Claudia Troise

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.

A mechanical fluid-dynamical model for ground movements at Campi Flegrei caldera

Abstract We present here a consistent model, which explains the mechanisms of unrest phenomena at Campi Flegrei (Italy), both at short-term (years) and at secular scales. The model consists basically of two effects: the first one is related to the elastic response of the shallow crust to increasing pressure within a shallow magma chamber; the second involves the fluid-dynamics of shallow aquifers in response to increasing pressure and/or temperature at depth. The most important roles in the proposed model are played by the effect of lateral stress–strain discontinuities marking the inner caldera borders and by the response of the geothermal system to the increase of pressure and/or temperature coming from the magma chamber. The model takes into account most results of recent research, and it is able to explain in a unitary way most of geophysical observations recently collected at Campi Flegrei. Some new results, presented in this paper, explain the mechanism of generation of seismicity in terms of Coulomb stress changes, and the difference between secular and short-term deformations, in terms of the non linear effect of the bordering discontinuities. The coupled effects of magma chamber stresses with the dynamics of shallow fluids provide results for a semi-quantitative interpretation of observed ground deformation, seismicity and of their time evolution. The model has very important implications for the hazard assessment during unrest phenomena. It represents, for the part involving the fluid-dynamical response, the ideal continuation of the basic, fundamental models and intuitions of Oliveri del Castillo and coworkers (Oliveri del Castillo, A., Quagliariello, M.T., 1969. Sulla genesi del bradisimo flegro. Atti Associazione Geofisica Italiana, 18th Congress, Napoli, pp. 557–594; Casterano, L., Oliveri del Castillo, A., Quagliariello, M.T., 1976. Hydromdynamics and geodynamics in the Phlegraean Fields area of Italy. Nature, 264, 161–154).

Seismicity at Somma-Vesuvius and its implications for the 3D tomography of the volcano

Abstract A research project to define the feeding system of the Vesuvius volcano and the upper crustal structure of the area was started in 1993. The core of the project is the high resolution seismic tomography study by using explosive sources. Results of a preliminary 2D seismic profile have been discussed by Zollo et al. (1998) [J. Volcanol. Geotherm. Res., this volume]. The study of local seismicity at Vesuvius has many implications for the determination of the substructure of the volcanic area. The shape and size of the seismic volume and the study of the focal mechanisms put important constraints on the stress field of the area. The local seismicity can greatly improve the knowledge of the tridimensional velocity distribution of both P and S seismic waves inside the volcano, using passive tomographic techniques. In this paper we obtain a first three-dimensional tomographic image of the volcanic structure, obtained by inversion of first P and S arrival times of local earthquakes. The results show the existence of a sharp velocity contrast, along a lineation oriented NW–SE, cutting the crater along the line separating the relict of the Somma caldera from the southwest part of the volcano. This study confirms, also for S wave velocities, the absence of indications for a magma chamber in the first 4–5 Km below the sea level, as already evidenced, just for P waves, by 2D tomography. Furthermore, we discuss the improvement of resolution obtainable by deploying a small set of three component seismic stations to get a better coverage of the area.

Coulomb stress changes at calderas: Modeling the seismicity of Campi Flegrei (southern Italy)

[1] The local seismicity occurring at Campi Flegrei caldera during a recent unrest (1982-1984) has been modeled in terms of Coulomb stress changes due to magma chamber overpressure, superimposed on a background tensile regional stress. The computations, performed by a finite element method in heterogeneous axial-symmetric elastic models, also take into account the effects of the caldera structure on stress distribution. Coulomb stress changes have been obtained for the best oriented focal mechanisms, determined for the total applied stress, i.e., local change plus background regional stress. The results demonstrate a very good correlation between positive Coulomb stress changes and earthquake locations. Furthermore, there is a good agreement between observed focal mechanisms and the optimally oriented mechanisms as predicted by the model. The obtained results represent not only a fundamental contribution toward understanding mechanisms of seismic generation at Campi Flegrei and other calderas but also demonstrate that Coulomb stress changes in volcanic areas can be very effective, because of the high intensity, and deserve more attention than generally paid.

Mechanisms of activity and unrest at large calderas

Large caldera collapses represent catastrophic natural events, second only to large meteoritic impacts. In addition, some calderas are densely populated, making the risk extreme, even for moderate eruptions. Understanding caldera mechanisms, unrest and the danger of eruption is therefore a crucial challenge for Earth sciences. Several key features of caldera behaviour have yet to be fully understood. Through a combination of case studies and theoretical modelling, the following topics are addressed in this volume: the conditions required to produce and to release large volumes of magma erupted during caldera formation; how magmatic feeding systems evolve before and after a caldera has formed; the processes that limit the behaviour of precursors to eruptions; how pre-emptive precursors can be distinguished from those that drive unrest without an eruption; and given that post-collapse eruptions may occur across a wide area, the optimum procedures for designing hazard maps and mitigation strategies.

Internal stress field at Mount Vesuvius: A model for background seismicity at a central volcano

We propose a model to explain the background seismicity occurring at Somma-Vesuvins in its present, mostly quiescent period. A finite element procedure has been used to simulate the stress field due to gravitational body forces in an axisymmetric volcano characterized by a central high-rigidity anomaly. Results emphasize the important effect of axial high-rigidity, which concentrates at its borders stresses resulting from the gravitational load of the volcanic edifice, as well as external (regional) stresses. The joint effect of the gravitational loading and of the presence of the anomaly produces stresses very close to or above the critical rupture threshold. The observed spatial concentrations of seismicity and moment. release correlate well with peaks of computed maximum shear stress. Seismicity is then interpreted as due to small stress perturbations concentrated around the high-rigidity core and added to a system already close to the failure threshold. This model can explain the widely observed occurrence of background seismicity at central volcanoes worldwide.

Seismicity and 3-D substructure at Somma^Vesuvius volcano:evidence for magma quenching

Abstract We present new results on the velocity structure of the Somma–Vesuvius volcano, obtained by joint inversion of P- and S-wave arrival times from both local earthquakes and shot data collected during the TOMOVES 1994 and 1996 experiments. The use of a large set of earthquakes, recorded over a period of ten years by both temporary and permanent seismic stations, allowed us to enhance the resolution of the structure beneath the Somma–Vesuvius down to 5 km depth. The results obtained show the presence of a high Vp and Vp/Vs anomaly located around the crater axis, between 0 and 5 km depth, involving the volcano edifice and the carbonate basement westward deepening from the adjacent Apenninic belt. The whole available seismic catalogue between 1987 to 2000 (1003 events) has been relocated in the obtained 3-D velocity model. Seismicity appears to be clustered around the anomalous high rigidity body. Laboratory experiments at high temperatures and pressures on 1944 eruption lava samples, taken representative in composition of the magma masses erupted through the cycle 1631–1944, support the interpretation of this anomaly in terms of magma quenching along the main conduit, because of the exsolution of magmatic volatiles. The effect of volatiles from the melt at the eruption onset and through its explosive phases is to increase the solidus temperature well above its eruptive temperature, causing the immediate quenching of the system. This paper shows a good example of how seismic tomography and experimental petrology constrain magmatic models. Results have important implications for the hazard assessment at Somma–Vesuvius, and at other volcanoes worldwide where similar seismological evidence has been recently observed.

A model for earthquake generation during unrest episodes at Campi Flegrei and Rabaul Calderas

We have analysed the seismicity that occurred at Campi Flegrei caldera (Italy) in the period 1982–1984, during an unrest episode. Earthquake locations describe a system of inward dipping ring faults. Focal mechanisms of such events show a normal dip component, which is not in agreement with the differential uplift of the inner caldera, as defined by static ground deformations. We have performed a simulation of the stress field generated by overpressure in a magma chamber in presence of lateral discontinuities, using a boundary element method. Lateral discontinuities simulate the ring fault system marking the border of the inner caldera collapse. Results allow us to hypothesize that reverse fault slip on the ring fault is mainly aseismic, and such aseismic movement is able to focus normal fault shear stress along the lateral discontinuities. Aseismic slip on the ring fault in response to static deformation is also supported by the low seismic moment released (M0≅1015 Nm), about two orders of magnitude lower than expected from the shear slip on the discontinuities needed to accomplish the total static surface deformation (1.8 m). Such results have been compared with observations at Rabaul caldera, during a similar unrest episode. In this area, the seismic moment release is in good agreement with shear slip produced on a system of outward dipping ring faults, and seismicity is much more focused on the fault structures. Such a different behaviour can be interpreted, in the framework of our model, as due to the different sign of the stress normal to the ring faults, for inward and outward dip. The comparison between the two areas shed new light about the dynamics of earthquakes in calderas, in terms of the role played by ring fault systems.

Probabilistic Location of Seismic Sequences in Heterogeneous Media

We have developed a method for the Bayesian location of seismic sequences in 3D heterogeneous velocity models. The method is based on a Bayesian algorithm for single earthquake location. Joint location of seismic sequences is performed by summing the probability density functions for individual earthquake locations. One of the main features of the method is the ability to accumulate travel times in the heterogeneous model, as a function of both the seismic stations and the grid points, simulating the seismogenic volume. The ability to compute travel times just once, separately from the location process, allows for fast 3D locations. This is a very attractive feature for real-time monitoring. Probabilistic location of seismic sequences, visualized in terms of contours of earthquake density, moment, and energy release, etc., can be obtained to give a much more seismotectonic insight than classical location algorithms. The continuous character of the output quantities is particularly indicated for seismic-network-testing purposes. It is in fact possible to compute the response of the location procedure, given the seismic network geometry, to various input earthquake distributions. Some applications of the method are also shown in this work, both to the location of synthetic earthquakes and of real sequences. Seismic sequences at Vesuvius and Campi Flegrei volcanic areas (southern Italy) have been located by the probability algorithm, allowing a better picture of the primary seismogenic structures. We also apply the method to assess the resolution of the operating monitoring networks at the two areas.