Local Variations of the Geomagnetic Field Components as Possible Precursors of the Paravan and Spitak Earthquakes

Abstract

We analyze variable geomagnetic field which characterizes the change in electrical conductivity in the Earth’s crust for the Armenia from 1986 to 1993. Possible geomagnetic precursors of the Paravan (13.05.1986, М=5.4) and Spitak (07.12.1988, М=7.0) earthquakes were obtained. About half a year before the Spitak earthquake, the parameters of the ratios of bay-like variations (30–60 and 5–25 min) of the horizontal and vertical components of the geomagnetic field decreased by about 30%. An analysis of the data two months before the Paravan earthquake also shows a sharp drop in the values of this parameter by 20–25%. In the interval between two earthquakes, the ratios of baylike variations lie in the range of ±10% with an average value of 1. For the Sq variations of the δD, δH, and δZ components of the geomagnetic field, a sharp drop was observed about a month before the Paravan earthquake from 1.2 to 0.8. After the Paravan earthquake, all three components partially returned to their preearthquake parameters (~1.2) within a few months. Further, a monotonous decrease in all three components of the geomagnetic field was observed up to the Spitak earthquake from 1.2 to 0.54. After this earthquake, the baylike and Sq variations returned to normal values between 1 and 1.2 within a few months. The difference in the behavior for the bay-like and Sq variations in the period between two earthquakes is possibly associated with the beginning of the preparation of the Spitak earthquake a few months after the Paravan earthquake.