ASMI Archivio Storico Macrosismico ItalianoItalian Archive of Historical Earthquake Data

On May 20, 2012, at 4:03 local time (2:03 UTC), a large part of the Po Valley between the cities of Ferrara, Modena and Mantova was struck by a damaging earthquake (ML 5.9). The epicenter was located by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) seismic network [ISIDe 2010] at 44.889 ˚N and 11.228 ˚E, approximately 30 km west of Ferrara (Figure 1). The event was preceded by a foreshock that occurred at 01:13 local time, with a magnitude of ML 4. The mainshock started an intense seismic sequence that lasted for weeks, counting more than 2,000 events, six of which had ML >5. The strongest earthquakes of this sequence occurred on May 29, 2012, with ML 5.8 and ML 5.3, recorded at 9:00 and 12:55 local time, respectively. The epicenters of the May 29, 2012, events were located at the westernmost part of the rupture zone of the May 20, 2012, earthquake (Figure 2). The May 20 and 29, 2012, earthquakes were felt through the whole of northern and central Italy, and as far as Switzerland, Slovenia, Croatia, Austria, south-eastern France and southern Germany. Historical information reveals that the seismic activity in the Po Valley is moderate; indeed, except for the event that struck Ferrara on November 17, 1570, the last version of the Parametric Catalogue of Italian Earthquakes (CPTI11) [Rovida et al. 2011] does not report many other significant earthquakes occurring in this area [see also Castelli et al. 2012, this volume]. At times, small-to-medium-magnitude earthquakes have occurred in the area of the 2012 sequence, such as the events of December 6, 1986 (MW 4.6), and May 2 and 8, 1987 (MW 4.7, 4.6, respectively), that struck the northern sector of the Modena Province [Locati et al. 2011, Rovida et al. 2011]. The Po Valley is also affected by more frequent seismic activity due to the interaction of the northern Apennines and the Piedmont belt of Emilia, between Parma and Modena, where events of moderate magnitude often occur. An example of this seismicity was the October 15, 1996, Correggio (Reggio-Emilia) earthquake [Rovida et al. 2011]: this MW 5.4 event produced effects that extended up to the most damaged areas of the 2012 sequence [Locati et al. 2011] Soon after the strong earthquake occurred on May 20, 2012, the INGV 'QUick Earthquake Survey Team' (QUEST) organized a field survey of the major damaged areas, to be able to assign macroseismic intensities according to the European Macroseismic Scale (EMS98) [Grünthal 1998]. To take into account the cumulative effects of the aftershocks and their westward displacements, the survey was continuously adjusted and updated, to follow the enlarging 'most damaged zone'. In this report, the preliminary results of the macroseismic survey are presented.

Most of the inhabitants of northern Italy were woken up during the night of May 20, 2012, by the Mw 6.1 earthquake [QRCMT 2012] that occurred in the eastern Po Plain. The mainshock was preceded a few hours before by a Mw 4.3 shock, and it was followed by a dozen Ml >4 aftershocks in May and June, amongst which 11 had Ml ≥4.5. On May 29, 2012, a second Mw 6.0 mainshock struck roughly the same area [QRCMT 2012], which resulted in further victims, most of whom were caught under the collapse of industrial warehouses. Such earthquakes are an unexpected event in this region, as testified by the lack of local epicenters in the Italian seismic catalog [Rovida et al. 2011: CPTI11 from now] and by the consequent low level of the local seismic classification (seismic zone 3) [DPC 2012]. Apart from the warehouses and hundreds of old, crumbling farmsteads, severe damage was focused on ancient, tall buildings, such as churches, bell towers, castles, towers and palaces. Residential buildings generally suffered only light and/or moderate effects, apart from some exceptional cases. Using the Mercalli–Cancani–Sieberg (MCS) scale [Sieberg 1930], we began a macroseismic survey in the early morning of May 20, 2012, that ultimately included visits to almost 200 localities, 52 of which were carried out before the second mainshock.