July 9, 1956 Tsunami: Cyclades, Greece

On July 9, 1956 at 03:11:38 GMT a strong shallow earthquake occurred, with epicenter about 20 km south of the islands of Amorgos. This is considered as the strongest shallow earthquake occurred in Greece in this century. 53 people were killed, 100 injured. The main shock was followed 13 later by an aftershock of Ms=7.2 located about 5 km ENE of Santorini (Perissoratis, r468). This earthquake activity was associated with a very large tsunami 25 m heigh, observed in the southern coasts of Amorgos, and which affected all other central and southern Aegean islands and the northern coasts of Crete, where its heigh was 1-2 m (Perissoratis, r468). The tsunami source was located in the deepest patt of the Strait of Amorgos (Soloviev, r643). The severity of the waves of July 9, 1956, was not uniform. It varied greatly from place to place and over rather short distances. Waves up to 15 m in height were observed and many casualties were reported. The area affected by the waves exceeded 40,000 square miles and the distrubance of the sea lasted over 24 hours (Ambraseys, r457). (ref #4139)

The tsunami caused the greatest damage on the Island of Kalymnos because the main settlements of the island were grouped on the shore facing the wave source. The sea in the town of Pothea at first receded, and then rushed onto the coast in the form of a large crest and flooded the town completely. The rise of water was 2.5m, but the pressure of the water current was very strong. The port constructions and houses within the 90 m wide coastal strip suffered significantly. Over 30 fishing boats and one large sailing boat were thrown out onto the dry land and smashed. The sea receded and rose three times during the first hour. Various fragments and objects were carried 1.5 km deep onto the island; one person was lifted up by the wave and carried 1 km onto the dry land, three people were drowned; a large area of topsoil was washed away. (ref #2130)

The 9 July 1956, M = 7.5 Amorgos earthquake triggered a local tsunami with effects on 16 islands across the Aegean Sea and considerable impact on the coastal areas of Cyclades, Dodecanese Islands, Crete, and Asia Minor. More than 80 small ships and row boats were wrecked, sunk, or vanished resulting in 1 drowned and 2 injured people. (ref #13873)

The 1956 Amorgos earthquake involved slip of between 5 and 8 m on the Amorgos Fault, and perhaps also on the Amorgos North Relay fault, with rupture extending from a depth of ∼15 km to the surface. Hydrodynamical calculations demonstrate that the tsunami generated by displacement of the seafloor during the earthquake was responsible for the runups of over 1 m up that were observed up to 100 km from the epicenter, and for runups of 5–20 m in the near field. There is no need to invoke widespread submarine landslides, as had previously been suggested (e.g., Ambraseys, 1960; Galanopoulos, 1957; Okal et al., 2009), although localized landsliding triggered by the earthquake remains a likely explanation for runup greater than 10m on the south coast of Folegandros. (ref #14083)

The Amorgos tsunamigenic earthquake remains one of the largest normal faulting earthquakes recorded globally and generated the largest tsunami in the Mediterranean Sea for the past two centuries. Our data provides the first geological evidence that the Amorgos fault was the host of this event […] This large vertical displacement [6.4-13.4 m] , occurring only 1 km from the Amorgos coastline, suggests that the tsunami could plausibly have been triggered solely by the rupture and sudden vertical displacement of the seafloor. This justifies a revision of the role of mass wastings in generating the highest tsunami waves, in this event, at least close to the epicentral area. While mass-wasting seafloor structures are identified in the vicinity, they are undated, and their link to the 1956 earthquake remains to be established. (ref #14234)