I think 10 seconds difference is not bad at all.  There are lots of 
sources for small errors.

The first is that the earthquake location and depth must be known 
accurately, and the first numbers put out are often not too 
accurate.  For significant quakes the locations and depths will get 
revised as more thorough analyses of many observations are processed 
and combined.

Then, probably more important, the wave-velocity models used in 
WinQuake, IASP91 and Jeffreys-Bullen, are 1-D models, which assume 
that the properties of the earth only vary with depth, being the same 
everywhere over the earth's surface, which is not a totally accurate 
assumption.  http://www.iris.edu/dms/products/emc/models/IASP91.htm
Though these models give surprisingly good results, the true timing 
will depend on what the earth is really like, between you and the quake.

Work has been going on to produce a 3-D velocity model of the earth, 
called the Reference Earth Model.  In this one, different places on 
the earth have differing velocity profiles with depth.  You can see 
one of the models which may be used to help create the REM, 
here  http://igppweb.ucsd.edu/~gabi/rem2.dir/mod.s362d1.gif
in which variations in shear wave velocity are mapped at different depths.
Eventually travel times may be able to be calculated more accurately 
using a 3-D model, but I wonder if the computations might challenge 
the home computers we use today.

Regarding P & S waves, I haven't noticed that P & S generally start 
in phase.  I see that there have been papers published about how to 
use the initial P & S motions to estimate the nature of the fault 
motion which caused the quake.  That says to me that how they behave 
will depend on the quake.

Brett



At 10:17 PM 1/5/2013, you wrote:
>On 1/5/2013 8:56 PM, Dave Nelson wrote:
>>
>>your wish was granted anyway  regardless of it you wanted it or not
>Yes and  it came at a good time....kids gone = a quiet 
>house...during the night and during and an even quieter period (no 
>little quakes getting in the way)....Now I am sitting here looking 
>at the data and some elementary questions rise up; like just where 
>do you set the arrival of P & S, and is P and S always in phase on a 
>vertical or not....and I am seeing a difference of maybe 10 seconds 
>on the supposed/actual arrival time of P and S according to WinQuake


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