The mechanics of Campi Flegrei unrests as related to plastic behaviour of the caldera borders

We present here a model which explains the mechanism of generation of unrest episodes at Campi Flegrei caldera from a mechanical point of view. The mechanism involves the effects of plastic zones at the borders of the inner collapsed area on both static deformations and seismicity. The large amount of ground uplift observed necessarily calls for plastic effects. These effects are interpreted as concentrated at the caldera borders: the generation of such plastic zones is simulated in terms of the mechanisms leading to the caldera collapse. In order to simulate the influence of such plastic zones on both ground deformations and seismicity we model them as surfaces of discontinuities free from shear stress within an elastic homogeneous half-space. The presence of such discontinuities allows the inner caldera block to move differentially from the outer areas, by slip along the plastic bordering zones. Such a differential uplift of the central block causes the concentration of the ground deformation. Our model explains a lot of puzzling observations at Campi Flegrei in terms of the effects of the caldera structure. The model is applicable to other caldera areas, which show typical evidence for the effects of such discontinuity zones, during unrest episodes.