Jialiang Si

Continuous Permeability Measurements Record Healing Inside the Wenchuan Earthquake Fault Zone

Water at the Bottom of a Well Earthquakes generate numerous fractures as they propagate through an underground fault zone. These fractures strongly influence the way in which fluids flow in the subsurface, and the permeability of fault zones is often used as a proxy for the extent of fracturing. Following the 2008 Mw 7.9 Wenchuan earthquake in central China, several wells were drilled in and around the fault zone to understand the mechanics of the earthquake. Because the bottoms of these deep boreholes were open, the water levels in the wells were sensitive to tidal forces acting on the surrounding rock. Through continuous measurements of water levels over 1.5 years, Xue et al. (p. 1555) found that the rate at which water was pumped in and out of the borehole was proportional to the permeability of the fault zone, providing a direct way to measure the evolution of the hydrologic properties of a fault zone following a major earthquake. Permeability decreased ∼25% during that time, suggesting that fractures generated in fault zones heal relatively rapidly. Measurements of permeability inside a fault zone after a major earthquake reveal rapid healing of fractures. Permeability controls fluid flow in fault zones and is a proxy for rock damage after an earthquake. We used the tidal response of water level in a deep borehole to track permeability for 18 months in the damage zone of the causative fault of the 2008 moment magnitude 7.9 Wenchuan earthquake. The unusually high measured hydraulic diffusivity of 2.4 × 10−2 square meters per second implies a major role for water circulation in the fault zone. For most of the observation period, the permeability decreased rapidly as the fault healed. The trend was interrupted by abrupt permeability increases attributable to shaking from remote earthquakes. These direct measurements of the fault zone reveal a process of punctuated recovery as healing and damage interact in the aftermath of a major earthquake.

Gouge graphitization and dynamic fault weakening during the 2008 Mw 7.9 Wenchuan earthquake

The Longmenshan fault that ruptured during the 2008 Mw 7.9 Wenchuan (China) earthquake was drilled to a depth of 1200 m, and fault rocks including those in the 2008 earthquake slip zone were recovered at a depth of 575–595 m. We report laboratory strength measurements and microstructural observations from samples of slip zone fault rocks at deformation conditions expected for coseismic slip at borehole depths. Results indicate that the Longmenshan fault at this locality is extremely weak at seismic slip rates. In situ synchrotron X-ray diffraction analysis indicates that graphite was formed along localized slip zones in the experimental products, similar to the occurrence of graphite in the natural principal slip zone of the 2008 Wenchuan rupture. We surmise that graphitization occurred due to frictional heating of carbonaceous minerals. Because graphitization was associated with strong dynamic weakening in the experiments, we further infer that the Longmenshan fault was extremely weak at borehole depths during the 2008 Wenchuan earthquake, and that enrichment of graphite along localized slip zones could be used as an indicator of transient frictional heating during seismic slip in the upper crust.

Long-term temperature records following the Mw 7.9 Wenchuan (China) earthquake are consistent with low friction

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Fault gouge graphitization as evidence of past seismic slip

One moderateto large-magnitude earthquake (M > 6) nucleates in Earth’s crust every three days on average, but the geological record of ancient fault slip at meters-per-second seismic velocities (as opposed to subseismic slow-slip creep) remains debated because of the lack of established fault-zone evidence of seismic slip. Here we show that the irreversible temperature-dependent transformation of carbonaceous material (CM, a constituent of many fault gouges) into graphite is a reliable tracer of seismic fault slip. We sheared CM-bearing fault rocks in the laboratory at just above subseismic and at seismic velocities under both water-rich and water-deficient conditions and modeled the temperature evolution with slip. By means of micro-Raman spectroscopy and focused-ion beam transmission electron microscopy, we detected graphite grains similar to those found in the principal slip zone of the A.D. 2008 Wenchuan (Mw 7.9) earthquake (southeast Tibet) only in experiments conducted at seismic velocities. The experimental evidence presented here suggests that high-temperature pulses associated with seismic slip induce graphitization of CM. Importantly, the occurrence of graphitized fault-zone CM may allow us to ascertain the seismogenic potential of faults in areas worldwide with incomplete historical earthquake catalogues. INTRODUCTION Fault rocks accommodate most of the slip during earthquakes (Sibson, 2003), but their record of deformation events occurring at typical seismic slip rates of ~1 m/s, as opposed to slow-slip and aseismic creep events, remains uncertain because of the lack of unequivocal characteristics (Cowan, 1999; Rowe and Griffith, 2015). Seismic slip is thought to be accommodated in centimeterto submillimeter-thick slipping zones, and localized frictional sliding may trigger processes such as flash heating and melting, dehydration and decarbonation reactions, and thermal decomposition of fault rocks (Sibson, 2003; Di Toro et al., 2011). Because of the relatively high seismic slip rates at seismogenic depths, the natural slipping zone should record an abrupt and transient increase in temperature during earthquakes. Importantly, disordered organic compounds or amorphous carbonaceous material (CM) can be progressively and irreversibly transformed into stable graphite through thermally activated graphitization (Buseck and Beyssac, 2014). Therefore, the progressive increase in crystallographic order of CM associated with graphitization is widely utilized as an indicator of the maximum temperatures achieved by sedimentary and metamorphic rocks (Barker and Goldstein, 1990; Beyssac et al., 2002). Because CM is also found in natural fault zones, its graphitization may provide valuable information on earthquake mechanics (Oohashi et al., 2012). Fault-zone graphitization has been proposed for the principal slip zone (PSZ) of the Longmenshan thrust fault that ruptured in a devastating A.D. 2008 Mw 7.9 Wenchuan earthquake in southeast Tibet (Kuo et al., 2014). According to data from the Wenchuan Earthquake Fault Scientific Drilling-1 project borehole 1 (WFSD-1), at 590 m depth, the active fault zone includes an ~54-cm-thick black gouge made of quartz, feldspar, clay minerals, plus graphite and CM, surrounded by an ~2-m-thick fault breccia made of quartz, feldspar, calcite, clay minerals, and CM (mainly poorly crystalline anthracite), but without graphite (Fig. 1; Li et al., 2013; Si et al., 2014). Wang et al. (2014) demonstrated that CM within the Wenchuan fault zone originated from adjacent host rocks (Late Triassic Xujiahe Formation). Kuo et al. (2014) speculated that gouge graphitization occurred within CM-bearing fault gouges during the 2008 Mw 7.9 Wenchuan earthquake. However, it remained unclear the process responsible for CM graphitization, under which ambient and deformation conditions it occurred, and, more relevant, if CM graphitization could be associated only with seismic slip. These crucial questions are addressed here, where we also demonstrate that the experimental products obtained at seismic slip rates are almost identical to those found in the PSZ of the Longmenshan fault, making CM graphitization a powerful tool to investigate the seismogenic potential of active faults, especially if cropping out in areas with incomplete historical earthquake catalogues. EXPERIMENTAL METHODS To investigate the graphitization process of the CM-bearing materials, we sheared with the rotary-shear machine SHIVA (Di Toro et al., 2010; Niemeijer et al., 2011) the graphite-free rocks of the fault breccia retrieved from 589.32 m depth. The bulk fault breccia was gently pulverized down to <250 μm in size and poured into a ring-shaped metal sample holder designed for confinement of non-cohesive materials (Smith et al., 2012). The gouges were sheared for 3 m of slip at equivalent slip rates, V, of 0.0003 m/s (simulating just above subseismic, referred as subseismic hereafter) and 3 m/s (seismic) under a normal stress of 8.5 MPa. The experiments were conducted on 5 g of gouges (corresponding to an initial thickness of ~3 mm) at room temperature and humidity and, by the addition of 0.5 g of distilled water, at water-dampened conditions. Deformed samples were collected for microanalytical investigations including micro-Raman spectroscopy, field-emission scanning electron microscopy (FESEM), and focused ion beam–transmission electron microscopy (FIB-TEM) with energy dispersive X-ray spectroscopy (EDS). The natural materials of the active fault zone (black gouge and breccia) were investigated with microRaman spectroscopy and compared with the experimental products (Fig. 1). RESULTS The mechanical data, consistent with previous studies (Oohashi et al., 2011; Rutter et al., 2013; Kuo et al., 2014; Kouketsu et al., 2017), resulted *E-mail: liweikuo@ncu.edu.tw GEOLOGY, November 2017; v. 45; no. 11; p. 1–4 | Data Repository item 2017329 | doi:10.1130/G39295.1 | Published online XX Month 2017