State of stress in the Southern Tyrrhenian subduction zone from fault-plane solutions

In this paper we present revised locations and original focal mechanisms computed for intermediate and deep earthquakes that occurred within the Southern Tyrrhenian subduction zone between 1988 and 1994, in order to improve our knowledge of the state of stress for this compressional margin. In particular, we define the stress distribution within a large portion of the descending slab, between 40 and about 450 km depth. The seismicity distribution reveals a continuous 40-50 km thick slab that abruptly increases its dip from subhorizontal in the Ionian Sea to a constant 70 degrees dip in the Tyrrhenian. We computed focal mechanisms for events with magnitudes ranging from 2.7 and 5.7, obtaining the distribution of P- and T-axes for many events for which centroid moment tensor (CMT) solutions are not available, thus enabling the sampling of a larger depth range compared to previous studies. We define three portions of the slab characterized by different distributions of P- and T-axes. A general down-dip compression is found between 165 and 370 km depth, whereas in the upper part of the slab (40-165 km depth) the fault-plane solutions are strongly heterogeneous. Below 370 km the P-axes of the few deep events located further to the north have a shallower dip and are not aligned with the 70 degrees dipping slab, possibly suggesting that they belong to a separated piece of subducted lithosphere. There is a good correspondence between the depth range in which the P-axes plunge closer to the slab dip (similar to 70 degrees) and the interval characterized by the highest seismic energy release (190-370 km).