Estimating tectonic stress rate and state with Landers aftershocks

A new method of analyzing the location and timing of aftershocks that makes possible estimates of the rate at which faults in the area are being loaded with stress has been developed. This new method is an application of a theory of aftershocks proposed by Dieterich [1994]. The stress step from the mainshock triggers the aftershocks, which release energy stored by tectonic loading and the mainshock, and the tectonic stressing rate is responsible for the background activity. Thus a comparison of the number of aftershocks with the rate of background activity as a function of stress step can provide an estimate of tectonic stress rate. This method requires analysis of a catalog which includes background activity as well as aftershocks. The method has been applied to the aftershocks of the 1992 Landers, California, earthquake. The tectonic stress rate estimated for the Mojave block is ≈1–2 Pa/d, which corresponds to a repeat time for large events of ≈14,000 years, and suggests that much of the observed strain in the Mojave is anelastic. The southern San Andreas fault system is found to have a stress fate of ≈100 Pa/d, which corresponds to a repeat time of ≈270 years and is consistent with primarily elastic strain accommodation. Inversions for the best fitting background stress state are consistent with fits of focal mechanisms and support an effective friction coefficient near 0.6. Fits to the aftershock decay provide estimates of the Dieterich friction parameter A D ranging from ≈0.02, if lithostatic pore fluids are present, to ≈0.0003 if they are not.