5:15 PM - 6:30 PM
[SSS10-P11] A seafloor survey using multiple methodologies to verify the existence of a village “Kuroda-gori” submerged after a historical Nankai Earthquake in Nomi Bay, Kochi Prefecture
★Invited Papers
Keywords:Underwater remain, Kuroda-gori, Wave-cut platform, Nankai earthquake, Structure from Motion (SfM), Back-scattering image
Nomi Bay in Susaki, Kochi Prefecture, has experienced numerous earthquakes in the past. The oral tradition of Kuroda-gori, a coastal village submerged under seawater due to the Hakuho Earthquake in 684 AD, is well known in this area. Since no scientific evidence of Kuroda-gori has been presented to date, a seafloor survey of Nomi Bay was undertaken to locate the footprint of the submerged village. In addition, three-dimensional seafloor recording using Structure from Motion (SfM) multi-view stereo photogrammetry and underwater video recording using a 360-degree underwater camera were conducted to evaluate the feasibility to investigate underwater remains.
The results revealed the existence of a plateau at a depth of 6 to 7 m measuring 0.09 km2 that sloped gently toward deeper water in the northeastern part of Heshima Island. This site was previously reported to contain the ruins of a freshwater well. The seafloor in the platform was mainly covered with a thin sand layer, with the amount of gravelly substrates increasing toward the shore. The sand layer is considered to overlie hard basement rocks. These topographic and geological features are consistent with a wave-cut platform. Based on relative sea level changes and vertical coseismic movements, the former platform was probably submerged by the Nankai earthquake. This submarine platform may have been suitable for habitation because there is little flat land around Heshima Island; however, no evidence of habitation was found. Despite not finding any evidence of Kuroda-gori, a multifaceted method for conducting underwater geoarchaeological research was developed. In particular, seafloor observation by interferometric sonar system and a high-resolution seafloor topographic map produced from underwater photos using Structure from Motion photogrammetric technology could be used to conduct detailed underwater surveys of structures, such as ruins, in the future.
Acknowledgements
This research is funded by JSPS programs (Grants-in-Aid for Scientific Research, 26560147, 16H03103, 20H04309).
The results revealed the existence of a plateau at a depth of 6 to 7 m measuring 0.09 km2 that sloped gently toward deeper water in the northeastern part of Heshima Island. This site was previously reported to contain the ruins of a freshwater well. The seafloor in the platform was mainly covered with a thin sand layer, with the amount of gravelly substrates increasing toward the shore. The sand layer is considered to overlie hard basement rocks. These topographic and geological features are consistent with a wave-cut platform. Based on relative sea level changes and vertical coseismic movements, the former platform was probably submerged by the Nankai earthquake. This submarine platform may have been suitable for habitation because there is little flat land around Heshima Island; however, no evidence of habitation was found. Despite not finding any evidence of Kuroda-gori, a multifaceted method for conducting underwater geoarchaeological research was developed. In particular, seafloor observation by interferometric sonar system and a high-resolution seafloor topographic map produced from underwater photos using Structure from Motion photogrammetric technology could be used to conduct detailed underwater surveys of structures, such as ruins, in the future.
Acknowledgements
This research is funded by JSPS programs (Grants-in-Aid for Scientific Research, 26560147, 16H03103, 20H04309).