Using Vp$V_p$ and Vs$V_s$ to characterize the spatial extent of shear strengthening under a longleaf pine

Abstract Tree roots modify the elastic and mechanical properties of soil. High‐resolution seismic refraction surveys offer a novel, noninvasive approach to characterize how roots impact the elastic properties of soil beneath trees. This study presents two perpendicular profiles of P‐wave () and transversely polarized S‐wave () velocities beneath a 31 m tall longleaf pine in South Carolina. The data showed no significant anomalies beneath the tree. In contrast, data revealed a distinct high‐velocity zone, with increases of approximately 150–250 m/s, extending to a depth of 1.5 m and matching the width of the tree. This increase in produced low () values (<1.5) directly beneath the tree. These low () values suggest that the soil is more resistant to shearing while remaining compressible. The structure of the () values resembled the generalized shear failure planes described by Prandtl's wedge. This similarity supports the hypothesis that trees may opportunistically grow roots to reinforce soil along shear failure planes to maintain stability. These findings provide new insights into root–soil interactions and their role in soil stability, emphasizing the need for interdisciplinary research to test this hypothesis across diverse tree species and environments.