1,459 research outputs found
Bayesian networks for enterprise risk assessment
According to different typologies of activity and priority, risks can assume
diverse meanings and it can be assessed in different ways. In general risk is
measured in terms of a probability combination of an event (frequency) and its
consequence (impact). To estimate the frequency and the impact (severity)
historical data or expert opinions (either qualitative or quantitative data)
are used. Moreover qualitative data must be converted in numerical values to be
used in the model. In the case of enterprise risk assessment the considered
risks are, for instance, strategic, operational, legal and of image, which many
times are difficult to be quantified. So in most cases only expert data,
gathered by scorecard approaches, are available for risk analysis. The Bayesian
Network is a useful tool to integrate different information and in particular
to study the risk's joint distribution by using data collected from experts. In
this paper we want to show a possible approach for building a Bayesian networks
in the particular case in which only prior probabilities of node states and
marginal correlations between nodes are available, and when the variables have
only two states
Forecasts for the detection of the magnetised cosmic web from cosmological simulations
The cosmic web contains a large fraction of the total gas mass in the
universe but is difficult to detect at most wavelengths. Synchrotron emission
from shock-accelerated electrons may offer the chance of imaging the cosmic web
at radio wavelengths. In this work we use 3D cosmological ENZO-MHD simulations
(combined with a post-processing renormalisation of the magnetic field to
bracket for missing physical ingredients and resolution effects) to produce
models of the radio emission from the cosmic web. In post-processing we study
the capabilities of 13 large radio surveys to detect this emission. We find
that surveys by LOFAR, SKA1-LOW and MWA have a chance of detecting the cosmic
web, provided that the magnetisation level of the tenuous medium in filaments
is of the order of 1% of the thermal gas energy.Comment: 19 pages, 18 figures. A&A accepted, in press. The public repository
of radio maps for the full volumes studied in this work is available at
http://www.hs.uni-hamburg.de/DE/Ins/Per/Vazza/projects/Public_data.htm
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
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
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
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
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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