Steinar---I am not sure what arithmetic would come out at 2cm motion!!!
If so you would be heading for open space rather than watching your 
pendulum.  Yes. your whole system was moving about, especially the vertical 
member supporting the mass--and you may well have seen motion on the boom.

Jim
----- Original Message ----- 
From: "Steinar Midtskogen" 
To: 
Sent: Sunday, March 13, 2011 2:35 AM
Subject: Re: Lehman seismometer movement


> "Jim and Connie Lehman"  writes:
>
>> Steinar--an interesting observation at 8200 km--Let's chalk it up to
>> half plausible, and half imagination.  This event is a powerful
>> exhibit of energy, and it will be interesting to see reports of motion
>> exhibited or detected by conditions other than seismographs.
>
> By extrapolating http://jclahr.com/science/psn/magnitude.html we
> should expect about 2 cm (!) ground movement at this distance, but I
> expect that this would be longer periods than what my Lehman was
> designed for.
>
> While my observation might be plausible I'm actually more impressed
> with what I saw on my 4.5 Hz geophones.  Here's what I get if I apply
> a 1 Hz lowpass filter to the N-S geophone:
>
> http://voksenlia.net/nytt/2011/jordskjelv-20110311.jpg
>
> And here's the raw unfiltered seismometer plot:
>
> http://voksenlia.net/nytt/2011/jordskjelv-20110311b.jpg
>
> It's badly clipped, so it's less useful to study.
>
> -- 
> Steinar
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