associated with ionospheric changes. A good overview is the article
by Friedemann Freund, "Predicting Earthquakes," in The Economist, 14
In the laboratory, the crushing of rock crystalline structures generates
electromagnetic fields. The theory is that similar events in the Earth
can affect the ionosphere and thus show up as precursors to large
earthquakes. This research is still controversial and, if there are
effects, they may be too subtle for the SID instruments to pick up.
However, at least one research group claims to have found unusual sunset
signatures associated with the devastating earthquake and tsunami
of December 2004. The paper is: Unusual Sunset Terminator behavior
of VLF signals at 17kHz during the Earthquake episode of Dec., 2004
found the sunset signature was shifted later by 9 minutes, a significant
change. One would think that, for a monitor to pick up these changes,
the epicenter of a large quake would need to fall on or near the line
between the transmitter and receiver. However, these researchers and
the transmitter were in India, a good ways from the epicenter.
Here are some additional references:
Fraser-Smith, A. C., A. Bernardi, P. R. McGill, M. E. Ladd,
R. A. Helliwell, and O. G. Villard, Jr., "Low-Frequency Magnetic Field
Measurements near the Epicenter of the Ms 7.1 Loma Prieta Earthquake,"
Geophys. Res. Letters, 17,1465-1468, 1990
Hayakawa, M, O.A. Molchanov, T. Ondoh, & E. Kawai, Precursory signature of the Kobe
earthquake on VLF subionospheric signal. J Atmos Electr, 16, p. 247,
Molchanov, O. A., and M. Hayakawa, "Subionospheric VLF
signal perturbations possibly related to earthquakes" J. Geophys. Res.,
vol. 103, p. 17 489, 1998.
Chakrabarti, S K, M Saha, R Khan, S Mandal, K Acharyya, R Saha.
"Unusual sunset terminator behavior of VLF signals at 17 kHz during
the Earthquake episode of Dec. 2004". URSI General Assembly, 2005.