Two subparallel rift zones in the South Aegean Sea hosting the Santorini and Nisyros volcanic centres

The South Aegean Sea hosts two ENE–WSW rift zones with sedimentary basins of 500–700 m depth: the Christiana–Santorini–Kolumbo–Amorgos (CSKA) and the Astypalaea–Nisyros–Kos (ANK) zones. These form discontinuous graben structures with sediment thicknesses of 600–700 m from the Late Pliocene to Quaternary. Both rift zones comprise active volcanic centres: Santorini–Kolumbo in the CSKA and Nisyros in the ANK. Segmented by transverse NW–SE fault lines, these zones produce 25–40 km fault blocks, limiting maximum earthquake sizes per segment. High seismic activity includes the 7.4 magnitude 1956 Amorgos earthquake in the CSKA and several ∼6.5 events near Kos in the ANK (1933, 2017). Volcanic activity is stronger in the southwestern CSKA, with eruptions in Santorini continuing from the Minoan event (∼3.6 ka) to 1950, while Nisyros last erupted in 1878–85. The CSKA rift is also marked by a dyke system along the Santorini caldera and aligned submarine domes extending NE from Kolumbo. Both rifts are slightly oblique to the Aegean volcanic arc, with NW–SE crustal extension confirmed by Global Positioning System, showing 4–5 mm a −1 extension between the eastern Cycladic and southern Dodekanese islands. These rift geometries reflect deep tectonic processes related to magma generation and shallow crustal deformation above the East Mediterranean subducted slab.