LOW SLIP RATE FOR SAN GORGONIO PASS SECTION OF THE SAN ANDREAS FAULT INFERRED FROM GEODESY

We estimated fault slip rates using campaign and continuous GPS data from a transect across the Pacific-North American plate boundary from SanClemente/Oceanside, California through San Gorgonio Pass and the eastern San Bernardino Mountains to Primm, Nevada. Tens of thousands of possible slip rate combinations for nine sub-parallel faults within the transect were tested using one-dimensional elastic modeling to determine which of these slip rate combinations best fit with the observed GPS site velocity profile. The best-fitting model (using the χ² criterion) yielded the following slip rates (mm/yr): Elsinore fault, 0; San Jacinto fault, 20; San Andreas fault, 2; Johnson Valley fault, 8; Copper Mountain fault, 5; Calico fault, 4; Pisgah-Bullion fault, 0; Ludlow fault, 0; South Bristol Mountains fault, 0. Both the San Jacinto fault and the Eastern California Shear Zone appear to be more active than the San Andreas fault within the San Gorgonio Pass region. Other models that fit the observed GPS profile reasonably well allowed the San Andreas fault slip rate to be as high as 14 mm/yr, but no higher. Our best-fitting slip rate of 2 mm/yr for the San Andreas fault in San Gorgonio Pass is slower than published geologic and geodetic slip rate estimates, but is consistent with published studies that show a southeastward decrease in San Andreas fault slip rate from Cajon Pass to San Gorgonio Pass. This supports previously proposed models in which the San Gorgonio Pass section of the San Andreas fault is only a minor player in the plate boundary, with slip on the San Jacinto fault contributing to a higher San Andreas fault slip rate north of Cajon Pass, and slip in the Eastern California Shear Zone contributing to a higher San Andreas fault slip rate south of Indio.