Thursday, 21 July 2016

The Cretan Earthquake of AD 365

Earthquakes are not an uncommon occurrence in the eastern Mediterranean. Every now and then the earth trembles, but sometimes the earth trembles a lot.
Historian Ammianus Marcellinus (ca AD 325-391) documented the devastating effects of a tsunami hitting Alexandria, Egypt, on July 21, AD 365. He wrote: "For a little before sunrise there was a terrible earthquake, preceded by incessant and furious lightning. The sea was driven backwards ... [then] the waves ...rose ... beat upon the islands and the extended coasts of the mainland, leveling cities and houses wherever they encountered them. ... vessels of great size were driven on shore by the violence of the wind, and cast upon the house-tops. … and some were even driven two miles inland[1].

Based on geophysical surveys and sediment cores from the Ionian Sea research shows that the 20–25 m thick layer of sediment was triggered, not by the perhaps better known Santorini caldera collapse, but by the 365 AD Cretan earthquake cum tsunami[2].
The magnitude of this quake is estimated at 8.3–8.5, which makes it a quite a large one. What happens is that such an extreme earthquake will not 'only' produce a devastating tsunami, but the earth itself can move in unexpected directions. Western parts of Crete rose as much as 9 meters, leaving harbours high and dry.

Can such an event happen again in our lifetime? The processes that resulted in the AD 365 earthquake are still at work. Plus, they also occur along the rest of the Hellenic subduction zone. The 1303 AD earthquake (also one of about magnitude 8) and tsunami are thought to have originated near Rhodes, so the entire Hellenic subduction zone may represent a tsunami hazard for the eastern Mediterranean. We should expect an 365-like earthquake every ~ 800 years[3]. That there has been only one other such event (in AD 1303) in the past 1,650 years should focus our attention on the modern-day earthquake and tsunami hazards in the Eastern Mediterranean. Remember that 1303 plus 800 makes 2103, which is already perilously close to our own era.

[1] Marcellinus: Res Gestae: 26.10.16–19. See here.
[2] Polonia et al: Mediterranean megaturbidite triggered by the AD 365 Crete earthquake and tsunami in Scientific Reports – 2013
[3] Shaw et al: Eastern Mediterranean tectonics and tsunami hazard inferred from the AD 365 earthquake in Nature – 2008. See here.

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