GEOLOGIC PRINCIPLE§ 

FOR 

PRUDENT LAND USE 

A DecisionmakeJr,? s Guide 

for the San Francisco Bay Region 

RoBERT Do BROWN7 JRo 7 

and 

W ][LL][AM J o KocKELMAN 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 946 

WASHINGTON • UNITED STATES GOVERNMENT PRINTING OFFICE • 1983 
UNITED STATES DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing 1983 

Second printing 1985 

Third printing 1990 

Library of Congress Catalog in Publication Data 

Brown, Robert David, 1925-

Geologic principles for prudent land use: A decisionmaker's guide for the San Francisco Bay region. 

(Geologic Survey professional paper 946) 

Bibliography: p. 94-97 

Supt. of Doc. no.: I 19.16:946 

1. Geology-California-San Francisco Bay region. I. Kockelman, William J. II. Title III. Series: United States. Geological 

Survey. Professional Paper 946. 

QE90.S12B76 1983 333.7'09794'6 83-600285 

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

Washington, D.C. 20402 
CONTENTS 

Page 

THE NEED .......................................................................................... 2 

A PERSPECTIVE ................................................................................. 3 

ACKNOWLEDGMENTS .................................................................... 5 

THE PROVINCES OF LAND-USE CONCERN ................................ 6 

The estuary . . .. . . .. . . .. . . .. . .. . .. . . .. . . .. . . .. . .. . . .. . .. . . . .. . .. .. . .. .. . .. . .. .. . .. .. .. . .. .. . .. 7 

The coastal strip .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 14 

The lowlands.............................................................................. 22 

The hillsides and uplands .... .......... .... .... .... .. .. ...... .. ...... .. .. ........ 30 

FAULTS AND EARTHQUAKES ..................................................... 40 

THE USE OF EARTH-SCIENCE INFORMATION........................ 56 

1. An entire State ...... .. .... .... .... .. ...... .. ........ ...... .. .. ...... .. ............. 58 

2. A nine-county metropolitan area .. .... .... .. .. .. .. ...... .. .. .... .. ..... 63 

3. A bay-and-shoreline district ................................................ 68 

4. A 1,312-mi2 county .............................................................. 71 

5. A city of 55,000 people ....................................................... 78 

6. Individual lots and acreages offered for sale .... .. .. .... .. .. ... 84 

CONCLUDING COMMENTS ..................... .................................... 92 

REFERENCES CITED ....................................................................... 94 

Ill 
ESTUARY 

LOWLANDS 
FAULTS AND EARTHQUAKES 

T his report is written for those who decide how land is to be used in 

the San Francisco Bay region: Elected and appointed officials in State 

and local government, corporate executives, and those individuals 

who own or develop land. Its purpose is to remind these people of the intimate relation between geology and land use, and to inform them so that they 

are better equipped to ask the right questions, to address such questions to 

those best qualified to answer, and to demand responsive answers that are 

based on valid technical or scientific evidence. To achieve this purpose, we 

describe some common problems that arise in converting the land in the bay 

region to more intensive use and then review several major issues that illustrate how knowledge of the Earth sciences has contributed to improving 

land-use decisions. Furthermore, our viewpoint is that the experience gained 

in the bay region can serve as a basis, with appropriate changes, for the acquisition of knowledge and the deployment of strategies of land use elsewhere. 
2 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

THE NEED 

All of us have cause to desire wise use of 

the land, for all of us pay the price of misuse in 

higher taxes, increased cost of goods and services, and loss of environmental quality. Mismanagement of land or water resources may 

even endanger our health or threaten our lives. 

For these reasons, major decisions on land use 

are regularly reviewed and approved by public 

officials-mayors, city councilpersons, or county 

supervisors-who are as vital in the decisionmaking process as are the landowners, developers, bankers, and corporate executives who propose, finance, or implement the desired changes. 

Interests and motives may differ, but both public 

and private decisionmakers benefit from good 

decisions and suffer from bad ones. 

The distinction between good and bad landuse decisions often depends on how well the decisionmaker understands the land itself-its surface form and drainage, the physical properties 

of its rock and soil, and the geologic processes 

that shape it. Where Nature is neglected, it may 

become a formidable opponent, and most communities bear the scars of such past mistakes in 

the form of housing tracts that are periodically 

flooded; roads, sewers, or waterlines that are cut 

or endangered by landslides; or ponds of 

effluent that seep from poorly sited septic-tank 

drainfields. The list is long and varied, but these 

and other geologically related difficulties can all 

be avoided. 

To alleviate such problems, yet provide for 

growth and change, decisionmakers need not 

necessarily be scientists or engineers; they do, 

however, require some knowledge of rock, 

soil, and geologic processes. A few of them, 

chiefly ranchers and farmers, gain this knowledge chiefly from folklore or from daily observation. Many, however, live in cities, where 

pavement and buildings obscure the natural 

processes and materials; others are new residents, still unaccustomed to the natural setting 

and unfamiliar with the local environmental 

lessons to be learned. For these people, much 

of the information needed to anticipate and 

mitigate geologic hazards, though available, is 

scattered and difficult to interpret. Gathered 

by geologists, hydrologists, and seismologists, 

it is published in scientific journals, consultants' reports, and publications of the Federal 

and State geological surveys. Paradoxically, 

the very richness and diversity of this data 

bank lessen its value to nonscientists. The 

number of accessible sources and their specialized subject matter complicate their application to most decisions, even routine ones. The 

scientist's cautious language--or, worse, his 

jargon-may obscure significant findings .and 

permit sharply divergent views, especially 

over controversial issues. 

This report is a simplified presentation, 

discussing the nature of land-use problems in 

the bay region, showing how these problems can 

be identified and evaluated, and, finally, describing some of the methods used to ensure 

careful appraisal of land-use options. Its brevity 

and limited scope sacrifice many technical details; for example, it omits all but fleeting mention of ground water, of construction materials, 

and of saltwater intrusion of freshwater aquifers. 

The reader may also note other omissions, many 

of which, however, are considered in the publications cited in the text and listed at the end of 

this report. These references document more 

thoroughly the facts, scientific inferences, and 

experience that we summarize here, and include 

many typical examples that show how knowledge of the Earth sciences has solved land-use 

problems. Indeed, one purpose of the report is to 

inform decisionmakers that these published reports and maps are available, that they contain 

much useful information which is specifically related to everyday issues, and that they can be 

successfully used to improve land-use decisions. 

In this regard, the report might be considered a 

signpost, directing its readers toward appropriate sources of information and tested methods of 

evaluating the potential of the land in the bay 

region to support new uses. 

Many of the published maps and reports 

cited here were prepared between 1971 and 

1976 and were issued by the U.S. Geological 

Survey as part of the San Francisco Bay Region 

Environment and Resources Planning Study. 

Funded jointly by the Survey and the U.S. Department of Housing and Urban Development, 

this study produced more than 100 maps andre-
A PERSPECTIVE 3 

ports in which the geology, hydrology, 

geochemistry, and geophysics of the San Francisco Bay region were related to regional, 

county, and local land-use issues. Much of this 

information now helps to shape the plans and 

programs of both government and private enterprise. We describe a few of these applications in 

the final section of this report and briefly mention others elsewhere in the text. Our examples 

show that although Earth-science information is 

widely used, its effective application is still far 

from uniform, and some bay-region decisionmakers remain less well informed than others 

about how Nature restricts our use of the land. 

A PERSPECTIVE 

The nine counties that border the San 

Francisco Bay region make up a 7 ,400-squaremile (mi2

) area containing 93 cities and 5 million 

people. More than half these people have arrived since 1950. The annual rate of population 

increase in the bay region, which reached a 

maximum of about 41/2 percent between 1940 

and 1950, is now about 1 percent. This growth, 

however, has been far from uniform. Some 

urban centers, such as San Francisco, have lost 

population since 1950, and some rural communities show little change, but many cities and 

some counties have sustained decade-long 

growth rates that are more than twice the regional average. The counties that border the 

southern San Francisco Bay (fig. 1) have supported most of this past growth and now support 

most of the present population. 

The inflow of newcomers and the migration 

of city dwellers to the suburbs have steadily expanded the demand for housing and public services. This demand has been met by new construction in existing towns and suburbs and by 

extensive development of nearby rural areas. 

Most of the established communities were originally located on gently sloping well-drained alluvial plains, where site conditions presented 

few major problems. As development spread 

outward from these centers, however, the builders discovered different terrain with more complex and more difficult site conditions. As 

ridgecrests, hillsides, and marshes were converted to residential and commercial uses, an 

array of costly and unfamiliar problems harassed 

builders, homeowners, and public ojficials alike. 

Floods, drainage problems, and settling 

foundations beset some of the developments located near the bay and in stream valleys. Hillside projects were mostly free of these problems 

but faced others-among them landslides, failures in cut slopes and manmade fills, structural 

damage due to clays that swell when they are 

wet, and unforeseen difficulties in grading or excavating. Coastal residents lost roads, lots, and 

structures to the sea. In some places, coastal erosion accelerated when local harbor improvements altered wave patterns and sediment supply along miles of the coastline. 

Vl 

'Z 

0 

:::; 

_, 

~ 

?; 

z 

0 

i= 

4 

3 

<( 

_, 2 

:::> 

0.. 

0 

0.. 

0 

1940 1950 1960 1970 

YEAR 

Santa Clara 

Alameda 

San Francisco 

Contra Costa 

San Mateo 

Sonoma 

Marin 

Solano 

Napa 

1978 

FIGURE !.-Population growth, by county, in the San Francisco Bay region, 1940-78. 
4 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

These problems are visible, troublesome, 

and costly; yet all of them together are overshadowed by the threat of another major earthquake. Since the great San Francisco earthquake of 1906, more than 75 years of relative 

quiet has blunted our awareness of earthquake 

hazards. Housing tracts, schools, and hospitals 

now stand on or near the three largest faults that 

have ripped the landscape apart five times in the 

past 150 years. Along these same faults, dams 

and reservoirs that date from the turn of the century loom over bedroom communities in the lo 

flood plains below. In the cities, many tall buildings and other major structures were built to 

meet codes that we now know seriously underestimated the intensity of earthquake shaking; 

some buildings predate even the earliest earthquake codes. 

Many of the problems that arise during land 

use and development can be foreseen and either 

remedied or avoided. In the past, some probably 

were foreseen but were discounted because they 

seemed to be acceptable risks when weighed 

against short-term economic gains. Others were 

unrecognized or seriously underestimated because so few people understood how geology 

and hydrology can be used to predict the consequences of land-use decisions. Still others actually were not foreseen as problems but were perceived as such only when new research results 

broadened our knowledge. 

Many of the problems that stem from 

geologic causes are serious, indeed, and if they 

are ignored or unattended, they may increase in 

scope and severity. For example, a single small 

landslide on a %- to %-acre plot of developed 

land threatens not only the buildings, roads, and 

utility lines on the landslide itself but also those 

adjacent to it, as well as on any downstream 

property that receives the excess sediment or 

runoff derived from the slide area. Such landslides can easily cost many hundreds of 

thousands of dollars, even where only one residential lot is directly affected. Although the loss 

of the dwelling and lot is usually borne by the 

homeowner, most of the total costs are borne by 

the community and thus, ultimately, by the taxpayer. These public costs include: Cleanup work 

to remove and dispose of debris, restoration of 

roads and essential services, investigation of the 

soil and geology at the site to determine the 

cause of sliding, and measures taken to stabilize 

the slide. Some public costs may also result from 

the loss of tax revenues. Together, public costs 

may well amount to 5 to 10 times the preslide 

value of the lot and dwelling. 

By contrast, geologic studies to identify potential slide areas before and during site preparation cost little and can be part of a comprehensive geologic evaluation. For multiunit 

developments, the cost of geologic site investigations, when passed on to the individual home 

purchaser, is likely to be less than $300 per lot 

and rarely amounts to more than a negligible 

percentage of the total purchase price of the 

house and lot. 

Despite the numerous geologic hazards in 

the bay region, those that are most likely to 

occur in any one place are comparatively few 

and rather easy to identify. This localization permits us to define, for the San Francisco Bay region, four provinces, each of which presents a 

typical set of problems for those concerned with 

land development: (1) The estuary, (2) the coastal strip, (3) the lowlands, and (4) the hillsides 

and uplands. As listed in table 1, the last two 

provinces together make up more than 90 percent of the total land area in the San Francisco 

Bay region. 

TABLE i.-Distribution of land in the San Francisco Bay 

region, by province 

Province 

Estuary ......................................... . 

Coastal strip ................................ . 

Lowlands ......... ........................... .. 

Hillsides and uplands ................ . 

240 

175 

2,260 

4,293 

Total.... ................................ .. . 6,968 

Percentage 

of total 

area 

3 

3 

32 

62 

100 
ACKNOWLEDGMENTS 5 

Although most of our discussion concerns 

these four natural provinces, we also include a 

fifth topic that is common to the entire bay region-faults and earthquakes. Whereas some 

earthquake hazards are confined chiefly to specific provinces, others can occur in all of these 

provinces; in addition, the nature and severity of 

each such hazard can commonly be determined 

on the basis of its presence or absence in a particular province. 

After identifying the problems within each 

province, we summarize the geologic relations 

and processes that characterize each province 

and then suggest actions that may be taken to 

assure that new development is both cost effective and consistent with acceptable standards of 

public health and safety. Some of these problems are so common or so serious that State and 

local governments have already adopted formal 

measures to minimize losses and reduce hazards. Several of these measures are discussed 

briefly in the last section entitled "Examples of 

the Use of Earth-Science Information." 

ACKNOWLEDGMENTS 

We gratefully acknowledge the many helpful comments of those who reviewed the manuscript: Victor Calvo, former California Assemblyman, 21st District; Nancy Robertson and Jon 

Silver, councilpersons, Town of Portola Valley, 

Calif.; Priscilla Grew, former director, California 

Department of Conservation; James Davis, 

California State Geologist; Eric Carruthurs, alternate member, San Francisco Bay Conservation and Development Commission; and G. D. 

Robinson, Robert Page, and Robert Schuster, 

geologists, and Andrew Spieker, hydrologist (all 

of the U.S. Geological Survey). 

Substantial contributions, exceeding the 

normal scope of a review, were made by six 

people who had special knowledge of, or responsibility for, the six examples discussed in 

the final section of this report: Earl Hart, senior 

geologist, California Division of Mines and Geology; Jeanne Perkins, regional planner III, Association of Bay Area Governments; George 

Reed, senior planner, San Francisco Bay Conservation and Development Commission; James 

Berkland, Santa Clara County Geologist; 

Charles Gyselbrecht, former Chief Building Inspector, Redwood City; and John Freidlington, 

associate counsel, California Association of Realtors. 

The concept for this report, and much of its 

scope and content, emerged from our many discussions with three people: James Balsley, 

former Assistant Director for Research, U.S. 

Geological Survey; Arthur Zeizel, former environmental scientist in the Office of the Assistant Secretary for Policy Development and Research, U.S. Department of Housing and Urban 

Development, and currently with the Office of 

Natural and Technological Hazards, Federal 

Emergency Management Agency; and Andrew 

Spieker, former Deputy Director of the San Francisco Bay Region Environment and Resources 

Planning Study. 
THE PROVINCES OF 

"LAND-USE CONCERN 
Natural marshland and sloughs near Palo Alto yacht harbor, San Mateo 

County. The island and the channel margin (upper left) retain the intricate 

natural drainage systems and the cordgrass and pickleweed vegetation of 

the extensive marshlands that once fringed the bay. Bay mud, which underlies the marshland and tideflats, is water saturated and low in compressive 

and shear strength; at many sites it tends to amplify and prolong earthquake shaking. Where it contains saturated fine sand, it may liquefy during 

strong shaking, to create gaping fractures and cause lateral spreading of the 

ground surface. Photograph by Norman Prime, U.S. Geological Survey. 

Reclaimed and natural marshlands along the South Fork of Gallinas 

Creek near Santa Venetia, Marin County. All of the valley floor is underlain 

by bay mud and was formerly marshland similar to that bordering the 

straight segment of the creek on the left. Though diked and drained, such 

sites are within a few feet of sea level and retain many ofthe natural characteristics of marshland, among which are near-surface ground water and susceptibility to tidal, stream, and local flooding. Successful development of reclaimed marshlands depends on recognizing and solving the problems that 

accompany construction on bay mud. Photograph by Norman Prime, U.S. 

Geological Survey. 

THE ESTUARY 

Open water, tidal marshlands, and saltponds now cover 611 mi2, or 

about 8 percent, of the total surface area of the San Francisco bay region. Because of its size and central location, the estuary acts as a regional air conditioner, cooling the surrounding land areas in summer and 

moderating the chill in winter. Its waters provide harbors, shipping lanes, 

and a habitat for fish and shellfish; its marshlands are a source of salt and 

peat, and a hospitable environment for wildlife and waterfowl. 

7 
8 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

THE PROBLEM 

The San Francisco Bay and its bordering 

marshlands (fig. 2) make up the estuarine system, in which freshwater from inland rivers 

mixes with seawater. The estuary tempers the 

climate, provides waterways, and supports a 

complex biologic system, but it can easily be degraded by pollution, by changes in the seasonal 

-

-

inflow of freshwater, or by filling, which reduces 

its size and impairs normal estuarine processes. 

The natural and reclaimed (historical) marshlands bordering the bay lie at or near sea level 

and are underlain by unconsolidated water-saturated organic mud. Thus, much bay-margin land 

is susceptible to flooding and saltwater corrosion, as well as to foundation failure or settlement due to liquefaction and strong ground motion during earthquakes. 

5 

I 

I 

5 

0 

I 

0 

I 

5 MILES 

I 

5 KILOMETERS 

N 

t 

FIGURE 2.-Part of the San Francisco Bay discussed in this report; eastern part of estuary 

extends beyond map area into inland valleys that are outside bay region. Shaded 

areas are underlain by bay mud and approximate original extent of saltmarsh (now 

greatly reduced). A, Alviso; G, Grizzly Bay; GG, Golden Gate; S, Suisun Bay. Adapted 

from Schlocker (1970). 
THE PROVINCES OF lAND-USE CONCERN 9 

GEOLOGY AND PROCESSES 

The estuary formed when melting ice from 

the last great glaciation caused a worldwide rise 

in sea level (Atwater and others, 1977; Atwater, 

1979). Toward the end of the glacial period, 

about 15,000 years ago, sea level off the Golden 

Gate was more than 300 feet (ft) lower than 

today. The shoreline then lay 20 to 30 miles (mi) 

off the present coast and a few miles to the west 

of the Farallon Islands. As sea level rose, the 

shoreline moved landward. It reached the Golden Gate about 10,000 years ago, and seawater 

then invaded the branching valleys that now 

form the San Francisco, San Pablo, and Suisun 

Bays. The rate of marine incursion slowed about 

6,000 years ago, but flooding of the estuary by 

seawater continued up to historical time. 

As it grew in area and depth, the estuary 

formed a settling basin for fine sediment carried 

into it by streams and rivers. This sediment, 

chiefly clay and silty clay, was first deposited 

along the course of the primitive estuary as bottom mud. Later, as sea level stabilized near its 

present position, salt-marsh deposits accumulated between the level of high and low tides. 

These deposits grew outward from the shore 

until a belt of marshland, several miles wide, 

fringed much of the prehistoric bay (Atwater and 

others, 1979). Over time, as much as 120 ft of 

mud accumulated in some places (Helley and 

others, 1979, p. 21). Because it is geologically 

very young and has never been deeply buried or 

deformed, this mud is still unconsolidated and 

saturated with water; much of it is rich in organic material, and locally it contains lenses or 

beds of well-sorted silt and sand, as well as beds 

of peat. 

Within the estuary, saltwater from the ocean 

mixes with freshwater from inland streams. Depending on salinity, tide, and streamflow, estuarine water may flow seaward or landward; 

surface water commonly moves in one directio{h 

and the denser, more saline bottom water in 

another (Conomos, 1979). Seawater enters the 

estuary at the Golden Gate, driven by tides that 

have a maximum range there of about 8 ft. 

(Maximum tidal ranges elsewhere in the bay are 

greater; for example, at Alviso the range is about 

14 ft.) Freshwater enters the bay from many 

streams, but by far the greatest volume comes 

from the Sacramento and San Joaquin Rivers, 

which enter the estuary together at its east end. 

Because saltwater is denser than freshwater, the 

water near the bottom of the bay is more saline 

than that near the surface. Thus, at any one time, 

the salinity of the water varies with both location 

and depth. 

Salinity also varies over time. Minor 

changes result from the ebb and flow of the tide 

or from mixing by winds; major seasonal 

changes in salinity follow changes in the discharge of freshwater from the Sacramento and 

San Joaquin Rivers (fig. 3). The maximum discharge of these two rivers depends on rainfall 

Surface and bottom currents Salinity gradient 

South bay 

Stage 1-Maximum Sacramento River discharge. 

Duration, approximately 3 months 

Stage 2-Decreasing Sacramento River discharge. 

Duration, approximately 3 months 

Stage 3-Low Sacramento River discharge. 

Duration, approximately 6 months 

FIGURE 3.-Diagrammatic model of salinity gradients and 

net surface and bottom currents through annual seasonal stages in the San Francisco Bay. Lengths of arrows represent relative rates of flow. Curved lines on 

right-hand figures are salinity contours: H, high salinity; L, low salinity (from McCulloch and others, 1970, 

p. A12). 
10 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

and snowmelt and comes during late winter or 

spring. It may be 5 to 7 times the minimum discharge, which commonly occurs in late summer 

or early fall. The annual inflow of freshwater reduces the salinity of estuarine water and alters 

the density layering within the estuary; this inflow is especially important in flushing the 

southern San Francisco Bay (McCulloch and 

others, 1970), which receives only a minor 

amount of freshwater from its own small drainage basin. 

ACTIONS TO ENHANCE LAND USE 

The estuary (fig. 2) is a unique and irreplaceable part of the San Francisco Bay region. 

However, this resource has not always been 

managed wisely. In the past 120 years, the area 

of open water has decreased from 4 76 to 423 mi2 , 

or by about 11 percent, and of the marshland 

from 313 to 125 mi2 , or by about 60 percent 

(Nichols and Wright, 1971, p. 6). These changes 

are mostly manmade; they result from landfill, 

disposal of dredge spoil, diking and reclamation 

of marshland, and accelerated rates of sedimentation due, for example, to hydraulic gold mining 

in the Sierra Nevada. Historically, the estuary 

has also served as a receptacle for sewage, industrial wastes, and other pollutants. As early as 

1912, residents of the bay region recognized that 

a decline in the productivity of bay fisheries was 

caused by human activities and that the estuary 

needed protection (Nichols, 1973, p. 1). 

Attempts to protect the estuary and to ensure consistent management of its resources 

were long hampered by the conflicting priorities 

and overlapping jurisdictions of the numerous 

counties, cities, and towns along its perimeter. 

Pollution and sewer discharges were gradually 

controlled and regulated under State and Federal law, but filling of the inlets and marshes remained a problem until 1969, when the San 

Francisco Bay Conservation and Development 

Commission (BCDC) was given permanent 

status by the California Legislature. This commission had been established as a temporary 

body in 1965 in response to citizen concern, and 

in its first 4 years it prepared the San Francisco 

Bay Plan, a management program based on detailed findings and explicit policies. The 27 commissioners represent the public (7), the State (5), 

the Federal Government (2), the counties around 

the bay (9), and selected cities (4). Under the law 

and the Bay Plan, the commission regulates filling, dredging, and other changes in existing 

land use in the San Francisco, San Pablo, and 

Suisun Bays, as well as in the sloughs that are 

part of the bay system. It has limited jurisdiction 

over land use within a 100-ft-wide strip inland 

from the bay and over any proposed filling of 

saltponds or managed wetlands. A 1977 law extended the commission's jurisdiction to protect 

the Suisun Marsh (San Francisco Bay Conservation and Development Commission, 1978). 

The estuary and its bordering tidal lands 

confront planners and decisionmakers with two 

difficult problems: How to use the marshlands 

safely and prudently, and how to protect the 

quality of water. Both of these problems are related, and neither can be dealt with in isolation; 

for convenience, however, we discuss them here 

separately. 
THE PROVINCES OF LAND-USE CONCERN 11 

MARSHLANDS 

Several characteristics of the marshlands 

make them difficult or costly to convert to intensive use. The marshland surface lies between 

high and low tide and is subject to periodic 

flooding, which can cause trouble when peak 

storm runoff from local streams coincides with 

high tides. This flooding can be partly controlled 

by dikes or landfills, but it cannot be completely 

eliminated because the land is flat, the dikes are 

·vulnerable to failure, and, in most places, 

ground-water levels are within a foot or so of the 

surface. Near-surface ground water also creates 

other problems: It makes drainage difficult, it requires special foundation designs for certain 

structures, and, where it is saline, it is corrosive. 

' Flow_./" t "'-.._FlOW./ 

Mud Compaction Mud 

Bay mud underlies existing, reclaimed, and 

filled marshlands and ranges in thickness from a 

few inches to many tens of feet. Because this 

mud is unconsolidated, rich in organic matter, 

and saturated with water, its bearing strength is 

so low that it fails under even modest load; in 

some places, a foundation pile will sink into bay 

mud without being driven. The high water content and low strength of the mud, in its natural 

state, make it susceptible to settlement-gradual 

downward movement of an engineered structure 

as the unconsolidated material below the foundation compacts under load (fig. 4). It is somewhat firmer when drained, but draining reduces 

its volume and causes subsidence. The surfaces 

of many drained bay-region marshlands have already sunk several feet below their original 

levels. Subsidence and consolidation are even 

B 

FIGURE 4.-Typical settlement of engineered structures placed on artificial fill over mud with a low shear strength. A, 

Uniform settlement over a homogeneous layer of mud. B, Differential settlement over mud containing a sand body 

with a relatively high shear strength. From Helley and others (1979, p. 50). 
12 GEOlOGIC PRINCIPlES FOR PRUDENT LAND USE 

greater where peat, which is common in some 

marshes, is drained and exposed to the air (fig. 

5). 

Earthquakes cause other problems in sites 

underlain by bay mud. These sites shake longer 

and harder than those on bedrock, and where 

sand or silt beds in the mud are are saturated, 

the sediment may liquefy and cause ground failure. These problems are discussed further in the 

section below entitled "Faults and Earthquakes" 

(see also Youd, 1973; Youd and Hoose, 1978; 

H~lley and others, 1979). 

Some of the difficulties presented by the 

marshlands can be overcome by special engineering practices if geologic investigations 

have determined the thickness of the mud, its 

properties, and whether it contains lenses or 

beds of liquefiable sediment. Besides diking and 

filling, which are commonly used to mitigate 

flood hazards, some marshland sites have been 

drained and compacted before and during filling. Such attempts to control subsidence have 

SEA 

LEVEL 

"' -1 

~ Q) -2 

E 

I- 0 

w::: 

w .... 

-3 

u. Q) 

> -4 

z c 

0 

(.) -5 

z 0 0 .... 

-~ -6 t-O 

<{~ 

>O -7 w > ....J..O 

w > -8 

a. 

.... -9 :::> 

~-10 

-11 

1922 1926 1930 1934 1938 1942 1946 

YEAR 

FIGURE 5.-Progressive subsidence of Mildred, Bacon, and 

Jones Islands in the Sacramento-San Joaquin delta, 

primarily owing to peat oxidation, between 1922 and 

1946. These islands, diked to protect them from flooding, are in eastern part of estuary (not shown in fig. 2). 

From Weir (1950) and Helley and others (1979, p. 49). 

succeeded in some places, but estimation of the 

total amount of subsidence and prediction of the 

rate at which it occurs are difficult. Many large 

or heavy structures on bay mud avoid these 

problems because they are built on pilings or 

piers that penetrate through the mud to bedrock. 

Investigation of the geology of a site, preparation of the land, design and construction of 

special foundations, and installation of utility 

services add significantly to the costs of development on bay mud (Helley and others, 1979, p. 

76-77, 82-83). Public costs are also high (Laird 

and others, 1979, p. 61-89) because roads, storm 

and sewer pipelines, and other public improvements require more maintenance and because 

the risk of loss from earthquake and flood damage is greater in the marshlands than elsewhere 

in the bay region. 
THE PROVINCES OF LAND-USE CONCERN 13 

BAY WATERS 

Some land-use practices profoundly affect 

the quality of the estuarine water: Either they 

pollute it, or they interfere with such natural estuarine processes as the exchange of freshwater 

and saltwater or the production of oxygen. 

The San Francisco Bay Plan (San Francisco 

Bay Conservation and Development Commission, 1969, p. 10) summarized the findings concerning water pollution as follows: 

a. San Francisco Bay receives a variety of municipal, industrial, and agricultural wastes from sources throughout 

its tributary drainage area. Pollution occurs when 

waste discharges cause water quality conditions that 

damage or destroy varied uses of the Bay. Such conditions can result from toxic (poisonous) substances, 

from residues that unduly stimulate organic growth in 

the Bay, and from sewage that consumes oxygen in 

the water as it disintegrates. Polluted waters may be 

unsafe for human contact or use, offensive to the 

senses, damaging or lethal to marine life, and even 

unsuitable for industrial use. 

b. Compared to rivers and estuaries in other parts of the 

country, San Francisco Bay is relatively unpolluted. 

In recent years, extensive improvements in the treatment of industrial and municipal wastes have greatly 

reduced the pollution that once existed in the Bay. 

But some parts, especially in the South Bay, are still 

polluted at certain times of the year. As long as the 

Bay continues to receive wastes from an expanding 

population and industry, there must be constant improvement in waste management to upgrade presently polluted areas and prevent pollution problems 

in the future . 

c. While waste disposal poses a continuing threat to water 

quality in the Bay, this use of Bay waters will continue 

for some time. Pollution of Bay waters from these 

wastes can be prevented by: (1) transporting wastes 

directly to the ocean (but without allowing waste discharges to damage the ocean's marine life); (2) prohibiting the discharge into the Bay of toxic wastes 

(poisons) that do not break down; (3) adequate treatment of wastes before discharge into the Bay; and (4) 

natural breakdown of any biodegradable wastes 

placed in the Bay, which can be encouraged by maintaining adequate flushing action and an adequate 

supply of dissolved oxygen in the Bay. 

The San Francisco Bay Plan (San Francisco 

Bay Conservation and Development Commission, 1969, p. 9-13, 27) also recognizes the importance of natural estuarine processes in maintaining the quality of the water in the estuary. 

Historically, these processes have been impaired 

by the loss of both estuarine volume and surface 

area; either loss modifies the circulation and 

mixing of freshwater and saltwater and causes 

significant and generally detrimental increases 

in salinity. Moreover, most of the lost area was 

marshlands and mudflats, where grasses and 

algae are important sources of oxygen; thus, the 

loss of these wetlands deprived the estuarine 

system not only of important biologic habitats 

but also of part of its natural oxygen supply. The 

laws and regulations that now control filling of 

the estuary are, in part, specifically aimed at 

maintaining its surface area and volume. 

Also important to a biologically healthy estuary is the inflow of sufficient volumes of freshwater from the Sacramento and San Joaquin 

Rivers (San Francisco Bay Conservation and Development Commission, 1969, p. 10, 12; McCulloch and others, 1970). Recent plans to divert 

part of this flow from the Sacramento River 

drainage system to southern California and the 

San Joaquin Valley by way of a peripheral canal 

around the east side of the Sacramento River 

delta raise the difficult question of how much 

water can be diverted without degrading the estuary. The benefits of such diversion would accrue chiefly to water users in the San Joaquin 

Valley and in southern California; the most evident adverse effects-diminished flow, loss of 

circulation, and increased salinity-will be felt 

downstream and in the estuary. Although the 

scientific evidence needed to answer this question is beyond the scope of this report, this issue 

illustrates the important point that land uses and 

resources in the bay region can be seriously and 

permanently affected by decisions that are made 

outside the region. 
Point Montara lighthouse, San Mateo County. Resistant granitic rocks in 

the surf zone defend the shoreline, but the sea erodes and gullies the 

marine deposits underlying the coastal terrac&-the nearly flat land surface 

checkered with tilled fields. As they slowly retreat landward, the bluffs along 

the shore will eventually deliver buildings, roads, and other structures to the 

sea. Coastal erosion here averages less than a foot per year; only a few 

miles to the south, however, near Half Moon Bay, the average rate of coastal 

erosion is as much as 6.5 feet per year. Photograph by Earl E. Brabb, U.S. 

Geological Survey. 

California Highway 1 at the Devils Slide, San Mateo County. Hillslopes 

above and below the highway are mantled with landslide debris or detached landslide masses. The most active part of the slope is the debriscovered chute near the center, but the brushy hillside above the highway on 

the right also exhibits landslide topography and one well-defined landslide 

scarp. Bedded dark-gray shale and sandstone, the underlying bedrock here, 

dip to the left in the cliffs beyond the slide; granitic rock beneath the shale 

and sandstone crops out in the rugged cliffs at extreme right. This part of 

Highway 1 is frequently closed by slides, and because storm waves continue to erode the base of the cliff every winter, the stability of the slope diminishes over time. Ultimately this segment of the coastal highway may 

have to be abandoned. Vestiges of the Ocean Shore Railroad on the left 

below the highway mark an early, unsuccessful attempt to link San Francisco with coastal towns to the south. Constructed between 1905 and 1908, 

the railroad was severely damaged by landslides during the 1906 San Francisco earthquake and was abandoned in 1921. Photograph by Earl E. Brabb, 

U.S. Geological Survey. 

THE COASTAL STRIP 

T he Pacific Ocean bounds the San Francisco Bay region from the 

mouth of the Gualala River on the north to Point Aiio Nuevo on the 

south. No well-defined feature marks the landward extent of the coastal strip, and so, for the purposes of this discussion, we define it as extending 

inland 1 mi from the coast. 

14 
THE PROVINCES OF LAND-USE CONCERN 15 

THE PROBLEM 

The coastline, constantly under attack by 

the sea, retreats inland over time. Under natural 

conditions, some coastal bluffs retreat at rates of 

a few feet per year. These rates may increase 

rapidly where man interferes with coastal processes; for example, at Half Moon Bay, construction of a breakwater resulted in a nearly tenfold 

jump in local erosion rates to 20 ft per year. Cliff 

retreat and coastal landslides adversely affect 

development and are major problems along 

those parts of the coast that are underlain by 

sheared or poorly consolidated sedimentary 

rocks. Seacliffs of resistant rock, such as the 

bluffs of granite or ancient lava flows that defend most of the headlands, are more stable and 

enduring. Furthermore, because of its proximity 

to the San Gregorio and San Andreas faults, all 

the coastal strip is subject to severe earthquake 

hazards, of which strong ground shaking and 

earthquake-induced ground failure are the most 

prevalent. 

GEOLOGY AND PROCESSES 

The shape and trend of the Pacific 

coastline (fig. 6) provide important clues for 

planning because they show how various parts 

of the coast respond to attack by the sea. The 

general northwesterly trend of the coastline is 

interrupted by prominent headlands, by broad 

and gently concave embayments, and by narrower curving bays. Each of these features has 

resulted from the pounding of the sea against 

rock masses that differ in their resistance to 

marine erosion. Granite, one of the most resistant rocks in the bay region, defends Bodega 

Head, Point Reyes, and Point Montara; volcanic 

rocks defend Pigeon Point. Between these headlands the long reentrants in the coastline, such 

as those between Fort Ross and Bodega Head 

and between Point Reyes and Point Montara, are 

carved into less resistant sedimentary rocks. 

Smaller embayments at Drakes Bay, Bolinas 

Bay, and Half Moon Bay have a more complex 

origin. Each of these bays occupies the south 

side of a headland, and each owes its shape and 

position to the processes exemplified at Half 

Moon Bay. There, waves driven by the prevailing northwesterly wind approach the coast from 

the northwest, but they slow and change course 

as they pass the beadland at Pillar Point and 

meet south- or southeast-trending rocky shoals 

on the leeward side. These refracted waves approach the north shore of the bay from the west 

or southwest, attack the easily eroded rocks behind the headland, and carve out an arcuate 

bay. 

Beach sand protects parts of the coastline 

from marine erosion. It accumulates in the beach 

and surf zones, moves to and fro with breaking 

waves and runoff from the beach, and consumes 

much of the energy directed toward the shore. 

Winter storm waves erode the beach and carry 

sand from it to the surf zone. Winter beaches are, 

therefore, narrower than summer beaches, but in 

winter the submerged sand deposits cause the 

waves to break farther out, so that they expend 

much of their energy in turbulence before they 

reach the shore. 

The daily and seasonal to-and-fro movement of beach sand is superimposed on a 

broader pattern-the slow drift of sand for many 

miles along the coast. South- or southeast-flowing longshore currents form as waves from the 

northwest strike the shoreline at an oblique 

angle. These currents gradually move the sand 

that is brought to the sea by rivers or is eroded 

from the seacliffs. This littoral drift, combined 

with the refraction of waves on the leeward side 

of headlands, accounts for many of the beaches 

and bars rimming such south-facing bays as 

Drakes Bay and Bolinas Bay. 
16 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

EXPLANATION 

D 

Estuarine mud 

-

Alluvium 

D 

Unconsolidated or weakly consolidated 

sediments 

Sedimentary rocks 

Intensely 

-

sheared sedimentary and ultramafic rocks 

Volcanic rocks 

Granitic rocks 

Fault- Dashed where 

approximately located 

0 10 20 30MILES 

I I I I I 

0 10 20 30 40 KILOMETERS 

FIGURE 6.--Generalized geologic map of the San Francisco Bay region, showing prominent coastal landmarks. Nearly 

all irregularities in the coastline are related to the geology, but at this scale only large features and a few major 

rock types can be shown. Modified from Schlocker (1970). 
THE PROVINCES OF LAND-USE CONCERN 17 

ACTIONS TO ENHANCE LAND USE 

The coastal strip is scenic and relatively 

unspoiled. It contains much of the Point Reyes 

National Seashore and nearly a score of State 

and county parks and beaches. It is intensively 

developed only on the west side of the San Francisco peninsula, where the land is relatively flat; 

elsewhere, development has been slowed by the 

rugged topography, the distance from inland 

cities, and the relatively inhospitable climate. 

Fog shrouded and damp for much of the 

year, the coastal strip was long considered a less 

desirable place to live than the sunnier, drier 

areas east of the coastal mountains. Since about 

1950-60, however, more and more coastal land 

has been subdivided or converted to commercial 

and industrial use. This trend, which has taken 

place along almost the entire California coast, 

has raised statewide concern that a valuable agricultural, recreational, and esthetic resource 

might be threatened. 

Expressing this concern, California voters, in 

a 1972 referendum, mandated a comprehensive 

plan for the use of resources in the coastal zone. 

This plan was ratified by the State legislature as 

the California Coastal Act of 1976. Through 

mandatory implementation of its policies by 

local government, the act guarantees public access to the shore, insures that coastal developments serve the public as a whole, protects and 

restores coastal marshes, preserves coastal farmlands, and provides stringent environmental 

safeguards for coastal developments. The coastal 

plan is in part funded by the Federal Coastal 

Zone Management Act of 1972, and, like the San 

Francisco Bay Plan, it uses Earth-science knowledge and principles as a basis for decisions. 

Our description of the coastal strip stresses 

chiefly those features and processes that are 

unique to the shoreline environment. We emphasize them because the shoreline changes so 

rapidly and because the forces of changeespecially marine processes-are so great that 

they defy our attempts to control them. These 

forces are concentrated at the shore, and their 

effects diminish inland. The multitude of natural 

processes along the coastal strip, and the force 

and intensity of some of these processes, make 

decisions on coastal land use as challenging and 

as difficult as those in any other province of the 

bay region. 

LANDSLIDING 

Regardless of its local character, the coastal 

strip is geologically young, a result of the same 

postglacial rise in sea level that formed the San 

Francisco Bay. Thus, many coastal landforms are 

still being shaped by the sea, and some seacliffs 

are retreating at average rates of more than a 

foot per year (fig. 7). Past generations of planners, engineers, and developers, accustomed to 

the more moderate pace ofinland geologic processes, seriously underestimated .the erosive 

power of the sea. Many of the highways, streets, 

railroads, and private dwellings built along the 

coast in past years have been partly or wholly 

destroyed. For example, State Highway 1, the 

coastal route, crosses many landslides caused by 

marine erosion. Parts of this highway south of 

Fort Ross and at Devils Slide on the San Fran-

. cisco peninsula require almost constant maintenance; at Thornton Beach, in northern San 

Mateo County, the original right-of-way has 

been abandoned, and the highway relocated. 

Large landslides abound along exposed, 

precipitous stretches of coast, especially where 

the seacliffs have been cut into unconsolidated 

sediment or bedded, jointed, or sheared 

sedimentary rocks. Landslides here resemble 

those on the hillsides, but because they are 

under continuous attack by waves and surf, 

many coastal landslides are more unstable than 

their inland counterparts. 

Identifying landslides is critical in coastal 

planning. Some large active slides are so obvious that they are easily recognized in the field; 

less obvious or inactive ones may be shown on 

published geologic maps. The stability of seacliffs can be evaluated either by site investigations or by study of detailed geologic maps that 

show the distribution of rock types and the 

orientation of potential failure surfaces, such as 

bedding, joints, or shears. These active and potentially active coastal landslides are far more 

difficult to stabilize than their inland counterparts, chiefly because they are triggered by 
18 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

\ 37° 30' 

', 

\ 

': 

\ 

,. 

I ·I 

78 2 MILES 

I 

;,~ KILOMETERS 

AVERAGE RATE OF EROSION 

~ Greater than 1 foot (0.3 m) per year 

Rate, in feet per year, at locality 

5.0- indicated by arrow 

FIGURE 7.-How the rate of marine erosion during historical time can be determined by comparing the present coastline 

with its former position as shown on old maps and photographs. These rates depend on the resistance of the seacliffs and on the orientation of the shoreline with respect to the direction of prevailing swells. Modified from Atwater (1978, p. 15). 

marine processes that are difficult or impossible 

to control. Moreover, where coastal landslides 

are part of a general, long-term retreat of the 

coast, stabilization is, at best, a temporary solution; in such places, the only practical way to reduce the hazard from landslides is to avoid 

building on or near them. 

SEACLIFF RETREAT 

Some massive, homogeneous rocks successfully resist large-scale failure by sliding but fail 

piecemeal as waves and rain wash them away 

grain by grain or remove small fragments of 

rock. The relatively steady retreat of seacliffs 

eroded in this way is less spectacular than a 

large landslide, but the long-term rate of retreat 

may be comparable. For example, at Moss Beach 

in northern San Mateo County, historical records 

and photographs covering the past 105 years 

document 165 feet of seacliff retreat (Tinsley, 

1972, p . 63)-an average of 11

/2 ft per year. Both 

landsliding and gradual erosion contribute to 

this retreat, the rate of which has increased to 

more than 3ft per year since 1965 (Leighton and 

Associates, 1971). 

Partly as a result of experience at Moss 

Beach, San Mateo County now regulates blufftop development in this and other coastal areas. 

Depending on the erosion resistance of the rnaterials and the historical rate of erosion, the 

county may prohibit building, require that stability be demonstrated by a geotechnical study, or 

allow use only after a standard geotechnical report has been prepared (fig. 8). 
THE PROVINCES OF LAND-USE CONCERN 

122°30' 

\ 

37° 30' 

_ 2 KILOMETERS 

EXPLANATION OF SETBACKS 

EX -structures excluded 

DS -demonstrate stability in 

geotechnical reports 

N R -normal geotechnical reports 

for proposed land use 

19 

FIGURE 8.-How San Mateo County regulates coastal development according to the stability of seacliffs, from least 

stable and most closely regulated (A) to most stable and least development constrained (q. Stability of seacliffs 

can be interpreted from historical data (see fig. 7). from resistance of cliff materials to erosion, and from degree 

of natural protection from waves. Modified from Atwater (1978, p. 15). 
20 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

BEACH EROSION 

Beaches, like seacliffs, are easily eroded by 

the sea. Many beaches temporarily change their 

shape and size with the seasons, but they may 

change permanently if the natural equilibrium 

between sea and shore is disturbed. Manmade 

structures that reduce the supply of sand or 

obstruct its movement along the shore disturb 

this balance and may destroy the beaches. Both 

sect'Walls and inland dams reduce the supply of 

sand-seawalls by blocking the erosion of sand 

from seacliffs, and dams by preventing stream 

sediment from reaching the ocean. Jetties and 

breakwaters interfere with the longshore movement of sand (fig. 9) and may also divert wave 

energy toward vulnerable seacliffs or beaches. 

Natural sand transport 

Waves striking the shore obliquely 

cause beach sand to move. pa -

ralle l to the shore 

Jetty-obstructed sand transport 

Sand accumulates on one side of the jetty, starving 

the beach on the other side, and resulting erosion 

threatens the house 

FIGURE 9.-Manmade obstructions, which interfere with 

natural marine processes, can cause permanent 

changes in beaches, seacliffs, and bottom conditions. 

From Atwater (1978, p. 14). 

Tinsley (1979, p. 49-50) described how construction of a breakwater at Pillar Point in northern San Mateo County caused drastic and rapid 

changes in the shoreline. The breakwater, on the 

north side of Half Moon Bay (fig. 7), was built 

between 1959 and 1961 to shelter the harbor 

from the southwesterly swells that accompany 

some winter storms. During its construction, the 

nearby shoreline began to erode rapidly, and 

erosion rates, which formerly had ranged from 2 

to 3 ft per year, increased to 20 ft per year between 1959 and 1964. The most rapid increase 

was within a few hundred feet southeast of the 

east end of the breakwater, but, in all, a strip of 

land nearly a mile long and about a 100 ft wide 

was lost to the sea. 

Recent surveys show that this segment of 

the coast is still being eroded more rapidly than 

it was before 1959. Tinsley (1979) attributed the 

accelerated erosion to the impact of the breakwater on the following marine processes: 

1. The wave energy that formerly was refracted 

into the north end of Half Moon Bay and 

dissipated there, now is reflected off the 

breakwater and is directed toward that part 

of the shoreline where erosion rates are the 

highest. 

2. The southward longshore movement of sand 

by littoral currents has been stopped by the 

breakwater; the beach is deprived of its 

northern source of sand and is replenished 

only by sand eroded from the seacliff. 

3. Because no protective beach shields the cliff 

from the waves, erosion of unconsolidated 

sediment in the seacliff is rapid. 

Although the erosion caused by construction of 

the breakwater can be checked by engineering 

techniques, these techniques, too, will alter 

marine processes and introduce new imbalances 

into the shoreline regime. 
THE PROVINCES OF LAND-USE CONCERN 21 

OTHER PROCESSES 

Although erosion and deposition may be the 

most evident coastal processes, for good planning other processes must also be recognized 

and provided for. For example, the strength and 

direction of ocean currents limit the choice of 

sites for offshore disposal of solid and liquid 

wastes and dredge spoils; they also limit the 

choice of areas for those tanker operations that 

may cause offshore oilspills. Moreover, because 

surface currents, driven by the wind, commonly 

flow in different directions from those on the bottom (Conomos and others, 1970, 1971), disposal 

sites that are acceptable for wastes which sink to 

the bottom may be unacceptable for wastes that 

float or remain suspended in the water. 

Coastal planners must also recognize and 

provide for earthquake hazards. Except for the 

outer part of the Point Reyes peninsula, all of the 

bay-region shoreline is less than 5 mi from 

known active faults-the San Andreas fault in 

Sonoma, Marin, and northern San Mateo Counties; and the San Gregorio fault in southern and 

central San Mateo County. Both these faults 

have caused large, damaging earthquakes in 

the past, and both are capable of doing so 

again. Because of its proximity to these faults, 

the coastal strip is especially vulnerable to 

strong ground motion, and because so much of 

it is fringed with active or potentially active 

landslides, it also is vulnerable to earthquaketriggered landslides. These hazards, which are 

common to all the provinces of the bay region, 

are discussed more fully below in the section 

entitled 11 Faults and Earthquakes. 11 
San Francisco, noted for its hills, grew chiefly on lowlands of gravel, 

sand, and silt deposited by the sea, the bay, and local streams. In this view, 

the westerly sun reflects brightly from buildings north of Market Street. 

Building facades south of Market are shaded and dark because of the different orientation of the streets there. The industrial area between Potrero 

Point (extreme left) and China Basin (extreme right) rests on reclaimed 

marshlands underlain by bay mud and on artificial fill. The more extensive 

and more distant lowland development, especially north of Market, is on 

beach sand, dune sand, or alluvium deposited from streams. The most evident bedrock areas are both north of Market Street: Russian Hill and Nob 

Hill on the right and Pacific Heights in the center, beneath the centerspan of 

the Golden Gate Bridge. The varied geology beneath the city demands special care in investigating and designing foundations for building, but it also 

permits us to forecast where earthquake damage will be most intense and 

the nature of the processes causing that damage. Photograph by Norman 

Prime, U.S. Geological Survey. 

THE LOWLANDS 

K n this report, we consider the lowlands to be the terrain that is less than 

1,000 ft above mean sea level, that has slopes of no more than 15 percent 

(15 ft of elevation change in 100 horizontal feet), and that is above the 

level of tidal effects. Most of this land is in stream valleys or in alluvial slopes 

near the bay; some is in the marine terraces of coastal San Mateo and Marin 

Counties. 

22 
THE PROVINCES OF LAND-USE CONCERN 23 

THE PROBLEM 

Although the lowlands occupy only about 

a third of the land area in the San Francisco 

Bay region, they are by far the most intensely 

developed and most heavily populated province. Underlain chiefly by compacted, porous, 

and permeable deposits of gravel, sand, and 

silt, they provide level building sites, good 

drainage, and stable foundations. Some lowland areas periodically flood, but these areas 

are easily identified, and many different measures can be taken to reduce flood losses. Because lowland sites are valued for agriculture, 

housing, construction materials (for example, 

sand and gravel), and waste-disposal facilities, 

the competition for them is keen, and, in some 

places, such essential resources as construction 

materials must be protected. for future use. 

GEOLOGY AND PROCE,SSES 

The smooth, nearly level surface of the 

lowlands and the unconsolidated gravel, sand, 

and silt beneath this surface result from the deposition of sediment by running water. Most 

streams begin in the hills, follow steep gradients, 

and flow rapidly until they emerge from the hills 

EXPLANATION 

Stream deposits 

1@:~-£1 Sand and silt 

onto inland valleys or alluvial plains. They erode 

and carry sediment derived from hill slopes, but 

their erosive and carrying capacities diminish in 

the lowlands, where stream gradients flatten and 

stream velocities decrease (fig. 10). These 

changes in erosive and carrying capacities are 

recorded by the composition of the sediment deposited along the stream course. Where streams 

emerge from the hills, they construct alluvial 

fans-sloping fan-shaped, deposits that contain 

lenses of poorly sorted gravel and coarse sand. 

Downstream, where streamflow further slackens, finer, better sorted, and more regularly bedded deposits of gravel, sand, and silt blanket valley floors and lowland plains. As outlined on a 

geologic map, these deposits of gravel, sand, 

and silt show where stream deposition is the 

dominant geologic process. Some of these deposits in the San Francisco Bay region are hundreds of feet thick and represent tens of 

thousands of years of stream deposition. From 

such evidence, we can conclude that stream deposition and flooding are established natural 

processes, and that they will continue to affect 

our use of the lowlands. 

Stream flooding, deposition of sediment, 

and erosion are common and related geologic 

processes in the lowlands. Sediment is deposited, and banks and channels are eroded, during normal streamflow, but rates of deposition 

FIGURE 10.-The lowlands are underlain by deposits of stream sediment carried 

from the hills and deposited where stream gradients flatten and stream velocities decrease. Depositional processes predominate in the lowlands, and 

erosional processes in the hills. 
24 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

and erosion increase greatly during floods. Thus, 

for convenience, all three processes are here discussed together. Other, sporadic lowland processes, including earthquake shaking, faulting, and 

liquefaction, are discussed elsewhere in this report. 

Stream flooding is probably the most common and one of the most costly geologic processes in the lowlands. Its relative impact on various 

kinds of land use was shown (table 2) in a landcapability study, prepared by Laird and others 

(1979), to evaluate the costs of converting land to 

new, more intensive uses. The U.S. Water Resources Council (1978) estimated that annual 

flood damages in the California region-California plus Klamath County, Oregon-amount to 

$417 million and that $269 million, or 62 percent 

of this total, is in urban regions. The amount of 

flood losses has steadily increased as a result of 

increases in property values, in the size of floods, 

and in the number of buildings and other structures on flood-plain lands (Waananen and 

others, 1977, p. 6). 

Most major floods in the San Francisco Bay 

region affect only parts of the region; they tend 

to be localized both by the effects of topography 

and by storm tracks. Floods occur during the wet 

season, generally from November through April; 

and because prolonged rainfall and saturatedsoil conditions increase the possibility of flooding, they are most likely later in the season. 

Flooding results when streamflow exceeds the 

capacity of the stream channel. Because flooding 

TABLE 2.-Expected costs associated with stream flooding 

[Amounts shown are future costs due to flooding, discounted to their value 

in 1975 dollars. From Laird and others (1979, p. 39)) 

Land use 

Rural or agricultural ........................................... . 

Semirural residential .......................................... . 

Single-family residential .................................... . 

Multifamily residential ....................................... . 

Regional shopping centers ................................ . 

Downtown commercial ...................................... . 

Industrial ............................................................. . 

Expected cost 

per acre 

$ 200 

700 

9,000 

40,000 

40,000 

50,000 

40,000 

recurs so frequently, the lowland valleys are 

floored with the deposits of innumerable earlier 

floods. Where the surface of the land is relatively 

undisturbed, flood-prone areas can be recognized by a well-defined natural flood plain (fig. 

11), by natural levees along streambanks, by alluvial fans at the mouths of major canyons, or by 

the distinctive soil types that are associated with 

flood plains. 

Although these features may identify a potential flood hazard to an alert observer, accurate mapping of flood-prone areas depends on 

knowledge of the height and extent of past 

floods and of the normal patterns of streamflow. 

Detailed records of peak discharge (the highest 

rate of streamflow at a given point, measured in 

cubic feet per second) are used to estimate how 

FIGURE 11.-Many flood-prone areas are easily recognized because they coincide with 

a well-defined natural flood plain. 
THE PROVINCES OF LAND-USE CONCERN 25 

2090 60 

59 

2000 

0 0 

z z 50 0 .8 1740 49 (.) 

·W ww W(f) 

C)(/) 

II II 1500 ~II <(w 40 <( w I a.. I a.. 

()I- ()(f) 

~w (f) II -w ow 01- u. ~ 30~ w <( ~ 1000 <(~ 

wm wu a..:> a.._ CD (.) ::::> 

~ 20 (.) 

500 ~ 

5 20 30 50 

RECURRENCE INTERVAL, IN YEARS 

FIGURE 12.-Floodl-frequency curve for a hypothetical 

basin in the San. Francisco Bay region (from Waananen 

and others, 1977, p. 9). 

often major floods are likely to recur (fig. 12) and 

how large an area will be inundated. From such 

estimates has come the useful concept of the 

100-year flood: This flood will, on the average, 

happen once every 100 years; or, in a given year, 

the odds are 1 in 100 that this large a flood will 

occur. The areal extent, depth of water, and 

other characteristics of the 1 00-year flood are 

commonly used in river-basin planning, for regulatory purposes, and in administering the National Flood Insurance Program. Despite its wide 

acceptance as a planning tool, however, it is not 

always an appropriate or safe concept. Even 

larger floods are possible, and so dams and other 

major structures are commonly designed to accommodate larger and less frequent flooding. 

Moreover, the extent of the 100-year flood is estimated from records of historical events; its 

value as a reliable predictive tool diminishes 

where runoff and discharge characteristics have 

been modified by recent changes in land use or 

land cover. 

ACTIONS ·ro ENHANCE LAND USE 

Early settlers in the bay region chose to live 

in the lowlands because of convenience and accessibility. The level or gently sloping surface 

and the low relief provided good building sites, 

good farmland, and direct, nearly level routes for 

roads. Streams and, later, shallow wells provided 

a supply of freshwater. Lowland areas ~ear the 

bay or along navigable streams provided access 

to transportation-an important added incentive 

for many enterprises. Thus, most early cities and 

towns were ports or agricultural centers, and 

from these early lowland communities, later 

growth and developmenthas spread. 

The premium value attached to lowland 

sites is greater now than ever. Together, the inland valleys and plains and the coastal terraces 

provide nearly all the prime agricultural soils in 

the region. The lowlands provide optimum sites 

for residences and for commercial and industrial 

facilities, provided that the land is outside flood 

plains and free from other stream-related problems, such as erosion and deposition. Extensive 

interior lowlands, such as the Santa Clara and 

Livermore Valleys, contain relatively permeable 

sand and gravel deposits with important 

supplies of ground water, some of which is developed for domestic and agricultural use. The 

same properties that permit ground water to accumulate in the rocks also permit the underground storage of other fluids, so that some 

stream deposits are favored sites for the storage 

of liquid wastes. And, finally, the sand and 

gravel beneath the lowlands are important 

sources of construction materials, critically 

needed both for new development and for renovation of older structures. 

Our society demands level, accessible land 

for many critical needs, of which housing, industry, airports, mass-transit routes, and railroads 

are but a few. The intense competition for lowland sites brings change and conflict, as is evident where farms, ranches, or orchards give way 

to suburban housing, or where a major change 

in land use adversely affects the value of nearby 

property. The intensity of competition and the 

opportunities for change and conflict are unlikely to subside. They call for prudent and informed decisions by government, business, and 

individual citizens. These decisions are complex, 

but many of them depend critically on knowledge of the geologic processes operating in the 

lowlands anq of the materials beneath the lowland surface. 
26 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

FLOODING 

Manmade changes in drainage basins profoundly alter flood characteristics. Logging, 

conversion of grass- or brush -covered land to 

agriculture, grading or paving for urban and 

suburban use, and similar actions all diminish 

the capacity of land to absorb and retain rainfall, increase the rate of runoff, and increase 

the probability and size of floods (fig. 13). Removal of vegetation and extensive grading may 

also increase local rates of erosion and thereby 

lead to downstream deposition of sediment, to 

reduced downstream channel capacity, and 

thus to further risk of flooding. Similarly, construction ori flood plains and obstructions in 

stream channels impede the free flow of floodwaters, reduce stream velocities, and result in 

higher crests and more extensive flooding. 

Changes in land use may be especially 

troublesome where they are upstream from established lowland communities. Storm drains, 

culverts, and regulatory flood plains that were 

adequate for floodflow from a natural watershed 

may be unable to carry an increased flow. Be-

~3 

(/) 

<( 

CD 

0 .w 

~ N 

o2 

...J <( 

u..CD 

~a: 

<(::> 

wz 

a.. ::> 2 

u.. (/) 

0 ::> 

(/) 

0 a: - w 

~ > a: 0 

w 

N 

z <( 

CD 

a: 

50-year recurrence interval 

::> 1~~--~------~----~~-----L----~ 

0 60 80 100 

PERCENTAGE OF BASIN URBANIZED 

FIGURE 13.-Ratios of peak flows for 50- and 100-year 

floods in urbanized and unurbanized basins in the San 

Francisco Bay region (from Waananen and others, 1977, 

p. 12). For a basin that is 80 percent urbanized, peak 

flow for a 100-year flood is twice the peak flow in a 

similar but unurbanized basin. 

cause much new development in the bay region 

is encroaching on the steeper slopes upstream 

from established communities, flood hazards in 

some areas may be greater now than ever before. 

Even attempts to reduce flood hazards may 

produce mixed results. Some methods of controlling floods utilize such structural measures as 

dams, levees, and channel improvements to impound floodwaters or to confine them. Although 

these measures may successfully achieve their 

immediate objective of protecting a designated 

section of a stream from flood losses, they alter 

the natural streamflow and cause new problems, 

such as silting, accelerated erosion, or flooding 

in areas that previously were free of floods. 

Structural measures are likely to be most effective and to create the fewest unanticipated problems where they are part of a comprehensive 

river-basin plan. Such plans consider the hydrology and projected land use of the entire basin, 

together with other water-related objectives, 

such as irrigation, erosion control, and water 

quality. 

Despite the difficulties, flood losses can be 

greatly reduced. Roods obey fundamental physical laws that are well understood, and in most 

large streams, flood size and frequency can be 

predicted from data on streamflow. Where these 

data are unavailable, reliable estimates can be 

made by analogy with similar, well-measured 

drainage basins. All these predictions are probabilistic and nonspecific, but if their limitations 

are recognized, they can at least provide a rational basis for planning. 

Planners employ several methods to control 

flood losses. Some of these methods (table 3) 

were discussed more fully elsewhere (Kockelman, 1977, p. 23). Such methods most effectively 

reduce losses where the possibility of flooding is 

recognized early and evaluated carefully. 

Recognition and evaluation of flood hazards 

require careful review of land-use plans and development proposals, as well as knowledge of 

stream-basin characteristics. Such review must 

be tailored to specific plans or proposals, but 

some of the most common review questions are: 

1. Is the plan or proposal for an area that is subject to flooding? (If not, see question 5). If 

the area is subject to flooding: 
THE PROVINCES OF lAND-USE CONCERN 

TABLE 3.-Techniques to prevent flood losses 

(From W. J. Kockelman (in Waananen and others, 1977, p. 24)) 

Purpose 

Protection of existing development. 

Removal or conversion of existing 

development. 

Discouragement of development. 

Regulation of flood-plain uses. 

A. On the basis of existing data, what is 

the degree of hazard to the proposed development? 

B. How will the plan or proposed use affect flood characteristics and normal streamflow at the site; will it 

tend to increase or decrease flood 

crests and extent, or will these be 

unaffected? 

C. How will the plan or proposal affect 

flood characteristics and normal 

streamflow upstream and downstream from the site? 

Techniques 

Flood-control works: 

Reservoirs 

Channel improvements 

Diversions 

Flood warning and evacuation 

Flood proofing 

Public acquisition 

Urban redevelopment 

Public-nuisance abatement 

Nonconforming uses 

Conversion of use or occupancy 

Public-facility reconstruction 

Public information 

Warning signs 

Recordation of hazard 

Tax-assessment practices 

Financing policies 

Public-facility extensions 

Flood-insurance costs 

Zoning ordinance districts 

Special flood-plain regulations 

Subdivision ordinances 

Building ordinances 

2. Does the plan or proposal incorporate, as an 

essential component, structural measures 

for flood protection or control of 

streamflow? 

A. If so, are these measures a consistent 

part of a broader watershed-management plan? 

B. If they are not part of such a plan, 

how will they affect (i) streamflow 

above and below the site, (ii) erosion and sedimentation rates above 

and below the site, and (iii) other 

important uses of the water and the 

waterway? 

27 
28 GEOLOGIC PRINCIPLES FOR PRUDENT lAND USE 

3. Is the plan or proposal likely to lead to additional but as yet unplanned development in 

the area subject to flooding? 

4. What liability does the community assume in 

permitting the planned or proposed use 

within an area of recognized flood hazard? 

5. If the plan or proposal is in a flood-free area, 

will it require extensive changes in vegetation, slope r or surface permeability? 

A. How will these changes affect runoff 

and downstream flood characteristics? 

B. How will these changes affect local 

erosion rates and downstream 

sedimentation processes in the 

channels and flood plains that help 

determine flood characteristics? 

6. Will the plan or proposal, through its effects 

on flood characteristics, make obsolete any 

measures that protect existing development? 

LOWLAND SEDIMENT 

The stream deposits of gravel, sand, and silt 

beneath the lowland surface possess several 

properties of importance in land-use decisions. 

These deposits, compacted but not yet turned to 

rock, are in beds or lenses, a few inches to tens 

of feet thick and hundreds to thousands of feet 

wide and long. The layers of sediment are nearly 

flat; at most, they dip a few degrees downstream 

or toward the bay. Their continuity and nearhorizontal attitude are interrupted in some 

places by such active faults as the San Andreas 

or Hayward, and in others by buried bedrock 

ridges across which the layers of sediment are 

draped and mildly deformed. Most beds and 

lenses contain well-sorted sediment with a narrow range of grain sizes. Well-sorted sand or 

gravel contains voids or pore space, which may 

account for 25 percent or more of the total rock 

volume and which permit this sediment to store 

ground water or to serve as a conduit for flowing 

ground water. 

These properties make the lowland surface 

and the underlying deposits attractive to many 

users. The flat surface, left when sediment was 

deposited from flowing water, is the most obvious advantage. But the land is also well drained. 

In most areas and during most seasons, the 

water table lies below the depths reached by 

human activities. Although the deposits of 

gravel, sand, and silt are firm and well compacted, most are uncemented to very weakly 

cemented. Land underlain by these deposits is 

stable, firm, and strong enough to support normal structural loads without failing r yet it is far 

more easily excavated than hillside sites that are 

underlain by bedrock. 

Because the subsurface layers of gravel, 

sand, and silt were deposited by running water, 

most are sorted by particle size. Gravel is concentrated in some beds, sand in others, and even 

where mixing occurs, one grain size tends to 

dominate. The degree of sorting enhances the 

value of these deposits as commercial sources of 

sand and gravel, and because good sorting increases both the amount of void space and the 

ability to transmit fluids, many lowland beds of 

sand or gravel are important sources of freshwater. Porous and permeable layers also conduct 

subsurface drainage down the inclined surface, 

or dip, of the beds to the bay or to the ocean. 

Percolating water thus drains away to leave a 

zone of aerated, well-drained soil and sediment. 

Although fluids readily pass through most of the 

water-laid deposits, some natural barriers inhibit 

the flow of ground water and other fluids, and 

some water-bearing beds, or aquifers, are confined and will sustain artesian flow. For example, before the extensive development of 

ground-water resources in the Santa Clara Valley, many of the water wells there were artesian. 

The properties of lowland sediment differ 

greatly from those of the rocks that underlie the 

hills, although these lowland deposits are by no 

means uniform. Wells drilled at one lowland site 

may penetrate scores of feet of gravel and coarse 

sand before entering bedrock; wells a few miles 

away may penetrate only a few feet of silt, clay, 

or fine sand, even though the sediment layers in 

both drill holes are of the same age and were deposited by the same stream. Such local variations in grain size and bed thickness are typical 

of stream deposits and show how stream processes varied during deposition. 
THE PROVINCES OF LAND-USE CONCERN 29 

In the bay region, however, the properties of 

stream-laid sediment also vary with the age of 

the deposits. The youngest, and most extensive, 

stream deposits are those near present stream 

courses and those which fringe the bay. These 

young deposits are less consolidated, weaker 

under load, more likely to have a shallower 

water table, and support younger and less mature soils than does older stream sediment. The 

older deposits of sediment are graded to an iceage sea level that was more than 300 ft lower 

than the present stand of the sea (Atwater and 

others, 1977); they are exposed in narrow bands 

near the hills and underlie the younger deposits 

near the bay and in the valleys of most large 

streams. 

Because such properties as rock strength, 

depth of weathering, and porosity change with 

geologic age, sediment size by itself is an incomplete measure of the adequacy of a site for a specific use. Stated another way, the suitability of a 

site for a given land use depends on the geologic 

age of the underlying material as well as on its 

grain size. The extent of lowland deposits, their 

significant properties, and how these properties 

may affect major land-use decisions were discussed more fully by Helley and others (1979). 

Competition for space in the lowlands intensifies the need for informed and thoughtful decisions on land use. In the past, land-use decisions 

were strongly influenced by short-term 

economics; that is, those options that most 

quickly produced the greatest economic gain 

were favored. This strategy commonly encouraged land development for residences, commerce, and industry at the expense of agriculture, construction materials, ground-water resources, and potential waste-disposal sites. But 

as more and more lowland is converted to urban 

and suburban use, it becomes prudent to plan 

carefully for future needs: To conserve ground 

water, to reserve sites that are adaptable to the 

disposal of liquid or solid wastes, and to protect 

sources of essential construction materials. Such 

planning demands specific knowledge of the 

lowlands, of the materials beneath the surface, 

and of the changes in those materials from place 

to place. An example of how geologic and hydrologic knowledge is used to select waste-disposal sites is given below in the section entitled 

"Identifying Potential Waste-Disposal Sites." 
30 

Landslide on Van Cleave Way in Oakland, March 1958. This slope, less 

than half a mile northeast of the Hayward fault, is underlain by sheared bedrock of diverse origin; much of the rock exposed nearby is serpentinite, 

which is commonly unstable in steep slopes or where it is intensely sheared 

and altered. Slides such as this have a major lasting impact beyond their immediate limits: They diminish the stability of slopes above and alongside 

the failure scar; the displaced slide mass is potentially unstable and may 

threaten downslope structures; the changes in topography caused by the 

slide alter drainage and runoff, which may accelerate local erosion; and the 

unstable debris in the slide mass may choke downstream drainage systems 

with sediment and cause flooding. Most bay-region cities and counties now 

require professional geologic studies of proposed hillside developments to 

identify potentially unstable slopes before grading and construction. Photograph by the Oakland Tribune. 

Coalescing active landslides at Lomerias Muertas, 5 miles north of San 

Juan Bautista and near the boundary between San Benito and Santa Clara 

Counties. Scarps, ponds, hummocky topography, and primitive drainage record recent movement and instability in the central and youngest lobe, but 

nearly all of the land surface in this view has failed in the past and is very 

likely to do so again. In such terrain, grading, filling, or other construction 

is likely to trigger or accelerate slope failure. Even untrained observers 

would probably recognize the problems with this slope, but many unstable 

hillslopes are much more difficult to evaluate and require modern methods 

of geologic and geotechnical analysis as well as skill and experience in applying those m&thods. Photograph by Earl E. Brabb, U.S. Geological Survey. 
U.S. Interstate Highway 80 near El Sobrante, Contra Costa County. Although only embankment fill is visible in the scarp and the slide mass 

below, the failure extended into and probably began in the underlying bedrock. The initial failure, on May 11, 1969, followed a wet winter, and later 

exploratory drilling disclosed that the slip surfaces beneath the slide were 

saturated with water. Successive failures at the head of the slide caused all 

lanes of the highway to be closed by May 15 and thus cut the main highway 

link from the bay region to the east. Slip during the first stages of sliding 

also severed an aviation gasoline pipeline; leakage from this pipeline 

triggered emergency procedures because of the threat of fire or explosion 

to nearby residential areas in El Sobrante. 

This fill was constructed in 1958 at a cost of $7.1 million. Today, relatively modest geologic and geotechnical investigation, using modern knowledge and techniques, would probably identify potential site problems before construction and thus lead to appropriate mitigative measures. Photograph by Norman Prime, U.S. Geological Survey. 

THE HILLSIDES AND UPLANDS 

T he hillsides and uplands occupy more than half the land area in the 

San Francisco Bay region. Nearly all of the hillsides and uplands were 

used for timber, orchards, and grazing until the 1950's, when residential hillside developments began to grow in number and size. Today, residential growth continues unabated in the hills. People move to the hills for 

both economic and esthetic reasons. Economic incentives include the scarcity and high cost of lowland sites relative to the lower cost of undeveloped 

hillside land, and the ease and economy with which modern heavy equipment can excavate and grade the slopes. Esthetic incentives include unusual 

or unique homesites, opportunities for views, and proximity to open space. 

Moreover, some hillside sites are high enough to be above the inversion 

layer and in relatively clean, smog-free air. 

31 
32 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

THE PROBLEM 

Although the hillsides offer many advantages, developers face more numerous and different problems in the hillsides than in the lowlands-problems that, if unrecognized, can 

cause substantial loss of property and spell financial ruin for builders or homeowners. Among 

the most common of these problems are: 

-high costs for grading or excavating because 

of the unanticipated properties of near-surface rock; 

-failure of natural slopes due either to natural 

processes or to manmade changes in drainage, vegetative cover, or load. 

-failure of manmade slopes or fills; 

-poor drainage because of a shallow water 

table or impermeable rock at or very near 

the surface; 

-erosion, deposition, or flooding caused by removal of vegetative or soil cover, or by modification of the natural drainage system; 

-unstable foundation conditions caused by 

swelling clays, which change volume seasonally as they absorb or lose moisture. 

Some of these problems appear as rural land 

is cleared or graded for suburban use; where 

recognized early, -many of them can be remedied. Other problems, such as slope failure, poor 

drainage, and swelling clay, commonly remain 

unrecognized or underestimated until much 

later; they may not be discovered until years 

after a development is completed and occupied. 

Thus, in spite of the low initial cost of undeveloped hillside land, long-term costs due to 

geologic problems can make hillside land much 

more costly to develop than the alluviated nontidallowlands (Laird and others, 1979, p. 51-54). 

In this report, we consider the hillsides and 

uplands to include all slopes steeper than 15 percent and those gentler slopes that are at least 

1,000 ft above sea level. This terrain differs from 

the lowlands in three special ways: (1) It is underlain by bedrock and the weathered products 

of bedrock, (2) its surface slopes moderately to 

steeply, and (3) it is shaped and modified by erosional processes. 

GEOLOGY AND PROCESSES 

As we have seen, the lowlands are covered 

with gravel, sand, and silt-unconsolidated deposits that record stream processes during the 

past 20,000 to 40,000 years. By contrast, the hills 

and uplands are underlain by harder, consolidated rocks. These rocks have a longer geologic 

history spanning more than 200 million years. 

The rocks we find in the hills today represent ancient beaches, fans, and oozes of the deep 

ocean; submarine lava flows; terrestrial flows of 

lava and ash; gravel and sand from ancient rivers and streams; and crystalline bodies that 

formed deep within the Earth's crust. Many of 

these rocks were deeply buried, folded, and 

faulted. These differences in origin and in degree and style of deformation yield an assortment of bedrock types that vary widely in 

strength, in chemical composition, and in the 

spacing and orientation of fractures. 

Bedrock near the surface decays, is leached, 

and breaks down into a mantle of weathered 

rock and soil. In most places, this mantle is no 

more than a few feet thick, although it may be 

several times thicker at the foot of some slopes. 

The mantle is weaker and more permeable than 

the bedrock beneath it; it is more easily excavated than bedrock, and some of it is permeable 

enough to be suitable for septic-tank drainfields. 
THE PROVINCES OF lAND-USE CONCERN 33 

ACTIONS TO ENHANCE LAND USE 

BEDROCK AND MANTLE 

The heterogeneity of bedrock and mantle 

defies simple rules for safe, economical hillside 

development. Despite impressive advances in 

geologic knowledge and engineering technology, satisfactory development still depends on 

careful geologic site investigations and effective 

communication between geologists and engineers. Although many of the problems caused 

by the diversity of bedrock and mantle characteristics are technically complex (Wentworth and 

others, 1983), we describe a few representative 

examples below. 

MATERIAL STRENGTH 

The strength of rock or soil determines 

whether a slope will stand or fail under the load 

of overlying material. Most rocks are stronger 

than soils, and some can stand in nearly vertical 

faces hundreds of feet high; however, such rocks 

are found in only a few parts of the bay region. 

The strength of rock or soil may be described in various ways. Although shear strength 

most directly limits the height of cut slopes and 

the load imposed on foundations by structures, 

we commonly refer to unconfined compressive 

strength because it is more easily measured by 

simple laboratory tests and because it is applicable to the many routine problems that arise in 

hillside development: It measures the maximum 

load that can be supported without failure under 

unconfined conditions. Worldwide, unconfined 

compressive strengths for soils range from less 

than 3.5 to about 50 lb of force per square inch 

( lbf/in2), and for rocks from about 50 to more 

than 32,000 1bf/in2 . Although little information 

on this subject is available for the bay region, 

empirical evidence and approximate field tests 

suggest that unconfined compressive strengths 

for rocks and soil here range from about 5 to 

about 15,000 lbf/in2• 

Differences in rock and soil strengths depend on many variables, including parent bedrock, degree of weathering, and depth beneath 

the surface of the land. Where deep cuts intersect fresh hard rock, excavation becomes difficult. On the other hand, even shallow cuts in 

mantle materials of low strength require special 

care to ensure stability. Highway and freeway 

cuts are good places to compare rock strengths. 

Some near-vertical cuts in the volcanic rocks of 

eastern Sonoma County are stable, but more 

gently inclined cuts are needed for stability in 

most roadside materials. Although other factors 

also affect stability, rock or soil strength. can be 

critical. 

FRACTURES 

Also important for stability, especially in 

hard, unweathered rock a:o:d in some soils, are 

the spacing and orientation of fractures, joints, 

and bedding. Many of these planar discontinuities in rock or soil form potential failure 

surfaces; whether they contribute to the actual 

failure of slopes or cuts depends on their dimensions and orientation. Thus, joints that are 

vertical or dip into a slope are less likely to fail 

than those that dip outward (fig. 14). 

FIGURE 14.-The orientation of joints or other breaks 

may affect the stability of a slope. Vertical joints in 

block A and inclined joints that dip into the slope 

in block B do not significantly reduce stability. Outward-dipping joints in block C define planar failure 

surfaces, one of which (abed) is outlined. Most 

jointed or fractured rocks exhibit more than one set 

of breaks; intersection of fracture sets having different orientations can cause other, nonplanar types of 

failures. 
34 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

EXPANSIVITY 

Many soils and weathered rocks swell when 

wet and shrink when dry. They do so because 

they contain swelling clays, which expand as 

they absorb moisture. If small cracks in plaster 

and wallboard open and close seasonally, as 

they do in many homes in the bay region, this 

may mean that the soil beneath the foundation 

contains some swelling clay. In most such 

homes, patches and paint will repair the damage. However, serious damage results if structures or facilities rest on clay with a high expansivity or if they straddle the contact between 

nonexpansive rock or soil and beds with a high 

content of swelling clay. Claystone beds can 

cause swelling pressures and volume changes 

that are capable of breaking sewers, streets, and 

foundations as well as causing unsightly damage 

to interiors. 

Although simple tests help to identify and 

evaluate problems with swelling clays, engineering solutions are costly and not always 

completely successful. In 1975, engineering 

measures to mitigate a major problem of swelling clay in one community on the San Francisco 

peninsula added about 5 percent to the cost of 

new residences (Meehan and others, 1975, p. 

946). 

GROUND-WATER LEVEL 

Water, too, affects stability, as is shown almost annually by the rash of landslides and 

slope failures that follow heavy storms (Nilsen 

and Turner, 1975; Nilsen and others, 1976). Most 

of these failures are on slopes underlain by 

permeable, water-saturated rocks or mantle in 

which the pore spaces or fractures and joints are 

filled with ground water (fig. 15). Many slopes 

fail simply because the weight of rock or soil, 

plus the added load imposed by pore water, exceeds the bearing strength. Others fail because 

the pressure of pore water reduces the effective 

strength across joints, fractures, or bedding surfaces. If they are anticipated and correctly 

evaluated, many potential water-induced slope 

failures can be avoided by corrective engineering measures. Successful engineering measures 

FIGURE 15.-Slopes, whether cut or natural, may be stable 

at normal ground-water levels (n, ruled) but unstable 

during wet seasons when water levels are higher (s, 

dashed). Failure on such a surface as a-b may be 

caused by the added weight of pore water, by the loss 

of effective strength due to pore-water pressure along 

the failure surface, or by both. 

depend on knowledge of the permeability of 

rock or soil, of seasonal fluctuations in groundwater levels, and of the mechanisms of failure. 

The bedrock and mantle properties we have 

just discussed-material strength, fractures, expansivity, and ground-water level-are among 

the most important if hillside land is to be developed successfully. They are, or should be, 

carefully evaluated on site. For preliminary planning and for evaluating larger areas, these properties can be estimated from information available on modern geologic maps that are prepared 

with land-use issues in mind. These maps depict 

major rock masses, or formations, tha.t are distinguished by their geologic characteristics. Many 

of these geologic characteristics determine the 

properties that are critical in hillside development. Methods of recognizing these properties 

from conventional geologic maps have been discussed by Wentworth and others (1983). 

SLOPES 

The slope of the land surface is easily seen 

and measured. Measurement of slope is expressed as an angle, as the ratio of horizontal 

distance to elevation change, or as the percentage of elevation change per unit of distance (fig. 

16) Slope can be read directly from slope maps, 

such as those released by the U.S. Geological 

Survey; it can be calculated from distances and 
THE PROVINCES OF LAND-USE CONCERN 35 

elevations measured on contour maps; or, where 

precise slope measurements are needed, it can 

be determined accurately by surveying methods. 

Site planning usually demands surveying 

methods, but slope information derived from 

maps is useful for more general city and county 

planning. 

Steep slopes significantly increase the cost 

and difficulty of land use because they require 

more extensive grading for roads, utilities, and 

construction sites. Grading on steep slopes generates large volumes of earth or rock and leaves 

cuts, which must be carefully engineered if they 

are to remain intact. Manmade fills on steep 

slopes utilize the waste rock from nearby cuts, 

but, for lasting stability, deep fills must consist of 

carefully selected materials and be well engineered. Figure 17, which illustrates the relation between slope, height of cut, and volume of 

earth removed, shows vertical cuts. Such cuts 

minimize the height of the cut and the volume of 

earth or rock to be moved, but they are practical 

only in strong, unjointed rock. 

z 

0 

·------~·~~-----------2-----------------~~1~ .~-~~~~'~n~\ /t>-"';.,1 26.6'' 

~""~ 

DISTANCE 

FIGURE 16.-Slope can be expressed as (1) a vertical angle, 

measured in degrees from the horizontal (26.6°); (2) a 

ratio of horizontal distance to elevation change (2:1); or 

(3) a percentage, measured by dividing elevation 

change by horizontal distance (50 percent). 

Slope=3:1, 18.4°,0r331'3 percent 

Slope= 2:1, 26.6° or 50 percent 

H =25ft 

7 

I 

I 

H

1

f5o tt 

Slope= 1:1, 45° or 100 percent """ 

63° 

,......,......,......A,......=,......1__...2~50_ft_2 _ _., I ~ 

FIGURE 17.-Heights of cuts (H) and cross-sectional areas 

(A) of cut or fill (shaded) increase on steeper slopes. 

Horizontal graded surface, 100ft wide, is shown as half 

cut and half fill. Volume of earth or rock removed is a 

product of the length of the cut and its average crosssectional area. Vertical cut slopes and fill margins are 

diagrammatic. In the San Francisco Bay region, most 

surface rocks and all unretained fills are unstable in 

vertical faces. Inclination of the cut face to achieve stability, as indicated by dashed line where the slope is 

1:1, significantly increases both the height of the cut 

and the volume of earth or rock to be removed. 

Most cuts in the San Francisco Bay region 

must be inclined from the vertical to minimize 

the possibility of cut-slope failure. Greater departure from the vertical is needed in strongly 

jointed rocks or those with unusually low bearing or shear strength. For the 1: 1 slope shown in 

figure 17, reduction of the angle of the cut from 

vertical to about 63° doubles its height and more 

than doubles the volume of material that must be 

removed. Fills, too, must be designed for the 

slope, the rock or soil on which they rest, and the 

properties of the material used as fill. To insure 

stability, fills commonly employ gently sloping 

outer surfaces, retaining walls, and engineered 

drainage systems. 
36 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

Although steep slopes require more ex cavation than do gentle ones, slope alone is an unreliable measure of stability. Some natural slopes 

-of 20° or less fail; others much steeper than 45° 

are stable. Manmade cut slopes have similar 

ranges in stability. Some vertical cuts stand 

without engineered retaining walls, but some 

gently inclined cuts fail. Clearly, properties other 

than slope control the stability; these properties 

include: The shear strength of the rock or earth; 

the number and orientation of joints, bedding 

planes, and faults; the load imposed by the mass 

of ~arth or rock above the potential failure surface; and the degree to which pore spaces and 

fractures are filled with water. All these factors, 

together with the slope of the land surface, must 

be considered in analyzing slope stability. 

EROSION 

Erosion constantly shapes and modifies the 

hills and uplands, and gradually wears away the 

land surface and reduces the steepness of slopes. 

Gravity and runoff from storms carry rock and 

soil debris downslope and deliver it to streams, 

which ultimately deposit it in the lowlands or in 

the bay. The importance of erosional processes is 

commonly underestimated because, except for 

large landslides and flood deposits, evidence of 

erosion is not obvious. Nonetheless, streams that 

drain upland areas carry immense loads of 

eroded debris both as suspended sediment and 

as sediment that moves along the bottom. Brown 

and Jackson (1973) showed that, over the 7-year 

period from 1965 to 1971, the Russian River at 

Guerneville, Calif., carried a mean annual load 

of nearly 4 million tons of suspended sediment, 

equivalent to more than 2,800 tons of sediment 

per square mile of drainage area, which indicates a basinwide lowering of the land surface of 

about 0.02 in. per year. 

Rates of erosion vary in other drainage basins within the bay region. High rates can be 

found in areas with heavy rainfall, high relief, a 

paucity of vegetation, and weak, intensely fractured, or unconsolidated rock or soil, although 

the relation among these variables is complex 

and not fully known. Abnormally high rates of 

erosion may also be caused by urbanization, as 

was noted for the Colma Creek basin, south of 

San Francisco (Knott, 1973). Accelerated erosion 

during urbanization can be attributed chiefly to 

the removal of vegetation, to extensive grading, 

and to increased runoff, which also is caused by 

urbanization. Lightly developed or pristine hillslopes are constantly eroded by such processes 

as rillwash, surface creep of rock or soil, gullying, down cutting by streams, stream bank erosion, and landsliding. 

RILL WASH 

Rillwash attacks barren slopes of loose sand 

or silt. It erodes furrows on debris slopes, cut 

banks, and newly graded surfaces, but it is not a 

major problem in hillside development. 

CREEP 

Downslope creep of rock or soil on steep 

slopes (fig. 18) is due to gravity and to wetting 

and drying of the slope debris. Creep moves 

large volumes of soil and surface rock. On most 

slopes, the zone of creep is only a few feet thick, 

and rates of movement are so slow that creep is 

not an important problem for hillside development. Locally, however, creep rates of 0.5 in. per 

year are observed on slopes as low as 8° (Heming, 1972); such rates may require attention in 

site planning. 

FIGURE 18.-Creep of rock or soil, caused by gravity, 

slowly moves material downhill. Creep deforms the 

bedding near the surface and tilts or displaces fenceposts, power poles, and other manmade structures. 
THE PROVINCES OF lAND-USE CONCERN 37 

GULLYING 

Gullies (fig. 19) are steep-walled ravines that 

result from abnormally rapid downcutting of 

weak, easily eroded rock or soil by running 

water. Gullying signifies an increase in the erosive power of a stream; it may be induced by increased runoff, by lowering of the base level of 

the stream, or by deprivation of the stream of its 

normal load of sediment. Common causes of gullying are: (1) Overgrazing, which increases 

runoff; (2) addition to runoff by diversion of 

nearby drainages or storm sewers into the gully; 

(3) grading below the normal gradient of the 

stream, which lowers the local base level and induces downcutting upstream; and (4) damming 

of the stream, which traps sediment and increases the erosiveness of water discharged from 

the reservoir. Unless it is controlled, gullying 

moves upstream into tributaries as the base level 

of the main stream is lowered. 

Gullying is a minor problem in some parts of 

the bay region, but it can ordinarily be controlled 

when its cause is known. 

FIGURE 19.-A gullied slope. Rapid downcutting of soil 

and weathered rock has incised a small drainage below 

its normal course. Gullying moves upstream along small 

tributaries, but erosion slows where firm rock is encountered. 

DOWNCUTIING 

Most perennial streams in the hills gradually 

erode their channels and slowly lower the 

streambed; they are in equilibrium, automatically adjusting to changes in flow and sediment 

load. For these streams the rate of downcutting 

is low, and their gradient from source to mouth is 

relatively stable. 

The rate of downcutting by streams may be 

changed by dams or major diversions of 

streamflow. Such changes, which result from 

large engineering projects, are beyond the scope 

of this report, but most of these projects include, 

as an essential part of their design, an analysis of 

their effect on normal streamflow and provisions 

to minimize those effects. 

STREAMBANK EROSION 

Bank erosion by streams is a common and, in 

some places, serious problem. Streams erode 

their banks where they impinge on natural 

slopes or artificial fill, especially at the outer 

margins of curves or meanders (fig. 20), where 

water velocity and erosive power are greatest. 

During floods, high water levels and increased 

velocity accelerate bank erosion, and at these 

times the effects are most easily visible: Fills fail, 

roads are undermined, utility and service systems are lost or damaged, and, less frequently, 

dwellings and other structures are lost or damaged owing to undermining of their foundations. 

Roads are common victims of streambank erosion because many roads in the hills follow narrow stream valleys. Where a stream flows at the 

FIGURE 20.-Streams erode laterally on the outer margins 

of curves and meanders, leaving steep cut banks that 

are subject to failure. Sediment is deposited on the 

inner margins of curves. 
38 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

base of a landslide, bank erosion can remove 

support at the toe of the slide and cause failure 

and renewed sliding that may extend hundreds 

of feet upslope from the eroded bank. 

Many bank-erosion problems stem from a 

failure to recognize that rare or infrequent floods 

can greatly increase the volume of water and its 

velocity. Other problems appear when people Qr 

Nature diverts the flow against a bank that was 

previously stable. Such diversions are commonly 

caused by dumped fill, landslide deposits, or 

bridge piers. Most bank-erosion problems can 

be .. avoided or reduced to manageable proportions if streamflow data and basic hydrologic 

principles are taken into account in the design of 

structures or facilities located near the streamcourse. 

lANDSLIDING 

Historically, landslides are the most costly 

and pervasive geologic problem in the San Francisco Bay region; annual losses can exceed $25 

million (Nilsen and others, 1979, p. 7) and are 

likely to increase in the future as more hillside 

land is developed. 

Landsliding carves distinctive hillside landforms throughout the bay region. Landslides 

commonly move as debris slides, slumps, earth 

flows, and rock falls; rock slides and rock topples 

are less common in the bay region. Each of these 

terms describes a different mechanism of slope 

failure (Varnes, 1978, p. 11). Although each of 

these mechanisms is separately important to 

geologists and engineers, here we discuss them 

all together as landslides. 

Recent landslides provide clues to slope failure. Headwall scarps, bulging toes, and jumbled 

and poorly drained terrain (fig. 21) delineate 

slide deposits and leave telltale signs of internal 

disorder. Also, small ponds, springs, or wet 

areas-the results of interrupted drainage-are 

associated with many landslide deposits. Over 

time, erosion modifies and eventually destroys 

all these features, but landslide deposits as old 

as 10,000 to 20,000 years still retain traces of 

their origin. Geologists use such clues to detect 

and map landslide deposits. Landslide-inventory 

maps, prepared in this way, show that landslide 

deposits can cover hundreds of square miles of 

hillside terrain (Nilsen and others, 1979). 

If we compare landslide-inventory maps 

with geologic maps of the same area, we note 

that some geologic units host more or larger 

landslides than others and are, therefore, relatively unstable. A comparison of landslide-inventory maps with slope maps shows a similar 

relation between the number and size of landslides and the steepness of the land surface. 

Combining information on slope, landslide distribution, and geology, Brabb and others (1972) 

mapped the varying degrees of landslide susceptibility in San Mateo County. Nilsen and 

others (1979) modified this method and applied 

it to the entire bay region. These landslide-susceptibility maps, which were designed chiefly 

for preliminary planning or for the evaluation of 

large areas, also indicate possible problem areas 

for site planners and developers. 

FIGURE 21.-Landslides exhibit headwall scarps (h), bulging toes (t), and jumbled, poorly drained terrain (jt); 

ponds (p), springs, and wet areas on the surface of a 

landslide deposit indicate a shallow water table, poor 

drainage, or both. Slides are most abundant in weak 

rock or soil and where bedding or other potential failure surfaces dip outward. 
THE PROVINCES OF LAND-USE CONCERN 

The possibility of a landslide at a specific 

site can best be judged by investigating the surface materials, the bedrock and structure beneath the surface, and the ground water. Sites 

on or near recent or ancient landslide deposits, 

those in areas of high landslide susceptibility, 

and those on geologic units of known low stability warrant careful investigation by an experienced engineering geologist. Such investigations should precede site planning and continue 

as grading provides additional geologic information. 

The importance of competent geologic guidance can be illustrated by examining two common misconceptions. The first concerns soil tests, 

which measure the strength of surface materials 

and their ability to support structures without 

failing. Favorable tests of surface soil are commonly, though erroneously, cited as evidence 

that a site is free of landslide problems. Although 

some slides are shallow, many result from failure 

on surfaces that lie many feet or tens of feet beneath the surface-far deeper than standard soil 

tests. Thus, tests of surface materials, though 

useful in estimating foundation stability, do not 

necessarily assure us that a slope is stable. 

The second misconception concerns ancient 

landslide deposits, especially those showing no 

evidence of movement in historical time. Because the slope has already failed and because 

the landslide debris is in seeming equilibrium 

with its surroundings, the deposit appears to be 

stable and thus as suitab,le for development as 

are other areas. This view is invalid because it 

neglects two important principles: (1) Any landslide deposit is, at best, only temporarily stable, 

and relatively minor changes in load, slope, or 

ground-water level may generate renewed sliding; and (2) development on a landslide deposit 

necessarily alters many of the site characteristics 

that control stability. Although some ancient 

landslide deposits can be modified and developed safely, they should be viewed as potentially sensitive sites until thorough geologic investigation has proved them to be otherwise. 

39 
A water tank on the Hayward fault sits above a hillside residential district in the eastern part of Hayward. The fault scarp, formed by repeated 

episodes of horizontal slip, interrupts the regular slope of the hills and is 

most evident below the fence to the right of the water tank. This segment 

of the Hayward fault broke during the magnitude 6.7 earthquake of October 

21, 1868, which caused major damage throughout the East Bay. This view 

also shows how cut-and-fill grading is used to modify natural slopes and 

provide flat surfaces for housing and streets. Photograph by Norman Prime, 

U.S, Geological Survey. 

40 

\ 

This barn on the old Skinner Ranch, 0.5 miles northwest of Olema in 

Marin County, straddled 15ft of horizontal displacement during the 1906 

San Francisco earthquake; although it remained standing, its utility as a 

cowbarn was clearly reduced. The site of the Skinner Ranch is now the 

headquarters for Point Reyes National Seashore, and a modern version of 

the Skinner cowbarn still sits on the fault trace. Elsewhere along the fault, 

near Olema, several well-built wood-frame houses within a few feet of the 

fault break survived the earthquake with substantial damage; repaired, 

some of these remained in use for more than half a century. The impact of 

this earthquake in rural parts of Marin, San Mateo, Santa Clara, and Santa 

Cruz Counties contrasted sharply with its devastating effect in nearby towns 

and cities. Photograph by G. K. Gilbert, U.S. Geological Survey. 

Landslide on the San Andreas fault about 4 miles northwest of Bolinas 

Lagoon, Marin County. This small mass of earth and soil failed in March 

1907, nearly a year after major faulting along this segment of the San Andreas fault. Many similar slides moved during the winter and spring after 

the 1906 San Francisco earthquake. These failures, scattered along the fault 

zone, occurred where rainfall and runoff from winter storms saturated 

cracked and broken ground formed during fault displacement. 

Throughout northern California, many other slopes failed during the 

April 18, 1906, earthquake. That earthquake followed several months of 

heavy rainfall, and even for north-coastal California, March 1906 was unusually wet, with rainfall amounts from 50 to 200 percent above normal. The 

coincidence of strong ground shaking and water-saturated hillslopes 

created hundreds of massive slides and earth flows, but because most of 

northern California was still sparsely settled, the damage caused by slope 

failure was less apparent than that due to shaking. A repeat of this scenario 

today would result in major damage and financial loss because of widespread development on the hillslopes and uplands. Photograph by G. K. Gilbert, U.S. Geological Survey. 
Ground failure on Bluxome Street near Sixth Street in San Francisco 

after the 1906 earthquake. This and many other parts of San Francisco near 

the waterfront were formerly part of the bay and, after the Gold Rush, were 

reclaimed by artificial fill. These early fills, consisting of garbage, debris, 

and the hulks of abandoned wooden ships, were notoriously unstable and 

exhibited local compaction and subsidence even before the earthquake. 

Some of the most spectacular ground failures during the earthquake were 

in such filled areas, and in many places, liquefaction in the fill or in the underlying bay mud contributed to surface damage. These arcuate fractures 

may be due to compaction during ground shaking or to liquefaction. The 

two high points on the skyline to the right of the power pole are the Twin 

Peaks. Photograph by G. K. Gilbert, U.S. Geological Survey. 

FAULTS AND EARTHQUAKES 

THE PROBLEM 

Every community in the San Francisco Bay region is subject to earthquake damage, and many have experienced one or more destructive 

shocks. Surface faulting, the tearing apart of adjoining blocks of land 

at the Earth's surface, accompanies earthquakes of magnitude 6 and greater. 

Because the style of faulting is comparable throughout the bay region, the 

effects of surface fault displacement are comparable from place to place. In 

contrast, the severity of ground motion varies with distance from the fault 

and with local geologic conditions. Thick deposits of unconsolidated sediment, such as bay mud or alluvial gravel, sand, or silt, may amplify and prolong the shaking; thus, the hazard from shaking is greater around the margins of the bay and in some alluviated valleys than on hillside land that may 

be closer to the earthquake source. Whereas earthquake-induced landslides 

are a significant hazard in the hillsides and uplands, ground failure caused 

by liquefaction in soft, water-saturated sediment is more important in natural 

or reclaimed marshlands or in alluviated valleys. 

41 
42 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

SONOMA 

Bolinas Bay 

0 

EXPLANATION 

Fault with known historical 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 

10 20MILES 

10 20 KILOMffiRS 

\ 

\ 

" '"": 

L.., 

NAPA COUNTY \ 

' 

\ ' -.---- r·-._/ --·: 

I I s \ I ~ 

\~ \ I g ~~ --_) ' 'L --7 ~ 

~\ ~ SOLANO COUNTY • 

I 

0 sacramento 

' ,...J 

l 

\ 

\._('· 

FIGURE 22.-Faults in the San Francisco Bay region that may cause damaging earthquakes or surface displacement, 

or both. Some of these faults have slipped during historical time (past 150 years), some during Holocene time 

(past 10,000 years), and some during Quaternary time (past 3 million years). Most are members of the San Andreas fault system. Adapted from Borcherdt (1975, p. A7, fig. 3). 
FAULTS AND EARTHQUAKES 43 

GEOLOGY AND PROCESSES 

Enormous amounts of rock waste are continually being eroded from the hills and deposited in the lowlands of the San Francisco Bay 

region. Were no other processes acting, this continual erosion and deposition would eventually 

bring the hills and the lowlands to a common, 

nearly flat surface. No such regional surface is 

evident because the Earth's crust warps and deforms at rates that equal or exceed the rates of 

erosion and deposition. Some deforming 

mechanisms, such as tilting and folding, are 

gradual or intermittent, and in the bay region 

their effects, if any, are almost imperceptible. 

Faulting and earthquakes, however, are frequent, readily perceived, and potentially catastrophic. 

Nearly all destructive earthquakes in the 

San Francisco Bay region originate on faults in 

the San Andreas system (fig. 22), a set of northwest-trending fractures that extends more than 

BOO mi from the Gulf of California to Cape Mendocino. The nine bay counties straddle this fault 

system. Movement along faults of the San Andreas system juxtaposes strikingly dissimilar 

rock masses in the upper 10 mi of the Earth's 

crust, separating the North American continental 

plate from the Pacific oceanic plate and accommodating much of the motion as the continental 

plate episodically slips southeastward, past the 

oceanic plate. The long-term average rate of 

movement across the San Andreas fault system 

is about 2 in. per year. The forces that drive the 

plates elastically deform the Earth's crust near 

the plate boundary until the frictional forces resisting fault slippage are overcome. When this 

happens, the energy stored in the deformed 

crust is converted into seismic energy and 

radiates from the fault as seismic waves, which 

we feel as ground motion and record as wavy 

lines on a seismogram. We can see the fault-the 

failure surface or break between displaced rock 

masses-only if it extends to the surface and 

offsets the ground or disrupts manmade structures. 

Earthquakes come in all sizes, or magnitudes. The presently used method of measuring magnitude is that of C. F. Richter (fig. 23). 

1

,_----- S-P time, 24 s 

I 

I 

I 50 

I 

I 40 I 

\ 

\ 

' .... 30 .... 

20 

10 

6 

4 

2 

DISTANCE (km) I S-P TIME (s) 

6 

20 

4 

5 

3 2 

2 

0 AMPLITUDE (mm) 

MAGNITUDE SCALE 

FIGURE 23.-Determ.ination of Richter magnitude. Using 

the maximum amplitude of the seismogram and the 

difference in arrival times of the P and S waves, the 

value of magnitude can be read from this nomogram 

(from Hays, 1980, p. 28, fig. 24). 

The amplitude, or height, of a seismic wave recorded by a seismograph is measured and adjusted to compensate for differences in the sensitivity of individual seismographs and for the 

distance from the earthquake to the recording 

instrument. The magnitude determined in this 

way is a measure of the ground motion at a 

standard distance from the earthquake source; it 

is a single value for each earthquake. Because 

magnitude varies logarithmically, each step in 

the magnitude scale-for example, from 5 to 6-

represents a thirtyfold increase in the energy released and a substantial increase in the potential 

for damage. 
44 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

In the San Francisco Bay region, most earthquakes smaller than about magnitude 3 are detected only by seismographs, those greater than 

magnitude 3 are generally felt, and those greater 

than magnitude 5 can cause damage in urban or 

developed areas. Table 4 lists the magnitudes 

TABLE 4.-Historical earthquakes of magnitude greater than 

5. 0 in the San Francisco Bay region 

[Magnitude data before 1945 are incomplete, and some earthquakes of magnitude greater than 5.0 may have gone unreported. Data from Bolt and 

Miller (1975), Borcherdt (1975), Lester and Meagher (1978), and Toppozada and others (1981)] 

Date Locality Magnitude 

Jan. 1980 ............. .. livermore............................. 5.8 

Aug. 1979 .............. . Gilroy.................................... 5.7 

Nov. 1974 ............. .. Hollister................................ 5.1 

Oct. 1969 ............. .. Santa Rosa ........................... 5.6 

Dec. 1967 .............. . Watsonville .. .. . . .... . . .. . . .. ........ 5.3 

Sept. 1963 ............. .. do...................................... 5.4 

Apr. 1961 ............. .. Hollister................................ 5.6 

Mar. 1959 ............. .. Gilroy.................................... 5.3 

Mar. 1957 ............. .. San Francisco .. . .. .. . .. . . .. . . . . . .. . 5.3 

Sept. 1955 ............. .. San Jose............................... 5.5 

Oct. 1955 .............. . Concord-Walnut Creek...... 5.4 

Apr. 1954 ............. .. Watsonville.......................... 5.3 

Mar. 1949 .............. . Hollister................................ 5.2 

June 1938 .............. . do...................................... 5.5 

July 1911 ............. .. Coyote.................................. 6.6 

Apr. 1906 ............. .. San Francisco . . . . . . .... . . . .. .. .. .. . 8.3 

June 1899 ............. .. do...................................... 5.5 

Apr. 1899 .............. . Watsonville.......................... 5.7 

Mar. 1898 .............. . Vallejo . . .. ... .. . . . . . . . .. . . . . . . .. .. . . . . .. 6.3 

Apr. 1892 .............. . Winters................................. 6.6 

Apr. 1892 .............. . Vacaville . . .. .. . . . .. . . . . . .. . . .. .. . . . . .. 6.4 

Oct. 1891 .............. . Napa..................................... 5.6 

Jan. 1891 ............. .. San Jose............................... 5.1 

Apr. 1890 ............. .. San Juan Bautista............... 5.9 

July 1889 ............. .. Oakland................................ 5.1 

May 1889 ............. .. Antioch................................. 5.6 

Mar. 1884 .............. . Santa Cruz........................... 5.3 

Mar. 1883 ............. .. Gilroy.................................... 5.7 

Apr. 1870 ............. .. Oakland................................ 5.1 

Oct. 1868 .............. . Hayward............................... 6. 7 

Oct. 1865 ............. .. Watsonville.......................... 6.2 

Mar. 1865 .............. . Santa Rosa . ... . .. . . . . . . . .. .. .... . . . .. 5.3 

July 1864 .............. . Fremont................................ 5.4 

Mar. 1864 .............. . Milpitas .. . .. . . . .. . . .. .. .. .. . . .. . . . . .. . . 5.3 

July 1861 ............. .. Dublin................................... 5.3 

Nov. 1858 ............. .. Fremont................................ 5.9 

Feb. 1856 ............. .. San Mateo............................ 5.9 

Jan. 1856 ............. .. San Francisco .. .. . ... . . . .. .. .. ... .. 5.3 

Aug. 1855 .............. . Sonoma................................. 5.1 

June 1838 .............. . Woodside.............................. 7.0 

June 1836 ............. .. Hayward............................... 6. 7 

and dates of occurrence of historical earthquakes in the bay region. The largest, of magnitude 8.3, was the great San Francisco earthquake of April 18, 1906. Worldwide, the largest 

earthquakes attain magnitudes of about 8.9; 

larger ones are unlikely because the breaking 

strength of the strongest rocks limits the amount 

of strain energy that can be stored in the Earth's 

crust. 

Earthquake damage results (1) from faulting, (2) from ground motion or shaking, and (3) 

from ground failure induced by strong shaking; 

ground failure includes both landslides and failures caused by liquefaction of soft, water-saturated sediment. Borcherdt (1975), Borcherdt and 

others (1979), and Archuleta and others (1979) 

described methods of identifying damage-prone 

areas in the bay region, and Youd and Hoose 

(1978) discussed various earthquake-induced 

ground failures in northern California. Here, we 

discuss only briefly these three sources of earthquake damage and refer the interested reader to 

the above-mentioned reports for more complete 

information. 

FAULTING 

Geologic maps show hundreds of faults 

crisscrossing the San Francisco Bay region. Most 

of these faults, locked and immobile for millions 

of years, are unlikely to cause a major earthquake. Others, however, exhibit historical slip, 

currently generate small earthquakes, or show 

other evidence (such as offset soils or offset topographic features) of movement within therecent geologic past. These faults may suddenly 

shift several inches or feet and cause a damaging earthquake, or they may creep gradually, a 

fraction of an inch at a time, without any perceptible earthquake shocks. Such faults are considered active if their history, as determined by 

geologic and seismologic evidence, indicates 

that they are likely to move again during the 

lifetime of manmade structures. 

Criteria for defining an active fault differ 

with the kind of structure and with the Federal 

or State agency responsible for its safety. For 

city and county planning, the State of California defines as active those faults that have 
FAULTS AND EARTHQUAKES 45 

moved during the past 11,000 years. (State regulations regarding development on active faults 

are discussed on p. 58-62 of this report.) For 

dams, nuclear powerplants, and other critical 

facilities, less frequent-hence, older-movement is accepted as evidence that a fault is active. In this report, because we are concerned 

chiefly with city and county planning, our 

usage of the term 11 active fault" is closest to 

that of the State of California, although the 

latest movement on some faults that we consider active may not yet be accurately determined. 

Most active faults in the bay region belong 

to the northwest-trending San Andreas fault system and move in accordance with the plateboundary mechanism discussed above. The 

Hayward, Calaveras, and San Andreas faults are 

familiar active members of the San Andreas system; less well known, but also active, are the 

San Gregorio, Rodgers Creek, and Green Valley 

faults; all are nearly vertical, and all are rightlateral strike-slip faults. As we look along the 

fault trace of a right-lateral strike-slip fault, the 

block to our right moves toward us, and the slip 

is almost wholly in the horizontal plane, parallel 

to the strike, or surface trace, of the fault (fig. 

24). 

Right -latera I 

strike slip 

Dip slip 

(reverse) 

Left-lateral 

strike slip 

Dip slip 

(normal) 

FIGURE 24.-Four types of fault movement, characterized 

by movement relative to the fault and to the horizontal. 

Most faults in the San Francisco Bay region show rightlateral strike slip-the characteristic movement for the 

San Andreas fault (from Wesson and others, 1975, p. 

A12, fig. 4). 

Most of these faults have generated one or 

more damaging earthquakes. In those places 

where surface displacement has accompanied 

an earthquake, the sense of the displacement indicates right-lateral strike slip. Surface displacements accompanied the earthquakes of 1861, 

1868, and 1906 (see table 4); it was also reported 

for the earthquakes near Hayward in 1836 and 

near Woodside in 1838. 

A few bay-region active faults depart as 

much as 45° from vertical, and many have a 

large component of vertical slip (fig. 24). These, 

too, are part of the San Andreas system. Because 

they are more difficult to identify and map than 

vertical strike-slip faults, more of these dip-slip 

faults may still be undiscovered. 

Most active faults display clear evidence of 

repeated past movements (fig. 25), and because 

they consist of zones of crushed and broken 

rock, they are weak and likely to fail again. Hazards from surface faulting exist where buildings 

or other structures straddle the fault. Folklore 

and movie scripts commonly depict faults as 

gaping chasms that indiscriminately swallow 

both people and buildings. Such effects are unrealistic; nevertheless, fault displacement within 

the foundation of a building or major structure, 

such as a dam or powerplant, can cause appalling damage. Strike-slip displacements at the 

surface of as much as 16 ft accompanied the 

1906 San Francisco earthquake, and even several inches of displacement can seriously damage many buildings. 
46 GEOLOGIC PRINCIPLES FOR PRUDENT lAND USE 

Linear valley 

Offset drainage 

S chann~ 

carp Scarp 

Bench S . pnng 

Offset drainage 

channel 

Shutter 

ridge Linear valley 

or trough 

FIGURE 25.-Distinctive landforms and drainage patterns alined along strike-slip fault are visible evidence that fault 

movement is recent enough to have interrupted the more gradual processes of erosion and deposition. 

GROUND MOTION 

Shaking, due to the passage of seismic 

waves through the Earth, causes most of the 

damage in earthquakes. Several different waves 

propagate from the rupture surface and are reflected and refracted by layers with varying 

physical properties in the Earth's crust and 

deeper interior. Compressional, or P, waves and 

shear, or S, waves traverse the Earth's interior; 

surface waves travel along the Earth's surface 

layers. Each of these waves travels at a different 

velocity (compressional waves are fastest, surface waves slowest), possesses typical amplitude 

and frequency patterns, and attenuates at varying rates with distance from the fault, or earthquake source. 

The most accurate and detailed information 

on ground motion comes from the records written by strong-motion seismographs (fig. 26). 

These instruments record ground motion when 

they are triggered by a shock above a predetermined threshold level of acceleration, generally 

about 0.01 g (1 g=32 ft!s2). Estimates of the expected peak acceleration of the ground at a distance of 3 mi from a magnitude 6.5 earthquake 

range from about 0.3 to more than 1.0 g (Boore 

and others, 1978). The records obtained provide 

precise information about the amplitude, frequency, and duration of shaking. Unfortunately, 

very few instrumental recordings of damaging 

levels of ground motion are available, and nearly 

all the strong-motion data from earthquakes 

larger than magnitude 6 come from instruments 
FAULTS AND EARTHQUAKES 47 

1.5 

<:), S. 16° E. component, ~ 1.0 

z" ground acceleration 

0 0.5 i= 

<( 

0:: 

LU 0 ...J 

LU 

u 

u -0.5 <( 

-1.0 

c 150 z 

0 

u S. 16°E. component zLU 100 -en ground velocity ·a: 

~~ 50 uen 

Oa: 

...JLU 0 LUI-

>LU 

:::!: -50 i= 

z 

~ -100 

50 • en 1-a: S. 16° E. component, ZLU 

LUI-

:!:LU 

LU:i: 

u- 0 <(I-

...JZ 

a..LU 

enu 

Ci~ 

-50 

0 10 15 

TIME, IN SECONDS 

FIGURE 26.-Strong-motion seismograph records of horizontal ground motion at Pacoima damsite during the 

San Fernando, Calif., earthquake of February 9, 1971 

(after Trifunac and Hudson, 1971, p. 1406, fig. 13). Velocity (middle) and displacement (bottom) records were 

obtained by integrating the acceleration record (top) 

once and twice, respectively. This record is one of very 

few from an instrument less than 5 mi from a fault 

break. 

that are more than 5 mi away from the fault 

break. No records have been obtained from 

within 50 mi of a magnitude 8 or greater earthquake. Because of the absence of instrumental 

data, much uncertainty accompanies estimates 

of the severity and dynamics of shaking close to 

the fault, where damage is greatest. 

The severity of ground motion diminishes 

with distance from the earthquake source, and, 

where this source is a vertical fault and the surrounding geology is uniform, the pattern of 

shaking is simple and regular. In the absence of 

instrumental data, earthquake intensity is commonly used as a measure of shaking. Earthquake 

intensity is a number describing the effects of an 

earthquake on people, structures, and the 

Earth's surface. Though expressed quantitatively, intensity is actually a subjective measure 

(table 5). Intensity maps-that is, maps that show 

the severity of shaking in terms of observed 

earthquake effects-typically display nearly circular or elliptical bands of equal intensity of 

shaking surrounding the fault. On most of these 

maps, the strongest effects are nearest the fault, 

and intensities are progressively lower outward 

from the fault (fig. 27). T)lis simple distance-intensity relation is complicated where the earthquake fault dips at a low angle or where the 

geology is complex and nonuniform. 

The effect of local geology on ground motion appears on many intensity maps as pockets 

and irregular areas of severe damage that 

closely match the surface extent of certain 

geologic units. Areas underlain by thick deposits of uncompacted artificial fill, by soft, 

water-saturated mud, or by unconsolidated 

stream sediment shake longer and harder than 

areas underlain by bedrock, especially where 

these unconsolidated deposits are more than 50 

ft thick and overlie firm bedrock (Borcherdt, 

1975). Thus, some lowland areas 5 to 10 mi 

from an active fault may undergo as much 

shaking damage as bedrock sites much closer 

to it. Fortunately, most lowland areas in the bay 

region are underlain by well-compacted, welldrained sediment that behaves much like bedrock. The areas in which local geology is most 

likely to amplify the shaking are those underlain by bay mud, by old artificial fill resting on 

bay mud, and by stream sediment within a mile 

or two of the original landward boundary of the 

bay marshlands. Because of the possibility of 

dike failures during strong shaking, reclaimed 

marshlands underlain by these geologic units 

are especially vulnerable. 
48 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

TABLE 5.-Modified Mercalli scale of earthquake intensity 

(Modified from Richter (1958, p. 137-138). See Uniform Building Code for specifications on the quality of 

masonry construction (International Conference of Building Officials, 1976)] 

Intensity Description 

I Not felt. 

II Felt by persons at rest, on upper floors, or favorably placed. 

III Felt indoors. Hanging objects swing. Vibration like passing of light 

trucks. Duration estimated. May not be recognized as an earthquake. 

N Hanging objects swing. Vibration like passing of heavy trucks, or sensation of jolt like a heavy ball striking the walls. Standing automobiles 

rock. Windows, dishes, doors rattle. Wooden walls and frame may creak. 

V Felt outdoors; direction estimated. Sleepers wakened. liquids disturbed, 

some spilled. Small unstable objects displaced or upset. Doors swing. 

Shutters, pictures move. Pendulum clocks stop, start, change rate. 

VI Felt by all. Many frightened and run outdoors. Persons walk unsteadily. 

Windows, dishes, glassware broken. Knickknacks, books, and so on 

knocked off shelves. Pictures fall off walls. Furniture moved or 

overturned. Weak plaster and masonry D cracked. 

VII Difficult to stand. Noticed by drivers of automobiles. Hanging objects 

quiver. Furniture broken. Weak chimneys broken at roofline. 

Damage to masonry D, including cracks; fall of plaster, loose bricks, 

stones, titles, and unbraced parapets. Small slides and cave-ins 

along sand or gravel banks. Large bells ring. 

VIII Steering of automobiles affected. Damage to masonry C; partial collapse. 

Some damage to masonry B, none to masonry A. Fall of stucco and some 

masonry walls. Twisting, fall of chimneys, factory stacks, monuments, 

towers, elevated tanks. Frame houses moved on foundations if not 

bolted down; loose panel walls thrown out. Decayed pilings broken 

off. Branches broken from trees. Changes in flow or temperature of 

springs and wells. Cracks in wet ground and on steep slopes. 

IX General panic. Masonry D destroyed; masonry C heavily damaged, 

sometimes with complete collapse; masonry B seriously damaged. 

General damage to foundations. Frame structures, if not bolted, 

shifted off foundations. Frames racked. Serious damage to reservoirs. 

Underground pipes broken. Conspicuous cracks in ground 

and liquefaction. 

X Most masonry and frame structures destroyed with their foundations. 

Some well-built wooden structures and bridges destroyed. Serious 

damage to dams, dikes, and embankments. Large landslides. Water 

thrown onto banks of canals, rivers, lakes, and so on. Sand and 

mud shifted horizontally on beaches and flatland. Rails bent 

slightly. 

XI Rails bent greatly. Underground pipelines completely out of service. 

XII Damage nearly total. Large rock masses displaced. Unes of sight and 

level distorted. Objects thrown into the air. 
FAULTS AND EARTHQUAKES 49 

123° 122° 121° 

• 

• • 

• • • 0 • • • 

• • • • 

• • • Ukiah ti • • • • • • • 

39° • 

• 

eo 

• 0 

0 

0 

0 • 

0 0 

0 

0 • 

..0 • • • 

7 

38° (") 

..,.. • ~ 

..,.. • • 

(") San • • • 

• 0 

(") • I-IV ~ • 

'7 • 0 

~ 0 0@ 

San Jose 

0 

Limits of felt area 

• 

• 

37" • 

• 

20:56:46.5 P.s.t.,October 1,1969 

0 20 40 MILES I I I I I I I 

0 20 40 KILOMETERS 

FIGURE 27.-Modified Mercalli intensity map of the Santa Rosa, Calif., earthquake of October 1, 1969. Small circles 

denote where intensity observations (table 5) were reported; solid dots denote where earthquake was not felt. 

This magnitude 5.6 earthquake, which lasted 10 to 15 seconds, caused building damage of $6 million and damage to dwelling contents of $1.25 million (Steinbrugge and others, 197Q, p. 94). Adapted from Steinbrugge and 

others (1970, p. 95). 
50 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

GROUND FAILURE 

Earthquake shaking also imposes cyclic 

vertical and horizontal forces on both rock and 

soil. Most California earthquakes larger than 

about magnitude 5 shake the ground long 

enough to trigger at least a few small landslides, 

and still larger earthquakes may trigger hundreds. During the great 1906 San Francisco 

earthquake, landslides and other kinds of 

ground failure were reported from Salinas northward to Eureka and across nearly the entire 

width of the Coast Ranges. Earth flows, debris 

slides, and slumps blocked roads and railroads, 

damaged dwellings and other structures, overrode agricultural land, and dammed flowing 

streams. After the earthquake, the slide scars 

and deposits remained as the most obvious evidence of violent shaking. Still visible today, they 

remind us that earthquake-induced landslides 

are a major hazard throughout the bay region. 

lANDSLIDING 

The pervasive landsliding in 1906 is attributed to three main causes: Abundant unstable 

slopes, high ground-water levels, and unusually 

strong shaking. Hillslopes in the northern 

California Coast Ranges, which make up most of 

the bay region, are covered with both ancient 

and active landslide deposits; and even slopes 

that appear stable can be reactivated if they are 

shaken or overloaded with ground water. At the 

time of the great San Francisco earthquake, most 

potential landslide areas were heavily charged 

with ground water because of a period of unusually heavy rainfall during the preceding March 

(Youd and Hoose, 1978). Ground motion during 

this magnitude 8.3 earthquake was both severe 

and of long duration, so that the forces resisting 

ground failure were overcome. 

Although we cannot predict the magnitude 

of the next bay-region earthquake or tell 

whether it will come during a period of excess 

ground moisture, potential landslides are presently at least as numerous now as they were in 

1906. The risk to life and property, of course, is 

much greater now. Hillsides where cattle and 

sheep grazed in 1906 are today completely built 

over. Streams that drain unstable or marginally 

stable slopes now contain reservoirs; other 

streams, which earthquake-induced landslides 

may temporarily and insecurely dam, flow 

downstream through urbanized lowlands. And 

at countless points, complex networks of roads, 

railroads, and utility and communication lines 

cross terrain of doubtful stability. 

LIQUEFACTION 

Although earthquake-induced landsliding is 

chiefly a hillside process, earthquakes also cause 

ground failures in the lowlands. Some lowland 

ground failures resemble hillside landslides and 

result from the failure of steep slopes along 

streambanks or in manmade cuts. Others, however, occur on nearly flat ground and are accompanied by lateral spreading, by settlement, or by 

the ejection of fine sand from beneath the surface. Many of these flat-ground failures are 

caused by liquefaction-a process that Youd 

(1973, p. 3; Youd and others, 1975) defined as 

"the transformation of a granular material from a 

solid state into a liquefied state as a consequence of increased pore-water pressures." 

Saturated sediment may also liquefy and change 

its shape under static loading, but liquefaction is 

most apt to take place during earthquake shaking. 

Lateral spreads, the most common evidence 

of liquefaction, cause arcuate or linear ground 

fractures. These fractures are typically tens or 

hundreds of feet long, a few feet deep, and a few 

inches to a few feet wide at the surface. They 

separate masses of earth or soil that are slightly 

tilted or that have shifted horizontally as though 

they were pulled apart. Water may be visible in 

the deeper fractures, and even where it is not, 

sand boils-small inverted cones of clean sand 1 

or 2 ft in diameter-may remain as evidence that 

subsurface sediment has flowed to the surface. 

Settlement due to liquefaction is indicated 

by relative downward movement of the land surface adjacent to such fixed objects as well casings and pilings. It may amount to several feet if 

large volumes of subsurface sediment liquefy 

and flow laterally. 

During earthquakes, sediment liquefies and 

causes significant ground failures only if all four 

of the following geologic conditions are present: 
FAULTS AND EARTHQUAKES 51 

( 1) A potentially liquefiable bed or lens of porous 

well-sorted sand, (2) saturation of the intergranular pore spaces in the bed or lens by water, 

(3) confinement of pore water by impermeable 

layers above and below the liquefiable bed, and 

(4) proximity of the liquefiable bed to the surface 

(50ft or less). 

In the bay region, few geologic settings 

meet all four of these conditions. Those that do, 

occur locally where sand layers are interbedded 

either with bay mud or with flood-plain silt and 

clay along stream valleys (fig. 28). Some landslides on hillslopes also move at least partially by 

liquefaction, but in this report we do not differentiate these from other landslides. 

Although the possibility of damage from 

liquefaction is more localized than that from 

ground shaking or landsliding, it is a hazard at 

some sites on the reclaimed marshlands fringing 

the bay, especially where land at or below mean 

sea level is protected by dikes. In such places, 

lateral spreading or settlement may contribute to 

dike failure and lead to extensive flooding. Even 

without dike failures, the ground failures that result from liquefaction ca.n damage structures and 

roads and interrupt sewer, water, and utility service. 
52 

37"45' 

30' 

GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

122°30' 15' 

LIQUEFACTION POTENTIAL 

ZONE GEOLOGIC UNIT OF CLAY-FREE GRANULAR 

LAYERS 

• 

W/}~'!1 

[J] 

Bay mud and 

underlying 

sediment 

Holocene 

alluvium 

Holocene 

alluvium 

Generally moderate, locally high where clean granular 

layers are present in bay mud 

Moderate, water table normally 

above 3 meters (10 feet) 

Moderate, water table normally 

below 3 meters (10 feet) 

Late Pleistocene 

alluvium Generally low 

0 2 4 6 MILES 

I I 0 I 2 4 I 

G KILOMETERS 

122°00' 

FIGURE 28.-Preliminary map showing liquefaction potential in the southern San Francisco Bay region. The map shows 

the generalized liquefaction potential of granular layers in each map zone but does not delineate the locations of 

these layers. Thus, the map is useful for designating zones where special consideration should be given to the possibility of liquefaction, but is not valid for assessing a given site (from Youd and others, 1975, p. A73, fig. 50). 
FAULTS AND EARTHQUAKES 53 

ACTIONS TO ENHANCE LAND USE 

FAULTING 

State law now regulates new building for 

human occupancy along active faults. To the 

extent that this regulation has slowed the pace 

of development in active fault zones, it will undoubtedly save lives and minimize damage 

during future earthquakes. Fair and consistent 

administration of these regulations, however, is 

difficult because few faults exhibit the unity 

and simple continuity that their names imply. 

Most, like the San Andreas, form a series of 

sharp, well-defined breaks a few inches to a 

few feet wide and from 1 to 3 mi long. Individual breaks vary somewhat in trend, in 

length, and in their relation to neighboring 

breaks; some overlap, some split into branches, 

and others die out into broad, ill-defined fracture zones (fig. 29). Collectively these breaks 

define a zone that ranges from about 100 to 

more than 1,000 ft in width. It is the continuity 

of this zone that warrants naming a fault as a 

single continuous feature. 

Evaluation of the possibilities for surface 

faulting at sites within this zone requires careful 

geologic appraisal. In most places, trenches or 

test pits must be dug or holes drilled to confirm 

known or suspected fault breaks, to locate 

branch faults, and to identify relatively safe sites 

in unbroken rock. 

GROUND MOTION 

Most conventional dwellings and low-rise 

buildings possess natural periods of vibration in 

the range from 0.1 to 1 s. S waves typically induce horizontal shaking of the ground in this 

period range. When ground shaking is in tune 

with the natural period of structures and is prolonged, buildings begin to sway and vibrate. 

Unless structures are designed and built to 

withstand the horizontal forces imposed by this 

shaking, as well as the vertical forces that result from the weight of the structure and its 

contents, they will fail. Inexpensive methods of 

protecting conventional houses include 

plywood sheathing and diagonal bracing of 

frames (Yanev, 1974}; variations on these 

methods, described in the Uniform Building 

Code (International Conference of Building Officials, 1976), protect most modern residences 

in the bay region. Tall buildings, dams, and 

other massive or unusually large structures are 

most sensitive to longer period ground motion. 

Because of their cost and the consequences of 

failure, such structures require more sophisticated analysis, as well as more care in siting 

and in the engineering design and choice of 

building materials. 

Design of structures to survive earthquakes 

requires knowledge of ground shaking over a 

critical range of frequencies or periods. This information may be obtained from the analysis of 

strong-motion records or, where such analysis is 

impractical, from records of earthquake intensities. Intensity data exist for historical earthquakes that occurred before the deployment of 

strong-motion seismographs and thus constitute 

a valuable data base for setting design criteria 

for earthquake-resistant construction. 

GROUND FAILURE 

Earthquake-induced landslides resemble 

those caused by other processes and the same 

strategies for reducing their impact apply. These 

strategies were briefly discussed in the preceding section entitled "The Hillsides and Uplands." Of the many hazards related to earthquake-induced landslides, the most serious are 

those that cause the sudden release of impounded water. Slides may undermine or 

weaken the foundation of a dam and cause sudden failure. Large volumes of debris may slide 

into a reservoir and generate giant waves; where 

such waves overtop earthfill dams, they may 

trigger catastrophic failure of the dam. Slide debris may temporarily block natural stream 

courses, cause rapid impoundment, and threaten 

sudden and unpredictable flooding. Examples of 

such catastrophes and near-catastrophes are 

well documented from various parts of the 

United States and from other countries as well. 

None has thus far occurred in the bay region, but 

our future safety depends on knowledge of the 

degree of hazard and on exercising vigilance 

and caution in developing our hillside lands. 
54 

------ Fault breaks 

Solid line, obvious photogeologlc or field evidence of recent movement shown 

by scarp lines, trenches, sag ponds, systematically diverted drainages, and 

historical records of fault displacement'; dashed line, less obvious photogeologic or field evidence of recent movement, but very probably a fault 

break 

Areas probably underlain by landslide deposits 

Enclosed by solid line where limit is well defined, by dashed .line where limit 

is approximate or uncertain. Arrows show generalized direction of movement 

f'rne brief notes along the fault breaks indicate locations where the features mentioned are especially clear. 

Visible fault-trace features are not limited to the locations noted but are present to some degree all along 

the mapped fault lines 

0 1 MILE 

0 1 KI LO M ETER 

122° 22'30" 

FIGURE 29.-The individual breaks that make up an active fault zone branch, overlap, and die out, as on 

this map of the San Andreas fault in northwestern Sonoma County. Adapted from Brown and Wolfe 

(1972). 
FAULTS AND EARTHQUAKES 55 

LIQUEFACTION 

Geologists and geotechnical engineers ernploy drilling and other subsurface geologic and 

geophysical methods to determine whether potentially liquefiable sand underlies a site. A welldesigned, well-executed geologic exploration 

program distinguishes hazardous areas from 

those that are relatively safe and provides a 

basis for prudent site development. However, 

such programs have come into use only in the 

past 15 years, after liquefaction began to be recognized as a separate earthquake hazard. Until 

about 1965, few if any, geologic site studies in 

the reclaimed marshlands around the bay employed specific testing to determine whether 

earthquake-induced liquefaction was possible; 

thus, older developments near the bay may still 

be susceptible to this kind, of damage. 
THE USE OF EARTH-SCIENCE 

INFORMATION 
The following six examples illustrate some of the range and types of 

uses of Earth-science information by decisionmakers in the San Francisco Bay region. Included among these decisionmakers are State legislators, State agencies, regional commissioners, county board supervisors, 

mayors, councilpersons, municipal engineers, building inspectors, and realestate sellers. The examples affect an entire State, a nine-county metropolitan area, a bay-and-shoreline district, a 1,312-mi2 county, a city of 55,000 

people, and individual lots and acreages offered for sale. Similar examples 

from the bay region have been reported elsewhere (Kockelman, 1975, 1976, 

1979; Robinson and Spieker, 1978; Kockelman and Brabb, 1979). Each example contains: A summary of the problems or needs faced by the decisionmakers, the Earth-science information needed and available, the specific decisions or actions taken, the methods and procedures used to carry out each 

decision, and brief comments on the impact of each decision and its adaptation to other problems faced by other decisionmakers. Illustrations have been 

modified from the decisionmakers' original documents to show their actual 

use of Earth-science information. 

57 
58 GEOLOGIC PRINCIPLES FOR PRUDENT lAND USE 

Example 1. An entire State 

REGULATING DEVELOPMENT IN ACTIVE FAULT ZONES 

The trace of an active fault cannot always 

be seen at the surface. It may be concealed, and 

a geologist may have to approximate its location. 

Displacements do not always occur along a 

single fault trace; branching segments, braided, 

and echelon faults may result in wide areas of 

disturbance (fig. 30). Therefore, regulatory 

measures for avoiding or reducing the hazards of 

fault rupture commonly require detailed 

geologic investigations to accurately identify 

and evaluate all the strands of the faults. Once 

FIGURE 30.-Diagram of hypothetical fault traces, showing 

possible complexities of faulting that demonstrate the 

necessity for detailed geologic investigations within a 

broad zone astride a known fault-rupture trace. 

these fault strands are located, specific regulations-prohibiting certain uses or requiring specific buildings to be set back from the active 

strands-can be applied. 

Much of the damage associated with fault 

rupture can be limited by regulating construction on active faults. Utility lines and transportation facilities can be located, designed, and operated in such a way as to reduce outages and 

other disruptions. 

INFORMATION AVAILABLE 

In California, many potentially active and 

recently active faults have been identified and 

mapped at various scales; fault maps by the U.S. 

Geological Survey and the California Division of 

Mines and Geology are available at a scale of 

1:24,000 (1 in.= 2,000 ft) or larger. Evidence for 

surface fault displacement, magnitudes of the 

largest historical earthquakes, and estimated 

recurrence intervals for maximum earthquakes 

were summarized for 25 faults in a report on 

seismic zonation edited by Borcherdt (1975) 

that included discussions of patterns of surface 

faulting, fault-zone widths, and amounts of displacement. Some of the methods for using 

Earth-science information and hazard mapping 

in land-use planning and regulations were discussed by Blair and Spangle (1979). 

DECISION 

In response to public concern and because 

of the availability of scientific information, the 

California Legislature (1972a) enacted the Alquist-Priolo Special Studies Zones Act. This act 

provides for public safety in areas subject to fault 

rupture. In addition, the act provides for: 

Geologic reports, project approval by cities and 

counties, exemptions for altering or adding to 

existing structures, disclosure of hazards by realestate sellers and their agents, and the charging 

of reasonable application fees. 
THE USE OF EARTH-SCIENCE INFORMATION 59 

To assist the various cities and counties, the 

act requires the State Geologist of the California 

Division of Mines and Geology to delineate Special Studies Zones that include all "potentially 

and recently active" traces of the San Andreas, 

Calaveras, Hayward, and San Jacinto faults and 

other faults he deems "sufficiently active and 

well-defined" to constitute a potential hazard. 

For the purposes of the act, a fault is deemed 

"sufficiently active" if there is evidence of surface displacement along one or more of its seg0 10 20 MILES 

I I I I I 

37" 0 10 20 KILOMETERS 

FIGURE 31.-Part of Special Studies Zones index map 

(Hart, 1980), showing faults zoned for special geologic 

studies by the State Geologist. The quadrangle name of 

the State official map and the year issued or revised are 

indicated. Part of shaded quadrangle is shown in figure 

32. Surface traces of well-known faults have been dements or branches during the past 11,000 years, 

and a fault is considered "well-defined" if its 

trace is clearly detectable by a trained geologist 

as a physical feature at or just below the ground 

surface (Hart, 1980, p. 5-6). 

The State Geologist initially delineated 

zones about a quarter-mile wide; currently, the 

zones delineated are about 400 to 600 ft wide. 

Surface traces of all faults are shown on topographic maps, on the basis of the best information 

available (fig. 31). Zones are established by 

122° 

lineated on about 300 topographic maps at a scale of 

1:24,000 (1 in.= 2,000 ft). Information about the availability of these maps can be obtained from the Office of 

the State Geologist, California Division of Mines and 

Geology, Room 1341, 1416 Ninth Street, Sacramento, 

CA 95814. 
60 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

selecting turning points located at obvious features about an eighth-mile or less on either side 

of the fault trace (fig. 32); the zone boundaries 

are drafted as straight lines connecting these 

points. Because fault traces vary-some have 

branching segments, curved or discontinuous 

traces, and wide areas of crushed rock (fig. 30)-

the zones are irregular and may exceed a quarter-mile in width. Maps similar to figure 32 show 

faults, historical offsets (indicated by year), displacements caused by creep, and lineaments 

seen on aerial photographs, as well as the 

boundaries of zones. 

In addition, the California Division of Mines 

and Geology staff is presently conducting a 10-

year fault-evaluation program. This program has 

as its objective the evaluation of potentially active faults relative to the potential hazard of surface fault rupturing. The results, methods of 

evaluation, recommended zoning and zone revisions, and some of the problems discovered were 

summarized by Hart and others (1977, 1978, 

1979). 

APPLICATION 

The State Geologist uses U.S. Geological 

Survey 11/2-minute quadrangles (topographic 

series), at a scale of 1:24,000 (1 in.= 2,000 ft), as 

the base for delineating the Special Studies 

Zones. Information from fault and geologic maps 

is transferred to these quadrangle maps. Each 

Special Studies Zones quadrangle map contains 

specific references to the sources of the scientific 

information. 

The Special Studies Zones Act provides that 

cities and counties shall require, before the approval of a project in a Special Studies Zone, "a 

geologic report defining and delineating any 

hazard of surface fault rupture," and that approval shall be in accordance with the policies 

and criteria established by the California Mining 

and Geology Board (California Legislature, 

1972a, sec. 2623). The California Mining and 

Geology Board (in Hart, 1980, app. B) has prepared and adopted specific and detailed criteria. 

In addition, the California Division of Mines and 

Geology provides information on the availability 

of: Waiver forms, guidelines for evaluating surface fault ruptures, maps showing Special 

Studies Zones, indexes to the zone maps, and indexes to geologic reports within the zones. 

The board's criteria prohibit specific development within Special Studies Zones until a 

registered geologist retained by each city and 

county has evaluated geologic reports prepared 

by another registered geologist. The fault information shown on a quadrangle map (fig. 32) is 

not sufficient to meet the requirement for a 

"geologiC report. " In addition, the cities and 

counties must require that developers evaluate 

sites within Special Studies Zones to determine 

whether potential hazards from any faults exist. 

If a city or county finds that no undue hazard 

exists for a specific site, the required geologic report may be waived with the approval of the 

State Geologist. The act and the criteria provide 

that cities and counties may establish more restrictive policies and criteria if they so desire. 

One criterion initially adopted by the board 

provided that "No structure for human occupancy • • • shall be • • • placed across the trace 

of an active fault" or within 50ft of it. The area 

within 50 ft is assumed to be underlain by active 

branches until proved otherwise by a geologic 

investigation by a registered geologist. 

In 1976, the California Legislature (1972a, 

sec. 2621.6(a)) subsequently amended the original Special Studies Zones Act to exclude developments consisting of as many as three 

"single-family wood-frame dwellings" and, 

therefore, removed such buildings from the 

board's criteria. However, many cities and counties retain the 50-ft setback for all structures for 

human occupancy; others require even greater 

setbacks (fig. 33). 
THE USE OF EARTH-SCIENCE INFORMATION 61 

12n2'30'' 

3JD 22' ..,.,.,,..,.. ....... T'""I:<Ir-II'!T"ft""7"-r-77"-.-,..,...--;:---..,----,-..,.,.,.._~,.-------,-...-::>7"--:-::-,.,.,.,...,::-17.:::--r;,.....-:""'7"""''"'1""""'1'"~-\"\\--;-r---;-;-t-r---7~;--r~r--;-pr-Tr::--7"1 

0 

0 

FIGURE 32.-Part of the Mindego Hill quadrangle, onginally mapped at a scale of 1:24,000 (1 in.= 2,000 ft) by 

the California Division of Mines and Geology (1974), 

showing a Special Studies Zone along a section of the 

San Andreas fault. Faults are indicated by a solid line 

where accurately located, by a long dash where approximately located, by a short dash where inferred, 

1 MILE 

1 KILOMETER 

and by dots where concealed. The map also shows zone 

boundaries with turning points. Part of the shaded area 

is shown in figure 33 at a large scale. As of January 1, 

1982, Special Studies Zones affect 25 counties and more 

than 70 cities in California. Reproducible masters have 

been provided for each city and county. 
62 GEOlOGIC PRINCIPlES FOR PRUDENT lAND USE 

EXPLANATION 

0 500 FEET 

Fault trace,location known I 1 

r 

1 1

1 1 I 

0 150 METERS 

Fault trace, location inferred 

FIGURE 33.-Example of minimum easements required for 

building setbacks from active fault traces, as mandated 

by the Portola Valley Town Council (1973). All new 

building construction is prohibited within the 100-ftwide lightly shaded zone (50ft on each side of the accurately located section of the San Andreas fault}; strucCOMMENTS 

This example illustrates how geologic and 

seismic information is used by State legislators, 

State geologists, city and county officials, and 

consulting geologists to avoid fault-rupture areas 

throughout California. The act's provisions, the 

board's criteria, and local ordinances deter the 

tures housing more than a single family are required to 

be 125ft from the fault trace (dark shading). Where the 

location of the fault trace is less well known, more conservative setbacks of 100ft for single-family residences 

and of 17 5 ft for structures with larger occupancies are 

required (modified from Mader and others, 1972, fig. 5). 

placement of public and private buildings over 

faults that may creep, or move suddenly during 

a major earthquake. This method of providing 

for public safety can be adapted to other types of 

potential ground failures, including landslides 

and liquefaction, and to other States where similar hazards exist and where adequate scientific 

information can be obtained. 
THE USE OF EARTH-SCIENCE INFORMATION 63 

Example 2. A nine-county metropolitan area 

IDENTIFYING POTENTIAL WASTE-DISPOSAL SITES 

According to the Association of Bay Area 

Governments (ABAG) (1978b, p. V-17), about 

11.5 million tons of solid waste was produced in 

the San Francisco Bay region in 1975, of which 

about 820,000 tons was hazardous industrial 

waste. According to Perkins and others (1977), 

these figures will probably increase by about 5 

percent annually, and existing techniques of 

landfill and waste disposal may be the only effective methods of dealing with the problem. 

Many wastes are long lived and chemically 

complex and, when mixed, may form new compounds whose effects on health are largely unknown. Ground water contaminated by such 

wastes may remain in an unusable or hazardous 

condition for decades or even centuries. Pettyjohn ( 1979) reported that such contamination 

has occurred at Keizer, Oreg., Niagara Falls, 

N.Y., Barstow, Calif., Bellevue, Ohio, and in 

many other areas. Waste-disposal impacts on the 

San Francisco Bay have been discussed by Russell and others, Luoma and Cloern, and Hall (in 

Kockelman and others, 1982, p. 127-162). 

The four most common types of solid-waste 

disposal---open dumps, sanitary landfills, incineration, and onsite storage-carry an inherent 

potential for pollution. Seepage of rainwater 

through the wastes leaches out constituents that 

can reach the ground water; this leachate is generally contaminated both biologically and chemically. The extent of pollution from the leachate 

depends on the hydrogeology of the disposal 

site. The possibility of pollution is greatest where 

waste-disposal sites are located in flood-prone 

areas, near fault traces, in landslide areas, on 

permeable soil and rocks, on steep slopes, in 

areas of high precipitation, in areas where the 

water table is high, or in recharge areas. 

INFORMATION AVAILABLE 

Most of the basic information needed to 

make wise decisions concerning safe locations 

for waste-disposal sites in the bay region is currently available-information on flood-prone 

areas, geology, relative intensities of ground 

shaking, surface traces of faults, landslide deposits, areas susceptible to landsliding, slopes, 

annual precipitation, maximum probable well 

yield, areas with high water table, and historical 

marshlands. 

Sources of pollution, critical resources, critical geologic and hydrologic conditions, and 

suitability ratings for the various types of wastedisposal sites were discussed by Hines (1973). 

The development costs associated with geologic 

and hydrologic constraints and resources for several land uses were demonstrated in a quantitative land -capability analysis by Laird and others 

(1979). 

A wide range of hydrologic and geologic information is needed to identify safe waste-disposal sites. Although large-scale and detailed information is necessary for identifying specific 

sites, smaller scale and less detailed information, 

if accurate and clear, can help identify acceptable areas. The use of a wide range of information requires the selection and systematic application of specific criteria relating to surface- and 

ground-water quality. These criteria and their 

application can assist decisionmakers in understanding and using the results to identify and 

regulate specific sites. 

DECISION 

Hazardous-waste management was identified as a regional concern in seven of the nine 

bay-area county plans and by the nine-county 

group charged with solid-waste-plan coordination (Perkins and others, 1977). Because of this 

concern, ABAG identified potential Class I sites 

(Perkins, 1978) as part of a regional solid-waste~ 

management plan. Class I sites are disposal 

areas for such hazardous wastes as toxic chemicals, soluble industrial wastes, saline brines, and 

unquenched incineration ashes. The California 
64 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

Hazardous Waste Control Law (California Legislature, 1978a) defines ~~hazardous waste" as: 

• • • a waste, or combination of wastes, which because of its 

quantity, concentration, or physical, chemical, or infectious 

characteristics may either: (a) Cause, or significantly contribute to, an increase in mortality or an increase in serious 

irreversible, or incapacitating reversible, illness. (b) Pose a 

substantial present or potential hazard to human health or 

environment when improperly treated, stored, transported, 

disposed of, or otherwise managed. 

The ABAG study identifies areas that warrant further study for use as disposal sites for 

toxic and hazardous wastes, and recommends 

that these disposal sites and facilities be located 

so as not to adversely affect human health and 

safety, air and water quality, wildlife, critical environmental resources, and urbanized areas 

(Perkins and others, 1977). 

APPLICATION 

The hydrogeologic criteria used by ABAG 

for evaluating waste-disposal sites have been 

adapted from those of the California Water Resources Control Board (1976) and those 

suggested by Hines (1973). The board also requires all Class I sites to have a natural barrier to 

prevent downward movement of the wastes to 

usable ground water. Sites that may be subject 

to inundation, washout, faulting, liquefaction, 

landsliding, or accelerated erosion are not acceptable. The criteria used by ABAG have been 

divided into the three groups listed in table 6. 

TABLE 6.-List of criteria used to select Class I waste-disposal sites 

[Modified from Perkins (1978, table 2). The factors have been divided into three classes: Strict, graded, and acceptability. The strict factors eliminate those 

areas that are hydrologically and geologically unsuitable for sites; the graded factors identify the remaining areas as most likely, moderately likely, or unlikely to be found suitable; and the acceptability factors attempt to identify site limitations that are unrelated to hydrology or geology, or those that require onsite investigation) 

Factors 

Out of flood-prone areas ............... . 

Not in areas that average more 

than 30-in. of rain. 

Not in active earthquake area ...... . 

Not on unconsolidated materials ... 

Not on unstable materials or on 

slopes greater than 15 percent. 

Source 

Strict 

Areas on 100-year flood plains 

(Limerinos and others, 1973). 

Areas within 30-in. isohyet (Rantz, 

1971). 

Areas within 0.2 km of a fault 

capable of producing ground 

shaking (Association of Bay Area 

Governments, 1978a). 

Areas shown as Quaternary or 

Quaternary/Tertiary on maps by 

Schlocker (1970) and California 

Division of Mines and Geology. 

Areas shown as categories 1 or 2 on 

map by Nilsen and others (1979). 

Graded 

Amount of precipitation.................. Assign "3" to 0 to 20 in. and "2" 

to 20 to 30 in., as shown on map 

by Rantz (1971). 

Yield from wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Assign "3" to category A, "1" to 

category B, and "0" to categories 

C and D, as shown on map by 

Webster (1972). 

Rock types........................................ Assign "1" to the Franciscan 

Complex and granitic rocks, 

and "3" to other Tertiary 

or older rocks, as shown 

on maps by Schlocker (1970) 

(1970) and California Division 

of Mines and Geology. 

Soil permeability . .. .. . .... .. . .. .. .. . .. .... .. . Assign "3" to extremely impermeable 

soils, "2" to very impermeable 

soils, "1" to moderately permeable 

soils, and "0" to permeable soils, 

Relative slope stability and soil 

erosion. 

as shown on U.S. Soil Conservation 

Service map of soil associations. 

Assign "3" to category 1 and "1" 

to category 2, as shown on map 

by Nilsen and others (1979). 

Factors Source 

Acceptability 

Not in or adjacent to developed 

areas or areas with development 

potential. 

Not publicly owned for parks, 

recreation, and so on. 

Not in ecologically sensitive 

areas. 

Not affecting significant 

agricultural crops. 

Reasonably accessible by truck ..... 

Acceptable to the public and the 

Government (to the extent 

possible before public 

workshops). 

Located on shale or sandstone, 

not on highly sheared materials. 

Away from waters used for 

drinking or recreation. 

Developed lands and lands with 

development potential, as shown on 

ABAG's Local Policy Survey 

Summary Map. 

Road maps (various scales); 

local plans when applicable. 

USGS topographic quadrangles 

(scale, 1:24,000) and ABAG's 

Areas of Critical Environmental 

Concern (Association of Bay Area 

Governments, 1976). 

No available maps; general 

information from ABAG's San 

Mateo Coast Corridor Evaluation 

(Association of Bay Area 

Governments, 1975) and Areas 

of Critical Environmental 

Concern (Association of Bay 

Area Governments, 1976). 

Road maps (various scales) and ABAG 

base map. 

Based on discussions with selected 

county staff. 

Various geologic maps. 

ABAG base map. 
THE USE OF EARTH-SCIENCE INFORMATION 65 

Map information from the sources listed in 

table 6 was digitized and converted into grid 

cells (1/4 km2 , or approx 62 acres) to create eight 

computer files. These computer files were then 

combined so that a shaded map including both 

the "strict" and "graded" factors could be drawn 

by the printer-plotter (fig. 34). Cells with similar 

characteristics in each county were grouped, 

and the "acceptability" factors manually applied 

(table 7). As a result, several cells located in several counties were identified as possibly acceptable for use as Class I waste-disposal sites (fig. 

35). More than 350 mi2 of the San Francisco Bay 

region meets the strict hydrologic and geologic 

factors. Of this area, more than 85 mi2 is only 

marginally acceptable, and more than 54 mi2 

possibly acceptable. The other areas are in or 

near urbanized areas, or fail to meet one or more 

of the other "acceptability" factors. The general 

areas shown in figure 35 as possibly acceptable 

would be the focus of any ::;ite investigation (Perkins, 1978). 

0 1 2 3 4 MILES 

I I 11 1

1 I I I 

0 1 2 3 4 KILOMETERS 

FIGURE 34.-Part of a map from Perkins (1978) produced by a printer-plotter after a minicomputer was used to combine 

individual digitized maps and apply strict and graded factors. Shaded grid cells indicate areas that meet strict and 

graded factors (table 6) for waste-disposal sites. Letters designate areas (table 7) having similar social and environmental characteristics to which the acceptability factors were applied manually. 
66 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

COMMENTS 

adverse effects on surface- and ground-water 

supplies. 

This example shows how a wide range of 

hydrologic and geologic information, as well as 

physical, social, and political data, can be digitized and evaluated together through the use 

of a computer. The study, by a nine-county regional planning agency, covered a 7,000-mi2 

area. Selection of future sites for the disposal of 

hazardous wastes on the basis of the study's 

criteria will substantially reduce the likelihood of 

These criteria and this method of protecting 

water quality (and public health) also can be 

adapted where similar wastes must be disposed 

of on land and where adequate hydrologic and 

geologic information is available. The value to 

other counties and regions is increased because 

the California Legislature (1972b) requires solidwaste management plans, and section 208 of the 

Federal Water Pollution Control Act (U.S. Congress, 1977) requires waste-treatment-management plans. 

TABLE 1.-Application of waste-disposal-site factors to several areas in Contra Costa County 

(From Perkins (1918, app. B). Codes refer to some of the circled areas in figure 34. Acceptability tables for each designated area in each county within the 

San Francisco Bay region are available from: Association of Bay Area Governments, Hotel Claremont, Berkeley, CA 94105] 

Size Nearest Overall 

Code Location Present use Adjacent uses(s) Transportation Geologic surface Other areal (cells) (cells) access materials water issues acceptability 

F 15 Lime Ridge Undeveloped Urban, undeveloped, Interstate High- Cretaceous Drainage Probably not Probably unand (15). and open space. way 680; Ygna- and Ter- through politically acceptable for 

Concord cio Valley Road tiary sand- Walnut Creek acceptable; all but one 

Hills. through residen- stone and and Concord one cell is scattered cell 

tial areas. shale. to Carquinez scattered. that is unacStraits. ceptable. 

G 11 East of San Undeveloped Urban and undevel- Interstate High- Cretaceous Drainage to ······················ Probably unRamon (11). oped, grazing. way 680; poor and Ter- SanRamon acceptable 

Valley. to outlying tiary sand- Creek. for all but 

areas. stone and five scattered 

shale, cells that are 

largely unacceptable. 

unconsolida ted. 

3 Hills south Development Urban and unde- State Highway Cretaceous Drainage Scattered Unacceptable. 

of Pitts- potential veloped. 4;Buchanan and Ter- through cells. 

burg. (1), unde- and Kirker tiary sand- Pittsburg; 

veloped (2), Pass Roads. stone and Kirker 

grazing. shale. Creek to 

delta. 

J 56 Northern Urban (1), Urban and un- State Highways Tertiary Drainage ...................... Probably unacCentral develop- developed, graz- 4 and 160. sandstone through ceptable for 

Valley ment po- ing and crop and shale. Contra undeveloped 

foothills. tential rows. Costa; Sand cells. 

(28), un- and Marsh 

developed Creeks to 

(21). Suisun 

Marsh. 

K 121 Southern Undeveloped Undeveloped, State Highway Cretaceous Drainage ······················ Possibly acCentral (121). grazing and 4 and Byron and Ter- through ceptable. 

Valley crop rows. Highway; tiary sand- Marsh and 

foothills. to outlying stone and Kellogg 

areas. shale. Creeks to 

San Joaquin 

River. 

L 16 North of Undeveloped Undeveloped, State Highway Largely Drainage ...................... Do. 

Altamont (16). grazing. 4 and Vasco Creta- through 

Pass. Road; poor to ceo us Kellogg 

outlying areas. sandstone Creek to 

and shale. San Joaquin 

River. 
THE USE OF EARTH-SCIENCE INFORMATION 

_ _j _______ -.......l_ t:'-..-.......... _j \. ') • 

.... 

. \ \. ) \ 

\ / \ '...__ ;_., - '- '.1·-· I 

I ·~ 

l-, 

\ 

SONOMA COUNTY ' 

COUNTY \ .----·-...... 

0 

·-·, -, 

.santa 

Rosa 

A 

A ''-., 0 Petaluma 

,'- B ... ~.---, 

c '""'· 

10 20 MILES 

10 20 KILOMETERS 

B )-..,/ ---: 

F( A \ 

\ B \ 

E 

C CONTRA COSTAl 

D 

•concor'H .walnut 

Creek F 

G 

C G 

.Hayward 

ALAMEDA 

D 

.Fremont 

F I L__? 

F I 

l_-. / __________ :y 

A ') 

\ __ 

( • Sunnyvale 

• San 

Jose 0 ' ,-.1 

c F ( 

SANTA CLARA COUNTY \ I 

67 

FIGURE 35.-Part of an index map from Perkins (1978, pL 

1) showing areas (letters) evaluated in the acceptability 

tables (table 7) and areas (shaded) found to be possibly 

acceptable for hazardous-waste disposal (Class I sites). 

The possibly acceptable areas in Alameda County are 

in the vicinity of Altamont Pass; those in Contra Costa 

County are in the eastern part of the county on the edge 

of the Central Valley; those in Napa County border the 

hills in the southern part of the county; and those in 

Solano County are in the hills south of Vacaville, in the 

hills northwest of Fairfield, in the western Montezuma 

Hills, in the hills near Suisun and Denverton, and in the 

hills north of Benicia and Vallejo. 
68 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

Example 3. A bay-and-shoreline district 

PROTECTING AN AQUATIC AND WILDLIFE HABITAT 

The Suisun Marsh (fig. 36), the largest remaining wetland area near San Francisco, encompasses approximately 85,000 acres of tidal 

marsh, managed wetland, and waterwaysmore than 10 percent of California's remaining 

wetlands. It is a wildlife habitat of international 

as well as national importance and provides an 

irrl'portant resting place for waterfowl on the 

Pacific flyway from Canada to Mexico. 

INFORMATION AVAILABLE 

Most of the Earth-science information necessary for preparing and implementing a protection plan for the Suisun Marsh is currently available, much of it at relatively large scales 

(1 :24,000 [1 in.= 2,000 ft]). This information includes topography, the distribution of-and potential for-landslides, the locations of active 

faults, the extent of historical marshlands, and 

data on land use, flooding, and slope. The 

geologic and seismic history, physical and engineering properties, development problems, 

and general land-use capabilities of bay mud 

were discussed in the reports on flatland deposits by Helley and others (1979), on floodprone areas by Waananen and others (1977), and 

on relative slope stability by Nilsen and others 

(1979). 

Aquatic and wildlife habitats can be preserved and enlarged by acquiring marshlands, 

regulating the surrounding uplands, and proper 

management. Information regarding the location 

and character of the marshlands, geologic hazards, existing land uses, and natural resources is 

necessary to secure public and legislative support before preparing a protection plan. Furthermore, accurate, clear, and large-scale maps 

showing the boundaries of the lands to be acquired or regulated are prerequisite to effective 

and legal implementation of such a plan. 

5 MILES 

5 KILOMETERS 

"" - N 

t 

FIGURE 36.-The San Francisco Bay Conservation and Development Commission has jurisdiction over the San 

Francisco Bay system, including San Pablo Bay and 

Suisun Bay; all sloughs, marshlands, tidelands, and 

submerged lands; a 100-ft-wide shoreline strip inland 

from the bay system; diked saltponds; managed wetlands; and certain tributary waterways. 

DECISION 

The California Legislature in 1974 required the San Francisco Bay Conservation and 

Development Commission (BCDC) to submit a 

protection plan for the Suisun Marsh to the 

Governor and the California Legislature (1974). 

In response, the BCDC (San Francisco Bay 

Conservation and Development Commission, 

1976) prepared a proposal for the preservation 

and enhancement of the large aquatic and 

wildlife habitat, entitled the Suisun Marsh Protection Plan (figs. 37, 38). This plan calls for the 

creation of a primary management area that 
THE USE OF EARTH-SCIENCE INFORMATION 69 

EXPLANATION 

~ Landslide deposits 

J}i:;:;:::J Bay mud 

r77/1 

~ 

Area of high landslide 

potential 

-Acti ve fault 

2 MILES 

2 KILOMETERS 

FIGURE 37 .-Part of the Natural Factors Map used in preparing the Suisun Marsh Protection Plan. The San Francisco Bay Conservation and Development Commission 

(1976) derived the locations of the primary and secondary management areas (shown in part in fig. 38) from 

the natural factors indicated here. 

encompasses 89,000 acres within which existing uses-generally duck hunting, limited 

grain production, and cattle grazing-will continue. The BCDC has the major regulatory responsibility in the primary management area. 

To insulate this primary management area from 

incompatible upland land uses and agriculture, 

the plan also calls for the creation of a secondary management area encompassing 22,500 

acres surrounding the primary management 

area and consisting of upland grasslands and 

cultivated land. In this area, where the major 

regulatory responsibility rests with local government, such existing agricultural uses as 

grain production and grazing also would continue. 

The plan includes a statement of the policies 

designed to preserve and enhance the quality 

and diversity of the marsh's aquatic and wildlife 

habitat and to ensure that upland areas adjacent 

2 MILES 

2 KILOMETERS 

FIGURE 38.-Part of the Suisun Marsh Protection Plan map 

prepared by the San Francisco Bay Conservation and 

Development Commission (1976), showing parts of the 

management areas. 

to the marsh will be retained in uses compatible 

with its protection. Specific findings and recommendations concerning marsh environment, 

water supply and quality, natural-gas resources, 

utilities, transportation, recreation, public access, 

industry, land use, assessment practices, marsh 

acquisition, and marsh management are included in the plan. 

The San Francisco Bay segment of the coastal zone, including the Suisun Marsh, was placed 

under BCDC jurisdiction by the California Legislature (1977, sec. 29205). The BCDC management program for the bay system (San Francisco 

Bay Conservation and Development Commission, 1969), including the marsh-protection plan, 

was approved by the U.S. Secretary of Commerce in February 1977, pursuant to the Federal 

Coastal Zone Management Act (U.S. Congress, 

1972). 
70 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

APPLICATION 

Information on landslide deposits, historical marshlands, landslide susceptibility, land 

use, flood-prone areas, steep slopes, and active 

faults was obtained from published and unpublished sources, transferred to regional topographic workmaps, and then summarized on a 

Natural Factors Map (fig. 37). From this map, 

the marsh-protection-plan map (fig. 38) was 

prepared. Recommendations for land-use practi~es in the Benicia Hills are also based on 

Earth-science information, as were some of the 

legal boundaries and areas of jurisdiction (fig. 

36) for the BCDC' s planning and regulatory 

functions (Kockelman, 1979, p. 102). 

The California Legislature (1977) declared 

the Suisun Marsh a unique and irreplaceable resource, approved most of the marsh-protectionplan recommendations, provided for implementation of the plan, and assigned to the BCDC the 

primary State responsibility for carrying out the 

plan. The implementation process includes: (1) 

Requiring local governments-Solano County, 

the cities of Benicia, Fairfield, and Suisun City, 

and special districts-to develop a local protection program and controls consistent with the 

plan; (2) acquiring lands for public use or resource management through the California 

Wildlife Conservation Board; (3) regulating development within the primary management area 

through permits issued by the BCDC; (4) regulating development within the secondary management area through permits issued by the 

local governments; (5) managing fish and 

wildlife through the California Department of 

Fish and Game ; (6) creating the Suisun Resource 

Conservation District, and requiring it to regulate and improve water-management practices 

on privately owned lands; (7) requiring preferential tax-assessment practices; (8) providing for 

cease-and-desist orders; and (9) appealing the 

issuance of marsh-development permits by local 

governments to the BCDC. 

The development to be regulated in both the 

primary and secondary management areas includes: Placement of any material or erection of 

any structure, discharge or disposal of any material, grading or removal of any material, change 

in the density or intensity of land use, land division, alteration or demolition of any structure, 

and removal or harvesting of vegetation other 

than for agricultural purposes. 

A detailed map, prepared by the BCDC at a 

scale of 1: 24,000 ( 1 in. = 2, 000 ft) , shows both the 

primary and secondary management areas. 

Guidelines (San Francisco Bay Conservation and 

Development Commission, 1978) define the local 

protection program that the county, cities, and 

special districts are preparing for commission review and certification. 

About $4 million was made available to the 

California Wildlife Conservation Board for acquiring and improving lands in the marsh. Almost 1,400 acres has already been acquired, and 

additional acreage is being considered for acquisition. 

The BCDC has authority to grant, deny, or 

grant subject to conditions, permits to place fill, 

extract materials, or make any substantial 

change in the use of any water, land, or structure 

within its area of jurisdiction (California Legislature, 1965, sec. 66632). Pursuant to this authority, the BCDC has adopted an application and 

permit system and has prescribed the content 

and the procedures required for obtaining permits. It has integrated the marsh-developmentpermit system (a system which required that any 

proposed development be consistent with the 

BCDC's plan or local protection programs) into 

the existing bay-development-permit system. 

Under its enforcement powers, the BCDC monitors development in its area of jurisdiction (fig. 

36) and has issued orders to cease and desist filling and other work in the marsh area. In addition, it reviews and comments on environmentalimpact reports for actions affecting the marsh 

and has filed lawsuits where reports were believed to be inadequate. 
THE USE OF EARTH-SCIENCE INFORMATION 71 

COMMENTS 

This example shows how both basic and interpretative scientific information can be employed to protect an irreplaceable aquatic and 

wildlife habitat encompassing an area of more 

than 110,000 acres. The combination of regulations, land acquisition, local controls, fish and 

wildlife management, water management, and 

preferential tax assessments has preserved specific resources. 

Scientific information can be applied similarly to resource-protection planning and plan 

implementation in other estuarine environments, 

including those in the coastal areas of the United 

States affected by the Federal Coastal Zone 

Management Act (U.S. Congress, 1972). 

Example 4. A 1,312-mi2 county 

REQUIRING SITE INVESTIGATIONS IN HAZARDOUS AREAS 

Santa Clara County is subject to numerous 

earthquake hazards, including ground displacement, landslides, liquefaction of bay mud and 

other materials, and flooding (fig. 39). The 

county is traversed by several major active 

faults, and it has been shaken by many earthquakes, some of which caused severe damage. 

Santa Clara County has more than 1% million people and is one of the fastest growing 

counties in the United States. Some urban development, involving major gas and electric 

lines, transportation facilities, major water conduits, and some emergency-service facilities, has 

taken place in areas now known to be geologically hazardous (figs. 40, 41). However, the 

county is still predominately rural, and most of 

the potentially hazardous areas are undeveloped. 

In many parts of the county, landslides are 

caused by such factors as weak rock or soil conditions, high moisture content, and steep slopes 

(fig. 39). Landslides also commonly accompany 

earthquakes, as occurred during the great 1906 

San Francisco earthquake. In addition, slides are 

triggered by such human activities as steepening 

of slopes, removal of vegetation and downslope 

support, increase of upslope weight, disruption 

of surface drainage, and addition of fluids by 

watering of lawns and discharge of septic tanks. 

Landslides have made it necessary for the 

county to relocate roads, repair houses and lots, 

replace utility services, remove reservoir sediment, and lower property assessments (Taylor 

and Brabb, 1972). 

Natural and filled marshlands in Santa Clara 

County are underlain by soft bay mud that in 

places contains water-saturated fine silt and 

sand (fig. 39). The areas of clay-free alluvium 

below the water table are subject to seismically 

induced ground failure by liquefaction, lurching, 

lateral spreading, or differential settlement. In 

1906, such ground failures were observed to result in displacements of more than 6 ft, with 

cracks as much as 5 ft wide, 6 ft deep, and 100 

ft long (Youd and Hoose, 1978, p. 114). 

In addition, if the dike system that borders 

the bay were to fail during strong earthquake 

shaking, storm waves, high tides, and, possibly 

seismic seawaves could cause extensive saltwater flooding. The area inundated would depend 

on the tidal level. 
37"1 5' 

EXPLANATION 

Bay mud 

122" 

~~}.' Slope greater than 30 percent 

~>"' '-t'c\c;o; Potential ground failure along 

sloughs in baylands 

_ -·· Fault- Dashed where approximately located; 

dotted where concealed 

Epicenters X Magnitude 2.5 or greater 

+ Magnitude 1.5- 2.5 

+ Magnitude 0.5- 1.5 

0 5 MILES 

0 5 KILO M ETE RS 

FIGURE 39.-Part of a map showing some of the earthquake hazards in Santa Clara County. The map was 

prepared and used by the Santa Clara County Planning 

Department (1973) for public-information purposes. 

Preparation and distribution of the map took place before adoption of the county's seismic safety plan and ordinance enforcing onsite geologic investigations. Its 

wide distribution made the general public more aware 

of earthquake hazards and partly contributed to the unanimous adoption of both the plan and ordinance 

(Eleanor Young, former senior planner, Santa Clara 

County Planning Department, oral commun., Oct. 3, 

1979). 

l:j 

C) 

m 

0 

r0 

C) 

n 

""C 

;c 

z 

n 

""C 

rm 

V'l 

-n 

0 

;c 

""C 

;c 

c 

0 

m 

z 

-1 

s: 

z 

0 

c 

V'l 

m 
EXPLANATION 

• Where geologic investigation is normally required 

- Where geologic investigation may be required 

Where geologic investigation is not normally required 

0 5 MILES 

0 5 KILOMETERS 

FIGURE 40 .-Part of the San Clara County Relative Seismic Stability Map prepared by the Williams and Rogers (1974) and revised by the county 

in 1978. Shaded patterns indicate degrees of hazards and areas in which site investigations may be required. Larger scale maps of the 

county showing hazard areas and property boundaries are also available (see fig. 41). 

-1 

::t 

m 

c 

Vl 

m 

0 

"TI 

m 

> 

~ ::t 

I 

Vl 

n 

m 

z 

n 

m 

z 

"TI 

0 

;il:l 

~ 

~ 0 

z 

'.J 

w 
74 

- Where geologic investigatio n is 

normally required 

Where geologi c invest igation may 

be required 

Whe re geologic investigation is 

not norma lly required 

~~ Gas-tra nsmission Ji nes 

Electric powe rlines 

- 500 kV 

- 230 kV 

60-115 kV 

0 1 I I I I I I I 0 1 2 3 4 

3 4 5 M ILES 

1

1 I I 

5 KILOM ETERS 

GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

FIGURE 4L-Part of a map prepared by the Santa Clara County Planning Department (1975). showing major 

gas and electric lines located in potentially hazardous areas. The Planning Department observes that the 

capability of such lines to withstand ground failures caused by earthquakes needs to be evaluated. Similar 

maps show freeways, railroads, bridges, hospitals, and fire stations located in potentially hazardous areas. 

Repair or replacement of some of these facilities is recommended by the planning department only if the 

hazards can be reduced. 
THE USE OF EARTH-SCIENCE INFORMATION 75 

INFORMATION AVAILABLE DECISION 

Most of the seismic, geologic, and other information that is needed in preparing safety 

plans for counties in the San Francisco Bay region is currently available-topographic maps, 

land-use maps, and maps showing the locations 

of active fault breaks, landslide deposits and susceptibility, historical marshlands, liquefaction 

potential, relative intensities of ground shaking, 

geology, and steepness of slopes. Some of this 

information is available at a scale of 1:24,000 (1 

in.= 2,000 ft). 

The geologic and seismic histories, physical 

and engineering properties, development problems, and general land-use capabilities of areas 

involving bay mud, landslides, and active faults 

were discussed in the reports on seismic zonation edited by Borcherdt (1975), on relative slope 

stability by Nilsen and others (1979), on flatlands 

by Helley and others (1979), and on hillsides by 

Wentworth and others (1983). This information, 

however, is not commonly available at the scale 

or degree of detail needed for the administration 

of local land-development ordinances that would 

affect individual building sites. Detailed information, ordinarily at scales of 1:2,400 to 1:6,000 

(1 in.= 200-500 ft), must be gathered for individual sites as part of carefully designed 

geologic investigations. 

Large-scale hazard maps of the entire community, at a scale of 1:24,000 (1 in.= 2,000 ft), 

are a prerequisite to requiring site investigations. More detailed maps, at scales of 1:6,000 to 

1:12,000 (1 in.=500-1,000 ft), that show hazards 

related to property boundaries are desirable. 

Once the site investigation and geologic report 

are completed, development can be guided to 

safe, stable parts of the site, or remedial measures can be required. All such maps should be 

readable and readily available to land developers, homebuilders, real-estate salespersons, appraisers, assessors, lot purchasers, lending institutions, and insurance companies. 

In compliance with the State law requiring 

all cities and counties to prepare and adopt a 

seismic-safety element as part of their general 

plan for physical development (California Legislature, 1978b, sec. 65302(f)), the Santa Clara 

County Planning Department (1975) prepared 

such a plan. This plan includes two major recommendations to the county: Retain the services of 

an engineering geologist, and require onsite 

geologic investigations before construction. 

The County Planning Department combined 

all the potential earthquake hazards-liquefaction, lurching, lateral spreading, differential settlement, ground displacement, landslides, and 

flooding due to dike failure-on a seismic-stability map. Three zones, shown in red, yellow, and 

green on the map, were then delineated to indicate three different degrees of need for detailed 

site investigations, as determined by the level of 

hazards (fig. 40). Large-scale maps (fig. 42) show 

potential hazards in relation to property boundaries. 

The seismic-safety plan, unanimously 

adopted by the County Planning Commission 

and Board of Supervisors, is now implemented 

under the county geologic ordinance (Santa 

Clara County Board of Supervisors, 1978). This 

ordinance affects the county's other land-development ordinances-building, subdivision, 

grading, and zoning. The ordinance cites the 

seismic-stability map as one of the county's official hazard maps and includes the statement 

"Development within a known geologic hazard 

area will be discouraged." 

APPLICATION 

Existing land uses, such as urban development and major gas and electric lines, were 

superimposed on the seismic-stability map (fig. 

41). Citizens, planners, and decisionmakers are 
76 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

\,."¥'\ US C E - L U P E 

, ...,. 1951 

" \ 

\ ", '~ 

0 

I 

0 

, __ 

500 

I I I 

1000 FEET 

I 

500 1000 METERS 

FIGURE 42.-Part of one of Santa Clara County's cadastral earthquake-induced landslides; E/F, areas of low to mod-

(property boundary) maps, showing geologic hazards. erate potential for any geologic hazard. Information on 

Shaded area indicates a zone where site investigations the availability of these maps can be obtained from the 

are required because of an active or potentially active Office of the County Geologist, Department of Land Default. Letters indicate specific hazards that need to be in- velopment Engineering and Surveying, County of Santa 

vestigated and evaluated: Dr, area of high potential for Clara, 70 West Hedding Street, San Jose, CA 95110. 

ground displacement; Ds, area of high potential for 
THE USE OF EARTH-SCIENCE INFORMATION 77 

made aware of potential damage when they can 

see homes, freeways, railroads, bridges, 

pipelines, powerlines, hospitals, and fire stations 

located in the red hazard zones on the map. Recommendations on how to minimize possible 

losses of life and property are made for each element of the general plan. 

The county established a position for a 

State-certified engineering geologist within its 

Department of Land Development Engineering 

and Surveying. He helped prepare the seismicsafety plan and the geologic ordinance, and has 

major responsibility for administering the ordinance. The seismic-safety plan and seismic-stability map were prepared with the assistance of, 

or contributions from, consultants and members 

of the California Division of Mines and Geology, 

and from many other sources. 

The county geologist uses the seismic-stability map, along with other geologic information, 

when he reviews land-development and landuse proposals to determine whether a site investigation and detailed geologic report should be 

required. The geologic report, based partly on 

site investigations, describes the geology of the 

site and its surroundings, identifies specific 

problems of the site, and specifies remedial 

measures necessary to make the proposed development reasonably safe. Review and approval of the geologic report by the county 

geologist must precede any final action on applications for building sites and mobile homes, final 

approval of subdivisions, issuance of grading 

permits, and approval of use permits and rezonings. The geologic report and the county 

geologist's recommendations serve as the basis 

for approval or disapproval of specific land-use 

and land-development proposals or for any special conditions that may be imposed. 

If the geologic report indicates unusually severe geologic constraints, development may proceed only after the property owner signs a statement which acknowledges that he or she has 

been informed of the existence of the specific 

hazards, accepts the risks, and relieves the 

county of liability. This statement is recorded in 

the county recorder's office but may be expunged if subsequent information-approved by 

the county geologist-indicates that the hazard 

no longer exists or has been reduced. Generally, 

however, no new structures for human occupancy can be located on active fault traces or on 

active landslides that have not been stabilized 

by acceptable engineering procedures. 

The county geologic ordinance provides for 

a waiver of the site investigation and geologic 

report in rare situations. This waiver must be 

signed by the property owner, acknowledged 

and recorded in the office of the county recorder, 

and state that the property owner accepts all 

risks and consequences; the waiver can be expunged upon approval of a favorable geologic 

report by the county geologist. The ordinance 

also provides that a seller of real property located within a major hazard zone shall disclose 

to the buyer- by a written statement-the existence of the geologic risk. These statements, 

waivers, and acknowledgments serve to inform 

all parties to a real-estate transaction-the 

buyer, seller, agents, builder, lender, and insurer, as well as local building and zoning officials-that the property is subject to earthquake 

hazards. 

COMMENTS 

This example illustrates how earthquakehazard information can be used by a county before development to determine where site investigations and detailed geologic reports are 

necessary. The public understanding and buyer 

awareness that derive from the county's seismic-safety plan, seismic-stability map, and 

geologic ordinance will help guide development toward less hazardous areas, encourage 

remedial measures in other areas, and generally promote reasonably safe development in 

one of the United States' most populous and 

seismically active counties. 

These methods for avoiding earthquake 

hazards and reducing potential damage can also 

be adapted to other natural hazards and to other 

counties where hazard information is available 

and where seismic-safety plans are required by 

the California Legislature (1978b). 
78 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

Example 5. A city of 55,000 people 

SUPPLEMENTING BUILDING STANDARDS ON UNSTABLE BAYLANDS 

Recent estuarine deposits, called bay mud, 

underlie the San Francisco Bay and the present 

and former tidal marshes. The physical properties of bay mud are described above in the section of this report entitled "The Estuary." These 

properties, together with the high water table 

and location at or below mean sea level, result in 

several potential problems for urban development, such as flooding, ponding, foundation 

shifting, uneven settlement, liquefaction, and 

amplification of seismically induced ground 

shaking (Helley and others, 1979, p. 72). Therefore, retention of the natural estuarine environment-as has been recommended by the BCDC 

and by several cities-is generally the more economical and practical alternative. 

Nonetheless, the levelness of these lands, 

their proximity to freeways, and their position 

near a scenic and recreational estuarine environment has resulted in intense development pressures on many of the communities that border 

the San Francisco Bay. Development proposals 

include plans for single-family and multifamily 

dwellings, office buildings, shopping centers, 

hotels, airport and seaport facilities, industries, 

and amusement parks. More than 30 cities in the 

bay region have existing or historical tidal 

marshes underlain by bay mud within their jurisdiction. 

Some bay-region cities have planned and 

provided for urban development of these lands 

by dewatering and demulching the bay mud and 

by reconditioning diked land. For example, more 

than half of Redwood City (population, about 

55,000; located 25 mi south of San Francisco) lies 

on bay mud. At present, less than 10 percent of 

the more than 6,000 acres underlain by bay mud 

is developed. The comprehensive general plan 

prepared by the Redwood City Planning Department ( 197 5) proposes residential, commercial, 

institutional, industrial, public-facility, openspace, and unclassified uses for these undeveloped lands. Although the land-use plan being 

implemented by a recent zoning ordinance (Redwood City Council, 1978) permits urban use in 

some areas, most of the undeveloped baylands 

are presently zoned for tidal-plain uses. 

Even where dikes and fills reduce the risk 

from flooding, a major earthquake could cause 

severe damage to certain types of urban development as a result of liquefaction and subsequent ground failure. Youd and Hoose (1978) 

reported that ground failures in San Francisco 

have been limited mainly to areas underlain by 

filled-over marsh and bay-mud deposits, filled-in 

ravines, loose-sand deposits, sand dunes, and 

steep slopes. 

The effects of ground shaking are likely to 

be especially severe in areas underlain by bay 

mud and alluvium. In historical earthquakes, 

building damage from liquefaction and ground 

shaking included failure of foundations or footings, collapse of unreinforced-masonry walls and 

chimneys, movement of houses off their foundations, and toppling of unanchored machinery, 

storage tanks, and electrical equipment along 

with rupture of the lines. 

INFORMATION AVAILABLE 

Information on the location and general 

seismic response of bay mud, necessary for supplementing the building standards, is currently 

available, including information on historical 

marshlands, liquefaction potential, and the relative intensities of ground shaking. In addition, 

more detailed maps, at a scale of 1:24,000 (1 

in.= 2,000 ft), of the historical marshlands are 

available at the U.S. Geological Survey Western 

Region library in Menlo Park. 

The responses of bay mud to ground shaking, to ground failure associated with liquefaction, and to the predicted geologic effects of a 

postulated earthquake were discussed in the report on seismic zonation edited by Borcherdt 
THE USE OF EARTH-SCIENCE INFORMATION 79 

(1975). In addition, the geologic and seismic histories, physical and engineering properties, development problems, and general land-use 

capabilities of bay mud were discussed in the report on flatland deposits by Helley and others 

(1979), which includes maps of flatland deposits 

useful to public and private decisionmakers. 

Damage due to ground failure can be mitigated by first investigating and then designing 

for recognized site problems. Properly engineered fills, carefully designed foundations 

and structures, reinforced masonry, anchored 

machinery and equipment, and well-supervised 

construction are some of the supplemental measures that can be incorporated into building regulations to reduce damage from earthquake shaking. 

Information as to seismic response, examples of damage to be expected, and the locations 

of the unstable lands are necessary for public information before the adoption of supplemental 

building standards. An accurate, clear, and 

large-scale map showing the boundaries of the 

unstable lands is essential for effective and legal 

administration of such standards. 

DECISION 

To ensure community safety and welfare, 

the Redwood City Council (1974, 1977a) adopted 

an ordinance that provides for special seismic requirements relating to design and construction 

standards. These standards supplement those recommended by the International Conference of 

Building Officials (1976, sees. 1807(k), 2313, 

3704(c)) for structures in seismic zone 4 under 

the Uniform Building Code-the code adopted 

by the city as its own building code. The adoption of this ordinance was accompanied by a resolution by the Redwood City Council (1977b) 

expressly determining the necessity of the additional requirements because 

' ' ' local seismic and geologic conditions, experience, and 

comprehensive engineering, geological, seismic and soils 

analyses have shown that implementation of these detailed 

requirements in the specified areas can greatly minimize 

differential settlement of structures and provide increased 

structural integrity with respect to seismic safety. 

This ordinance is consistent with the city's 

initial Seismic Safety Element (Redwood City 

Planning Department, 1974), which had placed 

the bay mud in a moderately high risk zone and 

recommended that the Uniform Building Code 

be reviewed and amended as "frequently as 

may be prudent." The ordinance also follows the 

recommendations of the Redwood City Seismic 

Advisory Board (1972) with respect to foundation 

design, building design, and equipment anchorage. 

The ordinance was unanimously adopted 

and is presently being administered by the 

city's building department. The supplemental 

structural-design and construction standards 

(fig. 43) called for in the ordinance relate to 

special foundation-design criteria, design provisions for greater lateral force, foundation 

systems to resist settlement, wood-frame sheathing, moment-resisting frames, response 

spectrum, reinforced-masonry construction, 

elements of structural redundancy, and reinforcement of structural members. These standards apply only to those lands within the city 

that are underlain by bay mud, as shown on a 

map adopted by reference in the ordinance 

(fig. 44). The last part of the ordinance provides that within any structure that would be 

subject to earthquake hazard, all equipment 

shall be securely anchored. This part of the ordinance affects new installations throughout 

the entire city, including those on bay mud, alluvium, or hillside materials. 
80 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

DIVISION 2. AREAS OF REDWOOD CITY UNDERLAIN BY 

YOUNGER BAY MUD 

Sec. 9.121. Applicability of provisions. 

The provisions of this division shall be applicable to that 

portion of the City of Redwood City underlain by younger bay 

mud as indicated on that map prepared by the building department of the City of Redwood City entitled "Area of Redwood City llnderlain by Younger Bay Mud," and on file in the 

office of the city clerk . (Ord. No. 1727, § 2, 11-28-77) 

Sec. 9.122. One and two story structures. 

For one and two story residential buildings or structures 

only, all lot grading, soils design, foundation design, and construction design including lateral force analysis, shall be in 

accordance with the recommendations of the "Recommended 

Foundation Design Criteria for One and Two Story Residences" (dated April 15, 1974) as prepared by Rutherford and 

Chekene, Consulting Structural Engineers and as approved 

by the Redwood City Building Department, which document 

is on file in the office of the city clerk. (Ord. N(l. 1727, § 2, 

11-28-77) 

Sec. 9.123. Structural design. 

All structural design shall be in accordance with the building 

code including lateral force provisions for earthquakes. Structural design for major structures in the longer range period 

(three-fourths (%) of a second or more) shall use the "Recommended Base Shear Coefficient 'CS' for Lateral Force Design," as shown on Plate V-1, as revised, October 6, 1977, 

Sec. 9.124. Foundation systems. 

Foundation systems shall consist of mat, grill, piles or a 

similar system with a demonstrated ability to resist differential settlement and for tying the foundation elements together. The minimum tie strength shall be at least ten (10) 

per cent of the greatest load imposed on a foundation or 

foundation element. (Ord. No. 1727, § 2, 11-28-77) 

Sec. 9.125. Sheathing on exterior frame of wood frame buildings. 

All wood frame buildings shall be provided with five-sixteenths (5/16) inch plywood sheathing on the exterior frame 

in accordance with the "Design Criteria" prescribed by section 9.122 herein unless structural design and calculations 

prescribe lateral force parameters equal to or greater than the 

"Design Criteria Recommendations" of section 9.122. (Ord. 

No.1727, § 2, 11-28-77) 

Sec. 9.126. Walls or frames to resist lateral loads. 

"Frame only" (space frame-ductile moment resi~ting and 

space frame-moment resisting a~ per the building code) structures where K = 0.67 are not permitted. Shear walls or braced 

frames are required to resist the entire lateral load. In buildings of more than one hundred sixty ( 160) feet in height a 

moment-resisting ductible frame is required; this frame shall 

be capable of resisting twenty-five (25) per cent of the required lateral load. (Ord. No. 1727, § 2, 11-28-77) 

Sec. 9.127. Unit masonry structures. 

Unit masonry structures (concrete block, brick, unitized 

precast, prestressed concrete) are not permitted unless the 

project design engineer can demonstrate, by thorough analysis and / or tests, that the strength and ductility (including the 

effects of temperature, foundation settlement, shrinkage and 

creep) are equal to that of monolithic construction. (Ord. No. 

1727, § 2, 11-28-77) 

Sec. 9.128. Design of certain structures to incorporate sufficient elements of redunancy. 

For all structures more than four (4) stories irrespective 

of height or which contain more than twenty thousand (20,-

000) square feet of floor area, the design shall incorporate 

sufficient elements of redundancy such that complete failure 

of any one bracing element will not reduce the bracing capacity 

of the structures by more than seventy-five (75) per cent. 

(Ord. No. 1727, § 2, 11-28-77) 

Sec. 9.129. Design of structures based on response spectrums. 

The design of buildings or structures more than six (6) 

stories must, irrespective of height, be based on a response 

spectrum computed for the site. This response spectrum shall 

be compared with the average spectrum in accordance with 

Plate V-1 "Recommended Base Shear Coefficients 'CS' for 

Lateral Force Design," as revised October 6, 1977, following 

page 75A, 1972 Seismic Advisory Board Report (on file in the 

office of the city clerk) and the higher valued spectrum incorporated in the design. (Ord. No. 1727, § 2, 11-28-77) 

DIVISION 3. REDWOOD CITY GENERALLY 

Sec. 9.135. Provisions cumulative; applicability. 

The provisions of this division shall be applicable throughout Redwood City and shall be in addition to all other applicable provisions of the building code. (Ord. No. 1727, § 2, 

11-28-77) 

Sec. 9.136. Anchoring of machinery or equipment. 

Whenever connected to, part of, or housed within a building 

or structure, towers, tanks, storage-type water heaters, lighting fixtures, power transformers, machinery or other equipment that would constitute or contribute to earthquake hazards shall be securely anchored in accordance with Table 23J, 

item 4 of the building code. Exception: Domestic storage-type 

water heaters installed in one and two story residential buildings shall be anchored as recommended in the "Design Criteria" 

referenced in section 9.122. (Ord. No. 1727, § 2, 11-28-77) 

FIGURE 43.-Part of the Redwood City Council (1977a) building code supplementing the design and construction 

standards for lands underlain by bay mud. Distribution of bay mud is shown in figure 44. 
THE USE OF EARTH-SCIENCE INFORMATION 81 

-

BAIR ISLAND 

FIGURE 44.-Part of a map showing the area in Redwood City underlain by bay mud. The map is attached to the building code (Redwood City Council, 1977a), which requires supplemental structural-design and constructio_!l standards 

for all new development. Bay mud is indicated by shading, and its southwest boundary by a dashed line. Unshaded 

area along the Bayshore Freeway (U.S. Highway 101) lies outside the city's jurisdiction. 
82 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

APPLICATION 

Redwood City's map showing bay mud (fig. 

44) was based on the published map by Nichols 

and Wright (1971) that shows the historical margins of marshlands, but more detailed information from their unpublished materials was also 

incorporated (Robert Bruce, former building official, Redwood City, oral commun., July 13, 

1979). 

During preconstruction conferences with developers and builders, the city building-department staff ascertains the location of the proposed 

development, advises on any additional seismic 

requirement for development on bay mud, recommends foundation-design criteria (Rutherford 

and Chekene, 1974a, b) for one- and two-story 

residences, and, in some cases, provides visual 

guides similar to that shown in figure 45. A site 

investigation and soils report by a registered 

geologist or soils engineer, and the retention of 

an architect, civil engineer, or structural engineer, are required for any development on bay 

mud (Charles Gyselbrecht, former chief building 

official, Redwood City, oral commun., July 18, 

1979). 

Before issuing a building permit, the city 

staff verifies that the plans and specifications 

contain the appropriate structural-design and 

construction standards. The staff inspects the 

work during construction to ensure that the standards are complied with before their final approval of the completed work. 

In addition, the city's Public Works Department requires supplemental design and construction standards on bay mud for public works, 

such as storm drains, streets, sanitary sewers, 

and water supply (Charles Csicsman, project engineer, Redwood City, oral commun., July 12, 

1979). These standards include: Preparation of 

sites by dredging, demucking, excavation, filling, and compaction; determination of construction grades to obtain the "ultimate" grades 

needed for hydraulic flows after settlement; use 

of drainpipes made of noncorrosive materials; 

requirement of gates at outfalls to tidal waterways; and reduction of underground pipe 

lengths and increase of underground pipe 

grades over old slough crossings to reduce pipe 

damage during settlement. 

COMMENTS 

This example illustrates how knowledge of 

the locations of geologic units, their engineering 

properties, and their response to earthquake 

shaking can be used by a city to supplement 

building-design and construction standards. 

Buildings designed, erected, and equipped 

under this type of ordinance should have fewer 

foundation problems, withstand greater seismic 

shaking, and sustain less damage from earthquakes. 

The supplemental design and construction 

standards required for development on bay mud 

can be adapted for other types of unstable lands 

and can be applied in other cities where development on estuarine deposits or unstable 

lands may result in a threat to the public safety 

and welfare. 
THE USE OF EARTH-SCIENCE INFORMATION 

REDWOOD CITY BUILDING DEPARTMENT 

MINIMUM EARTHQUAKE ANCHORAGE WATER HEATER 

+ f A 

Heater 

X 

ro 

2 

+ N "' 

A 

Base anchor may be subst1tuted for 

lower strap. See figure D 2 

A 

F1gure 02 

+ A 

.~ 

E 

~ 

+ 

A 

1-%" X 3" lag bolt 

with washer each 

end of strap to stud. 

Heater 

16 ga. min galvanized 

steel strap "plumbers 

tape" 

TYPICAL STRAP ANCHOR 

SECTION A 

CORNER POSITION ANCHORAGE 

NOTE: Detail shown is m1nimum anchorage for water heaters up to 

50 ga-llons capacity. Anchor heaters to resist a lateral force 

equal to heater weight plus contents. 

WALL ANCHORAGE 

TYPICAL STRAP ANCHOR 

SECTION A 

FIGURE 45.-Part of the earthquake anchorage guide prepared and distributed by the Redwood City Building Department (1974) to applicants for water-heater installations. Use of the guide ensures compliance with section 

9.136 of the building code, shown in figure 43. 

83 
84 GEOlOGIC PRINCIPlES FOR PRUDENT lAND USE 

Example 6. Individual lots and acreages offered for sale 

DISCLOSING POTENTIAL HAZARDS TO REAL-ESTATE BUYERS 

The population of the nine-county San 

Francisco Bay region was more than 3.6 million 

in 1960 and surpassed 4.5 million in 1970. The 

California Department of Finances' Population 

Research Unit (1979) estimated the population to 

be almost 5 million on January 1, 1979, and the 

Association of Bay Area Governments (ABAG) 

(1Q80) projected about 5.2, 5.7, and 6.2 million 

people in the years 1980, 1990, and 2000, respectively. In 1975, these people lived in an estimated 1, 770,000 dwelling units (Association of 

Bay Area Governments, 1980). 

The construction of dwellings and other development continues in the bay region. Subdivision activity during the 4-year period ended 

June 1979 shows a rising trend in residential 

construction. The Real Estate Research Council 

of Northern California (1979) reported that the 

number of lots cleared for building in the bay region during two 12-month periods increased 

from 19,787 in 1975 to 32,356 in 1978. In addition, the total building valuation for the ninecounty bay region increased from $1.71 billion in 

1975 to $3.37 billion in 1978 (Security Pacific National Bank Research Department, 1978). The 

California Association of Realtors 1 advised that 

about 140,000 residential transactions (sales, exchanges, and other transfers) occur annually in 

the bay region (Joel Singer, California Association of Realtors, oral commun., Sept. 11, 1979). 

Some of these new lots, dwellings, and other 

developments have been located or constructed 

in areas subject to flooding, slope failure, or fault 

rupture. Many of the lots and dwellings have 

been purchased or repurchased without the sellers, buyers, or their agents even being aware of 

the potential hazards that may affect the use or 

value of the property. 

'The term "Realtors" denotes members of the National Association of Real 

Estate Boards. 

INFORMATION AVAILABLE 

Much information on hydrologic, seismic, 

and other geologic hazards is currently available 

for the San Francisco Bay region. Flood-pronearea, landslide-susceptibility, and fault-rupture 

maps have been prepared in a form understandable to, and at relatively large scales usable by, 

buyers who have no education or training in science or engineering. 

Flood-plain characteristics, maps of floodprone areas, measures to prevent and reduce 

flood loss, and the National Flood-Insurance Program were discussed in the report on floodprone areas and land-use planning by Waananen and others (1977); indexes to flood-plain 

maps and other flood information were included. 

Types of landslides, the factors causing landslides, and landslide mapping were discussed in 

the report on relative slope stability and landuse planning by Nilsen and others (1979); aregional map showing six categories of susceptibility to slope failure was included. Evidence for 

surface fault displacement, magnitude of the 

largest historical earthquake, and the estimated 

recurrence interval for a maximum earthquake 

were presented for 25 faults in the report on seismic zonation edited by Borcherdt (1975); discussions of patterns of surface faulting, fault-zone 

width, and amount of displacement were included. 

In addition, the Federal Insurance Administrator has identified communities in the bay region subject to flood hazards and has prepared 

maps showing the flood-hazard boundaries or 

the flood-insurance rates for many of these communities at scales ranging from 1:4,800 to 

1:24,000 (1 in. =400-2,000 ft). The State 

Geologist has prepared Special Studies Zones 
THE USE OF EARTH-SCIENCE INFORMATION 85 

maps showing fault-rupture traces at the relatively large scale of 1:24,000 (1 in.= 2,000 ft). At 

least one county in the bay region has transferred flood-prone, landslide, and fault-rupture 

areas onto cadastral (property boundary) maps 

at the large scale of 1:12,000 (1 in.= 1,000 ft) (see 

fig. 42). 

General information about hazards can be 

obtained by buyers of lots, dwellings, or other 

real estate through school classes, briefings, university courses, adult-education programs, special workshops, field trips, displays, regional 

conferences, lectures, and publications. More 

specific information can be obtained by contacting local building and zoning officials, local, 

State, and Federal geologists, or geotechnical 

consultants. However, an awareness that hydrologic, seismic, and other geologic hazards exist 

and may endanger the property being purchased is prerequisite to obtaining such information. One way to make property owners aware 

that their property may be affected by floods, 

landslides, or fault rupture is to disclose such 

hazards at the time of purchase. 

DECISION 

To provide for protection against flood 

losses through a Federally subsidized flood-insurance program, the U.S. Congress (1974) requires lenders to notify prospective borrowers 

that the real estate being mortgaged is located in 

a flood-hazard area, as identified by the Federal 

Insurance Administrator. 

To provide for the public safety from fault 

rupture through the Special Studies Zones Act, 

the California Legislature (1972a) requires a seller or his agent to inform the prospective buyer 

that the real estate is located within a fault-rupture zone, as delineated by the State Geologist. 

In an ordinance enforcing onsite geologic 

investigations before construction, the Santa 

Clara County Board of Supervisors (1978) also 

requires all sellers of real estate lying partly or 

wholly within the county's flood, landslide, and 

fault-rupture zones to provide the buyer with a 

written statement of the geologic risk. 

To assist them in complying with these Federal, State, and county laws, five local boards of 

Realtors in the bay region prepared colored 

street-index maps showing some or all of the designated flood, landslide, and fault-rupture 

zones. These five maps together cover one entire 

county and parts of three others, and include 

more than 50 cities; the maps show the floodhazard and fault-rupture zones (fig. 46) designated and mapped by the Federal Insurance Administrator and the California State Geologist, 

respectively. In addition, two maps show a 

county-designated fault-rupture zone; one of 

these maps (San Jose Board of Realtors, 1977) 

delineates areas of possible differential settlement of compressible soils, landslides, and saltwater flooding due to seismically induced dike 

failure. Another map delineates the locations of 

landslide deposits and faults in the Livermore 

Valley. The Federal, State, and county disclosure 

laws and the use of the map by Realtors directly 

affect buyers, sellers, and their agents and help 

interconnect the work of lenders, appraisers, 

builders, developers, insurance firms, local 

building and zoning officials, and geotechnical 

consulting firms. 
86 

0 2800 FEET 

0 600 METERS 

FIGURE 46.- Part of the Mid-Peninsula Cities streets index map prepared for the San Mateo-Burlingame Board of 

Realtors (1979), showing hazard zones. Lightly shaded areas denote flood-prone areas; darker shaded areas denote fault-rupture zones. Color maps are available from the publisher: Barclay Maps, 1206 Panoche Avenue, San 

Jose, CA 95122. 
THE USE OF EARTH-SCIENCE INFORMATION 87 

APPLICATION 

The publisher of the street-index maps used 

U.S. Geological Survey topographic maps, at a 

scale of 1:24,000 (1 in.= 2,000 ft), as the base for 

all of these five maps. The fault-rupture zones 

were taken directly from the State Geologist's 

Special Studies Zones maps, the flood-hazard 

areas from Federal Insurance Administration 

publications, and the county fault-rupture zone 

from the county geologist. All the potential hazards mapped, including differential settlement, 

landslides, saltwater flooding, landslide frequency, and Livermore Valley faults, were based 

on information from various Federal, State, and 

local agencies. 

Individual members of the five boards of 

Realtors have either received notice of and have 

access to, or have received free copies of, the 

maps for their particular jurisdiction; for example, almost all the 20,000 maps for the Menlo 

Park-Atherton area were distributed free. These 

maps may be used by Realtors to indicate, on the 

form that lists or offers it for sale, that the real estate is located in a flood-hazard or fault-rupture 

zone. (A listing form is one of the primary techniques used by a seller's agent to make other 

Realtors aware of real property that is being offered for sale.) Also, the maps may be used by 

Realtors as a general reference for advising prospective buyers on the presence of officially recognized hazards, as required by State and 

county disclosure laws. 

Because these types and scales of maps are 

not lot and site specific, the Realtors either 

merely alert buyers to the potential hazards or 

request supplemental advice from consulting 

geologists, county geologists, or local building 

officials. The publisher has placed a caveat on 

each of the maps, one of which reads: 

The primary purpose of this map is to provide a ready reference for quick identification of areas within officially 

adopted hazard zones. The zone boundaries were compiled 

from official maps available at the time of preparation and 

cannot be guaranteed as to accuracy due to changes in map 

scale. Also, periodic revisions of the official maps are made 

by responsible agencies • • •. Therefore, questions involving specific parcels at or near zone boundaries should be 

answered by consulting the appropriate official maps or 

contacting local firms offering determination services. 

In Santa Clara County, Realtors have begun 

to use new cadastral maps, at a scale of 1:12,000 

(1 in.= 1,000 ft), on which the County Geologist 

shows fault-rupture zones, flood-hazard zones, 

and areas of possible ground failure (fig. 42). 

The California Association of Realtors (1977) 

published an instruction booklet on the legal obligations of Realtors to disclose the geologic hazards that relate to the use of real estate. The 

California Association of Realtors (1981) provides, in its real-estate purchase contract form 

(fig. 47), space for attaching information about 

flood (hazard) insurance and Special Studies 

(fault rupture) Zones. The California Association 

of Realtors (1978) also prepared a disclosure 

form for Special Studies Zones (fig. 48) that can 

be attached to the contract. The last paragraph 

of this form provides space for entering the 

number of days a prospective buyer has, from 

the time of the seller's acceptance, within which 

to make further inquiries concerning the use of 

the property under the Special Studies Zones 

Act and provides that where inquiry discloses 

conditions unsatisfactory to the buyer, the buyer 

may cancel the contract. In addition, some Realtors in the bay region use other forms that can be 

attached to the contract and which disclose to 

prospective buyers that the property is located in 

areas subject to floods, fault rupture, or other 

geologic hazards (fig. 49). 
88 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

CALIFORNIA ASSOCIATION Of REALTORS® STANOAAO FOAM 

REAL ESTATE PURCHASE CONTRACT 

AND RECEIPT FOR DEPOSIT "l' 

THIS IS MORE THAN A RECEIPT FOR MONEY.IT IS INTENDED TO BE A LEGALLY BINDING CONTRACT. READ IT CAREFULLY. 

-----------------------------. California.-------------. 19----

Received from----------------------------------------------

herein called Buyer, the sum of ___________________________ Dollars$ ________ _ 

evidenced by cash 0, cashier's check 0, or 0, personal check 0 payable to---------

-----------------to be held uncashed until acceptance of this offer, as deposit on account of purchase price of 

---------------------------------------Dollars$ ________ _ 

for the purchase of property, situated in _______________ County of ______________ , California, 

described as follows: 

1. Buyer will deposit in escrow with ---------------------the balance of purchase price as follows: 

Set forth above any terms and conditions of a factual nature applicable to this sale, such as financing, prior sale of other property, the 

matter of structural pest control inspection, r~pairs and personal property to be included in the sale. 

2. Deposit witt 0 will not 0 be increased by $ to$ within days of 

acceptance of this offer. 

3. Buyer does 0 does not 0 intend to occupy subject property as his residence. 

4. The supplements initialed below are incorporated as part of this agreement. 

Other 

- Structural Pest Contf"Oi Certification Agreement 

- - Special Studt~ Zone Disclosure 

__ Occupancy Agreement 

--VA Amendment 

_ Flood Insurance Disclosure --FHA Amendment 

5. Buyer and Seller acknowledge receipt of a copy of this page. which constitutes Page 1 of __ Pages. 

X X 8UYER 7SE~L~L~E~R~----------------------

FIGURE 47.-Part of a real-estate purchase-contract form 

prepared by the California Association of Realtors (1981) 

and approved by the State Bar of California. This form or 

a similar one is used by many Realtors in the bay region 

to legally bind the buyer to his offer and the seller to his 

acceptance. Item 4 provides for supplemental disclosure 

forms (see fig. 48). These forms are available from the 

California Association of Realtors, 525 South Virgil Avenue, Los Angeles, CA 90020. Reprinted by permission; 

endorsement not implied. 
THE USE OF EARTH-SCIENCE INFORMATION 89 

This Addendum is attached as Page of ________ Pages to the Real Estate Purchase Contract and Receipt for Deposit 

dated 19 in which ------------------------------

is referred to as Buyer and -------------------------------------

---------------------------------------is referred to as Seller. 

The property which is the subject of the contract is situated in a Special Study Zone as designated under Sections 2621·2625, inclusive, of 

the California Public Resources Code; and, as such, the construction or development on this property of any structure for human 

occupancy may be subject to the findings of a geologic report prepared by a geologist registered in the State of California, unless such 

report is waived by the city or county under the terms of that act. No representations on the subject are made by Seller or Agent, and the 

Buyer should make his/ her own inquiry or investigation. · , 

Note: California Public Resources Code #2621.5 excludes structures in existence prior to May 4, 1975; 

·California Public Resources Code #2621 .6 excludes wood frame dwellings not exceeding two (2) stories in height and mobil homes over 

eight (8) feet in width ; 

California Public Resources Code #2621.7 excludes conversion of existing apartment houses into condominiums; 

California Public Resources Code #2621.8 excludes alterations and additions under 50% of value of structure from the Special Studies 

Zone Act. 

;.-. 

Buyer is allowed days from date of Seller's acceptance to make further inquiries at appropriate governmental agencies 

concerning the use of the subject property under the terms of the Special Study Zone Act and local building, zoning, fire, health and safety 

codes. When such inquiries disclose conditions or information unsatisfactory to the Buyer, Buyer may cancel this agreement. If notice in 

writing has not been delivered within such time, this condition shall be deemed waived. 

Receipt of a copy is hereby acknowledged. 

DATED: _______________________ ,19 ______ _ 

FIGURE 48.-Part of a Special Studies (fault rupture) Zone 

disclosure form prepared by the California Association of 

Realtors (1978). This form is designed to be attached to 

a real-estate purchase contract, as shown in figure 47, 

and is used by many Realtors in the bay region to comply 

BUYER: 

with the disclosure provisions of the California Legislature (1972a). The form is available from the California 

Association of Realtors, 525 South Virgil Avenue, Los 

Angeles, CA 90020. Reprinted by permission; endorsement not implied. 
90 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

2. NATIONAL FLOOD CONTROL ACT DISCLOSURE: The property which is the subject of this contract may be located in an area which has been 

identified as having special flood and/or mudslide hazards by the Secretary of Housing and Urban Development pursuant to Title 42 of the United 

States Code Annotated, Sections 400 and following. 

In the event said property is situated within such an area, the Buyer of said property will be required to purchase, in addition to other insurance, flood 

insurance as a condition to obtaining financing through a federally backed mortgage or through federally supervised, regulated or insured agencies or institutions. 

3. SPECIAL STUDIES ZONE ACT DISCLOSURE: The property which is the subject of the contract is or may be situated in a Special Studies Zone as 

designated under the Alquist-Priolo Special Studies Zone Act, Sections 2621-2625, inclusive,-of the California Public Resources Code; and, as such the 

construction or development on this property of any structure for human occupancy may be subject to the findings of a geologic report prepared by a 

geologist registered in the State of California, unless such report is waived by the city or county under the terms of that act. No representations on the 

subject are made by Seller or Agent, and the Buyer should make his own inquiry or investigation. This act provides certain exemptions from 1he 

necessity of obtaining a geologic report. These exemptions are set out as follows: 

California Publ icResources Code §2621.5 provides in part as follows: "This chapter is applicable to any project, as defined in § 2621 .6 .... " 

California Public Resources Code §2621.6 excludes from the definition of "project" 

11 Uses which do not contemplate the eventual construction of structures for human occupancy subject to the Subdivision Map Act; 

2) Single family woodrame dwellings not exceeding two stories in height (which is further defined to include mobilehomeswhosebody 

width exceeds eight feet), unless located as part of a development of four or more such dwellings constructed by a single person •. individual 

partnership or other organization; 

California Public Resources Code § 2621.7 excludes from the definition of "project" the conversion of an existing apartment complex to 

a condominium; 

California Public Resources Code § 2621.8 excludes from the definition of "project" the alteration or addition to any structure within a 

Special Studies Zone, the value of which does not exceed 50% of the value of the structure; 

California Public Resources Code § 2621.8 excludes from the definition of "project" properties in which a previous geologic report has 

been approved or waiver granted provided such new geological data warranting further investigation is not 'recorded'; 

California Public Resources Code § 2621 .5 provides that the provisions of the act do not apply to any development or structures in existence prior to May 4, 1975. 

The above is a summary of exemptions available under the California Public Resources Code. In the event further information is desired, you are directed 

to Chapter 7.5 of Division 2 of the California Public Resources Code ( § § 2621 et seq.). 

For Further Information Contact Approprrate City or County Agencies 

4. OTHER MAJOR GEOTECH~IICAL HAZARD ZONES DISCLOSURE (REQUIRED BY SANTA CLARA COUNTY) - Other than Alquist-Priolo 

Special Studies Zones: The property which is the subj<!ct of this contract is or may be situated in a zone of high geologic hazard (other than AlquistPriolo Special Studies Zones) as shown on the Santa Clara County Relative Seismic Stability Map, as revised. Such zones are designated on the described 

map, as follows: (Place a check mark in the appropriate box indicating the zone or zones in which the subject property is or may be situated.) 

D DC -areas of high potential for liquefaction and differential settlement. 

0 DR -areas of high potential for ground displacement along fault traces believed to be possibly active, but not presently in 

an Alquist-Priolo Special Studies Zone. 

D DS - areas of high potential for earthquake-induced landslides. 

D OF - areas of high potential for salt water flooding from failure of dikes. 

For further information. contact County Geologist, telephone No. 299-2871. 

Seller---------~------------------ Buyer 

FIGURE 49.-Part of a form prepared for the San Jose Board 

of Realtors (1978). This form is designed to be attached 

to a real-estate contract, as shown in figure 47, and is 

used by many local Realtors in the board's jurisdiction to 

comply with the disclosure provisions of the California 

State Legislature (1972a) and the Santa Clara County 

Board of Supervisors (1978). Items 2 through 4 provide 

for disclosure of flood, fault-rupture, and other geologic 

hazards. Reprinted by permission; endorsement not implied. 
THE USE OF EARTH-SCIENCE INFORMATION 91 

COMMENTS 

This example shows that complex hydrologic, seismic, and other geologic information 

can be conveyed to real-estate buyers before the 

sale. The five maps of single-line indexed streets 

with color overlays showing hazard zones provide easy reference and quick identification. 

Furthermore, they have unusually wide distribution throughout four counties in one of the most 

seismically active regions in the United States. 

Presenting scientific information in the form 

of relative degrees of hazard; passing Federal, 

State, and local disclosure laws; providing realestate-contract disclosure forms; and preparing 

and distributing hazard maps have resulted in 

making some buyers at least aware of, if not actually knowledgeable about, floods, fault rupture, and other geologic hazards. Prerequisites 

for effective disclosure of potential hazards by 

real-estate sellers include the sellers' or real-estate agents' knowledge and objectivity, the 

buyers' realization of the possible danger or financial loss before making the commitment to 

purchase, and the buyers' concern about hazards in relation to their other priorities. Palm's 

(1981) study of the disclosures of fault-rupture 

hazards by real-estate agents in Berkeley and 

Contra Costa County, indicates that these prerequisites are not always met. 

The method of conveying scientific information on these geologic hazards to real-estate 

buyers can be applied to other areas within the 

bay region and to other geologic hazards in 

areas where interpretative information exists. 

Because of the uniqueness of each real-estate 

transaction, however, decisionmakers cannot 

rely solely on sellers for effective disclosure of 

potential hazards or on buyers for a proper response to such hazards. 
92 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

CONCLUDING COMMENTS 

The six examples presented at the end of 

this report illustrate typical problems faced by 

planners and decisionmakers, along with innovative responses that were based on Earth-science information and designed to avoid geologic 

hazards, protect natural resources, and reduce 

property damage. Each decision was influenced 

by many of the same factors-a geologically hazardous environment, general public awareness, 

strong community interest, Federal or State enabling legislation, availability of scientific information, and the ability of geologists, engineers, 

planners, and lawyers to incorporate this information into a plan, program, or regulation. The 

collective effect of their decisions is to provide 

for greater public safety and the health and welfare of their constituents and their communities. 

Many other examples of hazard avoidance 

and resource protection could be cited in addition to the six examples presented above. The 

California Legislature (1975) enacted the Surface Mining and Reclamation Act to assure mineral-resource conservation in areas subject to irreversible land uses incompatible with mining. 

In accord with California Mining and Geology 

Board (1979) guidelines, the State Geologist is 

currently classifying lands and gravel-resource 

areas in the San Francisco Bay region; the final 

classification will resemble the sand and gravel 

classification by the California Division of Mines 

and Geology (1979) for the Los Angeles area. 

Decisionmakers-both public and privatelive and work in a complex geologic environment. This geologic environment, however, is 

just one aspect of the surroundings that affect a 

decisionmaker' s life and work; other aspects are 

social, economic, political, and esthetic, some of 

which are more apparent or more important than 

others to individual decisionmakers or their constituents. The crises faced by decisionmakers 

who fail to accommodate to a geologic environment affected by urban growth include: (1) The 

danger and trauma that accompany major earthquakes, landslides, and floods; (2) the contamination or loss of natural resources caused by pollution or incompatible land uses; and (3) the 

economic loss caused by damage or disruption to 

public facilities, utilities; and private property 

that are located in potentially hazardous areas. 

Many adverse geologic processes can be 

triggered by human activities simply because 

people lack an awareness or an appreciation of 

the specific hazard or resource. For example, 

watering landslide areas or draining septic systems into aquifers can cause property damage 

and resource contamination. 

Much of the scientific information needed 

for prudent land use in the San Francisco Bay region is currently available to decisionmakers and 

their staffs. Some of this information has been 

published at the detail and scale (1:24,000 [1 

in.= 2,000 ft ]) needed for general decisionmaking. However, greater detail and larger scales, 

ranging from 1:1,200 to 1:12,000 (1 in.=100-

1,000 ft), are needed for detailed development 

planning, site investigations, ordinance administration, project review, and permit issuance. 

Public staffs, private consultants, and applicants 

for permits generally can provide decisionmakers with information in the greater detail and at 

the larger scales needed. 

Decisionmakers, however, cannot be expected to have the requisite training or experience to understand and use scientific information. Therefore, to enable nonscientists to use 

the information available, it must be interpreted 

and placed on readable maps that display such 

information as: Recurrence intervals for 

maximum earthquakes and specific floods, relative intensities of ground shaking, susceptibility 

to landsliding, suitability ratings for waste-disposal sites, general land-use capabilities of bay 

mud, locations of landslides or active faults, potential for liquefaction, and predicted geologic 

effects of postulated earthquakes. 
CONCLUDING COMMENTS 93 

The Earth-science information and decisions 

described here for the San Francisco Bay region 

are applicable to areas outside the region where 

similar geologic hazards and land developments 

exist. Of course, any such use of this report 

would depend on the level of public awareness, 

enabling legislation, hazard and resource issues, 

order of priorities, community interest, innovativeness of decisionmakers, and staff abilities. 

Usually, additional scientific information related 

to each area would be needed to provide a 

sound basis for decisionmaking. In the end, 

however, the lasting effectiveness of any such 

decisions depends on many other factors, including: 

-continued awareness and interest by the public and their decisionmakers, 

-meticulous updating of hazard information 

and maps by geologists or geotechnical engineers, 

-careful revision of enabling legislation (if 

needed) by legislative bodies, 

-accurate site investigations by registered 

geologists or geotechnical engineers, 

-conscientious administration of regulations by 

inspectors and effective disclosure by real-estate sellers, 

-consistent enforcement by government attorneys, 

-sustained support of inspection and enforcement officials by political leaders, 

-judicious adjustment of regulations by administrative appeal bodies, 

-skillful advocacy (if challenged) and informed 

interpretation by the courts, and 

-concern for individual, family, and community 

health, safety, and welfare by real-estate 

buyers and developers. 
94 GEOLOGIC PRINCIPLES FOR PRUDENT LAND USE 

REFERENCES CITED 

Archuleta, R. J., Joyner, W. B., and Boore, D. M., 1979, A 

methodology for predicting ground motion at specific 

sites, in Brabb, E. E., ed., Progress on seismic zonation in the San Francisco Bay region: U.S. Geological 

Survey Circular 807, p. 26-36. 

Association of Bay Area Governments, 1975, San Mateo 

coast corridor evaluation: Berkeley, Calif., 10 p. 

--1976, Areas of critical environmental concern: Report to the Regional Planning Committee by the Re-

.. gional Open Space Task Force: Berkeley, Calif., 88 p. 

--1978a, Earthquake intensity and related costs in the 

San Francisco Bay area: Earthquake preparedness 

program: Berkeley, Calif., 12 p. 

--1978b, San Francisco Bay area environmental management plan, volume 1: Berkeley, Calif., 563 p. 

--1980, Projections 79-Population, employment, 

housing, 1980-2000: Berkeley, Calif., 111 p. 

Atwater, Brian, 1978, Central San Mateo County, California-land-use controls arising from erosion of seacliffs, landsliding, and fault movement, in Robinson, 

G. D., and Spieker, A. M., eds., "Nature to be commanded ... ": Earth-science maps applied to land and 

water management: U.S. Geological Survey Professional Paper 950, p. 11-21. 

--1979, Ancient processes at the site of southern San 

Francisco Bay: Movement of crust and changes in sea 

level, in Conomos, T. J., ed., San Francisco Bay: The 

urbanized estuary: San Francisco, California 

Academy of Sciences, p. 31-45. 

Atwater, Brian, Conard, S. G., Dowden, J. N., Hedel, C. 

W., MacDonald, R. L., and Savage, Wayne, 1979, History, landforms, and vegetation of the estuary's tidal 

marshes, in Conomos, T. J., ed., San Francisco Bay: 

The urbanized estuary: San Francisco, California 

Academy of Sciences, p. 347-385. 

Atwater, Brian, Hedel, C. W., and Helley, E. J., 1977, Late 

Quaternary depositional history, Holocene sea-level 

changes, and vertical crustal movement, southern 

San Francisco Bay, California: U.S. Geological Survey 

Professional Paper 1014, 15 p. 

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

land-use planning-selected examples from California: U.S. Geological Survey Professional Paper 941-B, 

p. B1-B82. 

Bolt, B. A., and Miller, R. D., 1975, Catalogue of earthquakes in northern California and adjoining areas, 

January 1910-31 December 1972: Berkeley, University of California, Seismographic Station, 567 p. 

Boore, D. M., Joyner, W. B., Oliver, A. A., III, and Page, 

R. A., 1978, Estimation of ground motion parameters: 

U.S. Geological Survey Circular 795, 43 p. 

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

the San Francisco Bay region: U.S. Geological Survey 

Professional Paper 941-A, p. A1-A102. 

Borcherdt, R. D., Gibbs, J. F., and Fumal, T. E., 1979, 

Progress on ground motion predictions for the San 

Francisco Bay region, California, in Brabb, E. E., ed., 

Progress on seismic zonation in the San Francisco Bay 

Region: U.S. Geological Survey Circular 807, p. 26-36. 

Brabb, E. E., Pampeyan, E. H., and Bonilla, M. G., 1972, 

Landslide susceptibility in San Mateo County, 

California: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-360, scale 1:62,500. 

Brown, R. D., Jr., and Wolfe, E. W., 1972, Map showing 

recently active breaks along the San Andreas fault 

between Point Delgada and Bolinas Bay, California: 

U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-692, scale 1:24,000, 2 sheets. 

Brown, W. M., Ill, and Jackson, L. E., Jr., 1973, Erosional 

and depositional provinces and sediment transport in 

the south and central part of the San Francisco Bay 

region, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-515, 21 p., scale 

1:125,000. 

California Association of Realtors, 1977, Disclosure of 

geologic hazards: Los Angeles, 43 p. 

---1978, Special studies zone disclosure: Standard 

Form SSD-FHD 11: Los Angeles. 

--1981, Real estate purchase contract and receipt for 

deposit: Standard form D-11-1 NCR: Los Angeles, 2 p. 

California Department of Finance, Population Research 

Unit, 1979, Population estimates of California, cities 

and counties: Report 79-E-1, 5 p. 

California Division of Mines and Geology, 1974, State of 

California Special studies zones, Mindego Hill quadrangle: Sacramento, scale 1:24,000. 

---1979, Mineral land classification of the greater Los 

Angeles area: Special Report 143, 79 p. 

California Legislature, 1965, McAteer-Petris Act, as 

amended: California Government Code, sees. 66600 

ff. 

---1972a, Alquist-Priolo Special Studies Zones Act, as 

amended: California Public Resources Code, sees. 

2621 ff. 

--1972b, Nejedly-Z'berg-Dills Solid Waste Management and Resource Recovery Act, as amended: 

California Government Code, sees. 66780 ff. 

---1974, Nejedly-Bagley-Z'berg Suisun Marsh Preservation Act of 1974, as amended: California Fish and 

Game Code, sees. 1850 ff. 

---1975, Surface Mining and Reclamation Act of 1975: 

California Public Resources Code, sees. 2710 ff. 

---1977, Suisun Marsh Preservation Act of 1977: 

California Public Resources Code, sees. 29000 ff. 

---1978a, Hazardous Waste Control Law: California 

Health and Safety Code sec. 25117. 

---1978b, Planning and Zoning Law: California Government Code, sees. 65300 ff. 
REFERENCES CITED 95 

California Mining and Geology Board, 1979, Guidelines 

for classification and designation of mineral lands, pt. 

2 of California surface mining and reclamation policies and procedures (revised ed.): Special Publication 

51, p. 21-36. 

California Water Resources Control Board, 1976, Waste 

discharge requirements for nonsewerable waste disposal to land; disposal site design and operation information: Sacramento, 60 p. 

Conomos, T. J., 1979, Properties and circulation of San 

Francisco Bay waters, in Conomos, T. J., ed., San 

Francisco Bay: The urbanized estuary: San Francisco, 

California Academy of Sciences, p. 47-84. 

Conomos, T. J., McCulloch, D. S., Peterson, D. H., and 

Carlson, P. R., 1971, Drift of surface and near-bottom 

waters of the San Francisco Bay system-March 1970 

through April 1971: U.S. Geological Survey open-file 

map, scale 1:625,000, 2 sheets. 

Conomos, T. J., Peterson, D. H., Carlson, P. R., and 

McCulloch, D. S., 1970, Movement of seabed drifters 

in the San Francisco Bay estuary and the adjacent 

Pacific Ocean: U.S. Geological Survey Circular 637··B, 

p. B1-B8. 

Fleming, R. W., 1972, Soil creep in the vicinity of Stanford 

University: Stanford, Calif., Stanford University, Ph. 

D. thesis, 148 p. 

Hart, E. W., 1980, Fault-rupture hazard zones in California (revised ed.): California Division of Mines and 

Geology Special Publication 42, 25 p. 

Hart, E. W., Bortugno, E. J., and Smith, T. C., 1977, Summary report: Fault evaluation program, 1976 area 

(western Transverse Ranges): California Division of 

Mines and Geology Open-File Report 77-8 SF, 8 p. 

Hart, E. W., Smith, D. P., and Saul, R. B., 1979, Summary 

report: Fault evaluation program, 1978 area (Peninsular Ranges-Salton Trough region): California Division of Mines and Geology Open-File Report 79-10 

SF, 10 p. 

Hart, E. W., Smith, D. P., and Smith, T. C., 1978, Summary 

report: Fault evaluation program, 1977 area (Los 

Angeles basin region): California Division of Mines 

and Geology Open-File Report 78-10 SF, 8 p. 

Hays, W. W., 1980, Procedures for estimating ground motions: U.S. Geological Survey Professional Paper 

1114, 77 p. 

Helley, E. J., LaJoie, K. R., Spangle, W. E., and Blair, M. 

L., 1979, Flatlands deposits of the San Francisco Bay 

region, California-their geology and engineering 

properties, and their importance to comprehensive 

planning: U.S. Geological Survey Professional Paper 

943, 88 p. 

Hines, W. G., 1973, Evaluating pollution potential of landbased waste disposal, Santa Clara County, California: 

An application of earth-science data for planning: 

U.S. Geological Survey Water Resources Investigations 31-73 (San Francisco Bay Region Environment 

and Resources Planning Study Interpretive Report 6), 

21 p. 

International Conference of Building Officials, 1976, Uniform building code: Whittier, Calif., 728 p. 

Knott, J. M., 1973, Effects of urbanization on sedimentation and floodflows in Colma Creek basin, California: 

U.S. Geological Survey Open-File Report, 54 p. 

Kockelman, W. J., 1975, Use of USGS earth science products by city planning agencies in the San Francisco 

Bay region, California: U.S. GeologiCal Survey OpenFile Report 75-276, 110 p. 

--1976, Use of USGS earth-science products by county 

planning agencies in the San Francisco Bay region, 

California: U.S. Geological Survey Open-File Report 

76-547, 186 p. 

---1977, Flood-loss prevention and reduction measures, in Waananen, A. 0., Limerinos, J. T., Kockelman, W. J., Spangle, W. E., and Blair, M. L., Floodprone areas and land-use planning·-selected examples from the San Francisco Bay region, California: 

U.S. Geological Survey Professional Paper 942, p. 23-

30. 

---1979, Use of USGS earth-science products by 

selected regional agencies in the San Francisco Bay 

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

Report 79-221, 173 p. 

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

seismic zonation in the San Francisco Bay region, in 

Brabb, E. E., ed., Progress on seismic zonation in the 

San Francisco Bay region: U.S. Geological Survey 

Circular 807, 91 p. 

Kockelman, W. J., Conomos, T. J., and Leviton, A. E., 

eds., 1982, San Francisco Bay; use and protection: 

San Francisco, American Association for the Advancement of Science, Pacific Division, 310 p. 

Laird, R. T., Perkins, J. B., Bainbridge, D. A., Baker, J. B., 

Boyd, R. T., Huntsman, Daniel, Staub, P. E., and 

Zucker, M. B., 1979, QuantitativE~ land-capability 

analysis: U.S. Geological Survey Professional Paper 

945, 115 p. 

Leighton, F. B., and Associates, 1971, Geologic report of 

Seal Cove-Moss Beach area, County of San Mateo: Irvine, Calif., 78 p. 

·Lester, F. W., and Meagher, K. L., 1978, Catalog of earthquakes along the San Andreas fault system in central 

California for the year 1974: U.S. Geological Survey 

Open-File Report 78-1010, 89 p. 

Limerinos, J. T., Lee, K. W., and Lugo, P. E., 1973, Floodprone areas in the San Francisco Bay region: U.S. 

Geological Survey Water Resources Investigations 3-

73 (San Francisco Bay Region Environment and Resources Planning Study Interpretive Report 4), scale 

1:125,000, 3 sheets. 

Little, A. D., Inc., 1975, An evaluation of the San Francisco Bay Region Environment and Resources Planning Study: Report to U.S. Department of Housing 

and Urban Development, Office of Policy Development and Research, 93 p. 

Mader, G. G., Danehy, E. A., Cummings, J. C., and Dickinson, W. ~., 1972, Land use restrictions along the 
96 GEOLOGIC PRINCIPLES FOR PRUDENT lAND USE 

San Andreas fault in Portola Valley, California: International Conference on Microzonation for Safer Construction Research and Application, Seattle, Wash., 

1912,Proceedings,v.2,p.845-851. 

McCulloch, D. S., Peterson, D. H., Carlson, P. R., and 

Conomos, T. J., 1910, A preliminary study of the effects of water circulation in the San Francisco Bay estuary: U.S. Geological Survey Circular 631. 

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p. 933-948. 

Nichols, D. R., and Wright, N. A., 1911, Preliminary map 

of historic margins of marshlands, San Francisco Bay, 

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

scale 1:125,000, 29 sheets. 

Nichols, F. H., 1913, A review of benthic faunal surveys 

in San Francisco Bay: U.S. Geological Survey Circlar 

611, 20 p. 

Nilsen, T. H., Taylor, F. A., and Brabb, E. E., 1916, Recent 

landslides in Alameda County, California (1940-11): 

An estimate of economic losses and correlations with 

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Geological Survey Bulletin 1398, 21 p. 

Nilsen, T. H., and Turner, B. L., 1915, Influence of rainfall 

and ancient landslide deposits on recent landslides 

(1950-11) in urban areas of Contra Costa County, 

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Nilsen, T. H., Wright, R. H., Vlasic, T. C., and Spangle, W. 

E., 1919, Relative slope stability and land-use planning in the San Francisco Bay region, California: U.S. 

Geological Survey Professional Paper 944, 96 p. 

Palm, R., 1981, Real estate agents and the dissemination 

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26, p. 112-182. 

Perkins, J. B., 1918, Identification of possible class I site 

areas, solid waste management plan: Berkeley, Calif., 

Association of Bay Area Governments Technical 

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92 p. 

Pettyjohn, W. A., 1919, Ground-water pollution, an imminent disaster, in National Ground Water Quality 

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Portola Valley Town Council, 1913, Zoning regulations: 

Portola Valley, Calif., Ordinance No. 1961-80, sec. 

6209.2. 

Rantz, S. E., 1911, Mean annual precipitation and precipitation depth-duration-frequency data for the San 

Francisco Bay region, California: U.S. Geological Survey Open-File Report, 23 p. 

Real Estate Research Council of Northern California, 

1919, Real estate and mortgage finance trends: San 

Francisco, no. 91, 2 p. 

Redwood City Building Department, 1914, Minimum 

earthquake anchorage-water heater: Redwood City, 

Calif., 2 p. 

Redwood City Council, 1914, Ordinance No.1613: Building code, sees. 9.120 ff.: Redwood City, Calif., 12 p. 

---1911a, Ordinance No. 1121: Building code, sees. 

9.120 ff.: Redwood City, Calif., 15 p. 

--1911b, Resolution No. 1933: Redwood City, Calif., 

3p. 

--1918, Zoning ordinance: Ordinance No. 1130 with 

revisions to Dec. 31, 1918: Redwood City, Calif., 120 p. 

Redwood City Planning Department, 1914, Seismic safety 

element: Redwood City, Calif., 26 p. 

--1915, Comprehensive general plan, including A 

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