The potential for a major earthquake in the Shumagin seismic gap, and the tsunami it could generate, was reported in 1971. However, while potentially tsunamigenic splay faults in the adjacent Unimak and Semidi earthquake segments are known, such features along the Shumagin segment were undocumented until recently. To investigate margin structure and search for splay faults, we reprocessed six legacy seismic records and also processed seismic data acquired by RV Langseth during the ALEUT project (cf. Bécel et al., 2017). All records show splay faults separating the frontal prism from the margin framework. A ridge uplifted by the splay fault hanging wall extends along the entire segment. At the plate interface, the splay fault cuts across subducted sediment strata in some images, whereas in others, the plate interface sediment cuts across the fault. Splay fault zones are commonly associated with subducting lower-plate relief.
Along the upper slope, beneath a sediment cover, major normal faults dipping landward and seaward border a ridge of basement rock. The faults displace a regional unconformity that elsewhere received Oligocene–Miocene sediment. Low seafloor scarps above some normal faults indicate recent tectonism. The buried ridge is a continuation of the Unimak Ridge structure that extends NE of the Unimak/Shumagin segment boundary. Some geological characteristics of the Shumagin segment differ from those of other Alaskan earthquake segments, but a causal link to the proposed Shumagin creeping seismic behavior is equivocal.