Abstract: The data of 5 ionospheric vertical sounding stations before 4 earthquakes in Japan are analyzed. It is found that most of the critical frequencies of the E sporadic layer tended to decrease and the only few tended to increase around the epicenters before and after the earthquakes. The analytical results of ionospheric stations near 55 earthquake epicenters during International Geophysical Year (IGY) show that the statistical feature of fo F2 disturbances is positive tendency before earthquakes. This paper discussed the mechanism of influence of source electric field on ionosphere. This research indicates that there is possibility of ionospheric anomalies before earthquakes as a precursor.

Key words: Earthquake precursor, Iono sphere, Critical frequency