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