A secular variation anomaly has been discovered at the north-east part of the Fergana vdey by repeated measurements every year or less. The change of total field ΔF at the ‘magnetic epicentre’ was 9 nT 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 ΔF increased in the northern part up to 5.2 nT, and decreased by 4.7n T 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 ΔF differences with magnitude ± 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; ΔF 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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Shapiro et al. (1982) studied this question.