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

Abstract

The 2008 Mw 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.

Fault Weakening During Short Seismic Slip Pulse Experiments: The Role of Pressurized Water and Implications for Induced Earthquakes in the Groningen Gas Field

“…We note in the first place that the thickness changes during the subsequent seismic slip pulse tests were much smaller (<0.1 mm), sometimes even showing a dilatation. The pre‐shear compaction was inferred to mostly stem from the formation of shear bands, characterized by strong grain size reduction and slip localization (cf., Kuo et al., 2022; Rempe et al., 2020). As revealed later by the microstructural analysis, the larger contraction (i.e., in Carboniferous shale gouges) was indicative of more compact gouge layers.…”

“…The pre‐shear test is expected to accomplish comminution, compaction and shear band formation (Faulkner et al., 2018). Its purpose is to suppress the fluid pressurization effects due to shear‐enhanced gouge compaction during the high‐velocity slip pulse (Chen et al., 2017; Kuo et al., 2022; Rempe et al., 2020).…”

“…A dramatic drop in apparent friction coefficient (i.e., shear stress/applied effective normal stress) to values around 0.1 is always observed in such experiments, regardless of the lithology, and is interpreted to be related to temperature rise due to frictional heating (Di Toro et al., 2011; Rice, 2006). Because of the sealing difficulties, most previous HVF studies on water‐saturated gouges report the frictional strength as the apparent friction coefficient, with poor knowledge of the pore pressures and thus the solid‐to‐solid friction (Kuo et al., 2021, 2022; Yao et al., 2018). To resolve the hydro‐thermo‐mechanical processes responsible for the observed weakening, recent efforts have been made to enable the monitoring of the thermal, dilatant and fluid drainage conditions of the simulated fault zones (Aretusini et al., 2021; Oohashi et al., 2015; Violay et al., 2014).…”