Stress and strain tensors based on focal mechanisms in the seismotectonic framework of the Friuli-Venezia Giulia region (northeastern Italy)

The principal axes of stress and strain are inferred for the seismotectonic zones of the Friuli-Venezia Giulia region, which forms the eastern part of the European Southern Alps. This region corresponds to a polyphase deformational belt formed during the Meso- and Neoalpine compressions (from the Eocene to Plio-Quaternary). Several distinct seismotectonic zones are recognized by comparing seismological and tectonic characteristics. Inference of the stress and strain tensors is based on 243 fault-plane solutions, corresponding to earthquakes occurring between 1984 and 2001 (magnitude range: 2.1-5.6). The analysis reveals two main regional stress patterns: a strike-slip regime active in the eastern and northwestern parts of the area under study and dominant thrusting processes in the central and western zones. The orientation of the maximum principal stress varies from north-northwest-south-southeast to north-northeast-south-southwest. The stress magnitude ratio of the calculated stress tensors varies from 0.4 to 0.7. The orientation of the maximum shortening axis is generally north-northwest-south-southeast, changing to north-northeast-south-southwest in the southeastern part of the region. For most seismotectonic domains, the inferred strain pattern is quite concordant with the surface tectonics. The orientations of the principal axes of stress and strain are in most cases similar. The complex regional tectonic structure, consisting of two indented tectonic wedges, appears to control the patterns of horizontal stress and strain.