LANDSLIDE PROCESSES CONTROL THE FATE OF SEDIMENT AFTER LARGE EARTHQUAKES (Invited Presentation)

Large continental earthquakes can generate 10⁴-10⁵ years of erosion in a geologically instantaneous moment through coseismic landslide generation. Evacuation of this landslide sediment can result in higher sediment fluxes immediately after earthquakes but they typically return to background levels within a decade resulting in an unknown volume of earthquake-derived sediment remaining in mountainous landscapes for centuries to millennia. The presence of relict landslide deposits, aggraded channels, and landslide dams suggests that this remaining sediment may affect the topographic evolution and sediment export from an orogen long after the earthquake shaking stops. We examine the processes by which the 3km³ of sediment generated by landslides associated with the 2008 M_w7.9 Wenchuan Earthquake have moved within and through catchments. Sediment export by fluvial erosion, debris flows, and overland flow was measured using a satellite-derived multi-temporal landslide inventory of the 10 years since the earthquake and literature-derived values. Less than 3% of the sediment generated by the earthquake has transited tributary catchments (of order <5) to enter the major axial rivers draining the orogen. Within the tributary catchments, the delivery of sediment to both the axial river and the tributary channel is controlled by the frequency and magnitude of debris flows. 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. Our observations suggest that the long term evolution of channels in these range front catchments may be governed by the stochastic delivery of earthquake derived sediment by landslide processes for centuries after the earthquake.