Acoustic emission and creep in rock at high confining pressure and differential stress

Abstract Two samples each of Weber Sandstone and Westerly Granite were tested under triaxial compression at 1 kb confining pressure. Axial load was increased in steps, and the acoustic emission generated in the samples during primary creep was monitored. The rate v of acoustic emission events was found to decrease exponentially at each stress level, obeying the law log v = β − αN, where N is the total number of microseismic events that have occurred and α and β are constants. By assuming that acoustic emission is proportional to inelastic deformation, this relation can be compared to empirical creep laws. It is similar to the relation given by Lomnitz and fits the data more closely than other creep laws that were functions of time rather than number of acoustic emission events. The value of α was found to decrease systematically with increasing differential stress, and in one experiment became negative before sample failure.