Prehistoric fault offsets of the Hilina fault system, south flank of Kilauea Volcano, Hawaii

dHistorical accounts of earthquakes on the Island of Hawaii date only to 1823 bur lava flows as old as 1500-3000 years B.P. contain fault offsets from prehistoric earthquakes. The M7.2 1975 Kalapana earthquake produced over 25 km of fault rupture along the Hilina fault system. We compare fault offsets in prehistoric lava flows with Kalapana earthquake fault offsets in neighboring 1969-1974 Mauna Ulu lava flows to estimate the frequency of prehistoric major earthquakes on the Hilina fault system. Horizontal and vertical fault offset rates across the Hilina fault system are 4.0 to 12.0 and -2.0 to -20.0 mm/yr, respectively, based on lava flows (750 years B.P. age. Assuming prehistoric earthquakes produced similar fault offsets compared to the Kalapana earthquake, three to five events are recorded in 400-750 years B.P. old lava flows yielding recurrence intervals of 260-80 years. Vertical fault offsets in prehistoric lava flows suggest that hanging wall rotation of the Hilina fault system contributes to fault offset. The assumption that past faulting resulted from prehistoric earthquakes with similar magnitude and fault offset to the Kalapana earthquake is an oversimplification. Large south flank earthquakes most likely do not have uniform recurrence intervals.