Estimating the seismicity from geological structure for seismic-risk studies

abstract Starting with geological data, this paper estimates the seismicity for applications in seismic risk studies. The rate at which seismic moment is released can be estimated on a fault when the slip rate is known. It can also be estimated in a region of crustal convergence (without subduction) or divergence when the rate at which opposite sides of the zone are converging or the regional strain rate is known. Then, provided all of the deformation is released seismically, by assuming the relative frequency of different sizes of earthquakes, the absolute frequency of events can be obtained. The procedure is used to estimate seismicity in southern California. A review of geological literature has provided preliminary estimates of slip rates on many important faults. The estimates of the seismicity resulting from these slip rates are consistent with historical records of earthquake occurrences for southern California taken as a whole. For smaller regions or individual faults in southern California, the seismicity estimated from slip rates may differ from historical rates of seismicity by a factor of two or more. In the western basin and range region, the historical seismicity is also consistent with an estimate for the strain rate. Because of this agreement in larger regions, where many faults are involved, it is inferred that the geological data is also useful for studies of smaller regions, even though on this scale the model cannot be tested because of the too short historical record of earthquake occurrences.