Effect of Foreshocks of the 2016 Kumamoto Earthquakes on the Aso-Bridge Slope

This paper presents an examination the Aso-bridge slope subjected to the foreshocks of the 2016 Kumamoto earthquakes. While the Aso-bridge slope was marginal stable in the absence of earthquake, it remained stable during the foreshocks that struck on April 14 and 15 2016, with magnitudes of 6.5 and 6.4 M_w, respectively. To validate that the slope did not fail during the foreshocks and the applicability of an enhanced FS method capable of determining the initiation time of co-seismic landslides, relationships between the Aso-bridge slope and the nearest recorded seismic signals, including the main-shock and the foreshocks, were investigated. It was found that the two foreshocks, having PGAs of 43.0 and 129.7 gal, were not strong enough to cause failure of the slope. They were not strong enough mainly because of the smaller magnitude as compared to the main-shock of 7.0 M_w with a PGA of 508.9 gal, leading to a significant difference in the PGAs. The study demonstrates that the enhanced FS method can be one efficient and practical alternative for determining large-scale co-seismic landslides.