Seismic Attenuation and Velocity Measurements of the Uppermost Mantle Beneath the Central and Eastern United States and Implications for the Temperature of the North American Lithosphere

摘要

We constructed a detailed map of the uppermost mantle seismic structure beneath the Central and Eastern United States (CEUS) using the International Seismological Centre Bulletin and seismic waveforms from USArray. Identical Pn and Sn travel time data sets were inverted to tomographically image the uppermost mantle P wave (Pn) velocity, S wave (Sn) velocity, and the velocity ratio (V-Pn/V-Sn). Furthermore, we modified the two-station method in order to limit the contributions of site response and estimate the effective attenuation of Sn phase (Q(-1)Sn). Lower Q values generally correspond with lower velocities in terms of both Pn and Sn wave speeds (e.g., New England and the Mississippi Embayment), but some regions, including southern Georgia, eastern South Carolina, and the New Madrid Seismic Zone, show high velocity and low Q(Sn) values. This can be explained by scattering attenuation of the Sn phase. To estimate the uppermost mantle temperature, a constrained grid-search algorithm was conducted using the observed V-Sn, V-Pn, and Q(Sn) with the calculated velocities of specific compositional models. The uppermost mantle temperature result shows similar to 300-500 degrees C beneath the northern midcontinent and 1100 degrees C beneath New England. Although our temperature results appear to be well resolved, we found that V-Pn, V-Sn, and Q(Sn) are not enough to constrain the detailed uppermost mantle composition model. Our results highlight significant temperature heterogeneity in the uppermost mantle across the CEUS and is consistent with there not being any melt within the uppermost mantle beneath the CEUS.