Cover. Insert, ground-shaking damage from the 1949 Puget Sound earthquake to unreinforced masonry in Seattle, Wash. Photograph by George Cankonen, Seattle Times. Background, landslide damage to the railbed between Olympia and Tumwater, Wash., in the

1965 Puget Sound earthquake. Photograph by Greg Gilbert, Daily Olympian.

Cover design by Carol A. Quesenberry.
M A R C H 3, 18 4 9

.S. DEPARTMENT OF THE INTERIOR

U

Assessing Earthquake Hazards and

Reducing Risk in the Pacific Northwest

Albert M. Rogers, Timothy J. Walsh, William J. Kockelman, and

George R. Priest, Editors

Volume 2

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1560

An investigation of the earthquake potential in the

Pacific Northwest and examination of the measures

necessary to reduce seismic hazards

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1998
For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

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

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Assessing earthquake hazards and reducing risk in the Pacific Northwest / Albert M. 

Rogers...[et al.], editors.

p. cm.—(U.S. Geological Survey professional paper ; 1560)

Includes bibliographical references.

Supt. of Docs. no.: I 19.16:1560

1. Earthquake hazard analysis—Northwest, Pacific. 2. Earthquakes—Northwest, 

Pacific. I. Rogers, A.M. II. Series.

QE535.2.U6A825 1997

363.3′495—dc20

ISBN 0-607-89262-5 95–37559

CIP
III

 CONTENTS

VOLUME 2

EARTHQUAKE HAZARDS

An Introduction to Predicting Earthquake Hazards and Losses in the

 Pacific Northwest 

By Albert M. Rogers and George R. Priest................................................ 307

GROUND-MOTION PREDICTION

Engineering Characterization of Earthquake Strong Ground Motions in the

 Pacific Northwest

By Walter J. Silva, Ivan G. Wong, and Robert B. Darragh ......................... 313

Simulated Strong Ground Motions for Magnitude 8 Earthquakes on the

 Cascadia Subduction Zone

By Brian Cohee and Paul Somerville.......................................................... 325

Earthquake Ground-Response Studies in West and South Seattle, Washington

By David L. Carver, Kenneth W. King, Robert A. Williams, and

David M. Worley ................................................................................. 345

Earthquake-Hazard Geologic Maps of the Portland, Oregon, Metropolitan Area

By Ian P. Madin ........................................................................................ 355

GROUND FAILURE

Ground Failure Associated with the Puget Sound Region Earthquakes of April

13, 1949, and April 29, 1965

By Alan F. Chleborad and Robert L. Schuster ............................................ 373

Evaluation of Liquefaction Potential in Seattle, Washington

By W. Paul Grant, William J. Perkins, and T. Leslie Youd.......................... 441

IMPLEMENTATION

Reducing Earthquake Hazards—An Introduction

By William J. Kockelman............................................................................ 477

Techniques for Reducing Earthquake Hazards

By William J. Kockelman............................................................................ 479

Earthquake Risk-Reduction Prospects for the Puget Sound and Portland, 

Oregon, Areas

By Peter J. May ........................................................................................... 497

Integrated Tsunami-Hazard Assessment for a Coastal Community,

Grays Harbor, Washington

By Jane Preuss and Gerald T. Hebenstreit .................................................. 517

Approaches to Seismic-Hazard Mitigation by Local Governments—An Example

from King County, Washington

By Derek B. Booth and John P. Bethel ....................................................... 537

Liability for Earthquake Hazards or Losses and its Impacts on the Cities and

Counties of Washington

By Jeanne B. Perkins and Kenneth K. Moy................................................ 543
IV CONTENTS

TABLE OF CONVERSION FACTORS

Multiply By To obtain

Area

square meter (m2) 10.76 square foot (ft2)

square kilometer (km2) 0.3861 square mile (mi2)

Density

gram per cubic centimeter (g/cm3) 62.43 pound per cubic foot (lb/ft3)

Length

millimeter (mm) 0.03937 inch (in.)

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi)

Volume

cubic centimeter (cm3) 0.06102 cubic inch (in.3)

cubic meter (m3) 35.31 cubic foot (ft3)

cubic kilometer (km3) 0.2399 cubic mile (mi3)

Abbreviations: ka, thousand years ago; Ma, million years ago
EARTHQUAKE HAZARDS
Preceding page. Insert, debris left by the April 29, 1965, Seattle, Wash., earthquake.

Photograph courtesy of NOAA/EDIS. Background, destruction caused by the fall of an

unbraced masonry parapet in downtown Klamath Falls, Oreg., during the Sept. 20, 1993,

M 5.9 and M 6.0 earthquakes. Photograph by Lou Sennick of the Herald and News,

Klamath Falls, Oreg. (from Dewey, J.W., 1993, Damages from the 20 September earthquakes near Klamath Falls, Oregon: Earthquakes & Volcanoes, v. 24, no. 3, p. 121–128).
307

AN INTRODUCTION TO PREDICTING

EARTHQUAKE HAZARDS AND LOSSES IN

THE PACIFIC NORTHWEST

By Albert M. Rogers1 and George R. Priest2

INTRODUCTION

The articles in the Earthquake Hazards section of the

professional paper discuss ground-shaking and

ground-failure hazards and the estimation of losses associated with these hazards. Ground shaking and ground failure

are the major factors in loss of life and property during earthquakes. The delineation of these hazards by mapping and

site-specific prediction techniques is an important step in the

process of reducing the effects of earthquakes. Ground-shaking and ground-failure maps, for example, can be valuable in

land-use-policy development, siting or relocation of local

government emergency facilities, and urban-renewal decisions. Furthermore, hazard maps can aid in the design and

siting of lifelines (see volume 1, Glossary) and ordinary

structures and in emergency planning and response, each

requiring advance information about the likelihood of earthquake damage to infrastructure. Estimates of the magnitude

of economic losses and mortality during future earthquakes

are also partly based on hazard maps. Loss estimates are not

only useful in planning for earthquakes but also serve decision makers in establishing preventive actions and determining the rate at which resources should be expended to reduce

earthquake effects.

Preparation of hazard maps requires extensive regional

development of databases, a process that is far from

complete in the Pacific Northwest. Useful hazard maps, for

example, are based on detailed mapping of geologic

deposits and measurement of their physical properties and

on topography, sediment thickness, basin geometry,

water-table depth, attenuation of ground motion, and the

seismic and geologic mapping of young faults capable of

producing damaging earthquakes. A new earth-science field

called paleoseismology has emerged that provides information about recurrence rates on faults and the time since the

last earthquake, data that can be incorporated into probabilistic ground-motion maps or used to forecast the probability of the next earthquake during a chosen time interval.

Database development will also be of great value in continued research to improve hazards-prediction methodology.

Site-specific hazard estimates are most useful in the

design of critical facilities and high-occupancy buildings

such as bridges, power plants, hospitals, and high-rise

structures. The information used for site-specific estimates

is generally more detailed than that required to produce hazard maps. Such estimates are commonly based on borehole

measurements at the site and modeling of local ground

motions based on these measurements and also on data such

as regional ground-motion attenuation and fault locations.

Estimating ground-shaking and ground-failure hazards

in this region is complicated by three factors. First, several

types of earthquakes are likely (see Rogers and others, volume 1, for a discussion of earthquake types), and each type is

expected to produce damage that differs in geographic distribution and level of intensity. Second, very few

strong-motion records exist that would permit calibration of

models or generalization about the characteristics of ground

shaking for the region. Furthermore, no records exist for the

types of Pacific Northwest earthquakes having the greatest

potential for destruction, that is, the great Cascadia

thrust-fault earthquake and the shallow continental-crust

earthquake. Third, the types of data needed to produce

microzonation maps for strong motion, ground failure, and

losses are not yet available except in limited areas.

Nevertheless, some estimates of these factors are

possible. This volume presents work to develop several

types of databases, research to predict site-specific estimates

of ground shaking for several earthquake types, research to

map limited areas that depict some types of ground failure,

and the estimation of one type of economic loss. In the

following, we review the reports that contribute to the

________________ 1U.S. Geological Survey, Box 25046, MS 966, Federal Center, Denver,

CO 80225. 2Oregon Department of Geoscience, 910 State Office Building, 1400

S.W. 5th Ave., Portland, OR 97201.
308 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

understanding of these problems. A review of earlier work

on this topic is presented in the introduction to this professional paper (Rogers and others, volume 1).

GROUND FAILURE

One of the first steps in the development of a

ground-failure prediction capability is the assessment of past

ground failures. This assessment is important in two

respects. First, we know from experience elsewhere that

ground failure is likely to recur in many of the same locations

during successive earthquakes, given the same conditions.

Second, past ground failures tell us about the local conditions, earthquake magnitudes, and epicenter distances likely

to produce ground failures in the future. Chleborad and

Schuster (this volume) present a study of ground failures

associated with the 1949 and 1965 Puget Sound earthquakes.

Failures from these events were spread across a large part of

the Puget Sound region and northwestern Oregon and

included landslides, ground settlement, ground cracks, and

liquefaction effects such as sand boils and lateral spreading.

Other miscellaneous effects that may have been related to

ground failure, such as broken and bent underground pipes,

are also mapped. These maps demonstrate the widespread

susceptibility of this region to earthquake-induced ground

failure and the concentration of these effects near bodies of

water, along bluffs, and in lowland areas with a high water

table. For larger earthquakes or earthquakes with shallower

focal depths than the 1949 and 1965 events, the effects are

expected to be more widespread and intense.

Grant and others (this volume) present a map showing

the estimated liquefaction hazard for Seattle based on

standard penetrometer measurements in more than 300

borings, depth to the water table, geologic maps of surficial

units, and unit thickness. They develop two sets of criteria

for liquefaction that in combination permit the delineation of

areas of high, moderate, low, and very low liquefaction

potential. This map shows that the zones of high and

moderate liquefaction are concentrated along shoreline

areas, in areas of fill, and along the Duwamish River tide flats

and Interbay (Smith’s Cove) areas. The map should help in

development of land-use policies, estimation of future losses,

recognition of the need for site-specific studies for some

types of structures, disaster planning, and other decision

making by local governments and citizens.

GROUND-MOTION ESTIMATION

DATABASE DEVELOPMENT

That near-surface geologic conditions can influence the

level of strong ground shaking is well known (for example,

see Kanai, 1952; Gutenberg, 1957; Borcherdt and others,

1975; and Rogers and others, 1985). The effects can be large

at some places; consequently, any attempt to estimate future

ground motions must account for the influence of site conditions in some manner. Several methods are available to predict these effects. In one method, detailed geologic maps are

prepared that discriminate between the youngest geologic

units that are most likely to influence shaking levels. From

such information, hazard maps can be prepared that depict

relative changes in expected shaking levels that can be

expressed in terms such as low, medium, and high. Other

methods might express these changes in terms of maps of

Modified Mercalli intensity increments (or any other earthquake-intensity measure) associated with specific surficial

sediments or Modified Mercalli intensity for hypothetical

earthquakes on specific faults. Linear or nonlinear models of

sediment responses can be used to calculate ground-shaking

levels relative to rock if data for these calculations are available. Measurement of the actual response of each geologic

unit using strong-motion recordings or recordings of local

earthquakes and blasts can also provide a measure of spectral

levels relative to rock. If calculations or measurements of

this kind are available for enough sites that can be correlated

with surficial geologic conditions, maps can be constructed

depicting the relative changes.

King and others (1990) and Carver and others (this volume) have collected data that partly establish the influence of

near-surface sediments on the level of ground shaking in

Olympia and Seattle, Wash. In these studies, local earthquakes and blasts were recorded on a variety of geologic

units and at sites for which the intensity of shaking in the

1949 and 1965 Puget Sound earthquakes could be established. The studies establish a correlation between shaking

intensity in damaging earthquakes and relative spectral levels for typical sediment types. Amplification of shaking by

as much as 800 percent has been observed in the Pacific

Northwest on some alluvium types for some spectral bands.

Because soils respond nonlinearly when subjected to very

strong motion, such large amplifications are not commonly

experienced in large earthquakes. However, these levels can

be used to determine a qualitative measure of the relative

geographic variation in shaking. For some alluvium types

and for low to moderate shaking levels, relative factors determined from small regional earthquakes and blasts may accurately predict variations in strong-motion levels (for

example, see Borcherdt and others, 1975; Borcherdt and

Glassmoyer, 1992; and Rogers and others, 1985).

Madin (this volume) compiles maps of faulting and

thickness of surficial sediments in the Portland, Oreg., region

that depict alluvial units likely to affect ground-shaking levels. These maps have as a basis compilations of earlier geologic mapping combined with existing borehole logs. Such

maps are the first step in studies to evaluate the

ground-shaking, ground-failure, and faulting potential in this

region. Based on this work, maps depicting these hazards
AN INTRODUCTION TO PREDICTING EARTHQUAKE HAZARDS AND LOSSES, PACIFIC NORTHWEST 309

are, in fact, in progress for the Portland area (George R.

Priest, Oregon Department of Geology and Mineral

Industries, written commun., 1992).

SITE-SPECIFIC GROUND-SHAKING

ESTIMATES

Cohee and others (this volume) compute

strong-motion time histories and spectra at sites underlain

by both rock and soil for a moment magnitude (Mw) 8

subduction-zone rupture on the Cascadia thrust fault using a

theoretical earthquake source model. The calculations were

made on a geographic grid that includes the Portland and

Seattle regions for two independent hypothetical Mw 8

earthquakes, one west of Seattle and the other west of Portland. These postulated earthquakes would produce damaging ground motions over large areas of western Oregon and

Washington. Although the highest values, equaling about

0.6g (see Acceleration in the glossary) at soil sites, are

computed near the outer coast of Oregon and the Olympic

Peninsula, significant damaging ground motions of about

0.1g near Portland to 0.3g near Seattle are indicated at sites

underlain by alluvium. These calculations show that strong

motions at this level can be expected to last 10 seconds or

more. Unfortunately, the calculations do not model the part

of the ground-motion spectrum that would affect structures

taller than about 10 stories. These long-period ground

motions are dependent on sediment geometry and

near-surface sediment properties, which are only partly

modeled in the calculations. Considerable research is

needed to evaluate the long-period shaking levels in the

major urban areas in a manner that realistically accounts for

the effects of geologic deposits and basin geometry.

Silva and others (this volume) model both

Benioff-zone and Cascadia thrust-fault ruptures using a

band-limited white-noise earthquake source. The model is

tested by comparing model predictions against the

strong-motion records available from the 1949 and 1965

Puget Sound earthquakes. In modeling the thrust-fault

earthquake, Silva and others (this volume) assume a Mw 8.5

event. This simulation yields peak acceleration values of

about 0.15g at Seattle, considerably smaller than that determined by Cohee and others (this volume). This discrepancy

is due to assumptions about the extent of downdip rupture.

Silva and others (this volume) assume downdip rupture is

limited to a point near the outer coast, whereas Cohee and

others (this volume) assume rupture occurs farther downdip,

to points well within the Olympic Peninsula. This result

demonstrates the importance of improving our understanding of thrust-fault properties and slip mechanisms. At

present, these issues are unresolved.

These studies provide a valuable first step toward our

goal to evaluate the degree of hazard and risk due to earthquakes in the Pacific Northwest. Nevertheless, it is clear

that much additional work is needed. With respect to

ground-failure and site effects, more detailed geologic maps

of young deposits are needed in urban areas by measuring

the properties of such geologic units using standard penetrometer measurements or borehole shear velocities. The

estimation of site effects in records of regional earthquakes

recorded at different basin locations and on different geologic units should also be continued to fully understand the

effects of geology on shaking levels. These records would

also serve as a database for modeling such effects in order to

estimate ground motions at other locations. Probabilistic

ground-shaking estimates for this region need revision to

incorporate the potential Cascadia thrust-fault earthquake,

newly discovered continental-crust earthquake sources, and

new information on paleoseismicity recurrence rates.

LOSS ESTIMATES

In a demonstration of Geographic Information System

(GIS) techniques for loss estimation, Wang and others

(1991) calculated earthquake-induced losses to water and

sewer systems in Portland. Their study includes development of methods for inventory of facilities and formulation

of an empirical loss-estimation algorithm that depends on

earthquake shaking and liquefaction effects. They modeled

two possible earthquakes, a surface-wave magnitude (Ms)

8.4 subduction-zone event and a Ms 6.5 local event, for their

demonstration project. Dollar losses were tabulated by

sewer- and water-pipe size, type of construction, materials,

shaking intensity, and soil conditions. The subduction-zone

earthquake was predicted to cause more than $4 million

damage to both water and sewer pipelines in one drainage

basin. Just as important, however, is that this type of study

and methodology can quickly show the location of probable

damage in map form, which can be important for emergency

planning and eventual system redesign.

Much additional work is needed to extend the study of

potential earthquake damage to water and sewer systems in

other urban areas and to begin studies of damage to other

infrastructure elements such as roads, power-distribution

systems, bridges, pipelines, and other facilities. In addition,

updated regional loss studies are needed in light of much

new data concerning the earthquake hazards in the Pacific

Northwest (for example, see Rogers and others, volume 1).

CONCLUSION

The studies reported in this volume, though only partial,

show some of the types of research that can increase our

understanding of earthquake effects and expected losses in

the Pacific Northwest. It is clear from such studies that

economic loss, life loss, and disruption of urban infrastructure is expected to be high for most earthquake-occurrence
310 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

hypotheses. It is important to continue such studies in order

to understand the hazards and to make realistic plans that

minimize the effects and facilitate responses to future earthquakes. The long-term effect of mitigation based on these

plans will be to reduce the economic burden of damaging

earthquakes and to save lives.

REFERENCES CITED

Borcherdt, R.D., and Glassmoyer, Gary, 1992, On the characteristics of local geology and their influence on ground motions

generated by the Loma Prieta earthquake in the San Francisco

Bay region, California: Seismological Society of America Bulletin, v. 82, no. 2, p. 603–641.

Borcherdt, R.D., Joyner, W.B., Warrick, R.E., and Gibbs, J.F.,

1975, Response of local geologic units to ground shaking, in

Borcherdt, R.D., ed., Studies for seismic zonation of the San

Francisco Bay region: U.S. Geological Survey Professional

Paper 941–A, p. A52–A67.

Gutenberg, Beno, 1957, Effects of ground on earthquake motion:

Seismological Society of America Bulletin, v. 47, no. 3, p.

221–250.

Kanai, K., 1952, Relation between the nature of surface layer and

the amplitude of earthquake motion: University of Tokyo,

Earthquake Research Institute Bulletin, v. 30, p. 31–37.

King, K.W., Tarr, A.C., Carver, D.L., Williams, R.A., and Worley,

D.M., 1990, Seismic ground response studies in Olympia,

Washington, and vicinity: Seismological Society of America

Bulletin, v. 80, no. 5, p. 1057–1078.

Rogers, A.M., Tinsley, J.C., and Borcherdt, R.D., 1985, Predicting

relative ground response, in Ziony, J.I., ed., Evaluating earthquake hazards in the Los Angeles region—An earth-science

perspective: U.S. Geological Survey Professional Paper 1360,

p. 221–248.

Wang, L.R.L., Wang, J.C.C., and Ishibashi, Isao, 1991, GIS applications in seismic loss estimation model for Portland, Oregon

water and sewer systems: U.S. Geological Survey Open-File

Report 91–441F, 71 p.
Ground-Motion Prediction
AN INTRODUCTION TO PREDICTING EARTHQUAKE HAZARDS AND LOSSES, PACIFIC NORTHWEST 307

Preceding page. Insert, damage from the 1949 Puget Sound earthquake. Photograph by

William P. Conser, by permission of the Olympia Heritage Commission. Background,

destruction caused by the fall of an unbraced masonry parapet in downtown Klamath

Falls, Oreg., during the Sept. 20, 1993, M 5.9 and M 6.0 earthquakes. Photograph by Lou

Sennick of the Herald and News, Klamath Falls, Oreg. (from Dewey, J.W., 1993, Damages

from the 20 September earthquakes near Klamath Falls, Oregon: Earthquakes &

Volcanoes, v. 24, no. 3, p. 121–128).
313

 ABSTRACT

Strong-motion recordings of earthquakes in the Pacific

Northwest are few in number and nonexistent for events along

the megathrust fault of the Cascadia subduction zone. The prediction of strong ground shaking from future large earthquakes

in the region based on an empirical approach is hampered by

this lack of data. In this study, strong ground motions for the

1949 surface-wave magnitude (Ms) 7.1 Olympia and 1965 Ms

6.5 Seattle-Tacoma earthquakes in Washington have been

computed in terms of acceleration response spectra based on

a numerical modeling technique that incorporates the bandlimited-white-noise earthquake source model and randomvibration theory. The estimates compare favorably with the

actual records of these earthquakes, as recorded at two soil sites

in Seattle and Olympia, although the model underpredicts the

motions for the latter. Based on this calibration of the technique, acceleration response spectra for a hypothetical

moment magnitude (Mw) 8.5 Cascadia subduction-zone

earthquake have also been predicted for the same two soil sites.

At source-to-site distances of 101 km and 146 km for Olympia

and Seattle, respectively, the estimated peak horizontal ground

accelerations are 0.15g and 0.14g (where g is 980 cm/s2).

Because these values strongly depend upon the assumed

crustal damping beneath western Washington and the location

of the eastern extent of rupture of a potential Cascadia subduction-zone earthquake, they should be viewed as approximations. Based on our analysis, the effects of near-surface

soils and the properties of the underlying rock will likely be significant factors controlling strong ground motions in the Puget

Sound region and other geologically similar regions in the

Pacific Northwest such as the Willamette Valley.

INTRODUCTION

An essential element in the seismic design of engineered

structures is a quantitative estimate of the characteristics of

strong ground motion. Of particular importance is a specification of the peak levels of ground motion as well as spectral

content as typically characterized by response spectra. The

spectral content is reasonably well defined for crustal earthquakes of approximate moment magnitude (Mw) 6–7 occurring in western North America (Mohraz, 1976; Seed and

others, 1976; Joyner and Boore, 1988). However, recent observations of strong ground motions in other tectonic regimes

have revealed significant differences in the spectral content

of earthquakes recorded at rock sites. Ground motions

recorded in stable tectonic regimes typical of eastern North

America may have significantly higher frequency content and

larger peak values than corresponding motions typical of

active regimes like western North America (Boore and Atkinson, 1987; Toro and McGuire, 1987; Silva and others, 1989;

Silva and Darragh, 1995). In the seismotectonic setting of the

Pacific Northwest, which includes the Cascadia subduction

zone, ground motions may also be unique. However, few

strong-motion records exist and the prediction of strong

ground shaking must rely on data from other regions, including

other subduction zones, if traditional empirical techniques are

to be used.

In the past decade, numerical modeling techniques have

been developed in an effort to provide alternative approaches

to ground-motion prediction. Such techniques have been used

in evaluating strong ground motions in the Pacific Northwest

including the Puget Sound region (Langston, 1981; Langston

and Lee, 1983; Ihnen and Hadley, 1986; Cohee and others,

1991; Wong and Silva, 1994) and the Portland, Oreg., area

(Wong and others, 1990, 1993). One such technique

incorporates the use of a stochastic earthquake source model

called the band-limited-white-noise (BLWN) model and

random-vibration theory (RVT). This approach has been

remarkably successful in predicting peak ground motion

ENGINEERING CHARACTERIZATION OF

EARTHQUAKE STRONG GROUND MOTIONS IN

THE PACIFIC NORTHWEST

By Walter J. Silva,1 Ivan G. Wong,2 and Robert B. Darragh3

________________ 1Pacific Engineering & Analysis, 311 Pomona Ave., El Cerrito, CA

94530. 2Woodward-Clyde Federal Services, 500 12th Street, Oakland, CA

94607. 3California Division of Mines and Geology, Sacramento, CA 95816.
314 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

values as well as spectral ordinates in different tectonic

regimes (Hanks and McGuire, 1981; Boore, 1983; Boore and

Atkinson, 1987; Silva and others, 1989; Schneider and others,

1993; Wong and Silva, 1993).

In this study, we have applied the BLWN-RVT methodology to compute 5-percent-damped acceleration

response spectra to compare with the 1949 surface-wave

magnitude (Ms) 7.1 Olympia and 1965 Ms 6.5 SeattleTacoma earthquakes in Washington as recorded by the

strong-motion instruments in the Highway Test Office in

Olympia and the Federal Office Building in Seattle (1965

earthquake only). Both earthquakes occurred within the

subducting Juan de Fuca plate. Site-specific shear-wave

velocity and density data for these two sites and the source

parameters of the two earthquakes were used in the

analysis.

Of particular importance to seismic hazards in the Pacific

Northwest is the possibility of a great earthquake (Mw greater

than 8) occurring along the megathrust of the Cascadia subduction zone beneath western Washington and Oregon. Based

on the calibration of the BLWN-RVT approach using the 1949

and 1965 events, 5-percent-damped acceleration response

spectra for a postulated Mw 8.5 megathrust earthquake have

also been estimated for the Olympia Highway Test Office and

Seattle Federal Office Building sites.

The focus of this study is to incorporate the effects of

appropriate source, region-specific path, and site-specific

parameters in the evaluation of ground motions. Such effects

influence ground motions at periods of greatest engineering

significance, between 0.1 and 1.0 s. Large-scale two- and

three-dimensional effects on ground motions such as those due

to basin geometry, however, have not been incorporated into

our analysis. Basin effects, as suggested by Langston (1981)

and Ihnen and Hadley (1986), can be significant for longperiod ground motions in the Puget Sound.

ACKNOWLEDGMENTS

This study was supported by the Professional Development Programs of Woodward-Clyde Federal Services and

Pacific Engineering & Analysis. Sue Penn, Anita Quintana,

and Sylvia Li assisted in the preparation of this paper. Our

thanks to Bill Joyner and Kenneth Campbell for their

thoughtful reviews of an early version of this paper and Al

Rogers for his review of the final version.

APPROACH USED IN THE STUDY

The BLWN-RVT ground-motion methodology has

been applied to a worldwide data set of earthquakes in the

range of Mw 1.5–8.1 in an analysis of rock motions

recorded at distances generally less than 50 km (Silva and

Darragh, 1995). This included four earthquakes, among

them the September 19, 1985, Ms 8.1 Michoacan

mainshock, that occurred in the subduction zone along the

coast of western Mexico and were recorded by the Guerrero strong-motion network. The spectral content of these

events has been modeled quite well for periods of 0.03–4 s

at distances to the rupture surface as close as 16 km (Silva

and Darragh, 1995). The technique has also shown that the

controlling factors in the specification of strong ground

motion at rock sites for engineering design are moment

magnitude, source-to-site distance, and the rock properties

directly beneath the site. Specifically, the near-surface

attenuation modeled through the approximate parameter κ

exerts a predominant effect upon spectral composition for

frequencies greater than about 3 Hz. Below this frequency,

Mw or seismic moment through corner frequency (see

equation 3) controls spectral shapes in the BLWN-RVT

ground-motion methodology.

An additional advantage of the BLWN-RVT methodology is the ability to easily incorporate site-specific nonlinear

soil response directly into the ground-motion analyses using

RVT in an equivalent-linear formulation. This is an important

consideration in seismic-hazard evaluations in the Pacific

Northwest because of widespread alluvial deposits beneath

most of the major urban areas in the Puget Sound region and

the Willamette Valley of Oregon.

The BLWN ground-motion model first developed by

Hanks and McGuire (1981) assumes a point source with

energy distributed randomly over the duration of the source.

The model assumes an ω−2 source model (Brune, 1970,

1971) with a single corner frequency and a constant stress

drop (Boore, 1983). The acceleration spectral density, a(f),

is given by

where f is frequency;

Mo is seismic moment;

R is distance to the equivalent point source;

βo is shear-wave velocity at the source;

Q(f) is the frequency-dependent quality factor;

A(f) are near-surface amplification factors;

P(f) is the high-frequency truncation filter;

fc is source corner frequency; and

where ρo is the density at the source (fig. 147). C is a constant

that accounts for the free-surface effect (factor of 2), the Swave source radiation pattern averaged over a sphere (0.55)

(Boore, 1986), and the partition of energy into two horizontal

components . In order to compute peak-time domain

C

o o

= 1 2 0 55 1

2 3 ρ β ( )( ) . π ) ) ) ) (2)

af C (1) f M

R Pf Af e o Q f o ( ) =

+

− 2

1

β ()() ( )

f

f

c) ) 2

π fR

(1 2 )
ENGINEERING CHARACTERIZATION, EARTHQUAKE STRONG GROUND MOTIONS, PACIFIC NORTHWEST 315

values, that is, peak acceleration and peak oscillator

response, RVT is used to relate root-mean-square computations to peak value estimates (Boore, 1983; Boore and

Joyner, 1984).

Source scaling is provided by specifying two independent parameters, Mo and the stress drop (∆σ) (fig. 147). The

stress drop relates fc to Mo through the relation

The spectral shape of the single-corner-frequency ω−2

source model is then described by the two free parameters

Mo and ∆σ (Silva, 1991). The corner frequency increases

with the shear-wave velocity and stress drop, both of which

are region dependent.

The P(f) filter models the observation that acceleration

spectral density appears to fall off rapidly beyond some

region-dependent maximum frequency. This observed phenomenon truncates the high-frequency part of the spectrum

and is responsible for the band-limited nature of the model.

In the Anderson and Hough (1984) attenuation model, the

form of the P(f) filter is

The function κ(r) is a site- and distance-dependent

parameter that represents the effect of intrinsic attenuation

on the seismic waves as they propagate through the crust

from source to receiver. The parameter κ depends weakly on

the epicentral distance (r) and on both the shear-wave velocity (Vs) and quality factor (Qs) averaged over a depth of H

beneath the receiver or site. At zero epicentral distance, κ is

given by

The value of κ(0) (herein referred to as kappa) is attributed to attenuation in the very shallow crust directly beneath

the site (Hough and Anderson, 1988). Silva and Darragh

(1995) suggested that the predominant kappa effects extend

from the surface down to several hundred meters and possibly as deep as 1–2 km. The intrinsic attenuation along this

part of the path is thought to be frequency independent but

site dependent (Hough and others, 1988). For a typical western North America rock site, kappa values are in the range of

about 0.02–0.06 s (Boore, 1986; Silva and Darragh, 1995).

The acceleration spectral density, a(f), models direct

shear waves in a homogeneous half-space (with effects of a

velocity gradient through the A(f) filter). For vertically

heterogeneous layered structures, the plane-wave

propagators of Silva (1976) are used to propagate SH or

P–SV motion through the layered structure.

In a half-space model, the near-surface amplification

factors, A(f), account for the increase in amplitude as the

seismic energy travels through lower velocity crustal

Figure 147. Schematic diagram of the band-limited-white-noise and random-vibration-theory approach used in this study to

estimate ground motions due to earthquakes. Small arrows show relative motion across fault surface. Mo, seismic moment; f,

frequency; fc, source corner frequency; βo, shear-wave velocity at the source; ∆σ, stress parameter; R, distance to the equivalent point

source; Q(f), frequency-dependent quality factor where Qo and η are model parameters; A(f), near-surface amplification factors; κ,

near-surface seismic-wave attenuation parameter where κ(0) is the attenuation directly beneath the site; H, depth; Vs, shear-wave

velocity; Qs, shear-wave damping; and ρ, density in the site geologic profile.

(3)

Pf e f (r) = −π κ ( )

κ 0 = H

v Qs s

( ) (5)

(4)

f M c o

o

= β ∆σ

8 44

1 3

. ) )

Geologic

profile

( V ρ) s,Qs,

S-waves

S-waves

Finite fault

(summation of

point sources)

Rupture

initiation

PATH OPERATOR:

POINT SOURCE:

SITE OPERATOR:

A(f) e –πκf

where κ(0) =

where

Mof 2

1+(f/fc)2

where Q(f) = Qo f η

-πfR

βo Q(f)

R

e

∆σ 8.44 Mo ( )1/3

H

VsQs - -

fc=βo
316 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

materials near the surface. These factors depend on average

crustal and near-surface shear-wave velocity and density.

Western United States amplification factors developed by

Boore (1986) have typically been used in the past to account

for the amplification by near-surface velocity gradients. If

detailed shear-wave velocity data are available for a site, it is

more desirable to use such information instead of

amplification factors.

The anelastic path attenuation from the source to just

below the site is modeled with the frequency-dependent

quality factor Q(f) where Q(f)=Qo f η and Qo and η are model

parameters. Geometric attenuation is taken as l/R or 

for distances greater than 100 km.

In order to accommodate the effects of site-specific soil

response, the BLWN power spectrum of the rock outcrop

motion is propagated through the one-dimensional soil

profile using the plane-wave propagators of Silva (1976). In

this formulation, only SH waves are considered. Arbitrary

angles of incidence may be specified, but normal incidence

is used throughout the present analyses.

In order to deal with possible material nonlinearities,

the equivalent-linear formulation is used. RVT is used to

predict peak time-domain values of shear strain based upon

the shear-strain power spectrum. In this sense, the procedure

is analogous to the computer program SHAKE (Schnabel

and others, 1972) except that peak shear strains in SHAKE

are measured in the time domain. The purely frequencydomain RVT approach obviates a time-domain control

motion and, perhaps just as significantly, eliminates the need

for a suite of analyses based on different input motions.

FINITE-FAULT METHODOLOGY

A methodology that combines aspects of finite earthquake source modeling (Hartzell, 1978; Irikura, 1983) with

the BLWN point-source model has also been developed to

produce response spectra as well as time histories appropriate for engineering design (Silva and others, 1990). The

approach is very similar to the empirical Green’s-function

summation methodology introduced by Hartzell (1978) and

Irikura (1983). In this case, however, the BLWN point

source is substituted for the empirical Green’s function.

Peak accelerations, peak velocities, and response spectra

(when time histories are not produced) are estimated using

RVT. The model can accommodate a region-specific Q(f),

Green’s-function sources of arbitrary seismic moment or

stress drop, and site-specific kappa values. A detailed

description of the methodology is contained in Schneider

and others (1993) and Wong and Silva (1993).

STRONG-MOTION DATA AND INPUT 

PARAMETERS

The strongest earthquakes to have shaken the Puget

Sound region this century occurred in 1949 and 1965.

Fortunately, both events were recorded by at least one

strong-motion station (either the Olympia Highway Test

Office or the Seattle Federal Office Building), and these

records largely constitute the available empirical data for

earthquakes in the region. As such, several investigators (for

example, Langston, 1981) have used these records to evaluate strong ground motions in the Puget Sound region. It

must be noted, however, that neither strong-motion station is

in the desired free field. The Olympia instrument is located

in a one-story wood-framed building. The Seattle accelerograph is located in the subbasement of an eight-story

structure, 4.6 m below grade. Despite these possible complications, we have attempted to model these earthquakes with

the intent of calibrating our approach.

INTRAPLATE EARTHQUAKES

The 1949 Olympia earthquake occurred at a depth of 54

km and at an epicentral distance of about 5 km from the

Olympia Highway Test Office (Baker and Langston, 1987)

(fig. 148). Peak horizontal ground accelerations of 0.16g

and 0.28g were recorded at this site (table 22). The 1965

Seattle-Tacoma earthquake occurred at a depth of 60 km and

at epicentral distances of 21 km and 61 km from the Seattle

Federal Office Building and Olympia Highway Test Office,

respectively (Langston and Blum, 1977) (fig. 148). Peak

accelerations of 0.06g and 0.08g were recorded as horizontal

components at the Federal Office Building, whereas horizontal values of 0.14g and 0.20g were recorded at the Highway Test Office. As also noted by others (for example,

Langston, 1981), larger peak horizontal accelerations were

recorded at the Highway Test Office than at the Federal

Office Building although the earthquake was closer to the

latter site.

Ground motions for the 1949 and 1965 earthquakes

were calculated using the BLWN-RVT point source

approach. Given the relatively long distances to Seattle or

Olympia and the estimated source dimensions for either

earthquake, a point source for both earthquakes was

assumed valid. The input parameters required in the modeling are as follows: (1) earthquake source parameters including Mw and stress drop; (2) distance between the site and a

point-source representation of the fault-rupture plane; (3)

propagation-path parameters (assuming a half-space)

including βo, ρo, Qo, and η; and (4) site parameters such as

Vs and ρ specified as a function of depth, kappa, and appropriate shear-modulus reduction and damping curves for the

soil and unconsolidated sediments overlying rock at each

site.

Stress drops have not been estimated for either the 1949

or 1965 earthquakes. A stress drop of 100 bars, typical of

western North America earthquakes (see Hanks and

McGuire, 1981) was assumed in the modeling of the

intraplate events.

(1 R )
ENGINEERING CHARACTERIZATION, EARTHQUAKE STRONG GROUND MOTIONS, PACIFIC NORTHWEST 317

The propagation path for the intraplate events was characterized by a βo of 4.5 km/s and ρo of 3.05 g/cm3 based on

the P-wave crustal model for western Washington used in

routine earthquake locations (Ludwin and others, 1991). A

Poisson’s ratio of 0.25 was used to determine S-wave velocities. A Qo of 380 and η of 0.39 for the Cascadia subduction

zone were assumed based on estimates by Atkinson (1995).

The hypocentral distances from the 1949 and 1965

earthquakes to the Olympia Highway Test Office and the

Seattle Federal Office Building were adopted as the pointsource-to-site distances in the BLWN-RVT modeling (table

22).

Geologic profiles for the two recording sites were

developed based on downhole data collected by Shannon &

Wilson, Inc., and Agbabian Associates (1978) (fig. 149). At

Olympia, a 156-m-deep borehole was almost entirely within

Figure 148. Cross section of the Cascadia subduction zone at latitude of southern Puget Sound in Washington. Divisions of subduction

zone are adopted from Hyndman and Wang (1993). The rupture of a great megathrust earthquake is assumed to extend into one-third of

the transition zone. Arrows show relative motions of plates. Temperatures (T) represent lower and upper bounds for transition zone in

which stable sliding occurs. The zone separates the locked zone, where stick-slip sliding occurs and earthquakes may nucleate, from the

ductile zone, where plastic deformation takes place due to high temperatures and where no earthquake rupture can occur.

SEA

LEVEL

20

40

60

80

100

Megathrust rupture

Pacific Ocean

OLYMPIC

MOUNTAINS

Washington

coast

T = 350°C

T = 450°C

Locked zone Transition zone

Puget

Sound

Olympia

CASCADE

RANGE

Major concentration

of crustal seismicity

NORTH AMERICA PLATE Ductile zone

DEPTH, IN KILOMETERS

JUAN DE FUCA PLATE

Seattle

1949

earthquake

1965 earthquake

0 25 50 KILOMETERS

Table 22. Observed and predicted median peak horizontal ground accelerations for earthquakes in the Puget Sound region.

[OHT, Olympia Highway Test Office; FED, Seattle Federal Office Building; (obs), observed; (pred), predicted; (--), not required]

________________________________________________________________________________________________________________________________________________________

 Horizontal1 Source-to-site2

 Recording Earthquake Depth distance distance Magnitude Stress drop Peak horizontal

 station (kilometers) (kilometers) (kilometers) (bars) acceleration (g) ________________________________________________________________________________________________________________________________________________________

________________________________________________________________________________________________________________________________________________________

 1For the 1949 and 1965 earthquakes, this distance is equivalent to epicentral distance. For a Cascadia subduction-zone event, this is the shortest horizontal

distance to the vertical projection of the rupture plane on the Earth’s surface.

 2For the 1949 and 1965 earthquakes, this distance is equivalent to hypocentral distance. For a Cascadia subduction-zone event, this is the shortest distance

to the rupture plane.

OHT

OHT

FED

OHT

FED

April 13, 1949

April 29, 1965

April 29, 1965

Cascadia subduction zone

Cascadia subduction zone

54

60

60

25

25

 5

 61

 21

 98

144

 54

 85.6

 63.6

101

146

MS 7.1

MS 6.5

MS 6.5

MW 8.5

MW 8.5

100

100

100

--

--

0.16, 0.28 (obs)

0.15 (pred)

0.20, 0.14 (obs)

0.07 (pred)

0.06, 0.08 (obs)

0.10 (pred)

0.15 (pred)

0.14 (pred)
318 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

glacial sediments. Low-strain shear-wave velocities ranged

from 165 m/s at the surface to 1,000 m/s at a depth of nearly

152 m (fig. 149). At Seattle, fill material was found to a

depth of 7 m. The natural deposits at the site have velocities

ranging from 198 to 1,000 m/s to a depth of 122 m (fig. 149).

Kappa values of 0.04 s and 0.06 s were assumed appropriate

for the rock underlying the soils and unconsolidated sediments at the Olympia and Seattle sites, respectively, based

on comparisons with similar rock types where site-specific

kappa values have been estimated (Silva and Darragh, 1995).

Rock in the geologic profiles was assumed to occur below

the depths of 144 m at Olympia and 101 m at Seattle. Shearmodulus reduction and damping curves appropriate for soils

comprising sands, gravels, and low-plasticity-index clays

were used (Electric Power Research Institute, 1993). The

degradation curves accommodate the effects of confining

pressure on modulus reduction and damping and are

implemented for profiles extending to 305 m (Electric Power

Research Institute, 1993).

CASCADIA MEGATHRUST EARTHQUAKE

For the finite-fault modeling of the Cascadia subduction-zone earthquake, the rupture plane was defined as eastward-dipping (average of 7°), 280 km long, and 120 km

wide. The rupture width was estimated based on the model

of the megathrust proposed by Hyndman and Wang (1993)

for offshore northern Washington. We assume the rupture

will not only include the locked portion of the megathrust but

will also extend downdip about one-third of the width of the

transition zone (Wong and others, 1993) (fig. 148). The

rupture length was constrained to a value where the rupture

area (length times width) would be appropriate for a Mw 8.5

Figure 149. Geologic profiles for the Olympia Highway Test Office and Seattle Federal Office Building strong-motion recording sites

in Washington. Vs is shear-wave velocity, and ρ is density.

DEPTH

(m) GEOLOGY DESCRIPTION

OLYMPIA HIGHWAY TEST OFFICE

GEOLOGY DEPTH

(m) DESCRIPTION ρ

(g/cm3

)

Fill

Loose to medium

 clayey, silty,

 fine sand

Very dense,

 silty, gravelly

 fine sand

Very dense

 silty clay

 with gravel

152 1.5

1.5

1.6

411

427

Hard silty clay

 with some

 gravel

503 1.7

Very dense,

 silty, sandy,

 fine gravel and

 silty, gravelly,

 fine to coarse

 sand with some

 cobbles

762 1.9

Quaternary

 glacial drift

 deposits

SEATTLE FEDERAL OFFICE BUILDING

Loose sand 165 1.5

Medium dense

 fine to medium

 sand

220 1.5

330 1.5

Interbedded

 very stiff to

 hard fine sandy

 silt and very

 dense silty fine

 to medium sand

Very dense

 fine to

 medium

 sand

350 1.5

450 1.6

500 1.6

575 1.6

975 2.0

270 1.5

VS (m/s)

9

7

16

33

47

65

101

122

198 1.5

610 1.8

1,000 2.0

Interbedded

 hard silty clay

 and very dense

 silty fine sand

1,000 2.0

ρ

(g/cm3

)

VS (m/s)

Fill

Glacio-

 fluvial (?)

 deposits

Glacio-

 lacustrine (?)

 deposits

Glacio-

 fluvial (?)

 deposits

Glacio-

 lacustrine (?)

 deposits

3

12

41

65

93

110

126

144

156

20
ENGINEERING CHARACTERIZATION, EARTHQUAKE STRONG GROUND MOTIONS, PACIFIC NORTHWEST 319

earthquake based on the empirical relationship between rupture area and magnitude by Wells and Coppersmith (1994).

Both the Seattle Federal Office Building and Olympia

Highway Test Office sites were assumed to be located

approximately in the middle along the strike of the rupture

plane model, given our lack of knowledge on the possible

segmentation of the Cascadia subduction zone.

A total of 50 randomized slip models (fig. 150) were

used to estimate ground motions, given the absence of information on the actual slip distribution of a future Cascadia

megathrust earthquake. The randomized slip models were

generated in the spatial domain using a process that preserves the area and number of asperities based on the slip

models derived from a number of large earthquakes. Large

slips near the edges of the rupture plane were suppressed by

applying a cosine taper. Fifteen elements were taken along

strike and eight elements along dip, giving a 280 km by 120

km rupture plane. Slip is initiated across the fault using a

constant rupture velocity (circular rupture front) of 3.04 km/

s. Because we do not know where rupture may nucleate in

a future Cascadia megathrust earthquake, points of rupture

initiation (foci) were also randomized along a 224-km-long

zone (80 percent of the length of the rupture zone) centered

in the deeper, east half of the rupture plane.

For the megathrust earthquake, a βo of 3.9 km/s and ρo

of 2.8 g/cm3 were assumed appropriate for the source region

based on the western Washington P-wave crustal model of

Ludwin and others (1991). The Qo and η values used in

modeling the attenuation for the intraplate earthquakes were

also assumed appropriate for the megathrust event. The

source-to-site distances to Olympia and Seattle were 101

km and 146 km, respectively, although we estimate these

values may be uncertain by as much as several tens of kilometers. These distances extend from the sites to the east

edge of the megathrust rupture, which is located at a depth

of about 25 km (fig. 148).

RESULTS AND DISCUSSION

Based on the above input parameters, acceleration

response spectra were computed for (1) the 1949 earthquake

at the Olympia Highway Test Office, (2) the 1965 earthquake at both the Olympia Highway Test Office and the

Seattle Federal Office Building, and (3) a Mw 8.5 Cascadia

megathrust earthquake at both sites. Figure 151 compares

the recorded and predicted motions for the 1949 event at the

Olympia Highway Test Office in terms of 5-percentdamped spectral acceleration (Sa) normalized by the peak

horizontal acceleration (amax). The use of this parameter,

Sa/amax, allows for a direct comparison of the spectral

shapes. The 5-percent-damped recorded spectral shape is

the average between the two horizontal components.

The match between the two spectral shapes is relatively good although there is an underprediction at short

periods and an overprediction at periods longer than 0.6 s.

Strong velocity contrasts in the geologic profiles (fig. 150)

are responsible for the peaks in the computed motions. The

large overprediction is likely a result of poorly known

properties in the deeper part of the profile and points out the

need to incorporate uncertainties by randomizing soil properties in modeling ground motions. The predicted peak horizontal acceleration for the 1949 event at the Olympia

Highway Test Office is 0.15g compared to the average

recorded value of 0.22g (table 22). A somewhat higher

stress drop of about 130 bars, very much within the range of

stress drops for western North America earthquakes, would

result in matching the observed peak acceleration.

Comparisons between the 5-percent-damped spectral

shapes of the recorded 1965 earthquake at both the Olympia

Highway Test Office and the Seattle Federal Office Building

and as predicted by the BLWN-RVT model are shown in

figure 152. As with the 1949 event, the spectral shape of the

predicted 1965 motions shows an underprediction at a

period of about 0.1 s and an overprediction at periods

beyond 0.2 s compared to the actual recorded motions at the

Olympia site. The predicted peak horizontal acceleration is

0.07g compared to an average value of 0.17g for the

recorded motions. The reason for this underprediction is

unknown although this difference has been noted by other

investigators (for example, Langston, 1981). Shakal and

Toksoz (1979) suggested that higher values of Qs are characteristic of the Olympia site as compared with the Seattle

site. More detailed borehole and upper crustal information

on vs and Qs is needed to resolve this inconsistency.

In contrast, the predicted peak horizontal acceleration

0.10g for the Seattle site is slightly higher than the actual,

average value of 0.07g (table 22). In general, the spectral

shapes of the recorded 1965 earthquake and the model prediction compare very favorably, with a slight model underprediction at long periods. Whereas these differences are

likely due to site effects, the site-specific modeling captures

quite well the large overall differences in spectral composition seen in the motions recorded at the two sites.

Based on this calibration of the BLWN-RVT model

and path and site parameters, we have computed ground

motions for a postulated Mw 8.5 Cascadia megathrust earthquake. Figure 153 shows the absolute 5-percent-damped

acceleration response spectra for both the Olympia and Seattle sites. Although lacking the site resonant peaks exhibited

by the 1965 earthquake, the differences in the spectral

shapes of the Seattle and Olympia sites (fig. 153) again illustrate the significant site response effects of the near-surface

geology. At a source-to-site distance of 101 km, the predicted median peak horizontal acceleration is 0.15g at Olympia (fig. 153, table 22). For Seattle, the model-predicted

median peak horizontal acceleration is 0.14g. The Seattle

site exhibits a greater high-frequency site amplification than

the Olympia site, as exemplified by its peak horizontal

acceleration, even though it is 45 km farther from the rupture
320 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST Figure 150. Four of the 50 randomized slip-distribution models used in the estimation of ground motions for the hypothesized 

Mw 8.5 Cascadia subduction-zone earthquake.

Contours represent coseismic slip along the rupture planes, normalized to the maximum slip. Contour interval is 0.2. Areas of large slip are called asperities.

0

40

80

120

0 40 80 120 160 200 240 280

0

40

80

120

0 40 80 120 160 200 240 280

0

40

80

120

0 40 80 120 160 200 240 280

0

40

80

120

0 40 80 120 160 200 240 280

LENGTH OF RUPTURE, IN KILOMETERS

DOWNDIP WIDTH OF RUPTURE, IN KILOMETERS

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1 0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5 0.5

0.5

0.5

0.5

0.5

0.5

0.7

0.7 0.7

0.7 0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.9

0.9

0.9

0.9

0.9

0.9

0.7
ENGINEERING CHARACTERIZATION, EARTHQUAKE STRONG GROUND MOTIONS, PACIFIC NORTHWEST 321

101

101

100

100

10-1

10-1

10-2

10-2

PERIOD, IN SECONDS

MAGNIFICATION (Sa/amax)

EXPLANATION

Average of two recorded horizontal

 components (5 percent damping)

Spectral shape predicted by the model

 (5 percent damping)

101

101

100

100

10-1

10-1

10-2

10-2

MAGNIFICATION (Sa/amax)

EXPLANATION

Average of two recorded horizontal

 components (5 percent damping)

Spectral shape predicted by the model

 (5 percent damping)

101

101

100

100

10-1

10-1

10-2

10-2

PERIOD, IN SECONDS

MAGNIFICATION (Sa/amax)

EXPLANATION

Average of two recorded horizontal

 components (5 percent damping)

Spectral shape predicted by the model

 (5 percent damping)

OLYMPIA

SEATTLE

zone of the megathrust earthquake. Based on an empirical

attenuation relationship for subduction-zone earthquakes

and rock-site conditions (Youngs and others, 1988), median

peak horizontal accelerations for a Mw 8.5 megathrust event

at distances of 101 km and 146 km are estimated to be 0.12g

and 0.09g, respectively.

An important aspect of any numerical modeling

approach is a proper statistical estimate of uncertainty. Total

uncertainty is a combination of modeling and parametric

uncertainties. A quantitative assessment of the modeling

uncertainty associated with both the BLWN-RVT point

source and finite-fault approach has been computed based

on analyses of the 1989 Mw 7.0 Loma Prieta, Calif., earthquake (Schneider and others, 1993). The parametric uncertainties permit a rapid and cost-effective means of assessing

which source, path, and site parameters are controlling the

ground motions for a particular application.

The parametric uncertainties for the predictions of the

megathrust earthquake are illustrated in figure 154 and listed

in table 23. At all frequencies, the site profiles at both sites

dominate the uncertainties in the ground motions, suggesting that a reduction in uncertainty is attainable with detailed

site investigations. Source effects (focus and slip) are also

large at low frequencies, with slip variation (asperity location) tending to remain constant with frequency (fig. 154).

The path damping parameter Qo is not a major contributor to

the uncertainty in the ground-motion predictions because of

the low attenuation in the Cascadia subduction zone, as

suggested by Atkinson (1995). Because the motions are low,

the effects of soil nonlinearity are insignificant, showing a

near-zero uncertainty for modulus reduction and damping.

SUMMARY

Predicted peak accelerations and acceleration response

spectra for the 1949 and 1965 Puget Sound earthquakes

based on the BLWN-RVT ground-motion methodology

Figure 151. Observed and predicted 5-percent-damped acceleration response spectral shapes of the April 13, 1949, earthquake recorded at the Olympia Highway Test Office site. Spectral shape is

defined by the parameter Sa/amax, where Sa is spectral acceleration

and amax is peak horizontal acceleration.

Figure 152. Observed and predicted 5-percent-damped acceleration response spectral shapes of the April 29, 1965, earthquake recorded at the Olympia Highway Test Office and Seattle Federal

Office Building sites.
322 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

0.71

102

0.50

101

0.25

100 0

10-1

0.75

102

0.50

101

0.25

100 0

10-1

FREQUENCY, IN HERTZ

OLYMPIA

SEATTLE

NATURAL LOG OF THE STANDARD ERROR NATURAL LOG OF THE STANDARD ERROR

EXPLANATION

Variation of focus

Variation of η

Variation of κ

Variation of shear

 modulus

Variation of slip

Variation of damping

Variation of shear-

 wave profile

Variation of Qo

EXPLANATION

Variation of focus

Variation of η

Variation of κ

Variation of shear

 modulus

Variation of slip

Variation of damping

Variation of shear-

 wave profile

Variation of Qo

100

101

10-1

100

10-2

10-1

10-3

10-2

SPECTRAL ACCELERATION (g)

100

101

10-1

100 10-1 10-2

PERIOD, IN SECONDS

OLYMPIA

SEATTLE

EXPLANATION

Median horizontal spectrum (5 percent damping)

Median minus one standard error

 horizontal spectrum (5 percent damping)

 —16th percentile

Median plus one standard error

 horizontal spectrum (5 percent damped)

 —84th percentile

EXPLANATION

Median horizontal spectrum (5 percent damping)

Median minus one standard error

 horizontal spectrum (5 percent damping)

 —16th percentile

Median plus one standard error

 horizontal spectrum (5 percent damped)

 —84th percentile

SPECTRAL ACCELERATION (g) 10-2

10-3

were calibrated against actual recordings made at the

Olympia Highway Test Office and Seattle Federal Office

Building. An application of the methodology predicts

median peak horizontal accelerations of 0.14g for the downtown Seattle site and 0.15g for the Olympia site from a

postulated Mw 8.5 Cascadia subduction-zone earthquake.

The uncertainty in these values is large and is driven by

uncertainties in the shear-wave velocity profile beneath each

site as well as the source-to site distance of the rupture zone

of a future megathrust event. The need for more site-specific

studies is exemplified by the observation that the site

response at the Olympia Highway Test Office differs significantly from the site response at the Seattle Federal Office

Building and by our inability to match, in absolute terms, the

1965 ground motions at Olympia. Based on this analysis, the

effects of near-surface soils and the properties of the underlying rock likely exert a dominant influence on strong ground

motions in the Puget Sound region and probably the

Willamette Valley in Oregon.

Figure 153. Predicted 5-percent-damped acceleration response

spectra of a hypothetical Mw 8.5 Cascadia subduction-zone earthquake at the Olympia Highway Test Office and Seattle Federal

Office Building sites.

Figure 154. Parametric uncertainties in the band-limited-whitenoise and random-vibration-theory computations of ground

motions for a hypothetical Mw 8.5 Cascadia subduction-zone

earthquake, as predicted for the Olympia Highway Test Office

and Seattle Federal Office Building sites. The uncertainties are a

result of varying each parameter shown in table 23.
ENGINEERING CHARACTERIZATION, EARTHQUAKE STRONG GROUND MOTIONS, PACIFIC NORTHWEST 323

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Table 23. Model input parameters and uncertainties in parametric variations for a hypothetical Mw 8.5 Cascadia

subduction-zone earthquake.

[(--), not required]

____________________________________________________________________________________________________________________________________________

 Parameter Mean or median value Distribution Standard error (σ)1

____________________________________________________________________________________________________________________________________________

____________________________________________________________________________________________________________________________________________

 1For lognormal distributions, σ is actually σln.

 2Approximately lognormal based on a correlation model for velocity and layer thickness.

Focus

Slip

Attenuation parameter (Qo)

Attenuation parameter (η)

Near-surface attenuation (κ)

Shear-wave profile2

Shear-modulus reduction

Shear-wave damping

Randomized in nucleation zone

Randomized models

380

0.39

0.04 s (Olympia)

0.06 s (Seattle)

See figure 149

See Electric Power Research 

Institute (1993, p. 7–A.41)

See Electric Power Research 

Institute (1993, p. 7–A.42)

--

--

Lognormal

Normal

Lognormal

Lognormal

Lognormal

Lognormal

--

--

0.18

0.05

0.30

0.40

0.35

0.35
324 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

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———1991, Global characteristics and site geometry, in NSF/

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Schneider, J.F., and Chiou, S.-J., 1990, A methodology to estimate design response spectra in the near-source region of large

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325

SIMULATED STRONG GROUND MOTIONS FOR 

MAGNITUDE 8 EARTHQUAKES ON THE

CASCADIA SUBDUCTION ZONE

By Brian Cohee1 and Paul Somerville2

ABSTRACT

Strong ground motions from subduction-zone thrust

earthquakes in western Washington and Oregon are

estimated using a semi-empirical simulation procedure. The

procedure is validated for large subduction earthquakes by

modeling recorded acceleration seismograms and response

spectra from the magnitude (M) 8 Michoacán, Mexico, and

Valparaíso, Chile, earthquakes of 1985. We find that slipdistribution models derived from strong-motion and

teleseismic velocity seismograms of these two earthquakes

also explain higher frequency motions of the recorded

accelerograms. Quantitative measures of the misfit between

recorded and simulated response spectra are used to estimate

the modeling uncertainty.

Ground motions are computed for two fault models

representative of the two different subduction-zone

geometries beneath Washington and Oregon. The most

critical geometrical source parameter controlling ground

motions in the urban regions of Puget Sound and Portland is

the depth of the rupture on the plate interface. We used a

geometry based on earthquake locations that places the

downdip limit of rupture about 50 km west of both Seattle

and Portland. A geometry in which the plate interface is

arched at depths shallower than 40 km beneath western

Washington would place the downdip limit beneath Seattle

and result in larger ground-motion estimates in the Puget

Sound area. Given the assumed fault location, the largest

cause of uncertainty in the estimated ground motions is due

to the distribution of slip (asperities) with depth on the fault

plane. As their depth increases, the asperities approach the

urban regions of Puget Sound and Portland and the ground

motions increase. Also, with increasing asperity depth, the

attenuation of peak acceleration with distance is more

gradual. The fault dip of the Oregon model is about twice

that of the Washington model, but the estimated ground

motions are similar.

The attenuation of horizontal peak acceleration (PGA)

with distance r from the fault asperity is given by

ln(PGA)=15.6–3.34ln(r+128)+0.79γ

where ln is the natural logarithm and γ is a site term (0 for

rock and 1 for soil). This relation is appropriate for r greater

than 25 km but less than 175 km and for M 8. When r is

defined as the closest distance to the fault plane, the attenuation relation is

ln(PGA)=2.8–1.26ln(r)+0.79γ

This relation is appropriate for r greater than 30 km but less

than 100 km and for M 8.

Formal estimates of uncertainty in the calculated

ground motions are obtained by considering both parametric

uncertainty (estimated from the range of source models of

hypothetical Cascadia subduction-zone earthquakes) and

modeling and random uncertainty (estimated from the misfit

between recorded and simulated ground motions of the

Michoacán and Valparaíso earthquakes). For periods less

than 1 s, the estimated response spectral velocities at soil

sites in the Seattle-Olympia region are about twice those

recorded during the 1949 M 7.1 Olympia and 1965 M 6.5

Seattle earthquakes, and the durations of strong motion are

significantly longer (45–60 s versus 10–15 s for rock sites).

INTRODUCTION

Strong-motion simulation procedures complement

conventional empirical methods of estimating strong ground

motion for seismic-hazard analyses. They augment the

relatively sparse set of on-scale recordings close to large

earthquakes, giving more confidence in the prediction of

ground-motion characteristics for hazard evaluations. This

additional information is especially valuable in regions such

as the Pacific Northwest, where there had been no historical

subduction-zone thrust earthquakes and, hence, no recorded

________________ 1Department of Geophysics, Stanford University, Stanford, Calif.

2Woodward-Clyde Federal Services, Pasadena, Calif.
326 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

strong ground motions prior to the magnitude (M) 7 Cape

Mendocino, Calif., earthquake of April 25, 1992

(Oppenheimer and others, 1993). Simulation procedures

also provide a means of estimating the dependence of ground

motion on variations in specific fault parameters; in the

empirical data, these dependencies are difficult to isolate

from the many factors that determine strong-motion characteristics (Crouse, 1991). The uncertainty in ground-motion

predictions from the modeling procedure can be quantified

by comparisons between recorded and simulated motions

and by parametric studies (Abrahamson and others, 1990).

Simulation procedures also allow specific information to be

included about the earthquake source, the wave-propagation

path between the source and site, and the local site response.

Seismograms can be generated whose wave composition,

duration, and frequency content reflect these specific

conditions rather than those contained in the empirical data.

Such time series are of growing importance as methods for

analyzing the nonlinear response of structures are developed.

When estimating seismic hazards in western Washington and Oregon, three earthquake source zones are relevant:

(1) the shallow plate interface—if it has the potential to generate large subduction earthquakes, significant ground

motions will occur throughout the region; (2) the intraslab

(Wadati-Benioff) zone, the source of the largest earthquakes

(the 1949 M 7.1 Olympia and 1965 M 6.5 Seattle events) and

strongest ground motions in the region during this century;

and (3) the shallow crustal zone, the likely source of the 1872

M 7.4 North Cascades earthquake. Ground motions from

seismic events on the shallow plate interface are the subject

of this report.

There is uncertainty in the size of the largest earthquakes that can occur on the Cascadia subduction zone.

Comparison of Cascadia subduction characteristics with

those of other subduction zones in the world led McCaffrey

and Goldfinger (1995) to suggest that M 8 earthquakes are

likely more characteristic of Cascadia subduction than less

frequent M 9 megathrust earthquakes. Their conclusion is

based on the oblique convergence angle between the plates,

lateral segmentation of the downgoing slab, and inelastic

deformation of the overriding plate.

Although the seismic potential of the Cascadia subduction zone remains uncertain, the plate interface did generate

the M 7 earthquake beneath Cape Mendocino (Oppenheimer

and others, 1993), and it may be able to generate great earthquakes (Heaton and Kanamori, 1984; Atwater, 1987; Rogers,

1988; Adams, 1990). In this report, we estimate ground

motions in western Washington and Oregon from hypothetical M 8 earthquakes on the Cascadia subduction-zone plate

interface. To prepare for the simulations and to estimate the

modeling and random uncertainty, we modeled strongmotion recordings of the 1985 M 8 Michoacán, Mexico, and

Valparaíso, Chile, subduction earthquakes (Somerville and

others, 1991). Ground-motion estimates were then made for

M 8 earthquakes on the Cascadia plate interface using a wide

range of source parameters. These estimates are described in

detail by Cohee and others (1991a, b). This report summarizes the method and uncertainty analysis from Somerville

and others (1991) and the ground-motion estimates from

Cohee and others (1991a, b), and identifies those aspects of

the earthquake source that most strongly influence the estimates and their uncertainty.

The two reasons for selecting the Michoacán and

Valparaíso earthquakes to estimate the modeling and random

uncertainty are the similarity in subduction-zone geometry

with that of the Cascadia subduction zone and the availability

of near-source acceleration seismograms. In both earthquakes, the shallow plate interface dips at a relatively

shallow angle, and much of the fault-rupture surface underlies land, which is similar to the Cascadia subduction zone.

In addition to this geometrical compatibility, both earthquakes were well recorded by a large number of low-gain

seismometers. Also, much is known about the earthquake

source characteristics, including estimates of the rupture

timing, slip-velocity function, and distribution of slip

amplitude over the fault surface.

Several different methods for simulating strong ground

motions have been tested against recordings of subductionzone earthquakes by other investigators. The most common

approach has been an empirical Green’s-function summation

method in which strong-motion recordings of smaller earthquakes are summed to simulate the rupture of a much larger

earthquake (Kamiyama, 1988; Takemura and Ikeura, 1988).

In this approach, the empirical Green’s functions include

both the source and wave-propagation effects. This

approach is most appropriate when recordings suitable for

use as empirical Green’s functions are available in the region

where the motions of the larger earthquake are to be

simulated. Heaton and Hartzell (1989) applied this method

to the Cascadia subduction zone using empirical source

functions from Japanese subduction earthquakes.

In regions where no strong-motion recordings are

available for use as empirical Green’s functions, it is

advantageous to calculate the wave-propagation effects

using a crustal velocity model specific to the region of

interest (Hadley and others, 1982). This approach was

used by Day and others (1988) to simulate strong-motion

recordings of the Michoacán earthquake and to estimate

strong motions from Cascadia subduction earthquakes.

They used ray theory to calculate Green’s functions for a

layered velocity model, and their source model was

composed of a series of cracks represented by theoretical

dislocation functions. The greatest shortcoming of this

purely theoretical approach is that observed complexity in

the recorded ground motion is not fully described by the

theory. In our semi-empirical approach used to estimate

ground motions, known aspects of the wave propagation

are modeled theoretically and unknown aspects are

modeled using stochastic effects and recorded seismograms

(corrected empirical Green’s functions).
SIMULATED STRONG GROUND MOTIONS, MAGNITUDE 8 EARTHQUAKES, CASCADIA SUBDUCTION ZONE 327

D

The ground motion estimates in this report are specific

to the velocity structure and fault geometry of the Cascadia

subduction zone but do not include the local site response,

which can be extremely important for sites on low-velocity

material. The objective of the study is to predict response

spectra for rock sites in the study area from a M 8 earthquake

and to identify those earthquake parameters that lead to the

largest variations in predicted motions. Estimates of

response spectra for soil sites must include local site and

basin effects for periods greater than about 1 s (for example,

see King and others, 1990; and Ho and others, 1991).

ACKNOWLEDGMENTS

This work was supported by the U.S. Geological Survey

under award #14–08–0001–G1516. Dr. Norman Abrahamson evaluated the uncertainty and Dr. Mrinal Sen assisted in

the modeling of the Valparaíso earthquake. Reviews of the

manuscript by Drs. Stephen Hartzell, Robert Youngs, and

Albert Rogers are gratefully acknowledged.

SIMULATION PROCEDURE AND 

UNCERTAINTY

There are several methods for simulating highfrequency ground motion close to a fault. Some of these

methods are described in Joyner and Boore (1988). They can

be broadly classified into three categories: deterministic,

stochastic, and hybrid. The deterministic methods (Aki,

1968; Haskell, 1969) use kinematic source models and

require specification of the distribution of slip amplitude on

the fault. This approach has been successful at long periods

(greater than 1 s). The stochastic methods model the earthquake acceleration as random noise in the spectral band

between the corner frequency and fmax, and the spectral

shape is given by the Brune spectrum (Hanks and McGuire,

1981). These methods are computationally efficient but do

not include wave-propagation effects rigorously. The hybrid

approach draws on the particular strengths of the other two

methods—known aspects of the wave propagation are

modeled deterministically and unknown aspects are modeled

stochastically.

The empirical Green’s-function method described by

Hartzell (1978) was the first of many hybrid techniques.

Rupture propagation and radiation pattern are specified

deterministically, and source radiation and propagation

effects are included empirically by assuming that recorded

aftershock seismograms contain this information. The

methods proposed by Hadley and Helmberger (1980), Irikura

(1983), and Heaton and Hartzell (1989) are modifications of

Hartzell’s (1978) method of summing empirical Green’s

functions. These methods require an adequate sampling in

both distance and source depth of empirical Green’s

functions (Cohee and others, 1994), which is very difficult

to obtain in most regions. In order to overcome this

limitation, Hadley and others (1982) proposed an approach

that uses empirical source functions convolved with

theoretical Green’s functions. The approach described

below is derived from their work and uses simplified

Green’s functions that include the effects of geometrical

spreading and exclude the effects of radiation pattern and

receiver function.

The procedure is illustrated schematically in figure

155. The fault surface is divided into discrete elements, and

the motions from these elements are summed at appropriate

times to simulate the rupture propagation across the fault

surface. The rupture front propagates at a fixed fraction of

the shear-wave velocity; however, a stochastic component is

included in the rupture velocity and the slip-velocity function to simulate heterogeneity in rupture dynamics. Largescale asperities are introduced by varying the amplitude of

slip on the fault surface.

The starting equation for the method is the representation theorem (Aki and Richards, 1980, equation 14.1) that

describes the elastic displacement u(x,t) from a displacement discontinuity [u(ξ,τ)] across an internal surface ∑ as

(1)

where cjkpq are the elastic constants, Gip is the impulse

response of the medium (displacement Green’s function),

and ν is unit normal to the fault surface. The ξ represents a

point on the fault plane, and x is the observation point. In the

far field, the above integral can be written as

where L is the fault length, W is the fault width, is the slip

function, and * represents a convolution. When we use this

equation to calculate ground motion from a large earthquake,

we divide the large fault plane into small elements called

subfaults. If we divide the fault plane into l subfaults (of

length ∆L) along the length and m subfaults (of width ∆W)

along the width of the fault plane, the integral reduces to

where is the time required for the rupture front to propagate from the hypocenter to the i,j subfault. The propagation time from the subfault to the observation point is

included in the Green’s function for that propagation path.

In most cases, the dislocation rise time increases with

earthquake size (Hadley and Helmberger, 1980; Irikura,

τ r 

ij

(2)

.

(3)

U( ,) x t D ̇ ξ 1 ξ 2 ( , , )t  ̇

0

W

∫0

L

∫ = G x ξ 1 ξ 2 t)dξ 1dξ 2 ( , , , *

G x ξ 1 ξ 2 t)dξ 1dξ 2 ( , , , *

U( ,) x t D ̇

0

∆W

∫ 0

∆L

∫

j = 1

m

∑

i = 1

l

∑ ξ 1 ξ 2 t τ

ij

r = (, ) ,( ) –

cjkpqGip,q (, ) x t;( ) ξ, τ ν ui( ) x t , τ [ ] uj(,) ξ τ k dΣ

Σ

∫ d –∞

∞

= ∫
328 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

1983). Based on a Haskell-type self-similar kinematic

source model, we need to add as many sources per subfault

as the ratio of the rise time of the large earthquake to that of

the small earthquake from which the source function is

derived. Then equation 3 reduces to the following form:

where nsrc is the number of subsources per subfault given by

nsrc=τ0/τe, and τ0 and τe represent the rise time of the large

and small earthquake, respectively.

The simulated motion of the large earthquake is then the

summation of contributions from l fault elements along

strike, m fault elements down dip, and nsrc time-lagged

source functions on each subfault. If l, m, and nsrc are

numerically identical, then the small and large earthquake

have self-similar scaling (Irikura, 1983). In our simulations

of the Michoacán and Valparaíso earthquakes, we do not use

this constraint because it would greatly degrade the data fit;

however, for both events, we maintain self similarity in the

spectral shape between the small and large earthquakes using

an ω2 scaling relation (Joyner and Boore, 1986). To accomplish this scaling, the product of l, m, and nsrc must be the

moment ratio of the large earthquake to the small earthquake

taken to the four-thirds power.

Although the form of the radiated source spectrum may

be specified deterministically, high-frequency details are

generally unknown and are therefore included empirically

using the empirical source function. The empirical source

function is a strong-motion recording of an aftershock that

has been corrected using a simplified Green’s function. The

correction that is most consistent with the intended use of the

source function is a deconvolution of the recording with a

theoretical Green’s function; however, due to zeros in the

spectrum, deconvolution is often unstable. The Green’s

function potential is usually very simple (a step function)

because the aftershock recordings are from horizontal distances less than one source depth, and most of the energy is

in the direct arrival. The deconvolution is then approximately equivalent to division by scalar G0, which is the

amplitude of the displacement Green’s function G(t). For a

recorded seismogram S(t), the corrected empirical source

function is then

(5)

G0 can be measured directly, but it is better to use the ratio of

the maximum seismogram amplitude to the maximum

amplitude after convolution with G(t):

(6)

The empirical source functions must also be corrected

for the surface velocity and incidence angles found in the

aftershock geometry. That is, they include the angular

decomposition of the P, SV, and SH wavefields into vertical,

radial, and tangential components (PZ, PR, SVZ, SVR, SHT) as

well as amplification due to the surface velocity. These traits

of the aftershock recordings are inappropriate and must be

removed, and the appropriate receiver function must be

Empirical source function

(corrected for propagation)

Simulated accelerogram

at recording station

Green's function

(path effect)

Fault surface

Figure 155. Schematic description of the strong ground motion simulation procedure. Contributions from

the subfault elements (dots) are lagged and summed to produce a simulated accelerogram at a receiver at

ground surface (triangle). The contribution from each subfault is the convolution of a corrected empirical

source function with a Green’s function that describes the wave propagation using the simulation geometry

and velocity model.

(4)

SE( )t S t( )

G0

= ---------

G0

max(S t( )

max( ) S t( ) =------------------------- G t( ) ) *

U( ,) x t D

 ̇

0

∆W

∫ 0

∆L

∫

k = 1

nsrc

∑

j = 1

m

∑

i = 1

l

∑ ξ 1 ξ 2 t τ

ij

r = (, ) ,( ) –

* G x ξ1 ξ2 t)dξ1dξ2 ( , , ,

. –(k–1) τe )
SIMULATED STRONG GROUND MOTIONS, MAGNITUDE 8 EARTHQUAKES, CASCADIA SUBDUCTION ZONE 329

introduced. This correction is performed by multiplying the

empirical source function by the ratio of theoretical wholespace receiver functions. For empirical source functions

denoted by PZ, PR, SVZ, SVR, and SHT, the corrected source

functions are simply

(7)

where α1, β1, and i1 are the P and S velocities and incidence

angle at the source-function receiver, and α2, β2, and i2 are

the same parameters at the site recording the simulated

earthquake.

The empirical source function contains other effects in

addition to that of the source, including anelastic absorption, unmodeled wave-propagation effects such as scattering, and characteristics of the local site response. Because

our knowledge of the attenuation structure of subduction

zones is sufficiently poor and the dependence of attenuation on path length is sufficiently weak, we assume that

attenuation characteristics of the region where the empirical recording was made are similar to those of the simulation region. The site effect is represented empirically by

using source functions recorded on the desired site

conditions (rock or soil).

Green’s functions are computed using generalized ray

theory (Helmberger and Harkrider, 1978). The response is

computed for a point source located at the center of each

subfault with propagation through a layered medium.

Contributions from the direct P and S waves and primary

reflections from each layer interface beneath the source are

included. Mode conversions, reverberations, and surface

waves are not included. The Green’s functions do not

include the radiation pattern and receiver function because

they are implicitly included in the corrected empirical

source function. The Green’s functions are thus the

response of the medium for P, SV, and SH potentials

described by the following equations:

(8)

and for SH,

where , c=α or β, and the quantity in 

brackets is evaluated over the Cagniard contour for each of

the n rays. Three step responses (P, SV, and SH) are computed for each propagation path.

The radiation pattern is not included in the Green’s

function for two reasons. First, the empirical source functions are recorded at distances comparable to the aftershock's

fault dimensions and thus already contain an averaged radiation pattern. Second, there is evidence that the coherence of

the radiation pattern becomes weaker as periods become

shorter than about 0.5 s (Liu and Helmberger, 1985) due to

scattering and short-wavelength heterogeneity.

Each subfault begins to radiate energy when the rupture

front reaches it. In order to avoid artificial periodicities due

to the discretization of the fault surface and to introduce

small irregularities in the velocity of rupture propagation, we

include a stochastic variable in the rupture time of each subfault. As schematically illustrated in figure 156, the turn-on

time t0, is a random number taken from a Gaussian probability distribution centered about (tb – ta)/2, where ta and tb are

the arrival and departure times of the rupture front, respectively. The probability that T0 is between ta and tb was set at

the three-standard-deviation level, or at a confidence level of

99.7 percent.

The form of the slip function at each point on the fault

is a ramp described by the slip amplitude and the rise time

(following Haskell, 1964). The slip function of the simulated

earthquake is built up by summing nsrc empirical source

functions as described above. A stochastic component is

included in the slip function in order to avoid undesirable

periodicity corresponding to the source function rise time

and to simulate roughness in the slip function. Accordingly,

in addition to lagging the source functions at integer multiples of the source function rise time Te, a random perturbation is also included, as illustrated in figure 156. The

probability that the initiation time tsi of a given source

function lies between ti and ti+1 is also set at the 99.7 percent

confidence level.

Implementation of the simulation procedure requires

estimates of several source parameters, some of which (such

as rupture dimensions) are known for the specific earthquake

in question and others of which (such as rupture velocity) are

assigned reasonable values. In principle, there are no free

for P, Φ( )t ;

M0

4π2ρ0

--------------- 2

r

-- 1

t = ----- * Im p

η α

-------Π( ) p dp

dt -----



i 



i = 1

n

∑

for SV,

Ω( )t ;

M0

4π2ρ0

--------------- 2

r

-- 1

t = ----- * Im p

η β

-------Π( ) p dp

dt -----



i 



i = 1

n

∑ ;

χ( )t

M0

4π2ρ0

--------------- 2

r

-- 1

t = ----- * Im p

ηc

-------Π( ) p dp

dt -----



i 



i = 1

n

∑

ηc

1

c

--



2



 p2 = –

PZ

1

2

-- α1 i

2 cos

α2 i

1 cos -------------------

 PZ

1

2

-- α1 i

2 cos

α2 i

1 sin -------------------     PR + 

 =

PR

1

2

-- α1 i

2 sin

α2 i

1 cos -------------------

 PZ

1

2

-- α1 i

2 sin

α2 i

1 sin ------------------     PR + 

 =

SVZ

1

2

-- β 1 i

2 sin

β 2 i

1 sin ------------------

 SVZ

1

2

-- β 1 i

2 sin

β 2 i

1 cos -------------------     SVR + 

 =

SVR

1

2

-- β 1 i

2 cos

β 2 i

1 sin -------------------

 SVZ

1

2

-- β 1 i

2 cos

β 2 i

1 cos -------------------     SVR + 

 =

SHT

β 1 i

2 sin

β 2 i

1 sin ------------------

  SHT  =

'

'

'

'

'
330 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

parameters involved in the procedure. This was true for the

Michoacán earthquake, where estimates of source

parameters (including rise time) were available from other

studies. There was no independent estimate of the rise time

of the Valparaíso earthquake, so it was treated as a free

parameter to be optimized.

To use predicted strong ground motions computed by

the simulation procedure in engineering analyses, an

estimate of their uncertainty is required that is analogous to

that given for empirical ground-motion-prediction equations. The total uncertainty in the predicted strong ground

motion should include all sources of uncertainty: modeling

uncertainty (describing the adequacy of the simulation

procedure in representing the processes that generate the

strong ground motions), random uncertainty (describing

earthquake-to-earthquake and station-to-station variability

that is not accounted for in the source and wave-propagation

models), and parametric uncertainty (describing uncertainty

in the source parameters of future earthquakes).

A procedure for estimating these uncertainties was

introduced by Abrahamson and others (1990). The modeling

and random component of uncertainty is estimated from the

misfit between recorded and simulated ground motions for

the Michoacán and Valparaíso earthquakes. The contribution of parametric uncertainty to the misfit between recorded

and simulated motions is assumed to be negligible if the

gross fault parameters of the earthquakes are known. The

parametric uncertainty is determined in the process of simulating ground motions. It is measured from the variability in

the predicted ground motions caused by varying unknown

earthquake source parameters. The overall uncertainty is

obtained by combining the modeling and random uncertainty

with the parametric uncertainty.

MODELING AND RANDOM 

UNCERTAINTY—SIMULATION OF 

MICHOACÁN AND VALPARAÍSO 

EARTHQUAKES

Before using the numerical model to simulate M 8 thrust

earthquakes in the Cascadia subduction zone, we tested the

method against recorded strong ground motions of the M 8

Michoacán and Valparaíso subduction-zone earthquakes.

Recordings of M 7 aftershocks of both earthquakes, corrected for wave propagation, were used as the empirical

source functions. We discovered that slip amplitude distribution models derived from near-source and teleseismic

velocity seismograms by Mendoza and Hartzell (1988) for

the Michoacán earthquake and Houston (1987) for the Valparaíso earthquake are for the most part consistent with the

higher frequency motions of the near-source accelerations.

The five near-source recordings of the Michoacán

earthquake (fig. 157) are from rock sites. The station-to-station accelerogram variability observed for this earthquake is

mostly reproduced in the simulated accelerograms, reflecting

the influence of the heterogeneous slip distribution and the

southeasterly direction of rupture propagation. These simulations use the Caleta de Campos source function and show

good agreement with the recorded duration, frequency content, and peak acceleration. Not surprisingly, simulations

using a constant slip model do not produce the two largeamplitude wave packets most obvious on the La Villita accelerogram (at 15 s and 40 s), but these are produced by two

asperities in the Mendoza and Hartzell (1988) slip model. A

A

B

RUPTURE VELOCITY

SLIP FUNCTION

Fault segment

Arriving

rupture

front

Departing

rupture

front

SLIP

TIME

t

a

t

0

t

b

t

0

 = R(ta, tb)

t

0 t

i t

i+1

tsi

Te

TR

t

i = t

0 + (i-1)*Te, i=1, nsrc

tsi

 = R(t

i,

t

i+1), i=1, nsrc

nsrc = Te

TR

Figure 156. Schematic diagrams illustrating the random component of rupture velocity and the slip function. A, The rupture onset

time for each fault element (t0) is a random number (R) taken from

a Gaussian distribution centered between arrival (ta) and departure

(tb) times of the rupture front. B, The total number of source functions (nsrc) that are summed for each fault element is determined

by ratio of rise time of the simulated event (TR) to rise time of the

empirical source function (Te). Initiation time for each source

function (tsi) is a random number from a Gaussian distribution centered on evenly spaced intervals of Te, shown in figure as dashed

vertical lines.
SIMULATED STRONG GROUND MOTIONS, MAGNITUDE 8 EARTHQUAKES, CASCADIA SUBDUCTION ZONE 331

Caleta de Campos–observed Caleta de Campos–simulated

0.091g

0.141g

0.144g

0.084g

0.143g

0.184g

La Villita–observed La Villita–simulated

0.059g

0.127g

0.122g

0.082g

0.142g

0.116g

La Union–observed La Union–simulated

0.126g

0.168g

0.150g

0.169g

0.179g

0.231g

Zihuatenejo–observed Zihuatenejo–simulated

0.106g

0.105g

0.164g

0.097g

0.149g

0.098g

Papanoa–observed Papanoa–simulated

0.083g

0.153g

0.110g

0.049g

0.066g

0.048g

Z

N

E

Z

N

E

Z

N

E

Z

S

W

Z

S

W

0 10 20 30 40 50 60 70 80 90

SECONDS

0 10 20 30 40 50 60 70 80 90

SECONDS

Figure 157. Comparison of

recorded and simulated threecomponent acceleration time

series for the September 19,

1985, Michoacán earthquake

at rock sites Caleta de

Campos, La Villita, La

Union, Zihuatanejo, and

Papanoa, Mexico. The peak

accelerations are expressed as

a fraction of acceleration due

to gravity (g). Note good

agreement in peak acceleration and duration between

observed and simulated

accelerograms. The three

components of motion are

labeled Z for vertical, N or S

for north-south horizontal,

and E or W for east-west

horizontal.
332 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

uniform slip model yields longer duration accelerograms

with lower peak amplitudes. A comparison of the recorded

and simulated response spectra, averaged over the five rocksite stations, shows good agreement over the 0.1–3 s period

(fig. 158A).

In contrast with the Michoacán earthquake, the Valparaíso earthquake was mostly recorded on soil sites. The

mean soil-site amplitudes are about twice those of the one

rock site. This is seen in the recorded spectral acceleration

for the rock site, shown in figure 158B, and the mean of the

five soil sites, shown in figure 158C. The simulated seismograms are generated using a rock-site source function for the

one rock site and a soil-site source function for the five soil

sites. The peak at 0.2 s in the simulated spectrum of figure

158B is due to a peak in the rock-site source spectrum at the

same period. The peak in the spectrum at 0.5 s in figure 158C

is also due to a peak in the soil-site source spectrum at the

same period. As before, a uniform slip model produces

accelerograms with longer durations and lower peak amplitudes than those in the recorded data.

The misfit between recorded and simulated response

spectra is used as an estimate of the modeling and random

uncertainty. The response spectra of the simulated motions

have little or no significant bias in the period range of 0.05–2

s for either earthquake, and the peak acceleration, duration,

and envelope shape of the acceleration seismograms are in

good agreement with the recorded seismograms. We found

that rock-site source functions recorded in Mexico or Chile

could be used interchangeably with comparable uncertainty—the uncertainty was not appreciably different when a

source function from one subduction zone was used to simulate ground motions in the other subduction zone. The standard error associated with the use of each empirical source

function, expressed as the natural logarithm of the spectral

acceleration, is shown in figure 159. More details about the

EXPLANATION

Simulated spectrum

Recorded spectrum

0.02 0.1 1.0 5.0

0.02 0.1 1.0 5.0

0.02 0.1 1.0 5.0

0.6

0.4

0.2

0

0.6

0.4

0.2

0

0.6

0.4

0.2

0

0.8

1.0

PERIOD, IN SECONDS

SPECTRAL ACCELERATION, IN FRACTION OF 

g

A

B

C

Figure 158. Comparison of recorded and simulated acceleration response spectra at 5-percent damping for A, the 1985 Michoacán earthquake at five rock sites in coastal Mexico (averaged

values); B, the 1985 Valparaíso earthquake at the one rock site in

Chile; and C, the 1985 Valparaíso earthquake at five soil sites in

Chile (averaged values).

0.01 0.1 1.0 10.0

0.5

0

1.0

PERIOD, IN SECONDS

STANDARD ERROR

EXPLANATION

VINA

CAL

VALU

Figure 159. Estimates of the standard error (natural logarithm

of spectral acceleration) in the simulation procedure that is

associated with using each of the three empirical source functions,

two from rock sites (VALU in Chile, CAL in Mexico) and one

from a soil site (VINA in Chile).
SIMULATED STRONG GROUND MOTIONS, MAGNITUDE 8 EARTHQUAKES, CASCADIA SUBDUCTION ZONE 333

modeling of the Michoacán and Valparaíso strong ground

motions and the windowed time series, response spectra, and

model bias for each of the empirical source functions is given

by Somerville and others (1991).

PREDICTION OF GROUND MOTIONS 

FOR WESTERN WASHINGTON AND 

OREGON

The simulation procedure was used to predict strong

ground motions from hypothesized M 8 thrust earthquakes

on the Cascadia subduction zone. Plausible fault models for

the western Washington and Oregon regions and also seismic-velocity models were adapted from published regional

studies (fig. 160). The surface projections of the two fault

geometries, subdivided into subfault elements, are shown in

figure 160A. A cross section perpendicular to the strike of

the western Washington geometry shows the plate interface

dipping at 11°(fig. 160B). The fault surfaces that ruptured in

the Michoacán and Valparaíso earthquakes are superimposed on the cross section. The western Washington model

has a dip similar to the 14° dip of the Michoacán fault plane,

and the Oregon model has a dip of 21°, similar to the

Valparaíso fault plane (25°). The western Washington and

Oregon fault models have different downdip widths, fault

lengths, and depths to the top of the fault (17 km and 13 km,

respectively).

The actual plate interface is not imaged well because it

has been historically aseismic. Instead, the geometry of the

plate interface is inferred from refraction profiles (Taber and

Lewis, 1986) and the inclined zone of hypocenters within the

shallow part of the downgoing Juan de Fuca plate. The most

critical source parameter influencing the ground motions for

line f (fig. 160A), which includes the most heavily urbanized

regions in the study area, is the depth extent of rupture.

There is general agreement that the downgoing Juan de Fuca

plate is arched at depth beneath Puget Sound, as shown by

the 54-km-depth contour in figure 160A (Crosson and

Owens, 1987; Weaver and Baker, 1988). To infer the plate

interface at shallower depth, we used the model of Weaver

and Baker (1988) that is based on hypocentral locations

within the downgoing slab. In their model, the 40-km-depth

contour is not arched. If we assume this seismicity is confined beneath the oceanic Moho, then the 40-km-depth contour, inferred to be the easternmost edge of the rupture

surface, lies about 50 km west of Seattle (fig. 160). However, Crosson and Owens (1987) prefer an interpretation

where the shallow plate interface is also arched. Using their

interpretation places the eastern extent of the fault beneath

Puget Sound. The ground motion estimates that result from

using the Crosson and Owens (1987) model may be approximated by simply shifting the station grid (described below)

50 km to the east.

A two-dimensional velocity profile across the subduction zone, adapted from Spence and others (1985), is shown

in figure 161. To compute the ray-theory Green’s functions,

we approximate this two-dimensional model with a different

layered model for each subfault depth. These one-dimensional models have an oceanic Moho 6 km below the source.

The adequacy of approximating the more realistic twodimensional model with a series of one-dimensional models

was tested by comparing ray-theory seismograms computed

in the layered models with finite-difference seismograms for

the two-dimensional model. In both calculations, which are

shown in figure 162, the largest phases are the direct S-wave

and the S-wave critically reflected from the oceanic Moho of

the subducting plate (SmS). The ray-theory approximation

does an adequate job of reproducing the seismic response of

the Spence and others (1985) model in the passband of

interest.

Although the dip of the western Oregon fault model is

about twice that of the western Washington model, the

ground motions for the two regions are very similar. Given

the assumed location of the potential rupture surface, the

largest cause of uncertainty in the predicted ground motions

is due to uncertainty in the distribution of slip amplitude with

depth on the plate interface. We performed simulations for

three generalized asperity models in which 60 percent of the

total moment is released in the upper, middle, and lower

third of the fault (termed the shallow, middle, and deep slip

models). The size and strength of the asperities and their

distribution along strike are similar to the pattern of slip

amplitude variation in the Michoacán and Valparaíso

earthquakes.

Ground-motion simulations are computed at the points

shown as open squares on the grid in figure 160A. The grid

is made up of seven lines, labeled a–g, parallel to the strike

of the fault and 20 km apart. The dependence of the

predicted ground motions on the asperity depth is shown in

figure 163 for the Washington geometry and in figure 164

for the Oregon geometry. These accelerograms are from an

east-west profile of stations that intersect the surface projection of the fault near the maximum slip amplitude and are

representative of maximum ground motions that could be

used in a deterministic hazard analysis. As the asperities

become deeper, the portion of the fault radiating the strongest energy becomes closer to the station grid, causing larger

accelerations. With increasing asperity depth, the attenuation of peak acceleration with distance becomes more gradual. The variability of peak ground acceleration across rows

a–g of the Oregon grid is shown in figure 165.

The attenuation of peak acceleration can be described

by the equation

ln(PGA)=a+bln(r+c)+dγ

where r is the distance to the nearest asperity in kilometers,

PGA is the maximum horizontal acceleration (in fraction of

g), and γ is the site term (1 for soil and 0 for rock). The leastsquares solution, averaged over the two fault geometries and

(9)
334 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

0 40 80 KILOMETERS

Sea

Level

20

40

60

80

DEPTH, IN KILOMETERS

B

A

Cascadia

subduction zone

trench axis Coast

Olympic

Mountains abcd e f g

Olympia Seattle

Puget Sound

Michoacan Valparaiso

NO VERTICAL EXAGGERATION

P=3.0 S=1.5 P=4.6 S=2.3

P=5.6 S=3.2

P=6.8 S=3.9

P=6.5 S=3.8

P=8.1 S=4.7

49°

48°

47°

46°

45°

44°

126° 125° 124° 123° 122° 121°

UNITED STATES

CANADA

WASHINGTON

OREGON

Vancouver

Island

Olympic

Mountains

17 km

40 km

54 km

13 km

Cascadia

subduction zone trench axis

PACIFIC

OCEAN CO LUMBIA RIVER

Seattle

Tacoma

Olympia

Portland

Puget Sound

0 50 KILOMETERS

a b c d e f g

Figure 160. Earthquake source and receiver

geometry for two hypothetical M 8 earthquakes

in the Pacific Northwest. A, Map view showing

the surface projection of the mainshock fault

planes and subfault elements (shaded rectangular

grids) and the receiver array (small squares) for

western Washington (slab dip is 11°) and Oregon

(slab dip is 21°) fault models. Receivers

equidistant from fault plane are grouped in lines

a–g. Assumed depth to the plate interface is

labeled at 13, 17, 40, and 54 km. Triangles are

stratovolcanoes. B, Cross section through the

center of the western Washington fault model

showing assumed location of rupture (dark solid

line) and a simplified velocity structure modified

from Taber and Lewis (1986); superimposed are

the rupture zones of the 1985 Michoacán,

Mexico, and Valparaíso, Chile, earthquakes. Pand S-wave velocities are shown at right in km/s.

Locations of the 1949 M 7.1 Olympia (54 km

depth) and 1965 M 6.5 Seattle (60 km depth)

intraslab earthquakes are shown by stars.
SIMULATED STRONG GROUND MOTIONS, MAGNITUDE 8 EARTHQUAKES, CASCADIA SUBDUCTION ZONE 335

DEPTH, IN KILOMETERS

DISTANCE, IN KILOMETERS

NO VERTICAL EXAGGERATION

the three slip models, yields a=15.6, b=–3.34, c=128, and

d=0.79, with a standard error of 0.32 (natural logarithm of

the peak acceleration). All parameters are significant at the

95-percent confidence level. This relation is appropriate for

r greater than 25 km but less than 175 km and for Mw 8.

Combining the modeling and random standard error (0.51)

derived from the fit to the Michoacán and Valparaíso earthquakes with the parametric uncertainty (0.32) yields a total

standard error of 0.60.

It is difficult to use an attenuation relation when the distance is measured to the nearest asperity because it is impossible to anticipate the distribution of moment release on the

fault surface. The closest distance to the fault is a more

useful distance measure although it is accompanied by

greater uncertainty in predicting the ground motion. A

regression using the same functional form yields a=2.8, b=

–1.26, c=0, and d=0.79, with a standard error of 0.40. The

increase in the standard error from 0.32 to 0.40 reflects the

increased variability in the ground-motion amplitude due to

variability in the depth of slip. In this case, the total standard

error is 0.65. This result is appropriate for r greater than 30

km but less than 100 km and for Mw 8. We do not present

results for shorter distances because the fault is shallow only

in the offshore region. At distances less than 30 km, the

slope of the attenuation relation would likely decrease, as

suggested by the trend of peak acceleration for r less than 40

km in figure 166.

The median values of the three asperity models are

compared with an empirical attenuation relation (based on

average properties of global recordings of subduction

earthquakes) in figure 166. Youngs and others (1988) and

Washington Public Power Supply System (1988) present

attenuation relations for rock and soil sites, respectively.

The separation between the empirical rock and soil curves is

in close agreement with the site term (e0.79) in our

attenuation relations (soil-site accelerations are 2.2 times

larger than rock-site accelerations), but the slopes of the

attenuation relations are different. Our predicted PGA values at close distance (30–40 km) are larger than the empirical

result, and the decay of PGA with distance is more rapid in

the numerical simulations. The western Oregon fault model

has slightly larger motions than the western Washington

model for a given value of closest distance to the fault. This

small difference is due to the steeper dip and narrower fault

width of the Oregon geometry, which brings more of the

fault closer to a surface site.

Figure 167 shows smoothed, median, 5-percent

damped, pseudospectral-velocity (PSV) curves for the

Washington and Oregon fault models. These results are

averages of separate simulations using the three slip models

and the two rock-site source functions for each line of

stations (a–g) shown in figure 160A. We estimate the

uncertainty in the response spectral estimates using the procedure outlined by Abrahamson and others (1990). An

estimate of parametric uncertainty is derived from the

ground-motion variability due to differences in the slip

model and the position of each site relative to the asperities.

The modeling and random uncertainty is derived from the

misfit between recorded and simulated motions of the

Michoacán and Valparaíso earthquakes, which is

summarized in figure 159. The overall uncertainty, which

combines the two estimates, is used to determine the 84thpercentile response spectra shown in figure 167. The

median and 84th-percentile values are listed in tables 24

and 25.

Figure 161. Generalized east-west velocity model of Cascadia subduction zone (adopted from Spence and others, 1985) used in finitedifference computations for the three seismic source locations at 18, 27, and 36 km depth (stars). The diagonal polygons represent the downgoing slab of the Juan de Fuca plate. Numbers in italics are S-wave velocities for each polygon, in km/s. Receiver locations are shown by

inverted triangles.
336 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST HORIZONTAL DISTANCE FROM SOURCE, IN KILOMETERS

18-KM SOURCE DEPTH 27-KM SOURCE DEPTH 36-KM SOURCE DEPTH

PEAK ACCELERATION, IN CENTIMETERS PER SECOND SQUARED

S SmS

TIME, IN SECONDS

A

D EF

B C

Figure 162. Comparison between two-dimensional finite-difference (A–C) and one-dimensional ray-theory (D–F) theoretical seismograms for the 15 receiver locations and 3 sources shown in figure 161; source depths are 18 km (A and D), 27 km (B and E), and 36 km (C

and F). The finite-difference seismograms are for the velocity model shown in figure 161, and the ray-theory seismograms are for the simplified velocity model shown in figure 160B. Note increase in the Moho-reflected S (SmS) amplitude as the wave field turns postcritical.

The finite-difference seismograms contain greater complexity, but amplitude and timing of the S and SmS arrivals are similar using either

method.

In figure 168A, we compare the simulated soil-site

response spectrum with estimates from two empirical

studies. The first was derived by Crouse and others (1988)

using regression methods (30-km source depth, 120 km to the

center of energy release). The second was derived by Heaton

and Hartzell (1989) from averages of strong-motion recordings from M 7.6–8.2 subduction earthquakes in the two epicentral-distance ranges of 50–100 km and 100–150 km. The

simulated estimates are for the shallow slip model, the deep

slip model, and the log mean of the three asperity models

from Washington line f (fig. 160A), which includes Seattle.

For this comparison, we use a soil-site source function. The

empirical estimates span the range between the estimates for

the average and shallow asperity models obtained from

numerical modeling. We cannot predict motions at periods

longer than 1 s for soil sites without information about the

site-response and modeling of long-period wave propagation

in realistic three-dimensional velocity structures.

Strong-motion recordings were made of two Benioffzone earthquakes in 1949 and 1965. The locations of these
SIMULATED STRONG GROUND MOTIONS, MAGNITUDE 8 EARTHQUAKES, CASCADIA SUBDUCTION ZONE 337

a

b

cde

f

g

a

b

cde

f

g

a

b

c

de

f

g

0.305

g

0.300

g

0.231

g

0.228

g

0.197

g

0.190

g

0.143

g

0.311

g

0.248

g

0.223

g

0.153

g

0.132

g

0.145

g

0.089g 

0.295

g

0.196

g

0.168

g

0.104

g

0.083

g

0.051

g

0.041

g

0 15 30 45 60 SECONDS 0 15 30 45 60 SECONDS 0 15 30 45 60 SECONDS N-S comp station 3 N-S comp station 3 N-S comp station 3

SHALLOW ASPERITY

Puget Sound model (dip=11)

rock site (caleta src)

MIDDLE ASPERITY

Puget Sound model (dip=11)

rock site (caleta src)

DEEP ASPERITY

Puget Sound model (dip=11)

rock site (caleta src)

Figure 163. Horizontal acceleration seismograms for shallow, middle, and deep slip distributions using the western Washington fault model and a rock-site empirical source function. The seismograms are from receivers in lines a–g shown in figure 160A. In this example, the receivers are in close proximity to the areas of greatest slip amplitude on the fault

and, thus, the waveforms are characteristic of the largest amplitudes predicted for rock sites in Washington. Numbers to right are peak accelerations, in fraction of g.
338 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

a

b

cd

e

f

g

a

bcd

e

f

g

a

b

c

de

f

g

0.243

g

0.440

g

0.262

g

0.330

g

0.184

g

0.140

g

0.156

g

0.481

g

0.558

g

0.194

g

0.243

g

0.137

g

0.127

g

0.087

g

0.717

g

0.378

g

0.293

g

0.196

g

0.110

g

0.058

g

0.042

g

0 15 30 45 60 SECONDS 0 15 30 45 60 SECONDS 0 15 30 45 60 SECONDS N-S comp station 4 N-S comp station 4 N-S comp station 4

SHALLOW ASPERITY

Portland region model (dip=21)

rock site (caleta src)

MIDDLE ASPERITY

Portland region model (dip=21)

rock site (caleta src)

DEEP ASPERITY

Portland region model (dip=21)

rock site (caleta src)

Figure 164. Example horizontal accelerograms for shallow, middle, and deep slip distributions using the western Oregon fault model and a rock-site empirical source function. The

seismograms are from receivers in lines a–g shown in figure 160A. The receivers are in close proximity to the areas of greatest slip on the fault and, therefore, the waveforms are

characteristic of the largest amplitudes predicted for rock sites in Oregon. Numbers to right are peak accelerations, in fraction of g.
SIMULATED STRONG GROUND MOTIONS, MAGNITUDE 8 EARTHQUAKES, CASCADIA SUBDUCTION ZONE 339

.719g

a

b

c d

e

f

g

Shallow

a .501g b

c d e

f

g

Middle

.434g

a b c d e f

g

Deep

earthquakes are shown by stars in the cross section of the

subduction zone in figure 160B. They occurred within the

lower part of the subducting Juan de Fuca plate beneath the

cities of Seattle and Tacoma at depths greater than the

downdip limit of rupture (40 km) in our fault models.

Their recorded motions are compared with the simulated

motions for a M 8 earthquake in figure 168B. The

averaged PSV response of 10 horizontal recordings from

the two earthquakes were derived from Seattle and

Olympia records for April 13, 1949 (M 6.5), and Seattle,

Tacoma, and Olympia records for April 29, 1965 (M 7.1).

The simulated PSV curve is the mean for the three

locations weighted appropriately using a soil-site source

function. For periods less than 1 s, the estimated response

Figure 165. Mean horizontal peak acceleration for each receiver location in the Oregon fault model for shallow, middle, and deep slipdistribution models. The lines represent receiver lines a–g shown in figure 160A. The widest range of peak accelerations is produced using

the shallow model, when slip is concentrated in top third of fault plane. The deep slip model produces more uniform amplitudes across the

receiver array. Accelerations (in fraction of g) are plotted on a common scale.

1.0

0.1

0.0310 100

 DISTANCE TO FAULT, IN KILOMETERS

PEAK ACCELERATION, IN FRACTION OF g

EXPLANATION

Washington model

Oregon model

ROCK SITES

1.0

0.1

0.0310 100

EXPLANATION

Washington model

Oregon model

SOIL SITES

Figure 166. Comparison of peak horizontal acceleration (PGA) attenuation relations derived in this study with relations derived for,

left, rock sites (long-dashed line, data from Youngs and others, 1988) and right, soil sites (long-dashed line, data from Washington Public

Power Supply System, 1988) in the Pacific Northwest. Circles, logarithmic mean PGA values for western Washington model; diamonds,

logarithmic mean PGA values for Oregon fault model. Standard deviation of the mean is less than 5 percent. Standard deviation of each

population shown by the vertical error bars.
340 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

spectral velocities in the Seattle-Olympia region are about

twice those recorded during the 1949 and 1965 earthquakes,

and the durations of strong motion are significantly longer

(45–60 s versus 10–15 s).

CONCLUSIONS

Slip-distribution models of the Michoacán and

Valparaíso earthquakes derived from near-source and

teleseismic velocity seismograms are generally consistent

with the higher frequency motions of recorded accelerograms. Simulated ground motions show close agreement

in the overall duration of strong motion, the frequency

content, and the peak accelerations of the recorded

motions. Measurements of the misfit between the simulated and recorded response spectra are used to estimate

modeling and random uncertainty in the simulation

procedure.

Determining the depth extent of rupture on the plate

interface is the most critical geometrical source parameter

influencing ground motions, as this extent controls the

1,000

100

10

1

1,000

100

10

1

VELOCITY, IN CENTIMETERS PER SECOND

1,000

100

10

1

1,000

100

10

1

0.01 0.1 1 10 0.01 0.1 1 10

PERIOD, IN SECONDS

OREGON FAULT MODEL

ROCK SITE: MEDIAN

 

WASHINGTON FAULT MODEL

ROCK SITE: MEDIAN

WASHINGTON FAULT MODEL

ROCK SITE: 84th PERCENTILE

OREGON FAULT MODEL

ROCK SITE: 84th PERCENTILE

a

b

c

d

e

f

g

a

b

c

d

e

f

g

a

b

c

d

e

f

g

a

b

c

d

e

f

g

Figure 167. Median and 84th-percentile pseudovelocity response spectra (at 5-percent damping) for each line of stations (a–g) shown in

figure 160A.
SIMULATED STRONG GROUND MOTIONS, MAGNITUDE 8 EARTHQUAKES, CASCADIA SUBDUCTION ZONE 341

closest approach of the rupture surface to urban districts in

the Puget Sound area. We use the model of Weaver and

Baker (1988), which places the 40-km depth contour on the

plate interface (inferred to be the easternmost edge of the

rupture surface) 50 km west of Puget Sound. An alternative

model proposed by Crosson and Owens (1987) places the

eastern margin of the rupture surface directly beneath Puget

Sound and produces larger ground motions in the Puget

Sound area.

Given the assumed location of the rupture surface, the

largest cause of uncertainty in the estimated motions is due

to the distribution of slip amplitude with depth on the fault

surface. As the large slip moves deeper, it approaches the

urban areas of Puget Sound and Portland, causing larger

Table 24. Spectral velocities at rock sites from a simulated M 8 Cascadia subduction-zone earthquake in the western Washington

fault model.

[Velocities are in centimeters per second]

____________________________________________________________________________________________________________________________________________________________

 Velocity at specified line of receivers in model array1

Period ______________________________________________________________________________________________________________

 (seconds) Line a Line b Line c Line d Line e Line f Line g ____________________________________________________________________________________________________________________________________________________________

____________________________________________________________________________________________________________________________________________________________

1See figure 160A for location of model and orientation of receiver array.

Median

 84th

percentile

0.030

.045

.056

.069

.087

.108

.135

.168

.210

.261

.326

.407

.507

.632

.789

.984

1.227

1.530

1.908

2.380

2.968

3.701

.030

.045

.056

.069

.087

.108

.135

.168

.210

.261

.326

.407

.507

.632

.789

.984

1.227

1.530

1.908

2.380

2.968

3.701

0.93

1.69

2.40

3.43

4.90

7.08

10.14

13.85

17.72

21.51

24.58

25.90

26.05

26.66

27.93

29.09

30.11

30.20

27.68

24.00

20.98

16.16

1.73

3.36

4.87

7.03

10.15

14.72

20.77

27.37

34.17

41.57

48.12

50.56

49.29

48.19

49.03

51.75

56.18

59.08

54.91

46.41

39.83

33.42

0.77

1.37

1.95

2.79

3.99

5.73

8.14

11.08

14.29

17.56

20.20

21.39

21.92

23.01

24.32

25.12

25.80

25.76

23.44

20.21

17.59

13.58

1.46

2.59

3.78

5.48

7.93

11.45

16.03

21.04

26.55

32.75

37.88

39.60

38.93

38.49

39.09

41.02

44.54

47.02

44.14

37.97

32.84

27.63

0.64

1.13

1.61

2.31

3.33

4.83

6.89

9.30

11.90

14.64

16.90

17.92

18.35

19.33

20.65

21.36

21.56

21.37

19.76

17.26

15.13

11.73

1.22

2.13

3.12

4.56

6.66

9.73

13.66

17.79

22.14

27.14

31.37

32.69

31.83

31.25

31.99

33.71

35.98

37.72

36.04

31.57

27.82

23.78

0.52

.93

1.34

1.91

2.73

3.91

5.53

7.45

9.46

11.50

13.20

14.00

14.36

15.17

16.30

17.15

17.59

17.31

15.64

13.29

11.56

9.13

1.01

1.90

2.78

4.05

5.87

8.48

11.85

15.48

19.25

23.34

26.68

27.52

26.54

25.92

26.61

28.23

29.75

30.00

27.69

23.68

20.96

18.47

0.40

.71

1.01

1.45

2.09

3.03

4.33

5.84

7.44

9.10

10.50

11.15

11.47

12.23

13.18

13.73

14.14

14.43

13.52

11.64

9.95

7.56

.78

1.36

1.99

2.90

4.25

6.21

8.72

11.33

14.00

16.99

19.57

20.37

19.90

19.79

20.45

21.64

23.43

25.05

23.96

20.31

17.33

14.76

0.33

.59

.85

1.23

1.77

2.56

3.65

4.90

6.17

7.41

8.43

8.89

9.09

9.58

10.22

10.56

10.87

11.21

10.78

9.61

8.36

6.37

.67

1.19

1.75

2.57

3.76

5.46

7.65

9.93

12.20

14.60

16.53

16.91

16.26

15.88

16.11

16.74

17.89

19.25

19.16

17.33

15.46

13.11

0.25

.45

.65

.93

1.34

1.94

2.74

3.68

4.65

5.67

6.56

7.02

7.33

7.91

8.44

8.51

8.57

8.94

8.90

8.07

7.02

5.35

.51

.87

1.27

1.86

2.71

3.93

5.49

7.13

8.83

10.75

12.42

13.02

12.99

13.27

13.78

14.15

14.90

16.23

16.50

14.81

12.82

10.74
342 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

ground motions. Deeper slip models produce more gradual

attenuation of peak acceleration with distance. The dip of the

western Oregon fault model is about twice that of the western

Washington model, but the predicted ground motions for the

two regions are not very different.

Quantifying the uncertainty allows ground-motion estimates to be used in probabilistic seismic-hazard calculations.

An estimate of the parametric uncertainty is derived from the

variability in the ground motions due to differences in the

position of a station along fault strike and in the along-strike

Table 25. Spectral velocities at rock sites from a simulated M 8 Cascadia subduction-zone earthquake in the western Oregon fault

model.

[Velocities are in centimeters per second]

_____________________________________________________________________________________________________________________________________________________________

 Velocity at specified line of receivers in model array1

Period ______________________________________________________________________________________________________________

 (seconds) Line a Line b Line c Line d Line e Line f Line g _____________________________________________________________________________________________________________________________________________________________

_____________________________________________________________________________________________________________________________________________________________

1See figure 160A for location of model and orientation of receiver array.

Median

 84th

percentile

0.030

.045

.056

.069

.087

.108

.135

.168

.210

.261

.326

.407

.507

.632

.789

.984

1.227

1.530

1.908

2.380

2.968

3.701

.030

.045

.056

.069

.087

.108

.135

.168

.210

.261

.326

.407

.507

.632

.789

.984

1.227

1.530

1.908

2.380

2.968

3.701

1.22

2.15

3.08

4.43

6.38

9.30

13.40

18.33

23.58

28.86

33.04

34.63

35.06

36.55

39.07

41.43

43.53

44.21

40.32

34.10

29.36

22.79

2.44

4.31

6.28

9.13

13.30

19.48

27.69

36.62

45.94

56.19

64.88

67.68

66.28

65.73

68.12

73.31

80.93

86.11

79.64

65.44

54.94

46.37

1.16

2.09

2.97

4.23

6.05

8.75

12.51

16.94

21.43

25.65

28.95

30.52

31.37

32.96

34.78

35.69

36.32

36.37

33.67

29.33

25.43

19.26

2.21

3.95

5.75

8.33

12.07

17.53

24.63

32.08

39.58

47.40

53.82

56.05

55.30

54.67

55.38

57.74

62.26

66.41

63.96

55.66

47.95

39.60

0.81

1.47

2.10

3.00

4.26

6.10

8.65

11.72

15.02

18.29

20.93

22.27

23.13

24.80

27.02

28.57

29.60

29.72

26.74

22.07

18.42

13.95

1.53

2.80

4.09

5.93

8.53

12.25

17.12

22.37

27.85

33.84

38.90

40.79

40.33

40.30

41.99

45.13

49.39

52.29

48.56

40.19

33.68

28.03

0.63

1.14

1.60

2.27

3.26

4.72

6.76

9.23

11.83

14.45

16.62

17.67

18.21

19.30

20.78

21.91

22.79

22.87

20.62

17.19

14.52

11.06

1.19

2.32

3.34

4.82

7.00

10.20

14.43

19.05

23.82

28.93

33.17

34.52

33.76

33.43

34.60

36.63

38.96

39.90

36.63

30.58

26.15

22.19

0.44

.78

1.11

1.58

2.26

3.24

4.60

6.23

8.01

9.87

11.47

12.36

13.05

14.36

15.97

16.97

17.49

17.49

15.89

13.39

11.31

8.52

.83

1.50

2.18

3.16

4.58

6.60

9.20

11.99

14.98

18.37

21.39

22.66

22.79

23.48

25.14

27.13

29.27

30.54

28.31

23.57

19.89

16.70

0.33

.58

.82

1.16

1.68

2.44

3.49

4.71

5.99

7.32

8.48

9.14

9.61

10.44

11.41

12.04

12.58

13.01

12.30

10.57

8.89

6.68

.64

1.16

1.67

2.42

3.53

5.16

7.26

9.46

11.72

14.26

16.49

17.37

17.25

17.43

18.13

19.19

20.80

22.44

21.91

19.02

16.24

13.55

0.26

.45

.64

.91

1.30

1.88

2.67

3.61

4.57

5.54

6.40

6.92

7.30

8.06

9.07

9.81

10.29

10.59

10.09

8.89

7.70

5.85

.51

.86

1.26

1.83

2.65

3.85

5.40

7.04

8.69

10.48

12.09

12.86

13.04

13.66

14.91

16.34

17.88

19.26

18.84

16.47

14.17

11.85
SIMULATED STRONG GROUND MOTIONS, MAGNITUDE 8 EARTHQUAKES, CASCADIA SUBDUCTION ZONE 343

and downdip location of slip on the fault. An estimate of

modeling and random uncertainty is derived from the misfit

between recorded and simulated motions of the Michoacán

and Valparaíso earthquakes. The total uncertainty is

represented by the 84th-percentile response spectra.

REFERENCES CITED

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Uncertainty in numerical strong motion predictions: U.S.

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Adams, John, 1990, Paleoseismicity of the Cascadia subduction

zone—Evidence from turbidites off the Oregon-Washington

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Aki, Keiiti, and Richards, P.G., 1980, Quantitative seismology—Theory and methods: San Francisco, W.H. Freeman and

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Cohee, B.P., Beroza, G.C., and Dodge, D.A., 1994, Modeling Loma

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———1991b, Errata: Seismological Society of America Bulletin,

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100

10

1

100

10

1

0.01 0.1 1 10

0.01 0.1 1 10

PERIOD, IN SECONDS

VELOCITY, IN CENTIMETERS PER SECOND

Mean of 10 

horizontal recordings

Simulation

B

A

Crouse and others

(1988): 120 km

Heaton and Hartzell

(1989): 50-100 km

Mean Shallow

Deep

Heaton and Hartzell

(1989): 100-150 km

Figure 168. A, Comparison of simulated horizontal response

spectra for M 8 subduction earthquakes at soil sites in Seattle with

estimates derived from globally recorded data. The three simulated

results (solid lines) are obtained using three different slip models

(shallow, deep, and the log mean of shallow, middle, and deep results). The empirical estimates from two studies (dashed and dotted

lines, labeled with the epicentral-distance ranges) are obtained from

recordings of M ~ 8 subduction earthquakes. B, Comparison of

simulated horizontal response spectra for soil sites in SeattleOlympia region for a M 8 earthquake with mean of 10 horizontal

components recorded in Seattle, Tacoma, and Olympia during the

1949 (M 7.1) and 1965 (M 6.5) earthquakes.
344 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Helmberger, D.V., and Harkrider, D.G., 1978, Modeling earthquakes with generalized ray theory, in Miklowitz, Julius, and

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345

EARTHQUAKE GROUND-RESPONSE STUDIES IN WEST 

AND SOUTH SEATTLE, WASHINGTON

By David L. Carver,1 Kenneth W. King,1 Robert A. Williams,1 and David M. Worley1

ABSTRACT

The characteristics of seismic ground response in West

and South Seattle were determined using recordings of three

local earthquakes at temporary seismograph sites. The

recording sites were located at or near those that experienced

Modified Mercalli VIII intensities during the 1965 Seattle

earthquake. The highest ground-response functions (GRFs)

calculated ranged from 5 to 12 at Harbor Island, which had a

Modified Mercalli intensity of VIII during the 1965 earthquake. Harbor Island is underlain by manmade fill. A similar relationship between GRF values and 1965 earthquake

intensities was observed at manmade fill sites in Olympia,

Wash. GRFs for other sites in West and South Seattle were

much lower, ranging from 2.1 to 4.4. No site in Olympia

experienced an intensity of VII or more in the 1965 earthquake without a GRF of 5.7 or greater. We conclude that

ground amplification probably did not cause the anomalously high intensities in West Seattle during the 1965 earthquake. Almost all of the observations that led to West Seattle

receiving a Modified Mercalli intensity of VIII were based

on damage to chimneys. Close inspection of chimneys in the

area of greatest damage revealed many with extremely deteriorated mortar, which perhaps contributed to the damage

during the 1965 earthquake and, thus, inflated the local intensity values for West Seattle. Therefore, the original Modified Mercalli intensity of VIII at West Seattle was probably

more in the range of intensity VII, which includes damage to

weak masonry.

INTRODUCTION

Urban areas in the Puget Sound, Washington, area are

subject to significant seismic risk. The largest recorded

earthquakes in the Puget Sound area (fig. 169) occurred in

1946, 1949, and 1965 (Thorsen, 1986). The 1949 earthquake

near Olympia caused a Modified Mercalli (MM) intensity of

VIII in both Seattle and Olympia. MM intensity VIII

includes damage to unreinforced masonry, fall of stucco and

some masonry walls, and twisting or falling chimneys. The

epicenter of the 1965 earthquake was about 25 km south of

Seattle (Algermissen and Harding, 1965). It caused widespread damage, although the maximum MM intensity of VIII

was concentrated in the Harbor Island and West Seattle areas

(fig. 170) (Hake and Cloud, 1967). Mullineaux and others

(1967) suggested that Harbor Island and West Seattle may

have also experienced higher intensities than surrounding

areas during the 1949 earthquake.

This study was designed to determine if near-surface

geophysical factors exist that may cause Harbor Island and

West Seattle to experience greater ground shaking during

earthquakes than other nearby areas. Six ground-response

monitoring sites were set up (fig. 170), and we recorded

ground motion from three small earthquakes (fig. 169).

These records were used to compute ground-response functions (GRFs) for the six sites. GRF is the amplification of

ground motion at a site of interest (usually underlain by

unconsolidated sediments) relative to ground motion

recorded at a standard, or reference, site (usually on rock).

This method has been applied in several areas including Los

Angeles, San Francisco, and Olympia (Borcherdt and Gibbs,

1976; Rogers and others, 1979, 1985; and King and others,

1990). Seismic-refraction lines were also recorded at each of

the six sites. The refraction lines provided information on the

compressional-wave seismic velocity characteristics of the

site from 0 to 90 m depth. Finally, to answer the question of

whether the houses and chimneys in West Seattle were more

susceptible to earthquake shaking than those in surrounding

areas, we determined the natural frequencies and damping

coefficients of 10 local one-story residences.

DATA COLLECTION AND 

PROCESSING

From November 1987 to December 1988, we operated

six portable digital seismographs in West and South Seattle

to record site-specific ground response. The recording sites

________________ 1U.S. Geological Survey, Box 25046, MS 966, Federal Center, Denver,

CO 80225.
346 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

124° 122°

48°

47°

0 20 40 KILOMETERS

101

5

5

90

A

B

C

1946

1965

1949

Seattle

Tacoma

Olympia

Yakima

Edmonds

Bremerton

WASHINGTON

PACIFIC OCEAN

Puget

Sound

were primarily chosen because they were at or near documented 1965 earthquake intensity observations. We also

wanted sites that represented different surficial geologic

units and a broad spacial distribution across the West and

South Seattle areas. Another consideration in recordingsite selection was finding sites that had sufficient space to

allow seismic reflection and refraction lines.

All sites except the one used as the standard (SEW)

were located at or near locations where MM intensities of

V, VI, and VIII were reported as a result of the 1965 earthquake. There were no historical data for the SEW site

because no inhabited structures existed there at the time;

however, surrounding areas within two blocks of the site

had intensity reports of IV or less. It is assumed that the

probable intensity at the SEW site, which was placed

directly on Tertiary fine-grained sandstone, would also

have been IV or less (Hake and Cloud, 1967). Sites HAR

and HIA experienced MM intensity of VIII; at site HIG,

intensity values of VI had been reported; and sites JEF and

LIN had experienced intensity V (fig. 170).

Our seismographs registered the three small earthquakes that were also recorded and located by the University of Washington permanent seismograph network. Two

local-magnitude (ML) 2.8 earthquakes were located in the

Puget Sound area and the third, registering ML 4.1,

occurred near Yakima, Wash. (fig. 169, table 26)

(University of Washington hypocenter computer data file,

unpub. data).

Figure 169. Map showing epicenters (circled letters) of three small earthquakes in Washington used as

seismic-shaking sources for computation of ground-response functions. Locations were calculated by the

University of Washington permanent seismograph network and are listed in table 26. Stars, epicenters of

the 1946, 1949, and 1965 earthquakes.

Table 26. Epicenter locations of small earthquakes in Washington detected by seismograph sites

during the study. Data are from the University of Washington hypocenter computer data file.

[ML, local magnitude]

___________________________________________________________________________________________________________________________

Earthquake Latitude Longitude Depth Size General location

N. W. (kilometers) (ML)

___________________________________________________________________________________________________________________________

___________________________________________________________________________________________________________________________

A

B

C

47o49.04'

46o40.49'

47o32.94'

122o21.76'

122o41.03'

122o44.59'

50

18

19

2.8

4.1

2.8

Edmonds, Wash.

Yakima, Wash.

Bremerton, Wash.
EARTHQUAKE GROUND-RESPONSE STUDIES IN WEST AND SOUTH SEATTLE, WASHINGTON 347

The portable digital seismographs used triaxial

velocity-sensitive transducers with a natural frequency of

1.7 Hz and a damping coefficient of 0.6 of critical. The

seismometers were leveled, oriented, and calibrated at each

site using standardized procedures outlined in Carver and

others (1986). The seismographs used an internal trigger

Figure 170. Surficial geologic map of West and South Seattle area (modified from Waldron and others, 1962). Dots and threeletter codes, locations of ground-response recording sites. Roman numerals, Modified Mercalli intensities observed at these locations

during the 1965 earthquake.

Lake

Washington

N

Elliott

Bay

PUGET

SOUND Duwamish

Head

Alki

Point

Harbor

Island

JEF

V

HAR

VIII

HIA

VIII

LIN

V

HIG

VI

West

Seattle

South Seattle

SEW

IV

0 1 2 KILOMETERS

Artificial fill, f and modified land, m—Modified land:

 areas leveled by cut and fill and fill frequency

Alluvium—Chiefly sand and silt but includes clay and

 peat. South of Renton and Tukwila these sediments

 are 15–25 ft thick and overlie sand and gravel.

 Mostly sand and gravel in Cedar River valley

Beach deposits—Chiefly sand; may include interbedded

 organic material; deposits in places are veneers of

 sand and gravel less than 2 ft thick on older deposits

Lacustrine sediments—Chiefly unconsolidated silt, clay,

 and fine sand generally less than 10 ft thick

Younger gravel—Chiefly sand and pebble gravel;

 commonly overlies Qt; as much as 100 ft thick

Vashon till—Compact, concrete-like mixture of silt,

 sand, gravel and clay, as much as 150 ft thick, but

 generally less than 50 ft. Upper 2–5 ft generally a

 loose, silty sand and gravel

Older gravel—Chiefly sand and pebble gravel; lies

 beneath Qt; as much as 200 ft thick

Older sand—Chiefly medium sand; loose, as much as

 300 ft thick

Older clay, till, and gravel—Silt, clay, fine sand, and

 till, very compact; locally includes lenticular sand and

 gravel shown as Ocg where mappable

Sedimentary rocks of Oligocene age—Chiefly

 tuffaceous sandstone and conglomerate; compact

 but poorly cemented; at least 2,500 ft thick

EXPLANATION

Bailey

Peninsula
348 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

algorithm that discriminates between ground shaking

induced by earthquakes and that caused by local disturbances such as traffic. Data were recorded digitally on cassette tapes that were subsequently played back into a

computer for analysis using spectral-analysis software

developed by Cranswick and others (1989).

The earthquake records were first displayed as amplitude-normalized seismograms to allow inspection and selection of a standardized portion of the time series for analysis.

This window was the same for all of the records; therefore,

it was unnecessary to normalize spectral amplitudes for window length. A 20-s time window was chosen beginning with

the P-wave arrival and including most of the coda. The data

time window was tapered with a whole-cosine bell (Hanning

window) before being transformed by a standardized Fast

Fourier Transform (FFT) computer program. Spectral

amplitudes and ratios were smoothed using a moving-average window with a Hanning taper and a width of 0.15 Hz.

The earthquake smoothed spectra were then compared with

the pre-event smoothed spectra to determine the signal-tonoise ratio. All of the data used in this report had a signalto-pre-event noise ratio of at least 1.5. The GRF was

calculated, using smoothed spectra, by the following

equations:

GRFia=0.5(Ri,2,a+Ri,3,a) (1)

and

Ri,j,a=(Si,j,a)/(So,j,a) (2)

where i is the recording site on unconsolidated sediments;

o is the standard recording site (SEW);

j is the horizontal component (2=north-south, 3=eastwest);

R is the spectral ratio;

S is the smoothed Fourier amplitude spectrum; and

a is the frequency band (0.5–1.0 Hz; 2–4 Hz; and 4–8

Hz).

Figures 171, 172, and 173 show the process of deriving the GRF. Figure 171 shows typical seismograms with

amplitudes normalized so that all components are displayed

at the same scale. Figure 172 shows smoothed Fourier

amplitude spectra for the sites on unconsolidated sediments

and standard rock site SEW. Figure 173 and table 27 summarize the GRF, the surficial geology, the MM intensity

observed from the 1965 earthquake for each site, and the

seismic-velocity structure.

Both earthquakes A and C were recorded at sites LIN

and SEW. Figure 174 shows the derived spectral ratios for

these earthquakes. The similarity of the two plots indicates

a high degree of repeatability of GRF values for different

shaking sources.

BUILDING STUDIES

Immediately following the 1965 earthquake,

Algermissen and Harding (1965) conducted a block by

block survey of West Seattle in which they calculated the

ratio of damaged to undamaged chimneys resulting from

the earthquake. One possible explanation for the large

number of damaged chimneys is that they were tuned to the

same frequency as the earthquake shaking at that location.

We selected 10 houses within two blocks of site HIA (fig.

170) in the area of maximum former chimney damage and

installed seismometers on chimneys, roof tops, and at the

midpoints of bearing walls. Several minutes of ambient

Table 27. Calculated ground-response functions (GRFs) for three small Washington earthquakes detected by seismograph sites in West

and South Seattle during the study.

[All values given relative to site SEW. Tb, Tertiary sedimentary rocks, sandstone; Qt, Vashon till, compact silt, sand, gravel; Qos, Vashon outwash, older sand, medium sand,

loose; af, artificial fill]

_____________________________________________________________________________________________________________________________________________________________________

 Average horizontal Modified Mercalli Average P-wave Depth

Site Earthquake ground-response function Geology1 intensity2 velocity interval

 _______________________________________________________ (1965 earthquake) (meters/second) (meters)

 0.5–10 Hz 1–2 Hz 2–4 Hz 4–8 Hz _____________________________________________________________________________________________________________________________________________________________________

_____________________________________________________________________________________________________________________________________________________________________

 1From Waldron and others (1962).

 2Margaret Hopper, U.S. Geological Survey, written commun.

SEW

JEF

HIG

LIN

LIN

HIA

HAR

 A, B, C

 A

 C

 A

 C

 B

 B

1

 3.7

 3.3

 3.8

 3.5

 4.4

 11.7

1

 3.5

 2.9

 3.2

 3.0

 3.9

 8.8

1

 2.8

 2.1

 2.3

 3.2

 4.0

 4.9

1

 2.3

 2.2

 2.3

 2.4

 2.5

 4.8

Tb

Qt

Qt

Qt

Qt

Qos

af

IV

V

VI

V

V

VIII

VIII

2,600

2,200

1,470

1,460

1,460

1,520

1,370

1–15

1–55

1–10

1–14

1–14

1–10

1–90
EARTHQUAKE GROUND-RESPONSE STUDIES IN WEST AND SOUTH SEATTLE, WASHINGTON 349

EARTHQUAKE B EARTHQUAKE C

NORMALIZED AMPLITUDE

0 5 10 15 20 25 30 0 5 10 15 20 25 30

TIME, IN SECONDS

SEW Vert.

N-S

E-W

SEW

LIN

HIG

JEF

Vert.

N-S

E-W

Vert.

N-S

E-W

Vert.

N-S

E-W

Vert.

N-S

E-W

Vert.

N-S

E-W

Vert.

N-S

E-W

0.05 cm/sec

HIA

HAR

Figure 171. Representative seismograms recorded at the West and South Seattle recording sites for earthquakes B and C (epicenters are

given in table 26). Three components of motion are shown for each recording site. Vert., vertical; N-S, north-south; E-W, east-west.

(first mode) was determined by calculating an FFT on the

time-series data (examples are shown in fig. 175). The

seismic background noise and several man-induced

vibration events were recorded. The natural frequency

Table 28. Natural frequencies and damping coefficients of West Seattle houses constructed on Quaternary unconsolidated

sediments.

[All buildings are one-story structures located within two blocks of seismograph site HIA. Data are derived from seismic and man-induced vibration sources]

__________________________________________________________________________________________________________________________________________________

 Building Natural frequency, Natural frequency, Damping coefficient Natural frequency,

 long axis (Hz) short axis (Hz) (percent critical) chimney __________________________________________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________________________________________

 1

 2

 3

 4

 5

 6

 7

 8

 9

10

 7.2

 8.2

 5.5

 6.8

 8.6

10.2

 7.0

 9.8

10.5

 5.3

11.4

 8.6

 5.4

 7.0

11.1

13.6

14.0

10.6

14.8

11.7

3.5

1.4

3.0

2.5

1.8

2.2

2.5

3.8

2.4

2.9

 7.0

 6.2

 8.6

10.0

13.7

12.5

7.8–11.7

 9.4

9.0–10.2

6.1–9.8
350 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

damping ratio was obtained by using

D=1/(2π)[–ln(Xn+1/Xn)] (3)

where D is the percent of critical damping and Xn is the

velocity amplitude of the nth cycle of motion (Dowding and

others, 1980). The results of these tests are shown in table

28.

SITE INVESTIGATIONS USING SEISMIC 

REFRACTION

High-resolution P-wave seismic-refraction profiles

were acquired at the six sites to characterize their nearsurface velocity characteristics. A 12-gauge shotgun or

gasoline-powered earth tamper was used as the seismic

source, and the signal was recorded by a 24-channel digital

seismograph with 100-Hz geophones spaced 1.5–3.0 m

apart. The slope-intercept method of analysis was used to

interpret the recordings. The results provide information on

the compressional-wave seismic-velocity structure of a site

at the depth range of 0–10 m for the shorter profiles and

down to 90 m for longer profiles. Because surface-wave

velocity is only a few percent slower than shear-wave

velocity (Aki and Richards, 1980), we used the surface

waves generated by the 12-gauge and tamper sources at some

of the sites to estimate the near-surface S-wave velocity. The

surface wave was identified on the vertical-component

refraction records as a high-amplitude dispersed wavetrain

on each record. The surface-wave velocity was then determined from the slope of the highest velocity (first arrival

group) surface wave on a time versus distance seismogram.

For S-wave velocity calculations, we assumed that the surface-wave velocity was 10 percent slower than the S-wave

velocity. We also assumed that this surface-wave velocity

applies to a depth of about one wavelength of the surface

wave (Sheriff and Geldart, 1982). Because we only had vertical-component refraction data, it’s possible that we were

measuring the S-wave direct arrival and not the highest

velocity surface wave. Therefore, we may have overestimated the S-wave velocity by about 10 percent; however, the

relative differences in S-wave velocity between the sites

would remain the same. Results of the refraction profiles are

summarized in table 27.

10-1

10-2

10-3

10-4

Station HAR

0.5 2 4 6 8 10

10-1

10-2

10-3

10-4

Station SEW

0.5 2 4 6 8 10

10-1

10-2

10-3

10-4

Station HIA

0.5 2 4 6 8 10

AMPLITUDE, IN CENTIMETER-SECOND PER SECOND

FREQUENCY, IN HERTZ

Figure 172. Horizontal, north-south components of velocity

spectra for the three recording stations in West and South Seattle

that recorded earthquake B. The ground-response functions are the

result of computing the ratios of the spectra of sites on

unconsolidated sediments (HAR and HIA) to the standard site

(SEW) located on rock.
EARTHQUAKE GROUND-RESPONSE STUDIES IN WEST AND SOUTH SEATTLE, WASHINGTON 351

Standard site SEW, in Seward Park on Bailey Peninsula, was underlain by a fine-grained sandstone with a compressional-wave velocity of about 2,600 m/s to at least 15 m

depth. This is the highest P-wave velocity observed at any

of the recording sites.

Site HAR was located on fill material in the Duwamish

River waterway with a uniform 1,371 m/s P-wave seismic

velocity to a minimum depth of 90 m. The surface-wave

data translate to a very slow 150 m/s S-wave velocity in the

upper 6 m of fill.

Sites JEF and LIN were located on Vashon till, which

in this area is a graded mixture of clay to gravel. The seismic-velocity data indicate that these two sites are located on

relatively firm ground with P-wave seismic velocities

increasing from 1,470 m/s at about 2 m depth up to 2,220 m/

s at 30–55 m depth. The S-wave velocity at LIN was about

300 m/s at 5 m depth whereas at JEF it was about 740 m/s at

about 15 m depth. Site HIG was located on a well-sorted and

poorly graded gravel that has a P-wave seismic-velocity

structure similar to LIN. Site HIA was located on older

uncemented sand of Quaternary age with a P-wave seismicvelocity structure also similar to LIN.

DISCUSSION AND CONCLUSIONS

The low velocities at site HAR confirmed that the fill

material is unconsolidated, saturated, and probably forms a

high impedance contrast at its base, which would influence

seismic-wave amplification. Not surprisingly, site HAR

had a GRF value 5–12 times greater than standard rock site

SEW. This result is in accordance with the MM intensity

VIII damage experienced at Harbor Island during the 1965

earthquake. Similar sites in Olympia (King and others,

1990), underlain by artificial fill and unconsolidated saturated sediments, also have very low seismic velocities and

high GRF values and have experienced MM intensity VIII

damage from the 1965 earthquake.

There has been considerable interest in explaining

why West Seattle experienced MM intensity VIII shaking

in the 1965 earthquake. Some investigators (Langston and

Lee, 1983; Ihnen and Hadley, 1986) have used velocity

models and ray tracing to create synthetic accelerograms

for the Puget Sound region. They concluded that the shaking was enhanced in both West Seattle and Harbor Island

by basin-geometry wave focusing as well as by nearsurface ground response.

50

10

1

Station JEF—earthquake A

2.8 3.5 3.7

50

10

1

Station HIA—earthquake B

4.0 3.9

4.4

50

10

1

Station LIN—earthquake

 A (solid lines) and earthquake

 C (dashed lines)

3.2

2.3

3.2

3.0

3.8

3.5

50

10

1

Station HIG—earthquake C

 

2.1

2.9

3.3

50

10

1

0.5 1 2 3 4 5

Station HAR—earthquake B

4.9

8.8

11.7

FREQUENCY, IN HERTZ

GROUND-RESPONSE FUNCTION

Figure 173. Average horizontal ground-response functions calculated from seismograms for three small earthquakes recorded at

all the monitoring sites in West and South Seattle. Values are spectral ratios relative to standard site SEW. Site LIN was the only one

other than the standard site to record more than one earthquake.
352 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

We located site HIA in West Seattle, in the center of

the MM intensity VIII damage from the 1965 earthquake.

The GRFs recorded at HIA are not significantly higher

than those at the other West and South Seattle sites that

experienced MM intensity V and VI shaking. Compared

to HAR, all of the other sites had relatively low GRF values, and their P-wave seismic-velocity structures were similar. Furthermore, no sites in Olympia experienced MM

intensity VII or greater without having a GRF of at least

5.7 (King and others, 1990). Thus, the near-surface geophysical data we have collected indicates that ground

response was probably not a factor in producing higher

intensities near site HIA in West Seattle compared with

those sites in the surrounding neighborhoods.

Tests of residential structures in West Seattle show

that the natural frequencies at which their walls and

chimneys are most sensitive to damage range from 5 to 15

Hz (table 28). The frequency range of the building

resonance only marginally overlaps with our earthquake

data frequency band. However, the GRF for site HIA

suggests that the houses are subjected to ground-motion

amplification factors of about 2.5 near their resonance

frequencies (table 27). Again, our data from Olympia

indicate that this GRF would not be high enough to explain

the MM intensity VIII reported for this neighborhood.

A possible explanation for the high MM intensity

became apparent as we talked with homeowners during the

building-vibration phase of our fieldwork. We asked

people who experienced the 1965 earthquake what other

effects they had observed. None of them, even those

whose houses had chimney damage, said that they had

experienced any other damage. One man who had been a

building contractor in the area stated that a poor grade of

mortar containing salty sand had originally been used in

many of the chimneys of West Seattle. Of 15 houses on

which we tried to measure chimney vibration, 5 had

chimneys in such poor condition that we deemed it unsafe

to place a small single-component seismometer on the top.

Of the 10 houses we did test, one-half of the chimneys had

significant deterioration of the mortar. Two of the 10

chimneys exhibited loose bricks when we installed the

seismometer. We believe that the original intensity rating

could have been biased because many chimneys were in

very poor condition before the earthquake occurred. The

MM intensity VII standard (Richter, 1958) includes

“damage to masonry D (weak materials, poor mortar)

including cracks. Weak chimneys broken off at roof line.”

The true intensity of shaking at West Seattle might therefore be better characterized by an MM intensity of VII.

10

1

0.1

0 2 4 6 8 10

FREQUENCY, IN HERTZ

AMPLITUDE RATIO

EXPLANATION

Earthquake A

Earthquake C

Figure 174. Average horizontal velocity spectral ratios of two earthquakes recorded at site LIN in West

Seattle relative to standard site SEW in South Seattle. Similarity of the curves shows reproducibility of

the ground-response functions despite different shaking sources.
EARTHQUAKE GROUND-RESPONSE STUDIES IN WEST AND SOUTH SEATTLE, WASHINGTON 353

0 4 5

SECONDS

0 4 5

SECONDS

0 5

SECONDS

0 1 2 3 4 5

SECONDS

0 4 5

SECONDS

1 4 8 12 16 20

FREQUENCY (HZ)

0 5

SECONDS

0 5

SECONDS

0 1 2 3 4 5

SECONDS

BLDG. 01–CHIMNEY

BLDG. 01–1 STORY

BLDG. 02–CHIMNEY BLDG. 03–CHIMNEY BLDG. 04–CHIMNEY

BLDG. 02–1 STORY BLDG. 03–1 STORY BLDG. 04–1 STORY

1 4 8 12 16 20

FREQUENCY (HZ)

1 4 8 12 16 20

FREQUENCY (HZ) FREQUENCY (HZ)

1 4 8 12 16 20

FREQUENCY (HZ)

1 4 8 12 16 20

FREQUENCY (HZ)

1 4 8 12 16 20

FREQUENCY (HZ)

1 4 8 12 16 20 25

FREQUENCY (HZ)

1 4 8 12 16 20 25

7.0

6.2 8.6

10.0

11.4

8.6

5.4 7.0

1 2 3 4

1 2 3 4

1 2 3

1 2 3 4

1 2 3

1 2 3

TIME

HISTORIES RELATIVE VELOCITY SPECTRA

TIME

HISTORIES RELATIVE VELOCITY SPECTRA

TIME

HISTORIES RELATIVE VELOCITY SPECTRA

TIME

HISTORIES RELATIVE VELOCITY SPECTRA

Figure 175. Seismograms and velocity spectra used to derive natural frequencies and damping coefficients of four representative houses in West Seattle. Seismometers were placed

on chimneys and bearing walls. The time history seismograms graph velocity versus time. Results for all buildings tested are shown in table 28.
354 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

REFERENCES CITED

Aki, Keiiti, and Richards, P.G., 1980, Quantitative

seismology—Theory and methods (v. 1): San Francisco,

Calif., W.H. Freeman and Company, p. 161.

Algermissen, S.T., and Harding, S.T., 1965, Preliminary seismological report, in The Puget Sound, Washington, earthquake of

April 29, 1965: Washington, D.C., U.S. Coast and Geodetic

Survey, 26 p.

Borcherdt, R.D., and Gibbs, J.R., 1976, Effects of local geologic

conditions in the San Francisco Bay region on ground motions

and the intensities of the 1906 earthquake: Seismological

Society of America Bulletin, v. 66, p. 467–500.

Carver, D.L., Cunningham, D.R., and King, K.W., 1986,

Calibration and acceptance testing of the DR–200 digital

seismograph: U.S. Geological Survey Open-File Report

86–340, 28 p.

Cranswick, Edward, King, K.W., and Banfill, Robert, 1989, A

program to perform time and frequency domain analysis of

vector timeseries recorded by portable autonomous digital

seismographs: U.S. Geological Survey Open-File Report

89–172, 62 p.

Dowding, C.H., Cummings, R.A., and Kendorski, F.S., 1980,

Seismic characteristics, dynamic behavior, and long term

vibration stability of erosional features: U.S. Department of

the Interior, National Park Service, EI No. 1047, p. 56.

Hake, C.A. von, and Cloud, W.K., 1967, United States earthquakes,

1965: U.S. Department of Commerce, Coast and Geodetic

Survey, 91 p.

Ihnen, S.M., and Hadley, D.M., 1986, Prediction of strong ground

motion in the Puget Sound region—The 1965 Seattle

earthquake: Seismological Society of America Bulletin, v. 76,

no. 4, p. 905–922.

King, K.W., Tarr, A.C., Carver, D.L., Williams, R.A., and Worley,

D.M., 1990, Seismic ground-response studies in Olympia,

Washington, and vicinity: Seismological Society of America

Bulletin, v. 80, no. 5, p. 1057–1078.

Langston, C.A., and Lee, J.J., 1983, Effect of structure geometry on

strong ground motions—The Duwamish River valley, Seattle,

Washington: Seismological Society of America Bulletin, v.

73, no. 6, p. 1851–1865.

Mullineaux, D.R., Bonilla, M.G., and Schlocker, Julius, 1967,

Relation of building damage to geology in Seattle, Wash.,

during the April 1965 earthquake, in Geological Survey

research 1967: U.S. Geological Survey Professional Paper

575–D, p. D183–D191.

Richter, C.F., 1958, Elementary seismology: San Francisco, Calif.,

W.H. Freeman and Company, p. 137.

Rogers, A.M., Tinsley, J.C., and Borcherdt, R.D., 1985, Predicting

relative ground response, in Ziony, J.I., ed., Evaluating earthquake hazards in the Los Angeles region—An earth-science

perspective: U.S. Geological Survey Professional Paper 1360,

p. 221–248.

Rogers, A.M., Tinsley, J.C., Hays, W.W., and King, K.W., 1979,

Evaluation of the relation between near-surface geological

units and ground response in the vicinity of Long Beach,

California: Seismological Society of America Bulletin, v. 61,

p. 1603–1622.

Sheriff, R.E., and Geldart, L.P., 1982, History, theory, and data

acquisition, volume 1 of Exploration seismology: Cambridge,

England, Cambridge University Press, p. 49.

Thorsen, G.W., 1986, The Puget lowland earthquakes of 1949 and

1965: Olympia, Washington Division of Geology and Earth

Resources, Information Circular 81, 113 p.

Waldron, H.H., Liesch, B.A., Mullineaux, D.R., and Crandell,

D.R., 1962, Preliminary geologic map of Seattle and vicinity,

Washington: U.S. Geological Survey Miscellaneous Geologic

Investigations Map I–354, scale 1:31,680.
355

EARTHQUAKE-HAZARD GEOLOGIC MAPS OF THE 

PORTLAND, OREGON, METROPOLITAN AREA

By Ian P. Madin1

ABSTRACT

Earthquake-hazard geologic maps have been produced

based on eight 1:24,000-scale quadrangle maps covering

most of the Portland, Oregon, metropolitan area. The map

data are derived from published and unpublished geologic

mapping and interpretation of several thousand borehole

logs. The maps depict the distribution and thickness of four

units of poorly consolidated fine-grained Quaternary sedimentary materials that may amplify ground shaking or liquefy during earthquakes. The four units are Pleistocene

catastrophic flood deposits, fine-grained facies; Holocene

alluvium; Holocene artificial fill; and Pleistocene loess.

The maps also depict other Quaternary and bedrock

geologic units, faults, and contoured depth-to-basement data.

They show that the northwest-trending Portland and Tualatin

basins are bordered by faults on their margins and separated

by the folded and faulted basement rocks of the Tualatin

Mountains (Portland Hills). Northwest-trending anticlines

of the Portland Hills are cut by parallel and transverse highangle faults and by southwest-dipping thrust faults.

Although numerous northwest- and northeast-trending faults

have been mapped in the area, none have yet been shown to

cut Holocene deposits. However, some faults do cut Pleistocene rocks.

INTRODUCTION

In 1987, the Oregon Department of Geology and Mineral Industries (DOGAMI) began a 5-year program to assess

earthquake hazards in the Portland, Oreg., metropolitan area

with funding provided by the U.S. Geological Survey

through the National Earthquake Hazards Reduction Program. In recognition of the importance of local geology in

assessing earthquake hazards, a mapping program with two

major goals has been carried out. The first goal of the

mapping program was to identify faults that may cut young

geologic deposits; the second goal was to map the distribution

and thickness of fine-grained unconsolidated sedimentary

deposits that may amplify ground shaking or liquefy during

an earthquake. The final maps are based on compilation of

existing maps, new surface mapping by the author, M.H. Beeson, and T.L. Tolan, and examination and interpretation of

more than 5,000 borehole logs by the author. The maps (figs.

177–185, beginning on p. 362) are reductions of the U.S.

Geological Survey’s Beaverton, Gladstone, Hillsboro, Lake

Oswego, Linnton, Mt. Tabor, Portland, and Scholls 1:24,000-

scale topographic quadrangles (fig. 176).

The maps depict all major geologic units and identify

Quaternary sedimentary units with high earthquake-hazard

potential. Where there are sufficient data, the thickness of

these sedimentary deposits is depicted with isopach lines.

The maps also depict contoured depth-to-basement data and

faults inferred from subsurface data.

All of the area covered in this study has been previously

mapped at a variety of scales. The earliest small-scale work

was a map by Treasher (1942) at a scale of 1:62,500. This

was followed by a detailed map at the same scale by Trimble

(1963). Several subsequent maps have involved compilation

of existing surface data with water-well data (Mundorff,

1964; Hart and Newcomb, 1965; Hogenson and Foxworthy,

1965). The southwestern part of the area was first mapped in

detail by Schlicker and Deacon (1967), and the Gladstone and

Lake Oswego quadrangles (fig. 176) were compiled at

1:24,000 by Schlicker and Finlayson (1979). Parts of the area

have been included in a 1:100,000-scale compilation by the

Washington Department of Geology and Earth Resources

(Phillips, 1987).

The mapping presented in this report differs from the

previous mapping in varying degrees. On some quadrangles

(Gladstone, fig. 185), there are significant changes in stratigraphy and structure. Other maps (Hillsboro and Scholls, figs.

178 and 182, respectively) differ little from the previous mapping. Significant departures from the previous mapping

occur only where there is good field or subsurface evidence

for the change.

________________ 1Oregon Department of Geology and Mineral Industries, 1831 First

St., Baker City, OR 97814.
356 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

ACKNOWLEDGMENTS

The following organizations contributed immensely to

this project by allowing access to their files of subsurface data:

City of Portland Bureau of Buildings, Oregon Department of

Transportation (Region 1, Office and Bridge Division), U.S.

Geological Survey Water Resources Division, Port of Portland, Dames and Moore, Shannon & Wilson, RittenhouseZeman Associates, Kelly-Strazer Associates, L.R. Squier

Associates, and Geotechnical Resources Inc. Dr. Marvin H.

Beeson and Mr. Terry L. Tolan provided unpublished geologic mapping data for the southwest corner of the Gladstone

quadrangle. Evan Thoms and Tom Popowski assisted in field

work in the Washington County areas. Reviews by Rod

Swanson, Bob Deacon, Marv Beeson, Terry Tolan, Ken Robbins, Steve Personius, John Tinsley, and George Priest were

greatly appreciated. Paul Staub provided valuable cartographic assistance. This work was funded by U.S. Geological

Survey cooperative agreement No. 14–08–0001–A0512 as

part of the National Earthquake Hazards Reduction Program.

123°00' 122°45' 122°30' 122°15'

45°

45'

45°

30'

45°

15'

COLUMBIA

RIVER

WILLAMETTE

RIVER

Sa ndy River

Tualatin River

Lake

Oswego

Clackamas River

Oregon City

Beaverton

Hillsboro

PORTLAND

DIXIE MTN SAUVIE ISLAND VANCOUVER ORCHARDS LACAMAS LAKE LARCH MTN

MT TABOR CAMAS WASHOUGAL

PORTLAND

HILLSBORO LINNTON

SCHOLLS BEAVERTON LAKE OSWEGO GLADSTONE DAMASCUS SANDY

OREGON CITY REDLANDS ESTACADA CANBY NEWBERG SHERWOOD

0 5 10 KILOMETERS

5

5





26

205

405 84

VANCOUVER TUALATIN MOUNTAINS

TUALATIN BASIN

CHEHALEM MOUNTAINS

PORTLAND

BASIN

CASCADE

RANGE

Figure 176. Index map of the Portland, Oreg., metropolitan area showing major cities, highways, and geographic features. The grid

represents the U.S. Geological Survey’s 1:24,000-scale topographic map coverage of the area. The quadrangle names are in the upper lefthand corner of each rectangle.
EARTHQUAKE-HAZARD GEOLOGIC MAPS OF THE PORTLAND, OREGON, METROPOLITAN AREA 357

STRATIGRAPHY AND 

NOMENCLATURE

The stratigraphy of the Neogene sedimentary rocks that

fill the Portland and Tualatin basins is still very poorly

understood, and the nomenclature of these units is an unresolved issue. The nomenclature and stratigraphy used in this

report are more completely discussed by Madin (1990).

Nomenclature for the volcanic rocks of the region follows

Beeson, Tolan, and Anderson (1989) except that the rocks of

the Columbia River Basalt Group are not subdivided in this

report.

DEPOSITS WITH HIGH 

EARTHQUAKE-HAZARD POTENTIAL

Poorly consolidated Quaternary deposits commonly

amplify ground motions or fail due to liquefaction during

major earthquakes. These phenomena enhanced the damage

caused by many recent earthquakes such as those in Mexico

City in 1985 (Seed and others, 1988), Armenia in 1988

(Borcherdt and others, 1989), and San Francisco in 1989

(Plafker and Galloway, 1989). Poorly consolidated deposits

of sand, silt, or clay that overlie more competent materials

are most likely to amplify ground shaking. Deposits of

poorly consolidated saturated sand and silt are most likely to

liquefy during strong ground shaking. Both types of deposits are widely distributed in the Portland area, as originally

recognized by Schlicker and others (1964). The groundshaking amplification at any site will depend on the thickness and seismic-velocity profile of the sedimentary column

beneath the site. Earthquake-induced liquefaction of the

sedimentary materials at any site will depend on the strength

and duration of local ground shaking as well as hydrologic

conditions at the site. The data in this report provide one

essential element for the quantitative modeling of groundshaking amplification or earthquake-induced liquefaction,

but these data alone are not sufficient to make a reliable,

quantitative estimate of potential earthquake hazards at any

site.

Many of the deposits in the Portland area may be susceptible to earthquake-induced landsliding. However, the

mapping of potentially unstable slopes or existing landslides

that may reactivate during an earthquake is beyond the scope

of this report.

The following four Quaternary geologic units in the

Portland metropolitan area may have high amplification or

liquefaction potential:

• Pleistocene loess

• Pleistocene catastrophic flood deposits, fine-grained

facies

• Holocene alluvium

• Holocene artificial fill

Of the four units, only the fine-grained facies of the catastrophic flood deposits and the alluvium are sufficiently

uniform in thickness to be mapped using isopachs. The

remaining geologic units on the maps (Pleistocene gravels

and older sedimentary and volcanic rocks) typically have relatively low potential for amplification of earthquake ground

motion or liquefaction. The nature and distribution of the

units with high earthquake-hazard potential are described

below and in figure 177.

PLEISTOCENE LOESS (Ql)

This unit consists of loessal silt that mantles higher

slopes in the Portland area. The loess is difficult to distinguish from the silts of the Pleistocene catastrophic flood

deposits (Qff), and the lower boundary of the unit is mapped

on the assumption that the loess is either buried by unit Qff

or has been eroded by catastrophic flooding below an altitude of 90 m. Previous workers (Trimble, 1963; Schlicker

and Deacon, 1967; Lentz, 1977) have generally depicted the

loess as thicker on ridgecrests and thinner on valley walls

and floors. Field work for this study indicates that the valley

walls of minor drainages are typically covered with in-place

or colluvial loess and that exposure of underlying bedrock

units is rare except in stream channels. Limited subsurface

data indicate that the loess is widely variable in thickness,

reaching a maximum of 30 m along the crest of the Tualatin

Mountains (Portland Hills). Loess 6–12 m thick is common

along the slopes of the Tualatin Mountains and substantially

thinner deposits are present on the Chehalem Mountains (fig.

182), Mt. Scott (fig. 185), and Cooper and Bull Mountains

(figs. 182 and 183). As a result of the variable distribution

and thickness of the loess, it has been mapped over the

underlying bedrock units only in the areas in which significant (1.5 m or greater) loess can be expected to occur. The

loess is notoriously landslide prone when saturated and represents a significant earthquake-induced landslide hazard.

PLEISTOCENE CATASTROPHIC FLOOD 

DEPOSITS, FINE-GRAINED FACIES (Qff)

This unit consists of crudely to complexly layered,

poorly consolidated, coarse sand to silt deposited by one or

more phases of catastrophic floods from late Pleistocene

Glacial Lake Missoula. The catastrophic flood deposits

occur along the Willamette and Columbia Rivers and

throughout the Tualatin basin. The thickness of the

catastrophic flood deposits is typically 9–18 m, with a maximum thickness of 55 m in the map area. The catastrophic

flood sediments were deposited beneath regionally ponded

floodwaters, the highest of which reached an elevation of

approximately 122 m above sea level, based on the distribution of ice-rafted erratics (Allison, 1935). However, the
358 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

catastrophic flood deposits are typically found no higher than

75–90 m above sea level. Ponding of floodwaters to 122 m

above sea level may not have happened sufficiently often or

for a sufficient length of time to allow significant sediment

deposition at higher elevation. It is difficult to distinguish the

catastrophic flood deposits from loess in most outcrops and

well logs, so the contact between the two units is commonly

drawn following the 90 m contour in the absence of sitespecific data.

Evidence of liquefaction is commonly observed in good

exposures of unit Qff in the form of silt and sand dikes. Some

of the liquefaction dikes cut earlier dikes and bedding planes

and paleosol layers in the catastrophic flood deposits. It is

not clear whether liquefaction occurred during multiple

catastrophic flood events, subsequent earthquakes, or both.

HOLOCENE ALLUVIUM (Qal)

Quaternary alluvium consists of poorly consolidated

sand, silt, clay, and gravel deposited by the Columbia and

Willamette Rivers and their tributaries. In the Willamette

and Columbia Rivers, sand and silt predominate although

organic material and clay are locally abundant, and gravel

deposits are rare with the exception of Ross Island, a major

gravel bar in the Willamette River (figs. 180 and 184), and

scattered gravels at the base of deposits in the Columbia and

Willamette Rivers. The channels of the Columbia and Willamette Rivers have been filled with as much as 55 m of finegrained alluvium. The upper limit of the fine-grained alluvial deposits is apparently restricted to a maximum elevation

of 10 m above modern sea level, an elevation that corresponds to the maximum level of historical floods. Alluvium

deposited by the Clackamas River (fig. 185) is dominantly

volcaniclastic gravel and sand. Alluvium deposited by the

Tualatin River (figs. 182–184) is predominantly sand, silt,

clay, and organic material. In both the Clackamas and Tualatin River drainages, alluvial deposits are largely restricted to

channels incised into the catastrophic flood deposits.

Limited subsurface data suggest that alluvium deposited by

minor tributaries thins rapidly away from the stream channels and is generally less than 1–2 m thick on floodplains.

The alluvium deposited by these tributaries has been omitted

from the maps in this report for the sake of clarity.

HOLOCENE ARTIFICIAL FILL (Qaf)

Artificial fill is widespread in developed areas along the

floodplains of the Columbia and Willamette Rivers and in

gullies in the downtown and East Portland areas. Although

the most common material is sand dredged from the river,

older fills contain significant amounts of construction rubble,

mill ends, and sawdust. Unit Qaf is mapped only where fill

material has eliminated lakes, sloughs, or gullies delineated

during the 1898 survey for the earliest topographic map of

Portland (U.S. Geological Survey, 1905). This unit was

mapped by comparing the 1:24,000-scale U.S. Geological

Survey topographic maps with the 1:62,500-scale U.S. Geological Survey topographic map (U.S. Geological Survey,

1905) and is therefore only depicted on the Portland (fig.

180), Mt. Tabor (fig. 181), and Linnton (fig. 179) quadrangles. Artificial fill 1.5–5 m thick is common in the developed

areas of the Columbia and Willamette River floodplains, but

its thickness and distribution are highly variable and cannot

be accurately depicted at the scale of these maps.

STRUCTURE

A primary goal of this program was to identify potentially active faults in the Portland area. Two general classes

of faults have been mapped in this study. Faults designated

on the maps by a long dash pattern are inferred from the offset of well-defined stratigraphic units, and these faults are

largely confined to the Lake Oswego quadrangle (fig. 184)

and the southwest quadrant of the Gladstone quadrangle (fig.

185) where the stratigraphy of the Columbia River Basalt

Group has been mapped in detail (M.H. Beeson and T.L.

Tolan, unpublished mapping, 1988; Beeson, Tolan, and

Madin, 1989). Faults designated with dotted lines in these

areas represent buried faults mapped by Beeson, Tolan, and

Madin (1989). Faults designated by dotted lines on the other

quadrangles are mapped along relatively abrupt changes in

elevation of a single contact. There is clearly significant erosional relief on many of the major contact surfaces, particularly the top of the Columbia River Basalt Group that is

incised by buried paleochannels up to 152 m deep. The interpretation of any particular change in contact elevation as a

fault rather than buried paleotopography is based on

examination of regional trends, neighboring structures, and

geomorphology.

The sense of vertical offset on faults is generally obvious from the change in the elevation of the contact. The

amounts of fault offset indicated by depth-to-bedrock contours on the maps are estimates based on limited borehole

data. The vertical offset on many faults appears to die out or

change sense along strike.

Horizontal offset cannot be proved or disproved on any

of the mapped faults. Exposed fault planes are rare, but subhorizontal slickensides have been found on fault planes in the

Columbia River Basalt Group in the southeastern part of the

Lake Oswego quadrangle (fig. 184) (M.H. Beeson, oral commun., 1989).

All of the faults mapped clearly cut the Columbia River

Basalt Group and, therefore, have been active since middle to

late Miocene time. Several faults cut the upper Miocene to

Pliocene Sandy River Mudstone and Troutdale Formation.

The youngest faulted rocks in the area are the upper Pliocene

to Pleistocene basalt flows of the Boring Lavas. The exact
EARTHQUAKE-HAZARD GEOLOGIC MAPS OF THE PORTLAND, OREGON, METROPOLITAN AREA 359

age of faulted Boring Lavas flows in the map area is not yet

known. Outside the map area, flows in the Damascus quadrangle (fig. 176) as young as 510±23 ka are faulted (Madin,

1994). Offsets of the Pleistocene loess, the fine-grained

facies of the Pleistocene catastrophic flood deposits, or

Holocene alluvium have not yet been determined.

The Tualatin Mountains are a narrow northwest-trending range that rises about 305 m above the adjacent basins.

Mapping in the Lake Oswego quadrangle (fig. 184) (Beeson,

Tolan, and Madin, 1989) has shown the range to be a folded

and complexly faulted structural high composed of the

Columbia River Basalt Group. Broad anticlinal folds are cut

by numerous high-angle faults both transverse and parallel to

the main northwest structural axis. The structural high is

also cut by several southeast-dipping thrust faults. Schlicker

and others (1964) and Balsillie and Benson (1971) suggested

that the Tualatin Mountains were the result of uplift along a

hypothetical Portland Hills Fault. The mapping of Beeson,

Tolan, and Madin (1989) and Beeson, Tolan, and Madin

(1991) indicates that the Tualatin Mountains may best be

considered a major fault zone.

The large-scale structure of the Portland and Tualatin

basins is fairly well understood, and this report only adds

detail to the models previously proposed by Swanson and

others (1993), Hammond and others (1974), and Hart and

Newcomb (1965).

The Tualatin basin is a broad northwest-trending syncline that is faulted along the eastern margin where it meets

the Tualatin Mountains (figs. 179 and 183). Intrabasin faulting occurs along the north and east edges of the Bull and

Cooper Mountain anticlines (figs. 182 and 183).

The Portland basin is clearly fault bounded along its

west edge from the Clackamas River as far north as downtown Portland (figs. 180, 184, and 185). A fairly abrupt step

of about 100 m occurs in the top of the basement (units Tcr

and Twh) as far north as downtown Portland (fig. 180).

Northwest of downtown Portland, this abrupt step is poorly

defined or absent. However, the unusually steep and straight

front of the Tualatin Mountains and also gravity data by Beeson and others (1975) imply a continuation of a fault in this

area, as suggested by Schlicker and others (1964) and Balsillie and Benson (1971). The east edge of the Portland basin

is outside the area of this study, but it also appears to be fault

bounded with an abrupt step in the top of the basement along

much of its margin (Mundorff, 1964; Davis, 1986; Hartford

and McFarland, 1989; Swanson and others, 1993). Data on

the depth to bedrock are absent in the center of the basin, but

limited gravity data (Perrtu, 1980) and proprietary seismic

data suggest that the top of the basement is fairly flat through

the center of the basin and about 487 m deep. A northwesttrending basement high (Hogenson and Foxworthy, 1965;

Swanson and others, 1993; Madin, 1994) occurs south and

east of the map area (Damascus quadrangle, fig. 176). Gravity data and limited subsurface data suggest that this high

extends to the northwest beneath Mt. Scott, Kelly Butte, Mt.

Tabor, and Rocky Butte. The amount of structural relief on

this buried feature has been estimated at 91 m based on

gravity data of Perrtu (1980). Faulting identified on the Mt.

Tabor quadrangle near Rocky Butte and Mt. Tabor (fig. 181)

is probably associated with this feature. No depth to basement data are available for the Mt. Tabor or Gladstone

quadrangles (figs. 181 and 185, respectively) except along

the westernmost edges of both. Depth to bedrock on the

remainder of these two quadrangles is a matter of conjecture,

and contours have not been drawn.

CONCLUSIONS

The earthquake-hazard geologic maps described in this

report indicate that large portions of the Portland metropolitan area are covered by poorly consolidated Quaternary

sand, silt, and clay deposits. These deposits may significantly amplify ground shaking or liquefy in future earthquakes, and the maps provide the basic geologic data

necessary to estimate the potential for these damage-enhancing effects. Earthquake hazard maps based on these geologic

maps have been published for the Portland (Mabey and others, 1993), Mt. Tabor (Mabey, Madin, Meier, and Palmer,

1995), Gladstone (Mabey, Madin, and Meier, 1995b), Lake

Oswego (Mabey, Madin, and Meier, 1995c), and Beaverton

(Mabey, Madin, and Meier, 1995a) quadrangles.

The earthquake-hazard geologic maps also demonstrate

that there are numerous faults in the Portland metropolitan

area, although none have been shown to be Holocene in age.

The faults depicted by these maps should not necessarily be

considered hazardous without further investigation.

SELECTED REFERENCES

Allen, J.E., Burns, Marjorie, and Sargent, S.C., 1986, Cataclysms

on the Columbia: Portland, Oreg., Timber Press, 211 p.

Allison, I.S., 1935, Glacial erratics in the Willamette Valley: Geological Society of America Bulletin, v. 46, no. 4, p. 615–632.

Baker, V.R., and Nummedal, D., eds., 1978, The channeled

scabland—A guide to the geomorphology of the Columbia

basin: Washington, D.C., National Aeronautics and Space

Administration, 186 p.

Balsillie, J.H., and Benson, G.T., 1971, Evidence for the Portland

Hills fault: Ore Bin, v. 33, no. 6, p. 109–118.

Beeson, M.H., Johnson, A.G., and Moran, M.R., 1975, Portland

environmental geology—Fault identification: Portland, Oreg.,

Portland State University, Geology Department, Earthquake

Hazards Reduction Program Final Technical Report to the U.S.

Geological Survey, Contract 14–08–0001–14832, 107 p.

Beeson, M.H., Tolan, T.L., and Anderson, J.L., 1989, The Columbia River Basalt Group in western Oregon—Geologic structures and other factors that controlled flow emplacement

patterns, in Reidel, S.P., and Hooper, P.R., eds., Volcanism and
360 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

tectonism in the Columbia River flood-basalt province: Geological Society of America Special Paper 239, p. 223–246.

Beeson, M.H., Tolan, T.L., and Madin, I.P., 1989, Geologic map of

the Lake Oswego quadrangle, Clackamas, Multnomah, and

Washington Counties, Oregon: Oregon Department of Geology and Mineral Industries Map GMS–59, scale 1:24,000.

———1991, Geologic map of the Portland quadrangle, Multnomah

and Washington Counties, Oregon: Oregon Department of

Geology and Mineral Industries Map GMS–75, scale 1:24,000.

Berggren, W.A., Kent, D.V., Flynn, J.J., and Couvering, J.A. van,

1985, Cenozoic geochronology: Geological Society of America Bulletin, v. 96, no. 11, p. 1407–1418.

Borcherdt, R.D., Glassmoyer, Gary, Andrews, M.C., and Cranswick, Edward, 1989, Effect of site conditions on ground

motion and damage, in Wyllie, L.A., Jr., and Filson, J.R., eds.,

Armenia earthquake reconnaissance report: Earthquake Spectra, Special Supplement, August 1989, p. 23–42.

Bretz, J.H., Smith, H.T.U., and Neff, G.E., 1956, Channeled

scabland of Washington—New data and interpretations: Geological Society of America Bulletin, v. 67, no. 8, p. 957–1049.

Davis, Stephen, 1986, An analysis of the eastern margin of the Portland basin using gravity surveys: Portland, Oreg., Portland

State University M.S. thesis, 135 p.

Hammond, P.E., Benson, G.T., Cash, D.J., Palmer, L.A., Donovan,

J., and Gannon, B., 1974, A preliminary geological investigation of the ground effects of earthquakes in the Portland metropolitan area, Oregon: Oregon Department of Geology and

Mineral Industries Open-File Report O–74–1, 40 p.

Hart, D.H., and Newcomb, R.C., 1965, Geology and groundwater

of the Tualatin Valley, Oregon: U.S. Geological Survey

Water-Supply Paper 1697, 169 p.

Hartford, S.V., and McFarland, W.D., 1989, Lithology, thickness,

and extent of hydrogeologic units underlying the East Portland

area, Oregon: U.S. Geological Survey Water-Resources Investigations Report 88-4110, 23 p., 6 sheets, scale 1:24,000.

Hogenson, G.M., and Foxworthy, B.L., 1965, Ground water in the

East Portland area, Oregon: U.S. Geological Survey WaterSupply Paper 1793, 78 p.

Hooper, P.R., 1982, The Columbia River basalts: Science, v. 215,

no. 4539, p. 1463–1468.

Lenz, R.T., 1977, The petrology and stratigraphy of the Portland

Hills Silt: Portland, Oreg., Portland State University M.S. thesis, 144 p.

Lowry, W.D., and Baldwin, E.M., 1952, Late Cenozoic geology of

the lower Columbia River Valley, Oregon and Washington:

Geological Society of America Bulletin, v. 63, no. 1, p. 1–24.

Mabey, M.A., Madin, I.P., and Meier, D.B., 1995a, Relative earthquake hazard map of the Beaverton quadrangle, Washington

County, Oregon: Oregon Department of Geology and Mineral

Industries Map GMS–90, scale 1:24,000.

———1995b, Relative earthquake hazard map of the Gladstone

quadrangle, Clackamas and Multnomah Counties, Oregon:

Oregon Department of Geology and Mineral Industries Map

GMS–92, scale 1:24,000.

———1995c, Relative earthquake hazard map of the Lake Oswego

quadrangle, Clackamas, Multnomah, and Washington Counties, Oregon: Oregon Department of Geology and Mineral

Industries Map GMS–91, scale 1:24,000.

Mabey, M.A., Madin, I.P., Meier, D.B., and Palmer, S.P., 1995,

Relative earthquake hazard map of the Mount Tabor

quadrangle, Multnomah County, Oregon, and Clark County,

Washington: Oregon Department of Geology and Mineral

Industries Map GMS–89, scale 1:24,000.

Mabey, M.A., Madin, I.P., Youd, T.L., and Jones, C.F., 1993, Lateral spread displacement, relative ground motion amplification,

and relative dynamic slope instability maps of the Portland

quadrangle, Oregon, in Earthquake hazard maps of the Portland

quadrangle, Multnomah and Washington Counties, Oregon:

Oregon Department of Geology and Mineral Industries Map

GMS–79, scale 1:24,000.

Madin, I.P., 1990. Earthquake-hazard geology maps of the Portland

metropolitan area: Oregon Department of Geology and

Mineral Industries Open-File Report 90–2, 21 p.

———1994, Geologic map of the Damascus quadrangle, Clackamas and Multnomah Counties, Oregon: Oregon Department of

Geology and Mineral Industries Map GMS–60, scale 1:24,000.

Mullineaux, D.R., Wilcox, R.E., Ebaugh, W.R., Fryxell, R., and

Rubin, Meyer, 1978, Age of the last major scabland flood of the

Columbia Plateau in eastern Washington: Quaternary

Research, v. 10, no. 2, p. 171–180.

Mundorff, M.J., 1964, Geology and ground-water conditions of

Clark County, Washington, with a description of a major alluvial aquifer along the Columbia River: U.S. Geological Survey

Water-Supply Paper 1600, 268 p.

Perttu, J.D., 1980, An analysis of the gravity surveys in the Portland

basin, Oregon: Portland, Oreg., Portland State University M.S.

thesis, 123 p.

Phillips, W.M., 1987, Geologic map of the Vancouver quadrangle,

Washington: Washington Division of Geology and Earth

Resources Open-File Report 87–10, 26 p.

Plafker, George, and Galloway, J.P., eds., 1989, Lessons learned

from the Loma Prieta, California, earthquake of October 17,

1989: U.S. Geological Survey Circular 1045, 48 p.

Schlicker, H.G., and Deacon, R.J., 1967, Engineering geology of

the Tualatin Valley region, Oregon: Oregon Department of

Geology and Mineral Industries Bulletin 60, 103 p.

Schlicker, H.G., Deacon, R.J., and Twelker, N.H., 1964, Earthquake geology of the Portland area, Oregon: Ore Bin, v. 26, no.

12, p. 209–230.

Schlicker, H.G., and Finlayson, C.T., 1979, Geology and geologic

hazards of northwest Clackamas County, Oregon: Oregon

Department of Geology and Mineral Industries Bulletin 99, 79

p.

Seed, H.B., Romo, M.P., Sun, J.I., Jaime, Alberto, and Lysmer, J.,

1988, The Mexico City earthquake of September 19,

1985—Relationships between soil conditions and earthquake

ground motions: Earthquake Spectra, v. 4, no. 4, p. 687–729.

Swanson, R.D., 1986, A stratigraphic-geochemical study of the

Troutdale Formation and Sandy River Mudstone in the Portland basin and lower Columbia River Gorge: Portland, Oreg.,

Portland State University M.S. thesis, 103 p.

Swanson, R.D., McFarland, W.D., Gonthier, J.B., and Wilkinson,

J.M., 1993, A description of hydrogeologic units in the Portland basin, Oregon and Washington: U.S. Geological Survey

Water-Resources Investigations Report 90–4196, 56 p.

Swanson, D.A., Wright, T.L., Hooper, P.R., and Bentley, R.D.,

1979, Revisions in stratigraphic nomenclature of the Columbia

River Basalt Group: U.S. Geological Survey Bulletin 1457–G,

59 p.
EARTHQUAKE-HAZARD GEOLOGIC MAPS OF THE PORTLAND, OREGON, METROPOLITAN AREA 361

Treasher, R.C., 1942, Geologic map of the Portland area, Oregon:

Oregon Department of Geology and Mineral Industries Map

GM–9, scale 1:62,500.

Trimble, D.E., 1963, Geology of Portland, Oregon, and adjacent

areas: U.S. Geological Survey Bulletin 1119, 119 p.

U.S. Geological Survey, 1905, Topographic map, Portland quadrangle, Oregon and Washington: U.S. Geological Survey,

scale 1:62,500.

Waitt, R.B., Jr., 1985, Case for periodic, colossal jökulhlaups from

Pleistocene Glacial Lake Missoula: Geological Society of

America Bulletin, v. 96, no. 10, p. 1271–1286.

———1987, Evidence for dozens of stupendous floods from Glacial Lake Missoula in eastern Washington, Idaho, and Montana, in Hill, M.L., ed., Cordilleran section of the Geological

Society of America: Boulder, Colo., Geological Society of

America Centennial Field Guide, v. 1, p. 345–350.
362 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Millions

of years

0.01

1.6

5.3

23.7

36.6

52.0 1Time scale after Berggren and others (1985) CENOZOIC TERTIARY QUATERNARY Holo. Pleistocene Pliocene Miocene late early late middle early Oligocene Eocenelate mid

?

?

? ?

?

?

?

Qal

Qff Qfc Qfch

Qaf

Ql

Qt1

Qt2

QTt

QTb

Tsr

QTm

Tcr

Twh

CORRELATION OF MAP UNITS

Contact—Approximately located

Area overlain by more than 1.5 m of Ql

Subsurface data points

Strike and dip of beds 5

Anticline—Arrowhead on axis indicates direction of local plunge

Fault—Inferred from offset of well-defined stratigraphy; bar and

 ball on downthrown side

Thrust fault—Inferred from offset of well-defined stratigraphy;

 dotted where concealed; sawteeth on upper plate

Fault—Inferred from offset of a single contact (on the Lake

 Oswego quadrangle and southwest corner of the Gladstone

 quadrangle, buried fault inferred from offset of well-defined

 stratigraphy); bar and ball on downthrown side

30 Isopach of Qff or Qal—Measurement in feet

300 Depth to basement contour—Measurement in feet

Borehole bottoms in units Tcr or Twh

Borehole bottoms in units Qal, Qff, or Ql

Borehole bottoms in units Qfc, Qfch, QTt, QTm, QTb, or Tsr

Artificial fill (Holocene)—Sand and silt fill that locally includes rock, 

gravel, construction rubble, sawdust, and mill ends

Qal 

Qaf 

Qff 

Qfc 

Qt1

Qt2 

Ql 

Alluvium (Holocene)—Poorly consolidated river and stream deposits 

of silt, sand, clay, and gravel confined to channels and floodplains 

of the major rivers and tributary streams

Catastrophic flood deposits (Pleistocene)—Boulders, gravels, sandy 

gravels, and sands containing high percentages of clasts derived 

from the Columbia River Basalt Group and representing highenergy, subfluvial deposition during catastrophic floods caused by 

repeated failure of the ice dam that impounded Glacial Lake 

Missoula (Bretz and others, 1956; Baker and Nummedal, 1978; 

Waitt, 1985; Allen and others, 1986). The age of the most recent 

catastrophic flood is estimated as 15,500–13,000 yr B.P. 

(Mullineaux and others, 1978; Waitt, 1987; Beeson, Tolan, and 

Madin, 1989). Within the study area, catastrophic flood sediments 

are subdivided into three facies:

Clackamas River terrace surfaces (Pleistocene)—Erosional terrace 

surfaces cut by the Clackamas River across semiconsolidated 

Pleistocene conglomerates of unit QTt. The surfaces are 

differentiated by their height relative to the modern Clackamas 

River. Unit Qt2 surfaces are higher than unit Qt1 surfaces. Includes 

the Estacada Formation of Trimble (1963)

Loess (Pleistocene)—Poorly to moderately consolidated, massive, 

brown to red-brown or gray, quartzomicaceous silt. The exact age 

is uncertain; Lenz (1977) considered its age as between 700 and 34 

DESCRIPTION OF MAP UNITS

(Condensed and revised from Madin, 1990)

QTt 

QTb 

QTm 

Tsr 

Tcr 

Twh 

Troutdale Formation (Pleistocene? and Pliocene)—Moderately to 

well-lithified conglomerates with minor interbeds of sandstone, 

siltstone, and claystone and volcanic ash and debris flows. The 

conglomerates typically consist of well-rounded pebbles and cobbles 

derived from the Columbia River Basalt Group, high-alumina basalt 

from the High Cascades and Boring Lavas, andesite, dacite, and 

exotic metamorphic and plutonic rocks. The sand and silt 

conglomerate matrix and interbeds contain varying amounts of 

feldspathic, quartzomicaceous, and volcanic lithic and vitric 

sedimentary materials. Includes the Gresham and Walters Hill 

Formations of Trimble (1963) and unnamed conglomerate of 

Beeson, Tolan, and Madin (1989)

Boring Lavas (Pleistocene to Pliocene)—Light-gray to gray, 

diktytaxitic, olivine-(less commonly, plagioclase-)phyric basalt and 

basaltic andesite flows erupted from a series of local vents. 

Swanson (1986) reports K/Ar age dates for Boring Lavas of 1.33 

Ma (fig. 181, Rocky Butte, Mt. Tabor quadrangle) and 2.6 Ma (fig. 

176, Oregon City area). Madin (1994) reports an age for Boring 

Lavas from the Damascus and Gladstone quadrangles (figs. 176 and 

185) of 3,146±62 to 510±8 ka

Mudstone and sandstone (Pleistocene? to middle 

Miocene?)—Moderately to poorly lithified mudstone and sandstone 

that fills the Tualatin basin; lithologically equivalent to the Sandy 

River Mudstone. The unit is poorly exposed, but cuttings from a 

few deep wells consist of blue-gray and brown quartzomicaceous silt 

and very fine sand. Well logs typically describe blue-gray and brown 

or red-brown sand and clay and, rarely, gravel. Includes the 

Troutdale Formation and Sandy River Mudstone of Trimble (1963), 

Helvetia Formation of Schlicker and Deacon (1967), and 

undifferentiated sediments of Beeson, Tolan, and Madin (1989)

Sandy River Mudstone (Pliocene)—Moderately to poorly lithified 

quartzomicaceous mudstone and sandstone in the Portland basin. 

Organic material is common, including branches and logs. Volcanic 

ash layers and pumice sands occur locally. Rocks are commonly 

finely laminated and locally ripple laminated and cross bedded

Columbia River Basalt Group (middle Miocene)—Tholeiitic floodbasalt flows that were erupted from long linear fissure systems in 

northeastern Oregon, eastern Washington, and western Idaho from 

about 17 to 6 Ma (Swanson and others, 1979; Hooper, 1982). 

Members and units belonging to the Wanapum and Grande Ronde 

Basalts, two of the five formations of the Columbia River Basalt 

Group, are present within the mapped area

Basalt of Waverly Heights and associated undifferentiated 

sedimentary rocks (Eocene)—Subaerial basaltic lava flows and 

associated sediments that uncomformably underlie flows of the 

Columbia River Basalt Group. Two flows have yielded K/Ar dates 

of about 40 Ma (Beeson, Tolan, and Anderson, 1989)

ka based on relations with the Boring Lavas and catastrophic flood 

deposits. Includes the upland silt of Schlicker and Deacon (1967) 

and undifferentiated sediments of Beeson, Tolan, and Madin (1989)

Qfch Channel facies—Poorly consolidated, complexly interlayered and

 variable silts, sands, and gravels deposited in major floodways by

 catastrophic flood events.

Fine-grained facies—Coarse sand to silt deposited by catastrophic

floods. Finer sedimentary materials are predominantly quartz

and feldspar and also contain white mica. Coarser sedimentary

materials are mostly fragments derived from the Columbia River

Basalt Group 

Coarse-grained facies—Poorly consolidated pebble to boulder gravel

with silt and coarse sand matrix. Coarse sedimentary materials

are poorly sorted and moderately to well rounded. Coarse deposits

range from openwork gravel to gravel with considerable finegrained matrix

Figure 177. Explanation for earthquake-hazard geologic quadrangle maps in figures 178–185.
EARTHQUAKE-HAZARD GEOLOGIC MAPS OF THE PORTLAND, OREGON, METROPOLITAN AREA 363

Figure 178. Geologic map of the U.S. Geological Survey’s 1:24,000-series topographic Hillsboro quadrangle. From

Madin (1990).

Hillsboro

North Plains

Hillsboro

Airport

Qff

Qff

Qff

Qff

Qff

Qal

Qff

Tcr

30

30

30

150

60

150

60

150

60

90

90

90

90

90

120

120

1350

1050

1200

1350

1200

1050

900

300

450

600

750

900

450

300

750

600

600

750

450

300

900

123°00'00" 122°52'30"

45°

37'

30"

45°

30'

00"

26

26

219 8

Mountains

Tualatin

Tcr

Tcr

Qtm

Qtm

Tcr

0 1 2 KILOMETERS

8
364 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Cedar Mill

Tualatin

Mountains

Qff

Qff

QTb

QTm

QTm

600

750

900

1200

1050

60

90

QTb

Tcr

30

300

450

30

600

450

Qff

Tcr

Tcr

Tcr

QTb

Qff

QTm

30

150

300

300

60

450

300

Qff

Qff

Qal Qaf

Qal

Qaf

Qaf

Qal

Qal

Qal

30

60

90

300

120

150

60

30

Qaf

120

Tcr

Tcr

90

30 60

60 90

150

90

122°52'30" 122°45'00"

45°

37'

30"

45°

30'

00"

26

0 1 2 KILOMETERS

WILLAMETTE RIVER

217

Figure 179. Geologic map of the U.S. Geological Survey’s 1:24,000-series topographic Linnton quadrangle. From

Madin (1990).
EARTHQUAKE-HAZARD GEOLOGIC MAPS OF THE PORTLAND, OREGON, METROPOLITAN AREA 365

Tcr

Tcr

Tcr

QTt

QTm

QTm

QTm

QTm

QTm

QTm

QTb

Qfc

Qfc

Qff

Qff

Qal

Qfc

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qal

Qaf

Qaf

Qaf

Qaf Qaf

Qaf

Qaf

Qal

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qal

Qaf

Qaf

Qaf

Qaf

QTg

QTg

Qal

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qaf

Qff

QTt 

Qls

Qls

Qls

Qls

Qaf

Qal

QTt

Qff

Qff

Qaf

Qfch

Qff

Qal

Qff

Qal

Qls

QTm

COLUMBIA RIVER

Qaf

Qaf

Qff

Portland

30

150

60

90

120

150

120

90

90

60

30

60

90

120

120

30

30

90

60

600

450

300

90

150

60

60

90

1200

450

300

30

90

1050

900

750

600

750

600

30

60

900

30

60

90

60

90

150

60

30

60

60

30

90 120

60 90

120

90

150

60

90

30

30

120

150

30

120

90

30

30

60

120

150

Qal

Qaf

122°45'00" 122°37'30"

45°

37'

30"

45°

30'

00"

26

0 1 2 KILOMETERS

14

Qal

WILLAMETTE

RIVER





84 405

5

Figure 180. Geologic map of the U.S. Geological Survey’s 1:24,000-series topographic Portland quadrangle. From

Madin (1990).
366 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

60

30

180

120

150

180

210

210

30

60

90

240

30

60

30

60

90 120 180 150

120

COLUMBIA RIVER

18

30

90

120

150

180

150

120

90

60

Qal

Qal

Qal

Qal

30

Qal Qaf

Qfc

60

QTb

Qff

Qff

Qaf

Qfc

Qff

Qaf Qaf

30

90

60

Qff

Qfc

Qfc

Qff

Qfc QTt

QTb

QTb

QTt

QTt

QTt

Qfc

122°37'30" 122°30'00"

45°

37'

30"

45°

30'

00"

30

0 1 2 KILOMETERS

GOVERNMENT

ISLAND

ROCKY

BUTTE

MT.

TABOR

205

84



14

205

PORTLAND

INTERNATIONAL

AIRPORT

Figure 181. Geologic map of the U.S. Geological Survey’s 1:24,000-series topographic Mt. Tabor quadrangle. From

Madin (1990).
EARTHQUAKE-HAZARD GEOLOGIC MAPS OF THE PORTLAND, OREGON, METROPOLITAN AREA 367

Scholls

5

6

Qff

Qff

Qff

Tcr

Tcr Tcr

Tcr

Tcr

30

30

30

Qff

60

60

60

60

30

90

90

150

150

300

300

450

60

30

90

90

90

150

150

300

300

60

60

300

450

600

750

900

1050

1200

1350

90

90

30

1050

1200

900

150

750

450 600

Qal

Qal

Qal

Qff

Qff

Qff

Qal

Qal

123°00'00" 122°52'30"

45°

30'

00"

45°

22'

30" 0 1 2 KILOMETERS

TUALATIN

RIVER

TUALATIN

RIVER

COOPER

MOUNTAIN

Tcr

MOUNTAINS

CHEHALEM

Tcr

Tcr Tcr

Tcr

219

8

Figure 182. Geologic map of the U.S. Geological Survey’s 1:24,000-series topographic Scholls quadrangle. From

Madin (1990).
368 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Qff

Qff

Qff

Qff

Qff

Qff

Tigard

Beaverton

Tualatin

Qff

15

6

4

Qff

Qal

Qal

Qal

Qal

Tcr

Tcr

Tcr

Qfc

QTb

QTb

Qff

Qtm

QTb

30

300

450

60

1200

1050

60

600

750

900

30

30

600

600

450

60

90

150

300

30

150

450

300

150

30

150

300

60

30

30

30

300

150

60

150 300

450

450

300

450

450

600

60

90

300

60

30

300

450

90

90

30

60

300

300

60

90

150

30

450

150

150

122°52'30" 122°45'00"

45°

30'

00"

45°

22'

30" 0 1 2 KILOMETERS

99

TUALATIN RIVER

60

BULL

MOUNTAIN

COOPER

MOUNTAIN

Qtm

Tcr

Tcr

Tcr

Tcr

QTb QTb

Qtm



8

5

10

217

Figure 183. Geologic map of the U.S. Geological Survey’s 1:24,000-series topographic Beaverton quadrangle. From

Madin (1990).
EARTHQUAKE-HAZARD GEOLOGIC MAPS OF THE PORTLAND, OREGON, METROPOLITAN AREA 369

Milwaukie

Lake Oswego

MT. SYLVANIA

Qff

Qff

Qff

Qff

Qff

Qff

Qff

Qff

Qff

Qff

Qff

Qff

Qff

Qal

Qal

Qal

Qal

Qal

Qal

QTb

QTb

QTb

QTb

QTm

QTm

QTm

QTm

QTm

QTm

QTm

 QTt

QTt

Qfc

Qfc

Qfc

Qfc

Qfc

Qfc

Qfc Qfc

Qfc

Qfc

Qfch

Qfch

Twh

Twh

Twh

Twh

Twh

Twh

Tcr

Tcr

Tcr Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Tcr

Qff

30

Qff

Qff

QTb

Qal

QTb

150

300

300

150

300

150

450

600

450 

600

30

150

60 150

90

300

300

150

150

30

300

90 60

450

120

300

60

30

150

30

90 150 90

60

30

60

30

600

30

900

450

60

30

30

30

30

300

Qfch

30

450

300

30

Tcr

Tcr

Tcr

Tcr

Qal Tcr

Twh

450

750

122°45'00" 122°37'30"

45°

30'

00"

45°

22'

30" 0 1 2 KILOMETERS

WILLAMETTE

RIV ER

TUALATIN RIVER

5

5

10

217

99W

Figure 184. Geologic map of the U.S. Geological Survey’s 1:24,000-series topographic Lake Oswego quadrangle. From

Madin (1990).
370 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

MT SCOTT

GLADSTONE

CLACKAMAS

1200

1050

900

30

750

30

60

60

1050

1200

30

450

600

750

30

60

30

30

30

900

30

60

30

30

30

30

600

450

150

60

Tcr

Tcr

Tcr

Tcr

Tcr

Qfch

Qfch

Qfc

Qff

Qff

Qff

Qff

Qff

QTb

Qfc

Qfc

Qfc

Qff

QTt

Qff

QTb

QTt

QTb

QTt

30

Qff

Qal

QTt

Qal

Tsr

QTt

Qal

Qt2

QTb

Qal

QTb

Qt1

QTt

QTb

QTb

QTt

QTt

Qff

Qff

QTt

QTt QTt 

QTb

Tsr

QTt

QTb

QTb

Tcr

QTb

QTt

QTt

Qt1

QTt

QTt

QTt

150

300

450

122°37'30" 122°30'00"

45°

30'

00"

45°

22'

30" 0 1 2 KILOMETERS

QTt

CLACKAMAS

RIVER

WILLAMETTE

RIV ER

205

212

224

212

224

Qff

Figure 185. Geologic map of the U.S. Geological Survey’s 1:24,000-series topographic Gladstone quadrangle. From

Madin (1990).
Ground Failure
Preceding page. Insert, rescue of a motorist trapped by fallen bricks moments after the

April 29, 1965, Seattle, Wash., earthquake. Photograph by Ken Harris of the Seattle PostIntelligencer. Background, destruction caused by the fall of an unbraced masonry parapet

in downtown Klamath Falls, Oreg., during the Sept. 20, 1993, M 5.9 and M 6.0 earthquakes. Photograph by Lou Sennick of the Herald and News, Klamath Falls, Oreg. (from

Dewey, J.W., 1993, Damages from the 20 September earthquakes near Klamath Falls,

Oregon: Earthquakes & Volcanoes, v. 24, no. 3, p. 121–128).
373

GROUND FAILURE ASSOCIATED WITH THE

PUGET SOUND REGION EARTHQUAKES OF

APRIL 13, 1949, AND APRIL 29, 1965

By Alan F. Chleborad1 and Robert L. Schuster1

ABSTRACT

Landslides, ground settlement, and surface cracks generated during earthquakes have been a major cause of property damage and casualties in earthquake-prone regions of

the world. The historical record suggests that during future

large earthquakes, such ground failures will most likely

occur at the same or geologically similar locations as those

of previous earthquakes. At many of these locations, the

potential for losses has increased due to rapid population

growth and extensive urban development. Consequently,

detailed information on historical earthquake-induced

ground failures, including location, topography, hydrology,

and geologic materials, is essential for understanding the

nature and extent of the hazard and for responsible land-use

planning.

The Puget Sound region earthquakes of April 13, 1949

(magnitude 7.1), and April 29, 1965 (magnitude 6.5), the

two largest historical Puget Sound earthquakes, produced

ground failures over a large area that includes most of western Washington and part of northwestern Oregon. Because

information on the locations and nature of ground failures

associated with earthquakes prior to the 1949 event is meager, the many reports on the 1949 and 1965 occurrences are

an important source of data for predicting ground-failure

hazards from future earthquakes. Our investigation of

ground failures produced by the 1949 and 1965 earthquakes

consisted of a review of published information, interviews

with residents and local officials having information on the

ground failures, and a field study of selected ground-failure

sites. This collection of new data and the verification and

refinement of reported data have resulted in a substantial

increase in the quantity and quality of ground-failure

information for the region.

————————

1U.S. Geological Survey, Box 25046, MS 966, Federal Center,

Denver, CO 80225.

Many of the 1949 and 1965 ground failures involved

artificial fill underlain by alluvial, deltaic, lacustrine, or glacial deposits. Landslides, including slumps, slides, and flow

failures, were common on bluffs overlooking Puget Sound,

along roads and railroads, and along the banks of rivers and

lakes. Rock falls, rock slides, and rock avalanches occurred

almost exclusively in the Cascade Range and along the western Columbia River Valley. Reports of sand boils and other

evidence that suggests liquefaction of sediments as the probable cause of ground failure are associated principally with

failures in the lowlands involving Holocene fluvial deposits,

deltaic deposits, beach deposits, tidal-flat muds, or artificial

fill. In a few cases, evidence of possible liquefaction was

reported for failures in poorly consolidated Pleistocene glacial deposits.

In this report, we provide topographic maps of the

region showing the locations of all known landslides, ground

cracks, liquefaction features, and miscellaneous effects

related to ground failure from the 1949 and 1965 earthquakes. These maps allow quick visual comparison of the

1949 and 1965 data regarding types of ground failures and

their distribution. In addition, we provide tables of information on selected ground failures keyed by number to locations

on the maps. These tables include information on ground

failure type, geographic location, estimates of location accuracy, and descriptive information from reports and field studies. Also included are numerous photos of ground failures,

some published here for the first time.

INTRODUCTION

Many of the losses from a future large earthquake in the

western Washington–northwestern Oregon region would

likely be caused by ground failure. This prediction is supported by the local historical record and by studies of the

effects of past earthquakes in other earthquake-prone regions

of the world. The great Alaskan earthquake of 1964, for
374 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

example, spawned major landslides and other ground failures

that accounted for 48 deaths (more than one-third of the total

fatalities) and 60 percent of the estimated $300 million in

total damage (Youd, 1978; Keefer, 1984). Ground shaking

during the Alaskan earthquake caused water-saturated sediment beneath roads, railroads, bridges, and port facilities to

liquefy and lose strength; resulting ground failures destroyed

waterfronts at three coastal communities and buckled or

compressed more than 250 bridges (Committee on Earthquake Engineering, 1985). In the same year, a magnitude 7.5

earthquake in Niigata, Japan, produced spectacular groundfailure damage. During that earthquake, saturated sand liquefied beneath a large apartment complex, causing multistory buildings to sink and tilt as much as 60 degrees (Youd,

1978).

Occasionally, earthquakes generate ground failures that

result in catastrophic loss of life. Since the year 1700, at least

250,000 people have been killed by earthquake-induced

landslides in People’s Republic of China (Li, 1990). In a single event in 1920 in Gansu Province, China, massive earthquake-triggered landslides in loess resulted in as many as

100,000 fatalities (Close and McCormick, 1922; Varnes,

1978). Yet another example is the devastating earthquakeinduced rock and ice avalanche that occurred in 1970 on

Nevados Huascaran, the highest mountain in Peru. In that

disaster, the failed mass evolved into a high-velocity debris

avalanche that swept downvalley, burying the town of Yungay and part of Ranrahirca and killing at least 18,000 people

(Plafker and others, 1971).

Fortunately, ground failures generated by historical

earthquakes in the western Washington–northwestern Oregon region have not resulted in catastrophic losses. The

potential for greater losses in future earthquakes, however,

has increased due to increased population and extensive

urban development. About 2.6 million people and $93 billion in property are exposed to the earthquake hazard in the

Seattle-Portland region alone (Hays and others, 1988). In

addition, some studies suggest that an earthquake of magnitude 8 or greater, larger than any historical Washington or

Oregon earthquake, is possible along the Cascadia subduction zone (Heaton and Hartzell, 1986). Such an earthquake

would be expected to cause relatively strong shaking and

numerous damaging ground failures over a large area that

would probably include the heavily populated Puget Sound

and Willamette Valley regions.

The historical record also suggests that ground failures

caused by future large earthquakes will most likely occur at

the same or geologically similar locations as those generated

during previous earthquakes (Youd and Hoose, 1978). This

was recently demonstrated by the magnitude 7.1 Loma Prieta, northern California earthquake of October 17, 1989

(Youd and Hoose, 1978; U.S. Geological Survey Staff, 1990;

Benuska, 1990). In the area of San Francisco Bay, damaging

ground failures occurred in zones of artificial fill located

along the waterfront, just as in the 1906 San Francisco

earthquake. In the Salinas–Santa Cruz area, many cataloged

1906 failures in flood-plain deposits were reactivated by the

1989 earthquake. In addition, some areas in the Santa Cruz

Mountains that developed landslides during the 1906 earthquake were again hard hit by landslides triggered by the 1989

event. Thus, studies to determine the distribution, nature,

and geologic environments of historical earthquake-induced

ground failures are essential to efforts aimed at reducing

earthquake hazards.

PURPOSE AND METHOD OF STUDY

The purpose of this study is to better define the distribution and characteristics of ground failures generated by two

large Pacific Northwest earthquakes—the Puget Sound

earthquakes of April 13, 1949, and April 29, 1965. This

information is intended to further develop an understanding

of the probable location and nature of future earthquakeinduced ground failure, knowledge needed for earthquakehazard reduction and effective land-use planning in the western Washington–northwestern Oregon region. In order to

verify and refine reported data and to expand the existing

fund of ground-failure information, a study was undertaken

consisting of a review of published information (newspaper

and technical journal articles, and governmental agency

accounts), interviews with residents and local officials having information on ground failures related to the 1949 and

1965 earthquakes, and a field study of selected earthquakeinduced ground-failure sites.

Locations of all known ground failures triggered by the

1949 and 1965 events are plotted on topographic base maps

(pls. 4–8), allowing quick visual comparison of regional

ground-failure distributions of the two earthquakes. Descriptions of selected ground failures are provided in tables

29–33. Estimates of the accuracy with which the selected

ground-failure sites can be relocated, given the available

information, are also provided in the tables.

ACKNOWLEDGMENTS

The authors thank Margaret Hopper of the U.S. Geological Survey for providing ground-failure data files compiled

during her study of earthquake effects in the Puget Sound

region, and Gerald Thorsen, Geological Consultant (formerly with the Washington Division of Geology and Earth

Resources), who generously contributed much helpful information, including photographs, of ground failures that

occurred in the vicinity of Olympia, Wash. Steve Palmer

(Washington Division of Geology and Earth Resources) and

Gerald Thorsen reviewed the manuscript and provided valuable suggestions and comments. We gratefully acknowledge

the cooperation and technical assistance of personnel of the

Washington Division of Geology and Earth Resources, the
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 375

Oregon Department of Geology and Mineral Industries, and

the City and County governments in western Washington

and northwestern Oregon that supplied helpful information

on ground-failure locations. Newspaper organizations in the

region were especially helpful in supplying photographs of

ground failures. In addition, we thank the many individuals

who supplied detailed information and photographs essential

to the success of this study.

TYPES OF EARTHQUAKE-INDUCED 

GROUND FAILURE

Ground failure is movement of an earth mass (soil,

rock, or debris) that results in permanent displacement or

disruption of the ground surface. For the purpose of this

report, earthquake-induced ground failure is defined as

ground failure triggered by seismic shaking. Landslides,

ground settlement, and ground cracks are the major types of

earthquake-induced ground failure.

Landslides include rotational slides (slumps); translational slides; rock falls; soil falls; lateral spreads; mud, earth,

and debris flows; and rock, soil, and debris avalanches (landslide nomenclature after Varnes, 1978). Three principal

effects of earthquake wave propagation trigger landslides

(Crozier, 1987): (1) the direct mechanical effect of horizontal acceleration that, at high shaking intensity, may exceed

acceleration due to gravity; (2) cyclic loading (repeated

events of compressive loading related to earthquake shaking) in clays, sands, and silts with weak interparticle bonding; and (3) reduction, by sudden shock, of intergranular

bonding afforded by cohesion and internal friction, irrespective of the degree of saturation.

Ground settlement is defined as vertical displacement

of the ground surface due to consolidation of sediments, subsurface sediment flow, landsliding, or a combination of those

processes.

Ground cracks are fissures or openings in the ground

produced by seismic shaking. Ground cracks are commonly associated with other forms of ground failure such as

landsliding and ground settlement.

Sand boils or sand blows, though not technically a form

of ground failure, are considered because of their relation to

some occurrences of landsliding, settlement, and ground

cracking. Sand boils are discussed in more detail in the following section on liquefaction and related ground failure.

Miscellaneous effects that often are related to earthquake-induced ground failure, but are not in themselves conclusive evidence of ground failure, include broken or

damaged underground utility lines, permanent bridge and

piling displacements, bent or broken well pipe, disruption or

change in water-well or spring flow, and formation of dust

clouds in areas susceptible to rockfalls, rockslides, or rock

avalanches.

Types of earthquake-induced ground failure associated

with earthquakes in other areas but not with the 1949 or 1965

Puget Sound earthquakes include ground cracking directly

related to tectonic faulting, and regional tectonic subsidence

or settlement. Although these forms of ground failure have

not been identified among those produced by the 1949 and

1965 earthquakes and thus are not discussed in this report,

they are possible hazards from future earthquakes.

LIQUEFACTION AND RELATED

GROUND FAILURE

Damaging earthquake-induced ground failure is often

the result of liquefaction within a deposit of loose, saturated

sand or other granular material. Liquefaction is defined as

“the transformation of a granular material from a solid state

into a liquefied state as a consequence of increased porepressures” (Youd, 1973). Ground shaking or cyclic loading

during an earthquake initiates liquefaction in a loose, saturated granular soil by disrupting grain-to-grain contacts;

ensuing soil consolidation and transfer of overlying load

from the grains to the pore water between the grains results

in increased pore pressure and nearly complete loss of shear

strength. The strength loss can result in lateral displacement

of large surficial blocks of soil on gently sloping or nearly

level ground (lateral spreads), flow failures on moderate to

steep slopes (mud, earth, and debris flows), ground settlement, loss of bearing capacity, and sand boils. The following descriptions of liquefaction-related ground failures are

excerpted, in large part, from a report by the Committee on

Earthquake Engineering (1985).

Lateral spreads generally develop on very gentle slopes

(most commonly between 0.3° and 3°) and move toward a

free face, such as an incised stream channel. Lateral displacements range up to a few meters and, in particularly susceptible conditions, to tens of meters, accompanied by

ground cracking and differential vertical displacement

(Youd, 1978). Lateral spreads often disrupt the foundations

of buildings or other structures, rupture pipelines and other

utilities within the failure mass, and compress engineering

structures at the toe of the mass.

Flow failures develop in loose saturated sands or silts

on natural or man-made slopes steeper than 3°. Flows may

consist of completely liquefied soils or of blocks of intact

material riding on layers of liquefied soil. They commonly

displace large masses of material for tens of meters at velocities as great as tens of kilometers per hour.

Consolidation and settlement of saturated sediments is

commonly associated with and enhanced by liquefaction.

Settlement of the ground surface at Portage, Alaska, due to

the 1964 earthquake was so great that houses and highway

and railroad grades were inundated at high tide.

Loss of bearing capacity occurs when the soil supporting a building or other structure liquefies and loses strength.
376 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

This process results in large soil deformations under load,

allowing buildings or other structures to founder and sink

into the ground. The previously cited instance involving

multistory apartment houses that tilted during the 1964 earthquake in Niigata, Japan, is a classic example of bearingcapacity failure due to liquefaction.

Sand boils are formed by water venting to the ground

surface from zones of high pore pressure generated at shallow depth by the consolidation of saturated granular soils

during seismic shaking. The ejected or vented water usually

carries suspended sediment to the surface; this sediment is

deposited in a conical shape around the vent. Sand boils are

not strictly a form of ground failure because alone they do not

cause ground deformation; they are, however, diagnostic evidence that liquefaction has occurred.

GROUND FAILURE CAUSED BY 

HISTORICAL EARTHQUAKES 

IN THE WESTERN 

WASHINGTON–NORTHWESTERN 

OREGON REGION

It is estimated that a magnitude 4 earthquake is the

smallest event likely to cause landsliding (Keefer, 1984).

Magnitude 5 earthquakes are considered the minimum for

soil liquefaction and liquefaction-generated lateral spreads

and flows (Kuribayashi and Tatsuoka, 1975, 1977; Youd,

1977; Keefer, 1984). The western Washington–northwestern Oregon region has experienced many historical earthquakes capable of causing ground failures. Noson and others

(1988) listed the 23 known earthquakes felt in Washington

having magnitudes greater than 4.7. Of those, 16 occurred in

the Puget Sound region between Olympia and the Canadian

border, in the Cascade Range, and along the WashingtonOregon border.

Noson and others (1988) noted 14 earthquakes from

1872 to 1980 that are known to have triggered landslides in

Washington. However, information on the distribution and

nature of landslides and other ground failures related to historical western Washington earthquakes prior to the 1949

Olympia earthquake is meager. For example, credible

accounts of landslides generated by the 1872 North Cascades

earthquake, the largest known seismic event in Washington

or Oregon (magnitude 7.0–7.5), are limited to the few reports

along the Columbia River east of the Cascades, along the

shores of Lake Chelan north of Wenatchee, Wash., and in

southern British Columbia in the vicinity of Fort Shepard and

at Lake Okanagan (Coombs and others, 1977). An assertion,

based on contemporaneous and later accounts, that the

13,000,000 m3 Ribbon Cliffs rock slide along the Columbia

River north of Wenatchee was triggered by the 1872 event

(Coombs and others, 1977) is presently disputed. Kienle and

others (1978) concluded, on the basis of tree and tree-stump

dating, that the slide did not occur during the 1872 earthquake and that any significant movement of the landslide

debris took place more than 215 years ago. In comparison to

the few reports of ground failure due to the 1872 earthquake,

several hundred to many thousands of landslides have been

reported for earthquakes of similar magnitude in other areas

of the world (Keefer, 1984), suggesting that few of the landslides triggered by the 1872 North Cascades earthquake were

reported. Sparse population and difficult travel and communication at the time may account for the apparent lack of

reported ground failures.

Landslides on Mount Rainier were attributed to earthquakes in 1894, 1903, and 1917 (Townley and Allen, 1939);

however, few details are known about the extent and nature

of those ground failures. A well-known and well-documented rock slide/debris avalanche, the world’s largest historical landslide (volume of 2.7 km3), was triggered by a

magnitude 5 earthquake associated with the 1980 eruption of

Mount St. Helens. The landslide swept about 22.5 km down

the valley of the North Fork Toutle River, destroying public

and private buildings, State Highway 504, U.S. Forest Service and logging company roads, and several bridges

(Schuster, 1983).

During the period from 1872 to 1989, the greatest number of recorded earthquake-induced ground failures occurred

as a result of the magnitude 7.1 Olympia earthquake of April

13, 1949, and the magnitude 6.5 Seattle-Tacoma earthquake

of April 29, 1965. Consequently, ground failures associated

with those two events were chosen for detailed study and are

discussed in the following sections.

PREVIOUS STUDIES

Previous investigators (Hopper, 1981; Keefer, 1983)

have identified types of ground failures associated with the

1949 and 1965 Puget Sound earthquakes, described their distribution, and defined geologic environments in the Puget

Sound region that have high susceptibility to earthquakeinduced ground failure. Their studies were based on published and unpublished data including extensive information

derived from written responses to University of Washington

intensity-survey questionnaires by local inhabitants of the

damaged areas. Hopper (1981) noted that liquefaction phenomena were confined primarily to areas that experienced a

Modified Mercalli intensity (MMI) of VIII (with a few

occurrences in intensity VII areas) and that reports of landslides and settlement came from areas with shaking intensities as low as MMI V (isoseismal maps for the 1949 and 1965

earthquakes are shown in fig. 186). Keefer (1983) identified

the following geologic environments in the Puget Sound

region as having high susceptibility to earthquake-induced

ground failure: areas of poorly compacted artificial fill; areas

of Holocene alluvium, lacustrine sediments, and beach sediments; deltas of rivers emptying into Puget Sound; and rock

or soil slopes steeper than 35° in the Puget Sound lowland or

adjacent mountains.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 377

WASHINGTON

WASHINGTON

IDAHO

IDAHO

MONTANA

MONTANA

OREGON

OREGON

CANADA

CANADA

Spokane

Portland

Seattle

Tacoma

OLYMPIA

Spokane

Portland

Seattle

Tacoma

OLYMPIA

PACIFIC OCEAN

PACIFIC OCEAN

LIMIT OF FELT AREA

FELT

NOT FELT

LIMIT OF FELT AREA

I-IV

I-IV

I-IV

I-IV

V

V

V

V

V

VI

VI

VI

VI

VI

VI

VII

VII

VII

VII

VII

VIII

124° 122° 120° 118° 116°

48°

46°

44°

EARTHQUAKE OF APRIL 13, 1949

124° 122° 120° 118° 116°

46°

48°

EARTHQUAKE OF APRIL 29, 1965

0 50 100 150 200 250 KILOMETERS

Figure 186. Modified Mercalli intensity maps for the Puget Sound earthquakes of April 13, 1949

(modified from Ulrich, 1949), and April 29, 1965 (modified from Algermissen and Harding, 1965).

Stars indicate the earthquake epicenters.
378 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

from north of the study area to a hilly area between Olympia

and Centralia, Wash., on the south and from the Olympic

Mountains on the west to the Cascade Range on the east.

These glacial deposits are generally unconsolidated but vary

in degree of compactness. Deposits overridden by the thick

glacial ice are more highly consolidated than other glacial

and postglacial sediments, resulting in varying physical

properties. Compared to bedrock and postglacial deposits,

glacial sediments overridden by glacial ice are intermediate

in both density and water content (Mullineaux and others,

1967). The erosive action of rivers and glaciers has sculpted

the glacial fill, leaving the troughs of Puget Sound, lake

basins, broad valleys, and intervening land areas that stand as

high as a few hundred meters above sea level. In many

places, erosion of the glacial sediments has produced steep

slopes that are susceptible to landsliding.

Numerous areas underlain by deltaic sediments, tidalflat muds, or beach deposits border the many kilometers of

Puget Sound shoreline. Also, local to fairly extensive

deposits of artificial fill are common along transportation

corridors and in residential, business, and industrial areas.

Valley floors in the lowland are underlain chiefly by

postglacial alluvial deposits of unconsolidated sand, silt,

DATA LIMITATIONS

The quality and completeness of available information

on 1949 and 1965 ground failures are highly variable. Youd

and Hoose (1978), in a similar study, cited several reasons

for limited and incomplete data, two of which apply here: (1)

most postearthquake investigations have been designed to

assess the damage to structures; hence, ground failures not

affecting constructed works often have been neglected; and

(2) the areal coverage of postearthquake investigations has

been uneven—areas in and near centers of population and

along major transportation routes have received much more

attention than less developed or remote areas. In addition,

location information is sometimes vague or of questionable

accuracy. In other cases, damage descriptions lack sufficient

information to determine whether damage to structures

resulted from ground failure or solely from some other cause

such as ground shaking. An obvious disadvantage in a study

such as this is the inability to examine reported ground failures immediately after their occurrence. Many years have

passed since the 1949 and 1965 earthquakes, and field evidence of many of the ground failures is obscure or nonexistent. Despite these limitations, much reliable data on the

location and characteristics of ground failures was obtained,

which can be used to help delineate areas and geologic environments susceptible to earthquake-induced ground failure.

CHARACTERISTICS OF THE

STUDY AREA

Ground failures associated with the 1949 earthquake

were scattered over an area of about 28,500 km2 and those of

the 1965 earthquake over an area of about 20,700 km2. The

total area considered in this study encompasses these two

areas from near the Canadian border on the north to just

beyond Portland, Oreg., on the south and from the Cascade

Range on the east to the Pacific Ocean on the west (fig. 187).

The physiography and geology of the total area affected

by 1949 and 1965 ground failures are highly varied. A broad

lowland, bordered on the east by the Cascade Range and on

the west by the Coast Range, is underlain predominantly by

Pleistocene glacial sediments associated with advances of

the Puget Lobe of the Cordilleran ice sheet. These sediments

include glacial till, outwash sand and gravel, and glacial lake

deposits of sand, silt, and clay. Nonglacial sediments interbedded with the glacial deposits provide a record of repeated

advances of the Puget Lobe into the Puget Sound region

(Mullineaux, 1970). In some areas of the lowland, the thickness of unconsolidated sediments exceeds 900 m (Hall and

Othberg, 1974). The Pleistocene sediments cover Tertiary

sedimentary and volcanic bedrock in all but a few areas

where the bedrock is covered by younger sediments or is

exposed at the surface. Continental glacial sediments extend

125° 124° 123° 122° 121° 120° 49°

48°

47°

46°

45°

0 40 80 120 160 KILOMETERS 

PACIFIC OCEAN

Columbia River

OLYMPIA

Portland

Tacoma

Seattle

WASHINGTON

OREGON

CANADA

CAS

CA

DE RA

N

GE

1965 epicenter

1949 epicenter

Puget

Sound

Figure 187. Areas of reported ground failures related to the Puget

Sound earthquakes of April 13, 1949 (solid line), and April 29, 1965

(dashed line). Modified from Keefer (1983).
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 379

clay, and gravel up to a few hundred meters thick. Prehistoric mud/debris flows that originated on the flanks of Mount

Rainier deposited sediment beyond the mountain front onto

nearby valley floors and other lowland areas. One of these,

the Osceola “Mudflow,” spread material over an area of

about 170 km2 to depths as great as 23 m (Crandell, 1963).

The rugged Cascade Range, in the eastern part of the

study area, rises from about 200 m above sea level on its

western border with lowland areas to as high as 4,389 m at

the summit of Mount Rainier. In many areas, the terrain of

the Cascade Range is characterized by steep slopes and sheer

cliffs that are prone to various types of landsliding.

In northern Washington, the Cascade Range is composed of a variety of rock types including metamorphic

rocks of Paleozoic age, Paleozoic and Mesozoic sedimentary

and volcanic rocks of marine origin, and volcanic and plutonic rocks of various ages including those of the Mount

Baker and Glacier Peak volcanoes. The northern Cascades

were extensively glaciated during the Pleistocene; as a

result, glacial till is ubiquitous, and outwash and glacial-margin deposits are common in the river valleys and along the

mountain front (Fiksdal and Brunengo, 1981).

Most of the Cascades south of the Skykomish River,

within the study area, are composed of Tertiary and Quaternary volcanic rocks. Smaller areas are underlain by intrusive

rocks such as granodiorite, rhyolite, and granite, most of

which are interspersed among widespread volcanic deposits

(Walsh and others, 1987). In a few scattered areas, metamorphic rocks and marine and continental sedimentary rocks of

various ages are present.

Towering, snowclad volcanoes such as Mount Rainier

dominate the landscape with their steep rock faces and precipitous cliffs formed by the action of glaciers and other erosive agents. Jointed volcanic strata on the sides of the

volcanoes typically dip downslope, which adds to other factors that can cause instability.

Alpine glaciers, fed by extensive ice fields, moved

down major valleys in the Cascades at various times during

the Pleistocene. Many valleys were glaciated in the central

part of the Washington Cascades, including those of the

Lewis, Kalama, Toutle, Cowlitz, Nisqually, Puyallup,

White, Green, and Cedar Rivers. Alpine glacial drift, including till, and glaciofluvial and glaciolacustrine sediment are

widespread in several of the mountain valleys occupied by

those rivers. In some places on the west side of the range,

the drift extends onto the adjoining lowland (Crandell, 1965;

Fiksdal and Brunengo, 1981; Walsh and others, 1987). Artificial fill is present locally in developed areas, along transportation routes, and where it has been used in the

construction of earthfill dams or other engineered structures.

Other lowlands within the study area are situated south

or southwest of the Puget Sound lowlands in southwestern

Washington and northwestern Oregon. These include the

valleys of the Chehalis, Cowlitz, Columbia, and Willamette

Rivers. Valley floors in these lowland areas are underlain

chiefly by unconsolidated Quaternary alluvium composed of

silt, sand, gravel, and clay (Trimble, 1963; Walsh and others,

1987). Local areas of artificial fill are common in some

towns and urban areas, along railroads and highways, and at

the sites of engineered structures such as earthfill dams.

Hills and mountains rise from valley bottoms and

coastal areas in southwestern Washington and northwestern

Oregon to form the upland areas of the remaining part of the

study area. Most of the upland areas range in elevation from

a few hundred to more than a thousand meters above sea

level. The Coast Range of southwesternmost Washington

and northwesternmost Oregon is composed predominantly

of marine and nonmarine sedimentary rocks, various types

of volcanic deposits, and basic intrusive rocks, all of Tertiary

age (Warren and others, 1945; Walsh and others, 1987).

Unlike many glaciated areas of the Cascades, the Coast

Range of southwestern Washington and northwestern Oregon typically has a very thick weathering profile related to

millions of years of exposure.

Upland areas between the Coast Range and the Cascades in southwestern Washington and northwestern Oregon

are also underlain by a variety of rock types including volcanic rocks, continental sedimentary rocks, alpine glacial outwash deposits, and periglacial flood deposits. As in other

parts of the study area, artificial fill is present locally in

developed areas and along transportation corridors. Also,

beach deposits that include fine to coarse beach sands and

gravels, dune sands, and estuarine muds and sands are common in the coastal areas of the region.

The study region includes large areas that are particularly susceptible to landsliding due to a number of causes.

The general character, distribution, and causes of landslides

in the various physiographic regions of Washington and Oregon, including the area of this study, have been reviewed by

the authors in a previous report (Schuster and Chleborad,

1989).

PRECIPITATION AND GROUND WATER

Precipitation is an important factor affecting the susceptibility of a given area to earthquake-induced ground failure. This is particularly true for areas underlain by sediment

types susceptible to liquefaction because saturation or near

saturation of sediments is required before liquefaction can

occur. In addition, elevated pore pressures associated with

saturated slope conditions can reduce slope stability, thus

increasing the probability of slope failure during earthquakes.

Western Washington and northwestern Oregon are

areas of relatively high precipitation (fig. 188), with the wet

season extending from September through April. In the area

of study, annual precipitation means range from

approximately 122 cm/yr in the area of the Puget-Willamette

lowland to more than 254 cm/yr in the high, snowy parts of

the Cascade Range. Precipitation records for the Seattle-
380 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Tacoma area for the water years 1948–1949 and 1964–1965

(those associated with the 1949 and 1965 earthquakes) are

shown in figure 189. These water years were slightly below

normal; 8.66 cm below normal in 1948–1949 and 6.60 cm

below normal in 1964–1965. For the purpose of comparison, precipitation records from the Seattle-Tacoma area for

the water year 1971–1972 (an unusually wet year) are also

shown in figure 189. Precipitation during that year was 40.6

cm above normal. High precipitation was a major cause of

numerous damaging landslides that occurred in the Seattle

area during the spring of 1972 (Tubbs, 1974). Because the

1949 and 1965 earthquakes occurred well into the wet seasons, water tables were probably at or near their yearly highs,

increasing the probability of some types of ground failure;

however, had amounts of precipitation been similar to those

in the 1971–1972 water year, it is likely that many more

ground failures would have have been triggered by the two

earthquakes.

24

32

24 32

16 

8

16

24

32

24

24

24

32

16

16

96

96

96

96

96

16

16

16

64

64

64

64

64

64

48

48

48

48

32

32

24

24

80

80

80

80

80

128

128

96

96

80

80

48

32

32

64

96 

96 

96 

24

WASHINGTON

OREGON

0 50 100 150 200 KILOMETERS

Figure 188. Contour map showing mean annual precipitation (1 inch equals 2.54 cm) in Washington

and Oregon for the period 1931–1960 (modified from U.S. Geological Survey, 1970).
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 381

Figure 189. Graphs showing precipitation (1 inch equals 2.54

cm) in the Seattle-Tacoma area for the water years 1948–1949,

1964–1965, and 1971–1972 (U.S. Department of Commerce, 1948,

1949, 1964, 1965, 1971, and 1972).

DISTRIBUTION AND CHARACTER OF 

GROUND FAILURES TRIGGERED BY 

THE PUGET SOUND EARTHQUAKES

Data compiled on individual ground failures are presented in plates 4–8, which show ground-failure locations

(1949, closed symbols; 1965, open symbols), and in tables

29–33, which provide descriptions of selected ground

failures. Entries in the tables include the following: (1) relatively extensive ground failures and ground failures exhibiting relatively large displacements, (2) all known landslides,

(3) all known sand boils and other ground failures that

appear related to liquefaction of sediments, (4) significant

ground failures at locations affected by both the 1949 and

1965 earthquakes, (5) significant ground failures at locations

showing concentrations or trends of ground failure, and (6)

ground failures that have affected engineered structures such

as dams and bridges. Excluded from the tables are the

numerous minor settlements and ground cracks that are not

clearly related to landsliding or liquefaction phenomena.

However, these smaller ground failures are included on the

plates in order to show the overall distribution of ground

effects and to help identify significant trends or concentrations of ground failures that may be present. Copies of published and unpublished information used in the development

of tables 29–33 and plates 4–8 are on file in the offices of the

U.S. Geological Survey in Golden, Colorado.

For the purpose of showing ground-failure distributions

and for ease of discussion, the study area was divided into

four regions. The regions were selected on the basis of the

number or density of ground-failure locations in a given

area, the availability of base maps at appropriate scales, and,

as much as possible, on similarities in topography and geology. The following discussion considers the distribution and

character of ground failures in each of the four regions.

CENTRAL AND SOUTHERN PUGET

LOWLAND REGION

The central and southern Puget lowland region (pls. 4

and 5) includes much of the Puget Sound lowland. About 85

percent of the reported ground failures in the 1965 earthquake and 50 percent in the 1949 earthquake occurred in this

region. Some areas experienced Modified Mercalli intensities as high as MMI VIII during both earthquakes (Ulrich,

1949; Algermissen and Harding, 1965; Hake and Cloud,

1967) (fig. 186). Ground cracks and settlement were the

most numerous and widespread of the reported ground failures. Most were isolated occurrences of limited extent that

caused only minor damage to roads, buildings, foundations,

driveways, utilities, and so forth. Some, however, contributed to the considerable structural damage that occurred in

areas of relatively intense ground shaking. Landslides in the

region were most common on hillsides and coastal bluffs and

along the banks of rivers, lakes, and other bodies of water.

Many caused damage to roads, railroads, buildings, waterfront facilities, utility lines, and recreational facilities. One

large debris avalanche, with an estimated volume of 500,000

m3, occurred along the eastern bluff of the Tacoma Narrows

3 days after the April 13, 1949, earthquake. A few flow failures related to the 1965 event also occurred in the region. A

significant number of ground failures developed in





























7

6

5

4

3

2

1

0

PRECIPITATION, IN INCHES

1948 1949 

1948-1949 WATER YEAR

JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN

7

8

9

10

6

5

4

3

2

1

0

PRECIPITATION, IN INCHES

1964 1965 

1964-1965 WATER YEAR

JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN

7

8

9

6

5

4

3

2

1

0

PRECIPITATION, IN INCHES

1971 1972 

1971-1972 WATER YEAR

JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN

EXPLANATION

Precipitation Normal
382 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

environments thought to be conducive to liquefaction failures, as suggested by the presence of sediment types susceptible to liquefaction, high water tables, and in some cases the

occurrence of sand boils in the immediate vicinity.

NORTHERN HALF OF THE CENTRAL AND SOUTHERN 

PUGET LOWLAND REGION

The northern half of the central and southern Puget

lowland region (pl. 4) includes the parts of King, Snohomish, Kitsap, Island, Jefferson, Mason, and Clallam Counties

bounded by lat 48°00' N. and lat 47°30' N. and by long

122°00' W. and long 123°00' W.; this area includes Seattle.

Plate 4 shows a conspicuous concentration of ground failures related to both the 1949 and 1965 earthquakes in West

Seattle (locs. 7–13) and the adjacent Duwamish

River–Harbor Island–Elliott Bay waterfront area (locs.

14–25, and 28).

In West Seattle, the number of reported ground failures

for the 1965 earthquake far exceeded those of the 1949 event.

One of the most destructive 1949 ground failures was a

slump that occurred on moderately sloping ground in artificial fill overlying the Esperance Sand Member of the Vashon

Drift (loc. 10; fig. 190). The failure reportedly damaged six

homes, including four that were under construction (see

damage description, table 29). Hillside residential property

(loc. 13) in West Seattle was the site of minor ground cracking in 1949 and ground cracking and settlement in 1965. The

proximity of that site to a steep bluff located near the head of

a large mapped landslide (Waldron and others, 1962) suggests an incipient slope failure, apparently initiated by the

1949 quake and reactivated in 1965. Minor sliding also

occurred in 1965 at a few other locations on steep to moderately sloping bluff areas on the east and west sides of West

Seattle (locs. 8, 9, 11, and 12). Two of the 1965 slope failures

were wholly or partly in artificial fill (locs. 8 and 11); the

other three (locs. 9, 12, and 13) apparently involved the Esperance Sand Member and (or) overlying surficial debris. Most

of the bluff areas that border West Seattle have been categorized as unstable with regard to slope stability (Washington

Department of Ecology, 1979a). A common slope-failure

process that occurs on the bluffs in West Seattle and in other

parts of the city, especially during wet periods, involves the

Esperance Sand Member and the underlying Lawton Clay

Member of the Vashon Drift (Tubbs, 1974; Tubbs and

Dunne, 1977). Ground water percolates downward through

the Esperance Sand Member until it reaches relatively

Figure 190. Slumping and settlement in West Seattle (pl. 4, loc. 10) induced by the April 13, 1949, earthquake. The

slumping exposed artificial fill (lower left) that underlies the site. Photograph from the Seattle Post-Intelligencer

Collection, courtesy of the Museum of History and Industry, Seattle, Wash.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 383

impermeable alternating sand and clay layers at the top of the

Lawton Clay Member; the water then changes direction and

moves laterally until it reaches the hillside, where it saturates

surface debris and seeps out onto the ground surface.

Slumping of the Esperance Sand Member occurs near the

Esperance-Lawton contact, usually due to the development

of pore-water pressure in the sand (Tubbs, 1974). Other

slides develop as slide debris accumulates and seepage

undermines the toes of slopes (Tubbs and Dunne, 1977; fig.

191). Yount (1983) suggested that the water-saturated condition along the Esperance-Lawton contact may be a contributing factor to intensified ground shaking during

earthquakes, particularly in the vicinity of Tertiary bedrock

at Alki Point, a short distance from the bluffs (Waldron and

others, 1962).

Ejection of ground water (a form of sand boil) in the

Alki Point beach area (closed and open circles at loc. 7) that

occurred during both the 1949 and 1965 earthquakes is a

probable indication of liquefaction. The area is underlain

chiefly by postglacial beach deposits (Waldron and others,

1962; Mullineaux and others, 1967). Similar deposits in

other areas have been found to have moderate to high susceptibility to liquefaction and liquefaction ground failure

(Youd and Perkins, 1978; Keefer, 1984). Closer to Alki

Point, in the vicinity of Alki Monument (loc. 7, open-square

symbol), about 15 cm of settlement occurred in the 1965

earthquake along an extensive stretch of promenade located

behind a seawall. Many waterline breaks, some of which

may have been caused by unreported or undetected ground

failure, also occurred in the Alki Point area in 1965.

According to Mullineaux and others (1967), waterline

breaks were more numerous at Alki Point than anywhere

else in Seattle.

Other 1965 reports of ground failures in the West

Seattle area consist of numerous instances of minor ground

cracking and settlement that often resulted in damage to

foundations, basement floors, sidewalks, driveways, bulkheads, and so on, or jamming and sticking of windows and

doors. Some of those failures probably were related to consolidation of sediments caused by vibration, although Hopper (1981) found little or no correlation between damage

related to settling reports and reports of chimney damage

caused by vibration. Hopper (1981) also noted that nearly

the entire cluster of 1965 settling reports for West Seattle is

located on the Esperance Sand Member (called the older

sand by Waldron and others, 1962) but concluded that the

relationship may be coincidental because the same geologic

unit appears at other places in the city without any concentrations of reported ground settlement. In addition, a study

of the distribution of chimney damage in the 1965 earthquake with respect to the various Pleistocene deposits in

West Seattle demonstrated that the damage pattern is not

related to recognizable differences in the underlying deposits

(Mullineaux and others, 1967). Some incidents of minor settling and ground cracking may have been caused by incipient

sliding, particularly those on the steep bluff areas that partly

surround West Seattle. The occurrence of ground failures in

West Seattle during both the 1965 and 1949 earthquakes

indicates that the area is highly susceptible to ground failure

caused by intense seismic shaking.

0

20

40

60

ELEVATION, IN METERS

0 10 20 30 METERS

Esperance

Sand Member

of Vashon Drift

Water table

Upper transition zone of

Lawton Clay Member

(clayey phase)

(sandy phase)

Lawton Clay Member of Vashon Drift

Kitsap Formation of former usage

Groundwater seepage undermines

upper slope; sand and silt collapse

onto bench

Bench at top of impermeable

clay; debris collects here and

slides off periodically

Exposed Lawton Clay Member

is softened by weathering and

is eroded

Road

Loose fill and

landslide debris

Landslide debris

and mudflow deposits

Figure 191. Cross section of a West Seattle bluff showing conditions associated with slope failure (from Tubbs and Dunne, 1977).
384 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Figure 192. Maps showing the sequence of development of

the former tidal flats area at the mouth of the Duwamish River,

Seattle, between the years 1893 and 1973. Since 1893, the tidal

flats have been extensively covered by artificial fill to allow waterfront development. The area was the site of many earthquakeinduced ground failures and considerable property damage in

both 1949 and 1965. From U.S. Geological Survey (1894, 1909,

and 1949 (photorevised 1968, 1973)).
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 385

The Duwamish River–Harbor Island–Elliott Bay waterfront area lies just to the east of West Seattle (pl. 4). The

area, much of which is a former tidal flat (fig. 192), was

extensively artificially filled (Waldron and others, 1962) to

provide real estate for industrial and commercial interests

along the waterfront. Structural damage was substantial in

this area in both the 1949 and 1965 earthquakes (Edwards,

1951; Mullineaux and others, 1967). The heaviest building

damage in Seattle related to the 1965 earthquake occurred in

this area, from the lower downtown business district south to

about Spokane Street (Mullineaux and others, 1967). A considerable amount of the 1949 and 1965 damage was related

to ground failure (table 29, locs. 14–25 and 28). Groundfailure displacements ranged from a few centimeters to about

a meter, but most were less than 30 cm. Direct evidence of

liquefaction of sediments is provided by reports of sand boils

produced by the 1949 earthquake on the west side of the

West Waterway (loc. 19), on Harbor Island (loc. 21), and

adjacent to the Elliott Bay waterfront (loc. 25), and by the

1965 earthquake at locations 24 and 25 adjacent to Elliott

Bay. Incipient lateral spreading or slumping, indicated by

lateral as well as vertical displacements, occurred at locations 15–17 north and south of Spokane Street (fig. 193) in

1949 and north of Spokane Street (locs. 18 and 22) in 1965.

There were many reports of settlement in the waterfront area

in both 1949 and 1965 (pl. 4; table 29) that may have resulted

from consolidation of sediments, incipient landsliding, or

both. All this evidence indicates that many locations within

the Duwamish River–Harbor Island–Elliott Bay waterfront

area are highly susceptible to damaging ground failure during intense seismic shaking and that there is a high susceptibility to liquefaction of artificial fill and (or) underlying

postglacial sediments in the former tidal flats area. Similar

types of ground failure causing equal or greater damage can

be expected in the event of future large earthquakes that

cause intense ground shaking in the area.

Other locations in Seattle experiencing ground failure

in both the 1949 and 1965 earthquakes include the

University of Washington practice field area just west of

Union Bay at Lake Washington (pl. 4, loc. 31), the area just

south of Green Lake (loc. 33), and two locations along the

west shore of Lake Washington (locs. 4 and 27). Similar to

ground failures in the Duwamish River–Harbor

Island–Elliott Bay waterfront area, locations 4, 31, and 33

are also in areas of artificial fill underlain by postglacial

lacustrine or alluvial sediments (Waldron and others, 1962;

Figure 193. Ground failure along Spokane Street approach to West Seattle (pl. 4, loc. 16) triggered by the April 13,

1949, earthquake. Pavement cracks and slight slump are in a road embankment composed of artificial fill. Note arcuate

scarp and tilted light pole at right. Photograph from the Seattle-Post Intelligencer Collection, courtesy of the Museum of

History and Industry, Seattle, Wash.
386 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Figure 194. Slump along the shore of Green Lake, north of downtown Seattle. This 1949 earthquakeinduced ground failure occurred near the south end of the lake (pl. 4, loc. 33) in an area that subsequently

experienced ground failure during the April 29, 1965, earthquake. Photograph from the Seattle PostIntelligencer Collection, courtesy of the Museum of History and Industry, Seattle, Wash.

Figure 195. Ground cracks on the southeast side of Green Lake in north Seattle (pl. 4, loc. 33). The cracks

opened during or shortly after the April 13, 1949, earthquake and probably resulted from subsidence and

lateral movement of ground downslope toward the lake (out of sight to the left of view). Photograph from

the Seattle Post-Intelligencer Collection, courtesy of the Museum of History and Industry, Seattle, Wash.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 387

Mullineaux and others, 1967; Hale Lowry, oral commun.,

1988). Ground-failure location 27 is locally underlain by

artificial fill (Steele Lindsay, written commun., 1965).

In the Union Bay area of Lake Washington (locs. 31

and 32), vertical and horizontal displacements as great as 0.3

m were reported in both 1949 and 1965; damage caused by

these ground failures was minor, however. As in the waterfront areas, the occurrence of sand boils and associated

ground cracking during the1965 earthquake is evidence of

the susceptibility of the artificial fill and (or) underlying

alluvial and lacustrine sediments to liquefaction and liquefaction-related ground failure.

Slumping and possible lateral spreading along the south

shore of Green Lake (loc. 33; figs. 194 and 195) in 1949

apparently did little damage. Ground failure in the same area

in 1965 damaged a small building, fractured walks and paving, and broke utility lines (Mullineaux and others, 1967).

Possible liquefaction of sediments at the south end of Green

Lake is suggested by the presence of loose granular sediments, the probability of a high water table in sediments at

or below lake level, and the characteristics of some of the

1949 and 1965 ground failures that suggest incipient lateral

spreading.

A hillside failure in north Seattle, triggered by the 1965

earthquake, evolved into a debris flow that covered a road in

Carkeek Park (loc. 36, fig. 196). The flow originated in a

large ravine north of Piper Creek. The sides of the ravine are

underlain by the Esperance Sand Member and undifferentiated nonglacial sediments (Washington Department of Ecology, 1979a). The Seattle Times (1965c) stated that the slide

“opened a pool of ground water that roared down the creek,”

flooding the road with debris. Water from a small creek that

flows down the ravine may also have mixed with the slide

debris, adding to its mobility. The slide resulted in only

minor damage to the park road.

Another 1965 flow failure occurred in the city of

Edmonds, north of Seattle (loc. 40; fig. 197). The failure,

which took place at the head of a small ravine on moderately

sloping ground, left a cavity tens of meters across and 3–5 m

deep. Slide debris flowed downslope for several hundred

meters. The site of the failure is within an area underlain by

advance outwash deposits of the Vashon Drift, mostly clean

sand and gravel (Minard, 1983). According to local residents, the site is also directly underlain by artificial fill. An

eyewitness to the event reported that the slide uncovered a

stream that mixed with the slide material, transforming the

slide “into a muck the consistency of wet cement.” The earth

flow destroyed an abandoned artesian well and watershed

located on the property.

At Port Orchard, on the Kitsap Peninsula west of Seattle

(loc. 1), ground failure triggered by the 1965 earthquake

badly damaged an asphalt-covered parking lot in a business

Figure 196. Mudflow covering Carkeek Park Road in northwest Seattle (pl. 4, loc. 36). This mudflow

developed as material from an earth slide triggered by the April 29, 1965, earthquake mixed with water and

flowed down a Piper Creek tributary ravine. Photograph used by permission of the Seattle Times Co.
388 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Figure 197. Earth flow in Edmonds, Wash. (pl. 4, loc. 40), induced by the April 29, 1965, earthquake.

The landslide began at the head of a small ravine and flowed downslope about 180 m, destroying an

abandoned city water-well shed in its path. Photograph from the Seattle Post-Intelligencer Collection,

courtesy of the Museum of History and Industry, Seattle, Wash.

Figure 198. Damage to Country Club Road, southwest of Port Orchard, Wash. (pl. 4, loc. 2), due to

slumping induced by the April 29, 1965, earthquake. This slump occurred in granular fill underlain by

glacial deposits of the Vashon Drift. Photograph used by permission of the Bremerton Sun.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 389

area adjacent to Sinclair Inlet. The failure occurred in a

waterfront area of artificial fill (Washington Department of

Ecology, 1979b). Spatial relationships with adjacent geologic units (Washington Department of Ecology, 1979b)

suggest that the artificial fill is underlain by the Esperance

Sand Member. A newspaper photo of part of the damaged

area (Bremerton Sun, 1965a) shows vertical displacements

of the asphalt and underlying fill as great as 0.6 m. A slight

lateral displacement was indicated in the direction of the

inlet.

Another small slide in artificial fill during the1965

earthquake extensively damaged a paved road south of Port

Orchard (loc. 2; fig. 198). Movement was downslope in the

direction of a small ravine adjacent to the road embankment.

Glacial deposits of the Vashon Drift underlie the artificial fill

at this location.

Location 39 (pl. 4) at Suquamish was the site of a 1965

coastal-bluff landslide that badly damaged a house and

uprooted trees. According to a press report, the shoreline

heaved as much as 4.6 m in some places (Hake and Cloud,

1967). The 15-m-high bluff is underlain by the Vashon Drift

(till and outwash sand and gravel) (Washington Department

of Ecology, 1979b). Field studies in 1988 revealed zones of

seepage near midslope in the reported area of sliding. The

ground failure resulted in a 30-m-long cracked and raised

area on the beach, parallel to the shoreline.

A 10 m section of highway fill, 5 km west of Kingston

(loc. 41), slumped about 1 m as a result of the 1965 tremor.

At that location, the highway crosses a low, wet area (a

ravine and small creek). Geologic mapping by Garling and

others (1965) indicates that the failure site is part of an area

underlain by recessional outwash deposits of the Vashon

Drift. Damage caused by the slide closed one lane to traffic

until repairs could be made.

SOUTHERN HALF OF THE CENTRAL AND 

SOUTHERN PUGET LOWLAND REGION

The southern half of the central and southern Puget

lowland region (pl. 5) includes parts of King, Pierce, Kitsap,

Mason, and Thurston Counties bounded by lat 47°30' N. and

lat 47°00' N., and by long 122°00' W. and long 123°00' W.

In this area, reported landslides, ground cracks, and settlement related to the 1949 and 1965 earthquakes were numerous and widespread (pl. 5; table 30).

Sand boils and other ground-failure effects indicating

liquefaction were common on broad valley floors extending

from the former tidal flat area at Tacoma (area of locs.

77–79) southeast to Puyallup (locs. 60–71), and from Sumner (locs. 57–59) to just north of Kent (locs. 106 and 107).

The valley floors are underlain generally by postglacial

alluvial sediments and locally by artificial fill (Mullineaux,

1965a, 1965b; Walsh and others, 1987).

On the former Tacoma tidal flat (area of locs. 77–79),

sand boils, ground cracks, and settlement occurred as a result

of both the 1949 and 1965 earthquakes. Reported displacements were less than 0.3 m, but ground cracks developed that

were as long as 300 m. In one instance in 1949, displacement (indicated by a ground crack) in the direction of the

waterfront suggests incipient lateral spreading or slumping

(loc. 77). Many waterline breaks occurred in the area in

1949, and streets and other paved areas were damaged by

ground cracks and settlement in both 1949 and 1965. The

area of the former tidal flat is underlain mostly by fine sand

and silt that are extensively covered by artificial fill.

Reports of sand boils produced by the 1949 earthquake

were most numerous in and near the city of Puyallup. Much

of our data comes from Shulene (1990), who presented information on the location and nature of sand-boil occurrences in

the Puyallup area. In northwest Puyallup, sand-boil activity

was concentrated in areas north and south of Stewart Avenue

(locs. 63–69), such as the 20-square-block area north of

Stewart Avenue running west from 5th Street NW to 9th

Street NW and from Stewart Avenue north to 8th Avenue

NW (loc. 66). Sand boils reportedly pushed through the concrete basement floor of one home and forced timber foundation through the flooring of another, which caused

considerable damage. Also, liquefied sand came up through

a basement floor and floated a furnace that was apparently

unsecured. In addition, city water mains were broken and

ground cracks opened across some streets and walks. In one

case, vertical displacement associated with a ground crack

that crossed a street was great enough to cause a drop that

was noticeable when driven over by a car. In general, reports

of sand boils in this area did not include detailed descriptions

of the amounts and types of sediment involved; however, at

two of the sites, several small mounds (maximum dimension

less than 0.6 m) of black sand were reported. Damage to the

basements described above, however, suggests considerably

larger volumes of ejected material at those locations.

Another concentration of sand-boil activity occurred in

an area just south of Stewart Avenue and west of 12th Street

NW (loc. 68). Ground cracks several meters long and several centimeters wide developed, and some reported “geysers” (sand boils) deposited many yards of sediment on the

surface and caused flooding (figs. 199–201). At one location, as many as 20 sand boils appeared that left “cone

shaped” piles of light-colored sand as much as 18 cm in

diameter and 20 cm high; at another “sand pile,” diameters

were estimated as 0.6–0.9 m. At yet another location, sand

boils producing a blue-gray sand were reported. The only

sand-boil activity reported for the 1965 earthquake in or near

Puyallup occurred within this same area (loc. 68). The 1965

occurrence took place on the Aylen Junior High School playfield and reportedly produced a “considerable amount of

water.”

Five or six “geysers” produced by the 1949 earthquake

were reported to have deposited mounds as much as 0.3 m
390 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Figure 199. Sand boil in Puyallup, Wash. (pl. 5, loc. 68), produced by the April 13, 1949, earthquake.

The earthquake generated many sand boils in northwest Puyallup. Some, such as this one, deposited considerable volumes of sedinment on the surface; others were associated with damage to homes in the area.

The sand boil vent (collapsed area in the center) is 2–3 m long. Photograph by Richard Six, Puyallup, Wash.

Figure 200. Ground crack and ejected sediment (sand boil) produced by the April 13, 1949, earthquake. This ground failure in

northwest Puyallup, Wash. (pl. 5, loc. 68), may indicate incipient

lateral spreading due to liquefaction of sediments. Shadow of

individual (lower left) shows scale. Photograph by Richard Six,

Puyallup, Wash.

Figure 201. Flooding in northwest Puyallup, Wash. (pl. 5, loc.

68), at the time of the April 13, 1949, earthquake. According to

local residents, the flooding could not be attributed to broken water

mains and may have resulted from the ejection of ground water

from the many sand boils that occurred in the area. Photograph by

Richard Six, Puyallup, Wash.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 391

high at a location along West Main Street (loc. 63) presently occupied by the Puyallup High School Gymnasium.

To the west (loc. 69), a basement was “pushed up” on one

end so that the house teetered, and liquefied sand ejected

into a basement attained a depth of approximately 1.2 m.

Also at this location, 0.3-m-high sand mounds developed

“all over” a cultivated field. In addition, a report of ground

cracks and a “shift” in a road adjacent to the Puyallup River

in Puyallup (loc. 67) suggests slumping or lateral spreading

at that location.

Northwest of Puyallup, along the west side of Clarks

Creek (loc. 71), a 60- to 100-m-long crack developed during

the 1949 earthquake and exhibited as much as a few meters

of horizontal and vertical displacement (fig. 202). Lateral

movement was in the direction of the creek, indicating lateral

spreading or slumping of the alluvial soil. The foundation of

a nearby house on the east side of Clarks Creek was reportedly destroyed, probably as a result of ground failure.

Sand boils occurred at several locations north of the

Puyallup River. Included among these were the reports of a

sand-boil deposit as great as 5 m in diameter (loc. 72), “dozens” of sand boils composed of light-colored sand (loc. 74),

and sand boils 0.3–0.5 m in diameter and 9–12 m apart made

up of black, clean sand (loc. 75).

The 1949 earthquake generated many small sand boils

with clean, black sand at one location just east of Sumner

(loc. 58); detailed descriptions of sediment type were not

reported for two other locations to the southeast (locs. 56 and

57).

Farther north, near the King-Pierce County line (loc.

100), sand boils in 1949 reportedly deposited fine sand at

various points along a long fissure and at various locations in

a cultivated field; in 1965, ejected sand formed many piles

over two fields. In this same area in 1949, a fissure reported

to be about 30 m long, 0.3 m wide, and as much as 2 m deep

developed in a cultivated field. Immediately after the event,

water was observed covering the bottom of the fissure. The

Figure 202. Ground crack along Clarks

Creek, northwest of Puyallup, Wash., associated

with slumping or lateral spreading of alluvial

sediments (pl. 5, loc. 71). This 1949

earthquake-induced ground failure extended

along 60–90 m of the west bank of Clarks

Creek; lateral movement was in the direction of

the creek (out of sight to the left of view).

Photograph from the Richards Photographic

Studio Photo Collection, courtesy of the

Tacoma Public Library, Tacoma, Wash.
392 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

fissure was located near a swale immediately to the west,

which suggests possible lateral movement in that direction.

Drilling of a well in the area revealed a sequence of 3.0–3.5

m of clean sand overlying 30–35 cm of peat, followed by

more clean sand (Richard Madole, U.S. Geological Survey,

oral commun., 1989).

Two sand boils appeared in a cultivated field on the valley floor northwest of Kent near the Green River (loc. 107).

A newspaper photograph of one of the sand boils (Kent

News-Journal, 1965) shows an open vent, approximately 0.3

m in diameter, surrounded by a circular apron of ejected sediment several centimeters deep and a few meters in diameter.

Landslides were generated along the bluffs overlooking

Puget Sound, on hillsides, in road embankments, and along

the banks of rivers, lakes, and other bodies of water. Landslides were triggered on the bluffs overlooking Puget Sound

at Tacoma (locs. 80–83), on the east and west sides of

Vashon Island (locs. 92, 94, and 95), on Fox Island (loc. 84),

and west of the Seattle-Tacoma Airport near Three Tree

Point (loc. 109). The largest reported landslide triggered by

either of the Puget Sound earthquakes occurred on a steep,

90-m-high bluff on the eastern shore of the Tacoma Narrows

3 days after the April 13, 1949, earthquake (loc. 81; fig. 203).

The bluff failed catastrophically, sending about 500,000 m3

of sand, gravel, trees, and other debris plunging into the

waters of Puget Sound. The slide narrowly missed nearby

beach homes and created a 2.5-m-high wave front that did

minor damage to small docks and boats moored nearby. At

the site, thick layers of unconsolidated sand and sand and

gravel overlie a firm clayey silt base. Seepage occurs near

the bottom of the slope at the gradational contact between the

clayey silt base and overlying sand. Geologic mapping indicates that the bluff area is underlain (from top to bottom) by

deposits of the Vashon Drift (glacial outwash), Esperance

Sand Member, and undifferentiated Pleistocene sediments

(Washington Department of Ecology, 1979c). A report of

white sand boiling up through a deep crack a short distance

from the cliff’s edge (Vogel, 1949) suggests that liquefaction

of sediments within the hillside may have weakened the

slope at the time of the 1949 earthquake, 3 days prior to the

failure (Chleborad and Schuster, 1989). Modeling and analysis of the landslide have been performed by Chleborad

(1994). Also in 1949, in a similar geologic setting (pl. 5, loc.

84 on Fox Island), a 15- to 30-m-high bluff failed, dumping

a small house into Puget Sound. The bluff at that location is

composed of the Esperance Sand Member and underlying

Figure 203. Tacoma, Wash., landslide, April 16, 1949 (pl. 5, loc. 81). This landslide was probably

initiated by the 1949 earthquake that occurred 3 days previously. The large mass narrowly missed homes

to the south on Salmon Beach as it plunged into the waters of the Tacoma Narrows. The resulting 2.5 m

tidal wave damaged small boats and nearby dock facilities. Photograph used by permission of the

Associated Press.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 393

undifferentiated Pleistocene sediments (Washington Department of Ecology, 1979c). Two slumps that caused minor

damage along the bluffs in Tacoma (locs. 80 and 82) showed

activity in 1949 and 1965; both are in areas underlain by a

sequence of Pleistocene sediments that includes the Esperance Sand Member.

A landslide triggered by the 1965 earthquake on a

steep bluff on the west side of Vashon Island near Sanford

Point (pl. 5, loc. 92) reportedly involved “tons of sand.”

Pleistocene sand deposits older than the Vashon Drift (till)

underlie the bluffs of that area (Washington Department of

Ecology, 1979a). In 1965, at Klahanie on the east side of

the island (loc. 94), slumping occurred along about 60 m of

hillside underlain by Vashon advance outwash deposits and

the Esperance Sand Member (of the Vashon). The slope

failure resulted in minor damage to a waterfront cottage

and retaining wall. Farther south on the island, near Burton

(loc. 95), a slide that originated on a steep bluff covered the

main highway between Burton and Tahlequah. Vashon till

and the Esperance Sand Member are the mapped geologic

units that underlie that area (Washington Department of

Ecology, 1979a). East of Vashon Island, near Three Tree

Point (loc. 109), an incipient slope failure related to the

1965 seismic event is suggested by the report of a ground

crack 8–10 cm wide and at least 60 m long. The ground

crack was located in the upper part of a steep 100-m-high

bluff overlooking Puget Sound. The crack was oriented

parallel to the strike of the slope. A short distance to the

north, the 1965 earthquake triggered a slope failure that

damaged a retaining wall. Both ground failures are in an

area underlain by a large landslide deposit in Vashon glacial sediments (Waldron and others, 1962; Washington

Department of Ecology, 1979a).

Hillside landslides involving glacial materials also

occurred at locations inland from the bluffs of Puget Sound.

East of Auburn, along the Big Soos Creek drainage (loc.

102), a hillside underlain by deposits of mass wasting,

including relatively large block slides (Mullineaux, 1965a),

showed evidence of earthquake-induced slope movement in

1949 and 1965. Several miles to the north along the Big

Soos Creek drainage (loc. 105), about 30 m of hillside above

a small reservoir slumped slightly during and immediately

after the 1949 earthquake. Ejection of sediment and additional water from a spring on the hillside indicate possible

liquefaction of sediments.

To the south, near Orting (pl. 5, loc. 54), a 1965 ground

crack reported to be 120–150 m long and 15 cm wide and

probably related to incipient sliding opened along the brow

of a hill overlooking Kapowsin Creek. Geologic mapping by

Crandell (1963) indicated that the hillside is underlain by

proglacial lacustrine sand.

The 1949 and 1965 earthquakes generated a number of

slumps in road embankments composed of artificial fill. A

1949 road failure south of Port Orchard (loc. 87) occurred at

the same location as a previous ground failure triggered by

an earlier seismic event, the magnitude 6.2 Puget Sound

earthquake of November 12, 1939. The 1939 ground failure

was described by Coombs and Barksdale (1942) as follows:

Investigation of this crack showed it to be on the surface of a fill approximately 260 yards [240 m] in length and 4 yards [3.7 m] thick in the center.

A bed of quicksand fed continuously by many springs a short distance

away on the uphill side underlies the sand and gravel of the fill. So unstable a foundation might well be expected to give way under the effects of

earthquake motion.

Geologic mapping by Garling and others (1965) indicated that the area is underlain by what they called the Colvos Sand (of the Vashon Drift). The upper part of the Colvos

Sand is probably correlative with the Esperance Sand Member (of the Vashon) in some areas according to the Washington Department of Ecology (1979b).

A slump in a 6-m-high embankment of artificial fill

adjacent to Case Inlet (loc. 86) in 1949 resulted in ground

cracks and as much as 15 cm of vertical displacement along

30 m of highway; the slump was reactivated by the 1965

earthquake. The site is part of an area underlain by Pleistocene sand older than till of the Vashon Drift (Washington

Department of Ecology, 1980a).

Failures involving artificial fill overlying postglacial

alluvial sediments damaged roads in the Duwamish River

valley in the vicinity of Kent (locs. 104 and 108) in 1965.

Small slumps at those locations resulted in displacements of

a few centimeters (loc. 104) to about a meter (loc. 108); damage was minor.

A 1965 failure (loc. 83) in artificial fill undermined the

roadway on the mainland side of the approach to Day Island

Bridge on the east side of the Tacoma Narrows. It was

reported by the press that the slide sent trees and earth cascading into a gully.

As a result of the 1965 tremor, road-embankment failures in artificial fill overlying tidal-flat muds occurred at Gig

Harbor (loc. 89), southwest of Grapeview at McLane Cove

(loc. 85), and in Olympia (loc. 48). At location 85 southwest

of Grapeview, a 9-m-high road embankment crossing a

small arm of McClane Cove reportedly split down the middle and moved out along the cove as “a kind of flow.” At Gig

Harbor (loc. 89), a slump along 6.1 m of roadway caused fill

material to move into a small creek and tidal pool.

Extensive slumping of artificial fill on tidal flat muds

occurred along a 0.8 km stretch of the Deschutes Parkway on

the west side of Capitol Lake in Olympia (pl. 5, loc. 48; fig.

204). Sand boils in the immediate vicinity (fig. 205) suggested that the ground failure was liquefaction induced.

The 1965 earthquake also generated a slump in artificial

fill on a steep slope on the east side of Capitol Lake in Tumwater (loc. 45). The failure badly damaged 45.7 m of Union

Pacific freight-line track (fig. 206) and broke Tumwater’s

main sewer line, causing sewage to spill into Capitol Lake.

The hillside is underlain by outwash sand of the Vashon

Drift (Walsh and others, 1987).
394 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Figure 204. Damage to

Deschutes Parkway, Olympia, Wash., resulting from a

1965 earthquake-induced

ground failure (pl. 5, loc.

48). The parkway was

constructed on granular fill

placed on tidal flat muds

that are now within the

limits of Capitol Lake;

failure was probably due to

liquefaction. Photograph

by Gerald W. Thorsen,

Division of Geology and

Earth Resources, Washington Department of Natural

Resources, Olympia, Wash.

Figure 205. Sand boils near Capitol Lake in Olympia, Wash.,

related to the earthquake of April 29, 1965 (pl. 5, loc. 48). The sand

boils are evidence of soil liquefaction at this ground failure

location. Sidewalk slab in background shows scale. Photograph by

Gerald W. Thorsen, Division of Geology and Earth Resources,

Washington Department of Natural Resources, Olympia, Wash.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 395

Figure 206. Damage to Union Pacific Railroad tracks near Tumwater, Wash. (pl. 5, loc. 45), due to a 1965 earthquakeinduced slope failure in artificial fill. Photograph by Gerald W. Thorsen, Division of Geology and Earth Resources,

Washington Department of Natural Resources, Olympia, Wash.

Figure 207. Damage to Black Lake Road south of Olympia, Wash. (pl. 5, loc. 44), caused by failure of a

steep lake bank during the earthquake of April 13, 1949. Photograph used by permission of Jack Goldsby,

Tumwater, Wash.
396 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Southwest of Olympia (pl. 5, loc. 44; fig. 207), about 30

m of one lane of Black Lake Road was damaged in 1949 by

slumping. The failure occurred on a steep 8-m-high lake

bank underlain by Vashon outwash sand and possibly some

artificial fill.

About 45 m of the sandspit at Cooper’s Point, north of

Olympia (pl. 5, loc. 51, fig. 208), slid into Puget Sound during the 1949 earthquake. An eyewitness reported seeing the

mass of land sinking beneath the waves. Soundings taken

afterward at high tide indicated water depths of 15 m in an

area where formerly the spit was 1.5 m above water at high

tide. The sandspit is a Holocene beach deposit (Washington

Department of Ecology, 1980b) that includes medium to

coarse sand and minor gravel.

In Olympia, extensive settling was reported in 1949 in

the Port of Olympia area (loc. 50) and in a residential area

three to eight blocks east of the State Capitol (loc. 46). The

entire port area, underlain by artificial fill, was reported to

have settled 13 cm. Pipelines were broken and asphalt was

bulged up over pilings. Settlement east of the State Capitol,

which reportedly occurred in an area underlain by a peat soil,

damaged several residences.

CASCADE RANGE, WASHINGTON, REGION

The Cascade Range of Washington (pl. 6) includes the

area bounded by lat 49°00' N. and lat 46°00' N. and by long

121°00' W. and long 122°00' W. The region includes central

and eastern parts of King, Skagit, Snohomish, and Whatcom

Counties; eastern parts of Lewis and Pierce Counties; northcentral and northeastern Skamania County; and far western

parts of Chelan, Kittitas, and Yakima Counties. Nearly all of

the reported 1949 and 1965 ground failures are located on the

western side of the Cascade Range (pl. 6, table 31). Ground

failures produced by the 1949 and 1965 earthquakes in this

mostly mountainous region include (in order of abundance)

rock falls and rock slides on steep cliffs and rock faces,

slumps and other slides in alluvial valleys along roads and

river banks, and minor ground cracks and (or) settlement in

alluvial areas on valley floors and in embankment areas of

dams.

A cluster of ground-failure activity was generated by

the 1949 earthquake along the Cowlitz River Valley near

Randle (locs. 117–122). Blocks of volcanic rocks fell from

steep cliffs east of Randall onto Cline Road (locs. 120 and

121) and onto the old Randle-Packwood Highway (loc. 122).

The blocks were reported to be as large as 6 m on a side. On

the valley floor, southwest of Randle (loc. 117), a road

embankment composed of artificial fill underlain by loose,

liquefiable sand slumped into a nearby field. Road damage

caused by the slump is shown in figure 209. Simultaneously

on the same property (loc. 118), numerous sand boils

erupted. To the east and closer to the Cowlitz River (loc.

119), small slumps were triggered along the bank of an

abandoned river channel.

Figure 208. View of Cooper’s Point, located 11 km north of Olympia, Wash. (pl. 5, loc. 51). About 45

m of the end of the sand spit failed and disappeared into Puget Sound during the April 13, 1949, earthquake.

Photograph was taken in 1988.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 397

Rock slides on Mount Rainier were reported for both

the 1949 and 1965 earthquakes. In 1949, a “major” rock

slide occurred on the slopes above Nisqually Glacier and, in

1965, large amounts of rock and soil reportedly fell or slid in

the vicinity of Kautz Glacier (pl. 6, loc. 126). Closer to the

community of Longmire, the 1949 tremor induced rock falls

and (or) rock slides of undetermined size on the cliffs of

Eagle Peak (loc. 123) and Rampart Ridge (loc. 124). Also,

incipient sliding of flood debris in the Kautz Creek drainage

was reported (loc. 125). Numerous instances of ground failure on and near Mount Rainier, related to the 1949 and 1965

earthquakes and to several other historical earthquakes prior

to the 1949 event (Noson and others, 1988), strongly indicate

a high susceptibility to earthquake-induced landsliding.

Jointing and layering of the volcanic rocks that underlie the

steep slopes of Mount Rainier probably contribute to their

seismic instability.

Mount Si (fig. 210; pl. 6, loc. 130) was the site of earthquake-triggered rock slides in 1945, 1949, and 1965. On

April 29, 1945, an earthquake with a felt magnitude of 5.9

(Noson and others, 1988) triggered landsliding on Mount Si

that was described by Bodle and Murphy (1947), as follows:

“At the Mount Si Ranger Station, near North Bend, the earth

buckled and heaved and tons of rock and earth cascaded

down the 4,000 ft [1,220 m] cliffs of Mount Si.” In 1949, a

large dust cloud caused by a rock slide was observed rising

from the west side of the mountain at about the 600 m level

and, in 1965, a slide on the southwest slope of Mount Si was

described as “extensive.” The 1945, 1949, and 1965 ground

failures demonstrate the high susceptibility of steep rock

slopes on Mount Si to earthquake-induced rock falls, rock

slides, and rock avalanches. Metavolcanic rocks (metamorphosed basic lavas), some of which are foliated, form the

bedrock surface over much of the western part of Mount Si

(Kremer, 1959). The rocks on the western part of the mountain dip very steeply to the west, and much of the rock is fractured. Fractures and bedding planes with unfavorable

orientations commonly contribute to the instability of rock

slopes. As stated by Piteau and Peckover (1978), “The stability of rock slopes depends largely on the presence and

nature of defective planes or discontinuities within the rock

mass.”

Minor ground cracking was reported on Mud Mountain

Dam (loc. 127) in 1949 and 1965 and on Howard Hanson

Dam (loc. 128) in 1965. In both instances, the dams were not

endangered. Small slides in the vicinity of Mud Mountain

Dam were also reported for both the 1949 and 1965 earthquakes. In 1949, sliding occurred in an old slide area downstream from the dam and, in 1965, a slope near the Vista

House was affected by small slides.

Figure 209. Slump in a 1- to 1.2-m-high road embankment in the Cowlitz River Valley, about 1.6 km

southwest of Randle, Wash. (pl. 6, loc. 118). This April 13, 1949, earthquake-induced ground failure

occurred in artificial fill underlain by loose liquefiable sand. The earthquake also produced sand boils in an

adjacent field. Photograph by Alice Peters, Randle, Wash.
398 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

CENTRAL-SOUTHWESTERN

WASHINGTON REGION

The central-southwestern Washington region (pl. 7)

extends from lat 47°30' N. to lat 46°22' N. and from long

122°00' W. to the Pacific Ocean (excluding the area of pl.

4). Included in the region is all of Lewis County west of

long 122°00' W., the area of southwestern Pierce County

south of lat 47°00' N. and west of long 122°00' W., all of

Thurston County south of lat 47°00' N., Mason County

south of lat 47°30' N. and west of long 123°00' W., all of

Grays Harbor County south of lat 47°30' N., and all but the

southwesternmost part of Pacific County. In this region,

nearly all of the reported ground failures were produced by

the 1949 earthquake; most occurred in lowlands, including

the valley bottoms of major rivers and adjacent hillsides,

and in glaciated areas bordering the southern terminus of

Puget Sound. Ground-failure locations in this region are

shown on plate 7; reports describing selected ground failures

are listed in table 32.

Reports of sand boils and related ground cracking produced by the 1949 earthquake came from several locations

in the central part of the region (locs. 141 and 146–149).

As many as 12 sand boils deposited “patches” of sand in a

swampy pasture near Deep Lake, east of Maytown (loc.

141). An eyewitness reported that mud was ejected first,

followed by clean, white sand. The ejected sediment

erupted along a north-south-trending line over a zone 3 m

wide, forming “patches” on the ground as wide as 3 m. The

area is underlain by glacial deposits of the Vashon Drift

(Walsh and others, 1987); a gravel pit at the south end of

the property, a short distance from the sand boils, exposes

poorly sorted sand.

On a low terrace of the Chehalis River, just south of

Centralia (loc. 146), about 20 ground cracks appeared in a

farmer’s field; from some of these (figs. 211 and 212), geysers of water and sediment spouted as much as 45 cm above

the ground. It was reported that the geysers brought up what

appeared to be “clean ocean sand.” It was also reported that

water continued to run slowly from the fissures for about a

week.

On the flood plain of the Chehalis River northwest of

Centralia (loc. 149), sand boils erupted from a ground crack

approximately 2.5 cm wide and 5 m long. In the process,

“several buckets full” of pure white sand were deposited on

the ground. A 1949 sand boil south of Chehalis (loc. 147) is

reported to have “heaved some ground” 0.6 m in the vicinity

of a spring, forming a mound from which “muddy water”

poured. Available information was not sufficient to determine the exact location of the sand boil or the type of sediment involved.

One other sand-boil occurrence in the region is suggested by the report of a fissure on a road near Ceres (loc.

148) that produced a “wet and soft spot in the roadbed where

it was previously dry.” Artificial fill underlies the road at

that location.

Ground settlement related to the 1949 earthquake

caused extensive damage to several buildings in Centralia

(loc. 145), including a church that had to be condemned.

Water mains in the city also were damaged. Foundation

damage and broken water pipes in the Hanaford Valley area

(loc. 142) were probably related to ground failure. Both

Figure 210. Mount Si near North Bend, Wash. (pl. 6, loc. 130). “Rockslides” on Mount Si were reported

for the Puget Sound area earthquakes of April 29, 1945, April 13, 1949, and April 29, 1965. Photograph

was taken in 1988.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 399

areas are underlain by alluvial sediments that are locally

covered by artificial fill.

In the areas adjoining the central Puget Sound lowland,

landslides were triggered on hillsides, along roads and railroads, and on lake and river banks. In the LaGrande-Eatonville-Kapowsin area (locs. 136–139), landslides and ground

cracks were associated with both the 1949 and 1965 earthquakes. Although little information is available regarding

the exact locations and nature of ground failures in the vicinity of LaGrande (pl. 7, loc. 136), it was reported that the 1949

earthquake generated ground cracks on “side hills” in the

area and, in 1965, that “slides went into rivers and onto

roads.”

A landslide triggered by the 1949 earthquake at Ohop

Lake (loc. 138) is particularly interesting because, like the

Tacoma Narrows landslide (pl. 5, loc. 81), it demonstrates

the continuing hazard of some earthquake-induced ground

failures beyond the time of the earthquake. At the time of the

April 13, 1949, tremor, a 200-m-long and 8- to 10-cm-wide

crack appeared on a county road located several feet above

lake level on the northwest side of the lake. One month later,

on May 13, complete failure occurred as the unstable mass

suddenly slumped and (or) flowed about 30.5 m into the

lake. The failure, which involved artificial fill underlain by

alluvial sediments, left a scarp on the roadway about 1.5 m

high (fig. 213). Summer cottages and supports for telephone

and power lines were damaged by the slide.

In 1949, a smaller slide damaged about 30 m of an

approach to a railroad overpass southeast of Eatonville (loc.

137). Artificial fill was also involved in that failure.

The 1949 earthquake induced a slump along about 30 m

of county road adjacent to the Cowlitz River southeast of

Morton (loc. 134). The slide material (glacial sand and

gravel) dammed the river for a short time. Closer to Morton,

the same event triggered a rock fall (loc. 135) from a nearvertical exposure of Tertiary-age sedimentary rocks. It was

reported that a block of bedrock 8 m high fell from the cliff,

frightening homeowners below.

Reported landslides in the vicinity of Centralia and

Chehalis (locs. 143 and 144) were later determined to be

small slumps with displacements of 0.6 m or less. One of the

slides occurred on the side of an old gravel pit north of Centralia (loc. 143) in a glacial outwash deposit of sand and

gravel; the other, located on the north shore of Plummer

Lake (loc. 144), probably involved Quaternary alluvial sediments. Small slumps in 1949 also damaged roads near

Rochester (loc. 150) and Oakville (loc. 151); both involved

artificial fill on poor alluvial foundations. Horizontal and

vertical displacements of 0.6 m or less were noted.

An 85-m-long section of a railroad embankment

slumped into Patterson Lake south of Lacey (loc. 140) during or shortly after the 1949 earthquake. The slide is

reported to have caused a wave surge that rose 25–30 cm

over a nearby dock. About 19,000 m3 of material was

required to repair the damage (U.S. Army Corps of Engineers, 1949). The failure involved artificial fill over glacial

sediments of the Vashon Drift.

Near Shelton (pl. 7, loc. 156), one lane of U.S. Highway

101 was damaged by a minor slump triggered by the 1965

earthquake. The slump, which was in artificial fill underlain

by glacial sediments, dropped a part of the road approximately 0.3 m for a distance of about 46 m. Also, slides generated by the 1965 event were reported along the west side

of Oakland Bay near Shelton (loc. 155). However, available

information was insufficient to determine the exact locations

and characteristics of those ground failures.

Numerous broken water mains and sidewalk cracks in

Hoquiam and Aberdeen (locs. 152 and 153) associated with

the 1949 earthquake may have resulted from minor settlement or incipient landsliding. Much of the Hoquiam-Aberdeen area adjacent to Grays Harbor is underlain by

Quaternary alluvium (Walsh and others, 1987), and artificial

fill is present locally. Ground failure induced by the 1949

Figure 211. Alignment of sand boils in a field 1.6 km southwest

of Centralia, Wash. (pl. 7, loc. 146), is related to linear ground crack

from which the water and sand were emitted during a 1949 liquefaction incident. Photograph by Ted Dorn, Centralia, Wash.
400 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Figure 212. Sand boils on the floodplain of the Chehalis River, about 1.6 km southwest of Centralia,

Wash. (pl. 7, loc 146). Water and sand spouted from several ground cracks at this location at the time of

the April 13, 1949, earthquake. Photograph by Ted Dorn, Centralia, Wash.

Figure 213. Scarp created by a landslide on the west side of Ohop Lake, Pierce County, Wash. (pl. 7, loc.

138). Incipient failure induced by the April 13, 1949, earthquake resulted in the development of a large

ground crack. On May 13, complete failure occurred as the unstable mass suddenly slumped into the lake,

damaging the roadway, utility lines, and cottages near the shore. Photograph used by permission of the

Tacoma News Tribune.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 401

and 1965 earthquakes has produced similar effects in other

areas with similar geologic settings.

WESTERN COLUMBIA RIVER REGION, 

OREGON AND WASHINGTON

Included in the western Columbia River region (pl. 8)

are the following Washington counties: Cowlitz and

Wahkiakum Counties, western and southern parts of Skamania County west of long 121°00' W. and south of lat 46°00'

N., part of western Klickitat County west of long 121°00' W.

and south of lat 46°00' N, and southwestern Pacific County

south of lat 46°23' N. Included in Oregon are Clatsop,

Columbia, Washington, Multnomah, and Hood River Counties; the northwestern part of Wasco County west of long

121°00' W. and north of lat 45°14' N.; the northern parts of

Clackamas and Yamhill Counties north of lat 45°14' N; and

all of Tillamook County except the southwest corner south

of lat 45°14' N.

In this region, the 1949 earthquake spawned numerous

ground failures along and near the Columbia River; these

included rock falls, rock slides, or rock falls/rock avalanches

on steep cliffs and rock faces, and ground cracks, settlement,

and sliding probably related to liquefaction in alluvial areas

on the valley floor.

A landslide (probably a rock fall/rock avalanche) that

originated on a steep head scarp of the 36 km2 Bonneville

landslide west of Stevenson, Wash. (loc. 160), reportedly

created a dust cloud that was seen for miles along the Columbia Gorge. Rock falls on the steep scarps are a common,

ongoing mass-wasting process that is probably easily accelerated by seismic activity. According to Palmer (1977),

“The head scarps are actively raveling, with large blocks

more than 15 ft [4.6 m] in diameter crashing down from time

to time, so that it is unsafe at the base of the cliffs.” The head

scarp of the Bonneville slide (fig. 214) exposes about 370 m

of the Eagle Creek Formation (conglomerate, sandstone, and

minor tuff) overlain by basalt (Korosec, 1987). It was

reported that a “considerable” area of fresh surface on the

rock cliff was exposed by the 1949 landslide and that abandoned buildings, including a small school house, were damaged or destroyed. Post-1949 airphotos reveal an area of

slide debris, possibly related to the 1949 earthquake, as

much as several tens of meters wide and a few hundred

meters long at the base of a steep cliff on the south side of

Table Mountain (loc. 160).

At Blue Lake, east of Portland, Oreg. (loc. 158), a 1949

rock fall from a near-vertical rockface on the south side of

the lake reportedly endangered a boathouse. A 9- to 15-mhigh cliff at that location exposes part of the Troutdale Formation (conglomerate and sandstone) (Trimble, 1963).

Rock falls involving highly jointed volcanic rocks

occurred along the Spokane-Portland-Seattle railway about

1.6 km east of Mayger, Oreg. (loc. 166), and along the Ocean

Beach Highway just west of Stella, Wash. (loc. 168). The

rock fall east of Mayger damaged the railroad, but it was

reported that only a few hours were required to clear the slide

and repair the damage. The rock fall at location 168 originated on the near-vertical face of a 15- to 25-m-high bluff

that extends for about 0.8 km along the highway west of

Stella. Part of the rock fall reportedly crossed the highway

and spilled into the Columbia River. According to an eyewitness, the Washington State Department of Highways had

to “shoot” (use explosives) to remove a block of rock “as

large as a house.”

Sand boils and associated ground cracks indicating liquefaction of sediments occurred at Longview, Wash., in

1949 near the junction of N.W. Nichols Street and Ocean

Beach Highway (pl. 8, loc. 163) and near 40th Avenue (loc.

164). Geysers of water and sand 1–1.2 m high developed in

a yard and, nearby, sand boils and settlement caused considerable structural damage to a local residence. The area is

part of a flat, low-lying terrace or flood plain of the Columbia River underlain by Quaternary alluvium. Artificial fill is

present locally. The 1949 ground failure near 40th Avenue

took the form of a 6-m-long ground crack from which

“black, fine sand bubbled.” The site is part of a sandy flood

plain; Quaternary alluvial sediments underlie the area

(Walsh and others, 1987). Local residents who lived in the

vicinity in 1949 say the ground-water table was almost at the

surface at the time of the earthquake.

Other effects of the 1949 earthquake, possibly related to

liquefaction of Columbia River sediments, include the following: a slide involving artificial fill overlying alluvium

that damaged 100 m of the Longview dike road (loc. 165);

damage to a building at Mayger, Oreg. (loc. 167), related to

shifting of pilings toward the Washington side of the river;

sliding or flowing of sediments on Puget Island in the

Columbia River (loc. 170), indicated by well pipes all bent

in the same direction; and settling, ground cracks, and bent

pipes in a flat-lying, swampy alluvial area at Skamokawa,

Wash. (loc. 171).

OBSERVATIONS AND CONCLUSIONS

In this study, data on ground failures generated by the

April 13, 1949, and April 29, 1965, Puget Sound earthquakes

were obtained by (1) review of published and unpublished

information, (2) interviews with local residents and State and

local officials, and (3) field study of selected ground-failure

sites. These data include new and previously unpublished

ground-failure information, particularly on landslides,

related to the 1949 and 1965 events. The data support conclusions by Hopper (1981) and Keefer (1984) regarding the

general distribution and character of the 1949 and 1965

ground failures and the relative susceptibility of geologic

environments in the Puget Sound region to earthquakeinduced ground failure. The following observations and
402 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

conclusions are based, in large part, on the perspective

gained by the compilation of new and existing information

on ground-failure characteristics and by study and comparison of ground-failure distributions made possible by the plotting of 1949 and 1965 ground failures on regional

topographic maps (pls. 4–8).

Ground failures triggered by the 1949 and 1965 earthquakes were scattered over areas of about 28,500 km2 and

20,700 km2, respectively (fig. 187). Ground failures in areas

affected by both earthquakes were similar, taking the form of

landslides, settlement, and ground cracks. Although most of

the reported ground failures were small settlements and

ground cracks that resulted in only minor damage to roads,

sidewalks, buildings, utility lines, and other structures, many

caused significant damage in areas of intense ground shaking. Many damaging ground failures occurred in environments thought to be conducive to liquefaction failures, as

suggested by the presence of sediment types susceptible to

liquefaction, high water tables, and in some cases the occurrence of sand boils in the immediate vicinity.

Liquefaction of sediments, indicated by sand boils,

was most common on flood plains or low terraces of valley

floors extensively underlain by Quaternary alluvium and

locally by artificial fill and at the mouths of rivers where

deltaic sediments and tidal flat muds are extensively overlain by artificial fill. Many slides, mostly small slumps

with estimated volumes of less than 1,500 m3 (Chleborad

and Schuster, 1989), involved artificial fill underlain by

granular alluvial, deltaic, lacustrine, or glacial deposits.

Most of these occurred in embankments along primary and

secondary roads and railroads and along the banks of rivers,

lakes, and other bodies of water. Also, slumps and other

slides of undetermined type were common on steep bluffs

along Puget Sound. Many of the slides along these bluffs

occurred within a stratigraphic sequence composed of glacial sediments of the Vashon Drift that included its Esperance Sand Member or its lithologic equivalent underlain by

impermeable silts and clays. Such conditions often give

rise to naturally unstable slopes because of high groundwater conditions. The importance of local ground-water

conditions in the development of earthquake-induced flows,

slumps, and other slides is suggested by the proximity of

many of the slope failures to rivers, lakes, streams, or other

bodies of water and by reports of associated springs,

swampy areas, and “opened pools of ground water,” all of

which indicate a probable high water table and saturated

Figure 214. Bonneville landslide area along the Columbia River west of Stevenson, Wash. (pl. 8, loc. 160). Rock fall/

rock avalanche activity triggered by the April 13, 1949, earthquake occurred along the steep head scarp (arrows), creating

a dust cloud visible for many kilometers along the Columbia River Gorge. Photograph by Derek Cornforth, Landslide

Technology, Portland, Oreg.
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 403

ground conditions. Coupled with the presence of loose

granular sediments, such conditions often indicate a high

susceptibility to liquefaction-induced ground failure.

Rock falls, rock slides, and rock avalanches reported

for the 1949 and 1965 earthquakes occurred almost exclusively in the mountainous terrain of the Cascade Range and

along the western Columbia River valley. Typically, these

failures originated on very steep slopes (slopes of 45° or

more) in areas of highly jointed volcanic, metamorphic, or

sedimentary rocks and in areas where such failures occur

naturally even under nonseismic conditions.

Examination of ground-failure distributions in each of

the four regions discussed reveals that a high percentage of

the reported failures occurred in and near the more populated

and heavily traveled areas, which suggests that many ground

failures that occurred in less populated and inaccessible

areas were not reported. This notion is supported, to some

extent, by the paucity of reports of landsliding in several

large areas of landslide-susceptible terrain in western Washington and northwestern Oregon, such as the Cascade and

Coast Ranges (Radbruch-Hall and others, 1982; Schuster

and Chleborad, 1989), where intense ground shaking (MMI

V or greater) might be expected to generate such failures.

Future large earthquakes in the western Washington–northwestern Oregon region can be expected to generate

similar types of ground failures in the same or geologically

similar environments as those that occurred in the 1949 and

1965 earthquakes. The location and extent of losses caused

by future earthquake-induced ground failures will depend on

many factors, including the magnitude and location of the

earthquake, ground-water conditions at the time of the earthquake, and the susceptibility of populated and developed

areas to damaging ground failure during intense seismic

shaking. Recognition of geologic environments susceptible

to earthquake-induced ground failure is an important step in

efforts to reduce the earthquake hazard faced by residents of

the western Washington–northwestern Oregon region.

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404 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

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U.S. Geological Survey, 1894, Seattle [edition of December 1984]:

Washington, D.C., U.S. Government Printing Office, scale

1:62,500.

———1909, Seattle [reprinted 1942]: Washington, D.C., U.S.

Government Printing Office, scale 1:62,500.

———1949, Seattle South [photorevised 1968 and 1973]: Washington, D.C., U.S. Government Printing Office, scale 1:24,000.

———1970, Mean annual precipitation, in The National Atlas of

the United States of America: Washington, D.C., U.S. Geological Survey, p. 97, scale 1:17,000,000.

U.S. Geological Survey Staff, 1990, The Loma Prieta, California,

earthquake—An anticipated event: Science, v. 247, p.

286–293.

Varnes, D.J., 1978, Slope movement types and processes, in

Schuster, R.L., and Krizek, R.J., eds., Landslides—Analysis

and control: Washington, D.C., National Academy of Sciences, Transportation Research Board, Special Report 176, p.

12–33.

Vogel, Elmer, 1949, Cliff topples into Sound—Vast slide laid to

quake—Slide stirs tidal wave: Tacoma, Wash., Tacoma News

Tribune, April 18, p. 1, 2.

Voight, Barry, Glicken, Harry, Janda, R.J., and Douglass, P.M.,

1981, Catastrophic rockslide avalanche of May 18, in Lipman,

P.W., and Mullineaux, D.R., eds., The 1980 eruptions of Mount

St. Helens, Washington: U.S. Geological Survey Professional

Paper 1250, p. 347–377.

Waldron, H.H., Liesch, B.A., Mullineaux, D.R., and Crandell, D.R.,

1962, Preliminary geologic map of Seattle and vicinity,

Washington: U.S. Geological Survey Miscellaneous Geologic

Investigations Map I–354, scale 1:31,680.

Walsh, T.J., Korosec, M.A., Phillips, W.M., Logan, R.L., and

Schasse, H.W., 1987, Geologic map of Washington—Southwest quadrant: Olympia, Washington Division of Geology and

Earth Resources Geologic Map GM–34, 2 sheets, scale

1:250,000, 28 p. text.

Warren, W.C., Norbisrath, Hans, and Grivetti, R.M., 1945, Geology

of northwestern Oregon west of Willamette River and north of

latitude 45°15': U.S. Geological Survey Oil and Gas Investigations Preliminary Map 42, scale 1:145,728.

Washington Department of Ecology, 1978, Coastal zone atlas of

Washington, Jefferson County: Olympia, Washington Department of Ecology, v. 11, scale 1:24,000.

———1979a, Coastal zone atlas of Washington, King County:

Olympia, Washington Department of Ecology, v. 6, scale

1:24,000.

———1979b, Coastal zone atlas of Washington, Kitsap County:

Olympia, Washington Department of Ecology, v. 10, scale

1:24,000.

———1979c, Coastal zone atlas of Washington, Pierce County:

Olympia, Washington Department of Ecology, v. 7, scale

1:24,000.

———1980a, Coastal zone atlas of Washington, Mason County:

Olympia, Washington Department of Ecology, v. 9, scale

1:24,000.

———1980b, Coastal zone atlas of Washington, Thurston County:

Olympia, Washington Department of Ecology, v. 8, scale

1:24,000.

Washington Division of Geology and Earth Resources, 1984, “Sand

boils” or “mud volcanoes” along Capitol Lake [photo caption]:

Olympia, Washington Division of Geology and Earth Resources, Washington Geologic Newsletter, v. 12, no. 2, p. 5.

Washington Highways, 1965, All the fear a word can make—Earthquake!: Washington Highways, May, p. 14.

Wenatchee Daily World, 1965, Earthquake: Wenatchee, Wash.,

Wenatchee Daily World, April 29, p. 10.

Youd, T.L., 1973, Liquefaction, flow, and associated ground failure: U.S. Geological Survey Circular 688, 12 p.

———1977, Discussion of “Brief review of liquefaction during

earthquakes in Japan” by Eiichi Kuribayashi and Fumio Tatsuoka [1975] in Soils and Foundations, v. 15, no. 4, p. 81–92:

Soils and Foundations, v. 17, no. 1, p. 82–85.

———1978, Major cause of earthquake damage is ground failure:

Civil Engineering, v. 48, no. 4, p. 47–51.

Youd, T.L., and Hoose, S.N., 1978, Historic ground failures in

northern California associated with earthquakes: U.S. Geological Survey Professional Paper 993, 177 p.

Youd, T.L., and Perkins, D.M., 1978, Mapping liquefactioninduced ground failure potential: American Society of Civil

Engineers, Journal of the Geotechnical Engineering Division,

v. 104, no. GT4, p. 433–446.

Yount, J.C., 1983, Geologic units that likely control seismic ground

shaking in the greater Seattle area, in Yount, J.C., and Crosson,

R.S., eds., and Jacobsen, Muriel, comp., Proceedings of Workshop XIV, Earthquake hazards of the Puget Sound region,

Washington, October 13–15, 1980, Lake Wilderness, Wash.:

U.S. Geological Survey Open-File Report 83–19, p. 268–273.
TABLES 29–33

Descriptions of ground failures in the Puget Sound region caused by the April 13, 1949, and April 29, 1965, earthquakes
408 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 409
410 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 411
412 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 413
414 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 415
416 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 417
418 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 419
420 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 421
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GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 423
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GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 425
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GROUND FAILURE, PUGET SOUND REGION EARTHQUAKES, APRIL 1949 AND APRIL 1965 439
441

EVALUATION OF LIQUEFACTION POTENTIAL IN 

SEATTLE, WASHINGTON

By W. Paul Grant,1 William J. Perkins,2 and T. Leslie Youd3

ABSTRACT

Seattle has experienced significant damage related to

liquefaction during historical earthquakes and may be subjected to even greater damage in the future due to increased

development of the area and the potential occurrence of a

subduction-zone earthquake. The methodologies and results

of two research studies that evaluated the liquefaction hazard to Seattle are discussed and compiled in a single map

indicating the local liquefaction-hazard potential. Delineation of the liquefaction hazard in the area will benefit landuse planning, future building development, and planning for

disaster response.

All liquefaction evaluations were based upon existing

data from more than 350 boring logs and an empirical procedure that relates Standard Penetration Test N-values to

threshold ground accelerations needed to initiate liquefaction. A computerized database was developed to facilitate

storage and retrieval of the boring data for subsequent analyses. The liquefaction potential was evaluated using two

procedures. One procedure grouped similar geologic units

and assigned relative rankings to the liquefaction potential

based upon the percentage of Standard Penetration Test Nvalues that fell below a threshold N-value needed to resist

liquefaction resulting from a 0.30g (where g is equal to 9.8

meters per second) ground acceleration. The second procedure assigned relative liquefaction-potential rankings based

upon the computed thickness of material in individual borings that would liquefy for ground accelerations of 0.15g

and 0.30g. A review of all major geologic units within the

study area for liquefaction potential, using both criteria and

assigned hazard ratings of high, moderate, low, or very low,

showed that fills and Holocene alluvial deposits at the mouth

of the Duwamish River have a high liquefaction potential;

however, Holocene alluvium and beach deposits elsewhere

were given a moderate liquefaction rating. Pleistocene

alluvial sedimentary deposits were given a low liquefaction

rating, and glacially consolidated Pleistocene sedimentary

deposits were assigned a very low potential.

INTRODUCTION

Earthquake-induced liquefaction and related ground

failures have caused substantial casualties and major property losses in various parts of the world. Property losses in

excess of $800 million have been attributed to liquefactionrelated ground failures that occurred during the 1964 Niigata,

Japan, earthquake (Keefer, 1983). Also, property losses

related to liquefaction-induced ground failures were estimated to have exceeded $200 million in the March 27, 1964,

Alaska earthquake (Keefer, 1983). During the Alaska earthquake, soil liquefaction induced lateral spreads that compressed or buckled more than 250 bridges, disrupting railroad

and vehicular traffic. Liquefaction also generated subaqueous landslides that destroyed sections of the waterfronts of

Valdez, Seward, and Whittier.

Earthquake-induced ground failures during the 1949

and 1965 Puget Sound, Washington, earthquakes resulted in

substantial damage to buildings, bridges, highways, railroads, water distribution systems, and marine facilities.

Property damage from the 1949 and 1965 events totaled $25

million and $12 million, respectively. Grant (1986) estimated that 25–50 percent of the total damage from these

earthquakes may be attributed to earthquake-induced ground

failures such as liquefaction. Whereas this amount of

damage may seem relatively minor when compared with

other major earthquakes, the damage is consistent with the

relatively low levels of ground acceleration (typically less

than 0.10g, where g is equal to 9.8 m/s) that were recorded in

Seattle during these events. Should Puget Sound experience

a major Cascadia subduction-zone earthquake of magnitude

8.0 or greater, as postulated by Heaton and Kanamori (1984),

damage from earthquake-induced ground failures could

easily be an order of magnitude higher than the damage

experienced in past events.

________________ 1Kleinfelder, 2405 140th Ave., N.E., Suite A101, Bellevue, WA

98005-1877. 2Shannon & Wilson, Inc., 400 N. 34th Street, Seattle, WA 98103.

3Brigham Young University, Department of Civil Engineering, Provo,

UT 84602.
442 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

This report summarizes the results of two research

studies (Grant, 1990; Perkins, 1991) that evaluated the liquefaction-hazard potential in Seattle (fig. 215). Grant (1990)

was a U.S. Geological Survey (USGS) sponsored study in

which a computer database of existing borings in Seattle was

developed and used to evaluate liquefaction potential based

on empirical procedures by Seed and Idriss (1971), Seed and

others (1983), and Seed and others (1984). Liquefactionhazard categories were differentiated on the basis of comparing SPT (Standard Penetration Test) N values for a geologic

unit with the N values required to resist liquefaction during

an earthquake with a 0.30g ground acceleration. Perkins

90

N

5

5

Port

Townsend

Everett

STILLAQUAMISH RIVER

STRAIT OF JUAN DE FUCA

OLYMPIC MOUNTAINS

Study area

SNOQUALMIE RIVER

Seattle

Issaquah

Renton

LAKE

WASHINGTON HOOD CANAL

PUGET

SOUND

LAKE

CUSHMAN

Shelton

Tacoma

OLYMPIA

Black Hills

CHEHA LIS RIVER

DU WAMISH / GREEN RIVER

CASCADE RANGE

Puget

lowland

WASHINGTON

0

0 10

10 20 MILES

20 KILOMETERS

Figure 215. The Puget lowland and location of the study area, Washington.
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 443

(1991) used the same database but established liquefactionhazard categories based upon the cumulative thickness of

material liquefying from either 0.15g or 0.30g ground accelerations. The results from both of these studies were combined, resulting in the single liquefaction-hazard map

contained on plate 9.

Identifying areas where liquefaction may potentially

occur within Seattle provides a tool to aid government agencies in land-use planning, building development, and planning for disaster response. The liquefaction-potential map

developed as a result of this study may be used by engineers,

city officials, and planners to assess the need for changes in

zoning ordinances or building codes to mitigate the hazard.

For example, building codes could be modified to require

site-specific liquefaction assessments and appropriate foundation designs for structures located in high-risk areas. The

map also could be used by engineering departments within

various governmental agencies and the insurance industry to

estimate the damage potential to existing buildings during a

future earthquake, information that may be used to prioritize

structures for seismic retrofitting. Finally, the liquefactionhazard map could be used by emergency-response planners

to anticipate areas within the city that may sustain high damage and casualties or where the infrastructure (for example,

roads, bridges and water-supply lines) may be particularly

vulnerable, affecting emergency response efforts.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support and

sponsorship of this project by the USGS under the Earthquake Hazards Reduction Program. The support provided

by Ms. Deborah Walsh, the contracting officer with the

USGS, is greatly appreciated. We also wish to acknowledge

the efforts of Ms. Joanmarie Gorans-Eggert of Shannon &

Wilson, Inc., in creating the computerized database for

subsurface boring data and the help of Mr. Steven Bartlett, a

research assistant at Brigham Young University, in providing the computer software to evaluate the thickness of liquefied sediments. Finally, we appreciate the valuable technical

review of this paper by Dr. Steve Obermeier of the USGS

and Dr. Steve Palmer of the Washington State Department of

Natural Resources.

THE LIQUEFACTION PHENOMENON

Liquefaction is a phenomenon in which saturated, cohesionless soils are temporarily transformed into a liquid state,

most commonly as a result of earthquake-induced ground

shaking. Liquefaction occurs as a result of the buildup of

excess pore-water pressures during the shaking. When the

pore-water pressure exceeds the grain-to-grain (effective)

contact pressure of the soil, the soil particles lose contact

with each other and the soil essentially behaves as a liquid.

Pore-water pressure in a liquefied soil may become so great

as to result in small geysers from which water is ejected,

leaving sedimentary features commonly termed sand boils.

The development of liquefaction is controlled by a

number of complex and interrelated factors. These factors

can be generally related to the following parameters: (1) the

strength of the underlying soil deposit, (2) the location of the

water table, and (3) the severity of earthquake ground shaking. Historically, clean sands and silty sands are the materials most susceptible to liquefaction. In addition, the

liquefaction resistance of any particular soil is affected by its

density, fabric, prior earthquake history, and in-place stress

conditions. The depth of the water table is a controlling factor because a soil must be saturated or located below the

water table for liquefaction to occur. Within any given soil

deposit, liquefaction is more likely to occur where the water

table is shallow as opposed to conditions where the water

table is depressed. Finally, development of liquefaction is

dependent upon the magnitude of the earthquake stresses

induced in the soil deposit and also the duration of ground

shaking. The stresses and duration of ground shaking are, in

turn, affected by the size and location of the earthquake, the

travel path of the earthquake motions to the site, and any

local amplification of ground motions that may occur within

the soil column.

Depending on site-specific factors, liquefied soil can

cause various types of ground failures that may damage

overlying structures. A common result is loss of bearing

capacity for shallow foundations located over the liquefied

soil. Other adverse effects include flow failures, buoyant

rise of buried structures, ground settlement, failure of retaining walls due to an increase of lateral pressures, and lateral

spreading.

HISTORICAL LIQUEFACTION IN 

SEATTLE

Historical records provide valuable information for

assessing earthquake-induced liquefaction potential. Specifically, areas that have experienced liquefaction during past

earthquakes may liquefy again during a future event. For

example, reconnaissance reports from the 1989 Loma Prieta

earthquake (U.S. Geological Survey, 1989) indicate that

many of the areas that experienced liquefaction and unusually severe ground shaking during the 1906 San Francisco

earthquake experienced similar damage patterns in the

recent event. In both of these earthquakes, locations of

uncontrolled, random fills significantly correlated with

severely damaged areas. Therefore, in our study of the liquefaction potential of soils in Seattle, we first reviewed

accounts of historical liquefaction in the area and then correlated these accounts with historical maps that show tideland
444 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

reclamation along shoreline areas. This information would

likely identify the soil conditions having the highest relative

liquefaction potential.

Table 34 provides accounts of liquefaction occurrences

in Seattle during the 1949 and 1965 Puget Sound earthquakes. The April 13, 1949, event was located about 63 km

south-southwest of Seattle, near Olympia, and had a surfacewave magnitude (Ms) of 7.1 (Weaver and Baker, 1988). The

body-wave magnitude (mb) 6.5 earthquake of April 29, 1965,

occurred about 23 km south of Seattle (Weaver and Baker,

1988). As indicated in table 34, liquefaction during these

earthquakes typically resulted in differential settlement of

buildings, lateral movement of bulkheads, and cracking of

basement walls.

Because uncontrolled fills have been particularly susceptible to earthquake-induced liquefaction during historical

time, old topographic maps of Seattle were reviewed to

delineate fill areas. The changes to Seattle shorelines and

their associated fills are shown in figure 216. Additionally,

the figure shows the locations of sites that liquefied during

the 1949 and 1965 earthquakes, the majority of which coincide with shoreline areas that were filled during the early

development of Seattle. Experience from earthquakes in

other regions also suggests that areas of uncontrolled fill may

be particularly vulnerable to liquefaction during a future

event.

A final factor that is needed in understanding the historical accounts of liquefaction in Seattle is the level of ground

shaking, or peak ground acceleration, that occurred locally

during the 1949 and 1965 earthquakes. Fortunately, both

events were locally recorded on strong-ground-motion accelerographs (Shannon & Wilson, Inc., and Agbabian Associates, 1980), which were located close to areas that

experienced liquefaction (fig. 216). These accelerographs

recorded peak ground accelerations of about 0.10g during

both the 1949 and 1965 earthquakes (California Institute of

Technology , 1976; Shannon & Wilson, Inc., and Agbabian

Associates, 1980). Based upon seismicity studies conducted

by Shannon & Wilson, Inc. (1980) for various sites within

the Puget Sound region, it is estimated that this level of

ground acceleration may have a 20- to 40-year recurrence

interval.

GEOLOGIC FRAMEWORK

Because liquefaction resistance can generally be correlated with the age of a geologic deposit, understanding the

origins and ages of the different geologic units in the study

area helps establish a framework for categorizing potentially

liquefiable soils. In the following discussion, primary

emphasis is given to the glacial deposits in the region.

REGIONAL GEOLOGY

Seattle is located in the Puget lowland, which is a

slightly arcuate, convex-eastward basin lying between the

Cascade Range on the east and the Olympic Mountains on

the west (fig. 215). The basin is open on the north to the

Georgia depression and the Strait of Juan de Fuca, the latter

connecting Puget Sound with the Pacific Ocean. Beneath the

Puget lowland, nonlithified Quaternary sedimentary deposits

of varying thickness generally unconformably overlie Tertiary bedrock. These sedimentary deposits are both glacial

and nonglacial in origin.

The incursion of Pleistocene continental ice into the

basin is well documented (Willis, 1898; Bretz, 1913;

Mackin, 1941; Crandell and others, 1958; Armstrong and

others, 1965; Crandell, 1965; Easterbrook and others, 1967;

Mullineaux, 1970; Crandell and Miller, 1974; Blunt and others, 1987). Ice originating in the coast and insular mountains

of western British Columbia, Canada, coalesced in the Georgia depression and moved south across the 49th parallel to

the south end of the Puget lowland, about 80 km south of

Seattle. At least four major advances and several partial

advances have been identified. Although highly complex,

each advance left a sequence of lacustrine, advance outwash,

glaciomarine drift, till, and recessional outwash deposits.

The nonglacial intervals generated combinations of finegrained deposits of fluvial and lacustrine origin and also

some organic deposits, except along the basin margins,

where coarse fluvial deposits and mudflows predominate.

The trend of the existing ridges, valleys, and deep inlets of

Puget Sound is north-south, with the valleys being scoured to

great depths. The thickness of the total unconsolidated basin

fill varies from trace amounts in scattered locations throughout the lowland to over 1,100 m in the central part of the

basin near downtown Seattle (Hall and Othberg, 1974; Yount

and others, 1985) (fig. 217).

The bedrock underlying the Puget lowland is not well

understood because of the thick and pervasive mantle of

Pleistocene deposits. The sparse knowledge of the configuration of the bedrock surface has been interpreted from geophysical data, a few deep borings, projection of surface

exposures along the basin flanks, and several bedrock ridges

that partly cross the basin along northwest trends. The rocks

are mostly folded, faulted, and deeply eroded Tertiary marine

and estuarine sedimentary materials; volcanic materials consisting of basalt, andesite, and volcaniclastic rocks; and terrigenous deposits such as sandstone, shale, and

conglomerate, including extensive interbedded coal seams

that lie along the Cascade Range flank east and south of Seattle. Gravity- and magnetic-survey data show high differentials in the bedrock elevations, a result most likely related to

major faulting. In fact, some of the steepest gravity gradients

in the United States have been measured in Seattle (Danes

and others, 1965; Rogers, 1970). One of these steep gravity

gradients coincides with the Olympic-Wallowa lineament, a
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 445
446 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 447
448 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 449

major west-northwest-trending structural zone that cuts

through the Cascade Range and Columbia Plateau to the

east. South of this lineament, bedrock is exposed in scattered

outcrops in southeast Seattle, at Alki Point, and in a series of

prominent ridges to the east, collectively called the

Newcastle Hills, that trend eastward to the Cascade Range.

LOCAL GEOLOGY

Seattle consists of several north-south-trending elongated ridges and drift uplands. The hills and uplands are separated by large Pleistocene glacial troughs and outwash

channels that are now occupied by tidal waters and large

lakes or alluviated by streams that occupied the troughs

about 13,500 years ago following the retreat of the last glaciation. Major troughs lie beneath the main body of Puget

Sound, the Duwamish River valley, and Lake Washington.

The symbols used to identify the geologic units in the following discussion correspond to those used on plate 9 and

those used by Waldron and others (1962), who originally

mapped the surficial geology of the area.

BEDROCK

A broad band of Tertiary sedimentary and volcanicintrusive rocks forms the Newcastle Hills promontory

between Renton and Issaquah, east of Seattle. This westnorthwest-plunging promontory crosses the southern part of

the city, with bedrock exposed or subcropping in the

southern part of Beacon Hill, locally in the southern Duwamish River valley, and at Alki Point in West Seattle. These

rocks are folded into northwest to west-northwest-trending

anticlines and synclines that are broken by northeast-trending left-lateral faults (Weaver, 1937; Mullineaux, 1970).

The faults were most recently active in early to middle Tertiary time (Gower and others, 1985).

Two Tertiary bedrock units lie beneath and sporadically

crop out within the study area. Waldron and others (1962)

identified the oldest unit as middle Eocene sedimentary conglomerate, sandstone, siltstone, and shale chiefly with volcanic clasts (Td). This older unit crops out along the southern

edge of the study area, along the sides and below the Duwamish River valley. The younger bedrock unit (Tb) contains

Oligocene marine to estuarine sandstone and shale with subordinate amounts of conglomerate, mostly of volcanic origin. The unit is part of the Blakely Formation (Weaver,

1937; Waldron, 1962; and Livingston, 1971). This unit

crops out across the southern part of Seattle and at Alki

Point.

Immediately north of this band of Tertiary rocks, the

bedrock surface drops abruptly to depths of more than 1,100

m below sea level in a horizontal distance of less than 1.6

km, as shown in figure 217. Present data suggest that the

bedrock surface rises from its lowest point in downtown

Seattle gradually to the northeast to about 400 m below sea

level at the north end of Lake Washington (Hall and Othberg, 1974; Yount and others, 1985).

PLEISTOCENE DEPOSITS

Nonlithified, glacially overridden sedimentary deposits

generally lie unconformably above the Tertiary bedrock (pl.

9). This sedimentary material is both glacial and nonglacial

in origin. The youngest of these sedimentary deposits, the

lower parts of the Vashon Drift, were deposited as the Vashon ice lobe advanced southward during the Vashon stade of

the Frasier glaciation, about 15,000 yr B.P. (Mullineaux and

others, 1965). At its greatest extent, the lobe advanced to a

position about 80 km south of Seattle (Booth, 1987). Covering Seattle with an estimated 900-m-thick layer of ice, the

weight of the glacier greatly overconsolidated the underlying

sediment to various degrees, including the lower parts of the

Vashon Drift. In the Seattle area, these very dense sedimentary materials underlie most upland areas, including major

hills and ridges.

As the Vashon ice lobe retreated northward, it left

behind recessional outwash deposits (Qys, Qyg) largely

consisting of mixtures of gravel and sand. The outwash

was generally confined to the major glacial troughs but was

also irregularly distributed on the drift uplands. Recessional deposits also include coarse-grained outwash deltas,

kame terraces and other ice-contact deposits along certain

ridge flanks, local fine sand and silt deposited in ephemeral

ice-marginal lakes, and sands and gravels in local outwash

channels. Locally, recessional deposits attain thicknesses

of 30 m or more in major outwash deltas. The younger

sands (Qys) are fine to medium grained and generally less

than 3 m thick in the upland channels. The younger gravels

(Qyg) are composed of sand and pebble-size gravel and are

also about 30 m thick.

HOLOCENE DEPOSITS

Holocene deposits in the Seattle area include alluvium

(Qa) in the Duwamish River, Rainier, and Interbay valleys;

beach and adjacent marine deposits (Qb) along shorelines;

colluvial and landslide deposits (Ql); and peat (Qp) and

lacustrine deposits (Qsc) in upland depressions and along

low-lying lakes. Alluvial deposits (Qa) consist of fine sand,

silty fine sand, fine sandy silt, and nonplastic silt, with local

pockets or stringers of organic materials. Owing to shifting

of depositional channels, individual beds of uniform grain

size are rarely laterally continuous over large areas, and

interfingering of different soil units is common. Typically,

the Holocene deposits consist of very loose to loose granular

soils within about 10 m of the ground surface. Subsurface
450 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST























17

8

18

11

19

3

4

20

21

22

22

23

5 23

22

6

9

7

24

25

Young's

Cove

Harbor

Island

(Duwamish

tideflats)

E. Marginal Way

Spokane St.

E. Waterway

W. Waterway

122°27'25" 122°15'00"

47°

37'

40"

47°

37'

20"

West

Point

Lake Washington

ship canal

Aurora Ave. N.

Interbay

(Smith's Cove)

Existing

shoreline

Green

 Lake 

Salmon

Bay

Lake

Union

Union

Bay

Existing

shoreline

Portage

Bay

Sand

Point

Mud

NE. 45th St.

1

12

13

5

E. Madison St.

Yester Way

LAKE

WASHINGTON

SEATTLE

Denny Way

Olive St.

Denny

Regrade

14

15 2

16

Duwamish

Head

Alki

Point

10

26 Admiral Way

WEST

SEATTLE

Duwamish River

22

Rainier Ave. S.

MT. BAKER RIDGE

RAINIER VALLEY

BEACON

HILL

PIONEER

SQUARE

Elliott

Bay

*

0 1 2 4 KILOMETERS 3
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 451

explorations for the West Seattle Bridge indicated that the

alluvium near the mouth of the Duwamish River generally

extends to a depth of about 55 m and locally extends to

depths of 75 m.

Lacustrine deposits (Qsc) and very soft to soft peat

(Qp) also occur in numerous closed depressions on the surface of the drift uplands and along the shorelines of lakes.

Lacustrine sedimentary deposits are composed of silt, clay,

and fine sand and are usually less than 3 m thick. The peat

ranges from fibrous to peaty silt (muck). Both the upland

and lowland peats contain a pervasive 2.5- to 5.0-cm-thick

layer of ash related to the eruption of Mount Mazama (Crater

Lake, Oreg.) 6,800 years ago (Wilcox and Power, 1964;

Curran, 1965).

Colluvium is the veneer of loose to medium-dense soil

that drapes the sides and toes of slopes throughout the city

and its environs. The deposits consist of mixtures of the

materials composing the slopes and, hence, the grain size of

these colluvial deposits can range from fine-grained clay and

silt to boulder-size clasts. Processes forming colluvium

range from very slow creep (the imperceptible movement of

only fractions of a centimeter per year) to catastrophic landslides. The areal extent of landslide deposits (Ql) is relatively small; these deposits lie near the base of steep hills,

ridges, and uplands. Slide material is especially common at

or below the contact of the Esperance Sand and Lawton Clay

Members of the Vashon Drift (Tubbs, 1974). On steep

slopes (greater than 40°), the colluvial veneer is generally

very thin (1 m or less), whereas near the toe of the hillside,

where slope angles are 10°–20°, thicknesses of colluvium

generally range from 5 to 10 m.

FILLS AND LAND MODIFICATIONS

Major fills (f) and drainage modifications (m) in the

city have resulted from engineering projects during the first

two decades of the 20th century (Phelps, 1978). Areas of

major tideland reclamation are shown in figure 216. In the

early 1900’s, shallow tidal areas of the Duwamish River

delta were filled with material that was largely sluiced from

adjacent drift uplands to improve the usability of the seaport

and obtain an area for industrial development. As a result of

these operations, the mouth of the Duwamish River was

extended about 0.80 km northwest to its present location.

Also during this period, the sinuous, meandering course of

the lower Duwamish River was straightened and deepened to

what is now the Duwamish Waterway. Additionally, Harbor

Island was built of hydraulic fill placed on tidelands at the

river mouth when the East and West Waterways were

dredged. A tidal marsh in the Pioneer Square area was filled

with soil and with organic debris from nearby lumber mills.

Glacial soils from the Jackson and Dearborn Regrades were

sluiced via flumes and pipes to the Duwamish tidal flats

south of the Pioneer Square area, where it accumulated to

depths up to 12 m in the period between 1909 and 1910. The

water-laden soil was washed into a series of diked ponds so

that the fine particles could settle out of the slurry (Phelps,

1978). The tidal marsh at Smith’s Cove (Interbay) was

filled, as was the delta of Longfellow Creek (Young’s Cove)

in West Seattle. These projects provided extensive areas for

seaport facilities and industrial expansion.

Logs of geotechnical borings show that these fills are

highly variable in composition, ranging from sand to silt to

clay and often containing sawdust, bricks, logs, wood fragments, cinders, and other debris. Yount (1983) reported that

later fills are generally of better quality (more compact material consisting of medium to coarse sand) than the older fills.

The fills are typically 3–5 m thick but can be as much as 10

m thick.

Regrading of the downtown Seattle hillsides was

accomplished in two major phases between 1903 and 1928

to facilitate expansion and ease access within the central

business district. This included removing Denny Hill

entirely, which resulted in an excavation that was locally in

excess of 30 m and covered a 62-city-block area (Sale, 1976;

Morse, 1989). Glacial soils removed from Denny Hill were

either sluiced or dumped by barge into shallow areas of

Elliott Bay.

Between 1911 and 1916, the Lake Washington Ship

Canal was constructed, linking Lake Washington to Puget

Sound. This construction resulted in lowering of the surface

of Lake Washington a nominal 3 m to the level of Lake

Union. Additionally, the canal construction, which also

includes a set of locks, resulted in raising the water surface

in Salmon Bay to the level of Lake Union. The lowering of

the water surface in Lake Washington eliminated the Black

River, which drained from the south end of Lake

Figure 216 (facing page and above). Sites of historical liquefaction in Seattle. The numbers correspond to the location numbers

used in table 34 for descriptions of liquefaction. Base map from

Waldron and others (1962).

ACCELEROGRAPH STATIONS AND LOCATIONS

USGS accelerograph station 2170 (discontinued) that recorded

 the April 13, 1949 earthquake;

 4735 E. Marginal Way ( 47.55 N.; 122.34 W.)

USGS accelerograph station 2102 (existing) that recorded the

 April 29, 1965 earthquake;

 909 1st Ave. ( 47.604 N.; 122.335 W.)

*

1949 earthquake

1965 earthquake

Approximate location of liquefaction site

SITES OF HISTORICAL LIQUEFACTION

HISTORICAL SHORELINES

U.S. Coast Survey (1879)

U.S. Geological Survey (1897)



Marsh or swamp

Tidal flat

EXPLANATION
452 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST118 75 22 55 52 69 20 25 58 31 23 28 82 69 14 16 18 70 12 73

114

55

15

6 6

6

116

95

376

65

1139

1073

85

174

166

612

576

522

351

360

432

477

477

531

558

549

603

621 630

504

576

540

630

531

540

567

585

477

729

756 819

846

900

981

1080

1107

1089

1044 387

351

405

45

0

135

279

351

15

79

93

541

612

450

441

315

450

495

477

414 90

N

685
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 453

Washington to the Duwamish River at Tukwila. Also as

part of the canal construction, the Cedar River was diverted

into Lake Washington (Chrzastowski, 1983).

The lowering of Lake Washington left a gently sloping

terrace underlain by loose sedimentary materials around the

lake’s periphery, part of which has been retained as park

land and part of which has been privately developed (Galster, 1989). The northern portion of Union Bay was filled

subsequent to the lowering of Lake Washington. Originally,

the site was a landfill that was later capped with fill material.

A peat bog to the north of N.E. 45th Street was partially

removed for peat and then filled with granular materials.

During construction of the Sand Point Naval Air Station, the site was extensively graded. Glacial soils from the

central portion of the site were excavated and used to fill a

small embayment on the north side and at Mud Lake.

Smaller areas of fill, as shown in figure 216, are located

at the south end and northwest corner of Green Lake, the

south and west sides of Lake Union, and the southern portion of Salmon Bay. Smaller fills were also placed in oxbow features along the Duwamish and Green Rivers.

GROUND WATER

Most of the normally consolidated soil units in the

Seattle area lie in alluvial valleys or along lakes and bays

where ground-water levels are relatively high. Static water

levels recorded or estimated from borings in various areas of

the city are summarized in table 35. As shown in the table,

the average depth to ground water generally ranges from 0.6

to 3 m except in the upland outwash gravels. Although

available boring data for the upland gravels suggest the

absence of near-surface ground water, the near-surface presence of water cannot be precluded because of the paucity of

data. High ground-water levels are likely where this gravel

lies adjacent to lakes or ponds in the upland areas. Additionally, perched ground-water conditions may locally occur.

REGIONAL SEISMICITY

Examining the historical record concerning seismicity,

earthquake source mechanisms, and postulated levels of

peak ground motion for the Puget Sound region is essential

to defining the liquefaction hazard of the Seattle area

because the strength of the earthquake and the duration of the

ground shaking directly affect the development of

liquefaction.

HISTORICAL SEISMICITY

Seattle is located in a moderately active tectonic province that has been subjected to earthquakes of low to moderate strength and occasionally to strong shocks during the

160-year historical record of the Pacific Northwest. The

largest historical earthquakes in the region are believed to be

associated with deep-seated plate-tectonic activity (U.S.

Geological Survey, 1975). Major mapped faults in the

region (within 88 km of Seattle) have not been active in the

Holocene and, consequently, none are known to be associated with historical seismicity. The nearest faults known to

be active are small faults on the Olympic Peninsula, about 65

km west of Seattle.

The more significant historical earthquakes (those of

Modified Mercalli intensity VI or greater) that have occurred

in the Seattle area are listed in table 36. Of the 18 events

listed, 5 had intensities of VII or greater. The largest of these

were the April 13, 1949, Ms 7.1 intensity VIII shock and the

April 29, 1965, mb 6.5 intensity VII–VIII event. These earthquakes, which were respectively centered 63 km and 23 km

from Seattle, caused considerable property damage in the

city.

Other large historical earthquakes that have affected

Seattle include one in the North Cascades of Washington and

two in western British Columbia. The North Cascades earthquake of December 15, 1872, appears to have been one of the

largest in the Pacific Northwest, as it was felt over an area of

about 1,295,000 km2. It has been estimated that this major

shock had a magnitude of about 7 and an MMI of VIII.

Although the epicenter of this event is uncertain owing to the

sparse population of the area at that time, it apparently

occurred somewhere in the northern Cascade Range.

In Canada, major earthquakes occurred on Vancouver

Island on June 23, 1946, and in the Queen Charlotte Islands

on August 21, 1949 (Coffman and Hake, 1973). The Vancouver Island event had a magnitude of 7.3 and a maximum

intensity of VIII. Although the magnitude 8.1 Queen Charlotte Islands earthquake was felt over an area of more than

5,180,000 km2, damage was minor because of the sparse

population in the epicentral area.

Figure 217 (facing page and above). Map showing

depth to bedrock in the Seattle area. From Yount and

others (1985).

EXPLANATION

100 Contours showing depth to bedrock; in meters

Area of bedrock outcrop

Major structural or geophysical lineament

Marine seismic reflection profile 432

Water well 174

Geotechnical boring; depth to rock 95

Exploration not reaching rock 15
454 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

EARTHQUAKE SOURCE MECHANISMS

Earthquake source mechanisms, which have been

correlated with the observed historical seismicity, include

shallow crustal events and deep subcrustal events. Maximum magnitudes of about 6.0 and 7.5 have been postulated

for these two source zones, respectively (Rasmussen and others, 1974; U.S. Geological Survey, 1975). The deeper events

are believed to be associated with faulting or release of

extensional stresses in the subducted slab of the Pacific plate

beneath the Puget lowland area (Taber and Smith, 1985;

Weaver and Baker, 1988). The two major earthquakes in the
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 455

region, the 1949 and 1965 events, both had focal depths in

excess of 40 km, which is consistent with the deep-sourcemechanism hypothesis. The majority of historical events,

however, occurred at relatively shallow depths of about 24

km or less, which is consistent with the shallow-earthquakemechanism hypothesis.

A third source mechanism, which is currently being

debated within the scientific community, is the possible

occurrence of a major earthquake on the Cascadia subduction zone off the coast of the Pacific Northwest (Heaton and

Kanamori, 1984). Presently, the Cascadia subduction zone

is quiet, with only scattered and diffuse seismicity, and no

large subduction-zone earthquakes have occurred during

historical times. However, Atwater (1987) has introduced

geologic information that would suggest the possible occurrence of several subduction-zone events during the past

2,000 years.

POSTULATED GROUND MOTIONS

Estimates of seismic peak ground acceleration for the

Seattle area have been postulated from regional studies conducted by the USGS and from local microzonational studies

conducted by other researchers. Information on the ground

acceleration of the area is an essential parameter in conducting a liquefaction-hazard evaluation.

The USGS has performed several regional studies on

seismic hazards in the Pacific Northwest (Algermissen and

others, 1982; Algermissen, 1988a, b). Figure 218 presents a

regional, probabilistic evaluation of peak ground accelerations that could occur on rock within the Pacific Northwest.

The accelerations shown in this figure have a 10 percent

probability of being exceeded in a 50-year period, which

corresponds to a 475-year seismic return period. Figure 219

compares the seismic exposure of Seattle to other areas of

the United States. This figure presents ground accelerations

on rock that have a 10 percent chance of being exceeded during the indicated time intervals. Both figures indicate that

Seattle may be subjected to a ground acceleration of 0.30g

an average of every 475 years.

Whereas the ground-motion estimates presented in

figures 218 and 219 are based upon conventional earthquake source mechanisms (shallow and deep), recent work

by Algermissen (1988a) suggests that ground accelerations

in Seattle from a large subduction-zone earthquake occurring off the coast of Washington would not vary appreciably from the 475-year accelerations estimated from the

conventional earthquake sources. However, the duration of

ground shaking for a subduction-zone earthquake may be

several times greater than that associated with more conventional earthquake source mechanisms. Increased duration of ground shaking would tend to increase the areal

extent of liquefaction.

On a site-specific basis, Langston and Lee (1983) and

Ihnen and Hadley (1987) performed ray-tracing studies to

investigate the local variations in ground response in the

Puget Sound region. Whereas Langston and Lee (1983) specifically evaluated amplification of ground motion in the

Duwamish River valley, Ihnen and Hadley (1987) developed a seismic hazard map for the entire Puget Sound region

that included considerations for ground-motion amplification due to soil type and wave-focusing effects. Results

from both of these studies indicated that ground motions

along the Duwamish River could be 50–100 percent greater

than adjacent elevated areas. Both studies, however, indicated that the computed values of ground motion were

highly dependent upon the focal mechanism and location of

the generating earthquake. Because of the speculative

nature and high degree of sensitivity associated with the

results of the local microzonational studies, the results from

these local studies have not been widely accepted or used for

seismic design within the local engineering community.

STUDY METHODOLOGY

TECHNIQUE

Methods for evaluating liquefaction potential on a

regional basis range from empirical techniques relating

general liquefaction susceptibility to underlying geologic

125° 120°

45°

40°

Seattle

OLYMPIA

10

4

10

4

10

20

10

10

10

20 10

20

60

40

21

6

6

6

30

9 9

15

8

4 4

5 4

18

62

<4

<4

0 100 200 300 400 KILOMETERS

Figure 218. Earthquake peak ground accelerations for the

Pacific Northwest. Numbers are peak accelerations in percent of

g on rock with a 10 percent chance of exceedance within 50 years

(475-year return interval). Contours hachured to indicate closed

low. From Algermissen (1988a).
456 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

conditions (Youd and Perkins, 1978), to more elaborate

probabilistic, analytical evaluations (Power and others,

1986). These techniques have been applied to sites in southern California (Lee, 1977; Youd and others, 1979; Power and

others, 1982; Tinsley and others, 1985; and Power and others, 1986), northern California (Youd and others, 1975; Blair

and Spangle, 1979; Davis and others, 1982; Youd, 1982;

Kavazanjian and others, 1985; Youd and Perkins, 1987; and

Power and others, 1988), and other locations in the Western

United States (Anderson and Keaton, 1982, 1986; Moriwaki

and Idriss, 1987) and Eastern United States (Budhu and

others, 1987; Hadj-hamou and Elton, 1989; Elton and Hadjhamou, 1990).

Our liquefaction study of Seattle used empirical relations developed by Seed and Idriss (1971), Seed and others

(1983), and Seed and others (1984) to establish the

liquefaction potential of the various geologic units in the

area. The procedures were used because of their acceptance

and wide use in engineering practice. Furthermore, the use

of these procedures permits a better conceptual understanding of the liquefaction phenomenon and the interrelation of

the various parameters, such as subsurface geology and SPT

N-values, that affect the occurrence of liquefaction.

The first step required the development of a database

containing ground-water levels and SPT N-values for the

various geologic units within the study area. This

information was obtained from more than 350 borings in

Seattle. The SPT N-values of the soils within the various

geologic units were then compared with the threshold SPT

N-values needed to resist liquefaction. The relative liquefaction hazard of the particular geologic unit was then assessed

on the basis of the percentage of SPT values falling below the

threshold SPT N-values. Additionally, the liquefaction hazard was assessed on the basis of the computed cumulative

thickness of potentially liquefiable soil within the borings.

DATABASE

Because liquefaction susceptibility is affected by the

geologic origin, depth, relative density, and gradation of the

soil and the depth of the water table, a computerized database

was developed to facilitate the storage and retrieval of

subsurface data for subsequent use in the liquefaction evaluation (Grant, 1990). The database, which includes the logs

of more than 350 borings, allows sorting of the data corresponding to various parameters including geographic location, drilling method, geology, and individual SPT N-value.

Data recorded for each boring include UTM (Universal

Transverse Mercator) coordinates, location description, date

drilled, drilling method, surface elevation of the boring,

static ground-water depth, and SPT N-value as a function of

100

80

60

40

20

 0 10 50 100 150 200 250

EXPOSURE TIME, IN YEARS

ACCELERATION, IN PERCENT GRAVITY

SAN FRANCISCO

SAN DIEGO

ANCHORAGE

SAN BERNARDINO

NEW MADRID

WASATCH FRONT

SEATTLE

MEMPHIS

BOSTON

ST. LOUIS

ALBUQUERQUE

CHARLESTON

EL PASO

WHITE SANDS, NM

Figure 219. Seismic exposure map

indicating peak accelerations on rock in

various locations in the United States and

corresponding to a 10 percent probability

of occurrence for the indicated exposure

times. From Algermissen (1988b).
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 457

depth. Each SPT sample in a boring was assigned a code

corresponding to the geologic unit of the material as well as

a separate code describing the composition of the materials

within the sample. The data corresponding to a particular

geologic unit or material were subsequently retrieved to

determine liquefaction susceptibility. By including individual SPT data from each boring, we were able to statistically

account for variability of the SPT values within individual

borings or within an entire geologic unit. This assessment is

discussed subsequently in the evaluation-criteria section.

PEAK GROUND ACCELERATION

A key parameter in the liquefaction evaluation is the

selection of a peak ground acceleration value for use in the

numerical computations of the liquefaction potential. The

following factors were considered in selecting the peak

ground acceleration for the liquefaction study:

• Scenario earthquake or probabilistic assessment

• Criteria for probabilistic determination

• Uniform risk or site-specific studies

The first factor considered was whether to base the

evaluation upon a scenario earthquake, such as a repeat of

the 1949 or 1965 Puget Sound earthquakes, or to conduct the

evaluation based upon a probabilistic risk assessment of the

study area. One advantage of selecting a scenario earthquake is that other studies (U.S. Geological Survey, 1975;

Langston and Lee, 1983; Ihnen and Hadley, 1987) have been

conducted using such scenario events. Additionally, the

results from a scenario-earthquake evaluation may be compared with historical earthquake damage in the area. The

disadvantage to a scenario-earthquake study is that the earthquake sources in the area are not constrained to well-defined,

known faults with surface rupture. Hence, it is quite probable that future earthquakes could occur at any location within

the Puget Sound region and not at the epicenters of past

events. Accordingly, it was decided to conduct the liquefaction evaluation based upon a probabilistic assessment of the

earthquake hazard in the study area.

In selecting a probabilistic approach, it is necessary to

establish the criteria for defining the design earthquake. In

this regard, the design-earthquake ground acceleration was

selected to correspond with motions having a 10-percent

probability of being exceeded within 50 years. This approximately corresponds to a 475-year return interval. The criterion was selected to be consistent with local building

practice in Seattle, which is based upon the Uniform Building Code (International Congress of Building Officials,

1991). Thus, the 475-year return interval provides consistency between the liquefaction-hazard map and nationally

recognized standards for earthquake design of buildings.

The third factor considered in the liquefaction evaluation was whether to assume that the seismic risk or groundshaking potential was uniform throughout the entire study

area or whether the level of peak ground acceleration

should be varied throughout the study area, considering

amplification from topographic effects or subsurface soil

conditions. Clearly, one would expect variations in ground

accelerations throughout the study area for any given earthquake. These variations could be attributed to differences

in subsurface geology or geometric attenuation of energy

from the earthquake source. In fact, studies have been conducted to evaluate the local influence of such effects (Langston and Lee, 1983; Ihnen and Hadley, 1987). However,

one limitation of microzonational studies of local ground

motion is that the subsurface conditions throughout the

study area are not perfectly known. Furthermore, the

results of local studies of ground-motion effects in the Seattle area (Langston and Lee, 1983; Ihnen and Hadley, 1987)

have shown that the calculated results were highly dependent upon the focal mechanism and location of the generating earthquake. Thus, it would appear that whereas

techniques are available for computing ground motions on

a microzonational level, these computed ground motions

may be highly speculative and their application may be

limited, considering the unconstrained location of future

earthquakes in the Puget Sound region.

To avoid introducing additional uncertainties that are

associated with the calculation of site-specific earthquake

ground motions, it was decided to base the liquefaction evaluation upon a single level of ground acceleration. This

would imply a uniform seismic risk throughout the entire

study area. Although in reality ground motions may vary

throughout the study area, there are several reasons that support selection of a single value of ground-surface acceleration. First, the fact that earthquakes in the Puget Sound

region are not constrained to well-known structural features

indicates that future earthquakes will likely occur at random

in the region. This factor is consistent with the assumption

of a uniform seismic risk. A second factor supporting the

selection of a single ground-acceleration value is that the

study area is predominantly underlain by similar soil conditions. Specifically, about 80 percent of the study area is

underlain by glacially consolidated sedimentary deposits

that would be categorized as “stiff soils,” or “S2” soils using

the Uniform Building Code (International Conference of

Building Officials, 1991) soil classification scheme. The

remaining 20 percent of the study area is underlain by alluvial soils that may have a somewhat greater potential for

ground-motion amplification. These alluvial soils, however,

generally do not include the thick sequences of clay that have

characteristically resulted in large ground-motion amplifications in other areas, such as the San Francisco Bay region,

during prior earthquakes. Thus, based upon the random

location of future earthquakes in the Puget Sound region and

the predominance of a single soil type underlying the study

area, it was concluded that it is reasonable to use a single

ground-acceleration value to represent the seismic risk in the

liquefaction evaluation.

Based upon a 10-percent probability of exceedance

during a 50-year interval, it was decided to use a peak ground
458 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

acceleration of 0.30g for the liquefaction-hazard evaluation.

This acceleration corresponds to the bedrock acceleration

that is indicated in figures 218 and 219. Additionally, it is

consistent with the seismic-hazard map recently developed

by the USGS (Building Seismic Safety Council, 1991) for

sites in the United States that are underlain by “stiff soils,” or

“S2” soils as defined in the Uniform Building Code (International Conference of Building Officials, 1991). Thus, this

acceleration would likely apply to at least 80 percent of the

study area. Furthermore, it is assumed in the liquefaction

analysis that this acceleration would correspond to an earthquake having a magnitude of about 71

/2. This level of acceleration was used in the liquefaction studies of both Grant

(1990) and Perkins (1991). In addition, Perkins (1991) used

a peak ground acceleration of 0.15g to evaluate the effects of

liquefaction from a smaller earthquake that may have a

higher probability of occurrence.

EVALUATION CRITERIA

The final and, perhaps, most important factor in the liquefaction study was the selection of criteria for assigning the

relative hazard ranking to the various geologic units in the

study area. Selection of an appropriate hazard ranking

scheme is complicated by the fact that no one criterion has

been consistently used in prior liquefaction studies. Consequently, any liquefaction evaluation criteria used in a mapping study may appear arbitrary and require adjustments to

reconcile the predicted performance with past observations

of liquefaction. For example, the liquefaction study of San

Mateo County, Calif. (Youd and Perkins, 1987), includes an

adjustment factor of 10 to reconcile the study results with

damage resulting from the 1906 San Francisco earthquake.

Consequently, rating criteria developed for other geographic

locations may not necessarily be applicable to the Pacific

Northwest.

Two criteria were selected to assess the relative hazard

rankings of the local geologic units: threshold and thickness.

Threshold is based upon the relative percentage of SPT Nvalues in a geologic unit that would signify liquefaction during the 0.30g earthquake. Thickness differentiates the liquefaction hazard on the basis of the computed thickness of a

geologic unit that may liquefy during a 0.15g and a 0.30g

earthquake. Both the threshold and thickness criteria were

selected to provide a reasonable segregation of the liquefaction hazard of the different geologic units in the area.

THRESHOLD CRITERION

The threshold liquefaction criterion (Grant, 1990) is

based on evaluating the liquefaction resistance of a

geologic unit, defined by the SPT N-values for the unit, as

compared with a minimum SPT N-value needed to resist

liquefaction for a 0.30g peak ground acceleration.

Minimum SPT N-values needed to resist liquefaction, with

appropriate adjustments for fines content, were determined

from the following equation based on empirical correlations by Seed and others (1984):

(1)

where

Nuncorr is the uncorrected SPT value,

(N1)60 are the corrected SPT values adjusted for fines

content (Seed and others, 1984),

CN is the correction factor for overburden pressure

(Seed and others, 1984),

Amax is the peak ground acceleration (0.30g),

σ is the total overburden pressure,

σ' is the effective overburden pressure,

rd is the reduction factor for depth (Seed and others,

1984),

rm is the factor for earthquake magnitude (Seed and

others, 1984), and

g is the gravitational acceleration

The liquefaction evaluations primarily concentrated on

the materials within 13 m of the ground surface because

historical accounts of substantial damage from liquefaction

have been concentrated within this depth range. The

uncorrected SPT N-values characterizing a particular

geologic unit were compared with the minimum SPT Nvalues to resist liquefaction for each 1.5-m depth interval of

that unit. This incremental evaluation would account for

potential variability of the N-values with depth within the

geologic unit.

The following rating scheme was used to differentiate

the hazard potential of the soils in the study area:

____________________________________________

Percent of N-values below Hazard

the 0.30g threshold criteria rating

____________________________________________

>50 High

25–50 Moderate

10–25 Low

<10 Very low

____________________________________________

The percentage cutoff levels in the above tabulation

were selected in an attempt to provide reasonable segregation

of the data. Whether other cutoff values may be used, too

stringent criteria could result in all of the soils falling in the

high-hazard rating, and too lax criteria could result in all soils

having a very low liquefaction potential. Thus, it is more

important to develop rating criteria that segregate the data

than it is to use criteria from other locations that may not

adequately describe the relative local hazard.

Nuncorr ( ) N1 60 0.65( )σ Amax r = = d

CN gσ′

'rmCN
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 459

THICKNESS CRITERION

The liquefaction potential of the various geologic units

in the area was also evaluated using the thickness criterion

(Perkins, 1991), which is based on not only a threshold

acceleration but also a minimum thickness of liquefiable

material. The total amount (cumulative thickness) of potentially liquefiable soil in each boring was computed using

equation 1 and the peak ground accelerations of 0.15g and

0.30g. The calculations were completed for borings that

were typically less than 16 m deep. Liquefaction was

defined to be significant at locations where a minimum of 3

m of soil (cumulative thickness) would liquefy in the 0.30g

earthquake and a minimum of 0.3 m soil would liquefy in the

0.15g earthquake. Although these thickness values are

somewhat arbitrary, when combined with the 0.30g and

0.15g acceleration levels this criterion provides a basis for

segregating the performance of the underlying geologic

units under conditions of a large earthquake and a more

common, but smaller event.

The following classification scheme was selected to

differentiate the hazard potential of the soils in the study area

using the thickness criterion:

_____________________________________

Percent of borings with Hazard

computed liquefaction1 rating

______________________________________

>50 High

25–50 Moderate

<25 Low

_____________________________________

 13 meters liquefaction—0.30g; 0.3 meters liquefaction—0.15g.

SPT BIAS

One potential concern in the liquefaction evaluation

was that the drilling method may have a significant effect on

the SPT N-values obtained in the borings. Whereas rotary

techniques have been recommended as a standard procedure

in liquefaction evaluations (Seed and others, 1984), the vast

majority of borings drilled in the Puget Sound region were

advanced using hollow-stem auger drill rigs.

To evaluate the potential effect of drilling procedures

upon the resulting SPT N-values, a comparative study was

made of N-values at sites at the mouth of the Duwamish

River, where there is a high concentration of both hollowstem auger and rotary borings in a relatively confined area.

The results from this study, which are presented in figure

220, indicate that the N-values obtained in the hollow-stem

auger borings are about 6–7 blows per foot less than the Nvalues from the rotary borings. Additionally, the data presented in figure 220 indicate that the mean N-values from

the hollow-stem auger borings reasonably approximate the

lower quartile N-values from the rotary borings.

On the basis of these results, it is concluded that whenever feasible, the rotary-boring data set should be used in the

liquefaction evaluations. Additionally, it was assumed that

the mean N-values from the hollow-stem auger data would

reasonably approximate the lower quartile N-values if all

data were obtained using rotary techniques. This assumption

is an integral part of our evaluation because of the lack of

coverage of rotary borings within some of the geologic units.

STUDY RESULTS

As previously indicated, two separate but parallel

studies (Grant, 1990; Perkins, 1991) were conducted to

delineate the liquefaction hazard of the soils in Seattle.

Although different criteria were used in these studies, the

results of both research efforts were quite similar. Because

of this similarity, a single liquefaction-hazard map (pl. 9)

has been developed representing both research efforts and

using the previously described methods and database. Both

background studies focused upon ranking the relative liquefaction hazard of the major geologic units in the study area

because it was assumed that units having the same general

depositional characteristics should also have the same liquefaction resistance, provided all other factors are equal,

such as the ground-water depth and assumed level of earthquake ground shaking. Three geologic groupings were

evaluated for liquefaction resistance: fills, Holocene deposits, and Pleistocene deposits. The three groups were primarily differentiated by age because it was assumed that the

youngest deposits would likely have the highest liquefaction potential and the oldest deposits would have the least

potential. The areal extent of these geologic units and the

assigned hazard rankings are indicated on plate 9.

FILLS

DUWAMISH TIDEFLATS

The liquefaction resistance of the fill and underlying

alluvial soils in the Duwamish River tideflats was evaluated

because this area represents the largest uncontrolled fill in

Seattle (fig. 216). The area is bounded on the north by

Elliott Bay, on the east by Beacon Hill, on the west by

West Seattle, and on the south by Orcas Street. The ground

surface is at about 3 m elevation (city of Seattle datum),

and the ground-water table is typically present at depths

ranging between 0.6 and 4 m below the ground surface.

The subsurface geology typically consists of 3–5 m of fill

materials, chiefly sands, underlain by alluvial deposits that

are also predominantly sand. The fill material within the

tideflat area has largely been deposited using hydraulic

techniques. It is estimated that the fill consists of about 70

percent clean sand, 10 percent silty sand, and the remainder
460 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

sandy silt and clayey silt. Based on the logs in the study

area, the underlying alluvial materials are composed of

about 50 percent clean sand and 20 percent silty sand. The

remaining materials range from sandy silt to clayey silt.

The threshold criterion was applied to the fills and

alluvial soils in the tideflats to evaluate their liquefaction

susceptibility. The minimum SPT N-values needed to resist

liquefaction from the 0.30g threshold earthquake are

indicated, together with the SPT values for the underlying

soils, in figure 220. Although the data in figure 220 have

been segregated into both clean sand and silty sand units, the

most significant characterization of the data is the composite

plot of rotary boring data that includes not only clean and

silty sands but also SPT N-values that were excluded from

the other plots because the SPT N-values exceeded 40 blows/

foot. These high SPT N-values were initially excluded from

the data set because of a belief that any N-value of 40 blows/

foot or greater may be the result of driving the sampler on a

rock. However, a more detailed review of the logs indicated

that very few rocks are present in the underlying soils in the

EXPLANATION

Rotary drilling

Hollow-stem auger (HSA) drilling

Rotary drilling—Combined clean and silty

 sand and other very dense sand 

Minimum N values to resist liquefaction

 for clean and silty sands

25th percentile N-values (rotary borings)

Elevation of water table

0 20 40 0 20 40 0

10

20

30

40

50

0

10

20

30

40

50

DEPTH, IN FEET

DEPTH, IN METERS

DEPTH, IN FEET

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

DEPTH, IN METERS

0.30g 0.30g

CLEAN SAND SILTY SAND

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

No. of

samples Rotary HSA

No. of

samples Rotary HSA

26 36

69 38

55 54

45 59

55 43

49 38

41 36

30 37

21 29

17 30

 4 40

13 54

14 40

18 31

19 30

14 25

18 25

16 24

11 21

10 18

Figure 220. Liquefaction evaluation of the Duwamish River tideflats fill using the threshold criterion. The drilling data represent mean

SPT values.
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 461

tideflats area and that excluding N-values above 40 blows/

foot would bias the data set. Thus, the composite data set

represents the most accurate data set for evaluating the liquefaction resistance of the underlying soils.

Using the threshold criterion and the composite data,

figure 220 indicates that the mean (50 percentile) SPT N-values fall below the threshold level in the zone within 10 m of

the ground surface. This condition corresponds to a high

hazard rating. Because 25–50 percent of the composite SPT

N-values of soils below a depth of 10 m fall below the

threshold level, it is concluded that the underlying soils

would have a moderate liquefaction rating. The high liquefaction-potential rating given to the topmost 10 m of soil in

the tideflats is consistent with the site-specific studies for the

West Seattle Freeway Bridge replacement (Shannon & Wilson, 1980), which similarly showed depths of liquefaction in

this area to be on the order of 6–9 m for an earthquake with

a ground acceleration of about 0.30g.

Data supporting the thickness criterion evaluation of

the liquefaction susceptibility of the tideflats fill are presented in figure 221A. The curves in the figure represent the

percentage of borings in the Duwamish tideflats data set that

would experience liquefaction over an interval ranging from

0 to 8 m as a result of earthquake ground shaking with peak

accelerations of 0.15g and 0.30g. The cumulative thickness

of liquefaction computed for each boring does not necessarily represent a continuous zone of liquefaction. The intent of

these evaluations is to further quantify the liquefaction

potential and qualitatively indicate the areal extent where

liquefaction may occur to a significant degree.

On the basis of the data presented in figure 221A and the

thickness criterion previously discussed (Perkins, 1991), we

conclude that the fill soils along the Duwamish River tideflats have a high liquefaction potential. This conclusion is

based on the observation that a cumulative thickness of 3 m

of liquefaction would occur in about 65 percent of the borings for a 0.30g earthquake. Similarly, it is observed that a

cumulative thickness of 0.3 m of liquefaction would occur in

68 percent of the borings for a 0.15g earthquake. A high liquefaction-hazard rating would apply to the tideflats fill soils

because the computed cumulative thickness of potentially

liquefiable soils would exceed the minimum thickness criterion in over 50 percent of the borings.

We conclude that the liquefaction potential of the soils

in the filled Duwamish River tideflats is high on the basis

of the criteria used in both methods of evaluation. This

conclusion is in reasonable agreement with the historical

record (fig. 216), which indicates that instances of reported

liquefaction primarily occurred in the tideflats area. Furthermore, the high hazard rating of this area is consistent

with the findings of the site-specific liquefaction study for

the West Seattle Freeway Bridge replacement (Shannon &

Wilson, 1980). Whereas the fill in the tideflats has been

assigned a high liquefaction-hazard rating (pl. 9), areas on

the tideflats within about 60 m of open bodies of water

would have an even higher liquefaction potential and the

potential for lateral spreading, based on the historical

performance of the area.

INTERBAY

A liquefaction evaluation was performed for the fill soils

found in the Interbay area, which is bounded by Salmon Bay

on the north, Elliott Bay on the south, Queen Anne Hill on

the east, and Magnolia on the west. This location was also

identified for special study because it contains a significant

amount of uncontrolled fill that was deposited during the

early 1900’s. Ground-surface elevations in this area typically

range between 3 and 6 m (city of Seattle datum), and groundwater levels commonly are about 3 m below the ground surface. Soils in the Interbay area can include as much as 6–9

m of fill soils overlying alluvial deposits. The fill soils may

have a variable composition including clean sand, silty sand,

garbage, and construction debris or rubble. The underlying

native soils range from clean sand to clayey silt.

Data relevant to the threshold evaluation of the liquefaction hazard of the Interbay fill are presented in figure 222.

Conclusions drawn from the figure may be compromised

somewhat because the data set is relatively small and consists

exclusively of hollow-stem auger borings. Nevertheless,

because the mean (50 percentile) SPT N-values for the Interbay fill soils fall below the threshold criterion for liquefaction

corresponding to a 0.30g earthquake, it is concluded that the

Interbay fills have a high liquefaction-hazard rating. Because

the mean SPT N-values from the hollow-stem auger data set

are typically 10–15 blows per foot below the threshold criterion, the high hazard ranking would not be changed if the hollow-stem auger data were increased by 6–7 blows per foot to

provide equivalency with rotary borings (see fig. 220).

Data supporting the thickness evaluation of the liquefaction susceptibility of the Interbay fills are presented in

figure 221B. About 65 percent of the borings were calculated to have 3 m and 68 percent to have 0.3 m of sediment

that may liquefy during 0.30g and 0.15g events, respectively.

Using the thickness criterion previously discussed, the soils

in the Interbay area have high liquefaction potential.

Based on the application of both criteria, the Interbay

area is judged to have a high liquefaction potential. This liquefaction rating, however, may be somewhat conservative

when compared with the high hazard rating also given to the

Duwamish River tideflats fill because historical liquefaction

has not been reported at Interbay whereas numerous locations of liquefaction have been reported along the tideflats.

Although this would not preclude liquefaction in the Interbay area, it does demonstrate a higher hazard potential for

the Duwamish River tideflats. Nevertheless, considering the

potential variability of soil conditions in the Interbay area,

the Interbay fills were assigned a high liquefaction-hazard

rating (see pl. 9).
462 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

25

20

15

10

5

0

0 10 20 30 40 50 60 70 80 90 100

0

1

2

3

4

5

6

7

NORMALIZED PERCENTAGE OF BORINGS WITH

COMPUTED LIQUEFACTION

CUMULATIVE THICKNESS OF LIQUEFACTION, IN FEET

A

172 BORINGS IN

DATA SET

0.15g

0.30g

25

20

15

10

5

0

0 10 20 30 40 50 60 70 80 90 100

0

1

2

3

4

5

6

7

NORMALIZED PERCENTAGE OF BORINGS WITH

COMPUTED LIQUEFACTION

CUMULATIVE THICKNESS OF LIQUEFACTION, IN FEET

B

21 BORINGS IN

DATA SET

0.15g 0.30g

25

20

15

10

5

0

0 10 20 30 40 50 60 70 80 90 100

0

1

2

3

4

5

6

7

NORMALIZED PERCENTAGE OF BORINGS WITH

COMPUTED LIQUEFACTION

D

CUMULATIVE THICKNESS OF LIQUEFACTION, IN METERS

23 BORINGS IN

DATA SET

0.15g

0.30g

25

20

15

10

5

0

0 10 20 30 40 50 60 70 80 90 100

0

1

2

3

4

5

6

7

NORMALIZED PERCENTAGE OF BORINGS WITH

COMPUTED LIQUEFACTION

C

CUMULATIVE THICKNESS OF LIQUEFACTION, IN METERS

111 BORINGS IN

DATA SET

0.15g 0.30g

Figure 221. Liquefaction evaluation of soils in the Seattle area using the thickness criterion. Each curve represents the cumulative

thickness of liquefaction for the indicated seismic ground acceleration. A, Duwamish River tideflats fill; B, Interbay fill; C, Duwamish River

valley Holocene alluvium; D, Alki Point Holocene beach deposits.

during the 1965 Puget Sound earthquake. Therefore, all

significant fills mapped by Waldron and others (1962) have

been designated as having a high liquefaction-hazard rating

(see pl. 9).

HOLOCENE DEPOSITS

ALLUVIUM

The most significant deposit of Holocene alluvium

within Seattle consists of flood-plain material (hereafter

OTHER FILLS

Other fills have been mapped by Waldron and others

(1962) throughout the Seattle area. Although boring information was sparse or not available for these fills, it was

judged prudent to conservatively represent these materials

as having a high potential for liquefaction, considering their

variable composition and density. This high hazard rating is

partly substantiated by the performance of fills at the

University of Washington athletic fields and at the south end

of Green Lake (fig. 216), which experienced liquefaction
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 463

EXPLANATION

Hollow-stem auger (HSA) drilling

 (mean SPT values)

Minimum N values to resist liquefaction

 for clean and silty sands

Elevation of water table

0 20 40 0 20 40

0

10

20

30

40

50

0

10

20

30

40

50

DEPTH, IN FEET

DEPTH, IN METERS

DEPTH, IN FEET

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

DEPTH, IN METERS

0.30g 0.30g

CLEAN SAND SILTY SAND

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

No. of

samples HSA

No. of

samples HSA

2

4

1

8

5

6 

4

4

3

2

1

4

3

2 

0

1

2

0

0

0

Fill

Alluvium(?)

beach

? ?

Figure 222. Liquefaction evaluation of the low-lying Interbay area fills using the threshold criterion.

alluvial material in the upper parts of the Duwamish alluvium contains a somewhat larger percentage of silt compared with the alluvial materials underlying the Duwamish

tideflats.

Data relevant to the threshold evaluation of the

liquefaction hazard of the Duwamish alluvium are presented

in figure 223. The N-values in this plot have been segregated

based upon drilling technique as well as the material

encountered within the sampling depth. As indicated in the

figure, there is a relatively small percentage of rotary borings

informally referred to as the Duwamish alluvium for simplicity) in the Duwamish River valley. The valley typically

extends several hundred to several thousand meters on either

side of the Duwamish River. This zone is characterized by a

relatively flat lying area with ground surface elevations

ranging between 3 and 6 m (city of Seattle datum) and

ground-water levels 0.6–3 m below the ground surface.

Typically, soils within the area consist of shallow fill overlying alluvial deposits that may contain about 60 percent clean

to silty sand and about 40 percent sandy to clayey silts. The
464 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

in the data set, with most of the information obtained from

hollow-stem auger drilling borings. Additionally, the smaller

set of rotary-boring SPT data is somewhat suspect because

the consistency that existed between the larger data set of

rotary and hollow-stem auger borings in the Duwamish River

tideflats (fig. 220) is absent in the data from the Duwamish

alluvium. Furthermore, figure 223 shows the mean SPT data

from the rotary borings to be erratic whereas the mean SPT

data from the hollow-stem auger borings are less variable and

similar to the data shown for the tideflats fill (fig. 220).

Due to these inconsistencies, it was decided to evaluate the liquefaction potential of the Duwamish alluvium

based upon the hollow-stem auger data set. Because the

SPT N-values shown in figure 223 suggest that between 25

and 50 percent of the data would fall below the 0.30g

threshold criterion, it was concluded that the Duwamish

alluvium has a moderate liquefaction potential. This conclusion is based on the assumption that the mean SPT Nvalues from the hollow-stem auger borings correspond to

the 25-percentile values of the equivalent rotary data and

0 20 40 0 20 40 0

10

20

30

40

50

0

10

20

30

40

50

DEPTH, IN FEET

DEPTH, IN METERS

DEPTH, IN FEET

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

DEPTH, IN METERS

0.30g 0.30g

CLEAN SAND SILTY SAND

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

No. of

samples Rotary HSA

No. of

samples Rotary HSA

 2 18

10 27

12 24

11 36

10 24

13 13

 8 19

 8 10

 5 14

 5 7

 1 9

 5 10

 4 17

 4 18

 3 7

 1 9

 1 6

 0 9

 2 5

 1 9

EXPLANATION

Rotary drilling

Hollow-stem auger (HSA) drilling

Minimum N values to resist liquefaction

 for clean and silty sands

Elevation of water table

Figure 223. Liquefaction evaluation of Holocene alluvium in the Duwamish River valley using the threshold criterion. The drilling data

represent mean SPT values.
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 465

that there would be a differential of about 6 blows/foot

between the 25- and 50-percentile values (see fig. 220).

Data supporting the thickness criterion evaluation of

the liquefaction susceptibility of the Duwamish alluvium are

presented in figure 221C. As shown in this figure, about 40

percent of the borings in the area were calculated as having

at least 3 m of sediment that may liquefy during a 0.30g

earthquake. Also, about 38 percent of the borings were calculated as having at least 0.3 m of sediment that may liquefy

during a 0.15g event. Thus, we conclude from the thickness

criterion evaluation that the soils in the upper part of the

Duwamish alluvium have a moderate liquefaction rating.

The moderate liquefaction rating of the Duwamish alluvium appears to be reasonably consistent with the rating

given to the materials within the Duwamish tideflats area

because the materials in the upper parts of the Duwamish

alluvium appear to have higher SPT values, on the average,

compared with the materials in the tideflats area. This conclusion is based on reviewing the SPT N-values from hollow-stem auger borings advanced in both areas where the

SPT N-values in the upper part of the Duwamish alluvium

are about 7 or 8 blows/foot higher than the values obtained

at the mouth of the Duwamish River. Similarly, this liquefaction rating appears to be consistent with the historical performance of the area during the 1949 and 1965 earthquakes,

which indicated relatively few instances of liquefaction in

the upper Duwamish portion of the study area.

Whereas alluvial materials are present in areas other

than along the Duwamish River, the boring coverage that

exists in these areas is too sparse to support a significant

evaluation of liquefaction resistance. Therefore, based on

the Duwamish River valley data, all Holocene alluvial

deposits in the study area were classified as having a moderate liquefaction-potential rating (see pl. 9).

BEACH DEPOSITS

Beach deposits within the Seattle area are primarily

found along Puget Sound at West Point, west of Magnolia,

and by Alki Point in West Seattle. These deposits form relatively local zones where there may be either residential

development or municipal treatment plant facilities such as

at West Point. These zones typically have ground-surface

elevations ranging between 3 and 9 m (city of Seattle datum)

and ground-water levels typically on the order of 3 m or

more beneath the ground surface. The beach deposits are

predominantly clean, fine sands.

Data relevant to the threshold evaluation of the liquefaction hazard of these Holocene beach deposits are presented in figure 224. Whereas there are relatively few

borings in the data set, the data show that the mean SPT Nvalues from the rotary borings are consistently higher than

the hollow-stem auger data, similar to the trend observed in

the larger data set of the Duwamish tideflats area (fig. 220).

Furthermore, the mean SPT N-values from both the hollowstem auger and rotary data show consistency (absence of

erratic N-values) at various depths below the ground surface.

Thus, it was concluded that the rotary boring data presented

in figure 224 are applicable for the liquefaction evaluation.

Because the SPT N-values of the rotary borings in the figure

indicate that between 25 and 50 percent of the data fall below

the 0.30g threshold criterion (mean SPT N-values are above

the threshold), it was concluded that the Holocene beach

deposits have a moderate liquefaction potential.

A similar conclusion was derived using the thickness

criterion and the data presented in figure 221D for the Alki

Point area. As shown in this figure, about 35 percent of borings were found to have at least 0.3 m of sedimentary materials that may liquefy during a 0.15g event. This percentage

corresponds to a moderate liquefaction rating. However,

only 13 percent of borings were calculated to have at least 3

m of potentially liquefiable soil during a 0.30g earthquake,

which would correspond to a low rating. In light of this variance, a moderate liquefaction-hazard rating was conservatively assigned to the Holocene beach deposits.

The moderate liquefaction-susceptibility rating of the

beach deposits is reasonable when considering that only two

instances of liquefaction of beach deposits were reported

during the 1949 and 1965 Puget Sound earthquakes. Both

observations occurred at the same residence in the Alki Point

area. Liquefaction was not reported at West Point following

either event. However, excavations conducted for the West

Point treatment plant have encountered materials that would

suggest ancient liquefaction (paleoliquefaction). Thus, on

the basis of these limited and scattered observations, we conclude that the moderate liquefaction-hazard rating conservatively but accurately represents the relative hazard of the

beach deposits. This rating is consistent when compared

with the high hazard rating given to the Duwamish tideflats,

where numerous instances of liquefaction occurred during

prior historical earthquakes.

OTHER SEDIMENTARY DEPOSITS

Other Holocene sedimentary deposits mapped by Waldron and others (1962), such as their lacustrine sediments

(Qsc) and peat (Qp) units, underlie small, isolated parts of

the study area. Unfortunately, there is relatively little information in the database to characterize the liquefaction susceptibility of these units. Considering that these units may

be largely composed of cohesive sediments, it is believed the

liquefaction potential for these soils is relatively low. However, because the composition of these units is largely

unsubstantiated by the information contained in the

database, these materials were conservatively assigned a

moderate liquefaction-potential rating, as shown on plate 9.
466 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

PLEISTOCENE DEPOSITS

NORMALLY CONSOLIDATED ALLUVIUM

Normally consolidated Pleistocene alluvial deposits

typically are found at higher elevations (above 6 m) in scattered locations throughout the study area. Significant deposits of Pleistocene alluvium exist in West Seattle and have

been described by Waldron and others (1962) as deposits of

sand or gravel. These materials may typically include up to

about 70 percent of clean to silty sand, with the remaining

materials consisting of silt or gravel. Because these materials are recessional outwash deposits, they have not been glacially consolidated. Ground-water levels within these

deposits may be quite variable, considering that perched

water tables exist at higher elevations in the Seattle area.

Shallow ground-water conditions would be anticipated

within these deposits in areas adjacent to creeks or lakes.

Data relevant to the threshold evaluation of the

liquefaction hazard of the Pleistocene alluvial deposits are

0 20 40 0 20 40 0

10

20

30

40

50

0

10

20

30

40

50

DEPTH, IN FEET

DEPTH, IN METERS

DEPTH, IN FEET

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

DEPTH, IN METERS

0.30g 0.30g

CLEAN SAND SILTY SAND

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

No. of

samples Rotary HSA

No. of

samples Rotary HSA

 1 5

 0 10

 3 6

15 8

 6 2

 2 6

 1 2

 1 5

 0 2

 0 2

 0 0

 1 1

 4 0

 1 1

 1 0

 2 1

 1 2

 0 0

 0 0

 0 0

EXPLANATION

Rotary drilling

Hollow-stem auger (HSA) drilling

Minimum N values to resist liquefaction

 for clean and silty sands

Elevation of water table

Figure 224. Liquefaction evaluation of Holocene beach deposits found in all borings in the database using the threshold criterion. The

drilling data represent mean SPT values.
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 467

presented in figure 225. Because of the size of the data set

and some of the inconsistencies noted in the rotary boring

data (such as erratic N-values and SPT resistance values

lower than the hollow-stem auger data), it was decided to

base the liquefaction evaluation on the hollow-stem auger

data. The data in figure 225 generally show that the mean

SPT values from the hollow-stem auger installations, which

are assumed to be equivalent to the lower quartile values

from rotary borings, are typically near the minimum SPT

required to resist liquefaction during a 0.30g earthquake.

Thus, these high SPT values, combined with the anticipated

relatively low ground-water levels, lead to the conclusion

that the deposits will be best categorized as having a low

liquefaction-potential rating. This rating also is consistent

with the observed historical performance of these materials

during the 1949 and 1965 earthquakes, as no instances of

liquefaction were reported within Pleistocene alluvial

deposits.

0 20 40 0 20 40 0

10

20

30

40

50

0

10

20

30

40

50

DEPTH, IN FEET

DEPTH, IN METERS

DEPTH, IN FEET

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

DEPTH, IN METERS

0.30g 0.30g

CLEAN SAND SILTY SAND

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

STANDARD

PENETRATION RESISTANCE,

IN BLOWS PER FOOT

No. of

samples Rotary HSA

No. of

samples Rotary HSA

 1 9

 3 14

 2 7

 2 7

 2 6

 3 3

 2 6

 3 2

 2 1

 1 0

 0 7

 3 7

 0 5

 1 6

 1 6

 3 2 

 2 2

 2 2

 1 0

 0 1

EXPLANATION

Rotary drilling

Hollow-stem auger (HSA) drilling

Minimum N values to resist liquefaction

 for clean and silty sands

Elevation of water table

Figure 225. Liquefaction evaluation using the threshold criterion for Pleistocene alluvium found in borings where ground surface

elevations exceed 6 m in the Seattle area. The drilling data represent mean SPT values.
468 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

GLACIALLY CONSOLIDATED

SEDIMENTARY DEPOSITS

The majority of the soils within the study area are composed of glacially consolidated sedimentary materials ranging from till to sand and gravel. These materials are found

throughout the city, typically at higher elevations, with variable perched ground-water tables.

SPT data obtained from these glacially consolidated

sedimentary deposits are summarized in table 37. All the

SPT data were obtained using hollow-stem auger drilling

techniques. The data in the table have been differentiated on

the basis of undetermined-origin material, till, and glaciolacustrine deposits. High blow counts were obtained in practically all cases, and the 10-percentile values are greater than

the minimum SPT values required to resist liquefaction

during a 0.30g earthquake. As such, we conclude that these

materials would have a very low liquefaction-potential rating

and that the cohesive soils within this grouping, such as

glacial till, are not susceptible to liquefaction.

DISCUSSION

The liquefaction-hazard ratings that were developed for

each of the generalized geologic units in the study area were

based upon the data presented in figures 220–225 as applied

to the threshold and thickness ranking schemes. To provide

a means of evaluating the internal and external consistency of

the results of the liquefaction study, the rankings of each of

the individual geologic units have been summarized in table

38 along with information on their liquefaction performance

and the relative liquefaction ranking that would be assigned

to the deposits using the liquefaction classification system of

Youd and Perkins (1978). Agreement between these rating

schemes increases the confidence of the findings of the Seattle liquefaction study.

Several trends are apparent in the liquefaction hazard

rankings shown in table 38. First, although the threshold

and thickness criteria are not necessarily mutually inclusive,

the hazard rankings developed from both criteria are identical except for the low rating for the Holocene beach deposits corresponding to a 0.30g earthquake. Additionally, the

assigned relative hazard rankings are in agreement with the

liquefaction performance of the soils. Specifically, (1)

areas assigned a high hazard rating frequently had numerous instances of reported liquefaction, (2) areas assigned a

moderate rating had minor, scattered occurrences of liquefaction, and (3) areas assigned a low hazard rating had no

reported liquefaction. The final external consistency check

is the comparison of the assigned hazard ratings and those

that would have been assigned using the ranking scheme of

Youd and Perkins (1978). As shown in the table, the

assigned liquefaction rankings are identical to those that

would be determined from the Youd and Perkins (1978)

classificational scheme with the exception of fill soils that

Youd and Perkins (1978) have ranked as having a very high

hazard. On the basis of these favorable internal and external comparisons, we conclude that the liquefaction potential

map presented in plate 9 provides reasonable and realistic

seismic hazard rankings.

SUMMARY AND CONCLUSIONS

Seattle is located in a tectonic and geologic environment

that is conducive to the development of liquefaction during
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 469

relatively strong earthquakes. Instances of liquefaction have

been reported during the two largest historical earthquakes in

the region, the April 13, 1949, magnitude 7.1 Olympia earthquake and the April 29, 1965, magnitude 6.5 Seattle-Tacoma

earthquake. Although only moderate damage occurred in

Seattle as a result of these earthquakes, the damage level is

consistent with the low level of acceleration recorded locally

(approximately 0.10g) during both these events. This historical damage, however, may not accurately represent the

potential hazard in Seattle because the area may likely experience an earthquake with a ground acceleration of 0.30g

(Algermissen, 1988b; International Conference of Building

Officials, 1991). The liquefaction-hazard potential of the

area may be even greater during a subduction-zone earthquake because the duration of such an earthquake may be

several times greater than any historical event experienced in

the Puget Sound region.

The methodology used for evaluating and mapping the

local liquefaction hazard assumed that the entire study area

would be subjected to a uniform peak ground acceleration of

either 0.15g or 0.30g. The 0.30g level of acceleration has a

10-percent chance of being exceeded in 50 years (475-year

return period), and this acceleration is consistent with either

bedrock (Algermissen, 1988b) or stiff soil deposits (Building Seismic Safety Council, 1991). The 0.30g acceleration

is also consistent with local practice for the seismic design of

buildings (International Conference of Building Officials,

1991). This level of acceleration was coupled with the

empirical liquefaction procedures of Seed and Idriss (1971),

Seed and others (1983), and Seed and others (1984) to determine the liquefaction resistance of the various geologic units

in the area. A generalized liquefaction-hazard rating for

each geologic unit was evaluated using threshold performance and thickness criteria. The threshold criterion ranked

the relative liquefaction potential on the basis of the percentage of SPT N-values falling below the minimum N-value

required to resist liquefaction during the 0.30g event. A high

liquefaction potential was assigned to units with a mean (50

percentile) SPT N-value below the threshold value. A low

liquefaction-potential rating was given to materials with

lower quartile N-values than the 0.30g threshold level. Intermediate values were ranked as having a moderate liquefaction potential.

The second evaluation method was based on the thickness, or vertical extent, of soils that would potentially liquefy

in 0.15g and 0.30g earthquakes. A high hazard rating was

given to geologic units in which the thickness of the liquefied layer was predicted to exceed 3 m during a 0.30g earthquake and 0.3 m during a 0.15g earthquake in at least 50

percent of the borings. A low rating was assigned to units in

which these thicknesses would develop in less than 25 percent of the borings. Intermediate values were ranked as having a moderate liquefaction potential.

On the basis of the methodologies and criteria

described, a liquefaction-hazard map was developed for the

Seattle area (pl. 9). We conclude that fill soils and underlying alluvial deposits, particularly in the Duwamish River

tideflats and the Interbay area, have a high potential for liquefaction during a 0.30g earthquake. Deposits having a

moderate liquefaction potential at this seismic level include

Holocene alluvium, beach deposits, and other sedimentary

materials. The most significant Holocene alluvial deposits
470 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

occur along the Duwamish River valley. Pleistocene alluvial

deposits, which have not been glacially consolidated, were

given a low liquefaction-potential rating. Pleistocene glacially consolidated sedimentary deposits received a very low

liquefaction-potential rating.

The results from these studies are intended to provide

a regional assessment of liquefaction potential and should

not be considered as a substitute for site-specific studies for

individual buildings or other structures. Because conditions

vary locally, site-specific geotechnical investigations are

required to accurately assess liquefaction potential at any

given location.

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I–1257–G, scale 1:62,500.

Youd, T.L., Tinsley, J.C., Perkins, D.M., King, E.J., and Preston,

R.F., 1979, Liquefaction potential map of San Fernando
EVALUATION OF LIQUEFACTION POTENTIAL IN SEATTLE, WASHINGTON 473

Valley, California, in Brabb, E.E., ed., Progress on seismic

zonation in San Francisco Bay region: U.S. Geological Survey Circular 807, p. 37–48.

Yount, J.C., 1983, Geologic units that likely control seismic

ground shaking in the greater Seattle area, in Yount, J.C., and

Crosson, R.S., eds., and Jacobsen, Muriel, comp., Proceedings

of Workshop XIV, Earthquake hazards of the Puget Sound

region, Washington, October 13–15, 1980, Lake Wilderness,

Wash.: U.S. Geological Survey Open-File Report 83–19, p.

268–279.

Yount, J.C., Dembroff, G.R., and Barats, G.M., 1985, Map showing depth to bedrock in the Seattle 30' by 60' quadrangle,

Washington: U.S. Geological Survey Miscellaneous Field

Studies Map MF–1692, scale 1:100,000.
IMPLEMENTATION
Preceding page. Insert, masonry damage caused by the April 29, 1965, Seattle, Wash.,

earthquake. Photograph courtesy of NOAA/EDIS. Background, damage to unreinforced

masonry wall and parapet in downtown Klamath Falls, Oreg., during the Sept. 20, 1993,

M 5.9 and M 6.0 earthquakes (from Dewey, J.W., 1993, Damages from the 20 September

earthquakes near Klamath Falls, Oregon: Earthquakes & Volcanoes, v. 24, no. 3, p.

121–128).
476

REDUCING EARTHQUAKE HAZARDS—

AN INTRODUCTION

By William J. Kockelman1

Any effective program having earthquake-hazard

reduction as a goal requires (1) scientific and engineering

studies identifying and assessing the hazards; (2) translation of such studies for nontechnical users; (3) transfer of

this translated information to those who will or should use

it; (4) use of appropriate hazard-reduction techniques; and

(5) review and revision of such techniques to ensure their

effectiveness. Each is a prerequisite for its successor.

The preceding articles under the headings “Tectonic

Setting” (volume 1 of this report) and “Earthquake Hazards” (this volume) are a major contribution to scientific

and engineering studies and their translation. The publication and widespread distribution of this professional paper

will contribute substantially to the transfer of this information. The following articles are a part of this transfer component, namely, an introduction to and discussion of

hazard-reduction strategies and techniques. These reports

are addressed to the potential users listed in table 9 of the

Introduction to the report (Rogers and others, volume 1, p.

40).

May (this volume) addresses the prospects and

strategies for reducing earthquake hazards. The article is

based upon interviews with State and local officials in the

Puget Sound and Portland areas and on analysis of current

hazard-reduction policies and techniques. The author, a

political scientist, observed that officials are generally

aware of earthquake risk but that their concern is limited

because of more pressing problems. He takes a closer look

at the local political and economic factors that relate to

reduction of earthquake hazards in the future. The

relatively vulnerable cities are divided into “pacesetter,”

“resourceful,” and “restrained” groups and the less vulnerable cities into “new sophisticate,” “measured,” and “nonplayer” groups. Counties are divided into “leader,”

“transitional,” and “rural” groups. The understanding of

these diverse groups provides a basis for considering the

following four strategies whose strengths and limitations

are discussed:

• Disseminating translated scientific information.

• Seeking improvements in State-level building codes or

land-use mandates.

• Improving the practices of local building and planning

departments.

• Improving the practices of the private engineering and

design communities.

Preuss and Hebenstreit (this volume) develop a method

for assessing multiple hazards created by tsunamis and apply

that method to estimating impacts on coastal communities.

The method includes (1) identifying the tsunami hazard using

numerical simulation of wave direction and height; (2) analyzing the vulnerabilities of populations and land uses; (3)

identifying secondary hazards—subsidence, battering, fire,

and air contamination; and (4) delineating the hazard zones

of tsunami impact, flooding, and subsidence, including the

coastal highway system. Identifying the hazards and delineating the zones is difficult and requires specialized technical

treatment. The authors, a planning consultant and an oceanographer, then relate these hazard zones to various

impacts—casualties, damage, and socioeconomic disruption—and possible mitigation, response, and recovery techniques. Their work uses Grays Harbor, Washington, as a

case study and is applicable to other areas and hazards.

Booth and Bethel (this volume) discuss four regulatory

techniques for reducing earthquake hazards on undeveloped

land: comprehensive land-use and development plans and

policies, functional land-use plans, building codes, and

zoning overlays. The authors, King County, Washington,

geologists, identify and discuss five elements needed for any

regulation to reduce earthquake hazards:

• Defining the hazard.

• Evaluating specific hazardous site conditions.

• Mapping of the hazard zones.

• Screening of development proposals.

• Adding appropriate conditions to the basic ordinance

requirements.

King County used all four reduction techniques, and

the authors were able to evaluate the effectiveness of these

five elements. They conclude that the mapping and sitespecific evaluations are particularly weak and are likely to ________________ 1Deceased.
477 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

be so in other jurisdictions. They then make pertinent

recommendations to improve each of the above elements.

Perkins and Moy (this volume) examine the potential

liability of local governments in Washington for losses in an

earthquake. The Washington Tort Claims Act is analyzed, as

well as legal concepts such as “act of God,” negligence,

discretionary immunity, and the public-duty doctrine. Pertinent case law is cited. The authors, a regional planner and an

attorney, observe that the most likely source of liability for a

local government is losses caused by the dangerous condition

of its own property—hospitals, city hall, jails, and other

public works. They make the following recommendations to

local governments:

• Comply with all State and Federal regulations.

• Inspect, repair, and maintain public buildings and

facilities.

• Use local government risk managers as allies in promoting

safety.

• Do not assume that public liability exists for programs

affecting private property.

• Act reasonably to promote public safety and welfare.

Kockelman (this volume) introduces 36 types of earthquake-hazard-reduction techniques and describes the following six, citing examples: preparing redevelopment

plans, creating regulatory zones, securing nonstructural

building components, informing the public, strengthening

unreinforced masonry buildings, and making loss estimates.

The specific objectives of these techniques are to create an

awareness of, avoidance of, resistance to, response to, or

recovery from the effects of an earthquake on people and

their land uses, structures, and socioeconomic activities.

The author, a planner, emphasizes that scientific and engineering studies are a prerequisite to the use of these reduction techniques. Thus, it is not prudent for urban planners

to develop land-use regulations for civil engineers when

designing structures and for lenders and public-works directors when adopting policies reducing earthquake hazards

without scientific and engineering studies that assess the

hazards.
479

TECHNIQUES FOR REDUCING

EARTHQUAKE HAZARDS

By William J. Kockelman1

ABSTRACT

Six earthquake-hazard-reduction techniques are discussed: preparing redevelopment plans, creating regulatory

zones, securing nonstructural building components, informing the public, strengthening unreinforced-masonry buildings, and making loss estimates. An overview of these

techniques is useful to planners who prepare hazard-reduction programs, engineers who serve as advisors to local or

State governments, and decisionmakers who select the most

appropriate technique for a given situation.

INTRODUCTION

Many techniques for reducing earthquake hazards are

available to planners, engineers, and decisionmakers. This

report identifies 36 techniques and describes 6 in detail using

examples. Some of these techniques, such as public acquisition of hazardous areas, are well known to the planning

profession. Others, such as design of resistant structures, are

commonly used by engineers. Still others, such as installation of warning systems and emergency preparedness, are

obvious and practical but require maintenance and persistence in their implementation.

As an overview, the various techniques are listed in

table 39. They are divided into six groups related to specific

objectives; however, they can also be grouped in other ways,

such as the following chronological order:

• Pre-earthquake mitigation techniques, which may take

1–20 years;

• Preparedness measures, which may take 1–20 weeks;

• Response during and immediately after an earthquake;

• Recovery operations after an earthquake, which may take

1–20 weeks; and

• Postearthquake reconstruction activities, which may take

1–20 years

These estimated time periods would vary depending upon the

postulated or actual size of the earthquake, the damage it

causes, and the resources available to a State, its communities, its corporations, and its citizens.

The specific objectives of the techniques listed in table

39 are to create an awareness of, avoidance of, resistance to,

response to, or recovery from the effects of earthquake phenomena on people and their land uses, structures, and activities. The general goal of these objectives is to reduce human

casualties, property damage, and socioeconomic interruptions. Many of the reduction techniques are complex, interconnected, and require special skills—legal, financial,

legislative, design, economic, communications, educational,

political, and engineering.

Many of the hazard-reduction techniques have been discussed and illustrated by Blair and Spangle (1979), Kockelman and Brabb (1979), Brown and Kockelman (1983),

Kockelman (1985, 1986), Jochim and others (1988), Mader

and Blair-Tyler (1988), Blair-Tyler and Gregory (1988), and

the United Nations Office of the Disaster Relief Coordinator

(Lohman and others, 1988).

Prerequisite to the use of these reduction techniques are

scientific and engineering studies. Such studies are vital

because, in the words of former U.S. Geological Survey

director Walter C. Mendenhall, “There can be no applied science unless there is science to apply.” Experience has shown

that it is not prudent for urban planners to develop land-use

regulations, civil engineers to design structures, and lenders

and public-works directors to adopt policies reducing earthquake hazards without reliable scientific and engineering

assessments.

The following six earthquake-hazard-reduction techniques and examples of how they are applied are detailed in

this report:

1. Preparing redevelopment plans 

2. Creating regulatory zones 

3. Securing nonstructural building components 

4. Informing the public 

5. Strengthening unreinforced-masonry buildings 

6. Estimating casualties, damage, and interruptions 

________________ 1Deceased.
480 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

These techniques are discussed and illustrated for

nontechnical readers. The references cited for each technique will provide both scholars and practitioners with more

details and examples.

ACKNOWLEDGMENTS

Earl Brabb and Albert Rogers of the U.S. Geological

Survey and Jeanne Perkins of the Association of Bay Area

Governments provided many valuable suggestions and critical comments that have improved this chapter. Special

thanks are extended to Alice Olsen for word processing of

the manuscript and to Barbara Hillier for editing.

PREPARING REDEVELOPMENT 

PLANS

Incorporating earthquake-hazard information into

designs for the development or redevelopment of a community’s land use, housing, transportation, and other public

facilities is a common natural-hazard-reduction technique.

One of the designs is the redevelopment plan. Most States

authorize the creation of public redevelopment agencies that

provide for the preparation and adoption of redevelopment

plans; the acquisition, clearance, disposal, reconstruction,

and rehabilitation of blighted (including damaged) areas; and

the relocation of persons. These redevelopment agencies

usually are empowered to issue bonds, receive part of the

taxes levied on property included in the project, and use

grants or loans available under various State and Federal programs. Redevelopment plans may be divided into three categories:

1. Those that incorporate damaged areas into redevelopment

plans that had been created prior to damaging earthquakes 

2. Those that include vulnerable structures (identified prior

to an earthquake) in the redevelopment plans 

3. Those that include damaged areas in redevelopment plans

that are created after an earthquake 

Santa Rosa, Calif., a city of about 50,000 people, has a

redevelopment plan that illustrates the first category. Two

earthquakes struck within 2 hours in 1969, and many old

unreinforced-masonry buildings were damaged. Mader and

others (1980, p. C1–C15) reported the following:

In 1961, Santa Rosa embarked on a redevelopment project covering part of

the downtown area. Just prior to the earthquakes, the city had adopted a

central business district plan which covered an area adjacent to the redevelopment area. After the earthquakes, this area, with a high percentage of

damaged buildings, was added to the original redevelopment area.
TECHNIQUES FOR REDUCING EARTHQUAKE HAZARDS 481

BENTON

CARRILLO

COLLEGE

LINCOLN

101

WILSON

SANTA ROSA

AVENUE

B

STREET

HEALDSBURG MENDOCINO AVE

SLATER

HUMBOLDT

ORCHARD

BEAVER

KING

WRIGHT

MONROE

MCDONALD

SPRING

LELAND FAIR OAKS

5th

4th

3rd

2nd

E ST 

SONOMA AVE WALNUT

CLARK

SANTA

ROSA CR E EK

METANZ A S

CREEK

0 100 200 300 400 500 METERS

CALIFORNIA

Santa

Rosa

N

Figure 226. Part of the Santa

Rosa, Calif., urban-renewal

project area. Light shade, Phase

I—the original project area;

medium shade, Phase II—the area

added following two 1969 earthquakes; black, Phase III—the area

surveyed for additional land

required for a regional shopping

center. Modified from Mader and

others (1980, p. C–8, fig. 7).

The time and effort required to get the revised redevelopment project funded and underway after the earthquakes

was significantly shortened because of the existence of an

up-to-date plan (fig. 226) adopted prior to the earthquakes.

Spangle and others (1987, app. A) described the second

category—incorporating vulnerable structures identified

prior to an earthquake into the redevelopment plan. It is a

new technique called pre-earthquake planning for

postearthquake rebuilding (PEPPER). It includes four preearthquake activities: evaluation of vulnerability to damage;

organization for preparedness and response; mitigation of

hazards; and planning for postearthquake response. Spangle

and others (1987) believed it possible to develop damage

estimates sufficiently accurate for pre-earthquake programming of postearthquake recovery activities and to define the

nature of the postearthquake recovery organization needed.

The Whittier City Redevelopment Agency (1987)

adopted a plan for Whittier, Calif., that represents the third

category—redevelopment plans created after an earthquake.

The plan provides for redevelopment powers to be used for

projects to maintain, repair, restore, demolish, or replace

property or facilities damaged or destroyed as a result of an

earthquake in 1987. The earthquake damage in Whittier

exceeded $70 million in 1987 dollars.

Preparing and implementing redevelopment plans that

recognize and reduce earthquake hazards is very important

because postearthquake reconstruction commonly takes

place in the same hazardous areas. Youd and others (1978,

p. 1111), for example, observed that after the 1971 San

Fernando earthquake, “***buildings had been repaired, new

buildings have been built, and a freeway interchange has

been constructed across the trace of the 1971 fault rupture.”

CREATING REGULATORY ZONES

Various types of land-use and land-development

regulations meant to reduce earthquake hazards are available

to State and local governments. Controlling use and development with regulatory zones can be one of the most

economical and effective means available to government

agencies. The regulations can be used to reduce damage

from earthquake hazards such as a surface-fault rupture,

ground shaking, liquefaction, landslides, and tsunamis.

They may be divided into four categories:

1. Regulations requiring site investigations and building

setbacks 

2. Regulations reducing the density of development or the

number of occupants 

3. Regulations permitting only less vulnerable land uses and

land development

4. Regulations requiring special seismic-design and

construction standards

The first category can be illustrated by the AlquistPriolo Special Studies Zone Act enacted by the California
482 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Legislature (1972). The act provides for public safety by

restricting development near or over the surface traces of

active faults (fig. 227). In addition, the act requires or

provides for geologic reports, approval of projects by cities

and counties, and the charging of reasonable fees for

administrative costs. The California State Geologist

delineates appropriately wide zones that include “all

potentially and recently active traces” of faults that “he

deems sufficiently active and well defined to constitute a

potential hazard to structures from surface faulting or fault

creep” (Hart, 1988, app. A).

Before approval of any project in these zones, cities and

counties must require “a geologic report defining and

delineating any hazard of surface fault rupture.” The legislature defines a project as including structures for human

occupancy and subdivisions that contemplate the eventual

construction of structures for human occupancy. The legislature exempts single-family wood-frame buildings (including mobile homes) not exceeding two stories when not part

of a development of four or more dwellings. The approval of

a project must be in accord with the policies and criteria

established by the California Mining and Geology Board.

The board prohibits a project from crossing the trace of an

active fault (Hart, 1988, app. B), requires a geologic report if

a project lies within 15 m of an active fault trace, and requires

a registered geologist retained by the city or county to evaluate such reports. The act also allows cities and counties to

establish more restrictive policies and criteria. Some local

jurisdictions, such as the Portola Valley Town Council

(1973), require multifamily buildings to be set back 40 m or

more from fault traces.

The San Mateo County Board of Supervisors (1973) is

using the second category—reducing the density of development. San Mateo County, Calif., has a resource-management

zoning district that also carries out the objectives and policies

of the county’s open-space and resource-conservation plans.

The district regulations limit the number of dwellings in

zones with a surface-fault-rupture hazard, flood hazard, or

unstable slopes to one unit per 16 hectares (40 acres) and

require geologic site investigations to ensure that the reduced

development is in nonhazardous areas (fig. 228). The lower

net number of dwellings permitted may then be clustered at

a higher density in the nonhazardous areas.

An example of the third category—permitting only less

vulnerable land uses—can be seen in Colorado, where geologic hazards have been declared by the State legislature to

be matters of State interest. To assist communities in designing land-use regulations, the Colorado Geological Survey

prepared model geologic-hazard-area control regulations for

adoption by local governments. The regulations permit only

the following uses in areas designated geologically hazardous: (1) agricultural uses such as general farming, grazing,

truck farming, forestry, sod farming, and wild-crop harvesting; (2) industrial/commercial uses such as loading areas,

parking areas not requiring extensive grading or impervious

paving, and storage yards for equipment or machinery easily

moved or not subject to geologic-hazard damage; and (3)

public and private recreational uses not requiring permanent

structures designed for human habitation such as parks, natural swimming areas, golf courses, driving ranges, picnic

grounds, wildlife and nature preserves, game farms, shooting

preserves, target ranges, trap and skeet ranges, and hunting,

fishing, skiing, and hiking areas, if such uses do not cause

concentrations of people.

The fourth category is illustrated by a Redwood City

Council (1974, 1977) ordinance in California that requires

special seismic design and construction standards. These

standards supplement those recommended by the International Conference of Building Officials (1991) for structures

in seismic zone 4 under the Uniform Building Code—the

code adopted by Redwood City as its own building code.

Figure 227. Hypothetical surface-fault-rupture regulatory zone

in California showing different ways that the ground may break.

The figure shows the complex faulting (solid, dashed, and dotted

lines), the minimum 300-m-wide investigation zone required by

the Alquist-Priolo Special Studies Zones Act, and the 15-m building setbacks required by the California Mining and Geology

Board. From Brown and Kockelman (1983, p. 8, fig. 30).

Surface trace

Building

setbacks

Investigation zone

Approximate

location

Concealed

Crushed rock

Echelon

Braided

Branching

Subparallel
TECHNIQUES FOR REDUCING EARTHQUAKE HAZARDS 483

The ordinance is consistent with the city’s initial Seismic

Safety Element (Redwood City Planning Department,

1974), which had placed the mud around the margins of San

Francisco Bay in a moderately high-seismic-risk zone and

recommended that the Uniform Building Code be reviewed

and amended as “frequently as may be prudent.” The supplemental standards called for in the city’s ordinance relate

to special foundation-design criteria, design provisions for

greater lateral force, foundation systems to resist settlement,

wood-frame sheathing, moment-resisting frames, response

spectrum, reinforced-masonry construction, elements of

structural redundancy, and reinforcement of structural

members. These standards apply only to those lands within

the city that are underlain by bay mud, as shown on a map

adopted for the ordinance (fig. 229).

SECURING NONSTRUCTURAL 

BUILDING COMPONENTS

Proper siting, design, and construction of structures are

well-known techniques for reducing earthquake casualties

and damage, but often the contents and other nonstructural

components of buildings are overlooked. People have been

injured by falling light fixtures, flying glass, overturned

shelves, and spilled chemicals. The Federal Emergency

Management Agency (1981, table 2) (FEMA) estimates that

one-third of the property lost in future earthquakes will be

building contents. Such contents are only one part of the

nonstructural components of buildings.

Nonstructural damage is caused by object inertia or distortion of the structure. For example, if an office computer

is shaken, only friction will restrain it from sliding toward its

user. As a structure bends or distorts, its windows, partitions, and other items are stressed, causing them to shatter,

crack, or spring out of place. There are measures that can be

taken to protect nonstructural components, among which are

the following:

• Attaching sharp or heavy office equipment and fixtures to

the floor or walls 

• Attaching artwork securely to walls 

• Connecting filing cabinets at their tops and to a wall 

• Arranging free-standing, movable partitions in a zigzag

pattern 

• Installing locks on cupboards 

• Boxing large containers that contain hazardous chemicals 

• Strapping hot-water heaters to wall studs 

An excellent guidebook on reducing the risk of nonstructural earthquake damage was prepared by Reitherman

(1983). He described typical conditions found in office,

retail, and government buildings. Measures are suggested

for restraining more than 20 nonstructural building components such as office machines, electrical equipment, file cabinets, partitions, suspended ceilings, exterior ornamentation,

elevators, piping, stairways, and parapets. Each component

is rated for existing and upgraded vulnerability to life-safety

hazards, percent of replacement-value damaged, and

postearthquake outages for three levels of shaking intensity

(fig. 230).

A second guidebook (Washington State Superintendent

of Public Instruction, 1989) focuses on procedures for reducing nonstructural hazards in schools. This guidebook, issued

by the Washington State Superintendent of Public Instruction, contains drawings of methods for securing hazardous

objects commonly found in schools. The objects include

ceiling panels, chemicals, doors, exterior chimneys, exterior

Figure 228. Regulatory zones in a hypothetical area of seismic

and other geologic constraints. In San Mateo County, Calif.,

dwellings in floodplain, fault, and unstable slope zones are limited

to one per 16 hectares (40 acres) by the San Mateo County Board

of Supervisors (1973). Modified from Kockelman and Brabb

(1979, p. 82, fig. 6).

  















  





  

 

 

 

  



        

  





 

 





 

     

   



  



Road

Creek

0–15

16–30

31–50

Over 50

PERCENTAGE OF SLOPE Floodplain

Fault zone

Unstable slope

EXPLANATION
484 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

masonry, parapets, furniture, file cabinets, windows, mirrors,

skylights, heaters, light fixtures, partitions, and water

heaters. A general estimate of the earthquake risk for each

object and the cost to secure each are provided. In addition,

checklists for school administrators and custodians are

included for both interior hazards, such as ceilings, floors,

walls, boiler rooms, cafeterias, halls, stairways, and

laboratories, and exterior hazards such as chimneys,

ornaments, and parapets.

The application of such techniques to another type of

public building was made by the city of Mountain View,

Calif. Blair-Tyler and Gregory (1988, p. 19) observed that

101

Main Street

Broadway

101

0

0 3000 FEET

500 METERS

Marine

World

Ralston Avenue

Holly

Street

El Camino Real

Whipple Road

Woodside Expressway

Bayshore Freeway

San Carlos Airport

Veterans

Boulevard

Salt Evaporator

Greco Island

Redwood City

Marina

Harbor Blvd

Port of

Redwood City

Marina

Marina

Salt Evaporator

Salt Evaporator

BAIR ISLAND Salt Evaporator Redwood

Point

First Slough

Slough

Westpoint

Redwood Creek

Corkscrew

Smith

Slough

Deep

Water Slough

Bay Slough

Steinberger Slough

SAN FRANCISCO BAY

N

Figure 229. Map included with the Redwood City, Calif., ordinance that requires special seismic-design and construction standards

for all new development on San Francisco Bay mud deposits. Bay mud is indicated by shading; its southwestern boundary is the dashed

line. The unshaded areas lie outside the city’s jurisdiction. From Redwood City Council (1977).
TECHNIQUES FOR REDUCING EARTHQUAKE HAZARDS 485

the city had consultants prepare a room-by-room inventory

of nonstructural hazards in the Emergency Operations

Center—an alternate City Hall that would be used in the

aftermath of an earthquake. Blair-Tyler and Gregory (1988)

reported the following:

Communications equipment was braced and interior glass is being

replaced with safety glass or covered with a safety film. The City’s maintenance staff is providing the estimated 320 man-hours to complete the nonstructural work during the next year. Any structural strengthening will be

done by an outside contractor. Information gained from this experience

will be used to reduce nonstructural hazards in the design of Mountain

View’s new Library and City Hall.

INFORMING THE PUBLIC

A fourth earthquake-hazard-reduction technique

involves bringing earthquake-hazard information to the

attention of the public. Both pre-earthquake and

postearthquake hazard-reduction programs depend on the

understanding and support of an informed public. Responsible developers and prudent citizens, when told of possible

earthquake hazards, would not wish to risk property losses or

expose their clients or families to danger and trauma.

Preparing, announcing, and disseminating information on

possible earthquake damage, risk, and hazard-reduction

techniques can be accomplished in many ways (see

examples in tables 40 and 41).

STRENGTHENING UNREINFORCEDMASONRY BUILDINGS

Many techniques for strengthening, converting, or

removing unsafe structures are available to State and local

governments. One practice, strengthening unreinforcedmasonry buildings, has been used by several communities.

California has begun the first phase of this process, the

identification of unsafe buildings by cities and counties.

Figure 230. An example of how to reduce the risk of earthquake damage to one type of nonstructural building component.

From Reitherman (1983, p. 39).
486 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

These structures include unreinforced-masonry bearing-wall

buildings and steel- and concrete-frame buildings with infill

walls of unreinforced masonry. According to the California

Seismic Safety Commission (1987, p. 2), these structures

typically have four weaknesses:

1. Masonry walls, lacking reinforcement, cannot resist

earthquake shaking without degrading, which sometimes leads to collapse.

2. The practice of not structurally tying the walls to the roof

and floor can allow excessive movements in the

walls, which may lead to collapse.

3. Ground floors with open fronts and little crosswise bracing may allow excessive movement and twisting

motions, damaging the building.

4. Unbraced parapets may fall into the street.

An ordinance adopted by the Los Angeles City Council

(1981) outlines procedures and standards for identifying and

classifying buildings having unreinforced-masonry bearing

walls; these procedures and standards are based on a building’s present use and occupancy (fig. 231). Priorities, deadlines, and standards are also established under which

buildings are required to be structurally analyzed and

anchored. Whenever analysis determines deficiencies, the

ordinance requires that the building be strengthened or

removed. The ordinance applies to all buildings having bearing walls of unreinforced masonry that were constructed

before 1933 or those structures for which a building permit

was issued prior to 1933, the effective date of the city’s first

seismic building code. The ordinance does not apply to

detached one- or two-story single-family dwellings and

detached apartment houses containing less than five dwelling

units and used solely for residential purposes.

Affected buildings are classified according to type of

function and occupancy as essential, high risk, medium risk,

and low risk (see SEC. 91.6803 in fig. 231). The strengthening standards and time schedules for notification and compliance vary with the risk category. A structural analysis of

each building is also required in order to determine the remedial measures necessary to meet the appropriate standards.

The city provides a specific time schedule.

An alternative compliance schedule, intended to lessen

the financial and social impacts of the ordinance, gives a

building owner the option of performing part of the remedial

work within 1 year of notification in exchange for a longer

time in which to reach full compliance. The work to be performed within that year involves the anchoring of all unreinforced-masonry walls to roof and floor with bolts and

washers. According to the Los Angeles City Planning

Department (1979, p. 5), this procedure yields an immediate

and substantial improvement in safety for perhaps one-fifth

the cost of full compliance.

Using the knowledge and experience gained from the

procedures used in Los Angeles, the California Legislature

(1986) requires all cities and counties in seismic zone 4 to

identify hazardous unreinforced-masonry buildings, notify

the building owners, and establish a mitigation program.

Local building departments are authorized to establish fees to

recover the costs of identification. The California Legislature (1986) specifies that the mitigation program may include

the following:

The adoption by ordinance of a hazardous buildings program, measures to

strengthen buildings, measures to change the use to acceptable occupancy

levels or to demolish the building, tax incentives available for seismic rehabilitation, low-cost seismic rehabilitation loans, * * * application of structural standards necessary to provide for life safety above current code

requirements, and other incentives to repair the buildings which are available from federal, state, and local programs.

Compliance with a hazardous-buildings ordinance or

mitigation program is the responsibility of building owners.

Nothing in the law makes any local government responsible

for paying the cost of strengthening a privately owned

structure, reducing the occupancy, demolishing a structure,

preparing engineering or architectural analyses, conducting

investigations, or other costs associated with compliance of

locally adopted mitigation programs.

A guidebook addressing the hazards of unreinforcedmasonry buildings has been developed by the California

Seismic Safety Commission (1987). The guidebook contains

a series of steps for both identifying potentially hazardous

buildings and developing and implementing a hazard-mitigation program, including a model ordinance that provides for

strengthening or removal of unsafe buildings. Other discussions include costs to local government, costs to building

owners, incentives, and sources of information.

Some of the advantages of such ordinances are that

deaths and injuries will be substantially reduced, economically obsolete buildings may eventually be removed, land

will be reused more efficiently, and repair or demolition

will provide work for the construction industry. Some of

the disadvantages of such ordinances are that some lowincome housing may be lost, tenants probably will have to

be relocated, and businesses could be interrupted.

ESTIMATING CASUALTIES, DAMAGE, 

AND INTERRUPTIONS

Several techniques to assist State and local governments in preparing for, responding to, and recovering from

earthquake emergencies and disasters are available. One of

the techniques is commonly called a loss estimate. A

National Research Council (1989) panel defines an earthquake loss estimate as “a forecast of the effects of a hypothetical earthquake. Depending on its purpose, a loss study

may include estimates of deaths and injuries; property

losses; loss of function in industries, lifelines, and emergency facilities; homelessness; and economic impacts.”

These estimates are effective techniques in creating public

awareness of hazards and support for preparedness

measures and response and recovery operations. Three

examples of loss estimates follow.
TECHNIQUES FOR REDUCING EARTHQUAKE HAZARDS 487
488 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
TECHNIQUES FOR REDUCING EARTHQUAKE HAZARDS 489

The Federal Emergency Management Agency (1981)

estimated the number of dead and hospitalized, the number

of injured but not hospitalized, and losses to buildings and

their contents resulting from postulated earthquakes in four

California locations (table 42). In addition, damage to or

impact on selected facilities was discussed as were postseismic needs; these included temporary housing, key communication facilities, military command circuits, all

transportation modes, businesses, and industries. FEMA

and the California Office of Emergency Services then conducted an analysis of readiness for each of these categories

and discussed Federal, State, and local responses and

response planning.

The second example of a loss estimate was prepared by

Davis and others (1982). They created a planning scenario

for a postulated earthquake in the Los Angeles region that

can be used to gauge the severe impact on this urban area by

assessing the effects on principal lifelines for emergency

planning purposes. An analysis of readiness can then be

used to provide planning insights, recommend further work,

and serve as a basis for making or improving emergency preparedness, response, recovery, and reconstruction plans.

Davis and others (1982) included individual scenarios

that show damage to lifelines such as highways, airports,

railroads, marine facilities, communication lines, watersupply and waste-disposal facilities, and electrical power,

natural gas, and petroleum lines. The scenarios are based on

evaluation of earthquake-engineering literature, comments

Figure 231 (overleaf). Part of the Los Angeles, Calif., earthquake-hazard-reduction ordinance requiring owners of buildings

having unreinforced-masonry bearing walls constructed before

1933 to obtain a structural analysis. If the building does not meet

the minimum standards, the owner is required to strengthen or remove it according to a specific time schedule. From Los Angeles

City Council (1981).

.
490 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST
TECHNIQUES FOR REDUCING EARTHQUAKE HAZARDS 491

by numerous engineers and officials of public agencies, and

judgments by the authors. The assessment evaluated the

postearthquake performance of lifeline segments throughout

the region. Figure 232 shows the communications map,

which assesses telephone-system performance following the

postulated earthquake. Other maps (water-supply and

waste-disposal facilities, for example) show the locations

and estimates of damage to specific facilities. Most of these

planning maps contain notations that are explained further in

their text, such as “Water deliveries through the MWD

Upper Feeder will be temporarily interrupted by pipe rupture

where this major transmission line crosses the Santa Ana

River.” The scenarios indicate that most of the lifelines will

sustain significant damage that could require a major emergency-response effort. Each scenario map is accompanied

by a discussion of the general patterns of earthquake effects,

as in the following:

Interstate 5 from the San Joaquin Valley and Interstate 15 through Cajon

Pass will be closed, leaving U.S. 101 along the coast as the only major viable route open from the north. Highway connections with San Diego will

remain open.

And,

Not all of the [telephone] systems in the greater Los Angeles region are set

up to process emergency calls automatically on previously established priority bases. Thus overloading of equipment still in service could be very

significant.

Similar scenarios have been prepared for other postulated

earthquakes, such as on the Hayward fault in the San Francisco Bay region, by Steinbrugge and others (1987).

The third example of a loss estimate was one prepared

by the U.S. Geological Survey (1975). It postulated earthquakes for two locations in the Puget Sound, Wash., region

and concluded that under the worst circumstances, there

could be as many as 2,200 dead, 8,700 injured, and 23,500

homeless. Anticipated damage patterns for five counties in

the region were also estimated for both events. The degree

of impairment was assigned to selected critical facilities,

equipment, or supplies (fig. 233). Detailed assessments are

included, as in the following:

• Damage to hospitals having capacities of 50 or more beds

• Physician and nurse fatalities at nonhospital locations

• Losses of stock at retail drugstores and pharmacies

• Damage to railroad bridges and tunnels

• Probability of fatalities based upon siting of schools in

areas of high damage intensities

These loss estimates, damage scenarios, and degrees of

impairment are intended for planning purposes only, and

some may consider them overly pessimistic. However, in

emergency planning, it is important to plan for the most

severe levels of casualties and socioeconomic disruption in

order to be better able to prepare, respond, and recover.

PREREQUISITES FOR EARTHQUAKEHAZARD-REDUCTION TECHNIQUES

Prerequisites for the selection and implementation of an

appropriate earthquake-hazard-reduction technique from

table 39 are as follows:

• Conducting scientific and engineering studies of the physical processes of earthquake phenomena—source,

location, size, likelihood of occurrence, triggering

mechanism, path, ground response, structure

response, and equipment response

• Translating the results of such studies into reports and onto

maps at an appropriate scale so that the nature and

extent of the hazards and their effects are understood

by nontechnical users

• Transferring this information to those who will need to use

it and then assisting and encouraging them in its use

SCIENTIFIC AND ENGINEERING STUDIES

It is not prudent for planners to develop land-use regulations, engineers to design structures, and lenders and public-works directors to adopt policies reducing earthquake

hazards without adequate and reliable scientific and engineering assessments. As an overview of some of these

assessments, the nontechnical reader is referred to Hays

(1989, p. 193–194, list 1). Many studies were envisioned

and are described in the “Regional Earthquake Hazards

Assessments” draft work plan for the Pacific Northwest by

Hays (1988, p. 12–33).
492 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Figure 232. Planning-scenario map showing the impact of a postulated earthquake on the telephone system for part of the Los Angeles,

Calif., metropolitan area. This compilation by Davis and others (1982) shows the percentage of telephone-system effectiveness in four zones

(designated A, B, C, and D) as much as 3 days after the event. For example, in zone D near San Bernardino, only about 25 percent of the

telephone system would be in operation 3 days after the earthquake.
TECHNIQUES FOR REDUCING EARTHQUAKE HAZARDS 493

TRANSLATION OF RESULTS FOR 

NONTECHNICAL USERS

Translating scientific and engineering data into formats

that can be readily used by lay people provides those users

with an awareness that a hazard exists that may affect them

or their interests; gives them information that they can easily

present to their superiors, clients, or constituents; and

provides them with materials that can be directly used as

hazard-reduction techniques. My experience with reducing

potential natural hazards indicates that for the seismichazard information to be successfully used by nontechnical

users, it must have the following three elements in one form

or another:

1. The likelihood of occurrence of an earthquake that will

cause casualties, damage, or disruption

2. The location and areal extent of ground effects by the

earthquake

EXPLANATION

Modified Mercalli

Intensity

IX

VIII

VII

VI

EARTHQUAKE A EARTHQUAKE B Population...........1,143,800

Area in km2.................5,512

DEGREE OF IMPAIRMENT

EARTHQUAKE A EARTHQUAKE B

ESTIMATED LOSSES

EARTHQUAKE A EARTHQUAKE B

VITAL NEEDS Minimal Minor Major Minimal Minor Major

Communications......................................

Fire............................................................

Police.........................................................

 Electric power...........................................

Water.........................................................

Access roadways.....................................

Manpower...............................................

Hospitals.................................................

Ambulances............................................

Blood bank..............................................

Supplies..................................................

Food supplies...........................................

Deaths........................................................

Schools (as shelters)................................

Serious injuries.........................................

Homeless...................................................

1,500

6,000

7,130

1,650

6,600

18,630

Medical:

Figure 233. Anticipated damage and casualties from two postulated earthquakes in King County, Wash. The black square indicates the

general location of Seattle. From U.S. Geological Survey (1975, p. 5, table 2).
494 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

3. The estimated severity of the ground effects and effects on

structures and equipment

These elements are needed because engineers, planners, and

decisionmakers usually will not be concerned with a

potential hazard if its likelihood is rare, its location is

unknown, or its severity is slight, and neither will lenders,

politicians, or citizens.

TRANSFER OF INFORMATION TO

NONTECHNICAL USERS

The objective of transferring hazard information to nontechnical users is to assist in and encourage its use in order to

reduce losses from future earthquakes. Translated hazard

information is a prerequisite for transfer to nontechnical

users. A comprehensive example of both the translation and

transfer of geologic information for use by county planners

and decisionmakers is provided by Brabb (1987).

Various terms are used to convey information transfer

to users, including “dissemination,” “communication,”

“circulation,” “promulgation,” and “distribution.” These

terms are often interpreted conservatively, such as when

merely issuing a press release on hazards or distributing

research information to potential users. This level of activity

usually fails to result in the formulation of effective hazardreduction techniques and may even fail to make users aware

of the hazard. Therefore, I suggest that we use the term

“transfer” to mean the delivery of a translated product

(report, map, video, or poster) in a usable format at a scale

appropriate to its use by a specific person or group interested

in or responsible for hazard reduction. In delivering such a

product, assistance and encouragement in its use must also be

provided.

EVALUATION AND REVISION OF 

TECHNIQUES

The effectiveness of each hazard-reduction technique

varies with the time, place, and persons involved. Therefore,

it is prudent to include a continuing systematic evaluation as

part of any comprehensive earthquake-hazard-reduction program. An inventory of uses made of the information, reports

of interviews with the users, and analysis of the results and

responses will also result in identifying new users and innovative uses as well as any problems concerning the scientific

and engineering studies themselves and their translation,

transfer, and use. The evaluation will be helpful, even

necessary, to those involved in producing, translating, transferring, and using the research information as well as to those

funding and managing the program.

Performing the studies and then translating and

transferring the research information is expensive and

difficult because of the limited number of scientists and

geotechnicians, particularly when considering the number of

communities faced with possible seismic hazards throughout

the United States. The adoption and enforcement of an

appropriate hazard-reduction technique is time consuming

and requires many skills—planning, engineering, legal, and

political—as well as strong and consistent public support.

Scarce financial and staff resources must be committed, and persistent and difficult actions must be taken to

enact laws, adopt policies, or administer hazard-reduction

programs for long periods of time. To discover later that

the specific hazard-reduction technique selected is ineffective or unenforceable or its cost is greatly disproportionate

to its benefits not only is disheartening but also may subject

the persons involved to the criticism and loss of financial

support!

CONCLUSIONS

The earthquake-hazard-reduction techniques presented

in this report include preparing redevelopment plans, creating regulatory zones, securing nonstructural building components, informing the public, strengthening unreinforcedmasonry buildings, and estimating casualties, damage, and

interruptions. These techniques are designed to provide

greater public safety, health, and welfare for individuals and

their communities. The decision to adopt a technique is

influenced by many factors—the nature of the earthquake

hazard, public concern, strong community interest, Stateenabling legislation, the availability of scientific and engineering information, and the ability of geologists, engineers,

planners, and lawyers to incorporate the information into an

effective hazard-reduction technique.

Some of the geologic and seismologic information

needed for land-use and general planning in the Pacific

Northwest region is available but generally not at the level of

detail and scale needed for engineering and decisionmaking.

Even greater detail at larger map scales ranging from 1:1,200

to 1:12,000 is needed for development planning, site investigation, ordinance administration, project review, and permit

issuance.

Earthquake-hazard research is continuing, the information base is growing, new methods for evaluating hazards are

being developed, and hazard-reduction techniques may be

tested. Planners, engineers, and decisionmakers (both public

and private) need to recognize these facts and use the latest

information, methods, and techniques. However, they cannot be expected to have the training or experience necessary

to understand and use untranslated scientific and engineering

information. Therefore, if nontechnical users are to benefit

from this information, it must be translated and transferred to

them before effective hazard-reduction techniques can be

adopted.

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TECHNIQUES FOR REDUCING EARTHQUAKE HAZARDS 495

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497

EARTHQUAKE RISK-REDUCTION PROSPECTS

FOR THE PUGET SOUND AND

PORTLAND, OREGON, AREAS

By Peter J. May1

ABSTRACT

This chapter addresses current efforts and future prospects for earthquake risk reduction among major cities and

counties within the the Puget Sound and Portland, Oregon,

areas. There is a clear distinction among leading and lagging

jurisdictions within the region with respect to existing and

future efforts for earthquake risk reduction. The differences

result not only from different risk profiles, but also from differing political and economic circumstances. The understanding of differing situations leads to four State- or

regional-level risk-reduction strategies that might be undertaken as new information about seismic risk is produced.

These strategies range from relatively passive hazards-information dissemination to more active efforts to seek policy

reforms or to influence design practices.

INTRODUCTION

One of the important lessons of the U.S. Geological

Survey’s hazard-assessment program in Utah and California

is that simply providing technical information about earthquake risks will not stimulate changes in local practices or

policies. The design of State- or regional-level risk-reduction strategies requires identification of key features of the

political and economic environment that are likely to shape

future opportunities for and obstacles to local risk reduction.

This chapter develops such an understanding for major cities

and counties in the Puget Sound and Portland, Oreg., areas.

It is important to remember that the scientific study of seismic risk in the Puget Sound–Portland region is still underway. This chapter also suggests how risk-reduction efforts

might be altered as more is learned about the extent of seismic hazards and the risks they pose.

ACKNOWLEDGMENTS

The author thanks Patricia Bolton of Battelle Human

Affairs Research Centers, Thomas Yellin of the U.S. Geological Survey, and William Kockelman of the U.S. Geological

Survey for reviews of earlier drafts of this report, and is

grateful for assistance provided by Edward Fox and Nancy

Stark. Funding for the research and preparation of this report

was provided by the U.S. Geological Survey under awards

14-08-001-G1535 and 14-08-0001-G1778.

JURISDICTIONS STUDIED

Thirteen counties within the Puget Sound area (Clark,

Cowlitz, Grays Harbor, Island, King, Kitsap, Jefferson,

Mason, Pierce, Skagit, Snohomish, Thurston, and Whatcom)

and 6 counties within the Willamette Valley area (Clackamas, Marion, Multnomah, Polk, Washington, and Yamhill)

were selected for this study. There are 43 cities of more than

10,000 population within these 19 counties (table 43), which

are referred to as a single region in the following discussion.

_________________ 1Professor, Department of Political Science DO-30, University of

Washington, Seattle, WA 98195.
498 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Within the 19-county region are 97 incorporated cities,

more than 200 school districts, more than 200 special districts, and 22 port districts. The special districts consist of

separately governed entities such as water and sewer districts, public utility districts, drainage districts, road districts,

soil-conservation districts, and metropolitan councils of government. Within many of these cities and other local jurisdictions are multiple agencies with responsibility and

overlapping authority for addressing public safety. Earthquake-risk-reduction efforts in this region include more than

500 jurisdictions and 4.7 million people as of 1987.

The potential vulnerability of this region to earthquake

damage is indicated by statistics about the region. In 1988,

the total value of all property in the region was estimated to

be $180 billion. The value of new construction of apartment,

office, commercial, and industrial buildings in 1987 was

reported by the U.S. Census Bureau to be nearly $1.5 billion.

This amount is an almost-50-percent increase since 1980.

Because many newcomers, some who have left earthquakeprone areas of California, are moving to the region, the population and assessed property values will certainly grow over

the coming decade.

The region’s counties, utilities, port districts, and 43

largest cities were chosen for inclusion in the study because

they have the greatest populations and property values.

Within the 13 Washington counties studied are 27 cities of

more than 10,000 population, including Bellingham, Bellevue, Everett, Olympia, Seattle, Tacoma, and Vancouver.

Some 3.3 million people lived in these 13 counties as of

1987. Within the six Oregon counties studied are 16 major

cities, including Beaverton, Gresham, Portland, and Salem,

having a 1987 combined population of 1.4 million.

The study is based on interviews with 177 persons, conducted over a period of 6 months, beginning in September

1988. The following were interviewed:

• Those directly responsible for land-use and building regulation in the 43 cities and 19 counties

• Public-works or other utility personnel responsible for

water and sewer functions for municipal systems in

41 of the cities

• Engineering directors of six of the larger port districts

(Bellingham, Everett, Olympia, Portland, Salem, and

Tacoma)

The interviews of personnel in cities and counties

entailed talking with the planning director and the chief

building official (or an individual they designated). The

study’s focus on risk reduction rather than response to seismic events led to the decision to exclude emergency-services

personnel except in a few jurisdictions where such personnel

had broader responsibilities. In many jurisdictions, personnel with public safety responsibilities in the office of the

mayor, city manager, or county administrator were also interviewed. Additional interviews were conducted with State

officials in State-level building-regulation and land-use

agencies. In-person interviews lasting 30 minutes to 1 hour

were conducted with officials at the State level and within the

larger jurisdictions. The remaining interviews were by telephone.

The interviews were used to develop profiles of the

earthquake-related policies and practices of the cities and

counties and to develop the information that follows. Draft

profiles were shared with key officials in each jurisdiction

and were revised to correct factual errors.

DIFFERING RISK PERCEPTIONS

One aspect of the study characterized different risk perceptions within the jurisdictions. One set of questions

addressed elected officials’ perception of the likelihood of

significant property damage, deaths, or injuries from an

earthquake in the next 20–30 years.2 Another set of questions was addressed to building officials and engineering

directors to obtain their perceptions of the vulnerability to a

major earthquake of the building stock or facilities within

their jurisdictions.

RISK PERCEPTIONS OF ELECTED

OFFICIALS

Information about risk perceptions of elected officials

helps to depict the broad political context of earthquake risk

reduction. The study results show a general awareness of

seismic risks in the region, with a somewhat greater awareness by local officials in Washington. Some respondents

reported reading in local newspapers about the U.S. Geological Survey research on seismic risk, commented about

media reports of earthquake swarms, or recalled the 1965 or

1949 earthquakes. On the other hand, some respondents

seemed to discount the risk. Whatever the risk perception,

earthquake hazards appeared to be of relatively little concern to elected officials, because of other, more pressing

concerns.

These results are consistent with similar investigations

in other areas of moderate to high seismic risk, including California (for example, Berke and Wilhite, 1988; Drabek and

others, 1983; Mushkatel and Nigg, 1987). Earthquake risk

falls at the less dreaded and more accepted ends of the spectrum of attention to various risks by politicians and the general public.

The profile of responses by elected officials (fig. 234)

suggests a general awareness of the potential for a moderate

____________ 2The responses for elected officials were based on asking an appropriate administrative-level employee in each jurisdiction: “On a scale of 0 to

100, how likely do you think most officials in this jurisdiction think the

chances are in the next 20 to 30 years of significant property damage, injuries, or loss of life are from each of the following***”
EARTHQUAKE RISK REDUCTION, PUGET SOUND AND PORTLAND, OREGON, AREAS 499

earthquake and a lesser sense of the potential for a major

earthquake.3 Because the absolute risks posed by these hazards are unknown, it is more appropriate to refer to relative

comparisons of risks as represented by deviation scores.4 In

addition, the tendency for some respondents to rate all risks

very highly and other respondents to rate all risks less high

is accommodated by using relative scores.

Chemical spills are considered by elected officials to be

the greatest risk (fig. 234). Moderate earthquakes were rated

the second greatest risk by Washington city officials and the

third greatest risk by Oregon city officials. Major earthquakes are considered less significant risks than major landslides but more significant than dam failures. Although

major earthquakes are presumed to be more damaging than

moderate earthquakes, elected officials appeared to discount

the likelihood that major earthquakes will occur and thus

rated them a lesser risk.

The results for county elected officials (not shown) are

somewhat different than those for city officials. For both

Oregon and Washington counties, flooding is considered to

be the greatest risk, followed in decreasing order by

chemical spills, landslides, moderate earthquakes, major

earthquakes, and dam failures.

These results for elected officials suggest an awareness

level that corresponds to some degree with the way in which

seismic risks have been characterized in this region in the

past. Earthquake risks are perceived to be lower in Oregon

than in Washington. Yet, officials in both Oregon and

Washington discount the potential for significant damage or

loss of life, whereas earthquake engineers have said that

even moderate events in this region can be very damaging.

In both States, moderate earthquakes were perceived to

be more probable than major earthquakes. On the absolute

rating scale (0 to 100), the mean rating by Oregon city officials for the likelihood of significant effects of a moderate

earthquake is 28 and of a major earthquake is 18. The corresponding figures for Washington city officials are 48 and 34.

Earthquake risks were generally perceived by county officials to be lower. On the same rating scale, the mean likelihood rating by Oregon county officials for significant effects

of a moderate earthquake is 29 and of a major earthquake is

14. The corresponding figures for Washington county

officials are 31 and 15.

DAMAGE PERCEPTIONS OF BUILDING

OFFICIALS

Perhaps more important barometers for future riskreduction efforts are the perceptions of building officials.

This study attempted to learn these views in two ways. First,

building officials were asked about potential earthquakevulnerable classes of buildings in each jurisdiction. Because

none of the jurisdictions maintain building inventories, the

responses only reflect officials’ impressions of vulnerability.

Second, building officials and engineering directors were

asked to assess the impacts of various earthquakes upon the

building stock (or facilities) within their jurisdiction.

Three classes of buildings were considered to be potentially vulnerable: (1) unreinforced masonry buildings; (2)

40

30

20

10

0

-10

-20

RELATIVE RISK SCORE

Dam

failure Majorearthquake

(magnitude

6.6–7.5)

Moderate

earthquake

(magnitude

5.5–6.5)

Major

landslide

Major

flood

Chemical

spill

POTENTIAL HAZARDS

EXPLANATION

Risk perception for 16 Oregon cities

Risk perception for 27 Washington cities

Figure 234. Hazard-risk perceptions of elected officials of Oregon and Washington cities in the

study area. Relative risk scores are deviations from mean rating of risks posed by the combined

hazards.

_____________ 3A “moderate” earthquake was defined as Richter magnitude 5.5–6.5

and a “major” earthquake as Richter magnitude 6.6–7.5. Duration, depth,

and location were not specified. Most respondents seemed to understand the

labels “moderate” and “major” better than information about potential magnitude of an event. “Great” earthquakes (greater than magnitude 7.5) were

not addressed. 4Deviation scores were calculated for each respondent by subtracting

the average rating of all hazards for that respondent from the respondent’s

ratings for each hazard.
500 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

tilt-up concrete buildings built before 1975, typically used as

warehouses, as light-industrial facilities, or for other similar

purposes; and (3) reinforced-concrete frame buildings built

before the early 1960’s, typically 3–10 stories, used as office

buildings, schools, or apartments. Unreinforced masonry

buildings (table 44) appear to pose the most widespread

existing risk in the region’s cities. Tilt-up concrete buildings

built before the mid-1970’s are about equally prevalent

among cities in the two States. Reinforced-concrete frame

buildings are more common in Washington cities than in

Oregon cities. Unincorporated areas of counties are somewhat less at risk. Most unincorporated county areas are rural

or suburban and have mainly single-family homes and newer

commercial buildings. The most common hazardous-building type, reported to be “somewhat common” or “very common” in one-third of the counties, is tilt-up concrete

buildings built before 1975.

To understand the risks that potentially hazardous

buildings pose, building officials were asked to think about

the damage that might follow from various earthquakes.5

Because respondents were asked to assess damage as a result

of a particular earthquake, their responses do not entail judgments about likelihood of an earthquake. Both lack of specific information about the location and duration of the

seismic events and the difficulty that they had in envisioning

the degree of damage that might follow constrained their

responses.

City building officials perceived that the greatest potential for damage would be from major earthquakes, followed,

in order, by moderate earthquakes, major flooding, and

major landslides (fig. 235). On the absolute rating scale (0 to

100), the mean of responses by city building officials in

Washington of the likelihood of significant damage from a

major earthquake in the region was 46 (range from 10 to 90).

The mean of responses for a moderate earthquake was 27

(range from 0 to 75). The corresponding mean for Oregon

cities for a major earthquake was 44 (range from 15 to 85),

and for a moderate earthquake event the mean was 21 (range

from 5 to 60). County building officials in Washington and

Oregon had similar damage perceptions.

The building officials’ perception of prospective damage is consistent with their perception of the prevalence of

the different types of hazardous buildings. For cities, the

Pearson correlation (r) between prevalence of unreinforced

masonry buildings and likelihood of significant damage from

a major earthquake is a moderately strong +0.41. The

corresponding Pearson correlations between the prevalence

of tilt-up concrete buildings and prospective damage is +0.46

and between the prevalence of reinforced concrete frame

buildings and prospective damage is +0.14.

DAMAGE PERCEPTIONS OF UTILITIES AND 

PORT OFFICIALS

Utility and port officials viewed earthquake hazards as

a potentially broader risk than flooding or landslides. Landslides are likely to affect only part of the utility system,

whereas earthquake-related ground shaking would affect

the whole system. As part of risk management, utilities and

ports seek to protect the more valuable aspects of their

facilities—reservoirs, storage facilities, and treatment plants

for water or sewer utilities, and piers and cranes for ports.

About one-third of the respondents from water utilities

identified reservoirs as potentially vulnerable to a major

earthquake. An equal number were concerned about disruption of major water-transmission lines within their

system.

Utility personnel had perceptions of damage potential

comparable to those of building officials. Both Oregon and

Washington utility respondents rated major earthquakes as

having the highest damage potential, followed, in order, by

moderate earthquakes, major flooding, and major landslides. Oregon utility personnel as a group reported slightly

higher likelihood of damage from moderate earthquakes

(mean 22) and major earthquakes (mean 47) than the corresponding likelihood reported by Washington utility personnel. Washington respondents had much broader ranges of

likelihood of damages.

Utility personnel reported considerably more experience with occasional flooding and landslide damage, such

as transmission breaks in parts of their systems, broken connections to households, and so forth, than with earthquake

damage. Only Seattle and Kent municipal water system

officials reported more than minor damage from the 1965

earthquake. The response to flooding and landslides, particularly by the larger utilities, has been to build greater

redundancy into their systems. Because of this redundancy

and the localized nature of flood and landslide impacts, it is

not surprising that personnel of larger utility systems tended

to perceive less likelihood of significant damage from

major flooding (Pearson r= –0.41) or from landslides

_____________ 5Specifically, they were asked: “Please think about what damage

would follow if the following events actually occurred. On a scale of 0 to

100, how would you characterize the potential for damage to a significant

number of structures from ***”
EARTHQUAKE RISK REDUCTION, PUGET SOUND AND PORTLAND, OREGON, AREAS 501

(Pearson r= –0.31) than officials of smaller utilities.6 In

contrast, the perceived likelihood of significant damage

tended to increase with utility size for moderate earthquakes (Pearson r= +0.27) and for major earthquakes

(Pearson r= +0.16).

Engineering personnel at the six major ports that were

part of this study reported a likelihood of damage that generally corresponded to the perceptions of municipal utility

and building personnel. However, the likelihood data and

the qualitative responses to the interviews indicated a lower

perception of the probability of prospective earthquake damage to port facilities. The likelihood of prospective damage

from major earthquakes averaged 40 (on the 0 to 100 absolute scale) with a range of 20–50. Port personnel tended to

be more confident than the utility officials that seismic forces

had been adequately considered as part of facility design.

CURRENT RISK-REDUCTION 

POLICIES AND PRACTICES

Policies (as the term is used here) are officially adopted

laws or regulations at the State level and local ordinances at

the local level (May and Williams, 1986). Policies may be

general in establishing broad goals, or they may be specific

in specifying standards or in referencing other sources of

standards as part of legislation. They may be strongly

directive in mandating particular actions by other levels of

government. Policies commonly have associated administrative regulations that provide specific guidelines about

how to comply with the policies. At the State level, such

regulations are part of administrative codes subject to Statelevel rule-making procedures.

An important category of policies considered here is

State mandates that either require local governments to adopt

policies consistent with certain State goals or require local

governments to comply with State policies. The State

mandate may allow discretion in the way local governments

formulate policy, or the local policy may be highly prescribed. For example, Oregon’s statewide land-use planning

mandate requires local adoption of comprehensive plans

consistent with 19 State planning goals. The State planning

mandate establishes broad planning goals, and associated

regulations specify the steps for local compliance with the

mandate. The local comprehensive plans officially adopted

by local governments establish local land-use policies.

Practices (as the term is used here) are the actions of

local building officials, land-use administrators, and other

government officials in carrying out their functions. Practices are governed by policies and their implementing regulations. The policy or associated regulations may be vague

enough to allow discretion in carrying out the policy. As a

result, there can be differences between written policies and

actual practice. For some situations, no applicable policy

may exist. Therefore, the realities of practice are often difficult to study.

BUILDING REGULATION

Building regulation is a shared governmental function

for which Oregon and Washington specify State building

20

15

10

5 

0

-5

-10

-15

RELATIVE DAMAGE SCORE

Major earthquake

(magnitude 6.6–7.5)

Moderate earthquake

(magnitude 5.5–6.5)

Major

landslide

Major

flood

POTENTIAL HAZARDS

EXPLANATION

Damage perception for 16 Oregon cities

Damage perception for 27 Washington cities

Figure 235. Expected-damage perceptions of building officials of Oregon and

Washington cities in the study area. Relative damage scores are deviations from

mean rating of expected damage from the combined hazards.

_____________ 6The correlations reported in this paragraph were calculated by using

deviation scores for risk perceptions and omitting relevant outliers. Previous flood experience and future damage potential were very weakly related

(Pearsons r=+0.04) and previous landslide experience and future landslide

damage potential were weakly related (Pearsons r=+0.28). Insufficient

variation in earthquake-damage experience prevented analysis of the relationship between past earthquake experience and future damage perception.
502 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

codes, and local governments implement and enforce the

codes through local ordinances and building regulatory practices. The State codes are based on the UBC (Uniform Building Code) (International Conference of Building Officials,

1988) as amended in each State. Discretion in building regulation comes both through local amendments to the State

code (prohibited in Oregon) and through building officials’

exercise of discretion as permitted by amended UBC provisions and relevant local ordinances.

Perhaps the greatest limit to the increase in earthquakehazard risks in this region has been the adoption by Oregon

in 1974 and by Washington in 1975 of State laws and associated administrative provisions establishing State building

codes.7 These may not seem like big steps forward, because

some 80 percent of Washington cities and 60 percent of Oregon cities in this study had building codes with seismic provisions prior to these mandates, which were typically

adopted in the 1950’s or 1960’s. Fewer counties had building codes prior to the State mandates. Yet, the establishment

of State codes was important for jurisdictions where no

building codes of any kind existed and for establishing a way

to update the codes to reflect changes in UBC provisions.

Neither State has enacted or even considered State policy

mandates governing retrofit of existing hazardous buildings

beyond the relevant UBC provisions concerning change in

use or occupancy.

The two States’ building-code mandates (table 45) are

similar in approach but are different in some important specifics. Both mandates establish State building codes, require

local compliance with those codes, and create means for

updating the codes and monitoring local code compliance.

The States adopt relevant structural code provisions by referencing and amending provisions of the Uniform Building

Code.8 As newer versions of the UBC have been produced,

both States have adopted them with amendments. The 1988

edition has been recently adopted with amendments as the

basis for each State’s structural building code.

Because of differing seismic-zone delineations for Oregon (now designated zone “2B”) and Washington (now designated zones “2B” and “3”) and differing State-level

amendments, there are important differences in seismic provisions between the two States. Oregon also has adopted the

Council of American Building Officials One and Two Family Dwelling Code (Council of American Building Officials,

1986) as a basis for State code governing residential

construction.

The main differences between the two mandates are in

the degree of local discretion permitted in amending State

provisions. Oregon does not permit any amendment by local

jurisdictions. The Oregon code is a genuine State code for

which the implementing responsibility is delegated by the

State to local governments.9 Washington permits amendments to the State building code as long as they do not

weaken code provisions. Since 1986, State-level review and

approval of amendments affecting one- to four-unit residential buildings has been required. The local discretion in

amending the code combined with municipal building codes

that existed prior to the State mandate has resulted in a

greater variety of local codes in Washington than in Oregon.

Although local updating of ordinances to correspond

with new versions of State building codes may lag, localities

seem to have policies that comply with State mandates. The

critical question about the effectiveness of the State building

codes is the extent to which seismic provisions are enforced.

Enforcement relates to the capabilities of the relevant local

building departments (the quality of plans review and inspection) and the use of discretion by local building officials

where permitted by the codes. Oregon has attempted to

strengthen enforcement capabilities for all aspects of the

code by requiring certification of building officials, plans

examiners, and inspectors.

Data concerning local building regulation enforcement

are very sketchy. A recently completed Washington study of

code enforcement noted “a perception, and even public

acknowledgment, that the level of building code enforcement

varies throughout the state” (Washington State Building

Code Council, 1989).10 Some building officials who were

interviewed as part of this study hinted at political pressures

on them to be less stringent in interpreting codes. Others,

including some building officials in relatively small jurisdictions, were very aggressive in code enforcement. The overall

impression from the interviews was that current levels of

enforcement for new commercial construction are high.

Considerably more variation in code enforcement was suggested for code provisions regarding renovation of commercial structures. This variation reflects differences both in

building officials’ attitudes and in their use of discretion. 

Data about building-department staffing provide a

limited basis for assessing enforcement capabilities with

respect to seismic-hazard provisions. Only 15 percent of the

Washington cities and 20 percent of the Oregon cities in this

study reported having structural engineers as part of the

___________ 7Each State has laws that specify seismic design for public facilities

and “public places of assembly” dating from 1963 in Oregon (Oregon Revised Statutes 456.965 “Public Structures”) and 1955 in Washington (Revised Code of Washington, Chapter 70.86, “Earthquake Resistant

Standards”). Each of these laws has been superseded by State building-code

provisions. The extent to which these provisions were enforced in the

1950’s and 1960’s is unknown but likely to have been limited because of the

lack of State-level enforcement mechanisms at that time. 8Other codes govern electrical, mechanical, fire, and other aspects of

construction. 9The delegation of responsibility for codes follows a hierarchy in Oregon (Oregon Building Codes Agency, 1988). If a municipality elects not to

enforce the State code, the responsibility falls to the county. If the county

elects not to enforce the State code, the responsibility falls to the State. Enforcement of the structural code has been delegated to the cities and counties

in this study. 10The limited nature of the data is underscored by the fact that only 17

people gave public testimony at the five regional hearings held by the Washington State Building Code Council on this topic.
EARTHQUAKE RISK REDUCTION, PUGET SOUND AND PORTLAND, OREGON, AREAS 503

building-department staff. (The corresponding figures for

the counties in this study are 12 percent in Washington and

33 percent in Oregon.) Seattle’s building department has the

greatest staff capability, consisting of a construction review

staff of 19, of which 6 are licensed structural engineers, 4

have other engineering certification, and 9 have buildingconstruction or architectural degrees. Some 60 percent of

cities in the region reported sending complex drawings for

plan review to the regional office of the International Conference of Building Officials or hiring consultants for the

review. Not surprisingly, the smaller jurisdictions reported

more limited ability to interpret seismic-hazard code provisions. This difficulty was summarized by one building official who stated, “I often have questions about what the

seismic provisions are based on; it is hard for a non-engineer

to tell.”

The code-enforcement gaps in some of the smaller

jurisdictions and more rural counties prior to the establishment of the State codes are illustrated by the following

quotes from interviews with building officials:

• “We were derelict prior to 1975 or so in keeping up with

UBC changes, but we are now tightly tied to State

law.”

• “There were blatant problems in the 1970’s with bad engineering practice that we were unable to address.”

• “Shifting the building function back and forth between the

city and county made consistent code compliance

difficult.”

The existence of State building codes in Oregon and

Washington may seem to make local building regulatory

policies and practices less important aspects to consider for

future earthquake-risk-reduction strategies in the region. For

newly constructed, engineered buildings this is probably a

reasonable conclusion. Given the apparent relatively good

enforcement of seismic provisions as applied to such buildings, the degree of risk for engineered buildings built since

the early 1970’s (or even the early 1960’s in larger cities) is

essentially determined by UBC seismic provisions in place

at the time the buildings were constructed. The judgment

that seismic risk is low for these buildings presumes that

good engineering practices were followed in building design

and that the seismic zones were properly designated in the

codes in place at the time of construction.11

Local policies and practices are especially important

concerning (1) existing, potentially hazardous buildings,

such as unreinforced masonry buildings (URM), tilt-up concrete buildings, and reinforced concrete frame buildings

described earlier in this chapter; and (2) building and excavation provisions for steep slopes and unstable soils. The

variation in local policies and practices concerning these

hazards stems from multiple sources of code provisions and

latitude within those codes.

____________ 11Oregon’s zone classification has been changed somewhat, particularly in the 1988 edition of the UBC. This in itself provides a basis for wondering about risk posed even by the “newer” engineered buildings.
504 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Two relevant UBC discretionary provisions define the

trigger for applying seismic-code provisions to buildings

undergoing renovation. Section 104 states that “additions,

alterations, or repairs” may be made without requiring the

existing building to comply with all of the code requirements. In addition, section 502 provides an exemption for

proposed uses that are less hazardous than existing use for

buildings undergoing a change in “character of occupancy or

use.” These provisions allow for interpretation for buildings

that are remodeled in stages or for where the change of use

can be debated. This is less of a problem for tilt-up concrete

buildings, for which changes in use are less frequent than for

other categories of potentially hazardous buildings.

The main UBC discretionary exemption that applies to

older, potentially earthquake prone buildings consists of

“historic building” provisions of the UBC Section 104(f).

This section allows discretion in meeting current code

requirements for buildings certified as having special architectural or historical significance. The key provision is that

the restored building or structure be no more hazardous than

the original.

Local ordinances referencing suggested model codes

for other aspects of building regulation are an additional

source of differing code-enforcement policies and practices

in Washington and to a lesser extent in Oregon. Some jurisdictions incorporated provisions of the Uniform Code for

Abatement of Dangerous Buildings in ordinances about dangerous buildings. That code allows building officials discretion concerning dangerous buildings. A few building

officials reported using such ordinances to require demolition of earthquake-vulnerable buildings, but such actions

were rare. In addition, the ordinances of some jurisdictions

specifically refer to the Uniform Code for Building Conservation in establishing guidelines for renovation of buildings.

Another source of differences in local regulation of

existing buildings is jurisdiction-specific building codes.

Seattle has done the most in this regard (Perbix and Burke,

1989; Skolnik and Wood, 1975), beginning in 1973 with the

enactment of amendments to the Seattle Building Code,

which states:

In cases where total compliance with all the requirements of this building

code is physically impossible and/or impracticable, the applicant may

arrange a pre-design conference with the design team and the building official to identify design solutions which will provide equivalent protection.

The building official may waive specific requirements in this building code

which he/she has determined to be impracticable. (Sec. 104 of the 1985 edition of the Seattle Building Code)

The only other examples of locally adopted seismic policies for existing buildings among the jurisdictions in this

study were parapet provisions enacted in Tacoma and Seattle. (UBC seismic provisions concerning parapets and chimneys are in section 3704(c) of the 1988 edition.) Tacoma’s

ordinance was adopted after the 1965 earthquake (ordinance

17842, May 18, 1965). The ordinance made it possible to

declare buildings with unanchored parapets dangerous and

therefore a hazard requiring abatement. Seattle’s provisions,

in a section of the Seattle Building Code, are similar in making it possible to declare buildings with unanchored parapets

unsafe. There have been various spurts of enforcement of

these parapet provisions. Seattle, for example, reportedly

looked actively for problem parapets in 1975 after a large

chunk of a parapet fell from a now-demolished hotel. Some

200 building owners were cited for unsafe parapets at the

time.

Important differences in practices concerning existing

buildings involve negotiations for seismic-safety upgrading

of buildings undergoing renovation. Seattle’s building code

appears to codify the existing practice in many jurisdictions.

Seattle, Tacoma, and Portland appear to have done the most

to establish administrative procedures for negotiating seismic-safety requirements in building renovation. Portland

was particularly innovative in establishing a Structural Advisory Committee, composed of external structural engineers,

for advising the city’s building official about appropriate

requirements for design changes as part of major renovations. The volume of renovations of unreinforced masonry

buildings alone has been enough in these jurisdictions, about

20–30 per year in Seattle, 2 or 3 per year in Tacoma, and 20

per year in Portland, to justify having such procedures.

Within jurisdictions having less experience with major renovations of potentially earthquake hazardous buildings, the

practices are less formalized but are sometimes guided by the

Uniform Code for Building Conservation.

LAND-USE AND SECONDARY HAZARDS

The contrast between State-level land-use planning

mandates in Oregon and Washington is between a single, relatively strong Oregon land-use mandate and a set of relatively weak, more general environmental mandates in

Washington. Despite differences in the written mandates,

the differences in local practices between Oregon and Washington jurisdictions are not that great concerning seismic

hazards other than landslides.

The primary State-level land-use planning mandate in

Oregon relevant to earthquake hazards comes from provisions contained in legislation first enacted in 1973 as the Oregon Land Use Act. This legislation established 19 statewide

planning goals and required local governments to develop

comprehensive plans to be reviewed for consistency with the

statewide planning goals (Oregon Land Conservation and

Development Commission, 1986). Goal Seven is a statewide

goal “to protect life and property from natural disasters and

hazards.” The guidelines for Goal Seven call for identification of areas subject to natural hazards and the development

of “appropriate safeguards” as part of the planning process.

The Oregon mandate and associated administrative

guidelines provide discretion in implementing Goal Seven.

The Oregon jurisdictions in this study fully complied with
EARTHQUAKE RISK REDUCTION, PUGET SOUND AND PORTLAND, OREGON, AREAS 505

the State mandate provisions requiring a State-approved

local comprehensive plan. Goal Seven compliance appears

to be very strong with respect to flood hazards (in complying

with Federal Emergency Management Agency flood-insurance mandates), moderately strong for landslide hazards (in

identifying areas of potential landslides and considering

compatible land uses), and weak for other earthquake hazards. Planners cited lack of awareness of earthquake hazards

and difficulties in identifying areas other than steep slopes as

possibly subject to earthquake hazards as a primary limitation of their ability to apply land-use measures to earthquake

risk reduction. State personnel who review local comprehensive land-use plans reported very few plans in which

seismic hazards other than landslides were even mentioned.

The most relevant Washington State mandate for consideration of earthquake hazards is the State Environmental

Policy Act of 1971 (SEPA).12 This legislation and associated administrative regulations require cities and counties to

adopt procedures for environmental review and designation

of mitigating actions for certain categories of development

(for example, planned developments, annexations, shoreline

development) likely to have “significant” environmental

impacts. Because many categories of development are

exempted and the nature of mitigating actions is not specified, the SEPA process is not a very comprehensive

approach for regulating development in areas subject to seismic hazards.

Administrative amendments, enacted in 1984, to the

SEPA regulations allowed cities and counties to designate

“environmentally sensitive areas,” a procedure which would

allow for review of land-use and development proposals that

might normally be exempt from SEPA review. This more

recent SEPA provision has been the basis for several of the

“sensitive-areas ordinances” discussed in the following

section.

The Federal flood-insurance program, authorized by

Congress in 1968 and substantially strengthened in 1973 by

providing penalties for communities failing to participate,

provides a different set of mandates for local adoption of

ordinances for management of land use and development

within flood-prone areas. In this discussion, the floodinsurance provisions are relevant to localities potentially

vulnerable to earthquake-induced water waves (tsunami).

These provisions are potentially relevant to some coastal

communities in Oregon and Washington, but relatively few

of the relevant jurisdictions were included in this study

(Urban Regional Research, 1988).

One of the central points in evaluating policies addressing secondary earthquake hazards is that, with relatively few

exceptions, the relevant local ordinances do not prohibit

development in potentially hazardous areas; they only control it to some degree. The typical approach is to require

appropriate engineering in the form of special pilings or

strengthened foundations for structures along steep slopes or

in areas with expansive soils. This requirement is more or

less routine as many building officials reported acting daily

on geotechnical or soils reports they require under UBC provisions. More extensive approaches to mitigation of secondary hazards entail special review processes for development

in areas with steep slopes or that are vulnerable to other seismic hazards.

The most specific local policies concern landslide hazards. Nearly 60 percent of the cities reported having some

form of steep-slope or landslide-hazard ordinance. (All the

Oregon counties and more than half the Washington counties

reported having landslide regulations.) Those jurisdictions

without specific steep-slope ordinances cited UBC provisions regarding excavation and foundations (UBC sections

2905 and 2910) as one basis for allowing building officials

to request soils or geotechnical reports for proposed buildings. Excavation and grading ordinances based on appendix

Chapter 70 of the UBC serve as the basis for some local ordinances requiring mitigating actions in areas subject to landslide hazards. Other jurisdictions have specific steep-slope

ordinances that specify limits on construction and (or)

requirements for geotechnical reports and mitigating actions.

And yet other jurisdictions had more generic “sensitiveareas ordinances” that included special review and permit

processes for development in designated areas subject to

landslide hazards.

Liquefaction and subsidence hazards appear to be dealt

with less directly. The UBC excavation and foundation

provisions (section 2905) make general reference to “expansive soils,” thereby providing building officials with a basis

for requiring mitigating actions for areas subject to liquefaction and subsidence. However, building officials reported

considerably more difficulty in deciding when to require the

reports that trigger engineering solutions. Some officials

appear to be very aggressive (for example, in requesting

reports when in doubt), whereas others seem to be more

restrained.

Sensitive-areas ordinances are more comprehensive

approaches used by jurisdictions in establishing overlays on

land-use maps that designate areas subject to such hazards as

expansive soils, steep slopes, or subsidence. When development is proposed within the designated sensitive areas,

special review processes that include consultant reports or

_____________ 12Subsequent to completion of this study, Washington enacted in 1990

and further amended in 1991 the State Growth Management Act. This act

requires every county and city in the State to designate critical areas within

its planning jurisdiction, including areas with geologic hazards. Other provisions of the act require the faster growing counties, including those in this

study, to protect the designated critical areas from incompatible land uses.

In practice, the critical-areas provisions of the State Growth Management

Act supersede the sensitive-areas provisions of the State Environmental Policy Act. The State Shoreline Management Act of 1971 is less directly relevant to earthquake-risk reduction. It establishes a cooperative program

between the State and local governments under which specified local

governments with coastal or riverine shorelines must establish plans for

managing the shorelines.
506 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

other actions are required. The specific requirements for

development in sensitive areas vary among the ordinances.

Two different examples are the King County sensitive-areas

ordinance and Bellevue’s “natural determinants code.” King

County’s ordinance requires geotechnical reports and

negotiated mitigating actions. Bellevue’s code is more prescriptive in specifying such things as the type of footings

required for foundations of buildings in sensitive areas and in

establishing a transfer of development credits to other sites

for developers who agree to limit development in sensitive

areas. Personnel involved with both programs report noteworthy success in modifying development in areas having

steep slopes, moderate success in areas subject to subsidence,

and limited attention to other seismic hazards.

Seismic hazards are only indirectly considered as part of

longer range zoning or land-use decisions. Many Washington jurisdictions reported having comprehensive plans that

generally refer to seismic hazards. Oregon jurisdictions are

required by the State land-use planning mandate to evaluate

natural hazards as part of their planning processes. In practice, most Oregon jurisdictions’ land-use plans appear to be

fairly general evaluations of earthquake potential.

The only way that jurisdictions in this study appeared to

use land-use planning as a longer term response to seismic

risks was public land acquisition so that the acquired

property could remain as open space. For example, Bellingham planners cited their extensive open-space acquisition

program as a long-term means for addressing seismic risks.

In jurisdictions where such open-space acquisition was done,

seismic concerns were a relatively minor impetus. These

programs tend to be undertaken to protect wetlands, limit

vulnerability to landslides, protect marine bluffs, or to reduce

flood risk.

UTILITIES AND PORTS

The utilities and ports engineers who participated in this

study reported considering seismic hazards as normal practice in constructing new facilities or upgrading existing ones.

Good engineering practices were presumed to provide adequate seismic-risk resistance. Interviewees reported only

rare instances when seismic-risk considerations entered into

siting or land-use decisions. Only a few of the utilities

included in this study have initiated formal reviews of the

earthquake vulnerability of their systems.

The design of facilities for a water system or a port is

obviously very different than the design of a major building.

Utility personnel design a range of facilities including reservoirs, storage tanks, pumping stations, control centers, and

transmission lines. Port personnel design piers, cranes, loading and storage facilities, drydocks, and other ship repair

facilities. Because of the variety of facilities involved, ports

and utilities depend greatly upon experienced and knowledgeable engineering consultants for all aspects of facility

design. The larger ports and utilities have very sophisticated

engineering units and spend large amounts (several reported

hundreds of thousands to millions of dollars per year) on geotechnical and other engineering consultants for facility

design.

Engineering practices relating to seismic-risk considerations for such facility design might be considered more an

art than a science. Engineers who design such facilities do

not have a single reference source like the UBC to guide

sesismic-risk design. Instead, nearly a dozen professional

associations provide guidance or standards concerning

design elements for these facilities.13 “Good” engineering

results from knowing what guidelines might be helpful, what

seismic forces to design for, and how to extrapolate from

existing codes or standards to the particular design situation.

Experience and professional knowledge are at the heart of

this process.

This study provides little information about the adequacy of engineering practices for seismic safety in this

region. Utility and port engineering personnel who were

interviewed for this study reported confidence in the consultants they had worked with over the years. Earthquake hazards appeared to be only one of many engineering design

concerns, of less concern than high winds to many of the

respondents (particularly among ports). Although they

acknowledged potential vulnerability of parts of water systems or port facilities to seismic hazards, most interviewees

thought that seismic hazards were adequately addressed, particularly in newer facilities.

Only three utilities in this study, the Everett, Portland,

and Seattle water departments, have initiated formal reviews

of the seismic vulnerability of their systems. For two of these

utilities, an engineering director who had previous experience with seismic design considerations in California provided an impetus for the review. Some of the other utilities

had addressed seismic risk in isolated aspects of their systems, such as slope stability around some reservoirs, foundations in older pumping stations, and spillways for dams.

These seismic-risk-reduction efforts typically emerged from

facility reviews undertaken for other purposes.

Many water utilities reported ongoing renovation programs for their major pipelines and other water mains. In

some jurisdictions, the water system contains major elements

that are quite old. Tacoma has a water pipeline dating from

1915; Everett has one dating from 1929. Even newer pipelines constructed in the 1930’s or 1940’s were reported by

utilities for which cast iron and brittle cement piping is

common. Replacement of older pipes with ductile iron pipes

_____________ 13For example, guidelines prepared by the Technical Council on Lifeline Engineering of the American Society of Civil Engineering address oil

and gas piping systems, buried pipelines, and numerous other aspects of

lifeline engineering. The American Water Works Association has created

standards for welded-steel water-storage tanks. The American Concrete Association provides standards for concrete sanitary storage facilities used in

treatment plants. Other compilations of existing practices have been published by the Structural Engineers Association of California and by the

Building Seismic Safety Council.
EARTHQUAKE RISK REDUCTION, PUGET SOUND AND PORTLAND, OREGON, AREAS 507

and use of flexible piping for joints are common practices as

part of renovations. Although such renovations are primarily to reduce future maintenance costs, they also increase

seismic resistance.

Ports appear to be in a somewhat different situation

regarding older structures. Modernizing facilities has been

an important part of the business because of competitive

pressures. All the ports in this study had substantially

upgraded or replaced piers over the past few decades, except

for one older wooden pier used by one of the ports. Most of

the remaining older structures are warehouses that are

planned to be replaced or whose use is limited to lower valued purposes.

Perhaps the greatest potential seismic vulnerability for

ports is liquefaction. Port facilities in this region are mostly

built on alluvial plains and extensive filled areas. The primary response to the liquefaction hazard has been incorporation of the potential in engineering designs. The liquefaction

hazard was seldom cited by interviewees as an important

land-use consideration. The examples respondents cited

were of a proposed warehouse and other buildings for which

more suitable sites were selected. The engineering directors

commenting on these siting decisions were confident that

engineering designs could have been developed to address

the hazard, but it made more economic sense to put the buildings elsewhere. Such choices are more limited when it

comes to the design of piers or other water-dependent elements of port operations.

THE POLITICAL AND ECONOMIC 

ENVIRONMENT FOR RISK 

REDUCTION

Risk reduction policies and practices are shaped by a

host of local political and economic factors that are

influenced by State and other mandates. Earthquake risks

and the political-economic situation of the jurisdictions

studied vary considerably. In the following discussion, cities

and counties are categorized according to existing and

potential risk-reduction efforts.

RELATIVELY VULNERABLE CITIES

Thirteen cities in the study can be classified as relatively

vulnerable to earthquake hazards (table 46). The vulnerability of these cities is a function of the scale of existing development and of population size. Consideration of existing

earthquake-risk-reduction practices and the political-economic environment for future risk-reduction efforts leads to

further delineation of three categories of future risk-reduction

prospects among the 13 more vulnerable cities (table 46).

The three largest cities in the region are in this grouping. The population of this group of cities has been fairly stable (median in 1987 of 44,000) with a median population

growth from 1980 to 1987 of less than 2 percent. Valuable

buildings are being constructed in these cities (median 1987

commercial building-permit values of about $20 million

with a range of $1 million to $500 million). However, the

value of new construction is a relatively small percentage of

the total value of the building stock.

The extent of existing development and lack of substantial population growth of these cities have four main implications:

• Unreinforced masonry buildings, reinforced concrete

frame buildings built before the 1960’s, and concrete

tilt-up buildings built before the mid-1970’s constitute considerable potential risk

• The potential for property loss is substantial because of the

amount of development in these cities
508 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

• Land-use policies and practices with respect to seismic

hazards are relatively weak at present and are likely

to be relatively ineffective in the future. There is very

little undeveloped land for which to apply traditional

land-use risk-reduction measures

• The degree of earthquake-risk reduction in these areas is

and will be largely determined by policies and practices concerning existing buildings and their

renovation

PACESETTER CITIES

This category comprises the three largest cities in the

region, Portland, Seattle, and Tacoma (average 1987 population of 360,000). These cities have the most knowledgeable

building departments in the region and the most experience

in negotiating renovation of existing potentially hazardous

buildings. Seismic provisions in the building codes of these

cities date to the late 1940’s or early 1950’s, although these

early provisions were very minimal compared to today’s

standards. These cities are also leaders in developing advisory groups for negotiating the particulars of seismic-resistant building renovations, in establishing parapet ordinances,

and in Tacoma, instituting a strong-motion instrument program for measuring earthquake forces in buildings.

The capabilities and experience of the building departments in these cities provide confidence that these cities are

doing as good a job as any major city in the country in assuring adherence to current seismic standards for new construction.14 Because these cities have their own building codes,

local strengthening of code provisions is possible, except in

Portland whose code is constrained by State law.

Modification of codes would not necessarily be done

readily. One policy official, incorrectly referencing current

practice, commented about the U.S. Geological Survey

hazard assessment program:

We’ve got conservative codes. They provide considerable protection.

Right now, we’re designing to what California has been designing for the

past 20 years. But we’re not going to start changing the codes just because

of some new reports. There’d have to be considerable evidence before we

make any changes. And if seismology has become an exact science, that’s

news to us.15

Building officials interviewed from these cities as a

group gave the highest ratings for the likelihood of potential

significant damage to buildings in their cities from moderate

earthquakes (mean of 50 for the 0 to 100 scale used in the

building officials’ risk assessment) or major earthquakes

(mean of 80 for the 0 to 100 scale). In contrast, elected officials from these cities appeared to have the same degree of

concern (or indifference) to earthquake hazards as officials

from other cities. The political-economic environment is

such that building officials for these jurisdictions doubted

that retrofit programs requiring seismic upgrading could be

instituted. As one official stated: “We have kicked the idea

[of a retrofit program] around at administrative levels, but it

won’t fly politically, and there are technical difficulties in

establishing appropriate standards.”

These cities tend to be much less innovative with traditional land-use practices such as the use of overlay zones to

land-use plans, open-space zoning, or other measures to limit

seismic risk. The comment of one planning director summarized the approach of this category of cities: “Besides landslide controls, we have done very little [in the way of landuse measures]. The building department is in the driver’s

seat since they handle geotechnical reviews and building regulation.”

RESOURCEFUL CITIES

This category (table 46) comprises five cities of moderate population (average 1987 population 42,000) that are distinguished more by having initiated earthquake-riskreduction practices than by having innovative seismic-hazard-reduction policies or sophisticated building departments.

For example, Aberdeen personnel have participated in tsunami-warning studies, and at the time of our interviews were

funding a study of risks posed by unreinforced masonry

buildings. Bremerton officials have condemned potentially

earthquake vulnerable buildings as unsafe and have

upgraded school buildings for seismic safety (both undertakings were apparently controversial for the personnel

involved). The building departments of Olympia, Renton,

and Salem reportedly have been relatively aggressive in

enforcing seismic-risk-abatement requirements, particularly

regarding homes.

The label “resourceful” is applied to this category of cities because the impetus for risk reduction appears to have

come through individual efforts to affect building practices

rather than from external mandates or policy decisions. In

most instances, a building official willing to endure criticism

for being too stringent was a key factor in initiating riskreduction efforts. For this category of cities, a strong base of

support for pursuing risk reduction seems to exist in relevant

administrative levels of government. However, the political

climate, broadly defined in terms of elected officials and the

building community, and the economic climate do not necessarily support further risk-reduction efforts. As one official

said, “This jurisdiction struggled to institute its only retroactive ordinance—requiring smoke detectors in certain situations.” Because of the tenuousness of these risk-reduction

efforts, there is a potential that they could decline, moving

these cities into the “restrained” category (see following) of

___________ 14The Building Seismic Safety Council trial-designs review of

implications of designing different classes of new buildings according to the

council’s recommended provisions showed that following these new provisions in Seattle would result in lower construction costs than those incurred

by following current Seattle building codes (Webber, 1985). Tacoma and

Portland were not included in the trial-design program. 15Statement of the Director of Seattle’s Department of Construction

and Land Use, quoted from “Seattle Could Face ‘Great’ Quake But Building

Officials Skeptical,” Seattle Business Journal, September 10, 1984, p. 20.
EARTHQUAKE RISK REDUCTION, PUGET SOUND AND PORTLAND, OREGON, AREAS 509

relatively vulnerable cities. Practices change quickly as new

directions are established by elected officials, as key

employees leave, or as budgets are further constrained.

RESTRAINED CITIES

This category comprises five moderately populated

(1987 average population 38,000), low-growth cities for

which State-mandated actions appear to be well integrated

with building regulatory and planning practices, but sustained earthquake-specific risk-reduction policies or practices have not been initiated. Yet, these communities have

potentially noteworthy earthquake risk because of the nature

of the building stock.

Many respondents from cities in this category indicated

that earthquake-risk-reduction initiatives are constrained by

political-economic circumstances that vary among jurisdictions, for example, resistant policy officials, unreinforced

masonry buildings with concentrated ownership, vacant

buildings that are not being renovated because of depressed

economic conditions, and so forth. A component of the

restrained response to seismic risk is the relatively depressed

economies of these cities, as reflected by an average population growth from 1987 to 1988 of less than 1 percent and a

1987 median value of building permits issued for apartments, office buildings, and industrial buildings of only

about $3 million.

The challenge for future risk-reduction efforts for this

group of cities is building both a supportive political and

economic climate and the capacity to undertake needed measures. As a group, elected officials of these five cities gave

the lowest average ratings of the likelihood of significant

deaths, injuries, or damage in the next 20–30 years from a

moderate earthquake (average score 19) as well as the lowest

average ratings of the likelihood of significant losses from a

major earthquake (average score 13). These ratings are

about one-half the corresponding averages reported in the

other two categories of relatively vulnerable cities.

LESS VULNERABLE CITIES

The category of less vulnerable cities includes 30 cities

for which earthquake risks are lower when compared with

the more highly populated, more developed cities discussed

previously. This lower risk is relative, as these cities still

have considerable earthquake risks. As for the relatively

vulnerable cities, this less vulnerable group can be further

differentiated into three categories (table 47) in terms of

existing risk-reduction practices and the general political and

economic evironment.

The median population as of 1987 for these cities was

19,100. Newer development is indicated by the rapid growth

rates of these cities, which had a median population growth

rate of 13 percent and a median growth rate in commercial

building permits of 19 percent from 1980 to 1987. The

recent development of these cities and the lower population

sizes have several implications in comparing these less vulnerable cities to the relatively vulnerable cities:

• Although there is an existing hazard posed by tilt-up concrete buildings and unreinforced masonry buildings

in at least some of the cities, the rate of development

forces attention to seismic considerations for new

construction
510 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

• Because most of these cities still have vacant land for

further development, land-use measures can be used

to reduce risk

As many of these cities cope with growth-control

issues, land-use regulation is especially volatile. Thus both

opportunities and problems exist: opportunities for linking

earthquake risk reduction to growth controls and problems in

adding risk-reduction measures to a highly charged political

environment.

NEW-SOPHISTICATE CITIES

This category comprises 10 of the fastest growing moderate-sized cities in the region. The average 1987 population

was 37,000, and the average population growth from 1980 to

1987 was 32 percent. The average growth in value of building permits issued for apartment, commercial, and industrial

buildings during this period was 105 percent. The rapid

growth, accompanied by development of commercial and

manufacturing facilities in many of these cities, has stimulated an increase in the capacity of building and planning

departments. Building and planning departments in these

cities are relatively advanced compared to those of similarsized cities of no or little growth.

Private engineering and design consultants have been

important in increasing the sophistication of new construction within these cities. For example, Gresham city building

department employees described how Japanese electronic

firms and their California structural consultants brought

them greater understanding of the need for seismic design.

Auburn, Bellevue, Beaverton, Kent, Kirkland, Redmond,

and Tigard have all had substantial growth in commercial

development. Mercer Island and Lake Oswego are more residential in character and have lower growth rates. Their

increased attention to seismic risk comes from a combination

of building-department awareness and more custom

designed homes than most communities have.

Because of the high growth rate of the cities in this category, land-use issues are more controversial for this category. All of the cities have some form of steep-slope

regulation, typically involving some form of map-overlay

zone designating areas where geotechnical reports are

required prior to development. This category in general has

the greatest potential for linking future land-use risk-reduction measures to growth controls.

MEASURED CITIES

This category (table 47) consists of eight smaller moderate-growth cities for which State-mandated earthquakerisk-reduction efforts appear to be integrated into normal

practice. However, other risk-reduction policies or practices

are limited. The primary reasons for designating these cities

as less vulnerable are the smaller populations and lesser

value of new commercial development. The average 1987

population for these eight cities was 16,500. Most of them

have at least a few unreinforced masonry buildings in historic

downtown areas. Building officials of cities in this category

gave a somewhat higher average rating of the likelihood of

significant damage from a major earthquake (57 on the 0 to

100 scale) than the corresponding rating of building officials

in the new-sophisticate category (average rating of 39). The

reported seismic-risk perception of elected officials was

about the same for the two categories.

Cities in this category have potential for joining the

new-sophisticate category, but that potential appears to be

limited by economic circumstances. These cities do not have

the new commercial development or redevelopment that the

new-sophisticate category has. The median value of building

permits issued in 1987 for apartment, commercial, and industrial buildings was $1.3 million, compared with a value for

the “new sophisticates” of $17.3 million. These cities are not

as sophisticated in their approach to land use and its relationship to risk reduction as are the “new sophisticates.” Because

growth is not as great in cities of this category, there has been

less pressure to develop land-use controls that concurrently

reduce earthquake risk.

Relevant city personnel seem to have a reasonably

strong recognition of earthquake risks. However, their ability to act is severely constrained by economic circumstances.

A change in economic circumstances, which might move

some of these cities into the “new sophisticates” category, is

difficult to forecast.

NONPLAYER CITIES

This category (table 47) comprises somewhat smaller

cities (average 1987 population 16,000) for which seismic

hazards appear to be of lesser concern to both elected and

building officials. The “nonplayer” name indicates the perception of officials in these cities that seismic risks are not

particularly relevant to them. The label does not imply that

these cities are not enforcing seismic components of building

codes or following reasonable planning practices.

Building officials for nonplayer cities had the lowest

average ratings for the likelihood of significant losses from a

moderate earthquake (14 on the 0 to 100 scale) and from a

major earthquake (average rating of 30): These ratings are

about one-half the corresponding ratings of building officials

in each of the other categories of cities. Whether relative

indifference is justified is difficult to judge—thus the need

and purpose of the overall U.S. Geological Survey hazardassessment process. Building officials from these cities

consider their areas at lower risk because they are primarily

residential, comprise wood-frame buildings, and have only

newer commercial development.
EARTHQUAKE RISK REDUCTION, PUGET SOUND AND PORTLAND, OREGON, AREAS 511

The challenge for future risk-reduction efforts for this

category of cities is to document the extent of earthquake

hazards and then, if the documented hazard is serious, to

convince relevant personnel that the hazard should be

addressed.

COUNTIES

Relevant county risk-reduction policies and practices

apply, with a few minor exceptions, only to unincorporated

areas within the counties. Compared to their municipal

counterparts, the unincorporated areas have lower population, less developed building stocks, and much lower densities of both population and structures. Although these

factors no doubt have produced a lower sense of earthquake

vulnerability, the recurring flooding and landslides in unincorporated areas of many counties are reminders that these

areas are not insulated from natural hazards.

The unincorporated areas of the counties in the study

area are undergoing the most change. Unincorporated

fringes of urban areas are rapidly becoming urbanized, and

rural areas are rapidly becoming suburbanized. The extent

of development of the counties and how they have responded

to increased pressure for both building and land-use regulation define the three categories of risk-reduction prospects

for counties discussed in this section (table 48).

LEADING COUNTIES

This category (table 48) comprises the three most populated counties that surround the largest urban areas in the

region. Nearly 1 million people (1987 estimate) live in unincorporated areas of these counties. The growth in these

unincorporated areas can be characterized as increased

urbanization accompanied by development of commercial,

professional, and light manufacturing facilities. The counties are called “leaders” because they have relatively older

building codes (since the 1950’s and into the 1960’s), relatively larger building departments than other counties, and

some land-use provisions concerning seismic hazards.16

Compared to the other two categories of counties, this

category has a greater stock of existing, potentially earthquake vulnerable buildings. These buildings are primarily

tilt-up concrete buildings constructed before the mid-1970’s.

Building officials for these counties rated the likelihood of

significant damage from a moderate earthquake (average rating of 48 on the 0 to 100 scale) as twice as likely as the corresponding rating for the other categories of counties. They

rated the likelihood of significant damage from a major

earthquake (average rating 65) as 1.5 times greater than the

other two categories of counties. These counties have large

building departments with experience in reviewing complex

structures, but the in-house structural-review capacity (that

is, the number of structural engineers on staff) is more limited than for the counterpart “pacesetter” cities.

The leading counties have paid more attention to landuse considerations than have “pacesetter” cities. Each

county in the “leader” category has some form of landslide

land-use controls in effect. King County has an ordinance

that is commonly cited as an innovation in regulating landuse practices for risk reduction and preservation of environmentally sensitive areas. Land-use issues have been important considerations in debates about development, and

seismic risk was considered in some decisions. For example,

seismic factors relating to landslides reportedly were considered in Multnomah County’s decision to deny a permit for a

metropolitan-area landfill. King County officials have been

embroiled in debates over delineation of environmentally

sensitive areas. As one means of controlling growth, these

counties are moving toward strengthening land-use policies

that may also have beneficial side effects for earthquake risk

reduction. These improved policies provide the best prospects for addressing earthquake risks in these counties.

TRANSITIONAL COUNTIES

This category (table 48) of 10 counties comprises those

unincorporated areas for which growth has entailed the

greatest transitions. Large unincorporated areas in these

counties are rapidly changing from rural to suburban. The

median growth rate in population of unincorporated areas of

these counties from 1980 to 1987 was 17 percent. Building

and planning departments are trying to catch up with the

growth rate by increasing their capacity to address future

growth pressures.

For some of these counties, catching up has consisted of

establishing building departments and instituting land-use

practices. As noted by interviewees:

• “In 1974 over one-half of the county was unzoned and

there was no building code.”

• “There was no building code until 1971. Since then,

enforcement has been progressive. You can’t just

institute such regulation overnight.”

• “Our first major commercial building was built in 1980.

There was no county-wide zoning until 1980.”

Other counties in this category are much further along

in developing building departments and in land-use planning. For these, the main issues have been strengthening

building-code enforcement and land-use practices.

Mixed reaction to growth pressures is shown by the

variation in the status of current building regulatory and

land-use practices. For example, a proposed comprehensive

land-use plan for Pierce County that contained zoning and

steep-slope provisions was defeated in a county-wide

____________ 16Because of the urbanized nature of Multnomah County, building regulation is somewhat different from that of the other two counties in this category. The incorporated areas that make up the county regulate building

practices.
512 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

referendum in 1986. Many voters apparently thought the

plan went too far in regulating land use. Planning officials in

three transitional counties specifically mentioned political

pressures to go slow in regulating growth.

The future prospects for risk reduction appear to be

somewhat mixed for this category of counties. Growth

issues will inevitably continue to be an important factor

pressing future land-use and building regulatory decisions.

However, the political climate, broadly defined, does not

necessarily support strong land-use regulation.

RURAL COUNTIES

This category (table 48) consists of seven counties in

which growth is occurring but whose unincorporated areas

have remained predominantly rural. Officials in these counties have thus not had to confront the growth issues that have

affected policies and practices of the other two categories of

counties. One building official noted that “we have not had

a subdivision of any kind,” and another pointed out that “this

is a one-man building department.” The median population

growth rate of unincorporated areas of these counties was 5

percent from 1980 to 1987. The median population of unincorporated areas of these counties was 11,600 in 1987.

Despite the rural nature of these counties, they have had

some noteworthy experiences with natural hazards. Grays

Harbor County officials evacuated 14,000 people in 1986 in

response to a tsunami warning (flooding but no tsunami

occurred). Jefferson and Skagit County officials have been

involved in lawsuits stemming from deaths caused by landslides in logged areas. Cowlitz County officials have been at

the center of a series of negotiations concerning flood and

debris control relating to Mount St. Helens. Jefferson

County contains Port Townsend, an incorporated area with

many unreinforced masonry buildings.

The constraints for future risk-reduction efforts for this

category of counties appear to have less to do with recognition of risks than with the costs involved. The exception to

this observation are the Oregon counties in this group for

which there is little perceived seismic risk. The rural

counties simply do not have resources to undertake more

extensive risk-reduction efforts.

FUTURE EARTHQUAKE-RISK 

REDUCTION STRATEGIES

The preceding discussion provides a basis for considering risk-reduction strategies that might be undertaken as new

information about earthquake risks is developed for this

region. Ongoing research may provide a better understanding of the earthquake hazard in this region. Assumptions

about presumed degree of risk will change dramatically if

research indicates a high probability of a great (M>8.0) earthquake. Such information is potentially relevant to all jurisdictions studied, especially the “nonplayer” category of

cities.

Prospective risk-reduction strategies range from a relatively passive one of hazards-information dissemination to

more active efforts to seek policy reforms or to influence practices (table 49). These strategies are not mutually exclusive:

pursuing one does not preclude pursuing another. However,

because of limited resources, choices must be made as to the

strategies to emphasize. Evaluating the strengths and limitations of the strategies entails envisioning the likely responses

to the implementation of each strategy. The following assessments are based on the understanding of the political and

economic environment described previously.
EARTHQUAKE RISK REDUCTION, PUGET SOUND AND PORTLAND, OREGON, AREAS 513

DISSEMINATE HAZARDS INFORMATION

This strategy consists of dissemination of new scientific

information about earthquake hazards in the region through

professional publications, newsletters, and meetings. The

information might be presented as hazards maps prepared at

a scale that would show information relevant to at least the

major jurisdictions in the region. Demonstration uses of

Geographical Information Systems that translate hazards

into the risks posed for people or structures might also be

undertaken.

One strength of hazards-information dissemination is

that it is fairly easy to implement. Several activities are

already being jointly undertaken by the Federal Emergency

Management Agency and the U.S. Geological Survey. This

approach assumes that credible scientific information is

essential for making the case for revising building regulatory

or land-use policies.

The key uncertainty for this strategy concerns who is

likely to act upon the new information. Although the details

depend on both the substance and the form of the information, this study suggests that only a few categories of cities

and counties are either capable of acting or willing to act

upon such information without further external efforts to

influence risk-reduction practices. Only the “pacesetter” and

“new-sophisticate” cities, and “leading” counties appear to

have both the technical in-house capacity and at least some

willingness to respond to such information. The “nonplayer” cities may respond to new information about earthquake risks, but to get them to respond with risk-reduction

measures would probably take more than simply providing

the information. For the other categories of cities and counties, too many other factors constrain response.

Other potentially responsive audiences are attentive

professional groups such as the structural engineers associations of Washington and Oregon. These groups effectively

translate the information into new engineering or design

practices. Depending on the extent of change in risk

assumptions and the credibility of the information, the professional groups might use the information to lobby State

building agencies and private code-writing authorities for

changes in building standards.

Clearly, better hazards and risk information is necessary. However, this study of risk-reduction prospects suggests that mere dissemination of such information, without

other efforts to influence policies or practices, will produce

limited reduction of earthquake-hazard risk. The more

capable cities and counties may make use of the information, whereas little will likely happen in the remaining

jurisdictions. Some structural engineers, geotechnical consultants, and others who design structures may use the new

information as part of their practice.

SEEK MANDATE REVISIONS

This strategy consists of directly seeking revisions in

State-level building-code or land-use mandates. The

specific changes sought will depend upon the nature of the

information developed from the scientific research. These

revisions might include new seismic-zone delineation, new

design standards, special code provisions for particular
514 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

categories of buildings, or better delineation of seismic hazards within land-use mandates. Building codes could be

changed either by State amendments or through the coderevision process of the International Conference of Building

Officials. Neither procedure would be easy, and both would

require considerable technical justification.

Because State building-code and land-use mandates

establish the foundation for local risk-reduction policies, the

mandate-revision strategy could lead to desired changes in

local policies. Moreover, the changes would be more or less

uniform within each of the States. However, this strategy for

influencing risk reduction has three main limitations. The

first is the difficulty of achieving changes, particularly

signficant ones, in State building codes, private codes, or

State land-use mandates. The second limitation is that

changed policies will only address future development and

construction. Even with substantiation of sizeable risks,

retroactive State-level policies concerning seismic-risk

review or retrofit of existing, potentially hazardous buildings

are very unlikely to be enacted in Oregon or Washington.

The third limitation is that although local policies closely

mirror State mandates, local practices still vary considerably.

Implementation of the policies and the discretion used by

building and land-use officials depends on the broader

political and economic environment.

In sum, appropriate changes will likely be made for new

construction (or renovation) throughout the region. To the

extent that the new provisions allow discretion in interpretation or implementation, there will be a varied response that

reflects differing political and economic factors among the

categories of cities and counties. Overall risk reduction in

the region can be expected to advance, but the categorization

of cities and counties is unlikely to change.

INFLUENCE LOCAL GOVERNMENT

PRACTICES

This strategy consists of efforts to influence the practices of building and planning departments in carrying out

State mandates and local policies. This influence might

entail providing jurisdiction-specific seminars on seismic

risk, preparing guidelines for using discretionary buildingcode and land-use judgments, providing technical assistance

in land-use planning or construction-plan review, or funding

geologists or structural engineers as part of local staffs.

These actions could be targeted to specific jurisdictions or

classes of jurisdictions.

This strategy has several strengths. The targeting of

assistance would respond to the varied situations of jurisdictions in this region. The emphasis on practice, as opposed to

policy, would influence important discretionary judgments

concerning such things as renovation of buildings. Also, several model risk-reduction efforts could be developed that

could potentially be transferred to other jurisdictions.

The limitations of this strategy result from the constraints imposed by State mandates on local exceptions to

those mandates. The Washington State building code allows

local jurisdictions to enact stronger provisions than State

mandates with appropriate State-level review. The Oregon

State building code does not permit exceptions. Therefore,

the effectiveness of this strategy is a function of the amount

of discretion that exists within existing codes. This strategy

would improve risk-reduction efforts the most among those

jurisdictions in which current risk-reduction efforts are the

weakest. It will do little in the “pacesetter” or “new

sophisticate” cities, or in the “leading” counties.

INFLUENCE PRIVATE PROFESSIONAL 

PRACTICES

This strategy consists of efforts to influence the practices

of the private-sector engineering and building-design community. This influence might entail providing special seminars on seismic risk and earthquake engineering, funding

creation of special guidelines by professional associations, or

providing some other form of professional development

opportunities. These actions could be targeted to different

types of engineers and building-design professionals.

The strength of this strategy is in the prospective direct

influence on the design and engineering recommendations of

this community in reducing earthquake risks. The results are

likely to be greatest in those sectors that rely extensively on

the judgments of design and engineering professionals, such

as the utilities and ports in this study. Indirect benefits of

changes in practices or knowledge might lead to interest in

lobbying for code changes.

Obvious implementation difficulties for this strategy are

identifying and reaching appropriate professionals and then

convincing them of the need for changes in practice. If these

difficulties were overcome, the main limits to the effectiveness of this strategy are the constraints under which the

design and engineering community practices its professions.

Without code changes, competitive pressures and client

desire to reduce costs may restrict the extent to which practices exceed minimum code requirements. The main

beneficiaries of this strategy may be the professionals and

supportive clients who already are doing the most to address

earthquake risks. Insurance companies and financial institutions could potentially be important in endorsing or requiring

new seismic-design practices. However, competition and

other factors have limited the influence of insurance companies and financial institutions in stimulating stronger riskreduction efforts.

CONCLUSIONS

The results of this study of earthquake-risk perceptions,

existing policies and practices, and the political and
EARTHQUAKE RISK REDUCTION, PUGET SOUND AND PORTLAND, OREGON, AREAS 515

economic environment allow substantially different interpretations of the current state of earthquake risk reduction in

the region. A positive assessment includes the general

awareness of damage potential by building officials, the relatively advanced building and land-use policies in some of

the more populous jurisdictions, and the growth in earthquake-engineering experience among private design professionals in this region. A negative assessment might include

the seeming indifference of elected officials to earthquake

risks, the limited attention to seismic hazards of several categories of cities and counties identified in this study, and the

inevitability of a major earthquake that building officials

acknowledge will likely lead to significant losses.

Also, any summary evaluation is likely to mask the

variation in situations that was discussed in this report. The

discussion not only shows the extent to which jurisdictions

have different relative risks, but also demonstrates the

importance of considering political and economic factors

when designing future risk-reduction strategies. The current

situation and future prospects for risk reduction are very

different between “pacesetter” cities and “restrained” cities,

and between “leading” counties and “rural” counties.

SELECTED REFERENCES

Berke, Philip, and Wilhite, Susan, 1988, Local mitigation planning

response to earthquake hazards—Results of a national survey:

College Station, Texas, Texas A&M University, Hazard

Reduction and Recovery Center, 37 p.

Council of American Building Officials, 1986, CABO one and two

family dwelling code: Falls Church, Va., 342 p.

Drabek, T.E., Mushkatel, A.H., and Kilijanek, T.S., 1983, Earthquake mitigation policy—The experience of two states: Boulder, Colo., University of Colorado Institute of Behavioral

Science, 238 p.

Friday, W.M., 1977, Oregon’s experience in statewide code uniformity—Third year, in Cooke, P.W., ed., Research and innovation in the building regulatory process: U.S. Department of

Commerce, National Bureau of Standards Special Publication

473, p. 225–240.

International Conference of Building Officials, 1988, Uniform

building code, 1988: Whittier, Calif., 885 p.

May, P.J., and Noson, L.L., 1986, Reducing earthquake

risks—Seismic safety policy: Seattle, Wash., University of

Washington, Institute for Public Policy and Management,

Washington Public Policy Notes, v. 14, 6 p.

May, P.J. and Williams, Walter, 1986, Disaster policy implementation—Managing programs under shared governance: New

York, Plenum Publishing, 198 p.

Mushkatel, Alvin, and Nigg, J.M., 1987, Opinion congruence and

the formulation of seismic safety policies: Policy Studies

Review, v. 6, p. 645–656.

Northwest Institute for Historic Preservation, 1985, Seismic hazards in unreinforced masonry buildings in the Pacific Northwest: Seattle, Wash., Northwest Institute for Historic

Preservation, 85 p.

Oregon Building Codes Agency, 1988, Oregon directory of responsibilities for building code enforcement—1988–89: Salem,

Oreg., Building Codes Agency, 114 p.

Oregon Land Conservation and Development Commission, 1986,

Oregon’s statewide planning program: Salem, Oreg., Land

Conservation and Development Commission, 24 p.

Perbix, T.W., and Burke, Padraic, 1989, Toward a philosophy for

seismic retrofit—The Seattle experience: Earthquake Spectra,

v. 5, p. 557–569.

Skolnik, A.M., and Wood, E.G., 1975, Preservation legislation for

Pioneer Square historic district, in Preservation and Building

Codes: Washington, D.C., National Trust for Historic Preservation, p. 41–46.

Urban Regional Research, 1988, Planning for risk—Comprehensive planning for tsunami hazard areas: Seattle, Wash., Urban

Regional Research, 246 p.

Washington State Building Code Council, 1989, A study of code

enforcement in Washington State—Report to the Governor

and Washington State Legislature: Olympia, Wash., Washington Department of Community Development, 9 p.

Webber, S.F., 1985, Cost impact of the NEHRP recommended provisions on the design and construction of buildings, in Social

implications—Selected readings, FEMA-84: Washington,

D.C., U.S. Federal Emergency Management Agency, p. 1–19.
517

INTEGRATED TSUNAMI-HAZARD ASSESSMENT FOR 

A COASTAL COMMUNITY, GRAYS HARBOR, 

WASHINGTON

By Jane Preuss1 and Gerald T. Hebenstreit2

ABSTRACT

The Grays Harbor region of southwestern Washington

lies at the east edge of the Cascadia subduction zone. This

study was conducted to examine the potential hazards to

Grays Harbor arising from tsunamis generated just offshore

by possible thrust earthquakes involving the Juan de Fuca

plate. A series of numerical simulations were conducted to

delineate the effects of tsunamis, either in the form of flooding or as high waves, in the populated areas around the shores

of Grays Harbor and along the outer coast to the north and

south of the harbor. Once the basic tsunami threat potential

was identified, the soil structure and land-use patterns in and

around Grays Harbor were examined to establish what hazards could arise from secondary tsunami and earthquake

effects. These effects include damage to storage containers

due to wave impacts, collapse of coastline structures due to

wave scour and erosion, and flotation of storage containers

(both moored and free standing) due to flooding. Attention

was given to the possibility of the coincidence of tsunami

waves and riverine flooding due to runoff. In the analysis,

the Grays Harbor area was represented by microzones, each

characterized by the type of hazard peculiar to its soil and

land-use patterns. The study shows that such an integrated

hazard-analysis methodology can be of great value in helping communities to plan for specific disasters rather than a

broad spectrum of possibilities.

INTRODUCTION

This study was conducted to develop and apply a methodology for making an integrated hazard assessment that

treats an earthquake-generated tsunami as the initiator of a

suite of interrelated hazards rather than as the sole threat to a

community. Through such an integrated approach to hazard

assessment, relatively accurate loss estimates and subsequent

mitigation efforts can be made effectively.

Because vulnerability to specific risk factors varies from

community to community, a risk-based urban planning

approach was developed that balances the needs of waterfront

activities (industrial and resort) with safety and preparedness

requirements in coastal areas vulnerable to tsunamis and

earthquake-induced flooding. The study had four objectives:

1. Identify the threat of tsunamis. Characteristics and

dimensions of the potential tsunami threat to a coastal community were defined. Numerical simulations of locally generated tsunamis arising from offshore earthquakes were used

to define the direction of energy and wave heights.

2. Delineate the vulnerability zone. Patterns of vulnerability to tsunamis were defined based on land use and population distribution. Field work was used to make these

determinations.

3. Identify secondary hazards. Secondary hazards that

could result from earthquake ground motion and (or) impact

of a tsunami or flood were defined. Specific attention was

directed to the potential for release of hazardous materials. A

combination of field inventory, multidisciplinary data analysis, and an air-dispersion model was used.

4. Microzonation. Primary and secondary hazards were

correlated with vulnerability patterns, and a system of microzonation was proposed. Interactive analysis was used.

Grays Harbor, Wash. (fig. 236), was the location used to

characterize earthquake-hazard risks along the Washington

coast and to calculate the geographic area of vulnerability to

earthquake hazards. The study area generally corresponds to

an area of sand lenses discovered on the outer Washington

coast at Willapa Bay and Grays Harbor (Atwater, 1987;

Bourgeois and Reinhart, 1988) that has been interpreted as

having been subjected to multiple tsunami impacts from great

subduction events in the past.

ACKNOWLEDGMENTS

This study was sponsored by the U.S. Geological Survey

(Grant No. 14-080001-G1346) under the Earthquake Hazards

_________ 1Urban Regional Research, Suite 1000, Tower Building, 1809 Seventh

Avenue, Seattle, WA 98101.

2Science Applications International Corp., McLean, VA 22102.
518 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Reduction Program and was based on a project conducted

jointly by Science Applications International Corp. and

Urban Regional Research. The authors thank their two

reviewers, Peter May and James F. Lander, whose comments

served to significantly strengthen the presentation of the

results of the project.

BACKGROUND OF AND APPROACH 

TO THE STUDY

In the past decade, considerable interest has developed

in the possible occurrence of major subduction-related earthquakes along the Juan de Fuca plate boundary in the Pacific

Northwest (for example, see Heaton and Hartzell, 1986,

1987; Heaton and Kanamori, 1984; Rogers, 1988). Evidence

from recent investigations (Bourgeois and Reinhart, 1988;

Atwater, 1987) and from Indian legends (Heaton and Snavely, 1985) indicates that the coasts of Washington and Oregon are vulnerable to tsunamis. Atwater (1987) reported at

least six episodes of coastal subsidence due to earthquakes in

the last 7,000 years, with vegetated coastal lowlands being

buried by intertidal mud. In three of the episodes, patterns of

sand sheets lying atop the buried lowlands could be

explained by inundation due to tsunamis and the resulting

shoreward transport of sand. Other research (Reinhart and

Bourgeois, 1987; Atwater and others, 1987; Johnson, written

commun.) cited additional evidence for subsidence and possible tsunami-related flooding in the past 1,000 years.

Although the hazards most commonly expected from

major earthquakes are ground movement and failure attributable to the seismic motion, an earthquake occurring offshore

in a subduction zone always has the potential to generate a

destructive tsunami, which could cause considerable damage

JUA

N

DE FU

CA

PLATE

126° 124° 122°

49°

48°

47°

46°

Seattle

Tacoma

Olympia

Aberdeen

Longview

Astoria

Vancouver

Portland

Cannon Beach

PACIFIC

OCEAN

Strait of Juan de Fuca

Grays

Harbor

VANCOUVER

ISLAND CANADA

UNITED STATES

WASHINGTON

OREGON

Columbia River

0 50 100 KILOMETERS

Study area

Figure 236. Index map showing the Grays Harbor study area (shaded). Heavy dashed line

indicates shoreward boundary of Juan de Fuca plate. The south Cascadia portion of the

Cascadia subduction zone extends from just north of Grays Harbor to southern Oregon.
INTEGRATED TSUNAMI-HAZARD ASSESSMENT, GRAYS HARBOR, WASHINGTON 519

to inhabited areas of coastlines. Tsunamis in 1946, 1952,

and 1957 originated in the Aleutian Islands region of the

North Pacific; a 1960 tsunami originated in Chile; and a

1964 tsunami resulted from an Alaskan earthquake. The

recorded data (table 50), although spotty, show that significant waves struck the Pacific coast during these events. The

coasts of Washington and Oregon were significantly damanged by the 1964 Prince William Sound, Alaska, earthquake and tsunami (Hogan and others, 1964) (table 51). A

wave of about 3 m was observed at Ocean Shores, Wash.,

just north of the entrance to Grays Harbor.

Analysis of the 1964 Alaska earthquake damage in

Seward, Alaska (fig. 237) indicates that there were four relatively distinct causes of tsunami damage. Primary causes

were from direct water forces, including hydrodynamic

forces, buoyancy, and hydrostatic pressures, and from loss of

ground support through subsidence, compaction, erosion,

liquefaction, and (or) sand transport. Secondary causes of

damage were from direct water forces acting on land uses,

including impacts from floating debris (logs, buildings, vehicles, boats), fire, and contamination of land and water from

oil, fuel, and other stored materials. Most of the damage was

due to the secondary impact of the tsunami. For example,

buildings weakened by water-pressure forces were subsequently dislodged when foundations were scoured by the

erosive actions of the drawdown following the incoming

waves. Thus, even if water levels were not high, damage

was still severe.

It is also important to note that considerable tsunami

damage in coastal areas can occur away from the shoreline.

Spaeth and Berkman (1972) described such damage in Oregon from the 1964 tsunami. The town of Seaside sustained

about $275,000 in damage to residential and commercial

areas several blocks from the shore. The waves surged up

both the Necanicum River and Neawanna Creek, damaging

bridges and structures set back from the coast.

These historical data do not establish the likelihood of

locally generated tsunamis (source within 200 km of the

shore), but they do indicate that the offshore topography of

the region does not provide any natural protective barrier

to incoming waves. This lack of natural protection means

that a locally produced tsunami could indeed be a serious

threat.

A recent study (Hebenstreit and Murty, 1989) used

numerical modeling techniques to examine the potential

threat to the coasts of Washington, Oregon, and British

Columbia from tsunamis caused by earthquakes within the

Juan de Fuca plate. The results of the study indicated that

certain areas of the coastline were more susceptible to concentrated wave energy than others because of variations in

offshore topography (similar to a finding by Hebenstreit

and Bernard, 1985, for the Hawaiian Islands).

Tsunamis, like earthquakes, vary in magnitude and

intensity. In addition, the nature of the tsunami risk is profoundly influenced by characteristics of land use in the

inundated areas. The level of risk is examined by combining the likelihood of tsunami impact in a given geographic

area with an inventory of the types of land-use-related hazards that could be exposed to that impact. The first step in

mitigation-based land-use planning is to develop a clear

understanding of scientific criteria for delineation of the

hazard. Subsequently, land-use decisions can be based on

specific vulnerability to distinct and definable risks.

The hazard analysis for this study consists of two

parts. One is the identification of the geographic areas vulnerable to direct tsunami damage. Numerical simulation

was used to identify this risk. The second part is the identification of potential secondary hazards caused by the earthquake and (or) the interaction of earthquake effects and

tsunami. Identification of these potential hazards was

based on secondary sources such as soils data from the

Washington State Department of Natural Resources.
520 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

ANALYSIS OF THE TSUNAMI HAZARD

The first step in analyzing the potential tsunami hazard

in the study area was to carry out relatively coarse numerical

simulations of the likely tsunami propagation patterns due to

possible thrust-type earthquakes occurring offshore. These

simulations were made in an earlier study (Hebenstreit and

Murty, 1989) of the general threat to the Pacific coast from

hypothesized subduction-related earthquakes in the Juan de

Fuca plate. The plate was subdivided into a number of seismic source regions, including the southern Cascadia subduction zone off the coast of Washington and Oregon.

Earthquake-source parameters in each region were postulated from an examination of such factors as the probable

length of the fault plane in each section of the plate, the width

of the plate, and the depth and dip angle of historical earthquakes in the area. This information was used to produce

reasonable depictions of seafloor uplift due to large thrust

earthquakes. Several arbitrary vertical-thrust values were

used to provide a realistic range of possible tsunami intensities. The various uplift patterns were then translated into

resulting sea-surface displacement fields that could be used

as the starting point for simulations of wave propagation

toward the local coastlines. The wave-propagation simulations were imposed on actual seafloor bathymetric data to

include the effects of variations in the ocean bottom on propagation patterns.

These simulations, based on reasonable assumptions

concerning the nature of possible earthquakes, formulated a

model that helped identify the parts of the Pacific coast most

susceptible to tsunamis originating in the different specific

sections of the plate. In all the simulations, the model indicated that seafloor uplift would occur offshore and some subsidence would occur on land. Dominant wave-energy

distribution would be confined to the immediate source area;

that is, the most extreme wave heights would be found along

the coastal areas within the source uplift zone. Also, the elevations would tend to decline (although not uniformly) north

and south of the immediate area of the uplift. Tsunamis originating in the southern Cascadia subduction zone would

likely focus a large part of their energy on the southwestern

coast of Washington, including Grays Harbor.
INTEGRATED TSUNAMI-HAZARD ASSESSMENT, GRAYS HARBOR, WASHINGTON 521

To characterize the effects of tsunamis on the coast, a

series of recording stations were specified in the model grid,

and wave elevations at those points in the grid were stored

for every simulation time step (fig. 238). The calculated

mean value of simulated wave heights in the southern Cascadia subduction zone is just under 6 m above MLLW (mean

lower low water—the sea-level datum used by NOAA

(National Oceanographic and Atmospheric Administration)

bathymetric charts). However, the coast between Newport,

Oreg., and Grays Harbor is projected to experience mean

wave heights of 8–9 m.

Once the wide-geographic-area simulations had been

used to identify likely severe tsunami-impact areas, a more

detailed site-specific wave-propagation model was used to

investigate wave behavior in regions of high energy

concentration. This model, called SURGE II, simulates the

behavior of long waves (tsunamis) running toward and

onto a coastline and can also delineate flooding in lowlying areas along the shore. The model has been used for

many tsunami simulations, including an extensive study

for Valparaiso, Chile (Hebenstreit and Gonzalez, 1985).

The simulations are obtained by means of an explicit

finite-difference algorithm for numerically solving linearized long-wave equations on a Cartesian (x,y as opposed to

latitude, longitude) grid. Bottom friction is included in the

model by means of a quadratic term. Inundation of coastal

areas is computed by means of a weir-overtopping scheme,

but wave runup is not calculated. Radiation boundary conditions are applied to open-ocean boundaries to ensure that

wave energy leaving the simulation grid is only minimally

reflected.

The procedure used in the detailed tsunami simulations is similar to that used in the wide-area simulations:

1. A seismologically realistic earthquake source is

developed using historical evidence to specify parameters

such as length, depth, width, and dip angle of the fault

plane.

2. These parameters are used in the model of

Mansinha and Smylie (1971) that predicts the movement of

the seafloor that such an earthquake would produce.

3. This seafloor motion is translated directly into a

disturbance of the sea surface that propagates toward the

shoreline as a long wave (tsunami).

BUILDINGS MARINE

STRUCTURES LIFELINES OTHER

WOOD FRAME

MASONRY/

CONCRETE

STEEL FRAME

WHARVESDOCKSPIERS

JETTIESBREAKWATERS

ROADSAIRSTRIPS

RAIL LINES

BRIDGES

UTILITY PLANTSUTILITY LINESBOATS

OIL TANKS

PRIMARY CAUSES

OF DAMAGE

DIRECT WATER FORCES

LOSS OF GROUND SUPPORT

FLOATING DEBRIS

FIRE AND CONTAMINATION

SECONDARY CAUSES

OF DAMAGE

Hydrodynamic forces

Buoyancy

Hydrostatic pressure

Subsidence

Compaction

Erosion

Liquefaction

Sand transport

Cars

Boats

Logs/Stored materials

Buildings

Oil and fuel storage

Vehicular/Railroad

Electrical

Stored materials

EXPLANATION

Least severe

Most severe

Figure 237. Categories and levels of tsunami damage to structures and other features in Seward, Alaska, due to the

1964 earthquake. From Urban Regional Research (1988).
522 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

4. The long waves are allowed to interact with the

coastal area, and water-level time series are recorded at

specific points. Locations on the grid where flooding is

indicated are noted.

The tsunami sources used in the simulations were

offshore of Grays Harbor. One source was located about 200

km from the coast (the approximate location of the surface

expression of a fault at depth under the continental slope).

127° 126° 125° 124° 123° 49°

48°

47°

46°

45°

44°

43° 0 3 6 9 12

PROJECTED MAXIMUM WAVE

HEIGHT ABOVE MEAN LOWER

LOW WATER, IN METERS

0 50 100 KILOMETERS

JUAN

D

E FU

C

A PLATE

AVERAGE WAVE HEIGHT

PACIFIC

OCEAN

Strait of Juan de Fuca

Vancouver

Island

Port Angeles

Ocean Shores Aberdeen

South Bend

WASHINGTON

Astoria

Seaside

Cannon Beach

Tillamook

Pacific City

Lincoln City

Newport

Reedsport

Coos Bay

OREGON

CANADA

Figure 238. Projected near-shore (about 20 m depth) wave heights calculated for a major thrust earthquake in the southern Cascadia subduction zone of the Juan de Fuca plate (dotted line shows shoreward

boundary of plate). Earthquake fault plane extends from north of Aberdeen, Wash., to Coos Bay, Oreg.

Line with tick marks indicates model grid recording stations where wave elevations were stored.
INTEGRATED TSUNAMI-HAZARD ASSESSMENT, GRAYS HARBOR, WASHINGTON 523

The second was only 100 km offshore, to include possible

onshore subsidence effects in the simulation. Because the

model is essentially linear, both sources produced about the

same results, varying only slightly in the magnitude of wave

heights calculated along the coast.

Seafloor (and hence sea surface) uplift from the more

distant source (fig. 239) was calculated using the Mansinha

and Smylie (1971) source-displacement model with the

parameters listed in table 52.

The source zone specified for this earthquake lies in the

southern part of the Cascadia subduction zone, along the

coast from near Coos Bay, Oreg., to just north of Grays Harbor. This source zone was used in an earlier study (Hebenstreit, 1988) of the generalized tsunami threat from

movement of the Juan de Fuca plate. The uplift pattern

shown in figure 239 is the north end of a larger pattern from

the earlier study.

The site-specific SURGE II model uses a numericalcomputation grid with variable seafloor and land topography. A realistic rendition of actual conditions is vital

because the process of interaction between long surface

waves (tsunamis) and the shoreline is heavily influenced by

changes in water depth, as in coastal flooding. Simulated

topography in the Grays Harbor model area is shown in figure 240. The contour line labeled “0” marks the approximate location of the shoreline.

One of the dominant landforms in the Grays Harbor

area is the extensive mud flats. A large part of the harbor is

so shallow that some of the bottom is exposed at low tide.

A central channel has been dredged to allow seagoing vessels to reach Hoquiam and Aberdeen.

A series of recording points was identified in the

model to facilitate examination of the waves during the

course of the simulations (fig. 241A). Time series for several sets of these points are shown in figure 241B (stations

on the outer coast), figure 241C (stations roughly along the

axis of the main channel in the harbor), and figure 241D

(stations at the far east end of Grays Harbor).

In the simulation, high-amplitude waves of 7–8 m

height strike the outer coast soon after the uplift (fig.

241B). The recording stations closest to the mouth of the

harbor (stations 14, 15, 19, fig. 241C) are initially subjected

to high-amplitude waves that rapidly damp down to lowamplitude, high-frequency waves. Farther into the channel,

initial amplitudes are greatly reduced. By the time the

waves reach the Hoquiam-Aberdeen area (stations 4–9, fig.

126° 125° 124° 48°

47°

46°

0 25 50 KILOMETERS

Grays

Harbor

Willapa

Bay

-0.4

-0.4

-0.4

-1.2

-1.2

-2

0

0

0.4

0.4

1.2

1.2

2

2

2.8

2.8

2.8

Figure 239. Contours showing relative vertical seafloor response to a possible earthquake in

Grays Harbor region. Positive values indicate

upward motion of seafloor; negative values indicate downward motion. Irregularities in contours are due to plotting software. Contour

interval 0.4 m.
524 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

241D), there is only a minor variation in water level (about

0.5 m).

Except for some low-lying areas near the mouth of the

harbor, especially around Westport, all the areas predicted to

be flooded by tsunamis (fig. 242) are on the outer coast,

which in this part of Washington is largely sandy beaches

with dune barriers on the shoreward side. This simulation

seems to be substantiated at Willapa Bay, south of Grays

Harbor, where recent sediment-dating studies seem to indicate the possibility of subsidence events and tsunami inundation in the recent geologic past (for example, see Atwater,

1987).

The simulations indicate that the interior of Grays Harbor is relatively well protected from a serious tsunami threat,

for several reasons. For example, Grays Harbor is primarily

diamond shaped (fig. 241); it is 2.2 km wide at the mouth,

widens to about 21 km, and then narrows at the mouth of the

Chehalis River at the east end. This configuration is

expected to cause the wave to break at the mouth, dissipating

its energy. Another factor contributing to the relatively moderate level of tsunami threat is that the extensive shallow mud

flats will quickly dissipate a large part of the wave energy,

reducing wave height inside the harbor. The wave amplitude

would continue to decline as the wave travels inland across

the harbor to the Chehalis River. An initial wave amplitude

of 2–3 m above the existing tide level at the mouth of the

harbor would diminish to 0.5 m by the time the wave reaches

Aberdeen. Because other tsunami waves would break at the

mouth of the harbor, energy would be considerably reduced.

Therefore, the simulation indicates that the wave would have

relatively low velocity within Grays Harbor. A wave period

of about 20 minutes is anticipated.

However, even relatively small tsunamis can cause considerable damage to coastal areas. The drawdown of the sea

surface can expose normally submerged bottom areas to erosion and slumping. Boats and ships moored at coastal installations can be severely damaged by anomalous surface

motions such as drawdown or seiching (oscillation at the resonant frequency of the basin). In addition, objects torn from

their moorings can become dangerous floating projectiles. If

a small tsunami occurs during a time of severe storm seas,

anomalously high tides, or river flooding, it can cause greater

destruction, because under these conditions, surface waves

can propagate much farther inland. Finally, if the surface

resonance within the bay coincides with the period of the tsunami, the wave would be amplified by an unknown factor

instead of being dissipated.

COASTAL FLOODING CONDITIONS AS 

AN ADDITIONAL HAZARD

Although the simulations did not factor in river- and

rain-induced flood conditions, historical coastal flooding is

well documented. Flooding in the Aberdeen area is generally the result of high river flow caused by winter rainfall generated by Pacific weather fronts combining with

tidal flows. The tidal influence in Grays Harbor extends

126° 125° 124° 48°

47°

46°

0 25 50 KILOMETERS

Grays

Harbor

Willapa

Bay

-1500

-1500

-2000

-2500

-1000

-1000

-500

-500

-200

-200

-100

-100

-50

-50

-10

-10

-10

0

0

20

20

20

Figure 240. Topography used in numerical

simulations of Grays Harbor area. Contour

values in meters relative to MLLW (mean lower

low water). Small outlines indicate location of

small undersea features significantly shallower

than surrounding water.
INTEGRATED TSUNAMI-HAZARD ASSESSMENT, GRAYS HARBOR, WASHINGTON 525

up the Chehalis and Wishkah Rivers and may coincide

with high river flows to increase flooding. These conditions can be aggravated during rainstorms by backups in

the city’s storm-drainage system, when heavy local runoff is prevented from entering the rivers because of high

water levels.

The highest river and harbor-water stages in the Aberdeen area result from a combination of anomalous high

astronomic tides (which occur frequently), low barometric

pressure, strong onshore winds, and heavy rains. This combination of conditions has produced extensive water damage

to homes, businesses, and public property many times in the

130 135 140 145 150 155 160 165

85

80

75

70

65

60

55

MODEL GRID VALUES

MODEL GRID VALUES

1

2

12

11 10

17

16

15 19

14

13

3

4

5 18

9

8

6 7

Westport

Hoquiam Aberdeen

Chehalis River

N

A PACIFIC OCEAN

Grays

Harbor

EXPLANATION

Hydrograph station

0 5 10 KILOMETERS

14

ELAPSED TIME, IN HOURS

WAVE HEIGHT, IN METERS

B

12

10

8

6

4

2

0

-2

0 1 2 3 4 5678

Station 13

Station 12

Station 2

Station 1

ELAPSED TIME, IN HOURS

WAVE HEIGHT, IN METERS

C

12

10

8

6

4

2

0

-2

0 1 2 3 4 5678

Station 15

Station 18

Station 17

Station 16

Station 19

Station 14

ELAPSED TIME, IN HOURS

WAVE HEIGHT, IN METERS

D

12

10

8

6

4

2

0

-2

0 1 2 3 4 5678

Station 9

Station 7

Station 6

Station 8

Station 5

Station 4

Figure 241. Simulated time-series data for a hypothetical tsunami impacting Grays Harbor area. A, Locations of modeled hydrograph

recording stations at which time series of waves were recorded. Values on edge of map are part of the model grid. B, Time series of

computed waves at stations just outside Grays Harbor. C, Time series for stations at west end of Grays Harbor. D, Time series for stations

at east end of Grays Harbor. For clarity, each time-series curve above lowest is vertically offset by +2 m on each graph.
526 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

past (table 53). Flooding along the lower sections of the

small streams in Aberdeen is primarily caused by high water

in the rivers backing up into the creeks and inundating adjacent low areas. Also, the Wilson Creek drainage basin was

first clearcut in 1974. Additional logging operations have

caused an increase in the volume of water that comes down

this creek during rainstorms.

On average, Pacific coast tidal action causes the Chehalis River at Aberdeen to vary about 3 m in elevation, from

–1.53 m at MLLW (mean lower low water) to 1.56 m at

MHHW (mean higher high water). Flooding problems

develop first near the southeastern city boundary when the

Chehalis River reaches elevations between 2 and 2.3 m

above mean sea level. This flooding is primarily on land that

is undeveloped. When the water level reaches 2.3 to 2.6 m

height, flooding spreads inland to residential and commercial

properties in southeastern South Aberdeen along State Highway 101. Also, properties along the Wishkah River in North

Aberdeen are affected. A 10-year flood level for this area is

estimated to be 2.7 m; at a height of 2.8 m, general flooding

occurs because the dikes protecting South Aberdeen are

overtopped in many places when the water rises to between

2.6 and 2.8 m. At that level, water enters the downtown area

of Aberdeen from the Wishkah River immediately to the

east. Above 2.9 m, which is equivalent to a 25-year flood,

major flooding occurs throughout the city. Flood-water

velocity becomes a problem at this stage because overtopped

dikes can fail due to saturation and scouring.

If extraordinarily high tides or tide surges are accompanied by heavy rainfall, flooding likely will occur even earlier

than in the previous scenario. Because the peak astronomic

tide for Grays Harbor coincides with the greatest threat of

winter storm surge and rainfall for the area, the combinations

of factors that can potentially result in flooding occur every

125

115

105

95

85

75

65

55

45

 35

25

15

MODEL GRID VALUES

0 10 20 30 KILOMETERS

110 120 130 140 150 160

MODEL GRID VALUES

PACIFIC OCEAN

Grays

Harbor

Willapa

Bay

Ocean Shores

Westport

Figure 242. Approximate areas of coastal inundation (rectangles)

computed by the tsunami-impact model for Grays Harbor region.

Values on edge of map are part of the model grid.
INTEGRATED TSUNAMI-HAZARD ASSESSMENT, GRAYS HARBOR, WASHINGTON 527

year between November and February. Ordinary high tides

are about 1.6 m at MHHW; an additional 0.5 m of tsunami

inundation would raise the level to 2.1 m. Flooding has

occurred at 2–2.3 m. The extra 0.5 m of water from a tsunami during winter flood conditions of 2.3–2.6 m would

inundate the downtown areas as well as the coastal highway.

Throughout the city, storm-water runoff is directed into

adjacent rivers and sloughs. The storm-water-drainage system in North Aberdeen consists of underground drains; in

South Aberdeen, the system is open ditches. Storm drains

overflow when tide gates at the storm-drain outlets close due

to high river levels. This storm-water flooding can occur

throughout the area, and storm-water ponding remains as

long as high river levels persist. Another problem that can

develop is sewer overflow. The dikes protecting the area can

be overtopped at a water level of 1.9 m MHHW, which is

significantly below the 10-year flood level. If the 0.5 m tsunami flood level is added to the estimated 1.5–2.15 m of

potential subsidence (fig. 243) during an earthquake, then

the flood level would be raised by roughly 2.3 m, a damaging

level even at low tide. These conditions could result in a

serious contamination problem.

A tsunami occurring at high tide or during near-flood

levels would arrive when the harbor is significantly deeper

than normal. Under these conditions it would carry more

wave energy into the Hoquiam-Aberdeen area. Thus, if the

tsunami occurs during high winter tide conditions, the additional 0.5 m of projected wave height could easily overtop

and (or) weaken the dikes protecting Aberdeen. The drawdown from the first tsunami wave can be expected to cause

severe scouring of the inland sides of the dikes; the second

tsunami wave could destroy them.

VULNERABILITY OF PEOPLE AND 

PROPERTY

The vulnerability analysis conducted in this study consisted of two primary components. One was the definition of

the population at risk within the coastal hazard zone (generally defined as being below 6.1 m above mean sea level,

based on the National Geodetic Vertical Datum of 1929).

The other was the definition of land-use patterns and specific

characteristics of those uses that could result in secondary

hazards. For example, the presence of hazardous materials

stored at or frequently transported to or through a site could

be a secondary hazard.

POPULATION AT RISK

The population at risk from tsunami hazards in the

Grays Harbor area varies seasonally. If a tsunami occurs

between late October and late March, it could coincide with

periods of elevated river heights due to heavy rain, which

would magnify the potential for extensive property damage;

population levels would, however, be relatively low during

that period. If a tsunami occurs during the summer months,

the beach and resort communities along the open coasts

would have high populations.

The largest year-round population center in Grays Harbor County is the Aberdeen-Hoquiam-Cosmopolis area,

with a population of 14,241 (1990 Census of Population and

Housing—Washington). This urban complex lies at the east

end of Grays Harbor, fed by several rivers (including the

Chehalis and the Wishkah) and open to the sea through the

channel flanked by Westport on the south and Ocean Shores

on the north.

Average year-round population levels of the coastal

communities are low, but there are large seasonal fluctuations. Winter populations tend to be below 5,000 residents.

During the summer, however, the wide sandy beaches of the

Washington and Oregon coast are popular destinations for

both Seattle-Tacoma and Portland urban populations. The

1990 population for the greater Seattle-Tacoma consolidated

metropolitan area was 2,259,164, and Portland, Oreg.–Vancouver, Wash. had a metropolitan population of 1,286,222.

Thus virtually the entire coast can be heavily populated during the summer months by campers and tourists staying in

the many beach-front communities, which would make the

potential for loss of life from a local tsunami very high. For

example, whereas the year-round population of Ocean

Shores is about 5,000, about 35,000 people attended a 1-day

sand-castle-building contest in a nearby community during

the summer. A large number of people in the urbanized

Aberdeen-Hoquiam area could also be at risk, even though

their residences are within Grays Harbor.

LAND-USE RISKS AND TOPOGRAPHY

Projections of land-use disruption must rely on an estimate of potential flood areas, which to a large extent is a

function of ground elevation. This analysis, therefore,

encompasses all the area below 6.1 m elevation (above

mean sea level), the slope of which is 0–2 percent. A threedimensional terrain model was used as the basis for the

land-use analysis (fig. 244). Correlation of topography

with land use permits rapid assessment of geographically

based vulnerability to flooding. The urbanized area of

Ocean Shores, a community of predominantly second

homes, is entirely below 3 m elevation. Land-use patterns

indicate that the urban and industrial areas in the central

business districts of Hoquiam and Aberdeen are on coastal

lowlands, virtually all of which are unconsolidated fills.

Grays Harbor has several industrial complexes located

along the shore. About 25 percent of the work force in

Grays Harbor County is employed in manufacturing activities. ITT-Rayonier and Grays Harbor Paper have a combined pulp and fine-paper production facility on the

waterfront in Hoquiam. A large Weyerhauser wood-pulp

facility is in South Aberdeen. The harbor has several port
528 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

7.5

7.2

7.9

7.4 7.4

9.0 7.7

8.2

8.7

9.0

8.1

9.2

7.9

9.1

8.7

6.9

6.5

8.8

6.3

7.9

8.1

5.8

N

CHEHALIS RIVER

OVERTOPPING OF DIKE

DURING 10 YEAR FLOOD

(8.5 ft/2.7m)

Elevations above mean sea level

Dikes

EXPLANATION

facilities; in 1985, 5.8 million tons of materials were

shipped through the harbor, about 55 percent of which was

logs (U.S. Department of the Interior, 1988). In addition,

Westport is a commercial fishing center.

In summary, the Grays Harbor area, although not an

exceptionally large economic center, contains a significant

investment in both commodities handled and infrastructure

required to serve the industries present. It is the busiest port

in the Pacific Northwest with respect to distribution of

regionally produced lumber.

The area of potential flooding encompasses all the

industrial areas, many bridges, and the State highway

Figure 243. Critical flood levels for Aberdeen (north side of Chehalis River) and South Aberdeen (south side of river). Low-lying

areas will be subject to flooding during low tide; during high tide, the entire urbanized area will be subject to extensive flooding that,

because of deeper water level, could have relatively high velocity. One hundred year flood boundary defined by FEMA in conjunction with NFIP (National Flood Insurance Program) is the 10-foot contour. From Urban Regional Research (1986).
INTEGRATED TSUNAMI-HAZARD ASSESSMENT, GRAYS HARBOR, WASHINGTON 529

linking Grays Harbor with points to the north and south. In

addition, the central business districts of both Aberdeen and

Hoquiam (fig. 245), as well as residential areas in both

communities, are within potential inundation areas. The

headquarters of the fire department in Hoquiam and both

fire stations in Aberdeen are within the coastal hazard zone.

Two principal highways, State Routes (SR) 12 and

101, serve the area from the east, north, and south. SR 12,

a four-lane highway, connects Aberdeen and Hoquiam with

the north-south Interstate 5 corridor. SR 101, which is

basically a two-lane highway, serves the Olympic Peninsula and southwestern Washington. Two-lane routes connect Ocean Shores and Westport and points north and south

along the Pacific coast. Highway routes are characterized

by soft soils, so there is a high probability that transportation, such as response and rescue efforts and fire fighting,

would be interrupted.

Data collected in conjunction with the analysis of

damage from the 1964 Alaska earthquake tsunami specifically mention damage to four bridges in the southern

Washington–northern Oregon region. Damage to bridges

from a projected tsunami in the Grays Harbor area could be

even greater in the industrialized area. Many bridges cross

rivers emptying into Grays Harbor, including three drawbridges over the Chehalis and Wishkah Rivers and one

drawbridge crossing the Chehalis River between Aberdeen

and South Aberdeen–Cosmopolis.

SECONDARY HAZARDS

SUBSIDENCE

An earthquake-related threat that could increase the

destructiveness of a tsunami is subsidence caused by compaction of soils under strong ground motion and (or) tectonic

displacement. Coastal subsidence commonly accompanies

great subduction earthquakes, primarily along an onshore

belt flanked by a mostly offshore zone of coseismic uplift

(Atwater, 1987). Estuarine deposits of late Holocene age

near Washington’s outer coast indicate that submergence

and shoaling have occurred in cycles that resemble, at least

superficially, the known and inferred cycles of coseismic

submergence and postseismic shoaling in great-earthquake

regions of Alaska and Chile. The amount of subsidence is

estimated to be about 1.6–2 m respectively (Curt Peterson,

Portland State University, oral commun., 1990).

The seismic effects of the 1964 Alaska subduction

earthquake along the Washington coast are well documented. These effects were particularly severe in areas

where subsidence was increased by shaking-induced settlement when seismic vibration caused consolidation of loose

granular materials. Rearrangement of constituent particles

aided by ejection of interstitial water through water spouts

or mud spouts caused compaction and local differential

subsidence of the ground surface. Lateral spreading, too,

caused lowering of the ground surface in places (Plafker,

    

 

Second homes

Residential

Commercial

Industrial

Rural

EXPLANATION

N

Figure 244. Grays Harbor land-use patterns. Patterns extend past land boundaries for clarity.
530 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

1969). In coastal areas where local subsidence was superimposed on regional tectonic subsidence, the damaging

effects were magnified.

Two examples from the 1964 Alaska earthquake illustrate the combined effects of tectonic displacement and subsidence. On Kodiak Island, local subsidence of as much as 3

m was widespread in noncohesive granular deposits through

compaction, flow, and sliding that resulted from vibratory

loading during the earthquake. Subsidence of more than 1.8

m occurred throughout the northern part of the region. This

subsidence, which was largely restricted to saturated beach

and alluvial deposits or artificial fill, was locally accompanied by extensive cracking of the ground and ejection of

water and water-sediment mixtures. Within the affected

area, tectonic subsidence, which was locally augmented by

surficial subsidence of unconsolidated deposits, caused

widespread inundation of shorelines and damage to intertidal

organisms, near-shore terrestrial vegetation, and salmonspawning areas (Plafker and Kachadoorian, 1966). Subsidence also occurred in the Cook Inlet area, which was downwarped. At the head of Cook Inlet near Portage, estuarine silt

buried 18 km2 of lowland that had subsided 1.6 m and settled

an additional 0.8 m, totaling 2.4 m (Atwater, 1987).

As these examples of Alaskan subsidence indicate, a

critical variable in projecting inundation and risk is a

determination of the areas prone to subsidence. These areas

can reasonably be expected to be composed of soft and

highly saturated material, such as the alluvium in virtually

the entire urbanized Hoquiam and Aberdeen areas (fig. 245)

(Washington State Department of Natural Resources, 1987).

Soils in the Grays Harbor flood plain are primarily alluvial silt and fine sand, locally with organic material (fig.

246). Some areas are mantled by artificial fill. These soils

are about 1.5–1.8 m deep and range from moderately well

drained to somewhat excessively well drained, to excessively

well drained on the diked tidelands. This soil type formed in

sandy and loamy river dredgings. The other type of soil,

found primarily in the flood plain of South Aberdeen, is a

silty clay loam. It is a deep, artificially drained soil found on

flood plains and deltas that are protected from tidal overflow.

This soil type formed in clayey alluvium deposited in quiet

waters of coastal bays.

Close to the fairly abrupt boundary between the flood

plain and the adjacent uplands are zones of coarse sand and

gravel. These zones are probably interbedded with finer

grained materials (Washington State Department of Natural

Resources, 1987).

Existing soils are predominantly nonengineered fill and

(or) highly saturated alluvium; both soil types are prone to

compaction. If it is assumed that a combination of tectonic

 

    

  

20

20

South Channel

North Channel

Chehalis River GRAYS HARBOR

Hoquiam

Aberdeen

South

Aberdeen

Central

business

district

Residential

area

Residential

area

Industrial

area

Industrial

area

Industrial

area

 Terraced sediments

Alluvium

Older alluvium

State highway

Police station

Fire station

EXPLANATION

0 1 2 4 5 KILOMETERS 3

20 10

Figure 245. Detailed land use and geology for the Aberdeen (including South Aberdeen) and Hoquiam area of Grays Harbor.
INTEGRATED TSUNAMI-HAZARD ASSESSMENT, GRAYS HARBOR, WASHINGTON 531

displacement, subsidence, and consolidation will occur to an

extent about equivalent to that in the 1960 Chile and 1969

Alaska earthquakes and postulated for Holocene Puget

Sound earthquakes, then subsidence of 1.5–2 m would be

likely to occur. Under this assumption, all the industrial

areas, the majority of the commercial centers, and a significant part of the residential areas are at risk. Loss of the

industrial facilities could have a long-term devastating effect

on the economy of the area.

In the event of subsidence, wave scouring could further

erode foundations and lead to structural failures. Furthermore, much of Grays Harbor and environs consists of sand

and sandy clay, which could liquefy under severe shaking,

causing foundations to sink differentially and breaking buried pipes and storage structures. Finally, ground saturated

by flooding will lose its bearing capacity, and there would be

a high incidence of foundation failures and buildings floating

off their foundations.

Because subsidence appears to have occurred in the

past and seems likely to occur again, a simulation was made

to project inundation as a result of subsidence. This simulation found that the flood-prone area would extend somewhat

further inland than would occur without subsidence.

BATTERING

A sudden rise in water level can produce extensive

damage from flooding and wave action on structures. It can

also produce less direct effects such as battering by objects

(logs, boats, railway freight cars, vehicles, and storage tanks)

which become floating projectiles.

In a fishing enclave, the greatest water-related hazard is

the fishing boats themselves. Any dockside complex such as

the marina in Westport contains boats that, if torn from their

moorings by currents created by rapid rising and falling of

sea level, could easily become floating projectiles capable of

damaging not only each other but also coastal structures such

as industrial and commercial buildings, hotels, and processing plants. Spaeth and Berkman (1972) and Wilson and

Törum (1968) cited instances of this type of damage from the

1964 Alaska tsunami.

A wood-pulp facility such as the Weyerhauser plant in

South Aberdeen has many types of materials that could

become battering hazards. Most notable are the logs piled

near the water’s edge awaiting processing. Smaller piles

could float under high-water conditions and, like the fishing

boats, could become projectiles. Vessels moored at the

  



 







 

 

 

 

 



 



N

EXPLANATION

Fine to medium sand

Silt and clay

Silty sand

Clayey soil

Coarse sand

Sandy soil

Peat and marsh

Silty loam







Fill

Figure 246. Soil and sediment structure in Grays Harbor area. Source of map data is Grays Harbor Estuary

Management Program.
532 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

plant’s dock could also become projectiles or could be battered by other vessels and (or) the logs. As it is, every year

during storm conditions people are killed by errant logs.

Under tsunami conditions, the increased water velocities will

turn these log storage areas into highly hazardous areas. In

addition, the ITT-Rayonier plant in Hoquiam, at the mouth of

the Hoquiam River, is serviced by a railroad. The rail cars on

the tracks are also potential floating hazards during flooding.

Another potential industrial hazard is toxic materials

stored at plant sites. For example, piles of waste materials on

the grounds of a paper plant could be a long-term health and

contamination hazard if they were dislodged by water-borne

objects and then dispersed by flooding.

AIR CONTAMINATION AND FIRE

The Hoquiam-Aberdeen population is at risk from possible fires and airborne contamination that could originate in

the industrial port area and spread to the neighboring residential and commercial areas. Although the precise causes and

dimensions of fire or air contamination have not been predicted, it is clear that toxic chemicals could be a devastating

hazard if their storage containers were to be breached by

ground motion, wave action, or floating.

Grays Harbor is a principal port for Pacific Northwest

forest-products industries. Manufacturing of a wide range of

wood products (for example, wood shakes) involves the use

of an extensive range of chemicals as preservatives, fire

retardants, and other related agents. Fire is always a problem

when chemical storage tanks are breached; toxic-material

storage facilities constitute a major threat to public safety

when they are damaged and facilities are disrupted. Surfacewater, ground-water, and airborne contamination could

occur in the flooded areas.

The U.S. Environmental Protection Agency SERA Title

III Program requires mandatory public disclosure of stored

hazardous materials exceeding a specific amount for each

chemical. Among the toxic substances stored in industrial

facilities in the Grays Harbor area are ammonia, chlorine,

nitric acid, sulfur dioxide, hydrogen peroxide, propane, and

formaldehyde gas. In order to demonstrate vulnerability, the

release and spread of selected toxic materials from an industrial site were simulated in the study area. The ComputerAided Management of Emergency Operations (CAMEOTM

II) program, developed by the U.S. National Oceanic and

Atmospheric Administration (NOAA) Hazardous Materials

Response Branch, was used to project the geographic extent

of vulnerability to the spread of toxic materials. This model

is designed to help emergency planners and first-response

personnel plan for and safely handle chemical accidents.

CAMEO’s air-dispersion model can simulate potential

dispersion of toxic gases under a variety of wind and weather

conditions. It simulates the extent and footprint of a chemical downwind from a chemical spill, using a diagram drawn

by the computer from a defined location on a base map. For

this analysis, project base maps were used to facilitate correlation of the air-dispersion analysis with land-use patterns.

Because concentrations differ for each chemical, the predicted adverse effects vary widely. Two hypotheses were

developed by the project to illustrate the pattern of possible

air contamination:

Hypothesis 1. Small chemical spill. Release of threshold limits (45 kg or 227 kg) of ammonia and chlorine projected.

Hypothesis 2. Large chemical spill. Release by partial

rupture of an average railroad tank-car load (2,270 kg) of

chlorine.

Other variables were based on average wind and temperature

conditions during summer versus winter (table 54).

Downwind chemical concentrations from a chemical

accident were simulated under two base-time conditions:

1. IDLH: Immediately dangerous to life and health.

This condition exists in the immediate vicinity of the spill

within minutes after it occurs. Under IDLH conditions, the

gases remain concentrated and pose a serious threat. Just

after a spill, no one should enter an IDLH atmosphere without a self-contained breathing apparatus.

2. TLV-TWA: Threshold limit value—time weight

average. This condition occurs within the first 30 minutes

after the spill. In TLV-TWA conditions, the gases are more

dispersed and pose a less general but still serious threat.

The extent of potential airborne contamination from a

release of 45 kg of chlorine gas at the ITT-Rayonier plant

under both summer and winter average wind conditions is

shown in figure 247. When reviewing the simulation results,

it is important to know that the CAMEO program tends to

underestimate the dispersal of heavy gases such as chlorine

by a factor of about 2. Thus the IDLH is really more

extensive than shown in the figure.

Under summer conditions (fig. 247A), the threatened

areas are primarily residential, and the fire station is within
INTEGRATED TSUNAMI-HAZARD ASSESSMENT, GRAYS HARBOR, WASHINGTON 533

the potential contamination zone. The threat is exacerbated

because when the lives of fire personnel are at risk, the ability of the community to respond to the emergency is reduced.

INTEGRATED HAZARDS 

MANAGEMENT

Once the base data for delineation of the tsunami threat

and the characteristics of vulnerability were developed, it

was possible to correlate these two factors. The physical

threat, including flooding, strong currents, and potential for

ground subsidence, could be correlated with land-use characteristics. An integrated physical-social-economic data

base was thereby created for estimating potential damage

caused by floating debris, fire, and contamination from hazardous substances. In essence, this integrated methodology

treats the tsunami threat as a system rather than a single

physical process. The analysis also highlights the reality that

hazards are cumulative, that is, whereas the immediate water

hazard is dangerous, it can precipitate still other hazards with

even greater consequences.

The geographic location, land use, and underlying

soils constitute a system of base perimeters of variable

vulnerability. Defining tsunami-hazard boundaries can

serve two purposes. One is to define possibly vulnerable

areas in order to plan for damage mitigation; the other is to

define evacuation procedures. Once at-risk locations are

Figure 247. Potential dispersion patterns for chlorine gas spreading from a simulated spill (45 kg) at ITT-Rayonier pulp and

paper plant in Hoquiam. Overlapping circles, extent of airborne plume; dashed lines, wider envelopes of potential hazard under

IDLH (immediately dangerous to life and health) and TLV-TWA (threshold limit value–time weighted average) scenarios.

IDLH concentrations assumed as 30 ppm (parts per million); TLV-TWA concentrations assumed as 1 ppm. A, Dispersion under

summer temperature and wind conditions. B, Dispersion under typical winter temperature and wind conditions.

A IDLH TLV-TWA

IDLH TLV-TWA

EXPLANATION

Extent of airborne plume

TLV-TWA hazard

IDLH hazard

B

Hoquiam River

Hoquiam River

Hoquiam River

Hoquiam

River
534 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

defined, a risk-reduction program can be developed that

organizes the specific characteristics of vulnerability

according to subdivided areas of risk. Analyses of risks in

the Grays Harbor area indicate that they are primarily a

function of land use. For example, the apparent lack of a

high-impact tsunami threat within the harbor indicates that

the predominant threat for which Grays Harbor must be

prepared is secondary hazards. These secondary hazards

include battering, erosion and scouring, loss of soil

strength, fire, and air contamination. During high tide and

(or) riverine flood conditions, major flooding may exacerbate high-water conditions created by the tsunami.

MICROZONATION OF THE RISK AREA

The specific nature of the tsunami threat is determined

mostly by underlying soil conditions and by land-use patterns. Once the soil conditions are clearly defined, an integrated hazard-management system organized into a series of

microzones can be applied that reflect these conditions. This

hazard analysis constitutes the basis for geographically

delineating the risk area. The next step in the hazard-mitigation planning process requires more specific data upon which

actual design can be based. The microzones defined by this

study (fig. 248) are discussed following.

TSUNAMI HIGH-IMPACT ZONE

Area Delineation.—Based on the flooding simulations,

only the outer coastal areas encompassing Ocean Shores

and Westport are vulnerable to direct tsunami impact.

Because the elevation of both communities is about 3 m

above mean sea level, most of the residential areas are

within the high-impact hazard zone. The rest of the study

area does not appear to be vulnerable to direct high-level,

high-velocity tsunami impact.

Preparedness and Mitigation.—In Ocean Shores, the

critical planning issues are warning and evacuation. Population levels in this second-home community fluctuate seasonally; a tsunami that occurs in the summer could result in

comparatively high life loss, whereas the risks during winter would be considerably less.

In Westport, primary concerns relate to the presence of

the fishing fleet. A major cause of destruction from the

1964 Alaska tsunami and earthquake was boats being

swamped, battered, and (or) thrown inland against nearby

structures. Although boats are normally moored to withstand prevailing currents, tsunami-induced flows may overpower mooring lines. Boat owners who have any warning

of an approaching tsunami tend to move their vessels from

their anchorage and travel toward the open sea. Of the preventable deaths in Alaska during the 1964 tsunami, many

resulted when fishermen tried to save their boats. In Westport, similar damage patterns could be expected to occur.

Boat owners and radio-dispatch operators in the Westport

area should be informed of proper procedures to be followed in the event of a tsunami warning. It may be necessary to prevent boat owners from entering the marina area if

the warning time is not sufficient for evacuation before the

first several waves arrive.

N

Ocean

Shores

Second

homes

Westport CBD/

Small-boat harbor

Port/Industry

State

Route

109

Aberdeen CBD

 Hoquiam CBD

South Aberdeen

Wave

impact

Emergency response

impacts/potential

road washout

Wave impact

Boat battering

Subsidence

Structure damage

Flooding

Fire

Air contamination

Figure 248. Grays Harbor microzones (polygons) defining hazards arising from tsunamis and earthquakes. Dashed

line, major State route. A smaller roadway along south shore of harbor provides local access between Aberdeen and

Westport. CBD, central business district. Base prepared by Urban Regional Research.
INTEGRATED TSUNAMI-HAZARD ASSESSMENT, GRAYS HARBOR, WASHINGTON 535

ZONE OF FLOODING AND HIGH 

PROBABILITY OF SUBSIDENCE

Area Delineation.—The majority of the urbanized

Hoquiam-Aberdeen central business district and port is less

than 5.5–6.1 m above mean sea level and is reported to be

built on fill and (or) soft alluvial soils. This type of soil may

amplify ground motion and is also prone to subsidence.

Based on the effects of historical great subduction earthquakes in the Pacific Northwest and elsewhere, about 0.5 m

of subsidence from compaction could accompany about 1.5

m of tectonic subsidence, resulting in a total flood elevation

of 2 m above the existing MHHW of 1.7 m. Under these

assumptions, the land vulnerable to flooding encompasses

all the commercial and industrial areas inland to the 6.1 m

elevation.

Preparedness and Mitigation.—Two categories of

issues based on use have been identified in the urbanized

areas. In the commercial zone, preparedness and mitigation

must address conditions related to risk from damage and collapse of structures. In the industrial zone, a critical issue is

vulnerability of hazardous materials, either transported into

or stored on site, which could be dispersed either by flood

waters or by wind. Preparedness plans consider two zones

of risk: (1) the immediate flooded area, which contains airborne contamination-hazard zones under IDLH conditions;

(2) the larger area vulnerable to TLV-TWA (airborne)

spread as well as the area disrupted by interruption of major

transportation routes.

EMERGENCY PREPAREDNESS PRIORITY 

AREA

Area Delineation.—The outer coastal areas of Washington and Oregon are linked to the Interstate highway system by State highways. Significant parts of these routes,

which include an extensive network of bridges and roads, lie

within the zone projected for flooding. Inevitably, coastal

routes of the State highway system will be disrupted.

Preparedness and Mitigation Issues.—The preparedness process must assume that in the event of a major earthquake, road access will be disrupted and outside assistance

will not be able to reach the area for several days. A self-reliance contingency plan for search and rescue, emergency

medical, and repair should assume a delay of 2 days before

adequate outside resources through mutual aid can arrive.

CONCLUSIONS

Integrated hazard assessments based on the tsunami

threat have not, to our knowledge, been undertaken previously; instead, studies have focused almost exclusively on

flooding as a threat. It is clear from this study that an

examination of the interconnection of many potential hazards can lead to more fruitful analysis of a multifaceted

threat. Additional specific studies should be made, including

the following:

1. Collection of more data concerning the foundation

design and condition of structures in the flood-hazard zone.

2. Calculations to describe motions of merchant vessels

moored in the harbor and subject to tsunami-induced current

motions. Such studies should include examination of preventive docking practices.

3. Determination of the location of toxic-material storage facilities in areas prone to tsunami-induced flooding.

The design of these facilities must be reviewed because of

the potential for breaching of the containers, thereby releasing toxic chemicals into the air and water.

4. Assessment of the impact of the primary (tsunami)

and secondary threats on community-response capabilities

(immediately after the event and in the postdisaster time

frame, both in geographic terms and in terms of response

resources (manpower and equipment).

SELECTED REFERENCES

Atwater, B.F., 1987, Evidence for great Holocene earthquakes

along the outer coast of Washington State: Science, v. 236, p.

942–944.

Atwater, B.F., Hull, A.G., and Bevis, K.A., 1987, Aperiodic

Holocene recurrence of widespread, probably coseismic subsidence in southwestern Washington [abs.]: Eos, v. 68, no. 44,

p. 1468.

Berkman, S.C., and Symonds, J.M., no date, The tsunami of May

22, 1960, as recorded at tide stations: Washington, D.C., U.S.

Department of Commerce Coast and Geodetic Survey, unpublished report. [Copies available from second author.]

Bourgeois, Joanne, and Reinhart, M.A., 1988, Potentially

damaging waves associated with earthquakes, coastal Washington, in Hays, W.W., ed., Proceedings of Conference XLII;

a workshop on evaluation of earthquake hazards and risk in

Puget Sound and Portland areas: U.S. Geological Survey

Open-File Report 88-541, p. 96–99.

Heaton, T.H., and Hartzell, S.H., 1986, Source characteristics of

hypothetical subduction earthquakes in the northwestern United States: Seismological Society of America Bulletin, v. 76,

no. 3, p. 675–708.

———1987, Earthquake hazards on the Cascadia Subduction

Zone: Science, v. 236, p. 162–168.

Heaton, T.H., and Kanamori, Hiroo, 1984, Seismic potential

associated with subduction in the northwestern United States:

Seismological Society of America Bulletin, v. 74, no. 3, p.

933–941.

Heaton, T.H., and Snavely, P.D., Jr., 1985, Possible tsunami along

the northwestern coast of the United States inferred from Indian traditions: Seismological Society of America Bulletin, v.

75, no. 5, p. 1455–1460.

Hebenstreit, G.T., 1988, Local tsunami hazard assessment for the

Juan de Fuca Plate area: McLean, VA 22102, Science Applications International Corporation, Technical Report, 60 p.
536 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Hebenstreit, G.T., and Bernard, E.N., 1985, Azimuthal variations in

tsunami interactions with multiple-island systems: Journal of

Geophysical Research, v. 90, no. C2, p. 3353–3360.

Hebenstreit, G.T., and Gonzalez, F.X., 1985, Preliminary model

results of the 1985 Chilean tsunami [abs.]: Eos, v. 66, p. 962.

Hebenstreit, G.T., and Murty, T.S., 1989, Tsunami amplitudes from

local earthquakes in the Pacific Northwest Region of North

America—Part I, The outer coast: McLean, VA 22102, Science Applications International Corporation, Technical Report

[prepared jointly with Institute of Ocean Sciences, Sydney,

B.C., Canada].

Hogan, D.W., Whipple, W., and Lundy, C., 1964, Tsunami of 27

and 28 March 1964, State of Washington coastline Seattle, U.S.

Army Corps of Engineers, cited in Wilson, Basil, and Törum,

Alf, 1972, Runup heights of the major tsunami on North American coasts, in The great Alaska earthquake of 1964, oceanography and coastal engineering: Washington, D.C., National

Academy of Sciences. [Pages not given.]

Hull, A.G., 1987, Buried lowland soils from Willapa Bay, southwest Washington—Further evidence for recurrence of large

earthquakes during the last 5,000 years [abs.]: Eos, v. 68, no.

44, p. 1468–1469.

Mansinha, L., and Smylie, D.E., 1971, The displacement fields of

inclined faults: Seismological Society of America Bulletin, v.

61, p. 1433–1440.

Murty, T.S., and Hebenstreit, G.T., 1989, Tsunami amplitudes from

local earthquakes in the Pacific Northwest region of North

America—Part II, Strait of Georgia, Juan de Fuca Strait, and

Puget Sound: Sydney, B.C., Institude of Ocean Sciences,

Department of Fisheries and Oceans [joint report with Science

Applications International Corporation, McLean, Va.]. [Pages

not given.]

Plafker, George, 1969, Tectonics of the March 27, 1964, Alaska

earthquake: U.S. Geological Survey Professional Paper 543–I,

74 p.

Plafker, George, and Kachadoorian, Reuben, 1966, Geologic

effects of the March 1964 earthquake and associated seismic

sea waves on Kodiak and nearby islands, Alaska: U.S. Geological Survey Professional Paper 543–D.

Preuss, Jane, 1987, Coastal high hazard mitigation—Comprehensive planning for areas vulnerable to tsunamis, in American

Society of Flood Plain Managers, Proceedings: p. 317–322.

———1988a, Coastal effects of a great subduction earthquake—Regional land use implications, in Workshop on Earthquake Hazards in Puget Sound, Proceedings: U.S. Geological

Survey Open-File Report 88-541, p. 234–245.

———1988b, Planning for risk—Comprehensive planning for tsunami hazard areas: Seattle, Wash., Urban Regional Research,

for the National Science Foundation. [Pages not given.]

Preuss, Jane, and Hebenstreit, G.T., 1989, The tsunami threat in the

Pacific Northwest under today’s land use conditions, in Third

Annual Puget Sound/Portland Area Workshop on Earthquake

Hazards, Proceedings: p. 114–123.

Reinhart, T.J., and Bourgeois, Joanne, 1987, Distribution of anomalous sand at Wallapa Bay, Washington—Evidence for lage

scale landward-directed processes [abs.]: Eos, v. 68, no. 44, p.

1469.

Rogers, G.C., 1988, Seismic potential of the Cascadia Subduction

Zone: Nature, v. 332, p. 17.

Salsman, G.G., 1959, The tsunami of March 9, 1957, as recorded at

tide stations: Washington, D.C., U.S. Department of Commerce, Coast and Geodetic Survey, Technical Bulletin 6.

[Copies available from second author.]

Spaeth, M.G., and Berkman, S.C., 1972, The tsunamis as recorded

at tide stations and the seismic seawave warning system, in The

great Alaska earthquake of 1964—Oceanography and coastal

engineering: Washington, D.C., National Academy of Sciences, p. 28–110.

State of Washington Forecasting Division F87–08, 1987, Population trends for Washington State, 1988.

Symons, J.M., and Zetler, B.D., no date, The tsunami of May 22,

1960, as recorded at tide stations, preliminary report: Washington, D.C., U.S. Department of Commerce, Coast and Geodetic

Survey. [Copies available from second author.)

Thorsen, G.W., 1988, Overview of earthquake-induced water

waves in Washington and Oregon, in Workshop on Evaluation

of Earthquake Hazards and Risk in the Puget Sound and Portland Areas, Proceedings: p. 83–94.

Urban Regional Research, 1986, Hazard mitigation and preparedness plan—City of Aberdeen, for State of Washington Department of Emergency Management, Federal Emergency

Management Agency. [Pages not given.]

U.S. Bureau of the Census, 1990, 1990 census of population, Supplementary report, Metropolitan statistical areas, PC 80-S1-18.

U.S. Department of Commerce, 1953, The tsunami of November 4,

1952, as recorded at tide stations: Washington, D.C., U.S.

Department of Commerce, Coast and Geodetic Survey, Special

Publication 300. [Copies available from second author.]

U.S. Department of the Interior, 1988, Baseline socioeconomic profiles of coastal counties in the Washington-Oregon planning

area, Outer Continental Shelf (OCS): Washington, D.C., Minerals Management Service, Study MMS 88-0084.

Washington State Department of Natural Resources, 1987, Geologic map of Washington, southwest quadrant, prepared by T.

Walsh, M. Konsec, W. Phillips, R. Logan, and H. Schasse.

Wilson, B.W., and Törum, Alf, 1963, The tsunami of the Alaska

earthquake, 1964—Emergency evaluation: U.S. Army Corps

of Engineers Coastal Engineering Research Center, Technical

Memorandum 25, 444 p.
537

APPROACHES TO SEISMIC-HAZARD MITIGATION BY 

LOCAL GOVERNMENTS—AN EXAMPLE FROM

KING COUNTY, WASHINGTON

By Derek B. Booth1 and John P. Bethel2

ABSTRACT

In areas of rapid urban development, local governments

have the opportunity to effectively reduce seismic risk by

regulation. Basic risk-reduction strategies in hazardous

areas either limit the intensity of land use or apply more

stringent building requirements to the development that

occurs. These two strategies can be implemented through a

combination of methods, including policy-setting comprehensive plans, policy-implementing functional plans, building codes, and hazard-area delineation and regulation.

King County, Washington, has utilized all these methods in addressing seismic hazards. The county’s experience

with hazard-area delineation and regulation, through its Sensitive Areas Ordinance, is a particularly relevant example for

other jurisdictions to study. Major weaknesses in present

efforts to address seismic hazards include the poor data available from which to map hazard areas and the uneven quality

of site-specific reports by private consultants. Recommendations to any local government contemplating a similar

effort include a clear articulation of policies regarding basic

strategy; zoning designations that are explicitly derived from

functional plans and that reflect the stated policy; regional

hazard mapping that is conservative but credible; and sufficient staff geotechnical expertise to adequately establish and

implement an effective hazard-reduction program in cooperation with private geotechnical and design consultants.

INTRODUCTION

Local governments are pivotal in the mitigation of seismic hazards. They establish land-use policies, apply zoning,

and review all new development within their jurisdictions.

Whether intentionally or not, the policies and ordinances

administered by these governments determine the vulnerability of all new urban construction to damage from earthquakes. In established cities, the influence over new

construction may affect only a small proportion of the total

number of structures. Here, risk reduction may require more

aggressive efforts to upgrade existing buildings and ensure

postearthquake function of utilities and emergency operations. In rapidly growing regions, however, new development may become a significant, or even the predominant,

component of the built environment. We focus our attention

on these regions because the opportunities for effective risk

reduction are most promising and most attainable.

Once seismic hazards are recognized, reducing them

for new developments can involve one of two broad strategies. If the seismic risk is perceived as severe and cannot

be reduced, then intensive development can be prohibited

through zoning regulations. If the risk is perceived as

minor or can be mitigated to that level, then development

with appropriate conditions may be allowed. The choice of

strategies reflects a social rather than a purely technical

judgment about the ultimate severity of that risk. That

judgment also is likely to be influenced by the size of the

area affected, the certainty of the available hazard data, and

the economic impacts of the alternative strategies. In general, even in the most hazard-prone areas of North America,

only the latter strategy of development modification has

been actively pursued.

Within both of the strategies for hazard reduction, four

tools are available to local governments. They apply at different stages and in different ways to land development, but

each may be part of an overall effort to reduce seismic

hazards.

1. Comprehensive Plans. These documents establish landuse and land-development policy throughout a

region. They do not regulate land use themselves;

indeed, the area of a comprehensive plan developed

by a county may include incorporated cities over

which the county has no jurisdiction. These plans are

intended to establish the policy by which parcelspecific zoning decisions will subsequently be made.

________________ 1King County Basin Planning, 770 Dexter Horton Building, Seattle,

WA 98104, and Department of Geological Sciences, University of Washington, AJ–20, Seattle, WA 98195. 2King County Building and Land Development Division, Eastgate

Plaza, Bellevue, WA 98004.
538 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

They therefore define which of the two basic strategies, prohibition or modification, will guide future

efforts at seismic-hazard mitigation.

2. Functional Plans. These documents apply zoning regulations over large regions of the issuing jurisdiction.

They control allowable land uses and intensity of

development for specific parcels within these

regions. In theory, functional plans simply implement policies articulated in the comprehensive plan.

In practice, they may postdate the relevant policy

document to the extent that what is actually implemented reflects subsequent community or political

evolution.

3. Building Codes. Most municipalities in the Western

United States have adopted the Uniform Building

Code (UBC) as the basis for their building regulations. The seismic provisions of the UBC set standards for new structures. Where no other attention to

seismic hazards is given, the UBC is implicitly

assumed to satisfy any stated (or unstated) policies

regarding the minimum acceptable level of safety to

be provided to the public. Yet the seismic-design

section of the UBC addresses only one element of

seismic risk, lateral acceleration. Any seismic hazards other than those related specifically to lateral

forces on the structure need to be addressed through

other measures.

4. Zoning Overlays. Where a jurisdiction knows of or suspects the existence of specific areas that have an

enhanced risk of seismic damage, it may choose to

control development in those areas. This approach

requires the designation of specific hazard zones by

means of an overlay (that is, an area-specific change

to established zoning restrictions) and a procedure to

evaluate and condition development proposals that

lie in the areas so designated. One such effort, King

County’s Sensitive Areas Ordinance, is discussed in

detail in the following section.

ACKNOWLEDGMENTS

We thank our colleagues in King County’s Building and

Land Development Division, particularly geologist Steve

Bottheim, for development of the ideas in this chapter.

David Masters, a senior planner in the county’s Parks Planning and Resources Department, offered us valuable clarifications of zoning theory and practice, and improved the

discussions in the text through his review. Reviewer Martha

Blair-Tyler made extensive recommendations that have

allowed us to better focus our discussion. We hope that the

result will be of value to other jurisdictions contemplating a

similar program.

KING COUNTY—A CASE STUDY

INTRODUCTION

In the Puget Sound region, unincorporated King County

(fig. 249) has probably progressed furthest in specifically

addressing seismic hazards on undeveloped land (May,

1989). The county has used, to varying degrees, each of the

four basic approaches to hazard reduction listed in the previous section. The ultimate goal is defined by two policies of

the King County Comprehensive Plan, adopted in 1985:

E–308. In areas with severe seismic hazards, special building design and construction measures should be used to

minimize the risk of structural damage, fire, and injury

to occupants, and to prevent postseismic collapse. 

E–309. Prior to development in severe seismic hazard areas,

builders should conduct special studies to evaluate seismic risks and should use appropriate measures to reduce

the risks.

These policies mandate, at most, modification but not

prohibition of development, reflecting the prevailing local

attitude towards seismic risk. This approach contrasts with

the treatment afforded certain other types of geologic hazards, such as active landsliding or coal-mine subsidence. In

areas of such hazards, policies and subsequently adopted

restrictions effectively prohibit most development on the

constrained parts of the sites. Subsequent functional plans

have reiterated these two seismic-hazard policies and do not

implement land-use restrictions on the basis of seismic risk.

King County uses both area-wide and site-specific

approaches to reduce seismic hazards. These approaches

predate the 1985 Comprehensive Plan, and thus the Comprehensive Plan policy does not guide but simply reiterates

124° 122° 120° 118°

49°

48°

47°

46°

0

0 100 KILOMETERS

50 100 MILES

50

Seattle

King

County

Puget

Sound

WASHINGTON

CASCADE

RANGE

OLYMPIC

MOUNTAINS

Figure 249. Index map of Washington showing location of King

County.
SEISMIC-HAZARD MITIGATION, KING COUNTY, WASHINGTON 539

established practice. Area-wide control is provided by the

seismic provisions in the UBC. Site-specific risk-reduction

measures for hazards not addressed by the UBC have been

provided by the county’s Sensitive Areas Ordinance,

adopted in 1979 and substantially revised in 1990. That

ordinance also designates landslide, erosion, and coal-minehazard areas and provides for studies and site-specific mitigation to avoid the worst consequences of development in

geologically hazardous areas.

The Sensitive Areas Ordinance has proven more complex in its administration and far-reaching in its implications

than simple building codes. We therefore describe its elements and its application in some detail, because such an

approach to seismic-hazard reduction is probably most feasible for a wide range of local governments.

Several components are necessary for any regulatory

effort designed to mitigate any seismic or geologic hazard.

These include (1) definition of the hazard; (2) characterization of a set of hazardous site conditions; (3) delineation of

the hazard zones on a map; (4) screening of proposed development; and (5) review and conditioning of developments in

mapped hazard areas. Each component is described herein,

both in a general context and in light of King County’s specific experience.

HAZARD DEFINITION

Seismic hazards take a variety of forms. They are generally divided into the direct and indirect effects of earthquakes. Direct effects include immediate ground shaking

and displacement, ground rupture, differential settlement,

and liquefaction. Indirect effects, often as damaging or more

so, include landslides, tsunamis and seiches (ocean and lake

waves), floods from damaged dams or levees, and fire.

Planning efforts are typically motivated by past earthquakes; therefore, experience usually guides the choice of

relevant concerns in a particular region. In the Puget Sound

area, the 1949 and 1965 earthquakes (Thorsen, 1986) indicate that direct effects, particularly shaking-induced ground

failure and shaking of buildings, are the primary concerns in

this region. The indirect effects of landslides, seiches, and

liquefaction were reported in several localities as well but

were generally less severe.

CHARACTERIZATION OF HAZARDOUS

SITE CONDITIONS

Characterizing hazardous site conditions for seismic

hazards primarily involves attempting to recognize those

areas where earthquake damage will be anomalously high.

Any map of damage after a single earthquake shows some

areas where damage is as high as in other areas much closer

to the epicenter, and other areas where the effects appear

anomalously mild relative to the surrounding region (for

example, see Plafker and Galloway, 1989).

Conditions that will control the spatial variability of

earthquake-related damage include: (1) proximity to active

faults; (2) proximity to, and characteristics of, nearby water

bodies; (3) thickness, character, and stratification of surficial

deposits; (4) depth to ground water; and (5) site topography.

Any of these factors could in theory be made a part of

the basis for seismic zonation of an area (that is, the discrimination of areas of differing seismic hazard or risk). In practice, some of these determinants are more applicable or

usable than others.

In King County, only soil conditions and slope angle

are presently used to identify hazardous areas; other potential criteria are not applied. Historical earthquakes here have

been relatively deep seated, and no surface trace of active

faults in this part of the Puget Lowland has been unequivocally identified, so proximity to known faults is not relevant

(despite a few local examples of building setbacks from

inactive Tertiary-age faults). Tsunamis and seiches have not

caused significant damage in historical earthquakes.

Soil and substrate characteristics have long been

accepted as primary determinants of earthquake damage.

Areas underlain by thick deposits of low-strength, low-density soils have commonly been associated with severe earthquake damage (for example, see Bolt, 1988). Such damage

may result from liquefaction or amplification of low-frequency seismic waves. In King County, most of the soil has

been consolidated to a high density by multiple glacial episodes. The most extensive low-density deposits are therefore in areas where postglacial sedimentation has filled

valleys or depressions in the glaciated ground surface. The

Sensitive Areas Ordinance therefore identifies “recent alluvium and organic soils” as indicators of high seismic hazard.

Steep slopes have a potential for landslides during and

immediately following an earthquake. Therefore, the seismic provision of the Sensitive Areas Ordinance originally

included all slopes steeper than 15 percent as seismically

hazardous. Unfortunately, this attempt to include areas of

both low-density soil and potential slope instability as a single, undifferentiated hazard area on a single map reduced the

usefulness of the hazard mapping. For this reason, the 1990

revision to the ordinance deleted sloping areas and instead

treats seismically triggered landsliding as a part of the landslide-hazard review process.

MAPPING OF HAZARD ZONES

Ideally, the representation of seismic-hazard zones

would be based on complete topographic, hydrologic, geologic, and seismologic information. The risk from the direct

effects of ground shaking might be quantified by the maximum horizontal ground acceleration for an earthquake of

given energy release. These data could be mapped and

contoured based largely on soil and substrate properties.
540 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

Other, indirect effects then could be overlaid on this map

where relevant in order to define overall levels of risk.

In practice, the data and the resources are rarely available to make such detailed estimates. New mapping is

beyond the means of most local jurisdictions, and existing

soils and geologic maps are not specifically prepared to identify seismically hazardous soils. Although a complete data

source would show and identify the known types of seismic

hazards, including artificial fills, recent alluvial soils, lowdensity organic soils, thick unconsolidated deposits, and

landslide susceptibility, more commonly the information

available consists only of surface soil types (for example,

county soil surveys) and topography. The result is a much

more generalized hazard map, discriminating only relatively

“good” land from land that is more likely “bad.” King

County has this kind of generalized hazard map (fig. 250),

where the presence of unfavorable soils (alluvial or organic)

solely defines the hazard zones. About 10 percent of the land

area within the actively developing parts of the county is so

categorized.

Despite these deficiencies, the actual determinants of

seismic response in most regions correlate fairly well with

soils and slope information. Deep, unconsolidated deposits

are most common beneath surfaces of alluvial sediment,

which typically include areas of loose organic soil as well.

Saturation of these sediments is also common. Steeper

slopes correlate fairly well with landslide hazards. Yet, use

of soils maps may also identify areas where no increased

seismic hazard exists, such as shallow pockets of peat on an

undulating till surface or moderate-gradient hillslopes underlain by competent bedrock. Conversely, other seismic hazards may pass unnoticed, such as low-lying shorelines and

areas of recent artificial fill.

SCREENING OF DEVELOPMENT PROPOSALS

Once a map is prepared, affected development proposals must be screened. In King County, that authority was

created by the Sensitive Areas Ordinance, which requires

that virtually all proposals requiring a permit be checked

against a map showing so-called hazardous and nonhazardous areas. The process is quite straightforward; the location

of the project is checked on a 1:62,500-scale map of hazard

areas by the intake permit technician (for building permits) or

lead planner (for subdivisions or other large projects). If the

project falls within a hazard area, it is referred to a staff

geotechnical specialist for further review.

0

0

1

1

2 MILES

0.5 2 KILOMETERS

0.5

122°05'00" 122°02'30" 122°00'00" 121°57'30"

Sammamish Lake

35 36

3 2 1 5

9 10 11 8

47°35'

Figure 250. Part of a seismic-hazard map of King County, Washington. Class III areas (dark shade) are defined and regulated under the county’s

Sensitive Areas Ordinance (King County, 1990). Actual maps are at a scale of 1:62,500.
SEISMIC-HAZARD MITIGATION, KING COUNTY, WASHINGTON 541

REVIEW AND CONDITIONING OF

PROPOSALS

Once a project has been identified to be in a seismichazard zone, the geotechnical specialist must typically

choose among the following alternatives to determine if any

additional review of the project is needed:

1. Because of the nature of the project, no concern is warranted (for example, kitchen remodeling without any

structural change to the building).

2. Despite the project’s apparent location within a mapped

hazard zone, no concern is warranted (for example,

the site is not actually in the hazard zone because of

known mapping error or map-reading error).

3. The project lies in a seismic-hazard zone, but the seismicity concerns will be adequately addressed in solving

other, more severe, site constraints (such as excessive depth to bearing soil, or active landslide threat).

This alternative is most commonly chosen for

projects in the seismic-hazard zones in King County.

4. The seismic hazard is in fact a significant concern for the

project and requires specific mitigation.

A local government will typically proceed in a similar

fashion for either of the last two options, where conditions or

requirements beyond the standard zoning and building codes

are deemed necessary. The applicant will be directed to hire

a professional consultant, normally a geologist or an engineer, to perform a detailed site evaluation and to design an

appropriate solution that will be submitted for review (usually) to the local jurisdiction. Detailed site evaluations are

routinely required because the existing information regarding site conditions is seldom enough to develop appropriate

mitigation. Site evaluations typically characterize the

ground-water conditions and address the depth, density, and

texture of the subgrade soils. For seismic hazards in King

County, proposed mitigations have included subgrade

replacement, alternative foundation systems, or improved

site drainage. At many sites, these efforts also represent

engineering solutions to other nonseismic problems that

reduce the seismic hazard to a level equivalent to

nonhazardous areas.

EVALUATION OF KING COUNTY’S

MITIGATION EFFORTS

King County’s primary effort to reduce seismic hazards

has several key components. A zoning overlay has been

established that defines a method for requiring geotechnical

evaluation, thus achieving additional engineering mitigation.

No change (that is, no reduction) in the intensity of land use

is intended or achieved. Relevant seismic hazards have been

identified, namely landsliding and ground failure. A map of

these hazard zones has been prepared to screen development

proposals. Special engineering studies, prepared by the

applicant’s consultants, assess any seismic risk and

necessary mitigation. Finally, geotechnical review by the

county’s staff maintains consistency and minimum competency of the mitigation procedures finally adopted.

In this process, two elements are particularly weak.

The first is the mapping of hazard areas. Critical because of

the sheer volume of development activity (more than 10,000

permits processed in King County in 1989), the seismic-hazard map is imperfectly correlated with zones of actual seismic hazard. King County’s current seismic-hazard map

displays the extent of several Soil Conservation Services soil

types that have been identified as being seismically sensitive

(Rasmussen and others, 1974). In practice, it has become

apparent that many areas designated as hazardous on this

map are not particularly hazardous. Other potentially relevant determinants of seismic hazards have not been fully

considered. For example, liquefaction potential is identified

only by surface soil types; subregional variability in earthquake intensity because of focusing effects or particularly

thick, unconsolidated deposits is nowhere identified. Other

potential hazards, particularly seiches or dam breaks, are

simply not included in the mapping of any geologic hazard.

The second weak element is the reliance on special

engineering studies for specific mitigation strategies. The

structural and geotechnical engineering community has a

broad range of experience and knowledge in addressing seismic hazards. In the Pacific Northwest, there is little consensus in the geotechnical community on a standard of practice

for evaluating site-specific seismic hazards. This lack of

consensus is especially apparent in reviewing geotechnical

reports for small to moderate-sized projects (residences or

small commercial structures). The areas where geotechnical

practice is most variable include selection of a design earthquake (namely, the size of the largest earthquake of concern), the scope of adequate subsurface exploration, and

appropriate mitigation measures for identified hazards. King

County is fortunate in having staff for review, but reliance on

these outside studies for design is unavoidable. Currently,

the county staff consists of three engineering geologists for

all aspects of geotechnical review of development proposals.

However, more than 25 years has passed since the last major

earthquake in the region. Thus, the experience of local consultants is commonly limited, resulting in reports that vary

widely in scope, analytical methodology, and design

recommendations.

RECOMMENDATIONS

King County has more than 10 years of experience in

implementing a program of seismic-hazard reduction

through regulation of land use and building construction.

The following recommendations are largely based on this

experience and are offered for consideration by other local

jurisdictions contemplating a similar program. Their value,

however, will be known only after the next large earthquake
542 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

in the Puget Lowland, followed by a review of developments

that were built under this program.

1. Establish Clear Policy. The jurisdiction’s comprehensive

land-use plan needs to define clearly the policy

towards land development in seismically active

areas. Without this foundation, subsequent efforts at

hazard mitigation will either lack consistency or

establish only informal policy. Under the authority

of a comprehensive plan, the existence and significance of the seismic threat should be stated clearly,

and the types of seismic hazards specific to the jurisdiction should be identified explicitly. Finally, a general framework for hazard mitigation should be

established for ultimate implementation through

functional plans, building codes, and zoning

overlays.

2. Use Policy and Zoning to Minimize Risk. Functional

plans, which implement the land-use policies of the

comprehensive plan, should reflect both the policy

towards and the nature of the seismic hazards. If the

hazard can be mitigated during development, then the

seismic-hazard delineation should be a factor weighing against intensive land uses but not precluding all

uses. This kind of decision would apply to areas subject to liquefaction or settlement of uncontrolled fill,

for example. Even hazards that can be mitigated

should factor into decisions on locating intensive

land uses because of the additional cost of public service to such areas and the potential that mitigation

may not be effective. If the hazard (for example, that

of potential inundation by tsunamis) cannot be effectively mitigated during development, the hazard

should preclude intensive structural land uses (for

example, see Nichols and Buchanan-Banks, 1974).

Such areas should be set aside for agriculture, recreation, natural-resource production, or other uses that

minimize life and property risks.

3. Map Accurately and Conservatively. Although maps

associated with zoning overlays are vital to efficient

implementation, community-wide seismic-hazard

maps are less detailed and less accurate than site-specific studies. For this reason, hazard mapping should

be represented and understood as a guideline to the

general distribution of seismically sensitive areas

rather than as a definitive delineation of such areas.

Because the hazard mapping will be approximate, it

should err on the side of including too much area in

the hazard zone. Errors of this type can be identified

during site-specific evaluations. The mapping, however, should not be so conservative that it loses credibility as a useful hazard predictor. It should also

seek to incorporate data beyond soil surveys, and it

should be updated as new information becomes

available.

4. Encourage Uniform Standards for Study Scope and Quality. Jurisdictions should encourage a more uniform

approach to seismic-hazard evaluation by working

with engineering design professionals and technical

experts to establish some baseline hazard-evaluation

criteria. In particular, these criteria may include designation of an appropriate design earthquake and

establishment of a minimum scope of study for sites

in designated seismic-hazard areas. Recent revisions

to King County’s Sensitive Areas Ordinance provide

that authority and also allow more stringent criteria

for certain critical structures such as schools, hospitals, and emergency centers.

5. Provide In-House Expertise. Effective implementation of

a seismic-hazard-mitigation program requires geotechnical expertise within the jurisdiction as well by

the applicant’s consultant. Larger governmental bodies, such as the City of Seattle and King County, can

justify maintaining a full-time geotechnical staff.

This staff is available to assist in all phases of permit

processing in seismic-hazard areas, from initial

screening to review of construction inspection

reports. Smaller municipalities will contract geotechnical review to private consultants, whose overall

role in permit processing typically will be more limited. The one step in the permit-review process

where geotechnical expertise is most clearly required

is the evaluation of geotechnical studies submitted by

the applicant. This is the stage at which adherence to

a consistent minimum standard of practice must be

assured. Yet, without established, well-founded criteria for such a standard, the final results may be far

short of needs.

REFERENCES CITED

Bolt, B.A., 1988, Earthquakes: New York, W.H. Freeman and Co.,

282 p.

King County, 1990, Sensitive areas map folio: Seattle, King County Department of Parks, Planning and Resources, scale

1:62,500.

May, P.J., 1989, Anticipating earthquakes—Risk reduction policies

and practices in the Puget Sound and Portland areas: Seattle,

University of Washington Institute for Public Policy and Management, 31 p.

Nichols, D.R., and Buchanan-Banks, J.M., 1974, Seismic hazards

and land-use planning: U.S. Geological Survey Circular 690,

33 p.

Plafker, George, and Galloway, J.P., eds., 1989, Lessons learned

from the Loma Prieta, California, earthquake of October 17,

1989: U.S. Geological Survey Circular 1045, 48 p.

Rasmussen, N.H., Millard, R.C., and Smith, S.W., 1974, Earthquake hazard evaluation of the Puget Sound region, Washington State: Seattle, University of Washington Geophysics

Program, 99 p.

Thorsen, G.W., 1986, The Puget Lowland earthquakes of 1949 and

1965: Olympia, Washington Division of Geology and Earth

Resources, Information Circular 81, 113 p.
543

LIABILITY FOR EARTHQUAKE HAZARDS OR LOSSES 

AND ITS IMPACTS ON THE CITIES AND

COUNTIES OF WASHINGTON

By Jeanne B. Perkins1 and Kenneth K. Moy2

ABSTRACT

Based on a legal analysis of Washington State tort law,

local governments can be held liable for some earthquake

losses. The most likely sources of liability are injuries or

damage caused by the dangerous condition of governments’

own properties. Local governments also have potential liability for injuries on private properties, although the exposure is less than on public property. Discretionary immunity

is the only defense for losses resulting from the issuing of

earthquake warnings or from emergency response activities.

The act of God defense may prevail in only two very limited

situations: (1) if the earthquake was of such type or size as to

be unforeseeable, and the local government did not act negligently with respect to dealing with a foreseeable earthquake; or (2) if the earthquake was foreseeable and the local

government took all reasonable actions to prevent harm but

damage still occurred. Local government employees in

Washington, California, Utah, and Alaska rank potential liability as one of the top five factors motivating earthquakehazard-reduction programs.

THE EXTENT OF LOCAL 

GOVERNMENT LIABILITY IN 

WASHINGTON

Local governments can be held liable for some earthquake losses. The Washington Tort Claims Act provides

that all political subdivisions and municipal corporations of

the State are held liable for damages for their or their

employees’ tortious conduct to the same extent as if they

were a private person or a corporation (Revised Code of

Washington, Sec. 4.96.010). Thus, sovereign immunity in

Washington is abolished without any statutory exceptions.

The legal defense that earthquakes are an act of God

will no longer prevail in most circumstances. In the legal

sense, an act of God is a natural event, causing damage,

over which people have no control. Although in one sense

an earthquake is beyond our control, the facts are that damaging earthquakes occur in Washington, and scientists are

gaining increased knowledge about them and their processes. Earthquakes and the damage resulting from earthquakes may be foreseeable, and under some circumstances

the losses can be at least partly mitigated. Therefore, the

act of God defense to tort liability may prevail in only two

very limited situations: (1) if the earthquake was of such

type or size as to be unforeseeable, and the local government did not act negligently with respect to dealing with a

foreseeable earthquake; or (2) if the earthquake was foreseeable and the local government took all reasonable

actions to prevent harm but damage still occurred.

SOME KEY LEGAL CONCEPTS

A tort is a civil (as opposed to criminal) wrong, other

than a breach of contract, for which courts award damages.

A tort has four elements: (1) A pertinent duty must be

imposed on the defendant (local government); (2) the defendant (local government) must have violated that duty; (3) the

victim must have been injured or suffered damages; and (4)

there must be a causal connection between the negligence

and the harm suffered by the victim. 

Negligence is the usual standard by which a defendant’s

actions are judged in order to determine whether a duty was

violated. The concept of negligence is commonly based on

the rule of reasonableness. How would a reasonable person

have acted under similar circumstances? Could the injury or

loss have been foreseen? What was the apparent magnitude

of the risk? What were the relative costs and benefits of

action versus inaction? Has the defendant complied with

applicable statutory or regulatory standards?

________________ 1Earthquake Program Manager, Association of Bay Area Governments, Oakland, Calif. 2Legal Counsel for Association of Bay Area Governments, Moy and

Lesser, Berkeley, Calif.
544 ASSESSING EARTHQUAKE HAZARDS AND REDUCING RISK IN THE PACIFIC NORTHWEST

If negligence is established, the local government can

raise applicable defenses, the most important of which is the

discretionary immunity. The discretionary immunity applies

to basic policy decisions that have a planning, rather than an

operational, character. For example, a city council’s decision to enact a law requiring a landowner to disclose geologic

and soil conditions prior to selling their property or building

on it would be immune to liability as a discretionary function.

The city manager’s decision to waive the requirement may or

may not be immune, depending on the language of the ordinance, the factors used to make the decision, and the State in

which the manager is located. The clerk who issues a building permit without requiring the disclosure document is not

immune under the discretionary immunity.

The discretionary immunity was created by the Washington judiciary as an exception to the rule of general liability

of local governments (see Evangelical United Brethren v.

State, 67 Wn. 2d 246, 407 P.2d 440 (1965)). To avail itself

of the immunity, the act of a local government must meet a

four-part test. The challenged act must (1) involve a basic

governmental policy, program, or objective; (2) be essential

to the realization or accomplishment of that same policy, program, or objective; (3) require the exercise of basic policy

evaluation, judgment, and expertise; and (4) be performed by

the government agency having the requisite authority and

duty to perform the act. In addition, the municipality must

demonstrate that it actually exercised discretion (see King v.

Seattle, 84 Wn. 2d 239, 525 P.2d 228 (1974)). Finally,

Washington appears to give some weight to the position of

the decision maker in the governmental hierarchy in assessing whether the decision was truly discretionary (ChambersCastanas v. King County, 100 Wn. 2d 275, 869 P.2d 451

(1983)). However, the issue of position in the hierarchy is

less important than in some other States, such as California.

A duty may be imposed on the local government under

the public-duty doctrine, which applies when a so-called

special relationship exists between the local government

and the victim. If the enabling statute for the governmental

action states a clear legislative intent to protect an identifiable class of persons and a member of the class is injured,

then a special relationship exists (see Baerlein v. State, 92

Wn. 2d 229, 595 P.2d 930 (1979)). A general duty to regulate private-sector activity for the benefit of the general

public does not create a special relationship. If a statute

obligates a local entity to abate a special known and dangerous condition, failure to do so will create a special relationship between a plaintiff and a defendant (see Campbell v.

City of Bellevue, 85 Wn. 2d 1, 530 P.2d 234 (1975)). If an

injured party relies on expressed or implied assurances by a

governmental agency with whom the party had direct contact, a special relationship also may be created.

It is also important to understand the role of judges and

juries in a jury trial. Questions of fact (concerning whether

something is factually true or untrue or whether something

did or did not occur) are the province of the jury except under

extreme circumstances. Questions of law (requiring that the

law be interpreted or applied) are the exclusive province of

the judge. However, it is the jury that decides (as a matter of

fact) whether or not it would have been reasonable to do

more, thereby determining (as a matter of law) that negligence exists. The legal community refers to such questions

as mixed questions of fact and law.

LIABILITY VARIES WITH THE 

CIRCUMSTANCES

The most likely sources of liability for a local government in Washington are injuries or damages caused by the

dangerous condition of its own property—its hospitals, city

halls, jails, and public works. In most situations, the injured

party will rely on traditional tort analysis in establishing liability. The possible use of the discretionary immunity for

certain decisions regarding public facilities (such as decisions to build and siting) is undeveloped as of 1992.

Washington’s local governments also have potential liability for injuries on private property, although the exposure

is less than on public property. Local governments in most

States are immune from liability for most actions relating to

the issuing of permits or inspection activities. Washington

courts, on the other hand, have consistently found liability for

building-inspection and permitting activities so long as the

injured party can establish a special relationship under Washington’s public-duty doctrine.

For losses resulting from emergency-response activities, discretionary immunity is the only defense for local

governments in Washington. Emergency-response field

decisions are probably not immune. Therefore, the question

of whether the public entity owes a duty to the injured party

is pivotal. However, this question is difficult to analyze

without knowing the specific circumstances of the situation.

In Washington, there is no statute granting immunity for

the issuance of an earthquake warning. The discretionaryfunction immunity is the only defense to liability.

THE IMPACT OF LIABILITY ON 

LOCAL-GOVERNMENT

DECISION MAKING

Using written questionnaires and in-person interviews

with local-government staff in Washington, Alaska, California, and Utah during 1978, we arrived at several conclusions

about the effects of liability on the creation and motivation of

earthquake mitigation programs. Even though a higher risk

of earthquakes and earthquake-produced hazards events can

trigger the initiation of programs, mere higher risk is an

incomplete picture of the motivation process. Local-government staff in the four States ranked 10 factors motivating
LIABILITY FOR EARTHQUAKE HAZARDS OR LOSSES AND ITS IMPACTS ON WASHINGTON 545

earthquake-hazard-reduction programs in their jurisdictions.

The top five (in order) were:

1. Leadership of a staff member or elected official.

2. The need to maintain local government functions.

3. Concern for potential liability.

4. Improved public safety.

5. A State or Federal government requirement.

The top five of Washington government staff were (there

was a tie for third place):

1. A State or Federal government requirement.

2. Leadership of a staff member or elected official.

3. The need to maintain local government functions;

 Avoiding employee injury.

5. Concern for potential liability.

Liability was ranked even higher among those local

governments with most active earthquake programs in the

four States surveyed. However, it was listed first by very

few local governments when compared to the other top motivators; it ranks high because it is pervasive. There is no single motivator for earthquake-hazard-reduction programs; the

motivators are as diverse as the jurisdictions themselves.

More than 90 percent of local governments surveyed in

the four-State area (and 80 percent of those in Washington)

believe that the law is at least sometimes uncertain. In the

four-State area, half of those noting uncertainty think that

this has little or no effect on their jurisdiction, one-quarter

think that it encourages aggressive hazard-reduction programs, and one-quarter think that it discourages programs.

In Washington, about 40 percent think that the uncertainty

has little or no effect on their jurisdiction, 30 percent think

that it encourages action, and 30 percent think that it discourages action. Managers from jurisdictions that have the most

comprehensive earthquake-mitigation programs are more

likely to believe that this uncertainty encourages action.

Managers from jurisdictions with active earthquakehazard-reduction programs do not perceive significantly

more or less liability exposure than the entire group surveyed. Thus, there is no indication that any major change in

rules governing liability or immunity would result in more

active earthquake-hazard-reduction programs. A general

concern for liability rather than a specific perception of the

degree of liability exposure appears to motivate earthquakehazard-reduction programs. About two-thirds of those surveyed reported concern for liability for earthquake hazards

within their jurisdictions, double the number of a similar survey conducted by ABAG (Association of Bay Area Governments) in 1978.

Jurisdictions with active earthquake-hazard-reduction

programs tend to be self insured with active risk-management programs created to control losses associated with general liability claims. However, we concluded that the

existence of active risk-management programs and active

earthquake-hazard-reduction programs are the result of a

progressive top management and stable elected bodies promoting safety awareness, rather than risk management

somehow causing the earthquake-hazard-reduction program

to be more active.

PROMOTING SAFETY WHILE COPING 

WITH LIABILITY—SOME ADVICE 

FOR LOCAL GOVERNMENTS

As a result of this study, the following recommendations are made for local governments to promote public

safety and cope with liability:

1. Local governments should comply with any statutory or

regulatory standards imposed by State or Federal

governments.

2. A program should be developed to inspect, repair, and

maintain public buildings and facilities.

3. The local government’s risk manager can help promote

increased earthquake-hazard safety in all public

facilities and buildings.

4. Local-government staff should not assume that liability

exposure exists for any mitigation program involving private property. Ask advice from your legal

counsel. 

5. Act to promote the safety and welfare of the people in

your community. If you act reasonably, your liability

exposure can be minimized.

The preceding summary highlights the findings of a

research project conducted by Perkins and Moy (1989). In

addition, a companion document (Perkins and Moy, 1988)

contains the background legal research, summaries of case

law and statutes, and the results of the survey of localgovernment behavior. 

ACKNOWLEDGMENTS

The authors wish to thank Professor David Jung, Hastings College of the Law, University of California, San

Francisco, and Crane Miller, attorney and legal researcher,

Washington, D.C., for their thoughtful comments on this

report.

REFERENCES CITED

Perkins, J.B., and Moy, Kenneth, 1988, Liability of local governments for earthquake hazards and losses—Background and

research reports: Available from the Association of Bay Area

Governments, P.O. Box 2050, Oakland, CA 94604–2050,

250 p.

———1989, Liability of local governments for earthquake hazards

and losses—A guide to the law and its impacts in the states of

California, Alaska, Utah, and Washington: Available from the

Association of Bay Area Governments, P.O. Box 2050,

Oakland, CA 94604–2050, 52 p.