Source analysis of acoustic emissions in Aue granite cores under symmetricand asymmetric compressive loads

The effect of stress anisotropy on the brittle failure of granite is invest igated under uniaxial compression. Non-standard asymmetric compression test s are performed on cores of Aue granite (diameter 52 mm, length 100 mm), in which 20 per cent of the core top surface remains unloaded. The edge of th e asymmetric steel loading plate acts as a stress concentrator, from where a shear rupture is initiated. The propagation of the fracture-related proce ss zone from top to bottom of the core is mapped by microcrack-induced acou stic emissions. Compared to standard uniaxial tests with symmetric loading, in the asymmetric tests both a greater quantity and more localized distrib utions of emission event hypocentres are observed. The maximum event densit y doubles for asymmetric (20 events per 10(-6) m(3)) compared to symmetric tests. The cluster correlation coefficient, a measure of strain localizatio n in the faulting process, reaches 0.15 for symmetric and 0.30 for asymmetr ic tests. The clustering of events, however, is found post-failure only. Th ree different amplitudes are used to determine b-values discussed as a poss ible failure precursor. Focal amplitudes determined at a 10 mm source dista nce and maximum amplitudes measured at eight piezoceramic sensors lead to b -values that drop before rock failure. First-pulse amplitudes automatically picked from emission wavelets show no anomaly. First-motion polarity stati stics of amplitudes indicate that a shear-crack-type radiation pattern is r esponsible for 70 per cent of the failure of granite, irrespective of stres s boundary conditions. For type-S events with an equal percentage of dilata tional and compressional first motions, focal mechanisms are determined by fitting measured first-pulse amplitudes to an assumed double-couple radiati on pattern. While hypocentres of large type-S events align parallel to the later fracture plane, their fault plane solutions show no coherent pattern. Spatial views of fracture planes reconstructed from X-ray computed tomogra ms reveal local small-scale changes in fracture plane orientation. Nodal pl anes from average fault plane solutions of the microscopic acoustic emissio n events coincide with the overall orientation of the macroscopic fracture plane azimuth (strike angle) determined from thin sections and tomograms.