Caldera Collapse Geometry Revealed by Near-Field GPS Displacements at Klauea Volcano in 2018

We employ near-field GPS data to determine the subsurface geometry of a collapsing caldera during the 2018 Klauea eruption. Collapse occurred in 62 discrete events, with "inflationary" deformation external to the collapse, similar to previous basaltic collapses. We take advantage of GPS data from the collapsing block and independent constraints on the magma chamber geometry from inversion of deflation prior to collapse onset. This provides an unparalleled opportunity to constrain the collapse geometry. Employing an axisymmetric finite element model, the co-collapse displacements are best explained by piston-like subsidence along a high angle (similar to 85 degrees) normal ring fault that may steepen to vertical with depth. Reservoir magma has compressibility of 2 -> 15x10(-10)Pa(-1), indicating bubble volume fractions from 1% to 7% (lower if fault steepens with depth). Magma pressure increases during collapses are 1 to 3MPa, depending on compressibility. Depressurization of a triaxial point source in a homogeneous half-space fits the data well but provides a biased representation of the source depth and process. Plain Language Summary When large volumes of magma erupt rapidly, the rock overlying the subsurface reservoir founders, producing a caldera. During the 2018 eruption of Klauea Volcano, Hawai'i, collapse occurred in over 60 events, each lasting 5 to 10s. We analyze GPS data collected during the last 32 of these events to determine the geometry of the ring fault system bounding the caldera block and the properties of the underlying magma. The faults are on average very steep but dip slightly inward at shallow depth. Inferred pressure increases during collapse events constrain the compressibility of the magma and imply an exsolved gas phase with 1% to 7 % bubbles by volume.