Disastrous earthquakes happen 100 to 200 times a year. World community carries out
seismological, geophysical, tectonomagnetic as well as non-traditional investigations
to make possible the earthquake prediction. It is believed that due to a wide web
of seismic stations and earthquake prediction test sites the problem of long-term
earthquake forecast has been basically solved. This means that we are able to answer
rather closely where the earthquake may occur but not when. There is a chance that
the data of ionospheric plasma observations hold an answer to the latter question.
Satellites and ground-based facilities may detect earthquake precursors in the ionosphere
a few hours or days before the main shock. Although ionospheric parameters are
primarily controlled by the helio-physical processes the perturbations which propagate
from the Earth are also detectable. As it became clear last decade, both the tropospheric
perturbations (storms, cyclones, peculiarities of near-Earth air circulation) and
the tectonic processes (earthquakes, active fractures, subduction zones) are locally
reflected in the ionospheric state. Let us consider the most reliable manifestations
of seismo-ionosphere relations. The first example of the night sky luminescence
related to the earthquake was described in 373 B. C. in Rome. This phenomenon has
been observed above Tashkent (1966) and Spitac (1988) before the earthquakes at the
atomic oxygen spectral lines 6300 and 5577 A. Long-term observations of the air glow
have confirmed its correlation to earthquake-events with an advance time from a few
hours to a day. Another set of data has been obtained by the means of vertical
ionospheric sounding. Considerable variations in the effective heights of signal reflection,
changing value of the critical frequencies (i.e., of the peak electron density), increasing
of the number of ionograms with signatures of irregular and random anomalous radio
reflections, sporadic E-layer "blurring", and other effects indicate that
the preparation of earthquake is accompanied by the enhancement of plasma turbulence
as well as growing amount of ionospheric plasma inhomogeneities over the seismoactive
region As far as we know the first space-born observation of earthquake related
phenomena has been carried out aboard the Soviet "Intercosmos-19" satellite.
Then a great deal of data have been obtained by "OGO-6", "Nimbus",
"Aureol-3", "Geos-1", "Geos-2", "Intercosmos-Bulgaria-1300",
"DE-2" and "Intercosmos-24" missions. A specific noise-like
VLF electrostatic emission has been detected in range of tens KHz over the earthquake
epicenter within 200 to 300 km in the latitudinal direction and over a sufficiently
long distance in the longitudinal direction. This emission originates about 10 to
20 hours before the main shock and lasts over about the same time after it reaching
the intensity peak at the instant of the main shock. One of proposed explanations
of this phenomenon anticipates the penetration of electromagnetic field from the earthquake
focus into magnetosphere, interaction of this field with high-energy particles of
the Earth radiation belts and subsequent excitation of VLF plasma noise. Such interpretation
is supported by the particle measurements of "Aureol-3" satellite which
has passed narrow region of significant increases in the precipitation of high energy
(100 KeV) electrons and protons. Further seismogenic phenomena detected by satellites
is local decreasing (about 20 per cent) of the ambient ion density. A possible interpretation
suggests the generation of so called ionospheric bubbles (rare plasma regions) at
geomagnetic field lines based on the epicenter of shocks. ULF "Hz"-oscillations
have been recorded several hours before the main shock in a narrow magnetic field
tube (40...100 km along the satellite trajectory) conjugated with epicenter of the
earthquake. In the case of moderate earthquake (M = 3.5...5) the magnetic field disturbances
have reached magnitudes of 0.2...0.5 nT/Hz at altitudes of 800 to 900 km and frequency
about 8 Hz. Probably, these disturbances propagate through the ionospheric plasma
as Alfven waves. The history of the investigations under review may be conventionally
divided into three chronological stages. At the beginning anomalous of the ionospheric
plasma parameters had been noted in connection with seismic activity. The next stage
involved statistical verification of each phenomenon. In result, real existence of
seismogenic effect upon the ionosphere has been confirmed (we emphasize this point
because there are teams of investigators who prove the absence of such effect). At
the moment the main theoretical problem is to explain the mechanisms of seismo-ionospheric
coupling. Physics of the earthquake precursors generation is not well understood today
and only qualitative speculations are used for some explanation of observation results.
Last modified March 15, 1996 Mail to V.M.Ivchenko G.Lizunov