Superposition of different stress orientations in the western sector of the northern Apennines (Italy)

Previous studies of the stress regime in the northern Apennines report compression in the outer sector and extension in the inner sector. In this study we focus on the relationships between the two regimes and, by making use of 54 focal mechanism solutions, we try to shed some more light on a tectonic setting that appears more complex than the previous models. The focal mechanisms, computed using the first onset technique, are inverted for stress parameters. The method applied requires subdividing the volume to be investigated into homogeneous sectors: to comply with the complexity of the area under study, we included depth as a parameter for sub-zoning. Our results show that the shallow regime (0–10 km of depth) is transtensive even in the sector previously reported to be compressive. In fact, at a shallow depth, very few thrust focal solutions lie in a spatially limited sector in the eastern part of the area under study. Just below 10 km in depth, the stress regime converts to transpressional: stress axes are approximately inverted. Deeper than 45 km, thrust solutions are found. They are not numerous enough to perform an inversion but indicate the existence of a compressional regime at depth. It is worth noting that a gap of seismicity is observed in the layer 30–45 km.