A low-angle normal fault in the Umbria region (Central Italy): a mechanical model for the related microseismicity

Abstract

In the Northern Apennines, the upper crust is thinned by a set of east-dipping low-angle normal faults (LANFs): the easternmost and more recent of these LANFs is the Altotiberina Fault (ATF) located in Northern Umbria. The geometry of the ATF has been reconstructed matching surface geology with seismic reflection profiles and borehole data. The fault, whose average dip is ∼20°, borders the Upper Pliocene–Quaternary Tiber basin and has a displacement of about 8 km. The deeper portion of the ATF is located below the axial zone of the Northern Apennines where the strongest instrumental and historical seismicity is recorded; the microseismicity of the region ( M<3.0), located between 4 and 14 km in depth, shows a fairly good correlation with the trajectory of the ATF. We apply frictional reactivation theory, under vertical trajectories of σ 1, to evaluate the boundary conditions for the brittle activity of the ATF. The fault can be reactivated for low values of differential stress ( σ 1− σ 3<28 MPa), relatively high values of tensile strength of the fault surrounding rocks ( T∼10 MPa), and tensile fluid overpressure P f> σ 3 (e.g. λ v>0.93). In the peculiar situation of the Northern Apennines, the deep emissions of large amounts of CO 2 documented in the area can be entrapped in their ascent by structural seals (e.g. ATF) favouring localised fluid overpressures. The impossibility of sustaining P f> σ 3 for wide fault portions, counteracted by hydraulic fracturing, increased permeability under low effective stress and load weakening behaviour for normal faulting, would prevent the nucleation of moderate ruptures along the fault. The short-lived attainment of P f> σ 3 along small fault portions can account for the microseismic activity located along the ATF, which occurs on rupture surfaces in the range of 10 −1–10 −3 km 2.