Subduction Zone Interface Structure beneath Kodiak Island, Alaska: Constraints from Receiver Functions Across a Spatially Dense Node Array

Kodiak Island is an exposed part of an accretionary complex along the Alaska-Aleutian subduction zone that formed as the Pacific plate subducted below the North American plate. Subduction of the Pacific plate beneath Alaska has produced more >M8 earthquakes than any other plate boundary system during the last ~100 yrs, including the 1964 M9.2 Great Alaska earthquake. Kodiak Island lies on a section of the subduction zone that ruptured in the 1964 event and experiences seismic tremor, suggesting multiple modes of plate interface slip. Unconsolidated sediment thickness and fluid distribution along the subduction zone interface are some of the factors thought to affect slip stability. We use Ps receiver functions to identify potential low seismic velocity anomalies and estimate Vp/Vs within the oceanic crust and the overlying plate to constrain metamorphism and fluid pressure. We test the hypothesis: Two distinct layers will compose the plate interface beneath Kodiak Island, a shallower low-velocity layer related to sediment subduction, and an oceanic layer that has higher velocity than the sediment but lower velocity than the underlying oceanic mantle. In May 2019, we deployed 398 nodal geophones on Kodiak Island as part of the Alaska Amphibious Community Seismic Experiment. The dense lines consisted of nodes at approximately 200-m spacing adjacent to the ~40 km-long Chiniak Highway system across northeastern Kodiak Island. The full array recorded continuously for about 25 days. Receiver functions show a west dipping positive conversion at ~40 km depth that represents the Pacific plate subducted slab Moho. We also observe negative conversions at ~30 km consistent with computed synthetic receiver functions which produce negative conversions at around the same depth when low-velocity layers are added to the models. These results suggest that there is a low-velocity layer, perhaps subducted sediments, between higher velocity continental and oceanic plates. Finally, we observe both coherent positive arrivals and a chaotic mix of positive and negative arrivals across the profile between 0 and 30 km depths. These shallow conversions may represent coherent sections of layered turbidites and mélange terranes that characterize deformation associated with underthrusting beneath the Kodiak accretionary complex.