Pn tomography of the western United States using USArray

摘要

USArray has now provided several years of high-quality seismic data and improved ray coverage for much of the western United States, which will enable increased resolution for studies of the lithospheric and deeper structure of the North American continent.Here we analyze Pn arrival times from the transportable stations of USArray to resolve crustal thickness and uppermost mantle structure. We use 123,008 Pn picks from April 2004 to October 2009 as measured by the Array Network Facility at epicentral distances from 180 to 1450 km. These picks are derived from 778 stations at ～70 km spacing and 7903 earthquakes and quarry blasts. Applying the classic time-term method, we use a regularized least squares inversion to estimate crustal thickness variations and image velocity perturbations in the uppermost mantle just below the Moho. We also consider upper mantle anisotropy and describe the velocity perturbations with a cos 2φ azimuthal variation. Our crustal thickness map generally agrees with receiver function results from other researchers but differs in some details. We obtain an average upper mantle velocity of 7.93 km/s, with higher velocities beneath eastern Washington and northern Idaho, and lower velocities near the California-Mexico border, the Sierra Nevada, the northern coastal California region, and the greater Yellowstone area. We observe large anisotropic anomalies in southern California as well as in the Snake River Plain area. These results should complement other seismic studies (e.g., body and surface wave tomography and shear wave splitting) to provide information about composition, temperature, and tectonic processes in the western United States.