Laboratory Investigations Into the Dynamic Nonlinear Elastic Behaviour of Fault and Volcanic Rocks Under Realistic Subsurface Conditions

Abstract

Rocks display a range of remarkable nonlinear elastic phenomena when disturbed by elastic waves or other dynamic stresses. This dynamic nonlinear elasticity manifests primarily as a reduction in the elastic modulus, followed by a logarithmic recovery over time after the perturbation has stopped. Such variations in the elastic modulus of rocks have significant implications for processes in Earth’s crust, and may explain how earthquakes and volcanic eruptions can be dynamically triggered by passing seismic waves. However, no experiments to date have explored the dynamic nonlinear elasticity of intact rocks from fault zones and volcanoes under realistic subsurface conditions. In this thesis, we perform laboratory experiments to investigate dynamic nonlinear elasticity in fault and volcanic rocks under conditions which replicate the realistic subsurface environment. We study a suite of rocks from the Alpine Fault and Whakaari/White Island, New Zealand. By progressively increasing the amplitude of laser-generated ultrasonic waves in our samples, we are able to measure the decrease in shear modulus as a function of dynamic strain. Results demonstrate that the nature and magnitude of nonlinear elasticity are controlled by the degree of damage in a rock. Increasing confining pressure suppresses nonlinear softening, while increasing temperature significantly increases dynamic nonlinear softening. When the effects of increasing pressure and temperature are combined, nonlinear softening persists to equivalent depths of at least 1000 m. Dynamic acoustoelastic testing (DAET) experiments at elevated temperatures demonstrate that nonlinear elasticity is observed in these rocks at perturbation frequencies approaching those of seismic waves. Additionally, experiments on a Berea sandstone show that changes in temperature induce nonlinear elasticity due to the breaking and recreation of microscopic contacts. The work presented in this thesis expands our understanding of the nature and mechanisms of dynamic nonlinear elasticity in rocks and demonstrates that nonlinear elasticity can occur to greater depths than previously thought, especially in localised areas with damaged rocks and high temperatures at the core of faults and volcanoes. This has significant implications for understanding how dynamic nonlinear elasticity in rocks influences processes such as the triggering of earthquakes and volcanic eruptions by seismic waves.