Construction of a Precise Gravity Anomaly Map over the Chubu and Kinki Districts, Central Japan, and Formation of Gravity Data-Base.

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Present study was executed to develop the works of gravity survey in the Southwest Japan, conducted by Nagoya University since 1978. By adding more than 8, 800 new data, 19, 555 points of our original data were updated, and compiling ones from other sources, a gravity data-base consisted of more than sixty thousands of data were formed. 2/3 of whole Japan, the southern part from 38.5ﾟ N, were covered by this data-base. The area among it, ranged from 133ﾟ E to Fossa-Magna, has very dense distribution of measurement points. Total revisions both for gravity value reductions and coordinates readings on all data file were performed. Our data-base garantees 1-0.5 mgal of comprehensive resolution and makes it possible to draw any kinds of very precise gravity anomaly maps in the whole range.Very characteristic gravity profiles are delineated and following factors are recognized : 1)Distinct linearments are delineated along major, geological tectonic lines such as Median Tectonic Line, Itoigawa-Shizuoka Geotectonic Line and so on, and active faults such as Atera Fault. 2)A very large scale of steep gradient belt of gravity is running from Suruga Bay to north-eastern Shikoku through Chita Peninsula by forming two large arcs. This belt extends to the Aso Graven in Kyushu. 3)Very strong steep gradient belts of gravity of middle to small scale are found in various places. One of the most characteristic among them is that with 15 km long along the seismic focal region of the 1984 Western Nagano Prefecture Earthquake. 4)The Kinki Triangle marks very characteristic strong negative anomaly. 5)Central Mountain Ranges have no or too small negative low anomalies. This suggests that their bodies are supported by some tectonic forces other than isostatic mechanism.