Characteristic features of local breakdown near a crack-tip in the transition zone from nucleation to unstable rupture during stick-slip shear failure

Abstract

Characteristic features of local breakdown near a propagating crack-tip during slip failure nucleation and its transition process to unstable rupture under mode II conditions are experimentally studied using a rock sample with a simulated fault, the size of which is large compared with the size of the breakdown zone. In addition, distinctive features in the nucleation zone are compared with those in the zone of steady, dynamic rupture propagation. It is found that the local breakdown stress drop, the local dynamic stress drop, the local stress increase immediately before the slip-weakening process, the dimensionless parameter S defined by Das and Aki, and the shear fracture energy are functions of crack length in the transition zone from nucleation to unstable rupture; i.e., they all increase with crack growth in the nucleation zone. This is contrasted with the observations in the zone of steady dynamic rupture propagation, where these physical parameters do not depend on crack length. An increase in the magnitudes of these parameters with crack growth in the nucleation zone results in increasing resistance to crack extension with crack growth in the nucleation zone. The reason for the increase in crack growth resistance in this zone is that a slip failure nucleus is formed at a point (or zone) where the local strength and the shear fracture energy are at a minimum. This basic physical information can provide the key to earthquake prediction, since the nucleation itself is a precursory phenomenon. The ratio of the size of the breakdown zone to the crack length decreases abruptly beyond the critical crack length. Rupture velocity v accelerates with crack growth, and reaches the shear wave velocity or super-shear, and v is found to be expressed empirically as a function of crack length in the nucleation zone. A positive correlation is found between S and v in the nucleation zone ( v increases with increasing S ). However, no such correlation was observed in the zone of steady, dynamic rupture propagation, where v seems to have reached the shear wave velocity of super-shear for S = 0.5. This is consistent with Das and Aki's numerical result. The interrelationships between shear stress, slip displacement, slip velocity and acceleration near the crack-tip during the breakdown in the nucleation zone are revealed, and they are compared with those in the zone of dynamic rupture propagation. The present experiments suggest that the relationship between shear stress and slip displacement is more fundamental during the breakdown process than the relationship between shear stress and slip velocity. A model is presented to describe the breakdown process in the nucleation zone