No abstract

Our 3‐D electrical resistivity model clearly detects particular subsurface features for magmatism associated with subduction of the Philippine Sea Plate (PSP) in three regions: a southern and a northern volcanic region, and a nonvolcanic region on the island of Kyushu. We apply 3‐D inversion analyses for geomagnetic transfer function data of a short‐period band, in combination with results of a previous 3‐D model that was determined by using Network‐Magnetotelluric response function data of a longer‐period band as an initial model in the present inversion to improve resolution at shallow depths; specifically, a two‐stage inversion is used instead of a joint inversion. In contrast to the previous model, the presented model clearly reveals a conductive block on the back‐arc side of Kirishima volcano at shallow depths of ~50 km; the block is associated with hydrothermal fluids and hydrothermal alteration zones related to the formation of epithermal gold deposits. A second feature revealed by the model is another conductive block regarded as upwelling fluids, extending from the upper surface of the PSP in the mantle under Kirishima volcano in the southern volcanic region. Third, a resistive crustal layer, which confines the conductive block in the mantle, is distributed beneath the nonvolcanic region. Fourth, our model reveals a significant resistive block, which extends below the continental Moho at the fore‐arc side of the volcanic front and extends into the nonvolcanic region in central Kyushu.

Abstract. Geomagnetic depth soundings have been made in the area covering Kyushu island and its neighboring islands in the western sea of Kyushu to investigate the back-arc conductivity structure. The in-phase induction arrows derived from geomagnetic variations are found to point roughly southwestwards at most sites on Kyushu island for the period range 300-7200 sec. This is quite different from some other areas of Japan.

A magnetometer system for unmanned long-term observation in equatorial regions has been developed and is available. Because this system is designed for the study of both micropulsations and the equatorial electrojet, it must have accurate timing, high sensitivity, a wide measurement range, and a long-term recording capability. The equatorial regions we are interested in are quite remote far from sufficient technical and human support, and the breakdown of a public power supply is frequent. So, the system is requested to be compact and work successively in low power consumption with battery backup under equatorial environments. This system is successfully developed in low cost compared with commercial magnetometer system with similar properties.