Abstract
There have been published many papers on VLF (very low frequency) characteristics to study seismo-ionospheric perturbations. Usually VLF records (amplitude and/or phase) are used to investigate mainly the temporal evolution of VLF propagation anomalies with special attention to one particular propagation path. The most important advantage of this paper is the simultaneous use of several propagation paths. A succession of earthquakes (EQs) happened in the Kumamoto area in Kyusyu Island; two strong foreshocks with magnitude of 6.5 and 6.4 on 14 April (UT) and the main shock with magnitude 7.3 on 15 April (UT). Because the EQ epicenters are not far from the VLF transmitter (with the call sign of JJI in Miyazaki prefecture), we can utilize simultaneously 8 observing stations of our network all over Japan. Together with the use of theoretical computations based on wave-hop theory, we try to trace both the temporal and spatial evolutions of the ionospheric perturbation associated with this succession of EQs. It is found that the ionospheric perturbation begins to appear about two weeks before the EQs, and this perturbation becomes most developed 5 - 3 days before the main shock. When the perturbation is most disturbed, the maximum change in vertical direction is depletion in the VLF effective ionospheric height of the order of 10 km, and its horizontal scale (or its radius) is about 1000 km. These spatio-temporal changes of the seismo-ionospheric perturbation will be investigated in details in the discus-sion, a comparison has made with the VLF characteristics of the 1995 Kobe with the same magnitude and of the same fault-type, and a brief discussion on the generation mechanism of seismo-ionospheric perturbation is finally made.
Highlights
There has been an enormous progress on the studies of precursors to earthquakes (EQs) during the last two decades since the 1995 Kobe EQ
Together with the use of theoretical computations based on wave-hop theory, we try to trace both the temporal and spatial evolutions of the ionospheric perturbation associated with this succession of EQs
It is found that the ionospheric perturbation begins to appear about two weeks before the EQs, and this perturbation becomes most developed 5 - 3 days before the main shock
Summary
There has been an enormous progress on the studies of precursors to earthquakes (EQs) during the last two decades since the 1995 Kobe EQ. The amplitude (and/or phase) of subionospheric signals from any VLF/LF transmitters are continuously monitored, and the observed signal parameters are mainly determined by the position of reflection height which depends on the value and gradient of electron density. It is typically 80 km in daytime and is about 90 km at night. This VLF/LF method can provide us with the information on the perturbation in the lower ionosphere, which has been found to be very promising for the short-term EQ prediction [3], because such VLF propagation anomalies tend to appear about one week before an EQ. Since the pioneering works by Russian and Japanese [11] [12] [13], there have been published many papers on the use of VLF/LF method for the study of the seismo-ionospheric perturbations (see, our latest review [14]), and this VLF method is becoming a world trend for short-term EQ prediction as understood from the establishments of VLF networks in different countries [15] [16] [17] being stimulated by our Japanese network [3] [5]
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