A QUANTITATIVE-EVALUATION OF THE CONTRIBUTION OF CRUSTAL ROCKS TO THESHEAR-WAVE SPLITTING OF TELESEISMIC SKS WAVES

The seismic (elastic) properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commo nly measured petrofabrics. This modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic SKS -waves. Mafic rocks such as anorthosite or pyroxene-bearing gabbros ha ve complex S-waves properties, i.e. with small shear-wave anisotropies (< 3%) and without clear relationships between the fast shear-wave po larization plane orientations and the fabric. On the other hand foliat ed felsic rocks such as gneisses or schists and mafic rocks such as am phibole-bearing gabbros are strongly birefringent (anisotropies greate r than 5%) and exhibit clear relationships with the structure. Biotite and amphibole preferred orientations clearly play a major role in she ar-wave splitting in crustal rocks. We show strong correlations betwee n the orientation of the foliation and the amplitude of the delay time . Maximum delay time is systematically observed for waves propagating parallel to the foliation. The fast shear-wave is furthermore polarize d parallel to the foliation plane. We calculate a delay time of about 0.1-0.2 s per 10 km of crustal rocks.