Locking depths estimated from geodesy and seismology along the San Andreas Fault System: Implications for seismic moment release

The depth of the seismogenic zone is a critical parameter for earthquake hazard models. Independent observations from seismology and geodesy can provide insight into the depths of faulting, but these depths do not always agree. Here we inspect variations in fault depths of 12 segments of the southern San Andreas Fault System derived from over 1000 GPS velocities and 66,000 relocated earthquake hypocenters. Geodetically determined locking depths range from 6 to 22 km, while seismogenic thicknesses are largely limited to depths of 11-20 km. These seismogenic depths best match the geodetic locking depths when estimated at the 95% cutoff depth in seismicity, and most fault segment depths agree to within 2 km. However, the Imperial, Coyote Creek, and Borrego segments have significant discrepancies. In these cases the geodetically inferred locking depths are much shallower than the seismogenic depths. We also examine variations in seismic moment accumulation rate per unit fault length as suggested by seismicity and geodesy and find that both approaches yield high rates ( 1.5-1.8 x 10(13) Nm/yr/km) along the Mojave and Carrizo segments and low rates (similar to 0.2 x 1013 Nm/yr/km) along several San Jacinto segments. The largest difference in seismic moment between models is calculated for the Imperial segment, where the moment rate from seismic depths is a factor of similar to 2.5 larger than that from geodetic depths. Such variability has important implications for the accuracy to which future major earthquake magnitudes can be estimated.