The Effect of Undrained Fluid Boundary Conditions on Fault Stability

Pore fluids are ubiquitous throughout the lithosphere and are a major part of the stress distribution along faults. Elevated pore fluid pressure reduces the effective normal stress, allowing slip and potentially changing the mode of faulting.  Mature brittle faults are characterized by deca- to hecto-meter damage zones composed of gouge and complex shear localization fabrics that can host zones of low, anisotropic permeability. Such zones can include undrained pore fluid conditions that may result in a spectrum of slip behaviors including slow slip events. Despite the obvious importance of pore fluids for fault mechanics, their role in dictating fault stability is poorly constrained. Early results for rate-strengthening accretionary wedge materials suggest pore fluid has a stabilizing effect, as the friction parameter (a - b) increases in response to increased pore fluid pressure (P_f ). Here, we describe early stages of a laboratory investigation of the role of fluid pressure on friction rate and memory effects. We present experimental results from rate-weakening synthetic gouge samples at a range of pore fluid pressure conditions. Experiments use a servo-controlled biaxial load frame enclosing a pressure vessel to apply a true triaxial stress-state with pore fluid pressures. Samples are assembled in a double-direct shear configuration with two uniform 3-millimeter-thick gouge layers. Sample forcing blocks include shear wave piezoelectric transducers for ultrasonic monitoring of shear wave amplitude and velocity. We conducted stable sliding experiments at both drained and undrained conditions to explore role of pore fluids on the RSF parameters. Undrained stick-slip experiments were also done at a range of pore fluid pressures to investigate the role of fluid pressure on the nature of fault slip. We explore differences between the drained and undrained conditions with particular attention on changes from rate-weakening to rate-strengthing friction behavior due to localized overpressure. Additionally, we evaluate interplay between the modes of fault slip, due to poroelastic processes which will result in changes in the transmitted shear wave amplitude and velocity. In subduction environments pore fluid pressure can approach lithostatic pressures leading to localized overpressure. Therefore, it is important to understand the contributions of fluids and effective stress state on frictional stability and the mode of fault slip, whether it be aseismic creep, slow slip, or earthquake rupture.