Holocene Faulting and Earthquake Recurrence along the Northern Agua Tibia-Earthquake Valley Fault Zone and Implications for Slip Distribution in Southern California

Slip rates determined along the Elsinore and San Jacinto faults indicate that south of 33.5 degrees N, the slip rate along the Elsinore fault decreases by similar to 2.5 mm/yr, whereas the slip rate along the San Jacinto fault increases by nearly 6 mm/yr. A transfer of slip from the Elsinore to the San Jacinto faults is the likely explanation, but a fault with Holocene activity had yet to be identified to support this hypothesis. We report the results of new mapping and paleoseismic work along the Earthquake Valley fault, the eastern strand of the Elsinore fault, in Warner basin. Our paleoseismic investigation at Big Lake documents three stratigraphic horizons at which there is evidence of coeval shattering and displacements. The uppermost interpreted event breaks up to within 20 cm of a historical horizon and suggests a very young surface rupture. We attribute this event (E1) to either the 1890 or 1892 earthquakes. The ages of the two previous earthquakes are constrained to be between 486 C.E. and 1125 C.E., and between 158 B.C.E. and 235 C.E. These data suggest individual interevent recurrence intervals of 1106 +/- 340 yr (E1-E2) and 760 +/- 406 yr (E2-E3), respectively. The minimum slip rate of 1.95 +/- 0.4 mm/yr is estimated for the Earthquake Valley fault from the offset of two main drainages that cut into the sedimentary fill of Warner basin, which contains the Bishop tuff. We propose that with its geomorphological expression, late Holocene earthquake record, and slip rate estimate, the Earthquake Valley fault is the main structure responsible for transferring slip from the northern Elsinore fault to the southern San Jacinto fault. Given the recency of a surface rupture and our recurrence interval calculations, however, a large earthquake along the Earthquake Valley fault in the near future is unlikely, if time dependence is considered.