Possible Short-Term Ionospheric Precursors of Strong Crustal Earthquakes

We have studied changes in the ionosphere prior to crustal earthquakes with magnitudes of M >= 6.5 based on the data from the ground-based stations of vertical ionospheric sounding in Japan for the period 1968-2004 (32 cases). The data are analyzed based on hourly measurements of the virtual height and frequency parameters of the sporadic E layer and critical frequency of the regular F2 layer for the period of three days prior to the earthquakes provided that strong geomagnetic disturbances are absent. In the studied intervals of time before all earthquakes, anomalous changes were discovered both in the frequency parameters of the Es and F2 ionospheric layers and in the virtual height of the sporadic E layer. The changes were observed on the same day at stations spaced apart by several hundred kilometers. It is found that the lead-time of these ionospheric anomalies preceding the seismic impact is sensitive to the magnitude of the subsequent earthquakes and it tends to increase with the rise of the earthquake magnitude. It is concluded that such ionospheric disturbances can be short-term ionospheric precursors of earthquakes.