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, 6th).

************** 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 11月 6日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2025-11-06(Thursday), 13:30~
Place：Uji Campus Main Building E232D

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Speaker（発表者）:　船曵 祐輝 (Yuki Funabiki)
Title（題目）:
Diverse DAS-wavefields and their rupture processes of small-to-moderate earthquakes

Abstract（要旨）:
The rupture process complexity of large earthquakes (Mw > 5) are well recognized from waveform inversion and back-projection. On the other hand, it is hard to understand the rupture process of small-to-moderate earthquakes because high-frequency signals, which contains source information of such earthquakes, are more strongly affected by local structure heterogeneities and do not directly reflect the source clearly, and these events are often assumed as simple rupture style to analyze in frequency domain. However, many previous studies suggest that such earthquakes is as complicated as large earthquakes (e.g., Yamada et al., 2005; Yoshida & Kanamori, 2023), and high-density observation is required to investigate source complexity.
Distributed Acoustic Sensing, (i.e., DAS) is an emerging technology that has great potential in seismology. DAS enables extremely high-density and spatially distributed seismic observations by using optical fiber cables themselves as seismic sensors, which is impossible with conventional seismograms. Leveraging this advantage, Li et al. (2023) identified 4 subevents in M6 crustal earthquake from back-projection of DAS records. Their results suggest that DAS records contain at least non-negligible source information.
The earthquake swarms has been reported in Noto peninsula since 2020. o monitor this activity, DAS observations were conducted during Jan–Mar and Jul–Sep 2023 (Tanaka et al., JpGU 2023). Examining the observed DAS wavefields, we find that even seismic waves from earthquakes occurring in nearby areas with similar JMA magnitudes show diverse characteristics: some record their maximum amplitude almost exactly at the P-wave arrival, some appear to involve two events about 0.5 seconds apart, and others show weak vibrations for about 0.2 seconds after the P-wave arrival before reaching the maximum amplitude. Importantly, these features appear consistently across most channels in a way that aligns with the expected P-wave arrival time differences, suggesting that they directly reflect the source process.
To extract source processes from the DAS records, we made the following assumptions: The direct P-wave from the source arrives at each channel with a travel-time difference equal to the P-wave arrival, while the scattered waves near the surface, excited by the direct P-wave, behave as random noise and cancel out when stacked. Based on these assumptions, we applied slant stacking along the P-wave arrival to the DAS records of approximately 150 consecutive Channels (about 800m cable length). From the resulting slant-stacked waveform, we discuss the source processes of earthquakes that exhibited characteristic DAS wavefields.