On variations of foF2 and F-spread before strong earthquakes in Japan
Liperovskaya , E. V.
Parrot , M.
Bogdanov , V. V.
Meister , C.-V.
Rodkin , M. V.
Liperovsky , V. A.
- Publisher: Copernicus Publications on behalf of the European Geosciences Union
Natural Hazards and Earth System Sciences
(issn: 1561-8633, eissn: 1684-9981)
[ SDU.STU ] Sciences of the Universe [physics]/Earth Sciences | [ SDU.ENVI ] Sciences of the Universe [physics]/Continental interfaces, environment | [ SDU.OCEAN ] Sciences of the Universe [physics]/Ocean, Atmosphere | G | GE1-350 | Geography. Anthropology. Recreation | QE1-996.5 | Environmental technology. Sanitary engineering | Environmental sciences | Geology | TD1-1066
The statistical analysis of the variations of the dayly-mean frequency of the maximum ionospheric electron density <i>fo</i>F2 is performed in connection with the occurrence of (more than 60) earthquakes with magnitudes <i>M</i>>6.0, depths <i>h</i><80 km and distances from the vertical sounding station <i>R</i><1000 km. For the study, data of the Tokyo sounding station are used, which were registered every hour in the years 1957–1990. It is shown that, on the average, <i>fo</i>F2 decreases before the earthquakes. One day before the shock the decrease amounts to about 5%. The statistical reliability of this phenomenon is obtained to be better than 0.95. <P> Further, the variations of the occurrence probability of the turbulization of the F-layer (F spread) are investigated for (more than 260) earthquakes with <i>M</i>>5.5, <i>h</i><80 km, <i>R</i><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 seismo-ionospheric effects, here periods of time with weak heliogeomagnetic disturbances are considered. For the <i>fo</i>F2 analysis, the Wolf number is less than 100 and the index Σ<i>K<sub>p</sub></i> is smaller than 30, and in case of the F-spread study a Wolf number less than 80 and Σ<i>K<sub>p</sub></i> smaller than 17 are chosen.