COVER PHOTOGRAPH of San Francisco Bay Region taken April 14, 1972, 

at altitude of 65,000 feet from U-2 aircraft. Courtesy National Aeronautics 

and Space Administration (Ames Research Center, Moffett Field, Calif.) Front 

shows city of San Francisco and Golden Gate at bottom, San Francisco Bay and 

city of Oakland in middle, Sacramento-San Joaquin Delta and crest of Sierra 

Nevada at top. Back shows Bolinas lagoon and trace of San Andreas fault at 

bottom, San Pablo Bay in middle, Sacramento valley and crest of Sierra Nevada 

at top. 
Studies for Seismic Zonation of 

the San Francisco Bay Region 

Edited by R. D. BORCHERDT 

BASIS FOR REDUCTION OF EARTHQUAKE HAZARDS, 

SAN FRANCISCO BAY REGION, CALIFORNIA 

G E 0 L 0 G I C A L S U R V E Y P R 0 FE S SI 0 N A L PAPER 9 4 1- A 

A series of close(ry related earth science studies that 

define the nature and severity of earthquake hazards 

associated with geologic conditions 

jointly supported by the U.S. Geological Survey 

and the Department of Housing & Urban Development, 

Office of Policy Development and Research, 

as a part of a program to develop and apply earth-science 

information in support of land-use planning and decisionmaking. 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:l975 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 75-15239 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock Number 0~4-00 1-02676-7 
FOREWORD 

Earthquake hazard reduction program 

This report represents a milestone 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 delineated in sufficient detail 

to serve as a guide to community officials and 

developers. Even if the fault data had been available, 

standard procedures 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 entire 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 in 

an experimental field laboratory along the San Andreas 

fault to study the basic mechanisms of earthquakes and 

the patterns of energy radiation and attenuation as 

earthquake waves 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, requiring data 

and research in seismology, geology, soil mechanics, 

geophysics, hydrology, and engineering. Nevertheless, 

if earthquake hazards are to be reduced, earth-science 

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 January. 1970, is 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 

participates in the study and provides a liaison and 

communication link with other regional planning 

agencies and with county and local governments.· 

Although the study focuses on the nine-county 

7,400-mi2 San Francisco Bay region, it bears on 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 complexity, 

however, can be greatly reduced 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 earth-science information are ill suited to their needs. 

The study is 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 comIII 
IV FOREWORD 

munication between scientists and users, and (5) 

exploring alternate ways of applying earth-science 

information in planning and decisionmaking. 

Since the study was started in 1970, it has produced 

more than 70 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. 

Studies for seismic zonation of the San Francisco Bay 

region 

This report brings together the results of a number of 

earth-science studies that provide a basis for reducing 

earthquake hazards. 

The enormous amounts of energy released during 

large, or even moderate, earthquakes produce a 

complex chain of effects, most of which are potentially 

hazardous to man and his works. Many of these effects, 

such as fault displacement and ground shaking, are 

direct results of the earthquake. Others, like landsliding and liquefaction, result from the action of ground 

motion generated by earthquakes on unstable geologic 

units or structures. Still other effects result from the 

reaction of manmade structures to earthquake forces. 

Moreover, many of these effects are complexly interrelated in ways that make analysis difficult; ground 

shaking, for example, may be amplified or reduced by 

local geologic conditions, and the level of ground 

shaking may determine whether or not landslides are 

triggered or liquefaction induced. 

The diversity and complexity of earthquake effects 

make the reduction of earthquake hazards extraordinarily difficult and require adoption of a coordinated 

and disciplined plan of attack. Such a plan must 

incorporate the efforts of many individuals and the 

skills and techniques of several professions. Earth 

scientists, structural and civil engineers, professional 

planners, elected officials, and private citizens are 

among those who are essential participants in the effort. 

This report is designed to provide the earth-science 

basis for such a comprehensive approach to reducing 

earthquake hazards. It brings together and correlates 

significant results from several fields of geology and 

from seismology and engineering seismology. Because 

these results are derived from the natural processes 

brought into play by a damaging earthquake, they are a 

logical starting point for an attack on the problem. 

The method outlined here for seismic zonation is 

applicable, with modifications, throughout the San 

Francisco Bay region and elsewhere in regions of high 

earthquake hazard. Its effectiveness, however, depends 

on the degree to which these results are used or applied. 

Although many of the research findings can be applied 

directly in hazard-reduction programs now underway, 

others suggest the need for continuing communication 

among participants in the hazard-reduction process and 

for the conduct of related research. Thus, while few 

readers will be prepared to apply all that is presented 

here, the contents should assist them in determining 

where additional expertise is needed. 
CONTENTS 

Page 

Foreword ------------------------------------------------ III 

Page 

Borcherdt, W. B. Joyner, R. E. Warrick, and J. F. Gibbs ____ A52 

Abstract __ _ ____ ___ _ _ __ _ ___ _ _ ___ _ ____ ___ __ _ ___ ____ _ ____ ____ A1 Liquefaction potential, by T. L. Youd, D. R. Nichols, E. J. Helley, 

Introduction ---------------------------------------------- 2 

Faults and future earthquakes, by R. L. Wesson, E. J. Helley, 

and K. R. Lajoie ---------------------------------------- 68 

Landslides, by T. H. Nilsen and E. E. Brabb -~-------------- 75 

K. R. Lajoie, and C. M. Wentworth ______________________ 5 Predicted geologic effects of a postulated earthquake, by R. D. 

Estimation of bedrock motion at the ground surface, by R. A. Borcherdt, E. E. Brabb, W. B. Joyner, E. J. He1ley, K. R. Lajoie, 

Page, D. M. Boore, and J. H. Dieterich____________________ 31 

Differentiation of sedimentary deposits for purposes of seismic 

R. A. Page, R. L. Wesson, and T. L. Youd ________________ 88 

General conclusions ______________ -------------------------- 95 

zon~tion, by K. R. Lajoie and E. J. Helley ________________ 39 1906 intensity scale for San Francisco ---------------------- 96 

Response of local geologic units to ground shaking, by R. D. References cited __________________________________________ - 97 

FIGURE 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17-24. 

ILLUSTRATIONS 

Page 

Map showing apparent intensity of the 1906 earthquake in San Francisco----------------------------------------- A3 

Generalized geologic map of San Francisco _______________________________________ ------------------------------ 4 

Maps showing faults that may cause earthquakes or surface displacement in the San Francisco Bay region and sources of data on recent faulting ________________________________________________________________________ --- ___ --- 7 

Diagrams showing four types of fault movement----------------------------------------------------------------- 12 

Map showing zones of surface fault displacement associated with earthquakes during historic times, San Francisco 

Bay region ----------------------------------------------------------------------------------------------- 14 

Photographs showing examples of surface displacement along the San Andreas fault, 1906 earthquake ---------- 15 

Photograph showing curb offset by creep on Calaveras fault ----------------------------------------------------- 15 

Map showing distribution of documented fault creep in the San Francisco Bay region ----------------------------- 16 

Map showing epicenters of earthquakes of magnitude greater than or equal to 1, San Francisco Bay region, 1969-72, 

and block diagram showing relation between fault surface, earthquake epicenter, and earthquake focus -------- 18 

Sketch showing vertical deformation of young sedimentary deposits in trench across Coyote Creek fault, southern 

California _________________________________________________________________________________________________ 20 

Block diagram showing landforms developed along recently active strike-slip faults ------------------------------- 21 

Aerial photograph and map showing topographic features along a segment of the San Andreas fault near the Carrizo 

Plain, southern California _________________________________________________________________________________ 22 

Photograph and sketch of faulted wave-cut platform exposed in sea cliff near Point Aiio Nuevo ---------------------24 

Graph showing length of surface fault rupture in relation to earthquake magnitude -------------- _________________ 24 

Map showing surface rupture associated with the 1906 earthquake along part of the San Andreas fault near Fort 

Ross, Calif. _______________________________________________________________________________________________ 26 

Map showing surface ruptures associated with the 1971 San Fernando earthquake--------------------------------- 27 

Graphs showing: 

17. Horizontal ground motion recorded at Pacoima damsite, 1971 San Fernando earthquake ----------------- 33 

18. Comparison of spectra for horizontal gr{)und motion at Pacoima damsite, 1971 San Fernando earthquake ---- 34 

19. Peak horizontal ground a~celeration in relation to shortest distance to slipped fault as a function of earthquake magnitude --------------------------------------------------------------------------------- 34 

20. Peak horizontal ground acceleration in relation to shortest distance to slipped fault for earthquakes of magnitude 5.0 to 5.9 as a function1 of site geology----------------------------------------------------------- 35 

21. Peak horizontal ground acceleration in relation to shortest distance to slipped fault for earthquakes of magnitude 6.0 to 6.9 as a function of site geology----------------------------------------------------------- 35 

22. Peak horizontal ground velocity in relation to shortest distance to slipped fault as a function of earthquake 

magnitude and site geology ______________ -------------------------------- ___________________________ 36 

23. Peak horizontal ground displacement in relation to shortest distance to slipped fault as a function of earthquake magnitude and site geology--------------------------------~---------------------------------- 36 

24. Comparison of attenuation curves for four representative finite-element models of strike-slip faulting with 

data. on peak acceleration from figure 19------------------------------------------------------------- 38 

25-29. Maps showing: 

25. Location of detailed study area ____________________________________________________________ --- __________ 40 

26. Topography of Mountain View-Sunnyvale area ___________________________________________________________ 41 

v 
VI CONTENTS 

FIGURES 25-29. Maps showing-Continued Page 

27. Soil units in Mountain View-Sunnyvale area ----------------------------------------------------------- 42 

28. Alluvial deposits in Mountain View-Sunnyvale area ----------------------------------------------------- 44 

29. Geology of Mountain View-Sunnyvale area ______________________________ ------------------------------- 46 

30. Exa.mple of unpublished engineering data from which thickness and physical properties of alluvial de posits were partly 

derived --------------------------------------------------------------------------------------------------- 48 

31. Schematic cross section of southern San Francisco Bay region and description of certain physical properties of the 

generalized geologic units ______________________________________ --- _____ ------------------- ______________ -- 49 

32. Correlation diagram showing groupings of geologic units for evaluating ground response and liquefaction potential __ 50 

33. Map of San Francisco Bay region showing distribution of generalized geologic units and locations where ground motions generated by nuclear explosions were recorded ___ _ _ __ _ _ __ _ ___ _ _ _ __ _ ___ _ _ __ _ ___ _ _ ___ _ ___ ___ _ _____ _ __ _ ___ 53 

34. Recordings of horizontal ground motion generated by two nuclear explosions _______________________________________ 54 

35. Graphs showing horizontal spectral amplifications computed for 13 recording sites in San Francisco _________________ 57 

36. Histograms of average horizontal spectral amplification values computed for sites underlain by bay mud, alluvium 

and either granitic rocks or rocks of Franciscan Formation --------------------------------------------------- 58 

37. Graphs showing horizontal and vertical spectral amplification curves computed from recordings of a nuclear explosion 

and recordings of the San Francisco earthquake of March 22, 1957 ------------------------------------------- 59 

38. Graph showing observed intensity for sites underlain by bedrock as a function of distance to zone of surface rupture for 

1906 earthquake ----------------------------------------------------------------------------------------- 60 

39. Graph showing increments in 1906 intensity as a function of average horizontal spectral amplification computed at 

corresponding sites from recordings of nuclear explosions----------------------------------------------------- 61 

40. Map showing maximum earthquake intensities predicted for San Francisco --------------------------------------- 62 

41. Schematic geologic section at site of down-hole seismometer array near margin of San Francisco Bay _________________ 63 

42. Recordings of horizontal ground motion from a down-hole seismometer array ------------------------------------- 64 

43. Records showing comparison of observed and computed surface ground motions determined from motions observed in 

bedrock at a depth of 186m ------------------------------------------------------------------------------- 65 

44-4 7. Graphs showing: 

44. Computed and observed spectral amplification of ground motion at site of down-hole seismometer array ____ 67 

45. Comparison of high- and low-strain spectral amplification for site underlain by 220 m of alluvium (input 

scaled to maximum velocity of 31 cm/s) ___ _ ___ _ _ _ __ _ _ __ _ _ _ __ _ _ _ __ ____ __ _ _ ___ _ __ __ _ _ __ _ ___ _ __ __ _ ___ _ _ 67 

46. Comparison of high- and low-strain spectral amplification for site underlain by 11 m of bay mud and 173 m 

of alluvium (input scaled to maximum velocity of 19 cm/s) ------------------------------------------- 67 

4 7. Comparison of high- and low-strain spectral amplification for site underlain by 11 m of bay mud and 173 m 

of alluvium (input scaled to maximum velocity of 12 cm/s) ------------------------------------------- 67 

48. Graph showing estimated stress ratio required to produce liquefaction under field conditions during 30 cycles of seismic loading ----------------------------------------------------------------------------------------------- 70 

49. Graph showing criteria used for estimating liquefaction potential in the field ------------------------------------- 71 

50. Preliminary map showing liquefaction potential for the southern San Francisco Bay region _________________________ 73 

51-59. Photographs showing: 

51. Earthquake-generated landslide, Hebgen Lake, Mont. --------------------------------------------------- 75 

52. Aerial view of Turnagain Heights landslide, Anchorage, Alaska------------------------------------------- 75 

53. Aerial view of debris avalanche that destroyed towns of Yungay and Ranrahirca, western Peru ____________ 76 

54. Aerial view of landslides in Lopez Canyon area north of San Fernando, Calif. _____________________________ 76 

55. Transverse cracks that formed in upper part of old landslide near Paicines, Calif., after a moderate 

earthquake --------------------------------------------------------------------------------------- 76 

56. Earthflow-type landsliciPs triggered by earthquake of Aprill8, 1906, near HalfMoon Bay, Calif._ ________________ 77 

57. Roadbed of Ocean Shore railroad, south of San Francisco ------------------------------------------------- 77 

58. Landslide that occurred about 6 km north of Bolinas Lagoon, Marin County, Calif., in March 1907------------ 77 

59. Landslides in artificial fill along the shore of Lake Merced, near San Francisco, Calf., triggered by the San 

Francisco earthquake of March 22, 1957 ______________________________________________________________ 78 

60. Block diagram showing nomenclature of parts of a landslide ----------------------------------------------------- 78 

61. Block diagrams showing common types of landslides in the San Francisco Bay region ----------------------------- 79 

62. Map showing distribution oflandslides in San Mateo County, Calif.----------------------------------------------- 82 

6~66. Maps of part of northeastern Contra Costa County, Calif., showing: 

63. Distribution of landslide deposits _______________________________________________________________________ 83 

64. Generalized slope ------------------------------------------------------------------------------------~ 84 

65. Distribution of geologic units susceptible to landsliding --------------------------------------------------- 85 

66. Relative slope stability _______________________ ---------------------------------------------------------- 86 

67. Map of part of San Francisco Bay region showing location of demonstration profile and estimated length of surface 

rupture along the San Andreas fault for a postulated earthquake of magnitude 6.5----------------------------- 89 

68. Diagrammatic section showing predicted geologic effects of a postulated earthquake of magnitude 6.5 ____________ 91 

69. Diagrammatic section showing Fourier amplitude spectra of ground shaking estimated for an earthquake of magnitude 6.5 _ _ _ ___ _ _ ___ _ ____ _ ____ _ ___ _ _ __ _ __ ___ ___ _ ___ _ ___ _ _ ___ __ .. __ _ _ _ _ _ ___ _ ___ _ __ __ ________ _ _ _ _ _ _ ___ _ ___ 92 
CONTENTS VII 

TABLES 

Page 

TABLE 1. Faults with Quaternary displacement ____________________________________________________________ -- ___________ A10 

2. Historic surface fault displacements associated with earthquakes in the San Francisco Bay region----------------- 13 

3. Width of the main rupture along the Coyote Creek fault resulting from the 1968 Borrego Mountain earthquake ______ 28 

4. Statistics for samples of low-strain amplifications and intensity increments with respect to the Franciscan Formation 

for various geologic units _________________________________________________________________________ --_------- 61 

5. Summary of the analysis ofliquefaction potential using standard penetration data and criteria plotted in figure 49 ---- 72 

6. Parameters for estimating amplitude spectra for shaking of bedrock _______________________________________________ 93 

BASIS FOR REDUCTION OF EARTHQUAKE HAZARDS, 

SAN FRANCISCO BAY REGION, CALIFORNIA 

STUDIES FOR SEISMIC ZONATION OF THE 

SAN FRANCISCO BAY REGION 

ABSTRACT 

Studies by 15 researchers in various earth-science and engineering 

disciplines suggest that seismic zonation of the San Francisco Bay 

region is feasible using existing geologic and geophysical knowledge. 

Seismic zonation is defined as the delineation of geographical areas 

with different potentials for surface faulting, ground shaking, 

flooding, liquefaction, and landsliding during future earthquakes of 

specific size and location. Seismic zonation, as defined, is a necessary 

foundation for the development of regional land-use policies to 

minimize future losses during earthquakes. The need for seismic 

zonation was clearly demonstrated by the large variations in damage 

resulting from the great California earthquake of April 18, 1906. In 

some areas of the San Francisco Bay region, the losses of life and 

property were catastrophic, whereas in other areas the losses were 

minor. 

In an integrated sequence of papers, data required for seismic 

zonation are compiled and analyzed. Methodologies are emphasized 

for constructing the necessary tools from data currently available on a 

regional scale. Basic tools derived for seismic zonation are (1) a map 

showing active faults, (2} data on attenuation of shaking in bedrock, 

(3) geologic data, (4} a map showing qualitative estimates of ground 

response, (5) a map showing areas of potential inundation by 

tsunamis, (6} a map showing liquefaction potential, and (7) a map 

showing landslide susceptibility. 

The map showing active faults delineates areas of potential faulting 

of the ground surface and the location of potential sources of strong 

ground shaking. Categorizing the faults according to geologic and 

geophysical evidence for recency of movement and fault length 

permits crude estimates of earthquake potential, maximum earthquake magnitude, and characteristics of future ground deformation. 

The San Andreas, Hayward, and Calaveras faults are considered to 

have the highest potential for large earthquakes, with estimates of 

maximum magnitude of 8.5, 8.2, and 6.5, respectively. 

The data on attenuation of shaking with distance from the source 

suggest that duration and peak-amplitude parameters of ground 

motion can be predicted from empirical relations for sites on bedrock 

and firm alluvium at distances greater than 10, 20, and 40 km (6, 12, 

and 25 mi) for earthquakes ofmagnitu.de 5.0 to 5.9, 6.0 to 6.9, and 7.0 

to 7.9, respectively. Data are not available for smaller distances and 

larger magnitudes; hence extrapolation based on numerical models of 

faulting is required. 

Geologic data provide the basis for extrapolating results oflocal site 

studies to larger areas for purposes of seismic zonation. The 

unconsolidated sedimentary deposits of the San Francisco Bay region 

are differentiated into five geologic-genetic units based on geomorphic 

relations, soils, physical properties, fossils, and radiocarbon dates. 

Regrouping these units according to common sets of physical 

parameters such as thickness, density, sorting, data from standard 

penetration tests, and shear-wave velocities shows that certain 

groups of units are important for studying ground response, others for 

studing liquefaction, and still others for studying slope stability. 

The map showing qualitative estimates of ground response 

delineates on a regional basis those areas for which the effects of 

ground shaking, as amplified by surficial deposits, are expected to be 

least, intermediate, and greatest. These expectations are based on 

analysis of the observed 1906 California earthquake intensities, 

accelerograms recorded from the 1957 San Francisco earthquake, 

amplifications of ground shaking measured at 99 sites, numerical 

models of ground response, and geologic data. Combining the data on 

the potential location and magnitude of future earthquakes, on the 

attenuation of strong shaking on bedrock, and on the estimated 

high-strain response of surficial deposits permits quantitative 

predictions of ground shaking at specific sites. Such predictions 

suggest that certain geologic units substantially amplify frequencies 

of ground shaking near the fundamental mode of the unit and that the 

peak amplification in some instances can be at least as high at 

high-strain levels as that at low-strain levels. 

The map showing areas of potential inundation by tsunamis was 

prepared earlier by Ritter and Dupre (1972) and is mentioned here for 

completeness. The map delineates coastal areas and areas along the 

margins of San Francisco Bay likely to be inundated by an earthquake-generated wave of 6 m (20ft) at Golden Gate Bridge. 

The map showing landslide susceptibility delineates general areas 

where landsliding is considered likely. The map is based on the 

present distribution oflandslide deposits, bedrock geology, and degree 

of slope. The parts of the San Francisco Bay region having the greatest 

susceptibility to landsliding are hilly areas underlain by weak 

bedrock units of slope greater than 15 percent. 

Application of these seven basic tools along a demonstration profile 

for a postulated magnitude 6.5 earthquake on the San Andreas fault 

illustrates a methodology for seismic zonation of the San Francisco 

Bay region at the current state of the art. Pending completion of this 

suggested seismic zonation, a map showing maximum intensities 

delineates areas with potential earthquake problems, and the seven 

basic tools help identify the problems and their possible severity. 

A1 
A2 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

INTRODUCTION 

The San Francisco Bay region (nine bay area 

counties) includes three major active faults and seven 

minor faults. Historically, 4 violent earthquakes and 33 

smaller, but damaging, earthquakes have occurred on 

these faults. As a result, the San Francisco Bay region is 

considered one of the most earthquake-prone urbanized 

areas in the United States. 

The most devastating earthquake to hit the San 

Francisco Bay region was that of April 18, 1906. 

Damage to property from the earthquake and the 

ensuing fires was estimated at $400 million (1906 

value). Approximately 700 people lost their lives. The 

amount of damage from the earthquake was strongly 

dependent on the geologic character of the ground. For 

example, in the Telegraph Hill area ofSan Francisco, 

where rock is exposed at the surface, the effects of the 

earthquake were ~~weak," with ~~occasional fall of chimneys and damage to plaster, partitions, plumbing, and 

the like." But at a distance ofless than one-quarter mile, 

in an area underlain by artificial fill and watersaturated mud, the effects of the earthquake were Hviolent," with ~~fairly general collapse of brick and frame 

structures when not unusually strong" (Wood, 1908). 

Comparison of the 1906 distribution of intensity (fig. 1) 

and the geologic map of San Francisco (fig. 2) demonstrates the need for zoning the region to account for 

variations in earthquake hazards originating from variations in geologic conditions. 

The principal hazards to life and property from earthquakes in the San Francisco Bay region are 

potential failures of manmade structures, such as 

buildings, dams, waterlines, and bridges. Current 

urbanization implies that the effects of another 1906-

type earthquake would be catastrophic. A recent study 

(Algermissen, 1972) pre<licts loss of life ranging from 

2,300 to more than 100,000 people depending on time of 

day and the number of dam failures. The potential loss 

of property is estimated to be billions of dollars; the loss 

in productivity and earnings, substantially larger. Such 

potential losses can be minimized if manmade structures are adequately engineered to withstand future 

earthquakes. 

Construction of earthquake-resistant structures requires the prudent and conscientious application of 

advanced engineering design techniques that consider 

the geologic setting of the surrounding region. In the 

past, the geologic setting has been considered principally on a site by site basis and only for major 

structures (such as high-rise buildings and nuclear 

power plants). For most structures, no consideration has 

been given to the influence of the geologic setting on 

potential earthquake damage. This situation is due 

partly to a past lack of knowledge, partly to the expense 

of assessing the geologic setting on a site by site basis, 

and partly to a lack of appropriate public policy to 

incorporate and enforce developments in the earth 

sciences and engineering. 

The geologic setting of a region influences earthquake 

damage by controlling (1) the potential location and size 

of damaging earthquakes, (2) the potential for rupture 

of the ground surface by faulting, both slow creep and 

sudden movement, (3) the potential for damaging levels 

of ground shaking on different geologic units at various 

distances from the source of the earthquake, ( 4) the 

potential for flooding from dam failures, tsunamis, 

seiches, and tectonic changes of land level, and (5) the 

potential for shaking-induced ground failures such as 

landslides and those related to liquefaction. 

This study summarizes the state-of-the-art for assessing these potential earthquake effects on a regional 

scale for purposes of seismic zonation. Such an 

evaluation of the geologic setting on a regional scale 

provides the necessary foundation for developing 

policies that will minimize future earthquake losses. It 

also provides a basis for incorporating geologic factors 

into codes for the routine design of earthquake-resistant 

structures. 

The different sections of this report examine (1) faults 

and their earthquake potential, (2) estimation of 

bedrock motion at the ground surface, (3) geologic 

parameters for seismic zonation, (4) response of various 

geologic units to shaking, (5) liquefaction potential, and 

(6) landslide susceptibility. Seismic zonation requires 

the composite application of results from these studies 

for potential earthquakes of specific size and location. 

Such an application is demonstrated in terms of a profile 

showing effects predicted for a postulated earthquake 

(magnitude =6.5) on the San Andreas fault. This 

example illustrates the extent to which such effects as 

surface faulting, ground shaking, flooding, liquefaction, 

and landsliding can be predicted quantitatively on a 

regional scale for purposes of seismic zonation. 

The nature of the seismic zonation problem suggests a 

broad audience ranging from the research earth 

scientist and engineer to the practicing land-use 

planner, engineer, and politician. Accordingly, the 

authors have directed much of their discussion toward 

a broad. audience, with many results spanning a wide 

range of interests. The compilations and analyses 

presented in the first six papers are of most interest to 

earth scientists and engineers involved in research. The 

methodology presented in the seventh paper is of most 

interest to those involved in developing and implementing appropriate land-use and construction practices. 
122°30' 

37°48' 

~ 

;:t>. 

Cl 

..... 

'":tj 

..... 

Cl 

37°42' 

.. Very violent 

.. Violent 

STUDIES FOR SEISMIC ZONATION 

EXPLANATION 

(:-::-::::-{:\:1 Very strong 

k :: :1 Strong 

0 2 

0 2 3 KILOMETRES 

A3 

122°22' 

~ ;> 

~ p 

;> 

"' .,.>. 

"' "' c 

ttl 

p 

•.s::: 

c:::::J Weak 

3 MILES 

FIGURE I.-Distribution of apparent intensity of the 1906 earthquake in San Francisco, Calif. (after Wood, 1908). Detailed description 

of 1906 intensity scale for San Francisco is presented at end of report. Compare with figure 2, which shows distribution of geologic 

report units. 
A4 

3]042' 

REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

"'\; 

~ 

Cl 

...... 

'">) 

...... 

Cl 

EXPLANATION 

r1• Bay mud (in places covered by artificial fill as of 1906) 

[}::)}J Alluvium (>30m (100ft) thick) 

CJ] Alluvium (<30m (100ft) thick) 

~Bedrock ~ 

0 

0 2 

en. 

., 

;;s 

~ ., 

;;s 

("') 

..... 

00 

("') 

0 

0:1 

~ 

2 3 MILES 

3 KILOMETRES 

FIGURE 2.- Generalized geologic map, San Francisco, Calif. (compiled by K. R. Lajoie from data ofSchlocker and others, 1958). 
FAULTS AND FUTURE EARTHQUAKES 

By R. L. WESSON, E. J. HELLEY, K. R. LAJOIE, and C. l\1. WENTWORTH 

INTRODUCTION 

The San Francisco Bay region is located within a 

broad complex of faults associated with the San Andreas 

fault system. Movement along these faults and the 

associated geologic deformation have produced the 

mountains and valleys of the California Coast Ranges 

that help make the bay region a scenic place to live. But 

this tectonic deformation is not entirely an asset. It 

continues today, producing small earthquakes that 

wake sleeping residents and earth movements that 

crack sidewalks and buildings. More importantly, 

continued deformation holds potential for generating 

catastrophic earthquakes, which result from the sudden 

movement of blocks of the earth's crust along faults. 

All faults-and they are numerous in the bay 

region-have been surfaces of movement at least once 

in the geologic past. But which faults are likely to 

sustain future movement? And what will be the 

characteristics of this movement? The answers to these 

questions are critical to seismic zonation. If the answers 

can be found, land-use regulation and design and 

construction practices can be instituted to minimize the 

consequences of future movement. Our present methods 

are primarily empirical because physical laws governing earthquake behavior are still inadequately understood. Our judgment of whether or not a fault is likely to 

move in the near future is based on whether or not it is 

moving today or has moved in the recent geologic past. 

This determination is complicated because the geologic 

record of past movement is incomplete. 

It is not now possible to determine with certainty if a 

fault will sustain movement in the future. We must 

assume that if a fault has been active over a considerable length of time (millions of years) and has been historically active or shows evidence of movement in the 

geologically recent past, it will most likely sustain 

movement in the future. Evidence from historically active faults both here and abroad indicates that this 

assumption is generally valid. Extensive studies of the 

geologic and tectonic settings of historic earthquakes, 

particularly those in California such as the great 

California earthquake of 1906, the Kern County earthquake of 1952, the Parkfield earthquake of 1966, and 

the San Fernando earthquake of 1971, reveal that the 

faults responsible for these earthquakes were characterized by at least one or more of the following features: 

(1) historic earthquakes with or without surface fault 

displacement, (2) ephemeral physiographic features 

such as sag ponds, offset streams, and linear ridges that 

suggest recent fault displacement, and (3) offset 

Holocene and Pleistocene deposits and geomorphic features. The presence of these ~haracteristics provides a 

basis for determining which faults in the San Francisco 

Bay region are likely to sustain future movement. 

Faults along which these characteristics are developed 

are commonly termed ~~active faults." 

Faults in the San Francisco Bay region that display 

features like those mentioned above are shown in figure 

3A. They include the well-known San Andreas, Hayward, and Calaveras faults and many other less 

well-known faults potentially capable of causing substantial damage. The faults are classified according to 

the evidence available for recent or current movement. 

This evidence is summarized in table 1 and the sources 

of data are identified on figure 3B. 

All the faults in the San Francisco Bay region are part 

of what is broadly termed the San Andreas fault system. 

Most of these faults trend northwestward, and most 

display a similar sense of movement. This movement 

shifts the rock mass on the southwest side of each fault 

relatively toward the northwest. Fault displacements 

occur suddenly during earthquakes or very slowly by a 

process called fault creep, and for most northwesttrending bay region faults, present-day movement is 

almost entirely horizontal. This kind of fault movement 

occurs on the San Andreas, Hayward, and Calaveras 

faults. It is technically described as strike slip, and 

faults with predominantly horizontal movement are 

termed ~~strike-slip" faults. Most strike-slip faults in the 

San Andreas system, including those just named, 

exhibit a right-lateral sense of movement; that is, to an 

observer looking along the fault zone, the rock mass to 

the right of the fault moves toward him (fig. 4). For 

left-lateral strike-slip faults, the horizontal movement 

is opposite in sense-the right-hand rock mass moves 

away from the observer. 

A different sense of movement characterizes dip-slip 

or vertical-slip faults. Movement on them is predominantly vertical, and the rock mass on one side of a fault 

A5 
A6 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

surface is elevated relative to the opposite mass. 

Depending on the geometry of the fault surface and the 

sense of movement, these faults are termed ~~normal" or 

~~reverse" (or thrust) faults (fig. 4). Although less 

common than strike slip, some dip-slip faults are 

recognized in the bay region. Many bay region faults are 

not yet well enough known to identify the sense of 

movement positively and unequivocally. 

The faults in the San Francisco Bay region are 

grouped geographically and numbered in figure 3B and 

table 1. The groups include the following: 

San Andreas fault-The San Andreas fault (1) trends 

through the Santa Cruz Mountains in the southern part 

of the bay region and along the coastal margin in the 

northern part. The 1906 earthquake made this one of 

the best known active faults in the world. Within the 

last few years, both local and State governments, 

through land-use regulation, have recognized the 

potential danger of this fault. 

Haywarq and related faults-The Hayward fault (2) 

trends northwestward along the base of the hills behind 

the East Bay cities from Fremont northwest to 

Richmond. North of San Pablo Bay, the Rodgers Creek 

and Healdsburg faults (3) continue along much the 

same trend. Farther north, three faults in the northeast 

of Alexander Valley ( 4, 5, and 6) continue the same 

trend. Segments of this fault system were responsible 

for damaging earthquakes in 1836, 1868, and 1969. 

Calaveras and related faults-The Calaveras fault (7) 

diverges northward from the San Andreas fault !South of 

Hollister and continues northward along the eastern 

margin of the Santa Clara Valley and into the Diablo 

Range. Related faults, some of which may be connected 

with the Calaveras, include the Pleasanton fault (8) 

near Pleasanton, the Concord fault (9) through Concord, 

the Green Valley fault (10) north of Suisun Bay, faults 

on the west side of Napa Valley (11), and the Silver 

Creek fault (12) southeast of San Jose. 

Faults west of the San Andreas-Faults west of the 

San Andreas include the Zayante (13), San Gregorio 

(14), and the Seal Cove (15) faults. The Zayante lies west 

of the San Andreas but trends more westerly. The San 

Gregorio trends northward across the mouth of Monterey Bay and along the coast ofSan Mateo County. The 

Seal Cove and associated faults (15) north of HalfMoon 

Bay may represent a northward continuation of this 

zone. The Pilarcitos fault (16) branches westward from 

the San Andreas fault on the San Francisco peninsula 

and may join the San Andreas beneath the Pacific 

Ocean south of San Francisco. 

Faults along the east margin of the Santa Cruz 

Mountains-This group of faults is poorly exposed; it 

includes the Sargent (18), Black Mountain (19), 

Berrocal (20), Serra (21), and Vasona (22) and occurs in 

an irregular band roughly parallel to, and about 5 km (3 

mi) east of, the San Andreas fault. Several of these are 

probably thrust faults with southwest-dipping fault 

surfaces. 

Faults along the west margin of the Great ValleyFaults along the west margin of the Great Valley 

include the Rio Vista fault (24) and the Antioch fault 

(25), which trends through the town of Antioch. The 

Antioch fault may extend northwestward to the 

Montezuma Hills fault (26). 

Faults in the Livermore Valley-Known faults in the 

Livermore Valley include the Livermore, Tesla, and 

Grenville faults (23). 

Faults between San Andreas and HealdsburgRodgers Creek faults-Faults with trends more westerly than the San Andreas fault occur in the San 

Anselmo, Petaluma, and Santa Rosa areas (27, 28, 29, 

and 30). 

Our knowledge of young fault movement in the San 

Francisco Bay region is still incomplete. Some of these 

faults may have moved more recently than is recognized, and other faults with as much potential for 

causing damaging earthquakes may be as yet unrecognized. This chapter is therefore a progress report, not a 

final definitive statement. 

EVIDENCE SUGGESTING FUTURE MOVEMENT 

ALONG FAULTS 

The process that leads earth scientists to believe that 

movement may occur along particular faults is one of 

determining which identifying characteristics result 

from historic fault movement and using these characteristics to evaluate the possibility of movement on 

those faults. What characteristics of the San Andreas 

fault were known prior to the 1906 earthquake? First, 

the fault was known to have produced surface displacement in earlier earthquakes in 1838 and 1890 (Lawson 

and others, 1908; Louderback, 194 7). Second, the trace 

of the fault was characterized by such physiographic 

features as linear ridges and depressions, sag ponds, 

and scarps (Schuyler, 1898; Anderson, 1899). Third, the 

fault was known to offset geologic deposits of Pleistocene age (Lawson, 1893, 1895), formed within the last 

3 million years. These and related characteristics are 

found worldwide along faults with historic movement. 

They are now accepted as evidence that a fault is likely 

to sustain future movement. 

This evidence can be divided on the basis of its age 

into three categories: (1) historic fault displacement-at 

the surface, either suddenly in association with earthquakes or gradually as fault creep, or at depth, inferred 

from earthquakes that can ·be reliably attributed to the 

fault; (2) displacement during Holocene time (last 

10,000 years)-faulted Holocene deposits and faultproduced topography of Holocene age; and (3) displacement during Quaternary time (last 3 million years)-

faulted Pleistocene deposits and fault-produced topog-
STUDIES FOR SEISMIC ZONATION 

380 -

37° 

0 

0 

SAN FRANCISC~f \' 

\ 

EXPLANATION 

Fault with known historic movement 

Historic seismicity, surface rupture, or creep 

Fault with known Holocene movement 

(past 10,000 years) 

Well-defined fault topography or patterns 

of alluvial deposits incompatible with 

surficial processes 

Fault with known Quaternary movement 

(past 3 million years) 

Offset Quaternary strata; bedrock faults 

associated with Quaternary faults 

0 I I I I I I I I I 10 

I 

20 

I 

30 MILES 

I 

0 10 

I 

20 

I 

30 40 KILOMETRES 

A 

/ tV 

'., -, ( 

, SACRAMEN'TIO. b -.1.~- · 

'-:"'\·'' 

l " ',,' "'_' ' ··~ \., ' l-6 

~ 

I 

_;:; F'\~ 

I 4 \ / 

( ; 

.-~-

-' 

A7 

FIGURE 3.-A, Faults that may cause damaging earthquakes or surface displacement in the San Frau~:Isco Bay reg1vn . '1 he mo~t recent 

displacement on these faults is known to have occurred during historic time (past 150 years), during Holocene time (0-10,000 years 

before present), or during Quaternary time (0-3 million years before present). B , Sources of data, with explanation on facing page. 

Numbers on boxes are keyed to table 1, which summarizes the evidence for recent displacement. 
A8 

38° 

37" 

REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

+ Point 

Reyes 

0 I I I I I I I I I 

0 10 

10 

I I 

20 

20 

I I 

30 

I 

30 MILES 

I 

40 KILOMETRES 

B 

FIGURE 3.-Continued. 

........ "\. 

""~ ' '\ ,, '- ..!> '"' ..... 

' . " 

\ \ .r 

~) ' ;. . ~ 

" 

/ 

; 

/ '-
STUDIES FOR SEISMIC ZONATION A9 

raphy of Pleistocene age. (The Holocene Epoch and 

preceding Pleis"ocenP Epoch constitute the Quaternary 

Period.) 

These categories form a framework that encompasses 

geophysical data, data on prehistoric and historic 

events, and geologic data. This framework can be used 

to distinguish recency of faulting on the basis of 

Map Fault name 

No. or locality 

1 . San Andreas 

2. Hayward 

3. HealdsburgRodgers Creek 

Sources of Data 

A. Sarna-Wojcicki, A., and Pampeyan, 

E. H. (unpub. data) 

B. Brown (1972) 

C. Brown and Wolfe (1972) 

A. Radbruch 0 967) 

B. Dibblee (l972b, c, d) 

A. Brown (1970a); Gealey (1951); 

Weaver (I 949 

L. ~Ielley, E. J. (unpub. data) 

4. Northeast of Radbruch-Hall, D. H. (unpub. data); 

Alexander Wentworth, C. W., and Rizzell, 

V. A., Jr. (unpub. data) 

5. Alexander Valley Helley, E. J. (unpub. data) 

6. Big Sulphur Creek McLaughlin, R. J. (unpub. data). 

7. Calaveras A. Brown (1970a) 

B. Dibblee (1972a, b, d; 1973a, b, c, e) 

8. Pleasanton Radbruch (I 968a) 

9. Concord Sharp (19 7 3) 

10. Green Valley Brown (1970a); Brown, R. D .. Jr., 

and Rizzell, V. A., Jr.(unpub.data) 

11. West side Napa Fox and others ( 197 3) 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

22. 

23. 

24. 

Valley 

Silver Creek 

Zayante 

San Gregorio 

Seal Cove 

Pilarcitos 

Monterey Bay 

Sargent 

Black Mountain 

Berro cal 

Serra 

Vasona 

Livermore 

Valley 

Rio Vista 

Dibblee (1972b; 1973c) 

HalL N. T. (unpub. data) 

A. Brown (1972) 

B. Weber and Lajoie (1974) 

Lajoie, K. L., and Brown, R. D., Jr. 

(unpub. data) 

Smith (1960); Lajoie, K. L.(unpub. 

data) 

Greene and others (197 3) 

McLaughlin (19 7 3) 

McLaughlin and others ( 1971 ): 

McLaughlin, R. J.(unpub. data) 

McLaughlin and others ( 1971 ): 

McLaughlin, R. J. (unpub. data) 

Bonilla (1965) 

McLaughlin, R. J. (unpub. data) 

A. California Department of Water 

Resources (1966) 

B. Burke, D. B. (unpub. data) 

Reiche (1950) 

25. Antioch Burke and Helley (197 3) 

26. Montezuma Hills A. Burke, D. B. (unpub. data) 

B. Sims and others (1973) 

27. Southeast of Helley, E. J., and Fox, K. F., Jr. 

Santa Rosa (unpub. data) 

28. Burdell Mountain Wentworth, C. W. (unpub. data) 

29. 

30. 

San Geronimo 

Valley 

Tolay 

Wright, R. H., and Sorg, D. 

(unpub. data) 

Wentworth, C. W. (unpub. data) 

FIGURE 3.-Continued. 

stratigraphic evidence, in the fashion of Wentworth, 

Ziony, and Buchanan (1970) and Ziony, Wentworth, and 

Buchanan (1973). The time span of each category 

overlaps younger ones, emphasizing that displacement 

more recent than the youngest identified may have 

occurred. Full use of this framework is possible only 

where much information is available, as in the San 

Francisco Bay region. In many places, information may 

be limited largely to historic data, topographic features, 

and sparse stratigraphic data. 

EVIDENCE FOR HISTORIC DISPLACEMENT 

Evidence for historic displacement along faults in the 

bay region comes from the historic record of surface 

faulting and studies of recent seismicity. These studies 

are based partly on data collected from networks of 

closely spaced seismograph stations and are providing 

precise locations and focal-mechanism solutions for 

small earthquakes as well as accurate measurements of 

fault creep. 

SURFACE FAULT DISPLACEMENT DURING EARTHQUAKES 

Six earthquakes in the San Francisco Bay area have 

been accompanied by documented surface fault displacement (table 2). These earthquakes occurred on ths 

San Andreas, Hayward, and Calaveras faults (fig. 5). By 

far the most extensive and best described ground 

rupture occurred with the 1906 earthquake along the 

San Andreas fault (Lawson and others, 1908). During 

this earthquake, surface displacement across the fault 

was as much as 5 m (16 ft) 1 (fig. 6A ). The pattern of 

disturbed ground along the fault, typical of surface 

displacement during earthquakes on strike-slip faults, 

is frequently described as a giant mole track (fig. 6B). 

Ground rupture along the Hayward fault accompanied 

the earthquake of 1868 (Lawson and others, 1908; 

Radbruch, 1967), and surface displacement due to fault 

slip accompanied four other earthquakes in 1836, 1838, 

1861, and 1890 (table 2). At least three additional 

earthquakes in the San Francisco Bay area (1800, 1865, 

1911) were large enough to have caused surface 

displacement, although none was described. 

FAULT CREEP 

Tectonic fault creep consists of gradual relative 

movement across a fault at rates as large as a few 

centimetres (an inch or two) per year. It is less 

1 A larger displacement, 6 m (20ft), is commonly cited as the maximum. This figure appears 

to be based on observations by G. K. Gilbert (in Lawson and others, 1908\ of fault 

displacement of a road near the south end of Tomales Bay. Gilbert's field notes and his 

published description of this locality (Lawson and others, 1908, p. 71 l suggest that some of 

this displacement was non tectonic and resulted from the shifting of road fill resting on 

marshy ground. Four unequivocal fault offsets located about 2 km (1112 mi) south of the road 

locality averaged about 4. 70 m (15.25 ftl according to Gilbert (Lawson and others, 1908, p. 

711. 
AlO REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

TABLE 1.-Faults with 

Evidence for displacement 

Map number <fig. 3BI 

Fault or group of faults Historic displacement Holocene displacement 

6 

9 

10 

11 

12 

13 

14 

15 

16 

17 

Hl 

19 

20 

21 

22 

San Andreas 

Hayward and related faults: Hayward_ 

Healdsburg--

Rodgers Creek. 

Northeast of Alexander Valley. 

Alexander Valley 

Surface fault displacement associated with earthquakes 

Yes (see Lawson and others (19081; 

Louderback, (194 7 l; Brown and others (196711. 

Yes (see Lawson and others (190RI; Louderback (194 7 l; Radbruch (196711. None known 

None known 

None known 

Big Sulfur Creek. _ _ _ None known_ Calaveras and related C?i faults: Calaveras Yes (see Radbruch (1968ail.' Pleasanton _ _ None observed 

Concord None known 

Green Valley None known __ 

Western side None known_ of Napa Valley. 

Silver Creek None known 

Faults west ofthe San Andreas: Zayante None known 

San Gregorio 

Seal Cove 

Pilarcitos 

Faults in Monterey Bay. 

Faults along the eastern margin of Santa Cruz Mountains: Sargent_ 

None known 

None known_ 

Possibly (see Lawson and others (190811. 

None known _____________ _ 

None known_ 

Fault creep 

Yes (see Steinbrugge and Zacher (19601; Tocher (1960!1. 

Yes (see Radbruch and others 0966 l; Cluff and Steinbrugge 

(1966). 

None known 

Small earthquakes 

Yes (see Brown and Lee (]971JJ. 

Yes (see Brown and Lee (197111. 

Yes (see Unger and Eaton ( 1970 l; 

McEvilly (197011. 

Yes 

Offset Holocene deposits 

Yes (see Helley, Lajoie, and Burke <1972; 

unpub. datal). 

Yes <E. J. Helley 

(unpub. data)). 

Probablv (D. H. Radbruch- Possibly (see Wesson and None observed Hall (unpub. others (1972a, b; 

datal). 1973a, b)). 

None knowu 

None known 

Yes (see Radbruch (1968a)). 1 

Possibly (see Radbruch (1968ai; Gibson and Wollenberg 

(1968)). 

Yes (see Sharp (197311. 

Possibly <M. G. Bonilla, R. D. Brown, and C. M. Wentworth (unpub. datal; V. A. Frizzell, Jr., and R. V. Sharp (unpub. datal; 

Dooley (197311. None known _ 

Possibly <Lowney/ Kaldveer Assoc. (unpub. report, 1971)1. 

None known 

None known __________ _ 

None known_ 

None known 

None known_ 

None known __ 

None known_ None observed 

_ __ Yes __ 

Yes (see Brown and Lee C1971ll. Possibly (see Lee and others (1971)1. 

Yes _________________ _ 

Possibly (see Gibson and Wollenberg (196811. 

Possibly (see Sharp (19731; Yes (E. J. Helley Lee and others (unpub. datal). 

(1972a,b,c, l; 

Wesson and others, (1972a,b; 1973)). 

Possibly (see Lee and others (1972a,b,cJ; Wesson and others (1972a,b,c; 19731). 

Possibly, but not within resolution of data (see Lee and others (1972a,b,cl; Wesson and others (1972a,b, c) I. 

Possibly (see Wesson and others (1972a,b; 19731; Lee and others (1972a,b,c)). 

Possibly (see Wesson and others (1972a,b; 1973 )). 

Yes (see Greene and 

others (19731; 

Griggs (19731). 

Possibly (M. G. Bonilla, R. D. Brown, and C. M. Wentworth (unpub. datal). 

Yes (see Fox and others (19731; E. J. Helley 

(unpub. data)). 

Possibly (Lowney/ Kaldveer Assoc. (unpub. report, 197111. 

Possibly (see Hall and others (in press, 19751) 

Yes (see Greene and others (19731; G. Weber (unpub. datal). 

None known ___________ None known on main 

strand; yes on associated strand (K. R. Lajoie, J. Tinsley, and G. Weber (unpub. datal). None known_______ None known _____ _ 

Possibly (see Greene and Yes (see Greene and and others <1973 l; others (1973 II. Griggs (19731). 

Yes (see Brown and Lee (197111. Yes (see McLaughlin 

(1973)1. 

Black Mountain ______ None known ______ None known Probably (see Lee and None observed 

Berrocal_ None known ___ _ _______ None known_ 

Serra ____________ _ None known __ None known 

Vasona _ None known __ _ None known 

others (1972a,b,c); W. H. K. Lee and P. Bauer (unpub. d,ta)l. 

Possibly (see Lee and None observed_ others (1972a,b,c); Wesson and others (1972a,b,c; 1973)). None known___________ None observed 

Possibly _______________ _ None observed 

23 Livermore Valley (several faults in the eastern Livermore Valley show various types of evidence for Quaternary displacement, including 

ground-water anomalies in young alluvium, creep, and small earthquakes. At present, however, these faults are poorly delineated. (See Bernreuter and Tokarz ( 1972 I; John Blume and Associates ( unpub. report); California Department ofWater Resources ( 1966 l; Hansen (1964 I; M.G. Bonilla and J. E. Schoellhamer (unpub. datal; Gibson and Wollenberg (19681. 

Geomorphic 

features 

Yes (see Brown (1970a,bJ; 

Brown and Wolfe (19721; U.S. Geological Survey 

(unpub. data)). 

Yes (see Radbruch (1968a)). 

Yes <R. D. Brown unpub. data)l. 

Yes (C. M. Wentworth and V. A. Frizzell, Jr. (unpub. datal I. 

Yes (E. J. Helley (unpub. 

data)). 

Yes 

Yes (see Radbruch (1968a)). 

Yes (see Brown (1970aJ; Gibson and Wollenberg 

(19681). 

Yes (see Sharp (1973)). 

Yes (see Brown (1970aJ; 

Dooley (1973); V. A. Frizzell, Jr. (unpub. 

data)). 

Yes (see Fox and others (19731; E. J. Helley 

<unpub. datal). 

Yes (see Brown (1970a)l. 

Yes ___________________ _ 

Yes (see Greene and others (19731; G. Weber, J. Tinsley, and K. R. Lajoi-· 

(unpub. datal). 

Yes (see Glen, (1959); 

Jack (1968); K. R. Lajoie, J. Tinsley, and G. Weber (unpub. data)). 

Yes (see Smith (1960); K. R. Lajoie 

(unpub. data)). Yes <see Greene and others (19731). 

Yes (see Allen ( 1946 l; Brown (1970al; McLaughlin (1973)). 

None observed _________ _ 

Yes (R. J. McLaughlin 

(unpub. datal). 

None observed 

None observed ______ _ 

Faults along the western margin of Great Valley: 24 Rio Vista ____ None known None known None known _________ _ Yes (see Reiche C1950ll. Yes (see Reiche (1950ll. 

25 Antioch None known_ 

Footnotes are at end of table. 

Probably (see Burke and Helley (19731). 

Probably (see McEvilly and Yes <see Burke and Casaday (19671). Helley (197311. Yes (see Burke and Helley (197311. 
STUDIES FOR SEISMIC ZONATION 

Quaternary displacement 

Quaternary displacement 

Offset Quaternary 

deposits 

Yes lsee Cummings 11968!1. 

Yes 

Yes IR. D. Brown and E. J. 

Helley lunpub. data11. 

None observed 

None observed 

Yes IR. McLaughlin 

lunpub. data11. Yes 

Yes 1 see Gibson and 

Wollenberg 1196811. 

Nom• observed_ 

Yes IM. G. Bonilla and 

C. M. Wentworth lunpub. datal; 

Dooley 1197311. 

Yes lsee Fox and 

others 119731; E. J. Helley 

lunpub. datal!. 

Yes I see Dibblee (1972a,b,c 1 I. 

Yes 

Additional factors for assessing 

earthquake potential 

Estimated recurrence 

interval (in years! for maximum earthquake, Magnitude of largest 

inferred from geologic historic earthquake 

slip rate7 

100--1,000 8.3 lsee Lawson and 

lfor magnitude 7-8+ I others 1190811.2, 3 

10--100 7±'.2 lsee Slemmons 1 for magnitude &- 7 I 1196711 2 • 3 

5.7 lsee McEvilly (197011.4 

3--4 lsee Lee and others 11972a,b,c I; 

Wesson and others 11972a,b; 197311. 

None known2 

None known12 _ 

10--100 6 

I for magnitude &-7 1 4.3 lsee Lee and 

others 1197111. 4 

Total known fault 

length lin kilometres! 

I estimate of maximum 

magnitude earthquake 

in parentheses" 1 

1,200 181

/219 

72 17.01 

72 17.01 

35 16.61 

13 1?1 

6.4 

115 1 Hollister to 

San Ramon! 17.31 

9 1?1 

18 (?I 

22 I includes extension 

Geyserville 

to Milpitas, 

163 17.51 

5.4 (see Sharp 11973 I; 

Murphy and Cloud 1195711.5 across Carquinez straits I 16.31 

2-3 (see Lee and 38 16.61 others 11972a,b,ci; 

Wesson and others 11972a, b: 19731).5 

4-5, on possible northward estension IR. L. Wesson (unpub. data11. 5 

2-3 (see Lee and others 17 1?1 

11972a,b,c I; 

Wesson and others 11972a, b; 197311. 5 

3.5 (R. L. Wesson and 20 (minimum estimate! 

others (unpub. datall 4 (6.21 

Not known 82 (7.41 

200 I inc! udes 

Present ability to predict pattern of 

surface faulting 

Generally good, locally 

very good. 

Generally good, locally 

very good. 

Generally good, locally 

very good. 

Locally very good, 

abundant evidence, fault not well mapped. 

Locally very good. Fault not well mapped, 

Locally very good. 

Generally fair, locally 

very good to very poor. 

Locally very good. 

Locally very good. 

Poor. 

Poor. 

Poor. 

Yes (see Greene and 

others 119731; Brabb (197011. 

Yes lsee Jack 119681; 

6.1 (see Richter 1195811s,Io 135 17.41 

None known12 _ 3 1?1 

} 

possible north- {Locally very good. ward extensiOn to San Andreas 

fault, connecting at Bolinas! Locally very good. 

Cooper 11971 I; K. R.Lajoie, J. Tinsley, and G. Weber 

lunpub. datal!. 

Yes lsee Cummings 11968 II. 

Yes (see Greene and 

others 1197311. 

Yes I see McLaughlin 1197311. 

Yes lsee Dibblee (19661; 

Pampeyan 119701; 

R. McLaughlin lunpub. datal I. 

Yes IR. J. McLaughlin and _ D. Sorg lunpub. datal I. 

Yes I see Bonilla 11965 II. 

Possibly 

Yes lsee Reiche (195011. 

Yes lsee Burke and Helley 1197311. 

(7.6111 

None known12 _ - - 43 (6.71 

6.1 (see Richter 11958ils,Io 42 (across entire bayl 16.71 

5.0 (see McEvilly 1196611. 4 95 (Portola Valley to Hollister! (7.4 1 

33 (Mount Madonna to 

Hollister I 16.71 

55 (Lake Eisman to Hollister! (6.91 3.6 (see Lee and 31 (Portola Valley to 

others 1197211.4 Los Gatos! (6.71 

4.5? IR. L. Wesson 33 (Los Gatos to (unpub. datall. 5 Mount Madonna! (6.71 

None known12 

2-3 (see Lee and 

• others 11972a,b,ci; Wesson and others (1972a,b; 197311. 5 

4.9 (see McEvilly and 

Casaday ( 1967 I; 

1899 earthquake6 ; 

Tocher (195911. 4 

4 1?1 

14 1?1 

5 1?1 

14 (?I 

37 (including en echelon 

northward extension! (6.61 

Locally good. 

Poor. 

Poor. 

Very good. 

Poor. 

Poor. 

Locally very good. 

All 

Comments 

Right-lateral strike-slip 

fault, maximum displacement in 1906, 6 m 

120 ftl. 

Right-lateral strike-slip 

faults. 

Right-lateral strike-slip 

faults. Northward 

extension of Green Valley 

probable. 

Northward extensiOn toward San Jose not well 

known. 

Right-lateral strike-slip 

fault. Southward extension. 

Southward extension on 

shore probable. 

Steep south~est-d1ppirig 

right-lateral fault, up on 

southwest side, dip decreases to northwest. 

Westward-dipping thrust 

fault. 

Westward-dipping thrust 

fault. 

Westward-dipping thrust fault. 

No longer exposed, buried by 

dredged material. 

Right-lateral strike-slip 

fault. 
A12 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

TABLE 1.-Faults with 

Evidence lor displacement 

Map number (fig. 381 

Fault or group of 

faults Historic displacement Holocene displacement 

26 Montezuma Hills _____ _ 

Faults between San Andreas and Healdsburg-Rodgers Creek faults: 

Surface fault displacement associated with earthquakes 

None known ___________ _ 

Southeast of None known ____ _ 

Santa Rosa. 

Fault creep 

None known_ 

None known_ 

Small earthquakes Offset Holocene deposits 

Yes (see Wesson and Possibly ________________ _ 

others (1972a,b,c: 19731). 

None known ___________ _ None observed 

Geomorphic 

features 

Yes m. B. Burke (unpub. 

datal). 

Yes (E. J. Helley (unpub. 

data)). 27 

28 Burdell Mountain ____ None known _______ _ None known ___________ None known __________ _ None observed __ Yes (C. M. Wentworth (unpub. datal!. 

29 

30 

San Geronimo Valley. 

None known____________ None known____________ None known ___________ _ None observed _________ _ Yes !R. H. Wright (unpub. 

datal; Berkland (196911. Tolay _______________ _ None known_____________ None known __ 

1Cited paper contains numerous additional references. 2Magnitude uncertain or estimated. 3Ground rupture along surface trace of fault associated with earthquake. 

None known ___ _ None observed Yes (C. M. Wentworth (unpub, datal!. 

4lnstrumental location of epicenter and focal mechanism suggest occurrence on named fault. 5lnstrumentallocation of epicenter compatible with, but not compelling evidence for, location on named fault. 6Location of epicenter based on data from felt earthquakes and is compatible with occurrence on named fault. 7Recurrence intervals for maximum magnitudes (from Wallace, 1970). "Maximum magnitude estimated assuming (1) total length of fault is known and (21 half the total length would break in maximum earthquake. The maxi mum magnitude is calculated 

Right-lateral 

strike slip 

Dip slip 

(reverse) 

Left-lateral 

strike slip 

Dip slip 

(normal) 

FIGURE 4.-Four types of fault movement, characterized by the sense 

of movement relative to the fault and to the horizontal. Most faults 

in the bay region show right-lateral strike slip, the characteristic 

sense of movement for the San Andreas fault. Movement on 

oblique-slip faults has both strike- and dip-slip components. 

spectacular than the sudden fault movements that 

accompany earthquakes. Fault creep is most obvious 

where it breaks or offsets streets, curbs, sidewalks, and 

other structures (fig. 7). Such breaks and offsets, where 

mapped through populated areas, define linear trends 

that approximate those of underlying bedrock faults. 

Fault creep was first measured in central California in 

1956 along the San Andreas fault south of Hollister 

(Steinbrugge and Zacher, 1960; Tocher, 1960), although 

its existence was predicted many years before (Louderback, 1942). Fault creep has now been documented for 

long sections of the San Andreas fault (Brown and 

Wallace, 1968; Nason, 1971; Savage and Burford, 1973) 

and the Hayward fault (Cluff and Steinbrugge, 1966; 

Bonilla, 1966; Blanchard and Laverty, 1966; Radbruch 

and Lennert, 1966; Bolt and Marion, 1966; Radbruch, 

1968b; Nason, 1971) and at several localities along the 

Calaveras fault (Rogers and Nason, 1971). Evidence for 

fault creep has been described along the Concord fault 

(Sharp, 1973) and along the Antioch fault (Burke and 

Helley, 1973). Fault creep is now recognized as a 

common and widespread characteristic of active faults 

in the San Francisco Bay region (fig. 8). 

SMALL EARTHQUAKES 

Small earthquakes provide convincing evidence of 

contemporary fault movement where (1) many are 

alined along a fault, (2) the sense of movement inferred 

from them is systematic, and (3) that sense of movement 

agrees with the sense of movement derived from 

geologic data. 

An abundance of small earthquakes does not, by 

itself, demonstrate that a fault has the potential to 

generate a large earthquake, but in California several 

recent earthquakes in the magnitude range from 5.0 to 

7. 7 were preceded by many small earthquakes clustered 

near the larger one (Wesson and Ellsworth, 1973). 

To demonstrate that small earthquakes are related to 

a given fault requires both relatively large numbers of 

earthquakes and the ability to locate them accurately. 

Accurate location depends primarily on the number and 

distribution of seismograph stations and also on the 

ability to correct for the complexities of wave propagation in the earth's crust (Wesson and others, 1973b). 

Most of the accurately located earthquakes in the San 

Francisco Bay region can be assigned to faults with 

historic or geologic evidence for recent movement 

(fig. 9). 
STUDIES FOR SEISMIC ZONATION A13 

Quaternary displacement 

Add ittonal factors for assessing earthquake potential 

Quater nary displacement 

Oflset Quaternary deposits 

Estimated recurrence 

inte rval (in years I for 

ma.ximum earthquake. 

infe rred from geo logic 

Magn 1tude of largest 

h 1storic ea rthquake 

Tota l known fa ull 

length (in kilometres I 

(est imate of maximum 

magmtude earthq uake 

Present ability to predict pattern of 

surface fau lt ing 

Commen ts 

slip ra t e7 in parentheses"! 

Yes (D. B. Burke (unpub. data l). 

Yes (E. J. Helley 

(unpub. data)). 

Yes (C. M. Wentworth and E. J . Helley (unpu b. 

da ta l!. None observed __ 

None observed __ _ 

None known 

None known 12 

None known12 

None known 12 

None known 12 __ __ _ _ __ _ 

16 (?) 

27 16.4! 

19 (?) 

15 (?I 

11 (?I 

Locally good. 

Poor. 

Locally good. 

Locally good. Fault 

not well mapped. Fau lt not well mapped. 

} 

Primarily right-lateral strike-slip fa ults, but also have significant reverse 

displacement . Northeast 

side up a long more westerly reaches. 

by tak ing the arithmetic average of three estima tes using t he empirical relations of Tocher ( 19581 M -0.9 log.o iLJ +5.6 , !ida (19651 M - 0.76 log.o iLJ + 6.07, and either Bonilla and Buchanan (1970! M 2.57 log.o (LI + 2.79 for strike-slip faults or Bon ill a and Buchanan (1 9701 M 2.96 logw (LJ + 1.85 for other faults, where M is magnitude and L is one-half the 

fa ult length in kilometres. Both the assumptions and the empirical magnitude-fault lengt h re lations are to some degree unce rtain ; therefore, the estimated maximum magnit ude 

must be regarded as no more than a crude estimate. Only fa ults longer than 20 km (1 2 m i ) .-\ere considered. 9Max imum magnit ude for the San Andreas fault is assumed to be equal to the magnitude of a historic eart hquake in 1906. 10Uncertai nty in location of ep_icenter ~r its assignment of eart hquake ~tobe r 22, 1926) to either the M?nter ey Bay or San Gregorio fa ult zone. 11Greene and others (1 973), us1 ng a vanety of magni tude-fau lt length relatiOns, estimated maximum magmt udes between 7.2 and 7.9 for this fault zone . 12No significant earthq uake can be assigned reliabl y to th is fau lt on basis of present data . 

T ABLE 2.- H istoric surface l ault displacements associated with earthquakes in the S an Francisco Bay regwn 

Date F ault Rupture length Locality References 

June 10, 1836 __________ Hayward ____________ Unknown ________ _____ ___ Hayward _____ __________ _ Louderback (194 7) 

Louderback (1947). 

Brewer (1930); 

Trask (1964); 

Witney (1865); 

Late June, 1838 ________ San Andreas ___ ___ __ Unknown ________ ___ _____ Woodside ___ __ __________ _ 

July 3, 1861 ____________ Calaveras __________ Unknown ______ __ ____ ___ 29 km (18 mi) northwest 

of Calaveras Reservoir, 

west side San Ramon 

Valley, Dublin. Lawson and others (1908); 

Radbruch (1968b). 

October 22, 1868 _____ ___ Hayward ___ ______ ___ >30 km (20 mi) ______ ____ Warm Springs northward Lawson and others (1908). 

to Sa n Leandro, possibly 

as fa r north as Berkeley. 

April 24, 1890 __________ San Andreas ___ __ __ _ > 10 km? (6 mil ________ __ San Juan Bautista to Lawson and others (1 908). 

Pajaro Gap?. 

April 18, 1906 __ ____ __ __ San Andreas ___ _____ > 430 km (270 mi) ________ San Juan Bautista north- Lawson and others (1908). 

EVIDENCE FOR HOLOCENE DISPLACEMENT 

Holocene deposits are those formed during the last 

10,000 years, during which climatic and sea level 

conditions have been similar to those now prevailing. 

These deposits are common where depositional processes are still active, in such places as stream flood plains 

and terraces, alluvial fan surfaces, low coastal terraces, 

marshes, and beaches. The age of these deposits is 

determined in various ways, but the most reliable age 

assignments depend ultimately on radiocarbon dating. 

Where Holocene deposits are displaced or offset by 

faults, the fault surface-and the movement that 

produced it-must be younger than the deposits. 

Although the age of landforms such as hills, terraces, 

valleys, and stream channels is more difficult to 

determine, some can be shown to have formed during 

Holocene time. Where landforms of known Holocene 

age are cut or displaced by a fault, they too provide 

evidence that establishes the time of fault movement. 

ward to Shelter Cove 

or Point Delgada. 

Faults that cut or displace Holocene deposits or 

Holocene landforms must have moved within the last 

10,000 years, a long time by conventional calendars but 

a brief and very recent episode in geologic time. Major 

geologic processes like faulting are long lived, lasting 

millions or tens of millions of years. Thus it is prudent to 

consider a fault that has moved within the past 10,000 

years as still active and as a factor to be weighed 

carefully in planning for the future. 

OFFSET HOLOCENE DEPOSITS 

Holocene deposits in the San Francisco Bay region 

consist primarily of stream and marine terrace deposits, 

alluvial fan deposits, the muds deposited in San 

Francisco Bay, beach deposits, and slope wash or 

colluvium. These deposits can be identified and dated by 

their surface morphology and the type of soil profiles 

developed upon them, by the presence of shells or bones 

from modern species, by the presence of aboriginal 
Al4 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

38° 

0 

EXPLANATION 

Fault with historic surface displacement 

Trian,qle next to date indicates termination 

of known surface rupture 

37. 

Quaternary fault 

Displacement occurred during past 

3 million years 

0 10 I I I I I r 1 1 1 1 

10 

I 

20 

20 

I I 

30 

I 

30 MILES 

I 

40 KILOMETRES 

_, · --~ ... ...-·--

FIGURE 5.-Zones of surface fault displacement associated with earthquakes in the San Francisco Bay region during historic times 

(see table 2). See figure 3A for explanation of symbols used for Quaternary faults. 
STUDIES FOR SEISMIC ZONATION A15 

FIGURE 6.-Examples of surface displacement that accompanied the 

1906 earthquake. A, Fence offset 2112 m (8 ft) by right-lateral 

displacement on San Andreas fault. Trace of fault approximately 

perpendicular to fence. One kilometre (V2 mi) northwest of 

Woodville, Marin County. Camera was alined with straight part of 

fence at right (in middle ground) to illustrate the zone of flexure 

beyond the abrupt offset. Total offset including flexure was about 

3.5 m (11ft) (from Lawson and others, 1908, plate 49A). B, "Mole 

track" produced by right-lateral displacement on San Andreas fault 

1 Y2 km (1 mi) northwest of Olema, Marin County (from Lawson and 

others, 1908, plate 40A). View northwest. 

artifacts or skeletal remains, or by radiocarbon dating 

of enclosed organic materials (Helley and Brabb, 1971; 

FIGURE 7.-North curb on Sixth Street in Hollister, Calif. , offset by 

right-lateral fault creep on the Calaveras fault. General trend of 

fault trace indicated by dashed line . Date of street construction is 

1925±2 years. Fault trend and date of street construction from 

Rogers and Nason (1971, figure 7). View east. 

Helley and others, 1972; Wright, 1971). Holocene 

faulting of these deposits is expressed as linear scarps 

on modern flood plains, anomalously straight contacts 

of fluvial deposits, and the disruption of surface and 

subsurface hydrologic processes along relatively 

straight lines. The study of trenches dug across 

suspected fault zones is one of the most useful 

techniques for determining the recency of fault movement; however, it is not infallible and does require 

careful attention in locating the trench, preparing the 

trench walls, logging the exposed stratigraphic and 

structural relations, and dating any amenable material. Most important, a trench must expose geologic 

relations that provide unequivocal evidence of the 

relative age of fault movement. 

A sketch of a trench wall (fig. 10) through the fault 

break of the 1968 Borrego Mountain earthquake in 

southern California provides an example of this 

technique. The relative movement along this fault zone 

is primarily strike slip, with a small dip-slip component. 

This dip-slip movement offsets the deposits vertically 

across the fault. The amount of this offset was measured 

at four stratigraphic levels in the trench. The progres-
A16 

37° 

REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

-

-

EXPLANATION 

Fault on which creep has occurred during 

historic time 

Fault with known Quaternary movement 

• 0.5 cm/yr 

(27 yr) 

0.5 cm/yr Average annual creep rate 

(27 yr) Length of record 

0 I I l l I I I I I 

10 

I 

20 

I 

I 

30 MILES 

I 

10 

I 

20 

I 

30 40 KILOMETRES 

' \ 

• < < " ~ 

,, 'sACRAMEN'ROfl 

)c: \ 

\ 

I } l 

, I· 

) (.I'. 

! 

FIGURE B.-Distribution of documented fault creep along faults in the San Francisco Bay region. Creep rate on the Hayward, Calaveras, 

and San Andreas faults from Nason (1971). SPC, offset curb in San Pablo; CC, offset curb in Concord; A, Antioch fault; BT, offset 

tunnel in Berkeley; HAC, offset curb in Hayward; IB, offset building in Irvington; SS, deformed survey array in Sunol; ABR, offset 

bridge at Anderson reservoir; HC, offset curb in Hollister; and SJB-N, offset fence north of San Juan Bautista. Creep rate on 

Concord fault from Sharp (1973). No data available for creep rate on Antioch fault (Burke and Reiley, 1973). See figure 3A for 

explanation of symbols used for Quaternary faults. 
STUDIES FOR SEISMIC ZONATION A17 

sively older beds show greater offset, and a graph of the 

offsets against radiometric age indicates a rate of 

vertical deformatiP'l. Analysis of this data not only 

provided abundant evidence for pre-1968 Holocene 

movement, but also permitted a quantitative estimate 

of the rate of deformation. 

PHYSIOGRAPHIC FEAT URES 

Faults that are surfaces of contemporary or youthful 

movement can also be identified by how they interact 

with other geologic processes. A fault undergoing 

tectonic creep or one with recent episodes of abrupt 

displacement causes subtle but distinctive changes in 

the terrain it crosses. For a series of similar repetitive 

events, these changes are additive. Anomalous and very 

distinctive patterns are produced where active faults 

cross streams, landslides, basins that are concurrently 

undergoing deposition, and other ongoing geologic 

processes. The alined features that produce such 

patterns include scarps, trenches, notches, ridges, 

stream offsets, sag ponds, and lines of springs or 

vegetation (figs. 11, 12). Some of these features are 

direct results of fault movememt, but some have more 

complex origins. 

The absence of identifiable fault topography in places 

along the fault zone does not necessarily imply lack of 

recent displacement. The preservation of such features 

depends on the local rates of erosion and deposition, 

which vary greatly from place to place. Landslides and 

downslope movement of soil, as well as many other 

geologic processes, can effectively bury or erase the 

physiographic evidence of fault displacement within a 

few years. 

Although topographic features caused by fault 

movement seldom can be dated precisely, many can be 

dated approximately by geologic interpretation. In the 

bay region, many certainly were formed during 

Holocene time and therefore indicate the youngest 

category of prehistoric fault movement. Recently, the 

dating of sediments in some sag ponds along the San 

Andreas fault in the San Francisco Bay region (Andrei 

Sarna-Wojcicki, written commun., 1973) has confirmed 

their previously tentative age assignment as Holocene. 

EVIDENCE FOR QUATERNARY DISPLACEMENT 

Displaced deposits of Pleistocene age and some 

fault-produced physiographic features record fault 

movements that took place between 10,000 and 3 

million years ago. Faults that display such evidence are 

here considered Quaternary faults (fig. 3A)-they 

clearly have moved in Pleistocene time and may have 

moved during Holocene time. 

Pleistocene deposits in the San Francisco Bay region 

consist of alluvial fan and stream terrace deposits 

10,000 years old and older, marine terrace deposits 

70,000 to 1 million(?) years old, older San Francisco Bay 

mud, and semi-indurated, structurally deformed continental sand and gravel deposits. The Pleistocene 

deposits are differentiated and dated approximately by 

their relation to present drainage, degree of erosional 

dissection, relative development of soil profile, degree of 

induration and weathering, fossil assemblages and 

radiometric ages. Many faults that cut these deposits 

are expressed as subdued linear surface features or as 

near-surface ground-water barriers. 

Emergent wave-cut marine terraces and their associated deposits record Quaternary faulting and 

associated warping along the coast. In some places 

approximate rates of deformation can be inferred from 

the relations shown by sequences of deformed terraces 

(fig. 13). 

ESTIMATING THE RATE OF FUTURE 

MOVEMENT ALONG' FAULTS AND THE 

CHARACTERISTICS OF ASSOCIATED 

EARTHQUAKES 

When the evidence suggests that future fault movement is likely, questions arise about the frequency and 

characteristics of the expected earthquakes. What 

proportion of the future movement will be associated 

with large earthquakes, and what proportion will occur 

as creep? What will be the characteristics of surface 

fault displacement? How much displacement may occur 

in a given event? But even more important are 

questions regarding the size of future earthquakes and 

the intensity of shaking. Until earthquake processes 

are better understood, the past must be used as the 

primary guide to the future in considering these 

questions. 

Present estimates ofthe maximum size and frequency 

of earthquakes along a given fault are based on (1) the 

geologically determined rate of slip and historic records 

of ground deformation, (2) the seismic history of the 

fault and the surrounding tectonic regime, (3) geologic 

evaluation of the tectonic setting, and (4) the empirically derived relation between magnitude of earthquakes and fault length or other parameters. 

GEOLOGICALLY DETERMINED SLIP RATES 

The offset of distinctive rock units establishes the rate 

of fault movement only within fairly wide bounds. 

Commonly these offsets average the rate of movement 

over millions of years and cannot be used to distinguish 

between sudden slip and creep. Data for the San 

Andreas fault suggest an average slip rate of 1-2 cm/yr 

(0.4-0.8 in./yr) over the last 20 million years (Dickinson 

arid Grantz, 1968). But to predict movements in the 

immediate future, the most recent few hundreds to 

thousands of yea.rs are the most important. 

The history and rate of fault movement have been 

obtained within this brief time period in a few special 

circumstances in southern California using absolute 

age dating techniques. Clark, Grantz, and Rubin (1972) 
A18 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

+ 

/ ' - ,f{ Point 

Reyes 

EXPLANATION 

Quaternary fault 

+ 

Earthquake epicenter 

0 I I I I I I I I I 

10 

10 

I I 

20 

+ 

20 

I I 

30 

I 

30 MILES 

I 

40 KILOMETRES 

A 
STUDIES FOR SEISMIC ZONATION A19 

were able to establish closely the slip rate along the 

Coyote Creek fault in southern California by radiometrically dating sediments of Holocene Lake Cahuilla (fig. 

10). The 1971 San Fernando earthquake led to similar 

investigations (Bonilla, 1974) that provide evidence 

bearing on recurrence intervals on another fault. Such 

investigations can allow joint estimates of the magnitude and frequency of prehistoric earthquakes. For 

example, Clark, Grantz, and Rubin (1972) estimated 

that an earthquake with a similar amount and type of 

displacement, and presumably a similar magnitude, to 

that of the 1968 earthquake would be required along 

each segment of the Coyote Creek fault about every 200 

years to account for the movement accumulated over 

the past 3,000 years. Comparable observations are not 

yet available in the San Francisco Bay region but are 

likely as more data are obtained. 

A major problem in estimating magnitudes and 

recurrence intervals is to allow for fault movement due 

to creep or to other aseismic processes. Wallace (1970) 

estimated magnitudes and recurrence intervals for 

maximum expectable earthquakes along parts of the 

San Andreas fault system by relating to a particular 

tectonic model the long-term slip rate inferred from 

geologic observations and the measured offsets from 

earthquakes and creep. This model assumes that the 

presently observed creep rates represent the longer 

term rates and predicts that the higher the rate of 

B 

tectonic creep, the longer the recurrence interval for an 

earthquake of a given magnitude. The recurrence 

intervals and maximum magnitudes obtained in this 

way for the San Andreas (100--1,000 years for magnitude 8+) and Calaveras (10--100 years for magnitude 

6-7) faults are no more than a general guide to the 

frequency of potentially hazardous earthquakes; they 

represent only one of several possible interpretations of 

the creep-earthquake relation, but they are a step 

toward answering the questions of expected magnitude 

and frequency. Eventually it may be possible to combine 

the geologic data on past rates of movement and fault 

creep with geodetic measurements of contemporary 

movements (for example, Savage and Burford, 1973) to 

obtain more accurate estimates of recurrence intervals. 

SEISMIO HISTORY 

The historic record of seismicity helps in estimating 

characteristics of future earthquakes by providing a 

measure of the magnitude of earthquakes that can occur 

along a given fault. In California the 200 years of 

written history and 40 years of reasonably good 

instrumental records are a small sample of the events 

on all active faults, but the usefulness of the available 

record can be expanded by assuming that similar faults 

are capable of sustaining similar earthquakes. Thus one 

can assume that an earthquake with a magnitude 

comparable to the 1868 earthquake along the Hayward 

f 

FIGURE 9.-A, Epicenters of earthquakes in San Francisco Bay region, 1969--72, with magnitudes ~1.0 (Lee and others, 1972 (three 

references); Wesson and others, 1972a and b, 1973a, 197 4). The pattern of epicenters is incomplete because distribution of seismograph stations is uneven and has changed with time. Most epicenters lying in elongate concentrations parallel to major faults 

were most likely associated with earthquakes on these faults. Thelocationoftheseepicentersoffthe fault traces stems from the 

necessarily simplified crustal model used in the location procedure. See figure 3A for explanation of Quaternary fault symbols. 

B, Relation between fault surface, earthquake epicenter, and earthquake focus. F, earthquake focus or hypocenter (that point, 

generally at some depth within the earth's crust, from which the seismic energy appears to radiate); E, earthquake epicenter (a 

point on the earth's surface that is vertically above the focus); f, fault surface (shaded) (a fracture surface along which failure and 

accompanying dislocation have displaced adjacent blocks of the earth's crust). In both diagrams arrows indicate a right-lateral 

strike-slip sense of movement, as on the San Andreas fault. 
A20 

(/) 

LLJ 

0::: 

1-

LLJ 

~ 

EAST 

1 

0 

~ 2 

I 

1-

a.. 

w 

Cl 

3 

4 

2 4 

REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

WEST 

2 

........ · ... 0 

740 mm (29.2 in.) 

1.7 m (67 in.) 

HORIZONTAL DISTANCE, IN FEET 

6 8 10 12 14 16 18 20 22 24 26 28 

. ·· . . . . . 

30 

2 

4 ILLJ 

LLJ 

u.. 

6 ~ 

:i 

Ia.. 

LLJ 

8 Cl 

10 

12 

14 

5L----L--~~--~~--~--~-----L--~L----L,---~--~----~-,-~L-__ _, ____ ~--~~ 

0 2 3 4 5 6 7 8 9 

HORIZONTAL DISTANCE, IN METRES 

2000 80 

1600 

z 60 z- 0 0 

~f{3 ~ <( 

~a:: 1200 ~(/) 0:::1- O:::LLJ Ol.LJ OI u..~ 

w- 40 t:::iU oj 0~ 

<t!~ .....JZ 800 <(-

~z () 

~--- ~ 0::: 0::: LLJ LLJ 

> 20 > 

400 

0 0 

4400 
STUDIES FOR SEISMIC ZONATION A21 

Offset drainage channel 

Linear valley 

Bench Scarp Sag pond 

Scarp Spring 

Linear 

ridge 

Offset drainage channel 

Linear valley 

or trough 

FIGURE 11.-Block diagram showing landforms developed along recently active strike-slip faults. 

fault could occur along the San Gregorio fault because of active parts of the world (Iran, Japan, Turkey) where 

similarities in tectonic regime, fault geometry, surface historic data cover a much greater time span than in 

expression, and seismic characteristics (Greene and California. 

others, 1973). 

This kind of assumption is given added credibility by 

historical records of earthquakes in other seismically 

FIGURE 10. - Simplified sketch of trench wall showing vertical deformation of initially fiat-lying sediments and sedimentary contacts 

associated with predominantly horizontal movement on the Coyote 

Creek fault, southern California (from Clark and others, 1972, fig. 

83). The trench, dug shortly after the 1968 Borrego Mountain 

earthquake, crosses a branching break of the fault zone along which 

about 50 mm (2 in.) of vertical displacement and about the same 

amount of horizontal displacement took place during the earthquake. Deposits at points A, B, and C were dated radiometrically. 

The vertical displacement of the sedimentary contacts plotted 

against the age of the corresponding deposits yields an average rate 

of vertical deformation of about 0.5 mm/yr (0.02 in./yr) for the past 

3,000 years. This suggests a recurrence interval for earthquakes the 

size of the 1968 event of about 200 years (Clark and others, 1972). 

EMPIRICAL MAGNITUDE-FAULT LENGTH RELATIONS 

Several empirical relations between earthquake 

magnitude and the length of associated surface ruptures along faults have been derived (Bonilla, 1970; 

Bonilla and Buchanan, 1970; Albee and Smith, 1967; 

lida, 1965; Tocher, 1958; Bolt, 1973). These magnitudefault length relations may be used as crude estimates of 

the maximum-magnitude earthquake that might be 

expected from a particular fault, if the length of the 

fault is well known (fig. 14). 

The broad scatter shown by the data is partly due to 

(1) the variety of field conditions in the areas where the 

observations of faulting were made, (2) the inadequacy 

ofthe length of surface fault rupture as a measure ofthe 

length offaulting at depth, and (3) theoretical considerations (Dieterich, 1973; Thatcher and Hanks, 1973). 
A22 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

Another difficult problem is the inability to determine, 

for faults lacking historic ruptures, the proportion of a 

given fault zone that might be involved in any one 

earthquake (Wentworth and others, 1973). Despite 

these serious limitations, magnitude-fault length relations are widely used to estimate the size of the. 

maximum expectable earthquake on known faults. 

Estimates of maximum magnitude for various San 

A 

Francisco Bay region faults are shown in table 1. These 

estimates are based on the assumption that half the 

total fault length can break in any one earthquake; 

however, some of the faults present special problems. 

For example, lengths of 135 km (84 mi) or 200 km (125 

mi) can be assumed for the San Gregorio fault (No. 14, 

fig. 3), depending on whether the Seal Cove fault and its 

offshore extension (No. 15, fig. 3) are considered as part 

FIGURE 12.-Physiographic features of recent faulting along a segment of the San Andreas fault near the Carrizo Plain, southern California. 

A, Aerial view west-southwestward. Low sun angle enhances physiographic features. See B for location. B, Part of strip map of San 

Andreas fault showing topographic features such as offset drainage channels, linear ridges and valleys, and linear scarps associated 

with recent faulting. View inA shown by stippled area (from Vedder and Wallace, 1970). 
\ 

STUDIES FOR SEISMIC ZONATION 

APPROXIMATE MEAN 

DECLINATION, \975 

EXPLANATION 

------ Fault traces 

Solid line, field ar plwtogoologic evilknce of recent movement slwwn 

by scarp lines, trenclws, sag ponds, spring lines, and vegetation 

contrasts 

Daslwd line, less obvious evilknce of recent movement, but very probably a fault break 

Brief notes along fault breaks indicate /ooations wlwrefeatures mentioned are especially clear. Visible fault features are not limited 

to tM locations noted, but are present to some degree all along tM 

mapped fault lines 

0 

0 

/ 

B 

FIGURE 12.- Continued. 

A23 

liz 1 MILE 

.5 1 KILOMETRE 

24 
A24 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

' . ::- ~ # _, . ~~~:.;: ~:~~- ~:~~:~ :;;...~~ ~~ .; •• •• 

B 

FIGURE 13.-A, Faulted wave-cut platform of probable Sangamon age 

(70,000-120,000 years before present) near Point Aiio Nuevo, San 

Mateo County, Calif. B, Sketch showing geologic relations. 

Movement along strand of San Gregorio fault (No.14, fig. 3 and table 

1) has displaced wave-cut platform (B) about 5.2 m (17ft) and thrust 

Miocene siliceous shale (CJ; over unconsolidated Sangamon marine 

terrace deposits (D) and highly sheared Miocene shale (E). Terrace 

deposits overlying the steeply dipping siliceous shales in the 

upthrown block (C) have been removed by erosion. Neither the age 

of the most recent displacement nor the amount of lateral 

displacement associated with the thrust displacement on this fault 

is known. The youngest features displaced are the wave-cut 

platform and overlying terrace deposits, which are probably 

70,000-120,000 years old. Therefore, the displacement is younger 

than that age. Displacement along a related subparallel fault 

strand 1.2 km (0.8 mi) east of this site has also offset this same 

(/) 

w 

0:: 

f--

w 

::;: 

0 

-' 

,:: 

z 

~ 100 - ::::l 

Lt 

z 

;;: 

::;: 

u.. 

0 

w 

0:: 

::::l 

f--

Q. 

::::l 

0:: 

w 

u 

<t 

u.. 

0:: 

::::l 

(/) 

u.. 

0 

I 

f--

(!) 

z 

w 

-' 

10 

5 6 7 

EARTHQUAKE MAGNITUDE 

(/) 

w 

-' 

::;: 

z 

100 ~ ::::l 

<t 

u.. 

z 

;;: 

::;: 

u.. 

0 

w 

0:: 

::::l 

f--

Q. 

0 ::::l 

0 0:: 

w 10 u 

0 <t 

u.. 0 0:: 

::::l 

0 (/) 

u.. 

0 

I 

f--

(!) 

z 

w 

-' 

8 9 

FIGURE 14.-Length of observed surface rupture in relation to 

earthquake magnitude (Bonilla and Buchanan, 1970). Observations of rupture length often underestimate the actual source 

dimension of the earthquake because (1) the rupture expressed 

at the surface may represent only a small part of the total 

rupture or (2) the surface rupture may be obscured by vegetation 

or water. Shown for comparison are four suggested magnitudefault length relations: Bonilla and Buchanan (1970), best fit to 

all the data plotted (world); Bonilla and Buchanan (1970), best 

fit to all the data from strike-slip faults; and Iida (1965) and 

Tocher (1958), best fit to subsets of the data. 

story, because damaging earthquakes of lesser magnitude commonly occur with greater frequency. In fact, 

statistics indicate that over the last 40 years the 

number of earthquakes smaller than a given magnitude 

increases by a factor of about 10 for each unit decrease in 

magnitude. Thus, the frequency of occurrence of various 

sizes of earthquakes is also required to appraise the 

potential hazards of various faults. 

wave-cut platform and deformed alluvial deposits dated at 

9,510±140 years before present (Weber and Lajoie, 1974). ESTIMATING THE CHARACTERISTICS OF 

of the San Gregorio. These alternate assumptions give 

estimates of about magnitude 7.4 and 7. 7, respectively 

(Greene and others, 1973). 

In summary, accurate estimates of the maximummagnitude earthquake for any given fault zone cannot 

be made at present. Consideration of the available data 

in the manner described above yields reasonable 

approximations. Maximum magnitude is not the whole 

GROUND DEFORMATION ASSOCIATED WITH 

FUTURE MOVEMENT ALONG FAULTS 

At the time of the great California earthquake in 

1906, the San Andreas fault broke almost instantaneously along at least 430 km (270 mi) of its length and 

offset the ground surface as much as 5 m (16 ft). The 

main line of rupture followed preexisting faultcontrolled topography and was accompanied by subsidiary faulting and tectonic distortion of the ground 
STUDIES FOR SEISMIC ZONATION A25 

that at least locally extended as far as 1 km to several 

kilometres (thousands offeet to several miles) from the 

main fault (Lawson and others, 1908). Data from this 

and numerous other historic faulting events in the 

world give a basis for estimating the location, character, 

and maximum amount of ground deformation along 

many of the faults in the San Francisco Bay region. 

Of principal concern in fault-related ground deformation are (1) detailed prediction of the pattern of surface 

faulting, especially the width ofthe zone, (2) the amount 

of displacement across the surface traces of faults, and 

(3) tectonic distortion of the ground, including uplift, 

subsidence, and horizontal distortion. 

PATTERN OF SURFACE FAULTING 

The pattern of surface faulting, especially along the 

strike-slip faults, involves a main fault zone of varying 

but generally narrow width along which the principal 

offsets occur and lesser branch and secondary faults 

that extend to, or occur at, considerable distance from 

the main zone (figs. 12, 15). Reverse faults commonly 

produce more complex rupture zones, and the zones 

typically are broader and less regular in plan (fig. 16). 

Major displacements can be expected along lineaments defined by recognizable fault-caused topographic 

features (figs. 12, 15). Studies of several surface faulting 

events indicate that historic ground ruptures closely 

follow mappable geomorphic features that delineate 

preexisting fault traces (1857 Fort Tejon-Wallace, 

1968; 1906 San Francisco-Lawson and others, 1908, 

Wallace, 1969; 1966 Parkfield-Brown and Vedder, 

1967; 1968 Borrego Mountain-Clark and others, 1972, 

Clark, 1972; 1971 San Fernando-Yerkes and others, 

1974; 1973 Managua- Brown and others, 1973); these 

observations suggest that patterns of surface faulting 

are predictable. Clark (1972) estimated, for example, 

that along about 50 percent of the length of the surface 

rupture from the Borrego Mountain earthquake of 

1968, the position of the main surface fractures could 

have been predicted to within about 100 m (300 ft) 

before the earthquake. In the San Francisco Bay region, 

the San Andreas, Hayward, Concord, Antioch, and a few 

other faults are mapped in sufficient detail to accurately 

show the location of fault traces and of the expected 

future displacements (Brown and Wolfe, 1972; Brown, 

1972; Radbruch, 1968a; McLaughlin, 1971; Sharp, 

1973; Burke and Helley, 1973). Much of this map · 

information is adequate to influence decisions on 

structural design and land use. 

The confidence with which surface traces can be 

mapped at a scale of 1:24,000 (1 cm=240 m; 1 

inch=2,000 feet) varies considerably depending on frequency and amount of Quarternary displacement, the 

style of fault movement, and rate of destruction of 

geomorphic features (controlled largely by climate and 

local geology and topography). Recent fault traces along 

strike-slip faults such as the San Andreas can be mapped more confidently than those on faults with dip-slip 

movement. Consequently, dip-slip faults with youthful 

movement are only now being recognized in regions 

where active strike-slip faults have long been known. 

ZONE WIDTH 

Although the most obvious fault displacement tends 

to be localized along recognizable and mappable fault 

lineaments, some permanent ground deformation from 

fault movement extends outward from the main fault 

trace. This deformation, manifested as fractures, 

relatively small surface faults, and local warping, 

defines an irregular zone that parallels and includes the 

more obvious and more continuous traces of the main 

fault. 

The width of this zone of surface deformation varies 

with the type of faulting, earthquake magnitude, the 

local geologic setting, and perhaps other factors. An 

exa::nple of this variation for strike-slip faulting 

associated with the Borrego Mountain earthquake is 

shown in table 3 (Clark, 1972). Because the zone width 

is so variable and because it seldom can be well defined 

by surface morphology prior to a major fault event, 

detailed site studies are usually required for accurate 

delineationof the zone. Such detailed site information 

is not yet widely available. In its absence, estimated 

zone widths are often based on comparison with known 

patterns of deformation associated with welldocumented modern fault geometry accompanying 

major earthquakes. 

Data on zone widths for North American earthquakes 

in the magnitude range from 5.5 to about 8.5 were 

analyzed by Bonilla (1970). The data are sparse because 

only a few events are well documented, but they 

indicate the general range in width of zones that can be 

anticipated. For strike-slip faults, the maximum halfwidth of the zone, from the centerline ofthe main fault 

zone to the outer edge of the deformation zone, is about 

92 m (300 ft). For dip-slip faults the zone is as much as 

900 m (3,000 ft). These values are probably conservative 

estimates except for very large earthquakes. They have 

been suggested as the basis for some kinds of planning 

decisions (Brown, 1972; Hall and others, 197 4), but they 

should be used cautiously and where possible should be 

supplemented by site investigations. Some evidence 

from studies of worldwide data suggests that maximum 

zone width for strike-slip faults may be significantly 

greater than that cited above and that deformation 

zones of strike-slip faults may be as wide as those 

associated with dip-slip faults (U.S. Geological Survey, 

1971b, p. A169). 
A26 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

The designation of deformation zones by these 

criteria is relatively simple where a single fault trace 

can be used as the center of the zone. Where multiple, 

parallel, or overlapping fault traces are recognized, the 

zone width is established by measuring out a half-width 

from the two outermost fault traces. 

Surface offset may also extend well beyond the main 

0 1000 FEET 

0 300 METRES 

CONTOUR INTERVAL 25 FEET 

N 

123° 13'30" 

+ 38°30' 

rupture zone as branch faults and secondary ruptures 

on faults that are not visibly connected to the main zone 

and at considerable distance from it. Large branch and 

secondary faults probably will be recognized as problem 

faults in their own right on the basis of geologic 

evidence, but smaller ones or those with less frequent 

displacement may not. Secondary and branch faults 

EXPLANATION 

Houses -· 

~ Pond 

Stream 

Marsh 

Fault traces 

Furrow or crack ,,,,,,,,,,,, 

Scarp 

FIGURE 15.---Surface rupture associated with the 1906 earthquake along part ofthe San Andreas fault near Fort 

Ross, Calif., showing primary and secondary fault segments. Map simplified from P. Matthes (Lawson and 

others, 1908). 
STUDIES FOR SEISMIC ZONATION 

118°25" 

EXPLANATION 

Tectonic rupture, showing dip 

Queried where tectonic origin is doubtful; dotted where 

surface expression is poor 

Boundaries of zone containing main tectonic ruptures in Sylmar segment of San Fernando fault 

zone 

0.30C 

Magnitude of shortening across tectonic ruptures 

or other compressional features, in metres 

O.BOL 0.04R 

Left Right 

Magnitude of lateral component of movement, 

in metres 

L or R witlwut number sign fies o.ffset less than 

O.Olm 

Direction of shortening at tectonic ruptures or 

component of shortening on features other than 

surface ruptures 

0.5 

D 

Vertical com~onent of relative movement 

Dot or D on downthrown side of rupture, U on upthrown 

side. Magnitude of vertical component of movement 

given in metres 

A27 

FIGURE 16.-Part of detailed map of surface ruptures associated with the 1971 San Fernando earthquake (U.S. Geological Survey, 1971a, 

fig. 2). The greater complexity of this pattern relative to that oft.he 1906 earthquake (fig. 15) reflects the difference in type of faulting 

(reverse slip for 1971, strike slip for 1906) and the properties of the near-surface materials. 
A28 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

TABLE 3.-W idth of the main rupture zone along the Coyote Creek fault 

resulting from the 1968 Borrego Mountain earthquake 

Width of rupture Cumulat ive length Percentage of t.otal 

length of rupture 

m ft km mi 

0-50 0- 150 19.7 12.2 64 

50- 100 150- 300 2.7 1.7 9 

100- 500 300- 1.600 5.1 3.4 17 

500-1.000 1.600-3,300 1.0 0.6 3 

> 1.000 >3,300 2.2 1.4 

historically have shown measurable displacements at 

distances of as much as 10 km (6 mi) from the main fault 

zone. 

AMOUNT OF DISPLACEMENT 

Approximate limits on the amount of surface fault 

displacement associated with future earthquakes can 

be estimated from data on past events and estimates of 

the maximum earthquake magnitude of which a fault is 

capable. Maximum offsets in historic faulting events in 

the San Francisco Bay region range from a centimetre 

(lh in.) or so in some creep events to 5 m (16ft) on the San 

Andreas fault in 1906 (Lawson and others, 1908). 

Wallace (1968) suggested that offset on the San Andreas 

fault in southern California during the 1857 Fort Tejon 

earthquake was about 10 m (30 ft), and so 5 m (16 ft) 

cannot be assumed as the maximum possible for 

magnitude 8 earthquakes in the San Francisco region. 

Other documented evidence in the bay region is limited 

to the 0.9 m (3ft) of horizontal and 0.3 m (1ft) of vertical 

offset that occurred on the Hayward fault in 1868. None 

of the reverse or thrust faults in the region are known to 

have undergone historic surface offset that can be used 

as a guide to future events. 

These data together with other historic data comparing earthquake magnitude and maximum fault offset 

(Bonilla and Buchanan, 1970; Clark, 1972; U.S. 

Geological Survey, 1971b) suggest upper bounds for 

maximum displacements on strike-slip faults of 10 m 

(30ft) for magnitude 8, 6 m (20ft) for magnitude 7, 2m 

(6ft) for magnitude 6, and 0.5 m (2ft) for magnitude 5. 

The available historic data for the strike-slip faults in 

the San Francisco Bay region (Bonilla, 1970) suggest 

that the vertical displacement along these faults may be 

at most about one-third of these values but commonly is 

less than one-tenth. 

TECTONIC GROUND DISTORTION 

In addition to discrete rupture along fault traces, 

large areas of the ground surface can be more subtly, but 

also permanently, affected by vertical and horizontal 

distortions of the earth's crust, including gross uplift 

and subsidence. These effects are related geometrically 

to the main fault offset but may extend for tens of 

kilometres (or tens of miles) from the fault. These 

distortions include bending, warping, and changes in 

elevation (Bonilla, 1970; Sharp and Clark, 1972). The 

1964 Alaska earthquake (magnitude 8.5) caused crustal 

deformation over an area of perhaps 285,000 km2 

(11,000 mi2 ), producing a maximum uplift of12 m (38ft) 

and a maximum downwarp of 21/2 m (7lfz ft) (Plafker, 

1969). This may be an extreme example, and the 

amount of distortion may reflect the fact that it was 

related to thrust faulting; however, similar distortions 

of smaller magnitude and extent have been observed 

with several other earthquakes. Studies of the widespread surface distortion associated with the 1971 San 

Fernando earthquake suggest that its general character could have been predicted from the Quaternary 

geologic history of the area (Yerkes and others, 197 4). 

Commonly the maximum distortion is adjacent to the 

fault, and it gradually decreases with distance away 

from the fault. 

Such distortions must be expected to accompany any 

large earthquake in the San Francisco Bay region. 

Because the most hazardous faults in the bay region are 

characterized by predominantly horizontal movement, 

the amount oftectonic uplift or subsidence probably will 

be less than a metre (about 3ft). However, significant 

changes in elevation or slope can be caused by dip-slip 

faulting or by other mechanisms of surface distortion 

such as landslides. IMPLICATIONS FOR REDUCING 

EARTHQUAKE HAZARDS 

Which faults are capable of generating damaging 

earthquakes? What will be the frequency and character 

of the earthquakes? What will be the pattern and 

character of surface fault displacement associated with 

earthquakes? These questions are central to the 

practical aspects of reducing the hazards associated 

with earthquakes. The answers are still incomplete, but 

the information available now can substantially reduce 

loss of life and property in future large earthquakes. 

Nevertheless, the San Francisco Bay region faults 

discussed are all believed capable of future movement. 

Future loss can be reduced through planning that 

recognizes (1) that earthquakes on these faults will be 

the sources of locally severe and widespread shaking 

and (2) that surface fault displacement and associated 

deformation will be localized along these faults. 

Current knowledge provides an ample basis for this 

kind of land-use planning, the formulation of local 

building codes, and similar governmental decisions. 

Because the most severe ground deformation is typically restricted to a zone several tens to hundreds of 

metres wide along the fault trace, limitations in land 

use or requirements for special engineering design to 

reduce loss need affect only a small area, compared with 
STUDIES FOR SEISMIC ZONATION A29 

the area for which precautions against severe seismic 

shaking should be taken. 

The response that follows recognition of a particular 

fault hazard can vary greatly. One sort of response may 

be appropriate for facilities such as nuclear reactors, 

hospitals, and schools, quite another for parks, recreation areas, or other less potentially hazardous uses. For 

potentially hazardous land uses, prudence demands a 

conservative response even though the evidence for 

current and youthful fault movement is relatively 

weak. But any fault showing evidence for late Quaternary or more recent movement must be regarded as 

capable of future movement and should be carefully 

evaluated in any kind of land-use or planning decision. 

Faults showing evidence for older Quaternary displacement must be regarded with extreme caution, 

especially if they are near sites under consideration for 

potentially hazardous facilities. 

The likelihood that earthquakes of various sizes will 

occur may be taken into account by considering several 

levels of performance of structures during an earthquake. One level might be represented by survival of 

the structures without impairment of function or safety, 

and another level might permit substantial structural 

damage but not so much as to represent catastrophic 

failure. In the siting and design of nuclear power 

reactors, for example, an expectable ((operating basis 

earthquake" and a maximum expectable ((safe shutdown earthquake" are considered (Atomic Energy 

Commission, 1973) so that capital investment and 

generating capacity are protected at reasonable cost but 

so that catastrophe can be avoided in the event that full 

potentials of the fault are realized during the life of the 

structure. Similar procedures can be applied to other 

structures with less extreme impact on public safety. 

The immediate implications of fault creep for any 

engineering structure astride the trace of a creeping 

fault are clear; many structures in the bay region, 

particularly where the Hayward fault passes through 

the densely populated east side of the bay, exhibit 

damage from fault creep. But fault creep also may or 

may not imply a high level of stress in the earth's crust. 

What this means in terms of the potential for a large 

earthquake is not yet known and is currently the subject 

of vigorous debate (see, for example, Savage and 

Burford, 1973; Nason, 1971). Whether tectonic creep 

sufficiently relieves stress to inhibit the occurrence of a 

large earthquake, whether creep is a precursor of a 

large earthquake, or whether the actual situation is 

some combination of these is uncertain at present. But 

fault creep and large earthquakes can occur on the same 

segment of a fault; the sections of the Hayward fault 

responsible for large damaging earthquakes in 1836 

and 1868 have been creeping at an average rate of about 

1 em (Y2 in.) per year for at least the last 54 years (Nason_ 

1971). 

Planning and land-use decisions .at site, local, and 

regional scales should take into account the broader 

zone of deformation as well as fault traces the1nselves. 

Until proved otherwise by geologic site investigations, 

prudence suggests zone widths of 184m (600ft) for the 

largest strike-slip faults and 1,800 m (6,000 ft) for the 

largest dip-slip faults. In the San Francisco Bay region, 

most dip-slip faults are relatively short (less than 16 km 

or 10 mi), and for these, narrower zone widths are 

appropriate. Hall, Sarna-Wojcicki, and Dupre (197 4) 

have assigned zone widths of 850 m (2,800 ft) to the 

Zayante fault, an oblique-slip fault in Santa Cruz 

County. 

The level of potential hazard from branch and 

secondary faults is less than that associated with the 

main fault, but for some facilities it may be essential to 

perform geologic site investigations to insure that 

branch and secondary faults are not a problem. 

Tentative upper bounds on the maximum surface 

fault displacement for an earthquake of a given 

magnitude are 10m for magnitude 8, 6 m for magnitude 

7, 2 m for magnitude 6, and 0.5 m for magnitude 5. 

In the San Francisco Bay region, the impact of 

permanent tectonic distortion of the earth's surface may 

be small in comparison with the damage caused by 

shaking, landslides, and other effects. Vertical movements present problems along shorelines and on canals 

and pipelines. Horizontal distortions may cause problems for pipelines as well but may also cause havoc in 

the definitions of land ownership. 

SUMMARY 

A well-designed program to reduce hazards from 

earthquakes requires concerted and coordinated effort 

both by the scientific and engineering disciplines and by 

those public agents having social and political responsibilities. It also requires an information base that can 

be used as input to engineering analysis and to policy 

decisions. The most fundamental segment of that 

information base deals with (1) the location of faults 

capable of generating damaging earthquakes, (2) the 

magnitude of earthquakes anticipated on these faults, 

(3) the amount of fault displacement anticipated, (4) the 

nature and areal distribution of deformation accompanying earthquakes or fault movement, and (5) the 

frequency of recurrence of earthquakes on a known 

fault. 

The evaluation presented in this chapter reflects our 

current assessment of these topics for the San Francisco 

Bay region, and it suggests an approach that can be 

applied to other areas with high earthquake risk. The 

results are encouraging for those who wish to imple-
A30 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

ment a program of earthquake hazard reduction, but 

they also indicate a need for further effort and for 

attention to new discoveries. 

About 30 faults in the bay region are potentially 

capable of producing damaging earthquakes. Most of 

these can be accurately located, and those that are the 

largest and potentially most destructive can be very 

well located. Detailed maps, suitable for most planning 

and decisionmaking purposes, are available for many of 

these faults. 

Magnitudes of historic earthquakes are known for 

more than half of the recognized faults. These data 

indicate that at least eight moderate or largemagnitude events have occurred on known bay region 

faults and that one very large earthquake (magnitude 

8.3) was located on the San Andreas fault. Current 

methods of estimating maximum magnitude in the 

absence of historic data are still crude, but they provide 

an approximate measure of the size of earthquake that 

can be expected on faults that have no historic record of 

damaging earthquakes. 

Fault displacement of as much as 5 m (16 ft) was 

recorded after the 1906 earthquake on the San Andreas, 

and maximum horizontal displacement of as much as 10 

m (30 ft) is judged possible with a magnitude 8 

earthquake on a strike-slip fault. Estimated upper 

bounds for displacement (horizontal) accompanying 

smaller earthquakes on strike-slip faults in the bay 

region are 6 m (20 ft) for magnitude 7, 2 m (6 ft) for 

magnitude 6, and 0.5 m (2ft) for magnitude 5. Vertical 

displacements associated with earthquakes on strikeslip faults are likely to be less than one-third of the 

horizontal displacement. Displacement associated with 

dip-slip faults is more difficult to evaluate, but these 

evidently are fewer and shorter in the bay region than 

strike-slip faults. 

The nature and areal distribution of deformation 

related to (fault movement includes (1) permanent 

ground deformation localized as a zone along the fault 

and (2) systematic deformation ofthe earth's surface on 

a regional or subregional scale. 

Accurate delineation of the width of the zone of 

deformation along the fault is best accomplished 

through careful geologic site studies including, where 

necessary, trenching, excavation, or other subsurface 

investigations. Where such data are not available, zone 

width can be crudely estimated by analogy with 

measured zones of deformation that have accompanied 

historic faulting. This method suggests that, for 

strike-slip faults, permanent ground deformation may 

be expected to extend for 92 m (300ft) on either side of a 

recognizable strike-slip fault trace and 425 m (1400 ft) 

on either side of a recognizable dip-slip fault trace (Hall 

and others, 1974). Designation of deformation zones on 

this basis is admittedly a stop-gap measure and should 

be re-evaluated with the availability of new geologic 

data at the site. 

Regional or subregional deformation of the earth's 

crust commonly accompanies major earthquakes. It is 

manifested predominantly as upwarping or subsidence 

for dip-slip faults and predominantly as horizontal 

distortion for strike-slip faults. In the bay region 

horizontal distortion appears to be the predominant 

process, although some local vertical warping (about 0.5 

m or 1.5 ft) accompanied the 1906 earthquake on the 

San Andreas. The magnitude of this process and its 

potential hazard for the bay region are not completely 

known, but it appears to be less important in evaluation 

of earthquake hazards than other earthquake effects. 

The frequency of recurrence of earthquakes is 

perhaps the most difficult to assess of all these topics. 

Until more geologic data are available, recurrence 

estimates are tentative at best and depend heavily on 

our knowledge of recurrence of historic earthquakes. 

The historic record in the bay region is little more than 

150 years old, a woefully inadequate sample for faults 

that have been active for millions or tens of millions of 

years. But even that record shows a crude pattern of 

damaging earthquakes on major bay region faults. 

Attempts to determine recurrence intervals for bay 

region earthquakes are further complicated by the 

unresolved relation between fault creep and damaging 

earthquakes because several bay region faults exhibit 

fault creep along parts of their length. Despite the need 

for more accurate data on frequency of recurrence, the 

phenomenon of recurrence is well established. 

Many important questions are still unanswered, but 

enough is known now to move positively toward 

reducing the hazard from future earthquakes. Some 

steps in this direction are obvious. All residents would 

agree that schools and hospitals should not be located 

astride the traces of major faults; most would accept 

requirements for geologic site studies in the deformation zones along major faults; and many would agree on 

siting restrictions that would locate major highway 

interchanges, dams, or power plants away from faults 

that may generate earthquakes. These kinds of actions 

are ultimately a product of the democratic process, and 

they depend as much on social and economic values as 

on our scientific knowledge. 

Other steps toward reducing earthquake hazards 

cannot be taken without more information than is given 

here. Building codes, for example, are an important 

mechanism for protecting life and property from 

earthquakes. But such codes require specific information on the nature of seismic shaking, possible modes of 

structural response, and other factors that go far beyond 

the initial geologic process that causes the earthquake. 

These and other problems relating to hazard reduction 

are treated in subsequent sections of this report. 
ESTIMATION OF BEDROCK MOTION AT 

THE GROUND SURFACE 

By R. A. PAGE, D. M. BooRE, and J. H. DIETERICH 

INTRODUCTION 

In terms of human and economic losses, seismic shaking is the most significant factor contributing to the 

overall earthquake hazard. Shaking contributes to 

losses not only directly through vibratory damage to 

manmade structures but also indirectly through triggering of secondary effects such as landslides or other 

modes of ground failure. Thus, an important element in 

seismic zonation on a regional basis is the geographical 

assessment of potential ground shaking. 

The intensity and character of ground shaking depends upon earthquake source parameters such as 

magnitude, driving stress causing the fault to slip, and 

dimensions of the slip surface, as well as upon distance 

from the fault. In addition, experience shows that surficial geologic materials may influence the level and nature of ground motion. Hence, the problem of evaluating 

the potential of seismic shaking divides naturally into 

two parts: estimation of bedrock motion at the ground 

surface, which is the subject of this paper, and estimation of the response of surficial geologic units to bedrock 

motion, which is discussed by Borcherdt, Joyner, and 

others (this report). 

FACTORS INFLUENCING DAMAGE POTENTIAL 

OF GROUND MOTION 

Three factors, amplitude, frequency content, and 

duration, govern the damage potential of ground motion 

and thus must be included in any scheme to characterize 

ground motion for purposes of design or hazard 

assessment. Damage tends to increase with the 

amplitude of bedrock motion; however, the relation 

between damage and amplitude is generally complex 

because of the response of surficial geologic deposits and 

manmade structures to large ground motions. Frequency content is a critical factor because structures, 

and in some cases surficial deposits, may respond in a 

resonant manner depending upon the frequency content 

of the ground motion. Relatively large deformations and 

stresses can occur in a structure or unconsolidated 

surficial deposit if the shaking includes significant 

amounts of energy at frequencies close to the natural 

resonant frequencies of the system. Duration of shaking, which is perhaps the least widely recognized factor 

influencing damage from shaking, is important because 

failure mechanisms in structures and uncdnsolidated 

surficial deposits commonly are dependent upon the 

cumulative number of induced stress cycles as well as 

the amplitudes of the stress. For example, had the 

duration of strong shaking during the San Fernando, 

Calif., earthquake of 1971 been longer, the earthquake 

damage and loss of life caused by shaking would have 

been greater than it was, as several critical structures 

were very near failure (Housner and Jennings, 1972). 

CHARACTERIZATION OF BEDROCK MOTION 

Ground motion can be characterized in a number of 

ways. The most complete characterization is a time 

history of ground movement, that is, a specification of 

ground motion in three independent spatial coordinates 

for every instant of time in terms of acceleration, 

velocity, or displacement. As basic data for the dynamic 

analysis of structural designs, time histories are a 

valuable tool to the seismic engineer. For purposes of 

seismic zonation, however, a more compact characterization of the ground motion is desirable. For sites 

underlain by bedrock, one such characterization 

employs four physical parameters scaled or computed 

from standard strong-motion seismograms: maximum 

ground acceleration, maximum ground velocity, 

maximum ground displacement, and duration of shaking above some threshold amplitude. On a typical 

· strong-motion recording of an earthquake at distances 

within which damage is sustained (see fig. 17), the peak 

ground acceleration commonly occurs at frequencies in 

the range 2 to 10Hz, whereas the dominant frequencies 

of velocity and displacement are in the ranges 0.5 to 2 

Hz and 0.06 to 0.5 Hz, respectively. Thus, specification 

of the peak amplitudes for acceleration, velocity, and 

displacement also conveys information about the 

frequency content of the motion. 

The strategy for predicting ground motion (Borcherdt, Brabb, and others, this report) involves two 

A31 
A32 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

steps: first, estimation of bedrock motion at the ground 

surface and, second, modification of that motion to account for the dynamic response of surficial geologic 

deposits. For use as input to the second step, we want to 

express ground motion in terms of the Fourier 

amplitude spectrum of ground acceleration. We cannot 

estimate the Fourier spectrum directly from peak 

ground-motion parameters and duration of ground 

motion; however, an upper bound to the spectrum is 

given by the zero-damped velocity response spectrum 

(Hudson, 1962), for which a smooth estimate can be 

obtained from the peak ground-motion parameters 

using an empirical technique described by Newmark 

and Hall (1969). 

The velocity response spectrum is a common tool for 

seismic design. It is defined by the maximum response 

velocities of a suite of linear, damped, single-degree-offreedom oscillators subjected to a specified time history 

of motion. A velocity response spectrum for a given level 

of damping is thus a plot of maximum velocity as a 

function of oscillator period or frequency (see fig. 18). 

The usefulness of the response spectrum for design 

purposes stems from the ability to model structures by 

comparable oscillators and to estimate stresses induced 

by the ground motion from knowledge of the response 

spectrum and of the equivalent natural frequencies and 

damping of the structure. 

The relation between the Fourier and zero-damped 

velocity response spectra is illustrated (fig. 18) for one 

horizontal component of the Pacoima damsite recording 

of the 1971 San Fernando earthquake. Figure 18 also 

illustrates the graphical method of Newmark and Hall 

(1969) for constructing smooth response spectra from 

peak ground-motion parameters. The first step is to plot 

lines of constant acceleration, velocity, and displacement (dot-dashed lines, fig. 18) equal to the maximum 

values of ground motion (1.25 g, 115 cm/s (43 in./s), and 

43 em (17.8 in.), respectively). To obtain the smooth 

response spectfum, these lines are then shifted upward 

(dashed lines, fig. 18) on the plot by multiplying the 

ground-motion· values by factors (4.8, 3.0, and 1.7, 

respectively) that reflect the dynamic amplification of 

the ground motion by the oscillator and the duration of 

ground motion. The amplification factors are dependent 

on the level of damping of the oscillator and, at low 

levels of damping, on the duration of shaking. In this 

example, the amplification factors are chosen to produce 

the illustrated fit between the smooth tripartite 

response spectrum (dashed line) and the computed 

zero-damped response spectrum (heavy solid line). 

These factors are derived for a magnitude 6.6 earthquake; larger factors would be appropriate for a larger 

magnitude, and hence longer duration, earthquake. 

SCALING OF GROUND-MOTION PARAMETERS 

WITH DISTANCE AND EARTHQUAKE 

MAGNITUDE 

Since 1966, the increase in the amount and quality of 

strong-motion data obtained within 50 km (31 mi) of the 

causative fault during moderate-sized earthquakes 

(magnitude 5.0 to 6.9) has made it possible to predict on a 

statistical basis peak ground-motion values for earthquakes smaller than magnitude 7.0 at distances greater 

than 10 to 20 km (6 to 12 mi) for sites on competent 

geologic materials ranging from bedrock to fi:m 

alluvium. The increase in the quantity ofstrong-motwn 

data comes largely from increased numbers of strongmotion recorders placed in operation in recent years by 

the Seismological Field Survey of the National Oceanic 

and Atmospheric Administration (now the Seismic Engineering Branch of the U.S. Geological Survey). Even 

with the greater number of recorders, however, no 

strong-motion seismograms have been obtained within 

40 km (25 mi) of a magnitude 7 earthquake and within 

more than 100 km (62 mi) of a magnitude 8 shock. The 

quality of the strong-motion data also has improved in 

the sense that detailed aftershock investigations and 

field studies after recent moderate-sized earthquakes 

have delineated the inferred slip surface for the main 

earthquake and have made possible more accurate estimates of distances of strong-motion sites from the 

causative fault. 

To determine the scaling factors for ground-motion 

parameters with distance, we must know ~he.distance 

from a recording site to the source of the se1sm1c energy 

responsible for the peak recorded motion. La~king such 

information for most earthquakes, we approximate that 

quantity by the shortest distance to the slipped fault 

surface. Close to a fault the approximation may be poor 

because the source of peak motions may not be the point 

on the fault closest to the recording site (Lindh and 

Boore 1973· Boore and Zoback, 1974; Hanks, 1974; 

Trifu~ac, 19'7 4). For this reason, there is considerable 

uncertainty in empirical relations between groundmotion parameters and distance close to the fault (at 

distances less than the width or depth of the fault), even 

for earthquakes for which some near-fault data exist 

(shocks smaller than magnitude 7). 

PEAK GROUND ACCELERATION 

A plot of peak horizontal ground acceleration against 

shortest distance to the slipped fault (fig. 19) reveals 

that acceleration increases with magnitude at all 

distances for which data exist and that the rate of 

attenuation with distance is similar for all magnitudes 

at distances beyond 10 to 40 km (6 to 25 mi). Although 
STUDIES FOR SEISMIC ZONATION A33 

1000 

Ground acceleration 400 

0 

0 

z 

0 

(.) 

LLJ 

en 

~0:: ~W 

ZCL 

Qo 

1-Z 

<(0 

ffi&3 

ijjen 

(.)C:: 

(.)LLJ 

<(CL 

en 

w 

I 

(.) 

1~~-----------------------L----------------------~--------------------~400 z 

Ground velocity 50 

100 

100 

50 

50 20 

Ground displacement 

en 

LLJ 

I 

(.) 

~ 

~ 

.,..: 0 z 

w 

~ 

w 

(.) 

<( 

....J 

CL 

en 

0 

50-L-----------------------L--------~-------------L----------------~--~ 20 

0 5 10 15 

TIME, IN SECONDS 

FIGURE 17.-Records of N. 14° E. component of horizontal ground motion at Pacoima damsite for San Fernando, Calif., earthquake 

of February 9, 1971 (after Trifunac and Hudson, 1971). Velocity (center) and displacement (bottom) records are obtained by 

integrating acceleration record (top) once and twice, respectively. 

there is considerable scatter in the data for a particular 

range of distance and magnitude, the systematic trends 

are obvious. Distances are known to an accuracy of 

about 2 to 3 km (1 to 2 mi) for most events of magnitude 

5.0-5.9 and 6.0-6.9, and to within 5 km (3 mi) for events 

of magnitude 7.0-7.9, with one exception for which a 

minimum value is plotted. If peak-acceleration data 

taken from the literature are plotted without regard to 

the measure of distance (that is, whether the distance is 

to the epicenter-, hypocenter, or closest point on the 

slipped surface) or without regard to accuracy of the 

distance, the scatter at distances less than 30 km (20 mi) 
A34 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

Cl 

zz -0 

>--(.) 

1-LLJ 

-en 

(.)0:: 

OLLJ 

Ljja.. 

>en 

LLJW 

eno:: zlow 

ll..~ 

~~ o::Z LLJ 

(.) 

100 

10 

1.0 

FREQUENCY, IN HERTZ 

10.0 

FIGURE 18.-Comparison of spectra for N. 14° E. component of ground 

motion at Pacoima damsite for San Fernando, Calif., earthquake of 

February 9, 1971. Amplitude at a given frequency may be read in 

terms of velocity (vertical axis), acceleration (bottom left to top right 

axis), or displacement (bottom right to top left axis). Zero-damped 

velocity response spectrum (heavy solid line) envelopes Fourier 

spectrum of ground acceleration (light solid line). Derivation of 

smooth tripartite response spectrum (dashed lines) from peak 

ground-motion values (dot-dashed lines) is described in text. 

is at least twice that observed in figure 19 (compare 

Page and others, 1972, fig. 4) and tends to obscure the 

marked attenuation of acceleration within this distance 

range. 

The sit~ conditions corresponding to the data shown 

in figure 19 range from crystalline rock to thick sections 

of firm alluvium. To estimate the effect of surficial 

conditions on peak ground acceleration, the data are 

replotted separately for the magnitude intervals 5.0-5.9 

and 6.0-6.9 (figs. 20, 21) with a twofold classification of 

site geology. A site underlain by less than 5 m (16ft) of 

alluvium is classified as a rock site, whereas a site 

underlain by a greater thickness is labeled an alluvium 

site. Within the range of surficial materials and levels of 

ground motion sampled (figs. 20, 21), site geology 

appears to contribute no more to the observed variation 

in peak acceleration than other factors that are yet to be 

thoroughly investigated, such as variations in driving 

stress and dimensions of the fault surface for earthquakes of equivalent magnitudes and variations in the 

propagation of seismic waves. At sites close to the fault, 

§ 

z 

0 

~ <( 

0:: 

LLJ 

....J 

LLJ 

(.) 

(.) 

<( 

0.1 

0.01 

0 

MAGNITUDE 

• 5.0-5.9 

0 6.0-6.9 

• 7.0-7.9 

x& 

DISTANCE, IN MILES 

10 100 

0 00 

X 

10 

X 

0 

• 00 

0 

• 

• 

• 0 

0 <DO 0 

0 8 00 

• 0~ § • • 0 

. - 0 

0 • • .a ooo 

0 0 CP 0 0 

• • 

0 

00 

100 

DISTANCE, IN KILOMETRES 

FIGURE 19.-Peak horizontal ground acceleration in relation to 

shortest distance to slipped fault as a function of magnitude. 

Acceleration expressed in terms of gravitational acceleration, g. 

Data from free-field sites, including buildings up to two stories high. 

Most data from Page, Boore, Joyner, and Coulter (1972, table 4). 

Arrow pointing to left signifies distance to fault is actually smaller 

than shown; arrow to right signifies distance is greater. Crosses are 

interpolated values for postulated earthquakes (see Borcherdt, 

Brabb, and others, this report). 

where ground shaking is sufficient to cause significant 

damage to ordinary structures, surficial deposits may 

substantially influence peak accelerations. As more 

near-fault strong-motion records of damaging levels of 

shaking are obtained, we anticipate that the effects of 

site geology upon peak acceleration will become clear. 

For example, on weak foundation materials such as the 

bay mud and Holocene alluvium in the San Francisco 

Bay region (see Lajoie and Helley, and Borcherdt, 

Joyner, and others, this report), we expect that for some 

sites peak levels of acceleration will be limited by the 

ability of the geologic materials to transmit the intense 

motion from the bedrock to the ground surface. 

Although the peak accelerations, which are generally a 

measure of the ground-motion amplitude at the higher 

frequencies, may be reduced for such sites from those 
STUDIES FOR SEISMIC ZONATION A35 

0 0 

• 

§ 0.1 

z 

0 

i= 

<( 

0:: 

w 

.....J 

w 

(.) 

(.) 

<( 

0.01 

• 

0 

10 

DISTANCE, IN MILES 

10 100 

• • 

• • 

• 

• 

• 

MAGNITUDE 5.0-5.9 

• Rock 

o Alluvium 

• 0 

• 0 

• 0 

0 • 

.:> 

100 

DISTANCE, IN KILOMETRES 

FIGURE 20.-Peak horizontal ground acceleration in relation to 

shortest distance to slipped fault for earthquakes of magnitude 5.0 

to 5.9 as a function of site geology. Data from figure 19. Rock site is 

underlain by less than 5 m (16ft) of alluvium; alluvium site by more 

than 5 m (16 ft). Arrow as in figure 19. 

expected on bedrock, the overall damage potential is 

likely to be greater on the soft ground than on rock, 

because of possible ground failure (see Youd and others, 

this report), possible extended duration of shaking, and 

possible ground-motion amplification at the lower 

frequencies (see Borcherdt, Joyner, and others, this 

report). 

From the data presented, it is clear that the dependence of peak acceleration a upon distance r can be 

represented by an inverse power law,a = kr-f3, wherek 

is a factor depending on magnitude. Within the uncertainty of the data, the exponent f3 is independent of 

magnitude and ranges from 1.4 to 1. 7. This relation 

applies outside the immediate vicinity of the fault, that 

is, at distances greater than one fault depth or width. 

Within the immediate vicinity of the fault, the rate of 

attenuation must be less. Although there is considerable scatter in the peak-acceleration data (figs. 19, 20, 

21), it is clear that useful statistical predictions of peak 

• 1.0 

§ 0.1 

z 

0 

~ c:: 

L.t..J 

....J 

L.t..J 

(.) 

(.) 

<C 

0.01 

0 

10 

DISTANCE, IN MILES 

10 

•• 

o•• 

f# • 

.. 0 

0 0 0 

• • 0 

100 

MAGNITUDE 

6.0-6.9 

• Rock 

o Alluvium 

C» 0 

o ro• <) 0. CD 

. • ' • 

•• 0 

~ 0 .. 

100 

0 • ., 0 0 

• eo 

DISTANCE, IN KILOMETRES 

FIGURE 21.-Peak horizontal ground acceleration in relation to 

shortest distance to slipped fault for earthquakes of magnitude 6.0 

to 6.9 as a function of site geology. Data from figure 19. Rock site is 

underlain by less than 5 m (16ft) of alluvium; alluvium site by more 

than 5 m (16 ft). 

acceleration can be made now for sites on rock and 

competent alluvium at distances greater than 10, 20, 

and 40 km (6, 12, and 25 mi) for magnitude 5.0-5.9, 

6.0-6.9, and 7.0-7.9 earthquakes, respectively. 

PEAK GROUND VELOCITY 

Standard strong-motion seismographs produce accelerograms, that is, graphical records of ground 

acceleration as a function of time. Ground velocity and 

displacement are calculated by integrating accelerograms once or twice, respectively. Because of the data 

processing involved, both velocity and displacement 

data are scarce compared with acceleration data, which 

are scaled directly from accelerograms. Recently established programs for routine computer processing of 

digitized accelerograms (Hudson and others, 1971) are 

beginning to provide the velocity and displacement data 

required to establish the dependence of peak ground 

velocity and displacement upon magnitude, distance, 
A36 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

DISTANCE, IN MILES 

10 100 

MAGNITUDE SITE GEOLOGY 

5.0-5.9 & Rock 6 Alluvium 100 

6.0-6.9 • 0 

0 • z 7.0-7.9 0 

0 0 u z LiJ 0 (f) u 0::: LiJ 

LiJ 0 en 0.. ,6 • 0::: en ,6 X LiJ 

LiJ • 0 10 0.. 0::: 

1- • xe o• ~r=fh en LiJ 0 LiJ 

::2: • 0 I 

u i= ,6 z X ~ 

u 

LiJ ., @ ~ 

~ 0 ~ 6 eo 1-

~ • • 00 {) 1- 0 {) o• ...J 

LiJ 0 > ...J 

LiJ O(J:> > 

0 

• • 

10 100 

DISTANCE, IN KILOMETRES 

FIGURE 22.-Peak horizontal ground velocity in relation to shortest 

distance to slipped fault as a function of magnitude and site geology. 

Data from free-field sites, including buildings up to two stories high. 

Solid symbols for rock sites (underlain by less than 5 m (16 ft) of 

alluvium); open symbols for alluvium sites (more than 5 m (16ft) of 

alluvium). Large symbols for velocities from integrated accelerograms; small symbols for velocities estimated by approximate 

integration techniques typically accurate to 10 to 20 percent. 

Distances accurate to 5 km (3 mi) or less with one exception, for 

which the minimum value is plotted. Crosses are interpolated 

values for postulated earthquake (see Borcherdt, Brabb, and others, 

this report). Arrow as in figure 19. 

and site conditions. 

In many respects the behavior of peak horizontal 

ground ~elocity parallels that of peak acceleration. 

Peak velocity increases with magnitude at all distances 

for which data are available and attenuates with 

distance from the slipped fault (fig. 22) following an 

inverse power law. The rate of attenuation, however, is 

slightly less than that for peak acceleration. The scatter 

in the velocity data is comparable to that observed in the 

acceleration data (compare fig. 19). Within the data 

shown, which includes sites on rock and firm alluvium 

only and corresponds only to low-strain levels of ground 

motion, there is no clear dependence of peak velocity 

upon site geology. 

PEAK GROUND DISPLACEMENT 

The displacement parameter discussed in this chapter is the peak horizontal dynamic ground displacement 

DISTANCE, IN MILES 

10 100 

MAGNITUDE SITE GEOLOGY en 5.0-5.9 • Rock w • :c 6.0-6.9 • 0 

7.0-7.9 0 

(.) 

~ 10 ~ 0 ....,.: 

z X w 0 

0 ::?; 

w 

(.) 

<( X • 0 6 • ...J 

a.. 

en 6 6 • 0 X 0 

0 

S:? 

::?; l <( 

z 

>- 0 

0 

10 100 

DISTANCE, IN KILOMETRES 

FIGURE 23.-Peak horizontal ground displacement in relation to 

shortest distance to slipped fault as a function of magnitude and 

site geology. Data from free-field sites, including buildings up to 

two stories high. Solid symbols for rock sites (underlain by less 

than 5 m (16 ft) of alluvium), open symbols for soil sites (more 

than 5 m (16 ft) of alluvium). Large symbols for displacements 

from twice-integrated accelerograms; small symbols for values 

from 10-second displacement meters. Distances accurate to 5 km 

(3 mi) or less. Crosses are interpolated values for postulated 

earthquake (see Borcherdt, Brabb, and others, this report). 

Arrow as in figure 19. 

obtained from double integration of accelerograms or 

recorded directly by strong-motion displacement meters 

with natural periods of 10 s (seconds) and does not 

contain spectral components longer than about 10-15 s. 

Because the resonant periods of structures are usually 

less than a few seconds, even for very large structures, 

no significant information is lost by using this measure 

of displacement rather than the actual ground displacement. 

A plot of peak dynamic displacement versus distance 

as a function of magnitude (fig. 23) shows that peak 

dynamic displacement increases with magnitude and 

attenuates with increasing distance from the fault at a 

rate probably less than those observed for the acceleration and velocity data. The data are scanty, and no 

inference is made regarding the effects of site geology. 

DURATION OF SHAKING 

Within the seismic engineering literature, no single 

measure of duration of shaking is in common usage and, 

in fact, discussion of duration is often not based on a 

quantitative definition. A crude but useful measure of 

duration is the time interval between the first and last 

peaks equal to or greater th~m 0.05 g on the accelero-
STUDIES FOR SEISMIC ZONATION A37 

gram. This measure roughly corresponds to the ((intense" or ~(strong" phase of shaking witnessed close to 

the fault during moderate-sized earthquakes and 

defines the time interval during which significant 

damage results from shaking. Durations of shaking for 

several earthquakes in the magnitude range 5.~ 7.9 

increase with magnitude and with decreasing distance 

to the fault (Page and others, 1972). The dependence of 

duration on magnitude reflects the increase in fault 

length with magnitude and the finite velocity at which 

rupture propagates along the fault. 

EXTRAPOLATION BEYOND EMPIRICAL DATA BASE 

The existing strong-motion data provide an empirical 

basis for predicting surface bedrock motion from 

earthquakes in the magnitude range 5.~6.9 at distances greater than 10 to 20 km (6 to 12 mi) and at 

distances beyond 40 km (25 mi) for magnitude 7.~ 7.9 

shocks. There is, however, little observational data from 

which to predict motion in the immediate vicinity of the 

causative fault for earthquakes in the magnitude range 

5.~6.9 and no observational basis for predicting motion 

within 40 km (25 mi) of a magnitude 7 .~ 7.9 earthquake 

or within more than 100 km (60 mi) of a magnitude 8 

shock. Thus for purposes of design or zonation, it is 

necessary to extrapolate from the existing base of data 

to small distances and to larger magnitudes. We briefly 

discuss some of the theoretical and numerical studies of 

the fault process and of the attenuation of ground 

motion with distance that can be used to guide 

extrapolations of recorded data. 

Extrapolation of the apparent power-law attenuation 

of peak acceleration, velocity, and displacement with 

distance (figs. 19 through 23) to within a kilometre of 

the fault surface suggests unrealistically large ground 

motions; hence the attenuation curves must flatten 

close to the fault to reflect finite limits of motion at the 

fault surface. Values for such limits have been obtained 

for various simplified models of the fault mechanism 

(Housner, 1965; Ambraseys, 1969; Brune, 1970; 

Dieterich, 1973; Ida, 1973) and are shown in normalized 

form in figure 24. Peak velocity depends on the density 

and rigidity of the material surrounding the fault and 

on the driving stress (also referred to as stress drop) 

causing the fault to slip. Peak acceleration depends on 

these parameters and also on the high-frequency limit 

or cutoff in the frequency content of the motion. The 

high-frequency cutoff may arise from the mechanics of 

rupture (Ida, 1973) and the inelastic absorption of 

energy in highly sheared rock present in major fault 

zones (Boore, 1973). Estimates of the driving stress 

operating during earthquakes are as much as a few 

hundred bars and according to various fault models (for 

example, Brune, 1970) suggest peak velocities at the 

fault surface in excess of 100 cm/s (40 in./s) and peak 

accelerations in excess of 2 g as recorded on a standard 

strong-motion accelerometer with a natural frequency 

of 16 hertz. Whether these peak values of ground motion 

will occur at a particular site depends on the shear 

strength of the underlying geologic material. On 

competent rock such values are expectable, whereas on 

unconsolidated alluvium the strength may be insufficient to transmit such intense motion to the surface 

(Ambraseys, 1973). 

The attenuation of these ground motions with 

distance close to the fault can be studied with numerical 

simulations of the faulting process using finite-element 

models. The results of such a study (Dieterich, 1973) 

have been used to derive scaling laws that relate peak 

ground-motion parameters to the stresses actirlg at the 

fault surface and to the dimensions of the fault. The 

acceleration data (fig. 19), normalized accordingly, are 

plotted together with attenuation curves computed 

from several different finite-element models (fig. 24). 

The model results are in reasonable agreement with the 

data. The more rapid rate of attenuation in the 

empirical data at distances greater than a few 

minimum fault dimensions reflects inelastic absorption 

of energy, a process that is not included in the model. 

The use of this numerical model for the prediction of 

ground motion from an assumed earthquake requires a 

knowledge of both the stresses acting at the fault 

surface and the dimensions of the fault surface. At 

present, stress estimates for a given earthquake are 

uncertain by factors as large as five; as knowledge of the 

mechanics of faulting increases, the uncertainty in 

stress estimates should decrease. Although the intensity of ground motion is physically dependent upon the 

stresses acting at the fault surface, the current 

uncertainties in stress estimates are such that magnitude is currently a more satisfactory parameter than 

stress for scaling ground motion for purposes of seismic 

design. 

SUMMARY 

Strong-motion recordings of earthquakes currently 

provide a suitable basis for predicting peak parameters 

and duration of ground motion at sites on rock and firm 

alluvium at distances greater than 10, 20, and 40 km (6, 

12, and 25 mi) for earthquakes of magnitudes 5.~5.9, 

6.~6.9, and 7.~7.9, respectively. There are still, 

however, very few recordings of ground motion from the 

critical region close to the fault, where earthquake 

damage is intense, and there are no recordings from 

within 100 km. (60 mi) of an earthquake larger than 

magnitude 8.0. Simplified physical models of faulting 

provide theoretical estimates of ground motion close to 

the fault inside the distance range of existing observa-
A38 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

0.01 

(/) 

c::: 

<( 

co Zz 0-

i=O: 

~~ ~0 

wen 

(_)(/) 

uw 

<(C::: ..... 

(/) 

0.001 

Numerical model 

Earthquake 

magnitude 

• 5.0-5.9 

• 6.0-6.9 

• 7.0-7.9 

0.0001 

0.01 

• 

0.1 

• 

• 

• 

• 

• 

. 

•••• .. ... -• 

• 

• • 

• 

• • • • • • • • ... ••• • • ~ • -• • • • •• • • 

-... •• 

• • 

10 100 

DISTANCE FROM FAULT (ARBITRARY UNITS) 

MINIMUM FAULT DIMENSION 

FIGURE 24.-Comparison of attenuation curves for four representative finite-element models of strike-slip faulting with peak acceleration 

data of :figure 19. The data are normalized by dividing ground acceleration by stress drop and distance to the fault by minimum dimension 

offaulting (Lmin) as suggested by the scaling relation of Dieterich (1973). Attenuation curves give peak accelerations along lines oriented 

perpendicular to fault and originating at point of initial surface rupture and assume a 10-hertz high-frequency cutoff of the ground 

motion. Locations of grid points at which accelerations were computed are shown as X. At distances less than 0.2 Lmin all curves extend 

uniformly to fault. Differences between attenuation curves reflect differences in source geometry and point of origin -for the four models. 

tional data; however, the reliability of such estimates is 

determin~d by the accuracy of both the model and its 

input parameters and can be ultimately tested only by 

comparison with observational data. 

There are two indications that the intensity of ground 

motion close to the fault in the zone of destructive 

shaking is significantly greater than that which was 

widely assumed for seismic design prior to 1971. One is 

the sparse but growing number of accelerograms 

recorded close to the fault, which have caused significant upward revision of acceleration-distance relations (compare Seed and others, 1969; Schnabel and 

Seed, 1973). The other indication is the extensive 

damage caused by recent moderate-sized earthquakes 

occurring in or on the edge of urban areas. For example, 

the 1969 Santa Rosa, Calif., earthquakes (magnitudes 

5.6 and 5. 7) caused $6 million damage to buildings 

(Steinbrugge and others, 1970), and the San Fernando, 

Calif., earthquake (magnitude 6.6) resulted in $500 

million damage (Housner and Jennings, 1972). Another 

sobering example of extensive damage in an urban area 

is the Managua, Nicaragua, earthquake of 1972 

(Brown, Ward, and Plafker, 1973). 

In the next several years estimates of surface 

bedrock motion near to the fault where damage is 

intense will become more reliable as strong-motion 

recordings are collected at a growing rate and as more 

refined and complete theoretical and numerical models 

of the faulting processes are developed. Advances in 

techniques of processing strong-motion records will 

provide many more reliable data on velocity, displacement, and duration. 
DIFFERENTIATION OF SEDIMENTARY DEPOSITS 

FOR PURPOSES OF SEISMIC ZONATION 

By K. R. LAJOIE and E. J. HELLEY 

INTRODUCTION 

Geologic data are the basis for special-purpose 

interpretive maps such as ground-response maps, 

liquefaction-potential maps, and slope-stability maps. 

However, most standard geologic maps do not, in 

themselves, contain sufficient data for these purposes, 

particularly in areas underlain by unconsolidated 

sedimentary deposits. For example, most geologic maps 

differentiate bedrock units in considerable detail but 

only crudely differentiate young, unconsolidated deposits. Yet, large historic earthquakes (for example, 

those that occurred near San Francisco, in Mexico City, 

and in Anchorage) demonstrate that some of the 

greatest structural damage and resultant loss oflife due 

to high amplitudes of ground shaking and extensive 

ground failure occur in areas underlain by unconsolidated sedimentary deposits. The potential for such 

damage is greatest in flat lowlands because these areas 

generally are underlain by thick unconsolidated 

sedimentary deposits are are often highly developed 

and densely populated; parts of the San Francisco Bay 

region are examples of such areas. 

The present distribution and physical properties of 

the various unconsolidated sedimentary deposits are 

controlled by their age and depositional environment. 

Geologic units defined and mapped on the basis of 

temporal, genetic, and physical criteria, therefore, can 

be used to outline regions of potential earthquakeinduced hazards such as liquefaction, seismic amplification, and ground failure . The primary physical properties used to differentiate, map, and then regroup the 

unconsolidated deposits into broader units with similar 

seismic behavior include thickness, bedding, density, 

induration, texture (grain size), and porosity. These 

primary parameters control secondary parameters, 

such as seismic velocities and penetrometer resistance, 

which are useful to predict general behavior during 

earthquakes. Because none of these parameters were 

measured systematically in the present reconnaissance 

mapping project and some were estimated, the range of 

physical properties within each geologic unit is not 

precisely known. Therefore, the derivative maps based 

on these units, for example, the liquefaction-potential 

rna p (fig. 50) shows only in a general manner those areas 

where a particular seismic hazard most likely exists. 

Presently, the main applications of such maps are to call 

attention to areas where land users and planners should 

consider certain problems and to provide a base for 

future, more rigorous studies of seismic behavior. 

This paper describes the reconnaissance techniques 

used for rapidly differentiating and mapping unconsolidated sedimentary deposits in the San Francisco Bay 

region and briefly discusses the physical parameters 

used to regroup these deposits for delineating areas 

where liquefaction and ground-motion amplification 

might occur. 

GEOLOGIC MAPPING TECHNIQUES 

One ofthe main efforts in the current geologic study of 

the San Francisco Bay region has been to differentiate 

into geologically distinct and seismically significant 

units the alluvial deposits underlying the gently 

sloping sedimentary plain between the bay and the 

surrounding hills. Shortcuts and specialized mapping 

techniques have been used because of the large area 

(approximately 19,300 km2 (7,450 mi2 )) and short time 

(3 years) involved. The alluvial units are defined by 

various combinations of geologic and genetic criteria 

such as depositional environment, geomorphic expression, soil-profile development, age, induration, compaction, and texture. The distribution of the units is 

determined primarily from topographic maps, published soil series maps, and aerial photographs. The 

evolution of such a geologic map for the area of detailed 

study shown in figure 25 is illustrated in figures 26, 27, 

28, and 29. 

The contour lines on the topographic map (fig. 26) 

clearly reveal the major geomorphic features such as 

the hilly uplands, the flat marshlands adjacent to the 

modern bay, and the broad alluvial plain sloping gently 

from the hills to the bay. The contour lines also reveal 

smaller geomorphic features such as distinct alluvial 

fans, stream levees, and flood plains, all of which were 

formed by separate but closely related alluvial processes and which reveal the distribution of genetically 

related deposits. 

On the basis of relative soil-profile development, the 

18 alluvial soil se.ries described in the Soil Conservation 

Service report on the region (fig. 27) fall into two distinct 

groups that reflect some basic difference in the deposits 

A39 
A40 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

FIGURE 25.--Location of detailed study area. 

on which they are developed. The soil units having 

strongly developed weathering profiles constitute one 

group, and those having weakly to moderately developed weath~ring profiles constitute the other. Soil 

profiles are developed by physical and chemical weathering processes at the surface of the earth; therefore, 

well-developed soil profiles generally indicate that the 

materials on which they are formed have been exposed 

to either intense weathering conditions or moderate 

weathering conditions for a considerable length oftime. 

Because the weathering conditions in the bay region are 

moderate and relatively uniform, the alluvial deposits 

having strongly developed weathering profiles were 

inferred to be significantly older than the deposits 

having weakly to moderately developed profiles. This 

inferred age relation was used to differentiate younger 

and older alluvial deposits (fig. 28). 

The younger deposits make up the alluvial fans being 

formed under the existing hydrologic regime. The 

streams forming these young fans are graded to present 

sea level. These younger deposits and the bay mud into 

which they grade are informally referred to as Holocene 

deposits (fig. 29). The older alluvial deposits, now partly 

covered by the Holocene deposits, make up alluvial fans 

formed by these same streams when they were graded to 

lower stands of sea level during the late Pleistocene 

(prior to about 10,000 years before present). These older 

deposits are informally referred to as late Pleistocene 

alluvium (fig. 29). 

The Holocene alluvium is differentiated further into 

(1) depositional facies (fig. 29) on the basis of textural 

characteristics (that is, gravel, sand, silt, and clay) 

derived primarily from published soil reports and 

unpublished engineering foundation reports and (2) 

depositional environment (that is, stream levees and 

flood basins) determined from geomorphic expression as 

revealed on topographic maps and aerial photographs. 

These facies grade from coarse-grained gravel and sand 

deposits, which form prominent stream levees at the 

highest parts of the alluvial fans, into medium-grained 

sand and silt deposits, which form broad flood plains and 

subdued levees along the lower margins of the alluvial 

fans. These stream deposits grade into and interfinger 

with fine-grained silt and clay deposits that form the fiat 

floors of flood basins between stream levees on the outer 

margins of the alluvial fans directly adjacent to the bay 

marshlands. These fine-grained basin deposits and 

some of the medium-grained levee deposits interfinger 

with and grade into the bay mud, the carbonaceous silty 

clay deposited in the marshes and on the mudflats of San 

Francisco Bay during Holocene time (approximately 

the past 10,000 years). 

This gradation from coarse-grained to fine-grained 

sediment in the Holocene alluvium is a natural 

consequence of very recent stream erosion, transportation, and deposition. The coarsest rock debris eroded 

from the bedrock uplands is deposited near the base of 

the hills where the rapidly flowing streams enter the 

broad, gently sloping alluvial plain. Only the finer 

grained debris is carried by the ever-slackening water 

to the lower parts of the alluvial fans and eventually 

into the bay itself, where it is deposited as bay mud. The 

landward extent of the saturated plastic bay mud 

underlying the former marshes and tidal mudflats of 

San Francisco Bay (fig. 26) was inferred from early (ca. 

1850) U.S. Coast and Geodetic Survey hydrographic 

charts (Nichols and Wright, 1971) rather than from 

direct field observation because cultural activity over 

the past 50 years has obscured its original distribution. 

Long exposure to erosion and weathering processes 

has altered the original geomorphic expression and 

physical character of the late Pleistocene alluvium, 

thus it has not been separated into depositional facies 
STUDIES FOR SEISMIC ZONATION 

122°05' 

0 1 MILE 

0 1 KILOMETRE 

NATURAl 

·FLOOD BASIN 

122°00' 

A41 

FIGURE 26.- Topographic map of the Mountain View-Sunnyvale area, Mountain View ?\12-minute quadrangle. The enhanced contour 

lines show the irregular topography of the bedrock uplands and fluvial geomorphic features on the broad alluvial plain such as 

a lluvial fans, stream levees, and floo(i basins. Distribution of former tidal marshland from Nichols and Wright (1971). 
A42 

~I 

·;: 1. 

-+.;-

REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

-,-,. 

! 

.• :t"'· ... 

0 

0 

Sv 

1 MILE 

1 KILOMETRES 

... 

Tf 
STUDIES FOR SEISMIC ZONATION A43 

similar to those of the Holocene alluvial deposits. 

In the areas of the bay region where detailed soils 

data are not available, the alluvial deposits have been 

differentiated using geomorphic and genetic criteria 

EXPLANATION 

Soil Series in the Mountain View-Sunnyvale area* 

Alluvial soil series with weakly to moderately 

developed weathering profiles: 

An Alviso clay 

Ba Bayshore clay loam 

Ca Campbell silty clay loam 

Cc Campbell silty clay loam, clay substrate 

Cf Castro clay 

Ch Clear Lake clay, drained 

Cra Cropley clay, 0- to 2-percent slopes 

GbB Garretson gravelly loam, 0- to 5-percent slopes 

Pf Pacheco loams, clay substrate 

Sv Sunnyvale silty clay, drained 

YaA Yolo loam, 0- to 2-percent slopes 

YeA Yolo silty clay loam, 0- to 2-percent slopes 

Zbz Zamora clay loam, 0- to 2-percent slopes 

Alluvial soil series with strongly developed weathering profiles: 

PoA Pleasanton loam, 0- to 2-percent slopes 

PoC Cropley clay loam, 0- to 2-percent slopes 

PpA Pleasanton gravelly loam, 0- to 2-percent slopes 

PpC Pleasanton gravelly clay loam, Z. to 9-percent slopes 

SdA San Ysidro loam, 0- 2-percent slopes 

Upland soil series (nonalluvial soils): 

AsE Ayer clay, 15- to 30-percent slopes 

AuG Azule clay loam, 30- to 75-percent slopes 

AvE Azule silty clay loam, 15- to 30-percent slopes 

AvD2 Azule silty clay loam, 9- to 15-percent slopes 

AvE2 Azule silty clay loam, 15- to 30-percent slopes 

DaD Diablo clay, 9- to 15-percent slopes 

DaE Diablo clay, 15- to 30-percent slopes 

FbG Felton-Ben Lomond complex, 50- to 70-percent slopes 

LGE Los Gatos clay loam 

LGE2 Los Gatos clay loam, 15- to 30-percent slopes 

LKG3 Los Gatos and Maymen, 50- to 75-percent slopes 

MEF2 Maymen fine sandy loam, 15- to 30-percent slopes 

PhG3 Permanente stony loam, 50- to 75-percent slopes 

PRC Positas-Saratoga loam, Z. to 9-percent slopes 

PRD Positas-Saratoga loam, 9- to 15-percent slopes 

SgC Saratoga-Positas loam, Z. to 9-percent slopes 

ShE2 Soper gravelly loam, 15- to 30-percent slopes 

ShF Soper gravelly loam, 30- to 50-percent slopes 

Miscellaneous map symbols: 

KfB Kitchen middens, archeological site 

LfF Landslides 

Ma Made land 

Tf Tidal flats 

*From U.S. Soil Conservation Service (1968) 

FIGURE 27.----Soil units in Mountain View- Sunnyvale area. Soil units 

are defined primarily on the basis of profile development, texture, 

and slope. Soil-profile development is controlled by many factors 

including time, climate, parent material, slope, and biological activity. In an area such as this, where weathering conditions and parent 

material are relatively uniform, the time factor is clearly expressed 

by relative development of soil profiles, which can be used as a 

means of differentiating alluvial deposits on the basis of relative 

age (fig. 28). 

derived from aerial photographs and extrapolated from 

areas where detailed soils data provide the main and 

most reliable means of recognition. Fossils, archeological remains, and radiometric ages corroborate the 

relative ages and correlations based on these limited 

data. The upper part of the late Pleistocene alluvium 

contains a Rancholabrean fossil vertebrate fauna 

containing mainly extinct species (for example, camel, 

bison, mammoth, and ground sloth), whereas the 

Holocene alluvial deposits contain a fossil fauna 

completely modern in aspect (for example, deer and elk). 

The upper part of the late Pleistocene alluvium 

contains fossil wood and fresh-water molluscan shells 

that yield radiocarbon ages of about 22,000 years before 

present. Holocene alluvial deposits contain fpssil wood, 

shells, and archeological remains that yield radiocarbon ages of about 5,000 years before present and 

younger. Peat and shell deposits in the bay mud yield 

radiocarbon ages that range from about 9,600 years 

before present at the base of the unit in the lowest parts 

of the basin to modern at the top of the unit. These ages 

from the bay mud date the latest marine transgression 

into the basin, which agrees very well with the 

post-Pleistocene rise in sea level established from 

worldwide data (Milliman and Emery, 1968). In 

general, the partial flooding of the basin caused by this 

rise in sea level raised the base level ofthe local streams 

and began the present depositional cycle represented by 

the Holocene alluvial deposits. The fact that the 

Holocene alluvial deposits around the margins of the 

bay are graded to the present sea level is used as one 

means of identifying these deposits and adds credence to 

the relative ages and correlations based on other 

criteria. 

Most of the age control used for establishing an 

absolute chronology and for correlating sedimentary 

deposits in the bay region was gleaned from published 

and unpublished geologic, archeologic, and engineering 

reports. These diverse and readily available sources 

provided the data for constructing an initial geologic 

model and determined where subsequent dating control 

was needed. Many of the samples dated specifically for 

this study were obtained from the numerous drill cores 

collected over a period of 25 years for engineering 

foundation studies of various proposed or existing 

transbay bridges. 

PHYSICAL PARAMETERS OF GEOLOGIC UNITS 

Physical parameters of the various geologic units 

such as texture, thickness, bulk density, induration, 

and seismic velocity are needed to assess their 
A44 

... 

):~.~t 

REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

0 

0 

,:c 

I 

j .• ·• - ~-·· 

-. \ 

SOFT BAY MUD ,, 

·~·· 

.\:.~!:: .. .. '~: .. · ··' 

1 MILE 

1 KILOMETRE 

--~ --= .- .. , 

,, 

'I ·- ,: *-·~ r : 

'\fllit ~;;: t 

- . .- . .... -··--::-: 

-;.-- . I 

.( 

122°00' 
STUDIES FOR SEISMIC ZONATION A45 

seismic-hazard potential. These parameters were compiled and interpreted from readily available sources 

(fig. 30) that were augmented by direct measurement 

where no data existed or where verification of existing 

data was needed. This information also was used to 

differentiate units where soil series data were ambiguous or did not exist. 

Very few data are available on the total thickness of 

the unconsolidated and semiconsolidated deposits 

filling the bay basin. A few deep drill holes and several 

seismic profiles (Hazelwood, 1974) indicate that these 

sediments are probably more than 600 m (1,970 ft) thick 

in San Jose and thin irregularly northward from 60 to 

90 m (200-300 ft) near San Francisco (see generalized 

cross section in figure 68 for data on sediment thickness 

in the Palo Alto-Coyote Hills area). 

The thickness of the younger units is fairly well 

known from numerous shallow bore holes described in 

engineering reports (fig. 30) and from shallow seismic 

surveys where bore-hole data are lacking. The Holocene 

alluvium (fig. 29) generally ranges in thickness from 

about 15 m (50 ft) near the heads of alluvial fans to 

about 3 m (10 ft) near the margins of the bay. The 

Holocene bay mud ranges in thickness from 0 to as much 

as 37 m (120 ft). 

The thickness of the late Pleistocene alluvium (fig. 

29) is not precisely known because its base is not well 

defined in the thick sedimentary section beneath the 

bay and the surrounding alluvial plain. Where the base 

of the late Pleistocene alluvium can be seen on stream 

terraces in narrow valleys, these sediments are about 3 

m (10ft) thick. They are probably as much as 46 m (150 

ft) thick beneath the bay where they overlie old 

estuarine mud, as identified by saltwater fossils 

brought up from that depth in a drill sample. Still older 

FIGURE 28.-Distribution of younger and older alluvial deposits in 

Mountain View-Sunnyvale area as determined primarily from relative development of soil profiles, based on soil series as mapped by 

Soil Conservation Service (fig. 27). Alluvial deposits on which weak 

to moderate weathering profiles are developed were initially inferred to be younger than alluvial deposits on which strong weathering 

profiles are developed. Radiocarbon and fossil data have confirmed 

this relative age classification. The younger alluvial deposits contain modern vertebrate and invertebrate fossils and organic remains that yield radiocarbon ages of about 5,000 years before present and younger. Therefore, these deposits and the bay mud with 

which they interfinger are informally referred to as Holocene deposits (see figure 29). The older alluvial deposits locally contain 

extinct late Pleistocene vertebrate fossils such as camel, sloth, bison, and mastodon and organic remains that yield radiocarbon ages 

of about 20,000 years before present. Therefore, these older deposits 

are informally referred to as late Pleistocene alluvium (see figure 

29). 

Pleistocene alluvial deposits probably underlie these 

deeply buried estuarine muds, but their total thickness 

is not known. In the southern bay area these deposits 

may grade downward into Pliocene and early Pleistocene alluvial deposits of the Santa Clara Formation or 

may lie unconformably on them. 

Published soils maps and unpublished engineering 

reports (fig. 30) are the main sources of data on the 

physical properties of the various geologic units. Each 

unit generally has a distinctive range of values for 

properties such as grain size, sorting, bulk density, 

compaction, induration, and moisture content. These 

properties have been used in defining and delineating 

some of the units, in particular the late Pleistocene 

alluvium and the three facies of Holocene alluvium (see 

figs. 29, 30). 

The primary physical properties such as grain size 

and sorting are controlled by the depositional environment, whereas the secondary properties such as 

induration, compaction, and bulk density are related to 

(and generally increase with) the age of the geologic 

units. This variation of the secondary physical properLies with age is reflected in the resistance to penetration, which increases from low values for the Holocene 

deposits to high values for the various bedrock units 

(col. 4, fig. 31). The very low resistance to penetration of 

the bay mud is attributable to its extremely high water 

content and loose packing, which reflect its youthful age. 

Older deeply buried estuarine muds would have higher 

resistances to penetration owing to compaction. Penetrometer resistance can be used to estimate relative 

densities that, with data on grain size, sorting, and 

moisture content, can be used to evaluate liquefaction 

potential in shallow, unconsolidated deposits (see Youd 

and others, this report). 

The velocities of seismic waves in geologic materials 

are determined by various primary and secondary physical properties and therefore can be used as a rough 

index of these properties and as a means of evaluating 

seismic behavior. The seismic wave velocities in the 

sedimentary units and some bedrock units are listed in 

figure 31. The compressional, or P-wave, velocities (Vp) 

were obtained primarily from shallow seismic refraction surveys conducted to identify or determine stratigraphic thicknesses of the younger sedimentary units 

and from deep seismic refraction surveys conducted to 

determine the total thickness of sedimentary material 

in the bay basin (Hazelwood, 1974). The shear, or 

S-wave, velocities (Vs) of the bay mud and alluvial units 

were derived from limited surface surveys by the authors and down-hole experiments by Warrick (1974; 
A46 

:~ • .= •• ' "'' 

~,: 

,()1~ 

~ ~-; 

.:.......;-·-. 

Mzf 

REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

0 

0 

• ~- ·t i .. -\. ~ r: " 1 .1: f -~ 

Qpa 

1 MILE 

1 KILOMETRE 

· .... ~ 

~::.~~~ 

FIGURE 29.-Geologic map of the Mountain View- Sunnyvale area. 

Qhbm 

...... 

. ·-: . .,..-; 

I .;... •:. "It~ l'ar~IL.m~ ~. j\ _";;I 

)_----
STUDIES FOR SEISMIC ZONATION A47 

oral commun., 1974). The S-wave velocities of the bedrock units were not measured directly but were estiEXPLANATION 

DESCRIPTION OF MAP UNITS 

Holocene deposits (less than 10,000 years old): 

Holocene estuarine deposits (0-9,000 years old): 

Qhbm Bay mud. Water-saturated estuarine mud; predominantly 

clay and silty clay underlying marshlands and tidal mudflats of San Francisco Bay. Occasional fenses of well-sorted 

fine sand and silt; occasional shelly and peaty layers. Interfingers with and grades into fine-grained and mediumgrained alluvium; generally overlies early Holocene alluvium 

or late Pleistocene alluvium 0-40 m (0-120 ft) thick 

Holocene alluvial deposits (0-5,000 years old): 

Qhaf Fine-grained alluvium. Plastic, poorly sorted carbonaceous 

clay and silty clay in poorly drained interfluvial basins marginal to bay marshlands. Locally contains thin beds of wellsorted silt, sand, and fine gravel; contains modern vertebrate 

fossils and freshwater gastropod and pelecypod shells. Interfingers with and grades into bay mud and mediumgrained alluvium; overlies late Pleistocene alluvium. Generally less than 5 m (15ft) thick 

Qham Medium-grained alluvium. Loose, moderately drained, moderately sorted sand forming alluvial plains and stream 

levees. Locally contains beds of well-sorted clay, silt, and 

gravel; contains modern vertebrate fossils and fresh water 

gastropod and pelecypod shells. Intermediate in character 

and lateral extent between fine-grained and coarse-grained 

alluvium with which it interfingers; generally overlies late 

Pleistocene alluvium. Generally less than 7 m (21ft) thick. 

Qhac Coarse-grained alluvium. Loose, well-drained, moderately 

sorted, permeable sand and gravel forming stream levees 

and flood plains on higher parts of alluvial fans; gravel becomes dominant toward fan heads. Locally contains beds 

of well-sorted silt, sand, and gravel; contains modern vertebrate fossils and fresh water pelecypod and gastropod shells. 

Thickness ranges from as much as 15m (50ft) at fan heads 

to 6 m (20 ft) where these deposits interfinger with and 

grade into medium-grained alluvium; overlies late Pleistocene alluvium and bedrock 

Pleistocene deposits (10,000-3,000,000 years old): 

Qpa Late Pleistocene alluvium (10,000-70,000? years old). Weathered, slightly consolidated and indurated alluvial fan deposits consisting primarily of gravel and sand with some 

silt. Less permeable than Holocene alluvium. Locally contains fresh water pelecypod and gastropod shells and extinct 

late Pleistocene vertebrate fossils. Overlain by Holocene 

deposits on lower parts of alluvial plain; incised by channels 

that are partly filled with Holocene alluvium on higher parts 

of alluvial plain. Maximum thickness unknown but at least 

45 m (150 ft) near margins of present bay where these 

deposits overlie deeply buried Pleistocene estuarine deposits 

Bedrock: 

QTa Pliocene and early Pleistocene alluvium. Tectonically deformed alluvial fan deposits with local minor amounts of 

shallow-water marine deposits. Weakly to moderately indurated gravel, sand, and silt with subordinate amounts of 

lacustrine silt and clay; local thin tuff beds; contains late 

Pliocene and early Pleistocene vertebrate fossils. Underlies 

late Pleistocene alluvium; overlies or is in fault contact with 

Franciscan Formation. Consists of the Santa Clara Formation in southwestern part of bay area 

Mzf Mesozoic Franciscan Formation. Well-indurated sandstone, 

chert, and altered volcanic rocks. In map area underlies or 

is in fault contact with Pliocene and early Pleistocene alluvium 

FIGURE 29.-Continued. 

mated using the relation Vs=Vp/2. 

The P-wave velocity is low in loosely packed materials such as the Holocene alluvial deposits, which can 

be easily compressed, and high in hard materials such 

as the Franciscan Formation, which cannot be easily 

compressed. Within the Holocene alluvium the P-wave 

velocities are strongly dependent on water content. 

Above the water table theP-wave velocity is lower than 

that of water, whereas below the water table it is virtually that of water. The S-wave velocity is low in materials with low shear strengths, such as the saturated 

bay mud, and high in materials with high shear 

strengths, such as the well-indurated Franciscan rocks. 

In general, theP-wave andS-wave velocities systematically increase with induration and compaction of the 

geologic materials, and because these two properties 

generally increase with age of the geologic) units, the 

seismic velocities increase in ·a similar manner. Also, 

within an individual geologic unit both the P-wave and 

S-wave velocities tend to increase with depth owing to 

increasing compaction and induration. 

DELINEATING AREAS OF SEISMIC HAZARDS 

Once the unconsolidated deposits have been differentiated into map units whose distribution and general 

physical properties are known or can be reasonably 

inferred, these units can be combined in_ various ways on 

the basis of particular similarities for specific seismic 

zonation purposes. Figure 32 shows how the sedimentary units delineated in this chapter have been recombined to reflect liquefaction potential and possible relative ground response. 

LIQUEFACTION POTENTIAL 

Liquefaction is defined as the transformation of a 

granular material from a solid state to a liquefied state 

as a consequence of increased pore-water pressure 

(Y oud and others, this report). If shear stresses resulting from sloping terrain or nonuniform loading are present, the liquefied sediment may flow, generating 

ground failures that could result in serious damage to 

manmade structures. One cause of liquefaction is 

ground shaking during earthquakes. Seismic shaking 

tends to compact granular sediments, which causes a 

transfer of load from intergranular contacts to the interstitial pore water, thereby increasing the pore-water 

pressure. 

Seismically induced liquefaction is most likely to occur in beds of loose, water-saturated, well-sorted silt 

and sand within 30 m (100 ft) of the ground surface. 

Geologic and engineering data indicate that these conditions exist to varying degrees in all five Holocene and 
A48 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

late Pleistocene sedimentary units (fig. 29). These five 

units were recombined to delineate three zones of different liquefaction potential in the southern bay region 

(fig. 50). 

Beds of loose well-sorted silt and sand within and 

directly beneath the bay mud have the highest liquefaction potential because they lie below sea level and are 

permanently saturated. The distribution of bay mud 

therefore defines a zone of moderate to high liquefaction 

potential (zone 1, fig. 50). Similar beds occur in the 

Holocene alluvium but are not permanently saturated 

LO 

~ 

~ N ·c. + "0 

~ cY) 

Q) 1.0 tlD 

tlD 00 ro 8-3 10 0 

....I U5 """ El. 29·8 

because of the fluctuating ground water table. The distribution of the Holocene alluvium therefore defines a 

zone of moderate liquefaction potential (zone 2, fig. 50) 

that is divided into two subzones (2a and 2b, fig. 50) on 

the basis of depth to the water table. In subzone 2a the 

depth to the water table is less than 3m (10ft). These 

areas are underlain by the fine-grained Holocene alluvium and medium-grained alluvium that form the 

low, poorly drained parts of the Holocene alluvial fans. 

In subzone 2b the depth to the water table is greater 

than 3 m (1 0 ft). These areas are underlain by the 

,..... 

~ 00 

+ .,.; cY) 

1.0 ....I 

~ cci 8-4 cY) U5 

30 

Soft dark gray silty clay 

Soft to stiff tan silt 20 

5 GWS. El. 15·2 Dense brown silty sand7-31-56 some pebbles 

10 

Soft brown to blue-gray 

sand and clayey siltsome pebbles and roots MEAN 

0 0 

SEA LEVEL 

1-

10 LLJ 

LLJ 

en Dense brown sand silty sand u. 

LLI 20 0:: 5 TONS and small gravel ILLI 

~ 20 

Stiff tan to gray clayey 

silt-some pebbles 

Stiff and slightly compact 30 

10 gray clayey fine sand 400 300 200 100 0 

SECONDS PER FOOT Stiff tan and gray mottled 

clayey silt 40 

EXPLANATION c: 

~ Slightly compact sandy silt «J 

.., 

15 _g 8-Number with thin sand leases 

Size of sampler (inches) ~··: Top hole elevation 50 .·,·t Hard blue-gray clayey silt Blows per foot 140 pound ..... : 

hammer with 30" drop , , 

Unconfined compressive 3.4 35 1.4 ~ GWS Elevation 

strength ( T 1ft2) / 60 / 

20 Shear strength (lbtft2) 

Percent 

Moisture 

FIGURE 30.-An example of unpublished engineering data from which thickness and physical properties of alluvial deposits were 

partly derived. The abrupt decrease in driving rate of the penetrometer at about 6 m (20ft) below sea level is interpreted to reflect 

the stratigraphic contact between the Holocene alluvium and the late Pleistocene alluvium. 
STUDIES FOR SEISMIC ZONATION A49 

coarse-grained Holocene alluvium and some of the 

medium-grained Holocene alluvium that together form 

the high, well-drained parts of the Holocene alluvial 

fans. These deposits are only seasonally saturated and 

are not subject to liquefaction during much of the year. 

Therefore, the beds of well-sorted silt and clay that may 

exist in subzone 2a are water saturated for a greater 

part of the year and have a slightly higher liquefaction 

potential than similar beds that may exist in subzone 

2b. 

Most beds of well-sorted silt and sand in the late 

Pleistocene alluvium are slightly compacted or chemically altered and are not as likely to liquefy as similar 

deposits in the loose Holocene alluvium. The surface 

distribution of Pleistocene alluvium therefore defines a 

zone (zone 3, fig. 50) of low liquefaction potential. The 

bedrock units are generally too well indurated to 

liquefy, and so their distribution delineates a zone of 

negligible liquefaction potential. 

WEST 

Ts 

LOW 

The liquefaction-potential map based on the above 

criteria does not outline areas where liquefaction, with 

or without resultant ground failure, will occur. It 

merely outlines those areas where units occur that may 

contain potentially liquefiable materials and where 

liquefaction and resultant ground failure may be expected during moderate or large earthquakes. Therefore, this map highlights those areas where liquefaction 

potential exists and should be evaluated prior to various 

types of land use. 

GROUND RESPONSE 

Medvedev (1965) showed that seismic impedance 

(defined as the product of S-wave velocity and bulk 

density, Vs p) can be used to make rough estimates of 

relative ground response. The potential for seismic amplification increases as the impedance contrast between 

an overlying and an underlying unit increases if other 

parameters such as stratigraphic thickness are conEAST 

IMPEDANCE CONTRAST 

Thickness Relative bulk Penetration P-wave 

Unit density, p resistance1 velocity, VP (m) (g/cm3) (blows/ft) (m/sec) 

Qhbm 0-36 1.3-1.7 0 1400 4 

Qha 0-15 1.9 20-80 300-600 3 

Qpa 10-45 2.1 100 1500-2100 

QTa 0-250? 2.0 100-refusal 2500 

Ts 0-300 2.4 Refu~al 1500-3300 

Mzf ·----- 2.7 Refusal 2800-4000 

1 Test used 140-lb hammer dropped 30 in. 2From Warrick (1974) 3 Above water table. Below water table 'I P = 1500-1700 4 Figures in italics are estimated values 5

Warrick (oral commun., 1974) 

S-wave Impedance, velocity, V5 

(m/sec) Vs p 

90-130 2 117-153 

200-300 380-570 

200-400 5 420-630 

1200 2400 

500-1400 1200-3360 

1400-2000 3780-5400 

FIGURE 31.-Schematic cross section of southern San Francisco Bay region and description of 

certain physical properties of the generalized geologic units. Qhbm, bay mud; Qha, 

Holocene alluvium; Qpa, late Pleistocene alluvium; QTa, early Pleistocene and Pliocene 

alluvium; Ts, Tertiary sandstone; Mzf, Franciscan Formation. 
A 50 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

Lajoie and Helley, Borcherdt, Joyner, Youd and others, Geologic unit and others, this report this report 

this report 

Liquefaction 

potential 

Holocene I Qhbm IQhbm I zone 

Bay mud Qhbm bay mud Bay mud Bay mud 1 

Alluvium Qal 

Holocene alluvium Qha 

Fine grained Qhaf Holocene I Qhaf l Qal 

Holocene I 2a 

Medium grained Qham alluvium Qham alluvium 

Coarse grained Qhac Qhac Alluvium 2b 

Late Pleistocene I Q 

Qal 

Late Pleistocene I Late Pleistocene alluvium Qpa 3 alluvium pa alluvium 

Bedrock Tl\llz b 

Pliocene-Pleistocene alluvium QTa QTa 

Merced Formation QTm Santa 

Santa Clara Formation QTsc Clara I 

Tertiary rock Ts Bedrock Ts Formation I Bedrock 

Pre-Tertiary 

Pre-Tertiary rock l\llzb and Tertiary 

Granite l\llzg bedrock 

Great Valley sequence l\l1z gv 

Franciscan Formation l\llzf Franciscan I l\llzf 

Formation 

FIGURE 32.-Correlation diagram showing groupings of geologic units for evaluating ground response (Borcherdt, Joyner, and others, this 

report) and liquefaction potential (Youd and others, this report). 

stant. The table in figure 31 lists the generalized 

geologic units in the southwestern bay area in increasing age, which is roughly proportional to density and 

S-wave velocity and therefore to impedance (column 7). 

The Holocene bay mud has the lowest impedance owing 

to both its low density and its lowS-wave velocity. The 

three facies of the loose Holocene alluvium (fig. 29) have 

very similar physical properties and therefore are combined into one geologic unit that has only slightly lower 

impedance values than the weakly consolidated late 

Pleistocene alluvium. The variability of physical properties within and between bedrock units is reflected in 

their relatively wide range of moderate to high impedance values. 

The schematic cross section (fig. 31) shows the 

generalized stratigraphic relations in the southwestern 

bay region with the high contrasts in impedance between units represented by slightly heavier contact 

lines. Considering only the impedance contrasts, amplification of bedrock motion is expected to be highest 

where bay mud overlies late Pleistocene alluvium or 

where Holocene alluvium overlies Pliocene and early 

Pleistocene alluvium. The impedance data suggest that 

the highest levels of amplification would occur where 

thick deposits of bay mud directly overlie Franciscan 

bedrock. 

Because seismic amplification is dependent on frequency, and therefore controlled by other factors such as 

stratigraphic thickness, predicted amplification potential using only impedance data is neither very precise 

nor directly applicable to engineering design. These 

crude predictions are consistent, however, with comparative low-strain ground-motion measurements 

(Borcherdt, Joyner, and others, this report) that show 

that the highest amplifications occur on bay mud sites. 

It is probably significant that four of the generalized 

geologic units with distinct low-strain amplifications 

(table 5) roughly correspond to the four groups of 

geologic units with similar impedance values (groups 

separated by heavy lines in column 7 of fig. 31; correlation shown in fig. 32). 

Combining the geologic units into groups with similar impedance values provides a useful means of 

evaluating data on earthquake intensity and lowstrain-level response. For example, if deposits with 

similar impedance values behave differently in an 

earthquake, other parameters such as variation in 

stratigraphic thickness might be investigated as the 

causative factor. SUMMARY 

The alluvial deposits in the San Francisco Bay region, 

which in the past were usually treated as one geologic 
STUDIES FOR SEISMIC ZONATION A51 

unit, are differentiated into two main units, Holocene 

alluvium and late Pleistocene alluvium, primarily on 

the basis of soil profile development. The Holocene alluvium is further differentiated into three textural 

units, coarse-, medium-, and fine-grained alluvium, on 

the basis of depositional environment as determined 

from aerial photographs, soils reports, and engineering 

data. The fine-grained alluvium interfingers with and 

grades into the Holocene bay mud. Physical properties 

such as relative bulk density, compaction, and induration as expressed by penetrometer resistance and seismic velocities are generally lowest in the bay mud, intermediate in the Holocene alluvium, and highest in the 

late Pleistocene alluvium. 

These sedimentary units may be regrouped in various 

ways for purposes of seismic zonation on the basis of 

similar physical properties. For example, all the unconsolidated sediments contain some potentially liquefiable beds of loose well-sorted fine sand and silt. Therefore, zones of different liquefaction potential based on 

the moisture content, relative compaction, and distribution of the geologic units can be delineated. The 

Holocene bay mud delineates a zone of high liquefaction 

potential because the beds of loose sand and silt within 

these estuarine sediments are permanently saturated. 

The beds of loose silt and sand in the Holocene alluvium 

are only seasonally saturated, and these three geologic 

units are grouped together. Their distribution delineates a zone of moderate liquefaction potential. This 

zone is subdivided into two subzones; one where the 

ground water table is less than 3 m (10 ft) deep has 

slightly higher liquefaction potential than the other. 

The beds of well-sorted silt and fine sand within the late 

Pleistocene alluvium are slightly compacted and indurated, and so they are not as susceptible to liquefaction 

as similar beds in the Holocene deposits. The distribution of the late Pleistocene alluvium therefore delineates a zone of low 1iquefaction potential (Y oud and 

others, this report). 

For purposes of comparing ground amplification, the 

sedimentary deposits and bedrock units can be grouped 

into four units according to similarities in seismic impedance (V8 p). The bay mud forms a unit with the 

lowest impedance, and the late Pleistocene and 

Holocene alluvium are combined to form a unit oflow to 

moderate impedance. The bedrock units in the south 

bay region fall into two groups with higher seismic 

impedances than the sedimentary deposits. The early 

Pleistocene and Tertiary sandstones form a unit with 

moderate to high impedance, and the Franciscan Formation forms a unit with the highest impedance (fig. 

31). Medvedev (1965) pointed out that the highest levels 

of seismic amplification can be expected where the impedance contrast between overlying and underlying 

geologic units is greatest. This relation suggests that 

the highest seismic amplifications in the bay region 

would be expected where thick deposits of bay mud 

directly overlie Franciscan bedrock. In the southern bay 

region the highest seismic amplifications would be expected where bay mud overlies the late Pleistocene alluvium or where deposits of late Pleistocene and 

Holocene alluvium overlie bedrock. This appraisal does 

not consider other factors, such as stratigraphic thickness, that will greatly affect seismic amplification. Data 

on contrasts in impedance do, however, provide means 

of evaluating these other parameters. 
RESPONSE OF LOCAL GEOLOGIC UNITS TO GROUND SHAKING 

By R. D. BoRCHERDT, W. B. JOYNER, R. E. WARRICK and]. F. GIBBS 

INTRODUCTION 

The most widespread earthquake damage to manmade structures is generally a direct result of ground 

shaking. Local geologic conditions can change the 

characteristics of earthquake ground shaking. In particular, the intensity of shaking in certain frequency 

bands can be amplified by thick deposits of unconsolidated materials. Such materials exist over a large proportion of the San Francisco Bay region, and after the 

1906 earthquake, effects of exaggerated ground shaking were documented at sites underlain by these materials (Lawson, 1908). For example, violent effects were 

observed on the muds near San Francisco Bay and on 

the thick alluvial deposits underlying San Jose and 

Santa Rosa. Lawson (1908, p. 160--253) reported evidence for exaggerated shaking on alluvial deposits in 18 

other communities. This phenomenon is not considered 

in present building codes, partly because observations 

of damage are not sufficiently quantitative to be incorporated easily into designs for earthquake-resistant 

structures and partly because data on the seismic response of different geologic units are limited. However, 

recent increases in the number of comparative seismic 

recordings and advances in numerical models for the 

dynamic response of surficial geologic deposits are 

yielding improved quantitative data. This chapter 

sl,lmmarizes these data for the San Francisco Bay region 

and examines their usefulness for purposes of seismic 

zonation. 

OBSERVED GROUND-MOTION 

AMPLIFICATIONS FROM NUCLEAR EXPLOSIONS 

IN NEVADA 

Comparative ground-motion measurements of a 

single seismic event provide quantitative estimates for 

the effects of various geologic units on ground shaking. 

Comparative measurements of ground motion generated by distant nuclear explosions in Nevada have been 

made at 99 sites in the San Francisco Bay region (fig. 

33). Nuclear explosions in Nevada are especially useful 

for such studies in the bay area, since at these distances 

(approximately 530 km (330 mi)) source characteristics 

and travel paths are nearly the same for each recording 

site. In addition, most of the ground-motion energy is in 

A52 

the frequency band for which the effects of the local 

geologic units are greatest. This coincidence of frequency bands causes the amplification effects of the 

geologic units to be readily apparent on the analog recordings of ground velocity. 

Detailed analyses of these ground-motion data were 

presented by Borcherdt (1970) and Gibbs and Borcherdt 

(1974). Results ofthese analyses are summarized here. 

A brief summary of the geology aids in understanding 

these results. The numerous geologic units in the region 

can be grouped into three general categories on the 

basis of gross physical properties (see Lajoie and others, 

this report, for a discussion of methods for differentia~­

ing sedimentary deposits and a discussion of the physical parameters used in regrouping th~m~. The ~hree 

general categories determined to have distinctly different seismic properties (Borcherdt, 1970) are as follows: 

1. Bay mud (equivalent to the younger bay mud unit of 

Borcherdt, 1970) consists mostly of recently deposited soft plastic carbonaceous clay, silt, and minor 

sand containing more than 50 weight percent water; thickness as much as 40 m (130 ft); shear 

velocities 90 to 130 m/s (290 to 430 ft/s). 

2. Alluvium (equivalent to the older bay sediment unit 

of Borcherdt (1970) and the late Pleistocene and 

Holocene alluvium of Lajoie and Reiley (Lajoie 

and Reiley, this report) consists mostly of silty 

sandy clay, silty clayey sand, and sand and gravel 

with less than 40 weight percent water; thickness 

as much as 600 m (2,000 ft); shear velocity approximately 200 m/s (660 ft/s) at the surface, increasing 

3. 

with depth. 

Bedrock consists of Pliocene and early Pleistocene 

alluvium of Lajoie and Reiley (this report), which 

includes the Santa Clara Formation, consisting of 

semiindurated and indurated sandstone, siltstone, 

and mudstone; Tertiary rocks, consisting of 

marine sandstone and shale of Eocene, Miocene, 

and Pliocene age; the Page Mill Basalt, lava flows 

and pyroclastic rocks of Miocene age; and preTertiary rocks, which include the Franciscan 

Formation, consisting mostly of sandstone, shale, 

radiolarian chert, and greenstone (volcanic rocks), 

minor amounts of granitic rocks, and the Great 

Valley sequence, consisting of indurated 
37' 

45' 

37' 

22' 

30" 

122'22'30" 

STUDIES FOR SEISMIC ZONATION 

... 0. .. .. . . . ... . . _._,_ -~· ... . . . . . . . . . . .... . . .. ...... . .. . . . . . . . . . . . . . . . . . . . . . . . . . 

EXPLANATION 

Bay 

-

mud 

Alluvium 

~ 

~ 

Bedrock 

• 

Recording location 

A 53 

121 '52'30" 

•• • • •• ••• • 0 . ••• ·'!· .. · 

·N. 

10 MILES 

10 KILOMETRES 

FIGURE 33.-Distribution of generalized geologic units and locations where three components of ground motions generated by nuclear 

explosions were recorded. 

sandstone and siltstone; thicknesses vary; shear 

velocities estimated to range from 500 to 2,000 m/s 

(1,600-6,600 ft/s). 

Examples of. horizontal ground-velocity recordings 

obtained on each of these three units are shown on 

figure 34A. All the recordings were made at sites within 
0 

z 

0 

u 

w 

en 

en 

w 

~~ 

a:: 

fw 

:E 

0 

a:: 

u 

~I H7 

lO 

~ 

~ 

R7 

0 

en 

~I .. ~ 

> 0 

a:: 

:E 

8 

~ Ql ~ 

0 

aS 

..... 17 

I 

J7 

BAY MUD 

ALLUVIUM 

BEDROCK 

I 10 seconds 1 I ---- A 

FIGURE 34.-Recordings of horizontal ground motion generated by two nuclear explosions. The recordings are grouped according to the type of geologic unit at the site. A , Equiscaled recordings of nuclear explosion 7 at sites in San Fran cisco. B, Equiscaled recordings of nuclear explosion 11. 

> 01 

.;:.. 

:::0 

trl 

t:l 

c: 

C"'l 

>-3 

-0 

z 

0 

"'l 

trl 

> 

~ ::r: 

£) 

c: 

> 

~ 

trl 

::r: 

> N 

> :::0 

t:l 

sn 

rn 

> z 

"'l 

:::0 

> z 

(J 

-rn 

(J 

0 

tp 

> 

><: 

:::0 

trl 

0 -0 

z 
2: 

:::> 

> 

:::> 

...J 

...J 

<( 

STUDIES FOR SEISMIC ZONATION 

........ 

........ 

...., 

' 

:::e:::: 

(.) 

0 

c::: 

0 

L1J 

(D 

ON0~3S ~3d S3~1311\10~~111\1 Oil ~ Sll0/\0~~111\1 OOO'vi = ~ 

l 

A 55 

Vl .,; "C 

c CD 

0 ::s () .s Q) +> Vl r::: 

0 0 

........ 

~ ~ ~ et:l 

1":1:1 

~ 

::::> 

S! 

r... 
A 56 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

the city limits of San Francisco from the same nuclear 

event. Comparison of the recordings clearly illustrates 

the large amplifications of horizontal ground velocity 

due to differences in local geology. The maximum 

amplitudes recorded on sites underlain by bay mud are 

five to eight times larger than those recorded on nearby 

bedrock sites. Recordings from another nuclear event 

(fig. 34B) show similar large amplifications for sites 

underlain by bay mud and thick sections of alluvium 

(see Borcherdt, 1970, for geologic profiles of some of the 

sites). 

Similar analyses have been made of vertical ground 

motion recorded at each of the sites. Most of the 

observed variations in vertical ground motion are less 

than the observed variations in horizontal motion 

(Borcherdt, 1970). As a result, this summary is 

concerned principally with horizontal motion, which is 

of most interest for the design of earthquake-resistant 

structures. 

The characteristics of the seismic amplitude response 

of local geologic units can be approximated by 

computing the ratio of the absolute value of the Fourier 

transform for a recording to that obtained from a 

simultaneous recording at a nearby bedrock location. 

The ratios over the frequency band for which there is a 

good signal-to-noise ratio provide an estimate of the 

response characteristics of the local geologic unit with 

the effects of the source, travel path, and the recording 

instruments removed. The spectral ratios computed for 

the 13 recording sites in San Francisco (fig. 35) are 

grouped according to the type of underlying geologic 

unit. 

The characteristics of the spectral ratios show several 

correlations with the type of underlying geologic unit. 

For sites underlain by bedrock, the spectral 

amplification curves are approximately constant as a 

function of frequency, with minor variations about 1. 

For sites underlain by alluvium, the spectral 

amplification curves are generally greater than 1, with 

irregular variations as a function of frequency and with 

peak values that may exceed 5. For sites underlain by 

bay mud, pronounced peaks exist in the spectral 

amplification curves, with much larger peak values 

than observed on either the bedrock sites or the alluvial 

sites. 

To summarize these data, average horizontal spectral 

amplifications (AHSA) have been computed over the 

frequency band for which the signal-to-seismicbackground-noise ratio is greater than 2. (The seismic 

background noise was determined from spectral 

analyses of the seismic noise recorded immediately 

prior to the arrival of the seismic energy generated by 

the nuclear explosions.) The AHSA values for all sites 

have been normalized by the average value obtained 

from sites underlain by granitic rock. Histograms for 

these average values (fig. 36) show that there are significant differences in the seismic response of the bay 

mud unit, the alluvial unit, and the bedrock unit. The 

mean anp standard deviations for the three samples are, 

respectively, 1.3 and 0.6 for bedrock, 4.4 and 1.9 for 

alluvium, and 11.3 and 6.0 for bay mud. The standard 

deviations are partly dependent on variations in amplification caused by variations in thickness of the corresponding geologic units. 

In summary, the spectral amplifications show that 

there are significant differences between the seismic 

responses of the three types of geologic units in the San 

Francisco Bay region. Of particular interest for the 

design of earthquake-resistant structures are the 

predominant periods and corresponding large 

amplifications observed on nearly all the bay mud sites 

and some of the alluvial sites. 

For purposes of seismic zonation it is important to 

know to what extent these spectral amplifications can 

be extrapolated to describe quantitatively the 

amplification effects of local geologic units in the event 

of another large earthquake. To examine this question, 

the results from the nuclear explosions are compared 

with those from the March 22, 1957, San Francisco 

earthquake, data from the April 18, 1906, California 

earthquake, and numerical models of the dynamic 

response of surficial geologic units at high strain levels. 

COMPARISON OF NUCLEAR EXPLOSION DATA 

WITH THE 1957 AND 1906 EARTHQUAKE DATA 

The epicenter of the San Francisco earthquake of 

March 22, 1957, (magnitude 5.3) was located approximately 17 km (10 mi) southwest of downtown San Francisco. Four strong-motion recordings were obtained in 

San Francisco, one on bedrock, two on alluvium, and one 

on bay mud. An estimated $1 million of damage, limited 

principally to single-family dwellings in the immediate 

vicinity of the epicenter (Westlake-Palisades area), resulted from the earthquake (Oakeshott, 1959). Varying 

degrees of minor damage were reported in the buildings 

containing the strong-motion instruments. As part of 

this investigation, a nuclear explosion was recorded at 

each of the four locations occupied by the strong-motion 

recorders. The instruments used to record the nuclear 

explosion were placed within 3 m (10 ft) of the strongmotion instruments. 

Samples of the spectral amplifications computed from 

the two events for the site underlain by bay mud (fig. 37) 

differ in detail, but their gross features are similar. In 

particular, the predominant frequencies agree to within 

0.2 hertz, the average amplification to within 32 percent, and the maximum spectral amplification to within 

36 percent. These comparisons suggest that the princi-
STUDIES FOR SEISMIC ZONATION A57 

BEDROCK 

:~-~~ 

17 

J7 

,l .......... 

Lll 

:[-==~ 

~[~ 

~[~ ~f" _,.........._..,.,I 

0 1 2 3 4 

z 

0 

~ 0 ALLUVIUM u:: 

~:r~.-.· <( 

_J 

<( 

0::: 

1-

K7 

Jll ~ o5r e, L~"A.., 

10 1 

;r~~":- 0 1 2 3 

FREQUENCY, IN HERTZ 

Tll 

4 

BAY MUD 

1: ~ H7 

..- J' I \ 

z 

0 

~ 

l:t~ P7 

\• 

(.) 

u:: 20 ::::i 

ll.. 

~ T7 

<( 

_J 10 

<( 

0::: 

1-

(.) 

w 

ll.. 0 (/) 

20 Q5 

10 

OL..---.L____J _ __JL__---L---'-------'--

0 1 2 3 

FREQUENC~ IN HERTZ 

FIGURE 35. Horizontal spectral amplification curves computed for 13 recording sites in San Francisco. 

pal amplification effects of the local geologic deposits 

could have been predicted quantitatively for this earthquake by analyzing the nuclear explosion recordings. 

The other set of observational data for the San Francisco Bay region that bears on the extrapolation problem is the intensity data from the 1906 earthquake. 

Wide variations in the amounts of damage were observed in the city of San Francisco after the 1906 earthquake (fig. 1). Wood (1908) attributed these wide variations to the geologic character of the ground. Wood concluded ((***Where the surface was of solid rock, the 

shock produced little damage; whereas upon made land 

great violence was manifested***." This dependence on 

the geologic character of the ground can be due to both 

increased levels of shaking and an increased number of 

ground failures. In many places, the surficial geologic 

deposits most likely to amplify shaking are also those 

with least strength. Buildings on such deposits are 

especially susceptible to failure during earthquakes. 

The 1906 intensity maps prepared for the San Francisco Bay region (maps 19, 21, and 22, Lawson, 1908) 

show that for similar geologic units the observed intensity values generally decrease with increasing perpendicular distance from the San Andreas fault. Borcherdt 
A 58 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

Cf) 

w 

len 

LL.. 

0 5 

a:: 

w 

co 

~ 

::J 

z 

5 10 15 

EXPLANATION 

CIJ 

Granitic rocks and rocks 

of Franciscan Formation 

Alluvium 

EJ 

Bay mud 

20 25 

AVERAGE HORIZONTAL SPECTRAL AMPLIFICATION 

(WITH RESPECT TO GRANITE) 

FIGURE 36.-Histograms of average horizontal spectral amplification values computed for sites underlain by bay mud, alluvium, and 

either granitic rocks or rocks of Franciscan Formation. Dashed line indicates mean value for each geologic unit. 

and Gibbs (1975) derived an empirical relation between 

the 1906 intensities and distance for sites underlain by 

the Franciscan Formation (fig. 38). The resulting relation (intensity= 2.69-1.90 log (distance (km))), derived 

from only those sites (approximately one square city 

block in size) for which there was good evidence for the 

ascribed degree of intensity, shows that intensity for 

sites on the Franciscan Formation generally decreases 

as the logarithm of distance (fig. 38). 

For each site Borcherdt and Gibbs (1975) determined 

an intensity increment between the observed 1906 intensity value and that predicted at the same distance by 

the logarithmic relation for the Franciscan Formation. 

These intensity increments were plotted as a function of 

the AHSA values computed from the nuclear data at 

corresponding sites (fig. 39). (The plotted AHSA values 

were normalized to the average value determined for 

the Franciscan Formation.) 

The correlation coefficient of 0.95 computed for the 

empirical relation (S/ =0.27 +2. 70 log(AHSA)) shows 

that a strong correlation exists between the computed 

intensity increments and amplifications observed at 

low-strain levels. The physical basis of this empirical 

relation is complex and does not necessarily indicate 

that amplifications observed at low-strain levels can be 

extrapolated directly to high-strain levels. However, 

two interpretations of the amplifications obtained from 

small motions may be useful for predicting areas of high 

intensity in future large earthquakes: (1) For levels of 

ground shaking that would not cause ground failure, 

the higher amplifications characterize those sites most 

likely to sustain the higher levels of ground shaking and 

(2) for levels of ground shaking that would induce 

ground failure, the higher amplifications characterize 
STUDIES FOR SEISMIC ZONATION A 59 

20 

(.!J 

z 15 

C5 

....J 

:::) 

c:l 

S:2:::s:::: 10 I..L-0:: 

-<( 

~a.. O...LLJ 

z~ 

ffi<.!J 5 ::r:Z t-LLJ 

:::)0 

o.....J 

C/)0 

1-(.!J 

<(0 0 ztot- -U 1-W 15 <(Q.. 

(.)(/) 

- w !::!::o::: 

.....J::r: 

a.. I- :::E-

<(~ 10 ....J 

<( 

0::: 

1-

(.) 

LLJ 

0... 

C/) 5 

0.5 

A 

II 

:~Nuclear- 5 

HORIZONTAL A 

,. I I 

11 I I 11 I I I \1 I 

I " I 

I I o\ 

II\ 

tl I 

~ I "' I I 1 v I 

Earthpuake 

~I\ 

1v' 

I I 

I I 

I I I I r, I I I I v I 

I 

\ 

I 

\ 

VERTICAL 

1.0 

rl 

,, 

,, 

Nuclear 5-; I 

I I 

I I 

I I 

I I 

I 

I 

I 

,, \/ ,, 

;J 

1.5 2.0 

FREQUENCY, IN HERTZ 

2.5 

FIGURE 37.-Comparison of spectral amplification curves computed 

from recordings of a nuclear explosion with those computed from 

recordings at the same sites of the San Francisco earthquake of 

March 22, 195 7. The curves show the spectral amplification of a bay 

mud site (Southern Pacific Building) with respect to a bedrock site 

(Franciscan Formation; Golden Gate Park). A, Spectral amplification curves computed for horizontal motions from the N. 45° W. 

recordings at the Southern Pacific Building and the S. 80° E. recordings at Golden Gate Park. B, Spectral amplification curves computed from the recordings of vertical motion at the two sites. 

those sites most susceptible to ground failure (see Youd 

and others, this report). Such predictions are especially 

useful for evaluating the general earthquake hazard in 

currently urbanized areas not developed at the time of 

the 1906 earthquake. 

On the basis of the empirical relation M=0.27+2.70 

log(AHSA), which is based only on intensity data in San 

Francisco for which there is ((unequivocal evidence," 

Borcherdt and Gibbs (1975) predicted intensity increments for each of the 99 sites at which amplification 

values have been measured from the nuclear explosion 

data. The resulting average intensity increments, together with the average low-strain amplifications computed for each of the various geologic units with respect 

to the Franciscan Formation, are given in table 4. The 

average intensity increments range from -0.29 for 

granite to 2.43 for bay mud. 

Utilizing the computed average intensity increments 

for the various geologic units (table 4), the empirical 

relation between intensity and distance (fig. 38), and a 

generalized geologic map (K. R. Lajoie, oral commun., 

1974), Borcherdt and Gibbs (1975) predicted intensities 

on a regional scale ( 1: 125,000) for the San Francisco Bay 

region. An insert from their map shows the intensities 

predicted for San Francisco (fig. 40). The map shows the 

maximum intensity predicted for a site that might result from an earthquake in the San Francisco Bay region on either the San Andreas fault or the Hayward 

fault. The map is useful for delineating general earthquake problem areas in the San Francisco Bay region; 

hence, the map is a preliminary form of seismic zonation 

that can be used to develop general land-use policies for 

reduction of future earthquake losses. For a more rigorous zonation of the San Francisco Bay region where 

additional data are available, the maps developed in 

other papers of this report can be used to further define 

the nature and potential severity of the problems in the 

various areas. 

The design of critical structures such as hospitals, 

schools, and nuclear power plants requires quantitative 

estimates of ground shaking that the site might experience during a future strong earthquake. For soil sites 

such estimates require quantitative estimates of the 

shaking response of the underlying deposits. The 

analysis of the 1957 earthquake recordings suggests 

that the spectral amplification curves computed from 

the nuclear explosions could be used for such quantitative estimates of strain levels as much as approximately 

10-4 in soil. However, for higher strain levels as occurred during the 1906 earthquake, there are no 

strong-motion records for a similar analysis. Another 

approach for developing improved quantitative estimates is through the use of numerical models and 

laboratory data on the behavior of surficial geologic 

units at high strain levels. 

NUMERICAL MODELS FOR THE RESPONSE OF 

SURFICIAL GEOLOGIC UNITS 

To develop improved numerical models, threecomponent seismometers were emplaced in drill holes 

at a site near the margin of San Francisco Bay. The 

three-component systems were placed in bedrock, at an 

intermediate depth in the alluvium, at the base of the 

bay mud, and at the surface (fig. 41). The output of the 

systems is recorded continuously on magnetic tape and 

used to analyze ground motions generated both by earthquakes and nuclear explosions. By using the motion 

recorded at depth in the bedrock as input, numerical 

models can pre9ict the motions at other depths that in 

turn can be compared with the observed motions. 

Horizontal ground motion produced by two 

earthquakes--one relatively near the recording site, 
A60 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

DISTANCE, IN MILES 

0 2 3 4 5 6 7 8 9 10 

4 

0 

0 

15 

H 

13 1 7 

1-----l H H 

33 

5 

27 

10 

• 78 

H 

1 

H 

2 

H 1----l 2 7 

H H 

1 11 

15 

DISTANCE, IN KILOMETRES 

FIGURE 38.~0bserved earthquake intensities for sites (one square city block in size) underlain by rocks of the Franciscan Formation as a 

function of perpendicular distance from the zone of surface rupture during the 1906 earthquake. For sites with "unequivocal evidence" 

in San Francisco (map 19, Lawson, 1908) the number of observed intensity values is shown below the corresponding distance interval. 

For sites intersected by an "examined route" south of San Francisco (maps 21 and 22, Lawson, 1908) the number of observed values is 

shown above the corresponding distance interval. The observed 1906 intensity values are expressed in terms ofthe 1906 San Francisco 

intensity scale with the letters A-E corresponding respectively to the numbers 4-0 (see section "1906 Intensity Scale for San Francisco"). 

the other farther away-was recorded on the down-hole 

systems. The recordings of both earthquakes (fig. 42) 

show that horizontal ground motion at the surface was 

substantially larger than that in bedrock. However, the 

depth range over which amplification took place differed for the two earthquakes. This observation is easily 

explained by the difference in the relative frequency 

content of the bedrock motions for the two events. The 

ground motion generated by the local earthquake was 

richer in higher frequencies, because the seismic waves 

did not travel as far from the source as they did for the 

San Fernando earthquake. The differences in the nature of the amplification, as apparent on the analog 

records for the two earthquakes, illustrate the need for 

characterizing the seismic response of local geologic 

units by spectral amplification curves rather than by 

ratios of maximum amplitudes measured from analog 

records. The ratios of maximum amplitude in general 

d~pend on the nature of the source, the length of the 

travel path, and the frequency response of both the local 

geologic units and the recording instruments. In contrast, the spectral amplifications are to a first approximation dependent only on the frequency response of the 

local geologic unit. 

A comparison between the predicted and observed 

horizontal surface motions for the two earthquakes is 

shown in figure 43. Even though the relative frequency 

content of the bedrock motions for the two earthquakes 

is considerably different, the numerically predicted 

time histories agree reasonably well in both amplitude 

and relative frequency content with those actually observed. 

The predictions of the numerical model are based on 

the solution for incident homogeneous plane SH -waves 

in plane-layered viscoelastic materials (Kanai, 1952). 

Kanai's solution was extended to multiple layers using 

the Thompson-Haskell (Haskell, 1953) matrix formulation and a viscoelastic constitutive law proposed by 

Chae (1968). The material parameters for the constitutive relation used in these calculations were determined 

from in situ seismic experiments (Warrick, 197 4). 

To characterize the seismic response at the surface 
STUDIES FOR SEISMIC ZONATION A61 

4 81 =0.27 +2.70 log (AHSA) 

_.... 81 =0.19+2.971og (AHSA):7/ 

LJ.J 3 /. ~ 

t-<1: h 

~&l 0 

~0 2 0 

LJ.JU O:::Ul 0 u- zu _z 

>-<I: t::ff: 0 

Ulz •o 0 z<t 

~(/) • 

Z\0 0 ~ 0 8 -o ~0 0 0'1 

- ?' • ......, /"o 

-1 / ro o / 0 

-2 

0.2 0.5 5 10 20 30 

AVERAGE HORIZONTAL SPECTRAL AMPLIFICATION 

FIGURE 39.-Increments in 1906 intensities as a function of average 

horizontal spectral amplification computed at corresponding sites 

from recordings of nuclear explosions. Both the intensity increment 

values (IH) and the average spectral amplification values (AHSA) 

were computed with respect to the corresponding average value 

determined for sites underlain by rocks of the Franciscan Formation. The empirical relation (81 = 0.19+2.97log(AHSA)) is based 

on the data from all sites for which there was an observed 1906 

intensity value (circles and dots). The empirical relation ( 81 = 

0.27+2.70 log(AHSA)) is based on only the data from sites in the 

city of San Francisco (dots) for which there was unequivocal evidence for the ascribed degree of intensity. (The second empirical 

relation is preferred, although intensity increments predicted from 

either relation differ by less than two-tenths of an intensity increment.) 

TABLE 4.-Statistics for samples of low-strain amplifications and 

intensity increments with respect to Franciscan Formation for 

various geologic units 

Geologic unit Average horizontal Intensity increment 

spectral amplification (1906 San Francisco scale) 

Mean Standard Mean Standard 

deviation deviation 

Granite ____ ----- 0.63 0.11 -0.29 0.21 

Franciscan 

Formation ------ 1.00 0.38 0.19 0.47 

Great Valley 

sequence --- ---- - 1.42 0.45 0.64 0.34 

Santa Clara 

Formation 1.70 0.64 0.82 0.48 

Alluvium ---- ------ 2.76 1.16 1.34 0.58 

Bay mud - - -------- 7.06 3.78 2.43 0.58 

with respect to the bedrock, spectral amplifications 

were predicted with the numerical model. This prediction is compared with the amplifications observed from 

the recordings of the distant San Fernando earthquake 

(fig. 44). The fundamental frequency and the frequency 

of the first two higher modes for the computed response 

agree to within about 10 percent of those observed. The 

amount of amplification is also in reasonable agreement. The maximum computed amplification is about 

35 percent less than the maximum observed. Such comparisons of low-strain amplifications (max. strain = 

10-6) are useful for understanding the amplification 

phenomena and for the development of numerical model!'. The corresponding low-strain parameters for the 

unconsolidated deposits can be determined satisfactorily both in the laboratory and in situ. However, estimates of high-strain parameters at present must result 

largely from laboratory measurement of soil parameters. 

Using laboratory data on the dynamic behavior ofbay 

mud and alluvium, a quasilinear procedure was applied 

to estimate the high-strain response of the surficial 

geologic deposits. The procedure is based on the assumption that the linear low-strain model is applicable, 

provided the model parameters are chosen in accord 

with high-strain data from laboratory studies. This assumption has been applied previously by Idriss and 

Seed (1968). The high-strain parameters were determined from curves presented by Hardin and Dernevich 

(1972) and from data on the properties of the unconsolidated deposits compiled by Harold Olsen (written commun., 1972). 

Comparisons of the high-strain spectral response and 

the low-strain response for three sites are shown in 

figures 45, 46, and 4 7. The higher strain responses were 

computed using the bedrock motion from the San Fernando earthquake of February 9, 1971 at Pacoima Dam. 

The accelerogram was scaled according to the perpendicular distance ofthe sites from the San Andreas fault. 

The model calculations suggest that the principal effect of high strain is to shift the frequencies of the higher 

modes to lower values and to reduce the amplification 

corresponding to these modes. For frequencies in the 

vicinity of the fundamental mode, there is only a slight 

change in the amount of amplification. 

The high-strain responses of the two bay· mud sites 

(figs. 46, 4 7) are distinctly different, even though the 

low-strain seismic models used for these two sites were 

identical. This difference is due to the difference in the 

input strain level at the two sites. The higher strain 

level (fig. 46) has two effects: (1) to increase the impedance contrasts at the bedrock-alluvium and 

alluvium-mud interfaces and (2) to increase the damping of the seismic waves. The first effect tends to increase the amount of amplification, and the second 

tends to decrease it. However, since the attenuation 

increases with frequency, the higher level of input 

strain results in less amplification at the higher modes 

but increased amplification for the fundamental mode. 
A62 

37"45' 

37°37'30" 

REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

EXPLANATION 

PREDICTED MAXIMUM 

EARTHQUAKE INTENSITY 

(1906 SAN FRANCISCO 

SCALE) 

Very violent 

Violent 

Very strong 

r:::::l 

L;__:__j 

Strong 

Weak 

San Andreas fault 

R 

Reservoir 

N 

3 KILOMETRES 

FIGURE 40.-Maxirnum earthquake intensities predicted for San Francisco. Each value is the maximum of those predicted assuming a 

large earthquake on the San Andreas fault or the Hayward fault. The intensity values are predicted from the empirical relations 

(figs. 38, 39) based on only the good intensity data for the 1906 earthquake together with a generalized geologic map compiled by 

K. R. Lajoie (written commun., 1974). Letters A- E indicate grades of the San Francisco intensity scale (see section "1906 Intensity 

Scale for San Francisco"). 
STUDIES FOR SEISMIC ZONATION A63 

Bay mud 

-_,.....--t--+---------------11 m (36ft) 

12m 

(39ft) 

40m 

(131 ft) 

Alluvium 

FIGURE 41.-Schematic geologic section at the site of the down-hole 

seismometer array. 

In brief, these calculations show that an increase in 

level of input strain does not necessarily mean a decrease in amplification for all frequencies. 

The model predicts that for the higher frequencies 

(frequencies more than about three times the fundamental) the corresponding levels of ground shaking can 

be substantially reduced. Part of this reduction in 

amplitude may be an artifact of the quasilinear model 

because the high-strain parameters are chosen for the 

dominant frequency in the strain history. This possibility has been raised by other researchers (for example, 

Dobry and others, 1971). Recent work with nonlinear 

models (W. B. Joyner, unpub. data) suggests that 

ground motions calculated using the quasilinear model 

are adequate for frequencies near the fundamental frequency, but at the higher frequencies the ground motions are underestimated by the quasilinear model. 

Further investigation of the problem is needed. 

With the assumption that the high-strain modeling 

procedure is valid, it becomes possible to predict 

surface ground motions from postulated future earthquakes. Such predictions have been made at four different sites along a profile perpendicular to the San Andreas fault and are described in Borcherdt, Brabb, and 

others (this report). 

SUMMARY 

The amount of damage in San Francisco from the 

1906 earthquake was observed to depend strongly on 

the geologic character of the ground. This dependence 

suggests the need for zonation maps to reduce losses 

from future earthquakes. 

Comparative measurements of ground shaking generated by nuclear explosions and the 1957 earthquake 

show that there is a significant and consistent difference 

in the response to shaking of different geologic units in 

the San Francisco Bay region. Comparison of the measured amplifications with the 1906 intensities show 

that an increase in amplification corresponds to an increase in intensity. This correlation suggests that 

equidistant sites 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 in another 1906-type earthquake on 

either the San Andreas fault or the Hayward fault (fig. 

40; Borcherdt and Gibbs, 1975). Such a zonation, properly interpreted, is a useful first step toward predicting 

the earthquake hazard associated with various geologic 

conditions. Such a zonation does not provide quantitative estimates of ground shaking, nor does it necessarily 

define the nature of the problems in the various areas, 

such as surface faulting, liquefaction, or landsliding. It 

does delineate many potentially hazardous areas and 

provides a basis for development of general land .. use 

policies to reduce the hazards of future earthquakes. 

The data from the 1957 earthquake suggest that the 

response to shaking of the various geologic units, as 

based on data from nuclear explosions, can be extrapolated quantitatively to maximum strain levels of approximately 10-4• For higher strain levels, the results 

of the numerical model suggest that there can be large 

amplification effects near the fundamental frequency of 

a site. However, further analysis of numerical procedures and more field data on the dynamic behavior of 

surficial geologic units at high-strain levels are needed 

before these preliminary model predictions can be used 

quantitatively with confidence in the design of 

earthquake-resistant structures. 

In summary, certain qualitative conclusions can be 

drawn regarding expected intensities of shaking on a 

regional scale from future earthquakes. The -data currently available for the San Francisco Bay region 

suggest that the level of ground shaking will vary substantially depending on the type of underlying geologic 

deposit. For sites equidistant from the fault, excluding 
A64 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

LOCAL EARTHQUAKE A 

z Surface 0 

~ 

0 

~ 

_J 

~ 12m (39ft) 

z 

0 

N 

~ 40 m (131 ft) 0 

I 

186m (610ft) 

0 5.0 10.0 15.0 20.0 25.0 

TIME, IN SECONDS 

z 

0 

~ 

0 

~ 

_J 

~ z 

0 

!::::! 

a:: 

0 

I 

0 

DISTANT EARTHQUAKE 

10.0 20.0 30.0 

TIME, IN SECONDS 

B 

Surface 

12m (39ft) 

40 m (131ft) 

186m (610ft) 

40.0 50.0 

FIGURE 42.-Recordings of horizontal ground motion (N. 40° W. component) from a down-hole seismometer array near the margin of San 

Francisco Bay. Vertical scale within each set of records is the same to illustrate amplification effects ofthe soil profile. A, Local earthquake, magnitude 3.5, 79 km (49 mi) from recording site. B, Distant earthquake, magnitude 6.3, 480 km (298 mi) from site (San 

Fernando earthquake of February 9, 1971 ). 

those in the immediate fault zone, the effects of amplified ground shaking are expected to be least for those 

sites underlain by bedrock, intermediate for those sites 

underlain by alluvium, and greatest for those sites underlain by artificial fill and bay mud. This qualitative 

classification of geologic units, together with the 

geologic maps of the San Francisco Bay region described 

in Lajoie and Helley (this report) provides a qualitative 

ground-response map of the San Francisco Bay region. 

The general areas for which the effects of amplified 

ground shaking are expected to be the greatest are also 

those areas that are generally considered to be most 

susceptible to liquefaction (see Y oud and others, this 

report). 
Computed horizontal motion at surface 

Observed horizontal motion at surface 

Observed horizontal motion at a depth of 186 m (610ft) 

0 5.0 10.0 15.0 20.0 25.0 30.0 

TIME, IN SECONDS 

A 

FIGURE 43.-Comparison of observed and computed surface ground motions determined from motion observed in bedrock at a depth of 186 m (610ft). A, The observed motions 

were generated by a local earthquake (fig. 42A).B, The observed motions were generated by the distant San Fernando earthquake (fig. 428). 

UJ 

t-3 

c::: 

t:::; 

~ 

t:z:j 

UJ 

~ 

0 

~ 

UJ 

t:z:j 

00 

s:: 

~ 

0 

N 

0 

z 

> t-3 

8 

z 

> m 

01 
Computed horizontal motion at surface 

Observed horizontal motion at a depth of 186m (610ft) 

0 5.0 10.0 15.0 20.0 

TIME, IN SECONDS 

B 

FIGURE 43.-Continued. 

25.0 30.0 

> O':l 

O':l 

~ 

t_:l:j 

t:1 

e 

C1 

1-:3 

~ 

0 

z 

0 

l-%j 

t_:l:j 

> 

~ 

1-:3 

::c: 

p:) 

e 

> 

~ 

t_:l:j 

::r: 

> N 

~ t:1 

sn 

r:n 

~ l-%j 

~ 

> z 

C1 

~ 

r:n 

C1 

0 

td 

> 

~ 

~ 

t_:l:j 

0 

~ 

0 

z 
STUDIES FOR SEISMIC ZONATION A67 

15 

10 

5 z 

0 

~ 0 (.) 

u:: 

::::i 

a.. 

:::2: 

20 

<( 

.....1 

<( 15 0:: 

f-

(.) 

w 

a.. 

CJ) 10 

5 

5 

FREQUENCY, IN HERTZ 

FIGURE 44.-Computed and observed spectral amplification curves 

for the site at the down-hole seismometer array. The curves were 

determined using the horizontal motion recorded in bedrock at a 

depth of 186 m (610 ft) from the San Fernando earthquake (fig. 

428). 

10 

9 

8 

7 

z 

0 

~ 6 (.) 

u:: 

::::i 

a.. 

:::2: 5 <( 

.....1 

<( 

0:: 

f- 4 I (.) 

w I 

a.. I CJ) I 

I 

3 I 

I 

I 

I 

I 

2 I 

I 

I 

I 

'/ 

0 

0 

METRES FEET 

0 ::Bay mud 

200 

Alluvium 

400 ,. 

I I 

I I 

I I 

600 I 

200 Bedrock 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 ,_ , 

1 2 

FREQUENCY, IN HERTZ 

I 

I 

I 

\ 

I 

\ 

\ 

\ 

\ 

3 

FIGURE 46.-Comparison of high- and low-strain spectral amplification for a site underlain by 11 m (36ft) ofbay mud and 173 

m (567 ft) of alluvium. The high-strain spectral amplification 

was computed by using as input the bedrock motion recorded at 

Pacoima Dam, (San Fernando earthquake of February 9, 1971) 

scaled to a maximum velocity of 19 cm/s (8 in./s). 

z 

0 

(.) 

~ 

u:: 

::::i 

a.. 

:::2: 

<( 

.....1 

<( 

0:: 

f-

(.) 

w 

a.. 

CJ) 

6 

5 

4 

3 

2 

Alluvium 

600 

n ~o 

/ \ Bedrock 

I \ 

I \ ,-. I \ I \ 

I \ I \ I \ I \ I \ I \ I \ 

\ / ' ,,_; ' ......... 

OL------.....1------~-------L-------L------~----~ 

0 2 

FREQUENCY, IN HERTZ 

3 

FIGURE 45.-Comparison of high- and low-strain spectral amplification for a site underlain by 220m (722ft) of alluvium. The 

high-strain spectral amplification was computed by using as 

input the bedrock motion recorded at Pacoima Dam, (San Fernando earthquake of February 9, 1971) scaled to a maximum 

velocity of 31 cm/s (12 in./s). 

9 

8 

7 

z 

0 

~ 6 

(.) 

u:: 

::::i 

a.. 

:::2: 5 <( 

.....1 

<( 

0:: 

f- 4 (.) 

w 

a.. 

CJ) 

3 

2 

METRES FEET 

0 

Alluvium 

400 ,. 

I ' I I 

I I 

600 I 

200 Bedrock I 

I ,, I I I I 

I \ I I ' I 

I I I 

I ' I 

I ' \ I ' \ I 

' I 

\ I 

\ I 

\ I ,_., 

2 

FREQUENCY, IN HERTZ 

I 

' I 

\ 

' \ 

' \ 

\ 

\ 

\ 

,, 

I 

I ' I ' I ' I 

I \ 

I I ._/ \ 

\ 

\ 

\ 

\ 

\ 

\ 

3 

FIGURE 47.-Comparison of high- and low-strain spectral amplification for a site underlain by 11 m (36ft) of bay mud and 173 

m (567 ft) of alluvium. The high-strain spectral amplification 

was computed by using as input the bedrock motion recorded at 

the Pacoima Dam, (San Fernando earthquake of February 9, 

1971) scaled to a maximum velocity of 12 cm/s (5 in./s). 
LIQUEFACTION POTENTIAL 

By T. L. Youo, D. R. NICHOLS, E. J. HELLEY, and K. R. LAJOIE 

INTRODUCTION 

Liquefaction of clay-free granular layers has produced abundant and sometimes catastrophic ground 

failures during earthquakes and hence must be considered in assessing seismic risk or hazard. Conditions 

requisite for seismically induced liquefactionsaturated unconsolidated deposits and high 

seismicity-are widespread in the San Francisco Bay 

region. Evaluation of the liquefaction potential of these 

deposits thus forms an important element in mapping 

seismic hazards of the area. 

This paper describes how a preliminary 

liquefaction-potential map of part of the San Francisco 

Bay region was made and describes types of ground 

failure commonly associated with liquefaction that 

might be expected to occur in that region. Map zones are 

based on detailed geologic studies of the unconsolidated 

sediments (Helley and Brabb, 1971; Helley and others, 

1972; Lajoie and others, 1974; Nichols and Wright, 

1971). Liquefaction potential is estimated from an 

analysis of maximum horizontal surface accelerations, 

duration of ground motion, depth of water table, and 

depth and standard penetration resistance of clay-free 

granular sediments. The analysis is based on the usimplified procedure for evaluating liquefaction potential" 

developed by Seed and ldriss (1971). The results were 

statistically averaged to provide an estimate of 

liquefaction potential for each zone. 

Liquefaction is defined here as the transformation of a 

granular material from a solid state into a liquefied 

state as a consequence of increased pore-water pressures (Youd, 1973). This definition distinguishes 

liquefaction as a transformation process rather than 

liquefied flow or a type of ground failure. Hence, a potential for liquefaction does not necessarily indicate a similar potential for ground failure. However, ground failures are common consequences of liquefaction and 

hence can be expected to occur in areas susceptible to 

liquefaction. 

GEOLOGY AND SEISMICITY OF STUDY AREA 

the east the Santa Cruz Mountains on the west, and by 

the citie~ of Oakland on the northeast, San Francisco on 

the northwest, and San Jose on the south. The area 

contains the broad alluvial plain surrounding the 

southern part of San Francisco Bay. This plain is unde.rlain by late Cenozoic sediments that vary greatly In 

density and degree of consolidation. The sediments ar.e 

subdivided into units with generally similar geotechnical properties, chosen for mapping liquefaction potential (see Lajoie and Helley, this report). Those sedime~ts 

whose grain-size distribution (clay-free sand and silt) 

and degree of lithification (completely uncemented) 

make them potentially liquefiable occur within two 

units· the older of these was deposited during late Pleistocen~ time and the younger, during Holocene time. 

Environme~ts of deposition during both the late Pleistocene and Holocene were similar to those of today except that marine and estuarine conditions were absent 

during parts of the late Pleistocene. . 

The older deposits are denser and more consolidated 

and tend to be coarser grained. Because they have been 

long exposed to weathering processes and ~hanging 

climatic regimes, they commonly contain welldeveloped soil profiles. Where exposed, the older deposits are expressed geomorphically as sl~ghtly d~ssected alluvial fans and aprons generally lying at higher 

altitudes near the margins of the plains, where they 

gradually merge into the surrounding foothills:· ~ecause these fans are in the highest part of the plain, 

ground-water levels are generally deep but may be temporarily high during wet seasons. 

The younger alluvial deposits, which are much loos~r, 

wetter and less consolidated than the older fan deposits 

on whi~h they rest, grade into the modern sediments of 

San Francisco Bay. The interfingering of alluvial and 

estuarine (bay) sediments in these younger deposits 

reflects the post-Wisconsin marine transgression into 

the basin of San Francisco Bay. 

The San Francisco Bay region is very active seismically, having been subjected to large historic earthquakes originating nearby on both the San Andreas and 

Hayward faults. For example, the 1906 earthquake 

The mapped area is bounded by the East Bay hills on (magnitude 8.2) was accompanied by a continuous 306 

A68 
STUDIES FOR SEISMIC ZONATION A69 

km (190 mi) surface rupture on the San Andreas fault. 

The 1868 (and possibly the 1836) earthquake on the 

Hayward fault also produced significant surface ruptures. 

DESIGN EARTHQUAKES 

Sediments are classified by liquefaction potential as 

follows: (1) Sediments likely to liquefy in the event of a 

moderate earthquake (magnitude 6.5) originating 

nearby on the San Andreas, Hayward, or other local 

fault are considered to have a high liquefaction potential; (2) sediments unlikely to liquefy even in the event 

of a major earthquake (magnitude 8.0) nearby on the 

San Andreas fault are considered to have a low liquefaction potential; and (3) sediments between these two 

extremes are considered to have moderate liquefaction 

potential dependent on earthquake size and duration 

and sediment properties such as grain size and degree of 

sorting. A moderate-size event would be characterized 

by approximately 10 significant strong-motion cycles 

(Seed and Idriss, 1971) with maximum horizontal surface accelerations of 0.2 g or greater (Page and others, 

1972) over much of the area, and a large event by as 

many as 30 significant strong-motion cycles (Seed and 

Idriss, 1971) with maximum horizontal surface accelerations of0.5 g or greater (Page and others, 1972). These 

parameters are used in the following analyses. 

METHOD OF EVALUATING LIQUEFACTION 

POTENTIAL 

The method used to estimate liquefaction potential is 

based on the ~~simplified procedure for evaluating 

liquefaction potential," which was developed for materials that underlie relatively level surfaces (Seed and 

Idriss, 1971, p. 1249) and have relative densities (Dr) 1 

less than about 80 percent (p. 1256). Because slopes on 

the alluvial plain surrounding San Francisco Bay are 

small, the method can be applied over most of the area. 

However, because the large design earthquake could 

possibly produce liquefaction in sediments with relative 

densities greater than 80 percent, the procedure of Seed 

and Idriss was extended to permit evaluation of 

liquefaction potential for these extreme conditions. 

The simplified procedure is based on two basic relations. First, the average cyclic shear stress Crav), developed during a given earthquake at a depth (h), beneath a level surface is estimated from the equation 

T av = 0.65r dyh(amaxlg), (1) 

1Relative density <Drl, in percent, is defined as 

emax- e 

Dr=--- (100), 

emax- emin 

where emax and emin are void ratios of a given granular material in its loosest and densest 

states, respectively, and e is the void ratio of the material at the density in question. 

where rd is an empirically determined stress reduction 

coefficient, y is the unit weight of the soil, a max is the 

maximum horizontal surface acceler~tion, and g is the 

acceleration of gravity. (Equation 1 is equation 4 of Seed 

and Idriss, 1971, p. 1256.) Second, the ratio of in situ 

cyclic shear stress (r) required to produce liquefaction in 

a given number of cycles (l) on laboratory samples 

molded at the in situ relative density (Dr) to the effective 

overburden pressure (0"0 ') is related to results of 

laboratory cyclical triaxial compression tests as 

(r/0"0 ')lDr = Cr (O"dci20"a)l50 (Dr /50) for Dr <80 percent,(2) 

where Cr is a correction coefficient applied to triaxial 

compression test results, O" de is the cyclic deviator stress 

producing liquefaction in l cycles on a remolded sample 

of the in situ or similar material at a relative density of 

50 percent, and O"a is the initial effective confining pressure. (Equation 2 is equation 6 of Seed and ldriss, 1971, 

p. 1258.) Empirical curves have been constructed by 

Seed and ldriss (1971) for estimatingCr and (O"dci20"a)so 

from density state and gradational properties of the soil 

and the number of significant strong-motion cycles. 

Thus, by comparing the average cyclic shear stress Crav) 

developed at any given depth (equation 1) with the 

cyclic shear stress (r) required to produce liquefaction of 

the materials at that depth (equation 2), a criterion is 

established for assessing liquefaction potential. 

For the moderate-size design earthquake, the following parameters were used in equations 1 and 2 to estimate limiting values of relative density at which 

liquefaction would be likely to occur: 

amax = 0.2 g, 

l = 10 cycles, 

0.8<rd <1.0, 

0.6<Cr<0.7, 

1.44 g/cm3 <p <1.76 g/cm3 (90 lb/ft3 < y < 110 

lb/ft3), 

0.2<(0"dci20"a)zso < 0.3, and 

0 m <hw <3m (0 ft < hw <10ft), 

where pis the density ofthe material andhw is depth to 

the water table. Substitution of reasonable, combinations of these values into the equations shows that most 

granular soils with relative densities less than 65 percent that are located beneath the free water surface 

would have a high potential for liquefaction during the 

estimated moderate-earthquake conditions. 

To evaluate liquefaction potential for the large design 

earthquake, it was necessary to extend the simplified 

procedure to relative densities greater than 80 percent. 

To do this, data presented by Peacock and Seed (1968) 

were used to evaluate stress conditions causing 

liquefaction at these higher densities. Average shear 

stress ratios (r/0"0 ') producing liquefaction in 30 loading 

cycles (simple shear) on samples with relative densities 
A70 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

ranging from 50 to 90 percent (Peacock and Seed, 1968, 

p. 701) are 

r/a-0 ' =0.060 for Dr=50 percent, 

rla-0 '=0.093 for Dr=80 percent, and 

r/a-0 '=0.152 for Dr=90 percent. 

According to Seed and Peacock (1971, p. 1102), these 

stress ratio values should be increased 35 percent because of subsequent !!improvements in sample preparation and cap seating techniques." They also stated (p. 

1102) that the results should be increased by a compounded 15 percent to agree with results using rough 

platen~. Even after applying these corrections, Seed and 

Peacock (1971, p. 1111) further increased the values by 

an additional factor (which varies with density) to bring 

the results into agreement with estimated field behavior. These factors (interpolated from data given by 

Seed and Peacock (1971, p. 1112)) are 

22 percent for Dr=50 percent, 

46 percent for Dr=80 percent, and 

55 percent for Dr=90 percent. 

Thus, estimated stress ratios required to produce 

liquefaction in the field during 30 cycles of ground motion are 

rla-0 ' =0.114 for Dr=50 percent, 

r/a0 '=0.211 for Dr=80 percent, and 

rla-0 ' =0.366 for Dr=90 percent. 

These data are plotted and a curve (reconstructed curve) 

drawn through them (fig. 48). A plot of equation 2 also is 

drawn on figure 48 for comparison. Parametric values 

used in constructing the latter curve include 30 cycles of 

ground motion, (a-dcl2a-a)=0.18, a minimal (worst condition) value, (see Seed and Idriss, 1971, p. 1257), and Cr 

values taken directly from curves given by Seed and 

Idriss (1971, p. 1258). 

To facilitate the use of the reconstructed curve for 

evaluating the liquefaction potential of granular sediments at relative densities greater than 80 percent, 

equation 2 was modified to 

(rla-0 ')ZDr=M for Dr >80 percent, (3) 

where the value of Mistaken directly from the reconstructed curve. Next, r in equation 3 was equated with 

Tav in equation 1 to solve for limiting M values at which 

liquefaction could occur: 

1\1=0.65 rd(yh/a-0 ') (amaxlg). (4) 

For the large design earthquake, the following 

parametric values were used: amax=0.5g, l =30 cycles, 

and other values as given for the moderate-size design 

earthquake. Substitution of reasonable combinations of 

these values into equation 4 yields limiting M values 

between 0.3 and 1.0. This corresponds to relative densities ranging from 87 to 92 percent. (The range is narrow because of the steep slope of the reconstructed 

curve, figure 48.) Thus, a limiting density of90 percent 

was selected as the maximum at which liquefaction 

0.5 

0 0.4 

~ 0::: 

Cf) 0.3 Cf) 

w 

0::: 

fen 0.2 

0.1 

30 40 50 60 70 80 90 100 

RELATIVE DENSITY, IN PERCENT 

FIGURE 48.-Estimated stress ratio, (r/cr0 ')wr• required to produce 

liquefaction under field conditions during 30 cycles of seismic loading. Curve A, plotofequation2 (equation 6 ofSeedandidriss, 1971). 

Curve B, reconstructed curve plotted from data given in Seed and 

Peacock (1971). 

might be expected to occur during a large earthquake in 

the San Francisco Bay region. 

The liquefaction-potential criteria can now be summarized as follows: Saturated clay-free granular sediments with relative densities less than 65 percent are 

considered to have high liquefaction potential, even in a 

moderate earthquake; clay-free granular sediments 

with relative densities greater than 90 percent are considered to have low liquefaction potential, even in a 

major earthquake; and saturated clay-free granular 

sediments with relative densities between 65 and 90 

percent have moderate liquefaction potential that depends on intensity and duration of ground shaking and 

textural properties of the sediments. 

To facilitate application of the liquefaction criteria to 

field observations, relative densities were estimated 

from data on standard penetration tests using relations 

developed by Gibbs and Holtz (1957). This procedure 

also was used by Seed and Idriss (1971). Standard 

penetration versus depth curves taken from the relations of Gibbs and Holtz are plotted in figure 49 for 

relative densities of. 65 and 90 percent. Assumed 

parameters used in constructing these curves include a 

water-table depth of 3 m (10ft) at the time of drilling 

and a dry density of 1.6 g/cm3 (dry unit weight=100 

lb/ft3 ). 

MAPPING OF LIQUEFACTION POTENTIAL 

Boring logs from throughout the study area were collected from numerous private consultants and governmental agencies. Standard penetration data from 

clay-free granular deposits within 15 m (50 ft) of the 
STUDIES FOR SEISMIC ZONATION A71 

I=' 

LLJ 

LLJ 

\t, 

en 

LLJ 

3 

(10) 

6 

(20) 

~ 9 

:g (30) 

~ 

::C 

1-

a.. 

LLJ 

Cl 

12 

(40) 

15 

(50) 

18 

(60)o 

High 

Liquefaction 

potential 

Moderate 

20 40 

Low 

STANDARD PENETRATION RESISTANCE, 

IN BLOWS PER 0.3 METRE (1.0 FOOT) 

60 

FIGURE 49. Criteria used for estimating liquefaction potential in 

the field. Based on a correlation between standard penetration 

resistance and relative density developed by Gibbs and Holtz 

(1957). Curves are based on the assumption that the water table 

was 3m (10ft) deep when penetration data were obtained from 

exploratory borings. 

surface were compiled and statistically analyzed in each 

of the generalized map zones shown in figure 50. These 

zones were derived from geologic maps of unconsolidated sediments (Helley and Brabb, 1971; Helley and 

others, 1972; Nichols and Wright, 1971; Lajoie and 

others, 1974.) Zone 1 is an area ofbay mud (9,000 years 

and younger) overlying Holocene and late Pleistocene 

alluvium. Clay-free granular layers are not generally 

present in the bay mud but do occur locally near present 

and former stream channels. Clean granular layers are 

rather common, however, in the Holocene alluvium and 

the late Pleistocene alluvium beneath the bay mud. The 

surficial deposits in zone 2 are Holocene alluvium (less 

than 10,000 years old) overlying late Pleistocene alluvium. This zone is subdivided on the basis of watertable depth. Areas in which the water table is normally 

at 3m (10ft) or less are labeled 2a. Areas with a deeper 

water table are labeled 2b. The Holocene alluvium is 

generally greater than 3 m (10ft) thick in subzone 2a 

and thus extends below the water table. Hence, the 

Holocene alluvium in this zone has a continual potential for liquefaction. In subzone 2b, the Holocene alluvium is generally less than 3 m (10ft) thick and thus 

is normally above the water table. It therefore has at 

most only a seasonal potential for liquefaction. Zone 3 is 

an area of late Pleistocene alluvium consisting of overconsolidated alluvial-fan deposits that extend to a considerable depth. 

Table 5 summarizes the percentages of available 

standard penetration test data plotting in each category 

of liquefaction potential for each geologic unit. The data 

show that granular layers within the bay mud (zone 1) 

have a generally high potential for liquefaction. 

Seventy-three percent of the penetration data from 

these layers indicate high potential for liquefaction, and 

an additional 21 percent indicate moderate potential. 

Only 6 percent of the penetration data indicate low 

potential. 

The granular layers beneath the bay mud (zone 1) 

show a much lower, but still significant, potential for 

liquefaction. Thirty-three percent of the data from these 

layers indicate high potential for liquefaction, and an 

additional 28 percent suggest moderate potential. The 

overall liquefaction potential of this unit is classed as 

moderate. Virtually all the bay mud and the deposits 

underlying the bay mud lie below the normal groundwater table; thus, they present a persistent potential for 

liquefaction. 

Clay-free granular layers within the Holocene alluvium in zone 2 show on the average less potential for 

liquefaction than those within or beneath the bay mud 

because of both their greater density and the greater 

depth of the water table. Their potential, however, is 

still classed as moderate. Twenty-two percent of the 

penetration values for these sediments indicate high 

potential for liquefaction, an additional33 percent indicate moderate liquefaction potential, and 45 percent 

indicate low potential. The Holocene alluvium of subzone 2b would be, at most, seasonally or intermittently 

liquefiable because it is normally above the water table. 

Although not specifically shown in figure 50 or table 5, 

granular layers in the relatively recent channel and 
A72 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

overbank deposits along present drainageways are 

generally characterized by lower penetration resistance 

and, consequently, higher liquefaction potential than 

deposits in the adjacent alluvial plains. 

Liquefaction ·potential of granular layers in the late 

Pleistocene alluvium (zone 3) is generally low. Only 11 

percent of the penetration values from these sediments 

indicate high potential for liquefaction, whereas 60 percent indicate low potential. Liquefaction potential in 

this zone is further diminished by a relatively high 

topographic position and hence a deep water table. 

TABLE 5.-Summary of the analysis of liquefaction potential using 

standard penetration data and criteria plotted in figure 49 

[Two probable local earthquakes are considered; (1) a moderate event (magnitude "'6.5) and 

(2) a large event (magnitude 8.0 ). Sediments likely to liquefy during a moderate event are 

classified as having high liquefaction potential; those unlikely to liquefy during a large 

event are classified as having low liquefaction potential; and those between these two 

categories are classified as having moderate liquefaction potential] 

Zone Sedimentary unit 

Deposits within 

bay mud--------·-

Deposits underlying bay mud ____ 

2a, Holocene 

2b alluvium --------

3 Late Pleistocene 

alluvium 

Standard penetration test data 

Percent indicating 

Dr<65 percent 65 percent<Dr Dr>90 percent 

(high <90 percent (low 

liquefaction (moderate lique- liquefaction 

potential) faction potential) potential) 

73 21 6 

33 28 39 

22 33 45 

11 29 60 

Number 

of 

tests 

53 

155 

708 

357 

GROUND FAILURES ASSOCIATED WITH 

LIQUEFACTION 

Three types of ground failure are commonly associated 

with liquefaction (Seed, 1968; Youd, 1973). (1) Flow 

landslides are failures that generally occur on moderate 

to steep slopes underlain by loose granular deposits. In 

this case, once liquefaction has occurred, flow deformation commences and continues unabated until the driving shear forces are reduced (as by slope reduction) to a 

value less than the viscous shear resistance of the 

liquefied soil. When that state is reached, the material 

stops flowing and solidifies, usually far from the point of 

origin. Loose granular deposits on moderately to steeply 

sloping hillsides in the San Francisco Bay region could 

be susceptible to this type of failure if they were saturated. Such failures occurred on San Bruno Mountain 

near Colma and near Half Moon Bay during the 1906 

San Francisco earthquake (Crandall, in Lawson, 1908, 

p. 249; Anderson, in Lawson, 1908, p. 395). Because of 

the generally small slopes, it is unlikely that this type of 

failure would occur on the broad alluvial plain surrounding San Francisco Bay. 

(2) Lateral-spreading landslides are failures that occur most commonly on gentle to nearly horizontal slopes 

underlain by loose to moderately dense granular deposits or layers. In this type of failure, liquefaction occurs and flow commences; however, after a finite displacement, flow is arrested by a drop in pore-water pressure resulting from the tendency for all but very loose 

granular sediments to dilate during shear. Continued 

shaking may cause reliquefaction (provided the shaking causes reversals in shear stress (Seed and Lee, 1969; 

Youd, 1973)), and a second episode of flow displacement 

may occur followed by restabilization. This sequence 

may continue as long as strong shaking continues. Displacements ranging from nearly zero to tens of metres 

have been produced by these kinds of failures (Varnes, 

1958; Youd, 1973). Factors that contribute to greater 

disp1acement include greater duration of shaking, loose 

sediments, and optimal slope conditions. (Slopes that 

are too flat inhibit movement, and slopes that are too 

steep inhibit reversals in shear stress necessary for the 

generation of repeated episodes of liquefaction (Y oud, 

1973).) Cracks, fissures, and differential settlement are 

common on, and especially at the margins of, lateralspreading failures. Although these features and accompanying slide movements may appear rather inconsequential in open terrain, they have proved to be very 

damaging and disruptive to structures and utilities constructed across, on, or within the slide mass. 

Lateral spreading probably would be the most pervasive type of ground failure associated with liquefaction 

on the broad alluvial plain surrounding San Francisco 

Bay. Sediments containing granular layers, especially 

the bay mud and recent channel and overbank deposits 

in the Holocene alluvium, probably would be the materials most susceptible to this type of failure because of 

their greater potential for liquefaction and generally 

uncompacted state, which would permit greater slide 

movement after liquefaction. Least susceptible to this 

type of failure would be the late Pleistocene alluvium 

(zone 1) because of its low potential for liquefaction and 

generally dense state, which in turn would prevent significant displacements from occurring even if liquefaction should develop. 

Evidence of lateral spreading was reported at several 

places within the study area during the 1906 San Francisco earthquake. Most of these slides occurred in the 

susceptible areas listed above. For example, lateral 

ground movements, some as large as 2m (6ft), occurred 

in several areas of San Francisco where artificial fill is 

underlain by bay mud (zone 1) (Wood, in Lawson, 1908, 

p. 220--245). In addition, lateral displacement of floodplain deposits toward the depressions of Alameda and 

Coyote Creeks was mentioned specifically (Lawson, 

1908, p. 400). Many lateral-spreading landslides generated by the 1906 earthquake may not have been re-
STUDIES FOR SEISMIC ZONATION A73 

30' 

LIQUEFACTION POTENTIAL 

ZONE GEOLOGIC UNIT OF CLAY-FREE GRANULAR 

LAYERS 

~ 

VJ'#.bfl 

Bay mud and 

underlying 

sediments 

Holocene 

alluvium 

Holocene 

alluvium 

Generally moderate, locally 

high where clean granular 

layers are present in bay mud 

Moderate, water table normally . above 3 metres (10 feet) 

Moderate, water table normally 

below 3 metres (10 feet) 

F::l Late Pleistocene Li.:_j alluvium Generally low 

0 2 4 6 MILES 

0 2 4 6 KILOMETRES 

FIGURE 50.-Preliminary map showing liquefaction potential for the southern San Francisco Bay region. The map shows generalized liquefaction potential of granular layers in each map zone but does not delineate locations of these layers. Hence, the map is useful for designating zones where special consideration should be given to the possibility ofliquefaction but is not valid for assessing the liquefaction 

potential of a given site. 

corded, especially in undeveloped areas near the bay 

that were not thoroughly investigated. 

(3) Quick-condition failures have occurred historically most often in flat areas with high water tables and 

loose to moderately dense granular sediments extending from near the surface to substantial depths. In this 

situation liquefaction may lead to a quick condition and 

often to the loss of bearing capacity with the result that 

structures, embankments, or other loads founded on the 

surface sink into the liquefied sediments. At the same 

time buried tanks or other vessels may rise buoyantly. 

Other than subsidence of several roadway fills in San 
A74 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

Francisco, which may or may not have been due to 

liquefaction, the authors have found no reports that this 

type of failure occurred in the study area during the 

1906 San Francisco earthquake. 

SUMMARY 

A preliminary map ofliquefaction potential has been 

compiled for the southern San Francisco Bay region. 

This map delineates zones in which existent clay-free 

granular layers are estimated to have low, moderate, or 

high liquefaction potential. The map zones are derived 

from detailed geologic studies of the unconsolidated sediments in the region. Liquefaction potential of the 

granular layers is estimated from an analysis of 

lithologic, water table, standard penetration, and seismic data. 

Areas underlain by bay mud containing clay-free 

granular layers have a generally high potential for 

liquefaction. Granular layers underlying the bay mud 

have a significant but lower potential, which is classed 

herein as moderate . Granular layers within the 

Holocene alluvium have an even lower, but still moderate, potential; furthermore, most ofthis unit is normally 

above the water table and thus, at most, is only seasonally or intermittently susceptible to liquefaction. Clayfree granular layers within channel and recent overbank deposits of the Holocene alluvial unit generally 

are characterized by a greater liquefaction potential 

than adjacent deposits in the alluvial plain. Granular 

layers in the late Pleistocene alluvium generally have 

low potential for liquefaction. 

Zones delineated in this study as having significant 

liquefaction potential indicate areas in which the 

liquefaction process may occur in existing clay-free 

granular layers; unfortunately, insufficient data are 

presently available to plot the actual locations of these 

layers. The data also give no indication of type or 

amount of ground failure, if any, that might follow 

liquefaction. However, lateral-spreading landslides are 

a common consequence of liquefaction beneath gentle 

slopes. Hence, in the event of a major earthquake, this 

type of failure is likely to be a result of liquefaction 

beneath the alluvial plain surrounding San Francisco 

Bay. Reports from the 1906 San Francisco earthquake 

verify this conclusion. 

The criteria used for evaluating liquefaction potential are based on empirical procedures formulated by 

Seed and Idriss (1971) and approximate estimates of 

ground-motion parameters and geotechnical properties. 

In addition, the estimated potential of each zone is based 

on somewhat limited data generalized to include the 

entire map unit. Thus, the map ofliquefaction potential 

must be considered preliminary and approximate and 

not valid for direct determination ofliquefaction potential at any specific site. However, despite its limitations, 

the map should serve the intended purpose of generally 

delineating areas where the probability that liquefaction will occur during a major earthquake is greatest 

and hence areas where special attention is required. 
LANDSLIDES 

By T. H. NILSEN and E. E. BRABB 

INTRODUCTION 

Landslides are characteristically abundant in areas 

of high seismicity and steep slopes. Landslides associated with earthquakes may cause as many or more 

fatalities as the initial fault rupture and shaking of the 

gro~nd . They may also occur long after an earthquake, 

havmg been caused or aided by the loosening, shaking, 

and disruption of the deposits on slopes during the 

earthquake. As a result, the landslide hazards related to 

earthquakes may persist long after the ground has 

stopped shaking and therefore can be a long-term problem. 

Some of the major earthquakes that have occurred 

during the past 15 years have vividly demonstrated the 

hazards of seismically triggered landslides. The Hebgen 

Lake, ~ont ., earthquake of 1959 triggered a very large 

landslide (fig. 51) that killed and injured many people, 

formed a temporary lake, and blocked travel in the area 

(Hadley, 1964, fig. 54). The Anchorage, Alaska, earthquake of 1964 triggered both extensive subaerial (fig. 

52) and submarine landslides; tsunamis (seismic sea 

waves) generated by the submarine landslides caused 

extensive damage and many fatalities in coastal areas 

(Hansen and others, 1966). The earthquake in western 

FIGU~E 51.-Earthquake-generated landslide at Hebgen Lake, Mont. 

This landslide, one of the largest ever recorded in the United States 

was responsible for 26 fatalities and many injuries during the Au: 

gust 17, 1959, earthquake (magnitude 7.1). Photograph by U.S. 

Forest Service. 

Peru in 1970 triggered a massive debris avalanche (fig. 

53) that destroyed the cities ofYungay and Ranrahirca· 

it caused probably about one-half of the 38,000 fatalitie~ attributed to the earthquake (Plafker and others, 1971). 

The Sa~ Fernando, Calif., earthquake of February 9, 

1971, tnggered more than 6,000 individual landslides 

in the surrounding upland areas, most of them of small 

size (fig. 54); however, only a few damaged manmade 

structures because residential and industrial development had been restricted almost wholly to the relatively 

flat floor of the San Fernando Valley (Morton, 1971; oral 

commun., September 1973). 

Each of the major earthquakes described above had 

magnitudes greater than 6.5. Although smaller earthquakes may cause less damage (or none) to manmade 

str~ctures by ground shaking, they are capable oftriggermg slope failures in hillside areas, especially renewed movements of old, marginally stable landslide 

d~posits. F~r example, an earthquake in February 1972, 

w1th magmtude 5.0 and epicenter located 37 km (23 mi) 

south of Hollister, Calif., triggered considerable 

downslope movement on a large old landslide deposit on 

Halls Ranch, near Paicines (fig. 55; Rogers, 1972). Had 

FIGURE 52.-0blique aerial view of Turnagain Heights landslide in 

A.nchorage, Alaska, triggered by earthquake of March 27, 1964 (magmtude 8.4). The landslide destroyed more than 75 homes. Photograph 

by U.S. Army. 

A75 
A76 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

FIGURE 53.-0blique aerial view of debris avalanche that destroyed 

the towns of Yungay and Ranrahirca in western Peru during the 

earthquake of May 31, 1970. The avalanche originated at an altitude between 5,500 and 6,400 m (18,000 and 21,000 ft) and moved 

2.4 km (1.5 mi) downslope to the Rio Santa at a speed in excess of280 

km!hr (175 milhr) (Plafker and others, 1971, p. 550--558). More than 

18,000 people were killed by the avalanche, and an additional 

20,000 persons were killed by other effects of the earthquake. 

the hillside area been developed for residential purposes 

at the time of the earthquake, considerable damage 

might have resulted. 

The most extensive records of seismically triggered 

landslides in the San Francisco Bay region are descriptions of the California earthquake of April18, 1906, and 

the San Francisco earthquake of March 22, 1957. The 

1906 earthquake triggered many landslides throughout 

the bay region (Lawson and others, 1908, p. 389-399). 

Photographic evidence and eyewitness accounts indicate that the earthquake triggered many different types 

of landslides, resulting in several fatalities and major 

damage to nearby manmade structures. Fortunately, in 

1906 most of the residential and industrial development 

was concentrated along the gently sloping margins of 

FIGURE 54.-0blique aerial view of landslides (white patches) in 

Lopez Canyon area, north of San Fernando, Calif., triggered by the 

earthquake of February 9, 1971 (magnitude 6.6). Photograph by 

D. M. Morton. 

FIGURE 55.-Transverse cracks formed in upper part of old landslide 

deposit on Halls Ranch near Paicines, central California, by renewed movement du!'ing a moderate earthquake (magnitude 5.0) in 

February 1972. Photograph by T. H. Nilsen. 

San Francisco Bay and in some interior valleys, and so 

the total amount of damage caused by the numerous 

landslides was not catastrophic. 

Some landslides that occurred in 1906 provide useful 

background data for predicting slope failures in future 

earthquakes. Earthfl.ow-type landslides were triggered 

locally on very gently sloping surfaces where the ground 

moisture content was high (fig. 56A); under normal 

conditions, very few landslides in the San Francisco Bay 

region develop on slopes ofless than 15 percent (Bonilla, 

1960a; Brabb and others, 1972; Nilsen and others, 

197 4). Other earthfl.ow-type landslides developed on 

steeper slopes, and some were observed to have moved 

during short periods oftime (figs. 56B, C). Because the 
STUDIES FOR SEISMIC ZONATION A77 

FIGURE 56.-Earthflow-type landslides triggered by the earthquake 

of April 18, 1906 (magnitude 8.3), near Half Moon Bay, Calif. A, 

Landslide on very gentle slope in water-saturated ground. The 

scarp is about 3 m (10 ft) high. Photograph 66 of J. C. Branner 

collection. B, Landslide on steeper slope in same general area as A. 

Photograph 69 of J. C. Branner collection. C, Landslide at Nunez 

Ranch. The flow originated about 150 m (500 ft) above the valley 

floor and took about half an hour to reach the base of the hill 

(Lawson and others, 1908, p. 397). Photograph 64 of J. C. Branner 

collection. Note: This photograph and others from the Branner and 

Gilbert collections are available for inspection at the U.S. Geological Survey library, Menlo Park, Calif. 

1906 earthquake occurred in April, at the end of the 

rainy season, the ground probably was particularly susceptible to failure by earthfiow processes because of 

high moisture content. 

Other landslides triggered in 1906 damaged manmade structures such as railroads located in coastal 

areas (fig. 57). Some landslides that occurred a year or 

more after the earthquake were probably initiated by 

the earthquake. In one such example from the northern 

San Francisco Bay region (fig. 58), open fractures in the 

ground appeared immediately after the earthquake, but 

massive failure did not occur until the next rainy season. 

The 1957 earthquake occurred near Daly City. Although the earthquake was of only moderate magnitude 

(5.3), it triggered landslides that caused considerable 

damage to manmade structures (fig. 59; Bonilla, 1959). 

Landsliding is a severe and continual problem in the 

FIGURE 57.- Roadbed of the coastal Ocean Shore railroad, south of 

San Francisco, damaged by landslides triggered by the 1906 earthquake. Photograph 78 of J. C. Branner collection. 

FIGURE 58.-Landslide located about 6 km (4 mi) north of Bolinas 

Lagoon, Marin County, Calif., generated in March 1907. Cracks 

that formed during the earthquake of April1906 contributed to the 

triggering of this landslide. Photograph 34 of the G. K. Gilbert 

collection. 
A78 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

FIGURE 59.-Landslides in artificial fill along the shore of Lake 

Merced, near San Francisco, triggered by the San Francisco earthquake of March 22, 1957 (magnitude 5.3). Photograph by ·M. G. 

Bonilla. 

San Francisco Bay region, as indicated by the high 

annual costs of landslide damage. Taylor and Brabb 

(1972) estimated an overall public and private cost in 

the bay region of more than $25 million from landslides 

generated during the rainy season of 1968-69. In 

another study Nilsen and Brabb (1972) determined that 

a single landslide, which has been active over a period of 

at least 10 years, cost the city of San Jose more than 

$750,000. 

This brief review of the effects and history oflandsliding in the San Francisco Bay region and otper areas 

points out the severity of the problem and the magnitude of the potential hazard. However, we are only 

beginning to understand many aspects of the problem 

and are only in the early phases of developing methods 

for predicting the location, distribution, and types of 

landslides that would be triggered by a major earthquake in the bay region. Prediction of an individual 

slope failure for a given earthquake requires an understanding of all the factors that contribute to the slopefailure process and detailed investigations of specific 

site characteristics. Such detailed information is not 

availab_le on a regional scale in the San Francisco Bay 

region. Our work has included (1) historical studies of 

landsliding during past earthquakes, (2) mapping of old 

landslide deposits, (3) study of bedrock units that are 

susceptible to landsliding, (4) studies of recent landsliding and its relation to slope, bedrock geology, rainfall, 

and areas underlain by old landslide deposits, (5) the 

effects of development and construction activities on 

landsliding, and (6) some of the economic costs associated with landsliding. 

This report briefly describes landslide processes, discusses basic data pertinent to analyzing the landslide 

problem on a regional scale, and utilizes these data to 

delineate areas within the San Francisco Bay region 

with various levels of landslide susceptibility during 

earthquakes. 

PREVIOUS WORK 

Most previous geologic studies in the bay region have 

focused on bedrock units and their structural and 

stratigraphic relations rather than on surficial deposits 

and their slope-stability characteristics. Consulting engineering geologists have examined the slope-stability 

characteristics of many small parcels of land in detail, 

but little ofthis work has been published; moreover, few 

regional studies have been undertaken by either consultants or governmental agencies. Some of the earlier 

studies concerned with landslides, slope-stability 

characteristics, and engineering properties of bedrock 

and surficial units provided data and valuable contributions to our studies. These include studies by Schlocker, 

Bonilla, and Radbruch (1958), Bonilla (1960a, 1960b, 

1971), Radbruch (1957, 1969), Radbruch and Weiler 

(1963), Kojan, Foggin, and Rice (1968), Harding (1969), 

Clague (1969), Pampeyan (1970), Rogers (1971), Waltz 

(1971 ), Huffman (1972a, b), Rice and Strand (1972), 

Burnett (1972), Radbruch and Wentworth (1971), 

Taylor and Brabb (1972), Nilsen and Turner (1974), and 

Nilsen, Taylor, and Brabb (1974). These and current 

studies have yielded important data for analyzing the 

landslide problem on a regional scale. 

LANDSLIDE PROCESSES 

Landslides are the downward and outward movement 

of slope-forming materials composed of natural rock, 

soils, artificial fill, or combinations thereof (Eckel, 1958, 

pl. 1). They move along surfaces of separation by falling, 

sliding, and flowing, giving rise to many characteristic 

features (fig. 60). 

Landslide deposits range in appearance from clearly 

discernible, largely unweathered and uneroded topographic features to indistinct, highly weathered and 

eroded features recognizable only by their characterisFIGURE 60.-Nomenclature ofparts of a landslide (from Eckel, 1958). 
STUDIES FOR SEISMIC ZONATION A79 

tic topographic configurations. Topographic and associated features useful in recognizing landslide deposits include (1) small isolated ponds, lakes, and other 

closed depressions, (2) abundant natural springs, (3) 

abrupt and irregular changes in slope and drainage 

pattern, (4) hummocky irregular surfaces and fiat or 

backtilted areas, (5) smaller landslide deposits that are 

commonly younger and form within older and larger 

landslide deposits, (6) steep, arcuate scarps at the upper 

edge of the deposit, (7) irregular soil and vegetation 

patterns, and (8) disturbed vegetation. 

Landslides commonly are classified by the type of 

material underlying the slope before it moved, by the 

DEBRIS SLIDE 

Incoherent or broken masses of rock and other 

debris that move downslope by sliding on a surface that underlies the deposit 

Coherent or intact masses that move downslope 

by rotational slip on surfaces that underlie as 

well as penetrate the landslide deposit 

type of movement, and by the amount of water in the 

material. Four common types oflandslides found in the 

San Francisco Bay region are shown in figure 61. 

The formation ofla11dslides under national conditions 

is affected by (1) type of earth materialsunconsolidated, soft sediments or surficial deposits will 

move downslope easier than consolidated, hard bedrock; 

(2) structural properties of earth materials-the orientation of the layering of some rocks and sediments relative to slope directions, as well as the extent and type of 

fracturing and crushing of the materials, will affect 

landslide potential; (3) steepness of slopes,-landslides 

occur more readily on steeper slopes; (4) waterEARTH FLOW 

Colluvial materials that move downslope in a 

manner similar to a viscous fluid 

ROCKFALL 

Rock that has moved primarily by falling through 

the air 

FIGURE 61.-Common types of landslides in the San Francisco Bay region. 
ABO REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

landsliding is generally more frequent in areas of seasonally high rainfall because the addition of water to 

earth materials commonly decreases their resistance to 

sliding; water decreases cohesive forces that bind clay 

minerals together, lubricates surfaces along which slippage may occur, adds weight to surficial deposits and 

bedrock, reacts with some clay minerals, causing 

volume changes in the material, and mixes with finegrained unconsolidated materials to produce wet, unstable slurries; (5) type of vegetation-trees with deep 

penetrating roots tend to hold bedrock and surficial deposits together, thereby increasing ground stability; (6) 

proximity to areas undergoing active erosion -rapid 

undercutting and downcutting along stream courses 

and shorelines makes slopes in these areas particularly 

susceptible to landsliding; (7) earthquake-generated 

ground shaking-strong ground shaking can trigger 

failures at the time of the earthquake and can jar and 

loosen hillside materials leading to failure at some later 

time. 

These are some of the many complex interrelated 

factors that may contribute to the formation of landslides, with earthquake-generated ground shaking 

being only one of several possible triggering mechanisms. The prediction of a slope failure at a specific site 

from a specified level of ground shaking requires an 

analysis of all such factors, as well as very detailed and 

expensive onsite investigations. Because such information is not available on a regional scale and we are 

only beginning to understand the landslide problem, we 

have approached the study of earthquake-induced landslides on a regional scale by trying to delineate relative 

slope stability on the basis of physical properties. Because landslides triggered by the 1906 earthquake occurred throughout the San Francisco Bay region, delineation of unstable areas according to their physical 

characteristics provides a guide to those areas most 

susceptible to earthquake-induced landsliding. 

SLOPE STABILITY ANALYSIS ON A REGIONAL 

SCALE 

Relative slope stability maps can be prepared in various ways and from various types of information. No 

generally applicable formula or technique has been developed that covers all situations and all areas. Different techniques have been used to prepare relative slope 

stability maps for different areas, at different scales, for 

different purposes, and from different types of information. Many examples of the widely divergent form and 

style of such maps have been published in recent years 

for California (Blanc and Cleveland, 1968; Johnson and 

Ellen, 1965; Johnson and Lobo-Guerrero, 1968; Rogers, 

1971; Brabb and others, 1972; Rice and Strand, 1972; 

Huffman, 1972a, 1972b, 1974; Burnett, 1972; Frame, 

1973; Radbruch and Wentworth, 1971; Saul, 1972) and 

for other parts of the United States (Bailey, 1971; Van 

Horn, 1972a, b, c; Williams, 1972; Scott, 1972; Maberry, 

1972a, 1972b; Simpson, 1973a, 1973b). 

Complexly interrelated factors contribute to the generation of landslides, and engineering geologists commonly spend months preparing sophisticated analyses 

of soil and rock-strength parameters, precipitation records, slope geometry, and other factors to determine 

the causes of individual landslides. To cover an area as 

large as the San Francisco Bay region (19,300 km2) 

(7,450 mi2 ), no such detailed analysis could be made 

owing to limitations of time, personnel, and available 

data. Instead we utilized data that (1) were available at 

the present, (2) were available throughout the entire map 

area, (3) could be incorporated easily into the slopestability analysis, and (4) yielded information about 

some of the most important factors that control slope 

stability. The only data that met all these criteria were 

landslide distribution, slope, and bedrock geology. 

LANDSLIDE DISTRIBUTION 

Numerous studies in the bay region and elsewhere 

have shown that most landslides in a particular year 

occur in areas of previous landsliding (Nilsen and 

Turner, 1974; Kojan, 1973; Bailey, 1971). Commonly 

the new landsliding consists of renewed movements of 

old landslides triggered by earthquakes, unusually intense rainfall, and (or) man's activities. Hence, the preparation of maps showing the distributuion of present 

landslide deposits is a first step toward delineating areas 

likely to fail in the future. In addition, such maps are 

useful for identifying major factors contributing to the 

formation of landslides. 

Maps showing the distribution oflandslides in most of 

the San Francisco Bay region have been prepared for 

this purpose at a scale of1:62,500, primarily by photointerpretation with a minimum of field checking. This 

technique is necessary because of the large size of the 

area, the inaccessibility of much of it, and time limitations. The photointerpretive techniques depend upon 

the recognition of scarps, anomalous bulges and lumps, 

hummocky topography, ridge-top depressions and 

trenches, terraced or backtilted slopes, abrupt changes 

in slope, altered stream courses, discontinuous drainage 

patterns, closed depressions, springs, and anomalous 

color, texture, shade, vegetation, and bedrock patterns. 

A number of maps have been published, and many 

others are in preparation (Brabb and Pampeyan, 1972; 

Nilsen, 1971, 1972a, b, c, d, 1973a, b, c; Sims and Nilsen, 

1972; Burnett, 1972; Rice and Strand, 1972). 

An example of the landslide distribution in San 
STUDIES FOR SEISMIC ZONATION A81 

Mateo County as mapped by Brabb and Pampeyan 

(1972) is shown in figure 62. The inventory shows the 

presence of more than a thousand landslide deposits in 

the region and indicates that landsliding is one of the 

major erosional processes. Recently, a map of the distribution of landslide deposits in the nine-county San 

FranciEeo Bay region was compiled at a scale of 

1:125,000 for a slope-stability analysis of the entire 

region by Nilsen and Wright (unpub. data). An example 

from the northeastern bay region is shown in figure 63. 

Although the type of movement, date of most recent 

activity, and nature of landslide materials were not 

determined, the maps by themselves can be used as a 

general guide to areas where landslides may be a problem; they provide a regional picture of the past history of 

landsliding, and they are useful to planners and engineering geologists in making preliminary appraisals 

of building sites. 

In addition to the landslide-distribution maps, an 

isopleth map has been prepared for the southern San 

Francisco Bay region (Wright and Nilsen, 1974; Wright 

and others, 1974). This map shows contours depicting 

variations in geographic density of landslide deposits 

and permits rapid, quantitative evaluations of the 

abundance of landslide deposits in different areas. 

Landslide deposits are abundant along some active 

faults or parts of active faults and uncommon along 

others. Where abundant, they may be related to either 

seismicity along the fault zone, the weak, crushed rocks 

found in the fault zone, or both. Abundant landslide 

deposits have been mapped along the Calaveras fault 

and along some parts of the San Andreas and Hayward 

faults. The maps of landslide deposits permit correlations and comparisons of the distribution of landslide 

deposits with other factors, but so far they have not in 

themselves permitted the recognition of those landslide 

deposits that were originally triggered by earthquakes. 

SLOPE 

Degree of slope is an important parameter controlling 

the stability of hillside materials. A slope map of the 

San Francisco Bay region has been prepared by the U.S. 

Geological Survey (1972) at a scale of 1:125,000. 

Studies by Bonilla (1960a), Brabb, Pampeyan, and 

Bonilla (1972), and Nilsen, Taylor, and Brabb (1974) 

showed that in the bay region most landslides occur on 

slopes greater than 15 percent, with very few on slopes 

of 5-15 percent and virtually none on slopes less than 5 

percent. As part of the current study, Nilsen and Wright 

(unpub. data) have prepared a generalized slope map of 

the San Francisco Bay region at a scale of 1:125,000 

showing the slope intervals {}-.5 percent, 5-15 percent, 

and greater than 15 percent. (An insert from their map is 

shown in fig. 64.) The generalized slope map shows only 

areas wider than about 300 m (1 ,000 ft) and eliminates 

the thousands of very small, discontinuous areas shown 

on the original slope map. The generalized map provides 

an important element for evaluating slope stability on a 

regional scale. 

BEDROCK GEOLOGY 

Certain bedrock units are more susceptible to landsliding than others because of their physical and chemical characteristics, as well as the types and thicknesses 

of soils that tend to develop over these rock types. Thus, 

two adjacent areas that may appear to be similar in 

most resepcts may differ greatly in landslide susceptibility because of the type of bedrock underlying them 

(Radbruch and Weiler, 1963; Radbruch and Case, 1967; 

Brabb and others, 1971). 

For purposes of evaluating slope stability, a map 

showing the distribution of geologic units in the San 

Francisco Bay region was prepared at a scale of 

1:125,000. On the basis of discussions with geologists at 

the U.S. Geological Survey responsible for bedrock 

mapping and research into the physical properties of 

hillside materials, certain of these units were judged to 

be especially susceptible to landsliding; these units are 

shown in figure 65 for an area in the northeastern bay 

region. 

RELATIVE SLOPE STABILITY OF THE 

SAN FRANCISCO BAY REGION 

Nilsen and Wright (unpub. data) prepared a slope 

stability map of the nine-county San Francisco Bay 

region at a scale of 1:125,000. An example for a part of 

the northeastern bay region is shown in figure 66. The 

map delineates areas according to five categories of 

relative slope stability ranging from generally highly 

stable to generally unstable. The five categories are 

defined and tabulated (fig. 66). 

The map incorporates the most recent data on mapped 

landslide deposits, slope, and bedrock geology (refer to 

figs. 63, 64, 65, respectively) and delineates areas in 

more detail than an earlier map showing relative abundance of landslides at a scale of 1:500,000 (Radbruch 

and Wentworth, 1971); it does not incorporate as much 

detailed data, especially from field investigations, as 

earlier slope-stability maps of smaller areas prepared at 

larger scales (for example, Brabb, Pampeyan, and Bonilla, 1972). The map can be used only for regional scale 

investigations and not for slope-stability analyses of individual lots or small subdivision areas. 

The map provides a generalized regional representation of relative slope stability in the San Francisco Bay 

region. Although it does not predict that particular 

slopes will fail during future earthquakes of specific size 
A82 

37"30' 

37"15' 

REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

122°30' 122°15' 

I 

\ 

"i.-. 1 

-'t--::;1~;.:=-- ....::~~·------\ 

\ 

' ,.,-.\ 

.p ' 7 \ 

,1-> \ 

C> " ..-" ' "lnt<ernaltiio< nail Airport <P \ 

C> ' 0 

0 

0 

EXPLANATION 

Landslide 

-

deposit 

0 4 MILES 

I I 11 

0 4 KILOMETRES 

N 

1 

FIGURE 62.-Landslides in San Mateo County, Calif. More than 1,000 landslides have been mapped in this area. From 

Brabb and Pampeyan (1972). 
STUDIES FOR SEISMIC ZONATION A83 

and location, the map is useful for delineating areas 

according to their potential for earthquake-induced 

landslides. It defines the relative slope stability of general areas on the basis of existing physical characteristics. Because the 1906 earthquake generated landslides 

throughout the region and because most of the more 

SCALE 1:125 000 

4 KILOMETRES 

unstable areas are concentrated along the ridges paralleling the San Andreas and Hayward faults, these areas 

must be considered most susceptible. to landsliding during moderate to large earthquakes. Delineation of such 

areas is a basic tool for the development of land-use 

policies for reducing the hazards associated with 

4 MILES 

EXPLANATION 

Landslide deposit larger than 150 m 

(500 ft) in longest dimension 

Landslide deposit smaller than 150 m 

(500ft) in longest dimension 

FIGURE 63.-Distribution of landslide deposits in part of northeastern Contra Costa County, Calif. After Sims and 

Nilsen (1972) and Nilsen (1971). 
A84 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

earthquake-induced landslides. Because the relativeslope-stability map is based on bedrock geology, slope, 

and the distribution of past landslide deposits, the map 

also defines general areas susceptible to landsliding 

triggered by other mechanisms such as rainfall and 

man's activities. 

SCALE 1: 125 000 

4 KILOMETRES 

SUMMARY 

Landsliding is continually a major hazard to life and 

property in the San Francisco Bay region. Studies of 

landslide co.:-;t showed that at least $25 million was 

spent in 1968-69 on landslides triggered by processes 

4 MILES 

EXPLANATION 

D 

Area of 0- to 5-percent slope 

Area of 5- to 15-percent slope 

Area of slope greater than 

15 percent 

FIGURE 64.-Generalized slope map of part of northeastern Contra Costa County, Calif. Modified from U.S. Geol. Survey (1972) 

by T. H. Nilsen and R. H. Wright. 
STUDIES FOR SEISMIC ZONATION A85 

such as rainfall and man's construction activities. The 

number oflandslides generated by the 1906 earthquake 

suggests that in the event of another such earthquake 

thousands of additional landslides would be triggered, 

costing possibly billions of dollars and untold loss of life 

because of extensive development of the hillside areas. 

Evaluation of the landslide problem on a regional 

38' 

oo- ~~ ~~~~~~~~~~7:~~,~~~ 

JO' 

SCALE 1:125 000 

4 KILOMETRES 

scale requires more generalized data and permits less 

specific conclusions than evaluation of the problem at 

specific sites. At present it is not possible to predict 

particular slope failures on a regional scale from specified levels of earthquake-induced ground shaking. 

Nevertheless, sufficient data exist in the San Francisco 

Bay region on a regional scale to provide a basis for 

4 MILES 

EXPLANATION 

Area underlain hv unstable 

bedrock u~its 

Area underlain by stable bedrock units 

or un('onsolidated sedi ments at low 

angles of slope 

FIGURE 65.-Distribution of geologic units susceptible to landsliding in part of northeastern Contra Costa County, Calif. 
A86 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

substantially reducing the hazards associated with 

landsliding, both that occurring on a continuing basis 

and that triggered by earthquakes. 

The relative-slope-stability map compiled at a scale of 

1:125,000 of the San Francisco Bay region delineates 

areas according to five categories of estimated relative 

slope stability (Nilsen and Wright, unpub. data; fig. 66). 

<I 

:;2' 

30" 

The map indicates the stability of areas on the basis of 

landslide distribution, slope, and bedrock geology. The 

least stable areas are located mainly on the steep slopes 

of the ridges roughly paralleling the major San Andreas 

and Hayward faults. The map is preliminary and represents an initial attempt at analyzing the landslide problem of the entire region on a scale useful for land-use 

EXPLANATION 

D 

Category 1 

Generally highly stable. Areas of 0- to 5-percent slope, generally Mt underlain by landslide deposits 

Category lA 

Subject to liquefaction. Areas of 0- to 5-percent 

slope that are underlain by soft, unconsolidated bay mud that may be subject to liquefaction and lateral spreading (see Youd 

and others, this report) 

Category 2 

Generally stable. Areas uf 5- to 15-percent 

slope, generally nut underlain by landslide 

deposits 

D 

Category 3 

Marginally stable or generally stable to locally 

unstable. Areas of greater than 15-percent 

slope, gene,rally nut underlain by landslide 

deposits or bed ruck units susceptible t,o landsliding 

Category 4 

Potentially unstable areas. Areas of greater 

than 15-percent slope that are underlain by 

bedrock units highly su ~ceptible to landsliding 

Category 5 

Generally unstable areas. Areas underlain 

by or immediately adjacent to landslide. 

deposits and ranging in slope from 0 to 90° 

Small landslide deposit 

SCALE 1:125 000 

2 4 MILES 

Generally less than 150m {500ft) but greater 

than 60 to 90 m (200 to 300ft) in longest 

dimension; only shown where solitary or 

isolated and more than 300m (1000 ft) from 

nearest adjacent landslide deposit I 

4 KILOMETRES 

FIGURE 66.-Relative slope stability of part of northeastern Contra Costa County, Calif 
STUDIES FOR SEISMIC ZONATION A87 

planning. In addition to those regional factors incorporated into the map, numerous other factors are known to 

influence slope stability, This preliminary map provides 

a basis for analyzing the importance of these factors and 

for developing future maps which incorporate improved 

data and techniques. The present map, even with its 

limitations, is a first step toward delineating .. hose general areas most susceptible to landsliding during future 

earthquakes and, as such, provides an essential tool for 

seismic zonation of the San Francisco Bay region. 
PREDICTED GEOLOGIC EFFECTS OF A POSTULATED EARTHQUAKE 

By R. D. BoRCHERDT, E. E. BRABB, W. B. JoYNER, E. J. HELLEY, K. R. LAJOIE, R. A. PAGE, 

R. L. WEssoN, and T. L. Youn 

INTRODUCTION 

The analyses presented in the preceding six papers 

show that the geologic setting of the San Francisco Bay 

region has a dominant influence on potential earthquake hazards. The strong correlation between geologic 

conditions and the amount of earthquake damage in 

1906 emphasizes the importance of this influence and 

demonstrates the need for seismic zonation. Seismic 

zonation can provide the logical basis for preparation of 

special-purpose land-use maps that, with appropriate 

public policy, would be a significant step toward reducing the currently expected catastrophic effects of 

another great earthquake (Algermissen, 1972). 

Seismic zonation requires a set of integrated multidisciplinary predictions about the geologic effects of 

potential earthquakes. To illustrate a strategy for making such predictions, a demonstration profile has been 

chosen perpendicular to a segment of the San Andreas 

fault, along which a magnitude 6.5 earthquake has been 

postulated. This profile includes a wide variety of 

geologic conditions and provides a means for application 

of the analyses presented in the preceding six papers. 

Earthquake hazards to life and property originate 

from (1) surface faulting, (2) ground shaking, (3) flooding, (4) liquefaction, and (5) landsliding. The extent to 

which each of these geologic effects can be predicted for 

an earthquake of a given magnitude and location depends on the current state of the art. To illustrate 

techniques and data currently available, each effect is 

considered for the postulated earthquake. 

This paper does not provide final estimates for the 

total earthquake hazard along the demonstration 

profile. Such an objective would require analysis of individual manmade structures as well as consideration 

of earthquakes of other sizes and locations. Instead, this 

paper illustrates the extent to which the above effects 

can be predicted for an earthquake of this given size and 

location. Such an analysis demonstrates a methodology 

for seismic zonation based on available data. 

A POSTULATED EARTHQUAKE 

For illustrative purposes, an earthquake of magnitude 6.5 is assumed to occur on the San Andreas fault. 

A88 

The location and estimated length of fault rupture are 

shown in figure 67. Previous large earthquakes a~­

sociated with rupture of the ground surface along this 

section of the fault occurred in 1838 (magnitude >6.5) 

and in 1906 (magnitude 8.3). The moderate 1957 earthquake (magnitude 5.3) occurred approximately 25 ~m 

(15 mi) north of the postulated surface rupture, but with 

no associated surface faulting on land. 

The assumed 6.5 magnitude is moderate in comparison with the 8.3 magnitude of the 1906 earthquake. 

However, the damage resulting from the recent San 

Fernando and Managua earthquakes suggests that 

damage from a moderate earthquake can be very intense but of smaller areal extent that that of a great 

earthquake. 

GEOLOGY ALONG DEMONSTRATION PROFILE 

The demonstration profile extends northeasterly 

from the community of Sky Londa, across the San Andreas fault zone, through the city of Menlo Park, and 

across San Francisco Bay to the southern tip of Coyote 

Hills (fig. 67). A brief summary of the geology along the 

profile will aid in understanding the potential geologic 

effects of the postulated earthquake. . 

The demonstration profile includes seven geologic 

units that are grouped into five units on the basis of 

physical properties. In order of increasing age, these units are as follows: 

1. Bay mud; mostly recently deposited soft clay, ~ilt, 

and minor sand; contains more than 50 weight 

percent water; 

2. Holocene alluvium; poorly consolidated clayey silt, 

sand, and gravel; contains less than 40 weight percent water; 

3. Late Pleistocene alluvium; primarily same material 

composition as Holocene alluvium, but contains 

less water and is more consolidated; in some places 

overconsolidated (soil-engineering sense); 

4. Pliocene and early Pleistocene deposits; primarily 

continental Santa Clara and marine Merced Formations consisting of semiconsolidated and consolidated sandstone, siltstone, and mudstone; and 

5. Pre-Tertiary and Tertiary bedrock; includes Franciscan Formation, consisting mostly of sandstone 
30' 

15' 

STUDIES FOR SEISMIC ZONATION 

30' 

PACIFIC 

OCEAN 

EXPLANATION 

Estimated surface rupture for postulated 

earthquake (magnitude 6.5) 

.. ...__ .. " 

Fault trace 

Demonstration profile 

... Site for which ground shaking was calculated 

for postulated earthquake (magnitude 6.5) 

0 5 10 MILES 

0 5 10 KILOMETRES 

N 

A89 

FIGURE 67 .-Location of demonstration profile and estimated length of surface rupture associated with a postulated earthquake of magnitude 

= 6.5 on the San Andreas fault, southwestern San Francisco Bay region. 
A90 STUDIES FOR SEISMIC ZONATION 

and shale with lesser amounts of radiolarian chert, 

greenstone, limestone, and serpentine; marine 

sandstone and shale of Eocene, Miocene, and 

Pliocene age; and Page Mill Basalt, consisting of 

lava flows and pyroclastic rocks of Miocene age. 

The general stratigraphic relations of the five geologic 

units are illustrated in figure 68 (more detailed descriptions are given in Lajoie and Helley, this report). 

PREDICTED GEOLOGIC EFFECTS 

The potential effects of the postulated earthquake are 

dependent on the distribution of the five geologic units 

with respect to the San Andreas fault. A generalized 

prediction for each geologic effect along the profile is 

presented in figure 68 and discussed in the following' 

sections. 

SURFACE FAULTING 

Historically, the larger earthquakes of the San Francisco Bay region have been associated with surface ruptures localized along the main surface traces of strikeslip faults. The resulting displacements of the ground 

surface along the faults have been mainly horizontal, 

with only minor vertical displacements. They are typically expressed as an en echelon pattern of ground fractures that trend obliquely to the overall fault trace. The 

en echelon fractures have exhibited displacements 

ranging from a few centimetres (a few inches) to a few 

metres (several feet) and have defined a surface fault 

zone ranging in width from a few metres (several feet) to 

several tens of metres (about 200 ft). 

On the basis of past observation, the postulated magnitude 6.5 earthquake probably would be associated 

with right-lateral surface displacement along the San 

Andreas fault that may be as great as 1 m (3ft) (fig. 67). 

The length of estimated surface rupture is 40 km (25 mi) 

plus or minus about 10 km (6 mi). This displacement is 

likely to be predominantly horizontal with the land 

west of the fault shifting toward the northwest relative 

to the land east of the fault. The main zone of surface 

rupture will range in width from a few metres (several 

feet) to several tens of metres (about 200 ft), but small 

fractures and permanent ground distortion may extend 

to much greater distances. Locally, branch and subsidiary faults, such as the Black Mountain fault, the 

Cupertino fault, and the Canada fault, may also move, 

but movements on such lesser faults are mu..::h more 

difficult to predict. If sympathetic surface movements do 

occur along these lesser faults, they are expected to be 

less than those on the main fault rupture. 

During the 1906 earthquake, horizontal displacements as large &s 2.6 m (8.5 ft) were observed along this 

segrr:ent of the San Andreas fault. Branner (1908) reported for the 1906 earthquake possible evidence for 

slight sympathetic movement on the Black Mountain 

fault and prevalent ground cracking extending several 

tens of metres (a few hundred feet) from the main trace 

of the San Andreas fault. Most of the ground cracking 

was associated with small local ground failures. The 

maximum reported fault displacement on the San Andreas of 6.4 m (21 ft) occurred farther north in Marin 

County. 

GROUND SHAKING 

The characteristics of the ground shaking expected 

from the postulated earthquake depend on many factors. They depend on (1) characteristics of the earthquake source (for example, type of offset, magnitude, 

_location _ . .- , stress drop, and size of associated rupture surface), (2) distance from associated rupture surface, and 

(3) characteristics ofthe local geologic materials. Existing data on these factors are presented in the first four 

pages of this report. To illustrate a strategy for predicting ground shaking on the basis of these data, four sites 

along the demonstration profile (fig. 68) were selected to 

illustrate responses of the several geologic units to 

ground shaking. 

The first step in predicting ground shaking for the 

postulated earthquake is to estimate the bedrock shaking at each site. The amplitude spectra for bedrock 

shaking (fig. 69) were approximated by using a 

technique proposed by Newmark and Hall (1969). (For 

discussion of technique see Page and others, this report.) The acceleration, velocity, and displacement values used in constructing the spectra (see table 6) were 

taken from a preliminary set of data (see Page and 

others, this report) and probably are more representative of a magnitude 6.0 earthquake than one of magnitude 6.5. These values are included to illustrate 

techniques, and it was not considered necessary torepeat the calculations for the larger values more appropriate for a magnitude 6.5 earthquake. 

The second step is to estimate the response to shaking 

of the unconsolidated deposits at each site. By using the 

techniques discussed in Borcherdt, Joyner, and others 

(this report) these responses were estimated from the 

Pacoima Dam accelerogram (San Fernando earthquake, Feb. 9, 1971), scaled to the peak velocity values 

used in construction of the bedrock spectra. The final 

amplitude spectra for ground shaking at the surface 

were obtained at each site (fig. 69) by multiplying the 

amplitude response spectra of surficial deposits, computed at the appropriate high-strain levels, by the approximated bedrock spectra. 

The estimates of ground shaking for the postulated 

earthquake (fig. 69) are tentative; however, they do 

suggest some general conclusions for the sites considered: 

1. The bay mud and alluvium deposits substantially 
About 1 m (3ft) right-lateral offset likely 

SURFACE FAULTING 

~ t Offset possible No surface faulting likely 

-·------------------------------------------------------------------. 

GROUND SHAKING 

Strong bedrock shaking (75-125 em Is; 30-50 in./s) decreasing with distance to moderate bedrock shaking (10-30 em Is; 5-15 in. IS) 

Several large and small 

landslides likely 

(j) 

w 

0::: 

1-

w 

~ 

z 

z 

0 

i= 

~ w 

_J 

w 

400 

200 

EXPLANATION 

- Baymud 

lillilliTITII Holocene alluvium 

ED Late Pleistocene alluvium 

(j) 

w 

0::: 

1-

w 

~ 

~ 

z 

0 

~ > w 

_J 

w 

D Pliocene and early Pleistocene 

deposits 

D Pre-Tertiary and 

Tertiary bedrock 

~ Landslide deposit 

Water table 

/ / Fault 

200 

100 

0 

100 

200 

Amplification for frequencies < 1. 5 Hz Increased amplification for frequencies <1.5 Hz 

FLOODING 

Not possible Partial inundation possible 

--------------------------------- --------

LIQUEFACTION 

Hirlh ootential (where clean granular layers exist) 

LANDSLIDING 

Several small landslides likely Few small landslides likely 

Lateral spreading associated with liquefaction 

ANDREAS FAULT ZONE 

500 

1-

w San Francisco Bay w 

LJ.. 

0 ~ 

~ 

z 

0 

~ > w 

_J 

w 

500 

0 5 10 15 MILES 

0 5 10 15 20 24 KILOMETRES 

DISTANCE 

FIGURE 68.-Predicted geologic effects of a postulated earthquake (magnitude =6.5) on the San Andreas fault (see fig. 67 for location of demonstration profile and estimated length 

of surface rupture). The severity of each earthquake effect is indicated qualitatively by thickness of underlining and quantified to the extent permitted by the current state 

of the art for seismic zonation on a regional scale. The severity of the predicted earthquake effects generally depend on the type of underlying geologic materiaL Geologic 

cross section compiled by K. R. Lajoie. 

~ c::: 

0 

"""'" tr::l 

rn 

l-%j 

0 

~ 

rn 

tr::l 

~ -

0 

N 

0 

z 

~ 

0 

z 

> 1:.0 

f-l 
200 

100 

(J) 

Cl 

~6 300 

u.i<-> 

ow 

::::>(J) 

~a::: 

~~ 200 

:::;E(J) 

<(W 

0:::0::: 

w~ 0::2 100 

5~ 

~~ (.) 

Bedrock 

____ j __ _ 

0'--~---L-.L...___L_---L.---' 

0 1 2 

FREQUENCY, 

IN HERTZ 

SAN ANDREAS FAULT ZONE 

3 0 

Bedrock 

___ j __ 

1 2 

FREQUENCY, 

IN HERTZ 

3 0 

Bay mud/ 

/alluvium 

2 3 

FREQUENCY, 

IN HERTZ 

0 

Bedrock 

1 2 

FREQUENCY, 

IN HERTZ 

100 

50 

30 

r\l_A J~ ~ \ \ I ~ 

z 

-c 

u.iz 

CIO 

::::><..> 

~w 

:::::i(J) 

a. a::: :::;EW 

<(0. 

a:::(J) 

ww 

-I 

0:::(.) 

=>z 

o-

~ 

500 

~ ~ ~ w ~ ~ 

z ~ 

-_ 0 0 z z 0 0 -

~ ~ ~ ~ w ~ 

~ w w 

100 :: ::::: .... :::::::::~·:::::::::·: ···::::::::::::: 

~~ ~~~~~ ::::·· ~~~~~~~~~~~~~~~~~~~:~ ~;·:;:::: __ ::~~~~~~~~ .. .... .... . .................. ·· ·········· .... . ::::: ..... :::::::::::::::::: .:: :::::::::::::::.:.::. 500 

~~i :h::: u:~ uu:lu::: ::::::~::::y::; :~:·:::::;::::::::::: 

200 

0 5 10 15 MILES 

0 5 10 15 20 24 KILOMETRES 

DISTANCE 

FIGURE 69.-Fourier amplitude spectra of surface ground shaking predicted at four sites along the demonstration profile for a postulated earthquake (magnitude =6.5). The estimated spectra suggest that even at high levels of strain local geologic units may substantially amplify frequencies of ground shaking 

near the fundamental frequency of the site (see Page and others; Borcherdt, Joyner, and others; Borcherdt, Brabb, and others, this report, for descriptions 

of techniques; see fig. 68 for explanation of geologic symbols). 

> 

~ 

l'V 

~ 

l'%j 

t;:; 

e 

0 

t-:3 

0 

z 

0 

~ 

l'%j 

> 

~ 

t-:3 

::1:1 

.(:) 

e 

~ l'%j 

::1:1 

~ 

~ t;:; 

~(/). 

~ z 

~ 

~ z 

0 

m 

0 

0 

tJ:j 

> 

~ 

~ 

l'%j 

e 

0 

z 
STUDIES FOR SEISMIC ZONATION A93 

TABLE 6.--Parameters for estimating amplitude spectra for 

shaking of bedrock 

A. Time history parameters 

Site Distance Acceleration Velocity Displacement 

(kml (mi) (g) (cm/s) (in./s) (em) (in.) 

1 ------- 9 5.6 0.40 31 12.2 14 5.5 

2 ------- 14 8.7 0.22 19 7.5 9 3.5 

3, 4 ______ 21 13.1 0.12 12 4.7 6 2.4 

B. Estimated spectral parameters 

Acceleration (g) Velocity Displacement 

Site (factor 3.0) (factor 1. 7) (factor 4.8 I (cm/sl (in./s) (em I (in.) 

------------ 1.92 93 36.6 24 9.4 

1.06 57 22.4 15 5.9 

3,4 - 0.58 36 14.2 10 3.9 

amplify particular frequencies of bedrock shaking 

and attenuate others; 

2. The amplification effect of the unconsolidated deposits is strongly dependent on frequency, which is 

partly due to input-strain level (compare the responses for the two sites on bay mud that have 

identical low-strain seismic models); 

3. The amount of amplification near the fundamental 

frequency of the site in some places actually increases with increasing strain level; and 

4. The amplification effect of the unconsolidated deposits in certain frequency bands is greater than 

the attenuation caused by increasing distance. 

In general, strong shaking (50-125 cm/s (20-50 in./s)) 

could be expected from the postulated earthquake for all 

surface bedrock sites along the profile west of the bay 

plain (fig. 68). The model calculations suggest that a 

substantial amplification of bedrock shaking in the frequency range below 1.5 hertz could be expected for all 

parts of the demonstration profile underlain by alluvial 

deposits, with increased amplifications for the parts underlain by bay mud. The predicted amplifications are 

large enough to suggest that ground shaking for frequencies below 1.5 hertz may be stronger at the sites 

underlain by bay mud and alluvium than at sites underlain by bedrock much closer to the fault. Manmade 

structures with natural periods coinciding with those of 

the underlying unconsolidated geologic deposits are 

particularly susceptible to damage. 

FLOODING 

Areas of potential flooding have not been delineated 

in the San Francisco Bay region, except for the areas 

likely to be inundated by a tsunami-generated runup of 

6.1 m (20ft) (Ritter and Dupre, 1972). Analysis of the 

problem is not presented in the first six papers in this 

report; however, for completeness in considering those 

earthquake effects influenced by geologic conditions, a 

brief description of the problem is presented for the 

demonstration profile. For the postulated earthquake, 

the most probable cause of inundation by water is the 

failure of dams or dikes. Flood water from such failures 

could originate from either San Francisco Bay or upland 

reservoirs. 

Southern San Francisco Bay is surrounded by a large 

number of dikes constructed mostly of fine-grained sediments dredged from the bottom of the bay. The continuous repair work necessary on many of the dikes 

suggests their vulnerability to failure, and their location on bay mud increases their vulnerability. Bay mud 

has both a high potential for amplifying particular frequencies of ground shaking (Borcherdt, Joyner, and 

others, this report) and a high potential for ground failure due to liquefaction (Y oud and others, this report). 

The area likely to be inundated because of possible 

dike failures depends on the tidal level at the time ofthe 

earthquake. The tidal range at the Dumbarton Bridge 

extends as much as 3.4 m (11ft) above mean lower low 

water. For the postulated earthquake, partial inundation appears likely up to the 1850 shoreline (Nichols and 

Wright, 1971) and even farther in certain areas where 

the ground has subsided in recent years owing to the 

withdrawal of ground water. At the southern tip of the 

bay, for example, flooding could extend as much as 2 km 

(1.2 mi) beyond the 1850 shoreline. 

Possible upland sources of flooding during the postulated earthquake are the Upper and Lower Crystal 

Springs Reservoirs and San Andreas Lake. Each of 

these bodies of water is in the San Andreas rift valley, 

which is drained by San Mateo Creek about 20 km (12 

mi) north of the demonstration profile. Without a detailed engineering analysis of the associated dams, 

which is beyond the scope of this study, it is not possible 

to assess the resistance of the dams to the postulated 

earthquake. However, the areas of possible inundation 

can be outlined on a topographic map. None ofthe areas 

of possible inundation intersect the demonstration 

profile, even though they are of considerable size. 

For the postulated moderate earthquake, the likelihood of a large vertical offset of the sea floor or large 

submarine landslide necessary to generate a tsunami 

seems remote. Ritter and Dupre (1972) reported that 19 

tsunamis were recorded by the tide gage at the Golden 

Gate during the past 100 years. The maximum recorded 

wave height from a tsunami was only 2.3 m (7.5 ft). 

Tidal records within San Francisco Bay show that 

waves attenuate rapidly to less than 50 percent of their 

original height by the time they reach the area of the 

demonstration profile. For flooding to result from the 

unlikely possibility of a tsunami in San Francisco Bay, 

the corresponding earthquake would have to occur dur-
A94 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

ing high tide when the tops of some of the dikes are less 

than 1 m (3 ft) above water level. 

The postulated earthquake of magnitude 6.5 probably 

is not large enough to generate a seiche in San Francisco 

Bay. However, seiches could be generated in reservoirs 

close to the postulated surface faulting; thus, seiches 

might be generated in the Upper and Lower Crystal 

Springs Reservoirs. 

The tectonic setting of the San Francisco Bay region 

suggests that large tectonic changes of land level and 

associated significant changes in sea level (such as occurred during the 1964 Alaska earthquake) are very 

unlikely to accompany the postulated earthquake. Only 

minor local changes in vertical elevation of about 0.3 m 

(1ft) were produced by the 1906 earthquake. 

LIQUEFACTION 

By using the procedures discussed in Y oud and others 

(this report), a cross section showing liquefaction potential was constructed along the demonstration profile for 

the postulated earthquake (fig. 68). Sediments with the 

greatest potential for liquefaction are the clay-free 

granular layers within the bay mud unit. Holocene alluvium has a generally moderate potential with locally 

high potential in some recent channel and overbank 

deposits. Late Pleistocene alluvium is generally dense 

and has a low potential for liquefaction. In addition, 

much of the Holocene and the late Pleistocene alluvium 

is normally above water table and thus has, at most, a 

seasonal potential for liquefaction. 

The most common type of ground failure expected to 

result from liquefaction along the profile is that of 

lateral-spreading landslides (Youd and others, this report). In brief, this type of failure consists of movement 

of a soil mass down a mild slope with resulting cracks, 

fissures, and differential settlements within and near 

the margins of the slide mass. Relative displacements as 

large as tens of metres have been observed for such 

ground failures. Areas of the profile with the highest 

potential for this type of ground failure from the postulated earthquake are underlain by bay mud along the 

western margin of San Francisco Bay (fig. 68). 

Evidence for lateral spreading during the 1906 earthquake was reported at several locations. For example, 

lateral ground movements as large as 2m (6ft) occurred 

in virtually every arm of bay mud extending beneath 

the city of San Francisco. Many more such ground failures may have been generated near the margins of 

southern San Francisco Bay but went unreported. Much 

of this area was marshland in 1906. 

LANDSLIDING 

The postulated magnitude 6.5 earthquake could be 

expected to generate several landslides along the demonstration profile. By using the techniques presented in 

Nilsen and Brabb (this report), a cross section showing 

general landslide susceptibility along the demonstration profile has been prepared (fig. 68). These approximate landslide susceptibilities are not specifically 

dependent on the postulated magnitude of 6.5 and 

would be equally applicable to any moderate earthquake in the same general location. 

If the postulated earthquake were to take place during a wet season and high ground-water levels, many 

landslides could be expected along the profile. A few 

large (more than 150m (500ft) in maximum dimension) 

landslides and several small (10-150 m (30-500 ft) in 

maximum dimension) landslides are likely on the steep 

slopes between Sky Londa and the San Andreas fault. In 

this area, existing landslide deposits could be reactivated. Several small landslides could be generated in 

the area between the San Andreas fault and the western 

margin of the bay plain. Lateral-spreading landslides 

associated with liquefaction could be expected near the 

margins of San Francisco Bay. 

If the postulated earthquake were to occur during a 

dry season and low ground-water levels, the amount of 

landsliding is expected to be much less. Some landsliding still could be expected between Sky Londa and the 

San Andreas fault, a few small landslides probably 

would occur in the other hilly areas along the profile, 

and lateral-spreading landslides associated with 

liquefaction still could be expected near the margins of 

San Francisco Bay. 

The 1906 earthquake apparently generated hundreds 

of landslides throughout the hilly regions along the 

western margin of the bay plain. Numerous landslides 

were reported in the hilly regions between the San Andreas fault and Sky Londa. An exceptionally large mass 

(0.8 km (0.5 mi) across) near Black Mountain moved in 

1906. 

The epicenter for the 1957 earthquake (magnitude 

5.3) was located approximately 25 km (15 mi) north of 

the location for the postulated earthquake. It generated 

about 15 small landslides along the steep coastal bluffs 

in the Daly City-San Francisco area (Bonilla, 1960a). 

SUMMARY 

Earthquake hazards to life and property originate 

from surface faulting, ground shaking, flooding, 

liquefaction, and landsliding. The extent to which the 

currently available data permit quantitative prediction 

of these effects on a regional scale, for an earthquake of 

a given size and location, is illustrated in figure 68. 

Along the demonstration profile, the predicted severity 

of the effects varies substantially depending on local 

geologic conditions. 
STUDIES FOR SEISMIC ZONATION A95 

In brief, and as shown in figure 68, the principal losses 

from the postulated earthquake could be expected to 

originate from the following factors: 

1. Surface faulting, ground shaking, and landsliding in 

the distance interval 0 to 10 m (0 to 30 ft), underlain by pre-Tertiary and Tertiary bedrock and 

Pliocene and early Pleistocene deposits immediately adjacent to the San Andreas fault; 

2. Ground shaking and landsliding in the distance interval10 m to 5.2 km (30ft to 3.2 mi), underlain by 

pre-Tertiary and Tertiary bedrock and Pliocene 

and early Pleistocene deposits; 

3. Ground shaking in the distance interval 5.2 to 9.3 

km (3.2 to 5.8 mi), underlain by Pliocene and early 

Pleistocene deposits and late Pleistocene alluvium; 

4. Ground shaking and landsliding of the lateralspreading type associated with liquefaction in the 

distance interval 9.3 to 12.0 km (5.8 to 7.4 mi), 

underlain by Holocene alluvium; 

5. Ground shaking, flooding, and landsliding of the 

lateral-spreading type associated with liquefaction in the distance interval12.0 to 21.0 km (7.4 to 

13 mi), underlain by bay mud; 

6. Landsliding in the distance interval21.6 to 22.1 km 

(13.4 to 13.7 mi), underlain by pre-Tertiary and 

Tertiary bedrock; and 

7. Ground shaking, flooding, and landsliding of the 

lateral-spreading type associated with liquefaction in the distance interval 22.1 to 22.8 km (13. 7 

to 14.2 mi), underlain by bay mud. 

The strong dependence of the predicted effects on 

geology, together with the availability of extensive 

geologic and geophysical data, suggests the feasibility 

of extending this analysis to the entire San Francisco 

Bay region for other potential earthquakes. Such a regional analysis would provide a preliminary seismic 

zonation of the region from which special-purpose 

land-use maps could be constructed and then used to 

reduce earthquake hazards. 

GENERAL CONCLUSIONS 

The great California earthquake of April 18, 1906, 

demonstrated large variations in earthquake vulnerability for various areas of the San Francisco Bay.region 

and provided basic data for the study of earthquakes 

and their effects. Extensive research facilities have 

been developed within the last decade to study the basic 

physical processes associated with earthquakes. These 

facilities have yielded important new geological and 

geophysical data, which whenincorporated with data 

from the 1906 earthquake, provide the basis for development of guidelines to reduce future earthquake 

losses. 

The analyses presented in this report show that the 

geologic setting of the San Francisco Bay region has a 

dominant influence on potential earthquake hazards. 

The geologic setting is shown to control the potential 

severity of the various earthquake effects from which 

losses of life and property originate; namely, surface 

faulting, ground shaking, flooding, liquefaction, and 

landsliding. The first step required to reduce earthquake hazards is seismic zonation, which requires 

prediction of the potential severity of these various 

geologic effects on a regional scale for future earthquakes of specific size and location. An example of 

such prediction as illustrated by a demonstration profile 

for a postulated earthquake (Borcherdt, Brabb, and 

others, this report) suggests that seismic zonation of the 

San Francisco Bay region is feasible using existing 

geological and geophysical knowledge. This example 

illustrates a methodology for seismic zonation at the 

current state of the art and demonstrates the extent to 

which the various effects can be predicted quantitatively on a regional scale using existing data. 

Tools derived and discussed as a basis for seismic 

zonation are (1) a map showing active faults (fig. 3A), (2) 

data on attenuation of shaking in bedrock, (3) geologic 

data, (4) a map showing ground response, (5) a map 

showing liquefaction potential (fig. 50), (6) a map showing landslide susceptibility (fig. 66), and (7) a map showing areas that might be inundated by tsunamis (Ritter 

and Dupre, 1972). (This last map represents only part of 

the analyses necessary for a map of potential flooding.) 

Application of these tools to the problem of seismic 

zonation shows that predictions on a regional scale for 

earthquakes of specific size and location are less quantitative than those that can be made at specific sites 

where additional data are available. Nevertheless, such 

predictions are useful for the development of regional 

land-use policies to reduce losses from future earthquakes. For example, although it is not possible to 

predict on a regional scale all those sites that will incur 

landsliding during the next earthquake of specific size 

and location, it is possible to delineate those general 

areas most susceptible to landsliding and hence those 

areas where special additional studies may be required 

to evaluate the landslide hazard for specific types of 

structures. 

Until more detailed treatments of seismic zonation 

are avilable, it is hoped that these basic tools and 

analyses will be useful to a wide variety of users con-
A96 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

cerned with reducing earthquake losses in the bay region. In general, the predicted maximum-intensity (fig. 

40) map serves to delineate areas of potential earthquake problems, and the basic tools mentioned above 

help identify the nature and possible severity of the 

problems in each area. In particular and as an informal 

summary, the analyses presented in the various papers 

are useful for the following purposes: 

1. Estimating the maximum size and location of future 

damaging earthquakes and delineating areas of 

potential surface faulting and the locations of potential sources of strong ground shaking (Wesson 

and others). 

2. Estimating the degree of bedrock shaking at various 

distances from earthquake sources of various sizes 

(Page and others). 

3. Interpreting basic geologic data pertinent to extrapolating results of specific site studies to aregional scale (Lajoie and Helley). 

4. Estimating the shaking response of various geologic 

units at specific sites and delineating those 

geologic units (defined by Lajoie and Helley) for 

which the effects of amplified ground shaking at 

equidistant sites are likely to be least, intermediate, and greatest (Borcherdt, Joyner, and 

others). 

5. Estimating the general liquefaction potential of various geologic units and delineating those geologic 

units (defined by Lajoie and Helley) for which the 

liquefaction potential of existent clay-free granular layers is low, moderate, and high (Y oud and 

others). 

6. Evaluating on a regional scale the vulnerability of 

various areas to landsliding and delineating those 

areas with various degrees of landslide susceptibility (Nilsen and Brabb). 

7. Applying the various basic tools to the problem of 

predicting the geologic effects of potential earthquakes of specific size and location for purposes of 

seismic zonation (Borcherdt, Brabb, and others). 

These analyses are preliminary and represent a first 

attempt at the required multidisciplinary analysis of 

the seismic-zonation problem. There are still many 

problems associated with seismic zonation, and the 

state of the art is rapidly changing. Nevertheless, it is 

hoped that this effort will serve as a useful first step 

toward the reduction of earthquake hazards on a regional scale. 

1906 INTENSITY SCALE FOR SAN FRANCISCO 

The following grades of apparent intensity were used 

by Wood (1908, pp. 224-225) in the city of San Francisco 

after the California earthquake of April 18, 1906: 

Grade A. Very violent. Comprises the rending and 

shearing of rock masses, earth, turf, and all 

structures along the line of faulting; the 

fall of rock from mountainsides; numerous 

landslips of great magnitude; consistent, 

deep, and extended fissuring in natural 

earth; some structures totally destroyed. 

Grade B. Violent. Comprises fairly general collapse of 

brick and frame buildings when not unusually strong; serious cracking of 

brickwork and masonry in excellent structures; the formation of fissures, step faults, 

sharp compression anticlines, and broad, 

wavelike folds in paved and asphalt-coated 

streets, accompanied by the ragged fissuring of asphalt; the destruction of foundation walls and underpinning structures by 

the undulation of the ground; the breaking 

of sewers and water mains; the lateral displacement of streets; and the compression, 

distension, and lateral waving or displacement of well-ballasted streetcar 

tracks. 

Grade C. Very strong. Comprises brickwork and 

masonry badly cracked, with occasional 

collapse; some brick and masonry gables 

thrown down; frame buildings lurched or 

listed on fair or weak underpinning structures, with occasional falling from underpinning or collapse; general destruction of 

chimneys and of masonry, brick, or cement 

veneers; considerable cracking or crushing 

of foundation walls. 

Grade D. Strong. Comprises general but not universal 

fall of chimneys; cracks in masonry and 

brickwork; cracks in foundation walls, retaining walls, and curbing; a few isolated 

cases of lurching or listing of frame buildings built upon weak underpinning structures. 

Grade E. Weak. Comprises occasional fall of chimneys 

and damage to plaster, partitions, plumbing, and the like. 
STUDIES FOR SEISMIC ZONATION A97 

REFERENCES CITED 

Albee, A. L., and Smith, J. L., 1967, Geologic criteria for nuclear power 

plant location: Soc. Mining Engineers Trans., v. 238, p. 430--434. 

Algermissen, S. T. (principal investigator), 1972, A study of the earthquake losses in the San Francisco Bay area: Natl. Oceanic Atmospheric Admin., U.S. Dept. Commerce, 220 p. 

Allen, J. E., 1946, Geology of the San Juan Bautista quadrangle, 

California: California Div. Mines Bull. 133, 112 p., map scale 

1:62,500. 

Ambraseys, N. N., 1969, Maximum intensity of ground movements 

caused by faulting: World Conference on Earthquake Engineering, 4th, Santiago, Chile, Proc., v. 1, p. 154-171. 

---1973, Dynamics and response of foundation materials in epicentral regions of strong earthquakes: World Conference on 

Earthquake Engineering, 5th, Rome, Italy, Proc., 23 p. 

Anderson, F. M., 1899, The geology of the Point Reyes peninsula: 

California Univ. Pubs. Geol. Sci., v. 2, p. 11~153. 

Atomic Energy Cpmmission, 1973, Nuclear power plants, seismic and 

geologic siting criteria: U.S. Federal Register, v. 38, p. 31279-

31285. 

Bailey, R. G., 1971, Landslide hazards related to land-use planning in 

Teton National Forest, northwest Wyoming: U.S. Dept. Agriculture, Forest Service, Intermountain region, Ogden, Utah, 131 p. 

Berkland, J. 0., 1969, Geology of the Novato quadrangle, Marin 

County, California: San Jose State College, unpub. M.S. thesis, 

146 p. 

Bernreuter, D. L., and Tokarz, F. J., 1972, Design basis earthquakes 

for the Lawrence Livermore Laboratory site: California Univ. 

Lawrence Livermore Lab. Pub. UCRL-51193. 

Blanc, R. P., and Cleveland, G. B., 1968, Natural slope stability as 

related to geology, San Clemente area, Orange and San Diego 

Counties, California: California Div. Mines and Geology Spec. 

Rept. 98, 19 p. 

Blanchard, F. B., and Laverty, G. L., 1966, Displacements in the 

Claremont Water Tunnel at the intersection with the Hayward 

fault: Seismol. Soc. America Bull., v. 56, p. 291-293. 

Bolt, B. A., 1973, Duration of strong ground motion: World Conference 

on Earthquake Engineering, 5th, Rome, Italy, Proc., p. 1304-

1313. 

Bolt, B. A., and Marion, W. C., 1966, Instrumental measurement of 

slippage on the Hayward fault: Seismol. Soc. America Bull., v. 56, 

p. 305--316. 

Bonilla, M. G., 1959, Geologic observations in the epicentral area of 

the San Francisco earthquake of March 22, 1957: California Div. 

Mines and Geology Spec. Rept. 57, p. 25--38. 

---1960a, Landslides in the San Francisco South quadrangle, 

California: U.S. Geol. Survey open-file rept., 44 p. 

---1960b, A sample of California Coast Range landslides, in 

Geological Survey research 1960: U.S. Geol. Survey Prof. Paper 

400--B, p. B149. 

---1965, Geologic map of the San Francisco South quadrangle, 

California: U.S. Geol. Survey open-file map, scale 1:24,000. 

---1966, Deformation of railroad tracks by slippage on the Hayward fault in the Niles district of Fremont, California: Seismol. 

Soc. America Bull., v. 56, p. 281-289. 

---1970, Surface faulting and related effects, in R. L. Wiegel, ed., 

Earthquake engineering: Englewood Cliffs, N.J., Prentice-Hall, 

p. 47-74. 

---1971, Preliminary geologic map of the San Francisco South and 

part of the Hunters Point quadrangles: U.S. Geol. Survey Misc. 

Field Studies Map MF-311, scale 1:24,000. 

---1975, Trench exposures of parts ofthe February 9, 1971, surface 

faults in the San Fernando fault zone, in Geological and Geophysical Studies, v. 3, of San Fernando, California, earthquake ofFeb. 

9, 1971: Natl. Oceanic Atmospheric Admin., U.S. Dept. Commerce (in press). 

Bonilla, M. G., and Buchanan, J. M., 1970, Interim report on world 

wide historic surface faulting: U.S. Geol. Survey open-file rept., 

32 p. 

Boore, D. M., 1973, Empirical and theoretical studyofnear-fault wave 

propagation: World Conference on Earthquake Engineering, 5th, 

Rome, Italy, Proc., p. 2397-2408. 

Boore, D. M., and Zoback, M. D., 1975, Two-dimensional kinematic 

fault modeling of the Pacoima Dam strong-motion recordings of 

the February 9, 1971, San Fernando earthquake: Seismol. Soc. 

America Bull. (in press). 

Borcherdt, R. D., 1970, Effects oflocal geology on ground motion near 

San Francisco Bay: Seismol. Soc. America Bull., v. 60, p. 2~61. 

Borcherdt, R. D., and Gibbs, J. F., 1975, Prediction of maximum 

earthquake intensities for the San Francisco Bay region: U.S. 

Geol. Survey Misc. Field Studies Map (in press). 

Brabb, E. E., 1970, Preliminary geologic map of the central Santa 

Cruz Mountains, California: U.S. Geol. Survey open-file map, 3 

sheets, scale 1:62,500. 

Brabb, E. E., and Pampeyan, E. H., 1972, Preliminary maps of land-. 

slides in San Mateo County, California: U.S. Geol. Survey Misc. 

Field Studies Map MF-344, scale 1:62,500. 

Brabb, E. E., and Pampeyan, E. H., and Bonilla, M. G., 1972, Landslide susceptibility in San Mateo County, California: U.S. Geol. 

Survey Misc. Field Studies Map MF-360, scale 1:62,500. 

Branner, J. C., 1908, The earth movement on the fault of April 18, 

1906; Crystal Springs Lake to Congress Springs: Carnegie Inst. 

Washington Pub. 87, 104 p. 

Brewer, W. H., 1930, Up and down California in 1860--64; the journal 

of William H. Brewer (F. P. Farquahar, ed.): New Haven, Yale 

Univ. Press, 601 p. 

Brown, R. D., Jr., 1970a, Faults that are historically active or that 

shows evidence of geologically young surface displacements, San 

Francisco Bay region; A progress report-October 1970: U.S. 

Geol. Survey Misc. Field Studies Map MF-331, 2 sheets, scale 

1:250,000. 

---1970b, Map showing recently active breaks along the San Andreas and related faults between the northern Gabilan Range and 

Cholame Valley, California: U.S. Geol. Survey Misc. Geol. Inv. 

Map 1-575, scale 1:62,500. 

---1972, Active faults, probable active faults, and associated fracture zones, San Mateo County, California: U.S. Geol. Survey 

Misc. Field Studies Map MF-355, scale 1:62,500. 

Brown, R. D., Jr., and Lee, W. H. K., 1971, Active faults and preliminary earthquake epicenters in the southern part ofthe San Francisco Bay region: U.S. Geol. Survey Misc. Field Studies Map 

MF-307, scale 1:250,000. 

Brown, R. D., Jr., and Vedder, J. G., 1967, Surface tectonic fractures 

along the San Andreas fault, in The Parkfield-Cholame, earthquakes of June-August 1966: U.S. Geol. Survey Prof. Paper 579, 

p. 2-22. 

Brown, R. D., Jr., Vedder, J. G., Wallace, R. E., Roth, E. F. Yerkes, R. 

F., Castle, R. 0. Waananen, A. 0., Page, R. W., and Eaton, J.P., 

1967, The Parkfield-Cholame, California, earthquakes of JuneAugust 1966-Surface geologic effects, water-resources aspects, 

and preliminary seismic data: U.S. Geol. Survey Prof. Paper 579, 

66 p. 

Brown, R. D., Jr., and Wallace, R. E., 1968, Current and historic fault 

movement along the San Andreas fault between Paicines and 

Camp Dix, California, in Dickinson, W. R., and Grantz, Arthur, 
A98 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

eds., Proceedings of conference on geologic problems of San Andreas fault system: Stanford Univ. Pubs. Geol. Sci., v. 11, p. 

22-41. 

Brown, R. D., Jr., Ward, P. L., and Plafker, George, 1973, Geologic and 

seismologic aspects of the Managua, Nicaragua, earthquakes of 

December 23, 1972: U.S. Geol. Survey Prof. Paper 838, 34 p. 

Brown, R. D., Jr., and Wolfe, E. W., 1972, Map showing recently active 

breaks along the San Andreas fault between Pt. Delgada and 

Bolinas Bay, California: U.S. Geol. Survey Misc. Geol. Inv. Map 

1-692, scale 1:24,000. 

Brune, J. N., 1970, Tectonic stress and the spectra of seismic shear 

waves from earthquakes: Jour. Geophys. Research, v. 75, p. 

4997-5009. 

Burke, D. B., and Helley, E. J., 1973, Map showing evidence for recent 

fault activity in the vicinity of Antioch, Contra Costa County, 

California: U.S. Geol. Survey Misc. Field Studies Map MF-533, 

scale 1:24,000. 

Burnett, J. L., 1972, Geologic and slope stability maps of the Hayward 

hills, California: Hayward, California Planning Dept., Hayward 

Hills Study Area, scale 1:24,000. 

Califorr.ia Department of Water Resources, 1966, Evaluation of 

ground water resources, Livermore and Sunol Valleys, Appendix 

A, Geology: California Dept. Water Resources Bull. 118-2, 153 p. 

Chae, Y. S., 1968, Viscoelastic properties of snow and sand: Am. Soc. 

Civil Engineers Proc., Jour. Eng. Mechanics Div., v. 94, p. 1379-

1394. 

Clag~e, J. J., 1969, Landslides of southern Point Reyes National 

Seashore: California Div. Mines and Geology Mineral Inf. Service, v. 22, p. 107-110, 11~118. 

Clark, M. M., 1972, Surface rupture along the Coyote Creek fault, in 

The Borrego Mountain earthquake of April 9, 1968: U.S. Geol. 

Survey Prof. Paper 787, p. 55-86. 

Clark, M. M., Grantz, Arthur, and Rubin, Meyer, 1972, Holocene 

activity ofthe Coyote Creek fault as recorded in the sediments of 

Lake Cahuilla, in The Borrego Mountain earthquake of April 9, 

1968: U.S. Geol. Survey Prof. Paper 787, p. 1112-1130. 

Cluff, L. S., and Steinbrugge, K. V., 1966, Hayward fault slippage in 

the Irvington-Niles district of Fremont, California: Seismol. Soc. 

America Bull., v. 56, p. 257-279. 

Cooper, Allan, 1971, Structure of the continental shelf west of San 

Francisco, California: California State Univ. San Jose, unpub. 

M.S. thesis. 

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 

Univ. Pubs. Geol. Sci., v. 11, p. 191-207. 

Dibblee, T. W., Jr., 1966, Geology of Palo Alto quadrangle, Santa 

Clara and San Mateo Counties, California: California Div. Mines 

and Geology,Map Sheet 8, scale 1:62,500. 

---1972a, Preliminary geologic map of the Lick Observatory 

quadrangle, Santa Clara County, California: U.S. Geol. Survey 

open-file map, scale 1:24,000. 

---1972b, Preliminary geologic map of the San Jose East quadrangle, Santa Clara County, California: U.S. Geol. Survey openfile map, scale 1:24,000. 

---1972c, Preliminary geologic map of the Milpitas quadrangle, 

Alameda and Santa Clara Counties, California: U.S. Geol. Survey open-file map, scale 1:24,000. 

---1972d, Preliminary geologic map of the Calaveras Reservoir, 

Santa Clara County, California: U.S. Geol. Survey open-file map, 

scale 1:24,000. 

---1973a, Preliminary geologic map of the Gilroy quadrangle, 

Santa Clara County, California: U.S. Geol. Survey open-file map, 

scale 1:24,000. 

--1973b, Preliminary geologic map of the Gilroy Hot Springs 

quadrangle, Santa Clara County, California: U.S. Geol. Survey 

open-file map, scale 1:24,000. 

---1973c, Preliminary geologic map of the Morgan Hill quadrangle, Santa Clara County, California: U.S. Geol. Survey open-file 

map, scale 1:24,000. 

---1973d, Preliminary geologic map of the Mt. Madonna quadrangle, Santa Clara and Santa Cruz Counties, California: U.S. 

Geol. Survey open-file map, scale 1:24,000. 

---1973e, Preliminary geologic map of the Mt. Sizer quadrangle, 

Santa Clara County, California: U.S. Geol. Survey open-file map, 

scale 1:24,GOO. 

Dickinson, W. R., and Grantz, Arthur, 1968, Proceedings of conference 

on geologic problems of San Andreas fault system: Stanford Univ. 

Pubs. Geol. Sci., v. 11, o. 117-120. 

Dieterich, J. H., 1973, A deterministic near-field source model: World 

Conference on Earthquake Engineering, 5th, Rome, Italy, Proc., 

p. 2385-2396. 

Dobry, R. Whittman, R. V., andRoeset,J. M., 1971, Soil properties and 

the one-dimensional theory of earthquake amplification: Mas-. 

sachusettslnst.Tech.Research Rept.R71-18, Soils Pub. 275,347 p. 

Dooley, R. L., 1973, Geology and land use considerations in the vicinity of the Green Valley fault, Solano County, California: California Univ. Davis, unpub. M.S. thesis, 47 p. 

Eckel, E. B., ed., 1958, Landslides and engineering practice, in National Research Council, Highway Research Board Spec. Rept. 29: 

Dept. ofSoils, Geology and Foundations, NAS-NRC Pub. 544, 232 

p. 

Fox, K. F., Jr., Sims, J. D., Bartow, J. A., and Helley, E. J., 1973, 

Preliminary geologic map of eastern Sonoma County and western 

Napa County, California: U.S. Geol. Survey, Misc. Field Studies 

Map MF-483, 4 sheets, scale 1:62,500. 

Frame, P. A., 1974, Landslides in the Mt. Sizer area, Santa Clara 

County, California: California State Univ. San Jose, M.S. thesis, 

67 p. 

Gealey, W. K., 1951, Geology of the Healdsburg quadrangle, California: California Div. Mines and Geology Bull. 161, 76 p. 

Gibbs, J. F., and Borcherdt, R D., 1974, Effects of local geology on 

ground motion in the San Francisco Bay region-a continuing 

study: U.S. Geol. Survey open-file rept., 162 p. 

Gibbs, H. J., and Holtz, W. G., 1957, Research on determining the 

density of sand by spoon penetration test: Internat. Conf. Soil 

Mechanics and Foundations Eng., 4th, London, Proc., v. 1, p. 

35-39. 

Gibson, W. M., and Wollenberg, H. A., 1968, Investigations for ground 

stability in the vicinity of the Calaveras fault, Livermore and 

Amador Valleys, Alameda County, California: Geol. Soc. 

America Bull., v. 79, p. 627-638. 

Glen, William, 1959, Pliocene and lower Pleistocene of the western 

part of the San Francisc9 pE>ninsula: California Univ. Pubs. Geol. 

Sci., v. 36, p. 147-198. 

Greene, H. G., Lee, W. H. K., McCulloch, D. S., and Brabb, E. E., 1973, 

Faults a,nd earthquakes in the Monterey Bay region, California: 

U.S. Geol. Survey Misc. Field Studies Map MF-518, 4 sheets, 

scale 1:250,000. 

Griggs, G. B., 1973, Earthquake activity between Monterey and l{alf 

Moon Bay, California: California Geology, v. 26, p. 103-110. 

Hadley, J. B., 1964, Landslides and related phenomena accompanying 

the Hebgen Lake earthquake of August 17, 1959: U.S. Geol: 

Survey Prof. Paper 435, p. 107-138. 

Hall, Nelson T., Sarna-Wojcicki, Andre, and Dupre, William, 1974, 

Faults and their potehtial hazard in Santa Cruz County, California: U.S. Geol. Survey Misc. Field Inv. MF-626, scale 1:62,500. 

Hanks, T. C., 1975, The faulting mechanism of the San Fernando 

earthquake: Jour. Geophys. Research (in press). 
STUDIES FOR SEISMIC ZONATION A99 

Hansen, W. R., 1964, Sewage effluent disposal and its effects on 

ground water beneath Livermore V ~lley: Assoc. Eng. Geologists 

Bull., v.- 1, p. 22-36. 

Hansen, W. R., Eckel, E. B., Schaem, W. E., Lyle, R. E., George, 

Warren, and Chance, Genie, 1966, The Alaska earthquake, 

March 27, 1964: Field investigations and reconstruction effort: 

U.S. Geol. Survey Prof. Paper 541, 111 p. 

Hardin, B. 0., and Dernevich, V. D., 1972, Shear modulus and damping in soils; design equations and curves: Am. Soc. Civil Engineers Proc. Soil Mechanics and Found. Div., p. 667-692. 

Harding, R. C., 1969, Landslides-A continuing problem for Bay area 

development, in Danehy, E. A. (ed.), Urban environmental geology in San Francisco Bay region: Assoc. Eng. Geologists, San 

Francisco Section, Spec. Pub., p. 65-74. 

Haskell, N. A., 1953, The disperson of surface waves on multilayered 

media: Seismol. Soc. America Bull., v. 43, p. 17-34. 

Hazelwood, R. M., 1974, Preliminary report of seismic refraction survey along the east side of San Francisco Bay, Alameda County, 

California: U.S. Geological Survey open-file rept., 10 p. 

Helley, E. J., and Brabb, E. E., 1971, Geologic map oflate Cenozoic 

deposits, Santa Clara County, California: U.S. Geol. Survey Misc. 

Field Studies Map MF-335, 3 sheets, scale 1:62,500. 

Helley, E. J., Lajoie, K. R., and Burke, D. B., 1972, Geologic map oflate 

Cenozoic deposits, Alameda County, California: U.S. Geol. Survey Misc. Field Studies Map MF-429, scale 1:62,500. 

Hudson, D. E., Brady, A. G., Trifunac, M.D., and Vijayaraghavan, A., 

1971, Strong-motion earthquake accelerograms, corrected accelerograms and integrated ground velocity and displacement 

curves: California Inst. Tech., Earthquake Eng. Research Lab., 

EERL 71-51, v. 2, 321 p. 

Huffman, M. E., 1972a, Geology for planning on the Sonoma County 

coast between the Russian and Gualala Rivers: California Div. 

Mines and Geology Prelim. Rept. 16, 38 p., scale 1:24,000. 

---1972b, Geology for planning in the Sonoma Mountain and 

Mark-Riebli Road areas, Sonoma County, California: California 

Div. Mines and Geology open-file rept. 72-25, scale 1:24,000. 

---1974, Geology for planning on the Sonoma County coast between the Russian River and Estero Americana: California Div. 

Mines and Geology Prelim. Rept. 20, 36 p. 

Hausner, G. W., 1965, Intensity of earthquake ground shaking near 

the causative fault: World Conference on Earthquake Engineering, 3rd, Wellington, New Zealand, Proc., v. 1, p. III-94-III-109. 

Hausner, G. W., and Jennings, P. C., 1972, The San Fernando, 

California earthquake: Earthquake Eng. and Structural 

Dynamics 1, p. 5-31. 

Hudson, D. E., 1962, Some problems in the application of spectrum 

techniques to strong-motion earthquake analysis: Seismol. Soc. 

America Bull., v. 52, p. 417-430. 

Ida, Y., 1973, The maximum acceleration of seismic ground motion: 

Seismol. Soc. America Bull., v. 63, p. 959-968. 

Idriss, I. M., and Seed, H. B., 1968, Seismic response of horizontal soil 

layers: Am. Soc. Civil Engineers Proc., Soil Mechanics and 

Found. Div., v. 94, p. 1003-1031. 

Iida, Numizi, 1965, Earthquake magnitude, earthquake fault and 

source dimensions: Nagoya Univ. Jour. Earth Sci., v. 13, p. 115-

132. 

Jack, Robert, 1968, Quaternary sediments of the Montara Mountain 

area, San Mateo County: California Univ., Berkeley, unpub. M.A. 

thesis. 

Jahns, R. H., and Hamilton, D. H., 1971, Geology of the Mendocino 

Power Plant, in Mendocino Power Plant Preliminary Safety 

Analysis Report: U.S. Atomic Energy Comm., v. 1, appendix 2.5A, 

61 p. 

Johnson, A.M., and Ellen, S.D., 1965, Preliminary evaluation of the 

interaction between engineering development and natural 

geologic processes on the Bovet property, town of Portola Valley, 

California-with a section on the San Andreas fault by W. R. 

Dickinson: Report prepared for City of Portola Valley, California, 

scale 1:2,400. 

Johnson, A.M., and Lobo~Guerrero, A. U., 1968, Preliminary evaluation of the relative stability of ground, Marianni property, town of 

Portola Valley, California-with a section on the San Andreas 

fault by W. R. Dickinson: Report prepared for the City of Portola 

Valley, California, 22 p. 

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

amplitudes of earthquake motions: Tokyo Univ. Earthquake Research Inst. Bull., v. 30, p. 31-37. 

Kojan, E., Foggin, G. T., III, and Rice, R. M., 1972, Prediction and 

analysis of debris slide incidence by photogrammetry, Santa 

Ynez-San Rafael Mountains, California: Internatl. Geol. Congress, 24th, sec. 13, p. 124-131. 

Lajoie, K. R., Helley, E. J., Nichols, D. R., and Burke, D. B., 1974, 

Unconsolidated and semi-unconsolidated sedimentary deposits, 

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

Field Studies Map MF-575, scale 1:62,500. 

Lawson, A. C., 1893, The post-Pliocene diastrophism of the coast of 

southern California: California Univ. Pubs. Geol. Sci., v. 1, p. 

115-160. 

---1895, Sketch of the geology of the San Francisco peninsula, 

Extract from the Fifteenth Annual Report of the U.S. Geol. Survey, 1893-94, Washington, Govt. Printing Office, p. 401-476. 

Lawson, A. C., and others, 1908, The Californi~1 earthquake of April 

18, 1906-Report of the State Earthquake Investigation Commission: Carnegie Inst. Washington Pub. 87, 2 vols. 

Lee, W. H. K., Eaton, M. S., and Brabb, E. E., 1971, The earthquake 

sequence near Danville, California, 1970: Seismol. Soc. America 

Bull., v. 61, p. 1771-1794. 

Lee, W. H. K., Meagher, K. L., Bennett, R. E., and Matamoros, E. E., 

1972, Catalog of earthquakes along the San Andreas fault system 

in central California for the year 1971: U.S. Geol. Survey open-file 

rept., 67 p. 

Lee, W. H. K., Roller, J. C., Bauer, P. G., and Johnson, J. D., 1972, 

Catalog of earthquakes along the San Andreas fault system in 

central California for the year 1969: U.S. Geol. Survey open-file 

rept., 48 p. 

Lee, W. H. K., Roller, J. C., Meagher, K. L., and Bennett, R. E., 1972, 

Catalog of earthquakes along the San Andreas fault system in 

central California for the year 1970: U.S. Geol. Survey open-file 

rept., 73 p. 

Lindh, A. G., and Boore, D. M., 1973, Another look at the Parkfield 

earthquake using strong-motion instruments as a seismic array 

(abs.): Seismol. Soc. America, Earthquake Notes 44, p. 24. 

Louderback, G. D., 1942, Faults and earthquakes: Seismol. Soc. 

America Bull., v. 32, p. 305-330. 

---1947, Central California earthquakes of the 1830's: Seismol. 

Soc. America Bull., v. 37, p. 33-74. 

Maberry, J. 0., 1972a, Map showing landslide deposits and areas of 

potential landsliding in the Parker quadrangle, Arapahoe and 

Douglas Counties, Colorado: U.S. Geol. Survey Misc. Geol. Inv. 

Map 1-770-E, scale 1:24,000. 

---1972b, Slope map of the Parker quadrangle, Arapahoe and 

Douglas Counties, Colorado: U.S. Geol. Survey Misc. Geol. Inv. 

Map 1-770-F, scale 1:24,000. 

McEvilly, T. V., 1966, The earthquake sequence of November 1964 

near Corralitos, California: Seismol. Soc. America Bull., v. 56, p. 

755-773. 

---1970, Magnitudes, epicenters, and fault plane solutions, in The 

Santa Rosa earthquakes of October 1969: Mineral Inf. Service, v. 

23, p. 44-48. 

McEvilly, T.V., andCasaday,K. B., 1967, The earthquake sequence of 
AlOO REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

September 1965 near Antioch, California: Seismol. Soc. America 

Bull., v. 57, p. 11~124. 

McLaughlin, R. J., 1971, Geologic map of the Sargent fault zone in the 

vicinity of Mount Madonna, Santa Clara County, California: U.S. 

Geol. Survey open-file map, scale 1:12,000. 

---1973, Geology of the Sargent fault zone in the vicinity of Mount 

Madonna, Santa Clara and Santa Cruz Counties, California: 

California State Univ. San Jose, unpub. M. S. thesis, 

131 p. 

McLaughlin, R. J., Simoni, T. R., Osbun, E. D., and Bauer, P. G., 1971, 

Preliminary geologic map of the Lorna Prieta-Mount Madonna 

area, Santa Clara and Santa Cruz Counties, California: U.S. 

Geol. Survey open-file map, scale 1:24,000. 

Medvedev, S. V., 1965, Engineering seismology: National Technical 

Inf. Service, NTIS No. TT65-50011, 260 p. 

Milliman, J.D., and Emery, K. 0., 1968, Sea levels during the past 

35,000 years: Science, v. 162, p. 1121. 

Morton, D. M., 1971, Seismically triggered landslides in the area 

above the San Fernando Valley, in The San Fernando, California, 

earthquake of February 9, 1971: U.S. Geol. Survey Prof. Paper 

733, p. 99-104. 

Murphy, L. M., and Cloud, W. K., 1957, United States earthquakes, 

1955: U.S. Coast and Geodetic Survey, 83 p. 

Nason, R. D., 1971, Measurements and theory offault creep slippage 

in central California: Royal Soc. New Zealand Trans., v. 9, p. 

181-187. 

Newmark, N. M., and Hall, W. J., 1969, Seismic design criteria for 

nuclear reactor facilities: World Conference Earthquake Engineering, 4th, Santiago, Chile, Proc., v. 2, p. 37-50. 

Nichols, D. R., and Wright, N. A., 1971, Preliminary map of historic 

margins ofmarshlands, San Francisco Bay, California: U.S. Geol. 

Survey open-file map, scale 1:125,000. 

Nilsen, T. H., 1971, Preliminary photointerpretation map oflandslide 

and other surficial deposits of the Mount Diablo area, Contra 

Costa and Alameda Counties, California: U.S. Geol. Survey Misc. 

Field Studies Map MF-310, scale 1:62,500. 

---1972a, Preliminary photointerpretation map of landslide and 

other surficial deposits of parts of the Altamont and Carbona 

15-minute quadrangles, Alameda County, California: U.S. Geol. 

Survey Misc. Field Studies Map MF-321, scale 1:62,500. 

---1972b, Preliminary photointerpretation map of landslide and 

other surficial deposits of the Byron area, Contra Costa and 

Alameda Counties, California: U.S. Geol. Survey Misc. Field 

Studies Map MF-338, scale 1:62,500. 

---1972c, Preliminary photointerpretation map of landslide and 

other surficial deposits of the Mount Hamilton quadrangle and 

parts of the Mount Boardman and San Jose quadrangles, 

Alameda and Santa Clara Counties, California: U.S. Geol. Survey Misc. Field Studies Map MF-339, scale 1:62,500. 

---1972d, Preliminary photointerpretation map of landslides and 

other surficial deposits of parts of the Los Gatos, Morgan Hill, 

Gilroy Hot Springs, Pacheco Pass, Quien Sabe, and Hollister 

15-minute quadrangles, Santa Clara County, California: U.S. 

Geol. Survey Misc. Field Studies Map MF-416, scale 1:62,500. 

---1973a, Current slope stability studies by the U.S. Geological 

Survey in the San Francisco Bay region: Landslide, The Slope 

Stability Review, v. 1, no. 1, p. 2-10. 

---1973b, Preliminary photointerpretation map of landslide and 

other surficial deposits of the Livermore and part ofthe Hayward 

15-minute quadrangles, Alameda and Contra Costa Counties, 

California: U.S. Geol. Survey Misc. Field Studies Map MF-519, 

scale 1:62,500. 

---1973c, Preliminary photointerpretation map of landslide and 

other surficial deposits of the Concord 15-minute quadrangle and 

the Oakland West, Richmond, and part ofthe San Quentin 7.5-

minute quadrangles, Contra Costa and Alameda Counties, 

California: U.S. Geol. Survey Misc. Field Studies Map MF-493, 

scale 1:62,500. 

Nilsen, T. H., and Brabb, E. E., 1972, Preliminary photointerpretation 

and damage maps of landslide and other surficial deposits in 

northeastern San Jose, California: U.S. Geol. Survey Misc. Field 

Studies Map MF-361, scales 1:12,000 and 1:24,000. 

Nilsen, T. H., Taylor, F. A., and Brabb, E. E., 1975, Structurally 

damaging landslides in Alameda County, California, from 1940 

to 1971-economic analysis and influence of ancient landslide 

deposits: U.S. Geol. Survey Bull. 1398 (in press). 

Nilsen, T. H., and Turner, B. L., 1974, The influence of rainfall and 

ancient landslide deposits on recent landslides (1950-1971) in 

urban areas of Contra Costa County, California: U.S. Geol. Survey Bull. 1388. 

Oakeshott, G. B., ed., 1959, San Francisco earthquake of March 1957: 

California Div. Mines Spec. Rept., v. 57, p. 7~123. 

Page, R. A., Boore, D. M., Joyner, W. B., and Coulter, H. W., 1972, 

Ground motion values for use in the seismic design of the transAlaska pipeline system: U.S. Geol. Survey Circ. 672, 23 p. 

Pampeyan, E. H., 1970, Geologic map of the Palo Alto 7.5-minute 

quadrangle, San Mateo and Santa Clara Counties, California: 

U.S. Geol. Survey open-file rept., scale 1:24,000. 

Peacock, W. H., and Seed, H. B., 1968, Sand liquefaction under cyclic 

loading simple shear conditions: Am. Soc. Civil Engineers Proc., 

Jour. Soil Mechanics and Found. Div., v. 94, no. SM3, p. 689-708. 

Plafker, George, 1969, Tectonics of the March 27, 1964 Alaska 

earthquake: U.S. Geol. Survey Prof. Paper 54~1. J4 p. 

Plafker, G., Erickson, G. E., and Concha, F. J., 1971, Geological 

aspects of the May 31, 1970, Peru earthquake: Seismol. Soc. 

America Bull., v. 61, p. 54~578. 

Radbruch, D. H., 1957, Areal and engineering geology of the Oakland 

West quadrangle, California: U.S. Geol. Survey Misc. Geol. lnv. 

Map 1-239, scale 1:24,000. 

---1967, Approximate location offault traces and historic surface 

ruptures within the Hayward fault zone between San Pablo and 

Warm Springs, California: U.S. Geol. Survey Misc. Geol. Inv. 

Map 1-522, scale 1:62,500. 

---1968a, Map showing recently active breaks along the Hayward 

fault zone and southern part ofthe Calaveras fault zone, California: U.S. Geol. Survey open-file map, 2 sheets, scale 1:24,000. 

---1968b, New evidence of historic fault activity in Alameda, 

Contra Costa, and Santa Clara Counties, California, in Dickinson, W. R., and Grantz, Arthur, eds., Proceedings of conference on 

geologic problems of San Andreas fault system: Stanford Univ. 

Pubs. Geol. Sci., v. 11, p. 46-54. 

---1969, Areal and engineering geology of the Oakland East quadrangle, California: U.S. Geol. Survey Geol. Quad. Map GQ-769, 

scale 1:24,000. 

Radbruch, D. H., Bonilla, M.G., and others, 1966, Tectonic creep in 

the Hayward fault zone, California: U.S. Geol. Survey Circ. 525, 

13 p. 

Radbruch, D. H., and Case, J. E., 1967, Preliminary geologic map and 

engineering geologic information, Oakland and vicinity, California: U.S. Geol. Survey open-file map, scale 1:24,000. 

Radbruch, D. H., and Lennert, B. J., 1966, Damage to culvert under 

Memorial Stadium University ofCalifornia, Berkeley, caused by 

slippage in the Hayward fault zone: Seismol. Soc. America Bull., 

v. 56, p. 295-305. 

Radbruch, D. H., and Weiler, L. M., 1963, Preliminary report on 

landslides in a part of the Orinda Formation, Contra Costa County, California: U.S. Geol. Survey open-file rept., 49 p. 

Radbruch, D. H., and Wentworth, C. M., 1971, Estimated relative 

abundance oflandslides in the San Francisco Bay region, California: U.S. Geol. Survey open-file rept., scale 1:500,000. 

Rice, S. J., and Strand, R. G., 1972, Geologic and slope stability maps 

of the Tennessee Valley, Lucas Valley and north coastal areas, 
STUDIES FOR SEISMIC ZONATION AlOl 

Marin County, California: California Div. Mines and Geology 

open-file rept. 72-22, scale 1:12,000. 

Richter, C. F., 1958, Elementary seismology: San Francisco, W. H. 

Freeman, 768 p. 

Reiche, Parry, 1950, Rio Vista, California, fault scarp (abs.): Geol. Soc. 

America Bull., v. 61, p. 1529--1530. 

Ritter, J. R., and Dupre, W. R., 1972, Map showing areas of potential 

inundation by tsunamis in the San Francisco Bay Region, 

California: U.S. Geol. Survey Misc. Field Inv. Map MF-480, scale 

1:125,000. 

Rogers, T. H., 1971, Environmental geologic analysis of the Santa 

Cruz Mountain study area, Santa Clara County, California: 

California Div. Mines and Geology, San Jose, Calif., Santa Clara 

County Planning Dept., 64 p. 

--1972, Bear VaHey-Melendy Ranch earthquakes of 24-27 February 1972: California Geology, v. 25, p. 138. 

Rogers, T. H., and Nason, R. D., 1971, Active displacement on the 

Calaveras fault zone at Hollister, California: Seismol. Soc. 

America Bull., v. 61, p. 399--416. 

Saul, R. B., 1972, Geology and slope stability of the southwest quarter 

of the Walnut Creek quadrangle, Contra Costa County, California: California Div. Mines and Geology Map Sheet 16, scale 

1:12,000. 

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

relative plate motion in central California: Jour. Geophy. Research, v. 78, p. 832-845. 

Schlocker, J., Bonilla, M.G., and Radbruch, D. H., 1958, Geology of 

the San Francisco north quadrangle, California: U.S. Geol. Survey Misc. Geol. Inv. Map I-272, scale 1:24,000. 

Schnabel, P. B., and Seed, H. B., 1973, Accelerations in rock for 

earthquakes in the western United States: Seismol. Soc. America 

Bull., v. 63, p. 501-516. 

Schuyler, J.D., 1898, Reservoirs for irrigation, in Eighteenth Annual 

Report of the U.S. Geol. Survey for 1896--1897, Part IV: 

Washington, D. C., p. 711-713. 

Scott, G. R., 1972, Map showing landslides and areas susceptible to 

landsliding in the Morrison quadrangle, Jefferson County, Colorado: U.S. Geol. Survey Misc. Geol. Inv. Map I-790-B, scale 

1:24,000. 

Seed, H. B., 1968, Landslides during earthquakes due to ~oil liquefaction: Am. Soc. Civil Engineers Proc., Jour. Soil Mechanics and 

Found. Div., v. 94, p. 1053-1122. 

Seed, H. B., and Idriss, I. M., 1971, Simplified procedure for evaluating 

soil liquefaction potential: Am. Soc. Civil Engineers Proc., Jour. 

Soil Mechanics and Found. Div., v. 97, p. 1249--1273. 

Seed, H. B., Idriss, I. M., and Kiefer, F. W., 1969, Characteristics of 

rock motions during earthquakes: Am. Soc. Civil Engineers Proc., 

Jour. Soil Mechanics and Found. Div., v. 95, p. 1199--1218. 

Seed, H. B., and Lee, K. L., 1969, Pore-water pressures in earth slopes 

under seismic loading conditions: World Conf. on Earthquake 

Engineering, 4th, Santiago, Chile, Proc. V. 3, p. A-5-1 to A-5-11. 

Seed, H. B., and Peacock, W. H., 1971, Test procedures for measuring 

soil liquefaction characteristics: Am. Soc. Civil Engineers Proc., 

Jour. Soil Mechanics and Found. Div., v. 97, p. 1099--1119. 

Sharp, R. V., 1973, Map showing recent tectonic movement on the 

Concord fault, Contra Costa and Solano Counties, California: 

U.S. Geol. Survey Misc. Field Studies Map MF-505, scale 

1:24,000. 

Sharp, R. V., and Clark, M. M., 1972, Geologic evidence of previous 

faulting near the 1968 rupture on the Coyote Creek fault, in The 

Borrego Mountain earthquake of April9, 1968: U.S. Geol. Survey 

Prof. Paper 787, p. 131-140. 

Simpson, H. E., 1973a, Map showing landslides in the Golden quadrangle, Jefferson County, Colorado: U.S. Geol. Survey Misc. Geol. 

Inv. Map I-761-B, scale 1:24,000. 

---1973b, Map showing areas of potential rockfalls in the Golden 

quadrangle, Jefferson County, Colorado: U.S. Geol. Survey Misc. 

Geol. Inv. Map I-761-C, scale 1:24,000. 

Sims, J. D., Fox, K. F., Jr., Bartow, J. A., and Helley, E. J., 1973, 

Preliminary geologic map of Solano County and parts of Napa, 

Contra Costa, Marin, and Yolo Counties, California: U. S. Geol. 

Survey Misc. Field Studies Map MF-484, 5 sheets, scale 1:62,500. 

Sims, J. D., and Nilsen, T. H., 1972, Preliminary photointerpretation 

map of landslides and other surficial deposits of parts of the 

Pittsburg and Rio Vista 15-minute quadrangles, Contra Costa 

and Solano Counties, California: U.S. Geol. Survey Misc. Field 

Studies Map MF-322, scale 1:62,500. 

Slemmons, D. B., 1967, Pliocene, and Quaternary crustal movements 

of the Basin-and-Range Province, USA: Osaka City Univ. Jour. 

Geosciences, vol. 10, p. 1-11. 

Smith, D. D., 1960, Geomorphology of the San Francisco peninsula: 

Stanford Univ., unpub. Ph.b. thesis, 433 p. 

Steinbrugge, K. V., Cloud, W. K., and Scott, N.H., 1970, The Santa 

Rosa, California earthquakes of October 1, 1969: Washington, 

U.S. Govt. Printing Office, 99 p. 

Steinbrugge, K. V., and Zacher, E. G., 1960, Fault creep and property 

damage, in Creep on the San Andreas fault: Seismol. Soc. 

America Bull., v. 50, p. 389--396. 

Taylor, F. A., and Brabb, E. E., 1972, Map showing distribution and 

cost by counties of structurally damaging landslides in the San 

Francisco Bay region, California, winter of 1968-69: U.S. Geol. 

Survey Misc. Field Studies Map MF-327, scale 1:1,000,000. 

Thatcher, W., and Hanks, T. C., 1973, Source parameters of southern 

California earthquakes: Jour. Geophys. Research, v. 78, p. 8547-

8576. 

Tocher, Don, 1958, Earthquake energy and ground breakage: Seismol. Soc. America Bull., v. 48, p. 147-153. 

---1959, Seismic history of the San Francisco Bay region, in San 

Francisco earthquakes of March 1957: California Div. Mines 

Spec. Rept. 57, p. 39--48. 

---1960, Creep on the San Andreas fault-creep rate and related 

measurements at Vineyard, California, in Creep on the San Andreas fault: Seismol. Soc. America Bull., v. 50, p. 396-404. 

Trask, J. B., 1964, Earthquakes in California from 1800 to 1864: 

California Acad. Sci. Proc., v. 3, p. 130-153. 

Trifunac, M.D., 1975, A three-dimensional dislocation model for the 

San Fernando, California, earthquake of February 9, 1971: Seismol. Soc. America Bull. (in press). 

Trifunac, M. D., and Hudson, D. E., 1971, Analysis of the Pacoima dam 

accelerogram-San Fernando, California, earthquake of 1971: 

Seismol. Soc. America Bull. v. 61, p. 1393-1411. 

Unger, J. D., and Eaton, J. P., 1970, Aftershocks of the October 1, 

1969, Santa Rosa, California, earthquakes (abs): Geol. Soc. 

America Abs. with Programs, v. 2, p. 155. 

U.S. Geological Survey, 1971a, Surface faulting, in The San Fernando 

California earthquake of February 9, 1971: U.S. Geol. Survey 

Prof. Paper 733, p. 55-76. 

---1971b, Worldwide correlation of surface displacement and 

Richter magnitude, in Geological Survey research 1971; U.S. 

Geol. Survey Prof. Paper 750-A, p. 168. 

---1972, Slope map of the San Francisco Bay region: U.S. Govt. 

Printing Office, 3 sheets, scale 1:125,000. 

U.S. Soil Conservation Service, 1968, Soils of Santa Clara County, 

California: Washington, D. C., U.S. Govt. Printing Office, 227 p. 

Van Horn, Richard, 1972a, Slope map ofthe Sugar House quadrangle, 

Salt Lake County, Utah: U.S. Geol. Survey Misc. Geol. Inv. Map 

I-766-C, scale 1:24,000. 

---1972b, Landslide and associated deposit map of the Sugar 

House quadrangle, Salt Lake County, Utah: U.S. Geol. Survey 

Misc. Geol. Inv. Map I-766-D, scale 1:24,000. 

---1972c, Relative slope stability of the Sugar House quadrangle, 

Salt Lake County, Utah: U.S. Geol. Survey Misc. Geol. Inv. Map 
A102 REDUCTION OF EARTHQUAKE HAZARDS, SAN FRANCISCO BAY REGION 

1-76&-E, scale 1:24,000. 

Varnes, D. J., 1958, Landslide types and processes, chap. 3, in Eckel, 

E. B., ed., Landslides and engineering practice: Natl. Research 

Council, Highway Research Board Spec. Rept. 29, NAS-NRC 

Pub. 544, p. 20-47. 

Vedder, J. G., and Wallace, R. E., 1970, Map showing recently active 

breaks along San Andreas fault between Cholame Valley and 

Tejon Pass, California: U.S. Geol. Survey Misc. lnv. Map 1-574, 

scale 1:24,000. 

Wallace, R. E., 1968, Notes on stream channels offset by the San 

Andreas fault, southern Coast Ranges, California, in Dickinson, 

W. R., and Grantz, Arthur, eds., Proceedings of conference on 

geologic problems of San Andreas fault system: Stanford Univ. 

Pubs. Geol. Sci., v. 11, p. &-21. 

--1969, Planning along active strands of the San Andreas fault: 

Assoc. Eng. Geologists, National Meetings, San Francisco, 1969, 

Field Trips (Guidebook), p. E5-E6. 

--1970, Earthquake recurrence intervals on the San Andreas 

fault: Geol. Soc. America Bull., v. 81, p. 2875-2890. 

Waltz, J.P., 1971, An analysis of selected landslides in Alameda and 

Contra Costa Counties, California: Assoc. Eng. Geologists Bull., 

v. 8, p. 153-163. 

Warrick, R. E., 1974, Seismic investigation of a San Francisco Bay 

mud site: Seismol. Soc. America Bull., v. 64, p. 

Weaver, C. E., 1949, Geology of the Coast Ranges immediately north 

of the San Francisco Bay region, California: Geol. Soc. America 

Mem. 33, 242 p. 

Weber, G. E., and Lajoie, K. R., 1974, Holocene movement on the San 

Gregorio fault zone near Aiio Nuevo, San Mateo County, California (abs.): Geol. Soc. America Abs. with Programs, v. 6, p. 273. 

Wentworth, C. M., Bonilla, M. C., and Buchanan, J. M., 1973, Seismic 

environment ofthe Burro Flats site, Ventura County, California: 

U.S. Geol. Survey open-file rept., 35 p. 

Wentworth, C. M., Ziony, J. 1., and Buchanan, J. M., 1970, Preliminary geologic environmental map of the greater Los Angeles 

area, California: U.S. Atomic Energy Comm., TID-25363, 41 p., 

scale 1:250,000. 

Wesson, R. L., Bennett, R. E., and Lester, F. W., 1972a, Catalog of 

earthquakes along the San Andreas fault system in central 

California, April-June, 1972: U.S. Geol. Survey open-file rept. 42 

p. 

Wesson, R. L., Bennett, R. E., and Meagher, K. L., 1972b, Catalog of 

earthquakes along the San Andreas fault system in central 

California, January -March, 1972: U.S. Geol. Survey open-file 

rept. 60 p. 

Wesson, R. L., and Ellsworth, W. L., 1973, Seismicity preceding moderate earthquakes in California: Jour. Geophys. Research, v. 78, 

p. 8527-8546. 

Wesson, R. L., Lester, F. W., and Meagher, K. L., 1974, Catalog of 

earthquakes along the San Andreas fault system in central 

California, October-December 1972: U.S. Geol. Survey open-file 

rept., 46 p. 

Wesson, R. L., Meagher, K. L., and Lester, F. W., 1973a, Catalog of 

earthquakes along the San Andreas fault system in central 

California, July-September, 1972: U.S. Geol. Survey open-file 

rept., 49 p. 

Wesson, R. L., Roller, J. C., and Lee, W. H. K., 1973b, Time-term 

analysis and geological interpretation of seismic travel-time data 

from the Coast Ranges of central California: Seismol. Soc. 

America Bull., v. 63, p. 1447-1471. 

Whitney, J. D., 1865, Geology of California, vol. I, part I. Geology of 

the Coast Ranges: California Geol. Survey, p. 1-197. 

Williams, P. L., 1972, Map showing landslides and areas of potential 

landsliding in the Salina quadrangle, Utah: U.S. Geol. Survey 

Misc. Geol. Inv. Map 1-591-L, scale 1:250,000. 

Wood, H. 0., 1908, Distribution of apparent intensity in San Francisco, in The California earthquake of April18, 1906, Report of the 

State Earthquake Investigation Commission: Carnegie Inst. 

Washington Pub. 87, p. 220-245. 

Wright, R. H., 1971, Map showing locations of samples dated by 

radiocarbon methods in the San Francisco Bay region: U.S. Geol. 

Survey Misc. Field Studies Map MF-317, scale 1:500,000. 

Wright, R. H., Campbell, R. H., and Nilsen, T. H., 1974, The preparation and use of isopleth maps oflandslide deposits: Geology, Oct. 

1974, p. 483-485. 

Wright, R. H., and Nilsen, T. H., 1974, Isopleth map of landslide 

deposits, southern San Francisco Bay region, California: U.S. 

Geol. Survey Misc. Field Studies Map MF-550, scale 1:125,000. 

Yerkes, R. F., Bonilla, M. G., Youd, T. L., and Sims, J. D., 1974, 

Geologic environment of the Van Norman reservoirs area, northern San Fernando Valley, California: U.S. Geol. Survey Circ., 

691, pt. 1., 35 p. 

Youd, T. L., 1973, Liquefaction, flow and associated ground failure: 

U.S. Geol. Survey Circ. 688, 12 p. 

Ziony, J. 1., Wentworth, C. M., and Buchanan, J. M., 1973, Recency of 

faulting; A widely applicable criterion for assessing the activity of 

faults: World Conference on Earthquake Engineering, 5th, Rome, 

Italy, Proc., p. 1680-1683. 

l:} U.S. GOVERNMENT PRINTING OFFICE: 1975-0-690-036/7