Slow Faulting Induced by Subcritical Crack Growth at High Pore Fluid Pressures

Zachary Zega, & Wen-lu Zhu

Published August 16, 2021, SCEC Contribution #11554, 2021 SCEC Annual Meeting Poster #163

Under drained conditions, low-temperature brittle deformation depends primarily on the difference between the confining and pore fluid pressures (differential pressure) and not the magnitude pore fluid pressure. Laboratory experiments have shown that high pore fluid pressure causes slow faulting in compact rocks deformed at the same differential pressure (French and Zhu, 2017). The mechanism of the observed stabilizing effect was thought to be dilatant hardening; if the deformation becomes undrained, a propagating fault causes the pore fluid pressure to drop, resulting an increase in effective normal stress that impedes further fault growth. We investigated the effect of pore fluid pressure on fault propagation in porous Darley Dale sandstone samples. At a strain rate of 10-5 s-1, faulting and slip behaviors were comparable under a constant differential pressure, regardless the magnitude of pore fluid pressure, indicating the deformation was drained and dilatant hardening was negligible. At a slower strain rate of 10-6 s-1, fault propagation under higher pressures was slower despite the constant differential pressure. Based on microstructural analysis of the deformed samples, we propose that enhanced subcritical crack growth at high pore fluid pressures was responsible for the observed slow faulting under well-drained conditions.

To further test this idea, we observed brittle faulting in Darley Dale sandstone under a slower strain rate of 10-7 s-1 using a constant differential pressure of 10 MPa with pore fluid pressures ranging from 2 to 180 MPa. Samples were initially loaded at an axial strain rate of 10-5 s-1 until the sample reached roughly 95% of the brittle strength. Subsequently, the strain rate was lowered to 10-7s-1 for the remainder of the experiment. In samples deformed at high pore fluid pressures, weakening proceeded in a stepwise fashion where stress corrosion processes caused rapid stress relaxation followed by subsequent re-strengthening of the sample as the sample approached frictional stress. At low pore fluid pressure, weakening occurred as one stress drop. Our experimental data show the under drained conditions, high pore fluid pressure facilitates slow faulting when the failure is primarily driven by stress corrosion. Time dependent brittle rock deformation is prevalent in Earth’s crust. The observed stabilizing effect of pore fluid pressure has important implications for understanding rupture processes of natural faults.

Key Words
subcritical cracking, creep, pore fluid pressure, slow slip, rock deformation,

Citation
Zega, Z., & Zhu, W. (2021, 08). Slow Faulting Induced by Subcritical Crack Growth at High Pore Fluid Pressures . Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)