A Synthetic Seismicity Model for Southern California: Cycles, Probabilities, Hazards

Steven N. Ward

Published October 10, 1996, SCEC Contribution #253

The absence of a long historical catalog of observed seismicity with which to constrain earthquake recurrence behaviors is a fundamental stumbling block to earthquake prediction in California. Conceding that this limitation is not likely to relax in the foreseeable future, alternative approaches must be sought to extend the catalog artificially. In this article, I evaluate the long-term behaviors of earthquakes on a map-like set of southern California faults through computer simulations that incorporate the physics of earthquake stress transfer and are constrained by excellent, but restricted, bodies of geological and seismological data. I find that model seismicity fluctuates on both short (decades) and long (centuries) timescales but that it possesses a well-defined mean and standard deviation. Seismicity fluctuations correlate across different magnitudes, and the long-term cycles of smaller events seem to lead cycles of larger events. Short-period seismicity fluctuations do not exhibit this tendency, and short-term changes in low-magnitude (M5 +) seismicity are not likely to be an effective predictor of future large events, at least for the region as a whole. As do real faults, the model faults produce characteristic and power law quakes in variable ratios with diverse periodic and nonperiodic behaviors. Generally, larger events tend to occur quasi-periodically, and smaller ones tend to cluster; however, only for a few earthquake classes and certain locations is recurrence notably non-Poissonian. An important use of synthetic seismicity is in the construction of earthquake hazard maps because it firmly grounds previously ad hoc assumptions regarding frequency-magnitude distributions, multiple-segment failure statistics, and rupture extents, while satisfying a spectrum of geological constraints such as fault slip rate, segment recurrence interval, and slip per event. With its depth of temporal and spatial coverage, synthetic seismicity also provides a means to investigate the time dependence of seismic hazard. Because hazard likelihood is a concatenation of the recurrence statistics from many seismic sources, in only about 40–50% of the regions near the major faults do sequences of 0.1 g or 0.2 g exceedances differ from Poissonian.

Ward, S. N. (1996). A Synthetic Seismicity Model for Southern California: Cycles, Probabilities, Hazards. Journal of Geophysical Research, 101(B10), 22393-22418. doi: 10.1029/96JB02116.