In the Mediterranean Sea, the probability that a large earthquake-triggered tsunami will occur in the coming decades is high. Historical tsunami database informs us on their geographical occurrence but their sources, i.e., the faults that slipped during earthquakes and displaced the seafloor to generate tsunamis, are often unknown. Here we identify the submarine rupture of the Amorgos earthquake that on July 9, 1956, triggered the largest mediterranean tsunami in the past two centuries. Using submarines, we explored major normal faults in the epicentral area, and discovered a large surface rupture along the 75-km long Amorgos fault. The 9.8-16.8-m large seafloor offset is compatible with a Mw7.5 event. This finding prompts a reassessment of the largest (≥20 m) tsunami wave origin, previously attributed to earthquake-triggered submarine mass-wasting. It demonstrates that tsunami source can be determined several decades after an event, a key information to better assess future seismic and tsunami hazards.

Surface ruptures generated by large continental earthquakes, whether historical 1,2 or recent 3 , are today systematically mapped through fieldwork, satellite data, and high-resolution topographic analyses. They provide key information that allows the seismic hazard of a region to be evaluated, in particular through the identification of the causative fault, the extent of the rupture, and the amount of slip generated by the earthquake. Such work was performed in only a few weeks following the 2023 Turkey-Syria earthquake 4 for example. When the earthquake occurs offshore, similar analyses are challenging to perform, but they are of prime importance to understand the triggering of tsunamis and evaluate the seismic hazard along submarine faults 5 . The recent deployment of submarine vehicles (such as Remotely Operated Vehicles ROVs), however, offers new opportunities to image undersea fault scarps 6-9 , identify fault planes that have been recently exhumed by slip during an earthquake 10 , and map and quantify the related surface rupture 11 . With such a vehicle, we investigated the faults around the proposed epicenter of the 1956 Amorgos earthquake to identify seafloor ruptures and determine the fault responsible for this event.