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, 28).

************** 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 11月 28日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2024-11-28, 13:30~
Place：Uji Campus Main Building E232D

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Speaker 1（発表者）:　Yutaro Ishihara

Title（題目）:
Detection of Very Low-Frequency Earthquakes off the Southeast Coast of the KiiÄb0Peninsula Using Waveform Correlation

Abstract（要旨）:
Very low-frequency earthquakes (VLFEs) have been observed in subduction zones such as the Nankai Trough and the Japan Trench, both known for hosting megathrust earthquakes along the plate interface. In Nankai Trough, the VLFEs are detected in both the updip and downdip portions of the seismogenic zone, suggesting that VLFE activity, serving as a proxy for slow earthquakes, may be linked to the stress accumulation process in areas prone to future coseismic slip. Therefore, it is essential to monitor the spatio-temporal pattern of VLFE activity. Previous studies have employed various methods to obtain the distribution of VLFEs. For instance, on the updip side of the seismogenic zone, VLFEs exhibit a limited distribution in certain areas, showing clear hypocenter migration. In contrast, VLFEs in the downdip portion are distributed in a belt-like pattern. However, detailed spatio-temporal distributions of VLFEs remain unclear due to the limited number of seismic stations and the duration of data collection with highly dense seismic networks. This gap highlights the need for further research into VLFEs to elucidate their spatio-temporal pattern and calls for the development of novel methods to monitor these patterns over the long-term, even with sparse seismic networks.
In this study, we develop a new approach to detect VLFEs using waveform correlations with sparse networks. This method involves calculating waveform correlation for pairs of events and detecting VLFEs based on the differences between observed and calculated trave-times derived from these correlations. We specifically focus on the Kumano-nada area, off the Kii Peninsula, using F-net, a nationwide broadband seismic network, to detect shallow VLFEs. The primary objective of this study is to capture the spatio-temporal characteristics of the detected VLFEs.

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Speaker 2（発表者）:　Kentaro Kuniyoshi

Locating tectonic tremors accurately is important to understand slip behavior of plate interfaces. In general, the signal of tremors has no clear phases unlike regular earthquakes. Therefore, it is difficult to locate tremors using the same method as for regular earthquakes, and a common approach is using the envelope cross correlation method. (e.g., Obara, 2002). Inspired by the Obara (2002)'s envelope correlation method, several modified versions have been proposed (e.g., Mizuno and Ide, 2019).
In the south coast of Guerrero, Mexico, there is a seismic gap called “Guerrero seismic gap” (e.g., Suarez et al., 1990). Plata-Martinez et al. (2021) reported the existence of shallow tremors that occurred in the northwestern part of the gap from 2017 to 2018. However, tremors have not been observed in the southeastern part of the gap due to lack of ocean bottom seismic observation covering the region.
Therefore, this study aims to detect and locate tremors to reveal the spatiotemporal features of regular earthquakes and possible shallow tremors in the southeastern part of the gap from 2022 to 2023. To avoid possible influence that the different envelope correlation methods have, we analyzed same dataset based on two types of envelope correlation methods; the conventional one based on Obara (2002) and the modified version proposed by Mizuno and Ide (2019). Revealed tremor activities have both common and unique characteristics between each method, and it is suggested that each method has its own advantages and disadvantages in terms of the station network and regionally dominant waveforms. We can also see a “silent zone” where neither tremors nor regular earthquakes occurred as suggested in the northwestern part of the gap by Plata-Martinez et al. (2021). In this presentation, I will show the spatiotemporal features of tremors by two methods.

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