The July 17, 2011, ML 4.7, Po Plain (northern Italy) earthquake: strong-motion observations from the RAIS network

On July 17, 2011, at 18:30:23 UTC, a ML 4.7 earthquake occurred on the east side of the Po Plain (northern Italy), between the towns of Ferrara and Rovigo. The epicentral coordinates provided by the National Earthquake Center of the Istituto Nazionale di Geofisica e Vulcanologia (National Institute of Geophysics and Volcanology, INGV) were 45.01 N and 11.41 E (http://iside.rm.ingv.it/iside). The depth of the hypocenter was constrained at 8.1 km, corresponding to a buried active source that existed in the area. The source of the event was characterized by a predominant left-transverse focal mechanism, even if there was also an important reverse component. Although it did not produce relevant damage, the earthquake was clearly felt in an area of about 50 km radius around the epicenter. The maximum observed intensity was V on the Mercalli-Cancani-Sieberg (MCS) scale, with a predominant distribution of damage towards the north-west. This study provides an overview of the strong-motion waveforms of the mainshock as recorded by the RAIS (Rete Accelerometrica Italia Settentrionale) strong-motion network, in particular focusing on the recordings provided by the stations located in the central part of the basin, which were installed in correspondence with hundreds of soft sediments. The preliminary results show the relevant influence of the basin on the seismic wavefield, highlighting in particular a possible site-amplification phenomena, and also affecting the ground motion at long periods (T >1 s). The systematic underestimations provided by the empirical ground-motion predictive models calibrated for Italy in terms of acceleration response spectra up to 2.0 s support this hypothesis. The sharing of the 24 waveforms (in raw sac and ascii formats) recorded by RAIS is assured by the availability of the data at the ftp site: ftp://ftp.mi.ingv.it/download/RAIS-FR_rel01/.