Weakening Mechanisms of Alpine Fault Gouge in High‐Velocity Shear Experiments

Abstract

AbstractThe Alpine Fault, New Zealand, is a large plate boundary fault with history of major seismic events that frequently ruptured to the surface. To better understand earthquake slip at shallow depth, we analyzed the frictional properties of gouge samples collected at three field exposures distributed along 40 km of the fault trace. The samples are rich in phyllosilicates (30–55%) and quartz/feldspar (41–64%), with small amounts of calcite (4–12%). The gouge samples were sheared in a confined rotary cell under room conditions at a normal stress of 2–3 MPa, under alternating slip velocity steps from 0.002 to 1.47 m/s. We found initial friction coefficients of 0.6–0.8 and gentle velocity strengthening during early slip with short displacement and low velocity. A drastic weakening of more than 50% reduction of the frictional strength occurred after displacements of 2–5 m as slip velocity exceeds ~0.2 m/s, and there was no strength recovery even when slip velocity reduced back to low levels. This weakening was associated with marked temperature rise and intense CO2 and H2O emissions. Based on continuous monitoring of gas pressure, CO2, and H2O concentrations within the gouge chamber, our analysis revealed that the dynamic weakening was triggered by gouge‐zone pressurization, which was driven by thermal decomposition of calcite and dehydration of absorbed water. The pressurization process was enhanced by slip localization and establishment of a low‐permeability shear zone. We further envision that the gouge zone pressurization is likely to cause local overpressure and internal fluidization within of the fine‐grain gouge layer that leads to dynamic weakening.