Investigating Dynamic Weakening of Fault Core Rocks Under In Situ Pressures and Temperatures Using Laser Ultrasonics

Nonlinear softening of the elastic modulus due to strong shaking can contribute toward dynamic earthquake triggering or fault weakening during rupture propagation. The effective pressure and temperature of the rock in the subsurface strongly influence the magnitude of this nonlinear softening. Thus, in order to assess the potential for dynamic triggering or fault weakening, it is important to understand the effect of the in situ environment on nonlinear softening of real fault core rocks. In this study, we evaluate the nonlinear softening of cataclasites from the core of the plate boundary Alpine Fault in South Island, New Zealand. Experiments are performed with a novel apparatus replicating a range of shallow crustal pressures and temperatures. Large-amplitude ultrasonic waves are generated and recorded in the samples using entirely non-contacting laser ultrasonics, allowing us to record the nonlinear softening up to 200^oC at 20 MPa confining pressure without any mechanical interference. By using coda wave interferometry to detect velocity changes due to nonlinear softening of the rocks, we can accurately quantify the decrease in modulus with increasing source amplitude. Our initial results show a 1.5% reduction in shear modulus at transient strains of 10^-6 under atmospheric conditions, with smaller decreases at higher pressures. These quantitative estimates of nonlinear softening in real fault core rocks under realistic subsurface conditions are important for furthering our understanding of dynamic weakening processes and their role in earthquake rupture.