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タイトル: Mapping of an Unusual Crustal Discontinuity by Microearthquake Reflections in the Earthquake Swarm Area near Ashio, Northwestern Part of Tochigi Prefecture, Central Japan
その他のタイトル: 栃木県北西部,足尾周辺の群発地震活動地域での微小地震反射波による地殻内不連続面の検出とその特異性について
著者: Mizoue, Megumi
Nakamura, Isao
Yokota, Takashi
著者(別言語): 溝上, 恵
中村, 功
横田, 崇
発行日: 1986年3月31日
出版者: 東京大学地震研究所
掲載誌情報: 東京大学地震研究所彙報. 第57冊第4号, 1986.3.31, pp. 653-686
抄録: Microearthquake seismograms recorded by the stations in the earthquake swarm area near Ashio on the westren border of Tochigi Prefecture, Central Japan, frequently have two sharp phases following the direct S-wave. They were identified as near vertical S to P-wave (SxP) and S to S-wave (SxS) reflections from an unusual deep crustal discontinuity. Reflection points calculated on the basis of hypocentral coordinates in combination with the traval time data of the reflections were mapped with contour lines of a lkm interval. The discontinuity was estimated to have a depth of about 14 km beneath Ashio dipping southeastward with considerable local irregularities. The ratio of observed amplitudes of SxP to SxS varies from 0.045 to 0.34 for the incident angle of less than 23°. The theoretical amplitude ratio of the reflections agrees well with the observational results when the S-wave velocity decreases from 3.35 km/sec to 0.00 km/sec across the discontinuity. A horizontal extension of the unusual discontinuity strikes in a NE-SW direction along the general trend of the volcanic front in the area. It is suggested from the analysis that the discontinuity is underlain by a material of very low rigidity and the existence of an extensive magma body may be a plausible explanation of the reflections.
URI: http://hdl.handle.net/2261/12862
ISSN: 00408972


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