Marine geology and earthquake hazards of the San Pedro Shelf region, southern California

High-resolution seismic-reflection data have been com-
bined with a variety of other geophysical and geological data
to interpret the offshore structure and earthquake hazards of
the San Pedro Shelf, near Los Angeles, California. Prominent
structures investigated include the Wilmington Graben, the
Palos Verdes Fault Zone, various faults below the western part
of the shelf and slope, and the deep-water San Pedro Basin.
The structure of the Palos Verdes Fault Zone changes mark-
edly southeastward across the San Pedro Shelf and slope.
Under the northern part of the shelf, this fault zone includes
several strands, but the main strand dips west and is probably
an oblique-slip fault. Under the slope, this fault zone con-
sists of several fault strands having normal separation, most
of which dip moderately east. To the southeast near Lasuen
Knoll, the Palos Verdes Fault Zone locally is a low-angle fault
that dips east, but elsewhere near this knoll the fault appears to
dip steeply. Fresh sea-floor scarps near Lasuen Knoll indi-
cate recent fault movement. The observed regional structural
variation along the Palos Verdes Fault Zone is explained as the
result of changes in strike and fault geometry along a master
strike-slip fault at depth. The shallow summit and possible
wavecut terraces on Lasuen knoll indicate subaerial exposure
during the last sea-level lowstand. Modeling of aeromagnetic
data indicates the presence of a large magnetic body under
the western part of the San Pedro Shelf and upper slope. This
is interpreted to be a thick body of basalt of Miocene(?) age.
Reflective sedimentary rocks overlying the basalt are tightly
folded, whereas folds in sedimentary rocks east of the basalt
have longer wavelengths. This difference might mean that the
basalt was more competent during folding than the encasing
sedimentary rocks. West of the Palos Verdes Fault Zone, other
northwest-striking faults deform the outer shelf and slope.
Evidence for recent movement along these faults is equivocal,
because age dates on deformed or offset sediment are lacking.