Sn velocities in western and eastern North America

We have investigated the use of the new delay time data set of Engdahl et al. [1997] for the construction of tomographic images of Sn velocity in North America. The delays show a significant difference in average delay between paths located west and east of 105 degrees W. This difference is larger when paths longer than 10 degrees are included, indicating a positive gradient for the S velocity in eastern North America. Discarding outliers, two separate regions in the western and eastern United States and Canada where imaged. Low velocities were found under the Colorado Plateau, extending westwards towards the Gulf of California. In the eastern U.S. the Michigan Basin stands out with low Sn velocity, and the Grenville province is characterized by a positive anomaly.