On the precursory signature of Kobe earthquake on VLF subionospheric signals

The subionospheric VLF Omega signal transmitted from Tsushima, Japan (geographic coordinates: 34/spl deg/37'N, 129/spl deg/27'E) is continuously received at Inubo (35/spl deg/42'N, 140/spl deg/52'E), and the propagation characteristics (especially phase) of this signal have exhibited abnormal behaviour (especially around the sunrise and sunset local times) a few days before the main shock of the 1995 Kobe earthquake. A statistical study based on long-term (four months before and four months after the earthquake) observational data has strongly suggested that this anomaly was not coincidental, but it is highly likely to be related to the earthquake. We have found from a computer simulation of VLF signal propagation that this observed effect can be explained by a decrease of about 1.5 km in the VLF reflection height. Plausible mechanisms for this decrease are discussed.