LOCAL VERSUS REGIONAL ACTIVE STRESS FIELD IN 5900m SAN GREGORIO MAGNO 1WELL (SOUTHERN APENNINES, ITALY)

Abstract

The southern Apennines are a NE-verging fold-and-thrust belt, which formed from late Oligocene to Pleistocene times in response to deformation processes induced by the convergence between the African and European plates. The post-collisional phase includes the early Pleistocene development of strike-slip faults, responsible of lateral variations and of the segmentation of the belt. The last tectonic phase that affected the belt is relative to an extensional regime characterized by NW-SE faults and is still acting. Present-day stress state can be assessed by different techniques, such as borehole breakouts, focal mechanism solutions, active faults, hydrofracturing, overcoring, crustal deformation and differential strain. Our goals are to compare the local versus regional active stress in Irpinia region and to identify active shear zones along a deep well using borehole breakout and downhole log data. The selected area is characterized by diffuse low magnitude seismicity, although in historical times it was repeatedly struck by moderate to large earthquakes. On 23rd November 1980 a strong earthquake (M=6.9) occurred in this area producing the first unequivocal historical surface faulting ever documented in Italy. The mainshock enucleated on a 38 km-long normal fault, 308° striking and 60°-70° northeast-dipping, named Irpinia fault. The surface trace of this fault is very close to the San Gregorio Magno 1 deep oil well which should cross it approximately within an interval depth of 1500m. To discriminate the presence of the Irpinia fault and other possible active shear zones and to define the present-day stress along San Gregorio Magno 1 well, we have analyzed in detail borehole breakout and downhole geophysical data. Our analysis of stress-induced wellbore breakouts shows a direction of minimum horizontal stress N18°±24°, quite consistent to the regional Shmin trend (N44°±20°). Although some breakout zones with a different trend from the regional one have been identified, these have been related to slip on nearby faults. Comparing the breakout rotations with the downhole logs we have defined two most probable intervals where the Irpinia fault crosses the borehole around the depth of 2300 and 3800m. We conclude by considering the more general implications of our data for this area considered one of the regions with the highest probability (25%) of occurrence of an earthquake (M>5.5) for the next 10 years.