A likely VLF subionospheric signal effect related to seismic activity was first reported by Hayakawa et al. [1996a, b] in association with the great Kobe earthquake. We have analyzed similar data during periods around 10 other great earthquakes (magnitude M>6) in order to understand the main features of such an effect. The following characteristics emerged from our analysis: The effect appears as a transient oscillation with a 5‐ to 10‐day period, which is initiated a few days before a large earthquake and decays over a few days to weeks after it. It is mainly related to crustal earthquakes. It appears when resonant atmospheric oscillations with periods in a range of 5–11 days exist before the earthquake. The seismic influence on the VLF signal is probably explained by the generation of long‐period gravity waves during the earthquake process and their intensification at heights of 70–90 km.
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Molchanov et al. (1998) studied this question.
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