GNGTS 2013 - Atti del 32° Convegno Nazionale

areas and does not implies any monitoring activities except for some ambient noise single station measurements. This limitation is implicit in the definition of MS1 that is only dedicated to defining, mainly on the base of geological information, the areas suitable of seismic amplification with a homogenous behaviour without evaluating any amplification factor. As already pointed out the Civil Defence Department (DPC) promoted MS1 for the areas characterized by a high seismic risk in Italy. The Lazio County is part of DPC MS program and many municipalities, including the city of Arpino, the object of this paper, were already investigated by MS1. Historical seismicity. Arpino, located at the Southeastern edge of Lazio County, on the hillside area of the Lazio-Abruzzi Apennine chain, experienced in the past several seismic events that caused important damages to the urban structures. In particular the 1915 Mw 7.0 Avezzano event was felt with a MCS macroseismic intensity 8, and the 1654 Sorano-Marsica Mw 6.3 event induced a MCS intensity 9. The epicentral distances for these events were of about 30 and 6 kilometres respectively. On February 2013 the area of Sora (about 8 kilometers North-West of Arpino) was hit by a seismic sequence of low to moderate magnitude events with the mainshock of February 16th with Mw of 4.8 according to the data recorded by the Italian Seismic Network (RSNC) (http:// iside.rm.ingv.it ). The sequence lasted for more than two months and generated several tenths of aftershocks. The mainshock of February 16th was clearly felt by the citizens of Arpino, as testified by the MCS map produced thanks to the 4543 questionnaires compiled on line after the event. Despite to its low magnitude the event was able to generate some very light damage in Arpino probably due to the high vulnerability of buildings (http://www.haisentitoilterremoto . it). The focal mechanism associated to the mainshock is essentially normal with strike oriented in the NW-SE direction in agreement with the general NE-SW extensional trend of the area. Seismic data collection and geological setting. The high seismicity rate observed during the seismic sequence offered the opportunity to record, in a very short lapse time, weak motion seismic data in the epicentral area. For this reason four seismic stations (herein after AR stations) were installed in the Arpino municipality starting from February 23, and active until April 4. The installed instruments were based on high sensitivity Reftek 130 data loggers connected to three component seismometers Lennartz LE3D-5s with eigenfrequency of 0.2 Hz and sensitivity of 400 v/m/s. Stations were operating in a continuous recording mode with a sample frequency of 125 samples per second. Absolute timing was ensured by GPS synchronization. The selection of the recording sites was based upon the MS1 results, in particular on the distribution of the so-called “MOPS” that are intended as areas characterized by an homogeneous seismic behaviour. In MS1 the MOPS are defined starting from the geological and lithotechnical maps of the investigated area. The geological setting of Arpino territory is clearly connected to the evolution of the Lazio-Abruzzi Apennine chain mostly constituted by Cretaceous-Paleocene calcareous sequence. This unit represents the geological and seismic bedrock for the overlying continental deposits. In the MOPS map (Fig. 1) the area were this calcareous bedrock outcrops is indicated as stable and named as S1if it presents a slope less 30%. According to the Italian building code (NTC, 2008) the same unit is considered suitable for topographic amplification and is indicated as SA1 when its slope is higher than 30%. The most widely diffused deposit outcropping in the Arpino area is the Plio-Pleistocenic so- called Santopadre formation (Angelucci, 1970). This Formation, having a thickness up to about some hundreds metres, consist of two members: the lower one is composed of coarse to fine– grained silty sandstone and clayey siltstones, deposited in a fluvio-lacustrine and palustrine environment; the upper one is composed mainly of old alluvial sediments constituted by 390 GNGTS 2013 S essione 2.3