Active stress from earthquake focal mechanisms along the Padan–Adriatic side of the Northern Apennines (Italy), with considerations on stress magnitudes and pore-fluid pressures

The active tectonic regime along the outer Northern Apennines (Padan–Adriatic area) is a matter of debate.We analyse the active tectonic regime by systematically inverting earthquake focal mechanisms in terms of their driving stress field, comparing two different stress inversion methods. Earthquakes within the area often deviate from Andersonian conditions, being characterized by reverse or transpressional slip on high-angle faults even if the regime is almost purely thrust faulting (e.g. Reggio Emilia 1996 and Faenza 2000 earthquakes). We analyse the stress conditions at faulting for the Reggio Emilia and Faenza earthquakes in order to infer the stress magnitudes and the possible role of fluid pressures. The stress analysis defines a consistent pattern of sub-horizontal active deviatoric compression arranged nearly perpendicular to the eastern front of the Padan–Adriatic fold-and-thrust system, independent of the stress inversion method used. The results are consistent with active compression operating within the Padan– Adriatic belt. The stress field is thrust faulting (sub-vertical σ3), except for the Cesena–Forlì and Ancona areas, where a strike–slip regime (sub-vertical or steeply-plunging σ2) operates. The strike–slip regimes are interpreted as being caused by the superposition of local tensional stresses due to oroclinal bending (i.e. rotations of the belt about vertical axes) on the regional compressional stress field. Kinematic complexities characterize the 1996 Reggio Emilia seismic sequence. The distribution of these complexities is not random, suggesting that they are due to local variations of the regional stress field within the unfaulted rocks surrounding the coseismic rupture. The stress conditions at faulting for the Reggio Emilia 1996 and Faenza 2000 earthquakes, coupled with the observation that seismicity in the Padan–Adriatic area often occurs in swarms, suggest that high pore-fluid pressures (Pf≥70% of the lithostatic load) operate within the compressed crust. The estimated stress difference (σ1–σ3) is ≤460–560 MPa at 15–20 km depth.