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AuthorsMariucci, M. T.* 
Montone, P.* 
TitleContemporary stress in northern Italy: how leak off data depict a compressive tectonic setting
Issue Date20-Aug-2013
KeywordsContemporary stress field
Leak-off test
Northern Italy
Subject Classification04. Solid Earth::04.07. Tectonophysics::04.07.05. Stress 
AbstractThe knowledge of present day stress magnitude is often crucial to understand the behavior of natural systems. Not only relative magnitudes but also absolute values are important parameters that help the improvement of physical models, notwithstanding their uncertainties. The Ml 5.9 2012 Emilia earthquake sequence (northern Italy) gave us the chance to focus on present day stress of that region. The area where the sequence occurred is in the Po Plain, a flat area in front of the outcropping Apennine thrust and fold N-verging belt and on the top of its buried front. Active blind structures are responsible of the earthquakes in this area although until the recent sequence only a few earthquakes occurred in the area in historical times. The focal mechanisms of the 2012 sequence suggest a thrust regime with maximum horizontal stress about NNE oriented, in agreement with the other stress indicators in the area and geomorphologic studies which pointed out the presence of active blind thrusts. We analysed in detail well data in order to compute the magnitude of the stress tensor principal axes in the crust. We used leak-off test data, performed in deep oil wells at depth between 200 and 5400m, in order to achieve the magnitude of the least stress component. Other well data (density log, sonic log) were used to infer the vertical stress magnitude, and, following the formula currently used, we computed the third principal stress component. From a comparison of the three principal stress values we were able to estimate the complete stress tensor. Consistently with previous studies the magnitude of the vertical stress and the least principal stress are very close, indicating that a thrust regime is acting. Nevertheless, some results suggest that locally the regime can became strike-slip, due to a variation at depth, but also to the arc shape of main structures which may vary the stress orientation and relative magnitudes. The consistency of leak off analysis and focal mechanisms allow us to affirm the reliability of the leak off method in characterizing the stress regime at depth, even when few data of the test are available.
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