Abstract
Automation of the throw backstripping method has proven to be an effective tool for the determination of the evolution of tectonic activity in wide fault zones. This method has been applied to the Al Hoceima Bay (southwesternmost Mediterranean, Alboran Sea) for a time period covering the last 280 kyr on 672 faults imaged on 265 high-resolution seismic reflection profiles. This area was affected by major earthquakes and corresponds to a transtensional basin deformed by growth faults. The automated application of throw backstripping allowed for a faster deciphering of the migration of tectonic activity. Results show a westward migration of the deformation with quickly increasing deformation rates in the most recent time frames near Al Hoceima, one of the most populated cities. This migration is in agreement with the current seismicity, the GPS data, and recent brittle deformation data. Vertical throw rates of up to 0.47 mm/year have been calculated, for the most recent time periods, in segments of the Bokkoya fault zone. The westward migration of the deformation fits with the reconstruction suggested by the westernmost Mediterranean geodynamic models during the Pleistocene epoch, and it might be the consequence of the interaction between the northwest (NW) movement of the South Alboran indenter and the back Rif south-westward displacement. The highly accurate constraints of the evolution of the tectonic activity offered by this automation will substantially improve the seismic hazard assessment.
Highlights
Active faults associated with earthquakes are generally characterized by the deformation of very recent deposits and geomorphological signatures, such as continuous scarps, linear valleys, or low sinuosity mountain fronts (e.g., Wesnousky, 1986; Keller and Pinter, 1996)
The results clearly provide an evidence that during the Middle Pleistocene, the extension through the Al Hoceima Bay was active and distributed between the Bokkoya fault and the Trougout fault
By taking into account of the possible overestimation, a change from 0.07 to 0.48 would imply an acceleration of the deformation rate in the Bokkoya fault zone. This deformation could be related to recent coseismic deformation, since the western area of the Bay is where the seismicity is concentrated (Figure 2). These results show that the fault activity during the Late Pleistocene has increased and migrated westward in the Al Hoceima Bay
Summary
Active faults associated with earthquakes are generally characterized by the deformation of very recent deposits and geomorphological signatures, such as continuous scarps, linear valleys, or low sinuosity mountain fronts (e.g., Wesnousky, 1986; Keller and Pinter, 1996). Recent small-scale and blind faults are not usually the target of palaeoseismic studies which generally focus on faults with a well-preserved record of major earthquakes, even if their most recent activity is decreased (Cornell, 1968; McCalpin, 2009 and references therein). In faulty submarine basins with considerable sedimentation rates, growth faults are the best candidates for study Their activity creates significant vertical movement affecting deposits that can be quantified using deformed regional reference markers in both fault blocks (e.g., Worrall and Snelson, 1989; Childs et al, 1995; Mansfield and Cartwright, 1996; Cartwright et al, 1998; Back et al, 2006; Baudon and Cartwright, 2008). In the study of the evolution of fault systems, the latter has been employed as a useful tool in many geological settings (Jackson et al, 2017, and references therein).The throw backstripping has been applied over long periods of time (several millions of years) in fault systems (Nicol et al, 1997; Meyer et al, 2002; Walsh et al, 2003; Giba et al, 2010; Phillips et al, 2018) and over short periods of time (Nicol et al, 2005, 2006)
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