Medium-term ionospheric precursors to strong earthquakes

Abstract

[1] An attempt is made to determine the most typical changes in parameters of the ionospheric F2 and Es layers during the 7 days prior to an earthquake. Earthquakes with magnitudes M > 6 in the Pacific region in the vicinity of Japan are considered. Hourly measurement data at Kokubunji ionospheric station for 40 earthquakes with epicenter distances R < 1000 km are used. Seismoionospheric disturbances are detected on the basis of analysis of the derivations of current values of the ionospheric layer parameters from the running medians. A method of identification of precursors on the basis of some set of morphological signs is proposed. It is shown that different times of appearance of precursors are caused by their dependence on the magnitude of the coming earthquakes and the distance from the observational point to the epicenter. For M = 6 − 7 and R < 600 km this time is from 1 to 6 days. Empirical dependencies of the time of appearance of ionospheric precursors and the value of seismogenic disturbances in ionospheric layer parameters are obtained and may be used for prediction purposes.