Seismic and Geodetic Progression of the 2018 Caldera Collapse of Kīlauea Volcano

Abstract:

The 2018 eruption of Kīlauea volcano, Hawaiʻi, resulted in a major collapse of the summit caldera along with an effusive eruption in the lower East Rift Zone. The caldera collapse comprised 62 highly similar collapse cycles of strong ground deformation and earthquake swarms ending with a magnitude 5 collapse event plus one partial cycle without a collapse event. We analyzed geodetic and seismic data to better understand the progression of caldera collapse over 3 months of activity, focusing on the cyclical activity. We identified 3 main phases of collapse. The draining of the summit magma reservoir starting in early May initiated ring-fault activation and small explosions by mid-May (Phase 1), which was accompanied by relatively low earthquake activity and GPS displacement. From late May through early June (Phase 2), increases in GPS displacements on the eastern side of the collapse region coincident with decreases on the western side and shifts in the seismicity indicate that initial failure occurred asymmetrically, with the west failing first. Major changes in the displacement trends and earthquake swarm behavior around June 9 suggested that a central piston had formed and become mobile. At the beginning of Phase 3 (early to late June), the piston dropped downward and recoupled with the magma reservoir. This led to a stable piston mechanism until the eruption’s end in early August, indicated by relatively steady GPS displacement and earthquake behavior. We also interpret the formation of a major peripheral fault on the eastern side of the collapse caldera in late June from temporary decreases in eastern GPS displacements along with changes in the earthquake swarm parameters, such as the most active locations and a peaking of the cumulative moment. Changes in both the displacement directions and earthquake locations suggest that the collapse may have had an eastward-component of motion after the ring fault system had formed. The cyclical seismic and geodetic character showed no obvious signs that the collapse was coming to an end, with the only notable change being a significant increase in the ratio of cyclical displacement to co-collapse displacement observed during the last complete cycle on GNSS stations outside the caldera region.