Synthetic seismicity models of multiple interacting faults

Abstract

A synthetic seismicity model for multiple, interacting faults, of any strike and dip in a three‐dimensional elastic half‐space has been developed. The ultimate purpose is to determine how hazard is modified by elastic interactions of events in complex fault systems and to examine the synthetic seismicity for unusual behavior before large events that might be observable in the real world. Each fault is subdivided into some number of equal‐sized patches, and a coefficient of static friction, random within a specified range, is assigned to each. The patch static strength is then the product of friction and differential normal stress. Stress (both shear and normal) accumulates due to some predefined tectonic driving force until one or more patches fail and slip instantaneously. The slip can be in any direction within the fault plane and its magnitude is determined by a specified fractional stress drop. This initial slip induces shear stress and strength changes on all other patches and faults and perhaps results in other patches failing. Patches can slip more than once; once having failed, their strength is reduced, resulting in an overshoot that approximates the effect of dynamic stress enhancement. An event is over when all patches are stable. Although present computational constraints do not allow models with more than about 1500 patches, or a large number of faults, experiments with simple models of two parallel strike‐slip faults (25 km length, 15 km depth, 694 m × 714 m patch size), driven at different rates and separated by 10 or 15 km, give some indicative results. A range of friction between 0.1 and 0.2, with a stress drop of 10% (about 3 MPa or 30 bars), produces a b value (500,000 events) close to 1. The models generate a distinct class of large “characteristic” events that rupture the full fault plane. The distribution of interevent times for the background seismicity is very similar to that for a Poisson process, while the characteristic events are quasiperiodic. Nearly half the larger events directly induce some activity on the other fault. The effect of a characteristic event on the occurrence of another characteristic event on the opposite fault is small and depends on the separation: at 10 km distance other events are retarded while at 15 km distance they are advanced. At 10‐km separation the probability of a pair of characteristic events within 1 year is reduced to 0.3% from 1.7% for the case with no interaction. Consideration of interactions between slip on the faults and the driving mechanism, as well as higher stress drops and more overshoot, would increase the magnitude of the interaction effects.