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In subduction zones, along‐strike and downdip variations in megathrust slip behavior are linked to changes in properties of the subducting and overriding plates. Although marine geophysical methods provide insights into subduction zone structures, most surveys consist of sparse 2D profiles, limiting our understanding of first‐order controls. Here, we use active‐source seismic data to derive a 3D crustal‐scale P‐wave velocity model of the Alaska Peninsula subduction zone that encompasses both plates and spans the Semidi segment and SW Kodiak asperity. Our results reveal modest variations within the incoming plate, attributed to a series of fracture zones, seamounts and their associated basement swell, collectively contributing to plate hydration. Basement swell appears to modulate the distribution and type of sediment entering the trench, likely impacting observed variations in slip behavior. The overriding plate exhibits significant heterogeneity. The updip limit and width of the dynamic backstop are similar between the SW Kodiak asperity and eastern Semidi segment, but differ significantly from the Western Semidi segment. These distinctions likely account for differences in earthquake rupture patterns and interseismic coupling among these segments. Additionally, high‐velocities in the mid‐lower forearc crust coincide with the location of megathrust slip during the Mw 8.2 2021 Chignik event. We interpret these velocities as intracrustal intrusions that contributed to the deep rupture of the 2021 event. Our findings suggest that the contrasting structural and material properties of both the incoming and overriding plates influence the spatially complex and semi‐persistent segmentation of the megathrust offshore the Alaska Peninsula. Plain Language Summary: Differences in styles and timing of earthquakes in subduction zones are often attributed to different geometry and properties of the plate boundary fault or megathrust, including the composition and properties of the surrounding rocks. Marine geophysical data allow us to determine structural and material differences within the subducting and overriding plates that contribute to these differences, but are typically limited to two dimensions and insufficient to properly characterize the transitions between segments with different rupture patterns and plate locking. We use recently acquired active source seismic data to create a 3D model of compressional wave velocity that encompasses both plates of the Alaska Peninsula subduction zones and spans the eastern and western Semidi segments and the SW Kodiak asperity. Our results reveal links between large structures and hydration state of the subducting plate, differences in the architecture and composition of rocks belonging to the overriding plate, and observed differences in slip behavior both along and perpendicular to the margin. These results underscore the importance of using 3D data sets to extract the combination of structures and properties of both plates that contribute to observed megathrust segmentation. Key Points: Our 3D Vp model reveals long‐wavelength crustal‐scale variations in the subducting Pacific plate and overriding North American plateSubducting seamounts and swell, and fracture zones modulate sediment distribution, plate hydration and influence changes in slip behaviorVariations in backstop architecture and composition contribute to variable megathrust slip behavior both along‐strike and downdip [ABSTRACT FROM AUTHOR]

Laboratory experiments report that detectable seismic velocity changes should occur in the vicinity of fault zones prior to earthquakes. However, operating permanent active seismic sources to monitor natural faults at seismogenic depth is found to be nearly impossible to achieve. We show that seismic noise generated by vehicle traffic, and especially heavy freight trains, can be turned into a powerful repetitive seismic source to continuously probe the Earth's crust at a few kilometers depth. Results of an exploratory seismic experiment in Southern California demonstrate that correlations of train‐generated seismic signals allow daily reconstruction of direct P body waves probing the San Jacinto Fault down to 4‐km depth. This new approach may facilitate monitoring most of the San Andreas Fault system using the railway and highway network of California. Plain Language Summary: Even though laboratory experiments report that they should be preceded by detectable precursors, earthquakes remain unpredictable. Indeed, contrary to the lab, scanning natural faults at a few kilometers depth where earthquakes initiate requires operating high‐energy seismic sources continuously in time, which is found to be nearly impossible. In this study, we show that large freight trains generate sufficient seismic energy to travel down to a few kilometers depth and be detected at tens of kilometers from railways. We demonstrate that we can turn this apparently random source of seismic signal into an impulsive virtual seismic source to monitor active faults. We finally estimate that this new approach can be used for monitoring most of the San Andreas Fault system using the railway and highway network of California. Key Points: Freight trains in Southern California are locally equivalent to a magnitude 2.2 earthquake every dayWe use train noise to reconstruct repetitive virtual sources of P waves crossing the San Jacinto Fault at 4‐km depthPredictions of train noise across California show potential for passive monitoring of most of the San Andreas Fault system [ABSTRACT FROM AUTHOR]