lattice 

USGS

forachaitging world

• epared m~cb6peratibri~with the Nuclear Regulatory Commission

lecurrence Intervals for

ireat Earthquakes of the Past 3,500 Years at

ortheastern Willapa Bay, Washington

Professional Paper 1576

epartment of the interior 

'eological Survey
AVAILABILITY OF PUBLICATIONS OF THE U.S. GEOLOGICAL SURVEY

Order U.S. Geological Survey (USGS) publications from the offices listed below. Detailed ordering instructions, along with prices of 

the last offerings, are given in the current-year issues of the catalog "New Publications of the U.S. Geological Survey."

BOOKS, MAPS, AND OTHER PUBLICATIONS 

By Mail

Books, maps, and other publications are available by mail 

from—

USGS Information Services 

Box 25286, Federal Center, Denver, CO 80225

Publications include Professional Papers, Bulletins, Water-Supply 

Papers, Techniques of Water-Resources Investigations, Circulars, 

Fact Sheets, publications of general interest, single copies of 

permanent USGS catalogs, and topographic and thematic maps.

Over the Counter

Books, maps, and other publications of the U.S. Geological 

Survey are available over the counter at the following USGS Earth 

Science Information Centers (ESIC's), all of which are authorized 

agents of the Superintendent of Documents:

• Anchorage, Alaska—Rm. 101,4230 University Dr.

• Denver, Colorado—Bldg. 810, Federal Center

• Menlo Park, California—Rm. 3128, Bldg. 3, 

345 Middlefield Rd.

• Reston, Virginia—Rm. IC402, USGS National Center, 

12201 Sunrise Valley Dr.

• Salt Lake City, Utah—2222 West, 2300 South

• Spokane, Washington—Rm. 135, U.S. Post Office 

Building, 904 West Riverside Ave.

• Washington, D.C.—Rm. 2650, Main Interior Bldg., 

18th and CSts., NW.

Maps only may be purchased over the counter at the 

following USGS office:

• Rolla, Missouri—1400 Independence Rd. 

Electronically

Some USGS publications, including the catalog "New 

Publications of the U.S. Geological Survey" are also available 

electronically on the USGS's World Wide Web home page at

http://www.usgs.gov

PRELIMINARY DETERMINATION OF EPICENTERS

Subscriptions to the periodical "Preliminary Determination 

of Epicenters" can be obtained only from the Superintendent of

Documents. Check or money order must be payable to the 

Superintendent of Documents. Order by mail from—

Superintendent of Documents

Government Printing Office

Washington, DC 20402

INFORMATION PERIODICALS

Many Information Periodicals products are available through 

the systems or formats listed below:

Printed products: Printed copies of the Minerals Yearbook and 

the Mineral Commodities Summaries can be ordered from the 

Superintendent of Documents, Government Printing Office 

(address above). Printed copies of Metal Industry Indicators and 

Mineral Industry Surveys can be ordered from the Center for 

Disease Control and Prevention, National Institute for 

Occupational Safety and Health, Pittsburgh Research Center, P.O. 

Box 18070, Pittsburgh, PA 15236-0070.

Mines FaxBack: Return fax service.

1. Use the touch-tone handset attached to your fax machine's 

telephone jack. (ISDN [digital] telephones cannot be used with 

fax machines.)

2. Dial (703) 648^999.

3. Listen to the menu options and punch in the number of your 

selection, using the touch-tone telephone.

4. After completing your selection, press the start button on your 

fax machine.

CD-ROM: A disc containing chapters of the Minerals Yearbook 

(1993-95), the Mineral Commodity Summaries (1995-97), a 

statistical compendium (1970-90), and other publications is 

updated three times a year and sold by the Superintendent of 

Documents, Government Printing Office (address above).

World Wide Web: http://minerals.er.usgs.gov/minerals/

SUBSCRIPTION TO THE CATALOG "NEW 

PUBLICATIONS OF THE U.S. GEOLOGICAL SURVEY"

Those wishing to be placed on a free subscription list for the 

catalog "New Publications of the U.S. Geological Survey" should 

write to the U.S. Geological Survey, 903 National Center, Reston, 

VA20192.
Recurrence Intervals for Great Earthquakes 

Of the Past 3,500 Years at Northeastern 

Willapa Bay, Washington

By Brian F. Atwater and Eileen Hemphill-Haley

A geologic history of earthquakes 

at the Cascadia subduction zone

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1576

Prepared in cooperation with the 

Nuclear Regulatory Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1997
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

MARK SCHAEFER, Interim Director

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center, Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

Published in the Western Region, Menlo Park, Calif. 

Manuscript approved for publication August 21,1997.

Library of Congress Cataloging in Publication Data

Atwater, Brian F.

Recurrence intervals for great earthquakes of the past 3,500 years at northeastern Willapa

Bay, Washington / by Brian F. Atwater and Eileen Hemphill-Haley. 

p. cm. — (U.S. Geological Survey professional paper; 1576) 

"A geologic history of earthquakes at the Cascadia subduction zone." 

"Prepared in cooperation with the Nuclear Regulatory Commission." 

Includes bibliographical references. 

Supt. of Docs, no.: 119.16: 1576 

1. Paleoseismology—Holocene. 2. Paleoseismology—Washington (State)—Willapa Bay

Region. I. Hemphill-Haley, Eileen. II. U.S. Nuclear Regulatory Commission. III.

Title. IV. Series. 

QE539.2.P34A88 1997

551.22'09797'92—dc21 97-34929

CIP
CONTENTS

Abstract........................................................................................................................ 1

Introduction.................................................................................................................. 2

Acknowledgments........................................................................................................ 5

Previous estimates of recurrence intervals for plate-boundary earthquakes at the

Cascadiasubductionzone................................................................................ 7

Turbidites in deep-sea channels off Washington and Oregon.............................. 7

Buried soils at Humboldt Bay, California............................................................ 7

Buried soils at estuaries in northern Oregon........................................................ 7

Interbedded peat and mud at Coos Bay, Oregon.................................................. 8

B uried scarps west of Willapa B ay near Long Beach, Washington..................... 8

Buried soils at Grays Harbor, Washington........................................................... 8

Structural setting and modern tidal wetlands of northeastern Willapa B ay................. 8

Geologic structure................................................................................................ 8

Tides..................................................................................................................... 11

Tidal-wetland environments................................................................................. 11

Uplands................................................................................................................ 14

Methods........................................................................................................................ 14

Delineation of stratigraphic units......................................................................... 14

Identification of buried soils........................................................................ 14

Letter names for buried soils........................................................................ 17

Continuity of stratigraphic features ............................................................. 20

Borings and depth to Pleistocene deposits................................................... 20

Particle size.................................................................................................. 22

Organic content............................................................................................ 22

Horizontal and vertical datums.................................................................... 22

Fossils................................................................................................................... 22

Vascular plants............................................................................................. 22

Diatoms........................................................................................................ 22

Criteria for inferring upland conditions............................................... 22

Significance oiParalia sulcata....... ..................................................... 23

Comparisons between diatoms and vascular plants..................................... 23

Radiocarbon dating.............................................................................................. 24

Kinds of years.............................................................................................. 24

Analytical errors in dating .......................................................................... 24

Geologic errors in dating ............................................................................. 24

Sequences of buried soils............................................................................................. 25

Overview of evidence for stratigraphic correlation and decomposition.............. 25

Regional sequence........................................................................................ 25

Decomposition after burial........................................................................... 25

Niawiakum River................................................................................................. 26

Redtail locality............................................................................................. 26

Buried soils.......................................................................................... 26

Tsunami deposit................................................................................... 27

Fossil diatoms...................................................................................... 27

Growth-position fossils of vascular plants........................................... 29

Cultural deposits.................................................................................. 30

Age....................................................................................................... 32

ill
IV

Oyster locality.............................................................................................. 36

Buried soils.......................................................................................... 37

Tsunami deposit................................................................................... 45

Growth-position fossils of vascular plants........................................... 48

Fossil diatoms...................................................................................... 48

Age and correlation.............................................................................. 52

Pool locality................................................................................................. 63

Heron locality.............................................................................................. 63

Willapa River....................................................................................................... 65

Jensen locality.............................................................................................. 65

Buried soils.......................................................................................... 67

Growth-position fossils of vascular plants........................................... 67

Age and correlation.............................................................................. 67

Sewer locality............................................................................................... 69

Buried soils.......................................................................................... 69

Growth-position fossils of vascular plants........................................... 69

Cultural deposits.................................................................................. 69

Fossil diatoms...................................................................................... 70

Age and correlation.............................................................................. 71

Airport locality............................................................................................. 72

Distinction between tectonic subsidence during earthquakes and other ways of

producing the buried soils................................................................................ 73

Cut and fill by tidal streams................................................................................. 73

Deposition by storms or floods............................................................................ 73

Fluctuations in sea level....................................................................................... 73

Abrupt changes in herbaceous fossils.......................................................... 75

Abrupt changes in microscopic fossils........................................................ 75

Tsunamis...................................................................................................... 77

Gradual submergence elsewhere in the region ............................................ 79

Breaching of bay-mouth barriers......................................................................... 79

Settlement from compaction during shaking....................................................... 80

Tectonic subsidence not attended by seismic shaking......................................... 80

Distinction between plate-boundary and upper-plate sources for the earthquakes...... 80

Sense of land-level change................................................................................... 82

Amount of land-level change............................................................................... 82

Vascular-plant assemblages......................................................................... 82

Thickness of mud between soils.................................................................. 82

Numerical estimates of subsidence.............................................................. 83

Coevality of land-level change............................................................................. 83

Possibly unusual changes in land level 1,000-1,100 years ago ........................... 83

Sizes of the earthquakes............................................................................................... 85

Dimensions of plate-boundary ruptures............................................................... 85

Rupture width.............................................................................................. 85

Rupture length.............................................................................................. 88

Tradeoff between size and frequency .................................................................. 88

Number and ages of earthquakes................................................................................. 90

Events recorded by buried soils........................................................................... 91

Soil J............................................................................................................ 91

SoilL............................................................................................................ 91

Soil N........................................................................................................... 91

SoilS............................................................................................................ 91

SoilU........................................................................................................... 93

Soil W.......................................................................................................... 93

SoilY........................................................................................................... 94

Additional, unrecorded events............................................................................. 94
CONTENTS

Minimum time for formation of a peaty soil............................................ 95

Events for which soils were destroyed...................................................... 95

Intervals between great earthquakes......................................................................... 95

Uncertainties in the estimated intervals............................................................ 97

Does every buried soil represent a great earthquake?............................... 97

Is every great earthquake represented by a buried soil?............................ 97

How precisely are the earthquakes dated?................................................ 97

How accurately are the earthquakes dated?.............................................. 97

Individual intervals............................................................................................ 98

Intervals between events marked by soils J, L, and N .............................. 98

Interval between events marked by soils N and S..................................... 98

Interval between events marked by soils S and U..................................... 98

Interval between events marked by soils U and W................................... 98

Interval between events marked by soils W and Y................................... 99

Average interval................................................................................................ 99

Variability among individual intervals.............................................................. 99

Comparison with intervals estimated from buried soils at other coastal

estuaries................................................................................................. 100

Comparison with intervals estimated from deep-sea turbidites........................ 100

Average interval........................................................................................ 100

Variability among intervals....................................................................... 102

Intervals implied by deposits between turbidites.............................. 102

Intervals implied by burrows in turbidites ........................................ 102

Pattern of recurrence......................................................................................... 103

Speculations about the next great earthquake in southern coastal Washington........ 103

References cited........................................................................................................ 103

FIGURES

1. Map of the Cascadia subduction zone.......................................................................................................................... 3

2. Diagrams showing inferred origin of the main coastal features cited as evidence for prehistoric earthquakes

at the Cascadia subduction zone................................................................................................................................... 4

3. Diagram showing comparison among average recurrence intervals estimated from geologic evidence for

prehistoric earthquakes at the Cascadia subduction zone............................................................................................. 5

4. Diagrams showing sources of error in using stratigraphic records to estimate recurrence intervals for plateboundary earthquakes at the Cascadia subduction zone............................................................................................... 6

5. Map of Cenozoic folds in relation to some of the sites having evidence for coseismic subsidence of

Quaternary age in southern coastal Washington........................................................................................................... 9

6. Index map of northeastern Willapa Bay ....................................................................................................................... 10

7. Index maps showing location and setting of outcrops and transects surveyed along the Niawiakum and

Willapa Rivers............................................................................................................................................................... 12

8. Diagrams of vertical zonation of vascular plants in tidal wetlands of Willapa Bay..................................................... 13

9. Photographs of diatom species most commonly found in high marshes or in both high and low marshes ................. 15

10. Photographs of diatom species most commonly found in low marshes or in tidal flats, tidal sloughs, and

shallow subtidal channels ............................................................................................................................................. 16

11. Diagram showing brackish intertidal diatom index for modern samples from western Washington........................... 20

12. Diagrams showing variation in brackish intertidal diatom index along leveled transects from high marsh to

channel at the Niawiakum River................................................................................................................................... 21

13. Diagrams of stratigraphy at the Redtail locality, Niawiakum River............................................................................. 28

14. Photographs of the Redtail locality, Niawiakum River................................................................................................. 30

15. Graph showing summary of diatom assemblages at the Redtail locality, Niawiakum River....................................... 33

16. Graph showing relative abundances of diatom species grouped by inferred depositional environment at the

Redtail locality, Niawiakum River................................................................................................................................ 34
VI CONTENTS

17. Graphs showing abundance of the diatom Paralia sulcata in stratigraphic sections at the Redtail and Oyster

localities, Niawiakum River......................................................................................................................................... 37

18. Diagram of stratigraphy at the Oyster locality, Niawiakum River.............................................................................. 44

19. Photographs of the Oyster locality, Niawiakum River................................................................................................ 46

20. Graphs showing summaries of diatom assemblages at the Oyster locality, Niawiakum River................................... 49

21. Graph showing relative abundances of diatom species grouped by inferred depositional environment near

the northwest end of the Oyster locality, Niawiakum River........................................................................................ 50

22. Graph showing relative abundances of diatom species grouped by inferred depositional environment near

the southeast end of the Oyster locality, Niawiakum River......................................................................................... 56

23. Photographs comparing diatom assemblages in soil U at horizontal coordinates 25 and 165 m, Oyster

locality, Niawiakum River........................................................................................................................................... 62

24. Diagram of stratigraphy at the Pool locality, Niawiakum River.................................................................................. 64

25. Diagrams of stratigraphy at the Heron locality, Niawiakum River.............................................................................. 66

26. Photograph of onlapping soil at the Heron locality, Niawiakum River....................................................................... 68

27. Diagram of stratigraphy at the Jensen locality, Willapa River..................................................................................... 70

28. Photographs of the Jensen locality, Willapa River ...................................................................................................... 72

29. Diagrams of stratigraphy at the Sewer locality, Willapa River.................................................................................... 74

30. Photographs of the Sewer locality, Willapa River....................................................................................................... 76

31. Diagram of stratigraphy at the Airport locality, Willapa River................................................................................... 78

32. Diagrams showing inferred correlations among buried Holocene soils at estuarine localities in southern

coastal Washington and at Puget Sound...................................................................................................................... 84

33. Diagram comparing high-precision radiocarbon ages for the most recent coseismic subsidence at four

estuaries along the Cascadia subduction zone............................................................................................................. 89

34. Maps showing the distribution of well-exposed evidence for earthquakes in the past 3,500 years in

southern coastal Washington ....................................................................................................................................... 92

35. Diagrammatic history of land-level change, vegetation, and decomposition inferred from buried soils

at the Redtail and Oyster localities, Niawiakum River................................................................................................ 96

36. Graph showing inferred history of great earthquakes at northeastern Willapa Bay in the past 3,500 years............... 99

37. Diagram showing turbidite stratigraphy of lower Cascadia Channel and ages for individual turbidites .................... 101

TABLES

1. Buried soils in Pleistocene deposits near Willapa Bay................................................................................................. 11

2. Diatom taxa used for the brackish intertidal diatom index........................................................................................... 17

3. Brackish intertidal diatom index of modern surface samples from Willapa Bay and Puget Sound............................. 18

4. Numbers of diatoms counted at the Redtail locality..................................................................................................... 38

5. Numbers of diatoms counted at the Oyster locality, horizontal coordinates 15 and 25 m........................................... 53

6. Numbers of diatoms counted at the Oyster locality, horizontal coordinate 165 m....................................................... 59

7. Radiocarbon ages of samples from the Oyster, Redtail, Jensen, Sewer, and Airport localities,

northeastern Willapa Bay.............................................................................................................................................. 86

8. Hypothetical sizes and numbers of earthquakes resulting from 300, 500, and 1,000 years of plate

convergence at the Cascadia subduction zone.............................................................................................................. 90
Recurrence Intervals for Great Earthquakes of the Past 3,500 Years 

at Northeastern Willapa Bay, Washington

By Brian F. Atwater and Eileen Hemphill-Haley

ABSTRACT

Plate-boundary earthquakes have occurred repeatedly 

in the past several thousand years at the Cascadia subduction zone, where they are widely recorded by buried marsh 

and forest soils beneath estuarine wetlands. This report adds 

to previous accounts of such soils along the Pacific coast of 

southern Washington State. Our new evidence comes from 

outcrop surveys, diatom analyses, and radiocarbon dating of 

soils exposed about 10 km apart in banks of the Niawiakum 

and Willapa Rivers, tidal arms of Willapa Bay. This new 

evidence clarifies the timing of great (magnitude 8 or larger) 

earthquakes during the past 3,500 years at this part of the 

Cascadia subduction zone.

All the surveyed outcrops display buried soils that 

probably record tectonic subsidence during earthquakes. 

Although in many cases we cannot rule out every alternative 

to such coseismic subsidence, we found no buried soil that 

is better explained by stream migration, storm, river flood, 

sea-level rise, barrier breaching, or sediment compaction.

Each of the soils probably represents a marsh or forest 

that suddenly became a tidal flat and consequently was buried by tidal mud. In nearly every case the soils have too 

much lateral continuity and too little relief to record cutting 

and filling by tidal streams. Vascular-plant fossils preserved 

within and above many of the soils show that storms or 

floods, if unaccompanied by lasting submergence, cannot 

account for burial of the soils. Where their remains are preserved, plants that had lived on the soils belong to species 

indicative of high parts of tidal marshes, or of tidal swamps 

or uplands. By contrast, the main vascular-plant species preserved in mud above the soils is Triglochin maritimum, a 

colonist of saltwater mudflats in southern coastal Washington. Assemblages of diatoms verify vascular-plant evidence 

for lasting submergence and show that such submergence is 

recorded also by buried soils with which vascular-plant fossils are not preserved. In an outcrop along the Niawiakum 

River, for example, diatoms show that each of six successive 

soils represents a high marsh or an upland, and that mud 

above each of these soils represents an intertidal or subtidal 

mudflat.

Fossils further show that the change from high marsh 

or upland to mudflat probably happened too fast to have 

resulted from a gradual rise in sea level. For every soil studied for fossil diatoms, diatom assemblages imply that this 

change happened suddenly, without transition through low 

marsh. Gradual sea-level rise is further precluded for some 

soils by remains of vascular plants that had lived on buried 

soils. These remains include stems and leaves that were surrounded by mudflat deposits before they had time to decompose.

Other alternatives to coseismic subsidence can be discounted as well. Submergence from breaching of a baymouth barrier is ruled out by soils of brackish marshes that 

require tidal connection with the sea. Localized settlement 

from earthquake-induced compaction of unconsolidated 

Holocene deposits does not explain the presence of buried 

soils directly above well-consolidated Pleistocene deposits. 

Evidence against alternative explanations at one outcrop can 

be extrapolated to other outcrops by radiocarbon and stratigraphic correlation of buried soils.

Plate-boundary earthquakes probably account for all 

the subsidence events. The plate boundary is the only recognized fault common to all areas having evidence for coseismic subsidence in southern coastal Washington. Although 

some of these areas coincide with mapped late Cenozoic 

synclines, where coseismic subsidence might accompany 

earthquakes on faults in the North America plate, others are 

outside such synclines.

The coseismically subsided areas include part of welt 

of Eocene basement rock 40 km long and 15 km wide, 

herein termed the South Bend antiform. This structural 

high, the largest in southern coastal Washington, has probably grown in late Cenozoic time. The South Bend antiform 

should subside during plate-boundary earthquakes that flex 

the North America plate throughout southern coastal Washington. By contrast, the antiform might grow upward during 

an upper-plate earthquake that produces subsidence only in 

late Cenozoic synclines that flank the antiform. With these 

expectations in mind, we compared outcrops off the South 

Bend antiform (Niawiakum River) with outcrops on the 

antiform (Willapa River). We found no difference in sense
RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

or timing of earthquake-induced changes in land level. Like 

the surveyed outcrops along the Niawiakum River, the surveyed outcrops along the Willapa River contain buried soils 

indicative of earthquake-induced subsidence. The longest 

stratigraphic sequences exposed along both streams contain 

six or seven soils less than 3,500 years old, and these 

sequences correlate with one another on the basis of soil 

horizons, spruce roots, and radiocarbon ages.

Though every buried soil identified in the surveyed 

outcrops probably records a plate-boundary earthquake, not 

every plate-boundary earthquake is widely recorded by a 

buried soil. One buried soil was obliterated at many sites 

through centuries of decomposition, probably because shallow burial left the soil high in the weathering profile of a 

succeeding soil. Such decomposition also destroyed most of 

the organic matter associated with two other soils. In addition, a widespread soil might not be available for burial if an 

earthquake occurs too soon after its predecessor for much 

rebuilding of tidal marshes. Despite these limitations, the 

composite record from buried soils in large outcrops of 

northeastern Willapa Bay probably includes every earthquake in the area during the past 3,500 years that caused at 

least Vi m of widespread coseismic subsidence and followed 

the preceding great earthquake by more than a century.

Willapa Bay's earthquake history in the past 3,500 

years probably includes seven events, each comprising a 

single rupture or multiple contiguous ruptures on the Washington part of the Cascadia plate boundary. Each event probably included at least one great earthquake, as judged from 

likely rupture widths inferred from modern geophysical evidence, likely rupture lengths inferred from coastwise correlation of buried soils, extensive sea-floor displacement 

inferred from a tsunami in Japan, and seismic-moment 

release deduced from plate motions and average recurrence 

intervals. The history begins with three events between 

3,300-3,500 years ago and 2,400-2,800 years ago (ranges 

include estimated 95-percent confidence interval). The next 

recorded events occurred 1,500-1,700 and 1,130-1,350 

years ago. They were followed by a poorly dated event that 

probably occurred before 900 years ago and may have been 

associated with rupture on inland faults in the North America plate 1,000-1,100 years ago. The most recent of the 

events happened close to 300 years ago, probably in January 

1700.

The six intervals between events in this inferred history 

average 500-540 years but range from about one to three 

centuries to about a millenium. The earliest two intervals 

sum to 540-1,100 years. The first of them may have been 

the longer, as judged from spruce roots that may indicate 

prolonged interseismic emergence. Next came an interval of 

700-1,300 years, when spruce forests spread onto emerging 

tidal marshes and decomposition largely destroyed an 

underlying buried soil. This exceptionally long interval was 

followed by two short ones that together spanned no more 

than 800 years. The most recent complete interval, marked 

by another spreading of spruce forests and decomposition of 

earlier buried soils, lasted 600-1,000 years.

This pattern of long and short recurrence intervals at 

Willapa Bay may match the pattern of intervals between turbidity currents in Cascadia Channel, on the abyssal sea floor 

200 km off the central Oregon coast. Previous work showed 

that these currents largely originated at submarine canyon 

heads about 50 km west of Willapa Bay, and that great 

earthquakes may have generated 13 currents in the past 

7,500 years. Although pelagic layers between the turbidites 

have been interpreted as evidence for recurrence intervals of 

fairly uniform duration, burrows in the turbidites suggest 

variability in recurrence intervals and allow one-for-one 

correlation with recurrence intervals inferred from buried 

soils at Willapa Bay.

INTRODUCTION

Plate-boundary earthquakes at the Cascadia subduction 

zone are known primarily from geologic evidence for prehistoric land-level changes, tsunamis, and shaking (figs. 1, 

2). Average recurrence intervals estimated from this geologic evidence, as summarized in figure 3, range from a few 

hundred to more than one thousand years (Adams, 1990; 

Clarke and Carver, 1992; Atwater and others, 1995; Geomatrix Consultants, 1995; Darienzo and Peterson, 1995; Nelson and others, 1996a).

Much of this broad range in average recurrence intervals is due to problems with identifying and dating earthquakes (fig. 4). Such problems affect the strongest of the 

evidence for prehistoric plate-boundary earthquakes at the 

Cascadia subduction zone—buried marsh and swamp soils 

at estuaries along the Pacific coast between southern British 

Columbia and northern California. Many of these soils represent sudden tectonic subsidence of coastal land during 

plate-boundary earthquakes; the subsidence caused frequent 

tidal submergence and consequent burial of the soils by 

estuarine mud and sand (Atwater and others, 1995). However, burial of some of the soils may have resulted from 

nonseismic events, such as storms or rapid sea-level rise 

(fig. 45) (Nelson and others, 1996a, b), or from earthquakes 

on faults in the North America plate (fig. 4C) (Nelson, 

1992a; Clarke and Carver, 1992; McCaffrey and Goldfinger, 

1995). Still other soils may have disappeared through 

decomposition or erosion, or may have escaped burial 

through insufficient tidal submergence or sediment supply 

(fig. 4D). Moreover, the combined geologic and analytical 

errors in radiocarbon dating of the earthquakes can 

approach or exceed the lengths of time between them, with 

consequently large uncertainty in the duration of recurrence 

intervals (fig.4£) (Nelson, 1992b; Atwater, 1992).

We reduced such uncertainties in identifying and dating earthquakes by studying sequences of buried soils in 

low-tide outcrops at northeastern Willapa Bay (figs. 5, 6). 

We took advantage of large outcrops to make stratigraphic 

and paleontologic tests of alternatives to sudden tectonic 

subsidence and to identify poorly preserved soils that might 

be overlooked in isolated borings. We also used the varied
INTRODUCTION

120°

50°

BRITISH COLUMBIA

Canada 

U.S.A.

PUGETSOUND 

•"T

Area of detail in figure 5 

WASHINGTON

North

America

plate

AJsea Bay|« rrm «••

EXPLANATION rrm

V Thrust fault at seaward

edge of subduction zone 

Other fault 

Spreading ridge Coquille;RiviBr|«. rrm'Y

uiLJ Marine terrace

Thick tidal-marsh peat 

nm Buried soil 

•••• Sand layer 

T Liquefaction feature 

zzO Landslide

Turbidity-current deposit 

G2l] Fault rupture at surface 

Radiocaibon-dated plant 

killed by submergence 

after sudden subsidence 

JL Tree 

Herb

IllCape.Blanco

Humbbldt Bay|» rrm

CALIFORNIA

Cape Mendocino

San Andreas fault

45°

130°

125°

Figure 1. Cascadia subduction zone. Symbols explained at lower left depict plate boundaries, faults active or probably 

active in the Quaternary, and evidence bearing on occurrence of Holocene earthquakes. Modified from compilations by 

Atwater and others (1995) and, for the Puget Sound area, by Bucknam and others (1992). Includes additions from Benson 

and others (1997) and unpublished data from Oregon provided by Alan R. Nelson.
RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Subsidence Buried soil 

of subsided 

marsh and 

forest

B Tsunami Sand layer 

on buried soil

Liquefaction Sand

intruded into 

and

extruded onto 

buried soil

Before 

earthquake

Minutes to hours 

after earthquake

Months to years 

after earthquake

Decades to centuries 

after earthquake

Figure 2. Inferred origin of the main coastal features cited as evidence for prehistoric earthquakes at the Cascadia 

subduction zone. A, Forest soil buried by tidal mud after subsidence during an earthquake allows tidal water to 

inundate the soil frequently. B, Sand sheet is deposited on a subsided soil by a tsunami that comes ashore minutes 

to hours after an earthquake. C, Liquefied sand erupts through and onto a subsided soil as a result of seismic 

shaking. After Atwater and others (1995).
ACKNOWLEDGMENTS

130° 120V

Seaward edge of subduction zone 

Spreading ridge 

Transform fault

--— Deep-sea channel

B

Range inferred for Willapa Bay

P^~I

Grays Harbor 

Willapa Bay - 

Long Beach -

Deep-sea channels — 

Northern Oregon bays 

Coos Bay —————

Humboldt Bay

WfSxS S£lfi£S iHHHSK

"llilllPllll

0 4000

Age range

of recurrence record, 

in calendric years 

before A.D. 2000

8000 200 400 600 800 1000

Range of average

recurrence intervals,

in calendric years

<—• Limiting maximum

5 Number of inferred earthquakes

. i latest earliest 

Age range oT earthquake earthquake

Figure 3. Comparison among average recurrence intervals estimated from geologic evidence for prehistoric earthquakes at the Cascadia 

subduction zone. In B, narrow lines denote total age range of the inferred earthquakes, and wide lines denote age ranges of latest and earliest of 

the inferred earthquakes. For details see section "Previous Estimates of Recurrence Intervals for Plate-Boundary Earthquakes at the Cascadia 

Subduction Zone" (p. 7). Sources: Grays Harbor, Shennan and others (1996); Willapa Bay, this report; deep-sea channels, Adams (1990); 

northern Oregon bays, Darienzo and Peterson (1995); Coos Bay, Nelson and others (1996a); Humboldt Bay, Clarke and Carver (1992).

structural setting of the outcrops to check for earthquakes on 

faults in the North America plate. We found that all the buried soils in the outcrops can be explained most simply by 

submergence and burial following regional coseismic subsidence from plate-boundary earthquakes. This finding, 

along with several new radiocarbon ages having unusually 

small geologic and analytical errors, shows that about 500 

years probably elapsed, on average, between earthquakes of 

magnitude 8 or larger (great earthquakes) in the past 3,500 

years at northeastern Willapa Bay. The buried-soil record of 

these earthquakes, however, is locally incomplete. We also 

checked the possibility that the earthquakes happened at 

regular intervals as diagrammed in figure 4F. We found 

instead that the earthquakes probably occurred at variable 

intervals (fig. 4G) ranging from a few centuries to about 

1,000 years.

ACKNOWLEDGMENTS

We thank David Lewis, John Shulene, Jamie Hill, 

Boyd Benson, Mark Hemphill-Haley, Alan Carter Mortimer, Judith Boughner, Roger Lewis, Maria Ferrier, Patricia 

Atwater, Yumei Wang, and Bernice Wuetnrich for help in 

surveys of stratigraphy and modern vegetation; Minze 

Stuiver, Phil Wilkinson, and Paula Reimer for high-precision radiocarbon ages; Hansjb'rg Streif for a reference to 

decomposed peat horizons in Germany; Mary Ann Reinhart 

and Steve McMullen for measurement of oriented plant fossils along the Niawiakum River; Chris Goldfinger, Lisa 

McNeill, George Plafker, Thomas Yelin, Ray Wells, and 

Robert Yeats for discussion of tectonics; Marie Hanak for 

photographic prints; and Alan Nelson, Harvey Kelsey, John 

Adams, lan Shennan, John Tinsley, Kathleen Sayce, and
RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

EXPLANATION

* <&

Earthquake Earthquake Storm 

on plate on fault 

boundary in North

America plate

Event marker; 

a buried soil in F and G

Sequence Stratigraphic 

of events record

A Complete for plate-boundary 

earthquakes

Sequence Stratigraphic 

of events record

t

B Mix of plate-boundary 

earthquakes and a storm

Sequence Stratigraphic 

of events record

C Mix of plate-boundary 

earthquakes and an 

earthquake in the North 

America plate

Sequence Stratigraphic 

of events record

D Incomplete for plateboundary earthquakes

Sequence Stratigraphic 

of events record

dating

E Ages of earthquakes 

poorly known

t

Sequence Stratigraphic 

of events record

Sequence Stratigraphic 

of events record

Soils having

similar 

preservation

and 

vegetation

F Periodic, characteristic 

earthquakes

Figure 4. Sources of error in using Stratigraphic records to estimate 

recurrence intervals for plate-boundary earthquakes at the Cascadia 

subduction zone. A, One-for-one correspondence between plateboundary earthquakes and Stratigraphic markers. B, C, Records 

contaminated by Stratigraphic markers for events other than plateboundary earthquakes. D, Record lacking Stratigraphic marker for

Soils differ

from one

another in

preservation

and 

vegetation

trees

G Aperiodic, not necessarily characteristic 

earthquakes

one of the earthquakes. E, Complete record having large 

uncertainties in dating. F, Complete record of periodic, 

characteristic earthquakes. G, Complete record of aperiodic 

earthquakes for which passage of interseismic time promotes change 

in vegetation and decomposition of soils; see figure 35 for inferred 

examples.
PREVIOUS ESTIMATES OF RECURRENCE INTERVALS

Jeffrey Troll for critical reviews. The work was funded by 

earthquake programs of the U.S. Geological Survey and the 

Nuclear Regulatory Commission.

PREVIOUS ESTIMATES OF

RECURRENCE INTERVALS FOR

PLATE-BOUNDARY EARTHQUAKES

AT THE CASCADIA SUBDUCTION ZONE

Geologic attempts to estimate recurrence intervals for 

great (magnitude 8 or larger) Cascadia earthquakes began in 

the middle 1980's with work by Adams (1990), who used 

deposits in deep-sea channels as recorders of earthquakes. 

Estimates of earthquake-recurrence intervals at coastal estuaries began to be made in the late 1980's and have appeared 

in refereed journals since the early 1990's, as reviewed by 

Atwater and others (1995). Atwater and Hemphill-Haley 

(1996) made preliminary estimates for northeastern Willapa 

Bay that are superseded in this report.

TURBIDITES IN DEEP-SEA CHANNELS OFF 

WASHINGTON AND OREGON

Adams (1990, 1996) cited deep-sea turbidites as evidence that great earthquakes off the Washington and Oregon 

coast have recurred at an average interval close to 600 years. 

This estimate was based on counting, in cores from sites in 

deep-sea channels (fig. 1), turbidites younger than Mazama 

ash, which erupted about 7,500 calendric years ago. Many 

of these cores contain 13 or 14 post-Mazama turbidites, 

both above and below junctions of tributary channels 

(Griggs and Kulm, 1970, p. 1375-1376). Griggs and Kulm 

(1970, p. 1382) ascribed these turbidites to "periodic earthquakes or severe storms" acting on unstable deposits at 

heads of submarine canyons. Adams (1990) argued that 

great earthquakes provide the simplest explanation for the 

uniformity in number of post-Mazama turbidites above and 

below channel junctions, and for the many-century intervals 

between turbidites.

Adams inferred a one-for-one correspondence between 

turbidites and great plate-boundary earthquakes. He 

acknowledged, however, that some great earthquakes may 

have been preceded by too little time for much sediment to 

accumulate in canyon heads (Adams, 1990, p. 580). He further stated that a few turbidites in some cores may have 

resulted from lesser earthquakes on faults in the North 

America plate. Such earthquakes, he argued, may account 

for tributary-channel cores that contain 14, 15, or 16 postMazama turbidites.

If the turbidites and plate-boundary earthquakes correspond one for one, the mean recurrence interval for 13 postMazama earthquakes is between about 550 and 630 years. 

This range is based on the assumption that the earliest postMazama turbidite—the lowest one containing Mazama ash 

accumulated between 6900 and 7800 years ago (Adams, 

1990, p. 576), and that the most recent turbidite formed 

close to 300 years ago (Adams, 1990, p. 580). The 13 intervals inferred by Adams have a standard deviation of 170 

years (Adams, 1990, 1996) or less (Weichert and Adams, 

1995), as reviewed in the section beginning on page 102.

BURIED SOILS AT HUMBOLDT BAY, CALIFORNIA

Clarke and Carver (1992) cited buried soils exposed at 

low tide along Mad River Slough—an arm of Humboldt 

Bay—as evidence for earthquake recurrence intervals that 

average less than 500 years. They reported evidence for four 

or five earthquakes between about 1,450-1,750 years ago 

and about 300 years ago.

Clarke and Carver inferred that three of these events 

were accompanied by surface rupture of a nearby thrust 

fault in the North America plate. However, they proposed 

that this thrust faulting usually accompanies great earthquakes on the underlying plate boundary.

BURIED SOILS AT ESTUARIES IN 

NORTHERN OREGON

Darienzo and Peterson (1995) used buried soils less 

than 3,000 years old to estimate average recurrence intervals 

less than 600 calendric years for great earthquakes between 

the Necanicum River and Alsea Bay (Peterson and 

Darienzo, 1996). They reported that the average interval 

could be as little as about 200 calendric years.

The minimum average interval that we calculate from 

their data—about 400 years (fig. 3)—is longer for two reasons:

1. We use high-precision (quoted errors ±10-15 14C yr) 

radiocarbon ages that effectively lengthen the time 

between the most recent earthquake and its predecessors. 

The high-precision ages show that the most recent plateboundary earthquake, or earthquake series, occurred 

close to 300 years ago along most or all of the Cascadia 

subduction zone (Nelson and others, 1995) (see p. 81). 

Darienzo and Peterson (1995) used less precise radiocarbon ages, according to which this seismicity occurred as 

much as 680 years ago.

2. We average the last five of the recurrence intervals 

inferred by Darienzo and Peterson (1995). The shortest 

recurrence intervals estimated by Darienzo and Peterson 

(1995) are based on only the last four intervals. These 

intervals exclude the interval between their events 5 and 

6—the longest interval implied by their radiocarbon ages 

(Darienzo and Peterson, 1995, their fig. 4).
RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

INTERBEDDED PEAT AND MUD AT 

COOS BAY, OREGON

Nelson and others (1996a) presented evidence for a 

broad range of earthquake-recurrence intervals at Winchester Creek, which is part of the South Slough arm of Coos 

Bay. They evaluated 10 couplets of peat and mud, observed 

in cores, as evidence for coseismic subsidence in the past 

4,290-4,930 years.

Nelson and his coworkers inferred coseismic subsidence if a couplet met three main criteria: it records at least 

Vi m of relative sea-level rise, as judged from qualitative and 

quantitative analysis of foraminifera and diatoms; the submergence happened suddenly, as shown by abrupt changes 

in lithology and in microfossil assemblages; and the submergence also happened widely, as shown by stratigraphic 

correlation among cores and radiocarbon correlation among 

estuaries. They judged three of the couplets indicative of 

earthquakes but found insufficient evidence to determine 

whether six additional couplets represent earthquakes.

Nelson and others (1996a, p. 152) stated that an 

unknown fraction of the earthquakes occurred on the plate 

boundary; others may have occurred independently on 

faults in the North America plate.

All three of the confidently inferred earthquakes date 

from the past 2,350-3,240 years, and two other peaty layers 

that might or might not represent earthquakes also fall 

within this range. The corresponding average recurrence 

interval ranges from about 500 to 1,500 years (fig. 3). An 

average interval of at least 500 years is also indicated if the 

entire sequence of peat and mud records nine or fewer plateboundary earthquakes between about 300 and 4,290-4,930 

years ago.

BURIED SCARPS WEST OF WILLAPA BAY NEAR 

LONG BEACH, WASHINGTON

Meyers and others (1996) interpreted Pacific Ocean 

beach deposits as evidence for six plate-boundary earthquakes in the past 4,500 years. Using ground-penetrating 

radar, they identified eight different buried scarps near Long 

Beach, west of southern Willapa Bay (fig. 5). Meyers and 

others (1996) attributed each of these scarps to storm erosion consequent to earthquake-induced subsidence.

Six of the scarps are probably less than 4,500 years 

old; the other two, the earliest, are undated. Meyers and others (1996) acknowledged that their inferred record of earthquakes is incomplete if erosion following one earthquake 

removed the scarp that resulted from a previous earthquake. 

If the most recent scarp originated about 300 years ago, and 

if each of the six dated scarps records an earthquake, the 

average recurrence interval is no greater than 840 years.

BURIED SOILS AT GRAYS HARBOR, 

WASHINGTON

Shennan and others (1996) reported evidence for eight 

coseismic subsidence events less than 5,000-5,400 years

old at the Johns River, an arm of Grays Harbor (fig. 5). They 

used assemblages of fossil diatoms, pollen, and foraminifera—all compared with assemblages on modern Johns 

River wetlands—to show that each of eight successive buried soils records sudden tidal submergence. The youngest 

five of these soils are preserved widely enough to provide a 

reliable record of earthquake recurrence. They can be seen 

in an outcrop several tens of meters long and in five correlated cores distributed along a transect that extends 250 m 

landward from the outcrop.

Shennan and others (1996) dated the subsidence events 

by obtaining radiocarbon ages on peat in the soils. Such peat 

can be younger or centuries older than the subsidence event 

that the peat is meant to date (Nelson, 1992b). Shennan and 

others (1996) interpreted the ages as "not dissimilar to others reported from the Cascadia subduction zone." The most 

recent of the eight earthquakes inferred by Shennan and others (1996) occurred close to 300 years ago, and the earliest 

of the earthquakes probably occurred no more than 5,000- 

5,400 years ago. This history implies an average recurrence 

interval no greater than 670-730 years.

STRUCTURAL SETTING AND MODERN

TIDAL WETLANDS OF NORTHEASTERN

WILLAPA BAY

GEOLOGIC STRUCTURE

The northeastern part of Willapa Bay flanks and 

crosses a late Cenozoic uplift herein termed the South Bend 

antiform (fig. 5). This structural high, which extends about 

40 km in a north-northwestward direction, is defined mainly 

by an exposed core of Crescent Formation—Eocene pillow 

basalt, basaltic breccia, and basaltic sedimentary rock (Wagner, 1967a, b). Near the northern and southern ends of the 

antiform the Crescent is flanked to the east and west by 

younger sedimentary strata of Eocene and Oligocene age. 

North of Raymond, in the eastern part of the antiform, sedimentary rocks within the Crescent dip to the east and northeast, approximately parallel to dips in strata that probably 

rest conformably on the Crescent (Wagner, 1967a). The 

antiform may be bounded on the west by an east-dipping 

thrust fault (Cowan and Potter, 1986; Palmer and Lingley, 

1989, p. 11; Snavely and Wells, 1996, p. 163). Small but 

abundant outcrops of the Crescent near the Willapa River 

demonstrate that the antiform crosses the Willapa River, 

which forms the easternmost arm of Willapa Bay (fig. 6).

The South Bend antiform has probably grown in postMiocene time and has undergone net uplift, with or without 

continued folding, in the late Quaternary. Flanks of the antiform trend parallel to synclines that deform rocks as young 

as Miocene (fig. 5). Net Quaternary uplift is shown by estuarine and shallow marine deposits of probable Pleistocene 

age; Wagner (1967a, b) mapped such deposits on the antiform to altitudes of 180 m. However, this net uplift extended 

beyond the antiform into other areas bordering Willapa Bay
STRUCTURAL SETTING AND MODERN TIDAL WETLANDS OF NORTHEASTERN WILLAPA BAY

124°

47C

Copalis 

River

': iiiftii$iiii

10km

5Sy-- 

t«Efoff"^lPC^

Slough Blue 

Slough

Area of

detail

in figure 6 =

X

EXPLANATION

MAP UNITS 

Tidal-wetland deposits (Holocene)

Crescent Formation (Eocene)

FOLDS

Anticline

Syncline

Age of youngest rocks mapped as 

folded—Miocene unless labeled O 

(Oligocene) or E (Eocene)

EVIDENCE FOR COSEISMIC 

SUBSIDENCE

Buried soil or soils in Holocene estuarine

deposits

T Dated by radiocarbon methodsChiefly from Atwater (1992)

• Dated by matching of ring-width 

patterns—Redcedar snags 

(Yamaguchi and others, 1997) 

Uppermost soil covered by tsunami

• deposits—Reinhart and Bourgeois

(1987)

In large outcrops far from buried 

O soils that have been dated or are

covered by tsunami deposits—

B.F. Atwater and M.A. Reinhart

(unpub. data)

•k Buried scarp in Holocene beach depositsMeyers and others (1996) 

V Buried soil or soils in Pleistocene estuarine

deposits—Details in table 1

MODERN DIATOM SAMPLE 

S Locality sampled by Hemphill-Haley 

(1993,1995a; our table 2)»B, Bay 

Center; L, Leadbetter Point; S, Stony 

Point; T, Toke Point; W, Willapa River. 

Additional samples from Querquelin 

locality and Niawiakum River (fig. 6)

Figure 5. Cenozoic folds in relation to some of the sites having evidence for coseismic subsidence of Quaternary age in 

southern coastal Washington. Most sites plotted on the map comprise at least one outcrop tens of meters long that widely 

displays one or more successive buried soils. Many additional sites not plotted, mainly in areas of modern tidal wetlands. 

Folds in Tertiary rocks and limits of Crescent Formation are from compilation by Walsh and others (1987).

and Grays Harbor, as shown by Pleistocene marine and estuarine deposits tens of meters above present sea level (Walsh 

and others, 1987). Such areas include a Neogene syncline 

along the Naselle River (fig. 5), where Wells (1989) mapped 

a terrace of Pleistocene deposits. Consequently, the presence of uplifted Pleistocene deposits on the South Bend 

antiform need not signify that the antiform has grown in the 

late Quaternary.

We say "net" Quaternary uplift because estuarine 

deposits at Willapa Bay contain buried soils indicative of 

sudden subsidence. The soils, located both on and beside

the South Bend antiform (fig. 5), demonstrate repeated 

coseismic subsidence of Holocene age (p. 73) and may also 

record repeated coseismic subsidence of Pleistocene age 

(table 1; Peterson and Darienzo, 1989). Such subsidence is 

compatible with net uplift if cumulative interseismic uplift 

exceeds cumulative coseismic subsidence (Atwater, 1987). 

Rates of present-day interseismic uplift at Willapa Bay are 

poorly known; geodetic estimates for the past 50 years 

range from near zero (Holdahl and others, 1989; Mitchell 

and others, 1994) to several millimeters per year (Ando and 

Balazs, 1979; Hyndman and Wang, 1995).
10 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

123° 52.5' 123° 45'

46° 37.5'

EXPLANATION

Estuarine water and tidal flat >^ Strike and dip of bedding

D Town O Other locality 

N

Tidal marsh and tidal swamp

Floodplain

Upland underlain by:

Estuarine and fluvial deposits (Pleistocene) 

Sedimentary rocks (Eocene and Oligocene) 

Crescent Formation (Eocene)

Road

0 5 km

Figure 6. Index map of northeastern Willapa Bay. Geology generalized from Wagner (1967a, b).
STRUCTURAL SETTING AND MODERN TIDAL WETLANDS OF NORTHEASTERN WILLAPA BAY 11

Table 1. Buried soils in Pleistocene deposits near Willapa Bay 

[Listed north to south, corresponding to open triangles in fig. 5 except for third entry, which is near solid triangle labeled B in fig. 5]

Location Diagnostic features

NE/4 NE/4 sec. 33 and SE/4 SE/4 sec. 28, T.14N., R.10W. 

Bay Center 7.5-minute quadrangle; lowermost 2 m of 

bayside cliff

W/2 SW/4 sec. 1, T.13N., R.10W, South Bend 7.5-minute 

quadrangle; roadcuts along west side of South Bend - 

Palix River Road

NW/4 SW/4 sec. 4, T.13N., R.10W., Bay Center 7.5- 

minute quadrangle (H.E. Clifton, oral cornmun., 1989); 

bayside cliff

In sec. 37, T. 13N., R. 10W., 820 m south-southeast of 

Goose Point, Bay Center 7.5-minute quadrangle; 

lowermost 3 m of bayside cliff

NE/4 NW/4 SW/4 sec. 3, T.12N., R.10.W., Neman 7.5- 

minute quadrangle; lowermost 2 m of bayside cliff

SE/4 SE/4 SW/4 sec. 3, T.12N., R.10W., Neman 7.5- 

minute quadrangle; 2-6 m above base of bayside cliff

NW/4 SE/4 sec. 10, T.12N., R.10W., Neman 7.5-minute 

quadrangle; 5 m above base of bayside cliff

Woody organic horizon 5-15 cm thick, about 300 m long, abruptly overlain by 

mud; this mud contains a dark gray layer about 1 cm thick, perhaps the faint 

surficial horizon of a second buried soil

At least two superposed carbonaceous layers 2-5 cm thick, separated from one 

another by mud

Mud with many traces of roots and rhizomes, conformably overlain by lightercolored mud that lacks such traces (Clifton, 1983, his fig. 9)

Mud containing many rhizomes of Triglochin maritimum, abruptly overlain by 

locally woody mud that lacks rhizomes

Three dark, probably carbonaceous layers 2-4 cm thick and about0.5 m apart, 

separated from one another by mud. Horizontal laminations in the mud 

resemble those of the tidal-marsh deposits photographed by Clifton (1983, 

his figs. 9,14); much of the mud also contains rhizomes of Triglochin 

maritimum

Dark, probably carbonaceous layers, at least two of them below and another 

above a surface of nondeposition marked by a woody, probably upland soil. 

Lowest layer conformably overlies silt and clay having low-angle crossbeds 

consistent with intertidal or subtidal origin

Two successive beds, each about 0.3 m thick, in which mud with rhizomes 

grades upward into peaty mud or peat whose upper contact is abrupt. Lower 

bed abruptly overlies the A horizon of an upland soil. Bioturbated mud, 

probably deposited on a tideflat, conformably overlies the upper bed

TIDES

Willapa Bay is subject to two daily low tides and two 

daily high tides of unequal height. Mean lower low water, 

the standard datum for nautical charts in the western United 

States, is the average of the lower of the two daily low tides; 

mean higher high water similarly represents the average of 

the higher of the two daily high tides.

The typical difference between mean lower low water 

and mean higher high water is about 3 m. Extreme astronomical tides expand the height of the intertidal zone to as 

much as 4.5 m.

TIDAL-WETLAND ENVIRONMENTS

Many of the buried soils described in this report represent vegetated wetlands dominated by herbaceous plants 

(marshes) or by trees (swamps). Modern analogs for these 

wetlands can be found high in the intertidal zone along 

streams tributary to Willapa Bay (fig. 7).

Most modern tidal marshes at Willapa Bay form nearly 

horizontal surfaces at or slightly above mean higher high 

water (high marsh). Additional tidal marsh of lower altitude

(low marsh) descends to tidal flats and channels (fig. 8). 

The high marsh is mostly about 1 m higher than tidal mud 

flats being colonized by Triglochin maritimum, a dominant 

low-marsh plant. This difference in level is labeled "D" in 

figure 8.

Tidal swamps of modern Willapa Bay are restricted to 

mostly freshwater reaches of the bay's tidal streams. 

Though they resemble uplands in some of their vegetation, 

they are not much if any higher in the intertidal zone than 

are high marshes farther downstream. We found no such difference in level by surveying the tidal marsh and tidal 

spruce swamp along the Niawiakum River near the Pool 

locality (fig. 8).

Vascular-plant species of the modern wetlands are 

commonly zoned with respect to tide level and salinity 

(Weinmann and others, 1984; Kunze and Cornelius, 1982). 

On brackish-water high marshes the dominant species are 

tufted hair-grass (Deschampsia caespitosa), Baltic rush 

(Juncus balticus), and Pacific silverweed (Potentilla pacified). Pickleweed (Salicornia virginica) is common in saline 

parts of high marshes and in the transition between high 

marshes and low marshes. A mainly upland tree, Sitka 

spruce (Picea sitchensis), is joined by western crabapple
12 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Niawiakum River

Oyster and Redtail localities (figs. 13-23)

123° 55'

46° 38' -

Heron and Pool localities (figs. 24-26)

123° 54'

46° 37'

South Bend - 

Palix River Road

Line of cross section 

in Atwater (1987)

Spruce

QL-4405, -4406J

Pool

46° 42'

46° 40'

EXPLANATION 

Upland

Tidal marsh or tidal swamp—

Along Willapa River, includes 

areas diked or filled historically

Estuarine water and tidal flat 

£-—^ Seaward direction of river flow

—— Surveyed outcrop

•—• Vegetation transect (fig. 8)

•QL 14c locality of Atwater and others (1991)

-•-. Road

46° 41'

500 m r

Willapa River

Airport locality (fig. 31)

123°50'

Sewer locality (figs. 29-30)

123° 48'

Jensen locality (figs. 27-28)

123° 46'

North boundary 

""*•••..... of historical tidal marsh 

**••.approximately located., 

west of here

J ^^

Figure 7. Location and setting of outcrops and transects surveyed along the Willapa and Niawiakum Rivers.
STRUCTURAL SETTING AND MODERN TIDAL WETLANDS OF NORTHEASTERN WILLAPA BAY 13

Willapa River

Sewer

D = 1.0-1.6 m

+0.46,16 July 1994 

+0.21,15 July 1994 

+0.06, 27 July 1994

-0.70, 22 July 1994

-0.18, 26 July 1994

Oyster

0 = 0.8-1.4m

.,+0.34, 28 July 1994

,-0.24, 13 July 1994 

-0.43, 12 July 1994 

x -0.67, 28 July 1994

Airport

0 = 0.8-1.1 m

-0.52, 8 July 1994

Niawiakum River

Redtail

D = 0.4-1.5m

Bone River 

0 = 0.8-1.5m

-V -y-

+0.67,1 July 1994

0

i, 1b

Pool

D = 1.1-1.8 m

-0.40, 4 July 1992 

^-0.61,3 July 1992

-0.52,11 July 1994

-0.73,2 July 1992

EXPLANATION

Peaty 

soil \

Range of tides

1ml

Sitka spruce

Crabapple

viKHr- High-marsh herbs

y I ^ -MTriglochin 

maritimumMud

Vertical range of

Z=- High marsh and spruce swamp 

— Low marsh

— Tidal flat

Height range of land surface

Oi Number of height measurements 

a, algal mat 

b, bank of high marsh 

i, interior of high marsh 

s, spruce swamp 

T, lowest Triglochin maritimum

rj Difference in height between high marsh 

and lowest Triglochin maritimum

— +2.01, 2 Aug 1994 Low or high tide from which 

low-water datum on y-axis is estimated. 

Number in hundredths denotes 

predicted tide level, in meters above 

mean lower low water at Aberdeen

On

tidal

wetland

Figure 8. Vertical zonation of vascular plants in six tidal marshes and swamps at Willapa Bay. Zonation measured at surveyed 

outcrops and also, along the Niawiakum River, at transects plotted in figure 7. Topographic profiles schematic. Datum for each profile 

is mean lower low water estimated from tides tabulated under each profile.
14 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

(Pyrus fused) in tidal swamps served mainly by freshwater. 

Seaside arrowgrass (Triglochin maritimuni) is the chief vascular plant native to salt- and brackish-water low marshes; 

upstream it is supplanted by Lyngby's sedge (Carex lyngbyei), which is also common in brackish-water high 

marshes.

Diatoms also define vertical zones in wetlands at the 

Cascadia subduction zone, both in central Oregon (Nelson 

and Kashima,1993) and at Willapa Bay (Hemphill-Haley, 

1995a). Diatom assemblages at Willapa Bay differ between 

marshes and tidal flats, and they also differ between low 

marshes and high marshes. Examples of important members 

of these assemblages are shown in figures 9 and 10.

Several statistical methods have been used to relate 

assemblages of microscopic organisms to vertical position 

in the intertidal zone at the Cascadia subduction zone (Jennings and Nelson, 1992; Nelson and Kashima, 1993; 

Mathewes and Clague, 1994; Hemphill-Haley, 1995a; Guilbault and others, 1995; Shennan and others, 1996). We use a 

simple ratio, termed the brackish intertidal diatom index 

(BDDI), that is consistent with results of Hemphill-Haley's 

(1995a) Q-mode factor analysis of samples from modern 

marsh deposits at Willapa Bay.

The index, which ranges from 0 to 1, summarizes proportions of taxa found mainly in modern high marshes (H), 

low marshes (L), both high and low marshes (B), or tidal 

flats and shallow subtidal channels (T) (table 2):

BIDI = (H + B + 0.5L)/(H + B + L + T).

The value for low-marsh taxa receives partial weight in the 

numerator to treat low marsh as transitional between tidal 

flat and high marsh.

The index ranges from 0 to 0.29 in tidal flats and tidal 

channels, 0.29 to 0.65 in low marshes, and 0.65 to 1.00 in 

high marshes. These ranges come from 63 intertidal sites at 

Willapa Bay and Puget Sound (table 3; fig. 11). One tidalflat sample contained a low-marsh assemblage, probably 

because of diatoms transported from a nearby tidal marsh 

(table 3, sample SFW-1). Some of the sites are located 

along leveled transects at the Niawiakum River (fig. 12).

UPLANDS

We use "upland" to denote typically forested land at or 

above extreme high water. Unlike tidal marshes, such 

uplands at Willapa Bay are too shady and too dry in summer 

to support large numbers of herbaceous plants and diatoms.

The few diatoms found on uplands adjoining the bay's 

modern tidal marshes are poorly preserved marsh and tidalflat species. Such assemblages were found at 10 upland sites 

at Willapa Bay, 8 of which adjoin tidal marshes of the Niawiakum River (table 3). Each site is located at the edge of 

tidal wetland, where it is probably subject to inundation by 

the highest astronomical tides, particularly those augmented 

by storm surges. The meager assemblages of diatoms at

these upland sites contrast with abundant freshwater diatoms in freshwater tidal swamps along the Willapa River, 

upstream of Raymond (fig. 6).

METHODS

We made detailed studies of earthquake recurrence 

along two tidal streams: the Niawiakum River, at the western edge of the South Bend antiform; and the Willapa River 

where it crosses the antiform (fig. 6). Along each stream we 

studied buried soils in low-tide outcrops several meters high 

and tens or hundreds of meters long. Most of the field work 

was done in July 1994 except along the Niawiakum River at 

the Redtail locality (studied intermittently 1986-1992) and 

the Pool and Heron localities (surveyed 1995). From several 

of the outcrops we collected samples for study of fossil diatoms, and from most of the outcrops we also collected samples for radiocarbon dating. Results of the surveys, and of 

subsequent laboratory studies, are shown in figures 13-32.

DELINEATION OF STRATIGRAPfflC UNITS

We surveyed stratigraphy at each of seven localities. 

Details from these surveys are shown in outcrop diagrams 

(figs. 7, 13, 18, 24, 25, 27, 29, 31). We paid most attention 

to the identification and delineation of buried soils. We also 

noted where the soils are truncated by channel fills—bodies 

of mud that locally contain much woody detritus, inclined 

bedding, or both. In addition, at four localities we noted 

depths to the top of Pleistocene deposits, which we identified as mud that is markedly stiffer and greener or yellower 

than any of the overlying estuarine deposits of Holocene 

age.

IDENTIFICATION OF BURIED SOILS

Buried soils of former marshes and swamps punctuate 

the stratigraphy of the surveyed outcrops. Each buried soil 

has a dark uppermost layer interpreted as a buried A or O 

horizon. We refer to this layer as a "surficial horizon." We 

call a soil "bold" if its surficial horizon contains peaty mud 

or peat 2-10 cm thick. In a "faint" soil, by contrast, the surficial horizon consists of 1-2 cm of mud that is merely 

darker, by one value unit in the Munsell color system, than 

mud above and below it.

The interpretive term "soil" applies without question 

where a surficial horizon contains growth-position roots 

attached to tree stumps or herbaceous stems and leaves 

that are entombed in mud above the surficial horizon. Such 

fossils identify the surficial horizon as material on which 

plants grew.

Other, more subtle features justify the use of "soil" in 

faint soils, which typically lack growth-position fossils 

other than rootlets injected from higher in the stratigraphic 

section. Most of the faint soils resemble bold soils in lateral
METHODS 15

Figure 9. Examples of diatom species most commonly found in high marshes (1-5) or in both high and low marshes (6-13).
16 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

'-*' v;

Melosira nummuloides

Trachysphenia australis

Figure 10. Examples of diatom species most commonly found in low marshes (1-8) or in tidal flats, tidal sloughs, and shallow subtidal 

channels (9-18).
METHODS 17

Table 2. Diatom taxa used for the brackish intertidal diatom index

Diatoms representative of high marshes

Cosmioneis pusilla (Wm. Smith) Mann and Stickle in

Round and others, 1990 

Denticula subtilis Grunow, 1862 

Navicula tenelloides Hustedt, 1937 

Nitzschia brevissima Grunow in Van Heurck, 1881 

Nitzschia pusilla (Kiitzing) Grunow, 1862 

Nitzschia terrestris (Peterson) Hustedt, 1934 

Pinnularia lagerstedtii (Cleve) Cleve-Euler, 1934 

Tryblionella aerophila (Hustedt) Mann in Round and

others, 1990

Diatoms representative of high or low marshes

Colonels hoc ilium (Grunow) Cleve, 1894

Diploneis pseudovalis Hustedt, 1930

Frustulia vulgaris (Thwaites) De Toni, 1891

Luticola mutica (Kiitzing) Mann in Round and others, 1990

Navicula cincta (Ehrenberg) Ralfs in Pritchard, 1861

Nitzschia commutata Grunow in Cleve and Grunow, 1880

Nitzschia fasciculata (Grunow) Grunow in Van Heurck,

1881 

Tryblionella debills Arnott in O'Meara, 1873

Diatoms representative of low marshes

Caloneis westii (W. Smith) Hendey, 1964

Diploneis interrupta (Kiitzing) Cleve, 1894

Diploneis smithii var. rhombica Mereschkowsky, 1902

Frustulia linkei Hustedt, 1952

Gyrosigma eximium (Thwaites) Boyer, 1927

Mastogloia exigua Lewis, 1862

Navicula cryptotenella Lange-Bertalot in Krammer and

Lange-Bertalot, 1985 

Nitzschia bilobata W. Smith, 1853 

Nitzschia scapelliformis Grunow in Cleve and Grunow,

1880

Rhopaloda tnusculus (Kutzing) Miiller, 1899 

Scoliopleura tumida (Brebisson ex Kutzing) Rabenhorst,

1864 

Tryblionella navicularis (de Brebisson ex Kutzing) Ralfs in

Pritchard, 1861

Diatoms representative of muddy intertidal flats, 

sloughs, and shallow subtidal areas

Bacillaris paradoxa Gmelin, 1791

Campylodiscus echineis Ehrenberg, 1840

Catenula adhaerens Mereschkowsky, 1903

Cerataulus turgidus (Ehrenberg) Ehrenberg, 1843

Cocconeis scutellum Ehrenberg, 1838

Cocconeis scutellum var. parva Grunow ex Cleve, 1895

Gyrosigma acuminatum (Kutzing) Rabenhorst, 1853

Gyrosigma balticum (Ehrenberg) Rabenhorst, 1853

Melosiramoniliformis (Miiller) Agardh, 1824

Melosira nummuloides Agardh, 1824

Navicula digitoradiata (Gregory) Ralfs in Pritchard, 1861

Navicula tripunctata (Miiller) Bory, 1824

Nitzschia sigma (Kutzing) W. Smith, 1853

Nitzschia socialis Gregory, 1857

Odontella aurita (Lyngbye) Agardh, 1832

Opephora parva Krasske, 1939

Plagiogramma staurophorum (Gregory) Heiberg, 1863

Rhaphoneis amphiceros (Ehrenberg) Ehrenberg, 1844

Rhaphoneis psammicola Riznyk, 1973

Synedrafasciculata (Agardh) Kiitzing, 1844

Trachyneis aspera (Ehrenberg) Cleve, 1894

Tryblionella acuminata W. Smith, 1853

Tryblionella apiculata Gregory, 1857

Tryblionella coarctata (Grunow) Mann in Round and

others, 1990

Tryblionella levidensis Wm. Smith, 1856 

Tryblionella littoralis (Grunow) Mann in Round and

others, 1990 

Tryblionellaplana (W. Smith) Mann in Round and others,

1990

Tryblionellapunctata W. Smith 1853 

Tryblionella vexans (Grunow) Mann in Round and others,

1990

Diatoms representative of sandy intertidal flats, sloughs, 

and shallow subtidal areas

Achnanthes delicatula (Kiitzing) Grunow in Cleve and

Grunow, 1880

Amphora proteus Gregory, 1857 

Amphora ventricosa (Gregory) Hendey, 1951 

Cocconeis diminuta Pantocksek, 1902 

Cocconeis peltoides Hustedt, 1939

Dimeregramma minor (Gregory) Ralfs in Pritchard, 1861 

Fallacia cryptolyra (Brockmann) Stickle and Mann in

Round and others, 1990 

Lyrellla lyra (Ehrenberg) Karayeva, 1978 

Navicula cancellata Donkin, 1872 

Petroneis granulata (Bailey) Mann in Round and others,

1990

Petroneis marina (Ralfs) Mann in Round and others, 1990 

Trachyneis australis (Ehrenberg) Cleve, 1894

extent and relief (figs. 13, 18, 24, 25, 27, 29, 31). Some 

grade laterally into bold soils in which vascular-plant fossils 

are preserved (p. 45, 69). In sections studied for fossil diatoms, faint soils contain diatom assemblages typical of tidal 

marshes (figs. 15, 20).

LETTER NAMES FOR BURIED SOILS

We assign each surveyed soil a name, such as "soil S," 

in which the capital letter denotes inferred position in a

sequence of soils that we regard as correlative throughout 

the study area (p. 83; fig. 32). For the youngest soil we use a 

letter (Y) near the end of the alphabet; for progressively 

older soils we use letters nearer the front of the alphabet.

Our complete sequence of letter names is H, J, L, N, S, 

U, W, and Y. Gaps in the sequence of letters leave room for 

soils that we may have overlooked—a possibility evaluated 

in the section entitled "Additional, Unrecorded Events" 

(P- 94).
18 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON 

Table 3. Brackish intertidal diatom index (BIDI) of modern surface samples from Willapa Bay and Puget Sound

BIDI Sample Region Site (figure number for location 

map or graph, in parentheses)

Environment

0.00

0.00

0.00

0.00

0.00

0.01

0.01

0.01

0.03

0.03

0.03

0.03

0.08

0.09

0.12

0.12

0.12

0.15

0.18

0.21

0.23

0.27

0.28

0.30

0.30

0.34

0.34

0.44

0.53

0.56

0.56

0.56

0.58

0.61

0.62

0.62

0.62

0.64

0.64

0.72

0.75

0.76

BC-1

BC-2

BC-4

LC-3

SP-2

BR-1

LC-1

T4-S

BR-2

LC-1 7

LC-7

SP-1

LP-1

T3-S

SP-3

T3-1

TP-1

T3-2

SFW-2

T3-3

LC-1 4

T4-1

T4-2

LP-2

SP-4

SFW-1

TP-2

TP-3

LC-11

LC-5

LP-3

T3-4

T4-3

PL-2

LC-10

LC-9

PL-1

LC-8

SFW-3

LC-1 5

LC-1 2

T4-4

Willapa Bay

Willapa Bay

Willapa Bay

Puget Sound

Willapa Bay

Willapa Bay

Puget Sound

Willapa Bay

Willapa Bay

Puget Sound

Puget Sound

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Puget Sound

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Puget Sound

Puget Sound

Willapa Bay

Willapa Bay

Willapa Bay

Puget Sound

Puget Sound

Puget Sound

Puget Sound

Puget Sound

Willapa Bay

Puget Sound

Puget Sound

Willapa Bay

Bay Center (6)

Bay Center (6)

Bay Center (6)

Lynch Cove (1)

Stony Point (5)

Querquelin (6)

Lynch Cove (1)

upper Niawiakum River (7, 12)

Querquelin (6)

Lynch Cove (1)

Lynch Cove (1)

Stony Point (5)

Leadbetter Point (5)

lower Niawiakum River (7, 12)

Stony Point (5)

lower Niawiakum River (7, 12)

Toke Point (5)

lower Niawiakum River (7, 12)

South Fork Willapa River (5)

lower Niawiakum River (7, 12)

Lynch Cove (1)

upper Niawiakum River (7, 12)

upper Niawiakum River (7, 12)

Leadbetter Point (5)

Stony Point (5)

South Fork Willapa River (5)

Toke Point (5)

Toke Point (5)

Lynch Cove (1)

Lynch Cove (1)

Leadbetter Point (5)

lower Niawiakum River (7, 12)

upper Niawiakum River (7, 12)

Potlatch(l)

Lynch Cove(l)

Lynch Cove (1)

Potlatch(l)

Lynch Cove (1)

South Fork Willapa River (5)

Lynch Cove (1)

Lynch Cove(l)

upper Niawiakum River (7, 12)

Sandy tidal flat

Sandy tidal Hat

Sandy tidal Hat

Sandy tidal flat 10m from edge of marsh

Muddy tidal flat

Muddy tidal flat

Sandy tidal flat

Shallow (~ 0.5 m) subtidal channel

Muddy tidal flat

Floor of tidal slough

Floor of tidal slough

Muddy tidal flat

Tidal flat mud

Shallow (—3 m) subtidal channel

Muddy tidal flat

Muddy bank with Zostera marina

Muddy tidal flat

Muddy bank of tidal stream

Muddy bank of tidal stream

Muddy bank with Zostera japonica

Tidal slough with Triglochin maritimum on nearby slump

block

Muddy bank of tidal stream

Muddy bank of tidal stream

Triglochin maritimum low marsh at edge of tidal flat

Low marsh with Triglochin maritimum and Salicornia

virginica

Muddy bank of tidal stream

Low marsh with Triglochin maritimum and Salicornia

virginica

Low marsh with Triglochin maritimum and Salicornia

virginica

Low marsh with Distichlis spicata and Salicornia

virginica

Low marsh with Distichlis spicata and Salicornia

virginica

Low marsh with Distichlis spicata and Salicornia

virginica

Low marsh on steep bank with Salicornia and Jaumea

Low marsh on steep bank with Carex lyngbyei

Low marsh with Salicornia, Distichlis, and Jaumea

Low marsh with Distichlis spicata and Salicornia

virginica

Low marsh with Distichlis spicata and Salicornia

virginica

Low marsh with Salicornia and Jaumea

Low marsh with Distichlis spicata

Low marsh on steep bank with Carex lyngbyei

Marsh transitional between low and high; Juncus balticus

and Distichlis spicata

Marsh transitional between low and high; Juncus balticus

and Distichlis spicata

Marsh transitional between low and high; Carex lyngbyei,

Deschampsia caespitosa, and Potentilla pacifica
METHODS 19 

Table 3. Brackish intertidal diatom index (BIDI) of modern surface samples from Willapa Bay and Puget Sound—Continued

BIDI Sample Region Site (figure number for location Environment

0.79

0.84

0.84

0.86

0.89

0.90

0.91

0.92

0.93

0.93

0.94

0.96

0.97

0.97

0.97

0.97

0.98

0.99

0.99

0.99

1.00

none

none

none

none

none

none

none

none

none

none

L

S

S

S

T

S

T

S

S

T

T

T

L

L

r

T

T

L

L

PI

L(

t

T

T

T

R

R

H

H

T

T

map ui giapu, 111 pai v^uiuv^^a ;

:-is

W-5

W-7

>-5

-8

W-4

-5

W-8

W-6

-6

1-5

i-6

:-2i

>-4

!-7

^-7

k8

:T-S

>-5

.-3

:T-6

-Ul

-U2

-U3

-U4

r-i

r-2

i

2

-Ul

-U2

Puget Sound

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Puget Sound

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Puget Sound

Willapa Bay

Puget Sound

Puget Sound

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Willapa Bay

Lynch Cove (1)

South Fork Willapa River (5)

South Fork Willapa River (5)

Stony Point (5)

lower Niawiakum River (7, 12)

South Fork Willapa River (5)

upper Niawiakum River (7, 12)

South Fork Willapa River (5)

South Fork Willapa River (5)

upper Niawiakum River (7, 12)

lower Niawiakum River (7, 12)

lower Niawiakum River (7, 12)

Lynch Cove(l)

Leadbetter Point (5)

lower Niawiakum River (7, 12)

upper Niawiakum River (7, 12)

upper Niawiakum River (7, 12)

Lynch Cove (1)

Leadbetter Point (5)

Potlatch (1)

Lynch Cove (1)

Niawiakum River, near Oyster

locality (7)

Niawiakum River, near Oyster

locality (7)

Niawiakum River, near Oyster

locality (7)

Niawiakum River, near Oyster

locality (7)

Niawiakum River, Redtail

locality (7)

Niawiakum River, Redtail

locality (7)

Niawiakum River, Heron locality

(7)

Niawiakum River, Heron locality

(7)

Willapa River about 5 km east of

Raymond (6)

Willapa River about 5 km east of

Raymond (6)

Marsh transitional between low and high; Juncus balticus

and Distichlis spicata

High marsh of Carex lyngbyei

High marsh of Carex lyngbyei

High marsh of Potentilla pacifica

Landward edge of high marsh; Carex obnuta, Achillea

borealis

Landward edge of high marsh; Potentilla, Aster, grasses

High marsh dominated by Potentilla pacifica and

Deschampsia caespitosa

High marsh; Carex lyngbyei, Potentilla pacifica, Aster,

Deschampsia

High marsh dominated by Potentilla pacifica and

Deschampsia caespitosa

High marsh dominated by Potentilla pacifica and

Deschampsia caespitosa

High marsh dominated by Distichlis spicata and

Deschampsia caespitosa

High marsh of Deschampsia caespitosa

High marsh dominated by Juncus and Aster

High marsh; Carex lyngbyei and Distichlis spicata

High marsh; Potentilla pacifica and Distichlis spicata

High marsh; Potentilla pacifica and Deschampsia

caespitosa

High marsh; Potentilla pacifica and Juncus balticus

Grassy meadow lacking halophytes but containing

driftwood

High marsh; Potentilla pacifica and Deschampsia

caespitosa

High marsh; Juncus balticus and Deschampsia caespitosa

Grassy meadow lacking halophytes but containing

driftwood

Upland forest <0.5 m above and <5 m landward from

landward edge of high marsh

Upland forest <0.5 m above and <5 m landward from

landward edge of high marsh

Upland forest <0.5 m above and <5 m landward from

landward edge of high marsh

Upland forest <0.5 m above and <5 m landward from

landward edge of high marsh

Upland forest <0.5 m above and <5 m landward from

landward edge of high marsh

Upland forest <0.5 m above and <5 m landward from

landward edge of high marsh

Upland forest <0.5 m above and <5 m landward from

landward edge of high marsh

Upland forest <0.5 m above and <5 m landward from

landward edge of high marsh

Upland forest <0.5 m above and <5 m landward from

landward edge of tidal swamp

Upland forest <0.5 m above and <5 m landward from

landward edge of tidal swamp
20 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Q

fi

X

0)

o

3

"CO

-g 

tr

0) "c

^i

CO

none 

0.90-1.00 

0.80-0.89 

0.66-0.79 

0.60-0.65 

0.50-0.59 

0.40-0.49 

0.30-0.39 

0.20-0.29 

0.10-0.19 

0-0.09

Upland

High marsh

Low marsh

Tidal flat,

tidal slough, or

subtidal channel

5 10 15 

Number of Observations

1.00

0.65

0.29

20

Upland samples

High-marsh 

samples

Low-marsh samples

Tidal-flat, tidal-slough,

or shallow-subtidal samples

Figure 11. Brackish intertidal diatom index for modern samples from western Washington (table 3).

CONTINUITY OF STRATIGRAPHIC FEATURES

We observed stratigraphic features between some surveyed points and inferred their existence between others—a 

distinction shown by contrasting symbols in the outcrop diagrams. Soils depicted as continuously observed were apparent from differential erosion in natural outcrop (for 

example, soils J, L, S, and Y in fig. 145; soil S in fig. 195- 

D) or were exposed where cleaned with a shovel (soil U in 

fig. 14C; soil Y in fig. 19D, E\ soils S, U, W, and Y in fig. 

285). Otherwise, we interpolated stratigraphic contacts 

between isolated exposures or borings.

We observed lateral continuity between bold and faint 

parts of soil N at the Redtail locality (fig. 13), soil U at the 

Oyster locality (fig. 18), and soil L at the Jensen locality 

(fig. 27). Although the outcrop diagrams show the change 

from bold to faint as abrupt, the changes are gradational 

across a few to many meters.

BORINGS AND DEPTH TO PLEISTOCENE DEPOSITS

We supplemented observations made in outcrop by 

using a gouge corer—a steel half-cylinder 2 cm in diameter 

and 1 m long—to find features not exposed at even the lowest tides. These features include soil H at the Redtail locality

(fig. 13), soils H and J at the Pool locality (fig. 24) and at the 

southern part of the Jensen locality (fig. 27), parts of soils N 

and L and all of soil H at the Heron locality (fig. 25), and 

Pleistocene deposits at all these localities.

We did not core to Pleistocene deposits more than a 

few meters below the low tide levels. Deep cores have been 

taken, however, for a highway bridge near the Oyster locality (p. 45). We use these highway borings, along with our 

shallower cores of Pleistocene deposits (figs. 13, 24, 25, 27) 

and downward projection of valley walls, to infer differences in depth to Pleistocene deposits and to deduce consequent differences in settlement from gradual compaction of 

Holocene mud (see section "Thickness of Mud Between 

Soils," p. 82).

Figure 12. Variation in brackish intertidal diatom index 

along leveled transects from high marsh to channel at the 

Niawiakum River. MHHW, mean higher high water; MLLW, 

mean lower low water; S, lowest sample. A, Transect Nl, 

which coincides with the vegetation transect at the Oyster 

locality (figs. 7, 8). B, Transect Nu, located about 0.5 km 

northwest of the Pool locality, just north of the South BendPalix River Road and on the east side of the Niawiakum 

River.
METHODS 21

Northeast

Transect Nl - lower Niawiakum River at Oyster locality

Southwest

10

Distance along transect, in meters

Sample location

High-marsh plants, 

principally Deschamps/a 

and Potentilla

1 Low-marsh plants, 

principally Triglochin 

and Salicomia

Diatom assemblages

High marsh 

Low marsh

TidaTflat, tidal slough, or

subtidal channel 

s

B

East West

Transect Nu - upper Niawiakum River 0.5 km NW of Pool locality

2 

1 

0

§ -1 

^ -2

-3

-4

MHHW

-MLLW

i i i i i 

10 20 30 40 50 

Distance along transect, in meters

60

Diatom assemblages

High marsh

Low marsh

Tidal flat, tidal slough, or 

subtidal channel
22 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

PARTICLE SIZE

Mud above some of the soils—particularly soils S and 

Y—commonly contains more silt than does mud below their 

surficial horizons. This contrast is evident in well-washed 

outcrops in which layers of silt and sandy silt have been 

etched by water (fig. 30B). The contrast also can be seen on 

outcrops freshly faced with a shovel or machete; whereas 

the finer mud smears, the coarser mud peels roughly. This 

contrast aids in identification of buried soils.

Particle size was measured for samples from the Redtail locality (fig. 15). The samples, collected at an average 

interval of 6.5 cm, came from slabs sampled also for fossil 

diatoms. Organic matter was removed in 30 percent H2O2. 

Counts were made with a Cimax hydrophotometer and 

rapid settling analyzer at the U.S. Geological Survey in Palo 

Alto, California. Precision, estimated from calibration tests, 

is ±13 percent for the hydrophotometer and ±15 percent for 

the rapid settling analyzer. Particle-size distributions were 

determined with the technique of Torresan (1987).

ORGANIC CONTENT

Although we did not measure organic contents of the 

surficial horizons of the soils, such measurements have been 

made for a stratigraphic section containing three bold soils 

(probably correlative with soils L, S, and Y) and two faint 

soils (probably N and U) along the Johns River (figs. 5, 32; 

locality 14 of Atwater, 1992). The measurements, by means 

of loss on ignition, were made independently by James B. 

Phipps of Grays Harbor College (written commun., 1988) 

and by Shennan and others (1996, their fig. 3).

Both Phipps and the Shennan team reported organic 

contents of 30-50 percent of dry weight for 10 samples 

among bold soils L, S, and Y (horizons V, in, and I of Shennan and others, 1996). By contrast, they reported organic 

contents less than 10 percent for two samples each from 

faint soils N and U (horizons TV and II). They found little or 

no difference in organic content between the faint soils and 

the mud immediately above or below.

HORIZONTAL AND VERTICAL DATUMS

We used arbitrary points particular to each of the outcrops as datums for measuring positions of soils, channel 

fills, Pleistocene deposits, and fossils. Horizontal measurements were made by tape measure, vertical measurements 

with a tripod-mounted level.

The vertical measurements include the heights of one 

or more low slack tides at each locality. These tidal heights, 

and the tide level at Aberdeen (near Elliott Slough, fig. 5) as 

predicted in tide tables of the National Oceanic and Atmospheric Administration, are plotted on the outcrop diagrams.

FOSSILS

We depend mainly on vascular-plant fossils and diatom 

assemblages to assign ancient deposits to one of three depositional environments: high marsh, low marsh, and tidal flats 

or channels. We also use such evidence to estimate whether 

coseismic subsidence exceeded 1 m (p. 83).

VASCULAR PLANTS

Atwater and several coworkers identified vascularplant fossils in the field, mainly in their growth position. 

They include woody roots, chiefly of Sitka spruce; formerly 

above-ground stems and basal leaves of Deschampsia caespitosa, Potentilla pacifica, Juncus balticus, and Salicornia 

virginica; and below-ground stems (rhizomes) of Triglochin 

maritimum, Carex (probably Carex lyngbyei), and Distichlis 

spicata (figs. 13, 18, 24, 25, 27, 29, 31). For species names 

we used the taxonomy of Hitchcock and Cronquist (1973).

DIATOMS

Hemphill-Haley studied diatom assemblages from vertical slabs collected at the Oyster, Redtail, and Sewer localities. The diatoms were sampled, separated, and identified by 

procedures previously described (Hemphill-Haley, 1995a, 

b). For the Oyster and Redtail localities, Hemphill-Haley 

counted an average of 388 diatom valves per sample, of 

which an average of at least 100 proved useful for inferring 

depositional environment.

Results of the diatom counts are listed in tables 4-6. In 

each of these tables the diatoms are listed alphabetically in 

two categories, benthic and planktonic. Each table concludes with summary data that include the brackish intertidal diatom index (BIDI). This index is defined on page 14 

and is plotted in figures 15 and 20. Counts used to calculate 

the index are plotted in figures 16, 21, and 22, and photographs of important taxa are shown in figures 9 and 10.

CRITERIA FOR INFERRING UPLAND CONDITIONS

By analogy with diatom assemblages of modern 

uplands (p. 14), we interpret many fossil assemblages of 

sparse, poorly preserved diatoms as indicators of uplands 

that bordered tidal wetlands. Species typical of these assemblages have thick, sturdy valves that make them more resistant to breakage and dissolution than other diatoms. Some 

are species that live today in marshes, such as Cosmioneis 

pusilla, Pinnularia lagerstedii, and Tryblionella debilis. 

Others are typical of tidal flats—Delphineis cf. D. surirella, 

Cocconeis scutellum, Tryblionella granulata, and Paralia 

sulcata.

Several kinds of evidence generally confirm that such 

assemblages of fossil diatoms represent uplands at the Redtail locality. First, as discussed below under "Comparisons 

Between Diatoms and Vascular Plants" (p. 23), sparse,
METHODS 23

poorly preserved diatoms coincide with spruce roots in soils 

S and Y and with an archaeological site in soil Y. Second, 

upland conditions inferred for soils N and Y continue an 

upsection trend toward progressively higher land. This 

upsection trend is shown by upsection gradation from tidalflat assemblages, through low-marsh assemblages, to highmarsh assemblages (figs. 15, 16). The same trend may be 

present for soil S if a 12-cm gap between samples accounts 

for the lack of a high-marsh assemblage between low-marsh 

and upland assemblages.

Without such independent evidence, however, assemblages of sparse, poorly preserved diatoms need not signify 

upland conditions. Fully estuarine deposits may contain 

sparse, poorly preserved diatoms where conditions during 

or after deposition favor removal of diatoms (Denys, 1984; 

Sherrod and others, 1989).

SIGNIFICANCE OF PARALIA SULCATA

Paralia sulcata, which abounds in most of our fossil 

samples, shows little about vertical position at the time of 

deposition but may serve as an indicator of later dissolution. 

Though the species lives mainly on tidal flats, its valves 

may be found at all intertidal levels and on uplands as well 

(Nelson and Kashima, 1994; Hemphill-Haley, 1995a; Shennan and others, 1996). This large vertical range is due partly 

to ease of transport; the valves form chains easily suspended 

by tides. The abundance of Paralia sulcata also may reflect 

resistance to dissolution. Having sturdy, heavily silicified 

valves, Paralia sulcata may become concentrated relative to 

delicate, more soluble valves of other diatom species. The 

resulting predominance of Paralia sulcata in marsh and 

upland deposits may be mistaken as evidence for tidal-flat 

conditions (Hemphill-Haley, 1995b).

These points are illustrated by Paralia sulcata in samples from stratigraphic sections at the Redtail and Oyster 

localities. For these samples we plotted relative abundance 

of Paralia sulcata valves, defined as their percentage of the 

total number of diatoms counted (fig. 17).

In each of three sections the abundance of Paralia sulcata shows no overall correlation with the depositional environment inferred from other diatom species (fig. 17A-C). 

These environments include upland at the Redtail locality 

(fig. 17A). On a sample-by-sample basis the environmental 

significance of Paralia sulcata also is murky in two respects 

(tables 4-6):

1. Paralia sulcata at the Redtail locality is less abundant in 

tidal-flat deposits than in buried soils that formed high 

above the levels where the species would have lived. The 

species makes up about half the valves in tidal-flat 

deposits at 0.66, 0.70, 0.74, and 1.89 m. By contrast, 

Paralia sulcata makes up nearly three-quarters of the 

valves high in soils N (1.97 m) and S (1.68 m). Spruce 

roots and a scarcity of other diatoms show that the sample from soil S represents a spruce upland, where Paralia 

sulcata probably was deposited by storm surges and later

concentrated, relative to other species, by dissolution of 

the valves of other taxa. The sample from soil N connotes an upland or a high marsh beside an upland (p. 23). 

2. At the Oyster locality, Paralia sulcata is most abundant 

(82 percent) in a tidal-flat deposit at depth 1.18 m and 

horizontal coordinate 25 m. However, it is also abundant 

(about 70 percent) in high-marsh deposits at 0.95, 0.98, 

and 1.44m.

The abundance of Paralia sulcata varies widely 

depending on whether the diatom preservation is good 

(most valves intact, dissolution not evident), moderate 

(many valves broken, dissolution evident but minimal), or 

poor (most valves broken, dissolution severe) (fig. 17D-E). 

Where preservation is good or moderate, very abundant 

(>70 percent) Paralia sulcata may reflect high production 

of the species. Poor preservation of other diatoms, however, 

may also make Paralia sulcata relatively abundant, as 

shown by soil U at the Oyster locality (figs. 18, 20). Where 

soil U is bold and vascular-plant fossils well-preserved (horizontal coordinate 165 m), diatoms are abundant and 

diverse, and Paralia sulcata is correspondingly minor (6 

percent at depth 2.01 m; table 6). By contrast, where soil U 

is faint and vascular-plant fossils are lacking (horizontal 

coordinate 25 m), diatom preservation is similarly poor, and 

Paralia sulcata predominates (69 percent at depth 1.44 m; 

table 5).

COMPARISONS BETWEEN DIATOMS AND VASCULAR PLANTS

Environments inferred from the fossil diatom assemblages resemble those inferred from vascular-plant fossils in 

most cases where the inferences can be compared (figs. 15, 

20). Such comparisons can be made for fossils inferred to 

record upland conditions at the Redtail locality, high marsh 

at the Oyster locality, and transitions from tidal flat to low 

marsh at both these localities.

Diatom assemblages suggestive of uplands accord with 

vascular-plant fossils in two of three cases at the Redtail 

locality (fig. 15). The upland assemblages, marked by 

valves that are scarce and fragmented, are consistent with 

the presence of many spruce roots in growth position in 

soils S and Y (figs. 13, 145). The diatom assemblage in soil 

Y further accords with archaeological evidence for a fishing 

camp on the soil (p. 30). However, the lack of tree roots in 

soil N seems inconsistent with the presence of only scarce, 

fragmented diatoms high in this soil. The lack of roots cannot be explained by decomposition after burial of soil N 

because, despite its typical faintness, soil N contains or is 

immediately overlain by well-preserved spruce cones, as 

shown by radiocarbon samples plotted in figure 13. Moreover, soil N locally retains a peaty surficial horizon—at the 

site of the cones dated 2,475±23 14C yr B.P. (fig. 13).

Comparisons between diatoms and vascular-plant fossils show general agreement in the inference of high marsh 

for soils S, U, and Y at the Oyster locality (fig. 20). 

Although soils bearing high-marsh herbs locally have
24 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

brackish intertidal diatoms indices transitional between low 

marsh and high marsh, these indices are due to probably 

allochthonous valves of heavily silicified and easily 

reworked tidal flat diatoms (p. 48). Diatoms in soil U accord 

with vascular-plant evidence that soil U represents a slightly 

lower-level high marsh than do soils S and Y (p. 48).

Transitions from tidal flat to low marsh can be inferred 

both from diatom assemblages and from vascular-plant fossils in mud above soils S, U, and Y at the Oyster locality 

(fig. 20). Each transition is marked in outcrop by the first 

appearance of Trigochin maritimum rhizomes in soft gray 

mud above the buried soil. The mud lacks other vascularplant fossils in growth position but contains rhythmic layers 

of organic mud ascribed to filamentous algae found today 

mainly at the transition between tidal flat and low marsh (p. 

48). Diatom assemblages independently suggest that mud 

below the rhizomes represents a tidal flat or shallow subtidal conditions. However, the diatom assemblages, as summarized by the brackish intertidal diatom index, place the 

transition to low marsh several tenths of a meter above the 

lowest Triglochin maritimum rhizomes. This discrepancy 

may reflect patchy colonization of the tidal flat, which could 

have supported dominantly tidal-flat assemblages of diatoms until Triglochin maritimum patches fully coalesced. 

The discrepancy may also result from errors inherent in 

using diatom assemblages to estimate the point of transition 

from tidal flat to low marsh (Hemphill-Haley, 1995b).

Inferences from rhizomes and diatoms differ somewhat 

for a tidal-flat-to-low-marsh transition between soils J and L 

at the Redtail locality (fig. 15). Diatoms found with the lowest Triglochin maritimum rhizomes (at depth 310 cm) 

include such low-marsh species as Caloneis westii and 

Diploneis interrupta. However, the assemblage contains 

many more valves of the high-marsh diatom Cosmioneis 

pusilla var. 1. These valves, which could have been 

reworked from high marsh that may have fringed the upland 

beside the Redtail locality (fig. 7), push the diatom index 

into the high-marsh range (fig. 15).

RADIOCARBON DATING

Radiocarbon ages have been measured on samples of 

wood, peat, and rhizomes from the Oyster, Redtail, Pool, 

Jensen, and Sewer localities. Most of the ages were measured by liquid scintillation methods and were calculated for 

an assumed fractionation (513C) of-25 parts per thousand 

(%o) (table 7, lab code Beta). Other samples were dated by 

proportional gas methods (lab codes QL and USGS). 513C 

was measured for all the QL ages and was assumed to be 

-25 %o for the USGS ages. Still other samples were dated 

by accelerator mass spectrometry (RIDDL, GX) and calculated with measured 513C.

KINDS OF YEARS

Table 7 and the outcrop diagrams give ages in conventional radiocarbon years before A.D. 1950 ( 14C yr B.P.). In 

the text and in interpretive diagrams, however, we mainly 

use calendric years before A.D. 2,000 (years ago). We converted radiocarbon ages to calendric-year ranges by means 

of the calibration data of Stuiver and Becker (1993) and version 3.0.3 of the radiocarbon calibration program of Stuiver 

and Reimer( 1993).

We do not use "cal yr B.P.," the standard unit for calibrated radiocarbon ages (Stuiver and Pearson, 1986), 

because the "B.P." connotes "before present" but denotes 

"before A.D. 1950." Thus we describe A.D. 1700 as about 

300 years ago, not as 250 cal yr B.P.

ANALYTICAL ERRORS IN DATING

The error quoted with each age, as listed in table 7, 

does not necessarily equal one standard deviation of the age 

measurement. Such underestimation of analytical uncertainties is common in radiocarbon dating (Scott and others, 

1990). The quoted error probably exceeds 0.6-0.8 standard 

deviation for the QL ages (Stuiver and Pearson, 1986) but 

may approximate 1.0 standard deviation for the GX ages 

(Nelson and others, 1995). We lack such figures for the 

Beta, RIDDL, and USGS ages.

Figure 32 shows two calendric-age ranges for each 14C 

age. The narrower range equals the 95-percent confidence 

interval if the error quoted by the lab equals one standard 

deviation of the age measurement. The wider range incorporates a published, maximum error multiplier of 1.6 for ages 

measured at the University of Washington (lab code QL; 

Stuiver and Pearson, 1986) and an assumed and probably 

excessive error multiplier of 2.0 for the other ages. In other 

illustrations and in the text we use the wider range, and in 

the text we commonly use a still broader range rounded outward to the century.

GEOLOGIC ERRORS IN DATING

The material dated formed before or after the inferred 

earthquakes that we sought to date. For several samples of 

forest-floor litter this difference is probably negligible (fig. 

32, "closely limiting" ages for soils J and N), and for some 

tree-root samples the difference has been corrected by 

means of tree-ring counts (Atwater and others, 1991) (fig. 

32, "closely limiting" ages for soils S and Y). For other 

samples, particularly of peat, the difference may amount to 

decades or centuries (Nelson, 1992b). Arrows in figure 32 

identify ages subject to such differences. These include ages 

on small woody roots that may have died long before an 

inferred earthquake (arrow pointing upward), ages on rhizomes that may have formed long after the earthquake 

(arrow pointing downward), and ages on peat and sticks that
SEQUENCES OF BURIED SOILS 25

may either predate or postdate the earthquake (arrows pointing both directions).

Even the carefully selected and exhaustively cleaned 

stems of perennial herbaceous plants appear subject to geologic errors in dating of earthquakes. Ages measured on 

Potentilla pacifica and Juncus balticus rooted in soil Y at 

the Oyster locality (table 7) average 40-50 radiocarbon 

years greater than the likely radiocarbon age of the earthquake (Nelson and others, 1995). Reasons for the differences may include:

1. The dated material, coming from perennial plants, 

largely originated decades before the earthquake.

2. Air used in photosynthesis contained old CO2 derived 

from decay of organic matter in the marsh soil, including 

litter at the ground surface. Such uptake of old GO2, suggested to us by Minze Stuiver (oral commun., 1995), 

may account for part of the photosynthate in the grass 

Spartina alterniflora at a South Carolina salt marsh 

(Morris and Whiting, 1986).

3. Soil water contained old carbon, in such compounds as 

amino acids, that was taken up by the plants (Kathleen 

Sayce, written commun., 1996).

4. Ages may differ systematically between radiocarbon laboratories (Scott and others, 1990).

Nelson and others (1995) found that neither the first 

nor the fourth of these possibilities fully explains the relatively great ages obtained from the herbaceous fossils 

rooted in soil Y and its inferred correlatives.

SEQUENCES OF BURIED SOILS

OVERVIEW OF EVIDENCE FOR STRATIGRAPfflC 

CORRELATION AND DECOMPOSITION

Buried soils at Willapa Bay are part of a regional 

sequence that is largely defined by a pattern of alternating 

bold and faint soils. This pattern is probably due, in part, to 

differences in decomposition of buried A or O horizons 

after burial.

REGIONAL SEQUENCE

Low-tide outcrops in southern coastal Washington 

resemble one another in their sequences of buried marsh and 

swamp soils of late Holocene age. The similarities, first 

noted in reconnaissance in 1987 (Atwater and others, 1987), 

create a pattern, like a bar code. In this bar code three typically bold soils (L, S, Y) alternate with faint soils (N, U). At 

many sites an additional faint soil (W; overlooked in 1987) 

is barely visible between soils U and Y.

Additional properties of the soils support such stratigraphic correlation (p. 83). Tree roots are most common in 

soils S and Y, and they are scarce or absent in soils N, U, 

and W—even where soils N and U have peaty surficial horizons. Radiocarbon ages are similar among soils correlated 

by abundance of peat and tree roots (fig. 32).

The combined evidence for stratigraphic correlation 

leads us to treat soils as mappable units that can be assigned 

the same name at various localities in southern coastal 

Washington and beyond. We also use the inferred extent of 

correlative soils as evidence that earthquake ruptures 

occurred on the Cascadia plate boundary (p. 83) and that the 

ruptures were long enough to have resulted from earthquakes of magnitude 8 or larger (p. 85).

DECOMPOSITION AFTER BURIAL

Decomposition of organic matter accounts for much of 

the typical faintness of soils N, U, and W, and it probably 

also explains much of the absence of soil W. We further 

infer that this decomposition took place after burial, in the 

weathering profile of soil S (in the case of soil N) and in the 

profile of soil Y (for soils U and W). We summarize these 

inferences here because they bear on the correlation and 

consequent naming of buried soils at the localities described 

below. Differences in decomposition also suggest that some 

recurrence intervals were much longer than others (p. 98- 

99).

Decomposition of organic matter is evident in modern 

tidal-marsh soils at Willapa Bay. Most of these soils are oxidized (Munsell colors 2.5Y to 10YR 5/4) to depths of ¥2 m 

and commonly contain rust-colored mottles to depths as 

great as % m. Fossil rhizomes of Triglochin maritimum are 

rotten or absent in the oxidized mud. Moreover, the oxidized 

mud is firmer (probably, more desiccated) than underlying, 

gray mud that contains well-preserved fossils of vascular 

plants. The oxidation may occur, in large part, during neap 

tides. Such is inferred to be the case in a Maryland salt 

marsh having a mean tide range of 3 m, similar to the range 

at Willapa Bay. In the Maryland marsh, neap tides diminish 

pore-water pressure in marsh soil by failing to overtop it, the 

pressure falls below the threshold for entry of air into the 

soil, air probably enters along pathways created by plant 

roots, and soil thereby contains as much as 2 percent air 

(Hemond and Chen, 1990).

Several lines of evidence confirm that faint soils at 

Willapa Bay initially contained organic matter that later 

decomposed in weathering profiles from overlying soils:

1. Soils are faint wherever they—and the mud above and 

below them—are firm and lack fossil rhizomes or plant 

detritus. Soils that are typically bold become faint where 

directly overlain by firm mud without rhizomes: soil S at 

the Redtail locality (p. 27), soil L at the Jensen locality 

(p. 67), and soil Y at the Sewer locality (p. 69). Similarly 

at the Oyster locality, soil U is faint where associated 

mud is firm and vascular-plant fossils are lacking, but 

bold where associated with soft mud and well-preserved 

roots and rhizomes (p. 45).

2. Assemblages of environmentally indicative diatoms in 

soil U at the Oyster locality do not differ greatly between 

a section where the soil is bold and located 1.0 m below 

the top of soil Y (horizontal coordinate 165 m; BIDI =
26 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

0.80) and a section where the soil is faint and located 0.3 

m below the top of soil Y (horizontal coordinate 25 m; 

BIDI = 0.71) (fig. 20; tables 5, 6). This similarity in diatom assemblages implies that differences in later weathering, not differences in environment of formation, 

account for the variation from bold to faint for soil U at 

the Oyster locality (p. 45). Differences in weathering of 

soil U at the Oyster locality are further suggested by the 

differences in diatom preservation (fig. 23) and by the 

abundance ofParalia sulcata, whose sturdy valves probably resist decomposition (p. 21). The ratio of Paralia 

sulcata to all other taxa is less than 1:10 where soil U is 

bold (table 6, depth 2.01 m) but about 1:2 where soil U is 

faint (table 5, depth 1.44 m).

3. Typically faint soils N and U at the Redtail locality 

formed in a high marsh (U) or perhaps in the transition 

from marsh to upland (N) (p. 27), as judged from diatom 

assemblages. In those settings, vascular plants should 

have rained much organic matter onto each soil and 

injected additional organic matter into it, making the soil 

bold before burial.

Horizons of decomposed peat have also been reported 

from the Holocene of coastal Germany. According to Streif 

(1982, p. 36), the horizons can be ascribed to "temporary 

lowering of the groundwater table" and consequent "oxidation of peaty matter."

NIAWIAKUM RIVER

The Niawiakum River drains 5 km2 of hills composed 

of Pleistocene estuarine deposits and the Crescent Formation. Brackish water extends far upstream; usual freshwater 

inflow in late summer is probably about 0.1 m3/s. Brackish 

marshes therefore extend far upstream as well, grading into 

a tidal swamp of Sitka spruce and western crabapple 0.5 km 

northeast of the Pool locality (fig. 7).

Buried soils along the tidal Niawiakum River have provided much previously reported evidence for repeated prehistoric earthquakes on the Cascadia plate boundary. Gouge 

cores penetrated as many as eight successive buried soils 

along a cross section about 0.5 km northwest of the Pool 

locality (Atwater, 1987; location of cross section shown in 

our fig. 7). Downstream from this cross section the uppermost buried soil (soil Y) is widely covered by tsunami 

deposits that are commonly several centimeters thick 

(Atwater, 1987, 1996; Reinhart and Bourgeois, 1987, 1989; 

Hemphill-Haley, 1995b, 1996). Herbaceous stems and 

leaves rooted in soil Y and in another, lower soil (soil S) 

show that both these soils record subsidence that occurred 

suddenly (Atwater and Yamaguchi, 1991, their figs. 4, 5). 

Fossil diatoms at the Oyster and Redtail localities, and at 

two other sites farther upstream, independently confirm that 

soil Y suddenly subsided (Hemphill-Haley, 1995b; her sites 

1, 2, and 3 are at the Oyster, Redtail, and Heron localities, 

respectively). Radiocarbon evidence for extensive plateboundary rupture at the Cascadia subduction zone includes

high-precision ages of spruce stumps rooted in soil Y near 

the Pool locality (Atwater and others, 1991; locations of 

dated stumps shown by QL symbols in our fig. 7) and accelerator-mass-spectrometry (AMS) ages on herbaceous stems 

and leaves rooted in soil Y at the Oyster locality (Nelson 

and others, 1995). The AMS ages are listed in table 7, and 

their approximate sample location is plotted in figure 18.

We surveyed successive buried soils along the Niawiakum River at four places where they are especially well 

exposed—the Redtail, Oyster, Pool, and Heron localities 

(figs. 6, 7). At very low tides the Redtail locality displays 

six buried soils from the past 3,500 years, whereas the Oyster locality has four buried soils from the past 2,000 years. 

Probable correlatives of five of the soils at the Redtail locality crop out at the Pool and Heron localities. We discuss the 

more thoroughly studied outcrops first (Redtail and Oyster 

localities) and the less studied outcrops second (Pool and 

Heron localities).

REDTAIL LOCALITY

The Redtail locality comprises 65 m of stream bank— 

about 30 m along the Niawiakum River, the rest along an 

unnamed tributary (figs. 13,14A, E). The bank is capped by 

tidal marsh except at its west end, where it intersects a forested upland. The upland is made of Pleistocene estuarine 

deposits that were deeply dissected by the glacial-age Niawiakum River and its tributaries. These deposits form a stiff 

muddy foundation that descends eastward beneath a 

Holocene section containing seven buried soils.

BURIED SOILS

Six buried soils are exposed at the Redtail locality at 

tides below mean lower low water. Four of these soils are 

commonly peaty and (or) woody enough to form prominent 

ledges (fig. 14A-C, soils J, L, S, and Y); the other two, 

which are characteristically faint, lack such topographic 

expression (soils N and U). A seventh soil (H), observed 

only in core, underlies soil J near the forested upland.

All the soils dip away from the Pleistocene deposits 

onto which they lap. Such dips are greatest for the oldest 

soils (figs. 13, 14#; soils H, J, and L). Soil J is 1.7 m lower 

near the unnamed tributary than it is on Pleistocene deposits—a difference more than three times greater than undulatory relief on the nearly horizontal modern marsh along the 

Niawiakum River (fig. 13).

Most of the 1.7 m of relief on soil J, and most of the 

northeastward dip of other buried soils as well, was probably caused by settlement through compaction of soft 

Holocene mud. Such mud is probably many meters thick 

above the last-glacial thalweg of the unnamed tributary; during the last glaciation the lower Niawiakum River flowed 

tens of meters below modern sea level, as shown by highway borings near the Oyster locality (p. 45). Settlement 

caused by compaction of peat and mud is common in tidal
SEQUENCES OF BURIED SOILS 27

wetlands, as recently shown for modern salt marshes of the 

southeastern United States (Cahoon and others, 1995).

Most of the soils are interrupted by muddy channel fills 

in the northeastern half of the outcrop (fig. 13). The channel 

fills are 2-10 m wide in the plane of the outcrop. We 

observed truncation of buried soils on one or both margins 

of each fill. The fills consist of soft gray mud; in one of 

them, near horizontal coordinate 35 m, the mud contains 

much detrital wood.

Abundance of peat in the surficial horizon varies 

greatly for many of the soils, as summarized simplistically 

by the symbols for bold and faint soils in figure 13. In soil J 

the surficial horizon consists of woody peat 10 cm thick 

near the forested upland but grades, along 20 m of continuous outcrop beside the Niawiakum River, into slightly peaty 

mud 2 cm thick. The woody peat abounds in forest-floor litter—sticks, spruce cones, needles. Soil L consistently contains 20 cm of muddy peat along the Niawiakum River but 

lacks such peat in 9 m of outcrop along the unnamed tributary. There, the surficial horizon of soil L is made of slightly 

peaty mud having many undulations 5-10 cm high and 5-20 

cm long—elk or deer hoofprints, perhaps. The surficial 

horizon of soil N, though typically limited to dark brownish 

gray mud 2 cm thick, is composed of woody peat 5 cm thick 

for several meters along the Niawiakum River. Like soil J, 

this peat contains many sticks and spruce cones. Soil S, 

which typically contains a surficial horizon of woody peat 

5-10 cm thick (fig. 14C), becomes faint where it laps onto 

Pleistocene deposits and thereby enters the weathering profile of soil Y.

These variations in abundance of peat are probably due 

to initial composition in some cases and later decomposition 

in others. An initial abundance of forest-floor litter in soils J 

and N probably accounts for the greater abundance of peat 

near the forested upland; these soils logically received most 

such litter near the forested upland. Later decomposition in 

the profile of soil S probably accounts for the widespread 

faintness of soil N, and decomposition in the profile of soil 

Y probably accounts for the general faintness of U and the 

localized faintness of soil S (p. 25-26).

TSUNAMI DEPOSIT

A tsunami deposit discontinuously overlies soil Y at 

the Redtail locality. The deposit consists of micaceous 

sandy silt about 1 mm thick. Most of the sand is very fine. 

The sandy layer is separated from the underlying soil by as 

much as 3 mm of gray mud. Such mud layers are commonly 

part of the tsunami deposit on soil Y elsewhere along the 

Niawiakum River (Reinhart and Bourgeois, 1987; Atwater 

and Yamaguchi, 1991, their fig. 55), as illustrated at the 

Oyster locality (p. 45,47; fig. 19E).

Several points show that a tsunami probably deposited 

the sandy layer:

1. The layer resulted from deposition by an unusually 

strong current. Other than cracked rock in cultural deposits (p. 35), the sandy layer is the coarsest inorganic material in the outcrop.

2. The unusual current approximately coincided with subsidence recorded by intertidal submergence and burial of 

soil Y. This coincidence, shown by the position of the 

sandy layer just above the top of soil Y, would not be 

expected of a storm but would be expected of a tsunami 

generated by a plate-boundary earthquake at the southern 

Washington part of the Cascadia subduction zone (Atwater, 1987). The coincidence is further demonstrated by 

the preservation of herbaceous stems and leaves, in 

growth position, in the sandy deposit on soil Y at the 

Oyster locality (p. 47^8).

3. The current entailed flow up the Niawiakum River valley, as shown by variation in thickness of correlative sand 

elsewhere along the Niawiakum River (Atwater, 1987, 

1996; Reinhart and Bourgeois, 1987), by oriented plant 

fossils at the Oyster locality (p. 47^48), and by diatoms—found immediately above soil Y at four sites 

between the Oyster and Pool localities—that were 

derived from sandy tidal flats (Hemphill-Haley, 1995b, 

1996).

4. The current had multiple pulses, as shown by the presence of five or six sandy layers within the sandy deposit 

on soil Y at the Oyster locality (p. 45, 47; fig. 19£). A 

tsunami typically comprises a train of waves; each of the 

sandy layers can be ascribed to a separate wave in a tsunami wave train.

5. Currents from a storm or seiche are unlikely to have been 

swift enough to account for the extent and grain-size 

trends of sandy deposits on soil Y along the Niawiakum 

River, according to an unpublished sediment-transport 

model by Reinhart and Bourgeois (1989). However, a 

storm surge at least 2.5 m high deposited sand, silt, and 

clay on tidal marshes along several kilometers of a Florida river in 1993 (Goodbred and Mine, 1995).

FOSSIL DIATOMS

Diatoms show that each of the six buried soils seen in 

outcrop at the Redtail locality formed higher in the intertidal 

zone than did mud above the soil (figs. 15, 16). In addition, 

diatoms provide further evidence for tsunami deposition on 

soil Y, and they hint at the presence of an otherwise invisible 

soil between soils U and Y.

Each of the six soils contains diatoms indicative of 

conditions at or above very high tides (figs. 15, 16). Diatoms of soils J, L, and U are dominated by species—particularly Cosmioneis pusilla and Pinnularia lagerstedtii (table 

4)—indicative of tidal marsh above mean higher high water. 

Even higher conditions, transitional to upland, are suggested 

by the small number and poor preservation of diatoms in the 

uppermost parts of soils S and Y. Such conditions are consistent with the abundant spruce roots in these soils (p. 29) 

and with the presence of Paralia sulcata valves (fig. 17; see 

section "Significance of Paralia sulcata" p. 23). Soil N
West-southwest

9 73

E«

c o

Z-—

O) §

Area shown in figure 14D

" / "" \ / x \

Area of archaeological detail

V

- Soil U not shown on this enlargement

630 ± 90 (charcoal) Shading denotes black Nspruce root with >60 annual rings

peat and muddy peat grown jnto cultural lens 

with abrupt top and bottom

J_________ \ _______I_________I________

22 24 26 

Distance, in meters from local datum

Highest flotsam 

from winter 

of 1991-1992 x

£2 

I

j= t3- 

°> t4'

<D

X

6 -

8 -1

Outcrop shown in figure 14B

West-southwest Bend in section Northeast 

Niawiakum River \ Unnamed tributary

Section sampled

for fossil diatoms Area of archaeological detail shown above 

(figs. 15-17) / Tidal creek <1 m wide

.... / / Cleaned outcrop shown in figure 14C

ds^fi^^f^

630 ± 90; (charcoal) j i

2475 ± 23 (cones on soil) 

2510 ± 80 (stick) 

2590 ± 60 (stick)

230 ± 70 (root)

250 ± 60 (root)

t1

12

t3

t4

3165± 

3166±

3180 +

'&&

1 7 (cones on soil) />"%;

18 (sticks on soil) f/&

30 (stick on soil) //-.

/ -A

3570 ± 70 (rhizomes) M

-0.40,

-0.61 ,

-0.52,

-0.73,

'*h 

iiv"( i 

\

4•A

pi

i

'H

•-•

',s2^— 3320 ± 80' ' 

lff(S^fs) Many

.--\-|~ —— T^^~.' - - -

3230 ± 60

1 (s

i

ick)

4 July 1992

'"- 5-20 c

'• \

undulations

m high and j — r-c

shown schematically I

3 July 1 992 ' ' 11 July 1994 0 20 40

i ——— i ——— i —

60

"- 2945 + 30 (SO

3190 ± 110 (

. 3063 ± 80 (stic

3730 ± 21 0 (so

8

2 July 1 992 Distance, in meters from local datum

1720 ± 40 

1740 ± 70 

1830 ± 40 

2590 ± 80 

2258 ± 75 

2780 ± 45 

2840 ± 70 (ro

Figure 13. Statigraphy at Redtail locality, Niawiakum River.
SEQUENCES OF BURIED SOILS 29

s

-0,

<

.s o.S rt

11!

W) CUQ o

0

U. 0 S

fa U

•

o 

o

B:

5

%

5 "8

« &

^1to X)

^§ 

C O T3

|gp,l £°

M

i

•al io o

.

ai

contains an upland assemblage of diatoms but lacks tree 

roots that should accompany such an assemblage (p. 23).

For all six buried soils the transition from soil to overlying mud coincides with an abrupt shift to assemblages of 

diverse diatom species indicative of intertidal flats or shallow subtidal areas (table 4). For soils J, L, and U this shift 

bypassed low marsh, and for soils S and Y (and perhaps also 

soil N) it bypassed high marsh as well. The corresponding 

rise in relative sea level, as inferred from modern wetlands 

and their diatom assemblages (figs. 8, 12), is probably about 

1 m or more in each case.

A vestige of a soil, invisible in outcrop, may be present 

between soils U and Y at the outcrop shown in figure 14C. 

Assemblages of poorly preserved diatoms are all that 

remain in the oxidized profile of soil Y at depths of 94-104 

cm below the top of this outcrop. However, a possible tidalflat assemblage at 96 cm overlies a sparse assemblage of 

poorly preserved high-marsh species at 98-100 cm — an 

interval located 18 cm above the top of soil U and 22 cm 

below the top of soil Y This sequence provides inconclusive 

evidence for an otherwise invisible soil between soils U and 

Y. The invisible soil may be soil W, which can be seen in 

outcrop 1 km downstream at the Oyster locality but only 

where it is more than 25 cm below the top of soil Y (fig. 18).

Diatoms are part of the evidence that a tsunami deposited the discontinuous lamina of very fine sandy silt that are 

located a few millimeters above soil Y at the Redtail locality 

(p. 27). A sample from deposits just above soil Y contains 

well-preserved estuarine tidal flat species and lacks freshwater diatoms (table 4, sample from depth 0.74 m). More 

than half the tidal-flat species are diatoms typical of sandy 

flats of Willapa Bay. These species show that the sand was 

not washed down the Niawiakum River by a flood of freshwater.

Diatoms provide the only direct evidence that a tsunami overran soils S and N at the Niawiakum River. 

Although neither of these soils is overlain by a bed as sandy 

or as close to the soil as the sandy bed above soil Y, both are 

overlain by mud that contains high percentages of species 

typical of sandy tidal flats.

GROWTH-POSITION FOSSILS OF VASCULAR PLANTS

l8l*s '"!Siinii

I

6

2

Ifa

Four of the buried soils contain spruce roots suggestive 

of conditions within or above the upper Vz m of the intertidal 

zone. The roots are most widespread in soils S and Y (figs. 

13, 14A-Q. Spruce roots are also present in soil L, both at 

its juncture with Pleistocene deposits (fig. 14B) and along 

the tributary slough near the northeastern end of the surveyed outcrop (fig. 13). Soil J contains spruce roots only 

where it laps onto Pleistocene deposits (fig. 14B).

Mud above soils H and J contains rhizomes indicative 

of conditions lower in the intertidal zone. All these rhizomes are of Triglochin maritimum except above soil H, 

where the mud also contains rhizomes of Carex. The lack of 

rhizomes of mud above younger soils is probably due to
30 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Figure 14. Redtail locality, Niawiakum River (fig. 13). A, 

Overview to north, across the Niawiakum River; tributary creek 

enters at right. Trees in background are rooted on upland, above 

level of highest tides. Tidal marsh high in intertidal zone (high 

marsh) forms meadow in foreground and in front of trees. Scale 

shown by people at center, low on far bank of river. June 1987. B, 

Closer view of outcrop along Niawiakum River. Soils J and L dip

eastward, away from firm substrate of Pleistocene deposits onto 

which they lap. Buried soil S is prominent because of its peaty 

surficial horizon, abundant spruce roots, and lack of concealment 

by modern vegetation. Soil Y is mostly concealed by modern 

plants. Landward end of soils J and L marked by spruce stumps 

(st) rooted in them at left. August 1986.

decomposition. Mud beneath the modern marsh soil is 

brown and firm to within a few centimeters of the top of soil 

Y. We rarely found fossil rhizomes in such oxidized, desiccated mud, and where we did, they are mostly decomposed. 

When soil Y was at the surface, decomposition in a deeply 

oxidized, desiccated profile of soil Y probably destroyed 

rhizomes in mud above soils S and U. Such decomposition 

in the profile of soil S likewise helps to explain the lack of 

rhizomes in mud above soils N and L.

CULTURAL DEPOSITS

Remains of a human campsite at the Redtail locality 

add to diatom and spruce-root evidence for nearly upland 

conditions on soil Y. The campsite, which extended along 

the Niawiakum River part of the outcrop (fig. 13), is marked 

by charcoal, cracked rock, heated bone, and reddish-brown 

lenses probably composed of ash from wood fires (Cole and 

others, 1996) (fig. 14£>).
SEQUENCES OF BURIED SOILS 31

Figure 14. C, Contrast in distinctness between faint soil U and bold soils S and Y. Shovel at horizontal coordinate 42 m; handle is 0.5 m 

long.

The campsite at the Redtail locality is one of several 

archaeological sites discovered in soil Y in southern Washington and in probably correlative soils in northern Oregon. 

The discoveries began in Oregon (Grant and Minor, 1991;

Minor and Grant, 1996). The sites in Washington include 

one at the Copalis River (Cole and others, 1996) and two 

previously unreported sites along the Willapa River—at the 

Sewer locality (p. 69), and northwest of the Jensen locality
32 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Figure 14. D, Black peaty horizon of soil Y, containing reddish-brown lens (light tone, in right half of photo) of probable fire ash and a 

mammal bone (white dot at center). Soil U faint, as in C. Shovel handle near horizontal coordinate 21 m; stripes on handle are 0.1 m long.

(fig. 7). The site at the Redtail locality has been assigned, by 

the Washington State archaeologist, Smithsonian number 

45-PC-102. Stratigraphy at each of the sites shows that an 

earthquake about 300 years ago caused the lasting tidal submergence of land previously high enough to have been used 

as a campsite.

AGE

High-precision ages can be assigned to the oldest and 

youngest buried soils recorded in outcrop at the Redtail 

locality (fig. 32). Radiocarbon ages on forest-floor litter at 

the Redtail locality (table 7, lab numbers QL-^716, -4717, 

and -4718) show that the burial of soil J began 3,300-3,500

calendric years ago. Radiocarbon ages on spruce roots from 

nearby sites—a beach at Bay Center (fig. 6) and river banks 

near the Pool locality (fig. 7)—show that the burial of soil Y 

began about 300 years ago (Atwater and others, 1991), 

probably between A.D. 1700 and 1720 (Nelson and others, 

1995).

Additional radiocarbon ages from the Redtail locality 

suggest broad calendric ranges during which some of the 

other burial events probably occurred (fig. 32): 2,400-4,000 

years ago for soil L, 2,400-2,800 years ago for soil N, and 

1,400-2,100 years ago for soil S.

A charcoal age from soil Y shows that the Redtail 

locality was used by people as early as A.D. 1000, while 

another charcoal age and a tree-ring count show that
SEQUENCES OF BURIED SOILS 33

.£"5.

CD

Q

-2 1

o c *

Simplified

stratigraphic

column

Diatom assemblage

0 +

100

200

300

U

N

° w

1 1

F-S 11 

12

TO

O

TO

E

0)

±

T3

C

TO 

Q.

0 1 

Brackish intertidal diatom index

Particle size 

of inorganic sediment

U

N

50 

Percent of dry weight

100

EXPLANATION FOR SIMPLIFIED STRATIGRAPHIC COLUMN

Vascular-plant fossil

Spruce stump rooted in soil 

Rhizomes rooted in mud

Surface of modern tidal marsh

Surficial horizon of soil

Bold soil 

Faint soil

Mud

Tsunami deposit

Figure 15. Summary of diatom assemblages at Redtail locality, Niawiakum River. Samples collected from section at horizontal 

coordinate 18m (fig. 13).
34 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

re

•'


-U_L

x

0) •O

O -D

.O <D 7~ w

CO CO

o

CD

-O

o 2

CM O

-o o

^

0)

o

qsjeiu MOI 

'q6no|s '

e-

-a 

S

T3 

T3

T3

8,

I

8,

Ul

•31

•a 

1

sjejeuj ui 'eoejjns ujepouj MO|0q ij)deQ
SEQUENCES OF BURIED SOILS 35

(/) re

5ol

\

««>

LjSJBW MO"]

lepjiqns 'i|6nojs v

r

r

r

r

I .,

" i . In

.

r , ,„

r . ..

r . ...

F

F . .1.

F

F ,1..

r .

f .

F. 1 ...

f ,.

r

r .

: .1.

r .

F

-

r

. i ii

F

' 1 . .

r

><

,

1

,11

k.

=>

. 1.

.1

, II

, 1

,1,

III

Cfl

,

1

1

, .

,

L^

•

z

. 1

_

.1

. 1

,

.,1 1 ,

,

h

I

1, .1

\ --

i

J iF

. . F

F

. . . . F

..,lk.

, . ., F

F

. F

....1

f

f

1 1 ,.\.,t. \

. . ... i

[

.. , f

.. i

III i

. . . .. i

. 1

, —————— ^J

. . 1

Ir" •

i,. ..

1

1

. . 1,11

X

I

:§ I

^o .2

• o «^ 

o

0) 

Q.

sj9)9iu uj '

CM

ujepoiu MO|eq

rO _— O TJ £ re *u v

2-| o u

=5 5

£
36 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

lepjiqns 'i|6n6'|s"'ie|j

O i- OJ CO

O sjejeiu uj 'eoejjns ujepoiu M0|eq

humans may have abandoned the site decades before its 

intertidal burial in the early 1700's. The older of the dated 

charcoal samples—630190 14C yr B.P., or A.D. 1000-1600 

(fig. 32)—came from a cultural lens that is separated from 

the top of soil Y by 10 cm of archaeologically sterile peat. 

The younger charcoal sample gave an age of 250±60 14C yr 

B.P., which corresponds to a calendric range that includes 

A.D. 1700. Although this younger sample came from a fire 

pit that extends nearly to the top of soil Y, a spruce root having at least 60 annual rings grew into this fire pit (fig. 13). 

This spruce, like the others represented by roots in soil Y, 

probably died from submergence that initiated the burial of

soil Y. In that case, the fire pit predates the submergence and 

initial burial of soil Y by at least 60 years.

OYSTER LOCALITY

The Oyster locality, located 1 km west-northwest of 

the Redtail locality, comprises the northeast bank of the 

Niawiakum River about 100-300 m upriver from U.S. 

Highway 101. The top of the bank is high marsh except for 

a mudflat 50 m wide at the confluence of two tidal creeks 

tributary to the river (figs. 18, 19A, B). Since 1986 the upper
SEQUENCES OF BURIED SOILS 37

15

(0

•2 

1

? 

o

<5

CL

CD

!_ C

O CgS

1

(0

E

— TJ 

'

100

"g 

.S 

;§-

10-

80- 

60- 

40- 

20-

A &

D Good 

preservation

100 

80- 

60- 

40- 

20-

100

80- 

60- 

40- 

20- 

0

0.0 0.2 0.4 0.6 0.8 1.0 

Brackish intertidal diatom index (BIDI)

1.2 20 40 60 80 

Percent Paralia sulcata

Figure 17. Abundance of the diatom Paralia sulcata in stratigraphic sections at the Redtail and Oyster localities, Niawiakum River. 

Abundance expressed as percentage of total number of diatoms counted. Data from tables 4-6. A-C, Comparison with depositional 

environment inferred from brackish intertidal diatom index at (A) Redtail locality, (E) Oyster locality, horizontal coordinates 15 and 25 m, 

and (O Oyster locality, horizontal coordinate 165 m. D-F, Comparison with qualitative measures of diatom preservation.

half of the bank has been invaded by Spartina alterniflora 

(fig. 19C% an Atlantic-coast grass that began to spread rapidly at Willapa Bay in the early 1980's (Sayce, 1988; Mumford and others, 1990).

BURIED SOILS

Four buried soils are widely exposed at the Oyster 

locality. The oldest and youngest of these have ledge-forming peaty horizons (fig. \9B-D). The two intervening soils

are characteristically faint and lack topographic expression 

except on well-washed, nearly vertical faces, from which 

they protrude a few centimeters more than does mud immediately above them. For reasons discussed under "Age and 

Correlation" (p. 52), we equate the bold soils with soils S 

and Y of the Redtail locality, and the lower of the faint soils 

with Redtail soil U. At the Redtail locality we did not notice 

evidence for a correlative of soil W, the other faint soil at the 

Oyster locality, except for two samples dominated by highmarsh diatoms (p. 29).
38 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON 

Table 4. Numbers of diatoms counted at the Redtail locality

Uppermost 1 cm 

of soil Y

Uppermost 1 cm 

of soil U

Uppermost 1 cm 

of soils

Uppermost 1 cm 

of soil N

Uppermost 1 cm 

of soil L

Uppermost 1 cm 

of soil J

Depth below 

modern

surface (m)

0.01

0.15

0.35

0.55

0.66

0.70

0.74

0.76

0.90

0.99

1.09

1.14

1.17

1.22

1.40

1.50

1.54

1.58

1.68

1.80

1.89

1.95

1.97

2.05

2.10

2.15

L 2.18

2.45

2.58

2.65

2.67

2.75

2.90

3.00

3.10

3.20

3.23

3.27

3.31

3.35

3.39

3.41

3.45

Diatom sample

ID

OS 1 Ox-75

OS 1 Ox-60

OS 1 Ox-40

OS 1 Ox-20

OS8-1

OS8-3

OS8-6

OS8-8

OS 10-24

OS10-32

OS10-42

OS7-10

OS 10-56

OS 10-61

930718B3-20

930718 B3-30

OS6-4

OS6-8

OS6-18

930718B4-25

OS5-1

OS5-7

OS5-9

OS5-18

OS5-22

OS5-27

OS5-30

93071 8 B7-1

OS4-1

OS4-8

OS4-10

OS4-18

930718 B8-1

930718 B8-10

930718B8-20

OS2-1

OS2-4

OS2-8

OS2-12

OS2-16

OS2-20

930718B9-23

OS2-26

|enthic diatoms 

PQ

~~| -hanthes hauckiana 

•«:

l

4

2

1

2

2

3

2

6

3

1

\"hnanthes cf. minutissima 

•*.

11

1

1

1

3

2

2

7

delicatula -hnanthes 

•«:

22

3

1

3

32

2

1

20

12

17

2

1

18

1

1

1

1

1

4

8

1

1

1

36

6

12

11

23

20

1

4

|lanceolata -hnanthes 

•«:

l

2

1

1

1

2

1 

1

•<

4

8

7

2

5

10

4

4

\-hnathes brevipes 

•«:

4

4

2

2

1

2

5

1

2

2

—

\libyca nphora

*<

4

2

\nphora roteus p 

•<

2

4

2

1

1 

so -«?•

^

3

4

1

1

2

1

4

i

\nphora ventricosa 

•<

l

1

l

|nphora ventricosa 

•«:

2

|ddulphia spp.

CQ

1

2

1

2

_

<3 

•1

£

03

2

\lloneis bacillum 

a

2

6

6

13

7

12

4

4

1

1

3

2

10

\lloneis permagna 

U

1

|lloneis spp. 

U

2

1

1

2

1

3

2

2

2

4

1«n '53 

J

u

1

4

14

23

7

9

1

2

1

17

2

46

10

6

3

8

2

1

11

7

13

1

3

7

16

37

26

19

18

7

2

2

\impylodiscus bicostatus 

U

1

impylodiscus echineis

0

2

4

5

8

1

5

10

24

0.5

1

6

3

8

1

1

3

1

3

1

\adhaerens itenula 

0

2

2

1

1

1

5

1

2

1

1

\zrataulus turgidus 

<J

7

1

8

3

6

2

1

1

diminuta jcconeis 

<J

4

4

3

6

3

6

2

6

16

20

8

3

1

64

39

7

4

5

2

3

25

33

7

4

6

1

2

6

11

1

peltoides jcconeis 

U

4

16

2

1

19

2

1

9

7

22

3

7

i>

4

6

3

\placentula jcconeis 

U

4

2

5

1

4

2

4

2

1

1

1

1

1

2

4

3

o

\scutellum jcconeis 

U

1

1

5

1

2

3

1

3

2

1

scutellum jcconeis parva v. 

U

2

5

2

2

5

1

1

1

2

2

1

2

1

2

1

6

10

1

|jcconeis spp. 

U

1

4

2

1

1

1

6

2

2

pusilla jsmioneis 

U

10

10

4

43

59

61

39

4

10

5

1

21)

3b

16

10

36

14

24

5

'25

1

1

1

6

41

6U
SEQUENCES OF BURIED SOILS 

Table 4. Numbers of diatoms counted at the Redtail locality—Continued

39

Uppermost 1 cm 

of soil Y

Uppermost 1 cm 

of soil U

Uppermost 1 cm 

of soil S

Uppermost 1 cm 

of soil N

Uppermost 1 cm 

of soil L

Uppermost 1 cm 

of soil J

Depth below

modern

surface (m)

0.01

0.15

0.35

0.55

0.66

0.70

0.74

0.76

0.90

0.99

1.09

1.14

1.17

1.22

1.40

1.50

1.54

1.58

1.68

1.80

1.89

1.95

1.97

2.05

2.10

2.15

2.18

2.45

2.58

2.65

2.67

2.75

2.90

3.00

3.10

3.20

3.23

3.27

3.31

3.35

3.39

3.41

3.45

\pusillavar. lioneis 1

§

1

2

2

1

12

5

9

6

61

2

5

1halophila 'cula 

p

U

2

2

1

8

3

3

1

2

|

1

4

3

2

4

3

7

3

2

2

1

2

2

2

2

2

1

1

1

\cf. karstenii hineis 

-SQ

3

9

9

6

4

12

4

3

2

4

5

2

6

9

60

7

6

2

10

4

2

\margaritalimbata fa'raw c/ 

Q

8

4

1

1

10

2

1

1

7

2

3

1

8

3

2

1

1

4

2

c/ surirella fa'rais 

Q

1

6

4

12

18

3

2

5

9

5

2

1

3

6

3

2

2

4

1

3

12

43

30

53

16

8

surirella hineis 

Q

1

15

5

10

7

5

3

2

\subtilis icua

§

Q

2

1

1

2

4

minor regramma 

Q

1

1

2

14

3

9

2

1

2

2

4

12

3

•S

"S. i

Q

2

2

1

1

meis interrupta

"S.

Q

2

21

125

3

1

1

3

28

3

2

2

1

1

60

3

8

2

5

7

16

2

2

2

1

oblongella meis 

Q

1

1

10

1

pseudavalis meis 

~§ (

Q

1

2

2

5

14

\smithii rhombica mesv.

"3.

Q

1

1

2

5

mies stromi

"a.

Q

2

1

i*3

33

8

13

9

11

5

4

3

2

2

6

4

1

2

1

5

1

3

1

2

1

3

2

i

3att3"

l

!

1

1

go 

2 

•S

•9*

i

3

3

18

3

4

7

4

3

6

1

1

2

2

3

2

6

3

6

1

3

\pectinalis tia 

g

t»a

1

2

1

i .g^

t*3

1

1

\cryptolyra cia 

•^

£

3

1

8

8

12

10

1

1

2

13

1

2

3

2

2

4

5

3

11

4

\ciaforcipata 

4

1

\ciapygmaea 

=3

£

4

1

2

1

1

1

\'laria construens venterv. 

trI*.

4

11

1

1

3

'laria pinnta

^

i*.

1

2

1

1

2

2

20

4

-S

s?

I*.

2

1

1

\lewisiana ea

£

k.

2

1M/ta //nfe

§

I*.

1

3

2

1

\ulia vulgris 

§

fe.

5

1

10

1

5

\parvulum fhonema 

O

2

1

2

1

1

1

10

\jhonema spp. 

O

1

2

2

1

1

2

\imatophora oceanica 

2

4

1

1

2

1

1

2

1

1

2

1

2

|s/£/n# acuminatum 

2

l

2

2

5

3

2

5

1

8

6

1

1

6

2

1

i

\balticum s/g/na 

£G'

5

1

3

14

2

2

5

1

1

7

5

1

2

88

38

37

23

9

4
40 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Table 4, Numbers of diatoms counted at the Redtail locality—Continued

Uppermost 1 cm 

of soil Y

Uppermost 1 cm 

of soil U

Uppermost 1 cm 

of soil S

Uppermost 1 cm 

of soil N

Uppermost 1 cm 

of soil L

Uppermost 1 cm 

of soil J

Depth below 

modern

surface (m)

0.01

0.15

0.35

0.55

0.66

0.70

0.74

0.76

0.90

0.99

1.09

1.14

1.17

1.22

1.40

1.50

1.54

1.58

1.68

1.80

1.89

1.95

1.97

2.05

2.10

2.15

2.18

2.45

2.58

2.65

2.67

2.75

2.90

3.00

3.10

3.20

3.23

3.27

3.31

3.35

3.39

3.41

3.45

yrosigma eximium

O

7

2

5

1

1

77

10

13

1

4

5

1

2

\wansbeckii yrosigma 

O

3

3

\arcuatum annaea 

^

1

1antzschia amphioxys 

a;

i

i

2

1

1

4

5

1

1

antzschia virgata

^

2

yalodiscus laevis

^

1

3

1

1

8

10

4

10

1

1

2

30

1

1

2

1

\yalodiscus scoticus 

^

3

8

1

iticola mutica

3

18

17

2

8

7

5

2

4

4

4

10

3

3

1

1

6

2

5

3

1

14

4

43

iticola muticoides

3

2

5

1

9

2

1

22

17

1

1

4

5 ,£•>

-S "§

£

2

2

2

2

astogloia exigua

^

1

4

2

13

1

1

2

1

3

5

15

10

4

7

1

\astogloia smithii 

^.

2

\elosira moniliformis 

§

8

33

1

1

2

1

2

4

4

1

1

4

3

5

elosira nummuloides

^

8

15

2

5

7

2

2

5

2

2

4

1

4

7

ivicula cancellata

^

1

1

37

2

1

1

ivicula cincta

^

197

3

3

1

2

4

5

6

2

11

2

2

35

4

1

13

14

5

3

2

1

2

4

10

ivicula cryptotenella

^

2

2

1

1

3

2

13

13

27

10

\ivicula digitoradiata 

^

4

2

2

1

5

2

2

2

1

2

2

4

2

3

1

1ivicula directa 

^

1

-

1ivicula elginensis 

^

1

1ivicula gregaria 

^

2

1

ivicula libonensis

%

1

1

- c

00

K%) a. 

,0 "3

•y1

4

1

1

8

ivicula peregrina kefvingensis v. 

*.

3

6

11

\hyllepta ivicula p 

*.

5

ivicula rhyncocephala

^

4

1

5

5

31

1

2

\salinarum ivicula 

*.

2

ivicula spp.

*

1

1

1

4

8

1

4

2

2

2

2

10

3

1

8

1

1

2

1

2

4

2

\ivicula tenelloides 

*.

2

\bilobata tzschia 

*.

1

14

4

1

2

\'tzschia brevissima 

*

1

1

tzschia cf. nana

*.

2

2

2

1

5

1

1

2

2

2

2

1

3

4

10

3

2

|tzschia commutata 

^

2

4

'tzschia fasciculata

*

1

2

2

1

2

1

2

2

1

2
SEQUENCES OF BURIED SOILS 

Table 4. Numbers of diatoms counted at the Redtail locality—Continued

41

Uppermost 1 cm 

of soil Y

Uppermost 1 cm 

of soil U

Uppermost 1 cm 

of soil S

Uppermost 1 cm 

of soil N

Uppermost 1 cm 

of soil L

Uppermost 1 cm 

of soil J

Depth below

modern

surface (m)

0.01

0.15

0.35

0.55

0.66

0.70

0.74

0.76

0.90

0.99

1.09

1.14

1.17

1.22

1.40

1.50

1.54

1.58

1.68

1.80

1.89

1.95

1.97

2.05

2.10

2.15

2.18

2.45

2.58

2.65

2.67

2.75

2.90

3.00

3.10

3.20

3.23

3.27

3.31

3.35

3.39

3.41

3.45

lorenziana :hi 

3

^

1

20

3

1

\^hiaperminuta 

3

^

1

1

2

2

2

5

5

1

,<^

!> ~3 §• 

s

1

S

^

3

1

3

2

3

2

1

0.5

1

1

*hia sigma

S

?

1

15

10

6

6

4

10

3

6.5

3

1.5

0.5

12

14

23

4

4

1.5

8.5

13.5

6

5

26.5

4

10

4

11

4

8

\socialis ha

S

^

1

8

13

2

1

1

1

1

i

§^

U

2

3

5

1

1

14

2

2

8

1

8

5

3

5

17

13

6

51

2

6

5

1

7

11

hia terrestris

§

^

4

13

9

4

1

2

7.5

16

1

\tella aurita

1O

1

1

5

1

13

2

1

2

2

2

1

1

1

5

1

1

1

3

\tella longicruris 

1

O

2

1

2

\tella obnuta 

1

O

5

4

1

! §. t?

I

8-

A

o

i

5

1

2

24

15

4

2

2

2

1fzora .spp. 

6-

o

2

2

6

3

4

sulcata ia 

5

5

26

30

216

59

161

185

149

30

38

75

26

56

66

30

60

16

5

53

18

114

54

79

11

93

19

26

23

115

70

11

2

16

8

24

37

62

32

61

41

123

20

58

—V

g>

^2.

"3!

i

•S

S

5

66

122

551

155

186

71

146

5

23

88

77

96

147

198

106

110

83

9

299

150

197

78

91

60

222

56

98

49

109

102

96

18

137

58

77

31

37

5

50

39

122

38

75

neis granulata

£

$

1

1

2

1

5

4

2

2

1

1

1

1

marina nes 

£

S

2

1

2

2

2

13

1

\laria borealis 

s

&,

1

1

1

\lagerstedtii laria 

K

&,

15

10

9

1

42

17

102

41

3

2

4

5

30

16

4

11

2

56

36

^

1

1

4

33

52

laria spp.

K

$ft.

16

1

1

1

1

4

3

2

11

19

4

2

\ian'a viridis 

K

<£

1

2

\staurophorum ogramma 

'So 

«

s;

5

5

salinarum osigma 

i~

«j

=C

1

10

Ionemaarcuatum

1

OS

6

2

1

4

2

1

2

12

1

1

!

§5 S

!

<X

1

4

5

3

4

1

2

2

1

2

4

1

1

1

1

3

2

1

2

1

1

\psammicola honeis 

§

OS

1

4

24

6

4

1

7

1

1

1

3

\osphenia abbreviata 

§

§

1

2

1

1

2

8

2

3

2

2

2

\2/orf/a acuminata 

$

OS

3

6

6

1

1

1

2

5

3

1

1

2

1

1

1

\gibba j/odfa 

8-

OS

2

\ilodia musculus 

§•

OS

1

6

5

1

3

8

1

3

1

\ilodia operculata 

§•

OS

3

\ipleura tumida 

"S

X

5

4

10

15

5

18

14

4

3

7

2

1

2

4

6

4

15

3

39

88

43

20

10

3

1

.tn1

3

£

3

CO

2

1

4

1
42 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON 

Table 4. Numbers of diatoms counted at the Redtail locality—Continued

Uppermost 1 cm 

of soil Y

Uppermost 1 cm 

of soil U

Uppermost 1 cm 

of soils

Uppermost 1 cm 

of soil N

Uppermost 1 cm 

of soil L

Uppermost 1 cm 

of soil J

Depth below

modern 

surface (m)

0.01

0.15

0.35

0.55

0.66

0.70

0.74

0.76

0.90

0.99

1.09

1.14

1.17

1.22

1.40

1.50

1.54

1.58

1.68

1.80

1.89

1.95

1.97

2.05

2.10

2.15

2.18

2.45

2.58

2.65

2.67

2.75

2.90

3.00

3.10

3.20

3.23

3.27

3.31

3.35

3.39

3.41

3.45

$

-2

!5

1

i

2

2

1

2

2

1

4

1

2

1

isciculata

«,

S

^

1

1

12

2

13

8

6

2

1

2.5

1

4

6.5

3

3.5

1

7

2

4.5

7

3

1

2.5

1

3

3

4.5

8

20

13

10

2.5

1

&W" 

1

^

&

1

1

1

5

ifenestrata

3

X

S

2

20

1americana 

gS

2

2

1

4

S *)

1

>n

*, 

*

•ts

s

1

29

2

3

1

16

20

12

1

2

1

2

5

1australis tenia 

1

•ts

1

1

1

1

\la acuminata 

«U

-a 

£

1

1

la aerophila

«3 

S

1

17

5

1

2

1

1

\/a apiculata 

<u

•O

£

2

2

4

2

3

2

1

2

2

4

4

1

2

1

1

2

|la coarctata 

%j

-a 

£

1

2

1

1

1

1

2

\ladebilis 

«

S

-a 

£

38

10

2

2

2

2

4

5

12

8

7

10

18

2

1

2

1

1

2

15

1fo granulata 

%j

-a 

£

4

18

33

10

8

9

2

40

9

6

26

10

2

8

14

24

5

3

2

8

8

2

10

17

5

2

2

3

4

27

12

14

3

12

\la levidensis 

<&

1

1

1

2

3

1

4

3

2

1

1

1

3

\la littoralis 

«

S

-ft£

1

1

2

\/a navicularis 

<»

•Ci

£

2

2

2

1 

.3

<a

-a

£

5

1

2

10

1

4

1

2

\lapunctata 

<a

1

1

4

2

1

1

1

2

1

1

1la vexans 

«U

1

1

1

1

3

1

1

1

2

2

1diatoms c

|

1

1curvatulus us

&"5 

o•<

1

3

5

3

1

2

1

1

1

1

1

4

\kutzingii us 

&•S

tj •"C

2

11

2

9

4

3

2

1

1

1

4

4

2

1

1

1

1

2

1

2

4

5

3

i 

s^

.Sj

&o•<

3

1

2

7

1

1

2

1

7

1

1

3

*hus senarius

5

R-

-I

(J

•<

2

9

7

10

5

4

2

6

1

7

7

1

1

2

1

2

3

14

39

19

8

li

1

i 

•e§

£

R-

•I

0

•^

2

1

1

2

1

1

2

7

4

2

1

1

tn "C

J>"3

i •e

£

*

•Stj•"C

1

2

1

i ?

i£s

•<

2

3

11

6

22

3

8

6

37

77

40

26

56

10

10

1

9

18

8

2

1

2

10

2

2

11

6

5

2

7

4

i

8

i

1

o

1

9

2

6

2

2

2

1

2

i tn1i

i

2

1

7

14

18

5

5

1

8

2

1

2

1

1

1

1

1

2

1

1

2

3

8

10

i

.§"3

o

£

i

8

2

6

2

3

3

3

1

2

2

3

2

2

1

2

4

1

1

3

\hiemalis wa 

o

C) •W

§

1

\liscus spp. 

8-

05

i

i

i

i

i

4

2

1

1

13&§,

'!•

<>5

4

1

1
SEQUENCES OF BURIED SOILS 

Table 4. Numbers of diatoms counted at the Redtail locality—Continued

43

Uppermost 1 cm 

of soil Y

Uppermost 1 cm 

of soil U

Uppermost 1 cm 

of soil S

Uppermost 1 cm 

of soil N

Uppermost 1 cm 

of soil L

Uppermost 1 cm 

of soil J

Depth below 

modern 

surface (m)

0.01

0.15

0.35

0.55

0.66

0.70

0.74

0.76

0.90

0.99

1.09

1.14

1.17

1.22

1.40

1.50

1.54

1.58

1.68

1.80

1.89

1.95

1.97

2.05

2.10

2.15

2.18

2.45

2.58

2.65

2.67

2.75

2.90

3.00

3.10

3.20

3.23

3.27

3.31

3.35

3.39

3.41

3.45

Thalassionema nitzschioides

5

1

5

3

2

1

1

3

8.5

0.5

1

12

4.5

2

2.5

4

30

1.5

5

2

1

3

3

5

6

14

11

1

\Thalassiosira eccentrica 

1

5

1

1

2

1

2

4

1

4

3

4

\Thalassiosirapacifica 

2

5

3

1

2

1

2

4

1

1

2

3

4

6

5

2

\Thalassiosira spp. 

1

10

4

11

13

5

1

6

10

8

5

1

2

3

3

1

4

4

10

14

1

4

3

1

1

22

5

19

17

23

2

6

1Unidentified diatoms 

4

7

2

4

7

4

5

1

2

2

1

14

1

8

3

10

4

12

7

7

!

G

G

P

P

M

M

G

VP

P

M

P

P

M

M

M

M-G

G

VP

P

P-M

M

M

P

G

M

M-G

M-G

M-G

G

G

M

G

M

M

P-M

M

M

M

M

M

M

M

M

1Total diatomvalves counted 

462

464

1034

571

655

464

487

14

242

364

307

309

438

450

416

419

300

43

426

320

518

367

208

211

517

323

331

303

433

435

322

340

302

206

303

441

431

405

428

405

564

381

471

diatoms(l) Total high marsh 

29

39

1

13

4

1

1

101

77

173

88

2

7

24

15

7

60

53

20

13

12

100.5

67

37

12

86

5

2

1

14

88

118

marsh Totallow/high diatoms (1)

255

30

3

3

2

1

10

11

10

8

10

6

2

7

2

19

2

33

38

33

18

21

2

23

47

19

11

7

11

3

1

19

29

89

(1) Total lowmarsh diatoms 

6

13

50

156

28

25

7

2

4

48

20

3

68

22

27

1.5

4

71

9

3

1

5

3

105

33

41

15

4

7

21

43

57

74

116

91

51

45

23

5

9

(1) Total tidaflat diatoms 

27

112

46

57

67

75

94

1

6

8

18

57

2.5

1

108

94

82.5

11

8

3

115.5

124.5

9

12.5

16

33

86

81

103.5

99

17.5

9

23

35.5

17

205

105

95

104

99

101

25

21

(BIDI) Total usedfor indexdiatoms 

317

194

100

229

101

102

101

#

119

100

66

77

185.5

100

186

124

111.5

#

#

100

124.5

161.5

#

110.5

110

191

150

155

120.5

103

141

130

100

101.5

167

295

226

186

155

146

157

147

237

BIDI value

0.91

0.39

0.29

0.41

0.20

0.14

0.03

#

0.94

0.90

0.36

0.13

0.99

0.98

0.24

0.15

0.14

#

#

0.62

0.04

0.22

#

0.86

0.84

0.55

0.32

0.35

0.08

0.02

0.88

0.90

0.67

0.44

0.73

0.18

0.28

0.24

0.16

0.17

0.28

0.81

0.89

(1) Individual taxa listed in Table 2

KEY:

G

M

P

VP

#

Good preservation

Vtoderate preservation

3oor preservation

Very poor preservation

Upland deposit inferred from

rare reworked diatoms
44 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

§

umjcp (toot «Aoq« sie^eui IN '

jcSCS
SEQUENCES OF BURIED SOILS 45

§

F-l

i

n <

^

S

1O

a 

£

a

bO

13 

M

y

S

O 13 

g S £.8

T3 T° 

(U 73 •g-8

•° §

°l

<U ^T

tl

.•a -b

, es5

ifllls

:!N!St!i! •I MHj§ggep3^w ill! l^U

A 2 a g .!i U S ^

1

o 

U

I

All the soils at the Oyster locality dip gently southeastward (figs. 18, 19B), diagonally away from the nearest 

upland. The oldest soils dip most steeply; soils S and U are 

nearly 1 m lower on the southeast than on the northwest. As 

at the Redtail locality (p. 26), most of this relief is probably 

due to compaction of soft Holocene mud, which may 

thicken southeastward beneath the Oyster locality, the direction toward the middle of the Niawiakum River valley (fig. 

1). Such thickening of soft mud has been observed in foundation borings along nearby U.S. 101 (Washington State 

Department of Transportation, Olympia, unpub. data, 1986). 

In these borings, the mud is thickest—about 30 m—about 

midway across the valley. Slumping along the Niawiakum 

River may have also contributed to relief on the buried soils 

at the Oyster locality. Such slumping is indicated by nearly 

vertical faults having less than 1A m offset and exposed on 

the modern bench between soils S and W in the southeastern half of the outcrop.

Soil U varies greatly in abundance of peat at the Oyster 

locality. At the northwest end of the outcrop the surficial 

horizon of soil U is barely recognizable as a band of mud 

about 1 cm thick that is slightly darker (by one unit of value 

in the Munsell color system) than the mud above and below 

it (fig. 19E). To the southeast, however, the surficial horizon 

of soil U grades laterally through peaty mud into muddy 

peat 5 cm thick. This southeastward gradation is accompanied by the appearance of vascular-plant fossils—rhizomes 

below and above the soil, stems and leaves of herbaceous 

plants rooted within the soil—that are absent to the northwest (fig. 18). The southeastward gradation is further 

accompanied by softening of the mud above and below soil 

U. It is not accompanied, however, by any major change in 

diatom assemblage (fig. 20), with one main exception—the 

sturdy valves of Paralia sulcata are much more abundant, 

and the preservation of more delicate valves noticeably 

poorer, where the soil is faint than where the soil is peaty (p. 

23; fig. 23). We infer that the lateral changes in vascularplant and diatom fossils reflect differential decomposition 

and dissolution during the time when soil Y was at the land 

surface. Such diagenesis is more likely to have attacked the 

northwest end of the outcrop, where soil U is 0.3 m below 

the top of soil Y, than the southeast end, where soil U is 1.0 

m below the top of soil Y.

Soil W is everywhere fainter than the underlying part 

of soil U. At the northwest end of the outcrop, soil W is not 

generally visible (fig. 19E), probably because of decomposition in the profile of soil Y, the top of which is 0.2 m above 

soil W (fig. 18). Like soil U, soil W becomes more noticeable to the southeast, where it descends to as much as 0.4 m 

below the top of soil Y Nowhere at the Oyster locality, however, is soil W peaty.

TSUNAMI DEPOSIT

A tsunami deposit overlies soil Y at the Oyster locality 

(fig. 19E). The deposit comprises five or six sandy laminae
46 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Figure 19. Oyster locality, Niawiakum River (fig. 18). A, 

Overview to the southeast (upvalley), showing tidal marsh at left 

and tidal flat at right. Trees in background are rooted on upland, 

above level of highest tides. August 1987. B, Closer, eastward 

view of surveyed outcrop. Buried soil S, which has apparent dip to 

the southeast, is interrupted by channel fill (c) centered near

horizontal coordinate 60 m. May 1995. C, Closer view of channel 

fill shown in B. Horizontal layer at man's feet is sandy bed 

probably correlative with tsunami deposit on soil Y. Difference in 

lighting makes area of channel fill appear lighter toned than in B. 

Sp, Spartina foliosa, an introduced salt-marsh grass, on 

accretionary bank. Tide higher than shown in A and B. July 1994.
SEQUENCES OF BURIED SOILS 47

Figure 19. D, Buried soils S and Y downstream from channel fill. 

Rhizomes of Triglochin maritimum account for resistant layer (r) 

just above water line. Stakes on top of bank are 10 m apart; middle 

stake denotes horizontal coordinate 10m. View across Niawiakum 

River; upstream to right. £, Tsunami deposit on soil Y at 

horizontal coordinate 10 m. Deposit contains sandy laminae that 

appear dark from shadow in recess etched by water. Above 15-cmlong scale at center the upper several laminae turn upward against 

tuft of Deschampsia caespitosa, whose leaves are flopped over to 

the right, upvalley. Faint soil W, which is noticeable between soils 

U and Y farther upstream at the Oyster locality, is too faint to show 

in the photograph. Shovel and centimeter scale are in same 

position as in D.

that alternate with mud (Atwater and Yamaguchi, 1991, 

their fig. 5). The lowest of these laminae, which is also the 

least distinct and least continuous, rests directly on the soil. 

The thickest and coarsest lamina is micaceous, silty, very 

fine sand 1 cm thick. The two uppermost laminae are a pair

of silt partings. The total thickness of the deposit is mainly 

5-7 cm.

The deposit accumulated around herbaceous plants 

rooted in soil Y. At the northwest end of the outcrop the surrounded tufts of Deschampsia caespitosa are flopped over
48 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

to the east (fig. 19£)—upvalley (fig. 6)—as are the stems 

and basal leaves of Potentilla pacifica. The measured azimuths of 14 of these flopovers near the northwest end of the 

outcrop range from 065° to 120° and average 082° (M.A. 

Reinhart and S. McMullen, written commun., 1987). These 

orientations imply currents directed eastward, up the Niawiakum River valley (fig. 7).

There are no other sandy beds in the outcrop except in 

a probably correlative lens within the channel fill that 

underlies the mudflat at the confluence of tidal creeks (figs. 

18, 19C). This sandy lens, as much as 15 cm thick, consists 

of micaceous, silty, very fine to fine sand interbedded with 

mud. Several of the sand beds in the lens abound in flatlying plant fragments—mainly bark-free twigs, pieces of 

bark as much as 1 cm thick, and a few leaf bases of Triglochin maritimum. The twigs may have been recycled from 

an older deposit, as shown by their lack of bark and their 

radiocarbon age (p. 63).

Reasons for inferring that a tsunami deposited the sand 

on soil Y at the Oyster locality are listed on page 27.

GROWTH-POSITION FOSSILS OF VASCULAR PLANTS

Vascular-plant fossils and algal layers imply that burial 

of soils S, U, and Y at the Oyster locality resulted from lasting tidal submergence. Rooted in each soil are stems and 

leaves of high-marsh plants. By contrast, mud above each 

soil contains growth-position rhizomes of the predominantly low-marsh species Triglochin maritimum. The rhizomes imply a position near the lower growth limit of 

Triglochin maritimum for two reasons:

1. Though well preserved, the lowest Triglochin maritimum 

rhizomes above each of the three soils lack associated 

fossils of high-marsh plants.

2. These lowest rhizomes are underlain, above each of the 

three soils, by mud containing rhythmic organic laminae 

similar to those produced by filamentous algae on mudflats below the lowest Triglochin martima at the Oyster 

locality. Such laminae, which are among the geologic 

recorders of fully intertidal conditions at Willapa Bay 

(Clifton, 1983, p. 362), form today about 1.0-1.5 m 

below the level of high marsh at the Oyster locality (figs. 

8, 18, "algal mat").

Herbaceous plants rooted in soils S and Y probably 

lived higher in the intertidal zone than did the plants rooted 

in soil U. All three soils bear tufts of the high-marsh grass 

Deschampsia caespitosa, as shown in photographs of soils S 

(Atwater and Yamaguchi, 1991, their fig. 4B) and Y (our 

fig. 19£). But only soil U bears stems of the pickleweed Salicornia virginica, which tolerates higher soil salinity than do 

other vascular plants of tidal marshes in the western United 

States (Mall, 1969; Barbour and Davis, 1970). Conversely, 

the less saline high-marsh species Juncus balticus and 

Potentilla pacifica are commonly rooted in soils S and Y but 

appear absent from soil U. Soil U probably was inundated 

by salty tidewater about as frequently as the modern high

marsh at the Oyster locality, on which Salicornia is common within 50 m of the Niawiakum River. The lack of Salicornia on soils S and Y implies less frequent inundation by 

salty tidewater. This interpretation is supported by differences in diatom assemblages (see next section).

Soil W represents a marsh above the lower growth 

limit of Triglochin maritimum. We found no growth-position remains of vascular plants that might have been rooted 

on soil W or which might have colonized the overlying 

mud. However, rhizomes of Triglochin maritimum are 

present 35 cm below the top of soil W near the southeastern 

end of the Oyster locality (fig. 18). These rhizomes probably formed near the lower limit of Triglochin maritimum. As 

just noted, they are underlain by algal laminae commonly 

found near that limit; moreover, mud among the rhizomes 

contains diatom assemblages indicative of the transition 

from tidal flat to low marsh (fig. 20, section at horizontal 

coordinate 165 m). After Triglochin maritimum became 

established, the marsh probably aggraded to a position 

higher in the intertidal zone, as shown by an overall shoaling trend in diatom assemblages between soils U and W 

(fig. 20).

FOSSIL DIATOMS

Diatom assemblages show that the four soils exposed 

at the Oyster locality represent tidal marshes, and that mud 

above each of these soils was deposited lower in the intertidal zone or perhaps in subtidal water (figs. 20-22). The 

diatoms further suggest that soils S and Y represent positions higher in the intertidal zone than do soils U and W, and 

that soil W represents the lowest position of any of the four 

soils.

Diatoms in soils S and Y are dominated by high-marsh 

species. However, they also contain large numbers of 

heavily silicified and easily reworked tidal flat diatoms 

(tables 5, 6). These probably allochthonous valves make the 

brackish intertidal diatom index lower than would be predicted for soils S and Y on the basis of vascular-plant fossils.

Diatoms in soil U imply a slightly lower-level high 

marsh than do diatoms for soils S and Y because, unlike 

soils S and Y, soil U contains a well-preserved assemblage 

of low-marsh species that require regular tidal inundation, 

particularly Gyrosigma eximium. Such an assemblage can 

be found today in stands of Salicornia virginica near mean 

higher high water on the riverward part of the modern marsh 

at the Oyster locality (Hemphill-Haley, 1995a). The fossil 

diatom assemblage thereby accords with the vascular-plant 

assemblage rooted in soil U; the vascular plants rooted in 

soil U are dominated by Salicornia virginica.

Soil W contains low-marsh assemblages of diatoms 

(fig. 20). The most important of its low-marsh taxa is Diploneis interrupta. Although uncommon in modern samples 

from Willapa Bay, D. interrupta is a dominant constituent 

of Salicornia and Distichlis low marshes in the Puget Sound
SEQUENCES OF BURIED SOILS 49

Horizontal coordinates 15 and 25 m

3 o

*3

0) 

O

c 

oT

«

(0

£ 

(0

o

I

Q. 

O 

O

100--

200--

300--

Simplified

stratigraphic

column

T3 

•|

m

W 

U

Diatom assemblage

F-5

Horizontal coordinate 165 m

Simplified

stratigraphic

column

w

u

0 1

Brackish intertidal diatom index

Diatom assemblage

+ o

- 100 o>

I

(0

JC

i£ 

I

o

T3 

O

_o

o

a. 

o> O

fis

300-i

200

0 1

Brackish intertidal diatom index

JL EXPLANATION FOR SIMPLIFIED STRATIGRAPHIC COLUMNS 

Surface of modern tidal marsh Vascular-plant fossil

Top of soil

Bold soil

Faint soil—Line dashed where soil 

barely visible

Mud

High-marsh herb rooted in soil

Rhizomes ofTriglochin martimum 

rooted in mud

•?•?!?•?•' Tsunami deposit

Figure 20. Summary of diatom assemblages at Oyster locality, Niawiakum River. Samples collected from sections at horizontal 

coordinates 20 m and 165 m (fig. 18).
• 1 

U

I

W

<5

|

o"3

a a> 

o

CO

1

CO

Oi 12

Diatoms found in 

high marshes

Diatoms found in 

low and 

marshes

Diatoms found in 

low marshes

0 0.5 1.0 

Brackish 

intertidal 

diatom index

Soil

W

tr

0 20 40 60 80 100 

Percent of total diatoms used in index

?o

8ffl2H <

3JC

g

w

CO

i

Figure 21. Relative abundances of diatom species grouped by inferred depositional environment for a stratigraphic section at horizontal coordinates 15 and 25 m, Oyster locality, 

Niawiakum River.
ca -a Diatoms found in 

tidal flats, sloughs,

and shallow subtidal areas

JC / 0* 

D) / . . V . . A

B

oo

0

E 

c

of

0 -j ,

t

CO

c

•8o

o . E

5

o

O

•

if! ////&f/$ * // ////* / + ^i§ I •? j /^ ^^-^^/ <$ ***?* / / c/V ^ P /X*'X^VV 

Hi i \/^^^^/^y/^^^^^s

i— . — • -^ — » I ,,vvvvwvv > x^x/v\ N \\ N N vv^^^^^ >>> 

V

^^^v

p

••K

^p

v^

•^^

^•••|r

^^^^v

V

1

[

pBH^

0 0.5 1.0

1 1 ' 1 ' 1

—

1 1 ' 1 ' 1 -1

^1

in

^rn

•

—

TH

^i

-1

^TH

-

-

^

——

^

__

r

•

-

-n "n

• i • i • i

•

-

__

»

^

' 1 ' 1 " 1

-

• i • i • i

• i • i

—

——

n.... .

• 1 • \

-

•

~—

k

^i

•n

—

•

-

-m

^n

—

-r,

^i

•

h

^n

*....

~

-m

^rn

'

-

^

^Tn

PTTT..

k^

^1

•

•

m

————

k

-

-1

^1

-

-1

•

h

•

•

•

fc1. .

~

;

H

•

•

:

i..

^n

• i • i ' i

^_

-

i^^«pi^

—

1 1 • i ' i

-

-

-

•

-•

•

h

brackish i • • i • i • i • i • i • i • i • i • i

intertidal 0 20 40 60 80 100

diatom index n . . . . , .. . . . . . „

Figure 21. Continued.
52 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

m ujapoui Mojeq i|)dea

region at Lynch Cove (E. Hemphill-Haley, unpub. data; 

location shown in fig. 1).

The tsunami deposit overlying soil Y at the Oyster 

locality (p. 45) contains many well-preserved diatoms typical of sandy tidal flats in Willapa Bay (sample depth 0.89 m 

in table 5, 1.09 m in table 6). Chief among these species are 

Achnanthes delicatula and Cocconeis diminuta.

Changes in diatom assemblages across the top of soil 

W probably did not result primarily from a tsunami. The 

assemblage changes from low marsh (soil W) to tidal flat 

(the overlying mud). Although the assemblage reverts to 

tidal marsh barely 10 cm above soil W (fig. 20), the mud on 

soil W contains few diatoms that might have been derived 

from sandy tidal flats of the open bay.

AGE AND CORRELATION

A radiocarbon age shows that tidal-marsh deposits 

exposed near the southeastern end of the Oyster locality

began to form between about 2,050 and 2,350 years ago. 

The age was measured on Triglochin maritimum rhizomes 

collected about 50 cm below the top of soil S (age D in fig. 

18). It serves as a limiting minimum age for any soil earlier 

than soil S, for the interval between the dated sample and 

the top of soil S contains no such soil yet retains many vascular-plant fossils. These fossils suggest that the interval 

would retain a buried soil had one ever been present.

Additional radiocarbon ages have been measured on 

vascular-plant fossils associated with the burial of soils S, 

U, and Y at the Oyster locality (table 7; figs. 18, 32).

Burial of soil S at the Oyster locality began shortly 

before 1,400-1,600 years ago. This estimate corresponds to 

the more precise of two concordant radiocarbon ages on rhizomes 45-55 cm above the soil. We assume that less than a 

century elapsed between the onset of burial of the soil and 

the inception of low marsh marked by the dated rhizomes. 

This assumption is supported by documentary evidence for 

rapid postearthquake reconstruction of tidal marshes at
SEQUENCES OF BURffiD SOILS 53

•dds rnioung

vptSjtt) tn

1 -ds vfootpug

VOlUUlf SWUO\d\Q

-p si9utifdj9(j

I •J

Vfltsnd siduonuso^

•dds snosipouiosoj

v\ivd

saptojpd swiia33O3

snptSjnj snjnvjvjaj

uinjjpvq swu0\V3 

•ds tmfdjnpptg

mcopojtvd vuviipvq

VSOOUJI&A vjoifduiy

•dds vjoifduiy

snajojd vjoifdtuy

snuvuds sn

ssdtt&jq samtannfoy

CQ oa oa oa oa CQ CQ oa oa oa

3

> e

>

'o

*8

P: !

^^
54 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

vsonivf trusting

vjoynuiuvsd si9uoi{dm^

•dds vtiDjnuutj

HjpatsjaStTj vur^numj

sijoyjoq Dtitqnuuij

(uuofjjtnus) t>jD3jns tnjvsvj

tqvzjns tnjvjvj

vAJvd

SUJS9JJ9) Vttj^&JM

sjjtnzos vnjzsZjttf

stuuofijpdnzs vtqzsZjiN

vjvqojiq vnj3s2jifj

sapiojnututnu vjiso^fy

sruuoftjiuaut vjisojdffl

sifxottfdum vii{3S2ji

lunnutxz muSisoj/ff)

V31UB330 VJOlfdojVUOUVJf)

stivSjnA tnjnjsruj

tqnzids tnimsruj

ermost Yof soil

1 tW

U cm

Upperm of scm
Table 5. Numbers of diatoms counted at the Oyster locality, horizontal coordinates 15 and 25 m—Continued

Uppermost 1 

cm of soil Y

Uppermost 1 

cm of soil W

Uppermost 1 

cm of soil U

Uppermost 1 

cm of soil S

Depth 

below 

modern 

surface (m)

0.20

0.45

0.65

0.75

0.80

0.87

0.89

0.90

0.95

0.98

1.08

1.11

1.13

1.16

1.18

1.19

1.20

1.20

1.22

1.23

1.27

1.28

1.36

1.42

1.44

2.21

2.27

2.32

2.37

2.40

2.48

Synedrafasciculata

1

1

9

11

2

0.5

12

13

3

1

2

0.5

9.5

8.5

28

21

1

9

7

9

11

0.5

0.5

Trachyneis aspera

1

4

2

2

1

1

6

9

4

2

2

1

1

Trachysphenia australis

2

0.5

4

1

2

1

Tropidonfis spp.

1

4

13

2

onella acuminata

I

1

1

1

0.5

1

2

1

4

1

onella aerophila

*

2

onella apiculata

*

2

2

2

5

4

5

2

onella debiis

r^

2

1

4

onella granulata

jl

20

3

14

11

6

6

6

4

20

26

15

2

38

52

10

7

5

4

6

1

36

5

16

16

22

8

10

5

3

3

onella levidensis

jl

1

1

1

1

1

2

onella littoralis \

^

1

onella navicularis

|

5

2

4

7

0.5

2

a, 

.3

5

1"

1

onella punctata

J

2

2

1

1

2

onella vexans

|

2

1

Planktonic diatoms

Actinocyclus curvatulus \

1

2

2

1

2

1

2

2

Actinocyclus ochotensis

9

5

12

3

12

2

7

Actinocyclus kutzingii

2

2

1

1

1

3

4

1

1

Aulacoseira spp. \

24

25

30

14

18

9

6

10

93

2

2

Chaetoceros spp. \

3

2

1

Cyclotella sp.\

3

Cyclotella spp.

2

1

6

8

2

1

7

1

2

2

6

4

1

4

1

34

8

1

1

2

2

Stephanodiscus spp.\

1

4

3

1

3

2

4

1

1

Thalassiosira spp.

2

2

8

14

13

6

1.5

2

1

6

10

2

2

2

6

2

2

21

26

34

42

2

Thalassionema nitzschioides

1

4

5

1

1

0.5

1

5

3

3

2

2

Preservation

M

M

M

M

M

M

M

M

M

P

M

M-G

M

M

M

M-P

M-G

P

P

M-G

P

M-G

P

P

P

M

M

M

M

M

M

Total diatomvalves counted

415

295

667

542

476

423

428

239

936

595

306

135

265

458

688

420

146

220

196

134

342

203

408

637

458

212

194

231

272

142

192

Total high marsh diatoms (1)

3

13

2

2

3

18

83

72

63

50

40

30

14

4

4

8

3

4

4

2.5

8

40

1

2

39

29

Total low/high marsh diatoms (1)

37

2

5

2

23

5

21

10

5

1

4

2

4

2

14

3

1

33

5

Total lowmarsh diatoms (1)

31

64

59

66

25

10

14

6

9

3

30

53

62

28

16

34

89

86

90

95

61

40.5

30

18

34

2.5

6

17

12

18

55

\ 

Total tidaflat diatoms (1) \

29

23

39

32

70

155

132

12

14

12.5

10

2

8

57

76

62

3

9

2

0.5

36.5

59.5

70

72

12

97.5

90

80

88

10

11

Total diatoms usedfor index(BIDI)

100

100

100

100

100

170

166

124

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

100

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0
Diatoms found in 

high marshes

Diatoms found in

low and high

marshes

Diato

low 

o

!

w

1«

JO

f 

o 

O

0 0.5 1.0

Brackish

intertidal

diatom index

0 20 40 60 80 100

Percent of total diatoms used in index

Figure 22. Relative abundances of diatom species grouped by inferred depositional environment for a stratigraphic section at horizontal coordinate 1
F,

ijsjeiu

MO"|

lepuqns 'q6no|s"iB|i"|Bp!i

SEQUENCES OF BURIED SOILS

i I

_L*i. 1 I . I. I

.11 i .

r

JF

IF

F

IF

IF

IF

IF

JF

IF

.IF

IF

U£

IF

, iF

JF

IF

jp

57

00

804 2 6

'-o

ndex

diatoms used Percent oftotal 

•a

I

o 

U

sjejeiu uj 'eoe|jns ujepoui MO|eq
58 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

qSJBLU MO~|

'q6no|s'

u; 'eoejjns ujepoiu MOjeq q)deQ

2

I

Willapa Bay in the past 300 years (see section "Minimum 

Time for Formation of a Peaty Soil" on p. 95). Moreover, 

onset of burial shortly before 1,400-1,600 years ago is consistent with radiocarbon ages from other localities. Burial 

began between 1,520 and 1,700 years ago at the Pool locality, as judged from a high-precision age on inner rings of a 

red-cedar root in soil S (p. 63), and it began sometime 

between 1,400 and 2,100 years ago at the Redtail locality, as 

judged from conventional radiocarbon ages on roots and 

peat in the soil (fig. 32).

Two high-precision ages have been measured for soil U 

at the Oyster locality—one on Salicornia virginica stems 

rooted in the soil, the other on Triglochin maritimum rhizomes 20 cm above the soil. The ages, which are nearly 

identical, imply that burial of the soil began between about 

1,130 and 1,350 years ago.

Burial of soil Y began no earlier than A.D. 1660, as 

shown by radiocarbon ages of Potentialla pacifica and Juncus balticus rooted in soil Y and surrounded by the overlying tsunami deposit (Nelson and others, 1995). The 

individual ages are listed in table 7, and ranges of calendric 

ages corresponding to their weighted mean (161±15 14C yr 

B.P.) are plotted in figure 32.

Several additional lines of evidence tend to confirm 

that the oldest and youngest buried soils exposed at the Oyster locality correlate with soils S and Y, respectively, at the 

nearby Redtail locality. Both soils are characteristically bold 

at both localities. The youngest buried soil is the only one 

covered by sand at either locality. Diatom assemblages at 

both localities suggest that soils S and Y formed higher in 

the intertidal zone than did the intervening soil or soils (p. 

48). Finally, vascular-plant fossils rooted in both soils at 

both localities imply lower soil salinity than do the plants of 

modern marshes. At the Oyster locality, Salicornia is common on modern high marsh near the river but is absent from 

soils S and Y. Similarly at the Redtail locality, Sitka spruce
SEQUENCES OF BURIED SOILS 59

vjvununov mu8isoi£r)

V31UO33O VJOlfdoJVUtlUDJf)

suvS\nA. tnjmsruj

tqnoids tnjnjsruj

vjvdpj of tnov]]vj

•dds mjoung

I -

voiqwoifj -A inarms swuo\diq

-p stzuttjdpQ

-p st?utifdp(j

vflisnd vi

VfllSnd S19UOHUSOJ

•dds snosipouijso^

viuvd -A tunjpjnos swuooooj

snptSjnj snjnvjvjdj

iijsdM stduojvj

•ds tntfdjnppjg

siuuofiyv

suiojeip 3iq|U9g

B « m m
60 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

wruoifdo

•dds vutqnuuij

njp93SJ9St>j vutqnutttj

(uuoj jjtnus) vjvojns tnjvuvj

muSts vn{3S2mq

wnutuudd vn/ysljifj

vjmununuoo vtijJ&WN

vjvqopq mi{3S2)W

vsoipvj ojnoiAvpj

SIJtTWJOU V]nO1AV[q

snostuaui v^nomviq

viinpvjioiiStp -Jo ojnoiAvpj

swuofynioui vjisojaffl

S

U cm

1 ostoil Y

U cm U 

soil Uofcm
SEQUENCES OF BURIED SOILS 61

•8

en|BA|aiQ S

(iQia) xepuj joj pesn swopp JBJOI S

(0 swojBip pnepimoi

(0 SWOJBJP IJSJBUJ M0| |B)01

(0 SUJOJBIp IJSJBW Ij6m//V\0| |B}01

(0 SLUOJBJP qsjBiu i|6|ij IBJOI

pejUnOO S6A|BA UJOIBJP |BIOJ. 8 8 s S

UO!)BAJ8S8Jd a a a O

•dds vjisoissvjtnjj,

suvjnyiiwu vipuoijqLix

vjznpojd s&uojnvjs

snjnysmu vipojodoyy

vtpojvdoijii

Illli

Up cm

Uppermost of soilYcm 
62 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Figure 23. Differences in diatom preservation in soil U at the Oyster locality. Scale bar at upper right applies to all 

three photographs. A, Good preservation at horizontal coordinate 165 m, marked by diverse assemblage of largely 

intact valves. B, C, Poor preservation at horizontal coordinate 25 m, marked by sparse assemblage of largely broken 

valves. 1, Gyrosigma eximium; 2, Nitzschia sp.; 3, Navicula gregaria; 4, Rhopalodia musculus\ 5, Fallacia pygmaea; 

6, Tryblionella debilis; 7, Pinnularia lagerstedtii; 8, Colonels westii', 9, Diploneis interrupta; 10, Caloneis bacillum; 

11, Cosmioneis pusilla.
SEQUENCES OF BURIED SOILS 63

is absent from the modern marsh but abundant in soils S 

and Y.

The lower of the characteristically faint soils between 

soils S and Y at the Oyster locality (soil U) probably correlates with the only faint soil preserved between soils S and 

Y at the Redtail locality (soil U). The upper of the faint soils 

between soils S and Y at the Oyster locality (soil W) was 

probably destroyed by weathering at the Redtail locality: 

soil U at the Redtail locality is as high in the profile of soil 

Y as it is at the northwest end of the Oyster locality (fig. 18), 

where soil W was probably also destroyed while soil Y was 

at the ground surface (p. 45).

Twigs from the sand lens in the channel fill at the Oyster locality yielded radiocarbon ages that may be centuries 

greater than the age of the lens. Several tens of twigs collectively gave an age of about 500-1,000 years ago, and a single twig gave an age of about 500-800 years ago. However, 

the lens accumulated only 300 years ago if it correlates with 

the sandy interval on soil Y. We infer that a tsunami along 

the Niawiakum River about 300 years ago exhumed and 

redeposited twigs that had first been deposited centuries 

before. Such recycling of twigs and sticks best explains discordant radiocarbon ages from a tsunami deposit about 300 

years old at Port Alberni, British Columbia (Clague and 

Bobrowsky, 1994).

Atwater, 1996, his fig. 22C). Hemphill-Haley (1996b) reasoned that a tsunami would carry diatom valves farther 

up valley than it would carry sand grains.

Inner rings of one of the redcedar roots in soil S gave a 

high-precision age that places the initial burial of soil S 

about 1,520-1,700 years ago. We selected 10 consecutive 

rings that formed 60-69 tree-ring years before the root 

stopped growing. The age obtained for this group of rings— 

1,740±15 14C yr B.P. (QL-4796)—corresponds to A.D. 

242-408. If the tree died 65 years later, and if tree death 

coincided with initial burial of the soil, then burial of soil S 

began sometime between A.D. 307 and 473—that is, 

between about 1,520 and 1,700 years ago. This estimate 

accords with the age of 1,400-1,600 years ago inferred for 

the onset of burial of soil S at the Oyster locality (p. 52, 58).

High-precision radiocarbon ages were previously 

reported for several samples from soil Y near the Pool locality (Atwater and others, 1991). The nearby samples are from 

Sitka spruce stumps rooted in the soil at locations plotted in 

figure 7 and identified there by laboratory number (QL-). 

For one of these stumps, separate ages for inner and outer 

rings (QL-4405 and QL-4406) reduce ambiguity in converting radiocarbon ages to calendric ages; the ages limit the 

probable time of tree death to the first few decades of the 

1700's (Nelson and others, 1995).

POOL LOCALITY

The Pool locality comprises 20 m of bank about 10m 

from an upland (fig. 7). The bank is fully exposed at many 

low tides because the minimum level of river water at the 

Pool locality is about 1 m above the level of mean lower low 

water (fig. 24). The water cannot drop further because, in its 

upper tidal reaches, the bed of the Niawiakum River is 

higher than the level of very low summer tides. During such 

tides the river trickles beneath the South Bend-Palix River 

Road.

Though not tall, the bank exposes five buried soils. In 

addition, we noted two underlying soils in a gouge core near 

the southwest end of the outcrop, and we found Pleistocene 

deposits about 1 m below the lower of these soils (fig. 24).

The sequence of five soils resembles the sequence of 

the five uppermost buried soils at the Redtail locality: three 

bold soils contain tree roots in growth position; and these 

soils (L, S, and Y) alternate with faint soils (N and U). Most 

of the tree roots at the Pool locality belong to Sitka spruce, 

but soil S also contains roots of western redcedar (fig. 24, 

horizontal coordinate 3 m).

Mud deposited by a tsunami probably mantles soil Y in 

the vicinity of the Pool locality (Hemphill-Haley, 1995b). 

This mud, like the sandy tsunami deposit on soil Y at the 

Redtail locality, contains diatoms typical of sandy tidal flats. 

The nearest sandy tsunami deposits on soil Y are nearly 0.5 

km downvalley from the Pool locality, according to mapping by Atwater (1987) and by Mary Ann Reinhart and 

Kenneth A. Bevis (written commun., 1987; summarized by

HERON LOCALITY

The Heron locality includes 35 m of bank between an 

upland and a tidal creek (figs. 7, 25). We identified eight 

buried soils in and beneath this bank. The widely recognized—soils J, L, N, S, and Y—spread apart from one 

another as they extend northward, away from the upland.

The south end of the Heron locality exposes onlap of 

soil Y onto stiff Pleistocene mud that makes up the upland 

(fig. 26). The peaty part of soil Y turns upward onto this 

mud (or onto colluvium derived from it) and is continuous 

with the O horizon of the modern upland soil.

Soil N at the Heron locality contains peat more widely 

than does soil N at the Redtail locality. Much of the exposed 

part of soil N at the Heron locality consists of peaty mud 

about 5 cm thick (fig. 25, horizontal coordinates 106-112 

m). The soil can be correlated confidently with soil N at the 

Redtail locality, however, not only by stratigraphic position 

beneath bold soil S, but also by the presence of several partings of micaceous silt in the lowest few centimeters of mud 

above soil N. The mapped extent of these silt partings at the 

Heron locality is shown by dashed lines in the upper part of 

figure 25.

Hemphill-Haley (1995b, p. 372 and her fig. 6) 

described diatom assemblages associated with soil Y a 

meter or two north of the area shown in figure 26. Diatoms 

in the peaty surficial horizon of the soil are few and poorly 

preserved, as also seen in soils S and Y at the Redtail locality. This assemblage, combined with abundant spruce roots, 

implies conditions at or above extreme high tides. By con-
Southwest 

10

•1625 ± 15 (inner rings of redcedar root)

Southwest

(0

IB

E

.£

Q)

LW — 

6

5 7 

Distance, in meters from local datum

Area of detail

shown above Northeast

t,+2.01,

— 2 August 1994

M, minimum 

water level 

_ beside outcrop 

at low tide 

and low 

river flow

— LW, level of 

mean lower 

low water 

estimated from 

measured tide 

of 2 August 

1994 7yy

Vertical

exaggeration

x10

0 20

Distance, in meters from local datum

Figure 24. Stratigraphy at Pool locality, Niawiakum River.
EXPLANATION

O

Top of soil—Shown by solid line where continuity 

observed; shown only by top of shading where 

continuity inferred 

Surveyed point 

Bold soil 

Faint soil

Letter name of buried soil 

Top of Pleistocene deposits—Marked by hard

angular aggregates of greenish mud 

Fossil root—Spruce except for western redcedar 

labeled in soil S 

Shown to scale 

Not shown to scale

1625 ±15

t,+2.01, 2 August 1994 -

Figure 24. Continued.

5 & rt a

£} p" n> P-

>2 i— ^ o>

S N

to

3^0 

Cf- ' 3 

» 3 °

5: ° 3*

S S1 Si C/J fTv fT

3 a.

C3 on 

3' on

II

w

O S O «"

f §3|

S a§. s N— a o> o 

o 3 p. n>

|S §.R

i^' P ^ CTtfj B OQ "

OJ •-*>

to c*

S TM

B:

.

- s-s»

°

O> on on S

It^ll

Live herbaceo

Radiocarbon a

(table 7) 

Low tide—Hei

meters relativ

Boring—Made 

OQ

0 

»-t

on

cr

o

1 

sn>

on

^'

P 

on

^5

^

o

o.

CL

n> 

^ 

o

on

0 

t— ^

gr

3* 

Son

on 

O_

O

?
North 

.5) 

d)

X

t1 —

12 —

O —

No vertical exaggeration

100

t1 +0.76, 20 July 1995 

t2 -0.36, 13Aug 1995

110 120 

Distance, in meters from local datum

South

60 80 100 

Distance, in meters from local datum

120 140

Figure 25. Stratigraphy at Heron locality, Niawiakum River.
67

* s

-o g

(U T3 B

IBS ^ XI TS

nr c/i o

ill Si*8

0 M M

Is!

2 H ft

S3 re 0;

8 a§ 1 is

gH

C bQ liSP *- re o

it

di Upper e — scale Low— 

II*

* TM VI

• S Q 1*

$& N °

J* -i-H N

i^^ K -M 

« tt

R w o

"45 "§> £ 5^ ^ ° ^ ^ ^^

"> t+H «+H .£f> o o 

^ e e 

2-8.8 0 -5 -5

» «« S

laa I-o-S

z =

fcj

« P - o

* C g 43

fc. ft ^ M

,*3JSI«

£ 

J

&&-S.£ 

l^ 5?^

5/2

&&,ff|'»i tS :f

££6£l'e £bl) Kzi<g^i«i;f.g ^ o S o

V9

£

O taS <-> O tH O

04^ J tt

O <

in

en 

en

§ 

U

vi

B

cut. The bottom of this additional outcrop intersects a modern mudflat about 1 m above lowest tides.

BURIED SOILS

Eight buried soils crop out at the Jensen locality. Four 

are typically bold enough to form horizontal ledges that 

extend without interruption for tens of meters along the 

dredge cut—especially soil S, which has a peaty surficial 

horizon 15-20 cm thick. A fifth soil, termed soil L, is bold 

only near the spruce roots shown in figure 28A. The other 

three soils are everywhere faint and have little topographic 

expression.

The horizontality and close vertical spacing of the buried soils at the Jensen locality reflect a shallow, almost horizontal foundation of Pleistocene deposits. These deposits 

consist of stiff yellowish mud that resembles weathered 

Pleistocene estuarine deposits elsewhere in the Willapa Bay 

area. This mud lies less than % m below the top of soil H 

and grades upward, through firm brownish gray mud, into 

the peaty surficial horizon of soil H (fig. 27). We speculate 

that the brownish gray mud was deposited by floods or 

extremely high tides that barely surmounted the Pleistocene 

deposits shortly before 3,500-4,500 years ago, which is the 

approximate time of burial for soil H at the Jensen and Redtail localities.

Faint soils N and W are the least traceable of the buried 

soils at the Jensen locality. They fade laterally like soil W at 

the northwest end of the Oyster locality, where soil W 

becomes too faint to identify readily high in the profile of 

the succeeding bold soil Y (fig. 19E). Soils N and W at the 

Jensen locality are high in the profiles of succeeding bold 

soils S and Y, respectively. Faint soils U and W are clearly 

exposed, bracketed by bold soils S and Y, near horizontal 

coordinate 30 m (fig. 28Q.

GROWTH-POSITION FOSSILS OF VASCULAR PLANTS

Soils H, J, L, S, and Y are associated with woody roots 

and (or) with herbaceous rhizomes (fig. 27). Soils H and Y 

contain woody roots indicative of conditions high in the 

intertidal zone or above highest tides; in addition, mud 

above each of these soils contains rhizomes of Triglochin 

maritimum, an indicator of conditions lower in the intertidal zone. Soil L contains radiating roots attributable to a 

single Sitka spruce. Soil S lacks woody roots in growth 

position but is overlain by mud that contains rhizomes 

doubtfully identified as Carex lyngbyei. Soil J also lacks 

woody roots, but mud above it contains rhizomes of Triglochin maritimum.

AGE AND CORRELATION

The sequence of soils and intervening mud at the 

Jensen locality spans the past 3,500-4,500 years. This estimate is based on the age of a stick associated with soil H— 

3,640±80 14C yr B.P. This stick undulated across the top of
68 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Figure 26. Onlap of soil Y, and of the intertidal mud that buried it, onto upland at the south end of the Heron locality (fig. 25, upper 

diagram). Shovel handle 0.5 m long.

the soil. Its age shows that burial of soil H began no later 

than 3,500-4,500 years ago. Similarly, soil H at the Redtail 

locality was buried shortly before 3,570±70 14C yr B.R, the 

age of Triglochin maritimum rhizomes above the soil (table 

7; fig. 27).

Three points suggest that typically faint soil L at the 

Jensen locality correlates with typically bold soil L at the 

Redtail locality, despite the greater abundance of peat in the 

latter:

1. The outer 25 rings of a spruce root in soil L at the Jensen 

locality gave a radiocarbon age—2,820±50 14C yr B.R— 

similar to three of the four ages for soil L at the Redtail 

locality (table 7; fig. 32). This age is geologically reliable 

because the outermost of the dated rings probably 

formed during the year before the tree died from earthquake-induced submergence: this ring adjoins bark; and, 

like several tens of preceding rings, it is wide enough to 

imply vigorous tree growth up to the time of an earthquake.

2. In containing spruce roots, soil L at the Jensen locality 

resembles soil L, but not the root-free soil N, at the Redtail locality.

3. The typical faintness of soil L at the Jensen locality may 

be due to weathering in the profile of soil S. Whereas the 

top of soil S is at least 1 m above the top of soil L at the 

Redtail locality (fig. 13), the corresponding distance is 

0.5 m at the Jensen locality.

These correlations of soils H and L are consistent with 

overall similarities among the Jensen, Redtail, and South 

Fork localities in the sequence and age of bold and faint 

soils (fig. 32). The sequence at the Jensen locality differs 

only in the presence of a faint soil—soil W—not recognized 

at the Redtail and South Fork localities. But as inferred on 

pages 45 and 63, poor preservation limits the traceable 

extent of soil W at the Oyster locality and probably accounts 

for the absence of soil W at the Redtail locality. The survey 

of the South Fork locality was completed in 1987, before 

findings elsewhere—especially at Elliott Slough (figs. 5, 32)
SEQUENCES OF BURIED SOILS 69

(Atwater, 1996, 1996, his fig. 235) and the Oyster locality—alerted us to possible destruction of soils high in the 

profile of soil Y.

SEWER LOCALITY

The northeast bank of the Willapa River displays buried soils for much of the 5-km distance between the South 

Bend sewer ponds and the Willapa Harbor airport. We surveyed outcrops near the South Bend sewer ponds (the Sewer 

locality) and near the airport (the Airport locality) (fig.7). 

The buried soils at both these localities are exposed mainly 

in steep banks less than 2 m high. Additional outcrop below 

these banks slopes gently riverward and is commonly 

obscured by modern mud and (or) cut by bank-parallel fractures.

The surveyed outcrop at the Sewer locality extends 500 

m northwestward from the pipe that carries effluent to the 

sewer ponds, which are situated on former tidal marsh north 

of the river. We noted an uppermost buried soil, soil Y, along 

the entire length of this outcrop, and we found a lower buried soil, termed soil S, along its northwestern half. Figure 29 

depicts the outcrop's central 180 m, which contains representative stratigraphy and all the archaeological features 

noted at the locality.

BURIED SOILS

The northwestern part of the Sewer locality contains 

four buried soils. Only the lowest of these, soil S, is characteristically peaty. Soil Y is the next-most noticeable but 

mainly because the lowest 10-30 cm of the overlying mud 

is distinctively laminated with silt that etches on wavewashed faces (fig. 30A, B). Both the remaining, intervening 

soils (soils U and W) are faint, particularly the upper one. 

Like soil S, soils U and W are probably not exposed in the 

southeastern 1A km of the outcrop.

We did not observe continuity of the buried soils as 

widely at the Sewer locality as at the Redtail, Oyster, and 

Jensen localities. In general, the soils at the Sewer locality 

cannot be seen where outcrop has not been scraped clean. 

None of the soils form widely visible breaks in the slope of 

the outcrop: soil S is commonly at or near the bottom of 

steep outcrop; none of the other soils are typically peaty; 

and steep outcrop is commonly cluttered by modern slumps 

and plants.

Where seen in outcrop, soils S and Y have maximum 

relief of ¥2 m. Between horizontal coordinates 360 and 290 

m, soil Y rises southeastward to an enigmatic crest on the 

northwest margin of a tidal creek. This crest may be a natural levee or it may mimic relief from underlying archaeological material. However, neither of these explanations 

accounts for the similar height of soil Y between coordinates 260 and 180 m, where nearby tidal creeks are lacking 

and cultural deposits are scarce (fig. 29).

The surficial horizon of soil Y ranges from peaty mud 

1-2 cm thick to dark mud as little as 0.5 cm thick, probably 

because of variable decomposition after burial. The soil is 

peatiest at low altitude, where the lowest part of the overlying mud locally contains Triglochin maritimum rhizomes. 

At high altitude the surficial horizon of the soil contains no 

peat (fig. 30A, B), the overlying mud lacks Triglochin maritimum rhizomes, and mud both above and below the surficial 

horizon of the soil is commonly firmer and more oxidized 

(redder) than at low altitude. These differences imply that 

where soil Y is high and faint, the weathering profile of the 

modern marsh has extended through the surficial horizon of 

the underlying buried soil and has contributed to the decomposition of any peat or overlying rhizomes that may have 

been present soon after burial.

GROWTH-POSITION FOSSILS OF VASCULAR PLANTS

Growth-position fossils show that soil Y formed at 

higher altitude than did mud that overlies the soil. Whereas 

the soil contains sparse stumps of shrubs (perhaps western 

crabapple; fig.30#), the overlying mud locally contains 

growth-position rhizomes of Triglochin maritimum.

Vascular-plant fossils in growth position are absent 

within, and sparse or absent immediately above, the other 

buried soils. Mud above soil S contains rhizomes of Triglochin martima at several surveyed points, as does mud 

above soil U at one such point. However, we did not find 

remains of plants that had lived on either of these soils, nor 

did we find any vascular-plant fossils within or immediately 

above faint soil W. Like the widespread lack of preserved 

peat and vascular-plant fossils in overlying soil Y, and also 

in soil U at the Oyster locality (p. 45), this paucity or 

absence of vascular-plant fossils probably reflects decomposition after burial, not initial absence.

CULTURAL DEPOSITS

The Sewer locality served as a prehistoric fishing camp 

(Smithsonian site number 45-PC-103). Cultural deposits 

underlie soil Y, and stubs of probably contemporaneous fish 

weirs remain visible on an adjacent tidal flat.

The cultural deposits are 5-20 cm below the top of soil 

Y, from which they are separated by mud (figs. 29, 30A, B). 

The cultural deposits, in bodies as much as 18 m long and 

0.4 m thick, consist of black carbonaceous mud and sand. 

Within this mud and sand are charcoal, cracked rock, and 

lenses of reddish-brown fire ash containing fish vertebrae. 

In places the carbonaceous deposits continue downward, 

into underlying mud, as filled holes about 5 cm in diameter 

(figs. 29, 30A). The holes were probably left by stakes: we 

found laminae bent downward beside one of the holes, and 

we found the charred, nearly vertical lower tip of a stake 

about 3 cm in diameter near a probable fire pit (fig. 29, horizontal coordinates 298.5 and 295.5, respectively).
70 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

2

re

uinjep |eoo| aAoqe sja;aiu u;'

A fish weir was built or repaired at the Sewer locality 

between A.D. 1400 and 1650 (p. 54). The dated weir and a 

second, similar feature are located on a tidal flat 10-25 m 

southwest of (riverward from) the steep riverbank outcrop 

and 25-100 m southeast of (upriver from) the cultural 

deposits (fig. 30C, £>). Their remains comprise hundreds of 

vertical sticks arrayed in V-shaped lines 5-15 m long. The 

sticks retain bark and sound wood below the soupy modern 

mud that veneers the tidal flat, and they protrude as rotten, 

worm-bored stubs as much as 15 cm above the flat.

FOSSIL DIATOMS

Diatom assemblages across the top of soil Y, sampled 

at horizontal coordinate 299 m (fig. 29), show that the soil 

formed at higher altitude than did the overlying mud. The 

faint surficial horizon of soil Y contains diatom fragments 

and abundant pollen grains suggestive of conditions near or 

above the level of highest tides. By contrast, mud above the 

soil contains many tidal-flat diatoms (for example, Rhaphoneis psammicola, Synedra fasciculata, Thalassionema nitzs-
15

Top of soil—Shown by solid line where continuity 

observed; shown only by top of shading where continuity inferred 

Surveyed point 

Bold soil Faint soil

Letter name of buried soil 

Top of Pleistocene deposits—Marked by hard

angular aggregates of greenish mud 

Live herbaceous plants of modern marsh 

Fossil spruce root—Numerals denote diameter, in 

centimeters

o

A

D

2820 ± 50

t1 -0.40, 25 July 1994 

- Fossil rhizomes of Triglochin maritimum Surveyed position of highest rhizomes Surveyed position of lowest rhizomes Fossil rhizomes probably of Carex lyngbyei Radiocarbon age—In 14C years before A.D. 1950 (table 7) Low tide—Height as predicted in tide tables; unit is

meters relative to mean lower low water 

Boring—Made with gouge corer 2 cm in diameter

Figure 27. Continued.

tSJ

S

^^s,S

'

a. 3t ft) 3 § i c« i -r 3-8

5s

intzschia amphio

1

P3id Cosmione5" 

S

uticoides.

oo1

(XI0)3*

(XI rr 5

brackish so

B* 5'

i

y

oc*5' & 

00

Cn

S

rirella, Dimeregr

& 3.^^' o

Sn> oc^

B

g. D. 

^

O 22. t^N*«

1s;

o S'i/j

/"N 

?

i

ta

tIT

§

honeis psam

3S'

o S"

t) ^

fS'

S5"

oMI

&

posit andthetop

S,(XI O_

5 1 

&3i'rS:d bytidal-fl

P^*

^ 

9

3

3 

§r*

^ ft)

f

D.

1

(XI 3'3 

o.CT 0)

?83

5- 

n>

o£I^^

•<1 §i/j

1 

S

Ssr&«S'8i/j cr. 

oo>-h

«<depositiona

^^

1.i

Diatom assem

a; ET •85 centimeter:

w

o*&

S1 5

ol-tj

i/j O_ ^^

0

the northwest.

^

iglochin maritimi

K 

33-

izomesn mi

o. 

&

1

§fi/j O.

^^J^

O3

3-

ioides, Nitzschia

Oa <*>' §

2), consisten

i-f 

|.&

5- 

n>

>T3

3i/j

1Oft)O>-*>

n mCX3o TI03 SO

CX3 OP CX3
72 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Figure 28. Jensen locality, Willapa River (fig. 27). A, Overview of outcrop along dredge cut (fig.7). View to southeast; horizontal 

coordinate 40 m is near right edge of area shown. The most extensive, resistant layer visible in the outcrop is soil S. B, Bold soils S and Y 

and intervening faint soils U and W near horizontal coordinate 32 m. Shovel handle is 0.5 m long.

AIRPORT LOCALITY

Two buried soils are widely exposed at the Airport 

locality and a third, intervening soil is exposed in the southeastern part of this locality (fig. 31). The uppermost soil,

equivalent to soil Y at the other localities, is commonly 

overlain by mud that contains Triglochin maritimum rhizomes. Such rhizomes also overlie the lowest soil at the 

edge of a filled channel. This lowest soil, equivalent to soil 

U, is typically faint but locally retains a peaty surficial hori-
DISTINCTION AMONG WAYS OF PRODUCING THE BURIED SOILS 73

zon and rhizomes of the high-marsh plant Juncus balticus. 

The faintness and stratigraphic position of the intervening 

soil suggest correlation with soil W.

Triglochin maritimum rhizomes above soil U at the 

Airport locality gave a radiocarbon age of 1,247±22 14C yr 

B.P. (table 7; fig. 31). The age resembles that obtained from 

stratigraphically similar rhizomes at the Oyster locality, 

1,300±25 14C yr B.P. (fig. 18, age A).

DISTINCTION BETWEEN TECTONIC

SUBSIDENCE DURING EARTHQUAKES

AND OTHER WAYS OF PRODUCING THE

BURIED SOILS

Buried soils at estuaries can record tectonic subsidence 

that accompanies earthquakes (fig. 2A). Near Anchorage, 

Alaska, subsidence during the great 1964 Alaska earthquake 

lowered marshes and forests to the level of tidal mudflats. 

As a result of this subsidence, peaty soils of the marshes and 

forests became covered by 1-2 m of tidal-flat mud by the 

early 1970's (Ovenshine and others, 1976).

Similar buried soils might also be produced, however, 

by other means (Nelson and others, 1996b; Peterson and 

Darienzo, 1996). These include cut and fill by migrating 

tidal streams, deposition by storms and floods, a fluctuating 

rise in sea level combined with variable sedimentation rates, 

breaching of bay-mouth barriers, settlement from earthquake-induced compaction, and rapid aseismic folding.

We found no evidence that alternatives to coseismic 

tectonic subsidence better explain any of the buried soils 

that we surveyed. However, criteria for identifying such 

subsidence can be applied more securely to some buried 

soils than to others. We ruled out cut and fill for most buried 

soils in most of the surveyed outcrops. In many cases we 

also ruled out deposition by storms or floods; in some cases 

we ruled out rise in tide level from sea-level rise and barrier 

breaching; and in some cases we can also show that subsidence had a tectonic, probably coseismic component. Finally, 

we used similarities in stratigraphic sequence and radiocarbon age to extrapolate from these various cases to all soils 

correlated among the surveyed outcrops.

CUT AND FILL BY TIDAL STREAMS

Most of the buried soils have less relief and more lateral continuity than is likely for a mud-over-peat contact 

from fluvial cutting and filling. Relief on the buried soils 

resembles the typically low relief on modern tidal marshes. 

This low relief contrasts with the unconformities observed 

for modern and filled channels (figs. 13,18, 19A-C, 29, 31). 

In addition, the preservation of herbaceous stems and leaves 

rooted in some of the soils—particularly in soils S and Y at 

the Oyster locality—shows that the tops of those soils 

underwent little or no erosion before burial. The sole candidate for cut and fill is faint soil W at the Jensen locality, and

this only because the soil is so poorly preserved that its 

relief and continuity are unclear.

Lateral continuity has also been checked through drilling of boreholes away from modern tidal streams at several 

localities in southern coastal Washington. Most such work 

has been done to check for tsunami deposits on soil Y along 

the Niawiakum River—about 50 borings in reconnaissance 

(Atwater, 1987) and about 200 additional borings made by 

M.A. Reinhart and several coworkers (compiled by Atwater, 

1996, his fig. 22C). Borings have also demonstrated continuity of buried soils beneath marshes along the Johns River, 

at Grays Harbor (fig. 5) (M.A. Reinhart, written commun., 

1993; Shennan and others, 1996).

DEPOSITION BY STORMS OR FLOODS

Fossils show that storms or floods alone cannot 

account for the burial of many of the soils. Burial of a soil 

by storm or flood deposits builds land higher. By contrast, 

fossils in mud above many of the buried soils record conditions of lower altitude than do fossils within the soils (fig. 

32). Such a contrast is shown by vascular-plant fossils in 

growth position associated with soil J at the Redtail locality; 

soils S, U, and Y at the Oyster locality; soils H and Y at the 

Jensen locality; soil Y at the Sewer locality; and soil U at 

the Airport locality. Contrasting altitudes are also implied 

by diatom assemblages associated with soils J, L, N, S, U, 

and Y at the Redtail locality; by diatom assemblages associated with soils S, U, and Y at the Oyster locality; and, less 

dramatically, by diatom assemblages associated with soil W 

at the Oyster locality (p. 52). Many additional soils are associated with half the vascular-plant evidence for change in 

altitude: either they contain growth-position fossils of highmarsh or upland plants, or they are overlain by mud having 

fossils of low-marsh plants.

If the soils correlate as inferred in figure 32, the storm 

and flood hypotheses can be ruled out for every buried soil 

shown in figures 13 through 31. On the basis of preserved 

vascular-plant fossils alone, correlation would rule out the 

storm and flood hypotheses for soils H, J, S, U, and Y. 

Storm and flood can be further ruled out for these soils, and 

for soils N and W as well, by correlation with soils associated with contrasting diatom assemblages at the Redtail 

locality (soils J, L, N, S, U, and Y; fig. 15) and the Oyster 

locality (soils S, U, W, and Y; fig. 20). Similarly, the storm 

and flood hypotheses are precluded by changes in fossil diatom and pollen assemblages that Shennan and others (1996) 

found associated with soils at the Johns River (fig. 5) that 

probably correlate with soils L, N, S, U, and Y at Willapa 

Bay (fig. 32).

FLUCTUATIONS IN SEA LEVEL

Fluctuations in sea level have been implicated in the 

interbedding of lowland peat and tidal-flat mud in tectonically stable areas. Late Holocene peat layers intercalated
Area shown in figure 30X\

Filled stake hole

Stratigraphy not shown below cultural deposits 

No vertical exaggeration

Laminae bent downward 

beside filled stake hole

neters above lo

x

. 8

•- IU

_c

ra 

X

ri

8 '

t1 x__

t2x_ 

t3^~

15 /.

4

y • S^"^— '-W-AWMW!

^ Y • •»? S*'-p-*"ts*'55W> ¥ v^rf-rt

••..:----w. 

•-• •••-•• xj.,

MWsSwas^S^

Soils Y and S

302

^Y **<^ B55HS55! T WWUWilt5lH.iii!>T.^«*KvK-:v.TM £'•'-' "''"'

*!B!'"-" 90 ± 60 (root) 

\ .._.- ^g

.. Poorly preserved ^^**

~ % 1990 ±70 (rhizomes) 1

^•W:.^»:*x^^

traced for another 1 65 m

300 298 296 

Area of archaeological detail shown above Area shown 

-^'^'••"'•T *•*""'..... r»-, ^1' V.... y -/-^ 

_ ^ 1 1

JHJIhl ! - - •-'•' ""' -"""Y-—— ---n

TM 8^ ! I, ^S8£* * 

I

' I' 1 .... v • ..,,,.,,,.,.,,,,,,,,,,,,,. ^ ...,,,,,,,,,.

\ ;;j 1:1 u? \;;;;r;::::::::::;::::::::;;;;: '////,'/,^^^//. '//,', ',

tT' 7-1 0 + 70 (stake)'i V :::;::::::::::;::;;;:;:::;:::::;::;::::: fTrr?-; very 

1 ''''•'* X * . ^ /^' '//////S,,,,,S/t',ff,/fff , f f , S 'f< S ' ' ' ' S IS S S / ?S, 

-In : - -- ---'", ^i-- - ;;:;::;::::::;; ;;;;; vi;;;;:;:::;;;;M-

pf : : : •/./> - - ; - l|s ^ ^V ; ; ;;;;;:;;;;;;;;; '////,y±///////////////

[; ; ;;;;;; ;;;F' :-^S^-:-:::S«K.- « - ; :: :::xm;,:-i^----c/-"^</----;.:;^:: ::

..:L................................................................_^^ ---

Outcrop below this line covered by modern mud and (or) cut by bank-parallel fractures; . . 

x denotes surveyed point Vertical range of we

on tidal flat; dot denote

Vertical exaggeration x10 surveyed poin

————— i ————— i ————— . ————— i ————— i ——— .. j ————— i ——— ... — i —— ._......! , ——— i — . ,.._ . i — i ——— ... ....i..., —

360 340 320

t1 +0.46, 16 July 1994

12 +0.21, 15 July 1994

t3 +0.06, 27 July 1994

t4 -0.70, 22 July 1994

15 -0.18, 16 July 1994

300 280 260

Distance, in meters northwest of sewer line

240 220 

Figure 29. Stratigraphy at Sewer locality, Willapa River.
DISTINCTION AMONG WAYS OF PRODUCING THE BURIED SOILS 75

on 4« 3 o

i!"!

•a &

"S-a

e a

13 T3 C tt)

§ -*

o

13 

.1

1^§ a &ail

13 =» o ^

13-g|-

o

*N rt

s« 

5

o

V}

ON

Q ^

J«t3 « ^

« "^

O O

o »-i

I" Cd s'-»-; a S'^^

?^ o5 ^V is ^

I-^

C 1/1 -

^^^t

c ^

^ 8 i

o &, WD

C p At

* ^ e

^C BB a A I _. '-35 J3^ 

'S 8 Z£

cte

c/] O <D

S II

•~ o *& •—'

o G

p<g ^ s £2bO D

IIi~a

S i2 -S S

fe (L>

3

I

v

13 

O

1

^O

CM 

O 

V

Is2 |

1 :| "€ i

with tidal-flat deposits are widespread in northwestern 

Europe (Jelgersma, 1961; Streif, 1972; Devoy, 1979) and 

the eastern United States (Johnson, 1925, his fig. 259B; van 

de Plassche, 1991;Varekamp and others, 1992; Fletcher and 

others, 1993a, b). As at Willapa Bay, peat layers locally 

form soils of submerged forests (Reid, 1913, p. 6-8). Also 

as at Willapa Bay, peat layers can be correlated regionally 

by stratigraphic sequence (Jelgersma and others, 1979, p. 

131-134) and radiocarbon age (Geyh, 1980; Shennan and 

others, 1983). Sea-level fluctuations are commonly invoked 

to explain the alternation between peat and mud, as illustrated by Meyerson (1972), Devoy (1979, p. 388), Behre 

and others (1979, p. 102-103), Shennan (1986), van de 

Plassche (1991), Varekamp and others (1992), and Fletcher 

and others (1993a, b).

None of the buried soils in our surveyed outcrops, 

however, suggest fluctuations in sea level. Such an origin 

for the soils tends to be ruled out by abrupt changes in herbaceous and microscopic fossils, by the occurrence of tsunamis at the onset of burial, and by tidal-marsh deposits that 

record gradual submergence elsewhere in the region.

•<J 'S

2

for stratigrap point urveyed 

C/l

Top of soil

cultural deposit of Top 

i «•

of cultural deposit Bottom 

E

fC

O

O

VH

^

S

soilold ofShown libpy — 

H

ao

£

c? ••-i

WD

*0

observed; shown only bytop

continuity inferred 

Bold soilFaint soi

$.

|

if

8

1•g

!>

1

• *^

of buriedsoil ettername fill so 'hannel Bounded by — 

J w

cc —

-

1T3

^

1T3

I«

5 6

unconformable edg observe! line whereunconformity infe

— <••**

X

AM 0

§1S i

E*

If

ive ofherbaceous plants mo

J (J

V

> C

a s

^ g o .5

3|

: 1

ossil rhizomes ofTriglochin

H fc

)

1/1 3 S

S c

S o 

1H •P -9H"5

rj^ -A— >

Surveyed position of highe; Surveyed position of lowes

0 0

^

Large of rhizomes mass 

', w 

s. >.

li *>

(U

.s

G

O

U

ON

1

ABRUPT CHANGES IN HERBACEOUS FOSSILS

Only extremely rapid rise in sea level is likely to have 

allowed preservation of the herbaceous fossils that are 

rooted in soils S, U, and Y at the Oyster locality (fig. 18), in 

probable correlatives of soils L and S at the Johns River, or 

in soil Y at the Copalis River (Atwater, 1992, p. 1908- 

1909). These fossils, consisting of stems and leaves rooted 

in the buried soil and entombed in overlying tidal-flat mud, 

show that tidal submergence and consequent burial were 

completed before the stems and leaves had time to decompose. From casual inspection we estimate that such decomposition takes place within a few years on modern high 

marsh at Willapa Bay. The fossils thereby imply submergence too fast for most sea-level rise in the past few thousand years (Atwater and Yamaguchi, 1991; Jacoby and 

others, 1995).

There remains a possibility that rapid but nontectonic 

rise in sea level would allow preservation of herbaceous 

stems and leaves that had been above ground and in growth 

position as the sea began to rise. Such fossils might be 

present in Delaware, where "stems and roots" and "rhizomes and stalks" are preserved at a mud-over-peat contact 

in Holocene tidal-marsh deposits (Fletcher and others, 

1993a, p. 123; Fletcher and others, 1993b, p. 185). Fletcher 

and his coworkers ascribed the contact to a rapid, nontectonic rise in sea level. They did not, however, show that the 

fossils had been subaerial when the inferred rise began.

ABRUPT CHANGES IN MICROSCOPIC FOSSILS

Sea-level rise also appears inconsistent with abrupt 

changes in microfossil assemblages at the contact between 

buried soils and overlying mud. Such changes have been
76 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Figure 30. Sewer locality, Willapa River (fig. 29). A, B, Faint soil 

Y and associated cultural deposits. Top of soil Y marked by shovel 

blade at left in A; mud above soil contains laminae of silt that are 

particularly distinct in B. Modern tidal marsh at top. Darkest 

deposit in both photographs is carbonaceous mud and sandy mud 

(c) containing charcoal, cracked rock, and (in A) a reddish-brown

lens (rb) probably made largely of ash from wood fires. These 

cultural deposits form a bed 25 cm thick in A, a lens as much as 15 

cm thick below the shovel in B, and the fill of stake holes (sh) in A. 

Roots of a shrub, perhaps western crabapple, protrude from soil Y 

at left in B. Centers of photographs near horizontal coordinates 299 

m (A) and 228 m (B).
DISTINCTION AMONG WAYS OF PRODUCING THE BURIED SOILS 77

Figure 30. C, D, Weirs on tidal flat adjacent to outcrop. View in C is to the southeast, upriver; outcrop is vertical bank at left, weirs are 

lines of stakes at center and right. View in D is to the southeast along long arm of weir centered near horizontal coordinate 205 m.

documented not only for diatoms at the Redtail and Oyster 

localities (figs. 15, 20) but also for diatoms, pollen, and foramimfera at the Johns River (Shennan and others, 1996). 

Shennan and his coworkers used samples collected at 1-cm 

intervals in the uppermost 5-15 cm of each of six buried 

soils and in the lowest 5 cm of mud immediately above each 

of these soils.

The abrupt changes in microfossil assemblages cannot 

have resulted from gradual rise in sea level unless gradual 

transitions in assemblages have been removed by widespread erosion. We recognized no widespread unconformities at the surveyed localities. Moreover, erosion can be 

ruled out where formerly subaerial stems and leaves of herbaceous plants are preserved in growth position on the soils.

Such fossils are associated with diatom evidence for sudden 

submergence of soils S, U, and Y at the Oyster locality (fig. 

20) and of soil L at the Johns River (Atwater, 1992, his table 

1, locality 14, depth 3.0 m; Shennan and others, 1996, their 

horizon V).

TSUNAMIS

At least three of the buried soils are associated with 

evidence that a tsunami coincided with the event that caused 

the soil to undergo lasting submergence and consequent 

burial. Such coincidence would be expected of coseismic 

subsidence because tsunamis typically result from sudden
-J

00

Northwest Southeast ft

I

-§ 10

s

_o 

to g.a(0

43

.£ 8- 

.raX

t1 -RFine sand, Sandy dredge spoil /' probably dredge spoil \>c\:. y Y v inset against bank ...o^-A "' " '•'"'* •MWB89 — — - ••••^••••••••••r / \ N//' -•••••wwflBS^'S!?'"'^''*-* ""' •^•^'••jiiSWs**^,^^ MI IT mil / \ 1 1 f 1 1 mm m | ---------- I /X\ 1247 + 22 -,;, L' « ii - " '- V^ l.^^^^^ •-"•"•-"'"" •-.....,.•.•••'••••••••- U .......,..,,,,r^..TM....v_^ ^ ^^U-**ww^^'"'"C' \Peatymud Peaty mud / \: '///.'/,',' .. . .................. . ........ •• ........ .. ....... .. ...... withrhizomes ••• with rhizomes v ;;:::::; Generalized lower limit of outcrop of J|/nc|/s 6a/&-ct/s Qf ^^ fia;ffc|/s Vertical exaggeration x10 nCD?3% 55m — j>HX~)

20 40 60 80 100 

Distance, in meters from local datum

120 140 160

EXPLANATION

!''•''! <*m

Top of soil—Shown by solid line where continuity 

observed; shown only by top of shading where continuity inferred

Surveyed point

Bold soil

Faint soilLetter name of buried soil 

Channel fill—Bounded by solid line where 

unconformable edge observed; bounded by dashed line where unconformity inferred

V

1 247 ± 22

t1 -0.52, 8 July 1 994 — Live herbaceous plants of modern marsh Fossil rhizomes of Triglochin maritimum Surveyed position Large mass of rhizomes Radiocarbon age—In I4C years before A.D. 1950 (table 7) Low tide—Height as predicted in tide tables; unit is 

meters relative to mean lower low water

>5

I

2!

Figure 31. Stratigraphy at Airport locality, Willapa River.
DISTINCTION AMONG WAYS OF PRODUCING THE BURIED SOILS 79

sea-floor displacement. By contrast, a tsunami need not 

coincide with an aseismic change in sea level.

A tsunami deposit can record coincidence with coseismic subsidence by resting on a suddenly subsided soil and 

by surrounding the remains of plants that had been living on 

the soil before it subsided (fig. 25). Such is the case for the 

sand sheet on soil Y along the Niawiakum River (p. 27). 

Moreover, this sand sheet probably grades upvalley into 

mud that contains diatoms probably transported by tsunami 

(p. 63). Similar mud implies that tsunamis coincided with 

the subsidence marked by soils N and S along the Niawiakum River (p. 29).

The lack of recognized tsunami deposits above other 

buried soils in the surveyed outcrops is open to many explanations other than aseismic subsidence of the soils. Tsunami 

deposition should depend on the size of the tsunami, which 

is likely to vary with the location of earthquake ruptures 

(Geist and Yoshioka, 1996) and perhaps also with the size 

and location of coseismic submarine landslides. Tsunami 

deposition along the Niawiakum and Willapa Rivers may 

also vary with the shape of channels connecting these 

streams to the Pacific Ocean. Moreover, both the Niawiakum River and the Willapa River may be too far inland to 

have received much sand from most tsunamis generated at 

the Cascadia subduction zone. They are much farther from 

the coast than are sites having multiple sandy tsunami 

deposits: the Copalis River, where a probable correlative of 

soil S is widely overlain by sand (fig. 32; Atwater, 1992); 

several estuaries in northern Oregon where sand is common 

on such possible correlatives of soils S and U (Darienzo and 

others, 1994; Darienzo and Peterson, 1995); and a coastal 

lake near Cape Blanco, Oregon, that has been invaded by 

tsunami as many as 14 times in the past 7,500 years (Nelson 

and others, 1996c).

GRADUAL SUBMERGENCE ELSEWHERE IN THE REGION

If buried soils at Willapa Bay are due to nontectonic 

fluctuations of sea level in the past 3,500 years, the fluctuations did not extend into areas of Washington and Oregon 

that contain thick deposits of tidal-marsh peat from the past 

several thousand years. Such areas include a site at northern 

Puget Sound (Eronen and others, 1987) and sites along the 

south-central coast of Oregon (Nelson, 1992a; Nelson and 

others, 1996b).

The peat in these areas, which formed during the past 

several thousand years, probably resembles classic evidence 

for gradual submergence in Massachusetts. There, tidalmarsh peat several meters thick is pervaded by growth-position roots and rhizomes of plants that thrive only near hightide levels; submergence occurred gradually enough for 

marshes to build upward apace with the sea (Mudge, 1862; 

Redfield, 1972, p. 210).

While gradual submergence at Puget Sound and southern Oregon was allowing tidal marshes to persist, jerky submergence in southern coastal Washington abruptly changed

tidal marshes into tidal flats. The coexistence of these two 

styles of submergence can be explained most simply by 

gradual submergence throughout the region and by coseismic subsidence of more limited extent (Atwater, 1987).

BREACHING OF BAY-MOUTH BARRIERS

If Willapa Bay had a barrier at its mouth that limited 

the tidal range behind it, the breaching of such a barrier 

might cause high-tide levels to rise, thereby producing buried soils similar to those produced by subsidence during 

earthquakes. Such tide-level rise "has, for a long time, been 

the mechanism used to explain the intercalated peats and 

clays" of northwestern Europe (Shennan, 1986, p. 121), as 

illustrated by Jelgersma and others (1979, p. 139). It has 

also been invoked for some of the interbedding of peat and 

mud at Coos Bay, Oregon (Nelson and others, 1996a, p. 

151-152). But neither historical records nor fossils imply 

that barrier breaching contributed to the episodic late 

Holocene submergence and burial of wetland soils in southern coastal Washington.

Historical records provide no basis for inferring past 

blockage of the largest estuaries in southern coastal Washington. Although associated with treacherous offshore bars, 

Willapa Bay, Grays Harbor, and the Columbia River have 

stayed fully open to the sea since Euro-Americans first 

entered them about 200 years ago. A likely explanation is 

that the estuaries discharge too much freshwater to the sea, 

and tidally exchange too much water with it, to have been 

blocked by barrier sand. The tidal prisms are huge—¥2 km3 

for Willapa Bay and Grays Harbor, 1 km3 for the Columbia 

River (Johnson, 1973)—and so is the freshwater discharge 

of the Columbia River.

Fossils preclude bay-mouth blockage if such blockage 

would have turned Willapa Bay into a freshwater lake in the 

years just before buried soils were submerged and buried. 

Instead of freshwater plants like cattails and willows, soils S 

and Y at the Oyster locality supported the brackish-water 

tidal assemblage of Deschampsia caespitosa, Potentilla 

pacifica, and Juncus balticus. Moreover, soil U at the Oyster 

locality was largely covered by the halophyte Salicornia virginica. Inflow of seawater is further implied by diatom 

assemblages in the surficial horizon of soils S, U, W, and Y 

at the Oyster locality; in the surficial horizon of soils J, L, 

and U at the Redtail locality; and just beneath the surficial 

horizon of soil N at the Redtail locality. None of the diatom 

assemblages studied at either of these localities are consistent with prevalence of standing freshwater.

Fossils similarly imply seawater inflow to the Copalis 

River and Grays Harbor just prior to the burial of one or 

more soils at these estuaries. Deschampsia caespitosa, 

Potentilla pacifica, and Juncus balticus were living along 

the Copalis River when a probable correlative of soil Y was 

suddenly submerged (Atwater, 1992, p. 1908-1909). These 

species also lived on probable correlatives of soils L and Y 

at Grays Harbor, along the Johns River (Atwater, 1992,
80 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

localities 13 and 14 of his table 1 and his figs. 6, 7). Moreover, the mouth of the Johns River was also bordered by a 

Triglochin maritimum low marsh at the time of the submergence marked by soil Y (Atwater, 1992, locality 12 in 

his table 1 and his figs. 6, 7). Annual freshwater inflow to 

Grays Harbor averages about 10 km3 (Peterson and others, 

1984, p. 88), 10 times the high-tide volume of Grays Harbor 

(Barrick, 1975, p. A-6). If a barrier greatly limited seawater inflow to Grays Harbor before sudden submergence 

of soil Y, salinity along the Johns River should have been 

low enough to favor freshwater plants over Triglochin 

maritimum.

SETTLEMENT FROM COMPACTION DURING 

SHAKING

Seismic shaking can produce localized submergence 

through compaction and consequent settlement of unconsolidated deposits. Such settlement from the 1964 Alaska 

earthquake widely affected estuarine and alluvial deposits 

(Plafker and Kachadoorian, 1966, p. D13-D18; Plafker, 

1969, his plate 4B and fig. 27) and locally doubled the total 

subsidence near Portage (McCulloch and Bonilla, 1970, p. 

D81). Settlement may also have contributed to coastal submergence in Chile during the 1960 Chile earthquake 

(Plafker and Savage, 1970, p. 1006, their table 1, and their 

fig. 5A).

Although settlement may have augmented tectonic 

subsidence in southern coastal Washington, it cannot 

account for all the evidence for subsidence. If most or all the 

subsidence were due to settlement, buried soils would 

merge where they are underlain, at shallow depth, by wellconsolidated Pleistocene deposits. Such is not the case, 

however, at the Jensen locality, where successive, distinct 

soils have a shallow, subhorizontal foundation of stiff Pleistocene mud (fig. 27). Moreover, buried soils remain distinct 

where they lap onto uplands made of well-consolidated 

Pleistocene deposits, as shown along the Niawiakum and 

Copalis Rivers (Atwater, 1992, p. 1909). Such onlap is conspicuous at the Heron locality (fig. 26; p. 63), where soil Y, 

covered by tidal deposits, ascends a valley wall composed of 

stiff Pleistocene mud. This evidence shows that the upland, 

as well as soil Y, subsided into the intertidal zone, consistent 

with tectonic subsidence but not with subsidence limited to 

poorly consolidated valley fill.

TECTONIC SUBSIDENCE NOT ATTENDED BY 

SEISMIC SHAKING

Two lines of evidence suggest that none of the buried 

soils at northeastern Willapa Bay are likely to record rapid 

tectonic subsidence that was not attended by seismic shaking. First, as discussed on pages 100 and 102, subsidence 

marked by soils J and younger may correlate with turbidity 

currents that probably resulted from shaking about 50 km 

west of Willapa Bay. Second, as discussed here, subsidence 

marked by buried soils Y and S probably correlates with liquefaction a few tens of kilometers to the south. Liquefaction 

has also been inferred to correlate with the subsidence 

marked by soil W, but shaking was not necessarily involved 

in this case.

A buried soil probably correlative with soil Y is widely 

associated with liquefaction features along the lower 

Columbia River about 30-45 km east of the Pacific coast. 

The features, discovered by Obermeier (1995), are further 

documented in observations compiled by Atwater (1994). 

Although most of the liquefaction features are dikes that do 

not extend to the top of the soil, some of the intrusions 

appear continuous with sand lenses that rest directly on the 

soil. The stratigraphic position of the sand lenses, which 

resembles that of the tsunami deposits at the Oyster and 

Redtail localities, implies that the sand was vented during or 

soon after the land subsided (fig.2C).

A buried soil probably correlative with soil S is associated with several gravelly dikes along the Naselle River of 

southern Willapa Bay. These dikes, discovered in 1995 by 

Alan Mortimer, Judith Boughner, and Brian Atwater, are a 

few tens of meters downstream from locality 20 of Atwater 

(1992), which is the outcrop described by Ota and Umitsu 

(1995). At least one of the dikes is low in a mud-filled crack 

that extends upward to, but not above, a buried tidal flat 

continuous with the probable correlative of soil S. Mudfilled cracks continuous with dikes below are among the 

geologic features near Anchorage, Alaska, that record liquefaction from the 1964 Alaska earthquake (Walsh and others, 

1995, their figs. 15,17, 21, 24, 33, 34, and 55). We infer that 

the similar crack at the Naselle River records lateral spreading that coincided with the sudden subsidence marked by 

soil S.

Enigmatic sand bodies mistakenly ascribed to liquefaction are present along the Copalis River, where they formed 

between 900 and 1,300 years ago (Atwater, 1992). Although 

probably caused by an earthquake and perhaps correlative 

with submergence marked elsewhere by soil W and its correlatives, these sand bodies do not appear to record seismic 

shaking (p. 83).

DISTINCTION BETWEEN PLATEBOUNDARY AND UPPER-PLATE 

SOURCES FOR THE EARTHQUAKES

Geophysics provides starting points for implicating 

plate-boundary earthquakes in coseismic subsidence along 

the Pacific coast at the Cascadia subduction zone:

1. Plate-boundary earthquakes could have caused widespread, sudden subsidence by elastically thinning the 

North America plate. Possible analogs include the sudden subsidence of coastal areas hundreds of kilometers 

long during great historical earthquakes at subduction 

zones in Alaska, Chile, and Japan (Heaton and Hartzell, 

1986).

2. The Cascadia plate boundary resembles, in age of subducting lithosphere and rate of convergence, some plate
DISTINCTION BETWEEN PLATE-BOUNDARY AND UPPER-PLATE SOURCES FOR THE EARTHQUAKES 81

boundaries elsewhere on which great earthquakes have 

occurred historically (Heaton and Hartzell, 1987; Rogers, 1988).

3. A large fraction of the shallow slip between the Juan de 

Fuca and North America plates should occur during 

earthquakes, according to a recent hypothesis about 

forces from plate motions (Scholz and Campos, 1995, p. 

22112).

4. The Cascadia subduction zone is likely to be accumulating energy for release in future plate-boundary earthquakes, according to interpretation of geodetic surveys 

and heat-flow models (Savage and others, 1991; Hyndman and Wang, 1993, 1995; Dragert and others, 1994; 

Mitchell and others, 1994; Dragert and Hyndman, 1995). 

The alternative of upper-plate earthquakes arises where 

sudden coastal subsidence may have been limited to areas 

along faults or folds in the North America plate. Such localized subsidence has been documented for a syncline in 

northern California that trends approximately parallel to 

nearby thrust faults having Holocene slip (Clarke and 

Carver, 1992). Farther north, subsidence during the past few 

thousand years may have been localized along coastal synclines in south-central Oregon (Mclnelly and Kelsey, 1990; 

Nelson, 1992a; Nelson and Personius, 1996). In addition, 

upper-plate faults off the Pacific coast of Oregon and Washington have slipped in the past 10,000 years, and some of 

the estuaries having evidence for sudden subsidence have 

been correlated with such youthful structures mapped offshore (Goldfinger and others, 1992a, b; McNeill and others, 

1994; McCaffrey and Goldfinger, 1995). The subduction 

zone along the North Island, New Zealand, may contain 

analogs for land-level change during upper-plate earthquakes at the Cascadia subduction zone. Some of the prehistoric, Holocene marine terraces on the North Island 

represent coseismic uplift that was not necessarily accompanied by seismic slip on the subduction boundary between 

the Pacific and Australian plates (Berryman and others, 

1989;Berryman, 1993).

Coastal geology provides at least two criteria for distinguishing between plate-boundary earthquakes and upperplate earthquakes as causes of coseismic subsidence at the 

Cascadia subduction zone. One of these is the extent of subsidence having the same approximate age. Unlike a single 

plate-boundary rupture at the Cascadia subduction zone, a 

series of earthquakes on faults in the North America plate 

could be distributed through enough time for radiocarbon 

detection of differences in dates of subsidence. Such differences have not been detected for the best dated and most 

recent of the prehistoric sudden subsidence (Nelson and 

others, 1995). Instead, high-precision radiocarbon dating 

shows that the most recent sudden subsidence at Willapa 

and Humboldt Bays, 625 km apart along the subduction 

zone, occurred during the same few decades in the early 

1700's (fig. 33). Additional radiocarbon dating of lower resolution, employing subsidence-killed herbaceous plants, 

shows that the most recent coseismic subsidence postdates 

A.D. 1660 at the six estuaries where such plants have been

dated: the Copalis River and Willapa Bay, Washington; and 

Netarts Bay and the Nehalem, Salmon, and Coquille Rivers, 

Oregon (localities shown by herb symbols in fig. 1).

The sense of coseismic land-level change on anticlines 

provides a second criterion for distinguishing between 

plate-boundary earthquakes and upper-plate earthquakes. 

This criterion is based on two assumptions:

1. An anticline can subside during a plate-boundary earthquake at a subduction zone by participating passively in 

the coseismic thinning of the upper plate. In a well-studied example, a magnitude-9.2 earthquake on the subduction zone in Alaska in 1964 was accompanied by 

subsidence throughout an Alaskan area 800 km long and 

more than 100 km wide (Plafker, 1969), apparently irrespective of many late Cenozoic folds and faults (Plafker 

and others, 1994).

2. An anticline is unlikely to subside during an upper-plate 

earthquake that causes localized subsidence along adjacent synclines of similar age and trend. Rather, the anticline may undergo uplift, as during the folding at Puget 

Sound during one or more earthquakes on faults in the 

North America plate about 1,000-1,100 years ago 

(Bucknam and others, 1992).

The Cascadia subduction zone contains at least three 

anticlines having buried soils indicative of coseismic subsidence. One of these, located near Cape Blanco, Oregon, 

deforms Pleistocene marine terraces that have been uplifted 

at net rates of nearly 1 m per 1,000 years, which is among 

the highest rates of net Quaternary uplift along the Pacific 

coast at the Cascadia subduction zone (Kelsey, 1990; Kelsey 

and others 1994, their fig. 4). Despite this history of late 

Quaternary uplift, the south flank of the anticline contains 

several successive buried soils suggestive of coseismic subsidence during the past 5,000 years (Kelsey and others, 

1993, 1996).

The Coquille River estuary, about 30 km north-northeast of Cape Blanco, shows evidence for coseismic subsidence that also may have occurred on a Quaternary anticline. 

The evidence for subsidence was described by Nelson 

(1992a, p. 298) as the best yet found in south-central Oregon. It consists of a buried soil that contains rooted stumps 

and herbs entombed in overlying mud. The subsidence 

recorded by the soil has been dated to sometime after A.D. 

1660 and probably resulted from the same earthquake, or 

swift series of earthquakes, as the subsidence marked by soil 

Y in southern coastal Washington (p. 81; Nelson and others, 

1995). The subsided site is located in line with the southern 

projection of the axis of an anticline that warps a marine terrace about 100,000 years old (Mclnelly and Kelsey, 1990, 

their fig. 5).

Buried soils also show that coseismic subsidence has 

occurred in the Holocene on the largest structural uplift in 

southern Washington—the South Bend antiform (fig. 5; p. 

8). Buried soils further imply that coseismic uplift has not 

occurred on this structure in the past 3,500 years or more. 

The soils are present not only in the Holocene deposits surveyed at the Jensen, Sewer, and Airport localities (figs. 27-
82 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

31) but also in estuarine deposits of Pleistocene age (p. 9; 

table 1). Structural setting has had little effect on the amount 

or timing of coseismic subsidence in southern coastal Washington, except perhaps about 1,000-1,100 years ago. These 

findings, summarized below, show that most or all coseismic subsidence in southern coastal Washington results from 

plate-boundary earthquakes, not from earthquakes on faults 

in the North America plate.

SENSE OF LAND-LEVEL CHANGE

The South Bend antiform is similar to other parts of 

southern coastal Washington in its evidence for coseismic 

subsidence. Buried soils are present in estuarine deposits of 

Pleistocene age (table 1) and Holocene age (fig. 34), both on 

and off the antiform. Where preserved, vascular-plant fossils in Holocene soils on the antiform show that lasting tidal 

submergence, not just storms or floods, caused burial of the 

soils (p. 73). The probable correlation between buried 

Holocene soils on and off the antiform (fig. 32) further 

implies that this submergence resulted from sudden tectonic 

subsidence, at least in the case of soils L, S, U, and Y; herbaceous fossils rooted in these soils off the antiform demonstrate sudden submergence followed by rapid burial (p. 75). 

Although we did not find such herbaceous fossils rooted in 

any buried soil on the South Bend antiform, this lack of 

entombed herbs is probably due to decomposition after 

burial (p. 29-30) rather than lack of coseismic subsidence 

on the antiform. For example, we found no entombed herbs 

rooted in any of the buried soils at the Redtail locality, nor 

did we find them rooted in soils J, N, or W at any other 

locality off the antiform.

We recognized no evidence for coseismic uplift at Willapa Bay, either on or off the South Bend antiform. As little 

as 0.5 m of uplift would suffice to change tidal marshes into 

upland forests containing not only Sitka spruce but also 

other trees typical of uplands beside Willapa Bay: red alder, 

big leaf maple, western redcedar, western hemlock. On the 

antiform, however, growth-position remains of trees of any 

species are sparse or absent in all buried soils at the Airport, 

Sewer, and Jensen localities (figs. 27, 29, 31). Most or all of 

the woody roots and stumps at the Jensen locality are of 

Sitka spruce (p. 67), the dominant tree of tidal wetlands in 

southern coastal Washington (p. 11). Roots and stumps at 

the Sewer locality belong to an unidentified shrub that may 

be western crabapple (p. 69), which is also a tidal-wetland 

species (p. 11). Off the antiform, soils S and Y were wooded 

by Sitka spruce beside uplands (Redtail and Heron localities; figs. 13, 25) and along the upper reaches of tidal 

streams. In addition, forests on these soils contained western redcedar among some of the Sitka spruce (at and near 

the Pool locality, for example; p. 63). But nearer the sea, 

even soils S and Y represent brackish-water tidal marshes, 

as at the Oyster locality (fig. 18).

Coseismic subsidence on the South Bend antiform 

might result from earthquakes in the North America plate if

a Quaternary syncline crosses the antiform near the Willapa 

River. The only reported candidate for such a structure is a 

shallow syncline several tens of kilometers wide that 

McNeill and others (1994) projected toward Willapa Bay 

from seismic-reflection profiles about 10 km off the Pacific 

coast (Goldfinger and others, 1997). If this syncline controls 

coseismic subsidence at Willapa Bay, the syncline should be 

bounded to the north and south by relatively uplifted areas 

that escaped coseismic subsidence. There is little room, 

however, to thread such areas among coseismically subsided 

sites in southern coastal Washington (fig. 34).

AMOUNT OF LAND-LEVEL CHANGE

The South Bend antiform probably resembles adjacent 

areas not only in subsiding during earthquakes but also in 

the amount of this subsidence. The amount is difficult to 

determine because the indicator fossils have large vertical 

ranges (fig. 8; Hemphill-Haley 1995a) and because local 

settlement can augment tectonic subsidence (p. 80). However, if the amount of subsidence depended on location with 

respect to the South Bend antiform, so would vascular-plant 

assemblages and the thickness of mud between soils. We 

found no such dependence on location with respect to the 

South Bend antiform.

VASCULAR-PLANT ASSEMBLAGES

Where vascular-plant fossils are preserved in growth 

position both within a buried soil and in mud above the soil, 

the fossils show that a high marsh, or a forest near extreme 

high water, became a tidal flat or a low marsh—both on and 

off the antiform. Examples include soil Y on the antiform at 

the Jensen, Sewer, and Airport localities, and off the antiform at the Oyster locality; soil U at the Airport and Oyster 

localities; and soil H at the Jensen locality.

Soil S at the Oyster locality may record greater subsidence than does soil S at other localities; several tens of centimeters of tidal-flat mud accumulated on this soil before 

Triglochin maritimum became established (fig. 20). However, this greater subsidence may be due to settlement from 

compaction of tens of meters of soft Holocene fill (p. 45).

THICKNESS OF MUD BETWEEN SOILS

The thickness of mud between buried soils at the surveyed localities varies mainly with the presence or absence 

of Pleistocene deposits at shallow depth beneath them. Mud 

between soils is thin where their Pleistocene foundation lies 

just a few meters beneath, both on the antiform (Jensen 

locality; fig. 27) and off the antiform (west end of Redtail 

locality, south end of Heron locality; figs. 13, 25). Conversely for sites probably underlain by many meters of soft 

Holocene mud, buried soils are more widely spaced, 

whether on the antiform at the Sewer and Airport localities 

(figs. 29, 31), or off the antiform at the Oyster locality (fig.
DISTINCTION BETWEEN PLATE-BOUNDARY AND UPPER-PLATE SOURCES FOR THE EARTHQUAKES 83

18) and away from uplands at the Redtail and Heron localities (figs. 13, 25).

The thickness of intertidal mud above a buried soil can 

serve as a rough indicator of the amount of coseismic subsidence because subsidence makes room for tides to deposit 

mud on the soil. The thickness of mud is unlikely to equal 

the amount of coseismic subsidence, however, if tidal inundation subsequently decreases during interseismic uplift or 

increases during interseismic subsidence (Darienzo and 

Peterson, 1990; Hemphill-Haley, 1995b, p. 377; Guilbault 

and others, 1995).

NUMERICAL ESTIMATES OF SUBSIDENCE

We do not compare structural setting with numerical 

estimates of subsidence because, at present, such estimates 

would be unreliable for the South Bend antiform. Estimates 

of coseismic subsidence at the Cascadia subduction zone 

are best made at sites studied carefully for microscopic fossils (Mathewes and Clague, 1994; Hemphill-Haley, 1995b; 

Guilbault and others, 1995; Nelson and others, 1996a, b; 

Shennan and others, 1996). Microfossils have not yet been 

studied for most soils on the South Bend antiform. However, they have been studied abundantly for soils off the 

antiform, along the Niawiakum River.

Subsidence of at least 1 m was previously inferred for 

soil Y at the Redtail and Oyster localities, mainly on the 

basis of diatom assemblages (Hemphill-Haley, 1995b). Here 

we extend this estimate to most soils at the Redtail and Oyster localities. The estimates are based on inferred differences in depositional environment between buried soils and 

the mud immediately above them (figs. 15, 20). We inferred 

the environmental differences from stratigraphy and from 

species of vascular plants and diatoms that are vertically 

zoned in modern tidal wetlands (figs. 8,11, 12).

Subsidence of at least 1 m is probably recorded by soils 

J, L, N, S, U, and Y at the Redtail locality because all these 

soils probably contain high marsh or upland assemblages of 

diatoms and because mud above each of them contains diatom assemblages suggestive of widely unvegetated mudflats 

(p. 29). The modern difference between these environments 

is rarely less than 0.5 m and commonly exceeds 1 m (fig. 8, 

parameter D). The evidence for at least 1 m of subsidence is 

strongest for soils S and Y, whose spruce roots corroborate 

diatom evidence for conditions transitional to upland (p. 

29).

The Oyster locality also has strong evidence that soils 

S and Y record at least 1 m of subsidence (p. 48). This evidence consists of high-marsh plants rooted in the soils, algal 

lamination and Triglochin maritimum rhizomes in mud 

above the soils, and diatom assemblages compatible with 

these features.

Only soil W has evidence against subsidence greater 

than 1 m. At the Oyster locality this soil represents a tidal 

marsh above the lower limit of Triglochin maritimum but 

probably below the transition from low marsh to high marsh

(p. 48). Moreover, after soil W subsided it did not remain a 

tidal flat for long, as judged from the thickness of mud dominated by tidal-flat diatoms (p. 52). In that case it may have 

subsided only through the vertical range of low marsh, or as 

little as about 0.5 m.

COEVALITY OF LAND-LEVEL CHANGE

Buried soils of the past 2,000 years on the South Bend 

antiform do not appear to differ—either in stratigraphic 

sequence or in radiocarbon age—from buried soils at nearby 

sites off the antiform (fig. 32). All these soils appear to be 

part of a regionally correlative sequence previously reported 

from many estuarine localities along the southern 100 km of 

the Washington coast (Atwater, 1992). In this sequence, two 

bold soils (herein, soils S and Y) bracket one or two faint 

soils (U and W). Soils S and Y commonly contain more fossil tree roots than do soils U and W, and only soil Y is commonly covered by a tsunami deposit at Willapa Bay and 

Grays Harbor. Ages determined by radiocarbon dating 

(chiefly for soils S and Y; Atwater, 1992) and by tree-ring 

pattern matching (soil Y only; Yamaguchi and others, 1997) 

are consistent with correlations inferred from these various 

stratigraphic clues. Additional radiocarbon dating of soils in 

the stratigraphic position of soils S, U, and Y is consistent 

with correlation between a typical southern-Washington 

sequence of buried soils at the Naselle River (fig. 5) and a 

sequence of buried soils at Netarts Bay, Oregon (fig. 1) 

(Nelson and others, 1996b, their fig. 6).

Buried soils 2,000-3,500 years old also show no major 

differences in stratigraphy or approximate age between the 

South Bend antiform and the handful of sites at which such 

soils have been dated in southern coastal Washington. The 

four lowest soils on the antiform at the Jensen locality probably correlate with soils H, J, L, and N off the antiform at 

the Redtail locality (p. 67-69). Soils low in the section at the 

Jensen locality probably also correlate with soils off the 

anticline at Willapa Bay (South Fork locality) and Grays 

Harbor (Johns River and Blue Slough localities) (fig. 32).

POSSIBLY UNUSUAL CHANGES IN LAND LEVEL 

1,000-1,100 YEARS AGO

The subsidence marked by soil W at Willapa Bay may 

have coincided, to the nearest century or less, with unusual 

faulting in the North America plate. Such faulting probably 

occurred some 50 km to the north, at the Copalis River. 

There, vented sand covers a soil, possibly correlative with 

soil W (fig. 32), that underwent little or no subsidence at the 

time of venting. This lack of subsidence implies localized 

deformation in the North America plate because the venting, which is not readily explained by shaking-induced liquefaction, probably resulted from faulting or folding of an 

aquifer beneath the Copalis River (Atwater, 1992, p. 1917, 

"Note added in proof). Such faulting or folding could have 

been part of widespread movement on structures high in the
84 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

SOUTH BEND 

•"" ANTIFORM ~"

West NORTHEASTERN WILLAPA BAY I East 

Niawiakum River, — Willapa River.—

Bay Oyster Red- Pool : Air- Sewer Jen-; South

Center tail ; port sen ; Fork

beach + + • < • • • <

GRAYS HARBOR COPALIS PUGET 

RIVER SOUND

Johns Elliott Blue 

- River- Slough Slough

West 

Point

...-.--. -.• . •..-.--:>-: »Vi«Sx!8i*SSiSSiSSBBS>SixKs^

^

.. Buried 

soil

I J

4000

4000 H
SIZES OF THE EARTHQUAKES 85

<— Figure 32. Inferred correlations among buried Holocene soils at estuaries in southern coastal Washington and at Puget Sound. 

Individual age ranges calculated from radiocarbon ages in Atwater and others (1991), Atwater (1992), Atwater and Moore (1992), and 

table 7 of this report. Vertically ruled bands denote likely age ranges explained in section "Number and Ages of Earthquakes" (p. 91). 

Age range for soil Y at Oyster locality corresponds to weighted mean age of 161 ±15 14C yr B.P.

d

D

•

m

m

EXPLANATION

Columnar sections

Section described in this report—Column 

generalized from outcrop drawing (figs. 13,18, 

24, 27,29, and 31)

Soil—Categorized by surficial horizon 

Bold—Peaty mud or peat 

Faint—Dark mud 

Deposits between successive soils 

Mud

Probable tsunami deposit on soil 

Vented sand on soil—Erupted from >35 m

below land surface (Atwater, 1992, p. 1917) 

Muddy peat—>0.3 m thick; records gradual 

submergence in the past 1000 years (Atwater and 

Moore, 1992)

Vascular-plant fossils in growth position 

Rhizomes in mud above soil—Triglochin

maritimum or, at South Fork, Carex lyngbyei 

Plant remains rooted in buried soil—Herbs,

shrubs, or trees

Microscopic fossils—Diatoms at Redtail and 

Oyster localities (figs. 15,16, and 20-23); 

diatoms and pollen at Johns River (Shennan and 

others, 1996)

Unconformity on modern beach 

Correlations—Dashed where drawn through 

section in which soil is not recognized; queried 

where speculative. Letter gives name for soils 

inferred to correlate with one another 

Soil typically bold 

Soil typically faint

Ages of inferred earthquakes

Thick line gives 95-percent confidence interval

calculated from error reported by laboratory; thin

line increases this error by 1.6 or 2.0 (see text

section "Radiocarbon Dating")

Closely limiting age—Obtained from earthquakekilled plants (soils S, U, W, Y) or forest litter 

from uppermost 0.5 cm of buried soil (soils J, 

N); ages for soils S and Y denote inferred time 

of plant death

Minimum age—Measured on rhizomes of 

Triglochin maritimum above soil

Maximum age—Measured on woody root, plant 

detritus, or archaeological charcoal in buried soil

Minimum or maximum age—Measured on peat 

in soil or on plant detritus in mud less than 20 

cm above soil

Likely age range for inferred earthquake or 

earthquake series

Figure 32. Continued.

i 

t

North America plate in western Washington 1,000-1,100 

years ago. At that time, movement on one or more faults in 

the North America plate farther inland, in the Puget Sound 

area, generated 5-7 m of coseismic uplift near Seattle and 

several meters of coseismic uplift in an area 30-50 km to 

the southwest (Bucknam and others, 1992).

Despite this evidence for upper-plate seismicity, soil W 

at Willapa Bay probably represents an earthquake on the 

plate boundary. Radiocarbon-dated soils show that coseismic subsidence occurred elsewhere along the Washington 

coast between 900 and 1,300 years ago (Atwater, 1992, 

his figs. 6, 7)—at Neah Bay (location shown in fig. 1), at 

Grays Harbor along Elliott Slough, and at southern Willapa 

Bay along the Bear River (locations shown in fig. 5). We 

speculate that movement on faults and folds in the North 

America plate of western Washington accompanied—or 

shortly preceded or followed—one or more plate-boundary 

earthquakes 1,000-1,100 years ago, and that this plateboundary seismicity is recorded at northeastern Willapa Bay 

by soil W.

SIZES OF THE EARTHQUAKES

Although their sizes are difficult to estimate, most or 

all of the earthquakes represented by buried soils H through 

Y were probably great earthquakes—of moment magnitude 

8 or larger. Such magnitudes are suggested by geologic evidence for large ruptures and by a tradeoff between size and 

frequency.

DIMENSIONS OF PLATE-BOUNDARY RUPTURES 

RUPTURE WIDTH

Wide ruptures are likely because, as shown in the section "Distinction Between Plate-Boundary and Upper-Plate 

Sources for the Earthquakes" (p. 80), the earthquakes 

marked by the buried soils probably occurred on the boundary between the Juan de Fuca plate and the North America 

plate. Ruptures on the Washington part of this plate boundary are likely to exceed 50 km in width, according to interpretation of geodetic and heat-flow data (Savage and others, 

1991; Dragert and others, 1994; Hyndman and Wang, 

1995). A potential rupture width of more than 100 km has 

been estimated from satellite measurements of deformation 

between 1992 and 1994 near the border between Washington and British Columbia (Dragert and Hyndman, 1995, p. 

756).

High temperatures and fluid pressures may preclude 

ruptures much wider than 100 km on the Washington part of
86 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON 

Table 7. Radiocarbon ages of samples from the Oyster, Redtail, Jensen, Sewer, and Airport localities, northeastern Willapa Bay

Age ± quoted 

error1

90

120

135

139

155

156

162

174

230

250

250

272

360

380

601

630

710

720

1,247

1,260

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

±

60

41

42

40

47

43

49

43

70

60

60

47

70

50

55

90

70

70

22

14

Material

Outer part of bark-bearing root 

10 cm in diameter without 

conspicuous rings; probably 

western crabapple

Leaf base of Potentilla pacifica

Leaf base of Potentilla pacifica

Leaf base of Potentilla pacifica

Leaf base of Potentilla pacifica

Leaf base of Potentilla pacifica

Culm base ofJuncus balticus

Leaf base of Potentilla pacifica

Spruce root 1.0-1.5 cm diameter

Spruce root 1.0-1.5 cm diameter

Charcoal

Culm base ofJuncus balticus

Spruce root 3.5 cm diameter, 

containing about 1 5 rings

Stave. Dated portion — 

consisting of branched, barkcovered stick — was 4 cm in

maximum diameter and

contained about 1 5 annual

rings

Bark-free twig

Charcoal

Stake 5 cm in diameter; lower 

end charred, hard, and sharp

Bark-free twigs

Triglochin maritimum rhizomes

Salicornia Virginia stems and

Soil

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

—

Y

"

-

U

U

Stratigraphic setting

10 cm below top of soil; root 

attached to bark-bearing 

trunk about 1 5 cm in 

diameter that extends into 

silt above soil

Rooted in soil and entombed 

in overlying tsunami deposit

Rooted in soil and entombed 

in overlying tsunami deposit

Rooted in soil and entombed 

in overlying tsunami deposit

Rooted in soil and entombed 

in overlying tsunami deposit

Rooted in soil and entombed 

in overlying tsunami deposit

Rooted in soil and entombed 

in overlying tsunami deposit

Rooted in soil and entombed 

in overlying tsunami deposit

Uppermost 1 cm of soil; from 

same tree as Beta-19840?

Uppermost 1 cm of soil; from 

same tree as Beta- 1983 9?

Fire pit that extends to top of 

soil

Rooted in soil and entombed 

in overlying tsunami deposit

Top of root 1 cm below top of 

soil

Weir on mudflat

Probable tsunami deposit in 

channel fill

Fire pit 10-20 cm below top of 

soil

Vertical in mud to depth of 

1.0m below top of soil Y; 

charred end down

Probable tsunami deposit in 

channel fill

In mud above filled channel

Stems in mud 0-3 cm above

Locality

Sewer

Oyster

Oyster

Oyster

Oyster

Oyster

Oyster

Oyster

Redtail

Redtail

Redtail

Oyster

Redtail

Sewer

Oyster

Redtail

Sewer

Oyster

Airport

Oyster

River

Wil

Nia

Nia

Nia

Nia

Nia

Nia

Nia

Nia

Nia

Nia

Nia

Nia

Wil

Nia

Nia

Wil

Nia

Wil

Nia

Lab number 2

Beta

GX

GX

GX

GX

GX

GX

GX

Beta

Beta

Beta

GX

Beta

Beta

GX

Beta

Beta

Beta

QL

QL

76289

17927

17852

17946

17846

17838

17828

17942

19839

19840

63220

17933

22368

74541

20307- 

AMS

49195

76290

76685

4822

4795

Field 

number

J13A

ARN91-37

ARN91-36

ARN91-40

ARN91-34

ARN91-32

ARN91-31

ARN91-39

SL86-106O

SL86-106P

F14B

ARN91-38

A1F

J4A

J10A2

Al

J14A

J10A1

J2A

J9C

attached roots top of soil; roots in peaty 

mud 0-2 cm below top of 

soil
SIZES OF THE EARTHQUAKES 87

Table 7. Radiocarbon ages of samples from the Oyster, Redtail, Jensen, Sewer, and Airport localities, northeastern Willapa Bay— 

Continued

Age ± quoted 

error1

1,302 ± 

1,598 ±

21 

23

Material

Triglochin maritimum rhizomes 

Triglochin maritimum

Soil

U 

S

Stratigraphic setting

In mud about 20 cm above top 

of soil

In mud 45-55 cm above top of

Locality

Oyster 

Oyster

River

Nia 

Nia

Lab number 2

QL 

QL

4798 

4797

Field 

number

J9D 

J9B

rhizomes; split into roughly 

identical sample for Beta76633

1,630 ± 70 Triglochin maritimum

rhizomes; split into roughly 

identical sample for QL-4797

1,720 ± 40 Root or stick 3 cm diameter; 

split into more or less 

identical sample for Beta19837

1,740 ± 15 Thuja plicata (western

redcedar) root; rings number 

60-69, where ring number 1 

adjoins bark

1,740

1,800

1,830

1,990

2,170

2,258

2,475

2,510

2,530

2,590

2,590

2,780

2,790

2,820

2,840

2,945

3,063

3,165

3,166

3,180

3,190

3,230

±

±

±

±

±

±

±

±

±

±

±

±

±

+

+

±

±

+

±

±

Hh

±

70

70

40

70

25

75

23

80

100

80

60

45

120

50

70

30

80

17

18

30

110

60

Root or stick 2 cm diameter

Root or stick 3 cm diameter;

split into more or less 

identical sample for USGS2540

Root or stick

Triglochin maritimum rhizomes

Triglochin maritimum rhizomes

Rotten stick 0.5-1. Ocm

diameter

Spruce cones

Stick

Sticks and spruce cones

Sticks

Stick

Peat

Peat

Spruce root, outer 25 rings

Root

Peat

Bark-free twig 0.2 cm diameter

Spruce cones

Twigs

Stick

Root

Stick

S

S

S

S

S

N

N

N

N

N

N

N

L

L

L

L

J

J

J

J

L

J

soil

S In mud 45-55 cm above top of Oyster Nia 

soil

S Top of soil

One of a forked pair of 

redcedar roots high in soil

S Top of soil 

S Top of soil

Uppermost 1 cm of soil

Mud 20-30 cm above top of 

soil

Mud 50 cm below top of soil 

Uppermost 0.5 cm of soil

Resting on top of soil; 

probably part of forest-edge 

litter at time of subsidence

Mud 1-2 cm above top of soil 

Uppermost 1 cm of soil 

Top of soil

Mud 1-2 cm above top of soil 

Uppermost 1 cm of soil 

Uppermost 1-2 cm of soil

Part of a large root system in 

soil (fig. 135)

In soil

Uppermost 1 cm of soil

Top of soil

Resting on top of soil; probabl; 

part of forest-edge litter at 

time of subsidence

Identical to QL4718 

Identical to QL4718 

In soil 

Top of soil

Beta 76633 J9B1

Redtail

Pool

Redtail

Redtail

Redtail

Sewer

Oyster

Redtail

Redtail

Redtail

Redtail

Redtail

Redtail

Redtail

Redtail

Jensen

Redtail

Redtail

Redtail

Redtail

Redtail

Redtail

Redtail

Redtail

Nia

Nia

Nia

Nia

Nia

Wil

Wil

Nia

Nia

Nia

Nia

Nia

Nia

Nia

Nia

Wil

Nia

Nia

Nia

Nia

Nia

Nia

Nia

Nia

USGS

QL

Beta

Beta

USGS

Beta

QL

RIDDL

QL

Beta

Beta

Beta

Beta

USGS

Beta

Beta

Beta

USGS

RIDDL

QL

QL

QL

Beta

Beta

2540

4796

19838

19837

2541

76291

4823

686

4715

22371

21826

19836

22370

2539

21824

81551

19835

2538

684

4718

4717

4716

19834

21822

SL86-

106K2

J12A

SL86-106L

SL86-

106K1

SL86-106M

J18A

J9A

SL86-106I

F19A1

A1N2

A2A+B

SL86-106H

A1N1

SL86-106J

AID

J8A

SL86-106G

SL86-106E

SL86-106B

F16A1

F16A2

F16C

SL86-106F

A-1B
88 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Table 7. Radiocarbon ages of samples from the Oyster, Redtail, Jensen, Sewer, and Airport localities, northeastern Willapa Bay— 

Continued

Age ± quoted 

error 1

Material Soil Stratigraphic setting Locality River Lab number 2 Field 

number

3,262 ± 80 Bark-free twig 0.5 cm diameter J Uppermost 0.5 cm of soil Redtail Nia RIDDL 685 SL86-106C

3,320 ± 80 Rhizomes J Mud 20-40 cm above top of Redtail Nia Beta 21823 A1C

soil

H Mud 20-30 cm below top of Redtail Nia Beta 21825 A1E 

soil J and about 30 cm above 

top of soil H

H In upper few centimeters of soil Jensen Wil Beta 76288 J8E1 

and in lower few centimeters 

of overlying mud

J Uppermost 1-2 cm of soil Redtail Nia USGS 2537 SL86-106A

3,570 ± 70 Rhizomes

3,640 ± 80 Bark-free stick

3,730 ± 210 Peat

1 Ages in radiocarbon years before A.D. 1950 (yr B.P.)

2 Beta, Beta Analytic, Inc., Coral Gables, Florida. GX, target prepared by Geochron Laboratories, Cambridge, Massachusetts; age measured by accelerator mass 

spectrometry by Institute of Geology and Nuclear Sciences, Ltd., Lower Hutt, New Zealand. QL, Quaternary Isotope Lab, University of Washington, Seattle, 

Washington. RIDDL, Radioisotope Direct Detection Laboratory, McMaster University, Hamilton, Ontario.

the Cascadia subduction zone. High temperatures at depth 

should limit the landward extent of brittle failure (Hyndman 

and Wang, 1993). High fluid pressures may limit release of 

seismic energy from displacement on the seaward part of 

the plate boundary (Byrne and others, 1992; Goldfinger and 

others, 1996).

RUPTURE LENGTH

Ruptures at least 50 km wide at the Cascadia subduction zone can be as short as 200 km and still have enough 

area to produce a magnitude-8 earthquake, if a magnitudearea correlation for mostly smaller ruptures (Wells and Coppersmith, 1994) applies to the Cascadia subduction zone.

The most recent coseismic subsidence, which probably 

dates to 300 years ago, struck estuaries as much as 625 km 

apart during the same few decades (fig. 33; Nelson and others, 1995). This degree of coevality implies either a single 

rupture at least 600 km long or a swift series of shorter ruptures. A long single rupture at Cascadia best explains a tsunami in Japan in January 1700 (Satake and others, 1996). 

This correlation is supported by tree-ring dating in southern 

Washington, as summarized on page 94.

Earlier coseismic subsidence, though not so precisely 

dated, may have also resulted from long ruptures. Largely 

overlapping age ranges spanning about 150-250 calendric 

years, obtained from multiple radiocarbon ages on earthquake-killed herbs or delicate plant detritus on probable correlatives of soils S, U, and Y (Nelson and others, 1996b, 

their fig. 6), suggest correlation of two Stratigraphic 

sequences 110 km apart—at the Naselle River, Washington 

(fig. 5) and at Netarts Bay, Oregon (fig. 1). Widespread subsidence is further suggested by similarities among ages for

soil S throughout southern coastal Washington (fig. 32). 

Moreover, a long apparent interval without buried soils, centered about 2100 years ago, is indicated by a gap in radiocarbon ages both in southern Washington (see section 

"Interval Between Events Marked by Soils N and S," p. 98) 

and in northern Oregon (fig. 4 of Darienzo and Peterson, 

1995; gap between ages for their events 5 and 6).

TRADEOFF BETWEEN SIZE AND FREQUENCY

If plate-boundary earthquakes at the Cascadia subduction zone are typically smaller than moment magnitude 

(Mw) 8, the earthquakes should happen more often than is 

indicated by historical seismic quiescence or implied by 

geologic estimates of recurrence intervals. Table 8 illustrates this tradeoff between size and frequency of earthquakes for various degrees of coupling at the plate 

boundary. From this table we conclude that maximum earthquakes of Mw 7.5 or 8.0 would require many more such 

events than are allowed by historical and geological records. 

We further infer that this problem cannot be solved through 

creep at the plate boundary or permanent deformation 

within the North America and Juan de Fuca plates.

Plate convergence creates seismic moment, which is 

related to seismic energy and is defined as the product of 

rigidity, rupture area, and average fault displacement (Hanks 

and Kanamori, 1979). Given estimates of rigidity and rupture area, one can deduce combinations of earthquake size 

and frequency that produce average displacement consistent 

with plate convergence at the Cascadia subduction zone. 

Only some of these combinations, however, are consistent 

with the lack of thrust earthquakes on most of the Cascadia 

plate boundary in the past 150-200 years, and with geologic
SIZES OF THE EARTHQUAKES 89

130'

Copalis River

Washington! willapa Bay

Je

*S£ 

£ « o

Inferred time of subsidence, 

in calendric years A.D.

1720 1700

hH hM hH

55

100

Nehalem RiverHumboldt Bay

525

KH

h-DH

0 100 

Measured age

of sample, 

in 14CyrB.P.

200 300

h[H Ijl hjH hyH

1680 1660

Inferred age

of inner-ring sample,

in calendric years A.D.

EXPLANATION

Seaward edge of subduction zone 

Spreading ridge 

Transform fault

Figure 33. Comparison of high-precision radiocarbon ages 

measured on inner and outer rings from spruce stumps in 

southern Washington, northern Oregon, and northern 

California. Washington ages tabulated by Atwater and others 

(1991); Oregon ages, Nelson and Atwater (1993); California 

ages, Carver and others (1992). All ages summarized by 

Nelson and others (1995). Calendric time scale at bottom, 

based on calibration data of Stuiver and Becker (1993), 

applies to inner-ring samples if they formed before the early 

1700's. Such timing has been demonstrated for one stump at 

Willapa Bay (Atwater and others, 1991, their fig. 3b) and for

Average number of tree-ring years before death, 

for tree probably killed by effects of subsidence 

° 5 — Outer-ring sample 

• 40 — Inner-ring sample

Error bars for sample ages and for calendric-year scales 

represent at least 0.6-0.8 standard deviation

one stump at Humboldt Bay (Nelson and others, 1995, thenfig. 2). For these stumps, calendric dates younger than the 

early 1700's have been disallowed through matching of 

distinctive wiggles in radiocarbon time scales. Years for 

calendric time scale at top are 40 years later than those for 

the calendric scale at bottom because the trees died 40 treering years after formation of the middle ring in the inner-ring 

samples. Error bars on calendric scales show two-standarddeviation uncertainty in converting from radiocarbon years 

to calendric years for wood formed between 1650 and 1680.

evidence that earthquakes on this plate boundary recur hundreds of years apart.

We obtained minimum estimates of seismic moment 

by using several estimates treated here as constants. For 

rigidity we use 3xl010 N/m2, at the low end of the range that 

Kanamori (1977, p. 2983) used to calculate sizes of great 

earthquakes. For rupture area we use 50,000 km2, well 

under the 90,000 km2 estimated by Hyndman and Wang 

(1995, p. 22152) on the basis of geodetic and heat-flow 

data. For plate convergence we use 3.5 cm/yr, the component orthogonal to the strike of the plate boundary off Oregon and Washington; the component parallel to strike is 

excluded because most or all of this component is probably

expended on permanent deformation in the North America 

plate (McCaffrey and Goldfinger, 1995). Orthogonal convergence probably exceeds 3.5 cm/yr off northern Washington and southern British Columbia, where the convergence 

direction is nearly orthogonal (Wilson, 1993, p. 16069), and 

off northern California, where the orthogonal component of 

plate convergence is increased by spreading in the Basin and 

Range province (Pezzopane and Weldon, 1993, p. 1164- 

1165). Modern earthquakes in the North America and Juan 

de Fuca plates account for no more than about one-tenth of 

the convergence (Hyndman and Weichert, 1983).

The calculated seismic moment varies largely with 

coupling—the fraction of plate convergence that becomes
90 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Table 8. Hypothetical sizes and numbers of earthquakes resulting from 300, 500, and 1,000 years of plate 

convergence at the Casadia subduction zone 

[For assumptions see section, "Tradeoff Between Size and Frequency"]

Coupling1 Seismic moment2, in 

1029 dyne-cm

Corresponding size of

single earthquake

(Mw)

Corresponding number 

of lesser earthquakes

MW 8.5 MW 8.0

Fraction of plate convergence that becomes seismic slip at the plate boundary.

Product of rigidity, rupture area, and average displacement (Hanks and Kanamori, 1979).

Rounded to nearest whole number.

MW 7.5

1

3/4

1/2

1/4

1

3/4

1/2

1/4

1

3/4

1/2

1/4

1.56

1.18

0.79

0.39

2.63

1.97

1.31

0.66

5.25

3.94

2.63

1.31

300 years of plate convergence

8.8

8.7

8.6

8.4

500 years of plate convergence

8.9

8.8

8.7

8.5

1,000 years of plate convergence

9.1

9.0

8.9

8.7

2

2

1

1

4

3

2

1

8

6

4

2

14

11

7

4

23

18

12

6

47

35

23

12

79

59

39

20

132

99

66

33

263

197

132

66

seismic slip on the plate boundary. At many subduction 

zones this fraction has been shown to be less than onefourth (Pacheco and others, 1993), and such weak coupling 

in northern Japan has been shown to reflect aseismic creep 

in the first year after a large plate-boundary earthquake 

(Heki and others, 1997). Coupling has not been measured 

for the Cascadia subduction zone, owing to the lack of 

plate-boundary earthquakes in the zone's instrumental history. However, because of their motions with respect to the 

mantle, the Juan de Fuca and North America plates may be 

strongly coupled (Scholz and Campos, 1995, p. 22112). 

Strong coupling at Cascadia is consistent with heat-flow and 

geodetic data (Savage and others, 1991; Hyndman and 

Wang, 1993, 1995; Dragert and others, 1994). It is also consistent with the lack of small modern earthquakes on the 

plate boundary—earthquakes that could demonstrate 

present-day creep (Heaton and Hartzell, 1987; Rogers, 

1988).

The other main variable in table 8 is the amount of time 

during which the seismic moment accumulates. We illustrate three values: 300, 500, and 1,000 years. Three hundred 

years approximates the time since the most recent great 

earthquake inferred for the Cascadia subduction zone (see 

section "Soil Y" on p. 94) as well as the shortest average 

recurrence interval estimated for plate-boundary earthquakes at Cascadia (p. 7; fig. 3). Five hundred years approximates the average of six intervals between great plateboundary earthquakes of the past 3,500 years at northeastern Willapa Bay, as inferred in this report (see p. 99). The 

longest of these intervals—between earthquakes marked by 

soils N and S—spanned about 1,000 years (p. 98).

The three right-hand columns in table 8 show that the 

seismic moment from centuries of plate convergence at the 

Cascadia subduction zone would produce very large numbers of earthquakes if the maximum earthquake size is Mw 

7.5 or 8.0. The Mw 7.5 earthquakes are so numerous, even 

for just 300 years of plate convergence, that many such 

earthquakes should have occurred historically if plateboundary earthquakes at the Cascadia subduction zone do 

not exceed Mw 7.5. The numbers of Mw 8 earthquakes are 

also too large to agree with recurrence intervals of 300-500 

years unless only about one-quarter of the plate convergence 

becomes seismic slip at the plate boundary. In that case, all 

the convergence could be accounted for by a series of four 

to six adjacent Mw 8 ruptures each about 200 km long and 

adjoined like sausage links along the 1,000-km-long subduction zone. The stronger coupling inferred above requires 

earthquakes of Mw 8.5 or larger to obviate average recurrence intervals shorter than those summarized in figure 3.

NUMBER AND AGES OF EARTHQUAKES

Seven great plate-boundary earthquakes probably 

struck southern coastal Washington between 3,300-3,500 

years ago and 300 years ago. A great earthquake, or a pair
NUMBER AND AGES OF EARTHQUAKES 91

of such earthquakes having adjacent ruptures on the southern Washington part of the Cascadia plate boundary, probably corresponds to each buried soil that we observed in 

outcrop along the Niawiakum River (figs. 13-26) and correlated with buried soils elsewhere in southern coastal Washington (figs. 32, 34). More than seven great earthquakes (or 

series) may have struck southern coastal Washington in the 

past 3,500 years if the buried-soil record is incomplete.

EVENTS RECORDED BY BURIED SOILS

For convenience we use "event" for the earthquake or 

series of earthquakes represented by a buried soil and its 

probable correlatives. The term is useful because latitudinally adjacent parts of the Cascadia plate boundary may 

rupture all together in a single earthquake, or they may rupture individually in an earthquake series that spans too little 

time for the individual ruptures to be distinguished from one 

another by conventional geologic dating.

Such variable rupture—sometimes a single earthquake, 

other times a series of smaller earthquakes—is common at 

subduction zones (Thatcher, 1989), including several zones 

that have been likened to Cascadia (Heaton and Hartzell, 

1986, 1987). A single earthquake ruptured about 500 km of 

subduction zone off Columbia and Ecuador in 1906, but this 

same area then ruptured piecemeal in a series of three earthquakes between 1942 and 1979 (Kanamori and McNally, 

1982). Similarly at the Nankai Trough off southwest Japan, 

about 500 km of subduction zone ruptured as a whole in 

1707 but piecemeal, in a pair of adjacent earthquakes 32 

hours apart in 1854 and in another pair of earthquakes two 

years apart in the 1940's (Ando, 1975). In the language used 

below, the Nankai example comprises three events—the 

first in 1707, the second in 1854, the third in the 1940's.

SOILJ

The earliest well-dated event in southern coastal Washington is marked by soil J. We observed this soil in outcrop 

at the Redtail locality (figs. 13, 14B) and saw probable correlatives exposed at two other sites in northeastern Willapa 

Bay—at the Jensen locality (figs. 27, 28A) and along the 

South Fork Willapa River (Hull, 1987). We have also found 

a probable correlative of soil J along the Naselle River, 

about 200 m downstream from the outcrop studied by Ota 

and Umitsu( 1995).

Low altitude, not small rupture area, probably accounts 

for the limited extent of soil J depicted in figure 34. At most 

places the present altitude of the soil is too low for exposure 

at even the lowest of tides. We infer that soil J exists in 

many areas where it is below lowest tides and therefore can 

only be seen in core. One such area may be the Johns River 

(fig. 5), where a possible correlative of soil J was observed 

in borings by Shennan and others (1996).

The event marked by soil J probably occurred between 

3320 and 3,500 years ago (fig. 34). This estimate is based

on three concordant ages on forest-floor litter at the Redtail 

locality (p. 32).

SOILL

Soil L, the oldest soil widely exposed at Willapa Bay 

and Grays Harbor, records a poorly dated event that probably happened between 2,800 and 3,300 years ago. The soil, 

which is typically bold and commonly contains spruce roots 

seaward of modern spruce swamps, is conspicuous in outcrop at Willapa Bay along the Niawiakum and Willapa Rivers, and at Grays Harbor along the Johns River and Blue 

Slough (fig. 34). A probable correlative of soil L also crops 

out along the Naselle River, about 200 m downstream from 

the outcrop studied by Ota and Umitsu (1995).

The estimated age of 2,800-3,300 years is based on 

three kinds of evidence: conventional radiocarbon dating of 

materials that did not necessarily form close to the time of 

the event; conventional radiocarbon dating of wood that 

probably formed in the last 25 years before the event; and 

stratigraphic position between the two better-dated soils that 

bracket soil L. The conventional ages—on peat and sticks, 

and on small woody roots sampled without regard to the 

number or position of rings—together provide broadly limiting ages of 2,200 and 3,800 years ago (fig. 32). The outer 

25 rings of a bark-bearing spruce root at the Jensen locality 

gave a conventional age (p. 68) that narrows the range to 

2,800-3,300 years ago. This narrower range fits in the gap 

between events marked by soil J (3,300-3,500 years ago) 

and soil N (2,400-2,800 years ago).

SOILN

Soil N has about the same mapped distribution in 

southern coastal Washington as does soil L (fig. 34). Typically faint, soil N rarely contains growth-position remains of 

woody plants, even where its surficial horizon retains peat, 

as shown at the Redtail and Heron localities (figs. 13,25).

Nearly all the ages for the event marked by soil N were 

measured at the Redtail locality and nearly all of these are 

concordant (fig. 32). One is a high-precision age on forestfloor litter (2,475±23 14C yr B.P.; table 7). Its corresponding 

range of calendric ages is broad—between 2,400 and 2,780 

years ago—because of a plateau in radiocarbon ages for that 

period of calendric time (Stuiver and Pearson, 1986). At the 

Oyster locality, an age on rhizomes gives soil N a limiting 

minimum age between about 2,050 and 2,350 years ago (p. 

52).

SOILS

Bold soil S is widely and conspicuously exposed not 

only at Willapa Bay and Grays Harbor but also at estuaries 

having smaller tidal ranges. Likely correlatives of soil S 

have been documented in outcrop at the Copalis River (fig. 

34; Atwater, 1992, his "lower bold soil"), at Netarts Bay, 

Oregon (Darienzo and others, 1994; Nelson and others,
92 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

47C

124°

abdhsR 

• Copalis 

River

10km

• rv

•d

br.

o br

abdhR 

bdhr

SoilW SoilU

°ab

abdhr

abr

SoilY SoilN SoilL

sr

ob br

•abdr

br

abr

abdhir

SoilS

'abr

SoilJ

Figure 34. Geographic distribution of well-exposed evidence for earthquakes in the past 3,500 years in southern 

coastal Washington. Sources: Atwater (1987), Reinhart and Bourgeois (1987), Atwater and Yamaguchi (1991), 

Atwater and others (1991), Atwater (1992), Hemphill-Haley (1995b), Ota and Umitsu (1995), Shennan and others 

(1996), B.F. Atwater, M.A. Reinhart, and D.K. Yamaguchi (unpub. data, 1987-1990), and this report.

1996b), and perhaps as far south as Coos Bay, Oregon (Nelson and others, 1996a, their couplet 4) and Humboldt Bay, 

California (Clark and Carver, 1992). The soil is typically the 

most conspicuous of all the buried soils seen in many Washington outcrops, as illustrated at the Redtail (fig. 14A, B), 

Oyster (fig. 195), and Jensen (fig. 28A) localities. A common characteristic is spruce roots in growth position at least 

as far downstream as any other exposed soil, and farther 

downstream than modern spruce swamps.

The event marked by soil S probably occurred between 

1,500 and 1,700 years ago. This estimate is based on concordant high-precision ages of two samples collected along 

the Niawiakum River—rhizomes that probably formed soon 

after the event (p. 52, 58), and tree rings that probably 

formed about 65 years before the event (p. 63).

The range of 1,500-1,700 years accords with conventional radiocarbon ages on material associated with soil S in 

southern coastal Washington. Most of these ages were measured on the peaty surficial horizon of the soil, on rings of 

unknown position in roots within the soil, and on sticks and

cones directly above the soil (table 7; Atwater, 1992, his 

table 1). Such material provides maximum limiting ages for 

the event; these ages are between 1,400 and 2,100 years ago 

(fig. 32).

The range of 1,500-1,700 years ago probably accords 

with problematic ages of five carefully selected samples 

from the Naselle River of southeastern Willapa Bay (fig. 5). 

These ages, reported by Nelson and others (1996b), were 

measured on thoroughly cleaned stems and (or) leaves of 

herbaceous fossils (four Potentilla pacifica, one Juncus balticus) rooted in a probable correlative of soil S. The individual ages range from 1,680±40 14C yr B.P. to 1,910±50 14C yr 

B.P (table 7 of Atwater and Hemphill-Haley, 1996). The 

weighted mean age, 1,797±23 14C yr B.P., corresponds to 

about 1,600-1,900 years ago. But the mean is questionable 

because the five ages are significantly different at the 95- 

percent level, according to the chi-squared test of contemporaneity of Ward and Wilson (1978).

We obtained another potentially conflicting age from 

the Sewer locality. This age—1,990±70 14C yr B.P. (table
NUMBER AND AGES OF EARTHQUAKES 93

EXPLANATION

i'HSi^^^5^^i^^^S^S^^lT^^^^SSSfSSSiS5aS:: S?S?Ss¥g"::i;:;;sfl|»a

:-:>:;;:*s;:;s;:;s:* w&w&w w*:;ss;ssssss? v^S?!Sf!i^KmmmKSSSSBHs:s s s-s ;]• i

Evidence for Holocene earthquakes

Buried soil observed in 

outcrop—Typically in one or more lowtide exposures at least 30 m long. 

Letters beside dot denote associated 

evidence for subsidence, tsunami, 

shaking, and stratigraphic correlation 

• From published report 

o From unpublished data

Subsidence—Inferred from paleontologic 

evidence that mud above soil represents a 

lower altitude than does the soil itself 

a Remains of herbs rooted in mud

above buried soil—Species typical of 

low parts of tidal marshes 

b Remains of herbs or trees rooted in 

buried soil—Species typical of high 

parts of tidal wetland or of upland. 

h Locally include herbaceous stems and 

leaves entombed in tsunami and (or) 

tidal-flat deposits above soil (h) 

d Microscopic fossils in soil and in 

mud—Diatoms, pollen, and 

foraminifera at Johns River (Shennan 

and others, 1996); diatoms only at 

other localities

Figure 34. Continued.

Tsunami—Indicated by sand sheet on buried

soil

Shaking—Inferred from intrusive and 

extrusive sand. Excludes sand that vented 

from depths probably too great for ordinary 

earthquake-induced liquefaction (v) 

Correlation of buried soils—Tested by 

radiocarbon dating 

Conventional dating—Quoted error 

typically 50-100 radiocarbon years 

High-precision dating—Quoted error 10- 

25 radiocarbon years

Evidence for net Cenozoic uplift or 

subsidence

Structural high—Eocene Crescent Formation

mapped at surface 

Syncline—Mapped in rocks as young as

Miocene

7)—was measured on rotten, possibly contaminated rhizomes above soil S (p. 71).

SOILU

Soil U appears to be widespread in southern Washington (fig. 34) and may have correlatives at least as far south 

as Netarts Bay, Oregon (Darienzo and others, 1994; Nelson 

and others, 1996b). The soil is typically faint and rarely contains woody roots in growth position, even where peaty or in 

an area of modern tidal spruce.

The event recorded by soil U probably occurred 

between 1,130 and 1,350 years ago. This range corresponds 

to two high-precision ages on growth-position herbaceous 

fossils at the Oyster locality—1,260±14 and 1,300±25 14C 

yr B.P. (p. 58). These ages are compatible with the age of 

1,247±22 14C yr B.P. obtained on rhizomes above soil U at 

the Airport locality (p. 73). They are also close to 1,364±21 

14C yr B.P., the weighted mean of six concordant radiocarbon ages each measured on a different herbaceous plant 

rooted in a probable correlative of soil U at the Naselle

River (Nelson and others, 1996b; individual ages listed in 

table 7 of Atwater and Hemphill-Haley, 1996). The mean 

Naselle River age corresponds to a calendric range of about 

1,200-1,400 years ago.

Ages greater than 1,400 years ago were reported by 

Atwater (1992) for conventionally dated material associated 

with soil U at Grays Harbor and the Copalis River (fig. 32). 

The conventional ages, however, were measured on peat 

(Elliott Slough and Copalis River) and on an upright but 

possibly detrital stump (Blue Slough). Both the peat and the 

stump could have originated centuries before the event 

recorded by the soil.

SOILW

Soil W is barely preserved at some Washington localities and is absent, probably because of decomposition (p. 

25), at some localities where soils U and Y are both present 

(fig.'34).
94 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

The event marked by soil W probably occurred 

between 900 and 1,300 years ago. This range is consistent 

with conventional ages on peaty mud at Grays Harbor, on a 

spruce root at southern Willapa Bay, and on sticks and a willow(?) root along a tributary to the Columbia River (Atwater, 1992, his table 1). It is further consistent with ages on 

peat and roots in a soil covered by vented sand at the Copalis River (fig. 32).

As speculated on page 83, the event marked by soil W 

may have occurred at or about a time of unusual faulting in 

the North America plate at Puget Sound. High-precision 

dating of earthquake-killed herbs and trees shows that this 

Puget Sound seismicity occurred between 1,000 and 1,100 

years ago (Bucknam and others, 1992; Atwater and Moore, 

1992; Jacoby and others, 1992; Huntley and Clague, 1996, 

their table 1).

SOILY

Soil Y is distinctive, like soil S, for containing spruce 

roots far downstream from modern spruce swamps (Atwater, 1992). It is further marked at Willapa Bay by a widespread mantle of tsunami-laid sand (Atwater, 1987, 1996; 

Reinhart and Bourgeois, 1987, 1989). Probable correlatives 

of soil Y have been identified along at least 900 km of the 

Cascadia subduction zone (Nelson and others, 1995).

The likely time of the event marked by soil Y is close 

to 300 years ago. Conventional radiocarbon ages show that 

the event happened after about A.D. 1500 (fig. 32). This 

window was narrowed to post-1680 by Yamaguchi and others (1989, 1997), who matched ring-width patterns in western redcedar snags rooted in soil Y in southern coastal 

Washington. Atwater and others (1991) obtained high-precision ages on selected rings of earthquake-killed spruce 

along the Niawiakum River, at nearby Bay Center, and at 

the Copalis River. The corresponding time of tree death is 

between about A.D. 1700 and 1720 (fig. 33), if calculated 

with the corrected radiocarbon calibration data of Stuiver 

and Decker (1993).

Among radiocarbon estimates for the time of the event 

marked by soil Y, the narrowest published ranges have been 

obtained from six ages for inner-ring samples of spruce 

stumps rooted in the soil (Atwater and others, 1991; Nelson 

and others, 1995). Each sample contains 10 or 20 rings that 

average 40 tree-ring years before tree death, and each comes 

from a different spruce stump: three stumps at the Copalis 

River, and three stumps 55 km to the south at Bay Center 

(two stumps) and the Niawiakum River (one stump). The 

six ages can be averaged if the trees probably died from the 

same earthquake or from a series of earthquakes within a 

few years of one another (Atwater and others, 1991). Their 

weighted mean—209±6 14C yr B.P.—has been converted to 

two different calendric age ranges intended to include the 

95-percent confidence interval for tree death. The first 

reported, A.D. 1695-1710 (Atwater and others, 1991), is 

based the calibration data of Stuiver and Becker (1986). In 

this calibration, 209±6 14C yr B.P. corresponds to a 95-percent confidence interval of A.D. 1655-1669 if ±6 14C yr 

equals at least 0.6 standard deviation. Stuiver and Becker 

(1993) corrected the calibration data for slight contamination by radon. With this corrected data the weighted mean 

age implies tree death between 1703 and 1715 (Nelson and 

others, 1995).

The event consisted of a single magnitude-9 earthquake close to 9 p.m. local time on January 26, 1700, 

according to Satake and others (1996). Satake and his 

coworkers studied written records of a tsunami that struck 

Japan 10 hours later. They showed that earthquakes capable 

of generating this tsunami probably did not occur in Japan, 

Kamchatka, Alaska, or South America during January 1700. 

Using a numerical model of tsunamis, they also showed that 

the tsunami in Japan could have resulted from an earthquake 

that ruptured most of the length of the Cascadia subduction 

zone.

Correlation of the January 1700 tsunami in Japan with 

a great earthquake at Cascadia was first tested at Cascadia 

through study of living, presently tidal Sitka spruce (Picea 

sitchensis) that survived the submergence about 300 years 

ago in southern Washington and northern Oregon. Several 

dozen such trees have been found at the transition between 

floodplain and tidal swamp at the Copalis River, Willapa 

Bay, and the Columbia River. Annual rings in a third of 

them show changes in width and anatomy consistent with 

disturbance—tilting, increased flooding, or both—in the 

first few years after 1699 (Jacoby and others, 1997).

Further support for correlation with the January 1700 

tsunami has come from tree-ring dating of redcedar that 

were killed or damaged by the most recent event in southern 

Washington. Among eight redcedar retaining bark despite 

centuries of decay, at least six died sometime between 

August 1699 and May 1700; they contain a complete 1699 

ring but no younger ring (Yamaguchi and others, 1997). The 

five trees come from four estuaries along nearly 100 km of 

the Cascadia subduction zone: Copalis River, Grays Harbor, 

Willapa Bay, and Columbia River.

ADDITIONAL, UNRECORDED EVENTS

Buried soils do not necessarily record all the great 

plate-boundary earthquakes that have struck southern Washington during the past 3,500 years. The buried-soil record is 

most likely to be incomplete for earthquakes comparable to 

the 1944 and 1946 earthquakes in southwest Japan. These 

earthquakes, which caused a maximum of Vi m of tectonic 

subsidence (Ando, 1975, p. 122), were probably close to Mw 

8.0 (Heaton and Hartzell, 1986, p. 693-694).

Such an earthquake could fail to register as a buried 

soil for two reasons:

1. If the earthquake occurs soon after a preceding great 

earthquake, there may be no peaty soil to bury; tidal flats 

resulting from the preceding earthquake may not have 

had time to grow upward into marshes and swamps having peaty soils.
INTERVALS BETWEEN GREAT EARTHQUAKES 95

2. If the earthquake is followed by a long period without 

great earthquakes in the area, the buried soil that records 

the earthquake may be destroyed by decomposition after 

burial (p. 27; fig. 35).

Both these problems probably contribute to the spotty 

distribution and typical faintness of soil W. Little time may 

have elapsed between the earthquake marked by soil U 

(1,130-1,350 years ago) and soil W (900-1,300 years ago, 

perhaps 1,000-1,100 years ago). Soil W, where studied at 

the Oyster locality, had not yet become a high marsh at its 

time of subsidence and burial (p. 48). For many centuries 

thereafter the soil resided high in the profile of soil Y, where 

it was subject to decomposition before being lowered far 

into the intertidal zone about 300 years ago. This decomposition allowed destruction of soil W at the northwestern part 

of the Oyster locality (p. 45) and throughout the Redtail 

locality (p. 29).

We doubt, however, that any great earthquake of the 

past 3,500 years at northeastern Willapa Bay completely 

failed to leave a trace. Rather, we infer that a buried soil 

should be preserved, at least locally, for every earthquake 

that caused more than l/2 m of widespread coseismic subsidence at northeastern Willapa Bay in the past 3,500 years— 

provided the soil had many decades to form.

MINIMUM TIME FOR FORMATION OF A PEATY SOIL

Nineteenth-century writings and maps suggest that 150 

years is ample for reconstructing high marshes from muddy 

tidal flats at Willapa Bay. The writings come from one of 

the first Euro-American settlers of Willapa Bay, James G. 

Swan. Swan resided at the marshy mouth of the Querquelin 

(now Bone) River (fig. 6) in the early 1850's, barely 150 

years after the most recent great earthquake in southern 

coastal Washington changed Willapa Bay's marshes into 

tidal flats. Though tinged with boosterism, Swan's account 

leaves little doubt that the bay's tidal marshes ("prairieland") had already become high marsh in those 150 years:

"...all the rivers, large and small, run through fine prairie-land, exceedingly rich. That portion nearest the Bay is liable to be overflowed one 

or twice during the highest tides of winter, and are termed tide lands. 

This overflowing is, however, of no great detriment, although the water 

is salt, as, wherever the lands have been properly cultivated, they have 

yielded heavy crops. These prairies are all covered with grass of an 

excellent quality, making good grazing for stock, or a nutritious fodder 

when cut and made into hay." (Swan, 1857, p. 28).

Swan's account is consistent with depiction of tidal 

marshes on l:10,000-scale plane-table maps of the bay's 

margins in the 1870's. These so-called T sheets, made by 

the U.S. Coast Survey, are on file with the National Oceanic 

and Atmospheric Administration in Rockville, Maryland; in 

addition they can be viewed and reproduced from microfilm 

at this agency's library in Seattle. The maps show tidal 

marshes similar in extent to those of modern Willapa Bay. 

Symbols on the maps further imply that most of these 

marshes stood about as high as they do today. The symbols 

include a solid line that separates marsh from mudflat 

except for a few small patches of marsh surrounded by mudflat. Whereas the patches of marsh consisted of Triglochin 

maritimum that was colonizing the mudflat in the 1870's, 

the solid line connotes the landward limit of ordinary high 

tides—tides that failed to cover most of the marshes.

EVENTS FOR WHICH SOILS WERE DESTROYED

Although centuries of decomposition may suffice to 

cause total destruction of shallowly buried soils at Willapa 

Bay, even 1,000 years is not enough for such destruction if 

the soil is more than about 0.3 m below the surface of the 

postearthquake marsh or swamp.

A postearthquake interval of 700-800 years is probably long enough to destroy a buried soil within 0.2-0.3 m of 

a high-marsh or spruce-swamp surface at northeastern Willapa Bay. Such an interval, as discussed on page 85, allowed 

destruction of soil W at part of the Oyster locality and 

throughout the Redtail locality.

Slightly deeper burial, however, allowed preservation 

of soil N during a postearthquake interval of about 1,000 

years. The soil, which is barely visible where 0.2-0.3 m 

below soil S at the Jensen locality, is locally bold where 0.4 

m below soil S at the Redtail locality (fig. 13, horizontal 

coordinate 18m) and widely bold where 0.5 m below soil S 

at the Heron locality (fig. 25, horizontal coordinate 106- 

112). The interval during which soil N could have been 

destroyed by decomposition began soon after 2,400-2,800 

years ago (the time of the event marked by soil N) and 

lasted until 1,500-1,700 years ago (the event marked by soil 

S).

Such minimum depths of burial to preserve a soil are 

probably smaller than the amounts of coseismic subsidence 

that the soil represents, for two reasons:

1. Amounts of subsidence along the Niawiakum River are 

probably 1 m or more for all but soil W, as inferred from 

the paleoecologic contrast between soil W and immediately overlying mud (p. 83).

2. Burial depth should be smaller than coseismic subsidence where interseismic uplift reduces the thickness of 

postseismic mud needed to rebuild a high marsh (p. 83). 

Interseismic uplift, which within 15 years undid as much 

as one-third of the 1964 subsidence in Alaska (Brown 

and others, 1977), should be the norm at northeastern 

Willapa Bay, particularly where soils cannot settle 

because they are underlain, at shallow depth, by scarcely 

compressible Pleistocene deposits. Pleistocene deposits 

at Willapa Bay record net late Quaternary uplift that cannot have been accomplished without interseismic uplift 

(p. 9).

INTERVALS BETWEEN GREAT 

EARTHQUAKES

The intervals between great earthquakes inferred from 

the buried soils can be portrayed simplistically as centuries 

of tectonic uplift and tidal sedimentation between instants 

of tectonic subsidence. We depict such history schemati-
96 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

A Inferred history of Oyster locality

Forest

Tidal marsh

Tidal flat

Soils J, L, and N not present at 

Oyster locality because site 

was probably a tidal flat or 

channel throughout the time 

when these soils formed in 

marshes and (or) forests 

elsewhere

Soils U and W weathered in profile of soil Y. 

Spacing between soils W and Y increased through 

settlement of soil W and consequent upbuilding of 

soil Y Rate of interseisrnic emergence reduced, 

relative to Redtail locality, by settlement

Deschampsia, 

Potentilla, \ Salicomia, 

Juncus \ Deschampsia

Coseismic subsidence augmented by 

settlement and (or) slumping of thick 

body of Holocene rnud inferred to 

underlie Oyster locality

Deschampsia, 

Potentilla,

modem 

soil

B Inferred history of Redtail locality

Upland

Tidal flat

Environment of soil N Soil N weathered 

as inferred from: In Profile Pf so" s

Diatom assemblage

Lack of stumps

Soil W destroyed

and soil U weathered

in profile of soil Y

J —

modem 

soil

N

4000 3000 2000 1000 

Age range of inferred earthquake, in calendric years before A.D. 2000

Vertical scale

for stratigraphic columns;

approximate vertical scale

for inferred changes

in relative land level

I

1 rn

Bold soil 

—Mud 

Faint soil

EXPLANATION

Land surface

High-marsh herbs--In A, observed

Sitka spruce-- 

Shown for soils 

containing many 

rooted stumps

Campsite—Marked 

by rock, charcoal, 

and bone in soil Y

as fossils rooted in soils S, U, and Y; 

main genera shown in italics. In B,

N Letter name deduced from diatom assemblages 

of buried soil W Triglochin maritimum—Shown only for

deposits hi which its rhizomes are preserved

Figure 35. Diagrammatic history of land-level change, vegetation, and decomposition inferred from buried soils at Niawiakum River at Oyster 

locality (A) and Redtail locality (B). Sawtooth curve displays land-level changes associated only with sedimentation, settlement, and tectonic 

uplift and subsidence directly related to earthquakes. Diagram ignores changes in sea level and permanent crustal uplift or subsidence. Time 

scale and explanation at bottom applies to both A and B. For details see first few paragraphs in section "Intervals Between Great Earthquakes" 

(p. 95,97).
INTERVALS BETWEEN GREAT EARTHQUAKES 97

cally for the Oyster and Redtail localities (fig. 35). In this 

figure the heavy sawtooth line for each locality represents 

wetland height relative to a sea-level datum. We inferred 

wetland height from vascular-plant fossils and diatom 

assemblages (p. 83). The graph shows no overall trend in 

wetland height; we neglect eustatic and isostatic changes in 

sea level, and we further neglect fall in sea level from permanent tectonic uplift. Rather, the graph shows only the 

combined effects of the elastic component of interseismic 

uplift, sedimentation during this uplift, settlement from 

interseismic compaction, and tectonic coseismic subsidence

We assume that the combined effect of interseismic 

uplift and sedimentation is rapid at first but slow later, in 

keeping with the high initial rates of interseismic earthquake 

uplift (Brown and others, 1977; Savage and Plafker, 1991) 

and sedimentation (Ovenshine and others, 1976; BartschWinkler and Garrow, 1982) documented for parts of Alaska 

that subsided during the 1964 Alaska earthquake. Rapid 

postearthquake sedimentation is further suggested by historical evidence from Willapa Bay, as discussed in the section 

"Minimum Time for Formation of a Peaty Soil" (p. 95).

The overall sawtooth shape of the land-level curves in 

figure 35 is based partly on analogy with cyclic vertical 

deformation related to great earthquakes in southwest Japan 

(Thatcher, 1984). The gentler interseismic slope of the wetland-level line for the Oyster locality represents settlement 

from interseismic compaction of peat and mud (p. 26-27). 

We infer that such settlement has counteracted tectonic 

uplift and sedimentation more effectively at the Oyster 

locality, where soft underlying mud may be as much as 30 

m thick (p. 45), than at the Redtail locality, where buried 

soils lap onto stiff Pleistocene deposits (fig. 13).

UNCERTAINTIES IN THE ESTIMATED INTERVALS

Our estimates for recurrence intervals may contain 

errors from buried soils that do not represent great earthquakes, from earthquakes not recorded by buried soils, and 

from problems with dating the inferred earthquakes (fig. 4).

DOES EVERY BURIED SOIL REPRESENT A 

GREAT EARTHQUAKE?

As shown schematically in figure 4B and C, some buried soils may represent events other than great earthquakes. 

We evaluate this possibility in the sections "Distinction 

Between Tectonic Subsidence During Earthquakes and 

Other Ways of Producing the Buried Soils" (p. 73) and 

"Distinction Between Plate-Boundary and Upper-Plate 

Sources for the Earthquakes" (p. 80). We conclude that a 

great plate-boundary earthquake probably accounts for each 

of the buried soils in the surveyed outcrops.

This general conclusion does not apply equally to all 

soils. The case for a great plate-boundary earthquake at 

northeastern Willapa Bay is strongest for soil Y. Its shallow 

depth allows alternative explanations to be tested more

readily than for less accessible soils; it is the only soil 

shown to be widely accompanied by evidence for tsunami 

and liquefaction in southern coastal Washington; and it is 

the only soil for which rupture length has been investigated 

through high-precision radiocarbon dating. The case is 

weakest for soil J, whose great depth limits its exposure and 

dating to a few outcrops, and for soil W, owing to its faintness and widespread absence.

IS EVERY GREAT EARTHQUAKE REPRESENTED BY 

A BURIED SOIL?

We evaluate possible incompleteness from long intervals and short intervals in the section "Additional, Unrecorded Events" (p. 94). There, we conclude that the buriedsoil record at northeastern Willapa Bay is likely to include 

every great earthquake that caused widespread coseismic 

subsidence in the area during the past 3,500 years, except 

for earthquakes close to Mw 8.0 that caused less than about 

0.5 m of subsidence and occurred within the first decades 

after a preceding great earthquake in the same area.

Some pairs of earthquakes may be represented by a 

single, widely correlated soil. Such would be the case if a 

soil, as correlated widely in southern coastal Washington, 

records a pair of great earthquakes—say, one from a rupture 

mainly north of the latitude of central Willapa Bay, the other 

from a rupture to the south of that latitude. To cover this 

possibility we refer to each soil as recording an "event," 

each of which comprises one or more great earthquakes 

(p. 91).

HOW PRECISELY ARE THE EARTHQUAKES DATED?

The age range assigned to each event on the basis of 

radiocarbon dating is summarized in the section "Events 

Recorded by Buried Soils" (p. 91). Each such range probably includes the 95-percent confidence interval (p. 24). The 

range spans about 400 years for the events marked by soils 

L, N, and W; about 200 years for soils J, S, and U; and about 

20 years for soil Y. The broad range for soil W can be narrowed to 100 years if the event represented by soil W correlates with a better dated earthquake at Puget Sound (p. 94). 

The event marked by soil Y has been tree-ring dated to an 

interval less than a year and correlated with a tsunami that 

pins an earthquake to the day and approximate hour (p. 94).

To estimate recurrence intervals we compare full 

ranges of allowable age, not midpoints of ranges. Such comparison yields awkwardly broad ranges of recurrence intervals but propagates numerical uncertainties in dating.

HOW ACCURATELY ARE THE EARTHQUAKES DATED?

The accuracy of our numerical estimates of individual 

recurrence intervals can be checked, to some degree, against 

relative lengths of intervals. To infer relative lengths we use 

extent of trees and preservation of buried soils as guides to 

the duration of interseismic time. The approach is based on
98 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

two premises summarized in figure 4G and illustrated in figure 35:

1. Passage of time at Willapa Bay helps change a tidal flat 

into a spruce forest rarely inundated by tides. Such vegetational succession is likely because Willapa Bay is subject to tectonic uplift between earthquakes (p. 9). The 

rate of uplift, if as high as estimated for the past 50 years 

. by Hyndman and Wang (1995), is several times the average rate of late Holocene submergence, which is close to 

1 mm/yr at northern Puget Sound (Eronen and others, 

1987). The net result should be interseismic emergence, 

at least at sites where shallow depth to Pleistocene 

deposits permits little interseismic settlement from compaction of Holocene deposits. Such sites include the 

Jensen locality (fig. 27) and landward parts of the Redtail 

and Heron localities (figs. 13, 25).

2. Passage of time provides greater opportunity for a buried 

soil to decompose in the profile of the active soil above 

it. Such decomposition probably accounts for much of 

the scarcity of organic matter and lack of large vascularplant fossils in the faint soils (p. 25).

We thus presume that the extent of trees and decomposition of buried soils are related more to the passage of time 

than to the occurrence of unusually rapid uplift. This presumption is supported by radiocarbon dating and perhaps 

also by correlation with turbidites.

Radiocarbon estimates of recurrence intervals correspond well with the extent of trees and the decomposition of 

buried soils, as shown in figure 35. The longest intervals 

inferred from numerical ages correspond to the longest 

intervals implied by abundance of trees and decomposition 

of soils—the intervals between the events marked by soils N 

and S and between W and Y. Soils S and Y were more 

widely forested than any of the others, and they are the only 

soils widely underlain by one or more faint soils. Conversely for a short interval—the one between the events 

marked by soils S and U—vascular-plant fossils and diatoms indicate tidal inundation more frequent for soil U than 

for soils S and Y (p. 48), and underlying soil S is well preserved. During what may be the shortest interval—the time 

between the events marked by soils U and W—soil W 

merely reached the transition between low marsh and high 

marsh (p. 48). Underlying soil U is rarely well preserved, 

but this poor preservation probably resulted from prolonged 

weathering high in soil Y.

Maximum extent of trees and decomposition of soils at 

Willapa Bay may correlate with maximum burrowing into 

turbidites on the floor of Cascadia Channel. This correlation 

is discussed in the section "Comparison with Intervals Estimated from Deep-Sea Turbdites" (p. 100).

INDIVIDUAL INTERVALS

Six individual recurrence intervals can be estimated 

from the seven inferred events—earthquake or earthquake 

series—marked by soils J, L, N, S, U, W, and Y.

INTERVALS BETWEEN EVENTS MARKED BY SOILS 

J, L, AND N

The two intervals among these events together span 

somewhere between 540 and 1,100 years; their average 

length is between 270 and 550 years.

The first of the intervals was probably longer than the 

second. Such a difference cannot be discerned from numerical ages, owing to large uncertainties for the events marked 

by soils L and N (p. 91). However, roots at the Redtail, Pool, 

Heron, and Jensen localities imply that spruce may have 

been more widespread on soil L than on soil N (figs. 13, 24, 

25, 27)—evidence for greater interseismic emergence of 

soil L. Greater emergence of soil L is also suggested by 

microscopic fossils in probable correlatives of soils L and N 

at the Johns River (Shennan and others, 1996, their horizons 

V and IV). It appears to conflict, however, with the presence 

of an upland diatom assemblage in soil N at the Redtail 

locality (p. 23).

INTERVAL BETWEEN EVENTS MARKED BY SOILS N AND S

This interval, the longest one that we infer, spans 

somewhere between 700 and 1,300 years. In keeping with 

such a duration, soil S has fossils indicative of prolonged 

interseismic emergence: abundant spruce roots at many 

localities and a nearly upland assemblage of diatoms documented at the Redtail locality. Moreover, underlying soil N 

is commonly faint, as should be the case if it weathered for a 

long time in the profile of soil S.

INTERVAL BETWEEN EVENTS MARKED BY SOILS S AND U

A much shorter interval, of somewhere between 150 

and 570 years, separated the events marked by soils S and 

U. The interval was long enough for rebuilding of a high 

marsh at the Oyster and Redtail localities (figs. 15, 20). 

However, at the Oyster locality it was not long enough for 

Salicornia virginica to be replaced by the fresher-water 

assemblage of vascular plants found rooted there in soils S 

and Y (p. 48).

A relatively short interval between soils S and U can 

also be inferred from microscopic fossils at the Johns River. 

Shennan and others (1996), citing assemblages of diatoms, 

pollen, and foraminifera at the Johns River, inferred that a 

soil U (their horizon II) represents a lower level in the intertidal zone than do soils S and Y (their horizons I and HI).

INTERVAL BETWEEN EVENTS MARKED BY SOILS U AND W

An interval no longer than 450 years separates the 

events marked by soils U and W. The interval spans somewhere between 30 and 350 years if the event marked by soil 

W coincided, as speculated on page 94, with faulting in the 

Puget Sound region 1,000-1,100 years ago.

Fossils in soil W at the Oyster locality imply that the 

interval lasted no more than a century or two. The fossils 

imply that the event marked by soil U produced a tidal flat
INTERVALS BETWEEN GREAT EARTHQUAKES 99

4000 2000

Age range of inferred earthquake, 

in calendric years before A.D. 2000

EXPLANATION

Age from Willapa Bay

Age of possibly correlative 

earthquake on Seattle fault

Figure 36. Inferred history of great 

earthquakes at northeastern Willapa Bay in 

the past 3,500 years. Age ranges described 

in section "Events Recorded by Buried 

Soils" (p. 91).

that shoaled into a Triglochin maritimum low marsh but did 

not become a high marsh before the event marked by soil W 

(p. 48). By contrast, the event marked by soil Y produced a 

tidal flat that probably became a high marsh within 150 

years (see section "Minimum Time for Formation of a Peaty 

Soil" on p. 95).

INTERVAL BETWEEN EVENTS MARKED BY SOILS W AND Y

The final recorded interval lasted as little as 600 years 

and as much as about 1,000 years. If the event marked by 

soil W occurred 1,000-1,100 years ago, the interval lasted 

somewhere between 700 and 800 years.

A long interval before the event marked by soil Y is 

consistent with several kinds of evidence for emergence that 

could have been accomplished through prolonged interseismic uplift: spruce rooted in the soil in upstream and forestedge areas where spruce is absent today (as near the Pool 

locality and at the Redtail and Heron localities); western 

redcedar similarly rooted downstream of its modern limits 

(Atwater, 1992, his fig. 5); diatom assemblages in the soil 

that suggest conditions transitional to upland (fig. 15); 

and remains of fishing camps high in the profile of the soil 

(Cole and others, 1996; our p. 30, 69, and figs. 13, 14D 29, 

30A, B}.

AVERAGE INTERVAL

The six recurrence intervals since 3,500 years ago 

have an average length of 500-540 years (fig. 36). This 

range includes one-sixth of 3,020-3,200 years, which is the 

difference in ages between the event marked by soil J 

(3,320-3,500 years ago) and the event marked by soil Y 

(300 years ago).

The average is sensitive to uncertainties in the counting 

of earthquakes (see sections "Does Every Buried Soil Represent a Great Earthquake?" and "Is Every Great Earthquake Represented by a Buried Soil?" on p. 97). The 

average increases to 600-645 years if soil W does not represent a great plate-boundary earthquake. Conversely, the 

average decreases to 430-460 years if one such earthquake 

left no lasting soil.

VARIABILITY AMONG INDIVIDUAL INTERVALS

Individual intervals between great plate-boundary 

earthquakes (or earthquake series) recorded by buried soils 

at northeastern Willapa Bay have been far from uniform 

during the past 3,500 years. Intervals between recorded 

events range from about one to three centuries (the interval 

between events marked by soils U and W) to about a millenium (the interval between events marked by soils N and S).

This variability might be reduced, but probably not 

much, by a great earthquake that was nowhere recorded by a 

lasting buried soil. Such an unrecorded earthquake would 

probably occur within a century of the preceding great 

earthquake (see section "Additional, Unrecorded Events", p. 

94). Consequently, the unrecorded earthquake need not 

greatly shorten a long recurrence interval, and it would 

increase variability among recurrence intervals if it followed an especially short recurrence interval.

The event marked by soil W illustrates this problem 

with using unrecorded events to reduce the inferred variability among recurrence intervals. Soil W represents an event 

that is widely unrecorded, probably owing to decomposition 

of a shallowly buried soil during a long interval before the 

event marked by soil Y. Without soil W the time between 

the events marked by soils U and Y could be mistaken for a 

single interval about 1,000 years long. Soil W divides this 

time into two shorter intervals. But such division increases 

variability among intervals because the interval between 

events marked by soils U and W lasted several centuries less 

than the average interval.
100 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

COMPARISON WITH INTERVALS ESTIMATED

FROM BURIED SOILS AT OTHER COASTAL

ESTUARIES

The average interval of 500-540 years at Willapa Bay 

overlaps with all the ranges estimated from buried soils at 

other coastal estuaries with one exception (fig. 3). This 

exception is at Humboldt Bay, where the average interval 

for the past 1,450-1,750 years ranges from about 300 to 500 

years. The possible explanations include:

1. Some of the inferred earthquakes at Humboldt Bay were 

limited to faults in the North America plate (Clarke and 

Carver, 1992).

2. Great earthquakes happen more often on the California 

part of the plate boundary than elsewhere on the plate 

boundary.

3. In the past 1,750 years, great Cascadia earthquakes have 

occurred more often than usual, regardless of location 

along the subduction zone. For example, the average of 

the three full recurrence intervals in the past 1,750 years 

at Willapa Bay is between 400 and 480 years (intervals 

between the events marked by soils S and U, soils U and 

W, and soils W and Y).

The exceptionally long interval between events marked 

by soils N and S may correlate with the longest recurrence 

interval inferred from buried soils at estuaries in northern 

Oregon. This Oregon interval, perhaps 1,000 years long and 

centered about 2,000 years ago, separates events 5 and 6 of 

Darienzo and Peterson (1995).

COMPARISON WITH INTERVALS ESTIMATED 

FROM DEEP-SEA TURBIDITES

A great plate-boundary earthquake accompanied by 

tectonic subsidence at Willapa Bay is well situated to send a 

turbidity current down Cascadia Channel (fig. 37). Though 

200 km off the Oregon coast, Holocene turbidites in Cascadia Channel were derived primarily from canyon heads a 

mere 50 km west of Willapa Bay (Griggs and Kulm, 1970, 

p. 1381-1382). These canyon heads probably shook hard 

during great Holocene earthquakes on the southern Washington part of the Cascadia plate boundary: the canyon 

heads are located directly above the probable area of shallow rupture on the Cascadia plate boundary (Hyndman and 

Wang, 1995).

Given this connection, recurrence intervals recorded by 

buried soils at Willapa Bay ought to resemble recurrence 

intervals recorded by turbidites in Cascadia Channel. 

Adams (1990, p. 579) noted such a resemblance in average 

recurrence interval. Citing preliminary studies of buried 

soils, he estimated an average of 625 years for the last five 

to eight times of coseismic subsidence at Willapa Bay and 

likened it to his average of 590 years for the last 13 turbidity 

currents down lower Cascadia channel. Though the average 

interval at Willapa Bay now appears somewhat shorter than

625 years, it is probably compatible with the turbidite average, as we show in the next section "Average Interval."

We then consider evidence for and against one-for-one 

correspondence between recurrence intervals for coseismic 

subsidence at Willapa Bay and recurrence intervals for turbidity currents in Cascadia channel. Adams (1990) interpreted similar thicknesses of pelagic clay between turbidites 

as evidence for earthquake-recurrence intervals less varied 

than those we now infer for Willapa Bay. However, successive turbidites differ from one another in abundance of burrows, and these differences suggest varied recurrence 

intervals that individually correlate with recurrence intervals 

at Willapa Bay.

AVERAGE INTERVAL

The average interval of 500-540 years at Willapa Bay 

is slightly less than the average that Adams (1990) inferred 

from turbidites in Cascadia Channel and other deep-sea 

channels off the Washington and Oregon coast (figs. 1, 3). 

Adams's 590-year average is plotted in figure 3 as a range 

of 550-630 years to express uncertainties in age of the earliest of the 13 great earthquakes that he inferred (see section 

"Turbidites in Deep-Sea Channels Off Washington and Oregon," p. 7).

Several possibilities could explain why the Willapa 

Bay average is slightly less than the offshore turbidite average. Two arise from the difference in period of record— 

about 7,500 years for the turbidites ascribed to earthquakes 

by Adams (1990), versus 3,500 years for well-dated buried 

soils at Willapa Bay (fig. 36). Perhaps great Cascadia earthquakes have occurred more often in the past 3,500 years 

than in the preceding 4,000 years. Or perhaps Cascadia 

earthquakes were recorded by turbidites more completely in 

the past 3,500 years than in the preceding 4,000 years. 

Either of these possibilities is consistent with a postMazama increase in frequency of turbidity currents. As 

noted by Griggs and Kulm (1970, p. 1375-1376), average 

pre-Mazama intervals between Holocene turbidites were 

about 750-1,500 years in the same areas where postMazama intervals are less than 600 years.

Another possibility is that the turbidite record of great 

earthquakes in Cascadia Channel is incomplete because 

some earthquake-induced turbidites failed to travel beyond 

its tributaries. Average intervals between turbidity currents 

are about 100 years shorter in tributary channels; conversely, they exceed 1,000 years in the lowermost reaches of 

Cascadia Channel (Griggs and Kulm, 1970, p. 1375-1376), 

beyond the southern limit of the channel shown in figure 37. 

Cores from tributary channels contain as many as 16 turbidites less than about 7,500 years old (Adams, 1990, his fig. 

1). If these tributary-channel sequences record 14-16 great 

earthquakes, the average recurrence intervals overlap with 

the Willapa Bay range of 500-540 years.

Finally, the buried-soil record at Willapa Bay may 

include an earthquake too small to produce any of the 13
INTERVALS BETWEEN GREAT EARTHQUAKES 101

Turbidite age

inferred

from thickness

of pelagic layers

in core K1

Turbidite

stratigraphy

in core 27

Age of possibly

correlative event

marked by

buried soil

at Willapa Bay

1000

EXPLANATION

Seaward edge of subduction zone 

Deep-sea channel

I '.

26 Core

Turbidite 

Pelagic layer

Burrows 

in turbidite

J Letter name

of buried soil 

4 Turbidite number

4645 ± 190 Age of detrital

organic matter, 

~~ in 14CyrB.P.

Each line between 

stratigraphic column 

and time scale refers to 

turbidite above the line

1 m Vertical scale 

in core 27

Figure 37. Turbidite stratigraphy of lower Cascadia Channel 

and ages inferred for individual turbidites. Burrows in 

turbidites suggest variability among recurrence intervals and 

correlation with recurrence intervals inferred from buried soils 

at Willapa Bay (p. 102). Stratigraphic column from Griggs and 

others (1969, their fig. 6). Ages based on pelagic-layer

4000 l^mm^mi ^mmm

thickness are from Adams (1990, his table 3); all were 

estimated from pelagic layers in core Kl except for the age of 

the youngest turbidite, whose age Adams estimated from 

pelagic layers in six other cores (his table 2). Ages of events 

marked by buried soils at Willapa Bay are from figures 32 and 

36.
102 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

turbidites in the turbidite record of Griggs and Kulm (1970) 

and Adams (1990). Soil W might represent such an earthquake because fossils associated with the soil imply less 

than 1 m of coseismic subsidence (p. 83).

VARIABILITY AMONG INTERVALS

INTERVALS IMPLIED BY DEPOSITS BETWEEN TURBIDITES

Turbidites of Cascadia Channel are separated from one 

another by layers of gray clay—termed pelagic (Griggs and 

others, 1969; Adams, 1990) or hemipelagic (Griggs and 

Kulm, 1970)—each of which probably represents at least 

the early part of the time between successive turbidity currents. The thickness of these pelagic layers is fairly uniform: 

on average, at least three-quarters of the pelagic layers in a 

single core differ in thickness by no more than a factor of 

two (Griggs and Kulm, 1970, p. 1375). Griggs and Kulm 

therefore inferred that the "time intervals between flows 

were of a similar duration."

Adams (1990, p. 578) used thickness of pelagic layers 

as evidence that only one of the intervals in the past 7,500 

years is likely to be shorter than 300 years or longer than 

900 years. He deduced this range from a standard deviation 

of 170 years, calculated from average rates of pelagic sedimentation and from the standard deviation of pelagic thickness in three cores selected for their pelagic layers of fairly 

uniform thickness.

Such statistics do not necessarily express uncertainties 

in measuring pelagic thickness in cores and in accounting 

for thickness lost to erosion. These uncertainties may be 

large (Griggs and others, 1969, p. 167). However, Adams 

(1990, p. 577) viewed them as tending to add to the real 

variability among recurrence intervals. He inferred this variability to be small (p. 576), in accord with his presumption 

that earthquakes on a plate-boundary recur at regular intervals and resemble one another in size (p. 578).

INTERVALS IMPLIED BY BURROWS IN TURBIDITES

Griggs and others (1969) showed that the abundance of 

burrows varies among successive turbidites of Cascadia 

Channel. They found such variability in three cores along 65 

km of channel—cores 23, 26, and 27 in figure 37. They 

used the variability to correlate individual turbidites among 

these cores. A sketch of core 27, reproduced in figure 37, 

illustrates the correlative variability: "The second, fifth and 

seventh graded layers [turbidites 2, 5, and 7 in fig.37] contain abundant and distinct burrows to depths of 25 cm or 

greater. In intervening flows, however, they are much less 

abundant and are restricted primarily to the upper 5 or 10 

cm" (Griggs and others, 1969, p. 165).

The abundance of burrows in these turbidites may 

increase with the time between turbidites. Griggs and others 

(1969, p. 167) contrasted this hypothesis with the uniform 

thickness of pelagic layers. But they also cited grooves

eroded into many of the pelagic layers as evidence that preserved pelagic thickness represents only part of the time 

between successive turbidity currents. Further, Griggs and 

his coworkers described the turbidites as too similar to one 

another for variable abundance of burrows to be explained 

by differences in texture and organic content of the burrowed sediment.

If burrows in the turbidites became more abundant with 

time, recurrence intervals implied by the turbidite sequence 

of Griggs and others (1969) may correlate, one for one, with 

intervals inferred from buried soils at Willapa Bay (fig. 37). 

By this correlation, turbidites 5 and 2 were heavily burrowed in lower Cascadia Channel during the exceptionally 

long intervals when soils S and Y, respectively, formed at 

Willapa Bay. Conversely, light burrowing of turbidites 4 and 

3 coincided with the short intervals preceding the events 

marked by soils U and W. Heavy burrowing of turbidite 7 

would have occurred during the interval between events J 

and L, whose duration is poorly known but may be long on 

the basis of fossils (p. 98).

These correlations between Cascadia Channel and Willapa Bay are consistent with the sole available radiocarbon 

age from the sequence of correlated turbidites. The age, 

reported by Griggs and others (1969, their fig. 6), was measured on organic silt containing plant fragments probably 

recycled from older deposits. The age shows that turbidite 8 

formed no earlier than 4,645±190 14C yr B.P., or no more 

than about 5,000-6,000 calendric years ago. If generated by 

the event marked by soil H, turbidite 8 should be 3,500- 

4,500 calendric years old (fig. 32). Alternatively, turbidite 8 

correlates with an earlier buried soil (Adams, 1990, p. 579). 

Peat from this soil, which underlies soil H at the South Fork 

locality (fig. 6), gave an age of 4,290±80 14C yr B.P. (Atwater, 1992, his table 2).

Our correlations between Cascadia Channel and Willapa Bay are inconsistent with many of the ages and individual recurrence intervals estimated by Adams (1990, p. 581) 

from core Kl (fig. 37). One of the largest inconsistencies 

involves the youngest complete recurrence interval. Adams, 

citing a pelagic layer 19-25 mm thick, estimated that this 

interval lasted about 300-500 years. By contrast, we estimate that the youngest complete recurrence interval at Willapa Bay lasted 600-1,000 years—the interval between the 

events marked by soils W (900-1,300 years ago) and Y (300 

years ago) (p. 99; fig. 36). We speculate that the pelagic 

layer was much thicker than 19-25 mm before it was eroded 

by the current that deposited the youngest turbidite at the 

site of core Kl.

John Adams (written commun., 1996) suggested to us 

that highly varied recurrence intervals might produce fairly 

uniform pelagic thickness if the depth of erosion of a 

pelagic layer increases with the size of the turbidity current, 

which in turn may increase with the length of the recurrence 

interval. Adams also pointed out, however, that some 

pelagic layers should have been removed altogether had 

deep erosion occurred widely in Cascadia Channel. Perhaps 

the typical pelagic layer accumulated more rapidly at first,
SPECULATIONS ABOUT THE NEXT GREAT EARTHQUAKE 103

while the water column retained particles from the preceding turbidite. In that case, the lower part of the layer might 

be compact enough to resist erosion, however short or long 

the interval during which the rest of the layer accumulates.

PATTERN OF RECURRENCE

The likely variability among recurrence intervals of the 

past 3,500 years shows that great earthquakes at Willapa 

Bay do not happen on a regular schedule. The least regularity is implied by the three successive intervals between soils 

N and S, S and U, and U and W. Whereas the first of these 

intervals spanned 700-1,300 years, the other two together 

spanned no more than 800 years. The interval between the 

events marked by soils J and L may also have been much 

longer than the succeeding interval, between the events 

marked by soils L and N (fig. 36).

Such highly irregular recurrence of earthquakes on one 

part of a fault has been inferred for the southern San 

Andreas fault (Sieh and others, 1989), for a thrust fault in 

Algeria (Swan, 1987), and for normal faults near the Dead 

Sea (Marco and others, 1996) but not for great plate-boundary earthquakes at subduction zones other than Cascadia. 

For most subduction zones the reported record of great 

earthquakes spans only one or a few earthquake cycles 

(Thatcher, 1989), where "cycle" denotes the interval 

between "events" as defined on page 91. The main exception is southwest Japan, where historical records since A.D. 

684 suggest cycle lengths of 90-264 years but no strong 

clustering of the cycles (Ando, 1975). Identification and 

dating of earthquakes are too uncertain to show whether 

recurrence intervals vary greatly in the region of the 1964 

Alaska earthquake (Plafker and others, 1992).

SPECULATIONS ABOUT THE NEXT

GREAT EARTHQUAKE IN SOUTHERN

COASTAL WASHINGTON

The next great earthquake at northeastern Willapa Bay 

need not be as large as the largest Cascadia earthquakes of 

the past 3,500 years. This speculation is based on several 

kinds of evidence that soils S and Y represent exceptionally 

great earthquakes:

Exceptional subsidence during the events marked by 

soils S and Y.—Because they were more widely forested 

than other soils of the past 3,500 years in southern coastal 

Washington, soils S and Y had to subside farther to become 

tidal flats (figs. 15,20,35).

A long rupture during the event marked by soil Y.—A 

single Mw 9 earthquake best explains radiocarbon and treering correlation with a tsunami that damaged Japan in January 1700 (p. 94).

Long recurrence intervals.—The events marked by 

soils S and Y followed recurrence intervals long enough to 

generate earthquakes of Mw 9 on a strongly coupled Cascadia plate boundary, under conservative assumptions stated 

in the section "Tradeoff Between Size and Frequency" (p. 

90).

Alternatively, the event marked by soil Y failed to 

expend a large fraction of the plate convergence accumulated in the preceding centuries, in which case the stored 

convergence could allow the next great Cascadia earthquake 

to be as large as any Cascadia earthquake of the past 3,500 

years. Such storage probably contributed to the size of the 

Mw 9.5 1960 Chile earthquake. Plate-boundary slip during 

this earthquake widely exceeded 20 m, according to interpretation of land-level changes that accompanied the earthquake (Plafker and Savage, 1970; Barrientos and Ward, 

1990). However, the preceding great earthquake occurred 

only 123 years earlier (Lomnitz, 1970). This interval is too 

short for accumulation of 20 m of slip at 0.1 m/yr, the average rate of plate convergence in the region of the 1960 

earthquake (DeMets and others, 1990).

REFERENCES CITED

Adams, J., 1990, Paleoseismicity of the Cascadia subduction zone: 

evidence from turbidites off the Oregon-Washington margin: 

Tectonics, v. 9, p. 569-583.

————Great earthquakes recorded by turbidites off the OregonWashington coast, in Rogers, A.M., Walsh, T.J., Kockelman, 

W.J., and Priest, G.R., eds., Assessing earthquake hazards 

and reducing risk in the Pacific Northwest: U.S. Geological 

Survey Professional Paper 1560, p. 147-158.

Ando, M., 1975, Source mechanism and tectonic significance of 

historic earthquakes along the Nankai Trough, Japan: Tectonophysics, v. 27, p. 119-140.

Ando, M., and Balazs, E.I., 1979, Geodetic evidence for aseismic 

subduction of the Juan de Fuca plate: Journal of Geophysical 

Research, v. 84, p. 3023-3028.

Atwater, B.F., 1987, Evidence for great Holocene earthquakes 

along the outer coast of Washington State: Science, v. 236, p. 

942-944.

————1992, Geologic evidence for earthquakes during the past 

2000 years along the Copalis River, southern coastal Washington: Journal of Geophysical Research, v. 97, p. 1901- 

1919.

————compiler, 1994, Geology of liquefaction features about 300 

years old along the lower Columbia River at Marsh, Brush, 

Price, Hunting, and Wallace Islands, Oregon and Washington: U.S. Geological Survey Open-File Report 94-209, 64 p. 

-1996, Coastal evidence for great earthquakes in western

Washington, in Rogers, A.M., Walsh, T.J., Kockelman, W.J., 

and Priest, G.R., eds., Assessing earthquake hazards and 

reducing risk in the Pacific Northwest: U.S. Geological Survey Professional Paper 1560, p. 77-90.

Atwater, B.F., and Hemphill-Haley, E., 1996, Preliminary estimates of recurrence intervals for great earthquakes of the past 

3500 years at northeastern Willapa Bay, Washington: U.S. 

Geological Survey Open-File Report 96-001, 88 p.

Atwater, B.F., Hull, A.G., and Bevis, K.A., 1987, Aperiodic 

Holocene recurrence of widespread, probably coseismic subsidence in southwestern Washington [abstract]: Eos, v. 70, p. 

1330-1331.
104 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Atwater, B.F., and Moore, A.L., 1992, A tsunami 1000 years ago in 

Puget Sound, Washington: Science, v. 258, p. 1614-1617.

Atwater, B.F., Nelson, A.R., Clague, J.J., Carver, G.A., Yamaguchi, D.K., Bobrowsky, P.T., Bourgeois, J., Darienzo, M.E., 

Grant, W.C., Hemphill-Haley, E., Kelsey, H.M., Jacoby, G.C., 

Nishenko, S.P., Palmer, S.P., Peterson, C.D., and Reinhart, 

M.A., 1995, Summary of coastal geologic evidence for past 

great earthquakes at the Cascadia subduction zone: Earthquake Spectra, v. 11, p. 1-18.

Atwater, B.F., Stuiver, M., and Yamaguchi, D.K., 1991, Radiocarbon test of earthquake magnitude at the Cascadia subduction 

zone: Nature, v. 353, p. 156-158.

Atwater, B.F., and Yamaguchi, D.K, 1991, Sudden, probably 

coseismic submergence of Holocene trees and grass in coastal 

Washington State: Geology, v. 19, p. 706-709.

Barbour, M.G., and Davis, C.B., 1970, Salt tolerance of five California salt marsh plants: American Midland Naturalist, v. 84, 

p. 262-265.

Barrick, R.C., 1975, Hydrodynamics of Grays Harbor estuary, in 

Maintenance dredging and the environment of Grays Harbor, 

Washington: Seattle, U.S. Army Corps of Engineers, Appendix A.

Barrientos, S.E., and Ward, S.N., 1990, The 1960 Chile earthquake: inversion for slip distribution from surface deformation: Geophysical Journal International, v. 103, p. 589-598.

Bartsch-Winkler, S.R., and Garrow, H.C., 1982, Depositional systems approaching maturity at Portage Flats, in Coonrad, W.L., 

ed., The United States Geological Survey in Alaska: accomplishments during 1980: U.S. Geological Survey Circular 

844, p. 115-117.

Behre, K.-E., Menke, B., and Streif, H., 1979, The Quaternary 

geological development of the German part of the North Sea, 

in Oele, E., Schttenhelm, R.T.E., and Wiggers, A.J., eds., The 

Quaternary History of the North Sea: Acta Universitatis 

Upsaliensis, Symposia Universitatis Upsaliensis Annun Quingentesimum Celebrantis, v. 2, p. 85-113.

Benson, B.E., Grimm, K.A., and Clague, J.J., 1997, Tsunami 

deposits beneath tidal marshes on northwestern Vancouver 

Island, British Colombia: Quaternary Research (in press).

Berryman, K.R., 1993, Age, height, and deformation of Holocene 

marine terraces at Mania Peninsula, Hikurangi subduction 

margin, New Zealand: Tectonics, v. 12, p. 1347-1364.

Berryman, K.R., Ota, Y, and Hull, A.G., 1989, Holocene paleoseismicity in the fold and thrust belts of the Hikurangi subduction zone, eastern North Island, New Zealand: 

Tectonophysics, v.163, p. 185-195.

Brown, L.D., Reilinger, R.E., Holdahl, S.R., and Balazs, E.I. S 

1977, Postseismic crustal uplift near Anchorage, Alaska: 

Journal of Geophysical Research, v. 82, p. 3369-3378.

Bucknam, R.C., Hemphill-Haley, E., and Leopold, E.B., 1992, 

Abrupt uplift within the past 1700 years at southern Puget 

Sound, Washington: Science, v. 258, p. 1611-1614.

Byrne, D.E., Sykes, L.R., and Davis, D.M., 1992, Great thrust 

earthquakes and aseismic slip along the plate boundary of the 

Makran subduction zone: Journal of Geophysical Research, 

v.97, p. 449-478,

Cahoon, D.R., Reed, D.J., and Day, J.W, Jr., 1995, Estimating 

shallow subsidence in microtidal salt marshes of the southeastern United States: Kaye and Barghoom revisited: Marine 

Geology,v. 128, p. 1-10.

Carver, G.A., Stuiver, M., and Atwater, B.F., 1992, Radiocarbon 

ages of earthquake-killed trees at Humboldt Bay, California 

[abstract]: Eos, v. 73, no. 43, p. 398.

Clague, J.J., and Bobrowsky, P.T., 1994, Tsunami deposits beneath 

tidal marshes on Vancouver Island, British Columbia: Geological Society of America Bulletin, v. 106, p. 1293-1303.

Clarke, S.H., Jr., and Carver, G.A., 1992, Late Holocene tectonics 

and paleoseismicity, southern Cascadia subduction zone: Science, v. 255, p. 188-192.

Clifton, H.E., 1983, Discrimination between subtidal and intertidal 

facies in Pleistocene deposits, Willapa Bay, Washington: 

Journal of Sedimentary Petrology, v. 53, p. 353-369.

Cole, S.C., Atwater, B.F, McCutcheon, P.T., Stein, J.K., and 

Hemphill-Haley, E., 1996, Earthquake-induced burial of 

archaeological sites along the southern Washington coast 

about A.D. 1700: Geoarchaeology, v. 11, p. 165-177.

Cowan, D.S., and Potter, C.J., 1986, Juan de Fuca spreading ridge 

to Montana thrust belt: Geological Society of America Continent-ocean transect B3.

Darienzo, M.E., and Peterson, C.D., 1990, Episodic tectonic subsidence of late Holocene salt marshes, northern Oregon coast, 

central Cascadia margin, U.S.A.: Tectonics, v. 9, p. 1-22.

————1995, Magnitude and frequency of subduction-zone earthquakes along the Oregon coast in the past 3,000 years: Oregon Geology, v. 57, p. 3-12.

Darienzo, M.E., Peterson, C.D., and Clough, C., 1994, Stratigraphic evidence for great subduction-zone earthquakes at 

four estuaries in northern Oregon: Journal of Coastal 

Research, v. 10, p. 850-876.

DeMets, C., Gordon, R., Argus, D.F, and Stein, S., 1990, Current 

plate motions: Geophysical Journal International, v. 101, p. 

425^78.

Denys, L., 1984, Diatom analysis of coastal deposits: methodological aspects: Bulletin van de Belgische Vereniging voor Geologic, v. 93, p. 291-295.

Devoy, R.J.N., 1979, Flandrian sea level changes and vegetational 

history of the lower Thames estuary: Philosophical Transactions of the Royal Society of London, B, v. 285, p. 355-407.

Dragert, H., and Hyndman, R.D., 1995, Continuous GPS monitoring of elastic strain in the northern Cascadia subduction zone: 

Geophysical Research Letters, v. 22, p. 755-758.

Dragert, H., Hyndman, R.D., Rogers, G.C., and Wang, K., 1994, 

Current deformation and the width of the seismogenic zone of 

the northern Cascadia subduction zone: Journal of Geophysical Research, v. 99, p. 653-668.

Eronen, M., Kankainen, T, and Tsukada, M., 1987, Late Holocene 

sea-level record in a core from the Puget lowland, Washington: Quaternary Research, v. 27, p. 147-159.

Fletcher, C.H., III, Pizzuto, J.E., John, S., and van Pelt, J.E., 1993a, 

Sea-level rise acceleration and the drowning of the Delaware 

Bay coast at 1.8 ka: Geology, v. 21, p. 121-124.

Fletcher, C.H., III, van Pelt, J.E., Brush, G.S., and Sherman, J., 

1993b, High-resolution stratigraphic framework, Holocene 

sea-level and climate history, and evolution of a tidal wetland: 

Palaeogeography, Palaeoclimatology, Palaeoecology, v. 102, 

p. 177-213.

Geomatrix Consultants, 1995, Seismic design mapping: prepared 

for Oregon Department of Transportation, Salem, Oregon, 

personal services contract no. 11688, project no. 2442, various pagination.
REFERENCES CITED 105

Geist, E., and Shoichi, Y., 1996, Source parameters controlling the 

generation and propagation of potential local tsunamis along 

the Cascadia margin: Natural Hazards, v. 13, p. 151-177.

Geyh, M.A., 1980, Holocene sea-level history: case study of the 

statistical evaluation of 14C dates: Radiocarbon, v. 22, p. 695- 

704.

Goldfinger, C, Kulm, L.D., Yeats, R.S., Appelgate, B., MacKay, 

M., and Moore, G.F., 1992a, Transverse structural trends 

along the Oregon convergent margin: implications for Cascadia earthquake potential: Geology, v. 20, p. 141-144.

Goldfinger, C., Kulm, L.D., Yeats, R.S., Mitchell, C., Weldon, R., 

Peterson, C.D., Darienzo, M.E., Grant, W.C., and Priest, 

G.R., 1992b, Neotectonic map of the Oregon continental margin and adjacent abyssal plain: Oregon Department of Geology and Mineral Industries Open-File Report 0-92-4, scale 

1:500,000.

Goldfinger, C., Kulm, L.D., Yeats, R.S., McNeill, L., and Hummon, C., 1997, Oblique strike-slip faulting of the central Cascadia submarine forearc: Journal of Geophysical Research, v. 

102, p. 8217-8243.

Goldfinger, C., McNeill, L.C., Kulm, L.D., and Yeats, R.S., 1996, 

Width of the seismogenic plate boundary in Cascadia: structural indicators of strong and weak coupling [abstract]: Geological Society of America, Abstracts with Programs, v. 28, 

no. 5, p. 69.

Goodbred, S.L., Jr., and Hine, A.C., 1995, Coastal storm deposition: salt-marsh response to a severe extratropical storm, 

March 1993, west-central Florida: Geology, v. 23, p. 679- 

682.

Grant, W.C., and Minor, R., 1991, Paleoseismic evidence and prehistoric occupation associated with late Holocene sudden submergence, northern Oregon coast [abstract]: Eos, v. 72, no. 

44, p. 313.

Griggs, G.B., Carey, A.G., and Kulm, L.D., 1969, Deep-sea sedimentation and sediment-fauna interaction in Cascadia Channel and on Cascadia Abyssal Plain: Deep-Sea Research, v. 16, 

p. 157-170.

Griggs, G.B., and Kulm, L.D., 1970, Sedimentation in Cascadia 

deep-sea channel: Geological Society of America Bulletin, v. 

81,p.1361-1384.

Guilbault, J.-P, Clague, J.J., and Lapointe, M., 1995, Amount of 

subsidence during a late Holocene earthquake—evidence 

from fossil tidal marsh foraminifera at Vancouver Island, west 

coast of Canada: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 118, p. 49-71.

Hanks, T.C., and Kanamori, H., 1979, A moment magnitude scale: 

Journal of Geophysical Research, v. 84, p. 2348-2350.

Heaton, T.H., and Hartzell, S.H., 1986, Source characteristics of 

hypothetical subduction earthquakes in the northwestern 

United States: Bulletin of the Seismological Society of America, v. 76, p. 675-708.

————1987, Earthquake hazards on the Cascadia subduction 

zone: Science, v. 236, p. 162-168.

Heki, K., Miyazaki, S., and Tsuji, H., 1997, Silent fault slip following an interplate thrust earthquake at the Japan Trench: 

Nature, v. 386, p. 595-598.

Hemphill-Haley, E., 1993, Occurrences of Recent and Holocene 

intertidal diatoms (Bacillariophyta) in northern Willapa Bay, 

Washington: U.S. Geological Survey Open-File Report 93- 

284,94 p.

—1995a, Intertidal diatoms from Willapa Bay, Washington: 

application to studies of small sea-level changes: Northwest 

Science, v. 69, p. 29-45.

—1995b, Diatom evidence for earthquake-induced subsidence and tsunami 300 yr ago in southern coastal Washington: 

Geological Society of America Bulletin, v. 107, p. 367-378. 

-1996, Diatoms as an aid in identifying late-Holocene tsunami deposits: The Holocene, v. 6, p. 439-448.

Hemond, H.F., and Chen, D.G., 1990, Air entry in salt marsh sediments: Soil Science, v. 150, p. 459-468.

Hitchcock, C.L., and Cronquist, A., 1973, Flora of the Pacific 

Northwest: Seattle, University of Washington Press, 730 p.

Holdahl, S.R., Faucher, F, and Dragert, H., 1989, Contemporary 

vertical crustal motion in the Pacific Northwest, in Cohen, 

S.C., and Vanicek, P., eds., Slow Deformation and Transmission of Stress in the Earth: American Geophysical Union 

Geophysical Monograph 49, p. 17-29.

Hull, A.G., 1987, Buried lowland soils from Willapa Bay, southwest Washington: further evidence for recurrence of large 

earthquakes during the past 5000 years [abstract]: Eos, v. 68, 

p. 1469.

Huntley, D.J., and Clague, J.J., 1996, Optical dating of tsunamilaid sands: Quaternary Research, v. 46, p. 127-140.

Hyndman, R.D., and Wang, K., 1993, Thermal constraints on the 

zone of major thrust earthquake failure: the Cascadia subduction zone: Journal of Geophysical Research, v. 98, p. 2039- 

2060.

————1995, The rupture zone of Cascadia great earthquakes from 

current deformation and thermal regime: Journal of Geophysical Research, v. 100, p. 22133-22154.

Hyndman, R.D., and Weichert, D.H., 1983, Seismicity and rates of 

relative motion on plate boundaries of western North America: Geophysical Journal of the Royal Astronomical Society, 

v. 72, p. 59-82.

Jacoby, G.C., Bunker, D.E., and Benson, B.E., 1997, Tree-ring evidence for an A.D. 1700 Cascadia earthquake in Washington 

and northern Oregon: Geology, v. 25, p. 999-1002.

Jacoby, G.C., Carver, G., and Wagner, W, 1995, Trees and herbs 

killed by an earthquake -300 yr ago at Humboldt Bay, California: Geology, v. 23, p. 77-80.

Jacoby, G.C., Williams, PL., and Buckley, B.M., 1992, Tree ring 

correlation between prehistoric landslides and abrupt tectonic 

events in Seattle, Washington: Science, v. 258, p. 1621-1623.

Jelgersma, S., 1961, Holocene sea level changes in the Netherlands: Mededelingen van de Geologische Stichting, Serie C, 

VI, v. 7, p. 1-100.

Jelgersma, S., Oele, E., and Wiggers, A.J., 1979, Depositional history and coastal development in the Netherlands and the adjacent North Sea since the Eemian, in Oele, E., Schttenhelm, 

R.T.E., and Wiggers, A.J., eds., The Quaternary History of the 

North Sea: Acta Universitatis Upsaliensis, Symposia Universitatis Upsaliensis Annun Quingentesimum Celebrantis, v. 2, 

p. 115-142.

Jennings, A.E., and Nelson, A.R., 1992, Foraminiferal assemblage 

zones in Oregon tidal marshes—relation to marsh floral zones 

and sea level: Journal of Foraminiferal Research, v. 22, p. 13- 

29.

Johnson, D.W., 1925, The New England-Acadian Shoreline: New 

York, John Wiley, 608 p.

Johnson, J.W, 1973, Characteristics and behavior of Pacific Coast 

tidal inlets: Journal of the Waterways, Harbors, and Coastal
106 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Engineering Division, Proceedings of the American Society 

of Civil Engineers, v. 99, p. 325-339.

Kanamori, H., 1977, The energy release in great earthquakes: Journal of Geophysical Research, v. 82, p. 2981-2987.

Kanamori, H., and McNally, K.C., 1982, Variable rupture mode of 

the subduction zone along the Ecuador-Colombia coast: Bulletin of the Seismological Society of America, v. 72, p. 1241- 

1253.

Karlin, R.E., and Abella, S.E.B., 1992, Paleoearthquakes in the 

Puget Sound region recorded in sediments of Lake Washington, U.S.A.: Science, v. 258, p. 1617-1620.

Kelsey, H.M., 1990, Late Quaternary deformation of marine terraces on the Cascadia subduction zone near Cape Blanco, 

Oregon: Tectonics, v. 9, p. 983-1014.

Kelsey, H.M., Engebretson, D.C., Mitchell, C.E., and Ticknor, 

R.L., 1994, Topographic form of the Coast Ranges of the Cascadia Margin in relation to coastal uplift rates and plate subduction: Journal of Geophysical Research, v. 99, p. 12245- 

12255.

Kelsey, H.M., Witter, R.C., and Hemphill-Haley, E., 1996, Record 

of plate boundary earthquakes near Cape Blanco, Oregon 

[abstract]: Geological Society of America Abstracts with Programs, v. 28, no. 5, p. 80.

Kelsey, H.M, Witter, R.C., and Polenz, M., 1993, Cascadia paleoseismic record derived from late Holocene fluvial and lake 

sediments, Sixes River valley, Cape Blanco, south coastal 

Oregon [abstract]: Eos, v. 74, no. 43, p. 199.

Kunze, L.M., and Cornelius, L.C., 1982, Baseline inventory of 

rare, threatened and endangered plant species/communities 

along Washington's Pacific coast: Washington Department of 

Ecology, National Oceanic and Atmospheric Administration 

Coastal Zone Management Grant G68-029.

Lomnitz, C., 1970, Major earthquakes and tsunamis in Chile during the period 1535 to 1955: Geologische Rundschau, Band 

59, p. 938-960.

Mall, R.E., 1969, Soil-water-salt relationships of waterfowl food 

plants in the Suisun marsh of California: California Department of Fish and Game, Wildlife Bulletin 1,59 p.

Marco, S., Stein, M., Agnon, A., and Ron, H., 1996, Long-term 

earthquake clustering: a 50,000-year paleoseismic record in 

the Dead Sea Graben: Journal of Geophysical Research, v. 

101, p. 6179-6191.

Mathewes, R.W, and Clague, J.J., 1994, Detection of large prehistoric earthquakes in the Pacific Northwest by microfossil 

analysis: Science, v. 264, p. 688-691.

McCaffrey, R., and Goldfmger, C., 1995, Forearc deformation and 

great subduction earthquakes: implications for Cascadia offshore earthquake potential: Science, v. 267, p. 856-859.

McCulloch, D.S., and Bonilla, M.G., 1970, Effects of the earthquake of March 27,1964, on The Alaska Railroad: U.S. Geological Survey Professional Paper 545-D, 161 p.

Mclnelly, G.W, and Kelsey, H.M.,1990, Late Quaternary tectonic 

deformation in the Cape Arago-Bandon region of coastal 

Oregon as deduced from wave-cut platforms: Journal of Geophysical Research, v. 95, p. 6699-6714.

McNeill, L.C., Goldfmger, C., Kulm, L.D., and Yeats, R.S., 1994, 

Tectonics of the Washington continental margin, Cascadia 

subduction zone [abstract]: Geological Society of America 

Abstracts with Programs, Annual Meeting 1994, p. A-523.

Meyers, R.A., Smith, D.G., Jol, H.M., and Peterson, C.D., 1996, 

Evidence for eight great earthquake-subsidence events

detected with ground-penetrating radar, Willapa barrier, 

Washington: Geology, v. 24, p. 99-102.

Meyerson, A.L., 1972, Pollen and paleosalinity analyses from a 

Holocene tidal marsh sequence, Cape May County, New Jersey: Marine Geology, v. 12, p. 335-357.

Miles, M.M., Wiggins, W.D., Ruppert, G.P., Smith, R.R., Reed, 

L.L., Hubbard, L.E., and Courts, M.L., 1994, Water resources 

data Washington water year 1993: U.S. Geological Survey 

Water-Data Report WA-93-1,408 p.

Minor, R., and Grant, W.C., 1996, Earthquake-induced subsidence 

and burial of late Holocene archaeological sites, northern 

Oregon coast: American Antiquities, v. 61, p. 772-781.

Mitchell, C.E., Vincent, P., and Weldon, R.J., 1994, Present-day 

vertical deformation of the Cascadia margin, Pacific Northwest, United States: Journal of Geophysical Research, v. 99, 

p. 12257-12277.

Morris, J.T., and Whiting, G.J., 1986, Emission of gaseous carbon 

dioxide from salt-marsh sediments and its relation to other 

carbon losses: Estuaries, v. 9, p. 1-19.

Mudge, B.F., 1862, The salt marsh formations of Lynn: Proceedings of the Essex Institute, v. 2, p. 117-119.

Mumford, T.F., Jr., Peyton, P., Sayce, K.R., and Harbell, S. (eds.). 

1990. Spartina workshop record. Seattle, Washington, University of Washington, Washington Sea Grant Program, 

WSG-WO 91-01, 73 p.

Nelson, A.R., 1992a, Holocene tidal-marsh stratigraphy in southcentral Oregon evidence for localized sudden submergence in 

the Cascadia subduction zone, in Fletcher, C.P, and Wehmiller, J.F., eds., Quaternary coasts of the United States: 

marine and lacustrine systems: Society for Sedimentary Geology Special Publication 48, p. 287-301.

———1992b, Discordant 14C ages from buried tidal-marsh soils 

in the Cascadia subduction zone, southern Oregon coast: 

Quaternary Research, v. 38, p. 75-90.

Nelson, A.R., and Atwater, B.F., 1993, Radiocarbon ages of earthquake-killed plants along the Cascadia subduction zone 

[abstract]: Eos, v. 74, no. 43, p. 199-200.

Nelson, A.R., Atwater, B.F., Bobrowsky, P.T., Bradley, L.-A., 

Clague, J.J., Carver, G.A., Darienzo, M.E., Grant, W.C., 

Krueger, H.W, Sparks, R., Stafford, T.W., and Stuiver, M., 

1995, Radiocarbon evidence for extensive plate-boundary 

rupture about 300 years ago at the Cascadia subduction zone: 

Nature, v. 378, p. 371-374.

Nelson, A.R., Hemphill-Haley, E., and Witter, R.C., 1996c, A 

7500-year lake record of Cascadia tsunamis in southern 

coastal Oregon [abstract]: Geological Society of America, 

Abstracts with Programs, v. 28, no. 5, p. 95.

Nelson, A.R., Jennings, A.E., and Kashima, K., 1996a, Holocene 

intertidal stratigraphy, microfossils, rapid submergence, and 

earthquake recurrence at Coos Bay, southern coastal Oregon, 

USA: Geological Society of America Bulletin, v. 108, p. 141- 

154.

Nelson, A.R., and Kashima, K., 1993, Diatom zonation in southern 

Oregon tidal marshes relative to vascular plants, foraminifera, 

and sea level: Journal of Coastal Research, v. 9, p. 673-697.

Nelson, A.R., and Personius, S.F., 1996, The potential for great 

earthquakes in Oregon and Washington—an overview of 

recent geologic studies and their bearing on segmentation of 

Holocene ruptures, central Cascadia subduction zone, in Rogers, A.M., Walsh, T.J., Kockelman, W.J., and Priest, G.R., 

eds., Assessing earthquake hazards and reducing risk in the
REFERENCES CITED 107

Pacific Northwest: U.S. Geological Survey Professional 

Paper 1560, p. 91-114.

Nelson, A.R., Shennan, I., and Long, A.J., 1996b, Identifying 

coseismic subsidence in tidal-wetland stratigraphic sequences 

at the Cascadia subduction zone of western North America: 

Journal of Geophysical Research, v. 101, p. 6115-6135.

Obermeier, S.F., 1995, Preliminary limits for the strength of shaking for the Columbia River valley and the southern half of 

coastal Washington for the Cascadia subduction zone earthquake of about 300 years ago: U.S. Geological Survey OpenFile Report 94-589.

Ota, Y, and Umitsu, M., 1995, Stratigraphic, radiocarbon and diatom indicators of sudden submergence along the Nesselle 

[sic] River mouth, Washington, USA: Journal of Geography, 

v. 104, p. 107-112 (in Japanese).

Ovenshine, A.T., Lawson, D.E., and Bartsch-Winkler, S.R., 1976, 

The Placer River Silt—an intertidal deposit caused by the 

1964 Alaska earthquake: Journal of Research of the U.S.Geological Survey, v. 4, p. 151-162.

Pacheco, J.F., Sykes, L.R., and Scholz, C.H., 1993, Nature of seismic coupling along simple plate boundaries of the subduction 

type: Journal of Geophysical Research, v. 98, p. 14133- 

14159.

Palmer, S.P., and Lingley, W.S., Jr., 1989, An assessment of the oil 

and gas potential of the Washington outer continental shelf: 

Washington Sea Grant Program, University of Washington, 

Seattle, 83 p.

Peterson, C.D., and Darienzo, M.E., 1989, Potential evidence of 

subduction zone tectonics from stacked peat horizons in late 

Pleistocene coastal terraces of the northern Cascadia margin 

[abstract]: Eos, v. 70, p. 1331.

————1996, Discrimination of climatic, oceanic, and tectonic 

mechanisms of cyclic marsh burial, Alsea Bay, Oregon, in 

Rogers, A.M., Walsh, T.J., Kockelman, W.J., and Priest, G.R., 

eds., Assessing earthquake hazards and reducing risk in the 

Pacific Northwest: U.S. Geological Survey Professional 

Paper 1560, p. 115-146.

Peterson, C.D., Scheidegger, K., Komar, P., and Neim, W, 1984, 

Sediment composition and hydrography in six high-gradient 

estuaries of the northwestern United States: Journal of Sedimentary Petrology, v. 54, p. 86-97.

Pezzopane, S.K., and Weldon, R.J., II, 1993, Tectonic role of 

active faulting in central Oregon: Tectonics, v. 12, p. 1140- 

1169.

Plafker, G., 1969, Tectonics of the March 27, 1964 Alaska earthquake: U.S. Geological Survey Professional Paper 543-1, 

74 p.

Plafker, G., and Savage, J.C., 1970, Mechanism of the Chilean 

earthquakes of May 21 and 22, 1960: Geological Society of 

America Bulletin, v. 81, p. 1001-1030.

Plafker, G., Gilpin, L.M., and Lahr, J.C., 1994, Neotectonic map of 

Alaska, in Plafker, George, and Berg, H.C., eds., The Geology 

of Alaska: Geological Society of America, The Geology of 

North America, v. Gl, Plate 12, scale 1:2,500,000 with 3 

tables and text.

Plafker, G., and Kachadoorian, R., 1966, Geologic effects of the 

March 1964 earthquake and associated seismic sea waves on 

Kodiak and nearby island, Alaska: U.S. Geological Survey 

Professional Paper 543-D, 46 p.

Plafker, G., Lajoie, K.R., and Rubin, M., 1992, Determining recurrence intervals of great subduction zone earthquakes in southern Alaska by radiocarbon dating, in Taylor, R.E., Long, A.,

and Kra, R.S., eds., Radiocarbon after four decades, an interdisciplinary perspective: New York, Springer-Verlag, p. 436- 

453.

Redfield, A.C., 1972, Development of a New England salt marsh: 

Ecological Monographs, v. 42, p. 201-237.

Reid, C., 1913, Submerged forests: Cambridge, University Press, 

129 p.

Reinhart, M.A., and Bourgeois, J., 1987, Distribution of anomalous sand at Willapa Bay, Washington—evidence for largescale landward-directed processes [abstract]: Eos, v. 68, p. 

1469.

————1989, Tsunami favored over storm or seiche for sand 

deposit overlying buried Holocene peat, Willapa Bay, WA 

[abstract]: Eos, v. 70, p. 1331.

Rogers, G.C., 1988, An assessment of the megathrust earthquake 

potential of the Cascadia subduction zone: Canadian Journal 

of Earth Sciences, v. 25, p. 844-852.

Satake, K., Shimazaki, K., Tsuji, Y, and Ueda, K., 1996, Time and 

size of a giant earthquake in Cascadia inferred from Japanese 

tsunami record of January 1700: Nature, v. 379, p. 246-249.

Savage, J.C., Lisowski, M., and Prescott, W.H., 1991, Strain accumulation in western Washington: Journal of Geophysical 

Research, v. 96, p. 14493-14507.

Savage, J.C., and Plafker, G., 1991, Tide gage measurements of 

uplift along the south coast of Alaska: Journal of Geophysical 

Research, v. 96, p. 4325^336.

Sayce, K., 1988, Introduced cordgrass, Spartina altemiflora 

Loisel. in saltmarshes and tidelands of Willapa Bay, Washington: flwaco, Washington, Willapa National Wildlife Refuge, 

U.S. Fish and Wildlife Service contract FWSI-87058(TS), 

70 p.

Scholz, C.H., and Campos, J., 1995, On the mechanism of seismic 

decoupling and back arc spreading at subduction zones: Journal of Geophysical Research, v. 100, p. 22103-22115.

Scott, E.M., Aitchison, T.C., Harkness, D.D., Cook, G.T., and Baxter, M.S., 1990, An overview of all three stages of the international radiocarbon intercomparison: Radiocarbon, v. 32, p. 

309-319.

Shennan, I., 1986, Flandrian sea-level changes in the Fenland. I: 

The geographical setting and evidence of relative sea-level 

changes: Journal of Quaternary Science, v. 1, p. 119-154.

Shennan, I., Long, A.J., Rutherford, M.M., Green, P.M., Innes, 

J.B., Lloyd, J.M., Zong, Y, and Walker, K.J, 1996, Tidal 

marsh stratigraphy, sea-level change and large earthquakes, I: 

a 5000 year record in Washington, USA: Quaternary Science 

Reviews, v. 15, p. 1023-1059.

Shennan, I., Tooley, M.J., Davis, M.J., and Haggart, B.A., 1983, 

Analysis and interpretation of Holocene sea-level data: 

Nature, v. 302, p. 404-406.

Sherrod, B.L., Rollins, H.B., and Kennedy, S.K., 1989, Subrecent 

intertidal diatoms from St. Catherine Island, Georgia: taphonomic implications: Journal of Coastal Research, v. 5, p. 665- 

677.

Sieh, K., Stuiver, M., and Brillinger, D., 1989, A more precise 

chronology of earthquakes produced by the San Andreas fault 

in southern California: Journal of Geophysical Research, v. 

94, p. 603-623.

Snavely, P.D., Jr., and Wells, R.E., 1996, Cenozoic evolution of the 

continental margin of Oregon and Washington, in Rogers, 

A.M., Walsh, T.J., Kockelman, W.J., and Priest, G.R., eds., 

Assessing earthquake hazards and reducing risk in the Pacific
108 RECURRENCE INTERVALS FOR GREAT EARTHQUAKES AT WILLAPA BAY, WASHINGTON

Northwest: U.S. Geological Survey Professional Paper 1560, 

p. 161-182.

Streif, H., 1972, The results of stratigraphical and facial investigations in the coastal Holocene of Woltzeten/Ostfriesland, Germany: Geologiska Foreningens i Stockholm Forhandlingar, v. 

94, p. 280-299.

————1982, Review of chapter 1: The occurrence and significance of peat in the Holocene deposits of the German North 

Sea coast, in de Bakker, H., and van den Berg, M.W., eds., 

Proceedings of the symposium on peat lands below sea level: 

Wageningen, The Netherlands, International Institute for 

Land Reclamation and Improvement, p. 31-41.

Stuiver, M, and Becker, B., 1986, High-precision decadal calibration of the radiocarbon time scale, AD 1950-6000 BC: 

Radiocarbon, v. 28, p. 863-910.

————1993, High-precision decadal calibration of the radiocarbon time scale, AD 1950-6000 BC: Radiocarbon, v. 35, p. 

35-67.

Stuiver, M., and Pearson, G.W., 1986, High-precision calibration 

of the radiocarbon time scale AD 1950-500 BC: Radiocarbon, v. 28, p. 805-838.

Stuiver, M., and Reimer, P.J., 1993, Extended 14C data base and 

revised CALIB 3.0 14C age calibration program: Radiocarbon, v. 35, p. 215-230.

Swan, F.H., 1987, Temporal clustering of paleoseismic events on 

the Oued Fodda fault, Algeria, in Crone, A.J., and Omdahl, 

E.M., eds., Directions in Paleoseismology: U.S. Geological 

Survey Open-File Report 87-673, p. 239-248.

Swan, J.G., 1857, The Northwest Coast or, Three Years' Residence 

in Washington Territory: Harper & Brothers, 435 p. (reprinted 

in 1972 by University of Washington Press, Seattle).

Thatcher, W, 1984, The earthquake deformation cycle at the Nankai Trough, southwest Japan: Journal of Geophysical 

Research, v. 89, p. 3087-3101.

————1989, Earthquake recurrence and risk assessment in circum-Pacific seismic gaps: Nature, v. 341, p. 432-434.

Torresan, M.E., 1987, Review and comparison of hydrophotometer 

and pipette method in analysis of fine grained sediment: U.S. 

Geological Survey Open-File Report 87-514, p. 1-38.

van de Plassche, O., 1991, Late Holocene sea-level fluctuations on 

the shore of Connecticut inferred from transgressive and 

regressive overlap boundaries in salt-marsh deposits: Journal 

of Coastal Research, special issue 11, p. 159-179.

Varekamp, J.C., Thomas, E., and Van de Plassche, O., 1992, Relative sea-level rise and climate change over the last 1500 years,

in Wezel, F.-C, ed., Global Change Special Issue: Terra Nova, 

v. 4, p. 293-304.

Wagner, H.C., 1967a, Preliminary geologic map of the Raymond 

quadrangle, Pacific County, Washington: U.S. Geological 

Survey Open-File Report 67-265, scale 1:62,500.

————1967b, Preliminary geologic map of the South Bend quadrangle, Pacific County, Washington: U.S. Geological Survey 

Open-File Report 67-266, scale 1:62,500.

Walsh, T.J., Combellick, R.A., and Black, G.L., 1995, Liquefaction 

features from a subduction zone earthquake: preserved examples from the 1964 Alaska earthquake: Washington Division 

of Geology and Earth Resources, Report of Investigations 32, 

80 p.

Walsh, T.J., Korosec, M.A., Phillips, W.M., Logan, R.L., and 

Schasse, H.W, 1987, Geologic map of Washington—southwest quadrant: Washington Division of Geology and Earth 

Resources Geologic Map GM-34, scale 1:250,000.

Ward, O.K., and Wilson, S.R., 1978, Procedures for comparing and 

combining radiocarbon age determinations: a critique: 

Archaeometry, v. 20, p. 19-31.

Weichert, D.H., and Adams, J., 1995, Non-parametric estimates of 

Cascadia earthquake hazard [abstract]: Seismological 

Research Letters, v. 66, no. 2, p. 49.

Weinmann, F, Boule, M., Brunner, K., Malek, J., and Voshino, V., 

1984, Wetland plants of the Pacific Northwest: Seattle, Washington, U.S. Army Corps of Engineers, 85 p. and others, 

1984.

Wells, D.R., and Coppersmith, K.J., 1994, New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement: Bulletin of the 

Seismological Society of America, v. 84, p. 974-1002.

Wells, R.E., 1989, Geologic map of the Cape DisappointmentNaselle River area, Pacific and Wahkiakum Counties, Washington: U.S. Geological Survey Miscellaneous Investigations 

Map 1-1832, scale 1:62,500.

Wilson, D.S., 1993, Confidence intervals for motion and deformation of the Juan de Fuca plate: Journal of Geophysical 

Research, v. 98, p. 16053-16071.

Yamaguchi, O.K., Atwater, B.F., Bunker, D.E., Benson, B.E., and 

Reid, M.S., 1997, Tree-ring dating the 1700 Cascadia earthquake: Nature, v. 389, p. 922-923.

Yamaguchi, O.K., Woodhouse, C.A., and Reid, M.S., 1989, Treering, evidence for synchronous rapid submergence of the 

southwestern Washington coast about 300 years ago 

[abstract]: Eos, v. 70, p. 1332.

U.S. GOVERNMENT PRINTING OFFICE: 1997 - 673-048 / 20088 REGION NO. 8


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

BOOKS AND OTHER PUBLICATIONS

Professional Papers report scientific data and interpretations 

of lasting scientific interest that cover all facets of USGS investigations and research.

Bulletins contain significant data and interpretations that are 

of lasting scientific interest but are generally more limited in scope 

or geographic coverage than Professional Papers.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of 

wide interest to professional geologists, hydrologists, and engineers. The series covers investigations in all phases of hydrology, 

including hydrogeology, availability of water, quality of water, and 

use of water.

Circulars are reports of programmatic or scientific information of an ephemeral nature; many present important scientific 

information of wide popular interest. Circulars are distributed at no 

cost to the public.

Fact Sheets communicate a wide variety of timely information on USGS programs, projects, and research. They commonly 

address issues of public interest. Fact Sheets generally are two or 

four pages long and are distributed at no cost to the public.

Reports in the Digital Data Series (DDS) distribute large 

amounts of data through digital media, including compact 

disc-read-only memory (CD-ROM). They are high-quality, interpretive publications designed as self-contained packages for 

viewing and interpreting data and typically contain data sets, 

software to view the data, and explanatory text.

Water-Resources Investigations Reports are papers of an 

interpretive nature made available to the public outside the formal 

USGS publications series. Copies are produced on request (unlike 

formal USGS publications) and are also available for public inspection at depositories indicated in USGS catalogs.

Open-File Reports can consist of basic data, preliminary 

reports, and a wide range of scientific documents on USGS investigations. Open-File Reports are designed for fast release and are 

available for public consultation at depositories.

MAPS

Geologic Quadrangle Maps (GQ's) are multicolor geologic 

maps on topographic bases in 7.5- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, 

surficial, or engineering geology. Maps generally include brief 

texts; some maps include structure and columnar sections only.

Geophysical Investigations Maps (GP's) are on topographic 

or planimetric bases at various scales. They show results of geophysical investigations using gravity, magnetic, seismic, or radioactivity surveys, which provide data on subsurface structures that 

are of economic or geologic significance.

Miscellaneous Investigations Series Maps or Geologic 

Investigations Series (I's) are on planimetric or topographic bases 

at various scales; they present a wide variety of format and subject 

matter. The series also includes 7.5-minute quadrangle photogeologic maps on planimetric bases and planetary maps.

INFORMATION PERIODICALS

Metal Industry Indicators (Mil's) is a free monthly newsletter that analyzes and forecasts the economic health of five metal 

industries with composite leading and coincident indexes: primary 

metals, steel, copper, primary and secondary aluminum, and aluminum mill products.

Mineral Industry Surveys (MIS's) are free periodic statistical and economic reports designed to provide timely statistical data 

on production, distribution, stocks, and consumption of significant 

mineral commodities. The surveys are issued monthly, quarterly, 

annually, or at other regular intervals, depending on the need for 

current data. The MIS's are published by commodity as well as by 

State. A series of international MIS's is also available.

Published on an annual basis, Mineral Commodity Summaries is the earliest Government publication to furnish estimates 

covering nonfuel mineral industry data. Data sheets contain information on the domestic industry structure. Government programs, 

tariffs, and 5-year salient statistics for more than 90 individual 

minerals and materials.

The Minerals Yearbook discusses the performance of the 

worldwide minerals and materials industry during a calendar year, 

and it provides background information to assist in interpreting 

that performance. The Minerals Yearbook consists of three volumes. Volume I, Metals and Minerals, contains chapters about 

virtually all metallic and industrial mineral commodities important 

to the U.S. economy. Volume II, Area Reports: Domestic, contains 

a chapter on the minerals industry of each of the 50 States and 

Puerto Rico and the Administered Islands. Volume III, Area Reports: International, is published as four separate reports. These 

reports collectively contain the latest available mineral data on 

more than 190 foreign countries and discuss the importance of 

minerals to the economies of these nations and the United States.

PERMANENT CATALOGS

"Publications of the U.S. Geological Survey, 1879-1961" 

and "Publications of the U.S. Geological Survey, 1962-1970" are

available in paperback book form and as a set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" is

available in paperback book form (two volumes, publications 

listing and index) and as a set of microfiche.

Annual supplements for 1982, 1983, 1984, 1985, 1986, and 

subsequent years are available in paperback book form.