The Santorini-Amorgos zone is located in the Cyclades islands group which is in the center of the Hellenic volcanic arc. This zone is affected by the extension in the Aegean domain as the result of slab rollback combined with gravitational spreading. Due to volcanic activity and extension, the Santorini-Amorgos zone has high seismic activity that has produced two large earthquakes which were followed by a tsunami. These earthquakes occurred in the southeastern part of the island of Amorgos on 9 July 1956 and had 7.4 and 7.2 surface wave magnitude. We utilized modern seismological methods to understand better the seismic activity in the Santorini-Amorgos zone. We used data from temporary (CYCNET and EGELADOS) and permanent (HUSN) seismic networks. The data period of CYCNET is from September 2002 to July 2004, and EGELADOS is from October 2005 to March 2007. The data period of HUSN is from January 2011 to December 2018. We found 1868 earthquakes which occurred in the Santorini-Amorgos zone. The absolute location of these earthquakes was obtained by using NonLinLoc which implements a non-linear approach of earthquake location calculation. The one-dimensional crustal velocity model of the southeast Aegean produced better absolute locations for these earthquakes than the model of the Santorini-Kolumbo volcanic zone. We relocated 83.4% of the earthquakes (1558 earthquakes) using hypoDD which implements the double-difference algorithm. The horizontal error of relative location is 0.03-0.05 km, and the vertical error is 0.05 km. The relocated earthquakes concentrated in Kolumbo volcanic reef and the Santorini-Anafi fault zone, including the Anydros basin. The 489 relocated earthquakes occurred in Kolumbo volcanic reef, and most of them concentrated at depths 3-15 km beneath Kolumbo submarine volcano. The relocated earthquakes in the Santorini-Anafi fault zone occurred in a southwest-northeast direction along the horst of Santorini-Anafi fault. We calculated the moment tensor solutions of the earthquakes which had local magnitude above 3.5 based on the National Observatory of Athens earthquake catalog using the ISOLA package. Most of the moment tensor solutions showed oblique normal faulting mechanism striking in the southwest-northeast direction. One of them had a significant compensated linear vector dipole (CLVD) component probably caused by rupture complexity in this zone. The moment tensor solution of an earthquake that occurred beneath Kolumbo volcano had a significant double-couple (DC) component representing a normal fault that probably links with the Kameni-Kolumbo zone.
