Fluid pressure, stress field and propagation style of coalescing thrusts from the analysis of the 20 May 2012 ML 5.9 Emilia earthquake (Northern Apennines, Italy)

Terra Nova, 25, 72–78, 2013 Abstract Two major earthquakes of M L 5.9 and 5.8 and hundreds of aftershocks affected the Emilia region (Po Plain, Italy) between 20 May and June 2012. The events concentrate in the uppermost 10 km of the crust with few events up to 30 km. Two buried, sub‐parallel N100°E striking thrusts of the Northern Apennines belt are reactivated. These thrusts coalesce at the interface between the metamorphic basement of the belt and the overlying, about 10‐km thick, sedimentary succession. Focal mechanisms indicate a compressive stress field with a sub‐horizontal, roughly N‐S striking σ 1 . This stress field is consistent with active shortening in Northern Apennines. Suprahydrostatic pore pressure and σ 1 ∼ σ 2 ≠σ 3 are required to reactivate the thrusts. The involved fluids could be gas and brines hosted in the folded sedimentary successions. The time and spatial evolution of the seismicity indicates a foreland to hinterland propagation of the ruptures and a thin‐skinned deformation of Northern Apennines.