U.S. GEOLOGICAL SURVEY CIRCULAR 1067 

Program for Earthquake 

Hazards Assessment 

in the Pacific Northwest 
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Program for Earthquake 

Hazards Assessment 

in the Pacific Northwest 

By KAYE M. SHEDLOCK and CRAIG S. WEAVER 

U.S. GEOLOGICAL SURVEY CIRCULAR 1067 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing, 1991. 

Second printing, 1993. 

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. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Shedlock, Kaye M. 

Program for earthquake hazards assessment in the Pacific Northwest I by 

Kaye M. Shedlock and Craig S. Weaver. 

p. cm.-(U.S. Geological Survey circular; 1 067) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.412:CIRC 1067 

1. Earthquake hazard analysis-Northwest, Pacific. 2. National Earthquake 

Hazards Reduction Program (U.S.) I. Weaver, Craig S. II. Title. Ill. Series. 

QE535.2.N6S54 1991 

551.2'2'09795-dc20 91-20675 

CIP 
CONTENTS 

Summary 1 

Introduction 1 

Geologic and tectonic settings 3 

Earthquake hazards and risk assessment in the Pacific Northwest 5 

Earthquake sources 6 

Crustal earthquakes 6 

Intraplate earthquakes 11 

Interplate earthquakes 11 

Structure of the crust and lithosphere 12 

Deformation record 14 

Geodetics 15 

Ground failure 17 

Strong ground-motion and structural response 18 

Framework for NEHRP studies 20 

Necessary program components 20 

Monitoring seismicity and deformation in the Pacific Northwest 20 

Tectonic framework studies 22 

Improve seismic hazard and risk assessments 22 

Cooperative hazard mitigation studies 25 

Relation to the USGS Earthquake Hazards Reduction Program Z5 

Conclusions 25 

Acknowledgments 26 

References cited 26 

Appendix-Pacific Northwest Science Plan Workshop participants 29 

FIGURES 

1-6. Maps showing: 

1. Boundaries and physiographic provinces of the Pacific Northwest 

region 2 

2. Locations of earthquakes in the Pacific Northwest with magnitudes 

estimated to be greater than 6, 1870-1990 4 

3. Distribution of crustal earthquake activity, 1980-1990 7 

4. Distribution of earthquakes deeper than 30 km 7 

5. Distribution of Modified Mercalli Intensities, 1872 earthquake 8 

6. Seismicity in eastern Washington, 1970-1987, magnitude 1.0 and 

larger 9 

7. Diagram of inferred crustal structure, eastern Washington 10 

8-10. Maps showing: 

8. Major damaging earthquakes in Oregon and Washington 11 

9. Distribution of Modified Mercalli Intensities, 1964 Prince William 

Sound, Alaska, earthquake 12 

10. Contours of crustal thickness, Pacific Northwest 13 

11. Photograph and map showing evidence for subsidence and areas of marsh 

stratigraphy studies 15 

Contents Ill 
12-14. Maps showing: 

12. Average crustal strain accumulation along the Cascadia subduction 

zone 16 

13. Locations of GPS strain networks along the Cascadia subduction 

zone 17 

14. Extent of the Osceola debris flow 18 

15. Photograph of ground-failure damage in Olympia, Washington, induced by 

the 1965 earthquake 19 

16-19. Maps showing: 

16. Regional seismic network stations in the Pacific Northwest 21 

17. Strong-motion accelerometer stations operating in the Pacific 

Northwest 21 

18. Proposed locations of crustal structure profiles 23 

19. Areas of special hazards or special studies 24 

IV Contents 
Program for Earthquake Hazards Assessment 

in the Pacific Northwest 

By Kaye M. Shedlock and Craig S. Weaver 

Summary 

The Pacific Northwest (including the part of California 

north of Cape Mendocino, Oregon, Washington, and part of 

Briti~ Columbia) may be capable of producing a great 

subduction-zone earthquake of magnitude 8-9. While no 

earthquake of this size has historically occurred here, there is 

mounting geologic, geomorphologic, and geophysical evidence suggesting that such an occurrence is a strong possibility. Large crustal and intraplate earthquakes (magnitude 

= 6-7) have historically occurred in the Pacific Northwest and 

are expected to continue to occur. Furthermore, the level of 

instrumentally recorded seismicity, although not uniform 

across the region, is one of the highest in the United States. 

Potential losses in the Pacific Northwest, as a result of 

either a great subduction-zone earthquake or crustal and (or) 

intraplate earthquakes near population centers, are expected 

to be large for a number of reasons. First, the population has 

grown considerably over the last few decades and continues to 

increase rapidly. Second, most buildings, bridges, lifelines, 

and transportation facilities were not designed or built to 

withstand large earthquakes. Third, the population centers 

(and, hence, the areas containing the highest concentrations of 

buildings) in the Pacific Northwest tend to be located in basins 

or in low-lying coastal areas-these are areas of poorly 

consolidated sedimentary rocks that will likely amplify ground 

motions or that are susceptible to local ground failure. A great 

subduction-zone earthquake in this area could cost about 

$8.6 billion and result in the loss of hundreds of lives. A crustal 

or intraplate earthquake of magnitude 6-7 near Seattle could 

cause losses in the range of tens of millions of dollars 

(S.T. Algermissen, written commun., 1990). 

Losses due to earthquakes can be reduced by means of 

effective mitigation measures, but these mitigation measures 

cannot be implemented without reliable characterization of 

earthquake hazards. The likely types, sizes, frequencies, 

locations, and effects of future earthquakes must be well 

understood. This program for earthquake hazard assessment in 

the Pacific Northwest is designed to address the necessary 

questions and to develop strategies needed to both characterize earthquake hazards and to quantify the associated risks. 

Manuscript approved for publication March 21, 1991. 

INTRODUCTION 

The National Earthquake Hazards Reduction P-ogram 

(NEHRP) was authorized by the Earthquake Hazards 

Reduction Act of 1977 to provide information abo·1t both 

earthquake hazards and risk to concerned citizens, public 

officials, and planners in order to ( 1) minimize loss of life 

and property that can result from ea11hquakes and 

earthquake-related hazards, and (2) to minimize economic 

hardship and social disruption that can result from the 

occurrence of an earthquake and its related hazards" 

As a participating agency in the NEHRP, tt'~ U.S. 

Geological Survey (USGS) is charged with the leadership 

role in conducting fundamental research studies designed to 

delineate, evaluate, and document earthquake hazards in the 

United States. Thus, the USGS is the lead agency for studies 

of earthquake potential; earthquake prediction; earthquake 

information and data services; earthquake hazards a'ld risk 

assessments; strong ground-motion data collection, dissemination, and estimation; and technical assistance in developing methods to implement loss-reduction measures. The 

NEHRP specifies seismically active geographic regions 

within the United States for program emphasis. This document describes the current plans and priorities for future 

research of this USGS program in one of the regions of 

current emphasis: the Pacific Northwest. 

Within the context of the current NEHRP program, 

the Pacific Northwest is understood to include northern 

California, Oregon, and Washington, with particular emphasis on the subduction-zone system that reaches from Cape 

Mendocino in northern California northward alo'lg the 

Pacific coast to northern Vancouver Island (fig. 1 ). Our 

study area includes the Cascade Range and adjacent regions 

of the backarc provinces, particularly the Pasco bafin and 

adjacent parts of the Columbia Plateau and southwestern 

British Columbia (fig. 1 ). 

The Pacific Northwest is the only location in the 

United States where crustal and lithospheric evolution may 

be traced from an active mid-ocean ridge to a stable 

continental platform in a distance of only 1,200 km (fig. I). 

Thus, the USGS program must include sc:~ntific 

lntroductio"' 
132°0' 

51 oo·r~~E~~~~i'f'i'"=..-==~~===-====-~=-====-=r=r==--.......,...~--l 

Juan de Fuca 

Plate 

0 200 

I I I 

Kilometers 

(!) 

South 

Gorda <!> 

Plate 

~ 

Figure 1. Map showing plate boundaries and physiographic provinces of the Pacific Northwest region. Octagons denote 

epicenters of earthquakes of magni~ude 4 or larger that are listed in the National Oceanic and Atmospheric Administration 

(NOAA) catalog through 1985. Location of the 1949 south Puget Sound earthquake, the largest instrumentally recorded event 

in Washington or Oregon, is indicated by the largest octagon and is labeled "1949." Open triangles represent Quaternary 

stratovolcanoes: M, Meagher Mountain; B, Mount Baker; G, Glacier Peak; R, Mount Rainier; S, Mount St. He'ens; A, Mount 

Adams; H, Mount Hood;), Mount Jefferson; N, Newberry Volcano; ML, Medicine Lake Volcano; Sh, Mount Sbasta; L, lassen 

Peak; CL, Crater Lake. From Ludwin and others (in press). 

investigations covering a broad spectrum of earthquake 

hazards and a wide range of tectonic conditions. Within the 

last decade, the population living in the Pacific Northwest 

has experienced a number of significant geologic hazards: 

Mount St. Helens continues an eruptive sequence (accompanied by seismic activity) that began in 1980; the largest 

crustal earthquake in Oregon or Washington since 1937 

(magnitude 5.5) occurred north of Mount St. Helens in 

1981; and the largest (possibly earthquake-related) landslide 

on Mount Rainier since 1963, involving about 1-2 million 

cubic meters, occurred during the summer of 1989. The 

possibilities of large to great earthquakes in this region are 

most important in USGS geologic hazard studies. Some 

investigators (e.g., Heaton and Hartzell, 19~6) have suggested that an earthquake as large as magnitu1e 9 (similar to 

the 1960 Chilean or 1964 Alaskan events) could occur along 

the coasts of northern California, Oregon, Washington, and 

southern British Columbia. The staggering size of the 

source regions for such great earthquakes (fig. 1) requires 

that our hazard assessments be on an equa11:; large scale. 

2 Program for Earthquake Hazards Assessment in the Pacific Northwest 
Because the USGS recognizes the implications to 

society should it be proven that the Pacific Northwest is 

capable of generating great earthquakes, it is contributing to 

a comprehensive earthquake hazards reduction program 

that, by necessity, has a heavy emphasis on earthquake 

hazards assessments. Such a program needs to encourage 

major scientific advances in understanding the nature of the 

entire subduction margin and forearc region and should 

offer the means to coordinate these new perspectives into 

hazard studies throughout the region. By the end of the 

program outlined here, scientists from the USGS, other 

government agencies, and universities will have collected 

and analyzed a broad suite of data that is an essential 

component in our effort to assess potential earthquake 

hazards associated with the subduction interface. 

Within this large geographic area, most regional-scale 

USGS/NEHRP studies will relate primarily to sources of 

and potential for earthquakes--detailed studies of hazards 

and associated risks will be confined largely to the urban 

areas surrounding Puget Sound, and in the Pasco basin, the 

Willamette Lowland, and coastal areas. We include British 

Columbia in the study area both because the subduction 

zone continues northward to central Vancouver Island and 

because we believe that our program should take advantage 

of joint research opportunities with the Geological Survey 

of Canada. 

Because of the complex geologic setting of the 

Pacific Northwest, scientific objectives central to providing 

the tectonic framework necessary for a rational assessment 

of eatthquake hazards must be broadly stated. The central 

goal of the proposed program is to answer these questions: 

Can the Cascadia subduction zone produce great 

thrust-zone earthquakes? 

What are the expected distributions; source characteristics; and effects of shallow, crustal earthquakes in 

the Pacific Northwest? 

What are the principal seismic hazards in the Pacific 

Northwest? 

What combination of short-term (i.e., relating to the 

repeat of a known earthquake) and long-term (i.e., 

relating to a large, crustal earthquake in an urban area 

or a thrust-zone earthquake) risk-reduction measures 

should be pursued in the Pacific Northwest? 

GEOLOGIC AND TECTONIC SETTINGS 

Washington, Oregon, and northern California have 

features typical of convergent boundaries in other parts of 

the world. Offshore, on the continental shelf, deformed 

Quaternary sediments are found where converging plates 

meet (Barnard, 1978). A convergence rate of 4-4.: cm/yr 

between these two plates and a convergence direction of 

approximately N. 50° E. have been estimated by Riddihough ( 1977, 1984 ). The regional free-air and Bouguer 

gravity anomalies show a pattern common in subduction 

zones: an elongated gravity low along the toe of the 

continental slope and a gravity high inland over the coastal 

mountains. An active andesitic volcanic arc, dating from the 

middle Miocene, covers the area from northern California to 

southern British Columbia and is consistent w:th the 

presence of a subducting plate at a depth of about 100 km. 

Beneath western Washington, a zone of earthquakes at 

depths of 35-80 km, distinct from crustal earthquakes, 

inclines down to the east from the coast (Crosson, 1983); 

this has been interpreted as an intraplate Benioff zone 

(Crosson, 1983; Taber and Smith, 1985). Similarly, a 

dipping Benioff zone has been defined beneath n':lrthern 

California (Cockerham, 1984; Walter, 1986). Most of the 

larger earthquakes in Oregon and Washington since the 

mid-1800's have probably occurred within this deep zone 

(Rogers, 1983a); however, in northern California, the 

largest historical events have been located offshore, in the 

Gorda plate. 

Onshore, the surficial geology of northern California, 

Oregon, and Washington is complex, reflecting in part 

long-tetm convergent tectonics and plate margin accretion. 

The Klamath Mountains, North Cascades, an~ Blue 

Mountain provinces (fig. 1) are largely pre-Cenozoic terranes that include folded and metamorphosed compl~xes of 

continental basements and sediments (Misch, 1966). Volcanism in the Cenozoic Era was extensive and voluninous. 

The Western Cascades, High Cascades, and Columbia 

Plateau provinces experienced repeated eruptions of basaltic 

and (or) andesitic lavas. During the Quaternary Period, 

volcanism has been more limited in extent. South of the 

latitude of Mount Adams, Quaternary volcanic rocks are 

exposed almost continuously in the High Casczdes in 

Oregon and northern California. North of Mount Adams, 

Quaternary volcanism has been confined to the large stratovolcanoes of the Cascade Range. The Olympic Mo·mtains 

and the Coast Range of Washington and Oregon consist 

mainly of Tertiary oceanic sediments and basaltic volcanics 

(Cady, 1975). These rocks appear to have been accreted to 

the continent in the subduction process (Tabor, 1972). The 

Olympic Mountains have been interpreted as oceanic 

basement obducted onto the continental margin (Snavely, 

1988). Paleomagnetic data from the Coast Range have been 

used to infer a significant rotation for this p:--ovince 

compared to inland provinces (Magill and others.. 1981, 

1982; Beck and Engebretson, 1982). The Puget-Willamette 

Lowland is an extensive depression covered by thick 

sedimentary sequences eroded from the Olympic 

Mountains, the Coast Range, and the Cascade Range during 

Miocene uplift (Tabor, 1972; Cady, 1975). 

Geologic and Tectonic Settin~s 3 
The distribution of Pacific Northwest earthquakes 

having magnitudes greater than 4.0 for the period 

1960--1985 (fig. 1) only weakly reflects the convergent 

framework. Most of the events were located offshore: major 

activity was located primarily at the transform fault 

boundaries such as the Blanco fracture zone. Although there 

was moderate activity near the Gorda and Explorer ridges, 

the major ridge in the region (the Juan de Fuca) is 

seismically quiet. Likewise, the subduction zone and 

continental margin of Washington and Oregon are seismically quiet compared to the offshore transform faults. Very 

few earthquakes have occurred in Oregon: this fact agrees 

with an early observation by Montessus de Ballore (1906), 

who noted an absence of damaging earthquakes between 

Portland, Oregon, and Crescent City, California. The sparse 

distribution of earthquakes onshore in Washington and 

Oregon contrasts markedly with the distribution of 

seismicity reported in most active subduction zones where 

numerous earthquakes occur at the interface between the 

two plates, within the subducting plate, and within the 

overriding plate (Uyeda and Kanamori, 1979). For the 

Oregon-Washington portion of the Cascadia subduction 

zone, no subduction or thrust earthquakes have been 

identified along the plate interface at any magnitude level; a 

few small-magnitude thrust earthquakes that might have 

occurred along the plate interface have tentatively been 

identified offshore of northern California (McPherson, 

1989). 

One of the significant problems in assessing the 

earthquake hazards of the Pacific Northwest is the discordance between the historical earthquake record and the 

record available from modern seismic instrumentation 

(dating from about 1950). The historical record is essential 

because, in Washington and Oregon, the only earthquake 

greater than magnitude 6 since the World Wide Standard 

Seismic Network began recording in 1962 is the 1965 event 

near Seattle. Since 1962, all events greater than magnitude 

6 in California north of Cape Mendocino have been located 

offshore in the Gorda plate. Thus, one objective of the 

earthquake hazards assessment program must be to place 

historical, larger magnitude earthquakes within the regional 

tectonic framework. This task is made more difficult in 

western Oregon and Washington by the lack of known 

faults with late Holocene surface offsets. 

Since 1870, earthquakes estimated to be larger than 

magnitude 6 are almost totally restricted to northwestern 

Washington and northwestern California, where most of the 

larger events are located offshore (fig. 2). The 1872 North 

Cascades earthquake is generally considered to be the 

largest earthquake known in the Pacific Northwest (Milne, 

1956): it had an estimated magnitude of 7.4 (Malone and 

Bor, 1979). This earthquake was felt over an area of more 

than 1,010,000 km2

, including Washington, central to 

northern Oregon, northern Idaho, western Montana, and 

southern British Columbia. The earthquake was followed by 

Pacific 

Plate 

0 

North AmP.rican 

PlatE: 

Nevada 

1915 

r'e) 

39oo• Kilometers 13~1°:0.~~----------~~~~~~--~~~~~ 

Figure 2. Map showing locations of earth":luakes in the 

Pacific Northwest with magnitudes estimated to be greater 

than 6 for the years 1870-1990 (octagons). Open triangles 

represent Quaternary stratovolcanoes. Except for the Cape 

Mendocino area, offshore earthquakes clearly related to the 

ridge system have not been shown. Earthquak:>s greater than 

magnitude 7 are labeled with year of occurrence and are 

discussed in the text. Earthquake magnitudes and locations for 

northern California from Ellsworth (1990); Vancouver Island 

data from Rogers and Hasegawa (1978) and frc m Cassidy and 

others (1988); Oregon and Washington data from Ludwin and 

others (in press). 

an extensive aftershock sequence (Milne, 1956). Study of 

damage reports suggests that the maximum intensity (on the 

Modified Mercalli scale) exceeded VII and nay have been 

as high as IX (Milne, 1956). Both the locatio, and depth of 

the North Cascades earthquake are subjects of controversy. 

The location shown in figure 2 was determined by Malone 

and Bor (1979) based on the intensity pattern: these authors 

also summarize other suggested locations of the event. 

Hopper and others ( 1982) have suggested a shallow depth 

based upon intensity contours and the extemive aftershock 

sequence. All instrumentally located earttquakes (since 

1970) near the epicenter proposed by Mc.lone and Bor 

( 1979) are shallower than 25 km. The depth of this 

earthquake is, therefore, unresolved: a shallow earthquake 

of magnitude 7.4 should have a rupture area large enough so 

that primary surface faulting would be evid~nt-none has 

been noted (Shannon and Wilson, Inc., 1977). 

An earthquake estimated to have a rragnitude of at 

least 6.75 (Toppozada and others, 1981) occurred in 1873 

near the Oregon-California border (fig. 2). The earthquake 

was felt from near Portland, Oregon, to Sacramento, 

California, but was felt with intensity VIII only in a small 

4 Program for Earthquake Hazards Assessment in the Pacific Northwest 
area north of Crescent City, California (Toppozada and 

others, 1981 ). There is no mention of felt aftershocks 

accompanying the mainshock in newspaper accounts of the 

event. This observation suggests that either the event was 

sufficiently far offshore so that aftershocks were not felt or 

that it was deep, possibly within the subducting Gorda plate. 

The 1936 Milton-Freewater earthquake is the largest 

known event in the eastern Washington region. Maximum 

intensity was reported as VII (Coffman and others, 1982), 

and the surface wave magnitude has been calculated to be 

5.75 (Gutenberg and Richter, 1954); the magnitude estimated from felt area has been calculated to be 6.4 by Noson 

and others ( 1988). Because numerous aftershocks were felt 

in 1936 and because all earthquakes in eastern Washington 

located since 1970 are in the crust, the 1936 event is also 

assumed to be crustal. 

Between 1939 and 1965, four earthquakes (in 1939, 

1946, 1949, and 1965) greater than magnitude 6 occurred in 

the southern Puget Sound basin (fig. 2; Rogers, 1983b ). The 

1949 (magnitude= 7.1) and the 1965 (magnitude= 6.5) 

earthquakes caused significant damage in the Puget Sound 

region (Murphy and Ulrich, 1951; Nuttli, 1952; Algermissen and others, 1965) and had instrumentally determined 

hypocentral depths of 54 km and 60 km, respectively 

(Algermissen and others, 1965; Baker and Langston, 1987). 

No aftershocks were felt or recorded after the 1949 

earthquake (instrumentation available at the time would 

have detected events larger than magnitude 4.5). Similarly, 

following the 1965 earthquake, no aftershocks were felt, 

and an examination of seismograms recorded on stations 

operating within the region failed to identify any aftershocks greater than magnitude 2.5. 

Moderate earthquakes off the northern California 

coast have been frequent during the last 120 years (fig. 2), 

but damage from these events has been of the non-structural 

type. The largest offshore event (magnitude 7.3) occurred in 

1923, and shaking damaged a number of chimneys in small 

towns north of Cape Mendocino (Toppozada and others, 

1981; Ellsworth, 1990). In 1980, a magnitude 7.0 event 

occurred within the Gorda plate, just west of the deformation front (fig. 2) but did little damage to structures. The 

most damaging earthquake in northwestern California to 

date is the 1954 Eureka event (fig. 2). This event was 

estimated to be of magnitude 6.5; one person was killed 

during this earthquake, and there was considerable nonstructural damage (Ellsworth, 1990). 

At the northern end of the Juan de Fuca plate, there is 

a concentration of larger magnitude events across central 

Vancouver Island (fig. 1). This area has been subjected to 

two earthquakes greater than magnitude 7: an event estimated to be of magnitude 7.2 in 1918 and a second event of 

magnitude 7.2 in 1946; in addition, there was an event of 

about magnitude 6 in 1957. Because central Vancouver 

Island is lightly populated and because the hypocentral 

depths of the 1918 and 1946 events have been estimated to 

be about 20 and 30 km, respectively, actual damage from 

these earthquakes was very slight. Nevertheless, because 

these events occurred within the crust of the North 

American plate (Cassidy and others, 1988), they raise 

serious questions as to the possibility of similar eartl''Juakes 

occurring in heavily populated urban areas of the Pacific 

Nmthwest. 

Actual losses from earthquakes occurring in the 

Pacific Northwest were greatest from the 1949 ard 1965 

events. Eight people were killed in the 1949 earthquake 

(Ulrich, 1949) and six died in the 1965 earthquake (Algermissen and others, 1965). In terms of 1984 dollars, $150 

million in damage occurred in 1949; the 196~ event 

produced an estimated $50 million in damage (No~on and 

others, 1988). The effects of the 1949 and 1965 earthquakes 

are currently used as the basis for engineering design, 

emergency response planning, and damage and loss 

estimates in the Puget Sound region (Hopper and others, 

1975). Since the 1975 report was issued, the population of 

the Puget Sound region has increased by more than EOO,OOO, 

and many new structures have been built. More importantly, 

the 1975 study considered only the effects c-t deep 

earthquakes and did not consider the effect of either a 

shallow, crustal earthquake (like the 1872 event) or the 

possible effects of a great earthquake on the megathrust. 

Thus, there is wide agreement in the scientific community, 

in engineering groups, in political bodies, and among 

residents of the region that a updated discussion of the 

sources, the hazards posed by these sources, and estimates 

of earthquake losses is urgently needed in the Pacific 

Northwest. 

EARTHQUAKE HAZARDS AND RISK 

ASSESSMENT IN THE PACIFIC NORTHWEST 

The first step in providing an assessment of 

earthquake hazards and risk within a region requires that the 

nature and distribution of earthquake sources be und~rstood. 

During the past I 0 years of the NEHRP program, much of 

the effort in the Pacific Northwest has focused on definition 

of the nature of the hazards rather than on methods of 

immediately reducing risk. In this regard, it is interesting to 

take two measures of our enhanced understanding of the 

earthquake hazards. First, Hopper and others (1975) 

suggested that the Juan de Fuca plate may be attached to the 

North American plate, thereby making it unlikely tLat great 

earthquakes could occur on the interface between the two 

plates. Fifteen years later, most earth scientists acknowledge 

the potential of thrust earthquakes on this interface at least 

as large as magnitude 8.0, and many scientists argue that 

earthquakes greater than magnitude 9.0 may occur here. 

Second, Perkins and others (1980) issued a revised estimate 

of maximum horizontal ground-acceleration for Oregon and 

Washington in which it was necessary to regionalize the 

Earthquake Hazards and Risk Assessment in the Pacific Northwest 5 
maximum expected earthquake: in the southwestern 

Washington Cascade Range, near Mount St. Helens, the 

magnitude of the estimated maximum earthquake used in 

the analysis was 5.1. By 1985, the St. Helens seismic zone 

had been identified, and a maximum magnitude event of 6.8 

was adopted for engineering-design purposes for several 

structures built to help control the level of debris-dammed 

lakes that formed in the aftermath of the May 18, 1980, 

eruption. 

After reviewing the accomplishments of the NEHRP 

in the Pacific Northwest, we recognize that the region is not 

yet in the position of grappling with measures to reduce the 

effects of great earthquakes along the coast; however, we 

are taking the first steps toward an understanding of the 

physical processes that cause these earthquakes. We are also 

not yet at the point of issuing probabilities for the next 

major earthquake. These two items (the lack of a policy for 

risk reduction and the lack of probability estimates) reflect 

a difference between the Pacific Northwest and California. 

In the Pacific Northwest, only a few earthquakes greater 

than magnitude 6 have occurred (fig. 2). Exposed faults that 

have late Holocene offsets are rare, and the instrumental 

record of seismicity is not in complete agreement with 

either the distribution of historical earthquakes or mapped 

evidence of faulting. Thus, in this region, we are not yet at 

the point of making intermediate-term forecasts concerning 

possibly damaging earthquake activity. Nevertheless, over 

the past decade, much progress has been made in the Pacific 

Northwest toward resolving many of the uncertainties of 

earthquake hazards. In turn, as our ability to quantify 

hazards has improved, we are beginning to see added 

emphasis on a better understanding of the risks posed. 

The state of knowledge of the seismotectonics of the 

Pacific Northwest contrasts sharply with the current focus in 

California on issuing probabilities of future seismic activity. 

Central and southern California are dominated by strike-slip 

tectonics, and three independent data sets are available to 

integrate into probability estimates: first, there is a historical 

record that is liberally sprinkled with magnitude 6 and 

greater earthquakes; in some cases there are repeat 

earthquakes available in the historical record. Second, there 

are geological data for surface faults that allow for 

independent assessment of previous large earthquakes. 

Third, modern, instrumental seismicity shows a pattern 

consistent with both the geological record and historical 

earthquake data-this has allowed all three data sets to be 

unified under a comprehensive plate tectonic framework. 

Thus, in California, the USGS and other groups are in the 

position of beginning to estimate intermediate-term probabilities (10 years to a few decades) for the occurrence of 

large earthquakes along major fault systems (U.S. Geological Survey, 1988, 1990). 

The estimation of probabilities for future earthquake 

occurrence is but one of many necessary steps that are part 

of regional risk-assessment and mitigation programs. Clear 

components of risk assessment include careful mapping of 

local soil and near-surface bedrock condithns and estimating how local sites will be affected by strong ground 

shaking. The design of mitigation programs relies directly 

on the ability of the earth sciences community to reliably 

communicate a clear statement of earthquake hazards to the 

general public. However, the final step (actual implementation of revised building codes, land-use zoning 

changes, building retrofitting, etc.) will likely only be 

achieved if the earthquake hazards program has been 

successful in developing the scientific case sc that political 

decisions will be made that will help to mitigate the effects 

of future earthquakes. Ultimately, tradeoffs will occur in the 

political arena between risk-reduction measures that are 

tailored for the largest conceivable earthquake a region may 

face and actions designed to prevent losses from perhaps 

more frequent, smaller earthquakes. 

In the remainder of this section, w~ provide a 

commentary on important results from the last 10 years and 

on some of the outstanding problems and opp1rtunities that 

must be faced over the next 10 years. 

Earthquake Sources 

There are three distinct sources of earth'luakes in the 

Pacific Northwest: (1) crustal earthquakes that occur within 

the overriding North American plate, (2) int-aplate earthquakes that occur within the subducting Juan de Fuca and 

Gorda plates, and (3) interplate earthqua1

,..es that are 

expected to occur at the interface between the Juan de Fuca 

(and Gorda) plate and the North American plate (subduction 

or thrust events). West of the Cascade arc, th~ distribution 

of earthquake-source-type reflects a combination of factors: 

the geometry of the subducting Juan de Fuca and Gorda 

plates; crustal structure within the over:-iding North 

American plate; and tectonic interactions among the North 

American, Pacific, and Juan de Fuca plates. East of the 

Cascades, it is likely that earthquake distribution primarily 

reflects the tectonics and structure of only the North 

American plate. 

There are common questions for ea~h particular 

source region: how large might earthquakes be. where might 

they occur, how often do they occur, and what are the 

expected ground responses from each source type? Beyond 

these common questions, there is a wide range of questions 

appropriate for each source type. Most of th~se questions 

have yet to be addressed in any systematic way. 

Crustal Earthquakes 

The distribution of crustal earthquakes is not uniform 

across the Pacific Northwest (figs. 3 and 4). Weaver and 

others (1990) used sharp changes in seismicity to divide the 

Cascade Range and adjacent areas into four segments. In the 

6 Program for Earthquake Hazards Assessment in the Pacific Northwest 
125°0' 119°0' 

49°0' r==~~rer~w;;===r====f""""""='t 49°0' 

~ 

(!) (!) (!) 

~J (!) 

Blue Mts. (!) 

0 

cv ~TS 

8 ~ 6N Basin 

~CL 

M~~ 

40°0' '===~~~!!!!l.;;!llb....,=J!a==o.:!!! 

125°0' 

0 

Kilometers 

& 

Range 

ti 

200 

I 

\] {J 

Figure 3. Map showing distribution of crustal earthquake 

activity, 1980-1990. Earthquakes shown are less than 30 km 

deep; location errors estimated to be less than 5 km; rootmean-square error (RMS) less than 0.35 seconds; more than 6 

stations reporting. SHZ, St. Helens zone. Open triangles 

represent Quaternary stratovolcanoes: M, Meagher Mountain; 

B, Mount Baker; G, Glacier Peak; R, Mount Rainier; S, Mount 

St. Helens; A, Mount Adams; H, Mount Hood; J, Mount 

Jefferson; N, Newberry Volcano; ML, Medicine Lake Volcano; 

SH, Mount Shasta; L, Lassen Peak; TS, Three Sisters; CL, Crater 

Lake. From Weaver and others (1990). 

first segment, nearly all of the well-located crustal 

earthquakes are confined to the region between the eastern 

Olympic Mountains and the western edge of the North 

Cascades (fig. 3); there are few events near Quaternary 

stratovolcanoes. The second segment defined by Weaver 

and others (1990), from Mount Rainier to Mount Hood, is 

the most seismically active segment. In southern 

Pacific 

Plate 

0 200 

British Columbia 

Figure 4. Map showing distribution of earthquakes deeper 

than 30 km. Location errors estimated to be less than 5 krn; 

root-mean-square error (RMS) less than 0.35 second'; more 

than 6 stations reporting. The 60-km line shows the approximate easternmost extent of earthquake distribution at this 

depth. Although most of these earthquakes are intraplate 

earthquakes occurring within the Juan de Fuca plate, there are 

a few crustal earthquakes that occur at depths between 30 and 

35 km below central Puget Sound. 

Washington, the St. Helens zone (SHZ) is prominent within 

this segment. Although there is an absence of activit~' east of 

the SHZ, seismicity continues in a broad zone from the area 

immediately west of the SHZ into southeastern Washington 

(fig. 3). The third segment, from south of Mount Hood to 

just nmth of Mount Shasta, is seismically very quiet (fig. 3). 

Although this entire segment has not been monitored 

continuously, very few earthquakes were observed during 

the two years (1980-1982) of continuous operati<Jn of a 

32-station network in the Cascade Range or since stations 

were reinstalled in the Cascade Range of central Oregon in 

1987 ~Ludwin and others, in press). There is a marked 

increase in earthquake activity in the fourth segment 

(between Mount Shasta and Lassen Peak) and also along the 

northern California coast (fig. 3). At the present time, we do 

not understand why earthquakes are distributed in this 

pattern, even though the answer to this question is 

fundamental to our understanding of earthquake hazards in 

the Pacific Northwest. 

Intensity patterns associated with the 1872 North 

Cascades earthquake (fig. 5), an extensive af+ershock 

sequence, and the shallow depths of all earthquakes 

instrumentally recorded near the 1872 epicenter provide 

evidence for a shallow hypocenter for the 1872 event 

(Milne, 1956; Hopper and others, 1982), although no 

Earthquake Hazards and Risk Assessment in the Pacific Northwest 7 
0 200 Kilometers ~-----'-----' 

Figure 5. Map showing Modified Mercalli Intensity (MMI) 

plot of the 1872 earthquake. Based on this intensity pattern, 

researchers have inferred that the epicenter of the 1872 

earthquake was located in the vicinity of Lake Chelan. 

Intensity VIII corresponds to structural damage (cracked walls, 

downed chimneys). Intensity VI (which would include all of 

the urban areas along Puget Sound) corresponds to shaking 

severe enough to cause items to fall from shelves. Modified 

from unpublished figure prepared by M.G. Hopper. 

evidence of surface faulting has been noted (Shannon and 

Wilson, Inc., 1977). Two earthquakes of magnitude 7 or 

greater have occurred during this century (in 1918 and 

1946) in central Vancouver Island. The existence of these 

events, which were most likely crustal, raises the issue of 

how much of Oregon, Washington, and northern California 

could be involved in such large, crustal events. Current 

crustal seismicity in the Puget Sound basin does not occur 

along linear fault zones but is distributed throughout the 

crust (fig. 3). Mapped Quaternary faulting in the basin is 

sparse (Gower and others, 1985), although newly collected 

shallow reflection data may support the interpretation of 

shallow Quaternary faulting within Puget Sound (Harding 

and others, 1988). Geologic mapping provides evidence of 

abrupt uplift of marine terraces at two locations within the 

basin: Restoration Point (5 km west of Seattle) and Belfair 

(35 km southwest of Restoration Point). These uplifts may 

be due to one or more earthquakes (Bucknam and Barnhard, 

1989). These authors also state that "No comparable 

coseismic deformation has been observed historically in the 

region despite the occurrence of earthquakes as large as 

magnitude 7.2 (1949), suggesting the sudden uplift was 

p.roduced by an earthquake source and mech"nism greatly 

different from any observed to date" (Bucknam and Barnard, 1989, p. 1,332). The 1949 earthquake wa: 54 km deep; 

it was not, therefore, a shallow, crustal earthquake. The 

available record of seismicity in the Puget S0und basin is 

usually interpreted to suggest that the expected maximum 

magnitude crustal earthquake is less than that expected in 

either southwestern or southeastern Washington (Ludwin 

and others, in press). The recent discover:' of uplifted 

terraces of Holocene age by Bucknam and Barnhard (1989) 

raises serious doubts about these other interpretations. 

An important advance in understanding the hazards 

posed by crustal earthquakes has been the result of a series 

of studies along the St. Helens zone, a 130-km-long zone of 

moderate-magnitude earthquakes that strikes north-northwest through Mount St. Helens. The SHZ is a right-lateral 

strike-slip zone: nearly all of the focal mechanisms 

calculated to date have had nearly pure strike-slip solutions 

(Ludwin and others, in press). Although the largest known 

earthquake in the SHZ had a magnitude 5.5, work by the 

USGS, the A1my Corps of Engineers, and the State of 

Oregon Department of Geology and Mineral Industries all 

concluded that a much larger event (magnitude 6.2-6.8) 

could occur along the SHZ (Grant and Weaver, in press). 

This assessment of the SHZ has clear implications for the 

Portland area, where recent study of the seismicity has 

concluded that there may be a series of en echelon fault 

zones southwest of the SHZ (Yelin and Patton, 1991). The 

Portland Hills fault, a few km west of downtown Portland 

(Balsillie and Benson, 1971) is viewed as one of these 

zones. Because Portland has a history of moderatemagnitude emthquakes (approximately magritude 5 and 

above), it is clear that the area between the SHZ and the 

Portland Hills fault zone requires continued, dose seismic 

monitoring to test this hypothesis. 

One of the major issues for which virtually no work 

has yet been done is the relation between crustal structure 

and the distribution of crustal earthquakes. G~nerally, the 

crustal structure in Oregon and Washington is poorly known 

because there are few high-quality reversed refraction 

profiles of the region. However, a crustal refn:ction profile 

across the Columbia Plateau through the Pasco basin 

coincides with an area of relatively high backarc crustal 

seismicity-this allows a comparison between the details of 

crustal structure and the distribution of seis11icity. This 

refraction line (C-C', fig. 6) crossed a series of east-west 

striking thrust faults at acute angles and passec through the 

center of the Pasco basin where deposits of tb~ Columbia 

River Basalt Group are thought to be thickest. 

8 Program for Earthquake Hazards Assessment in the Pacific Northwest 
121 °30' 11 poo• 

48°0' r-------..,.....-~-__,..1"'9""""""--..----....,..,..---.,-~--, 48°0' 

... 

' :: .. :+ 

~ , 

fdge 

• OfCoJ. • urnb;a R: • tvere::~.,_ 

-...:t.hG I roup , 

. . . .. ,. .... -~:it 

. : . "' 

0 50 

Kilometers 

... 

. .. 

\ •... 

' _d_d d 

46°0'----~~~~~~~~~~--~------~--------~~~~----~~~~-;--~~ 121 °30' 

Figure 6. Map showing seismicity in eastern Washington, 1970-1987, magnitude 1.0 and larger. Earthquake magnitudes are 

shown by two symbol sizes: events less than magnitude 4 denoted by dots; events greater than magnitude 4 denoted by open 

circles. Heavy lines are mapped faults of post-Miocene age (d, downthrown side); light lines are axes of major anticlines. The 

western margin of the Columbia River Basalt Group is indicated by hachures; Quaternary volcanics near Mount Adams are 

shown by stippled "v" pattern. Location of cross section C-C', (fig. 7) is shown. From Ludwin and others (in press). 

The crustal structure, interpreted by Catchings and 

Mooney (1988) along line C-C' on figure 7, contains four 

features of interest with respect to earthquake distribution: 

(1) a series of alternating layers of high and low P-wave 

velocity (5.85 and 5.3 km/s, respectively) comprises the 

Columbia River Basalt Group. High-velocity layers 

represent Miocene basalts, whereas low-velocity layers 

represent sedimentary deposits between flows (Catchings 

and Mooney, 1988). This integrated structure reaches its 

maximum depth near the center of the Pasco basin (fig. 7). 

(2) A low-velocity layer beneath the Columbia River Basalt 

Group is interpreted by Catchings and Mooney ( 1988) as a 

pre-Miocene sedimentary basin. (3) A low-velocity layer 

(having a P-wave velocity of about 5.0 km/s) is contained 

within a graben structure that is evident in the mid and 

upper crust (fig. 7). The graben structure is about 70 km 

wide and has a vertical offset of about 4 km between the 

central axis and either side; the low-velocity secimentary 

fill is thickest within the graben axis. (4) A low~r crustal 

upwelling that has a seismic velocity of 7.5 kmls occurs 

beneath the entire graben structure and reaches its highest 

crustal level of about 25 km below sea level directly beneath 

the center of the graben. Taken together, these four elements 

are typical of the structure of continental rifts (Catchings 

and Mooney, 1988). 

The earthquake distribution (fig. 7) clearly shows a 

concentration of events within the Columbia River Basalt 

Group. Most of the shallow, crustal events within the 

Columbia River Basalt Group are near th~ Saddle 

Mountains, which occur over the northeastern edge of the 

Earthquake Hazards and Risk Assessment in the Pacific Northwest 9 
C C' .~~-------------------------N_.3_5__E_. ________________________ _, 

c 

0 

-~ 

> 

Q) 

w Columbia River 

Basalt Group 

"V=6.1 

V=6.3 " 

Pasco 

Basin 

V=6.1 e 

"' "V=6.3 kn/s 

V=8.40 -500~----~------~~=---------~------------~----------~ 100 150 

Distance, in Kilometers 

Figure 7. Crustal cross section (C-C') in eastern Washington. Location of line of section shown on figure 6. Vertical 

exaggeration of topography is 10:1; vertical exaggeration of subsurface is 1.8:1. CRBG, Columbia River Basalt Group; V, P-wave 

velocity of layer in kilometers per second. Earthquake hypocenters (since 1970), regardless of magnitude, shown by single 

symbol size. Earthquakes 50 km on either side of line C-C' (fig. 6) are projected onto a vertical plane striking N. 35° E. From 

Ludwin and others (in press). Structural information from Catchings and Mooney (1988). 

upper-crustal graben. The low-velocity sediments (having a 

P-wave velocity of about 5.0 km/s) are nearly devoid of 

earthquake hypocenters: the deeper crustal events are 

occurring in what is probably a granitic layer having a 

P-wave velocity of 6.1-6.4 km/s. The deepest crustal events 

(which are approximately 22 km deep) occur over the 

shallowest extent of the lower-crustal upwelling. Thus, the 

seismicity seems to be distributed unevenly about the 

upper-crustal graben: most events occur on the northeast 

side of the graben. The graben, though pre-dating the 

Columbia River Basalt Group and contemporary tectonics, 

apparently exerts some influence on the shallow crust and 

produces (or localizes) the Saddle Mountain system and 

contemporary seismicity. Although the processes responsible for localizing the shallow thrusts and earthquakes in 

the Pasco basin are unknown, there is a similar tendency for 

most of the earthquakes below the Columbia River Basalt 

Group to occur beneath the northeastern half of the graben 

(fig. 7). 

Another example of the relation between crustal 

structure and earthquakes is the SHZ in muthwestern 

Washington. Here, a combination of seismological, aeromagnetic, magnetotelluric, and volcanological studies have 

concluded that the SHZ represents a suture betw~en Tertiary 

marine volcanics to the west and a compre~sed marine 

forearc basin to the east (Stanley and others, 1990). The 

forearc basin is distinctive because it contains a huge 

conductivity anomaly (> 10,000 siemens/m); it is surrounded on three sides by a major, regional aeromagnetic 

low. Quaternary volcanism is voluminous on the edges of 

the basin but is extremely sparse within it; earthquake 

activity is concentrated predominantly along th~ boundaries 

of the basin. The SHZ occurs along the western boundary of 

the conductivity anomaly. The relation between the SHZ 

and other regional structures is an important component of 

a complete assessment of the hazards along the>: St. Helens 

zone; yet, no detailed seismic profiling of the crust has been 

done in this area. 

10 Program for Earthquake Hazards Assessment in the Pacific Northwest 
The earthquake hazards implications derived from the 

relation between crustal events and crustal structure have 

never been properly addressed in the Pacific Northwest. 

Results in the Pasco basin and along the SHZ show, 

however, the clear need for crustal structure studies to be 

integrated into the tectonic framework across the region. 

Intraplate Earthquakes 

Intraplate earthquakes (events within the subducting 

plate) are the most frequently observed of the large 

earthquakes in the historical record (magnitudes ~ 6). At 

least six large intraplate earthquakes are known to have 

occurred: in 1873, along the coast near the OregonCalifornia border, and in 1909, 1939, 1946, 1949, and 1965, 

all within the Puget Sound basin (fig. 8). The source 

characteristics of intraplate earthquakes in the Pacific 

Northwest are fairly well known, but the spatial distribution 

of these events is uncertain. Intraplate events are generally 

believed to be caused by gravitational forces within subducting plates. Thus, the source region reflects the subducting plate depth and geometry. The depths of the largemagnitude intraplate earthquakes, based on the historical 

record, are estimated to be between 45 and 60 km (fig. 4). 

125°0' 

49°0' 

I yancouver ·~ ,)M1909 1872 _.- / 

· :~:~a ~\~san'Juan • North ( -.r '-u.~~ -~ Is. Cascades \ · "'}\;~::>Puget _ __,~l___ ~' , N',:n, 1 Bas1n -,j 

·. \ 193J( ~Seattle Spokane© 

1946, .. 1965 

~ trOiympia 1949 <Zl Washington 

R f "~ cr l - 1~6 

~ ., 

1

\ co1urnb ~~ ~ ~ ,~ ~ -;-"-~~ Mllton-Freewater 

Q / Portland 1:5 

-~·!' !l !"~' . ' Ct1 (3 u 

&:. 

; 

·< 

.. ~ I 

.\1873 

. ·-----

Oregon 

0 

~ 

100 

Kilometers 

1- '" u :~ California · Nevada 

49°0' 

41 oo• . I Crescent City 

12~5°~0~.~--~--~--~---L--~--~--~ 

Figure 8. Map showing major damaging earthquakes in 

Oregon and Washington and year of occurrence. Small 

octagons are events having magnitudes estimated from felt 

areas to be between 6.0 and 6.9; large octagons are events 

with magnitudes estimated to be greater than 7. From Ludwin 

and others (in press). 

The distribution of intraplate earthquake hy"Jocenters 

indicates that the subducting Juan de Fuca plate arches 

upward beneath southern and central Puget Sound. Beneath 

southwestern Washington, the plate dips to the eastsoutheast, changing to a northeast-dipping plate beneath 

northwestern Washington (Weaver and Baker, 1988). Most 

of the large intraplate earthquakes (magnitude ~ 6) have 

been located in the southern Puget Sound basin; this leads to 

hypotheses that these events may not occur e1sewhere. 

However, because the plate is thought to be continuous 

everywhere between the trench and volcanic arc, intraplate 

events could occur anywhere that intraplate stresses reach 

some critical level. The major unresolved issue for intraplate earthquakes is whether they can occur be1eath the 

southern and central Oregon Coast Range, beneath northernmost California, and beneath central Vancouver Island. The 

plate geometry beneath most of Oregon is eith~r poorly 

resolved or unknown because of low rates of seismicity and 

the sparse distribution of seismic network station'3. 

lnterplate earthquakes 

Some investigators (e.g., Heaton and Hartzell, 1986) 

have suggested that an earthquake as large as magnitude 9 

(similar to the 1960 Chilean or 1964 Alaskan events) could 

occur along the coasts of northern California, Oregon, 

Washington, and southern British Columbia (fig. 9). In 

contrast to the crustal and intraplate earthquakes that have 

been instrumentally or historically recorded, the oxurrence 

of interplate events must be inferred from the geologic 

record because no recorded earthquakes have occurred 

along the interface between the plates. Ongoint; work in 

intertidal marshes along the Oregon, Washington, and 

northern California coasts has shown that the marsh 

stratigraphy includes alternating layers of buried peat and 

intertidal mud (Atwater, 1987). The sharp contacts between 

peat and overlying intertidal muds have been intetJreted as 

evidence for rapid coseismic subsidence of the tidcl marshes 

in response to a large subduction earthquake. This evidence 

supports the concept that the Cascadia subduction zone has 

a number of similarities with other subduction zon~s around 

the world that have experienced frequent tr'"ust-zone 

activity. In addition, the marsh studies may supply evidence 

of a locally generated tsunami coincident with a large 

earthquake . 

The paradox in Oregon and Washington is that, 

despite the existence of a subduction zone, the'"e are no 

contemporary thrust emthquakes on the shallow-dipping 

interface. The Cascadia subduction zone differs fmm nearly 

all convergent margins throughout the world because there 

is no great earthquake (magnitude ~ 8) in the historical 

record. However, studies of subduction-zone characteristics 

(Heaton and Kanamori, 1984; Heaton and Hartzell, 1986), 

strain accumulation in the region (Savage and others, 1981), 

and marsh stratigraphy along the Washington and Oregon 

Earthquake Hazards and Risk Assessment in the Pacific Northwest 11 
Figure 9. Map showing distribution of Modified Mercalli Intensities (MMI) for the 1964 Prince William So·md, Alaska, 

earthquake. MMI =VII indicates the beginning of structural damage: cracked masonry; chimneys broken at the roo+line; plaster, 

tile, and loose bricks fall; irrigation ditches damaged; etc. (The MMI = VII contour encloses about 270,000 km2 .) MMI = X 

represents an intensity sufficient to destroy most masonry and frame structures as well as bridges. MMI =XI indicates destruction 

severe enough that lifelines and transportation systems are completely out of service. Modified from Espinosa and ethers (1986). 

coasts (Atwater, 1987; Grant and others, 1989) provide 

evidence that the entire Cascadia subduction zone should be 

regarded as capable of generating large, interplate earthquakes. These observations spark debate about whether the 

thrust interface can be absolutely quiet at some point in the 

subduction cycle: Heaton and Kanamori (1984) compared 

the seismic quiesence along the Cascadia subduction zone 

with that on the northern section of the San Andreas fault 

that broke in 1906. 

Structure of the crust and lithosphere 

The composition of the crust of the Pacific Northwest 

has been investigated using geologic and geophysical 

methods, but there are few reversed, high-resolution refraction or wide-angle reflection profiles in the region. Mooney 

and Weaver (1989) summarized ex1stmg studies with a 

contour map of estimated crustal thickness beneath Washington, Oregon, and northern California (fig. 10). The 

sparse number of seismic lines emphasizes the need for 

deep seismic control in many areas. 

There are two noteworthy features of fg:ure 10 that 

relate to a working hypothesis for the configuration of the 

Moho. The first feature is the pronounced east\'lard increase 

in crustal thickness from 16 km at the continental margin to 

about 40 km beneath the western flank of the Cascade 

Range. Gravity modeling along two profiles ir Oregon and 

preliminary interpretations of electrical and magnetic data 

collected by the EMSLAB experiment (EMSLAB Group, 

1988) along a profile perpendicular to the northern Oregon 

coast are consistent with crustal thickening. The contours 

shown on figure 10 beneath the Klamath Mountains in 

12 Program for Earthquake Hazards Assessment in the Pacific Northwest 
Pacific 

Ocean 

Washington 

~ 

-

Q. 

Q) 

J 

0 

32 

~ 0 100 

I 

Kilometers 

Figure 10. Contour map of crustal thickness (in kilometers) in the Pacific Northwest. Solid lines show locations of reversed 

refraction profiles (sometimes with other supporting geophysical data); dashed lines show locations of gravity or electroMagnetic 

profiles; dotted lines show location of crustal reflection profiles; solid triangles, Quaternary stratovolcanoes. Areas enc 1

osed by 

dashed lines have had estimates of crustal thickness made from seismic array studies. From Mooney and Weaver (1989). 

Earthquake Hazards and Risk Assessment in the Pacific Northwe~~ 13 
southwestern Oregon and northwestern California lack 

seismic control. Nevertheless, the known, thin oceanic crust 

and thick Cascade Range crust support the general trends 

represented by the contours. The second major feature of the 

crustal thickness map is the presence of thick crust beneath 

the Cascade Range, the Puget Sound basin, and the 

Columbia Plateau (fig. 10). Crustal thickness is estimated to 

be at least 38 km over this entire region and locally reaches 

46 km in the Cascade Range of southern Oregon. A gradual 

eastward thinning of the crust occurs beneath the Basin and 

Range of southeastern Oregon and northeastern California 

(fig. 1 0). The Moho shallows beneath the Okanogan 

Highlands in northeastern Washington, where a reflection 

profile (line 5, fig. 10) has been interpreted as indicating a 

flat Moho at a depth of about 36 km beneath most of this 

province (Potter and others, 1986). 

There are several unresolved scientific issues with 

respect to crustal structure. The first has to do with crustal 

earthquakes. In Washington, the deepest crustal events 

(approximately 30 km deep) are found near the point where 

the dip of the Juan de Fuca plate begins to steepen-generally, this is the area where plate depth is between 40 and 

60 km. The magnitude 7 and larger events on central 

Vancouver Island have source depths estimated to be in the 

mid to lower crust (Rogers and Hasegawa, 1978~ Cassidy 

and others, 1988). These source depths have about the same 

relation to the subducting plate beneath Vancouver Island as 

that observed between the crustal activity in Puget Sound 

and the Juan de Fuca plate. Thus, better resolution of the 

deep crustal architecture is essential to determine if there are 

common features of the deep crust, presumably related to 

the subduction process, that are controlling the deep, seismogenic volume. Second, the SHZ is thought to represent 

the suture between Tertiary volcanic rocks to the west and 

a compressed marine forearc basin to the east; this boundary 

then acts to localize regional seismic slip along along its 

boundaries (Weaver and Malone, 1987). Crustal structure 

investigations along the SHZ would test this hypothesis and 

would allow insight into the distribution of earthquakes 

across much of southwestern Washington. Third, in the 

Puget Sound basin, crustal structure studies are needed to 

examine possible links between surface marine-terrace 

uplift, large Bouguer gravity anomolies in the central basin, 

and background seismicity at mid-crustal depths. Fourth, 

there is a need for a thorough examination of the geometry 

of the interface between the Juan de Fuca and North 

American plates. There are remaining questions concerning: 

( 1) whether the interface is the actual slip surface for great 

megathrust events, (2) the landward extent of possible slip 

surfaces, and (3) whether major discontinuities might exist 

along the interface that could act as barriers to the 

generation of great earthquakes. 

Deformation Record 

Significant coseismic and permanent. vertical deformation often accompany the occurrence of great (magnitude 

~ 8) subduction-zone thrust earthquakes. General characteristics of subduction-zone coseismic deformation may be 

summarized as follows (West, 1989, p. 36-37): 

Great earthquakes (magnitude ~ 8) g~nerally affect 

more than 400 km of coastline (lir~ar distance); 

smaller magnitude earthquakes affect shorter lengths. 

Maximum uplift is between 2 and 6 m and occurs 

within 100-120 km of the trench; maximum subsidence is between 0.5 and 2.0 m and may occur as far 

as 275 km from the trench. 

''Multiple uplifted Holocene marine features (wavecut platforms, terraces, strandlines) are common to 

30 m elevation in the zone of coseismic uplift and are 

evidence of past thrust events.'' 

Average rates of uplift vary between 0.2 and 4.0 

mm/yr. 

·'Geologically-derived recurrence intervals for great 

earthquakes indicate they have return periods of 

300--2000 yr while historic records suggest the 

periods may be shorter (50--500 yr)." 

West (1989) noted that the Pacific Northwest coastline includes five uplifted Pleistocene terraces but does not 

include uplifted Holocene features indicative of great 

subduction-zone earthquakes. 

Several researchers (Atwater, 1987; Nelson, 1987; 

Peterson, 1989) have infened periods of sutsidence based 

on episodes of marsh burial (fig. 11A). These authors infer 

that abrupt changes in microfossil assemblages, nonerosional upper contacts, and the thicknesse0 of overlying 

sand deposits favor tectonic subsidence and tsunamis as 

agents rather than storms or seiches. Evidence of episodes 

of subsidence that are likely coincident in time (and, hence, 

possibly coseismic with respect to a great subduction-zone 

earthquake) have been documented along the coast from 

southern Washington to northern California (fig. liB). 

Localized subsidence and uplift have also bee1 documented 

in the accretionary wedge of southern Oregon and northern 

California (Grant and others, 1989). 

The lack of Holocene terrace uplift has been cited as 

evidence against repeated great subduction-zone earthquakes in the Pacific Northwest during the Holocene, while 

the areal extent and possible coincidence in time of rapid 

subsidence along the coasts of Washington and Oregon 

provide evidence for repeated earthquakes during Holocene 

time. This discrepancy may be explained by long recurrence 

intervals for great subduction-zone earthquakes (magnitude 

~ 8) interspersed with smaller (magnitude = 6-7) thrust 

events-however, if this is the case, then the coast is in a 

unusually long period of seismic quiescence at the 

magnitude 6-7 level. A second possible explanation is that 

14 Program for Earthquake Hazards Assessment in the Pacific Northwest 
Figure 11. A, Photograph showing evidence for coastal subsidence, southwestern Washington. A ghost forest of cedar 

snags protrudes through an intertidal marsh along the Copalis 

River, Washington. The forest was probably killed after subsiding suddenly into the intertidal zone about 300 years ago. 

Man in lower left-hand corner of photograph is approximately 

1.8 m tall. Photograph by B.F. Atwater; 8, Location of sites 

(shown by sol id circles) in Washington, Oregon, and northern 

California where coastal marsh stratigraphy studies are 

underway. Open triangles, Quaternary stratovolcanoes. 

the behavior of coastal terraces is related to both the dip of 

the subducting plate and to details of the contact area 

between the two plates. It is clear that, although marsh 

stratigraphy may provide evidence of past, great thrust 

earthquakes, there is a pressing need to place coastal marsh 

sites into the proper regional geologic and tectonic 

framework. 

Geodetics 

Geodetic measurements indicate that crustal strain is 

accumulating in northwestern California, in the Puget 

Sound basin, and across the Strait of Georgia (fig. 12). The 

direction of principal contraction for each of the networks is 

generally close to the direction of relative plate convergence 

(N. 50° E.) expected from the theoretical plate-motion 

117°0' 

R"~~~~;;;--;o,....,..-----.-----,5 0' 

Pacific 

Plate 

Kilometers 

EUGENE Oregon 

. "' 

. "' 

studies of Riddihough (1984). However, the rate of strain 

accumulation is slow, and, with the estimated uncertainty, 

the interpretation of contraction in the direction of relative 

convergence is not accepted unequivocally (Ando and 

Balazs, 1979; Sbar, 1983). Most modeling attempts (Savage 

Earthquake Hazards and Risk Assessment in the Pacific Northwest 15 
132°0' 116°0' 

52°0' .------r------....,..,.-----r-----r------r---.,-----,-....._--, 52°0' 

0 

I 

Kilometers 

British 

Columbia 

Juan de Fuca 1892-1987 

t= 0.11 ±0.03 

~ =N82°E± s• 

Olympic 1982-1986 

£.1 = 0.07 ± 0.06 

~ = --{).09 ± 0.06 

-- --~ = J'!'5S·~ ± 8~-------··----~-----1 

! 

! Seattle 1972-1985 

£1 = 0.03 ± 0.02 

Ez = --{).03 ± 0.01 

~ =N69°E ±7° 

I 

i 

i ! _;w' l. . -~----- ------\ . v~ ' .. .__ __ ..r----....--H~ford 1972-1983 ) 

El=--{).02±0.01 I 

£2 = --{).02 ± 0.01 • 

~=N87•E±34° , / 

Oregon 

Northern Calif. 1919-1979 

r= o.16 ±0.o1 

\ 

i 

i 

i 

~ = N25°E ± 14• 

Eureka 1942-1989 : 

~ -----1. .,-_..,ll .... ,'-.I.Y,'ILL----------_ _L_ __ 

Nevada 

£. 1 = --{).05 ± 0.03 

£.2 = --{).19 ± 0.03 

~= N31°E ± s• 

39°0'L-----~------~------J-------~-----+------~------~----~ 

132°0' 

Prlndpal Strain Rates (from length changes) 

£.2 

+

-----f • "!; 

t; ' ' ~- 1 I .g_ I I ":; 

I I 

1 ---r-· 

O.l11strain/yr 

Devlatorlc 

Principal Strain Rates (from angle changes) 

'Y=t; -~ 

:-----~-----, 

r-------1 

1 I I 

I I 

:ei ,....~ G=t=l:>< 

I 

I - - - ;y = o.z- -, ---- -.y=-0:3~yf' 

Figure 12. Map showing average strain accumulation rates (in parts per mi llion per year) along the Cascadia subduction zone; 

calculated from repeated geodetic surveys. Areas covered by geodetic networks are shaded; average deviatoric or principal strain 

rates are shown w ith arrows. The network labels give the times of the initial and final surveys; the principal strain rates (£1 , £2 , if 

distance changes are measured) or the total shear (y, if angle changes are measured); and the direction of maximum contraction 

(~) All results except those for northern California (Drew and Snay, 1989), Hanford (Prescott and Savage, 1984), and Gold River 

(Lisowski, 1985) are from unpublished USGS data. Rate and direction of convergence of the juan de Fuca plate relative to the 

North American plate from DeMets and others (1987). Figure courtesy of M . Lisowski. 

and others, 1981; Melosh, 1989) have been done for data 

collected over the arch in the Juan de Fuca plate; thus, it 

remains uncertain how appropriate the two-dimensional 

models are for a region of complicated three-dimensional 

structure. A particular problem is that, in the central Puget 

Sound basin, most earthquake focal mechanisms have 

P-axes striking nearly north-south, rather than N. 50° E., 

and inversions for crustal stress directions show that the 

direction of maximum horizontal stress is north-south (Ma, 

1988). Weaver and Smith (1983) suggested that there may 

be more than one crustal stress province in western 

Washington, perhaps because of major geological structures 

in the crust. 

The uncertainty surrounding crustal strain in the 

Pacific Northwest may be resolved by improved network 

coverage and by the advent of the Global Position System 

16 Program for Earthquake Hazards Assessment in the Pacific Northwest 
1 16' 0' 

~--r--"J?"--.-~------r----r __,r.:"""'~ 52' 0' 

Pacific 

Plate 

39°0' Kilometer 

132' 0' 

Juan de Fuca 

Plate 

300 

North American 

Plate 

·- ·: -

~----\ .. ~ .. -.. ~· ) 

Oregon 

/ 

( 

I 

\ 

\ 

,,_ .. __ ,_ .. - ... - .. -··-.. J .. - Caiifornia \ Nevada 

Figure 13. Map showing the locations of Global Positioning 

System (CPS) strain networks along the Cascadia subduction 

zone. Sites occupied with CPS receivers, solid squares. 

Surveys of the juan de Fuca network were made in 1986, 

1987, and 1989. Columbia and Eureka networks were 

surveyed in 1989. Many of these CPS measurements were 

made in order to recover the position of historical triangulation 

stations. Figure courtesy of M. Lisowski. 

(GPS) for making very high resolution geodetic surveys. 

The current distribution of GPS stations is shown along the 

coast of the Pacific Northwest in figure 13; this network will 

have to be expanded both along the coast and inland to the 

volcanic arc. In fact, it is a desirable program goal to 

integrate a regional GPS network designed to examine 

plate-scale strain accumulation with additional dense 

networks of stations around selected volcanic areas (Mount 

Rainier, Mount St. Helens, Medicine Lake, Mount Shasta, 

Lassen Peak, and Mount Hood) and other active, tectonic 

structures. 

Ground Failure 

One of the most disastrous natural hazards scenarios 

that one might imagine in Washington involves the repeat of 

the Osceola debris flow of about 5,700 years ago (fig. 14). 

Originating high on the slopes of Mount Rainier, this debris 

flow covered approximately 70 km2 of the flood plain of the 

White River and nearly reached Puget Sound at Tacoma 

(Crandell, 1971 ). Despite the fact that the source zone was 

a volcano, it is possible that this debris flow may not have 

been of volcanic origin (R.L. Schuster, oral commun., 

1989). The rockslide/debris avalanche (volume = 2.8 km3) 

associated with the 1980 eruption of Mount St. Helens 

destroyed much of the public and private property along the 

sparsely populated North Fork of the Toutle River valley 

(Schuster, 1983). Earthquake-induced ground failures 

(primarily landslides and liquefaction events) occurred 

throughout an area of about 30,000 km2 as a result of the 

1949 earthquake; the 1965 earthquake caused ground 

failures throughout an area of about 20,000 km2 (Keefer, 

1983). 

Earthquake-induced landslides in Washington State 

have been documented for 14 emthquakes between 1872 

and 1980 (Noson and others, 1988). There is an ongoing 

controversy concerning the extent of landslides associated 

with the 1872 earthquake (the largest known earthquake in 

Oregon or Washington). One group of researchers has 

suggested that the Ribbon Cliffs rockslide, a prominent 

feature along the Columbia River north of Wenatchee, was 

triggered by the 1872 earthquake and temporarily dammed 

the Columbia River in north-central Washington (Weston 

Geophysical Research, Inc., 1976). A study by Kienle and 

others (1978) suggested that the Ribbon Cliffs rockslide 

may not have been activated by the 1872 earthquake: it may, 

instead, have had a longer history of activity. Given the 

nature of the morphology and the materials involved, the 

slide is most likely earthquake induced (R.L. Schuster, oral 

commun., 1989). Landslides triggered by the 1949 and 1965 

earthquakes significantly damaged lifelines and property in 

the Puget Sound basin (Chleborad and Schuster, 1989). 

Given the similarities in the climate, geology, and 

physiography of Washington and Oregon, the lower level of 

seismicity in Oregon is primarily responsible for the lower 

level of losses due to earthquake-induced landslides. An 

increase in seismicity would be accompanied by increased 

landslide activity. Though the losses associated with 

historical earthquake-induced landslides in the Pacific 

Northwest have not been catastrophic, the risk to lifelines 

and rescue efforts cannot be underestimated. 

Liquefaction (the loss of internal coherency in 

shallow layers of sand buried beneath the water table) is 

another type of ground failure induced by earthquakes 

(fig. 15). Most commonly, liquefaction causes loss of 

bearing capacity and results in differential settlement, 

buoyancy, or lateral displacement of engineered structures 

such as buildings, bridges, towers, storage tanks, and 

roadways. Much of the loss during the 1964 Alaska and 

1989 Loma Prieta earthquakes was attributed to liquefaction. Liquefaction susceptibility is highest in young, alluvial 

deposits or in artificial fills in areas with high ground-water 

levels (i.e., adjacent to waterways). Thus, most of the major 

urban areas in the Pacific Northwest have areas of high 

liquefaction susceptibility. Soil liquefaction as far as 120 

km from the epicenter has been documented for the 1949 

Puget Sound basin earthquake and as far as 36 km from the 

1965 earthquake epicenter (Keefer, 1983). Liquefaction 

effects have occurred in areas experiencing Modified 

Mercalli Intensities (MMI) as low as VII (Keefer, 1983). 

Earthquake Hazards and Risk Assessment in the Pacific Northwest 17 
Figure 14. Map showing the Osceola debris flow, which originated high on the slopes of Mount Rainier about 5,700 years ago 

and flowed down both branches of the White River. A repeat of this debris flow would essential ly bury the cities of Puyallup, 

Sumner, Buckley, Enumclaw, and Auburn, along with several highways and railroad tracks; this would seriously hamper any 

response efforts. 

The occurrence of a great subduction-zone earthquake, or 

even crustal earthquakes with magnitudes of about 6-7 near 

urban areas in the Pacific Northwest, could result in high 

liquefaction losses. 

Strong Ground-Motion and 

Structural Response 

Quantitative estimates of the characteristics of strong 

motion and structural response are essential structural 

engineering components of regional seismic hazard and risk 

assessments. Estimates of ground motion are made for a 

variety of applications: earthquake-resistant design provisions for building codes, disaster mitigation, land-use 

planning, as input to the assessment of earthquake-related 

ground-failure hazards, and as input to evaluations of 

earthquake risk. 

Source depths are the largest factors controlling 

quantitative strong ground-motion in the Pacific Northwest. 

Strong ground-motion records exist from the 1949 and 1965 

Puget Sound basin earthquakes, which had source depths of 

54 and 60 km, respectively (i.e., these were intraplate 

earthquakes). Langston (1981) attempted to model observed 

18 Program for Earthquake Hazards Assessment in the Pacific Northwest 
Figure 15. Photograph showing damage to Deschutes 

Parkway, Olympia, Washington, resulting from ground 

failure induced by the 1965 earthquake. The Parkway was 

constructed on granular fill placed on tidal-flat muds; failure 

was probably due to liquefaction. Streetlights in photograph 

are approximately 8 m high. Photograph by G.W. Thorsen, 

Division of Geology, Washington Department of Natura l 

Resources. 

strong motion velocities and displacements recorded at 

three locations within the Puget Sound basin for the 1965 

earthquake. He illustrated that strong motion in the basin 

generally exhibited the characteristics of near-vertical wave 

propagation in layered structures but also exhibited complex 

site amplitude behaviors. Higher attenuation beneath Seattle 

and Tacoma relative to Olympia, though not strictly 

required, explains this complexity. Using teleseismic data to 

help constrain the velocity structure, Langston (1981) found 

evidence of dipping, large velocity-contrast interfaces and 

other heterogeneity beneath Olympia. The maximum 

acceleration observed anywhere in the basin was about 0.2 

g (this is relatively low for a magnitude 6.5 earthquake 

when compared to accelerations > 1.0 g observed for the 

1971 San Fernando and 1979 Imperial Valley earthquakes). 

Langston (1981, p. 90 I) attributes this to depth, noting that, 

If it is reasonably assumed that the 1965 accelerations 

are dominated by body waves, then they may be 

approximately corrected back to a point near the source 

using geometrical spreading. If the source was I 0 km 

from Olympia rather than about 100 km, accelerations 

could have been as high as 2 g using this simple 

technique. Accelerations for the 1949 event show 

values slightly larger than those observed for the 1965 

event. If the source was shallower at I 0 km rather than 

70 (N uttli, 1952), the geometric spreading correction 

again gives values greater than 2 g. 

Based on these analyses, Langston ( 1981) summarized that 

attenuation and lateral heterogeneity are second-order 

wave-propagation considerations compared to source depth; 

this biases strong ground-motion estimation for the Pacific 

Northwest. 

In the first of a sequence of papers leading to 

estimates of ground motions from plausible Cascadia 

subduction-zone earthquakes (i.e., interplate earthquakes), 

Heaton and Kanamori (1984) illustrated that the Cascadia 

subduction zone shares many tectonic characteristics with 

other strongly coupled subduction zones worldwide. Thus, 

Heaton and Kanamori (1984) concluded that the Cascadia 

subduction zone may be capable of producing great 

(magnitude :2: 8) shallow earthquakes. 

Noting that few strong ground-motion records exist 

for great subduction-zone earthquakes, Hartzell and Heaton 

( 1985) examined teleseismic source time functions for 

earthquakes in subduction zones worldwide. They illustrated that these functions may be used to constrain earthquake source parameters in similar subduction zones for 

which little or no strong ground-motion has been recorded. 

These source descriptions may then be used to synthesize 

near-source strong-motion records-estimates of frequency 

content and seismic radiation necessary for seismic-design 

purposes may then be made. 

Heaton and Hartzell (1986) analyzed trench bathymetry, free-air gravity profiles, seismicity, subductionzone length, width of continental margin, heat flow, Quaternary volcanism, and other characteristics of subduction 

zones and dete1mined that the Cascadia subduction zone is 

most similar to the subduction zones of southern Chile, 

southwestern Japan, and Colombia. Unlike the Cascadia 

subduction zone, however, the historical seismicity record 

of each of these three similar subduction zones contains at 

least one great (magnitude :2: 8) shallow subduction-zone 

earthquake. Heaton and Hartzell (1986) then proposed 

several hypothetical earthquake sequences for the Cascadia 

subduction zone. 

Heaton and Hartzell ( 1989) used the empirical 

Green's function technique to synthesize strong groundmotions that could result from the occurrence of these 

hypothetical earthquakes on the Cascadia subduction zone. 

They illustrated that relatively strong shaking occurs as far 

as several hundred km from magnitude 8 earthquakes. The 

great subd uction-zone earthquake synthetic responsespectra produced for the coast ranges are, at all frequencies, 

comparable to or smaller than the response spectrum from 

the 1971 San Fernando earthquake, wh ich is among the 

largest motions recorded for any earthquake. Thus, Heaton 

and Hartzell (1989) have inferred that a great subductionzone earthquake in the Pacific Northwest will produce 

strong shaking over a large region. 

Cohee and others (1991) extended the above 

methodology to simulate more regionally specific ground 

motions from a great subduction-zone earthquake at an 

array of stations covering coastal Washington and Oregon. 

Earthquake Hazards and Risk Assessment in the Pacific Northwest 19 
Their approach allowed estimation of the variability of 

ground motion due to differences in the location of the 

hypocenter and rupture surface. They assumed a velocity 

structure for Cascadia adapted from a geophysical transect 

across the Cascadia margin; fault geometries were adapted 

from several researchers. They simulated strong groundmotions at both rock (i.e., Coast Range) and soil (i.e., 

Seattle) sites in this regionally specific crust/lithosphere 

structure. They illustrated that stations close to the subduction zone will experience large ground motions for a 

relatively brief time and more distant stations will 

experience moderate ground motion for longer intervals 

(about 1 minute). Somerville and others (1989) further 

refined the method and concluded that, (1) the response 

spectral velocities in the Seattle-Portland region would be 

about twice those recorded during the 1949 and 1965 

earthquakes, (2) the duration of strong motion would be 

longer (60s instead of 10-20 s), and, (3) ground motions at 

periods longer than 1 s would be substantially larger. Thus, 

structures with long natural periods (i.e., multistory 

buildings) or structures harmonically responsive to long 

durations (i.e., bridges, storage tank farms, etc.) would be 

severely damaged, if not completely destroyed, by the 

occurrence of a great subduction-zone earthquake. 

Site effects (subsurface geologic conditions, in 

particular) play a complicated role in ground shaking and 

building damage in the Pacific Northwest (Yount, 1983). 

Ground-motion amplification is common at sites underlain 

by unconsolidated deposits over bedrock (Idriss and Seed, 

1968). Yount (1983) examined the subsurface geology of 

the Seattle area and the ground shaking effects reported for 

the 1965 earthquake. He reported that areas underlain by 

alluvium, fill, and water-saturated sand (low-impedance 

units) showed strong shaking effects where bedrock was 

near the unit but that shaking was attenuated where thick 

(> 100m) sequences of semiconsolidated Quaternary sediments separated bedrock and low-impedance units. 

FRAMEWORK FOR NEHRP STUDIES 

The characterization of geologic hazards in the 

Pacific Northwest faces a fundamental obstacle: here, more 

clearly than anywhere else in the conterminous 48 states, 

there is an overlap between hazards posed by volcanos, 

earthquakes, and landslides. Further, the fundamental tools 

to study the processes responsible for these hazards, such as 

seismic networks and Quaternary mapping, cut across 

existing program responsibilities. The overlap atuong 

geologic hazards occurs, in part, as a result of the muHiple 

nature of an initial hazard (for example, an earthquake) 'that 

may subsequently trigger secondary hazards (typically 

ground failure, a tsunami, and, unique to the Pa¢ific 

Northwest, perhaps a volcanic eruption) and, in 11>art, 

because common sources may also trigger multiple hazards. 

Heavy precipitation may cause flooding and debris and !(or) 

mud flows. Despite the multiple nature of both processes 

and hazards, geologic hazard studies and mitigation 

programs are commonly divided into studies of individual 

hazards. 

The separation of hazard studies by program or 

discipline is particularly limiting in areas like the Pacific 

Northwest where, for example, earthquake hazards are 

poorly understood and a variety of scientific opinion exists 

surrounding individual aspects of the hazards problem. 

Nonetheless, the USGS must contribute to detailed studies 

of hazards and risk assessment in urban areas, to framework 

studies on regional and local scales, and to plate-scale 

studies of the subduction environment. There are four 

necessary components of the USGS/NEHRP program 

designed to address, in varying degrees, the spatial characteristics, expected magnitudes, recurrence intervals, 

ambient strain, source characteristics, site response, and 

attenuation associated with the three types of earthquakes 

known to occur in subduction zones like those in the Pacific 

Northwest. The immediate need for hazards and risk 

assessments in urban areas is obvious, and a variety of 

regional and local geologic and geophysical studies are 

therefore necessary. Furthermore, a comprehensive assessment of the capability of the subduction interface to produce 

earthquakes possibly as large as magnitude 9 makes platescale experiments mandatory for hazard determinations. In 

addition, the complementary study of the nature and 

evolution of magmatic arcs, central to the subduction 

regime of this region, allows development of comprehensive tectonic models that can anchor hazards 

assessments. 

Necessary Program Components 

I. Monitoring seismicity and deformation in the 

Pacific Northwest 

• Upgrade, expand, and maintain the seismic network 

to monitor earthquakes in the Pacific Northwest. 

• Deploy a network to monitor crustal deformation in 

the Pacific Northwest. 

The coordinated operation and maintenance of 

expanded regional seismic networks, appropriate U.S. 

National Seismograph Network (USNSN) stations, and an 

array of strong-motion stations from Cape Mendocino to the 

Canadian border and from the coast inland across the 

Cascade Range are necessary. The Earthquake Program 

should fund an appropriate portion of the network, 

including the installation of enough new stations in 

southwestern Oregon to provide monitoring capability of 

the entire subduction zone (fig. 16). Appropriate USNSN 

stations should be installed, and the data from these stations 

should be made available to academic institutions in the 

Pacific Northwest. Regional monitoring of Cascade Range 

20 Program for Earthquake Hazards Assessment in the Pacific Northwest 
132°0' 

51°0' 

0 

Pacific Plate 

British Columbia 

39°30'-;---.~,---'-.:.==.:!::::___l]~~1!&;L-A~~U...U__,j,- 13200' 

Figure 16. Map showing regional seismic network stations in 

the Pacific Northwest. Solid triangles represent stations 

operating on August 1, 1990. Most stations in British Columbia 

are operated by the Geological Survey of Canada in Sidney; 

stations in Washington and Oregon are operated by the 

University of Washington; stations in Nevada are part of the 

University of Nevada at Reno network; most stations in 

California are part of the U.S. Geological Survey "Calnet" 

operated in Menlo Park. 

volcanoes, except Mount St. Helens, falls within this 

objective. 

Free-field strong-motion instrumentation needs to be 

installed at new sites in Oregon, additional sites in 

Washington (particularly in the Pasco basin and along the 

SHZ), as well as in representative coastal areas of northern 

California (fig. 17). This objective includes expanding the 

existing short-period network (operated by the University of 

Washington) in Oregon and Washington and linking it with 

the northern California portion of the USGS network 

operated in Menlo Park. Data from these short-period 

networks, as well as the network operated by the Geological 

Survey of Canada, should utilize the real-time, digital 

telemetry and storage systems being designed for the 

USNSN. A regionally based research center that would 

maintain the seismic and strong-motion networks, archive 

the data collected, perform routine processing, and provide 

basic data to all researchers should be formally established. 

The University of Washington performs most of these 

functions currently-thus, expansion of their program with 

resources to provide better linkage to State surveys and 

interested regional universities and the f01mal designation 

of the University of Washington as a regional earthquake 

study center should be considered. 

The purpose of this monitoring is to obtain a much 

better description of potential earthquake sources in the 

WASHINGTON 

OREGON 

_______ l _____ _ 

CALIFORNIA NEVADA 

0 200 Kilometers 

Figure 17. Map showing strong-motion accelerometer 

stations operating in the Pacific Northwest. Open tri::~.ngles 

mark locations of stations installed either within or on 

engineered structures; solid triangles are free-field or ground 

sites. Accelerometers are generally set to trigger when ground 

acceleration exceeds 0.01 g; this corresponds to a local 

earthquake of approximate magnitude 4.5 or larger. ~trongmotion stations do not record initial P-wave motions; instead, 

they record subsequent motions of the building or ground. 

Pacific Northwest. This description by necessity will recognize the three different source zones. Expanded seismic 

monitoring will determine if the Juan de Fuca plate l'·~neath 

central and southern Oregon is truly aseismic. 

The development of a crustal deformation network to 

identify and (or) quantify accumulated strain should be 

encouraged. Initial deployment probably should be 

undertaken along the strike of the subduction zore. The 

development of Global Positioning System (GPS) technology makes this a realistic goal, and such an effort is 

required by the large source region of potential subductionzone earthquakes. The present GPS network (fig. 17) does 

Framework for NEHRP StudieC' 21 
not unequivocally provide the data necessary to distinguish 

between the possibility of a great subduction zone on the 

interface (involving source dimensions of hundreds of 

kilometers) and more local events (perhaps in the magnitude 

7 and above range). This objective includes the need for 

detailed monitoring of deformation at selected Cascade 

Range volcanoes and other active tectonic structures as they 

are identified. Close coordination with and participation in 

experiments with the Geological Survey of Canada is 

necessary; the current experiment across the Strait of 

Georgia should be continued. 

II. Tectonic Framework Studies 

Integrate geologic and geophysical studies to better 

understand the tectonic regime of the Pacific 

Northwest. 

Define the geometry of the subduction-zone 

interface. 

Determine the spatial, temporal, and source 

characteristics of damaging earthquakes in the 

Pacific Northwest. 

The tectonics of the Pacific Northwest subduction 

zone must be well understood. Despite comparisons 

between the Cascadia subduction zone and other zones 

around the world and geological evidence of movement 

during Holocene time along the Washington and Oregon 

coasts, the lack of seismicity on the thrust interface between 

the Juan de Fuca and the North American plates makes 

analysis of the potential for great thrust earthquakes 

equivocal. The data recorded by the expanded network, 

combined with the additional broad-band data collected by 

the USNSN, will provide the data base necessary for 

sophisticated structural modeling of the Juan de Fuca-North 

American plate interaction. In the absence of earthquake 

activity on the thrust interface, other data that will allow the 

definition of active processes occurring in the subduction 

zone are needed for a complete assessment of the hazards 

facing this region. Processes such as tectonic underplating 

in the thrust interface zone, splay faulting in coastal 

margins, active magmatism of the andesitic arc, or the 

interaction of backarc extension in the Basin and Range 

with subduction tectonics of the forearc region need to be 

understood to allow earthquake hazards of the Cascadia 

subduction zone to be placed in the proper plate tectonic 

framework. Central to these data is an understanding of the 

long-term effects of convergent margin tectonics on the 

crust of North America and on the interface between the two 

plates. In particular, bedrock mapping of critical areas at a 

1:250,000 scale (i.e., Bellingham and Vancouver, Washington) must be completed. Tectonic syntheses of seismic, 

geodetic, geologic, gravity, and other data must be 

undertaken; these studies should include Geographical 

Information Systems (GIS) approaches. A regional GIS data 

base must be established. 

This component of the program includes Quaternary 

studies in the Puget Sound basin and Willamette Lowland. 

All of the ongoing studies of Quaternary geology (geologic 

mapping, stratigraphy, paleoseismicity) need to be continued and expanded appropriately to realistically constrain 

ground-motion studies and associated shaking hazards. This 

component also includes investigations of the nature of the 

late Quaternary record of deformation. This effort is currently underway along numerous portions of the Pacific 

coast and is largely responsible for the rapidly growing 

acceptance of the possibility of great thrust earthquakes in 

the Pacific Northwest. The observations and interpretations 

of coastal marsh-zone stratigraphy need to be rigorously 

tested and debated. This component has recently been 

expanded into the Puget Sound basin and includes field 

studies of evidence of deformation, the modeling of such 

evidence, and the incorporation of these obfervations and 

models into the larger plate tectonic and structural setting. 

The Earthquake Program must suppc:t a comprehensive series of seismic, magnetotelluric, and geologic 

studies aimed at understanding the crustal architecture. The 

new Deep Continental Studies (DCS) Pacific Northwest 

study provides a vehicle for Earthquake Program funding to 

help define the issues of interest to the DCS p~ogram and to 

considerably broaden the scope of DCS invest:gations in the 

region. Earthquake Program funding should be targeted to 

investigate the nature of the thrust interface l'y means of a 

series of north-south profiles in Washington and northern 

Oregon (fig. 17); these profiles would serve as strike-lines 

for an east-west transect from the Grays Harbor, Washington, area eastward across the Columbia Plateau and 

could possibly be conducted in conjunction with offshore 

experiments under the EDGE program of the National 

Science Foundation. The Earthquake Program should take 

advantage of the unique prospect of seismic and magnetotelluric profiling in northern California across the landward 

extension of the fold and thrust belt that forms the offshore 

deformation front. Finally, crustal-structure investigations 

are needed of local features such as the central Puget Sound 

basin, where recent evidence of late Holocene uplift of 

marine terraces may indicate large, crustal earthquake 

activity in heavily populated urban areas. 

Ill. Improve Seismic Hazard and Risk Asr~ssments 

Delineate zones corresponding to each cf the three 

types of seismic sources. 

Identify areas expected to experience strong ground 

shaking and (or) amplification from each of the 

different types seismic sources. 

Identify areas subject to ground failure, including 

liquefaction. 

Encourage loss-estimation studies, including 

inventories and responses of man-made structures. 

22 Program for Earthquake Hazards Assessment in the Pacific Northwest 
126°7' 

49°15' ...,..----~~""'!"! 

115°0' 

..,n-------,------r.----...,..-.., 49°25' 

. . 

-FY91 

---- FY92 

•••••••• FY93 

-·-· FY94 111111111 FY95 

---- FY96 

FY97 

Refraction, MT 

Refraction,MT 

Marine Reflecton 

Refraction,MT 

Refraction.MT 

Refraction,MT 

Open 

39°50' L.-_,__ ___ __,__....;,. __ ,__ ___ _._ ______ __. 

125°0' 

Figure 18. Map showing proposed locations of crustal stru ctu re profiles to be funded e ither completely or in part by the 

Earthquake Program. These profil es wi ll be planned in conj unction with the USGS Deep Contine ntal Studies Program, Pacific 

Northwest experiment. 

This component is geographically restricted to major 

urban centers in the Pacific Northwest, particularly the 

Willamette Valley of Oregon and the Puget Sound region in 

Washington, as well as the St. Helens seismic zone and the 

Pasco basin (fig. 18). The principle objective of this 

component is to revise existing assessments of earthquake 

hazards and risk in urban areas. Included in this objective 

are: the collection of seismic and geological data necessary 

to produce models of strong ground-motions and complete 

the earthquake history, the calculation of estimates of these 

Framework for NEHRP Studies 23 
strong ground-motions, estimation of expected ground 

failures, and deterministic and probabilistic estimation of 

risk. An updated catalog of all strong ground-motion 

monitoring sites in the Pacific Northwest should be prepared and kept current as the number of stations changes. 

The deployment of a free-field strong ground-motion 

network should be encouraged in the populated regions of 

the Pacific Northwest. The data from this network should be 

supplemented with building monitoring using a portable 

network. 

Providing estimates of losses in urban regions from 

earthquakes and, if appropriate, from earthquake-induced 

landslides is also included in this component. Methods for 

obtaining inventories of buildings, lifelines, and transportation systems should be established. Strong ground 

shaking and ground failures would severly impact these 

125°0' 116°0' 

49°0'r===;-~~~~==.====;====r===r===r=~)f[==--, 49°0' 

· Washington 

(!) • 

\L!i.___/17"7"'-, M 0 UN T.. . 

~~-tTl· RAINIER. !~ . Cl,, e River 

·~mbia River 

PORTLAND 

. "'i):D, 

if c,~ • 

' PASCO 

~ASI N 

... 

Oregon 

It\ Newberry 

L.::>. Volcano 

0 100 

Kilometers 

Magnitude Scale 

2.5- 3.9 

(1) 4.0- 4.9 

(1) 5.0 - 5.9 

C) 6.0- 6.9 

C) 7.0- 7.9 

eJ 0 

..c: 

~ - •' 

Q) 

Figure 19. Map showing areas of special hazards or special studies. The Puget Sound, Portland, and Eugene urban 

areas are emphasized. Open triangles, Quaternary stratovolcanoes; open octagons, earthquake epicenter locations. The 

St. Helens zone (5HZ), Mount St. Helens, and Mount Rainier areas are identified hazards that require continued 

detailed monitoring and ongoing study. The degree of earthquake hazard in the Pasco basin is uncertain; however, 

considerable risk from earthquake shaking effects may exist for the people living in the area. 

24 Program for Earthquake Hazards Assessment in the Pacific Northwest 
facilities, which, generally, have not been designed to resist 

earthquakes. 

Another objective of this component is to encourage 

the establishment and maintenance of systems to monitor 

and provide appropriate warnings to the population in areas 

where geologic hazards are particularly well-defined. For 

example, Mount St. Helens represents a clearly defined 

hazard that requires continued USGS monitoring and 

communication of results to the public. This objective 

should contribute to development of integrated computerbased information systems, mentioned previously. These 

systems must be designed to meet the needs of engineering 

and scientific users but should be flexible enough to allow 

nontechnical users to view typical end-product results. The 

data bases and display systems should be as machine 

independent as possible, considering existing technology 

and realistic considerations of funding. 

A necessary cooperative project for this component is 

to complete an inventory of ground failure hazards in the 

Puget Sound and Willamette Valley areas. This inventory 

should be funded by the Landslide Program, but it should 

rely, as necessary, on cooperation from NEHRP scientists. 

This objective recognizes the need to produce comprehensive susceptiblity maps for ground failures of local 

origin within the study areas (such as landslides and 

liquefaction) as well as to investigate the hazards associated 

with large-volume rock slides and debris flows originating 

on Mount Rainier or other high-elevation areas outside of 

urban centers. 

IV. Cooperative Hazard Mitigation Studies 

Encourage implementation of earthquake-hazard 

mitigation measures. 

Encourage preparedness and response programs. 

The fourth component of the USGS/NEHRP program 

in the Pacific Northwest must be the integration of research 

activities into preparedness, response, recovery, and mitigation activities. Scientific results must be communicated to 

the engineering and planning communities in formats that 

are understandable and usable. The primary responsibility 

of the USGS under this component of the program is to 

perform feasibility studies that will develop and (or) test 

methods for quantifying earthquake hazards and risks. Once 

a method is developed and (or) tested, it is necessary for the 

USGS to translate the results of these scientific studies into 

reports and onto maps so that the nature and extent of 

hazards and risks may be understood by nontechnical users. 

Then, end users (usually other Federal agencies, States, or 

municipalities) may decide what methods they wish to 

adopt for complete regional or local programs of implementation, preparednes's, and mitigation. This element will 

require a new cooperative effort between the USGS and 

State agencies because many of the final products are best 

described, overseen, and checked by local scientists who are 

most capable of communicating with user groups in 

Oregon, Washington, and California. Working togetb~r. the 

USGS and representatives from Oregon, Washingtcn, and 

California will develop the pilot products each State c~sires; 

methods of large-scale implementation will then be d~vised. 

We will try to convene a scientific oversight committee that 

includes knowledgeable USGS scientists who will be 

available to help evaluate locally directed work unc~r this 

component. Projects under this component will be subject to 

peer review, but they will have different criteria than those 

used for the three scientific components. 

Included in this component are meetings or workshops that focus on current science and issues in the Pacific 

Northwest. We include in this component meetings of 

scientists working under other components-this will provide 

State groups with the opportunity to be kept fully informed 

on the relatively rapid changes that are occurring in the 

scientific understanding of earthquake hazards of this 

region. These workshops should be jointly designed by the 

USGS and the States of Oregon, Washington, and 

California. 

Relation to the USGS Earthquake Hazar~s 

Reduction Program 

In November 1990, the USGS adopted a revised 

earthquake plan to guide agency investigations for fiscal 

years 1991 to 1995. Scientific work under the new earthquake plan is organized around four goals: (I) understanding 

the earthquake source, (II) evaluating the earthquake 

potential, (III) predicting the effects of earthquakes, and 

(IV) utilizing research results. The four necessar)' components in our plan for the Pacific Northwe~t are, 

appropriately, subsets of the goals of the new national plan. 

Our components I and II largely span the objective0 listed 

under goal II of the national plan, and our components III 

and IV are direct equivalents of goals III and IV of the 

national plan. The specific objectives of goal I of the 

national plan (determining the physical and mec'l.anical 

behavior of active, crustal fault zones and developing 

quantitative models of the physics of the earthquake process) are not region specific. 

CONCLUSIONS 

A great subduction-zone earthquake that could 

devastate the Pacific Northwest is possible; crustal and 

intraplate earthquakes near population centers of the Pacific 

Northwest are probable. Either type of earthquake would 

severely impact the region. Loss of life could be high and 

damages could easily run into the billions of dollars. Thus, 

Conclusions 25 
the principle scientific objectives of the USGS/NEHRP 

program in the Pacific Northwest are to establish estimates 

of the probabilities of the occurrences of crustal, intraplate, 

and great subduction-zone earthquakes and to establish 

estimates of the effects and damage due to each type of 

earthquake. Since earthquake hazards in the Pacific Northwest are both regional and local in scale, the earthquake 

hazard assessment program must be both regional and local 

in scale. The program must include: (1) regional and local 

monitoring of seismicity and deformation, (2) tectonic 

framework studies, (3) improved seismic hazard and risk 

assessments, and ( 4) cooperative hazard-mitigation studies. 

The resulting scientific information will provide the 

foundation necessary for State governments, local governments, and the private sector to implement effective programs for earthquake hazard mitigation, preparedness, and 

response on regional and local scales. 

ACKNOWLEDGMENTS 

We are grateful to the U.S. Geological Survey Branch 

of Central Technical reports, in particular Rick Scott, for the 

outstanding preparation of this Circular on very short 

notice. We thank Randy Updike and Bob Schuster for their 

detailed reviews. We also thank Rob Wesson for providing 

the impetus for us to write this Circular. 

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1978, Investigation of the Ribbon Cliff landslide, Entiat, 

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28 Program for Earthquake Hazards Assessment in the Pacific Northwest 
APPENDIX-PACIFIC NORTHWEST SCIENCE PLAN WORKSHOP PARTICIPANTS 

The Office of Earthquake, Volcanoes, and 

Engineering of the U.S. Geological Survey initiated the 

process of formulating a broad science plan for the Pacific 

Northwest with a workshop in Golden, Colorado, on March 

7 and 8, 1989. Although the results of that meeting have 

already been published elsewhere (Shedlock and Weaver, 

1989), many of the ideas expressed there are included in this 

document. Participants in that workshop were: 

• Algermissen, S.T. (U.S. Geological Survey) 

• Bucknam, Robert C. (U.S. Geological Survey) 

• Heaton, Thomas H. (U.S. Geological Survey) 

• Lisowski, Michael (U.S. Geological Survey) 

• Madole, Richard F. (U.S. Geological Survey) 

• Muffler, L.J. Patrick (U.S. Geological Survey) 

• Priest, George R. (Department of Geology and 

Mineral Industries, Oregon) 

• Rogers, Garry C. (Geological Survey of Canad"') 

• Schuster, Robert L. (U.S. Geological Survey) 

• Shedlock, Kaye M. (U.S. Geological Survey) 

• Stanley, William D. (U.S. Geological Survey) 

• Tabor, Roland W. (U.S. Geological Survey) 

• Updike, Randall G. (U.S. Geological Survey) 

• Walsh, Timothy J. (Department of Natural Reso•trces, 

Washington) 

*U.S. GOVERNMENT PRINTING OFFICE: 1993-774-049/66046 Appendix-Pacific Northwest Science Plan Workshop Participants 29 



SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS 

Periodicals 

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly). 

Technical Books and Repons 

Professional Papers are mainly comprehensive scientific reports of 

wide and lasting interest and importance to professional scientists and engineers. Included are reports on the results of resource studies. and of 

topographic, hydrologic, and geologic investigations. They also mclude 

collections of related papers addressing different aspects of a single sci entific topic. 

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. They include the results 

of resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic. 

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. 

Cb'culars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest. 

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs. 

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information. 

Maps 

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 7 1/2- or 15-minutequadrangleformats (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. 

Geo.physical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology. 

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photo geologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon. 

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas. 

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential. 

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations. 

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data ofbothregular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1 :250,000 scale or smaller. 

Catalogs 

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List) 

"Publkations of the Geological Survey, 1879- 1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche. 

"Publications of the Geological Survey, 1962- 1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche. 

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche. 

Supplements for 1982, 1983, 1984, 1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form. 

State catalogs, "List of U.S. Geological Survey Geologi~ and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only 

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only. 

Selected copies of a monthly catalog "New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092. 

Note.--Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.