The fate of landslide debris after large earthquakes, Wenchuan, China
Large earthquakes are significant geomorphic events in mountain ranges;they can cause uplift while also triggering huge volumes of bedrock landsliding. Constraining theerosion of coseismic landslide deposits is important forunderstandingthe long-term impact of earthquakes on landscape evolution and natural hazards. The 2008 Mw7.9 Wenchuan earthquake provides an excellent opportunity to constrain the processes and rates that erode landslide deposits within and out of mountain catchments. I constructed a large multitemporal landslide inventory of the epicentral regionof the earthquaketo map the erosion of landslide deposits in the 10 years after the earthquake. In these 10 years less than 13% of the sediment hadbeen deposited into the stores associated with the main trunk of the orogen draining river leaving up to 88% stored on the hillslope. Debris flows were the key remobilising process, transporting over 50% of all the sediment that is mobilised. Using slope stability modelling and analysis of the landslide inventory I identified most of the debris flows were triggered on the hill slope and their frequency decreased rapidly through time. Initially debris flows were triggered in small deposits with low drainage areasbut through time the landslides closer to channel network become more likely to be the source of debris flow activity, a pattern that highlights a change in the physical properties of the deposits. I evaluate several theories that may explain this pattern and highlight strategies to identify the stabilising processes. Finally, I present a 0-Dimensional model investigatingthe impact of storage of coseismic sediment on landscape evolution. This model suggests coseismic sediment can remain in the mountain range for timescales greater than the recurrence time of large earthquakes causing a decrease in exhumation ratesas sediment is remobilised. The long storage time of sediment in orogens reduces the impact of earthquakes on long term (>1000 years) exhumation records so that single earthquakes are only recorded in the most local or highest resolution records.