COMPARISON OF P-STATION AND S-STATION CORRECTIONS AND THEIR RELATIONSHIP TO UPPER-MANTLE STRUCTURE

We use International Seismological Centre data between the years 1964 and 1994 to derive a data set of P and S station corrections. By restr icting the raw P and S data to those which share the same source to re ceiver paths, we minimize any bias to the station corrections from dif ferential P and S data distributions. We find that both static and azi muth dependent terms are highly correlated. In addition, we perform sy nthetic tests to determine the sensitivity of our data to mantle struc ture and show that static terms are sensitive to structure, at least t hroughout the upper mantle. In determining the regression slope of the static terms, we allow for regional offsets and find a best-fit globa l slope of S to P correction a = 2.85 +/- 0.19. This is lower than tha t obtained in previous station correction studies but is in agreement with results in the upper mantle using differential travel times and w ith values derived for the upper parts of the lower mantle. Regionaliz ing the data by tectonics does not give statistically distinct ratios of S to P station correction. The result for shield areas, however, is significantly smaller than previously believed, probably because of o ur ability to derive station corrections from similar data sets. Varia tions in the second azimuthal terms are similar in both fast direction and relative magnitude. In another synthetic experiment we find that their general trends can be predicted well by isotropic models of hete rogeneous velocity structure. From this result and also because they d o not correlate with SKS splitting data, we conclude that station corr ection second azimuthal terms should not be interpreted in terms of an isotropy.