Elucidating the water content in the oceanic mantle using the ocean bottom seismometers

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Our research aims to elucidate the hydrated regions under the Pacific plate of various ages and the subduction beneath Japan by locating low-velocity zones with in situ observations. However, the ocean bottom seismometer (OBS) records were difficult to use to gain a reliable analysis. The highly complex record due to the structure (water and slow velocity sediment) was a challenge. This research aims to exactly understand the challenges and further present methods that can overcome the difficulties.In the second year of FY2021, our work that discusses the effects of slow seafloor sediment and suggests a way to overcome the influence using the data in off Tohoku region has been published in an international journal. The work shows the low-velocity zone of the subduction zone extends beneath the shallow depths than previously reported. To discuss the variability along the trench, we collected additional OBS arrays in the off Aomori and off Ibaraki regions to extend the imaging region. The data has been pre-processed using active source records for determining component azimuths of the OBSs.In addition, a method to constrain seafloor sediment structures using passive records is developed. Research progress during FY2020 and 2021 helped us to realize that identifying sediment phases in the OBS records is important to do a structural analysis of deeper depths of 100 km. During this year, we attempted to use body waves to constrain sediment structures in high resolution and succeeded. The result of this work is planned to be presented at conferences in FY2022.