Received October 21, 1999L. Kh. Shatashvili, D. I. Sikharulidze, and N. G. Khazaradze
Geophysical Institute, Tbilisi, Georgia
Received October 21, 1999
6.0 for the period 1958-1988. According to the
histograms, the
probability of occurrence of large destructive earthquakes is
periodic, with approximately an 11-year cycle.
In recent years the efforts of many researchers [Gokhberg et al., 1998; Shatashvili et al., 1999; Sobolev et al., 1988; Sytinskiy, 1989, 1990] have been devoted to the search for extraterrestrial phenomena responsible for creating conditions for earthquakes. In our opinion, of major interest is the work of Sytinskiy and Oborin [1997] who showed that all the disturbed solar plasma fluxes which are, on the average, 1 day ahead of strong geomagnetic storms accompanying earthquakes are geoeffective for earthquakes. Sobolev et al. [1988] put forward the hypothesis that the seismic activity level changes during a geoeffective solar flare; and Sasaki and Tsarev [1997] pointed out that it is necessary to study the generation mechanisms of the electromagnetic radiation for seismically active regions of the Earth. Some of great destructive earthquakes are associated with bursts of cosmic rays in the far regions of the Universe during explosions of supernovas [Iu-Then-Dong, 1985]. Asatryan et al. [1991] noted that a 100-% increase in the intensity of the rigid component of cosmic rays (CR) was observed in the stratosphere above Erevan 30 minutes prior to the well-known Spitak earthquake. The prospects of studies the lunar CR variations due to tidal phenomena in the upper atmosphere were reviewed by Duggal [1977]. The work was based on the evidence given by Naskidashvili and Shatashvili [1977]. Volodichev et al. [1997] and Dunbar et al. [1992] stated that the particle fluxes detected by neutron monitors have not only the solar-galactic, but also the terrestrial origin.
Shatashvili et al. [1999] has given a brief review of the most important investigations of many researchers who believe that major earthquakes are caused exclusively by physical and chemical processes in the interior of the Earth which give rise to motion of tectonic plates. However, this idea should be reconsidered. The author also described the preliminary results of studies carried out using a small amount of statistical material for the observation period 1957-1961 [Dunbar et al., 1992; Mansurov, 1984].
From the analysis of complex diagrams demonstrating the correlation between the events of decrease in the An/Am ratio (where An and Am are the amplitudes of daily variations in the neutron and meson components of the CR intensity, respectively), and the moments of the IMF neutral sheet crossings, Full Moon and New Moon, and also of perigee and apogee, Shatashvili et al. [1999] concluded that large destructive earthquakes often take place when several of the factors mentioned above superimpose.
Using a considerably larger statistical material for 1958-1998, this work shows that, of all space environment phenomena directly or indirectly associated with destructive earthquakes, the most powerful factor is crossing of the IMF neutral sheet by the Earth.
Similar to Shatashvili et al. [1999], we suppose that the near-Earth environment phenomena which, in principle, can generate large destructive earthquakes and, generally, affect the processes at the Earth must act first of all through the channels which are in immediate contact with the magnetosphere of the Earth. The physical processes taking place in the vicinity of the Earth's magnetosphere and propagating through its surface reach the upper layers of the atmosphere and interact with it. We suppose that crossing of the IMF neutral sheet by the Earth, that is, a drastic change in the IMF direction, must significantly affect the Earth's magnetosphere. The geoefficiency of this process must be very high, and it is quite probable that not only the solid shell of the Earth may be affected, but also tectonic plate displacements may take place.
We analyzed the data on earthquakes [Dunbar et al., 1992; Mansurov, 1984] for the period 1958-1988. We started from the assumption that not every change of the IMF sign near the Earth is followed by an earthquake because, in our opinion, this change is only a triggering mechanism that comes into action when the conditions for the earthquake already exist. Therefore, the zero time in the diagrams was taken to be an initial moment of a large destructive earthquake with a magnitude greater than 6. We built a large number of histograms showing the number of crossings of the IMF neutral sheet by the Earth with respect to the earthquake occurrence. The histograms were plotted for each successive year and also for every 4-5 years using two procedures. 1. The moments of crossings of the neutral sheet by the Earth both prior to the earthquakes and after them were considered. 2. Only one event with the shortest time interval between the crossing and the earthquake was included for each earthquake. The resulting histograms were found to be alike.
Figure 1 shows, as an example, histograms of the number of
crossings of the IMF neutral sheets with respect to large
destructive earthquakes with a magnitude
6.0 for the
observation period 1965-1969. The data of
Dunbar et al. [1992]
and
Mansurov [1984]
were used.
Figure 1a
presents the number of crossings of the IMF neutral sheet by the
Earth both prior to the earthquake and after it. Figure 1b shows
the number of crossings of the IMF neutral sheet with the shortest
time interval between the crossing and the earthquake either prior
to the earthquake or after it. The moment of an earthquake is
marked by
"0" on the abscissa axis, negative numbers "
-1 ", "
-2 "
etc. indicate the days of crossings of the IMF neutral sheet prior
to the earthquake, and positive numbers show the days of crossings
after the earthquake.
To reveal the periodicity (with approximately an 11-year cycle) in the probability of coincidence between the occurrence of large destructive earthquakes and crossings of the IMF neutral sheet by the Earth, we selected from the histograms the intervals of the highest probability of large destructive earthquakes. The portion of the sum of the earthquakes coinciding with the crossing ("0") and occurring in a day after the crossing (" -1 ") in the total number of earthquakes per year (in percent) was determined, and then the graph of variations in the relative numbers of large destructive earthquakes was plotted. To show more visually the cyclic character of this graph, the obtained successive dependences were smoothed by the running mean method (with an averaging period of 5 years).
The results of these calculations are shown in Figure 2. It shows
cyclic changes in the extreme numbers of crossings of the IMF
neutral sheet by the Earth (in percent) averaged by the running
mean method (with an averaging period of 5 years) for 1958-1988.
The data of
Dunbar et al. [1992]
and
Mansurov [1984]
were used.
Analysis of Figures 1 and 2 shows that
1. Large destructive earthquakes occur a day before the IMF neutral sheet crossing by the Earth or at the day of crossing in nearly 75% of cases.
2. Large destructive earthquakes correlating with crossings of the IMF neutral sheet by the Earth show a tendency to have 11-year periodic cycles.
3. Since the time of appearance of the IMF neutral sheets can, in principle, be easily predicted, our results can be used for modeling the precursors of earthquakes.
The comparison of the calculations with the space environment
phenomena accompanying large destructive earthquakes
[Shatashvili et al., 1999;
Volodichev et al., 1997]
shows that
crossing of the IMF neutral sheet by the Earth plays an important
role as a triggering mechanism in providing the conditions for
earthquakes and in occurrence of the earthquakes with a magnitude
6.0.
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