We have developed IASP91 shear wave propagation path correction (PPC) surfaces for use
with the EvLoc [Bratt and Bache, 1988; Nagy, 1996] location algorithm. Using the shear wave
data set from Figure 2 (EDR and LANL Regional databases), PPCs were constructed from
travel time residuals for events that met the following:
 30 or more defining phases
 Azimuthal gap less than 180°
  Residual  ¾ 10 seconds
Corrections surfaces were constructed using modified Bayesian kriging [Schultz, et. al., 1998]
which provides wellbehaved interpolants, but requires adequate error estimates
associated with the travel time residuals. Shear wave PPCs were developed for 72 stations
from 667 events which had S phases meeting the above criteria.
Figure 3  Events with S phases which met the criteria for generating shear wave PPC surfaces. 
