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(October, 24).

************** 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：2024年 10月 24日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2024-10-24, 13:30~
Place：Uji Campus Main Building E232D

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Speaker（発表者）:　Shukei Ohyanagi

Title（題目）:
Annual shallow earthquake activation following tremor bursts near the Japan Trench, Off Fukushima

Abstract（要旨）:
Despite of contrasting rupture processes and separation in the source regions, synchronization of slow and fast earthquake activities have been obserrved. Due to limited observations of slow earthquakes synchronized with large or moderate-sized fast earthquakes, some researches focus on interaction between slow earthquake and microseismicity as their interactions are potentially occurring more frequent. Yamamoto et al. (2022) reported shallow SSEs in the Nankai subduction zone frequently accompany earthquake swarms. Similar synchronous activity also occurs in the Japan Trench subduction zone. Obana et al. (2020) reported bursts of shallow tremor often synchronized with earthquake activity in the Off Fukushima section, where is the southern proximity of the coseismic slip region of the 2011 Tohoku-Oki Earthquake (Iinuma et al., 2012; Kubota et al., 2023). However, their observation was made by short-term (4 months) Ocean-Bottom Seismometer (OBS) observation. Because of the limited observational period, it was not clear how frequent such synchronous activity of slow and fast seismicity is occuring in the Off Fukushima.
In order to investigate the correlation of the slow and fast earthquake in Off Fukushima near the Japan Trench, we compare the long-term (6 years; 2016 - 2022) catalogs of tectonic tremor and fast earthquakes. These two catalogs are constructed based on S-net (Aoi et al., 2020) stations. In particular, we chose eight OBSs that located around a known slow earthquake source region of Off Fukushima (Nishikawa et al., 2019; 2022; Matsuzawa et al., 2015; Ohta et al., 2019). Tectonic tremors are detected and located using a modified envelope correlation method (Mizuno and Ide, 2019). The tremor catalog constructed by the envelope correlation method is further refined by removing fast earthquake signals and airgun signals, as well as space-time clustering (e.g. Wech and Creager, 2008). Fast earthquakes are detected by the Earthquake Transformer (Mousavi et al., 2020), which is a deep learning-based module to detect a fast earthquake from a continuous waveform. Detected earthquakes are phase- associated by the REAL (Zha et al., 2019) and located by hypomh (Hirata and Matsu’ura, 1987) with a local 1D veolocity model.
Based on the constructed catalog, we identified a source of earthquake swarm that repeatedly activated after tremor bursts. The source of repetitive earthquake swarm is about 50 km apart from the tremor source region in along-trench direction, and embedded in the region where seisimicity is low relative to nearby region. During the analyzed period, at least 5 earthquake swarm sequence occurred in this region. While the tremor bursts itself recur every ~ 3 months, the earthquake swarms synchronous with the tremor bursts occur almost every ~ 1 year. This result indicates that even though tremor burst does not always accompany seismicity activation, episodic synchronization of slow and fast earthquakes taking place in the shallow part of the Japan Trench subduction zone.

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