Passive seismic tomography and 3-D velocity structure of the uppermost crust below Lesvos Island, Greece

The authors study the three dimensional velocity structure for the upper crust beneath Lesvos island in Northeast Aegean sea using a passive source seismic tomography method. The tomographic inversion is applied to 1,700 P- and S- phase observations from 99 local earthquakes recorded on a 10 vertical component local seismic network. Locations of hypocenters were initially determined using HYPO71 program based on a reference one dimensional layered velocity structure. Grid size for the inversion is 6 Km horizontally and 2 km or more vertically. The results from the inversion show a remarkable correlation with surface geology and known geophysical features. They indicate and delineate parts of the tectonic framework of the island where geothermal fluid circulation may occur. The available geological evidence for the areas of Polichnitos and Stipsi in conjunction with P- and S- velocity structure and V{sub p}/V{sub s} values detected in the upper 2 km are indicative of geothermal systems near the water steam transition. There is a strong evidence that the zones delineated by inversions of the seismic velocities represent weak boundaries, separating distinct geologic terrains, along which both geothermal and earthquake activity is observed. The results of this study are considered rather suggestive than conclusivemore » since the inversion is based on vertical data only.« less