Quasi-static fault growth and cracking in homogeneous brittle rock under triaxial compression using acoustic emission monitoring

This paper describes the localization of deformation acceleration in the pe riod prior to dynamic failure in hornblende schist rock under triaxial comp ression using acoustic emission (AE) monitoring. Rather than stabilize the failure process by controlling axial stress to maintain a constant rate of AE (for monitoring AE hypocenters) as in previous works [e.g., Lockner et a l., 1991], we have instead developed a rapid multichannel data collection s ystem. This enables us to elucidate the dynamics of fault nucleation under condition of constant stress (creep) loading, which is a better approximati on to low strain rate condition in the Earth and allows both quasi-static a nd dynamic crack growth to occur. The waveforms of more than 8000 AE events which occurred mainly during a 15 s period were recorded on 32 channels, w ith a sampling rate of 50 ns and mask time of 200 mu s. Hypocentral locatio ns of AE sources revealed that the fault initiated at one end of the core a nd then propagated into the unfaulted rock with a process zone (fault front ) of intense cracking. We found that there were two different processes ope rating during the quasi-static nucleation of a shear fault, namely a proces s zone in front of the fault tip and a "wake" of damage zone following the process zone. The process zone had the following features: (1) major tensil e cracking, (2) low b value and fewer larger events, and (3) strong self-ex citation. The mechanism of crack interaction and fault growth was, therefor e, a mutual enhancement` on dilatation due to tensile cracking, On the othe r hand, the damage zone was characterized by (1) major shear cracking, (2) low b value and more larger events, and (3) weak self-excitation, indicatin g that in the damage zone, following the development of a shear fault, link age between cracks became the major mechanism of crack interaction and faul t development. The mutual changes of b value and self-exciting strength obs erved in our experiments seem to occur as a result of the hierarchy of faul t growth, which was not observed under slowed down loading conditions. Ther efore our experimental results, under a realistic approximation of the dyna mic condition of the Earth, are meaningful for the problems of earthquakes as well as rock bursts.