Hypothesis for the mechanics and seismic behaviour of low-angle normal faults: the example of the Altotiberina fault Northern Apennines

Widespread mapping of low-angle normal faults in areas of former continental extension continues to prompt debate as to whether such structures may be seismically active at very low dips (• <30 °)in the upper continental crust.The Northern Apennines provide an example where an active low-angle normal fault (Altotiberina fault, ATF)can be studied.A set of commercial seismic reflection profiles plus deep boreholes have been used to constrain the geometry of the fault at depth.These data have been integrated with a microseismic survey showing that part of the microseismicity (M <3.0)is consistent with the geometry of the ATF as imaged by depth converted seismic reflection profiles.Frictional fault mechanics under Byerlee ’s friction coefficient and vertical • 1 (constrained from the inversion of the focal mechanisms)defines the peculiar condition for reactivation of the ATF:small values of differential stress,• 1 •• 3 <28 MPa,relatively high value of tensile strength of the fault surrounding rocks (T ~10 MPa)and tensile fluid overpressure P f >• 3 (i.e.• v >0.93).The short-lived attainment of P f >• 3 along small fault portions,in an area characterised by large amounts of CO2,account for the microseismic activity located along the ATF,which occurs on rupture surfaces in the range of 10 –•10 – 3 km 2..