Downdip variations in seismic reflection character: Implications for fault structure and seismogenic behavior in the Alaska subduction zone

Seismic reflection data collected offshore of Alaska Peninsula across the western edge of the Semidi segment show distinctive variations in reflection characteristics of the megathrust fault with depth, suggesting changes in structure that may relate to seismic behavior. From the trench to ∼40 km landward, two parallel reflections are observed, which we interpret as the top and bottom of the subducted sediment section. From ∼50 to 95 km from the trench, the plate interface appears as a thin (<400 ms) reflection band. Deeper and farther landward, the plate interface transitions to a thicker (1-1.5 s) package of reflections, where it appears to intersect the fore-arc mantle wedge based on our preferred interpretation of the continental Moho. Synthetic waveform modeling suggests that the thin reflection band is best explained by a single ∼100 to 250 m thick low-velocity zone, whereas the thick reflection band requires a 3 to 5 km thick zone of thin layers. The thin reflection band is located at the center of the 1938 M_w 8.2 Semidi earthquake rupture zone that now experiences little interplate seismicity. The thick reflection band starts at the downdip edge of the rupture zone, correlates with a dipping band of seismicity, and projects to the location of tremor at greater depth. We interpret the thin reflection band as a compacted sediment layer and/or localized shear zone. The thick reflection band could be caused by a wide deformation zone with branching faults and/or fluid-rich layers, representing a broad transition from stick-slip sliding to slow slip and tremor.