For two decades, many researchers have been claiming that pre-seismic electromagnetic phenomena may be useful in short-term earthquake prediction. Despite much effort, however, the relationship between anomalous electromagnetic signals and earthquakes has not been proven. In this paper, we examine LF- and VLF-range electromagnetic-pulse data accumulated by Kyoto University since 1981, with special emphasis on the influence of lightning. Our data on electromagnetic pulses were compared with available lightning data to evaluate the influence of lightning.
We examined how far lightning affects our observation system. Although maximum distance varies with time and frequency ranges, it was found our observation system is influenced by lightning as far away as about 250 km in the LF range and about 420 km in the VLF range. The alleged “seismic” electromagnetic signals, which were reported in previous studies, were then re-examined with the view that lightning in the above ranges could be a source of noise.
An increase in the number of the electromagnetic pulses was observed 50 minutes before the 17 January 1995 Hyogo-ken Nanbu earthquake. We analyzed the continuous VLF electromagnetic waveforms at that time. We classified the waveforms and compared them with the lightning data and searched for the origin of each waveform. Most of the waveforms were explained by lightning activities.
Moreover, in the cases of other earthquakes, the observed electromagnetic pulse anomalies may be considered as due to lightning. For the obser vation of electromagnetic pulses in LF and VLF ranges, the application of electromagnetic pulse anomalies to earthquake forecast requires a good deal more effort to achieve convincible results and cannot be considered promising.