The effect of cyclic differential stress on dilatancy in westerly granite under uniaxial and triaxial conditions

Laboratory experiments have been performed on samples of Westerly granite in which the differential stress was repeatedly cycled to 85% of the intact sample strength. The experiments have shown that under uniaxial conditions the onset of dilatancy is reduced to fairly low stress; however, under triaxial conditions dilatancy can be an apparently stable process, and the onset of dilatancy is not affected by the repeated cycling. Thus the implication for midcrustal earthquakes is that the onset of dilatancy repeatedly occurs at relatively high stress levels. For example, our results indicate that at typical focal depths of 2.5 and 10 km (corresponding to effective hydrostatic pressures of about 500 and 2000 bars) the onset of dilatancy repeatedly occurs at 1.8 and 3.0 kbar of differential compressive stress, respectively.