Hazards related to tectonic processes (1) are the most relevant in the Alboran
Sea. They are related to its complex tectonic context (Fig. 6.1b) caused by the
interaction between the Eurasian and African plates. This fact is evidenced by the
numerous faults and folds affecting the seabed, in addition to the related seismicity.
The central Alboran Sea presents the highest concentration of active tectonic structures (d’Acremont et al. 2014; Estrada et al. 2018; Galindo-Zaldívar et al. 2018)
(Figs. 6.1b and 6.7). Historical records suggest that Spanish, Moroccan, and Algerian populations bordering the Alboran Sea have been affected by more than
50 destructive earthquakes in the last 2000 years (Martín-Lechado et al. 2005).
The last was on January 25th, 2016, when there was an earthquake of magnitude
Mw ¼ 6.3 with its epicentre in the Alboran Sea (35.6
N, 3.81
W). This caused
material damage in Melilla and other Moroccan cities.
Hazards related to bottom current processes (2) are mainly related to the erosive
activity of the Atlantic and Mediterranean waters. This activity is revealed by the
erosive contouritic features. Published literature on the present-day characteristics of
oceanographic circulation points to the fact seafloor erosion could occur in the
vicinity of the Strait of Gibraltar, on both the Iberian and African margins. In this
area, the Atlantic and Mediterranean water masses accelerate (e.g. Kelling and
Stanley 1972; La Violette 1984; Parrilla et al. 1986; Parrilla and Kinder 1987;
Viúdez et al. 1996; Naranjo et al. 2012; Peliz et al. 2013; Sotillo et al. 2016). This
results in two hazardous effects: the reworking of slope terrace sediments, rock
outcropping, and the presence of a mobile seafloor at the entrance to the Strait of
Gibraltar (Kelling and Stanley 1972; Ercilla et al. 2016). Here, sand waves and
undefined sediment waves (metre-sized in relief, and a few to tens of metres in
length) have been mapped (Heezen and Johnson 1969).
Hazards related to mass-movement processes (3) is revealed by the presence of
landslides on the open slope and basins and on the steep walls of valley walls and
seamounts (Fig. 6.6a). In spite of the active tectonic context of the Alboran Sea, the
relationship between recent earthquake epicentres and submarine landslides is not
readily apparent in this basin (Figs. 6.1b and 6.6a). The correlation is only clear on
the northern side of the Francesc Pagès seamount and Alboran Ridge, where several
landslides have been mapped along the 2016–2017 ENE-WSW seismicity alignment
(Fig. 6.1b) (Ercilla et al. 2016; Galindo-Zaldívar et al. 2018). Most of these landslides are located on contourite deposits and the occurrence of alternating layers with
different grain sizes and mechanical behaviour could induce slope failures under
cyclic and/or static loading conditions. Slope instability may be also related to the
presence of a high content of gas bubbles in the interstitial pore water that contributes
reduced sediment shear strength. The gas content is mainly evidenced by the
presence of pockmarks, as in the case of the Baraza Slide (Fig. 6.6a) (Casas et al.
2011). Many other predisposing factors contribute to the triggering of submarine
landslides; for example the interaction of shallow canyon heads (e.g. Algeciras,
Guadiaro and Carchuna canyons) (Figs. 6.3b and 6.10a, b) with coastal and fluviomarine processes that undercut the canyon sidewalls, leading to a progressive slope
over-steepening of the canyon heads. On the other hand, sedimentary processes
related to the generation of hyperpycnal flows due to the torrential nature of the
6 Seafloor Morphology and Processes in the Alboran Sea
193
Sea. They are related to its complex tectonic context (Fig. 6.1b) caused by the
interaction between the Eurasian and African plates. This fact is evidenced by the
numerous faults and folds affecting the seabed, in addition to the related seismicity.
The central Alboran Sea presents the highest concentration of active tectonic structures (d’Acremont et al. 2014; Estrada et al. 2018; Galindo-Zaldívar et al. 2018)
(Figs. 6.1b and 6.7). Historical records suggest that Spanish, Moroccan, and Algerian populations bordering the Alboran Sea have been affected by more than
50 destructive earthquakes in the last 2000 years (Martín-Lechado et al. 2005).
The last was on January 25th, 2016, when there was an earthquake of magnitude
Mw ¼ 6.3 with its epicentre in the Alboran Sea (35.6
N, 3.81
W). This caused
material damage in Melilla and other Moroccan cities.
Hazards related to bottom current processes (2) are mainly related to the erosive
activity of the Atlantic and Mediterranean waters. This activity is revealed by the
erosive contouritic features. Published literature on the present-day characteristics of
oceanographic circulation points to the fact seafloor erosion could occur in the
vicinity of the Strait of Gibraltar, on both the Iberian and African margins. In this
area, the Atlantic and Mediterranean water masses accelerate (e.g. Kelling and
Stanley 1972; La Violette 1984; Parrilla et al. 1986; Parrilla and Kinder 1987;
Viúdez et al. 1996; Naranjo et al. 2012; Peliz et al. 2013; Sotillo et al. 2016). This
results in two hazardous effects: the reworking of slope terrace sediments, rock
outcropping, and the presence of a mobile seafloor at the entrance to the Strait of
Gibraltar (Kelling and Stanley 1972; Ercilla et al. 2016). Here, sand waves and
undefined sediment waves (metre-sized in relief, and a few to tens of metres in
length) have been mapped (Heezen and Johnson 1969).
Hazards related to mass-movement processes (3) is revealed by the presence of
landslides on the open slope and basins and on the steep walls of valley walls and
seamounts (Fig. 6.6a). In spite of the active tectonic context of the Alboran Sea, the
relationship between recent earthquake epicentres and submarine landslides is not
readily apparent in this basin (Figs. 6.1b and 6.6a). The correlation is only clear on
the northern side of the Francesc Pagès seamount and Alboran Ridge, where several
landslides have been mapped along the 2016–2017 ENE-WSW seismicity alignment
(Fig. 6.1b) (Ercilla et al. 2016; Galindo-Zaldívar et al. 2018). Most of these landslides are located on contourite deposits and the occurrence of alternating layers with
different grain sizes and mechanical behaviour could induce slope failures under
cyclic and/or static loading conditions. Slope instability may be also related to the
presence of a high content of gas bubbles in the interstitial pore water that contributes
reduced sediment shear strength. The gas content is mainly evidenced by the
presence of pockmarks, as in the case of the Baraza Slide (Fig. 6.6a) (Casas et al.
2011). Many other predisposing factors contribute to the triggering of submarine
landslides; for example the interaction of shallow canyon heads (e.g. Algeciras,
Guadiaro and Carchuna canyons) (Figs. 6.3b and 6.10a, b) with coastal and fluviomarine processes that undercut the canyon sidewalls, leading to a progressive slope
over-steepening of the canyon heads. On the other hand, sedimentary processes
related to the generation of hyperpycnal flows due to the torrential nature of the
6 Seafloor Morphology and Processes in the Alboran Sea
193