5:15 PM - 6:45 PM
[U15-P89] "Underwater drone" exploration of a submarine slide caused by the 2024 Noto Peninsula Earthquake in the Toyama Bay, northern Central Japan
Keywords:The 2024 Noto Peninsula earthquake, Toyama Bay, submarine slide, ROV, headwall scraps
The tsunami caused by the magnitude 7.6 earthquake in the Noto Peninsula on January 1, 2024 (the 2024 Noto Peninsula Earthquake) propagated not only to the Noto Peninsula coast but also to Toyama Bay (The Headquarters for Earthquake Research Promotion, 2024). The first wave arrived at the Toyama Port, located 90 km south from the epicenter and in inner part of the Toyama Bay, about 3 minutes after the earthquake (The Headquarters for Earthquake Research Promotion, 2024). Since this arrival time is more than 30 minutes earlier than that of the Nanao Port, which is closer to the epicenter, it is unlikely that the tsunami in the Toyama Port was caused by changes in water level due to the fault movement causing the main earthquake. However, it is pointed out that a nearby tsunami wave source should be expected as a possible cause of the earlier arrival of the tsunami at the Toyama Port (The Headquarters for Earthquake Research Promotion, 2024).
The Japan Coast Guard conducted bathymetric surveys by survey vessels from January 15 to 17, 2024 and from February 27 to 28, 2024, and reported that the slopes of the submarine canyon off the Jinzu River had collapsed over a wide area (Japan Coast Guard, 2024a, 2024b). In particular, the seafloor surface in the area (about 500 m long and 80 m wide; water depth 260-330 m) about 4 km off from the mouth of the Jinzu River, was found to be about 40 m deeper than in 2010 (Japan Coast Guard, 2024a). However, the causal relationship between the collapse of the slopes of the submarine canyon and the recent earthquake was unclear, because the hypothesis was based on the comparison of survey results in 2010 and 2024.
In this study, we conducted "underwater drone" survey to reveal whether the collapse occurred recently or not. An "underwater drone" (Remotely Operated Vehicle: ROV) was used for the exploration. The drone: DiveUnit 300 (FullDepth Co. Ltd.) can dive to the depth of up to 300 m, and shoot Full HD video (1080p, 30 fps), and record the information such as water depth, water temperature, and orientation of the drone.
As a result of the survey, cliffs and cracks were observed on the west-facing slope at a water depth of approximately 250 m, which were interpreted as headwall scarps and crown cracks. Below them (at a water depth of approximately 270 m), the strata that originally form the slope were scattered there as blocks and/or debris. These characters are consistent with the characteristics of submarine slides described by Bull et al. (2009) and Kawamura et al. (2017). The blocks were angular in shape, and the absence of any sessile organisms, such as sponges, cnidarians, or algae, on the block and seafloor surface suggests that the formation of these blocks occurred very recently. Furthermore, although the abundant benthoses which live on or move under the seafloor surfaces, such as brittle stars and irregular echinoids, were observed during a pre-earthquake ROV survey near the study area in the Toyama Bay (Sano et al., 2022), these benthoses are absent in the areas where the blocks were scattered. These facts indicate that the observed collapse of the slope of the submarine canyon is a very recent submarine slide, most likely caused by the 2024 Noto Peninsula Earthquake.
It is confirmed that the ROV surveys can reveal the seafloor topography, the nature of the sediments and benthic biota, and that it is possible to distinguish the type of collapse and its age (older or recent ones). In the future, the combination of bathymetric survey and direct observation of seafloor topography and sediments by ROV will contribute the studies of the topography, geology, and biota on the seafloor and their changes more easily and more in detail, which could be studied by only big projects using deep-submergence vehicles, such as Shinkai 6500 previously. Thus, “underwater drone” exploration probably makes a breakthrough for the geographical, geological and biological researches in ocean very soon.
The Japan Coast Guard conducted bathymetric surveys by survey vessels from January 15 to 17, 2024 and from February 27 to 28, 2024, and reported that the slopes of the submarine canyon off the Jinzu River had collapsed over a wide area (Japan Coast Guard, 2024a, 2024b). In particular, the seafloor surface in the area (about 500 m long and 80 m wide; water depth 260-330 m) about 4 km off from the mouth of the Jinzu River, was found to be about 40 m deeper than in 2010 (Japan Coast Guard, 2024a). However, the causal relationship between the collapse of the slopes of the submarine canyon and the recent earthquake was unclear, because the hypothesis was based on the comparison of survey results in 2010 and 2024.
In this study, we conducted "underwater drone" survey to reveal whether the collapse occurred recently or not. An "underwater drone" (Remotely Operated Vehicle: ROV) was used for the exploration. The drone: DiveUnit 300 (FullDepth Co. Ltd.) can dive to the depth of up to 300 m, and shoot Full HD video (1080p, 30 fps), and record the information such as water depth, water temperature, and orientation of the drone.
As a result of the survey, cliffs and cracks were observed on the west-facing slope at a water depth of approximately 250 m, which were interpreted as headwall scarps and crown cracks. Below them (at a water depth of approximately 270 m), the strata that originally form the slope were scattered there as blocks and/or debris. These characters are consistent with the characteristics of submarine slides described by Bull et al. (2009) and Kawamura et al. (2017). The blocks were angular in shape, and the absence of any sessile organisms, such as sponges, cnidarians, or algae, on the block and seafloor surface suggests that the formation of these blocks occurred very recently. Furthermore, although the abundant benthoses which live on or move under the seafloor surfaces, such as brittle stars and irregular echinoids, were observed during a pre-earthquake ROV survey near the study area in the Toyama Bay (Sano et al., 2022), these benthoses are absent in the areas where the blocks were scattered. These facts indicate that the observed collapse of the slope of the submarine canyon is a very recent submarine slide, most likely caused by the 2024 Noto Peninsula Earthquake.
It is confirmed that the ROV surveys can reveal the seafloor topography, the nature of the sediments and benthic biota, and that it is possible to distinguish the type of collapse and its age (older or recent ones). In the future, the combination of bathymetric survey and direct observation of seafloor topography and sediments by ROV will contribute the studies of the topography, geology, and biota on the seafloor and their changes more easily and more in detail, which could be studied by only big projects using deep-submergence vehicles, such as Shinkai 6500 previously. Thus, “underwater drone” exploration probably makes a breakthrough for the geographical, geological and biological researches in ocean very soon.