日本地球惑星科学連合2019年大会

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[E] ポスター発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG48] Science of slow earthquakes: Toward unified understandings of whole earthquake process

2019年5月29日(水) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:井出 哲(東京大学大学院理学系研究科地球惑星科学専攻)、廣瀬 仁(神戸大学都市安全研究センター)、氏家 恒太郎(筑波大学生命環境系)、波多野 恭弘(東京大学地震研究所)

[SCG48-P14] Spatial distribution of the electrical resistivity beneath western part of Shikoku Island where long-term SSE events have repeatedly occurred

*上嶋 誠1畑 真紀1市原 寛2吉村 令慧3相澤 広記4 (1.東京大学地震研究所、2.名古屋大学大学院環境学研究科附属地震火山センター、3.京都大学防災研究所、4.九州大学大学院理学研究院附属地震火山観測研究センター)

キーワード:豊後水道スロー地震、3次元比抵抗構造、Network-MT観測

In the Bungo channel area at the western margin of the Nankai megathrust rupture zones, the long-term slow slip events (SSE) repeatedly occurred about every 6 or 7 years. Since the last large-scale SSE event occurred in 2010, we will have a next large event soon. In order to obtain the electrical resistivity structure in the vicinity of the SSE area and examine its temporal change, we have started the Network-MT survey in the western part of the Shikoku Island facing the Bungo channel since April, 2016. Just before beginning of the observation, a tiny-scale SSE event occurred in the Bungo-channel (Ozawa, 2017, Hirose et al., 2018). We selected 17 areas in the western part of the Shikoku Island and installed 3 or 4 electrodes in the respective areas. The electrical potential differences with typical electrode spacing of several km are measured by using metallic telephone line network. We also measure the geomagnetic field at two stations in the target region. With the aid of the BIRRP code (Chave and Thomson, 2004), we could estimate the frequency-domain Network-MT response functions of good quality between the voltage difference and the magnetic field. The Network-MT response functions were directly inverted to estimate the 3-D resistivity structure in this area with the aid of the DASOCC Network-MT 3-D inversion code (Siripunvaraporn, Uyeshima & Egbert, 2004).

Conductive portions are commonly detected in the mid-crust of the target region. Their roots are just on the upper surface of the subducting Philippine Sea slab. This may suggest dehydration there. Among the spatial distribution of resistivity, that of seismic P-wave attenuation (Kita and Matsubara, 2016), and that of SSE slips (e.g. Ozawa et al., 2013, Yoshioka et al., 2015, Hirose et al., 2018), some mutually related features are detected. Low Qp (high attenuation) areas tend to correspond to highly conductive areas. The low Qp and highly conductive area are detected just above the subducting slab and outside of the SSE area. Inside the SSE area, while Qp takes generally high values, a patchy conductive area is detected in overall resistive background structure.

This study is supported by JSPS KAKENHI Grand Number JP16H06475 in Scientific Research on Innovative Areas "Science of Slow Earthquakes". It is also partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program. We acknowledge staffs of the Nippon Telegraph and Telephone WEST Corporation for their cordial support in the survey. We also thank H. Abe, A. Takeuchi and Y. Suwa for their help in preparing and installing instruments.