STRESS TENSOR ORIENTATION DERIVED FROM FAULT PLANE SOLUTIONS IN THE SOUTHWESTERN ALPS

Gephart and Forsyth's method has been applied to estimate stress orien tations from earthquake fault plane solutions of the southwestern Alps , a region where the tectonic stress regime is known to be fairly comp lex. Fault plane solutions have been either taken from the literature or computed using data from local and. regional networks. Data refer t o seismic events of magnitude in the range 2.5-5.3 which occurred in t he last decades at depths between 0 and 25 km. Two zones with a differ ent 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 c orresponding to the crest of the alpine belt, where a high-dip maximum compressive stress is found, and the eastern zone (Alps chain to Po P lain transition), characterized by an almost horizontal E-W sigma(1) a nd a nearly vertical sigma(3). Hypocenters of earthquakes used for str ess inversion lie in the depth ranges 0-15 km and 5-25 km in the weste rn and eastern zones, respectively. The transition between the two str ess domains is very sharp; and this is also indicated by space distrib ution of earthquake individual misfits to the respective stress models . The findings of the present study are a good match for tectonic mode ls which assume E-W compression derived from the Adria-Europe interact ion and producing: (1) major thrusting processes in the eastern side d f the chain and (2) secondary tensional effects at very shallow depth beneath the alpine belt crest (western zone of the area studied in thi s work).