Evidence from recent earthquakes and explosions is presented showing substantial regional variations in the velocity of Pn in the United States. These variations have a significant effect on the location of epicenters by the least-squares method. We have included a contour map of Pn velocity in the United States. A new method for computer location of epicenters is described, and is shown to decrease the error of location for events in the western United States by as much as an order of magnitude.

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