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/