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The Cascadia Initiative (CI) Amphibious Array provides us an unprecedented opportunity to investigate seismologically the formation, alteration, and subduction of the downgoing Juan de Fuca and Gorda plates, as well as the fore‐arc and volcanic arc (Fig. 1). Formed at the spreading center, the oceanic crust undergoes normal faulting near the ridge axis. As the plate cools and moves away from the spreading center, the thermal stress and forces at the plate boundaries and the base of the plate may cause faulting and deformation in the plate interior. Near the subduction trench, the plate bends, forming cracks in the upper plate (e.g., Ranero et al., 2003). The faults and cracks provide pathways for fluid to penetrate deep into the crust and possibly mantle lithosphere (e.g., Nedimović et al., 2009), causing chemical reactions that alter the composition of the plate, including serpentinization of the mantle lithosphere (Melson and Thompson, 1971). As the plate subducts, fluid within the plate and in the entrained sediment is released progressively with increasing pressure and temperature, affecting the rheology and fault behavior at and near the plate interface and resulting in partial melting in the mantle wedge and volcanism along the arc (Ranero et al., 2003). To understand subduction zones, we must have a clear and complete picture of the fore‐arc structure and the incoming plate at the trench, which in turn requires a good understanding of the interior of the plate.