HIGH-RESOLUTION SEISMIC-REFLECTION PROFILING OF THE SANTA-MONICA FAULT ZONE, WEST LOS-ANGELES, CALIFORNIA

High-resolution seismic reflection data obtained across the Santa Moni ca fault in west Los Angeles reveal the near-surface geometry of this active, oblique-reverse-left-lateral fault. Although near-surface faul t dips as great as 55 degrees cannot be ruled out, we interpret the fa ult to dip northward at 30 degrees to 35 degrees in the upper few hund red meters, steepening to greater than or equal to 65 degrees at 1 to 2 km depth. A total of similar to 180 m of near-field thrust separatio n (fault slip plus drag folding) has occurred on the fault since the d evelopment of a prominent erosional surface atop similar to 1.2 Ma str ata. In the upper 20 to 40 m strain is partitioned between the north-d ipping main thrust strand and several closely spaced, near-vertical st rike-slip faults observed in paleoseismologic trenches. The main thrus t strand can be traced to within 20 m of the ground surface, suggestin g that it breaks through to the surface in large earthquakes. Uplift o f a similar to 50,000-year-old alluvial fan surface indicates a short- term, dip-slip rate of similar to 0.5 mm/yr, similar to the similar to 0.6 mm/yr dip-slip rate derived from vertical separation of the oxyge n isotope stage 5e marine terrace 3 km west of the study site. If the 0.6 mm/yr minimum, dip-slip-only rate characterizes the entire history of the fault, then the currently active strand of the Santa Monica fa ult probably began moving within the past similar to 300,000 years.