Superposition of different stress orientations in the western sector of the Northern Apennines

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.

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