PROGRESS REPORT ON DEVELOPMENT OF METHODS FOR RECOGNITION OF SEISMOGENIC FAULTS

Our work is motivated by the assumption that there is value in recognizing faults that may produce damaging earthquakes. Active-fault recognition contributes to public safety through the development and application of appropriate zoning and building codes. Our primary field area has been the central Santa Monica Mountains of southern California, although the methods are applicable elsewhere.

The process includes (1) geomorphic analysis using all available generations of aerial/satellite imagery and digital elevation models (DEMs) to locate features that may have developed as a result of faulting; (2) seismo-lineament analysis in which the intersection of a DEM and the nodal plane from an earthquake focal mechanism solution (FMS) is mapped; and (3) field work to evaluate the hypotheses generated during the initial analyses. Marine sub-bottom profiling and multibeam data along the Malibu coastline are used to extend DEM coverage offshore. The geospatial data, including relevant published geologic maps, are compiled in a GIS format to facilitate analysis. High-resolution geodesy will be incorporated as appropriate in the future.

From public sources, we have compiled an earthquake catalog for the central Santa Monica Mountains that contains >1000 events, including >150 with reported FMS. Each FMS resolves two mutually perpendicular nodal planes, one of which coincides with the fault that generated the earthquake. The error region associated with each nodal plane generates a swath across the DEM that indicates the most probable area within which a fault corresponding to the reported earthquake nodal plane might be found. For events that lack FMS, error regions around reported epicenters are analyzed to evaluate possible association with hypothetical fault planes. Older epicenter data without error estimates are collected for context, but are not suitable for quantitative analysis.

Preliminary tests of these methods are encouraging. Previously unmapped faults in the Santa Monica Mountains that correlate with earthquake nodal planes have been recognized. These methods hold the promise of permitting recognition of previously unmapped seismogenic faults, including faults located in highland areas that do not cut datable Holocene material at the ground surface.