The Shumagin Geomag at Sand Point, Popof, AK (Near Kodiak) D-field registered an extremely sharp 90 nT (Appx.) drop at 0015 UTC 09182007. Sharp drops and spikes similar to this appear to lead EQ's in the appx. area by 12 hours (appx.).
The method to estimate the location and magnitude is looking at the signals, comparing readings to the usual relative strength of normal field action in that area, then using the scaled signal to make an estimation of the location by triangulation.
Since there isn't a huge network of magnetometers throughout the US, it's kind of like triangulating a cell phone signal from sites perhaps thousands of miles away.

• Shumagin