Experiments to Characterize the Nonlinear Elastic Behavior of Rocks in Realistic Subsurface Environments

The nonlinear reduction of modulus in rocks and subsequent slow dynamics recovery caused by dynamic strain perturbations have the potential to affect processes such as earthquake nucleation, rupture propagation, and the initiation of volcanic eruptions. Experiments have demonstrated that several factors influence the magnitude of the nonlinear response, including the mineralogy and microstructure of the rock, its physical environment (e.g. pressure, temperature, saturation), and the nature of the perturbation. However, experiments which combine all these variables to accurately replicate subsurface geological environments are sparse. Moreover, experiments targeting nonlinearity on rocks from fault zones and volcanoes are seldom. In this work, we perform laboratory experiments to characterize the nonlinear response of cataclasites and volcanic breccias under realistic subsurface conditions. Our experiments involve controlling the temperature and pressure of rock samples over ranges of 20-200°C and 0-20 MPa, respectively, in order to recreate typical subsurface pressure and temperature profiles. The strain perturbations which induce nonclassical nonlinear behavior are provided by either a 1-2 microstrain thermoelastic pulse, or a sub-kilohertz sinusoidal pump akin to dynamic acoustoelastic testing (DAET) experiments. Using different perturbations allows us to measure how nonlinearity changes from seismic to ultrasonic frequencies. We monitor the evolution of ultrasonic velocity in the rocks during and after dynamic perturbations using a laser ultrasonics apparatus, and deduce changes in modulus by performing stretching coda wave interferometry on the waveforms. Our results reveal that the nonlinear reduction in shear modulus of damage-zone cataclasites increases threefold between 20 and 80°C, with a similar decrease in the nonlinear response over a 0-2 MPa increase in effective pressure. The findings of our experiments will contribute toward understanding how nonlinear elastic behavior operates in realistic subsurface environments and influences processes at faults and volcanoes.