253 research outputs found
Volcanic deformation and flank instability due to magmatic sources and frictional rheology: the case of Mount Etna
The overall picture of Mount Etna deformation emerging since a couple of decades of geodetic surveys shows effects of magma accumulation, characterized by inflation/deflation cycle, accompanied by a sliding instability of the southeast flank, whose manifestation is an increase in the horizontal deformation away from the volcano summit. This is a very interesting case to test whether advanced models, taking into account topography, internal structure and frictional rheology, may contribute to a better understanding of the complex interplay among mechanical response, magmatic activity and gravitational load occurring in a volcanic system. Using finite element numerical models we make predictions of surface displacements associated with a simple expansion source and with a dike like vertical discontinuity. A new methodology is
developed to initalize the lithostatic stress field according to the material and geometrical complexities of the models considered. Our results show that, while an amplification of the horizontal displacement can be easily obtained up to a maximum distance of 10 km from the source, we have not been able to find any onfiguration to extend further this signal. For the case of Mount Etna this suggests that the large horizontal displacements observed in the east flank along the coast cannot be directly related to magma accumulation below the volcano's summit
Perceived job insecurity and sustainable wellbeing: Do coping strategies help?
For workers, perceived job insecurity represents a threat and an obstacle towards achieving a decent and sustainable dimension of wellbeing at work and in life. Using the theoretical background of the psychology of sustainability and sustainable development, the aim of this study is to deepen the relation between subjective job insecurity, self-related health and life satisfaction considering the effect of personal resources, such as specific coping strategies that people may undertake facing job insecurity perceptions. The hypotheses were tested in a convenience sample of 769 employees in France. Data were collected with a self-report questionnaire and analyzed with the Statistical Package for the Social Science (IBM SPSS). Results showed that job insecurity was negatively related to self-reported health and life satisfaction; furthermore, problem-focused coping of the type of job, social support and training searching behaviors resulted in worsening this negative relationship. This study expanded the understanding of coping in the context of job insecurity and showed the limitations that proactive coping strategies have against job insecurity. These results encourage the debate on how far is it possible to attain sustainable wellbeing by relying on personal resources when job insecurity is experienced
Numerical inversion of deformation caused by pressure sources: application to Mount Etna (Italy)
The interpretation of geodetic data in volcanic areas is usually based on analytical deformation
models. Although numerical Finite Element modeling allows realistic features such as topography
and crustal heterogeneities to be included, the technique is not computationally convenient
for solving inverse problems using classical methods. In this paper we develop a general tool to
perform inversions of geodetic data by means of 3D FE models. The forward model is a library
of numerical displacement solutions, where each entry of the library is the surface displacement
due to a single stress component applied to an element of the grid. The final solution is
a weighted combination of the six stress components applied to a single element-source. The
precomputed forward models are implemented in a global search algorithm, followed by an
appraisal of the sampled solutions. After providing extended testing, we apply the method to
model the 1993-97 inflation phase at Mt. Etna, documented by GPS and EDM measurements.
We consider four different forward libraries, computed in models characterized by homogeneous/
heterogeneous medium and flat/topographic free surface. Our results suggest that the
elastic heterogeneities of the medium can significantly alter the position of the inferred source,
while the topography has minor effect
Analysis of driving seat vibrations in different transfer conditions
The vibration transmitted through the seat of a four-wheel drive tractor, developing 179 kW, and equipped with front suspension axle and shock absorber for the implement, were measured and analysed according to the ISO standard. Several tests were carried out in different conditions considering: type of operation (transfer with and without mounted implement); type of track (conglomerate bituminous road, country lane); connected and disconnected suspension and/or shock absorber; and forward speeds. For the transfer on bituminous conglomerate road, it was observed that the suspension always reduces acceleration av, in both tests
without and with implement, for all the considered forward speeds. For the tests conduced on country lane, the front axle suspension involved a reduction of the acceleration, but it was less evident than on the road Results showed that the forward speed and the mass distribution were the most important factors on tractor behaviour influencing the driver comfort. The daily exposure time which could compromise the driver's health increased
from 2.5 to 8 h decreasing the speed from 3.06 to 2.22 m/s during transfer in country lane without implement. In the other condition the time exposure is approximately more than a working day
On deformation sources in volcanic areas: Modeling the Campi Flegrei (Italy) 1982–84 unrest
Deformation sources in volcanic areas are generally modeled in terms of pressurized tri-axial ellipsoids or
pressurized cracks with simple geometrical shapes, embedded in a homogeneous half-space. However, the
assumption of a particular source mechanism and the neglect of medium heterogeneities bias significantly the
estimate of source parameters. A more general approach describes the deformation source in terms of a
suitable moment tensor. Ratios between moment tensor eigenvalues are shown to provide a strong diagnostic
tool for the physical interpretation of the deformation source and medium heterogeneities may be accounted
for through 3D finite element computations. Leveling and EDM data, collected during the 1982–84 unrest
episode at Campi Flegrei (Italy), are employed to retrieve the complete moment tensor according to a
Bayesian inversion procedure, considering the heterogeneous elastic structure of the volcanic area. Best fitting
moment tensors are found to be incompatible with any pressurized ellipsoid or crack. Taking into account the
deflation of a deeper magma reservoir, which accompanies the inflation of a shallower source, data fit
improves considerably but the retrieved moment tensor of the shallow source is found to be incompatible
with pressurized ellipsoids, still. Looking for alternative physical models of the dislocation source, we find that
the best fit moment tensor can be best interpreted in terms of a mixed mode (shear and tensile) dislocation at
5.5 km depth, striking EW and dipping by ~25°–30° to the North. Gravity changes are found to be compatible
with the intrusion of ~60–70·10^6 m^3 of volatile rich magma with density ~2400 kg/m^3
A new interpretation of the 1982-84 unrest episode at Campi Flegrei caldera (Italy) by numerical inversion
The 1982-84 unrest episode at Campi Flegrei was characterized by huge deformation (about 1.8 m uplift) located inside the caldera and significant gravity variations correlated with the elevation changes (about -213 Gal/m). Due to the bell shape of the uplift, the source is usually interpreted to have a fixed spherical shape.
In the present study, we combine simple point source mechanisms (dipoles and double couples) to represent arbitrary sources such as sphere, ellipsoid or sill.
The models are realized by Finite Element and the medium may be characterized by elastic heterogeneities.
We study the deformation detected by leveling and EDM techniques by coupling the FE forward models with an inversion procedure.
The potential point sources are contained in a volume of 888 km located beneath Pozzuoli, the site of maximum displacement.
We calculate the displacement field at each data point for each basic mechanism and we compare the result with the observed value.
From the inversion of geodetic data we retrieve the best-fitting source parameters, without fixing the shape a priori.
The best-fitting source is located beneath Pozzuoli at about 4.8 km b.s.l. and undergoes to horizontal compression and vertical dilatation
Geometrical and physical properties of the 1982-84 deformation source at Campi Flegrei - Italy
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
Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements
In a type-II superconductor the generation of higher harmonics in the
magnetic response to an alternating magnetic field is a consequence of the
non-linearity in the I-V relationship. The shape of the current-voltage (I-V)
curve is determined by the current dependence of the thermal activation energy
U(J) and is thus related to the dynamical regimes governing the vortex motion.
In order to investigate the vortex dynamics in MgB2 bulk superconductors we
have studied the fundamental (chi1) and third (chi3) harmonics of the ac
magnetic susceptibility. Measurements have been performed as a function of the
temperature and the dc magnetic field, up to 9 T, for various frequencies and
amplitudes of the ac field. We show that the analysis of the behaviour in
frequency of chi3(T) and chi3(B) curves can provide clear information about the
non-linearity in different regions of the I-V characteristic. By comparing the
experimental curves with numerical simulations of the non-linear diffusion
equation for the magnetic field we are able to resolve the crossover between a
dissipative regime governed by flux creep and one dominated by flux flow
phenomena.Comment: to be published in "Horizons in Superconductivity Research" (Nova
Science Publishers, Inc., NY, 2003
On deformation sources in volcanic areas: modeling the Campi Flegrei (Italy) 1982-84 unrest
Deformation sources in volcanic areas are generally modeled in terms of pressurized
tri-axial ellipsoids or other cavities with simple geometrical shapes embedded
in homogeneous half-spaces. However, the assumption of a particular source
mechanism and the neglect of medium heterogeneities bias significantly the estimate
of source parameters. Leveling and EDM data, collected during the 1982-84
unrest episode at Campi Flegrei (Italy), are employed to retrieve source parameters
according to a Bayesian inversion procedure, considering the heterogeneous
elastic structure of the volcanic area. We describe a general deformation source
in terms of a suitable moment tensor, through 3D finite element computations.
Best fitting moment tensors are found to be incompatible with any pressurized
ellipsoid. Taking into account the deflation of a deeper magma reservoir, which
accompanies the inflation of the shallower moment source, data fit improves considerably
but the retrieved moment tensor of the shallow source is found to be
incompatible with pressurized ellipsoids, still. Looking for alternative physical models of the deformation source, we find that the best fit moment tensor can be
best interpreted in terms of a mixed-mode (shear and tensile) dislocation at 5.5
km depth, striking EW and dipping by 30 to the North. Gravity changes are
found to be compatible with the intrusion of 60·106 m3 of volatile rich magma
with density 2000 kg/m3
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