Kay,

My first assumption was that they were referring to the whole-earth 
vibration modes, having periods from several minutes to roughly an 
hour, though I'd expect that you would usually see them after a big 
quake, not before.

Brett



At 10:26 PM 9/5/2010, you wrote:
>Mark,
>
>Can you explain the following sentence from your email...
>
>"Eigenmode activity was observed sporadically over the week leading up to the
>main quake."
>
>Sorry for my ignorance.  Thanks muchly!
>
>Kay


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