今週のうなぎセミナーとSSJのリハーサルについてお知らせいたします。

Here is information of the Unagi-seminar and two SSJ Meeting rehearsal(October, 26).

************** 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：2023年 10月 26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：2023-10-26, 13:30~
Place：Uji Campus Main Building E232D or Zoom (Hybrid)

　　　Please join the seminar on-site, especially students who need credit.

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Speaker（発表者）:　Shukei Ohyanagi

Title（題目）:　
Periodic swarms of tremor and earthquake in Off Fukushima revealed by the OBS network

Abstract:
　In recent years, an interaction of slow and fast earthquakes become subject of active researches. They are inspired by several observations of Slow Slip Events preceding large earthquake [e.g. 2011 Tohoku-Oki: Kato et al., 2012; Ito et al., 2013; 2014 Iquique: Ruiz et al., 2014; 2017 Guerrero: Cruz-Atienza et al., 2021]. However, such observations are still limited to date; most of the slow earthquakes do not accompany large earthquakes. In order to understand detailed spatiotemporal feature of synchronous slow and fast earthquake activities, it is crucial to target the region where they occur nearby to each other, since we may able to observe small-scale synchronizations more frequently. Off Fukushima near the Japan Trench is one of suitable regions for such study. In Off Fukushima, bursts of shallow tectonic tremor occur every ~ 3 months [Nishikawa et al., 2019; Ohta et al., 2019]. Based on campaign Ocean Bottom Seismometers (OBSs) observation, Obana et al.(2020) discovered some tremor bursts are followed by earthquake activity in Off Fukushima. However, their observation was limited to several months and only covers up-dip part of the tremor source region. Thus, Off Fukushima is an ideal place to study interaction of slow and fast earthquakes, but spatial extent and long-term temporal features of synchronous activity are still unclear.
　To understand spatial and temporal relationship between shallow slow and fast earthquake activities in Off Fukushima, we develop and compare catalogs of tectonic tremor and ordinally earthquake based on observations made by OBSs. For this study, I used continuous record of 8 S-net [Aoi et al., 2020] OBSs deployed around the targeted region. Tectonic tremors are detected and located by a modified envelope correlation method [Mizuno and Ide, 2019]. Non-tremor detections, such as earthquakes and air-gun shots produced during active seismic surveys are removed from an initial catalog. Then, the catalog is finalized by applying space-time clustering [e.g. Wech and Creager, 2008] after removing short duration and low signal-to-noise ratio events. Fast earthquakes are detected by Earthquake Transformer [Mousavi et al., 2020]. P-waves and S-waves picked at each OBS are associated by the REAL [Zhang et al., 2019] and located by hypomh [Hirata and Matsu’ura, 1987] using a local 1D velocity model. The fast earthquake catalog is further inspected to identify earthquake swarm sequences, which may related to slow earthquake activity [Nishikawa and Ide, 2018]. Earthquake swarms are searched based on the method proposed by Nishikawa and Ide (2017; 2018), which enable to identify earthquake swarms utilizing the epidemic-type aftershock sequence (ETAS) model [e.g. Ogata, 1988; Okutani and Ide, 2011].
　In this presentation, we compare slow and fast earthquakes occurred between August 2015 and June 2022. The identified earthquake swarms are distributed in the region surrounding the tectonic tremor. However, durations of their sequences are spatially heterogeneous. The earthquake swarms located at down-dip side of the tremor source region have short duration (1 - 3 days), while the earthquake swarms located at up-dip side or in along-trench neighbors tend to have long duration (4 < days). Those long-lasting earthquake swarms are predominantly preceded by tremor bursts with characteristic delay time of 3 - 5 days, which suggests SSE initiated at the tremor-genic region migrates to the neighboring region and trigger earthquake swarms. The long-lasting swarm collocated to the tremor source region along trench especially recur every ~ 1 year, indicating that periodic swarms of tremor and earthquake occur in the shallow portion of the Japan Trench, Off Fukushima.

SSJ Meeting rehearsal:
Speaker: Masaru Morinaga
Title: Application of an Earthquake Early Warning with the PLUM Method to the 2023 Turkiye-Syria Earthquake
Speaker: 森永 優
Title: 緊急地震速報（PLUM法）の2023年トルコ・シリア地震への適用

Speaker: Yuki Funabiki
Title: Observation of P-wave polarity with DAS recordings along Route 9
Speaker: 船曵祐輝
Title: 国道9号線沿いのDAS記録によるP波初動の観測

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