Abstract. This paper describes the localization of deformation acceleration in the period prior to dynamic failure in hornblende schist rock under triaxial compression 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 al., 1991 ], we have instead developed a rapid multichannel data collection system. This enables us to elucidate the dynamics of fault nucleation under condition of constant stress (creep) loading, which is a better approximation to low strain rate condition in the Earth and allows both quasi-static and 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, with a sampling rate of 50 ns and mask time of 200 gs. Hypocentral locations of AE sources revealed that the fault initiated at one end of the core and then propagated into the unfaulted rock with a process zone (fault front) of intense cracking. We found that there were two different processes operating during the quasi-static nucleation of a shear fault, namely, a process 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 tensile cracking, (2) low b value and fewer larger events, and (3) strong self-excitation. The mechanism of crack interaction and fault growth was, therefore, a mutual enhancement on dilatation due to tensile cracking. On the other hand, the damage zone was characterized by (!) major shear cracking, (2) low b value and more larger events, and (3) weak self-excitation, indicating that in the damage zone, following the development of a shear fault, linkage between cracks became the major mechanism of crack interaction and fault development. The mutual changes of b value and self-exciting strength observed in our experiments seem to occur as a result of the hierarchy of fault growth, which was not observed under slowed down loading conditions. Therefore our experimental results, under a realistic approximation of the dynamic condition of the Earth, are meaningful for the problems of earthquakes as well as rock bursts.

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Coarse-grained Inada granite (grain size 5 mm in average) and fine-grained Oshima granite (grain size < 2 mm) samples were deformed in triaxial compression experiments. Acoustic emission (AE) was monitored using 20 transducers in real time. In each experiment, hypocenters of some thousands of events were determined using anisotropic velocity model based on measured data. The spatial distributions of AE hypocenters in both rocks had fractal structure. Their fractal dimensions were 2.3 and 2.7 in average for Inada granite and Oshima granite, respectively. Focal mechanisms of AE showed an important difference between two granites. In Inada granite, type-S (emitted by shear fracturing) was dominant throughout the fracturing process. However, in Oshima granite, fracture types were dependent on stress levels. At lower stress stage, type-C (assigned to implosive fracturing) was dominant (although only few in number). At the stress level below 80% of the fracture strength, type-T (emitted by tensile fracturing) was dominant, whereas above this stress level, type-S gradually became dominant.

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