We report on our observation of pulse-like electromagnetic signals in the frequency range 1∼10 kHz that we associate with earthquakes. The severest difficulty in separating earthquake-associated VLF signals from those originating in lightning discharges stems from the circumstance that the latter signals are overwhelming in number compared with the former. While claims have often been made of observation of electromagnetic signals in association with earthquakes, most of the claims, excepting a few, heavily rely on temporal correlation. By means of simple instrumentation and data processing software, our method by and large enables us to isolate VLF signals whose direction of arrival is well focussed at the epicenter direction. In this preliminary report we present several examples that positively demonstrate the existence of a class of VLF signals having a peak frequency of occurrence 1∼4 days prior to earthquakes of Magnitude 4∼6. With an accumulation of experience the technique described in this paper would seem to offer a promising approach towards earthquake prediction.
Abstract:It has been reported by many authors that the number of pulse-like electromagnetic signals per unit time increases in the LF, VLF and HF ranges during the period of several days preceding the occurrence of an earthquake in land or under shallow sea close to land. However, with the reported observations whether or not these signals originate in the epicentral area is not clear, because in most cases the epicenterto-receiver distances are many tens of kilometers or more. We have conducted an experiment to directly detect the electromagnetic emissions from earthquake source by making radio-noise observation in conjunction with aftershocks of the 17 January 1995 Hyogo-ken Nanbu (Kobe) earthquake. The motivation behind this attempt was to investigate if the aftershocks of very small S-P times are accompanied by a type of electromagnetic signals different from those usually observed in association with distant large earthquakes. Observations were made from 21 to 27 January 1995, using radio receivers. During this period, 804 aftershocks were recorded. In 128 of these aftershocks, the S-P time was shorter than 0.7 sec, or the hypocentral distance was less than approximately 3.5 km. Further, 53 cases of these 128 aftershocks were accompanied by electromagnetic signals lasting approximately 1.5 to 4.0 min. Since the correlation between the occurrences of electromagnetic signals and aftershocks was found to be high, the two phenomena can be considered to have a physically close relationship.
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