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The Southwestern United States experiences active deformation, seismicity, and magmatism, remarkable in an intraplate setting. The Basin and Range and Colorado Plateau (CP) are inferred to differ in lithospheric thickness, but modeling geophysical properties of the lithosphere, in particular the depth of the Lithosphere‐Asthenosphere Boundary (LAB), across the entirety of the region, has proved challenging. Here, we introduce a new model of 1‐D depth profiles in shear wavespeed, determined through a probabilistic joint inversion of information from Sp receiver functions and Rayleigh wave phase velocity. From these profiles we quantify the locations and Vs contrast of wavespeed gradients that represent boundaries such as the Moho, the LAB, and intralithospheric discontinuities. We infer a lithosphere that is thinner and lower in Vs in the Basin and Range. In the CP and farther north, the LAB is more gradual, deeper, and intermittently observed. We also observe Mid‐Lithospheric Discontinuities (MLDs) near the boundaries between the CP, Wyoming Craton, and Northern Basin and Range, as well as within the Craton. When both an MLD and LAB are observed, the Vs gradient associated with the LAB is narrower than expected. Finally, we image Positive Velocity Gradients beneath areas of thinner lithosphere, which are consistent with recent global observations that have been attributed to the base of a partially molten zone below the lithosphere. Overall, the picture of the lithosphere‐asthenosphere system that emerges is one of considerable structural complexity with a strong dependency on tectonic regime and geological history. Plain Language Summary: We examine the properties of, and processes that shape, the lithosphere—the rigid outermost layer of the Earth, which participates in plate tectonics. Our focus is the southwestern United States where in fact the lithosphere does not behave like a simple, homogeneous, plate. Instead, in the Basin and Range Province, widespread deformation, frequent earthquakes, and recent volcanic activity are observed, all of which are rare for a location removed from plate boundaries. In the neighboring Colorado Plateau and the regions north and west of it, the lithosphere behaves more rigidly but volcanism is also observed. To understand how the lithosphere and mantle below vary, we produce a model of seismic wavespeed using data from surface waves, which travel along the Earth's surface, and converted body waves, which interact with interior boundaries such as the bottom of the lithosphere. This model enables us to find the depths of major boundaries and understand how the speed of seismic waves changes across them. We consistently observe decreases in seismic wavespeed that we associate with the bottom of the lithosphere or structures within the lithosphere. We also note wavespeed increases below the lithosphere that might indicate partial melting of rocks deep within the Earth. Key Points: We perform a joint inversion for Vs in the upper mantle of the Southwestern U.S., using data from body wave scattering and surface wavesThe lithosphere varies regionally: it is flat and thin in the Basin and Range, thicker in the Colorado Plateau, and complex in cratonic regionsMid‐Lithospheric Discontinuities are observed in cratonic regions, associated with sharp Vs gradients at the base of the lithosphere [ABSTRACT FROM AUTHOR]

Seismic deployments in the Alaska subduction zone provide dense sampling of the seismic wavefield that constrains thermal structure and subduction geometry. We measure P and S attenuation from pairwise amplitude and phase spectral ratios for teleseismic body waves at 206 stations from regional and short‐term arrays. Parallel teleseismic travel‐time measurements provide information on seismic velocities at the same scale. These data show consistently low attenuation over the forearc of subduction systems and high attenuation over the arc and backarc, similar to local‐earthquake attenuation studies but at 10× lower frequencies. The pattern is seen both across the area of normal Pacific subduction in Cook Inlet, and across the Wrangell Volcanic Field where subduction has been debated. These observations confirm subduction‐dominated thermal regime beneath the latter. Travel times show evidence for subducting lithosphere much deeper than seismicity, while attenuation measurements appear mostly reflective of mantle temperature less than 150 km deep, depths where the mantle is closest to its solidus and where subduction‐related melting may take place. Travel times show strong delays over thick sedimentary basins. Attenuation signals show no evidence of absorption by basins, although some basins show signals anomalously rich in high‐frequency energy, with consequent negative apparent attenuation. Outside of basins, these data are consistent with mantle attenuation in the upper 220 km that is quantitatively similar to observations from surface waves and local‐earthquake body waves. Differences between P and S attenuation suggest primarily shear‐modulus relaxation. Overall the attenuation measurements show consistent, coherent subduction‐related structure, complementary to travel times. Plain Language Summary: Seismic waves lose more energy passing through hot and partly molten volumes than cold regions. As a result, measurements of variation in their amplitudes, or attenuation, provides a tool for mapping out the upper mantle, complementing more traditional measurements of their variation in travel time. New high‐quality arrays across southern Alaska, along with recent methodological developments, now allow this measurement to be made systematically across the entire region. They show consistently low attenuation where subducting plates are near the surface or along paths that follow the cold subducting plates. These regions are in southernmost Alaska. By contrast, signals traveling beneath volcanic regions or north of them, where hot mantle flows toward subduction zones, show high attenuation. The attenuation patterns resemble those from travel time, but seem to show more sensitivity to the upper 150 km of the Earth while travel time delays more uniformly sample deeper. Sedimentary basins show confusing signals, with travel time delays as expected for low‐wavespeed sediments, but high amplitudes that are difficult to explain. These signals allow quantitative mapping of temperature and melt variations in the upper mantle, even in regions as complex as subduction zones where both properties vary rapidly over short distances. Key Points: Body‐wave attenuation measurements reveal upper mantle structure, including subducting plates and the Alaskan sub‐/back‐arcThe Wrangell Volcanic Field is underlain by Yakutat subduction, but geometry is complexSedimentary basins unexpectedly produce negative apparent attenuation in body‐wave spectra [ABSTRACT FROM AUTHOR]