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

It has been reported by many authors that the number of pulse-like electrom agnetic 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 la nd or under shallow sea close to land. However, with the reported observati ons whether or not these signals originate in the epicentral area is not cl ear, because in most cases the epicenter-to-receiver distances are many ten s 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-k en Nanbu (Kobe) earthquake. The motivation behind this attempt was to inves tigate if the aftershocks of very small S-P times are accompanied by a type of electromagnetic signals different from those usually observed in associ ation with distant large earthquakes. Observations were made from 21 to 27 January 1995, using radio receivers. During this period, 804 aftershocks we re 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. Furth er, 53 cases of these 128 aftershocks mere accompanied by electromagnetic s ignals 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 relati onship.