Geophysical Monitoring: Seismicity And Ground Deformation In The Kos-Nisyros-Yali Volcanic Field

A Vp velocity model of the sediments, crust and upper mantle of the Kos-Rhodos area, southeastern Aegean Sea, was obtained by evaluating wide aperture reflection refraction seismic data, collected by using 4C Ocean Bottom Seismographs (OBS) and stand-alone land stations along 8 seismic lines of 1000 km total length. Seismic energy was generated by air gun shots spaced at 120 m intervals fired by a tuned air gun array of 49 l volume. Velocity modelling by seismic tomography and ray tracing revealed the structure of the crust and sediments. Good Ps, Pg and PmP reflections were recorded at nearly all observed sections, allowing delineating the sediments and crust structure and defining crustal thickness. The sediments are 3-5 km thick. Four sedimentary layers were mapped with velocities ranging from 1.7 to 5.4 km/s. The lateral velocity variations in the sediments and the existence of several faults indicate intense fragmentation and the development of several individual basins, like the western and eastern Kos basins, the Kondelioussa basin, the north Karpathos basin, the south Nisyros basin, the Tilos basin and a number of basins west of Rhodos. The upper crust is approx. 6.5 km thick with Vp velocities ranging from 5.8 to 6.5 km/s. The lower crust is 10-12 km thick with Vp velocities 6.8-7.2 km/s. The crust is stretched continental, with the Moho discontinuity located at 17 km towards the Cretan Sea, 20 km below Kos-Nisyros area, thickening to 22 km under Rhodos and western Turkey. Pn velocities have a value of approx. 7.8 km/s, indicating that below the Dodecanese area a low velocity zone exists in the uppermost part of the mantle, limited in depth by a high velocity layer of approx. 8.1 km/s. A deep sub-Moho reflector at 31 km depth was identified, implying the solid nature of rocks in the upper mantle. Magmatic intrusions were identified in the broader Caldera region.