Conductivity Anomalies in the Southern Half of Hokkaido, Japan

Observations of geomagnetic variations were made at fourteen temporary stations in Hokkaido, Japan, by transferring a flux-gate type variometer from one place to another every two months. From data analyses and model calculations, the following results were obtained: 1) The anomalous behaviors of geomagnetic variations for 5 minute periods are interpreted by means of the heterogeneity of electrical conductivity due to the sea and sediments. 2) It is presumed from variations of 60 minute periods that the surface of the conducting layer of the upper mantle in the southwestern part of Hokkaido is located at a depth of about 40km whereas that on the eastern side of the Ishikari Plain is at a depth of 130km. These results are not in contradiction with other geophysical data. The high heat flow area corresponds to the highly conducting region. The low Pn velocity of the seismic wave is also seen in the conducting region.