Active normal faulting in the upper plate of the Alaska subduction zone

The Alaska subduction zone exhibits large along-strike variations in interseismic coupling and seismic behavior, yet the long-term structural response of the overriding forearc to these variations remains poorly constrained. We document active normal-sense faulting on Chirikof Island within the Semidi segment that lies between the contrasting plate-interface behavior and upper-plate deformation styles of the Kodiak and Shumagin segments. Geomorphic and geochronologic data indicate 4−9 m of cumulative slip and that fault-scarp development continued after ca. 6 ka, with evidence suggesting multiple episodes of fault reactivation. Bedrock structures and thermochronologic data indicate that this normal faulting represents a recent shift from prolonged shortening to extension in this part of the forearc. This deformation change coincides with the along-strike transition from persistent thrusting in the Kodiak segment to large-scale normal faulting in the Shumagin segment. We propose that variations in subducting-plate fabric orientation and inherited seafloor heterogeneities may influence plate-interface mechanics and the stress state of the upper plate, potentially contributing to the contrasting deformation styles. The normal-sense fault reactivation may enhance tsunami generation during large megathrust earthquakes.