著者

上田 拓 山谷 里奈 尾形 良彦 加藤 愛太郎

雑誌

JpGU-AGU Joint Meeting 2020

巻号頁・発行日

2020-03-13

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On June 18, 2019, a Mj6.7 earthquake occurred at Yamagata-oki. The source region of this earthquake is adjacent to that of the Mj7.5 earthquake which occurred on June 16, 1964, and in this region, there are few aftershocks right after the 1964 earthquake, and the seismicity rate in recent years is extremely low (Earthquake Research Committee, 2019). This observation suggests that the source region of the 2019 Yamagata-oki earthquake was not ruptured by the Niigata earthquake, but the cause has not been revealed. In order to elucidate the relationship between these two areas, this study compared the characteristics of seismicity between the two areas.We used the JMA catalog constructed by Japan Meteorological Agency (the Preliminary Determination of Epicenters). We applied HIST-ETAS (Hierarchical Space Time Epidemic Type Aftershock Sequence) model (e.g., Ogata, 2004) considering the spatial dependence of each parameter of the Space Time ETAS model (e.g., Ogata, 1998), to the hypocenter catalog (M1.8) from 1998 through 2019 in order to estimate the spatial distribution of background seismicity rate μand number of aftershock occurrences K. As a result, we find that μ-value is higher and K-value is lower in the source region of Yamagata-oki earthquake than in that of Niigata earthquake.In addition to these differences, we find that the b-value, which is one of the characteristics of the seismicity, is lower in the source region of Yamagata-oki earthquake than in that of Niigata earthquake. Moreover, comparing the seismic wave velocity structure obtained by Matsubara et al. (2019), the P wave velocity is lower in the source region of Yamagata-oki earthquake than in that of Niigata earthquake.The difference in seismic wave velocity and characteristics of seismicity between these two areas suggests that the macroscopic behavior in the source region of Yamagata-oki earthquake is more ductile than in that of Niigata earthquake. In more ductile area, microfracture is likely to proceed and it decreases seismic wave velocity. In addition, background seismicity rate (μ) decreases in more ductile area because of low brittleness. Moreover, the results of rock experiments and numerical simulation by Amitrano (2003) imply the increase in aftershock productivity (K) and the decrease in b-value in more ductile area. Focusing on the short-wavelength component of the linear strain rate distribution in the east-west direction (Meneses-Gutierrez and Sagiya , 2016), the different response for the 2011 off the pacific coast of Tohoku earthquake between the source regions of Yamagata-oki earthquake and Niigata earthquake is appeared. These differences may reflect different deformation styles between the two regions.

著者

上田 拓 山谷 里奈 加藤 愛太郎

出版者

日本地震学会

雑誌

日本地震学会2019年度秋季大会

巻号頁・発行日

2019-08-15

著者

吉田 真吾 上嶋 誠 中谷 正生 加藤 愛太郎 小河 勉

出版者

東京大学

雑誌

基盤研究(B)

巻号頁・発行日

2004

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最近,種々の構造探査により,縦波速度(Vp),横波速度(Vs),電気比抵抗などが同一断面上にマッピングされるようになってきた.それら観測可能なVp, Vs,電気伝導度,Qなどから,どのような物質がどのような状態にあり,どのような破壊・摩擦特性をもっているのか推定できるようになることを目指し,室内実験によりVp, Vs,電気伝導度などと,破壊・摩擦特性を様々な条件下で同時測定できる装置を開発した.同時測定が必要なのは,間隙の形状や連結性に依存する物性パラメターは,(特に高温で間隙水が存在する場合,化学反応が活発なので,)温度・圧力を与えても一意に定まるとは限らないからである.高温高圧下で岩石の電気伝導度を測定する場合,金属ジャケットで岩石試料を覆うことになる.そのような状態で岩石試料の伝導度を求めるのに,金属ジャケットを主に流れてきた電流と試料中心部を流れてきた電流を分離し,それぞれガードリングとセンター電極で測定するガードリング法を用いる.ガードリングに流れ込む電流とセンター電極に流れ込む電流を計算し,適切な配置を検討し,昨年度,ガードリングモジュールを設計・製作した.さらにその測定システムを用い,日高変成帯主帯の泥質変成岩類などの測定を行った.ガードリングを用いても金属ジャケットを用いる影響を完全には取り除けないので,見かけ抵抗から試料の真の電気伝導度を算出する補正係数を数値解から求めた.温度は室温から25℃ごと250℃まで,圧力は10MPaから250MPaまで,周波数は周波数1Hz~1MHzまで変化させて測定した.日高変成帯泥質ホルンフェルスの電気伝導度は黒雲母片岩などに比べ非常に高い.一定圧力のもとで,このホルンフェルスの比抵抗は温度上昇とともに増加し,75℃近傍で最大値をとり,その後減少することがわかった.

著者

中村 亮一 鶴岡 弘 加藤 愛太郎 酒井 慎一 平田 直

出版者

公益社団法人 日本地震工学会

雑誌

日本地震工学会論文集 (ISSN:18846246)

巻号頁・発行日

vol.20, no.1, pp.1_1-1_12, 2020 (Released:2020-01-31)

参考文献数

15

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関東地方には約300点の加速度計から構成される高密度なMeSO-netが展開されており,2008年から連続波形記録が蓄積されている.これら高密度観測記録を用いることで,より高分解能の三次元減衰構造を求めることができることが期待される.ただし,各地震計は地中約20mの深さに設置されており,観測波形記録には地表からの反射波の影響が含まれると考えられるため,これらの影響を考慮してゆく必要がある.そこで,まず波形記録のスペクトルに現れる特徴を調べた.その結果,地中設置のためスペクトルに谷が形成されていることが確認できた.次に,MeSO-netとK-NET及びKiK-net記録を用いた三次元Q値とサイト増幅特性の同時インバージョンを行い,地中設置による地盤増幅特性への影響について解析を実施した.ここで,地盤増幅は卓越周期からグループ化し,それぞれのグループで同じ増幅をもつと仮定する手法であり,K-NET及びKiK-netの地表観測地点は8グループに分け,MeSO-netの地中記録は,それとは別の2グループに分けた.その結果,減衰構造は先行研究と整合した結果が得られた.地中設置の場合でも,その増幅率を適切に考慮することにより減衰構造を求めることができることを確認した.また,平均的にみて地中記録の増幅特性は地表の岩盤サイトに類似しており,地表の地盤の差異による影響に比べて小さいことがわかった.

著者

加藤 愛太郎 上田 拓

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2019年大会

巻号頁・発行日

2019-03-14

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We reconstructed the spatiotemporal evolution of seismicity associated with the 2018 Mw 5.6 northern Osaka earthquake, Japan, to discuss the source fault model of the mainshock rupture, the possible link between this rupture and known active faults, and subsequent crustal deformation. We first relocated the hypocenters listed in the earthquake catalog determined by the Japan Meteorological Agency using a double-difference relocation algorithm. We then searched for the earthquake waveforms that closely resembled those of the relocated hypocenters by applying a matched filtering technique to the continuous waveform data. The relocated hypocenters revealed two distinct planar alignments with different fault geometries. A combination of the relocated hypocenters and focal mechanisms suggests that the mainshock rupture initiated on a NNW–SSE-striking thrust fault, dipping ~45° to the east, with the rupture propagating to an adjacent sub-vertical ENE–WSW-striking strike-slip fault ~0.3 s after the initial mainshock rupture, resulting in the simultaneous propagation of dynamic rupture along the two faults. The strike-slip fault is oblique to the strike of the Arima-Takatsuki Fault, indicating that blind strike-slip faulting occurred. While the east-dipping thrust fault is located deeper than the modeled extent of the Uemachi Fault, a simple extrapolation of the near-surface geometry of the Uemachi fault partially overlaps the mainshock rupture area. Although it is unclear as to whether a blind thrust fault or a deep portion of the Uemachi Fault ruptured during this mainshock–aftershock sequence, a mainshock rupture would have transferred a static stress change of >0.1 MPa to a portion of the east-dipping thrust fault system. Intensive aftershocks have persisted along the northern and southern edges of the source area, including moderate-magnitude events, whereas the seismicity in the central part of the source area has shown a rapid decay over time. Delayed triggered aftershocks were clearly identified along the northern extension of the rupture area. Because the background seismicity is predominant in this northern area, we interpret that aseismic deformation, such as cataclastic flow lubricated by crustal fluids, triggered this off-fault seismicity.

著者

上田 拓 加藤 愛太郎

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2019年大会

巻号頁・発行日

2019-03-14

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A growing body of evidence suggests that seismicity is seasonally modulated in a variety of tectonic environments (e.g., Gao et al., 2000; Heki, 2003; Bettinelli et al., 2008; Ben-Zion & Allam, 2013; Amos et al., 2014; Johnson et al., 2017). Identifying cyclic variations in seismicity leads to an improvement of our understanding about the physics of earthquake triggering.San-in district, southwest Japan, is an active seismicity zone characterized as high shear strain rate by geodetic measurement (Nishimura and Takada, 2017). Moreover, Ogata (1983) has pointed out a possibility of seasonal variations in seismicity rate in the Inner Zone of southwest Japan. We here focus on seasonal variations in crustal seismicity in San-in district.We used the JMA catalog (constructed by Japan Meteorological Agency) from 1975 through 2017 (magnitude M >= 3.0 and depth <= 20 km). We applied space-time Epidemic Type Aftershock Sequence (ETAS) model (e.g., Ogata, 1998; Zhuang et al., 2004) to the catalog and used a probability-based declustering procedure based on the work of Zhuang et al. (2002) to evaluate the significance of the seasonal variations, adopting uncertainties derived from the declustering scheme.We demonstrated that semiannual variations in background seismicity rate, which increases in spring and autumn, are statistically significant from 1980 through 2017. The distribution of large historic and modern earthquakes (from 1850 through 2017, magnitude M >= 6.2, constructed by Chronological Scientific Tables) shows a similar pattern to recent background seismicity, suggesting that seismicity in San-in district has shown seasonal variations for over 150 years. There is some correlation between the monthly averaged background rate shifted backward 2 months and monthly averaged rain amount in the studied region. These results infer that seasonal variations in seismicity in San-in district can be explained by increasing pore pressure within fault zones, caused by infiltration of rainfall in autumn and decreasing surface mass due to snow melting in spring. Some correlation between seismicity and precipitation suggests that modulation of precipitation may be a key ingredient to produce time-dependence of background seismicity.

著者

前田 裕太 寺川 寿子 加藤 愛太郎 山中 佳子 古本 宗充 山岡 耕春 堀川 信一郎 松廣 健二郎 奥田 隆

出版者

特定非営利活動法人日本火山学会

雑誌

日本火山学会講演予稿集

巻号頁・発行日

vol.2014, 2014-11-02