Continuous gravity reveals huge mass changes during the onset of intrusion, eruption, and collapse at Kīlauea Volcano, Hawaíi, April-May 2018

The onset of Kīlauea's 2018 lower East Rift Zone eruption was accompanied by magma withdrawal from two eruptive vents, at the summit and at Púu ´Ō´ō. Continuous gravimeters were perched on the rim of each of these vents and recorded gravity changes associated with ground subsidence and mass variations. At Púu ´Ō´ō, gravity decreased by 200 microgals over the first 8 minutes of the onset of transient activity, followed by an increase of 350 microgals over the subsequent 9 minutes. This increase plateaued for 45 minutes before a variable-rate decrease of 1400 microgals over 90 minutes, and an additional 1000 microgal drop over the 3 days that followed. At the summit, gravity decrease began slowly, correlating with deflationary tilt and lowering of the lava level. Over the course of 9 days, summit gravity dropped by about 1300 microgals. By mid-May, little gravity change was recorded at either location. The Púu ´Ō´ō gravimeter was recovered on June 7, and the summit gravimeter was consumed by the expansion of Halemáumáu crater on May 15.

The abrupt evacuation of the summit and Púu ´Ō´ō provided an opportunity to image the structural detail of both eruptive vents. Using these data, along with observations of lava levels and surface deformation over time, it should be possible to gain insight into the processes that caused gravity changes that are, by far, the strongest ever observed through continuous measurements. We hope to understand the onset of the new activity at Púu ´Ō´ō, which involved not only collapse of the crater floor, but also the opening of a small eruptive fissure on the west flank of the cone during a time when only a few ambiguous geological and geophysical observations were available. Continuous gravity may be able to provide unique information on the processes associated with the onset of the most significant volcanic event at Kīlauea of the past several hundred years.