Present activity and seismogenic potential of a low-angle normal fault system (Città di Castello, Italy): constraints from surface geology, seismic reflection data and seismicity.

We present new constraints on an active low-angle normal fault system in the Città di Castello–Sansepolcro basin (CSB) of the northern Apennines of Italy. New field data from the geological survey of the Carta Geologica d'Italia (CARG project) define the surface geometry of the normal fault system and lead to an interpretation of the CROP 03 deep-crust seismic reflection profile (Castiglion Fiorentino–Urbania segment), with particular attention paid to the geometry of the Plio-Quaternary extensional structures. Surface and sub-surface geological data are integrated with instrumental and historical seismicity in order to define the seismotectonics of the area. Low-angle east-dipping reflectors are the seismic expression of the well-known Altotiberina Fault (AF), a regional extensional detachment on which both east- and west-dipping high-angle faults, bounding the CSB, sole out. The AF breakaway zone is located ∼10 km west of the CSB. Within the extensional allochthon, synthetic east-dipping planes prevail. Displacement along the AF is ∼4.5 km, which agrees with the cumulative offset due to its synthetic splays. The evolution of the CSB has mainly been controlled by the eastdipping fault system, at least since Early Pleistocene time; this system is still active and responsible for the seismicity of the area. A low level of seismic activity was recorded instrumentally within the CSB, but several damaging earthquakes have occurred in historical times. The instrumental seismicity and the intensity data points of the largest historical earthquakes (5 events with maximum MCS intensity of IX to IX–X) allow us to propose two main seismogenic structures: the Monte Santa Maria Tiberina (Mmax=5.9) and Città di Castello (Mmax up to 6.5) normal faults. Both are synthetic splays of the AF detachment, dipping to the NE at moderate (45–50°) to low (25–30°) angles and cutting the upper crust up to the surface. This study suggests that low-angle normal faults (at least with dips of 25–30°) may be seismogenic.