1. Introduction

[2]  The events in December 2004 in the Indian Ocean showed that not enough attention has been paid until recently to studies of earthquakes and of their impact on the environment. It is especially true for strong carpet earthquakes with magnitude M > 6 causing the strongest damages to the population and economics of the countries located in epicenter zones. For a development of a working system of earthquake forecast, both the knowledge on the earthquake mechanism and revealing of their precursors according to the data of various geophysical fields, and in particular on the basis of ionospheric parameters measurements, are needed. The data obtained earlier on ionospheric precursors to earthquakes [Liperovsky et al., 1992; Silina et al., 2001; Strakhov and Liperovsky, 1998] provide information on their most general features that cannot be used for forecasting needs. According to the results of spectral analysis the disturbances in ionospheric parameters with periods 1.5-2 hours are singled out both in the F region [Korsunova et al., 1996; Liperovsky et al., 1992; Pulinets and Liu, 11998a] and in the sporadic E layer [Korsunova et al., 1999; Silina et al., 1998]. Just these perturbations being averaged for concrete hours demonstrates growth (or decrease) of the critical frequencies of ionospheric layers for tens percent with respect to the undisturbed state of the ionosphere. The discussed perturbations in ionospheric parameters are discovered mainly within 1-3 days before the earthquake and can serve as precursors to the event. It should be mentioned that these data were obtained for large amount of earthquakes with M < 6 and hence they are statistically meaningful. It is much more complicated to reveal precursors to earthquakes with M > 6, for which statistical analysis does not give reliable results [Pulinets, 1998].

[3]  Hobara and Parrot [2005] investigated the regular F2 layer for the period more than 80 days before the earthquake with M 8.3. Statistically meaningful anomalies were revealed only 1-2 days before the event. Similar result was obtained earlier for the earthquake with M > 7 in Argentine [Rios et al., 2004] and in Japan according to the E_s data [Ondoh, 2003]. However, all mentioned above anomalies in the ionosphere are related to short-term earthquake precursors. The absence of medium-term precursors in the study by Hobara and Parrot [2005] confirms that their mean-square amplitude deviation does not exceed 20%. So, they cannot be discovered by usually accepted statistical methods.

[4]  From the other side, in the measurements of the geophysical fields on the ground medium-term precursors are singled out reliably enough. For such precursors the period before the earthquake comprises from several days to one month [Sidorin, 1979]. The appearance of the medium-term precursors can be expected in the ionosphere also, taking into account the influence of the seismic processes on the ionosphere due, for example, of electric fields. Indeed, the link between quasistatic electric fields in the near Earth ionosphere with seismic events was proved experimentally [Smirnov, 2005], and the influence of the quasistatic electric fields on the ionosphere was discussed theoretically by Pulinets and Liu [1998b]. At last, complex experiment in which the E_z component of the electric field, radon concentration, and ionospheric parameters were measured simultaneously at Kamchatka, confirmed the existence of lithosphere-atmosphere-ionosphere interaction [Mikhailov et al., 2002].

[5]  We proposed a new approach to revealing earthquake precursors on the background of the total variability of the ionosphere. This approach uses the set of the data on simultaneous measurements of several parameters of ionospheric layers: the critical frequency of the regular F2 layer ( f_oF2 ), blanketing frequency of the sporadic E layer ( f _bE_s ), and its virtual height ( h'E_s ). This approach made it possible to find some regularity in the time of appearance of ionospheric precursors for various earthquakes, the regularity agreeing with the results of observations of surface geophysical fields. Moreover, empirical dependencies of the value of deviations of ionospheric parameters on the earthquake magnitude and the epicenter distance were obtained.

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