EARTHQUAKE RECURRENCE ON THE SOUTHERN SAN-ANDREAS MODULATED BY FAULT-NORMAL STRESS

Earthquake recurrence data from the Pallett Creek and Wrightwood paleo seismic sites on the San Andreas fault appear to show temporal variati ons in repeat interval. These sites are located near Cajon Pass, south ern California, where detailed mapping has revealed geomorphically and structurally expressed domains of alternating extension and contracti on respectively associated with releasing and restraining bends of the San Andreas fault. We investigate the interaction between strike-slip faults and auxiliary reverse and normal faults as a physical mechanis m capable of producing such variations. Under the assumption that faul t strength is a function of fault-normal stress (e.g. Byerlee's Law), failure of an auxiliary fault modifies the strength of the strike-slip fault, thereby modulating the recurrence interval for earthquakes. In our finite element model, auxiliary faults are driven by stress accum ulation near restraining and releasing bends of a strike-slip fault. E arthquakes occur when fault strength is exceeded and are incorporated as a stress drop which is dependent on fault-normal stress. The model is driven by a velocity boundary condition over many earthquake cycles . Resulting synthetic strike-slip earthquake recurrence data display t emporal variations similar to observed paleoseismic data within time w indows surrounding auxiliary fault failures. Although observed recurre nce data for the two paleoseismic sites are too short to be definitive about the temporal variations or the physical mechanism responsible f or it, our simple model supports the idea that interaction between a s trike-slip fault and auxiliary reverse or normal faults can modulate t he recurrence interval of events on the strike-slip fault, possibly pr oducing short term variations in earthquake recurrence interval.