抄録
The geomagnetic field variation was measured for about a month at two points on Tahiti Island in the south-east Pacific. Measurements were made with ocean bottom magnetometers which were set 1 meter under the ground. This method reduced the magnetic field noise originating from the diurnal temperature change by 1/20 of the surface value. The data indicated a typical anomaly of the geomagnetic field variation characteristic of the island effect.
The conductivity structure beneath Tahiti Island is estimated through comparison between observed response functions and expected ones from a number of numerical models. A reasonably well-constrained, four-layer model is summarized as follows; (1) The first, shallowest, layer is highly conductive at the depth of 5-10km with a conductivity of 1S/m. (2) The second layer is less conductive with a conductivity of 0.05S/m and is 40-50km thick. (3) The third layer has a characteristic high conductivity of 0.1 or 0.2S/m and is 9-10km thick at the depth of 50-60km. (4) The bottom layer is assumed to be uniform with a conductivity of 0.06S/m. We interpret that the lithosphere is composed of the first and second layers and that the asthenosphere corresponds to third layer beneath Tahiti Island. The thickness (50-60km) of the lithosphere is consistent with that postulated for a thickened plate under Tahiti Island at 70 m. y.
