Rapid post-seismic landslide evacuation boosted by dynamic river width and implications for sediment fluxes during the seismic cycle
Résumé
Mass wasting caused by large magnitude earthquakes choke mountain rivers with several cubic kilometers of sediment.
The timescale and mechanisms by which rivers evacuate the coarse fraction of small to gigantic landslide
deposits are poorly known, but are critical to predict post-seismic hydro-sedimentary hazards, interpret the signature
of earthquakes in sedimentary archives and decipher the coupling between erosion and tectonics. Here, we
use a new 2D hydro-sedimentary evolution model to demonstrate that river self-organization into a narrower alluvial
channel overlying the bedrock valley dramatically increases sediment transport capacity of coarse sediments
and reduces export time of gigantic landslides by orders of magnitude compared to existing theory. Predicted export
times obey a universal non-linear relationship function of landslide volume and pre-landslide valley transport
capacity. Dynamic alluvial channel narrowing is therefore a key, previously unrecognized, mechanism by which
mountain rivers rapidly digest extreme events and maintain their capacity to incise uplifted rocks. Upscaling these
results to realistic populations of landslides show that removing half of the total sediment volume introduced by
large earthquakes in the fluvial network would typically last 5 to 25 years in various tectonically active mountain
belts, with little impact of topography and climate. If several studies indicate a strong dependency of total landslide
volume to earthquake magnitude, our study show that the sediment export time of a landslide population is
not strongly impacted by earthquake magnitude or by the total volume of the landslide population. Building on
these new findings, we then investigate the dynamics of mountainous landscapes submitted to a series of earthquakes,
following either a Gutenberg-Richter distribution or a single large magnitude event. We infer the temporal
and spatial evolution of the number of active landslide deposits, of the sediment load along the fluvial network
and of the exported sediment flux throughout several seismic cycles. These results highlight how landscapes and
sediment fluxes respond on longer time scales to a succession of earthquakes able to trigger landslides