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Lushan, Sichuan Ms 7.0 earthquake: a typical event by a blind thrust fault

2014/5/27 13:24:22

The Lushan Ms7.0 earthquake on April 20, 2013 is another destructive event on the nappe tectonic zone of the Longmenshan Mountains after Wenchuan Ms8.0 earthquake of May 12, 2008. The focal mechanism solutions by multiple domestic and foreign scientific research institutions suggest that the Lushan Ms7.0 s earthquake is a typical thrust faulting event at the eastern tip of the eastward escape of the Bayanhar block in the central and eastern Tibetan Plateau, adjoinging the Sichuan Basin in the northwestern South China Block. The epicenter is in the territory of Lushan County (30.314°N, 102.934°E), the focal depth is 12.3 - 15 km, the fault strike of the hypocenter is N40°E, the dip is about 35°, the surface wave magnitude is 7.0 and the moment magnitude is about 6.6 (USGS, Event ID: usb000gcdd; Liu Chao, Xu Lisheng, Chen Yuntai, 2013). After occurrence of the earthquake, 16 scientific researchers from the Institute of Geology and the Institute of Crustal Dynamics of China Earthquake Administration and Sichuan Earthquake Bureau are jointly responsible for emergency scientific research of the earthquake. The main task is to determine distribution of earthquake surface rupture zones and distribution of coseismic displacement and analyze the seismogenic structure of the Lushan Ms 7.0 earthquake.

The emergency scientific research on the earthquake site was started on April 21, 2013 by four work groups. They studied geology of the possible seismogenic faults in the meizoseismal area as well as its neighboring areas, including: the Dayi fault, Shangli Town fault, Gaohe Town fault, Shuangshi–Dachuan fault, Yanjing – Wulong fault and other fractures as well as North Lushan – Longmen Township – Taiping Town strongly-affected area in a shape of stripe. So far, we have not found any real-sense earthquake surface rupture zone, while water spraying and sand emitting is often seen on low river terraces or flood plains. For example, water spraying and sand emitting in en echelon arrangement and an overall strike of about 75° is found on the flat ground at front of and behind Yu Youtao’s house at Yankan Group, Shuanghe Village, Shuangshi Town along Shuangshi – Dachun Fracture (Photo 1). The excavation of exploratory trenches reveals water spraying and sand emitting mainly occurs along SE tension cracks (Photo 2). Besides, there are also compressed ruptures (Photo 3) on cement pavement of the highway and bending of cement dike dam of roadside drain (Photo 4) along the NE extension line, forming an about 1km long surface disturbed zone. They are surface secondary ruptures induced by sand liquefaction and may also reflect that local crustal shortening in this area along the Shuangshi – Dachuan fault. Besides, along river embankments and steep slopes, extension cracks parallel to the bank and caused by vibration and gravity and relevant landslides (Photo 5 and Photo 6) are often seen. This is a section with the most obvious phenomena discovered during this inspection.  

It is reported on a website (http://www.fzxy.edu.cn/art/2013/4/25/art_52_52241.html) that there is a 500m-long earthquake surface rupture on and near the highway at Caojia Village, Daxi Township. After field verification, we think it is not an earthquake surface rupture in a real sense, instead a slope effect generated under the action of vibration and gravity: there are two opposite “quasi normal fault” scarps. There are arc extensional crack zones on high positions of a slope. They connect the two opposite “quasi normal faults”. These arc extensional crack zones gradually disappear after short extension. Therefore, we think they are not real-sense earthquake surface rupture zones, rather secondary slope effect generated under the action of vibration and gravity.

Besides, along other faults, extensional cracks on slopes are also developed and landslides or slope collapses are obvious. Particularly, on hillside on the two sides of Longmen Township and on the steep slopes with an elevation of about 1500m on the northern side of Shangli, Zhongli and Xiali, extensional cracks are widely distributed. They are noteworthy. Secondary geological hazards should be prevented.

Apparently, this earthquake didn’t create obvious earthquake surface rupture zones and is a typical blind thrust faulting event. Same as other thrust fault type earthquakes, its seismogenic fault does not reach the surface and is hidden under the ground.

The scientific researchers participating in this emergency scientific research include Xu Xiwei (leader), Wen Xueze, Han Zhujun, Zhang Shimin, Li Chuanyou, Zheng Wenjun, Chen Guihua, Ren Zhikun, Xu Chong, Tan Xibin, Wei Zhanyu, Liang Mingjian, Ren Junjie and Wang Mingming.

http://www.eq-igl.ac.cn/admin/upload/Photos/科研动态/S20130426082458750076.jpg

Photo 1: Water spraying and sand emitting on level-1 low terrace at Shuanghe Village, Shuangshi Town



http://www.eq-igl.ac.cn/admin/upload/Photos/科研动态/S20130426082457730074.jpg

Photo 2: Connected tension cracks under the water spraying and sand emitting point



http://www.eq-igl.ac.cn/admin/upload/Photos/科研动态/S20130426082458230075.jpg

Photo 3: Compressed rupture on cement pavement of a highway



http://www.eq-igl.ac.cn/admin/upload/Photos/科研动态/S20130426082456220071.jpg

Photo 4: Bending and shortening of cement dike dam



http://www.eq-igl.ac.cn/admin/upload/Photos/科研动态/S20130426084116360077.jpg

Photo 5: Tension crack formed under the action of vibration and gravity



http://www.eq-igl.ac.cn/admin/upload/Photos/科研动态/S20130426082457200073.jpg

Photo 6: Landslide mass and arc scarp on its rear border