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(June, 5th).

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

科目：地震学ゼミナールIV A, C / Seminar on Seismology IV A, C（修士・博士）
日時：2025年 6月 5日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2025-06-05(Thursday), 13:30~
Place：Uji Campus Main Building E232D

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Speaker 1（発表者）:　Yutaro Ishihara
Title（題目）:
Spatio-Temporal Variability of Very Low-Frequency Earthquakes: A Novel Detection Approach Using Waveform Correlation

Abstract（要旨）:
Very low-frequency earthquakes (VLFEs) have been observed along subduction zones, including the Nankai Trough and the Japan Trench, both of which host megathrust earthquakes along the plate interface. In the Nankai Trough, the VLFEs occur both updip and downdip of the seismogenic zone, suggesting that VLFE activity often regarded as a proxy for slow earthquakes may be associated with stress accumulation in regions susceptible to future coseismic slip. Therefore, continuous monitoring of the spatio-temporal evolution of VLFEs is crucial for understanding the underlying physical process in subduction zones.Previous studies have mapped the distribution of VLFEs using various detection methods. In the updip side of the seismogenic zone, VLFEs are localized within specific areas and exhibit showing clear hypocenter migration, while in the downdip side tend to form a belt-like distribution. However, detailed spatio-temporal characteristics of VLFEs remain poorly constrained due to the limited spatial coverage and duration of high-density seismic observations. Addressing this gap requires the development of novel detection techniques that enable long-term monitoring, even with relatively sparse seismic networks.In this study, we propose a new VLFE detection method based on direct waveform correlation. Unlike conventional approaches such as matched-filter and envelope-correlation techniques, our method calculates waveform similarity at individual stations, allowing for the detection of VLFEs with fewer constraints. To investigate the detailed spatio-temporal characteristics of VLFE activity, we applied this method to the nationwide broadband seismic network, F-net, and investigated shallow VLFE activity.
Our analysis focused on VLFE activity offshore of the Kii Peninsula in September 2004. We identified a marked increase in VLFE activity following the Mw 7.4 earthquake on September 5, 2004.

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Speaker 2（発表者）:　Takashi Nishizawa
Title（題目）:
Whats going on Beneath Central Lowland Zones? Insights into Tectonics from Repeating Earthquakes

Abstract（要旨）:
In subduction zones, a characteristic pattern of topography and gravity anomalies generally appears: subduction in oceanic trenches and uplift in island arcs (outer arc to volcanic arc). Matsu'ura & Sato (1989) applied a dislocation theory to plate subduction and calculate crustal deformation in subduction zones. Applying this method to long-term plate subduction, they successfully reproduced subsidence in oceanic trenches and uplift of island arcs. However, the most important internal structure in island arcs is the relative subsidence between the outer arc and the volcanic arc (Central lowland zone: Kaizuka, 1972), has not been reproduced. This means that while the general uplift of island arcs can be explained, it is not well understood why the lowland zones are observed in the Seto Naikai (Inland Sea) and the Kitakami-Abukuma in Japan as well as in the Cascadia and Nicaragua. However, it is interesting that some island arcs, such as Aleutian, Kermadec and Mariana do not have it. Therefor it is expected that different subduction parameters (dip angle, plate age, etc.) and mechanical properties (e.g., viscosity) in each region control its formation.
This study aims to understand the formation mechanism of the central lowland zone through a seismological approach. In particular, we focus on repeating earthquakes. One advantage of studying repeating earthquakes is that they provide information on the steady slip rate on the plate interface. Additionally, they contribute to a better understanding of the three-dimensional structure of the subducting slab. Therefore, I present the results of repeating earthquake analyses conducted in Nicaragua and northern Costa Rica. The event catalog was automatically generated using PhaseNet (Zhu & Beroza, 2019), GaMMA (Zhu et al., 2022), and hypomh (Hirata & Matsu’ura, 1987). From the detected events, we extracted candidates for repeating earthquakes based on waveform cross-correlation (Igarashi et al., 2003).
Furthermore, using a large number of detected earthquakes, we constructed a three-dimensional slab model for this region. Specifically, we used the depths of the seafloor topography and the earthquake catalog as input data and applied a basis function expansion using cubic B-splines under spatial smoothing constraints (Yabuki & Matsu’ura, 1992). The smoothing parameter was determined by minimizing the ABIC.
Based on these results, we will enjoy the discussion of the tectonics in Nicaragua and Northern Costa Rica.