Coseismic displacements and creeping along the Pernicana fault (Etna, Italy) in the last 17 years: a detailed study of a tectonic structure on a volcano

Abstract

The Pernicana fault is a tectonic fault located on Mt Etna (Sicily). Movements across this structure have been monitored, over the last 17 years, by a levelling network operated by the Osservatorio Vesuviano. During the monitoring period, the network has recorded coseismic displacements associated with earthquakes of magnitude 4–5, as well as aseismic displacements. We present a detailed analysis of the displacement data, aimed at modelling the geometry and mechanism of this fault, as well as the size and mechanisms of individual earthquakes. We use, for interpreting the displacement data, two types of inverse methods. The first is able to determine, from ground deformation data, the geometry, mechanism and location of a system rectangular faults with homogeneous dislocation; the second finds, for a fixed fault system geometry, the heterogeneous slip distribution giving the best fit to the data. The total recorded displacements during the last 17 years are well described by a single, homogeneous rectangular normal fault, with dislocation and size corresponding to a magnitude 5.7 earthquake, thus revealing intense dynamics along such a fault. Coseismic displacements for individual earthquakes of considerable magnitude account for about 30% of the total deformation. When interpreted in terms of an homogenous slip fault as inferred from total deformation, some aspects of the data are not well explained, thus calling for heterogeneous slip models. It is hence possible, with a certain approximation, to distinguish the slip distributions associated with individual earthquakes from the almost homogeneous slip resulting from the sum of coseismic and aseismic movements. The inferred seismic moments for the two larger earthquakes that occurred along this fault are considerable higher than reported in seismic bulletins (about 1 degree of magnitude). The results obtained here support the important role of levelling data for the detailed study of a peculiar kind of tectonic structure on a volcano, aimed at understanding the complex interaction between volcano dynamics and induced movements along well defined structures.