Sean-Thomas, John, and Angel--

Thanks all for the references and background information on P phases.  The Pb 
phase shows for regional events when I make a calculation of travel times on 
the USGS "Travel Times to Your Seismic Station" calculator 
(http://geohazards.cr.usgs.gov/tt/).  But, its position in relation to other 
arrivals does not agree with those calculated by Winquake.  For example, for 
a recent event in Ohio (389 km from my location) the USGS calculator shows Pn 
as first arrival and Pb is 7 seconds later.  When I used the regional tables 
in Winquake, the Pb and a P arrival were first at virtually the same time and 
Pn came in about a second later.  I guess I am now puzzled how Winquake 
calculates phase arrivals in this situation and why the differences between 
those determined by the USGS calculator.

Bob Laney
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