Active tectonics in Central Italy: Constraints from surface wave tomography and source moment tensor inversion

We investigate the lithosphere-asthenosphere structure and active tectonics of Central Italy, with emphasis on the Umbria-Marche area, by means of surface wave tomography and seismic moment tensor inversion. The data include: a large number of short period local and regional group velocity measurements sampling the Umbria-Marche Apennines and the Adria margin, respectively; incorporation of published phase velocity measurements sampling Italy and surroundings; results from deep seismic soundings which go through the Umbria-Marche area. The local group velocity maps, covering the area reactivated by the 1997-1998 Umbria-Marche earthquake sequence, suggest an intimate relationship between the lateral earth structure variations and the distribution of the active fault systems and related sedimentary basins. The upper crustal models reveal the importance of inherited compressional tectonics on the recent extensional deformation and associated seismic activity. Source inversion studies of the main events of the 1997 earthquake sequence show the dominance of normal faulting mechanisms, whereas selected aftershocks between the fault segments, at the step-over, reveal that the prevailing deformation is of strike-slip faulting type. At the regional scale, the crust exhibits clear layering and varies in thickness from about 25 km below the Tuscan Metamorphic Complex (TMC), to about 30 km below the Val Tiberina extensional thick sedimentary basin and reaches about 35 km below the Umbria-Marche geological domain (UMD). The lithospheric mantle (lid) is thin (about 30 km) below TMC, while it is about 70 km thick below UMD. A lithospheric root about 120 km wide, between the TMC and UMD, reaches a depth of at least 130 km. A low-velocity zone, defined mantle wedge (VS less than 4.2 km/s) in the uppermost mantle overlying the high velocity lid is detached. This wedge is about 20 km thick and decouples the underlying lid from the crust. The retrieved crust and upper mantle structure beneath Central Italy is in agreement with Bouguer anomaly and heat flow data and supports a delamination process. The high velocity upper mantle underlying the mantle wedge is inferred to be subcrustal lithospheric material delaminated from the overlying crust.