FAILURE ZONE DEVELOPMENT IN ANDESITE AS OBSERVED FROM ACOUSTIC-EMISSION LOCATIONS AND VELOCITY CHANGES

Three-dimensional (3-D) locations of acoustic emissions (AE) and P-wav e velocity anisotropy were investigated to analyse the failure zone de velopment in two Yugawara andesites. The results were obtained during the deformation of rock under incremental loading and short-duration c reep regimes at a confining pressure of 40 MPa. Creep tests were carri ed out at a constant differential stress of about 90-95% of short-term fracture strength of andesites for the single- and two-stage creep ex periments. In both the experiments, the AE activity during loading was mostly confined to two regions on the midplane of the rock sample. Du ring the creep regime, the fault plane nucleated abruptly from the reg ion where AE hypocentres concentrated near the cylindrical surface of the rock. Prior to fault nucleation, the localized zone of microcracks attained a spheroidal shape in one sample and planar shape in the oth er. P-wave velocity anisotropy increased to 44% as final failure appro ached in the rock. The loading rate and the magnitude of creep stress were found to have a profound influence on the details of failure zone development and times of failure of the rocks studied.