The tsunami of 26 December 2004 shocked the world. It killed over 225 000 people in 11 countries and served as a wake up call on the destructive capabilities of nature. The event also shook Europeans, especially those living in coastal areas, leaving them to wonder whether such an event could happen at home. The answer came in a recent study analysing the plate tectonics of the Mediterranean seafloor: it suggested that Europe may be in store for a tsunami a lot sooner than previously thought.
For people living in the Mediterranean, earthquakes are a common occurrence. In fact the Hellenic arc is considered by many seismologists as the most active seismic region of the transition zone between the African and Eurasian plates. Earthquakes are caused when these tectonic plates bump into or slide past each other.
Thanks to ancient historians there is documented evidence of the last tsunami that struck the region, on 21 July 365 AD. One such historian was Ammianus Marcellus, who wrote about what he saw when the tsunami struck the port city of Alexandria. So devastating and far reaching was the tsunami that it drowned thousands of people and destroyed cities from the Nile Delta in Egypt to Croatia up on the Adriatic Coast.
However, up until now the precise location and tectonic setting of this earthquake had been uncertain. The location of the earthquake is important as it can help seismologists to predict approximately when the next earthquake will occur. Thanks to new evidence based on radiocarbon data and field observations from the day of the tsunami, the location of the epicentre of the earthquake can been located. According to field observations, Western Crete was lifted as high as 10 metres above sea level. The distribution of uplift combined with modern data on seismicity suggests that the earthquake occurred not on the subduction interface beneath Crete, but on a fault dipping at about 30 degrees within the overriding plate.
A subduction is where two of the Earth's plates meet, with one plate riding over a second plate that is gliding downwards at an angle into the planet's mantle. Subduction zones usually have measurable creep of, say, a few centimetres a year. But as the rock becomes brittle and deformed at greater depths, these zones can also deliver titanic quakes, displacing so much land that, when the slippage occurs on the ocean floor, a killer wave is generated.
By measuring the crustal shortening of the plate near Crete, seismologists were able to estimate that a repeat of the tsunami of 365 AD would occur in approximately 5,000 years. However, if the new evidence is correct and the location of the previous earthquake occurred in the Hellenic subduction zone, then the overall recurrence time of strong earthquakes could be closer to 800 years. Up until now the 365 AD earthquake was thought to have been caused by the subduction zone beneath Crete, and a fault in the overriding plate.
The University of Cambridge’s Professor Beth Shaw was able to reconstruct the earthquake uplift from 365 AD and the propagation of the resulting tsunami from radiocarbon data, field observations and model simulations. The last tsunami to hit the eastern Mediterranean occurred on 8 August 1303 and is thought to have originated off the coast of Rhodes. This indicates that the whole Hellenic subduction zone may represent a tsunami hazard for the eastern Mediterranean. And if the analysis is correct, we can expect to see another earthquake on the scale of the one seen in 365 AD a lot sooner than when we previously thought.
The findings of the study are published in Nature Geoscience.