Abstract
A method of eliminating noises from geomagnetic observations for detecting local anomalous changes is proposed. Unlike the simple or weighted difference methods hitherto proposed, the method is based on an idea that most fluctuations in the geomagnetic total intensity are caused by those of equatorial ring current which produces the Dst field. It is possible to correct the observed values of the total force at a station for the Dst, that is to obtain the values that should have been observed when the equatorial Dst is zero.Applying the above method to the proton precession magnetometer data as obtained by the geomagnetic research group for earthquake prediction, isoporic lines of the total force over Japan are obtained for epoch 1970. 0. The pattern of the isoporic lines is completely different from those obtained for epoch 1965. 0.An anomalous secular change in the total intensity has been found at stations on Oshima Island, a volcanic one about 100km south of Tokyo. The change seems likely to have something to do with the volcanic activity there.
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