Modeling of subionospheric VLF signal perturbations associated with earthquakes

It has been reported that propagation of very low frequency (VLF) waves in the Earth-ionosphere waveguide might provide an indication of imminent eart hquakes [Hayakawa et al., 1996; Molchanov et at, 1998]. Narrow-band data fr om Inubo, Japan, suggested that transmissions from Omega Japan, 1000 km awa y, might be influenced by pre-earthquake processes. The terminator time (TT ) was defined as the time where a minimum occurred in the received phase (o r amplitude) during sunrise and sunset. A few days before an earthquake the TT was observed to deviate significantly from the monthly averages, produc ing a longer "VLF day." The TT effect has been explained through some rathe r simple modeling by a 1-2 km drop in the VLF reflection height at the lowe r ionospheric boundary. In this study we apply more realistic propagation m odels to show that the changes in VLF reflection height associated with ear thquakes would have to be considerably larger (similar to 4-11 km) than tho se suggested previously in order to produce the reported effect. If the rep orted TT changes were caused by alterations in the VLF reflection height as sociated in some manner with an imminent earthquake, these effects would be commensurate with the effects of a solar flare. However, this would lead t o changes in received amplitude (or phase) that would be significant at all times, and not just during the day/night transition. Hence it is not at al l clear that a simple height-lowering explanation for the TT effect is corr ect.