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

Abstract

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. Computer simulation of VLF signal propagation showed that the observed effect can be explained by a decrease of about 0.7 km of VLF reflection height at the atmosphere-ionosphere boundary.