Geoffry,

It relates to the direction of the ground oscillations caused by different 
phases.  P waves, more or less, vibrate at right angles to S waves.  So, 
*for a particular quake*, if you point your sensor in the direction of the 
arriving P waves in order to see them the best, you would then have to turn 
it by 90 degrees to pick up the S waves most strongly, something like 
aiming a directional radio antenna.

With two or three-axis sensors the pointing can be done mathematically with 
a computer, working on the saved data, rather than by actually rotating the 
sensor, which allows you to easily estimate the direction of the source of 
the quake.

Brett

At 07:37 AM 9/28/2009 -0700, you wrote:
>It seems silly to me that you can differentiate between
>various types of disturbances in any way except through
>frequencies. Even P waves can present themselves
>with motion or deformation in almost any direction.
>We live at a phase boundary between air and earth
>where waves will meet a melodramatic velocity change.
>Where P can become S and vice versa.
>
>
>----- Original Message ----- From: "GPayton" 
>To: "PSN Network List" 
>Sent: Sunday, September 27, 2009 11:14 AM
>Subject: Seismometer Siting
>
>
>>Over and over, I have read that normally a seismometer is sited for N-S, 
>>E-W, and Vertical sensing.
>>However, I just read a paragraph in An Introduction to Seismology, 
>>Earthquakes and Earth Structure about a seismometer in Hawaii that "was 
>>oriented to receive S phase signals."  How is that accomplished?
>>I could venture a guess, but probably would be wrong.
>>Thanks,.
>>Jerry Payton
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