Focal Mechanisms of Acoustic Emission Events During Fault Propagation and Frictional Sliding

Cylindrical samples of Aue granite with 50 mm diameter and 100 mm length were loaded to failure in triaxial compression tests. A localized shear fracture initiated at the sharp edge of a steel plate covering 2/3 of the top sample surface. Fault nucleation, propagation and post-failure frictional sliding were studied by monitoring the acoustic emission (AE) activity related to microcracking. Ten piezoceramic transducers were attached to the samples and used to locate events and determine focal mechanisms. The velocity of fault propagation and sliding was controlled by the AE rate. Crack microstructures of deformed samples were investigated using optical microscopy. Fault nucleation involves 10-30% of the total AE events observed up to stress drop. A localized process zone of AE surrounds the tip of the propagating fault. Spatial correlation of the AE is low during fault nucleation, but increases during failure propagation up to stress drop. In the sliding stage, spatial correlation remains relatively constant corresponding to D-values of 1.8-2.0. Initial P-axes orientation distribution of focal mechanisms from shear events is broad with two maxima during fault nucleation, but it narrows during fault propagation. For given time intervals, AE P-axes show a preferred orientation near the tip of the process zone, but are more irregular at the trailing edge. During experiments B-values range between 1.2-1.8, but are similar for different fault segments. After stress drop a single P-axis orientation dominates along the fault. The P-axis distribution corresponds to a preferred orientation of secondary shears at a small angle to the macroscopic fault. The shears develop during propagation at the fault tip and remain similar in orientation during sliding.