Variations of stress directions in the western Alpine arc

Abstract

The western Alpine arc originated during the Cretaceous orogenesis as a consequence of the continental collision between the European and Adriatic plates. The distribution of forces acting in this sector of the Alps is still somewhat uncertain. In the past, some efforts have been made to map the distribution of P and T axes but it is known that these can be substantially different from the principal stress directions. In recent work, we presented a first attempt to determine the directions of σ1 and σ3, which we could compute only for a ‘local’ regime at the level of the magnitude of the larger events that occurred in the area. To obtain the stress orientation, we applied the technique developed by Gephart and Forsyth to invert fault plane solutions. In this work we present the results of a detailed analysis performed on a larger area, applying the same methodology. A total of 86 earthquakes with magnitudes ranging from 2.5 to 5.3 has been used for inversion. The results confirm the impossibility of defining, within the available data, a regional stress field. In fact, different local behaviours have been demonstrated in four subregions. For the first subregion, namely the northern part of the western Alps, the inversion of 28 earthquakes, resulting in a misfit of 5.9°, revealed a distensive regime orientated N-S. For the second subregion, the outer part of the western Alps, the inversion of 16 earthquakes led to a misfit of 5.3° for a distensive E-W orientated regime. In the inner part of the chain, an opposite result was obtained by the inversion of 14 earthquakes, confirmed by a misfit of 4.7°. Finally, the region of the Ligurian Sea revealed an almost horizontal NW-SE orientated σ1, whereas σ3 is NE-SW orientated with a dip of around 30°–40°. The inversion for this subarea was carried out on a data set of 28 earthquakes and characterized by a misfit of 7.1°. The uncertainty of the stress axis orientation (90 per cent confidence limits) is, on average for all inversions, around 40°.