Compressive failure of mudstone samples containing quartz veins using rapid AE monitoring: the role of asperities

This paper presents the results of an ongoing experimental investigation of compressive failure of homogeneous and heterogeneous rocks. We used a rapi d data acquisition system to monitor the spatio-temporal distribution of ac oustic emission (AE) during fault nucleation under conditions of either con stant-rate loading or static loading. In order to examine the effect of asp erities on faulting, we conducted a series of experiments on mudstone with quartz veins. The bedding planes of the mudstone were oriented at an angle of 30 degrees with the maximum compressive stress and were expected as the fracture plane. The quartz veins are much stronger than the bedding plane o f mudstone and thus the samples model faults having strong asperities. Expe rimental results show that: (I)AE activity initiated close to the peak stre ss, and almost all AE hypocenters appeared at the intersections of the vein s and the fault planes, suggesting that the vein asperities control faultin g; (2) the b-value changed with time and shows multiple large and short-ter m fluctuations; (3) the change of the b-value correlated closely with the s patio-temporal hierarchy of the fracture process; and (4) fault segments al ong the bedding plane show behavior of slip having large compressive deform ation before the peak stress, while the vein asperities show large precurso r dilatancy prior to dynamic rupture. These experimental results are helpfu l for the understanding of seismic precursor phenomena associated with stra in localization, dilatancy, change in level of ground water, as well as the b-value. (C) 2000 Elsevier Science B.V. All rights reserved.