今週のうなぎセミナーについてお知らせいたします。
次回のうなぎセミナー（4/21）は松尾さん、小出さんの自己紹介と大柳さんの発表になります。

Here is information of the Unagi-seminar(April, 21).
On the seminar (April, 21), topic is self-introduction (Matsuo and Koide) and presentation (Ohyanagi).

************** 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：2022年 4月 21日 (木) 14:00~
場所：京都大学防災研究所 連携研究棟301号室（大セミナー室）または オンライン(Zoom)

Date and Time：2022-04-21, 14:00~
Place：Seminar Room #301, DPRI Collaborative Research Hub or Zoom (Hybrid)

https://www.kyoto-u.ac.jp/ja/access/campus/map6r-uji
（構内マップ中の77番の建物です。）

Presenter (発表者): Shukei Ohyanagi

Title (題目):

Shallow tectonic tremor activity in the Japan Trench from 2016 until now: development of fully automated monitoring system and its application to the Off Sanriku and the Off Fukushima tremor clusters

Abstract (要旨):

The Japan Trench subduction zone is a cradle of various seismic and aseismic activities, such as the Mw 9.0 Tohoku-Oki earthquake, slow slip event [Ito et al., 2013; Honsho et al., 2019], very low frequency earthquakes [Matsuzawa et al., 2015; Baba et al., 2020], and shallow tectonic tremor [Nishikawa et al., 2019; Ohta et al. 2019; Tanaka et al., 2019]. Nishikawa et al. (2019)’s tremor catalog has been widely used on investigating property of shallow tremor in the Japan Trench [e.g. Yabe et al., 2021; Takahashi et al., 2021]. However, the catalog only covers tremor activity until August 2018, and none of the research have yet been reported the tremor activity afterward.

To understand the up-to-date shallow tremor activity in the Japan Trench, we developed a fully automated monitoring system. The system composed of 3 steps; tremor candidate detection, false detection removal, and spatiotemporal clustering. The tremor candidates are detected and located by envelope correlation method [Mizuno et al., 2019]. The initial candidate catalog is expected to be contaminated with falsely detected earthquake, and air-gun pulses used for active source seismic survey. The contaminating earthquakes are removed utilizing Earthquake Transformer [Mousavi et al., 2020], and the air-gun pulses are removed based on identification of spectral feature [Ohta, 2021 SSJ]. At last, we perform spatiotemporal clustering and applied duration filter to finalize the tremor catalog.

The developed system is applied to stations of S-net cabled OBS network [Aoi et al., 2020] located in Off Sanriku and Off Fukushima, in order to understand the tremor activity in those regions. We construct a tremor catalog spanning August 2016 to April 2022. The constructed catalog shows anomalous change in recurrence period of tremor activity. In Off Sanriku, the tremor activity was characterized by frequent short bursts of tremors every 1 or 2 months. However, such bursts subsided around beginning of 2019, and the tremor activity is dominated by long bursts every 6 months or so since then. Similar extension in the recurrence period of tremor activity is also seen in Off Fukushima. In the presentation, we are also going to discuss briefly about possible earthquake swarms synchronizing with the identified tremor activities.