Some studies have suggested that the shaking and deformation associated with earthquakes would result in a temporary increase in hillslope erodibility. However very few data have been able to clarify what causes this transient state and what controls its temporal evolution.
We present integrated geomorphic data constraining an elevated landslide susceptibility to rainfall following 5 continental shallow earthquakes, the Mw 6.9 Finisterre (1993), the Mw 7.6 ChiChi (1999), the Mw 6.6 Niigata (2004), the Mw 6.8 Iwate-Miyagi (2008) and the Mw 7.9 Gorkha (2015) earthquakes. We constrained the magnitude (5 to 20 fold) and the recovery time (1 to 4 years) of this susceptibility change and associated it with subsurface damage caused by the strong shaking (Marc et al. 2015). The landslide data suggest that this ground strength weakening is not limited to the soil cover but also affects the shallow bedrock. Coseismic rock damage is supported by observations of shallow (0 to ~100m) seismic velocity drops constrained with ambient noise waveform correlations within the epicentral area of four of those earthquakes (e.g., Takagi etal. 2012, Hobiger et al. 2015). For most stations we observe a subsequent exponential velocity recovery (i.e. proportional to e−t/τ) with a τ value in fair agreement with the one estimated based on landslide observation. This recovery dynamic is also consistent with post-seismic processes, namely GPS post-seismic displacement and aftershocks decay (Fig. 1, Marc et al., in review). We analyzed strain time series in Japan and Taiwan and it appears inconsistent with the recovery of landslide susceptibility and shallow seismic velocities. In contrast, surface dynamic strain associated with ground shaking caused by aftershocks display similar relaxation time and may control the subsurface property recovery.
