IRIS Università degli Studi di Bolognahttps://cris.unibo.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Mon, 27 Sep 2021 13:12:22 GMT2021-09-27T13:12:22Z10621Modelling earthquake effects on groundwater levels: evidences from the 2012 Emilia earthquake (Italy)http://hdl.handle.net/11585/541687.7Titolo: Modelling earthquake effects on groundwater levels: evidences from the 2012 Emilia earthquake (Italy)
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11585/541687.72016-01-01T00:00:00ZModeling non-Andersonian fault growth following the energetic criterion: the creation of detachments and listric faultshttp://hdl.handle.net/11585/684939.2Titolo: Modeling non-Andersonian fault growth following the energetic criterion: the creation of detachments and listric faults
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11585/684939.22019-01-01T00:00:00ZBayesian source inference of the 1993-97 deformation at Mount Etna (Italy) by numerical solutions,http://hdl.handle.net/11585/67430Titolo: Bayesian source inference of the 1993-97 deformation at Mount Etna (Italy) by numerical solutions,
Abstract: Deformation data collected at Mount Etna from 1993 to 1997 show that the inflation of the
volcano edifice is accompanied by instability of the eastern flank. We propose a 3D finite
element model including topography and lateral variations of elastic constants. 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 addressed using a kinematic
approach, consisting of a rigid-body translation over a prescribed area. The aim
is to evaluate how source parameters are affected by sliding of the eastern flank. When
sliding is accounted for, the inferred source location shifts ∼ 1 km SE and its strength
decreases by ∼ 20%.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11585/674302009-01-01T00:00:00ZTIME DEPENDENT FLUID FLOW IN NEAR-CRITICAL PERMEABILITY MODELShttp://hdl.handle.net/11585/232675Titolo: TIME DEPENDENT FLUID FLOW IN NEAR-CRITICAL PERMEABILITY MODELS
Abstract: Several indications are being collected all over the world that fluids released from the mantle have important effects on seismic and volcanic activity. The upward migration of fluids extracted in this way from the asthenosphere is expected to take place at lithostatic pressure, up to the bottom of the brittle crust, where deviatoric stresses induced by tectonic motions become significant with respect to the gravity load. The steady or, maybe episodic, migration of these high pressure fluids, from below the brittle-ductile transition up to the hydrostatic aquifers, has significant effects on rock strength, according to the Coulomb failure criterion, and on the flow of resident fluids. Fluid flow within rocks is described in terms of rock permeability, according to Darcy law, and permeability is a complex function of the distribution of cavities in the rock, depending on their density, shape and degree of connectivity. Therefore, permeability varies considerably even among different samples of the same rock. Poro-elasticity is the discipline taking into account simultaneously the elastic property of the rock and the flow of compressible fluids filling its pores. Several conceptual models of a permeable rock have been proposed, which usually provide a proportionality between porosity and permeability, but the constant of proportionality may differ considerably among different models. Poro-elasticity allows for porosity variations due to changes of confining pressure and pore pressure, but permeability is usually assumed to be a constant rock property, which is clearly inappropriate. “Dual porosity models” may account for the very different permeabilities related to grain size pores and to macroscopic fractures in a rock, but model parameters are again assumed to be constant. However, dual porosity models have the important merit of distinguishing between a pervasive small scale permeability network and a larger scale network of fractures along which enhanced fluid motion takes place. A dislocation model is presented here which describes a pervious rock in terms of an “intrinsic permeability” (e.g. related to connected intergranular porosity) and a pressure dependent permeability, related to the opening of cracks when the pore pressure exceeds the confining pressure. The resulting “effective permeability” is found to be extremely sensitive to the parameters describing the distribution of cracks and to the pore pressure, which is, in turn, strongly controlled by variations of the effective permeability. If only the crack opening depends on pressure (while its length and density remain constant), the effective permeability has lower and upper bounds, which may easily differ by a few orders of magnitude. Moreover, in a near-critical stress state, when the stress intensity factor is close to failure conditions, crack propagation takes place, which is accompanied by pore pressure drop within the crack. The small scale pressure transients, taking place in the surrounding grain size porosity, are studied. An important result is that crack extension takes place in “jerks”, delayed by pore-pressure drop and suction takes place of fluids resident within the inter-granular pore space, toward the crack network, where their mobility is greatly enhanced. We show that the present permeability model provides a useful unifying tool for the understanding and the interpretation of several observations (seismic, geochemical and hydraulic) commonly made in tectonically active regions.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11585/2326752009-01-01T00:00:00ZGravity changes due to overpressure sources in 3D heterogeneous media: application to Campi Flegrei caldera, Italyhttp://hdl.handle.net/11585/48856Titolo: Gravity changes due to overpressure sources in 3D heterogeneous media: application to Campi Flegrei caldera, Italy
Abstract: Employing 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 1982-84 unrest episode. The source shape and the characteristics of the
medium have great influence in 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/m3 (supercritical water) to ∼ 3300 kg/m3 (higher than trachytic
basalts), with drastically different implications for risk assessment.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11585/488562008-01-01T00:00:00ZModeling the poro-elastic signature of two medium-large earthquakes, tips from GPS and InSAR datahttp://hdl.handle.net/11585/684937.2Titolo: Modeling the poro-elastic signature of two medium-large earthquakes, tips from GPS and InSAR data
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11585/684937.22019-01-01T00:00:00ZGeodetic constraints to the source mechanism of the 2011-2013 unrest
at Campi Flegrei (Italy) calderahttp://hdl.handle.net/11585/480166Titolo: Geodetic constraints to the source mechanism of the 2011-2013 unrest
at Campi Flegrei (Italy) caldera
Abstract: Campi Flegrei caldera (Italy) was affected by a new unrest phase during 2011–2013. We exploit
two COSMO-SkyMed data sets to map the deformation field, obtaining displacement rates reaching
9 cm/yr in 2012 in the caldera center. The resulting data set is fitted in a geophysical inversion framework
using finite element forward models to account for the 3-D heterogeneous medium. The best fit model is a
north dipping mixed-mode dislocation source lying at ~5km depth. The driving mechanism is ascribable to
magma input into the source of the large 1982–1984 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.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11585/4801662015-01-01T00:00:00ZEarthquake prediction research and the earthquakes of 2000 in the South Iceland Seismic Zonehttp://hdl.handle.net/11585/102128Titolo: Earthquake prediction research and the earthquakes of 2000 in the South Iceland Seismic Zone
Abstract: The purpose of this paper is to describe and to model long- and
short-term processes ahead of two large earthquakes that occurred in 2000 in the South Iceland Seismic Zone (SISZ). The results are based on some key findings from multinational earthquake-prediction research projects in the SISZ since 1988. It
involves a fusion of significant old and new results. The paper presents a new hypothesis for earthquake build-up processes in the region, followed by a discussion on how interseismic and preseismic observations can be explained by this model and a discussion about a plausible earthquake cycle in light of the new
hypothesis. The research described here started in 1988 with the South Iceland Lowland (SIL) project. A significant outcome was the development of a seismic acquisition and evaluation system, the SIL system, retrieving source information from earthquakes down to magnitude zero. The research continued with the use of more multidisciplinary and multinational projects, revealing new information about crustal processes related to large earthquakes in the area. The validity of the work was tested in 2000 when two magnitude 6.6 (Ms) earthquakes occurred in the central part of the SISZ. The earthquakes had a long-term assessment of place. A short-term warning (“within short”) about the location and size
of a probable impending earthquake was issued 25 hours before the second earthquake.
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/11585/1021282011-01-01T00:00:00ZMathematical and analogue models of fluid filled fracture propagation in layered elastic mediahttp://hdl.handle.net/11585/119810Titolo: Mathematical and analogue models of fluid filled fracture propagation in layered elastic media
Abstract: In order to overcome the practical impossibility of direct observation of dike propagation within the crust, we develop mathematical and analogue models to describe the physical processes involved in their dynamics.
We focus our attention on what happens when a dike approaches the boundary between two media with different rigidities. In our 2D mathematical models, a dike opens and propagates in an infinite elastic medium, made up of 2 welded half-spaces with different elastic parameters.
Dikes are modelled as boundary element fluid-filled cracks. The pressure gradient along the crack is proportional to the difference between the densities of the host rock and the fluid. We take into account the compressibility of the fluid and a variable density in order to conserve the mass of the intrusion during its motion. The mathematical model allows to set a tectonic stress field and an arbitrarily tilted boundary separating different media. The growth, arrest and direction of propagation of the crack is governed by an energetic criterion: the motion of the dike is driven by the minimization of the elastic deformation energy plus the gravitational energy. Propagation is allowed when the energy release during the motion exceeds a fracture threshold.
The output of this code gives us the path followed by the crack during the propagation, its shape and the stresses induced in the elastic medium.
Interestingly, the mathematical simulations provide a sort of refraction phenomenon, that is a sudden change in direction of propagation when the crack crosses the boundary separating different rigidities. In order to validate our mathematical results, we perform laboratory experiments of air filled cracks propagating in gelatine. Gelatine represents well an elastic medium: it is brittle at refrigerator temperature and varying the concentration of dry gel powder dissolved in water we can control its rigidity. By injecting air from the bottom of a trasparent cylinder containing the gelatine, we obtain an air filled crack, tilted with respect to the vertical and propagating upwards toward the rigidity transition surface.
The experiments confirm the main characteristics of the mathematical simulations.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11585/1198102008-01-01T00:00:00ZGeometrical and physical properties of the 1982-84 deformation source at Campi Flegrei - Italyhttp://hdl.handle.net/11585/119803Titolo: Geometrical and physical properties of the 1982-84 deformation source at Campi Flegrei - Italy
Abstract: Deformation of the ground surface in volcanic areas is generally recognized as a reliable indicator of unrest, possibly
resulting from the intrusion of fresh magma within the shallow rock layers. The intrusion process is usually
represented by a deformation source such as an ellipsoidal pressurized cavity, embedded within a homogeneous and
elastic half-space. Similar source models allow inferring the depth, the location and the (incremental) volume of the
intrusion, which are very important parameters for volcanic risk implications. However, assuming a homogeneous
and elastic rheology and, assigning a priori the shape and the mechanism of the source (within a very restricted
“library” of available solutions) may bias considerably the inference of source parameters. In complete generality,
any point source deformation, including overpressure sources, may be described in terms of a suitable moment
tensor, while the assumption of an overpressure source strongly restricts the variety of allowable moment tensors.
In particular, by assuming a pressurized cavity, we rule out the possibility that either shear failure may precede
magma emplacement (seismically induced intrusion) or may accompany it (mixed tensile and shear mode fracture).
Another possibility is that a pre-existent weakness plane may be chosen by the ascending magma (fracture
toughness heterogeneity). We perform joint inversion of levelling and EDM data (part of latter are unpublished),
collected during the 1982-84 unrest at Campi Flegrei caldera: a 43% misfit reduction is obtained for a general
moment source if the elastic heterogeneities computed from seismic tomography are accouted for. The inferred
source is at 5.2 km depth but cannot be interpreted as a simple pressurized cavity. Moreover, if mass conservation
is accounted for, magma emplaced within a shallow source must come from a (generally deeper) reservoir, which
is usually assumed to be deep enough to be simply neglected. At Campi Flegrei, seismic tomography indicates
that the “deep” magma source is rather shallow (at 7-8 km depth), so that its presence should be included in any
thorough attempt to source modeling. Taking into account a deflating source at 7.5 km depth (represented either as
a horizontal sill or as an isotropic cavity) and an inflating moment source, the fit of both levelling and EDM data
improves further (misfit reduction 80%), but still the best fitting moment source (at 5.5 km depth) falls outside the
range of pressurized ellipsoidal cavities. The shallow moment source may be decomposed in a tensile and a shear
dislocation. No clue is obtained that the shear and the tensile mechanisms should be located in different positions.
Our favourite interpretation is in terms of a crack opening in mixed tensile and shear mode, as would be provided
by fluid magma unwelding pre-stressed solid rock. Although this decomposition of the source is not unique, the
proposed solution is physically motivated by the minimum overpressure requirement. An important implication of
this new interpretation is that the magma emplaced in the shallow moment source during the 1982-84 unrest was
not added to already resident magma at the same position.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/11585/1198032010-01-01T00:00:00Z