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タイトル: Analyses of Gravity Values in Japan
その他のタイトル: 本邦重力測定値の解析
著者: Hagiwara, Yukio
著者(別言語): 萩原, 幸男
発行日: 1968年2月10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45冊第4号, 1968.2.10, pp. 1091-1228
抄録: Gravity values in Japan as obtained by the Geographical Survey Institute are analyzed from various standpoints. As large parts of Japan are occupied by mountainous areas, it is clear that terrain correction must be made in order to accurately obtain the Bouguer anomaly distribution in Japan. Topographic heights of Japan Islands and depths of the adjacent sea are first of all digitalized on punched cards. The data thus provided are subsequently used for the actual computation of terrain correction on a digital computer. Number of the data amounts to 160,000. Computation of terrain correction is made at the 12,000 gravity stations established by the Geographical Survey Institute. It is found that the statistical distributions of topographic heights above sea-level and of terrain correction values can generally be approximated by exponential curves. The highest correction value obtained reaches about 24 mgal. A terrain-corrected Bouguer anomaly map is drawn, and gravity-topography and -geology relations are discussed. In order to apply gravity values to geodetical problems, free-air anomaly distribution has to be used instead of that of Bouguer anomaly, because Bouguer correction slightly deforms the shape of the actual earth Since it is hard to calculate free-air anomaly directly from the gravity data over a mountainous area, the value of the Bouguer anomaly together with topographic elevation are read at a grid point of meshes covering the map, and then the free-air anomaly distribution is obtained. A method of free-air reduction to an equipotential surface above the geoid is also advanced by the auther. In the last half of this paper, the author deals with geodetical problems such as determination of the earth's flattening and computation of the vertical deflection from the gravity distribution obtained. The gravimetric deflections at stations, where astrogeodetic deflections have been observed by the Geographical Survey Institute, are calculated from the distribution of the free-air anomaly and from that of the Bouguer anomaly to which terrain corrections for the vertical deflection are added. Furthermore, undulation of the geoid is estimated. The conclusions reached are briefly given in the following: 1) The flattening as obtained in this paper ranges from 1/243.5 to 1/310.0. It is difficult to determine the flattening from gravity values in Japan bacause of the narrow latitude range occupied by Japan. Nevertheless, 1/303.1 seems likely to be the most reliable value among these flattenings, bacause it is derived from the free-air data. 2) The optimum components of the vertical deflection at the Tokyo datum station are estimated as ξ0=-11".06 and η0=7".77, which are consistent with the deflection values previously obtained by many other authors. 3) The amplitude of the geoidal undulation amounts to several meters and the highest of the geoid reaches about 7 meters above the level of the reference ellipsoid.
URI: http://hdl.handle.net/2261/12369
ISSN: 00408972


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