[S-IT24] Probing the Earth's interior with geophysical observation on seafloor

[SIT24-P03] Shear wave splitting observed in the Ontong Java Plateau and adjacent regions

*田中 聡¹、末次 大輔¹、塩原 肇²、杉岡 裕子³、伊藤 亜妃¹、一瀬 建日²、石原 靖⁴、大林 政行¹、利根川 貴志⁴、吉光 淳子¹、小林 拓史³ (1.海洋研究開発機構　地球深部ダイナミクス研究分野、2.東京大学　地震研究所、3.神戸大学理学研究科惑星学専攻、4.海洋研究開発機構 地震津波海域観測研究開発センター)

キーワード：マントル異方性、オントンジャワ海台

We measured shear wave splitting of SKS, SKKS, and PKS phases observed at broadband seismic stations on the sea floor and islands in the Ontong Java Plateau (OJP) and adjacent regions. Due to high noise level, we could use only 3 deep earthquakes with magnitudes greater than 7 occurred in the south America. A bandpass filter with cut-off frequencies between 0.02 and 0.2 Hz was applied to the observed seismograms, of which frequency range is a bit lower than the usual for continental areas. By using the eigenvalue method (Silver and Chan, 1991; Helffrich et al., 2000), we tried to estimate the splitting directions and delay times. However, except for three stations, null or node with N-S and E-W directions were obtained or many cases. The directions of the nodes did not coincide with the absolute plate motion of the Pacific plate (WNN-ESE, Argus et al., 2011). The exceptions were observed at the stations located in the southeast margin of the OJP near Stewart Basin and Ellice Basin, which showed that the fast direction was NNW-SSE or N-S and the delay time was 1–2 s. These observations suggest that the directions of mantle flows below the OJP are different from those in the southeastern outside.