Creep and locking of a low‐angle normal fault: Insights from the Altotiberina fault in the Northern Apennines (Italy)

Abstract

While low‐angle normal faults have been recognized worldwide from geological studies, whether these structures are active or capable of generating big earthquakes is still debated. We provide new constraints on the role and modes of the Altotiberina fault (ATF) in accommodating extension in the Northern Apennines. We model GPS velocities to study block kinematics, faults slip rates and interseismic coupling of the ATF, which is active and accounts, with its antithetic fault, for a large part of the observed chain normal 3 mm/yr tectonic extension. A wide portion of the ATF creeps at the long‐term slip rate (1.7 ± 0.3 mm/yr), but the shallow locked portions are compatible with M > 6.5 earthquakes. We suggest that positive stress accumulation due to ATF creep is most likely released by more favorable oriented splay faults, whose rupture may propagate downdip along low‐angle normal fault surface and reduce the probability of occurrence of a seismic rupture of the shallower locked portion.

On the mechanical behaviour of a low-angle normal fault: the Alto Tiberina fault (Northern Apennines, Italy) system case study

“…The first main outcome is that, in an elastic crust, the GPS observations cannot be explained without considering the ATF contribution to deformation, indicating that this fault, although misoriented, has to be considered tectonically active. Our findings also indicate the need for creeping behaviour along the ATF below 5 km depth, in agreement with the outcomes of Anderlini et al (2016). Our simulations show that using only the ATF does not exhaustively explain the GPS data, deserving the contribution of at least another active segment located in the ATF hanging wall volume such as the Gubbio fault (Fig.…”

“…1b) Thus, our work shows that the ATF plays an active role in the deformation processes of the region and that the interpretation of our results cannot exclude the occurrence of damaging earthquakes along the ATF fault system. These main outcomes obtained with a simple elastic modelling are in agreement with the results of Anderlini et al (2016), suggesting the existence of shallow and locked portions of the ATF potentially corresponding to a large earthquake (M w 6.7), even if such large events are not present in the historical seismic catalogue for this area.…”

“…The GPS data used for model calibration are available from Anderlini et al (2016), whereas the geometry used for the numerical simulations is available from Mirabella et al (2011).…”

Mixed‐Mode Slip Behavior of the Altotiberina Low‐Angle Normal Fault System (Northern Apennines, Italy) through High‐Resolution Earthquake Locations and Repeating Events

“…RE are seismic events characterized by near‐identical waveforms that repeatedly rupture the same fault patch and that have been interpreted as the signature of a creeping process (Nadeau et al, ). Then, we investigate the spatiotemporal distribution of RE to map those portions of the ATF characterized by distinctive frictional behavior, i.e., stick slip or stable sliding, as it has been suggested by geodetic data (Anderlini et al, ; Vadacca et al, ).…”

“…GPS data suggest that slip actively occurs along the ATF plane (Hreinsdottir & Bennett, ). The ATF accounts, with its antithetic Gubbio Fault (GuF) system, for a large part of the observed 3 mm/yr NE trending tectonic extension (Anderlini et al, ). In particular, the authors suggest a model in which about half of the ATF surface below 5 km of depth is characterized by creep, producing a long‐term slip rate of 1.7 ± 0.3 mm/yr, while the shallow portion of the same fault (depth < 5 km) is locked and it may be capable of generating M 6.5+ earthquakes.…”