Todesco, Micol

The Colli Albani is a quiescent volcano located nearby the city of Roma, characterised by the presence of an active geothermal system, periodic seismic swarms and intense diffuse degassing. Several accidents, some of which lethal, have occurred in recent years associated to episodes of more intense releases and outbursts of volcanic gases, dominantly CO2 and H2S. Gas emissions are presently the most hazardous phenomenon for the highly populated Colli Albani area, along with the potential occurrence of seismic activity. This chapter presents the numerical modeling of heat and fluid circulation applied to study the mechanisms which control the diffuse degassing at Colli Albani volcano. Multi-phase and multi-component simulations were carried out using the TOUGH2 geothermal simulator in a realistic geological context, which includes all available information on the stratigraphy and structure of the Colli Albani substrate, along with data on the total gas flux, the local geothermal gradient, the local hydrogeology, and the thermal characteristics of the rocks. The geothermal reservoir at Colli Albani is hosted by the 2-3000 m thick Mesozoic-Cenozoic carbonatic succession capped by Pliocene clays which act as aquiclude and are few hundreds to over 1000m thick, in turned covered by continental sedimentary and volcanic deposits, which host the shallow hydrogeological system. Numerical simulations evaluate the effects associated with the thickness of the carbonatic basement and its cap rock; the role of CO2 supply rate at depth; and the influence of permeable channelways through the cap rocks. Numerical simulations show that thickness of the geothermal reservoir hosted by the carbonatic basement and of its impervious cover control the vigor of the convection, the extent and depth (and hence temperature) of the lateral recharge area, and the distribution of the carbon dioxide within the system. This result suggests that the temperature distribution and diffuse degassing at surface do not simply reflect the characteristics of the heat and fluid source at depth, but also the specific structure and hydrological properties of the site where they are measured.

A full review of the 79 CE Plinian eruption of Vesuvius is presented through a multidisciplinary approach, exploiting the integration of historical, stratigraphic, sedimentological, petrological, geophysical, paleoclimatic, and modelling studies dedicated to this famous and devastating natural event. All studies have critically been reviewed and integrated with original data, spanning from proximal to ultradistal findings of the 79 CE eruption products throughout the Mediterranean. The work not only combines different investigation approaches (stratigraphic, petrological, geophysical, modelling), but also follows temporally the 79 CE eruptive and depo sitional events, from the magma chamber to the most distal tephras. This has allowed us first to compile a full database of all findings of those deposits, then to relate the products (the deposits) to the genetic thermo mechanical processes (the eruption), and lastly to better assess both the local and regional impacts of the 79 CE eruption in the environment. This information leads to a number of open issues (e.g., regional environmental impact vs. local pyroclastic current impact) that are worthy of further investigations, although the 79 CE eruption of Vesuvius is one of the best studied eruptions in volcanology. The structure of the work follows three macro-categories, the historical aspects, the products, and the processes of the 79 CE eruption. For each investigation approach (from stratigraphy to modelling), all dedicated studies and original data are discussed. The open issues are then synthesized in the discussion under a global view of Plinian eruptions, from the magma setting to its dispersion as pyroclasts flowing on the surface vs. falling from the volcanic plume. In this way, a lesson from the past, in particular from the well-studied 79 CE eruption of Vesuvius, will be of help for a better synchronization of processes and products in future developments. Lastly, various aspects for volcanic hazard assessment of Plinian eruptions are highlighted from the tephra distribution and modelling points of view, as these large natural phenomena can have a larger impact than previously thought, also at other active volcanoes.

Explosive events are commonly accompanied or followed by heavy rains. These eruption- induced storms together with the deposition of large amounts of ash contribute to destabilise the hydrological cycle in the areas affected by volcanic eruptions. Flooding of the region surround- ing the active volcano can easily follow, increasing the complexity of the volcanic crisis and its management. This is particularly true in the case of Vesuvius, that is not only characterized by a dramatic volcanic hazard, but it is also located within an area that is normally prone to flood hazard. A complete assessment of the impact associated with explosive volcanic eruptions should involve a flood hazard assessment for the region. This work represents a first attempt to address the problem: a topographically based rainfall-runoff model was here applied to the Vesuvian area where two main sub-basins were analysed. The model was applied to evaluate the role of selected parameters on the total discharge at the basins’ outlet. These parameters were chosen among those likely to be affected by an explosive event and were varied through a reasonable range. Results confirm that the deposition of large amounts of ash can affect the temporal evolution of the discharge and its maximum value, for a given precipitation event. The simulations presented outline the need for a detailed flood forecasting study for the Vesuvian area, that should be included within the hazard mitigation strategies.

The area known as Terre Calde (literally “hot lands”) in the plain of the Po River (Italy) is well known for unusual ground temperatures, and up to now, the cause o/f the heating has not been fully investigated. These higher-than-average temperatures are commonly associated with diffuse methane seepage. A detailed study of shallow stratigraphy, temperature profile, and associated gas concentrations and flow rates recently suggested that the observed anomaly could be related to the exothermic oxidation of biogenic methane, possibly rising from a shallow peat layer. In this work, a porous media flow simulator (Transport of Unsaturated Groundwater and Heat 2) was applied to verify a conceptual model of this phenomenon. The model describes a layered system, with a shallow unsaturated zone, where methane is continuously supplied along the base and heat is generated as a result of its oxidation above the water table. To mimic the oxidation process, heat sources are placed within the layer where oxidation takes place, and the heat generation is computed as a function of methane flux entering the layer. Numerical simulations were carried out imposing different methane flow rates along the base of the model. The simulations also explored the efficiency of methane oxidation, considering different heat generation rates and accounting for seasonal effects. The good match between observed and simulated temperature profiles suggests that the main features of the process are captured by the model and that the conceptual model devised on the base of available data is plausible from a physical point of view.

Volcanic eruptions increasingly present catastrophic natural risks with hundreds of mil- lions of people now living in areas of active volcanism and major conurbations around active eruptive centres. Interdisciplinary studies in disaster reduction have an important role in volcanic emergency management through advancing our understanding of the physical impacts of eruptive phenomena and the causes of death and injury in explosive eruptions. Numerical modelling of pyroclastic flows, amongst the most destructive of eruptive phenomena, provides new opportunities to improve the evaluation of the potential destructiveness of volcanic events and their human impacts in densely populated areas. In this work, the results of numerical modelling of pyroclastic flow propagation at Vesuvius have been analysed in terms of the physical parameters (temperature, ash in air concentra- tion, and dynamic pressure) that are most critical for human survival. Our numerical simulations of eruptions of Vesuvius indicate that a large area exists where total destruction may not be inevitable in small to medium scale events, a finding that has prompted us to explore further the implications for human survival as part of an interdisciplinary approach to disaster reduction. The lessons of mod- elling at Vesuvius should be integrated into civil protection plans for other urban centres threatened by volcanoes.

Ground deformation is commonly observed at active volcanoes, where it represents a reliable sign of unrest and a potential precursor of eruptive activity. The source of deformation, however, is not always unequivocally constrained. Magma ascent and differentiation are generally involved, but hydrothermal fluids may play a role, due to thermal expansion and pore pressure acting on rocks. The identification of mechanisms driving ground displacement bears important consequences for hazard evaluation. The aim of this work is to evaluate mechanical effects associated with pressurization and heating of hydrothermal fluids. We first simulate the heat and fluid flow driven by the arrival of magmatic fluids from greater depth. Then, we calcu- late the rock deformation arising from simulated pressure and temperature changes within a shallow hydrothermal system. We employ a mathematical model, based on the linear theory of thermo-poro-elasticity and on a system of distributed equivalent forces. Results show that stronger degassing of a magmatic source may cause several centimeters of uplift.

The Torre Alfina medium enthalpy geothermal field is located about 10 km north of the Bolsena caldera (Italy). The reservoir is a buried structural high consisting of fractured Meso-Cenozoic carbonate sequences and sealed by clayey flysch successions and Pliocene marine clays. We performed TOUGH2 numerical simulations, testing different model designs based on contrasting conceptual models. Results indicate that deep circulation is forced by the geometry of the reservoir and by the applied T and P gradients. We interpret the Torre Alfina field as a "blind" system, mostly recharged by lateral advection of heat and fluids from the Bolsena caldera deep high-enthalpy system, through the permeable caldera faults.

Controversies that stir the public debate on geological matters usually revolve around a few specific aspects, including the actual trigger of geological phenomena (i.e., natural vs. anthropogenic), their predictability, and the trustworthiness of the experts who provide information and advice on the phenomena. A typical example of such difficulties is the case of the 2012 Emilia, Italy, seismic sequence which struck an area of relatively moderate seismic hazard. In that period, geophysical prospecting was planned to assess the potential of a reservoir for gas storage, near the town of Rivara. The low frequency of important seismic events in the area, associated with the ongoing industrial planning prompted widespread rumors of an anthropogenic origin of the 2012 earthquakes. Controversy also arose about the actual size of the seismic events: earthquakes magnitude can be computed with different methods, and its value depends on the type, number, and geographical distribution of the available seismic stations. As a result, different institutions commonly release different estimates of the earthquake magnitude, casting doubts on the reliability of each estimate. Since 2012, public concern has also been caused by the repeated occurrence of unusual phenomena in the area, such as ground heating or bubbling well waters. Popular belief tends to establish a causal link between particular phenomena and seismic activity, reinforcing the false conviction that seismicity could be predicted. In this work we present and discuss some of the activities that INGV pursued through the years to contrast rumors and disseminate correct scientific information. In the aftermath of the 2012 seismic sequence, INGV worked in collaboration with the National Department of Civil Protection, the local administrations, the University Network of seismic engineering, the Regional Healthcare System and local volunteer organizations. The organization of public meetings, the collection and analysis of widespread rumors and the creation of ad hoc outreach materials all contributed to reinforce the mutual trust between our research institute and the local population. KEYWORDS

Hydrothermal activity is often associated with active volcanic systems. During quiescent times, geochemical monitoring of discharged fluids commonly is carried out to gain insights on the state of evolution of the whole volcanic system. The interest in geochemical monitoring derives from the observation that compositional variations of discharged fluids are commonly observed as a new eruptive phase is approaching. In particular, an increase in the gas components of direct magmatic origin may indicate a higher magma degassing rate at depth, potentially related to a renewal of explosive activity. Surveillance programs devoted to hazard mitigation in active volcanic areas generally include periodic analyses of discharged fluids at various locations on the volcanic edifice. Unfortunately, when significant changes are observed in gas composition, their correct interpretation in terms of system evolution is not always clear and straightforward. Several mechanisms may in fact be responsible for differences in the proportion of magmatic gases and shallower fluid components, and it is not always possible to recognize the magmatic gas fraction. Discrimination among fluids of different origin ideally is accomplished based on the isotope composition that each fluid acquires at the time of its generation. However, this isotope signature can be altered before the fluids reach the surface, because of mixing between fluids of different origin, or due to reactions that modify the original isotope composition. Thus, the interpretation of isotope data sometimes is quite complex and it can be highly misleading. In this work, the origin of the steam discharged at the hydrothermal system of Vulcano (Italy) was investigated by the means of a dual approach: first, the available isotope data on the discharged steam were analyzed. On the basis of on these and other data, a conceptual model of the hydrothermal system was developed and numerical simulations of a multiphase, multicomponent flow through a heterogeneous porous medium were carried out to evaluate the effects of various parameters on the system evolution. Results suggest that the hydrothermal system of Vulcano is controlled by the entrance of large quantities of seawater whose original isotope composition has been altered by water-rock interaction. Rock permeability and its distribution at depth were shown to be a primary parameter, controlling the overall system evolution, the fluid composition, and fluid discharge rate at the surface.

Monitoring of quiescent volcanoes, such as Campi Flegrei (Italy), involves the measurement of geochemical and geophysical parameters that are expected to change as eruptive conditions approach. Some of these changes are associated with the hydrothermal activity that is driven by the release of heat and magmatic fluids. This work focuses on the properties of the porous medium and on their effects on the signals generated by the circulating fluids. The TOUGH2 porous media flow model is applied to simulate a shallow hydrothermal system fed by a source of magmatic fluids. The simulated activity of the source, with periods of increased fluid discharge, generates changes in gas composition, gravity, and ground deformation. The same boundary conditions and source activity were applied to simulate the evolution of homogeneous and heterogeneous systems, characterized by different rock properties. Phase distribution, fluid composition, and the related signals depend on the nature and properties of the rock sequence through which the fluids propagate. Results show that the distribution of porosity and permeability affects all the observable parameters, controlling the timing and the amplitude of their changes through space and time. Preferential pathways for fluid ascent favor a faster evolution, with larger changes near permeable channels. Slower changes over wider areas characterize less permeable systems. These results imply that monitoring signals do not simply reflect the evolution of the magmatic system: intervening rocks leave a marked signature that should be taken into account when monitoring data are used to infer system conditions at depth.