著者

平野 史朗 川方 裕則 土井 一生

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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Revisiting earthquake catalogs have revealed that 40% or more of major earthquakes are accompanied by foreshock activities, at least in California [Abercrombie & Mori 1996 Nature] and Japan [Tamaribuchi et al. 2018 EPS].To investigate whether the foreshocks are magnification and activation of background seismicity, we have to compare waveforms due to the foreshocks and background events that might be sometimes uncataloged because of their small sizes.We can mine even small seismic signals similar to some template waveforms from continuous waveform records by using a matched-filter analysis based on cross-correlation coefficients (CC) between the template waveforms and continuous records.However, in the conventional analysis, we have to define a threshold of CC to detect similar seismic waveforms, which have been chosen subjectively and empirically.Then, we propose a threshold-free method to detect outliers from the empirical distribution of CC among seismic waveforms.In our framework, empirical distributions of the coefficients are modeled by the theory of extreme value statistics, and the detectability is automatically determined from Akaike's Information Criterion (AIC), depending on data.We applied the method of seismic signal detection to 2-years-continuous records before an M5.4 earthquake in Nagano, Japan (June 30, 2011) that followed 27 foreshocks cataloged by JMA.First, we found that the empirical frequency distribution of CC between the continuous records and foreshocks did not follow a normal distribution, which means that we cannot estimate the possibility of a false positive by assuming the normal distribution as a model.Instead, we also found that the maximum value of CC in every few seconds follow the Gumbel distribution after elimination of some outliers.The elimination can be achieved by comparing AICs of data including and excluding the outlier candidates.Given this method, we found a similar event ~2 months before the mainshock and 3 similar events 3-4 days before the mainshock.This result implies that the foreshocks were not similar to background events, and hence, not magnification or activation of them.

著者

王 功輝 土井 一生 釜井 俊孝 後藤 聡 千木良 雅弘

出版者

日本地球惑星科学連合

雑誌

JpGU-AGU Joint Meeting 2017

巻号頁・発行日

2017-03-10

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During the 2016 Kumamoto earthquake, numerous landslides had been triggered in Minamiaso Village. Most of the landslides originated on steep slopes, whereas some of them occurring on gentle slopes were fluidized and the displaced debris travelled long travel distance, resulting in causalities and severe damage to many houses on the downslope. In this study, we examined the geological features of these fluidized landslides occurring on gentle slopes, and performed both in-situ direct shear tests and dynamic ring shear tests on the soils taken from the sliding surface. During the tests, the samples were prepared at different initial water contents, and dynamic tests were performed by applying cyclic loadings with regular frequency and amplitude of shear stress, and also by coseismic loading referred from seismic motion recorded in a seismic station nearby. Based on these results, we finally analyzed the possible initiation and movement mechanisms of these fluidized landslides.

著者

豊本 大 川方 裕則 平野 史朗 土井 一生

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2016年大会

巻号頁・発行日

2016-03-10

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Recently, small foreshocks have been frequently detected using a cross-correlation technique (e.g., Bouchon et al., 2011, Science). For inland earthquakes, foreshocks whose hypocenters were estimated to be adjacent to the mainshock hypocenter were detected from several tens of minutes before the main shock occurrence (Doi and Kawakata, 2012, GRL; 2013, EPS). Toyomoto et al. (2015, SSJ) tried to detect foreshocks of an M 5.4 earthquake in central Nagano prefecture on June 30, 2011, in a similar manner to Doi and Kawakata (2013). Using the continuous waveforms of the vertical component at N.MWDH (Hi-net) station (the epicentral distance of the mainshock is 4.5 km), they newly detected 14 foreshocks with a cross-correlation criterion of 0.6, in addition to 27 foreshocks listed in the JMA (Japan Meteorological Agency) unified hypocenter catalogs. To efficiently detect small foreshocks for other inland earthquakes, it is necessary to design how to set the cross-correlation detection criterion for foreshock detection.In this study, we carried out foreshocks detection of the same earthquake in the same method as Toyomoto et al. (2015, SSJ) using the waveform record of N.MNYH (Hi-net) station (epicentral distance of main shock is 5.3km). In this case, the maximum correlation coefficients during one minute tended to be higher than those for records at N.MWDH station, and the result of detection strongly depends on a threshold employed in a cross-correlation method. This indicates that we should not use a universal threshold independent of data. One of alternative way is to use the standard deviation of cross-correlation coefficients. Then, we made a histogram of the cross-correlation coefficients of 1-day data. The histogram of N.MWDH data is Gaussian and the cross-correlation coefficients obey a normal distribution with the average of zero. Although the histogram of N.MNYH data is not Gaussian, so the cross-correlation coefficients have a large-deviation. In such a case, a criterion depending on the standard deviation is inadequate.Acknowledgments:We used continuous waveform records of NIED high-sensitivity seismograph network in Japan (Hi-net) and the JMA unified hypocenter catalogs.

著者

張 又心 Barbara 土井 一生

出版者

九州産業大学

雑誌

産業経営研究所報 (ISSN:02880059)

巻号頁・発行日

no.45, pp.47-62, 2013

著者

土井 一生

出版者

京都大学

雑誌

特別研究員奨励費

巻号頁・発行日

2007

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地震発生メカニズムを調べるため、本研究では、震源域下における不均質構造がどのように地震発生に影響を与えるのか、についての考察を与えることを目的として、内陸地震の地震発生域下に見られる不均質構造の共通因子を抽出し、それらの不均質構造が地震発生に果たした役割について考察する。まず、中国地方において、山陰地方で発生したマグニチュード6以上の地震が、深さ15-25kmの反射波の強度が高い領域の境界で発生していることが共通する特徴として抽出された。用いる波の周波数を変えた解析でも同じように検出され、不均質構造の特徴的なサイズは一意ではないことが伺えた。解の分解能から断定はできないが、断層が異なる物質の境界に位置し下部地殻までほぼ鉛直な断層の下部延長が存在する可能性を示唆するものと考えられる。続いてこうした特徴が他の似たテクトニックセッティングを持つ1948年にM7.1の福井地震が発生した北陸地方で見られるか検証した。本地域において同様の地殻内近地地震の波形記録を使用し、中国地方と同様の反射法解析を行うことにより、深さ0-100kmの不均質構造の抽出を行った。その結果、フィリピン海プレート、Moho面からの反射を検出することができ、同地域での深さがそれぞれ50-60km、40km程度と推定された。深さ20-25km程度にも明瞭な反射波層が検出された。深さ15-25kmで断層(の深部延長を含む領域)を横切る方向に反射波の強度が断層のはさんで大きく変化していることがわかった。以上により、山陰地方と北陸地方の両方で横ずれ断層を持つ震源が反射波の強度の高低の境界に位置することがわかった。2000年鳥取県西部地震ではこの境界面が断層面とほぼ一致したため、断層の深部延長として機能し、地震発生に結びついたという可能性が示唆される。