Interseismic fault healing at hydrothermal conditions

Abstract:

Pore fluid plays a critical role in fault zone processes. Elevated pore fluid pressures contribute to shear failure and the creation of shear and dilatant fractures can provide efficient fluid pathways, reducing the fluid pressure. At the same time, fluid-rock interactions result in temperature- and rate-dependent fault healing processes that can strengthen faults, decrease permeability, and promote overpressure. While there is significant evidence for fault healing in natural faults and in lab experiments, the thermal, hydraulic, mechanical, and chemical (THMC) interactions that influence healing are poorly understood. We present preliminary results of triaxial slide-hold-slide experiments to constrain rates and mechanisms of interseismic healing. Experiments were conducted on Westerly granite samples with a 1 mm thick granite gouge layer along a 30˚ sawcut. The tests were run at temperatures of 23˚C and 200˚C with confining and nominal pore pressures of 30 MPa and 10 MPa, respectively. We used an axial displacement rate of 0.1 𝞵m/s during sliding periods. Deionized water was continuously flowed along the inclined sawcut so we could determine in-plane permeability and relative aperture change (inferred from permeability) during the tests. In an experiment conducted at 200˚C, we measured an order of magnitude decrease in permeability, from 4.5x10^-18 to 0.41x10^-18 m², over the course of the 500-hour test. Residence time of fluid in the gouge layer increased from ~30 s to 200 s as flow rate decreased. Hold periods ranged in duration from 10³ s to 10⁶ s. We identified a general trend of increasing peak shear strength with increasing hold time. Following sliding we observed a ~15% recovery in permeability. In an experiment run at 23˚C over 220 hours, the permeability decreased from 1.9x10^-18 to 0.9x10^-18 m² and showed no clear evidence of fault re-strengthening. The differences between room and elevated temperature behavior suggest that the reduction in permeability and re-strengthening of the fault is strongly influenced by the THMC interactions occurring at elevated temperatures. Future work will include SEM analysis of the sawcut surface and gouge layer to look for evidence of the healing mechanisms. Future experiments will look at influence of rock type and gouge thickness as well as degree of fluid disequilibrium.