Shallow normal faulting and block rotation associated with the 1975 Kalapana earthquake, Kilauea Volcano, Hawaii

The Hilina fault system is a set of normal faults that accommodate extension of the mobile south flank of Kilauea Volcano. Large earthquakes (M 6) and aseismic fault slip transport the flank southeastward along a basal detachment at similar to8-10 km depth. Both the 1975 M 7.2 Kalapana and the 1868 M 7.9 Great Kau earthquakes produced slip on the Hilina faults. We compare Kalapana earthquake fault offsets, ground displacements derived from analysis of geodetic surveys, and model displacements from a dislocation model to evaluate whether the central Hilina fault slip associated with the Kalapana earthquake was due to (1) shallow normal faulting independent of basal detachment slip or (2) deep normal faulting directly linked to basal detachment slip. Our analysis shows that observed site motions at the coast are significantly greater than model displacements expected from a dislocation model of basal detachment slip alone. To explain ground displacements and fault offsets, we require fault slip on shallow normal faults (as deep as 2-3 km) triggered by slip on the basal detachment. Leveling data along the Chain of Craters Road and vertical fault offsets across the central Hilina fault system suggest that the Kalapana earthquake produced block rotation of the hanging-wall blocks.