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

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.