GNGTS 2013 - Atti del 32° Convegno Nazionale

lead to a total number of about 100 seismic stations acquired in real time, which makes the OGS the reference institute for seismic monitoring of North-eastern Italy (Bragato et al. , 2011). In 2012 in the Fadalto area in the Belluno province in Northeast Italy several strong rumbles were heard, which eventually scared population and concerned authorities. OGS and University of Firenze installed in cooperation a real time seismic and infrasound monitoring system in the area. A description of the technical system capabilities, together with the preliminary results of the analysis of several months of recordings will be here illustrated. The area. The Lapisina Valley is located in North-eastern Italy, Venetian Prealps, and separates the Belluno Prealps in the West from the Cansiglio Plateau in the Est. It is a typical glacial valley, with wide bottom, and steep longitudinal profile and represents the original course of the Piave River, abandoned during the Late Glacial epoch for the obstruction of the valley due to the Fadalto landslide, detached from the Costa-Millifret Mount (Pellegrini, 2000). The pre-Quaternary rock in the Fadalto Valley are all sedimentary (glacial, landslide, talus slope). Fadalto landslide is chaotic and consists of predominantly calcareous debris, in silty- sandy matrix (Autostrade SpA, 1985). The coarse grain decrease in size from North to South. At the edge and bottom of the valley there are Mesozoic formation which are transitional sequence among predominantly pelagic and escarpment faces. From the tectonic point of view the complex syncline at the bottom of the valley has been dislocated by a system of inverse and transcurrent faults, connected with the line ‘Longhene- Fadalto-Cadola’, striking almost NNE-SSW (Pellegrini and Surian, 1996). The Lapisina Valley does not posses a main stream, but from north to south is characterized by four lakes (the main lake of Santa Croce, and the smallest Dead, Restello and Negrisiola lakes, modified for the exploitation of hydroelectric resources, since the beginning of the twentieth century). From the hydro-geological point of view, the Sella of Fadalto represents the watershed between the basin of the Piave and Livenza rivers. Meschio is the only river that collects underground waters from karst relieves of Col Visentin. Farther East, sources give rise to Livenza (Gorgazzo, Santissima, Molinetto), picking up the circulation of karst Group Cansiglio-Cavallo. The position of the water table is heavily influenced by the regime of precipitation and water level of the lakes. The instrumentation. The seismic network installed by OGS to monitor the Fadalto area is illustrated in Fig. 1. The configuration changed over time according to findings in the actual data, while the initial configuration reflected the population perception of the rumbles (CRS staff, 2011). The seismic stations used were made of Reftek C130 acquisition units and Lennartz LE3D 1 second seismometers. The RefTek 130S-01 Broadband Seismic Recorder (http://www.reftek.com/products/seismic-recorders-130-01.htm) digitizes and stores analog input from a variety of external sensors, including seismometers, accelerometers, tiltmeters and other geophysical sensors. The 130S-01 hardware is optimized for field deployments and is designed to be easy to use: compact, light in weight, low power consumption, and requires few maintenance. The Lennartz LE-3Dlite (http://www.lennartz-electronic.de/index. php?option=com_content&view=article&id=50&Itemid=55) is a 1 Hz to 100 Hz passband, low-power, low-noise, rugged and compact seismometer for applications requiring short-period sensitivity, field worthiness and portability. The seismic network was com- pleted with the FADA station made of a Quanterra Q330 digital acquisition unit (orange box in Fig. 2, left) with 6 channels, 24 bit digitizer sampled at 200 sps and a Lennartz LE-3D Lite short period seismometer (bronze Fig. 2 – Seismic (left) and infrasound (right) sensors installed in Fadalto. 198 GNGTS 2013 S essione 3.3