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.

Tsunami risk in East Africa

The 26 December 2004 Indian Ocean tsunami was one of the deadliest natural disasters since the start of the 20th century, with an estimated death toll of around 230 thousand. Millions more were deeply traumatised, bereft of homes and possessions, rendered short of food and clean water, and threatened by disease. Together with that launched onto the seaboard of eastern Japan by the Sendai earthquake of 11 March 2011, it has spurred research into detecting the signs of older tsunamis left in coastal sedimentary deposits (see for instance: Doggerland and the Storegga tsunami, December 2020). In normally quiet coastal areas these tsunamites commonly take the form of sand sheets interbedded with terrestrial sediments, such as peaty soils. On shores fully exposed to the ocean the evidence may take the form of jumbles of large boulders that could not have been moved by even the worst storm waves.

Sand sheets attributed to a succession of tsunamis, interbedded with peaty soils deposited in a swamp on Phra Thong Island, Thailand. Note that a sand sheet deposited by the 2004 Indian Ocean tsunami is directly beneath the current swamp surface (Credit: US Geological Survey)

Most of the deaths and damage wrought by the 2004 tsunami were along coasts bordering the Bay of Bengal in Indonesia, Thailand, Myanmar, India and Sri Lanka, and the Nicobar Islands. Tsunami waves were recorded on the coastlines of Somalia, Kenya and Tanzania, but had far lower amplitudes and energy so that fatalities – several hundred – were restricted to coastal Somalia. East Africa was protected to a large extent by the Indian subcontinent taking much of the wave energy released by the magnitude 9.1 to 9.3 earthquake (the third largest recorded) beneath Aceh at the northernmost tip of the Indonesian island of Sumatra. Yet the subduction zone that failed there extends far to the southeast along the Sunda Arc. Earthquakes further along that active island arc might potentially expose parts of East Africa to far higher wave energy, because of less protection by intervening land masses.

This possibility, together with the lack of any estimate of tsunami risk for East Africa, drew a multinational team of geoscientists to the estuary of the Pangani River  in Tanzania (Maselli, V. and 12 others 2020. A 1000-yr-old tsunami in the Indian Ocean points to greater risk for East Africa. Geology, v. 48, p. 808-813; DOI: 10.1130/G47257.1). Archaeologists had previously examined excavations for fish farming ponds and discovered the relics of an ancient coastal village. Digging further pits revealed a tell-tale sheet of sand in a sequence of alluvial sediments and peaty silts and fine sands derived from mangrove swamps. The peats contained archaeological remains – sherds of pottery and even beads. The tsunamite sand sheet occurs within the mangrove facies. It contains pebbles of bedrock that also litter the open shoreline of this part of Tanzania. There are also fossils; mainly a mix of marine molluscs and foraminifera with terrestrial rodents fish, birds and amphibians. But throughout the sheet, scattered at random, are human skeletons and disarticulated bones of male and female adults, and children. Many have broken limb bones, but show no signs of blunt-force trauma or disease pathology. Moreover, there is no sign of ritual burial or weaponry; the corpses had not resulted from massacre or epidemic. The most likely conclusion is that they are victims of an earlier Indian Ocean tsunami. Radiocarbon dating shows that it occurred at some time between the 11th and 13th centuries CE. This tallies with evidence from Thailand, Sumatra, the Andaman and Maldive Islands, India and Sri Lanka for a major tsunami in 950 CE.

Computer modelling of tsunami propagation reveals that the Pangani River lies on a stretch of the Tanzanian coast that is likely to have been sheltered from most Indian Ocean tsunamis by Madagascar and the shallows around the Seychelles Archipelago. Seismic events on the Sunda Arc or the lesser, Makran subduction zone of eastern Iran may not have been capable of generating sufficient energy to raise tsunami waves at the latitudes of the Tanzanian coast much higher than those witnessed there in 2004, unless their arrival coincided with high tide – damage was prevented in 2004 because of low tide levels. However, the topography of the Pangani estuary may well amplify water level by constricting a surge. Such a mechanism can account for variations of destruction during the 2011 Tohoku-Sendai tsunami in NE Japan.

If coastal Tanzania is at high risk of tsunamis, that can only be confirmed by deeper excavation into coastal sediments to check for multiple sand sheets that characterise areas closer to the Sunda Arc. So far, that in the Pangani estuary is the only one recorded in East Africa