Shear-wave velocity models from Rayleigh-wave dispersion in the broader Aegean area

An analysis of Rayleigh waves generated by earthquakes in the broader Aegean is undertaken to estimate variations in the shear-wave velocity structure in the crust and upper mantle down to about 70 km depth. The study centres literally on the vertical-component long-period standard seismograph at Athens (ATH), and is only possible because this station has provided good-quality data over many years of continuous operation. Rayleigh waves from 81 earthquakes during 1963-87, with surface-wave magnitudes in the range 4.2 to 5.4 and epicentral distances of 300-500 km from Athens are analysed. These earthquake epicentres are selected in clusters defining 12 propagation paths across the broader Aegean and radial to Athens. The highest concentration of azimuthal sampling traverses the Hellenic Volcanic Are. The ensuing group-velocity curves over these radial paths are inverted to shear-wave velocity-depth models using linear and then Monte Carlo Hedgehog inversion schemes. The resulting velocity-depth solutions from both schemes, presented as individual path velocity-depth profiles, 3-D azimuthal perspectives and contoured velocity-depth panoramas over the whole broader Aegean, are corroborative. The western are of azimuths extending from Chalkidiki, north of Athens, to SW Crete, south of Athens, have velocity-depth profiles which consistently reach sub-Moho velocities at about 45 km depth. The most striking feature revealed by the contoured shear-wave velocity-depth panoramas is an extensive zone of relatively low velocity, about 3.8 km s(-1), centred around 30 km depth, which may extend from 20 to 40 km depth. This low-velocity zone is contained in the wedge of paths from Carpathos, Rhodes and SW Turkey which traverse the Hellenic Volcanic Are and subtend an angle of about 33 degrees at Athens. There is also slight evidence for anomalously low velocities at greater depths (around 4.1-4.2 km s(-1) at about 50-60 km depth) but these are at the limits of useful resolution of the available data; nevertheless, these indications are at their strongest in the same wedge below the Hellenic Volcanic Arc.