Preliminary maps of Quaternary deposits and liquefaction susceptibility, nine-county San Francisco Bay region, California: a digital database, USGS OFR 00-444

Map delineation of the different types and ages of Quaternary deposits
supports evaluation of susceptibility to liquefaction, the immediate
application of the work, but serves many other purposes as well. It provides a
framework for interpreting the architecture and history of the Quaternary
sedimentary basins, which is used in estimating earthquake shaking and modeling
the groundwater system. The mapping is also useful in constraining the ages and
histories of offsetting faults, in guiding geotechnical investigations, and in
other engineering, geologic, and archeological applications.

This report presents a preliminary map and database of Quaternary deposits and
liquefaction susceptibility for the nine-county San Francisco Bay region,
together with a digital compendium of ground effects associated with past
earthquakes in the region. The report consists of (1) a spatial database of
fivedata layers (Quaternary deposits, quadrangle index, and three ground
effects layers) and two text layers (a labels and leaders layer for Quaternary
deposits and for ground effects), (2) two small-scale colored maps (Quaternary
deposits and liquefaction susceptibility), (3) a text describing the Quaternary
map, liquefaction interpretation, and the ground effects compendium, and (4)
the databse description pamphlet.

The nine counties surrounding San Francisco Bay straddle the San Andreas fault
system, which exposes the region to serious earthquake hazard (Working Group on
California Earthquake Probabilities, 1999). Much of the land adjacent to the
Bay and the major rivers and streams is underlain by unconsolidated deposits
that are particularly vulnerable to earthquake shaking and liquefaction of
water-saturated granular sediment.

This new map provides a modern and regionally consistent treatment of
Quaternary surficial deposits that builds on the pioneering mapping of Helley
and Lajoie (Helley and others, 1979) and such intervening work as Atwater
(1982), Helley and others (1994), and Helley and Graymer (1997a and b). Like
these earlier studies, the current mapping uses geomorphic expression,
pedogenic soils, and inferred depositional environments to define and
distinguish the map units. In contrast to the twelve map units of Helley and
Lajoie, however, this new map uses a complex stratigraphy of some forty units,
which permits a more realistic portrayal of the Quaternary depositional system.
The two colored maps provide a regional summary of the new mapping at a scale
of 1:275,000, a scale that is sufficient to show the general distribution and
relationships of the map units but cannot distinguish the more detailed
elements that are present in the database.

The report is the product of years of cooperative work by the USGS National
Earthquake Hazards Reduction Program (NEHRP) and National Cooperative Geologic
Mapping Program, William Lettis and & Associates, Inc. (WLA) and, more
recently, by the California Division of Mines and Geology as well. An earlier
version was submitted to the Geological Survey by WLA as a final report for a
NEHRP grant (Knudsen and others, 2000). The mapping has been carried out by WLA
geologists under contract to the NEHRP Earthquake Program (Grants
#14-08-0001-G2129, 1434-94-G-2499, 1434-HQ-97-GR-03121, and 99-HQ-GR-0095) and
with other limited support from the County of Napa, and recently also by the
California Division of Mines and Geology. The current map consists of this new
mapping and revisions of previous USGS mapping.

This report has undergone two scientific peer reviews, one digital database
review, and review of the digital files for functionality.

This report presents a preliminary map and database of Quaternary deposits and
liquefaction susceptibility for the nine-county San Francisco Bay region,
together with a digital compendium of ground effects associated with past
earthquakes in the region. The report consists of (1) a spatial database of
five data layers (Quaternary deposits, quadrangle index, and three ground
effects layers) and two text layers (a labels and leaders layer for Quaternary
deposits and for ground effects), (2) two small-scale colored maps (Quaternary
deposits and liquefaction susceptibility), (3) a text describing the Quaternary
map, liquefaction interpretation, and the ground effects compendium, and (4)
the database description pamphlet. See the revision list (of00-444_2x) or
Open-File pamphlet (of00-444_1x) for further details.