Climate Regime Controls Fluvial Evacuation of Sediment Mobilized by Large Earthquakes

Abstract:

Large earthquakes in active mountain belts can trigger landslides which mobilize large volumes of clastic sediment. Delivery of this material to river channels may result in aggradation and flooding, while sediment residing on hillslopes may increase the likelihood of subsequent landslides and debris flows. Despite this recognition, the controls on the residence time of coseismic landslide sediment in river catchments remain poorly understood. Here we assess the residence time of fine-grained (<0.25 mm) landslide sediment mobilized by the 2008 M_w 7.9 Wenchuan earthquake, China, using suspended sediment fluxes measured in 16 river catchments from 2006-2012. Following the earthquake, suspended sediment flux was elevated 3 to 7 times, consistent with observations of dilution of ¹⁰Be concentrations in detrital quartz (West et al., 2014). However, the total 2008-2012 export was much less than input of fine-grained sediment by coseismic landslides determined by area-volume scaling and deposit grain-size distributions. Estimates of the residence time of fine-grained sediment in the affected river catchments range from <1 to >100 years at the present export rate. We show that the residence time is proportional to the extent of coseismic landsliding, and inversely proportional to the frequency of intense runoff events. Together with previously reported observations from the 1999 Chi-Chi earthquake in Taiwan, our results demonstrate the importance of climate in setting the length of time that river systems are impacted by large earthquakes.

References:

West et al., 2014, Earth Planet Sc. Lett., 396, 143-153.