[A-CG40] 大気・海洋観測の気候・海洋予測へのインパクト評価

[ACG40-P06] Impact of a major seismic event on seafloor gas seeps: Torigakubi Spur during the Noto Peninsula Earthquake

*Glen Snyder¹、Hitoshi Tomaru²、Yoshinori Ono²、Natsuki Kaneko²、Natsuki Oneda²、Yoshiki Honma²、Shinsuke Aoki³、Yasumasa Sakai⁵、Gen Takatori⁵、Shohei Ito⁴、Hashimoto Yuichi⁷、Ryo Matsumoto⁶ (1.Atmosphere and Ocean Research Institute, University of Tokyo、2.Department of Earth Sciences, Chiba University、3.Faculty of Agriculture, Kagawa University、4.FullDepth Co. LTD. 、5.Graduate School of Maritime Sciences, Kobe University、6.Gas Hydrate Research Lab, Meiji University、7.Toyama Bay Lab)

キーワード：Noto earthquake, gas hydrate, Torigakubi Spur, ADCP, CTD, seafloor lander

On October of 2023, UN Ocean Decade project COESS (Chemistry, Observation, Ecology of Submarine Seeps) deployed a Shallow Fugro Environmental Lander (SFEL) to carry out observations in Torigakubi Spur, Joetsu Basin. The COESS team consists of researchers from 14 research institutions in Japan, many who have had experience studying gas hydrate and seeps in the Japan Sea. Torigakubi Spur was selected by the group as an area with active biological communities, shallow gas hydrate, and active gas seeps. The SFEL was equipped with a variety of sensors, including CTD (Conductivity, Temperature, Depth) and ADCP (Acoustic Doppler Current Profiler). Continuous data was recorded every 10 minutes until lander recovery in November 2024. Torigakubi Spur is located 50-60 kilometers from the Noto Earthquake epicenter on January 1, 2024. The major seismic event as well as the subsequent aftershocks in the days that followed had an appreciable effect on the seafloor at the SFEL deployment site. The SFEL recorded an immediate shift in the pitch and roll of the lander itself at 16:10 JST when the earthquake occurred. This was accompanied by a positive spike in fluorescence, and negative spikes in salinity and pH. Turbidity suddenly increased and only gradually decreased as the aftershocks subsided around 21:00 JST the same day. Minor spikes in turbidity coincided with lesser aftershocks the following day. The ADCP revealed not only increases in vertical and horizontal current speed in the bottom 60m of the water column above the SFEL, but also increased turbulence in the hours following the earthquake.
In July of 2024 , the COESS team carried out a follow-up cruise, employing an Underwater Drone Prototype (FullDepth Inc.) to obtain high-resolution video footage of the lander and surrounding seafloor. The SFEL was covered with a fine sediment. Seafloor authigenic carbonate pavement was observed broken in many areas, and shallow gas hydrate was observed in one of the disturbed areas near the SFEL. It is likely that the increased turbulence and turbidity were a result of chunks of shallow hydrate breaking loose from the seafloor following the earthquake and floating upward in the water column.