A large granite sample containing a saw cut, modeling a natural fault, was triaxially loaded at confining pressures up to 1000 bars. Fourteen violent slip events accompanied by foreshock and aftershock sequences occurred under constant strain rate loading. From digitally recorded acoustic emission, locations and amplitudes were determined for nearly 8,000 microseismic events.

Plots of log amplitude versus log frequency of microseismic events were drawn for three periods between each slip event, termed foreshock, aftershock, and background. These plots indicate that the b-value is lower during foreshocks than for periods between events, implying increased average amplitude of microseismic activity just before slip. These experimental results suggest that it may be possible to devise an earthquake warning system based on changes in b-values in active tectonic regions. It has been suggested that according to the dilatancy-diffusion model, b-value would decrease prior to earthquakes. In our experiment, however, the rock was dry.

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