A preliminary map of liquefaction potential of unconsolidated
sediments has been made for the southern part of the San Francisco Bay
region. Map zones are established on the basis of recent detailed
geologic mapping. Liquefaction potential for each zone is estimated
from an analysis of lithologic, water-table and standard penetration
test data. Due to empirical aspects of the evaluation method,
approximations in establishing parameters and the generalizations of
limited test data to entire map zones, the liquefaction potential map
must be considered as~preliminary and approximate. Nevertheless, the
map should serve the intended purpose of generally delineating areas
where the probability for liquefaction during a major earthquake is
greatest and, hence, areas where special attention should be given
to the possibility of liquefaction.
Sediments with the highest potential for liquefaction are clean
granular layers within the younger bay sediments. These layers are not
everywhere present but commonly occur locally near present and former
stream channels. Clean granular beds and layers immediately beneath the
younger bay sediments generally have a lesser but significant potential
for liquefaction. Younger (Holocene) alluvial deposits have an even
lower potential, however, the potential can be locally high in recent
channel and overbank deposits. Part of the Holocene unit is normally
above water-table and thus would have only a seasonal potential for
liquefaction. Older (Pleistocene) alluvial fan deposits have a
generally low liquefaction potential. Lateral spreading landslides are
commonly associated with liquefaction beneath gentle slopes. Thus.
during a major earthquake. slides of this type would probably be the
most pervasive damaging effect of liquefaction on the broad alluvial
plain surrounding San Francisco Bay.
