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(Dec 10).

************** うなぎセミナーのご案内 (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：2020年12月10日（木）14:00～
場所：オンライン(Zoom)

Date and time：Dec 10 (Thursday), 14:00～
Site：Online by Zoom

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[発表者1 (Presenter1)]
森 祐太朗(Yutaro Mori)

[題目 (title)]
一般に、沈み込み帯では大陸側のリソスフェア（島弧）が海溝軸とほぼ平行に隆起し、島弧-海溝系が形成される。その物理メカニズムは2次元の数値モデルによって合理的に説明されている(Fukahata & Matsu’ura, 2016)。しかし、現実のプレート境界は3次元的な形状を持ち、例えば海溝軸の屈曲部に大きな重力の負異常が生じたりしているが、このような変形の物理メカニズムは未解明の問題として残っている。
本研究では、海溝軸が屈曲部を持つ場合について、定常沈み込みによるリソスフェアの変形を弾性-粘弾性層構造モデルを用いて数値計算した。その結果、海溝軸の屈曲が大きいほど屈曲部付近に大きな沈降のパターンが現れることが明らかになった。
今回の発表では、この変形の物理メカニズムの議論に加え、海溝軸の屈曲に対する沈み込み方向やプレート境界形状の非対称性が変形に与える影響についても紹介する。

[発表者2 (Presenter2)]
Atikul Haque Farazi

[題目 (title)]
The ambient noise H/V at the ocean bottom: implication for subsurface Vs structure estimation at the offshore Fukushima region

[要旨 (Abstract)]
For geophysical characterization of the solid earth, it is crucial to know the Vs structure of the subsurface. But estimation of Vs is not as straightforward as land environment in the ocean. So, in such circumstances, we need to rely on the empirical relationship of Vp and Vs, which is not always desired.
This research aims at deploying the H/V spectral ratio method using the OBS sensors at the deep ocean bottom (~2800 to ~4500 m below the MSL) for Vs structure estimation of the subsurface. The study area is the offshore Fukushima region of Japan, where a plate interface is well defined within 15 to 16 km depth along with occurrence of the Japan Trench, approximately in the 110 km east. Ambient seismic noise data has been used from 24-hours continuous seismic record for obtaining the H/V curves within frequency range of 0.01 to 1 Hz. 3 broadband and 16 short-period OBS sensors were used in this study, where for shortperiod sensors the H/V curve frequency range was limited to 0.2 to 10 Hz. Prior to the H/V computation, however, background seismic noise level was obtained following the method of McNamara and Buland (2004) to get insight into the noise level at lower frequency like 0.01 Hz. The noise model also played a role in verifying the behavior of the H/V spectra in presence of a thick water column. Later, the H/V spectra was inverted for Vs structure estimation beneath each station using the procedure and code developed by Agostiny et al (2015, 2019): it facilitates removal of the water layer effect: and under the diffuse field assumption (DFA) of ambient noise filed (Sanchez-Sesma, 2011).
In this work, the H/V frequency range has been lowered to as low as 0.01 Hz that is beyond traditional application of this method. Because of application of the H/V method for the first time to these OBS sensors, their spatial and temporal stability was also examined. Effect of the huge water column on the H/V spectrum was examined by forward modelling that removed the water layer effect. The behavior of the H/V spectra has also been verified from the noise models. We are hopeful that we have been able to detect the plate interface and Vs structure of the area, rendering novel application of the H/V spectral ratio method itself as well as novel results for the study area. A new frontier of marine seismology could be opened from the outcomes of this work.

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