Abstract The structure and properties of mature upper oceanic plates may evolve through mechanisms such as magmatism, hydrothermal circulation, and faulting. However, high‐resolution constraints, especially those involving both P‐ and S‐waves, remain scarce, limiting our ability to detect these processes and assess their impacts on crustal properties. We present high‐resolution P‐ and S‐wave velocity models from traveltime tomography of downward‐continued long‐offset streamer data acquired along a margin‐parallel profile in the outer rise of the Sumatra subduction zone. The data reveal high‐quality, doubly converted S‐wave arrivals from the upper crust. Layer 2A (uppermost crust) exhibits high and laterally uniform Poisson’s ratios (0.3–0.35), whereas the underlying Layer 2B is more heterogeneous with lower Poisson’s ratios (0.26–0.33). We interpret Layer 2B heterogeneity as reflecting widespread deformation within the Wharton Basin. In contrast, the more uniform and elevated Poisson’s ratio in Layer 2A likely indicates the opening of cracks by plate bending in the outer rise.

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