COMBINING EARTHQUAKE FOCAL DATA AND DIGITAL MAP ANALYSIS IN RECONNAISSANCE FOR ACTIVE FAULTS, CENTRAL SANTA MONICA MOUNTAINS AND NORTHERN SANTA MONICA BAY, CALIFORNIA

We are developing techniques to combine digital elevation data with other geological/geophysical data types using GIS to facilitate reconnaissance mapping of structures that have affected surface topography and may be active. Preliminary lineament analysis of a DEM-based terrain model of the central Santa Monica Mountains of southern California suggested an active structural influence on surface topography. Based on this preliminary analysis, field work subsequently verified existence of previously unmapped faults along some of the lineaments defined in the terrain analysis.

To augment this continuing work, we have compiled a database of >900 earthquakes with reported epicenters beneath the central and western Santa Monica Mountains and northern Santa Monica Bay of southern California. Of these, >150 have reported focal mechanism solutions, each of which provide two admissible fault plane solutions. No recorded earthquake has yet been correlated with observed fault displacement at the ground surface along the Malibu coastline; however, it may be possible to associate some well-located earthquakes with faults exposed at the surface. To test this idea, we are projecting the nodal planes from focal mechanism solutions upward to map the trace of their intersection with the ground surface. These traces are then compared with published geologic maps and with the lineament map derived from the digital terrain analysis to help focus our future field research and to refine our assessment of faults that may be potentially active.