DETAILED OBSERVATIONS OF CALIFORNIA FORESHOCK SEQUENCES - IMPLICATIONS FOR THE EARTHQUAKE INITIATION PROCESS

We find that foreshocks provide clear evidence for an extended nucleat ion process before some earthquakes. In this study, we examine in deta il the evolution of six California foreshock sequences, the 1986 Mount Lewis (M(L)=5.5), the 1986 Chalfant (M(L)=6.4), the 1986 Stone Canyon (M(L)=4.7), the 1990 Upland (M(L)=5.2), the 1992 Joshua Tree (M(W)=6. 1), and the 1992 Landers (M(W)=7.3) sequence. Typically, uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution. However, the similarity of location and focal mechanisms for the events in thes e sequences leads to similar foreshock waveforms that we cross correla te to obtain extremely accurate relative locations. We use these resul ts to identify small-scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mai nshock. In general, these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress tr ansfer. Instead, the foreshocks seem to concentrate near structural di scontinuities in the fault and may themselves be a product of an aseis mic nucleation process. Fault zone heterogeneity may also be important in controlling the number of foreshocks, i.e., the stronger the heter ogeneity, the greater the number of foreshocks. The size of the nuclea tion region, as measured by the extent of the foreshock sequence, appe ars to scale with mainshock moment in the same manner as determined in dependently by measurements of the seismic nucleation phase. We also f ind evidence for slip localization as predicted by some models of eart hquake nucleation.