TEST OF A METHOD FOR RECOGNIZING PREVIOUSLY UNMAPPED SEISMOGENIC FAULTS

The central Santa Monica Mountains form part of the active southern boundary of the Transverse Ranges Province of southern California. Current models indicate that the only potentially active fault in the immediate area is the Malibu Coast Fault Zone (MCFZ), thought to be a north-dipping left-reverse oblique fault extending along the E-W trending coastline. Only a very small strand of the MCFZ is zoned active within the Point Dume 7.5' Quadrangle, where our study is focused.

We are currently using data from 8 earthquakes with epicenters located in the Pt. Dume quad, and for which focal mechanism solutions (FMS) have been reported. Each FMS includes the orientation of two mutually perpendicular nodal planes, each with an associated hanging-wall slip vector. The orientation of one of the nodal planes generally coincides with the orientation of the fault that generated the earthquake. The area within which the nodal plane probably intersects the ground surface, as represented by the digital elevation model (DEM) of the Pt. Dume quad, is determined in a manner that incorporates the horizontal and vertical errors reported for the location of the earthquake focus.

Field work was initiated after nodal planes for several earthquakes were projected onto the Pt. Dume DEM. Due to dense chaparral vegetation, high relief and some private property constraints, we concentrated on looking for faults along the many road cuts, fire breaks and trails that traverse the mountains. Field reconnaissance yielded several localities where previously unmapped faults were observed that may correlate with reported earthquakes.

It is of particular interest that the seismo-lineaments investigated in this study, and the previously-unmapped faults encountered in the field, do not coincide with the MCFZ. This indicates that the MCFZ is not the only active fault structure in the Santa Monica Mountains. It is also interesting to note that the newly mapped faults traverse highland areas where they cut pre-Holocene formations exposed at the ground surface. An active fault in an eroding upland area may cut only pre-Holocene material at the surface, illustrating the limits associated with the practice of defining active faults only on the basis of trench studies or observed surface rupture during earthquakes.