The 1956 earthquake and tsunami in Amorgos, Greece

We conduct a comprehensive study of the Amorgos, Greece earthquake and tsunami of 1956 July 09, the largest such event in the Aegean Sea in the 20th century. Systematic relocation of the main shock and 34 associated events defines a rupture area measuring 75 × 40 km. The use of the Preliminary Determination of Focal Mechanism algorithm resolves the longstanding controversy about the focal geometry of the event, yielding a normal faulting mechanism along a plane dipping to the southeast, which expresses extensional tectonics in the back arc behind the Hellenic subduction zone. The seismic moment of 3.9 × 10²⁷ dyn cm is the largest measured in the past 100 yr in the Mediterranean Basin. A quantitative database of 68 values of tsunami run-up was built through the systematic interview, over the past 5 yr, of elderly eyewitness residents of 16 Aegean islands and the Turkish coast of Asia Minor. It confirms values of up to 20 m on the southern coast of Amorgos, 10 m on Astypalaia, and up to 14 m on the western coast of Folegandros, 80 km to the west of the epicentre. These values, largely in excess of the inferred seismic slip at the source, and their concentration along isolated segments of fault, are incompatible with the generation of the tsunami by the seismic dislocation, and require an ancillary source, in the form of a series of landslides triggered by the earthquake and/or its main aftershocks, a model confirmed by hydrodynamic simulations using both the dislocation source and models of landslide sources.