Landsliding and sediment dynamics following the 2008 Wenchuan Earthquake in the Beichuan area of China

Abstract

Extensive and widespread landsliding is a common feature in a post-earthquake mountainous environment. The intense seismic shaking of an earthquake leaves the ground destabilised and thus very susceptible to slope failure. In addition to co-seismic landsliding, many slopes retain the high potential to fail for a significant amount of time beyond seismic activity. Therefore there is a need to further develop our understanding of sediment dynamics of steep mountain environments once the shaking has stopped. The 2008 Wenchuan Earthquake in China resulted in widespread landsliding, generating large volumes of loose rock and soil. Examples from other recent large earthquakes warn of the potential secondary hazards associated with such loose material: up to 30m of river-bed aggradation was seen following the 1999 Chi-Chi Earthquake, Taiwan and it is thought that Sichuan may experience hazards of a similar magnitude. Preliminary reports and oblique photographs have displayed significant levels of sediment aggradation in certain areas and summer monsoonal rains continue to trigger further landslide failures. In addition to the associated hazards, this event has provided the opportunity to investigate sediment dynamics following a large earthquake (Mw = 7.9) in a unique area of heterogeneous lithology and wide ranging geophysical variables, which has been impacted upon by both seismic and post-seismic (rainfall) activity. This study uses a combination of desk-based and field-based research in order to examine the distribution and evolution of post-seismic landslide failures. Volume-area scaling laws are developed in order to allow erosion rates to be calculated and finally an innovative oblique photography technique is used to constrain the depth of sediment aggradation. The results demonstrate that as a source of material, the occurrence of landslides in this region is controlled by a combination of topographic, geologic and seismological factors. Resulting volume estimations and subsequent erosion rates indicate that the Wenchuan earthquake has potentially destroyed more material through erosion than it has built through surface uplift. To conclude the movement of sediment through a mountain catchment, levels of sediment aggradation show that a significant proportion of material from the hillslope is transported down into the valley bottom; this is seen to coincide with periods of intense rainfall. Overall, this research derives a unique assessment of sediment mobilisation in Sichuan in order to understand the controls on sediment remobilisation and secondary hazards. By constraining the extent of sediment sources and transfer, this research has the potential to aid the prediction of future post-earthquake hazards and landscape response in Sichuan, providing insight into the role of earthquakes in landscape evolution