Geodynamics Research Center, Ehime University
National Research Institute for Earth Science and Disaster Prevention
Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University
抄録
In this paper we present seismological evidence for the influences of fluids and arc magma on the generation of large earthquakes in the crust and the subducting oceanic slabs under the Japan Islands. The relationship between seismic tomography and large crustal earthquakes (M 5.7-8.0) in Japan during the period of 116 years from 1885 to 2000 is investigated, and it is found that most of the large crustal earthquakes occurred in or around zones of low seismic velocity. The low-velocity zones may represent weak sections of the seismogenic crust. Crustal weakening is closely related to the subduction process in this region. Along the volcanic front and in back-arc areas, crustal weakening may be caused by active volcanoes and arc magma, resulting from the convective circulation process in the mantle wedge and dehydration reactions in the subducting slab. Recent examples are the 1984 West Nagano earthquake (M 6.8) and the 2000 West Tottori earthquake (M 6.9), both occurred adjacent to volcanoes. In the forearc region of southwest Japan, fluids are detected in the 1995 Kobe earthquake source zone, which may have contributed to the rupture nucleation. The fluids may originate from the dehydration of the subducting Philippine Sea slab. The recent 2001 Geiyo earthquake (M6.4) occurred at 50 km depth within the subducting Philippine Sea slab, and it may also be related to the slab dehydration process. A detailed 3-D velocity structure is determined for the northeast Japan forearc region using data from 598 earthquakes that occurred under the Pacific Ocean with hypocenters well located with sP depth phases. The results show that strong lateral heterogeneities exist along the slab boundary, which may represent asperities and results of slab dehydration, and may affect the degree and the extent of interplate seismic coupling. These results indicate that large earthquakes do not strike anywhere, but only in anomalous areas that may be detected with geophysical methods. The generation of a large earthquake is not a pure mechanical process, but is closely related to the physical and chemical properties of materials in the crust and upper mantle, such as magma and fluids.
雑誌名
東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo
巻
76
号
3
ページ
271 - 289
発行年
2002-02-12
ISSN
00408972
書誌レコードID
AN00162258
フォーマット
application/pdf
日本十進分類法
453
出版者
東京大学地震研究所
出版者別名
Earthquake Research Institute, University of Tokyo
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Seismological Evidence for the Influence of Fluids and Magma on Earthquakes
ji0763001.pdf
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