TEMPORAL VARIATIONS IN SEISMICITY DURING QUASI-STATIC AND DYNAMIC ROCK FAILURE

A comprehensive model is presented which can explain temporal fluctuations in seismic b-values in the period leading to mechanical failure in terms of the underlying physical processes of time-varying applied stress and stress corrosion-enhanced crack growth under conditions of constant strain rate. The form of the b-value anomaly in the period leading to failure depends on the form of the stress/time relationship. For the case where dynamic failure occurs at peak stress after a period of strain hardening, the model predicts a single cusp-like b-value anomaly, reaching a critically low value of 0.5 at failure. For the physically most realistic case where dynamic failure is preceded by a period of precursory strain energy release during strain softening, the model predicts two minima in the b-value. separated by a temporary maximum or inflection point. These fluctuations in the b-value are consistent with reported "intermediate-term" and "short-term" earthquake precursors separated by a period of seismic quiescence. For the case of quasi-static cataclastic flow, the b-value mirrors the stress and never falls to the critical value because there is no critical rupture. New results from a series of controlled laboratory experiments are presented in which microseismic event rates and b-value were monitored contemporaneously with stress/time data for all variants of the stress/time relationship. Recent field observations of temporal changes in seismicity rates and b-value preceding major earthquakes are also reported. Both data sets exhibit b-value anomalies which are consistent with the model predictions. © 1990.