Comparing Estimates of Lithospheric Thickness from Sp Receiver Functions and Surface Wave Tomography in the Southwestern United States

The thickness of the lithosphere in the western United States has been found to be thinner than that of the stable continental interior of North America. Explanations for this thin lithosphere range from active tectonics to the presence of partial melt. Past studies of Sp receiver functions and tomography models in the western U.S., which broadly agree, have led to significant improvements in our understanding of mantle structure and related dynamic and tectonic processes, but significant uncertainty remains as the results have yet to be consistently integrated.

To better understand how constraints on lithospheric thickness vary between receiver functions and tomography, we first calculated Sp receiver functions using data from over 1000 broadband stations from more than 50 temporary and permanent networks, ranging from 31°N to 43°N and 112°W to 126°W. Using extended time multi-taper method deconvolution and the 1D velocity model AK135 for depth migration, we find the average depth of the lithosphere-asthenosphere boundary (LAB) phase across the region in our Sp receiver function study to be 65.3 km, but absolute depths vary from roughly 30 to 100 km.

Preliminary comparisons between our Sp receiver function results and the absolute shear velocity tomography model of Shen and Ritzwoller (2016) reveal that roughly 80% of the receiver function LAB depth picks fall within the absolute depth range of the negative velocity gradient, which we define as the potential LAB depth range from tomography. However, no correlation exists between receiver function LAB depth and other aspects of the tomography model, such as the depth of the maximum negative velocity gradient or the maximum or minimum velocity depths. Furthermore, no correlation appears with the magnitude of the tomographically inferred velocity gradient or gradient thickness and LAB amplitude from receiver functions. However, a potential (weak) correlation is seen between the receiver function LAB amplitude and magnitude of the difference between maximum and minimum velocities constrained from tomography, which appears to be largely controlled by the minimum velocity. Ongoing analysis is being done to better determine how Sp receiver functions and tomography models can be used together to further our understanding of the lithosphere-asthenosphere boundary.