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Abstract Strong tsunami excitation from slow rupture of shallow subduction zone faults is recognized as a key concern for tsunami hazard assessment. Three months after the 22 July 2020 magnitude 7.8 thrust earthquake struck the plate boundary below the Shumagin Islands, Alaska, a magnitude 7.6 aftershock ruptured with complex intraplate faulting. Despite the smaller size and predominantly strike-slip faulting mechanism inferred from seismic waves for the aftershock, it generated much larger tsunami waves than the mainshock. Here we show through detailed analysis of seismic, geodetic, and tsunami observations of the aftershock that the event implicated unprecedented source complexity, involving weakly tsunamigenic fast rupture of two intraplate faults located below and most likely above the plate boundary, along with induced strongly tsunamigenic slow thrust slip on a third fault near the shelf break likely striking nearly perpendicular to the trench. The thrust slip took over 5 min, giving no clear expression in seismic or geodetic observations while producing the sizeable far-field tsunami. 

The Kalapana, Hawaii,M_W7.7 earthquake on November 29, 1975 generated a local tsunami with at least 14.3 m runup on the southeast shore of Hawaii Island adjacent to Kilauea Volcano. This was the largest locally generated tsunami since the great 1868 Ka'u earthquake located along‐shore to the southwest. Well‐recorded tide gauge and runup observations provide a key benchmark for studies of statewide tsunami hazards from actively deforming southeast Hawaii Island. However, the source process of the earthquake remains controversial, with coastal landsliding and/or offshore normal or thrust faulting mechanisms having been proposed to reconcile features of seismic, geodetic, and tsunami observations. We utilize these diverse observations for the 1975 Kalapana earthquake to deduce a compound faulting model that accounts for the overall tsunamigenesis, involving both landslide block faulting along the shore and slip on the island basal décollement. Thrust slip of 4.5–8.0 m on the offshore décollement produces moderate near‐field runup but controls the far‐field tsunami. The slip distribution implies that residual strain energy was available for the May 4, 2018M_W7.2 thrust earthquake during the Kilauea‐East Rift Zone eruption. Local faulting below land contributes to geodetic and seismic observations, but is non‐tsunamigenic and not considered. Slip of 4–10 m on landslide‐like faults, which extend from the Hilina Fault Zone scarp to offshore shallowly dipping faults reaching near the seafloor, triples the near‐field tsunami runup. This compound model clarifies the roles of the faulting components in assessing tsunami hazards for the Hawaiian Islands.