We employ near‐field GPS data to determine the subsurface geometry of a collapsing caldera during the 2018 Kīlauea 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 (∼85°) normal ring fault that may steepen to vertical with depth. Reservoir magma has compressibility of 2→15 × 10−10 Pa−1, indicating bubble volume fractions from 1% to 7% (lower if fault steepens with depth). Magma pressure increases during collapses are 1 to 3 MPa, 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.

The supply, storage, and subsurface transport of magma are some of the most fundamental, yet least understood volcanic processes (Poland et al., 2014). These processes, along with eruptive dynamics, are modulated by the geometry and nature of the pathways connecting magmatic reservoirs (Keating et al., 2008). The geometry and dimensions of individual pathways can be constrained by inverting surface deformation with continuum mechanics based models (e.g., Owen et al., 2000;Montagna & Gonnermann, 2013). However, with multiple reservoirs and a network of magmatic pathways, estimating the dimensions of each pathway directly from deformation can be challenging. Because magma flux is proportional to the hydraulic conductivity of the pathway, and pressure change in a reservoir depends on magma flux, time dependent deformation associated with each reservoir may reveal the connectivity of a multi-reservoir system (e.g., Bato et al., 2018;Le Mével et al., 2016;Reverso et al., 2014). Here we demonstrate that, physics-based models, coupled with Bayesian inversion, can synthesize multi-reservoir conceptual models with geodetic measurements to quantitatively constrain the hydraulic connectivity of magmatic systems.Despite decades of research, the nature of Kīlauea's summit reservoirs and their connectivity to the East Rift Zone remains enigmatic (we reserve "East Rift Zone" for the geographic location and "ERZ" for the reservoir active in the observation period). Efforts to interpret summit deformation in terms of simple reservoir models yielded diverse reservoir locations and geometries (e.g., Baker & Amelung, 2012;Fiske & Kinoshita, 1969). Although modeled reservoirs cluster into two groups -a shallow Halema'uma'u (HMM) and a deeper South Caldera (SC) reservoir (e.g., Cervelli & Miklius, 2003;Poland et al., 2014), it has been suggested that the summit system represents a single irregularly shaped reservoir (Dieterich & Decker, 1975;Ryan, 1988). This ambiguity arises because deformation signals associated with these reservoirs are almost always of the same sign. The configuration of magmatic pathways connecting Kīlauea's summit reservoirs and ERZ is also elusive. Cervelli and Miklius (2003) argue that an "Γ shaped" pathway connecting the deeper SC reservoir to the shallower HMM reservoir, and then to ERZ, is required to explain the drainage Abstract From August 2018 to May 2019, Kīlauea's summit exhibited unique, simultaneous, inflation and deflation, apparent in both GPS time series and cumulative InSAR displacement maps. This deformation pattern provides clear evidence that the Halema'uma'u (HMM) and South Caldera (SC) reservoirs are distinct. Post-collapse inflation of the East Rift Zone (ERZ), as captured by InSAR, indicates concurrent magma transfer from the summit reservoirs to the ERZ. We present a physics-based model that couples pressure-driven flow between these magma reservoirs to simulate time dependent summit deformation. We take a two-step approach to quantitatively constrain Kīlauea's magmatic plumbing system. ...

[1] Mount Etna is characterized by a complex structural setting that influences its evolution. In order to understand how the interaction between dike-forming intrusions and faulting influences the kinematics of the volcanic edifice, we developed a numerical model. It takes account of the topography, the medium heterogeneities, the gravitational loading, and the most active crustal discontinuities. A parameterization of the apparent coefficient of friction, as a function of the depth, has been considered in the range from 0.01 to 0.5. The density values used, Young's modulus, and Poisson's ratio range from about 1700 to 3200 kg/m 3 , 8.5 to 140 GPa, and 0.15 to 0.35, respectively. The model was applied to the 2002-2003 Etna eruption. The resulting deformation pattern was in agreement with the data provided by the continuous GPS stations and the seismological knowledge, obtaining displacements up to 1 m. Some discrepancies between the recorded ground deformation field and the modeled displacements allowed us to make some hypotheses on the volcano-tectonic (e.g., for the Pernicana fault) or regional-tectonic origin (Acireale-S. Alfio fault system) of the crustal discontinuities. Moreover, our results suggested that the Pernicana fault is characterized by a very low apparent coefficient of friction, less than 0.1. Furthermore, we evaluated the contribution of the gravitational loading and found that it implies a variation of about 10% of the overall deformation pattern. Our results clearly showed that the weight of the volcanic edifice acts in opposition to the magmatic intrusions. Conversely, we found that the presence of medium heterogeneities may favor the eastern flank movements toward the SE and the spreading of the summit area, playing a fundamental role in ascending magma. Finally, we investigated the shallow sliding model and found that dike-forming intrusions produce negligible displacements, less than 1 mm, along the subhorizontal detachment surface. Therefore, the gravitational loading during dike-forming intrusions is not able to trigger sliding processes along this plane.

Simultaneous summit inflation and deflation constrain the location and geometry of Halema'uma'u (HMM) and South Caldera (SC) reservoirs.• A model with time dependent magma flux between reservoirs explains the postcollapse spatial-temporal deformation pattern.• Time dependent deformations require a HMM-East Rift Zone (ERZ) pathway and a significantly less hydraulically conductive SC-ERZ pathway.