Constraining incoming-plate upper mantle hydration at Cascadia, Alaska, and Nankai: distinguishing mineral-bound and free water using Vp and Vs

Seismic velocity is widely used to estimate water stored in subducting upper mantle, but the same velocity reduction can result from very different combinations of serpentinization and fluid-filled porosity. We develop a rock-physics framework that treats serpentinization as modification of the solid matrix and crack porosity as a separate fluid reservoir, while explicitly accounting for uncertainty in crack geometry. For single Vp or Vs observations, absolute water contents remain strongly nonunique, whereas relative velocity differences provide more robust hydration contrasts. When Vp and Vs are available together, their combined behavior restricts the crack geometries that can explain the observations, providing stronger constraints on water partitioning. Applications to Cascadia, Alaska, and Nankai reveal substantial differences in how water is stored in the incoming mantle: propagator wakes in Cascadia are more hydrated than surrounding mantle; Semidi permits appreciable serpentinization, whereas Shumagin’s very low velocities require little serpentinization and only trace water in very thin cracks; and Nankai data are consistent with appreciable free water in thicker fractures with only moderate serpentinization. These results show that low mantle velocity does not uniquely imply high water content and that distinguishing structurally bound from crack-hosted water is important for assessing water transport into subduction zones.