A new study published in AGU Advances highlights and reassesses the Crete earthquake, considered the strongest recorded earthquake in the Mediterranean, which occurred in 365 A.D.
Using new radiocarbon dates and historical records, researchers have shown how much the shorelines on the Greek island were raised during the earthquake.
The findings point to the importance of considering the sources of fault earthquakes in regions where tectonic plates collide, and guide future research to determine the extent of the hazard in more detail.
On July 21, 365 A.D., a huge earthquake struck the Mediterranean Sea region, so strong that it generated a tsunami that spread throughout the eastern Mediterranean basin. As a result, cities were destroyed and many people died. In Alexandria, the tsunami was so strong that the event has been celebrated for centuries as a “day of terror.
The Crete earthquake of 365 is generally believed to be the largest recorded earthquake in the Mediterranean, powerful enough to lift part of the island of Crete by several meters. This earthquake left behind fossil coastlines, which researchers have studied to re-evaluate the event.
Richard Ott, a scientist at the German Geosciences Research Center GFZ and lead author of the study, worked with his colleagues to refine the dating of the fossil shores and compared information from many samples with ancient earthquake records made by historical authors.
Radiocarbon analysis of fossils beneath the shoreline helped determine when the upheaval occurred. The team collected fossil samples of vermettites and corals from the Krios paleoport at eight different locations. The shells and skeletons of these dead organisms indicate when the event occurred because they died when the earthquake lifted them above sea level.
The results showed that the fossil coastline was most likely not raised by a single large earthquake, but by a series of powerful earthquakes around the island in the first centuries AD. This result is consistent with archaeological findings that the ancient harbor at Falasarna had already been abandoned after an earthquake in 66 AD.
Compared to previous studies, the results showed that normal faults may be behind this sequence of earthquakes.
Normal faults are a special type of fracture in the Earth’s crust that occurs when the crust slides apart. The researchers modeled tsunami propagation from earthquakes on these faults and found that their new type of model could explain well the tsunami report from Alexandria by historical writer Ammianus Marcellinus.
The results also suggest that earthquakes on such normal faults may be common and cause tsunamis in the Mediterranean. The Crete earthquake is rated below M8.0, which is lower than previous estimates of M8.3 to 8.5.
Nevertheless, the researchers noted that there is a risk of large earthquakes in the eastern Mediterranean, stressing the importance of educating locals on how to act in the event of such a disaster.
“Based on these results and a better fit with long-term data on crustal expansion in the region, we lean in favor of a normal fault origin for the 365 AD and earlier earthquakes,” the research team wrote.
“However, we note that more research, and especially geophysical surveying, is needed to adequately understand the tectonics and seismic hazard of the Greek subduction zone.”
Synopsis
At dawn on July 21, 365 A.D., a powerful earthquake measuring more than 8.5 on the Richter scale struck off the west coast of Crete. Not only did it cause considerable destruction, but it also generated a tsunami that devastated the coastal cities of the eastern Mediterranean.
Tsunami wave: wave length – 100 km, angle of incidence – 30°, wave height – 20 m, the height of the wave crest above the sea level on Crete – 10 m
The Roman historian Ammianus Marcellinus left the first reliable description of the tsunami in history:
“… Shortly after daybreak there was a great crash of thunder, followed one after another, and then the whole land began to tremble, the sea began to agitate, it rolled back from the shore, so that the opening depths were seen mired in the mud of the sea animals; then the huge valleys and mountains, eternally concealed in the depths of the sea, probably for the first time, saw the sunbeams. Many ships found themselves as if on dry ground, and people wandered freely through the remaining puddles, picking up fish and other sea animals with their hands. At the same time the waves of the sea, as if in anger at their removal, rose again and spilled with terrible force through shallow places across the islands, flooding large areas of the continent and flattening many buildings in cities and other places.
The whole face of the earth was covered by the unleashed struggle of the elements, and miraculous phenomena were observed everywhere. A terrible whirlpool caused the water mass to return to the shores, and when the excitement calmed down, it was found that many ships had perished, and the bodies of the shipwrecked men were lying on their backs and down on the ground. Huge ships were lifted by the surge of water and found themselves on top of buildings, as happened in Alexandria; some were abandoned two miles from the shore – such was the case near the town of Motona in Laconica, where I myself saw when I passed in those places a rotten and collapsed ship…”
