Anelastic structure of the Iberian Peninsula obtained from an automated regionalization algorithm and stochastic inversion

An automated regionalization of a reduced domain of the Earth's surface, wi thout a-priori seismotectonic information, and the corresponding anelastic structures are achieved by means of a sequential application of a principal component analysis, a clustering procedure and the stochastic inversion al gorithm to anelastic attenuation coefficients of Rayleigh waves. Our databa se is formed by path-averaged attenuation coefficients derived from fundame ntal mode Rayleigh waves crossing the Iberian Peninsula. The wavetrains wer e recorded at the broad-band stations installed some years ago in the Iberi an Peninsula for the ILIHA project. Before the sequential application of th e algorithms, the area covered by the seismic paths was characterized by 22 maps of local attenuation coefficients corresponding to the 10-120 s perio d range. All these maps were obtained by applying Yanovskaya's formulation for laterally heterogeneous media to the set of path-averaged attenuation c oefficients mentioned above. After that, we divided the Iberian Peninsula i nto six homogeneous regions in terms of the local attenuation coefficients for 22 different periods and by means of the principal component analysis a nd the clustering algorithm. We then obtained, by stochastic inversion, the respective anelastic structure down to a depth of 225 km for the six homog eneous regions. The coefficients vary from 1.0 to 2.0 x 10(-3) km(-1) and t he Q(beta)(-1) structures, ranging from 110 x 10(-3) to 40 x 10(-3), sugges t for five of the regions an asthenosphere with its upper and lower boundar ies close to 80 and 180 km depth, respectively. A shallow beginning of the asthenosphere is detected for the sixth homogeneous region. This range is v ery similar to the depths deduced from previous tomographic studies of the Iberian Peninsula based on Rayleigh wave phase and group velocities. Finall y, correlations obtained between the regions and some seismotectonic charac teristics of the Peninsula are discussed. It is noteworthy that we cannot a ssociate each of the six homogeneous regions with a single Hercynian, Alpin e or Neogene domain. (C) 1999 Elsevier Science B.V. All rights reserved.