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Geomorphic evaluation of the Santa Monica Fault Zone, northwestern Los Angeles Basin, southern California

Brian Olson

Published 2015, SCEC Contribution #7326

The California Geological Survey (CGS) is evaluating the Santa Monica Fault Zone (SMFZ) for the Alquist-Priolo Earthquake Fault Zoning program. The SMFZ is expressed as a series of <i>en echelon</i> scarps in the Quaternary alluvial fan deposits emanating from the Santa Monica Mountains. It extends easterly from the coast ~12 km through urbanized areas of Santa Monica, Beverly Hills, and western Los Angeles. The SMFZ is generally north-dipping and exhibits left-lateral reverse oblique motion. Many investigators believe the primary fault is a low-angle blind thrust and the surface scarps are associated with sub-vertical hanging wall normal faults. Limited geologic studies reveal both active and inactive strands of the SMFZ.

Geologic studies and geomorphic features suggest the fault is located 3-4 km south of the mountain front. Scarp heights typically range from 7-12 m along the fault zone. An exception is the alluvial fan surface just east of Santa Monica Canyon. Here, the fan surface appears to have been broadly uplifted maintaining an overall slope of about 1.5° to the south. The scarps to the east project directly toward this surface but it is not clear how the fault proceeds to the west. Within this fan surface are ridges, broader fold scarps, and discontinuous slope breaks stepping <i>en echelon</i> to the south. At the northeastern edge of this uplifted fan surface is the nearly 25-m high anomalous Brentwood Knoll (BK) with a low continuous curvilinear scarp and ridgeline extending to the west towards Santa Monica Canyon. Some researchers believe this represents an anticlinal ridge formed over a shallow blind thrust. The westernmost expression of the SMFZ is two aligned 3-5 m high scarps in the Stage 5e marine terrace surface. A fault exposure at the mouth of Potrero Canyon, directly on trend with these scarps, places Pliocene bedrock against the Pleistocene terrace deposits. The transition to the Hollywood Fault in the north is problematic.

Key Words
geomorphology, Transverse Ranges,

Olson, B. (2015). Geomorphic evaluation of the Santa Monica Fault Zone, northwestern Los Angeles Basin, southern California. Poster Presentation at Seismological Society of America annual meeting, Pasadena, CA, April 21-23, 2015.