Evidence for a Sharp, Negative Velocity Gradient at the Lithosphere Asthenosphere Boundary From Array Analysis of Ambient Noise Autocorrelograms Near Sweetwater, Texas

We use array analysis of auto‐ and cross‐correlations of ambient noise recorded by closely‐spaced station pairs in a dense array near Sweetwater, Texas to image the P‐wave reflectivity of Earth's upper mantle structure. Imaging was performed with 18 linear arrays, each comprised of 80–100 Fairfield Nodal vertical‐component instruments that recorded seismic noise continuously for 8–11 days in March 2014. A coherent signal at ∼26 s two‐way time (TWT) appears consistently across all profiles, including in separate analyses of daytime and nighttime records. This event persists even in gathers formed from single‐day recordings without any reported teleseismic events of magnitude ≥5.5, suggesting that teleseismic events are not the sole source of illumination. Analysis of the H/V ratio from a co‐located three‐component broadband station shows persistently low values over the deployment period, consistent with a wavefield that is dominated by body waves rather than surface waves. These conditions were likely critical to the successful extraction of the reflection response. We interpret the event at ∼26 s TWT, following a forward modeling exercise, as a PpP‐type reflection response from the Lithosphere‐Asthenosphere Boundary (LAB) at ∼90 km depth. We support this interpretation using teleseismic P‐wave coda autocorrelograms computed for nearby broadband stations. The presence of this signal in a relatively high frequency band 0.4–4.0 Hz with a positive polarity in autocorrelograms indicates a steep, negative velocity gradient at the LAB.