Frictional Properties of the Longmenshan Fault Belt Gouges From WFSD-3 and Implications for Earthquake Rupture Propagation

The 2008 M_w 7.9 Wenchuan earthquake generated ∼270 and ∼80 km long surface ruptures along the Longmenshan fault belt, namely the Yingxiu-Beichuan fault (YBF) and the Guanxian-Anxian faults (GAF), respectively. So far, most of the frictional investigations were performed on the YBF gouge materials. Here, we present the results of rotary shear friction experiments performed on the GAF gouges recovered from the depth of ∼1.25 km of the Wenchuan Earthquake Fault Scientific Drilling project-3 along the GAF. The fault gouges, mainly composed of quartz, illite, chlorite, and kaolinite, were sheared at slip velocities V ranging from 10⁻⁵ to 2 m/s and normal stresses from 8.5 to 10 MPa under both room humidity and wet conditions. At any imposed slip velocity, the wet gouges have an apparent friction coefficient lower than the room humidity one. In addition, enhanced velocity-strengthening behavior at intermediate velocities (10⁻² m/s < V ≤ 10⁻¹ m/s) was recognized. We characterized the products using field-emission scanning electron microscopy combined with synchrotron X-ray diffraction analysis. These microanalytical investigations evidence the formation of size-reduced particles (without mineral phase changes) and R- and Y-shears in the principal slip zone (PSZ). Regardless of the ambient conditions, the width of PSZ was proportional to the input frictional work density (the product of shear stress times displacement). Our results support the hypothesis that the GAF preferentially ruptures through wet fault gouges; however, the enhanced velocity-strengthening regime at intermediate velocities may act as a barrier to slip acceleration during fault rupture propagation.