Gravity anomalies and derived subterranean structure on/around Tokyo Bay and southern Kanto district.
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AbstractHydrographic Department, MS A, Japan, carried out sea gravity survey in Tokyo Bay from Feb to March, 1985 and in Dec,1985. Gravity field at sea surface was measured by KSS-30 type sea gravity meter, installed on S/V shoyo. The position of S/V shoyo was fixed with Decca-Transponder positioning system within the estimated error of 5 m. Through this survey, Free-air and Bouguer anomaly maps of Tokyo Bay were compiled and gravity basement depth was calculated by fourier intergral inversion method The gravity basement depth is fairly consistent with relief of Pre-Neogene basement structure. Free-air gravity anomaly map on/around sou them Kanto district was also compiled using sea gravity survey data measured by JHD, for study of tectonic situation of the region concerned. Three gravity anomaly profiles on/around Tokyo Bay were analysed with two-dimensional Talwani's method. Through the above study, followings become apparent. I. Negative Bouguer anomaly in the northern part of Tokyo Bay, is subdivided into two regions, one is extension from Anegasaki, north-eastern coast of Tokyo Bay,whose negative peak reaches 24 mGal and the other is from Yokohama, south-western coast of Tokyo Bay where negative peak reaches 21 mGal. 2 Gravity analyses on two profiles in Tokyo Bay reveal that the negative Bouguer anomaly should be ascribed to two sources, one is Pre-Neogene basement structure, and the other is slope of Moho boundary. This result also imply that the thickness of crust beneath Tokyo Bay become deeper than surroundings. 3. The relative Bouguer gravity low amounting more than 120 mGal is recognized over Tokyo subbottom channel, where gravity basement is depressed more than 4000 m. This feature suggests that Tokyo subbottom channel is one of the results of tectonic dynamic of southern Kanto district. 4 Gravity analysis on E-W profile along 35°30', from E 139°00’ near Tanzawa mountains to 141°30’，offing of Taito cape, reveal that subducting plate should be taken into considered, as well as Pre-Neogene basement structure. Density contrast of each part is as follows, 0.12 g/cc for Philippine plate, 0.25 g/cc for Pacific plate and 0.58 g/cc for Pre-Neogene basement. The density of Pacific plate could be reduced by taking account of the slope of Moho boundary.
JournalReport of Hydrographic and Oceanographic Researches