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

我们的研究旨在通过现场观测定位低速带来阐明不同年龄的太平洋板块下的水化区域和日本之下的俯冲。然而，海底地震仪(OBS)的记录很难得到可靠的分析。由于结构(水和低速沉积物)的高度复杂的记录是一个挑战。这项研究旨在准确地了解挑战，并进一步提出克服困难的方法。在2021财年的第二年，我们的工作讨论了缓慢海底沉积物的影响，并提出了一种利用东北外海地区的数据来克服影响的方法，这项工作已经发表在一份国际期刊上。这项工作表明，俯冲带的低速带延伸到了比之前报道的更浅的深度之下。为了讨论沿海沟的可变性，我们在青森附近和茨城附近收集了额外的OBS阵列，以扩展成像区域。利用主动源记录对数据进行了预处理，以确定OBS的成分方位。此外，还发展了一种利用被动记录来约束海底沉积物结构的方法。2020财年和2021财年期间的研究进展帮助我们认识到，在OBS记录中识别沉积物相对于进行100公里深处的结构分析是重要的。在这一年里，我们尝试使用体波高分辨率地约束沉积物结构，并取得了成功。这项工作的成果计划在2022财年的会议上公布。

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.