Here variations in crustal structure below the Basin and Range, Colorado Plateau, and Rocky Mountain provinces are revealed through mapping depths of compressional-to-shear wave conversions recorded by three-component broadband seismometers. This study combines data from several past passive source earthquake seismic experiments deployed in the western United States to image an extended region of crustal structure and determine how features vary regionally.

Using arrival times of seismic converted waves, and assuming a velocity model, we are able to image lateral crustal thickness variations. The thinnest crust in our study area is below the northern Basin and Range where the crust thins to as little as 30 km. The crust is 5-9 km thicker in the southern Basin and Range. Below the Rocky Mountains, we find 50 km thick crust. We also detect an arrival of a converted phase from a mid-crustal feature, possibly resulting from the Conrad discontinuity. The Conrad discontinuity is thought to mark the boundary between the felsic upper crust and more mafic lower crust. The lateral extent of this feature appears to be limited as we only observe mid-crustal arrivals from data recorded in northern Colorado. A localized shallow mid-crustal arrival near 20 km deep is also present below the Wasatch Front, where the crust is 30-40 km thick. The cause of this feature remains unclear. Below the Colorado Plateau, the crust appears be to 40 km thick and not vary greatly.

By comparing arrival times of both the direct compressional-to-shear wave arrival and arrivals of reverberations we are able to estimate the V_P/V_S ratio of the crust and can examine how these values vary geographically. The V_P/V_S ratio can be used to estimate Poisson’s ratio, an elastic parameter related to the composition of the crust. Knowledge of Poisson’s ratio helps determine the composition of crustal rocks. Comparing variations in Poisson’s ratio with crustal thickness information will help constrain possible mechanisms contributing to crustal thickness variations and surface topography. We find that the crust below the Colorado Plateau is characterized by a higher V_P/V_S ratio than that of the Basin and Range. This corroborates previous results suggestive of a stronger, more mafic crust below the plateau.