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Uplift and erosion driven by Wenchuan earthquake and their effects on geomorphic growth of Longmen Mountains:A case study of Hongchun gully in Yingxiu,China

LI Yong;ZHOU Rong-jun;ZHAO Guo-hua;LI Hai-bing;SU De-chen;DING Hai-rong;YAN Zhao-kun;YAN Liang;YUN Kun;MA Chao;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Chengdu University of Technology;Institute of Earthquake Engineering,Seismological Bureau of Sichuan Province;Institute of Geology,Chinese Academy of Geological Science;  
How the tectonic uplift,landslides,and debris flow erosion driven by the Wenchuan(Ms 8.0)earthquake affect the geomorphic growth of Mt.Longmen is the focus of the debate at present.The aim of this study is to make clear how the mass wasting triggered by the Wenchuan earthquake affect the growth of the Mt.Longmen.Field data,aerial photographs,and digital elevation data are used to conduct quantitative analyses of the tectonic uplift driven by the Wenchuan earthquake and of landslide and debris flow erosion processes,along with their effects on the geomorphic growth of Mt.Longmen.The preliminary insights obtained are as follows.(1)The Hongchun gully is located in alpine valleys within the earthquake epicenter area.The seismogenic fault- the Beichuan fault cuts through the gully.(2)The Wenchuan earthquake triggered the coseismic landslides of 3.800 1×106m3,being three times the volume before the earthquake.(3)The volume of the debris flow resulting from heavy rainfalls on August 13 and 18,2010 after the earthquake was 0.705×106 m3,indicating that 20% of the volume of the coseismic landslides after the earthquake was changed into debris flows because of heavy rainfalls.(4)The Hongchun gully debris flow discharged solid materials of 0.485×106 m3 into the Minjiang River(the main river),leading the watercourse to be narrowed,the riverbed elevated,and the riverbed gradient increased.(5)The conversion ratio between the sediments input into the watercourse and the coseismic landslide sediments was about 13%.The conversion ratio between the sediments input into the watercourse and the debris flow sediments was 70%.(6)The volume(3.800 1×106 m3)of the coseismic landslides in the Hongchun gully was less than that of the coseismic uplift(6.67×106 m3).Only about 57% of the uplift volume was converted into the landslide volume,indicating agreater volume of the tectonic uplift compared to the volume of the landslides resulting from the Wenchuan earthquake characterized by thrust and strike-slip motions.This has caused the new uplift and the geomorphic growth of the Mt.Longmen.
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