Sun, sand and sangria on the Mediterranean Costas – and tsunamis?

You can easily spot a tourist returning from a few summer weeks on the coast of the western Mediterranean, especially during 2022’s record-breaking heat wave and wildfires: sunburnt and with a smoky aroma that expensive après-sun lotion can’t mask. Judging from the seismic records, they may have felt the odd minor earthquake too, perhaps putting it down to drink, lack of sleep and an overdose of trance music. Data from the last 100 years show that southern Spain and north-west Africa have a generally uniform distribution of seismic events, mostly less than Magnitude 5. Yet there is a distinct submarine zone running NNE to SSW from Almeria to the coast of western Algeria. It crosses the Alboran Basin, and reveals significantly more events greater than M 5. Most earthquakes in the region occurred at depths less than 30 km mainly in the crust. Five geophysicists from Spain and another two from Algeria and Italy have analysed the known seismicity of the region in the light of its tectonics and lithospheric structure (Gómez de la Peña, L., et al. 2022. Evidence for a developing plate boundary in the western Mediterranean. Nature Communications, v. 13, article 4786; DOI: 10.1038/s41467-022-31895-z).

Topography of the Alboran Basin beneath the western Mediterranean. The colours grey through blue to purple indicate increasing depth of seawater. Grey circles indicate historic earthquakes, the smallest being M 3 to 4, the largest greater than M 6. Green arrows show plate motions in the area measured using GPS. Active faults are marked in red (see key for types of motion). (Credit: based on Fig 1 of Gómez de la Peña et al.)

The West Alboran Basin is underlain by thinner continental crust (orange on the inset to the map) than beneath southern Spain and western Algeria. Normal crust underpins the Southern Alboran Basin. To the east are the deeper East Alboran and Algero-Balearic Basins, the floor of the latter being true oceanic crust and that of the former created in a now extinct island arc. Running ENE to WSW across the Alboran Basin are two ridges on the sea floor. Tectonic motions determined using the Global Positioning System reveal that the African plate is moving slowly westwards at up to 1 cm yr-1, about 2 to 3 times faster than the European plate. This reflected by the dextral strike-slip along the active ~E-W Yusuf Fault (YSF). This bends southwards to roughly parallel the Alboran Ridge, and becomes a large thrust fault that shows up on ship borne seismic reflection sections. The reflection seismic survey also shows that the shallow crust beneath the Alboran Ridge is being buckled under compression above the thrust. The thrust extends to the base of the African continental crust, which is beginning to override the arc crust of the East Alboran basin. Effectively, this system of major faults seems to have become a plate boundary between Africa and Europe in the last 5 million years and has taken up about 25 km of convergence between the two plates. An estimated 16 km of this has taken place across the Alboran Ridge Thrust which has detached the overriding African crust from the mantle beneath.

The authors estimate an 8.5 to 10 km depth beneath the Alboran fault system at which the overriding crust changes from ductile to brittle deformation – the threshold for strains being taken up by earthquakes. By comparison with other areas of seismic activity, they reckon that there is a distinct chance of much larger earthquakes (up to M 8) in the geologically near future. A great earthquake in this region, where the Mediterranean narrows towards the Strait of Gibraltar, may generate a devastating tsunami. An extension of the Africa-Europe plate boundary into the Atlantic is believed to have generated a major earthquake that launched a tsunami to destroy Lisbon and batter the Atlantic coasts of Portugal, Spain and NW Africa on 1st November 1755. The situation of the active plate boundary in the Alboran Basin may well present a similar, if not worse, risk of devastation.

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