Krafla volcano, located in Iceland's North Volcanic Zone, has been extensively studied using static gravity surveys since 1975. This study integrates measurements acquired between 2022 and 2023 with legacy data to produce a new gravity map of the area. We produce gravity gradient maps to delineate shallow subsurface density features and invert them to develop a 3D density model that images key subsurface structures. The combined survey coverage, extending from Fremrinámar to the Þeistareykir Volcanic Systems, reveals a range of features with contrasting densities which reflect the tectonic and volcanic processes that have shaped the area. Denser materials are associated with mafic intrusions along the principal faults related to caldera‐subsidence and ancillary faults within the caldera. In the Iceland Deep Drilling Project 1 (IDDP‐1) area, located inside the caldera, we identify a positive density anomaly at depths corresponding to magma encountered during drilling at 2.1 km depth. Negative gravity anomalies, indicative of relatively low‐density materials, are distributed within and outside the Krafla caldera. Within the caldera, these anomalies are interpreted as felsic intrusions and highly fractured geothermal zones. Beyond the caldera, a prominent negative density anomaly corresponds to the graben structure associated with the Húsavík‐Flatey Fault. Although strong near‐surface gravity anomalies and the spatial distribution of our gravity measurements limit the identification of deeper structures (>5 km), this study offers valuable insights into the distribution of the magmatic system features, as well as large tectonic characteristics in the area. These insights improve our understanding of magmatic and tectonic processes, volcanic hazards, and the future development of geothermal production in the area.

Magnetotelluric investigations can reconstruct features within magmatic systems, such as the distribution of melt and fluid and the volcano-tectonic structures. Here, we use this approach to image the subsurface beneath the active Campi Flegrei caldera, providing new details on the crustal structure down to 20 kilometers. The imaging shows a multilevel magmatic architecture controlled by deep and shallow volcano-tectonic structures. The magma plumbing system primarily manifests as a low resistivity anomaly located at 8-20 kilometers depth and interpreted as a mid-lower crustal mush zone containing similar to 10% melt. This zone transitions upwards through a channel-like low resistivity structure containing similar to 8% of melt and a medium resistivity sector where small lenses of heterogeneously distributed magma accumulate and cool. Our new subsurface imaging attests to a transcrustal mush system that has been revealed beneath Campi Flegrei, with important implications for interpretation of monitoring data at one of the world's most hazardous caldera.

The Krafla Volcanic System (KVS) in the Northern Volcanic Zone (NVZ) in Iceland last erupted between 1975 and 1984, during an eruptive period called “the Krafla Fires”. The KVS is composed of a restless caldera, an array of scoria cones along a fissure swarm and is among the best-studied volcanic systems due to the exploitation of its geothermal potential. In 2009, the Icelandic Deep Drilling Project (IDDP) encountered a shallow rhyolitic magma body at 2.1 km depth beneath the caldera. To date, no geophysical method has been able to image this magma body at Krafla within the top 4 km of the crust. Here we present new micro-gravity data collected in June and July 2022 across a 14-station network of benchmarks in the KVS. Micro-gravimetry is a relative method that records changes in gravity between a reference and a series of benchmarks over both space and time to investigate subsurface mass or density changes via time-series analysis and modelling. Our 2022 survey highlights negative gravity differences of benchmarks located in the centre of the caldera with respect to a reference located to the south and outside the caldera. The most negative values are found in its eastern part. Positive gravity differences can be found south of the southern caldera wall along a set of past eruptive fissures. The next steps in data processing include data reduction for deformation effects to link the new data to previous joint deformation and micro-gravity surveys conducted at the KVS since 1965. This should enable us to quantify the long-term evolution of the KVS over more than 50 years providing unprecedented insights into its inner workings.

Mapping fluid accumulation in the crust is pertinent for numerous applications including volcanic hazard assessment, geothermal energy generation, and mineral exploration. Here, we use seismic attenuation tomography to map the distribution of fluids in the crust below Uturuncu volcano, Bolivia. Seismic P wave and S wave attenuation, as well as their ratio (Q(P)/Q(S)), constrain where the crust is partially and fully fluid-saturated. Seismic anisotropy observations further constrain the mechanism by which the fluids accumulate, predominantly along aligned faults and fractures in this case. Furthermore, subsurface pressure-temperature profiles and conductivity data allow us to identify the most likely fluid composition. We identify shallow regions of both dry and H2O/brine-saturated crust, as well as a deeper supercritical H2O/brine column directly beneath Uturuncu. Our observations provide a greater understanding of Uturuncu's transcrustal hydrothermal system, and act as an example of how such methods could be applied to map crustal fluid pathways and hydrothermal/geothermal systems elsewhere.

Pre‐eruptive signals at the crater lake‐bearing Mt. Ruapehu (New Zealand) are either absent or hard to identify. Here, we report on geophysical anomalies arising from hydrothermal unrest (HTU) and magmatic unrest (MU) using multiphysics numerical modeling. Distinct spatio‐temporal anomalies are revealed when jointly solving for ground displacements and changes in gravitational and electrical potential fields for a set of subsurface disturbances including magma recharge and anomalous hydrothermal flow. Protracted hydrothermal injections induce measurable surface displacements (>0.5 cm) at Ruapehu's summit plateau, while magmatic pressurization (5–20 MPa) results in ground displacements below detection limits. Source density changes of 10 kg/m 3 (MU simulations) and CO 2 fluxes between 2,150 and 3,600 t/d (HTU simulations) induce resolvable residual gravity changes between +8 and −8 μGal at the plateau. Absolute self‐potential (SP) anomalies are predicted to vary between 0.3 and 2.5 mV for all unrest simulations and exceed the detection limit of conventional electric surveying. Parameter space exploration indicates that variations of up to 400% in the Biot‐Willis coefficient produce negligible differences in surface displacement in MU simulations, but strongly impact surface displacement in HTU simulations. Our interpretation of the findings is that monitoring of changes in SP and gravity should permit insights into MU at Ruapehu, while HTU is best characterized using ground displacements, residual gravity changes and SP anomalies. Our findings are useful to inform multiparameter monitoring strategies at Ruapehu and other volcanoes hosting crater lakes.

EDITORIAL article Front. Earth Sci., 17 June 2022Sec. Volcanology https://doi.org/10.3389/feart.2022.949029

The deformation history of the Campi Flegrei caldera during the last decades consists of two large uplift events in 1970–1972 and 1982–1984, with ~3.5 m cumulative uplift, occurring at a rate of ~1 m/yr. Both events were accompanied by seismic activity, gravity changes and compositional variations of volcanic gases but no eruption took place. During the following decades, the area has been slowly subsiding but minor uplift episodes (~a few cm), seismic swarms and changes in degassing activity took place, showing that the area persisted in a near-critical state. Since November 2005, ground deformation resumed, although at a slower rate, totalling a displacement of ~0.77 m (February 2021). In this chapter, we present a retrospective analysis of ground deformation leading to a critical re-evaluation of the 1982–1984 uplift and of the following deflation, employing the most updated modelling techniques. Deformation and gravity data provide important constraints on depth, volume, mass density and dislocation mechanisms accompanying magma emplacement processes. The correct evaluation of these parameters is strongly conditioned by simplifying assumptions built in the different inversion procedures: in particular, the inferred source depth ranges from 5.5 km to less than 3 km and the intrusion density ranges from values pertinent to aqueous fluids to typical magmatic values. This review depicts the salient phases of the deformation history of this densely populated and high-risk volcanic area, helping to address debated issues, such as the role of the magmatic system, and their interaction with the shallower hydrothermal system. In spite of the mentioned difficulties, the following conclusion may be considered as firmly established: during 1982–1984 a magmatic intrusion took place, and the subsequent complex deformation history (1985–2010) was mainly controlled by the exsolution of volatiles with magmatic origin and their interaction with a shallow hydrothermal system. However, data collected after 2011 were modelled in terms of a deep inflating source of deformation, possibly resulting from a resumed magmatic recharge at depth.

As volcanoes undergo unrest, understanding the conditions and timescales required for magma reservoir failure, and the links to geodetic observations, are critical when evaluating the potential for magma migration to the surface and eruption. Inferring the dynamics of a pressurized magmatic system from episodes of surface deformation is heavily reliant on the assumed crustal rheology, typically represented by an elastic medium. Here, we use Finite Element models to identify the rheological response to reservoir pressurization within a temperature‐dependent Standard Linear Solid viscoelastic (“thermo‐viscoelastic”) domain. We assess the mechanical stability of a deforming reservoir by evaluating the overpressures required to initiate brittle failure along the reservoir wall, and the sensitivity to key parameters. Reservoir inflation facilitates compression of the ductile wall rock, due to the non‐uniform crustal viscosity, impacting the temporal evolution of the induced tensile stress. Thermo‐viscoelasticity enables a deforming reservoir to sustain greater overpressures prior to failure, compared to elastic analyses. High‐temperature (e.g., mafic) reservoirs fail at lower overpressures compared to low‐temperature (e.g., felsic) reservoirs, producing smaller coincident displacements at the ground surface. The impact of thermo‐viscoelasticity on reservoir failure is significant across a wide range of overpressure loading rates. By resisting mechanical failure on the reservoir wall, thermo‐viscoelasticity impacts dyke nucleation and formation of shear fractures. Numerical models may need to incorporate additional processes that act to promote failure, such as regional stresses (e.g., topographic and tectonic), external triggers (e.g., earthquake stress drops), or pre‐existing weaknesses along the reservoir wall.

The absence of precursory signals of recent eruptions at Mt. Ruapehu poses a problem for hazard assessment and risk mitigation at the popular Tongariro National Park. Ruapehu hosts an active hydrothermal system with volcanic unrest being driven by either migration of magma, hydrothermal fluids, or a combination of both. In our study, we develop a suite of 2D axisymmetric numerical models to study the detectability limit of precursory subsurface processes at Ruapehu to inform recommendations for monitoring protocols. In our models magmatic unrest (MU) results from pressurisation of a transcrustal elliptical mush zone due to the intrusion of juvenile magma which triggers a poroelastic response in the hydrothermal system. Hydrothermal unrest (HTU) is simulated by the injection of hot multicomponent and multiphase fluids (H2O and CO2) into Ruapehu’s hydrothermal system (HTS), where thermo-poroelastic responses are triggered. We simultaneously solve for ground displacement, self-potential (SP) anomalies and residual gravity changes resulting from the subsurface perturbations, with model parameterization adapted to Ruapehu. All models account for topography and subsurface mechanical and hydro-electric heterogeneities.For a plausible reference parameter set, we find that geophysical observables are markedly distinct in their magnitude and wavelength in both magmatic and hydrothermal unrest scenarios. Most geophysical anomalies show their largest magnitudes directly above the hydrothermal system, with signals falling off rapidly with distance. At Ruapehu’s summit plateau (500 m from the HTS) vertical displacement amplitudes for MU simulations are 1.5 times smaller than maximum magnitudes of 1.2 cm for HTU simulations, with the latter being above conventical detection limits (1 cm in the vertical). Maximum residual gravity changes on the plateau are -4 μGal for HTU simulations and hence below detection levels of standard field observations, while for MU simulations with a source density change of 10 kg/m3 resulting signal magnitude is twice as high. Modelled SP anomalies are predicted to exceed conventional detection levels of 0.1 mV with typical SP signals for HTU simulations attaining maximal amplitudes of 1.3 mV, which are ~3 times larger than those resulting from MU simulations.Parameter exploration shows that residual gravity changes for MU simulations are predominantly controlled by reservoir density changes, while SP polarity and magnitude strongly depends on the hydro-electric coupling coefficient for both unrest scenarios. Moreover, we find that the Biot-Willis coefficient (degree of poroelastic response) has the greatest influence on displacement amplitudes for HTU simulations, with negligible effect on displacement, SP and gravity changes resulting from MU simulations. Although gravity changes and displacements for reservoir strengths (volume/overpressure) > 7 km3/MPa are greater as for reference simulations, vertical displacement remains below detection levels. Magnitudes of all signals from HTU simulations correlate with fluid fluxes. Our interpretation of the findings is that magmatic unrest at Ruapehu should be identifiable by joint residual gravity and SP time series, whereas ground displacements >1 cm in the vertical and SP anomalies should be indicative of hydrothermal unrest.

Uturuncu volcano is situated in the Bolivian Andes, directly above the world's largest crustal body of silicic partial melt, the Altiplano-Puna Magma Body (APMB). Uturuncu last erupted 250,000 years ago, yet is seismically active and lies at the centre of a 70 km diameter uplifted region. Here, we analyse seismicity from 2009 to 2012. Our earthquake locations, using a newly developed velocity model, delineate the top and bottom of the APMB, reveal individual faults, and reconcile differences in depth distribution between previous studies. Spatial clustering analysis of these earthquakes reveals the orientations of the faults, which match stress orientations from seismic anisotropy. Earthquake b-values derived from moment magnitudes (1.44 +/- 0.06) differ significantly from those using local magnitude measurements (0.80 +/- 0.03). From these observations and theoretical justification, we suggest that, if possible, moment magnitudes should be used for accurate b-value analysis. We interpret b-values > 1 in terms of fluid-enhanced seismicity. Shallow seismicity local to Uturuncu yields b-values > 1.1 with some temporal variation, suggesting fluid migration along pre-existing faults in a shallow hydrothermal system, likely driven by advection from the APMB. Intriguingly, events deeper than the APMB also yield large b-values (1.4), mapping the ascent into the lower crust of fluids that we infer as originating from a subducting slab. Cumulatively, these results provide a picture of an active magmatic system, where fluids are exchanged across the more ductile APMB, feeding a shallow, fault-controlled hydrothermal system. Such pathways of fluid ascent may influence our understanding of arc volcanism, control future volcanic eruptions and promote the accumulation of shallow hydrothermal ore deposits. (C) 2021 Published by Elsevier B.V.

Uturuncu volcano in southern Bolivia is a member of a distinctive class of volcanoes – systems that show unrest despite not having erupted in the Holocene. Uturuncu has not erupted in 250 kyr, but has been deforming (uplift with a moat of subsidence) for several decades, along with seismic swarms and active, sulfur-encrusted fumaroles. Our work builds on previous geophysical imaging at Uturuncu by jointly analyzing multidisciplinary datasets, focusing on imaging the shallow (<15 km depth below surface) structure of the system with geophysical and geochemical data. Whereas previous research pointed to andesite melt at depths >15 km depth, results were ambiguous as to what proportions of melts vs. brines are present at shallower depths. Identifying fluids (melt, brine, etc.) and structures at shallow depths is key for evaluating the hazard potential of the volcano and understanding the source of the unrest. We present new results from gravimetry, seismology (hypocenter relocation, seismic velocity and attenuation tomography), gas geochemistry, and InSAR observations. The results point to an extensive and active hydrothermal system extending ~20 km laterally and ~10 km vertically from Uturuncu, with possible connections at depth to the deeper magmatic system. A combined view of the new density, seismic velocity and attenuation models, and the existing resistivity model is crucial for revealing key features of the hydrothermal system: a vapour-rich conduit beneath Uturuncu (low resistivity/high attenuation column extending from 1.5 to 12.5 km depth), an extensive alteration zone surrounding Uturuncu (complex zone of annular shaped anomalies surrounding Uturuncu from 1.5 to 12.5 km depth), and a possible zone of sulfide deposition just below the western flank of Uturuncu at 1.5 km depth (high density/low resistivity/high attenuation). High fluxes of diffuse CO2 degassing at sub-magmatic temperatures and a small area directly above a low resistivity anomaly subsiding from 2014 to 2017 show that the hydrothermal system is currently active. Analyzed jointly, this multidisciplinary data set suggests that current activity within the shallow structure at Uturuncu is dominated by hydrothermal, rather than magmatic processes.

Enigmatic large‐scale (>150 km wide) ground deformation in southern Bolivia has been ongoing for more than 50 year. Concurrent changes in gravity recorded between 2010 and 2018 imply minor changes in subsurface density in the absence of significant mass changes. Numerical modeling of the gravity changes and concurrent InSAR LOS displacements gives annual bulk density changes of 0.002 kg m −3 in the Altiplano‐Puna Magma Body (APMB) and −0.03 kg m −3 in a vertical bulge‐column ensemble beneath Uturuncu volcano. We propose that the transcrustal migration of fluids from the APMB to shallower crustal levels by compressible flow is the source of ground deformation. Localized ground subsidence south of Uturuncu can be best explained by a density decrease of 20 ± 5 kg m −3 between 2011 and 2013 in a hydrothermal reservoir. Our findings contribute to the growing recognition of transcrustal fluid migration as a source of volcanic unrest.

Dome-building volcanoes are particularly challenging for volcanic hazard assessment, where long-term eruptive episodes can be interspersed with periods of intra-eruptive repose. Defining the end of eruptive episodes is vitally important for the socio-economic recovery of affected communities, but highly problematic due to the potential for prolonged, seemingly low-risk, repose to rapidly transition to dangerous effusive or explosive activity. It is currently unclear what constitutes the end of repose and an eruptive episode. Here we show that analysis of surface deformation can characterise repose and help define an eruptive episode. At Soufrière Hills volcano (SHV) the observed long-term deformation requires the pressure in the magma system to increase with time; time-dependent stress relaxation or crustal creep cannot explain the deformation trends alone. Continued pressurisation within the magmatic system during repose could initiate a renewed eruption, qualifying as sustained unrest and therefore continuation of the eruptive episode. For SHV, persistent magma pressurisation highlights the need for sustained vigilance in the monitoring and management of the volcano and its surroundings, despite the last eruptive activity ending in 2010.

The recent identification of unrest at multiple volcanoes that have not erupted in over 10 kyr presents an intriguing scientific problem. How can we distinguish between unrest signaling impending eruption after kyr of repose and non-magmatic unrest at a waning volcanic system? After ca. 250 kyr without a known eruption, in recent decades Uturuncu volcano in Bolivia has exhibited multiple signs of unrest, making the classification of this system as “active”, “dormant”, or “extinct” a complex question. Previous work identified anomalous low resistivity zones at <10 km depth with ambiguous interpretations. We investigate subsurface structure at Uturuncu with new gravity data and analysis, and compare these data with existing geophysical data sets. We collected new gravity data on the edifice in November 2018 with 1.5 km spacing, ±15 μGal precision, and ±5 cm positioning precision, improving the resolution of existing gravity data at Uturuncu. This high quality data set permitted both gradient analysis and full 3-D geophysical inversion, revealing a 5 km diameter, positive density anomaly beneath the summit of Uturuncu (1.5–3.5 km depth) and a 20 km diameter arc-shaped negative density anomaly around the volcano (0.5–7.5 depth). These structures often align with resistivity anomalies previously detected beneath Uturuncu, although the relationship is complex, with the two models highlighting different components of a common structure. Based on a joint analysis of the density and resistivity models, we interpret the positive density anomaly as a zone of sulfide deposition with connected brines, and the negative density arc as a surrounding zone of hydrothermal alteration. Based on this analysis we suggest that the unrest at Uturuncu is unlikely to be pre-eruptive. This study shows the value of joint analysis of multiple types of geophysical data in evaluating volcanic subsurface structure at a waning volcanic center.

As volcanic systems undergo unrest, understanding the conditions required for reservoir failure, the associated timescales, and the links to geophysical observations are critical when evaluating the potential for eruption. The characteristics and dynamics of a pressurised magmatic system can be inferred from episodes of surface deformation, but this process is heavily reliant on the assumed crustal rheology. In volcanic regions, shallow or long-lived magmatic systems can significantly perturb the regional geothermal gradient, altering the rheology of the surrounding crustal rock. Viscoelasticity incorporates a time-dependent viscous deformation response, accounting for the increased ductility and thermomechanical heterogeneity induced by the modelled reservoir. Here, we investigate the influence of an imposed thermal regime on the critical reservoir overpressure (OPc) required to facilitate failure in elastic and viscoelastic models, alongside the predicted critical surface uplift (Uc). By evaluating tensile and Mohr-Coulomb failure criteria on the reservoir walls, we can determine the mechanical stability of the magma reservoir and identify the conditions that are susceptible to failure. We explore a range of reservoir temperatures (representing felsic, intermediate, and mafic magma compositions) and background geothermal gradients, to simulate varied volcanic regions, and use the Standard Linear Solid viscoelastic rheology together with a temperature-dependent viscosity structure, calculated from the thermal constraints. The models incorporate mechanical heterogeneity in the form of a temperature-dependent Young’s modulus, accounting for the thermal weakening of the surrounding crustal rock. We use an overpressure rate of 10 MPa yr-1, in excess of lithostatic pressure, that produces an average elastic volumetric strain rate of ~3-7x10-12 s-1, depending on the imposed thermal regime. We show that reservoir failure is systematically inhibited by incorporating viscoelasticity, with OPc for Mohr-Coulomb failure increasing by up to 65% with respect to the corresponding elastic model. The greatest increases in OPc, and Uc, are observed when pairing cool reservoir temperatures (i.e., felsic composition) with low background geothermal gradients. In contrast, stress partitioning due to the viscoelastic crustal rheology promotes failure at the ground surface, decreasing the required OPc for tensile failure by up to 32%. The greatest reductions in OPc are produced in models that couple a hot reservoir temperature (i.e., mafic composition) with low background geothermal gradients. By resisting mechanical failure on the reservoir wall, temperature-dependent viscoelasticity impacts the conditions required for dyke nucleation and propagation. Further to this, a viscoelastic crustal rheology dramatically reduces the timescales for throughgoing failure; complete brittle failure connecting the reservoir to the ground surface. This occurs much earlier than suggested by elastic models, which could have implications for interpreting the conditions, and onset, of a potential eruption.

Volcanoes undergoing unrest often produce displacements at the ground surface, providing an important window to interpret the dynamics of the underlying magmatic system. The thermomechanical properties of the surrounding host rock are expected to be highly heterogeneous, with key physical parameters having a strong dependence on temperature. Deformation models that incorporate nonelastic rheological behaviors are therefore heavily reliant on the assumed thermal conditions, and so it is critical to understand how the thermomechanical crustal structure affects the observed deformation field. Here, we use a series of thermo‐viscoelastic Finite Element models to explore how variations in thermal constraints (i.e., reservoir temperature and background geothermal gradient) affect surface displacement patterns when using the Maxwell and Standard Linear Solid (SLS) viscoelastic configurations. Our results demonstrate a strong variability in the viscoelastic deformation response when changing the imposed thermal constraints, caused by the partitioning of deformation and the dissipation of induced stresses. When using the SLS rheology, we identify that cumulative long‐term displacements can vary by over 20%, relative to a reference model with a reservoir temperature of 900°C and background geothermal gradient of 30 K km −1 . The relative change increases to a maximum of 35% when thermal weakening of the Young's modulus is also considered. Contrastingly, the deformation patterns of the Maxwell rheology are governed by unbounded displacements and complete stress relaxation. Ultimately, we outline that uncertainties in the thermal constraints can have a significant impact on best‐fit source parameters (e.g., size and depth) and overpressure/volume‐change loading histories inferred from thermo‐viscoelastic models.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Other. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Dissecting a Zombie: Joint analysis of density and resistivity models reveals shallow structure and possible sulfide deposition at Uturuncu Volcano, BoliviaAuthorsPatriciaMacQueeniDJoachimGottsmanniDMatthew EPritchardiDNicolaYoungFaustinoTiconaEusebioTiconaRubenTintayaSee all authors Patricia MacQueeniDCorresponding Author• Submitting AuthorCornell UniversityiDhttps://orcid.org/0000-0001-7692-3416view email addressThe email was not providedcopy email addressJoachim GottsmanniDUniversity of BristoliDhttps://orcid.org/0000-0001-9280-4011view email addressThe email was not providedcopy email addressMatthew E PritchardiDCornell UniversityiDhttps://orcid.org/0000-0003-3616-3373view email addressThe email was not providedcopy email addressNicola YoungUniversity of Bristolview email addressThe email was not providedcopy email addressFaustino TiconaEscuela Militar de Ingenieríaview email addressThe email was not providedcopy email addressEusebio TiconaUniversidad Mayor de San Andrésview email addressThe email was not providedcopy email addressRuben TintayaObservatorio San Calixtoview email addressThe email was not providedcopy email address

Uturuncu volcano in southern Bolivia is something of a “zombie” volcano – presumed dead, but showing signs of life. The volcano has not erupted in 250 kyr, but is exhibiting unrest in the form of ground deformation, seismicity, and active fumaroles. Elucidating the subsurface structure of the volcano is key for interpreting this recent unrest. Magnetotelluric measurements revealed alternating high and low resistivity anomalies at depths <10 km beneath the volcano, with a low-resistivity anomaly directly beneath Uturuncu. A key question is, what is the nature of this anomaly? To what extent is it partial melt, a hydrothermal brine reservoir, or a mature ore body? Knowing the density of this anomaly could distinguish between these scenarios, but existing density models of the area lack sufficient resolution. To address this issue, we collected additional gravity measurements on the Uturuncu edifice with 1.5 km spacing in November 2018. Gradient analysis and geophysical inversion of these data revealed several features: a 5 km diameter, high density anomaly beneath the summit of Uturuncu (1 – 3 km elev.), a 20 km diameter ring-shaped negative density anomaly around the volcano (-3 – 4 km elev.), a NNE trending, positive density anomaly northwest of the volcano (0 – 4 km elev.), and a NW trending, negative density anomaly to the southeast. These structures often (but not always) align with resistivity anomalies, features in new seismic tomography models, and relocated earthquake hypocenters. Based on a joint analysis of these data, we interpret the positive density anomaly as a crystallizing dacite pluton, and the negative density ring anomaly as a zone of hydrothermal alteration. Earthquakes around the edges of the crystallizing pluton may represent escaping fluids as the magma cools. The high density anomaly to the northwest likely represents a solidified pluton, and the low density anomaly to the southeast may represent a fractured fault zone. We posit that the alternating zones of high and low resistivity anomalies represent zones of low and high fluid/brine content, respectively. Based on this analysis we suggest that the unrest at Uturuncu is unlikely to be pre-eruptive. This study shows the value of joint analysis of multiple types of geophysical data in evaluating volcanic subsurface structure.

Data and models presented in the paper "Dissecting a Zombie: Joint analysis of density and resistivity models reveals shallow structure and possible sulfide deposition at Uturuncu Volcano, Bolivia". See file "README.txt" for detailed descriptions of each item.

The Campi Flegrei caldera (Italy) has been undergoing unrest over the past five decades including episodes of rapid ground deformation, seismicity, and variations in gas emissions. Hydrothermal fluids and gases are released most vigorously in the central sector of the caldera at the fumarolic fields of Solfatara volcano and Pisciarelli. We conducted a high-precision gravity survey coupled with inverse modeling to image the shallow (<2-km depth) structure of the hydrothermal feeder system. Results indicate the presence of three low density bodies beneath Pozzuoli, Astroni volcano and the Solfatara/Pisciarelli fumarolic fields. The first two are inferred to be sealed hydrothermal systems trapped beneath impermeable cap rock, while the latter depicts a plume-like geothermal feeder system reaching the surface via a combination of Solfatara's maar-diatreme structure and the intersection of NW-SE and NE-SW trending regional faults. The density contrasts of the reservoirs from background values are best explained by a multiphase mixture of caldera fill containing a secondary and interconnected void volume fraction of between 0.2 and 0.3 that hosts a vapor volume fraction psi(v) of between 0.38 and 1 and a liquid volume fraction psi(l) fraction of between 0 and 0.62. This work highlights the control of volcano-tectonic structures on fluid movement in the shallow crust of hydrothermally active volcanic systems undergoing sustained or periodic unrest.