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¡ã dip) located in the Northern Apennines of Italy. During
a temporary seismic experiment, we recorded ~2000 earthquakes with ML ¡Ý 3.1.
The micro-seismicity defines a 500 to 1000 m thick fault zone that cross-cuts
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 hangingwall the seismicity distributions highlight 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, ~
3 events per day (ML ¡Ü 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 (~75¡ã) 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 whilst micro-earthquakes occur in velocity weakening
patches. We propose that these short-lived frictional instabilities are
triggered by fluid overpressures related to the build-up of CO2¨Crich fluids as
documented by boreholes in the footwall of the ATF.