Controls of Pore Fluid Pressure on Fault Slip Weakening and Fracture Energy

William Marsh Rice University, Houston TX

Investigators

Abstract

In the past 15 years, measurements from the fields of geodesy and seismology have revealed that large faults along the boundaries of tectonic plates regularly slip at rates slower than earthquakes, but faster than the tectonic plates. These intermediate-rate slip events influence earthquake hazards through their potential to cause an earthquake on an adjacent segment of the fault or to control how big a future earthquake may become. Although the cause of these slip events is still debated, they are consistently observed in regions where the fluids filling pores in the rock are thought to be at very high pressure. Motivated by previous and preliminary laboratory observations that faulting becomes slower and more stable with increasing pore fluid pressure, scientists at Rice University are quantifying the effects of fluid pressure on the physics of faulting. This study is designed to provide inputs into models of regional seismic hazards, including along the San Andreas Fault and in Cascadia, where these slip events have been observed. This research is supporting the development of a laboratory by an early career researcher and the education of one graduate student and several undergraduate students. The researcher and students will incorporate this work into lectures and teaching activities on natural hazards. This research quantifies an experimental observation that is not explained by current models for faulting and, given the abundance of evidence for elevated fluid pressures in fault zones, has direct implications for the cause and characteristics of slow slip. Specifically, previous experimental observations show that the slip weakening distance and fracture energy of faulting increase with fluid pressure. Physical models for fluid pressure stabilization through a process known as dilatant hardening cannot predict the magnitude of this effect or the apparent monotonic decrease in failure rates with increasing fluid pressure. The purpose of this work is to (1) determine the processes that cause stabilization and (2) develop constitutive relations that include the effect of fluid pressure, which can then be incorporated into models of faulting. To this end, researcher and a graduate student are using experimental rock deformation to quantify the slip weakening distance and fracture energy of faulting as a function of pore fluid pressure magnitude and effective stress magnitude. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.