POTENTIAL EARTHQUAKE HAZARDS BENEATH MT. TAMALPAIS:MODELING FAULT INTERACTIONS

The major earthquake hazards in the San Francisco Bay area are the strike-slip faults that litter the region. However, the complex geometries and slip budgets of these faults imply that there is the potential for linkages between them that may have been previously unrecognized. Specifically, blind thrust structures may connect segments of strike-slip faults, completing regional slip budgets. These faults not only help explain the variable topography observed but also may serve as additional earthquake sources within the region.

Analysis of uplift patterns in the Marin County – Mt. Tamalpais region and best estimates of slip on major strike-slip faults in the area precludes fault-normal shortening as the primary mechanism generating the observed elevations. We propose that the Marin County – Mt. Tamalpais region is instead underlain by one or more blind thrust structures. These faults would connect the northern Hayward Fault to the San Andreas near Point Reyes. We test our hypothesis against a number of different fault geometries that have the potential to produce the present-day elevations in the region, taking into account realistic estimates of erosion. In evaluating the earthquake potential on the proposed blind thrusts, we have modeled an array of fault geometries, spatial extent, slip distribution, and linkages between strike-slip faults in the region. Additional keys to constraining the earthquake potential of such structures are improved estimates of slip rates and histories of major faults in the San Francisco Bay area, as well as a more complete record of current displacements and erosion rates in the region.