There are 6 ULF antennas - these things are very large and there is also interference to consider. (Below 3 Hz)

Earth quake Tracker is tying to monitor ELF - Extremely Low Frequencies. (3Hz-3000Hz)

Anthony Frasier-Smith is the first one to note this prior to the Northridge quake - his web site is:
http://www-star.stanford.edu/~acfs/

He wrote a couple of papers on it:
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.
and
Bernardi, A., A. C. Fraser-Smith, P. R. McGill, and O. G. Villard, Jr., "ULF Magnetic Field
Measurements near the Epicenter of the Ms 7.1 Loma Prieta Earthquake," Phys. of the Earth Planet. Interiors, 68,45-63, 1991.

More info can be found at:
http://www-star.stanford.edu/~ttliu/ - Thomas Tao-Ming Liu - worked with Frasier-Smith
and here:
http://www-star.stanford.edu/ - Star Laboratory

Here's another good article I stumbled upon about the subject: http://www.spie.org/web/oer/august/aug97/earthquakes.html

Canie