Effects in the sporadic E-layer during the preliminary stage of the earthquake on 27 August 2008 as deduced from observations in Irkutsk

We have carried out a search for possible ionospheric precursors of the 27 August 2008 strong crust earthquake with the magnitude Ms=6.1 whose epicenter was at about 100 km southward of Irkutsk. We have analyzed changes in diurnal variations of sporadic E-layer parameters in the summer of 2008, using 15-min data from Irkutsk vertical sounding station. Several cases of abnormal Es occurrence, which match the criteria of identification of ionospheric precursors of earthquakes, with different look-ahead periods AT (27, 13 and 1 days) have been found. These periods were demonstrated to be in agreement with the look-ahead periods for earthquake precursors of different duration obtained from results of other measurements during the preliminary stage of crust earthquakes with the same magnitude. Besides, we have revealed changes in Es for weaker earthquakes (Ms=4ч5) whose epicentral distances from the ionospheric station were less than 500 km.