SLAM -- A METHOD FOR FINDING SEISMOGENIC FAULTS, AND ITS APPLICATION TO THE CENTRAL SANTA MONICA MOUNTAINS AND NORTHEASTERN SANTA MONICA BAY, CALIFORNIA

This work is an extension of studies initiated by J. E. Slosson, and reflects his efforts to identify faults capable of generating earthquakes in California. In the Seismo-Lineament Analysis Method (SLAM), we use the reported location of an earthquake and the two potential fault-plane orientations derived from its focal-mechanism solution, along with the corresponding reported errors, to define an uncertainty volume for each fault-plane solution. The intersection of this uncertainty volume with the ground surface, represented by a digital elevation model (DEM), is called a “seismo-lineament.” A geomorphic analysis to identify features that may be related to faulting is conducted within each seismo-lineament by artificially illuminating the DEM from several directions that are nearly perpendicular to the strike of the corresponding fault-plane solution. Hypotheses are generated about possible locations of faults within the two seismo-lineaments associated with each focal mechanism solution, and fieldwork is undertaken to evaluate those hypotheses. Brittle-cataclastic faults whose orientation and slip characteristics are statistically indistinguishable from the corresponding nodal plane, and that are located within the seismo-lineament, are described as potentially seismogenic. SLAM provides a means to associate observed faults with recorded earthquakes.

SLAM has been applied to earthquakes reported in the Point Dume, Malibu Beach and Topanga 7.5 minute quadrangles in the Santa Monica Mountains. Several faults that correlate with earthquakes have been identified, most of which were previously unmapped faults. Some seismo-lineaments extend offshore into an area of the northeastern Santa Monica Bay surveyed by Jim Slosson, Bob Dill and their associates in the mid-1990s. Earthquakes used in this study have small reported magnitudes, and may be distant aftershocks of significant prehistoric earthquakes that ruptured the ground surface.