EXPERIMENTS TO LOCATE SOURCES OF EARTHQUAKE-RELATED VLF ELECTROMAGNETIC SIGNALS

Very low frequency (VLF) band electromagnetic signals(1)-11)) seem to be one of the most plausible precursory phenomena in the various kinds of anomalous electric, magnetic and electromagnetic field changes pre ceding earthquakes. In particular the number of VLF pulse-like signals has been known to increase clearly just before earthquakes(2),4),9),1 0)) possibly providing data on the occurrence time of impending earthq uake. However, there is a lack of knowledge as to how to predict where the earthquake will strike. Also, it is difficult to establish the me chanisms of emissions induced by natural geophysical activity: earthqu akes and volcanic eruptions. There are two main hypotheses regarding t he generation mechanism of the VLF waves: the first proposes that elec tromagnetic waves are directly emitted from the hypocentral region,(4) ,10),12)-15)) while the second proposes that the emission is a result of electric charge redistribution in the Earth's atmosphere. (16)-18)) Here, we present source region data for VLF pulse-like signals showin g that the VLF emission does not originate around the epicenter but fr om regions of atmosphere above the epicenter spreading horizontally in several hundreds kilometers. Our data suggest that the VLF emission i s produced by the electromagnetic processes in the Earth's atmosphere up to the ionosphere.