Discrepancy between earthquake rates implied by historic earthquakes and aconsensus geologic source model for California

We examine the difference between expected earthquake rates inferred from t he historical earthquake catalog and the geologic data that was used to dev elop the consensus seismic source characterization for the state of Califor nia [California Department of Conservation, Division of Mines and Geology ( CDMG) and U.S. Geological Survey (USGS) Petersen et al., 1996; Frankel et a l., 1996]. On average the historic earthquake catalog and the seismic sourc e model both indicate about one M 6 or greater earthquake per year in the s tate of California. However, the overall earthquake rates of earthquakes wi th magnitudes (M) between 6 and 7 in this seismic source model are higher, by at least a factor of 2, than the mean historic earthquake rates for both southern and northern California. The earthquake rate discrepancy results from a seismic source model that includes earthquakes with characteristic ( maximum) magnitudes that are primarily between M 6.4 and 7.1. Many of these faults are interpreted to accommodate high strain rates from geologic and geodetic data but have not ruptured in large earthquakes during historic ti me. Our sensitivity study indicates that the rate differences between magni tudes 6 and 7 can be reduced by adjusting the magnitude-frequency distribut ion of the source model to reflect more characteristic behavior, by decreas ing the moment rate available for seismogenic slip along faults, by increas ing the maximum magnitude of the earthquake on a fault, or by decreasing th e maximum magnitude of the background seismicity. However, no single parame ter can be adjusted, consistent with scientific consensus, to eliminate the earthquake rate discrepancy. Applying a combination of these parametric ad justments yields an alternative earthquake source model that is more compat ible with the historic data. The 475-year return period hazard for peak gro und and 1-sec spectral acceleration resulting from this alternative source model differs from the hazard resulting from the standard CDMG-USGS model b y less than 10% across most of California but is higher (generally about 10 % to 30%) within 20 km from some faults.