[15] Using the above described method, Korsunova and Khegai [2005] earlier processed the ionospheric data for 17 earthquakes under more hard conditions for their choice: the absence of strong geomagnetic disturbances in the earthquake preparation periods. The results obtained in this paper confirm the regularities in the appearance of medium-term ionospheric precursors of earthquakes found for magnetically quiet conditions: prevailing of anomalous positive spikes 2-3 hours long in all ionospheric parameters exceeding the chosen value for the deviations determined in the previous section. Most often the ionospheric precursors are detected around the noon and midnight, but the daytime is prevailing (29 cases out of 40). The disturbances in the regular F2 layer most often goes ahead of the disturbances in the sporadic E layer parameters (32 cases). No increase in the spikes amplitude is observed to the earthquake moment. The most important parameter characterizing the process of earthquake preparation is the time interval between the appearance of the anomalous spikes and the shock moment. We call it the advance time ( T ) of the appearance of an ionospheric precursor, the value of T being measured in diurnal intervals (24 hours).
[16] Table 1 shows the data on the term of appearance of ionospheric
precursor and corresponding values of the deviations in the
parameters of the
F2 and
Es layers for the epicenter distances
R 600 km in the conditions of the absence of strong geomagnetic
disturbances. The performed analysis showed that at large distance
and under disturbed magnetic field, the value of seismogenic
disturbances decreases considerably, especially in the
F2 layer, so
it is difficult to detect them on the background of the general
ionospheric variability. For powerful earthquakes with
M > 7, the
precursor effects are detected even at distances of the order of
1000 km. The value of the effect is different in the
F2 and
Es layers: in the
Es layer it is higher by a factor of 1.5-2.
Taking into
account the comments made, one may state that the analysis of the
changes in the critical frequency of the
F2 layer only cannot in the
real conditions provide complete enough description of the
ionospheric reaction to an earthquake preparation.
[17] On the other hand, the sporadic E layer parameters are in a significant degree influenced by the dynamic processes in the environment (IGW, turbulence) [Chimonas, 1974; Chavdarov et al., 1975; Ovezgel'dyev et al., 1977], so the use of the Es layer only for revealing of seismoionospheric effects is also able to lead to wrong conclusions. This is why we emphasize the need to consider simultaneously changes in the F2 and Es layer parameters. This makes it possible to identify the ionospheric effects of earthquake preparation with more reliability. In our study, changes in the height of the sporadic E layer are a determining factor satisfying the above presented requirements, as the virtual heights of the sporadic E layer are close to the real heights because of the small width of this layer.
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Figure 2 |
[19] The empirical dependencies for the assumed seismogenic
disturbances in the
Es and
F2 parameters shown in Figure 2
(second through fourth panels from the top) are obtained for the
first time. It follows from these dependencies that the stronger the
earthquake and the closer the epicenter the more the value of the
deviation in ionospheric parameters, this fact agreeing also with
the results for the time of precursor appearance
T. The slopes of
the approximating lines for all three parameters are fairly close to
each other ( 0.65 0.1 ) but are almost by a factor of 2 less than for
lg(TR). Moreover, it could be noted that the coefficients in the
right-hand side of the logarithmic dependencies for the frequency
parameters of the
F2 and
Es layers differ in a lesser degree than for
the virtual heights of
Es
and the time of precursor appearances. The
latter means that the same effects in
foF2 and
f bEs may be caused
by close but weak earthquakes or by strong but remote ones (see
lines 2 and 19 in Table 1). So for forecasting purposes, one can use
the empirical dependencies for
T,
Dh'Es, and
Df b/f m Es, whereas the
F2 -layer parameters are needed to control the correctness of the
ionospheric precursors identification. Having determined such
dependencies for magnetically quiet conditions and particular
observational point using known earthquakes, one can evaluate the
magnitude of the earthquake in preparation, the time of the shock,
and the distance from the observational point to the epicenter.
Approximate estimates of the epicenter location may be made on
the basis of a map of the Earth crust fractures where earthquake
hypocenters are located most often.
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