Seismic Hazard Assessment along the Northern Apennines front (Italy): Deterministic inputs from the mapping of active and capable faults

Abstract

Understanding how the brittle deformation pattern at the surface relates to active seismogenic sources at depth is one key element for accurate Seismic Hazard Assessment procedures based on deterministic inputs. Establishing a relationship between surface faulting and deep sources can, however, be very challenging, especially in areas where seismogenic structures lack obvious and readily interpretable geological evidence at the surface. Here, we present results of detailed structural and geological investigations from a field-based study of active and capable faults along the Northern Apennines front (Pedeapenninic margin) between Reggio Emilia and Bologna, in northern Italy. Those results are then implemented into a Probabilistic Seismic Hazard Assessment model (PSHA) that also relies on an accurate surface acceleration model computed by considering site effects from the local stratigraphic amplification factors. In the study area, the geological framework is characterized by two lithotectonic units: the Eocene-to-Miocene Epiligurian Units and the Pliocene-to-Present successions cropping out along the frontal Pedeapenninic margin. A compressive tectonic regime is currently dominant, with a regional-scale, NE-verging thrust system shaping the first-order architecture of the Pedeappenninic front. This thrust system is complex and is dissected by transverse normal and transpressive/transtensive faults. The architecture of the studied margin reflects exposed NE-verging thrusts within the Epiligurian Units in the more internal domains, and mostly blind thrusts below the Pliocene-to-Present units in the external domains. The Pliocene-to-Present units are also faulted and folded, indicating that tectonic activity is still in full swing, hence with significant seismogenic potential (as also documented by seismic archives). Top-to-NE and -SW normal faults are common in the area and deform the Pliocene-to-Present successions together with mostly NE-SW striking strike-slip and transpressional/transtensional faults. Based on these structural/stratigraphical constraints we produced a geological model that represents the deterministic input to improve our current knowledge of seismogenic sources in the study area. Regarding the seismic response at the surface in terms of the maximum expected acceleration, we computed the mean equivalent value of shear wave velocities in the uppermost 30 m of subsoil (VSeq) by using the available geognostic database of the area. The VSeq value allowed to calculate a specific stratigraphic amplification factor at each measurement point. In the study area, amplification varies, on average, from a 1.2 factor within the more rigid substrate to a 2 factor within the less consolidated, Pleistocene-Holocene in age, intra-valley deposits and in the Apennines foothills. This classification will be used as input to the Ground Motion Prediction Equations (GMPEs), as well as the Earthquake Source Model built through the combination of geological information about the sources (width, depth, slip rate, kinematics...) and historical/instrumental seismicity.