PALEOSEISMOLOGY ALONG THE OWENS VALLEY FAULT: ACCOUNTING FOR THE SAN ANDREAS DISCREPANCY

One of the largest historical earthquakes in California occurred in 1872 along the Owens Valley fault (OVF) located in the Eastern California Shear Zone (ECSZ). Recently published GPS studies indicate ~20-24% (10-12 mm/yr) of Pacific-North America plate boundary slip is accommodated across the northern part of the ECSZ, accounting for the San Andreas discrepancy. Published elastic half-space modeling of GPS measurements yield right-lateral slip rates across the OVF of ~7mm/yr suggesting that the OVF accounts for ~60-70% of the total shear within the ECSZ. Our new paleoseismic studies along the OVF suggest otherwise. New trenching studies yield evidence for two events—the 1872 and penultimate ruptures. OSL dating bracket the timing of the pre-1872 earthquake to between 3.3±0.6 and 4.0±0.4 ka. If we assume uniform return, a recurrence interval of ~3650 yrs is suggested. Exposed elsewhere are three beheaded stream channels that provide surface geomorphic evidence for the last three earthquake events along the OVF. These channels show cumulative right lateral offset of 4.9±0.4, 8.9±0.8, and 12.9±0.2 m, respectively, suggesting three 1872-like rupture events—4.9 m offset during the 1872 rupture and 4.0 m offset during each of the older events. Using the recurrence interval of ~3650 yrs determined from our trenching studies implies that the oldest of the three rupture events occurred ~7.3 ka yrs ago, indicating a minimum slip rate of 1.9±0.4 mm/yr. Alternatively, if we assume that the older two events occurred within a few years of each other between 3.3±0.5 and 4.0±0.4 ka, this implies a maximum late Holocene slip rate of 3.8±0.9 mm/yr. Our studies imply that the OVF accounts for ~13-47% of the total shear within the ECSZ. Our estimated geologic slip rate for the OVF is about half that based on elastic half-space modeling of GPS data. Alternatively, using a viscoelastic model, the recurrence interval estimated here, and GPS data from Dixon et al. (2000) yields a present-day slip rate of 1.7±1.0 mm/yr, in agreement with the geologic rate.