Melting of fault gouge at shallow depth during the 2008 MW 7.9 Wenchuan earthquake, China

Abstract

Typical rocks at shallow depths of seismogenic faults are fluid-rich gouges. During earthquakes, on-fault frictional heating may trigger thermal pressurization and dynamic fault weakening. We show that frictional melting, rather than thermal pressurization, occurred at shallow depths during the 2008 MW 7.9 Wenchuan earthquake, China. One year after the Wenchuan earthquake, we found an ~2-mm-thick, glass-bearing pseudotachylyte (solidified frictional melt) in the fault gouges retrieved at 732.6 m depth from the first borehole of the Wenchuan Earthquake Fault Scientific Drilling Project. The matrix of pseudotachylyte is enriched in barium and cut by barite-bearing veins, which provide evidence of co- and post-seismic fluid percolation. Because pseudotachylyte can be rapidly altered in the presence of percolating fluids, its preservation suggests that gouge melting occurred in a recent large earthquake, possibly the Wenchuan earthquake. Rock friction experiments on fluid-rich fault gouges deformed at conditions expected for seismic slip at borehole depths showed the generation of pseudotachylytes. This result, along with the presence of a second slip zone attributed to the Wenchuan earthquake at 589.2 m depth, implies that during large earthquakes, frictional melting can occur at shallow depths and that seismic slip can be accommodated by multiple faults. This conclusion is consistent with the evidence from surface faulting that multiple ruptures propagated during the Wenchuan earthquake.

Fluid Influx Promotes Local Strengthening of the Creeping Xianshuihe Fault, Eastern Tibet

“…Therefore, the significant fine‐size of angular quartz grains within the fault gouge, truncated calcite veins (Figure 6e), injection gouge veins containing angular particles (Figures 6e, 7b, and 7e), as well as the consistent position between the black fault gouge and fault trace, all suggest that the 3 to 5 cm‐thick black gouge is formed by seismic (or stick‐slip) fault behavior, and that it represents the principal slip zone (PSZ) of the Qianning segment of the Xianshuihe fault. Although the exact thickness of the PSZ associated with a single paleoseismic event is impossible to determine, the rapid implementation of scientific drillings in seismic fault zones after a large earthquake might provide some essential data: for example, the 50–300 μm or 1‐2 mm‐thick fault gouges formed by the 1999 Chi‐Chi earthquake ( M w 7.6) in Taiwan (e.g., Heermance et al., 2003; Kuo et al., 2011), and the ∼200 μm‐thick ultrafine‐grained slip zone and ∼2 mm‐thick glass‐bearing pseudotachylyte which were formed by the 2008 Wenchuan earthquake ( M w 7.9) in the Longmenshan's Yingxiu‐Beichuan fault zone (Kuo et al., 2014; H. Wang et al., 2023). Considering the reworked gouge clasts in BG 2 (Figures 7c and 7d) and the injection gouge vein wedging into BG 1 from BG 3 (Figure 7e), the 3 to 5 cm‐thick PSZ gouge in the Cunielongba exposure thus records repeated seismic events.…”

Coseismic Frictional Heating With Concomitant Hydrothermal Fluid Circulation Revealed by Rock Magnetic Properties of Fault Rocks From the Rupture of the 2008 Wenchuan Earthquake, China

“…Thanks to the presence of multiple generations of pseudotachylyte outcropped along the Yingxiu-Beichuan fault zone, H. Wang et al (2015) suggested that friction melt lubrication, thermal pressurization, and mechanical lubrication could operate together during the coseismic slip. Recently, however, with finding of melted fault gouge (present as glass-bearing pseudotachylytes) at shallow depth of the rupture zone, frictional melting, rather than thermal pressurization, is suggested to have occurred at shallow depths during the Wenchuan earthquake (H. Wang et al, 2023). Obviously, these make the dynamic weakening of the faults, especially the occurrence of thermal pressurization, still debated.…”

Experimental and Numerical Perspectives on The Role of Localization and Heating in Small-Magnitude Earthquakes

“…0.01-0.3) over a given slip weakening distance Dw (Di . They include flash heating and associated weakening at early stages of slip (Proctor et al, 2014;Yao et al, 2016a, b), frictional melting at grain contacts (Wang et al, 2023), thermal decomposition (De Paola et al, 2011), bulk thermo-mechanical pore fluid pressurization (TP mentioned above; Aretusini et al, 2021;Faulkner et al, 2011;Hunfeld et al, 2021;Togo et al, 2011), silica gel production and lubrication (Di Toro et al, 2004;Rowe et al, 2019), thermochemical pressurization (Brantut et al, 2010), thermally activated super-plasticity (Chen et al, 2021;De Paola et al, 2015), pore water phase transitions from the liquid to the gaseous or supercritical state (Chen et al, 2017;Ferri et al, 2010: Oohashi at al., 2011, and flash pressurization (combined flash heating and TP at asperity contacts; Chen et al, 2023;Yao et al, 2018). The activation of any of the above-mentioned weakening mechanisms is typically dependent on the amount of frictional heat generated, which is controlled by experimental conditions (e.g.…”

“…Feng et al, 2023;Gomila et al, 2021;Violay et al, 2013Violay et al, , 2014Violay et al, , 2015Violay et al, , 2019Yao et al, 2023) or shearing of mm-thick simulated fault gouge layers (e.g. Boulton et al, 2017;Chen et al, 2023;Hunfeld et al, 2021;Kuo et al, 2021Kuo et al, , 2022Wang et al, 2023;Yao et al, 2018). Across different lithologies, these studies have consistently reported the effectiveness of TP at scales ranging from the bulk shear zone to grain-to-grain asperity contacts.…”