Evaluation of the seismic site response using the recent earthquake swarm of Basilicata region (Southerns Appennines-Italy)

During the fall of 1998, an accelerometric digital (vertical component) station has been installed in two contigous sites located at the towns of Castelluccio Inferiore (CI) and Castelluccio Superiore (CS) (Calabrian Lucan Appennines, Southern Italy), with the aim of the evaluation of the seismic site response at Castelluccio Inferiore. This area is very important because in the last centuries has been interested by large earthquakes. At present the Lucanian Appenines represent a major boundary between two distinct seismotectonic areas: the Southern Appenines, characterized by NW-SE striking active normal faulting and the Calabrian-Sicilian area with EWE-WSW striking active normal faulting. Both areas are interested by earthquakes of magnitudo up to 6-7° Richter. (Turco et al.,1990; Selvaggi et al.,1997; Kiratzi, 1994; Hyppolite et al.1994). The accelerometric station was made by a piezoelettric vertical sensor with frequency response flat in 1-100 Hz. The analogic signal was digitalized by a 16 bit A/D board and finally recorded on the mass storage of a PC.The data was collected in a time interval that allowed the recording of a considerable number of seismic events in two different sites. At the first site (CS) the power spectra is referred to the bedrock because the sensor is located on lapideus rocks (limestones), at the other site (CI) the seismic response is filtered by a 25 m thick Plio-Pleistocenic overburden constituted by shale and clay with very low seismic rigidity (vp ~ 700 m/s; ρ ~ 1.1 g/cmc).