Geomagnetic anomaly analysis on the seamounts, Daiiti-Kasima and Katori, off Kashima-nada around the southern part of Japan Trench.

Abstract

As a part of “Basic Sea Map" project,and partly of the “Earthquake PredictionPlan" the detailed geomagnetic, hydrographic, geological, and gravitational survey, was carried out by the Hydrographic Department, Maritime Safety Agency of Japan, at sea in the southern part of Japan Trench from 1976 to 1977. Making use of some of these results, the geomagnetic anomalies of both the Daiit-Kasima and Katori seamounts were analyzed by means of the Talwanis method, with special reference to the subduction hypothesis of the Daiiti-Kasima seamount. Throughout these results, the following features were apparent. 1) The breakdown of the western half of Daiiti- Kasima seamount, which was proposed by Mogi and Nishizawa (1980a), may be considered as the depression of the magnetic body to the extent of more than 1000 m in line with the Japan Trench axis. The further consideration of its geomagnetic structure suggests us that the Daiiti-Kasima seamount has been suffering from "accretion process" during the subduction of the Pacific Plate. 2) A large amount of the uplift (3600 m to 5250 m) of the Daiiti- Kasima seamount is nonmagnetic as a whole, and the rest ( 5250 m to 7000 m) has a magnetization vector defined by dec= － 12.0°, inc= 20.1°, /J/=1072×10-2Am-1. 3) For the Katori seamount, the magnetization vector of the top portion ( 4200 m to 4750 m) is dec=-15.2°, inc= 18.3°, /J/=1390×10-2Am-1, and the rest (4750 m to 7500 m) is dec=170.0°, inc=19.5°, /J/=520×10-2Am-1. This shows that the Katori seamount has a memory of successive normal and reversal magnetic events. 4) The VGP (Virtual Geomagnetic Pole) for Daiiti-Kasima is located at N62°, E349。, and that of the top portion of Katori is at N60°, E354 °. These results are consistent with the mean VGP from other Cretaceous Pacific seamounts, and support the hypothesis of the northward drift of the Pacific Plate. Key words: Solid earth geomagnetism, Japan Trench, Seamount, Palcomagnetism.