MAPS DEPICTING QUATERNARY SURFICIAL DEPOSITS AND LIQUEFACTION SUSCEPTIBILITY (1:24,000-SCALE) IN THE SAN FRANCISCO BAY AREA

Observations of liquefaction-related ground failures caused by historical large-magnitude earthquakes in the San Francisco Bay (SF Bay) area demonstrate that the distribution of damaging effects is not random. Detailed hazard maps indicate that liquefaction occurs in areas underlain by saturated, unconsolidated sand, silt and uncompacted artificial fill. We use original 1:24,000-scale Quaternary geologic mapping, which distinguishes surficial materials by age and depositional environment, to produce a new liquefaction susceptibility map that covers areas of densest urban growth in the SF Bay area. Both maps encompass 68 quadrangles and improve upon existing regional mapping. Multiple active faults in the region are capable of producing large earthquakes that expose the entire study area to long-duration ground motions (PGAs >0.2g) sufficient to trigger liquefaction in highly susceptible materials. Development of the susceptibility map uses a criteria matrix to assign one of five liquefaction susceptibility categories to each geologic unit. The five categories range from very low to very high and reflect the relative likelihood that saturated, loose, granular materials occur within 40 ft of the surface. Criteria used to assign categories include, documented liquefaction-related effects, ground-water levels, and geotechnical analyses of regional boring data. The spatial distribution of historical liquefaction shows that the highest percentage of effects (35%) occurred in artificial fill over Bay mud. The observations also suggest future liquefaction occurrence of >76% in very high to high zones; ~20% in moderate zones; and <4% in low to very low zones. The map is not intended for site-specific studies and does not predict the type, magnitude or location of ground failure. However, ground failures may accompany liquefaction and are more likely to occur in higher susceptibility zones. Both maps will be published as a digital database to assist hazard mitigation efforts and reduce earthquake losses. For example, the maps provide baseline data for State mandated Seismic Hazard Zones of Required Investigation. The Quaternary geologic map will be integrated into the unified SF Bay region geologic map and provide age context for the northern California Quaternary fault database, both in progress by the USGS.