The Structure of the Upper Mantle under the United States from the Dispersion of Rayleigh Waves

Abstract

Summary Seismograms from vertical-component, long-period instruments of the World-Wide Standard Seismographic Network (WWSSN) within the United States have been analysed to determine the dispersion of Rayleigh waves by the two-station method. Phase velocities from fundamental mode Rayleigh waves over the period range 20–250s have been obtained to study the structure of the upper mantle of the continent. The phase velocities show systematic variations in specific sub-regions of the United States. The phase velocities observed along five representative paths have been inverted into sub-surface structural cross-sections by the Hedgehog method. The results of the inversion indicate that there are significant regional variations in S-wave velocities, particularly from the Moho down to about 400km. We make no statement about lateral variations in structure at greater depths. The aseismic south-central part of the continent and the tectonically active western Cordillera are characterized by a well-developed low-velocity channel for S waves in the upper mantle. The north-central United States has dispersion data which are consistent with structures which either have a low velocity channel of marginal properties or no low velocity channel at all; if a channel is present for this region, the velocity contrast with the layers of the mantle above and below it is very small.