Distributed Acoustic Sensing Observation Using Seafloor Cable Observation System in the Japan Sea

In 2010, seafloor cabled observation system with a total length of 25 km and 4 stations was installed near Awashima island in the Japan Sea. A single armored optical fiber cable were buried 1 m below the seafloor to avoid a conflict with fishing activity, and has 8 optical fibers. One fiber pair for data transmission reaches the furthest station without repeater. Recently Distributed Acoustic Sensing (DAS) is available for seismic observation. Spatially high-density data can be obtained for a long distance using DAS. Therefore, seismic observation using DAS technology on seafloor becomes popular at the present. We start observations of a DAS measurement from April 2023 using a fiber of the cable observation system in the Japan Sea. For the DAS measurement using the Japan Sea seafloor observation system, a fiber connecting the furthest station without repeater is used. An interrogator for the DAS measurement was installed in the landing station temporarily. The first observation was performed in April 2023 for approximately four days. As a result, micro-earthquakes occurring near the cable system was recorded, and we confirmed a good quality of DAS data. In September 2023, a DAS observation for about one-day was also carried out as a system evaluation after exchange of optical connectors in the landing station. The parameters of recording for the observation were identical to those for the first observation. We evaluated performance of DAS data from the first and second observations. The 2024 Noto-Hanto earthquake occurred on January 1st and many aftershocks followed. To record aftershocks, we made a DAS observation using the cable observation system in the Japan Sea from early February to late June, 2024. For aftershock observation, we used two sets of recording parameters for different recording periods. Reflecting an environment of the DAS observation, many aftershocks were recorded by the DAS observation.