Seafloor measurements of geomagnetic variations were undertaken at six sites in the Ryukyu trench-arc system in order to investigate the electrical conductivity structure of the crust and upper mantle with special reference to the subduction of the Philippine Sea plate. In addition to the conventional analysis in which transfer functions for the vertical component are derived, we show the effectiveness of another type of transfer functions through which seafloor horizontal fields relate to horizontal fields simultaneously recorded on an island near seafloor sites. Because of the high attenuation of geomagnetic variations at periods shorter than 30min at seafloor sites, analysis was possible only for the periods longer than 30min. We estimated both types of transfer functions for the period range of 30 to 240min. We attempted a two-dimensional modeling for a profile orthogonal to the strike of the Ryukyu trench-arc system, using a finite element method. The comparison of transfer functions derived from observations and calculations yields a model showing two columnar conductors in the mantle wedge; one at the depth range from 20 to 60km in the forearc region and the other at the depth range from 60 to 130km or more beneath the northwestern margin of the Okinawa Trough behind the arc. According to recent mineralogical studies, dehydration of some minerals in hydrated peridotite at depths of about 100km and 150km plays an important role on magma genesis in subduction zones. The columnar conductor found below the forearc region is likely to be a zone containing water released through dehydration of some minerals in the hydrated peridotite, whereas the columnar conductor beneath the northwestern margin of the Okinawa Trough seems to correspond to a zone of partial melting which may be triggered by water released through dehydration of some other minerals.

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The piezomagnetic field associated with the Mogi model was reexamined for the point source and the finite spherical source problems. In the point source case the question was how to deal with divergent stresses near the pressure source. In SASAI's (1979) method, the magnetized crust was divided into two layers, i.e. the upper half shallower than the source (0<z<D) and the lower half deeper than the source (D<z<H). The solution (type I) was simply the sum of the two contributions as calculated by the Fourier transform method. An alternative calculation (type II) integrates the whole magnetized area except for an infinitesimally small sphere involving the pressure source. This was achieved with the aid of a theorem for spherical harmonics. These two procedures gave different results. In order to identify the cause of the discrepancy, the finite source problem was further investigated. The piezomagnetic field was represented in the form of a one-dimensional integral containing complete elliptic integrals. It can be evaluated numerically with the aid of the double exponential formula (DEF). We called it the type III solution. The present analytic approach verifies the results of numerical 3-D integrations by DAVIS (1976), and SUZUKI and OSHIMAN (1990). The solution as the spherical radius diminishes to zero coincides with the type II solution. The type II solution had a gap across H=D. The potential value at H=D was just the average of the two limiting values for H=D-ε and H=D+ε. In the type I solution, the magnetization along a thin layer at H=D was ignored, which produced the pressure source term in the type II solution. Accordingly the type II solution was appropriate for the point source problem: the previous result by SASAI (1979) should be rejected.

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Aeromagnetic surveys over the Japanese Islands yielded a detailed magnetic anomaly map. Magnetic anomaly maps were also derived for sea regions surrounding the Japanese Islands from ship-borne magnetic surveys. We combined these maps into a unified magnetic anomaly map for the entire Japanese island arc region. The map enables us to find some characteristic features; in particular, three similar anomaly belts are clearly recognized. We attempted to combine these belts into one linear belt, although they are spatially separated and the direction of elongation is different from one belt to another. This process requires rotations of the blocks constituting the Japanese Islands. The result of our reconstruction of the blocks agrees with the result derived from paleomagnetic studies, supporting the opening of the Japan Sea. This implies that magnetic anomalies serve as markers for the original locations of blocks, in a sense similar to yet slightly different from oceanic magnetic lineations in plate reconstruction.

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