著者

長谷川 昭 中島 淳一 内田 直希 海野 徳仁

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.122, no.3, pp.398-417, 2013-06-25 (Released:2013-07-08)

参考文献数

60

被引用文献数

1 1

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Recent investigations based on seismic tomography, hypocenter determinations and focal mechanism analyses using dense seismic network data reveal the precise configurations of the Pacific (PAC) and Philippine Sea (PHS) plates subducting beneath the Tokyo metropolitan area. Estimated geometry shows a broad contact zone between the two plates located directly beneath the Kanto plain. The overlap with the PHS plate subducting above it provides the PAC plate with protection from being heated by the hot mantle wedge. Moreover, the fore-arc portion of the PHS plate, before its subduction beneath Kanto, had been cooled by the subduction of the PAC plate from the Izu-Bonin trench. These cause lower-temperature conditions within the two oceanic plates and the upper continental plate beneath the Tokyo metropolitan area. As a result, depth limits of seismic activities within the plates and along their boundaries are anomalously deep. Seismic tomography studies show that the easternmost portion of the PHS slab mantle is serpentinized. The PHS slab may have been torn in two along the western boundary of this serpentinized mantle, with the eastern portion being left behind relative to subduction of the western portion. This is accompanied by the generation of large intraslab earthquakes along the boundary. We need to take these observations into consideration to understand the mechanism generating M7-class earthquakes, which are anticipated to occur in the southern Kanto region with a high probability.

著者

長谷川 昭 中島 淳一 内田 直希 弘瀬 冬樹 北 佐枝子 松澤 暢

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.190-204, 2010-04-25 (Released:2010-07-06)

参考文献数

44

被引用文献数

13 10

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A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. We introduce a precise configuration of the Philippine Sea and Pacific plates subducting beneath the Japanese Islands, which was recently obtained by seismic tomographic imaging, precise earthquake hypocenter determinations, and focal mechanism studies. Seismic tomographic studies show that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the area north of the Izu Peninsula. The estimated geometry of the subducted Pacific and Philippine Sea slabs shows a broad contact zone between the two slabs located directly beneath the Kanto plain. It further shows the wavy configuration of the Philippine Sea slab subducting beneath the entire region of southwestern Japan. Contact between the Philippine Sea plate and the Pacific plate causes anomalously deep interplate and intraslab earthquake activity in Kanto. Moreover, the interplate coupling coefficient estimated from repeating earthquake data shows a distinct change across the northeastern edge of this slab contact zone, suggesting that the overlying plate controls large-scale interplate coupling. High-resolution studies of spatial variations of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittlement hypothesis for the generation of intraslab earthquakes.

著者

西川 友章 松澤 孝紀 太田 和晃 内田 直希 西村 卓也 井出 哲

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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Subduction zone megathrust earthquakes result from the interplay between fast dynamic rupture and slow deformation processes, which are directly observed as various slow earthquakes, including tectonic tremors, very low-frequency earthquake (VLFs) and slow slip events (SSEs), and indirectly suggested by a temporal change in the frequency of repeating earthquakes and the occurrence of episodic earthquake swarms. Some megathrust earthquakes have been preceded by slow earthquakes and terminated near the areas where slow earthquakes were frequently observed. While capturing the entire spectrum of slow earthquake activity is crucial for estimating the occurrence time and rupture extent of future megathrust earthquakes in a given subduction zone, such an observation is generally difficult, and slow earthquake activity is poorly understood in most subduction zones, including the Japan Trench, which hosted the 2011 Mw9.0 Tohoku-Oki earthquake. Here we reveal the slow earthquake activity in the Japan Trench in detail using tectonic tremors, which we detected in the seismograms of a new ocean-bottom seismograph network, VLFs, SSEs, repeating earthquakes, and earthquake swarms. We show that the slow earthquake distribution is complementary to the rupture area of the Tohoku-Oki earthquake and correlates with the structural heterogeneity along the Japan Trench. Concentrated slow earthquake activities were observed in the afterslip area of the Tohoku-Oki earthquake, which is located to the south of the fore-arc geological segment boundary. Our results suggest that the megathrust in the Japan Trench is divided into three segments that are characterised by different frictional properties, and that the rupture of the Tohoku-Oki earthquake, which nucleated in the central segment, was terminated by the two adjacent segments.

著者

内田 直希 松澤 暢 三浦 哲 平原 聡 長谷川 昭

出版者

SEISMOLOGICAL SOCIETY OF JAPAN

雑誌

地震. 2輯 (ISSN:00371114)

巻号頁・発行日

vol.59, no.4, pp.287-295, 2007-03-25

参考文献数

26

被引用文献数

5 4

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Spatio-temporal distribution of quasi-static slip on the plate boundary east off Miyagi and Fukushima prefectures, NE Honshu, Japan is estimated by using small repeating earthquake data. The analysis of small repeating earthquakes has advantages of relatively high spatial resolution, especially near the trench, and the availability of long-term data (22 years) compared to GPS data. The results show that the repeating earthquakes are distributed outside the coseismic slip areas (asperities) of large earthquakes, showing that fault creep is dominant outside the asperities. The cumulative slip (slip histories) of small repeating earthquake groups reveal the existence of many non-steady aseismic slip events. Most of the episodic quasi-static slip events are associated with <I>M</I>≥ 6 earthquakes and they are frequently seen in the areas near the Japan trench in particular. Minor afterslip (∼15cm) of the 2005 Miyagi-oki earthquake (<I>M</I>7.2) is also estimated in the area which encompasses the coseismic slip area of the 2005 earthquake.

著者

長谷川 昭 中島 淳一 内田 直希 梁田 高広 岡田 知己 趙 大鵬 松澤 暢 海野 徳仁

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.121, no.1, pp.128-160, 2012-02-25 (Released:2012-03-05)

参考文献数

120

被引用文献数

6 12

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The mechanisms that generate the three main types of earthquake in subduction zones are discussed addressing their relations to geofluids. Studies on the spatial distribution of earthquakes and seismic velocity structure within the subducted slab provide evidence that strongly supports the dehydration embrittlement hypothesis for the generation of intermediate-depth intraslab earthquakes. Detailed imaging of the seismic velocity structure in and around plate boundary zones suggests that interplate coupling is mainly controlled by local fluid over-pressure. Seismic tomography studies show the existence of inclined sheet-like seismic low-velocity zones in the mantle wedge, not only in Tohoku but also in other areas in Japan, which perhaps correspond to the upwelling flow of the subduction-induced convection system. These upwelling flows reach the Moho directly beneath the volcanic areas, suggesting that those volcanic areas are formed by the upwelling flows. Aqueous fluids derived from the slab are probably transported up through the upwelling flows to the arc crust, where they might weaken the surrounding crustal rocks and finally cause shallow inland earthquakes. All of these observations suggest that geofluids expelled from the subducting slab play an important role in the generation of earthquakes in subduction zones.

著者

細田 正洋 石川 徹夫 床次 眞司 金賀 愛子 伊藤 亮輔 内田 直希 荻原 俊輔 笠 喜洋 川内 隆行 石井 忠

出版者

日本保健物理学会

雑誌

保健物理 : hoken buturi (ISSN:03676110)

巻号頁・発行日

vol.42, no.3, pp.221-226, 2007-09

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The bio-kinetics of radon ingested from drinking water was investigated for eight Japanese subjects. A whole-body counter at National Institute of Radiological Sciences (NIRS) was used for the measurement of the ^<214>Bi transfer half-life from the stomach to the small intestine. The transfer half-life seemed to be affected by water quality (chemical/physical properties) as well as the meal ingested before the experiments. The shortest transfer half-life among the subjects was about 50 minutes. During the whole-body counting, the air expired by the subjects was sampled and its radon concentration was measured using two ionization chambers. The changes in radon concentration in the expired air could be classified into three patterns. (1) It decreased with time after ingestion. (2) A peak appeared at the 15 minutes after ingestion. (3) A peak appeared at the 25 minutes after ingestion. These patterns seemed to be related to the ^<214>Bi transfer half-life and the amount of ingested water.

著者

内田 直希 東 龍介 石上 朗 岡田 知己 高木 涼太 豊国 源知 海野 徳仁 太田 雄策 佐藤 真樹子 鈴木 秀市 高橋 秀暢 立岩 和也 趙 大鵬 中山 貴史 長谷川 昭 日野 亮太 平原 聡 松澤 暢 吉田 圭佑

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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沈み込み帯研究のフロンティアである前弧の海域下において,防災科学技術研究所は新たに日本海溝海底地震津波観測網(S-net)を構築した.S-netは東北日本の太平洋側の海岸から約200kmの範囲を海溝直交方向に約30km,海溝平行方向に50-60km間隔でカバーする150点の海底観測点からなり,その速度と加速度の連続データが,2018年10月より2016年8月に遡って公開された.観測空白域に設置されたこの観測網は,沈み込み帯の構造およびダイナミクスの解明に風穴をあける可能性がある.本発表ではこの新しいデータを用いた最初の研究を紹介する.まず,海底の速度計・加速度計の3軸の方向を,加速度計による重力加速度および遠地地震波形の振動軌跡を用いて推定した.その結果,2つの地震に伴って1°以上のケーブル軸周りの回転が推定されたが,それ以外には大きな時間変化は見られないことがわかった.また,センサーの方位は,5-10°の精度で推定できた.さらに得られた軸方向を用い,東西・南北・上下方向の波形を作成した(高木・他,本大会).海底観測に基づく震源決定で重要となる浅部の堆積層についての研究では,PS変換波を用いた推定により,ほとんどの観測点で,350-400mの厚さに相当する1.3 – 1.4 秒のPS-P 時間が観測された.ただし,千島-日本海溝の会合部海側と根室沖の海溝陸側では,さらに堆積層が厚い可能性がある(東・他,本大会).また,雑微動を用いた相関解析でも10秒以下の周期で1.5 km/s と0.3 km/sの2つの群速度で伝播するレイリー波が見られ,それぞれ堆積層と海水層にエネルギーを持つモードと推定された(高木・他,本大会).さらに,近地地震波形の読み取りによっても,堆積層およびプレート構造の影響を明らかにすることができた.1次元および3次元速度構造から期待される走時との比較により,それぞれ陸域の地震の海溝海側での観測で3秒程度(岡田・他,本大会),海域の地震で場所により2秒程度(豊国・他,本大会)の走時残差が見られた.これらは,震源決定や地震波トモグラフィーの際の観測点補正などとして用いることができる(岡田・他,本大会; 豊国・他,本大会).もう少し深い上盤の速度構造もS-netのデータにより明らかとなった.遠地地震の表面波の到達時間の差を用いた位相速度推定では,20-50sの周期について3.6-3.9km/sの位相速度を得ることができた.これはRayleigh波の位相速度として妥当な値である.また,得られた位相速度の空間分布は,宮城県・福島県沖の領域で周りに比べて高速度を示した(石上・高木,本大会).この高速度は,S-netを用いた近地地震の地震波トモグラフィーからも推定されている.また,このトモグラフィーでは,S-netの利用により海溝に近い場所までの速度構造がよく求まることが示された(豊国・他,本大会).雑微動解析によっても,周期30秒程度の長周期まで観測点間を伝播するレイリー波およびラブ波を抽出することができた.これらも地殻構造の推定に用いることができる(高木・他,本大会).また,海域の前弧上盤の構造についてはS-net 観測点を用いたS波スプリッティング解析によって速度異方性の特徴が明らかになった.プレート境界地震を用いた解析から,速いS波の振動方向は,海溝と平行な方向を向く傾向があり,マントルウエッジの鉱物の選択配向や上盤地殻のクラックの向きを表している可能性がある(内田・他,本大会).プレート境界においては,繰り返し地震がS-net速度波形によっても抽出できることが示された.プレート境界でのスロースリップの検出やプレート境界の位置推定に役立つ可能性がある(内田・他,本大会).さらに,S-net加速度計のデータの中には,潮汐と思われる変動が観測されるものもあり,プレート境界におけるスロースリップによる傾斜変動を捉えられる可能性があるかもしれない(高木・他,本大会).以上のように,東北日本の前弧海洋底における連続観測について,そのデータの特性が明らかになるとともに,浅部から深部にわたる沈み込み帯の構造や変動についての新たな知見が得られつつある.これらの研究は技術的にも内容的にもお互いに密接に関わっており,総合的な解析の推進がさらなるデータ活用につながると考えられる. 謝辞:S-netの構築・データ蓄積および公開に携わられた皆様に感謝いたします.

著者

川上 大喜 大久保 寛 内田 直希 竹内 伸直 松澤 暢

出版者

一般社団法人電子情報通信学会

雑誌

電子情報通信学会技術研究報告. SIP, 信号処理 (ISSN:09135685)

巻号頁・発行日

vol.114, no.474, pp.169-174, 2015-02-23

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相似地震(微小繰り返し地震)とは,同一のアスペリティ(固着域)が原因となって発生する地震のことを指す.相似地震は発生時刻が異なってもほぼ同一な波形,つまり相似な波形を伴う性質を持つことがわかっている.この相似地震を解析する事によって,個々の地震の原因となるアスペリティの特性を抽出することや,プレート境界で生じるすべりの時空間発展をモニタリングする事が可能となる.本研究では相似地震の検出法としてフーリエ位相相関法とその手法に対して地震の主要周波数を考慮して帯域制限を施したフーリエ帯域制限型位相相関法,直交3成分地震波データに用いたコヒーレンス関数を適用し,その有効性について検討した.また近年,演算能力の高さから関心を集めているGPU(Graphics Processing Unit)を利用した汎用計算アーキテクチャであるGPGPU(General Purpo Secomputation on GPU)を応用した並列計算処理による高速化についても検討し,処理に要する計算時間を比較した.加えて,これらの一次元信号の対する相関処理演算に対してマルチGPUによるGPGPUを適用し,その有効性についても併せて検討した.

著者

海野 徳仁 河野 俊夫 岡田 知己 中島 淳一 松澤 暢 内田 直希 長谷川 昭 田村 良明 青木 元

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.59, no.4, pp.325-337, 2007-03-25 (Released:2013-08-05)

参考文献数

21

被引用文献数

1 5

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Hypocenters of the main shocks and aftershocks of the 1933 Mjma 7.1, 1936 Mjma 7.4, 1937 Mjma 7.1, 1939 Mjma 6.9, 1978 Mjma 7.4, and 1981 Mjma 7.0 Miyagi-oki, NE Japan, earthquakes are relocated by using S-P times reported in the Seismological Bulletin of the Japan Meteorological Agency (JMA) and those re-read from original smoked-paper seismograms observed at Mizusawa station of National Astronomical Observatory of Japan (NAOJ) and at Mukaiyama station of Tohoku University. In order to avoid large errors caused by inaccuracies in the arrival times of P- and S-waves and the limited number of observation stations, we determined hypocenters by using a grid search method based on the assumption that these events occurred at the boundary between the subducting Pacific plate and the overriding plate. The main shock epicenters of the 1933, 1936, 1937, and 1978 earthquakes are determined close to each other, and distributions of their aftershocks show that aftershock areas of 1933, 1936, and 1937 events partly overlap with that of the 1978 event and occupy its easternmost, central, and westernmost portions, respectively. It is likely that the 1933, 1936, and 1937 events possibly ruptured a part of the source area of the 1978 event, that is the eastern, central, and western portions, respectively. Locations of the main shock and aftershock area of the 1939 event are adjacent to the eastern edge of the source area of 1978 event. After the 1978 event, the 1981 earthquake had occurred there following the slip on the asperities in the presumed Miyagi-oki earthquake source area.

著者

内田 直希 松澤 暢 三浦 哲 平原 聡 長谷川 昭

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.59, no.4, pp.287-295, 2007-03-25 (Released:2013-08-05)

参考文献数

26

被引用文献数

4

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Spatio-temporal distribution of quasi-static slip on the plate boundary east off Miyagi and Fukushima prefectures, NE Honshu, Japan is estimated by using small repeating earthquake data. The analysis of small repeating earthquakes has advantages of relatively high spatial resolution, especially near the trench, and the availability of long-term data (22 years) compared to GPS data. The results show that the repeating earthquakes are distributed outside the coseismic slip areas (asperities) of large earthquakes, showing that fault creep is dominant outside the asperities. The cumulative slip (slip histories) of small repeating earthquake groups reveal the existence of many non-steady aseismic slip events. Most of the episodic quasi-static slip events are associated with M≥ 6 earthquakes and they are frequently seen in the areas near the Japan trench in particular. Minor afterslip (∼15cm) of the 2005 Miyagi-oki earthquake (M7.2) is also estimated in the area which encompasses the coseismic slip area of the 2005 earthquake.

著者

松澤 暢 内田 直希 内田 直希 有吉 慶介 島村 浩平

出版者

東北大学

雑誌

基盤研究(C)

巻号頁・発行日

2007

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アスペリティ(地震性すべりを起こせる領域)の連鎖・連動破壊のしやすさが何に規定されるのかを調べた.その結果,別の大地震の余効滑り(地震の後のゆっくりした滑り)等の擾乱があると連鎖破壊がしやすくなること,二つのアスペリティが隣接していても,破壊の伝播方向と反対側に位置していれば連鎖・連動破壊は生じにくいこと,連動破壊したときのすべり量は個々のアスペリティの破壊履歴に依存することが明らかになった.