Variations in the Ionospheric F 2 Region Prior to the Catastrophic Earthquake in Alaska on March 28, 1964, According to the Data of the Ground-Based Stations of the Ionospheric Vertical Sounding

The variations in the ionospheric F 2 region prior to the catastrophic earthquake in Alaska on March 28, 1964, are analyzed. The evolution of the spatial distribution of the critical frequencies in the Alaska region a few hours prior to the earthquake, when the disturbances of the critical frequency in the vicinity of the epicenter were maximum has been studied. It is shown that, at least, a few hours prior to the earthquake, specific regional disturbances were observed in the ionosphere both near the epicenter of the impending earthquake ( ~100 km) and at a considerable distance from it ( ~1000-1500 km). It has been found that, according to the data of Anchor- age ground-based station of ionospheric vertical sounding located at a distance of 124 km from the earthquake epicenter, in the vicinity of this epicenter, the amplitude of the disturbed daytime values of foF 2 is maximum and reaches ~25% 4.5 h prior to the main underground shock.