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A Mutually Consistent Seismic-Hazard Source Model for Southern California

Edward H. Field, David D. Jackson, & James F. Dolan

Published June 1999, SCEC Contribution #453

A previous attempt to integrate geological, geodetic, and observed seismicity data into a probabilistic-hazard source model predicted a rate of magnitude 6 to 7 earthquakes significantly greater than that observed historically. One explanation was that the discrepancy, or apparent earthquake deficit, is an artifact of the upper magnitude limit built into the model. This was controversial, however, because removing the discrepancy required earthquakes larger than are seen in the geological record, and larger than implied from empirical relationships between fault dimension and magnitude. Although several papers have addressed this issue, an alternative, integrated source model without an apparent deficit has not yet appeared. We present a simple geologically based approach for constructing such a model which agrees well with the historical record and does not invoke any unsubstantiated phenomena. The following factors are found to be influential: the b-value and minimum maginitude applied to Gutenberg-Richter seismicity; the percentage of moment released in characteristic earthquakes; a round-off error in the moment-magnitude definition; bias due to historical catalog incompleteness; careful adherence to the conservation of seismic moment rate; uncertainty in magnitudes estimates obtained from empirical regressions; allowing multi-segment ruptures (cascades); and the time dependence of recurrence rates. The previous apparent deficit is shown to have resulted from a combination of these factors. None alone caused the problem, nor solves it. The model presented here is relatively robust with respect to these factors.

Key Words
san andreas fault, size earthquakes, gap hypothesis, moderate size, magnitude, evolution, length, slip

Field, E. H., Jackson, D. D., & Dolan, J. F. (1999). A Mutually Consistent Seismic-Hazard Source Model for Southern California. Bulletin of the Seismological Society of America, 89(3), 559-578.