M. Bonafede - Academia.edu (original) (raw)

Papers by M. Bonafede

Annals of Geophysics, 2009

Employing a 3D finite element method, we develop an algorithm to calculate gravity changes due to... more Employing a 3D finite element method, we develop an algorithm to calculate gravity changes due to pressurized sources of any shape in elastic and inelastic heterogeneous media. We consider different source models, such as sphere, spheroid and sill, dilating in elastic media (homogeneous and heterogeneous) and in elasto-plastic media. The models are oriented to reproduce the gravity changes and the surface deformation observed at Campi Flegrei caldera (Italy), during the 1982-1984 unrest episode. The source shape and the characteristics of the medium have great influence on the calculated gravity changes, leading to very different values for the source densities. Indeed, the gravity residual strongly depends upon the shape of the source. Non negligible contributions also come from density and rigidity heterogeneities within the medium. Furthermore, if the caldera is elasto-plastic, the resulting gravity changes exhibit a pattern similar to that provided by a low effective rigidity. Even if the variation of the source volumes is quite similar for most of the models considered, the density inferred for the source ranges from ∼400 kg/m 3 (super critical water) to ∼3300 kg/m 3 (higher than trachytic basalts), with drastically different implications for risk assessment.

Tectonophysics, 1990

Albarello, IX and Bonafede, M., 1990. Stress diffusion across laterally heterogeneous plates. In:... more Albarello, IX and Bonafede, M., 1990. Stress diffusion across laterally heterogeneous plates. In: E. Mantovani {Editor), Seismicity and Crustal Deformation. ~ecfo~o~~ysics, 179: 121-130. Stress diffusion following a major dislocation event may provide significant contributions to the stress field present in the neighbouring regions. Seismic regularities observed in the southern Adriatic region are used in the present paper to constrain possible physical models of stress diffusion. Lateral variations in the rheological structure of the lithosphere-asthenosphere system are considered. A diffusion equation with variable diffusivity is obtained using the Elsasser approximation, which describes migration of the stress field after a dislocation event. Approximate solutions of this equation are worked out in the long wavelength limit, showing that diffusion is faster where the lithosphere is thicker and slower where it is thinner. Lithosphere thinning accordingly provides a higher amplitude and longer duration diffusional stress following a dislocation event. This behaviour is particularly evident in a transient regime. It seems to be widely accepted that the space and time distribution of seismicity in seismic regions which are tectonically connected may have some degree of interdependence. Evidence of seismic migration or coupled earthquakes has been identified in many areas of the world (see e.g. Kasahara, 1979). In particular, a significant degree of correlation between intense seismic activities on the eastern and western margins of the Adriatic region (Fig. 1) is suggested by several statistical studies based on the Pearson Correlation (Muc

Journal of Volcanology and Geothermal Research, 2005

The 1982–1984 unrest episode at Campi Flegrei (CF) caldera, Italy, was characterized by huge defo... more The 1982–1984 unrest episode at Campi Flegrei (CF) caldera, Italy, was characterized by huge deformation (more than 1.5 m uplift) concentrated inside the caldera. According to point source isotropic models in homogeneous elastic and visco-elastic half-spaces, the source depth is very shallow (∼3 km). If the source radius is ∼1 km, this implies that magma is at a depth of ∼2 km depth. However, several independent observations show that the top of the magma chamber at CF must be deeper than ∼4 km. This paper investigates how the inferred source depth increases when rigidity heterogeneities (obtained through seismic tomography at CF) are considered and when the long-term deformation takes place under drained conditions. Finite element models indicate that overpressure needed at the source to reproduce the 1.5 m maximum uplift is however beyond typical rock strength values. This evidence, together with the high thermal anomalies, the presence of fluids and the low cohesion of tuffs filling the caldera, suggests the use of elasto-plastic constitutive laws. For elasto-plastic behavior, the same deformation is obtained using a deeper source (with center at ∼5 km depth) and a lower overpressure (than required by elastic models). The plastic deformation concentrates both at the source boundaries and above the source, where seismic activity has been recorded. These results indicate that the rheological properties of the shallow crust of CF have important implications for hazard estimate during unrest episodes.

Journal of Geophysical Research, 2010

Geophysical Journal International, 2010

We develop a mathematical model describing dyke propagation in proximity of an elastic discontinu... more We develop a mathematical model describing dyke propagation in proximity of an elastic discontinuity of the embedding medium. The dyke is modelled as a fluid-filled crack in plane strain configuration employing the boundary element method. The pressure gradient along the crack is assumed proportional to the difference between the densities of the host rock and the fluid. Mass conservation is imposed during propagation and fluid compressibility is taken into account. The path followed by the crack is found by maximizing the total energy release, given by the sum of the elastic and gravitational contributions. The mathematical simulations provide a sort of 'refraction phenomenon', that is a sudden change in the direction of propagation when the crack crosses the boundary separating different rigidities: if the dyke enters a softer medium, its path deviates towards the vertical, if the dyke enters a harder medium its path deviates away from the vertical and may even become arrested as a horizontal sill along the interface, if the rigidity contrast is large. Gravitational energy plays a major role during propagation; in particular, in proximity of layer boundaries, this role is enhanced by the shift of the centre of mass due to changes of dyke shape. Mathematical results were validated by laboratory experiments performed injecting tilted air-filled cracks through gelatin layers with different rigidities.

Geophysical Journal International, 2009

Deformation data collected at Mount Etna from 1993 to 1997 show that the volcano edifice inflatio... more Deformation data collected at Mount Etna from 1993 to 1997 show that the volcano edifice inflation is accompanied by instability of the eastern flank. We propose a 3D finite element model including lateral variations of material properties and topography. Source parameters of the inflating source are constrained by a direct search followed by an appraisal stage of the sampled solutions. The instability of the eastern flank is here addressed using a kinematic approach consisting in a rigid translation of a prescribed area. Our aim is to evaluate how the inflating source inference is affected by eastern flank sliding. Our results show that when sliding is accounted for, the inferred source location is shifted by ∼ 1 km toward SE and its strength decreased by ∼ 20%.

Geophysical Journal International, 2002

Analytic solutions are provided for the displacement and stress fields induced by an edge disloca... more Analytic solutions are provided for the displacement and stress fields induced by an edge dislocation in a layered medium composed of two welded half-spaces endowed with different elastic parameters. A plane strain configuration is considered, with the dislocation surface perpendicular to the interface between the two half-spaces. Two cases are considered: in case I the dislocation surface is entirely embedded in one half-space, in case II the dislocation surface cuts across the interface. From these elementary solutions, a closed Volterra dislocation (with constant slip) can be easily obtained: in model A the dislocation opens and closes within the same medium, in model B it opens in one half-space and closes in the other. These elementary solutions also provide the singular kernel of the integral equation governing the equilibrium configuration of a mode-II crack (in which the stress drop is assigned). The stress field induced by closed Volterra dislocations (models A and B) or by a crack with constant stress drop (model C) is computed and compared with the solutions in a homogeneous medium. Important differences appear in the overall pattern of the stress maps. Lower values are generally found for the vertical normal component and for the shear component near and within the softer half-space, which might be anticipated on intuitive grounds as due to the different rigidity of the two media in welded contact. However, unexpected differences are found in the normal stress component parallel to the interface (which needs not be continuous at the interface): this component shows wide regions of high stress on the hard side of the interface which, in models B and C, give rise to clear maxima and minima within two narrow lobes elongated along the interface. These interface stress concentrations extend up to distances comparable to the vertical extension of the dislocation surface and their amplitude is comparable to the stress change induced over the dislocation surface. The physical motivation for the presence of these maxima are discussed and their dependence is studied from the rigidity contrast between the two media and from the position of the dislocation surface relative to the interface.

Geophysical Journal International, 1999

The ground deformation produced by a spherical overpressure source in a heteroge- neous elastic a... more The ground deformation produced by a spherical overpressure source in a heteroge- neous elastic and/or viscoelastic medium is investigated by numerical models based on the finite element method. Sources are assumed to be located at different depths beneath Mt. Etna (Sicily), whose structure is approximated as axially symmetric. Fi- nite element modelling allows to incorporate in the analysis realistic features such as the laterally heterogeneous multi-layered structure inferred from seismic tomography and the topographic relief. An elastic analysis, performed initially, shows significant changes of the deformation field with respect to homogeneous half-space solutions: ground deformation is more confined to the proximity of the axis and its amplitude is strongly sensitive to the presence of low rigidity layers above the source. The ratio of maximum radial to vertical deformation is significantly larger for deep sources. A further develpoment of the model includes the study of inelastic properties assuming a Maxwell viscoelastic rheology for different layers. If the viscoelastic rheology is applied only to layers deeper than the source, the solutions are affected according to the distance of the source from the viscoelastic layer. If a viscoelastic layer is present above the source, a large amplification (by more than 100%) of the surface deforma- tion is predicted by the model. The most striking effects are observed when the source is embedded within a viscoelastic layer: in this case a static equilibrium configuration is not attained and, in the long term, both components of deformation revert their signs in proximity of the axis. Simple physical explanations are proposed for the different cases.

Geophysical Journal International, 2002

We study the effects of structural inhomogeneities on the stress and displacement fields induced ... more We study the effects of structural inhomogeneities on the stress and displacement fields induced by strike-slip faults in layered media. An elastic medium is considered, made up of an upper layer bounded by a free surface and welded to a lower half-space characterized by different elastic parameters. Shear cracks with assigned stress drop are employed as mathematical models of strike-slip faults, which are assumed to be vertical and planar. If the crack is entirely embedded within the lower medium (case A), a Cauchy-kernel integral equation is obtained, which is solved by employing an expansion of the dislocation density in Chebyshev polynomials. If the crack is within the lower medium but it terminates at the interface (case B), a generalized Cauchy singularity appears in the integral kernel. This singularity affects the singular behaviour of the dislocation density at the crack tip touching the interface. Finally, the case of a crack crossing the interface is considered (case C). The crack is split into two interacting sections, each placed in a homogeneous medium and both open at the interface. Two coupled generalized Cauchy equations are obtained and solved for the dislocation density distribution of each crack section. An asymptotic study near the intersection between the crack and the interface shows that the dislocation densities for each crack section are bounded at the interface, where a jump discontinuity is present. As a corollary, the stress drop must be discontinuous at the interface, with a jump proportional to the rigidity contrast between the adjoining media. This finding is shown to have important implications for the development of geometrical complexities within transform fault zones: planar strike-slip faults cutting across layer discontinuities with arbitrary stress drop values are shown to be admissible only if the interface between different layers becomes unwelded during the earthquake at the crack/interface junction. Planar strike-slip faulting may take place only in mature transform zones, where a repetitive earthquake cycle has already developed, if the rheology is perfectly elastic. Otherwise, the fault cannot be planar: we infer that strike-slip faulting at depth is plausibly accompanied by en-echelon surface breaks in a shallow sedimentary layer (where the stress drop is lower than prescribed by the discontinuity condition), while ductile deformation (or steady sliding) at depth may be accommodated by multiple fault branching or by antithetic faulting in the upper brittle layer (endowed with lower rigidity but higher stress).

Earth and Planetary Science Letters

Abstract We study the strain and stress fields produced by temperature and pore pressure increase... more Abstract We study the strain and stress fields produced by temperature and pore pressure increases within and outside a Thermo-Poro-Elastic (TPE) inclusion (the source region), embedded within a medium (the matrix) in isothermal drained conditions. This model is suitable to describe a crustal region in a volcanic environment pervaded by hot pressurized fluids released by an underlying magma chamber. After introducing the pertinent constitutive relations, a formal solution for the displacement field is provided in terms of the Green's function for an elastic medium with drained isothermal elastic moduli, employing a generalization of Eshelby (1961) procedure. If an unbounded medium is considered, a displacement potential can be introduced, obeying the Laplace equation within the source region and the Poisson equation within the matrix. If a spherically symmetric source region is considered, simple analytical solutions are obtained for the displacement, the strain and the stress fields, showing that thrust faulting mechanisms are promoted within the source region while normal faulting mechanisms prevail in the embedding matrix. Employing reasonable numerical values for the thermo-poro-elastic parameters, suitable to describe highly porous sedimentary rock, strain and stress variations are found to be significant even for moderate changes of temperature and pore pressure. Variations of the Coulomb failure function are high in the TPE region and are strongly dependent on the friction coefficient and pore pressure. Application of these results to the 1982-84 and 2011-13 unrest episodes at Campi Flegrei caldera (Italy) suggests that an oblique dike intrusion across a previously unfaulted TPE region took place with a mixed tensile-thrust dislocation mechanism in both events, as previously inferred from accurate inversion of geodetic data.

Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy

ABSTRACT

is a nested caldera in Italy, at the western edge of the Bay of Naples. Together with Vesuvius an... more is a nested caldera in Italy, at the western edge of the Bay of Naples. Together with Vesuvius and Mt Etna, it is one of the Italian GeoHazard Supersites. The area is characterized by one of the highest volcanic hazard in the world, due to the very high density of inhabitants (1800/km 2), the persistent activity of the system and the explosive character of volcanism. A major unrest episode took place in 1982-84, when the town of Pozzuoli, located at the caldera center, was uplifted by 1.80 m (∼1 m/yr). Minor uplifts of few cm, seismic swarms and degassing episodes took place in 1989, 2000 and 2004-06. Since 2005 Campi Flegrei is uplifting, reaching a ground velocity of 9 cm/yr in 2012, showing that the caldera is in a critical state on the verge of instability.

Il Nuovo Cimento C, 1978

ABSTRACT

Tectonophysics, 2009

The popular Mogi model, describing deformation due to a small pressurized spherical cavity embedd... more The popular Mogi model, describing deformation due to a small pressurized spherical cavity embedded in a homogeneous elastic half-space, is shortly reviewed, similarities and differences with other isotropic deformation models are discussed and the interpretation of the different volume changes accompanying source inflation is provided. The model is then generalized to viscoelastic rheology. The overpressure needed within the source to reproduce the large deformation, sometimes observed in volcanic regions, is found to be strongly dependent on rheological parameters, so that reasonably low values may be obtained if the relaxation time of the medium surrounding the source is short (months), instead of the extremely high values inferred from the elastic theory. If the inflation of the source is due to the input of new magma from remote distance, the resulting residual gravity change Δg may be easily computed, thanks to the spherical geometry of the source, and the density of the newly emplaced material may be inferred from the ratio of Δg/w, between the gravity change and the uplift. This ratio is independent of the overpressure and the radius of the source, which are generally ill defined parameters, and is slightly dependent on the relaxation time in the transient phase. Explicit solutions are shown for a step-like overpressure history and for a constant magma supply rate. A more realistic model is also proposed, in which the viscoelastic properties are restricted to a small spherical volume around the source, surrounded by an elastic medium. Even if an accurate interpretation of deformation and gravity changes in volcanic areas generally requires taking into account topography, elastic layering and the triaxial geometry of the inflation source, values computed according to the generalized Mogi model are compared with observations made during the 1982–84 uplift episode at Campi Flegrei caldera (Italy), for the sake of illustration.

Il Nuovo Cimento C, 1980

Summary Some earthquake models based on the elastic theory of dislocations are presented. Earthq... more Summary Some earthquake models based on the elastic theory of dislocations are presented. Earthquake occurrence is modelled as the opening of a crack in an infinite elastic medium triggered by the action of localized stress distributions. The fracture is modelled as a continuum of infinitesimal dislocations. This approach allows us to have complete information about the stress field and the displacement field and to make a first step towards understanding the relation between source mechanism and stress distributions.

Il Nuovo Cimento C, 1980

Summary The main features of seismic strong-motion accelerometers are discussed. A complete anal... more Summary The main features of seismic strong-motion accelerometers are discussed. A complete analysis of the «triggering process» activating the recorder shows that current devices are not best suited for this purpose and a new solution is proposed. An examination of applicability and effectiveness of spectral-analysis operation suggests that it can give misleading results. A complete analysis of errors related to the

Il Nuovo Cimento B, 1977

ABSTRACT

Annals of Geophysics, 2009

Employing a 3D finite element method, we develop an algorithm to calculate gravity changes due to... more Employing a 3D finite element method, we develop an algorithm to calculate gravity changes due to pressurized sources of any shape in elastic and inelastic heterogeneous media. We consider different source models, such as sphere, spheroid and sill, dilating in elastic media (homogeneous and heterogeneous) and in elasto-plastic media. The models are oriented to reproduce the gravity changes and the surface deformation observed at Campi Flegrei caldera (Italy), during the 1982-1984 unrest episode. The source shape and the characteristics of the medium have great influence on the calculated gravity changes, leading to very different values for the source densities. Indeed, the gravity residual strongly depends upon the shape of the source. Non negligible contributions also come from density and rigidity heterogeneities within the medium. Furthermore, if the caldera is elasto-plastic, the resulting gravity changes exhibit a pattern similar to that provided by a low effective rigidity. Even if the variation of the source volumes is quite similar for most of the models considered, the density inferred for the source ranges from ∼400 kg/m 3 (super critical water) to ∼3300 kg/m 3 (higher than trachytic basalts), with drastically different implications for risk assessment.

Tectonophysics, 1990

Albarello, IX and Bonafede, M., 1990. Stress diffusion across laterally heterogeneous plates. In:... more Albarello, IX and Bonafede, M., 1990. Stress diffusion across laterally heterogeneous plates. In: E. Mantovani {Editor), Seismicity and Crustal Deformation. ~ecfo~o~~ysics, 179: 121-130. Stress diffusion following a major dislocation event may provide significant contributions to the stress field present in the neighbouring regions. Seismic regularities observed in the southern Adriatic region are used in the present paper to constrain possible physical models of stress diffusion. Lateral variations in the rheological structure of the lithosphere-asthenosphere system are considered. A diffusion equation with variable diffusivity is obtained using the Elsasser approximation, which describes migration of the stress field after a dislocation event. Approximate solutions of this equation are worked out in the long wavelength limit, showing that diffusion is faster where the lithosphere is thicker and slower where it is thinner. Lithosphere thinning accordingly provides a higher amplitude and longer duration diffusional stress following a dislocation event. This behaviour is particularly evident in a transient regime. It seems to be widely accepted that the space and time distribution of seismicity in seismic regions which are tectonically connected may have some degree of interdependence. Evidence of seismic migration or coupled earthquakes has been identified in many areas of the world (see e.g. Kasahara, 1979). In particular, a significant degree of correlation between intense seismic activities on the eastern and western margins of the Adriatic region (Fig. 1) is suggested by several statistical studies based on the Pearson Correlation (Muc

Journal of Volcanology and Geothermal Research, 2005

The 1982–1984 unrest episode at Campi Flegrei (CF) caldera, Italy, was characterized by huge defo... more The 1982–1984 unrest episode at Campi Flegrei (CF) caldera, Italy, was characterized by huge deformation (more than 1.5 m uplift) concentrated inside the caldera. According to point source isotropic models in homogeneous elastic and visco-elastic half-spaces, the source depth is very shallow (∼3 km). If the source radius is ∼1 km, this implies that magma is at a depth of ∼2 km depth. However, several independent observations show that the top of the magma chamber at CF must be deeper than ∼4 km. This paper investigates how the inferred source depth increases when rigidity heterogeneities (obtained through seismic tomography at CF) are considered and when the long-term deformation takes place under drained conditions. Finite element models indicate that overpressure needed at the source to reproduce the 1.5 m maximum uplift is however beyond typical rock strength values. This evidence, together with the high thermal anomalies, the presence of fluids and the low cohesion of tuffs filling the caldera, suggests the use of elasto-plastic constitutive laws. For elasto-plastic behavior, the same deformation is obtained using a deeper source (with center at ∼5 km depth) and a lower overpressure (than required by elastic models). The plastic deformation concentrates both at the source boundaries and above the source, where seismic activity has been recorded. These results indicate that the rheological properties of the shallow crust of CF have important implications for hazard estimate during unrest episodes.

Journal of Geophysical Research, 2010

Geophysical Journal International, 2010

We develop a mathematical model describing dyke propagation in proximity of an elastic discontinu... more We develop a mathematical model describing dyke propagation in proximity of an elastic discontinuity of the embedding medium. The dyke is modelled as a fluid-filled crack in plane strain configuration employing the boundary element method. The pressure gradient along the crack is assumed proportional to the difference between the densities of the host rock and the fluid. Mass conservation is imposed during propagation and fluid compressibility is taken into account. The path followed by the crack is found by maximizing the total energy release, given by the sum of the elastic and gravitational contributions. The mathematical simulations provide a sort of 'refraction phenomenon', that is a sudden change in the direction of propagation when the crack crosses the boundary separating different rigidities: if the dyke enters a softer medium, its path deviates towards the vertical, if the dyke enters a harder medium its path deviates away from the vertical and may even become arrested as a horizontal sill along the interface, if the rigidity contrast is large. Gravitational energy plays a major role during propagation; in particular, in proximity of layer boundaries, this role is enhanced by the shift of the centre of mass due to changes of dyke shape. Mathematical results were validated by laboratory experiments performed injecting tilted air-filled cracks through gelatin layers with different rigidities.

Geophysical Journal International, 2009

Deformation data collected at Mount Etna from 1993 to 1997 show that the volcano edifice inflatio... more Deformation data collected at Mount Etna from 1993 to 1997 show that the volcano edifice inflation is accompanied by instability of the eastern flank. We propose a 3D finite element model including lateral variations of material properties and topography. Source parameters of the inflating source are constrained by a direct search followed by an appraisal stage of the sampled solutions. The instability of the eastern flank is here addressed using a kinematic approach consisting in a rigid translation of a prescribed area. Our aim is to evaluate how the inflating source inference is affected by eastern flank sliding. Our results show that when sliding is accounted for, the inferred source location is shifted by ∼ 1 km toward SE and its strength decreased by ∼ 20%.

Geophysical Journal International, 2002

Analytic solutions are provided for the displacement and stress fields induced by an edge disloca... more Analytic solutions are provided for the displacement and stress fields induced by an edge dislocation in a layered medium composed of two welded half-spaces endowed with different elastic parameters. A plane strain configuration is considered, with the dislocation surface perpendicular to the interface between the two half-spaces. Two cases are considered: in case I the dislocation surface is entirely embedded in one half-space, in case II the dislocation surface cuts across the interface. From these elementary solutions, a closed Volterra dislocation (with constant slip) can be easily obtained: in model A the dislocation opens and closes within the same medium, in model B it opens in one half-space and closes in the other. These elementary solutions also provide the singular kernel of the integral equation governing the equilibrium configuration of a mode-II crack (in which the stress drop is assigned). The stress field induced by closed Volterra dislocations (models A and B) or by a crack with constant stress drop (model C) is computed and compared with the solutions in a homogeneous medium. Important differences appear in the overall pattern of the stress maps. Lower values are generally found for the vertical normal component and for the shear component near and within the softer half-space, which might be anticipated on intuitive grounds as due to the different rigidity of the two media in welded contact. However, unexpected differences are found in the normal stress component parallel to the interface (which needs not be continuous at the interface): this component shows wide regions of high stress on the hard side of the interface which, in models B and C, give rise to clear maxima and minima within two narrow lobes elongated along the interface. These interface stress concentrations extend up to distances comparable to the vertical extension of the dislocation surface and their amplitude is comparable to the stress change induced over the dislocation surface. The physical motivation for the presence of these maxima are discussed and their dependence is studied from the rigidity contrast between the two media and from the position of the dislocation surface relative to the interface.

Geophysical Journal International, 1999

The ground deformation produced by a spherical overpressure source in a heteroge- neous elastic a... more The ground deformation produced by a spherical overpressure source in a heteroge- neous elastic and/or viscoelastic medium is investigated by numerical models based on the finite element method. Sources are assumed to be located at different depths beneath Mt. Etna (Sicily), whose structure is approximated as axially symmetric. Fi- nite element modelling allows to incorporate in the analysis realistic features such as the laterally heterogeneous multi-layered structure inferred from seismic tomography and the topographic relief. An elastic analysis, performed initially, shows significant changes of the deformation field with respect to homogeneous half-space solutions: ground deformation is more confined to the proximity of the axis and its amplitude is strongly sensitive to the presence of low rigidity layers above the source. The ratio of maximum radial to vertical deformation is significantly larger for deep sources. A further develpoment of the model includes the study of inelastic properties assuming a Maxwell viscoelastic rheology for different layers. If the viscoelastic rheology is applied only to layers deeper than the source, the solutions are affected according to the distance of the source from the viscoelastic layer. If a viscoelastic layer is present above the source, a large amplification (by more than 100%) of the surface deforma- tion is predicted by the model. The most striking effects are observed when the source is embedded within a viscoelastic layer: in this case a static equilibrium configuration is not attained and, in the long term, both components of deformation revert their signs in proximity of the axis. Simple physical explanations are proposed for the different cases.

Geophysical Journal International, 2002

We study the effects of structural inhomogeneities on the stress and displacement fields induced ... more We study the effects of structural inhomogeneities on the stress and displacement fields induced by strike-slip faults in layered media. An elastic medium is considered, made up of an upper layer bounded by a free surface and welded to a lower half-space characterized by different elastic parameters. Shear cracks with assigned stress drop are employed as mathematical models of strike-slip faults, which are assumed to be vertical and planar. If the crack is entirely embedded within the lower medium (case A), a Cauchy-kernel integral equation is obtained, which is solved by employing an expansion of the dislocation density in Chebyshev polynomials. If the crack is within the lower medium but it terminates at the interface (case B), a generalized Cauchy singularity appears in the integral kernel. This singularity affects the singular behaviour of the dislocation density at the crack tip touching the interface. Finally, the case of a crack crossing the interface is considered (case C). The crack is split into two interacting sections, each placed in a homogeneous medium and both open at the interface. Two coupled generalized Cauchy equations are obtained and solved for the dislocation density distribution of each crack section. An asymptotic study near the intersection between the crack and the interface shows that the dislocation densities for each crack section are bounded at the interface, where a jump discontinuity is present. As a corollary, the stress drop must be discontinuous at the interface, with a jump proportional to the rigidity contrast between the adjoining media. This finding is shown to have important implications for the development of geometrical complexities within transform fault zones: planar strike-slip faults cutting across layer discontinuities with arbitrary stress drop values are shown to be admissible only if the interface between different layers becomes unwelded during the earthquake at the crack/interface junction. Planar strike-slip faulting may take place only in mature transform zones, where a repetitive earthquake cycle has already developed, if the rheology is perfectly elastic. Otherwise, the fault cannot be planar: we infer that strike-slip faulting at depth is plausibly accompanied by en-echelon surface breaks in a shallow sedimentary layer (where the stress drop is lower than prescribed by the discontinuity condition), while ductile deformation (or steady sliding) at depth may be accommodated by multiple fault branching or by antithetic faulting in the upper brittle layer (endowed with lower rigidity but higher stress).

Earth and Planetary Science Letters

Abstract We study the strain and stress fields produced by temperature and pore pressure increase... more Abstract We study the strain and stress fields produced by temperature and pore pressure increases within and outside a Thermo-Poro-Elastic (TPE) inclusion (the source region), embedded within a medium (the matrix) in isothermal drained conditions. This model is suitable to describe a crustal region in a volcanic environment pervaded by hot pressurized fluids released by an underlying magma chamber. After introducing the pertinent constitutive relations, a formal solution for the displacement field is provided in terms of the Green's function for an elastic medium with drained isothermal elastic moduli, employing a generalization of Eshelby (1961) procedure. If an unbounded medium is considered, a displacement potential can be introduced, obeying the Laplace equation within the source region and the Poisson equation within the matrix. If a spherically symmetric source region is considered, simple analytical solutions are obtained for the displacement, the strain and the stress fields, showing that thrust faulting mechanisms are promoted within the source region while normal faulting mechanisms prevail in the embedding matrix. Employing reasonable numerical values for the thermo-poro-elastic parameters, suitable to describe highly porous sedimentary rock, strain and stress variations are found to be significant even for moderate changes of temperature and pore pressure. Variations of the Coulomb failure function are high in the TPE region and are strongly dependent on the friction coefficient and pore pressure. Application of these results to the 1982-84 and 2011-13 unrest episodes at Campi Flegrei caldera (Italy) suggests that an oblique dike intrusion across a previously unfaulted TPE region took place with a mixed tensile-thrust dislocation mechanism in both events, as previously inferred from accurate inversion of geodetic data.

Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy

ABSTRACT

is a nested caldera in Italy, at the western edge of the Bay of Naples. Together with Vesuvius an... more is a nested caldera in Italy, at the western edge of the Bay of Naples. Together with Vesuvius and Mt Etna, it is one of the Italian GeoHazard Supersites. The area is characterized by one of the highest volcanic hazard in the world, due to the very high density of inhabitants (1800/km 2), the persistent activity of the system and the explosive character of volcanism. A major unrest episode took place in 1982-84, when the town of Pozzuoli, located at the caldera center, was uplifted by 1.80 m (∼1 m/yr). Minor uplifts of few cm, seismic swarms and degassing episodes took place in 1989, 2000 and 2004-06. Since 2005 Campi Flegrei is uplifting, reaching a ground velocity of 9 cm/yr in 2012, showing that the caldera is in a critical state on the verge of instability.

Il Nuovo Cimento C, 1978

ABSTRACT

Tectonophysics, 2009

The popular Mogi model, describing deformation due to a small pressurized spherical cavity embedd... more The popular Mogi model, describing deformation due to a small pressurized spherical cavity embedded in a homogeneous elastic half-space, is shortly reviewed, similarities and differences with other isotropic deformation models are discussed and the interpretation of the different volume changes accompanying source inflation is provided. The model is then generalized to viscoelastic rheology. The overpressure needed within the source to reproduce the large deformation, sometimes observed in volcanic regions, is found to be strongly dependent on rheological parameters, so that reasonably low values may be obtained if the relaxation time of the medium surrounding the source is short (months), instead of the extremely high values inferred from the elastic theory. If the inflation of the source is due to the input of new magma from remote distance, the resulting residual gravity change Δg may be easily computed, thanks to the spherical geometry of the source, and the density of the newly emplaced material may be inferred from the ratio of Δg/w, between the gravity change and the uplift. This ratio is independent of the overpressure and the radius of the source, which are generally ill defined parameters, and is slightly dependent on the relaxation time in the transient phase. Explicit solutions are shown for a step-like overpressure history and for a constant magma supply rate. A more realistic model is also proposed, in which the viscoelastic properties are restricted to a small spherical volume around the source, surrounded by an elastic medium. Even if an accurate interpretation of deformation and gravity changes in volcanic areas generally requires taking into account topography, elastic layering and the triaxial geometry of the inflation source, values computed according to the generalized Mogi model are compared with observations made during the 1982–84 uplift episode at Campi Flegrei caldera (Italy), for the sake of illustration.

Il Nuovo Cimento C, 1980

Summary Some earthquake models based on the elastic theory of dislocations are presented. Earthq... more Summary Some earthquake models based on the elastic theory of dislocations are presented. Earthquake occurrence is modelled as the opening of a crack in an infinite elastic medium triggered by the action of localized stress distributions. The fracture is modelled as a continuum of infinitesimal dislocations. This approach allows us to have complete information about the stress field and the displacement field and to make a first step towards understanding the relation between source mechanism and stress distributions.

Il Nuovo Cimento C, 1980

Summary The main features of seismic strong-motion accelerometers are discussed. A complete anal... more Summary The main features of seismic strong-motion accelerometers are discussed. A complete analysis of the «triggering process» activating the recorder shows that current devices are not best suited for this purpose and a new solution is proposed. An examination of applicability and effectiveness of spectral-analysis operation suggests that it can give misleading results. A complete analysis of errors related to the

Il Nuovo Cimento B, 1977

ABSTRACT