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(December, 4th).

************** 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：2025年 12月 4日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2025-12-04(Thursday), 13:30~
Place：Uji Campus Main Building E232D

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Speaker 1（発表者）:　小松 理子（Riko KOMATSU）
Title（題目）:
Estimation of spatiotemporal variations in interplate coupling in the Hyuga-nada, southwest Japan, using ~30-year GNSS data

Abstract（要旨）:
The Hyuga-nada region is located at the western edge of the source area of M≥8 Nankai Trough earthquakes, and has also experienced M7–8 class events occurring independently in the past. Estimating interplate coupling in this region is therefore crucial for assessing its future seismic potential. Although interplate coupling in Hyuga-nada has been investigated previously (e.g., Wallace et al., 2009), those studies were limited by the sparse GNSS network at the time and by non-uniqueness in modeling upper-plate internal deformation.
In this study, we aim to clarify the spatial distribution and temporal evolution of interplate coupling in Hyuga-nada by using GNSS data (including university-operated stations) from 1996 to 2024, together with offshore GNSS-A measurements.
In this presentation, we estimate both the long-term average interplate coupling and the block motions using a block-fault model (McCaffrey, 2009) that assumes a simplified block geometry in which the continental plate is divided into the southern Kyushu block and the other block. The analysis uses GNSS data from March 1996 to September 2024, and GNSS-A data from January 2012 to September 2024.
We first removed co-seismic displacements associated with M≥6.0 earthquakes, and corrected postseismic deformation for the 2011 Tohoku-oki earthquake. To objectively extract temporal variations in interplate coupling, we then applied an L1 trend filter (e.g., Yano et al., 2022) to the land-based GNSS time series, divided the analysis period into eight subperiods, and conducted coupling inversions for each.
Our results show that in the source area of the 2024 M7.1 Hyuga-nada earthquake, significant interplate coupling was present during approximately 2004–2016, whereas little to no coupling is observed in the periods before and after. In contrast, in the source region of the 1968 M7.5 Hyuga-nada earthquake, interplate coupling appears to have persisted throughout 1996-2024, suggesting that strain may have been accumulating from the last earthquake. Furthermore, our results imply that this locked region may be mechanically separated from the locked zone off Shikoku.

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Speaker 2（発表者）:　石原 祐太朗（Yutaro ISHIHARA）
Title（題目）:
Spatio-temporal Variability of Very Low-frequency Earthquakes: A Novel Detection Approach Using Waveform Correlation

Abstract（要旨）:
Understanding the spatio-temporal characteristics of slow earthquake activity is a fundamental issue in seismology. Slow earthquakes have been widely observed in subduction zones around the world, including Nankai Trough and Ryukyu Trench, as well as in regions such as Cascadia and Mexico. Like fast earthquakes, slow earthquakes are thought to occur along plate boundaries, and their source regions are often located adjacent to seismogenic zones. Therefore, monitoring these events is essential for evaluating stress accumulation and the seismic potential along plate boundaries.
For example, in the Nankai Trough, very low-frequency earthquakes (VLFEs), a type of slow earthquake, are known to occur in both the shallow and deep portions of seismogenic zone. VLFEs are characterized by dominant frequencies in the range of 0.01-0.1 Hz and are primarily observed as surface waves. Because surface waves experience less attenuation than body waves, they can be detected at broadband onshore stations even when the source is located offshore. This makes it possible to monitor their activity using various seismic networks.
In this study, we propose a VLFE detection approach based on direct waveform correlation. While previous studies have employed methods such as matched-filter technique and envelope correlation method, our approach directly evaluates waveform similarity at individual stations. Moreover, by reducing assumptions related to factors such as velocity structure, this approach allows for more flexible event detection.
In the presentation, we apply the method to the nationwide F-net broadband seismic network in Japan and introduce results of VLFE detection.