A Very Broad Band Borehole Seismic Sensor in the Po Alluvial Basin near Ferrara (Italy) - Installation and Data Analysis

The Seismological Research Centre of the Italian National Institute for Oceanography and Experimental Geophysics (OGS), in Udine (Italy) after the strong earthquake of magnitude M=6.4 occurred in 1976 in the Italian Friuli-Venezia Giulia region, started to operate the North-eastern Italy Seismic Network: it currently consists of 20 very sensitive broad band and 22 simpler short period seismic stations, all telemetered to and acquired in real time at the OGS CRS data centre in Udine, (Bragato et al., 2011). OGS monitors the seismicity in cooperation also with local Civil Defence authority like in the Veneto region (Bragato et al., 2014). The south-western edge of the OGS seismic network in this region stands on the Po alluvial basin: earthquake localization and characterization in this area is affected by the presence of soft alluvial deposits. Following the ML=5.9 earthquake that struck the Emilia region around Ferrara in Northern Italy on May 20, 2012, a cooperation of Istituto Nazionale di Geofisica e Vulcanologia (INGV), OGS, the Ferrara Municipality and the Ferrara University lead to the reinstallation of a previously existing very broad band (VBB) borehole seismic station in Ferrara to record the local earthquakes that occurred during the seismic sequence. The aim of the OGS intervention was on one hand to extend its real time seismic monitoring capabilities toward South-West, including Ferrara and its surroundings, and on the other hand to evaluate seismic site responses in the area. We describe here details of the Ferrara VBB borehole seismic station installation and details of seismic site characterization with surface/borehole comparisons in terms of seismic noise, site amplification and resonance frequencies.