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Natural Hazards and Earth System Sciences (NHESS) (http://www.nathazardsearthsystsci.net/6/735/2006/)
Further, the variations of the occurrence probability of the turbulization of the Flayer (F spread) are investigated for (more than 260) earthquakes with M>5.5, h<80 km, R<1000 km. For the analysis, data of the Japanese station Akita from 1969–1990 are used, which were obtained every hour. It is found that before the earthquakes the occurrence probability of F spread decreases. In the week before the event, the decrease has values of more than 10%. The statistical reliability of this phenomenon is also larger than 0.95. Examining the seismoionospheric effects, here periods of time with weak heliogeomagnetic disturbances are considered. For the foF2 analysis, the Wolf number is less than 100 and the index ΣK_{p} is smaller than 30, and in case of the Fspread study a Wolf number less than 80 and ΣK_{p} smaller than 17 are chosen.