Jen:

There appears to be evidence that the precursor tones are worldwide.

In other words, you could hear a tone which may in fact Possibly be associated with ultra low wave energy which somehow causes EMF variations along the earth's crustal plates.

IMO, you should record every tone. A correlation study may be able to determine what correlations exist as to magnitude and range of earthqaukes which are part of the event(s).

One test you may consider is upon hearing a tone turn your head by 90 degrees to see if it goes away. This tests whether or not there is polarization in the signal (angular resonance). Also do the 90 degree turn in an up and down direction (in case it is directionally inbound from below you rather than from the side).

Who knows if hundreds get involved...maybe the distance separation of all who heard the same tone will scale to magnitude of the earthquake. There are other possibilities as well.

Have fun,

mark