Travel times of P waves to observatories at Tucson, Arizona, and at El Paso, Junction, and Lubbock, Texas, from earthquakes which occurred along the west coast of Mexico during 1963 through 1966 indicate that the low-velocity layer in the upper mantle beneath Mexico is at a depth of about 100 km and is about 70 km thick. The velocity in the layer is 7.5 km and the velocity contrast is greater than has been reported for adjacent areas to the north.

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