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(July 30).

************** うなぎセミナーのご案内 (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：2020年7月30日（木）14:00～
場所：オンライン(Zoom)

Date and time：July 30 (Thursday), 14:00～
Site：Online by Zoom

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*1)* [発表者 (Presenter)]
劉弋鋒 (Yifeng LIU)

[題目 (title)]
1992年ニカラグア津波地震と前震及び余震の遠地波形の特徴

[要旨 (Abstract)]
地震動から求められるマグニチュードの大きさに対して、大きな津波が観測される地震が津波地震である。津波地震は観測事例が少ないことから、その発生メカニズムはいまだ不明である。本研究では、津波地震と同様の特徴を示すM5−６程度の中規模の地震が本震前後に、震源域周辺で発生しているか調べた。1989年〜2020年にニカラグア付近の沈み込み帯で起こっている低角逆断層型の地震について、表面波とS波の最大振幅の比と、 モーメントマグニチュードと表面波マグニチュード（または表面波マグニチュードと実体波マグニチュード）を比較することで、津波地震と同様にマグニチュードの差が大きい地震を津波地震と同様の特徴を示す地震として抽出した。結果、1992年ニカラグア津波地震（1992年9月2日,Mw 7.6）の発生3日後に発生した余震（1992年9月5日, Mw 6.0）について、本震と同様の大きな振幅比が得られた。また、この地震は表面波と実体波からそれぞれ推定されたマグニチュードの差も0.8と他の地震と比べても大きい。この余震は本震の震央付近で発生している。その他、やや振幅比の大きな地震は本震の震央から南東に100km程度離れた領域にも分布することから、振幅比の大きな地震は面的に発生していたことがわかった。津波地震発生域周辺の前震および余震活動も調べた。ここでは1992年ニカラグア津波地震、2006年インドネシア・ジャワ島沖の津波地震2010年インドネシア・メンタワイ島沖の津波地震の本震前後の地震活動を調べた。これらの地震活動を小領域に分けて地震活動を調べた結果、いずれの津波地震についても、本震の発生前に震源域の周りで地震活動度が低下を確認した。津波地震の発生に、その周辺で地震の静穏が観測される可能性を示唆する。

*2)* [発表者 (Presenter)]
Mpuang Admore

[題目 (title)]
1-D layered seismic structure of the crust beneath Botswana

[要旨 (Abstract)]
There is limited knowledge of crustal structure beneath Botswana, with previous studies mostly focused on Kalahari craton and Okavango rift zone and assuming a simple crust. In an attempt to obtain detailed knowledge of the crustal structure beneath Botswana and its relation to tectonics and observed seismicity, 1-D shear wave velocity (Vs) models for layered crust are estimated for the first time from receiver functions. P-to-S receiver functions are computed for teleseismic events from epicentral distances between 30˚ and 85˚ with Mw>5.8 recorded at 35 stations in Botswana. Quality assessment is carried out to select the best quality radial receiver functions. Selected radial receiver functions are stacked in backazimuth as preliminary results to guide further analysis. Grouping and stacking of radial receiver functions is then done based on waveform similarity and relative difference in slowness. A genetic algorithm inversion technique (Sambridge and Drijkoningen, 1992; Shibutani et al, 1996) is applied to the newly stacked radial receiver functions to obtain 1-D Vs models for a horizontally layered isotropic crust. The preliminary stacks reveal converted phases from the Moho and mid-crustal interfaces, with some backazimuthal variation that may be caused by anisotropic and dipping structures. “Delayed” and “double-peaked” direct P arrivals observed at stations in mobile belts and sedimentary basins are caused by high Vs contrasts in the shallow structure. Resulting 1-D models reveal low velocity layers in the thinner crust (33.7 ± 2.4 km) of Okavango rift zone that may be caused by fluid presence in weak zones as suggested by (Leseane et al., 2015). Maximum crustal thinning is coincident with the location of the rift zone’s active fault system. While the thin crust seems to extend into central Botswana beneath Nosop Basin, thicker crust (~50km) is observed beneath SW end of Ghanzi-Chobe belt and Western border of Zimbabwe craton. A NW-SE trending region of higher Vs in the upper crust cuts across tectonic regions. Its coincidence with a previously reported higher gravity anomaly suggests the presence of mafic intrusions that may be related to the intrusions of the nearby Okavango dyke swarm. Similarity of velocity models in Damara belt and Congo craton may suggest an extension of the Damara belt beyond its currently presumed boundary with the Congo craton. Results of this study are within 4km of a 1-to-1 correlation in crustal thickness with previous studies. It is recommended that future studies consider detailed investigation of shallow structure beneath mobile belts and sedimentary basins, as well as anisotropic and dipping structures in the crust.

*3)* [発表者 (Presenter)]
Raymundo Plata-Martinez

[題目 (title)]
Shallow slow earthquakes and mechanical properties at the Guerrero Gap, Mexico

[要旨 (Abstract)]
For the past decades a proposed earthquake scenario for the Guerrero seismic gap, located along the Pacific coast of Mexico, has been one of a pending rupture of the entire gap capable of generating a large earthquake with Mw>8. The Guerrero subduction zone is also prone to slow earthquakes, unfortunately, these observations come only from onshore observations, limiting our knowledge of the shallow plate interface. So far, shallow slow earthquakes have not been observed at the Mesoamerican trench, for that reason we deployed an array of ocean bottom seismometers inside the northwest Guerrero seismic gap. We seek to answer questions such as, if slow earthquakes happen near the trench, what are their characteristics and most importantly, we look to improve our understanding of future large earthquakes in Guerrero. We searched for shallow tremors as first evidence of shallow slow earthquakes. Regular earthquakes and repeaters were also located and together with the residual gravity and residual bathymetry, were interpreted to infer mechanical properties at the plate interface. We found that subducted relief might contribute to produce highly heterogeneous frictional conditions that give rise to slow slip conveyed with episodic shallow tremor, repeaters and possible short-term slow slip events. Our observations indicate that the central portion of the Guerrero seismic gap is absent of a seismogenic zone and dominated by a low-coupling stable regime. This implies that the initiation of a large earthquake is less likely to occur in the northwest Guerrero seismic gap. However, we cannot discard the possibility that the seismic gap could ruptured as the results of a M>8 earthquake propagating from an adjacent unstable domain.

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