The cross correlation method response prior to earthquakes using foF2 data from various ionospheric stations

Abstract Considering the non-systematic aspect in detection of seismo-ionospheric precursors; we propose an attempt to characterize the nature of the perturbations in the F2 ionospheric layer prior to earthquakes using cross-correlation method. The effectiveness of cross-correlation method is not questioned; but could the use of more than two observation points undergoing identical influence of solar activity reveal the same perturbations for the same earthquake? The cross-correlation method is known for its ability to eliminate the solar activity effect when two measurements of the critical frequency of the F2 ionospheric layer (foF2) are correlated. As the observation points (ionosonde stations) used have similarity of geographical location they necessarily undergo the same influence of solar activity. Thus only the seismogenic aspect would remain in the cross-correlation coefficients. Therefore we have considered only earthquakes where there are several ionosonde stations satisfying the conditions of use of cross-correlation method. Using several stations should allow determine whether the cross-correlation coefficients vary in the same way for the same event or not. More than Fifty earthquakes from Mediterranean Basin and Japan with magnitude 5 ≤ M ≤ 6.6 and depth not exceeding 40 km are selected according to the availability of critical frequency data of the F2 ionospheric layer. Results vary from one earthquake to another. This is normal because each earthquake has its own characteristics. But the response of the cross-correlation technique is not always the same for the same event. Thus, there are cases where, for the same event, the variation of cross-correlation coefficients has the same behavior for the various pairs of stations considered and other cases where the response is not similar. Neither the magnitude of the earthquake nor the position of the internal station with respect to the epicenter, nor the position of the external stations relative to the internal station, could explain the difference in the behavior of the cross-correlation coefficients during the earthquake preparation process.