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A Compound Faulting Model for the 1975 Kalapana, Hawaii, Earthquake, Landslide, and Tsunami
Journal of Geophysical Research: Solid Earth  (IF3.848),  Pub Date : 2021-10-19, DOI: 10.1029/2021jb022488
Yoshiki Yamazaki, Thorne Lay, Kwok Fai Cheung

The Kalapana, Hawaii, MW 7.7 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, 2018 MW 7.2 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.