Mass changes at different levels revealed by micro-gravity and deformation measurements at Kilauea Volcano, Hawai’i. (Invited)

Using campaign micro-gravity measurements collected at Kilauea Volcano, Hawai’i (United States), between December 2009 and November 2012, we document significant mass variations at the summit of the volcano. These variations produce a maximum residual gravity change of 370 ± 14 μGal near the east margin of Halema’uma’u Crater, within Kilauea’s summit caldera, where in March 2008 a new eruptive vent opened. This vent has progressively enlarged through several collapses and now forms a 210x160 m elliptical cavity that is currently occupied by a lava lake whose surface fluctuates between 25 and 200 meters below the vent rim. Five micro-gravity surveys were performed using two Scintrex CG-5 gravimeters. Each survey was completed following a double-looping procedure, and each measurement was corrected for earth-tides, ocean loading, and instrument drift. Gravity changes at each station were then corrected for the free-air effect using vertical displacements calculated from combined ascending and descending InSAR measurements. InSAR data are from both the German Space Agency (DLR) TerraSAR-X satellite and the Italian Space Agency (ASI) Cosmo-SkyMed satellite-constellation. The spatial distribution of the gravity changes suggests that they are predominantly caused by the enlargement of the vent and variations in the height of the summit lava lake. The contribution to the gravity measurements caused by changes in lava level within the conduit feeding the lava lake is therefore estimated using a numerical model that takes into account its geometry, as inferred from visual and remotely sensed (LiDAR) observations, and lava height at the time of each gravity survey, determined from thermal camera data. These results can be used to make inferences on the density of the magma filling the lava lake, which we compare to values obtained using independent data from continuous gravimeters located near the campaign stations. Estimates of the lava level effect on the gravity allow us to compensate for it and assess changes measured across Kilauea’s summit that may be due to mass variations caused by subsurface magma accumulation or withdrawal. We observe that a positive anomaly (which magnitude varies depending on the density of the material used for the fluid filling the lava lake) is still present after this correction in the 20-month-long period following the March 5-9, 2011, east rift zone (ERZ) fissure eruption. InSAR data spanning the same interval show that these positive changes are associated with uplift of the summit area, which we model as the inflation of a shallow magma reservoir at ~1.5 km depth. The contemporary increase in volume, inferred from the deformation data, and in mass, inferred from the micro-gravity measurement, suggest that, after the 2011 ERZ fissure eruption, magma has been accumulating beneath the summit of Kilauea.