yam hi II Formation in the 

central part of the Oregon Coa t Range. Thi tu ff unit i 

locall y expo ed from Florence northward to Dolph, a di -

tance of more than I 50 km. The source of a h that form thi 

tuff bed wa mo t likely vents along an ancestral ascade 

arc. If thi interpretati on i con·ect, volcanism in the Oregon 

Ca cades tarted about 44 Ma. 

During the middle and late Eocene, trcams tran ported 

large quantities of arko ic and to a broad, low-lying coas tal 

pl ain that bordered the ea tern and outhern part of the 

forearc basin . Coal-bearing continental bed greater than 

3,500 m thick are repre ented by the Puget Group (Wolfe 

and others, I 96 1) and the Cowlitz and Skookumchuck Formations (Snavely and other , I 958) in Washington and the 

oaledo Formation (Allen and Baldwin, I 944; Dott. I 966) 

in southwe l Oregon. Some of thi coar c cia tic material 

und ubtedl y was transported al ng channe l into shelf 

basins and formed ·ubmarine fan and turbidite deposit . 

The e continental beds interfinger with both Cascade-arcd ri ed lava and with coas tal tholeiitic lava , thu tying the 

middle Eocene tholeiite to the continental shelf. 

FOREA RC DEFORMATION 

In the latest Eocene (about 37 M a), the forearc bas in 

was deformed across a broad front by obi ique convergence 

between the North Ameri ca and Farallon plates. Regional 

uplift that accompanied this event segmented and healed 

the forearc region to produce a number of shelf basin. that 

deepened wes tward. Eros ion of uplifted middle and upper 

Eocene ubaerial vo lcanic pile , such a the Tillamook Vol -

canic and the Yachat Basalt, produced thick deposit of 

fossi I i ferous basalt ic sandstone and conglomerate that 

fringed the ba alt highlands (Bukry and Snavely, I 988; 

na ely and other. , I 990a. b). In th · northern mo. t part of the 

forearc ba in. two contras ting styles of dcform ati n seem to 

support oblique . ubduction a the driving force. The first i 

north-south comprc sion, a. documented by major thru t 

faulting in the late middle ocenc along the rcsccnt and 

other maj r fault (fi g. 66) (MacLeod and other., I 977; Snavely and others, 1986). This transpression resulted from 

northward movement of the Olympic Mountains-Coa t 

Range Paleogene terrane again t the buttr·c. of pre-Terti ary 

rock of Vancouver I land . Resulting erosion of uplifted preTerti ary terrane on outhern Vancouver I land . upplied 

coar e cia tic debri to th northern part of the forearc basin 

to form the conglomerate-rich upper Eocene Lyre ormation 

(Brown and other , I 956: Snavely and other ·, I 986. 1989) 

along the northern Ol ympic Pen in ula. The second deformation tylc, which occurred at about the ame time, formed 

melange and broken formation of late Eo enc age that were 

underthrust along the we. t ide of the Olympic Peninsula 

(Snavely and Kvenvolden, I 989), impl ying ontinued subduction during n rth- outh com pre sion of the Coast Range. 

Sei mic-renection profil s and ubsurface data from 

deep te. t well on the Ore0 on continental . hclf (Snavely. 

Wagner, and Lander, I 980: Snavely and other. , I 9 2) al o 

how a regional unconformity at the base of upper Eocene 

lower Rcfugian iltstone. The e trata unconformably o erlie folded and faulted rocks as old a earl y Eocene and o erlap the Fu lmar fau lt (Sn avely, Wagner, and Lander, I 980: 

navely, 1984). l.o, in several areas in the northwesternme t part of the Ol ympic Pen in ula, strata of earl y Refu gian 

age on lap melange and broken formation of middle and late 

Eocene age ( navely and other , I 986). 

OLIGOCENE TO MIDDLE MIOCENE 

BA I DEYELOPME 'T 

Rapid ub idence occurred in the forearc basin during 

Oligocene and earl y Miocene time, and more than 2.500 m 

of bathya l tuffaceou iltstone and arko ic sandstone were 

depos ited in ax ial part of the basin. On the Oregon continental margin, these trata arc represented by tuffaceous silttone of the Alsea Formation of Oligocene age (Snavely and 

other , 1975) and the ye Mudstone of arly Miocene (Sauce ian Stage) age (Vokes and Snavely, 1948; navely and 

others, I 964). In the late Oligocene, high-energy deltaic 

depos its of pumiceou , coar e-grained sandstone such as the 

Yaquina Formati on (Snavely and Wagner, I 963: Goodwin. 

I 973) were depo ited in places along the eas tern margin of 

the ba in. In the deep margi nal Tofino-Fuca ba in along the 

north side of the Ol ympic Peninsula. more than 2,500 m of 

turbidite and tone and silt tone of the M akah and Py ht 

Formation were depo ited from the Oligocene to the earl y 

M iocene (Snavely and other., I 978; Snavely, iem, and others, I 980; Sn avely and other , I 986). The Makah Formation 
E OZOIC EVOL TIO OF THE CO Tl ENTAL MARGI OF OREGOt DWA HI GTO 167 

Figure 66. Generali zed geologic map of the region adjacent to Cape Flattery, Wash., howing I ati ns of major nshorc fault s and 

their offshore ex ten: ions. Ts, Terti ary sedimentary rock . . undivided: To. upper Oligocene ·and -tone and conglomerate: Tm, o crthru st 

pre-Tertiary to upper -ocene broken f rmation above Ozelle thru. t: Tc, highl y deformed Eo ene sandstone and siltstone and other rock~ 

of the Olympic core: Tv. Eocene pillow basalt, breccia. and asso iated dikes and ill of the Met hosin Volcanics (C lapp. 19 17) on Vancou cr Island and the e~cc nt Formation in Washington: pTim. pre-Tertiary igneou. and metamorphic rock , undivided. on Vancouver 

I. land and Point of the rchcs, Wash. Faults arc dashed ' here onccalcd: atTows show strike- lip displacemen t: sawtecth are on upper 

plate of thrust fault: bar and ball arc on down thrown ide of fault. M odified from MacLeod and others ( 1977) and unpublished maps by 

P.D. Snavely, Jr. 

includ . a major submarine landslid dcpo. it (the Jansen 

reck M ember) that is composed of sedimen t derived from 

. outhern Yancou cr I. land ( icm and oth r . 1989). Thi. 

bas in . hoa lcd in the early Mio enc, and near. horc coa l. 

sandston , and congl meratc of the lallam Formation were 

deposited (G wer. 1960; Addicot t, 1976). 

CO TJ E TAL-MA RG IN MAGMATI M 

Volcanic ac ti vity in the Ca. cade arc ontributed large 

quantities of ash and pumiceous tuff breccia to the f rcarc 

bas in . Cascade-deri ved Ol igocene mudfl w conglomera! 

and thick ( I 0 m) pumi ce beds occur a far we t a the pre. ent 

coa tline (Sn avely and other . 1975). Close to the late 

Eo ene to middle Oligocen strandlinc, whi ch lay ncar the 

present foot hi li s of the Oregon Cascade Range. andes iti c and 

daci tic tuff breccia and volcanicla ti bed arc complexl y 

in terca lated throughout much of the marine sequence. 

In the central Oregon oa t Range. mall vo lurnc of 

highl y evolved magma were erupt d and intruded into the 

marine trata. amptonites. nepheline yen ites . and 

ferrogabbros were emplaced between 34 and .)0 Ma. 

roughl y contemporaneous with the end of major late 

Eocene dextral lip faulting along the Fulmar fau lt and 

related fa ult to the cas t. This magmati sm represents the 

fi nal episode of ri ft-related magmati sm in the oa. t Range. 
16 A SESSI G EARTHQ AKE HAZARDS A D RED CI G RISK I THE PA IFIC ORTHWEST 

0~-----------------------------------------------------------------r 

0 5 KILO ~ETER 

------------------------------------------------------------------------------~ 

Figure 67. Geologic interpretation of a 24-channel scismic-renection profil e (USGS line W077- ll ) collect d off the central Orcg n 

coa. t near !at 45° . The famil y of cast-dipping thru t faults form fault-propagation fold that are considered to be normal fau lts reactivated during late middle Miocene tran pression. Qh, Holocene units; Qp, Pleistocene units; Tp, Pliocene units; Tmu. upper M iocenc 

unit. ; Tmm, middle Miocene units; Tml, lower Miocene unit s: To, Oligocene units; Te, late Eocene unit. ; Tidb, basalt now; Tim, basalt 

sill. Heavy lines are faults; arrows how direction of movement on thru. t faults; on vertical strike-. lip faults. T denotes movement toward 

viewer and A is movement away from viewer. Light line. are form line showing structure in the sedimentary sequence. Verti cal exaggerati on is approx imately 2: I . 

DEFOR lATIO 

Onshore deformation during the Oligocene and earl y 

Miocene occurred throughout we tern Oregon and Washington and was probabl y most intense in the Ol ympic M ountains. Pota sium-argon dating of low-grade metamorphi c 

rocks from the Ol ympic core rock by Tabor ( 1972) uggests 

regional uplift at about 29 M a. A mudflow unit of late Oligocene age that contains oli stos tromal block of basalt and 

graywacke deri ved from the Olympic core rock prov ide 

evidence of thi epi ode of uplift. The unit can be traced 

intermittently along the west side of the Olympic Peninsula 

from near Taholah north ward to the Point of the Arches, a 

di stance of more than 100 km (P.O. Snavely, Jr. , unpub. 

mapping, 1969). However, in the Tofino ba in along the 

north side of the pen in. ula, depo ition appears to have been 

continuous during the late Oligocene and early Miocene, 

with lenticular channel-fill deposits that contain conglomera! cia ts deri ved from Vancouver Island (Snavely. Niem, 

and others, 1980). 

AI ng the central Oregon coast, two tholeiitic ba alt 

unit. are interbedded with middle Miocene andstone and 

iltstone. The aide t- the Depoe Bay Ba. alt- i. petrochemicall y identica l to the Grande Ronde Ba alt, and the younger 

Cape Foulweather Basalt is petrochemica ll y identical to the 

Frenchman Spring M ember of the Wanapum Basalt of the 

Columbia Ri ver Basalt Group (Snavely and other , 1973). 

The Depoe Bay Basalt ex tends more than 16 km seaward and 

was penetrated in several of the tes t well s at depths of as 

much a 2,500 m (Snavely, 1984). The Cape Foulweather 

Basa lt, however, is re tricted to the inner shelf. Si lls and 

fl ows of the Depoe Bay Basalt are wide pread in the northern Oregon Coa t Range and on the continental shelf (Snavely and Well s, 1984; Snavely and M cClellan, 19 7; iem 

and oth r. , 1990). The tratigraphic and petrologic similarity 

between the coastal basalt and correlati ve units on the 

Columbia Pl ateau led some workers (Beeson and other. , 

1979) to uggest that the coa tal basalt are in vasive tongues 

of the Columbia River Basalt Group that erupted on the plateau. Thi s theory may explain most of the coas tal Miocene 

ba alt outcrop , although it is hard to expl ain intrusion of 

the Depoe Bay Basalt into volcanic rock as old as earl y 

Eocene in the central coa tal area of Oregon (Snavely and 

others, 1990b). 

Regional uplift in the forearc basin in the earl y Miocene 

(about 20 M a) restri cted marine deposition to the west fl ank 

of the Oregon Coa t Range and the adjacent continental shelf 

and to the Coos Bay, ewport, A toria, Grays Harbor, and 

Tofino-Fuca structural embaymen ts (fig. 64). ear hore deltaic and strandline and tone and silt tone depo its of the 

lower and middle Miocene Astoria Formati on (Cooper, 

198 1) grade westward into predomin antl y bathyal ilt tone 

in the deep marginal bas in off Oregon (Snavely and others, 

1982). A long the central and northern Oregon coast, the 
CE OZOIC EYOL TIO OF THE CO Tl E TAL MAR I OF OREGON A1 D W SH ! TO 169 

Astoria Form ati on re ts on strata ranging in age from late 

Eocene to early Miocene. In the deep marginal ba in on the 

Oregon continental shelf, ci mic-refl ec tion profi les and 

drill-hole data indi cate that sedime!llation was virtually con -

tinuous. and siltstone . trata correlative with the ye Mudtone and Astori a Formati on form a inglc ro k- trati graphic 

unit (Sna ely, Wagner, and L ander, 1980). 

On the Ol ympi c Penin. ula. lower and middle Miocen 

thick-bedded sand tone and thin-bedded siltstone and conglomerate of both helf and bathyal depositional environment. lap onto Eocene melange and broken formation (Rau. 

1975. 1979; Snavely and K venvo lden. 1989). The c dq osit 

were inten ely folded and faulted during pl ate convergence 

in the late middle Miocene. 

LATE MIDDLE MIOCENE TO PLIOCENE 

Regional deform ati on occurred in western Oregon and 

Wa hington and on the adja ent continental shelf in the late 

middle M iocenc, bet ween 15 and I 0 M a. Up! i ft of the oa t 

Range and Ol ympic M ountain formed highland areas that 

were rapidl y eroded and supplied large amounts of cia tic 

debris to elongate basin on the continemal shelf. On the 

Oregon inner he! f. strata as young a middle Miocene were 

folded and uplifted, trunca ted by eros ion. and sub equcmly 

downwarped and overl apped unconformabl y by upper 

Miocene . trata (Snavely, Wagner, and Lander. 1980). Uplift 

wa grcate. t on the Olympic Pen in. ula. perhaps partl y owing 

to i astatic uplift of the thick pri sm of melange and broken 

formation that wa ubducted during the late middle 

Miocene and partl y owing to northward moti on of the Coast 

Range. Alternatively, Brandon and Calderwood ( 1990) ugge ted that uplift of the core rocks may be a rcspon c to 

development of a fl ex ure in the subducting slab beneath the 

Olympic M ountain . 

Norm al faults on the Oregon shelf were reacti vated a. 

thru. t faults during late middle Mio cnc transprc .. ion and 

formed a famil y of land ward-dipping fa ult-propagati on fold 

(fig. 67). !though mos t fold. were truncated by the late 

Mi ccne unconformit y, mo em nt on som fa ul t. gentl y 

folded strata as young a Plei. toccnc. 

BASI OEYELOPl\IE 'T 

ln the late Miocene and Pliocene. episodic downwarping of a deep marginal ba. in off Oregon wa. virtuall y continuous. and more than 2,000 m of sand and ill ' ere 

deposited. ppcr Mio cnc and Pliocene deposit thin again. t 

the ca tern border of the marginal ba. in and again. ! older 

midshclf structural highs ( na ely. Wagner. and L ander, 

1980: Clarke and others, 198 1 ). hclf ba:ins formed landward of folded and thrust-faulted late ligocenc to late middle Miocene melange welts. On the central and southern 

Washington shelf, as much a. 2,000 m of upper Miocene and 

Pliocene sediment accumulated on a thick accretionary 

wcdoe of melange and broken formation of late 01 igoccnc(?) 

to late middle M io enc age (Rau, 1975. 1979; Sna cl and 

Wagner, 1982). The Miocene and Pliocene strata thin again. l 

growing anticlines or diapirs, th cor . f hich con. ist of 

upper Oligo ene to middle Mioc nc melange and brok n 

form ati n. Adja ent to the. e diapiri . tructur . , numerous 

unconformitie , growth fault . and gravit . !ide occur 

with in younger strata, all of which likely r fi e t episodic 

uplift (fi g. 68) (al o e Snavel and Wagner, 1982). 

Off northwes t Wa. hington and southern ancou cr 

Island, upper Miocene . trata re t unconformably on older 

Terti ary ro ks (Should icc, 197 1; navely and Wagner. 

1980). Although mos t strata of this ag ar rc. tricted to the 

hclf, upper Miocene shallow-water sandstone and iltstonc 

occur in a mall iso lated ba. in alono the we l sid of the 

Ol ympic Penin ula in the lower Boga hi I Ri cr alley 

(Rau, 1979) and in fault-bounded bl o ks al ng the c as t 

north of Taholah (Rau, 1970. 1975). ppcr Mio cne strata 

also crop out on the . outhcrn Oregon coa. l ncar oos Ba 

(Addi cotl. 1976; Arm ntroul. 19 0). 

In a fill ed trench along the base of the slope (fig. 69), 

about 3.500 m of strata o cur above an upper ioc nc o canic crust (Kulm and Fowler, 1974; navel , Wagner. and 

Lander, 19 0; Carl son and Nels n, 1987: navcly. 19 7). 

From analy i f the cismic vcloc itic of the cd im nts. it 

is es timated that a much a half of thi fill is of late 

Miocene and Pliocene age. 

PLEISTOCENE TO HOLO ENE 

Pleistocene and Holocene scdim -nt ex tend a r s 

most of the c minental shelf of Oregon and Washington, and 

eogenc strata arc exposed on th ca floor onl y on laro 

banks . uch a Stonewall , Heccta, and ehalcm (Kulm and 

Fowler. 1974; navely and Wagner. 19 0; navcly. Wagn r. 

and Lander. 1980; Clarke and others. 198 1; Carl son and N ion, 1987: navcly and M cClellan. 1987). The. c Qu atern ary 

deposit. of fi ne and and ilt are thickest (about 500 m) on 

the inner shelf and in ba in between c mpres. ional folds on 

the outer . helf and lower lope. S vera! unconformities 

occur within thi sequence. indicating epi odi c do' nwarping of the bas in a well a uplift of di apiric tructure during 

depo ilion (fig. 6 ). On the abyssal pl ain. more than 1,800 m 

of trala are mo. l likely f Plci toccne and H locene age 

(Kulm and Fowler, 1974; navely, Wagner. and Lander. 

1980; arl. on and el on. 19 7) 

Plei tocene uplift of the Ol ympic M ountain and the 

Or gon and Wa hington oa. l Range hed coarse cia. tic 

debri s that overwhelmed hemipelagic cdimcnt. along th 

ca. tern fl anks of the marginal shelf bas in . pli fted Pleistocene channel-fill depo its along the coa. l are a. much a 

60 m thick and contain bed of peat (Snavely. 1948). 

Glacioflu vial and and gravel from alpine glac iers in the 

Ol ympic M ountain and from the Fraser lobe (Easterbrook. 

1969) of the continental icc sheet were depo. itcd along the 

eaward margin of the Ol ym1 ic Peninsula and ex tend onto 

the inner shelf. 
170 

u.5 

::;;: 

UJV:: ~ 

>Cl 

<z 

"'0 

~--~ >-U'l 

<z ::: - 6 

3 

f0 TH 

SSESSI 1 G EARTHQUAKE HAZ RD D RED I G Rl K I 1 TH E PA IFI RTHWEST 

ORTH 

0 2 KILOMETER 

Figure 68. High-resolution single-channel sci. mic-rcfl ection profile ( G line WOS0- 1 ) across an asymmetri c di apiric structure off 

the mouth of 1he Columbia Ri ver, Oregon and Washington. Strata of Pleistocene age lap omo and thin against thi s growing structure. The 

marked unconformity on the north side of the diapiric structure refl ects epi sodic uplift of the core. which appea rs to be bounded by a growth 

fault on it. south side. ote the fault ncar the south side of the pr file th at offset. uppermost Plci. tocene strata but not sea-floor . cdimcnts. 

Verti ca l exaggeration i. approximately 12: I. From Snavely ( 19 7). 

Or--------------------------------------------------------------------------------------, 

0 10 KILOMETERS 

WEST E T 

0 Marginal ridge Second ridge _j 

UJ 

> 

UJ 

_, 

< i.U 

"'"' wo:: 

ow 

__. .... 4 ~:..!.l Qp 

:i3 2 

>0 <:d 

::: >£ 6 oz _,-

:.r.J 

"" J: 8 t 

UJ 

Cl 9 

Juan de Fuca plate Subducting scdirncnl 

Figure 69. Geologic interpretation of a migrated 24-channcl :,cismic-refl cction profil e (USGS line W076-5) across the lower continental . lop f central coastal Oregon showing filled-trench depos it. and a deformation front con. isting of a famil y of fault-propagation folds. 

Qu . Holocene unit. ; Qp. Pleistocene units;, Tp. Pliocene units; Tnw, upper Miocene oceanic crust; Tmo. Oligocene to middle M iocene 

melange and broken formation. In the lower panel. thrust faults arc indicated with sawtccth on the upper plate; arrows show direction of 

relative m vcmcnt. Verti ca l exaggeration i. 2.4: I in the upper panel. Lower panel from Snavel y and others ( 1987). 
CE OZOIC EVOL TIO OF TH E CO TI E TAL MARGI OF OREGON 0 WASHINGTO ' 17 1 

REGIONAL STRUCTURE 

The con crgent margin of Oregon and Wa hington con-

. i. ts of three maj or tec tonic element (fi g. 64 ): ( I ) the Coas t 

Range terrane. an oceanic basalt-floored terrane that compri ses mo t of the onshore region between Vancouver Island 

and the Kl amath M ountains and ex tend. westward beneath 

the inner continental shelf; (2) the Fulm ar terrane, a fault 

sliver on the outer continental helf, poss ibl y of continental 

deri at ion. that i empl aced sea \ ard of the oceanic terrane; 

and (3) the acc retionary a . . embl age. an imbricate . tructural 

complex of off craped and underplated edimentary rocks 

th at makes up the continental . lope and outer . hel f. The structure and arrangement of the e terranes refl ect a long hi tory 

of oblique convergence during whi ch margin-parallel trike 

. lip, oblique ex ten ion. and continental-margin com pres ion 

have all been a pan . The importance of dex tral- lip faulting 

along the Pacific orth wc t con crgcnt margin ha onl y 

recently been reali zed with the eli . co cry of offshor faults 

li ke the Fu lmar faul t ( na ely, Wagner, and L ander, 1980: 

Snavely. 19 7) and abundant paleomagnetic evidence for 

clockwi e rotati on of coa tal region · (sec Well and Heller, 

1988; and Well , 1989, for summary). 

COAST RANGE TERRANE 

The . tructurc of the oas t Range terrane i largely the 

result of the Paleocene to earl y Eocene oblique rifting of the 

margin (fi g. 65), > hich produced the oceanic ba alt ba cment of the terrane. and the ubsequent Eocene compresional event that folded the basalts aga inst the continental 

margin. On the ontincntal hclf, the tectonic boundary 

between the tholeiitic ba. alt Coa t Range ba ement and the 

pre-Tertiary conti nental ro ks i. exposed nca r Ros burg in 

the :outhcrn Ore0 on oas t Range and at the . outhcrn tip of 

ancouvcr I land (fig. 6-t). tl o cburg. the Pal o cne pillow basalt and ov rl y ing lower Eocene turbidite. are folded 

and th ru t southeas tward b 11 ath pre-Terti ary ac reted terrane. marginal to the Klamath M ountain. (Baldwin. 1974; 

Hel ler and Ryberg. 19 icm and iem. 1990). Th n nhcast-trending folds and fault. arc unconformabl y onlapp d 

by gently dipping middle Eo cne turbidites of the Tyee Formati on. thus limiting th time of ompre ion to about 50 

Ma. On Vancouver I sian I, lower Eo en tholeiitic ba. alt of 

the ctcho. in Form ation arc thru. t to the nonhea. t beneath 

the Leech Ri ver chi. t along the Lee h Ri ver fault ( lowe 

and others. 1987). The . chi. t has a late middle occnc potas-

. ium-argon c oling age of about 42 M a, whi ch has been 

interpreted as the time of accreti on of the oceanic terrane to 

the continent ( airchild and owan, 19 2). Thi s age is much 

younger than that inferred from . trati graphic relationship. in 

the . outh rn Oregon Coa. t Range and Kl amath M ountain. 

and implies a complex tec tonic hi story for the OrcgonWashinoton continental margin . 

The \ e t rn boundary of th occani ba. mcnt i. the 

Fulm ar fault on th outer continental shelf of Or gon. It is 

pre umed to ex tend at lea. t 200 km north-. outh on the ba i. 

of a linear aeromagnetic gradient (Bond and Zeitz, 1987) that 

Sn avely. Wagner. and Lander ( 19 0) int rprctcd to be the 

wes tem edge of Coa t Range ba altic ba cmcnt. ci micrefl ecti on profile and offshore deep test wel l ( navel and 

other , 1982) confirm that a . teep fault . parate. IO\ cr 

Eocene oceanic basalt n the east from Jo, er occne quartzose lithic and tone of continental . ource on the w t. Th 

Fulmar fault i interpreted to be a maj or Eoc n ri ght-lateral 

strike- lip fault that formed the we t 111 margin ran bliquc 

pull-aJ;> art ba in in which the oast Range ba. alts wer 

erupted (Well s and others, 1984; Sna el . 19 4, 1987). To 

the south, the fault intersect the coas t outh of oos Ba ju t 

n nh of Fi ve Mile Point and i in ferred :outh ard into 

northern Cali fomia either along the oquille Ri ver fault 

(Sn avely. 1987) or on the continental shelf ( Iarke. 1992). 

Interpretati on of multichannel seismic-rene tion profil e. 

indicate. that the fault ex tend along the middle . hclf northward to about lat 45° .. where di splacement may ha e b en 

tran. ferred toward the c a t to a comparable fault bounding 

the eastern margin f the Coast Range ba emcnt. However. 

the eas tern boundary of the oa. t Range ba cment is cover d by youn oer volcanic rocks of the a. cade ar . 

DEFOR A Tf01 OF THE COAST RANGE TERRA E 

Folds and fault that formed during accreti on of the 

oa t Range basement to the continent ha c . ystematic 

trends that ha c partl y controlled the structural grain of later 

deformation. Fold axe. in uplift · of pillow-ba alt bas ment 

generall y trend north as t in Oregon and north wc t in Washington, apparentl y refl ecting ubscqucnt, greater clockwi. c 

tectonic rotation of Oregon (70°) a compared to Wa bington (about 30°) (. ee Well s. 19 9. for a . ummary). Some of 

the e trans erse fold. and related thru t fault ha e been 

ac ti e into the late cnozoi and po ibl y arc boundari es for 

tc t nica ll y rotated bl ocks (Well s and Co . 19 5). In s uthwc t Washington. northwest-trending fold and thru. t faults 

ex tend ea tward into the wes tern Ca. cadc Range where they 

deform rock as y ung a Miocene ( navel and others, 

1958: Wal. h and others, 19 7). The northwe t-trending 

fold. are compati ble with moderat shortening of the forearc 

during oblique c nvcrgcnce of the Faral lon plate. In Oregon. 

northea. t-trcnding folds and boundary fault largely became 

inac ti ve after Eocene time and have been overprinted by 

cogenc north- to north west-trending fault and fold 

(Well. and other. . 1983 : Niem and iem. 19 5; navely and 

oth r . . 1990b) 

L ate enozoic north- outh com pre: ion of the oa, t 

Range-Ol ympic terrane is suggc ted by widely paced eastwes t r verse faults and related folds in rocks as young a 

Quaternary. These structure. include (among other. ) the 
172 SSES I G EARTHQ AKE HAZ RDS AND R D CI G RISK I TH E PA IFI ORTHWE T 

Clallam yncline and parallel, tight a ymmetric folds along 

the north flank of the Ol ympic M ountain (Brown and other , 1960; Gower, 1960). the Doty-Grays Ri ver-Columbia 

Ri ver fault zone in outhwest Wa. hingt n (Pea. e and 

Hoover. 1957; Snavely and other , 1958; Wolfe and M cKee, 

1968, 1972; Well . 198 1 ), and the ape Falcon fault zone in 

north we t Oregon ( iem and iem, 1985) that deforms middle Miocen flow of the Columbia Ri ver Ba alt Group. 

A coa. tal . y. tem of northwe !-trending dex tral fault 

and local conjugate northeas t-trending sini stral faults i coni tent > ith north- outh compre sion of the OregonWa hington oast Range terrane (Snavely and other . . 

1976a- c; Well s and oth rs, 1983; iem and Niem. 19 5; 

Well s and Coe, 1985). However, the mooth increa e in 

clockwi se rotation toward the coast (Well s, 1989) at. o ugge. ts that the coa tal region i under dex tral shear. probably 

a a r suit of long-term oblique ubduction along the plate 

boundary. Block rotation is probably accommodated by the 

abundant we. t-north we t- and northwe !-trending ini stral 

fau lt. mapped along the coa t (see Well s and oe. 1985 : 

Snavely and other , 1990a, b). The e faults cut the youngc. t 

unit (Pomona M ember of the Saddle M ountain. Ba. alt of 

th olumbia Ri ver Ba alt Group, dated at 12 M a) and common ly have well-developed subh ri zontal lickensides . 

Regionall y significant dextral faults ex tend eastward into the 

Yak ima foldbelt j ust ea t of the Ca. cade arc (Tolan and 

Reidel, 1989). Fault di spl acements are difficult to determine, 

but the in rca. e in tec tonic rotati on of the Columbia Ri ver 

Basalt Group toward the pl ate boundary (fig. 70) sugge. ts 

th at cumulati ve dex tral slip after 15 M a may exceed I 00 km 

between the coas t and the unrotated olumbia Pl ateau 300 

km to the east (England and Well , 199 1 ). Apparently, much 

of the late Cenozoic margin parallel component of oblique 

ubduction i. being converted to strike-s lip deformation and 

northward transport of the c ntinental margin . 

Definiti ve e idence for north-south tran pres ion during the Quaternary is documented in roadcut exposure 

along U .. Highway 101 on the ea t ide of M or e Creek, 6 

km ca. t of Port Angele . Wa h. Here. Oligocene iltstone 

bed of the Twin Ri ver Group are thru ·t northward over 

earl y(?) Pleistocene alpine drift (Brown and others, 1960: 

Snavely and other. 1978). Thi. poorl y bedded alpine drift. 

which is overturned and dips about 40° to the south, in turn 

i. unconformably overl ain by north-dipping (30- 50°). moderately well bedclccl . and, gravel, and till with scattered clas ts 

of Ol ig cenc silt. t ne eroded off the fault carp. Broadly 

channelized fl at-lying outwash gravel of the Fraser continental glac ier lob unconformab ly overlie the north warddipping outwash and till unit. In the Strait of Juan de Fuca. 

Wagner and Tomson ( 1987) used seismic-refle ti n data to 

map numerou west-trending folds and faults that in volve 

sed iments as you ng as Holocene. Th e . tructures refl ec t 

deformati on of a northward-mov ing oastal block against 

the pre-Terti ary Vancouver I. land buttress (Snavely, 1987). 

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\· E T LO GIT DE. IN DEGREES 

Figur·e 70. Clo kwise paleomao netie rotat ion of sample. of the 

Miocene Columbia Ri ver Basalt Group in regon and Wa. hingt n 

pi tted on an east-west profile. Rotati on · increase westward from 

the Columbia Plateau toward the orth America pl ate boundary, 

implying a driving force acting along the plate boundary. From England and Wells ( 199 1 ). 

The late Cenozoic north- outh compres ion i. compatible with compre. sion axe determined from present-cl ay 

cru stal earthqu ake and well-bore breakout. in the Pacific 

orthwest (Werner and other , 198 ; Zoback and Zoback, 

1989: Lud win and ther. , 199 1) and suggest. growing influence of the Pacifi c plate during late enozoic deformati n of 

western Oregon and Wa hington (Spence, 1989). 

FULMAR TERRANE 

The com po ition and ex tent of the Fulmar terrane can 

onl y be inferred from the seismic, aeromagnetic, and deepwell data. It is thought to be a continental fragmen t that ha. 

been eli placed north ward . seaward of the oast Range oceanic ba ement. It may represent a piece of southern Oregon 

or northern Cali fornia emplaced during regional dex tral-s lip 

faulting along the continental margin. The Fulm ar fault (fi g. 

64) conform to a regional pattern of latest Cretaceou. to 

Eocene right-lateral faulting throughout the Pac ific orth.-

wes t (Dav is and others, 1978; Ewi ng 1980; Tabor and others. 1984) and northern .-.ali fornia (Blake and Jay ko. 1986). 

Di spl acement on the Fulmar fault may be at leas t 200 km . 

ba ed on the northward ex tent of the Fulmar terrane; however, geochemica l similaritie between oa t Range basalts 

and age-equi valent lower Eocene tholeiite from the allochthonou Yakutat bl ock in the Gu lf of A laska (Davis and 
Plafker. 1986) sugge l that larger di. pl acement may be po.-

ible. Movement along thi s fault is contemporaneou with 

the formation of the Kul a pl ate (fig. 65) and it. rapid northward moti n wi th respec t to orth Ameri ca (Engebret on 

and other , 1985). The late Eocene end of motion on the Ful -

111 ar fault may correlate with the demi. c of the Kula pl ate at 

about 43 M a, or it may record the northward migrati on of the 

triple junction of the Kul a, Farallon, and 1orth Ameri ca 

plate. along the continental margin. M ore head-on convergence of the Farallon pl ate may have ended large-. ca l tran-

. current motion. and cau:ed thru. tin g of the marginal ba. in 

noor and its sedimentary cover beneath the continental margin. The younger apparent age for accreti on of the o canic 

basement in southern Vancouver I. land may refl ect proo rcsive accretion of the marginal basin to th continent in the 

wake of the northward-moving trip! juncti on. 

ACCRETIONARY-WEDGE TERRANE 

An imbri cate tack of thru t-boundcd folds of high! 

deformed Cenozoic sedimentary rock forms th outermost 

tectonic belt, the accreti onary wedge, of the OregonWashington convergent margin (fi g. 69). The maj ority of 

thi s assemblage i off. hore. and its tructurc is inferred from 

eismic-refl cction profi les and a few deep te. l well . Howe cr, in the Ol ympi · M ountains. the accreti onary a emblagc ex tend onshore, where it can be llldied ' ith 

traditional field techniques (for example. sec Stewart, 1974; 

Tabor. 1975 ; Rau, 1975 , 1979: Tabor and Cady, 1978a; Snavely and others, 1986; Sn avely and Kvenvolden, 19 9). 

The subducti on complex of the Ol ympic M ountains 

form s the core of a broad anticlinal uplift in whi ch imbricate 

a .. cmblagcs of E ccne to io enc turbidite rock. arc thru l 

beneath o eanic ba all f the rc. ccnl ormati on (fi g. 64). 

The boundary between the oceanic ba. em nl and the accretionary wedge i. a maj r thru. t fault that juxtapo. e middle('/) Eocene melange and brok n formation of the ceci le.-

Gray Wolf assemblage to the wc. t aga in t coherent strata of 

the peripheral rocks to the as t (Tabor and ady . 197 a; Snavely and thcr . . 19 6: Brandon and other . 19 ). Reflecti on 

and refraction profile. interpreted fr m the anadian lith -

prob program indicate that the. c Eocene trata underplatc 

the lower E ccnc M tcho in Formati on and pre-Terti ary 

rocks on outhcrn Vancouver Lland ( lowe and others. 

1987). The thrust-bounded sedimentary a scmblagc. arc 

younger to th southwe. t, and the upper Oli gocene to lower 

Miocene rock a sembl agc i. expo. ed along the coast (Rau. 

1975. 1979; Sna ely and Kvcnvoldcn. 1989). In the northwe. I Ol ympi c M ountain. , two coherent terranes \ ere thrust 

beneath the ba alts of the resc nt ormati on in the late middle Eocene- the Ozelle tcn·a n and the pre-Terti ary Point of 

th e Arches terrane (Sn avely and others, 1986). 

D WASHfNGTO 17 

TECT01 IC U DERPLATI G 

Ge logic mapping in accrctionar as. cmblag s f the 

wes tern Olympi c M ountain (Snavel and other. . 19 6. 

19 9: Snavel and K cnvolden. 19 9) and int rpretation of 

seismi profi les on the Vancou er Island margin ( navel 

and Wagner, 19 I; lowe and others. 1987 : Da i · and 

Hyndman. 19 9), on the Washington continental sh If ( navely and Wagner. 19 2). and on the Oregon ontin ntal 

. lope (Snavely and thers, 1985. 1987) indicate that . trata 

ranging in age from middle Eoc nc to Plioc nc ha c und rpl atcd older rock. along the con crgcnt margin . 

Reconnai sance geologic mapping along the wes t side 

of the Ol ympic Pcnin ula and intcq r tation of ~.: mi c profil es and . ub urface data in the Pan mcri an Oil o. P-0 14 1 

well (tabl e 17, well no. 12) on the . hclf led na I and 

Kvcn olden ( 19 9) to 1 cculate that the middle t upper 

Eocene alawah melange and broken formation undcrpl at 

the lower and middle Miocene rocks of the Hoh melange 

along the coast. Farther . outh on the middle . hclf off Grays 

Harbor. the upper Oligocene and middl Mi occ ~.: rocks of 

the Hoh melange arc interpreted from cismic-rcfl cc tion profil es to undcrplatc upper(?) Mi cenc and younger strata 

along a ma tcr . hear zone r d ' ollcmcnt as . hown on figure 

7 1. On the inner . helf. howe cr, the upper Oligocene and 

middle Mioc nc rock. of the Hoh melange appear to underplate middle Eocene ba alt(?) and middle and upper 

occne(?) . trata a. hown on the astern part of the profi I c. 

Diapiric structures that originate from ovcrprcs urcd 

melange developed below thi. Ill ga. h ar zone intrude the 

overl ying upper(?) Mio enc and Pliocene strata. land-sea 

geologic eros ection drawn ju t south of ra . Harbor 

( navely and Wagner. 1982) also show. th at the upper ligoccnc and middle Miocene rock of the Hoh melange 

undcrpl atc older ro ks ncar th oastlinc and may ex tend 

ca tward beneath Gray Harbor bas in. 

FA L T-PROPAGATIO I FOLD 

A D BLIJ D THR STS 

orthca. tward oblique ubduction of the Juan de Fuca 

plate ha. honened the accretionary-wedge trata younger 

than Miocene(?). producing a group of fault-propagati on 

fold . . Th c fault-bounded fold are well imaged on ei mic 

profile. acros the deformation front (fig. 64) along the continental . lope of Oregon and Wa hington. In Oregon. the 

vergence of these thrusts is ea twarcl . whereas along the 

vVa hingt n lope thevergencc i we tward ( navcl y. 19 7). 

The change in the direc ti on of ergencc occur ju. t south of 

the olumbia Ri ver. 

A west-trending multichannel ei. mic-rcfl ec ti on profi lc across the deformati on front at the ba e of the slope ff 

the mouth of the olumbia Ri ver (fi g. 72) wa gcnerou ly 

made available to P.O. Snavely, Jr. , by Exx n ompany. 
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Figure 71. Interpreted time secti on of migrated mul!i channel seismic-reflecti on profil e of the continental shelf off Grays Harbor, W ash. The upper Oligocene and middle 

Miocene Hoh melange is in ferred 10 underplate broadl y folded strata of late Mi ocene and Pli ocene age. Near the eastern edge of the profile, the Hoh melange appears to 

underplate middle(?) Eocene basalt and upper Eocene(?) strata. Qhp, Pleistocene and Holocene sediments, undivided; Tpm , upper Miocene and Pliocene strata, undivided ; 

T e, upper Eocene strata; T eb. middle Eocene basa lt. L arge arrows show directi on of tectoni c underplating. Heavy lines are faul!s; arrows show directi on of relati ve movement; 

T denotes movement toward the viewer and A is movement away from the viewer. L ight lines are form lines showing structure in the sedimentary sequence. Verti cal exaggerati on is about 2.6: I . 

WEST AST 

0 2 4 KILOMETERS 

Figure 72. Migrated multi channel seismi c-refl ection profi le across the deform ati on front on the lower continental slope off northwest Oregon. The fold-thrusi bell of uplifted abyssal 

strata of Pliocene and earl y Pleistocene age is underpl ated by an easi ward-thi ckening accreti onary wedge of I me Oligocene('l) and Miocene age. The top of the subducting Juan de Fuca 

plme ca n be traced from the base of 1he slope eas1ward for about 55 km . Qu, Quaternary sediments. undi vided; Q!U. T erti ary and Quaternary sediments. undi vided: Tp. Pl iocene s1rata : 

Tmu, upper Miocene strata; Tmb, Miocene oceanic basalt. Heavy lines are fau lts; arrows show directi on of rei ali ve movement: T denotes movemen t wward !he viewer and A is movement away from!he viewer. L arge atTows show directi on of undcrplaling. From Exxon Company. U.S.A .. w ith perm iss ion. 

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CE OZOIC EVOL TIO OF THE CONTI ENT L IARGIN OF OREGO AI D W SHI GTO 175 

.S.A .. for inclu. ion in thi report. Thi s profile clearl y 

i111 ages a group of fault-propagation fold on the continemal 

slope in strata of Pli ocene and earl y(?) Pleistocene age. Thi 

fold-thru. t belt is interpreted to be underpl ated by an ea !-

ward-thickening melange wedge of late Oligocene to late 

Miocene age. The decol lement between the Pliocene and 

Plei. toccne sequcnc and the melange i · sharply defi ned and 

truncates folds and faults in the upper plate. M os t likely. 

truncation of upper plate structure was by processe of subduction cro. ion as defined by choll and ther ( 19 0). The 

melange wedge ovcrli the gentl y dipping (about 3°), ubducting Juan de Fuca plate th at can be traced from the base 

of the lope eas tward for about 55 km. The thru st fault on 

the lower slope dip eastward . whereas the younge t thru. t at 

the base of the . lope at the west end of the profi le dip we twarcl. navcly ( 19 7) speculated that the change in direction 

of vergence of th ru st faults alono the contin ntal slope of 

Orcoon and Washington may have been controlled by the 

lope of the back ·top again. t w hich the melange wedge 

accreted. A steep back top is inferred to rc ult in seaward 

ergcncc. whcrca a gently dipping ba kstop result. in landward crgence. In figure 72. the tccp backstop formed by 

the strike-sl ip(?) fault ncar the a:tern end of the profile produced thrust. with s award ergencc, whereas the gentl y 

cl ipping backstop along the ba e of the slope produced the 

landward vergence of the westernmo t thru t. 

Fault-propagati on f I d. also occur ca t of the accretionary wedge on the Oregon continental shelf. where numerous 

blind thrust faul t off.et rocks a young as middle Miocene 

(fig. 67). These blind thrust die out in fault-propagati on 

fo lds, some of which gentl y warp trata a young as Pl eistocene. One . uch fold in the deep marginal ba in off central 

Oregon wa. the exploration target for the Standard Oil Co. 

Nau tilu. o. I P0-0 103 well (tab le 17, well no. 5). drilled in 

1964 (Snavely, Wagner. and Lander, 1980; Snavely. 1987). 

riti ca l to th timing of formation of the blind thru t 

and other structures on the continen tal shelf arc wide pread 

ubacrial basa lt flow. of middl io cnc age (about 16 M a) 

as. igncd to the Depoe Bay Ba alt ( navel and other . . 

1973). These rapidl y erupted flows can be traced' estward 

from the coa. t over a wide ar a of the Oregon on tinen tal 

shelf ( na ely and Well. , 198-t) and probabl fl owed onto 

the hclf fr m coastal ven ts during a global eu t<Hic low. tand 

(betwc n cy lcs 2.3 and 2.4 of . UJ ercycle TB2 of Haq and 

other.. 1987). Therefore. u ing the Depoe Bay fl ows as a 

time hori zon. an episode of major transprcs. ional tectoni s 

that occurred on the Oregon continental margin can be documented after miclcllc Miocene time and pri or to the regional 

unconformity at the ba. e of upper M ioccn trata (about I 0.5 

Ma). In addition, middle Miocene basalt . ill. and dikes arc 

common both nshor and off. horc ( i m and others. 1990). 

evcral sci mi c profi le. that cro. s the upp r continental 

. lope als indicate that the . hclf margin has co ll<IJ sed along 

wes t-clipping listri c faults (fig. 7 1 ). 1 aiTO\ ex tcn. ional halfgraben bas ins bounded by th . c fault. have been 

progres ivcl infillecl ' ith upper and 

Pliocene(?) sediment ( na ely and 

nconformiti cs betwe n the c . quen c. mo. l like! reflect 

epi ode of downslope movement along ba in-bounding li -

tric fault. owing to sed iment load ing. 

NEOTECTONICS 

The intense deformation recoonizccl in en zoic strata 

of the Or ,on-Wa. hington convcrg nt margin clear! 

record. a prolonged hi. tor of episo lie tectonism along th 

active plate boundary. The numcrou unc nf rmiti c. on 

growing tructure . . the record of giant modern and ancient 

submarine I ide . . uch a · the Oligocene Jan n reek M ember of the M akah Formation (Sna ely, icm. and other . 

1980; iem and others. 1989). and abund·mt d bri fl o, s in 

ba in-filling . cquenc all infer a hi. tory of . ci mic even ts. 

There i. ample ev idence that thi. cpi. od ic d formati n ontinues in the Quaternary. 

OFFSHORE STRUCTURES 

Sci. mic-reflccti on profi l s aero. the deformational 

front along the continental slope of Oreg n and Wa. hington 

. how that epi odic und nhru ting of the Juan de Fuca plate 

beneath the North America plate ha. produced a seri es of 

n nh- to nonh-northwes t-trendino elongate en-echelon anticl inal ridges bounded by thrust faul ts (Silver, 1972; arson, 

1977; na el . Wagner. and Lander. 1980; navely. 19 7). 

The. c ridg s have bathymetric ex pre .. i n. and they uplift 

Pleistocene ab : al cd imcnt. a much a. 1.100 m (fig. 69) 

(Byrne and others. 1966; ar on an I other . 1974; Kulm and 

Fo' lcr, 1974; navely, Wagner. and Lander, 19 0; Snavely 

and Wagner, 198 1 ). 

The profile off central Oregon (fig. 69) indicate that 

two lower slope ba. ins formed landward of anticl inal folds 

in Plci t ccnc strata. The larger ba.' in, which is near the base 

of the upp r lope. contain a much a 00 m of Quaternary 

sed iment. and an un onformity within these basin depo it . 

thus indi ating two di tinct period of uplift of the faultpropagation fold that bound the basin on the we t. Along the 

axi of the deep marginal shelf ba in off central Or gon, 

numerous unconformities o cur in the eogenc . equence. 

or particu lar interest i the fac t that the sea floor it. elf is al 0 

downwarped along the axi of the bas in. A family of northtrending fault along the eastern margin of the bas in off ets 

the sea fl oor and coastal terraced posit.. We peculate that 

transprc. ion acros the ba in epi od icall y wa rel ieved 

along thi .. et of faults to produce the tack d unconformitie 

along the axial part of the ba in (fig. 73). 

Other recent def rmation on the Oregon helf ugge ts 

a complex relation. hip between normal faulting and growth 

of diapirs. A high-resolution profile (fi g. 74) on the inner 
EAST 

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Figure 73. Hi gh-reso lu!i on seismic-refl ec1ion profile ex lending weslward just south of Newport , Oreg., across the ax is of the deep marg inal 

basin on the inner she lf. Stacked unconformiti es (best shown on the west side of the marginal basin) are due 10 episodi c periods of downwarping. The sea fl oor also appears to be downwarped, indicating a presentlranspressional regime. A 1/2-km-wide zone of faul!ing is present along 

the eastern margin of the basin. These north-trending faul!s are i nlerpretedto be dex tral stri ke-slip fa ults along whi ch the northeast-to-southwest 

transpression was re leased, perhaps resul!ing in recent small-magnitude earthquakes along this segment of the Oregon coast. Qh, Holocene 

stra ta; Qp. Ple istocene s1ra1a. The sea-fl oor mu l!i ple is an acousti ca l artifact that represents a repeat of the sea-fl oor acousti ca l signal. 

WEST 

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Figure 74. WesHrending hi gh-resoluti on seismi c-re fl ecti on profi le ac ross the inner she lf off north west Oregon. j us! south of 1he Co lu mbia Ri ver. The broad di apiric fold 

in the west-central part of the profi le is capped by pinnac le like features th at rise 5- 15m above the sea fl oor. Verti ca l dashed li nes are 5-minute seismogram time ti cks. 

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E OZOI EVOL TIO OF TH E ONTI E T L MARGI OF OREGON D W . HI GTO 177 

helf off northwe t Oregon how a broad 2- km-wide uplift 

of Pliocene(?) strata capped by a group of pinnaclclikc feature. th at ri se 5- 15 m above the ea fl oor. Ob ervati ons during a submersible di ve by L.D. Kulm (Oregon State 

University. written commun ., 198 ) indicate that th wall. of 

these features are nearl y vertical and that rockfall debri s is not 

pre. ent on the intervening flat floors. Al so, there apparentl y 

is little evidence of erosion that could have carved thi topograph , and the urface of some blocks have verti cal tri ati on 

(s licken ides?). A multichannel profile aero. s thi pinnaclecres ted fold indi cates that thi s fea ture is a faulted diapiric 

structure bounded on the ea t by a normal (,,rowth) fault. Our 

interpretati on i that the cres t of thi s di apir ha. been ubjected 

to ex tension over the 2 km fold, resulting in the development 

of horst and graben structures along the ere t of the fold. The 

lack of signifi ca nt erosion on the wa ll of the c blocks and 

the fac t that they have been upli fted above the level of the urrounding fl at eros ion urface, whi ch most l ikely was be eled 

during the late Pleistocene low. tand of ea level, all point to 

recent UJ!i ft and attendant ten i nal faulting. 

Multi hannel . eismic-retlecti n profil es off . outhern 

Vancouver Island arc imilar to tho. e off Oregon in that sediments of the ascadia forearc bas in have been uplifted a 

much as 600 m. The seaward cgmcnt of a profile that 

crosses the base of the slope clearl y show a major nonhea tdipping thru st fault (fi g. 75. fault A) that uplift virtuall y the 

entire ecti on of . trata above upper Miocene oceanic cru st. 

Fault A is overl ain by a small bas in that contains about 120 

m of upper Plci t ccne(?) and Holocene ediment . The 120-

m secti on of . ediment th at overli es fault A indicates that thi s 

fa ult ha. been inac ti ve incc latc('7) Pleistocene time. Thru tfault B. whi ch l ies seaward (.-outhwe t) of the principal fault 

A. also i. overl ain by a thi ck equcnce f late Plci. tocene and 

Holocene sediment . As the imbrica te thrust faults along the 

outer shelf and . lope of Oregon and Washington have 

migrated seaward ' ith time, the nex t maj or displa emcnt 

may be along fault B. Thi s relation of upper Pleistocene cdimcnt. depo ited o cr the trac of fault A in figure 75 and 

folded or tilted unconf rm itic. in lower lope ba. ins (fi g. 69) 

. uggcst. cpi odic rath r than ontinuous underthrusting at 

the base of the continental lope. 

ONSHORE FEATURE 

M aj or land. I ides . uch a those in the Ol ympi · ountain that eli idee! an icnt Lake resccnt imo the pre cnt 

two lakes- Lake Crc cent on the we. t and L ake Sutherland 

on the cast (Brown and others, 1960; Tabor. 1975)-may 

have been initiated by an earthquake during late Holocene 

time. Qui ll cutc Indian legend puq on s that during a great 

battle between the lallam and Quilleutc Indians. M ount 

torm King (on the south side of ancient Lake Crc. cent) 

became angry and took a great piece of roc k from hi crest 

and hurled it down the valley. killing all who w rc fi ghting 

(Tabor. 1975). !though th clebri that forms the older and 

maj or land !ide came largely from the north, a large slid 

a! o originated from the ridge ju. t ca t of M ount tonn 

King (Brown and other. , 1960). Th youthful nature of th 

older landslide. whi h i indicated by th Ia k of a thi k 

oil zone on the landslide dcbri and hummo ky topography with clo cd dcpre. sion that ontain little recent fill , 

ugge t that the older slide may have OCCUlT d rclati cl 

oon after the melting of the icc lobe that occupied the 

Lake re. cent-Lake utherland vall y. Studies n th Lak 

rcscent land. !ide clam by Logan and hu. ter ( 199 1) support a Holocene age for the land I id . They rep ned a 500-

to 550-year-old tree growing on r cks that form th clam. 

submerged tree on the we. t ide f the older landslide. 

apparentl y in an upright (growth) po. ition in Lak rescent, ha a l-l age of _,so yr. B.P. ( navely. 1987). However. thi s tree was probabl y transported into th lake by the 

younger . !ide. perh aps the one witnes eel by the Indian . 

Pleistocene and Holo cnc faults arc pre ent along the 

oa. tal zone and on the continental he! f. In the we. tern part 

of the 01 mpic Penin ula. upper Plci: to cnc glacial drift is 

hcared and tectOnicall y interleaved with siltst nc be I f 

Eocene age ( navely, 1983). nh f Ozette Lake, a 

Holoc nc . oil z neon upper Pleistocene outwa h gravels is 

offset a much as 2 m. To the southwest, on the continental 

she! f off Grays Harb r, Wash., sea-fl oor . edim nt are off. ct 

about 7 m by a trap-door type of fault (Snavely and other. , 

1977). On the inner . helf and coastal zone of central Oregon. 

a 75-km-long north-trending zone of : tccply dipping normal 

fault off. ct Holocene(?) sediments ( na ely, Wa5ncr. and 

Lander. 1980); onshore. the c fault offset upper Plci toccnc 

marinc-t rracc deposit. ( navcly and th rs. 1976a, c). 

The e faults are downthrown to the cast toward the uplifted 

Coa. t Range rath r than toward the offshore marginal basi n, 

as one would ex pt.:ct for . caward-vcrging thrust faults or 

grav ity faults. cveral earthquakes with M odified M ercalli 

intensities of Ill to IV have occurred in the vicinity of ewport (Berg and Baker. 1963) and may have been generated by 

movement along fault within thi zone. 

EARTHQUAKE POTENTIAL 

Although pre cnt . ismi c ac ti vity is low along the continental margin of Oregon and Wa hington, thi s regi n lies 

along a major ubducti on zone where the Juan de Fu a and 

North Ameri ca pl ate arc converging at 3. 5 em per year (Riddihough. 1977). In mo t subduction zone , epi sodic train 

r lea c has generated large earthquake. (Pi afker, 1972; Kanamori , 1977; H aton and Kanamori , 1984), but the absence 

of a well-defined megathrust along the Washington and Oregon convergent maroin has perplexed ge logists and 

eismologists. The have pro1 osed . cvcral models to 

account for the ismica lly quiescent pl ate boundary: 
178 ASSESS ! G EARTHQ AKE HAZARDS A D RED I G RISK I TH E PA IFI ORTHWEST 

SO THWEST ORTHEAST 

Figure 75. Twenty-four-channel seismic-reflecti on profile (U GS line 80- 7) ac ross the abyssal plain and lower continental slope off 

southwestern Vancouver Island, British olumbia. Landward-dipping thrust-fault A has folded and uplifted 700-m-thi k Pliocene and Pleistocene aby sa l sediment . This fau lt apparentl y has been inacti e during the Holocene because deposits of Holocene age in the small ba\in 

southwest of the thrust fold arc not offset by fault A. The ex istence of fault B i~1dicatc . that the deformation front at the base of the slope is 

migrating seaward. Velocities (kilometers per second) arc based on . onobuoy refract ion data. . sea-fl oor mu lt iple. Heavy lines arc faults. 

Line of x' s shows probable contact between Pliocene and Pleistocene deposit.. Vertical exaggerati on is about 3: I . 

I . Convergence has ceased between the Juan de Fuca 

and orth America pl ates. 

2. onvcrg nee is occurring along the plate boundaries 

but is ase ismi c becau c a young. warm , plastic ocean ic crust 

is being subductcd with ductile rather than bri ttle deformati on of the pl ate boundary. 

3. The Juan de Fuca and orth Ameri ca plate presently 

arc strongly coup led. form ing a . eismic gap along thi s segment of the northeast Paci fie Ocean. 

Geodeti c measurement. by Savage and others ( 198 1) 

indicated that cru stal train is accumulating in the Pugct 

Sound area and ha an average principal direc tion of contraction of . 7 1 ± 6° E. Thi s directi on of contracti on is in general 

agreemen t wi th the . 50° E. direc ti on of relati ve moti on 

between the Juan de Fuca and onh Ameri ca plates. It also 

agrees clos ly wi th the direction of tec tonic tran. port during 

the late middle Miocene period of plate convergence. as 

d duccd from drag folds in the upper pl ate of the Ozc!le 

thrust fau lt in the northwcstemmo t part of the Olympi c Penin sula (fi g. 66, unit Tm) ( navcly and others. 19 6). 

Land ward tiltin g in the Oregon and Washington Coa. t 

Range and the Olympic Peninsula has been documented by 

using tide gauges and by geodetic leveling of uplifted marine 

terraces (Adams, 1984). Thi s landward tilt, along with 

cru stal shortening shown by thrust faults and folds on the 

outer she! f and . lope of Oregon ( Kulm and Fowler. 1974: 

Seely. 1977; navely. Wagner, and Lander. 1980; Snavely. 

1987) and on the slope, continental shelf, and coa. tal zone of 

Washington ( ilvcr. 1972: Rau, 1975, 1979: Carson. 1977: 

Snavely and Wagner, 1982: Snavely and others. 1986; 

Well s. 1989). i. characteri sti c of many other subducti on 

zones where major thrust earthquakes have occurred. 

Detailed stratigraphic studi es of buried upper Holocene 

es tuarine deposit · in we ternmos t W ashington led Atwater 

( 1987; thi s volume) to conc lude th at co cismic ubsidcnce is 

responsible for their burial. At least ix epi odes of coseismic 

ubsidence may have occu rTcd in the last 7.000 year . . Individual episode. of subsidence ex tended for many ten of kilometer along the coast and at I as t 30 km inland, implying 

large Cascadia subduction-zone earthquakes. Similar in vestigations of coastal . alt marshc in Oregon by Peter. on and 

Darienzo ( 19 ) have cs tabl i hed late Holocene cpi odic tectonic subsidence that is interpreted as c idencc of abrupt 

strain release and intervening gradual strain accumulation 

along the . outhern Cascad ia subducti on zone during the last 

3,500 year . 

The epi od ic subsidence of coas tal lowland. and the 

evidence for epi odic thrust faul ting along the deformation 

front may both renee! intermittent coupling between the 

Juan de Fuca and orth Ameri ca pl ates. Presentl y. the thru. t 

interface may be . trongly coupled , and clasti c train may be 

accumulating across the continental margin (Heaton and 

Kan amori , 1984). Ba. eel upon their interpretati on of earthquake focal-mechani sm data. Weaver and Smith ( 1983) also 

concluded that the subducti on zone i. locked. As there is no 

histori ca l record of large, shallow earthquakes along the . ubducti on zone, a 900-km-long seism ic gap seems to be pre. ent 

along the subducti on zone off Oregon and Wa hington- the 

most remarkable gap to be found in the circum-Pacific sei. -

mic belt (Heaton and Kanamori , 1984). De pite the fact that 

present-day seismic ac ti vity is low, the potential for a maj or 

subducti orHypc earthquake cannot be di counted. 
CENOZOIC EVOLUTIO OF TH E Ot Tf E T L MARGI OF OREGO A 0 WA HI GTO 179 

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523-540. 
TECTONICS OF THE WILLAMETTE VALLEY, 

OREGON 

By RobertS. Yeat , 

1 Erik P. Graven, 1 • 2 Kenneth S. Werner, 1 • 3 Chri Goldfinger, 1 and 

Thoma A. Popowski 1 

ABSTRACT 

The Will amctte Valley is a lowland separating the Oregon Coast Range from the Ca. cade Range. Three separaLe 

ba ·in wi thin this lowland were studied: the southern Willameue Valley south of and including the Salem and Waldo 

Hill , the northern Will ameuc Valley between the alem and 

Waldo Hill s and the Chehalem M ountains, and the Tu alatin 

ba in northeast of the hehalcm M ountains and sou thwe t of 

the Tualatin M ountain . 

The rock of the Will amette Valley are. imilar to those 

of the Coast Range, beginni ng with oceanic ba alt of the 

Siletz Ri ver Volcanics f earl y and middle ocene age and 

deep-water turbidite strata of the Tyee Formation of middle 

Eocene age. Overl ying . trata of late Eocene and early Oligocene age grade we. tward from volcanogenic rocks to 

deep-water sedimentary rock . . . howing that arc volcani. m 

cas t of the Will amette Valley may have begun a earl y a 47 

Ma but not as earl y as 50 M a, the age of the Tyee Formation. 

1 on marine and marine strata as young as earl y Miocene 

were tilted westward pri or to 16- 14. 5 M a. when nows of the 

Col umbia Ri ver Ba all Group mo ed through a lowland in 

th a. cade Rang , across the northern Will amette Valley, 

and thence to the coa t a. intracanyon flow . . The olumbia 

River Ba. alt Group i overl ain. locall y with angular unconformity, by flu ial dcpo it. of the pr to-Will amcttc Ri ver 

and it. tributaries. the fir t . trata t be limited to the modern 

Willamcttc Valley. These dcpo. its arc poorly dat d but may 

range in age from late Miocene to Plei. toccnc. In the northern Will amettc Valley. these . trata are overl ain by ven ts and 

imruded by small stocks of the Boring Lavas. After a peri od 

of ero. ion, the flu vial dcpo. its were . uccccded by glacialou twash depo its of the Rowl and Formati on and by catastrophic fl ood deposits of the Willamcttc Formation, both of 

lat Pleistocene age. 

1

Dcpanmcn1 or coscicnccs. Oregon 1a1c nivcrsi1 . orvalli s. OR 

97331. 

2Unocal orp.. nchoragc. A K 995 19. 1Un cal orp .. Lafayette. LA 70505. 

Fault and fold. began to d velop in Eocene time, 

accompanying clockwi c rotation of ru stal bl cks. M o t 

prominent of these earl y . tructurcs i. the orvalli. fault. a 

low-anolc thrust fault with hori z ntal scparati n stimatcd 

to be 11 - 13 km . Fault. and folds affecting the lumbia 

Ri ver Basalt Group and younger flu vial depo it 

high-angl reacti va ti on of the or alli s fault. 

Creek fault. the HatTi . burg anticl ine. the ill reck fault , 

the Wald Hills range-front fault. the Gal s reck-M ount 

A ngel tructural zone. the Yamhill- hcrwood tructural 

zone. the torthcrn Willameuc downwarp, the Bcav non 

and Helveti a faul ts in the Tualatin va lley and faults at the 

northern margin of the Will amctte all y probably related 

to emplacement of Boring Lava . 

Fau lt s po tdating the Columbia Ri ver Ba alt Group 

trend predominantly northwc. t and northeas t. and f Ids trend 

predominantl y cas t-we t, compatible with the modern . tres 

field in which max imum hori zontal com pre. i c trcs. is oriented north- outh . Limited evidence su,sge ts rel ati vely I w 

slip rate . . probably lc . than 0.5 mm/ycar. lndi idual faults 

arc relati vely short . but brittle cru. tmay ex tend to depths a: 

great as 30 km , indi ca tin.s a capability of generating moderate-size earthquake. with long recurrence interval. . 

INTRODUCTION 

The Willamette Valley i part of a broad lowland eparating the Oregon ast Range from theCa cade Range (fig. 

76). The lowland i 120 km long and ex tends north from 

Eugene, Oreg .. to about 0 km north of Vancouver, Wash. 

The lowland is more than 0 km wide at the latitude of Portland. where it include th Portl and and Tualatin bas in ·, and 

onl y 30 km wide in the southern Will amette Valley south of 

Albany (fi g. 77). The lowl and contains four metropolitan 

area. , Portl and-Vancouver. alem, Corvalli. -Albany, and 

Eugene-Springfield. and many smaller town . . The lowland 

is di vided into separate basin by narrow ridge underl ain by 

the iocene Columbia Ri ver Basalt Group (fi g. 77). The 

Tualatin M ountain (P rtl and Hill. ) separate the Portl and 

I 3 
184 A SE SI NG EARTHQ A KE HAZA RD A D REDUCI G Rl K I TH E PACIFIC ORTHWEST 

50' 

45' 

40' 

0 

PACIFIC 

PLATE 

100 200 

130' 

' ' ' '• ' ' ' ' 

~ EXPLORER ' 

PLATE _,.,.. _...:. 

:<,....... \ 

"t>-~'0'-"' \ ...- ~~>" T 

\ 

\ 

\ 

\ 

r 

e> l 

I 

'ci, I 't·,. 

I 

'&_I 

I 

125' 

JUAN DE FUCA 

PLATE 

:z. I 

g ... 

GORDA 

SOUTH 

PLATE 

MENDOCINO 

FRACTURE ZONE 

400 KILOMETERS 

I 

('> 

J:> 

(/) 

() 

120' 

J:> 

0 

m 

-----...._/ 

_ ___....... 

,6. 

ORE GO 

:n )> 

z 

C> 

m 

-------- CALJFOR lA 1- ,6. I 

I 

\

z 

~ 

~e; 

\)> 

I 

I 

I 

' ' 

Figure 76. lla1c boundaric of the Pac ific Nonhwcs t showing locati ons of te ·tonic features and the Wil -

lamettc Valley tud y area. Heavy line. study-area boundary; hatched area. Willamcttc Valley and Pugct 

lowland; sawtecth denote upper plate of thrust fault. M ajor stratovolcanoes arc shown by open tri angles. 

Dot labeled .. M .. in northwestern Oregon is the Mist gas fi eld. 
124' 

45' 

44' 

TECTO l S OF TH E W!LLAMETTE VALLEY. OR EGO 

0 Tillamook 

,_ 

(/) 

<( 

0 

u 

OREGON 

TILLAMOOK 

HIGHLANDS 

EXPLANATION 

D Lowlands of Willamette Va lley 

123' 

WASHINGTON 

,

001 0 Vancouve r 

' ll/)16 

!?!!toer 

~ 

NORTHERN 

WILLAMETTE 

VALLEY 

0 Portl a nd 

PO RTLAND 

BASI N 

50 KILOMETERS 

D Upla nds 

Figure 77. Geographic and phy. iographi c features of the Willamett e Vall ey and P rtl and and Tualatin basins, 

northwe. tern Oregon. The squ are indicates the epicenter of the March 25, 1993. Scotts Mills earthquake. 

I 5 
186 AS ESSJ G EARTHQ AKE HAZARDS D RED CING Rl K I TH E PA IFI ORTHWE T 

and Tualatin ba. in. , the Chehalcm Mountain cparate the 

Tualatin ba in and the northern Willameue Valley, and the 

Salem and Waldo Hill eparate the northern Will amelle 

Valley and outhern Willamctte Valley. 

With re pect t the Cascadia ubduction zone and Cascade volcanic arc, the Willamette Valley ha the ame structural position as the Puget lowland (fi g. 76), but the two 

lowlands are not connected (fig. 77). orth of Vancou er 

and . outh of Eugene, cast-d ipping rock of the Coast Range 

abut directl y again t and are overl ain by rock · of theCa cadc 

Range. The pre-middle Miocene tratigraphic sequence 

underlying the Will amette Valley is similar t that of the 

oast Range, the upper part of the ection containing a 

higher percentage of volcanic and vo lcaniclastic rock s than 

the lower part, refl ecting the proximi ty of the alley to a -

cade arc volcanoe . 

The structure of northwestern Oregon i dominated by 

a broad, north-plunging anti clinorium centered over the 

Coa t Range. The we tern fl ank of thi. anticlinorium, including the Oregon coast, on tain strata of the same age a those 

of the eastern fl ank dipping into the Willamette Valley. The 

. trata dip ea l acros. the Will amettc Valley and into the western Cascade Range . . o that mo ·t of the trata that predate the 

Columbia Ri ver Basalt Group of the Will amette Valley are 

older than rocks of the we tern Cascade Range. The north 

pl unge of the anticlinorium permits correlation of strata as 

young a Miocene acros the Coast Range near the Columbia 

River ( icm and i m, 1985). Smaller scale . tructurcs 

include fau lts and open fold. in both the Coa t Range and 

Will amette Valley . Some of these structure involve the 

Miocene olumbia Ri ver Ba alt Group and younger trata, 

but other structures formed mainly in Eocene time. 

The oldc t expo cd rock: in the region are the Siletz 

River Volcanics, which are basalt flow and breccias of earl y 

and middle Eocene age. The lavas are similar in com1 o. it ion 

to ocean-ridge or oceanic-plateau ba alt and are interpreted 

to be a result f rifting and cx ten ion of an elongate ba in 

pri or to the depo ition of the Tyee Formation of middle 

-ocenc age (Well and others. 1984). The Tyee Formation, 

derived from pre-Tertiary plutonic rock s and a volcanic-arc 

t rrane to th . outh. prograded northward along a basin axi 

c mered on the ast Range. The Tyee consist. of di tallllrbidi tes in the latitude of the southern Will amettc Valley 

( han and Dott , 19 3). 

Th lower part of the upper Eocene Yamhil l Formation, 

largely fine grained, overl aps the Tyee Formation to re t 

directl y on th Siletz River Volcanic . . It i the oldest formati on in the Will amettc Valley that grade eas tward into volcani c rocks pos. ibl y related to the earl y western Cascade 

Range (Baker, 198 ). In the nonhem Coast Range, the Yamhill i overl ai n by and possibly interbedded with the 

Till amook Volcanics (Well s and others, 1983). In the Willamcttc Valley, the Yamhill is overlain by the . and-ri ch 

upper Eocene Spencer Formation. whi ch grade northward 

into the Cowlitz Formati on and . outheas tward into the 

volcani c-ri ch Fi sher Formation. The overl ying marine 

Eocene and Oligoccn ugcn · Formati n and the marine 

and nonmarine Oligocene and earl y ioccne S ott. Mills 

Form ation grad southca. tward into vo lcanic rock s of the 

western Ca. cade Range and rest unconformably on western 

Cascade olcanic r ck . The equi val nts of the Eugene and 

Scotts Mill. formations on the coa t arc the A lsea and 

Yaq uina Formati ons. 

About 16- 14.5 M a, basa lt flows of the Columbia Ri ver 

Ba. alt Group mo ed through a lowland in the ascade 

Range between the olumbia Ri ver and the lackama 

Ri ver into the northern Will amettc Va lley (Beeson, Tolan, 

and Anderson, 19 9). The 2 fl ov s of the Grande Ronde 

Basa lt and Ginkgo flows of the Frenchman Spring. M ember 

of the Wanapum Basa lt have identica l counterpart s on the 

Oregon coa t, indi cating that the c llov : also cro ed the 

Coast Range, probably a. intracanyon flows (Beeson, T ian, 

and Anderson , 1989). 

A llu vial depo it. that postdate the lumbia Ri ver 

Basalt Group arc the oldes t . trata to be onfinc I principally 

to the pr-sent lowland areas . The. e deposits include the 

laeu. trine (informal) M onroe clay of late Miocene to earl y 

Pli cene aoe (Robert . . 19 4; Roberts and Whitehead, 1984) 

in the southern Willamctte Valley and the Hel veti a FormaLi on ( chlickcr and Deacon, 1967), Sandy River Mud. tone 

(Trimble, 1963). and Troutdale Form ation (Lowry and Baldwin, 1952) in the northern Willamette Valley; the Troutdale 

Formati on wa deri ved in large pan from the ancestral 

Columbia Ri ver. In late Pli ocene to Plci. tocene time, the 

Boring Lavas were erupted from vent. in the Portl and basin, 

Tualatin ba in, and nonhcrnmo l Will amctte Valley (fig. 77) 

(Allen , 1975). Som volcanic center. in the we tern Cascade 

Range arc also of thi age. 

The Qu aternary hi tor of the area is characteri zed by 

allu vial depo it and land surface influenced by gla iati on 

in the ascade Range and change in sea level or in the 

longitudinal profile of the Col umbia Ri ver (M cDowell, 

199 1 ). ear the end of Plei. toe ne time, catas trophic glacial-outbur t flood fr m the Columbia Ri cr repeatedl y 

inundated the Will amcttc Valley a. far south as Eugene 

and deposited the Willamette Formation (Balster and Parons, 1969: Allison , 1978). 

We have compiled and field checked ex isting bedrock 

mapping (pl. 2) in and around the Will amctte Valley lowland 

except for the Portland ba in, which is being described separately by the Oregon Department of Geology and Mineral 

lndu trie . We mapped the ubsurfacc geology of the lowland it elf using data from oil-explorati on well s (pl otted on 

pl. 2A and 2B) and multi channel ei. mi c profil es that were 

part of an exploration campaign in the 1970' and 1980'. 

together with a network of gravity stations, the data from 

which were already in the public domain. The sub urface 

interpretati on of sediment. younger th an the Columbia Ri ver 

Basalt Group is based largely on data from water well s and 

borehole drilled for engineering purpo. es by the Oregon 

Department of Transportation. 
TECTO ICS OF THE WILLAM ETTE VALLEY. OREGO 187 

Re ponsibi lities for indi vidual part of the geologic map 

(pl. 2) arc: southern Will amette Valley, E. P. Graven; orvallis fault, Chri s Goldfinger; northern Will amette Valley, K. 

Werner; and Tua latin alley. T.A. Popowsk i. A more detailed 

description of the geology i in Goldfinger ( 1990), Graven 

( !990), Werner ( 1990), and Popowski ( 1995). 

ACKNOWLEDGMENTS 

Thi project was supported by grant 

14- 08-000 1- G 1522 from the Earthquake Hazard Reduction Program, .S. Geological urvey. Additional funding 

wa prov ided by A RCO Oil and Gas Co. and the Peter P. 

Johnson cholar hip ommittce. W thank ARCO Oil and 

Ga Co.. orth we. t atural Gas Co., Mobil Oi l Corp., and 

Seism logica l Data Services for allowing u to use proprietary data, including seismic profiles. Oil-explorati on-well 

record were prov ided by the Oregon Department of Geology and Mineral Industries, supplem nted by data from individual operator. . Water-well record were obtained from the 

Oregon Department of Water Re. ourcc . ore hole through 

the alluvial deposit. that postdate the Columbia Ri ver Ba. alt 

Group were 1 rov idcd by the Oregon Department of Tran:-

portation . Aeri al photographs taken in 1980 were provided 

by the Environmental Remote Sensing Applications Laboratory at Oregon State ni versity. Synthetic sei m grams arc 

based on a program wri tten by John hay. Drafting was done 

by independent contractors Paula Pitt and Linda Haygarth. 

and by Camel a arstarphen and argaret Mumford of Oregon State nivcr ity. 

We benefited much from eli cus. ion of Coast Range 

stratigraphy with Jack Meyer, A lan iem, Wendy iem, 

!rich Franz, and Ray Well s; of olumbia Ri ver Basa lt 

Group distribution wi th M arvin Bee. on; of we tern Cascade Range geology " ith George Pric. t, Dav id hen·od. 

and Ed\ ard Taylor: of Portland ba. in and ea. tern Tualati n 

ba. in geology wi th Jan ad in ; f Will amettc Valley alluvial . tratigraphy with M arshal l Gannett , Bernard Klcutsch. 

and Patricia M Do\ ell · and of Will amette Valley oravi t 

data with Ri chard ouch. Robert O'Malley. and Gerald 

onnard . M ar in Bee. on. Jan M ad in. and George Priest 

reviewed th manu cript. 

STRATIGRAPHY 

on·eiation of stratigraphi c units (fi g. 78) i based on 

rad iom tri c age. , benthic foraminifer , and ca lcareous nannoplankton. annoplank ton are correlated to zones that arc 

v idely di. tributcd in the northern Pa ific Ocean and are 

believed to be close to contcmporanc us throughou t thi s 

region. A preliminary zonati on for Paleogene strata based on 

coccoliths (Bukry and Snavely, 198 ) show some discordance with zonati on ba cd on ben thi c foraminifers. Benthic 

foraminiferal zones shown here arc those of Kl in pel! ( 1938) 

and M all or ( 1959) and are ba. ed on . tratigraphic . tion in 

California. These zon s are trongl influ n db bath metry and . cdimcn tary environments. om pari. on \ ith openocean pl anktic zone. show that the b nthi zon . ar tim 

transgre siv within ali fornia ba in ( rouch and Bukr , 

1979; Poor . 1976. 1980); ex tensi n of these zone. t Oregon add. additional uncertain! in time correlati n. 1oncthelcs , mo t of the fos il found in northwe. t regon arc 

benthic forami ni fer. , h nee the Kleinpcll and M allo1 

b nthic zones are the only ones a ail ablc f r biostratigraphic 

correlations of most urfac and sub. urfacc c tions. 

SILETZ RIVER VOLCANI S 

Oceani ba. alt. and intcrbcdd d basalt i s dimcntar 

rock. a signed to the ilctz River Vol ·ani . and dated as 

5 .I± 1.5 to 50.7±3.1 a (Duncan. 1982) form the ba. cmcnt 

that underlie. the Terti ary rocks of the oa t Range and Willamette Valley. The formation was originally named the 

Si letz Ri ver Volcani cS ri , by na ely and B·lldwin ( 1948) 

for exposures on the Siletz Ri r and it s tributari s in the 

central Oregon oa t Range. navcly and other ( 196 ) 

renamed the formation the iletz Ri er Volcan ic. and 

divided it into a lower unit consisting of submarine th leiitic, 

fine-grained, amygdaloidal pi llow ba alt and breccia and an 

upper unit of . ubmarinc and suba rial alkali ba alt, wit h the 

upper unit of much smaller volume than the lower unit. Sedimentary intcrb d yielded marine microfo. il. that na ely 

and other (1968) and cWilliam ( 19 0) r ferrcd to the 

Pcnutian and lati ian benthic . tagc. of M al lor ( 1959), in 

general agreement with the radi metri age. . o coliths 

from thi s formati on in the oast Rang- w st of the W i 1-

lamett Ri ver are referred to Subzone CP 10, e·timatcd as 

about55 .3- 53 .7 M a (Bukry and Snavely, 1988). 

The Siletz Ri ver Volcanics expo cd n nhwc. l of orvalli. belong main! to the lower unit of the formation a 

de cribed by navcly and other ( 1968). The volcanic rocks 

are overlain by, and part ly interbedded with, as much a 

I ,000 m of thin-bedded brown to gray tuffaceous marine silt-

. tone and hal ca lled th Kings Valley Si ltstone M ember 

( f theSilctz Ri ver Volcanic.)b Vokcsandother (1954). 

The Kin gs Valley ilt. tonc ember contains thin len. es of 

ba a! tic and. tone. a few thin layer. of white tuff, and rare 

foramini fer and carbonaceous debris. The andstone len. e 

contain lasts of basalt together with a terrigenous component derived from theca t. occoliths from the King. Valley 

ilt tone M ember. as well a imilar trata farther west, are 

r fcrred to Subzone CP I I , estimated as about 53.7- 52.5 M a 

(Bukry and navcly. 19 ). 

onhwe t of M cMinnville, the Siletz River Vol canics 

con. i t f c. icular ba alt flow . pillow basalt, fl ow breccia, 

and tuff breccia with interbeds of red to c~ n ca lcareou 

sandy tuff (Baldwin and other . 1955). The top of the unit 

contains medium- to dark -gray, ca lcareous, tuffaceous shale, 

siltstone, and sandstone (Brownfield and chlicker, 19 I a; 

Brownfi ld, 19 ' 2b) 
EPOCH 

PLIOCENE 

FORAMINIFERAL 

STAGE 

Delmontian 

UJ 1------1 

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Ulatisian I 50 

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Bulitian 

Coos Bay area 

Tarheel Formation 

Tunnel Point Ss. 

Bastendorff Shale 

Roseburg Formation 

"' c 

Eugene area CorvallisMonmouth area 

Columbia River Basa lt Group 

Ill 

Central Coast Range 

Nye Mudstone 

Yaquina Formation 

Alsea Formation 

SheridanMcMinnville area 

Siletz River Volca nics I Siletz River Volcanics I Siletz River Volcanics 

Oregon CityMolalla area Columbia County area 

Siletz River Volcan ics 

00 

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TE TO 'I S OF T HE WILLAM ETTE VALLEY. OREGON I 9 

The Siletz Ri ver Volcanic. were found in the Gulf Porter 1 and Humble Miller I exploratory well (pl. 2B) in the 

.outhern Will amette Valley, where they contain interbeds of 

marine strata 3- 30m thick. In the northern Will ametle Valley. the Rcichold Finn I well (pl. 2A) contain. 1.1 I 0 m of the 

iletz Ri ver Volcani cs, predominantly gray tuff. tuffac ou 

shale, iltstonc, and . and tone with Ulati sian (including late 

Ulati ian) microfoss il s indica ti ng lower middle bathyal to 

tropica l inner neriti c bathymetry (M cKeel, 1984). The Siletz 

Ri ver Volcanics in the Reserve Bruer I well (pl. 2A) con i. t 

of volcanic breccia and red and green tuff. Synthetic sei mograms f the Rc ervc Bruer I and Reichold Finn I well. 

show an abrupt increase in soni c velocity at the top of the 

Siletz Ri er Volcani sand a gradual increa e bel w, except 

for a . harp increase at the top f the volcanic breccia 

(Wern er. 1990). The seismic ex pre. sion of the top of the 

ilctz Ri ver Volcanics i. irregular or eli continuous, possibl y 

due to preservati on of original fl ow top or to ero. ion. as 

ugg steel for field exposure by Brownfield ( 1982b). 

The base of the iletz Ri ver Volcanics is neither 

expo. eel nor found in well s, so the thickness of the unit i 

unknown. although Sna ely and others ( 196 ) propo eel that 

its thickness exceeds 6 km. An ea t-we. t . eismic profile in 

the outhern Will amettc Valley ncar Bellfountain shows a 

zone of h ri zontal to gentl y west-dipping refl ector. beneath 

the Sil etz Ri ver Volcanics . uggc ting a maximum thicknes 

of 8 km , thinning eastward beneath the Will amette Valley 

(Keach and others, 1989). The Siletz Ri ver Volcanics have 

resisti vitic of around 100 ohm-meters; thi s re i ti ve unit is 

underl ain by a near-hori zontal unit of low rc i. tivity (Wannamaker and others. 1989). If thi s low-resistivit unit is not 

part of the Siletz Ri ver Volcanic , the thi cknes f the iletz 

Ri ver could be as little as 2 km . 

Tbc Siletz Ri ver Volcanics are correlati ve with basalts 

f the Ros burg Formation (Baldwin , 1974) in outhern 

Oregon, the rcsccnt Formati on of Washington. and the 

Mctcho. in Volcani s of uthcrn an ouvcr I land ( navc ly an I others. 196 ; Tabor and ady, 197 ): volcanic 

ro ks at the northern and southern ends of the outcrop 

areas of these unit arc older than thos lo. cr t the 

Columbia Ri ver (Duncan, 19 2). Taken together. these 

volcani c formation arc considered to be part of a seamount chain ( ilctzia) accr ted to orth Ameri ca prior to 

dcpo ition of the Tyee Formati on of middle Eocene age 

(Duncan. 1982) or. alternatively, they ar ba alts erupted 

Figure 7S (facing page). tratigraphic correlation chan for Tertiary rocks of western regon. Foramini feral tages from Kleinpell 

( 193 ) and M allory ( 1959). Tyrn. Miller sand of the Yamhill Formation: Tyv. volcanic rocks of the Yamhill Formation; M Od. dac ite 

and rh yodac ite of the Lillie Bulle Vol anies: MOb. basalt and basaltic andes ite of the Lilli Bulle V Jca nics; MOt. welded to non welded ash-now tuff of the Lillie Bulle Volcanics: m.y.B.P .. million 

year. before 1 resent. Modified fr m Armentrout and others ( 1983). 

during obliqu rifting of allo hthon u. terrane now found 

in . outhern Ia. ka (Well and oth r. , 19 4). The locati n 

of the ca tern boundary of thi. 

unknown ; it ma lie beneath the \ t rn 

A linear zone of high-freq uenc magneti 

beneath the we tcm Ca cade Range fo thill. mmittc 

for the M agnetic nomaly M ap of 

i. similar to the magn tic ignaturc of ophi olit and thu. 

may be cau. cd by mafi c and ultramafi c r k. markin g the 

suture zone between iletzia and orth Amcri a (Johnson 

and other . . 1990). 

TYEE FORMATION 

and t nc that form s the T cc Formation, fir. t 

d scribed by Diller ( 1896) in the southern Oregon oa. t 

Range, ex t nd for more than 260 km along the oast Range 

from the Rogue Ri ver nonh to the latitude or a! m 

(M olenaar. 1985). The Tyee Formati on consi ts of a deltaic 

facies to the south and a deep- ca fan facies to the north 

(Lovell , 1969; han and Dott, 19 "; Heller and Dick in. on, 

19 5). Its thi ckness i. 1.200 m ncar Eugene and decrca cs 

northward to 500 m west of Dall as. \ here mudstone and 

thin -graded :and. tone and . iltstone of the Tyee rmation 

abut a highland underl ain by the Siletz Ri cr Volcanic . Farther north. the Tyee i o erl apped by the Yamhill F rm ati n. 

The thickness of the Tyc Formation also decrea. c. ca:tward 

in the outhcrn Will amette Valley to 3 15 m in the Gulf Porter 

I well and S I min the Humble Miller I well (pl. 2B). In the 

Gulf Porter I well. strata a igncd t the Yamhill Formati on 

may be fine-grained equi alent. of the Tyee Formati on in 

the oas t Range. The M obil Ira Baker I well was terminated 

after penetrating 92 m of fos. ilifcrou_ iltstonc and arkos ic 

sand. tone correlated with the T cc Formation. T he age or 

the Tyee is lati sian or middle Eocene (M olenaar. 1985), 

and microf sil. in it mos t di tal northern exposure indica te dcpo ition in at lea. t middle bathyal water depth 

(M cKeel. 1985; Heller and Dickin on. 19 5). Coccoliths 

from the Tyee Form ation in the oa t Range we t of the Willamette Valley arc referred to ubzonc P 12a and 12b, 

resulting in an age c timate of 52. 5- 50 M a for the Tyee 

(Bukr and Sn a ely, 1988). 

The Tyee Formati on i. deri ved from the outh. and it 

show no ev idence of Cascade volcani sm to the ca t. The 

source was largely plutonic bur in luded a volcanic arc. also 

south of the Coa t Rang (Chan and Dott. 19 ). Snavely 

and others ( 1964) sugge ted that the source of Tyee andstone wa. the Kl amath M ountains or outhern Oregon and 

northern ali forni a. Heller and tilers ( 1985) . ubsequentl y 

found evidence of a Precambrian cru tal component in Tyee 

scdi m ntary materi als. and they concluded that the maj or 

source of the Tyee wa the Idaho batholith. now far away to 

the cas t but pre. umabl y closer to the Oregon Coast Range 

pri r to lock wi se rotation of the oa. t Range (Well and 

Heller. 19 8). 
190 SSES I G EARTHQ AKE HAZARDS A D REDUCI G RI K I TH E PA IFIC RTH\ EST 

YAMHILL FORMATION 

The Y amhill Formation was named by Baldwin and 

others ( 19-5) for expo ure in ill Creek, a tributary of the 

outh Yamhill Ri er sou thwes t of Sheridan. In the type 

locality, the formation consist of 150m of tu ffaceou ilLstone and hale overlain by 150m of basalti c andstone and 

silt tone and I ,050 m of micaceou mudstone and iltstone. 

In the . ub urfacc of the northern Will amcne Valley. the 

Yamh ill Formation consi ts largely of shale and silt tone 

with minor tuffaceou trata and fi ne-grained graded andstone showing partial Bouma. equences (Richard E. Thoms, 

Ponland State Uni er ity, oral com mun., 199 1). The Yamhill thicken. ea. tward acros coeval normal fau lt to at least 

932 m thick in the Reichold Bagdanoff 23- 2 well (pl. 2A). 

where it include ba. alt and lUff. 

The Yamhill Formation include the informally named 

Miller sand of 13 ruer and other. ( 1984 ), known on I y from the 

sub urfacc in the Will ame!le Valley outh of Salem. Thi i: 

a sandy to conglomeratic volcanicl astic unit overlain by and 

interfingering northwc tward wi th fainlly bedded micaceous 

si lt tone and mudstone (Baker, 1988). The Miller and 

pinche. out to the north west, suggesting coeval di splacement 

on the nearby Corvalli fault. The Miller and increa e. in 

thickne s sou thea tward a an underl ying mudstone unit 

decrea. es in thick ne s, although the overall thickness of the 

Yamhill orm ation incrca e a well . Farther outheast, both 

the Miller sand and the underlying mud. tone grade into a 

vo lcani c facie . The Gulf Porter I we ll (pl. 213 ) penetrated 

I ,055 m of volcanicl astic and. tone and silt. tone and ba altic conglomerate, and the M obil Ira Baker I well (pl. 2B) 

contain I ,96 1 m of basalt, andesite, dacite, tuff, and minor 

tuffaceous mud. tone, silt tone, volcanicla ti c and tone, and 

breccia, all probably equivalent to the Yamhill . 

The iller sand contains microfo. il indicating shallow-marin depo ition, whcrea the overlying and und rl ying mudstone member of th Yamhill Formati on were 

deposited in upper to middle bathyal water depths (M cKeel, 

1984, 19 5). Foramin iferal assemblage. in the type ecti on 

f the Yamhill are as igned to the earl y ari zian tage 

(M cWilli ams, 19 0; Brownfield, 1982b; D.R. McKeel. written comm., 19 7). Foramin ifer in the Reserve Bruer I and 

R ichold Finn I well s in the northern Willamene Valley are 

earl y ari zian in age at the base of the Yamhill and ari zian 

in age for th remainder of the formation. The Yamhill in the 

Reichold Bagdanoff 23- 28 well yielded earl y an z1an to 

late lati. ian foramin iferal a ·emblages (M cKeel, 19 4). In 

the Coast Range south of Eugene, the Elkton Formation and 

Lorane Si ltstone (Bird. 1967; Heller and Dickinson, 1985) 

have lati ian and earl y ari zian foraminifer . Coccoliths 

from the Yam hill Formati n in the Coa. t Range we t of the 

northern Will amcue Valley arc refeiTCd to ubzone CP 13c 

and P 14a, the ages of which are estim ated as 4 7 M a to 

about 42.5 M a (Bukry and Snavely, 1988). However, coccolith. from the Elkton Formation and Lorane Siltstone arc 

r ferred to ubzone P 12b, clo. e to the age of the Tyee 

Formation (Bukry and Snavely, 198 ), . uggcsting that these 

form ations arc older than the Yam hill Formati on despite the 

pre. nee of lati. ian and earl y ari zian foraminif rs in all 

three formations. The Yamhill change. facies ca:tward in 

the ubsurfacc of the Will amcnc Valley t arc! ike volcanic 

rocks (Baker, 1988). ·uggcsting either that the ascadc arc 

began t develop earli er than the 43-4 1. M a (Lux , 19 2; 

Priest and Vogt, 1983; Verplanck and Duncan, 1987; Taylor, 

1990) or 5 M a (Pric t. 1990) age · that arc generally 

accepted ba ed on urface geology or that the volcan i m that 

produced th larno Formation, widespread in eas tern Oregon, ex tended thi s far to the west. 

In the oast Range we t of the Tualatin va lley, the 

Yamhill Formation interfinger with and is overl ain by the 

Till amook Volcanic (Well. and other. , 1983). The Yamhill i. intruded by a sill of zcoliti zcd gabbro with a potaium-argon age on plagiocla c of 43.2± 1.8 M a (L.G. 

Pickthorn , in Bukry and navcly, 1988). Strata with ari zian microfo sil s underl ying the owlitz Formation in the 

Mi st gas field (fig. 76) arc also referred to the Yamhill Formati n by Bruer and other. ( 1984). but the. e bed. contain 

late ari zian microfossil s and overlie the Ti llamook Volcanics; they were described by iem and iem ( 1985) as 

their informal Hamlet formati n. The Yamhi ll Formation in 

the northern Willamcue Valley. where olcanics are not 

present. may include in it upper part trata eq uivalent to 

the informal Hamlet formation of the northern Oregon 

Coa. t Range. although no late ari zian microfoss il. have 

been found in th Yamhill Formation. 

TILLAMOOK VOLCANICS 

The Gales Peak area (pl. 2A) west of the Tualatin bas in 

is underl ain by ba. alt correlated by Wells and others ( 1983) 

with the Tillamook Volcanics of the northem Coast Range. 

Con tact relation with adjacent edimentary rocks arc 

unclear clue to faulting and poor exposure. In the ahama 

and Weagant Kl ohs I well (pl. 2A) in the Tu alatin bas in, 

Refugian trata overlie zeoliti zed ba alt that is interbedded 

with unfos iliferou marine silt tone, clay tone, and minor 

and tone; thi s basalt may either be the Till amook Volcanics, Goble Volcanics, or ba. alt of Waverl y Height . The 

Yamhill Formation and Till amook Volcanic may be interbedded in the Texaco C oper M ountain I well (pl. 2A) 

between 945 and 2,823 m well depth . The volcanic rock 

below 2, 124 m in the Richfield Barber I well (pl. 2A) may 

be the Till amook Volcani cs. 

In the Tillamook Highland (fig. 77), the Tillamook 

Volcanics consist of a lower unit of submarine basalt with 

sedimentary interbeds containing earl y ari zian microfossil s (W.W. Rau, in Wells and other , 1983) overlain by 

mostl y subaerial basalt. Potassium-argon age from the middle and lower 1 art s of the cquence are 46.0±0.9 to 42.7±0.5 

M a (M agill and others, 1981 ); an age of 33 .4±0.5 M a is 

reported in Well. and other ( 1983). 
TECTO lCS OF THE WJLLAMETTE VALLEY, OREGO 19 1 

BASALT OF WAVERLY HEIGHTS 

At Waverl y Heights in Milwauki e. along the Willamette Ri ver south of Portl and, the Columbia Ri ver Ba alt 

Group is underl ain with angular unconformity by a sequence 

of ubaeri al basalt flows and a sociated sed imentary rock . . 

probably marine (Beeson, Tolan, and M ad in, 1989) The top 

of the unit is marked by a thick soil zone. Potass ium-argon 

ages from two flow are dated about 40 M a (R.A. Duncan. in 

Beeson, Tolan, and Maclin, 1989), younger than the Siletz 

Ri ver Vol canics. and the unit is probabl y correlati ve with the 

Ti llamook Volcani cs. Thi s unit may be present in well s in 

the Tualatin bas in, but its presence has not been confirmed. 

SPENCER FORMATION AND 

CORRELATIVE UNITS 

(COWLITZ AND NESTUCCA 

FORMATIONS) 

The Spencer Formation, named by Turner ( 1938) for 

ex posures near Eugene, crops out along the western edge of 

the Willamette Valley from south of Eugene north to the 

Chehalem M ountain adjacent to the Tualatin ba. in . Microfossil s are referred to the late Nari zian. The Spencer is 

divided into two member (AI Azzaby, 1980; Baker. 19 

Richard . Thoms, Portland State Univer ity. oral commun ., 

199 1 ). The lower member con ists of micaceous, ark osic 

sand tone, silt tone, and minor coa l depo itcd in a strand line 

to middle-shelf environment. Wes t of the Tualatin basin, th 

lower member ranges in thicknes from 60 m near Henry 

Hagg Lake to about 300 m of predom inately clean arkosic 

sand south of Patton Valley (Richard E. Thom . Portland 

State Uni versity, oral commun., 199 1 ). In the Quintana Gath 

I, Linn County Oil Barr I American Quasar Hickey 9- 12, 

and Mobil Ira Baker I well s (pl. 2B) in the southern Willamette Valley, thi. member change to tuffaccou .. trata 

ea tward and interfinger. and grade. upward into vo l an ic 

rock. of ascade Range ori gin . In the American Quasar 

Wolverton 13-3 1 and Humble M iller I well. (pl. 2B) north 

of Albany , the lower member includes basal t. andesi te. and 

tufl Microfos il tudied by M cKeel ( 1984. 1985) indicate 

middle to inner neriti c water depths along the western side of 

the va lley and inner neriti c water depths to pos. ibl y nonmarine along the eas tern edge of the va lley (Hickey I well). 

Thickne. e in the uthern Will amcttc Va lley vary from 

230 m in the Reichold orthwe t atural Gas M errill I and 

Orcg n atural Ga Independence 12- 25 well s (pl. 2B) to 

310 m in the Ameri can Quasar Wolven n 13-3 1 well. 

The upper member consist of mudstone, siltstone. 

and subordinate sandston , grading eas tward to tuffaccou. 

strata and volcanic rocks (A meri can Quasar Wolverton 

13- 3 1 and Gu lf Porter I well s. pl. 2B). A long the ca. tern 

edge of the va lley (Quintana Gat h I , A meri can Quasar 

Hickey 9- 12, Linn Count Oil Barr I. and M obi l Ira Baker 

I well , pl. 2B). the upper m mber con. ists of volcanic and 

volcani lastic strata corrclati e in part , ith the lower part 

of the Fi her Formati on in th Eug nc area. The mud t n 

and siltstone of the upper member of the Sp n er Formati on in the southern Will amett c Valley ' cr dcpo ited in 

upper bathyal water depth deepenino upscc tion to middle 

bathyal depth (M cKeel, 1984, 1985). The thicknes. of the 

upper member in the southern Will amett Valle range. 

from I 00 m in the Am ri can Quasar M • I Fanns 33- --+ 

and American Quasar Wolverton 13- I well. to 178 m in 

the Gulf Porter I well (pl. 2B) 

ear the Corva lli s fault, the Spencer Formati on o crlics 

the Tyee Formation directl y wi th an angular unconf rmit of 

as much a 90° difference in dip. The Spencer Formation. 

where it is in fault con tac t wi th the iletz Ri cr olc·1 ni cs 

along the Corvalli s fault between orvalli s and Phil omath. 

con ist of fos. iliferou. tu ffaccou , ba altic sand. tone and 

conglomerate with clasts a much as 2 m in diameter dcri eel 

from the Siletz Ri er Volcanics. 

The Spencer Formation also on i. ts of a lower (predominantly sandstone) member and an upper (siltstone an I 

mudstone) member in the northern Willameue Valley. Th 

formati on thins north ward from 760 m near Dall as t 490 m 

in Yamhill and Washington Counties. and it thins eas tward 

from 325 m in the Reserve Bruer I well to 45 m in the Oregon Natural Ga DeShazer 13-22 well (pl. 2A). In the Tualatin ba in , the Spencer F nnation i 400 m thick nea r Henry 

Hagg Lake. orth of Forest Grove. trata orrelati vc wi th th 

Spencer Formati on arc rcferr d to the owlitz Formation. 

which rc t unconformably on the Tillamook Vol cani cs. The 

Cowlitz comprise a lower unit con isring of a ba. al conglomerate o erl ain by siltstone (the informal Hamlet formati on of Niem and iem. 1985). the C& W sandstone of local 

u age that produces ga. in the Mi. t ga · field, and an upper 

. iltstone member. The upper two members may be pre em in 

the Texaco Cooper M ountain I well (pl. 2A). 

The Ne tucca Formation. originall y de. cribcd by navely and Yoke ( 1949). overlies the Yamhill Formation wi th 

angular unconformi ty in the Coa. t Range wes t of M cMinnvi lie. It appear to be a deeper water facies equi valent of the 

pencer Formati on and correlate. wi th the in form al Ham let 

formati on of the northern Coast Range of Orc-=>o n. The estucca Formation con. ists of tuffaceous shale and silt tone 

and thin-bedded sand tone wi th interbeds of pill ow basalt. 

brecc ia, and tu ff (Baldwin and others. 1955) grad ing into the 

Yachats Basalt and the basa lt ofCa cade Head. Foramin ifer 

are ass igned to the late ari z. ian (W.W. Rau. in Well and 

other , 1983), and cocco lith are assigned to Subzones P 

!Sa and P ISb, giving an age es timate by Bukry and 

Snavely ( 1988) of about 38.5- 36.7 M a. 
192 ASSESS! G EARTHQ KE HAZARDS A D RED Cl G Rl K I TH E PACIFI ORTHWEST 

FISHER FORMATION 

onmarine volcaniclastic . trata and interfingering 

flows in the ugenc area as far north as Cox Butte, we. t of 

Jun tion Ci ty. arc mapped a the Fi sher Formati on (Vokes 

and others, 1951 ). Thi s formation i I ,680 m thick and consists of andesitic lapil li tuff and bre cia. tu ffaceou. sandstone and . ilt tone. and pebble to boulder conglomerate 

interbedded with fl ow of predominantl y andes ite and ubordinate basalt and dacite (Hoover. 1963). The c rocks extend 

. outh f Eugcn and may be stratigraphica ll y equi valent to 

the alapooya Formati n and Cole. tin Formation of the 

southern Oregon a cade Range (Well s and Waters, 1934; 

Peck and other , 1964). They make up unit T5 of Sherrod and 

Sm ith ( 1989), with an age estimated a 45- 35 M a. 

Foss il leaves in the lower part of the Fi ·her Form ati on 

. outh of Cottage Grove (25 km south of Eugene) suggest a 

late Eo ene age (R.W. Brown, in Hoover, 1963). Radiometri ages from basa lt and ba altic andesite near the top of the 

form ati on arc 40- 35 M a (Lu x. 1982), close to the youngest 

age in ferred for the arizian rage of M allory ( 1959). The 

Fi her appears to interfing r northward with the marine 

Eugene Form ati on of latest Eocene ( ari zian) and Oligocene (Refugian) age (Vokes and other. , 195 1 ). The basalt 

fl ows dated by Lu x ( 1982) were considered by Vokes and 

others ( 195 1) to over! ie the Fisher and Eugene Formation 

unconformabl y. but Walker and Duncan ( 1989) sugge l that 

these fl ows arc age e JUi va lents of (and thus part of) the 

Fi her Formati on. 

In the ubsurface of the southern Will amette Valley, 

the M obil Ira Baker I well (pl. 28 ) penetrated 143 m of 

vo lcanic and volcaniclas ti c rocks overl ying ari zian 

marine strata. Between the Salem Hills and Tualatin ba in, 

the marine equivalents of the Fi. her Formation are notably 

free of volcanic rocks. In the Quintana Gath I we ll (pl. 2B). 

a 440-m-th ick volcanic unit bracketed by trata with late 

ari zian and Refugian microfos il s may be equi valent to 

the Fi. h r Formation. St ill farther north , the Goble Volca -

nic. are interbedded with the Cowlitz Formation; the Goble 

Volcanics, like the Fisher Formation, are arc-related rock 

(Phi ll ips and other., 1989). 

EOCENE AND OLIGOCENE 

MARINE STRATA 

North and wes t of the Tua latin basin , marine . trata of 

ccnc and Oligocene age are mapped as the Keasey Formation and the vcrl ying Pittsburg Blu ff Formati on (Well s and 

others, 19 3). The Keasey Formation is predomi nantl y thick, 

light-gray tu ffaceous claystone and siltstone with minor 

mudstone and . and tone; foramini fers are late Nari zian and 

early Refugian in age (M cWilli ams, 1968. 197 3; Brownfield 

and Schlicker, 19 I a). onh of Henry Hag.s Lake, the Pittsbu rg Blu ff ormation consists of a. much as I ,400 m of 

greeni sh-gray to gray, tuffaceous, glauconitic and basaltic 

I itharen ite sandstone, siltstone. and min r conglomerate with 

foram ini fer. of the Refugian Stage (Richard E. Thom. , Portland State Uni versity, oral commun ., 199 1 ). 

W t of the Eola Hill. . the Keasey Formati on con. ists 

of sandy tu ffaceous ilt tone, and the Pitt sburg Bluff Formation con i. ts of tu ffa cous sandstone. tufT, and tuffaceous 

shal e and silt tone deposited in shallower water than the 

Keasey Form ation. The R ichold Werner 14- 2 1 and Oregon 

atural Ga Werner 4- 2 1 and DeShazer 13-22 well s (pl. 

2A) in the northern Willamelle Valley document an angular 

unconformity betweet1 the Keasey Formati on and the Pittsburg Bluff Formation. The two units together are about 715 

m thick in the Eola Hill s near Am ity (Brownfield, 1982b). 

In the southern Willamette Valley, feldspathi c, tu ffaceous and tone and siltst ne containing Refugian foramini fers of Eocene and Oligocene age constitute the Eugene 

Formation. The form ati on i 550 m thick in the hill s ca. t of 

Coburg, which include 343 m of strata penetrated in the 

M obil Ira Baker I well. more than 800 m thi ck in the Lebanon area. including . trata in the Ameri can Quasar Hickey 

9- 12 well , and 7 0 m thick beneath the Salem Hill , based 

on data from the Oregon Natural Ga I ndependencc 12-25 

and Reichold 1orth west Natural Gas M errill I well (pl. 

2B). The formati on is bes t exposed in the hill cas t of 

Coburg, at Peterson Butte, and at the base of the Salem Hill s. 

To the south, the Eugen Form ati on interfi nger with the 

nonmarine Fi sher Form ation. 

LITTLE BUTTE VOLCANICS 

The Little Butte Volcanic Seri es of Well ( 1956) and 

Peck and others ( 1964), whi ch i. h re renamed the Little 

Butte Volcanic in accordance with Article 38 of the ( 1983) 

orth A meri can Stratigraphic Code, makes up the base of 

the expo ed we tern Cascade Range eq uence on the ea tern 

margin of the outhern Will ameue Valley. where it overli es 

the Eugene Formati on, and the northern Will amette Valley, 

where it is overl ain unconformably by the Oligocene and 

Miocen Scott. Mill Formati on and the Miocene Columbia 

Ri ver Basalt Group (Miller and Orr, 198 ). 

We ubdi vide the Little Butte Volcanic into basa lt and 

ba alti andesite fl ow of Walker and Duncan ( 1989), porphyriti c andesite fl ow of Hampton ( 1972), dacite to rh yodacite vent complexes of Walker and Duncan ( 1989) and 

Bri stow ( 1959), and welded a h-fl ow tu ff in the foothill s of 

the Cascade Range northeas t of Eugene (Walker and Duncan, 1989). The age range of the Little Butte Volcanics is 

35- 17 M a (Sutter, 1978 ; Lu x, 1982; Walker and Duncan, 

1989), corresponding to the T4 and T 3 time units of Sherrod 

and Smith ( 1989). However, many of the rocks in the Eugene 

area described a. Little Butte Volcani cs by Lux ( 1982) yield 

radiometri c ages older than 35 M a. Lu x ( 1982) reported radiometri c ages of 4 1.5±0. 9 M a and 39.2±0. 5 M a eas t of Lebanon. However, Verplanck ( 1985) red a ted a sample from one 

of these localities as 3 1.7±0.4 M a. 
TECTO IC OF TH E WILLAMETT · VALLEY. OREGO 19 

In the Eugene area. Little Butte Volcanics unconform -

abl overlie the Eocene and Oligocene Fi her and Eugene 

Form ations (W. . Orr, in Armentrout and others, 1983). 

However. Peck and others ( 1964) found tongues of Eugene 

Formation interfingering with Little Butte Volcan ic outh 

of Brown. ville, Beaulieu ( 1974) noted similar relation. to 

the north in Linn ounty, and the Quintana Gath I well in 

.outhwc. tern M arion ounty ncar Salem contain. volcanic 

rock interbedded with the ugenc Form ati on. These vo lcanic rocks arc more likely correlated to the Fi sher Formation 

discus cd above rather than to the Little Butte Volcanics . 

The olcanic rocks are not found in the Rcichold 1 onhwc. t 

1 atural Gas M errill I , Oregon 1atural Gas Independence 

12- 25, and Ern tson 'chcrmachcr I well s (pl. 2B) to the we t. 

Following erupti on, the Little Bulle Volcanic. \ ere tilted 

gently to the ea t and eroded pri or to deposition of the call 

Mill Formation in the northern Will amcue Valley (di scussed below). Peck and others ( 1964) considered the vo lcanic . cqucncc to be between I ,300 and 2.600 m in thickne . 

throughout most f the we tern Ca cadc I angc. The vo lcanic rock appear to thin to the we t. car Wa. hburn Butte, 

about 750 m of volcanic rocks lie between the Eugene Formation and Miocene-Pliocene ande itc that probabl y overlies the Little Butte Volcani cs with anoular unconformit y. 

The Humble Wicks I well , 10 km outhea. t of Sil verton, 

penetrated I . 30 m of vol cani c rocks, whi ch may include 

units other than the Little Bulle Vol canic . . 

cdimentary formations correlati ve with the Lillie 

Butte Vol ani cs include the AI ea Formation and Yaquina 

Formati on near cwport and the informall y named 0 wald 

West formati on of N icm and others ( 19 5) of the northern 

Oregon coast. These formations are older than 25 M a and 

thu fall wi thin the time range f the Little Bulle Volcanic . 

The Al sea Form ation is referred to occolith Zon P 16 of 

36.5- 35 M a age (Buk ry and navely, 19 ). The Scott. 

Mil l. Formation and olalla Formati on (L owry and Baldwin , 1952) arc late Oligocene and earl y Mio cnc in age 

(Miller and Orr, 1988), also within the time range of the Little Butte olcanics. However, the c form ations o crlie th 

Li ttle Bulle V lcanics unconformably in the M olalla and ilcrton area (Miller and Orr. 19 ). 

INTRU IVE ROCKS OF THE 

OA T RANGE 

Gabbroic ill s, dikes, and laccoliths are common in the 

central and northern Oregon oa. t Range. The be. t known 

i the M arys Peak sill , 390 m thick, which imrude the 

Tyee Formati on near it basal contact with th King ValIcy Siltstone M ember f the il ctz. River Volcanics. The ill 

is a highl y differentiated, titanium-ri ch body of gran ph yri c 

oabbr and granophyri c diorite with abundant aplite dikes 

near it. upper chilled conta t (Roberts, I 53). The Marys 

Peak . ill was dated as 29.7± 1.2 M a (middle Olig ccnc) 

(P.O. Snavely, Jr .. in lark . 1969). Other Iaroe intru. ivc 

boclie. of imilar lithology are f unci . outh of Philomath 

and at Bald Hill . Dimple Hill. Vine ard Hi ll. ffin Bulle. 

Logsden Ridge, and Witham Hill (Snavel and Wagner. 

196 1; navely and others. 19 0). The reman nt magneti zati on of the M ary s Peak ill is normall y polari zed ( lark . 

1969). as it i for all the other intru ions field hcc kecl in 

the Corvalli s and Albany area. upporting the sugge tion of 

Snavely and Wagner ( 196 1) that th miclcll Jig enc 

intru. ivc epi sode wa of hon duration. Elongate likes 

. triking we t to wes t-northwest arc found n ar the or alli s 

fault. orthcast- triking dikes intrude the fault , and northeast- triking . ills o cupy fold hi noes parallel to th fault in 

the Tyee and Sp ncer Formation. , indicating that str mg 

folding predated intru ion. A dike intrude. the W'St-northwcs t-s triking Philom ath fault that off. ct. the orva lli s 

fault, but no other intrusion w rc found assoc iated wi th 

other northwe t-. triking faults off. tting the orva lli s fault. 

Sill s. presumabl y of · occnc age, intrude · o enc marine 

. trata of the Willameue Valley as young as the Eugene Formation. In the . outhern Will ameuc Valle , a sill intruding at 

the pcncer-Yamhill Formati on contac t was found in the 

Gulf P rtcr I well (pl. 28 ) and i m·1rk cd b a high-amplitude refl ec tor on a. eismi c profi le. There arc dik s in cral 

of the buttes on the eastern edge of the valley . An intrusion 

at Skinner Bulle in ugcnc was dated at 30.3±0.9 and 

29.4±0.9 M a (J.G. mith, in Walker and Dunca n, 1989), the 

same age a the M arys Peak si II. lntru. ion al ng the eas tern 

edge of the valley arc mapped as Oligocene to Mioccn in 

age (Beaulieu. 1974; Walker and Duncan, 1989) and prcumably fed volcanic r ck of the western ascad Ran c. 

SCOTTS MILL FORMATION 

M arine and nonmarine trata of the cou Mill Formati on (Miller and Orr, 1988) of late 01 igoccne and earl y 

Mi ccnc age arc exposed in the Cascade Range foothill s cast 

of il erton adjacent to the northern Willamctte Valle 

Millcr and0rr ( l 9 ) divide the cott Mills ormationinto 

the Marquam M ember, 300- 500 m thi ck, overl ain by and 

interfingering wi th the Abiqua M ember, 300m thick. which 

grade laterall y into the Crooked Finger M ember, 200 m 

thi k. The M arquam M ember unconform abl y overli es the 

Little Butte Volcanic on a urfacc with regional relief of a 

much a. I 00 m. The marine arquam M ember includes 

cr ss-stratified barnacle lime tone, fossiliferou conglomerate, burrowed claystone. tufTa eous sandstone, and graded 

mud tone. indicating depo ition along a rocky coa t (Miller 

and Orr. 1988). The biqua M ember is campo ·eel of marine 

and nonmarine olcanic arko e. and the rooked Finger 

M ember con. ists of nonmarine volcanic conglomerate and 

mudstone. The three member con titutc a prograding delta 

complex that developed southwc t of a volcanic headland 

underl ain by the Little Butte Volcanic . . The Scotts Mil ls 
194 I G ARTHQUAKE HAZARD A D RED Cl G Rl K I THE PA IFI ORTHWEST 

Formation is found in two well. drilled in the Waldo Hill s. 

the Humble Wicks I well (pl. 2B) that penetrated 239 m of 

volcanic rocks, claystone, . iltstone. andstonc, and rare volcanic conglomerate and the RI-1 Exploration Ander. on I well 

(pl. 2A). which penetrated 140 m of an upward-coar cning 

seq uence of sand tone, ilt tone, and claystone with vo lcanic 

fragment . These trata overlie the Little Butte Volcani in 

both well . The absence of the Scotts Mills in other wells to 

the we t is probably due toea tward tilting and erosion prior 

to depo ition of the Columbia River Ba. al t Group. 

It is unclear how the Scott Mills Formation i. related 

to other formati ons of the same age in western Oregon . 

MOLALLA FORMATION 

The Molalla Formation (Harper, 1946; Lowry and 

Baldwin , 1952) consi t of about 300 m of nonmarine tu ffaceou conglomerate, andstone, silt tone, and water-laid 

tu ff with paleo ol in the foothill of the northern Will amctte 

Valley eas t of Si lverton . The M olalla F rm ation i ex po ed 

in tream valleys in the Waldo Hills; it i not found in any 

exploratory wells. The lower part of the M olalla Formation 

re t unconformably on the Little Butte Volcan ic , is interbedded with the upper members of the Scotts Mills Formati on, and is overlain unconformably by the Columbia Ri vcr 

Basalt Group (Miller and Orr, 1988) volcanic rock. of the 

Sardine Formation. and the upper part of the M olalla Formation. trata included in the upper part of the M olalla Formation appear to be interb dded with the Columbia Ri ver 

Basa lt Group (Miller and Orr, 1988). Foss il leaves in the 

M olalla Formation were dated a earl y Mio ne (J.A. Wolfe. 

in Peck and others. 1964). Rad iometri ag s of tuff beds in 

the olalla are 15.9± 1.0 M a and 15.0±0.7 M a (Fiebelkorn 

and other., 1983). Thus, the age of the Molalla Formati n 

ranges from perhap. as old as late Oligocene to middle 

Miocene. onglomcratc mapped as part of the Trou tdale 

Fom1ati n by Peck and others ( 1964) in this area i considered t be pa rt of the M lalla Formation . 

CAPPOOSE FORMATION 

Fine-grained . hallow marine sedimentary depo it 

intcrfingered with nuvial sandstone, coal-bearing mudstone. 

and conglomerate make up the middle Miocene Scappoo e 

Formation (Van Atta and Kelty, 1985). The weak ly consolidated strata arc commonly expo. ed in steep slope capped by 

the re i tant olumbia Ri ver Ba alt Group. M ore than 275m 

of cappoosc strata disconformably overlie the Keascy and 

Pitt sburg Bluff Formation north of the Tualati n basin, and 

at leas t 335 m overlie Pittsburg Bluff . trata on the western 

flank of the hehalem Mountains. 

The cappoo. e orm ation was deposited in an cstua. 

rin or deltaic to hallow-marine en ironmen t over a di _ 

sec tcd paleotopography with rcli f f a. much as 245 rn . 

Ba. altic cong lomerate derived from low-magnesium fl ow 

of the Grande Ronde Ba. al t com monly occur ncar the ba e 

of the forma tion. The upper con tact is conformable wi th the 

Columbia River Ba. alt roup; cappoose strata frequently 

are intercalated with Grande Ronde flows and overl ain b 

flows of either the Grande Ronde Basalt or Wanapum Basalt 

(Frenchman pring. M ember) (Van Al ta and Kel ty, 1985). 

The Scappoose Formation is probably entirely middle 

Miocene in age, based on the occurrcnc of clas ts of the 

Grande Ronde Basalt within the ba. al conglomerate and 

flow of the Columbia Ri ver Basalt Group ovcrl yin the 

unit. Partl y corrclati v trata in the northern Coast Range and 

north rn Willamettc Valley ar the A toria and cotts Mills 

Formations. respectively. 

COLUMBIA RIVER BASALT GROUP 

Flood-basalt now. of the Columbia Ri ver Basa lt Group 

were erupted from fis ure in ea ·tern Oregon and Wash ington and western Idaho from 16.5- 6 Ma. Some of these flow 

traver ed the a cadc Range via the olumbia trans-arc lowland, which ex tended from the Columbia Ri ver 60 km south 

to the lackamas Ri ver (Bee on, Tolan, and Ander. on, 

1989; Bee. on and Tolan, 1990) and reached as far as the 

present-day Pacific coa t, where they are interbedded with 

marine strata. A noted by Bee on and other ( 1975) and 

Bee on. Tolan , and Anderson ( 1989), some of the broad 

folds and faults of the Oregon Cascade Range and Willamette Valley were acti ve during the emplacement of the 

olumbia Ri ver Ba. all ,roup. Th s folds and fau lts include 

the Portland Hill -Cia kama Ri ver tructu ral zone that limited ome of the flows of the Grande Ronde Basalt and 

Wanapum Ba alt into the Portland basin and the lower 

reaches of the Columbia Ri ver, and the Gales Creek-M ount 

Angel structural zone, which formed a barrier to ome flows 

of the Wanapum Basalt. Only the R2 and N2 flows of the 

Grand Ronde Ba alt (about 16- 15.6 M a) and the ba alt f 

Ginkgo, Silver Falls, Sand Hollow, and Sentinel Gap of the 

Wanapum Basalt (about 15.3 M a: Beeson and others, 19 5) 

cro. sed the Portland Hills-Ciackama River . tructural zone 

and entered the northern Will amette Valley. 

The Columbia Ri er Ba alt Group underlies nearly all 

of the Portl and, Tualatin, and northern Will amette Valley . 

Water we ll s penetrate more than 200 m of basalt beneath 

parts of the Tualatin basin (Popow ki, 1995). Erosionally 

re i tant ba alt comprises most of the exposures in the 

Waldo, Salem, and Eola Hill s, the Red Hills of Dundee, the 

Tualatin M ountains (Portl and Hills), Pete Mountain, Parrett 

M ountain, Cooper and Bull Mountain. , and the Chehalem 

M ountains. The Grande Ronde Basa lt makes up mos t of the 

volume of the Columbia Ri ver Ba. alt Group west of the 
TECTOt ICSOFTHE WILLA 1ETTEYALLEY. OR GOt 

ca cadc Range, a it doc in the Columbia Pl ateau to the 

ca t. ex tending into the Willamette alley a. far . uth a 

Franklin Butte, 3 km sou thea t of Scio. There were no acti ve 

Ca cade Range volcanic center in the lowland through 

which the fl ow of the Grande Ronde Ba. alt ro ed the Ca. -

cade (Bee on. Tolan, and A nder on. 1989). Wherca the 

nows of the Grande Ronde Basalt bl anketed mo t of the 

northern Will amette Valley, some of the fl ow units of th 

Wanapum Basalt tended to follow channel. . One chan nel of 

the Ginkgo fl ow crossed the a. cade Range beneath the 

future site of Mt. Hood and followed the south ward -convex 

arcofthe Waldo, alem, and Eola Hil l. (Beeson, Tolan, and 

Anderson, 1989). Ba alt on the Oregon coa t near ewport 

i geochemicall y identi ca l to the Ginkgo fl ows . uggc ting 

that the Ginkgo flow· continued aero. to the coa. t prior to 

mo t of the uplift of the Coa t Range. Another Ginkoo fl ow 

pas ed north of the Will amettc Valley in uch a way that the 

intervening northern Will amctte Valley contain no fl ow. of 

the Wanapum Ba alt, onl y those of the Grande Ronde Basalt 

(Beeson, Tolan, and Anderson, 19 9; Tolan and other . . 

1989; Well and other , 1989). imilarl y. the Wanapum 

Basalt i absent in the Tualatin ba in . The Sil ver Fa ll. and 

Sand Hollow basalt fl ow mo ed south we. t along the ax i of 

the Waldo Hill s, and the ba alt of Sand Hollow at Hungry 

Hills is the southernmo. t expo ure of the olumbia Ri ver 

Ba alt Group in the Will amette Valley (Bee on and others, 

1985; Bee on, Tolan, and Anderson. 1989). The Waldo, 

Salem, and Eola Hills show a rever. al in topography bccau c 

they were a low area during the time of Ginkgo erupti e 

acti vity. The olumbia Ri er Basalt Group i. 100- 1 0 m 

thi ck in the Salem Hills. 

The Columbia Ri ver Basalt Group re t with angular 

unconformity on older units: the M olalla Form ati on and 

Scotts M i li s Fom1 ati on cast of th Wi llamette Valley and the 

Eocene and Oligocene marine seq uence in the alcm and 

Eola Hill .. T hu., the homocli ne com po. ing the west rn Cascade Range dipped cas t pri or to the eru pti on of th lumbia 

River Basa lt Group (Pri est. 1990). 

SARDINE FORMATION 

Volcanic and volcanicla tic rock. of the western Ca -

cadc Range, erupted after emplacement of the olumbia 

Ri ver Ba. alt Group, were ailed the Sardine Formati on by 

Peck and thcrs ( 1964 ). The olcani c roc k. postdate a period 

of relati ve quiescence in the a. cadc Range between 17 and 

13.5 M a (Sherrod and Smith, 1989). b tween Epi odes I and 

2 of Priest ( 1990). The ardinc Formation is regarded as 

Miocene and Pliocene in age. The formati on i equi va lent to 

the Sardine Serie of Thayer ( 1939) and includes rocks 

de cribcd a the Fern Ridge Tuff by Thayer ( 1939). the 

Rhododendron Form ati on by Hodge ( 19"3), and the Outerson Basa lt by Hammond ( 1979) and Hamm ond and th ~ 

( 1980). In the stud y area . the Sardine Form ation includes 

basa lt at M ark Ridge northca. t of wcet H m . basaltic 

ande. ite at Wa. hburn Butt . nonmarine tuffacc u . trata 

o erl ying the Columbia Ri ver Basalt r up in the Wald 

Hill . and volcanic and volcaniclasti rock r . ting on ca tdipping ro k of the Little Butte ol anic. southca. t of th 

Waldo Hills. Adj acent to the north rn Will amcttc all y, 

breccia and tuff of the Rhododen Iron Formati on ar o crl ain 

unconformabl by p roxenc andc ite flow. , \ ith the un onfonnit marked by a laterite (Hampton, 1972). T hi s ol anism was more ca lc-alkaline than that pri or to erupti n of th 

Columbia Ri ver Basalt Group, and it con. i ts of Ia a fl ows 

and debri. flow of intermediate compo. iti n with locall 

abundqnt ba. alt and ba altic andesite (Pri st and Vogt. 

1983). The ba alt at M ark Ridge was dated at 4.5±0.28 M a 

(Episode 3 of Priest, 1990). and the ba. altic ande itc at 

Wa hburn Butte wa dated at 11 .9±0.3 M a ( crplan k, 

1985; Epi. ode 2 of Priest, 1990). 

NONMARINE FINE-GRAINED 

SEDIMENTARY DEPOSIT 

In the Portl and, Tu alatin. and Will amett basins. the 

olumbia Ri ver Basalt Group and older r ck. " rc deepl y 

eroded. developing a topographic . urfacc with a: much as 

250 m relief. These rocks arc o erl ain unconform abl by 

moderately to poorl y lithified . ilt tone,. andston . mud: tonc, 

and clay. tone with common woo I fragments and local v 1-

canic ash and pumice and. The . eq uencc cx po:cd along the 

lackama. and Sandy Ri v rs wa named the andy Ri ver 

Mudstone by Trimble ( 196 ). who considered the mode of 

deposition to be lacu trine. However, sedi mentary . tructu rcs 

along the lackamas Ri ver . uggcst a fl uvial origin ( .D. 

Peter. on, P rtl and St ate ni vcrsity. and A .R. icm, Oregon 

tate ni versity, oral com mun., 19 9). Deeply 1 ca thcrcd 

flu vial and I e al . ilts interbedded with gra els dominated 

by clast of the olumbia Ri ver Basalt Group in the Tualatin 

ba in were mapped a the Helveti a Form ation by Schlicker 

and Deacon ( 1967). These sed imentary depo it are at lea t 

240- 275 m thick in the Portland bas in, 350 m thick we. t of 

Bca erton. and 410 m thick benea th Hill sboro in the center 

of th Tualatin ba in. The pre cncc of clast of granite and 

quartzite together with abundant quart z and mi ca and more 

locally deri ved clast. sugge t that the greater part of thc:e 

sedimentary materi als wa depo ited by the ances tral Columbia Ri ver with omc contribution by . ide . tream draining the 

ascade Range and the ri sing Tualatin M ountains. 

South of the Portl and ba. in. correlative . cdimentary 

d po its are identified mainly in the ubsurfacc. where the 

arc known to water-well driller as the so-called blue clay. 

In the northern Willamcttc Valley (pl. 2B), the contac t 

between these deposit and the C lumbia Ri ver Ba alt 

Group is marked in om wells by a laterite. The sedimentary deposits increas in thick ne s northca tward . from 160 

m in the Rei chold Bagdanoff 23- 2 well and Reichold 
196 AS ES I G EARTHQ AK HAZ RDS A D REDUCI G Rl K I THE PACIFI ORTHWE T 

Werner 14-21 well to 265 m in the Oregon atural Gas 

De hazer 13- 22 well and 300 m in the Damon Stauffer 

Farms 35- 1 well. In the northern Will amctte Valley and 

Tualatin ba in , the sed imentary depo it are characteri zed 

on sci. mic profile a a lower sequence with low-amplitude 

renector and an upper seq uence with medium- to highamplitude rcnectors. Paleontologic analysis of borehole 

cu tting. from the Tualatin ba in indica tes that this upper 

sequence probably i late t Pliocene or early Plei. tocene in 

age ( nruh and others, 1994). 

In the . outhem Willamctte Valley ·outh of the Salem 

Hill , a sequence of clay with intercalated and and gravel 

was found to overlie marine Eocene strata along a . urfacc of 

moderate relief ( 1iem and other , 1987) and is a much a 

I 00 m thick ncar the center of the valley. Corehole DH 13- 8 

obtained by the Oregon State Highway Divi ion at Corvalli 

penetrated 42 m of gr eni sh-blue to blue-gray micaceous 

clay with minor interlayered and, dark -brown organic clay, 

and poorly developed pal ·osol with rootlets in growth po it ion, suggc ting a nuvial origin (fig. 79). In another corehole 

at orvallis, a multicolored paleo ol i at the ba e of the clay 

unit. In corehole DI-1 14-90 between Sublimity and Stayton, 

the Columbia River Basalt Group i overlain by blue to darkgray clay wi th inter alations of volcanic! a. tic and and with 

paleosols with rootlet in growth position (fig. 80). ear 

onroe, several corehole penetrated a clay unit informall y 

named the M onroe clay and dated a late Miocene to earl y 

PI iocene on the bas is of palynology (Robert and Whitehead, 

1984 ). Roberts and Whitehead ( 1984) interpreted the M nroe 

clay as lacu trine in origin, in contra t to the nuvial interpretation for clay from the Oregon State Highway Divi ion 

corehole. Toward theccnter ofthesouthern Willamette Valle , the clay is interbedded with sand and gravel that Graven 

( 1990) interpreted as part of the main channel of the 1 rotoWill amctte River (figs. 81 and 82). However, more detailed 

study of water wells by M arshall Gannett ( .S. Geological 

Survey, oral com mun ., 1993) and Paul renna (Oregon tate 

ni versity, oral commun., 1993) how that the coarser 

grained depos its arc part of glacionuvial fans from the Cascade Range. Drainage from a fan emerging from the drainage 

ba in of the orth Santiam River at Stayton (pl. 2B) appear 

to have no wed through a narrow water gap at the eastern margin of the Salem Hills now occupied by Mill Creek, an underfit . trcam. A channel beneath the pre ent-day Willamette 

River west of the Salem Hill i · filled by fine-grained sedimen tary deposits rather than the . and-and-gravel fan facies 

found in the channel beneath Mill reek. The main entry 

point i in the Eugene- pringfield area, where a 90-m-thick 

. equencc of sand and gravel wa. called the Springfield delta 

by Frank ( 197 ). A imilar gravel fan is found near Sa lem at 

the n rth end of the Mill reek watergap. eismic profil es in 

the northern Will amettc Valley and Tualatin bas in show a 

prominent seri es of rcncctors midway in the sedimentary 

sequence th at are coarse clastic units in the Tualatin ba in . 

based on water-well log . . Other side channels underlie the 

prc:cnt onh anti am Ri vcr and South anti am River on the 

ca. t side of the al ley and Long Tom Creek at the southwestern orncr of th valley. 

Depth 

(meters) 

0 -.:-:-::. =-=-==· =-:1 - -- ----·- · ---- -- -----

-·-·-----· ------ -- -- · --· .... 

_--\-:j --{- ~ --\-

20 --=-t..---= ::=-r 

-_ ....... _-_-...... -

=-~=-= -=-T-= 

30 - _-_-_-_-_-_-_-_-_ 

---------

-- - ------ 40 

-_.....,._-__ _ -_-_-_-___ _ 

50 - -=-=-=-=-=-=-=-=-

--------

Willameue Formation: brown, micaceous, clayey silt 

with some fine sand and a trace of organic material 

Rowland Formation (Diamond Hill M ember): gray, 

micaceous, fine-grained silty sand 

Rowland Formation (Linn Member): gray to brown 

sandy gravel and gravelly sand; many clasts greater 

than 3 em, predominantly basalt 

Unnamed fluvial sedimentary deposit s: blue-green to 

gray clay with faint laminations and some organic 

material; abundant slickensided fractures 

Small rootlets in growth position 

Large wood fragments greater than 3 em thick 

Small rootlets in growth position 

lnterlayered dark·green to gray clayey sand 

Dark-brown organic clay 

Large wood fragments, some more than 6 em thick 

(logs or roots) 

Spencer Formation: green-gray siltstone, and 

gradational contact with overlying clay 

o~o~o~Oc Basaltic sandy gravel 

Dark-gray sandstone and siltstone 

Figure 79. Graphic log of corehole DH 13- , drilled on the 

sou thea. t side of Corvallis, Oreg., just ca t of the inter ection of 

State Highway 34 and 99W (. ee pl. 3 and fi g. 82 for location of 

corehole). Total depth represented i 47.9 m. 

TROUTDALE FORMATION 

The moderately to wel l-indurated pebble to cobble conglomerate with a silt and sand matrix in the Portland basin is 

referred to the Troutdale Formation (Hodge, 1933: Trimble. 

1963). The conglomerate cia ts are predominantly derived 

from the Columbia River Ba alt Group with significant percentage of exotic clasts such as quartzite and grani te, indicating depos ition by the ance tral Columbia River (ancestral 

Columbia Ri ver facie of the Troutdale Formation of Tol an 

and Beeson, 1984). The upper part of the ance tral olumbia 

Ri ver fac ies include andstone contai ning basaltic gla . 

together with interbeds of conglomerate with cia. ts of highalumina basalt derived from the Boring Lavas and the informally named High Ca cade lava , and the subordinate 

forei gn cia t (Tolan and Beeson, 1984; Swan on, 1986). 
TECTO I S OF TH E WILLAM ETTE VALL Y, OREG01 197 

Depth 

(m eters) 

......-~~=-=:-:-=1 

10 

30 

40 

Clayey silt: mottled red brown and gray, minor wood 

fragments and decomposed pumice or lithic sand 

grains 

Clayey sand: red brown, with heavy mineral s 

Silty clay w ith some sand: blue gray and mottled brown; 

sand is fine to coarse grained, decomposed to clay; 

minor wood fragments 

Clay: green gray 

Silty clay with some sand: blue gray and mottled brown; 

sand is fine to coarse grained, decomposed to clay; 

~~~~~....__ \Nm~~i,;n~~o,'r,. w~ood fragments t- ash layer(?): mottled pale orange and dusky 

Clayey si lt: dark yellowish brown to olive black; some 

very fine sand with decomposed pumice or lithic 

grains 

Silty sand: olive black to olive gray; ba saltic; moderately 

decomposed; minor wood fragments 

Silty clay: mottled green gray to blue gray and yellow 

brown 

Clayey sand: dark green gray, fine to coa rse grained, 

basaltic; wood fragments 

Silty clay and clayey silt: mottled blue gray and brown; 

nodule layers 1- 2 em thick 

Subvertical root traces 

Silty sand: olive gray, fine to coarse grained 

Silty clay and clayey silt: mottled blue gray and brow n, 

§~~~~'- basaltic; nodule layers 1- 2 em thick Silty sand: olive gray, fine to coarse grained 

Silty clay and clayey silt: olive gray; thin (2- 8 em) layers 

of fine to coarse multicolored sand grains, volcanic 

and decomposed, fining upwards; abundant small 

wood fragments 

Rootlets in growth position 

Columbia River Basa lt Group: basalt, olive black. 

aphanitic, vesicular. fi ne to very fine ph enocrysts of 

plagioclase t2- 3 percent) 

Figure 80. Graphic log of corehole DH 14-90. drilled 1.4 km west 

of ublimity at State Highw, y 22 nonh of Mill reck on the . outhern edge of the Waldo Hill.. Oregon. showing unnamed nonmarine 

sedimentary deposits over! ing the olumbi a Ri er Basalt Group. 

orrclati ve conglomerate in the southca t part of the Portland ba in contain a predom inance of last derived from 

the Ca:cadc Range (Tolan and Beeson, 19 4: Hanf rd and 

McFarl and, 1989). Th Trou tdale overlie the andy Ri ver 

Mud. tone and locally may be interbedded with iL. Pl ant fo.-

sil. in the Troutdale arc of earl y Pli o ene age (Trimble. 

1963), but the upJ cr part of the formati on could be younger. 

BORING LAVA AND 

SNOW PEAK VOLCANO 

Vents and flow of high-alumina. diktytax iti . porphynuc olivine ba alt and basaltic ande itc ' ith subordinat 

pyroclastic rock , breccia, and ash in the Portland basin w r 

named the Boring L a a. by Trea. her ( 1942) and r named th 

Boring L a a. by Allen ( 1975). The. c Ia a. intrude th and 

Ri er Mud. tone and Troutdale Formation and form cone. of 

interlayercd cinders and Ia a(. uch a Mt. S lvania and 

Scott in the middle of urban Portland) on an eroded . urfa c 

of the Troutdale Formati on (Trimble, 1963) a. well a. forming stock from whi h the surrounding country rock has be n 

eroded (Rocky Butte). Ea. t f Portl and, high-alumina ba. alt 

flow overlie and arc interb ddcd \ ith th Troutdal Formation (Lowry and Baldwin. 1952: Tolan and Be son. 19 4: 

Tolan and others, 1989). The Boring La a. ar older than late t Plei. tocenc flood deposit. ( lien. 1975). In th as tern 

Tualatin bas in and the northern Will amettc ' all ey, ub. urface bodie isiblc on . cismic line. as bowing ur thc olumbia Ri ver Ba. alt Group arc intcrpr ted as basalt intru ion · 

correlated with the Boring Lavas . An intracanyon fl ow at 

Carver in the Jackamas Ri cr alley southeast of Portl and 

ha a potassium-argon age of 6 12±23 ka (R. . Duncan, Oregon State ni vcr ity. oral commun. to I.P. M aclin , 1989): 

other pota sium-argon age ar a. old a 5 M a (Luedke and 

Smith, 19 2: Swanson, 19 6). The Boring L avas of the Oregon City pl ateau are dated a. 2.6 M a (S wan. on. 1986). 

Snow Pca k,ju t ea. t of the . tudy area, consi. L of nearl y 

I ,000 m of ba altic ande itc flm · and breccia with minor 

basa lt (Beau! ieu, 1974 ). The. e rocks were ela ted by V !'-

planck ( 1985) a 3.3±0. 6 M a and 2.8±0. M a, equi va lent in 

age to the Boring Lavas. The shield olcano is deepl y dissected by -shaped va lley. ar eel by glaciers. 

PLEISTOCENE TERRACE GRAVELS 

Sedimentary material young r than the Boring Lavas, 

Troutdale Form ation. and nonmarine fine-grained edimcntary depo. its are de cribed by cDowell ( 199 1 ). Gravels in 

the eastern pan s of the Willamctte Vall y and Portland basin 

are glaciofluvial, deri ved from the Cascade Range. wherea. 

gravels on the we. t ide of the e ba:ins arc flu vial and 

derived from the Coa t Range. Alii. on ( 1953) de cribcd 

three terrace-gravel unit on the eastern margin of the southern Willamctte Valley; from oldest to youngest, these arc the 

Lacomb, Leffler, and Linn Gra cl.. The L acomb and Leffler 

Gravels are pre crved a high terrace at altitude of 70-200 

m along the edge of the valley (Alii. on, 1953: lli son and 

Felts, 1956). We do not . cparate the Lacomb and Leffler 

Gravels but in. tcad refer to the. e units an I other deepl y 

weathered gravels as high-terrace gravels (fi g. 8:'). Robert 

( 1984) suggc. ted that the high-terrace gravels are the constructi onal top of hi informall y named M nroc clay. Subsequent erosion was ace mpanicd by development of deep 
19 A SESSI G EARTHQ AKE HAZARDS A D RED G Rl KIN TH - PA IFIC ORTHWEST 

44" 

45' 

44" 

30' 

44" 

15' 

Location map 

0 

123"15' 

o . 

10 

/ 

/ 

/ 

123"00' 

0 Salem 

& 

lba~: • r! 

0 /. 

. / 

. / 

/ 

/ 

/ / 

/ / 

Lebanon 

. //.. . • ·· <;:::, 

/ / . " . . . 

. . 

EXPLANATION 

- 80- Structure contour, in 

me ters- Dashed where 

inferred 

~~~ Subcrop of clay beneath 

Pleistocene gravels 

122"45' 

20 KILOMETERS Wate r well used to dete rmine 

structure 

Figure 81. 1ruc1ure con10ur map of the 10p of the nonmarine sedimentary dep sits overl ying the Columbia Ri ver Basa l! Group, which is the contact with the base of the Pleistocene Rowland Formation 

(Balster and Par. ons, 1969), in the . outhern Willamene Valley, Oregon. H- /-1 ' is the li ne of s cti on for 

the cross secti on shown in fi gure 82. 
WEST 

H Oi :.:: 120 r- ~ UJ 

.,;oo UJ 

~~~ cr: 

Ut--

>~ _J_J 

Rowland :::=> sor- 86~ Formation 0~ 

V) 

a: 

40 

::: SEA 

~ LEVEL ~ 

t 

a._ 

~ -40 

-80 

-120 

Overbank facies 

: of unnamed : 

: sedimentary : 1=:: ::/, -- depOSitS - - =:: / 

(/) 

E= 

~ "' ~ ~ 

a._~ 

~~ · Willamette Formation 

Glaciofluvial fan facies of 

unnamed sedimentary deposits 

Rowland 

Form ation 

Eugene Formation 

0 2 4 KILOMETERS 

--~::_ _ 

Overbank facies of 

unnamed sedimentary deposits 

Oligocene and 

Miocene 

volcanic rocks 

? ............._ 

?............._ 

?'---.... 

? 

Vertical exaggeration 42:1 

EAST 

H' 

Figure 82. Structural cross secti on between Corvallis and Lebanon, Oreg., showing channel and overbank facies of unnamed nuvial sedimentary depos it s, high-terrace gravels, late 

Pleistocene outwash depos its of the Rowland Formati on, and catastrophic nood deposits of the Willamette Formati on. Data are from water wells. engineering bore holes, and petroleumex plorati on well s. 

-l 

rn 

n 

-l 

0 

z 

n 

en 

0 

-n 

-l 

:c 

rn 

< 

r 

r 

)> 

s:: 

rn 

:j 

rn 

< 

r 

r 

rn 

-< 

0 

;;v 

rn 

Cl 

0 

\() 

\() 
200 ASSES I G EARTHQ AKE HAZA RD D RED CI G Rl K I THE PA IFI ORTHWE T 

oil atop the Eola urface of Balster and Par ons ( 1968). 

evidence that the oil of the Eola urface are not correlati ve 

with the paleo ol on top of the Spencer Formation near Corvalli s or on top of the Columbia Ri ver Ba. alt Group in the 

northern Wil lamette Valley. 

The material forming the Linn Gra els. at or below the 

present Willamette Valley floor. were called the Rowland 

Formation by Bal ter and Par on ( 1969) and di idee! into 

two members in the southern Will amette Valley: the Linn 

Member, predominantly . ilt. and, and gravel , and the overlying Diamond Hill Member, predominantly and and silt 

capped by a palco ol. The Linn Member is 6 m thick at orvall i and thicken. to about 20 m along the eastern edge of 

the valley. Alii on ( 1953) suggested that the Linn Gravel 

are glacial outwa h ed imcnts deri eel from the Ca cacle 

Range, a view upportecl by a urface morphology of coalescing allu ial fan recognized by Piper ( 1942). The orth 

Santiam, South Santiam , Will amette, McKenzie, and alapooia Ri ver. all appear to have contributed to the e deposit . Radiocarbon date show that cl position of the Diamond 

Hill Member began before 36.000 year. B.P. and continued 

pa t 28,500 year B. P. ( J! cDowcll and Roberts, 1987). 

fn the Portl and basi n, Trimble ( 1963) mapped the 

Springwater, Gre ham. and Estacada Formations, con i ting 

largely of glacioflu vial boulder and cobble gravel int rbecldecl with muclflow and forming terrace along the Clackamas and Sandy Ri vers. Mad in ( 1990) found that the Es tacada 

Formation includes ·cveral terrace ·, and he has recommended that Trimble' s ( 1963) nomenclature be discontinued. 

The relation between the Willamette Valley terraces 

and Plei tocene glacial seq uence of the Cascade Range i. 

poorly understood. 

PORTLAND HILLS SILT 

Poorly indurated quartz- and mica- bearing silt mantling hills around the Portland and Tualatin basins wa 

named the Portl and HilL Silt [Member] of the Troutdale 

Formation by Lowry and Baldwin ( 1952); thi s unit wa 

subsequently rai. ed to forma ti onal rank a the Portland 

Hills Silt by Baldwi n ( 1964). The ilt i more than 30 m 

thick in the Portl and Hill. but thinner elsewhere, and it is 

ab. ent in the Red Hill s of Dundee and farther outh . Lentz 

( 198 1) demon trated that the Portland Hill Silt is predominantly Joe s derived from the olumbia basin east of the 

Cascade Range. Lentz ( 198 1) delineated as many as four 

silt units eparatccl by paleo ol , indicating that the Joe s 

accumulated over a con. iclerable time span in the Quaternary. At one lo ality, the ilt i. apparentl y interbedded with 

what entz ( 198 1) call the Boring Lava . 

CATASTROPHIC FLOOD DEPOSITS 

The Willam tte Valley fl oor as far south a: Eugene i 

mantled with horizontally b deled silt and gravel derived 

from the olumbia basin by glacial-outbur. t fl o d cau ed 

by the ata. trophic drainage of Glacial Lake Mi ssoula. an 

origin fir t recognized by Alii. on ( 1932. 1936, 1978). 

Trea ·her ( 1942) used the term " Wi llamctte Valley terrace." 

for light brown , homogeneous silt interbedded with coarser 

grained deposit . Alli son ( 1953) subsequently ref rred to 

thee clepo it as the Will am tte Sil ts, and Baldwin and other ( 1955) called them the Willam tte ilt. The unit contains 

boulder of exotic lithology found around the margi ns of the 

Willamette Valley, and a type ec tion at Iri sh Bend was 

de cribed by A IIi son ( 1953). Balster and Parsons ( 1969) 

used the name "Willam ttc Form ation' ' for the unit and subdivided it into four member . The ba. al Wy tt M mber con-

. ist of sand and silt that o rlic the Ro> land Formation 

unconformably a. localized channel fi ll. . The vcrl ying lri h 

Bend Member re ulted from multiple catastr phic flood. 

that clcpo ited . ilt in Jow-l ing area. across much of the Willamette Valley (fig. "). The coar. c-grainecl equi valent. or 

thee ilts in then rthcrn Will amcttc Valley were ca lled the 

Ri ver Bend Member (o f th Will am ttc Formati on) by Robens ( 19 4), and they consist of at lea. t 40 rhythmically 

clepo itcd beds of silt and fine . and. each apparentl y deposited by an incli idual cata trophic flood (Glenn, 1965). A 

paleosol separates the l ri sh Bend Member from the overlying M al pas Member. an ex ten i c but eli. continuou clay 

unit (Parsons and other. , 1968; Bal tcr and Parsons, 1969). 

The uppermo t Greenback Member con. ist. of i It accompanied by erratic boulder. draped o cr the landscape at altitude as high as 122 m by one ca tas trophic flood (fig. ) 

(Alii. on. 19- ). In the Portl and bas in. the flood deposit 

include boulder gravel. sandy gravel. and and with a 

Columbia Ba in provenance (Allen and others. 19 6). The 

flood occurred later than 15,000 year B.P. (Baker and Bunker, 1985; Wai tt. 19 5), and a bog overlying flood clcpo it 

near Portland i. elated a. 13.000 year. B. P. (Mullineaux and 

others, 1978; McDowell, 199 1 ). 

HOLOCENE DEPOSIT 

During the Holocene, the Will amette Ri ver has inci sed 

the main Calapooyia-Senecal urfacc (fig. 83) and cut and 

depo. itcd three add itional urface , the Winkle, Ingram, and 

Hor eshoe (Balster and Parsons, 1968). The surface reflect 

a change from a more braided channel pattern to the pre ent 

meandering channel pattern (M cDowell , 199 1 ). Holocene 

and, gravel, . ilt, and clay arc largely confined to channels 

and fl ood pl ains of major river and their tributari es. 
TECTO lCS OF THE Wl LLAM ETTE VALLEY, OREGO 

CUT AND FILL STAGE 

Holocene 

GREENBACK MEMBER STAGE 

Late Pleistocene 

(about 13,000 years before present) 

Balster and Parsons (1969) 

IRISH BEND MEMBER STAGE 

Late Pleistocene 

(after 28,000 years before present) 

Glenn (1965) 

LINN GRAVELS STAGE 

Middle to late Plei stocene 

Allison (1953) 

HIGH TE RRACE GRAVELS STAGE 

Early to middle Pleistocene 

Piper (1943) 

UNNAMED SEDIMENTARY DEPOSITS STAGE 

Late M iocene to early Pl eistocene 

Deep weathering 

/ (Eola surface) 

Fine-grained fluvial 

(or overbank) 

Irish Bend Member 

Linn Gravels 

Spencer or ~d os ~~i5~~@li~~~~~~~ Volcanics 

Tyee 

Formation 

Eugene Formation 

Figure 83. Depo. i1ional history of the sou thern Willamette Val ley. regon. after ~i oce e time. Modified from Roberts ( 19 4). 

20 1 
202 

STRUCTURE 

Faulting and folding have taken place in the Willamelle 

Valley and Coa t Range ince the emplacement of the Siletz 

Ri ver Volcanic . Angular unconformiti s arc common. The 

region ha. been near a. ubducti on z ne for the entire cnozoic era. with pl ate convergence rate decrea ing and the 

trike- lip component of subduction increasing from the 

Paleocene to the pre ent (Wells and other , 1984; Riddihough. 1984). Thi. hi tory ha resulted in clockwi e rotati on 

of tructural blocks, with the amount of rotation decrca. ing 

from ocene time to the pre ent and from wes t toea l. 

In thi s chapter. we focus on deformation affecting the 

Columbia Ri ver Ba alt Group and younger depo it . Older 

structures are examined briefl y; they are important to our 

di cu sion only b cau e in omc plac s they are zone of 

weakne s reacti vated by younger faulting and thu they may 

ha e seismogenic potential. Fault cutting Oligocene and 

older strata but having unknown age relation to the olumbia Ri ver Ba alt Group and younger depo its are discu sed 

by Graven ( 1990) and Wemer ( 1990). M o t faults are shown 

on plate 2A and 28 and in figure 84 and 85. 

DEFORMATION PREDATING THE 

COLUMBIA RIVER BASALT GROUP 

COAST RA GE A TICLI ORI M 

Pre ent-day exposure of the Si letz Ri ver Volcanic 

commonly correspond to ba ement highland that ex i ted 

during the time ocene trata were depo ited. The Tyee Formation onlapped a highland composed of the Siletz Ri ver 

Volcanic in the Val. etz area wes t of Dall as . The Si letz 

Ri ver Volcani cs in the hanging wall of the Corva lli fault 

occupied a po iti e area that cau. cd the in formall y named 

Miller and of the Yamhill Form ation to lens out toward the 

fault (Baker, 19 8; Gra en. 1990). Thi po iti ve area provided detritu to the upper Eocene Spencer Formation (Goldfinger, 1990). In the northern Will amette Valley, strata as 

young as the cott Mill Formation (Oligocene and 

Miocene) were tilted to the east prior to the formation of the 

olumbia Ri ver Basa lt Group, presumabl y reflecting uplift 

of the Coa t Range, yet the Coast Range was low enough that 

intracanyon flows of the Columbia Ri ver Ba alt Group were 

able to eros the range to the pre cnt-day coastline. 

CORVALLI FA LT 

The northea t-trending Corvalli s fault (fi g. 84; pl. 2B) 

at lea t 50 km long, and for part of it length , it i the we tern boundary of the outhern Will amelle Valley. The fault 

cannot be traced across the Will ametle Ri ver to the Salem 

Hil l., although the small-displacement Turner fault in the 

RTHWE T 

Salem Hill ha the . am trend and sen of di. placement. 

Two recent gravity profil es show that the primary fault is a 

thru t that dips about I 0° north we t (G ldfinger, 1990). Dip 

of strata in the hanging wall average 20° northwes t, and bed 

in both th hanging wall and the footwall arc overturned 

clos to the fault, uggcsting a fault-propagati on fold geometry. Verti cal separation on the rvalli fault at cismogenic depths is about 6.7 km, a figure obtained by adding 

the separati on on the fault at th urfacc and the . cparati on 

implied by considering surfac folding as caused by fault 

propagation (Yeats, 19 ). ing a fault dip of 20°, the horizontal hortcning is calculated a. 11 - 13 km . The fault wa. 

ac ti ve in late Eoc nc tim , an interpretati on based on isopachs n a map of the Miller and that . how thinning of the 

and in the dir lion of the or alii. fault (Baker, 198 ) and 

on sedimcntar breccia with cia. t. of the ilctz Ri v r Volcanics pre ent within the Spencer Form ation, adjacent to the 

fault we t of Corvalli_ ( ldfinger, 1990). A y ungcr highangle fault parallel t the orval li s thrust i xpo ·ed in a 

quarry 2 km northca t of Philomath and has left-lateral horizontal licken ·ide and mullion tructur . The main fault 

trace i off et by . cveral northwe. !-trending fault . Bccau e 

thi. high-angle fault may di pl ace Pleistocene edimcntary 

depos it , it is di cu. sed further below. 

EOLA HILL -AM ITY HILL ORMAL FA LTS 

A proprietar sci mi profile show. that the ilctz 

Ri cr Volcanics and the lower part of the Yamhi ll Formation 

in the Eola Hill and A mity Hill are cut by two normal fa ults 

bowing movement down to the eas t (fig. 5). Thi profi le 

and a residual gravity map (Werner, 1990) suggc. t that the 

faults strike north-northca. t. The Yamhill Formation 

increa e. in th icknes eastward aero s the fault from 950 to 

I ,450 ITl. The Spencer Formation and the upper part of the 

Yamhill Formation are onl y lightl y warped acres. the westernmost of the two fau lt . 

DEFORMATION YOUNGER THAN THE 

COLUMBIA RIVER BASALT GROUP 

COAST RA GE ANTICLI ORJ M A D 

WILLAMETTE VALLEY YNCLI ORl M 

Flow of the Columbia Ri ver Basalt Group can be 

traced to the western margin of the northern Will amette ValIcy where they are exposed in an ea t-dipping homoclinc. 

ubaerial fl ows and in vasive fl ow identical to tho e mapped 

in the Will amettc Valley are found on the Oregon coa t from 

Seal Rock north to the Columbia Ri ver. The ab ence of the 

Columbia Ri ver Ba. alt Group in the intervening Coast 

Range is due to younger warping of the Coast Range (Niem 

and 1 iem, 1985). As the Coast Range arched upward , the 
TECTONIC OF THE WILLAM ETTE VALLEY, OREGO 203 

\ illamette Valley ub ided and accumulated ediment of 

the proto-Willamette and proto-Columbia ri ver and major 

tributary side streams. At Monroe, the e edimentary deposit are dated by palynology a late Miocene to earl y Pliocene 

(Robert and Whitehead, 1984), but contact relations a 

young as 0.6 M a with the Boring Lava sugge t that the e 

edimentary materi al may be a young a Plei tocene. We 

suggest that the aggradati on in the Willamette Valley and 

adjacent Columbia Ri ver valley was cau ed by a relati ve 

change in ba e level as the Coast Range wa uplifted. 

Farther ca t, the we tern Ca cade Range underwent tilting (Prie t, 1989). Bee on Tolan, and Anderson ( 19 9) recogni zed more than 1,200 m of uplift of the Cascade Range 

near the Columbia Ri ver, based on the tructure oft he Columbia Ri ver Basalt Group and overly ing Troutdale Formation. 

CORVALLIS FA LT 

In the main Corvallis fault zone, hori zontal lickenside 

are found in rock s a young a Oligocene intru ions, sugge ting reactivation in a stre s field compatible with north- outh 

com pres ion. In Corvallis and along the lower reache of the 

Marys River. the contact between gravel pos ibl y correlated 

with the Rowl and Formation and the Willamette Form ation 

dips 6°- 12° ea t and outhea t. The gra vel is capped by the 

Quad surface of Bal ster and Parsons ( 1969), a probable continuation of their Calapooyia UJface. Adj acent to the Corvallis fau lt, thi s urface i 30-40 m higher than it i farther 

ea t, ugge ting to Bal ter and Parsons ( 1969) that the surface was uplifted by faulting. At the Mid Valley quarry 

bet\ een Philomath and Corvalli s, the contact between the 

Willamette Formation and the underlying gravels i at an 

altitude of I 07 m, near the highe t elevati on at which the 

Willamctte Fom1ation has been found (McDowell and R bert. , 1987). Thi contact i at an altitude f 6 m in the Willamcuc Ri r channel ea t f the Mid Valley quarr In 

add ition. prc-Ro> land orm ation crbank facie depo it. 

of the proto-Willamettc Ri ver . imilar t tho. c pre cnt at and 

ca. t of th Will amcttc River (fig . 79. I. and 2) arc absent 

beneath the gra cis at the Mid alley quarry. The relation 

suggc t ca. tward tilting or ea t-sidc-down faulting. ltcrnati ely, the gra cl at the Mid Valley quarry may be older 

than the Rowland Formation at the Willamette River. 

In north orvalli. between Walnut Boulevard and a 

sadd le at the entrance to hip Ross Park . south of Jack. on 

reck, a carp ranging from a few centimeter to I m in 

height mark the trace of the orva lli. fault. It ha the ame 

sen. c of di placement as the main fault, teeply clipping with 

the southca t ide clown, a ba. ed on relation. xposcd in a 

hand-dug pit. This carp could be a lump rather than a fault. 

Low-sun-angle aeri al photographs how north wc !-trending 

. carp that may be related to left step on the orvalli. fault. 

These scarp. occur in areas of outcrops of the Si letz River 

Volcanic. and high-terrace gravel . The n otcctoni c origin 

of these features is not confirmed (Goldfinger. 1990). 

Goldfinger ( 1990) identified three eanhquak that 

have been felt along the general trend of the Corvalli s fau lt, 

one of intensity III in 1957, one of in ten. it I ll- 1 in 1961. 

and one of in ten ity V probably in 1946 or 1947 (in orTe tl y 

reported as May 12, 1942, by Berg and Baker, 1963). 

OWL CREEK FA LT 

The Owl Creek fault (fi gs. 82 and 84) strike . I oo E., 

has re erse separation with the ea t ide up, and i a ociated 

with an anticline in the hanging wa ll. The Spencer Formation 

is 220 m thick on the ere t of this fold but 550 m thick ca t 

of the fold , sugge. ting growth during Eocene time (Graven, 

1990). Water-well data how that the bedrock urfac i. 115 

m higher on the ere t of the anticline than it i. to the ea. t and 

we. t (fig. 82). By compari on. the ba e of the Spencer Formation i 725 m higher on the ere t of the anti line than it i 

to the ea t and we t. In the upthrown block, protoWill amette Ri ver overbank depo it dip as t with re pe tto 

gravel of the Rowland Formation, and the Rowland Fomlation i eroded away near the fau lt that the Will am uc Formati on rest directl y on the Eugene Formation (fig. 2). 

Gra I in the Rowland Formation of Ca cadc Range provenance i xpo. ed in the banks of the Willamette River at Corvalli . we t of the fault, ugge ting that the Rowland 

Formati on wa depo ited over the Owl reek . tru turc. then 

uplifted and eroded from the hanging-wall block prior to the 

depo ition of the Will amctte Formation. which appears to 

postdate all faulting. 

HARRI B RG At TICLI E 

A broad ea t-nonhea t-trending anticl ine between 

orvalli and Eugene plunge ea t (fig. 4) and ha about 

I 00 111 rei ief on the Eugene-Spencer Formation contact. 

The ax i of the channel of the proto-Will amette Ri cr i 

warped upward about 50 111 where it cro ses the anticline at 

Harri sburg (fig. 6). There i no evidence of faulting as ociated with th i anticline. 

JEFFERSOr A T1 LI E 

A broad anticline (fig. 4. pl. 2B) north of Albany create th concave-to-the-west map trace of the ba e r the 

ugene Formati n. as expre ed by ·tructur contours. 

There i. no evidence of tratigraphic thinning of ocene 

. trata across the structure, indica ting that the fold de eloped 

later. Topogra1 h resulting from anticlinal growth may have 

diverted flows of the Columbia Ri ver Ba a it Group, particularly the inkgo flow of the Frenchman Springs Member of 

the Wanapum Ba alt. northwc tward around the anti cline. 
204 A SE I G EARTHQ A KE HAZ RD 

44° 

30' 

w 

<..9 

z 

<{ 

a: 

D REDU lNG Rl K I TH P ! FIC ORTHWE T 

w 

<..9 

z 

<{ 

a: 

w 

0 

<{ 

u 

(/) 

<{ 

u 

EXPLANATION 

---- Contact 

----'--- Fault-Bar and ball on 

downthrown side 

+--t--- Anticline-Showing 

direction of plunge 

+---t-- Syncline-Showing 

direction of plunge 

10 20 KILOMETERS 

Figure 84. Tectonic map of the southern Wi llamette Valley. Oregon. Areas underlain by alluvial and nuvial deposits that postdate the 

olumbia River Basa lt Group are unshaded; area. underl ain directl y by bedrock arc shaded. BCF, Beaver Creek fau lt; CRF, Calapooia Ri ver 

fau lt: EAF, ast Albany fau lt; GF, Glenbrook fau lt: HA, Harrisburg anticline; JA, Jefferson anticline; LF, Lebanon fault: MCF, Mi ll reek 

fau lt; 1F. Monroe fau lt; Ow F, Owl Creek fault; OkCF, Oak Creek fault: PCF, Pierce Creek fault; RBF, Ridgeway Butte fault ; TF, Turner 

fault: WHR FF, Waldo Hills range-front fault; WH . Waldo Hill. uplift. 
TECTO 1

1 S OF THE WILLAM ETTE VALLEY, OREG01 205 

123'30' 

45' 

15' 

45' 

00' 

123'15' 123'00' 

10 

PEM 

ff a Butte 

@ Aurora 

intrusion 

20 KILOMETERS 

SILVERTON 

HILLS 

EXPLA ATION 

---- Contact 

----'"-- Fault- Bar and ball on 

downthrown side 

+-----'--- Monocline- hewing 

direction of plunge 

+----+-- Anticline- hewing 

direction of plunge 

+---t-- Syncline-Showing 

direction of plunge 

Figure 85. Tectonic map of the nort hern Wi ll amelte Valley. Oreg. nshaded area are underlai n by alluvial and tlu ial depo its that postdate the olumbia Ri er Basalt Group. H. Ami ty Hil ls; 1. Chehalem Mountains: PAM. Pan·ett Moumain: PEM . Petes Mountain; RHO. 

Red Hill s of Dundee: F. unis fault: G F. Gales Creek fau lt: MA . Mt. Angel fau lt: M F. M ill reek fault: F, herwood fau lt: TF, 

Turner fault : WHRFF. Waldo Hills range-front faul t: YR F, Yamhill River fault. 

The ax i. of a channel of the proto-Willamcllc Riv r west of 

the alcrn Hill crosses a b dro k highland north and cas t of 

Albany including Spring Hill. cravcl Hill, and Hale Bulle 

(fi . 8 1: pl. 3). and it is li kely that thi s channel was warped 

upward across the Jefferson anticline. HO\ ever. the base of 

the Rowland ·orm ation doc not appear to be warped (fig. 

8 1 ). The JciTer on an ticline may b a northeast c ntinuation 

of Lhe hanging-wa ll block of the orval li s fault in whi ch the 

Siletz Ri ver Volcanics arc at the su rface. The northwes t-

. triking contac t of the il t7. River Vol canics wi th younger 

rock . i. cxprcs cd as a positi ve gravi ty an maly in rc idualgrav ity contours (Werner. 1990). 
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::1: Vertical exaggeration 100:1 

Figure 86. Longitudinal profi les of the modern and proto-Wi llamette Rivers, Oreg. A, profile of the modern ri ver. B. profi le of the proto-Willamette Ri ver showi ng deformation of the 

ax is of the main stream channel. See plate 3 for location of line of section P- P'. 

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TECTO S OF TH E WILL METTE ALLEY. OR EGO 207 

ORTH SA T IAM BASI 1 

The orth Santiam Ri ver crosses a ba. in bounded by 

the Waldo Hill. on the north , the Salem Hill s on the we t, 

and the wes tern Cascade Range on the east. The minimum 

tructural relief at the top of the Columbia River Basalt 

Group in the orth antiam basin is 260 m, based on the 

maximum depth to ba alt in water wel ls in the basin and al o 

the top of the olumbia Ri cr Basalt Group in the adjacent 

Waldo Hill . Except for the Mill reck fault discu sed 

below. no fault have been recognized in thi s ba in, perhap 

becau c of the difficulty of correlating horizon. in the .edimentary clepo its postdating the Colu mbia Ri ver Basalt 

Group acros the basin. 

MILL C REEK FA LT 

The southern edge of the Waldo Hill. i: marked by a 

fault that eli . place the olumbia Ri ver Basalt Group about 

100 m (pl. 2B). The base of the Columbia Ri ver Basalt 

Group i expo ed along the western end of the W aldo Hill 

near Turner. whereas the top of the group i near sea level in 

the 1 orth antiam basin . outh of the range front. The Mill 

Creek fault may be theca tern ex ten ion of the Turner fault 

in the Salem Hill s. 

WALDOH ILLSRA GE-FRONTFA LT 

The northern range front of the Waldo Hill s is marked 

by a pronounced northeas t-trending aerial-photogrc ph l incation that is on a trend with the Corva lli s fault to the 

outhwc t. The contac t b tween the Columbia Ri ver Ba alt 

Gr up and under! ing marine strata i. xpo eel near the 

rang fron t s uthea. t of Salem; . c era! w li s al o penetrate 

thi s contac t (fi g. 7). onhw st of the range front. water 

well s rca h the t p of the olumbia Ri cr Ba. alt Group. 

indica ting vcrti al . cparation of at least 50 111. The fault 

may xt net farther northeas t than . hown: crti ca l . paraLi on decreases to ard the northeas t. o clear cv idcnc 

ho' s that the fault cut. trata you nger than the Columbia 

Ri cr Ba alt Group. 

G LE. C REEK-MO T ANGEL 

TR CT RALZO E 

The ales reek-M ount ngc l . tructural zone is the 

southcmmost of . cvcral n rth wc. t-trcnding, seismica lly 

acti ve linear features in north west rn Oregon and south we tern Wa. hington (Wern er, 1990). Both th Gales reek -

M ount ngel structural zone and th Portl and Hill slac kamas Ri ver strucwral zone '· ere a ·ti c during f nnati on of the olumbia Ri cr Basalt roup. The M ount Ang I 

fault. part of the ale. reek-M ount ngc l structural zone. 

formed a barri r t three il er Falls flows of the rcnchman 

pring Member of the Wanapum Ba alt (Be son. Tolan. 

and Anderson. 1989). 

Geological evidence exist for three cgment. of the 

Gales Creek-Mount ngel structural zone: the Gale. re ' k 

fau lt. the ewbcrg fault , and the M ount Angel fault, whi ch 

have the ame strike but are off et (fi 0 . 5). Th Gale re k 

fault egment fol lows the Gale Creek alley between Gales 

Peak and Dav id Hill , juxtapo ing rock equi alen t to the 

Till amook ol anies to the ou thwc t with the olumbia 

Ri ver Ba alt Group on the northea. t. The fau lt ha been 

ex tended south toward Gaston. wher sci mic profi le. reveal 

a comp lex zone of deformati on extcndin o from Gaston to the 

ba e of the Chehalcm Mountains. M odeling fa gra ity prorile aero thi: zon . uggc. ts three fault sco mcnts ha ing a 

total verti cal separation. largely earlier than th olumbia 

Ri ver Ba. alt Group. of almost km , do n to the nonhca t 

(H.J. M eyer. Oregon atural Ga. orp .. oral c mmun., 

199 1 ). It i. unclear whether thi. zone connect. wi th the wberg fault eo m nt. 

Werner ( 1990) mapped the 'e' bero fault on th ba. i 

of water-well data (fig. ). 1onh of the fault. the base of the 

Columbia Ri er Ba. a it Group is expos don the south side of 

the Chehalcm ountain. and dips northeas t. South of th 

fault, the top of the Columbia Ri cr Ba alt Gr up is expo. cl 

in the Reel Hill of Dundc and al. o lip northeast. The 

group i juxtapo. cd aga in t Oli gocene and ioccne marine 

trata along the fault, though the apparent en e of verti cal 

off ct i oppo itc that cxpre cd in the Ga ton area 

(Popowsk i. 1995). Gradient ly ~.; . of aeromagnetic and 

gravi ty data support the fault lo at ion. A sci mi profi le 

aero the projection of the fault zone between cwbcrg and 

Woodbum hO\ no displa ement of the top of the olumbia Ri ver Ba. alt Group (Wcrn r. 1990). 

The M ount ngel fault (fi g. 5) wa fir t mapped ncar 

ount ngel by Hampton ( 1972) u ing water-well data. 

Based on additional data from sei mic profile and water 

well. , w ex t nd the M unt ngel faul t from the Waldo Hill 

northwest pas t Woodburn (fig . 9 ancl 90). Verti cal offset of 

the top of the olumbi a Ri ver Basalt Group increases to the 

outhea t from I 00 m in eismic sec tion A- A' (fig. 9 1) to 200 

m in ci. mic e tion 8- 8' (fig. 92). and at lea t 250 m in 

cross e ti n - '(fig. 93). The pre ence of the Frenchman 

Springs M ember of the Wanapum Basa lt at the top of M ount 

Angel (M.H. B e. on, Portl and tate ni ver ity, oral commun .. 1990) indica te that the top of the Columbia River 

Basalt Group ' ould have been clo. e to the pre ent summit 

prior to cr ·ion, constraining off ct to about 250 m. A sei -

mic reflec t r' ithin th overlying fluvial equence is off et 

about 40 m and the clip of the fault i 60°-70° E .. ba. eel on 

seismic secti on 8- 8 ' (fi"'. 92). On the . outhwes t ide of the 

fault, the top of the olumbia Ri ver Ba. alt Group i warped 

into a shal low . yn line that increase. in prominence northward as offse t on the M ount Angel fault cl crease (fi g. ). 

E idencc for the M ount Angel fault i. limited in the 

Waldo Hill s. although a inkgo intracanyon fl ow of the 
20 ASSESS! G EARTHQ A K HAZARDS D RED I 'G RI K I T HE PA IFIC ORTHWEST 

(/) 

a:: 

LU 

tu 

:::< 

?. 

u.i 

C) 

;::? 

5 

<( 

45° 

56' 

A 

NORTHWEST 

150 

100 

50 

SEA 

LEVEL 

122°58' 

B 

Ons 

05 

Ttl 

+ + + 

Waldo Hills 

range-front fault. ·· 

. \ .. ······· 

+ Tc + 

Ons 

WALDO HILLS 

Te 

0 2 KILOMETERS 

1 KILOMETER 

Te 

olumbia Ri ver Basalt Group in the Waldo Hill i off et 

about I km right laterall y aero the fault (M.H . Beeson, 

Portl and State Un iver. ity, oral com mun .. 1990). 

Six small earthquake wi th me (me is coda-length magnitude) of 2.0, 2.5, 2.4, 2.2. 2.4, and 1.4 occurred on Augu t 

14, 22, and 23. 1990. ncar Woodburn (Werner and others, 

1992). piccntcr locations (fig. 90) were determined using 

the broadband cismi c . tati on in Corvalli · (epicentral di -

tancc, 6 km ) and the Wa hington Regional Seismograph 

etwork . Three earthquake. in 1980 and 1983 hav ing me 

lcs than 1.7 occurred at the same locality. The waveform s 

for the six earthqu ake are so similar that the locati ons of all 

events arc con. idcrcd to be much closer together than shown 

in fi gure 90. The preferred focal mechani sm (fig. 94) is a 

SOUTHWEST 

Waldo Hills 

Tms 

Vertical exaggeration 4:1 

Figure 87 (above and faci ng column). Cross secti on and map 

of the Waldo Hill s rangc-fron1 fault , Or gon. A. cross-section interpretati on based on data from water well s (thin cnical lin s) 

and surfa c geology. Although the range front is linear, it is not 

known if the fault cut: the unnamed nonmarine fine-grained sediments. B. map showi ng location of cross secti on (solid line) and 

exposures of Eocene an I Oligocene sedimentary rocks (from 

Hampton. 1972). Bl ack squares denote locations of water wells 

used as structural control for the cro. s secti on. T he dotted line is 

the known location of the fau lt in the . ubsurfacc. bar and ball on 

the downthro' n side. Qn . . Holocene and PleistOcene nonmarine 

sedimentary deposits o crl ying mainl y Tfl : Tfl, Pli ccnc and 

Miocene flu vial and \acustrinc('l) . cdimentary deposits; Tc, 

olumbi a Ri ver Basalt Group (M iocene): Tms, Oligocene and 

Eocene marine sedi mentary rocks. 

right-lateral strike-slip fault (wi th a small norm al componen t) triking north- outh and dipping teepl y to the west. 

On M arch 25, 199", an earthquake wi th me of 5.6 

truck the Waldo Hill s near the town of Scous Mill : (fi g. 77) 

(Thomas and others, 1993), cau ing about $28 million in 

damage. The focal depth wa 15 km . A preliminary 

determinati on of the focal mechani sm ha one nodal plane 

with a trike of . 56° W. and a dip of 58° NE. A sub. et of 

after hock d fines a plane with a we t-northwest trike dipping 55°-60° NE. Rupwre was by right-lateral trike slip and 

reverse sli p, consi. tent wi th moti on on the M ount A ngel 

fa ult, although the earthqu ake oc urred ea t of the outhea. t 

proj ec ti on of the M ount Angel fault in the Waldo Hills. 
TECTO 11 SOFTHEWILLA ETT VALLEY.OR EGON _09 

45° 

20' 

123°00' 

0 2 KILOMETERS 

EXPLA ATIO 

---- Contact 

· · · · · · T · · · · · Concea led Fa ult- Bar and ball on down thrown side 

- 1 - - Homocline- howing direction of dip 

---- Structure contour of the top of Columbia Ri ver 

Basa lt Group- Altitude we t of contour i above 

sea level 

• 

Subcro p of conta ct be tween Columbia Ri ve r 

Ba a li Group a nd Eocene and Oligocene 

sedimentary rocks 

Wa te r we ii- Reache Columbia Ri ve r Basalt Goup . 

located to the quart r·quarter secilon 

Wat e r well- on trains altitude of top of Columbia 

River Ba alt Group. located to the quarter·quarter 

section 

& Wa te r we ll- onstrain altitude of top of olumbia 

River Ba alt Group. field located 

Fi >ure gs. Buried trace of the Newberg fau lt. ewbero. reg .. 

jux taposing marine . !rata on the northeast against the olumbia 

River Basalt roup on the southwest. ed iments postdating the olumbia River Ba>al t roup do not appear 10 be fau lted. M ap-u nit 

symbol s arc same as in figure 7. 

YAMH ILL-SHERWOOD STR T RAL ZONE 

Thi s northeas t-trending zone (fi g. 85) includes the 

Yamhill Ri r fau lt of Baldwin and others ( 19-5) and 

Brownfield ( 1982a, b). which juxtapo. cs the c. tucca 

Formation on the north against the Yamhill Formation nth 

south (pl. 2A) wit h max imum verti cal separation greater 

than 300 m (Bald\ in and other. 195-). The fau lt ma continue along the northern end of the mit Hill s. based on a 

proprietary . ei mic profil e. Farther northeas t. the Shen ood 

fault between Parr tt M ountain and the hehalcm M ountains ha. creat d I 00- 150 m of veni a! separation in the 

Columbia Ri er Basalt GrOLq (Hart 'llld ewcomb, 1965: 

Be son. Tolan, and Ander on, 1989), though it i poor! 

expressed in aeromagnetic and gra ity data. The h rw d 

fault appear. to be the outhw t continuati on of the northern 

margin of the olumbia tran -arc lowland through which the 

Columbia Ri ver Basalt Group tra er. ed the ascade Range 

(B c. on. Tolan, and nderson. 1989). The : truc ture ma b 

part of the Yamhill-Bonne ille l ineament , wh ich ma hav 

influ need the di stri bution of ent f the Borin o La as in 

the I ortl and area (A llen, 1975). 

NORTH ERN WILLAMETTE DO\•V WARP 

The northern Will amcttc Valley general! trends northca. t. but the northern end of th va ll y i. underlain by an 

cast-trending downwarp in which the t p f the olumbia 

Ri ver Basalt roup is as deep as 500 m bel \ sea level. The 

downwarp cut. acr . s the northern ex ten ion of the M unt 

Angel fault bu t is mo. t prominent cast of the fault. Th' 

northern fl ank is steeper than the . outhcrn flank ; thi may 

indi ca te upwarping inf'lu need by the empl acement of intruion related to the Boring La vas, such as the Aurora sto k 

(pl. 2A). cd imcntary dcp sit.' po. tdating the Columbia 

Ri ver Basa lt roup are warped to a le .. cr degree than the top 

of the group, as shown by pro1 ri etary seismi c data. Thi. 

downwarp may be pan of a we tern ex tension of the Yak ima 

fold belt. li ke other stru ture to th northea t, as sugge ted 

by Beeson. Tolan. and Andcr.on ( 19 9). 

FA L TS AT PARRETT MO TAl ', 

PETES 10 t TAl 1

, AND I THE ADJACENT 

NORTH ERN WILLA\1ETTE VALLEY 

Fault s at Parrett M ountain and Pctcs M ountain w r 

map1 cd by M arvin H. Bee on and Tcrr L. Tolan of Portland 

tate nivcrsity (oral com mun . to ian P. M aclin, Oregon 

D pan ment of Geology and ~in era! I ndustrie , 1990) based 

on jux taposition of flow. of the Columbi a Ri ver Basa lt 

Group identified u ing geochem ical data. Di spl acements arc 

commonl y ten. of meter.. To the south, cismic profile. 

show that the top of the olumbia Ri ver Ba. alt Group i 
210 

45° 

10' 

45° 

05' 

45° 

00' 

SSESS I1 G EARTHQ 

122°55' 

Qns 

Qns 

KE HAZARDS A D RED I G RI K I THE P CIFI 

122°45' 

.,_ 

5 KILOMETERS 

EXPLANATIO 

---- Cont ac t 

-- · · · · T-· · · · Concea led Fa ult-Bar and ball on down thrown 

side: queries mark limits of where the fault can be 

traced in the subsurface 

- - 50- S tructure conto ur on to p of basalt- Da tum i sea 

level 

Seismic-refl ec tion pro file line 

-\>- Pe troleum-explora tion we ll 

Wa te r well tha t reaches basa lt 

00 Deep wa te r we ll tha t does not reach basa lt 

ORTHW -·ST 

Qns 

Figure 89. 1ru lure comour map of the lop of ba ·all units near M oun! A ngel, Oreg. T he basa lt is primarily the top of the olumbia Ri ver Basa lt Group except near exposure. of the Miocene basalt and basa lti c 

nd e~i e around the M oun! Angel faul!. Qns. Holocene and Pleistocene nonmari ne sedimemary deposits; Ttl , 

Pliocene and Mi oce ne flu vial and lacustrine('/) sedimemary deposits; Te, Columbia Ri ver Basa l! Group (M iocene); Tsm. Miocene and Oligocene Scous M i lls Formation of M iller and Orr ( 1988); T lbb, Miocene basa lt 

and basaltic andes ite. 
45' 

10' 

45' 

05' 

45' 

00' 

122'55' 

Qns 

TECTON ICS OF THE WILLAMETTE VALLEY. OR GO 

Wood bum 

0 A'D 

·. 0 

122'50' 

,,. B' 

D·· .. ~~ .... 

Do~~ --._ D 8 ·. 

Hubbard 

0 

Ons 

122'45' 

0 Gervais 0 

~0- . 

Qns 

c.-·. 

~-. 

>'.j-. o·. 

. 

-<'17 ·. ~ 

0-; .. Tc 

· ~ount Angel 

• 

• 

c • 

0 5 KILOMETERS 

EXPLANATION 

---- Cont ac t 

. · . . . · T. · · · · Concealed Fa ult- Bar and ball on down thrown 

side: queries mark limits of where the fault can be 

traced in the subsurface 

Seismic-re fl ec tion profil e lines for sections 

shown in fi gures 91 a nd 92 

C C' Line of cross section (fig. 93)- Dots show 

constraining wat er 1vells 

EARTHQUAKE EPICENTERS 

0 Magnitude 1.1-1.5 

0 Magnitude 1.6-2.0 

D Magnitude 2 .1- 2 .5 

Figure 90. Locati ons of seismic and water-well cr ss secti ons, and pi ce nt ~ of earthqu akes th at occurred 

between 1980 and 1990 ncar Wo elburn. reg. Map-unit symbols arc same as in fi gur 9. 

2 11 
ui 

~ 

i= 

--'Cfl 

wo :;;:z 

a:O 

f-U 

>-w <!(f) 

~~ 

0 

~ 

ui 

~ 

i= 

--'Cfl 

wo :;;:z 

a:O 

t-U 

>-w <!(f) 

~~ 

0 

~ 

WEST 

A 

0.0 

0.2 

0.4 

0.6 

0.8 

0.0 

0.2 

0.4 

0.6 

0.8 

EAST 

A ' 

Figure 9 1. Seism ic secti on A- ll' across the M ount A ngel fau lt, Willamettc Valley. Oreg. II. uninterpreted secti on: B. interpreted secti on. Four strands of the Mount Angel 

fau lt arc recogni zed. The prominent reflector within unit Ttl is higher on the northeast than on the sou thwest side of the fault. Queries denote possible faults. Map-unit symbols 

arc same as in figure 89. Line of secti on is shown in figure 90. 

N 

N 

)> 

(/) 

(/) 

rn 

(/) 

(/) 

z 

C) 

rn 

)> 

:;v 

-l 

:r.: 

0 

c 

)> 

;;><; 

rn 

:r.: 

)> 

N 

)> 

:;v 

0 

(/) 

)> 

z 

0 

:;v 

rn 

0 

c 

n 

z 

C) 

:;v 

v; 

;;><; 

z 

-l 

:r.: 

rn 

v 

)> 

0 

:;v 

-l 

:r.: 

~ 

r:1 

(/) 

-l 
SOUTHWEST 

8 

Q.Q j 1 jj ~ill 1 1\1! 1 I 1 \ 1 tJ j,, 1 1 jj, j, 1 1 • 1 J I I J • all I I •! 1!1 ,j 1 jj ltjjjjj I j j jjjjjjjjjjj j tj i jjj j j jjjjjjjjj, I jjjjjjjjjt jj:! jjjj I I I l : j I I jj • I II j1j1 'j1'' t I!· j J I JJ' j 1 Jjjj j jt · j 1 j 'jjt 1 

u.i 

:2 

i= 

-' <fl wa 

> z 0.4 0 i-U 

>-w 

~(f) 

s:~ 

0 0.6 

~ 

0.0 ,,,, I ,,l l llli!llli.illlitltl I ,,li l lllillillllilill,tittillliillilillill i llll l llllilitllli.liilltttlltllllol.•ttliti ttt.t.t .!IIIII lo llll .. t .. t.i. 

u.i 

:2 

i= 

-'<fl 

wa 

>z 0.4 

~0 i-U 

>-w 

~(f) 

s:~ 

0 

~ 

Figure 92. Seismi c section 8- 8 ' across the Mount Angel faull, Will amellc Vallt:y, Oreg. A, unintcrpreted section: B. interpreted section. The top of the basa lt (unit Tc) is offset a 

greater amount than the unit Ttl reflector. Map-unit symbols arc same as in fi gure 89. Line of section is ; ho11 n in fi gure 90. 

-1 

m 

n 

-1 

0 

z 

n 

(/) 

0 

"T1 

-1 

:r: 

m 

< 

r 

r 

)> 

:s: 

m 

:j 

m 

< 

)> 

r 

r 

m 

:< 

0 

;;o 

m 

Cl 

0 

I 

\.;.} 
2 14 ASSE I G · ARTHQ KE HAZARD A D RED I G RISK I TH E PA IFIC RTHW 'T 

SOUTHWEST 

0 500 I II II I 

1000 METERS 1 

Tfl 

Tc 

Mount Angel 

MOUNT ANGEL FAULT 

Tc 

A T 

Tfl 

NORTHEAST 

C' 

Vertical exaggeration 4:1 - 300L_ ______________________________________ i-----------------------------------~ 

EXPLANATION 

------ Contact- Dashed where not closely constrained by 

water-well data 

--=:=:--T.;- Fault- Barbs show direction of relative movement ; 

A T. movement toward viewer: A. movement away 

from viewer 

l 

r Water well- Horizontal lines and shaded range 

show gravel intervals 

Figure 93. Cros. section C- C' of the Mount ngel fau lt. Willamcttc Valley. Oreg., constrai ned by water wells. Onl y th se par ~ of wells 

with useful well log are shown. Ba. e of unit Tc is not shown. The fau lt clip isba. eel on . ei. mic section B- 13 ' (fi g. 92). M ap-unit symbol. 

are same a. in figure 89. Line of secti on is . hown in figure 90. 

faulted and has undergone upward bulging presumably 

related to emplacement of intrusion of the Boring Lavas 

(Werner, 1990). A fault (fig. 95), mapped by Glenn ( 1965) 

along the cast bank of the M olalla Ri ver behind wan Lake 

Farm s ncar Canby, dips teepl y to the north . A mud tone bed 

(fi g. 95, un it I) beneath the unfaulted Will amette Formati on 

i. offset I m down to the north . The mud tone wa correlated 

by Glenn ( 1965) with rock now known a the Rowland Formation, although the characteri tic paleosol at the top of what 

i now called the Diamond Hill M ember i absent. Alternati vely. the mud tone could be part of the pre-Rowland Formation flu vial equence that is expo ed eas t of Canby. 

BEAVERTO FA LT ZONE 

The Beaverton fault zone ex tends from beneath downtown Beaverton about 14 km we tward along the northern 

fl ank of ooper M untain and the hill north of Farmington. 

The fault z nc wa originall y mapped a a single fau lt wi th 

. cparation down to the north by M ad in ( 1990) utilizing water 

well s. Proprietary seismic-reflection data and recentl y 

acquired acromagn ti c data ( nyd rand others, 1993) how 

tw . cparate trace in the area north of Cooper M ountain 

wi th a net verti ca l separati on f 285 m down to the north. 

B neath Al oha, the approx imately hori zontal flows of the 

olumbia Ri ver Basa lt Group arc offset 75 m down to the 

. outh ; basa lts of thi. age arc south dipping at 260 m below 

sea level north of Cooper M ountain. whcrca north-dipping 

fl ows arc exposed at I 00 m elevation on the northern fl ank 

of Cooper M ountain . A data gap in the seismic profile prevents prcci. e location of the . outhcrn trace of the fault. 

The westward ex tent of the fault zone i. poorly constrained due to the paucit y of deep water well s in the area 

north of Farmington but is interpreted to ex tend till farther 

along the northern flank f the hchalem M ou ntain s. Several(?) northea. t-trending fau lts ex tend ing from the hehalcm M ountain to the Beaven n fault zone, interpreted n 

the ba i. of aeromagnetic data and water well s. arc apparentl y truncated by the fau lt zone. 

ELMO ICA FA LT ZO E 

Two ubparallel faults, con. trained by cism i 

refl ec ti on profiles and aeromagneti data, ex tend westward 

at lea. t I I km from Elmoni ca to the confluence of Rock 

and Beaverton Creek outh of Orcnco (pl. 2A). The c.!-

ward ex tent of the fault zone i unknown due to the paucit 

of deep water well in the center of the basin. but interpretation of aeromagnetic data uggcsts the faul ts term inate 

eas t of Sewell . 

The net verti cal eparati on of the Columbia River 

Basa lt Group i between 5 and I I 0 m down to the north . 

Separation on the northern trace increase. we tward. from 

about I 0 m near Elmonica to I I 0 m south of Orenco, though 

po sible intru ion of the Boring Lavas visible on se i. mic 

profil es complicate the structure. Seismic-reflecti on data 

. how that folding of the refl ec tors in the upp r part of the 

overl ying sedimentary deposits increases we tward : the 

reflectors are off et about 20 m ncar the map1 ed we. tern end 

of the fault. Where the southern trace crosses eismi c profil e , the verti ca l separation is about 75 m, and the reflector. 

in the overlying sedimentary deposits arc not visibly off ct. 
TECTO lCS OF THE WlLLAM ETT VALLEY, OR GO 2 15 

HELVETIA FA LT 

The northwe. t-trending Helvetia fau lt (pl. 2A) off t 

the Columbia River Ba alt Group from the M cKay Creek 

va lley . outheas tward to north of Orenco, based on waterwell data. Separation i down to the southwest. The faul t 

may ex tend northwestward into the Tualatin Mountains. 

Water well s west of Hel vetia suggc t a much a 100m of 

vertical sepa ration. The Helveti a fault ha little aeromagnetic expres ion. 

ndul ation of the top of the Columbia Ri ver Basalt 

Group in the northern Tualatin ba. in uggests that similar 

faults may ex tend southeas tward from the valley containing the East and Wes t Fork. of Dairy re k, though the 

data are i neon I u i vc. 

T UALATI BAS I 

The Columbia Ri ver Ba. alt Group i folded and faulted 

into a northwe [-trending, faul t-bounded, nat-bottomed 

ba. in southwes t of the Tualatin M ountain and north of the 

Gale reek and 1cwberg faults. The northeas t-trending 

Sherwood fau lt (fig. 85) terminate the ba. in to the southcas t, and the Hcl ctia faul t and an unnamed fault on the nank 

of the Tualatin M ountain. bound the ba in to the northeast. 

The structure of the eastern part of the basin is com1 li catcd 

by the apparen t intrusion of tocks c mposed of the Boring 

Lava . oopcr M ountain and Bull M ountain . in the center of 

the basin outhwcst of Beav n on, are underlain by the 

Colu mbia Ri ver Ba alt Group folded into two ea. t-trending, 

doubl y plunging anti clines. 

Fill postdating the Columbia Ri ver Ba alt Group consi. t of mud tone . . ilt. t ne. and sandstone with len c. of 

pebbl . and and gra cl. Th abundance of quartz and mica 

in these :cdimcntary dCJ osits indicates a predominant ourcc 

from the olumbia Ri ver. with a . ub rdinatc local ource 

from the olumbia Ri ver Basa lt Group in highlands nanking 

the basin. The noor of the basin is downwarp d, and it. axis 

is 200- 300 m deep trending ca:t-wcs t. The thi kc. t . cqucncc 

of trata that postdat . the olumbia Ri ver Basalt Group 

mca. urc. 0m. ncar Hil l.boro. Pr prietary cismic data 

. how that th i dips much more gentl y than the underlying 

olumbia Ri ver Basalt Group. 

DISCU SION AND CONCLUSIONS 

AGE OF THE WILLAMETTE VALLEY 

AND COAST RANGE 

The Will amcttc Valley i common! referred to as a 

forearc ba in. However, thi s . tat rn cnt i. true only for th late 

Cenozoic after emplacement of the Columbia Ri ver Basa lt 

Group. trata that predate the olumbia Ri ver Basa lt GrOLl[ 

arc part of a forearc ba. in . but thi basin included the oa. t 

N 

• 

0 0 

\ ... \\ 

W f------ ~-1- -'---~...L-.,.:J E I ,..."" I .,..,,"" 

---- --- ---- -----~--- 1 • •••• 

............... \ ........ ··············~· ·· ·· \ l e I : 

\ ~ I : I : \ : I • 

\ 

s 

Figur·e 94. omposite foca l m' han ism for the ugust 

1990 earthquake seq uence at Woodburn. O reg. (J.L. abelek. Oregon tate Univer>ity. wriuen commun .. 1990). 

ircl es indicate dilatati on an I clots denote compressio n. 

Larger symbols indicate a stronger first moti on. T hree 

separate fo ·a I mechanisms based on wave-form analysis 

are indicated by . olid , dashed , and cloned lines: the 

dashed-line foca l me hanism i. preferr d. These mechanisms a re compatibl e with first-motion so lutions. 

Range as wel l a the Will amcttc Valle . For the mo t part, 

cdimen tary facie. deepen to the wes t aero .. the Will amctt 

Valley and oas t Range. a be. t d cumcnted b the fa ics 

boundary between the upper Eocene strandlinc dcp si ts of 

the pcnccr Formati n and deeper water deposit. of th Nestucca Formati on. Ther were ba. alti c highland to the we t, 

but the c did not join tO form a con tinuou. Coa t Range. 

The fir. t-order . tructurc of the Will amette Valley i an 

east-dipping homoclinc that de eloped after the dcpo ition of 

the Oligo cnc and Miocene Scotts Mill Formation and prior 

t the emplacement of the Columbia Ri ver Ba alt Group. The 

east dip of thi. homoc linc implic uplift of the oa t Range 

pri r to the mplaccmcn t of the olumbia River Ba alt 

Group. but thi s uplift i. not clearl y documented in facies 

change. of sedimentary rocks th at predate the group. The 

n w. of the Columbia Ri ver Basalt Group pa sed through the 

a. cadc Range via the lumbia trans-arc lowland of Beeson, Tolan. and ndcr. on ( 19 9) and continued to the coa. t 

a. nows filling broad va lley in an incipient oa t Range. 

There was no marked tendency for these now. to follow the 

north-south trend of the modem Will amette Valley . 

The fluvial d po its of the Will amcttc Ri ver and tribu -

tary drainag arc the Ide t strata to follow the present trend 

of th Will arn cttc Valley. The top of the Columbia Ri ver 

Basa lt Group beneath thc. c deposit. i comm nl y deeply 

weathered , . ugge ting that a long time elapsed after the 

cm1 lac mcnt of the group before these dcpo it began to 
216 AS ESSI 'G EA RTHQUAKE HAZARD AND RED Rl K I 1 THE P CIFI ORTHWE T 

Figure 95. Swan Island fault ex posed in the bank of the M olalla Ri ver. Will amette Valley. Oreg. The mudstone labeled unit I may be 

correlative wi th the Rowland Formation as described by Glenn ( 1965) at Ri ver Bend on the Will amette Ri ver. 5 km south-southwest of St. 

Paul ; it i. downdropped to the north. Uni t II. the over! ing Willamelle Formation. is unfaultcd. View to the northeast. Photograph counc. y 

of Jerry L. Glenn. 

aggrade in the Willamcuc Valley. A channel cu t to 150 m 

altitude between the ooper and Bull Mountain uplift and 

the Chchalem M ountains al. o ugge ts a sub tantial period 

of downcutting pri r to sed imentation. Aggradati on may 

have been a consequence of oast Range uplift, rai. ing the 

local base level for these nuvial edimcnts. Reacti ation of 

the Gales reek fault y. tem and new fau lting re ultcd in the 

deepening of the Tualatin ba in . The Willamettc Vall ey wa 

downwarped. resulting in a nattcning of homoclinal dips of 

older trata underneath the va lley and. locally, a rever al of 

dip that prod uced broad syncline. and antic line . 

AGE OF INITIATION OF ARC VOLCANISM 

EAST OF THE WILLAMETTE VALLEY 

The general view is that ascadc arc volcani sm began 

at about 43-42 M a, based on the age of vo lcanic rocks at 

the ba. c of the expo ed sequence in the we. tern Cascade 

Range. Data from well s in the Will amette Valley. how that 

v Jeanie rocks of the Fi sher Formation arc underl ain by the 

Yamhill Formation, wh ich grade outhca t from marine 

strata to vo lcanic and olcanicla ti c rock . The Yamhi ll 

Formation in the Will amcttc Valley contains earl y ari zian 

t lat latisian foraminifers, suggc ting an age of 48-45 

Ma. occolith. from the Yamhill Formati on of the Coast 

Range arc rcfcrrc I to ubzones P 13c and P 14a. with 

e tim ated ages of 47 __ 5 M a. 

The underl ying Tyee Formation show. no evidence of 

a nearby a. tern olcani c source . . uggesting that it predated 

the incepti on of a volcani c arc east of the Willamette Valley. 

Coccolith. from the Tyee Formati n are referred to ubzoncs CP 12a and CP J_b_ with an agee timated a 52.-- 50 

M a. Thu s. a volcanic arc wa. initiated later th an 50 M a, and 

arc volcani sm occurred during depo ition of the Yamhill 

Formati on a. earl y as 47 a. Thi s 50-47 Ma age i coeval 

with the deposition of the larno Formation cast of the a.-

cade Range. and the Yamhill-age arc volcani m may be 1 art 

of the larno arc. 

THE SO THERN WILLA METTE VALLEY 

AS A BROAD STRIKE VALLEY 

The older rocks of the wes t rn Cascade Range o ·cur in 

an eastward-dipping homocline that trikes nearl y n nh 

( hcrrod and Pickthorn , 19 9). The outcrop belt is trunca ted 

at a low angle by the range front of the wes tern ascadc 

Range such that the oldes t sequence. dated at 45- 35 M a, is 

truncated in the southern Will amcttc Valley south of Lebanon , and the nex t olde t, dated at 35- 25 M a, i. truncated 

against the Will amctte Va lley cast of Sa lem. Facies boundarie. between marine cdimcntary rock. and volcaniclastic 
TE TO 1

ICS OF THE WILLAMETTE VALL Y, OREGO _17 

rock. of late Eocene and Oligocene age , trike northnorthea t. Thi. di scordance between . trike direction and the 

western Ca cade Range front means that the outcrop belt of 

Eocene and Oligocene rock cut. aero. s the arc at a small 

angle o that the southern end of the outcrop belt con i. t of 

volcanic rocks and the northern end consist. of sed imentary 

rock (fi g. 96). 

The southern and eastern edges of the Will amette Valley appear to be controlled by the facies boundary between 

eros ionall y rc. istant volcanic and erosionally weak nonvolcanic rocks. The. outhern edge of the Willamette Valley corrc ponds to the northward termination of the Fi her 

Formation ncar Eugene. Volcanic rocks of the Little Butte 

Volcanic change faci es to edimcntary rocks at the ea. tern 

margin of the southern Willamettc Valley. Intrusive plug in 

the Eugene Formation form isolated hill.. including Peter on 

Butte and Bond Butte near the eastern edge of the valley. 

The western edge of the va lley i. controlled by the contact between ero. ionall y resi tan t andstones of the Tyee 

Formati on and the Sp ncer Formation on the west and erosionall y weak fine-grained strata of the Eugene Formati on to 

the ea t. M o t of the alley is underl ain by the Eugene Formation. The out rop belt is wide becau c of a dccrca e in the 

ca t dip of the Eugene Formation accompanying down warping of the Will amette Valley following emplacement of the 

Columbia Ri ver Ba alt Group. The . trike valley underl ain by 

the Eugene Formati n is a late Tertiary feature because it is 

co crcd by flu vial depo. it of the Willamette Ri ver . y. tem. 

STRUCT RAL CONTROL OF 

FAULT SYSTEMS 

W : tcm Oregon has undergone clo kwi. e r tat ion fr m 

the Eoc nc through at least Miocene time. wi th grea ter rotations in Lh oast Range than in the ascadc Range (WelL 

and Heller, 19 ). Thi s rotati on ha. led to th de clopmcn t 

of fault. wi th major di placement as earl y a the Eocene. th 

greatc ·t mea. urcd di spl acement b ing on the orvalli fault. 

Because the c fault s arc zones of wcak nc s. further movement is likely to rcac ti va t them, even if their orientation i 

not para! I I to the Jlanc of maximum shear strc .. . The clcarc:t example of thi. i: the orvalli fault , whi ch' a. part of a 

low-angle fold -thrust sy. tcm in Eo enc time but wa .. ubscqu ntl y reac tivated a a high-angle fault with a large component of strike sliJ (Goldfinger, 1990). The G·1 lcs Crc k fault 

aLo had major verti cal movemen t in Eocene tim (H.J . 

Meyer, Oregon atural Gas orp., oral commun., 199 1) and 

has been reacti vated since middle lfioccnc time. 

Wern er and other. ( 199 1) compared the modern . trcs. 

orientati on in western Oregon using borehole break ut ' ith 

stress oricntati ns ba. cd on earthqu ake focal mechanisms, 

alignment of volcanic cnts, and orientation of onjugate 

faults. They found that the max imum hori zon tal comprcs. ivc 

str ss is oriented about north , confirming earlier studic. 

N 

I 

Southern 

Willamette 

Valley 

0 10 

\ 

\ 

\ 

20 30 KILOMETERS 

Figure 96. M ap showing outcrop belts of we. tern as ade Range 

volcanic rock. (mod ified from Sherrod and Pick thorn. 1989) an I 

the north -northeast-trendi ng boundary between volcanic and sedimentary rock. (dashed line). The facies boundary con trols the position of the eastern edge of the Willamcttc Valley: ero. ionall y 

resistant volcanic rock are to the east-southeast ::111d weak sedimentar rocks. principall y the Eugene Formation, arc to the we tnonhwc. t beneath the valle 

based on lc s data. The fault-trend pattern s in the Will amettc 

Valley arc predominantly northwest and northca t. and fold 

invol ing the . ccti on that po. tdate the Columbia Ri ver 

Basa lt Group commonl y trike ca t. in agreement with a 

north- outh max imum hori zontal compre . i ve stre s. 

EARTHQUAKE HAZA RD FROM CRUSTAL 

FAULTS IN THE WILLAMETTE VALLEY 

The fluvi al deposits of the Will amette Ri ver and it tributari es are ut by cveral fault. ' ith ve rti ca l s paration as . 

much as 250m. In addition. these deposit. are warped into 

broad fold s that may be the surface cxpre ions f faults at 

. ismogcnic depths. The fluvial depo its are poorly dated ; 

paleobotani ca l ev idence from fo sil leave · and pollen favor 
2 18 ASSESS! G EARTHQ AKE HAZA RD A D RED I 1

G RI K I THE I ACIFI ORTHWE T 

a late Miocene to Pliocene age, and the deposits in the Portland basin are lder than the Boring Lava , > hich i a you ng 

as 600 ka (Jan P. M acl in. Oregon Department of Geology and 

Mineral Industrie, oral com mun. , 1990). Roberts ( 1984) 

suggested that the deeply > eathered high-terrace gravel. of 

the southern Will amette Valley are the con tructi onal top of 

the equence formed by the Will amcue Ri er, and that thi s 

sequence is a young a Plei. tocene. 

Dcpo its that po. tdate the olumbia Ri ver Basa lt 

Group in the Tualatin ba in are cut by the Beaverton faul t 

zone and the Helvetia faul t, though ome of the deformation 

could b auributed to intru ion of tock , po ibly of the Boring Lavas (Popow ki, 1995). The Owl Creek fault (pl. 2B) 

cu t the Rowland Formation, which i late Plei tocenc in 

age, beginning earlier than 36 ka and continuing past 28.5 ka. 

If the gravel. at the Mid-Valley rock quarry between Cor allis and Philomath are part of the Rowland Form ati on, the 

Corval li fau lt zone ha undergone at least 40 m of verti ca l 

separati on ince the deposition of the Rowland Formation. 

The Will amette Formation, dated as younger than 

15- 13 ka, how no ev idence of off ct by faulting, including 

by the Owl Creek fau lt. However. seism icity on the Mount 

Angel fault at Woodburn indicate that thi fau lt i active. 

Because the erti cal offsets of the proto-Wi llamene 

Ri ver depo. its are no greater than a few hundred meters for 

deposit that are older than 600 ka, slip rates on Will amct te 

Valley faul t. are probably small. A di splacement of 00 min 

600,000 year. repre ent. a max imum average erti ca l slip 

rate of 0.5 mm/year. If the northwest- and northeas t-trending 

faults are predominantly trike lip, the rate ofo.- mm/year 

would be a minimum. However, the M ount Angel fault, with 

a maximum verti cal offset of the top of the Columbia Ri ver 

Basalt Group of about 250 m, ha only a 1-km lateral offset 

of a Ginkgo intracanyon now of the Frenchman Spring 

M ember of the Wanapum Basalt. There is no evidence of the 

amount f hori zontal off ct of other fau lt . 

Faults in the Will amelle Valley are relati vely hort in 

length. The mapped length of the t. Angel fault i about 24 

km . the Waldo Hill range-front fault abou t 6 km. the Owl 

reek fault abou t 15 km , and the Corva llis fault at lea t 35 

km. The eismic moment of an earthquake wi th a lip of I 

mona fault 30 km long in a cru tin which the brittle-ducti le 

tran. it ion i at 30 km wou ld be 2.7x I 026 dyne em, as uming 

a shear modu lus of 3x I 0 11 dyne/em. 

It i. premature to c tim ate the earthquake poten tial from 

crustal fau lts in the Will amctte Valley because we do not 

know how much of a mapped fau lt would rupture in a ingle 

earthquake and we do not know what the ·lip would be in a 

single crustal earthquake. Earthquakes of moderate size have 

been generated in thi s century on the St. Helen sei mi c zone 

(Weaver and mith, 1983). on the Portland frontal-fault 

structure (Yelin and Patton, 199 1), and on the Mount Angel 

fault (Thoma and others. 1993). but we do not know if 

larger earthquakes are po . iblc. Finall y, we have no direc t 

palco eism logical information abou t recurrence hi story on 

crustal faults in we tern Oregon. 

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8 1, no. J.p. 109- 130. 
ACTIVE STRIKE-SLIP FAULTING AND FOLDING 

OF THE CASCADIA SUBDUCTION-ZONE 

PLATE BOUNDARY AND FOREARC IN 

CENTRAL AND NORTHERN OREGON 

By Chris Goldfin ger, 

1 LaVerne D. Kulm,

2 Robert S. Yeat , I Bruce pp !ga t ,3 

Mary E. MacKay 3 and Guy R. Cochrane4 

ABSTRACT 

Three we t-northwe !-trending left-lateral trike-. lip 

faults on the aby al plain off central Oregon between lat 

44°40' . and I at 45 ° 12' . have been mapped using eismic 

rencction , : idescan sonar. data from AL V I ubmersible 

dives , and caBeam bathymetry. The e oblique faults. t.he 

Wecoma fault and faults B and C, inter. cct the north-. outhtriking structures of the active accreti onary wedge. The 

fault cut the oceanic litho pherc of the ubducting Juan de 

Fuca pl ate and appear to cro s the plate boundary. The be t 

tudied of these, the Wecoma faul t, ex tend 18 km northwe tward across the aby al plain from the leformation front. 

Displacement on thi s fault is 5.5± 0.8 km at the deformation 

front, decreasing to zero at the nonhwe. tern fault tip. ba ed 

upon 1 ier ing-point off. ct. The Wecoma fault has been 

active for about 600±50 thousand year. and has an average 

slip rate of 7- 10 mm/ycar. The late Plci to enc-Holocenc 

slip rate is 5- 12 mm/y ar. The c faults arc ac tive. as indica ted 

by the ffsct of the you ngc. t . cdimcn tary unit , the 1 rescnce 

of surficial faul t . carps. and the venting of deep ourcc nuid . 

The Wccoma fault intersects the deformati on fron t in a complex stru tural zone onsi Ling of fault-bounded pop-up 

(upli fted asymmetrica l tri anoul ar pl ateau ), an emba ment in 

the deformation front, and a local reversal of vergence in the 

ba. al thru. 1 of the accreti onary wedge. 

1Department or Geosc iences . Oregon State niversi ty. orvalli s. 

Oregon 9733 1. 

2Collcge or Oceanogra1 hy. Oregon State ni ver>.ity. orvallis. 

Oregon 9733 1. 

1

School or Ocean and Earth Science and Technology. niversit or 

lawaii. Honolulu. Hawaii 96822. 

"Earth Sciences. ni versity or California at anta Cru L, alifornia 

95064. 

Linear gulli . and scarps in the Wecoma fault LOne 

ex tend in to the first everal thru st ridges of the continental 

slop . suggesting that thi s fault remain a ti ve beneath the 

accreti onary wedge. hangcs in fold ori ntati on and amount 

of honcning in th initial three to four thrust ridges of the 

accretionary wedge . ugg st that the strike-s lip faul t hav 

innuenced the development of coe al accretionary-wedge 

tru ture through differen ti al . lip of the downgoing plate. 

cveral west-north wes t-trending deformation zones 

ha e been mapJ eel on th ontinental I pc off northern ·1nd 

central Oregon. The zone arc composed of we. !-northwesttrending linear scarps, en echelon nonhw st to wes tnorthwes t-trending fold . and left-steppi ng and . igmoidall y 

bent fold . Oblique fold arc common ly fault-propagation 

and fault-bend fold de eloped ab ve high-angle fault~. 

ome of t he e a ti e faults and fold striking north-north we t 

to we t-northwe t are refolding ome' hat older north and 

nonh-northea !-trending fold . . Deformati n in the. e zone 

i. con. i tent with a left-lateral en e f shear. Three of the 

zone adjoi n the three a by al-plain strike-s lip fault , wherca 

at lea. t one zone and pos ibl y everal other are rc tricted to 

the continental . I pe. We po tulate that the e are shear zone 

develop d in the upper plate above the ubdu ted . trike-s lip 

fault or. alternati vely. that tri ke- lip faulting in the upper 

plate ha. propagated eaward into the ubducting plate. 

INTRODUCTION 

TheCa cad ia subduction zone (fig. 97) off Oregon and 

Washington i. one of the be. t tudi cd subduction zones in 

the world. Its proxim ity t major nited States port and 

three decades of study by oc anographic in titutions, g vcrnmcn t agcncie . and industry have produced an exten ·i e 

223 
224 A SE Sl G EARTHQUA KE H Z RD D REDUCI G RISK I TH E PA I IC 1 ORT H'V E T 

data et for u. e in geologic and geoph ysical in vesti gation . 

In recent year , attention ha been focu. ed on the anomalous 

sei mic qui e. cence of much of the Cascadia ubduction zone 

in compari on to other ubducti on zone . The moderate 

plate-convergence rate of 40 mm/year directed 062° (calculated from the Juan de Fuca- orth Ameri ca - ul r vec tor f 

DeM et and others, 1990) and the relati ve youth of the subducting litho phere ( I 0- 9 M a, according t Wil son and others, 1984) charac teri ze the Ca. cadi a subduction zone a a 

Chilean-tyJ e convergent margin (Heaton and Kanamori , 

1984). Because mo t hilean-type margins have had great 

(M greater than .4) earthquakes in hi stori cal time., the Juan 

de Fuca- onh Ameri ca plate boundary should also be capable of generating great earthquake . Geologic e idence for 

great earthquake. ha. been in ferred from buried mar h 

clepo. its of the coa tal bay. of Washington ( twater, 19 7; 

Atwater and Yamaguchi, 199 1 ). Oregon (Darienzo and 

Peter on. 1990), and northern Cali forni a (Vick. 1988 ; arver 

and others. 1989: Clarke and Carver. 19 9). Rapi d co ei mic 

submergen e in the bay , accompanied by t unami s. is 

inferred from peat layer overl ain by marine and (Atwater, 

1987). Pre entl y, however, the Oregon a. cacli a convergent 

margin may have the lowe t incidence of plate-boundary 

sei micity of any ubduction zone. Althouoh earthquake in 

the upper and lower pl ate are well monitored by the ni erity of Wa hington eismic network , no thrust-type earthquake at any magnitude level have been located on the pl ate 

interface (Ludwin and other , 199 1 ). The ab ence of hi storical great earthquakes on the plate boundary can be expl ained 

by long recurrence interval of 300- 1.000 year or more 

(Atwater, 1987: Carver and other. , 1989; Adam . . 1990: 

Darienzo and Peter on, 1990; Atwater and Yamaguchi. 

1991 ), yet the pl ate interface remain · singularl y quiet among 

the world ' s convergent margin . 

The low sei micity of the Cascad ia ubduction zone ha 

led in ve tigator to expl ore a variety of hypothese to ex1 lain 

the apparent sei mic gap along the Cascadia margin ( ee 

Ando and Balasz, 1979 : West and M e rumb, 198 ; ykes, 

1989). Our in ve Li gati on of the tructure of the pl ate boundary and forearc off the Oregon coa t wa moti a ted in part by 

the eli covery of three oblique strike- lip fault in the abyssal 

pl ain off central Oregon. The e faults cut the subducting Juan 

de Fuca pl ate and cross the pl ate boundary, extending ome 

di tance into the overriding North A meri ca pl ate. To our 

knowledge, faults of thi s type have not previou ly been 

described from other subduction zone around the world , 

although trike- lip faulting restricted to the upper pl ate ha 

been de cribed in both rthogonal and oblique con vergent 

ettings (Lewi s and others. 1988, and references therein ; 

Gei. t and others. 198 ). Thi s discovery is almo. t certainl y 

the result of the increased re elution of . ea-noor mapping 

tool in recent year. , but these faults may also represent 

something unique about thi s convergent margin. The di covery of these faults suggests that detailed structural in ves ti gations may offer new information about oblique subducti on in 

general and the a cadia subducti on zone in parti cular. 

48' 

45' 

42' 

JUAN 

DE 

FUCA 

PLATE 

PACIFIC 

PLATE 

120' 

OREGON 

CALIFORNIA 

0 100 200 300 KILOMETERS 

Figure 97. Index map showing tectonics of the Cascadia subduction zone. The dashed line is the plat -c nvergence boundary. The 

dot indicates Deep ea Drilling Project site 174A. 

The purpose of thi s chapter is to document the structu ral harac teri stic of the Wecoma fault. the large t and be t 

known of the three strike- lip faul t. di covered on the abysal plain off Oregon, and to di cu s the relationshi p of thi. 

fault to new mapping of a ti ve tectoni features of the continental slope. The long-term goal of thi. study is to better 

define the a ti ve tec tonic of the ascadia ubduction zone 

and thereby develop a better under tanding of the regional 

tectoni c and ei mic hazards of the Pac ific N rthwest. The 

chapter conclude with a di ·cu ion of pos ible ori gins for 

the blique faults, and the implicati on of the oblique faul t. 

and f Ids we have mapped for pl ate-boundary proce se. 

within the Cascadia convergent margin . 

ACKNOWLEDGMENTS 

We thank Ri chard Perry and Steven Mutula. 

National Oceanic and Atmo. pheri c dmini tration 

( 1 OAA), 1 ational Ocean Survey, Rockvill e, Md ., for preliminary copie of the SeaBeam bathymetry of the Oregon 

continental margin . They have al o allowed us to publi. h a 

part of a three-dimen ional-mesh di agram of the northern 

and central Oregon continental margin generated at their 

facility. We thank Sigmund Snel ·on and Dan Worrall of 

hell Oil ompany for the loan of sei. mic profiles of the 

Oregon and Washington continental shelf and for their 

effort in locating the. e profile in the company archi ve . . 

We also thank Parke D. Snavely, Jr., Samuel H. Clarke. Jr .. 
ACTIVESTRIKE-SLIPFAULTI GA DFOLDI GOFTHEPLATEBOU DARYA DFOR AR .ORE N 225 

and Dan L. Orange for thorough revicv of the manu. cript. 

The first two rev iewers do not necc .. aril y agree with all of 

our intervrctations of oblique fau lts and folds on the upper 

continen tal lop or hell; these conclusions are . olcly our 

responsibility. Thi re earch wa. supported by National Science Foundati on grants 0 E- 8 1273 1 (OS ) and 

OCE- 882 1577 ( H) and by the .S. Geological Sur ey 

National Earthquake Hazard Red ucti on Program under 

award 14- 08- 00 1- G 1800 (OS ). We thank the crews of the 

e cl. R. V. Wecoma. Digicon M. V. Ceo1ide, and R. V. Allan -

lis II, and also Margaret Mumford and Cheryl Hummon of 

Oregon State ni versity for drafting the illustrations. 

METHODS OF STUDY 

We arc studying the submerged plate boundary and 

forearc region off Oregon and Washington (fig. 97) using 

SeaMAR I A and GLORIA side can sonar imagery. caBeam swath bathymetry, ALVI submersible observa ti ons. 

and a dense network of inglc-channel and multichannel 

ei mic (MCS) surveys . We are mapping the OrcgonWa hington pl ate boundary and submarine forearc at a cale 

of I :500,000. We have used about 30,000 km of sci mi -

rencction profiles collected by Oregon State Uni versity. the 

nivcrsity of California at Santa Cru z, the ni versity of 

Hawaii . the University of Washington, the Scripps In titution of Oceanography, the . . Geologica l Survey. roAA, 

and the oil industry (fig. 98). 

The scismi c-renec ti on profiles vary wide! in quality. 

depth of penetration , and navigational accuracy. ranging 

from single-channel parker record navigated with Loran A 

to 144-chann I digital profile na igatccl with the Global 

Po ition ing y. tcm (GP ). Partly in preparati n for the 

ational cicnce F undation's Ocean Drilling Program 

(ODP) cg 146. a sit survey n isting of clo. el pa d. 

high-resolution S lines, caM A R I side. can mapping, caBcarn wath bathymetry , and ALVI . ubmcr. ible 

dive. focu sed on the plate boundary and accr ti onary wedge 

at the propo. eel drilling . it . ncar lat45° I (fig. 99). Within 

thi 6.000-km2 area. we were ab le to map submarine tructurc in onsidcrabl detail. 

Within the ODP si te- urvey area (fig. 99). po ition 

information from the 144-channel rene tion profile. wa. 

used a the datum for mapping that portion of the larger stud 

area . Procc si ng through time migration ' a. carried ou t at 

the University of Hawaii . evcral other .S. Geological Survey 24-channel and Oregon State Uni vcr ity inglc- hanncl 

lines al o constrain the ·tructural interpretati ons. The side -

can and bathymetric . ur ey. were n·tvigated with a combinati on of GP an I Tran it sa t llitc navigation. with Loran 

trackin g used between satellite fixes. 1 a igati on of the deeptowed . idcscan towfi h wa by the method described by 

Appel gate ( 1988). Where . patial mi sfits occurred. we 

ac!j u ted the ide can data to be t fit the GP -navigated M S 

line or the eaBeam bath metry where apt ropriatc. 

SeaMARC I A sidescan data were co lic ted ' ith a de ptowcd 0-kH z sy tcm capable of imagino 2-km- or 5-kmswath width having patial re ol ution. of I m and -.5 m. 

respectively. A magnetometer atta ·h d to the towfi. h 

recorded total field intensity over the sidesca n tracks. Our 

technique with ide. can survey. was to o r the : tud ar a 

at the wider 5-km swath width, then return to ar a of int rest 

and conduct detailed . urveys u. ing the higher rc. olution 2-

km . wath. Sidescan pro c . . ino, \ hich includ d ocometri 

and speed correction . orrccti on for towfi. h po. it ion. gcorcfcrencing of image pixel. to a latitud -longiwde grid . and 

image enhancement (a. d . cribcct in ppelgatc, 19 8), ' as 

done at the OAA imag -pro .. ing fa ·ility in ' port, 

Oreg. Outside the ODP . itc-sur e ar a. na igational ac uracy was more variabl . Loran-A-na igatcd profiles have 

maximum error on th order of 1- 3 km, Loran errors arc 

about 0- I .5 km. and Transit at llitc error range fr m ncar 

zcr up to hundreds of meter. . The den. c cov rage of rcncction profiles allowed adjustment of older track lines ' h rc 

rossed by atcllite-navigated line .. 

STRIKE-SLIP FAULTS ON THE 

JUAN DE FUCA PLATE 

A long the central Oregon convergen t margin . the thr c 

wcs t-nonhwe t-trcnding left-lateral trike-slip faults 

(Wccoma fault and faults B and in fig. I 00) ex tend from 

the base of the continental lope I 0- 18 km northwestward 

across the aby sal plain. First imaged using caMAR lA 

. ide can sonar in 1986. these fault s w re . urveycd in d tail 

in 1989 u ing a narrow- wath, high-re olution urvey 

(Appelgate and other , 1992) and were cro . ed . c era! time 

by M profile in I 989. In reflecti on profiles of the abyssal 

plain. the thr e fault how verti ca l eparati on of both the 

oceani basement and the 2- to 3-km-thick overl ying cdimcntary sec tion (Appelgate and other , 1992; M acKay and 

others, 1992: thi. tudy). The fault. . trike 290°- 295°, and all 

three are prim aril y strike-s lip with a minor up-to-the-north 

componen t of di splacement a evidenced by off ct surface 

and subsurface features. In rhi . paper, we eli cus primarily 

the Wecoma fault because thi fault i better overed by ei -

mic and ides an urvey . Fault. B and C appear to b similar in structural tylc to the Wecoma fault , but the spar e data 

a ail able n these fault. pre entl y preclude. a detai led 

a c. mcnt of th m. 

CHARACTERISTICS OF THE 

WECOMA FAULT 

The Wccoma fault i the longest and be. t developed of 

the three trike-slip faults on the abyssal plain (fig. I 00). Th 

general phy iography of the Wccoma fault, its a sociated 
226 SSE SI G EARTHQUAK H ZARDS A D RED CI G RISK IN TH PA IFI ORTHWE T 

125. 30' 125.00' 124.30' 

1/ 

I ( 

0 10 20 30 40 KILOMETERS 

124.00' 

_L 

CAPE 

FALCON 

Netarts Bay 

CAPE 

LOOKOUT 

Yaquina Bay 

-

Figure 98. Trackline map of the nonhwc. tern Oregon coast showing the location of all seismic-refl ection profiles used in thi s study. 

Trackline ti ck marks arc hourl y or quan r-hourly ship po. it ions. The deformation front of the acc retionary prism is ·hown. The box ~ the 

area in figure 99. 
ACTIVE STRIKE-SLIP FA LTI G A D FOLD! G OF THE PLATE BO I A RY A D FOREAR . OR EGO 227 

45" 125"40' 125"30' 125"20' 125"10' 125"00' 30' ~----~------~~------~~---- ---- ----~ 

45" 

1 0' 

45" 

00' 

44" 

50' 

44" 

40' 

0 

37 

10 20 KILOMETERS 

37 

Figure 99. Map showi ng scismic-renection 1rac klines and bat hymetry in the Ocean Drilling Program ( DP) 

site-survey area off the northwestern Oregon coast. The locati on of th is map is shown in fi gure 98. Depth s are in 

meters; contour inter al is 100m. Bold-numbered lines are referred to in the tex t. Bold segments of lines 37. 45. 

and 77- 11 arc sections of profi les shown in fi gures 104. 105, ani 11 7, respecti vely. 
22 

0 

0 

~ N 

0 

(Y) 

~ N 

0 

0 

i.n 

N 

0 

(Y) 

i.n 

N 

AS E SI G EARTHQ KE H ZARD D RED CI1 G RISK I TH P IFI ORTHWE T 

1;:) 

Ctl 

Q) 

·

---

t J 
A TIYESTRIK --SLIPFA LTI G D FOLDING OF THE PLATE 80 DARY NO F RE R , OREGO 

• • 

EXPLANATION 

Fault-Dashed where inferred, dotted where 

concealed; barbs show relalive 

motion where known; bar and ball 

on downthrown side where known 

Thrust fault-Dashed where inferred, 

dotted where concealed; saw teeth on 

upper plate 

--tl--~> Anticline-Showing direction of plunge; 

trace dashed where inferred, dotted 

where concealed 

Syncline-Trace dashed where inferred, 

dotted where concealed 

Monocline-Dashed where inferred, dotted 

where concealed 

Lineation-From GLORIA sidescan imagery 

Topographic scarp-Ticks on lower side 

Channel 

Figu•·e 100 (faci ng page and above). tructurc of 

the central Oregon continental margin. Thi s ma1 

emphasizes acti ve features: most structures shown 

cut or deform the sea n oor. The deformati on front 

is a thrust fault south of fault B and is the base of a 

seaward -dipping thrust ramp north of fault B. Structure offset. along fau lt are shown only where 

known. Miss ing offsets ac ross faults may indi cate 

( I) insufficient data, (2) no h ri t.ontal separati on. or 

(3) hori zontal scparati n not visible at thi s sca le. 

Areas showing sets of structures intersecting and 

crossing other structure. result from lar,c age differences in the two structural . cts. 

stru ture . and th adj acent aby. sa l plain and accrcti nary 

wedge are shown in fi gure I 0 I . The surface tra e and morpholoo ic feature a ·so iatcd \ ith the Wecoma fault. and the 

complex inter. ccti on of the fau lt wi th the deformation fron t 

arc . hown in fi gure I 02. We ha c int grated all of the a ailable gcophy. ica l data in 'l geol gic structure map of the 

Wecoma faul t on the abyssa l pl ain and adjacelll frontal 

accretionary wedge (fi g. 103). M igrated M profile. of the 

Wccoma fault rc ealthat it i. ncar! verti ca l. in . omc pl ace 

con. isting of a main strand with linlc other dcforrnati n 

(fig . . 104A , B). and in other pl ace chara tcri zcd by an 

upward-branching posit ive fl ower structure (fi g. I 05), as 

defined by Harding ( 19 5). Blo k. within th fl ower truelure m . tl y show minor rever. c motion. In both sci. mi c and 

. iclcscan records, the di splacement and ·urface express ion of 

the Wccoma fault on the aby. sal pl ain dimini sh toward the 

northwest. On the hi oh-rc. olution 2- km side. can swath, the 

trace of the fault is lo. t about 18 km west-north west of the 

deformati on front. An M profile (line 33 in fi g. 99) 

cro scs the along-strike proj ection of the fa ult 7 km we !-

north west of the point at which the scaq cli sapp ar. on the 

2- krn sidescan record . At the projected locati on. onl y minor 

deformation i. seen, sugge ting that thi. rrofi l eros. es n ar 

the fault tip. Two other rossing r fie tion profile . I cat d 

3.7 km and 5.5 km west of the lin -33 eros. ing of the fault. 

show that the fault doe not ext ncl \ es t of long 125°41 'W . 

Seismic-reflec tion profil es indi ate that the oma 

fault i a structural break in the ubducting .Ju an de Fu a 

plat . On M C line 37 ncar the d formation front (fi o. 104A, 

B), erti cal . paration of the ba. altic o cani cru. t is apparent, the northeastern block being upthrown ( pp !gate and 

other , 1992; thi study). A lthough the erti al . cparati on of 

the ba emenl . ccn in fi gure 10-+ i. not large, idcnti al ertica l 

separati on a! oc ur. in the o erl ying tratigraphic units. A 

corresp nding basem nt offse t is required to 1 rodu e the 

erti cal . eparation of the . cclim nt s. thu . . the obser eel basement off ct cannot be a hummock on the basemen t surface. 

The overall tri ke of the Wecoma fault from the base of 

the contin mal lope to it. north\ es tern terminu. is 293°. 

and the faul t ex tend I km ac ross th ab ssa l pl ain from the 

deformati on front. The tra c bends gent! sou tlw ard 7 km 

northwes t of the deformation fron t, then abruptl y l nninatcs 

along the . outhern boundary of 'l . tructural up> arp (pres. ur ~ 

ridge PR in fi gs. I 02 and I 05). Di. placcm nl is taken up by 

a second northwes t-trending, right-s! pping segmen t that 

originate on the northern flank of the up\ arp and trikes 

northwest to its tcrminu (fi g. I 02: sec also ppclgatc and 

other , 1992). Thi s up\ arp wa. initi ally thought to be a mud 

volcano, but deep-towed seismic-rei! ction data shows internall y coherent rellect rs, . uggcsting a . tructural ori gin 

( ochranc and Lewis. 19 8). The upwarp give. much information about the timin g and development of the Wee ma 

fault and is therefore di scussed here in om detai l. It is a 

c mplcx tructure th at is bounded on the . uth by th southern strand of the Wecoma fault and on the other three ides 

by minor reverse fault.. but it i. best described a a doubl y 

plunging north-northwcsHrcn ling anti line. The threcdimcn ional tructurc of the upwarp i. shown in figure 106. 

M agneti c modeling of the up\ arp and the Wccoma 

fault indicate that the ba. a! ti c basement i also upwarped 

beneath the fold in the sedimen tary section and is v nica ll y 

offset about I 00 m by the fault. north block up (Appelgatc 

and other , 1992). Based on the in !vement of the ba em nt 

and the right step in the fa ult. we interpret the upwarp a. a 

restraining-bend anticline (Sylves t r. 19 ) formed by compress ion aero s the right tcp in the Wecoma fault. Therefore, we refer to the upwarp a a pre ure ridge. 

Al tern ati vely. th ridge may be a trunca ted. earl y formed 

anticline of the type de cribcd b Wilcox and others ( 1973). 

If o, the anticline may have formed in the sedimentary sc -

ti n in rc ponse to left-lateral eli . placement of the underl y ing . 

Juan de Fuca plate and then was panly truncated by two en 

echelon egmcnts or the fault a. they propagated upward 

with inc rca. ing fault motion . If thi s model is correc t, the lack 

of a single throughgoing trace implies that the fault has not 
. "'I -Neo9e 

p.._cc(e\IOIIa.'' 

~ 

Viewing angle= 225· 

Elevation angle = 35· 

Vertical exaggeration= 6:1 

.........__ ' 

s·oo 

:J 

1'2-

DEPTH, IN 

KILOMETERS 

Figure 101. Perspecti ve mesh plot of SeaBeam swath bathymetry data ofT the central and northern Oregon coast. View from the southwest showi ng the physiography of the abyssal 

plain and accreti onary wedge nca r the Wccoma faul t (WF) and faul ts B and C. Figure 100 shows map location of faul ts. BSC, base-of-slope channel; PR. pressure ridge: PU, pop-up 

plateau; SD, slump debri s: CH. Cascade Head on the Oregon coast. Arrow shows locati on of figure 11 8. Diagram provided by the National Oceanic and Atmospheric Administration, 

National Ocean Survey. 

N 

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45° 

12' 

45° 

11 ' 

45° 

1 0' 

45° 

09' 

45° 

08' 

125°34' 125°32' 125°30' 125°28' 125°26' 125°24' 

Figure I 02. M osaic of SeaMARC- I A 5-km swaths across the Wecoma faul t off the central Oregon coast. PR. pressure ridge: WF. Wecoma fault; PU. the pop-up \ tructure that mark\ 

the intersection of the Wecorna fault with the deform ati on front (DF); A, apex of pop-up corresponding to point A in figure 11 3: OC, off~e t channel; OS. offset slump scar; S. fault ..,plays 

that occupy gullies on the seaward flank of the marginal ridge (MR). An embayment in the deformation front (discussed in tex t) is ju<,t cast of PU. Numbered lines show locati ons of 

multichannel seism ic survey (MCS) lines 37 and 45. 

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232 

125'30' 

45' 

12' 

45' 

10' 

~ I 

I 

r 

SSES I G EA RTHQ AKE HAZARD D REDU I G RISK I THE P CIFI NORTHWEST 

125' 28' 125'26' 125' 24' 

2 KILOMETERS 

EXPLANATION 

Fault-Dotted where concealed; barbs show 

relative motion where known; thinner lines 

show minor faults 

4 4 Thrust fault-Dashed where inferred, dotted 

where concealed; saw1eeth on upper plate 

! > Anticline 

<rCH 

Lineation-From SeaMARC 1A sidescan 

imagery 

Topographic scarp-Ticks on lower side 

Channel-Showing flow direction; dashed 

where inferred,queried where uncertain 

Figure 103. Structure map of the western pan of the Wccoma fau lt, adj acent abyssal plain . and frontal accreti onary 

wedge ofT the central Oregon coast. tructure compiled from migrated and unmigrated seismic-renection profile. , 

SeaBeam bath ymetry, and ScaM RC I A sidescan imagery. A , apex of pop-up (corresponds to A in fi g. 11 3). Structures shown as offset onl y where known. 
A T IY STR IKE-SLIP FA LTI GA D FOLD! G OF THE PLATE BO DARY A 

NORTH 

NORTH WECOMA 

FAULT 

DFORE R ,OREG 

SOUTH 

SOUTH 

3.5 

4.0 (/) 

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Figur·c 10-t Migrated multichannel ci mi c line 37 cro sing of the Wecoma fault between the pressure ridge and 

the deform ati on front off the ce ntral regan coast. The location of line 37 is . hown in bold in figure 99. A, uninterpreted secti on: /J . interpreted section. otc thi ckening of unit I and thinning of units 2 and on the down thrown 

(ri ght) block. 0 . oceanic crust: T. block moving tOward the viewer: A. block movin g away from viewer. 
234 SSESSI G EARTHQ AKE HAZARDS D RED Cl G Rl K I THE PA IFIC ORTHWE T 

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3.5 

4.0 ~ 

z 

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Figure 105. Pan of multichannel . cismic line -i5 . bowing the Wecoma fault and the pressure-ri dge anticline off the central Oregon coast. 

Location of line 45 i shown in b ld in figure 99. Rcnector mark the strat igraphic point in the. cction ab ve which seismic stratigraphic 

units thin over the ridge ere l. Below renector S. the stratigraphy d es not . how the inOuence of ri dge growth. This profi le crossed the 

faul t ju. t east of the eastern terminu s of the northern strand of the Wecoma fault. A small normal fault is located j ust nonh of th Wecoma 

fault ; barbs show direction of movement. 

completed it final tage of development into a single 

throughgoing trand as modeled in clay experiments and 

ob. crved in nature (Wil cox and other , 1973). An alternati ve 

explanati on of the ridge i that the upwarp is related to the 

growth of the accreti nary w clge (Cochrane and Lcwi . . 

19 ). If so. the ridge wou ld be linked t the ma. tcrdecollemcnt by an undetected basal fault. However. analysi. of the 

growth hi tory of the ridge. di scussed below. indicates that it 

is roughl y twice the age of th fir t ridge of the accreti onary 

wedge and began it. growth when it wa 2 -32 km west of 

the prc. cnt deformati on fr nt. u. ing the 40 mm/year Ju an de 

Fuca- onh mcri ca pl ate-convergence rate of DcM ets and 

others ( 1990). The relative longevity of the ridge and the evidence for a basemen t upwarp uggcst that the ridge's formation wa. unrelated to growth of the frontal thrust. of the 

accr ti onary wedge. However. recent growth probably ha. 

been augmented by horizontal omprcssion clue to the proximity of the plate bou ndary. 

AGE, ET SLIP, A D AVERAGE L IP RATE 

OF THE WECOMA FAULT 

trati graphic relati onships w ithin the abyssal-pl ain . cdimentary sec ti on ncar the Wecoma fault and the pre. urc 

ridge can be u ed to infer the growth hi story of the pressure 

ridge a well a the slip rate and horizontal separati on on the 

Wecoma fau lt. 

ABYSSAL-PLAI STRATIGRAPHY 

We have informall y divided the abyssal-pl ain sedimentary sec ti on into four seismi c-stratigraphi c units on the ba is 

of reflection charac ter and strati graphic coherence using the 

1989 MCS data (fi g. I 048 ). nit 4 is a weak ly refl ecting 

layer that direc tl y overlie the basa ltic rocks of the cean ic 

cru t. The poor refl ec ti vi ty of thi s unit is probably clu e to its 

lithologic homogeneit y. consisting of poorl y tratifi ccl carbonate oozes, ca lcareous mud tone, and ill turbidites at the 
MARGINAL RIDGE 

WECOMA FAULT 

0 5 KILOMETERS 

Vertical exaggeration = 1.6:1 

Figure 106. Compos ite block diagram of the pressure-ridge area and intersecti on of the Wecoma fault and the deformation front as viewed from the southwest. Other faults arc also 

shown. Migrated seism ic secti on' (two-way traveltime) arc shown with selected rcncctors enhanced . Vertical scale i' two-way traveltime, in seconds. AP, abyssal plain section; AF, 

~to ri a Fan (note thickenin g ac ross the fault and thinning over the pressure ridge): OC. oceanic crust: A. moti on away from viewer: T. moti on toward viewer; SV, seaward vergence; 

LV, landward vergence: DF. deformation front. 

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236 ASSESS! G EARTHQ AKE HAZ RD 0 REDUCI 1G Rl K I TH P I IC ORTHWE T 

ati onal Science oundation·s Deep ea Drilling Proj ec t 

(DSDP) drill . ite 174A on A. toria Fan (Kulm. Huenc, and 

other . 1973). The thickne s of the unit va ri c. because it fill s 

in the rough ba ement topograph , but it average ab ut 00 

m near the pre ure ridge. nits 2 and 3 arc defined somewhat arbitrari ly on the ba is of prominent reflectors and consist primarily of di tal thin-bedded ilty clay turbidites 

interbedded wi th hemipelagic clay (Kulm. Huene. and other . 1973). The units are an abyssal-plain . cquence that i 

thought to ha e either a Kl amath M ountains or Vancou cr 

Island source (Kuhn and Fowler, 1974). nits 2 and 3 both 

thicken uniformly eas tward in respon c to increasing rates of 

sed imentation and the ea tward lope of the 11 -9 M a cru. t 

pre ently being subducted (Wil on and other , 1984). 

ppcrmo t unit I i composed f overl apping lobe of the 

middle to late Pleistocene A toria Fan (Kulm , Huene, and 

others, 1973). The lithology of unit I is d minatcd by th into thick-bedded med ium to very fine grained sand interval 

with sharp lower contacts grad ing upward into . ilts and . ilty 

clay (Kulm , Hucne, and other .. 1973). A capping fifth unit 

consists of 1- 2 m of Holocene hemipelag ic ilt and clay (not 

visible in fig. I 048) ( elson, 1976; thi s ·tudy). 

TIMI G CONSTRA I TS 

Seismic profi les arc notoriou ly poor at re olving . Iratigraphic and timing relationships in trike-sl ip fau lt zones. 

However, the clo. ely spaced MCS lines near the Wecoma 

fault and the fortu it u pre. ence of criti cal timing indica tors 

allowed u to bracket the interval in the tratigraphic secti on 

at which verti ca l motion of both the fault and the pressure 

ridge beoan. On M line 37 (fi g. I 048), unit I clearl y shows 

thickening of the whole layer and al o the individual acou tic 

intervals on the down thrown (. outh) ide of the fault, indicating that the Wecoma fault has been acti e for mos t of the 

depositi nal hi story of the Astoria Fan. On line 45 (fig. I 05), 

the sa me units thin acros the pre ure ridge, indicating that 

verti ca l development of the rid0 e ccurred during the same 

time interva l. Thickne. s trends in unit lower in the section 

on both CS line are unaffected by the presence of the fault. 

indicating that they predate the Wccoma fault. A lternati vely, 

Appelgatc and others ( 1992) have interpreted apparent stratigraphic pinchout. again t the pre ure ridge as ugge ting an 

early hi. tory for the basement upwarp, in ferring that ridge 

formation occurred . honly after the formation of the cru t at 

the spreading ridge ( ee unit 4 in fi g. I 05). However, the marph logy of the basement upwarp a determined from magnetic modeling and MCS 1 rofile matches that of the folded 

sed imen tary . cction, indicating that folding occurred following deposition of mos t of the . ed imentary sec tion. Additionall y, the pinchout of unit 4 is onl y observed on so me 

r fl ection 1 rofil cs of the upwarp, whcrca on others it i. 

clearl y an artifac t. We therefore now interpret thi s apparent 

pinchout a. a side echo of the upwarped basement 

superimposed on unit 4. Oregon tate ni versity MCS line 

30 < nd .S. Geologica l urvcy lin 77- 12, neither of which 

cross the upwarp. show th i. uperimpo. cd side echo clearl y 

(sec fig. 99 for locati ons of these line ). 

On close in pection, the divi. ion between prcfaulting 

and synfaulting unit . ref! ctor S, can be iden ti fied at a depth 

of 4. 0 seconds on line 37 (fig. I 048, abyssal plain, upthrown 

. ide) and 3.82 second. on I ine 45 (fig. I 05, top of the pressure 

ridge). Refl ectorS wa. iden tified by examining the thicknc . 

of each trati graphic ubunit n these two line. and pick ing 

the refl ector that separated unit. . hawing fault-related thickne s trend from those that did not. nit. 2 and 3 . how the 

re ersc thickness relation hip, with thinner units on the 

dO\ nthrown ide of the fault. This relation hip is maintained 

along the length of the faul t, sugge ting that the fault wa. not 

acti ve in a enical en e during unit 2 and 3 deposition or that 

the . cnsc of verti cal moti on was south block up. Because the 

refle tor. con istently . how a. outh-bl ck-d wn separation, 

we can attribute the. c thickness chang a ro s the fault to 

left-lateral trike- lip di spl acement of the ca. twardthickcning wedges of . edimcnt (that i. , left lip moves thinncr wedges of sediment ea tward on the southern . ide of the 

fault relati ve to the north ide). otc that not all reflectors 

need how any verti cal eparation on . cismic 1 rofile. of 

strike- lip fau lts and. 1n fact. ome in figure I 05 do not even 

though other e idence discussed below demonstrate 5- 6 km 

of net slip on the fault. Growth of the pressure-ridge anticline 

appears to have been coeva l with sli p on the Wccoma fault. 

a the ci. mic reflector in both lines 37 and 45 (refl ector ) 

that separate prefaulting and synfaulting sedim nt. can be 

correlat dover the inter cning di tancc on M C line 32. The 

high 1uality of the data allows correlation over tens of kilometers with a high degree of confidence. 

In order to es tabli. h the time at which fault motion 

began, we have correlated a prominent sei mic reflector at 

the ba c of the A tori a Fan wi th the . amc interval drilled at 

DSDP sitc 174A,70 km southwe t of the pre urc ridgc(fig .. 

97 and 107). Fortuitou. ly, during the initial survey for si te 

174A. a sci. mi c-refl cc tion profile was made that provides a 

direct tic between the drill site and the pre sure-ridge an ticline (Kulm and others, 1973, fi g. 2, line A- C). The change 

in reflecti on character above thi s reflector (fi g. I 048, base of 

unit I) can also be traced from the pre sure ridge to the drill 

itc. A t the drill ite, the profound lithologic change between 

the sand turbidites of the fan and the silt turbidites of the abyssa l pl ain sequence was ob. erved at the depth of thi. refl ector 

in the cores (fi g. I 07). Matching reflectors with their respective lithologic changes in the cores also allows accurate alculation of seismic-interval veloci ti sat the drill itc. 

Ingle ( 1973) plotted the coiling directions of the pl ank -

tonic foraminifer Clobigerina pachyderma in the core. from 

site 174A (fig. 108). Notable are three dex tral coiling events 

at depths below the surface of 0-40 111 , 125- 135 111. and 

238- 295 111. The earli es t of these three event. brackets the 
SOUTHWEST 

SITE 174A 

3.5, I (f) 

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z 

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z 

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~ 4.5 

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<( 

s: 

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5.0 

5.5 

- --

.... .........._ - , .,.--... _.._..,.----

0 5 10 KILOMETERS 

126'30' 126'00' 125'30' 

45'30' 

45'00' 

_,--~ 

r ........ ...._ ............ ----

NORTHEAST 

PRESSURE RIDGE 

--.... 

Figur·c 107. Line drawing of a single-channel reflecti on profile (Oregon State Uni versity crui se YA LOC 70. Leg 5) linking Deep Sea Dri ll ing Project drill site 174A to the pressureridge anticline and the Wecoma faul t off the central Oregon coast. Upper heavy line marks the base of the Astori a Fan at the drill site. Lower heavy line is the top of the basaltic ru t. 

dated by micropaleontologic rechniques at 760±50 ka. Inset shows 1rackline of 1he seismic profile linking the drill site wi th 1he pressure ridge. 

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238 A SESSI G EARTHQ A KE HAZ RDS A D REDUCI G Rl K I TH E PA IFI ORTHWEST 

depth at which the base of the A tori a Fan . ecti on was found 

in the hole (284m). J .. Ingle (Stanford ni vcr ity. writt n 

com mun ., I 99 I) ha related these dex tral coiling event. to 

regional northea. tern Pacific paleo-oceanographic event . 

which are dated with magneto. tratigraphy and tephrochronology. By thi s method, the upper and lower bounds of the 

lowest coiling event arc dated at 600 ka and 800 ka, re pcctivcly. Interpolating from thee bracketing age. with sed imentation rate , the age of the ba e of the A tori a Fan ec tion 

at ite I 74A is 760±50 ka, with the margin of error reflecting 

uncertaintie in usi ng uniform . edimentati on rate and in the 

preci ion of determining the age of the coilin0 event . 

To e. tim ate the age of thi s ame refl ector ncar the 

Wecoma fault, we infer that there i no. ignificanttimc transgre sian between site I 74A and the pre ure ridge. We make 

this inference becau e no significant nlap r ofn ap relationship were ob erved within se era! hundred verti ca l meter 

of the base of the fan (fig. I 07); the reflector is nearl y horizontal, and the trend of the ei mic ection is about perpendicular to the ed iment transport direction. ing an average 

ed imentation rate of I I 0 em per I ,000 years calcu lated from 

the age, thick ne s. and ei mic velocity of the fan secti on at 

the Wccoma fault, the age of reflectorS, and thu the age of 

verti cal motion on the Wecoma fault, is e timated to be 

600±50 ka. Although add itional uncertainties ex i t in sedimentati on rates, age correlation of the base of the fan. and 

seism ic velocities, we use the arne margin of error here 

bccau. ewe are unable to quantify them independentl y. 

RETRODEFORMATIO OF FA LT MOVE IE T 

We have taken advantage of the geometry of the abyssal-plain units in order to ca lculate the net slip on the 

Wccoma fault. Sedimentary units trike mo tl y north- outh 

and thi ken unifonnl y ea tward, re ulting in ea !wardthickening edimentary wedge . Slip n the Wecoma fault 

has cut and juxtapo ed the e wedge so that thicknes 

change aero the fault are pronounced. On MCS line 37. 

units 2 and 3 (fig. I 048) are thinner on the outhern 

(down thrown) . ide. We attribute thi abrupt thinning aero 

the fault to be the re ult of left-lateral translation of the 

eastward-thickening wedges making up these two unit (we 

a. ume negligible differential loading of the unit due to 

thickne . change in unit I). In order to e timate the horizontal se1 aration along the Wecoma fault, we have restored 

the aby sal-plain section by rever ing the left-lateral 

motion on the fault (fi g. 109). The blocks were translated 

right-laterall y until the ea. tward-thickening wedge of sed iment matched. sing thi s method, the thickness match 

aero .. the fault wa made simultancou. ly with 18 indi idual reflectors within unit 2 and 3, reducing the error inherent in picking onl y a few reflector . The hori zontal distance 

needed to res tore the section is 5.5±0.8 km. 

Epoch Stratigraphic 

units 

Astoria 

Fan 

Depth 

(meters) 

0 -

50 -

tOO -

tSO -

200 -

Coi ling direction 

100 percent 

sinistral t OO percent dextral 

- ,-

...,.~--.,;--- 165 ka 

>.--225ka 

l 

-!--- 600 ka 

4 - --t--!---l--- 760 ka 

4---l--- 800 ka 

Figure 108. Coiling directions of Clobigerina pachydenna in 

core from Deep Sea Drilling Pr ~ect site 174A off the central Oregon coast. Lowermo. t dextral coiling event is dated by tephrochronology. Plot is dashed where inferred. M od ified from Ingle 

( 1973, fig. 4). 

We a! o pl otted isopach of unit 2 and 3 as an alternative method for determining hori zontal separation . Figure 

I I 0 is a pseudo-i. opach plot (contours in two-way traveltime) of unit 2, following smoothing of the data and 

removal of a veloc ity artifact from th pre. sure-ridge area. 

The plot reveal a pattern of left off et of individual 
ACTIVE STRIKE- LIP FAULT! G A D FOLD! G OF TH E PLATE 80 DARY A D F RE R , OREGO 2 9 

NORTHWEST 

position 

(600 ka) ' ' ' 

' ' ' ' 

/ 

' / 

Present ', / 

position 

/ 

/ 

Figure 109. Diagram illustrating the method used in retrocleformation of the Wecoma fault located off the central Oregon coa. t. 

Fault m ti on was rever. eel graphicall y by moving sei. mic time section. on both sides of the fault until eastward -thickening sediment 

wedge. of units 2 and 3 matched aero. s the fault. The technique indicates a present hori zontal separati on of the units of 5.5±0. km. 

Barbs show relati ve motion. 

contours decrca ing northwe. tward to zero at the fault tip 

17-20 km . caward of the dcfom1 ati on front. The off et of 

the contours near the deform ation front i 3.7±0.5 km , 

determined by averaging the three ea. ternmo t contour . 

We attri bute thi omewhat differ nt re ult to poor constraint on the contour uth of the fault and to disruption of 

thickne . trend. near the adjacent pop-up. Additionall y. th 

isopach offset depend on only 3 piercing point . whereas 

the rctrocleformati on u .. I . We thu infer that th retrodeformation method i th better method of e timating horizontal cparati on. and we u. c the value 5.5± 0. km in th 

lip-rate ca lculation below. eglecting the I 00 m of vertica l cparati on. thi alue rcprc. cnt the net slip on the 

Wecoma fault ncar the deformation front. 

A p tcntial source of error in the u c of time vcr u 

thickne is the po ibility of lateral veloc ity change. in the 

se lecte !units. However. lateral velocity change . if pre ent. 

are mo. t likely to be oriented norm al to the deformation front 

and thus have negligibl cff ct on rc. toration of the sedimentary section. nfortunatcly, the younge t unit (fi g. I 048. unit 

I) eann t be u eel for an independent estimate of net . lip during clcpo. it ion of the younge t part of the fan section because 

verti ca l movement on the Wecoma fault ha. innuenced 

thickncs · of thi s unit. 

Using the previou. ly calculatccl age of the fault and the 

net-s lip estimate abo c. we calculated that the a eragc slip 

rate of the Wccom a fault since its incepti on 600±50 thousand 

years ago has been 7- 10 mm/ycar ncar the pr sent deformation front. Thi s rate depend on the es tim ated age of th fault. 

125'45' 125' 35' 125' 25' 

MCS 31 · ... 

45' 

10' ... ,... 0 C') 

... 0 

'"! 

0 

Figur·e 11 0. P. cudo-isopach plot of abyssal-plain unit 2 orr the 

cemral Oreg n oa ·t. Contour. are in un it. of l\ o-wa tra chime 

(seconds) rather than thickness: s e figure I 04 for unit des ignati ons. Ole westward-decreasing left offset of unit 2 by the Wccoma fault (barbs show directi on of movement). The avcrnge offset 

of the 0.625, 0.600. and 0.575 co nt r~ i. about -1 km. to ur~ 

cl ashed where inferred. Dots are control p ints along multichannel 

. ei. mic (MCS) profiles. Line numbers correspond to the trac klines 

sh wn in figu re 99. 

whi ch may include a margin of error that i too small, as eli . -

cus eel abo e. Thu . the c·1Iculated . lip rate ma have a 

larger error than alculated here. 

LATE PLE1 TOCENE-HOLO E E SLIP RATE 

ScaMAR lA side can imagery how. that the 

Wccoma fault offset a late Pleist cenc el i. tribut ary channel 

and an older . lump scar on the A toria Fan (OC c nd OS in 

fi g. 102 : offset channel in fi g. Ill ) left laterall y. Hori zontal 

cparati ons are about 1-0±5 m and 350±10 m, respec ti ely 

(Appelgatc and other . 1992: thi. tudy). The e cro .. cutting 

relati ons can be used to obtain an inclcpcndcnt estim ate of the 

·lip rate during latest Plci. toccne and Holocene time. The 

off et channel in parti cular offer. the bes t opportunity for 

estimating the Holo cne slip rate. The channel i. blocked 18 

km upstream (to the north) by slump debri s from a 32 km 3 

. 101 fa ilure (fi gs. I 00 and I 0 I. labeled SO). The lump i 

apparentl y a bedding- pl ane slide from the wes tern nank of 

the leading landward-vcrgcnt acc reti nary thrust ridge. We 
240 ASSE Sl G EARTHQ KE HAZARDS A D RED Cl G RISK IN TH E PACIFIC RTHWEST 

. ' " :.:·11 ' .. • .,. 

... ~·. '· :~· ...... 

. ... · 

Figure 111, High-resolution SeaM A R I A sidescan image of the Wccoma fau lt offsetting the we. tern bank of a late Pleistocene distributary channel on the southeastern part of the Astori a Fan. Hori zontal separation is 120± I 0 m. Light tones rcprc em high backscatter: in. onification (direction of sonar illumination) is from the south. Vertical scarps (highest backscatter at right-cemcr of image) are the result of 

stri ke-s li p juxtaposition of irregu lar sca-noor topography. The fau lt strike i~ 293°. Barbs show relative moti on. Minor fau lts (dashed lin s) 

that also offset the channel wall arc interpreted as pan of the . urface cx 1 ression of the nowcr strucwrc . hown in fi gure I 06. 

es tim ate the age of thi. lump to be 24- 10 ka, ba. eel on a 14 

el ate from a grav ity core taken from a sediment drape on top 

of one of the lump block. and also on onl apping relati on 

ob erved on a high-resolution eismic record. In the reneeLi on profile, the lump debri s rests directly on the sea-lloor 

rene tor with no vi sible sediment onl ap or p nding around 

the slump blocks. sing a composite late PleistoceneHolocene . ed imentation rate derived from elson ( 1968) 

and from thi s study. we estimated that sediment accumul ati on following the slump could not continue for more than 

about 24,000 years without depo. iting a thick-enough unit to 

b detectable on the M S profil e, thus se tting a max imum 

age for the ·Jump. We derive I the minimum age for the 

slumJ from the 14 age of sediment at the bottom of the 

gravi t core. The core sampled onl y postslump hemipelagic 

edi ment and, thu . the I 0.300 rad iocarbon-years-beforepresent (R.C. Y.B.P.) date from the lowermost part of the 

core ets the minimum age of the slum p at about I 0 ka. 

Because the fault is older than either the slump or the 

channel, a slip rate can be estim ated from the channel offset 

shown in figure I I I . The channel wa ll mu st have been cut 

prior to bl ockage of the channel by the slump, e tabli shing 

the minimum age of the slump at about 10 ka. Similarl y. a 

max imum age for the channel wa ll is constraine I to be the 

sa me as the max imum age of the slump. Channel cutting 

should have ceased or been grea tl y reduced following the 

blockage; thus, younger fault moti on would off etthe channel wa ll. as presentl y observed, without modification by eroion. If channel cu tting and fault motion were simultaneous. 

the high frequency of late Plei. tocene turbid ity currents 
ACTIYESTRIKE- LIP FA LTI GANDFOLDI G OF THE PL TE BO t D RY D FOR R . OR -GO A I 

carr ing large quantities of sand and gra el (Duncan 196 : 

el. on. 196 : rigg and others. 1970) should ha e eroded 

the offse t to ·omcthing other than the cri sp separation . ccn 

in ide. can images (fi g. I I I). Seismic-refl ection record 

showing the trunca ti on of deep-sea channel wa ll s (Grigos 

and Kulm. 1973), and numerous . cdimcnt hiatu. c in ore. 

from the ax ial part of the. e channels (Grigg. and Kulm. 

1970) document the erosi ve character of the oar. c-grainccl 

late Pleistocene turbi dity current in thi region. Therefore. 

the age of the channel wall is probabl y 24- 10 ka, con. istent 

with incision during the last epi sode of high turbidit y-current 

acti vity durin o the latest Pleistocene sea-level low stand . 

sing thi s age range, we calculated a latest PleistoceneHolocene lip rate of 5- 12 mm/year, one comparabl e to the 

7- 10 mm/ycar average rate calculated from the net hori zonta l di. pl acement since incepti on of the fault. 

INTER ECTIO OF THE ECO 1IA FA LT 

A 'D THE ACCRETIO ARY WEDGE 

The compl ex structure of the area where the Wecoma 

fault inter. ec t. the initialthru. t ridge indicate. that thi s fault 

is not simpl y o crridclen by the accreti onary pri sm. Highre. olution SeaMARC I A idescan records of the intersec tion 

zone on the abyssal pl ain show that thi s area i dominated by 

a nested seri es of uplifted a ymmetrical tri angular plateaus. 

called pop-ups, bounded by reverse fault. (fi g . . I 02, I 03. 

11 2. and I 13). The single trace of the Wecoma fa ult . cparates into sc era! di verging spl ays at the we tern tip of the 

largest of the pop-ups (A), and the largest of the c splays is 

the . outhern bounding fault of the pop-up. Si de. can and 

1 S record. suggc. t that the main trace is buri d by . lump 

dcbri. along much of it. length in thi s ar a (Appclgat and 

others. 1992). The pop-up pl ateaus appear to be the rc ult of 

a complc intcra ti on between the strike-slip fa ult and the 

evolving deformati on fr nt. The mod I we prefer t expl ain 

their development is shO\ n in fi gure I 13. The pl ateau. were 

uplift ed b tween divergent splays of th main strike-slip 

fault an I possibl were dri ven we tward by hori zontal compres. ion bccau. e of their prox imity to the pl ate boundary. 

ppclgate and other ( 1992) . uggcstcclthat the high verti ca l 

·carps between the ofT ct channel and the wes tern tip of the 

pop-up (fi g. I 02) arc the result of 2.5 km of I ft-latcral di splacement of a part of the pop-up. 

INFL ENCE OF THE WECOMA FA LT 

ON THE ACCRETIONARY WEDGE 

The ca. tern end of the pop-up directl y abut the western 

fl ank of the first accreti onary ridge of the onh mcri ca 

plat (MR in fi g. I 02). On the wes tern side of the initial 

thru st ridge, the pl ate boundary form. an embayment corrc-

. ponding to the width of the a fjaccnt pop-up on the abyssal 

pl ain ( ppclgate and others. 1992). A scaward-vergcnt 

thrust scgm nt oc upics the area of the cmba 111 nt in an 

o erall landward-vcrgcnt thru. t ·elling descri bed b 

M ac Kay and others ( 1992). Thi configuration sugg 'St. that 

the local rc er. al of verocnce in the leading ac r· tionar 

ridge may be related to the cffc t. ofth strike-. lip fault sul -

du ting beneath the wedg . The me han ism for thi · ergcncc reversal may b a local reduction in pore fluid 1 r ssur 

due to fluid cnting by th Wccoma fault. uch a prcssur 

los. would incrca c the ba al shear stress along the cl oll cmcnt. promoting de clopmcnt of th sea' arc! - rgcnt segment (Tobin and others. 1993). The ridge form d k s than 

300,000 year. ago. based upon microfoss il c iclcn ·e from 

roc ks dredged from the ridge ( arson. 1977). The cmba -

mcnt is bounded b lin ar gullic. that cut upslope into the 

initial thrust ridge. and other . uch gullic lie within the 

embayment (fi g. 102). We int rprct thcs linear gullies as 

fault spl a . related to . lip on the Wccoma fault. bscrvations and ample from the submer. iblc L I 1 confirm th at 

the linear oullics vi. iblc in sides an records arc zones of 

acti obli que faulting. cvcral di ves (fi g. 11 4) showed that 

bedding i. ex tcnsi ely hcarcd, with multidirecti onal sli kensidcd . urfacc. and mullions (fi g. 11 5). h ar planes stri king we. t-northwc. t, roughly parallel to the Wccoma faul t. 

' ere observed in these gullie . Linear west-north wes ttrending scarp. along the strike f the Wccoma fault arc also 

pre cnt on the ca. tern fl ank of the initial thrust ridge, the 

crest of whi h ha. a small left-lateral off'. t (fi g. I 16). These 

lineati on. follow th Wecoma fault trend and arc int erpreted 

as fault sca rp bccau. e ( I ) th y lack the cl ndriti c drainage 

pattern common! observed in nearby cro. ional gu llies, (2) 

th y strike at an angle to the bathymetric contours. and ( ) 

they originate at the highe t point on the marginal ri dge. 

S line 29 cro se the area of the. e scarps and how a 

. trong verti cal el i ruption of the marginal ridge that we interpret a. the cause of the . ca-fl oor scarps. 

Roc k collec ted with the AL V I submersible from the 

gulli c. n the caward flank of th marginal ridge consist of 

hca rcd silt. tone. sand t nc. and pebble-to-cobble conglomerate (Sample and others, 1993 : Tobin and others, 1993) that 

probabl y ori ginated on the A tori a Fan and were rcc mcnted 

with a cconcla ry carbonate cement (Kulm and ue s, 1990). 

The carbonate cement , deri cd from fluid venting from the 

shear zones, bonds the sheared edimcntary fragments into 

an angular brecc ia that fill s the bottoms of th gullies. Isotopic analy c of these pore-fluid-dcri cd ca rbonate cements 

show that cements in the fault zone differ significantl y from 

those cl ·ewherc in the young accretion ary wedge. Faultzone cement. from the Wccoma fault have depleted 81 0 

va lues ranging from -4 to - 13%r (pem1il) Pccdec Belemnite , 

(PDB) and 81

3 aluc. from - I to - 25%o PDB ( ample and 

others. 1993). cments from an accrctionary-wcdg thru. t 

fault on the ir.t thru t ridge 38 km to the uth ha c 8 180 

values ranging from +4 t +7%c PDB and 813C alues from 
242 A SESS I G EARTHQUAKE HAZARDS AND RED CING RI K I TH PA IFI ORTHWEST 

(/) 

a: 

UJ 

fUJ 

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L-----------------------~------------------------------------------_j 

~ ~ ~ ~ 
ACTIVE STRIKE-SLIP FAULTING A D FOLD! 1G OF TH • PL TE B 1 DARY A D FORE R . RE 

• • 

EXPLANATION 

Fault-Dashed where inferred, dotted where 

concealed; barbs show relative motion 

where known; bar and ball on downthrown 

side where known; smaller symbols indicate 

minor faults 

Thrust fault-Dashed where inferred, dotted 

where concealed; sawteeth on upper plate; 

smaller symbols indicate minor thrust faults 

--tt-+> Anticline-Showing direction of plunge; trace 

dashed where inferred, dotted where 

concealed; smaller symbols indicate minor 

anticlines 

t 

-E- CH 

Syncline-Trace dashed where inferred; 

smaller symbols indicate minor synclines 

Monocline-Trace dotted where concealed 

Lineation-From SeaMARC 1A sidescan 

imagery 

Topographic scarp-Ticks on lower side 

Channel-Showing flow direction; dashed 

where inferred 

Figure J 12 (above and facing page). Structural interpretalion of the illlersection zone and pop-up strucwre at 

the i!llersecti on of the Wecoma faull and the deformalion 

frolll. OS. off. et scarp: PR . pressure ridge: OC. offset channel; P • pop-up; A. apex of pop-up (cOTTe pond. to A in 

figure I 13). 

-30 to - 52%o PDB (Kulm and Sue , 1990). Sample and other ( 1993) attributed the isotopic rati os within the Wecoma 

fau lt zone to higher nuid temperatures near the Wecoma 

fau lt, pcrhap as high a I 00°C. and a pr bable thcrm geni c 

methane source. The source of the c nuids probably lie 

deeply buried within the sed imentary d · po:it of the 

abys. al-plain accreti onary wed,e. In c ntra. t, i. otopic rati o 

from the thru t fau lt arc be. t explained by nuid deri ved 

from a relati vely . hallow biog ni c methane source (Kuhn 

and uc , 1990). 

The evidence from nuid chemi. try, ide can image . 

and ei mi profiles indicate. that the I ad ing edge of the 

accreti nary wcdoc i. di srupted and off ct by the Wecoma 

fau lt. We have wdied side an image , eaBeam bathymetry. and seismi c profil es of the fr ntal accretionary wedge in 

ord er to determine the ex tent to whi ch the. c effect can be 

ob erved on the ontinental . lope. In high-rc olution 

SeaMAR I A idcscan image (fi g. 11 6), scarps and . hear 

zones arc observed to affect the bathymetric expressi n of 

several thru t ridges 8- 15 km land ward of the deform ati on 

front. The scarps arc ubparallel to the Wccoma fau lt and lie 

along the proj ec ted trend of the fa ult into the accreti onary 

wedge. sing d tailed image-enhancement techniques . we 

observed that the Wccoma fault zone is composed of . evcral 

. ubparall cl we. t-northwc. t-tr nding l inear scarp and shear 

zon . These linear fea tures arc clearl y fau lt . carps cutting 

ac ross the erosional spur. and gullie. of the main escarpment 

Figur·e I L3. Block diagram illustrating structure of the pop-up at 

the inter. ccti on of the Wccoma faull and th deformation fron t. 

Blocks with in the fl ower stru ·wre of the Wccoma faul t arc forced 

upward and westward b increasing horii.Ontal omprcss ion as subducti on pro rc~scs. Apex A corresponds t A in fi urc I I:?.. Arrow 

in chan nel shows flow direction. Barbs show n;lativt: motion on 

faull s. Sawtceth arc on upper plates of thrust fau lt:: fau lt ~ arc 

clashed where infc1Tcd. dolled where concea led. M odifi ' d from 

Syl est r ( 19 . fi g. 22C). 

at nearly right angle . The scawardmo t . ynclinal ba in of 

the accreti nary ' edge i. cut by both a northern and southern trand of the Wecoma fault. Eight kil omt:tcrs south as t 

of the second thru st ridge and 22 km utheast of the deformation fron t. the projected trend of the Wccoma fau lt is 

eros cd by .. Geological Sur ey line 77- 11 (fig . 99 and 

11 7). Thi renection record hows a near-verti ca l fault at. the 

western edge of a bathymetric bench near the proj ected po iti on of the Wecoma fault . A urficial bump or scarp overl ie 

the fault. wh ich affect the upp m1ost nat-lying sed iments of 

a broad ba in . Rencct r deeper in the ec tion how a sharp 

clip reversal and los of coherence. The dip reversal might be 

re olvcd a a ba in if the line were migrat d, but we think it 

is too abrupt and the radiu of curvature f the bed too hon 

for thi. interpretation. Thus. we interpret thi s fea ture a a 

verti ca l fau lt. po. sibl y the landward ex ten ion of the 

Wecoma fault. 

Hav ing found pos. ible ev idence for an upper-pl ate continuati on of the Wccoma faul t I 0-20 km outhea tward in to 

the accreti onary wedge. we carched for vidence of either 

terminati on or ontinuati on of thi. fault a well as fault · B 

and higher n the continental slope. We u eel academ ic, 

U.S . Gcologi al Survey. OAA. and indu. try . eismicrcfl ec tion profiles a well a. GLORIA long-range sidescan 

images and 0 SeaBeam high-resolution bathymetry 

now available from the ab sa l plain land ward t abou t the 

continental-shelf break in northern Oregon. 
24-t SSESS I G E RTHQ A KE H ZARDS AND RED Cl G Rl K l 1 THE P lFl OR Til WEST 

45° 

os· r f) 

0 2 KILOMETERS 

I ,_! __ __.. __ ___, 

Figu•·c 114. Bath ymetry and A LVI submersible track line ~ ncar the imcrscction of the Wecoma fault and the deformation front. Several 

dives focused on the gullie. visible on the SeaBeam bathymetry and SeaMARC I A sidescan images (see also fig. I 02). P . pop-up: DF, 

deformati on front. omour imerval is 20 m. Ticks face downslope. T rack lines show di e numbers. Wecoma fault cloned where concealed. 

We ob. crvcd that several prominent west- northwest/ 

cas t-southeast bathymetric trend s cro s the continental slope 

off central Oreg n (fig. I 0 I). These trends are composed of 

north-northwest- to we. !-northwest-trending folds and 

scarps 5-40 km in length . They appear prominentl y on the 

slope because they cross the north- to northwc. t-trending 

grain of the a crcti onary wedge obliquely and bccau e the 

deformati n within these zones is more intense than in the 

surrounding wedge. In several places, the oblique folds arc 

accreti onary-wedge fold. whose axes have been sigmoidall y 

bent to the northwest. whereas others are fault-bend and 

fault-propagati on folds overl ying . tccply clipping oblique 

fault. . In map view. SeaBea m bathymetry and GLOR IA 

images . how left-stepping ~igmoicla l bending of fold axes 

along these trend , left offsets of fold axe , and linear westnorthwest-trending scarps (fi g. I I ). These features were 

mapped u ing continuou GLOR IA long-range iclescan on 

th continental slope and continuous SeaBeam bathymetry 

on the slope and outcrmo t she! f. The deformation of accretionary-wedge structures bserved in the pcr: pcc ti ve SeaBeam mesh plot (fig. I 0 I) and in map view (fi g. I I ) has a 

variety of forms but, generall y, older structures are systemati call y disrupted and younger structures have developed 

along these throughgoing zone to th ex t nt that they arc 

bathymctri call y well cxpre sed . 

Three of the west-northwest-trcn ling slope deformati on zones (WF, B. and in fig. I 0 I) adjoin the Wccorna 

fault and fault. B and and have . imilar trend . The zones 
ACTIVE TR IKE-SLIP FA LTI G A D FOLDI G OF THE PLATE BOUNDAI y A1 D FORE R . OR -GO 24.:-

Figure J I S. andy si ltstone samples collected using the AL VI submersible from one of the fault-occupied gullic' 

on the sea ward fla nk of the first accreti onary ridge ncar the intersecti on of the Wccoma fault and the deformation front. 

otc slickensided surfaces and mullions. 
125°24' 125°22' 125°20' 125°18' 125°16' 

•• ~·c:.rf~,>Jj,'f!"- ;:_~""\(h.~~' ~ t7:. ! .• ~-: ... ·;r:·M:. ~ v-~~· : '·:;:_ ' ~-· ~· ,....,._ 45° ~ ·: - 08' .''." . .i .... """lll:.J"':I .. l:i:.~' ._.," .. /. R•.e .• 1-'-4'""'".:--·1.;"~ • .':-:--~-~-~.t."'.F ...,. _ .._. .... " I __J 

sou ern 

45° strand) 06' 

Figure 11 6. SeaMARC I !\ sidesca n image o f the Wecoma fault c rossing the acc re ti onary wedge. The northe rn ' trand o f the Wecoma fault cuts the first 

thru 't ridge and sync lina l basin and develops into a -.hear tone at the second thrust ridge. The sou the rn strand is a shear tone a t the first thrust ridge and 

cuts the first synclinal basin as a single splay and the -,ccond thrust ridge as two splays. Sec figure 120 for location o r image. 

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<l: 

5: 

4 6 

5: 

f5 

1 (/) 

0 

z 

0 

u 

w 

(/) 

2 z 

w 

~ 

i= 

_J 

w 

3 > 

<l: 

([ 

f-

>-

<l: 

5: 

4 6 

5: 

f5 

Figure 11 7. nmigrated 24-channel sei smic profile of the Wecoma fault on the lo> er continental slope. 26 km landward 

of the deformati on front. AITOW shows fault location. A. uninterpreted section: B. int rpretcd secti on. ote near-verti ca l trace. 

the sharp cl ip reversal s across the fault. and the up-to-the-west (north block up) ' urface express ion. The vertica l exaggeration 

is 2. 7: I. Sec fi gures I 03 and I 19A for location. From na ve ly and M e lell an ( 1987. line 77- 11 ). 

_-l7 
248 ESSI G E RTHQ AKE H ZARD A D RED CI G Rl K I THE P IFI ORTHWE. T 

SHELL 

7380 

0 2 3 4 

. 

. 

5 KILOMETERS 

EXPLANATION 

= :::',~~;: Fault- Dashed where inferred; barbs show relative motion 

Jill"' where known 

_ _. .. __ Thrust lault - Sawteeth on upper plate; dashed where inferred + Anticline - Trace dotted where concealed 

+ Syncline - Trace dotted where concealed 

- 500 - Bathymetric contour 
A T IVE TRIKE- LIP F LTING AND FOLD! G OF THE PLATE BO NDAR A D FORE R . OREGO 1 2-+9 

begin on the middl e to lowermost part of th slope and continue southeas t ward to the hcl r break. where bath ymetri c 

express ion dies out (fig . I 00 and I 0 I ). Zone WF in figure 

101 ex tend s southeastward from the area where . carps and 

ofT ets related to the W coma rau lt were mapped \ ith idescan imagery (fig. I 16). Thi s deformation zone is cro. sed by 

everal seismi c-refl ec ti on lines. including four Oregon tate 

ni vcr ity lines, two .S. Geological Survey line . and two 

industry lines in our data set. The profil es clearl y . how the 

oblique fo lds ob cr cd on the SeaBeam bathymetry. and 

mo. t al. o show that th deform ati on zone includes . everal 

major and many minor high-angle fault . everal profiles 

reveal that the oblique fold. that define the bathymetric 

deformation zone arc fault-bend and fault-propagation folds. 

both landward and . eaward vcrgent, developed above highangle faults. Detailed mapping uggests that some of thcs 

oblique fold arc superimposed on somewhat older north -

we. t- to north -northwest-trending fold . . Tho e that overlie 

high-angle fau lts show by their trend. that the under! ing 

fau lts also trend northwest to we. t-nonhwcst. In seism icreflection cross ings, the high-angle faults are common! 

expressed as a nearl y verti cal main trace w ith a fl ower structure in the upper ection (fig. I 19A- D). Development of 

oblique folds and faults decrea. es or becomes more clistribtlled southeast\ ard (landward) toward the upper . lope and 

ou term o. t . hell w ithin the deform ation zone. 

FAULT ORIENTATION AND THE 

REGIONAL TECTONIC SETTING 

The \ ecoma fault strike 293° on the aby al plain. the 

same as faults Band C (fig. I 00). Thi s orientati on is irtuall 

the same as the _95o . trike of the Bl anco fracture zone. the 

tran:form fau lt that . cparatc. the Juan de Fuca and Pa ific 

plates and form the boundary between th · Gorda and main 

. cgmcnts of th Juan de Fuca plate (fig. 97). Detailed mapping of the continenta l lope off' W ashington and Oregon has 

al so documented an add itiona l ix obliqu deformation 

zones wi th simi lar strike . . three of which adjoi n abyssalplain ~trike lip faults ( hri. Goldfinger and L.D. Kulm . 

Figu•·c 118 (facing page). Shaded relief plot of . eaBcam 

bath metry and structure of a part of the cen tral continen tal slope 

off the cen tral Oregon coast crossed by the Wecoma fau lt. The fault 

is expressed as a ,: ide zone consisting of several fault strands and 

intervening fault-related folds. The sigmoidal patterns and offsets 

of fo ld ax~s indicate left-lateral offset on the Weeoma fault. The 

bold s gment of the hell 7380 trac k line is profiled in figure 119/J. 

The are~1 !>hown is located by the arrow in figure I 0 I . 

unpub. data. 1993). II th se fea tures . includino the Bl anco 

fra ture zone, arc oriented along small cir les of rotation or 

the Juan de Fuca plate with rc pcct to the Pa ifi plate. It i. 

pre. entl y unkno1 n if thi s i oincidcnta l or if th rc is a fundamental relati on. hip between plate motion and the ori nt -

tion of the strike-s lip faults. n aura ti vc ex pl anation for thi . 

simil arity mi oht be that the strike-s lip faults arc r a ti vat d 

minor tran. f rm s, former! generated at th . prcadi ng rid ge, 

that ar curTcntl y rc. pending to th subdu ·tion-relatcd stn:ss 

field at the deformation fr nl. Dctai led rccon. truct ion. of the 

. prcading hi. tor f the Juan de Fu a pl at by Wil son and 

other ( 19 4). however, indi at that the Juan de Fu a ridge 

(fig. 97) wa oriented nearl y north-south at th time the pre.-

cntl y subducting cru st was general d. ny r li ct transforms 

generated at that time should b ori nt d cast-\ st, nnking 

it unlikely that reacti va ted fa ults ar responsible for th 

ob er eel structure . . Al though th refl ction profi les have 

been interpreted to include th po:. ibi lit of a long (more 

than 6 million years) history forth W ccoma fault (Appclgatc and thcrs, 1992). our preferred interpretati on : uggcsts 

that the faults arc on the order of 600,000 years old, liminating a spread ing-ridge ori gin . The uth of these raults and 

their proximity to and influence on the plate boundar . u ,_ 

gc. t that they arc a subducti on-related phenomenon as 

OJ po. eel to rei ict structures or . tructurcs rei at d to regional 

Juan de Fuca plat su·cs. c . . lnclc d. limited seismologi ·al 

evidence that sugge t the regional . tress field may be 

rou hl y north-south in much of the Ju an de Fuca plate 

( pence, 1989) is incompatibl e with th orientati on and slip 

direction f the abys. al-plain strike- lip fault . 

STRIKE-SLIP FAULTS AND THE 

ACCRETIONARY WEDGE 

W e have sho1 n that the W ccoma fau lt innuencc. the 

seaward most acc retionary wedge. Seismic and side. can 

record. sh w that the inter. ec ti on of the fau lt and the deformation front is a complex zone of in terac tion bet• ecn the 

fron tal thrust and the trike-slip fault. Strands of the fault 

ex tend into the scawardmost three or four thru t ri lgcs on 

ide. can record., and these observations arc supported by 

direct ob erva tion from ALVIN and analysis of carb nate 

material derived from venting fluid on th marginal ridge. 

In add iti on to cismic-refl ccti on and side can evidence. 

we observed ignificant differen cs in th sty le and growth 

hi t ry of the youngc t thrust ridge. near the W ccoma fau lt. 

onsidcrably mor shortening has occurred on equi va lent 

structures immed iately outh of the projection of the· 

W ccoma fault than to the north. Both fault-bend and fau ltpropagation folds arc better developed to the outh. and an 
250 ASSE G EARTHQUAKE HAZA RD A D RED l G RISK l THE PAC!Fl ORT HWEST 

45' 

00' 

44' 

00' A 

WEST 

8 

0 

124 ' 30' 

Nestucca 

Bay 

Yaquina 

Bay 

20 40 KILOMETERS 

Anticline 

Syncline 

~ 

j 

Fault ~ 0 

EXPLANATION 

.,;:: --"7 Fault- Dashed where inferred; barbs show direction 

of relative motion 

__.., _ _...__ Thrust fault-Sawteeth on upper plate 

Profile line 

EAST 

-- 7<' 

-- 2 KILOMETERS 

0 .9 

1.0 

(f) 

0 

z 

0 

0 

w 

(f) 

z 

w 

:2 

f= 

_.J 

w 

> 

<! 

a: 

f-

>-

<! 

~ 

0 

~ fFigure 119 (above and facing page). Seismic profiles showi ng the Wecoma fault and deformation zone off the central Oregon coast. A. 

locati on of sci. mic profiles shown on figures 11 7 and 11 98-D. B, line drawing of part of Shel l 7380 track! inc. This _ ingle-channel sparker 

record cro. sc. the Wecoma fau lt and deformation zone and show a northwest-trending anticline and a truncated syncl ine separated by a 

high-angle fault, the central of three mapped trands of the Wecoma fau lt. Other dipping faults may b~.: minor splays. The anticline is typical 

of young oblique structure. in the deformation zone. C, unmigrated single-channel . eismic profi le of a strike-sli p fault within the Wecorna 

fau lt clcf'ormation zone on the upper continental slope. Arrow show fault location. D, interpreted version of profil e in fi gure 11 9C. otc 

ncar-vertical fau lt and sharp clip reversa ls of renector . Oregon State Uni vers ity line SP- 56. Vertical exaggeration i · 2.0: I . 

add itional thru. t ridge i. present to the outh that is not 

pr . en t to the north (fig. 120). Thi s change in fold and thrust 

development acros. the Wecoma fault is clear on refl ection 

pr files and can also be seen in the bathymetry, wh ich shows 

an abrupt bathymetric change aero. s the fau lt (fi g. 120; ee 

also M acKay and others, 1992). Thi s change corresponds to 

the scarp discussed above and hown in fi gure 11 6. Much of 

the bathymetric change is due to the abru pt increa e in development of th thrus t ridges on the southern side. 

The structure map of thi s area (fi g. I 03) shows that 

other change in accretionary-wedge structures al o occur 

along thi s bathymetric trend and c carpment. ome fold 

axe and thru t faults step or bend to the left , wherea others termin ate altogether. The left off. ets and sigmoidal 

bending are con. istent with a left-lateral shear zone. Similar relati onships between trench-parallel accreti onarywedge structu res and a conj ugate set of strike-s lip faults 

have been mapped in the Shum agin segment of the 
CTIVE S TRIKE-SLIP FA LTI GA D FOLD! G OF THE PLATE BO I D RY A 

-- --- -~ -- -- WEST 

c 

- 0 0 

- 0.0 

25 1 

(/) 

0 

z 

0 

0 

w 

(/) 

z 

w 

~ 

5 

w 

> 

<{ 

a: 

f-

>-

<{ 

3: 

0 

~ 

(/) 

0 

z 

0 

0 

w 

(/) 

z 

w 

~ 

0.5 5 

w 

> 

<{ 

a: 

f-

>-

<{ 

3: 

0 

~ 
252 A SES.'I 1G EARTHQ A KE HAZARDS A D RED I G RISK I THE PA IFI ORTHWCST 

5 KILOMETERS 

Viewing angle = 130' 

Elevation angle = 35' 

Vertical exaggeration= 6:1 

Base-of-slope 

channel 

Figure 120. SeaBeam-bath mctry perspecti ve iew along the strike of the Wecoma faul t as it cro. ses from the abyssal plain into the 

accretionary wedge. Grid spacing i. 250m. Note the morphologic change in the thru;t ridges from the 'outh block to the north block acro~s 

the Wccoma fault. ote also the ex tra thrust ri dge on the south block th at has no correlative to the north. Parallelogram shows locati on of 

figure 116. Solid and da~hcd lines are fau lt: : solid li ne. with . awtecth arc thru ~t fault' (sawtceth on upper plate). PR. pres. ure ridge: P 

pop-up: DF. deformati on front ; WF, Wecoma fau lt. 

Al eutian forearc (Lewis and other , 1988). The proj ected 

trend of the Wccoma fau lt also coincide with an abrupt 

change in stri ke of the . caward thru st ridge . orth of the 

fault. th ridges strike about north-south ; south of the fault , 

the . trike is 340°. The difference. in thrust-ridge development acr ss the projection f the Wccoma fault also . upport the presence of an active left-lateral sh ar zone. Given 

the pl at convergence rate of 40 mm/ycar (DcM cts and others, 1990) and a implc calcu lation of the shortening on 

these structures. we es timate that the four westcrnmo t 

thrust ridge. must have fonncd during the 600±50 thou. and 

years that the Wccoma fault has been ac ti c. We po tul atc 

that the difference in shorten ing is due to a local change in 

th subducti on rate acros the fault rc. ulting from . ubduction of the acti ve Wccoma fault. ing the net slip of 

5.5±0.8 km and the 600±50 ka age of the Wccoma fault. 

the difference in subduction across the fault has been 

.4- 10.0 km. or 4 1-44 percent of the 19- 24 km of normal 

convergence that ha. occurred during the life span of the 

Wccoma fault. Figure 12 1 illu. rratcs thi. geometri c resu lt 

and show. that the effecti ve loca l convergence rate is 28 

p rccnt greater south of the Wecoma fault than to the north. 

We postul ate that increased convergence south of the fau lt 

due to thi s difference has caused the ex tra shortening in the 

. cawa rd thru t ridges and probably al. o the earl y development of the ex tra thru st ridge (fi g. 120). 
A CT IVE STRIK -SLIP FA LTI 1

G A D FOLD! G OF TH E PLATE 80 DARY D FORE R . OREGO 25. 

KINEMATICS OF OBLIQUE 

FAULT -ACCRETIONARY 

WEDGE INTERACTION 

We suggest three hypotheses to explain the possible 

interacti on of the abys. al-pl ain faults and the accrcti onar 

wedge: ( I ) the trike-s lip faults ex tend from the aby .. al 

plain beneath the acc reti onary wedge and arc the source of 

the deformati on zones; (2) the strike-slip faults originate 

within the orth Ameri ca pl ate and propagate into the subjacent . ubductcd . lab and seaward into the abyssa l pl ain ; 

and (3) the deformation zones on the slope arc unrelated t 

the abys. al-plain faults. 

If the strike-. lip faults originate in the Juan de Fuca 

pl ate (hypothesis I ). the deform ation zone. in the accretionary wedge may have developed in respon. c to strike-slip 

motion of th Juan de Fuca pl ate beneath the accreti onary 

wedge and thi ck Tertiary . hclf . eq uencc. If . o. the presence 

of such deformation in both the overriding and downgoino 

plates present. a probl em in reconciling strike-. lip m ti n 

and obi ique con crgcncc o cr the last 600.000 year . . 

Oblique con ergencc of about 24 km directed 062° has 

occurred over the past 600.000 years. according to the plate 

vectors of DeM et. and other. ( 1990). I uring that time. the 

points at which the three stri ke-slip faults on the abyssal 

plain intersect the present deformati on front should have 

moved :om 20 km to the north (neglecting the unknown 

amount of advance of the lcform ation front). yet we 

observed little or no offset between the abys al plain and 

accreti onary-wedge . cgmcnts of these faults. The di screpancy could be clue to a very recent slowdown or cessation of 

. ubcl uction. lth ugh cessati on i. unli ke! given the preponderance of g ologic and gcoph sica! ev idence. a . ignificant 

slowdown cannot be ruled out and ould r clu e the mi ss ing 

component of convergence (Riddihough. 19 4). Thre stru -

tural explanations might also be considered : ( I) the ex istence 

of right-lateral strike-. lip fa ul t. in the upper pl ate to theca t 

of the youngest acc rcti onar thrust . Such fault can decoupl the seaward part of the accretionary complc · and accommodate the margin-parallel component of oblique 

conv rgencc (Fitch, 1972: Jarrard, 1986; Karig and others. 

1986). navcly ( 1987) has mapped such a fea ture (the Fu lmar fault) on the Oregon . he! f. M ore than _00 km of dex tral 

slip occurred on thi fault , primaril y in the Eocene. but minor 

offsets of younger unit s may indica te c ntinued or renewed 

acti vit y (Sna ely and others. 19 5: navely . 19 7): (2) a 

right-lateral component on the accr tionary thrust. them-

. elves could accommodate the mi sing north-south convergence component, distributing the ex pec ted off. ct of the 

strike-sli p faults o cr many crossing structures. The deformation zones on the continental slope do show a tendency to 

step or bend to the south. a requirement of thi s mechani sm: 

or (3) the subclucted strike-. lip faults leave progrcs. ivc 

deformati on b hind in the upper plate as they pa .. beneath it 

A 

8 

Figure 121. Vector diagram of the inferred local effe t of We· ma-fault slip on the plate ·onvergence vector. A. regional Juan de Fuca- onh 

America (JDF- AM ) pl ate ector of I cM cts and 

others ( 1990), oriented 062°. 13 , J DF- A vector 

(da. hcd) on the north block of the We<:oma fault. 

The short arrow is the local vector due to moti on 

on the Wecoma fault. oriented _92°. The new resultant, with a strik of 05 1°, and con ergence 

rate are . hown by the bold arrow. ·, JDF- AM 

vector (dash'd) on the sou th blo<:k of the Wccoma fault. The short arrO\ is the local vec tor due 

to moti on on the fault. The n w resultam, oriemed 072°. and convergence rate arc shown with the 

bold arrow. Increased convergence south of the 

fault ma b re ponsible for the greater shortening and topograph ic expression or the thru st 

faults south of th Wecoma fault. alculated 

from the Euler pole f DeM cts and others ( 1990), 

North America fi xed. 

to the nonhca. t. T wo obliqu deform ation zones limited to 

the ac rctionary wedge have been mapped on the continental 

lope and shelf. one in northern Oregon off the Till amook 

Bay area and one ofT southern Oregon north wes t of ape 

Blan o. This pr e . mi ght be analogous to the trail of volcanoes left on a lithospheri c pl ate as it passes over a fi xed 

mantle hot spot. A the Juan de Fuca pl ate i · subducted, 

lower pl ate fault s leave behind a trail of deformation in the 

accreti onary wedge a. they pa. s obliquely beneath the North. 

Ameri ca plate. 

I f the strike-slip fa ult. ori ginate with in the upper plate 

(hypothc. i. 2). cveral problem · with hypothesi I arc 

av idcd. Bccau c not all the mapped deformation zones on 
254 ASSES I G E RTH Q A KE HAZA RDS D RED Cl G Rl K I TH E PA IFI ORTHWE. T 

the slope ha c aby al-pl ain counterpart. , an upper-plate origin doe not req uire a eparatc explanation for those structures. It is also more mechanica ll y rca onable to envision 

faulL in the thi cker upper plate propagating acros a locked 

interface into the thin. relatively warm . ubducting plate than 

the reverse. The apparen t incompatibilit y of we !-northwesttrending left-lateral faul t with the foca l s luti ons and 

inferred nonh- outh principal . trc s of Spenc ( 19 9) for the 

Juan de Fuca plate i re olved if the aby sal-plain faults arc 

onl y the we tern ex ten ions of fault that origi nate within the 

upper plate to the eas t. If thi s i the ca e, the regional . tres 

field in the oceanic plate might coex i t wi th a somewhat different stress field within the upper plate and near the deformati on front, with a tran ition somewhere eaward of the 

aby. al-plain fau lt. . Las tl y, a domain of ubparallel leftlateral . trike- lip fau lt hou ld rotate clockwi e through time 

(Freund, 1974; Ron and others, 1984; Scotti and others. 

1991 ). Such rotation as a re ult of progres. ive . lip on these 

faults were uggested by Goldfinger and others ( 1992) and 

could rc:olve the question of the offs t aero the deform ation front expected in hypothes is I becau e clockwi e rotation of the upper pl ate could make up the mi ing northsouth component of pl ate convergence. 

Hypothc. i. 3, that there i no connecti on between the 

slope deformation zone and the abyssa l-pl ain fault . mu t be 

considered, as data po itivcly linking the e structu res is not 

avai lable at pre cnt. However, the trends, shear sense, and 

youth of the e structures all trongly uggc t at leas t a genetic 

connection, if not a physica l one. If these structure are not 

directl y linked, they may be separate but imilar re pon es by 

the interacting pl ates to interpl ate ubduction stres e . 

CONCLUSIONS 

Three we t-northw st-ll·cnding left-lateral trike-slip 

fault in the ascadia . ubduction z ne ffthe central Oregon 

a. t off et both the oceanic ba. ement and sedimentary 

cover of the Juan de Fuca plate. The Wccoma fau lt, the larges t of the th r e faults, has a mea ured net slip of 5- 6 km at 

the pl ate boundary, dying out in the Juan de Fuca plate 18 km 

to the north we. t of its intersec tion with the deformation 

front. Two independent es tim ates of the slip rate on this fault 

during two time peri od how similar rates of 5- 12 mm/year 

for the peri od 24- 10 ka, and 7- 12 mm/ycar over the 

600,000-year life pan of the fault. The Wecoma fau lt intersects the dcfonnation front in a complex area of tructural 

p p-up and an embayment in the deformation front. Sidescan images show that scarp and linear gullic extend along 

the proj ected strike of the subducted Wecoma fault about 15 

km into the accreti onary wedge. Slip on the Wecoma fault 

beneath the rth America plate ha influenced the development of th initial thru. t ridges in the accretionary wedge. 

Three deformation z ncs in the accretionary wedoc lie along 

the landward proj ec tions of the three abyssal-plain fault . 

The c zones arc compo. eel of we. t-northwc. t- to n nhwe 1_ 

trending fold s. sigmoidall y bent fold axes, and we tnonhwc Hrending linear scarps. The ·tylc of deform ation in 

the. e zone is consi: tcnt wi th left-lateral shear zones . uperimpo eel on the . tructural grain of th accreti onary wedge. 

We postulate that these deformation zones ar the rc ult of 

subdu ti on of the ac ti ve strike-s lip fa ult. or. altern ati vely. 

that the shear zone. originate in the upper plate and propagate acros the plat boundary into th Juan de Fuca plate. 

The orientation of the Wccoma fault and th other strike- lip 

fault s along mall circles of rotation of the Juan de FucaPacific plate sy tcm is not yet understood. 

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Ando. M a ataka, and Bala1.s, E. l., 1979, Geodetic c idcncc for 

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92 15- 9225. 
WESTERN WASHINGTON EARTHQUAKE FOCAL 

MECHANISMS AND THEIR RELATIONSHIP TO 

REGIONAL TECTONIC STRESS 

By Li Ma, 

1 Robert Cros on, 1 and Ru th Ludwin 1 

ABSTRACT 

Te toni c stress in we. tern Wa. hington has been in vestiga ted by inverting earthquake focal mcchani. m. to ascertain principal-s tres. directi on. . total f 19 1 wellconstrained focal mechani sm. were determined from data 

co llec ted by the Wa. hington Regional Seismogra1 h Network for 3 . patiall y di stinct groups of earthquakes: ( I ) shallow (cru sta l) Pugct Sound : (2) deep (sub ru tal) Puget 

Sound: and (3) shal low (cru. tal) M ount St. Helen. event.. 

Although focal mechani sm. clearl y di ffer among the three 

groups, it is less obv ious whether significant difference in 

the tectonic stre. s regime arc required to produce the 

ob erved fo al-mcchani. m vari ati ons. 

Comparing cru stal ea rthqu akes (depth lc s than 0 km) 

in the orth Ameri ca pl ate between the Puget ound and 

Mount St. Helens rcgi ns. the inglc bes t filling stre model 

for Pugct Sound has a nearly north- outh o 1 (maximum principa l compr s. ivc . tress) ori ntation with an ca. t-west 0 3 

(m inimum principal comprcs ive stress) orientation, 

whcrca. the best fitting mod I in the M ount St. Helen. area 

ha north-north a: t o 1 orientation and as t-southcast 0 3 rientation. t all confid nee level examined. the allowable 

stress orientation. f r th c two region overl ap. Therefore. 

we conclude that rcori cmation of . tres in the M oun t t. 

Helens regi n i. neither required nor precluded by the data. 

A uniform north-south compres. ive tec tonic trcs in the 

cru t can adequate! expl ain all of the observed foca l mechani sms in western Washington. 

In the subductcd Ju an de Fuca Jlatc. or lab. the state of 

stress is more omplex. The ori entations of P (apparent compre . ional) and T (apparent ten. ional) axe . . derived from 

focal mechanism of . ubcru stal earthquake (depth greater 

than 30 km), do not clu t r around a single directi on. 

although T-ax is ri cntati on. weakl y correlate with the 

1

Gcophysics Program. i\K- 50. iv~ it y or \V a~ n. Seattle. 

Wi\ 98 195. 

general ca tcrl direc ti on of plat subdu ti on. Inversion to 

ascertain th tres orientation indi ates that n sino! · se t of 

strcs. direc tion. adequate! fit s th ohscr ed focal mcchani. ms for sub ru. tal eanhquak s within the subductcd slab. 

The diver. ity of ori rllati on. of P and T axe: of th sc 

intras lab earthquake. may be con: i. tent with the mod I of an 

arched slab b neath Pug t oun I that produc s variati ons in 

the slab stress . tate. 

The north-. outh ompressi c tec toni stress in the 

onh Ameri ca plate and the dramati ·change in strcs .. tate 

between it and the subdu t d Juan de -u a slab indicat 

that the ru. tal stress is n t controlled in a impl c fa. hion 

by the proccs. e cau ing plate con crgcnce and subduction. 

We . uggest that the magnitude of pl ate coupling stress is 

significantly lower than the level of regional tectonic . tr ss 

from other causes. 

INTRODUCTION 

On ly a few g ophysi al mcth ds provide direct in sight 

imo the ta te of tec tonic . tress . tr s measurements using 

borehole technique. refl ect only near-surface conditions and 

few have been mad in the Pacific orthwes t. Geodetic strain 

mea urements provide informati n on changes and rate of 

change of . tre s indirect! through mea uremcnt of strain 

but, again. onl y limited measurements have been made in the 

Pacific orthwe t (Li owski and others. 19 7; avage and 

Lisowski , 1991 ). Earthquakes refl ect train within the Earth 

through slip on fault urfaccs . Thi process is control led by 

strcs and the yield propertie. of rock. By making reasonable 

assumptions about the relati onships between strc and fault 

sli p. we an infer the state of stress at midcru tal and greater 

depths th rough the study f earthquake focal mechani m . 

For each earthquake. we can estimate the orientation of P 

and T axes, which arc the apparent max imum and minimum 

principal-. tres. direc ti n. for that event. We arc ultimately 

interested in the orientation: oro 1 and 0 3. the maximum and 

257 
258 l1 G EA RTI-IQ KE HAZA RD A DR 

m11111num principal- tress Om] onent-. respecti vely. For 

vari ou rca. on . P and T orientati ons for any oi ven earthquake may depart sionificantl y from the true cr 1 and <J3 dire -

tion . With a relati el large number of arthqu akc , 

tati sti ca l uncen aint in our ability to determine ·tre. can be 

reduced. Midcrustal and . ubcru tal earthquakes are relati ve] 

abundant in we. t m Wa. hington. sing data obtained by the 

W a. hington Regional Sei. mograph ctwork (WR ). a 

powerful observati onal tool, we analyze focalmcchani m: to 

c tabli. h the orientation of regional tectonic tre . 

In the Pacific 1 onhwe. t. we c pec t the tec tonics to be 

. trongly innucnccd by the subducti on of the Ju an de Fuca 

pl ate beneath the orth mcrica plate along the a eadia 

ubducti on zone (CSZ) (for example. see Atwater. 1970; 

Riddihough. 19 4). In ubduction zone. wher the convergent pl ate arc . trongly coupled, the backarc i. ex1 ectc I to 

be charac teri zed by tectonic compre sion normal to the arc 

(for example, . ec yeda ,,ncl Kanamori , 1979). Because tectoni trc. s i the underl y ing phy ica l ource of earthquak . 

under tanding regional te tonic stre s in the Pac ific orthwest i very important. At hallow depths (less than 0--40 

km ). mo. t subclu tion zone worldwide have earthquakes 

cau. eel by direct slip between the l\ o pl ates (interpl ate earth -

quakes). Stressc interpreted from these earthquakes are consistent with compre sion 111 the directi n of plate 

convergence. At greater depth., earthquak es within the 

de cending pl ate, or . lab, commonl y indicate downdip tension or other tre se. within the . lab (for exampl e. ee !sack. 

and M olnar, 197 1 ). 

lnterpl ate (between pl ates) earthquakes have never 

been recorded along the CSZ, although ac ti ve pl ate con ergencc i widely accepted by cienti. t . Previou focalmechani. m tudi e indicate the pre alcnce of north- outh 

compre . ive tec tonic tress orientati on ba ed on earthquakes 

in both we tern and eastern Washington ( ros on. 1972: 

M alonc and othcr , 197 5; Yelin anciCro son. 19 2; Cro on, 

19 3). Thi pre alent north-south compre ion i not ea il y 

interpreted in terms of our understanding of plate con ergence in a . 50° . direction (Riddihough. 19 4). or with 

the '· 70° E. principal- tre s direc ti on predicted from a imple mechanica l model of pl ate interacti on (Savage and other . 19 l ). l f thc pl atc are trongly coupled, weexpect foca l 

mechani m of cru tal earthquake to indica te compres ion 

in the directi on of convergence. The north-south cr 1 orientati on obtained from focal mechani m al o doe not agree 

with the northeas tward principal compre sive strain deri ved 

from the geodeti c obser at ion of Savage and other ( 198 1 ), 

Li owski and other ( 19 7), and Savage and Li ow. ki 

( 199 1 ), although train rate mea. urecl so far in the Pacific 

onhwe. t are generall y of low magnitude compared tO tho e 

in many other tectonicall y ac ti ve regions. 

The ea rthquake and geodetic observation. might be reconciled by noting that the geodeti c measurements are 

Cl G Rl. K I TH · PA lr-1 ORTI-IWEST 

scnsiti 10 • mall chanocs in . train , wh rca. ea rth ]uak s 

should refl ect the absolute ambient lc c1. of stres in the 

cru. t ( bar. 1982). Thu., . train-rat measurement. may 

rene ·t . mall incremental change in strc ... upcrimpo eel 

upon a mu h larger ambient tc ·toni c stress field . and th two 

ty] e. of mea. urcmcnts need n l ao rcc in ori ntation. The 

cone pt impli es that the northeast-directed strain found by 

a vag and other. ( 19 I), Lisowski and oth rs ( 1987), and 

avagc and Li ow ·ki ( 199 1 ). whi ch i onsistcnt with the 

xpcctccl orientati on due to subducti on, is . mall in magnitude rclati c to the background ambient tc tonic . tress that 

focal mechani sm in li ate i. dire ted north- out h. Thi s docs 

not nc cs aril y mean that large . ubduction earthquakes annot be produced I thi s relati vel y low level of coupling 

stres . . but raJ her that the stress clue to subdu lion may be low 

in magnitude compared to regional ambi nt tectoni c stre .. . 

In th M ount t. Helen r gion. focal m chanism. from 

earthqu ake. along the t. Hcl ns . ci. mic zone (Weaver and 

mith. 19 3) how rotation of P axes from north to the northca t relati ve to other regions in wc:tcrn Wa. hi ngton. Thi s 

rotation was interpr ted by Wen cr and mith ( 19 ) to indica te a cr 1 affected more stronoly by pl at coupling in thi · 

region . . uggc. ting a variati on in pl ate coupling from north to 

outh alono the Z. In thi. chapter. we further examine thi s 

important is uc. which b ar. direc tl y on carthquak hazard . 

Our understanding of the strc cs producing intras lab 

earthquakes (th at i . . earthquakes occurring within the ubducting Juan de Fuca slab) i. far from clear. Bccau e the 

Pugct ound region i ulncrable to hazards from ubcru tal 

earthquake . we may xpect to learn more about the origin 

and nature of the c hazard. through the tudy of intraslab 

focal mcchani. m . . Although pa t f ca l-mechani sm studie · 

have sugge ted hanges in orientation of trc from the subductcd lab to the overl ying orth A m ri ca plate. the ex tent 

of thi dif~ renee ha not been clear! quanti fied. Furthermore. although do nclip cx ten ional tre . ha been a sociated with lab earthquake. (Taber and Smith, 1985). not all 

intra lab earthquake reflect thi type of stre , a we illu -

trate in thi chapter. Thu , an objecti ve of thi s wd y i to 

clari fy omc of the uncertaintie. in interpretation f intra lab 

focal mechanism and to learn if a coherent str s model can 

be determined from the e earthquake . 

ACKNOWLEDGMENTS 

We thank John Gephart of ornell ni ver ity for providing the ori ginal computer program. for thi analysis and 

for a si tancc in their usc. Thi s swdy wa supported by the 

U.S. Geological Survey under grant 14- 08- 0001 - GI080, 

14- 08- 0001 - G 1390, and 14-08-0001 - G 1803. 
WA 'HI GTON EA RTHQ AKE FOCAL ME HANISMS A D THEIR RELATIO SH IP TOR GIO A T TO I STRESS 259 

METHOD OF STRESS 

DETERMINATION 

trcs is usuall y described in term. of three mutually 

pcrpen li cular axes. whi ch arc called prin ipal- tress axe . 

The orientation of the. e axes i determined by the perpendicul ar directi on corrc pending to max imum (o 1) and minimum (o ) comprc. sive . lre. s s. The third ax i , 

corre pending to intermediate stre. s (o2), is determined by 

the orientation of the oth r l\ o axe . . In conventional focal -

mechani sm tudies. it i common to usc the pattern of P and 

T axes as es timated by individual foca l mechani sms to infer 

the ori cntati n of the regional tectonic 1 rincipal-. trcs. axe . 

S auercd directi on. of axe · may be averaged either qualitati ely or quanti tati v ly to es tablish the preferred directions 

of o 1 and o_. A poss ible difficulty with thi approach 

became clear' hen M cKenzie ( 1969) pointed out that if lip 

oc urs n preexisting zones of wcaknc .. , there may be substantial difference between the ori entati on. of seismic P and 

T axe. and the true principal axes of strcs . A. the Earth' s 

cru st i commonl y believed to have many fracwrc . fault , 

and zone. f arying trcngth , the simplified interpretation 

of P and T axes may lead to crroneou. con lu ions about the 

true orientati on of tc toni c trc s. cKcnzie ( 1969) established th theoreti ca l ba. is for analyzing thi s problem. Several in v tigat rs. among them Angelier ( 1979), 

ubsequcntl y de eloped quantitati e method of analyzing 

large amount. of geologic data. such as fault-movement stri -

ations. to correctl y estimate regional . trc. s. uch methods 

are necessaril y limited to ncar-surface conditi n . Ell sworth 

and Zhonghuai ( 19 0) ex tended the e method to the analysis of focal mechani sm . 

Gephart and or. yth ( 1984) and Gephart ( 1985. 

1990a. b) developed a complete and elf- onsi tent method 

of LL ing a group of focal me hani m to find the ori entation 

of the re gional tectonic . tr . :ax _ along with a quantitative 

indicator of the rclatin: magnillld :of th 1 rin ·ipal trc c . 

Thei r method i ba~cd on M ·Kcn;ic's ( 1969) principle and 

make~ usc of a ~ca ar rotati onal mi . fit between the theoretical strc. orientati on that is ·urrcn tl hcin0 tes ted and the 

strc. rientati n required to ac ti vate the ob. er d fau lt 

plane . . The sum of mi sfi ts i. the objecti ve fu nction that i. 

minimized to achieve a bes t fi tting . tr . _ model. as w II a. l 

c. tabli. h the ti ti ca l . <Htcr of po .. iblc I uti n . . The 

ac tual li p pl ane for each earthquake may ei ther be a. igned 

or elected objc ti vc ly by the proce .. ing algorithm . F rmal 

. tati. ti ca l confidence limit. arc establ ished for po .. ible tc -

tonic strcs. orientation. that arc con. i. tent with the b crvcd 

focal m chani sms. A central assumption of thi. method i. 

that the magnitudes and dir ctions of principal strc .. c do 

not chang over a region from whi ch the earthquakes to be 

analyzed arc . elec t d. Therefore, th m thod may be vi wed 

a. a t st of the hypothesi s that a sin gle uniform regional 

strcs. expl ains all oh. rvcd foca l mcchani. ms. 

SELECTION OF DATA 

The WRS compri se. over 120 bon-period verti calcomponent . tation. in Washington and northern Oregon 

(Qamar and other , 19 7). Real-time . ignals s nt to a central 

recording faci lit y have been digi tall y recorded . ince 1980. 

Two horizontal-component Wood-Anderson . ci mograph. 

are al. o operated a part of the network . I though the n twork operated f r about I 0 years prior to 19 0, onl y data 

acquired after that time ha been u. eel for thi s swdy b cau c 

of the quality improvement resulting from digital-data acq uisition. Thi improvement i pani ularl y important for focalmechani sm sllldies because P-wave pol aritie . . the main P< n 

of our data et, arc generally clear! identifiable from ligital 

data, whcrea they arc ommonly ob. cured on the older analog fi lm record . . 

The following criteri a were used to initial! . elec t 

earthquake for focal-me bani m . tudic ·: ( I ) an azimuthal 

gap of . tati on coverage of I 00° or lc. s, (2) an unweightcd 

root-mean-square tra eltimc re. idual of 0.3 . econd or lcs . 

and (3) a coda magnitude of 1.0 or greater. Earthquake location determined in norm al prelimi nary proccs. ing (for 

exampl e. Qamar and others. 19 7) were u eel for thi s stud . 

All earthquake with ei, ht or more iden tified p lariti c. wer 

examined for pos ible inclu ion in the data ct; sci mogram. 

of promi. ing e cnt were reread f r polarities, and known 

polarit y reversal. wcr correct d. Focal mechanism were 

then con tructcd by hand fitting. and acceptable solution. 

were retai ned for the in ver ion analysi If a nodal plane 

could be rotated 20° or more. or the ol u tion was deemed 

poorl y constrained due to polarity eli repancie r other factor , the earthquake wa reje ted. ubcru stal c< nhquake 

(within the subductcd Juan de Fuca slab) are rar r than 

cru tal earthquakes (Crosson. 19 3) and relati vely fc,. of 

thei r focal me hani sm. ha been determined. o pe ial 

effort wa. made to include all from thi s group. A total or 19 1 

high-quali ty fo al mechani sms were ultimately retained for 

further anal is. Th . e mechan i m and their a. oc iated 

polarit data arc li. tcd in the "Focal Me bani. m Listings and 

Plots'' secti on at the end of thi s hapter. The data set i 

divided into thrc~; patial group : ( I ) hallo' Pugct Sound 

earthquake ·; (2) de p (subcru tal) Puget ound ea rthqu ake : 

and (_) hallo' M ount 1. Helen. region ea rthquake . Ea h 

group wa indi viduall y analyzed. and we eli cus. the signifiancc of our rc. uiL in light of regional str . s orientation. 

Figure 12_ is an ca t-we t cro section of we tern 

Wa hington bowing hypocenters f' earthquake u ed in 

thi . tudy. The grouping of ubcru. tal (deep r than "0 km ) 

earthquakes. con ti tuting what i. termed the Wadati-Benioff 

zone, is clearl y s parated from the shallower cru stal earthquake. . ub ru. tal sci. micity is within the subducting lab 

and not at the plate interface. The Jab dip angle varies from 

bet\· cen 10° and 12° in the Puget S und ar a to bctw en 15° 

and _oo north an I . outh or Pugct ound (Green and othGrs. 

19 6: Keach and others. 19 6: Cro:son and Owens. 19 7). 
260 AKE H ZA RD 'D RED Cl G Rl. K I 1 TH E I A "IFI ORTIIWEST 

WEST EAST 

OLYMPIC MOUNTAINS 

CASCADE RANGE 

Coastl ine Pug et Sound 

Figure 122. 

0 

(/) 

a: 

w ross section acros. 

0~ 0 

1- \ c tt:rn Washington . hawin g hypocenters of earthquakes u. eel inthi swd 

proj ected onto an east-west pl ane. 

Dash d line shows approximate top of 

subductecl Juan de Fuca plate. 

w 

~ 

.... go~ifta Wadati-Beni off a.... ·--------

zo ---- 1Yd?>~~--------- 0 

t26° 

49° 

47° 

0 

PACIFIC 

OCEAN 

EARTHQUAKE 

MAG ITUDES 

0 1.0- 1.9 

0 2.0-2.9 

0 3.0-3.9 

50 

124 

0 

_J 

52 

~ 

:i 

1-

0.. 

w 

0 

100 

125 

122° t20° 

Figure 123. Epicenter. of all crustal earthquake. in the focalmechanism data set used for thi .. tudy. Different ve locity model. 

are used for locati ons of ea rthquake. in the region> labeled Cascade 

Range. Pugct ouncl. and M ount St. lelens. Velocit y models and 

model area. are gi en in Ludwin and th ~ ( 199 1 ). All earthquakes 

u. eel for the study of the M ount St. Helen. region arc located within 

the boxed area. 

124 

0 0 69° 0 --

0 0 

123 

WEST LONGITUDE, IN DEGREES 

EXPLA ATIO 

EARTHQUAKE MAG ITUD 

0 1.0 - 1.9 0 3.0 -3.9 

0 2.0 - 2 .9 0 4.0-4.9 

126' 124 

49° 

47" 

PACIFIC 

OCEA 

Strike-slip fault 

.. J , 

Thrust fault 

• ormal fault 

~ 

0 

121 

120' 

Figure 124. Lower-hemisphere, equal-area foe, I mcchani,ms of 

the 17 gc t Washington crustal ea rthqu e~ ccl in thi s tud y. 

aJnitucles ranged from 3.0 to 3.6. umber' refer to earthquakes 

listed in tables 19 and 20. Dot. arc ea rthquake epi centers. ompres. ional quadrants arc shaded. The T ax i' of each mcchani 'm 

runs thr ugh the middle of the compressional quadrant, and the P 

ax is is in the middle of the tensional quadrant. An explanati on of 

earthquake focal mechani sms can be found in Fowler ( 1990). 
WASH I GTO E RTHQ A KE FO AL M ECHAN ISM A D THEIR REL T IO SHIP TO REGI0 1 TE TO STRE S 26 1 

FOCAL MECHANISMS 

CRUSTAL EA RTHQUAKES 

Cru sta l ea rthquake foca l mechani sms (hypocentral 

depths lc .. than 30 km ; fi g. 123) were di vided into two 

groups. The hallow Pugct Sound group numbered 76 earthquakes in the central Puget ound and we tern Ca. cadc 

Range regions. The shallow M ount St. Helens group wa. 

campo. eel of 73 earthquakes in the M ount St. Helens region 

(fi g. 123). Of the combined 149 earthquakes, 17 were larger 

th an magnitude 3.0 (fig. 124). Focal mechani sms for earthquake located in the Puget Sound region have P axe. varying genenll y fr m north -north we t to north-northeast and 

ar of both thrust and . trike- lip type. On ly two of the earth -

quakes shown in fi gure 124 arc in the M oum St. Helens 

group (c cnts 48 and 140; table 20). Of these, event 48 has a 

mcchan ism simi Jar to that of the 198 1 Elk Lake, Wash .. main 

shock (Grant and other. . 19 4). Both of these earthquake 

ha c strike-s lip rn cchani. m that are typica l of thi s region 

but less common in the Puget S unci r gion. 

Fiourc 125 summari ze the . hallow Puget Sound group 

data set ' ith lower hemi sphere. equal-area compo ite pl ots 

of the P and T axes. The figure shows that mo t P axe. are 

nc< rl y north-. outh and that T axes are more uniforml y di stributed in a zone around the equator of the proj ection. 

In the M ount t. Helens region, 52 of the 73 earthquakes were along the northern part of the t. Helen. sci. mi c 

zone (Weaver and Smith. 19 3) and the remai nder were catt red thr ughout the res t of the region. In our analy i . we 

assumed that the slip pl ane of the. c 52 earthquake. were 

con. tra ined to align approx imately with the trike of the ei -

mic zone. P axes in the M ount t. Helen. region tend to be 

ori ente I northea. t (fig. 126). indicating apparent . trc s 

rotati on luc ei ther to the ex istence of the St. Helens . cismic 

zone itse lf or to larger regional tectonic variation . 

' B R TAL EARTHQ AKE 

o~ t ~ub ru:-tal canhq uakc. ,,. re on the w stern side 

of Pugc t und and be n ath the ca. tern . ide f the Olym J ic 

cni ~u la (fi g. 127). ocal mechani sms for . lcc ted large 

'-Uhcrustal ea rthqu ake~ :1 rc shm n in fi gure 12 . Events_ and 

I 16 (table 2 1) were larger than magn itude 4.0. and the 

remaining events w ·rc bet\ c n magnitude .. . 0 and 3.9. 

Both event. _ and I 16 have enica l P axe. and ne:J rl hori -

I.Ontal Taxes. !though tiles two normal earthquake. sugges t an ex t l cnvir nmcnt beneath th convcrg' nt 

plate margin, in agre mcnt with Taber an I mith ( 19 5), the 

two earthquake. have very diff r nt apparent . trcs. ori entati ons. Event . , in the northca. tern orn r of th OJ mpi 

Pen in. ula and at a depth of 4 .5 km . ha. a nearl y north-south 

Taxi s. On the other hand. vent I 16. at the south rn end of 

N 

.. . 

' ·•z .... ·. , ... .. 

+ 

. • • • • I ., .. . ... . ... ...... . P axes 

N 

Taxes 

Figure l 25. Lower hemisphere. equal-area composite projections of P and T axes for the 76 crustal earthquakes in the shallow 

Puget Sound data . ct. 

""' ... . 

N 

,·:·.-·' . . . ' . ' .- ... : ... 

P axes 

N 

,fl 

T axes 

Figure 126. Lower hemi sphere. equal-area comp si te projections of P and Taxes for the 73 crustal ea rthqu akes in the M ount t. 

Helens region data set. 

the lympi c P nin. ul a. ha a southeast-ori ented T ax i . 

carefu l examination of the f al mechani. m sho' n in figure 

12 reveal: the lack of any obvi us con. i tency in the subcru stal grou p. It is difficult to generalize about the tate of 

stress ' ithin the ubdu ted slab except to note that there may 

be a prevalen c of hori zontal T axe ·. 

san thcr way to vie' thi. variability . pi t of P and 

T ax : for all subcru tal earthquakes arc pl atte I in fi gure 

129. P axe. ary in plunge fr m ncar verti ca l to n ar 

hori zontal and azimuths vary from northea t to outheast and 

from north\ est 10 . outhwe. t. Bccau e ther ar no P a e 

oriented north -south. a. with the ru sral earthquakes. we 

conclude qualitati ve ly that the. c data arc not con i tent w ith 

the north-s uth compr s. ion noted earlier for cru stal earthquakes and that there i. a maj r change in stre .. state from 

the subductcd Juan de Fuca slab 10 the overl ying orth 

mcri a plate. 
262 SE G EARTHQ AK - H Z RDS D RED G RISK I THE PA IFI ORTI-I 'V ·ST 

126' 

49° 

47° 

45 

PACIFIC 

OCEA 

EXPLA ATION 

EARTHQUAKE 

MAG ITUDES 

0 1.0- 1.9 

0 2.0 -2.9 

0 3.0-3.9 

0 4.0-4 .9 

0 50 

120° 

Figure 127. Epicenters of all subcrustal (also called deep or intraslab) canhquakes in wcs1crn Washington used in thi s study. 

49 

47' 

Thrust 

• ormal 

0 ~' 0 50 

Figure 128. Lower hemi. phcre. equal-area focal mcchani. m. of 

eight larger magnitude subcru stal, or intras lab, eanhquake in we tern Washington. Th is fi gure illu trates the variabi l ity of focal 

me hani ms within the imras lab eanhquake group. umber. refer 

lO earthquakes listed in table 2 1. Dots arc earthquake epi cemers. 

N N 

.. 

. '. . . . ~· . . 

P axes T axes 

Figure 129. Lower hemi sphere. equal-area composite projections 

of I and T axes for the 42 subcrustal (intraslab) Puge1 Sound earthquakes shown in fi gure 127. 

STRESS ANALYSIS 

We applied th in ersion technique of Gephart and For-

. yth ( 19 4) separately to the three groups (shallo Pu get 

ound, Olllll l. Hel n. region. and ubdu ted ) of focal 

mechanisms in w tern Wa. hington. The output of the in version calculati ons include the bes t fitting principal-. tr . sax i. orientations for 0 1. 0 2. and 0 3, th di stri bution of acceptable orientations for ach ax i at the . pec ifi d confidence 

level. and the be. t fitting I? alu " h re I? is defined a. : 

I? = ( 0 , - 0 2) 

( 0 , - 0 3) 

Thu . I? is a measure of whether the int nn di atc 

principal . tre .. i near the max imum (/? - 0) or the minimum (/? - I) principal . tre. es. In addition. the mean 

va lue of misfit for the be t fitting alue is gi ven as an 

angular rotati on (Gephart and Forsy th, 19 '4). Th inverion calcul at ion in ol e a modified rid -~ea h method 

where we may predefine the allowabl e . earch ranges to 

i ncrea e effi ciency. 

SHALLOW PUGET 0 NO GROUP RE LT 

For thi: data . et, we limited the grid-search va lues for 

0 1 u ed in the in er i n because there is a general con i -

tency in the P and T axe. distributions. The azimuth range. 

selec ted were from . 60° W . to 1 . 60° E. and from . 57° 

E. to S. 63° E. The allowed range of plunge wa et from I o 

to 51 ° . An angular increment of 5° between grid points wa. 

used f r both azimuth and plunge. The result of setting the e 

parameters is that I I ,000 di screte stre . orientations arc 

tested for each I? value. where I? vari es between 0. I and 1.0 

in tepsofO. l. 
WASH INGTO EARTHQUA KE FOCAL MECHA ISMS A D THEIR RELATION HIP TO REGIO AL TECTO IC STRE S 263 

Table 18. Best-fi t principal-stre s-orientation models of western Washington earthquakes ba ed on inver ion analyse of fo al 

mechanisms. 

1o1

. max imum compressive stress ; a2• intermediate compressive stress; cr3, minimum compressive stress. Azimuth measured clockwi se from north: plunge measured down from 

the hori zontal. R, ratio comparing the intcrmcdirue compressive stress to the n1ax.irnum and minimum compressive stresses ~). Misfit, n'lC-an rotational difference 

between individual focal mechanisms and modeled stress orientati ons! 

OJ 

Eanhquake umber 

group of events Azimuth Plunge 

Shallow Puget Sound 76 356° 10 

Shallow Mounl St. Helens 73 203° jO 

Subcru stal Pu get Sound 42 245° 51 ° 

For the hallow Puget Sound data set, the best fitting 

model i (azimuth, plunge): o 1 = 356° , I 0 : o2 = 262° . 72° : 

and o3 = 86° , 18° (tab le 18). A zimuth is ex pressed in degrees 

counterclockwi se from north, and plunge is in degrees below 

horizontal. The va lue of R yielding the best fit is 0.6. Fi gure 

130 how. the all owabl e o 1 orientations at both 50-percent 

and 95-percenl confi dence levels. In agreement w ith our intuition based on the di tributions of P and T axes, the be t fitting o 1 is nearly hori zontal and oriented north-south. w ith 

onl y a narrow range of po sibl e o 1 axe. in the 50-p rcent confidence limit plot (fi g. I 0). For the be t fitting . tre. s-ax is 

model. the 76 earthquakes have an average mi sfit of 12.6° , 

and all but IS have misfi ts of les than 20°, ugge ting that 

th e single north-south compression model is a good representati on of the regional tress. Thi re ult support and refines 

prev ious findin gs for the region (Crosson, 1972; Cro son and 

Frank , 1975; ros. on and Lin , 1975; M alone and others. 

1975; Rogers, 1979; Sbar, 1982; Yelin, 1982 ; Yelin and 

Crosson, 1982). T o test the stability of the in ersion to outlier 

data, the computati on. were repea ted after indi viduall y 

remov ing foca l mechani sm w ith mi fit greater than 20° . 

The rc ults were virtuall y unchanged, suggesting that our 

data set i larg enough to produc . table and r bu. t results. 

MO NT ST. HELENS REGION 

GROUP RES LTS 

t tal of 73 foca l mcchani. m. wa. included in th 

Mount St. Helen region data . ct. We uscclthc amc ori entation gri d on. trai nts as clc. ribed for the shallow Pugct 

Sound group, and the sam increment · for R. !though the 

in version t chnique ' ill norm all y selec t the appropri ate . lip 

plane using a minimum rotation cri terion, wh n xtcrn al 

kno\ ledge of the . lip plane i. a ailablc. it can (and shoul d) 

be . pccified expli itl y. The ex istence of a nearl y lin ar zone 

of epicenters more th an 50 km long ex tending from M ount 

t. Helen. to the north-northwes t all owed u. to . lc t the slip 

plane in 59 of the arthquake analyzed under the assumpti on that thi s is a strik -s lip cru tal fault zone. For thi s group. 

0 2 0 ) 

R Mi. fit 

Azimuth Plunge Azimuth Plunge 

262° 72° 86° 18° 0.6 12.6° 

!06° 81° 293° 90 0.7 9. !

0 

13" 26° 117° 26° 0. 1 19.9" 

nodal plane w ith . trik . wi thin I 5° of north were judged to 

be ali gned along the t. Helen · . i mic z nc and were 

elected a fault planes. 

The ingle be t. fitting model found for all 73 M ount t. 

H elens focal mechani ms is (azimuth. plunge): o 1 = 20~ • 

I 0 : o2 = I 06° , 8 1 ° ; o3 = 293° , 9° ; and R = 0.7 (table I ). The 

allowable o 1 ax is orientations at the 50-percent and 95-pcrcen t confidence level. arc shown in figure 13 1. The avcraoc 

mi sfit for all earthquakes in thi s example is 9. 1 ° . and nine 

events have mi sfit greater than 20° . t both 95-perccnt and 

50-percent confidence le cis, the all wablc P-ax is orientations for the hallow M ount St. Helens region o crl ap those 

for the shallow Pugct Sound re0 ion. 

SUBDUCTED-SLAB GRO P RES L TS 

If we as ume that M cKenzie's ( 1969) theory of uniform stre s causing slip on pre xi ting fau lts can be equall y 

applied to earthquakes at subcru stal depths. then the 

N 

'::: ... ·. 

. . .. . ... . . . ii: :\\ 

50-percent confidence 

limit 

N 

·.:.:!it:. 

. .... . .. . . . ... . . . 

:::::: ... .. . .... . . : ..... . •::iii\\. 

95-percent confidence 

limit 

Figure 130. Lower hemisphere. equal-area plots showing the distribution of maximum principal compressive . tress (Ot ) axes. Stress 

was determin d from focal mechani sms u. ing 76 shallow Puget 

Sound t:a rth qu:~kes and Gephart and Forsyth· s ( 198-l) method. The 

best fi tt in,. mod I is listed in table I . The distributi on of acceptable 

axes is shown at the 50-per ·cnt and 95-I ereent confidence limits. 
264 A SES It G E RTHQ KE HAZARDS A D RED 

N 

..... ! : ••. : }l:·.~ .. 

·:.-:··. ..... ,.,./ I • 

... 

:.-.·.· ..... . ... ·.·::::: 

... 

50-percent confid ence 

limit 

N 

.. · ... ·:::·:·· ~',~.· .... 

I /J . • I . 95-percent confidence 

limit 

Figure 13 L Lower hemi. phere. equal-area plots showing the 

distri bu tion of maximum principal comprc. sive strcs (o 1) axes. 

Stress \ as determined from focal mechanism. u:ing 73 . hallow 

Mount St. Helen. area earthquakes and Gephart and Forsyth 's 

( 1984) method. The best fitting rno lei is listed in table 18. The 

eli tribution of acceptable axes i: shown at the 50-percent and 9--

percent confidence limits. 

in ver ion method can be u. ed to in vesti gate the stre state 

in the subducted Juan de Fuca slab. Compo itc plots of P 

and T axes from focal mechanisms of ubcru tal earthquake. in the Puget Sound area (fig. 129) do not how a 

clu tcred di. tribution of P and T axes o, for ub ru. tal 

earthquake ·. a tres grid with 0 1 directi on covering the 

entire focal sphere was used. 

The best fining model for subcru stal earthquakes i. 

(azi muth. plunge): 0 1 = 245°. 51 ° ; 0 2 = 13°. 26° : 0 3 = 11 7°. 

26° ; and R = 0.1 (table 18). The allowable 0 1 axi orientation s at the 50-percent and 95-perccnt confidence le el arc 

shown in fi gure 132. The low va lue of R means thatth intermediate and max imum principal tres e are clo c in magnitude, with the minimum principal tres ignificantl y smaller 

than the other t\ o. Thi s result generall y agree with the 

expected ex tensional stress state along the top surface of a 

bending slab (I ack. and M olnar, 197 1 ). Furthermore. the 

caner of allowable elutions at th 50-percent and 95-pcrccn t confidence lc els indicate. that the _ ingle tres. -

orientation model doe not ati. factorily explain the range of 

focal mechanism observed. We sugge tthc . lab i. in a hetcrogeneou. strc. tate, perhaps re ulting from the complications in . lab geometry noted by Cro _on and Owen ( 19 7) 

and Weaver and Baker ( 19 ). It is certain from these 

rc. ult . however. that the state of tress in the ubductcd lab 

ignificantl y more complex and different than the . tate of 

. tr ss in the continental crust of the orth Ameri ca plate. 

DISCUSSION OF RESULTS 

The result of thi. _ tudy, ba cd on a substantial amount 

of high-quality digi tal data, confirm the general north- outh 

hori zontal teet nic compre sion in the 1 orth America pl ate 

of Washingt n, a first ugge ted by Cr s on ( 1972). 

, RISK IN THE P IF! 0 1 THW · T 

!though the be. t fiu ino . tress m del for the shallow Mount 

St. Helen. area has 01 rotated abou t 27° to the northeast rclati to the shallow Pugct ound r gion, the overl ap of pre-

~ rred . tress model s at the 95-p rccnt confid nee level 

uggc ts that a rotation of tec tonic strcs bet we n the. c two 

regions i. not req uired. Howev r, such a rotation i: al. o not 

precl uded by the data. 

Our resul t. ofT r an alternati ve cxplanati n to the suggestion by Wea r and Smith ( 19 3) that the rotation of 

stre: s indicate. a clnngc in strc s coupling along the subduction zone. In th ab nee of evidence to the contrary, we 

sh uld assume that a more uniform north-south regional tectoni c comprcss i c stress is prcd minant and can explain the 

focal-me hani. m .data while introdu ing the fewest complicating a. sumptions. The apparent rotati on of tress in the 

hallow M ount St. Helen. area i. li kely to be a resu lt of preferred l ip on a preex istin g zone of cru stal weakn s:. We 

note that north-sou th compression is the predominant t cton ic characteristi c along the wes t coast of th ni ted tate 

(Z back and Zobac k, 19 0: bar, 1982). Be ause of the 

rather limited . ize of the Juan de Fuca plate in rel ati on to the 

ex ten. ive an Andreas and Que n harl otte right-lateral 

transform fau lt s . tcm that form most of the remainder of 

the we. tern plate boundar of onh America, it appear. that 

the tre -. state of the onh Ameri a plate in we. tern Wa. hington i. influenced mainl y by the large-. ale interaction 

bet ween the Paci fie and 1 nh America plate · (Zoba k and 

Zoback , 19 0). 

The accumulation of tectonic . train record d by geodetic measurement. ( avage and others. 198 1: Li. owski and 

others. 19 7: Savage and Li w ki . 199 1) app ar. to conflict 

with the north-. outh ·ompression interpret d fr m the focalmechani sm analy i . . It i. important to note, howe cr. that 

the geodcti cally ob. crved strain rate is very low (le s than 

0.10 tr ai n per y ar) (Li. ow. ki and other. _ 19 7; a age 

. 

. 

. 

. 

N 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 

50-percent confidence 

limit 

. 

. 

. 

N 

. . . . . . . . . . . . . . . 

. 

. 

. 

. 

. 

. . . . . . . . . . . ... . . . . . . . : . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. 

95-percent co nfidence 

limit 

Figure 132. Lower h mis1 here. equal-area plots hawing the distribution of max imum criti cal compress ive . trc. s (o 1) axe . . Stres. 

was determined from focal mechanism. usi ng 42 subcrustal Pugct 

Sound earthquakes and Gephart and For. yth ·. ( 1984) method. The 

best fi tt ing model is listed in table I . The distribution of acceptable 

axe: is . hown at the 50-percent and 95-percent confidence limits. 
\NA HI GTO E RTHQ AKE FO AL M E HA ISMS A D THEIR RELATIONSHIP TORE lOt AL TE TO I TR E 265 

and Li sow ki . 199 1) and that strain mea. urement reflect 

on ly incremental changes in tre s, whereas focal mechani m probably reflect the regional ambi ent tre . Furthermore. geodetic mea ur ments may reflect inela tic 

deformation. so that the principal-s train axes determined 

from geodcti mea ur ment. need not reflect the deep state 

of stress. If the explanati on for the difference between the 

orientation of stress determined from geodetic measurement and from focal mechani sm lies in the relative magnitude of ambient and incremental . tres . . then clearl y the 

plate-coupling (incremental) stre . produci ng the obser eel 

. train mu. t be much mailer in magnitude than the ambient 

tectoni tress that causes most intrapl ate earthquake . 

The tre state in th Juan de Fuca . lab apr cars to be 

complica ted and i. not well described by a single tress 

model. The initial assumption of Gephart and Forsy th 's 

( 198-4 ) method. that slip occur on preex isting plane of 

weaknc .. may not apply to the slab. However. the wide . cattcr of P- and T-axis direc tions suggests an inhomogencou. 

. tress field. From the compo ite plots of P and T axes (fi g. 

129), it is difficult to even es timate direc ti ons of apparent 

com pres: ion and ten ion. However. there arc two clear charac tcri . tic : ( I) no P axe arc oriented north- outh with . hallow plunge angle. ; (2) T axes are generall y ori ented 

northeast or outhca ·t with hallow plunge angle . . The differences between the P and T axes of subcrustal (fi g. 129) 

and cru stal earthquakes (fi gs . 125 and 126) indicate that 

there i. little direct mechan ical coupli ng between the cru t 

and the deeper 1 art of the subducted slab. Rather. the . lab 

tr s eli tributi on appears to be generated largely by processc. in. ide the slab it elf (for example. bending . tresse. or 

. trcssc. induced from phase change). Although the implication of these result in term. of intra lab earthquake hazard. 

i. not c;: l ar at thi. time, unders tanding the slab . trc . state 

. hould lead to improved e tim ate. of source parameters of 

fuwrc lab earthq uakes. The general spatial coincidence of 

the crus tal and subcru stal earthquake groups in th Pug t 

ound region (fi g. 122) i pu a ling in view of the lear diffcren in strc. s . tates that we have foun I from thi s study. 

The varic t of the dire ·tion. of tensional ax s in different 1 art. of thc . lab might result from superposition ofd wndip ten. ion \ ith stress rc. ulting from the arching of the Ju an 

d Fuca plate under the Pug t ound area. To tc t thi. po .. ibility. we examined the slab . tre .. -ax i: orientations in light 

of the be. t a ailablc knowledge of . lab geometry. M . t T 

axes, a~ shown in fi gur I . 3. seem to lie in the pl an of the 

1 lat arch or in a down-dip direc ti on. For hi.,her magnitude 

earthquake . . thi s tend ncy may be cv n m r pronounc d. 

For exampl e, fi c of the cioht earthquake. \ ith maonitudes 

of 3.0 or greater had T axes that closely parallel th arched 

. lab. Thcs results sugge. t that the arching coul I produce 

tensional . tress due to bending of th . lab. The on fi gurati on 

of P axe. shown in figur 134 is quit cl iff rent. wi th Pax . 

tending to be normal to the . lab surface. A qualit at i v compari:on of fi curcs I :n and 134 suggc. ts a general ontrol of 

the intra ·Jab arthquak es by slab bend ing or slab geometry. 

(f) 

a: 

w 

1-

w 

~ 

0 

..J 

;;2 

z 

I . 

NORTH SOUTH 

.-------------~--,---------.--------, 

b:: 90 AZIMUTH RA GE w 

0 oo azimuth 

goo azimuth 120 L_ ________ ~ ________ J_ ________ _L ________ _..J 

50 49 48 47 46 

NORTH LATITUDE, IN DEGREES 

Figure 133. T axes of the 42 subcru. tal western Washington 

earthquakes used in the wdy projected onto a no11h-south cross 

secti on. The numbered curves ar approximate contours at equal 

longitude va lue. (from the model of ro. son and Owen . . 1987). 

(f) 

a: 

w 

Iw 

~ 

0 

..J 

;;2 

~ 

£ 

Ic.. 

w 

0 

NORTH SOUTH 

~------~--------.--------,--------, 

30 

120 

50 

EXPLANATION 

AZIMUTH RANGE 

oo azimuth 

goo azimuth 

49 48 47 

NORTH LATITUDE, IN DEGREES 

46 

Figure 134. P axes of the 42 subcru. tal we tern \ a. hington 

earthquakes u ed in the stud y projected onto a nonh-:outh cross 

secti on. The numbered curves arc approxi mate cnmours at equal 

longitude va lue. (from the 111 del of ro:son and wen,. 1987). 

CONCLUSIONS 

Our . ludic. indica t that th Wa hington part of the 

N rth meri ca plate is like! to be in a tate of uni form 

regional north-south c mpre si c teet nic stress that doc 

not direct ly refl ct th ac ti e ubduction along the Cascadia 

subducti on zone. Pl ate-c upling . tr s between the Juan de 

Fuca and onh Ameri ca plat may be of much IO\ er magnitude than the re,ional north-. outh compres ive trc s. 

Based on focal-mechani m data. a rotati n of principal stress 

directions i. not required near ount t. Helen . although 

such a rotation i not excluded by the data . The state of. tress 

in the . ubducted Juan de Fuca pl ate is eli tinctly different 

from and more complex than the strc tate in the ovcrl ing 

Iorth Ameri ca plate. The lack of a uniform tre ... tate in th 

subduct d slab indi cat s that pl at -geometry variati ons may 

be important in ontrolling slab . trc: and cismicity. In 

iew of the apparent str . s dccoupling of cru. tal and ubcru stal earthquakes in the Pugct Sound region. the . patial 

co incidcn c of thcs two zones remains enigmati c. 
266 A I G E RTHQ AK HAZ RDS A D RED CI G RISK I TH E PA IFI ORTHWEST 

REFERENCES CITED 

Angelier. J .. 1979, Determination of the mean principal directions 

of stre . . es for a given fault popul ation: Tectonophys ics. v. 56. 

no. 3-4, p. T 17-T26. 

Atwater. Tan a, 1970, I mplication. of pl ate tectonics for the enozoic tectoni c olution of we. tern onh Ameri ca: Geological 

ocicty of America Bulletin . v. I, no. 12, p. 351 3-3535. 

Cro:. on. R.S .. 1972. mall earthquakes. strucwre. and tectoni cs of 

the Puget ound region: Seismological ocicty of Ameri ca 

Bulletin, v. 62, no. 5, p. I 133- 1 17 1. 

---198 . Review of seismicity in the Puget Sound region from 

1970 through 197 . in Yount, J.C .. and ro son. R.S .. eels. , 

Proceedings of Work shop X IV. Earthquake hazards of the 

Puget ouncl region. Washington : .S. Geological Survey 

Open-File Report - 19, p. 6- 1 

rosson. R .. , and rank . D.G .. 1975 . The Mt. Rainier earthquake 

of Jul y 18. 1973. and its tectonic significance: cismological 

Society of Ameri ca Bulletin. v. 65 . no. 2. p. 39.>-40 I . 

Cro. son, R.S .. and Lin, Jia-Wen, 1975. A note on the Mt. Rainier 

earthqu ake of April 20. 1974: Seismologica l Society of mcrica Bulletin. . 65, no. 2. p. 549- 556. 

ros. n. R.S, and Owens, T.J.. 19 7, lab geometry of the ascadia subduction zone beneath Washington from earthqu ake 

hypocenters and tele. eismic convened waves: Gcoph ys i al 

Research Letter , v. 14, no. 8. p. 824- 27 . 

Ellsworth, W.L.L., and Zhonghuai. X., 1980, Determination of the 

stres. tensor fr m foca l mechanism data lab .. 1: EOS !American Geophy ical nion Tran. actions], v. 6 1. no. 46, p. 111 7. 

Fowler, .M .R .. 1990. The solid Earth- n introduction to global 

geophysics : ew York. Cambridge ni ver ity Press. 472 p. 

Gephan. J.W., 1985, Principal stre s directions and the ambi, uity 

in fault pl ane identification from focal mechanisms: ei. mologica l Society of Ameri ca Bulletin .. 75. no. 2. p. 62 1-625. 

--- 1990a, FMSI- A FORTRA program for inverting fault/ 

slickenside and earthquake focal mechanism data to obtain the 

regional tres ten or: omputer and Gco. ciences. v. 16. no. 

7, p. 953-989. 

---1990b. tress and the direction of sli p on fault planes: Tectonics, v. 9, no. 4, p. 845- 85 

Gephan . J. W .. and Fors th. D. W .. 19 4. n improved method for 

dct rmining the regional stre. ten. or using earthqu ake focal 

mechanism data-Appli cation to the San Fernando earthquake 

sequence: Journal of Geoph y ical Re. ear h, v. 9. no. II . p. 

9305-9320. 

Grant, W.C .. Weaver, C.S .. and Zoll weg. J.E., 1984. The 14 February 19 I Elk Lake. Washington, eanhquake equence: eismologica l Society of America Bulletin. v. 74, no. 4, p. 12 9- 1309. 

Green. A.G., Clowes, R.M .. Yorath. C.J.. Spencer. C. P .. 

Kanasewich. E.R .. Brandon, M .T. , and Brown. A.S .. 1986, 

Seismic rene tion imaging of the subducting Juan de Fuca 

plate: ature, v. 3 19. no. 6050. p. 2 10-21 3. 

!sacks, Bryan. and M olnar, Peter, 1971. Distribution of stre .. es in 

the descending lithosphere from a global survey of focalmechanism s lution. of mantle earthqu akes: Geoph y. ic. and 

pace Phy ics Revicw ,v.9. no. l , p. I03- 174. 

Keach, R.W. , II , Potter, .J.. Oli ver, J .. , and Brown, L.D .. 1986, 

Cenozoic acti ve margin and shallow Ca ·cades strucwrc- COCORP results from western Oregon [ab:. j: San Antonio, Tex .. Geological Society of America Abstracts with 

Program . . v. I , no. 6, p. 652. 

Li sowski. Michael. Savage. J .. , Prescott , W.H., and Dragert, 

Herb. 1987 . train accumul ation al ng the ascadi a ubdu ction 7.onc in western W"lShington labs.]: EOS !America n Geophysical nion Transactions!. v. 68, no. 44, p. 1240. 

Ludwin, R .. . , Weaver, .S., and rosson, R .. , 199 1, cismicity of 

\· ashington and Oregon. in Iemmons. D.B., Engdahl, E.R., 

Zoback, M .D .. and Blackwell. D.O., eel s., eotcctonic. of 

onh Ameri a: Boulder, Colo .. Geological Society of America. Decad Map Volume I , p. 77- 98. 

M alone. .D .. Rothe, G.H .. and mith. .W .. 1975. Detail. of 

microcanhquakc swarms in the olumbia basin. Washington: 

eismol gical ociety of Ameri a Bulletin . v. 65. no. 4. p. 

855- 64. 

M cKenzie, D.P., 1969. The relation bet\ cen fault plane solutions 

for earthquakes and th dire ti ons of the prin ·ipal stresses: 

ci smologi al S ciet of Ameri ca Bulletin , v. 59, no. 2. p. 

59 1- 601. 

Qamar, A .l.. Ludwin, R . . , ros>on. R.S .. and Malone. , .D .. 19 7. 

Earthquake h pocentcrs in Washington and nonhcrn regan. 

1982- 1986: W11. hington Department of at ural Res urcc., 

Di vi. ion of eology and Earth Resources, Information ircular 4, 78 1. 

Riclclihough. R.P .. 19 4. Recent rnovemems of the Juan de Fuca 

plate system : Journal of cophy. ical Research, v. 89, no. B , 

p. 69 0-6994. 

Rogers, G.C .. 1979, Earthqu ake fault plane solutions ncar Vancouver Island: anacli an Journal of E, nh ci nee .. v. 16. no .. . 

pan I. p. 523- 53 1. 

avage. J . .. and Li so ski. ~i ac l. 199 1. train measurements 

and the potenti al for a gr at subduction earthqu ake off the coast 

of Washington: cicnce. v. _52, no. 5002. p. 10 1- 103 . 

Savage. J . .. Li sowski , Michael. and Prescott, W.H .. 198 1. Geodetic . train measurement. in Wa ·hington: Journal of eophy ·ical 

Rescar h, v. 6. no. B6, p. 4929 9-1 0. 

Sbar, M .L. , 19 2, Delineation and interpretation of scism tectonic 

domains in western onh America: Journal of Geoph ysica l 

Re. earch, v. 7, no. B5. p. 39 19- ~92 . 

Taber. J.J.. and mith. .W .. 19 . eismici t and focal mechanism. associated with the subduction of the Juan de Fuca plate 

beneath the Ol ympic Peninsula. Wa. hington: eismological 

Society of Ameri ca Bulletin. . 75. no. I . p. 237- -49. 

yeda. .. and Kanam ri , Hiroo. 1979. Back-arc opening and the 

mode of subduction: Journal f G oph y. ical Research. v. -1 , 

no. B3. p. I 049- 106 1. 

Weaver. C.S., and Baker, G.E., 19 . Geometr of the Juan de Fu ·a 

plate beneath Washington and northern Oregon from sei. micity: Sci. mologica l Society of Ameri a Bulletin. v. 7 , no. I , p. 

26 275. 

Weaver, C.S., and mith .. W .. 19 3. Regional teet nic and earthquake hazards implication. of a crustal fault zone in southwe>tern Washington: Journal of Geoph ys ica l Research. . , no. 

Bl 2, p. 1037 1- 103 . 

Yelin, T.S., 1982, The Seattle ca nhquake swarm s and Puget basin 

focalmechani. m. and their tectonic impli cation. : Seattle, University of Washington, M .S. thesi. , 9 p. 

Yelin, T .. , and r s. on, R ... 19 2, A note on the . outh Pugct 

Sound ba in magnitude 4.6 ea rthqu ake of II M arch 197 and 

its aftershock. : Sei smological Society of America Bulletin. v. 

72, no. 3, p. 1033- 1038. 

Zoback, M.L., and Zoback, M .D .. 19 0. tate of stress in the contermin ous nited States: Journal of Geoph ys ical Research. v. 

85, n . I I , p. 6 1 13-61 56. 
FOCAL-MECHANISM 

LISTINGS AND PLOTS 

All focal mechanisms used in thi study are li sted and ploned here. The mechanisms were constructed 

by hand from polarity data reread from the original digital seismogram . An explanation of earthquake focal 

mechanisms can be found in Fowler ( 1990). All focal-mechanism plot shown here arc lower hemisphere. equalarea sterconct projections. Symbols represent the first motion of vertical- omponcnt P-wavc arri als. Symbols 

th at intersect at the center. such as Xs and asterisks. represent compress ion: symbols open in the middle. su h as 

octagon and triangles, represent dilatation for vertical-component P-wave arrivals. The strength of first motion is 

indica ted as follows: strong compre ion is shown as an asterisk , less strong compression as a large X, weaker 

comprcs ion as a smaller a teri sk. and weakest compre sion as a small x: strong dilatation i. shown as an octagon. 

less strong dilatation as a triangle. weaker dilatation as a small octagon. and weakest dilatation as a small triangle. 

Corrections have been made for stations known to have been reversed. Takeoff angles at the focal sphere were 

computed usi ng a local velocity model with a linear increa e in velocity with depth. approximating the regional 

velocity model used for the earthquake locations. Th is procedure smoothes the distribution of takeoff angle . A 

small square in the center of any symbol shows that the ray leaves the ource at an upgoing angle. The Taxi of 

each mechani sm run through the middle of the compressional quadrant.' herem; the P axis is in the middle of the 

tensional quadrant. The tables and plot are separated into the three regions analyzed in the study. The event numers arc sequential in time. For planes A and Bin the focal mechanisms. the azimuth give the angular di. tancc in 

degrees measured ountercl ockwi se from north to the dip direc tion {thi s means that the azimuth is 90° from the 

strike of the plane). and the plunge angle give the dip of the plane in degree b I w the horizontal. 
268 AS ES lNG EARTHQ A KE HAZARD At D RED CI G RISK I THE PA IFI ORTHWE T 

Table 19. Shallow Puget Sound reg ion earthquake data. 

Event Date (year, 

number month, day) 

14 820 102 

17 820 123 

26 820303 

29 820310 

30 8203 10 

38 

41 

55 

58 

61 

63 

67 

70 

71 

72 

73 

75 

78 

80 

81 

84 

85 

90 

92 

94 

95 

97 

99 

100 

102 

IOJ 

104 

106 

108 

110 

Ill 

113 

115 

11 9 

120 

122 

124 

126 

127 

131 

132 

133 

135 

I:\9 

143 

820404 

204 14 

82071 

20926 

210 15 

82 111 2 

82 12 11 

2121 

82 1220 

821231 

830 124 

830 131 

830303 

8303 13 

303 15 

830407 

830409 

830424 

830504 

830516 

8305 19 

83052 1 

830525 

830605 

830726 

8J0728 

830728 

830 19 

83090 1 

830904 

8309 14 

830929 

83 1023 

83 1216 

840 104 

840 111 

8402 19 

8403 14 

840323 

840427 

840602 

840602 

8406 19 

840724 

840905 

Latitude 

47.37 

46.6 1 

45.99 

46.74 

47.33 

Longitude 

122.39 

121.43 

122.44 

122.20 

122.71 

46.57 122.48 

47.7 1 122.52 

46.58 121.39 

46.87 121 .12 

47.59 122.63 

4 7.69 122.69 

47.53 122.73 

4 7.89 122.53 

46.59 121.42 

47. 19 122.08 

47. 11 121.99 

46.67 122.33 

47.64 121.94 

46.24 122.69 

46.52 122.79 

46.63 122.42 

46.74 121.82 

46.54 121.45 

48.34 122. I 0 

47.49 122.58 

47.64 122.50 

47.36 121.49 

47.78 121.7 1 

46.54 122.73 

46.69 122.54 

46.06 122. I 

46.07 122.74 

47.43 122.74 

47.77 122.72 

47.89 122.63 

47.09 121.93 

47.34 122.72 

46.56 122.35 

47.34 122.03 

47.68 122.58 

46.91 121.64 

4 7.35 122.35 

47.84 122.36 

47 .75 122.69 

47.65 122.03 

47.49 122.7 1 

47 .50 122.72 

47.72 122.99 

47.77 122.45 

47.92 122.04 

Depth 

(kilometers) 

14.47 

3. 3 

11.78 

16.71 

26.79 

19.53 

27 .2 

6.4 

3.25 

27.52 

24.54 

20.22 

23 .17 

5.26 

14.27 

6.62 

17.9 1 

2.35 

15.40 

24.05 

16.07 

8.27 

4.97 

8.68 

24.17 

23.4 

11 .69 

10.72 

23 .73 

17.35 

16.06 

15.65 

23.12 

19.20 

22.78 

19.02 

27.12 

17.28 

12.9 1 

18.83 

5.94 

15.83 

22.68 

19.07 

9.77 

21.49 

22.60 

8.78 

21.28 

17.37 

Magnitude 

2.7 

3.2 

2.1 

2.4 

2.9 

1.9 

3.4 

2.9 

3.4 

3.0 

2. 

2.3 

2.8 

2.7 

2.4 

3.0 

2.2 

2.9 

2.9 

2.7 

1.9 

1.8 

2.7 

2.9 

2.0 

2.0 

2.8 

3.0 

2.3 

2.2 

2.4 

2.3 

2.2 

2.5 

2.6 

2.2 

2.7 

2.5 

3.0 

2.8 

2.2 

2.4 

2.7 

2.9 

2.9 

3.6 

2.8 

3.0 

2.7 

2.2 

P axi~ 

Azimuth Plunge 

162 4 

180 45 

30 0 

192 4 

16 0 

182 

6 

5 

192 

38 

6 

5 

10 

15 

26 

342 

35 

346 

18 

36 

20 

170 

354 

2 

335 

2 

163 

180 

198 

199 

20 

176 

214 

163 

164 

161 

184 

212 

214 

216 

202 

160 

170 

174 

147 

216 

26 

0 

4 

322 

38 

8 

0 

20 

18 

0 

17 

5 

37 

36 

15 

0 

0 

0 

46 

0 

7 

0 

30 

67 

0 

II 

16 

36 

3 

0 

3 

23 

0 

5 

60 

2 

7 

8 

13 

28 

10 

4 

0 

0 

4 

8 

0 

7 

5 

Taxi ~ 

Azimuth Plunge 

72 6 

90 0 

120 0 

285 38 

106 0 

89 

101 

95 

285 

284 

96 

101 

190 

108 

132 

162 

88 

76 

108 

127 

110 

272 

84 

266 

65 

II 

256 

275 

291 

98 

110 

79 

96 

73 

344 

295 

276 

Ill 

305 

11 9 

105 

347 

350 

84 

57 

308 

290 

90 

11 8 

212 

4 

33 

0 

8 

51 

30 

18 

85 

4 

2 1 

75 

66 

0 

0 

0 

0 

60 

0 

10 

0 

62 

14 

16 

4 

75 

57 

68 

47 

30 

5 

22 

40 

58 

10 

28 

13 

80 

86 

0 

13 

22 

38 

0 

73 

76 

.l:l..i.l.!:!l; 

Azimuth Plunge 

207 83 

215 60 

345 90 

322 61 

331 90 

218 

138 

320 

330 

89 

137 

143 

100 

158 

175 

72 

11 0 

30 1 

333 

182 

335 

289 

309 

40 

134 

143 

299 

317 

34 1 

274 

139 

244 

259 

212 

254 

353 

312 

272 

350 

26 1 

240 

249 

260 

39 

193 

350 

75 

315 

Ill 

38 

61 

61 

90 

70 

40 

69 

65 

40 

61 

48 

30 

so 

90 

90 

59 

90 

46 

90 

61 

49 

51 

72 

67 

62 

44 

54 

46 

37 

69 

40 

29 

61 

47 

77 

60 

61 

35 

41 

90 

81 

72 

58 

90 

40 

42 

Plane B 

Azimuth Plunge 

117 89 

325 60 

75 90 

65 67 

61 90 

322 

238 

50 

237 

334 

235 

233 

280 

56 

76 

252 

246 

31 

63 

71 

65 

56 

39 

137 

356 

273 

30 

231 

239 

123 

261 

106 

150 

11 4 

74 

220 

57 

147 

80 

165 

336 

71 

80 

309 

101 

84 

332 

45 

26 1 

244 

68 

73 

90 

82 

71 

69 

89 

50 

68 

81 

60 

50 

90 

90 

60 

90 

58 

90 

77 

49 

51 

88 

90 

69 

50 

54 

52 

76 

69 

50 

69 

65 

59 

89 

80 

0 

55 

49 

90 

81 

78 

70 

90 

54 

51 
WASH ! GTO EARTHQUAKE FOCAL M ECHA ISMS A D THEIR RELATIONSHIP TOR GIONAL TE TONIC STRE 269 

Table 19. Shallow Puget Sound region earthquake data-Continued 

Event Date (year, 

number month, day) 

146 840920 

148 84 1029 

IS I 84 11 20 

I 53 84 11 30 

I 54 84 1204 

156 

157 

158 

159 

160 

163 

164 

165 

167 

169 

170 

172 

176 

177 

178 

182 

183 

184 

185 

186 

191 

84 1220 

850 121 

850 123 

850 123 

850 129 

85032 1 

850330 

850330 

8504 17 

850426 

850430 

850509 

8506 16 

85062 1 

850706 

850914 

851006 

851014 

85 1017 

85 11 06 

85 1227 

Latitude 

47.55 

47.85 

47.95 

47.76 

46:55 

47.88 

46.9 1 

47 .77 

47.83 

47.48 

47.64 

46.70 

46.69 

47.70 

48.4 1 

48.40 

46.57 

47.44 

46.5 I 

47.77 

47.43 

47.93 

46.37 

47.46 

46.89 

46.97 

Longitude 

122.34 

122.43 

121.98 

122.24 

122.37 

122.46 

122,02 

122.47 

122.48 

121.83 

122.22 

122.20 

122.20 

122.25 

122.3 1 

122.32 

121.84 

121.87 

122.37 

122.27 

122.38 

122.90 

122.68 

123.00 

I 21.99 

121.94 

Depth 

(kilometers) 

26.52 

I 8.35 

16.64 

23.72 

19.94 

22.65 

12.80 

I 8.39 

18.44 

17.55 

7.86 

15.98 

16.69 

23.94 

18.2 1 

18. 16 

9.84 

17.03 

20,02 

I 7.97 

I 9.85 

19.96 

20. 16 

15.87 

7.6 1 

7.02 

Magnitude 

2.7 

2.0 

2.0 

2.7 

1.7 

2.2 

2.7 

2.6 

2.2 

2.7 

3.0 

2.8 

2.6 

1.9 

3.0 

2.4 

2.7 

3. I 

1.8 

3. I 

3.0 

2.8 

1.5 

2.6 

2.3 

3.0 

P axis 

Azimuth Plunge 

26 4 

168 3 

310 13 

178 28 

228 29 

5 

176 

18 I 

177 

359 

160 

148 

ISS 

184 

182 

32 

196 

314 

212 

157 

334 

322 

32 

178 

3 

346 

5 

3 

6 

6 

3 

12 

21 

0 

18 

35 

0 

I 

6 

40 

13 

IS 

0 

12 

0 

12 

3 

T ax is 

Azimuth Plunge 

293 37 

264 62 

212 30 

85 6 

137 2 

102 

296 

295 

300 

262 

257 

246 

65 

297 

2 

302 

14 

49 

310 

54 

240 

52 

134 

268 

248 

255 

54 

84 

75 

79 

67 

29 

21 

0 

50 

55 

0 

89 

37 

9 

44 

14 

70 

44 

67 

64 

19 

~ 

Azi muth Plunge 

76 62 

284 49 

355 59 

218 66 

268 68 

127 

272 

285 

277 

67 

295 

287 

20 

314 

272 

257 

286 

84 

359 

206 

17 

71 

162 

289 

66 

32 

51 

42 

41 

40 

47 

60 

60 

90 

40 

10 

90 

44 

60 

56 

49 

69 

48 

50 

49 

39 

74 

~ 

Azimuth Plunge 

333 6 

54 54 

2 79 

315 75 

6 72 

246 

81 

79 

79 

289 

197 

290 

68 

92 

167 

106 

187 

255 

98 

107 

213 

270 

67 

292 

299 

59 

4 

52 

52 

52 

79 

90 

90 

71 

80 

90 

46 

69 

70 

70 

89 

48 

70 

49 

61 

79 
270 ASSE SI G E RTHQ AKE H ZARDS t D RED CI G RISK I THE PA IFIC ORTHWEST 

N N 

N 

N 

Figure 135 (above a nd next three pages). Lower hemisphere, equal-area projection. of shallow Puget Sound earthquake 

focal mechani. ms. 
WAS H! GTO EARTHQ AKE FOCAL ME HA ISMS AND THEIR RELATIO HIP TO REGIO AL TECTO I STR - 27 1 

N 

N 

Figure 135. Lower hemispher . equal-area projecti ons of shall ow Pu get Sound earth 1uake focal mec hanisms- ontinuecl 
272 A. E Sl !G E RTHQ AKE HAZ RD At DR -D Cl G RISK I TilE PACIFI NORTIIWE. T 

N N 

N N 

N N 

Figure 135. Lower hcmi>phere, equal-area projections or shallow Puget Sound earthquake focal mechani ' m'- Continued 
WA HI 1

GTO EARTHQUAKE FO AL MECHA ISM AND THEIR RELATIONS HIP TO REGIONALTE TO TRESS 27 

N 

N 

N N 

N N N N 

Figure 135. Lower hcmi ·phere, equal-area projections or shal low Pugct Sound c, rthqu ake focal mechani sms ontinucd 
274 A SESSI G EARTHQUAKE HAZA RDS A D RED Cl G RI SK l THE PAC!Fl ORTHWE T 

Table 20. Shall ow Mount St. Helens region earthquake data. 

Event Da te (year. 

number month, day) 

16 20 123 

18 820 127 

19 20208 

20 202 17 

21 2030 1 

22 

23 

24 

25 

27 

2 

32 

33 

34 

5 

36 

37 

39 

40 

42 

43 

44 

45 

46 

47 

48 

49 

50 

51 

52 

53 

54 

56 

57 

59 

60 

64 

65 

66 

68 

69 

74 

77 

79 

82 

83 

86 

87 

88 

9 

82030 1 

82030 1 

820302 

820302 

820303 

20306 

8203 16 

820316 

820320 

820326 

82040 1 

820402 

8204 10 

8204 12 

8204 17 

820426 

82052 1 

820526 

820527 

20528 

82053 1 

82053 1 

820605 

820606 

820606 

820704 

8207 12 

820724 

8208 19 

82 1008 

82 1009 

821 11 3 

82 111 6 

821 128 

82 1212 

82 1212 

830 129 

83020 

830309 

8303 1 

8 0320 

8304 12 

8304 12 

830420 

830420 

La titude 

4639 

46.42 

46.52 

46.4 1 

46.40 

Longi tude 

122.28 

122.26 

122.28 

122.32 

122.30 

46.42 122.30 

46.40 122.30 

46.39 122.30 

46.4 1 122.29 

46.39 122.30 

46. 8 122.28 

46.4 1 122.33 

46.40 122.27 

46. 9 122.33 

46.40 122.3 I 

46.38 122.25 

46.27 122.29 

46.38 122.31 

46.38 122.28 

46.37 122.25 

46.43 122.26 

46.4 1 122.07 

46.4 1 122.3 1 

46.36 122.26 

46.40 122.32 

46.39 122.32 

46.40 122.28 

46.43 122.29 

46.4 1 122.25 

46.38 122.25 

46.35 122.30 

46.30 122.30 

46.31 122.26 

46.40 122.25 

46.28 122.08 

46.39 122.3 I 

46.39 122.29 

46.31 122.30 

46.34 122.28 

46.38 122.30 

46.28 122.50 

46.36 122.34 

46.44 122.33 

46.40 122.29 

46.4 1 122.24 

46.12 122.13 

46.4 1 122.33 

46.39 122.31 

46.4 1 122.32 

46.4 1 122.33 

Depth 

(kilometers) 

9.45 

.09 

4.02 

10.61 

10.97 

11 .33 

11 .48 

11 .40 

11.61 

11 .13 

11 .24 

12.65 

11 .22 

11 .60 

11.61 

11 .94 

10.90 

8.88 

12.01 

9. 14 

9.69 

2.92 

11.94 

7.52 

11.56 

12.04 

11.76 

10.44 

10.86 

5.45 

9.26 

11 .45 

9.25 

9.68 

0.77 

8.91 

10.97 

10.58 

11.93 

14.80 

15.85 

11 .50 

9.25 

11 .36 

7.65 

9. 12 

10.31 

8.61 

9.48 

9.27 

Mag- E...M.is. 

nitude Azimuth Plu nge 

2.9 40 0 

2.1 43 16 

2.3 246 4 

1.7 34 0 

2.7 3 34 

1.8 

1.0 

2.0 

1.8 

1.5 

2.6 

1.7 

2.1 

2.4 

1.9 

2.2 

1.2 

2.2 

2.0 

1.7 

2.7 

1.5 

I. 

1.7 

1.7 

3.0 

1.8 

1.5 

1.0 

1.4 

1.2 

1.1 

1.2 

2.3 

2.2 

1.7 

I. 

1.9 

2.6 

2.2 

1.9 

1.3 

1.2 

1.1 

1.5 

2.0 

1.5 

2.0 

2.3 

2.3 

212 

44 

40 

213 

20 

2 

24 

33 

208 

47 

3 

56 

240 

35 

45 

201 

75 

35 

34 

45 

42 

27 

298 

45 

45 

212 

45 

I 

38 

168 

58 

62 

38 

13 

26 

8 

154 

40 

200 

40 

207 

18 

32 

218 

38 

19 

4 

20 

3 

4 

28 

4 

0 

21 

35 

12 

6 

40 

0 

0 

14 

75 

0 

0 

0 

20 

32 

65 

0 

0 

33 

0 

14 

20 

75 

26 

47 

0 

23 

5 

13 

5 

13 

35 

12 

5 

25 

30 

16 

10 

I.Mis 

Azimuth Plunge 

130 0 

155 53 

342 57 

124 0 

151 33 

108 

136 

143 

307 

11 0 

I 4 

29 1 

123 

306 

166 

135 

147 

144 

125 

135 

299 

255 

125 

124 

135 

139 

139 

11 6 

135 

135 

328 

135 

284 

136 

348 

312 

31 

128 

126 

116 

98 

248 

133 

299 

132 

299 

120 

123 

124 

130 

35 

22 

33 

50 

4 

27 

34 

0 

20 

34 

0 

7 

8 

0 

0 

29 

IS 

0 

0 

0 

19 

31 

25 

0 

0 

34 

0 

14 

20 

IS 

29 

12 

0 

43 

4 

0 

36 

12 

12 

II 

22 

25 

I 

IS 

II 

Elilnl:..A 

Azimuth Plunge 

355 90 

170 41 

6 50 

349 90 

184 38 

255 

178 

178 

337 

ISS 

171 

73 

348 

347 

197 

173 

191 

274 

350 

0 

336 

345 

350 

349 

0 

181 

173 

204 

0 

0 

359 

0 

61 

177 

78 

96 

87 

353 

ISO 

161 

144 

285 

177 

345 

176 

34 1 

160 

171 

26 1 

174 

50 

72 

51 

55 

84 

49 

64 

90 

60 

6 

60 

I 

57 

90 

90 

59 

30 

90 

90 

90 

62 

42 

20 

90 

90 

39 

90 

70 

61 

30 

49 

48 

90 

41 

84 

81 

61 

72 

56 

74 

71 

51 

68 

68 

75 

~ 

Azimuth Plunge 

85 90 

28 68 

129 57 

79 90 

94 90 

157 

272 

274 

91 

65 

I 

333 

7 

257 

108 

73 

165 

0 

79 

90 

91 

83 

27 

90 

90 

90 

90 

156 

267 

258 

5 

19 

83 

254 

71 

52 

26 

86 

245 

86 

75 

256 

73 

35 1 

264 

80 

7 

82 

60 

90 

89 

70 

90 

89 

90 

7 

89 

69 

90 

90 

0 

60 

90 

90 

90 

90 

90 

70 

90 

90 

89 

90 

90 

90 

60 

88 

69 

90 

78 

89 

I 

69 

89 

75 

89 

78 

82 

70 

89 

89 
WA HI GTO EARTHQUAKE FO AL MECHA ISMS A D TH EIR RELATIO SHIP TO REGIO AL TE T IC STRE 275 

Table 20. Shallow Mount St. Helens earthquake data-Continued 

Event Date (year, 

number month, day) 

93 830506 

96 8305 19 

98 83052 1 

109 830902 

11 2 8309 15 

11 4 

11 7 

11 8 

123 

129 

138 

140 

144 

145 

147 

149 

150 

166 

17 1 

174 

179 

18 1 

188 

83 1002 

83 11 0 1 

83 12 13 

840 111 

840404 

8407 16 

840805 

840908 

8409 15 

84 10 16 

84 11 03 

84 11 03 

8504 14 

850430 

850523 

85080 1 

850905 

85 111 7 

Latitude 

46.4 1 

46.39 

46.38 

46.33 

46.53 

46.46 

46.34 

46.37 

46.4 1 

46.43 

46.49 

46.52 

46.29 

46.50 

46.43 

46.4 1 

46.4 1 

46.40 

46.4 1 

46.2 1 

46.26 

46.33 

46.43 

Longitude 

122.23 

122.30 

122.37 

122.53 

122.45 

122.33 

122.29 

122.26 

122.28 

122.32 

122.30 

122.32 

122.28 

122.40 

122.3 1 

122.32 

122.3 1 

122.25 

122.30 

122.2 1 

122.52 

122.23 

122.33 

Depth 

(kilometers) 

7.4 1 

8.98 

9.32 

15.94 

15.3 1 

9.20 

9.95 

9.42 

6.52 

9.2 1 

15.25 

11 .26 

7.00 

13.36 

10.58 

11.24 

11 .57 

8.03 

10.95 

0.72 

16.91 

7. 16 

11 .06 

Magnitude 

2.3 

1.5 

1.9 

1.0 

2.3 

2.2 

2.0 

1.8 

1.7 

2.4 

2.8 

3.1 

1.4 

1.1 

1.2 

1.9 

1. 1 

2.1 

2.0 

2.5 

1.4 

2.5 

2.9 

P axis 

Azimuth Plunge 

38 0 

40 18 

180 18 

49 45 

16 6 

38 

155 

238 

27 

24 

2 10 

22 

2 10 

26 

0 

0 

12 

25 

37 

190 

13 

9 

45 

0 

32 

15 

0 

10 

6 

20 

22 

12 

25 

25 

38 

8 

27 

0 

7 

II 

0 

I.JWs 

Azimuth Plunge 

128 0 

136 18 

87 10 

306 13 

110 36 

128 

259 

332 

11 7 

135 

303 

290 

11 7 

11 9 

180 

180 

106 

29 1 

143 

10 

277 

10 1 

135 

28 

2 1 

16 

0 

64 

25 

6 

8 

12 

65 

65 

5 

26 

29 

90 

40 

II 

0 

£lll.M.A 

Azimuth Plunge 

5. 90 

178 64 

222 70 

76 49 

147 6 1 

169 

300 

15 

342 

14 1 

344 

64 

25 1 

162 

90 

90 

156 

7 1 

180 

100 

63 

145 

0 

7 1 

5 1 

68 

90 

4 1 

6 

7 1 

69 

73 

20 

20 

60 

66 

49 

45 

57 

74 

90 

.Elru!Ui 

Azimuth Plunge 

83 90 

268 90 

3 14 84 

18 70 

249 70 

267 

20 

105 

72 

274 

79 

157 

345 

72 

270 

270 

53 

35 

27 1 

280 

3 18 

55 

90 

7 1 

83 

89 

90 

59 

77 

80 

I 

90 

70 

70 

6 

78 

9 

45 

68 

90 

90 
276 SSE SI G EARTHQ A K HAZARD 1 D REDUCING RISK I THE PACIFIC ORTHWEST 

N 

N N N 

N N 

figure 136 (above and next three pages). Lower hemisphere, equal-area projecti on. of shallow M ount St. Helens region 

earthqu ake focal mechani. ms. 
WASH I GTO EARTHQUAKE FOCAL MECHA ISMS A D THEIR RELATIO SHIP TO REGIO AL T TO I STR S 277 

N 

47 

N N 

51 

N 

59 

N 

Figure 136. Lower hemisphere, equal-area proj ections of . hallow M ount St. Helen. region earthquake focal 

mechani. ms- Conti nued 
278 ASSESS! G EARTHQ AKE HAZARD A D RED CI G RISK I TH E PACIFIC ORTHWEST 

N 

N 

N N 

N 

Figure 136. Lower hemi phere, equal-area projection of hallow M ount St. Helens region eanhquake focal 

mechani ms ontinued 
WASH I GTO EARTHQUAKE FOCAL MECHA ISMS A D THEIR RELATIONSHIP TO REGIO AL T CTO I STRESS 279 

N 

Figure 136. Lower hemi. phere. equ al-area projections f ll o~ Mount St. Helens region eanhquake foca l 

mechani. ms- ontinued 
2 0 l G EART HQ A KE HAZARDS A D RED Cl G Rl KI N THE PA IFIC ORTHWE. T 

Table 21. Subcru tal (slab) Puget Sound region earthquake data. 

Event Date (year. 

number month, day) 

I 800204 

2 8004 16 

3 800608 

4 8008 16 

5 800906 

6 

7 

8 

9 

10 

I I 

12 

13 

15 

3 1 

62 

76 

9 1 

10 1 

105 

107 

116 

12 1 

125 

128 

130 

134 

136 

137 

14 I 

142 

152 

155 

16 1 

162 

168 

173 

175 

180 

187 

189 

190 

80 11 06 

80 11 30 

8 10 1 I I 

8 10704 

8 10705 

8 10722 

8 10804 

8 1082 1 

820 11 4 

8203 13 

82 11 0 1 

830205 

830425 

830708 

830802 

30828 

83 103 1 

840 105 

840223 

840328 

840-\08 

840604 

84062 1 

840708 

8408 12 

840902 

4 1121 

84 12 17 

50306 

503 18 

850-\26 

85052 1 

85052. 

850822 

85 111 5 

85 1202 

85 1204 

Latitude 

47 .39 

4 .19 

47 .97 

47 .39 

47.54 

47 .9 1 

47 .35 

47 .39 

47 .86 

47 .56 

47 .96 

47 .69 

47.62 

4 . 10 

47 .06 

47 .55 

46.67 

47 .2 

47 .76 

47 .66 

48 .00 

47.35 

47.44 

47 .65 

-\7 .33 

46.80 

46.29 

48 .30 

47 .57 

47 .73 

48 .75 

46.98 

47 .3 I 

48.90 

47 .37 

47 .3 I 

47 .66 

47 .67 

47 .67 

47 .5 1 

49.06 

48.86 

Longitude 

12 1.65 

122.90 

123. 10 

123 .26 

123.42 

123 .17 

123 .34 

123.47 

122.73 

123 .68 

123.43 

I 23. 13 

123 .67 

I 22.8 1 

122. 17 

123 .38 

123 .04 

123 .56 

123 .02 

122.87 

122.87 

123.29 

122.28 

123 .04 

I 23 .13 

I 22.49 

I 23 .04 

122.97 

122. I 

123 .02 

123 .20 

123.69 

122.9 1 

122.82 

122 .64 

I 22.4 

123 .22 

123 .3 1 

122.9 1 

123 .59 

123 .58 

122.87 

Depth 

(kilometers) 

96 .35 

50.10 

48.54 

43. 11 

46.16 

45 .56 

43.8 1 

40.58 

52. 11 

39.13 

44. 18 

43 .85 

40.52 

55.95 

72.53 

45 .6 1 

52.22 

38.3 1 

47 .12 

4 .48 

5 1.50 

43 .36 

38.77 

46.32 

42 .89 

67 .03 

52.60 

46.47 

46.69 

46.32 

57 .0 1 

35.67 

46.30 

66.48 

53.30 

57. 3 

47 .22 

47 .04 

50.27 

42. 13 

59.40 

59.14 

Mag- P ax is 

nitude Azimuth Plunge 

2.4 55 85 

3.8 258 6 

4.2 272 84 

2.3 228 55 

2.8 173 6 

2.9 

2.6 

1.8 

2. 

2.0 

2.3 

1.9 

2. 

2.7 

1.9 

2.5 

2.2 

1.9 

2.4 

2.3 

3.9 

4.3 

1.9 

2.2 

1.9 

3.3 

3.7 

2.0 

2.2 

2.2 

2.6 

2.8 

3.2 

1.8 

3.5 

2.5 

2.8 

2.0 

I. 

2.7 

2.3 

2.2 

20 1 

130 

90 

303 

55 

227 

0 

180 

254 

266 

197 

158 

99 

144 

254 

75 

3 16 

108 

11 7 

136 

286 

184 

132 

307 

I 12 

3 17 

2 I I 

236 

4 1 

276 

27 1 

11 3 

33 

244 

270 

173 

240 

34 

20 

7 

3 1 

27 

64 

7 1 

5 

65 

45 

74 

65 

31 

55 

85 

16 

75 

4 

48 

50 

20 

3 1 

7 

35 

0 

12 

36 

29 

21 

34 

12 

22 

6 

25 

76 

62 

60 

T axi, 

Azimuth Plunge 

236 5 

164 36 

15 I 

II I 17 

62 8 

106 

32 

209 

47 

21 6 

89 

123 

0 

74 

86 

40 

43 

7 

28 1 

74 

345 

134 

287 

16 

249 

44 

88 

225 

128 

22 

137 

326 

13 1 

145 

20 

170 

2 13 

125 

62 

90 

60 

22 

8 

20 

76 

22 

62 

20 

I I 

5 

25 

45 

15 

II 

4 

27 

5 

0 

15 

86 

10 

18 

53 

10 

2 1 

55 

43 

78 

30 

25 

32 

20 

42 

24 

21 

65 

14 

12 

24 

£lill1iul 

ALimuth Plunge 

326 40 

307 6 1 

99 44 

238 38 

175 4 1 

238 

17 1 

194 

88 

164 

205 

194 

90 

164 

356 

139 

159 

138 

332 

164 

I 19 

223 

19 

143 

299 

55 

22 1 

266 

35 

166 

47 

I 

272 

179 

63 

320 

253 

167 

336 

180 

179 

8 1 

6 1 

6 1 

39 

5 1 

19 

29 

37 

40 

20 

90 

3 1 

39 

66 

25 

40 

79 

30 

4 1 

50 

40 

36 

6 1 

70 

10 

6 1 

33 

40 

so 

5 1 

so 

52 

56 

7 1 

20 

3 1 

40 

25 

~ 

Azimuth Plunge 

146 50 

205 70 

29 1 47 

358 68 

3 15 56 

338 

8 1 

349 

353 

3 18 

346 

46 

270 

344 

176 

305 

295 

237 

208 

344 

2 I I 

44 

18 

255 

185 

173 

3 19 

0 

217 

58 

227 

267 

4 

275 

326 

2 14 

344 

26 1 

153 

0 

3 10 

305 

73 

90 

53 

84 

72 

67 

57 

so 

70 

90 

60 

60 

7 1 

75 

50 

79 

60 

49 

66 

7 1 

7 1 

76 

80 

80 

6 1 

57 

6 

88 

83 

8 1 

71 

9 

80 

70 

59 

61 

7 1 
WASH I GTO EARTHQ AKE FOCAL MECHA ISMS A D THEIR RELATION HIP TO REG IONAL TE T I STRE 2 I 

N 

N 

N N 

31 

N 

101 

Figure 137 (above and next two pages). Lower hemi sphere, equal-area proj ections of subcrus!al (s lab) Puge1 Sound 

earthqu ake focal mechani. ms. 
2 2 ASSE SI G EA RTHQUAKE HAZ RDS A D REDUCI G RI K I TH PA IF! ORTHWE T 

N N 

N 

N N N 

N N 

N 

Figure 137. Lower hemi sphere. equal-area proj ecti on of subcrustal (s lab) Puget Sound earthquake fo al 

mechanisms ontinued 
WASHINGTO EARTHQUAKE FOCAL MECHA ISMS A D THEIR RELATIO SH IP TO REGIO AL TE TO IC STRESS 283 

Figure 137. Lower hemisphere, equal-area projections of 

subcrustal (slab) Pugct Sound earthquake foca l 

mechanisms- Continued 

ESTIMATES OF SEISMIC SOURCE REGIONS FROM 

THE EARTHQUAKE DISTRIBUTION AND REGIONAL 

TECTONICS IN THE PACIFIC NORTHWEST 

By Cra ig S. Wea e r1 and Kaye M . hedlock2 

ABSTRACT 

The tec tonic and geologic setting of the Pacific orthwest i that of an acti ve ubducti on zone. Wes t of the Cascade Range, there are three di tinct earthquake ource 

region s: ( I) at the interface between the Juan de Fuca and 

orth America plate ; (2) within the subducting Juan de 

Fuca and Gorda plate : and (3) within the cru. t of the overlying orth Ameri ca plate. Ea t of the a cadc Range, earthquakes occur only in the North Ameri ca plate. The record of 

hi tori al seismicity in the region indica tes few earthquakes 

of magnitude 7 or greater; all of the c event arc thought to 

have been within the subducting part of the Juan de Fuca or 

Gorda pl ates, wi thin the cru t of the orth Ameri ca plate, or 

off northern California in the fl at-lying ection of the Gorda 

plate. Thi limited hi tori cal record i. used for nearl y all c timates of the earthquake hazard in the region. nfortunately. 

the di tribution of eismicity determined u ing the modern 

eismic network is not in accord wi th the location of man of 

the largc. t hi torica l earthquake . . 

The be. t undcrst od earthquake source zone is wi thin 

the subducting Juan de Fuca and Gorda plate . M o t of the 

hi. tori ca lly damaging earthqu ake of the Pac ific onhw t 

have been in thi. zone, and data from the c event ha . ervcd 

as the corner. tone for earthquake-hazard. ass . ment in the 

Pugct ound region. Generally. thi type of event is thought 

to be au cd by gra itational forces wi thin the ubducting 

pl ate here the dip increa c. from about I 0° to mor than 

25°. l ntcq rctation of the currently a ai lable data, including 

earthqu ake hypocenter. and gcophy ical imaging of the ubducting pl at . , indica te that the e earthquake. sh uld be 

ex pec ted along the stri ke of the entire subduction zone. 

Because of the known plate geometry. mos t of these events 

I .S. Gcol gical Survey. Gcophysic' Program A K-50. niversity of 

Washington. eau lc. W A 9 t95. 

2 .S. Geological Survey, Box 2 0-!6. M S 966. Federal Center. 

Denver. ·o 0225. 

are expected to be at depth. of 45- 60 km and t hav magnitude as lea. t as large a those already XJ ri cnccd in the 

region (magnitude 7+ ). Th rate of occurrence of the. c earthquake is unknown but. in Oregon and Washington . . ix 

events since 1870 had c. timatcd magnitud . of 6 or greater. 

With the excepti on of the M cndo ino triple juncti on 

area. no earthquake recorded b mod rn scismi net\ orks 

have been located on the intcrfac bctw en th Juan de Fu ·a 

and orth Amcri a pl ate , and in the hi story of · uropcan settlement in the Pac ific orthwcst. no known large nt can 

be a ciatcd with most of the intcrfa c. However, th late 

Holocene geologic record of ub. idcncc in coas tal intertidal 

mar. he. pr ide ev idence consistent with the oc urrcn c of 

great ubduction-r hted earthquakes. Th avai lab le gc -

logi record uggc. ts that the return period f r these events 

is irregular. varying from I 00 years to longer than 1,200 

years: the length of the coast int rprcted to have subsided 

during a particular event i .. ufficient to have gen rated 

earthquakes of magnitude or greater. om tudie have 

suggc. ted that the entire coa t from northern Cali fornia to 

central Briti h olumbia could sliJ in a single earthquake of 

magnitude 9 or greater. Even though there are many 

unknov n detail concerning intraplate earthquakes, the general under tanding of large- cale plate-tectonic processes 

allows a reliable timati on of the potential source zone for 

. ubdu ti n earthquakes. The ource zone for the e events i 

very great, including the entire coast from the deformation 

front offshore to about the coa tline. 

The sour e zone for cru tal carthquak i very poorl y 

known . The hi stori cal record includes event greater than 

magnitud 7 in central Vancouver Island. the orth a -

ca lcs of Washington, and off hore northern aliforni a. Geologic ma1 pin g ha found evidence of surface di splacement 

that are consi. tent' ith those ex pected to accompany a magnitude 7 or greater earthquake in the shallow cru st of the central Pugct ound bas in. ntil the tec tonic cau e of the c · 

earthquakes arc better undeuood, it i not pos ible to reliably determine the source zones for these cru tal e cnts. 

285 
2 6 ASSESS! G EARTHQUAKE HAZ RDS A 10 REDUCI G Rl 'K I THE PA IFIC ORTHWEST 

INTRODUCTION 

The first step in asse. sing earthqu ake hazard s and ri ·ks 

wi th in a region require. that the nature and distri bution of 

earthquake source b understood. During the pa t I 0 y ar , 

con iderable study in the Pacific orthwest ha been focu ed 

on determining the di tributi on of earthquake sources and on 

placing the e sources within the regional tectonic euing. 

Two examples illustrate how success ful the e studie ha e 

been in changi ng the as e . ment of earthquake hazard s in the 

Pacific orthwes t. Fir t, in 1975 , Hopper and others ( 1975) 

sugge ted that the Juan de Fuca pl ate might be attach d to the 

orth A meri ca pl ate (fig. 138), thereby making it unlikely 

that great thrust earthquake (magnitude 8 or larg r) wou ld 

occur on the interface between the two pl ate . . At pre ent, 

mo t earth cienti st routinely accept convergence between 

the two plates and, as a consequence, most further accept the 

interpretation that subduction-zone ea rthquake on thi pl ate 

interface will be at least magnitude 8.0 (for example, Heaton, 

1990); some . cienti t even argue for earthquakes greater 

than magnitude 9.0 (Heaton and Hartzell , 1987). The most 

compelling ev idence that has caused thi. change in earth -

quake-hazards a sessments is found in the intertidal marshes 

along the coast. At some sites in Will apa Bay on the Washington coas t, at leas t eight discrete epi sodes of subsidence of 

the intertidal marsh surface are recorded in the geologic secti on of the Ia t 4,000 year (Grant and other , 1989). Each 

epi ode f ubsidence i thought to have been the result of a 

ubduction-zone earthquake (Atwater. 1987); thus, subduction -zone earthquakes appear freq uentl y in the geologic 

record (see Atwater, thi s volume; and Rogers and others, thi s 

volume, for more di cus ion). 

As a second example of how source sllldies have 

changed earthqu ake-hazard assessments, Perkins and others 

( 1980) issued a revi ed e. timate of the max imum horizontal 

grou nd acceleration for Oregon and Wa hington in which 

the maxi mum expected earthquake magni wde was e timated 

for subregions of the two tate . In the southern Wa hington 

Cascade Range near M ount St. Helens, the estimate u ed for 

the max imum expected magnitude was 5.1. By 1985, the St. 

Helen sei mi c zone had been identified , and everal studies 

had concluded that a max imum-magnitude event of 6.8 

. hould be adopted for engineering-de ign purposes in the 

region of thi s zone. Both of these changes in the asse smem 

of earthquake hazards relied heavily on under tanding the 

regional tectoni c framework, w ith the second example 

nearl y completely dependent on eismicity tud ies. 

Because of the subduction-zone regime, there are three 

di stinct sources of earthqu ake in the Pacific orth west: ( I) 

cru stal earthquakes that occur within the overriding North 

Ameri ca plate, (2) intraplate earthquakes that occur within 

the subducting Juan de Fuca and Gorda pl ates, and (3) interplate arthquake. that are expected to occur at the in terface 

between the Ju an de Fuca (and Gorda) pl ate and the North 

A merica plate (subduction, or thrust, events). We t of the 

a cade Range, the eli . tribution of earthquake . ource type 

reflects a combination of the geometry of the ubducti ng 

Juan de Fuca and Gorda plates, cru tal tructure within the 

o erriding orth A meri ca pl ate, and tec tonic interaction 

among the North America, Pacific, and Juan de Fuca pl ates. 

East of the Ca cade Range, it i likely that the earthquake 

di tri bu tion refl ects the tectonic. and tructure of onl y the 

orth meri ca pl ate. 

Of the three ource type , cru stal earthquakes in the 

orth Ameri ca pl ate and event wi thin the ubdu Ling plate 

(we refer to the latter as intrapl ate events, or Benioff-zone 

earthquakes) have been the ba ·is of earthquake-hazard 

as essments for the Paci fie orth we t (for example, Algermissen, 1988). In Washington and Oregon, where the hi, -

tori cal record is thought to be complete in e the I 70' at 

magnitude 6 and greater (Ludwin and others, 199 1 ), there 

were two earthquake that certainl y occurred in the crust 

and six earthquakes that are either con idered or known to 

have oc urred wi thin the subducting plate. One of the 

anomalies of theCa cadia ubduction zone is that, wi th the 

excepti on of the M endocino triple junction (where the 

orth America. Pacific, and Gorda pl ate meet) (Oppenheimer and others, 1993), there are no large histori cal earthquakes that are thought to have had their source on the 

plate interface. In most subduction zone , it i thi s interface 

that produces the great (magnitude 8+) thrust events, li ke 

the earthquake that struck the Prince William Sound area of 

southern Alaska in 1964. Recentl y, effort have been made 

to incorporate at lea t the poss ibi lity of great thru t-zone 

earthqu ake. in the Pacific onhwest into a regional hazards analy is (A ig rmi ssen, 1988). 

Thi chapter focu se. on the ex tent of the three earthquake source region of the Cascad ia subduction zone. In 

defining the source regions, we have relied on recent compilati ons of earthquake cata logs for Oregon and Wa hington, 

studie of regional sei motectonics, inves ti gations of 

coa tal-marsh tratigraphy and determinations of the depth 

and dip of the Juan de Fuca and Gorda pl ates beneath the 

orth America plate. It is clear that intraplate earthquake , 

the mo t frequently ob erved large-magnitude (6+) events in 

the hi tori cal record of Oregon and Wa hington, are understood well enough that the source region expected to produce 

such event in the future can be speci fied with great confidence. De pi te uncertainty surrounding the detail s of how 

and when great subduction-zone thrust events may occur on 

the interface, there i clearl y a growing acceptance of the 

past occurrence of these events. As the general forces that 

produce these earthquakes are understood from comparati ve 

studie with other subduction zones, the potential source 

regions can be defined fai rl y accurately. Fin ally, becau e the 

causes of large-magnitude hi stori cal cru stal earthquakes 

remain obscure, the part. of Washington and Oregon that 

may be subjec t to their effects remain uncertain . A noted 

elsewhere (Rogers and others, thi s volume; Shedlock and 

Weaver, 1989), reducing the uncertainty surrounding the 
SEISM IC SO R E REGIO S FROM EA RTHQ AKE OJ TR IB TIO NO REGIO L r TO I , _ 7 

0 

0 131 0 

51 

0 

49 

47 

0 

45 

0 

43 

0 

41 

0 

39 

0 

JUAN DE FUCA 

PACIFIC 

OCEAN 

PACIFIC 

PLATE 

GORDA 

PLATE 

PLATE 

0 

0 

"U 

r 

:t> 

--l 

m 

0 0 

122 119 

BRITISH COLUMBIA 

6. 

491 993 

WASHINGTON 

OREGON 

116 

0 

)> 

I 

0 

66. NEVADA 

CALIFORNIA 

6 , 

Lassen / 

Peak @ @) 

200 Kil ometers 

0 

Figure 138. Locations of ea nhquakes in the Pac ific onhwe. t hav ing estimate I magnitudes greater than 6 from I 70 to 

1993. Small circle. are earthqu akes with magnitudes 6.0- 6.9; large cir les denote magnitudes greater th an 7.0. The yea r of 

occurrence is shown for eanhqu akes greater than magnitude 7 and for four events greater th an magnitude 6.0 discussed in 

the tex t. Triangles show Quaternar stratovolcanoes in the ascade Range. Parall el lines indi cate spreading ridges. 

occurrence of great subducti on-zone event and determining 

whether the urban enters in the Puget S und basin and the 

Will amcttc Valleyaswell a tho ceast of thcCas adc Rangc 

are ubject to magnitude 7, shallow (lc s than 20 km) cru. tal 

carthquak s will require significant in v stment: of time and 

effort for new experiments, research. and modeling. 

Thi s chapter differs from recent review arti cle on the 

earthquake distributi on in thi s region in that we give an 

overview of the eli. tribu tion in the en tire Juan de FucaNorth Ameri ca plate sy. tcm. Our plot of . cismi city combine th re icw of Ludwin and others ( 1991 ), who summari zed sci mtct ty 111 Oregon and Washington . a~d 

hrharnmcr ( 199 1) and Hill and other. ( 1991). who di ·-

cussed the sei micity of northcm and central California, 

respec ti ve ly. To complete our hi stori al perspecti ve, we 

combine the ca talog of earthqu ake greater than magnitude 
288 AS ES I G EA RTHQ AKE HAZ RD A D REDU I G RISK I TH E P IFI ORTHWE T 

6 for al ifomi a (Ell sworth . 1990) \ ith similar-sized c cnt. 

li. ted by Ludwin and other. ( 199 1) for Wa. hi ngton and 

Oregon and event. listed by Rogers ( 19 3a) for southern 

Briti h Columbia. The . cope of our rev iew broaden that of 

Wea er and other ( 1990). who discussed the cru. tal earthquake di. tri bution associated with the ascade Range from 

La en Peak, Cali f., to M ount Baker, Wa. h. 

A comment concerning the terminology used in thi. 

chapter. nfortunately for mo t readers. eismologi ts u c 

several magnitude calc to measure the strength of an 

earthquake at its . ource (magnitudes are di tinct from the 

earthquake- haking effec t. at specific itcs. which are meaurcd by intensity). The different magnitude cale refl ec t 

the fac t that m t scales arc appropri ate for onl y a small 

part of the wide range of po. siblc magnitude . Although we 

pccify the appropri ate magnitude . ca lc for parti cular earthquakes (local magnitude, M L; body-wave magnitude, mb; 

urfacc-" ave magnitude, Ms; moment magnitude. M w: or 

coda magnitude. M e ). to avoid confusion we have tried to 

deemphas ize the differences between the variou .. calcs and 

commonly u e onl y the term "magnitude.·· Generall y, magnitudes greater than 8 arc clcterminecl u ing seismic 

moment, magnitude. greater than about 5 but less than 8 

are calculated using either body or surface waves. magnitude le. s than about 5.5 u uall y arc determined using a 

local magnitude ( ometimes ca lled a W ad-Anderson magnitude, after the instrument used to generate the seismogram), and magnitudes less than about 4.5 tend to be 

alculated from the coda length of the event. M ore di scu -

ion of the magnitude . ca lcs and definition. of seismologic 

terms can be found in the glo sary to thi. volume. 

ACKNOWLEDGMENTS 

Thi chapter wa improved by the thoughtful comments 

of I van W ng, Garry Rogers. and Timothy Wal sh. Two of 

the editor. , Timoth y Wal. h and Alben Roger , . howecl 

unu. ual patience during re ision of the ori ginal manu cript. 

Barbara Hillier helped refine the language throughout the 

chapter. The fi gures were clraftccl by Karen Meagher of the 

.. Geological Survey in Scattl . 

EARTHQUAKE DISTRIBUTION 

One of the . ignificant problem in assc. sing the earthquake hazard. of the Pacifi c Northwest is the eli corclance 

bet ween the hi tori ca l earthquake record and the record 

available fr m m clem . ci. mic in trumentation (. tarting 

about 1950). The histori cal record i. essential b cau e in 

Washington and Oregon. onl y the 1965 event near Seaule 

exceeded magnitude 6. ince the Worl d Wiele Standard Seismic ctwork bega n recording in 1962: . ince then, the large t 

event wa. the eptember 1993 Kl amath Fall earthquake of 

magnitude 6.0 in Oregon. Thu s, one g al of any earthquakehazard. as c . . mcnt program mu l be to pi a c the hi. tori cal, 

larger magnitude anhquakcs in the tee t ni framework. 

Thi s ta k is difficult in we tern Oregon and Washington 

becau e of the lac k of known surfa c fault with Pl eistocene 

to Holocene offset (Roger. and oth rs. thi. volume). 

EARTHQUAKES GREATER THAN 

MAGNITUDE 6 

Earthquakes that occulTed from I 70 through September 1993 that arc e timat clto be larger than magnitude 6 arc 

restricted to a relatively small area of the Pacifi c 1orthwcst. 

In mapping the earthquake (fi g. 13 ). we did not include 

ent · along the offshore ridge system other than those ne·tr 

the Gorda plate. The likelihood that all e cnts greater than 

magnitude 6 will be felt or noted dec rca c. w ith increa. ing 

di stance from the coast to the offshore ridge. and fracture 

zones. For the northwc tern nitcd States, the cismic catalog arc thought to be complete for magnitude 6 carthquak 

since I 70 (Ell sworth, 1990; Lu I win and other.. 199 1 ): in 

Briti h Columbia. the ca talog of magniwclc 6 event i · complete from 1899 (G . . Rogers. Gco l gi al Survey of Canada, 

wriuen commun .. 1990). 

t the outhcrn end of the Juan de Fuca- orth America 

pl ate :ystem, these larger earthquakes ha e occurred b th 

onshore and off. hore in northern ali fornia and ca. t of La -

sen Peak in Nevada. Large earthquake are found in the hi s-

! ri ca! record from north western Wa hington and central 

Vancouver Island but arc notabl y ab cnt in nearl y all of Oregon (fi g. 138). Within thi eli tributi on, the mos t important 

events that must be incorporated into hazard a . e .. ments arc 

tho. e earthquake grea ter th an magnitude 7 (nine events 

total, excluding the two c cnt in cvada) and a few events 

of magnitude lc than 7. 

The 1872 orth ascades earthquake in north-central 

W a hington i generall y con iderecl the largest hi. tori cal 

earthquake in the Pacific orthwe t (Milne, 1956). with an 

e timated felt-area magnitude of 7.4 (M alone and Bor. 

1979). It wa felt over an ar a of more than I ,0 I 0,000 km2, 

including Wa hington. northern and central Oregon. northern Idaho, wes tern M ontana, and southern Briti. h Columbi a. 

The earthquake was followed by an ex ten i vc after hoc k 

sequence (M i Inc. 1956). Study of damage reports sugge ts 

that the maximum intensity cxceeclecl V II and may have been 

as high as IX on the M odified M ercalli inten ity (MMI) scale 

(Milne, 1956). Because the available hi tori ca l data does not 

unequi vocall y allow the location and depth of the earthquake 

to be determined. both remain . ubjccts of eli cu . ion. The 

location shown in fi gure 13 was determined by M alone and 

Bor ( 1979) ba. eel on the intensity pattern ; these authors also 

ummari zed other possible locati on of the event. Hopper 

and others ( 1982) suggc. ted a shallow cru stal depth based 

upon the intensity report s and the extensive aftershock 
SEISM IC OURCE REGION FROM EARTHQ AKE Dl TRIB TI0 1 A D REGIO L TE TO I S 2R9 

. equence. A hal low d pth for the 1872 earthqu ak is consistent with the depth determined from all instrumentally 

located earthquakes (since 1970) near the epicenter propo. ed 

by Malone and Bor ( 1979); all ca lculated hypocentral depth 

are hall ower than 25 km. 

The 1949 . outhcrn Puget Sound earthquake (M s 7.1 ) is 

one of five earthquakes (the others were in 1909 1939, 1946. 

and 1965) of magnitude 6 or greater known to have occurred 

in the Puget Sound bas in (fi g. 13 ). The 1949 and 1965 (mb 

6.5) earthquake caused . ignificant damage in the Puget 

Sound region (Murphy and Ulri ch, 195 1; uttli. 1952: 

Algenni sen and other . 1965) and mo t of the major earthquake damage documented in the Pac ific orthwest. The. c 

two events had instrumentall y determined hypocentral 

depths of 54 km ( 1949) and 60 km ( 1965) (Baker and Langton, 1987; Algcnni s en and others, 1965). o after hocks 

were felt or recorded after the 1949 earthquake; instrument. 

avai lable at the time wou ld ha e detected events larger than 

magnitude 4.5 (M L)· Simil arly , no aftershocks were felt fol -

lowing the 1965 earthquake, and an exam ination of seismogram recorded on tat ions operating within the region fail ed 

to identify any aftershocks greater than magnitu le 2.5 (Me)· 

Becau e the e large earthquake. did not produce felt afterhock , Roger ( 19 3a) noted that lack of after hock. is one 

pos ible way of estimating whether hi torical earthquakes 

were within the cru t (where felt aftersho ks wou ld be 

xpected) or within the ubducting plate. 

Large earthquake occurred in central Vancou er I land 

in 191 and 1946 (fi g. 1"'8). The 191 8 event wa about magnitude 7 and had an exten ive aftershock sequen e (Ca idy 

and other , 1988). The 1946 event of magnitude 7.3 (M s) 

occurred in the middle to lower cru st (about 15- 25 km deep) 

and had almo. t no aftershock (Roger and Ha egawa. 1978). 

Be au. e of the spar c population of central Vancou er I land 

during the c earthquakes, the va lue of the damag wa not 

large. As thcs events were within the cru t of the nh 

Ameri ca pl ate ( assidy and other.. 1988). they rai c crious 

questi on a. to the pos ibility of similar future c ent in the 

h av ily popul ated urban areas in the Pac ific 1 orthwc. t. 

Larg earthquake. off the northern alifornia coas t 

have been frequent du ring the Ia. t 120 year. (fi g. 13 ) an I 

have caused structural damage to onshore facilitic . Five 

event. are e. timatcd to have been of magnitud 7 or greater; 

the largest offshore event of felt-area magnitude 7.3 

occurred in 1923. and shaking from thi s shock damaged 

ch imneys in the small towns north of Cape Mendocino (Toppozada and oth rs, 19 I ; Ell sworth. 1990). In 1980, a magnitude 7.0 (Ms ) event occurred within the Gorda plate ju t 

west of the deformation front (fi g. 138). A I though th re was 

relatively little damage to mo t tructure . a highway overpass with five pc pi c collapsed: fortunately no one was 

injured (S imon, 198 1 ). 

The most damaging earthquake in northern aliforni a 

through September 199 as the April _5, 1992. ap 

Mend cino event of magnitude 7. 1 (M s). Damage from the 

earthquake has been c. timatcd nt between _, 48 million an 1 

$66 million. but thcr w rc no deaths. lth h the o cu rrenee of magnitude 7 vents i. not unusual in th Cape 

endoc ino area (fig. 13 ). thi. c ent i. parti ·ularl not ' -

worthy be aus it app ar: to provid dire t nvidcn ·c of 

faulting on the 1 lat interface and it g nerated 't small 

nami alono the we. tern coast of orth mcrica ( pp'nhcimcr and oth rs. 1993). Howe cr. because thi. earthquake 

occurred at the M endocino triple jun ·tion. the t ctonic S'tting i complex. and onsiderabl more . tud is required 

before the impli ati on. of thi s s qucncc ·an b integrate I 

into regi nal ea rthquake-hazard s assessment s. 

Three other not·1ble earthqu ake. arc in the hi storica l 

record. An event cs timat d to ha e a felt -area magnitude of 

at least 6.75 (Toppozada and oth rs. 19 I) o cuiT 1 in I , 73 

ncar the Oregon- aliforni a border (fi n. 138). The ea nhquak • 

wa. felt over most of wes tern Or gon and as far south as a -

ramento. alif., but was felt with MMI VIII onl in a small 

area ncar the c. timatcd epicenter (Toppozada and others. 

19 I). Thcr is no mention in on temporary newspaper 

accoun ts of felt after. hocks accompanyino the main shock. 

Thi suggc. t. either that the event wa: : ufficicntl y far ofTshor that no after. hocks were felt or that it \ as deep. possibly within the subducting Gorda plate where after hock. arc 

not neccs. ari! y expected. The . ccond c cnt is the 1936 Milton-Frcewat r arthquakc, the largest known event in cast rn 

Wa. hingt n. M ax imum intensity wa. reported a. MMI VI I 

(Coffm an and other . . 1982), and the mao nitudc wa. ca lculated by Gutenberg and Ri chter ( 1954) as 5.75 (M ·). 1

oson 

and thcr. ( 19 ) estim ated a magnitude of6.4 for thi s arthquake ba. cd on the felt ar a. B cau e all ca rthquak s lo <Heel 

since 1970 in eas tern Washington CC LIIT d in th cru st. the 

1936 event is also assu med t be cru. tal. umerous aft er-

. hock. were felt. The 1954 Eureka event c urred further 

inland in northern California than mos t oth r large earthquake . That ea rthquake was c. timated to be of magnitude 

6.5 (Ell sworth, 1990): one person wa. ki lied, and there was 

considerable nonstructural damage. 

The greatest earthquake lo. ses in the Pacific orthwcst 

were cau ed by the 1949 and 1965 event ight people were 

killed in the 1949 earthquake ( !rich, 1949). and six died in 

the 1965 cv nt (Ai gcrm isscn and other ·. 1965). In terms of 

1984 dollar .. 150 million damag occurred in 1949: the 

196 - event cau. cd an e timatcd 50 million damage (I o ·on 

and other . . 1988). A. noted above, the 1992 ape M cndoino earthquake cau. eel between 48 million and . 66 million 

damage. In 1993. two earthquake equence. in Oregon did 

con. iderablc damage. magni tude 5.6 event occurred in 

March 1993 about 40 km outheast of Portl and. De. pitc 

being a moderate- magnitude earthquake in a rc!ati el 

lightly pOJ ulated area along the Cascade Range foothill. , 

thi s earthqu ake . till caused more than $28 million damage 

(M adin and others. 1993). The sc one! sequence took pl ace 

in Se1 temb r 1993 near Kl amath Fall in southern Oregon. 

Two events of magni tude 5. and 6.0 occurred about three 
290 AS ESSI EARTHQ AKE HAZARDS NO RED Cl G RISK I THE P 'IFI ORTHWE T 

hours apan; two people died from shaking effects. and pr -

liminary c. timatc. of damage exceeded S5 million. Most of 

the damage was to older, unrcinforccd-masonry buildings in 

the dowmown . ccti on of Klamath Falls. 

The effect. of the 1949 and 1965 earthquake arc currently u. cd as the basi. for engineering design. emcrgencyrcsp ns planning, and damage and loss e. tim atcs in the 

Pugct ou nd region (Hopper and others, 1975). incc Hopper and other.' ( 1975) report wa i .. ucd. the population of 

the Pugct Sound region has i ncrcasc I by more than 600.000, 

and many new buildings have been con tructed. More 

important, the 1975 . tudy considered only the effects of d cp 

eanh 1uakc and did not consider the effects of ei ther a hallow crustal earthquake (l ike the I 72 event) or a great megathrust earthquake. Thu. , there is wide agreement by 

scient ist. that an updated study of ea rthquake sou rces. the 

hazard. po. cd by these . ources. and rev ised esti mate. of 

earthquake lo sc arc urgently needed in the Pac ific onhwc l. Perhaps the 1993 earthquakes in Oregon wi ll convince 

residents of Oregon and Wa. hington of the need to update 

earthqu ake hazards and ri sk assc .. ment in the region . 

SUMMARY OF INSTRUMENTAL SEISMICITY 

Although a few seismic station. ha e operated in the 

Pacific orthwc. t since 1900 (Rogers, 19 3a; Ludwin and 

others, 1991 ), 1960 is common ly as. Lllncd to be the beginning of the modern instrumental-recording period for the 

region. By 1960, the distribution of seismi c stations and the 

population density facilitated the r porting and lo ation of 

earthquakes greater than magnitude 4 o that . incc then, for 

Wa ·hington and Oregon, the earthquake ca talog is considered com pl ete for even ts greater than magnitu lc 4.0 (ML) 

(Ludwin and others. 199 1 ): in California. the cata log has 

been complete at thi magnitude level since at least 1960 

( hrham mer. 199 1 ). Since 1960. the number of sci. mic stati ons operating in the Pac ific orthwes t has greatly 

increased . o that by 1990, much of the region wa. routinely 

monitored for earthquake acti ity lc .. than magnitude 2.5 

(Me). Detail. of the evolution of the sci. mic network can be 

found elsewhere (for example. Ludwin and others, 1991: 

Weaver and others. 1982. 1990: Rogers. 19 3b; Eaton. 

1989). Th di. tribution of short-peri od ( I Hz) cismic stations operating in late 1993 is . hown in figure 139. M o t of 

the stati ons in British Columbia arc operated by the Geologica l urvcy of anada, stati ons in Oregon and Washington 

arc nearl y all operated by the ni ver. it y of Washington, and 

most of the stations in ali fornia arc part of the .S. Geologica l Survey net work. 

For a plate-wide per. pcctivc of instrumcmally recorded 

. cism icity, we se lec ted well-located earthquake that 

oc urrcd from 1980 through Apri I 1990 that were greater 

than magnitud 2.0 (Me). and that met the following stati tica l criteri a: calculated hypoccntral standard error of le s 

than± km. at least . ix P' aves used in the olutions, and a 

root mean sq uare of the tra cltimc rc. idual les than 0.35 . 

M ost hypocenter . olutions considerably xceed these criteria. Because of the ignificant earthquakes in 1992 and 1993. 

we added all located events greater than magnitude 5 from 

April 1990 through cptcmber 1993 to our initial catalog 

selection .. For Washington and Oregon, ou r hypocentral data 

arc from the ni ersity of Wa hington earthquake catalog, 

and for California. the data i. from ca talogs maintained by the 

.S. Geological Survey. Hypocentral cata logs compiled by 

the Geological Survey f Canada for British Columbia are 

incorporated into the Univer ity of Wa hington catalog. 

In plotting the ci mi di. tribution of th combined catalog (fig . 140 and 14 1 ), we subdivided our data into two 

depth ranges, 0-30 km and deeper than 30 km. Earthquake. 

deep r than 30 km are nearl y all within the subducting Juan 

de Fuca plate system (fi g. 140). Earthquak s shallower than 

30 km (fig. 14 1) are within the orth America plate in Wa. hington or Oregon; howe er. along the northern California 

coa. t, most of th hallow ei micity i. probably within the 

subducting Gorda plate (Hi II and other , 199 1; a ton. 19 9). 

Inland from the California coas t, th shallow earthquakes arc 

within the orth merica plate. 

It is clear that the distri bution orb th the shallow and 

deep earthquakes is not uniform aero., the Pacific onhwc l. The deep event arc concentrated beneath northwestern Washington and northwc tern aliforni a, with a spar. cr 

eli. tribution of event. beneath outhwc. tern Wa. hington 

and northern Oregon (fig. 140). The absence of deep events 

beneath the southwes tern Oregon coa. t may rc ult in pan 

from a lack or sci mic stati ons (compare fig. 140 wilh fig. 

139); however. a. noted earlier, earthquakes greater than 

magnitude 4 would have been instrumentall y recorded 

. incc about 1960. Events of thi magnitude would have 

been felt along the Oregon coa. t throughout the 20th century . . o there i. little doubt that at the magnitude 4 level. 

thi. part of the ubducting Juan de Fuca and Gorda plate. is 

sei mically quiet. 

The di tribution of cru. tal earthquakes is likewise not 

uniform acro. s the Pacific orthwest (fig. 14 1). Weaver 

and other ( 1990) used the change in the crustal sei micit pattem to divide the ascade Range and adjacent areas 

into four cgments; in thi chapter. we expand those segment. westward to the coa t. ln the northcrnmo t segment. 

from M ount Baker to Mount Rainier, nearl y all of the welllocated crustal earthquakes arc confined to the region 

between the eastem Olympic M ountai n and the Quaternary Cascade Range stratovolcanoe ; there were few 

events ea. t of the stratovolcanoc . The second . egmcnt 

defined by Weaver and other ( 1990), from Mount Rainier 

to Mount Hood, is the mo t seismically acti ve pan of the 

cru st. In southern Washington , the St. Helens sei mi c zone 

(S HZ) i. a parti cularly prominent north-northwest-. triking 

alignment of earthquakes. Al though there is a hiatus of 

activi ty cast of the SHZ, in general, . cismicity con tinues in 
SEISM I SO RCE REGIO FROM EARTHQ AKE DI TRIB UTIO A DR ·GIO L T TO 

0 

0 

49 

47 

0 

45 

0 

43 

0 

41 

0 

39 

0 (J 

~ cc 

J 

i! 

if 

< § 

..., JUAN DE FUCA 

PACIFIC 

OCEAN 

PACIFIC 

PLATE 

PLATE 

0 

0 

~ 

~ 

~~ 

~ 

~ 

.. ~ 

.. 

0 0 

122 119 

BRITISH COLUMBIA 

.. 

.. .. WASHINGTON 

~ ~ ~ 

'\ ~ 

~ 

~ .. .. ~ ~ 

~ .... {~ ~ ~ ~ ~ ~ .. ~ .... 

~ 

.. ~ 

.. .. ~ 

.. .. .. .. ~ .. ~ ~ .. .. .. .. .. 

~ .. ~ ~· .. OREGON ~ 

~ .. 

- ~ ~ 

~ .. ~. ~ 

CALIFORNIA 

.. .. .. ~­

0 

116 

0 

)> 

:r: 

0 

.. 

~ 

.. .. .. ~ 

.. ~ ~ NEVADA ~ 

~ 

~ 

.. .. ~ 

.. ~ .. ...... .. 

200 Kilomerers 

~ 

.. 1 ~ 

.. 

~ 

Figure 139. Regional short-period seismic tat ions (tri angles) operating in the Pacific orth\ est and south we ·tern British 

olumbia. MF , Mendo ino fracture zone: SAF. an Andrea fau lt. 

a broad zone from the area immediately we t of the SHZ 

into southeastern Wa hington. The third cgmcnt, from 

south of M ount Hood to ju. t north of M ount Shasta, is 

ei micall y very quiet except for the 1993 earthquakes ea t 

of the Cascade Range volcanoes ncar Kl amath Fall s. Thi 

egment has not been monitored continuou ly. but very 

few earthquake were ob. ervcd during the two year 

( 1980- 82) of continuous OJ rati on of a 32-station network 

or since stations were reinstalled in the central Oregon 

Ca cade Range in 1987 (Ludwin and others, 199 1 ). There 

is a marked increa. e in earthquake acti vity in the fourth 

. cgmcnt. between M ount Sha. ta and La. sen Peak and also 

along the northern aliforni a CO<t. t. 

CR STAL THICKNESS AND 

PLATE GEOMETRY 

The composition of the cru t of the Pacific Northwc. t 

has been inve ti 0 ated u. ing geologic and geophysical methods, but there arc few reversed. high-rc. elution refracti on or 

wide-angle reflection profiles for the region. Mooney and 
292 AS ES I 1G EART HQ KE HAZ RDS A D RED I G RISK I T HE P IFI ORTHWE T 

0 

0 

49 

0 

47 

0 JUAN DE FUCA 

45 PLATE 

0 

43 PACIFIC 

41 

0 

39 

0 

OCEAN 

PACIFIC 

PLATE 

0 

0 

""0 

r 

l> 

-I 

m 

0 

1 19 0 11 6 122 

BRITISH COLUMBIA 

WASHINGTON 

0 

~ 

I 

0 

OREGON 

NEVADA 

200 Kilomerers 

0 

Figure 140. Di stribution of earthquakes (dots) deeper th an 30 km in the Pacific orth west from 1980 to September 1993. 

The heavy lines show the approx imate eas tern mo. t extent of earthquake. at a depth of 60 km (Weaver and Baker. 19 ). 

Triangles show Quaternary stratovolcanoe in the Cascade Range. M F , Mendocino fracture 1.one; AF, an Andreas fault. 

Weaver ( 1989) . ummari zed the ex isting studies with a contour map of estimated cru stal thi ck ne. s beneath Washington. 

regon. and northem Cali forni a. sing thi s map as a working hypothesis for the configurati on or the M oho ( ee the 

glossary). two noteworth y fea tures can be seen. The first is 

the pronounced eas tward increase in cru stal th ick ness from 

16 km at the continental margin 10 about 40 km beneath the 

wes tern !'l ank of the ascadc Range. Grav it y modeling along 

two profil es in Oregon and interpreta ti ons of elec tri cal and 

magneti c data coll ec ted by the Elec tr magneti c Sounding of 

the Lith osphere and Beyond (EMSLAB) ex periment 

( MSLA B Group. 1988) along a profile perpendicular to the 

northern Oregon coas t arc consistent with cru . tal thi ckening. 

The second maj or feature i. the pre ence of thi ck cru. t 

beneath the Cascade Range. the Puget Sound bas in. and the 

olumbia Pl ateau. Cru . tal thi cknc s is es tim ated to be at 

leas t 38 km over thi. entire region and locall y reache 46 km 

in the ·outhcrn Oregon Ca. cad Range. There i a gradual 
SEISM IC SO RCE REGIO 1S FROME RTHQUA KE DI TRIB TIO A D REG IO AL TECTO 'IC. 293 

0 131 

51 

0 

49 

0 

0 0 

47 

45 

0 

43 

0 

41 

0 

39 

0 JUAN DE FUCA 

.. PLATE 

PACIFIC 

PLATE 

-

0 

?-

0 

0 

'"'0 

r 

l> 

~ 

m o (f) 

Q) 

Ol 

c 

C"tl 

cr 

0 

. • .. 

0 

119 

OREGON 

200 Kilometer 

116 

0 

)> 

I 

0 

0 

NEVADA 

. 

' 

.. ol,c, oo 

.. 

Figure 141. Distribution of earthqu akes (dot.) sha llower than 30 km in the Pac ific nhwc~t from 19 0 to September 

1993. SHZ is the t. Helens :eismic w ne. Triangles show Quatern ar stratovolcanoes in the Cascade Range. B. Mount Baker: 

R. oun t Rainier: , Mount t. Helens: H. Mou nt Hood: H, Mount Shasta: L. Lassen Peak: OM. Olympic ~ountains: FS. 

Mendocino frac ture zone: SAF. San Andreas fault. 

eastward thinning of the crust beneath southeas tern Or gon 

and northeastern ali forn ia. The M oho shallow. at about a 

36- km depth beneath most of n rthcastcrn Wa. hin gton (Potter and other , 19 6). 

The geometry of the . ubducting Juan de Fuca and 

Gorda pl ates has been part ly in ferred from the location. of 

earthquakes occurring wi thi n the plates; com monl y. these 

event. arc referred to as Benioff-zone earthquakes. In the 

hi stori ca l record of large-magnitude earthquakes . it i. these 

c cnt. that arc most frequentl y observed. Di sregarding the 

concentration of earthquake off ape M endocino. sin c 

1870 at leas t . ix large earthquake · arc either known or 

thought to have been wi thin the subductin g plate. These 

events occurred in I 73 along the coas t ncar the OregonCali fornia border and in 1909. 1939. 1946, 1949. and 1965' 

within the Pu gct Sound basin (fig. 142). The di. tributi on of 

intra1 late earthquake hypo enters (largely those shown in 

fig. 140) ind icate that the . ubducting Juan de Fuca plate 
294 G EARTHQUA KE HAZA RDS D RED Cl G RI SK I THE PACIFI ORTHWEST 

0 0 

116 

909 e 1872 

San Juan 

47 

0 

Spokane 

WASHINGTON 0 

)> 

0 

45 

0 

43 

Portland 

Eugene 

1993 

OREGON 

:r: 

0 

1873 •o Kl amath Falls 

CALIFORN IA NEVADA 

0 200 Kilometers 

Figure 142. Locati ons of major ea rthqu akes in Oregon and Wa hington. Small circles are earthqu akes 

with magnitudes estimated between 6.0 and 6.9; large circles are earthquakes with magnitudes e timated 

as greater than 7. The 1872, 1936, and 1993 earthqu ake were crustal, whereas all others are either kn own 

or thought to have been withi n the subducting plate . y tern . 

arch upward beneath the southern and central Puget Sound 

ba. in ; beneath outhwes tern Washington, the pl ate dips to 

the eas t-southeas t, changing to a northeast dip beneath northwes t Washington (Weaver and Baker, 19 ). The structure 

of the arch can be seen in fi gure 140, where we have indicated the eas tern ex tent of the location of hypocenters at 60-

km depth (from Weaver and Baker, 1988). A lthough much 

of the shallow plate geometry beneath Oregon is either 

p orly resolved or unknown. it appears that the average dip 

of the pl ate (between the deformation front and the po ition 

where the plate reaches 60 km depth) mu st be greater in 

northern Oregon than in Washington. 

In Washington and northern Califomia, there is general 

agreement between areas of the thicke t cru t (greater than 

38 km) and areas of crustal seismi city in the orth Ameri ca 

pl ate. !ea rl y, however, there are area where thi a sociati on does not hold, parti cularl y from the Oregon Ca cade 

Range eastward- recall that the shallow seismicity mapped 
SEISM IC SOURCE R · GIO S FROM EARTHQ A KE Dl TRIB TIO A D REGI0 1 AL TE TO I 295 

off the northern Cali forni a coa t is within the Gorda plate. 

Thu . the egmentati on of the cru tal seismicity pal!ern i. 

probably not directl y related to cru stal thi kne s. nfortunately, the structure of th crust i too poorly known in thi. 

region to determine if the composition of the middle to upper 

cru t may pl ay a role in the di tribution of cru stal seismicity. 

The segmentation of the cru stal-seism icity paucrn also 

doe not match the variation of the geometry of the Juan de 

Fuca pl ate. The crustal ei mi city in the Puget Sound area 

lie above the arch in the Juan de Fuca plate. yet relat ively 

sharp vari ati ons in the distribution of the earthquakes ugge t the importance of unknown cru tal tructure rather than 

variati on of the subducting plate geometry. Guffan ti and 

W ea er ( 1988) noted that the high- cismicity segment, 

between M ount Rainier and M ount Hood. wa on the southea tern edge of the plate arch, wherca. the ei mica ll y quiet 

segment in Oregon was inland from the section of the Juan 

de Fuca pl ate that ha an increase in pl ate dip. However, in 

sugge ting these a. sociati on. , Guffant i and Weaver did not 

incorporate the hi stori cal sci micity. In pa rti cular. when the 

1872 earthquake (fi g. 142) is considered, it is clear that ei -

mic-moment release in north-central Washington dominates 

all of the cru tal ci mici ty south of the anad ian border 

(Shedlock and other . 19 9). 

Thi s Ia t observation point to an im portant difference 

between seismici ty in Oreoon and Wa hington and that 

throughou t Cali forni a (including the . ubducti on regime). In 

Oregon and Wa hington, sei micity shows a temporal variati on on the scale of at least a few decade (Lud win and others, 199 1) whereas in Californi a, Hill and others ( 199 1) 

noted that a few years of microca nhquakc monitoring coupled with known mapped fault prov ides a good rcprcs ntation of the long-term (clecaclcs t a few centurie. ) sei. micity 

pauern . The 1993 Kl amath Fall. earthquake . . occurring in 

an area lac king any ignificant known hi. tori cal . ei micity 

(Ludwin and other.. 199 1 ), ha e served as the mo. t rec nt 

reminder of the l imitati on. fthcearthquakc ca talog in Oregon and Washington with r . peel to under tandino sei mic 

ource zone. and earthquake hazard . . 

ESTIMATES OF THE 

SOURCE REGIONS 

PROBABLEZONEFORINTRAPLATE 

EARTHQUA KES 

The geometry of the subducting plate (shown by the 60-

km-depth line in fi g. 140) illustrate. how large earthquake. 

in the hi stori a! record ( 1949. 1965) can be related directl y 

to the plate configurati on (W aver and Baker. 1988). The Tax i from the fo al mechani sm calcul ated by Baker and 

Langston ( 19 7) for the 1949 outh Pugct und earthquake 

of magnitude 7.1 (M ) is oriented to the east-southeas t. and 

the 20° plunge of the T-ax is was shO\· n b Weav r and 

Baker ( 19 ) to ao r c well' ith th plate dip angl ' determined from the earthquake hypocenter.. Th ~.:, Weaver 

and Baker ( 1988) concluded that the 1949 carthquak' 

re ultecl from down-d ip ten. ional force. ' ithin th subducting Juan de Fu a pl ate, an intcrpr tati on con. ist nt ' ith 

ob ervations for many earthquakes in thi. depth range in 

other . ubducti on zones (I sa ks an I M lnar, 197 1 ). Roo rs 

( 1983a) reached a similar onclusion con, rning the for c. 

rcspon. ible for the 1965 south caul ' ea rthquak' and a 

. mailer event of magnitude 5. 1 (111 b) in 1976 otT the southeas tern end of ancou er I ·land . Both event s were at a depth 

of about 60 km . and focal me han isms ca l ·ulat 'd for both 

earthquake. indi ca ted norm al faulting with theTa · 'S strik -

ing northca. t and plunging dO\ n clip (Rogers, 1983a). 

Ba cd on the agr cment bet\ cen the dip of the Juan de 

Fuca plat as inferred from earthquake hypocenters d ' !ermined from the modern seismooraphic network and th ' clip 

of the T axe. calculated for th laro r ma,.,nitude hi stori ·al 

earthquakes, we can confident ly predict the intraplate earthquake ourcc r gion forthccntir platc(fi g. 143). W ' ·pet 

that fu ture large-magnitude (about 7) intraplate earthquakes 

wi ll occur wi th in the Juan de Fuca plat sy. tcm in the depth 

range of the 1949 and 1965 e cnts. A lthough the depths of 

these event. arc consid red to be well knO\ n. we have chosen to bracket our source region at a shallower depth . n 

examination of the Universi ty of Wa. hington s i. mie catalog for the years 1970 through 1990 hows that al l of the 

intraplate earthqu akes greater than magnitude 4 were b lo> 

45 km and that n nc hav been located de per th an the 1976 

event (60 km). Therefore. we have used the depth range of 

45-60 km for our es timate of the probable . ou rce region for 

intrap late event.. 

We cmpha ize that thi probabl e source region reprecnt the l ikely areal ex tent wi thin' hich an earth 1uakc may 

occur: the ac tual dimensions of the fau lt area a. ociated with 

an earthquake of approx imate magnitude 7 would be 

expected to be similar to the 40- km -long fault length e timatcd for the 1949 earthquake (Baker and Langston, 1987). 

The queried area in southern Oregon (fi g. 143) represent. the 

region of unknown plate geometry where no intraplate earthquake have been lo ated either becau c any events that did 

occur were not large enough to be detected by the ex i ling 

ci mi network or becaus no event_ ha c occurred. We 

note that ex pan i n of the cx i. ting se ismic network (fi g. 139) 

in s uthcrn Oregon will gr atl y help to resol ve the longtanding questi on concerning whether thi s pan f the Juan de 

Fu a plate i currentl y trul y a eismic. In northern al ifornia. 

Benioff-zone earthquakes also allow the pl ate depth to be 

e ti matcd from the ubducti on-z ne trench eastward to the 

western edge of the Cascade Range (Cockerham, 19 4; 

W alter. 1986), so ' c have shown (fig. 143) the probable 

ource region here between the . amc depth limits as in 

W a. hington and northern Oregon. 
296 ASSESS ! G EARTHQ KE HAZA RD A D REDUCI G RISK I THE PA IFIC ORTHWEST 

0 131 

51 

0 

0 0 

49 

0 

47 

0 

45 

0 

43 

0 

41 

0 

39 

iJ.J 

8 

~ 

J t? -

0 

/f ? 

,.... 

< § 

':) JUAN DE FUCA 

PLATE 

e~l1.tc 0~ 

IY"~c 

PACIFIC 

OCEAN 

PAC IFIC 

PLATE 

'~'&IT. 

t: ..>ol1.t$ 

GORDA 

PLATE 

MFS 

Cape 

Mendoc ino 

0 

0 

0 

0 0 

122 119 

BRITISH COLUMBIA 

WASHINGTON 

Eugene 

OREGON 

11 6 

0 

)> 

I 

0 

CALIFORNIA NEVADA 

200 Kilometers 

0 

Figure 143. Probable source regions for imraplatc. down-dip tens ional earthquakes in the Pacific onhwest. Largemagniwde earthquakes (7- 7.5) are cxpe ·ted anywhere within the patterned areas. Questi on marks indicate areas where no 

earthqu ake. were located within the Juan de Fuca plate and where the plate geometry is uncertain. MF , M endoci no frac ture 

zone; AF, San Andreas fault. 

PO SIBLE 0 RCE REGIONS FOR 

UBDUCTION-ZONE EARTHQUA KES 

At nearl y all convergent margins around the world, 

magn itude 8+ earthquakes arc known to occur; the Cascadia subducti on zone i unu ual in that there is no such 

earthquake in the hi tori ca l record. However, recent studies 

of . ubduction-zone characteri sti cs (Heaton and Kanamori. 

1984: Heaton and Hart zell, 19 6), cru stal-. train accumulati on in Washington ( avage and others. 198 1 ). cru stal 

earthquake in southwes tern Washington (Weaver and 

Smith, 1983), and the strati graphy of coastal marshes along 

the Washington and Oregon coast (Atwater, 1987; A twater. thi s vo lume; Grant and others, 1989) have all either 

concluded directl y or inferred that the Cascad ia subduction 

zone . hould be regarded a capable of generating grea t 

interface earthquakes. 

The geologic ev idence from the coas tal marshes is 

parti cularl y compelling for the occurrence of past great 

subducti on-zone earthquakes. The evidence is multiple 
SEISMIC SO RCE REGIO S FROM EA RTHQUAKE DISTRIB TIO NO REG IO AL TE TO' IC 

buried peat hori zon. in many bays along the coa t (Atwater, thi volume). The preferred interpretati on i. that each 

buried peat layer repre ent a previou urface oi I hori zon 

that wa uddenly ubmerged during a great earthquake. 

Subsequently, intertidal deposit accumulated on the submerged soils until the deposits reached a le el allowing the 

development of a new mar h surface. At ome itc in Willapa Bay, W ash., fine-grained and is deposited directl y on 

the ubmerged oil. . Atwater ( 1987: thi s volume) and 

Grant and other ( 1989) interpreted these ands a ha ing 

been deposited by t unamis, and becau e they lie directl y 

on submerged peat layers, they ar further interpreted as 

hav ing been locall y generated by the amc earthquake that 

produced the ub idence. 

The marsh data indicate that the return peri od for great 

earthquake in the late Holocene has been irregular. varying 

from less than I 00 years to more than I ,200 year. (Atwater. 

thi · olume; Grant and others. 19 9). t many sites along the 

coas t from Humboldt Bay, Calif., to the Copali s Ri er. 

Wash., considerable field ev idence indica te that the Ia t 

event oc urred about 300 year ago; both carbon- 14 and treering dating techniques are in reasonable agreement on thi s 

date (Atwater, thi s volum ). Much unccnainty remain s concerning both the repeat time for the e great earthquakes and 

the pr bable magnitude. Both i ue. rcquir . howing ynchronous submergence of. pccific marsh hori zon. at multiple 

site on the coas t. Although, geologic techniques ma never 

demonstrate proof of ynchronous motion for a gi en marsh 

hori zon at many . ite ·, they may be able to rule out the po. ibility that the entire ubdu ti on zone from Cape M endoci no 

to central Vancouver Island lipped in a single earthquake. 

The area where great subducti on-zone earthquake 

occur is n tin doubt, in that the . hal low-dipping interface 

between the subducting pl ate and the overl y ing plate i. th 

fau lt plane. The i uc · concerning the ource area for subduction-zone earthquake relate lO whether or not the entire 

length of the a. cadia . ubducti on zone gcn rate. the. 

earthquakes and to the width of the fault zone that . lip . A 

minimum length wa. provided by Heaton and Kanamori 

( 1984 ). On the ba i. of regrc sion analysi of plate age and 

convergence rate again t the ob ervcd magnitude of interface earthquake in other subducti on zones. they suggc ted 

that in the ascadia . ubclucti on zone, an event of about magnitude .3 (Mw) would be expec ted given the pl ate age and 

convergence rate measured there. Such an earthquake might 

be expected to rupture a length of about 150- 200 km along 

the . ubduction zone. After comparing additional Jlatc 

parameters uch a offshore bathymetry and grav ity and the 

hi storical rate of moderate (magnitude 5.7+) earthqu akes. 

Heaton and Hart ze ll ( 19 6) suggested th at the entire length 

of the ascadia subduction-zone ( I , I 00 km ) from ap 

M endoc ino to central Vancouver I. Janel might rupture in one 

great event of magnitude 9 or grea ter (Mw). 

A lthough. as noted above, all of the mar h data is not 

completely analyzed. the great extent of the coas t where 

ev idcn c of rapid sub idcnce has n w been documented 

(fi g. 144) indicates that v r larg parts of the oasl (at 

lea t several hundred to man hundreds of kil otn'l rs) 

' ere affec t d b prehi. tori c ea rthquakes. The imprcss iv 

number of . it c where multiple buried p at scqu nc . of 

similar age have no' be n found (fig. l.f.f) . ugg st. that 

the mo. t lik ly cause of the marsh subsidcn is ·os ismi · 

urface deform ation during a gr at earthquake. B au:,' of 

the ex tent of the marsh data along th oa:l and lh ' ar!!,u -

m nts of tectonic cttin o rai. ccl by Heaton and llart?cll 

( 19 6), web li cve that the entire length of th ascadia 

subducti on zon is capabl f generat ing , reat 'arthquakes . 

even though it is not ct lear if the entire length fail s during a single event. 

The second i. sue in c. timatin Q. source areas ncern. 

the ' idth of the rupture area perpendicular to the coast. Two 

mod Is. on in which the ruptur is large! offshor and one 

in which it i. largely b neath the olllincnt , bracket lh range 

f probable sour c-arca iclths. In the fir. t model. the rurturc ex tend from the deformati on-front trcn h down-dip 

along the interface to a depth of 30---tO km. Because of th ' 

plate geometry. thi width aries along the subducti on 1.011 ' 

from a max imum beneath nonhwc. tern Washington (about 

200 km) to less than I 00 km beneath central Or gon and 

area. further south (fi g. 140). If th end of the fau lt rupture 

zone were a. deep a 40 km , then a significant part of the 

rupture area would be benea th Wa. hington (fi g. 145). 

whcrca if the rupture end at about a 30-km d pth , the cas tern cx telll of the fault-ru pture area wou ld b ncar the coa:l 

practi call y everywhere. In the second m del , in area. li ke the 

Cascadia subduction zone that have a very hi oh rate of sed imen tation off. horc. Byrne and others ( 1988) ha c argued 

that a· the c sed iment. ar sub luctcd, they allow vcr poor 

coupling bctwc n the two plates from the trench land wa rd. 

pos. ibl y a far a the coa l. In thi s model. the potential 

s ur c area capable of generating subduction-zone in terface 

ea rth uakes i · greatl y reduced, con i ting approx imately of 

the area from about the coa t inland to where the subducting 

plate begin to dip steep! ea tward , perhap at a depth of 

-o- 60 km (Byrne and other . . 19 8). Because of the plate 

geometry. thi s area i parti cularl y small outh of the arch 

beneath Puget ound (fi g. 140). 

In our map of the source areas f r great . ubdu Li onzone earthquake. (fi g. 145). we have hown the fault-zone 

width c tending from the deformation front offshore to j ust 

beneath the coa tlinc. We chose thi s fault width becawc of 

deformati on-modeling rc: ult that indicate that it is difficult 

to expl ain the pattern of udden . ubsiclcnce recorded in the 

c a. tal-marsh stratigraphy without the rupture area ex t nding off. horc ( a age and others, 198 1 ). The pattern ed area 

in fi gure 145 illustrate the large ourcc areas for these events. 

Based on the r cord from other ubduction zones . interface 

earthquake can be expec ted to occur anywhere within the 'e 

. ourcc areas. The two hypothesized earthquakes would fill 

the en tire ubduction zone; however, other pos. ibilitics have 

been cl iscussed by Hea ton and Hartzell ( 19 6). To date. the 

geologic record from coa. tal interti dal marshes docs not rul 
29 ASSESS I G EARTHQ AKE HAZA RD A D RED Cl G RISK I TH E PA IFI ORTHWE T 

0 

128 

0 

47 

0 JUAN DE FUCA 

45 PLATE 

0 

43 PACIFIC 

0 

41 

0 

39 

OCEAN 

PACIFIC 

PLATE GORDA 

PLATE 

MFS 

Cape 

Mendocino 

0 

0 

0 

119 0 

122 

BRITISH COLUMBIA 

0 

Eugene 

WASHINGTON 

OREGON 

116 

0 

J> 

I 

0 

NEVADA 

CALIFORNIA 

200 Kilometers 

0 

Figure 144. Site. in Washington, Oregon, and northern Cali forni a (dots) where coa tal-mar h stratigraph y studies have 

found evidence of . uclclen sub: idence. Compiled from Grant and other. ( 19 9). M FS. Mendocino fracture zone; SAF, San 

Andreas fault. 

out the possibility that the entire length of the ubduction 

zone can fail in one very great earthquake. However, the 

sa me data do not yet allow unequi vocal interpretations of 

whether the zone might break in a scrie. of smaller earthquakes of approx imate magnitude 8- 8.5 (Mw). 

Th cry large area invol cd in any subduction-zone 

earthquake indica tes the need for large-scale studie of the 

propcrti c. f the Ca. cadia subducti on zone. For example, 

greatl y expanded regional train studi es would help address 

the issue of whether the plate boundary breaks in a single 

great earthqu ake or in a . cri es of smaller one . A second 

issue i that of the eas tern ex tent of the fault. Clearl y, if the 

ca. tern li mit of the fault zone for a great subducti on event 

was further westward than the li mit . hown in figure 145 , 

then it is pos ible that this would produce lower levels of 

strong ground haking in the maj or urban areas. 
SEI MIC SO RCE REG\0 S FROM E RTHQ KE 01 TRIB TIO A •D REG I ' L TECTO 

0 

0 

49 

0 

47 

0 

39 

& 

8 

Q:" 

0 

,z 

!!: 

~ 

0 0 

§ 

.., JUAN OE FUCA 

PLATE 

PA CIFIC 

OCEA 

PACIFIC 

PLATE 

GORDA 

PLATE 

MFS 

Cape Mendocino 

0 I 19 

122 

BRITISH COLUMBIA 

attle 

WASHINGTON 

Portland 

OREGON 

0 

Eugene 

CALIFORNIA 

0 

I 16 

0 

J> 

:r: 

0 

NEVADA 

0 200 Kilometers 

L--""--_l 

figu« \45. Sou«'"'" (P""'"cd) foe IWO po ,;b\e '"'""'"' e>rt<hq"'" oo 1he h•ilo• .dipPiog ' "'"''" bo<• " " <he 

'"'"de Fuoo '"' ' orth mecic> pio<<'· AppCO''"''" momcm ""gui<udc fo< ooeh c em i< <ho• o. O<he< eombioo<ioo< of 

,ouc<' '"" '" oi<O po<'ibl<. FS. eudoeioo '""""woe; A f. Soo Aod<<'' fou l<. 

interaction of the Explorer plate with the orth Amer

pl"'e (Roge< . 19 3b). The''" e of <he 1872 e cn«cma

KNOWN uncertain in that it occurred in an area with ery \inle cSOURCE REGIONS OF LARGE 

CRUSTAL EARTHQ AKES 

The known areas of large-magnitude (7+) crustal eanhqu•Ke in ihC , octh me< iCO pla<C in lhC p,cif<C orthWCSI 

are limited to central Vancouver I. land and the northern ascacle Range (fi g. I 3 ). The \1 ancou er Island events ' ere 

pcob• bly cela<cd 1 1hc rte.. cegimc gcoeca<ed b <he 

temporary seismicity and no ob ious . urface-fault exp

sion (Malone and Bor. I 979). The e large crustal earthquakes rai e the que tio

whether the mi ght occur within the urban areas f we 

Wa hington and Oregon. nfortunatel . the spar it 
300 A ESS I 1G E RTHQ AKE HAZARD A D RED Cl G RISK I THE PA IFI ORTHWE T 

known Quaternary faults (Gower and other .. 1985) and the 

cu rrent seismici ty distribution doe. little to an wer thi s que. -

tion. In the Puget Sound ba in. crustal earthquakes do not 

occur along . imple. linear fault zone. but appear to be di. -

tributcd throughout the crust (fig. 141 ). Zoll weg and John on 

( 1989) have r ccntly interpreted a equencc of earthquake 

on the western margin of the northern ascade Range as c idcncc of a south-dipping fau lt zone. the fir. t : uch zone identified in northwe. tern Washington. e crthclc .. , it remain 

impo siblc to in~ r the po. sibi l ity or. or argue concl u. i ely 

against. a future magni tude 7+ . hallow cru tal earthquake in 

the Puget ound area. 

Mapped Quatemary fau lts in the Pugct Sound ba in 

arc par c (Gower and others. 1985), although . hallowreflection data may indicate Quaternary faults within Pugcr 

Sound (Harding and other. , 19 8). ombined w ith the 

apparen t sparsity of Quaternary fau lts. the avail able ci -

micity record in the Puget Sound ba in is u. uall y interpreted a. sugge. ting that the expected maximum-magnitude 

cru stal earthquake i less than that expected in southern 

W a hington (Ludwin and others. 199 1 ). H wever, observati ns of upli ft on late Holocene terraces by Buck nam and 

others ( 1992). coupled with supporti ng ev idence for the 

generation of t. unami s in Puget Sound ( twater and 

M oore. 1992) and land. lidc. into lowland lake (Schuster 

and others. 1992; Jac I y and others, 1993), strongly indica te that magnitude 7 event. have occurred in the crust of 

the Puget Sound I a in. One again, the lim ited earthquake 

history make it difficult to meld the new findings for the 

Puget Sound ba in directly into earthquake-hazards a ses. -

mcnts. M any additional geologic investi gations are ne dcd 

to aucmpt to estimate po. siblc recurrence of large cru tal 

earthquake in the Puget Sound basin . 

In contra. t to the earthquake distribution in the Puget 

ound ba in, much of the earthquake ac tivity in ou th we tern Washington occurs along the SHZ. a right-lateral 

strike-s lip fault zone that is defined for more than 130 km 

( udwin and others. 199 1; Wca er and Smi th. 1983). T wo 

earthquakes greater than magnitude 5 have occurred on the 

HZ since 1960: in 1961. a magni tude 5. 1 (M L) even t 

o curred sou th of M ou nt St. Helen. and, in 198 1, a magnitude 5.5 (M JJ c en t o curTed on the HZ north of the volcano. M ount t. Helen directl y overl ies the SHZ where a 

small (a few kilometer) right-. tcpping offset occur 

(Weaver and others, 19 7). cvcral tudie have a sumed 

th at the magmatic . y tern beneath M ount t. Helen prev nts the entire 130-km length from ru pturi ng in a inglc 

ea rthquake (Weaver and Smith. 19 3). We com pared possibl . ourcc ar as along the SHZ north of M ount St. Helens 

with obser ati ons of both faul t area and magn itudes ca lculated from earthquakes on other strike-. li p fa ult zones and 

concluded that an earthquake in the magnitude range of 

6.2- 6.8 was the ex ccted max imum-magnitude event for 

the HZ north of Mount t. Helen . . 

Our map of cru stal earthquake source area. (fi g. 146) 

show the regions where the. e ea rthquakes occurred in the 

hi stori ca l record including the HZ, the northern end of the 

an A ndrea fault . ystem in aliforni a, and the central Puget 

Sound ba in near Seattle, where palco ei mic ev idence f 

upli ft ha. recentl y been found. The large area in northcentral Washington illu. tratcs the uncertainty about the cpicentral pos ition f the I 72 earthquake. Although M alone 

and Bor ( 1979) concluded that the event probabl y wa on the 

northea. tern fl ank of the Cascade Range. other tudies 

(Milne. 1956) have. uggc ted that the event was near the 

United States- anada border. problem highlighted by thi s 

map i that modern eismi ity eli. rributi on gives no indicati on of the pa. t large crustal earthquake in cen tral Vancouver I land and the North a cade . 

Figure 146 doe emphas ize the advantage of both accurate locati ns and an undenanding of the seism teet nics 

rc pon. ible for cru tal earthquakes because, on the SH Z, it i 

po sible to relate a large earthquake to a spec ific structure, as 

opposed to ha ing to consider eq uall y li kely that the c cnt 

may occur th roughout a gi ven 'trea . We empha. ize that figure 146 repre ents a very incomplete as essment of the 

ource region of large cru tal e ents. n idcrable study f 

regional geology, local Quatern ary tudie . . and data from 

regional- calc tra in network , as discu sed by hed lock and 

Weaver ( 19 9). will be required to red uce the uncertainty in 

definin g s urce regions for large cru stal ea rthquake. in the 

Paci fie orthwest. 

INTERPRETATION AND DISCUSSION 

OF SOURCE REGIONS 

Our definition of the . ource area for the three type of 

anhquake. expected in the Pac ific orth west ha relied 

heav il y on hi stori cal and in trumental sci. micity with reference to the regional tec tonic scuing. Thi reliance is most 

clear with re. pect to the area of po iblc intraplate earthquake . Our existing under. tanding of the geometry of the 

Juan de Fu a plate y tern is sufficient for us to thin k that 

event like the 1949 south Puget Sound earthquake may 

occur anywhere along the length of the . ubducting pl ate system. Current interpretati on. of marine seismi c-refl ecti on 

data (Hynd man and others. 1990; Couch and Riddihough, 

1989; Snavely. 1988) show that e erywhcre off. hore. the 

Juan de Fuca plate is dipping eastward at shal low angles 

(Jess than I 0°). Onshore, even in areas where the Ju an de 

Fuca plate i currentl y . eismicall y quiet. in er ion of 

tele ei mic P-wave arri va l times ha been interpreted to 

show that below depths of 40- 60 km . the subducting pl ate i 

nearl y verti cal beneath the Cascade Ranoc (Michae l on and 

Weaver. 1986; Rasmussen and Humphreys. 1988). Furthermore, ros. on and Owens ( 1987) have . uggested that 

tele eismi c waveforms recorded near the central Washington 
SEI MIC SO RCE REGIONS FROM EARTHQ AKE Dl TRIB TIO D REGIO L TE TO IJ S .01 

0 13 1 

51 

0 

0 0 

49 

0 & 47 f) 

iC 

cJ 

2 

!J 

< :J 0 'j JUAN DE FUCA 

45 PLATE 

sz.qt\tc 

o"'" -9-'~c ~'u.tr(O 

0 <ot\t(O 

-

0 

?.. 

43 PACIFIC 

OCEAN 

o PACIFIC 

41 PLATE 

0 

39 

GORDA 

PLATE 

MFS 

Cape 

Mendocino 

0 0 0 

122 119 

BRITISH COLUMBIA 

WASHINGTON 

iJ 

r 

l> Portland 

-l 

m 

0 Eugene 

OREGON 

CALIFORNIA 

0 200 Kilometers 

116 

0 

)> 

I 

0 

NEVADA 

0 

Figu•·c 146. Known source areas (patterned) for crustal earthquakes greater than a magnitude of about 6.5. SHZ. l. Helens 

seism ic zone; M FS. M endocino fracwrc zone: AF. San A ndrea. fault. 

c a. t arc compatible wi th the hypothe is that the Ju an de 

Fuca plate i. continuous (that i . . not faulted or offse t) south 

of the arch beneath nonhwe tern Oregon. Thus, desp ite variat ions in the dip of the upper pan of the 1 late, the known or 

interpreted characteri ti cs of the Ju an de Fuca plate argue 

strongly for the ex istence of a subducting plate capable of 

producing intraplate tensional stre:se along th d wn-dip 

ax is of the pl ate ( ec pence, 1987, for a discu. sion of pl at 

. tresses). Indeed, if the 1873 earthquake at the Oregon -

aliforn ia border (fig. 138) was an intrap late earthquake. a. 

. ugge. ted by Ludwin and others ( 199 1 ), then the curren tl y 

sei. micall y quiet pan of the subducting plate. ystem had the 

second large t Benioff-zone earthquake in hi tori ca l times. 

An l ink between the . egmen ted nature of the cru stalearthquake distri buti on and . ources for either ubductionzone events or cru tal earthquakes i. unclear. With resp ctto 

cru stal arthquake., the segmentation outlined by Weav r 

and other ( 1990) is . imilar to the division f th Cascade 

Range sugge ted by Guffan ti and Weaver ( 1988). These 

authors used difTerences in the distribution and compositi on 
302 EARTHQ KE HAZARDS D RED I G RISK I THE PA IFI ORTHWEST 

of late Cenozoic vol ani c vent. (less than 5 M a) to eli ide the 

ascacle Range into ri e . egment.. The e volcanic segments 

were the same a tho. c uggestccl here, except that Gu ffanti 

anci Wcavcr ( J98 ) haclt> oscgm Ill innorthcrn Californi a. 

In fac t. iso. tatic rcs iclual-grav it anomalies (Bl akely and 

Jachcn . . 1990) and the vari ations in the volume of Quaternar olcani sm (Sherrod and Smith, 1990) in the ascacle 

Range can be used to eli ide the a. cadc Range into . cgmcnt. identica l t those of Gu ffanti and Weaver ( 19 8). 

pparcntl y. the . cgmcntation suggested b the . eismicity 

pattern in the Pac ific North wes t mu. t be refl ec ting the . amc 

regional tectonic framework that fundamentall y . hap d th 

el i. tri bution and volume of late cnozoic volcanism. The 

components of thi. tec tonic framework ( ari ati on in cru. tal 

. trc. s, change. in cru. tal structure) have left their mark in the 

middle and upper cru st. ex pres eel today by the grav ity field 

and, to a lesser ex tent, by regional heat fl ow (Bl ack\ ell and 

others, 1990) 

The link between the regional seismotcctonic fabri c and 

late Cenozoic volcani sm i: likely to be parti cularl y imJ ortant 

in assc sing the earth 1uake hazards in areas of the Pacific 

1orth wcst with few earthquak es in the available hi stori ca l 

record. The Portl and, Oreg., area is a clear example of the 

need to understand thi s link . A recent . tud y of seismi city in 

the region around Portland has concluded that there may be 

a series of n echelon stri ke-. lip fault zones . outhwcst of the 

SHZ (Yclin and Patton, 199 1). The Portl and Hill s fault. a 

few kil ometers west of downtown Portl and, shows at lea t 

Pli ocene off cts (Sherrod and Pi ckthorn , 19 9) and is nearl y 

parallel to the possible en echelon sci. mi c zones. 

The Portl and area has a history of moderate-magnitude 

earthquakes (about 5+). A recent earthquake in the heav il y 

urbanized area occurred in 1962 (the mo. t recent modcratcmagnitucl event occurred southeas t of Portl and in 1993 

along the foothill s of t he a. cad Range: ee Rogers and oth-

~, thi. vo lume). T he 1962 earth 1uake was probabl, a normal fa ulting c cnt (Yclin and Patton. 199 1 ). compati ble with 

the hypothc. is that the Portl and bas in formed by cru stal 

cx tcn. ion. Locali zed cru stal ex tension may also expl ain the 

occurrence of late cnozoic basaltic volcani. m in the Portland urban area, far we. t of the a. cadc Range ax i . . In contras t to th is result favoring cru. tal ex ten ion. the preliminary 

foca l mechani sm calculatccl for the 1993 earthqu ake southcas t of Portl and has a ombinati on of thru. t and trike-. lip 

moti on (M aclin and other . 1993), with a north-striking directi on of max imum hori zontal com pre sion. The. c two focal 

mcchani ms underscore the ambiguou. relation among contemporary . ismi city eli . tributi on, structural features such as 

the Portl and basin and the Portl and Hill s fault zone, and 

basaltic volcani sm. It is cl ar that the area warrant. continued 

sc mi · monit ring to cletcrmin more completely the seismotcc tonic relations and the ass ciatecl earthquake hazard s. 

The scgmcntati n interpreted from the se ismi city pattern may pro ide additional justification for a umption 

that are made for modeling ground . baking from crustal 

earthquake ·. In area where fault s arc not well expo. ed at the 

. urface. a region mu. t be divided into subregions with different source characteri sti cs (for exampl e. Al g rmi sen and 

others. 19 2). Commonl y. these clivi. ion refl c t geologic 

bounclarie and the max imum-magnitude earthquake 

cxpcctccl within each : ubrcgion. If sei mic . egmentation is 

related to broad regional processc or tructures, th n these 

processes or tructure wi II be a further con traint that an be 

u cd in determining subregion for ground-moti n modeling. In 'tddition, the segmentation may determine bounclari e 

for the u e of different modeling technique .. For example, 

acros . outhwc tern Washington and northwes tern Oregon, 

the definition of di crete . ei mic zone. (as defined by ea rthquake.) will likely prov ide a framework within whi ch maximum-magnitude event. can be es timated from an 

int rprctation of ci micity pattern . geologic mapping, and 

crustal- tructurc tudies. In northwestern Wa hington. however, the Jack of recognized eismic zones makes like! a 

continued reliance largely on an areal approach to hazard 

e timati on. !earl y, a more complete charac teri za tion of the 

cru tal tru ture and regional tectonic is the key to understanding the generati on of th two wide! di sparate . cismicity pattern s found in northwe tern and southwes tern 

Wa hington. A second point concern the po . ible segmentation of the Cascadia ubducti on-zone thru t interface. If 

any of the proce se responsible for the segmentati on 

observed acros the ascaclc Range arc the rc ult of the 

direc t interacti on of the Juan de Fuca pl ate sy. tcm with the 

North Ameri ca pl at , then p rhaps some fundamental rupture length along the coa t might refl ect thi egmentati on. 

We emphasize that the ourcc region. ' e have hown 

for the three earthquake type. arc part of a maj or rev i. ion of 

the understanding of earthquake hazards fac ing the Pac i fie 

1orthwest. As noted by Heaton and Hartzell ( 1987). great 

subduction-zone earthquake will have shaking effects that 

arc felt vcr much of we tern Washington. western Oregon, 

north we tern Cali forni a, and . outhwestcrn British Columbi a. With thee earthquake will likely come I call y generated t un ami s that will wash over many of the coa tal 

communities, with only minute. of warnin g (Heaton and 

Hartzell , 1987). The pos ibility of these great . ubducti on 

earthquakes alone i enough to warrant a thorou0 h reexamination of building codes and earthquakc-prcparcdnes programs throughout the region . We believe that it is criti cally 

important that thi reexamination begin immediately in 

urban areas in the Willamette Vall ey uch a Eugene, Salem, 

and Portl and because these cities were not included in earlier 

tudi cs undertaken in the Puget Sound region (Hopper and 

other . 197 5) that at leas t have allowecll ocal governments to 
SEISM IC SO RCE REG IO S FROM EARTHQ AKE 01 TRIB TIO D REG IO 30. 

be aware of ome of the potential problems from earthquakes. The fact that Will amettc Valley citic hav not had 

the larger magnitude earthquakes found in other parts of the 

Pac ifi c orthwest (Puget Sound, orth Ca. ade. , northern 

alifornia) should n t b a ju. tifica tion for deJa ing rca.-

sessments of hazards. A the March 1993 c ent outheast of 

Portl and has clearl y shown, many citie in Oregon arc parti cularl y vu lnerabl , even to relatively small oround motion 

cau ed by moderate earthquakes. We reiterate that the shortterm sei. mic record in the Pacific orthwcst cannot be 

iewed as repre entati ve of the long-term ri k f large earthquake . The September 1993 earthquak es near Kl amath 

Fall (Rogers and other . thi s volume), occurring in an area 

e. cnti ally without hi storical ci micity. have . crved to 

empha. ize the . hortcoming. of the historica l record in the 

Pacific orth wcst. Al so, our judgmen t that the entire part of 

the subducting Juan de Fuca plate capable of producino 

intrapl ate earthquakes like th sc of 1949 and 1965 in the 

Puget Sound ba in, coupled with the uncertai nty. urrounding 

the ex tent of the area where large cru. tal earthquakes 

hould be expected. give. added urgency for a region-wide 

reexaminati on of the hazard posed by earthquake . 

Finall y. there may be other potential sei mic source 

zones in the Pac ific Northwc tthatmay ultimately be important in earthquake-hazard asses. mcnts. One . uch zone i 

we t of the deform ation front (fig. 138). in the nearly natly ing part of the Juan de Fuca and Gorda pl ates. arthquakes 

in thi s part of the tec toni plate were rcfcrrcd t a '·oceani c 

intrap ial •· by A sti z and other ( 19 8). Thi s zone i. parti cularly acti ve off the co a t of nonhcrn Cal ifornia. But, as noted 

above, oceani c intraplate earthquakes in the hi tori cal record 

ha e cau ed relati ely little damage to on. hore facilitic . . 

Ncverthcle. s, a the collapse f the highwa bridge during 

the 1980 Eureka earthquake indi ated, the. c event. mu. t be 

acknowledged in hazard · assessment. along the northern 

coast of alifomia. Earthquakes arc . par within the Juan 

de Fuca plate west of the deformati on front off the Oreg n 

and Washington coa. ts (Spence. 19 9). Bccau. c o few 

earthquakes are known off the Oregon and Washington 

coa. ts and the tectonic setting of the. e intraJiatc event. i 

very uncertain. it is not yet po sible to as. css how important 

e cnts west of the dcforrnation front may be in the overall 

earthqu ake-hazard s as cssmcnt of the Pacific onhwc. t. 

However. if the histori cal record of damage r suiting from 

earthquake in thi. zone in northern aliforni a is similar for 

Oregon and Wa. hington, combined with the fact that the di -

tancc between the deformation front and the coas tline 

steadily increase north ward from ape M endocino. then it 

is l ikely that oceanic intrap late event. wi ll be con. idcrably 

Jess important in most of the Pacifi c orth\ c. t as compared 

with the three zones di scu cd in thi chapter. 

SUMMARY 

In the convergent-margin . cuing of the ascadia subducti on zone, three distinct arthquakc sources arc possibl : 

( I) earthquake. at the interface bet\ ecn the Juan d uca and 

orth Ameri ca plate; (2) earthquake wit hin the cru st of the 

over! ing onh mcrica pl at : and (3) C'lrthquakes ' ithin 

the ubducting Juan de Fuca plate. The probable sou r ·· 

region for earthquake. within the Juan d Fuca plate is the 

best known, a we arc ab le to c mbine the hi stori ·al data 

from the 1949 and 1965 carthquak s ' ith th modern instrLImental record . The Iauer data ha c been u. ed to infer the 

geometr of the Ju ·1 n d Fuca plate. ' hereas the hi storica l 

data have been used to deduce that th large-magnitude 

earthquake occur at lea. t in part in re sponse to do' n-dip 

ten. ional forces wi thin the suhdu tin e. plat'. We suggest that 

the entire . ubducti on zone. at depths between 45 and 60 J..m. 

is capable of producin th . e event s. 

De pi tc many unrc. olvcd i. sues surrounding great subducti on-zone interface earthquakes, the a ail able geologi 

record , combined with the plate-tec tonic sClling. is interpreted as ev idence that these events ha c occurred in th ' late 

Holocene along the coas t. Because thes vent s o cur on the 

shallow-dipping part of the plate interface. their general location i well known. ub tuction-zone ea nhquak 'S represent 

a major threat to the popu lati on of the Pa ific 1 onh wcst. one 

that ha not been integrated into current hazard ass ss mcnts. 

A program to accompli sh thi s inte0 rati on must consider th ' 

large . izc of the source region for these earthqu akes. The 

possibility of large crustal earthquakes in the Pacific onhwes t urban areas also remains very poorly studied. 

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Edited hy John Synnefakis and Barbara Hillier 

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Photocomposition by Carol A. Quesenhen )', Mari Kauffmann, and Shelly Fields 

Book design by Carol A. Quesenbert)' 

(r U. S . OVERN~IENT PRINTING OFFICE, 1996 - 77 4 - 045 I 20058 REGION NO. 8