Liquefaction phenomena associated with the Emilia earthquake sequence of May-June 2012 (Northern Italy)

Abstract

In this paper we present the geological effects induced by the 2012 Emilia seismic sequence in the Po plain. Extensive liquefaction phenomena were observed over an area of ~1200 km2 following the May 20, Ml 5.9 and May 29, Ml 5.8 mainshocks, both occurred on about E-W trending, S dipping blind thrust faults. We collected the coseismic geological evidence through field and aerial surveys, reports from local people and web-based survey. On the basis of their morphologic and structural characteristics we grouped the 1362 effects surveyed into three main categories: liquefaction (485), fractures with liquefaction (768), fractures (109). We show that the quite uneven distribution of liquefaction effects, that appear concentrated and aligned, is mostly controlled by the presence of paleo-river beds, out-flow channels and fans of the main rivers crossing the area; these terrains are characterized by the pervasive presence of sandy layers in the uppermost 5 m, a local feature that, along with the presence of a high water table, greatly favours liquefaction. We also find that the maximum distance of observed liquefaction from the earthquake epicentre is ~30 km, in agreement with the regional empirical relations available for the Italian Peninsula. Finally, we observe that the contour of the liquefaction observations has an elongated shape almost coinciding with the aftershock area, the InSAR deformation area, and the I≥6 EMS area. This observation confirms the control of the earthquake source on the liquefaction distribution, and provides useful hints in the perspective of the characterization of the seismogenic source responsible for historical and pre-historical liquefactions.