Stress tensor orientation derived from fault plane solutions in the southwestern Alps

Gephart and Forsyth's method has been applied to estimate stress orientations from earthquake fault plane solutions of the southwestern Alps, a region where the tectonic stress regime is known to be fairly complex. Fault plane solutions have been either taken from the literature or computed using data from local and. regional networks. Data refer to seismic events of magnitude in the range 2.5-5.3 which occurred in the last decades at depths between 0 and 25 km. Two zones with a different stress orientation have been identified in the studied area (44.0 degrees-45.5 degrees N, 6.5 degrees-8.5 degrees E): the western zone corresponding to the crest of the alpine belt, where a high-dip maximum compressive stress is found, and the eastern zone (Alps chain to Po Plain transition), characterized by an almost horizontal E-W sigma(1) and a nearly vertical sigma(3). Hypocenters of earthquakes used for stress inversion lie in the depth ranges 0-15 km and 5-25 km in the western and eastern zones, respectively. The transition between the two stress domains is very sharp; and this is also indicated by space distribution of earthquake individual misfits to the respective stress models. The findings of the present study are a good match for tectonic models which assume E-W compression derived from the Adria-Europe interaction and producing: (1) major thrusting processes in the eastern side df the chain and (2) secondary tensional effects at very shallow depth beneath the alpine belt crest (western zone of the area studied in this work).