An increment of volcano collapse: Kinematics of the 1975 Kalapana, Hawaii, earthquake

Earthquakes on Kilauea Volcano can be even more hazardous than volcanic eruptions. The most recent large earthquake was the 1975 Kalapana quake, which generated a locally damaging tsunami, over 8 m of horizontal surface displacement and 3 m of subsidence at the coast. The seismic magnitude estimates for this earthquake range from Ms 7.2 to Mw 7.7. The complexity of the seismic data is matched by the complex surface deformation that was observed with a combination of leveling, tilt and Electronic Distance Measurements (EDM). The geodetic data shows evidence of collapse of the caldera due to magma withdrawal, localized subsidence over the rift zone, normal faulting associated with shallow slumping along the Hilina Pali, and displacement on the basal detachment fault. We model the summit caldera, rift zones and basal detachment fault as dislocations in an elastic half-space to better quantify the sources of deformation related to this devastating event. Using inversion techniques that allow us to let the data constrain the geometry and magnitude of these sources, we find 0.04 km3 of magma withdrawal at the summit, between 3–5 m of opening along the rift zone and 7.1 m of slip along the basal detachment at 8.3 km depth. Models that allow for finer spatial resolution of slip on the detachment show that the largest slip occurred west of the seismically recorded earthquake hypocenter, along the region of the fault below the coast, and that the majority of the fault slip occurred south of the region of microseismicity. Our best-fitting model has a geodetic moment of 4.1 × 1020 Nm (Mw 7.7), which is consistent with tsunami models and recent analysis of long period seismic data. The residual displacements of sites located in the hanging wall of the Hilina Pali slump system, which were not used in the inversion of the other sources, suggest that triggered shallow slumping contributed several meters of horizontal displacement to the coseismic displacement at these sites.