Total intensity data acquired by aeromagnetic surveys were decomposed into vector components, following a procedure proposed by LOURENCO and MORRISON (1973). It has been revealed that the Fossa Magna is a magnetic region. Intense magnetic anomalies distribute inside the region, and no conspicuous anomalies outside. In the north Fossa Magna, a narrow belt of positive anomaly is formed with the vertical component and the total intensity. In the south, on the other hand, the positive anomaly spreads widely covering the whole area. The boundaries of the Fossa Magna correspond to the boundaries of the magnetic region. Across the Itoigawa-Shizuoka tectonic line, the west boundary of the Fossa Magna, a clear contrast is seen with the vertical component. A conspicuous positive anomaly zone of the east component runs north-south along the tectonic line. The southeast boundary of the Fossa Magna is clearly recognized in the vertical component. This boundary causes negative anomalies both in the north and the east components. Besides these large scale features, there are anomalies of small scale with strong intensity. They are either of volcanic or intrusive origin, such as Mt. Fuji and Tanzawa Mountains.
The magnetic belt in the north Fossa Magna is probably related to the Miocene formation called the Central Uplift Zone (Chuo-Ryuki-tai), which contains large mass of intrusive rocks and volcanics. Positive Bouguer anomaly along this belt suggests that high density materials are responsible for producing the magnetic belt. In the south, on the other hand, the Bouguer anomaly is negative. Presumably the magnetic anomaly is caused by thick marine sediments of Miocene time with smaller density, which distribute widely in the south Fossa Magna. The thickness of the causative body has been estimated to be about 10km for the magnetization of 3×10^-3emu/cm³.

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An ocean bottom magnetometer was developed. It is a fluxgate type magnetometer with a triaxial ring-core sensor suspended by a gimbal, and capable of recording the three geomagnetic components in a cassette tape in digital form for more than two months when the sampling interval is 3min. The resolution is 0.8nT in this case. A pop-up system is adopted for recovery, mechanically releasing a weight at a preset time by explosion of gas.
Two sets of magnetometers were used at two sea floor stations on the continental shelf east of the Northeast Honshu in 1981. Of these two sets one is a prototype and the other is an improved version. In the experiment of 1981 the magnetometer of the improved version worked almost perfectly, giving very reliable data of three components of the geomagnetic field. In this paper the details of this successful magnetometer named OBM-C2 as well as the data obtained from the measurements are reported.

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