Slide
Kushida (1998), Sakai et al.(2002), Fukumoto et al.(2002) using FM over-horizon signal reception (76-78 MHz) have found intensification of
the signals and fluctuations of the background
1-7 days before EQs in Japan.
Fukumoto et al (2002) demonstrated this scattering occurs in the troposphere.
Miaki et al. (2002), Molchanov et al. (2001) found night-time VLF signal variations before EQs have periods in a range 10 minutes-2 hours, i.e. in the range of atmospheric internal gravity waves (AGW).