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[HCG21-P07] Submarine Landslides in Southern Toyama Bay Associated with the 2024 Noto Peninsula Earthquake: Findings from Underwater Drone Surveys
Keywords:Submarine Landslide, Toyama Bay, 2024 Noto Peninsula Earthquake
The first tsunami wave generated by the M7.6 earthquake that struck on January 1, 2024, reached the southern part of Toyama Bay at an unusually rapid pace. One possible explanation for this phenomenon is the occurrence of submarine landslides in the southern Toyama Bay area, as suggested by the Earthquake Research Committee, Headquarters for Earthquake Research Promotion.
The authors had the opportunity to participate in an underwater survey using remotely operated vehicles (ROVs) in the first half of 2024. This survey identified submarine landslide traces off the coast of Iwase and Mizuhashi in Toyama City (Umino et al., 2024, among others). In this report, we present the results of a follow-up survey conducted by the Toyama Bay Submarine Landslide Investigation and Research Group, comprising some of the authors. The survey focused on two submarine canyon regions: offshore Iwase in Toyama City and offshore Fushiki in Takaoka City. The investigation was conducted using the DiveUnit300 ROV manufactured by FullDepth Inc. and the Tripod Finder-II system.
Survey Results: Offshore Iwase, Toyama City
In the submarine canyon located approximately 4 km offshore from the mouth of the Jinzu River, where the Japan Coast Guard reported the presence of large-scale collapse topography, we performed continuous ROV surveys from the canyon floor to the westward-facing submarine slope at depths ranging from approximately 350 m to 270 m.
At the canyon floor, a relatively flat terrain covered with fine-grained, soft sediments was observed, along with the presence of marine organisms such as Pasiphaea japonica and Lycodes spp.. Near the base of the submarine slope, blocky fractured rocks—derived from exposed terrestrial clastic sedimentary layers on the slope—were scattered. Above these, nearly vertical cliffs with a height difference of several tens of meters was observed, consisting of exposed terrestrial clastic sedimentary layers. The rock blocks and sedimentary layers appeared semi-consolidated, with fresh fracture surfaces. Only minimal fine-grained sediment deposition was observed, and almost no sessile organisms were present.
Above the cliffs, at depths shallower than approximately 275 m, the seafloor was covered with fine-grained sediments, and numerous burrow-like holes were identified. Various benthic organisms, including Lycodes spp. and Ophiuroidea, were observed inhabiting the area.
Survey Results: Offshore Fushiki, Takaoka City
Based on post-earthquake bathymetric surveys conducted by the Shin-minato Fisheries Cooperative, ROV investigations were performed at multiple locations within the submarine canyon offshore from the Sho-gawa and Oyabegawa River mouths.
At Site I, continuous ROV surveys were conducted from the canyon floor to the west-facing submarine slope at depths ranging from approximately 270 m to 240 m. Large rock blocks scattered near the canyon floor and a cliff composed of terrestrial clastic sedimentary bedrock with a height difference of several tens of meters were observed. Similar to offshore Iwase, the cliff face exhibited fresh fractures with no visible sessile organisms.
At Site II, a large cliff was identified on a westward-facing submarine slope at depths of approximately 200 m to 170 m. Unlike Site I, the cliff face appeared weathered and was densely colonized by sessile organisms such as polychaete tube worms, bivalves, sponges, and an orange-colored surface deposit.
At Sites III and IV, fine-grained soft sediments were distributed among scattered rock blocks, and recent marine debris was found on the sediment surface.
Interpretation and Conclusions
At Site I offshore Fushiki and offshore Iwase, the entire cliff face—composed of semi-consolidated terrestrial clastic sedimentary rock—appears to have fall or deep-seated landslides. In contrast, the cliff at Site II offshore Fushiki appears to have collapsed several years or more ago, based on the presence of sessile organisms and surface deposits. Characteristics observed at other locations strongly suggest that the collapses occurred very recently, likely triggered by the 2024 Noto Peninsula Earthquake.
The Japan Coast Guard previously reported the presence of collapse topography in these regions based on bathymetric differences between 2010 and 2024. Our survey confirms that large-scale collapses did indeed occur, that they occurred very recently, and that the collapses occurred at more sites than previously reported.
The observed submarine landslide traces may be classified into several distinct patterns, and further investigations will be conducted to characterize the different failure mechanisms involved.
The authors had the opportunity to participate in an underwater survey using remotely operated vehicles (ROVs) in the first half of 2024. This survey identified submarine landslide traces off the coast of Iwase and Mizuhashi in Toyama City (Umino et al., 2024, among others). In this report, we present the results of a follow-up survey conducted by the Toyama Bay Submarine Landslide Investigation and Research Group, comprising some of the authors. The survey focused on two submarine canyon regions: offshore Iwase in Toyama City and offshore Fushiki in Takaoka City. The investigation was conducted using the DiveUnit300 ROV manufactured by FullDepth Inc. and the Tripod Finder-II system.
Survey Results: Offshore Iwase, Toyama City
In the submarine canyon located approximately 4 km offshore from the mouth of the Jinzu River, where the Japan Coast Guard reported the presence of large-scale collapse topography, we performed continuous ROV surveys from the canyon floor to the westward-facing submarine slope at depths ranging from approximately 350 m to 270 m.
At the canyon floor, a relatively flat terrain covered with fine-grained, soft sediments was observed, along with the presence of marine organisms such as Pasiphaea japonica and Lycodes spp.. Near the base of the submarine slope, blocky fractured rocks—derived from exposed terrestrial clastic sedimentary layers on the slope—were scattered. Above these, nearly vertical cliffs with a height difference of several tens of meters was observed, consisting of exposed terrestrial clastic sedimentary layers. The rock blocks and sedimentary layers appeared semi-consolidated, with fresh fracture surfaces. Only minimal fine-grained sediment deposition was observed, and almost no sessile organisms were present.
Above the cliffs, at depths shallower than approximately 275 m, the seafloor was covered with fine-grained sediments, and numerous burrow-like holes were identified. Various benthic organisms, including Lycodes spp. and Ophiuroidea, were observed inhabiting the area.
Survey Results: Offshore Fushiki, Takaoka City
Based on post-earthquake bathymetric surveys conducted by the Shin-minato Fisheries Cooperative, ROV investigations were performed at multiple locations within the submarine canyon offshore from the Sho-gawa and Oyabegawa River mouths.
At Site I, continuous ROV surveys were conducted from the canyon floor to the west-facing submarine slope at depths ranging from approximately 270 m to 240 m. Large rock blocks scattered near the canyon floor and a cliff composed of terrestrial clastic sedimentary bedrock with a height difference of several tens of meters were observed. Similar to offshore Iwase, the cliff face exhibited fresh fractures with no visible sessile organisms.
At Site II, a large cliff was identified on a westward-facing submarine slope at depths of approximately 200 m to 170 m. Unlike Site I, the cliff face appeared weathered and was densely colonized by sessile organisms such as polychaete tube worms, bivalves, sponges, and an orange-colored surface deposit.
At Sites III and IV, fine-grained soft sediments were distributed among scattered rock blocks, and recent marine debris was found on the sediment surface.
Interpretation and Conclusions
At Site I offshore Fushiki and offshore Iwase, the entire cliff face—composed of semi-consolidated terrestrial clastic sedimentary rock—appears to have fall or deep-seated landslides. In contrast, the cliff at Site II offshore Fushiki appears to have collapsed several years or more ago, based on the presence of sessile organisms and surface deposits. Characteristics observed at other locations strongly suggest that the collapses occurred very recently, likely triggered by the 2024 Noto Peninsula Earthquake.
The Japan Coast Guard previously reported the presence of collapse topography in these regions based on bathymetric differences between 2010 and 2024. Our survey confirms that large-scale collapses did indeed occur, that they occurred very recently, and that the collapses occurred at more sites than previously reported.
The observed submarine landslide traces may be classified into several distinct patterns, and further investigations will be conducted to characterize the different failure mechanisms involved.