Experiments to Locate Sources of Earthquake-related VLF Electromagnetic Signals

概要

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Very low frequency (VLF) band electromagnetic signals1)-11) seem to be one of the most plausible precursory phenomena in the various kinds of anomalous electric, magnetic and electromagnetic field changes preceding earthquakes. In particular the number of VLF pulse-like signals has been known to increase clearly just before earthquakes2), 4), 9), 10) possibly providing data on the occurrence time of impending earthquake. However, there is a lack of knowledge as to how to predict where the earthquake will strike. Also, it is difficult to establish the mechanisms of emissions induced by natural geophysical activity: earthquakes and volcanic eruptions. There are two main hypotheses regarding the generation mechanism of the VLF waves: the first proposes that electromagnetic 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 Earths atmosphere. 16)-18) Here, we present source region data for VLF pulse-like signals showing that the VLF emission does not originate around the epicenter but from regions of atmosphere above the epicenter spreading horizontally in several hundreds kilometers. Our data suggest that the VLF emission is produced by the electromagnetic processes in the Earths atmosphere up to the ionosphere.
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