Architecture and mechanics of an active low-angle normal fault: Alto Tiberina Fault, northern Apennines, Italy

We present seismological evidence for the existence of an actively slipping low-angle normal fault ( 15 degrees dip) located in the northern Apennines of Italy. During a temporary seismic experiment, we recorded similar to 2000 earthquakes with M-L >= 3.1. The microseismicity defines a 500 to 1000 m thick fault zone that crosscuts the upper crust from 4 km down to 16 km depth. The fault coincides with the geometry and location of the Alto Tiberina Fault ( ATF) as derived from geological observations and interpretation of depth-converted seismic reflection profiles. In the ATF hanging wall the seismicity distribution highlights minor synthetic and antithetic normal faults ( 4-5 km long) that sole into the detachment. The ATF-related seismicity shows a nearly constant rate of earthquake production, similar to 3 events per day ( M-L <= 2.3), and a higher b value ( 1.06) with respect to the fault hanging wall ( 0.85) which is characterized by a higher rate of seismicity. In the ATF zone we also observe the presence of clusters of earthquakes occurring with relatively short time delays and rupturing the same fault patch. To explain movements on the ATF, oriented at high angles (similar to 75 degrees) to the maximum vertical principal stress, we suggest an interpretative model in which crustal extension along the fault is mostly accommodated by aseismic slip in velocity strengthening areas while microearthquakes occur in velocity weakening patches. We propose that these short-lived frictional instabilities are triggered by fluid overpressures related to the buildup of CO2-rich fluids as documented by boreholes in the footwall of the ATF.