Frictional properties of natural gouges from Longmenshan fault zone ruptured during the Wenchuan Mw7.9 earthquake

Abstract

We investigated the properties of frictional sliding of natural gouges collected from Longmenshan fault zone which ruptured during the Wenchuan Mw7.9 earthquake. Based on XRD analysis, five gouge samples of different mineralogical assemblies and mineral contents are chosen for frictional sliding experiments with initial confining pressure of 50MPa and at different temperatures from 25°C to 150°C. To obtain the rate dependence of friction, shearing rates are stepped up and down in the range of 0.0488 to 6.1μm/s. While the friction coefficient of most gouge samples decreases with the clay content following a general trend consistent with previous studies, the organic-matter-bearing sample (PX-3) shows a much lower value than the prediction by the general trend, indicating further strength reduction due to the ~9% organic matter included. We also found a significant effect of temperature on the sliding behavior of organic-matter-bearing and carbonate-rich gouges. When temperature increases to 150°C, the PX-3 sample (with organic matter) exhibits unstable oscillation that indicates velocity-weakening frictional sliding behavior, in contrast to the velocity-strengthening behavior at lower temperatures. The velocity dependence of carbonate-rich samples (PX-1, PX-2) with more than 50wt.% carbonates is also sensitive to temperature and shearing rates, and both samples have smaller velocity dependence of friction (a−b) at higher temperatures. The (a−b) value of the PX-2 sample (with ~80% carbonates) even decreases to the velocity neutral point at 150°C and slow shearing rates.Based on the rate and state friction framework, fast slip nucleation is possible for faults associated with the organic-matter-bearing gouge at 150°C, whereas triggered seismic slips are possible in faults that include the carbonate-rich samples which have a small (a−b) value at 150°C. The temperature-dependent sliding behavior of carbonate-dominant and organic-matter-bearing gouges probably provides a new clue to the problem of transition from shallow aseismic zone to seismic zone in subduction interfaces of the oceanic plate.