Reader Services Curious Finds – Geological and historic evidence of tsunami hazard on Mediterranean coastlines.

This Curious Find comes from Reader Derek Mottershead. He writes:

Ammianus Marcellinus (above) lived in the fourth century AD and was a soldier and chronicler of events in the Roman Empire.  This Curious Find is his description of a major wave on 21 July AD 365 which ‘spread through the entire extent of the world’ specifically naming Alexandria in Egypt and Methoni, in Greece. This event unfolded as follows:-

• the sea withdrew exposing the topography of the sea bed, where boats had become stranded and marine creatures were now floundering about
• people walked out with curiosity over the seabed collecting fish
• the sea then returned with a large wave roaring landwards and drowning people
• it then advanced over the land, carrying ships ashore and destroying buildings
• ships came to rest atop buildings in Alexandria and some were stranded almost two (Roman) miles inland, some 3 kilometres from the coastline.

We would now recognise this sequence of events as diagnostic of a tsunami wave, commonly created by the displacement of a large mass of seawater by seismic or other submarine disturbance.  Such waves ripple outwards from their source, but on approaching shallow coastal water they become warped and the wave increases in height.  On reaching the coastline they then run onshore until they lose their energy. ‘Tsunami’ is a Japanese word which translates as ‘harbour wave’ since they achieve their maximum height at the coastline where they are capable of causing great damage, especially to harbours.

There is increasing evidence of historic damage to ancient historic harbours around the Mediterranean Sea. The branch of science known as geoarchaeology studies the interrelationship between sedimentary deposits consisting of naturally occurring materials and those containing human artefacts emplaced by flowing water, which demonstrate the interplay between human activities and the geological processes of erosion and deposition in historic and prehistoric time. Examples of recent geoarchaeological studies of such ancient harbours around the Mediterranean are those located at Alexandria (Egypt), Caesarea Maritima (Israel), Leptis Magna (Libya), Helike (Greece) and Phalasarna (Crete).

Seismicity in the region has had a range of consequences in respect of both landscape and human activity.  For example, the epicentre of the AD 365 earthquake was to the west of Crete, and one geological consequence was the uplift of western Crete by up to 9 metres, thus stranding the harbour at Phalasarna some 9 metres above sea level and 50 metres inland from the coast, whilst much of the foreshore at Phalasarna now consists of the now uplifted former sea bed. Geologists now think that the AD 365 earthquake with its consequent tsunami was the most far reaching seismic event in the Mediterranean domain within the past 10000 years. Because the wealth of the Romans relied substantially on trans-Mediterranean trade, this event may have had a significant impact on human activity. The destruction in AD 365 of so many harbours, key nodes in maritime trading, is thought by historians to have been a contributory factor to the decline of the Roman Empire.

Ammianus Marcellinus thus provides us with a contemporary account of a historic tsunami in the Mediterranean, providing information of great value to geologists, geoarchaeologists and historians alike.

Item Consulted:  Ammianus Marcellinus, Res Gestae Book XXVI.10, 15-19, in J.C.Rolfe (1937) Ammianus Marcellinus Vol II. Harvard University Press, Cambridge Massachusetts; William Heinemann Ltd.

Available at John Rylands Library Special Collections (R74369 )