STRESS-STRAIN CHANGES AND TRIGGERED SEISMICITY AT THE GEYSERS, CALIFORNIA

The principal results of this study of remotely triggered seismicity i n The Geysers geothermal field are the demonstration that triggering ( initiation of earthquake failure) depends on a critical strain thresho ld and that the threshold level increases with decreasing frequency or equivalently, depends on strain rate. This threshold function derives from (1) analyses of dynamic strains associated with surface waves of the triggering earthquakes, (2) statistically measured aftershock zon e dimensions, and (3) analytic functional representations of strains a ssociated with power production and tides. The thresholds also consist ent with triggering by static strain changes and implies that both sta tic and dynamic strains may cause aftershocks. The observation that tr iggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process. Assum ing the physical processes underlying earthquake nucleation to be the same, Gomberg [this issue] discusses seismicity triggered by the M(w) 7.3 Landers earthquake, its constraints on the variability of triggeri ng thresholds with site, and the implications of time delays between t riggering and triggered earthquakes. Our results enable us to reject t he hypothesis that dynamic strains simply nudge prestressed faults ove r a Coulomb failure threshold sooner than they would have otherwise. W e interpret the rate-dependent triggering threshold as evidence of sev eral competing processes with different time constants, the faster one (s) facilitating failure and the other(s) inhibiting it. Such competit ion is a common feature of theories of slip instability. All these res ults, not surprisingly, imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceeden ce of some constant threshold but also the requirements for generating instabilities.