Fig. 1: Seismotectonic map in the epicentral area of the July 9th, 1956 earthquake.

Several epicenters for the main shock (star) and twin shock (dot) are represented, as summarized in Brüstle et al.¹³, with two proposed focal mechanism^13,14 calculated for the epicenter marked with a star and an asterisk. Main faults are represented by black lines, with thicker traces for longer and taller faults, and are modified from previous works^19,29,33. S.-A. fault: Santorini-Amorgos fault. Portions of faults explored with the ROVs during the AMORGOS-23 cruise are localized by yellow and white rectangles. Submarine landslides identified²⁵ are in dashed red, while the observation¹⁹ of a probable fault mirror associated with the 1956 event along the Amorgos fault is a red dot. Measured run-ups¹⁴ of the 1956 tsunami are represented as blue bars. In the inset, other tsunami observations of the 1956 event are located by blue¹⁴ and white²⁰ dots. Light purple areas are onshore and offshore volcanoes. The purple arrows on Astypalaea and Santorini show their relative displacements southeastward and southwestward, respectively, at a rate of ~4 mm/yr, with respect to central Aegean (Naxos)³¹. Mount Kroukelos on Amorgos is denoted by a black triangle. The two archeological sites that demonstrate long-term subsidence of the northern coast of Amorgos¹⁸ are located in Katapola (K.) and Aegiali (A.).