Hello Steinar,

Your question is central to seismometry and to be able to answer it you need to calibrate your seismometer.  Once you do that you can convert volts or counts to real ground motion.  The basic idea is simple, you move your sensor a known distance and see what you get in volts or counts at the end of your digitizer.  If you do that at a bunch of frequencies you can then figure where you sensor is linear and where it is not.  The you can derive the transfer function for your system and then you are set to answer your question.

Modern systems can resolve nano meters (billionth of a meter) the diameter of a molecule.

here are some values for ground motion in microns (millions of meter)

Dist(km)   ML=0	ML=3		ML=6 
100 	0.000428 	0.428486 	428.4860279 
200 	0.000144 	0.1441606 	144.1606322 
300 	4.27E-05 	0.0427143 	42.71426138 
400 	1.8E-05 	0.0180201 	18.02007902 
500 	9.23E-06	 0.0092263 	9.226280458 
600 	5.34E-06 	0.0053393 	5.339282672 

Happy New Year

Angel




Tuesday, December 30, 2008, 8:34:41 PM, you wrote:

> I wonder, how big are the movements produced by distant quakes?  Since
> only the waves with very long periods are detectable, the shaking is
> of course impossible to feel since even movements of several centimeters
> would be difficult to feel if the period is around 20 seconds.

> So, if my seismometer picks up a 6.0 quake 15.000 km away, what kind
> of movement are we talking about?

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