A fluid-filled fracture as possible mechanism of ground deformation at Phlegraean Fields, Italy

Abstract

Two uplift episodes have recently been recorded at Phlegraean Fields, a volcanic region near Naples (south-central Italy). The first episode occurred in 1970 and lasted up to 1972; the second has begun at the end of 1982 and is still in progress. An attempt to model the ground deformations which occurred during the 1970–1972 event is made in this paper. The source has been assumed to be a two-dimensional fluid-filled fracture, similar to a sill. A good fit with experimental data has been obtained for a short (1–2 km long) shallow (3 km deep) source and a driving pressure ranging from 60 to 210 bars. Rigidity values have been fixed at 3.5–4.0 × 1010 c.g.s., with Poisson ratio equal to 0.3. This solution which differs from previous approaches byMogi (1958) andWalsh andDecker (1971) is non-unique, but the present results are in good agreement with observed horizontal and vertical displacements. Notwithstanding the oversimplification made in assuming a homogeneous elastic semi-infinite medium, the predicted stress field seems to be in agreement with the fault-plane solutions and the pattern of epicenters determined for the uplift-seismic swarm episode that is still in progress.