Anisotropy beneath the Iberian Peninsula: The Contribution of the ILIHA-NARS Broad-band Experiment

Abstract

The presence of anisotropy beneath the Iberian Peninsula and its main distinctive features can be established through the analysis of teleseismic shear-wave splitting observed in the ILIHA-NARS experiment. In this experiment, an homogeneous data set is provided by a network of 14 broad-band stations deployed over the entire peninsula for about one year. Even if technical problems led to an amount of data smaller than expected, significant variations in the inferred fast velocity direction are observed for stations located in different Iberian domains. The stations in Central and East Iberia show a fast velocity direction oriented roughly E-W, coincident with previous results in Toledo. A clearly different NE-SW direction is observed in the Ossa-Morena zone, supporting the image from a previous regional experiment. The observed delay times lie between 0.5 and 1 s. Although large-scale mechanisms, such as the absolute plate motion of Eurasia, can be invoked to explain the origin of anisotropic features in many sites, the regional variations observed in some domains imply that differentiated origins of the anisotropy have to be considered, probably related to the particular tectonics in the area. An interesting example of this fact is provided by the stations in the Betic chain; the fast velocity direction inferred for a station located in the limit of the External Betics (South Iberian domain), oriented N80°E, is clearly different from the N15–35°E direction observed in the Internal Betics (Alboran crustal domain), the origin of which has to be related to the Alpine building of the chain.