Yingxiu-Beichuan Fault Research Articles

High-angle fault responsible for the surface ruptures along the northern segment of the Wenchuan Earthquake Fault Zone: Evidence from the latest seismic reflection profiles

Abstract The inverse/right-lateral Yingxiu–Beichuan fault ruptured during the Wenchuan earthquake for ~ 270 km. We investigated the northeastern segment of that fault by analysing 3 newly acquired and 3 older seismic lines that cross the fault. We combine this analysis with that of the surface ruptures and of the regional geology in order to discuss the geometry, the style and the amount of motion of the fault. We conclude that the Yingxiu–Beichuan fault is very steep (~ 70°) at the surface and has still a dip > 45° at 6 km depth. The fault offsets all previous structures and particularly the Tangwangzhai nappes from which a vertical offset of 3 to 6 km is evaluated. The right-lateral component of motion along the Yingxiu–Beichuan fault appears to be larger at the surface than at depth and increases only slightly towards the NE. We suggest that the Yingxiu–Beichuan fault is continuous at depth between its northern and southern segments.

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.

Internal structure of the Wenchuan earthquake fault zone, revealed by surface outcrop and WFSD-1 drilling core investigation

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 ~ 240 m-wide fault zone was confirmed as the Yingxiu–Beichuan fault zone (YBF) at the Bajiaomiao outcrop, corresponding to the ~ 100 m 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 ~ 15–10 Ma.

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.

Rupture process of the Wenchuan earthquake (Mw 7.9) from surface ruptures and fault striations characteristics

On 12 May 2008, the Wenchuan earthquake (Mw 7.9) produced complicated thrust-type co-seismic surface rupture zones, which encompass the dextral-slip thrust of the Yingxiu–Beichuan fault, the approximately pure thrust of the Guanxian–Anxian fault, and the sinistral-slip thrust of the Xiaoyudong rupture zone located between the former two. In order to understand the faulting mechanism, we discuss the rupture process by examining the segmentation and kinematics of the surface rupture zones, together with the co-seismic fault striations at various sites. Based on the two along-strike main displacement peaks (6–6.5 m and 11–12 m) and on the different geometric and kinematic patterns for the southern and northern segments of the surface rupture zones, we find that the Wenchuan earthquake might have consisted of two rupture stages, which is in agreement with seismic wave inversion results. By comparing the kinematics of fault striations occurring in the Bajiaomiao and Beichuan areas, it suggests that during the first stage, thrusting along both the Yingxiu–Beichuan fault and Guanxian–Anxian fault produced the ~ 80–100 km-long Yingxiu–Qingping surface rupture segment and the ~ 80 km-long Guanxian–Anxian surface rupture zone, respectively. Then, faulting was triggered along the Yingxiu–Beichuan fault by the first rupture process, yielding the second rupture stage, which was characterized by dextral strike-slip (or dextral oblique thrusting). Due to the overlap between the two rupture stages, the southern segment (Yingxiu–Qingping) of the Yingxiu–Beichuan rupture zone comprises two different processes while the northern segment (Gaochuan–Beichuan–Shikan) only suggests one rupture phase.

Stress changes on major faults caused by 2013 Lushan earthquake and its relationship with 2008 Wenchuan earthquake

On April 20, 2013, an M s7.0 earthquake occurred in Ya’an-Lushan region, Sichuan Province, China, killing and injuring more than one thousand people. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settlement as to avoid future disasters. Based on the elastic dislocation theory and multi-layered lithospheric model, we calculate the co- and post-seismic stress changes caused by the Wenchuan and Lushan earthquakes to discuss the relationship between M w7.9 Wenchuan earthquake and M s7.0 Lushan earthquake, the influences on the distribution of aftershock caused by the Lushan earthquake, and the stress changes on major faults in this region. It is shown that the Coulomb failure stress increment on the hypocenter of Lushan earthquake caused by the Wenchuan earthquake is about 0.0037–0.0113 MPa. And the possible maximum value (0.0113 MPa) is larger than the threshold of stress triggering. Therefore, the occurrence of Lushan earthquake is probably effectively promoted by the Wenchuan earthquake. The aftershock distribution is well explained by the co-seismic stress changes of Lushan earthquake. By the two ends of the rupture of Lushan earthquake with increased Coulomb failure stress, a lack of aftershock recordings indicates the high seismic hazard. The stress accumulation and corresponding seismic hazard on the Kangding-Dafu segment of the Xinshuihe fault, the Beichuan-Yingxiu fault, the Pengxian-Guanxian fault, and the Ya’an fault are further increased by the Lushan earthquake and post-seismic process of Wenchuan earthquake.

Constructing the Longmen Shan eastern Tibetan Plateau margin: Insights from low‐temperature thermochronology

AbstractContrasting models of upper crustal shortening versus lower crustal flow have been proposed to explain the formation of thickened crust in the Longmen Shan (LMS), eastern Tibetan Plateau (TP) margin. These models require different structural kinematics along the LMS, whose structural geometry is defined by three parallel NW‐dipping fault zones. From foreland (southeast) to hinterland (northwest), they are the Guanxian‐Anxian Fault, Yingxiu‐Beichuan Fault (YBF), and Wenchuan‐Maowen Fault (WMF). Newly derived and previously published low‐temperature thermochronology data from the LMS were synthesized to constrain the spatial exhumation and test previous models. The results show that (1) exhumation increases abruptly across the range‐bounding YBF, suggesting the fault being the main thrust boundary between the LMS and the Sichuan Basin to the east; (2) Younger Late Cenozoic cooling ages are found on the hinterland WMF, where a dichotomy of ages on the hanging wall versus footwall suggests Late Cenozoic thrust activity; and (3) toward the hinterland to the west, exhumation rates decrease twofold over a distance of ~30–40 km. This exhumation pattern indicates a westward decrease of tectonic uplift, providing the regional topography approached a steady state, whereby exhumation is in balance with tectonic uplift. The observed exhumation estimates support an upper crustal configuration where thrusts in the LMS merge gradually into a gentle detachment seated at a depth of ~20–30 km. Results of this study support a revised upper crustal thrusting model.

Differences in magnetic properties of fragments and matrix of breccias from the rupture of the 2008 Wenchuan earthquake, China: Relationship to faulting

Rock magnetic and mineralogical analyses were performed on fragments and matrix of six fault breccias from the Zhaojiagou outcrop of Yingxiu–Beichuan Fault, which was the main fault ruptured during the 2008 Wenchuan Mw 7.9 earthquake, at Leigu Town, Beichuan County, Sichuan Province (China). The matrix, which is generally enriched in dolomite, feldspar and clay minerals, but depleted in calcite, has much higher low-field magnetic susceptibility and magnetization, and lower coercivity in comparison with fragments and bulk samples. Magnetic behavior of the bulk samples and fragments is dominated by dia-/paramagnetic components, in contrast, small amounts of partially oxidized magnetite and lepidocrocite are present in the matrix. A simple conceptual model associating faulting-related effects was proposed to explain these significant differences in magnetic properties of fragments and matrix. During coseismic slip, intense shear may crush the pre-existing magnetic grains in fault rocks into finer ones; while stress and frictional heating, to a little extent, induce magnetic changes and thermochemical alterations of magnetic mineralogy in fault breccias, respectively. During interseismic periods, meteoric fluids would infiltrate and percolate into fault zone, and cause dissolution, precipitation and recrystallization of Fe-bearing minerals. As the diverse permeability structure and grain size of fragments and matrix, these effects would modify their magnetic mineralogy at various levels. Consequently, faulting-related effects, especially the fluid movements, taking place over many previous earthquake cycles, would be the most likely reasons for the observed different magnetic properties in fragments and matrix. It further proposes that magnetic studies of fault breccias would provide clues to help understand seismic faulting and history of fault activity. • We present detailed magnetic properties of different components of fault breccia. • Fragments and matrix of breccia show marked differences in magnetic properties. • Faulting-related effects modify magnetic mineralogy in fragments and matrix. • Magnetic study of fault rock would help understand evolution of seismic fault.

GIS-based bivariate statistical modelling for earthquake-triggered landslides susceptibility mapping related to the 2008 Wenchuan earthquake, China

The main purpose of this research is to evaluate the modelling capability and predictive power of a bivariate statistical method for earthquake-triggered landslide susceptibility mapping. A weight index ( W i ) model was developed for the 2008 Wenchuan earthquake region in Sichuan Province, China, using a wide range of optical remote sensing data, and carried out on the basis of a geographic information system (GIS) platform. The 2008 Wenchuan earthquake triggered 196007 landslides, with a total area of 1150.43 km 2 , in an approximately oblong area around the Yingxiu–Beichuan coseismic surface fault-rupture (the Yingxiu–Beichuan fault). The landslides of the study area were mapped using visual interpretation of high-resolution satellite images and aerial photographs, both pre- and post-earthquake, and checked in the field at various locations. As a consequence, a nearly complete inventory of landslides triggered by the Wenchuan earthquake was constructed. Topographic and geological data and earthquake-related information were collected, processed and constructed into a spatial database using GIS and image processing technologies. A total of 10 controlling parameters associated with the earthquake-triggered landslides were selected, including elevation, slope angle, slope aspect, slope curvature, slope position, lithology, seismic intensity, peak ground acceleration (PGA), distance from the Yingxiu–Beichuan fault, and distance along this fault. To assist with the development of the model, the complete dataset of 196007 landslides was randomly partitioned into two subsets; a training dataset, which contains 70% of the data (137204 landslides, with a total area of 809.96 km 2 ), and a testing dataset accounting for 30% of the data (58803 landslides, with a total area of 340.47 km 2 ). A landslide susceptibility index map was generated using the training dataset, the 10 impact factors, and the W i model. In addition, for a conditionally dependent factor analysis, seven other factor-combination cases were also used to construct landslide susceptibility index maps. Finally, these eight landslide susceptibility maps were compared with the training data and testing data to obtain model capability (success rate) and predictive power (predictive rate) information. The validation results show that the success and predictive rates of the W i modelling exceeded 90% for the approaches that include the use of seismic factors. The final landslide susceptibility map can be used to identify and delineate unstable susceptibility-prone areas, and help planners to choose favourable locations for development schemes, such as infrastructure, construction and environmental protection schemes. The generic component of this research would allow application in other regions affected by high-intensity earthquakes and unstable terrain covering very large areas.

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.

Estimation of Recurrence Interval of Large Earthquakes on the Central Longmen Shan Fault Zone Based on Seismic Moment Accumulation/Release Model

Recurrence interval of large earthquake on an active fault zone is an important parameter in assessing seismic hazard. The 2008 Wenchuan earthquake (Mw 7.9) occurred on the central Longmen Shan fault zone and ruptured the Yingxiu-Beichuan fault (YBF) and the Guanxian-Jiangyou fault (GJF). However, there is a considerable discrepancy among recurrence intervals of large earthquake in preseismic and postseismic estimates based on slip rate and paleoseismologic results. Post-seismic trenches showed that the central Longmen Shan fault zone probably undertakes an event similar to the 2008 quake, suggesting a characteristic earthquake model. In this paper, we use the published seismogenic model of the 2008 earthquake based on Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data and construct a characteristic seismic moment accumulation/release model to estimate recurrence interval of large earthquakes on the central Longmen Shan fault zone. Our results show that the seismogenic zone accommodates a moment rate of (2.7 ± 0.3) × 1017 N m/yr, and a recurrence interval of 3900 ± 400 yrs is necessary for accumulation of strain energy equivalent to the 2008 earthquake. This study provides a preferred interval estimation of large earthquakes for seismic hazard analysis in the Longmen Shan region.

Along-strike variation of the frontal zone structural geometry of the Central Longmen Shan thrust belt revealed by seismic reflection profiles

The 2008 Ms 8.0 Wenchuan earthquake of China ruptured two large faults along the Beichuan-Yingxiu thrust belt and the Pengguan thrust belt at the Longmen Shan Orogenic belt, eastern margin of the Tibetan Plateau. Our study focused on the structural geometry at the Pengguan thrust belt located at the frontal zone of the Central Longmen Shan. In this paper, in order to identify the co-seismic fault at the Pengguan thrust belt many 2-D/3-D seismic reflection profiles have been interpreted by integrating with the geological maps, drilling data, DEM data and the surface ruptures. The results show that there were three major faults and other branch faults at the Pengguan thrust belt. The major faults have formed as imbricates and associated minor duplexes within the shallow strata. The Pengguan fault has a steep dip at the shallow surface and a gentle dip at depth. Both the Beichuan-Yingxiu fault and the Pengguan fault were developed on the shallow detachment in the central segment of Longmen Shan. The third (No. ③) Pengguan fault is coincident with the co-seismic surface ruptures. It was a pre-existing fault and re-activated during the Wenchuan earthquake. Three-dimensional geometries show that the No. ③ fault is heterogeneous and it has controlled the slip of co-seismic ruptures. Based on the regional geology, the 3-D fault geometry and the surface ruptures, the frontal zone of the Central Longmen Shan has been divided into the Bailu segment and the Hanwang segment. Our studies suggest that the Pengguan fault has been active since the Cenozoic. The frontal zone of the Central Longmen Shan thrust belt has potential for future earthquakes.

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.

Paleoseismic events and recurrence interval along the Beichuan–Yingxiu fault of Longmenshan fault zone, Yingxiu, Sichuan, China

Prior to the 2008 Wenchuan earthquake (Mw 7.9), Sichuan, China, no earthquake of magnitude larger than M 7.0 had been reported in the historical record for the Longmenshan region. To reveal paleoseismic behaviors in this region, we conducted a paleoseismic study along the 2008 Wenchuan earthquake fault scarp at the town of Yingxiu along the Beichuan–Yingxiu fault. Through a trench investigation, geomorphic analysis and radiocarbon dating, we interpret the occurrence of three paleoseismic events (including the Wenchuan earthquake) within the past 6000years. The cumulative vertical displacement resulting from the three seismic events is ~7.6m. The coseismic uplift for the Wenchuan earthquake is ~2.4m. Integrated with previous paleoseismic results along the fault, the age of the penultimate event is constrained between 3300 and 2300cal BP. We suggest the antepenultimate event occurred later than 5920–5730cal BP. An average vertical slip rate of 0.88–0.91mm/year is calculated from the vertical displacement of U2 since 5940–5730cal BP. We estimate that an average recurrence interval of large earthquakes is at most 3000years. Furthermore, the three paleoseismic events revealed at Yingxiu show similar coseismic vertical displacements. We propose that the strain accumulation and deformation across the Longmenshan fault zone is a relatively stable and continuous process since late Quaternary.

Complex surface rupturing and related formation mechanisms in the Xiaoyudong area for the 2008 Mw 7.9 Wenchuan Earthquake, China

The large oblique reverse slip shock of the 2008 Mw=7.9 Wenchuan earthquake, China, produced one of the longest and most complicated surface ruptures ever known. The complexity is particularly evident in the Xiaoyudong area, where three special phenomena occurred: the 7km long Xiaoyudong rupture perpendicular to the Beichuan-Yingxiu fault; the occurrence of two parallel faults rupturing simultaneously, and apparent discontinuity of the Beichuan-Yingxiu rupture. This paper systematically documents these co-seismic rupture phenomena for the Xiaoyudong area. The discussion and results are based on field investigations and analyses of faulting mechanisms and prevalent stress conditions. The results show that the Beichuan-Yingxiu fault formed a 3.5km wide restraining stepover at the Xiaoyudong area. The Xiaoyudong fault is not a tear fault suggested by previous researches, but a frontal reverse fault induced by the oblique compression at this stepover; it well accommodates the ‘deformation gap’ of the Beichuan-Yingxiu fault in the Xiaoyudong area. Further, stress along the Peng-Guan fault plane doubles due to a change in dip angle of the Beichuan-Yingxiu fault across the Xiaoyudong restraining stepover. This resulted in two faults rupturing the ground’s surface simultaneously, to the north of the Xiaoyudong area. These results are helpful in deepening our understanding of the dynamic processes that produced surface ruptures during the Wenchuan earthquake. Furthermore, the results suggest more attention be focused on the influence of dextral slip component, the change of the control fault’s attitude, and property differences in rocks on either side of faults when discussing the formation mechanism of surface ruptures.

Internal structures and high-velocity frictional properties of a bedding-parallel carbonate fault at Xiaojiaqiao outcrop activated by the 2008 Wenchuan earthquake

This paper reports internal structures of a bedding-parallel fault in Permian limestone at Xiaojiaqiao outcrop that was moved by about 0.5 m during the 2008 MW7.9 Wenchuan earthquake. The fault is located about 3 km to the south from the middle part of Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system that was moved during the earthquake. The outcrop is also located at Anxian transfer zone between the northern and central segments of Yingxiu-Beichuan fault where fault system is complex. Thus the fault is an example of subsidiary faults activated by Wenchuan earthquake. The fault has a strike of 243° or N63°E and a dip of 38°NW and is nearly optimally oriented for thrust motion, in contrast to high-angle coseismic faults at most places. Surface outcrop and two shallow drilling studies reveal that the fault zone is several centimeters wide at most and that the coseismic slip zone during Wenchuan earthquake is about 1 mm thick. Fault zone contains foliated cataclasite, fault breccia, black gouge and yellowish gouge. Many clasts of foliated cataclasite and black gouge contained in fault breccia indicate multiple slip events along this fault. But fossils on both sides of fault do not indicate clear age difference and overall displacement along this fault should not be large. We also report results from high-velocity friction experiments conducted on yellowish gouge from the fault zone using a rotary shear low to high-velocity frictional testing apparatus. Dry experiments at normal stresses of 0.4 to 1.8 MPa and at slip rates of 0.08 to 1.35 m/s reveal dramatic slip weakening from the peak friction coefficient of around 0.6 to very low steady-state friction coefficient of 0.1–0.2. Slip weakening parameters of this carbonate fault zone are similar to those of clayey fault gouge from Yingxiu-Beichuan fault at Hongkou outcrop and from Pingxi fault zone. Our experimental result will provide a condition for triggering movement of subsidiary faults or off-fault damage during a large earthquake.

The origin of the 2008 Wenchuan Earthquake determined by the analysis on the active Longmenshan nappe in terms of rockmass mechanics

On 12 May 2008, the magnitude 8.0 Wenchuan Earthquake occurred along the Longmen Shan nappe, Sichuan, China. This devastating earthquake led to a heavy death toll of greater than 80,000. The seismic origin of this earthquake is currently hotly debated. We suppose that it is a special type of intraplate earthquake called an activenappe-type earthquake. Using a holistic methodology, incorporating rockmass structure cybernetics and Byerlee’s law, we present a comprehensive study on the geological origin of macroseisms in the Longmen Shan area and the seismic origin of the 2008 Wenchuan earthquake. Previous studies of neotectonic activity indicate that the Longmen Shan nappe moves at a rate of 1∼3 mm/yr, due to horizontal compressive stress from the Tibetan Plateau. The difference between movement rates in the Bayankala block, Longmen Shan nappe and Sichuan Basin cause slow shear stress and strain accumulation in the Longmen Shan nappe. It is exhibited a relatively simple linear relations for the shear strength and the buried depth of the structural planes, and the detachment layer of the nappe has a higher shearing-sliding strength compared to the overlying fault planes and the underlying ductile shear belts, thus making it more prone to stick-slip deformation. Therefore, the detachment layer was the main section responsible for the Wenchuan earthquake. The initial rupture burst in the detachment layer under the Yingxiu-Beichuan fault, the rupture area nearly 1.4454 × 104 km2, encompassed the cross point of the Yingxiu and the Anxian-Guanxian faults with the detachment layer, then caused the Yingxiu — Beichuan and Anxian-Guanxian faults took an active part in this earthquake, so this earthquake might consist of three chain-like earthquake stages, totally increasing the duration of this earthquake an unusually large amount, to 120 s. The focal depth spanned range of 10–20km, consistent with the observed result of this focal depth by several agencies.

Transient water and sediment storage of the decaying landslide dams induced by the 2008 Wenchuan earthquake, China

Earthquake-triggered landslide dams are potentially dangerous disrupters of water and sediment flux in mountain rivers, and capable of releasing catastrophic outburst flows to downstream areas. We analyze an inventory of 828 landslide dams in the Longmen Shan mountains, China, triggered by the Mw 7.9 2008 Wenchuan earthquake. This database is unique in that it is the largest of its kind attributable to a single regional-scale triggering event: 501 of the spatially clustered landslides fully blocked rivers, while the remainder only partially obstructed or diverted channels in steep watersheds of the hanging wall of the Yingxiu-Beichuan Fault Zone. The size distributions of the earthquake-triggered landslides, landslide dams, and associated lakes (a) can be modeled by an inverse gamma distribution; (b) show that moderate-size slope failures caused the majority of blockages; and (c) allow a detailed assessment of seismically induced river-blockage effects on regional water and sediment storage. Monte Carlo simulations based on volumetric scaling relationships for soil and bedrock failures respectively indicate that 14% (18%) of the estimated total coseismic landslide volume of 6.4 (14.6)×109m3 was contained in landslide dams, representing only 1.4% of the >60,000 slope failures attributed to the earthquake. These dams have created storage capacity of ~0.6×109m3 for incoming water and sediment. About 25% of the dams containing 2% of the total river-blocking debris volume failed one week after the earthquake; these figures had risen to 60% (~20%), and >90% (>90%) within one month, and one year, respectively, thus also emptying ~92% of the total potential water and sediment storage behind these dams within one year following the earthquake. Currently only ~0.08×109m3 remain available as natural reservoirs for storing water and sediment, while ~0.19×109m3, i.e. about a third of the total river-blocking debris volume, has been eroded by rivers. Dam volume and upstream catchment area control to first order the longevity of the barriers, and bivariate domain plots are consistent with the observation that most earthquake-triggered landslide dams were ephemeral. We conclude that the river-blocking portion of coseismic slope failures disproportionately modulates the post-seismic sediment flux in the Longmen Shan on annual to decadal timescales.

Ground Surface Ruptures and Near‐Fault, Large‐Scale Displacements Caused by the Wenchuan Ms8.0 Earthquake Derived from Pixel Offset Tracking on Synthetic Aperture Radar Images

Abstract:The 12 May 2008 Wenchuan Ms8.0 earthquake produced surface displacements along the causative fault, the Yingxiu‐Beichuan Fault, which are up to several meters near the fault. Because of the large gradient, satellite synthetic aperture radar (SAR) interferometric data are strongly incoherent; the usual SAR interferometry method does not allow such displacements to be measured. In the present study, we employed another approach, the technique based on pixel offset tracking, to solve this problem. The used image data of six tracks are from the Advanced Land Observing Satellite, Phased Array type L‐band Synthetic Aperture Radar (ALOS/PALSAR) dataset of Japan. The results show that the entire surface rupture belt is 238 km long, extending almost linearly in a direction of 42° north–east. It is offset left laterally by a north–west‐striking fault at Xiaoyudong, and turns at Gaochuan, where the rupture belt shifts toward the south by 5 km, largely keeping the original trend. In terms of the features of the rupture traces, the rupture belt can be divided into five sections and three types. Among them, the Beichuan–Chaping and Hongkou–Yingxiu sections are relatively complex, with large widths and variable traces along the trend. The Pingtong–Nanba and Qingping–Jingtang sections appear uniform, characterized by straight traces and small widths. West of Yingxiu, the rupture traces are not clear. North of the rupture belt, surface displacements are 2.95 m on average, mostly 2–3.5 m, with 7–9 m the maximum near Beichuan. South of the rupture belt, the average displacement is 1.75 m, dominated by 1–2 m, with 3–4 m at a few sites. In the north, the displacements in the radar line of sight are of subsidence, and in the south, they are uplifted, in accordance with a right‐slip motion that moves the northern wall of the fault to the east, and the southern wall to the west, respectively. Along the Guanxian–Jiangyou Fault, there is a uplift zone in the radar line of sight, which is 66 km long, 1.5–6 km wide, and has vertical displacements of approximately 2 m, but no observable rupture traces.

汶川8.0级地震地表破裂迹线附近建筑物的震害分布 Distribution of Seismic Damaged Buildings along the Trace of Ground Surface Rupture Caused by the Ms 8.0 Wenchuan Earthquake

汶川地震地表破裂带主要分三部分，映秀–北川地表破裂带、通济–雎水地表破裂带、以及小鱼洞地表破裂带。映秀–北川地表破裂带是汶川地震的主要破裂带，处于龙门山中央断裂带上，全长达260 km以上；通济–雎水地表破裂带是汶川地震在前山断裂带上产生的地表破裂，全长约50 km；小鱼洞破裂带，是主断裂带上的一个分支，从小鱼洞经草坝，延伸至向峨，破裂带由北西向逐渐过渡到南西向，全长约6 km。已发现的最大同震位错位于主破裂带的都江堰虹口附近，最大垂直位错4.8 m、最大水平右旋位错4.7 m。本文以南坝镇地震地表破裂点附近的建(构)筑物震害调查为例展开，此段地表垂直位错0.5~2.5 m，水平位错1.3~2.0 m，地表强变形带宽度7.6~18 m；相应地，50 m范围内的房屋完好率为0，倒塌率为0.424，中等破坏与严重破坏率为0.576。沿地表破裂带上的15个乡镇的1699栋建(构)筑物震害表明，在距地表破裂迹线距离为150 m范围内的建筑物的倒塌率逐渐下降，即从0.33193降低到0.29264；在距地表破裂迹线距离为250 m范围内的建筑物的完好率逐渐上升，即从0.08824上升到0.1248；超过这样的区域，倒塌率和完好率趋于平缓。从震害指数角度，可以发现，在距地表破裂迹线距离为150 m范围内的建筑物的震害指数逐渐下降，即从0.72降低到0.47344；超过这样的区域，震害指数规律性不明。Field investigations of the Ms 8.0 Wenchuan Earthquake show that buildings on the ground surface rupture zone were severely damaged. The surface rupture zone consists of three parts. Two of the parts are respectively along the trace of the Beichuan-Yingxiu fault and the Guanxian-Jiangyou fault. The former part is about 260 km long, which starts at Yingxiu and terminates at Nanba; the latter part is about 50 km long which starts at Tongji and terminates at Jushui. The third part connects the Beichuan-Yingxiu fault and the Guanxian-Jiangyou fault from Xiao Yudong, to Tongji, which is about 6 km long. The maximum vertical and horizontal offset of 4.8 m and 4.7 m, respectively, were observed along the Beichuan-Yingxiu fault, whereas a maximum vertical offset of 3.5 m occurred along the Guanxian- Jiangyou fault. The distribution of damaged buildings along the trace of ground surface was investigated in many segments. The seismic damage distribution in Nanba area will be introduced considering that lots of rural residents live there. Vertical and horizontal offset of 0.5 - 2.5 m and 1.3 - 2.7 m, respectively, were observed along this segment, and the significant width of deformation caused by the offset is 7.6 - 18 m. In this segment, the building of intact ratio is zero, the ratio of medium to the serious damage is 0.576 and the collapse ratio is 0.424 within 50 m. Totally 1699 structures were investigated in 15 segments along the ground surface rupture zone. The collapse ratio decreases from 0.33193 to 0.29264 corresponding to the fault distance 0 to 150 m. Here the fault distance is defined as a distance from location of structure to the trace line of rupture points; the building of intact ratio increases from 0.08824 to 0.12480 corresponding to the fault distance 0 to 250 m. In other way, earthquake damage index of building structure decreases from 0.72 to 0.47344 corresponding to the fault distance 0 to 150 m. There is no significant regularity beyond 150 m.