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

---

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

---

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

長谷川 昭 中島 淳一 北 佐枝子 辻 優介 新居 恭平 岡田 知己 松澤 暢 趙 大鵬

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.117, no.1, pp.59-75, 2008-02-25 (Released:2010-02-10)

参考文献数

50

被引用文献数

5 8

---

Transportation of H2O from the slab to the arc crust by way of the mantle wedge is discussed based on seismic observations in the northeastern Japan subduction zone. A belt of intraslab seismicity, perhaps caused by dehydration of eclogite-forming phase transformations, has been found in the Pacific slab crust at depths of 70-90 km parallel to iso-depth contours of the plate interface, showing the major locations of slab dehydration. H2O thus released from the slab may be hosted by serpentine and chlorite just above the slab and is dragged downward. DD seismic tomography detected this layer of serpentine and chlorite as a thin S-wave low-velocity layer. Serpentine and chlorite thus brought down to a depth of 150-200 km should decompose there. H2O released by this dehydration decomposition is then transported upward and encounters the upwelling flow directly above, which perhaps causes partial melting of materials within the upwelling flow. Seismic tomography studies have clearly imaged this upwelling flow as an inclined sheet-like seismic low-velocity zone at depths of 30-150 km in the mantle wedge subparallel to the subducted slab. This upwelling flow finally meets the Moho below the volcanic front, and melts thus transported perhaps stagnate directly below the Moho. Some of them further migrate into the crust, and are also imaged by seismic tomography as low velocity areas. Their upward migration and repeated discharge to the surface form the volcanic front. Seismic tomography study of the mantle wedge further revealed along-arc variations of the inclined low-velocity zone: very low velocity areas appear periodically every ∼80 km along the strike of the arc in the backarc region of northeastern Japan above which clustering of Quaternary volcanoes and topography highs are located, suggesting that melts could segregate from these very low velocity areas in the upwelling flow and rise vertically to form volcanoes at the surface in the backarc region.

著者

土山 絢子 平 貴昭 中島 淳一 Roland Bürgmann

雑誌

JpGU-AGU Joint Meeting 2020

巻号頁・発行日

2020-03-13

---

Low-frequency earthquakes generally have relatively stronger spectral components in the lower frequency range, compared to what is expected based on their magnitude. This type of earthquakes has been considered to be generated in some specific environments, such as volcanic regions and subduction zones that are also generating slow slip events and tremors. They usually occur in deeper regions below ~ 15km; however, some recent studies show that they can also be observed among aftershocks of large inland earthquakes in Japan (i.e., Niigata in 2004, Iwate-Miyagi in 2008, Kumamoto in 2016) with very shallow depth (~5km). Here, we attempt to detect such ‘low frequency aftershocks’ associated with a very recent earthquake sequence that occurred in Ridgecrest in California in 2019 on the source of waveform data from Plate Boundary Observatory Borehole Networks and nearby instruments archived at NCEDC, SCEDC and IRIS. We are considering The Frequency Index (FI) as a way to identify candidate events. FI is defined as: FI = log10(AH/AL), where AH and AL represent the mean spectral amplitude within the high-frequency band (e.g., 10-15 Hz) and low-frequency band (e.g., 2-5 Hz), respectively. We will discuss our results and the methods used to identify low frequency aftershocks from their spectra.

著者

土山 絢子 中島 淳一

雑誌

JpGU-AGU Joint Meeting 2020

巻号頁・発行日

2020-03-13

---

Generation mechanism of deep earthquakes in subduction slabs is still enigmatic. Through cross-correlation analysis of earthquakes in central Japan, we identify five pairs and one cluster of deep earthquakes showing strong waveform similarities (coherence > 0.9) for a 40-s time window at more than three stations. For those clusters, we relocate hypocenters by hypoDD, determine the focal mechanisms solutions, and carry out spectral analysis to estimate physical parameters of seismic ruptures (i.e., stress drop, radiated energy and efficiency). The obtained results show that some similar deep earthquakes in each cluster occurred at a slightly different spot on a single fault and have almost the same stress drop. The scaled energy and radiation efficiency of the groups precisely estimated from high-quality similar waveforms show that seismic parameters are significantly different between faults. A wide range of source parameters that have been reported for deep-focus earthquakes may not represent the differences in generation mechanisms of earthquakes but reflect the diversity of inherent fault frictional properties.

著者

片山 郁夫 平内 健一 中島 淳一

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.205-223, 2010-04-25 (Released:2010-07-06)

参考文献数

70

被引用文献数

6 8 2

---

Water is known to play important roles in earthquake generation and volcanic activity. Consequently, the presence of water and its heterogeneous distribution in subduction zones may contribute to the variability of subduction seismicity and arc magmatism. In this study, we infer water distribution, including aqueous fluids and hydrous minerals, based on the seismic tomography beneath Japan, and discuss subduction processes in terms of water circulation in subduction zones. Two distinct oceanic plates, the Pacific plate and Philippine Sea plate, are subducting beneath Japan. These plates have quite different characteristics. The Pacific plate is old (∼130 Ma) and is subducting beneath northeast Japan as rapidly as 10 cm/year. In contrast, the Philippine Sea plate is relatively young (∼20-50 Ma) and is subducting at ∼3-5 cm/year beneath southwest Japan. The subducting old Pacific plate results in cold environments beneath northeast Japan, whereas the thermal structure beneath southwest Japan is relatively warm as a result of the young Philippine Sea plate subduction. Most water is released by eclogite transformation in the subducting oceanic crust, and the expelled water infiltrates into the mantle wedge, forming hydrous minerals such as serpentine and chlorite. The seismic tomography beneath northeast Japan shows that eclogite transformation occurs at depths of ∼80-100 km, and above these depths, a low-velocity anomaly and high Vp/Vs are detected in the mantle wedge. In southwest Japan, eclogite transformation occurs at much shallower depths (50-60 km) due to a warm subduction geotherm. The down-dip limit of interplate seismicity is likely to be controlled by a brittle-ductile transition in southwest Japan, whereas such a limit beneath northeast Japan coincides with the low-velocity anomaly at depths of 60-70 km, suggesting that the presence of serpentine inhibits earthquake activity at the plate interface. The double plane of intraplate seismicity is probably caused by dehydration of eclogite forming reactions in the upper plane and serpentine/chlorite dehydration in the lower plane, although seismic activity is absent at the mantle wedge where water is released by serpentine breakdown. Low-frequency tremors above the Philippine Sea plate are mostly located at the interface between island arc Moho and subducting plate surface. Abundant aqueous fluids in this region due to permeability contrasts may trigger low-frequency tremors. A slab parallel low-velocity zone beneath northeast Japan is interpreted as a melt-filled upwelling flow in the mantle wedge. Such an anomaly is not detected in southwest Japan, and slab melting of the subducting Philippine Sea plate is probably the source of the arc magmatism in this region.

著者

弘瀬 冬樹 中島 淳一 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.60, no.3, pp.123-138, 2008-02-15 (Released:2013-08-20)

参考文献数

46

被引用文献数

5 18

---

We estimated three-dimensional seismic velocity structure in and around the Philippine Sea plate subducting beneath the Kanto district in Japan by applying the double-difference tomography method to arrival time data of earthquakes obtained by the dense nationwide seismic network (Kiban-network). A low S-wave velocity and high Vp/Vs layer with several-km thick, which is shallowly inclined toward the subducting direction of the slab, has been clearly imaged. Comparison with the location of the upper surface of the Philippine Sea slab estimated from seismic refraction surveys, hypocenter distribution of relocated earthquakes, and focal mechanisms shows that this low S-wave velocity and high Vp/Vs layer corresponds to the crust of the Philippine Sea slab. Based on the presently obtained location of the low S-wave velocity and high Vp/Vs layer, hypocenter distribution, and focal mechanisms, we estimated the configuration of the upper surface of the Philippine Sea slab in the Kanto district. Presently estimated configuration of the Philippine Sea slab shows that the slab bends concavely and the depression is located eastward compared with those of the previous studies. Most of the earthquakes associated with the Philippine Sea slab occur along the plate boundary and/ or around the slab Moho. Prominent low S-wave velocity and high Vp/Vs layer was detected at depth of 30 km beneath the region along the latitude of 35.8 degrees, suggesting the serpentinization of the forearc mantle wedge due to dehydration of subducting slab.

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

---

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

中島 淳一 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.61, no.Supplement, pp.177-186, 2009-07-31 (Released:2013-11-21)

参考文献数

65

---

Since late 1990’s, a dense nationwide seismograph network has been constructed in the Japanese Islands with an average station separation of ∼ 20 km, which has produced the highest quality data in the world and contributed to enhance the understanding of seismotectonics and volcanotectonics there. Travel-time tomography using such high-quality data has provided two important constraints on water-transportation paths in subduction zones. One is the evidence for hydrous minerals in and immediately above the slab. The hydrated oceanic crust is imaged as a low-velocity layer to a depth of 40 km for the Philippine Sea slab and 70-130 km for the Pacific slab. Another low-velocity layer is revealed immediately above the Pacific slab down to a depth of ∼ 110 km, which might correspond to a hydrous layer through which water is carried to deeper depths. The other is seismological imaging of mantle upwelling. Mantle return flows induced by the subduction of the Pacific and Philippine Sea slabs are imaged in Tohoku and Kyushu, respectively, whereas that probably generated by the subduction of both the Pacific and Philippine Sea slabs is apparent in central Japan. A large upwelling in the upper mantle revealed in the Chugoku district might be the origin of Quaternary volcanism there.

著者

武村 俊介 利根川 貴志 中島 淳一 汐見 勝彦

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

---

海洋プレート上部に低速度層として存在する海洋性地殻は,沈み込みに伴う脱水反応によりエクロジャイト化することで,深い部分では海洋マントルと同程度の地震波速度となることが予想される(例えば,Fukao et al., 1983 Nature; Abers et al., 2003 GRL; Hacker et al., 2003 JGR).本研究では,海洋モホを伝播する屈折波を用いて,フィリピン海プレートの海洋性地殻がエクロジャイト化する深さを拘束することを試みる.海域で発生する海洋モホより浅い地震時に,陸域で観測される初動は海洋モホを伝播する屈折波となることが知られている(Takemura et al., 2016 EPS).例えば,2016年4月1日に三重県南東沖の地震時に陸域のHi-netで観測された初動走時は,震央距離70~200 kmにおいて見掛け速度7.2 km/s程度の屈折波PPHSが明瞭に確認できる.震央距離200 km以上では,見掛け速度8 km/s以上と海洋マントルに対応した初動走時となる.このような見掛け速度の距離変化は,マントル以深の地震波速度構造に起因すると考えられる.現実的な構造を仮定した2次元差分法による地震動シミュレーションにより,初動走時の特徴を調べる.地殻構造はF-net 1D構造(Kubo et al., 2002 Tectonophys.),フィリピン海プレート上面形状はHirose et al. (2008 JGR)を採用し,プレート上面から深さ方向に7 kmの領域を海洋性地殻としてシミュレーションを行った.海洋性地殻を深部まで低速度層として存在させると,PPHSが遠方まで初動として伝播し,観測された初動走時の特徴を説明できない.そこで,海洋性地殻が海洋マントルと同程度まで高速化し,海洋モホがある深さで消失したとしてシミュレーションを行った.海洋モホの消失する深さを様々に変えたシミュレーションから,海洋モホが消失する深さに依存して初動走時が変化することがわかった.海洋性地殻のエクロジャイト化により海洋モホが深さ52 km以深で速度コントラストを持たなくなり,初動がPPHSから海洋マントルに対応したP波に変わるため,初動走時が変化する.海洋モホの消失する深さを52 kmとした時に,初動走時の再現性が最も良くなった.この深さは,Kato et al. (2014 EPS)による紀伊半島下のフィリピン海プレートの海洋性地殻のエクロジャイト化する深さと整合的である.以上のことから,海域で発生した海洋モホより浅い地震とモデルシミュレーションの比較から,海洋モホの速度コントラストが消失する深さを拘束できることが期待される.海洋モホの消失は沈み込みに伴う脱水反応が原因と考えられ,海洋性地殻のエクロジャイト化と関連があると考えられ,沈み込む流体の移送過程を知るヒントとなる.謝辞F-net MTカタログとHi-net速度波形を利用したました.地震動計算には地球シミュレータを利用しました.

著者

弘瀬 冬樹 中島 淳一 長谷川 昭

出版者

日本地震学会

雑誌

地震. 2輯 (ISSN:00371114)

巻号頁・発行日

vol.60, no.3, pp.123-138, 2008-02-15

参考文献数

46

被引用文献数

6

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

---

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

増山 一光 中島 淳一郎 蓮池 和都

出版者

公益社団法人日本工学教育協会

雑誌

工学教育研究講演会講演論文集

巻号頁・発行日

vol.24, no.60, pp.474-475, 2012-08-22

著者

佐野 有司 高畑 直人 堀口 桂香 植木 貞夫 中島 淳一 長谷川 昭

出版者

一般社団法人日本地球化学会

雑誌

日本地球化学会年会要旨集 2006年度日本地球化学会第53回年会講演要旨集

巻号頁・発行日

pp.250, 2006 (Released:2007-11-01)

---

日本列島においても温泉や火山、天然ガスなどを通じて固体地球から揮発性元素が定常的に脱ガスされている。これらの中でマントル起源であることが明らかなヘリウムー3と地殻起源のヘリウムー4の比のマッピングをすることで、日本列島におけるテクトニックな場をヘリウム同位体比がどのように反映するか1980年代から研究されてきた。一方、地震波の高精度データが蓄積し、その解析法に進歩があり、地下の構造を示す地震波トモグラフィーの詳細な研究が行われつつある。本研究では、これまでに蓄積されたヘリウム同位体比のデータをコンパイルし、そのマッピングと地震波トモグラフィーのデータを比較検討し、日本列島における脱ガス過程と地下構造の関係について発表する。

著者

佐野 有司 高畑 直人 堀口 桂香 植木 貞夫 中島 淳一 長谷川 昭

出版者

日本地球化学会

雑誌

日本地球化学会年会要旨集

巻号頁・発行日

vol.53, pp.250-250, 2006

---

日本列島においても温泉や火山、天然ガスなどを通じて固体地球から揮発性元素が定常的に脱ガスされている。これらの中でマントル起源であることが明らかなヘリウムー3と地殻起源のヘリウムー4の比のマッピングをすることで、日本列島におけるテクトニックな場をヘリウム同位体比がどのように反映するか1980年代から研究されてきた。一方、地震波の高精度データが蓄積し、その解析法に進歩があり、地下の構造を示す地震波トモグラフィーの詳細な研究が行われつつある。本研究では、これまでに蓄積されたヘリウム同位体比のデータをコンパイルし、そのマッピングと地震波トモグラフィーのデータを比較検討し、日本列島における脱ガス過程と地下構造の関係について発表する。