GNGTS 2017 - 36° Convegno Nazionale

206 GNGTS 2017 S essione 1.3 The parasitic pyroclastic cones provide a great amount of information that can help in distinguishing the geometry of buried magma-feeding fractures (e.g. Corazzato and Tibaldi, 2006 and reference therein). Parasitic pyroclastic cones are usually studied through the reconstruction of their stratigraphic position, the geological and morphometric characteristics, and growing process evolution in relation to the underlying magma-feeding fracture and sub- surface morphology. Three main kind of parasitic pyroclastic cones (right panel in Fig. 1) can be distinguished: i) simple cones characterized by a unique vent, having a plan from perfectly circular to markedly elliptical, ii) multiple superimposed cones represented by overlapping cones related to the same eruption where craters are aligned but do not interfere with each other and iii) multiple rifted cones represented by strongly elongated monogenetic edifices which usually have large dimensions (hundreds of metres). The present study aims to provide, through a ambient vibrations survey, useful information to increase the knowledge on subsurface structure of Mt. Vetore pyroclastic cone which is located in the upper S rift zone. It is a simple cone with a unique elliptical shape vent having a maximum and minimum axis in the range 170-240 m (Fig. 2). In this first survey we deployed, in the vent area, 6 single station ambient vibrations recording sites and a L shape geometry array, having two branches with length of 85 m (N-S direction) and 40 m (E-W direction) (Fig. 2). Methodology�. A quick estimate of the surface geology influence on seismic motion is provided by the horizontal to vertical noise spectral ratio technique (HVSR). This technique firstly introduced by Nogoshi and Igarashi (1971), was put into practice by Nakamura (1989) and became in recent years widely used since it provides a reliable estimate of the fundamental frequency of soft soil deposits. The good agreement observed between results obtained using earthquake records and ambient noise has pointed out that microtremors are a valid tool to investigate ground motion polarization properties (Rigano et al. , 2008; Di Giulio et al. , 2009; Panzera et al., 2017). Fig. 1 - Spatial density of historical fissures and pyroclastic cones on Mt. Etna (modified from Crisci et al. , 2008). Right panel shows the parasitic pyroclastic cones classification (modified from Corazzato and Tibaldi, 2006). A) Simple cones, B) multiple superimposed cones and C) multiple rifted cones