Timing and acceleration of a landslide failure caused by the 2016 Mw 7.1 Kumamoto Earthquake

The Mw 7.1 Kumamoto earthquake occurred on 16 April 2016 in Kyushu Island, southwest Japan, at a depth of about 15 km resulting in extensive damage in the Kyushu Island, southwest Japan. This earthquake produced a 40 km surface rupture zone striking NE, mostly along the pre-existing Hinagu-Futagawa fault zone (Lin et al., 2016) and caused numerous landslides, debris, and slope failures in a wide area around the coseismic surface rupture zone. This study focuses on a typical landslide (called Ayukaenotaki landslide) caused by the Kumamoto earthquake, that occurred on a mountain slope, 0.7 km south of the Futagawa Fault that is seismogenic fault, 24 km northeast of the epicenter. Topographically, the gradient of landslide slope ranges from 29° to 35° (with an average of 32°), which a critical dip angle for landsliding. Field investigation and topographical and geological analyses reveal that most of landslide mass materials composed of alluvial deposits with large gravels of up to 110 cm in diameter, have been thrown a distance of up to 150 m over a valley. Using S and P wave velocity, the thrown down acceleration of landslide mass materials during the earthquake are estimated to be 1.48 km/s2 and 0.43 km/s2 for P and S-waves, respectively. Considering the P and S waves as possible sources that caused the Ayukaenotaki landslide the thrown down times taken for the landslide material to arrive down the site over the galley are estimated to be 13.9 s and 17.6 s for P and S waves, respectively. The 2016 Mw 7.1 Kumamoto earthquake therefore offers a rare opportunity to study the relationships between the timing of seismic rupturing and the landslide velocity during a large-magnitude earthquake. Our findings provide a rare example for further study on the acceleration failure of landslide during a large magnitude earthquake.