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A Model of Tectonic Stress State and Rate Using the 1994 Northridge Earthquake Sequence

Cooperative Institute for Research in the Environmental Sciences
University of Colorado
Boulder, Colorado, 80309
sjg{at}quake.colorado.edu

A model of the spatial and temporal distribution of seismicity both before and after the Northridge, California, earthquake has been used to estimate the background stress state, loading rate, and some parameters relating to fault friction. A detailed source model was used to calculate the stress change field from the Northridge mainshock, which was fit to the spatial distribution of seismicity. Agreement between the best-fitting background stress state from seismicity modeling and the background stress state inferred from focal mechanisms was found to be 99% significant, showing that the spatial distribution of seismicity contains information about the stress state. The stress rate found for the Northridge area (1.2 Pa/day [0.004 bar/a]) is approximately a factor of 9 less than the stress rate expected from observed strain rates, which implies that aseismic deformation is reducing seismic hazard in the Los Angeles basin. The resulting estimate of the friction parameter A_D 0.008 is similar to laboratory values if an effective normal stress much smaller than the overburden and similar to background stress magnitudes 70 MPa from the seismicity modeling is assumed to apply. Estimates of the magnitude of background stress, gradient of stress with depth and effective coefficient of friction are quite uncertain.