Introduction to the Special Issue on the 2008 Wenchuan, China, Earthquake (original) (raw)
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Uplift of the Longmen Shan range and the Wenchuan earthquake
Episodes, 2008
The 12 May 2008 Wenchuan earthquake (M s =8.0) struck on the Longmen Shan foreland thrust zone. The event took place within the context of long-term uplift of the Longmen Shan range as a result of the extensive eastward-extrusion of crustal materials from the Tibetan plateau against the rheologically strong crust of the Sichuan Basin. The Longmen Shan range is characterized by a Pre-Sinian crystalline complex constrained by the Maoxian-Wenchuan-Kangding ductile detachment at the western margin and the Yingxiu-Beichuan-Luding ductile thrust at the eastern margin. The Longmen Shan uplift was initiated by intracontinental subduction between the Songpan-Ganzi terrane and the Yangtze block during the Pre-Cenozoic. The uplift rate was increased considerably by the collision between the Indian and Eurasian plates since ~50 Ma. The Wenchuan earthquake resulted in two major NE-striking coseismic ruptures (i.e., the ~275 km long Yingxiu-Beichuan-Qingchuan fault and the ~100 km long Anxian-Guanxian fault). Field investigations combined with focal solutions and seismic reflection profiles suggest that the coseismic ruptures are steeply dipping close-topure reverse or right reverse oblique slip faults in thẽ 15 km thick upper crust. These faults are unfavorably oriented for frictional slip in the horizontally compressional regime, so that they need a long recurrence interval to accumulate the tectonic stress and fluid pressure to critically high levels for the formation of strong earthquakes at a given locality. It is also found that all the large earthquakes (M s >7.0) occurred in the fault zones across which the horizontal movement velocities measured by the GPS are markedly low (<3 mm/yr). The faults, which constitute the northeastern fronts of the enlarging Tibetan plateau against the strong Sichuan Basin, Ala Shan and Ordos blocks, are very destructive, although their average recurrence intervals are generally long.
Geochemistry, Geophysics, Geosystems, 2009
We determined detailed 3-D images of P and S wave velocity (Vp, Vs) and Poisson's ratio (s) in and around the Longmenshan (LMS) fault zone by using a large number of P and S wave arrival times from the aftershocks of the 2008 Wenchuan earthquake (Ms 8.0) and other local events. Our results show that the structure of the LMS fault zone north of the Wenchuan main shock is very different from that south of the main shock. The southern section of the LMS fault zone contains a broad zone with low-Vp, low-Vs, and highs anomalies, while the northern segment exhibits more scattered heterogeneities, corresponding to most of the aftershocks which occurred there. A prominent low-Vp, low-Vs, and highs anomaly exists directly beneath the Wenchuan main shock hypocenter, suggesting that in addition to compositional variations, fluid-filled fractured rock matrices may exist in the LMS fault zone, which may have influenced the generation of the large Wenchuan earthquake. Our tomographic results provide sound seismic evidence for the hypothesis that an upward intrusion of lower crustal flow occurred along the LMS fault zone. In addition, most small earthquakes before the 2008 Wenchuan main shock occurred around the Guanxian-Jiangyou fault, while the Wenchuan aftershocks were mainly concentrated on the Yingxiu-Beichuan fault, suggesting that the rupture process of the Wenchuan earthquake may belong to an out-of-sequence thrusting event, a suggestion which is in good agreement with the results from geological surveys and also quite similar to the rupture processes of the 1999 Chi-Chi earthquake (M 7.5) and the 2005 Kashmir earthquake (M 7.6). A few aftershocks occurred close to the blind Guangyuan-Dayi fault in the Sichuan basin, suggesting that this blind fault was also ruptured by the Wenchuan earthquake, consistent with geological surveys.
A preliminary report on the Great Wenchuan Earthquake
Earthquake Engineering and Engineering Vibration, 2008
The May 12, 2008 Great Wenchuan Earthquake has resulted in more than 68,858 deaths and losses in the hundreds of billions RMB as of May 30, 2008, and these numbers will undoubtedly increase as more information becomes available on the extent of the event. Immediately after the earthquake, the China Earthquake Administration (CEA) responded quickly by sending teams of experts to the affected region, eventually including over 60 staff members from the Institute of Engineering Mechanics (IEM). This paper reports preliminary information that has been gathered in the fi rst 18 days after the event, covering seismicity, search and rescue efforts, observed ground motions, and damage and loss estimates. The extensive fi eld investigation has revealed a number of valuable fi ndings that could be useful in improving research in earthquake engineering in the future. Once again, this earthquake has shown that the vertical component of ground motion is as signifi cant as horizontal ground motions in the near-source area. Finally, note that as more information is gathered, the numbers reported in this paper will need to be adjusted accordingly.
The Role of Late Quaternary Upper-Crustal Faults in the 12 May 2008 Wenchuan Earthquake
Bulletin of the Seismological Society of America, 2010
The role of upper-crustal faulting in building and maintaining the extreme relief of the Longmen Shan region at the eastern margin of the Tibetan Plateau has been strongly debated. The M w 7.9 Wenchuan earthquake of 12 May 2008 ruptured three distinct faults along the plateau margin and thus provides a unique insight into the mechanisms and kinematics of regional crustal deformation. Two of the faults, the northeast-striking Beichuan and Pengguan faults, had been previously recognized as active, with past surface-rupturing earthquakes in the latest Pleistocene to Holocene time. Some of the late Quaternary fault segments were activated in the Wenchuan earthquake, but others, including several with clear evidence of Holocene strike-slip displacement, were not. Instead, in almost all cases, the Wenchuan surface rupture followed geological faults mapped on the basis of bedrock lithology. We infer that active deformation along the plateau margin is accommodated by partial reactivation of a complex network of preexisting faults but that a single through-going structure has not been established-and may never be if the total deformation is strongly rotational and three-dimensional. The earthquake also illustrates that the Beichuan and Pengguan faults are kinematically linked, in part by the northwest-striking Xiaoyudong fault, and that both must be strongly listric in the upper few km of the crust. The complexity of the active (or potentially active) fault network in the Longmen Shan, coupled with rapid postearthquake modification of the surface rupture trace, poses serious difficulties for seismic hazard assessment across the region.
Earth and Planetary Science Letters, 2009
Longmen Shan thrust belt Tibetan plateau surface rupture co-seismic slip partitioning out-of-sequence thrusting earthquake The M s 8.0, Wenchuan earthquake, which devastated the mountainous western rim of the Sichuan basin in central China, produced a surface rupture over 200 km-long with oblique thrust/dextral slip and maximum scarp heights of~10 m. It thus ranks as one of the world's largest continental mega-thrust events in the last 150 yrs. Field investigation shows clear surface breaks along two of the main branches of the NE-trending Longmen Shan thrust fault system. The principal rupture, on the NW-dipping Beichuan fault, displays nearly equal amounts of thrust and right-lateral slip. Basin-ward of this rupture, another continuous surface break is observed for over 70 km on the parallel, more shallowly NW-dipping Pengguan fault. Slip on this latter fault was pure thrusting, with a maximum scarp height of~3.5 m. This is one of the very few reported instances of crustal-scale co-seismic slip partitioning on parallel thrusts. This out-of-sequence event, with distributed surface breaks on crustal mega-thrusts, highlights regional,~EW-directed, present day crustal shortening oblique to the Longmen Shan margin of Tibet. The long rupture and large offsets with strong horizontal shortening that characterize the Wenchuan earthquake herald a re-evaluation of tectonic models anticipating little or no active shortening of the upper crust along this edge of the plateau, and require a re-assessment of seismic hazard along potentially underrated active faults across the densely populated western Sichuan basin and mountains.
Tectonophysics, 2010
The 12 May 2008 Wenchuan earthquake (Ms 8.0) occurred on the Longmen Shan fault zone, where slip rates are low and earthquakes are infrequent in comparison with other major fault zones in the Songpan-Ganze region, eastern Tibet. We have investigated the evolution of strain energy along major faults in this region by comparing the accumulation and release of seismic moment. First, we calculated the slip rates on the Longmen Shan and other major faults in the region using a three-dimensional regional-scale block model, constrained by the latest GPS data. On the Longmen Shan fault, the predicted right-lateral and dip-slip rates are respectively 1.7 ± 0.8 mm/year and 1.2 ± 1.0 mm/year along the southwestern segments, and 1.4 ± 1.1 mm/year and 3.3 ± 1.3 mm/year on the northeastern segments. These slip rates are one order of magnitude lower than those on the Xianshuihe fault and other faults in the region. Second, using the earthquake catalog, we estimated the scalar moment released on major faults in the Songpan-Ganze region between 1879 and 2007. The released seismic moment was~63% of the regional scalar moment accumulated during this period. The moment deficits were found mainly on the western Xianshuihe and eastern Kunlun faults. The eastern Xianshuihe fault has been a focus of studies because of the high slip rates and frequent earthquakes, but a sequence of major quakes there since 1893 has reduced the moment deficit to 3.68 × 10 19 N m, barely enough for a Mw 7.0 event. It takes the Longmen Shan fault more than 1000 years to accumulate the seismic moment (1.15 × 10 21 N m) released during the 2008 Great Wenchuan earthquake, hence we conclude that a repeat of great earthquakes on the ruptured segment of the Longmen Shan fault is unlikely in the next few hundred years, but the unruptured southwestern segment of the Longmen Shan fault is capable of producing a Mw 7.7 earthquake in the next 50 years.
Rupture Process of the 2008 Wenchuan, China, Earthquake: A Review
Earthquake and Disaster Risk: Decade Retrospective of the Wenchuan Earthquake, 2019
The May 12, 2008, Wenchuan earthquake (MW 7.9, MS 8.1) is the largest continental intraplate event to strike globally in the last 60 years. It caused great destruction and loss of life along the steep eastern margin of the Tibetan Plateau, adjacent to the Sichuan Basin. The event ruptured multiple faults with a mix of thrust- and right-lateral strike-slip faulting along the northeast-trending Longmen Shan thrust belt, with an overall oblique compressional deformation. Surface displacements of up to ~11 m, the distribution of thousands of aftershocks and landslides, geodetic observations, and seismic wave imaging indicate a total rupture extent of ~280 km, extending unilaterally northeastward from the hypocenter. The primary slip has a patchy distribution along the segmented out-of-sequence Beichuan fault, with large-slip patches in the region from Yingxiu to Xiaoyudong, near Beichuan, and near Nanba. The southwestern segment near Yingxiu, where the hanging wall is comprised of the h...
Stress changes on major faults caused by M w7.9 Wenchuan earthquake, May 12, 2008
Science in China Series D: Earth Sciences, 2009
On May 12, 2008, a magnitude 7.9 earthquake ruptured the Longmenshan fault system in Sichuan Province, China, collapsing buildings and killing tens of thousands people. As predicted, aftershocks may last for at least one year, and moreover, large aftershocks are likely to occur. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settling people as to avoid future disasters. It is demonstrated that the redistribution of stress induced by an earthquake should trigger successive seismic activity. Based on static stress triggering theory, we calculated the coseismic stress changes on major faults induced by the Wenchuan earthquake, with elastic dislocation theory and the multilayered crustal model. We also discuss the stress distribution and its significance for future seismic activity under the impact of the Wenchuan earthquake. It is shown that coulomb failure stress (CFS) increases obviously on the Daofu-Kangding segment of the Xianshuihe Fault, the Maqu and Nanping segment of the Eastern Kunlun Fault, the Qingchuan Fault, southern segment of the Minjiang Fault, Pengxian-Guanxian Fault, Jiangyou-Guangyuan Fault, and Jiangyou-Guanxian Fault. The increased stress raises the probability of earthquake occurrence on these faults. Since these areas are highly populated, earthquake monitoring and early disaster alarm system are needed. CFS increases with a magnitude of 0.03-0.06 MPa on the Qingchuan Fault, which is close to the northern end of the rapture of Wenchuan earthquake. The occurrence of some strong aftershocks, including three events with magnitude higher than 5.0, indicates that the seismic activities have been triggered by the main shock. Aftershocks seem to migrate northwards. Since the CFS change on the Lueyang-Mianxian Fault located on the NEE of the Qingchuan Fault is rather small (±0.01 MPa), the migration of aftershocks might be terminated in the area near Hanzhong City. The CFS change on the western Qinling Fault is around 10 Pa, and the impact of static triggering can be neglected. The increment of CFS on the Pengxian-Guanxian Fault and Beichuan-Yingxiu Fault southwest to the main rupture is 0.005-0.015 MPa, which would facilitate earthquake triggering in these areas. Very few aftershocks in these areas indicate that the accumulated stress has not been released sufficiently. High seismic risk is predicated in these areas due to co-seismic CFS loading. The Wenchuan earthquake released the accumulated CFS on the Fubianhe Fault, the Huya Fault, the Ha'nan-Qingshanwan Fault, and the Diebu-Bailongjiang Fault. The decrement of CFS changes on the Longquanshan Fault east to Chengdu City is about 0.002 MPa. The seismic activity will be depressed by decrement of CFS on these faults.