P and S velocity structures of the Santorini–Coloumbo volcanic system (Aegean Sea, Greece) obtained by non-linear inversion of travel times and its tectonic implications

One of the most prominent tectonic features of the Eastern Mediterranean region is the Hellenic volcanic arc in the Southern Aegean Sea, with the Santorini Island being one of the most active volcanic centers. Recent seismic studies show that the main seismic activity of the Santorini volcanic center is strongly associated with the volcanic processes, as well as with the seismo-tectonic regime of the broader Southern Aegean Sea area. The main cluster of local seismicity is located near the northeastern edge of the Santorini Island, beneath the Coloumbo Reef, which is a submarine volcanic seamount of the Santorini Island volcanic system. The P and S wave velocity structures of the Santorini–Coloumbo volcanic system is studied by inverting travel times of local earthquakes recorded by two independent dense seismic arrays installed in the broader area of Santorini islands during the period of September 2002–September 2005. In particular, 137 local earthquakes with 1600 P phases and 1521 S phases recorded by 25 seismological stations have been selected for the inversion. The inversion technique applied is non-linear, since three-dimensional ray tracing is incorporated. The reliability of the final tomographic results is demonstrated through resolutions tests using synthetic seismic data. The obtained results confirm the strong variations of the P and S wave velocity structures in the area of Santorini–Coloumbo volcanic system, as well as the connection between the tectonic setting of the study area with the magmatic processes taking place beneath the two volcanoes. The tomographic models show that a low-velocity zone extends along the northeastern edge of the Santorini Island, parallel to the “Kameni–Coloumbo” fracture zone (NE–SW direction), which corresponds to the western termination of the major ENE–WSW Santorini–Amorgos Fault Zone. Evidence is presented that this structural lineament corresponds to a tecto-volcanic fracture zone, which probably links the volcanic center of Santorini with the submarine volcano at Coloumbo Reef. Furthermore, the tomographic results show indications that the magmatic chamber beneath the Coloumbo volcanic seamount is located at a depth of 6–7 km in good agreement with recent independent studies in the area, though this feature is not within the resolving power of the employed data set.