Hi Thomas,

Very probably, what you are seeing is microseisms.  I have also been 
seeing this activity on my horizontal SG sensor for the last half a day 
or so, and they also are showing up strongly on the web site run by 
Lamont-Doherty Earth Observatory.

My understanding is that the mechanism that produces these waves isn't 
completely understood, but that they are believed to be caused by ocean 
waves resulting from strong storms.  Being able to see them is a good 
thing.  It's a sign that your sensor is working well.

Larry Conklin
Liverpool, NY

Thomas Dick wrote:
> Here in southern Indiana I have seen peaks in the area of 
> .233Hz/4.338period for about twenty four hours. It is most pronounced on 
> the E-W Lehman but even visible on the AS-1. Weather is quiet right now: 
> little wind or fronts around -- even foggy. Has anyone else noticed this 
> on your units?
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