July 21, 365 C.E.

The Hellenic arc.

Greece is one of the world’s most seismically active countries. The ancient Greeks attributed earthquakes to the God of the Sea, Poseidon, perhaps because so many of them were centered under the waters. On today’s date at about sunrise, the “Cretan earthquake of 365”, an undersea earthquake with an assumed epicenter near Crete and estimated to have been higher than 8.0 on the present-day Richter Scale, occurred. It caused widespread destruction in central and southern Greece, northern Libya, Egypt, Cyprus, and Sicily. In Crete, nearly all towns were destroyed.

Of course, today we know that earthquakes are due to the movement of huge tectonic plates on the Earth’s surface. The Africa plate subducts beneath the Aegean Sea plate along the Hellenic arc (aka Hellenic trench or subduction zone), from the western Peloponnesus through Crete and Rhodes to western Turkey, at a rate of almost 40 mm/year. As a result, shallow-focus earthquakes (focal depths less than 50 km) occur on faults in the boundary-region of the two plates. In the twentieth century, the largest shallow-focus earthquakes to have occurred near the Hellenic-arc plate boundary had magnitudes of about 7.2, but the earthquake centered near Crete in 365 CE was much larger than any Hellenic arc earthquake of the twentieth century. In other parts of the world, convergent-plate tectonic environments similar to that of the Hellenic arc have produced earthquakes of magnitude 8 and larger.

The Crete earthquake was followed by a tsunami which devastated the southern and eastern coasts of the Mediterranean, particularly Libya, Alexandria, and the Nile Delta, killing thousands and hurling ships nearly two miles inland. The quake left a deep impression on the late antique mind, and numerous writers of the time referred in their works to the event.

The Roman historian Ammianus Marcellinus described in detail the tsunami hitting Alexandria and other places in the early hours of 21 July AD 365. His account is particularly noteworthy for clearly distinguishing the three main phases of a tsunami, namely an initial earthquake, the sudden retreat of the sea and an ensuing gigantic wave rolling inland:

Slightly after daybreak, and heralded by a thick succession of fiercely shaken thunderbolts, the solidity of the whole earth was made to shake and shudder, and the sea was driven away, its waves were rolled back, and it disappeared, so that the abyss of the depths was uncovered and many-shaped varieties of sea-creatures were seen stuck in the slime; the great wastes of those valleys and mountains, which the very creation had dismissed beneath the vast whirlpools, at that moment, as it was given to be believed, looked up at the sun’s rays. Many ships, then, were stranded as if on dry land, and people wandered at will about the paltry remains of the waters to collect fish and the like in their hands; then the roaring sea as if insulted by its repulse rises back in turn, and through the teeming shoals dashed itself violently on islands and extensive tracts of the mainland, and flattened innumerable buildings in towns or wherever they were found. Thus in the raging conflict of the elements, the face of the earth was changed to reveal wondrous sights. For the mass of waters returning when least expected killed many thousands by drowning, and with the tides whipped up to a height as they rushed back, some ships, after the anger of the watery element had grown old, were seen to have sunk, and the bodies of people killed in shipwrecks lay there, faces up or down. Other huge ships, thrust out by the mad blasts, perched on the roofs of houses, as happened at Alexandria, and others were hurled nearly two miles from the shore, like the Laconian vessel near the town of Methone which I saw when I passed by, yawning apart from long decay.

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