GNGTS 2013 - Atti del 32° Convegno Nazionale

several phases in which there has been the formation of folds associated with thrust systems that have fragmented and stacked the crystalline rocks and their sedimentary covers in different tectonic units. The deformations are still actives and result in thrust, within the chain, and crustal thinning in the Tyrrhenian area (Finetti, 1982; Giunta et al. , 2000). The geometry of thrusts are ramp-flat-ramp like (Nigro, 1994) or, in some cases, are the result of duplex structures (Nigro, 1994). Reverse fault systems (breaching) affected the area since the Upper Miocene resulting in moderate shortening (Giunta and Nigro, 1998). At the end of the Miocene the Tyrrhenian rifting produced a series of extensional low angle faults that resulted in a thinning of the chain. During Plio-Pleistocene times the area occupied by the Peloritani Mountains was affected by a strike-slip tectonic phase that generated two different systems: the first one synthetic with right cinematic oriented NW-SE and E-W; the second one antithetical, mainly sinistral, and oriented N-S and NE-SW (Ghisetti and Vezzani, 1977, 1984; Boccaletti et al. , 1986; Malinverno and Ryan, 1986; Giunta, 1991; Nigro and Renda, 2005). The neotectonic deformation is partly still active and to shear zones related to it are to be ascribed the seismic events and the high uplift rates (up to 5.5 mm/year) of the area (Di Stefano et al. , 2012). The morphological features of Oliveri town are represented mainly by an alluvial plain, where lies the town itself, and by a hilly landscape that surrounds the southern and western part of the plain. Both the hills and the plain are incised by two fluvial valleys (Torrente Castello and Torrente Elicona) that are partly filled by silty-sandy and pebbly sediments. The lithostratigraphic framework of the substrate of the urban area is made up by the metamorphic basement of the Aspromonte Unit that is covered unconformably by the arenaceus member of the Capo D’Orlando Flysch. Upwards the flyschoid sediments are tectonically overlain by the Argille Scagliose Antisicilide. The previous deposits are capped by sands and calcarenites of Plio-Pleistocene age. The most recent sediments consists of alluvial and beach deposits. Seismological history. The town of Oliveri is located in an area characterized by an intense seismic activity generated by seismogenic faults in the surrounding territory. Fig. 1 shows the distribution of the epicenters of earthquakes occurred in the area between 1981 and 2011 with an indication of the magnitude class. The seismic activity is characterized by a high frequency of events of low energy (only three events with epicenters far less than 20 km from Oliveri in the last 30 years are characterized by a local magnitude greater than 4 and hypocenter depths ranging from a few to some tens of kilometers. The first historical earthquake that interested Oliveri occurred in March 10, 1786. This seismic event was characterized by M W = 6.15, maximum macroseismic intensity equal to IX MCS, and its epicenter at the town of Oliveri. The earthquake particularly damaged the towns of the Gulf of Patti in northern Sicily and nearly destroyed the town (Guidoboni et al. , 2007). In more recent times Oliveri has suffered of macroseismic effects of the earthquake of April 15, 1978. This event of magnitude M W = 6.6 was located in the Gulf of Patti and in the municipality of Oliveri its mean macroseismic intensity was estimated as VII MCS. The shock caused serious damage in some buildings and significant soil fractures were also observed (Barbano et al. , 1979). Both the above mentioned earthquakes have been attributed to the “Patti - Aeolian” seismogenic structure that extends from the Peloritani Mounts to the Aeolian Islands (Basili et al. , 2008). HVSR measurements. Microtremor is a very convenient tool to estimate the effect of surface geology on seismic motion when you do not have a lot of geological information in a studied area. The single-station passive techniques are based on the resonance phenomenon, caused by the entrapment of thewaves between two surfaces with different acoustic impedances. 236 GNGTS 2013 S essione 2.2