In 2018, Kīlauea Volcano experienced its largest lower East Rift Zone (LERZ) eruption and caldera collapse in at least 200 years. After collapse of the Pu‘u ‘Ō‘ō vent on 30 April, magma propagated downrift. Eruptive fissures opened in the LERZ on 3 May, eventually extending ~6.8 kilometers. A 4 May earthquake [moment magnitude (Mw) 6.9] produced ~5 meters of fault slip. Lava erupted at rates exceeding 100 cubic meters per second, eventually covering 35.5 square kilometers. The summit magma system partially drained, producing minor explosions and near-daily collapses releasing energy equivalent toMw4.7 to 5.4 earthquakes. Activity declined rapidly on 4 August. Summit collapse and lava flow volume estimates are roughly equivalent—about 0.8 cubic kilometers. Careful historical observation and monitoring of Kīlauea enabled successful forecasting of hazardous events.

Seismic evidence has long indicated that the south flank of Kilauea Volcano is mobile. Examination of triangulation, trilateration, and leveling data obtained throughout the 20th century shows that the south flank of Kilauea has been displaced upward and away from the rift zones by as much as several metres. The amount of horizontal displacement approximates the probable amount of dilation that accompanies the intrusion of magma as dikes in the rift zones and is greatest for p~riods of most intense intrusive activity, as evidenced by the frequency of eruptions. Displacement and seismic events on the south flank take place soon after intrusive activity, indicating that the displacement is the result of forceful intrusion in the rift zones, not the cause of relatively passive intrusion. The dikes are thought to be nearly vertical or to dip steeply southward, on the basis of both interpretation of seismic and displacement data for Kilauea itself and comparison with dikes exposed in older, eroded Hawaiian shield volcanoes. In contrast to the south flank, seismic and geodetic data indicate that the north flank is virtually immobile. This contrast is believed to reflect the fact that Kilauea was built on the south slope of Mauna Loa and was consequently influenced by the gravitational stress system of Mauna Loa, which favors displacement away from the volcanic edifice. The north flank of Kilauea is effectively buttressed by Mauna Loa, whereas thr south flank is unbuttressed and free to move away from the edifice when prompted by forceful intrusion of magma. The active part of the east rift zone of Kilauea has apparently migrated several kilometres southward with time. This is shown by the location of recent vents and by the location of the axis of a positive gravity anomaly along the north edge of the active part of the rift zone. The southward migration helps explain several features of the geometry of the east rift zone, particularly its prominent bend near Kilauea Caldera. The southwest rift zone and the caldera also show some evidence of southward migration. The Hilina fault system is considered to be a gravity-controlled system not directly related to the rift zones. Gravitational instability resulting from uplift and seaward displacement is eventually relieved by normal faulting along the seaward part of the south flank. The Hilina faults are thought to bottom at shallow depth without interseeting magma reservoirs, except possibly along part of the lower east rift zone, where the fault system impinges upon the rift zone. Strains have been accumulating within the Hilina system throughout this century, and a high level of instability may have been reached. We anticipate a subsidence event in the not too distant future, possibly similar to the damaging events of 1823 and 1868. (While this paper was in press, such an event occurred on November 29, 1975.) I ACKNOWLEDG ENTS Most staff members of the Haw iian Volcano Observatory took part in the 1970, 197 , and 1973 surveys; we thank them all, especially R. T. Okamura, ...

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