Effect of earthquakes on lower ionosphere as found by subionospheric VLF propagation

Reliable VLF subionospheric signal effect related to the seismic activity was first reported by Hayakawa et al. (1996 a, b) in association with the great Kobe earthquake. We have analyzed the similar data around 10 other great earthquakes (magnitude M >6), in order to understand the main features of such an effect, and the following characteristics are emerged from our analysis: (a) This effect looks as transient oscillations with 5-10 days period, which are initiated a few days before a large earthquake and relaxed for a few days or weeks after it. (b) It is mainly related to crust earthquakes. (c) It cannot be observed outside the sensitivity zone of VLF transmitter (first Fresnel zone) even for very large earthquakes (M >7). (d) It appears when the resonant atmospheric oscillations with a period in a range of 5-11 days exist before an earthquake. Probably, the seismic influence on the VLF signal is explained by the generation of long-term gravity waves during an earthquake process and their intensification at the heights of 70-90 km (C) 2000 COSPAR. Published by Elsevier Science Ltd.