The Fate of Sediment After a Large Earthquake

Abstract:

Large continental earthquakes can produce 10⁴ – 10⁵ years of erosion in a geological instant through coseismic landslide generation. Local erosion rates increase by an order of magnitude immediately after an earthquake, but rapidly return to background levels. The short-lived nature of the enhanced erosion rates is insufficient to clear the orogen of coseismic landslide material, which can remain stored for centuries to millennia. The sediment which remains affects topographic evolution and potential hazards until it is removed from the orogen. We examine the processes by which the 3km³ of sediment, generated by the cosesimic landslides of the 2008 M_w7.9 Wenchuan Earthquake, move through and within catchments. Using 10 years of satellite imagery and literature derived values, we can, for the first time, describe and measure the export of landslide generated sediment by fluvial erosion, debris flows and overland flow. We find that less than 15% of the sediment, produced by the earthquake, has transitioned from the hillslope through tributary channels (of order <5) into the major orogen draining rivers. The transport of sediment through tributary channels is controlled by the frequency and magnitude of debris flows, which transport 60% of the sediment. Fluvial undercutting of landslide deposits plays a minor role in controlling sediment export, likely due to the low stream power and coarse nature of the sediment in tributary channels. The stochastic removal of landslide deposits by debris flows of varying transport lengths results in long storage times, potentially greater than the return period of large earthquakes, of coseismic landslide sediment. The combination of the long residence time and stochastic transport significantly reduces the likelihood of single earthquakes being recognised in long term stratigraphic records.