Giuseppe Mastrolorenzo

Genesis and evolution of unrest episodes at Campi Flegrei caldera : The role of thermal fluid-dynamical processes in the geothermal system

We develop a model for describing water flow in a porous medium under the effect of thermal and pressure gradients. The model simulates geothermal systems in calderas. Given the boundary conditions and the fluid-dynamical properties of the medium, the model allows computation, in fluid-dynamical stationary states, of parameters characterizing the flow, such as flow velocity and temperature and pressure distributions at depth. The model is applied to investigate the effects of the local geothermal system on the unrest episodes at Campi Flegrei caldera. Using experimentally determined fluid-dynamical parameters for the caldera rocks, we show that changes of water flow in shallow aquifers under the effect of pressure and/or temperature variations within the geothermal system can be very important in the genesis and evolution of unrest crises. In particular, they can strongly amplify the effect of pressure increase in the magma chamber on ground uplift. They can also explain the timescales of evolution of ground movements in terms of transit times of the water front and of the connected temperature fronts due to advective transport. On such grounds an integrated mechanic-thermal fluid-dynamical model was built, allowing us to give a semiquantitative, global explanation to the genesis and evolution of unrest phenomena. Results obtained here can be generalized to other similar calderas.

The Campi Flegrei caldera: unrest mechanisms and hazards

Abstract In the last four decades, Campi Flegrei caldera has been the world’s most active caldera characterized by intense unrest episodes involving huge ground deformation and seismicity, but, at the time of writing, has not culminated in an eruption. We present a careful review, with new analyses and interpretation, of all the data and recent research results. We deal with three main problems: the tentative reconstruction of the substructure; the modelling of unrest episodes to shed light on possible pre-eruptive scenarios; and the probabilistic estimation of the hazards from explosive pyroclastic products. The results show, for the first time at a volcano, that a very peculiar mechanism is generating episodes of unrest, involving mainly activation of the geothermal system from deeper magma reservoirs. The character and evolution of unrest episodes is strongly controlled by structural features, like the ring-fault system at the borders of the caldera collapse. The use of detailed volcanological, mathematical and statistical procedures also make it possible to obtain a detailed picture of eruptive hazards in the whole Neapolitan area. The complex behaviour of this caldera, involving interaction between magmatic and geothermal phenomena, sheds light on the dynamics of the most dangerous types of volcanoes in the world.

The 472 AD Pollena eruption of Somma-Vesuvius (Italy) and its environmental impact at the end of the Roman Empire

Abstract Catastrophic sedimentary processes associated with explosive eruptions represent a significant geologic hazard in volcanic areas. Here we report a striking historic example of an intermediate-scale explosive event whose environmental effects were strongly amplified by secondary rapid mass flows and hydrogeologic disasters. The 472 AD Pollena eruption of Somma-Vesuvius (Campania, Italy) took place in the critical period of the fall of the Western Roman Empire. On the basis of an integrated geologic–archaeologic study we point out evidence of human habitation at the time of the eruption, effects induced and recovery time in a wide territory of Campania, and how the eruption significantly accelerated the deterioration of the local society during the Late Ancient age. The eruption began with a pulsating, sustained eruption column, followed by pyroclastic surges and scoria flows. Hydromagmatism acted early in the event, different from the typical Plinian eruptions of Somma-Vesuvius. Specific facies associations of primary and secondary volcaniclastic deposits characterize three depositional domains, including the volcano slopes, the surrounding alluvial plains and the distal mountains of the Apennine Range. Both volcano slopes and distal mountain slopes supplied loose pyroclastic material to the hyperconcentrated floods and debris flows that spread across the alluvial plains. The great impact of secondary volcaniclastic processes arose from: (1) the high vulnerability of the territory due to its geomorphic context; (2) the humid climatic conditions; (3) the hydromagmatic character of the eruption; (4) the decline of land management at the end of the Roman Empire.

Numerical simulation of pyroclastic density currents on Campi Flegrei topography: a tool for statistical hazard estimation.

Abstract We describe a numerical simulation of both concentrated and dilute gravity-driven pyroclastic flows on a digital topographic model of the Campi Flegrei volcanic field. Families of numerical flows are generated by sampling a multi-dimensional matrix of vent coordinates, flow properties and dynamical parameters within a wide range of values. Hazard maps are constructed from the data base of simulated flows, using a mixed deterministic–statistical approach. The set of probable vents covers the area of recent eruptions. Results show the key role of topography in controlling the flow dispersion. The maximum hazard appears to be the NE sector of the caldera. Flows in the eastern sector, including the city of Naples, are shown to be efficiently hindered by the Posillipo and Camaldoli hills at the caldera borders, thus reducing the hazard. The results represent the first physically based estimate of hazard from pyroclastic flows in this densely populated area, and can be used for civil defence purposes.

Vesuvius 1906: a case study of a paroxysmal eruption and its relation to eruption cycles

Abstract The April 1906 effusive and explosive paroxysm of Vesuvius concluded a 34-year-long cycle, characterized by prevalent effusive activity. The eruption began on 4 April 1906 with limited summit explosions that preceded the lava effusion from vents between 1200 and 600 m a.s.l. on the southern flank of the cone. In the late evening of April 7 the explosive strombolian activity evolved to the paroxysmal phase with the formation of two-km-high lava fountains that continued with varying intensity for about 4 hours. This phase generated a moderately dispersed tephra-fall deposit consisting of strongly vesiculated black scoriae and rare lithic clasts. When the lava fountaining ended, the subterminal effusive activity increased On 8 April, at 12:37 a.m. a violent earthquake preceded a change in the eruptive style with the ejection of both strongly fragmented, incandescent, vesiculated juvenile material and lithic fragments generated from the violent cone demolition During this phase the eruptive columns progressively drifted toward the north-northeast causing severe damage to the town of Ottaviano. At 4 p.m. another drastic change in the eruptive style occurred, with the formation of a giant sustained gas jet with a low concentration of dense blocky hydrated glass (the intermediate gas phase reported by Perret). This phase was generated by a powerful interaction of phreatic water with a small amount of degassed magma. The paroxysmal eruption ended with a low column generated by the degassing of the hydrothermal system (dark ash phase of Perret). This last episode deposited only a thin ash layer constituted by non-vesiculated ash particles.

Computer simulations of pyroclastic flows on Somma–Vesuvius volcano

Abstract We describe a numerical simulation of pyroclastic flows on Somma–Vesuvius consisting of two integrated parts: forward and inverse approaches. The forward approach consists of the numerical generation of radial, one-dimensional flow lines from the vent, over a digitised topographic model of the Somma–Vesuvius area. The numerical flows are generated sampling a multi-dimensional matrix of dynamic and rheological parameters based on data from real eruptions. Analysis of the simulated flow patterns suggests that the pyroclastic flows of Vesuvius are compatible with a relatively dense flow model. The acceptable parameter values are confined within a relatively small range (moderate flow thickness and viscosity and initial velocity near 50 m/s). The diffusion of most of the pyroclastic flows, even with high initial velocity was mainly restricted to the southern sectors of the volcano by the Mt. Somma rim. The range of flow patterns generated by the direct approach was sampled for inverse analysis of eruptive conditions and rheology of the historical pyroclastic flows associated with the A.D. 79 and A.D. 1631 Plinian and sub-Plinian eruptions. The results of inversion suggest that flows were compatible with Bingham rheology (yield strength=600 Pa), initial velocity ranging between 50 and 100 m/s, viscosity ranging between 30 and 700 Pa s, and flow thickness ranging between 2 and 10 m.

Campi Flegrei unrest episodes and possible evolution towards critical phenomena

Mechanical and thermal-fluid-dynamical contributions to unrest phenomena at active calderas are quantitatively evaluated, and a mixed mechanical-thermal-fluid-dynamical model is developed, to explain the unrest phenomena at Campi Flegrei caldera. Mechanical modelling involves the use of analytical and finite element formulations. Thermal-fluid-dynamical effects are considered in the framework of a one-dimensional approximation, solved by an analytical method. The results shed new light about the coupled effect of mechanically and thermally induced perturbations and variations in the geothermal fluid circulation regime. Unrest episodes at Campi Flegrei are interpreted in terms of the joint effect of a first, essentially mechanical, phase due to overpressure in a magma chamber and to a progressive amplification and final decay due to the circulating water. The elastic response of the rocks to the migrating front of overpressure is shown to be strongly conditioned by the caldera structure, which produces a concentration of strain in the inner part, and a stress concentration, causing the local seismicity, around the caldera borders. Thermal-fluid-dynamical effects are also shown to have important implications on the evolution of the volcanic system towards critical phenomena, which must be considered for a meaningful evaluation of volcanic hazard.

Computer simulation of pyroclastic flow movement: An inverse approach

A computer simulation of the Campanian Ignimbrite (Southern Italy) utilizes both forward and inverse techniques. The forward technique uses a given set of parameters (eruptive vent, initial velocity and rheology of the pyroclastic mass) to compute the trajectories and limits of pyroclastic flow propagation. The inverse approach, based on the Bayes theorem, calculates the probability density of the input parameters based on the limits of actual deposits in the field. The inverse method allows definition of the most likely set of eruption parameters consistent with the emplacement of the Campania Ignimbrite. These simulations furnish the following results: a) the Campanian Ignimbrite formation is compatible with a purely gravitative mode of movement; b) the areal distribution is consistent with a radial flow pattern from a limited vent area north of Naples; c) the most probable initial flow velocity is about 160 m/s which corresponds to a possible eruptive column height near 1.3 km.

Volcanic hazard assessment at the Campi Flegrei caldera

Abstract Previous and new results from probabilistic approaches based on available volcanological data from real eruptions of Campi Flegrei, are assembled in a comprehensive assessment of volcanic hazards at the Campi Flegrei caldera, in order to compare the volcanic hazards related to the different types of events. Hazard maps based on a very wide set of numerical simulations, produced using field and laboratory data as input parameters relative to the whole range of fallout and pyroclastic-flow events and their relative occurrence, are presented. The results allow us to quantitatively evaluate and compare the hazard related to pyroclastic fallout and density currents (PDCs) in the Campi Flegrei area and its surroundings, including the city of Naples. Due to the dominant wind directions, the hazard from fallout mostly affects the area east of the caldera, and the caldera itself, with the level of probability and expected thickness decreasing with distance from the caldera and outside the eastern sectors. The hazard from PDCs decrease roughly radially with distance from the caldera centre and is strongly controlled by the topographic relief, which produces an effective barrier to propagation of PDCs to the east and northeast, areas which include metropolitan Naples. The main result is that the metropolitan area of Naples would be directly exposed to both fallout and PDCs. Moreover, the level of probability for critical tephra accumulation by fallout is relatively high, even for moderate-scale events, while, due to the presence of topographic barriers, the hazard from PDCs is only moderate and mostly associated with the largest events.

Probabilistic-numerical assessment of pyroclastic current hazard at Campi Flegrei and Naples city: Multi-VEI scenarios as a tool for "full-scale" risk management

The Campi Flegrei volcanic field (Italy) poses very high risk to the highly urbanized Neapolitan area. Eruptive history was dominated by explosive activity producing pyroclastic currents (hereon PCs; acronym for Pyroclastic Currents) ranging in scale from localized base surges to regional flows. Here we apply probabilistic numerical simulation approaches to produce PC hazard maps, based on a comprehensive spectrum of flow properties and vent locations. These maps are incorporated in a Geographic Information System (GIS) and provide all probable Volcanic Explosivity Index (VEI) scenarios from different source vents in the caldera, relevant for risk management planning. For each VEI scenario, we report the conditional probability for PCs (i.e., the probability for a given area to be affected by the passage of PCs in case of a PC-forming explosive event) and related dynamic pressure. Model results indicate that PCs from VEI<4 events would be confined within the Campi Flegrei caldera, PC propagation being impeded by the northern and eastern caldera walls. Conversely, PCs from VEI 4–5 events could invade a wide area beyond the northern caldera rim, as well as part of the Naples metropolitan area to the east. A major controlling factor of PC dispersal is represented by the location of the vent area. PCs from the potentially largest eruption scenarios (analogous to the ~15 ka, VEI 6 Neapolitan Yellow Tuff or even the ~39 ka, VEI 7 Campanian Ignimbrite extreme event) would affect a large part of the Campanian Plain to the north and the city of Naples to the east. Thus, in case of renewal of eruptive activity at Campi Flegrei, up to 3 million people will be potentially exposed to volcanic hazard, pointing out the urgency of an emergency plan. Considering the present level of uncertainty in forecasting the future eruption type, size and location (essentially based on statistical analysis of previous activity), we suggest that appropriate planning measures should face at least the VEI 5 reference scenario (at least 2 occurrences documented in the last 10 ka).