Upper-mantle structure in the central United States from P- and S-wave spectra

The upper-mantle structure down to about 220 km in the central United States has been inferred from long-period P- and S-wave spectra of deep earthquakes recorded at three WWSSN stations, by using the re-formulated transfer ratio method. This method has been experimentally shown to be a very powerful means for elucidating the fine configuration of the low-velocity zone. The strong advantage of this method is its ability to determine separately the extent of the velocity decrease and the depth to the bottom of this zone, which is more uncertain by the other methods.From the Interior Plain to the Gulf of Mexico the low-velocity zone shifts to a shallower depth while increasing its thickness and decreasing its velocities. This zone is made up of an approx. 50 km thick layer ranging in depth from about 150 to 200 km under the Interior Plain, an approx. 75 km thick layer from about 120 to 195 km under the Gulf Coastal Plain, and an approx. 80 km thick layer from about 95 to 175 km under the continental shelf of the Gulf of Mexico, all nearly along 89°N longitude. The decrease in S-wave velocity at the top of this zone is about 0.30, 0.45, and 0.70 km/sec under each of the above areas, although the last value may be somewhat an overestimate. Both boundaries of this zone are sharp, the transition occurring over at most ca. 10 km. In this region the existence of the high-velocity lid zone is possible.