Along-strike variations in intermediate-depth seismicity and arc magmatism along the Alaska Peninsula

Abstract The Alaska Peninsula section of the Aleutian-Alaska subduction zone exhibits significant along-strike variations in plate coupling, earthquakes, and arc magmatism. Here we use regional and teleseismic data from 1990 to 2018 to investigate intermediate-depth intra-plate seismicity beneath the Alaska Peninsula. The distribution of these events shows five distinct segments from southwest to northeast: Unimak, Pavlof, Shumagin, Semidi, and Kodiak. We identify two double seismic zones (DSZs) in the Pavlof and Semidi segments but not in others. The magnitude-frequency relationship of these earthquakes (b-value) also varies along strike, suggesting more fluids in the southwestern segments and closer to the slab top interface. We also evaluate trace elements of arc lavas in the Alaska Peninsula. Th/La ratios indicate a significant amount of sediments subducted in the Semidi segment that includes Aniakchak and Veniaminof, and the sediment signature peaks beneath the Black Peak volcano. In contrast, high Pb/Th but low Th/La ratios from southwestern volcanoes suggest high fluids released from the subducted slab. In sum, our observations suggest that the slab is more hydrated and thus undergoing more intensive dehydration in the southwest than in the northeast, whereas more sediments are subducted in the northeast than in the southwest. These along-arc changes are generally in agreement with previous studies of plate coupling and the crustal structure of the incoming plate. Moreover, the complexities beyond the monotonic changes imply that the intermediate-depth intra-plate seismicity and sub-arc melting are influenced by not only the pre-existing fabrics in the incoming plate, but also the subducted sediments and plate stress state.