The effect of temperature on the nonlinear elasticity of a fault rock in dynamic acoustoelastic testing (DAET) experiments

Dynamic nonlinear elasticity in rocks may play an important role in earth processes, such as earthquake nucleation. In order to understand how nonlinear elasticity occurs within the shallow crust, experiments are required that simulate the in situ conditions of intact crustal rocks. Additionally, exploring the behaviour of nonlinear elasticity in response to changes in external parameters (e.g. temperature and wave frequency) acts as a means to further illuminate the complex mechanisms which give rise to nonlinear elasticity in rocks. In this study, we perform dynamic acoustoelastic testing (DAET) experiments on an intact cataclasite from the damage zone of the Alpine Fault, New Zealand. By performing pump-probe DAET experiments inside a temperature-controlled chamber, we are able to investigate a rich variety of nonlinear behaviour as a function of temperature. We find that the magnitude of average softening, cubic nonlinearity, and hysteresis tend to increase as temperature increases from 20 to 110 & DEG;C. In contrast, quadratic nonlinearity decreases with increasing temperature. These observations support the hypothesis that at least two distinct mechanisms control nonlinear phenomena in rocks. Nonlinear parameters show little to no dependence on frequency over the 200-600 Hz pump range, although values of the nonlinear parameter & alpha; are found to be nearly two orders of magnitude smaller than those determined using ultrasonic perturbations. Additionally, an analysis using different time windows shows that the surface waves of the ultrasonic probe sense greater nonlinearity compared to the direct P- wave due to differences in the polarization and propagation paths. As well as providing further insight into the mechanisms responsible for nonlinear elasticity in rocks, our experiments show that nonlinear softening will increase as temperature increases in the damage zones of faults. This has potential implications for understanding earthquake nucleation processes.