Abstract
Abstract Ionospheric F-region disturbances and anomalous f 0 E s increases were separately observed within a few days before two great earthquakes in geomagnetic and solar quiet conditions. The ionospheric f 0 F 2 decreased from its monthly median at Wakkanai, 290 km north of the epicenter from 3 days before the M 7.8 earthquake onset of July 12, 1993 in the northern Japan to 3 days after it. Anomalous f 0 E s increases above 7 MHz were observed at Shigaraki, 100 km northeast of the epicenter in the daytime on January 15, 1995 before the M 7.2 Hyogoken-nambu earthquake onset of January 17, 1995. The F-region disturbances before the earthquake is discussed in terms of the ionospheric vertical movements caused by a seismo electric field on the assumption of the global ionosphere-earth seismo current system. The anomalous f 0 E s increases before the earthquake onset could be caused by unknown super-volt seismo discharges since there was no thunderstorm cloud over the observatories.
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