Surveys of the Geomagnetic Total Intensity in the Tokai District (1) : Secular Changes during the Period from 1971 to 1978

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Geomagnetic surveys have repeatedly been carried out in the Tokai District since 1971. Survey site selection has been hampered by magnetic noises due to stray currents from the electric railways. However, since 1979 twenty sites in good locations have been maintained for repeated surveys. At each site, the geomagnetic total intensity is measured every minute for 10-20 min by using the portable proton precession magnetometer which has an accuracy of 1 nT. The measured values are compared with the total intensity at the reference station, which in this case is the Yatsugatake Magnetic Observatory. Thus, secular changes of the total intensity in the Tokai area relative to the reference station can be investigated. From 1971 to the beginning of 1979, the survey has been carried out thirteen times, although in some cases measurements were made only at some of the sites. Because of anomalies of short-period geomagnetic variations in the survey area, the difference in the total intensity between a survey site and the reference station cannot be determined very accurately. In the present survey system therefore, an anomaly of local nature, covering only a few sites, cannot be detected significantly unless the magnitude of such an anomaly is fairly large. In view of the spatial extent of an anomaly to be expected in association with a seismic event in the Tokai area, much stress has been put on the examination of an anomaly having a rather regional nature. For this purpose the survey area was divided into four survey regions and for each survey region a regional secular change was obtained by averaging secular changes observed at the survey sites in the region. The results obtained for the period from 1971 to 1978 revealed secular changes, relative to the reference station, amounting to 1.5 nT/yr or so at most. Such changes are characterized by a clear trend that the pattern of change in one region is very similar to that in another and the magnitude of the change tends to be large towards the south. Thus, the dependence of relative secular changes on the latitude difference between a survey region and the reference station was disclosed in the Tokai area. However, a very similar dependence was also observed for a pair of stations in the Kanto District, and it is concluded that the secular changes observed in the Tokai area are unlikely to reflect anomalies of crustal origin. Such a dependence would be merely the manifestation of spatial dependence of secular changes having their origins in the earth's core or in the magnetosphere.
1980-11-15

Surveys of the Geomagnetic Total Intensity in the Tokai District (1) : Secular Changes during the Period from 1971 to 1978

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