Randy,

That seems totally consistent with the fact that earthquake waves do 
not radiate in circles or spheres, like ripples in a pond (as I think 
the Calculator assumes) but rather somewhat more like beams, usually 
in four directions, depending on the orientation and direction of 
motion of the fault.  Of course body waves get all messed up as they 
bounce around, but they still have significant variations in 
intensity related to your direction from the epicenter.

I seem to recall seeing some statistical EQ records that suggested 
that, depending on your direction, there might be as much as a 10:1 
difference in observed amplitudes at similar distances.

Brett




At 10:23 PM 4/18/2012, you wrote:
>Hi,
>
>I have used the USGS Arrival Time Calculator to compare the 
>amplitudes of the past week to what I measure.  The 1 second 
>amplitude I measure ranges from almost perfect agreement on some 
>events to about 1.6 times greater on others and as bad as 1/7th 
>predicted on others.  I use a 1 second high and low pass filter and 
>take the amplitude of the P grouping.  I have not included the 
>events in the shadow zone or events where I have a PKP and not the 
>P.  I understand this for a horizontal but I am using a vertical so 
>direction should not factor in.  Is this a normal range or do I need 
>to look for an instrument issue?
>
>Randy
>


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