Three-dimensional imaging of electrical conductivity in the mantle transition zone beneath the North Pacific Ocean by a semi-global induction study

The three-dimensional (3-D) electrical conductivity distribution in and around the mantle transition zone (MTZ) beneath the north Pacific is imaged by inverting revised geomagnetic responses at 13 geomagnetic observatories and cable responses of 8 long baseline submarine cables in the area. Two one-dimensional (1-D) models representing the average structure beneath the north Pacific were employed as the reference and initial models for the 3-D conductivity inversions. The features in the revised semi-global conductivity model in the MTZ are (1) a high conductivity anomaly beneath the Philippine Sea, (2) a high conductivity anomaly beneath the Hawaiian Islands, and (3) a low conductivity anomaly beneath and in the vicinity of northern Japan. Arrhenius plots of the conductivity and temperature estimated from global seismic tomography models indicate that the anomalies in the MTZ beneath northern Japan and most areas under Hawaii can be explained mainly by the temperature effect, but that the conductivity beneath the Philippine Sea requires an additional cause to explain the higher conductivity. If the cause is the effect of water, then the present result suggests high water content of 1 wt% or more throughout the MTZ beneath the Philippine Sea.