ALBERT

Description: Horizontal profiles of geomagnetic transfer function were obtained at the periods of 15, 30, 60 and 120 minutes by analysing 3-component geomagnetic variation data at 21 sites in the central part of Japan and 7 sites on the Philippine Sea floor. A two-dimensional conductivity model was constructed, by use of the finite element method, to represent the spatial and period dependence of these transfer functions. Characteristics of the observed transfer functions on the Pacific coast and continental shelf of the Philippine Sea were well explained by introducing a highly conductive layer beneath the Philippine Sea plate at a depth of about 30km in addition to a large conductivity contrast between land and seawater. In order to account for the anomalous phase variation observed at the northern half of the central Japan sites, conductive lower crust was required beneath this region. This lower crustal conductor also explained the reduction of the coast effect on the Japan sea coast, and the existence of free water in the lower crust was considered as its likely cause. Our result did not require the conductive upper mantle beneath the Japan Sea, at least 200km away from the coast, to explain the data, which may suggest that the plate motion is the possible cause of highly conductive asthenosphere as well as the partial melting. © 1986, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Description: A new method was developed to estimate the geomagnetic transfer functions on the basis of Sompi spectral analysis which gives higher frequency resolution than the conventional techniques. We extended Sompi spectral method from one dimensional time series to time series of three dimensional quantities and applied it to the spectral analysis of geomagnetic variation data. With this new technique, we can estimate the geomagnetic transfer functions up to longer periods than the conventional techniques from the same length of data. It is also possible to describe the source field polarization in the geomagnetic variations. This enables us to estimate the geomagnetic transfer functions according to the polarization direction. © 1988, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Description: A finite element method was applied to study the resistivity structure below the Tohoku district, the northern Honshu of Japan. An east-west profile across the island axis was determined by two dimensional modelling. Mesh elements were designed so that the main features of geophysical structure so far known were smoothly incorporated into the model. Resistivities of the crust, the upper and lower, and the upper mantle were determined by solving an electromagnetic induction problem for E-polarization, where the electric field changes in parallel with the coast line and the magnetic field perpendicular to it. In this area, geomagnetic variations in the vertical component are coherent to those in the east component. Transfer functions were computed for the data acquired on land and on the ocean floor. Resistivity of this model was adjusted to explain the observed B-values of the transfer functions for the periods of 15 and 60 minutes. It has resulted in that large B-values observed on the Pacific side of the Tohoku district and on the continental slope are explained by a resistivity contrast between the land and the Pacific Ocean, but that the distribution of the B-values from the volcanic front near the central part of the area to the Japan Sea coast is difficult to interpret simply by a coast effect due to the Japan Sea. Low resistivity (5ohm·m) of the lower crust is necessary to consider extending from the volcanic front to below the Japan Sea coast. © 1986, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.