Relative S-wave station residuals have been determined over the Canadian long-period seismograph network and have been augmented by S residuals to the south into the United States taken from Poupinet (1977). The thick craton of the Canadian Shield is outlined by the S-residual contours with an inferred rapid thinning of the craton to the south except for a narrow central extension into Kentucky and Missouri. A comparison of these S residuals with P-wave residuals of Buchbinder and Poupinet (1977) reveals a low-velocity region in the west, a high-velocity zone centrally, and an area in the east where the S arrivals are late while the P arrivals are early.

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