Retention and transport of coseismic landslide sediments induced by the Wenchuan earthquake, and its implication for the topographic evolution of the Longmen Shan

W Wang ,P Wang ,Chong Xu ,Junyi Zhang ,Fangzhou Liu ,Li Zg ,Xin Xu ,Vincent Godard ,J Liu ,Kj Xie

doi:10.6038/cjg2018m0074

Abstract

The 2008 M(w)7. 9 Wenchuan earthquake, China, ruptured the parallel Beichuan and Pengguan faults along the Longmen Shan, eastern margin of the Tibetan plateau, which induced a large amount of landslides, volume comparable to the coseismic uplift. However, there lacks systematic data and research to quantify the removal of these loose materials and temporal variation of fluvial erosion rates, which is central to understand the mass redistribution along the Longmen Shan and the role of such earthquake in building topography in active mountain ranges. Over the period of 2008-2013 after the Wenchuan earthquake, we collected a series of river sand samples along three tributaries of the Tuojiang River, including the Jianjiang, Shiting Jiang, and Mianyuan River, and measured the detrital quartz Be-10 concentration. Together with published river sand Be-10 concentration prior to the Wenchuan earthquake, our results show that; (1) Detrital Be-10 concentration declined dramatically after the Wenchuan earthquake, which shows the dilution effect of co-seismic landslides; (2) Fluvial erosion rate in the upstream of Mianyuan River increased to 1. 3 similar to 18. 5 times of the pre-earthquake level, which indicates there occurs an obvious increase of erosion rates shortly after the earthquake; (3) Under the assumption of constant erosion rate, a simple calculation indicates the river will take at least 100 similar to 4000 years to move away these loose materials, comparable to the return time of Wenchuan-type largemagnitude earthquake in the Longmen Shan. Considering the effect of isostasy, we think that the Wenchuan earthquake will lead to a net material surplus in the Longmen Shan; and (4) The vertical motion and denudation at surface during the interseismic period and cosesimic rupture have different spatial pattern, which together control the topographic evolution of eastern Tibet.

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