THE ELECTRICAL HIGH CONDUCTIVITY LAYER BENEATH THE NORTHERN OKINAWA TROUGH, INFERRED FROM GEOMAGNETIC DEPTH SOUNDING IN NORTHERN AND CENTRAL KYUSHU, JAPAN

Geomagnetic depth soundings and magnetotelluric observations have been made in northern and central Kyushu, Japan. The in-phase response arrows observed for a period of 10 min point towards the west at almost all sites in central Kyushu and tend to be rotated anti-clockwise as the period increases between 10 and 51 min. Using the finite element method, the electrical conductivity structure is determined across central Kyushu and the region to the west extension. It reveals that a high conductivity layer is embedded in the upper mantle beneath the northern Okinawa Trough; this is the northern part of the backarc basin of the Ryukyu trench-arc system. This result may imply that the temperature of the upper mantle is high and thus the asthenosphere is ascending beneath the northern Okinawa Trough.