NUCLEATION AND GROWTH OF FAULTS IN BRITTLE ROCKS

We present a model for the nucleation and growth of faults in intact b rittle rocks. The model is based on recent experiments that utilize ac oustic emission events to monitor faulting processes in Westerly grani te. In these experiments a fault initiated at one site without signifi cant preceding damage. The fault propagated in its own plane with a le ading zone of intense microcracking. We propose here that faults ire g ranites nucleate and propagate by the interaction of tensile microcrac ks in the following style. During early loading, tensile microcracking occurs randomly, with no significant crack interaction and with no re lation to the location or inclination of the future fault. As the load reaches the ultimate strength, nucleation initiates when a few tensil e microcracks interact and enhance the dilation of one another. They c reate a process zone that is a region with closely spaced microcracks. In highly loaded rock, the stress field associated with microcrack di lation forces crack interaction to spread in an unstable manner and re cursive geometry. Thus the process zone propagates unstably into the i ntact rock. As the process zone lengthens, its central part yields by shear and a fault nucleus forms. The fault nucleus grows in the wake o f the propagating process zone. The stress fields associated with shea r along the fault further enhance the microcrack dilation in the proce ss zone. The analysis shows that faults should propagate in their own plane, making an angle of 20 degrees-30 degrees with the maximum compr ession axis. This model provides a physical basis for ''internal frict ion,'' the empirical parameter of the Coulomb criterion.