COMPARATIVE ANALYSIS OF IONOSPHERIC VARIATIONS BEFORE STRONG EARTHQUAKES

At two ionospheric stations Petropavlovsk and Magadan the series of deviations of critical frequencies foF2 from their moving average values are considered before strong earthquakes. The irregular variations at both stations are compared to exclude the identical changes connected with geomagnetic disturbances and to reveal seismo-ionospheric effects. It has been found that positive deviations are predominated in the time of preparation of strong earthquakes. They become more intense with increase of magnitude of earthquakes. Introduction Anomalies of electronic concentration of the ionosphere at middle latitudes, which are connected with seismic activity, are usually less than ionospheric disturbances observed during global geomagnetic storms, therefore the seismic effects are quite often difficult to identify. The earthquakes occur both in geomagnetic quiet and disturbed periods. It is a note in this connection that geomagnetic storms are global phenomena, whereas premonitory ionospheric effects of earthquakes are essentially local. In general, characteristics of F2-layer are qualitatively identical at stations if they are located from each other on distances no more than 500-700 km. However at such distances one of the stations can be in a zone of seismic activity, and another outside of it, so that seismo-ionospheric disturbances should be marked at the first station and to be absent or be less intense at other. In order to reveal seismo-ionospheric disturbances, the comparative analysis of data at two stations of radiosounding Petropavlovsk and Magadan has been made. Earlier, data at two stations are compared with each other, and daily correlation coefficients of ionospheric parameters are calculated. It is received that before strong earthquakes the correlations are quite often broken (Gaivoronskaya et al., 2002; Gaivoronskaya, 2005). Now hourly variations of critical frequencies foF2 are examined in more detail. Methods There have been considered ten strong earthquakes in 1992-1994 with magnitudes from М=5.4 up to М=7.5 at seismic region Kamchatka. Zone of active preparation of earthquakes usually covers the area in radius of 200300 km (Bowman et al., 1998; Dobrovolsky et al., 1979), thus the earthquakes with epicenter on distances not farther than 250 km from Petropavlovsk-na-Kamchatke have been chosen. The ionospheric F2-layer is the most variable of regular layers, and its critical frequency foF2 is one of the most accurately measured parameters of ionosphere, therefore that frequency has been taken for analysis of data series obtained at stations located near earthquake areas. We have examined the series of ΔfoF2 deviations of critical frequencies of F2-layer from their moving average values at two ionospheric stations Petropavlovsk and Magadan. Usually the average values are calculated for a month, they are derived from the data for the previous and following 15 days. In case of forecasting of seismic events it is necessary to calculate average values only for previous period of 15 days when results of radiosounding are known. The hourly irregular variations received at two stations Petropavlovsk and Magadan are compared with each other: Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)