Co-seismic hillslope weakening

Reduction in shear strength (RSS) of hillslope materials due to earthquakes have been rarely discussed numerically in regional scale analyses. Despite the limited literature, an empirical relationship between peak ground acceleration (PGA) and RSS was proposed based on Newmark's permanent-deformation analysis coupled with static limit equilibrium approach. However, the empirical relationship is solely based on co- and post-seismic landslide inventories associated with the 2008 Wenchuan earthquake and transferability of the approach is yet to be tested. To address this issue, we apply the same method to areas affected by the 2015 Gorkha and 2018 Palu earthquakes. Our results showed a good agreement in variation of the RSS with respect to PGA. This suggests that the approach is transferable for the estimation of co-seismic hillslope weakening in other geographies. We also analyzed the RSS in sedimentary, metamorphic and igneous rocks. Our results showed that igneous rocks feature the highest RSS in response to given ground shaking and it is followed by metamorphic and sedimentary rocks. Ultimately, we also discussed the RSS likely caused by precipitation events. Our findings imply that the RSS caused by 0.1 g of ground shaking may be 14 times larger than RSS due to precipitation. This argument needs further analyses but overall, our findings provide new insights into hillslope weakening in relation to both earthquake and precipitation.