Finite element modelling of stress field perturbations and interseismic crustal deformation in the Val d'Agri region, southern Apennines, Italy

The Val d'Agri area provides the opportunity to analyse active structures in a seismic region for which a large amount of subsurface data is available. This area, which was struck in 1857 by one of themost destructive earthquakes in Italy (MW=7.03), represents a unique natural laboratory to gain new insights into geometry,modes and rates of faulting controlling crustal deformation in an actively extending orogen. In this study, a crustal geological section through the southern Apennines is discretized into a finite elementmodel (FEM).Wepresent a 2D elastoplastic FEM that reproduces stress perturbations and strain field around the Val d'Agri active fault system. The influence of fault strand activity on interseismic crustal deformation is tested by a series of computermodels, whose predictions are compared with the horizontal velocity components of continuous GPS sites in the region andwith stress directions and geological data. The best fitwith available geological and geophysical constraints is obtained with a 300 km long, 29 kmdeepmodel formed by amultilayer including three components having different rheological characteristics and including several shallow, locked fault segments,which branch into a freely slipping major basement fault at depth. Finite element modelling provides new insights into the controversial andwidely debated active tectonic setting of the study area, pointing out the fundamental role played by a structural reactivation process involving inherited, long-lived, mature fault systems at depth. Our FEM, reconciling apparently contrasting geological and geophysical constraints from the study area, points tomaximumstress build up and strain accumulation at a depth of 15±5 km. Such a depth range is suggested as themost likely one for the nucleation of large events such as the 1857 Val d'Agri earthquake.