Maps of Quaternary Deposits and Liquefaction 
Susceptibility in the Central San Francisco Bay 
Region, California



Part 3: Description of Mapping and 
        Liquefaction Interpretation

In cooperation with the California Geological Survey

Geology by    Robert C. Witter(1), Keith L. Knudsen(2), Janet M. Sowers(1), 
              Carl M. Wentworth(3), Richard D. Koehler(1), and Carolyn E. Randolph(1)

Digital database by   Carl M. Wentworth(3), Suzanna K. Brooks(3), and Kathleen D. Gans(3)


Author Affiliations    
(1) - William Lettis & Associates, Inc.; 
(2) - California Geological Survey; 
(3) - U.S. Geological Survey


Open-File Report 2006-1037



U.S. Department of the Interior
U.S. Geological Survey


Gale A. Norton, Secretary
U.S. Geological Survey

P. Patrick Leahy, Acting Director
U.S. Geological Survey, Reston, Virginia 2006

For more information on the USGS--the Federal source for science about the 
Earth, its natural and living resources, natural hazards, and the environment: 
World Wide Web:  http://www.usgs.gov
Telephone:  1-888-ASK-USGS




Suggested Citation: 
Witter, R.C., Knudsen, K.L, Sowers, J.M., Wentworth, C.M., Koehler, R.D., 
  Randolph, C.E., Brooks, S.K., and Gans, K.D., 2006, Maps of Quaternary 
  Deposits and Liquefaction Susceptibility in the Central San Francisco Bay 
  Region, California: U.S. Geological Survey Open-File Report 2006-1037 
  (http://pubs.usgs.gov/of/2006/1037/)


Any use of trade, product, or firm names is for descriptive 
purposes only and does not imply endorsement by the U.S. Government.

This database, identified as 'Maps of Quaternary deposits and liquefaction 
susceptibility in the central San Francisco Bay region, California', has been 
approved for release and publication by the Director of the USGS. Although this 
database has been reviewed and is substantially complete, the USGS reserves the 
right to revise the data pursuant to further analysis and review. This database 
is released on condition that neither the USGS nor the U.S. Government may be 
held liable for any damages resulting from its use.




TABLE OF CONTENTS


INTRODUCTION


ACKNOWLEDGMENTS


BACKGROUND


METHODS


RESULTS


Quaternary Geology


Historical Liquefaction in the Central San Francisco Bay Region


Ground Water


Liquefaction Susceptibility Categories


REFERENCES CITED



Tables

Table 1.  Correlation chart showing relationships between geologic map units 
            characterized in this study, and the stratigraphic units shown on previous 
            geologic maps
            

Table 2.  Documentation of mapping procedures and data sources


Table 3.  Criteria matrix for assigning liquefaction susceptibility categories to 
            Quaternary geologic units
            

Table 4.  Relations between regional liquefaction susceptibility by category and 
            spatial distribution of historical liquefaction occurrences
            

Table 5. Summary descriptions of liquefaction susceptibility categories



Figures

Figure 1.  The Central San Francisco Bay region, with major faults
Figure 2.  Correlation of geologic map units
Figure 3.  Index map to Table 2


Appendices

Appendix A.  Description of geologic units








INTRODUCTION
This report presents new mapping of Quaternary deposits in the central part of the 
9-county San Francisco Bay region (figure 1) and the resultant new map of liquefaction 
susceptibility. These supersede the equivalent area in the preliminary maps released 
five years ago for the whole 9-county region (Knudsen and others, 2000a). The 
susceptibility map is developed from the new 1:24,000-scale mapping of Quaternary 
deposits, historical observations of liquefaction-related ground failure, hydrologic 
information, and liquefaction analyses of geotechnical boring data. The study area 
includes parts of Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, 
Santa Clara, Solano, and Sonoma Counties. Active faults capable of producing large 
earthquakes cross the study area; these faults include the Calaveras, Concord, Green 
Valley, Hayward, San Andreas and San Gregorio Faults (Figure 1). Potential earthquakes 
on these faults expose the entire region to long-duration ground motions with peak 
ground accelerations exceeding 0.3g (Frankel and others, 2002; Cao and others, 2003), 
sufficient to trigger liquefaction in susceptible natural deposits and artificial fill. 

The new maps depict the regional distribution of Quaternary surficial deposits and 
relative liquefaction susceptibility zones in the central San Francisco Bay area and 
are appropriate for planning purposes. The maps lack the necessary resolution, however, 
for site-specific conclusions or design. The maps provide baseline data from which the 
California Geological Survey is developing State-mandated Seismic Hazard Zones of 
Required Investigation (CDMG, 1999).

Liquefaction-related ground failures caused by historical large-magnitude earthquakes 
in the San Francisco Bay area have resulted in loss of life and damage to property 
and lifelines. Spatial patterns of liquefaction effects produced during the 1906 San 
Francisco, 1989 Loma Prieta, and earlier earthquakes (Youd and Hoose, 1978; Tinsley 
and others, 1998), are not random. Instead, observations of surface deformation and 
damage produced by liquefaction indicate that the effects tend to occur in areas 
underlain by saturated, unconsolidated sand, silt and uncompacted artificial fill. 
Fortunately, areas susceptible to liquefaction can be identified through detailed 
geologic, geomorphic, and hydrologic mapping as demonstrated by hazard maps developed 
for the Monterey-Santa Cruz area (Dupr_ and Tinsley, 1980; and Dupr_, 1990), the greater 
Los Angeles urban area (Tinsley and others, 1985), and the San Francisco Bay region 
(Youd and Perkins, 1987; Knudsen and others, 1997; Sowers and others, 1998; Knudsen and 
others, 2000a; Holzer and others, 2002). These publicly available maps allow planners, 
emergency responders, and property owners to identify and mitigate hazards in efforts 
to reduce losses caused by earthquake-induced liquefaction.

The Quaternary geologic and liquefaction susceptibility maps of this report are 
presented both as small-scale map images (Sheets 1 and 2) and as a detailed digital 
spatial database in which the polygons are coded both for Quaternary map unit and 
liquefaction susceptibility (see separate description of the digital database in Part 1). 
The new mapping builds directly upon the existing regional compilation of Quaternary 
geology and liquefaction susceptibility for the nine-county San Francisco Bay area by 
Knudsen and others (2000a). The new Quaternary geologic map presented here provides 
detailed map revisions for a subset of 68 quadrangles within the original nine-county 
map area and improves the scale of mapping for 29 quadrangles originally compiled at 
1:100,000 scale by Knudsen and others (2000a) to 1:24,000. This next generation 
Quaternary geologic map builds upon the pioneering work of E.J. Helley and K.R. Lajoie 
(Helley and others, 1979) that provided the first region-wide characterization of surficial 
deposits in the San Francisco Bay area. Improvements in the new liquefaction susceptibility 
map include a more accurate depiction of liquefaction related hazards based on 
susceptibility categories assigned using a revised criteria matrix. The susceptibility 
analysis incorporates quantitative analyses of geotechnical information contained in a 
regional borehole database compiled by the California Geological Survey (CGS) for the 
Santa Clara valley, San Francisco and East Bay areas. The susceptibility map delineates 
areas that are underlain by materials that have different relative susceptibilities to 
liquefaction; the map does not predict liquefaction-related ground failures, although 
ground failures may accompany liquefaction and are more likely to occur in areas with 
higher liquefaction susceptibility (Tinsley and others, 1985).



 Acknowledgements

Conversations with John Baldwin and Keith Kelson, who shared their extensive knowledge and 
perspectives on the Quaternary geology in the San Francisco Bay region, significantly 
aided our mapping effort. Critical and constructive technical review comments from Kevin 
Clahan and William Lettis improved the maps. Jackie Bott, Anne Rosinski, and Mark Wiegers 
also provided useful input to the maps. The manuscript was clarified and focused through 
reviews by Russ Graymer, Tom Holzer, and John Tinsley. This research was supported by 
the U.S. Geological Survey (USGS) Earthquake Hazards Program and  NEHRP award number 
99-HQ-GR-0095, by the California Geological Surveys Seismic Hazards Mapping Program, and 
by the Professional Development fund of William Lettis & Associates, Inc. 


BACKGROUND 

Liquefaction is the transformation of saturated, unconsolidated granular material from a 
solid state to a liquid state as a consequence of increased pore pressures that reduce the 
effective strength of the material (Youd, 1973). Ground shaking from large earthquakes can 
produce increased pore pressures in unconsolidated deposits that may lead to localized 
liquefaction across much of western California. The process of liquefaction may or may not 
lead to ground deformation or related surface manifestations, including lateral spreading, 
ground settlement, bearing capacity failure, sand boils, and ground cracking. The effects 
of liquefaction often result in damage to the built environment and, in some cases, loss 
of life. Holzer (1998) and Youd and Hoose (1978) document liquefaction-related effects 
observed after previous large earthquakes in the San Francisco Bay region.

Liquefaction potential is a function of both the susceptibility of surficial deposits to 
liquefaction and the probability that earthquake ground motions will exceed a specified 
threshold level, or opportunity. A liquefaction susceptibility map reflects the 
distribution of surficial deposits with different physical properties and variations in 
hydrologic conditions. The opportunity for liquefaction is determined by the proximity of 
seismic sources, the magnitude and recurrence interval of earthquakes the seismic sources 
are capable of generating, and local site conditions that control the amplification or 
attenuation of shaking. A liquefaction potential map is the product of a liquefaction 
susceptibility map and a map depicting probabilistic ground motions (e.g., Frankel and 
others, 2002; Cao and others, 2003). Tinsley and others (1985) argued that liquefaction 
susceptibility maps for the Los Angeles region could serve as liquefaction potential maps 
because the opportunity for liquefaction across the region was relatively uniform. 
Although shaking potential is not uniformly distributed across the San Francisco Bay 
region, the use of liquefaction susceptibility maps as proxies for liquefaction potential 
maps is reasonable because probabilistic ground motions exceed levels sufficient to 
liquefy susceptible deposits across most of the region.

Sufficient liquefaction opportunity is justified for the San Francisco Bay region on the 
following grounds: (1) active faults capable of generating large-magnitude earthquakes 
are distributed throughout the study area (Figure 1); (2) no site is more than 30 km 
from an active fault capable of generating a magnitude 6.5 or larger earthquake and most
are within 15 km; and (3) earthquakes on the Hayward/Rodgers Creek Fault (0.27 probability 
of a Me 6.7 between 2002 and 2030), San Andreas Fault (0.21 probability), Calaveras fault 
(0.11 probability) and San Gregorio Fault (0.10 probability), will produce long duration 
ground motions in excess of 0.3g over most of the study area (Working Group on California 
Earthquake Probabilities, 2003). We recognize that ground response is highly dependent on 
site-specific variations in the duration (cycles), strength, and frequency (especially 
potential for amplified low frequencies) of ground motions. The assumption of sufficient 
liquefaction opportunity is conservative and valid, however, because most of these 
site-dependent variations in ground motion tend to enhance liquefaction. 


METHODS 


Previous evaluations of regional liquefaction hazards identify several geologic and 
hydrologic factors that influence the susceptibility of a deposit to liquefaction, including 
(1) the age and depositional environment of the deposit; (2) the relative consolidation of 
sands and silts; and (3) the local depth to ground water (Youd and others, 1973; Youd and 
others, 1975; Youd and Perkins, 1978; Tinsley and others, 1985). We developed a revised 
liquefaction susceptibility map by following the procedure outlined in previous assessments 
of liquefaction susceptibility in the San Francisco Bay area (Knudsen and others, 1997a, 
1997b; Sowers and others, 1998; Knudsen and others, 2000a):

    - Map surficial deposits on the basis of age and depositional environment;
    - Estimate typical depth to ground water for each geologic unit;
    - Evaluate historical liquefaction occurrences and relate observations of liquefaction to 
        the associated geologic map unit;
    - Develop a criteria matrix to classify all potential combinations of type and age of 
        deposit with ground-water depth. Calibrate with historical liquefaction occurrence data, 
        previous studies, and borehole (SPT) evaluations of liquefaction peak ground 
        accelerations (PGA) thresholds; and
    - Assign liquefaction susceptibility categories to geologic map units.

Quaternary deposits were characterized by age and depositional environment through the 
interpretation of historical topographic and coastal survey maps, aerial photography (both modern 
and historical), published soil surveys, review of previously published and unpublished geologic 
mapping, and limited field reconnaissance. Thirty-eight primary geologic map units that differ 
by age and depositional environment were identified in this study (Table 1; Figure 2). Mapping 
procedures and data sources used for each 7.5-minute quadrangle are listed in Table 2 and keyed 
to Figure 3. For each geologic map unit we assign a corresponding liquefaction susceptibility 
category (ranging from very low to very high) based on a criteria matrix that considers historical 
liquefaction occurrences, geotechnical analyses of limited borehole data, and estimated depth 
to ground water (Table 3). Note that this mapping procedure excludes Plio-Pleistocene deposits 
with poorly preserved geomorphic surfaces that have been largely destroyed through erosion 
or tectonic processes. Thus, early Quaternary and Pliocene deposits, such as the Montezuma 
and Santa Clara Formations, are not included in the map.

As a deposit ages, soil formation, weathering, diagenetic processes, and earthquake shaking 
lead to consolidation and cementation of the sediment. Older deposits are therefore generally 
less likely to liquefy. We used several criteria to assess the age of a deposit, including: 
(1) the relative degree of soil profile development; (2) relative extent of surface dissection 
or other surface modification; (3) relative topographic position and cross-cutting relations; 
(4) correlation to dated deposits in the region; and (5) correlation to the stratigraphic 
framework used by previous researchers (Table 1). Deposits that could not confidently be 
differentiated as either Holocene or latest Pleistocene are assigned an age that spans the 
Pleistocene-Holocene transition (11,800 years ago).  This age category is also used where 
Holocene deposits are inferred to interfinger with, or form a thin veneer (< 5-feet thick) 
over, latest Pleistocene deposits. We use the term 'latest Pleistocene' to refer to deposits 
younger than about 30,000 years -- a different meaning than the commonly accepted <125,000 year age 
for late Pleistocene deposits. This categorization was adopted to permit distinction of these 
relatively young, but still Pleistocene deposits from older, more consolidated Pleistocene 
deposits and thereby distinguish deposits with different liquefaction susceptibilities.

Published soil surveys provide some of the primary data used to interpret the relative ages 
of a deposits that occur in the region.  Quaternary geologic units of specific age and 
environment are characterized by individual soil series (e.g. Appendix B in Knudsen et al., 
2000a).  Soil surveys reviewed for the region include Bates and others (1977), Cosby (1941), 
Gardner and others (1958), Kashiwagi (1985), Kashiwagi and Hokholt (1991), Lindsey and Weisel 
(1974), Lambert and Kashiwagi (1978), Miller (1972), Wagner and Nelson (1961), and Welch 
(1977, 1981).

Data on the depth to ground water were acquired in several ways. Data on ground-water 
depths from boring logs for geotechnical studies were obtained from a regional boring 
database compiled by the California Geological Survey. Additional hydrologic information 
came from the California Department of Transportation, the Bay Area Rapid Transit District, 
and from reports on file with county and city governments in the study area. Data also 
were collected in the field by measuring the depth to the water surface in streams, creeks, 
and drainage ditches with respect to the adjacent terrace or fan surface. In making these 
measurements, we assumed that the stream level was representative of the level of shallow 
ground water in the area. We extrapolated water levels from field observations or boring 
log data to areas with little available ground-water data. In some cases we used the depth 
of stream incision as a maximum depth to ground water. Water levels in water wells were not 
used because the wells tap deep, in many places artesian, aquifers and the levels thus 
represent the potentiometric surface for the aquifer, not the depth to saturated sediment. 
We used historical high ground-water depths where available to be consistent with the 
approach used by the California Geological Survey in its Seismic Hazard Mapping Program 
(California Division of Mines and Geology, 1999).

________________________________________________

Table 2.  Documentation of mapping procedures and data sources.

1.	Interpretation of topographic contours, published soil surveys (e.g., Lambert and
          Kashiwagi, 1978; Miller, 1972), and aerial photography dating from 1942, 1953, 
          1968, 1973, 1976, 1984, and 1997, and limited field reconnaissance. Previous 
          mapping reviewed includes: Fox and others (1973); Jennings (1988); Nichols and 
          Wright (1971); and Sowers and others (1998).  Lead authors: Koehler, Randolph, 
          and Sowers.  Map scale: 1:24,000.

2.	Modified from Sowers and others (1997) and Bezore and others (2000).  Extensive 
          modifications were made based on interpretation of aerial photography dating 
          from 1942, topographic contours, and published soil surveys (e.g. Bates and others, 
          1977; Lambert and Kashiwagi, 1978). Previous mapping reviewed includes: Fox and 
          others (1973); Helley and others (1979); Nichols and Wright (1971); Sims and others 
          (1973) and Sowers and others (1997).  Lead authors: Sowers and Witter.  
          Map scale: 1:24,000.

3.	Interpretation of topographic contours shown on modern and/or historical (circa 1915) 
          U.S. Geological Survey maps, and published soil surveys (e.g. Bates and others, 
          1977; Cosby, 1941).  Previous mapping reviewed includes Helley and others (1979), 
          Helley and Harwood (1985), and Nichols and Wright (1971).  U.S. Geological Survey 
          orthophotoquads dated 1970 were reviewed for the Elmira quadrangle.  Lead authors: 
          Knudsen and Witter.  Map scale: 1:24,000.

4.	Interpretation of topographic contours, published soil surveys (e.g. Bates and others, 
          1977; Cosby, 1941; Kashiwagi, 1985; Welch, 1977), and aerial photography dating from 
          1942, 1939, 1946, 1974, 1984, and 1995, and limited field reconnaissance. Previous 
          mapping reviewed includes: Borchardt (1994); Clark and Brabb (1997); unpublished 
          mapping by Ron Crane; Dibblee (1980a-e, 1981a-b); Galloway (1977); Haydon (1995); 
          Helley and Graymer (1997a); Helley and others (1979); unpublished mapping by E.J. 
          Helley and J.S. Noller; Knudsen and others (1997); Nichols and Wright (1971); Sims 
          and others (1973); Radbruch (1969); Sowers (1995); and Sowers and others (1998). 
          Lead author: Witter.  Map scale: 1:24,000.

5.	Interpretation of topographic contours, published soil surveys (e.g., Lindsey and 
          Weisel, 1974; Welch, 1977, 1981), stereoscopic aerial photography dating from 1939 
          and 1949, historical wetlands data compiled by the Goals Project (1999), Sowers (1999), 
          other historical wetlands data, and limited field reconnaissance.  Previous mapping 
          reviewed includes: Helley and Wesling (1989, 1990); Helley and Miller (1992); Helley 
          and others (1994); Helley and Graymer (1997b); Kelson and others (1993); and Sawyer 
          (1996).  Lead author: Sowers.  Map scale: 1:24,000.

6.	Interpretation of topographic contours, published soil surveys (e.g., Kashiwagi and 
          Hokholt, 1991; Wagner and Nelson, 1961), and stereoscopic aerial photography dating 
          from 1943, and limited field reconnaissance. Previous mapping reviewed includes: 
          Bonilla (1998); Brabb and others (1998a); Helley and others (1979); Knudsen and others 
          (1997); and Pampeyan (1994). In these quadrangles Helley and Graymer (1997b) is based 
          on previous mapping by Herd (1977).  Mapping of marine terraces is modified from 
          Weber and others (1993), and Lajoie and others (1974,1979).  Lead author: Witter.  
          Map scale: 1:24,000.

7.	Interpretation of topographic contours, published soil surveys (e.g., Gardner and 
          others, 1958), and stereoscopic aerial photography dating from 1943, and limited 
          field reconnaissance.  Previous mapping reviewed includes: Angell and others (1997); 
          Brabb and others (1998b); Graymer, 1997; Helley and Brabb, 1971; Helley and others 
          (1994); and Wentworth and others (1998).  Lead author: Knudsen.  Map scale: 1:24,000.
________________________________________________


We use a criteria matrix to assign liquefaction susceptibility categories for each combination 
of geologic unit and depth to ground water that represent the relative likelihood that loose, 
saturated, granular materials are present (Table 3). Each of five categories, ranging from very 
low to very high, represents the relative liquefaction susceptibility of a geologic unit based 
on the combined assessment of historical observations of liquefaction, geotechnical analyses 
of borehole data, and estimated depth to groundwater. By intersecting the digital compilation 
of historical liquefaction-related ground effects for the San Francisco Bay area (Knudsen and 
others, 2000a, Appendix C) with the Quaternary geologic map, we evaluated whether historical 
liquefaction had occurred in each geologic unit and, if so, the number of liquefaction 
occurrences per square kilometer over which the unit is mapped (Table 3). Using procedures 
developed by Seed and Idriss (1971, 1982) and updated by Youd and others (2001), we calculated 
the percent of samples from each map unit that are susceptible to liquefaction (Table 3). 
This analysis used the 10% probability of exceedance in 50 years peak ground acceleration (PGA) 
from probabilistic hazard maps for the State of California (Cao and others, 2003). The PGA 
and earthquake magnitude are calculated for each boring location.  The earthquake magnitude 
used in the liquefaction analysis is from the deaggregated probabilistic map and represents 
the magnitude of the earthquake for the source with the maximum contribution to the 
probabilistic ground motion. These geotechnical analyses were performed on an unpublished 
borehole database developed by the Seismic Hazard Mapping Program of the California Geological 
Survey (http://gmw.consrv.ca.gov/shmp/). Borehole data used in this analysis come from 
western Alameda County, northwestern Santa Clara County and the City and County of San Francisco.


RESULTS

Quaternary Geology


Depositional processes and environments operating in the San Francisco Bay region reflect 
the influence of active tectonics and climatic fluctuation during the Quaternary (Helley 
and others, 1979). For example, northwest-trending valleys are aligned with major strike-
slip faults that contain sediments deposited by streams on flood plains, levees, alluvial 
fans, and basins. Broad piedmont alluvial fans are deposited along the flanks of northwest-
trending mountain ranges undergoing active tectonic uplift. Flights of stream terraces 
(Kelson and others, 1993), aligned fan apices and abrupt changes in stream gradients (Hitchcock 
and Kelson, 1999) have been attributed to ongoing uplift of the ranges caused by 
transpressional deformation (Aydin and Page, 1984; Unruh and Lettis, 1998). Climatic influence 
on depositional systems is evidenced by latest Pleistocene fans deposited by streams graded 
to lowered base levels and eolian dunes on the San Francisco Peninsula and in Alameda that 
indicate westward migration of the shoreline during Wisconsin sea-level low stands (Atwater 
and others, 1977). Younger, Holocene, alluvial fans record deposition by streams responding 
to the marine transgression that culminated with the present sea-level high stand and 
formation of San Francisco Bay. 

Deposits in coastal and estuarine environments also have been influenced by climate change 
and tectonics during the Quaternary. Step-like flights of late Pleistocene marine terraces 
tectonically raised above the modern shoreline along the San Mateo coast preserve sediments 
deposited in near-shore marine, lagoon, beach, dune and alluvial environments during past 
sea-level high stands. Sedimentary and erosional processes active along the coast are driven 
by wind, wave action and littoral currents. Lower energy processes control the deposition of 
fine-grained sediments in broad, low-lying marshes and distal alluvial fan-estuary complexes 
that fringe San Francisco Bay and the Sacramento/San Joaquin Delta. In southern San Francisco 
Bay, Holocene Bay mud deposits are subsiding due to tectonic and possibly isostatic forces 
(Atwater and others, 1977). Along San Pablo Bay, the Carquinez Straight and near Lake Merritt 
in Oakland, latest Pleistocene estuarine and bay margin deposits are preserved as Bay terraces 
that contain the articulated shells of oysters that lived in a bay formed by the stage 5e sea-
level high stand 112 to 142 thousand years ago (Helley and others, 1993).

Physiographic differences among valleys, watershed area, and variations in the distribution 
of precipitation in the San Francisco Bay region also exert some control over depositional 
environments. Additional environmental controls on the sedimentary characteristics of a 
deposit may include vegetation, sediment load and bedrock geology. In the northern part of 
the region, watersheds containing the Napa River, Russian River and Sonoma Creek experience 
higher mean annual precipitation compared to the Santa Clara Valley and stream terraces 
and active flood plains are confined to narrow valleys. Along the margins of these valleys, 
smaller tributary streams that flow from the adjacent ranges form relatively small, steep 
alluvial fans. In contrast, broad coalescing alluvial fans, fan levees and inter-levee 
basins characterize depositional environments along Coyote Creek and Guadalupe Creek in 
the Santa Clara Valley and Walnut Creek basin where precipitation is generally lower and 
the valleys are less confined.

The new Quaternary geologic map presents a complete depiction of surficial deposits at a 
larger scale (1:24,000) in the most urbanized central core of the San Francisco Bay area 
compared to the nine-county maps developed by Knudsen and others (2000a). For example, 
twenty-nine quadrangles that were compiled at 1:100,000-scale for the nine-county map 
have been remapped at 1:24,000-scale for this project and most mismatched map units along 
quadrangle borders present in the nine-county map have been resolved. Figure 2 shows the 
stratigraphic correlations among the units by age, and Table 1 compares the geologic units 
with those of previous researchers who mapped parts of the area. Detailed descriptions of 
the Quaternary geologic units, including the newly defined Pleistocene Bay terrace deposits 
(Qbt), are presented in Appendix A. The general material properties of these units are 
characterized from subsurface information included in unpublished geotechnical and 
engineering reports and in an extensive borehole database compiled by the California 
Geological Survey covering the areas of western Alameda County (Haydon and others, 1995; 
1999), San Francisco (DeLisle and Real, 1994) and the Santa Clara Valley (Clahan and others, 
2000a; 2000b; 2000c; 2000d).

Man-made deposits cover approximately 3% of the region.  These areas are mapped as either 
artificial fill (af), artificial fill over estuarine mud (afem), artificial levee fill 
(alf), artificial stream channel (ac), artificial channel fill (acf), artificial dams 
(adf), or gravel quarry and percolation pond (gq).  Artificial fill over estuarine mud 
(afem) is mapped where fill was placed over Bay mud (Qhbm), bayward of the early historical 
extent of marshlands and bay (Nichols and Wright, 1971; Goals Project, 1999).  Artificial 
levee fill (alf) is mapped where the map symbols for levees are shown on 7.5-minute USGS 
topographic quadrangles.  Artificial levee deposits (alf) may not differ significantly 
from the underlying material, which is commonly Holocene Bay mud (Qhbm) or Holocene to 
historical alluvial fan levees (Qhly, Qhl) and stream terrace (Qhty, Qht) deposits. 
Artificial stream channels (ac), identified through interpretation of aerial photography 
and historical topographic maps, include modified channels, flood control channels and 
concrete canals. Artificial channel fill (acf) consists of human-emplaced materials 
within historical stream channels identified through inspection of topographic maps, 
aerial photographs and as mapped by Sowers (1995, 1997, 1999). Artificial dams (adf) 
include earthen embankments, rock-fill dams and artificial levees that impound water 
reservoirs, settling/cooling ponds, artificial lakes and stock ponds. Dams were identified 
through analyses of topographic maps and aerial photography. Gravel quarries and 
percolation ponds (gq) typically were mapped using information shown on 7.5-minute USGS 
topographic or orthophoto quadrangles.


Historical Liquefaction in the Central San Francisco Bay Region


Records of liquefaction-induced ground failures in the region are available for several 
historical earthquakes, including the two most damaging events: the 1906 San Francisco 
earthquake (Mw=7.7-7.9) (Youd and Hoose, 1978), and the 1989 Loma Prieta earthquake (Mw=7) 
(Plafker and Galloway, 1989; Seed and others, 1990; Tinsley and others, 1998).  Other 
earthquakes that generated liquefaction failures in the study area include the 1838, 1865, 
1868, and 1957 earthquakes (Youd and Hoose, 1978). Much of the 1989 liquefaction-related 
ground failures occurred in areas of previous recorded liquefaction (Dupr_ and Tinsley, 
1998; Seed and others, 1990).  

Knudsen and others (2000a) developed a preliminary spatial database of historical 
earthquake-related ground failures in the San Francisco Bay Region (Appendix C) and 
compared these data with the digital nine-county Quaternary geologic and liquefaction 
susceptibility maps.  The ground-failure database contains historical observations of 
earthquake-triggered damage, as interpreted by Youd and Hoose (1978), and observations 
of liquefaction-related ground failures resulting from the 1989 Loma Prieta earthquake 
(Tinsley and others, 1998). Comparisons between the new Quaternary geologic map and the 
distribution of historical ground effects (Knudsen and others, 2000a) were used to 
calibrate the liquefaction susceptibility criteria matrix (Table 3) and as a predictive 
indicator for the likely distribution of future liquefaction-related ground effects 
(Table 4).

The greatest number of historical ground failures, 34% of the total observed effects in 
the core Bay area, occurred in the map unit artificial fill over estuarine mud (afem). 
In contrast, Knudsen and others (2000b) concluded that artificial fill over Bay mud 
hosted about 50% of all historical liquefaction occurrences in the nine-county San 
Francisco Bay area and about 80% of the liquefaction occurrences resulting from the 
Loma Prieta earthquake. The higher percentage of occurrences in artificial fill over 
Bay mud on the nine-county map reflects the high density of liquefaction-related ground 
failures documented in the San Francisco North quadrangle, which was not included within 
the map area for this study. For the Loma Prieta earthquake, the higher percentage may 
reflect the low accelerations necessary to trigger liquefaction in this unit and the 
relatively low accelerations produced by the Loma Prieta earthquake in much of the nine-
county area.  In many cases it is not known whether the failures were in the fill or in 
the underlying estuarine deposits.  The high percentage of failures occurring in 
artificial fill over estuarine mud also may reflect amplified shaking levels due to 
the physical characteristics of Bay mud.  



Ground Water


Estimated depths to ground water for each geologic unit (Table 3) come from regional 
borehole data compiled by CGS, field observations and published USDA soil surveys. 
The depth to ground water in areas underlain by Holocene alluvial, estuarine, and beach 
sediment is generally less than 10 feet throughout most of the study area.  In general, 
ground water is deeper beneath topographically higher parts of the landscape (e.g., 
uplifted and dissected Pleistocene alluvial fans), and closer to the surface of 
topographically lower parts of the landscape (e.g., Holocene basins and terraces).  
Pronounced seasonal changes in ground-water levels occur in the San Francisco Bay 
area, with variations as large as tens of feet.  When available, we use historical 
high ground-water levels measured during and soon after the rainy season of wetter 
years.  Small, isolated alluviated valleys and pockets within the bedrock hills appear 
to have fairly shallow ground-water levels, generally less than 10 to 15 feet.  Soil 
characteristics of wet environments are mapped in many of these valleys, and the few 
data available on depth to ground water indicate shallow ground-water levels. 

Depths to ground water beneath marine terraces and dune sand can be significantly 
greater than other Quaternary deposits in the region.  Ground water beneath uplifted 
marine terraces can be deeper than 40 feet, except where water is perched.  Ground 
water beneath coastal dunes that form or mantle hills can be as deep as 50 to 100 
feet, approximately equivalent to the elevation of the hills.


Liquefaction Susceptibility Categories


Liquefaction susceptibility categories assigned to each geologic map unit are judgments 
based on the evaluation of several factors: quantitative criteria presented in Table 3, 
correlations between historical liquefaction occurrences and geologic map units (Table 4), 
and qualitative considerations that relate the age, environment, and topographic setting 
of a deposit to its susceptibility to liquefaction. The five susceptibility categories 
(VERY LOW to VERY HIGH, Table 5) reflect the relative likelihood that a particular 
geologic map unit, with its assigned degree of water saturation (Table 3), will liquefy 
at a given threshold PGA. There is no explicit correlation, however, between each 
geologic unit and the quantitative data related to its liquefaction susceptibility, 
because of the regional focus of the liquefaction susceptibility map, the limited scope 
of the geotechnical and historical liquefaction occurrences datasets, and the application 
of geologic judgement in making the assignments. 

Analysis of available historical liquefaction occurrence data, for example, shows only 
incomplete correlation between susceptibility category and liquefaction occurrence per 
unit area of a deposit. Not all map units assigned to the very high or high 
susceptibility categories have evidence of historical liquefaction during large magnitude 
earthquakes in the central San Francisco Bay region. Regardless, because of other criteria 
indicative of susceptibility (Table 3), we considered most latest Holocene to historical, 
granular deposits with low (0.1 to 0.2 g) PGA triggering thresholds and most artificial 
materials to be highly or very highly susceptible to liquefaction. Conversely, some 
historical liquefaction occurrences during the 1989 Loma Prieta, 1906 San Francisco and 
earlier earthquakes plot within map units assigned to the low and very low susceptibility 
categories, including 15 documented occurrences of liquefaction that plot within pre-
Quaternary deposits and bedrock. These units are sufficiently consolidated that we 
question the association. Many of these localities probably reflect either liquefaction 
of highly susceptible materials in deposits too small to be delineated at 1:24,000 scale 
or inaccurate locations of the historical occurrences. The extremely small areal extents 
of several map units that have hosted historical liquefaction results in extremely high 
(and probably unrepresentative) statistics relating historical occurrence to unit area 
for these units (e.g., Qhbs and Qhed, Table 4). Some map units are composed of multiple 
components that conceptually should have different liquefaction susceptibilities (e.g. 
Qhbm, Qha, Qhf), and some have sufficient uncertainty about their character to warrant a 
higher susceptibility rating than low or very low (e.g., Qa, Qf, Qt). San Francisco Bay 
mud (Qhbm), for example, consists of estuarine mud with local stream-mouth deltas that 
 ideally would be mapped as a separate, very different, unit. These heterogeneous or 

 uncertain units are assigned to the MODERATE susceptibility category.


________________________________________________


Table 5. Summary descriptions of liquefaction susceptibility categories.



VERY LOW -- Expect less than 2 percent of future liquefaction effects to occur within 
geologic units assigned VERY LOW susceptibility (with about 1 occurrence in any future 
earthquake for every 3,000 square kilometers). Units within this category include early 
to late Pleistocene and pre-Quaternary deposits and bedrock. Together, units assigned V
ERY LOW susceptibility cover over 5,716 square kilometers in the central San Francisco 
Bay region.  About 1.4 percent of historical liquefaction occurrences fall within map 
units assigned VERY LOW susceptibility (0.0003 liquefaction occurrences per square 
kilometer) (Table 4). Results of quantitative liquefaction analyses (using borehole data) 
are not available for deposits with VERY LOW susceptibilities. An estimated PGA of greater 
than about 0.6 g is necessary to trigger liquefaction in deposits assigned VERY LOW 
susceptibility.


LOW -- Expect about 2 percent of future liquefaction effects to occur within geologic 
units assigned LOW susceptibility (with about 1 occurrence for every 100 square kilometers). 
Geologic map units within this category include Pleistocene marine and Bay terrace deposits, 
late Pleistocene deposits and Holocene to latest Pleistocene basin deposits. Artificial 
(historical) earthen dams (adf) are also assigned to this category. Together, units 
assigned LOW susceptibility cover over 458 square kilometers in the central San Francisco 
Bay region. About 2.1 percent of historical liquefaction occurrences are located within 
map units assigned LOW susceptibility (about 0.01 occurrences per square kilometer) 
(Table 4). Few results of quantitative liquefaction analyses using borehole data are 
available; in two units, however, 9 to 19 percent of borehole samples are expected to 
liquefy at the 10% probability of exceedance in 50 year PGA (Table 3). An estimated PGA 
of greater than about 0.5 g is necessary to trigger liquefaction, although a lower level 
of shaking may trigger liquefaction in latest Pleistocene to Holocene basin deposits.


MODERATE -- Expect about 20 to 30 percent of future liquefaction effects to occur within 
geologic units assigned MODERATE susceptibility (with about 1 occurrence for every 50 square 
kilometers). Geologic map units within this category include latest Pleistocene to Holocene 
deposits from a variety of environments. Gravel quarries and percolation ponds (historical) 
are also assigned to this category. Together, units assigned MODERATE susceptibility cover 
2,314 square kilometers of the central San Francisco Bay region. About 25 percent of 
historical liquefaction occurrences fall within map units assigned MODERATE susceptibility 
(about 0.02 occurrences per square kilometer) (Table 4). The results of quantitative 
liquefaction analyses of several units (using borehole data) indicate that 7 (Qhbm) to 29 
(Qhl) percent of samples are expected to liquefy at the 10% probability of exceedance in 50 
year PGA (Table 3), and that 39 percent of samples of Ql may liquefy. PGAs of greater than 
0.2 g to 0.3 g are necessary to trigger liquefaction of deposits assigned MODERATE 
susceptibility, although slightly lower and significantly higher shaking thresholds may 
control liquefaction in some deposits.


HIGH -- Expect about 20 to 30 percent of future liquefaction effects to occur within geologic 
units assigned HIGH susceptibility (with about 1 occurrence for every 6 square kilometers). 
Geologic map units within this category include latest Holocene to historical alluvial fan, 
stream and estuarine deposits and many artificial fills. Together, units assigned HIGH 
susceptibility cover over 241 square kilometers of the central San Francisco Bay region.  
About 26 percent of historical liquefaction occurrences fall within map units assigned 
HIGH susceptibility (about 0.16 occurrences per square kilometer) (Table 4). Quantitative 
liquefaction analyses of several map units (using borehole data) indicate that 15 to 32 
percent of samples are expected to liquefy at the 10% probability of exceedance in 50 
year PGA (Table 3), but only 7 percent of samples of Qhfy are expected to liquefy. PGAs of 
greater than about 0.1 g to 0.2 g are necessary to trigger liquefaction of deposits assigned 
HIGH susceptibility.


VERY HIGH -- Expect about 40 to 50 percent of future liquefaction effects to occur within 
geologic units assigned VERY HIGH susceptibility (with about 1 occurrence for every 4 square 
kilometers). Geologic map units within this category include latest Holocene to historical 
stream channel, natural levee and beach deposits and artificial fill placed over San 
Francisco Bay mud and historically active stream channels. Together, units with VERY HIGH 
susceptibility cover over 284 square kilometers in the central San Francisco Bay region.  
About 45 percent of historical liquefaction occurrences fall within map units assigned 
VERY HIGH susceptibility (about 0.23 occurrences per square kilometer) (Table 4). 
Quantitative liquefaction analyses performed for two map units (using borehole data) 
indicate that 40 (Qhc) to 62 (Qhly) percent of samples are expected to liquefy at the 
10% probability of exceedance in 50 year PGA (Table 3). PGAs of about 0.1 g are necessary 
to trigger liquefaction in deposits assigned VERY HIGH susceptibility.
________________________________________________



Only data on historical liquefaction occurrence within the map area was used in the analysis. 
Historical liquefaction occurrences in the San Francisco North quadrangle were thus excluded 
from the analysis, despite the numerous occurrences documented there by Youd and Hoose (1978) 
and Tinsley et al. (1998). Inclusion of the occurrences along the Bay shoreline of San Francisco, 
in particular, would undoubtedly have increased the number of liquefaction effects per unit 
area in artificial fill over estuarine mud (afem) reported in Table 4. We excluded documented 
effects described as hillside landslides and found no effective way to use locations where 
an absence of liquefaction-related ground failure was noted. Some of the historical effects 
were reported as lines or areas of relatively dense arrays of occurrences, rather than 
specific points, making it impossible to express these in terms of occurrence per square 
kilometer. Most of these effects depicted as lines or areas were used only qualitatively 
in our analysis with the following exceptions: seven line effects and five area effects were 
used in the analysis where the specific occurrences could be attributed to a single geologic 
unit (in all but two cases, artificial fill over estuarine mud) based on published reports 
(e.g., Youd and Hoose, 1978; Tinsley et al., 1998). 

Summary descriptions of each liquefaction susceptibility category are provided in Table 5. 
The descriptions include the spatial abundance of future liquefaction occurrences, the age 
range of the deposits, the abundance of historical liquefaction occurrence by susceptibility 
category, the total map area of the units assigned to the category, the results of quantitative 
liquefaction analyses of limited geotechnical borehole information, and the estimated PGA 
triggering thresholds for the included units (Table 3).



REFERENCES CITED

Angell, M., Hanson, K., and Crampton, T., 1997, Characterization of Quaternary contractional 
deformation adjacent to the San Andreas Fault, Palo Alto, California:  Unpublished consulting 
report prepared for the U.S. Geological Survey, National Earthquake Hazards Reduction Program, 
Award no. 1434-95-G-2586, 70 p. plus tables, figures and plates.


Atwater, B.F., 1982, Geologic maps of the Sacramento-San Joaquin Delta, California:  U.S. 
Geological Survey Miscellaneous Field Studies Map MF-1401.


Atwater, B.F., 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.


Atwater, B.F., Ross, B.E., and Wehmiller, J.F., 1981, Stratigraphy of late Quaternary 
estuarine deposits and amino acid stereochemistry of oyster shells beneath San Francisco Bay, 
California: Quaternary Research. v. 16, p. 181-200.


Aydin, A., and Page, B.M., 1984, Diverse Pliocene-Quaternary tectonics in a transform 
environment, San Francisco Bay region, California: Geological Society of America Bulletin, 
v. 95, p. 1303-1317.


Bates, L.A. and others, 1977, Soil Survey of Solano County: U.S. Department of Agriculture, 
Soil Conservation Service, 112 p., 54 map sheets at 1:24,000 scale.


Bezore, S.P., Sowers, J.M., and Randolph, C.E., 2000, Digital geologic map of the Fairfield 
South 7.5-minute quadrangle, Solano County, California: California Division of Mines and 
Geology Open-File Report in preparation, scale 1:24,000. 


Bonilla, M.G., 1971, Preliminary geologic map of the San Francisco South 7.5-minute 
quadrangle and part of the Hunters Point 7.5-minute quadrangle, California; U.S. Geological 
Survey Miscellaneous Field Studies Map MF-311; scale 1:24,000. 


Bonilla, M.G., 1998, Preliminary geologic map of the San Francisco South 7.5-minute 
quadrangle and part of the Hunters Point 7.5-minute quadrangle, California: a digital 
database: U.S. Geological Survey Open-File Report 98-354; scale 1:24,000. 


Borchardt, G., 1994, The Sangamon strandline in northern San Francisco Bay and its 
significance for paleoseismic studies [abs]: in Prentice, C.S., Schwartz, D.P., and Yeats, 
R.S., eds., Proceedings of the workshop on Paleoseismology, U.S. Geological Survey Open-File 
Report 94-568, p. 24-25.


Brabb, E.E., Graymer, R.W., and Jones, D.L., 1998a, Geology of the onshore part of San 
Mateo County, California: A digital database: U.S. Geological Survey Open-File Report 98-137.


Brabb, E.E., Graymer, R.W., and Jones, D.L., 1998b, Geology of Palo Alto 30 X 60 minute 
quadrangle, California: A digital database: U.S. Geological Survey Open-File Report 98-348.


Bryan, K., 1923, Geology and ground-water resources of Sacramento Valley, California: 
U.S. Geological Survey Water-supply Paper 495, 286 p. plus plates.


California Division of Mines and Geology, 1999, Recommended criteria for delineating 
seismic hazard zones in California, CDMG Special Publication 118, 12 p.


Cao, T., Bryant, W.A., Rowshandel, B., Branum, D., and Wills, C.J., 2003, The Revised 
2002 California Probabilistic Seismic Hazard Maps, June 2003: California Geological 
Survey web page, 
<http://www.consrv.ca.gov/cgs/rghm/psha/fault_parameters/pdf/2002_CA_Hazard_Maps.pdf> (
November 18, 2005).


Clahan, K.B., Mattison, E., and Knudsen, K.L., 2000a, Liquefaction zones in the San Jose 
East 7.5-minute quadrangle, Santa Clara County, California in Seismic hazard evaluation 
of the San Jose East 7.5-minute quadrangle: California Division of Mines and Geology 
Open-File Report 2000-010.


Clahan, K.B., Mattison, E., and Rosinski, A.M., 2000b, Liquefaction zones in the Calaveras 
Reservoir 7.5-minute quadrangle, Santa Clara County, California in Seismic hazard evaluation 
of the San Jose East 7.5-minute quadrangle: California Division of Mines and Geology 
Open-File Report 2000-048.


Clahan, K.B., Mattison, E., and Rosinski, A.M., 2000c, Liquefaction zones in the Milpitas 
7.5-minute quadrangle, Santa Clara County, California in Seismic hazard evaluation of 
the San Jose East 7.5-minute quadrangle: California Division of Mines and Geology Open-
File Report 2000-051.


Clahan, K.B., Mattison, E., and Rosinski, A.M., 2000d, Liquefaction zones in the San 
Jose West 7.5-minute quadrangle, Santa Clara County, California in Seismic hazard 
evaluation of the San Jose East 7.5-minute quadrangle: California Division of Mines and 
Geology Open-File Report 2000-058.


Clark, J.C., and Brabb, E.E., 1997, Geology of Point Reyes National Seashore and Vicinity, 
California: A Digital Database, U.S. Geological Survey Open-File Report 97-456.


Cosby, S.W., 1941, Soil survey of the Sacramento - San Joaquin Delta area, California: 
U.S. Department of Agriculture, Bureau of Chemistry and Soils, series 1935, no. 21, 48 p.


Dibblee, T.W., 1980a, Preliminary geologic map of the Benicia quadrangle, Contra Costa 
and Solano Counties, California: U.S. Geological Survey Open-File Report OF-80-400, 
scale 1:24,000.
Dibblee, T.W., 1980b, Preliminary geologic map of the Briones Valley quadrangle, Alameda 
and Contra Costa Counties, California: U.S. Geological Survey Open-File Report OF-80-539, 
scale 1:24,000.


Dibblee, T.W., 1980c, Preliminary geologic map of the Clayton quadrangle, Contra Costa 
County, California: U.S. Geological Survey Open-File Report OF-80-547, scale 1:24,000.
Dibblee, T.W., 1980d, Preliminary geologic map of the Richmond quadrangle, Alameda and 
Contra Costa Counties, California: U.S. Geological Survey Open-File Report OF-80-1100, 
scale 1:24,000.


Dibblee, T.W., 1980e, Preliminary geologic map of the Walnut Creek quadrangle, Contra 
Costa County, California: U.S. Geological Survey Open-File Report OF-80-351, scale 1:24,000.


Dibblee, T.W., 1981a, Preliminary geologic map of the Mare Island quadrangle, Solano 
and Contra Costa Counties, California: U.S. Geological Survey Open-File Report OF-81-234, 
scale 1:24,000.


Dibblee, T.W., 1981b, Preliminary geologic map of the Port Chicago quadrangle, Solano 
and Contra Costa Counties, California: U.S. Geological Survey Open-File Report OF-81-108, 
scale 1:24,000.


Dupr_, W.R., 1990, Maps showing geology and liquefaction susceptibility of Quaternary 
deposits in the Monterey, Seaside, Spreckels, and Carmel Valley quadrangles, Monterey, 
California:  U.S. Geological Survey, Map MF-2096.


Dupr_, W.R., and Tinsley, J.C., 1980, Maps showing geology and liquefaction potential 
of northern Monterey and southern Santa Cruz counties, California:  U.S. Geological Survey 
Miscellaneous Field Studies MF-1199.


Dupr_, W.R., and Tinsley, J.C., 1998, Evaluation of liquefaction-hazard mapping in the 
Monterey Bay region, central California: in Holzer, T.L., ed., The Loma Prieta, California, 
Earthquake of October 17, 1989 - Liquefaction: U.S. Geological Survey Professional 
Paper 1551-B.


Fox, K.F. Jr., Sims, J.D.,  Bartow, J.A., and Helley, E.J., 1973, Preliminary geologic 
map of eastern Sonoma County, and western Napa County: U.S. Geological Survey 
Miscellaneous Field Studies Map MF-483, scale 1:62,500. 


Frankel, A.D., M.D. Petersen, C.S. Mueller, K.M. Haller, R.L. Wheller, E.V. Leyendecker, 
R.L. Wesson, S.C. Harmsen, C.H. Cramer, D.M. Perkins, K.S. Rukstales, 2002, Documentation 
for the 2002 update of the national seismic hazard maps: U.S. Geological Survey Open-File 
Report 022-420, 33 p. Available at: 
"http://geohazards.cr.usgs.gov/eq/of02-420/OFR02-420.pdf" 
http://geohazards.cr.usgs.gov/eq/of02-420/OFR02-420.pdf.


Galloway, A.J., 1977, Geology of the Point Reyes Peninsula, Marin County, California: 
California Division of Mines and Geology Bulletin 202, 72 p. plus 1:48,000-scale plate. 


Gardner, R. A., Harradine, F. F., Hargreaves, G. H., Retzer, J. L., Bartholomew, O. F., 
and Glassey, T. W., 1958, Soil Survey of the Santa Clara Area, California, U. S. 
Department of Agriculture, Soil Conservation Service, Series 1914, No. 17, 202 p., 2 
map sheets at 1:50,000 scale. 


Goals Project, 1999, Bay lands ecosystem habitat goals.  A report of habitat 
recommendations prepared by the San Francisco Bay Are Wetlands Ecosystem Goals Project:  
U.S. Environmental Protection Agency, San Francisco, CA / San Francisco Bay Regional 
Water Quality Control Board, Oakland, CA, 209 p. plus appendices.


Goldman, H.B., ed., 1969, Geologic and engineering aspects of San Francisco Bay fill: 
California Division of Mines and Geology Special Report 97.
Graymer, R. W., 1997, Geology of the southernmost part of Santa Clara County, California: 
a digital database: U.S. Geological Survey open-File Report 97-710.


Haydon, W.D., 1995, Landslide hazards in the Martinez-Orinda-Walnut Creek area Contra 
Costa County, California: Landslide Hazard Identification Map No. 32: California 
Department of Conservation, Division of Mines and Geology Open-File Report 95-12, 
pp. 40, Plates 32A-32D. 


Haydon, W.D, Mattison, E., and Clahan, K.B., 1999, Liquefaction zones in Cities of 
Oakland and Piedmont, Alameda County, California in Seismic hazard evaluation of 
Cities of Oakland and Piedmont, Alameda County: California Division of Mines and Geology 
Open-File Report 1999-11.


Helley, E.J., and Brabb, E.E., 1971, Geologic map of late Cenozoic deposits, Santa 
Clara County, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-335.

Helley, E.J., Fitzpatrick, J.A., and Bischoff, J.L., 1993, Uranium-series dates on 
oyster shells from marine terraces of San Pablo Bay, California: U.S. Geological Survey 
Open-File Report 93-286, 6 p.


Helley, E.J., and Graymer, R.W., 1997a, Quaternary geology of Contra Costa, and 
surrounding parts of Alameda, Marin, Sonoma, Solano, Sacramento, and San Joaquin 
Counties, California: a digital database: U.S. Geological Survey Open-File Report 97-98.


Helley, E.J., and Graymer, R.W., 1997b, Quaternary geology of Alameda County and 
parts of Contra Costa, Santa Clara, San Mateo, San Francisco, Stanislaus, and San 
Joaquin Counties, California: a digital database: U.S. Geological Survey Open-File 
Report 97-97.


Helley, E.J., Graymer, R.W., Phelps, G.A., Showalter, P.K., and Wentworth, C.M., 1994, 
Preliminary Quaternary geologic maps of Santa Clara Valley, Santa Clara, Alameda, 
and San Mateo Counties, California, a digital database: U.S. Geological Survey Open-
File Report 94-231.


Helley, E.J., and Harwood, D.S., 1985, Geologic map of the Late Cenozoic deposits of 
the Sacramento Valley and northern Sierran Foothills, California: U.S. Geological 
Survey Miscellaneous Field Studies Map MF-1790, scale 1:62,500.


Helley, E.J., Lajoie, K.R., Spangle, W.E., and Blair, M.L., 1979, Flatland deposits 
of the San Francisco Bay Region, California - Their geology and engineering properties, 
and their importance to comprehensive planning:  U.S. Geological Survey Professional 
Paper 943, 88 p.


Helley, E.J., and Miller, D.M., 1992, Geologic map of the Newark 7.5 minute quadrangle, 
Alameda County, California: U.S. Geological Survey Open-File Report 92-312, scale 1:24,000.


Helley, E.J., and Wesling, J., 1989, Quaternary geologic map of the Milpitas quadrangle, 
California, 1:24,000: U.S. Geological Survey Open-File Report 89-671.


Helley, E.J., and Wesling, J.R., 1990, Quaternary geologic map of the San Jose East 
quadrangle, Santa Clara County, California:  U.S. Geological Survey Open-File 
Report 90-427.


Herd, D.G., 1977, Geologic map of the Las Positas, Greenville, and Verona Faults, 
eastern Alameda County, California: U.S. Geological Survey Open-file Report 77-689, 
1:24,000.


Hitchcock, C.S., and Kelson, K.I., 1999, Growth of late Quaternary folds in southwest 
Santa Clara Valley, San Francisco Bay area, California: Implications of triggered slip 
for seismic hazard and earthquake recurrence, Geology, 27, 5, 391-394.


Holzer, T.L., Tinsley, J.C., III, Bennett, M.J., and Mueller, C.J., 1994, Observed 
and predicted ground deformation - Miller Farm lateral spread, Watsonville, California:  
Buffalo, New York, National Center for Earthquake Engineering Research Technical 
Report NCEER-94-0026, p. 79-99.


Holzer, T.L., Bennett, M.J., Noce, T.E., Padovani, A.C., Tinsley, J.C., III, 2002, 
Liquefaction hazard and shaking amplification maps of Alameda, Berkeley, Emeryville, 
Oakland, and Piedmont, California: a digital database, US Geological Survey 
Open-File Report 2002-296.


Holzer, T.L., ed., 1998, The Loma Prieta, California, Earthquake of October 17, 
1989 - Liquefaction: U.S. Geological Survey Professional Paper 1551-B.


Jack, R.N., 1969, Quaternary sediments at Montara, San Mateo County, California: 
unpublished Masters Thesis, University of California, Berkeley, 133 p.


Jennings, C.W., 1988, Preliminary geology map of the northwest quarter of the Santa 
Rosa [7.5'] quadrangle, Sonoma County, California: California Division of Mines and 
Geology, Open-File Report 88-05, scale 1:24,000. 


Kashiwagi, J., 1985, Soil Survey of Marin County, California: U.S. Department of 
Agriculture, Soil Conservation Service, 229 p., 13 map sheets at 1:24,000 scale.


Kashiwagi, J.H. and Hokholt, L.A., 1991, Soil survey San Mateo County, eastern 
part, and San Francisco County, California: U.S. Department of Agriculture, Soil 
Conservation Service, 120 p., 12 map sheets at 1:24,000 scale.


Kelson, K.I., Simpson, G.D., Haraden, C.C., Sawyer, T.L., and Hemphill-Haley, M.A., 
1993, Late Quaternary surficial deformation of the southern East Bay Hills: Final 
Technical Report, National Earthquake Hazards Reduction Program, U.S. Geological 
Survey, Award #1434-92-2209.
Knudsen, K.L., and Lettis, W.R., 1997, Preliminary maps showing Quaternary geology 
of twenty 7.5-minute quadrangles, eastern Stockton, California, 1:100,000 quadrangle: 
National Earthquake Hazards Reduction Program, U.S. Geological Survey, Final Technical 
Report , Award #1434-94-G-2499.
Knudsen, K.L., Noller, J.S., Sowers, J.M., Lettis, W.R., Graham, S.E., Randolph, C.E., 
and May, T.E., 1997, Quaternary geology and liquefaction susceptibility, San Francisco, 
California 1:100,000 quadrangle: a digital database: U.S. Geological Survey Open-File 
Report 97-715, scale 1:100,000.


Knudsen, K.L., J.M. Sowers, R.C. Witter, C.M. Wentworth, and E.J. Helley, 2000a, 
Preliminary Maps of Quaternary Deposits and Liquefaction Susceptibility, Nine-County 
San Francisco Bay Region, California: A Digital Database, U.S. Geological Survey Open-
File Report 00-444. Digital Database by Wentworth, C.M., Nicholson, R.S., Wright, H.M., 
and Brown, K.H. Online Version 1.0.


Knudsen, K.L., DeLisle, M.D., Clahan, K.B., Mattison, E., Perkins, J.B., and 
Wentworth, C.M., 2000b, Applicability of Quaternary geologic mapping in assessing 
earthquake-induced liquefaction hazard: San Francisco Bay Region: Sixth International 
Conference on Seismic Zonation, Earthquake Engineering Research Institute, p. 154.


Lajoie, K.R., Helley, E.J., Nichols, D.R., and Burke, D.B., 1974, Geologic map of 
unconsolidated and moderately consolidated deposits of San Mateo County: U.S. Geological 
Survey Miscellaneous Field Studies Map MF-575, scale 1:62,500.


Lajoie, K.R., Weber, G.E., Mathieson, S., and Wallace, J., 1979, Quaternary tectonics 
of coastal Santa Cruz and San Mateo Counties, California, as indicated by deformed marine 
terraces and alluvial deposits, in Weber, G.E., Lajoie, K.R., and Griggs, G.B. eds., 
Coastal tectonics and coastal geologic hazards in Santa Cruz and San Mateo Counties, 
California: Field Trip Guide, Cordilleran Section of the Geological Society of America, 
75th Annual Meeting, p. 61-80.


Lambert, G., and Kashiwagi, J., 1978, Soil Survey of Napa County, California: U.S. 
Department of Agriculture, Soil Conservation Service, 104 p., 47 map sheets at 
1:24,000 scale.


Lindsey, W.D., and Weisel, C.J., 1974, Soil survey of the eastern Santa Clara area, 
California:  U.S. Department of Agriculture, Soil Conservation Service, 90 p., 49 map 
sheets at 1:24,000 scale.


Matti, J.C., and Carson, S.E., 1991, Liquefaction susceptibility in the San Bernardino 
Valley and vicinity, Southern California - A regional evaluation: U.S. Geological 
Survey Bulletin 1898, 53 p. plus plates.


Mejia, L.H., Hughes, D.K., and Sun, J.I., 1992, Liquefaction at Moss Landing during the 
1989 Loma Prieta Earthquake:  Proceedings of the Tenth World Conference, Earthquake 
Engineering, p. 1435-1440.


Miller, V.C., 1972, Soil survey of Sonoma County, California: U.S. Department of 
Agriculture, Soil Conservation Service, 188 p., 123 map sheets at 1:20,000 scale. 


Nichols, D.R., and Wright, N.A., 1971, Preliminary map of historical margins of 
marshland, San Francisco Bay, California: U.S. Geological Survey Open-File Report, 
Basic Data Contribution 9, scale 1:125,000. 


Pampeyan, E.H., 1993, Geologic map of the Palo Alto and part of the Redwood Point 
7-1/2 quadrangles, San Mateo and Santa Clara Counties, California: U.S. Geological 
Survey Miscellaneous Investigation Series I-2371, scale 1:24,000.


Pampeyan, E.H., 1994, Geologic map of the Montara Mountain and San Mateo 7-1/2 
quadrangles, San Mateo County, California: U.S. Geological Survey Miscellaneous 
Investigation Series I-2390, scale 1:24,000.


Plafker, G., and Galloway, J.P., eds., 1989, Lessons learned from the Loma Prieta, 
California, earthquake of October 17, 1989: U.S. Geological Survey Circular 1045, 48 p.


Radbruch, D.H., 1969, Areal and Engineering Geology of the Oakland East Quadrangle, 
California, Map GQ-769, U.S. Geological Survey, Washington D.C. 


Rogers, J.D., and Figuers, S.H., 1991, Engineering geologic site characterization 
of the greater Oakland-Alameda area, Alameda and San Francisco Counties, California: 
Final Report to the National Science Foundation, Grant no. BCS - 9003785.


Sarna, A., 1967, Aerial extent and thickness of the bay mud in northern Santa Clara 
County: unpublished report to Santa Clara County Planning Department.


Sawyer, T.L., 1996, Unpublished surficial mapping conducted in support of the 
following project:  Unruh, J.R. and Sawyer, T.L., 1997, Assessment of blind seismogenic 
sources, Livermore Valley, eastern San Francisco Bay region: Final Technical Report, 
National Earthquake Hazards Reduction Program, U.S. Geological Survey, Award #1434-95-G-2611.


Seed, H.B., and Idriss, I.M., 1971, Simplified procedure for evaluating soil 
liquefaction potential: Journal of the Soil Mechanics and Foundations Division of 
ASCE, v. 97: SM9, p. 1,249-1,273. 


Seed, H.B., and Idriss, I.M., 1982, Ground motions and soil liquefaction during 
earthquakes:  Earthquake Engineering Research Institute, Engineering Monograph v. 5, 134 p. 


Seed, R.B., Dickenson, S.E., Riemer, M.F., Bray, J.D., Sitar, N., Mitchell, J.K., Idriss, 
I.M., Kayen, R.E., Kropp, A., Harder, L.F., Jr., and Power, M.S., 1990, Preliminary 
report on the principal geotechnical aspects of the October 17, 1989 Loma Prieta 
earthquake, Earthquake Engineering Research Center, Report No. UCB/EERC-90/05. 


Sims, J.D., Fox, K.F. Jr., Bartow, J.A., and Helley, E.J., 1973, Preliminary geologic 
map of Solano County, and parts of Napa, Contra Costa, Marin, and Yolo Counties: U.S. 
Geological Survey Miscellaneous Field Studies Map MF-484, scale 1:62,500. 


Sowers, J.M., 1995, Creek & watershed map of Oakland and Berkeley: Oakland Museum of 
California, Oakland, California, scale approx. 1:26,000.


Sowers, J.M., 1997, Creek & watershed map of Hayward and San Leandro: Oakland Museum 
of California, Oakland, California, scale approx. 1:26,000.


Sowers, J.M., 1999, Creek and watershed map of Fremont and vicinity: Oakland Museum of 
California, Oakland, California, scale 1:26,000
Sowers, J.M., Noller, J.S., and Lettis, W.R., 1998, Quaternary geology and liquefaction 
susceptibility, Napa, California 1:100,000 quadrangle: a digital database: U.S. 
Geological Survey Open-File Report 98-460.
Thomasson, H.G., Olmsted, F.H., and LeRoux, E.F., 1960, Geology, Water Resources, 
and usable groundwater storage capacity of part of Solano County, California:  U.S. 
Geological Survey Water-Supply Paper 1464, p. 48.
Tinsley, J.C., III, Egan, J.A., Kayen, R.E., Bennett, M.J., Kropp, A., and Holzer, 
T.L., 1998, Appendix: Maps and descriptions of liquefaction and associated effects: 
in Holzer, T.L., ed., The Loma Prieta, California, Earthquake of October 17, 1989 - 
Liquefaction: U.S. Geological Survey Professional Paper 1551-B.


Tinsley, J.C., Youd, T.L., Perkins, D.M., and Chen, A.T.F, 1985, Evaluating Liquefaction 
Potential, in Ziony, J.I., ed., Evaluating earthquake hazards in the Los Angeles Region - 
an earth-science perspective:  U.S. Geological Survey Professional Paper 1360, p. 263-316.


Unruh, J.R. and Lettis, W.R., 1998, Seismogenic deformation field in the Mojave block 
and implications for tectonics of the eastern California shear zone: Journal of Geophysical 
Research, v. 101, p. 8335-8361.


Wagner, R.J. and Nelson, R.E., 1961, Soil Survey of San Mateo Area, California: U.S. 
Department of Agriculture, Soil Conservation Service, Series 1954, no. 13, 112 p., 
49 map sheets at 1:15,840 scale. 


Weber, G.E., Nolan, J.M., and Zinn, E.N., 1993, Determination of Late Pleistocene and 
Holocene slip rates along the San Gregorio Fault zone, San Mateo County, California: 
Final Technical Report, National Earthquake Hazards Reduction Program, U.S. Geological 
Survey, Award # 1434-93-G-2336.


Welch, L.E., 1977, Soil survey Contra Costa County, California: U.S. Department of 
Agriculture, Soil Conservation Service, National Cooperative Soil Survey, 122 p.,  
54 map sheets at 1:24,000 scale.


Welch, L.E., 1981, Soil survey Alameda County, California, western part: U.S. Department 
of Agriculture, Soil Conservation Service, 103 p., 8 map sheets at 1:24,000 scale.


Welch, L.E., Huff, R.C., Dierking, R.A., Cook, T.D., Bates, L.A., Andrews, W.F., 1966, 
Soil survey Alameda area, California: U.S. Department of Agriculture, Soil Conservation 
Service, Series 1961, No. 41, 95 p., 42 map sheets at 1:20,000 scale.


Wentworth, C.M., Blake, M.C., McLaughlin, R. J., and Graymer, R.W., 1998, Preliminary 
geologic map of the San Jose 30 x 60 minute quadrangle, California: a digital database: 
U.S. Geological Survey Open-File Report 98-795, map scale 1:100,000.


Wolff, R.G., 1971, Paleoecology of a late Pleistocene (Rancholabrean) vertebrate fauna 
from Rodeo, California: unpublished Doctoral Dissertation, University of California, 
Berkeley, 136 p.


Working Group on California Earthquake Probabilities , 2003, Earthquake probabilities in
the San Francisco Bay region: 2002-2031: U.S. Geological Survey Open-File Report 03-214.


Youd, T.L., 1973, Liquefaction, flow and associated ground failure: U.S. Geological 
Survey Circular 688, 12 pp.
Youd, T.L., and Hoose, S.N., 1978, Historical ground failures in Northern California 
triggered by earthquakes: U.S. Geological Survey Professional Paper 993.
Youd, T.L., Nichols, D.R., Helley, E.J., and Lajoie, K.R., 1975, Liquefaction potential, 
in Studies for seismic zonation of the San Francisco Bay region: U.S. Geological Survey 
Prof Paper 941-A, p. 68-74. Youd, T.L., and Idriss, I.M., 1997, editors, Proceedings of 
the NCEER workshop on evaluation of liquefaction resistance of soils: Nation Center for 
Earthquake Engineering Research Technical Report NCEER-97-0022, 276 p.


Youd, T. L., Idriss, I. M., Andrus, R. D., Arango, I., Castro, G., Christian, J. T., 
Dobry, R., Finn, W. D. L., Harder, L. F., Hynes, M. E., Ishihara, K., Koester, J. P., 
Liao, S. C., Marcuson, W. F., Martin, G. R., Mitchell, J. K., Moriwaki, Y., Power, M. S., 
Robertson, P. K., Seed, R. B. and Stokoe, K. H. (2001). "Liquefaction Resistance of Soils: 
Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of 
Liquefaction Resistance of Soils." Journal of Geotechnical and Geoenvironmental 
Engineering, 127 (10), 817-833.


Youd, T.L., and Perkins, D.M., 1978, Mapping liquefaction-induced ground failure 
potential: Journal of Geotechnical Engineering, American Society of Civil Engineers, 
v. 104, no. GT4, p. 433-446.


Youd, T.L., and Perkins, J.B., 1987, Map showing liquefaction susceptibility of San 
Mateo County, California:  U.S. Geological Survey, Map I-1257-G.





Appendix A



DESCRIPTION OF GEOLOGIC UNITS


Map Symbol			Unit Name and Description


HISTORICAL (<150 YEARS) DEPOSITS.

af	Artificial fill (historical)._ Material deposited by humans. Additional types 
	of artificial fill are mapped as separate geologic map units, including: artificial 
	fill over estuarine mud (afem), artificial levee fill (alf), dredge spoils (ads), 
	artificial channel fill (acf), artificial dam fill (adf), and gravel quarries and 
	percolation ponds (gq).  Fill may be engineered and/or non-engineered material; 
	each may occur within the same area on the map.  Most of the artificial fill shown 
	forms large highway and railroad embankments, consisting of engineered fill up to 
	approximately 100 feet thick. Large earthen dams are mapped separately as artificial 
	dam fill (adf).  Our mapping of artificial fill on road and railroad embankments 
	is based on interpretation of topographic contours on the most recent 7.5-minute 
	U.S. Geological Survey (USGS) topographic quadrangles.  Fill whose thickness is less 
	than the contour interval (typically 5 to 10 ft) and fill emplaced after the 
	topographic base maps were surveyed are not shown.  Small bodies of fill, such as 
	small road embankments and earthen dams for farm ponds, are not shown.  Included 
	within this unit are small areas of Holocene alluvial deposits that are too 
	small to be mapped at this scale.  On the San Francisco Peninsula, previous mapping 
	by Bonilla (1971, 1998), and Pampeyan (1993, 1994) provide the primary basis from 
	which to identify and map artificial fill.  Elsewhere, mapping of artificial fill 
	is based on inspection of topographic maps and aerial photographs._

	Liquefaction susceptibility of artificial fill (af) may be very high to low depending 
	on (1) the nature and thickness of the fill materials, (2) whether the fill was 
	engineered or non-engineered, (3) the susceptibility of the deposit over which the 
	fill lies, and (4) its depth of saturation.  Most fill placed in the last few decades 
	is engineered; older fill is less likely to be engineered.  A large percentage of 
	observed historical liquefaction in the area has occurred in artificial fill on the 
	margins of San Francisco Bay; however, most of these artificial fill bodies are 
	included within the map unit artificial fill over estuarine mud (afem).  Artificial 
	fill is assigned the susceptibility of the underlying deposit(s) with the exception 
	that fill overlying deposits with very low susceptibility is assigned low 
	susceptibility. Where more than one deposit with different liquefaction 
	susceptibilities underlie an artificial fill map unit, the unit is subdivided and 
	assigned susceptibilities to reflect the underlying geologic units.  Where artificial 
	fills are narrow, such as artificial levees, or road embankments, and overlie the 
	boundary between two geologic map units, the higher susceptibility is assigned to 
	the fill. Specific bodies of artificial fill (af) were assigned High or Very High 
	susceptibilities based on the historical occurrence of liquefaction within or near 
	these bodies.

afem	Artificial fill over estuarine mud (historical). Material deposited by humans over 
	sediments along the margins of San Francisco Bay and other estuarine deposits mapped 
	in the Sacramento/San Joaquin delta and along the outer Pacific coast.  Fill may be 
	engineered and/or non-engineered material; each may occur within the same area on 
	the map.  This mapped artificial fill overlies estuarine sediment and was placed to 
	form new land (e.g., Goldman, 1969).  Mapping of artificial fill over estuarine mud 
	is based on comparison of present shorelines with those of the mid 19th century as 
	shown by Nichols and Wright (1971), the Goals Project (1999) and Sowers (1995, 1997, 
	1999), and, to a limited extent, on inspection of topographic maps and aerial 
	photographs.  In many areas, a line delineating the middle 19th century (1850s-1860s) 
	extent of marshland (Nichols and Wright, 1971) was used to delineate the landward 
	boundary of this unit.  Artificial fill placed inland of the Nichols and Wright (1971) 
	line was mapped as artificial fill (af).  The thickness of the fill overlying 
	estuarine sediment is typically five to twenty feet.  Included within this unit are 
	small areas of estuarine deposits and Holocene alluvial deposits that are too small 
	to be mapped at this scale of this project.  Levees and dikes that overlie estuarine 
	deposits are mapped as artificial levee fill (alf) and differentiated based on levees 
	identified on USGS 7.5-minute topographic quadrangles.  The levee materials may not 
	differ from the adjacent artificial fill over estuarine mud material (afem).  Other 
	unmapped levees and dikes likely exist within mapped bodies of this unit (afem).  
	This unit includes artificial fill placed over alluvial fan-estuarine complex deposits 
	(Qhfe) and estuarine delta deposits (Qhed)._

	Liquefaction susceptibility is very high based on the numerous past occurrences of 
	liquefaction in this unit.  About 34% of all past occurrences of earthquake-induced 
	liquefaction in the central San Francisco Bay region occurred in artificial fill over 
	estuarine mud. Most fill emplaced over estuarine mud in the last few decades is 
	engineered; older fill is less likely to be engineered.  Many of the reports of damage 
	in the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake involved 
	failures in older fill that probably was not engineered; such fill was probably 
	hydraulically emplaced.  Where the material to be used as artificial fill was dredged 
	or suctioned from sandy areas (e.g. near Oakland and the Marina District of San 
	Francisco), the fill may be very susceptible to liquefaction.  Typically, ground-water 
	levels in this unit are close to the ground surface._

alf	Artificial levee fill (historical).  Constructed levees bordering rivers, streams, 
	salt ponds, sloughs, and delta islands for the purpose of containing flood or tidal 
	waters.  Some are compacted and quite firm, but levees built before 1965 (enactment 
	of the Uniform Building Code) are likely to be uncompacted and made of poor quality 
	fill.  Levees bordering waterways of the Sacramento/San Joaquin Delta, mudflats, and 
	large streams were first emplaced as much as 100 years ago.  The mapped distribution 
	of levee fill conforms to levees shown on the most recent USGS 7.5-minute topographic 
	quadrangles.  In some places this unit was compiled from recent mapping by R.W. Graymer 
	and published in Helley and Graymer (1997a, 1997b), Helley and others (1994), and 
	Brabb and others (1998a, 1998b).

	Liquefaction susceptibility is estimated to be very high to moderate for all artificial 
	levees, based on the abundance of older non-engineered levees, the nature of the fill 
	materials, the susceptibility of the underlying deposit, the possible proximity of 
	channel free faces vulnerable to lateral spreading, and their likelihood of saturation.  
	Additionally, levees often are placed in areas where the substrate is highly 
	susceptible to liquefaction. We assign a very high susceptibility to all artificial 
	levees that overlie estuarine deposits, including San Francisco Bay mud (Qhbm), 
	alluvial fan-estuarine complex deposits (Qhfe), and estuarine delta deposits (Qhed). 
	Other levees emplaced over upland deposits are assigned High liquefaction 
	susceptibility reflecting the susceptibility of underlying deposits. Artificial levees 
	emplaced over deposits assigned low to very low susceptibilities are assigned 
	moderate susceptibility.

acf	Artificial channel fill (historical). Artificial fill emplaced in historically active 
	stream channels. The stream flow has been re-routed either to a pipe or channel in 
	another location or in a pipe beneath or within the fill. The fill, emplaced to 
	create level land for urban development, may be engineered and/or non-engineered 
	material; each may occur within the same mapped unit. These human-emplaced materials 
	may overlie modern stream channel deposits (Qhc) that consist of loose, 
	unconsolidated, poorly to well-sorted sand, gravel and cobbles, with minor silt 
	and clay. Artificial channel fill was identified through comparison of early 
	twentieth century aerial photography and topographic maps with recent 7.5-minute 
	topographic quadrangles and field inspection. In the Oakland, San Leandro, and 
	Fremont areas of the East Bay, artificial channel fills were mapped from creek 
	and watershed maps developed by Sowers (1995, 1997, 1999). Artificially-filled 
	channels of very small streams are not delineated at 1:24,000 scale.

	Liquefaction susceptibility is very high due to the likely presence of late 
	Holocene, loose, granular stream channel deposits and high ground-water levels 
	underlying the artificial fill.  

adf	Artificial dam fill (historical).  Earth dams, rock-fill dams, embankments and 
	levees constructed to impound land-locked water bodies, including water reservoirs, 
	cooling/settling ponds, artificial lakes, and stock ponds. Fill may be engineered 
	and/or non-engineered material; each may occur within the same area on the map.  
	As with the mapping of other human-emplaced fill, artificial dam fills are mapped 
	through interpretation of topographic contours on the most recent 7.5-minute USGS 
	topographic quadrangles.  Dams with fill thicknesses that are less than the 
	contour interval (typically 5 to 10 ft) and embankments emplaced after the 
	topographic base maps were surveyed are not shown. This unit does not include 
	dams constructed of concrete (e.g., concrete arch and gravity dams). 

	Liquefaction susceptibility of artificial dam fill (adf) may be moderate to very 
	low. The variation in susceptibility of the materials relates to (1) the properties 
	and thickness of the fill; (2) whether the dam is an engineered embankment or 
	nonengineered; and (3) the depth of saturation, and, in some cases, (4) the nature 
	of the native material underlying the dam. Most earthen dams constructed within 
	the last few decades are engineered and have very low susceptibilities to 
	liquefaction. Small stock ponds and older dams impounding large water reservoirs, 
	particularly those constructed of hydraulically pumped fill built in the early 
	twentieth century, are likely more susceptible to liquefaction. Because of variations 
	in material and age of artificial dams and the uncertainty with regard to the 
	degree of engineering used during construction, we assign a low susceptibility to 
	all artificial dam fill (adf) units. Site-specific studies are required to assess 
	the liquefaction potential at individual dam sites.

gq	Gravel quarries and percolation ponds (historical).  This unit consists of 
	excavations, associated spoil piles, and disturbed ground in stream channels or 
	alluvial deposits that were or are being used for the purpose of extracting sand 
	and gravel.  Because many gravel pits are eventually used as recharge or 
	percolation ponds, we include percolation ponds within this map unit.  These 
	areas are identified through interpretation of 7.5-minute topographic quadrangles 
	and aerial photography.

	Liquefaction susceptibility is moderate owing to the fact that the material is 
	largely reworked late Holocene to historical stream channel deposits and likely 
	saturated.

ac	Artificial stream channel (historical).  Modified stream channels including 
	straightened or realigned channels, flood control channels, and concrete canals.  
	In most cases, artificial channels were differentiated from natural channels by 
	interpretation of 7.5-minute topographic quadrangles.  Additionally, field 
	inspection and interpretation of aerial photographs were used to identify 
	artificial channels. Deposits within artificial channels can range from almost 
	none in some concrete canals, to significant thicknesses of loose, unconsolidated 
	sand, gravel and cobbles, similar to deposits of modern stream channel deposits 
	(Qhc).  

	Liquefaction susceptibility is very high to low, varying with the design of the 
	channel and the nature of the channel and bank material.  Channels that contain 
	loose, sandy sediments like the Alameda Creek flood control channel on the Newark 
	quadrangle are highly susceptible to liquefaction. Adjacent levees or banks may 
	be subject to lateral spreading into the channel if not well engineered. 
	Artificial stream channels are assigned the susceptibility of the underlying 
	deposit(s). Where more than one deposit with different liquefaction 
	susceptibilities underlie an artificial stream channel map unit, the unit is 
	subdivided and assigned susceptibilities to reflect the underlying geologic 
	units.  Long, narrow artificial stream channels that overlie the boundary between 
	two geologic map units are assigned the higher susceptibility of the underlying 
	map units.

Qhc	Historical stream channel deposits.  Fluvial deposits within active, natural 
	stream channels.  Materials consist of loose, unconsolidated, poorly to well sorted 
	sand, gravel and cobbles, with minor silt and clay.  These deposits are reworked 
	by frequent flooding and exhibit no soil development.  These deposits, like most 
	other alluvial deposits, fine downstream (i.e. sediment is coarser upstream).  
	Mapping of modern stream channels is based on topographic map inspection augmented, 
	in places, by interpretation of aerial photography or orthophoto quadrangles.  
	Where available, we reviewed early twentieth century (1914-1916) topographic maps 
	to evaluate whether stream channels shown on recent 7.5-minute maps have been 
	altered since the early twentieth century.  If the channels appear on recent maps 
	as unchanged since the earlier maps, we map the channel and its banks as modern 
	stream channel deposits. Contacts generally are shown near the top of the bank on 
	either side of the channel, although the deposits actually lie near the bottom of 
	the channel.  Channels of very small streams are not delineated at the 1:24,000 
	map scale. 

	Liquefaction susceptibility is very high.  Tinsley and others (1985) present an 
	analysis of borehole data in the Los Angeles area that shows that 76 to 81% of 
	boreholes in latest Holocene alluvial deposits contain liquefiable materials, 
	assuming ground-water levels at the surface, compared to 34 to 54% of boreholes 
	in earlier Holocene alluvial deposits.  Matti and Carson (1991) show similar 
	relations pertain to the San Bernardino Valley of Southern California.  Dupr_ (1990), 
	Holzer and others (1994), and Mejia and others (1992) describe liquefaction along 
	the coast south and west of the 1989 Loma Prieta epicenter, most of which occurred 
	because of the presence of late Holocene, loose, granular sediment and high 
	ground-water levels.  Ground-water levels typically are at or near the surface in 
	modern stream channel deposits.


LATEST HOLOCENE (<1,000 YEARS).

Qhfy	Latest Holocene alluvial fan deposits.  Alluvial fan sediment judged to be latest 
	Holocene (<1,000 years) in age, based on records of historical inundation or the 
	presence of youthful braid bars and distributary channels.  Youthfulness of braid 
	bars and distributary channels is evaluated using aerial photographs and orthophoto 
	quadrangles.  Alluvial fan sediment is deposited by streams emanating from mountain 
	canyons onto alluvial valley floors or alluvial plains.  Most apices of the mapped 
	latest Holocene alluvial fan deposits occur partway down older piedmont alluvial 
	fan complexes.  The stream channel typically is incised into older fan deposits 
	near the fan apex, then gradually is less incised down fan, until the stream 
	becomes unconfined and distributes young sediment across the toe of the fan.  A 
	good example of these relationships is the Alameda Creek fan in Fremont and Union 
	City.  Sediment is moderately too poorly sorted and bedded, and may be composed 
	of gravel, sand, silt and clay.  Grain size generally fines down slope.  Soils 
	are minimally developed on this unit and include entisols and inceptisols.

	Liquefaction susceptibility is high because these deposits are very young, loose 
	and generally lack cohesion.  There have been a significant number of past 
	occurrences of liquefaction in this unit.  Ground water typically is less than 
	twenty feet below the surface because these deposits lie near active stream 
	channels.  If the stream channel is incised, a free face will be present and 
	lateral spreading is likely if liquefaction does occur.  However, lateral 
	spreading is probably less likely in alluvial fan deposits than in laterally 
	accreted fluvial deposits (J. Tinsley, pers. com., 2000).  

Qhly  	Latest Holocene alluvial fan levee deposits.  Natural levee deposits of alluvial 
	fans judged to be latest Holocene (<1,000 years) in age based on records of 
	historical inundation and/or the presence of youthful braid bars and distributary 
	channels.  This unit is mapped along the downstream reaches of Alameda Creek, 
	Coyote Creek, and Guadalupe River.  Levees are identified as long, low, sinuous 
	ridges oriented down fan ("channel ridges" of Bryan, 1923 and Thomasson and 
	others, 1960).  On these very young levees, the stream often runs down the levee 
	centerline.  Levees contain coarser material than adjoining interlevee areas, 
	being composed of overbank materials dropped as the stream spills over its banks 
	(Helley and others, 1979).  Soils are typically entisols (fluvents).  Ground 
	water typically is no deeper than the depth to flowing water in the stream.  

	Liquefaction susceptibility is very high because of the presence of very young, 
	loose, likely saturated deposits.  Additionally, there have been a significant 
	number of past occurrences of liquefaction in this unit.  If the stream channel 
	is incised, a free face will be present and lateral spreading is likely if 
	liquefaction does occur.

Qhty  	Latest Holocene stream terrace deposits.  Stream terrace deposits judged to be 
	latest Holocene (<1,000 years) in age based on records of historical inundation, 
	the identification of youthful meander scars and braid bars on aerial photographs 
	or orthophoto quadrangles, and/or geomorphic position (elevation) very close to 
	the stream channel.  Stream terraces are deposited as point bar and overbank 
	deposits by major streams such as the Napa River, Russian River, Coyote Creek, 
	and Alameda Creek.  Although very young stream terrace deposits also are found 
	along smaller streams, these may be too small in size to be shown at the 1:24,000 
	map scale and therefore are often included in the modern stream channel (Qhc) or 
	Holocene stream terrace (Qht) map units.  Stream terrace sediment includes sand, 
	gravel, silt, and minor clay, is moderately to well sorted, and is moderately to 
	well bedded. Where multiple stream terraces are identified, the younger deposits 
	(Qhty1) are distinguished from older deposits (Qhty2).

	Liquefaction susceptibility is high based on the abundance of sandy, cohesionless 
	sediment, relatively high ground-water levels, and the presence of a free face at 
	the channel banks, which makes lateral spreading likely if liquefaction occurs. 

Qhay	Latest Holocene alluvial deposits, undifferentiated.  Fluvial sediment judged to 
	be latest Holocene (<1000 years) in age based on records of historical inundation, 
	the identification of youthful meander scars and braid bars on aerial photographs 
	or orthophoto quadrangles, or geomorphic position very close in elevation to the 
	stream channel.  This sediment was deposited on modern flood plains, active stream 
	channels, active alluvial fans, and flood-prone areas.  Deposits are loose sand, 
	gravel, silt and clay.  This unit is mapped in areas that historically have been 
	inundated by sediment-bearing water.  Latest Holocene alluvial deposits may 
	include terrace deposits (Qhty), deposits of the active stream channel (Qhc), 
	alluvial fan deposits (Qhfy), basin deposits (Qhb), and levee deposits (Qhly).  
	However, the small size of individual deposits of these map units prevented 
	differentiation at the map scale used in this project.  Typical soils developed 
	on these deposits are entisols (fluvents).  

	Liquefaction susceptibility is high based on the presence of loose cohesionless 
	sediment near the active stream channel.  Proximity to the active stream channel 
	indicates that (1) ground-water levels likely are close to the surface, and 
	(2) a free face may be present.  

Qhbs	Latest Holocene beach sand. This unit includes active beaches in coastal 
	environments.  Beach deposits typically are well sorted fine to coarse sand with 
	some fine gravel.  Where the beach is adjacent to a sea cliff, beach sediment may 
	form a veneer over a bedrock platform.  In places, low unstable dunes and/or sandy 
	islands may be included within this map unit.  

	Liquefaction susceptibility is very high because beach sand is well sorted, 
	saturated, loose sand. 


HOLOCENE (<11,800 YEARS).

Qhds	Holocene dune sand. This unit includes active dunes along with recently 
	stabilized dunes in coastal environments.  Dune sand typically is very well 
	sorted fine to medium sand.  This unit commonly occurs near beaches, where 
	Holocene age for much of the deposit is likely.  Large latest Pleistocene 
	dune fields like the Antioch-Oakley dunes, the Merritt Sand, and most of 
	the dunes covering the northern San Francisco Peninsula, which are mapped 
	as latest Pleistocene to Holocene dune sand (Qds), likely contain areas of 
	unmapped Holocene dune sand.  Typical soils developed on this unit (Qhds) 
	are inceptisols.

	Liquefaction susceptibility is moderate because coastal dune environments 
	typically have deeper ground-water levels. However, in areas of high ground 
	water or perched water conditions, such as dunes near water bodies or low-lying 
	areas between dune crests, liquefaction susceptibility may be high or very high.  

Qhbm	Holocene San Francisco Bay mud.  Sediment deposited at or near sea level in the 
	San Francisco Bay estuary that is presently, or was historically tidal marsh, 
	mud flat or bay bottom.  Bay mud sediment typically has low bulk density and 
	includes silt, clay, peat, and fine sand (Atwater and others, 1977).  This unit 
	is time-transgressive and generally occupies the area between the modern 
	shoreline and the historical limits of tidal marsh, as shown on the compilations 
	by Nichols and Wright (1971), San Francisco Estuary Institute (1998), and Sowers 
	(1995, 1997, 1999) of historical surveys of tidal marshlands circa 1850.  We 
	include areas that are presently, or were recently, used as salt evaporation 
	ponds within this unit.  Also included within this map unit are small areas of 
	artificial fill and Holocene alluvial deposits too small to be mapped at the map 
	scale used in this project.  Especially relevant to the evaluation of 
	liquefaction susceptibility are the many small marsh channels that are too 
	small to map, yet likely contain sandy substrates and may be more susceptible 
	to liquefaction than the silt, clay and peat of the marsh deposits.  Bay mud 
	deposits near the mouths of larger streams likely contain more sand and silt 
	than the deposits that are distant from stream and river mouths. Soils developed 
	on estuarine deposits typically are histosols, aquic entisols or mollisols.  
	Bay mud is mainly late Holocene in age with many areas presently subject to 
	deposition and flooding.  Some areas have been diked for farming, salt 
	evaporators, or other purposes.  Bay mud deposits thin landward and may be as 
	thick as 40 m along the bay margin (Rogers and Figuers, 1991).  This unit is 
	texturally and genetically similar to Holocene San Joaquin/ Sacramento Delta mud 
	and peat, which Knudsen and others, (2000a) mapped upstream of the confluence 
	between the Sacramento and San Joaquin rivers.  

	Although Bay mud primarily consists of clay and silt, we assign a moderate 
	liquefaction susceptibility to this unit due to high ground-water levels (often 
	tidally influenced) and the possible presence of sand lenses within the mud and 
	peat.  The mud itself is unlikely to liquefy due to the abundance of clay.  
	Estuarine sediment near the mouths of major streams, such as Alameda Creek, 
	is probably the most susceptible to liquefaction because the streams regularly 
	deliver large volumes of sand and silt to the estuary.  About 3% of all 
	observed occurrences of liquefaction in the San Francisco Bay area have occurred 
	within this unit.  

Qhed	Holocene estuarine delta deposits.  Estuarine sediment deposited in a delta at 
	the mouths of tidally influenced coastal streams where fresh water mixes with 
	seawater. Estuarine sediment primarily consists of silt and clay deposited by 
	marine and fluvial processes with interbedded organic-rich layers composed of 
	peat or woody debris. However, in larger streams and rivers along the coast 
	where fluvial currents dominate tidal processes, deltaic sediments deposited 
	in brackish water may consist of well sorted sand and/or gravel, forming channel 
	bars and stream banks. Estuarine delta deposits mapped along the Pacific coast 
	typically consist of a heterogeneous mixture of coarse and fine material 
	generally coarser than estuarine deposits of the San Francisco Bay (Qhbm). 
	Holocene estuarine delta deposits also are mapped near the mouths of streams 
	along the northern margin of San Pablo Bay where small fluvial deltas have 
	prograded out over Holocene Bay mud deposits (Qhbm). Examples include units 
	mapped at the mouths of San Antonio Creek (Petaluma River quadrangle), and 
	Sonoma Creek (Sears Point quadrangle).

	Liquefaction susceptibility is high. The judgment that estuarine delta deposits 
	(Qhed) are relatively more susceptible to liquefaction than San Francisco Bay 
	mud (Qhbm, assigned moderate susceptibility) is based on the observation that 
	fluvially dominated coastal streams deposit large volumes of sand. The 
	combination of high ground water levels near the mouths of streams, the likely 
	presence of loose saturated sand, and the possible occurrence of free faces 
	along channel margins increase the relative susceptibility of these deposits 
	to liquefaction.

Qhb	Holocene basin deposits.  Sediment that accumulates from standing or slowly 
	moving water in topographic basins.  Basin deposits consist of fine-grained 
	alluvium with horizontal stratification.  These deposits can be interbedded 
	with lobes of coarser alluvium from streams that flow into the basin.  Interbeds 
	of peat also may be present. Identification of basin deposits is based on 
	surface morphology, topographic position, and soil type.  This unit is similar 
	to flood basin deposits (Qhfb) of the Sacramento/San Joaquin Delta area 
	(Atwater, 1982), and is similar in texture to Holocene alluvial fan, fine 
	facies (Qhff) deposits.  Ground water is high, often at the surface, especially 
	during the rainy season.  Many basins contain, or historically contained, 
	seasonal wetlands, for example, Lake Elizabeth in Fremont and Tulare Lake in 
	the Pleasanton area.  Typical soil series developed on basin deposits include 
	Alamitos, Sunnyvale, Willows, Sycamore, and Clear Lake.  These soils are clay 
	rich with mottled subsoils, and may be somewhat saline or calcareous. 

	Liquefaction susceptibility is moderate.  Although these deposits contain 
	abundant clay, they also may contain layers of sand and silt.  In a fluvial 
	environment, we expect the distribution of sand to be irregular and discontinuous.  
	Thus, we assume that layers of liquefiable material may be present within basins.  
	Ground water at or near the ground surface makes liquefaction of surficial basin 
	deposits possible.

Qhfe	Holocene alluvial fan-estuarine complex deposits.  Deposits that form in the 
	transition zone from distal fan and basin environments to the estuarine environment.  
	This unit is mapped along the southern San Francisco Bay margin between the 
	Guadalupe River and Coyote Creek within the Milpitas 7.5-minute quadrangle.  
	The deposits represent a transition zone from fluvial sand, silt, and clay 
	(Qhf, Qhfy, Qhl, Qhly, Qhff) to Bay mud (Qhbm).  Coarser fluvial sediment, some 
	of which may be historical, typically forms a veneer over the finer sediment (Qhff) 
	and may overlie or interfinger with Bay mud.  Bay mud in this area is distinguished 
	from fine-grained alluvial fan sediment (Qhff) by its compressibility, high water 
	content, and peat content (Sarna, 1967).

	Discontinuous sloughs oriented perpendicular to the bay margin are typical of this 
	zone and are interpreted to be segments of abandoned creek channels whose upper 
	reaches are filled by recent fluvial sediment.  The lower reaches are maintained 
	by a combination of ground-water seepage and tidal influx.  This unit includes 
	a tongue of Bay mud mapped by Sarna (1967) that extends up the Guadalupe River.  
	Borings in this area indicate the presence of both Bay mud and very young fluvial 
	sediment.  We do not, however, include within this map unit a tongue of Bay mud 
	mapped by Sarna (1967) along Coyote Creek.  Borings in this area are concentrated 
	along the creek channel where tidal influx within the channel itself may be 
	responsible for the deposition of Bay mud.  The area on either side of the Coyote 
	Creek channel is mapped as a natural levee (Qhly), based on the shape of the ten-
	foot contour line and the presence of Mocho loam, an alluvial soil (Gardner and 
	others, 1958).

	Soils within this transition zone are strongly to moderately saline or alkaline; 
	the native vegetation historically included salt grass and pickleweed (Goals 
	Project, 1999).  Soil series associated with this zone include the Alviso clay, 
	tidal marsh, and the Mocho loam and fine clay loam over basin clays (Gardener and 
	others, 1958).  Ground water is tidally influenced owing to a direct hydraulic 
	connection to deposits in stream channels and sloughs.

	Liquefaction susceptibility is high based on the presence of shallow ground water, 
	historical damage owing to liquefaction, and lenses of unconsolidated, very young, 
	sandy, alluvial material.  

Qhf	Holocene alluvial fan deposits.  Sediment deposited by streams emanating from 
	mountain canyons onto alluvial valley floors or alluvial plains, including debris 
	flow, hyperconcentrated mudflow, and braided stream deposits.  Alluvial fan sediment 
	includes sand, gravel, silt, and clay, and is moderately to poorly sorted, and 
	moderately to poorly bedded.  Sediment clast size and general particle size 
	typically decreases down slope from the fan apex.  Many Holocene alluvial fans 
	exhibit levee/interlevee topography, particularly the fans associated with creeks 
	flowing west from the East Bay hills [see Holocene alluvial fan levee deposits 
	(Qhl) below].  Alluvial fan surfaces are steepest near their apex at the valley 
	mouth, and slope gently basin ward, typically with gradually decreasing gradient.  
	Alluvial fan deposits are identified primarily on the basis of fan morphology and 
	topographic expression.  Holocene alluvial fans are relatively undissected when 
	compared to older alluvial fans.  In places, Holocene deposits may be only a thin 
	veneer over Pleistocene deposits.  Soils are typically entisols, inceptisols, 
	mollisols, and vertisols.  About 9% of the central San Francisco Bay area is 
	covered by Holocene alluvial fan deposits; it is the most extensive Quaternary 
	map unit in the region. In some areas of the map, the relative ages of younger 
	(Qhf1), intermediate (Qhf2), and older (Qhf3) alluvial fan deposits are 
	distinguished from one another.

	Liquefaction susceptibility is moderate where ground water is within fifteen feet 
	of the surface.  Deposits may be less susceptible where ground-water levels are 
	considerably lower such as near fan apices and near the range front along the East 
	Bay Hills. Where an active channel is present but is not mapped through the fan 
	because of the map scale, the liquefaction susceptibility may be underestimated.  

Qhff  	Holocene alluvial fan deposits, fine facies.  Fine-grained alluvial fan and flood 
	plain overbank deposits laid down in very gently sloping portions of the alluvial 
	fan or valley floor.  Slopes in these distal alluvial fan areas are generally less 
	than or equal to 0.5 degrees, soils are clay rich, and ground water is within 3 
	meters of the surface.  Deposits are dominated by clay and silt, with interbedded 
	lobes of coarser alluvium (sand and occasional gravel).  Deposits of coarse 
	material within these fine-grained materials are elongated in the down fan or down 
	valley direction.  These lobes are potential conduits for ground water flow.  
	The surface contact with relatively coarser facies, fan (Qhf) and levee (Qhl), 
	is both gradational and interfingering, thus is dashed.  These deposits are similar 
	to basin deposits mapped by Helley and Graymer (1997a, 1997b) and Helley and 
	others (1994).  Typical soil series developed on this unit include Sunnyvale, 
	Orestimba, Clear Lake, Pescadero, Pacheco and Willows.  These soils are clay rich 
	with mottled or calcareous subsoils. 

	Liquefaction susceptibility is moderate based on shallow ground water and the 
	presence of lenses of fine sand and silt.  

Qhl	Holocene alluvial fan levee deposits.  Natural levee deposits of alluvial fans are 
	formed by streams that overtop their banks and deposit sediment adjacent to the 
	channel.  Mapping of these deposits is based on interpretation of topography; 
	levees are identified as long, low ridges oriented down fan ['channel ridges' of 
	Thomasson and others (1960)].  They contain coarser material than adjoining 
	interlevee areas, especially adjacent to creek banks where the coarsest material 
	is deposited during floods (Helley and others, 1979).  Levee deposits are loose, 
	moderately to well sorted sand, silt and clay (Helley and Wesling, 1989).  Soils 
	are typically entisols, inceptisols, mollisols, and vertisols. In some parts of 
	the map, the relative ages of younger (Qhl1), intermediate (Qhl2), and older 
	(Qhl3) alluvial fan levee deposits are differentiated from one another. 

	Liquefaction susceptibility is moderate because of the presence of unconsolidated, 
	sandy materials adjacent to an active or formerly active stream channel. However, 
	the topographic elevation of natural levees above adjacent stream channels generally 
	is associated with relatively deeper (15 to 20 ft) ground-water levels.  Where 
	streams are incised and form a free face along the channel margin, these deposits 
	may be susceptible to lateral spreading.  

Qht	Holocene stream terrace deposits.  Stream terrace deposits that were deposited in 
	point bar and overbank settings.  Terrace deposits include sand, gravel, silt, and 
	minor clay, and are moderately to well-sorted, and moderately to well bedded.  
	This unit is mapped where relatively smooth, undissected terraces are less than 
	25 to thirty feet above the active channel.  Soils are typically entisols, 
	inceptisols, and mollisols. Terrace deposits that are too small in extent to be 
	shown at the map scale, such as those along small creeks, are included within 
	the undifferentiated alluvial deposits (Qha and Qa) mapping units. Where multiple 
	stream terraces are identified, the younger deposits (Qht1) are distinguished 
	from older deposits (Qht2).

	Liquefaction susceptibility is moderate because of the presence of loose, granular 
	deposits and shallow ground water within fifteen feet of the ground surface.  
	Should liquefaction occur, the presence of a free face and laterally extensive 
	point bar deposits makes lateral spreading likely.  Overbank deposits, which 
	typically overlie point bar deposits, probably are not as susceptible to 
	liquefaction, or to lateral spreading should liquefaction occur, as point bar deposits.

Qha	Holocene alluvial deposits, undifferentiated.  Alluvium deposited in fan, terrace, 
	or basin environments.  This unit is mapped where separate types of alluvial 
	deposits could not be delineated either due to complex interfingering of depositional 
	environments or the small size of the area.  Typically, undifferentiated alluvial 
	deposits are mapped in relatively flat, smooth valley bottoms along small- to 
	medium-sized streams.  The planar and smooth geomorphic surfaces, with little to 
	no dissection, indicate that there has been little post-stabilization 
	modification/dissection of the surface; thus, deposits with smooth surfaces are 
	interpreted to be Holocene in age.  Undifferentiated Holocene alluvial deposits 
	probably are intercalated sand, silt, and gravel that are poorly to moderately 
	sorted.  Soils are entisols, inceptisols, vertisols, and mollisols.  

	Liquefaction susceptibility is moderate, based on: (1) the presence of 
	undifferentiated late Holocene channels and deposits; (2) relatively high ground-
	water levels; and (3) similarities in material properties and liquefaction 
	susceptibility for fan, terrace, and basin deposits (Qhf, Qht, and Qhb).  


HOLOCENE TO LATEST PLEISTOCENE (<30,000 YEARS).

Qds	Latest Pleistocene to Holocene dune sand.  Very well sorted fine to medium grained 
	eolian sand (<30,000 years).  Holocene sand may discontinuously overlie latest 
	Pleistocene sand, both of which may form a mantle of varying thickness over older 
	materials.  Most of these deposits are thought to be associated with latest 
	Pleistocene to early Holocene low sea level stands and subsequent transgression, 
	during which large volumes of fluvial and glacially derived sediment from the 
	Sierra Nevada via the Sacramento and San Joaquin Rivers were blown into dunes 
	(Atwater and others, 1977).  The deposits include the Merritt Sand in the Oakland 
	area and the sand dunes that cover much of the northern San Francisco Peninsula.  
	These dunes consist of fine to medium sand that is semiconsolidated and weakly 
	cemented.  

	Liquefaction susceptibility is generally low, but may be high locally where ground 
	water is shallow and sand is Holocene in age; therefore, we have assigned these 
	deposits moderate liquefaction susceptibility. However, there were no reports of 
	liquefaction within the Merritt Sand for either the 1906 or 1989 earthquakes 
	(Youd and Hoose, 1978; Seed and others, 1990).  Geotechnical boring information 
	collected by CGS and Holzer and others (2002) indicate that some parts of the 
	Merritt Sand and the dunes on the San Francisco Peninsula may liquefy during a 
	large earthquake.

Qb	Latest Pleistocene to Holocene basin deposits.  Sediment deposited in topographic 
	lows, such as a closed or semi-enclosed basin.  These areas have a high ground-water 
	table and soils characterized as poorly drained.  Deposits are generally clay rich.  
	This unit is mapped in the axis of Kenwood valley where the presence of both 
	latest Pleistocene and Holocene deposits is suggested by a range in soil development 
	that includes vertisols (relatively youthful clay rich soils) and durixeralfs 
	(mature soils having a B horizon and hardpan).

	Liquefaction susceptibility is low because of the fine-grained nature of these 
	deposits and relative older age.

Qf	Latest Pleistocene to Holocene alluvial fan deposits.  This unit is mapped on 
	gently sloping, fan-shaped, relatively undissected alluvial surfaces where the age 
	of deposits is not known (either latest Pleistocene or Holocene in age) or where 
	the deposits consist of thin patches of Holocene sediment overlying latest 
	Pleistocene alluvial fan sediment.  Fan sediment includes sand, gravel, silt, 
	and clay, and is moderately to poorly sorted, and moderately to poorly bedded.  
	Soils formed on these deposits are typically mollisols, and alfisols.  This 
	unit includes active stream channels that are too narrow to show at the map 
	scale used in this project.  

	Liquefaction susceptibility is moderate too low.  Ground water is assumed to be 
	greater than fifteen feet below the surface.  However, because of the presence 
	of sediment in stream channels not differentiated within this unit, historical 
	observations of liquefaction within the map unit and geotechnical analyses of 
	boring data, we assign a moderate susceptibility to the unit (Qf). 

Qt	Latest Pleistocene to Holocene stream terrace deposits.  This unit is mapped on 
	relatively flat, undissected stream terraces where deposit age is uncertain.  
	Terrace deposits include sand, gravel, and silt, with minor clay, and are 
	moderately to well sorted, and moderately to well bedded.  Soils are typically 
	inceptisols, mollisols, and alfisols.  Ground-water depth is variable, but is 
	generally less than thirty feet.  This unit may include active stream channels, 
	consisting dominantly of gravel and sand, that are too narrow to show at the map 
	scale of this project. Where multiple stream terraces are identified, the 
	younger deposits (Qt1) are distinguished from older deposits (Qt2).

	Liquefaction susceptibility is moderate, where ground water is within twenty feet 
	of the surface.  The moderate susceptibility reflects the range or uncertainty in 
	age of the terrace deposits. Liquefaction susceptibility in undifferentiated 
	channels within these deposits, especially in point bar deposits, may be higher.

Qa	Latest Pleistocene to Holocene alluvial deposits, undifferentiated.  This unit is 
	mapped in small valleys where separate fan, basin, and terrace units could not be 
	delineated at the scale of this mapping, and where deposits might be of either 
	latest Pleistocene or Holocene age.  The unit includes flat, relatively 
	undissected fan, terrace, and basin deposits, and small active stream channels.  
	Soils formed on these deposits are mollisols, alfisols and vertisols.

	Liquefaction susceptibility is moderate.  Ground-water depth is variable, but is 
	generally less than twenty feet.  The moderate susceptibility assignment is a 
	reflection of uncertainties and local variability in both the nature and age of 
	these deposits.


LATEST PLEISTOCENE  (11,800 to 30,000 YEARS).

Qpf	Latest Pleistocene alluvial fan deposits.  This unit is mapped on alluvial fans 
	where latest Pleistocene age is indicated by greater dissection than is present on 
	Holocene fans, and/or the development of alfisols.  Latest Pleistocene alluvial 
	fan sediment was deposited by streams emanating from mountain canyons onto alluvial 
	valley floors or alluvial plains and includes debris flow, hyperconcentrated mudflow, 
	and braided stream deposits.  Alluvial fan sediment typically includes sand, gravel, 
	silt, and clay, and is moderately to poorly sorted, and moderately to poorly bedded.  
	Sediment clast size and general particle size decreases down slope from the fan apex.  
	Latest Pleistocene alluvial fan sediment is approximately 10% denser than Holocene 
	alluvial fan sediment and has penetration resistance values about 50% greater than 
	values for Holocene alluvial fan sediment (Clahan and others, 2000a, 2000b, 2000c, 
	2000d).  Latest Pleistocene alluvial fans may be overlain by thin unmapped Holocene 
	alluvial fan deposits. Where multiple alluvial fans are identified, the younger 
	deposits (Qpf1) are distinguished from older deposits (Qpf2). Along the west-facing 
	hills of Oakland and Berkeley, where latest Pleistocene alluvial fan deposits are 
	mapped, the age of these deposits is not well constrained and the deposits may 
	actually be a combination of early to middle Pleistocene alluvial fan and thin 
	pediment deposits, and latest Pleistocene alluvial fan deposits.  

	Liquefaction susceptibility is low.  Ground-water levels are variable, but generally 
	are more than twenty feet below the surface.  Deposits typically are very stiff to 
	hard or medium dense to very dense (Haydon and others, 1999; Clahan and others, 2000a, 
	2000b, 2000c, 2000d).  

Qpt	Latest Pleistocene stream terrace deposits.  This unit is mapped on relatively flat, 
	slightly dissected stream terraces where latest Pleistocene age is indicated by 
	the development of alfisols and height of the terrace above flood level.  Terrace 
	sediment includes sand, gravel, silt, with minor clay, and is moderately to well 
	sorted, and moderately to well bedded.  Terrace sediment typically was deposited 
	in point bar and overbank settings and has since been elevated above the creek 
	bottom by incision of the streambed.  Latest Pleistocene terrace deposits that 
	are too small in extent to be shown at the map scale, such as those along small 
	creeks, may be included within the undifferentiated latest Pleistocene and latest 
	Pleistocene to Holocene alluvial mapping units (Qpa and Qa).
	


	Liquefaction susceptibility is low because of the age of the deposits and the 

	likelihood that ground water is relatively deep.  Should liquefaction occur, the 

	presence of a free face makes lateral spreading likely.  

Qpa	Latest Pleistocene alluvial deposits, undifferentiated.  This unit is mapped on 
	gently sloping to level alluvial fan or terrace surfaces where latest Pleistocene 
	age is indicated by depth of stream incision, development of alfisols, and lack 
	of historical flooding.  Undifferentiated latest Pleistocene alluvial deposits 
	are mapped in small valleys where separate fan, basin, and terrace units could 
	not be delineated at the mapping scale of this project.  These undifferentiated 
	latest Pleistocene alluvial deposits probably are intercalated sand, silt, and 
	gravel that are poorly to moderately sorted.  

	Liquefaction susceptibility is low because of the age of the deposits and the 
	probability that ground water is relatively deep (10 to 30 feet).  


PLEISTOCENE (>11,800 to 1.8 MYRS).

Qmt	Pleistocene marine terrace deposits.  Deposits on uplifted marine abrasion 
	platforms along the Pacific Ocean coast.  Although, we have not evaluated the 
	relative ages of all terrace deposits (e.g. late versus middle or early Pleistocene), 
	for much of the western San Francisco Peninsula, we have assigned relative ages 
	by numbering the deposits 1 through 4.  However, the numbers identifying different 
	aged terraces have not been applied consistently to all quadrangles. Sediment 
	deposited on the strath platforms is typically greater than ten-feet thick and 
	consists of moderately to well sorted, moderately to well bedded sand and gravel, 
	which may be locally fossiliferous.  We compiled and modified mapping of marine 
	terrace deposits by Weber and others (1993), Jack (1969), and Lajoie and others 
	(1974, 1979) in the southwestern part of the San Francisco Peninsula south to 
	Santa Cruz County.  Unpublished mapping by K. Lajoie also was used in mapping 
	marine terraces along the Marin County coast. In some parts of the map, four 
	marine terraces are distinguished as youngest (Qmt1), younger (Qmt2), older (Qmt3), 
	and oldest (Qmt4) deposits.

	Liquefaction susceptibility is low.  Ground water is typically deeper than twenty 
	feet, though areas may have perched ground water where marine sediment overlies 
	relatively impermeable bedrock.  Marine terrace sediment is typically too dense 
	to liquefy.  

Qbt	Pleistocene bay terrace deposits.  Estuarine and deltaic sediment on wave-cut 
	platforms above present sea level along San Pablo Bay, the Carquinez Straight, 
	and wave-beveled strath terraces near Lake Merritt, Oakland.  Uranium-series 
	dates on articulated, in-place oyster shells (Ostrea lurida) from terrace deposits 
	around San Pablo Bay provide an age estimate from 112 to 142 ka correlative with 
	the  oxygen-isotope stage 5e sea-level highstand (Atwater and others, 1981; 
	Helley and others, 1993).  Vertebrate fossils preserved in terrace deposits at 
	Rodeo on the south side of San Pablo Bay indicate a prograding deltaic environment 
	entering a marine embayment (Wolff, 1971).  Marine and estuarine fossil species 
	in these deposits include sea otters (Enhydra lutris), oysters (Ostrea lurida), 
	and the salt marsh harvest mouse, (Reithrodontomys raviventris) (Wolff, 1971).  
	Terrace sediment up to 5.9-m-thick includes trough-cross bedded sand and rounded 
	gravel deposited in a bay delta, and sand and silty clay with 35 to 100-cm-thick 
	shell beds deposited in an estuarine environment (Borchardt, 1994).  Terrace 
	surfaces are planar to rounded and moderately to deeply dissected.

	Liquefaction susceptibility is low because of the age and density of the deposits.  
	Shallow ground water may be present where perched above less permeable bedrock 
	abrasion platforms.


EARLY TO LATE PLEISTOCENE (>30,000 to 1.8 MYRS).

Qop	Early to late Pleistocene pediment deposits.  Alluvial deposits that form a thin 
	veneer on broad, planar erosional surfaces cut on older sediment or bedrock.  
	These pediments typically occur tens to hundred(s) of meters above the present 
	stream channel and are extremely dissected.  Bedrock and/or older sediment are 
	exposed by dissecting channels at depths less than 5 meters beneath the alluvium, 
	and, in places, only sparse sediment may remain from the original deposits.  
	These deposits are mapped primarily based on their geomorphic expression as 
	interpreted from topographic maps.  These deposits are mapped on the west side 
	of the East Bay hills from Oakland to Richmond and the western end of the 
	Potrero Hills near Fairfield. Soils formed on these deposits typically are well 
	developed, and include alfisols and ultisols.

	Liquefaction susceptibility is very low because of the age of the deposits and 
	their density.  Because these deposits are typically at least tens of feet above 
	present stream channels, ground water commonly is not present in these deposits.

Qof 	Early to late Pleistocene alluvial fan deposits.  Moderately to deeply dissected 
	alluvial deposits capped by alfisols, ultisols, or soils containing a silicic or 
	calcic duripan.  Early to middle Pleistocene alluvial fan sediment was deposited 
	by streams emanating from mountain canyons onto alluvial valley floors or alluvial 
	plains and includes debris flow, hyperconcentrated mudflow, and braided stream 
	deposits.  Because of the age of these deposits, the streams responsible for 
	deposition of mapped bodies of early to middle Pleistocene alluvial fan sediment 
	may have evolved and no longer be readily evident in todays topography.  Alluvial 
	fan sediment typically includes sand, gravel, silt, and clay, and is moderately 
	to poorly sorted, and moderately to poorly bedded.  This unit differs from 
	undifferentiated early to middle Pleistocene alluvial deposits (Qoa) in that 
	some original fan surface morphology is preserved. 
	

	
Liquefaction susceptibility is very low because of the age and density of the 
	
sediment as well as large depths to ground water.

Qot	Early to late Pleistocene stream terrace deposits.  Moderately to deeply dissected 
	alluvial terrace deposits capped by alfisols, ultisols, or soils containing a 
	silicic or calcic hardpan.  Terrace sediment includes sand, gravel, and silt, 
	with minor clay, and is moderately to well sorted, and moderately to well bedded.  
	Terrace sediment typically was deposited in point bar and overbank settings and 
	has since been elevated above the creek bottom by incision of the streambed.  
	This unit differs from Qoa in that some terrace surface morphology is preserved.  



	Liquefaction susceptibility is very low because of the age and density of 

	the sediment.

Qoa	Early to late Pleistocene alluvial deposits, undifferentiated.  Moderately to 
	deeply dissected alluvial deposits capped by alfisols, ultisols, or soils 
	containing a silicic or calcic hardpan.  Topography often consists of gently 
	rolling hills with little or none of the original planar alluvial surface 
	preserved.  Deposits mapped within this map unit can include alluvial fan, 
	stream terrace, basin and channel deposits.  This unit includes the Colma 
	Formation on the San Francisco Peninsula (Bonilla, 1971), which has been 
	described as a marine, estuarine and fluvial, unconsolidated fine to medium 
	sand with silt and clay.  It also includes Plio-Pleistocene deposits shed off 
	the flanks of Mt Diablo that have been previously mapped by Helley and Graymer 
	(1997a, 1997b).  

	Liquefaction susceptibility is very low because of the age and density of the sediment.

br	Early Quaternary and older (>1.4 Ma) deposits and bedrock, undifferentiated.  
	Primarily Jurassic to Pliocene sedimentary, metamorphic, volcanic and plutonic rocks, 
	and poorly consolidated Tertiary sediment.  Includes some Pliocene to Pleistocene 
	sedimentary units such as the Glen Ellen Formation, Santa Clara Formation, Livermore 
	gravels, and Merced Formations. Unit also includes landslides, talus, other bodies 
	of colluvium, and small stream channel deposits in bedrock that could not be 
	delineated at the map scales used in this project.
	

	Liquefaction susceptibility is very low. Stream channels within areas mapped as 

	bedrock may contain small areas of Holocene deposits; susceptibility of these 

	isolated deposits may be low to very high.