ANALYSIS OF SEISMO-LINEAMENTS, DEMS AND FIELDWORK LEADS TO DIFFERENT IDEAS ABOUT ACTIVE DEFORMATION IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA

The Santa Monica Mountains (SMM) are a young, structurally active mountain range in southern California. Current published models for the SMM associate active faulting with only a few left-oblique reverse faults located directly adjacent to the coastline or just offshore. Reported earthquake locations and focal mechanism solutions do not support this limited view of the extent of active faulting.

The purpose of this study is to map probable seismogenic faults within the Malibu Beach 7.5' quadrangle. We compiled available earthquake data and projected fault-plane solutions onto a digital elevation model (DEM). The intersection of each fault-plane solution with the DEM yields a seismo-lineament whose width reflects the reported confidence interval associated with the earthquake location. Geomorphic lineaments were identified by illuminating the DEM at a 45 degree elevation from various azimuths. Digital geologic maps, 10- and 30-meter DEMS, geomorphic lineaments and seismo-lineament maps were compiled in a GIS database to facilitate field reconnaissance for faults. The lineament maps were used to generate hypotheses about the possible location of structures, which were evaluated in the field. Some previously mapped faults are expressed as geomorphic lineaments, and field work verified the existence of previously unmapped faults along other lineaments. Some faults are approximately coplanar with earthquake fault-plane solutions, and are tentatively considered seismogenic. Some faults appear to contribute to economically significant slope failures. Site means and the associated 95% confidence intervals were computed, using Fisher statistics, for the orientation of the bounding surfaces and slip striae of faults examined in the field. The mean fault orientations were projected back into the array of earthquake foci to identify possible correlations between surface faulting and reported earthquakes. Focal mechanism solutions and field observations suggest that strain is partitioned between several active faults, including some with pure strike-slip displacements.

These preliminary results suggest the need to modify current models of active deformation in the western Transverse Ranges, which are relevant to the assessment of seismic risk in the greater Los Angeles area.