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A secular variation anomaly has been discovered at the north-east part of the Fergana v d e y by repeated measurements every year or less. The change of total field AF at the 'magnetic epicentre' was 9nT in 1977 and 16 nT in 1978 relative to the level of 1973. In 1977 an anomalous region was recognized, where according to the data from 25 observation points AF increased in the northern part up to 5.2 nT, and decreased by 4.7 nT in the southern part according to a further 22 points. Permanent observations were begun at the epicentre in 1978 October. We normally observed variations of A F differences with magnitude k 2-3 nT, which were not correlated with worldwide magnetic activity. Anomalous variations appeared on October 26 and rose to a maximum value of + 23 nT on October 30. The decrease of this anomalous field began on October 31. This made it possible to predict a potential earthquake. The Alay earthquake with M = 7.0 occurred on November 2 six hours after the prediction was issued; AF then returned to the initial level. Thus, using the geomagnetic field variations in the Fergana region, geophysicists were able to predict the moment of a strong earthquake.

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A comparison of satellite and surface surveys of geomagnetic anomalies is performed. A special regional aeromagnetic survey was carried out at the Urals. Different models of the reference field for the region under investigation were studied. A reference field has been constructed using polynomial of 4-th order. The long-wavelength component of the anomalous magnetic field was selected. It is shown that when dividing the magnetic field of the Earth into a main and anomalous parts by spherical harmonic analysis, a large part of the magnetic field from the crust is included within the main field model. A model of the crustal magnetization of the Northern Eurasia is constructed.