Damage and Site Effects of the May 2012 Emilia-Romagna and Lombardia Earthquake, with Particular Reference to the “Oltrepò Mantovano” (Mantua) Territory

Abstract:

The seismic sequence that struck the Lombardia and Emilia-Romagna in May 2012 consisted of seven main events of magnitude greater than 5 followed by numerous aftershocks. The strongest earthquakes occurred on May 20 (M=5.9) and May 29 (M=5.8) near the border between Lombardia and Emilia-Romagna. The epicenters of the main events are aligned in east-west direction in a segment of approx. 50 km just south of the Po river. The area was considered a low to medium seismicity: the seismic hazard in the epicentral area, and in the whole damaged area, was estimated to be less than 0.15 g PGA for 10% exceedance in 50 years.

Significant damage occurred over an area greater than 1000 km², which is extremely large for earthquakes of magnitude less than 6, bearing in mind the low vulnerability level of the structures. As seen in detailed geological investigations the degree of damage and its areal extent is largely attributable to the particular conditions of the soil. We focus on the relationship between damage and soil conditions in the area of Oltrepò Mantovano, situated between the Po River and the epicentral area. The soil is largely composed of Quaternary deposits of sands, silty-clay and clay with a very deep bedrock (greater than 100 m) and Vs30 generally less than 500 m/s. According to the cards Aedes (official forms of the Italian Government to assess the state of damage of buildings) houses declared uninhabitable because of the earthquake were mainly concentrated in a few small towns: Moglia, Gonzaga, Quistello and San Giacomo delle Segnate (located approximately at 20 km, 27 km, 20 km and 14 km from the epicenters of the two main shocks, respectively) which reported 73% of the total of all uninhabitable buildings; Moglia 27 %, Gonzaga 14%, Quistello 20% and San Giacomo delle Segnate 12%. The hydrographic system has evolved considerably since the Middle Bronze Age with the result that the area is characterized by a complex geomorphology with the presence of fluvial paleochannels, fluvial ridges and abandoned river channels where numerous cases of soil liquefaction have been observed. The May 2012 earthquake has highlighted a close relationship between the sites where the damage was concentrated and the geological and geomorphological characteristics of the area.