Characterizing Landslide and River Bed Sediments Grain Size after a Large Earthquake with Insight into Post-earthquake Sediment Dynamics

Abstract:

Landslides are important fluvial sediment sources, often characterized by distinct grain size distributions that may be useful for tracking sediment transport and channel bed evolution and understanding associated size fractionation. Landslide grain sizes may also leave an imprint of past triggering events in sedimentary archives. Here we investigate the grain size characteristics of landslide and river bed sediments in the Min Jiang river basin draining the Longmen Shan range, in Sichuan, China. The large number of landslides triggered by the 2008 M_w7.9 Wenchuan earthquake provides a natural experiment for exploring sediment transport and grain size evolution. We report new grain size information for landslide and river bed sediments from field work (pit digging, sieving and photographing) and lab work (wet sieving, photo analysis and statistical processing). We constrain the grain size of sediment sources by combining previously published grain size data from >100 earthquake-triggered landslides with new data collected from river bed sediments from several low-order, small catchments. We track the spatial evolution of grain size in river sediment across the Longmen Shan range, from the upper Min Jiang with little influence from co-seismic landslides to the epicentral region with intensive landsliding, and we explore how landslides modulate the grain size signal carried by river sediment. Overall, this work contributes a key dataset for modeling post-earthquake sediment transport and sheds light on how sediment grain sizes evolve in response to a large earthquake.