Bonafede, Maurizio

The influence of the hydrothermal circulation on seismicity and uplift observed at the Campi Flegrei caldera (Italy) is a topic of great interest to the scientific community. Recently, Thermo-Poro-Elastic (TPE) inclusions were proposed as likely deformation sources. They are suitable to explain the mechanical effects induced by hot and pressurized hydrothermal fluids, possibly exsolved from underlying magma, and pervading an overlying brittle layer. Recent works show that a TPE inclusion located at approximately 2 km depth below the Campi Flegrei caldera significantly contributed to the large and rapid soil uplift observed during the ‘82-’84 unrest phase. In the present work we demonstrate that such a source of deformation is likely playing a role even in the current unrest phase, which is characterized by a much lower uplift-rate with respect to the one occurred in the previous unrest phase. We will show that the time-series of soil uplift observed in the last 18 years can be reproduced by assuming the reactivation of the same deformation source responsible of the ‘82-’84 unrest located within a shallow brittle layer at about 2 km depth. The presence of a brittle layer has been evidenced in the past by tomographic studies and is confirmed by a sharp variation of the b-value at the corresponding depth.We believe that our results provide very important insights and evidences, supporting the existence and the importance of an active thermo-poro-elastic deformation source, which can be useful for understanding the unrest of the Campi Flegrei caldera, from both a scientific and geohazard perspective.

In the present paper we introduce a numerical model for the representation of displacement, strain and stress due to single forces embedded in a layered elastic half-space. The code EFGRN/EFCMP (Elastic Forces GReeN functions/Elastic Forces CoMPutation) is able to represent the mechanical effects due to pre-assigned distributions of single forces. Even if internal deformation sources can be described by distributions of equivalent body forces with vanishing resultant and moment, single forces are employed in geophysics to represent hydraulic and/or lithostatic loads, effects of internal density anomalies, and even some kind of seismic events. A distribution of single forces is also used to describe the effects of an inelastic inclusion located inside an elastic medium. In fact, the recent literature shows that poro-elastic and thermo-elastic inclusions can be represented using single forces distributed on their boundaries. EFGRN/EFCMP shares the benefits of rapid and semi-analytical calculation offered by the parent code, EFGRN/EFCMP , which is instead suitable for the representation of extended dislocation sources, as seismic faults. The present code also provides an option for computing the effects of a distribution of single forces embedded in a homogeneous half-space, by using the analytical solutions of Mindlin. Accordingly, EFGRN/EFCMP can be a valid support both for the representation of forward models of deformation sources and for the procedures of inversion of geodetic data in a layered medium. We show some applications of the code and we provide several scripts in MATLAB language which help the user to quickly start using EFGRN/EFCMP

The deformation history of the Campi Flegrei caldera during the last decades consists of two large uplift events in 1970-72 and 1982-84, with ~ 3.5 m cumula-tive uplift, occurring at a rate of ~ 1 m/yr. Both events were accompanied by seismic activity, gravity changes and compositional variations of volcanic gasesbut no eruption took place. During the following decades,the area has been slowly subsiding but minor uplift episodes (~ a few cm), seismic swarms and changes in degassing activitytook place, showing that the area persistedin a near-critical state.Since 2004, ground deformation resumed, although at a slower rate, totaling a displacement of ~ 0.6 m at the time we write(2019). In this chapter,we present a retrospective analysis of ground deformationleading to acritical re-evaluationof the 1982-84 uplift and of the following deflation, employing the most updated modeling techniques. Deformation and gravity data provide important constraints on depth, volume, mass density and dislocation mechanisms accompanying mag-ma emplacement processes but the correct evaluation of these parameters is strongly conditioned by simplifying assumptions built in the different inversion procedures: in particular, the inferred depth ranges from 5.5 km to less than 3 km and the intrusion density ranges from values pertinent to aqueous fluids to typical magmatic values. This review depicts the salient phases of the deformationhistory of this densely populated and high risk volcanic area, helping to address debated issues, such as the role of the magmatic system, and theirinteraction with the shal-lower hydrothermal system.In spite of the mentioned difficulties, the following conclusion may be considered as firmly established: during 1982-84 a magmatic intrusion took place, and the subsequent complex deformation history (1985-2010) was mainly controlled by the exsolution of volatiles withmagmatic origin and their interaction with a shallow hydrothermal system. However, data collected af-ter 2011 were modelled in terms of a deeperinflating source of deformation, pos-sibly resulting froma resumed magmatic recharge at depth.

The Emilia-Romagna seismic sequence in May 2012 was characterized by two mainshocks which were close in time and space. Several authors already modeled the geodetic data in terms of the mechanical interaction of the events in the seismic sequence. Liquefaction has been extensively observed, suggesting an important role of fluids in the sequence. In this work, we focus on the poroelastic effects induced by the two mainshocks. In particular, the target of this work is to model the influence of fluids and pore-pressure changes on surface displacements and on the Coulomb failure function (CFF). The fluid flow and poroelastic modeling was performed in a 3D half-space whose elastic and hydraulic parameters are depth dependent, in accordance with the geology of the Emilia-Romagna subsoil. The model provides both the poroelastic displacements and the pore-pressure changes induced coseismically by the two mainshocks at subsequent periods and their evolution over time. Modeling results are then compared with postseismic InSAR and GPS displacement time series: the InSAR data consist of two SBAS series presented in previous works, while the GPS signal was detected adopting a variational Bayesian independent component analysis (vbICA) method. Thanks to the vbICA, we are able to separate the contribution of afterslip and poroelasticity on the horizontal surface displacements recorded by the GPS stations. The poroelastic GPS component is then compared to the modeled displacements and shown to be mainly due to drainage of the shallowest layers. Our results offer an estimation of the poroelastic effect magnitude that is small but not negligible and mostly confined in the near field of the two mainshocks. We also show that accounting for a 3D fault representation with a nonuniform slip distribution and the elastic-hydraulic layering of the half-space has an important role in the simulation results.

The 2012 Emilia Romagna (Italy) seismic sequence has been extensively studied given the occurrence of two mainshocks, both temporally and spatially close to each other. The recent literature accounts for several fault models, obtained with different inversion methods and different datasets. Several authors investigated the possibility that the second event was triggered by the first mainshock with elusive results. In this work, we consider all the available InSAR and GPS datasets and two planar fault geometries, which are based on both seismological and geological constraints. We account for a layered, elastic half-space hosting the dislocation and compare the slip distribution resulting from the inversion and the related changes in Coulomb Failure Function (CFF) obtained with both a homogeneous and layered half-space. Finally, we focus on the interaction between the two main events, discriminating the contributions of coseismic and early postseismic slip of the mainshock on the generation of the second event and discuss the spatio-temporal distribution of the seismic sequence. When accounting for both InSAR and GPS geodetic data we are able to reproduce a detailed coseismic slip distribution for the two mainshocks that is in accordance with the overall aftershock seismicity distribution. Furthermore, we see that an elastic medium with depth dependent rigidity better accounts for the lack of the shallow seismicity, amplifying, with respect to the homogeneous case, the mechanical interaction of the two mainshocks

Changes in water level are commonly reported in regions struck by a seismic event. The sign and amplitude of such changes depend on the relative position of measuring points with respect to the hypocenter, and on the poroelastic properties of the rock. We apply a porous media flow model (TOUGH2) to describe groundwater flow and water‐level changes associated with the first ML5.9 mainshock of the 2012 seismic sequence in Emilia (Italy). We represent the earthquake as an instantaneous pressure step, whose amplitude was inferred from the properties of the seismic source inverted from geodetic data. The results are consistent with the evolution recorded in both deep and shallow water wells in the area and suggest that our description of the seismic event is suitable to capture both timing and magnitude of water‐level changes. We draw some conclusions about the influence of material heterogeneity on the pore pressure evolution, and we show that to reproduce the observed maximum amplitude it is necessary to take into account compaction in the shallow layer.

The existence and survival of volcanic lakes require the accomplishment of a delicate balance between meteoric recharge, evaporation, and water loss by infiltration within the volcanic edifice, commonly referred to as seepage. A deep-seated, volcanic component may participate to a variable extent to the lake’s evolution, depending on volcanic activity. In this work, we apply a numerical model of hydrothermal fluid circulation to study the interaction between the hot volcanic gases and the shallow lake water. We focus on the conceptual model developed for Poás volcano (Costa Rica), where a shallow magma intrusion drives the hydrothermal activity underneath and around the crater lake. Numerical simulations are carried out to assess the role of relevant system properties, including rock permeability, reservoir conditions, lake geometry, and meteoric recharge. Our results suggest that vertical seepage can be severely hindered by the ascent of volcanic gases, whereas horizontal infiltration through the vertical lake walls may ensure a long-term water loss. Our simulations also show that the permeability distribution, especially around the lake, determines the overall pattern of circulation affecting the development and spatial distribution of hot springs and fumaroles, and ultimately controlling the evolution of the lake.

Pressurized cavities are commonly used to compute ground deformation in volcanic areas: the set of available solutions is limited and in some cases the moment tensors inferred from inversion of geodetic data cannot be associated with any of the available models. Two different source models (pure tensile source, TS and mixed tensile/shear source, MS) are studied using a boundary element approach for rectangular dislocations buried in a homogeneous elastic medium employing a new C/C++ code which provides a new implementation of the dc3d Okada fortran code. Pressurized triaxial cavities are obtained assigning the overpressure in the middle of each boundary element distributed over the cavity surface. The MS model shows a moment domain very similar to triaxial ellipsoidal cavities. The TS and MS models are also compared in terms of the total volume increment limiting the analysis to cubic sources: the observed discrepancy (~10%) is interpreted in terms of the different deformation of the source interior which provides significantly different internal contributions (~30%). Comparing the MS model with a Mogi source with the some volume, the overpressure of the latter must be ~37% greater than the former, in order to obtain the same surface deformation; however the outward expansion and the inner contraction separately differ by ~±10% and the total volume increments differ only by ~2%. Thus, the density estimations for the intrusion extracted from the MS model and the Mogi model are nearly identical.

Campi Flegrei caldera (Italy) was affected by a new unrest phase during 2011-13. We exploit two COSMO-SkyMed datasets to map the deformation field, obtaining displacement rates reaching 9 cm/yr in 2012 in the caldera center. The resulting dataset is fitted in a geophysical inversion framework using finite element forward models to account for the 3D heterogeneous medium. The best-fit model is a North dipping mixed-mode dislocation source lying at ~5 km depth. The driving mechanism is ascribable to magma input into the source of the large 1982-84 unrest (since similar source characteristics were inferred) that generates initial inflation followed by additional shear slip accompanying the extension of crack tips. The history and the current state of the system indicate that Campi Flegrei is able to erupt again, and the advanced techniques adopted provide useful information for short-term forecasting.

Dikes and sills are the moving building blocks of the plumbing system of volcanoes and play a fundamental role in the accretionary processes of the crust. They nucleate, propagate, halt, resume propagation, and sometimes change trajectory with drastic implications for the outcome of eruptions (Sigmundsson et al., 2010). Their dynamics is still poorly understood, in particular when different external influencing factors are interacting. Here we apply a boundary element model to study dike and sill formation, propagation and arrest in different scenarios. We model dikes as finite batches of compressible fluid magma, propagating quasi-statically in an elastic medium, and calculate their trajectories by maximising the energy release of the magma-rock system. We consider dike propagation in presence of density layering, of density plus rigidity layering, of a weakly welded interface between layers, under the action of an external stress field (of tectonic or topographic origin). Our simulations predict sill formation in several situations: i) when a horizontal weak interface is met by a propagating dike; ii) when a sufficiently high compressive tectonic environment is experienced by the ascending dike and iii) in case a dike, starting below a volcanic edifice, propagates away from the topographic load with a low dip angle. We find that dikes halt and stack when they become negatively buoyant and when they propagate with low overpressure at their upper tip toward a topographic load. Neutral buoyancy by itself cannot induce dikes to turn into sills, as previously suggested.