Calibration of the landsliding numerical model SLIPOS and prediction of the seismically induced erosion for several large earthquakes scenarios
Résumé
Coseismic landsliding is an important contributor to the long-term erosion of mountain belts. But if the scaling
between earthquakes magnitude and volume of sediments eroded is well known, the understanding of geomorphic
consequences as divide migration or valley infilling still poorly understood. Then, the prediction of the location of
landslides sources and deposits is a challenging issue. To progress in this topic, algorithms that resolves correctly
the interaction between landsliding and ground shaking are needed. Peak Ground Acceleration (PGA) have been
shown to control at first order the landslide density. But it can trigger landslides by two mechanisms: the direct
effect of seismic acceleration on forces balance, and a transient decrease in hillslope strength parameters. The
relative importance of both effects on slope stability is not well understood. We use SLIPOS, an algorithm of
bedrock landsliding based on a simple stability analysis applied at local scale. The model is capable to reproduce
the Area/Volume scaling and area distribution of natural landslides. We aim to include the effects of earthquakes
in SLIPOS by simulating the PGA effect via a spatially variable cohesion decrease. We run the model (i) on the
Mw 7.6 Chi-Chi earthquake (1999) to quantitatively test the accuracy of the predictions and (ii) on earthquakes
scenarios (Mw 6.5 to 8) on the New-Zealand Alpine fault to infer the volume of landslides associated with large
events. For the Chi-Chi earthquake, we predict the observed total landslides area within a factor of 2. Moreover,
we show with the New-Zealand fault case that the simulation of ground acceleration by cohesion decrease lead to
a realistic scaling between the volume of sediments and the earthquake magnitude