332 research outputs found
Microwave response of bulk MgB2 samples of different granularity
The microwave response of three high-density bulk MgB2 samples has been
investigated in the linear and nonlinear regimes. The three samples,
characterized by different mean size of grains, have been obtained by reactive
infiltration of liquid Mg in powdered B preforms. The linear response has been
studied by measuring the microwave surface impedance; the nonlinear response by
detecting the power radiated by the sample at the second-harmonic frequency of
the driving field. Our results suggest that bulk MgB2 prepared by the liquid Mg
infiltration technique is particularly promising for manufacturing resonant
cavities operating at microwave frequencies.Comment: 4 pages, 2 embedded figures; Proceedings of 7th EUCAS Conference
(11-15 September 2005, Vienna - Austria
Microwave Harmonic Emission in MgB2 Superconductor: Comparison with YBaCuO
We report results of microwave second-harmonic generation in ceramic samples
of MgB2, prepared by different methods. The SH signal has been investigated as
a function of the temperature and the static magnetic field. The results are
discussed in the framework of models reported in the literature. We show that
the peculiarities of the SH signal are related to the specific properties of
the sample. A comparison with the results obtained in ceramic and crystalline
YBa(2)Cu(3)O(7) shows that the second-harmonic emission in MgB2 is weaker than
that observed in ceramic YBa(2)Cu(3)O(7).Comment: 13 pages, 6 figures; Proceedings of Third Workshop on Metamaterials
and Special Materials for Electromagnetic Applications and TLC (Rome, 30-31
March, 2006
State-building and sub-national tensions in Afghanistan and Pakistan
The transition to a democratic system in Afghanistan and Pakistan has not silenced ethnic and religious tensions. The paper argues that sub-national identities in both countries have been politicized as a result of the unequal access to political power and economic resources and of foreign interference. While attempts to foster national cohesion by reference to a common religion may backfire, as the Pakistani case illustrates, more needs to be done to involve all elements of civil society, irrespective of their sub-national belonging, in the state-building process, to promote freedom of expression and purge textbooks from ethno- or religious-centric narratives
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
Democratization and the relevance of history: the case of Pakistan
Many political scientists have argued, on the basis of extensive quantitative data, that democratic governments result in greater spending in health and education and in a redistribution of resources in favour of vulnerable groups. Democratic governments are also often associated with a decrease in levels of interstate conflict and in internal violence. And yet, there are cases \u2013 such as Pakistan \u2013 in which this has not occurred. Why have these \u2018democracy dividends\u2019 not been realized in Pakistan? This paper addresses some of the reasons why democratic governance in Pakistan has not translated into social welfare and peace, and highlights that the impact of democracy is time and context-specific. Therefore, greater attention must be devoted to the historical and societal peculiarities of each country
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
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
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
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