A closer look at factors governing landslide recovery time in post-seismic periods

Earthquakes increase landslide susceptibility in post-seismic periods. The time required for restoring pre-earthquake susceptibility levels is defined as landslide recovery time. Overall, stronger earthquakes are associated with relatively long recovery times in the literature. However, the seismic effect does not explain the whole process. This paper provides insight into factors governing this process. We hypothesize that the dominant post-seismic landsliding process (i.e., new or remobilized landslides), which is governed by the interactions between co-seismic landslide deposits and seismotectonic, morphologic and climatic conditions of a site, is the key to understand the physics behind the landslide recovery time. To test this argument, we analyze 11 earthquake-affected areas associated with seismic variables (i.e., earthquake magnitude and peak ground acceleration) and four environmental factors: (1) the fraction of area affected by co-seismic landslides (FAAL), (2) local relief, (3) average daily precipitation of extreme events (mm/day) and (4) rainfall seasonality. Our findings show that in areas with limited co-seismic landslide deposits (FAAL <~1%) or not enough local relief (<300 m) allowing co-seismic materials to be deposited on hillslopes, then the recovery processes are mostly controlled by new landslides caused by strength reduction of hillslope materials. This mostly results in a relatively quick recovery (<~1 year), which is also correlated with the seasonality of precipitation. The prolonged precipitation makes the recovery process quicken. If the predisposing factors create large co-seismic landslide deposits on hillslopes (FAAL > ~1%), then remobilization of material takes the role of the dominant mechanism and recovery may take longer than a year.