Anisotropía sísmica en pizarras de techar del noroeste peninsularmedida directa y cálculo de velocidades a partir de orientación cristalográfica preferente

Abstract

Phyllosilicates are minerals that show a strong anisotropy to the propagation of seismic waves. When they are aligned crys- tallographically in a rock, and given their habit this orientation also applies to grain shapes, then the bulk rock will preserve a great part of the mineral anisotropy, even if they do not constitute a dominant mineral fraction. In addition to shear zones, there are large parts in the interior of orogens where rocks develop a tectonic foliation defined by among other minerals, phyllosilicates. Thus, the intrinsic properties of these rocks may affect the bulk properties of large parts of the continental crust. In this contribution, we study the anisotropy of 9 slate samples from roofing slate quarries in the variscan basement of the northwest of the Iberian peninsula. P and S waves were directly measured on slate cubic samples and velocities were also calculated from crystallographic orientation patterns obtained from all minerals in the same sample blocks. The velocities are high and very similar when measured along the foliation (6,4-6,9 km/s for P waves), but direct measurements with res- pect to calculated are much lower perpendicular to the foliation (3,8-4 km/s for P waves), thereby increasing the anisotropy in the rocks when they get closer to the surface at pressures not allowing the closure of intrinsic microfissures in the rock (associated to grain boundaries).