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.

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.

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 1201m long. In-situ stresses were measured in rock samples from depths between 424 and 1173m 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 35.3MPa at the depth of 1173m. The relations between horizontal and vertical stresses are vertical stress σv>maximum horizontal stress σH>minimum horizontal stress σh at depths above 424m, σH>σh>σv at depths from 424m to 800m, and σH>σv>σh below the depth of 800m. These indicate that the stress states are in a normal faulting stress regime above 424m; a reverse faulting regime from 424m to 800m and a dextral strike-slip regime below 800m. 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.

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 1201.15m-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 585.75m) is 1.85°C/100m, while that of the sedimentary Xujiahe Formation (below 585.75m) is 2.15°C/100m. 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 585.75–594.5m-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.

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 M w 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.

The in-situ stress state of seismogenic fault after a strong earthquake is one of the important parameters for understanding the mechanism of the earthquake.The Wenchuan earthquake Fault Scientific Drilling project(WFSD) is a rapid response to the 2008 Ms 8.0 Wenchuan earthquake,which provided the test rock core for recognizing the stress state of the depth of Longmenshan fault on the eastern margin of the Tibetan Plateau.This paper focuses on the measuring instruments and processes of the anelastic strain recovery method(ASR) for the deep in-situ stress measurement.The anelastic recovery strains of seven different deep cores are in the range of 424-1173m in WFSD-1.The direction and magnitude of principal stress were determined and estimated,respectively.The dominant azimuths of maximum principal stress are between NW69° and NW35°,and the magnitude of principal stress increases with the increasing depth.

Fault zones record a series of faulting events that have occurred under different physical conditions during their evolution. Therefore, it is essential to understand the internal structures of fault zones in order to better understand the mechanical behavior of faults. The internal structure of the Wenchuan earthquake fault zone that prevailed at the Bajiaomiao outcrop and in the WFSD-1drilling cores, located along the southern segment of the Yingxiu–Beichuan surface rupture in the Hongkou area, is described in details in this paper. Based on field surveys, X-ray diffraction analysis, microstructure and analysis of the drilling cores, an ~240m-wide fault zone was confirmed as the Yingxiu–Beichuan fault zone (YBF) at the Bajiaomiao outcrop, corresponding to the ~100m fault zone in the WFSD-1 drilling cores. Fault rocks, including fault breccia, fault gouge and cataclasite were identified in both the outcrop and drilling cores, while pseudotachylyte was only present at the outcrop. Two different types of gouge veins, formed by thermal pressurization and fluidization respectively, are observed in this area. The YBF possesses the characteristics of a multiple core model, and consists of 5 different fault rock units. From top to bottom, these are cataclasite zone, black fault gouge–breccia zone, gray fault breccia zone, dark-gray fault breccia zone and black fault gouge–breccia zone. Outcrop investigation and drilling core research show that the slip zone of the Wenchuan earthquake does not completely follow the ancient fault slip zone. The Wenchuan earthquake fault is a high angle thrust fault which crosses the YBF obliquely. The multi-layered fault rocks displayed in the research area might indicate that the YBF comes from the long-term fault activity and evolution over the last ~15–10Ma.

The 2008 Wenchuan earthquake ( M W 7. 9)occurred at the eastern margin of the Tibetan Plateau,producing 270km and 80km-long co-seismic surface ruptures with different kinematics features,along the Yingxiu-Beichuan and Guanxian-Anxian faults, respectively. Fault rocks are the products of fault activities. The characteristics of fault rocks provide information on fault activities and their evolution history. Here,we focus on the outcrop of the Yingxiu-Beichuan fault in Bajiaomiao village,Hongkou town and results from WFSD-1drill cores. Based on field survey,X-raydiffraction analysis,microstructure and drill cores analysis,a ～240m-wide fault zonewas confirmed as the Yingxiu-Beichuan fault zone from the outcrop,corresponding to the～105m-wide fault zone at depth, as determined from the WFSD-1 drill cores,which presents the characteristics of a multiple cores model. Fault rocks,including fault breccia,fault gouge and cataclasite were identified in both the outcrop and drill cores,while pseudotachylyte was only visible at the outcrop. The location of the Principal Slip Zone of the Wenchuan earthquake,which does not entirely follow the ancient fault zone, obliquely crossing the Yingxiu-Beichuan fault zone,might indicate that the Wenchuan earthquake fault and Yingxiu-Beichuan faultdo not belong to the same fault system. Based on fault rocks studies,there are four kinds of fault mechanisms along the Yingxiu-Beichuan fault zone, such as frictional melting, thermal pressurization, mechanical lubrication and elasto-hydrodynamic lubrication. Thermochronometry research indicates that the Yingxiu-Beichuan fault activity has controlled the rapid uplift of the Longmenshan since it formed at 15～10Ma ago. The fact that the internal structure shows five different fault rock units in the fault zone may be related to the different episodes of the Longmenshan uplift.

With the WFSD-2 drill hole as the object,the authors studied core logging,petrology and structural geology.The results show that the cores can be divided into six segments,which are in downward succession the Pengguan complex(0~599.31m),the second Member of the Xujiahe Formation(599.31~1211.49m),the Pengguan complex(1211.49~1679.51m),the third Member of the Xujiahe Formation(1679.51~1715.48m),the Pengguan complex(1715.48~2081.47m) and the fourth Member of the Xujiahe Formation(2081.47~2283.56m).The Pengguan complex mainly consists of granite and volcanic rocks,and the Xujiahe Formation comprises Triassic sedimentary rocks of sandstone,slitstone,mudstone,shale,coal beds(streaks) and conglomerate.Three sections of the Pengguan complex and three sections of the Xujiahe Formation occur in alternate repetition in the WFSD-2,indicating that the Longmenshan tectonic belt is composed of a series of thrust lithologic sheets.There are a lot of fault-related rocks in the WFSD-2 cores,such as fault breccia,cataclasites and fault gouge,reflecting the brittle deformation.Based on a statistical analysis of the fault-related rocks,the authors detected twenty fault zones in the WFSD-2,indicating that many earthquakes like 2008 Wenchuan Earthquake occurred in Longmenshan area.Of the nine primary fault zones of FZ600,FZ720,FZ782,FZ817,FZ922,FZ951,FZ1449,FZ1681,FZ2082 and FZ1681,the FZ1681 is the biggest primary fault zone in the whole WFSD-2.According to the association characteristics of fault-related rocks,the fault zones can be divided into symmetric and asymmetric fault zones,with fault gouge as the center.According to the dip angles of the trench and WSFD-1 primary slip zone(PSZ),the PSZ in the WFSD-2 should occur within one of the primary fault zones of FZ1134,FZ1681 and FZ1681,which all have fault gouge and reasonable geometrical interpretation among rupture zones in WFSD-1 and WFSD-2.An integrated analysis of four thrust rock segments,twenty fault zones and high breakage density shows that the Longmenshan area must have experienced powerful tectonic activities.

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 178 days after the earthquake, down to a final depth of 1201.15 m. 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 similar to 100 m. 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 759 m-depth). Instead, it was found at similar to 590 m-depth with 1 cm-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 similar to 65 degrees, 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. (C) 2012 Elsevier B.V. All rights reserved.

Wenchuan Earthquake Fault Zone Scientific Drilling(WFSD) was organized jointly by Ministry of Science and Technology,Ministry of Land and Resources and China Bureau of Seismology.The WFSD project plans to drill five scientific wells on the hanging wall of the co-seismic surface rupture zone,including the Yingxiu-Beichuan fault and the Anxian-Guanxian fault in Longmenshan area.The five drilling holes include WFSD-1,WFSD-2,WFSD-3,WFSD-3P and WFSD-4.The WFSD-3 and WFSD-3P drill holes are located on the hanging wall of the Anxian-Guanxian fault.Basic studies were carried out for the WFSD-3 drilling cores in such aspects as petrology,structural geology and borehole logging.The results show that the fault-rocks in the WFSF-3 contain fault breccia,cataclastic rock and fault gouge.The pseudotachylite does not appear in this well.Twenty-six fault zones were detected in this well,including the three main major fault zones of FZ634,FZ1215 and FZ1250,with the FZ1250 being the possible primary slip zone during the 2008 Wenchuan earthquake.The dip angles of the trench,WFSD-3P and WFSD-3 are respectively about 60°,46° and 38°.The variation of the dip angle indicates that the Anxian-Guanxian fault is a listric thrust fault with low dip angle.

文章以龙门山中央断裂带映秀-北川断裂带为研究对象,重点对汶川地震破裂带南段虹口乡八角庙地区地表断裂带进行了详细研究,并结合汶川地震科学钻探1号孔(WFSD-1)岩芯部分研究成果,探讨龙门山中央断裂带的物质组成及其结构特征.研究表明,映秀-北川断裂带由很多次级小规模断裂(破碎带)以及夹持其中的块体所构成,其中断层泥的厚度由几毫米到25cm不等,与WFSD-1岩芯记录的地震断裂现象基本一致.从虹口乡八角庙露头来看,映秀-北川断裂带整体宽约120m,分布有近80条含有断层泥的次级断裂带.以台湾车笼埔断裂钻探项目(TCDP)和汶川地震科学钻探(WFSD)研究来看,一次大地震只能形成几毫米至约2cm厚的断层泥,推断映秀-北川断裂带中每层断层泥至少发生过1次到13次地震,该区总厚度约150cm的断层泥中发生地震次数至少为183次,说明沿着映秀-北川断裂带重复发生过多次强地震活动.每次地震活动并不完全沿袭老的地震断裂主滑移带滑动,而是沿着断层泥边部区域滑动.从整个断裂带中断层泥分布特征来看,地震断裂活动具有向断裂下盘迁移的趋势,并且断层泥的厚度与断裂活动性成正比关系,表明断裂带宽度与地震活动次数及其演化历史有着成因上的直接联系,多次地震活动叠加可能是龙门山形成的主要成因.