In this paper, we report friction experiments performed on samples collected from the region hit by the 2008 Wenchuan earthquake in the Longmen Shan fault zone (LFZ) of Sichuan, southwestern China. The materials tested consisted of simulated gouges prepared from intact clay-rich mudstone and sandstone, a calcite limestone, plus a natural fault gouge from a trenched, surface rupture cutting the mudstone and sandstone. The clay-rich samples, including the natural gouge, were dominated by illite and quartz.
In our experiments, we sheared 1-mm-thick gouge layers between saw-cut driver blocks, using a triaxial testing machine at conditions corresponding to ∼2 km depth in the LFZ. Temperature was varied from 25°C to 150°C, and to investigate the velocity dependence of friction, we stepped the shear displacement rate between 1.22 and 0.122 μm/s. Our results show that the natural gouge was more illite-rich and much weaker than the protolith mudstone and sandstone and showed a steady-state friction coefficient of ∼0.4 compared with ∼0.6 for the latter. The limestone gouge displayed values of 0.6–0.7. All samples, except the limestone, showed stable, velocity-strengthening slip. The limestone showed velocity-strengthening at 25°C–50°C, but quasi-static oscillations at 100°C–150°C along with velocity-weakening behavior at 150°C. We apply our results to discuss the role of the sedimentary rocks studied during events such as the Wenchuan earthquake and argue that the clay-rich sediments of the region may have a damping effect upon ruptures propagating from depth, whereas the limestone may accelerate propagation, producing significant stress drops.