Abstract
Geomagnetic and gravity surveys in the Bonin island (Chichijima), located in the central part of Izu-Bonin island arc were carried out by the Geographical Survey Institute in December 1973 and March 1974, respectively. The recent first geomagnetic and gravity surveys in this island have been done by the members of the Kakioka Magnetic Observatory since January 1971 and the Geographical Survey Institute from January to February in 1971. From these two surveys, the following results are obtained, (1) Geomagnetic secular change of declination at the first order geomagnetic point of Mt. Shigureyama (No. 98) shows the consistent tendency of west increasing in parallel with the extension line of the secular change of declination obtained by the repetition surveys of Hydrographic Department followed after Tanakadate's results at Ogiura. (2) Free air anomaly shows the value of +360 mgal near the central part of the island. Bouguer anomaly shows the value of +340 mgal in the central and the northern parts of the island. This value may be the most maximum one in the island arcs. (3) Geomagnetic anomalies both total (F) and vertical (Z) intensities show the patterns of the distribution due to a underground dipole though these patterns show the inverse correlation in comparison with the intensities of geomagnetic and Bouguer anomalies. These results are compared with the survey results of Oshima (7) island and discussed about the island arc.
