Earthquake Fault Scientific Drilling Project Research Articles

Knowledge-based machine learning for mineral classification in a complex tectonic regime of Yingxiu-Beichuan fault zone, Sichuan basin

The present study aims to better understand the mineralogy and thermal structure of the Yingxiu-Beichuan fault zone (YBFZ), Sichuan basin, China, which was lacking previously. The previous research on the Wenchuan earthquake Fault Scientific Drilling (WFSD) project was focused on mineral classification, fault analysis, and geochemical research utilizing original logs using WFSD-1 well and at a shallow depth of 700 meters. No investigations are conducted at a depth of around 1550 meters utilizing multiple dimensionality reduced well-logs. Thereafter, we sought to categorize the minerals along the YBFZ using Machine Learning (ML) technologies and concentration-number (C–N) modeling using multiple WFSD-1 and WFSD-2 wells. In the categorization of rocks, three classifiers are discussed: Support Vector Machines (SVM), Feed Forward Back Propagation (BPNN), and Radial Basis Function Neural Networks (RBFN). Linear Discriminant Analysis (LDA) and Principal Component Analysis (PCA) are used to normalize the log data in a geologically complicated region. By using the BPNN classifier to PCA, LDA, and all 17 logs, we achieved respective accuracy rates of 75.54%, 86.32%, and 72.54%, which are higher than the accuracy rates of RBFN (76.03%, 77.01%, and 65.3%), and SVM (74.45%, 78.03%, and 70.06%). These results suggest that BPNN shows improved accuracy rates for mineral classification in a complex tectonic regime. In addition, concentration-number (C–N) fractal model technique and log-log plots are also used to characterize geothermal features and the results of C–N modeling supports the results of ML models. High gamma ray (GR) ranges of 349.7API, heat production (HP) ranges of 5.5 μW/m3, and low thermal conductivity (TC) ranges of 0.28 W/km show that the fault region is associated with comparatively strong radiogenic activity compared to its surroundings.

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

AbstractTypical 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 postseismic 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.

Creeping Along the Guanxian‐Anxian Fault of the 2008 Mw 7.9 Wenchuan Earthquake in the Longmen Shan, China

AbstractCrustal active faults can slip either steadily by aseismic creep, or abruptly by earthquake rupture. Creep can continuously relax the stress and reduce the magnitude of earthquakes. Identifying the slip behavior of active faults plays a crucial role in predicting and preventing earthquake disasters. Here we carried out multiscale structural analyses of fault rocks from the Guanxian‐Anxian fault zone (GAF) surface rupture of the 2008 Mw 7.9 Wenchuan earthquake (Longmen Shan thrust belt, China), and from the 3rd pilot borehole of the Wenchuan earthquake Fault Scientific Drilling project (WFSD‐3P). Results revealed that distributed R1 shears, pressure solution seams, partly dissolved clasts and mineral grains (e.g., quartz and albite), and newly formed phyllosilicate minerals all prevail in the clay‐rich fault rocks at different depths. Microstructural observations combined with X‐ray diffraction analysis show that the majority of clay minerals in the fault gouges were newly formed by pressure solution. The growth of new phyllosilicates prevents soluble grains from coalescing, thereby maintaining fast diffusive paths along solution seams, which promotes the formation of pressure solution. Both weak phyllosilicate minerals and pressure solution can control aseismic creep. Therefore, the slip behavior of the GAF is dominated by long‐term aseismic creep due to the interplay between new phyllosilicate minerals and pressure solution. Based on P wave velocity and strata distribution, we propose a model in which the GAF is creeping at shallow depths and locked at greater depths, as a possible explanation for the Wenchuan earthquake rupture mechanism.

Research on In-situ Stress Testing of Deephole Cores from Earthquake Faults Using the Inelastic Strain Recovery Method1

The Wenchuan Earthquake Fault Scientific Drilling Project, a scientific drilling project that first responded to the Earthquake, performed deep 3D in-situ stress measurements using the inelastic strain recovery of core method for the first time under conditions whereby the borehole breakout method and hydraulic fracturing method could not be applied. The inelastic deformation of the deephole cores under the action of 3D stresses was simulated based on the inelastic strain recovery of viscoelastic body theory and core testing theory and the direction & magnitude of 3 major stresses were calculated under the precondition of homogeneous isotropy. In the field laboratory of the WFSD Project, the testing system developed was used to test the normal strain of inelastic recovery strain of deephole cores in 6 independent directions and determine the direction & magnitude of 3D in-situ stresses of the test points. In order to verify the effectiveness of the inelastic strain recovery method, an analysis was given into the testing result of #1 hole of the WFSD Project, which perfectly matches the result obtained by means of focal mechanism solution and other stress measurement methods, indicating that the inelastic strain recovery method is reasonable and reliable and is more applicable under complex geologic conditions whereby other in-situ stress testing methods cannot be applied. The method provides a new means for performing in-situ stress measurements using the core method under large-depth drilling conditions.

Real-time hydrogen mud logging during the Wenchuan earthquake fault scientific drilling project (WFSD), holes 2 and 3 in SW China

We present hydrogen gas concentration data for drilling mud extracted during the Wenchuan Earthquake Fault Scientific Drilling (WFSD; holes 2 and 3), measured on-line during drilling in SW China. Hydrogen influx into the well at depth is largely dependent on lithology and fracture density. In hole WFSD-2, the average background concentration of hydrogen is lower in granitic rocks than in sedimentary formations. More than five major hydrogen influxes were detected while drilling through the sedimentary formations, and two hydrogen-rich zones were detected in the granite (1240–1243 m and 1383.5–1405 m depth). In hole WFSD-3, mud extracted from a tectonic breccia contains high hydrogen concentrations at depths of 600–1000 m depth, and relatively low concentrations below 1000 m depth. In both holes, we observe a lack of hydrogen in the center of the fault zone and high concentrations of hydrogen in fractured zones. Hydrogen concentration in both holes displays significant vertical heterogeneity, and is positively correlated with fracture density. Hydrogen was likely sourced from interaction between water and fresh silicate minerals surfaces that were exposed during faulting, from the mantle, and from faulting events. Fracture zones provide the main channels for the migration of hydrogen gas. The variations in background hydrogen concentrations relate to changes in porosity and permeability. The two hydrogen-rich zones (642.36–676.22 m, and 1383.5–1405 m) were likely caused by earthquake activity or far-field triggering during WFSD-2 drilling. The results of this study provide gas data that can be used to model fault activity. It is important to consider the relationship between hydrogen gas and the seismic cycle, and to use such data to identify seismic precursors.

A Formula of Potassium Polymer Sulfonated Drilling Fluid Developed Based on Rock Microscopic Analysis

During the rapid coring drilling in the Wenchuan earthquake Fault Scientific Drilling project, the deep fault zone is characterized with extremely broken strata and strong creeping, which made the drilling tasks very difficult. The key technology to drilling success is a suitable drilling fluid system. To design such a drilling fluid system with desired properties, the mineral composition of rock samples from deep fault zone was analyzed using X-Ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) absorption spectrum. Based on the results, a potassium-based polymer sulfonated drilling fluid was developed. This drilling fluid system possesses a variety of advantages including good quality of filter cake, strong anti-pollution ability, anti-collapse ability and inhibition ability, and great compatibility with treating agent. Also, the composition of this drilling fluid is simple, easy to prepare and at a relatively low cost. The study of potassium sulfonated drilling fluid system is of great significance for the rapid core drilling in deep fault zone.

Use of spectral gamma ray as a lithology guide for fault rocks: A case study from the Wenchuan Earthquake Fault Scientific Drilling project Borehole 4 (WFSD-4)

The main purpose of the Wenchuan Earthquake Fault Scientific drilling project (WFSD) was to produce an in-depth borehole into the Yingxiu–Beichuan (YBF) and Anxian–Guanxian faults in order to gain a much better understanding of the physical and chemical properties as well as the mechanical faulting involved. Five boreholes, namely WFSD-1, WFSD-2, WFSD-3P, WFSD-3 and WFSD-4, were drilled during the project entirety. This study, therefore, presents first-hand WFSD-4 data on the lithology (original rocks) and fault rocks that have been obtained from the WFSD project. In an attempt to determine the physical properties and the clay minerals of the lithology and fault rocks, this study analyzed the spectral gamma ray logs (Total gamma ray, Potassium, Thorium and Uranium) recorded in WFSD-4 borehole on the Northern segment of the YBF. The obtained results are presented as cross-plots and statistical multi log analysis. Both lithology and fault rocks show a variability of spectral gamma ray (SGR) logs responses and clay minerals. This study has shown the capabilities of the SGR logs for well-logging of earthquake faults and proves that SGR logs together with others logs in combination with drill hole core description is a useful method of lithology and fault rocks characterization.

Hydraulic properties of samples retrieved from the Wenchuan earthquake Fault Scientific Drilling Project Hole‐1 (WFSD‐1) and the surface rupture zone: Implications for coseismic slip weakening and fault healing

AbstractIn this study, we report the hydraulic properties of samples recovered from the first borehole of the Wenchuan earthquake Fault Scientific Drilling and from outcrops associated with the surface rupture zone of the 2008 Wenchuan earthquake. Compositional and microstructural analyses have also been performed on selected samples. Using the pore pressure oscillation method, the permeability measurements show that (1) fault gouge samples have low permeabilities, decreasing from 2 × 10−18 m2 at an effective pressure (Pe) of 10 MPa (equivalent to an in situ depth of 600 m) to 9 × 10−21 m2 at 155 MPa. (2) Intact and cemented samples are impermeable with permeabilities less than 2 × 10−20 m2 at 10 MPa. (3) Fractured samples have variable permeabilities, ranging from 3 × 10−15 to 1 × 10−20 m2 at 10 MPa, and are most insensitive to changes in the effective pressure. (4) Granitic cataclasites have a moderate permeability at low pressure (i.e., 10−16 to 10−17 m2 at 10 MPa); which decreases rapidly with increasing Pe. Hydraulic conduction of the fault is believed to be influenced by the permeability of the fractures developed, which is controlled by the density, aperture, and/or connectivity of the fractures. Microstructural and compositional analyses of the samples indicate that the fault zone heals through chemically mediated fracture closure related to mineral precipitation, possibly assisted by pressure solution of stressed fracture asperities. Although other weakening mechanisms remain possible, our laboratory measurements combined with numerical modeling reveal that thermal/thermochemical pressurization, perhaps leading to gouge fluidization, played an important role in the dynamic weakening of the Wenchuan earthquake, at least in the study area.

High‐velocity frictional strength of Longmenshan fault gouge and its comparison with an estimate of friction from the temperature anomaly in WFSD‐1 drill hole

AbstractThis paper addresses the issue of whether high‐velocity friction experiments yield frictional strengths of fault materials that are consistent with the level of friction estimated from the postseismic temperature anomaly measured across a coseismic fault zone. Experiments were conducted on gouge (composed of quartz, dolomite, illite, albite, and other clay minerals), collected from a large outcrop of the Yingxiu‐Beichuan fault zone and from Wenchuan Earthquake Fault Scientific Drilling project hole‐1 drill cores in Hongkou, Sichuan Province, China. This fault is a major fault of the Longmenshan fault system that caused the 2008 Mw7.9 Wenchuan earthquake. All experiments revealed dramatic weakening at high velocities with peak friction coefficient μp of 0.07~0.35 and steady state friction coefficient μss of 0.02~0.15 for wet gouge, as compared with μp = 0.49~0.86 and μss = 0.12~0.21 for dry gouge. The average friction coefficients over a displacement of 5.5 m (coseismic displacement) extrapolated to the normal stress in the fault zones are 0.1~0.05 and 0.06~0.03 for dry and wet gouges, respectively. The average friction coefficients of dry and wet gouges are within the upper and lower bounds of friction coefficients estimated from the temperature profiles, and there are overall agreements between the two sets of data that are completely independent. Dry and wet gouges are characterized by highly sheared slip zones often forming overlapping slip‐zone structures and by broad shear zones, respectively. Wet gouge textures are perhaps closer to those of natural fault zone, but repeated slip experiments are needed to determine the textural evolution of wet gouge.

Lithological and structural characterization of the Longmen Shan fault belt from the 3rd hole of the Wenchuan Earthquake Fault Scientific Drilling project (WFSD-3)

Drilling in an active fault quickly after a large earthquake is an effective way to study earthquake mechanisms. In order to better understand the mechanical, physical, and chemical characteristics of the faults that ruptured during the 2008 Wenchuan earthquake (Mw 7.9), six boreholes were drilled on the two main strands (Yingxiu–Beichuan and Guanxian–Anxian faults) by the Wenchuan earthquake Fault Scientific Drilling project (WFSD). This paper focuses on the cores from the WFSD-3 borehole which drilled across the Guanxian–Anxian fault. A detailed petrological study shows that fault gouge and fault breccia are developed in the WFSD-3 cores in the Late Triassic Xujiahe Formation. The thicknesses of fault gouge range from ~1 mm to ~2.3 m. According to the characteristics of the fault rock combinations and their distribution, at least 22 subsidiary fault zones were recognized in the WFSD-3 cores. The Guanxian–Anxian fault zone is composed of fault rocks from 1192 to 1250.09 m depth, with a real thickness of ~50 m (~60 m thick in the WFSD-3 cores), and an actual damage zone of ~160 m (~980–1192 m depth in the WFSD-3 cores), and shows characteristics of multiple high-strain fault cores. The damage zone is only present in the hanging wall. The actual total thickness of the Guanxian–Anxian fault zone is ~210 m. Based on the analyses of comprehensive logging data, characteristics of the fault gouge, and seismic fault structures, the principal slip zone for the Wenchuan earthquake is identified in the black fault gouge at 1249.95 m depth in the cores, which lies almost at the bottom of the Guanxian–Anxian fault zone, and is also confirmed by surface rupture zone observations. The slip plane of the Wenchuan earthquake is a low-angle thrust fault with a dip angle of ~38° as estimated from the results of the WFSD-3 core analyses. The results from WFSD-1 showed that the Yingxiu–Beichuan segment is a high-angle thrust fault striking NW with a dip angle of ~65°. These two fault segments have different thicknesses and fault structures, which may suggest different faulting mechanisms and evolution history.

Magnetic properties of the Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1), Sichuan Province, China

We measured the magnetic susceptibility of the core from the first borehole of the Wenchuan Earthquake (May 12, 2008, Mw 7.9) Fault Scientific Drilling Project (WFSD-1) at 1-cm intervals. The correlations between magnetic susceptibility anomalies and fault rock occurrence are shown by a few fault zones in the WFSD-1 core. The values for the mass and ferromagnetic material magnetic susceptibility for the sample at 589.25-m depth are higher than those for the other samples. All the thermomagnetic curves display a rapid increase in slope after 380°C, and a marked peak occurs at about 510°C in the heating curves. The cooling curves are clearly higher than the heating curves. The saturation magnetization (Ms) shows a significant peak at a depth of 589.25 m, as do the mass magnetic susceptibility and the ferrimagnetic magnetic susceptibility. The mechanism principally responsible for the high magnetic susceptibility at a depth of 589.25 m might be the production of new magnetite from iron-bearing silicates (e.g., chlorite) or clays caused by frictional heating during seismic slip. Therefore, we suggest that the presence of high magnetic susceptibility fault gouges in the same country rock can be considered as an indicator of earthquakes or seismic signatures.

Determination of three-dimensional in situ stresses by anelastic strain recovery in Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1)

Abstract The Wenchuan Earthquake Fault Scientific Drilling Project was implemented rapidly after the great 12 May 2008 earthquake (Mw 7.9) to better understand rupture mechanisms of the seismic faults. The first borehole of the project, WFSD-1 was located in Hongkou Township, Dujiangyan City, Sichuan Province, China on the hanging wall of the Yingxiu–Beichuan fault, which underwent large dextral and vertical displacement during the earthquake. The near-vertical borehole was 1201 m long. In-situ stresses were measured in rock samples from depths between 424 and 1173 m drilling depth by the anelastic strain recovery (ASR) method. The average trend of the maximum principal stress σ1 was N309° (ranging from N291° to N325°), rotated with increasing borehole depth from NW–SE to WNW–ESE. The magnitude of the in-situ maximum principal stress was estimated to be 35.3 MPa at the depth of 1173 m. The relations between horizontal and vertical stresses are vertical stress σv > maximum horizontal stress σH > minimum horizontal stress σh at depths above 424 m, σH > σh > σv at depths from 424 m to 800 m, and σH > σv > σh below the depth of 800 m. These indicate that the stress states are in a normal faulting stress regime above 424 m; a reverse faulting regime from 424 m to 800 m and a dextral strike-slip regime below 800 m. The stress measurements from WFSD-1 are consistent with the focal mechanism solution in which the mainshock was dominated by thrusting accompanied by dextral strike-slip motion. The orientations of σ1 are roughly consistent with the tectonic displacement direction of the Longmenshan area, which provides further evidence of NW–SE movements representing compression of the Songpan–Ganzi block toward the Sichuan basin.

Structural and physical property characterization in the Wenchuan earthquake Fault Scientific Drilling project — hole 1 (WFSD-1)

The Wenchuan earthquake Fault Scientific Drilling project (WFSD) started right after the 2008 Mw 7.9 Wenchuan earthquake to investigate its faulting mechanism. Hole 1 (WFSD-1) reached the Yingxiu–Beichuan fault (YBF), and core samples were recovered from 32 to 1201.15 m-depth. Core investigation and a suite of geophysical downhole logs (including P-wave velocity, natural gamma ray, self-potential, resistivity, density, porosity, temperature, magnetic susceptibility and ultrasound borehole images) were acquired in WFSD-1. Integrated studies of cores and logs facilitate qualitative and quantitative comparison of the structures and physical properties of rocks. Logging data revealed that the geothermal gradient of the volcanic Pengguan complex (above 585.75 m) is 1.85 °C/100 m, while that of the sedimentary Xujiahe Formation (below 585.75 m) is 2.15 °C/100 m. In general, natural gamma ray, resistivity, density, porosity, P-wave velocity and magnetic susceptibility primarily depend on the rock lithology. All major fault zones are characterized by high magnetic susceptibility, low density and high porosity, with mostly low resistivity, high natural gamma ray and sound wave velocity. The high magnetic susceptibility values most likely result from the transformation of magnetic minerals by frictional heating due to the earthquake. The YBF exposed in WFSD-1 can be subdivided into five different parts based on different logging responses, each of them corresponding to certain fault-rocks. The high gamma radiation, porosity and P-wave velocity, as well as low resistivity and temperature anomalies indicate that the Wenchuan earthquake fault zone is located at 585.75–594.5 m-depth, with an average inclination and dip angle of N305° and 71°, respectively. The fact that the fracture directions in the hanging wall and footwall are different suggests that their stress field direction is completely different, implying that the upper Pengguan complex may not be local.

Isotope geochemistry of mercury and its relation to earthquake in the Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1)

Based on the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD), we first investigated the distribution of the total mercury (THg) from 600 m to 1035 m of WFSD Hole-1 core (WFSD-1). The concentrations of THg in the fault gouge are significantly higher than in the sandstone, shale, siltstone, and cataclasite in WFSD-1. The THg concentrations in the main seismic fault zone range from 9.9 to 73.5 ng/g while those under the main fault range from 20.5 to 36 ng/g. To determine the source of Hg, typical rocks from different depths were chosen for Hg isotope analysis. The results show that δ202Hg in the main seismic fault zone range from − 0.11‰ to − 2.68‰, and, below the seismic fault zone, δ202Hg are from − 0.64‰ to − 1.33‰. The seismic fault rocks are more enriched in THg and have a larger variation of δ202Hg compared to the other rocks. Mercury has different isotopic compositions and fractionation mechanisms in the fault zone, which are mainly affected by the earthquake and the fluid. The mercury isotope compositions reflect the fluid activities at the main fault zone. We demonstrate that mercury stable isotope ratios could serve as an effective tool for tracing mercury sources and monitoring and predicting earthquakes.

Real time fluid analysis during drilling of the Wenchuan Earthquake Fault Scientific Drilling Project and its responding features

Abstract The Wenchuan Earthquake Fault Scientific Drilling project was established shortly after the Wenchuan Earthquake. Several on-site laboratories were built to perform the real time fluid analysis during drilling simultaneously. The concentrations of argon, methane, hydrogen, carbon dioxide, helium, nitrogen, oxygen and radon in drilling mud gas were determined during the entirely process of drilling. The setup for real time fluid analysis was stability for long time. The mud gas such as methane and radon yielded low concentrations above the Principal Slip Zone (PSZ), whereas yielded high concentrations under the PSZ. The real time fluid data might provide the real time information for identifying and validating of the PSZ in the deep fault zone. The gas concentration showed abnormal fluctuation during the Ms 4.0 earthquake on April 27, 2010. The abnormality occurred one hour before the earthquake, and ended half an hour after the earthquake. The real time fluid analysis during drilling might have captured the signal of the Ms 4.0 earthquake at a quarter past six on April 27, one strong and nearest aftershock.

Fracture analysis and determination of in-situ stress direction from resistivity and acoustic image logs and core data in the Wenchuan Earthquake Fault Scientific Drilling Borehole-2 (50–1370 m)

After the Wenchuan Earthquake on May 12th, 2008, the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD) was initiated in order to investigate the structure of the fault zones and the mechanism of the earthquake. The WFSD contains four boreholes (WFSD-1, WFSD-2, WFSD-3 and WFSD-4) lying at the maximum displacement locations along the Yingxiu-Beichuan fault zone and the Guanxian-Anxian fault zone, and WFSD-2 is the second borehole and is still being drilled. Core samples, resistivity and acoustic image logging data were acquired from 50 to 1370m. The natural fractures, borehole breakouts, drilling-induced fractures and drilling-enhanced natural fractures were identified from the cores and the image logs and were statistically analyzed. The strikes of the natural fractures systematically vary and can be sorted into four groups according to depth: (1) above 637m, mainly striking ENE–WSW; (2) in the interval of 637–932.6m, striking NNE–SSW; (3) in the interval of 932.6–1200m, directed ENE–WSW then to WNW–ESE, while striking NE–SW from 1030m to 1150m; (4) from 1200m to 1370m, maintaining a strike of WNW–ESE. The natural fractures from 50m to 637m seem to be reverse faults which strike approximately parallelly to the main fault. Two sets of conjugate fractures around 1002.4m indicating subvertical maximum principal paleo-stress direction may be a subordinate structure of the main fault caused by a local stress field, and it reveals the complex stress field of Yingxiu-Beichuan fault zone when the fractures formed. A total of 12 BOs, 2 sets of DIFs and one set of DEFs with an overall length of 30.4m were interpreted from 960m to 1370m in WFSD-2. The average SHmax orientation interpreted for WFSD-2 (960–1370m) is 120.7°–300.7°N (i.e. WNW–ESE) with the standard deviation of 9.2° and it is consistent with the stress status of Yingxiu-Beichuan fault zone which is one of the main fault zones in the 2008 Wenchuan Earthquake.

Characteristics of the fault-related rocks, fault zones and the principal slip zone in the Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1)

Scientific drilling in active faults after a large earthquake is ideal to study earthquake mechanisms. The Wenchuan earthquake Fault Scientific Drilling project (WFSD) is an extremely rapid response to the 2008 Ms 8.0 Wenchuan earthquake, which happened along the Longmenshan fault, eastern margin of the Tibetan Plateau. In order to better understand the fault mechanism and the physical and chemical characteristics of the rocks, the WFSD project will eventually drill 5 boreholes along the two main faults. This paper focuses on the first hole (WFSD-1), which started just 178days after the earthquake, down to a final depth of 1201.15m. Petrological and structural analyses of the cores allowed the identification of fault-related rocks in the Yingxiu–Beichuan fault (fault gouge, cataclasite, and fault breccia), and the Principle Slip Zone (PSZ) location of the Wenchuan earthquake was determined.We found 12 fault zones in the entire core profile, with at least 10, including the Yingxiu–Beichuan fault zone, with a multiple cores structure and minimum width of ~100m. The co-seismic slip plane of the Wenchuan earthquake at depth (corresponding to the Yingxiu–Beichuan fault zone at the outcrop), as well as its PSZ, was expected to be located at the bottom of the fault zone (at 759m-depth). Instead, it was found at ~590m-depth with 1cm-wide fresh fault gouge, as determined by logging data such as temperature, natural gamma ray, p-wave velocity and resistivity, combined with the fresh appearance, magnetic susceptibility, and microstructure of the gouge. The Wenchuan earthquake slip plane has a dip angle of ~65°, showing the high-angle thrust feature. The distribution of fault gouge with several meters thick, the location of the Wenchuan earthquake's PSZ and the thickness of fresh gouge all imply a correlation between the width of the fault zone and the number of seismic events.

Rock magnetic properties of fault rocks from the rupture of the 2008 Wenchuan earthquake, China and their implications: Preliminary results from the Zhaojiagou outcrop, Beichuan County (Sichuan)

Rock magnetic measurements were conducted on fault rocks from the Zhaojiagou outcrop at Leigu Town, Beichuan County, Sichuan Province (China), located on the Yingxiu–Beichuan Fault, the main fault ruptured in the 2008 Wenchuan Ms 8.0 earthquake. Hysteresis measurements indicate that fault rocks contain very low concentrations of ferrimagnetic minerals, and their magnetic behavior is dominated by para-/diamagnetic components. Temperature-dependent magnetic susceptibility increased significantly above 400°C in the heating run. Magnetic behavior reveals the presence of siderite, lepidocrocite, chlorite and smectite as the possible major Fe-bearing minerals in fault rocks. Their thermal decomposition to ferrimagnetic minerals (e.g., magnetite and/or maghemite) could be responsible for the high magnetic susceptibility during the thermal treatment. A kinetic model analysis indicates that frictional heating could increase the temperature within the fault zone up to 320–908°C at depth of 590m, and 335–952°C at depth of 618.3m, respectively, during the seismic slip of the Wenchuan earthquake, depending on shear stress. Such high temperatures are enough to drive thermal decomposition of Fe-bearing minerals in fault rocks (in particularly, in fault gouge), to form ferrimagnetic magnetite and/or maghemite. This is the most likely mechanism for the reported high magnetic susceptibility anomalies within the fault zone in the pilot hole of Wenchuan Earthquake Fault Scientific Drilling Project (WFSD-1).

High-velocity frictional behavior of Longmenshan fault gouge from Hongkou outcrop and its implications for dynamic weakening of fault during the 2008 Wenchuan earthquake

High-velocity friction experiments were conducted on clayey fault gouge collected from Hongkou outcrop of Beichuan fault, located at the southwestern part of Longmenshan fault system that caused the disastrous 2008 Wenchuan earthquake. The ultimate purpose of this study is to reproduce this earthquake by modeling based on measured frictional properties. Dry gouge of about 1 mm in thickness was deformed dry at slip rates of 0.01 to 1.3 m/s and at normal stresses of 0.61 to 3.04 MPa, using a rotary-shear high-velocity frictional testing machine. The gouge displays slip weakening behavior as initial peak friction decays towards steady-state values after a given displacement. Both peak friction and steady-state friction remain high at slow slip rates are examined and gouge only exhibits dramatic weakening at high slip rates, with steady-state friction coefficient values of about 0.1 to 0.2. Specific fracture energy ranges from 1 to 4 MN/m in our results and this is of the same order as seismically determined values. Low friction coefficients measured on experimental faults are in broad agreement with lack of thermal anomaly observed from temperature measurements in WFSD-1 drill hole (Wenchuan Earthquake Fault Scientific Drilling Project), which can be explained by even smaller friction coefficient for the Wenchuan earthquake fault. High-velocity friction experiments with pore water needs to be done to see if even smaller friction is attained or not. Shiny slickenside surfaces form at high slip rates, but not at slow slip rates. Slip zone with slickenside surface changes its color to dark brown and forms duplex-like microstructures, which are similar to those microstructures found in the fault gouges from the Hongkou outcrop. Detailed comparisons between experimentally deformed gouge samples and WFSD drill cores in the future will reveal how much we could reproduce the dynamic weakening processes in operation in fault zones during Wenchuan earthquake at present.

Internal structure of Longmenshan fault zone at Hongkou outcrop, Sichuan, China, that caused the 2008 Wenchuan earthquake

This paper reports the internal structures of the Beichuan fault zone of Longmenshan fault system that caused the 2008 Wenchuan earthquake, at an outcrop in Hongkou, Sichuan province, China. Present work is a part of comprehensive project of Institute of Geology, China Earthquake Administration, trying to understand deformation processes in Longmenshan fault zones and eventually to reproduce Wenchuan earthquake by modeling based on measured mechanical and transport properties. Outcrop studies could be integrated with those performed on samples recovered from fault zone drilling, during the Wenchuan Earthquake Fault Scientific Drilling (WFSD) Project, to understand along-fault and depth variation of fault zone properties. The hanging wall side of the fault zone consists of weakly-foliated, clayey fault gouge of about 1 m in width and of several fault breccia zones of 30–40 m in total width. We could not find any pseudotachylite at this outcrop. Displacement during the Wenchuan earthquake is highly localized within the fault gouge layer along narrower slipping-zones of about 10 to 20 mm in width. This is an important constraint for analyzing thermal pressurization, an important dynamic weakening mechanism of faults. Overlapping patterns of striations on slickenside surface suggest that seismic slip at a given time occurred in even narrower zone of a few to several millimeters, so that localization of deformation must have occurred within a slipping zone during coseismic fault motion. Fault breccia zones are bounded by thin black gouge layers containing amorphous carbon. Fault gouge contains illite and chlorite minerals, but not smectite. Clayey fault gouge next to coseismic slipping zone also contains amorphous carbon and small amounts of graphite. The structural observations and mineralogical data obtained from outcrop exposures of the fault zone of the Wenchuan earthquake can be compared with those obtained from the WFSD-1 and WFSD-2 boreholes, which have been drilled very close to the Hongkou outcrop. The presence of carbon and graphite, observed next to the slipping-zone, may affect the mechanical properties of the fault and also provide useful information about coseismic chemical changes.