Analysis of Kinematic Mechanism of Active Fold-Thrust Fault in front of the Southern Longmenshan Mountain: A Case Study of the Lushan Earthquake
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    Abstract:

    The Lushan earthquake (MS7.0, April 20, 2013)that occurred in the southern segment of the Longmenshan Fault was another destructive earthquake that followed the Wenchuan (MS8.0) earthquake that occurred in the middle-northern section of the Longmenshan Fault.The epicenter was 30.3° N, 103.0° E at a depth of about 13 km.The Longmenshan Fault is located on the eastern side of the Qinghai-Tibetan Plateau, which marks an area of abrupt geological, geomorphological, and climatological changes in western China.There are three main faults in the southern section of the Longmenshan Fault, i.e., the Gengda-Longdong, Yanjing-Wulong, and Shuangshi-Dachuan faults.The research area encompassed three levels of geomorphic units that from west to east comprised landforms of the Qinghai-Tibetan Plateau, the alpine zone of the southern section of the Longmenshan Fault, and a piedmont alluvial plain area (Southwest of Chengdu Basin).The three main faults are located in the alpine zone in the southern section of the Longmenshan Fault, which crosses the Qingyi River.In this paper, a preliminary analysis of the seismogenic tectonic mechanism of the Lushan earthquake is presented based on data of post-earthquake surface deformational characteristics, aftershocks, focal mechanism solutions, and oil seismic exploration profiles.The results indicate that the Lushan earthquake was a typical folding earthquake, the seismogenic fault of which might have been the buried fault in front of the Longmenshan Mountain.The shortening produced by tectonic compression can be absorbed by folding.Considering the Lushan earthquake as an example, analysts suggest that the southern segment of the Longmenshan frontal region has the kinematic characteristics of an active fold-thrust fault, which is indicated by the enlarged Longmenshan thrusting towards the inner Sichuan Basin.Young, rapidly growing active folding is related not only to the earthquake location and seismic structure but also to the product of continuous earthquakes.In the southern Longmenshan thrust belt, at the same time as the late-Cenozoic faulting, the thrust belt entered the western Sichuan Basin, and the Longquanshan, Xiongpo, and Qiongxi faults were formed. From the Shuangshi fracture eastward to the Longquanshan tectonic belt, there are several rows of fault-related folds over one decollement at the bottom of the middle and lower Triassic Plaster gypsum stratum.Research has shown that the Lushan earthquake was an active fault-folding earthquake that differed from the Wenchuan earthquake.These active folds with stick-slip mechanisms are called "earthquake drapes". Traditionally, it has been considered that fold structures are products of homogeneous stress states, and therefore, earthquakes that occur in active folds are considered non-dangerous.However, the the Lushan folding earthquake caused great damage.In the southern segment of the Longmenshan Fault and its front areas, there is less chance of a great earthquake like the Wenchaun earthquake occurring;however, stress will concentrate in the Sichuan Basin by the detachment fault, and therefore, a watchful eye must be maintained on the faults in that basin, especially the Pujing-Xinjin fault.

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History
  • Received:April 21,2014
  • Revised:
  • Adopted:
  • Online: May 28,2015
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