Petra:

Good point(s).

The link indicates earthquakes making sounds up into the 30 hz range which is the lower level for the "hum".

Hey...what if the 'sound' "heard" from an earthquake isn't really a 'real sound' after all (as discussed in the past)? Just a thought....

-----------------------------------see link
Earthquake
Typically, an earthquake produces a triangular-shaped signal on a spectrogram, (with the highest acoustic energy yellows and reds), occurring in the 5 to 30 Hz frequency range.

Click here (285 kb, 10x) to hear the sound produced by this earthquake. (Note: the bird like sounds that you hear in the background are actually humpback calls at ten times their normal speed!)

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• Earthquake sound at 10 x speed