Progress on the seismic anisotropy parameters knowledge beneath Iberia and Morocco: new results from the second Topoiberia-IberArray deployment

Abstract

In summer 2009 the dense Iberarray broad-band seismic network deployed in the framework of the large-scale TopoIberia project moved to its second footprint. Up to 55 stations covered the central part of the Iberian Peninsula until end 2010, distributed in a regular grid with a nominal spacing of 60 km. Up to 19 additional stations, active since late 2007, have remained operative in the Northern part of Morocco till summer 2010 and then moved southwards, to cover the Atlas belt. Continuous data from permanent broad-band stations have also been gathered to produce a complete database. We focus here in the results constraining the presence of anisotropy as evidenced from the analysis of splitted teleseismic phases. Few anisotropic results in the area covered by this IberArray deployment have been published till now, all of them coming from a scarce number of permanent stations. Beneath Iberia, this second deployment encompasses mainly the Variscan units of the Central Iberian Massif. To the East, the investigated area includes also the southern part of the Celtiberian Chain and reaches the Valencia Gulf, affected by a significant extensional episode in Neogene times. Beneath Morocco, the newly installed stations cover the Atlas belt, and area that seems to be associated with a significant lithospheric thinning, even if its geodynamic features are still poorly constrained. The results would extend the anisotropic map obtained from the first TopoIberia-Iberarray deployment in the Betics-Alboran zone (Díaz et al, 2010). The inferred fast polarization directions (FPD) have clearly documented a spectacular rotation along the Gibraltar arc, following the curvature of the Rif-Betic chain, from roughly N65E beneath the Betics to close to N65W beneath the Rif chain. The stations of that first deployment located in the Iberian Massif tent to present a relatively small amount of anisotropy and suggested complex anisotropy features, probably including two anisotropic layers. The new results seem to confirm these points and show a significant change in both FPD and delay times across the two main units of the Variscan domain, the Ossa-Morena and the Central Iberian zones.Beneath Eastern Iberia, the new data allow to investigate the anisotropic signature of the NE termination of the Alboran Arc System. At the NE Betics, abrupt changes in FPD, which would be symmetric to those evidenced at the Alhoceimas region of the Rif, have not been observed so far. Delays seem to increase from West to East when approaching the Valencia Through. The southernmost stations of the first deployment, located in the northern part of the Atlas belt, suggested that the FPD could be subparallel to the belt. The preliminary anisotropic results from the new deployment in Morocco do not seem to confirm this point. In fact, the anisotropic delays decrease strongly south of the Middle Atlas chain, both beneath the Moroccan Meseta and the High Atlas. Those small values, altogether with the great number of null measurements, make difficult to assess the FPD beneath that zone.