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, 6).

************** 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：2024年 6月 6日 (木) 13:30~
場所：京都大学 防災研究所 本館E-232D

Date and Time：2024-06-06, 13:30~
Place：Uji Campus Main Building E232D

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Speaker（発表者）1:　Yuki Funabiki

Title（題目）:
Focal mechanisms estimation with S/P amplitude ratio of DAS records

Abstract（要旨）:
Distributed Acoustic Sensing (i.e. DAS) is the novel tool of earthquake observation. By sending laser pulses and measuring the phase change of Rayleigh back-scattering from intrinsic fiber-cable impurities, a DAS interrogator unit measures the strain or strain-rate along the cable.
There is a previous study that estimated the focal mechanism using DAS. Li et al., 2023 presented the method of estimating focal mechanisms with DAS by measureing P-wave polarities of a DAS array based on cross-correlation between earthquake pairs. However, their methods cannnot estimate focal mechanisms just with DAS records strictly because they measure only P-wave polarity, and they need multiple events that occurred nearby.
Here we present the method of estimating focal mechanisms from S/P Amplitude Ratio of DAS records. We first cut the P- and S- window from their arrival time, measured each maximum amplitude of strain, and calculated S/P of DAS records. We then searched the focal mechanism that best explains this observed S/P and compared with initial polarity distribution obtained from manual picking of conventional seismometers. Surprisingly,the observed S/P of DAS well corresponds to the "ordinary" S/P, despite the fact that "DAS only records a component along the cable". We will discuss the reason why these surprising results were obtained.

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Speaker（発表者）2:　Takashi Nishizawa

Title（題目）:
中央沈降帯(内海)を作ろう！
Let‘s make Central lowland zones (Inland seas)

Abstract（要旨）:
In subduction zones, a characteristic pattern of topography and gravity anomalies (trench low and island arc high) generally appears. Matsu'ura & Sato (1989) developed a method to calculate crustal deformation in subduction zones based on elastic-viscoelastic dislocation theory. Applying this method to long-term plate subductions reproduces subsidence in oceanic trenches and uplift of island arcs. However, the most important internal structure of the island arc, 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 the island arc can be explained, it is not well understood why the central lowland zones are formed, such as the Seto Inland Sea and the lowland along the Kitakami-Abukuma Rivers. Similar topography is found in some island arcs, such as Cascades and Chile. However, it is not found in some island arcs such as Izu, Kermadec and the Southern Sandwich. Therefor it is expected that different subduction parameters (dip angle, thickness, etc.) in each region control its formation.
This study aims to understand the formation of the central lowland zone through a theoretical model and comparative subductology of observed subduction parameters. The former part, I will discuss the results of numerical experiments by using the Dislocation model for plate subduction. The latter part, I will introduce the framework of the statistical method to reveal the underlying causal relationships among observed parameters.

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