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

尾形 良彦

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.50, no.appendix, pp.115-127, 1998-03-31 (Released:2010-11-17)

参考文献数

64

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The anomaly in focus is relative quiescence which is defined as a significant decrease of earthquake activity compared with the predicted occurrence rate by the estimated statistical model for the standard seismicity of the region: we use a point-process model called Epidemic Type Aftershock-Sequences (ETAS) model. Regardless of the seismicity level, the relative quiescence can take place. Size of such quiescence is seen in cumulative and M-T diagrams of transformed occurrence times by using the estimated ETAS model. By this method, we recognized significant, relatively quiet stages in shallow seismicity over M 5 class in a very wide area preceding all studied great earthquakes of M 8 class in and around Japan and also those of M 9 class in the world. However, we saw no relative quiescence for about 20 years up to 1990 in the wide areas which include the Tokai and Boso gaps [OGATA (1992)]. On the other hand, a few authors recently showed a very significant quiescence for the last 20 years since early or mid 1970's in the seismicity of certain areas in or around Tokai region. However, it turns out that these quiescence are seeming ones owing to magnitude shifts which took place during 1975?76. The magnitudes below MJ5.0 are substantially underestimated after the period. This shift is found and estimated by a statistical comparison of magnitudes between the JMA and USGS catalogs. Nevertheless, the recent seismicity with level of MJ≥5.5 in a very wide region including central and western Japan shows a significant relative quiescence since 1992, which may be related to the 1995 Kobe Earthquake of MJ7.2 but seems to be further extended to late 1996 when many M 6 class events successively occurred about the boundaries of the quiet region within a half year span.

著者

尾形 良彦 種村 正美

出版者

京都大学数理解析研究所

雑誌

数理解析研究所講究録 (ISSN:18802818)

巻号頁・発行日

vol.418, pp.2-8, 1981-02

著者

尾形 良彦 種村 正美 遠田 晋次 庄 建倉 鶴岡 弘 田村 義保 佐藤 整尚 川崎 能典 島崎 邦彦 間瀬 茂 柴田 里程 ANDREA Llenos SEBASTIAN Hainzl JEFFREY J. DAVID Vere-Jones

出版者

統計数理研究所

雑誌

基盤研究(A)

巻号頁・発行日

2008

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ETAS(epidemic type aftershock sequence)などの統計的点過程モデルで各地の地震活動の確率的予測を行う同時に,モデルを物差しにして静穏化・活発化などの地震活動異常を検出できる解析手法を確立した.地震活動予測からの逸脱による地震活動異常の空間パタンから,断層内の非地震性すべりによる地殻のストレス変化との因果関係を実証研究した.このような地震活動の異常性が殻歪変化のセンサーとして有用である.さらに地震活動異常とGPSによる地殻変動データとの整合性を確かめ,大地震の前駆的なすべり現象の解明に迫った.