日本地球惑星科学連合2015年大会

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インターナショナルセッション(ポスター発表)

セッション記号 S (固体地球科学) » S-SS 地震学

[S-SS02] Frontier studies on subduction zone megathrust earthquakes and tsunamis

2015年5月25日(月) 18:15 〜 19:30 コンベンションホール (2F)

コンビーナ:*金川 久一(千葉大学大学院理学研究科)、Demian Saffer(Dept. of Geosciences, The Pennsylvania State University, USA)、Michael Strasser(Geological Institute, Seiss Federal Insitute of Technology ETH Zurich)、山田 泰広(京都大学工学研究科都市社会工学専攻)、小平 秀一(海洋研究開発機構 地球内部ダイナミクス領域)、日野 亮太(東北大学災害科学国際研究所)、氏家 恒太郎(筑波大学生命環境系)、伊藤 喜宏(京都大学防災研究所)

18:15 〜 19:30

[SSS02-P02] Epicenter determination by back-projecting onshore and offshore seismic data along the Japan Trench

*仲谷 幸浩1望月 公廣1篠原 雅尚1山田 知朗1 (1.東京大学地震研究所)

キーワード:back-projection, semblance, onshore and offshore seismic array

In the Japan Trench subduction zone, we have to pay attention to the uncertainty of hypocenters, especially in focal depths, listed in the existing catalog due to a large distance between hypocenters and the onshore seismic network. In addition, there is a significant reduction in the number of detected events over months after large earthquakes, such as the 2011 Tohoku earthquake. In order to overcome these problems and to understand seismic energy release associated with large earthquakes, it is essential to automatically determine hypocenters by also using offshore seismic data.
Therefore, we have investigated changes in seismicity before and after the 2011 Tohoku earthquake around its southern limit by back-projecting offshore seismic array data. Previous results suggest the large coseismic slip zone of the Tohoku earthquake may not have extended off Ibaraki.
In this study, we try to improve the method for epicenter determination by combining the back-projection results from both onshore and offshore seismic array data, and to discuss the seismic energy release in the source area of the Tohoku earthquake over a wider range. We used the NIED Hi-net data from stations located in the eastern Japan along the Pacific coast as the onshore array, and the 1-Hz ocean bottom seismometer (OBS) data deployed off Ibaraki as the offshore array. We applied a semblance analysis to each array data, and then identified the epicenter from both back-projected semblance values. As preliminary results, we determined some epicenters with higher accuracy even of earthquakes away (〜40 km) from the OBS array. These results indicate that it may be possible that we can put some constraints on the extension of the rupture area of the Tohoku earthquake over a wider range by investigating seismicity and its temporal changes. We continue to develop and improve our method so that the method is applicable to continuous waveform data.