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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS009 Poster presentation 2056
Day-time variations of foF2 connected to strong earthquakes
Dr. Mikhail Rodkin
Geophysical Center Russian Academy of Sciences IASPEI
Elena Liperovskaya, Vadim Bogdanov, Claudia-Veronica Meister, Victor Liperovsky
In the present work, the authors continue their search for earthquake precursors and study of the
behavior of some characteristic parameters of the ionosphere occurring during the earthquake
preparation processes and after the earthquakes. The F-layer is the most dynamic regular layer of the
ionosphere being considerably modified by different reasons. The foF2 frequency characterizing the
maximum of the electron concentration in this layer is one of the parameters of the ionosphere that is
regularly measured during vertical sounding experiments. Here the variability of the foF2 frequency
connected with the strong earthquakes is investigated statistically, more than 100 events with
magnitude M>6.0 are taken into examination. These earthquakes have a depth less than 80 km, and
they happened at a distance less than 1000 km from the vertical sounding station. The used foF2
frequency hourly data were registered at the stations: Wakkanai, Akita , Kokubunji, and Yamagawa ()
since 1957 until 1990. The authors find an increase in the variability of the foF2 frequency at time scale
1-2 hours during 5-10 days before the event and the decrease in the variability of the foF2 frequency
since the day before until a few days after an earthquake. Analyzing the diurnal behavior of the effect,
they find that it occurs at day-time in periodbetween 11 h and 15 h LT. Diurnal variability depends on
the season, on 11-years Solar variations circle, and on geomagnetic variations. In the work we use the
data recorded in time intervals with weak heliomagnetic disturbances (Wolf number below 100 and Σ
Kp< 30). The amplitude of the modification may be larger than 10%, and the statistical reliability of the
effect amounts to more than 0.95. No effect is obtained for the strong earthquakes with depths more
than 80 km. Also, no effect is found for weaker earthquakes with M