S U M M A R YThe Northern Apenninic arc belongs to the deformation zones surrounding the Adriatic plate, which behaves as a rigid block. It is a NE-verging fold-and-thrust belt, which developed since the Miocene with the migration of an extensional-compressional pair. Previous seismological data are roughly in agreement with this deformation picture, although the outer compressional front was not clearly defined by the few available solutions. In this work we confirm the existence of two adjacent zones of contemporaneous extension and compression in the Northern Apennines, defining the extent of these two zones and their interim1 deformation better than was previously done. This is accomplished through the analysis of 125 new focal mechanisms of earthquakes (2.6 < Md < 4.8) recorded by the national network of the lstituto Nazionale di Geofisica in the period 1988-1995. The two deformation zones are clearly separated and lie very close to each other with a partial overlapping in the Emilian-Tuscan Apennines. Strikeslip events are scattered in most of the study area. Thrust-faulting earthquakes are located in the internal part of the most recently active thrust front (middle-upper Pleistocene). There is a general trend of the compressional P-axes of thrust-fault and strike-slip solutions to be perpendicular to the Apenninic direction in the external zone, while the tensional axes of the normal-fault and strike-slip solutions in the belt do not show a uniform orientation. However, clearly evident is the -ENE direction of extension in the peri-Tyrrhenian region, consistent with the direction of Shmln in the Quaternary volcanoes of this region inferred from borehole breakouts and microearthquake fault-plane solutions. We determined the orientation of the stress tensor in the two main deformation zones by inversion of the 125 fault-plane solutions determined in this study. We found that the external belt is characterized by NE-SW compression (ol horizontal, oriented N45"E), while a normal-faulting regime with o3 -E-W oriented is characteristic of the Umbria-Tuscany (back-arc) region.

S U M M A R YIn 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 k m thick slab that abruptly increases its dip from subhorizontal in the Ionian Sea to a constant 70" 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 t o the north have a shallower dip and are not aligned with the 70" 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 (-70") and the interval characterized by the highest seismic energy release (190-370 km).

S U M M A R YOur study area is a ca 50 km long section of the central-southern Apennines tectonic belt that includes the Pergola-Melandro basin (PM) and the Agri valley (AV). This region is located between the areas affected by the 1980 Ms = 6.9 Irpinia and the 1857 M = 7.0 val d'Agri earthquakes and is characterized by rare historical events and very low and sparse background seismicity. In this study we provide new seismological and geophysical information to identify the characteristics of the seismotectonics in the area as the prevailing faulting mechanism and the fit of local to regional stress field. These data concern focal mechanisms from waveform modelling and P-wave polarities, analyses of borehole breakouts and detailed investigation of two seismic sequences. All the data cover a significantly broad range of magnitudes and depths and suggest that no important local variation in stress orientation seems to affect this area, which shows a NE-SW direction of extension consistent with that regionally observed in southern Italy. Such local homogeneity in the stress field pattern is peculiar to the study area; the variations of orientation and/or type of stress observed in the northern Apennines, or less than only 100 km toward the northwest within the same tectonic belt, are absent here. Furthermore, there is a suggestion for a northeastward sense of dip of the seismogenic faults in the region, an interesting constraint to the characterization of seismic sources.

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