Regional landslide susceptibility following the 2016 Kumamoto earthquake using back-calculated geomaterial strength parameters

The 2016 Kumamoto earthquake first occurred on April 14, 2016 with magnitude 6.5 in Kumamoto, Japan as a foreshock. Subsequently, after 28 h, an even larger earthquake occurred with magnitude 7.3 as the main shock on April 16, 2016. These earthquakes were caused by two active faults: the Futagawa and Hinagu faults. This paper proposes a landslide susceptibility calculation method that considers the geomaterial strength reduction from peak to residual state and ground motion directivity. Although there is a lack of information regarding the strength parameters of geomaterials in the slopes, a parametric analysis with various strength parameters of friction angle and cohesion was carried out. To simulate the actual landslides triggered by the 2016 Kumamoto earthquake, the best combination of friction angle and cohesion in each lithology was optimized by a proposed weighted prediction rate. Based on the calculated permanent seismic displacement, a landslide susceptibility map was produced to show the degree of susceptibility over a wide area comprising 100 km2. The proposed regional landslide susceptibility map will be valuable for estimating the locations of possible slope failures and the extent of damage, as well as for planning field reconnaissance and preventing secondary disasters immediately after earthquakes.