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Quantifying subsurface fluid flows and related heat and gas fluxes can provide essential clues for interpreting the evolution of volcanic unrest in volcanoes with active hydrothermal systems. To better constrain the distribution of current hydrothermal activity, we mapped diffuse soil CO 2 degassing, ground temperature and self-potential covering the summit of La Soufrière de Guadeloupe during 2022–2023. We identify areas of fluid recharge and the zones and extent of major ascending hydrothermal flows. This paper provides a first estimate for summit ground CO 2 flux of 4.20±0.86 t d - 1 , representing about half the CO 2 emissions from the summit fumaroles. We find an extensive area of ground heating of at least 22250±6900 m 2 in size and calculate a total ground heat flux of 2.93±0.78 MW, dominated by a convective flux of 2.25±0.46 MW. The prominent summit fractures exert significant control over hydrothermal fluid circulation and delimit a main active zone in the NE sector. The observed shift in subsurface fluid circulation towards this sector may be attributed to a changing ground permeability and may also be related to observed fault widening and the gravitational sliding of the dome’s SW flank. Our results indicate that the state of sealing of the dome may be inferred from the mapping of hydrothermal fluid fluxes, which may help evaluate potential hazards associated with fluid pressurisation. [ABSTRACT FROM AUTHOR]

La Fossa Caldera at Vulcano (Italy) has been showing signs of unrest since September 2021. To investigate this phenomenon, we conducted an analysis of geodetic and seismological data from July to December 2021. In particular, we analyzed Multi Temporal Interferometric Synthetic Aperture Radar and Global Navigation Satellite System data, showing a pronounced elliptical uplift signal, which we elaborated using analytical source modeling. Additionally, seismic data were used to identify seismicity associated with hydrothermal system activity and assess its temporal evolution. The results indicate that the observed deformation is consistent with the expansion of the hydrothermal system within the La Fossa Caldera. These findings align with the analysis of seismic data, revealing signals indicative of hydrothermal activity, such as Very Long Period events. The results suggest that the ongoing phenomenon since 2021 represents a hydrothermal unrest, similar to the one observed during the late 1970s to early 1990s. Plain Language Summary: La Fossa Caldera at Vulcano Island, part of the Aeolian Islands archipelago in Italy, has shown an increased volcanic activity since September 2021. This activity is characterized by an increase in fumarole temperatures, massive gas emissions, as well as a marked uplift of the crater area, accompanied by an increase in seismicity. To investigate the nature of these phenomena, an analysis of ground deformation data obtained from Multi Temporal Interferometric Synthetic Aperture Radar and Global Navigation Satellite System measurements is presented. Additionally, a detailed analysis of data recorded by the seismic network on Vulcano Island has been conducted. The results indicate that these anomalies can be attributed to the expansion of the hydrothermal system, a phenomenon previously observed in the late 1970s and early 1990s. Key Points: Multi Temporal Interferometric Synthetic Aperture Radar enabled investigating localized ground deformation in the La Fossa CalderaThe analysis of local seismicity indicates it is associated with the injection of fluids into conduit‐like structuresThe modeled source of ground deformation associated with the 2021 unrest is consistent with the pressurization of the hydrothermal system [ABSTRACT FROM AUTHOR]

Abstract La Fossa Caldera at Vulcano (Italy) has been showing signs of unrest since September 2021. To investigate this phenomenon, we conducted an analysis of geodetic and seismological data from July to December 2021. In particular, we analyzed Multi Temporal Interferometric Synthetic Aperture Radar and Global Navigation Satellite System data, showing a pronounced elliptical uplift signal, which we elaborated using analytical source modeling. Additionally, seismic data were used to identify seismicity associated with hydrothermal system activity and assess its temporal evolution. The results indicate that the observed deformation is consistent with the expansion of the hydrothermal system within the La Fossa Caldera. These findings align with the analysis of seismic data, revealing signals indicative of hydrothermal activity, such as Very Long Period events. The results suggest that the ongoing phenomenon since 2021 represents a hydrothermal unrest, similar to the one observed during the late 1970s to early 1990s.

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/m3 (MU simulations) and CO2 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. Plain Language Summary: Eruptions at Mt. Ruapehu in New Zealand often occur without any warning amid an absence of what are called pre‐eruptive geophysical signals. In order to study the detectability of relevant geophysical signals, we use physics‐based models to simulate two distinct subsurface processes at Mt. Ruapehu: magma accumulation and flow of hydrothermal fluids. Both processes involve fluid flow, density variations and pressure changes in the ground and have the potential to trigger eruptive activity. Here we identify distinct measurable sets of geophysical signals from either subsurface process: changes in the electrical and gravitational potential fields, or surface deformation. Our study highlights the benefit of computer models to provide useful information on the link between subsurface processes and measurable geophysical signals prior to eruptive activity. Our findings may have implications for volcano monitoring efforts at Mt. Ruapehu and other crater lake volcanoes. Key Points: Multiphysics simulations to study unrest processes at Mt. RuapehuSpatio‐temporal variations in self‐potential and gravity changes identified as indicators of magmatic unrestHydrothermal unrest induces resolvable ground displacements, and changes in gravity and self‐potential [ABSTRACT FROM AUTHOR]

Abstract 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/m3 (MU simulations) and CO2 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.

Fumarolic gas survey of dormant volcanoes in hydrothermal activity is crucial to detect compositional and mass flux changes in gas emissions that are potential precursors of violent phreatic or even magmatic eruptions. Here we report on new data for the chemical compositions (CO2, H2S, SO2) and fluxes of fumarolic gas emissions (97–104°C) from La Soufrière volcano in Guadeloupe (Lesser Antilles) obtained from both mobile MultiGas measurements and permanent MultiGas survey. This paper covers the period 2016–2020, encompassing a period of enhanced hydrothermal unrest including an abrupt seismic energy release (M 4.1) on April 27, 2018. Our dataset reveals fumarolic CO2/H2S and SO2/H2S gas trends correlated to the evolution of surface activity and to other geochemical and geophysical parameters. We demonstrate that, even under tropical conditions (high humidity and rainfall), MultiGas surveys of low-T fumarolic emissions permit to distinguish deeply sourced signals of volcanic unrest from secondary changes in degassing due to shallow forcing processes such as water-gas-rock interactions in the hydrothermal system and meteorological effects.