Abstract
Mean isoporic charts of the geomagnetic horizontal component, declination, vertical component and total force in and around Japan are constructed for epoch 1970.0 on the basis of the results of the 1st order magnetic survey carried out by the Geographical Survey Institute and the observations at magnetic observatories.A number of local anomalous areas, over which the secular change is larger than the normal one as determined from the overall isoporic charts by 2γ/yr or more, are found. Some of the areas seem to be correlated with tectonic activities such as earthquake occurrence and volcanism.In addition to the anomalies of local character, a rapid change in the pattern of the secular change distribution in Japan is found within the period of 1950' or an early stage of 1960'. In the same period or thereabout, the maximum of the secular change curve of the vertical component occurred successively in Japan, Siberia and Ural as if a regional magnetic anomaly source had traveled in a north-west direction with a speed on the surface of the earth's core of about 200km/yr. The rapid change in the distribution of isoporic lines in Japan within a period of 10 years or less may be caused by such a regional secular change source.
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