Rate and State of Background Stress Estimated from the Aftershocks of the 1989 Loma Prieta, California, Earthquake

Estimates of the tectonic stress state including loading rate and magnitude of background stress are derived from the spatial and temporal distribution of Loma Prieta aftershocks. This technique was previously applied to the Landers aftershock sequence Gross and Kisslinger, 1997] and is based upon the seismicity model of Dieterich 1994]. Dieterich's theory suggests that background seismicity should be proportional to stress rate and the number of aftershocks in an area should be proportional to the stress step experienced in that area. We used two independently derived source models to compute the stress step from the mainshock and to determine how eeective that stress step was in triggering aftershocks. A background stress state is then chosen which makes the stress steps at the aftershock locations most distinct from the stress steps at hypocenters of background seismicity. The best tting background stress state has its greatest compressive stress plunging 17 to N13 E, and an intermediate stress very close in magnitude to the least principal stress. The small shear stresses at depth and low coeecient of friction suggest that high-pressure pore uids may be present inside active faults. The estimated stress rate of 70 Pa/d (0.25 bar/yr) is comparable to the stress rate found for the southern San Andreas Fault system. We found a best tting \eeective" coeecient of friction 0 0:2 for the Loma Prieta area, signiicantly less than 0 0:6 estimated for Landers in previous work. Variations in aftershock decay rate within the Loma Prieta aftershock zone are correlated with static stresses caused by postseismic slip as modeled by B urgmann et al. 1997]. Some of the postseismic slip occurred on structures that did not slip during the mainshock, so the postseismic and coseismic stress step elds have diierent spatial distributions. The eeectiveness of slowly accumulated postseismic static stresses in triggering aftershocks is especially interesting because dynamic stresses are insigniicant in this case.