Academic literature on the topic 'Hydrothermal unrest'

Large calderas formed by explosive eruption are often characterized by a structural uplift of the caldera floor that has been named resurgent dome or block. The analysis of recent unrest at several calderas suggests that the resurgent dome is likely formed by the intrusion of magma at shallow depth below the light caldera infill. Campi Flegrei is an active volcano considered the highest risk volcano in Italy and Europe and has been in a state of unrest in the last 70 years. The analysis of the past volcanological history point to an unrest localized in a central resurgent block. Different or paired interpretations on the current unrest suggest either an intrusion of magma at shallow depth (3-5 km) or a deformation governed by the poro-elastic response of a shallow hydrothermal system to changes in fluid pressure and temperature. The invariance of the shape of the deformation, as well as the diffuse degassing of the Solfatara area, hint that the unrest is related with the uplift of the resurgent block driven by magma intrusion at shallow depth.

As our ability to detect volcanic unrest improves, we are increasingly confronted with the question of whether the unrest has a magmatic origin (magma on the move) or a non-magmatic origin from a change in the hydrothermal system (fluids that are not magma on the move) or tectonic processes. The cause of unrest has critical implications for the potential eruptive hazard (e.g. used in constructing Bayesian Event Trees), but is frequently the subject of debate, even at well-studied systems. Here, we propose a set of multi-disciplinary observations and numerical models that could be used to evaluate conceptual models about the cause of unrest. These include measurements of gas fluxes and compositions and the isotopic signature of some components (e.g. H 2 , He, C, SO 2 , H 2 O , CH 4 and CO 2 ), the spatial and temporal characteristics of ground deformation, thermal output, seismicity, changes in gravity, and whether there is topographic uplift or subsidence spanning hundreds to thousands of years. In several volcanic systems, both magmatic and non-magmatic unrest is occurring at the same time. While none of these observations or models is diagnostic on its own, we illustrate several examples where they have been used together to make a plausible conceptual model of one or more episodes of unrest and whether eruptions did or did not follow the unrest. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’.

Abstract. Ground deformation and gravity changes in active calderas during periods of unrest can signal an impending eruption and thus must be correctly interpreted for hazard evaluation. It is critical to differentiate variation of geophysical observables related to volume and pressure changes induced by magma migration from shallow hydrothermal activity associated with hot fluids of magmatic origin rising from depth. In this paper we present a numerical model to evaluate the thermo-poroelastic response of the hydrothermal system in a caldera setting by simulating pore pressure and thermal expansion associated with deep injection of hot fluids (water and carbon dioxide). Hydrothermal fluid circulation is simulated using TOUGH2, a multicomponent multiphase simulator of fluid flows in porous media. Changes in pore pressure and temperature are then evaluated and fed into a thermo-poroelastic model (one-way coupling), which is based on a finite-difference numerical method designed for axi-symmetric problems in unbounded domains. Based on data for the Campi Flegrei caldera (Italy), a series of simulations assess the influence of fluid injection rates and mechanical properties on the hydrothermal system, uplift and gravity. Heterogeneities in hydrological and mechanical properties associated with the presence of ring faults are a key determinant of the fluid flow pattern and consequently the geophysical observables. Peaks (in absolute value) of uplift and gravity change profiles computed at the ground surface are located close to injection points (namely at the centre of the model and fault areas). Temporal evolution of the ground deformation indicates that the contribution of thermal effects to the total uplift is almost negligible with respect to the pore pressure contribution during the first years of the unrest, but increases in time and becomes dominant after a long period of the simulation. After a transient increase over the first years of unrest, gravity changes become negative and decrease monotonically towards a steady state value. Since the physics of the investigated hydrothermal system is similar to any fluid-filled reservoir, such as oil fields or CO2 reservoirs produced by sequestration, the generic formulation of the model will allow it to be employed in monitoring and interpretation of deformation and gravity data associated with other geophysical hazards that pose a risk to human activity.

Abstract. Ground deformation and gravity changes in restless calderas during periods of unrest can signal an impending eruption and thus must be correctly interpreted for hazard evaluation. It is critical to differentiate variation of geophysical observables related to volume and pressure changes induced by magma migration from shallow hydrothermal activity associated with hot fluids of magmatic origin rising from depth. In this paper we present a numerical model to evaluate the thermo-poroelastic response of the hydrothermal system in a caldera setting by simulating pore pressure and thermal expansion associated with deep injection of hot fluids (water and carbon dioxide). Hydrothermal fluid circulation is simulated using TOUGH2, a multicomponent multiphase simulator of fluid flows in porous media. Changes in pore pressure and temperature are then evaluated and fed into a thermo-poroelastic model (one-way coupling), which is based on a finite-difference numerical method designed for axi-symmetric problems in unbounded domains.Informed by constraints available for the Campi Flegrei caldera (Italy), a series of simulations assess the influence of fluid injection rates and mechanical properties on the hydrothermal system, uplift and gravity. Heterogeneities in hydrological and mechanical properties associated with the presence of ring faults are a key determinant of the fluid flow pattern and consequently the geophysical observables. Peaks (in absolute value) of uplift and gravity change profiles computed at the ground surface are located close to injection points (namely at the centre of the model and fault areas). Temporal evolution of the ground deformation indicates that the contribution of thermal effects to the total uplift is almost negligible with respect to the pore pressure contribution during the first years of the unrest, but increases in time and becomes dominant after a long period of the simulation. After a transient increase over the first years of unrest, gravity changes become negative and decrease monotonically towards a steady-state value.Since the physics of the investigated hydrothermal system is similar to any fluid-filled reservoir, such as oil fields or CO2 reservoirs produced by sequestration, the generic formulation of the model will allow it to be employed in monitoring and interpretation of deformation and gravity data associated with other geophysical hazards that pose a risk to human activity.

Geohazards associated to submarine hydrothermal systems still represent a tricky enigma to face and solve for the scientific community. The poor knowledge of a submarine environment, the rare and scarce monitoring activities, and the expensive and sometimes complicated logistics are the main problems to deal with. The submarine low-energy explosion, which occurred last November 3, 2002, off the volcanic island of Panarea, highlighted the absence of any hazard scenario to be used to manage the volcanic crisis. The “unrest” of the volcanic activity was triggered by a sudden input of deep magmatic fluids, which caused boiling water at the sea surface with a massive CO2 release besides changes in the fluids’ geochemistry. That event dramatically pushed scientists to develop new methods to monitor the seafloor venting activity. Coupling the information from geochemical investigations and data collected during the unrest of volcanic activity, we were able to (a) develop theoretical models to gain a better insight on the submarine hydrothermal system and its relationships with the local volcanic and tectonic structures and (b) to develop a preliminary submarine volcanic hazard assessment connected to the Panarea system (Aeolian Islands). In order to mitigate the potential submarine volcanic hazard, three different scenarios are described here: (1) ordinary hydrothermal venting, (2) gas burst, and (3) volcanic eruption. The experience carried out at Panarea demonstrates that the best way to face any submarine volcanic-hydrothermal hazard is to improve the collection of data in near real-time mode by multidisciplinary seafloor observatories and to combine it with periodical sampling activity.

Katla is one of the most active volcanoes in Iceland and is characterised by persistent seismicity. It is partly covered by the Mýrdalsjökull glacier and its historic activity is dominated by phreatomagmatic eruptions within the caldera associated with catastrophic glacial floods. In July 2011 a sudden jökulhlaup was released from the glacier, associated with tremor, elevated seismicity inside the caldera and a new cluster of seismicity on the south flank. This was likely caused by a hydrothermal or magmatic event, possibly a small subglacial eruption. Similar unrests occurred in 1955 and 1999. We have identified changes of the seismicity pattern coinciding with the 2011 unrest, suggesting a modification in the volcanic system. It may be speculated that if the persistent seismicity at Katla is an indication of a pressurized magma system ready to erupt, small events like those of 1955, 1999 and 2011 may trigger larger eruptions in the future. We have also conducted a pilot study of the geology of the southern flank, where the new seismicity is recorded, and identified sources for flank eruptions in the recent eruptive history of Katla. These include rhyolitic domes and surtseyan craters. Therefore, a wide range of volcanic processes have to be taken into account as possible source for the new seismicity and volcanic hazard.

In recent decades, the Campi Flegrei caldera (Italy) showed unrest characterized by increases in seismicity, ground uplift, and hydrothermal activity. Currently, the seismic and hydrothermal phenomena are mostly concentrated in the Solfatara–Pisciarelli area, which presents a wide fumarolic field and mud emissions. The main fumarole in Pisciarelli is associated with a boiling mud pool. Recently, episodes of a sudden increase in hydrothermal activity and expansion of mud and gas emissions occurred in this area. During these episodes, which occurred in December 2018 and September 2020, Short Duration Events (SDEs), related to the intensity of mud pool boiling, were recorded in the fumarolic seismic tremor. We applied a Self-Organizing Map (SOM) neural network to recognize the occurrence of SDEs in the fumarolic tremor of Campi Flegrei, which provides important information on the state of activity of the hydrothermal system and about the possible phreatic activity. Our method, based on an ad hoc feature extraction procedure, effectively clustered the seismic signals containing SDEs and separated them from those representing the normal fumarolic tremor. This result is useful for improving the monitoring of the Solfatara–Pisciarelli hydrothermal area which is a high-risk zone in Campi Flegrei.

SUMMARY Unrest at Long Valley caldera (California) during the past few decades has been attributed to the ascent of hydrothermal fluids or magma recharge. The difference is critical for assessing volcanic hazard. To better constrain subsurface structures in the upper crust and to help distinguish between these two competing hypotheses for the origin of unrest, we model the 3-D seismic attenuation structure because attenuation is particularly sensitive to the presence of melt. We analyse more than 47 000 vertical component waveforms recorded from January 2000 through November 2016 obtained from the Northern California Earthquake Data Center. We then inverted the S-to-coda energy ratios using the coda normalization method and obtained an average Q of 250. Low attenuation anomalies are imaged in the fluid-rich western and eastern areas of the caldera, one of which corresponds to the location of an earthquake swarm that occurred in 2014. From a comparison with other geophysical images (magnetotellurics, seismic tomography) we attribute the high attenuation anomalies to hydrothermal systems. Average to high attenuation values are also observed at Mammoth Mountain (southwest of the caldera), and may also have a hydrothermal origin. A large high attenuation anomaly within the caldera extends from the surface to the depths we can resolve at 9 km. Shallow rocks here are cold and this is where earthquakes occur. Together, these observations imply that the high attenuation region is not imaging a large magma body at shallow depths nor do we image any isolated high attenuation bodies in the upper ≈8 km that would be clear-cut evidence for partially molten bodies such as sills or other magma bodies.

Abstract. Large calderas are among the Earth's major volcanic features. They are associated with large magma reservoirs and elevated geothermal gradients. Caldera-forming eruptions result from the withdrawal and collapse of the magma chambers and produce large-volume pyroclastic deposits and later-stage deformation related to post-caldera resurgence and volcanism. Unrest episodes are not always followed by an eruption; however, every eruption is preceded by unrest. The Campi Flegrei caldera (CFc), located along the eastern Tyrrhenian coastline in southern Italy, is close to the densely populated area of Naples. It is one of the most dangerous volcanoes on Earth and represents a key example of an active, resurgent caldera. It has been traditionally interpreted as a nested caldera formed by collapses during the 100–200 km3 Campanian Ignimbrite (CI) eruption at ∼39 ka and the 40 km3 eruption of the Neapolitan Yellow Tuff (NYT) at ∼15 ka. Recent studies have suggested that the CI may instead have been fed by a fissure eruption from the Campanian Plain, north of Campi Flegrei. A MagellanPlus workshop was held in Naples, Italy, on 25–28 February 2017 to explore the potential of the CFc as target for an amphibious drilling project within the International Ocean Discovery Program (IODP) and the International Continental Drilling Program (ICDP). It was agreed that Campi Flegrei is an ideal site to investigate the mechanisms of caldera formation and associated post-caldera dynamics and to analyze the still poorly understood interplay between hydrothermal and magmatic processes. A coordinated onshore–offshore drilling strategy has been developed to reconstruct the structure and evolution of Campi Flegrei and to investigate volcanic precursors by examining (a) the succession of volcanic and hydrothermal products and related processes, (b) the inner structure of the caldera resurgence, (c) the physical, chemical, and biological characteristics of the hydrothermal system and offshore sediments, and (d) the geological expression of the phreatic and hydromagmatic eruptions, hydrothermal degassing, sedimentary structures, and other records of these phenomena. The deployment of a multiparametric in situ monitoring system at depth will enable near-real-time tracking of changes in the magma reservoir and hydrothermal system.

Mt. Spurr is a volcano in proximity to Anchorage, Alaska and major airline routes making an eruption or episode of unrest potentially hazardous. Between 2004 and 2006, Mt. Spurr underwent such an episode of unrest involving increased seismic activity, CO2 emissions, ice melting, and debris flows, which was likely forecasted by the increased seismicity of Oct 2002. The timeline of events provide data to construct a model analyzing the thermal energy release and constraining subsurface magmatic and hydrothermal processes during the period of unrest. The results show that the ice cauldron formation and the increase of meltwater temperature could not have been caused by the observed CO2 release alone and suggest that enhanced hydrothermal heat transfer related to increased CO2 output could provide the thermal power necessary to drive the melting event. Scaling hydrothermal convection in terms of its Rayleigh number and using boundary layer analysis suggests that the mean permeability of the volcanic edifice prior to the unrest event was ~10-14 m2. CO2 release, most likely related to mechanical fracturing of the edifice by over-pressurized fluids at depth and signaled by increased seismicity likely enhanced the hydrothermal Rayleigh number and heat output by a combination of heating and increased permeability.
Master of Science

We investigated the relationship between seismo-volcanic events, recorded at La Fossa crater of Vulcano (Aeolian Islands, Italy) during 2004-2009, and the dynamics of the hydrothermal system. During the period of study, six episodes of increasing numbers of seismo-volcanic events took place at the same time as geothermal and geochemical anomalies were observed. These geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids. Four classes of seismic events (long period, high frequency, monochromatic and tornillos events), characterised by different spectral content and various similarity of the waveforms, have been recognised. These events, clustered mainly below La Fossa crater area at depths of 0.5 1.1 km b.s.l., were space-distributed according to the classes. Based on their features, we can infer that such events at Vulcano are related to two different source mechanisms: (1) fracturing processes of rocks and (2) resonance of cracks (or conduits) filled with hydrothermal fluid. In the light of these source mechanisms, the increase in the number of events, at the same time as geochemical and geothermal anomalies were observed, was interpreted as the result of an increasing magmatic component of the hydrothermal fluids, implying an increase of their flux. Indeed, such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids. Increased pore pressures gave rise to fracturing processes, while the increased fluid flux favoured resonance and vibration processes in cracks and conduits. Finally, a gradual temporal variation of the waveform of the hybrid events (one of the subclasses of long period events) was observed, likely caused by heating and drying of the hydrothermal system. After careful analysis of the seismo-volcanic events of the Aeolian Islands area, the attention was paid to the tectonic events, in order to find possible relationships with the volcanic activity in the area. The aim of this part of the thesis was to identify spatial clusters of earthquakes, locate active seismogenic zone and their relationships with the volcanic activity in the Aeolian Islands. High precision locations were performed in the present thesis, by applying the concept of the velocity model-hypocentres joint inversion and earthquake relocations, along with an analysis of the fault plane solutions. In order to improve our knowledge on the active seismo-tectonics areas we exploited a dataset encompassing 351 events recorded during a 17 year period (1993-2010). Overall, our results show that part of the seismicity is clustered along active seismogenic structures that concur with the main regional tectonic trends whose activity furnishes new elements to better understand the dynamics of the area. A cluster of 24 events in the northern part of Vulcano, NE-SW oriented, marks the presence of a structure that seems to play a key role in magma uprising at Vulcano. These earthquakes suggest the existence of a seismogenic structure (passing just below Vulcanello), which could be interpreted as a discontinuity linking the two magma accumulation zones, thereby representing a possible preferential pathway along which magma may intrude as well as being responsible for fluid migration toward the surface. The results presented in this thesis suggest that the comparison of seismic, ground deformation and temperature data can be useful for better understanding the dynamics of a complex volcano-hydrothermal system, including a better definition of the origin of a volcano unrest, and hence for improving the estimation of the level of the local volcanic hazard.

L'activité hydrothermale volcanique présente des risques permanents liés à l'émission de gaz toxiques provenant à la fois du sol et des fumerolles. Cependant, des risques plus difficilement prévisibles peuvent également survenir tels que les explosions phréatiques ou l'effondrement des flancs. La présence d'un système hydrothermal a des implications importantes pour l'interprétation des signaux provenant du système magmatique. Par conséquent, la distribution spatiale et l'évolution temporelle des signaux géophysiques et géochimiques sur les volcans hydrothermaux donnent des informations cruciales pour la détection des précurseurs de l'activité éruptive.La Soufrière de Guadeloupe est actuellement dans une phase de réactivation qui a débuté en 1992 et dont l'intensité a augmenté en 2018. Afin de mieux comprendre le système hydrothermal de La Soufrière, nous avons effectué plusieurs cartographies du dégazage diffus du CO2 de la température et du Potentiel Spontané (PS) au niveau du dôme entre 2021 et 2024. Ce travail représente la première cartographie du PS depuis plus d'une décennie et la première quantification du dégazage de CO2 sur le sommet. Il fournit une image actualisée de la distribution de la circulation des fluides souterrains et les flux de chaleur et de CO2 associés. Nous proposons également une méthodologie numérique basée sur un modèle physique et des images thermiques des panaches de fumerolles permettant d'améliorer la quantification des flux des principales fumerolles de La Soufrière.En comparant nos mesures entre elles et à celles des études antérieures, nous constatons que la circulation des fluides hydrothermaux dans le secteur nord-est du sommet a augmenté de manière significative au cours de ces dernières années. Les profondeurs de condensation des fluides hydrothermaux ascendants suggèrent que ce développement peut être dû à un changement de la distribution de la perméabilité souterraine liée aux déformations du dôme. En parallèle, nous avons étudié la dynamique des flux hydrothermaux à l'aide d'une série temporelle du PS sur deux ans. Cette analyse montre des variations diurnes et semi-diurnes liées aux marées atmosphériques. Enfin, nous analysons la réponse du système hydrothermal aux précipitations, à la sismicité et à la température des fumerolles. Les résultats obtenus montrent que le secteur nord-est du sommet est fortement interconnecté et met en évidence le contrôle de la dynamique du système hydrothermal par les principales fractures du sommet.Cette thèse propose une image de la distribution actuelle et de l'évolution spatiotemporelle de la circulation des fluides hydrothermaux à La Soufrière de Guadeloupe. Nos résultats ont permis d'identifier les zones pour la surveillance future. De plus, les jeux de données acquises permettront de mieux contraindre les modèles issus d'autres méthodes géophysiques afin de déterminer l'état interne du dôme et d'évaluer les risques potentiels liés au dégazage passif, à l'altération ou à la pressurisation des fluides
Volcanic hydrothermal activity poses unpredictable hazards like phreatic explosions or flank collapse, as well as pervasive hazards such as the emission of hot, toxic gases from steaming ground and fumaroles. The presence of a hydrothermal system has important implications for interpreting signals from the magmatic system. Therefore, the spatial distribution and temporal evolution of geophysical and geochemical signals at volcanoes with long-lived hydrothermal systems provide crucial information for detecting precursors of eruptive activity.La Soufrière de Guadeloupe volcano is currently undergoing a phase of unrest, which started in 1992 and saw an increase in intensity in 2018. To advance the understanding of the shallow hydrothermal system at La Soufrière, we repeatedly mapped diffuse CO2 degassing, ground temperature and self-potential across the dome summit from 2021 to 2024. This work represents the first mapping of self-potential in over a decade and the first quantification of CO2 degassing over the entire summit. It provides an up-to-date picture of the distribution of subsurface fluid circulation and the associated ground heat and CO2 fluxes. We also outline a numerical approach to improve the quantification of the fumarole fluxes based on a physical plume model and thermal images of the fumarole plumes and use this to calculate heat and mass fluxes from La Soufrière's major fumaroles.Our multi-parameter mappings, repeated self-potential profiles, and comparisonswith previous studies show that hydrothermal fluid circulation in the northeastern summit sector has significantly increased over the last decade. Estimated condensation depths of ascending hydrothermal fluids suggest that this development may be due to a change in the distribution of subsurface permeability, which is likely related to the dome displacement field. The short-term dynamics of hydrothermal fluid circulation are investigated using a two-year self-potential time series. We observe diurnal and semidiurnal variations linked to atmospheric tides. Finally, we analyse the response of the shallow hydrothermal system to precipitation, seismicity and fumarole temperature.This shows that the northeastern summit sector is highly interconnected and highlights the strong structural control of the hydrothermal system dynamics by the main summit fractures.This work provides a picture of the current distribution and spatiotemporal evolution of shallow hydrothermal fluid circulation at La Soufrière de Guadeloupe. This helps us to identify the preferred zones for future monitoring. The datasets generated will help to constrain models from other geophysical methods to infer the internal state of the dome and assess potential hazards related to passive degassing, alteration or fluid pressurisation

Kawah Ijen (2386 m) is a stratovolcano located within Ijen Caldera, at the easternmost

part of Java island in Indonesia. Since 2010, the volcano has been equipped with seismometers

and several sensors (temperature and level) have been immersed in its acidic lake waters and in the acidic river seeping on the volcano flanks. While finding instruments capable of resisting to such extreme conditions (pH~0) has been challenging, the coupling of lake monitoring techniques with seismic data improves the knowledge of the volcanic-hydrothermal dynamics. Moreover, the monitoring capabilities have been considerably

enhanced supporting the decision-making of the authorities in case of emergency.

Several methods and processing techniques were used to analyze the seismic data. Much effort has been given to implement the seismic velocities (Moving Window Cross Spectral Analysis (MWCSA)) calculations. At Kawah Ijen, the frequency band that is less affected by the volcanic tremor and the seasonal fluctuations at the source ranges between 0.5-1.0 Hz. Moreover, a stack of 5 days for the current CCF gives reliable results with low errors and allows to detect fluctuations which are missed using a 10-day stack.

The background seismic activity mostly consists in low frequency events and a continuous tremor of low amplitude. Fluctuations of the lake temperature and level result from the recharge of the hydrothermal system during the rainy season. Kawah Ijen lake waters are not perfectly mixed and a shallow stratification occurs during the rainy season, because meteoric waters are less dense than the lake fluids.

Different unrest occurred during our study. Some of them strongly affected the volcanic lake, while others did only weakly. In the first category, a strong unrest commenced in October 2011 with heightened VT (Volcano Tectonic) earthquakes and low frequency events activity, which culminated mid-December 2011. This unrest was correlated with an enhanced heat and hydrothermal fluids discharge to the crater and significant variations of the relative velocities (~1%). This suggests an important build-up of stress into the system. VT earthquakes opened pathways for the fluids to ascend, by increasing the permeability of the system, which latter allowed the initiation of monochromatic tremor (MT) when the steam/gases interacted with the shallow portions of the aquifer. Our calculations evidence a higher contribution of steam in March 2012 that might explain the increase of the MT frequency when bubbles were observed at the lake surface. This period was also characterized by short-lived but strong velocity variations, related to water level

rises containing important amount of bubbles, and important heat and mass discharges

into the lake. On the contrary, the second category of unrest did only slightly affect the

lake system. This could be explained by a dryer hydrothermal system and/or locations of

the seismic sources, which were not directly linked to the lake.

While a magmatic eruption will likely be preceded by a strong seismic activity, the major challenges remain to understand why the unrest we studied did not lead to an eruption and to identify precursory signs of a phreatic eruption. Even a small phreatic eruption would be devastating for the people working everyday in the crater and the ones

who live nearby the voluminous acidic lake.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished