The model study of amplitude-spectral variations of low-frequency signals propagating in the Earth-ionosphere waveguide when seismic disturbances occur on the upper wall of waveguide - in the lower ionosphere have been carried out. The possible characteristics of seismic disturbances in the lower ionosphere causing the observed amplitude variations of signals are considered on results of the comparison with experimental data of observations of VLF- signals. The results of calculations of the dependence of spectra of low-frequency signals on the parameters of the Earthionosphere waveguide propagating along paths with disturbances in the ionosphere show that the increases in the signal amplitude registered in the experiment during periods of strong seismic events can be explained by the increase in the steepness of the electron profile and/or the increase in the height of waveguide.
The specific features of a method for radiosounding the lower ionosphere over earthquake epicenters using LF electromagnetic signals of thunderstorm sources (atmospherics) have been considered. The effects of shallow focus earthquakes with magnitudes larger than 5.0 and their precursors manifest themselves in amplitude characteristics of atmospherics. The ratio of the amplitudes of electric and magnetic signal components, in the variations of which seismic effects before earthquakes can also be observed, has been considered. An analysis of the ratio makes it possible to increase the probability of predicting earthquakes when using the characteristics of the electromagnetic signals of lightning discharges as an additional method of complex monitoring of disturbances in the lower ionosphere caused by seismic processes.
KEYWORDS: Waveguides, Modeling, Light wave propagation, Radio propagation, Signal attenuation, Electromagnetism, Data modeling, Solar processes, Atmospheric physics, Atmospheric propagation
Modelling consideration of amplitude -spectral variations of the low-frequency signals extending in the earth-ionosphere wave guide at occurrence of seismic disturbances on the top wall of wave guide ( the lower ionosphere) is carried out. By results of a comparison with experimental data of VLF-signal observation, possible characteristics of seismic disturbances in the lower ionosphere causing observable amplitude variations of signals are considered.
Features of the remote sensing of seismic disturbances in the lower ionosphere are considered according to observation data of lightning electromagnetic signals passing over the earthquake epicenter, in Yakutsk. The technique has the ability to scan a large seismically active region or even several regions directly from one point, though in some azimuths there are limitations due to insufficiently high lightning activity. In last case, the receiving of signals at several spaced points is used.
Observation of low-frequency radio signals may be used as a tool of remote monitoring of seismic disturbances in the lower ionosphere. In the case of using a low frequency radio signal the most informative parameter is a signal phase. When using natural low-frequency radio emissions - electromagnetic signals of lightning discharges (atmospherics), the variation of the signal amplitude is analyzed. For strong (magnitude greater than 5) and not deep-focus (focal depth is less than 50 km) earthquakes it is found that the effects of earthquakes and their precursors in the signals of atmospherics appeared in the form of increasing amplitude, whose duration is generally a few hours. In this work peculiar properties of the effects of manifestations in the signals of atmospherics of deeper earthquakes are considered on the example of the events in the Malaysian-Indonesian region.
For distant monitoring of disturbances in the lower ionosphere signals of low-frequency radio stations are often used. In particular, the method is used for investigation of ionospheric responses (disturbances) caused by seismic activity. As a variant of this method for the detection of seismic disturbances in the ionosphere one can use natural radio signals - an electromagnetic radiation of thunderstorm discharges (atmospherics). The examples of the effects of three earthquakes in the variations of average amplitude of atmospherics received in Yakutsk (φ = 62.1° N, λ = 129.7° E) were considered. The increase of signal amplitude for some days before the events can be considered as a precursor. It allows to used observations of impulse electromagnetic thunderstorm signals as one of possible means of distant monitoring of strong earthquakes.
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