Electromagnetic signals associated with small aftershocks of the 1995 Hyogo-ken Nanbu earthquake

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 epicenter-to-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 0.7sec, or the hypocentral distance was less than approximately 3.5km. Further, 53 cases of these 128 aftershocks were accompanied by electromagnetic signals lasting approximately 1.5 to 4.0min. 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.