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Titel |
Geometrical and physical properties of the 1982-84 deformation source at Campi Flegrei - Italy |
VerfasserIn |
Maurizio Bonafede, Elisa Trasatti, Carlo Giunchi, Giovanna Berrino |
Konferenz |
EGU General Assembly 2010
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 12 (2010) |
Datensatznummer |
250035974
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Zusammenfassung |
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levelling and EDM data (part of latter are unpublished), collected during the 1982-84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. |
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