Abstract:
"Massive surface fracturing and sand ejection took place during the main shock of the May 20, 2012, earthquake (ML = 5.9) in the Emilia-Romagna region, northern Italy. These phenomena were induced by the liquefaction of water-saturated sand layers, and they damaged several buildings, as well as many roads and sidewalks. They were clustered between the villages of Sant’Agostino and Vigarano Mainarda, located along a paleo-reach of the Reno River [Papathanassiou et al. 2012, this volume]. The subsurface surrounding two major (several decameters long) ground ruptures was investigated using electrical resistivity tomographies (ERT), as resistivity is strongly affected by the chemico-physical conditions of loose sediments. Italian regulations require the Municipalities within seismically active areas to develop maps of the potential liquefaction risk. Not all of the territories that are under this kind of risk have been investigated to date. A strong effort to improve this knowledge is therefore needed. Noninvasive geophysical methods can help to fill this gap, as high-resolution techniques are available with good result-to-cost ratios. Among the available methodologies, the most suitable are the methods based on electrical resistivity and permittivity, as they are highly sensitive to the presence of underground water. The ERT method has been carried out successfully across active faults, providing crucial paleoseismological information [Caputo et al. 2003, 2007]. A few weeks after the main Emilia-Romagna seismic event, ERT profiles were carried out at several locations within the affected area. In this study, we discuss two sites: the first in an uncultivated field south of San Carlo (Figure 1, site 1), which is associated with a paleoseismological excavation [Caputo et al. 2012, this volume], and the second within the urbanised area of the same village (Figure 1, site 2). Both of these areas were affected by several ground ruptures, and particularly site 2, by sand volcanoes. The two sites are located in similar morphological, hydrogeological and stratigraphic settings, along the leeeve flank of the paleoReno River. Indeed, since Middle ages and up to the XVIII century the Reno River was flowing towards northeast creating a fluvial system characterized by a channel, natural levees and crevasse deposits. The natural (and partly artificial) levees were progressively urbanized to avoid the effects of the frequent flood events, which were definitely stopped when the river was diverted near Sant’Agostino towards the southeast thus flowing directly into the Adriatic Sea. The witnesses of the past hydrography are still marked in the local topography and well known from stratigraphic investigations of the broader area [e.g. Bondesan 1989, Cibin and Segadelli 2009]."
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