[S-SS02] Seismological advances in the ocean

[SSS02-P07] Application of the wavefield decomposition for the Oldest-1 array data

*川勝 均¹、一瀬 建日¹、竹内 希¹、塩原 肇¹、杉岡 裕子²、Kim YoungHee³、歌田 久司¹、Lee Sang-Mook³ (1.東京大学地震研究所、2.神戸大学理学研究科、3.ソウル大学)

We apply the wavefield decomposition method (e.g., Thorwart and Dahm, 2005; Bostock and Trehu, 2012) to the broadband ocean bottom seismic and the differential pressure gage (DPG) data of the Oldest-1 array deployed on the oldest (~170 Ma) seafloor ~1000 km off the Marian trench (e.g., Shiobara et al., 2020, JpGU). We follow the formulation and approach of Bostock and Trehu (2012) and first estimate the thickness of the water layer using DPG data and vertical component seismograms of teleseismic earthquakes and regional deep events. Estimated water depths are consistent with the reported water depths of the 11 seismic stations within few tens of meters and honor the 1D assumption of the local seafloor structure around each station site that allows further analysis assuming 1D structure beneath. We then apply the wavefield decomposition to resolve upgoing P- and S-wave using the vertical and radial component seismograms and DPG data employing both the acoustic and the elastic formulae of Bostock and Trehu (2012). The preliminary analysis indicates that, although both formulae give reasonable results, the acoustic formula gives more stable decomposition. The comparison of decomposed upgoing S-wave and radial component waveforms indicates that the radial component is dominated by S-waves consistent with the presence of the low S-wave velocity sediment at the station sites.