第30回ジオダイナミクスセミナー
　　　Geodynamics　Seminar

　“Seismic tomographic studies of northeast Japan
　　forearc region and Bhuj, Gujarat earthquake,
　　western India”

　　　講師：Mr. O. P. Mishra（愛媛大学博士
　　　　　　　　　　　　　　　後期課程１年）　　　　　　　
　　　　　　　
　　　　　主催　：　愛媛大学地球深部ダイナミクス研究センター
　　　　　日時　：　２００２年２月１日（金）　午後17時〜
　　　　　場所　：　愛媛大学理学部講義棟　２０１教室

　　　　　　　　

要　旨
　We have attempted to investigate the 3-D P and S-wave velocity structures under the Pacific Ocean between the Japan trench and the Pacific coast of northeast japan. Two groups of earthquake data, one occurred in the land area of NE Japan and other beneath the Pacific Ocean, recorded by a dense seismic network of Tohoku University, have been used in this study. The sP-depth phase technique is used to relocate the sub-oceanic earthquakes (Umino et al., 1995) while the land data are relocated during the tomographic inversion process (Zhao et al., 1992). Our results clearly reveal strong lateral heterogeneities in the velocity structures of the forearc region. Few large earthquakes occur within the zone of low-V, which may represent a decoupled areas with fluids while a majority of small earthquakes occur in the low-V zone. Most of the large interplate earthquakes are located in the areas of high or average velocity zones, which may represent the strong-coupled asperities along the slab boundary and it may play a significant role in controlling the degree and extent of interplate seismic ruptures .

A big and shallow earthquake (Mw 7.5) has occurred at Bhuj in the highest seismically active zone, demarcated as zone-V in western India on January 26, 2001, causing a catastrophic damage to both people and properties of the region. We have attempted to investigate the 3-D P and S-wave velocity structures using the after shock distribution of earthquakes, recorded at 12 temporary seismic stations, ascribed to Geological Survey of India’s seismic network. Our preliminary results reflect a large-scale variation of lateral heterogeneities in both P and S-wave velocity structures. It also indicates that it is a shallow crustal earthquakes which is triggered in a high static stress drop zone and is located diagonally opposite and adjacent to previous big earthquake of Anjar (1956, Mw 7.0).


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　　　 　　　　　　　　　　　　　　　　E-mail　　inoue@sci.ehime-u.ac.jp