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°37'N, 129°27'E) is continuously received at Inubo (35°42'N, 140°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 at the atmosphere-ionosphere boundary.