Abstract
Abstract. Geological and seismological observations have been used to parameterize 2-D numerical elastic models to simulate the interseismic deformation of a complex extensional fault system located in the Northern Apennines (Italy). The geological system is dominated by the presence of the Alto Tiberina fault (ATF), a large (60 km along strike) low-angle normal fault dipping 20° in the brittle crust (0–15 km). The ATF is currently characterized by a high and constant rate of microseismic activity, and no moderate-to-large magnitude earthquakes have been associated with this fault in the past 1000 years. Modelling results have been compared with GPS data in order to understand the mechanical behaviour of this fault and a suite of minor syn- and antithetic normal fault segments located in the main fault hanging wall. The results of the simulations demonstrate the active role played by the Alto Tiberina fault in accommodating the ongoing tectonic extension in this sector of the chain. The GPS velocity profile constructed through the fault system cannot be explained without including the ATF's contribution to deformation, indicating that this fault, although misoriented, has to be considered tectonically active and with a creeping behaviour below 5 km depth. The low-angle normal fault also shows a high degree of tectonic coupling with its main antithetic fault (the Gubbio fault), suggesting that creeping along the ATF may control the observed strain localization and the pattern of microseismic activity.
Highlights
The mechanical behaviour of low-angle normal faults (LANFs; dip angle < 30◦) is a critical issue in fault mechanics (Jackson and White, 1989; Buck, 1993; Westaway, 1999; Collettini and Sibson, 2001; Collettini, 2011), and it represents a paradox if the paradigm of faulting in a brittle, elastic, homogenous crust holds
Notwithstanding, the outcomes of these 2-D simulations contain original and definitive indications for an active LANF as well as for a tectonic coupling between the two major structures of this fault system. This tectonic coupling generates a negligible stress accumulation in the Alto Tiberina fault (ATF) footwall, coherently with the lack of seismic activity, and a redistribution of the stress build-up consistent with the high rate of microseismicity occurrence in ATF hanging wall volume, including the footwall of the Gubbio fault plane (GuF) (Chiaraluce et al, 2007; Fig. 1b) 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
In this work we integrate the results of 2-D numerical models with geological, seismological and geodetic observations in order to understand some of the key features on the mechanical behaviour of the Alto Tiberina fault and its associated fault system
Summary
The mechanical behaviour of low-angle normal faults (LANFs; dip angle < 30◦) is a critical issue in fault mechanics (Jackson and White, 1989; Buck, 1993; Westaway, 1999; Collettini and Sibson, 2001; Collettini, 2011), and it represents a paradox if the paradigm of faulting in a brittle, elastic, homogenous crust holds. Frictional fault reactivation theory (Anderson, 1951; Sibson, 1985) predicts that in extensional settings the maximum principal stress is vertical and no motion is possible on faults dipping less than 30◦ and sliding with a friction coefficient (μs) in the range of 0.6–0.85 (Byerlee, 1978) This theory is supported by the evidence that no moderate-to-large magnitude earthquakes have been documented worldwide nucleating on LANFs using positively discriminated slip planes from the focal mechanisms (Jackson and White, 1989; Collettini and Sibson, 2001).
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