The (de)hydration process and the amount of hydrated sediment carried by the downgoing oceanic plate play a key role in the subduction dynamics. The deformation and (de)hydration of the downgoing tectonic plates, as well as the seismic, tsunami, volcanic hazards, in Cascadia and the New Zealand regions are not fully understood, partly due to a lack of combined studies of onshore and offshore data. In order to address these questions, we developed a 3-D high-resolution shear wave velocity model beneath Cascadia, the North and the South Islands of New Zealand, extending from offshore to onshore, with the use of full-wave ambient noise tomographic method and joint inversion with active seismic data. We extracted the empirical Green’s functions from continuous seismic records on the vertical components between each station pair that provide high-quality Rayleigh-wave signals at periods of 4-50 s for Cascadia and 5-150 s for the New Zealand. We simulate wave propagation using finite-difference method to generate station Strain Green’s Tensors and synthetic waveforms. The phase delays of Rayleigh waves between the observed and synthetic data are measured at multiple period ranges. We then invert for the velocity perturbations from the reference model and progressively improve the model resolution. Our tomographic imaging shows many regional- and local-scale low-velocity features, which are possibly related to slab (de)hydration from the oceanic plate to the overriding plate. Moreover, seismic lows atop the plate interface beneath the forearc, indicating fluid-rich sediments subducted and overthrusted at the accretionary wedge. Furthermore, high shear wave velocity patches at the depths of 35-100 km beneath South Island of New Zealand, indicating the Pacific slab being segmented due to stretch during initiation of plate margins and due to current plate motions. Finally, the relatively low shear wave velocities at 100- 200 km depth beneath the South Island of New Zealand, indicating possible asthenospheric upwelling. Our full-wave tomographic models provide new constraints on our understanding of plate deformation, (de)hydration, slab segmentation, and asthenospheric upwelling beneath Cascadia and the New Zealand regions.