A landslide induced by the 2016 Kumamoto Earthquake adjacent to tectonic displacement - Generation mechanism and long-term monitoring

Abstract

The 2016 Kumamoto earthquake, which occurred in the central Kyushu region of Southwest Japan, caused numerous landslides. Mass movements from the events damaged many structures in the Minami-aso village residential areas. We conducted geological and geophysical surveys and monitoring of landslides in one of these residential areas. It was found that the landslide occurred directly adjacent to the earthquake's main surface fault-ruptures. While the landslide slip surface formed within a layer of hydrothermally altered rhyolite at about 15 m depth for the downslope side, the slip surface for the upslope side formed within intact tephra layers due to the shallow dip of the rhyolite layer. Geophysical survey results show that fault ruptures in the tephra layers along with the tectonic faulting induce a decrease in soil strength. It was inferred that the landslide head scarp and upper slip surface formed inside this weak soil. Long-term monitoring of the landslide demonstrated that deformation continued for at least four months after the main slide. Although the initial and the major slide was activated by the earthquake, the landslide did not immediately stabilize and continued to pose risks after the main event. Toward the mitigation for earthquake-induced landslides, post seismic monitoring as well as the relationship between surface fault traces and landslide blocks should be taken into consideration.