3D crustal structure from local earthquake tomography around the Gulf of Arta (Ionian region, NW Greece)

During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northwestern Greece by a dense temporary seismic network. O f the 441 local events observed at 37 stations, 232 well locatable events w ith a total of 2776 P-phase readings were selected applying the criteria of a minimum of 6 P-observations and an azimuthal gap less than 180 degrees. This data set is used to compute a minimum 1D velocity model for the region . Several tests are conducted to estimate model stability and hypocenter un certainties, leading to the conclusion that relative hypocenter location ac curacy is about 500 m in latitude and longitude and 1 km in depth. The mini mum 1D velocity model serves as initial model in the non-linear inversion f or three-dimensional P-velocity crustal structure by iteratively solving th e coupled hypocenter-velocity problem in a least-squares sense. Careful ana lysis of the resolution capability of our data set outlines the well resolv ed features for interpretation. The resulting 3D velocity model shows gener ally higher average crustal velocities in the east, and the well resolved a rea of the eastern Gulf of Arta exhibits a homogeneous velocity around 6 km /s for the whole upper crust, A pronounced north-south trending zone of low velocities in the upper 5-10 km is observed in the area of the Katouna fau lt zone (KFZ), At greater depths (below 10 km) the KFZ is underlain by high -velocity material. E-W profiles suggest a horst-graben structure associate d with the KFZ. (C) 1999 Elsevier Science B.V. All rights reserved.