Inflation and microearthquake activity of phlegraean fields, Italy

Abstract

The Phlegraean Fields are a densely inhabited volcanic area which includes part of the city of Napoli. During the past 2,000 years it has been subject to slow vertical movements (bradyseisms). A rapid uplift was observed in early 1970, which caused alarm in the population. Ground deformation started to be monitored by means of tide gauge observations and topographic levelling, making it possible to define the area interested in the phenomenon and to outline the pattern of deformation. Vertical deformation data are well fitted by radially symmetric sources, such as the Mogi’s model, while horizontal deformation data are best fitted by a linear source model. Contemporaneously, a tight seismic network was installed in the area. The frequency of seismic events and the released energies have shown that the seismicity of the volcanic area is very low. Seismic activity evolved both with regard to the relative occurrence rate of various kinds of shock and to the distribution of epicentres. The values of the Ishimoto-Iida coefficient, calculated for three kinds of shock, show that the focal medium is very heterogeneous down to a depth of a few kilometres. Observations suggest that the origin of the ground uplift can be attributed to the intrusion of magmatic masses. This hypothesis is coherent with data and is supported by structural information. Available data allow a rough estimation of the maximum magnitude possible for an earthquake with epicenter in the Phlegraean Fields.