Objective regionalization of Rayleigh-wave dispersion data by clustering algorithms

Abstract

An objective regionalization of a restricted domain of the Earth's surface in terms of phase-and group-velocity data, without previous tectonic information, is presented. We apply our working scheme to local Rayleigh-wave velocities, which were determined from the study of teleseismic events recorded at the broad-band stations of the network of autonomously recording seismographs (NARS) installed in the Iberian Peninsula during the ILIHA project. According to the phase-and group-velocity maps computed at different wave periods, each point of the domain studied is characterized by two dispersion curves corresponding, respectively, to phase and group velocities. The procedure that we propose consists of the application of principal component analysis (PCA) to those velocities defining the local dispersion characteristics of the medium, and then an average linkage (AL) algorithm. It is worthy of mention that the PCA procedure enhances the relationship between local phase and group velocities. In this way, we obtain a classification of the domain into 10 homogeneous regions. Each homogeneous area is almost coincident, from a seismotectonic point of view, with either Neogene, Alpine or Hercynian domains. After the classification, we are able to deduce by stochastic inversion eight different shear-velocity structures for eight of the 10 homogeneous regions. All the structures deduced suggest a well-developed lowvelocity channel from 80 to 180 km in depth for the whole Iberian Peninsula, but with a clearly distinct evolution of their shear velocities with depth. Some of the models reach shear velocities close to 4.75 km s-' at depths beneath this channel. The highest shear velocities, close to 5.0 km s-l, are obtained at a depth of 50 km, and down to 200 km for region 3 (Neogene), and two possible low-velocity channels, down to a depth of 225 km, appear for models 7 and 8 (Hercynian domains). The existence of low-velocity channels in some models at depths of less than 30 km is discussed in terms of the resolving kernels and the generation of compatible models. In short, the main realistic differences between the models come from the shear velocities modelling each low-velocity channel. As an example, the shear velocities for depths ranging from 80 to 180 km vary, according to the region, from 3.82 km s -l to 4.42 km s-'