The 9 july 1956 (Mw 7.6) amorgos earthquake, Southern Aegean: Seismotectonics of a large tsunamigenic event

On 9 July 1956 a large earthquake occurred near the island of Amorgos, southern Aegean, causing extensive damage while it was followed by a tsunami that inundated the coasts of nearby islands also reaching the Turkish coast. This chapter aims first at providing a thorough review of previous studies about this earthquake and then developing a seismotectonic model for its occurrence by combining all available information. Analysis of historical seismograms of the Amorgos event shows that its focal mechanism is a purely normal fault and that the fault plane exhibits a low-angle (< 30 degrees) dip. This analysis also revealed a relatively deep centroid (45 km) placing the high slip area inside the upper mantle, unlike previous estimates which suggested a shallow focus (~10 km). This relatively large depth seems to be supported by numerical modeling of the tsunami generation and propagation that points to submarine landslides rather than co-seismic rupture of the seafloor as its cause. In addition, the crustal rheology of the area implies that the Amorgos earthquake could have nucleated near the depth of peak strength (33 km) rupturing up to a depth of 7 km without breaking the seafloor. However, these considerations are at odds with geological observations of highangle faulting near the epicentral area of the earthquake. The geological history of the southern Aegean can offer clues so as to resolve this apparent controversy. Amorgos lies at the eastern edge of a metamorphic core complex that started being exhumed 23-19 Ma due to the rollback of the subducting African lithosphere. A direct consequence of this exhumation is the development of large scale detachment faults consisting of upper crust high-angle and lower crust shallow-angle normal faults. The existence of these faults is based on geological observations of exhumed metamorphic rocks and thermo-mechanical numerical models of the exhumation process. Such a detachment fault can explain the geometry and nucleation depth of the Amorgos event thus resolving all the aforementioned discrepancies.