Abstract
The 1982-1984 unrest episode at Campi Flegrei (CF) caldera, Italy,
was characterized by huge deformation (more than 1.5 m uplift) concentrated
inside the caldera. According to point source isotropic models in
homogeneous elastic and visco-elastic half-spaces, the source depth
is very shallow (3 km). If the source radius is 1 km, this implies
that magma is at a depth of 2 km depth. However, several independent
observations show that the top of the magma chamber at CF must be
deeper than 4 km. This paper investigates how the inferred source
depth increases when rigidity heterogeneities (obtained through seismic
tomography at CF) are considered and when the long-term deformation
takes place under drained conditions. Finite element models indicate
that overpressure needed at the source to reproduce the 1.5 m maximum
uplift is however beyond typical rock strength values. This evidence,
together with the high thermal anomalies, the presence of fluids
and the low cohesion of tuffs filling the caldera, suggests the use
of elasto-plastic constitutive laws. For elasto-plastic behavior,
the same deformation is obtained using a deeper source (with center
at 5 km depth) and a lower overpressure (than required by elastic
models). The plastic deformation concentrates both at the source
boundaries and above the source, where seismic activity has been
recorded. These results indicate that the rheological properties
of the shallow crust of CF have important implications for hazard
estimate during unrest episodes.
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