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Deep‐crustal magma plumbing at arc volcanoes controls the volume, frequency, and composition of magma being transported to and stored in the upper crust. However, the mid‐to‐lower crust remains a challenging region to image. We explore the mid‐to‐lower crustal velocity structure beneath the Christiana‐Santorini‐Kolumbo Volcanic Field (CSKVF) to better understand how an established stratovolcano and flanking volcano (Santorini and Kolumbo) are fed through the mid‐to‐lower crust. We use active‐source seismic data to obtain a P‐wave velocity model of the crust below the CSKVF. We invert direct and reflected P phases to cover the entire depth extent of the crust and solve for the Moho interface depth. Our model requires a curved Moho interface representative of crustal thickening via underplating. Results show a highV_panomaly in the lower crust under Santorini and a mid‐crustal lowV_panomaly offset from both Santorini and Kolumbo. We find that accumulation of magma is located under the local extensional basin in the upper mid‐crust (<10 km) but is offset at deeper depths. We find evidence for melt storage at 11–13 km depth feeding volcanism at the Kolumbo volcanic chain. This melt is also a plausible source for the 2025 seismic swarm and dike intrusion. Resolution is limited in the mid‐crust below the Santorini caldera, leaving Santorini's mid‐crustal magma plumbing unconstrained. We think it likely that Santorini and Kolumbo have entirely separate crustal plumbing systems and mantle sources, but allow the possibility of a connection in the mid or lower crust. 

This study addresses a significant gap in understanding the features of the south‐central Cascadia subduction zone, a region characterized by complex geologic, tectonic, and seismic transitions both offshore and onshore. Unlike other segments along this margin, this area lacks a 3‐D velocity model to delineate its structural and geological features on a fine scale. To address this void, we developed a high‐resolution 3‐D P‐wave velocity model using active source seismic data from ship‐borne seismic shots recorded on temporary and permanent onshore seismic stations and ocean‐bottom seismometers. Our model shows velocity variations across the region with distinct velocity‐depth profiles for the Siletz, Franciscan, and Klamath terranes in the overlying plate. We identified seaward dipping high‐velocity static backstops associated with the Siletz and Klamath terranes, situated near the shoreline and further inland, respectively. Regions of reduced crustal velocity are associated with crustal faults. Moreover, there is significant along‐strike depth variation in the subducting slab, which is about 4 km deeper near the thick, dense Siletz terrane and becomes shallower near the predominantly less‐dense Franciscan terrane. This highlights a sudden tectonic and geologic transition at the southern boundary of the Siletz terrane. Our velocity model also indicates slightly increased hydration, though still minimal, in both the oceanic crust and the upper mantle of the subducting plate compared to other parts of the margin.