Earthquakes, Geomorphic Analysis and Field Work Yield Previously Unmapped Faults and a New Understanding of Active Tectonics, Santa Monica Mountains, California

Seidman, Lauren E.¹, Mark A. Millard¹, Brian G. Bayliss¹, Vincent S. Cronin¹ (1) Baylor University, Waco, TX

We are developing new methods for identifying seismogenic faults, and testing these methods within the Santa Monica Mountains of southern California. We combined multibeam bathymetric data from Santa Monica Bay with digital elevation data from the Santa Monica Mountains, using a near-shore geophysical survey to bridge part of the 3 kilometer-wide gap between the two. The result is a unique composite digital elevation model of the area. In addition to traditional aerial-image interpretation, we illumined the composite digital elevation model from different directions to accentuate structural-geomorphic features.

Published models of this area correlate active faulting with a few left-oblique reverse faults located along the coastline or just offshore; however, reported earthquake locations and focal mechanism solutions do not support this interpretation. Using a new method to project fault-plane solutions from an earthquake focus to a digital elevation model, we defined a number of seismo-lineaments that were subsequently evaluated in the field. Some seismo-lineaments correlate with previously unmapped faults. One of these is a probable seismogenic strike-slip fault, a development that suggests the need for a new model for the active structure of the range. The presence of a seismogenic strike-slip fault may indicate strain partitioning among reverse and strike-slip faults similar to the structural context of the Santa Cruz Island fault zone, on-trend to the west. Improved understanding of the processes responsible for the active development of the Santa Monica Mountains and surrounding basins will aid future petroleum exploration in areas with similar structural history and style.