The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

The 1989 Loma Prieta, California, earthquake perturbed the static stress field over a large area of central California. The pattern of stress changes on major faults in the region predicted by models of the earthquake's dislocation agrees closely with changes in the regional seismicity rate after the earthquake. The agreement is best for models with low values of the coefficient of friction (0.1

Cordell and Grauch (1982, 1985) discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks. The final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes. We attempt to automate their final step. Our method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources. The method applies to both local surveys and to continent‐wide compilations of magnetic and gravity data (e.g., Zietz, 1982; Simpson et al., 1983a; Kane et al., 1982).

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A physically-motivated model for earthquake recurrence based on the Brownian relaxation oscillator is introduced. The renewal process defining this point process model can be described by the steady rise of a state variable from the ground state to failure threshold as modulated by Brownian motion. Failure times in this model follow the Brownian passage time (BPT) distribution, which is specified by the mean time to failure, µ, and the aperiodicity of the mean, α (equivalent to the familiar coefficient of variation). Analysis of 37 series of recurrent earthquakes, M-0.7 to 9.2, suggests a provisional generic value of α = 0.5. For this value of α, the hazard function (instantaneous failure rate of survivors) exceeds the mean rate for times > µ⁄2, and is ∼ ∼2 ⁄ µ for all times > µ. Application of this model to the next M 6 earthquake on the San Andreas fault at Parkfield, California suggests that the annual probability of the earthquake is between 1:10 and 1:13.