A new seismic tomography of Aigion area (Gulf of Corinth, Greece) from the 1991 data set.

A new three-dimensional delay traveltime tomography is performed to image the intermediate structure of the western Gulf of Corinth. A large data set, collected in 1991 during a two-month passive tomographic experiment, has been reanalysed for the reconstruction of detailedVpandVsimages. An improved tomography method, based on an accurate traveltime computation, is applied to invert simultaneously delayedPandSfirst-arrival traveltimes for both velocity and hypocentre parameters. We perform different synthetic tests to analyse the sensitivity of tomography results to the model parametrization and to the starting 1-D model selection. The analysis of the retrievedVpandVsmodels as well as deducedVp/ VsandVp· Vsimages allows us to interpret and delineate the distribution of lithological variation, porosity/crack content and fluid saturation in the upper 9–11 km of the crust beneath the gulf. The tomographic models image a rather complex crustal structure, which is characterized by a vertical change in both velocity features and seismicity distribution. We identify a shallower zone of the crust (0–5 km depth), in which velocity distributions seem to be controlled by the still active N–S extensional regime and a deeper zone (7–11 km depth), which matches the seismogenic zone. The correlation between this latter and a specific unit of the Hellenic mountain structure (the Pyllite–Quartzite series) allows us to suggest a possible explanation for seismicity concentration in a narrow band at 7–9 km depth. Finally, the occurrence of clusters showing low-angle normal fault mechanisms in areas characterized by highVp/ Vsvalues indicates a possible role of fluids in triggering brittle creep along the identified low-angle normal faults. [ABSTRACT FROM AUTHOR]