RC. 807 

Progress on Seismic Zonation in the 

San Francisco Bay Region 

GEOLOGICAL SURVEY CIRCULAR 807 

«

(, 

._ .. / . (, 

• 

Progress on Seismic Zonation 

in the San Francisco Bay Region 

E. E. Brabb, Editor 

GEOLOGICAL SURVEY CIR-CULAR 807 

1979 
United States Department of the Interior 

CECIL D. ANDRUS, Secretary 

Geological Survey 

H. William Menard, Director 

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202 
CONTENTS, ILLUSTRATIONS, AND TABLES 

Page 

Foreword------------------------------------------------------------ v 

Introduction and summary, by E. E. Brabb and 

R. D. Borcherdt---------------------------------------------------

Neotectonic framework of central coastal California and 

its implications to microzonation of the San Francisco 

Bay region, by D. G. Herd----------------------------------------- 3 

Figure 1. Map showing active faults--------------------------- 4 

2. Map showing fault creep----------------------------- 6 

3. Map showing plate tectonics------------------------- 7 

4. Diagram of fault recurrence intervals--------------- 9 

Table 1. Historic fault displacements------------------------ 5 

2. Probable magnitudes of earthquakes------------------ 10 

Progress on ground motion predictions for the San Francisco Bay 

region, by R. D. Borcherdt, J. F. Gibbs and~- E. Fumal----------- 13 

Figure 1. Map showing maximum earthquake intensities---------- 15 

2. Generalized geologic map---------------------------- 17 

3. Diagram showing a typical seismic and 

geologic log-------------------------------------- 18 

4. Diagram showing intensity in relation to shear 

wave velocity------------------------------------- 19 

5. Diagram showing ground motion amplification in 

relation to shear wave velocity------------------- 19 

6. Diagram showing shear wave velocities for 

unconsolidated sedimentary deposits--------------- 21 

1. Diagram showing shear wave velocities for 

bedrock materials--------------------------------- 22 

Table 1. Summary of shear wave velocities for 

geologic materials-------------------------------- 23 

A methodology for predicting ground motion at specific sites, 

by R. J. Archuleta, W. B. Joyner, and D. M. Boore----------------- 26 

Figure 1. Diagram of fault rupture surface and 

stress distribution------------------------------- 28 

2. Diagram showing ground motion for the 

1966 Parkfield earthquake------------------------- 29 

3. Diagram showing particle velocities 

on a fault surface-------------------------------- 31 

4. Diagram showing particle velocities on a 

free surface-------------------------------------- 33 

5. Diagram showing acceleration on a free 

surface------------------------------------------- 34 

Liquefaction potential map of San Fernando Valley, California, by 

T. L. Youd, J. C. Tinsley, D. M. Perkins, E. J. King, 

and R. F. Preston------------------------------------------------- 37 

Figure 1. Geologic map---------------------------------------- 39 

2. Map showing depth to ground water------------------- 41 

3. Map showing liquefaction susceptibility------------- 44 

4. Enlarged segment of liquefaction susceptibility map-------------------------------------- 45 

5. Map showing seismic source zones-------------------- 46 

6. Map showing return period for liquefaction 

opportunity--------------------------------------- 48 

1. Map showing liquefaction probability---------------- 48 
Table 1. Liquefaction susceptibility of geologic 

materials----------------------------------------- 42 

2. Criteria for compiling liquefaction 

susceptibility map-------------------------------- 43 

3. Seismic parameters for earthquake 

source zones-------------------------------------- 46 

Preliminary assessment of seismically induced landslide susceptibility, by D. K. Keefer, G. F. Weiczorek, E. L. Harp, and 

D. H. Tuel-------------------------------------------------------- 49 

Figure 1. Map showing earthquake-induced landslide 

susceptibility------------------------------------ 56 

Table 1. Landslides triggered by earthquakes----------------- 51 

2. Relative abundance of different types 

of landslides------------------------------------- 52 

3. Assessment of seismically induced landslide 

susceptibility------------------------------------ 57 

Earthquake losses to buildings in the San Francisco Bay area, 

by S. T. Algermissen and K. V. Steinbrugge------------------------ 61 

Figure 1. Index map------------------------------------------- 70 

2. Diagram relating intensity to building loss--------- 71 

3. Diagram showing how isoseismal maps are 

constructed--------------------------------------- 71 

Table 1. Building classification----------------------------- 63 

2. Parameters used in making isoseismal maps----------- 67 

3. Building losses based on historic seismicity-------- 69 

4. Building losses for two assumed earthquakes--------- 70 

Examples of seismic zonation in the San Francisco Bay region 

by W. J. Kockelman and E. E. Brabb-------------------------------- 73 

Figure 1. Map showing earthquake investigation 

zones for City of Mountain View------------------- 74 

2. Map showing expected building damage from 

severe earthquake in San Francisco area----------- 76 

3. Map of earthquake risk zones in the Novato area----- 78 

4. Part of seismic stability map for Santa 

Clara County-------------------------------------- 79 

5. Part of geotechnical hazard synthesis map for 

San Mateo County---------------------------------- 81 

6. Hypothetical map of seismic and geologic 

constraints in San Mateo County------------------- 82 

The use of earthquake and related information in regional 

planning--what we've done and where we're going, by 

J. B. Perkins----------------------------------------------------- 85 

Figure 1. Map showing maximum earthquake intensity------------ 87 

2. Map showing economic risk for earthquake 

damage-------------------------------------------- 88 

3. Map showing land capability for residential 

use in Santa Clara County------------------------- 89 
FOREWORD 

EARTHQUAKE HAZARD REDUCTION PROGRAM 

This report represents another step in the_ evolution of methods for 

reducing the hazards of earthquakes. The great Alaska earthquake of 1964 

triggered an awareness among public officials of the seriousness of the 

earthquake hazard to many of the Nation's major cities. If the effects of 

the Alaska earthquake are used as a gage, it is clear that when a major 

earthquake hits California cities such as Los Angeles or San Francisco, 

casualties could be in the tens of thousands and damage could be in the tens 

of billions of dollars. 

After the Alaska earthquake, the U.S. Geological Survey began to focus 

its diverse earth-science capabilities more specifically toward the goal of 

reducing earthquake hazards. The possible effectiveness of land-use planning to avoid the most serious hazards began to be recognized as a 

supplement to the common practice of incorporating earthquake-resistant 

designs into structures. For decades geologists had known, for example,-

that structures built astride the San Andreas fault were in jeopardy, but 

only in a few places had the fault been delineaced in sufficient detail to 

serve as a guide to community officials and developers. Even if the fault 

data had been available, standard procedu~es were inadequate for 

translating the data into land-use plans or actions. Indeed, land-use 

planning was, and still is, in an early phase of evolution in the United 

States. No national land-use policy has been adopted. 

In order to satisfy some of the most urgent needs for basic data, 

several projects were started after the Alaska earthquake. The entit"e 

1,400-km (868-mi) length of the San Andreas fault was mapped for the first 

time on the best available topographic base maps. Nets of closely spaced 

seismic instruments were installed at experimental field laboratories along 

part of the San Andreas fault to study the basic mechanisms of earthquakes 

and the patterns of energy radiation and attenuation as earthquake wave~ 

pass through different types of rocks and soil. New laboratory studies were 

initiated to explore the physical principles of earthquakes. Research 

demonstrated the feasibility of earthquake prediction and of earthquake 

control and modification. 

The science of earthquakes is complex, requi~ing data and research in 

seismology, geology, soil mechanics, geophysics, hydrology, and engineering. Nevertheless, if earthquake hazards are to be reduced, earth-scien~e 

data must be translated from scientific and technical language into a form 

that can be used effectively in the decisionmaking process. 

THE SAN FRANCISCO BAY REGION ENVIRONMENT AND RESOURCES PLANNING STUDY 

Out of this recognition of the need to use earth-science information in 

regional planning and decisionmaking came an experimental program--the San 

Francisco Bay Region Environment and Resources Planning Study. The study, 

begun in 1970 and completed in 1975, was jointly supported by the U.S. 

Geological Survey, Department of the Interior, and the Office of Policy 

Development and Research, Department of Housing and Urban Development. The 

Association of Bay Area Governments participated in the study and provided a 

liaison and communication link with other regional planning agencies and 

with county and local governments. 
Although the study focused on the nine-county 19,000 km2 San Francisco 

Bay region, it related to a difficult issue that is of national concern--how 

best to accommodate orderly development and growth while conserving our 

natural resource base, insuring public health and safety, and minimizing 

degradation of our natural and manmade environment. The issue, however, can 

only be approached if we understand the natural characteristics of the land, 

the processes that shape it, its resource potential, and its natural 

hazards. These subjects are chiefly within the domain of the earth 

sciences: geology, geophysics, hydrology, and the soil sciences. Appropriate earth-science information, if available, can be rationally applied in 

guiding growth and development, but the existence of the information does 

not assure its effective use in the day-to-day decisions that shape development. Planners, elected officials, and the public rarely have the training 

or experience needed to recognize the significance of basic earth-science 

information, and many of the conventional methods of communicating earthscience information are ill suited to the needs of that particular group of 

users. 

The study was intended to aid the planning and decisionmaking community 

by (1) identifying important problems that are rooted in the earth sciences 

and related to growth and development in the bay region, (2) providing the 

earth-science information that is needed to solve these problems, ( 3) 

interpreting and publishing findings in forms understandable to and usable 

by nonscientists, (4) establishing new avenues of communication between 

scientists and users, and (5) exploring alternate ways of applying earthscience information in planning and decisionmaking. 

The study produced more than 100 reports and maps. These cover a wide 

range of topics: reduction of flood and earthquake hazards, unstable 

slopes, engineering characteristics of hillside and lowland areas, mineral 

and water resources management, solid and liquid waste disposal, erosion and 

sedimentation problems, bay water circulation patterns, and others. The 

methods used· in the study and the results it has produced have elicited 

broad interest and a wide range of applications from planners, government 

officials, industry, universities, and the general public. 

SEISMIC ZONATION 

The-skills and knowledge developed during the Earthquake Hazard Reduction Program and the San Francisco Bay Region Environment and Resources 

Planning Study were focused in 1972 on an analysis of the state-of-the-art 

for-assessing potential earthquake effects on a regional scale for purposes 

of seismic zonation. The analysis was done by a group of 16 earth 

scientists representing several different disciplines and organizational 

units within the U.S. Geological Survey. The preliminary results of these 

studies were presented at the First International Conference on Microzonation, Seattle, Washington, in November, 1972, and were published in 1975 as 

U.S. Geological Survey Professional Paper 941-A. This report provided the 

earth-science basis for a comprehensive approach to reducing earthquake 

hazards in large areas such as the San Francisco Bay region. The report 

included an analysis of the direct effects of earthquakes, such as fault 

displacement and ground shaking as well as indirect effects of landsliding 

and liquefaction. To illustrate the analysis, an earthquake of magnitude 

6. 5 was assumed for the San Andreas fault, and the various effects were 

predicted on a profile through the southern San Francisco Bay region. 
Since 1975, the composition of the core group within the U.S. Geological Survey concerned with seismic zonation has evolved to include seismologists and engineers concerned with probabilistic approaches to earthquake 

damage, a planner to facilitate the information transfer ~ram scientists and 

engineers to city, county, and regional planning staffs charged with the 

responsibility for implementing earthquake hazard reduction measures, and 

representatives from the Association of Bay Area Governments, who are 

working on economic risk, transportation, and land capability maps for the 

San Francisco Bay region. The progress of this group was reported at the 

Second International Conference on Microzonation held in San Francisco in 

November, 1978; the papers in this report are taken verbatim from the 

Proceedings of the Conference. 

The entire Proceedings, consisting of 3 volumes and 1132 pages of 

reports, can be purchased for $65 by writing to: Dr. Mehmet A. Sherif, 

Microzonation Conference Chairman, 132 More Hall FX-1 0, University of 

Washington, Seattle, Washington 98195. 

Vll 

PROGRESS ON SEISMIC ZONATION IN THE SAN FRANCISCO BAY REGION 

INTRODUCTION AND SUMMARY 

by 

Earl E. Brabb and Roger D. Borcherdt 

Studies by 16 researchers in various earth-science and engineering 

disciplines were summarized at the First International Conference on Microzonation in 1972. These reports, published in expanded form as U.S. 

Geological Survey Professional Paper 941-A, established that seismic zonation of the San Francisco Bay region was feasible and showeathe necessity 

for a multidisciplinary approach to the problem. The reports emphasized 

methodologies for constructing seismic zonation maps from earth-science 

data that were currently availabl.e on a regional scale. The maps showed 

maximum earthquake intensity, active faults, geologic units, qualitative 

ground response, liquefaction susceptibility, landslide susceptibility, and 

areas of potential tsunami inundation. These maps served to delineate areas 

with potential earthquake problems, identify the problem, and indicate its 

possible severity. 

These basic tools and a number of other products developed as part of a 

cooperative project between the U.S. Geological Survey and the Department of 

Housing and Urban Development have been utilized by most of the 91 cities 

and all of the counties in the San Francisco Bay region. They have also been 

used in the preparation of seismic safety, public safety, conservation, and 

open-space elements of general plans together with ordinance administration 

policy and environmental impact statements. They are the basic building 

blocks for derivative maps prepared by the Association of Bay Area Governments for regional planning, such as the appropriate location for toxic 

waste disposal, industrial development, population concentration, and 

transportation. This wide application of the products has resulted in a 

significantly increased demand for new and improved earth-science data that 

can be used for development of policies for earthquake hazard reduction. 

Since the initial study, a number of new data have been collected and a 

number of new approaches have been developed concerning communication of 

these data to the planning communi ties. The following set of papers 

discusses some of these new results in detail. Some of the highlights are 

as follows: 

Discovery of three new potentially active fault systems; 

Mapping of all faults with Quaternary displacement in northern San 

Francisco Bay region at scales of 1:125,000 and 1:24,000; 

1 
Seismic and geologic logging of 59 drill holes in southern San Francisco Bay region to develop a data base for improved regional ground 

motion predictions; 

Development of new Methods utilizing synthetic seismograms to improve 

quantitative ground motion predictions; 

Mapping liquefaction susceptibility of Santa Cruz County at a scale of 

1:62,500 using new techniques; 

Development of new techniques for mapping regional slope stability 

during earthquakes; and 

Development of improved meth~ds for estimating earthquake-induced 

damage to buildings on a regional scale. 

The need for new and improved data in all metropolitan areas of high 

seismic risk is becoming increasingly apparent. The recent Earthquake 

Hazards Reduction Act of 1977 (P.L. 95-124) calls for the identification, 

evaluation and characterization of .seismic hazards in all areas of high or 

moderate risk and the development of means to coordinate information about 

seismic risk with land-use policy decisions. Some of the methods and 

policies developed in the San Francisco Bay region will be directly applicable to other areas of the United States; however, where earthquake 

problems and basic data sets are different, additional methods and policies 

will be needed. 

2 
NEOTECTONIC FRAMEWORK OF CENTRAL COASTAL CALIFORNIA 

AND ITS IMPLICATIONS TO MICROZONATION 

OF THE SAN FRANCISCO BAY REGION 

by 

Darrell G. Herd 

ABSTRACT 

Microzonation of the San Francisco Bay region must consider future 

earthquakes on several major northwest-trending faults. Principal among 

these, the San Andreas fault zone extends through the central Coast Ranges 

to San Francisco, and then north along the Pacific coastline. Paralleling 

it offshore to the west is the San Gregorio-Hosgri fault system, which 

joins with the San Andreas near San Francisco. At Hollister, the HaywardLake Mountain fault system branches eastward from the San Andreas, extending north beyond Eureka. The Calaveras-Sunol, Concord, and Green Valley 

faults form a line that splays from the Hayward-Lake Mountain fault system 

near San Jose. East of San Francisco, the San Joaquin fault zone bounds 

the east flank of the Coast Ranges. 

Large earthquakes (M>7) are credible on several fault zones in the San 

Francisco Bay area and have a basic recurrence of tens to hundreds of years 

on a few. 

INTRODUCTION 

Microzonation of the San Francisco Bay region for seismic shaking must 

consider future earthquakes that likely will occur along several predominantly northwest-trending faults. The San Francisco Bay region lies 

astride the San Andreas fault zone at its intersection with two other major 

fault systems. These faults, which constitute the neotectonic framework of 

central coastal California, have been repeatedly active throughout the 

Quaternary Period (last 1.8 m.y.). 

Most historic California earthquakes have originated along such recently active faults (1, 2). An assessment of the length, character, and 

rate of displacement along them, with reference to historic worldwide 

seismicity, can be used to estimate the magnitude and frequency of large 

earthquakes expectable in the San Francisco Bay area. 

NEOTECTONIC FRAMEWORK 

Recently active faults. The San Francisco Bay region (fig. 1) is cut 

by several major northwest-trending right-slip faults. These faults, which 

have been active in both historic (Table 1) and geologically recent time, 

sliver coastal California into narrow crustal blocks. Principal among 

these faults, the San Andreas fault zone extends from southern California 

through the central Coast Ranges of San Francisco. From there north, the 

San Andreas skirts the Pacific coastline to near Cape Mendocino (see 

fig. 3), where the fault zone intersects the Mendocino fracture zone. The 

San Andreas is paralleled offshore to the west by the San Gregorio-Hosgri 

fault system (3), which begins west of Point Conception and joins the San 

3 
~ 

~ 

'\: SANTA 

4t_, ~-z.. ROSA 

""· 0 

~ 

%~~~ 

POINT 

REYES 

BOLINAS 

SAN 

122o 0 10 20 30miles 

~~-. lS ....... -----tl 

0 ~ 20 30km 

SACRAMENTO 

MONTEREY 

BAY 

·. 

' 

SAN 

JOAQUIN 

F.Z. 

J Figure 1.1 

Figure I.--Principal recently active faults in San Francisco Bay region, 

showing zones of surface rupture associated with historic earthquakes 

(Table 1). Squares denote locally determined rates of geologic offset. 

4 
Andreas just west of San Francisco. Approximately 120 km southeast of San 

Francisco a second system of faults, the Hayward-Lake Mountain (4), 

branches eastward from the San Andreas fault zone at Hollister. This line 

of large en echelon, recently active right-slip fault zones parallels the 

San Andreas northward, passing east of Cape Mendocino. The fault system 

extends beyond Eureka onto the continental shelf southwest of Crescent 

City. A much shorter line of three faults, the Calaveras-Sunol, Concord, 

and Green Valley, splays from the Hayward-Lake Mountain fault system near 

San Jose. To the south, the Sargent fault zone joins the CalaverasPaicines fault zone and the San Andreas (5). 

Southwest of San Jose, an imbricate system of thrust faults (collectively referred to here as the Berrocal fault zone) abuts the northeast 

side of the San Andreas fault zone (5). East of San Francisco, the San 

Joaquin fault zone bounds the east flank of the Coast Ranges. The zone is 

predominantly normal in character (east side down), with local reverse 

faults. 

Faulting and plate tectonics. The northward movement of the Pacific 

plate relative to North America (fig. 3) is manifested in coastal 

California as slip along the San Andreas fault zone and the subsidiary 

faults. The relative rate of movement across the plate boundary is not 

uniform and is difficult to measure because the area is slivered by intersecting and branching fault systems. One of the crustal slivers, the 

Humboldt plate (4), moves independently of the Pacific and North American 

plates. This small plate, bounded on the east by the Hayward-Lake Mountain 

fault system and on the west by the San Andreas, converges northwestward 

with the Gorda plate. Near San Jose, the Humboldt plate locally overrides 

and is crushed by the Pacific plate (along the Berrocal thrust faults). 

The crustal extension at the east side of the Coast Ranges (evidenced by 

normal faulting along the San Joaquin fault zone) may be due to the northwestward movement of the Humboldt plate away from the North American plate. 

Fault slip. Movement on the recently active faults in the San 

Francisco Bay region occurs catastrophically in seismic slip events (as 

much as 5 m of right-lateral displacement was measured (2) across the San 

Andreas fault after the 1906 earthquake) as well as gradually by fault 

creep. 

An average of 3.7 cm/yr of long-term slip (determined from the rightlateral offset of a 3,000-year-old stream channel (6) in the Carrizo Plain) 

occurs along the San Andreas fault zone south of Hollister (fig. 1, inset). 

Table 1.--Historic surface fault displacements associated with earthquakes 

in the San Francisco Bay region (2) 

Date Fault Rupture length Magnitude 

Late June, 1838 San Andreas Unknown 

July 3, 1861 Calaveras-Sunol Unknown 

October 22, 1868 Hayward >30 km 7+1/2 (estimated) 

April 24, 1890 San Andreas >10 km? 

April 18, 1906 San Andreas ~430 km 8.3 

Surface faulting has previously been reported (2) for the June 10, 1836 

earthquake on the Hayward fault zone. However, re-examination of the 

original newspaper accounts (referenced in 7) does not support such an 

interpretation. The described earth fissures were probably due to ground 

shaking and slope failure rather than faulting. 

5 
San 

. 

. . 

Figure 2.1 

.. 

Figure 2.--Documented creep on faults in central coastal California. Data 

marked by* are from R. 0. Burford (8), **from Frizzell and Brown (9), 

***from Harsh and others (10). Other data from Wesson and others (2). 

6 
Gorda 

Plate 

~ 

Mendocino 

~ 

Fracture 

Zone 

Pacific 

Plate 

I Figure 

0 50 100 miles 

I I I 

I I 

0 50 100 km 

OREGON 

:r~c:~PinT City 

North American 

Plate 

CALIFORNIA 

\. •• ~ Voley F. Z. 

rs Creek F.Z. 

Figure 3.--Map showing plate tectonics of coastal northern California and 

Oregon (4). Recently active faults of coastal California are represented. 

Large black arrows show movement of Pacific and Gorda plates relative to 

the North American plate deduced from vector diagram in bottom center. 

Motion of the North American plate with respect to the Pacific plate (Ap) 

is assumed to be 5.8 cm/yr parallel to the San Andreas fault zone. MOtion 

of the Gorda plate with respect to the Pacific plate (Jp} is assumed to be 

5.8 cm/yr parallel to the Blanco fracture zone. The resultant motion of 

the Gorda plate with respect to the North American plate (Ja) is a 

compression in a north-northeast direction of 2.5 em/yr. 

7 
Part of the displacement (currently as much as 3.6 cm/yr) occurs locally as 

creep (fig. 2). North of Hollister, near San Francisco (fig. 1), only 

2 cm/yr of long-term slip has been documented along the San Andreas fault 

zone, 0.6-2.2 cm/yr in displaced Pliocene (1.8-5.0 m.y.) rocks (11), and 

1-3 cm/yr in offset deposits 1-3 m.y. old (12). 

Most of the 1.7 cm/yr of slip that is not carried northward along the 

San Andreas beyond Hollister is apparently transferred to the CalaverasPaicines fault zone, which branches from the San Andreas just south of 

Hollister (fig. 1). Although the actual long-term rate of slip along the 

Calaveras-Paicines fault zone is not known (a minimum of 0.14-0.71 cm/yr 

slip has been determined (13) from offset 3.5-m.y.-old volcanic rocks north 

of Hollister, fig. 1), the long-term rate is maybe at least 1.2 cm/yr and 

more probably about 1.5 em/yr. Southeast of San Jose (fig. 2), 1.0-1.2 

. cm/yr of creep has been documented on the Paicines-Calaveras fault zone. 

The rate of creep along the Calaveras-Paicines fault zone, like the San 

Andreas south of Hollister, is presumably equal to or less than the longterm slip rate (the difference being made up in catastrophic seismic slip 

events). 

Slip along the Calaveras-Paicines fault zone is apportioned at San 

Jose between the Hayward-Lake Mountain fault system and the CalaverasSunol--Concord--Green Valley fault system (fig. 1). Although no geologic 

rates of offset have been locally determined along either fault system, 

the measurement of 0.6 cm/yr of creep on both the Hayward and Concord fault 

zones at about the same latitude suggests that the 1.5 cm/yr (?) of slip 

along the Calaveras-Paicines is equally divided between the two. A marked 

diminution in long-term slip rate northward along the respective fault 

systems is suggested by only 0.2 cm/yr of creep along the Maacama fault 

zone at Willits (fig. 2), and 0.03 cm/yr of creep on the Green Valley 

fault zone. 

About 1 cm/yr (0.63-1.3 cm/yr) of movement for the last 200,000 years 

has been measured (14) across the San Gregorio fault zone at Afio Nuevo 

(fig. 1). This amount of slip is added to the San Andreas fault zone west 

of San Francisco, apparently increasing the long-term slip rate along the 

San Andreas north of Bolinas to about 3.0 em/yr. 

"BASIC" EARTHQUAKE RECURRENCE 

Curves of Wallace's (15) "basic" earthquake recurrence can be determined for earthquakes of different magnitude at given points on faults in 

the San Francisco Bay area if (a) the rate of displacement on the fault is 

known; (b) the slip rate is constant; (c) all the long-term offset or slip 

on the fault was the cumulative effect of sudden slips accompanying earthquakes, interspersed with periods of elastic strain build-up; and (d) all 

earthquakes are assumed to be of the same size. These curves (fig. 4) can 

be generated for the Hayward, Calaveras-Sunol, Calaveras-Paicines, and San 

Gregorio fault zones, and parts of the San Andreas (for which average 

geologic slip rates can be approximated) by the formula 

D 

~ =s8 
I Figure 4.1 

1000 

,. 

0 

~ 100 

~ :? : .= 

CD 

u 

c 

~ 10 u 

CD 

a::: 

1.0 

0.5 

3 4 5 8 9 

Earthquake Magnitude 

Figure 4.--Basic recurrence intervals (15) at a given point on fault zones 

in the San Francisco Bay region, assuming average displacement rates 

indicated. The earthquake magnitudes have been calculated from the 

empirical relation of Slemmons (17) M = 6.717 + 1.214 log10 (D), where M 

is magnitude and D is displacement in meters (n = 30, r2 = 0.408, 

s = 0.639). 

where = recurrence interval, at a point on the fault of an 

earthquake of magnitude M, 

D most probable surface displacement associated with 

an earthquake of magnitude M, determined from a 

linear regression of earthquake magnitude on 

surface displacement from data on historical 

magnitude and surface rupture, and 

S long-term strain rate. 

The curves suggest that large earthquakes (M~7) have a "basic" recurrence of tens to hundreds of years on the faults. However, the curves are 

only a first approximation of estimated earthquake recurrence since the energy released during earthquakes of other magnitudes is not deducted. Moreover, the effect of fault creep, which may be considered a noncatastrophic 

mode of fault slip and thus an inhibiting factor in the accumulation of 

elastic strain and the generation of earthquakes, is not considered. 

9 
1 

"MOST PROBABLE" EARTHQUAKE MAGNITUDES 

Estimates of the "most probable" earthquake magnitudes that can be 

expected along faults in the San Francisco Bay area if one-half the total 

fault length ruptured (Table 2) can be used bo calculate ground response 

for microzonation. These estimates, which have commonly been called 

"maximum credible" or "maximum expectable" earthquake magnitudes (16), 

are made from linear regressions of earthquake magnitude on length of 

surface rupture using historical earthquake magnitudes and lengths of 

surface ruptures (17, 18, 19). 

The earthquake magnitudes estimated in Table 2 imply that M>6 earthquakes are credible on most principal fault zones in the San Francisco Bay 

area. The values differ from those previously determined for faults in 

the Bay area (2) because they are based on newly mapped fault lengths, and 

because they are not calculated from regressions of fault length on 

earthquake magnitude (18). 

Table 2.--Most probable magnitudes of earthquakes that would be expected, 

provided that rupture of one-half the total length of faults or fault 

segments in the San Francisco Bay region occurred. 

Fault zone 

San Andreas 

Hollister - Cape Mendocino 

Hollister - Bolinas 

Bolinas - Cape Mendocino 

Paicines-Calaveras 

Hayward 

Rodgers Creek 

Maacama 

Calaveras-Sunol 

Concord 

Green Valley 

San Gregorio 

San Joaquin 

Sargent 

Berro cal 

Character of 

motion 

Right-slip 

do 

do 

do 

do 

do 

do 

do 

do 

do 

do 

Predominantly 

normal 

Right-slip 

Thrsut 

Length 

km 

430 

160 

270 

100 

90 

50 

140 

70 

20 

90 

140 

120 

60 

60 

(L) 

Most probable 

magnitude 

1/2 L 

7.8 

7.2 

7.5 

6.9 

6.9 

6.5 

7.1 

6.7 

6.0 

6.9 

7.1 

7.3 

6.6 

7.4 

The magnitudes are calculated using the empirical relations of Slemmons 

(17): M = 0.597 + 1.351 log10 (L) for strike-slip faults (n = 31, 

r2 = 0.601, s = 0.694); M = 1.845 + 1.151 log10 (L) for normal faults 

(n = 18, r2 = 0.331, s = 0.521); and M = 4.145 + 0.717 log10 (L) for 

reverse faults (n = 9, r2 = 0.869, s = 0.167), where M is magnitude and 

L is fault length in meters. 

10 
REFERENCES CITED 

1. Allen, C. R., St. Amand, P., Richter, C. F., and Nordquist, J. M., 

1965, Relationship between seismicity and geologic structure in 

the southern California region: Bulletin of the Seismological 

Society of America, v. 55, no. 4, p. 753-797. 

2. 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. 

3. Graham, S. A., and Dickinson, W. R., 1978, Evidence for 115 kilometers 

of right slip on the San Gregorio-Hosgri fault trend: Science, 

v. 199, no. 4325, p. 179-181. 

4. Herd, D. G., 1978, An intracontinental plate boundary east of Cape 

Mendocino, California: Geology, in press. 

5. 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: 

Journal of Research of the U.S. Geological Survey, v. 2, 

no. 5, p. 593-598. 

6. Hall, N. T., and Sieh, K. E., 1977, Late Holocene rate of slip on the 

San Andreas fault in the northern Carrizo Plain, San Luis Obispo 

County, California (abst.): Geological Society of America 

Abstracts with Programs, v. 9, no. 4, p. 428-429. 

7. Louderback, G. D., 1947, Central California earthquakes of the 1830's: 

Seismological Society of America Bulletin, v. 37, p. 33-74. 

8. Savage, J. C., and Burford, R. 0., 1973, Geodetic determination of 

relative plate motion in central California: Journal of 

Geophysical Research, v. 78, no. 5, p. 832-845. 

9. Frizzell, V. A., Jr., and Brown, R. D., Jr., 1976, Map showing 

recently active breaks along the Green Valley fault, Napa and 

Solano Counties, California: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-743, scale 1:24,000. 

10. Harsh, P. W., Pampeyan, E. H., and Coakley, J. M., 1978, Slip on the 

Willits fault, California (abst.): Seismological Society of 

America Earthquake Notes, v. 49, no. 1, p. 22. 

11. Addicott, W. A., 1969, Late Pliocene mollusks from San Francisco 

Peninsula, California, and their paleogeographic significance: 

Proceedings of the California Academy of Sciences, Fourth 

Series, v. 37, no. 3, p. 57-93. 

12. Cummings, J. C., 1968, The Santa Clara Formation and possible postPliocene slip on the San Andreas fault in central California, in 

Dickinson, W. R., and Grantz, Arthur, eds., Proceedings of 

conference on geologic problems of San Andreas fault system: 

Stanford University Publications in the Geological Sciences, 

v. 11, p. 191-207. 

11 
13. Nakata, J. K., 1977, Distribution and petrology of the Anderson-Coyote 

Reservoir volcanic rocks: San Jose State University, California, 

Master of Science Thesis, 105 p. 

14. Weber, G. E., and Lajoie, K. R., 1977, Late Pleistocene and Holocene 

tectonics of the San Gregorio fault zone between Moss Beach and 

Point Afto Nuevo, San Mateo County, California (abst.): 

Geological Society of America Abstracts with Programs, v. 9, 

no. 4, p. 524. 

15. Wallace, R. E., 1970, Earthquake recurrence intervals on the San 

Andreas fault: Geological Society of America Bulletin, v. 81, 

p. 2875-2890. 

16. Wesson, R. L., Page, R. A., Boore, D. M., and Yerkes, R. F., 1974, 

Expectable earthquakes in the Van Norman Reservoirs area: U.S. 

Geological Survey Circular 691-B, 9 p. 

17. Slemmons, D. B., 1977, State-of-the-art for assessing earthquake 

hazards in the United States: U.S. Army Engineer Waterways 

Experiment Station Miscellaneous Paper S-73-1, Report 6, var. 

pag. 

18. Mark, R. K., 1977, Application of linear statistical models of earthquake magnitude versus fault length in estimating maximum 

expectable earthquakes: Geology, v. 5, p. 464-466. 

19. Mark, R. K., and Bonilla, M. G., 1977, Regression analysis of earthquake magnitude and surface fault length using the 1970 data of 

Bonilla and Buchanan: U.S. Geological Survey Open-file Report 

77-614, 8 p. 

12 
PROGRESS ON GROUND MOTION PREDICTIONS 

FOR THE SAN FRANCISCO BAY REGION, CALIFORNIA 

by 

Roger D. Borcherdt , James F. Gibbs , and Thomas E. Fumal 

ABSTRACT 

The amount of damage in the San Francisco Bay region from the 1906 

earthquake depended strongly on the geologic character of the ground. 

This dependence indicates the need for seismic zonation maps of the region 

to outline areas where special earthquake resistant design is necessary 

to reduce losses from futur~ earthquakes. 

Current research is directed at defining methodologies for improved 

quantitative estimates of ground response on a regional scale. This 

research includes determination of seismic and geologic logs in 59 drill 

holes to a depth of 30 meters. 

Relations derived between site amplifications (Amp), 1906 earthquake 

intensity increments (oi), and shear-wave velocity are, respectively, 

Amp = -11.4 log (S-vel, m/s) + 33.6 

and 

oi = -0.0027 (S-vel, m/s) + 2.25 

Geotechnical parameters such as texture, standard penetration, and depth, 

for sediments, and fracture spacing and hardness, for rocks, show strong 

correlations with seismic velocities and provide a useful means of 

defining 13 units with distinct seismic characteristics. Utilizing the 

preceding empirical relations, quantitative estimates of ground response 

at 59 sites, recently developed numerical models, and the classification 

of seismically distinct units on the basis of geotechnical parameters, 

improved quantitative estimates of variations in ground shaking can be 

provided on a regional scale for seismic zonation of the San Francisco Bay 

region. In addition, the seismic velocity relations permit extrapolation 

of these data to other regions. 

Introduction 

The most widespread earthquake damage is generally due to ground 

shaking and is strongly dependent on the geologic character of the 

ground. After the 1906 earthquake, Lawson (12) reported evidence for 

increased damage due to geologic conditions in 18 California communities. 

This strong dependence of damage on the geological character of the ground 

defined a strong need for predictions of regional ground motion that can 

13 
be used for economical earthquake-resistant design. This paper describes 

a new data base for developing a methodology to prepare predictions of 

regional ground motion that account for variations in geologic conditions. 

The problem of predicting regional ground motion is quite distinct 

from that of predicting for specific sites. For specific sites (for 

example, siting of a nuclear power plant or a high-rise structure), 

detailed geologic and seismic data are available. As a result, recently 

developed numerical modeling procedures can be used to predict reasonably 

detailed time histories of ground motion. However, such detailed seismic 

and geologic information is not available on a regional basis, and 

predictions must necessarily be more generalized. 

Previous Work on Regional Problem 

At the time of the First International Conference on Microzonation, 

Borcherdt, et al., (5) reported on data available for regional ground 

motion predictions in the San Francisco Bay region. These data included 

observed 1906 earthquake intensities, recordings of the 1957 earthquake, 

comparative measurements at 99 sites of ground shaking generated by 

nuclear explosions, and high-strain laboratory measurements of dynamic 

soil properties. 

Comparative measurements of ground shaking generated by the nuclear 

explosions and the 1957 earthquake showed that a significant and 

consistent difference in the response to shaking exists between different 

geologic units in the San Francisco Bay region (Borcherdt, 1). Comparison 

of the measured amplifications with the high quality 1906 intensity data 

showed that an increase in amplification corresponds to an increase in 

intensity. This correlation suggested that sites at equal distance from 

the fault with large observed amplifications may also be sites of 

relatively high intensity in future earthquakes. These data together with 

available geologic information were used to predict the maximum intensity 

that sites in the San Francisco Bay region might sustain from large 

earthquakes on either the San Andreas fault or the Hayward fault 

(Borcherdt, Gibbs, and Lajoie, 4). (See Fig. 1 for map). 

The intensity map delineates general areas susceptible to problems 

from earthquakes in the San Francisco Bay region, and, when properly 

interpreted, it provides a preliminary form of seismic zonation. The map 

has been used in the required Seismic Safety Elements of several bay 

region communities and for development of general land-use policies 

designed. to reduce earthquake losses. The map does not provide 

quantitative estimates of ground shaking nor does it predict the nature 

and areal extent of such problems as surface faulting or liquefaction. It 

does delineate many potentially hazardous areas and provides a qualitative 

estimate of the overall hazard from shaking on a regional scale. In 

addition to their use for the maximum predicted intensity maps, the data 

available in the San Francisco Bay region were considered adequate to 

prepare a map showing that the expected effects of amplified ground 

shaking would be least on bedrock, intermediate on alluvium, and greatest 

on bay mud (Borch€rdt et al., 2). However, the data were not considered 

adequate to prepare more quantitative maps depicting such parameters as 

peak acceleration, velocity and displacement. Such predictions require 

not only detailed models of the earthquake source and the seismic wave 

transmission path, but also detailed knowledge of the geometry and 

configuration of near-surface geologic deposits. 

14 
37°45' 

0 37'30" 

0 

EXPLANATION 

PREDICTED MAXIMUM 

EARTHQUAKE INTENSITY 

(1906 SAN FRANCISCO 

SCALE) 

Very violent 

Violent 

Very strong 

Strong 

E~ 

Weak 

San Andreas fault 

R 

Reservoir 

N 

3 KILOMETRES 

122°22'30" 

3 MILES 

Figure 1.-Maximum earthquake intensities predicted for San Francisco (map 

is excerpt. from Borcherdt, Gibbs, and Lajoie, 4). Each value is maximum of those predicted assuming a large earthquake on San Andreas or 

Hayward fault. Intensity values are predicted from empirical relations 

based on only good intensity data for the 1906 earthquake together with 

a generalized geologic map compiled by K. R. Lajoie (written comm., 1974). 

Letters A-E indicate grades of San Francisco intensity scale. 

15 
New Data Base for Regional Problem 

To develop an improved data base for more quantitative predictions of 

ground motion on a regional scale, a program was undertaken by the U.S. 

Geological Survey to determine detailed seismic and geologic logs for a 

large· number of sites in all major geologic units in the San Francisco Bay 

region. To date, seismic velocity logs of P- and S- waves, together with 

geologic logs, have been determined for 59 sites in drill holes to a depth 

of 30 meters (Fig. 2) (Gibbs et al., 7, 8, 9, 10). Seismic velocities 

(Fig. 3) were measured at 2.5=iiieter intervals using an in-hole technique 

developed by Kobayashi (11) and Warrick (13). (See Gibbs et al., 7, for 

detailed description of technique). Interpretive geologic-rags were 

compiled for each hole using field data, including descriptions of three 

to six samples taken at lithologic contacts and at points where changes 1n 

physical properties were indicated (see Fumal, 6, for details). Drill 

hole sites were selected on the basis of available high-quality 1906 

intensity data, measured ground response from nuclear explosions, and 

detailed geologic mapping. 

Seismic Velocity vs. Intensity Increments 

To compare seismic velocities with the 1906 intensity data, the effect 

of distance on the observed 1906 intensities was removed by computing 

increments in observed intensity with respect to a mean attenuation curve 

for intensities observed on the Franciscan Formation (Borcherdt and Gibbs, 

3). Those intensity increments, based on the better 1906 data and 

collected at sites for which no ground failure was observed, are plotted 

as a function of average shear wave velocity to the bottom of the hole 

determined at the corresponding site (Fig. 4). The plot shows 

considerable scatter in the data, but a decrease in seismic shear wave 

velocity clearly corresponding to a decrease in observed intensity 

increment. The relations without regard to geologic setting suggests that 

sites equidistant from the fault with average shear wave velocities of 

about 250 m/s could expect to experience an intensity of approximately 2 

units higher than sites with velocities near 1000 m/s. In addition, the 

relations helps to establish that seismic velocity may be a significant 

parameter for evaluating seismic hazards. 

Seismic Velocity vs. Measured Ground Response 

To compare seismic velocities with ground response determined from 

nuclear explosions, the average of the horizontal spectral amplification 

curves (Borcherdt and Gibbs, 3) were plotted as a function of average 

shear wave velocity to the bottom of the hole (Fig. 5). The data show a 

strong correlation between shear wave velocity and measured 

amplification. In particular, at sites with average shear velocities of 

250 m/s, low-strain ground motions over the frequency band 0.5 to 2.5 Hz 

are likely to be about seven times greater than those at sites with 

velocities near 900 m/s. 

The data from drill holes more than 100 m from the site of the 

measured amplification show considerably more scatter, and this scatter 

suggests that amplification effects are very localized and that care is 

required in extrapolating site-specific measurements to a regional scale. 

The data suggest that the seismic shear wave velocity of the upper 30 

16 
37° 

45' 

37° 

22' 

30'' 

.......................... 

• • • • • • • • • • • • • • • • • • • 0 •••••• 

. . . . . . . . . . . . . . . . _ ..... . 

EXPLANATION 

Bay 

-

mud 

Al1uvium 

Bedrock 

• 

Recording I ocation 

Location of seismic 

and geologic I ogs 

121"52'::.>" 

............ 

···································~· 

10 MILES 

10 KILOMETRES 

Figure 2.-Distribution of generalized geologic units, locations where 

seismic and geologic logs have been compiled in drill holes to 30 m 

depth, and locations where ground response has been measured from a 

nuclear source. 

17 
o First P arrival 

c First S arrival 

/1 First S peak SILTY CLAY, dk grey 

soft 

CLAY, yellowish brown, 

hard 

Sl L TY CLAY, grey with 

olive mottles 

v. stiff 

Figure 3.-Example of traveltime curves for P and S waves and simplified 

geologic log. Two picks are shown for the S-wave group--first S 

arrival and first S peak. 

18 
3 

--•2 ~c; 

z~ liJ 

2: 0 liJ u a::.!! 

0 g z 0 -.t 

c >- 0 ~C/) 

u; U) 0 

zo 

~~ 

z-

• 0 

• 

0 

0 

Intensity data from 1 

• S. F . 

o S . F. Peninsula 

I 81 = -0.0027 (S-VEL l + 2.25 

• 

• 0 0 • 

0 

200 400 600 800 1000 1200 

AVERAGE SHEAR WAVE VELOCITY (MIS) 

Figure 4.-Intensity increment (ol) determined from data of 1906 San Francisco Earthquake vs. shear wave velocity averaged from surface to approximately 30 m depth. Dots are observed data using San Francisco intensity scale and circles are data using Rossi-Forel scale. Intensity 

increments are expressed in terms of the San Francisco intensity scale 

converting the letters A-E to 4-0, respectively. Observed data expressed in Rossi-Forel scale were converted to San Francisco scale using X-..A, 

rx~B, vrrr-rx~c, vrr-vrrr~D, and vr-vrr~E. 

200 

AVERAGE 

• Amplification measured within 1 

100 m. of drill site • 

500 m. of drill site o 

/Amp.• -11.4 log ( S -VEL l + 33.6 

0 

• 

400 600 800 1000 1400 

SHEAR WAVE VELOCITY ( M /S) 

Figure 5.-Amplification (Amp.) as determined from recordings of ground 

motion generated by nuclear explosions vs. shear wave velocity averaged 

from surface to approximately 30 m depth. Dots represent ground motion 

amplification measured within 100 m; and circles, within 500 m of drill 

site. 

19 
meters of a surficial deposit plays a significant role in changing the 

anticipated characteristics of seismic waves in certain frequency bands. 

Geologic and Physical Properties of Seismically Distinct Units 

Preparation of regional ground response maps requires utilization of 

data available on a regional scale. In most urban areas this data base is 

limited to standard geologic mapping. Most such maps, however, are 

compiled for purposes of inferring geologic history and are not 

immediately applicable to preparing special-purpose interpretive maps. 

Mapped units commonly contain materials with a wide range of physical 

properties and seismic characteristics. 

In order to adapt the geologic data base for purposes of regional 

seismic zonation, a detailed study was undertaken to investigate 

correlations between geologic and physical descriptions of the various 

units and seismic velocities (Fumal, 6). The study identified a suite of 

physical properties of geologic materials that can be used to identify 

seismically distinct units and can.be readily determined in the field and 

thus incorporated in geologic mapping schemes. 

Seismic wave velocities were measured in each of the geologic map 

units in the San Francisco Bay region. The range of seismic velocities 

for a given unit is dependent on the variety of materials included in the 

unit, which is largely a function of the age of the deposit. Each of the 

Holocene map units shows a distinct and relatively narrow range of shear 

wave velocity. Older sedimentary deposits and bedrock materials show 

relatively wide and overlapping velocity ranges. For these materials, age 

has been an important fac.tor ·in defining gBQlogic units. Differences in 

age, however, frequently do not correlate with significant variation in 

the physical properties that affect seismic velocities. 

For the unconsolidated to semiconsolidated sedimentary units, texture 

or relative grain size distribution was found to have the most significant 

.effect on seismic shear wave velocity. On the basis of texture alone, the 

unconsolidated sedimentary deposits in the San Francisco Bay region can be 

divided into four categories: (1) clay and silty clay, (2) sandy clay and 

silt loam, (3) sand, (4) and gravel. Utilizing sta~dard penetration 

resistance measurements (SPR), the clay and silty clay unit and the sand 

unit each can be subdivided into two additional units. Each unit 

identifj.ed according to physical properties is clearly identifiable 

seismically (lower- abscissa Fig. 6, Table 1) with.the exception of the 

sand unit, which was classified separately because it is easily 

distinguishable in the field and because its compaction varies over a 

broad range.. Each of the units identified according to texture and SPR is 

also easily c~ecognized in.the field and should be readily differentiated 

in areas that have existing geologic maps. 

For the bedrock materials in the San Francisco Bay region, fracture 

spacing was found to have the most significant effect on seismic shear 

wave·velocity for various rock types. Hardness has the second largest 

effect and lithology can be used to distinguish between hard sedimentary 

and igneous rocks like the sedimentary.ttnits, each bedrock unit identified 

according to physical properties is seismically distinct (lower abscissa, 

Fig. 7; Table 1). The seismically distinct bedrock units can not be so 

easily mapped as the unconsolidated sediments because fracture spacing 

20 
.:-

SHEAR WAVE VELOCITY 

0 100 200 300 400 

5 I 

10 

-E 

-

:I: 

(m/sec) 

500 

~ 

600 700 

II 

SEISMICALLY DISTINCT UNITS 

CLAY AND SILTY CLAY 

v. soft to soft CN S 4) 

1- 15 

a.. I lu 

l&J 

0 

20 

~~:I r IL: 

25 I 

30• I I I I I j I I ' "I I I j I I J"l I ~o] I I I LJ I 0 I I I Ill j j 

I 

• ~ 

II 

IV ....___..... I 

lim 

CLAY AND SILTY CLAY 

medium to hard (N >4) 

SANDY CLAY SILT LOAM 

INTERBEDDED COARSE 

AND FINE SEDIMENT 

till IV SAND 

loose to dense CN S 40) 

~ V ·sAND 

dense to v. dense ( N > 40) 

0 VI GRAVEL 

Figure 6.-Shear wave velocity for various depth intervals determined in drill holes for unconsolid~ted to 

semiconsolidated sedimentary deposits. Seismically distinct units are classified according to the physical 

properties of texture or relative grain size and standard penetration resistance. Means and standard deviations 

for each unit are shown along lower abscissa by dots and error bars, respectively. 
SHEAR WAVE VELOCITY (m/sec) 

300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 

0-1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 

5 

10 

e 

-

~ 

"'~ 15 ; 0.. 

UJ 

0 

20 

25~ 

I 

1~11111 Ill ~~ m ll tl ~~j ~l 

SEISMICALLY DISTINCT UNITS 

FRACTURE 

ROCK TYPE HARDNESS SPACING 

I• SANDSTONE FIRM TO 

SOFT 

I . IGNEOUS HARD TO 

SEDIMENTARY SOFT 

I Rl 18NEOUS HARD TO 

SANDSTONE FIRM 

SHALE 

~ IY IGNEOUS HARD TO 

SANDSTONE FIRM 

m V SANDSTONE FIRM TO 

CONGLOMERATE HARD 

MODERATE 

AND WIDER 

CLOSE TO 

VERY CLOSE 

CLOSE 

CLOSE TO 

MODERATE 

MODERATE 

AND WIDER 

~ VI SANDSTONE HARD TO MODERATE II II QUITE FIRM AND WIDER 

0 VII IGNEOUS HARD CLOSE TO 

MODERATE 

30 ..1 I I I I ' I I; I I I jl I l' I I I ' ~j I ~l:i j ~'!] to{ I I I I I I I I I I u I 

I II • IV - .. -- _., . - . ~ 

v Vf . - . VII ~ 

Figure 7.-Shear wave velocity for various depth intervals determined in drill holes for bedrock materials. 

Seismically distinct units are classified according to the physical properties of fracture spacing, hardness, 

and lithology. Means and standard deviations for each unit are shown along lower abscissa by dots and error 

bars, respectively. 
TABLE I 

SHEAR WAVE VELOCITIES IN GEOLOGIC MATERIALS 

IHstilict Shear Wave Intenilfy- Distinct Physical Properties Shear Wave Intenaity Seiaaic Physical Vel. (a/a) Std. Amp. Increment Seisaic Rock Fracture Vel. (a/a) Std. A.p. lncr.ent 

Units Properties Mean Dev. (Pred.) (Pred.) Units !z2e Hardnel8 seaci!!l Mean Dev. (Pred.) (Pred.) 

Sedimenta~ Deeosita ledrock Materials 

I Clay-Silty Clay 88 22 11.43 2.01 I Sand- lira to Moderate 470 48 3.14 0.98 

very soft-soft atone Soft and vicler 

(H S 4) 

II Igneous Bard to Close to 517 51 2.12 .69 

II Clay-Silty Clay 186 22 7.73 1. 75 Sedi- Soft very close 

aediua-bard (H ~ 4) aentary 

Ill Sandy Clay-Silt 265 32 5.97 1.53 III Igneous Hard to Close 751 46 .82 .22 ~ Loaa Interbedded Sanda- rira (J:) Coarse and Fine atone 

Sediment Shale 

IV Sand 206 36 7.22 1.69 IV Igneous Hard to Close to 923 48 -.20 -.24 

loose to dense Sandstone lira Moderate 

(H S 40) 

v Sand 366 84 4.38 1.26 v Sandstone lira to Moderate 1073 31 -.95 -.65 

dense to very dense Congloa- Barel and wider 

(H ~ 40) erate 

Vl Gravel 504 138 2.79 .89 VI Sandstone Bard to Moderate 1257 42 

quite and wider 

fira 

VII Igneous Hard Close to 1660 20 
and hardness vary widely within many map units. For purposes of mapping 

regional ground response, however, subdivision of the bedrock materials is 

not so important, and fewer subdivisions may be adequate for many areas. 

Methodology for Regional Maps 

Regional maps depicting expected variations in ground response must be 

based on data available on a regional scale. Geologic and physical 

property data can be readily tied to more quantitative estimates of 

ground response such as amplifications and intensity increments utilizing 

seismic velocity data. 

Intensity increments and amplifications are predicted for 11 of the 13 

units in the San Francisco Bay region using relations in figures 4 and 5 

(Table 1). Both the intensity increments and amplifications predicted are 

easily distinguishable for the various units and show that a considerable 

geographic variation can be expected in ground response to earthquake 

generated shaking. These predictions, together with appropriate bedrock 

attenuation curves from a potential earthquake source, provide a technique 

for developing a preliminary but improved regional ground motion map for 

the San Francisco Bay region. To prepare ground response maps for other 

areas where similar intensity and amplification data are not available, 

measurement of seismic velocities in a relatively few seismically distinct 

units would permit extrapolation of the San Francisco data. A program is 

currently under way to compare data in the San Francisco and Los Angeles 

regions. 

24 
REFERENCES 

1. Borcherdt, R.D. (1970), Effects of local geology on ground motion 

near San Francisco Bay, Bull. Seis. Soc. Am., v. 60, pp. 29-61. 

2. Borcherdt, R.D. (Editor) (1975), Studies for seismic zonation of the 

San Francisco Bay region, U.S. Geological Survey Professional 

Paper 941-A. 

3. Borcherdt, R.D. and Gibbs, J.F. (1976), Effects of local geological 

conditions in the San Francisco Bay region on ground motions and 

the intensities of the 1906 earthquake, Bull. Seis. Soc. Am., v. 

66, pp. 467-500. 

4. Borcherdt, R.D., Gibbs, J.F. and Lajoie, K.R. (1975), Prediction of 

maximum earthquake intensity in the San Francisco Bay region, 

California, for large earthquakes on the San Andreas and Hayward 

faults, U.S. Geological Survey, MF 709. 

5. Borcherdt, R.D., Joyner, W.B., Nichols, D.R., Chen, A.T.F., Warrick, 

R. E., and Gibbs, J. (1972), Ground motion predictions, abs., 

Proceedings of the international conference on microzonation, p. 

862, Seattle, WA. 

6. Fumal, T.E. (1978), Correlations between seismic wave velocities and 

physical properties of near-surface geologic materials in the 

southern San Francisco Bay region, U.S. Geological Survey, 

Open-file report (in progress). 

7. Gibbs, J.F., Fumal, T.E. and Borcherdt, R.D. (1975), In-situ 

measurements of seismic velocities at twelve locations in the San 

Francisco Bay region, U.S. Geological Survey, Open-file report 

75-564. 

8. Gibbs, J.F., Fumal, T.E. and Borcherdt, R.D. (1976), In-situ 

measurements of seismic velocities in the San Francisco Bay 

region ... Part II, U.S. Geological Survey, Open-file report 76-731. 

9. Gibbs, J.F., Fumal, T.E., and Borcherdt, R.D. (1978), In-situ 

measurements of -seismic velocities at 59 locations in the San 

Francisco Bay region, abs., 73rd Annual Meeting, Seis. Soc. of 

Am., Sparks, Nv. 

10. Gibbs, J.F., Fumal, T.E., Borcherdt, R.D. and Roth, E.F. (1977), 

in-situ measurements of seismic velocities in the San Francisco 

Bay region ... Part III, U.S. Geological Survey, Open-file report 

77-850. 

11. Kobayaski, N. (1959), A method of determining the underground 

structure by means of SH waves, Zisin, ser. 2, v. 12, pp. 19-24. 

12. Lawson, A.C. (Chairman) (1908), The California earthquake of April 18, 

1906, report of the state earthquake commission, Carnegie Inst. 

Washington, p. 160-253. 

13. Warrick, R.E. (1974), Seismic investigation of a San Francisco Bay mud 

site, Bull. Seism. Soc. Am., v. 64, pp. 375-385. 

25 
A METHODOLOGY FOR PREDICTING GROUND MOTION AT SPECIFIC SITES 

Ralph J. Archuleta , William B. Joyner and David M. Boore1 

ABSTRACT 

An important development in current research on earthquake ground 

motion is the synthesis of ground motion records based on the physics of a 

propagating fracture. Different techniques are used for generating 

synthetic records depending upon the frequency range of interest. For 

frequencies below about 1-2 Hertz we used a finite element method to 

simulate a propagating fracture; for higher frequencies we used a 

stochastic dislocation model. With the finite element method we have been 

able to simulate a dynamic earthquake in a fully three-dimensional 

geometry. We compute the ground motion from two hypothetical earthquakes 

that differ only in their shear stress distribution with depth. On the 

free surface we have contoured the maximum particle velocity. From such 

contours one could approximate the areas most likely to suffer damage 

during an earthquake. We have also used the fault slip generated by a 

propagating stress relaxation as input for the stochastic model. 

Acceleration is computed using a statistical source model in which the 

amplitude of the dislocation-time function varies randomly along the fault 

while the shape of the function and the rupture velocity are constant. 

INTRODUCTION 

One of the fundamental assumptions for any microzonation plan is 

that one can realistically estimate the ground motion resulting from an 

earthquake. We are develeping methods for computing complete time 

histories of earthquake ground motion from physical models of the source 

and propagation path. We expect that the time histories will be useful, 

not only in the detailed dynamic analysis of structures, but also in 

estimating ground motion parameters, e.g., peak particle velocity and peak 

particle acceleration, for microzonation purposes. 

A major difficulty in making estimates of the ground motion in the 

near field is that the frequencies of interest range from d.c. to tens of 

hertz. In order to span this wide range of frequencies we have modeled 

the earthquake source by using a three-dimensional, finite element model 

of a propagating stress relaxation (1) in combination with a ~tochastic 

propagating dislocation (2). The finite element provides estimates of the 

dislocation time history (2). In addition the finite element method 

1 Professor of Geophysics, Stanford University, Stanford, CA. 94305. 

26 
provides estimates of particle displacement and particle velocity while 

the stochastic dislocation gives some estimate of the particle 

acceleration. The finite element method has been used successfully to 

model real ground motion data from the 1966 Parkfield earthquake (3). In 

this paper we illustrate the method by applying it to two hypothetical 

earthquakes: the first has a shear prestress distribution that is uniform 

over the width of the fault; the second has a prestress distribution that 

varies with depth, Figure 1. A comparison illustrates the influence of 

one of the important aspects of the earthquake source model. 

METHODS 

To simulate an earthquake in a prestressed medium we use the method 

of Archuleta and Frazier (1) that allows the fracture to nucleate at a 

hypocenter and spread with a prescribed rupture velocity over a given 

finite-sized fault area embedded within a halfspace. As the fracture 

spreads, it relaxes the stress enclosed by its rupture front. This model 

was based on the generally accepted elastic rebound hypothesis (4) as the 

mechanism for shallow, tectonic earthquakes. This method is fully 

three-dimensional and the rupture surface may or may not intersect the 

traction-free surface of the halfspace. Driven by the stress relaxation 

the medium adjusts to the new stress state. The particle displacement and 

particle velocity can be computed everywhere including the rupture surface 

and the free surface. 

To demonstrate that their numerical method correctly simulated the 

physics of a propagating stress relaxation, Archuleta and Frazier (1) 

compared their numerically computed dislocations for a circular fault in a 

full space with the dislocations analytically determined by Kostrov (5) 

for a continuously expanding circular stress relaxation. Until the 

arrival of edge effects due to the finiteness of the fault in the 

numerical method, the numerical and analytical dislocations showed 

exceptional agreement including the square root be~avior of the 

dislocation at the arrival of the rupture front. 

As an example of using such an earthquake model, together with the 

response of a layered medium, Archuleta and Day (3) have computed 

synthetic seismograms to compare with those recorded during the 1966 

Parkfield earthquake at Stations 2, 5, 8 and 12 which were about .8, 3.4, 

9.1, and 14 km off the fault, respectively. A comparison of displacement 

time histories for Station 5 in the Cholame-Shandon array is shown in 

Figure 2. One can see that the match between components is close. The 

phases could probably be made to match better by adjusting the rupture 

velocity. The other stations showed similar agreements between synthetic 

and recorded displacements. 

The frequency resolution of waves propagated using the finite 

element method depends critically on the number of nodal points per 

wavelength (6). Thus the finite element (and finite difference) methods 

27 
1'-:> 

~ 

Plan View 

L ..... _______ Xa I I 

• 

x. Xz Xt 

0 5 10 15 20 

I J J J > I < I ... l I I I I • x, -1.0 -0.5 0 0.5 1.0 IS 2.0 • I ......._..,... I I I 

Xz 

Expanding Rupture on Fault 

Xz 

Stress Distribution with depth 

Figure 1. Schematic of fault, rupture surface and stress distribution. 
STATJ(}.J 5 

Figure 2. Comparison of synthetic (+) and recorded (-) ground motion 

of the 1966 Parkfield earthquake. 

29 
cannot economically resolve the high frequencies found in acceleration 

records. It can also be argued that on a scale of several hundred meters 

which corresponds to wavelengths associated with a 10 Hertz wave that we 

neither know the spatial variation of stress on the fault nor the 

inhomogeneities of the medium. Thus we have chosen to estimate the high 

frequencies from a propagating stochastic dislocation along a line at a 

fixed depth. A representative dislocation function is selected from the 

dislocations computed using the finite element stress relaxation. The 

dislocation is propagated with the same velocity from the same 

hypocenter. However, the amplitude of the dislocation is allowed to vary 

randomly about a mean value calculated from the stress relaxation model. 

The width of the fault is taken into account by assigning the dislocation 

function a weighting factor related to the width used in the stress 

relaxation problem. 

EARTHQUAKE MODEL 

We will model a strike-slip earthquake that occurs on a vertical 

fault that is 32 km in length and 8 km in width. The plane of the fault 

intersects the traction free surface of a homogeneous, isotropic\ linearly 

elastic halfspace. The halfspace is characterized by a compress1onal wave 

sp3ed (~) 6.0 km/sec, shear wave speed(~) 3.5 km/sec and density of 2.7 x 

10 kg/m . To designate the spatial positions we use a Cartesian 

coordinate system x1 , x2 , x3 with the origin at the midpoint of the 

strike of the fault at the traction free surface, Figure 1. Components of 

motion referred to as parallel, vertical and transverse are the components 

in the X1 , x2 , x3 directions, respectively. The fracture nucleates 

at (0., 5., 0.) and spreads radially over the fault surface with a rupture 

velocity of .9~ , Figure 1. As the fracture spreads it relaxes the u

31 component of stress. 

We consider two different shear prestress distributions on the fault 

as a function of depth (u31 (X2)); however both shear stress 

distributions have the same average value. We also assume that the 

sliding friction stress does not vary with depth. The shear prestress 

does not vary along the strike of the fault. The parameter u is the 

magnitude of the difference between the tractions on the faul~ before the 

fracture nucleates and the tractions on the fault during sliding. Because 

the sliding friction stress is uniform, uE varies with depth exactly as 

does the prestress. The amplitude of the particle motion scales directly 

with uE (7). 

RESULTS 

To illustrate the particle motion on the fault we show in Figure 3 

time histories of particle velocity for points starting at the hypocenter 

and progressively moving toward the end of the fault on a line of constant 

depth. An important feature for microzonation is that the amplitud~ of 

the particle velocity increases in the direction of rupture propagation. 

30 
(,;0 

1-' 

() 165 

~ 110 

' 55 2 (.) 0 

-

. 

u, 

Particle Velocity 

variable stress with depth 

u2 u3 

OI65L_L 

~ 110 

' 55 2 () 0 

-

0 2 4 ~ ~ 10 0 2 4 6 8 10 0 2 4 6 8 10 

TIME (SEC) 

Figure 3. Particle velocity time histories on the fault surface. 

POSITION 

(0,5,0) 

(3,5,0) 

(6,5,0) 

(9,5,0) 

(12,5,0) 

(15,5,0) 
As pointed out by Archuleta and Frazier (1) this focusing of energy is a 

combination of both the directivity associated with a moving source (8) 

and the buildup of stresses on the fault ahead of a subsonic rupture. 

This focusing can be an important consideration in microzonation because 

the largest amplitude ground shaking depends not only on the stress but 

also the rupture velocity and its direction of propagation. It should be 

mentioned that if the rupture nucleates at depth and propagates towards 

the free surface, the particle velocity will increase in amplitude as it 

approaches the free surface. The free surface has the additional effect 

of nearly doubling the amplitude of the particle velocity should the 

rupture front break through the surface (1). 

To see how the earth's surface might respond to an earthquake we 

have plotted contours of maximum horizontal particle velocity in Figure 4 

for the cases of uniform stress with depth and variable stress with 

depth. Maximum horizontal particle velocity is calculated by first taking 

the absolute value of the vector sum of the parallel and transverse 

components of particle velocity for each node on the free surface for 

every time step of the computation. The maximum value attained during the 

entire process is then contoured using a linear interpolation between 

adjacent nodes. Depending on what information is considered important 

other variables such as peak displacement or maxima of individual 

components of particle velocity could be contoured. 

In order to provide numerical estimates of the peak horizontal 

particle velocity we have assumed an average value for u of 55 bars for 

both stress distributions. uE can underestimate th~ sta~ic stress drop 

by about 20 to 25 percent (1) due to the iner5ial effects of a dynamic 

rupture (9). With uE of 55 bars,~= 3.3 X 10 bars, a= 6 km/sec and 

~= S 3.5 km/sec the contours are drawn at 0.9 m/sec, 0.8 m/sec, etc. One 

can see that in the case of variable stress with depth the areas near the 

ends of the fault have the highest values; whereas if the stress were 

uniform over the entire fault, the distribution of horizontal particle 

velocity is nearly uniform along the entire strike of the fault with a 

gradual flaring of the contours near the ends. It should be pointed out 

that the maximum values of horizontal particle velocity anywhere on the 

free surface were 1.0 m/sec and 1.9 m/sec for the variable stress and 

uniform stress, respectively. Thus the contour of 0.9 m/sec for a uniform 

prestr~~~ not only encloses a larger area but also encloses larger values 

of peak horizontal particle velocity. Although the uniform stress 

produces larger values near the fault and over a larger area, the 

differences in the contours between the uniform and variable stress cases 

decreases with distance from the fault. At a distance of approximately 

one fault depth the contours are almost indistinguishable. Although the 

finite element propagating stress-relaxation can be used to compute 

variables such as particle velocity and particle displacement, it is too 

expensive an approach for computing particle acceleration where the 

frequencies of interest are around 10 Hertz. 

32 
Contours of Maximum Horizontal Particle Velocity Uniform effective stress ( cr1 ) Fault 32 X8 Hypocenter ( 0, 5, 0 ) 

a= 6.0 Km /sec 13 = 3.5 Km /sec p= 2.7 Kg/m1 v = 0.9 13 

For cr1 = 55 bars, contours are drawn for 0.9 m/sec, 0.8m/ sec .... 

I;" I'~ -~~ ~··t- ~---" ..... ..... ~ 

~-- I'-1-r-- ~~ r--i' ~"""I1--1- r- 1-- /.,. -~ 

v 21"'-r--. 1--"r--r- !". 

L 1"'- / / r-t- ,....r- " "' .... r-1'- ioo-1"'" ... ~ 

v r-1-~ I-I-~-" " .... 

/ '// ~-~ er- :ol-t- ... ~ r-r-• -~-- ~ \. '\. 

1 ~t-o .... \ 

I I 

'" I 

- I r.L \ 

l 1 11. J 9 ~\ 1 

\ ~F"-" -==eE I 

'\. 1\. ' -· """'"~ -I"'"- r--fs -- r- V'/ / 

1\.. ~~--- ~'-t"--1"-oo 4r-r- / 

" r-

....... -I""'" - -~""' ..... ~~- r-r-....3 .,. ~ 

"" v 

['.. looo'~- r- .... 2 v 

"\ ...... ... v r-- r-r-...,.... ~~-- 1-" 

r-ot""' r-1--" I-t'~ ~~ t-r---!"'--I ..... ~ 

, ...... "' .... r..--~~"'" 

Contours of Maximum Horizontal Particle Velocity Variable effective stress ( cr1 ) Fault 32 X 8 Hypocenter ( 0 , 5 , 0 ) 

a= 6.0 Km/sec 13 = 3.5 Km/sec p= 2.7 Kg/m1 v = 0.913 

For cr1 = 55 bars, contours are drawn for 0. 9 m I sec , 0. 8 m I sec .... 

~~ ~~ ~--,.... ~ ..... ~,.....- --~ 'I"'- ~~- r-t--i'"' ~ r-r- i"r- ~--~ - r--..... 

~---~ .... t-1'- .,. r-1"'-~ ...... ~,.... "" 

l/ 2 ....... / 1/ r-1-1'- ...... ~-.- " r--... 

_L 

"""~ .... ~ .... 1\ 

v v ~'"'"I-"'" i"""'J"'oo 3 ~~- ~~-r- "" " 

..L .L ..... r- r-r-....., r-r- ... ~-.,. ,....r-r-•r- 1\ 1'\. 

IJ I -r-- -r-.... -r--,. ~- ~~-I""" ~- I 

l\ ~- ~:::::: ~- ..... ' r-......, ~- I'.. i/ .,. ..... _ ..... ~'-"~.-" ~t:: ~- :;;:::~ L 

'1 1-.......... ,....r""" v ..... .......... 't- ,\ 

1\ \ - -~&- ~~-- ~~ I"' ... -r~t""oo r--s r- ll I I 

'\.. \ ~ -~""" .-1- 3 t--""' -~- / / 

' ' ~~--- ~ ..... 'I'- -.... ~ L / 

' ~I""" .... v 

.:--.. ....... ...... - r-- ~ ~ 

1'\.. 2 / 

to... .... r--- r-.... t--- ...,.V" 

r-I-I""" rt--

r-.... r-r-r---1'- ~~- """~"""r-- ~~ 1-r--. ~""" ~r- v -t- ""'"L,.ooo ~ ..... ~ i'~ ~I-"' 

. 

33 

. 

cu 

u 

i 

:s 

en 

cu 

cu 

'-

~ 

cu 

.t:: 

+» 

c 

0 

._ 

~ 

u 

0 

.... 

cu 

> 

cu 

.... 

u 

._ 

+» 

., '-

Q. 

~ 

0 

., 

'-

:s 

.B 

c 

8 
vo 

~ 

SAMPLE NO. 1 

TRANSVERSE 

I - ~ .... .., .. 

z 

0 I lg 

H 

la: . il I 

~ • PARALLEL 

w 

_j 

w 

u 

u 

0:1 VERTICAL 

0 5 10 

TIME (SEC) 

15 

Figure 5. Components of acceleration computed for a particle on the free surface 

at (14.,0.,10.) 

20 
To calculate particle accelerations we have characterized the source as a 

propagating dislocation along a line at constant depth with the amplitude 

of the dislocation varying randomly from point to point. The form of the 

dislocation is derived from the dislocation computed in the stress 

relaxation problem. Since the method of Joyner and Boore {2) is strictly 

applicable only for the farfield, the dislocation rate rather than the 

dislocation is necessary to compute the accelerograms. We have taken the 

particle velocity function shown in Figure 3 at position (12., 5., 0.) as 

a representative for the entire faulting process. Using the Green's 

function for a full space and taking into account the free surface 

interaction by applying complex plane wave reflection coefficients we have 

computed accelerograms for the point(l4.,0.,10.). A representative of the 

ensemble of accelerograms based on the stochastic method is shown in 

Figure 5. The acceleration records have been operated on by both the 

instrument response of an accelerograph with a natural frequency of 20 

Hertz and damping 0.6 critical and by an attenuation operator (10) using a 

quality factor of 150 for shear values. The peak values of acceleration 

are 0.52g, 0.22g, and 0.19g, for the paralle~, transverse and vertical 

directions, respectively where g = 9.8 m/sec . 

The methods of computing ground motion presented in this paper are 

approximations to our understanding of the earthquake source and the 

propagation paths. Further refinement of these methods plus the 

development of new techniques should lead to even better estimates of 

ground motion near a propagating fracture. 

REFERENCES 

{1) Archuleta, R. J. and G. A. Frazier (1978). Three-dimensional 

numerical simulation of dynamic faulting in a halfspace, Bull. Seis. 

Soc. Am., 68, No. 3, p. 541-572. 

{2) Joyner, W. B. and D. M. Boore (1978). A statistical source model for 

synthetic strong motion seismograms, Earthquake~' 49, !£· 1, p. 

3. 

{3) Archuleta, R. J. and S. M. Day (1977). Near-field particle motion 

resulting from a propagating stress relaxation over a fault embedded 

within a layered medium, ~' 58, p. 445. 

{4) Reid, H. F., (1910). Mechanics of the Earthquake, in California 

Earthquake, of April 18, 1906, II: Carnegie Inst., of Wash1ngton D. 

C. (updated in 1969).---

(5) Kostrov, B. V. (1964). Self-similar problems of propagation of shear 

cracks, J. App. Math. Mech. 38, p. 1077-1087. 

(6) Day, S. M. (1977). Finite element analysis of seismic scattering 

problems, Ph.D. Dissertation, University of California, San Diego. 

(7) Madariaga, R. (1976). Dynamics of an expanding circular fault, Bull. 

~· Soc. Am. 66, p. 639-666. 

35 
REFERENCES (continued) 

(8) Ben-Menahem, A. (1962). Radiation of seismic body waves from a 

finite source in the Earth, ~- Geophys. Res. 67, p. 345-350. 

(9) Savage, J. C. and M. D. Wood (1971). The relation between apparent 

stress and stress drop, Bull. Seism. Soc. Am. ~' p. 1381-1388. 

(10) O'Neill, M. E. and D. P. Hill (1978). Frequency-dependent 

anelasticity and its effect on synthetic seismograms, Earthquake 

Notes 49, !£· 1, p. 54. 

36 
Liquefaction Potential Map of San Fernando Valley, California 

by 

T.L. Youd , J.C. Tinsley , D.M. Perkins , E.J. King 

and RoF. Preston 

ABSTRACT 

Ground failure caused by liquefaction is a primary hazard associated 

with earthquakes. A first step in avoiding or mitigating this hazard is 

to recognize where liquefaction is likely to occur. A liquefaction 

potential map has been compiled for the San Fernando Valley, California, 

showing areas where conditions may be favorable for the development of 

liquefaction. The map incorporates assessments of age and type of sedimentary deposits, ground water depth, and expected seismicity into the 

delineated zones. This map is useful to planners, building officials, 

engineers, and others responsible for minimizing seismic risk because it 

points out areas where potential hazards exist and where further investigation, regulation, zoning, or other measures might be required. The map 

is not sufficient for evaluation of the actual liquefaction potential at 

an individual site. Site-specific geotechnical investigations are required 

to make such an assessment. 

INTRODUCTION 

Ground failures generated by liquefaction have been a major cause of 

damage during past earthquakes and pose considerable potential for damage 

and injury during future temblors. For example, during the 1971 San 

Fernando, California, earthquake, liquefaction-induced ground failures 

inflicted irreparable damage to several buildings at the San Fernando 

Valley Juvenile Hall and caused major damage to the partly completed 

Jensen Water Filtration Plant. Such failures do not occur at random, but 

rather are limited to certain geologic and hydrologic settings and to 

certain types of materials. A threshold seismic shaking intensity is also 

required to generate ground failure. This paper presents an analysis of 

these factors and their geographical distribution in the San Fernando Valley 

and a map to show general areas where potential for liquefaction and associated ground failure may exist. Site-specific studies are required to 

evaluate the potential beneath any specific parcel of land. 

The procedure used to develop the liquefaction potential map is a 

combination of the techniques proposed by Youd and Perkins (9) and Youd 

and others (8). The technique of Youd and others was used to make a liquefaction potential map for the southern San Francisco Bay area. The procedure used here requires the development of two constituent maps, a liquefaction susceptibility map and a liquefaction opportunity map. The susceptibility map delineates areas where liquefiable materials are most 

37 
likely to occur. The opportunity map shows recurrence intervals for earthquake shaking strong enough to generate liquefaction in susceptible 

materials. These maps are then superimposed to form a liquefaction potential map. 

LIQUEFACTION SUSCEPTIBILITY MAP 

The liquefaction susceptibility map developed herein incorporates 

the following factors that affect liquefaction susceptibility: age and 

type of sedimentary deposits; standard penetration resistance of cohesionless sediments; and depth te perched or other ground water. 

Sedimentary Deposits 

Analyses of historical occurrences of liquefaction, in general, indicate that the more recently a sediment is deposited, the more likely it is 

to be susceptible to liquefaction, and that certain types of deposits, such 

as river channel and flood-plain deposits, are more susceptible to liquefaction than other deposits, such as alluvial fan deposits (9). Grain size 

distribution and packing also influence susceptibility. Sand and silty 

sand are the textural classes most likely to be adversely affected. In 

general, the more loosely the grains are packed, the more susceptible the 

sediment is to liquefaction. Standard penetration resistance is commonly 

used by engineers as an index of density of packing; the smaller the penetration resistance, the more likely the sediment will liquefy. 

The San Fernando Valley lies in the Transverse Ranges structural 

province of southern California. The valley is an asymmetric basin filled 

chiefly with Miocene and younger (less than 20 million years old) sedimentary rocks. The relatively flat surface of the valley is unde1l~i~ by 

unconsolidated sediments of middle to late Pleistocene or younger age that 

are as thick as 650 ft (200m) (7). The exposed unconsolidated sediments 

are herein subdivided into three units -- most recent Holocene sediments 

(R), other Holocene sediments (H), and late Pleistocene deposits (P). The 

distribution of these units is shown on the map in Fig. 1. The margins of 

the basin comprise undifferentiated Pliocene to middle Pleistocene deposits 

(TQ) which include the Saugus and Pacoima Formations of Oakeshott (4), 

undifferentiated Tertiary sedimentary rocks (T), undifferentiated Mesozoic 

sedimentary rocks (K), and pre-Tertiary igneous and metamorphic rocks in 

the mountains mapped collectively as basement complex (BC). 

Areas mapped as most recent Holocene sediments are chiefly those 

known to have been flooded historically, where the most recent deposition 

is known or presumed to have occurred. The primary data used to delineate 

the extent of flooding include field notes and unpublished maps prepared by 

the Los Angeles Flood Control District that show and describe areas inundated during floods in 1934, 1938, 1941, 1943, 1944, 1952, 1954, and 1956. 

Other data used include comparative photo-interpretive studies of 1928 and 

1938, aerial photography at a scale of 1:24,000 or larger. The 1938 photographs were taken three to five months after the major floods of March, 1938. 

Features on the photographs used to delineate areas of recent deposition 

or flooding include changes in patterns of distributary channels or 

alluvial fans, areas where row crops, roads and other cultural features 

were washed out, areas adjacent to streams that were incised to depths less 

than 5 ft (1.5 m), and areas characterized topographically by bar and swale 

channel deposits. We have specifically excluded from the most recent 

Holocene unit areas that were locally inundated along streets where culverts 

38 
V.:l 

1.0 

118° 40' 118° 15' 

34° 20' 

SANTA Explanation: N 

R Most recent 

BC wosll I --- -..._f"" -, Holocene sediments 

H Other Holocene 

K sediments 

c:, p Late Pleistocene deposits 

TQ Pliocene and 

Early Pleistocene deposits 

T Tertiary sedimentary rocks, 

undifferentiated 

K Cretaceous sedimentary rocks, 

undifferentiated 

SANTA BC Pre-Tertiary 

basement complex 

0 5~ 

0 IOkm 

~--------------------------------------------------------------L_ ______________________________ _j_34°05' 

Fig. 1. Map showing distribution of geologic units in the San Fernando Valley, Calif. 

Bedrock units (T,K,BC, and TQ) from Jennings and Strand (3); R,H,P, and some TQ from 

J.C. Tinsley (unpub. field data) 
plugged and water simply backed up. In all cases, interpretations reflect 

our efforts to identify and delineate the youngest stages of a very 

youthful depositional system because these are most likely to contain sediments susceptible to liquefaction. 

Areas mapped as other Holocene sediments are differentiated chiefly 

on the basis of lack of record of recent inundation, a topographic position 

slightly higher than the most recent Holocene unit and by undeveloped to 

very weakly developed cumulative pedogenic soil profiles (chiefly entisols, 

inceptisols, and vertisols) typical of Holocene deposits. Soil maps prepared by the U.So Bureau of Soils (2), and the U.S. Department of Agriculture 

(6) were the primary data used to delineate this Holocene unit. 

Areas mapped as late Pleistocene deposits were differentiated on the 

basis of morphology and pedological development. They generally are exposed 

as topographic benches and terraces near the margins of the valley and on 

the up-thrown side of thrust faults in the northern part of the valley. 

They are also characterized by surface soils with textural "B" horizons. 

Under present climatic conditions, to form such a soil profile requires a 

period of development that began in Pleistocene time. 

By the criteria set out in Youd and Perkins (9), water-saturated, 

clay-free sediments in the most recent Holocene unit generally are expected 

to have high susceptibility to liquefaction; clay-free sediments in the 

other Holocene unit generally would be expected to have moderate susceptibility, and clay-free sediments in the late Pleistocene unit would be 

expected to have low susceptibility. The qualitative assessments of liquefaction susceptibilities were further verified by comparing them with liquefaction susceptibilities determined from standard penetration data. The 

evaluation of liquefaction susceptibility from standard penetration data 

followed the procedure previously used to compile a liquefaction potential 

map for the southern San Francisco Bay region. Plots were made of standard 

penetration resistance versus depth for the three youngest (R, H, and P) 

sedimentary units. Thicknesses of the two Holocene units have been inferred 

from descriptive logs of boreholes by identifying subsurface features such 

as, (a) an oxidized clay-rich layer which may indicate an ancient soil 

horizon that likely formed on the late Pleistocene unit before deposition 

of the Holocene unit, (b) an abrupt increase in penetration resistance not 

caused by intersecting a gravel layer, and (c) an estimated maximum thickness of the most recent Holocene unit of about 12 ft (3.6 m), based on 

rates of vertical acretion. 

Using the criteria developed for the southern San Francisco Bay region 

(8), percentages of penetration data were grouped into high, moderate, and 

low liquefaction susceptibility categories for each sedimentary unit 

(Table 1). These categories are based on analyses developed in the 

simplified procedure for evaluating soil liquefaction potential formulated 

by Seed and Idriss (5) and analyses used by Youd and others (8) to map the 

San Francisco Bay area. The high susceptibility category represents saturated sediments that would be likely to liquefy in a nearby moderate 

(M=6.5) earthquake (energy source within 10 mi or 16 km) or a distant, very 

large (M=8) earthquake (energy source within about 60 mi or 100 km). The 

moderate susceptibility category indicates sediments that might liquefy 

during a nearby large (M=8) earthquake (energy source within 10 mi or 16 km)o 

Low susceptibility indicates sediments that should not liquefy even when 

shaken by a nearby large earthquake. Assumptions made in deriving these 

categories are the same as those used in the San Francisco Bay region (8) 

40 
~ 

-

118°40' 118° 15' 

~----------------------------------~--------------------------------------------------------------r34°20' 

N 

1 

., 

SANTA 

SANTA 

SUSANA 

.. ! 

\~ 

~! 

') \ 

·~~ \\, ~( ~\ u\ 

' '!\ 

MONICA 

0 5~ 

0 IOkm 

\ 

\ 

\ 

\ 

I 

\30 

I 

I 

I 

I 

.c: 

.. 

a• 

"( 

! 

' ~ 

c3{/l !-,/ 

f

o:' 

~ 

0..0 

\ 

'"" 

S-4~ 

"'ou~.,-4 8 -.osll -----0·--

Explanation: c30 

Depth at which 

ground water was 

found. Dashed 

where not tightly 

constrained; 

queried where 

speculative. 

L_--------------------------------------------------------------------------------------------~34°05' 

Fig. 2. Map showing approximate depth to ground water, including perched ground water, 

for the San Fernando Valley, California 
Table I.--Evaluation of liquefaction susceptibility in clay-free granular 

layers from standard penetration datao 

Sedimentary unit Percentage of standard penetration Number 

data in susceptibility categories of tests 

High Moderate Low 

Most recent Holocene 61 33 6 153 

Other Holocene 39 49 12 618 

Late Pleistocene 20 39 41 443 

and include unit weight of 100 lb/ft3 (1.6 gm/cm3), a maximum surface 

acceleration of 0.2 g and 10 significant loading cycles for the nearby 

moderate (M=6.5) earthquake, and a 0.5 g maximum surface acceleration and 

30 significant loading cycles for the nearby large (M=8) earthquake. The 

influence of depth to water table was taken into account in the calculation. 

These data (Table 1) show that saturated clay-free sediments in the youngest 

unit can be expected to have high susceptibility to liquefaction; similar 

sediments in the other Holocene unit can be expected to have moderate susceptibility; and saturated clay-free sediments in the late Pleistocene unit 

generally have moderate or low susceptibility to liquefaction. 

Ground Water Depth 

One of the primary factors controlling the distribution of liquefiable 

sediments in the San Fernando Valley is depth to ground water including 

perched ground water. Liquefaction susceptibility generally decreases with 

depth of the ground water tabl~ for two reasons: (a) The deeper the water 

table, the greater is the normal effective stress acting on saturated sediments at any given depth. Liquefaction susceptibility decreases with 

increased normal effective stress. (b) Age, cementation, and compactness 

of sediments generally increase with depth. Each of these factors also 

increases resistance to liquefaction. Thus, as depth to the water table 

increases, and as the saturated sediments become older, more cemented, 

more compact, and more stressed, the less likely they are to liquefy during 

an earthquake. 

A map showing depth to ground water, including perched ground water, 

was prepared for part of the San Fernando Valley (Fig. 2). The lines on 

the map are not contour lines in the usual connotation used by hydrologists, 

i.eo, an equipotential surface. Rather, they enclose areas where our data 

show that either perched or unconfined ground water has been recently found 

within the specified intervals of depth. This map was compiled from borehole data and well information supplied by the several agencies and firms 

acknowledged in this report. Because of sparsity of data in some areas 

and variations ~n amounts and elevations of ground water, construction of 

the map required considerable averaging, generalizing, interpolation, and 

extrapolationo Where depth lines are not tightly constrained, they are 

dashed; where they are speculative or positioned by factors other than 

borehole and well data, they are queried. The following problems are among 

those that arose in construction of the map. (a) Borings that intersect 

the water table are sparse in many areas. (b) Water levels in some deep 

wells may have been higher than the phreatic surface in the surrounding 

42 
soil because of artesian pressures at depth. (c) Some measurements, particularly along freeways and certain major storm drains, were recorded as 

much as 30 years ago, and water levels may have changed in the interim. 

Where several measurements have been made in one well over a period of 

years, the average of the measurements taken in the past seven years was 

used. (d) Seasonal fluctuations in water levels perturb the consistency of 

the data. (e) Irregular impermeable layers with poorly defined boundaries 

commonly retain perched water. (f) Errors may have been made in measuring 

water table levels in some boreholes. These difficulties were accounted 

for and corrected to the extent possible, but space does not allow enumeration here of these corrections. 

With respect to depth to ground water, the following criteria were 

applied to liquefaction susceptibility: For ground water depths less 

than 10ft (3o0m), maximum possible susceptibility is very high. For 

water depths between 10ft (3o0m) and 30 ft (9.1 m), maximum possible 

susceptibility is higho For depths between 30 ft (9.·1 m) and 50 ft (15.2 m) 

maximum susceptibility is low. For water depths greater than 50 ft (15.2 m) 

maximum possible susceptibility is very low. These criteria are based in 

part on criteria suggested by Youd and Perkins (9)o 

Map Compilation 

The criteria developed in the preceding section and summarized in 

Table 2 were used to compile a liquefaction susceptibility map for the 

San Fernando Valley. These criteria were applied to the data compiled 

on Figs. 1 and 2 to derive the susceptibility map in Fig. 3. In order to 

show better the detail with which this type of map can be constructed for 

areas where sufficient data are available, a segment of the susceptibility 

IDap is shown at larger scale in Fig. 4. Note that the susceptibility maps 

presented here represent estimated average climatic conditions. Ground 

water levels and hence susceptibilities are likely to be higher during wet 

seasons and lower during extremely dry cycles. 

Table 2.--Criteria used in compiling liquefaction susceptibility map 

Sedimentary unit 

Most recent Holocene 

Other Holocene 

Late Pleistocene 

Late Pliocene and 

early Pleistocene 

Tertiary 

Probable susceptibility of clay-free granular 

layers 

Ground water depth, ft(m) 

<30(9.1) 30(9.1)-50(15.2) >50(15.2) 

High Low Very Low 

Moderate Low Very Low 

Low Low Very Low 

Very Low Very Low Very Low 

Very Low Very Low Very Low 

43 
..;::... 

..;::... 

118° 40' 118° 15' 

~----------------------------------~--------------------------------------------------------------t-34°201 

SANTA SUSANA N 

I 

VL 

SANTA 

MONICA 

0 5mi~ 

0 IOkm 

o• 

.e 

. 

7 0~ 

' { L-VL 

=;) 

CD, 

\ 

·"'·· 

VAN NUYS 

G-48R1£L.:;/ 

~,/ 

~:! li 

~ 

4fou,r-4'Ns 

VL 

'flosll ----..._[---

Explanation: 

Probable susceptibility of clay-free 

layers to liquefaction 

H - High 

M - Moderate 

L - Low 

VL - Very Low 

~----------------------------------------------------------------------------------------------_j_34°05' 

Fig. 3. Zonation map for the San Fernando Vailey showing probable susceptibility to liquefaction. Site-specific geotechnical investigations are required to determine susceptibility at a given locality. This map is also 

a liquefaction potential map with a return period of 46 years for shaking strong enough to cause liquefaction 
-::;-.,- - - - 0 1km I 

I 

l:~atttPfl( ttd\t :i.\ \b .,. - " 

N 

):" ,,. .. ., lfrjtfR c: ;;. 

'(I 

=V 

: ... · ·.... .I ...... 

=-----

:-:. t 

A Lt 

y 

L E .. ··:·.··-···1 -- -- S ,..jl 

: . ·:: ~,, 

II 

. .l, 

'( 

• ---

91 

.· .. · .. • : .jr5 

:::i I 

I )> •• 

I ,.11 ~1 I<; 

.. I )o jlt f 

_ _ _ §. _ _HJ (i tj: __ , _ <i • 

•• ,. · 1 ·····: il};· ::_.· .. ··: · I t; • ;-:: i :I: · of l!!: ! .. II 

I :: : : ·I I ! : : ! • 

. ·,. . . : .. . ...... 1 J: ... l ··- A'""' :: I : . .. '~f.Dt• • ·-&.. I" ' np .. .:-, • jil 1 J.,'l!' '-.,. H:i 4 , .. , 

f I ; :.:.: IQ: •• ,. ·"' •-..,.!J.rn - - - T. J . - -. . _...,..... .~..._.._ I - ..... -· . . - . . - -., : • •. • • ltv! '~dl '- • "'' 

i i::.: ~! ••• ' 

\ analdt'n ..... \\ ... . •••• -:-; ... • .. "':"";" •• •,''

--

l>a~ 

· --:" --I •• - - ", -:-.. ~ . .J: . · --:-. • ~ 

· 

-=--·. 

.. • 

,. 

~ 

. . 

~ 

. . .. 

,.-JI I .... •... :. ... .; . .• ! ~ .. -:-·' .. • • f p .... ·. ·.·· · · .... · ··. 1a1zana •••• • 'I · ! Pf!C'f:IL •• • .. 1 :-;,d'"" .. H ·ft·w ···_._ .. _,·,.-..----!+==~ ' ;r" ' 4: .. -· . + ··~ ••• .. ·· ... · LJ \!, IYY 

l N ~~ ~ ........... ~ J). .. · . .'·1 .. 9 ... ~~ -~ :.~. J Eme~ St 

·. .. . ... .... . ,. ·.:: ......... ...... . .. :.:····. . . .. 

. . . ., . .. ~· ,. ~ • • ... ~ \.. f •• 

HA r1'F'?A q :I ... ·.· ....... ,.: .:·1 

.: · . . ~: i ·. : : . . •. ttl . . .. . . . . : t: ..... ~: 

: .... : . I : ·.. : : • • • .7: •· . . ... : . ... . 

Fig, 4. Enlarged liquefaction susceptibility map for Tarzana area of the 

San Fernando Valley, California. Susceptibility categories same as Fig. 3. 

45 
N 

0 I 

Fig. 5. Seismic source zones in the vicinity of the San Fernando Valley. 

Only zones 2,4,13, and 17 are significant to liquefaction opportunity in 

the San Fernando Valley. San Fernando Valley (hachured area) lies in the 

eastern corner of zone 4. San Andreas fault province corresponds to zone 4. 

Table 3.--Normalized seismic parameters for source zones 

Zone Number of intensit2 b-value for epi-2 No. V's/100 yrs/104 km central intensity 

N bi 

2 60.6 -.40 up to XI 

then flat 

3 12.5 -.45 

4 24.2 -.45 

13 129.65 -.45 

14 136.56 -.45 

15 -.53 

16 20.8 -.50 

17 70.4 - .45 

18 2.5 -.so 

1

Modified Mercalli Intensity 

2

b-value for magnitude (bM) = 0.6 bi 

Maximum 1 intensity 

I 

0 

XII 

XI 

XI 

XI 

XI 

VIII 

X 

XI 

X 

Liquefaction Opportunity Map 

Maximum 1 magnitude 

M 

c 

8.5 

7.9 

7.9 

7.9 

7.9 

6.1 

7.3 

7.9 

7.3 

A liquefaction opportunity map delineates recurrence intervals for 

earthquake shaking strong enough to produce liquefaccion in susceptible 

materials. Information needed to compile an opportunity map includes an 

estimate of frequencies, magnitudes, and location of expectable future 

earthquakes and a relation between earthquake source characteristics and 

the distance from the source to the oound of a zone encompassing sites 

where liquefaction could lie generated. The estimated earthquake sources 

and frequencies of occurrence in the San Fernando Valley region used in 

this study (Fig. 5, Taole 3) are those previously defined by Algermissen 

and Perkins (1). The distance relation used is that given by Youd and 

46 
Perkins (9). The liquefaction opportunity map plotted on Fig. 6 was 

derived from these two sets of information. The map shows the return 

period (inverse annual probability) for liquefaction opportunity from the 

San Fernando Valley north to the San Andreas fault. The liquefaction 

opportunity varies only about 20 percent over this area and is effectively 

constant across the San Fernando Valley; the return period is about 46 

years. Figure 7 shows the contribution of the various sources to the 

annual probability along a meridian section of the map in Figure 6. In 

general, earthquakes of magnitude less than 6 contribute an insignificant 

amount to the overall probability of liquefaction. Roughly half of the 

contribution comes from large~gnitude events occurring locally in the 

Transverse Ranges province (ione 17, fig. 5) and along the San Andreas 

fault (zone 2, fig. 5). This contribution is roughly constant across the 

San Fernando Valley. Large-magnitude events occurring near the Garlock 

fault to the north are balanced by large-magnitude events occurring south 

of the Transverse Ranges. As the effect of one region decreases across 

the San Fernando Valley, the effect of the other region increases. 

Liquefaction Potential Map 

Superposition of the maps in Figs. 3 and 4 with the map in Fig. 6 

yields a liquefaction potential map. In this instance, the return period 

is constant at 46 years across the area in question, and hence no return 

period contours appear on the San Fernando Valley area. Thus, for a 46-

year return period, the maps in Figs. 3 and 4 show the areas where strong 

ground shaking is likely to produce liquefaction in sediments with high 

susceptibility. 

SUMMARY AND CONCLUSIONS 

A liquefaction potential map has been compiled for the San Fernando 

Valley, California. The map incorporates assessments of age and type of 

sedimentary deposits, ground water depth, and expected seismicity into the 

delineated zones. This map is useful to planners, building officials, 

engineers, and others responsible for minimizing seismic risk by pointing 

out areas where potential hazards exist and where further investigation, 

regulation, zoning, or other measures might be required. The map is not 

sufficient for evaluation of the actual liquefaction potential at an 

individual site. Site specific geotechnical investigations are required 

to make such an assessment. 

ACKNOWLEDGMENTS 

Appreciation and acknowledgement are given to the following governmental agencies and private firms, Who freely provided information for use 

in this study: U.S. Army Corps of Engineers, California Department of 

Transportation, California Department of Water Resources, Los Angeles 

County Flood Control District, Los Angeles County Engineer. 

References Cited 

(1) Algermissen, S.T., and Perkins, D.M., 1976, A Proliabilistic Estimate 

of Maximum Ground Acceleration in the Contiguous United States; 

U.S. Geological Survey Open..,..·File Report 76-416. 

(2) Holmes, L.C., 1919, Soil Survey of the San Fernando Valley area, 

Calif., in Field Operations of the Bureau of Soils 1915; U.S. 

Department of Agriculture, Buro Soils, 17th repto, pl. 63, 

scale 1:62,500. 47 
119° 

(3} 

(4) 

(5) 

(6) 

(7) 

(8) 

(9) 

N 

Jennings, C.W. and Strand, R.G., 1969, Geologic Map of California, 

Los Angeles sheet (scale 1:250,000): Calif. Division of Mines 

and Geology. 

Oakeshott, GoB., 1958, Geology and Mineral Deposits of San Fernando 

Quadrangle, Los Angeles County, Calif: Calif. Division of Mines 

Bulletin 172, 147 Po, 3 pls. 

Seed, H~B., and Idriss, I.Mo, 1971, Simplified Procedure for 

Evaluating Soil Liquefaction Potential: Journal of the Soil 

Mechanics and Foundations Division, Amer. Soc. of Civil Engineers, 

v. 97, no. SM9, p. 1249-1273. 

U.S. Department of Agriculture, 1977, Soil Survey of Los Angeles 

County, Calif. west San Fernando Valley area: an interim, 

unedited report. 186 p. 5 pl. 

Wentworth, C.M. and Yerkes, R.Fo, Geologic Setting and Activity of 

Faults in the San Fernando Area, Calif; in The San Fernando, Calif. 

Earthquake of February 9_, 1971: u.s. Geological Survey Professional 

Paper 73J, p. 6-16. 

Youd, T.L., Nichols, D.R., Helley, E.J., and Lajoie, K.R., 1975, 

Liquefaction Potential, in Studies for Seismic Zonation of the 

San Francisco Bay Region: U.So Geological Survey Professional 

Paper 941-A, p. A68-A74. 

Youd, T.L., and Perkins, D.M., 1978, Mapping Liquefaction-Induced 

Ground Failure Potential: Journal of the Geotechnical Engineering 

Division, Amer. Soco of Civil Engineers, v. 104, No. GT4, p. 433-446. T 350 

'55' 

Annual probability x 105 

0 5 10 15 20 25 

~------------~---------------, 50' 

45' 

40' Son Andreas 

•••••••••••••• 

Fault 

35' 

301 

25' 

20' 

151 

10' 

5' 34° 2

1 

3011-...--..,1--f---+---~ 

48' 42' 36' 30' 24' 18' 12' 

0 40mi 

0 60km 

Fig. 6. Return period for liquefaction opportunity in the vicinity of the San Fernando Valley 

Fig. 7. Contributions to annual 

probability of liquefaction 

opportunity for sites along 188° 

27.5' meridian. Major contributions to annual probability are 

due to large magnitude (M>6.4) 

earthquakes located south of the 

Transverse Ranges (zone 17), along 

the San Andreas fault (zone 2), in 

the vicinity of the Garlock fault 

(zone 13), and in the vicinity of 

the Transverse Ranges 48 
PRELIMINARY ASSESSMENT OF SEISMICALLY INDUCED LANDSLIDE SUSCEPTIBILITY 

by 

D. K. Keefer , G. F. Wieczorek , E. L. Harp and D. H. Tuel 

ABSTRACT 

Earthquake-induced landslides have taken a large toll in loss of 

life and property. We are currently engaged in studies aimed at determining which types of landslides are most common during earthquakes and at 

establishing criteria for mapping susceptibility of slopes to earthquakeinduced landsliding. This preliminary summary of our findings deals primarily with the types and abundance of landslides that occurred during 

15 historic earthquakes. Preliminary criteria have also been developed 

for mapping susceptibility to same kinds of landsliding, and these 

criteria are used herein to prepare an experimental susceptibility map of 

an area near San Francisco, Calif_. 

INTRODUCTION 

On Sunday afternoon, May 31, 1970, an earthquake of magnitude 7.7 

struck Peru. The shock caused a large mass of ice and rock to break away 

from Mount Huascaran, the highest mountain in the Peruvian Andes. The 

debris cascaded 600 m down the steep upper slopes of the mountain and 

landed on the surface of a glacier. It accelerated as it slid across the 

glacier and poured into the Llanganuco Valley. Below it lay the tewns of 

Yungay and Ranrahircao Mixed with glacial ice, surface water, and saturated soil, the mass became a hug-e debris flow and swept down the valley 

at velocities of 280 to 400 km per hour (28). Eleven kilometers from its 

source, a relatively small lobe of debris splashed over a ridge 200 to 

250 m high and buried the town of Yungay. Almost simultaneously, the main 

debris lobe crashed into Ranrahirca, devastating it and the small villages 

around it. Within four minutes the landslide had killed more than 18,000 

people (28). 

Past Ranrahirca, the debris followed the Llanganuco Valley to its 

confluence with the Rio Santa. It then turned and flowed down the Santa 

Valley for mor-e than 50 km, inundating many agricultural fields and 

causing extensive damage to dwellings, highways, communication networks, 

and a hydroelectric facility. The Peruvian earthquake was one of the most 

destructive shocks in recent history, and this single landslide caused 

approximately 40% of the deaths and a large but undetermined portion of 

the property damage attributed to that shock (28). 

Geologic and historic evidence indicated that Yungay and Ranrahirca 

were in areas of severe landslide hazard. Eight years earlier, a debris 

flow not triggered by an earthquake had buried Ranrahirca, killing approx-

.imately 4,000 people. In fact, during the 30 years before the earthquake, 

eight destructive rock fall-avalanches and debris flows had occurred in 

the Santa Valley (28). None of these landslides were triggered by earthquakes indicating that the landslide hazard is acute there even under nonseismic conditions. According to geologic evidence, many large prehistoric 

49 
avalanches and flows also occurred in this area, and Yungay itself was 

probably built on a flow or avalanche deposit (28). If this evidence had 

been properly evaluated and acted on before the earthquake, the catastrophes at Yungay and Ranrahirca might have been averted by moving the 

communities to safer areas. 

Many hundreds of thousands of landslides have been triggered by 

earthquakes, and they have caused enormous losses of life and property. 

Many landslides occurred in areas where the historic evidence of landslide 

hazard was not as clear as in the Mount Huascaran area. In fact, many 

occurred in areas with little or no record of slope instability under 

nonseismic conditions. We are, therefore, presently studying data from 

historic earthquakes to determine which types of landslides are most 

commonly triggered by earthquakes and which geologic environments are most 

susceptible to seismically triggered slope failure. Our goal is to apply 

this kind of information to the development of criteria for mapping earthquake-induced landslide sus:ceptibili.ty. Our results are preliminary. 

This paper presents data on landslides in 15 historic earthquakes and an 

experimental susceptibility map prepared as a demonstration of how criteria developed in this study can be used. 

LANDSLIDE OCCURRENCE DURING HISTORIC EARTHQUAKES 

Landslide data have been compiled (Table 1) from 15 historic earthquakes which range in size from the great M 8.6 Kansu, China event of 1920 

to the moderate-size M 5.7 Fortuna-Rio Dell, Calif. shock of 1975. We 

have participated in field investi-:gations following the 1977 San Juan, 

Argentina, the 1976 Guatemala (11) and the 1975 Fortuna-Rio Dell (18) 

earthquakes. For the other earthquakes, we have relied almost exclusively 

on published reports. 

Landslide names used in Table 1 conform to those of Varnes (33), 

which is a revision of Varnes' earlier landslide classification system 

(32). One major difference in terminology between this report and that of 

Varnes (33) is in the use of the term "landslide." In this report, all 

types of slope failures, including falls, avalanches, and flows as well as 

true slides. are considered types of landslides. In Varnes (33), uslope 

movement" is the general tenn us.e.d L~ denote all types of 1Jlo-vement. In 

some of the categories in Table 1, two or more types of landslides are 

lumped together because the published data did not permit more subdivision. 

Only landslide~ that were clearly outlined and that moved far enough 

to form clearly identifiable deposits were included in Table 1. Thus, 

sets of cracks or fissures that may have been incipient landslides were 

not considered. Where more than one type of material or more than one 

mode of movement were involved in the landslide, it was classified by the 

predominant material and mode of movement. 

Abundances of landslides are grouped into three classes in Table 1. 

Data on landslide abundances were not always expressed quantitatively in 

published reports, so in many cases judgments were based on brief accounts 

of the geographic limits of the regions within which landsliding occurred 

and our estimates of the number of landslides per unit area in these 

regions. Geographic coverage of post-earthquake L1vestigations was probably not complete for the 1920 Kansu, 1960 Chile, 1931 Hawke's Bay, or 

1968 Inangahua earthquakes·, so the information in Table 1 may underestimate the true extent of landsliding for those events. Relative abundance 

50 
U1 

...... 

-- .. ------ --·--- - - -- --·-· .. 'a!-----

EARTHQUAKE NAME, DATE, AND MAGNITUDE 

~~ ~ 0 {il :z:::c 'tl i ~~ {il 

::r ~~ f t;~! ID ~~~~ ~t 

~~ ~a ~c \0111 ~~~~ \0 (XI ~g 

~~ \0 1\)£: 

o .. ~=- 01'!1 

~ f: "- -..J ~~ -..JC.. 0\1'1 0 ~g -..JE :s:9 :s:g Ill Ill ft 3: ~ x[Z' 3: :s:i ~!l: 3:111 3: .. 

~i (XI ~ 1"'- CXIIII -..J ~ -..J -..J 

;. :-'r; :-'~ ~ . () 

(7\ w~ :.., \0 ~tJ Ul ~oQ ~ 

~ fJ I H• 

i ~ 

... 

!11 

LANDSLIDE TYPEI 

A. LANDSLIDES IN ROCK 

ROCK FALL - AVALANCHES E s M s s 

SLUMPS AND BLOCK SLIDES M E M s s 

FALLS AND SHALLOW DISINTEGRATING SLIDES E M E M E E M E 

B. LANDSLIDES IN UNCONSOL'IDATED OR POORLY CONSOLIDATED DEPOSITS (ENGINEERING SOI :.S) 

FALLS, AVALANCHES, AND SHALLOW, DISINTEGRATING SLIDES E M M E 

SLUMPS AND BLOCK SLIDES E M E s 

RE-ACTIVATION OF DORMANT LANDSLIDES s s M 

LATERAL SPREADSII E M E M 

WET FLOWsii E M M 

SUB-AQUEOUS LANDSLIDESII E s 

c. LANDSLIDES INVOLVING ARTIFICIAL CUTS OR FILLS 

CUT-SLOPE FAILURES M M 

I LIQUEFACTION-INDUCED LANDSLIDES IN ARTIFICIAL FILLS s s 

_T..ANI)SLIDES IN ARTIFICIAL FILLS NOT CAUSED BY LIQUEFACTION s 

1

classifications after Varnes (33). 

11Liquefaction may be primary or 

contributing cause of failure. 

IIIsmall landslides are those judged 

to contain less than 10,000 m3 

of material. 

Explanation 

E - Large number of landslides 

> 5,000 sma11III landslides, or > 100 largeiV landslides, or 

Landslides common over >1000 km2 area 

M - Moderate number of landslides 

M M 

M s 

s s 

E E E 

M s 

s s 

s E s E 

s s 

s s 

M M s s 

s 

s - ~·-

References: 

Kansu, China: 5, 15 

Alaska: 19, 31 

Chile: 6, 26 

San Francisco: 36 

Hawke's Bay: 20 

Peru: 2, 8, 28 

New Madrid: 10 

Guatemala: 11, 12 

iii' {il 

~~ 8' ~r ""' 0 

~c~ ~ ... ~ ... \0 \01'1 \0111 IDrt ~ NoQ -..JI'!I 0\ID ~ID jji U:§ (XI'S 

~( :I Ill 

:tJ 3:~ I 

~8> 3:~ 

;.§ 0\1"'- U\0 

0 • 0 . () ~ .. rt 1\)~ -..Ji' Ill 

~ F" Ill ~ 

~ 1 !"' 1"'-

.... 0 0 Ill Ill ~ ~ 

1"'- 1"'-

:- !11 

s 

s M s 

M M s s s 

s M s s s 

s s s s 

s s 

s 

s 

s s s 

s s 

s s s 

IVLarge landslides are those judged 

to contain more than 10,000 m3 

500 to 5000 sma11II landslides, or 

10 to 100 largeiV landslides, or 

Landslides common over 100 to 1000 km2 area San Juan, Argentina:!. L. Youd, D. K. Keefer,and 

of material. S - Small number of landslides < 500 sma11III landslides, or 

< 10 largeiV landslides, or 

Landslides common over <100 km2 area 

No symbol - No landslides of this type reported 

others, unpub. data 

Hebgen Lake: 1, 4, 9, 25, 29, 30 

Inangahua: 21, 22, 23, 27 

San Fernando: 14, 16, 17, 35 

Borrego Mountain: 3 

Managua: 7 

Fortuna-Rio Dell: 18 
categories are defined in Tahle 1. B~cause of limitations in the data, 

numbers used to divide the categories should be regarded as first-order 

approximations. Classifications based on numbers of landslides rather 

than on area were applied wherever possible. 

The number of landslides triggered by an earthquake correlates 

strongly with earthquake magnitude. The M 8.4 Alaska earthquake of 1964 

probably triggered more landslides than any other seismic event in recent 

history. In Alaska, landslides were reported from an area of more than 

210,000 km2, which is equivalent in size to half the state of California 

(19, 31). At the other end of the scale in Table 1, theM 5.7 Fortuna-Rio 

Dell event of 1975 caused only one large rock fall and a few small debris 

slides. The Fortuna-Rio Dell earthquake should not be regarded as the 

lowest magnitude event to cause landsliding. SDaller shocks have triggered landslides, but we have not yet evaluated data from smaller earthquakes. 

In Table 2, landslides in the various categories are ranked on the 

basis of their total abundance in the earthquakes listed in Table 1. The 

ranking was developed by assigning a numerical value to the abundance 

classification of each landslide category in each earthquake: E=3, M=2, 

S=l, No symh0l=O~ For each category, these numbers were totaled for all 

15 earthquakes. The landslide categories were then ranked in order from 

highest total value through lowest. 

TABLE 2: RELATIVE ABUNDANCE OF DIFFERENT TYPES OF LANDSLIDES. 

Most Abundant Falls and shallow, disintegrating slides in rock 

Least Abundant 

Avalanches, falls, and shallow disintegrating slides 

in soil 

Lateral spreads 

Cut-slope failures 

Slumps and block slides in soil 

Slumps and block slides in rock 

Wet flows 

Rock fall-avalanches 1 Liquefaction-induced landslides in artificial fills 

Re-activation of dormant landslides1 

Landslides in artificial fills not due to liquefaction 

Sub-aqueous landslides 

The most abundant types of landslides were not necessarily the most 

damaging. The distribution of landslide damage in past earthquakes is 

partly due to the location of dwellings or other constructed works in 

certain landslide source areas or in the paths of certain landslides. 

However, landslides of several types do appear to have an inherently high 

potential for causing damage either because they tend to be large, because 

they tend to move long distances at high velocities, or both. On the 

basis of data from the 15 earthquakes listed in Table 1, the most hazardous landslides in natural materials are rock fall-avalanches, debris avalanches, wet flows; sub-aqueous landslides, and lateral spreads. Fortunately, some of these types of failures occur in relatively small numbers. 

I Identical total numerical value 

52 
LANDSLIDE DESCRIPTIONS 

Rock fall-avalanches: Rock fall-avalanches are large landslides 

that travel long distances at high velocities. The Mount Huascaran rock 

fall-avalanche contained 90 million m3 of Eaterial and flowed more than 

60 km at velocities up to 400 km per hour. Nearly all of the rock fallavalanches listed in Table 1 originated on steep mountain slopes that were 

hundreds of meters high. Most of these slopes were oversteepened by 

active fluvial or glacial erosion. The failures occurred in a wide 

variety of rock types. They were particularly common in closely jointed 

or deeply weathered rock, many hroke loose where planes of weakness 

dipped out of slopes, and many occurred in unsaturated materials. 

Slumps and block slides in rock: Deep-seated slumps and block 

slides are less common during earthquakes than rock falls and shallow, 

disintegrating rock slides. They commonly originate on steep slopes, but 

some continue to move on surprisingly gentle slopes. For example, two 

block slides triggered by the 1968 Inangahua earthquake moved on shear 

planes that dipped about 4n (21,22,23). Many slumps and block slides 

disintegrate into avalanches; but because of their relatively small size 

and short distance of travel, they generally are not as hazardous as rock 

fall-avalanches. 

Rock falls and shallow, disintegrating rock slides: Landslides of 

these types are the most abundant slope failures listed in Table 1. It 

also appears that most shallow rock slides and falls originate on slopes 

of 35° or steeper (11,30). These landslides are particularly common in 

rocks that are poorly cemented, closely jointed, or highly weathered. 

Many slopes where these landslides occur have talus accumulations at their 

bases below where the earthquake-induced landslides originate (19,30,31). 

Falls, avalanches, and shallow, iisintegrating slides in soil: On 

December 16) 1920, a large earthquake shook Kansu Province in China, parts 

of which were underlain by thick deposits of loess. Loess is a windblown 

silt held together by a clayey or calcareous binder. Under nonseismic 

conditions, it is capable of standing in high, nearly vertical slopes. 

The earthquake apparently disrupted the binder holding the loess particles 

together. The result was prob~bly the most spectacular and destructive 

series of earthquake-induced landslides in recent history. Whole villages 

and towns were buried by huge loess avalanches, and many valleys were 

filled with debris. These landslides were responsible for a large 

portion of the 200,000 deaths attributed to the shock (5). 

The 1976 Guatemala earthquake also triggered thousands of soil avalanches and slides in a weakly cemented material that forms steep, high 

slopes under nonseismic conditions (11). There, the susceptible material 

is a soil derived from volcanic pumice (11). In many other earthquakes, 

soil avalanches, falls, and shallow, disintegrating slides have occurred 

in unsaturated, sandy or silty soils with little or no cohesion. The 

landslides take place most commonly on steep stream banks, ridge flanks, 

and artificial cuts (3,7,8,10,16,17,19,26,28,31,36). 

Slumps and block slides in soil: Deep-seated soil slumps and block 

slides have also been triggered by many earthquakes listed in Table 1 (3, 

16,17,19,20,21,26,28,31,36). They take place in soils derived from many 

different types of rocks, and they originate on moderate to steep slopes. 

53 
Many earthquake-induced slumps and block slides occur in regions where 

landslide deposits are already abundant (3,16,17,36). 

Re-activation of dormant landslides in soil: A surprisingly low 

number of dormant landslides in soil were re-activated by the earthquakes 

listed in Table 1. In part, this low number reflects the lack of data on 

pre-earthquake landslides in many areas. However, the data are remarkably 

consistent and hold true even for areas such as Guatemala (11), San 

Fernando (16,17), and Fortuna-Rio Dell (18) where the pre-earthquake 

lands-lide distribution was known and where the landslide distribution was 

re-examined after the shock. It appears that earthquakes do not reactivate large numbers of dormant landslides in soil. 

Lateral spreads: Lateral spreads are characterized by coherent 

blocks of material that move laterally on layers of liquefied sand or silt 

or of weakened, sensitive clay. The blocks commonly break up along 

internal fissures, and they may also rotate or subside. The large lateral 

spread at Turnagain Heights in Anchorage, Alaska was one of the more destructive landslides triggered by the 1964 earthquake (19,24,31). It 

started at the edge of a steep, flat-topped, coastal bluff, 20 m high, and 

progressively retrograded back from the face. It caused irreparable 

damage to 72 houses. The lateral movement took place primarily on a 

layer of sensitive clay, but liquefaction in sand and silt lenses almost 

certainly contributed to the failure (19,24,31). Similar, large lateral 

spreads from steep, high bluffs also took place during the 1811 New Madrid 

(10) and 1960 Chile earthquakes (26). Lateral spreads also occurred in 

areas of generally low relief such as river flood plains and deltas 

during several earthquakes (10,19,20,26,31,35,36). 

Many seismically triggered lateral spreads, wet flows, and subaqueous landslides (see below) are caused by liquefaction. Techniques of 

assessing liquefaction susceptibility on a regional basis have been 

developed by others (37) and are briefly summarized here. Liquefaction 

commonly develops in saturated, cohesionless sediments such as sands and 

silts. Tne liquefaction susceptibility of a deposit is related to its age 

and depositional environment. River channel deposits, deltaic deposits, 

and uncompacted fills less than 500 years old have the highest liquefaction susceptibility. Other materials with a significant susceptibility 

to liquefaction include Holocene flood plain, fan delta, lacustrine, 

colluvial, dune, loess, and-. tephra deposits, and several materials deposited in coastal environments (37}. 

Wet flows: Wet flows are composed chiefly of saturated soils that 

lose much of their internal cohesion during earthquakes. Flows have been 

triggered by several earthquakes. Most originated in sandy or silty soils, 

and many formed where the slope was only a few degrees. Because they can 

travel long distances at high velocities, they are significant hazards in 

many areas. Methods of assessing the susceptibility of a soil to liquefaction-induced flow failure are discussed by Youd and Perkins (37). 

Sub-aqueous landslides: Sub-aqueous landslides triggered by the 

1959 Hebgen Lake (30), 1960 Chile (26), 1964 Alaska (19,31), and 1976 

Guatemala (12) earthquakes were complex features involving a combination 

of slumping, lateral spreading and flow. The most common failure environments were the fronts of deltas or fan deltas, and most sub-aqueous slides 

were caused, at least partially, by liquefaction. In the Alaska earthquake, massive sub-aqueous slides severely damaged the towns of Valdez, 

54 
Whittier, Seward, and Homer (19,31). At least five sub~aqueous landslides 

in the Valdez area, but not in Valdez proper, had occurred during earthquakes in the preceding 70 years. Largely as a result of the 1964 slide 

the town was relocated on a non-liquefiable site (19,31). The relatively 

small nmnher of reported sub-aqueous landslides during earthquakes (Table 

2) probably reflects, in part, the difficulty of observing landslides of 

this type. 

Cut-slope failures: Cuts increase landslide susceptibility by 

locally increasing slope steepness and by disturbing material adjacent to 

the cuts. Cut-slope failures are widespread during earthquakes (3,7,11, 

16,17,19,20,26,28,30,31,36). 

Landslides in artificial fills: The most damaging failures in artificial fills have been caused by liquefaction (26,36,37), and these have 

occurred most frequently in uncompacted saturated sandy fills (37). In 

addition, much damage to artificial fills has been caused by liquefaction 

in subjacent materials (19,31). Other landslides not induced by liquefaction have occurred in artificial fills during earthquakes (3,7,16,17,20, 

26,28,36). MOst failures of this type were small slumps or debris slides. 

The relatively small number of reported landslides in artificial fills is 

due, in part, to the fact that artificial fills are much less abundant 

than natural slopes. 

SUMMARY OF LANDSLIDE TYPES AND FAILURE ENVIRONMENTS 

Our study has identified several types of landslides that are 

triggered by earthquakes and some environments in which failures occur. 

Our preliminary findings are summarized in Table 3. As our study progresses, we anticipate that other environments with high susceptibilities 

to seismically induced landsliding will be identified and that categories listed in Table 3 will be refined and subdivided for better 

assessments. 

PRELIMINARY MAP OF SEISMICALLY INDUCED LANDSLIDE SUSCEPTIBILITY OF THE 

LA HONDA AREA 

Figure 1 is an experimental susceptibility map of an area near the 

town of La Honda, in the Santa Cruz Mountains, 40 km south of San Francisco, 

Calif. The map was prepared to demonstrate how the preliminary criteria 

summarized in Table 3 can he used -to evaluate susceptibility to earthquakeinduced landsliding. The preliminary La Honda map (Fig. 1) is subject to 

rev1s1on as our study of landsliding during historic earthquakes progresses. 

Furthermore, it is a regional map which shows only zones where unfavorable 

site conditions may exist. Local site conditions, not determined in this 

study, govern whether specific sites within these zones are actually susceptible to failure. Therefore, it is not intended for use in determining 

the susceptibility to failure of any given site. Any such site-specific 

assessment should be carried out by a professional-engineering geologist. 

Data on the La Honda area came from a detailed engineering geologic 

field investigation (34), a published map of unconsolidated Quaternary 

materials (13), and a computer-generated slope map. On the basis of this 

information and our review of landsliding during historic earthquakes, 

three types of zones with high susceptihilities to earthquake-induced 

landslides were identified (Fig. 1}. These are areas with slopes greater 

than 35°, areas underlain by saturated sandy and silty Holocene alluvium, 

55 
FIGURE 1: PRELIMINARY SEISMICALLY INDUCED LANDSLIDE SUSCEPTIBILITY 

MAP OF THE LA HONDA AREA 

CONTOUR INTERVAL 40 FEET 

0 1 KILOMETER 

0 1 MILE 

ZONES OF HIGH SUSCEPTIBILITY TO SEISMICALLY INDUCED LANDSLIDES 

SYMBOL POSSIBLE LANDSLIDE TYPES 

~ SOIL AVALANCHES;FALLS,SLUMPS,BLOCK 

~ SLIDES AND SHALLOW DISINTEGRATING 

SLIDES IN SOIL AND/OR ROCK 

~tl LATERAL SPREADS AND WET FLOWS 

~ SLUMPS AND BLOCK SLIDES IN SOIL 

AND/OR ROCK 

56 

MAPPING CRITERIA 

SLOPES 

STEEPER 

THAN 35° 

UNDERLAIN BY SATURATED SANDY AND 

SILTY HOLOCENE ALLUVIUM 

ACTIVE AND RECENTLY ACTIVE 

LANDSLIDES 
and areas underlain by active and recently active landslides (34). Landslides judged to be dormant (34) are not included in the zones of high 

susceptibility. 

TABLE 3: PRELIMINARY ASSESSMENT OF SEim.1ICALLY INDUCED 

LANDSLIDE SUSCEPTIBILITY 

LANDSLIDE TYPES SOME ENVIRONMENTS IN WHICH LANDSLIDES ARE LIKELY 

Rock fall-avalanches 

Slumps and block 

slides in rock 

Rock falls and 

shallow, disintegrating rock slides 

Falls, avalanches, 

and shallow, disintegrating slides 

in soil 

Slumps and block 

slides in soil 

Re-activation of 

dormant landslides 

in soil 

Lateral spreads 

Wet flows 

TO OCCUR 

Very steep, very high slopes that are composed 

chiefly of deeply weathered, closely jointed, 

or highly sheared rock and that are being oversteepened by active erosion 

MOderate to steep slopes in many different types 

of rock 

1. Rock slopes steeper than 35° 

2. Rock slopes with talus accumulations at their 

bases 

3o Slopes containing weathered, sheared, or 

closely jointed rock 

1. Steep, high slopes containing certain kinds of 

weakly cemented materials such as some kinds 

of loess and some volcanic pumice soils 

2. Steep ridge flanks and stream banks containing 

unsaturated sandy or silty soils with little 

or no cohesion 

Moderate to steep slopes in soils derived from 

many different types of rocks, particularly 

in areas with abundant, preexisting landslide 

deposits 

Only relatively small numbers of dormant landslides are re-activated by earthquakes 

1. Areas of generally low relief underlain by 

many different kinds of unconsolidated saturated Holocene sandy or silty materials 

2. Steep, high bluffs containing layers of sensitive clay or liquefiable sand or silt as 

defined in 1. above 

3. Uncompacted saturated sandy artificial fills 

Steep to very gentle slopes underlain by some 

kinds of saturated sandy or silty soils 

57 
TYPES (can't.) 

Sub-aqueous 

landslides 

Cut-slope failures 

Liquefactioninduced landslides 

in artificial fills 

Landslides in artificial fills not 

caused by liquefaction 

CONCLUSIONS 

TABLE 3: (can't.) 

ENVIRONMENT (con' t . ) 

Fronts of deltas or fan deltas, particularly in 

areas underlain by saturated, liquefiable 

sand or silt 

Artificial cuts 

Uncompacted, saturated, sandy fills 

Many different types of fills 

Predominant types of earthquake-induced landslides and some environments with high susceptibilities to failure have been identified from data 

about 15 historic earthquakes. Landslides that occurred in greatest 

numbers in the 15 earthquakes were: falls and shallow, disintegrating 

slides in rock; falls, avalanches, and shallow, disintegrating slides in 

soil; lateral spreads; cut-slope failures; slumps and block slides in soil; 

and slumps and block slides in rock. Many other kinds of earthquake-induced landslides occur relatively infrequently but have high inherent 

potentials for causing loss of life and property. Landslides that have 

caused extensive loss of life or property during historic earthquakes 

include rock fall-avalanches, soil avalanches, lateral spreads, wet flows, 

sub-aqueous landslides, and liquefaction-induced landslides in artificial 

fills. Some environments with high susceptibilities to seismically induced 

landsliding have been identified on a preliminary basis. As our studies 

progress, it is anticipated that these categories will he refined and that 

new environments with high susceptibilities will be identified. 

REFERENCES 

(1) Billings Geological Society, 1960, Earthquake papers: Billings, 

Montana, Eleventh Annual Field Conference Guidebook, p. 24-85. 

(2) Browning, J.M., 1973, Catastrophic rockslide, Mount Huascaran, Northcentral Peru, May 31~ 1970: American Association of Petroleum 

Geologists Bulletin, v. 57, no. 7, p. 1335-1341. 

(3) Castle, R.O., and Youd, T.L., 1972, Engineering geology, in The 

Borrego Mountain earthquake of April 9, 1968: U.S. Geological 

Survey Professional Paper 787, p. 158-174. 

(4) Christiansen, R.L., and Blank, H.R., 1972, Volcanic stratigraphy of 

-the Quaternary rhyolite plateau in Yellowstone National Park: 

U.S. Geological Survey Professional Paper 729-B, 18p. 

(5) Close, U. and McCormick, E., 1922, Where the nountains walked: 

National Geographic, v. 41, no. 5, p. 445-472. 

58 
(6) Duke, C.M., and Leeds, D.J., 1963, Response of soils, foundations, 

and earth structures: Seismological Society of America Bulletin, 

v. 53, no. 2, p. 309-357. 

(7) Earthquake Engineering Research Institute Conference Proceedings, 

1973, Managua, Nicaragua earthquake of December 23, 1972: v. 1, 

p. 8-264. 

(8) Earthquake Engineering Research Institute Earthquake Report Committee, 

1970, Peru earthquake of Nay 31, 1970, preliminary report: 

Earthquake Engineering Research Institute, 55 p. 

(9) Fischer, W.A., 1960, Highlights of Yellowstone geology with an interpretation of the 1959 earthquakes and their effects in Yellowstone 

National Park: Yellowstone National Park, 62 p. 

(10) Fuller, M., 1912, The New Madrid earthquake; U.S. Geological Survey 

Bulletin 394, 118 p. 

(11) Harp, E.L., Wieczorek, G.F., and Wilson, R.C., 1978, Earthquakeinduced landslides from the February 4, 1976, Guatemala earthquake 

and their implications for landslide hazard reduction: 

International Symposium on the February 4th, 1976 Guatemalan 

Earthquake and the Reconstruction Process, v. 1. 

(12) Hoose, S.N., Wilson, R.C., and Rosenfeld, J.H., 1978, Liquefactioncaused ground failure during the February 4, 1976, Guatemala 

earthquake: International Symposium on the February 4th, 1976 

Guatemalan Earthquake and the Reconstruction Process, v. 2. 

(13) Lajoie, K.R., Helley, E.J., Nichols, D.R., and Burke, D.B., 1974, 

Geologic map of unconsolidated and moderately consolidated deposits 

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

Miscellaneous Field Studies Map MF-575, Basic Data Contribution 6. 

(14) Marachi, N.D., 1973, Dynamic soil problems at the Joseph Jensen 

Filtration Plant in Murphy, L.M., scientific coordinator, San 

Fernando, California earthquake of February 9, 1971: National 

Oceanic and Atmospheric Administration, v. 1, part B, p. 815-820. 

(15) Meyers, H., and von Hake, C.A., 1976, Earthquake data summary: 

Boulder, Colorado, National Oceanic and Atmospheric Administration, 

Environmental Data Service, Key to Geophysical Records Documentation 

No. 5, 54 p. 

(16) Morton, D.M., 1971, Seismically triggered landslides in the area ahove 

the San Fernando Valley, in The San :Fernando 1 California earthquake 

of February 9, 1971: U.S. Geological Survey Profes.sional Paper 733~ 

p. 99-104 

(17) Morton, D.M., 1975, Seismically triggered landslides in the area 

above the San Fernando Valley, in The San Fernando, California 

earthquake of February 9, 1971: California Division of Mines and 

Geology Bulletin 196, p. 145-154 

(18) Nason, R., Harp, E.L., La Gresse, H., and Malley, R.P., 1975, 

Investigations of the 7 June 1975 earthquake at Humbolt County, 

California, U.S. Geological Survey Open-File Report 75-404, 37 p. 

(19) National Academy of Sciences, 1968-73, The great Alaska earthquake of 

1964: Washington, D.C., National Academy of Sciences Printing and 

Publishing Office, v. 1, Geology, and v. 3, Hydrology. 

(20) New Zealand Department of Scientific and Industrial Research, 1933, 

The Hawke's Bay earthquake of 3rd February, 1931: New Zealand 

Journal of Science and Technology, v. 15, no. 1, p. 1-116. 

(_21) New Zealand Department of Scientific and Industrial Research, 1968, 

Preliminary report on the Inangahua earthquake, New Zealand, May, 

1968: Research Bulletin 193, 39 p. 

59 
(22) New Zealand Society for Earthquake Engineering Bulletin, 1968, The 

1968 Inangahua earthquake (conference seminar papers): v. 2, 

no. 1, 148 p. 

(23) New Zealand Society for Earthquake Engineering Bulletin, 1969, 

Special Inangahua reports: v. 2 , no. 1, 148 p. 

(24) Seed, H.B., 1968, Landslides during earthquakes due to soil liquefaction: Journal Soil Mechanics and Foundations Div., American 

Society of Civil Engineers, v. 94, no. SM 5, p. 1055-1122. 

(25) Seismological Society of America Bulletin, 1962, The earthquake at 

Hebgen Lake, Montana on August 18, 1959 (GCT): v. 52, no. 2, 

p. 153-273. 

(26) Seismological Society of America Bulletin, 1963, Special issuP on 

Chilean earthquakes of May 1960: v. 53, no. 6, p. 1123-1441. 

(27) Seismological Society of America Bulletin, 1970, The 1968 Inangahua 

earthquake: Report of the University of Canterbury Survey team: 

v. 60, no. 5, p. 1561-1606. 

(28) Seismological Society of America Bulletin, 1971, Special_ issue on 

Peru earthquake of May 31, 1970: v. 61, no. 3, p. 511-640. 

(29) U.S. Coast and Geodetic Survey, 1961, Abstracts of earthquake reports 

for the Pacific Coast and Western Mountain region, MSA-103, July, 

August, September, 1959: p. 51-189. 

(30) U.S. Geological Survey, 1963, The Hebgen Lake, Montana earthquake of 

August 17, 1959: U.S. Geological Survey Professional Paper 435, 242p. 

(31) U.S. Geological Survey, 1965-1970, The Alaska earthquake, March 27, 

1964: U.S. Geological Survey Professional Papers 542-A, 542-B, 542-C, 

542-D, 542-E, 542-G, 543-B, 543-E, 543-F, 544-A, 544-B, 544-D, 545-A, 

545-C, 545-D. 

(32) Varnes, D.J., 1958, Landslide types and processes, in Eckel , E.B., 

ed., Landslides and engineering practice: Righway Research Board 

Special Report 29, National Academy of Science - National Research 

Council Publication 554, p. 20-47. 

(33) Varnes, D.J., in press, Slope movement types and processes: Chapter 2 

in Schuster, R.L., and Krizek, R.S., eds., Landslides--analysis and 

control: Transportation Research Board Special Report 176, 

National Academy of Sciences. 

(34) Wieczorek, G.F., 1978, Landslide susceptibility evaluation in the 

Santa Cruz Range, San Mateo County, California, Ph.D. thesis, 

University of California, Berkeley, 278 p. 

(35) Youd, T.L., 1971, Landsliding in the vicinity of the Van Norman Lakes 

in the San Fernando, California earthquake of February 9, 1971: 

U.S. Geological Survey Professional Paper 733, p. 105-109. 

(36) 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. 

(37) Youd, T.L., and Perkins, D.M., 1978, Mapping liquefaction-induced 

ground failure potential: Journal of the Geotechnical Engineering 

Div., American Society of Civil Engineers, v. 104, no. GT4, 

p. 433-446. 

60 
EARTHQUAKE LOSSES TO BUILDINGS 

IN THE 

SAN FRANCISCO BAY AREA 

by 

S. T. Algermissen and K. V. Steinbrugge1 

ABSTRACT 

Losses in the San Francisco Bay area are simulated for five broad 

classes of buildings which include the majority of building types found in 

the area. Losses are expressed in terms of the average percentage of the 

total actual cash value required to fully repair, in kind, any building in 

a particular building class. The inventory of dwellings was obtained from 

census data, and the inventory for buildings other than dwellings is derived 

from land-use classification in the San Francisco Bay area supplemented by 

minimal field work. Long-term annual losses for the various building 

classes range from 0.1 to 1.6 percent. For a large earthquake on the San 

Andreas fault, the range in percent loss is about 5.0 to 25 percent, 

depending on the building class. The 1970 value of dwellings in the ninecounty Bay area was $30.289 billion. Annual average losses to dwellings is 

estimated at $271 per dwelling. The average loss per dwelling as a result 

of a large earthquake on the San Andreas fault would be about $1,355; the 

total loss (1970), about $1.5 billion. 

INTRODUCTION 

The primary economic losses to buildings as a result of earthquakes 

depend on three principal factors: (1) the spatial distribution of the 

various kinds of buildings exposed to ground shaking and geological 

hazards (landslides, liquefaction, surface faulting, etc.); (2) the 

spatial distribution of earthquake shaking and geological hazards associated with an earthquake; and (3) the susceptibility of each building 

class to loss. The purpose of this study is to present a methodology for 

the estimation of losses to buildings other than dwellings to complement 

earlier studies of single-family dwellings and to provide a general 

technique for the estimation of (a) total losses from single large earthquakes and (b) average losses resulting from earthquakes over a period of 

time. The nine-county San Francisco Bay area (fig. 1) is used to t-e&t the 

methodology for the calculation of losses resulting from earthquakes. The 

losses computed are expressed in terms of percent loss by class of construction. The percent loss is defined here to mean the average percentage 

of the total actual cash value required to fully repair, in kind, any 

building of a particular class represented by a particular degree of 

Modified Mercalli Intensity Scale. Only losses associated with ground 

shaking are estimated. 

l}mnager, Earthquake Department, Insurance Services Office, San 

Francisco, California. 

61 
METHODOLOGY 

Losses to buildings other than one- to four-family dwellings have been 

calculated as follows: 

(1) The spatial distribution of buildings throughout the nine-county 

area is approximated by class of construction for each U.S. Census 

Tract in the study area. 

(2) Relationships are developed between percent loss (as previously 

defined) and the various degrees of the Modified Mercalli (MM) 

Intensity Scale--the measure of ground shaking used in this study. 

(3) MM intensities for each census tract are determined for (a) individual 

earthquakes of interest and (b) ensembles of earthquakes. 

(4) The results from (1), (2), and (3) are used to calculate the loss to 

each class of construction that can be attributed to the occurrence of 

a particular earthquake or series of earthquakes. 

For example, the percent loss to buildings in, say, class C, due to a 

particular earthquake, is 

where: 

P(C). 1 

(1) 

percent of the buildings of class C in the nine-county area 

that are located in census tract (i) 

percent loss to buildings of class C when shaken at MM 

intensity I in census tract (i). 

The summation is made over the census tracts of interest. 

Methods for the estimation of earthquake losses to dwellings in 

California were developed in earlier studies (references 1, 2, 3, 4), and 

so no detailed description of the methods will be given here. 

BUILDING CLASSIFICATION 

An abridged description of the building classification used in this 

earthquake-loss simulation study is shown in table 1. These classifications are similar to those used by the majority of the property-casualty 

insurance companies in the United States and have the advantage of over 50 

years experience, including testing after earthquakes. 

62 
Notation used 

in loss tables 

3 and 4 

lA 

18 

2A 

28 

3U 

3LB 

3LC 

3LD 

3~. 3HB, 

JHC, 3HD 

4U 

4U 

4LC 

4LD 

4LE 

4~. 4HB, 

4HC, 4HD, 4HE 

SA 

SB 

5C 

SD 

5E 

SF 

Table 1.--Building classification used in this study 

Brief description of subclasses of five broad building classes 

Wood-frame and frame-stucco dwellings. 

Wood-frame and frame-stucco buildings not qualifying under lA (usually large-area nonhabitational 

units); (not considered in this study). 

One story, all metal; floor area less than 20,000 feet 2 • 

All metal buildings not considered under 2A. 

Steel frame, superior damage-control features; less than four stories. 

Steel frame; ordinary damage-control features; less than four stories. 

Steel frame; intermediate damage-control features (between 3LA and 3LB); less than four stories. 

Floors and roofs not concrete; less than four stories. 

Descriptions are the same as for 3LA, 3LB, 3LC, and 3LD except that buildings have four or more 

stories. 

Reinforced concrete; superior damage-control features; less than four stories. 

Reinforced concrete; ordinary damage-control features; less than four stories. 

Reinforced concrete; intermediate damage-control features (between 4LA and 4LB); less than four 

stories. 

Precast reinforced concrete, lift slab, less than four stories. 

Floors and roofs not concrete, less than four stories. 

Descriptions are the same as for 4LA, 4LB, 4LC, 4LD, and 4LE except that buildings have four or 

more stories. 

Dwellings, not over two stories in height, constructed of (a) poured-in-place reinforced concrete, 

with roofs and second floors of wood frame or (b) adequately reinforced brick or hollow-concreteblock masonry, with roofs and floors of wood (not considered in this study). 

One-story buildings having superior earthquake damage-control features, including exterior walls of 

(a) poured-in-place reinforced concrete, and (or) (b) precast reinforced concrete, and (or) (c) 

reinforced brick masonry or reinforced-concrete brick masonry, and (or) (d) reinforced hollowconcrete-block masonry. Roofs and supported floors are of wood or metal-diaphragm assemblies. 

Interior bearing walls are of wood frame or any one, or a combination, of the aforementioned wall 

materials. 

One-story buildings having construction materials listed for Class SB, but with ordinary earthquake 

damage-control features. 

Buildings having reinforced concrete load-bearing walls and floors and roofs of wood, but not 

qualifying for Class 4E; and buildings of any height having Class 5B materials of construction, 

including wall reinforcement; also included are buildings with roofs and supported floors of reinforced concrete (precast or otherwise) not qualifying for Class 4. 

Buildings having unreinforced solid-unit masonry of unreinforced brick, unreinforced concrete brick, 

unreinforced stone, or unreinforced concrete, where the loads are carried in whole or in part by the 

walls and partitions. Interior partitions may be wood frame or any of the aforementioned materials. 

Roofs and floors may be of any material. Not qualifying are buildings having nonreinforcPd load 

walls of hollow tile or other hollow-unit-masonry, adobe, or cavity construction. 

Buildings having load-carrying walls of hollow tile or o~her hollow-unit-masonry construction, adobe. 

and cavity-wall construction, and any building not covered by any other class (not considered in this 

study). 

63 
BUILDING INVENTORY 

The inventory was developed in the following manner: 

(1) A direct correlation was assumed to exist between specific building 

classes and land-use designations. For example, large-area buildings 

that constitute subclass 2B, all-metal structures, are usually aircraft 

hangars, steel plants, major manufacturing facilities, or large warehouses, and, accordingly, are situated in land-use areas primarily 

zoned for industrial purposes. This assumption makes possible the 

determination of the geographic distribution of building classes 

throughout the study area. Modification of this assumption for any one 

building class or subclass was made on the basis of field sampling and 

personal knowledge. 

(2) All building classes and building values were assumed to be uniformly 

distributed within the designated mapped zones. An equal distribution 

of building value is reasonably consistent with policy assumed in 

zoning ordinances formulated by the respective county planning commissions and regional agencies. This assumption was modified when field 

sampling and personal knowledge of the authors indicated that it gave 

obviously incorrect results. 

(3) Nonconforming uses are not included in the geographic distribution of 

building classes. In addition, small isolated pockets of semicommercial developments in suburban areas are not considered. In comparison 

to the major commercial areas tabulated, the values of these semicommercial developments are relatively insignificant. Insofar as 

possible, their values are included in the nearest major commercial 

area and are accounted for by a factor related to population distribution based on the 1970 census. In any event, visual surveys indicate 

that, except for the all metal gasoline service stations located in 

these random pockets, the majority of these structures are wood-frame 

buildings. 

(4) Land-use data obtained from the "Atlas of Urban and Regional Change" 

(5) were plotted on the "Census Tract Outline Map" (6), and the landuse designations were converted to the appropriate building class. 

Data compatibility with the respective land-use maps provided by the 

various county planning commissions was confirmed by cross-checking 

data sources. Mapped results were partially verified through data 

collected from the detailed city and street maps available for urban 

centers located in the San Francisco Bay area. 

(5) Final mapping results were checked using visual field surveys of 

critical areas. 

(6) Special service areas found in the San Francisco Bay area, such as 

San Francisco Presidio, Port of San Francisco, Hamilton Air Force Base, 

Moffett Naval Air Field, U.S. Naval Magazines at Port Chicago and 

Concord, Oakland Army Base, Oakland Naval Supply Center, ~imitz 

Field Naval Air Station in Alameda, San Quentin Penitentiary, and Mare 

Island, among others, were not included in the mapping of building 

classes. 

64 
(7) The area within each census tract having a particular land-use code 

(equated to building class) was measured. The area of each building 

class in each census tract was then summed to determine the total area 

of a particular building class in the nine counties considered in this 

study. The percentage of any building class in any census tract is 

then: 

Area of the particular building class in the census tract 

Total area of that building class in the nine-county area 

xlOO 

A more complete discussion of the development of the building inventory, 

together with examples, may be found in (7). 

GROUND SHAKING - LOSS RELATIONSHIPS 

The estimation of losses resulting from earthquakes requires that 

relationships be known or developed between the intensity of ground shaking 

and the degree of damage to structures by class of construction (fig. 2). 

The measure of the intensity of ground shaking used in this study is the 

Modified Mercalli Intensity Scale (8). Limitations of the scale have been 

discussed in several papers (for example, 9, 10). 

Development of the loss-intensity relationships used in this study 

entailed three steps: (1) examination of loss for a number of earthquakes; 

(2) analysis of existing building cost data; and (3) integration of (1) and 

(2), using engineering judgment based on actual earthquake experience, into 

loss-intensity relationships. Because of the large number of classes of 

construction and the many construction components included in nondwelling 

classes, the present attempt to develop loss-intensity relationships must be 

considered as preliminary. 

The most useful published sources of loss data are found in the studies 

of the most recent earthquakes, although data extending back to the 1906 San 

Francisco shock still have substantial value. A review of several publications showed that the damage data in the publications are not usually 

compatible. Further, a more detailed review of all major sources shows that 

data are far from complete for all intensities for all building classes. It 

then follows that interpolation and judgment must be used with the published 

record of actual losses to produce loss values. 

Analysis of existing building cost data has shown that the variations 

in the cGst percentages among construction components for any particular 

building cla.ss permit only very approximate loss estimates when applying 

loss averages to any particular building. 

Th-e development of loss-intensity relationships, then, requires the 

integration of actual earthquake loss with current cost data. It also 

requires the interpretation of earthquake loss data and their relation to 

each class of construction in terms of the Modified Mercalli Intensity 

Scale. Actual earthquake losses must be analyzed with relation to isoseismal maps that have been prepared for recent earthquakes. It is 

65 
important to note that this step effectively amounts to a more definitive 

description of losses at each intensity level than exist in the original MM 

Intensity Scale. In this sense, development and loss-intensity relationships 

for the various construction classes represents a further definition or 

refinement of the MM scale based on an analysis of loss experience and cost. 

At the present time we believe that MM intensity maps, together with the 

loss-intensity relationships developed using relevant experienced judgment, 

are the best bases for this kind of study. Indeed it is the only basis for 

which extensive data are available. 

For damage-analysis purposes, the lower intensity limit of the Modified 

Mercalli Intensity Scale is the threshold of damage, which varies with the 

kind of building as well as with the kind of ground motion but is generally 

VI or VII. The threshold normally includes "imaginary" damage, which may 

decrease the actual lower limit by one intensity unit. By "imaginary" 

damage we mean damage that the owner/occupant believes occurred during the 

shock, but which was actually in existence before the earthquake. On the 

other hand, the upper intensity limit is determined by the intensity at 

which ground-vibration effects to buildings are overshadowed by geologic 

effects, such as landsliding, faulting, and failures of structurally poor 

ground. This upper limit is normally given as MM intensity IX in insurance 

practice. This limit of IX is somewhat arbitrary since vibrational effects 

on buildings will increase somewhat with increasing intensity, but becomes 

overshadowed by building damage resulting from geological effects. 

Percent loss-MM intensity relationships are, in general, not linear. 

The general, qualitative characteristics of loss-intensity curves for 

various building classes have been discussed elsewhere (2, 7); the preliminary nature of this study precluded the determination of the detailed 

shape of the intensity-loss curves. As usable approximations, linear 

relationships were developed. In figure 2, percent losses from 0 to 10 have 

been estimated to the nearest 1 percent. Percent losses above 10 percent 

are estimated to the nearest 5 percent. 

ESTIMATION OF GROUND SHAKING 

MM intensities were assigned to all census tracts for all earthquakes 

of interest. Two techniques were used to assign intensities to census 

tracts. For earthquakes having maximum epicentral intensities from VI to 

VIII, average isoseismal maps were used. For earthquakes having maximum 

intensities greater than VIII, isoseismal maps were constructed using data 

from special studies (11, 12). 

Average isoseismal maps (for earthquakes VI < I 0 < VIII) were constructed in the following manner: 

(1) The average area shaken at each intensity level was determined for each 

earthquake in the San Francisco Bay area for which an isoseismal map 

was available. The same general approach was used in an earlier study 

of single-family dwellings (3, 4), and all of the intensity data used 

in that study, together with additional new material, was used to 

construct average isoseismal maps. 

66 
(2) Isoseismal patterns were considered to be elongated in the direction of 

faulting. This holds true in the San Francisco Bay area because earthquakes are shallow (< 15 km) and strike-slip faulting predominates, at 

least in the larger earthquakes that are of most interest to this 

study. 

(3) Using M=l+2/3 I 0 , where I 0 is the maximum MM intensity and M is magnitude, and log L=-0.39+0.34 M, where L is fault rupture length (3) in 

kilometers, the shapes of average isoseismal maps were constructed 

using 

(see fig. 3) 

w~e:e AI=area enclosed by the intensity I isoseismal plus higher intens~t~es 

and 

w1=width of zone bounded by the intensity I isoseismal plus 

higher ~ntensities. 

(4) The orientation of the isoseismals (the strike of L in fig. 3) for any 

particular historical earthquake was taken to be the same as the 

strikes of faults in the San Francisco Bay area that are known to have 

been active near the earthquake epicenter during or since Quaternary 

time (13). Earthquakes that could not be associated with specific 

faults were assumed to have circular isoseismals. Table 2 gives the 

values of W (fig. 3) and L for elongated isoseismals and the radius R 

for circular isoseismals for all intensities associated with earthquakes 

of maximum intensities VI through VIII. 

Table 2.--Parameters for construction of average isoseismals 

for VI~Io~VIII 

Maximum intensity 

Io and associated Fault length Width 1 Radius 2 

intensities I L (km) w (km) R (km) 

VIII: 58 1.65 8.00 

VII 5.90 15.55 

VI 17.54 30.90 

VII: 35 3.17 8.92 

VI 9.98 17.84 

VI 20 3.95 8.18 

1The width (W) and radius (R) are the maximum distances fer each 

intensity. 

2The radius (R) is for the circular isoseismals, which are assigned to 

earthquakes not associated with specific faults. 

67 
(5) The isoseismals constructed for earthquakes having VI~Io~VIII average 

the effects on surficial materials over broad areas. Consequently, the 

intensity of shaking at individual sites may differ considerably from 

on the average intensity map. The effect of site amplification was, to 

some extent, taken into account by dividing the surficial geology 

throughout the area into three units. Incremental intensities were 

assigned to these three units and added to or subtracted from the 

average intensities. The units and incremental intensities are bay mud 

(+1.0); alluvium (0); and bedrock (-1.0). These units are defined in 

more detail and discussed by Borcherdt et al., (12). The incremental 

intensities assigned here differ slightly from, but are in general 

agreement with, those suggested in (12). 

LOSS CALCULATIONS 

Losses were calculated for each class of construction using earthquakes 

that occurred during the following time intervals: 1800-1974 (175 years); 

1800-1899 (100 years); 1900-1974 (75 years); and 1907-1974 (68 years). The 

losses are expressed as the average annual percent loss to buildings of each 

construction class in the nine-county San Francisco Bay area. The inventory, 

exclusive of wood-frame dwellings (Class lA), is updated to 1973. Class lA 

inventory is based on 1970 United States census data. The results are shown 

in table 3. Losses for two large earthquakes of interest in the Bay area--

one, a maximum intensity X on the San Andreas fault, and the other, a maximum 

intensity IX on the Hayward fault--are shown in table 4. 

DISCUSSION OF RESULTS 

The four samples of historical seismicity data shown in table 3 indicate 

that, in general, the largest losses are associated with the 100-year period 

1800-1899. This is a consequence of the three large earthquakes that 

occurred during the period: in 1838 on the San Andreas fault, and in 1836 

and 1868 on the Hayward fault. In addition, the percent losses to buildings 

are considered constant (at the intensity IX level) for intensities above 

IX. Actual losses increase for intensities greater than IX, but the losses 

are increasingly caused by geologic effects that have not been considered in 

this report. The earthquakes of 1836, 1838, and 1868 are believed not to 

have been as large as the 1906 earthquake, but they would be about as 

effective in causing vibration damage. It is interesting to note also that, 

even though the earthquake history of the San Francisco Bay area is probably 

not complete for damaging shocks for the 1800-1899 period, the losses are 

generally the highest of any time interval considered. Best long-term 

average loss estimates are probably obtained from the losses simulated using 

either the 175-year seismicity sample (1800-1974), the 100-year sample 

(1800-1899), or the 75-year sample (1900-1974) even though the average 

losses computed using these three seismicity samples are all low, but for 

different reasons: The time spans that include 19th century data are 

incomplete, whereas the 1900-1975 sample contains only one large shock 

(1906). Data for the 68-year period, 1907-1974, are reasonably complete for 

damaging shocks, but yield useful average losses only if one believes that 

large damaging earthquakes will not occur in the Bay area during some period 

of interest in the future. 

68 
The losses shown in table 4, simulated for a maximum MM intensity X 

earthquake on the San Andreas fault and a maximum intensity IX earthquake on 

the Hayward fault, seem to indicate that the percent losses for the two 

earthquakes would be very similar from the point of view of vibration 

damage. Nearly equal losses result partly from the flat shape of the 

intensity-loss curves for intensities above IX and partly from the geometry 

of the Bay area and the high concentration of buildings in the east Bay 

area. Dollar loss estimates are available only for one- to four-family 

dwellings, the value of the dwellings being obtained from census data as 

previously reported (1, 2, 3, 4). There were approximately 1.1175 million 

one- to four-family dwellings in the nine-county study area having a replacement cost value of $30.3 billion and an average value of $27,100 in 1970. A 

conservative long-term annual average loss (based on table 3) might be 1 

percent of the replacement cost value or about $271. Average total losses 

to dwellings as a result of a large earthquake would be about 5 percent of 

value, or $1.51 billion, or an average loss per dwelling of $1,360. 

Table 3.--Estimated average annual Eercent losses in the nine-count~ 

San Francisco Ba~ areaz based on the historical seismicit_l 

during four time intervals 

Building 175 years 100 years 75 years 68 years 

Subclass (1800-1974) (1800-1899) (1900-1974) (1907-1974) 

J:/lA 0.702 0.663 0.755 0.692 

2A 0.131 0.153 0.102 0.043 

2B 0.160 0.201 0.106 0.028 

3LA -Q.l66 0.198 0.123 0.032 

3LB 0.822 0.820 -0.826 0.652 

3LC 0.208 0.248 0.154 0.040 

3LD 0.822 0.820 o-. 826 0.652 

3HA 0.260 --o. 357 0.132 0.008 

3HB 0.539 0.705 0.317 0.098 

3HC 0.325 0.446 0.165 0.010 

3HD 0.539 0.705 0.317 0.098 

4LA 0.208 0.248 0.154 0.040 

4LB 1.175 1.171 1.180 0.931 

4LC 0.291 0.347 0.215 0.056 

4LD 1.410 1.406 1.416 1.117 

4LE 1.292 1.288 1.298 1.024 

4HA 0.325 0.·446 0.165 0.010 

4HB 0.769 1.007 0.453 0.139 

4-HC 0.539 0. 705 0.317 0.098 

4HD 0.923 1.208 0.543 0.167 

4HE 0.846 1.108 0.498 0.153 

5B 0.208 0.248 0.154 0.040 

5C 0.822 0.820 0.826 0.652 

5D 1.057 1.054 1.062 0.838 

5E 1.521 1.589 1.430 1.083 

I Class lA inventory is based on 1970 census data. 

69 
Table 4.--Estimated percent losses for two large 

earthquakes in the nine-county San 

Francisco Bay area 

Build in& 

Subclaaa 

lA 

2A 

3LA 

3LB 

3LC 

3LD 

3HA 

3HB 

3HC 

3HD 

4LA 

4LB 

4LC 

4LD 

4LE 

4HA 

4HB 

4HC 

4HD 

4HE 

5B 

5C 

5D 

5E 

Percent losses for 

HM intensity X on 

the San Andreas 

fault 

5.04 

4.17 

6.59 

13.07 

8.24 

13.07 

9.34 

16.73 

11.67 

16.73 

8.24 

18.68 

13.07 

22.47 

20.54 

11.67 

23.89 

16.73 

28.67 

26.28 

8.24 

13.07 

16.81 

25.41 

Percent loaaea for 

MH intensity IX on 

the Hayward fault 

4.03 

3.38 

5.15 

11.47 

6.44 

11.47 

9.42 

16.82 

11.78 

16.82 

6.44 

16.30 

11.47 

19.56 

20.65 

11.78 

24.04 

16.82 

28.84 

26.44 

6.44 

11.47 

14.74 

22.51 

I.O ~ 1'1 MILES i I I I 

10 20 J0 .. 0 KilOMETERS 

Figure 1.--Location map of northern California 

showing the nine-county area studies in this 

report. 

70 
35r---------~--------~--------.----------~ 

..._ 

z 

UJ 

u 15 0:: 

UJ 

a.. 

5 

v VI VII VIII 

MM INTENSITY 

Figure 2.--Modified Mercalli intensity--loss relationship 

(by class of construction) used in this study. Descriptions of the various classes may be found in table 1. 

High-(H) and low-(L)rise subclasses of class 3 and class 

4 have been combined. 

~--------------L----------------~ 

Figure 3.--Schematic showing the construction of average 

isoseismal.maps (see text for discussion). 

71 
REFERENCES 

1. U.S. Coast and Geodetic Survey, 1969, Summary and recommendations, in 

Studies in seismicity and earthquake damage statistics, 142 p. 

2. Steinbrugge, K. V., McClure, F. E., and Snow, A. J., 1969, Appendix A, 

in Studies in seismicity and earthquake damage statistics: U.S. 

Coast and Geodetic Survey, 142 p. 

3. Algermissen, S. T., Stepp, J. C., Rinehart, W. A., and Arnold, E. P., 

1969, Appendix B, in Studies in seismicity and earthquake damage 

statistics: U.S. Coast and Geodetic Survey, 68 p. 

4. Rinehart, W. A., Algermissen, S. T., and Gibbons, Mary, 1976, Estimation of earthquake losses in single-family dwellings: U.S. 

Geological Survey Open-File Report 76-156, 57 p., plus appendices. 

5. U.S. Geological Survey, 1973, Atlas of Urban and Regional Changes, 

San Francisco Bay area: U.S. Geological Survey Open-File Report, 

44 maps, 1:62,500. 

6. Western Economic Research Company, 1973, Census Tract Outline Map of 

the Bay Area and Vicinity: Sherman Oaks, Calif., Scale 1:125,000. 

7. Algermissen, S. T., Steinbrugge, K. V., and Lagorio, H. J., 1978, 

Estimation of earthquake losses to buildings (except single-family 

dwellings): U.S. Geological Survey Open-File Report 78-441, 146 

p., plus appendices. 

8. Wood, H. 0., and Neumann, Frank, 1931, Modified Mercalli Scale of 

1931: Seismological Society of America Bulletin, v. 21, 

p. 277-283. 

9. Voight, D. S., and Byerly, P., 1949, The intensity of earthquakes as 

rated from questionnaires: Seismological Society of America 

Bulletin, v. 39, p. 21-26. 

10. Richter, C. F., 1958, Elementary seismology: San Francisco, Calif., W. 

H. Freeman & Co., 768 p. 

11. Algermissen, S. T. (principal investigator), Rinehart, W. A., 

Dewey, J. W., Steinbrugge, K. V. (principal consultant), 

Lagorio, H. J., Degenkolb, H. J., Cluff, L. S., McClure, F. E., 

Scott, Stanely, and Gordon, R. F., 1972, A study of earthquake 

losses in the San Francisco Bay area--A report prepared for the 

Office of Emergency Preparedness: U.S. Department of Commerce, 

National Oceanic and Atmospheric Administration, Environmental 

Research Laboratories, 220 p. 

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

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

Studies for seismic zonation of the San Francisco Bay area: 

U.S. Geological Survey Professional Paper 941-A, p. A52-A67. 

13. Wesson, R. L., Helley, E. J., Lajoie, K. R., and Wentworth, C. M., 

1975, Faults and future earthquakes: U.S. Geological Survey 

Professional Paper 941-A, p. A5-A30. 

72 
EXAMPLES OF SEISMIC ZONATION IN THE SAN FRANCISCO BAY REGION 

by 

W. J. Kockelman and E. E. Brabb 

ABSTRACT 

Six examples of seismic zonation at various :scales by cities and 

counties in the San Francisco Bay region show that s:cientific information 

can be used effectively in avoiding earthquake hazards and mitigating 

damage. The zonation method involves postulating an earthquake, grouping 

geologic materials with similar physical properties, predicting the geologic effects of an earthquake, and combining the geologic effects on a map. 

The method has been used by the Cities of Mountain View, Novato, and San 

Francisco and the Counties of Marin, Santa Clara, and San Mateo to develop 

zones which were used as a basis for their general plans, seismic safety 

plans, development policies or ordinances. 

INTRODUCTION 

The proceedings of the International Conference on Microzonation for 

Safer Construction Research and Application, Seattle, 1972 included papers 

on the state of the art, various techniques, and a pr·eliminary analysis of 

zonation methods. The papers lack uniform definition of "seismic zones," 

lack consensus on the scale and detail of seismic zonE~ -maps, and reveal the 

need for an unambiguous zonation method (10). 

Three events have encouraged communities to attempt seismic zonation: 

passage of the Alquist-Priolo Act by the California Legislature requ1r1ng 

special studies in zones encompassing potentially and recently active 

faults {5), preparation and adoption of seismic safety plan elements by 

cities and counties as required by California statue (4), and further 

development of a seismic zonation method (2). The method has four steps: 

postulating an earthquake of a given size and location; grouping geologic 

materials with similar physical properties; predic!ting effects of the 

postulated earthquake for each geologic unit by type of hazard or failure, 

namely surface rupture, ground shaking, flooding, liquefaction potential, 

and landsliding; and combining geologic effects by zones on a map (1). 

The following six examples of seismic zonation by cities and counties 

in the San Francisco Bay region illustrate applicatio,ns of the method. 

INVESTIGATION ZONES 

Three "investigation zones" are derived and mapped at a scale of 

73 
0 1 

Kilometer 

Planning area 

boundary 

Figure 1.--City of Mountain View "investigation zones." The boundaries of 

the zones separate areas with different geologic properties. Zone A, for 

example, is underlain by water-saturated bay mud with a moderate to high 

liquefaction potential whereas zone B contains seasonally saturated alluvial fan deposits that may liquefy only when the water table is high (6, 

20). Different development policies have been adopted for each of the zones 

(8). 

74 
1 :48,000 (fig. 1) by the City of Mountain View Planning Department from its 

review of various earth-science information. Ground shaking and ground 

failure are considered the primary earthquake problems (8). 

An earthquake on either the San Andreas or Ha~~ard fault systems is 

assumed (7). Design earthquakes of magnitudes 8.0 and 6.5 may be implied 

from the Department's use of potential liquefaction <lata (6, 20). 

Each "investigation zone" contains geologic uni t~s with similar physical properties. Zone A is underlain mostly by water-saturated bay mud. 

Zone B is underlain by weakly consolidated mud, silt and sand that are 

seasonally saturated. Zone C is underlain by somE~what coarser grained 

alluvium that is generally well drained and unsaturated (8, 9). 

Geologic effects are predicted for each group of geologic units having 

similar properties. For example, "Moderate to substantial settlement 

and/or differential settlement may occur" in zone A, "Deep liquefaction and 

significant settlement ..• is possible" in zone B, and "Neither significant 

settlement nor ground failure is likely to be experie!nced" in zone C (8). 

With the aid of the zone map (fig. 1), areas of "relative seismic 

concern," such as those where loss of life, property damage and economic 

dislocation may occur, are delineated on the City's E!xisting "General Plan 

Map." Land uses are subdivided into categories by importance, such as 

hospitals and fire stations that must provide emergency services after an 

earthquake; by occupancy, such as schools and other buildings that contain 

large numbers of people; and by type of construction, such as single-family 

dwellings. Development policies, such as "open space and low intensity uses 

shall be encouraged for sites most susceptible to earthquake damage," and 

"Uses should be limited to those where risk of loss of life, property 

damage, and social and economic dislocations are acceptable if liquefaction 

or settlement takes place," are specified for each land use group in each 

zone (8). 

The zones, areas of "relative seismic concern," and land-use development policies have been incorporated into the City's Safety Element and 

adopted by the City Council as part of its general plan (8). 

BUILDING DAMAGE LEVELS 

Four levels of building damage aggregated on a block basis are estimated and mapped at a scale of 1:48,000 (fig. 2) for the San Francisco City 

Planning Department (3, 14). 

A repetition of the 1906 earthquake (magnitude 8 .. 3) originating on the 

San Andreas fault is assumed. Earthquakes with small~~r magnitudes are also 

considered (3). 

Soil and rock types are combined on the basis c•f similar engineering 

properties, and their susceptibility to earthquake failure is discussed 

(3). Their properties are considered in estimating the types of potential 

ground motion such as peak ground acceleration, spectral response acceleration, and predominant periods for deep or soft soil deposits (14). 

City Assessor's data on construction type, year built, number of 

75 
stories, floor area, and use of each building along with earthquake history 

were considered to obtain average damage factors for each block. The 

factors were then weighted to obtain the four levels of estimated building 

damage shown in figure 2 (14). 

Hazards such as seiches, tsunamis, landslides, and dam failures are 

considered and mapped but are not generally used in developing the four 

levels of damage. These hazards are to be treated as "additional hazards;" 

for example, a building with a "slight" damage rating (fig. 2) that is apt 

to slide down a hill with a landslide should have its rating changed to 

"severe" (3). 

0 

I 

Cl Recently Demolished or to be Demolished 

;:;:;:; Information not Available or not in Study 

5KM 

I 

Figure 2. --Estimated building damage for a severe earthquake in the San 

Francisco area. Abatement of hazards associated with these buildings is 

recommended in-San Francisco's Community Safety Plan (14). 

76 
Policies related to building damage levels such as "Initiate orderly 

abatement of hazards from existing buildings and structures" and "Abate 

existing hazards in all critical community facili1~ies" are recommended 

(14). 

The building damage levels, "additional hazards" and policies have 

been incorporated into the City's Community Safety Plan and adopted by the 

Board of Supervisors as part of the City's comprehen::sive plan ( 14). 

EARTHQUAKE RISK ZONES 

Five "earthquake risk" zones are developed and mapped at a scale of 

1:12,000 {fig. 3) for the Novato area by the CalifoJ•nia Division of Mines 

and Geology in cooperation with the County of Marin ~ind the City of Novato 

(13). Ground shaking, surface rupture, ground failw~e, and landslides ~e 

considered as having the highest potential for damage, whereas tsunamis are 

considered not to be significant potential hazards (12, 13). An earthquake 

of approximately magnitude 8 with an epicenter in the northern San Francisco 

Bay area is postulated (13). 

Geologic materials are grouped into five zones on the basis of similar 

physical properties; for example, "firm, relatively unweathered bedrock" in 

zone A, "relatively shallow compacted alluvium and colluvium" in zone B, 

"deep upslope landslide deposits" in zone D, and "bc:ly mud" in zone E. In 

addition, a symbol is placed over the trace of the potentially active 

Burdell Mountain fault {13). 

Some geologic effects of the postulated earthqlltake are predicted for 

each zone; for example, "subject to relatively high frequency vibrations" in 

zone A, "may be threatened by landsliding" in zone B:, and "rapid differential settlement" in zone E. In addition, each zone :Ls rated from probable 

low to probable high damage. The geologic materials, geologic effects, and 

damage ratings are combined and mapped by zone ofi figure 3 (13). 

Recommendations related to the risk zones, such as "large public 

structures ••• not be located on any demonstrated fault trace," "residential 

developments on fill may not be a fitting use" in zone E, and "Engineering 

geology reports, based on detailed geological mapping:, [should] be required" 

in zone D, are made (13). 

The "earthquake risk" zones and recommendations~ have been considered 

by the City of Novato and the County of Marin and used as a basis for their 

seismic safety plan elements. 

SEISMIC SAFETY ZONES 

Three "seismic safety zones" are developed and mapped at a scale of 

1:62,500 (fig. 4) by the Santa Clara County Planning Department with 

assistance or contributions from the California Division of Mines and 

Geology, private consultants, and members of the U.S. Geological Survey 

( 18) • Dam failures, tectonic creep, dike failure;s, tsunamis, s·eiches, 

landslides, ground-shaking and surface ruptures we1~e considered and the 

potentially hazardous ones were combined in zones on a "relative seismic 

stability" map (18). This map also includes nonseifsmic geologic hazards, 

such as landslides triggered by rainfall. The original map is printed in 

77 
A Areas of probable low damage. These areas are underlain by firm, relatively unweathered bedrock that crops out at the surface or is covered by only thin layers of 

soil or colluvium. 

8 Areas of probable low to moderate damage. These areas are underlain by relatively 

shallow, compacted alluvium and colluvium on flat or gently sloping surfaces. 

c 

D 

E 

Areas of probable low to moderate damage. 

bedrock. 

These areas are underlain by sheared 

Areas of potentially high damage. These areas are underlain by landslide deposits and 

by thick deposits of colluvium or deeply weathered bedrock on steep slopes. 

Areas of probable high damage. These areas are underlain by bay mud ranging in 

thickness from a few feet to more than 100 feet. 

-Fault 

Figure 3.--Earthquake risk zones 

fornia ( 13) . 

in the Novato area, 

78 

Marin County, Cali-
Subdivision, Building, and Grading Ordinance .AMnchoent 

The Board of Supervisors of the County of Santa 

Clara, State of California, do ordain as follova: 

SECTION 6: Buildioa Peralta. 

Section C3-36 of the Santa Clara County Ordinance Cnde 1a added to read: 

C3-36: Geolodc: Report.,, 

Section 30l(b)7 1a a•nded to read: 

Section 301 (b) 7. Give such other inforaation as reasonably aay be required by the Building 

Offic:ial, such a a a geologic: report, which shall be 

necessary where the County deterainea that such report is needed on the basis of the County hazard .. p •• 

SECTION 7: County Hazard Mapa. 

Article 3 is added to Chapter IV of the Santa 

Clara County Ordinance Cnde to read: 

Article 3. County Geolodc Hazard Mapa. 

Section Cl2-277. Definition. Whenever 

the land develop•nt regulat~ ro County 

hazards .. ps, the reference is to the official 

Santa Clara County geologic: hazards .. ps as herein 

adopted and which .. y be -nded fro. ti.e to tiM 

by resolution of the Board of Supervisors, which .. ps 

are the basis for deteraining whether a geologic: report shall be required. The adopted .. pe are iden- tified as follows: 

~ 

Hap lluaber 

Port of Ordinance No. NS 1203.31. 

EXPLANATION 

- Where geologic report normally is required 

~ Where geologic report may be required 

D Where geologic report normally is not required 

Figure 4.--Part of the official seismic stability map for Santa Clara County 

(18). Scale 1:250,000. 

stoplight colors--red, yellow, and green--to make the information more 

easily understood by the general public. 

A recurrence of the 1906 earthquake (magnitude 8.3) along the San 

Andreas fault zone in the San Francisco Bay area may be implied from the 

Department's discussion of the various seismic hazards (18). 

Geologic units are grouped on the basis of similar physical properties, 

and the geologic effects are predicted for each group. For example, finegrained water-saturated sand or silt within 6 meters of the ground surface 

would be considered to have a high potential for liquefaction and would 

normally require geologic investigation before any development is approved. 

The same materials within 6 to 15 meters of the surface are considered to 

have a moderate potential for liquefaction and might require a geologic 

investigation. In most cases, the county geologist decides whether and what 

level of geologic investigation is required. 

Maps of other county plan elements, such as utility, transportation, 

community facilities, and urban development, are compared with the zone map 

to indicate their relation to the seismic safety zones (18). Recommendations to minimize the possible loss of life and property are made for each 

79 
county plan element. For example, "Emergency operations center structures 

should be evaluated for seismic vulnerability and should be designed and 

constructed to assure the continuity of vital services following a damaging 

earthquake," and "Proposed transportation routes, facilities and structures 

should be evaluated for potential vulnerability and built only if problems 

can be sufficiently mitigated" (18). 

The three zones and recommendations are included in the County's 

Seismic Safety Plan which was unanimously adopted by the County Planning 

Commission and Board of Supervisors. In addition, the seismic safety zones 

have been adopted by the Board of Supervisors as the County's official 

geologic hazard maps. The County Ordinance Code has been amended so as to 

require site investigations and geologic reports based on the official 

geologic hazards map. Four sections of the code are affected: major 

subdivisions, minor land divisions, building sites, and grading. The 

amendment provides for site investigations and geologic reports so as to 

discourage development on, or adjacent to, known potentially hazardous 

areas. The reports are to be prepared by an engineering geologist registered in the State, be submit ted to the County for approval, and specify the 

remedial measures that will make a safe development (17). 

GEOTECHNICAL HAZARD SYNTHESIS 

Thirteen geologic materials and seven geologic hazards are compiled, 

combined, and mapped at a scale of 1:24,000 (fig. 5) by the San Mateo County 

Planning Department and its consultant. Primary consideration is given to 

the extent and location of faults and to liquefaction potential; however, 

landslides, dam failure, and tsunamis are also considered and mapped. 

Seiches are considered but not mapped (11). 

A magnitude 7.5-8.3 earthquake on the San Andreas fault is assumed for 

ground shaking. For fault displacement, a maximum expected magnitude 8.25 

earthquake for the San Andreas fault and magnitude 7.0 for the Seal CoveGregorio fault are used (11). 

Geologic materials are grouped on the basis of similar physical 

properties, for example, "younger estuarine mud and artifical fill" as no. 

1, "beach and dune sand" as no. 4 "colluvium" as no. 5, and various 

"bedrock" hardnesses as nos. 6-13 (11). 

Earthquake stability is predicted for each geologic material--"poor, 

fair, or good" depending upon geologic structure, slope angle and landslide 

and soil creep potential. Earthquake intensity is predicted for each 

geologic material unit--"strong, very strong, violent, very violent, and 

weak" depending upon the distance from the San Andreas fault ( 11) . 

The geotechnical hazard synthesis is part of the County's Seismic and 

Safety Elements of the General Plan. These elements include recommended 

policies and implementation programs regarding existing land use, future 

land use and development, zoning and division of land, and critical-use 

structures that are directly related to the geotechnical hazard synthesis 

maps; for example, "Determine the level of acceptable risk which can be 

borne, utilizing the ... Maps," and "Integrate geotechnical hazard data •.• 

into zoning and subdivision ordinances" (16). The elements, including the 

geotechnical hazard synthesis maps and recommended policies and implementa80 
0 lKM 

Figure 5. --Part of the geotechnical hazard synthesis map for San Mateo 

County ( 11) , scale 1 :24,000. The explanation for this map is complex, 

combining information on geologic processes, such as faulting, landsliding, 

coastal erosion, and liquefaction, with information on geologic materials, 

shown by numbers on the map. The material units are further subdivided by 

both seismic and engineering characteristics. For example, areas designated "2b" are underlain by alluvial fan deposits ranging in coarseness from 

silt to gravel, have poor to fair slope stability, moderate liquefaction 

potential, good to fair stability in terms of the intensity of ground 

shaking during a 7.5-8.3 M earthquake, and have good foundation properties. 

81 
0 

Flood plain 

Fault 

Slope instability 

PERCENTAGE 

OF SLOPE D o-15 

16-30 

over 50 

600 Meters 

Figure 6. --Hypothetical property in San Mateo County 

geologic constraints. Dwelling units in the fault zone 

with seismic and 

and on landslides 

Similar low dwelling-unit 

steep slopes (19). 

are limited to one per 16 hectares (40 acres). 

densities are required along flood plains and on 

82 
tion programs, have been adopted by the County Board of Supervisors (16). 

RESOURCE MANAGEMENT ZONING DISTRICT 

Eleven land categories are derived and mapped at various scales by the 

San Mateo County Planning Department. Seven of the categories (fig. 6) were 

considered serious seismic or geologic constraints to development (19). 

A 7.5-8.3 magnitude earthquake on the San Andreas fault is implied from 

the seismic hazard information used by the Department (19). 

The seismic and geologic constraints are grouped on the basis of 

similar physical properties--faults, slopes, flood plains, and slope instability. Geologic effects are predicted for two categories; "potential 

surface deformation due to fault movement," and high, moderate, and low 

"susceptibility of slopes to failure landslides" (19). 

The Department recommended that the permitted density of residential 

development proposed in each category be reduced to minimize risk exposure 

and to carry out the objectives and policies of the County's open-space and 

conservation plans (19). 

The Board of Supervisors adopted an ordinance creating a resource 

management zoning district (15). In addition to the principal uses permitted in the district, the maximum number of dwelling units is limited by 

special density regulations. The regulations are applied to each application for a zoning permit through the use of a density matrix worksheet (15, 

19). For example, 100 hectares (250 acres) of the hypothetical property 

shown on figure 6 would lie in the fault and slope instability zones where 

the number of dwellings would be only one per 16 hectares (40 acres). 

CONCLUSION 

The seismic zonation method can be applied by cities and counties to 

avoid earthquake hazards, to reduce loss of life, and to mitigate damage. 

The examples help clarify the definition, scale, and detail of "seismic 

zones" and provide a workable method for presenting earthquake effects in a 

form usable by scientists, engineers, land-use planners, decisionmakers, 

and other citizens. 

REFERENCES CITED 

(1) Borcherdt, R. D., Brabb, E. E., Joyner, W. B., Helley, E. J., Lajoie, 

K. R., Page, R. A., Wesson, R. L., and Youd, T. L., 1975, 

Predicted geologic effect of a postulated earthquake in 

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

Francisco Bay region: U.S. Geological Survey Professional Paper 

941-A, p. 88-94. 

(2) 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. 

(3) Blume, John A. and Associates, 1974, San Francisco seismic safety 

investigation: San Francisco, CA, 124 p. 

( 4) California Legislature, Amendment to the local planning chapter, 

83 
1972: California Government Code, sec. 65302 (f), West's 

annot. Codes. 

( 5) Alquist Priolo special studies zones act, 1975: California 

Public Resources Code, sees. 2621 et seq., West's annot. Codes. 

(6) City of Mountain View Planning Department, 1974, Safety element: 

Earth science data interpretive report, Background report I, 

Mountain View, CA, 31 p. 

(7) 1975, Relative concern zones, structural hazards, critical facilities and fire hazards: Background report II, Mountain View, 

CA, 47 p. 

(8) 1975, Safety element of the general plan: Resolution no. 10392, 

Amend. No. 16, adopted February 24, 1975, Mountain View, CA, 14 

p. 

(9) Helley, E. J., and Brabb, E. E., 1971, Geologic map of late Cenozoic 

deposits, Santa Clara County, California: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-335. 

( 10) International Conference on Microzonation for Safer Construction 

Research and Application, Seattle, 1972: Proceedings, Seattle, 

WA, Vols. I and II, 987 p. 

(11) Leighton & Associates and San Mateo County Planning Department, 1976, 

Geotechnical hazard synthesis map: Redwood City, CA, 7 sheets. 

(12) Rice, Salem J., 1973, Geology and geologic hazards of the Novato area: 

California Division of Mines and Geology, 47 p. 

( 13) 1975, Geology for planning - Novato area: California Division of 

Mines and Geology, 56 p., 6 pls. 

( 14) San Francisco Department of City Planning, 1974, Community safety 

plan - A proposal for citizen review: 68 p. 

( 15) San Mateo County Board of Supervisors, 1973, Ordinance No. 2229, 

adding a Resource Management District and regulations to the 

county zoning ordinance: Adopted December 20, 1973, 24 p. 

(16) San Mateo County Planning Department, 1976, Seismic and safety elements of the general plan: Redwood City, CA, Vol. I, 57 p., Vol. 

II, 141 p. 

( 17) Santa Clara County Board of Supervisors, 1974, Ordinance No. NS 

1203.31, amending the subdivision, building, and grading ordinances, and adding geologic hazard maps to the County Ordinance 

Code: Adopted November 6, 1974, 11 p. 

(18) Santa Clara County Planning Department, 1975, Seismic safety plan, An 

element of the general plan: San Jose, CA, 119 p. 

( 19) Woolfe, Donald A. , 1973, San Mateo County Planning Depart, Adoption of 

the resource conservation area density matrix and policy statement: Memorandum dated September 21, 1973, 32 p. 

(20) 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, p. 68-74. 

84 
THE USE OF EARTHQUAKE AND RELATED INFORMATION 

IN REGIONAL PLANNING--WHAT WE'VE DONE AND WHERE WE'RE GOING 

by 

Jeanne B. Perkins 1 

ABSTRACT 

ABAG has used several techniques for combining earth science maps. These techniques include land capability analysis and various methods of 

calculating maximum earthquake intensity and cumulative economic risk due 

to earthquake damage. The resulting maps and data are more easily used 

in ABAG's planning programs, including: 

o providing data on characteristics of large vacant industrial 

sites and potential seaports 

o locating areas deserving further stuqv for use as potential disposal sites for hazardous wastes 

o assessing the impacts of alternative future land use alternatives 

o reviewing regionally-significant development proposals on a 

continuing basis 

o providing information to city and county staff 

ABAG's earthquake program plans to refine the existing earthquake 

maps by experimenting with the ways in which other relationships among geology, faults, topography, earthquake recurrence intervals and damage affect these maps. ABAG also expects to relate these maps more system- atically to various land development patterns. 

INTRODUCTION 

ABAG is a regional comprehensive planning agency that is owned and 

operated by the local governments of the San Francisco Bay Area. It was 

established in 1961 to meet regional problems through the cooperative action of its member cities and counties. 

The Bay Area is one of the most seismically active areas in the 

United States. The effects of earthquakes also usually cross city and 

county boundaries. Consequently, earthquake preparedness is a major regional concern. 

SOME TECHNIQUES FOR COMBINING MAPS 

Because it is difficult for planners and elected officials to deal 

directly with the reams of social, environmental, and economic information available concerning any given area or issue, ways of consolidating 

1 Regional Planner, Association of Bay Area Governments, Berkeley, California 

85 
earth science data to slant that information to each application become 

valuable. Two techniques for combining earthquake and other earth science maps that ABAG staff have found useful are described below. 

Maximum Earthquake Intensity and Cumulative Economic Risk 

An earthquake intensity map groups together a variety of different 

causes of earthquake damage; although shaking is the dominant cause of 

damage, other factors such as liquefaction, landsliding, fault rupture, and changes in ground level can also contribute to intensity. Figure 1 

is an example of a maximum earthquake intensity map for the San Francisco 

Bay Area based on the techniques developed by Borcherdt, et al. {1). 

This map is not the only maximum intensity map that can bedeveloped; altering the way that intensity is related to distance from a fault can 

produce a different map. This type of map can be used with information 

on existing buildings to forecast locations of maximum damage for use in 

planning emergency response measures and for designating areas of critical concern. 

A cumulative economic risk map can relate the expected damage to 

particular types of buildings over time. Such maps rely on intensity information as well as information on the amount of damage that can be 

expected for each intensity and general type of building and information 

on how often a particular earthquake is likely to occur. Figures 2A - D 

are examples of risk maps. They indicate the total expected percent damage due to earthquakes resulting from any of the major active faults 

in part of the Bay Area for any given area for two types of small buildings: wood-frame (Figures 2A and C) and other types (Figures 2B and D). Risk maps may be used in evaluating the relative costs due to earthquakes for new buildings in various locations throughout the region and for 

designating areas where special precautions may be needed. However, the 

intensity-cost information is not a sufficient basis for engineering decisions at a specific site, for these require specific knowledge of the 

process causing damage. The graphic appearance of a risk map is very dependent on how often earthquakes are assumed to occur on each fault. 

Figures 2A and 8 use a different set of recurrence intervals than Figures 2C and D. 

Land Capability Analysis 

Any microzonation map, though a representation of the earthquake problem, cannot by itself represent the problems due to all geologic constraints, let alone all environmental, social or economic constraints. 

By relating each geologic problem to some common denominator, such as 

cost or lives lost, one can effectively compare the geologic concerns or 

can add these concerns together, using traditional or computerized map overlaying techniques. Figure 3 is an example of such a map, based on 

cost information for residential development in Santa Clara County. 

86 
flbi\JfZe 1 

MAXIMUM 

EARTHQUAKE INTENSITY 

FOR THE 

SAN FRANCISCO BAY AREA 

• San Francisco intensity A 

• San Francisco intensity B 

• San Francisco intensity C 

• 

lii!ir~ ~~ 

D 

San Francisco intensity D 

San Francisco intensity E 

Less than intensity E 

87 

10 MILES 

I 

I 

15 I<ILOMETEAS 
flbrUfZP ').. : 

SAMPLE CUMULATIVE 

ECONOMIC RISK MAPS 

FOR EARTHQUAKE DAMAGE 

SMALL WOOD-FRAME BUILDINGS 

MILES 

0 5 10 -

I II I I I I II I I I I I 

0 5 10 15 

KILOMETERS 

88 
o 5 Mlh0 I I I I I I I I I I I 

0 5 Klt£lMfflli'J<S 

89 
SAMPLE USES 

Solid Waste Planning 

The earthquake intensity map and other geologic and hydrologic maps 

were used with map-related criteria to screen the Bay Area to find those 

general areas that warranted further study for use as disposal sites for 

hazardous waste. 

After additional screening using other social and environmental criteria, ABAG staff discovered that only four of the nine Bay Area counties 

had areas worthy of additional study. 

Special Facilities 

The problem of overlaying maps may be avoided by simply providing a 

listing of the level of concern (high, moderate or low) for environmental 

safety issues in the form of a computer display. If land capability information is also available, a general indicator of overall concern for 

environmental safety also can be included. Separate displays have been 

prepared for approximately 450 vacant industrial sites and 173 proposed 

seaport sites in the Bay Area. 

Assessment of Development Patterns 

As part of assessing the environmental impacts of planning programs, 

it is important to know the relationship of geologic hazards to alternative land use or development patterns. Earthquake intensity and related 

information has been used in assessing the implications of policies that 

encourage more dense central city growth as opposed to less compact development in suburban and rural areas in the region. It has also been 

used in assessing development patterns intended to reduce commute distances for Santa Clara County. 

Implementing Regional Policies 

As the designated areawide clearinghouse, ABAG reviews local plans and projects that propose to use Federal and State funds. One of the 

review criteria used is the completeness of the description of geologic and seismic hazards affecting the site. Microzonation maps and information on other geologic hazards is used in the reviews. 

In addition, ABAG assists cities and counties in obtaining earthquake information and incorporating that information into their programs 

as time permits. 

FUTURE PLANS 

Updating Information 

Because earthquake microzonation is such a dynamic field, it is essential that all maps can be easily updated. All of the basic information, including geology and land use, the modeling techniques, and the 

90 
products, including maps and displays, are integrated in ABAG's computer- ized Bay Area Spatial Information System (BASIS). Thus, as better information on correlations between geologic materials and earthquake intensity 

or on recurrence intervals becomes available, the interpretive maps and 

the related information will be modified easily. The system also allows 

cities and counties to modify the maps for their own uses. Maps can be 

produced at non-standard scales, such as 1:60,000 for Santa Clara County. Requests can be fulfilled to group peat soils with Bay mud, rather than 

with alluvium, for generating a maximum intensity map for Contra Costa 

County. 

Future Applications 

Several interesting projects using earthquake and related information 

should be completed by December 1979. The displays for special facilities 

will be expanded to include existing and proposed airports and solid waste 

disposal sites. The work on assessment of development patterns should be 

dramatically improved when detailed information on current and projected land use is placed in BASIS. Such information also will be extremely valuable for planning emergency services. A more formal program will be instituted for ensuring that cities and counties more actively use the earthquake map information. 

BIBLIOGRAPHY 

1. Borcherdt, R.D., Gibbs, J.F., and Lajoie, K.R., 1975, Maps Showing Maximum Earthquake Intensity Predicted for Large Earthquakes on the 

San Andreas and Hayward Faults, Southern San Francisco Bay Region, California, U.S. Geological Survey Miscellaneous Field Studies Map MF-709. 

91