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 earthquakes [Hayakawa et al., 1996; Molchanov et at, 1998]. Narrow-band data from Inubo, Japan, suggested that transmissions from Omega Japan, 1000 km away, might be influenced by pre-earthquake processes. The terminator time (TT) was defined as the time where a minimum occurred in the received phase (or amplitude) during sunrise and sunset. A few days before an earthquake the TT was observed to deviate significantly from the monthly averages, producing a longer "VLF day." The TT effect has been explained through some rather simple modeling by a 1-2 km drop in the VLF reflection height at the lower ionospheric boundary. In this study we apply more realistic propagation models to show that the changes in VLF reflection height associated with earthquakes would have to be considerably larger (similar to 4-11 km) than those suggested previously in order to produce the reported effect. If the reported TT changes were caused by alterations in the VLF reflection height associated in some manner with an imminent earthquake, these effects would be commensurate with the effects of a solar flare. However, this would lead to 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 all clear that a simple height-lowering explanation for the TT effect is correct.