ROSS, STEPHANIE L., HOLLY F. RYAN, and ANDREW J. STEVENSON
all at U. S. Geological Survey, 345 Middlefield Rd. MS-999, Menlo Park,
CA, 94025
Abstract: Sidescan Sonar and Seismic Reflection Data Over
the San Gregorio Fault
Between Pillar Point and Pescadero
Although the San Gregorio fault
is the major active
fault
zone
west of the San Andreas
fault
in central California, its seismogenic structure
is less understood than most faults in the area because it lies primarily
offshore. To study the near-surface structure of the
fault
, we collected
300 km of 100-kHz sidescan and high resolution seismic reflection data
near Half Moon Bay, California, over the San Gregorio
fault
zone. The lines
extended from just offshore to about 9 km to the west with a line spacing
of 500m.
Structural features are discontinuous along strike; this could indicate
multiple surficial fault
strands. Some seismic reflection lines reveal
fault
features such as offsets and discontinuities while others show a
broad zone of deformation. Over much of the area, the zone of deformation
separates a block of gently seaward-dipping reflectors to the west from
a block of more steeply landward-dipping reflectors closer to shore. The
northern and southern sections of the sidescan image display numerous rock
outcrops but the central section appears mostly sediment covered. Some
lines show pronounced topographic relief across the
fault
while others
show buried
fault
structures.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California