Anomalies in the sub-ionospheric VLF signals for the 1995 Hyogo-ken Nanbu earthquake

The sub-ionospheric VLF Omega signal transmitted from Tsushima, Japan (geographic coordinates: 34 degrees 37'N, 129 degrees 27'E) is continuously received at Inubo (35 degrees 42'N, 140 degrees 52'E). The signal'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.