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The stress field in Campi Flegrei caldera using joint inversion of seismological and ground deformation data

Campi Flegrei caldera is an active volcanic complex characterized, in the last decades, by seismic activity, gas emissions and ground deformation. During the period 1982-84, a bradyseismic crisis, with a total uplift of about 1.77 m, accompanied by intense seismicity (more than 16000 events, with magnitudes up to 4.2), affected this area.

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Figure 1 : Focal mechanisms are represented both on a map (top) and along the B-B’ cross-section (bottom). The size of the beachballs is proportional to the magnitude (see inset on the top-left). The distribution of mechanisms types is shown in the triangular diagram within the inset on the top-right.

The aim of this work is to determine the spatial and temporal variations of the stress field within Campi Flegrei caldera by applying a joint inversion of geodetic and seismological data.
We first have determined the geometry and the temporal variation of the source of ground deformation using about 1100 optical leveling data measured in the interval 1982-1985. Our findings show that this source has a roughly rectangular shape, located at the center of the caldera, beneath the town of Pozzuoli, at a depth of about 2700 m.
The inflation of this sill-like structure (possibly because of the injection of magma) is the source of the observed ground deformation, but also of the stress field responsible of the occurrence of the earthquakes. To determine the spatial and temporal pattern of the stress field we used a seismological dataset consisting of 265 focal mechanisms related to the interval 1983-1984. Applying various stress-inversion methods we have determined stress field in that area considering both the volcanic source and a regional stress field.
Results show that most of the stress field of the area is mainly related to the volcanic source, showing a nearly vertical σ1 axis and a sub-horizontal σ3. A weak extensional N-S regional stress field drives the orientation of the σ3 axes.

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Figure 2 : results of the joint inversion. The left column shows the σ1 and σ3 axes (respectively in red and blue) projected along a S-N cross-section (B-B’). The projection of the volcanic deformation source is outlined in green. Black contours indicate the values of the σ1 magnitude (in MPa). The central and the right columns show respectively the stereographic projections of σ1 and σ3 axes within the seismogenetic volume (see text for details). Triangles represent the regional stress tensor axes. a) b) and c) refer to the interval 1/1/1982-1/1/1983. d) e) and f) to the interval 1/1/1982-1/7/1983. g) h) and i) the interval 1/1/1982-1/1/1984. j) k) and l) to the interval 1/1/1982-1/1/1985. m) n) and o) represent the stress pattern of the volcanic source alone.

Work presented in EGU2014 Supersites session, EGU2014-5530 abstract

Authors : Elena Cristiano (1), Luca D’Auria (1), Bruno Massa (2,1), Carlo Del Gaudio (1), Flora Giudicepietro (1), Giovanni Ricciardi (1), and Ciro Ricco (1)
(1) Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli, Italy,
(2) Dipartimento di Scienze e Tecnologie, Università degli Studi del Sannio, Benevento, Italy

Contact : Luca D’Auria (luca.dauria[at]ingv.it)

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Last update : 11-07-2016