WESTERN UNITED-STATES UPPER-MANTLE STRUCTURE

Regional three-dimensional inversions of upper mantle P wave velocity structure are created from teleseismic P wave travel time residuals recorded by many of the high-density high-frequency regional arrays operated within the western United States. These inversions are adjusted to a global (International Seismic Centre) reference and merged to obtain an image of the upper mantle beneath western United States. The P wave velocities in the upper mantle am slow on average, and the structure is very heterogeneous. Where resolution is good, coherent upper mantle structures are imaged that extend as deep as approximately 200 km (the Juan de Fuca and Gorda slabs, which penetrate to greater depths, are exceptions) and deviate from the average velocity (at a given depth) by as much as +/-4%. Lateral resolution of these structures usually is very good, although the magnitude of the actual seismic variations is probably greater than that imaged. The long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200-500 km. Within approximately 250 km of the Pacific Coast, these structures have a wavelength of approximately 250 km and trend parallel to the surface physiography and young tectonic structures Beneath the continental interior, where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage, structures have a wavelength of approximately 500 km and trend northeasterly. This northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity.