Quantitatively Study The Deformation Of River Terraces Above Qingyi Jiang River In The Southern Segment Of The Longmen Shan Fault Zone

On 20 April 2013,another destructive earthquake,the Lushan Ms7.0 earthquake,occurred in the southern segment of the Longmen Shan fault zone,the eastern margin of the Tibetan plateau in Sichuan,China.This earthquake has not produced a surface rupture zone,and its seismogenic structure is not clear.Due to lack of the Quaternary sediment in the southern segment of the Longmen Shan fault zone,the tectonic activity of this area is not clear.This paper try to improve the understanding of the tectonic activity of the southern segment of the Longmen Shan fault zone,and explore the seismogenic structure of the Lushan earthquake.Firstly, the longitudinal profile of the terraces above the Qingyijiang River in the south segment of the Longmen Shan fault zone was reconstructed,which was based on geological interpretation of high-resolution remote sensing images,continuous differential GPS surveying along the terrace surfaces and geomorphic field evidence.Secondly,based on studying the relationship between tectonics and climate changes,the abandoned age of each river terraces of Qingyijiang River was obtained.At last,combined with the data collected from the dynamic seismic station which located on the southern segment of the Longmenshan fault zone which was deployed by the Institute of Earthquake Science,CEA,from 2008 to 2009,the result of the aftershock relocation,the regional geological data and a lot of other data about this region,this article has quantitatively studied the late Quaternary tectonic activity of the southern segment of the Longmen Shan fault zone.The following conclusions are made:1)Six-level Late Quaternary terraces,T1,T2,T3,T4,T5,T6,are well developed along the Qingyijiang River.Terraces T1 are mainly fill terraces and they are well preserved.Terraces T2 to T6 are mainly cut terraces,whereas they are poorly preserved. Based on studying the relationship between tectonics and climate changes, the abandoned age of each river terrace above the Qingyijiang River is obtained,which are 8-15ka、20-27ka、50-57ka、95-105ka、130-140ka、750-770 ka respectively.Thus,the Late Quaternary geomorphic time ruler is reconstructed.2)The vertical thrust rate of the Yanjing-Wulong fault is nearly 0.7mm/a in the Late Pleistocene.3)The Dachuan-Shuangshi fault has not thrusted obviously since the accumulated time of terrace T2 and its activity has not tilted the middle block since the Late Pleistocene.4)The coverage rate of the anticline-like mountain in the front of the southern segment of Longmen Shan is nearly 0.015mm/a.5)The seismogenic structure of the Lushan earthquake is a structural system,which consist of the Dachuan-Shuangshi fault,the floor thrust,the Lushan seismogenic fault which is a frontal ramp,the Lushan back-thrust fault,and the pop up structure between the ramp and the back-thrust fault.If the GPS recorded maximum horizontal displacement of the Lushan earthquake can represent every horizontal displacement of earthquakes in this region,the recurrence of the Lushan earthquake is nearly 2000-2600 a.6)Although the hypocenter of the Lushan earthquake is just 50 km from the Wenchuan and it occurred just five years after the Wenchuan earthquake, the seismogenic structures of the two earthquakes are different.Therefore,it is not reasonable to hold that the Lushan earthquake has filled the gap of the Wenchuan earthquake in the southern segment of the Longmen Shan fault zone.The Yanjing-Wulong fault in the southern segment of Longmen Shan still have risk to generate a large earthquake.However,the accumulated stress in the southern segment of the Longmen Shan can also be released by smaller earthquakes which are generated in the front of the southern segment of the Longmen Shan,such as the Lushan earthquake.