今週のうなぎセミナーについてお知らせいたします。
木曜日は吉田キャンパスで授業を受ける学生がいるため、Zoomが追加されました。

Here is information of the Unagi-seminar(May, 2).
Because there will be students attending classes at the Yoshida Campus on Thursdays, Zoom has been added.

************** Seminar on Seismology IV A, C /地震学ゼミナールIV A, C (Unagi Seminar) **************

科目：地震学ゼミナールIV A, C / Seminar on Seismology IV A, C（修士・博士）
日時：2024年 5月 2日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D または オンライン(Zoom)

Date and Time：2024-05-02, 13:30~
Place：Uji Campus Main Building E232D or Zoom (Hybrid)

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Speaker（発表者）:　Shukei Ohyanagi

Title（題目）:　Does tsunami earthquake source region also can be a slow earthquake source?: case of Off Sanriku, near the Japan Trench

Abstract（要旨）:
　The Japan Trench is one of the subduction zones where the trench axis is far from the coast line. As the consequence, seismological monitoring of slow earthquake activity in the shallow portion of the subduction zone cannot be completed by onshore network. While very low frequency earthquake have been observed by onshore broadband seismometer network (Matsuzawa et al., 2015; Baba et al., 2020) and slow slip events have been inferred from repeating earthquakes (e.g. Uchida et al., 2016), observation of tectonic tremor could not be achieved without observation by Ocean Bottom Seismometers (OBSs). Development of the continuous, long-term seafloor observation by S-net (Aoi et al., 2020) resulted in the revealing the spatiotemporal distribution of tremor in the Japan Trench (Tanaka et al., 2019; Nishikawa et al., 2019; 2023; Takahashi et al., 2024), including the Off Sanriku region, which is the northern neighboring region of the 2011 Tohoku-Oki coseismic slip region (e.g. Iinuma et al., 2012).. However, the tremor activity near the trench axis in Off Sanriku region was not clear due to the limited network coverage of the S-net. The slow earthquake activity in this region was of particular interest because it corresponds to the source region of past tsunami earthquakes such as the 1896 Meiji Sanriku earthquake (e.g. Satake et al., 2017) or the 2011 Tohoku-Oki earthquake (e.g. Kubota et al., 2023).
　In order to monitor the possible slow earthquake activity in the source region of tsunami earthquakes, we performed a temporal OBS observation in Off Sanriku, near the trench axis of the Japan Trench. We deployed 3 mini-dense array sites and 3 single-station sites on the trench side of S-net stations to extend our monitoring capability toward the trench side. The observation was conducted between October 2020 and January 2022. We used 1 OBS from each array site and all single station sites. 16 S-net stations covering known slow earthquake regions were also incorporated in our study.
　To reveal slow earthquake activity near the trench, we detect and locate tremors based on the integrated OBS network. We apply the following procedures which were also employed in our previous studies (Ohyanagi et al. in prep.; 2022 AGU). First, we applied the modified envelope correlation method (Mizuno and Ide, 2019) to the OBS records filtered in 2 Hz - 8 Hz to detect the candidate tremor events. After detecting the candidate tremor events, we removed the detections that were not supposed to be tremors. First, we removed ordinary earthquakes by adopting Earthquake Transformer (Mousavi et al., 2020) and air-gun pulses generated by active source seismic surveys. Next, we removed detections with a signal-to-noise ratio of less than 2. Finally, the tremor catalog is finalized by removing short-duration (< 20 sec) events and applying space-time clustering (e.g. Wech and Cleager. 2008; Obara et al., 2010).
　The newly conducted OBS observation reveals the spatiotemporal distribution of tremors near the trench, which was not resolved only by S-net. However, they are sparsely distributed and not as active as main clusters distributed between 40 km and 60 km from the trench axis. Despite the use of dense OBSs, those detected tremor did not exhibit episodic swarm as observed in the main clusters. This indicates that tremor activity in the tsunami earthquake source region near the trench is inactive. According to the active seismic reflection and refraction survey (Tsuru et al., 2002; Ito et al., 2004), this sparse-tremor region corresponds to the thick sediment is accreted on the toe of the over-riding plate. The number of the tremor events drastically increase as the material overriding on the subducting plate transit from the accreted sediment to the upper crust. Such transition of the property may control the occurrence of the shallow slow earthquake in the Off Sanriku region of the Japan Trench.

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