Objective regionalization of Rayleigh wave dispersion data by clustering algorithms: an application to the Mediterranean basin

The main target of the present study is an objective and automated regional ization of Rayleigh wave dispersion data for the Mediterranean basin, witho ut a priori seismotectonic constraints, and to determine the corresponding regional shear-velocity structures. The database used is formed by almost 2 00 Rayleigh wavetrains corresponding to 42 regional events, with surface-wa ve magnitude greater than 4.5, recorded at the MedNet very-broad-band stati ons in the Mediterranean area. Path-averaged group velocities for the Rayle igh wave fundamental mode are derived for each available epicentre-station trajectory crossing the Mediterranean basin. After this, a principal compon ent analysis and a clustering process are applied to local group velocities , obtained for 13 different periods from 10 to 70 s, in order to classify t he Mediterranean basin into several homogeneous regions. The stochastic inv ersion of the averaged group velocity dispersion curve obtained for each re gion provides the respective shear-velocity structures, down to a depth of 150-160 km. The characteristics of these areas and their possible correlati on with the main seismotectonic features of the Mediterranean region are di scussed. The regional models reveal significant lateral changes in the elas tic structure, with the main differences concerning particularly the upper 35-40 km. Within this depth range, low shear velocities, varying from 2.8 t o 3.9 km s(-1), characterize the Eastern Mediterranean, whereas higher velo cities, ranging from 3.0 to 4.2 km s(-1), are deduced for the Western Medit erranean, These results suggest a thicker rust in the eastern Dart, but wit h a greater thickness of sedimentary layers. However, for depths of between 80 and 110 km, lower shear velocities are obtained in the Western part, wh ile higher shear velocities are derived for the Eastern Mediterranean Sea, in the Aegean Sea, Greece, the south of Italy, Sicily and Tunisia. This vel ocity pattern suggests an averaged thicker lithosphere under the latter are as, as the top of the asthenosphere is detected at a mean depth of 75 km fo r the remaining regions. This thicker lithosphere can be related to process es associated with the convergence of the Eurasian and African plates and s ubduction under the Calabrian and Hellenic Area, (C) 2001 Elsevier Science B.V. All rights reserved.