Rupture behaviors of a subduction megathrust defines its slip type, extent and associated tsunami hazard, but are challenging to be known precisely due to limited fault-zone observations. Here, we integrate GNSS, tsunami-waveforms, seismic-profiles, and earthquake-cycle modeling to delineate slip extent of the 2020 Mw 7.8 Simeonof (Shumagin) and the 2021 Mw 8.2 Chignik (Semidi) earthquake sequence, and to understand the possible mechanical control on the distinct rupture behaviors of the neighboring Shumagin-Semidi segments along the Aleutian-Alaska subduction zone. We reveal a compelling fact that both the Simeonof and Chignik earthquakes slip at depth between ~20 to 40 km on the megathrust, a depth range typically observed at these segments in seismic cycles. We observe a contradictory slip behavior and disentangle why the Semidi segment ruptures in multi-variable style of large earthquakes, while the Shumagin segment ruptures with uniformly small to moderate seismicity, by illuminating the sharp variation in morphology across them. We identify a mechanical-structure boundary beneath the Shumagin Island where it separates the megathrust into gentle-smooth less-serpentinized east portion and steep-rough highly-serpentinized west portion. We also enlighten an intriguing fact that the upper slip bounds of the Simeonof-Chignik earthquakes corresponding to the deep boundary of the subducted seafloor ridges at the shallow megathrust, which likely arrested the slip propagating across this boundary to shallower depths as it did for the 1938 Mw 8.2 event. We highlight that the along-strike structure variation at depths and shallow trench control rupture styles, resulting in low to moderate tsunami hazard at this region.