M. Hayakawa et al., ANOMALIES IN THE SUB-IONOSPHERIC VLF SIGNALS FOR THE 1995 HYOGO-KEN-NANBU EARTHQUAKE, Journal of Physics of the Earth, 44(4), 1996, pp. 413-418
The sub-ionospheric VLF Omega signal transmitted from Tsushima, Japan
(geographic coordinates: 34 degrees 37'N, 129 degrees 27'E) is continu
ously received at Inubo (35 degrees 42'N, 140 degrees 52'E). The signa
l's propagation characteristics (especially phase) exhibited abnormal
behavior (especially around the sunrise and sunset local times) a few
days before the main shock of the 1995 Hyogo-ken Nanbu earthquake. We
found from the computer simulation of VLF signal propagation that the
observed effect can be explained by a decrease of about 0.7 km of VLF
reflection height al the atmosphere-ionosphere boundary.