Shan Thrust Research Articles

Co-seismic landslide topographic analysis based on multi-temporal DEM—A case study of the Wenchuan earthquake

Hillslope instability has been thought to be one of the most important factors for landslide susceptibility. In this study, we apply geomorphic analysis using multi-temporal DEM data and shake intensity analysis to evaluate the topographic characteristics of the landslide areas. There are many geomorphologic analysis methods such as roughness, slope aspect, which are also as useful as slope analysis. The analyses indicate that most of the co-seismic landslides occurred in regions with roughness, hillslope and slope aspect of >1.2, >30, and between 90 and 270, respectively. However, the intersection regions from the above three methods are more accurate than that derived by applying single topographic analysis method. The ground motion data indicates that the co-seismic landslides mainly occurred on the hanging wall side of Longmen Shan Thrust Belt within the up-down and horizontal peak ground acceleration (PGA) contour of 150 PGA and 200 gal, respectively. The comparisons of pre- and post-earthquake DEM data indicate that the medium roughness and slope increased, the roughest and steepest regions decreased after the Wenchuan earthquake. However, slope aspects did not even change. Our results indicate that co-seismic landslides mainly occurred at specific regions of high roughness, southward and steep sloping areas under strong ground motion. Co-seismic landslides significantly modified the local topography, especially the hillslope and roughness. The roughest relief and steepest slope are significantly smoothed; however, the medium relief and slope become rougher and steeper, respectively.Electronic supplementary materialThe online version of this article (doi:10.1186/2193-1801-2-544) contains supplementary material, which is available to authorized users.

Structural and Tectonic Framework of the Qilian Shan‐Nan Shan Thrust belt, Northeastern Tibetan Plateau

Structural and Tectonic Framework of the Qilian Shan‐Nan Shan Thrust belt, Northeastern Tibetan Plateau

What drove continued continent-continent convergence after ocean closure? Insights from high-resolution seismic-reflection profiling across the Daba Shan in central China

We conducted deep seismic-reflection surveying across the Jurassic Daba Shan thrust belt of central China to investigate how and why the continued convergence between north and south China lasted ∼50 m.y. after the Triassic closure of their intervening oceans. Our study, together with surface geology, gravity surveying, and magnetic observations, indicates widespread occurrence of mafic plutons below the Daba Shan thrust belt. We propose that subduction of dominantly eclogitized mafic crust of northern south China provided the driving force for continued convergence between north and south China after ocean closure.

Late Quaternary slip rate of the South Heli Shan Fault (northern Hexi Corridor, NW China) and its implications for northeastward growth of the Tibetan Plateau

AbstractBased on field investigations, aerial‐photo morphological analysis, topographic profiling, and optically stimulated luminescence (OSL) dating of alluvial surfaces, we estimate vertical components of the slip rate along the South Heli Shan thrust fault, which lies on the northern margin of the Hexi Corridor and the northeastern edge of the Tibetan Plateau. The fault consists of three segments with scarp heights ranging from less than 1 m to more than 16 m. OSL dating indicates that most of the alluvial fans cut by fault scarps formed during the transition from the last glacial stage to the present interglacial stage from ~19 to ~9 ka along southern Heli Shan and from ~27 ka to ~22 ka along its northern margin. In addition, remnants of older alluvial fan have been abandoned after ~67 ka. Scarp heights increase from west to east and reach a maximum of more than 16 m near the eastern end. Using three approaches, we calculate late Quaternary slip rates for each of the three fault segments along the southern margin and the fault on the northern flank. These approaches yield maximum vertical slip rates from 0.18 to 0.2 mm/a for the western segment, 0.3 to 0.43 mm/a for the central segment, 0.36 to 0.53 mm/a for the eastern segment, and 0.21 mm/a for the Wutongjing Fault, which lies on the north side of the Heli Shan. For a range of likely fault dips, these correspond to 0.1–0.2 mm/a of average horizontal shortening for the western segment, and increase to 0.4–0.5 mm/a across the eastern segment of the southern Heli Shan Fault. Combining the height of the eastern parts of the Heli Shan (Daqing Peak) above the Hei He (a major river that incised the western end of the range) and the vertical component of the slip rate of the eastern segment, we suggest that the Heli Shan was uplifted by motion on the South Heli Shan Fault beginning sometime between 1 and 4 Ma, most likely since ~2 Ma. This age suggests that the Tibetan Plateau continues to grow northeastward across the Hexi Corridor.

Provenance analysis of the Mesozoic Hoh‐Xil‐Songpan‐Ganzi turbidites in northern Tibet: Implications for the tectonic evolution of the eastern Paleo‐Tethys Ocean

Mesozoic strata of the Hoh‐Xil‐Songpan‐Ganzi complex in northern Tibet are exposed in a vast (> 370,000 km2) triangle‐shaped orogenic belt bound by the Longmen Shan thrust belt in the east, the Kunlun terrane and North China block in the north, and the Qiangtang terrane and Yidun arc in the south. These strata consist of Middle–Upper Triassic submarine fan and deep marine facies rocks that were deposited in the Paleo‐Tethys Ocean. Late Triassic–Early Jurassic contractional deformation in the eastern Hoh‐Xil‐Songpan‐Ganzi complex marks the demise of the Paleo‐Tethys Ocean basin and the accretion of the Gondwana‐derived Qiangtang terrane to Eurasia. We conducted geological mapping, regional stratigraphic analyses, and U‐Pb geochronology of detrital zircons (n = 4128) on the Mesozoic sequences exposed in the Hoh‐Xil‐Songpan‐Ganzi complex, Kunlun terrane, and Qiangtang terrane. We identify for the first time marine silciclastic sandstone and shale of Jurassic age in the northwestern Hoh‐Xil‐Songpan‐Ganzi complex that unconformably overlie Upper Triassic turbidites. Zircon age data indicate that the Middle–Upper Triassic marine gravity‐flow deposits of the Hoh‐Xil‐Songpan‐Ganzi complex were shed from the North and South China blocks, and Middle–Late Triassic ultrahigh‐pressure Qinling–Dabie orogenic belt, as well as the Kunlun and Qiangtang terranes. In addition, the detrital zircon results suggest vast sediment source to sink distances (>1500 km) for the Middle–Upper Triassic Hoh‐Xil‐Songpan‐Ganzi strata, which is consistent with tectonic models for the Paleo‐Tethys Ocean basin that incorporate significant components of horizontal tectonic transport like opening of large back‐arc basins in response to oceanic slab rollback.

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.

Historic seismicity near the source zone of the great 2008 Wenchuan earthquake: Implications for seismic hazards

In the past 500years, 14 historical earthquakes, including one that caused a maximum intensity of IX, occurred over a distance of more than 300km along the Longmen Shan thrust belt, including portions that did not rupture during the devastating Wenchuan earthquake sequence of May 12, 2008. Estimated locations of epicenters and trends of faulting during historical events complement information gathered after the 2008 sequence. In particular, in addition to the two fault splays that ruptured in 2008, there are additional seismogenic faults across the Longmen Shan belt, including those in the hinterland and in the foreland. In the latter case, the occurrence of moderate-sized historical earthquakes and the disproportionally wide region of reported damage in the Sichuan basin from large events within the Longmen Shan belt, probably an effect of low attenuation in the stable basin, calls for attention to the potential of seismic hazard in the heavily populated Chengdu basin. Moreover, long recurrence-intervals of great events, estimated from trenching of colluvium along the ruptures of the 2008 sequence, should not be taken as reliable estimates for somewhat smaller, but nonetheless highly destructive events along the Longmen Shan thrust belt.

The Jinhe–Qinghe fault—An inactive branch of the Xianshuihe–Xiaojiang fault zone, Eastern Tibet

The Xianshuihe–Xiaojiang fault zone (XFZ) forms a prominent linear NW–SE trending tectonic feature along the southeastern margin of the Tibetan Plateau. The spatial and temporal evolution of the predominantly strike–slip movement along the fault remains unclear, as estimates for the initiation age of its different segments range from 5 to 13Ma. The Jinhe–Qinghe fault (JqF; the south part of the Longmen Shan thrust belt) provides a key for understanding the tectonic role and deformation history of the Xianshuihe–Xiaojiang fault zone in terms of timing, magnitude of displacement, and spatial–temporal evolution. This paper presents research on the structure and chronology of the JqF. The results of a geometric and kinematic study of the Jinhe–Qinghe fault show it to be a thrust fault with a left-lateral strike slip component; The strike slip offset extends at least 13–18km to southwest, based on the shortening of its folds and faulting, and based on stream deflection. Ten apatite fission track apparent ages show a pronounced change in age/elevation gradient at ~17±2Ma, consistent with an abrupt increase of exhumation due to uplift of the hanging wall of the JqF at that time. Combined with the age of deformed Tertiary strata in the footwall of the JqF and the geometric relationships of the JqF with the Chenghai fault and the XFZ, this indicates that the JqF was active between 17 and 5Ma. Based on the fault chronology, geometry, and movement, we suggest that the fault was a transpressional structure at the south end of the XFZ during the period from 17 to 5Ma.

Field evidence of rupture of the Qingchuan Fault during the 2008 Mw7.9 Wenchuan earthquake, northeastern segment of the Longmen Shan Thrust Belt, China

Although many studies of the 2008 Mw 7.9 Wenchuan earthquake have described the ground deformation features, rupture mechanism, and structural features of the seismogenic fault zone associated with this event, debate remains concerning the total length of the co-seismic surface rupture zone and whether the earthquake ruptured the Qingchuan Fault in the northeastern segment of the Longmen Shan Thrust Belt (LSTB), China. In this paper, we present new field evidence that the Qingchuan Fault was ruptured by the 2008 Wenchuan earthquake and that the total length of the co-seismic surface rupture zone is up to 285–300km. Field investigations reveal that the earthquake produced a ≥60-km-long surface rupture zone along the pre-existing Qingchuan Fault, with the offset being mainly right-lateral strike–slip and a distinct component of vertical slip. Co-seismic surface ruptures are characterized by faults and extensional cracks. Field measurements indicate co-seismic right-lateral strike–slip displacements along the Qingchuan Fault of 0.3–0.6m and vertical offsets of 0.2–0.5m, which differs to the displacements observed along the central and southwestern segments of the Wenchuan surface rupture zone in the displacement amount and sense. The change in slip sense from thrust-dominated slip in the central and southwestern segments of the LSTB to right-lateral strike–slip-dominated displacement along the Qingchuan Fault (northeastern segment of the LSTB) reflects a change in the orientation of compressive stress along the LSTB, associated with eastward extrusion of the Tibetan Plateau as it accommodates the ongoing penetration of the Indian Plate into the Eurasian Plate.

Spatial variation in Meso-Cenozoic exhumation history of the Longmen Shan thrust belt (eastern Tibetan Plateau) and the adjacent western Sichuan basin: Constraints from fission track thermochronology

The NE-trending Longmen Shan thrust belt, marking the eastern margin of the Tibetan Plateau adjacent to the Sichuan basin, is characterized by the paradox of high topographic gradients but low convergence rates, and thus critical for understanding of geodynamic processes involved in the eastward growth of the plateau. Many low-temperature thermochronological studies focused on the central and southern portions of the Longmen Shan and western Sichuan basin to investigate the Cenozoic exhumation history of the eastern Tibetan Plateau, but little work has been done on their northern portions. Furthermore, the spatial variation in both Cenozoic and Mesozoic thermal histories of this region has been poorly documented. In this paper, zircon (ZFT) and apatite (AFT) fission track analyses for samples collected from three transects and three boreholes covering the northern, central and southern Longmen Shan and the western Sichuan basin provide new data demonstrating several cooling events in this region since the Indosinian deformation. They occurred during the late Triassic (∼200Ma), at the end of early Cretaceous (∼100Ma), during the early Cenozoic (60–40Ma), the early Miocene (20–25Ma) and the late Miocene (9–14Ma), respectively. On regional scale the Longmen Shan experienced a slow cooling between the Mesozoic and early Cenozoic, followed by a rapid cooling during the late Cenozoic, when the rapid regional exhumation began at ∼10Ma, with a sharp increase in exhumation rate from <0.1mm/yr to >0.2mm/yr, up to 0.9mm/yr locally. Comparison of ZFT and AFT ages shows that during the Mesozoic the northern segment of the Longmen Shan belt cooled more rapidly than the central and southern segments, while younger AFT ages indicate more rapid exhumation in central and southern segments of the belt during the late Cenozoic. Similar trend is demonstrated within the western Sichuan basin, where the onset of exhumation changes from ∼45Ma in the northern portion to 20–25Ma in the central and southern portions of the basin. The thermochronological data reveal that the Yingxiu-Beichuan and Guanxian-Anxian faults have accommodated significant differential exhumation by thrusting activity and associated denudation, with a greater amount of differential exhumation from north to south. We propose that the major cause for the rise in topography and rapid exhumation in the Longmen Shan thrust belt during the late Cenozoic is the thrusting activities integrated with the regional uplift of the eastern Tibetan Plateau, a process unrelated to the channel flow within the lower crust.

Structural and geochronological constraints on the Mesozoic‐Cenozoic tectonic evolution of the Longmen Shan thrust belt, eastern Tibetan Plateau

The Longmen Shan thrust belt in the eastern margin of the Tibetan Plateau underwent deformation of D1 associated with the Late Triassic collision between the North and South China Blocks, followed by deformation of D2 and D3 related to the eastward growth of the Tibetan Plateau. The D1 is marked by moderately tight folds (f1), spaced cleavage (S1), mineral lineation (L1), multistage reactivated faults, and top‐to‐the south thrusting. This deformation initiated before 237 Ma and ended at 208 Ma. The 193–159 Ma D2 deformation is top‐to‐the‐southeast directed and normal ductile in nature and was associated with the formation of half‐graben basins. The Mesozoic deformation of the Longmen Shan belt was related to the subduction of the Paleo‐Tethys oceanic lithosphere, followed by the South China Block underneath the Songpan‐Ganze Terrane. The D1 was produced in the pro‐side of the overriding Songpan‐Ganze plate, most probably driven by slab corner flow above the retreating South China Block. The D2 and extensional Songpan‐Ganze turbidite basin were produced by roll‐back and migration of the overriding plate above the retreating subduction zone. This model for the deformation from D1 to D2 is similar to a Mediterranean‐style model. The D3 is characterized by SE‐ward thrusting with stepwise migration of rapid denudation and dextral strike‐slip faulting during the Late Cretaceous to present. Therefore, the Longmen Shan thrust belt involved Mesozoic compressional and extensional events that were overprinted by the Late Cretaceous to Cenozoic compressional event.

Seismic slip recorded by fluidized ultracataclastic veins formed in a coseismic shear zone during the 2008 Mw 7.9 Wenchuan earthquake

Dark, aphanitic material occurs as a thin ( w 7.9 Wenchuan, China, earthquake. It also forms injection veins in a >5-cm-wide coseismic shear zone within the Longmen Shan thrust belt. Powder X-ray diffraction data and microstructural analysis indicate that the veins contain little or no glassy material, and are composed of fine-grained fragments set in an ultrafine-grained matrix, all sourced from the country rock of interbedded siltstone and mudstone. On the basis of X-ray diffraction and mesostructural and microstructural features, it is concluded that these veins are ultracataclastic veins generated by coseismic comminution with little melting, accompanied by rapid fluidization in the coseismic shear zone during the 2008 Wenchuan earthquake. The findings support earlier suggestions that (1) ultracataclastic veins that resemble pseudotachylyte veins in appearance can be generated by crushing and rapid injection accompanied by fluidization of fine-grained material during seismic faulting, with little or no melting; and (2) ultracataclastic veins formed in this way may record seismic slip events in seismogenic fault zones, acting as earthquake fossils in much the same way as pseudotachylyte veins.

Eyewitness Accounts of Surface Thrusting and Folding during the 2008 Mw 7.9 Wenchuan Earthquake, China

Research Article| November 01, 2010 Eyewitness Accounts of Surface Thrusting and Folding during the 2008 Mw 7.9 Wenchuan Earthquake, China Aiming Lin; Aiming Lin 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Search for other works by this author on: GSW Google Scholar Zhikun Ren; Zhikun Ren 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Search for other works by this author on: GSW Google Scholar Gang Rao Gang Rao 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Search for other works by this author on: GSW Google Scholar Author and Article Information Aiming Lin 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Zhikun Ren 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Gang Rao 1 Institute of Geosciences, Faculty of Science Shizuoka University Shizuoka Ohya 836 422-8529, Japan slin@ipc.shizuoka.ac.jp (A. L.) Publisher: Seismological Society of America First Online: 09 Mar 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2010 by the Seismological Society of America Seismological Research Letters (2010) 81 (6): 884–891. https://doi.org/10.1785/gssrl.81.6.884 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Aiming Lin, Zhikun Ren, Gang Rao; Eyewitness Accounts of Surface Thrusting and Folding during the 2008 Mw 7.9 Wenchuan Earthquake, China. Seismological Research Letters 2010;; 81 (6): 884–891. doi: https://doi.org/10.1785/gssrl.81.6.884 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The Mw 7.9 Wenchuan earthquake, which occurred on 12 May 2008 in the Sichuan region of China (Figure 1), resulted in significant and widespread damage throughout a large area of central and western China. Official estimates of casualties released by the Chinese government include some 70,000 confirmed deaths, plus approximately 374,000 injured and 18,000 missing. Field investigations and seismic data show that the earthquake was triggered by active faults of the Longmen Shan thrust belt, located on the marginal zone between the high Tibetan plateau and the Sichuan basin. A 285-km-long surface rupture zone produced by the... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Width Distribution of the Surface Ruptures Associated with the Wenchuan Earthquake: Implication for the Setback Zone of the Seismogenic Faults in Postquake Reconstruction

The 12 May 2008 Wenchuan earthquake ( M w 7.9) produced a 240-km-long surface rupture zone along the Beichuan fault and a 72-km-long rupture along the Pengguan fault, which are two of the three subparallel secondary strands of the northeast-striking Longmen Shan thrust fault system in the province of Sichuan, China. Among a total of twenty coseismic surface ruptures surveyed in the field, seventeen are less than 40 m wide, about half of them are 10–30 m wide, and only 3 are over 40–59 m wide. Many buildings sitting directly on the path of the surface ruptures were either completely demolished or severely damaged, including those with a frame or reinforced concrete structure. Other buildings a short distance from the rupture zones were subjected to only minor or no damage. These observations reinforce the use of fault setback zone in the postquake reconstruction of Wenchuan. Considering the crustal shortening of the inverse faulting that triggered the Wenchuan earthquake and uncertainties in width survey of the surface ruptures, we estimate the setback distance of 25 m for the seismogenic faults, which is comparable to the 15–30-m setback distance used in the seismically active regions of California. It is therefore recommended that construction of public buildings and residential structures should not be permitted within the 25-m setback zone of the seismogenic faults in the postquake reconstruction of Wenchuan and its surrounding areas.

Surface Rupture of the 2008 Wenchuan, China, Earthquake in the Qingping Stepover Determined from Geomorphologic Surveying and Excavation, and Its Tectonic Implications

The Wenchuan Mw 7.9 earthquake of 12 May 2008 caused the rupture of the Longmen Shan thrust belt, which bounds the eastern margin of the Tibetan plateau, and generated a very complex surface rupture. The Beichuan-Yingxiu fault (BYF )w as themainseismogenicfaultandformedtwodistinctivelydifferentsurfacerupturezones separated by the Qingping and Gaochuan stepovers. Real-time kinematic (RTK) sur- veying of alluvial terrace sequences indicates that terraces T1-T3 and river floodplain T0 have the same vertical displacement associated with the Wenchuan earthquake. Trench excavation and optical stimulated luminescence (OSL) dating of alluvial depos- its indicate that only the deformation of the Wenchuan earthquake was recorded in the Qingping stepover since at least ∼20 ka. Other deformations since ∼20 ka probably occurred in other places or did not reach the surface. This can be meaningful to analyze the completeness of paleoearthquakes in trench excavation on thrust faults. The width of the Qingping stepover is ∼1 km and it is not strong enough to form a barrier to arrest the propagation of rupture in the 2008 Wenchuan earthquake. The Gaochuan stepover is ∼7 km wide and it is a barrier to separate the two surface rupture zones associated with the Wenchuan earthquake along the BYF. The empirical rela- tionship of rupture displacement and stepover dimensions on strike-slip faults is prob- ably suitable for the large thrust fault zone with a strike-slip component. This may be beneficial to the study of cascade rupture sections and seismic hazard assessment on large thrust faults.

Fault-Scarp Features and Cascading-Rupture Model for the Mw 7.9 Wenchuan Earthquake, Eastern Tibetan Plateau, China

On 12 May 2008, the M w 7.9 Wenchuan earthquake ruptured two northeast-striking imbricated reverse faults and one northwest-striking reverse fault along the middle Longmen Shan thrust belt, at the eastern margin of the Tibetan plateau. The morphology of the coseismic scarp varies drastically along strike. We distinguish eight different categories: (1) simple thrust scarp, (2) hanging-wall collapse scarp, (3) simple pressure ridge, (4) dextral pressure ridge, (5) fault-related fold scarp, (6) backthrust pressure ridge, (7) local normal fault scarp, and (8) pavement suprathrust scarp. The coseismic surface ruptures confirm that the Wenchuan earthquake is dominated by reverse faulting with some right-lateral component that varies from site to site. The surface rupture can be divided into two parts, the Yingxiu segment and the Beichuan segment. When the earthquake is split into two subevents accordingly, they correspond to an M w 7.8 event and an M w 7.6 event, respectively. These two segments can in turn be divided into four second-order subsegments, which are equivalent to four subevents of M w 7.5, M w 7.7, M w 7.0, and M w 7.5, respectively. The segmentation of the rupture is consistent with a cascading-rupture pattern, responsible for the total 110 s of the earthquake rupture. In addition to surface ruptures, the focal mechanisms determined for the aftershocks recorded by the local seismic network are used to constrain the fault geometry of the subsegments. They show that the dip of the fault responsible for the earthquake varies along strike, and the fault tends to flatten at depth. In addition, the fault plane gets steeper northward, enabling the fault to accommodate a larger strike-slip component along a lateral ramp. This major earthquake confirms that crustal shortening could be the main driving force for the growth of high topography along the eastern margin of the Tibetan Plateau and that lower crustal flow is not required.

Structural evolution of the Piqiang Fault Zone, NW Tarim Basin, China

The Piqiang Fault is a prominent strike-slip (tear) fault that laterally partitions the Keping Shan Thrust Belt in the NW Tarim Basin, China. In satellite images, the Piqiang Fault appears as a sharp, NW-trending lineament that can be traced for more than 70 km. It is oblique to the general structural trend of the thrust belt and subparallel to the thrust transport direction. This paper presents a structural analysis of the Piqiang Fault, based on satellite image interpretation and field data. A net loss of Late Paleozoic sediment across the fault zone implies that it was initiated as a major normal fault during the Early Permian, and corresponds to widespread extension and magmatism during this period. Differential erosion across the fault resulted in the subsequent removal of sediment from the east relative to the west. During the Middle to Late Cenozoic, contraction of the NW Tarim Basin and the formation of the Keping Shan Thrust Belt resulted in reactivation of the Piqiang Fault as a strike-slip (tear) fault. The fault has accommodated lateral differences in thrust density and spacing which have arisen due to the abrupt, pre-existing change in stratigraphic thickness across it. The Piqiang Fault provides an insight into the formation of oblique, strike-slip (tear) faults in contractional belts and demonstrates the importance of inherited basement structures in such settings.

Structural analysis of the coseismic shear zone of the 2008 M w 7.9 Wenchuan earthquake, China

Field investigations reveal that the surface rupture of the 2008 M w 7.9 Wenchuan earthquake, China, occurred along a pre-existing shear zone in the Longmen Shan Thrust Belt. Structural analyses of the coseismic fault zone and fault rocks show that i) the main coseismic shear zone consists of a fault core that includes a narrow fault gouge zone of <15 cm in width (generally 1–2 cm) and a fault breccia zone of <∼3 m in width, and a wide damage zone of >5 m in width that is composed of cataclastic rocks including fractures and subsidiary faults; ii) the foliations developed in the fault core and damage zones indicate a dominantly thrust slip sense, consistent with that indicated by the coseismic surface rupture; and iii) coseismic slip was largely localized to within a narrow fault gouge zone of <2–3 mm in width. The structural characteristics of the coseismic shear zone and cataclastic rocks indicate that the location of coseismic slip zone associated with the 2008 Wenchuan earthquake was controlled by a pre-existing shear zone and that the main active fault of the Longmen Shan Thrust Belt has moved as a thrust since the formation of cataclastic rocks along the fault during the late Miocene or early Pliocene.

Trenching exposures of the surface rupture of 2008 Mw 7.9 Wenchuan earthquake, China: Implications for coseismic deformation and paleoseismology along the Central Longmen Shan thrust fault

The May 12, 2008, Mw 7.9 Wenchuan earthquake was induced by failure of two of the major faults of the Longmen Shan thrust fault zone along the eastern margin of Tibet Plateau. Our study focused on trenches across the Yingxiu–Bichuan fault, the central fault in the Longmen Shan belt that has a coseismic surface break of more than 200 km long. Trenching excavation across the 2008 earthquake rupture on three representative sites reveals the styles and amounts of the deformation and paleoseismicity along the Longmen Shan fault. Styles of coseismic deformation along the 2008 earthquake rupture at these three sites represent three models of deformation along a thrust fault. Two of the three trench exposures reveal one pre-2008 earthquake event, which is coincident with the pre-existing scarps. Based on the observation of exposed stratigraphy and structures in the trenches and the geomorphic expressions on ground surface, we interpret the 2008 earthquake as a characteristic earthquake along this fault. The interval of reoccurrence of large earthquake events on the Central Longmen Shan fault (the Yingxiu–Beichuan fault) can be inferred to be about 11,000 years according to 14C and OSL dating. The amounts of the vertical displacement and shortening across the surface rupture during the 2008 earthquake are determined to be 1.0–2.8 m and 0.15–1.32 m, respectively. The shortening rate and uplift rate are then estimated to be 0.09–0.12 mm/yr and 0.18–0.2 mm/yr, respectively. It is indicated that the deformation is absorbed mainly not by shortening, but by uplift along the rupture during the 2008 earthquake.

Structural interpretation of the coseismic faults of the Wenchuan earthquake: Three‐dimensional modeling of the Longmen Shan fold‐and‐thrust belt

The 2008 Mw 7.9 Wenchuan earthquake is a result of ongoing India‐Tibet collision and reflects the growth of the Longmen Shan fold‐and‐thrust belt. In this paper, we construct a 3‐D structural model of the geometry of the coseismic faults and related structures of the Wenchuan earthquake by integrating geological investigations, relocated aftershocks, and seismic reflection profiles. In the 3‐D structural model, the differences between the southern and northern segments of the rupture are highlighted. The structural transition zone between the two segments contains a major geometric segment boundary, reflecting differences in the structural configuration of the thrust ramp and the tectonic evolution of the fault system, which appears to have localized significant damage from Anxian to Beichuan. Within the northern segment, we identify a transverse structure across which the Beichuan fault plunges under the Tangwangzhai syncline. This boundary corresponds to a marked change in the nature of the surface rupture and is illuminated by a microearthquake sequence perpendicular to the Longmen Shan thrust belt. In the southern segment, our investigations confirm that uplift due to active faulting and folding is largely responsible for the areas of steepest topography. On the basis of this association, the southwestern segment of the Longmen Shan, south of the Wenchuan earthquake, is likely active and presents a significant earthquake hazard, despite the lack of historical earthquakes in this region. This study illustrates the importance of building 3‐D models to study active faulting and folding, as well as to assess earthquake hazard.