Li-Wei Kuo

Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project

Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data. The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced large slip (8 to 10 metres) at or near the surface, which is accessible to borehole drilling and provides a rare opportunity to sample a fault that had large slip in a recent earthquake. Here we present the retrieved cores from the Taiwan Chelungpu-fault Drilling Project and identify the main slip zone associated with the Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault sample was directly compared to the seismic fracture energy determined from near-field seismic data. From the comparison, the contribution of gouge surface energy to the earthquake breakdown work is quantified to be 6 per cent.

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.

An earthquake slip zone is a magnetic recorder

During an earthquake, the physical and the chemical transformations along a slip zone lead to an intense deformation within the gouge layer of a mature fault zone. Because the gouge contains ferromagnetic minerals, it has the capacity to behave as a magnetic recorder during an earthquake. This constitutes a conceivable way to identify earthquakes slip zones. In this paper, we investigate the magnetic record of the Chelungpu fault gouge that hosts the principal slip zone of the Chi-Chi earthquake (Mw 7.6, 1999, Taiwan) using Taiwan Chelungpu-fault Drilling Project core samples. Rock magnetic investigation pinpoints the location of the Chi-Chi mm-thick principal slip zone within the 16-cm thick gouge at ~1 km depth. A modern magnetic dipole of Earth magnetic field is recovered throughout this gouge but not in the wall rocks nor in the two other adjacent fault zones. This magnetic record resides essentially in two magnetic minerals; magnetite in the principal slip zone, and neoformed goethite elsewhere in the gouge. We propose a model where magnetic record: 1) is preserved during inter-seismic time, 2) is erased during co-seismic time and 3) is imprinted during post-seismic time when fluids cooled down. We suggest that the identification of a stable magnetic record carried by neoformed goethite may be a signature of friction-heating process in seismic slip zone.

Fault mirrors in seismically active fault zones: A fossil of small earthquakes at shallow depths

Fault mirrors (FMs) are naturally polished and glossy fault slip surfaces that can record seismic deformation at shallow depths. They are important for investigating the processes controlling dynamic fault slip. We characterize FMs in borehole samples from the hanging wall damage zone of the active Hsiaotungshi reverse fault, Taiwan. Here we report the first documented occurrence of the combination of silica gel and melt patches coating FMs, with the silica gel resembling those observed on experimentally formed FMs that were cataclastically generated. In addition, the melt patches, which are unambiguous indicators of coseismic slip, suggest that the natural FMs were produced at seismic rates, presumably resulting from flash heating at asperities on the slip surfaces. Since flash heating is efficient at small slip, we propose that these natural FMs represent fossils of small earthquakes, formed in either coseismic faulting and folding or aftershock deformation in the active Taiwan fold-and-thrust belt.

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