今週のうなぎセミナーについてお知らせいたします。

Here is information of the Unagi-seminar(November, 17).

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

科目：地震学ゼミナールIV B, D / Seminar on Seismology IV B, D（修士・博士）
日時：2022年 11月 17日 (木) 14:00~
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：2022-11-17, 14:00~
Place：Uji Campus Main Building E232D or Zoom (Hybrid)

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Speaker 1: 岡田悠太郎（Yutaro OKADA）

Title:
GNSSデータを用いた日本海溝及び千島海溝における短期的スロースリップイベント活動の調査
Investigation on the activity of short-term slow slip events along the Japan and Kuril Trenches using GNSS data.

Abstract:
短期的スロースリップイベント(S-SSE) (e.g., Dragert et al., 2001)は数日間から数週間継続する非地震性の断層すべり現象であり，スロー地震や中小規模の通常地震と時空間的に同期する場合があることが知られている(Ozawa et al., 2003; Rogers & Dragert, 2003)．日本海溝及び千島海溝ではこれまでもSSEやスロー地震の活発な活動が報告されてきたが（e.g., Kawasaki et al., 1995; Nishikawa et al., 2019; Nishimura, 2021），解析対象地域全域，特に千島海溝におけるS-SSEの活動については不明な点が依然として多い．そこで本研究ではGNSSデータの解析を行うことで，この地域で発生するS-SSEの活動様式を解明することを目的とする．
　本研究では（１）解析地域全域におけるS-SSEの系統的検出，及び（２）十勝沖で発生するS-SSEの平均的な断層モデル推定の２つの解析を行った．（１）の解析の結果，検出されたS-SSEの多くは先行研究で活発な活動が報告されている関東地方沖に分布していた．また宮城県沖以北，特に東北地方沖で検出されたイベントは中規模の地震と考えられるものがほとんどであり，S-SSEの可能性があると考えられるイベントを検出することはできなかった．しかし北海道，特に襟裳岬の沖合では，S-SSEと考えられる地表変位イベントを数例検出した．一方（２）の解析の結果，S-SSEの平均的な断層モデルは十勝沖に推定され，超低周波地震（Baba et al., 2020）の発生域と空間的に対応していた．またS-SSEの断層は2003年十勝沖地震の地震時すべり域（Yagi, 2004）の浅部側に位置し，アフタースリップ域（Itoh et al., 2019）内に含まれていた．

Speaker 2: 大柳　修慧 (Shukei OHYANAGI)

Title: Shallow tectonic tremor activity near the Japan Trench from 2016 to 2022: development of fully automated monitoring system and its application to the Off Sanriku and Off Fukushima tremor clusters.

Abstract:
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]. Especially, the deployment of the S-net [Aoi et al., 2020], a cabled network of ocean bottom seismometers (OBSs) contributed to continuous monitoring of the offshore shallow tremor activity feasible. However, none of the publicly-available researches reported the shallow tremor activity occurring after August 2018. Thus, despite of continuous ocean bottom observation, shallow tremor activity for the last 4 years remains unknown.
　To understand up-to-date activity in the Japan Trench subduction zone, we developed a fully automated monitoring system of the shallow tremor. The system is composed of 3 steps; tremor candidate detection, false detection removal, and space-time clustering. The tremor candidates are detected and located by an envelope correlation method [Mizuno and Ide, 2019]. We made a minor modification on the Mizuno and Ide (2019)’s code to improve location accuracy. Contaminating non-tremor detections, such as earthquakes are removed using Earthquake Transformer [Mousavi et al., 2020], and the air-gun pulses associated with active-source seismic surveys are removed based on identification of spectral feature [Ohta, 2021 SSJ]. Finally, after performing spatiotemporal clustering, we eliminate the short-duration events to finalize the tremor catalog. Fusion of multiple processes in addition to space-time clustering enable efficient monitoring of tectonic tremor in the region near the seismically active area such as the Japan Trench subduction zone.
　The developed system is applied to S-net cabled OBSs located in Off Sanriku and Off Fukushima, which are regions neighboring to the large coseismic rupture region of the Mw 9.0 Tohoku-Oki earthquake [Iinuma et al., 2012]. We construct a tremor catalog spanning from August 2016 to July 2022. The tremor activity revealed by the new catalog is characterized by long-term decay throughout the period of analysis in Off Sanriku During 2016-2017, Off Sanriku has 11 hours of monthly tremor duration on average, but decreased to 7 hours as of 2021-2022. Decrease in the tremor activity may indicate deceleration of the afterslip of the Tohoku-Oki earthquake in the targeted regions. We also observed anomalous change in recurrence period of tremor activity, which possibly due to SSE event on 2015 [Uchida et al., 2016; Honsho et al., 2019; Fujiwara et al. 2022]. In the presentation, we are also going to discuss briefly about long-term relation of earthquake swarms synchronizing with the identified tremor activities.

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