A novel method of surveying submerged landslide ruins: Case study of the Nebukawa landslide in Japan

We investigated submerged ruins from the 1923 Nebukawa landslide, which was caused by the 1923 Kanto earthquake. The on-land areas affected by the landslide have been restored and evidence of the landslide is mostly gone, but huge structures that appear to be man-made have been observed by divers on the seafloor near the area of the landslide. We used a fish finder designed for leisure use and other low-cost equipment to conduct a marine acoustic survey. Because the affected area off Nebukawa is close to shore and shallow and the tools were sufficiently lightweight, we were able to use an inflatable raft for the survey. We created a bathymetric map and side-scan images showing features of the landslide mass and scattered huge structures exposed on the seafloor. After the acoustic investigations, we conducted a diving investigation and were able to ascertain that the structures were made of concrete and were most likely parts of the old Nebukawa station. The ruins were displaced about 260 to 320. m horizontally from the original station location (to 110-170. m offshore) and were mixed with coarse rock fragments from the landslide mass. The distribution of bottom materials suggests that the landslide struck the seafloor and then traveled as far as 460. m offshore from the coastline. The landslide had an equivalent coefficient of friction (H/L) of about 0.15, indicating high mobility. The landslide probably transformed into a turbulent flow mixed with basal sandy sediments, which propelled the debris farther offshor