TECTONIC FRAMEWORK
The formation of the Alboran basin is encompassed within the Alpine orogeny, a large-scale tectonic process active since the late Mesozoic and during most of the Cenozoic, that is characterized in the Mediterranean Sea domain by the convergence between the Eurasian and African plates. In this context, the Alboran microplate migrated westwards and tangentially collided with the Iberian plate. As a consequence, during the Miocene (late Cenozoic), there was simultaneously a compression process, causing the uplift of the surrounding mountain ranges in the Gibraltar Arc, and an extension process, responsible for the Alboran basin opening. The extension stress field produced the continental crust thinning and different magmatism and volcanism episodes during the late Miocene, especially in the southern and eastern zones. Therefore, the relief of the Alboran Sea seafloor is complex and rugged, and it is divided in sub-basins by seamounts and volcanic cones. The extensive phase ended up in the late Miocene and the compressive stress field became dominant in the region, accommodated by fold alignments perpendicular to the compression and NNE-SSW and WNW-ESE reverse and strike-slip faults.
Currently, the oblique convergence between the Eurasian and African plates is active, in NW-SE direction and shows a displacement rate around 5 mm/yr. Hence, the dominant stress field in the region is compressive with strike-slip component (transpressive regime).
The complex and intense tectonic activity in this region generated different structures like the Alboran Ridge Crest and the Pagès and Tofiño Banks which are bounded on the north and south by opposite dipping active reverse faults. Both conjugate reverse fault systems are NE-SW direction and correspond to the Alboran Ridge Crest faults (FARN and FARS) and the Tofiño faults (FTÑS and FTÑN under debate), being the faults in the southern side of the structures the ones dipping to the NW. The Alboran Ridge Crest was partially an active volcano during the late Miocene. This relief divides the Alboran Sea into three sub-basins: west (COA), east (COE) and south (CSA). These three domains are limited by some active faults, among which two conjugate strike-slip with reverse component fault systems are the most remarkable. The first system with left-lateral movement encompasses two main faults, the NNE-SSW Al-Idrissi (FAL) fault and the NE-SW Carboneras fault (FC), connecting the region of Almería with the north of Morocco. The second system with right-lateral displacement and NW-SE direction holds one main fault, the WNW-ESE Yusuf fault (FY) that it is composed of two segments around 100 km long each. Northwards, the parallel Averroes faults (FAV and FAVN) and the Adra fault (FA) in front of the Granada and Almería coasts are also worth mentioning. These two strike-slip fault groups concentrate most of the seismic activity in the region. In the southern area of the Alboran Sea (North of Morocco) it also stands out the N-S normal Trougout (FTR) fault.
REGIONAL OVERVIEW AND GEOLOGICAL CONTEXT
The Alboran basin is located in the western edge of the Mediterranean Sea. It is the inner zone of the Gibraltar Arc as it is surrounded by the arc-shaped mountain range formed by the Betic System in the Iberian Peninsula, the Strait of Gibraltar and the Rif mountains in Morocco.
The Alboran Sea bathes the southeastern coast of the Peninsula over this basin, the northeastern coast of Morocco and the northwestern coast of Algeria. It is 350 km long in the E-W direction and the max-width is 180 km in the N-S direction. The average water depth is 1000 m, but it is over 2000 m in the deepest zones. There are many seafloor reliefs, in the southern area, the Alboran Ridge Crest and the Francesc Pagès and Tofiño Banks, NE-SW submarine mountain ridges, and in the northern area, the Djibouti and Chella Banks, are the most relevant reliefs. Moreover, some islands emerge in the region, they are mostly volcanic islands, and the most important one is the Alboran island, the single outcrop above sea level in the Alboran Ridge Crest.
Geologically speaking, it is a complex region with different crustal domains. In a simplified manner, the Alboran Sea seafloor consist of thinned continental crust (12-20 km) formed mainly by paleozoic and triassic (early Mesozoic) strongly metamorphosed rocks. On top of this basement there is a tertiary (Neogene) and quaternary discordant sedimentary cover, and therefore, contemporary to the basin formation and the uplift of the mountain ranges of the Gibraltar Arc. The sediment cover thickness strongly varies from some areas to others, reaching maximum values of 8 km in the westernmost part.
HISTORICAL AND INSTRUMENTAL SEIMICITY
The Alboran domain is one of the most seismically active areas in the western Mediterranean Sea. It is characterized by shallow seismicity (<40 km) of low to moderate magnitude, although several large destructive historical earthquakes have also hit this region. This shallow seismicity is mainly controlled by the active fault systems that cross the basin, with predominant strike-slip focal mechanisms, though some reverse and normal type mechanisms are also observed.
Historically, it is worth mentioning the 1804 earthquake in Dalías (Almería), with maximum intensity VIII-IX and estimated magnitude of 6.4, which caused around 400 deaths and the destruction of most of the buildings in Dalías, Berja and Roquetas. Also noteworthy are several earthquakes of intensity VIII and VII-VIII and estimated magnitudes around 6.0, such as those occurred in Almería in 1487 and 1658, the 1494 earthquake located offshore Málaga, the 1578 earthquake in Melilla and two earthquakes in Adra, one in 1804 which may have generated a small tsunami that affected the Spanish and African coasts, and another one in 1910.
During the instrumental period, most of the seismicity is clustered in the southern part of the basin, around the Al-Idrisi fault (FAL) and north of the Moroccan coast, although many shallow earthquakes have also been registered in the northern Alboran basin, around the Adra and Carboneras faults (FA and FC, respectively). Since 1990, there have been three main seismic episodes. The first one in 1994 with a magnitude of 6 to the northwest of Alhoceima city (Morocco), felt with intensity V in Melilla. The second one in 2004, with a main event of magnitude 6.2 located to the south of Alhoceima (Morocco), where was felt with a maximum intensity of VIII causing numerous damages to buildings and a large number of casualties. And a third episode in 2016 located in the Alboran Sea, near the central-to-south segment of Al-Idrissi fault. This seismic series began with a foreshock of magnitude 5.1, followed by a main shock of magnitude 6.3, both located about 100 km to the NW of Melilla. This event was widely felt in the south of Andalucía, Ceuta and Melilla, where a maximum intensity of VI was observed, and slight damage was caused. This is the maximum magnitude known in this area. Since 2021, thousands small-magnitude (M<3.0) shallow earthquakes, without a clear main shock (seismic swarm), are being recorded near the 2016 series epicentral area but located to the east of Al-Idrissi fault. The largest events recorded between 2021 and 2023 are two earthquakes of magnitude slightly above 5.0.
There is also a significant set of earthquakes occurring at intermediate depths (40-150 km) in the western part of the Alboran basin, whose epicenters are aligned NNE-SSW from Málaga to the coast of Morocco. The existence of this seismicity has given rise to different theories about its origin on which there is still no agreement. Some of the most accepted are the existence of a subduction slab of Atlantic oceanic lithosphere under the Gibraltar Arc with an eastward dip that becomes practically vertical at depth, or the occurrence of different delamination processes under the Alboran Sea.
