Travel-time curves and upper-mantle structure from long period S waves

Abstract

Long-period S-wave travel-time data, including second and later arrivals, are presented for distances of 3° to 65° for focal depths of 33 and 120 km. Onset times were determined on the basis of particle motion diagrams and the product of the horizontal radial and vertical components of motion. Because the recording stations principally were LRSM units, the travel times represent data for an “average” United States earth model. An S-wave velocity distribution calculated for the upper mantle provides a satisfactory fit to the empirical travel-time data for focal depths of both 33 and 120 km. The proposed model contains a pronounced low-velocity layer at a depth of about 150-200 km, and secondary low-velocity layers at depths of 340-370 km and of 670-710 km. In addition, there are regions of rapidly increasing velocity beginning at depths of about 400 and 750 km, and constant velocity zones at 220-350 km and 400-670 km.