AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY 

Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the current-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior to the current year are listed in the most recent annual "Price and Availability List" Publications 

that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and 

Availability List" are no longer available. 

Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated monthly, which is for sale in microfiche from the U.S. Geological Survey, Books and Open-File Reports Section, Federal Center, Box 

25425, Denver, CO 80225. Reports released through the NTIS may be obtained by writing to the National Technical Information 

Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. 

Order U.S. Geological Survey publications by mail or over the counter from the offices given below. 

BY MAIL 

Books 

Professional Papers, Bulletins, Water-Supply Papers, Techniques 

of Water-Resources Investigations, Circulars, publications of general interest (such as leaflets, pamphlets, booklets), single copies of Earthquakes 

& Volcanoes, Preliminary Determination of Epicenters, and some miscellaneous reports, including some of the foregoing series that have gone 

out of print at the Superintendent of Documents, are obtainable by mail 

from 

U.S. Geological Survey, Books and Open-File Reports 

Federal Center, Box 25425 

Denver, CO 80225 

Subscriptions to periodicals (Earthquakes & Volcanoes and 

Preliminary Determination of Epicenters) can be obtained ONLY from 

the 

Superintendent of Documents 

Government Printing Office 

Washington, D.C. 20402 

(Check or money order must be payable to Superintendent of Documents.) 

Maps 

For maps, address mail orders to 

U.S. Geological Survey, Map Distribution 

Federal Center, Box 25286 

Denver, CO 80225 

Residents of Alaska may order maps from 

Alaska Distribution Section, U.S. Geological Survey, 

New Federal Building - Box 12 

101 Twelfth Ave., Fairbanks, AK 99701 

OVER THE COUNTER 

Books 

Books of the U.S. Geological Swvey are available over the 

counter at the following Geological Survey Public Inquiries Offices, all 

of which are authorized agents of the Superintendent of Documents: 

• WASHINGTON, D.C.--Main Interior Bldg., 2600 corridor, 

18th and C Sts., NW. 

• DENVER, Colorado--Federal Bldg., Rm. 169, 1961 Stout St. 

• LOS ANGELES, California--Federal Bldg., Rm. 7638, 300 N. 

Los Angeles St. 

• MENLO PARK, California--Bldg. 3 (Stop 533), Rm. 3128, 

345 Middlefield Rd. 

• RESTON, Vfrginia--503 National Center, Rm. 1C402, 12201 

Sunrise Valley Dr. 

• SALT LAKE CITY, Utah--Federal Bldg., Rm. 8105,125 

South State St. 

• SAN FRANCISCO, California--Customhouse, Rm. 504, 555 

Battery St. 

• SPOKANE, Washington--U.S. Courthouse, Rm. 678, West 

920 Riverside Ave .. 

• ANCHORAGE, Alaska--Rm. 101, 4230 University Dr. 

• ANCHORAGE, Alaska--Federal Bldg, Rm. E-146, 701 C St. 

Maps 

Maps may be purchased over the counter at the U.S. Geological Survey offices where books are sold (all addresses in above list) and 

at the following Geological Survey offices: 

• ROLLA, Missouri--1400 Independence Rd. 

• DENVER, Colorado--Map Distribution, Bldg. 810, Federal 

Center 

• FAIRBANKS, Alaska--New Federal Bldg., 101 Twelfth Ave. 
Lessons Learned from 

the Lorna Prieta, California, Earthquake 

of October 17, 1989 

George Plafker and John P. Galloway, Editors 

With contributions by 

Michael J. Bennett 

David M. Boore 

Roger D. Borchardt 

William M. Brown Ill 

Robert 0. Burford 

Mehmet Celebi 

Malcolm M. Clark 

James H. Dieterich 

Donna M. Eberhart-Phillips 

William L. Ellsworth 

Thomas C. Hanks 

Ralph A. Haugerud 

Thomas L. Holzer 

Malcolm J. Johnston 

David K. Keefer 

William J. Kockelman 

Kenneth R. LaJoie 

Michael Lisowski 

Andrew J. Michael 

Daniel J. Ponti 

CarolS. Prentice 

William H. Prescott 

Paul A. Reasenberg 

Michael J. Rymer 

James C. Savage 

David P. Schwartz 

Robert V. Sharp 

Paul C. Thenhaus 

John C. Tinsley Ill 

Peter L. Ward 

Ray E. Wells 

U.S. GEOLOGICAL SURVEY CIRCULAR 1045 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

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 

Catalog Card Number: 89-600786 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1989 

Free on Application to the 

Books and Open-File Reports Section, 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 
CONTENTS 

Introduction 1 

Geologic setting 4 

The earthquake and its aftershocks 5 

Surface deformation 9 

Surface expression of faulting 9 

Ground shaking 17 

Landslides 23 

Liquefaction and related effects 25 

Damage to buildings, transportation routes, and utilities 28 

The potential for future large earthquakes 35 

Reducing earthquake hazards 36 

Conclusion 40 

Acknowledgments 40 

References cited 41 

Appendix 1. Glossary 43 

Appendix 2. General references and sources of information on earthquakes 46 

FIGURES 

1. Map showing mainshock epicenter of Lorna Prieta earthquake and 

inferred fault rupture in relation to areas affected by the earthquake 2 

2. Block diagram illustrating relative motion of Pacific and North American 

plates and principal active strike-slip faults in San Francisco Bay 

region 4 

3. Block diagrams illustrating earthquake cycle for right-lateral strike-slip 

fault according to elastic rebound theory 5 

4. Schematic diagram showing inferred motion on San Andreas fault during 

Lorna Prieta earthquake 6 

5. Cross sections showing seismicity along San Andreas fault from north of 

San Francisco to south of Parkfield 6 

6. Map and cross sections showing spatial distribution of aftershocks of Lorna 

Prieta earthquake in relation to San Andreas fault 7 

7. Graph showing time distribution of magnitude 3 or greater aftershocks of 

Lorna Prieta earthquake 8 

8. Map showing geodolite lines and GPS vectors that were observed 

following Lorna Prieta earthquake 9 

9. Map showing prominent surface cracks and net-displacement vectors near 

Summit Road, Santa Cruz Mountains 10 

10. Photographs showing crack system near Summit Road, 1/2 mile southeast 

of Highway 17 11 

11. Photograph showing cracks near Summit Road along southwest edge of 

elongate, closed depression 13 

12. Diagram showing crack trends and displacement vectors for surface cracks 

in Summit Road area 13 

13. Photographs showing larger of two cracks that broke Morrill Road by leftlateral motion in both 1906 San Francisco earthquake and 1989 Lorna 

Prieta earthquake 14 

14. Map showing areas of ground cracks and compressional deformation in 

Los Gatos and vicinity 16 

Contents Ill 
IV Contents 

15. Photograph of sewer grate in Los Gatos showing effects of 

compression 17 

16. Preliminary map showing distribution of modified Mercalli intensity for 

Lorna Prieta earthquake 18 

17. Map showing free-field, peak horizontal acceleration of ground motion 

during Lorna Prieta earthquake 20 

18. Graphs showing peak horizontal accelerations of different geologic 

materials during Lorna Prieta earthquake 21 

19. Three-component accelerograms showing large difference in trace 

amplitudes between bedrock and unconsolidated deposits 22 

20. Aerial photograph of rock slide near summit of Santa Cruz 

Mountains 23 

21-26. Photographs showing: 

21. Small rock-fall deposit caused by earthquake at roadcut in Santa Cruz 

Mountains 24 

22. Scarps and cracks associated with movement of deep-seated 

slumps 26 

23. Lateral spreading along Pajaro River banks 28 

24. Structures damaged in Marina district 29 

25. Extension cracks in Marina district 30 

26. Sand boils in Marina district 31 

27. Map showing three temporary seismograph stations in Marina 

district 32 

28-31. Photographs showing: 

28. Sand boils at Oakland International Airport 33 

29. Sand boils in irrigated fields near Hollister 33 

30. Collapsed Cypress structure on Interstate 880 34 

31. Highway 1 bridge destroyed at Struve Slough 36 

32. Map of segments of San Andreas and Hayward faults showing chance of 

occurrence of an earthquake in the next 30 years and Santa Cruz segment 

of San Andreas fault that slipped during Lorna Prieta earthquake 38 

33. Map showing predicted maximum intensity of ground shaking from large 

earthquakes on San Andreas and Hayward faults 39 

TABLES 

1. Modified Mercalli intensity scale 19 

2. Chronology of historical earthquakes in the San Francisco Bay region 37 
Lessons Learned from the Lorna Prieta, California, Earthquake 

of October 17, 1989 

George Plafker and John P. Galloway, Editors 

INTRODUCTION 

The magnitude 7.1 Lorna Prieta earthquake (5:04 

p.m. P.d.t., October 17, 1989) is the largest earthquake to 

strike the San Francisco Bay region and environs (fig. 1 ), 

home to more than 5.9 million people, since the great San 

Francisco earthquake of 1906. It was felt over an area of 

approximately 400,000 square miles, from Los Angeles 

on the south to the Oregon-California State line on the 

north, and to western Nevada on the east. Within about 

15 seconds of seismic shaking of the region extending 

from Monterey Bay to northern San Francisco Bay, the 

Lorna Prieta earthquake resulted in: 

• 62 known deaths, 3,757 injuries, and more than 12,000 

people homeless.1 

• Over $6 billion property damage.1 

• Disrupted transportation, utilities, and communications. 

Losses in public and private property already place it 

near the top of the list of America's most expensive 

natural disasters, and it is the most costly earthquake 

since 1906. Coinciding with the third baseball game of 

the World Series in San Francisco, the first major league 

baseball game ever canceled on account of an earthquake, the Lorna Prieta earthquake set a record for 

playing to the largest television audience ever to witness 

the direct effects of an earthquake. The only larger 

earthquakes to have affected the United States since 

1906 are the magnitude 7.5 Kern County, California, 

earthquake of July 21, 1952 (12 dead; $49 million damage) and the magnitude 9.2 great Alaskan earthquake of 

March 27, 1964 (130 dead,$0.5 billion property damage 

in 1987 dollar-equivalent). 

Manuscript approved for publication, November 22, 1989. 1

State of California, Governor's Office of Emergency Setvices, 

written communication, November 21, 1989. 

The Lorna Prieta earthquake ruptured a segment of 

the San Andreas fault beneath the Santa Cruz Mountains-a segment that had been recognized as having the 

greatest chance (30 percent for the next 30 years) for 

producing a magnitude 6.5 to 7 earthquake of any fault 

segment north of the Mojave Desert in southern California (U.S. Geological Survey, 1988). 

Just as the location and size of this earthquake were 

no surprise to earthquake specialists, neither were its 

principal effects-although, as in all large earthquakes, 

there were unexpected consequences and new lessons to 

be learned. Principal but preliminary findings are: 

• Seismologic and geodetic data indicate that the earthquake in the Santa Cruz Mountains was accompanied 

by slip along a 25-mile-long segment of the San 

Andreas fault that ruptured the Earth's crust to a 

depth of about 11 miles. Displacement on the fault, 

which dips to the southwest at an angle of about 70°, 

amounted to about 6 feet horizontally and 4 feet 

vertically; the southwestern side moved northwestward and upward with respect to the northeastern 

side. The displacement differs significantly from the 

dominantly horizontal movements on near-vertical 

planes that have characterized seismicity and historical surface ruptures along most segments of the San 

Andreas fault. 

• There were no known short-term seismic or strain 

precursors to warn of the impending earthquake. 

• An unusual aspect of the earthquake is the absence of 

recognizable primary surface faulting; instead, a zone 

as much as 3 miles wide of numerous ground cracks 

along and near the surface trace of the San Andreas 

fault suggests strain was distributed over a broad area. 

Displacement across many of these cracks was large 

enough to damage houses and roads; many of them 

also appear to have been the locus for landsliding. 

Similar features were observed in this region in 1906. 

This type of distributed cracking constitutes a previously unappreciated category of earthquake hazard, 

one that can extend well beyond the well-defined and 

narrow fault trace that typifies most of the San Andreas fault. 

Introduction 
• As obseiVed in the 1906 earthquake, and predicted in 

maps of future earthquake effects, seismic shaking 

was locally amplified. Significant amplification oc37' 

30' 

37' 

00' 

122' 30' 

0 

122'00' 

10 MILES 

CONTOUR INTERVAL 

500 FEET 

curred in areas of unconsolidated deposits and manmade fill over unconsolidated deposits that are widely 

distributed around the margins of Monterey Bay and 

121'30' 

Figure 1. Mainshock epicenter of Lorna Prieta earthquake and inferred fault rupture relative to areas of 

larger aftershocks, abundant ground cracks, and landslides and to limits of structural damage. Also shown 

are locations of major damaged structures and principal areas of ground cracks and liquefaction. 

2 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
San Francisco Bay and along lakes, lagoons, and 

rivers. Amplified ground motion, which can affect 

structures on such deposits at great distances from the 

earthquake source, is suspected to be an important 

factor in damage to structures such as the Bay Bridge, 

the Cypress section of Interstate 880, and the Interstate 280 and Embarcadero viaducts. 

• Areas underlain by thick deposits of water-saturated 

unconsolidated sand and mud were not only strongly 

shaken but were also affected by compaction and loss 

of strength in sediment that liquefied during the 

shaking; many of these same areas experienced similar 

processes in the 1906 earthquake. Sinking, tilting, and 

cracking of such deposits contributed greatly to the 

destruction, mainly in the same areas of amplified 

ground motion. This earthquake dramatically demonstrated that certain types of poorly engineered landfill, 

such as that which underlies the heavily damaged part 

of the Marina district in San Francisco, can fail at 

much lower levels of seismic shaking than might have 

been predicted. 

• Seismic shaking triggered many landslides in areas of 

steep unstable slopes, including many manmade cuts 

and fills, throughout the region of strong ground 

motion. In addition to causing at least two deaths and 

extensive damage to buildings and utilities in rural 

mountain communities, landslides blocked many of 

the highways and roads, thereby hampering rescue 

and relief efforts. 

• Damage to houses and multistoried buildings with 

foundations on good ground was minimal outside the 

D 

u $ 

70 

EXPLANATION 

Fault rupture of Lorna Prieta earthquake-Arrows show relative horizontal movement; small arrow and numeral show direction and amount of 

dip; U, upthrown side; D, downthrown side 

Fault-Dashed where approximately 

located 

* Mainshock epicenter 

Area of aftershocks and abundant landslides 

////111 Area of ground cracks possibly related 

to faulting 

• • • • • • • • • • • • • • • • Approximate limit of structural damage 

- • - • - · - Approximate limit of landslides 

@ Prominent area of liquefaction 

@ Damaged bridge 

@ Damaged dam 

@ Damaged two-story viaduct 

Figure 1. Continued. 

epicentral region. Serious damage was largely restricted to older buildings that predate present building 

codes. Many failures can be linked to poor foundation 

connections, unreinforced masonry or brick facade 

construction, or multistory wood-frame buildings in 

which the first floor lacked sufficient lateral resistance 

to earthquake shaking. 

• Part of downtown Los Gatos was destroyed in an area 

that was otherwise relatively undamaged. Small areas 

of enhanced damage appear to be present in a discontinuous zone that extends along the northeastern 

foothills of the Santa Cruz Mountains from Los Gatos 

to Palo Alto. Some of the damage is localized in areas 

of numerous ground cracks and surface deformation. 

The distribution of the damage and its relation to 

areas of surface cracking and warping suggest that 

damage may be related to the underlying geology, 

although this relation has yet to be proved. 

This report summarizes our principal initial findings 

concerning the causes and effects of the Lorna Prieta 

earthquake. It is addressed to the general public as well 

as to the earth scientists and researchers in related fields 

to provide information on this earthquake and an overview of the state of knowledge concerning future earthquakes in this region and their anticipated effects. In 

writing this report, we have tried to minimize use of 

scientific or technical terms and, for the benefit of the 

nontechnical reader, to explain concepts and terms as 

they are introduced. We have also converted most units 

of measurement from metric to English. A glossary of the 

commonly used terms in earthquake seismology and 

geology (appendix 1) is included at the end of the report 

to assist the nontechnical reader. 

Many published reports, available from the U.S. 

Geological Survey and other sources, document risk 

from earthquake, landslide, and flood hazards. The list of 

references cited in this report and in appendix 2 includes 

a selected sample of these publications. In addition, 

appendix 2 cites publications of general interest on 

earthquakes as well as references of interest to engineers, 

planners, and decisionmakers and sources of the information. 

The earthquake information presented in this preliminary report is based on less than 1 month of intensive 

postearthquake investigations. It derives chiefly from 

work by the staff of the U.S. Geological Survey, some of 

whom worked with scientists and engineers from several 

universities and private and public organizations and 

agencies. Prompt response to the earthquake was undertaken in order to (1) monitor aftershocks and postearthquake distortions of the Earth's surface because these 

phenomena diminish rapidly, (2) record the field evidence for ground failures before they were destroyed by 

emergency repairs or natural processes, (3) identify 

hazardous geologic situations, and ( 4) provide immediIntroduction 3 
ate information on the earthquake and its effects to 

concerned citizens and public officials. 

Investigations of the Lorna Prieta earthquake will 

continue for years, for there is much to do to understand 

this reminder sent to citizens and scientists alike. Earthquakes of comparable size can be expected either on the 

segment of the San Andreas fault north of the Lorna 

Prieta segment or on the Hayward fault. Such earthquakes will be far more disastrous to this growing and 

densely urbanized region unless we proceed vigorously to 

reduce the hazards. 

GEOLOGIC SETTING 

The San Andreas fault, which produced the Lorna 

Prieta earthquake, first drew general attention in 1906 

when it broke along a 280-mile stretch, causing the great 

San Francisco earthquake and fire. Since then, geologic 

mapping over many decades and the recent theories of 

plate tectonics have resolved that the San Andreas fault, 

which is about 800 miles long and extends to depths of at 

least 11 miles, is the major crustal boundary along which 

vast regions of the Earth's crust (known as the Pacific and 

North American plates) move past one another at rates 

of a few inches per year (fig. 2). Since the formation of 

the Pacific plate about 30 million years ago and on the 

basis of the offset of correlative rocks of known age on 

either side of the fault, the Pacific plate has moved 

northwestward relative to the North American plate as 

much as about 200 miles. In the San Francisco Bay area, 

the plate boundary is a broad, complex zone in which the 

horizontal slip is distributed over the San Andreas, 

Hayward, and Calaveras faults (figs. 1, 2), with additional 

slip on faults both seaward from the San Andreas and 

landward from the Calaveras. As a consequence, the San 

Andreas in this region takes up only about 40 percent of 

the relative plate motion, or an average of slightly less 

than one inch per year. 

Sudden slippage that initiates large earthquakes usually happens on only one section of the fault at a time. 

Total offset along the fault accumulates unevenly, primarily by movement on first one and then another section of 

the fault. The widely accepted explanation of the process 

by which elastic strain gradually builds up and is suddenly 

released to produce an earthquake, the elastic rebound 

Figure 2. Block diagram illustrating relative motion of Pacific and North American plates and direction of movement on principal 

active strike-slip faults in San Francisco Bay region. Faults dashed where concealed. Modified from Alpha and others (1989). 

4 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
theory, was developed as a result of studies of the fault 

displacement that accompanied the 1906 earthquake 

(Reid, 1910). According to this theory (see fig. 3), the 

sections that produce large earthquakes remain locked 

and quiet over periods of tens to hundreds of years while 

strain builds up by gradual deformation of the crust 

adjacent to the fault; this strain is relieved periodically in 

sudden fault displacements that produce earthquakes. 

The break on the San Andreas fault that occurred 

during the Lorna Prieta earthquake is in a sparsely 

inhabited part of the southern Santa Cruz Mountains, 

which attain their highest elevation (3,791 feet) at the 

nearby Lorna Prieta (fig. 1 ). The Santa Cruz Mountains 

extend southeastward from San Francisco and separate 

the coastal communities of Santa Cruz and Watsonville 

from the San Francisco Bay area and the Santa Clara 

Valley. The mountains are underlain by deformed and 

variably consolidated bedrock, whereas the bay margins 

and the Santa Clara Valley are underlain by young 

unconsolidated sediment derived from the surrounding 

uplands (Helley and others, 1979). These stream deposits become progressively less compacted and less dense 

toward the bay, which is bordered and underlain by 

widely distributed estuarine deposits referred to as bay 

mud. During this century, man has placed many buildings, bridges, highways, airports, and other facilities on 

extensive areas of landfill over mud that can be especially 

susceptible to failure during seismic shaking. 

Slight divergence between the present directions of 

crustal plate motions and the San Andreas fault produces 

a small component of compression perpendicular to the 

fault that is evident in the uplift of mountains and 

deformation of young rocks within the Santa Cruz Mountains. The southern Santa Cruz Mountains are bounded 

CRUSTAL BLOCKS AT REST 

THE INSTANT OF RUPTURE 

DEFORMATION DURING STRESS 

BUILD-UP 

REBOUNDING TO A NEW 

EQUILIBRIUM 

Figure 3. Block diagrams illustrating earthquake cycle for 

a right-lateral strike-slip fault according to elastic rebound 

theory. 

on the northeast by thrust faults along which the mountain side has moved upward and northeastward relative 

to the valley side. Both stratigraphic and seismic data 

show these faults to be active (McLaughlin, 1974). It is 

possible that similar, as yet unidentified, faults are 

present beneath the alluvial deposits of the Santa Clara 

Valley and San Francisco Bay. 

THE EARTHQUAKE AND ITS AFTERSHOCKS 

THE MAINSHOCK 

The epicenter of the October 17 earthquake, at 

37°02' N. latitude, 12r53' W. longitude in the Santa 

Cruz Mountains, is about 10 miles east-northeast of the 

city of Santa Cruz and 60 miles southeast of San Francisco (fig. 1). The main rupture began at 4 minutes 15 

seconds after 5 p.m. (Pacific daylight time), at a depth of 

about 11 miles beneath the Earth's surface. During the 

next 7 to 10 seconds, the rupture spread about 25 miles 

northwest and southeast and upward about 8 miles, 

stopping about 3 to 4 miles below the surface. Although 

it failed to reach the Earth's surface, the earthquake 

rupture ultimately involved slip on a 190-square-mile 

area of the buried fault surface. 

The magnitude of the earthquake is calculated to be 

7.1 from surface waves recorded around the world, 

making it the largest earthquake on the San Andreas 

fault since 1906. The energy released as seismic waves by 

the earthquake (1022 ergs) was approximately equal to 

the total energy yield from one thermonuclear bomb 

(500,000 tons of TNT); additional energy was expended 

in fracturing rocks and in uplift of part of the Santa Cruz 

Mountains. 

The earthquake is believed to have reruptured the 

southernmost 25-mile-long segment of the 1906 fault 

break. Because of the bend in the San Andreas fault and 

the compressional component noted previously, it is not 

surprising to find seismic evidence showing that the 

earthquake involved approximately equal amounts of 

right-lateral and reverse slip on a steeply inclined fault 

plane, with the Pacific plate moving up and northwestward relative to the North American plate (fig. 4). The 

fault-plane orientation was estimated to strike N. 50° W. 

± 8°, or approximately northwest, and dip 70°± 10° southwest, with the direction of slip on this dipping plane 

estimated at 130°±15°, based on 267 observations of 

primary waves recorded in central California. Observations at Tsukuba, Japan, and at Pasadena, California, of 

surface and body waves give essentially the same result: 

strike N. 54° W., dip 72° southwest, and direction of slip 

in the fault plane of 132°. This orientation of slip is 

consistent with seismic observations of displaced survey 

monuments and with the 3-dimensional distribution of 

aftershocks. 

The Earthquake and Its Aftershocks 5 
Surface cracks 

showing direction SAN ANDREAS FAULT 

of movement 

"'-

~.< 1t ~· ,,~ 1t 

Fault slip · Hypocenter at depth in · 

this earthquake 

Figure 4. Schematic diagram showing inferred motion on 

San Andreas fault during Lorna Prieta earthquake. Along 

southern Santa Cruz Mountains segment of fault, Pacific and 

North American plates meet along inclined plane that dips 

approximately 70° southwest. Plate motion is mostly accommodated by about 6.2 feet of slip along strike of this plane 

and by 4.3 feet of reverse slip, in which Pacific plate moves 

up fault and overrides North American plate. Amounts of fault 

slip and vertical surface deformation were determined from 

geodetic data. Modified from figure by M. J. Rymer. 

0 

5 (I) 

w 10 ..J 

:i 15 

:!: 

x 

.... 

:~~ a. 

w 

Q 

B 

0 

u 

tJ) 

·u 

c:C: 

CI)U.. "'~ 

50 

LOMA PRIETA 

GAP 

100 

BACKGROUND SEISMICITY 

The earthquake occurred north of the creeping 

segment of the San Andreas fault in the locked segment 

that last had major movement in 1906. The contrasting 

behavior of the fault is apparent in the microseismicity 

recorded since 1969 (fig. 5). Between San Juan Bautista 

and Parkfield, the San Andreas fault moves by steadily 

creeping and produces numerous small earthquakes in 

an apparent absence of accumulated elastic strain. In 

contrast, north of Watsonville, the fault has remained 

relatively quiet, or aseismic, since 1906. What little 

seismicity that has occurred there over the last 20 years 

outlines an aseismic gap between about Watsonville and 

Los Gatos, a gap that was filled by aftershocks of the 

Lorna Prieta earthquake. The Lorna Prieta section is 

believed to be a transition zone, similar to the Parkfield 

segment, which begins about 124 miles to the south (fig. 

5). These transition zones flank the creeping segment 

and sustain more frequent, but smaller, earthquakes than 

their respective neighboring segments to the north and 

south, where California's larger earthquakes are produced. 

THE AFTERSHOCKS 

Faults that ruptured during and after the mainshock 

are defined by the 3-dimensional distribution of the 

aftershocks (fig. 6). These aftershocks are caused by 

redistribution of stress following slip on the San Andreas 

fault during the mainshock. The mainshock hypocenter 

lies at the bottom and center of the aftershock distribution (fig. 6, section A-A'). Most of the aftershocks are 

150 200 

DISTANCE, IN MILES 

Figure 5. Cross sections showing seismicity along San Andreas fault from north of San Francisco to south of Parkfield. A, 

Background seismicity recorded during 20-year period before earthquake. Dense zone of activity south of San Juan Bautista 

is creeping segment of San Andreas fault. North of San Juan Bautista, San Andreas fault has been virtually aseismic since 

1906. On Lorna Prieta segment, what little seismicity has occurred has effectively outlined aU-shaped area (Lorna Prieta gap) 

that has been virtually aseismic over the past 20 years. 8, Aftershocks and mainshock (largest circle) almost completely filled 

former quiet zone of Lorna Prieta gap. 

6 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
confined to a plane that extends upward and laterally 

from the mainshock hypocenter with a dip of 70° southwest (fig. 6, section B-B'). This plane, the mainshock 

plane, projects to the surface between the traces of the 

San Andreas and Sargent faults. However, the dense 

clusters of shocks between 2 and 4 miles depth mark the 

inferred upper extent of the mainshock rupture. Above 

this point, the complex pattern of aftershocks probably 

represents movement on multiple branching faults, including shallow vertical segments of the San Andreas and 

Sargent faults. Thus, from the geometry of the deeper 

aftershocks, the primary rupture occurred on a dipping, 

buried portion of the San Andreas fault. 

Southwest of the principal zone of aftershocks a 

distinct subparallel cluster of shocks was triggered by a 

magnitude 5.0 aftershock 33 hours after the mainshock. 

This cluster, which followed the second largest afterNW 

A 

0 

(/) 

w 

_l 5 ~ 

~ 

I1- 10 0... 

w 

0 

15 

0 10 20 30 40 

DISTANCE, IN MILES 

20' 

10' 

50' 

shock in the earthquake sequence, constitutes a secondary aftershock sequence associated with a separate 

episode of faulting in the upper (Pacific plate) block. The 

location of this aftershock cluster suggests the possibility 

of movement on the Zayante fault (see fig. 1); however, 

no displacement has yet been documented along the 

surface trace of this fault. 

The rate of aftershock activity decreased rapidly with 

time after the mainshock, a pattern typical for a California earthquake sequence (fig. 7). A total of 51 aftershocks of magnitude 3.0 and larger occurred the day after 

the mainshock and 16 occurred the following day. After 

21 days, 87 magnitude 3.0 and larger aftershocks had 

occurred. Extrapolation of the observed pattern, using a 

well-established law of aftershock decay, suggests that 

magnitude 3 or larger aftershocks will continue to be felt 

in the epicentral region for at least two years. 

SE sw NE 

A' B B' 

0 

(/) 

w 

_l 

~ 5 

~ 

I1- 10 0... 

w 

0 

15 

50 0 10 20 

DISTANCE, IN MILES 

MAGNITUDES 

0.0+ 

1.0+ 

0 2.0+ 

0 3.0+ 

0 4.0+ 

0 5.0+ 

0 6.0+ 

' 0 '\ 7.0+ " " 

' ' ' 

Figure 6. Spatial distribution of aftershocks of lorna Prieta earthquake in relation to San Andreas fault. Along-fault, or 

longitudinal, cross section A-A' and cross-fault, or transverse, cross section ~8' display depth distribution of aftershocks on 

vertical planes. Faults are dashed where approximately located, dotted where inferred, and queried where uncertain. 

The Earthquake and Its Aftershocks 7 
7 

w 

0 6 ::> 

t: 

z 

" < 5 ~ 

w 

::.:::: 

<( 

::> 4 0 

:r: 

Ia: 

<( 

w 3 

2 

0 5 10 15 

l 

DAYS AFTER MAINSHOCK 

1000 

• • B 

>- <( 

0 

a: 100 w 

c.. 

(/) 

::.:::: 

() 

0 

:r: 

(/) 

a: 

w 

I- 10 u.. 

<( 

(") 

/\I 

~ 

u.. 

0 • 

a: 

w 

co 

~ 

::> 

z 

• 

15 min 2.4 hr 1 day 10 days 100 days 

TIME AFTER MAINSHOCK 

Figure 7. Time distribution of aftershocks of magnitude (M) 3 or greater of Lorna Prieta earthquake. A, 

Aftershocks during first three weeks are represented by circles (~3), squares (~4), and stars (~5). 

Rate of aftershocks decreases rapidly in first week of sequence. 8, Rate of ~3 aftershocks as a function 

of time, shown on logarithmic scales. Points plotted represent rate observed in aftershock sequence. Solid 

line is theoretical curve representing expected aftershock rate. Earthquake sequence closely follows 

expected time behavior. 

8 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
SURFACE DEFORMATION 

Movement on the San Andreas fault during the 

Lorna Prieta earthquake distorted the surface of the 

Earth. Earthquake-related changes in the positions of 

permanently marked sites (geodetic stations) can be used 

to infer the amount and distribution of fault slip. 

The Lorna Prieta earthquake occurred along the 

southwestern end of the 1906 rupture within an extensive 

and frequently measured network of geodetic stations. 

Since the early 1970's, the U.S. Geological Survey has 

monitored crustal deformation in the region near Lorna 

Prieta by repeatedly measuring the distances between a 

network of geodetic stations. The distances, which range 

in length from 3 to 28 miles, are measured to an accuracy 

of better than 0.4 inch using a geodolite (a high-precision 

electro-optical distance-measuring instrument) according to procedures outlined by Savage and Prescott 

(1973). The network surveys were repeated at intervals 

of one to five years. Monthly measurements from Lorna 

Prieta to stations located on Eagle Rock, Mount Allison, 

and Mount Hamilton (fig. 8) began in 1980. In 1985, the 

geodolite observations were supplemented by monthly 

observations of the relative position vectors between 

these same stations by observing a system of surveying 

satellites in Earth orbit-the so-called Global Positioning 

System (GPS) (Prescott and others, 1989). Since the 

October 17th Lorna Prieta earthquake, much of this 

network has been, or is being, resurveyed. 

On the basis of a preliminary analysis of the change 

observed in 11 geodolite distances and 4 G PS vectors 

surveyed immediately after the event, we have inferred 

that the rupture included about 6 feet of right-lateral 

strike slip and 4 feet of reverse slip on a fault surface that 

strikes N. 48° W. and dips 70° to the southwest (fig. 4). 

The best-fitting rupture surface extends about 25 miles 

along strike and at a depth of about 4 to 11 miles. The 

data suggest a broad uplift of the surface to a maximum 

of about + 1.5 feet on the southwest block and a more 

limited area of subsidence to a maximum of -0.5 foot of 

the northeast block as depicted schematically by contours 

in figure 8. A buried rupture surface is consistent with 

the lack of surface offset associated with the earthquake. 

The slip in the plane of the fault, with a considerable 

component of reverse slip, is about the same as that 

inferred from the seismological data. The amount of 

reverse slip on the fault rupture is not well constrained by 

the geodolite and GPS data, which are most sensitive to 

horizontal deformation. A better estimate of the vertical 

deformation may be obtained from a resurvey of 124 

miles of leveling routes in the Lorna Prieta area. Most of 

the initial level surveys, several of which extend across the 

Santa Cruz Mountains near the fault rupture, were made 

in the 1940's and 1950's and, except for segments along 

the coast and in the Santa Clara Valley, none have been 

repeated. 

PREMONITORY DEFORMATION 

Although several monitoring systems for determining strain changes were operating continuously before, 

during, and after the Lorna Prieta earthquake, no premonitory signals were observed. 

SURFACE EXPRESSION OF FAULTING 

SAN ANDREAS FAULT ZONE 

Within 24 hours of the earthquake, U.S. Geological 

Survey personnel and others determined that no rightFigure 8. Geodolite lines and Global Positioning System 

(GPS) vectors that were observed following Lorna Prieta 

earthquake. Projection of rupture surface, which extends to 

depths of about 4 to 11 miles, is shown by shaded rectangle. 

Contours show inferred uplift (gray lines) and subsidence 

(barbed gray lines) of surface due to oblique slip on fault 

plane; contour interval about 2 inches. Maximum uplift and 

subsidence were 1.5 and 0.5 feet, respectively. Location of 

San Juan Bautista Dilatometer is shown by circle. Faults 

(from Jennings, 1975) are dashed where approximately 

located and dotted where inferred. 

Surface Expression of Faulting 9 
lateral surface faulting occurred above the 25-mile-long 

deep rupture on the San Andreas fault. The nearby 

Sargent and Zayante faults also lacked clear evidence of 

tectonic rupture. On the basis of historical earthquakes 

along the San Andreas fault and crustal earthquakes of 

comparable magnitude on other strike-slip faults worldwide, we had anticipated through-going surface faulting 

with 3 to 6 feet of right lateral displacement (Bonilla and 

others, 1984). 

In the aftershock zone, only local cracks and sets of 

cracks with small right-lateral offset were observed along 

the mapped trace of the San Andreas fault. The largest is 

near Mt. Madonna Road, where right-lateral en echelon 

cracks were traced for approximately 0.6 mile. Along the 

San Andreas fault, right-lateral offset measured in Mt. 

Madonna Road within 16 hours of the mainshock was 

approximately % inch. The cracks, which extend to the 

northwest from this site, consistently form left-stepping 

patterns within a 3- to 30-foot-wide zone. Repeated 

measurements made in the 17 days after the earthquake 

indicate slight additional right-lateral movement of about 

1/20 inch across the San Andreas fault at Mt. Madonna 

Road. 

AREAS OF EXTENSIONAL GROUND CRACKS 

Sets of ground cracks that appear to be related to 

regional deformation, rather than local slumping, occur 

on the relatively upthrown southwest side of the San 

Andreas fault south of Highway 17 in an area about 5 

miles long and 3 miles wide. A distinguishing characteristic of most of these cracks is that offsets across them are 

commonly combinations of extension and either leftlateral or right-lateral horizontal slip. 

In the Summit Road area a zone of extensional and 

left-lateral cracks occurs just southwest of the main trace 

of the fault about 71

/2 miles northwest of the epicenter. 

These cracks are distributed in a 5-mile-long, 1.5-milewide zone on a ridgetop traversed by Summit Road 

between Highway 17 and Old San Jose Road and on part 

of the next ridge to the south (fig. 9). Their continuity 

along trend, relatively large displacements, and (or) great 

length (up to 2,000 feet) distinguish them from abundant 

small cracks throughout the zone of strong ground 

shaking that are associated with local slumping of natural 

ground and pavement. These cracks trend northwesterly, 

Figure 9. Zones of prominent surface cracks (heavy lines) 

and net-displacement vectors (arrows) near Summit Road, 

Santa Cruz Mountains. Open cracks obviously related to 

local ground failure or landslides are not shown. Only vectors 

greater than 5 inches long and next to Summit Road are 

shown. Wide gray band is approximate location of main trace 

of San Andreas fault as shown by 1906 offset in WrightsLaurel tunnel and by topography along and beyond northwest and southeast boundaries of figure. 

and they vary from single large crack to en echelon, 

anastomosing, or discontinuous crack sets (figs. 10, 11 ). 

Displacement across these cracks is mainly extensional, 

generally with a component of left slip of as much as 2.5 

=== I 

Wrights- Lau rei 

tunnel 

0 

0 

v .• 

I 

I 

• 

~' ,• ·~, .- ( /(~ ( 

,. (' 

, ... ' )1 "a \ \ 

\ <I . I I 

' ' ~ •• 

)\ 0 

G> 

{11 

VECTOR SCALE 

1 MILE 

40 INCHES 

10 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
feet, and locally with a component of dip slip to 2 feet. 

Along some cracks the downslope side is consistently up, 

opposite the sense that would be expected for gravity 

A 

slides. The direction of separation of opposite sides of the 

cracks (slip vectors) are consistently oriented roughly 

normal to the crest of the ridge along Summit Road (fig. 

Figure 10. Crack system near Summit Road, 1,h mile southeast of Highway 17. A, View northwest of wide zone of dominantly 

extensional cracks passing several feet in front of house. 8, View southeast back toward house showing driveway dropped 

down relative to garage. C, Same crack where it crosses Summit Road; crack trends nearly north here rather than northwest 

and displays large component of left-lateral displacement. In general, upthrown block is on downhill side of this crack. Swale 

along crack on northeast side of Summit Road ridge in this area suggests similar displacement may have occurred in the past. 

Surface Exprett.Sion of Faulting 11 
8 

c 

Figure 10. Continued. 

12 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 
Figure 11. Cracks through corral near Summit Road, about 1.1 miles southeast of Highway 17, 

along southwest edge of elongate, closed depression. Cracks are also found along northeast 

edge of same depression. Displacements on both sets of cracks deepened depression. 

Figure 12. Crack trends and displacement vectors for 94 surface cracks in Summit Road area. 

Rose diagram (black) shows strike of cracks, 

which, on average, are subparallel to San Andreas fault. Dots are lower-hemisphere equalarea projection of slip vectors. Note that 

dominant sense of slip is within both mixed 

left-lateral and extensional (shaded) and rightlateral and extensional domains. LL, left lateral; 

RL, right lateral; EX, extensional. Modified from 

figure by Ze'ev Reches. 

LL 

Surface Expression of Faulting 13 
A 

8 

Figure 13. Larger of two cracks that broke Morrill 

Road by left-lateral motion in both 1906 San Francisco earthquake and 1989 Lorna Prieta earthquake. 

A, View southwest across Morrill Road in 1906 showing left-lateral displacement of 3.6 feet (Lawson, 

1908, plate 65A). 8, Same crack on October 18, 1989, 

morning after earthquake, before road repairs, showing 1.1 feet of extension, 1.2 feet of left lateral 

displacement, and 0.3 feet of vertical displacement. 

Yardstick is aligned in direction of movement of 

opposite sides of crack. C, View of present-day 

Morrill Road (October 1989) similar to 1906 view 

after asphalt road had been patched, showing leftlateral displacement of edge of road. 

14 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 
c 

Figure 13. Continued. 

12). Repeated measurements across these cracks have 

detected no postearthquake displacement in 1 month 

after the earthquake. 

Individual cracks within the zone generally follow 

existing topography and occur along slope breaks and 

linear ridges; some bound linear depressions. The 

present topography in the Summit Road ridge area 

appears to have formed as a result of repeated similar 

movements along these ruptures. Some of the ruptures 

closely follow linear topographic trends identified by 

Sarna-Wojcicki and others (1975) and interpreted by 

them as elements of a complex fault pattern associated 

with this section of the San Andreas fault. The 1906 

surface rupture and associated ground failure were poorly documented along this part of the San Andreas fault 

(Lawson and others, 1908). However, two of the 1989 

Summit Road cracks coincide exactly with left-lateral 

offsets mapped during the field investigations following 

the 1906 earthquake. The larger of the two cracks 

showed left slip of about 4 feet in 1906 but 1.2 feet in the 

Lorna Prieta earthquake (fig. 13). The 1989 cracks cross 

Morrill Road southeast of the axis of the now abandoned 

and closed Wrights-Laurel railroad tunnel, which passes 

700 feet beneath the crest of the ridge. After the 1906 

earthquake, the most reliable measurement of offset 

along this reach of the fault, 4.5 feet of right slip, was 

made in the tunnel about 3,000 feet north-northeast of 

these left-slip cracks. The offset, however, was not observed at the surface above the tunnel. 

Ground cracks in the Skyland Ridge area (fig. 9) 

exhibit nearly the same trend and sense of displacement 

as the cracks located along Summit Road. Displacement 

of the Skyland Ridge cracks is largely extensional but 

with a component of left slip. Displacement is generally 

Surface Expression of Faulting 15 
less than that observed near Summit Road, and the 

cracks are less continuous along trend. In contrast to the 

ridge-parallel cracks along Summit Road, many cracks in 

the Skyland area cut obliquely across the ridge crest. 

They tend to follow pre-existing linear scarps and 

troughs, suggesting repeated motion along the zone of 

cracking. 

Along the San Jose-Soquel Road, asphalt road surfaces that are not obviously involved in slope failure are 

extensively cracked and extended to a distance of as 

much as 3 miles southwest of the San Andreas fault. 

These cracks show combinations of extension and crackparallel left and right slip. Many cracks have the same 

combination of left slip and extension as those along 

Summit Road and Skyland Ridge, but they differ in that 

most of them trend northeasterly at a large angle to the 

San Andreas fault. 

ORIGIN OF THE EXTENSIONAL CRACKS 

The origin of the Summit Road cracks and similar 

cracks southwest of Summit road is complex. The large 

cracks are most abundant on northwest-trending ridgetops underlain by relatively soft, Tertiary sedimentary 

rocks. The cracks tend to follow the trend of bedding in 

the underlying strata and may be concentrated over 

/ 

I 

EXPLANATION 

weaker shale beds. Uplift of the block on the southwest 

side of the San Andreas fault resulted in extension or 

spreading near the surface of the block (as depicted 

schematically in figure 4), and slip on weak bedding 

planes in the Tertiary strata probably accommodated this 

extension. Cracking may have been enhanced by the 

severe ground shaking along some of the ridgetops 

(described in the following section). This strong shaking 

may have caused ridgetops to spread laterally as the 

slopes moved outward and downward toward the valleys 

under the influence of gravity. Ongoing detailed studies 

of the relation of the cracks to the underlying geology 

should help to further clarify the origin of these unusual 

features. 

LOS GATOS-PALO ALTO ZONE OF DEFORMATION 

Along the northeastern margin of the Santa Cruz 

Mountains there is a northwest-trending discontinuous 

zone of relatively intense damage to structures and local 

areas of ground cracks and deformation that extends 

northwestward from east of Los Gatos to Palo Alto (fig. 

1). 

In the town of Los Gatos, about 6.2 miles north of 

the Summit Road area, concrete sidewalks and curbs 

were systematically fractured and buckled on northeastllJo ~so 'lJ} h . ..t.7l/J 

-' 

Boundary between young alluvium (to north) 

and old alluvium and bedrock (to south) 

Area of failure, mostly compressional, 

of concrete sidewalks and curbs 

Figure 14. Areas of ground cracks and compressional deformation in Los Gatos and vicinity. 

16 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
Figure 15. Sewer grate in Los Gatos showing effects of compression perpendicular to long direction 

of grate. Ruler is about 6 inches long. 

trending streets throughout much of the downtown area 

(fig. 14). Broken concrete slabs lining Los Gatos Creek 

indicate about 10 inches of northeast-southwest compression. 

Two miles to the northeast of Los Gatos, starting 

near Vasona Dam, a 3.7-mile-long, east-southeasttrending zone of freshly broken concrete sidewalks and 

curbs defines a similar zone of compression. 

Pavement also broke in places along and near Interstate 280 as far northwest as Page Mill Road in Palo Alto. 

Some of these ruptures were at or near localities of 

isolated damage in areas that were otherwise relatively 

undamaged. 

The cause of the relatively more intense damage to 

buildings along the northeastern foot of the Santa Cruz 

Mountains from Blossom Hill to Los Altos Hills, including Los Gatos, is uncertain. The distribution and nature 

of the surface deformation, especially the regional compressional features in Los Gatos and vicinity (fig. 15), 

suggest that they may be related to small secondary 

movement on one or more of the southwest-dipping 

thrust faults that parallel the Santa Cruz Mountain front 

in this area (fig. 1). Known faults include the Berrocal 

and Stanford faults (McLaughlin, 1974; Jennings, 1975); 

there may well be others in this area of generally poor 

exposure. Slip on a thrust fault at shallow depth, possibly 

triggered by the mainshock, would move the southwest 

block upward and northeastward along the dipping fault 

plane with resulting warping, cracking, and northeastsouthwest compression of the surface and high intensity 

of shaking. Ongoing geologic studies, resurveys of geodetic control points, and a detailed analysis of the aftershocks in this region should resolve the origin of these 

enigmatic, but destructive, surface ruptures. 

GROUND SHAKING 

Seismic shaking at a given site from an earthquake is 

a complex function of distance to the earthquake source, 

size of the earthquake, and the type and thickness of the 

geologic materials that underlie the site (for instance, 

rock or unconsolidated deposits). Qualitative data on 

shaking intensity can be evaluated from observations of 

the effects on objects, buildings, and the ground as well as 

from eyewitness accounts using the modified Mercalli 

(MM) intensity scale. Because earthquake effects are 

pervasive, intensities can be ascribed to all localities 

within the region disturbed by the earthquake. In contrast, instrumental records of shaking are available only 

from those sites where accelerographs are located and 

thus from limited areas within the disturbed region. 

Quantitative records of the horizontal and vertical components of ground motion during large earthquakes are 

obtained with a special low-sensitivity seismograph (accelerograph). Understanding the response of the ground 

to strong shaking of the underlying bedrock is essential to 

Ground Shaking 17 
reducing earthquake hazards in general and to the 

earthquake-resistant design of critical structures in particular. 

SHAKING INTENSITY 

A preliminary assessment of the regional distribution of modified Mercalli intensity resulting from the 

Lorna Prieta earthquake is shown on figure 16. Except 

for the MM VI assessments in Brentwood, Banta, Manteca, and Vernalis, the evaluations are based on primary 

observations and data collected by the field parties. A 

secondary source of information for the above locations 

was early responses to a mail survey of postmasters, 

police, and fire departments routinely conducted by the 

0 2 3 4 5 MILES 

Mill Va lley 

Bolin a~ 6 

0 2 3 4 5 MILES 

National Earthquake Information Center following significant earthquakes. 

The modified Mercalli (MM) intensity scale subjectively groups observations on earthquake effects into 

similar qualitative levels of shaking and then ranks the 

shaking levels into ascending order I through XII (table 

1). Ground failure phenomena such as landsliding, sand 

blows, and liquefaction are generally relegated to MM 

level IX and higher. However, research and observations 

since the scale was developed shows that such effects can 

occur at lower shaking levels depending on a number of 

physical properties of the surficial materials, such as 

water content, permeability, and degree of consolidation, 

and on the slope angles of hillsides and bluffs. Therefore, 

MM intensity has been assessed on shaking damage to 

Brentwood 121 °30' 

6 

MantE'ca 

Bania 6 

6 

Vernalis 

6 

+ 

VI 

VI 

6 Gonzales 

0 10 20 30 MILES 

Figure 16. Preliminary map showing distribution of modified Mercalli intensity for Lorna Prieta earthquake. Numbers indicate 

intensity values for localities where observations were made; Roman numerals represent intensity level between isoseismical 

lines. Location of mainshock epicenter is shown by circled star. Insets show more detailed assessments in cities of San 

Francisco and Oakland. See table 1 for descriptions of intensity levels. 

18 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
Table 1. Modified Mercalli intensity scale 

[The modified Mercalli scale measures the intensity of ground shaking as determined from observations of an earthquake's effect on people, structures, and the Earth's surface. 

This scale assigns to an earthquake event a Roman numeral from I to XII as follows:] 

Not felt by people, except rarely under especially 

favorable circumstances. 

II Felt indoors only by persons at rest, especially on upper 

floors. Some hanging objects may swing. 

III Felt indoors by several. Hanging objects may swing 

slightly. Vibration like passing of light trucks. 

Duration estimated. May not be recognized as an 

earthquake. 

N Felt indoors by many, outdoors by few. Hanging objects 

swing. Vibration like passing of heavy trucks; or 

sensation of a jolt like a heavy ball striking the walls. 

Standing automobiles rock. Windows, dishes, doors 

rattle. Wooden walls and frame may creak. 

V Felt indoors and outdoors by nearly everyone; direction 

estimated. Sleepers wakened. Liquids disturbed, some 

spilled. Small unstable objects displaced or upset; 

some dishes and glassware broken. Doors swing; 

shutters, pictures move. Pendulum clocks stop, start, 

change rate. Swaying of tall trees and poles 

sometimes noticed. 

VI Felt by all. Damage slight. Many frightened and run 

outdoors. Persons walk unsteadily. Windows, dishes, 

glassware broken. Knickknacks and books fall off 

shelves; pictures off walls. Furniture moved or 

overturned. Weak plaster and masonry cracked. 

Vll Difficult to stand. Damage negligible in buildings of 

good design and construction; slight to moderate in 

well-built ordinary buildings; considerable in badly 

designed or poorly built buildings. Noticed by drivers 

of automobiles. Hanging objects quiver. Furniture 

broken. Weak chimneys broken. Damage to masonry; 

fall of plaster, loose bricks, stones, tiles, and unbraced 

parapets. Small slides and caving in along sand or 

gravel banks. Large bells ring. 

buildings and structures but intensities have not been 

assigned to sites of ground failure effects. This distinction 

becomes futile, however, where structural damage from 

ground failure and shaking are inextricably mixed, as was 

the case for certain structures located in the communities 

of Redwood Estates, Santa Cruz, and the Marina district 

of San Francisco. In these cases, the behavior of nearby 

buildings and other structures (such as water towers and 

telephone poles) apparently not influenced by local 

ground failure effects were used to corroborate our 

overall assessments. Damage resulting from the amplification of ground motion by surficial geologic materials 

and local topography is legitimately classed as shaking 

damage to the structures. 

Though the intensity of the earthquake is tentatively 

rated to be MM VIII based on substantial damage to 

wood-framed dwellings and unreinforced masonry buildings in communities near the epicenter, the highest 

intensity levels (MM IX) are assigned to isolated sites in 

San Francisco and Oakland. The collapse of the elevated 

portion of Interstate 880 in Oakland and the considerable damage to Interstate 280 in San Francisco warrant 

MM IX. Both of the reinforced concrete freeway strucVIII People frightened. Damage slight in specially designed 

structures; considerable in ordinary substantial 

buildings, partial collapse; great in poorly built 

structures. Steering of automobiles affected. Damage 

or partial collapse to some masonry and stucco. 

Failure of some chimneys, factory stacks, monuments, 

towers, elevated tanks. Frame houses moved on 

foundations if not bolted down; loose panel walls 

thrown out. Decayed pilings broken off. Branches 

broken from trees. Changes in flow or temperature of 

springs and wells. Cracks in wet ground and on steep 

slopes. 

1X General panic. Damage considerable in specially designed 

structures; great in substantial buildings, with some 

collapse. General damage to foundations; frame 

structures, if not bolted, shifted off foundations and 

thrown out of plumb. Serious damage to reservoirs. 

Underground pipes broken. Conspicuous cracks in 

ground; liquefaction. 

X Most masonry and frame structures destroyed with their 

foundations. Some well-built wooden structures and 

bridges destroyed. Serious damage to dams, dikes, 

embankments. Landslides on river banks and steep 

slopes considerable. Water splashed onto banks of 

canals, rivers, lakes. Sand and mud shifted 

horizontally on beaches and flat land. Rails bent 

slightly. 

XI Few, if any masonry structures remain standing. Bridges 

destroyed. Broad fissures in ground; earth slumps and 

landslides widespread. Underground pipelines 

completely out of service. Rails bent greatly. 

XII Damage nearly total. Waves seen on ground surfaces. 

Large rock masses displaced. Lines of sight and level 

distorted. Objects thrown upward into the air. 

tures were built under seismic design requirements of the 

then-existing building codes. The Marina district in 

northern San Francisco also is assigned MM IX. Ground 

failure and shaking both played an apparent role in some 

apartment collapses in the district. However, other collapses occurred in areas of no apparent ground failure. 

The collapse and widespread structural damage to these 

substantial buildings is reason for the tentative MM IX 

assignment to the Marina district. 

In all of the above areas, amplification of shaking on 

landfill and weak young deposits may have contributed 

significantly to the observed damage. Intensity levels in 

eastern and northern San Francisco underlain by alluvium and bay mud exhibit intensity levels one to three units 

higher than other areas of the city. Intensity levels in the 

extreme western margin of San Francisco, which also lies 

on thick sediments, are no higher than in the central area 

(fig. 16). 

The maximum intensity assigned to the earthquake 

source region will be reevaluated as more reports of 

shaking effects from the sparsely populated southern 

Santa Cruz Mountains are analyzed. Severe shaking was 

reported in the Summit Road area south of Highway 17. 

Ground Shaking 19 
Many large trees had their tops broken off due to strong 

shaking. Residents reported the displacement of all 

household furniture by several feet, including one builtin oven that was ejected from its cabinet. Four residents, 

at three different locations, described being thrown 

through the air several feet by "explosion-like" forces. 

These three locations are situated on a narrow ridge 

where topography could have amplified the shaking. 

A qualitative comparison of the Mercalli intensities 

from the Loma Prieta earthquake and those from the 

great 1906 earthquake clearly shows the much higher 

levels of shaking in 1906, particularly along the north 

Peninsula segment of the San Andreas fault and around 

the margins of San Francisco Bay. Direct comparisons 

are hampered by the fact that the 1906 damage was 

described in a different intensity scale which has to be 

converted to Mercalli intensity (J.F. Evernden, written 

commun., 1989; Borcherdt, 1975). For both earthquakes, 

intensity levels are comparable in the southern Santa 

Cruz Mountains and Monterey Bay region, areas closest 

to the source of the Loma Prieta earthquake. For the 

1906 earthquake the equivalent MM VIII zone, centered 

on the San Andreas fault rupture, extends northward to 

the coast in an area that mainly was MM VII in 1989. 

Major differences, however, are apparent around the 

margins of San Francisco Bay, where there was a broad 

zone of equivalent MM VIII-X in 1906, mainly in areas 

of bay mud; these same areas in the Loma Prieta 

earthquake experienced intensity levels of only MM 

VI-VII, except for local pockets of MM IX in San 

Francisco and Oakland. 

GROUND ACCELERA liON 

Instrumental records of the ground shaking from the 

Loma Prieta earthquake were obtained at more than 100 

122° 121° 38°.---------~--~~------~----------------------------------~----~ 

0 

I 

MILES 

10 

I 

~0 

San • 613 

Jose 

531:J1:.33 

e Los Gatos 

~7~ 1}. Jj,33 

28~42 50 55 37 37 

Santa 

Cruz e Watsonville Jj,29 

25 .. 38 

Figure 17. Free-field, peak horizontal acceleration, in percent gravity (32 feet per second per 

second), of ground motion during Lorna Prieta earthquake measured from stations shown by 

triangles. Value plotted is larger peak acceleration of the two horizontal components of motion; 

vectorial peak motion would be even larger. Lined region is fault plane of mainshock projected to 

Earth's surface. 

20 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 
sites within 120 miles of the epicenter by accelerographs 

maintained by several agencies and groups, principally 

the California Division of Mines and Geology and the 

U.S. Geological Survey. For all practical purposes, the 

amplitudes of recorded earthquake motions are directly 

proportional to acceleration; this allows the peak acceleration to be rapidly obtained and analyzed. A map of 

peak horizontal accelerations, expressed in percent of 

gravity (fig. 17), shows that a station on the uplifted block 

within the focal region recorded 64 percent gravity a few 

miles south of the Summit Road area, where extremely 

hard shaking was inferred from damage and eyewitness 

accounts. Very high accelerations (47 to 55 percent 

gravity) were recorded at several localities within 12.5 

miles of the focal region on both the relatively upthrown 

and downthrown sides of the San Andreas fault. 

In several areas, the peak motions varied considerably over short distances (fig. 18). As an example, 

motions near Oakland range from 8 to 29 percent of 

gravity. These variations are almost certainly due to local 

changes in the geologic materials underlying the recording sites. In the Oakland area, two of the three highest 

values come from sites underlain by bay mud, the other 

high value is from a site underlain by alluvium, and the 

low values come from sites underlain by bedrock. 

If all sites were underlain by similar materials, we 

would expect a map of peak ground motion to show a 

generally uniform decrease of motion with distance. 

Because of the masking effect of geologic variations, 

however, it is difficult to see the attenuation of peak 

0.6 0 Rock site 

§ 

z 

0 \ 

~ 0.4 \ a: 

w \ ....J 

w 0 u 

u \ 0 < \ 

~ 0 \ < w \ " a.. 0 0.2 " \ 0 ' 

0 

0 20 40 60 

acceleration from studying the map alone. This problem 

can be overcome by plotting the motions as a function of 

distance from the fault, after separating the recordings 

into three categories depending on the geologic materials 

underlying the recording sites. Such a plot is shown in 

figure 18, in which the equation of Joyner and Boore 

(1988) for the ground acceleration as a function of 

distance and magnitude serves as a convenient measure 

of the ground shaking of the Lorna Prieta earthquake. 

The Joyner and Boore equation is widely used and is 

based on data from many past earthquakes. 

As shown in figure 18, the accelerations from rock 

sites are in reasonable agreement with the predictions. 

Although there is considerable scatter in the data, which 

is an inherent feature of representing a complex physical 

process by a single number, the points cluster about the 

expected value (solid curve). The recordings at soil sites 

as a group are systematically greater than the predictions 

for rock sites, with the accelerations at bay mud sites 

much larger than those from most of the alluvium sites. 

Relative to rock sites, ground-motion records obtained 

on young, poorly consolidated, water-saturated alluvial 

deposits and bay mud tend to be deficient in highfrequency amplitudes and enriched in lower frequency 

motion, the frequency-dependent amplification being a 

function of rigidity contrasts in the local geology and 

basin geometry. The effect of local geologic conditions is 

shown in figure 19, where the record from 1295 Shafter 

Street in San Francisco is the only one written at a hard 

rock site. The Emeryville and 575 Market Street records 

X Alluvium site 

• Bay mud site 

X 

\ • 

• • X\ 

X\ X. 

X ., X 

X ' X X 

' X 

'- • X ............. X 

' X 

'- x-- -- --

0 20 40 60 

DISTANCE, IN MILES 

Figure 18. Peak horizontal accelerations of different geologic materials during Lorna Prieta earthquake plotted as a 

function of closest distance from recording station to surface projection of fault (shown in fig. 17). Solid line is prediction 

of Joyner and Boore (1988); dashed lines indicate expected variation in individual observations. About two-thirds of 

observations should theoretically lie between dashed lines. 

Ground Shaking 21 
are from sites underlain by bay mud and dune sands, 

respectively. The Foster City and Redwood City records 

are both from areas of engineered artificial fill overlying 

bay muds, fill that performed well during the Lorna 

Prieta earthquake. 

0.22 

0.06 

0.26 

0.08 

0.06 

Emeryville 50 miles 

F p 

San Francisco, 575 Market St. 49 miles 

The influence of the local geologic deposits on the 

amplitudes of the ground shaking and the extent of 

damage come as no surprise. Shortly after the 1906 San 

Francisco earthquake, H.O. Wood (1908) wrote 

" * * * the amount of damage produced by the 

UP 

UP - 0. 1 1 -------------------------------'~~--~~~~-~----~~~--~~~--~--~-------

:§ 

z 

0 

~ 0.11 

a: 

~ 0.05 w 

u 

u 

<t: 0.07 

~ 

<t: 

w 

~ 

0.12 

San Francisco, 1295 Shafter 45 miles 

UP 

Foster City 31 miles 

360° 

UP 

0.09 

0. 11 

----~------------~~~~~-M~.~~~~~~~~w~-~~--~-------------------------------

Redwood City, Apeel # 2 29 miles 

0.23 

0 . 0 8 - ..,.,.. pill; '1: 

0.28 

Time 

trace ·-·-·---·---·-·-·---·---------------·-------------·----~-----------------------·-------------

Figure 19. Three-component accelerograms from five stations, showing large difference in trace amplitudes between 

bedrock (1295 Shafter) and unconsolidated deposits (other 

sites). Stations arranged in order of decreasing distance from 

1 

1 SECOND 

fault plane of mainshock. Directions of ground acceleration 

for upward trace motion are given above each trace, and 

peak accelerations scaled from records are given to left of 

each trace. Short raised lines of time trace occur every 0.5 s. 

22 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 
earthquake * * * depends chiefly on the geological character of the ground. Where the surface was of solid rock, 

the shock produced little damage; whereas upon made 

land great violence was manifested * * * ." During the 

late 1960's and early 1970's, instrumental recordings of 

local earthquakes and distant nuclear explosions were 

made at many tens of sites in and around the San 

Francisco Bay area to quantify spectral amplification 

ratios and resonant periods for these various geologic 

units relative to bedrock in this region. 

Combined seismic and explosion data show that 

ground-motion amplitudes on firm alluvium can be amplified by factors of 2 to 4 in the frequency band of 

several seconds to several cycles per second, the frequency band of greatest engineering interest (Borcherdt, 

1970). Similarly, amplification factors for the softer bay 

muds and artificial fill can be 5 to 10 and in some cases 

Figure 20. Aerial photograph of rock slide near 

summit of Santa Cruz Mountains that has an estimated volume of 6,000 to 10,000 cubic yards. Slide 

nearly completely blocked northbound lanes of 

Highway 17 that links Santa Cruz and surrounding 

even more. Historical and instrumental data show that 

observed patterns of damage and strong ground motion 

for the Lorna Prieta earthquake were both predictable 

and predicted (Borcherdt, 1975). 

LANDSLIDES 

Strong ground motions during the Lorna Prieta 

earthquake triggered thousands of landslides throughout 

an area of 5,400 square miles. This region encompasses 

most of the San Francisco Bay area, the Santa CruzMonterey Bay area, adjacent parts of the California 

Coast Ranges, and the Big Sur coastline as far as 81 miles 

south of the epicenter (fig. 1). In addition to causing at 

least tens of millions of dollars of damage to houses, 

other structures, and utilities, landslides blocked many 

coastal area with San Francisco Bay region. This 

highway, which carried an estimated tens of thousands of commuters per day before Lama Prieta 

earthquake, was closed for more than 1 month for 

repairs. 

Landslides 23 
transportation routes (fig. 20), thus greatly hampering 

rescue and relief efforts. 

Landslides were most numerous around the earthquake source in the steep, rugged, and heavily vegetated 

Santa Cruz Mountains (fig. 1 ). These mountains, which 

receive as much as 60 inches of mean annual precipitation (Rantz, 1971), have historically produced abundant 

landslides both during earthquakes (Lawson and others, 

1908) and during heavy winter rains (Keefer and others, 

1987; Ellen and Wieczorek, 1988). Landslides were 

abundant during the Lorna Prieta earthquake despite its 

occurrence after two dry years and at the end of the dry 

season during which only about 2 inches of rain had 

fallen in 5 months. 

Landslides triggered during the earthquake included 

many types of downslope and lateral movements of the 

ground. The most common landslides were rock falls, 

rock slides, and soil slides, all of which were typically 

shallow (about 10 feet or less) and moved very rapidly 

down steep slopes, producing chaotic deposits of boulders and finer grained material (fig. 21). Most of these 

deposits have volumes of less than 100 cubic yards but 

several have volumes in the range of 1,000 to 10,000 

cubic yards, and at least one has a volume of more than 

52,000 cubic yards. Such landslides killed at least two 

people during the earthquake. Weakly cemented, deeply 

weathered, or intensely fractured rock materials exposed 

in roadcuts, near-vertical mountain cliffs, and coastal 

bluffs were especially prone to landslides of these types, 

as previous studies had predicted (Keefer, 1984). 

Deeper seated (commonly 10 to 100 feet deep), 

slower moving blocks of ground also detached in hundreds of places during the earthquake to form rotational 

slumps or translational block slides. These typically 

moved a few inches or feet during the strong shaking, 

leaving scarps and cracks in roadways, residences, and 

other structures, and in natural ground (fig. 22). Slumps 

were especially common in manmade fill and thus were a 

source of significant and continuing disruption of roads 

in the heavily shaken region. Road fill had also been 

identified as a major potential source of such failures 

during previous studies (Keefer, 1984). At least some of 

the slumps and block slides showed evidence of continuing or renewed movement as a result of as much as 6 

inches of rain that fell in the 10 days following the 

earthquake. Both existing slide blocks and adjacent 

slopes weakened by cracking, and removal of support are 

likely to be continuing hazards during large aftershocks 

or winter rains. The largest slump or block slide yet 

identified, located in the Santa Cruz Mountains near the 

Figure 21 . Small rock-fall deposit at roadcut, typical of those caused throughout Santa Cruz Mountains by Lorna Prieta 

earthquake. 

24 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
community of Laurel, encompasses more than 185 acres. 

This landslide contains within its borders several dozen 

houses, many of which have sustained substantial damage. 

Other types of landslides associated with the earthquake include at least one very fluid m udflow (generated 

by increased flow from a spring that saturated hillside 

materials) and numerous lateral spreading failures in 

nearly flat lowland areas, which are discussed below in 

the section "Liquefaction and Related Effects." 

LIQUEFACTION AND RELATED EFFECTS 

Liquefaction, the transformation of loose saturated 

sandy material into a fluidlike condition, locally caused 

substantial damage to structures over a widespread area 

during the Lorna Prieta earthquake (fig. 1 ). Particularly 

hard hit in the San Francisco Bay area were developments on manmade artificial sandy fills in San Francisco 

and Oakland. These areas included the Marina district 

and the area south of Market Street in San Francisco, the 

Oakland International Airport and port facility, the 

Alameda Naval Air Station, and Treasure Island. Elsewhere, damage caused by liquefaction was primarily in 

natural sediments. Most of this damage was south of the 

epicentral area in the flood plains of the Pajaro, San 

Benito, and Salinas Rivers and near the mouth of the San 

Lorenzo River in Santa Cruz. No liquefaction was noted 

either in the southern bay area in areas that liquefied in 

1906 or at the extensive Foster City and Redwood Shores 

developments, which are built on engineered landfill over 

bay mud. 

Damage from liquefaction results primarily from 

large horizontal and vertical displacements of the 

ground. These displacements occur because sands in a 

liquefied condition have virtually no strength and provide 

little or no resistance to compaction, lateral spreading, or 

downslope movement. Thus, ground on even the gentlest 

slopes can move toward free faces (or bluffs) such as 

shorelines, river banks, and manmade cuts. In addition to 

the downslope displacements, the ground above liquefied 

sediment commonly breaks into small blocks which may 

tilt and cause vertical displacements between adjacent 

blocks. This permanent movement of the land surface, 

known as lateral spreading (fig. 23), can be devastating to 

surface structures such as buildings, bridges, and river 

levees, as well as to buried underground utilities such as 

gas pipelines, water lines, and sewers. 

SAN FRANCISCO AREA 

The most intense damage from liquefaction occurred in the Marina district and in several multiblock 

areas south of Market Street, all of which were 50 miles 

or more from the closest part of the earthquake rupture 

zone. Many structures, including private residences, were 

damaged or destroyed (fig. 24). Although strong ground 

motion and vulnerable building design also may have 

contributed directly to damage in the Marina district, 

sand boils, which erupted into basements, streets, yards, 

and parks, and lateral spreading cracks demonstrate that 

liquefaction was widespread (figs. 25, 26). Presumably, 

most of the broken underground utilities, which left 

about a thousand homes without gas or water after the 

earthquake, were severed by movements associated with 

lateral spreading. One of the ironies of the devastation in 

the Marina district is that the heavily damaged part rests 

on fine sand fill that was hydraulically emplaced after the 

1906 earthquake. Debris from buildings destroyed in 

1906 may have been used as landfill also. This filled 

lagoon, where the district now stands, was the site of the 

International Exposition to celebrate San Francisco's 

postearthquake rejuvenation. On October 17, 1989, sand 

boils in the district erupted pieces of buried charred 

redwood, tar paper, and other debris in a poignant 

reminder of the city's earthquake history. 

Aftershock data shed some light on the problem of 

whether damage in the Marina district was due to strong 

shaking or to foundation failure related to permanent 

deformation within the artificial fill. This is shown in 

figure 27, where seismograms of a magnitude 4.6 aftershock are shown at three sites. The top trace is from a 

station at Fort Mason (MAS), underlain by a competent 

sandstone member of the Franciscan assemblage. The 

middle trace is from a site underlain by clean dune sand 

just onshore of the pre-fill shoreline, near the eastern 

edge of the Marina district (PUC). Significantly, the 

structure here is a two-story brick building with a massive 

turret on its northwest corner, the San Francisco Gas and 

Light building constructed in 1893. This is not the sort of 

construction that performs well during strong shaking, 

yet it rode through the recent earthquake without a 

crack-and the great 1906 earthquake as well. The lower 

trace is from a site just 11

/2 blocks away within the area 

covered by Marina bay fill (LMS). Here, houses are 

badly deformed by foundation failure, the north side of 

the street is now 1.6 feet lower than the south side. Both 

the LMS and PUC sites are amplified by comparable 

amounts relative to Fort Mason, but the local damage 

patterns are grossly different. This suggests that the 

problems in the Marina district are fundamentally due to 

permanent deformation of the manmade fill by liquefaction and are not the result of local amplification differences within the Marina district. 

EAST BAY AREA 

Substantial damage along the eastern margin of San 

Francisco Bay also occurred in areas underlain by sandy 

fill. Costly damage at the port facilities was due to 

compaction and lateral spreading of wharves. Sand boils 

erupted onto and around the major runway at Oakland 

Uquefactlon and Related Effects 25 
International Airport, and lateral spreading damaged the 

northern third of the runway (fig. 28). Liquefaction also 

occurred on Interstate 80 at the Bay Bridge toll plaza and 

the Alameda Naval Air Station, as shown by sand boils, 

ground cracks, and differential settlement of fill. 

A 

MONTEREY BAY AREA 

Liquefaction-related ground failure was widespread 

from Santa Cruz to near Salinas in areas underlain by 

saturated late Holocene unconsolidated deposits of the 

Figure 22. Scarps and cracks associated with movement of deep-seated slumps. A, 

Scarp at head of block slide in residential area of Brookdale in Santa Cruz Mountains. 

Block slide moved approximately 1.5 feet downslope (to right). Part of house to left of 

scarp remained on undisturbed ground while part to right moved downslope on slide 

block. Notebook on scarp is ax 10.5 inches. 8, Home destroyed by landslide 

movement. Note cracks, from internal fissuring of landslide block, in pavement. Part 

of house on right has separated from part on left and tilted downslope as a result of 

differential movement within landslide block. 

26 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 
8 

Figure 22. Continued. 

San Lorenzo, Pajaro, and Salinas Rivers and spits, bars, 

and tidal channels along the coast. Liquefaction destroyed or disrupted flood-control levees, pipelines, 

bridge approaches, abutments and piers, roads, homes 

and utilities, and irrigation works, including gradients of 

irrigation ditches and irrigated fields (fig. 29). 

Liquefaction began with the main shock, but several 

eye-witness accounts reported reactivation of sand boils 

during at least one aftershock. Liquefaction occurred 

primarily in geologically young deposits less than about 

5,000 years old (late Holocene) and in present floodplain areas of coastal basins within 160 to 550 feet of the 

principal streamcourses. These areas are distinguished 

geologically by the presence of silt and sand and by the 

presence of shallow ground water. Lateral spreads 

caused displacements ranging from fractions of an inch 

to about 6.5 feet. The most widespread damage was to 

levees of the Pajaro and San Lorenzo Rivers, which 

suffered cracking due to differential settlements and 

small translational displacements at many locations of 

liquefaction. 

Late Holocene river channels, now filled with fine 

sand and silt, proved to be especially prone to compaction. One zone of compaction that locally amounts to 

more than 1.5 feet across a zone tens to hundreds of feet 

in width extends in a meandering path for distances of up 

to about 1.2 miles through the town of Pajaro east of 

Watsonville. It is marked at the surface by a line of 

condemned structures, sand boils, low scarps, disrupted 

gradients in irrigated fields, and the damaged east abutment of the Main Street Bridge in Watsonville. 

COMPARISON WITH 1906 

Damage associated with liquefaction during the 

Lorna Prieta earthquake comes as no surprise and should 

serve as a reminder of the consequences of building on 

loose saturated sands. Many of the areas that experienced liquefaction and unusually severe shaking damage 

in 1989 are known to have experienced ground failure in 

1906 (Lawson and others, 1908; Youd and Hoose, 1978). 

This observation clearly demonstrates that liquefaction 

can affect the same areas in successive earthquakes. 

Areas of saturated sandy fill in San Francisco that failed 

in 1906 damaged the same types of structures and 

Uquefactlon and Related Effects 27 
underground utilities that were damaged in 1989. In fact, 

Seawthorn and O'Rourke (1989) have documented how 

the fire that destroyed much of the Market Street area in 

1906 raged out of control because watermains had been 

severed by lateral spreading. Again in 1989, the fires in 

the Marina district could not be fought with city water 

because of failed watermains. Fortunately there was no 

wind at the time and water could be pumped to the fire by 

a fireboat; otherwise the fire might have spread out of 

control. Similarly, liquefaction in the flood plains south of 

the epicentral area was anticipated. Maps of 1906 liquefaction areas outline the same areas that liquefied in 

1989. In addition, maps showing the geology and relative 

susceptibility to liquefaction of parts of Santa Cruz and 

Monterey Counties, prepared about a decade ago by U.S. 

Geological Survey geologists (Dupre, 1975; Dupre and 

Tinsley, 1980), clearly distinguish those areas that experienced liquefaction-related ground failure in 1989 from 

those areas that did not. 

DAMAGE TO BUILDINGS, 

TRANSPORTATION ROUTES, AND UTILITIES 

The Lorna Prieta earthquake is the first large seismic 

event to provide a real test for earthquake-resistant 

construction of buildings, transportation facilities, utilities, and communications systems in California and the 

San Francisco Bay area. Although the majority of facilities performed well, many failed the test. Of particular 

concern is the damage or failure of critical facilities at 

·, 

•" 

Figure 23. Lateral spreading along Pajaro River banks due to 

liquefaction. 

great distances from the earthquake source, such as 

those in San Francisco and Oakland. There are no major 

high-rise buildings, bridges, or freeway overpasses within 

about 7 miles of the mainshock rupture, and few within 

15 or 20 miles of where the ground shaking was most 

violent; hence, this earthquake provided only a limited 

test for these types of structures. 

Comprehensive studies are underway by many 

groups into the causes of structural failures. The mainshock provided numerous accelerograms from strongmotion instruments that were emplaced in a variety of 

major structures throughout the San Francisco Bay region. When analyzed, these recordings will provide invaluable information on the response of modern 

engineered structures to strong shaking. Unfortunately, 

no accelerographs were installed on some critical structures such as the Bay Bridge and the Cypress structure, 

as strong-motion records would be invaluable in reconstructing the cause of failure. 

The following subsections briefly highlight the more 

significant aspects of damage to the works of man that 

are not obviously related to failure of the ground on 

which they rest. Places referred to are shown on figure 1, 

unless otherwise indicated. 

BUILDINGS 

In general, the widespread shaking damage to buildings as far away as San Francisco and Oakland, about 50 

miles from the closest part of the earthquake rupture 

zone, is unusual for an earthquake of magnitude 7 .1. 

During the earthquake, no engineered structure built on 

the basis of the latest codes collapsed. However, there 

are many engineered structures throughout the 

earthquake-affected area that sustained damage without 

collapse, and a number of them are condemned and will 

have to be demolished. It should be realized that building 

codes aim to reduce, not prevent, damage to structures 

during the most severe shaking likely to occur in a region. 

Wood-framed single-family dwellings on solid 

ground outside the epicentral region generally came 

through the earthquake without structural damage, although many lost brick or stone chimneys. However, in 

many areas that were subjected to extremely strong 

ground motions close to the earthquake source zone, 

even newer homes built to code had serious failures, 

mainly by shearing of the structure off its foundation 

supports. In Santa Cruz, Watsonville, Hollister, and Los 

Gatos, residential homes and older business buildings in 

downtown areas were severely damaged-some collapsed and others are beyond repair. Most of these 

buildings were older structures vulnerable because of 

one or more of the following reasons: (1) deterioration of 

structure, (2) lack of ties to foundation, (3) unreinforced 

masonry (brick or stone), ( 4) lack of shear resistance in 

28 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
A 

8 

Figure 24. Structures damaged in Marina district. A, Damage due to ground failure of liquefied 

land fill. 8, First story of this three-story building in Marina district was damaged because of 

liquefaction; second story collapsed. What is seen is third story. 

Damage to Buildings, Transportation Routes, and Utilities 29 
ground floor, (5) pounding of adjacent structures, and 

( 6) timber diaphragms not tied to unreinforced masonry 

walls, which allowed separation or pushing out of the 

walls. 

In Watsonville, two adjacent buildings of a department store sustained extensive structural damage due to 

a weak first story, insufficient shear reinforcement of the 

columns, and possible pounding of the two structures. 

Recently constructed buildings with tilt-up walls in the 

area performed well. 

Most structures in the southern Santa Clara Valley 

performed well. An exception in San Jose was damage to 

the trusses supporting the roof of a crucial machine shop 

at the FMC Corporation, which caused temporary layoffs 

of about 500 workers for almost one month until repairs 

were made. 

At the Stanford University campus, 30 miles northwest of the epicenter, 60 buildings sustained varying 

degrees of damage, with an estimated repair cost of $160 

million. 

The new, three-winged 5-story Fluor Building in 

Redwood Shores on filled land about 35 miles from the 

closest part of the rupture zone had cracks in several of 

its shear walls. Nearby, the 22-story steel-framed building 

on fill at Foster City was not damaged. 

In downtown San Francisco, modern high-rise buildings to 50 stories escaped without structural damage 

although several of them had nonstructural damage, and 

many were not occupied for several days after the earthquake due to loss of electrical power and the need to 

check for gas leakage. Many of the high-rise buildings are 

built on fill and bay mud, but their pile footings extend 

into high-bearing-strength layers at depth. South of Market Street, several buildings between 5 and 10 stories 

high were damaged. Old masonry buildings were badly 

damaged, including a four-story warehouse where collapse of exterior walls killed five people in the street. An 

older, masonry-walled 8-story building on Battery Street 

was damaged and later demolished. 

The concentration of damage to 2- to 4-story homes 

in the Marina district received much attention. Although 

much of this damage can be attributed to liquefaction of 

the hydraulically emplaced silty sand foundation material, there were other contributing factors. Some had a 

structurally weak first story because the buildings had 

large garages without sufficient lateral resistance on the 

ground floor; others, particularly at the corners of the 

blocks, did not have the benefit of the stiffness and 

strength provided by adjacent structures on two sides. 

Notably, there were many nearby 2-story buildings that 

were not damaged (fig. 24B). Many buildings, such as 

12-story apartment buildings around Lake Merced and a 

dormitory (Verducci Hall) on the San Francisco State 

University campus, came through the earthquake with 

insignificant structural damage although they sustained 

serious nonstructural damage. 

Figure 25. Extension cracks in Marina district formed by lateral spreading of liquefied sandy landfill. 

30 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 
In Oakland, several midrise buildings were heavily 

damaged. Although none collapsed, some of them will 

have to be demolished. 

BRIDGES, VIADUCTS, AND HIGHWAYS 

Of the many arterial bridges in the San Francisco 

Bay area, only the double-deck Bay Bridge between San 

Francisco and Oakland was closed to traffic because of 

the collapse of one span of the upper deck. The cause of 

the collapse of the span is uncertain but appears to be 

due to differential lateral displacement of piers during 

shaking. In addition, many of the approaches to the Bay 

Bridge were damaged and were closed as of one month 

after the earthquake. 

In Oakland, the Cypress Street viaduct of the Interstate 880 (Nimitz) Freeway is another important arterial 

structure that collapsed. Almost a 1.5-mile length of the 

double-decked reinforced-concrete viaduct collapsed 

onto unusually light commuter traffic, killing 41 people 

and injuring many others. The disaster may be attributed 

to the design and construction of the joint between the 

lower deck and the upper deck columns, to inadequate 

Figure 26. Sand boils formed by liquefaction in Marina district. 

Damage to Buildings, Transportation Routes, and Utilities 31 
0 

0 .2 

z 

0 

u 

(.LJ 

f/l 

~ 

(.LJ 

0.. 

f/l 

(.LJ 

::r: 

~ .1 

~ 

;; 

f0 

0 

-l 

(.LJ 

> 0 

SAN FRANCISCO BAY 

EJ Artificial fill 

0 Modern beach deposits 

0 Dune sands 

PUC 

EXPLANATION 

CJ Alluvium, undivided 

[\ .J Franciscan assemblage-Sandstone 

• Seismic station 

~!IIN\Mti. 11\AMIUI I 

LMS 

0 10 20 

TIME, IN SECONDS 

30 

Figure 27. Three temporary seismograph stations in Marina district. A, Locations of stations and types of geologic material. 

8, Vertical velocities during a magnitude 4.6 aftershock on October 21, 1989, show amplification of ground motion in both 

damaged (LMS) and undamaged (PUC) areas in Marina district relative to ground motion on bedrock (MAS). Damage 

occurred in area underlain by artificial fill. 

32 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 
Figure 28. Sand boils at Oakland International Airport. 

Figure 29. Sand boils in irrigated fields near Hollister. 

Damage to Buildings, Transportation Routes, and Utilities 33 
shear reinforcement of the top end of the columns 

between the decks, and possibly to amplified ground 

shaking by local ground conditions (fig. 30). Retrofitting 

plans prepared prior to the earthquake were not implemented due to a lack of funds. 

In San Francisco, three double-decked viaducts 

(Embarcadero Freeway, Highway 101 at Fell Street, and 

Interstate 280) were severely damaged and may have to 

be demolished. Their construction is similar to the Cypress structure, but with continuous column joints between decks. 

The San Mateo Bridge was slightly damaged and was 

closed briefly for inspection and repairs immediately 

after the earthquake. The closure of this bridge, together 

with the Bay Bridge, added to the serious disruption of 

cross-bay traffic. 

South of the epicentral area, on Highway 1 at Struve 

Slough, vertical movement of the concrete bridge deck 

slabs caused some support columns to punch through the 

deck with consequent tilting and collapse of the bridge 

(fig. 31). 

Other transportation lifelines, although interrupted 

for inspection, performed well. The Bay Area Rapid 

A 

Transit commuter train tunnel, which carries a significant 

burden of the traffic between San Francisco and Oakland, was not damaged. However, it was inoperative 

immediately after the earthquake because of power 

outage and for damage inspections. 

AIRFIELDS, PIPELINES, AND STORAGE FACILITIES 

San Francisco International Airport was closed to 

traffic for about 13 hours after the earthquake because of 

damage to the control tower. Part of a boarding ramp in 

one of the terminals was damaged, and a poorly designed 

2-story structure was so badly damaged it will have to be 

demolished. There was extensive damage to nonstructural components, including rupture of sprinkler system 

water lines and collapse of ceilings. As previously discussed, runways at Oakland International Airport and 

the Alameda Naval Air Station were damaged by liquefaction and lateral spreading. 

Damage to two dams within and close to the focal 

region (fig. 1) was reported by the New Civil Engineer 

(1989). Fortunately, their reservoirs were nearly empty at 

the end of the dry season. The 184-foot-high rockfill dam 

Figure 30. Collapsed Cypress structure on Interstate 880. A, Second deck collapsed onto first deck. 8, Closeup of failed 

column that supported second deck. Only four 2-in.-diameter, steel reinforcing bars were used in this joint (foreground). 

34 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 19811 
at Lake Eisman was severely cracked and compacted and 

the spillway was damaged. Deep cracks developed in 

both shoulders of the dam that impounds Lexington 

Reservoir near Highway 17. No other dams in the region 

are known to have been damaged significantly. Peak 

accelerations at the crest of Anderson Dam (17 miles 

east-northeast from the epicenter) reached 43 percent of 

the acceleration of gravity. 

Widespread power outages occurred throughout the 

region, and some localities were without power for 

several days. The Moss Landing Pacific Gas and Electric 

fossil-fuel plant, which supplies much of the region, had 

damage to its electrical circuit breakers, and there was 

evidence of subsidence and liquefaction within the plant 

periphery. 

THE POTENTIAL FOR FUTURE LARGE 

EARTHQUAKES 

Without doubt, additional large earthquakes will 

occur along the major faults in the San Francisco Bay 

8 

Figure 30. Continued. 

region, just as they have in the past (table 2). Of these 

faults, the most important are the San Andreas, Hayward, Calaveras, and San Gregorio (fig. 2). A large 

earthquake relieves stress on the fault segment that slips. 

Consequently, following an earthquake, the likelihood 

for another large earthquake on that fault segment is low 

for many years. However, as the time since the previous 

large earthquake along a particular fault segment becomes greater, the likelihood of an earthquake on that 

segment increases. Although the section of the San 

Andreas fault zone that slipped in the Lorna Prieta 

earthquake is not expected to generate another major 

earthquake for many years, the Lorna Prieta earthquake 

has not reduced the potential for large earthquakes along 

other fault segments in the San Francisco Bay area. 

For well-studied fault segments, the long-term probabilities for occurrence of earthquakes can be computed. 

In 1988 a working group consisting of 12 scientists from 

the U.S. Geological Survey, academia, and private industry issued a report (U.S. Geological Survey, 1988) on the 

probabilities of large earthquakes on selected faults in 

coastal California. This report, which was reviewed and 

approved by the National Earthquake Prediction Evaluation Council, concluded that the segment of the San 

Andreas fault affected by the Lorna Prieta earthquake 

had a 0.3 probability for a magnitude 6.5 to 7.0 earthquake over the interval 1988-2018. (Probabilities are 

expressed in numbers that range from 0 to 1, where 1 

represents certainty that the event will occur and 0 

indicates certainty that the event will not occur. A 0.3 

probability corresponds to a 30 percent chance of the 

earthquake happening.) This was the highest probability 

the working group assigned to any fault segment in 

central California. Earlier studies by individual scientists 

(Lindh, 1983; Sykes and Nishenko, 1984) also concluded 

that this section of the San Andreas fault had a high 

probability for a large earthquake. For this fault segment, 

Lindh (1983) assigned a 0.47 probability for a magnitude 

6.5 earthquake in a 30-year period. 

Figure 32 shows long-term probabilities for large 

earthquakes on segments of the Hayward and San Andreas faults in the region, the only two faults for which 

the data are sufficient to support an estimate. Two 

sections of the Hayward fault, the northern and the 

southern segments, have been judged capable of producing magnitude 7 earthquakes comparable to the earthquakes of 1836 and 1868, respectively. The 30-year 

probability for each of those segments is 0.2. The San 

Francisco peninsula segment of the San Andreas fault 

also has a 30-year probability of 0.2 for a magnitude 7 

earthquake. Because of the very large fault displacements in the great earthquake of 1906, the section of the 

San Andreas fault to the north of the San Francisco 

peninsula segment currently has a low probability for a 

magnitude 8 earthquake (less than 0.1 in 30 years). The 

total probability for one or more of the magnitude 7 

The Potential for Future large Earthquakes 35 
earthquakes on the Hayward and San Andreas faults is 

0.5 in the next 30 years. 

In addition to the San Andreas and Hayward faults, 

there are several other faults in the region that could 

produce damaging earthquakes. Consequently, the overall risk of destructive earthquakes in the region may be 

significantly greater than that posed by the San Andreas 

fault alone. A partial list of other faults of concern 

includes the Calaveras, San Gregorio, Concord, Green 

Valley, Healdsburg-Rodgers Creek faults and a number 

of poorly known thrust faults such as the SargentBerrocal and Stanford faults. Information concerning 

these faults is presently insufficient to compute probabilistic forecasts of future activity. 

REDUCING EARTHQUAKE HAZARDS 

The Lorna Prieta earthquake amply illustrates that 

large earthquakes can seriously impact society and the 

Nation. To confront this problem, significant steps have 

been taken over the the last decade or two to help 

mitigate earthquake hazards (see appendix 2). With 

these steps substantial progress has been made toward 

better construction practice, improved building codes, 

identifying and strengthening dangerous structures, 

earthquake forecasting, mapping and evaluating 

earthquake-related geologic hazards, and better emergency preparedness and disaster response planning. 

However, important questions to be asked from an earth 

science perspective in the wake of the Lorna Prieta 

experience are "Are the measures developed on the basis 

of the earth sciences sufficient? Are they being implemented? If not, what additional steps are needed to 

encourage active programs to reduce earthquake hazards 

in the San Francisco Bay region?" 

An important lesson reaffirmed by the Lorna Prieta 

earthquake is that damage to structures varied both with 

the quality of design and construction and with the 

behavior of the underlying ground. Geology influences 

earthquake damage by controlling: 

• the potential location, size, and time of occurrence of 

damaging earthquakes; 

• the potential rupture of the ground surface by faulting; 

• the potential severity of ground shaking, including its 

intensity and duration; 

• the potential shaking-induced failures and deformation of the ground surface resulting from landsliding 

or liquefaction; and 

• the potential flooding from dike and dam failures, 

seiches, tsunamis, and tectonic changes of land level. 

With the exception of flooding, each of these conditions contributed to the damage associated with the 

Lorna Prieta earthquake. Consequently, success in reducing losses and damage in future earthquakes depends 

Figure 31. Highway 1 bridge destroyed by strong shaking and liquefaction of river deposits at Struve Slough. 

36 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
Table 2. Chronology of historical earthquakes in the San Francisco Bay region 

[--,no data; Do., ditto] 

Date location Magnitude 

1865 Santa Cruz Mmmtains 6.5 

1868 Hayward 6.8 

1892 Vacaville 6.8 

1998 Mare Island 6.5 

1906 San Francisco 8.3 

1955 Oakland-Walnut Creek 5.4 

1957 San Francisco 5.3 

1969 Santa Rosa 5.6 

1980 Livermore Valley 5.5 

1984 Morgan Hill 6.2 

1989 Lorna Prieta 7.1 

1 Equivalent to 20 billion 1987 dollars. 

lives 

lost 

6 

Tl 

1 

700 

1 

1 

1 

62 

Estimated 

damage 

(in thousands 

of dollars) 

500 

350 

225 

1,400 

15oo,ooo 

1,000 

1,000 

8,350 

3,934 

8,000 

>-6,000,000 

Reference 

Sherburne, 1981 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Bennett and Sherburne, 1984 

State of California. Governor's Office 

of Emergency Services, written 

communication, November 21, 1989 

Note: For a listing of historical earthquakes in California see Coffman, J.L., and von Hake, C.A., 1973, Earthquake history of the 

United States: U.S. Department of Commerce (NOAA), Publication 41-1 (revised edition through 1970), 208 p. 

not only on sound engineering and construction, but also 

on our ability to predict the location, likelihood, and 

severity of geologic hazards on both a regional and a 

site-by-site basis. 

SEISMIC ZONATION 

Studies in the San Francisco Bay region (Borcherdt, 

1975) showed that seismic zonation, or the delineation of 

geographical areas with different potentials for each of 

the various geologic hazards, was feasible. These studies, 

and the techniques developed therein, resulted in preparation of maps showing the potential for surface faulting, ground shaking, liquefaction, landsliding, and 

flooding; similar studies produced maps which formed 

the basis for the development of regional land-use policies to minimize future earthquake losses (Blair and 

Spangle, 1979). Some of this information has been incorporated into the public safety plans mandated by sec. 

65302(g) of the California Government Code enacted in 

1974 and into other plans of many cities and counties of 

the region. Examples of the use of the seismic zonation 

method for planning and regulation by three counties 

and three cities has been reported by Kockelman and 

Brabb (1979). 

Laws already enacted have been major steps toward 

developing land-use policies to reduce the loss of life and 

property during future earthquakes. A State law of 

particular significance for mitigating hazards specifically 

related to surface faulting is the Alquist-Priolo Act of 

1972 (California Public Resources Code, sec. 2621 and 

following). This law requires that a special-studies zone 

be prescribed along the traces of known active faults 

capable of earthquake offset. Unfortunately, much of the 

San Francisco Bay region growth took place prior to its 

enactment so that concentrated development exists on 

active traces of the Hayward fault and, to a lesser degree, 

on the San Andreas fault. Development also has spread 

to landfill areas underlain by bay mud, to other areas with 

high potentials for liquefaction, and to upland slopes 

subject to landsliding. Many buildings within this region 

predate modern building codes, which require 

earthquake-resistant design. Use of our current understanding of the causes of earthquake damage can reduce 

the impact of the next large earthquake. For example, 

many communities in this hilly region have implemented 

slope regulations that reduce the hazard of developing 

hillside sites. Few communities, however, have prepared 

similar guidelines for areas that are subject to unusually 

severe ground shaking, to ground deformation related to 

compaction or liquefaction, to flooding resulting from 

earthquake shaking, or to other causes. 

The degree of vulnerability of the most densely 

urbanized part of the San Francisco Bay region, for large 

earthquakes originating on either the San Andreas or 

Hayward faults, is shown in figure 33. This map, based on 

the damage distribution of the 1906 earthquake, recent 

geologic information, and quantitative comparative 

ground-motion measurements, indicates that the damage 

Reducing Earthquake Hazards 37 
from these large earthquakes will vary from weak to very 

violent depending on distance of the site from one of 

these major faults and on the type of underlying geologic 

unit. Such maps help identify the most vulnerable areas 

0 20 

MILES 

in the San Francisco Bay region and in turn those areas 

requiring special studies. A demonstration project conducted in San Mateo County has produced a folio of 

1 :62,500-scale maps showing the potential for surface 

122° 

M 

SACRAMENTO 

1 

Segment that slipped in I 

1989 Lorna Prieta earthquake 

Figure 32. Segments of San Andreas and Hayward faults (heavy lines) showing chance of occurrence of an earthquake in 

the next 30 years (U.S. Geological Survey, 1988). Faults dotted where concealed. 

38 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
:u 

CD 

Q. 

c 

() 

5' 

ca 

g' 

:::s. 

::1' 

.Jl 

c 

Q) 

~ 

::t 

I» 

~ 

a 

• 

w 

co 

37° 122°30' 

52' 30" I ~:YL ··" R£F!5.Qt;:._ .IJ.; tz:l"'Mit.{ 1.-.S: TLl •. ~--'~-·--\\\1 

48\\:;\: \ /• 

~?~><}/ "' A 9/ \";;\ 

•no;v.:..A\.J>=<);:ml /" ., ~J \~\ __ , ' 

\ ,2\_~{- I L 

U': <- ~~~~ lll~,me Po\nt • 1' r .:et~~41 "~;iz -::y~.:.:.'"""l.f\~" ' .4--/'"-':1', 1 Marina "'" :"-~'\fn; I ,g \ ,y 

l;.\ rt: i district I. '

0

' )/ / 

91 

I. • il I·-;-- ~n 

60 

122°15' 

'.-> 1j. I 

\ . ><~\ - .,¢ 9 

-~,~ ~/ 

,-·7],\~ _/ -,<9-.\~-\~-"'/ 

~ --,..r··;;~~:.__ 10 

~/& 

~~ d 

47 ~-

~ 

MAXIMUM PREDICTED EARTHQUAKE SHAKING 

1 A 1 s 1 c- 1 o 1 E 1 

Very violent Violent Very strong Strong Weak 

~0 

Figure 33. Predicted maximum intensity of ground shaking (lettered areas) from large earthquakes on San Andreas fault (heavy dashed line) and Hayward fault (heavy 

solid line). Map from Borchardt and others (1975). Areas marked A apply to earthquake on Hayward fault but not to more distant Lorna Prieta earthquake. Damage 

will be influenced not only by shaking but also by ground failure and design and construction quality of structures. 
faulting, ground shaking, liquefaction, landsliding, and 

damage to various types of structures (Wieczorek and 

others, 1985; Brabb and Olson, 1986; Thomson and 

Evernden, 1986; Youd and Perkins, 1987; Perkins, 1987). 

These maps, which indicate the nature of the hazard and 

its potential severity, provide a rational basis for land-use 

and construction policies aimed at reducing the earthquake threat to public health and safety and the region's 

economy. 

These geologic hazards are discussed and illustrated 

for the San Francisco Bay area by Brown and Kockelman 

(1983). In addition, six examples are presented on how 

various State, local, and private agencies have used this 

information to reduce hazards. Mader and Blair-Tyler 

(1988) recently described 30 actions that local governments can take to improve seismic safety. Many of them 

are drawn from innovative efforts already undertaken. 

Each action is divided into steps; sources of additional 

information are given. A brief but comprehensive checklist provides a format for assessing local preparedness 

and there are suggestions for selecting actions and combining them into a multiyear earthquake-safety program. 

CONCLUSION 

The recurring theme in this report on the Lorna 

Prieta earthquake is that geologic conditions strongly 

influence damage. In other words, the geology determines where fault ruptures are likely to be, how hard the 

ground will shake, where landslides will occur, and where 

the ground will sink and crack. 

A parallel theme is that the pattern of damage from 

shaking and geologic effects observed in 1989 is very 

similar to that witnessed in 1906. Thus, many of the 

lessons taught by the 1906 shock have been forgotten or 

ignored. As the philosopher-poet George Santayana aptly noted "Those who cannot remember the past are 

condemned to repeat it." 

We know that an earthquake as large as, or larger 

than, the Lorna Prieta earthquake is likely to shake the 

San Francisco Bay region within the lifetime of most of 

the present residents and of many of the existing buildings and facilities. We know that such an earthquake will 

probably occur in a more urbanized area, most likely on 

the Hayward fault or San Francisco Peninsula segment of 

the San Andreas fault, and that the casualties and damage are conservatively projected to be several times those 

just experienced. We know that shaking levels and geologic hazards of future earthquakes can be predicted with 

sufficient detail and confidence to guide policies and 

priorities for reducing future earthquake losses. 

Good science and engineering are not enough to 

ensure reduction of earthquake hazards. Meaningful 

hazard reduction can be achieved only when a wellinformed and well-prepared public insists upon such 

protection from government at local, county, State, and 

Federal levels. If society has the collective will, the effects 

of the next major earthquake can be minimized through 

wise land use, strengthening of weak structures, and 

proper design and construction practices. We can either 

proceed vigorously to apply the lessons learned from the 

Lorna Prieta earthquake or be condemned to relearn 

them from the next earthquake. 

ACKNOWLEDGMENTS 

In addition to those who contributed directly to 

writing this report (listed on title page), many others 

contributed indirectly through their active involvement in 

the geological and geophysical studies of the earthquake, 

in analysis of the data, and in the typing, drafting, editing, 

technical review, and other facets of the team effort 

required for timely preparation of this report for publication. These contributors, listed below, are all with the 

U.S. Geological Survey, except as otherwise noted. 

TECHNICAL STAFF: 

A. Acosta, S.T. Algermissen, D.J. Andrews, Mary Andrews, CA. Aviles, W.H. Bakun, F.W. Baldwin, R.L. 

Baum, J. Alan Bartow, J.D. Bicknell, Michael Blanpied, 

John Boatwright, Julian Bommer (Imperial College, 

London, England), M.G. Bonilla, G.A. Brady, Katherine 

Breckenridge, L.R. Brewer, Robert D. Brown, Jr., V. 

Brown, R. Bucknam, K.E. Budding, T.R. Burdette, Sandra Burford, F.L. Butler, R. Campbell, Mauro Cardinale 

(CNR, IRPI, Perugia, Italy), M.D. Carr, L. Carrol-Haar, 

D. Carver, K.W. Clark, J.A. Cotton, P. Cowil (LamontDoherty Observatory), E. Cranswick, C. Diass, C.M. 

Dietel, Lynn Dietz, Sam Donaldson, M.M. Donato, R.S. 

Dollar, J.P. Eaton, S.D. Ellen, F. Ellis, W.L. Ellsworth, 

E.C. Etheredge, F.G. Fischer, J.P. Fletcher, E.B. Fogelman, L. Foote, A. Frankel, P. Frieberg, G.S. Puis, T.E. 

Fumal, J.P. Gibbs, G.N. Glassmoyer, P. Goldstein, W.K. 

Gross, Z. Guoxia, W.D. Hall, K.M. Haller, G.O. Hamilton, J.A. Hamilton, S.L. Hanson, D.H. Harlow, E.L. 

Harp, J.E. Healy, J.W. Herriot, Stephen Hickman, D.P. 

Hill, B.F. Hirschorn, M.G. Hopper, P.W. Irwin, E.G. 

Jensen, R. Jibson, D. Johnson, W.B. Joyner, B.R. Julian, 

R.M. Kaderabek, V.G. Keller, J.T. Kempe, Chi-Yu King, 

K., King, S.H. Kirby, F.W. Klein, W.M. Kohler, A.H. 

Lachenbruch, J.C. Lahr, J.O. Langbein, W.H. Lee, F.W. 

Lester, E.V. Leyendecker, R. Liechti, J. Lienkaemper, 

A.G. Lindh, Hsi-Ping Liu, J.H. Luetgert, N. Luttinger, 

N.G. MacGregor-Scott, M. Machette, R.P. Maley, R.K. 

Mark, S.M. Marks, G. Marshall, Robert McCiearn, A.F. 

McGarr, RJ. McLaughlin, K. Meagher, Dan Meier, C.E. 

Mortenson, C.S. Mueller, R.J. Mueller, B.L. Murchey, 

D. Myron, J.E. Nielson, Tom Noce, Brendan O'Neill, 

40 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
Paul Okubo, J.A. Olson, E. Omodt, D.H. Oppenheimer, 

T. Parker, D.M. Perkins, S.F. Personius, D.W. Peterson, 

J.H. Pfluke, R. Porcella, B.G. Reagor, A. Records, D.B. 

Reneau, Pat Rice, James Rigby (Nevada Bureau of 

Mines and Geology), Anselmo Rodriquez, E. Roeloffs, 

M.J. Salsman, A.M. Sarna-Wojcicki, K.V. Schmidt, Linda Seekins, Paul Segall, E.D. Sembera, Joe Sena, Stan 

Silverman, R.W. Simpson, J.D. Sims, P .A. Spudich, J. 

Staude, R.S. Stein, C.D. Stephens, L. Stevens, S.W. 

Stewart, C.H. Stiffler, C.W. Stover, J.M. Sutton, G.L. 

Svarc, Joseph Svitek, J.C. Switzer, W.R. Thatcher, J.R. 

Van Schaack, R.E. Wallace, C. Walkingstick, A.W. Walter, S.R. Walter, R.E. Warrick, K.E. Wendt, Carl Wentworth, R.E. Westerlund, R.A. White, G. Wieczorek, R.C. 

Wilson, P. Wood (New Zealand Geological Survey), D. 

Worley, Ze'ev Reches (Stanford University), M.L. 

Zoback 

SUPPORT STAFF: 

G.L. Allen, N.E. Arp, R.K. Barnhart, L.E. Buchholz, 

R.M. Callahan, B.B. Charonnet, M.E. Coveau, CJ. Criley, P.J. Cuneo, R.R. Eis, D.D. Hirsch, L.D. Hodgen, 

L.M. Hollis, N.L. Hoskin, S. Hotop, T.C. Jackson, M.L. 

Jacobson, A.C. Jones, M.W. Kennedy, D.L. Knifong, 

L.W. Kohnitz, Linnea Larson, E.P. Lopez, J.S. Lull, 

A.M. Margrave, S.E. Mayfield, Lori Moore, C.C. Ramseyer, M.L. Sanders, G.M. Schumacher, S.L. Scott, W.H. 

Seiders, J.L. Siegman, Moses Smith, R.M. Trombley, 

R.E. Wood, J .F. Vigil 

TECHNICAL REVIEWERS: 

Robert D. Brown, Jr., and Robert A. Page 

Although it is impossible to individually acknowledge everyone, we sincerely thank the many residents 

who graciously assisted us with our field investigations by 

providing access to their property and for their eyewitness accounts that helped us to better understand the 

earthquake and its effects. 

REFERENCES CITED 

Alpha, Tau Rho, Lahr, J.C., and Page, R.A., 1989, Oblique map 

of the northern half of the Lorna Prieta, California 

earthquake rupture zone and environs: U.S. Geological 

Survey Open-File Report 89-633, 1 sheet. 

Blair, M.L., and Spangle, W.E., 1979, Seismic safety and 

land-use planning-Selected examples from California: 

U.S. Geological Survey Professional Papaer 941-B, 82 p. 

Bonilla M.G., Mark, R.K., and Lienkaemper, J.J., 1984, Statistical relations among earthquake magnitude, surface 

rupture length, and surface fault displacement: Bulletin 

of the Seismological Society of America, v. 74, no. 6, p. 

2379-2411. 

Borcherdt, R.D. 1970, Effects of local geology on ground 

motion near San Francisco Bay: Bulletin of the Seismological Society of America, v. 60, no. 29, p. 61. 

Borcherdt, R.D., ed., 1975, Studies for seismic zonation of the 

San Francisco Bay region: U.S. Geological Survey Professional Paper 941-A, 102 p. 

Borcherdt, R.D., Gibbs, J.F., and LaJoie, K.R., 1975, Maps 

showing maximum earthquake intensity predicted in the 

southern San Francisco Bay region, California, for large 

earthquakes on the San Andreas and Hayward faults: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF-709, scale 1:125,000. 

Brabb, E.E., and Olsen, J.A., 1986, Map showing faults and 

earthquake epicenters in San Mateo County, California: 

U.S. Geological Survey Miscellaneous Investigations Series Map 1-1257-F. 

Brown, R.D., Jr., and Kockleman, W.J., 1985, Geology for 

decisionmakers-Protecting life, property, and resources: Berkeley, Regents of the University of California, 

Bulletin of the Institute of Governmental Studies, Public 

Affairs Report, v. 26, no. 1, 11 p. 

Dieterich, J.H., 1983, Assessment of a prototype earthquake 

prediction network for southern California: U.S. Geological Survey Open-File Report 83-576, 27 p. 

Dupre, W.R., 1975, Maps showing geology and liquefaction 

potential of Quaternary deposits in Santa Cruz County, 

California: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-648, 2 sheets, scale 1:62,500. 

Dupre, W.R., and Tinsley, J.C., III, 1980, Maps showing 

geology and liquefaction potential of northern Monterey 

and southern Santa Cruz Counties, California: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-1199, 2 sheets, scale 1:62,500. 

Ellen, S.D., and Wieczorek, G.F., eds., 1988, Landslides, floods, 

and marine effects of the storm of January 3-5, 1982, in 

the San Francisco Bay region, California: U.S.Geological 

Survey Professional Paper 1434, 310 p., 14 plates. 

Belley, E.J., Lajoie, K.R., Spangle, W.E., and Blair, M.L., 1979, 

Flatland deposits of the San Francisco Bay region, 

California-Their geology and engineering properties, 

and their importance to comprehensive planning: U.S. 

Geological Survey Professional Paper 943, 88 p., 3 

plates. 

Jennings, C.W., 1975, Fault map of California, with locations of 

volcanoes, thermal springs, and thermal wells: California 

Division of Mines and Geology, Geologic Data Map 1, 

scale 1:750,000. 

Joyner, W.B., and Boore, D.M. 1988, Measurement, characterization, and prediction of strong ground motion: Proceedings of Earthquake Engineering and Soil Dynamics, 

Park City, June 27-30, 1988, American Society Civil 

Engineers, no. 43, p. 102. 

Keefer, D.K., 1984, Landslides caused by earthquakes: Geological Society of America Bulletin, v. 95, no. 4, p. 406-421. 

Keefer, D.K., Wilson, R.C., Mark, R.K., Brabb, E.E., Brown, 

W.M., III, Ellen, S.D., Harp, E.L., Wieczorek, G.F., 

Alger, C.S., and Zatkin, R.S., 1987, Real-time landslide 

warning during heavy rainfall: Science, v. 238, p. 

921-925. 

Kockelman, W.J., and Brabb, E.E., 1979, Examples of seismic 

zonation in the San Francisco Bay region: U.S. GeologReferences Cited 41 
ical Survey Circular 807, p. 73-84. 

Lawson, A.C., and others, 1908, The California earthquake of 

April 18, 1906: Report of the California State Earthquake Investigation Commission: Washington, Carnegie 

Institution, Publication 87, v. 1, 2 parts, 451 p. 

Lindh, A.G., 1983, Preliminary assessment of long-term probabilities for large earthquakes along selected segments of 

the San Andreas fault system in California: U.S. Geological Survey Open-File Report 83-63, p. 1-5. 

Mader, G.G., and Blair-Tyler, Martha, 1988, California at 

risk-Steps to earthquake safety for local governments: 

Sacramento, California, Seismic Safety Commission Report sse 88-01, 56 p. 

McLaughlin, R.J., 1974, The Sargent-Berrocal fault zone and 

its relation to the San Andreas fault system in the 

southern San Francisco Bay region and Santa Clara 

valley, California: U.S. Geological Survey Journal of 

Research, v. 2, no. 5, p. 593-598. 

New Civil Engineer, 1989, The race to repair dams: New Civil 

Engineer, no. 896, November 2, p. 26-27. 

Perkins, J .B., 1987, Maps showing cumulative damage potential 

from earthquake ground shaking, in San Mateo County, 

California: U.S. Geological Survey Miscellaneous Investigations Series Map I-1257-I. 

Prescott, W.H., Davis, J.L., and Svarc, J.L., 1989, Global 

positioning system measurements for crustal deformation: Precision and accuracy: Science, v. 244, p. 

1337-1340. 

Rantz, S.E., 1971, Mean annual precipitation and precipitation 

depth-duration-frequency data for the San Francisco Bay 

region, California: U.S. Geological Survey Open-File 

Report, 23 p., 1 plate. 

Reid, H.F., 1910, The California earthquake of April18, 1906: 

Report of the California State Earthquake Investigation 

Commission: Washington, Carnegie Institution, Publication 87, v. 2, The mechanics of the earthquake, 192 p. 

Sarna-Wojcicki, A.M., Pampeyan, E.H., and Hall, N.T., 1975, 

Recent active breaks along the San Andreas fault between the central Santa Cruz Mountains and northern 

Gabilan Range, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-650, scale 1:24,000. 

Savage, J.C., and Prescott, W.H., 1973, Precision of geodolite 

distance measurements for determining fault movement: 

Journal of Geophysical Research, v. 78, p. 6001-6008. 

Seawthorn, C., and O'Rourke, T.D., 1989, Effects of ground 

failure on water supply and fire following earthquake: 

The 1906 San Francisco earthquake: Proceedings of 2nd 

U.S.-Japan Workshop on Liquefaction, Large Ground 

Deformation and Their Effects on Lifeline Facilities, 

September 26-28, 1989, Buffalo, New York. 

Sherburne, R.W., 1981, Seismology Program: California Geology, v. 34, no. 1, p. 3-6. 

Sherburne, R.W., and Bennett, J.H., eds., 1984, The Morgan 

Hill, California, earthquake: Sacramento, California 

Division of Mines and Geology Special Publication 68, 

271 p. 

Sykes, L.R., and Nishenko, S.P ., 1984, Probabilities of occurrence of large plate-rupturing earthquakes for the San 

Andreas, San Jacinto, and Imperial faults, California, 

1983-2003: Journal of Geophysical Research, v. 89, p. 

5905-5927. 

Thatcher, W., 1975, Strain accumulation and release mechanism of the 1906 San Francisco earthquake: Journal of 

Geophysical Research, v. 84, p. 4862-4872. 

Thatcher, W., and Lisowski, M., 1987, Long-term seismic 

potential of the San Andreas fault southeast of San 

Francisco, California: Journal of Geophysical Research, 

v. 92,p.4771-4784. 

Thomson, J.M., and Evernden, J.F., 1986, Map showing predicted seismic-shaking intensities of an earthquake in 

San Mateo County, California: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-1257-H. 

U.S. Geological Survey, 1988, Probabilities of large earthquakes occurring in California on the San Andreas fault: 

U.S. Geological Survey Open-File Report 88-398, 62 p. 

Wieczorek, G.R., Wilson, R.C., and Harp, E.L., 1985, Map of 

slope stability during earthquakes in San Mateo County, 

California: U.S. Geological Survey Miscellaneous Investigations Map I-1257-E. 

Wood, H.O., 1908, Distribution of apparent intensity in San 

Francisco, in The California earthquake of April 18, 

1906: Washington, Carnegie Institution, Publication 87, 

v. 1, p. 220--246. 

Youd, T.L., and Perkins, J.B., 1987, Map showing liquefaction 

susceptibility of San Mateo County, California: U.S. 

Geological Survey Miscellaneous Investigations Series 

Map I-1257-G. 

42 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 
APPENDIX 1. GLOSSARY 

[Terms set in bold type are defined elsewhere in the glossary] 

Acceleration. The time rate of change of velocity of a reference 

point during an earthquake. Commonly expressed in percentage of gravity (g, equal to 32 feet per second per second. 

Active fault. A fault that is considered likely to undergo 

renewed movement within a period of concern to humans. Also 

referred to as a "capable" fault. 

Alluvium. Loosely compacted gravel, sand, silt, or clay deposited by streams. 

Aftershock. Secondary tremors that may follow the largest 

shock or mainshock of an earthquake sequence. Such tremors 

can extend over a period of weeks, months, or years. 

Amplification. An increase in seismic signal amplitude within 

some range of frequency as waves propagate through different 

earth materials. 

Amplitude. Zero-to-peak value of any wavelike disturbance. 

Aseismic. Not associated with an earthquake. 

Attenuation. A decrease in seismic signal amplitude as waves 

propagate from the seismic source. Attenuation is caused by 

geometrical spreading of seismic wave energy and by the 

absorption and scattering of seismic energy in different earth 

materials. 

Bedrock. Relatively hard, solid rock that commonly underlies 

softer rock, sediment, or soil. 

Body wave. A seismic wave that travels through the interior of 

the Earth and is not related to a boundary surface. Primary and 

secondary waves are examples of body waves. 

Creep. Slow, more or less continuous movement that may occur 

either along faults owing to ongoing tectonic deformation or 

along slopes owing to gravitational forces. 

Crust. The outermost major layer of the Earth, ranging from 

about 6 to 40 miles thick worldwide and about 12 miles thick in 

coastal California; characterized by primary wave velocities less 

than 5 miles per second. 

Dip. Inclination of a planar geologic surface (for example, a 

fault or a bed) from the horizontal. 

Displacement. The difference between the initial position of a 

reference point and any later position. (1) In seismology, 

displacement is typically calculated by integrating an accelerogram twice with respect to time and is expressed in centimeters. 

(2) In geology, displacement is the permanent offset of a 

geologic or manmade reference point along a fault or landslide. 

Earthquake. Groups of elastic waves propagating in the Earth, 

set up by a sudden disturbance of the elastic equilibrium of a 

portion of the Earth. 

Earthquake hazard. Any physical phenomenon associated with 

an earthquake that may produce adverse effects on human 

activities. 

Elastic rebound theory. In seismology, the theory stating that 

faulting arises from the sudden release of elastic energy which 

has slowly accumulated in the Earth. Just before the rupture, 

the energy released by the faulting is entirely potential energy 

stored as the elastic strain in the rocks. At the time of the 

rupture the rocks on either side of the fault spring back to a 

position of relatively little or no strain. 

Elastic wave. A wave that is propagated by some kind of elastic 

deformation, that is, a deformation that disappears when the 

forces are removed. A seismic wave is a type of elastic wave. 

Epicenter. That point on the Earth's surface vertically above 

the hypocenter of an earthquake (where a seismic rupture 

initiates). 

Fault. A fracture or fracture zone along which there has been 

displacement of the sides relative to one another parallel to the 

fracture plane or planes. 

Fault-plane solution. An analysis to determine the attitude of 

the causative fault and its direction of slip from the radiation 

pattern of seismic waves for an earthquake. The analysis most 

commonly uses the direction of first motion of primary waves 

recorded at numerous stations and yields two possible orientations for the fault rupture and the direction of seismogenic slip. 

From these data, inferences can be made concerning the 

principal axes of stress in the region of the earthquake. 

Fault-trace. Intersection of a fault with the ground surface; 

also, the line commonly plotted on geologic maps to represent 

a fault. 

Focus. The source of a given set of elastic waves. The true 

center of an earthquake, within which the strain energy is first 

converted to elastic wave energy. See also Hypocenter. 

Focal zone. The rupture zone of an earthquake. In the case of 

a great earthquake (magnitude> 7.5), the focal zone may 

extend several hundred miles in length. 

Foreshock. A small tremor that commonly precedes a larger 

earthquake or mainshock by seconds to weeks and that originates at or near the focus of the larger earthquake. 

Free face. A sloping surface exposed to air or water such that 

there is little or no resistance to lateral movement of adjacent 

earth material. 

Frequency. Number of cycles occurring in unit time. Hertz 

(Hz), the unit of frequency, is equal to the number of cycles per 

second. 

Appendix 1. Glossary 43 
Geodetic measurements. Controls on location (vertical or 

horizontal) of positions on the Earth's surface of a high order of 

accuracy, usually extended over large areas for surveying and 

mapping operations. 

Geophysical surveys. The use of one or more techniques of 

physical measurement to explore earth properties and processes. 

Geotechnical. Refers to the use of scientific methods and 

engineering principles to acquire, interpret, and apply knowledge of earth materials for solving engineering problems. 

Ground motion. General term referring to the qualitative or 

quantitative aspects of shaking of the Earth's surface from 

earthquakes or explosions. 

Hypocenter. The point within the Earth where an earthquake 

rupture initiates. See also Focus. 

Intensity. A subjective measure of the force of an earthquake 

at a particular place as determined by its effects on persons, 

structures, and earth materials. The principal scale used in the 

United States today is the modified Mercalli intensity scale (see 

table 1). 

Isoseismal. A line connecting points on the Earth's surface at 

which earthquake intensity is the same. It is usually a closed 

curve around the epicenter. 

Liquefaction. Process by which water-saturated sediment temporarily loses strength, usually because of strong shaking, and 

behaves as a fluid. 

Magnitude. A number that characterizes the size of an earthquake, usually based on measurement of the maximum amplitude recorded by a seismograph for earthquake waves of a 

particular frequency. Scales most commonly used are (1) local 

magnitude (ML, commonly referred to as "Richter magnitude"), (2) surface-wave magnitude (M8 ), and (3) body-wave 

magnitude (mb)· None of these scales satisfactorily measures 

the largest possible earthquakes because each relates to only 

certain frequencies of seismic waves and because the spectrum 

of radiated seismic energy changes with the earthquake size. 

The recently devised moment magnitude ( M) scale, based on 

the concept of seismic moment, is uniformly applicable to all 

sizes of earthquakes. 

Body-wave magnitude (mb): Measures the type of waves 

that pass through the interior-the body-of the planet 

and that have a period of between 1 to 10 seconds. 

Local magnitude (ML): A scale most accurately applied 

when dealing with California earthquakes. It is still quite 

useful today for describing smaller and more moderate 

earthquakes, but is not useful in larger earthquakes. 

Surface-wave magnitude (M8 ): Scale formulated to describe earthquakes at distant locations. The scale principally measures surface waves with a 20-second period, or 

a wavelength of approximately 37 miles. 

Moment magnitude ( M): This is today perhaps the most 

meaningful scale for large and great earthquakes, in that 

it measures total energy released. The measurement 

takes into account the surface area of the fault that 

moved to cause the earthquake, plus the average displacement of the fault plane, and the rigidity of the 

material of the fault. A seismic moment,M0 , is the result, 

and when that is combined with an energy-magnitude 

formula, the outcome is a common means of measuring 

the greatest earthquakes on the planet, such as in Alaska, 

1964, and Chile, 1960. This scale was developed very 

recently, which is why great earthquakes, such as that in 

Alaska in 1964, which were once related in the Ms 8.5 

range have been upgraded to an M rating in the low 9's. 

Major earthquake. An earthquake having a magnitude of 7 or 

greater on the Richter scale. 

Microseismic event. Earthquake or man-induced vibrations 

observable only with instruments. 

Plate tectonics. A widely accepted theory that considers the 

Earth's crust and upper mantle to be composed of a number of 

large, relatively thin and rigid plates that move relative to one 

another. Interaction along their boundaries commonly results 

in earthquakes and volcanic activity. 

Primary wave (P-wave ). That type of seismic body wave which 

is propagated by alternating compression and expansion of 

material in the direction of propagation. It is the fastest of the 

seismic waves (traveling 3.7 to 4.2 miles per second in the crust 

and 5 to 5.3 miles per second in the upper mantle below the 

crust), and it is the type which carries sound. The P stands for 

primary; it is so named because it arrives before the slower S 

wave (secondary wave). 

Remote sensing. The acquisition of information or measurement of some property of an object by a recording device that 

is not in physical or intimate contact with the object under 

study. The technique employs such devices as the camera, 

lasers, infrared and ultraviolet detectors, microwave, and radio 

frequency receivers, and radar systems. 

Reverse fault. A steeply to slightly inclined fault in which the 

block above the fault has moved relatively upward or over the 

block below the fault. 

Right-lateral movement. Generally horizontal movement in 

which the block across the fault from an observer has moved to 

the right. 

Sand boil. Sand and water ejected to the ground surface as the 

result of liquefaction at shallow depth; the conical or ridgeshaped sediment deposit that remains is evidence of liquefaction. 

Scarp. A cliff or steep slope formed by a fault or landslide, 

generally by one side moving up relative to the other. 

Secondary wave (S-wave). That type of seismic body wave 

which is propagated by a shearing motion of material, so that 

there is oscillation perpendicular to the direction of propagation. It does not travel through liquids or through the outer core 

44 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 
of the Earth. Its speed is 1.8 to 2.5 miles per second in the crust 

and 2.7 to 2.8 miles per second in the upper mantle below the 

crust. The S stands for secondary; it is so named because it 

arrives later than the faster P-wave. 

Seiche. Oscillation of the surface of an enclosed body of water 

owing to earthquake shaking. 

Seismic. Pertaining to an earthquake or earth VIbration, including those that are artificially induced. 

Seismic risk. The probability of social or economic consequences of an earthquake. 

Seismic wave. An elastic wave generated by an impulse such as 

an earthquake or an explosion. Seismic waves may propagate 

either along or near the Earth's surface or through the Earth's 

interior. 

Seismic zonation. Geographic delineation of areas having 

different potentials for hazardous effects from future earthquakes. Seismic zonation can be done at national, regional, and 

local scales. 

Seismicity. The geographical and historical distnbution of 

earthquakes. 

Seismogram. A record of ground motion or of vibration of a 

structure caused by an earthquake or an explosion. 

Seismograph. An instrument that scribes a permanent continuous record of earth vibrations. 

Separation. The distance between any two parts of a reference 

plane (for example, a sedimentary bed or a geomorphic surface) 

offset by a fault measured in any plane. Separation is the 

apparent amount of fault displacement and is nearly always less 

than the actual slip. 

Shear. A mode of failure whereby two adjacent parts of a solid 

slide past one another parallel to the plane of failure. 

Shear wave. A distortional, secondary or transverse wave. 

Slip rate. The average displacement at a point along a fault as 

determined from geodetic measurements, from offset manmade structures, or from offset geologic features whose age can 

be estimated. It is measured parallel to the dominant slip 

direction or estimated from the vertical or horizontal separation of geologic, geodetic, or other markers. 

Strain. The amount of any change in dimensions or shape of a 

body when subjected to deformation. 

Stress. Force per unit area acting on a surface within a body. Six 

values are required to characterize completely the stress point: 

three normal components and three shear components. 

Strike-slip fault. Fault in which movement is principally horizontal. 

Strong motion. Ground motion produced by a "strong" earthquake or one capable of producing damage to structures. The 

magnitude of such an earthquake may vary considerably according to the character of the earthquake and the nature of the 

ground. 

Subsidence. Downward settling of the Earth's surface with little 

or no horizontal motion. May be caused by natural geologic 

processes (such as sediment compaction or tectonic activity) or 

by human activity (such as mining or withdrawal of ground 

water or petroleum). 

Surface faulting. Displacement that reaches the ground (or sea 

floor) surface during slip along a fault. Commonly accompanying moderate and large earthquakes having focal depths to 12 

miles. Surface faulting also may accompany aseismic tectonic 

creep or natural or man-induced subsidence. 

Surface wave. Seismic wave that propagates along the Earth's 

surface. 

Tectonic. Refers to crustal rock-deformation processes that 

affect relatively large areas. 

Travel time curve. A graph of arrival times of primary or 

secondary waves recorded at different points as a function of 

distance from the seismic source. Seismic velocities can be 

computed from the slopes of the resulting curve. 

Water table. The upper surface of a body of unconfined ground 

water at which the water pressure is equal to the atmospheric 

pressure. 

SOURCES: 

Bates, R.L., and Jackson, J.A., editors, 1987, Glossary of 

Geology [3d ed.]: Alexandria, Virginia, American Geological Institute, 788 p. 

Nance, John, J., 1989, On shaky ground: America's earthquake 

alert: New York, Avon Books, 440 p. 

Ziony, J.l., ed., 1985 Evaluating earthquake hazards in the Los 

Angeles Region-An Earth-Science perspective: U.S. 

Geological Survey Professional Paper 1360. 

Appendix 1. Glossary 45 
APPENDIX 2. GENERAL REFERENCES AND SOURCES OF INFORMATION ON EARTHQUAKES 

Earthquakes, by Bruce Bolt, 1988, New York, W.H. 

Freeman, 282 pages. A primer on earthquakes-their 

causes, measurement, precursors, and effects written by a 

leading researcher and teacher at University of California, 

Berkeley. 

Earthquakes, by Don DeNevi, 1977, Millbrae, California, 

Celestial Arts, 230 pages. An ove1View of historic earthquakes, earthquake hazard reduction, and earthquake prediction. 

Earthquake Survival Guide: Emergency planning for 

family, home, workplace, and school, 1989, Prepared as a 

Public Service by Artichoke Joe's, 659 Huntington Avenue, San Bruno, Calif. 94066, Artichoke Enterprises, 

Inc., 2nd ed., 24 p. 

Earthquakes and Volcanoes, United States Geological 

Survey, bimonthly publication available yearly for $9.00 

from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402. This 

magazine provides short articles and many illustrations for 

nonscientists on different aspects of research on earthquakes and volcanoes. 

Earthquakes and Volcanoes, Readings from Scientific 

American, 1980, San Francisco, California, W .H., Freeman and Company 154 pages. Eleven articles on earthquake properties, earthquakes and earth structure, and 

volcanoes. 

Earthquake Country, by Robert Iacopi, 1971, A Sunset 

Book, Menlo Park, California, Lane Books, 160 pages. 

An explanation as to why California has earthquakes and 

a guide to faults in California. 

On Shaky Ground: America's Earthquake Alert, by J.J. 

Nance, 1989, New York, Avon Books, 440 pages. 

Peace of Mind in Earthquake Country, by Peter Yanev, 

1974, San Francisco, California, Chronicle Books, 304 

pages. Describes earthquake hazards and practical steps to 

take before, during, and after earthquakes. 

Seismicity Map of California 1808-1987, 1:1,000,000 

scale, U.S. Geological Survey Open-File Report 88-286, 

available for $5.00 per map plus $2.00 shipping from 

National Earthquake Information Center, U.S. Geological Survey, 1711 Illinois Avenue, Golden, Colorado 

80401. 

SOME LOCAL SOURCES FOR EARTHQUAKE 

INFORMATION 

The Community Access Pages of Your Local Telephone 

Book. 

Your Community Library. 

Your County or City Planning Office. 

Association of Bay Area Governments (ABAG), P.O. 

Box 2050, Oakland, California 94604-2050, phone ( 415) 

464-7900. Maps that show ground-shaking probabilities, 

technical assistance in planning, publications on preparedness, and training courses for businesses. 

Bay Area Regional Earthquake Preparedness Project 

(BAREPP), Metro Center, 101 8th Street, Suite 152, 

Oakland, California 94607, phone (415) 540-2713. Publications, videotapes, scripted slide shows, and lectures on 

earthquake preparedness. 

California Division of Mines and Geology, Department 

of Conservation, P .0. Box 2980, Sacramento, California 

95812-2980, phone (916) 445-5716. Earthquake planning scenarios, maps, and other publications. 

Earth Science Information Centers, U.S. Geological Survey, 555 Battery Street, Room 504 Customs House, San 

Francisco, California 94111, phone (415) 705-1010; 345 

Middlefield Road, Menlo Park, California 94025, phone 

(415) 329-4390. USGS publications and information on 

other products and data bases. 

Federal Emergency Management Agency (FEMA), 

Building 105, The Presidio, San Francisco, California 

94129, phone (415) 923-7100. Pamphlets on how to 

prepare for an earthquake and what to expect. 

OVERVIEW AND ISSUES 

[This is a modified list of references that recently appeared in an 

Earthquake Engineering Research Institute Newsletter (EERI), September, 1989, v. 23, no. 9] 

Archuleta, R.J., Joyner, W.B., and Boore, D.M., 1979, A 

methodology for predicting ground motion at specific 

sites, in Brabb, E.E., ed., Progress on seismic zonation in 

the San Francisco Bay region: U.S. Geological Survey 

Circular 807, p. 26-36. 

Brown, R.D., Jr., and Kockelman, W.J., 1983, Geologic 

principles for prudent land use: A decisionmakers guide 

for the San Francisco Bay region: U.S. Geological Survey 

Professional Paper 946, 97 p. 

--1985, Geology for decisionmakers: Protecting life, 

property and resources: Berkeley, University of California, Bulletin of the Institute of Governmental Studies, 

Public Affairs Report, v. 26, no. 1, 11 p. (Condensed 

version of USGS Professional Paper 946] 

46 Lessons Learned from the Loma Prieta, California, Earthquake of October 17, 1989 
Hank, T .C., 1985, The National Earthquake Hazards 

Reduction Program-Scientific Status: U.S. Geological 

Survey Bulletin 1659, 40 p. 

Nichols, D.R., and Buchanan-Banks, J.M., 1974, Seismic 

hazards and land-use planning: U.S. Geological Survey 

Circular 690, 33 p. 

U.S. Office of Science and Technology Policy, 1978, 

Earthquake hazards reduction: Issues for an implementation plan: Washington, D.C., Executive Office of the 

President, 231 p. 

SEISMIC ZONATION 

Arnold, Christopher, and Eisner, R.K., 1984, Planning 

information for earthquake hazard response and reductions: San Mateo Calif., Building Systems Development, 

Inc., 79 p. 

Blair, M.L., and Spangle, W.E., 1979, Seismic safety and 

land-use planning: Selected examples from California: 

U.S. Geological Survey Professional Paper 941-B, 83 p. 

Borcherdt, R.D., ed., 1975, Studies for seismic zonation 

of the San Francisco Bay region: U.S. Geological Survey 

Professional Paper 941-A, p. A1-A102. 

Brabb, E.E., Pampeyan, E.H., and Bonilla, M.G., 1972, 

Landslide susceptibility in San Mateo County, California: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF-360, scale 1:62,500. 

Brown, R.D., Jr., and Wolfe, E.W., 1972, Map showing 

recently active breaks along the San Andreas fault between Point Delgada and Bolinas Bay, California: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-692, 2 sheets, scale 1:24,000. 

Hays, W.W., 1980, Procedures for estimating ground 

motions: U.S. Geological Survey Professional Paper 

1114, 77 p. 

Jafee, Martin, Butler, JoAnn, and Thurow, Charles, 

1981, Reducing earthquake risks: A planner's guide: 

Chicago, American Planning Association, Planning Advisory Service Report 364, 82 p. 

Kockelman, W.J., and Brabb, E.E., 1979, Examples of 

seismic zonation in the San Francisco Bay region: U.S. 

Geological Survey Circular 807, p. 73-84. 

Perkins, J.B., 1987, The San Francisco Bay area: On 

shaky ground: Oakland, Calif., Association of Bay Area 

Governments, 32 p. 

Williams, J.W., and Rogers, T.H., 1978, Relative seismic 

stability map, Santa Clara County, California (revised 

ed.): San Jose, Calif., Santa Clara County Environmental 

Management Agency, scale 1:250,000. 

Wesson, R.L., Helley, E.J., Lajoie, K.R., and Wentworth, 

C.M., 1975, Faults and future earthquakes, in Borcherdt, 

R.D., ed., Studies for seismic zonation of the San Francisco Bay region: U.S. Geological Survey Professional 

Paper 941-A, p. A5-A30. 

Youd, T.L., and Hoose, S.N., 1978, Historic ground 

failures in northern California triggered by earthquakes: 

U.S. Geological Survey Professional Paper 993, 177 p. 

Youd, T.L., Nichols, D.R., Helley, E.J., and Lajoie, K.R., 

1975, Liquefaction potential, in Borcherdt, R.D., ed., 

Studies for seismic zonation of the San Francisco Bay 

region: U.S. Geological Survey Professional Paper 941-

A, A68-A74. 

SEISMIC SAFETY PLANNING 

Foster, H.D., 1980, Disaster planning: The preservation 

of life and property: New York, Springer-Verlag, 275 p. 

Helley, E.J., LaJoie, K.R., Spangle, W.E., and Blair, 

M.L., 1979, Flatlands deposits of the San Francisco Bay 

region, California-Their geology and engineering properties, and their importance to comprehensive planning: 

U.S. Geological Survey Professional Paper 943, 88 p. 

Mader, G.G., Spangle, W.E., Blair, M.L., Meehan, R.L., 

Bilodeau, S.W., Degenkolb, H.J., Duggar, G.S., and 

Williams, Norman, Jr., 1980, Land use planning after 

earthquakes: Portola Valley, Calif., William Spangle and 

Associates, Inc., 158 p. 

Nilsen, T.H., Wright, R.H., Vlasic, T.C., and Spangle, 

W.E., 1979, Relative slope stability and land-use planning in the San Francisco Bay region, California: U.S. 

Geological Survey Professional Paper 944, 96 p. 

PLAN IMPLEMENTATION 

Atwater, Brian, 1978, Central San Mateo County, 

California-Land-use controls arising from erosion of 

seacliffs, landsliding, and fault movement, in Robinson, 

G.D., and Spieker, A.M., eds., "Nature to be commanded ... ": Earth-science maps applied to land and water 

management: U.S. Geological Survey Professional Paper 

950, p. 11-21. 

Blair-Tyler, Martha, and Gregory, P.A., 1988, Putting 

seismic safety policies to work: Oakland, Calif., Bay Area 

Regional Earthquake Preparedness Project, 40 p. 

Bolton, P.A., Heikkala, S.G., Greene, M.R., May, P.J., 

and Wolfe, M.R., 1986, Land use planning for earthAppendix 2. General References and Sources of Information on Earthquakes 47 
quake hazard mitigation: A handbook for planners: 

Boulder, University of Colorado, Natural Hazards Applications Information Center Special Publication 14, 

121 p. 

Kockelman, W.J., 1985, Using earth-science information 

for earthquake hazard reduction, in Ziony, J.I., ed., 

Evaluating earthquake hazards in the Los Angeles region: U.S. Geological Survey Professional Paper 1360, p. 

443-469, 502-505. 

Mader, G.G., and Blair-Tyler, Martha, 1988, California 

at risk: Steps to earthquake safety for local government: 

Sacramento, California Seismic Safety Commission Report sse 88-01, 56 p. 

San Jose Board of Realtors, 1977, Map of Santa Clara 

County, California, special studies zones and flood hazard area: San Jose, Calif., Barclay Maps. 

San Mateo-Burlingame Board of Realtors, 1979, Midpeninsula cities map; special studies zones and flood 

hazard area: San Jose, Calif., Barclay Maps. 

EVALUATING EFFECTIVENESS 

Alesch, D.J., and Petak, W J., 1986, The politics and 

economics of earthquake hazard mitigation: Boulder, 

University of Colorado, Institute of Behavioral Science, 

Program on Environment and Behavior Monograph 43, 

273 p. 

Kockelman, W.J., 1979, Use of USGS earth-science 

products by selected regional agencies in the San Francisco Bay region, California: U.S. Geological Survey 

Open-File Report 79-221, 173 p. 

May, P.J., and Bolton, P.A., 1986, Reassessing earthquake hazard reduction measures: Chicago, American 

Planning Association Journal, v. 52, no. 4, p. 443-451. 

Mintier, J.L., and Stromberg, P.A., 1983, Seismic safety 

at the local level: Does planning make a difference?: 

California Geology, v. 36, no. 7, p. 148-154 

48 Lessons Learned from the Lorna Prieta, California, Earthquake of October 17, 1989 1. U.S. GOVERNMENT PRINTING OFFICE: 785 
SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS 

Periodicals 

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly). 

Technical Books and Reports 

Professional Papers are mainly comprehensive scientific reports of 

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

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic. 

Bulletins contain significant data and intetpretations that are of lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

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

as collections of short papers related to a specific topic. 

Water-Supply Papers are comprehensive reports that present significant intetpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water. 

Citculars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

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

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

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs. 

Open-Flle Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

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

cited in other publications as sources of information. 

Maps 

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 71/2- or 15-minutequadrangleformats (scales mainly 1:24,000 or 1 :62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only. 

Geophysical Investigations Maps are on topographic or planimetric 

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

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

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology. 

Miscellaneous Investigations Series Maps are on planimetric or 

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

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

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon. 

Coal Investigations Maps are geologic maps on topographic or 

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

Oil and Gas Investigations Charts show stratigraphic information 

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

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

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

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

environmental studies or wilderness mineral investigations. 

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

of geohydrologic data of both regular and irregular areas; principal scale 

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

Catalogs 

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

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

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

CO 80225. (See latest Price and Availability List) 

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

set of microfiche. 

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

set of microfiche. 

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

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

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

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

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

over the counter in paperback book form. 

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

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

and over the counter in paperback booklet form only. 

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

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

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

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

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

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

National Center, Reston, VA 22092. 

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

announcements, and publications may be incorrect. Therefore, the 

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

and publications. 
fiLE COpy