The potential link between ELF/VLF and the ULF (0.01 - 10 Hz) as a precursor to an earthquake.
https://ee.stanford.edu/~acfs/LomaPrietaPaper.pdf
This ties in with research carried out on the effects of the sun's magnetic field(s) on our planet, carried here by the solar wind.
Inequality of the interplanetary current sheet and the movement of the Earth as it orbits through open or broken sections of the interplanetary magnetic field are further compounded by the reversal of the Sun's magnetic polarity - twice - during each solar cycle.
One of the scientific instruments on the Parker Probe in determining how better to measure the changes remotely, which might lead to a reliable warning system. The trouble is the speed at which a repeatable result of such a low frequency can be reported before the impact.
http://cds.cern.ch/record/118596/files/CM-P00068442.pdf