Crustal structure and possible anisotropy in Turkey from seismic surface wave dispersion

Abstract

SUMMARY The velocity structure of two regions of Turkey are determined using single-station measurements of Rayleigh and Love wave group velocities in the period range 8-50s. A differential inversion scheme yields models for Turkey in which crustal and upper mantle shear-wave velocities are slower than those of most of Europe. Comparisons of upper mantle shear-wave velocities we have obtained with reported P,, velocities leads to Poisson's ratio values in the upper mantle between 0.29 and 0.30 for eastern Turkey and between 0.27 and 0.31 for western Turkey. Crustal velocities are slightly slower and upper mantle velocities are slightly faster in western Turkey than in eastern Turkey. The crust-mantle boundary obtained in our studies is gradational, but if a shear velocity of 4.2 km s-l is taken to define the upper mantle then the crust appears to be about 40 km thick throughout all of Turkey. A sharp crust-mantle boundary may occur, but cannot be resolved. The data of this study require neither a low-velocity zone in the upper mantle nor polarization anisotropy in the crust or upper mantle. Azimuthal variations of Rayleigh and Love wave group velocities in western Turkey are consistent with velocities predicted by an azimuthally anisotropic upper crust in which vertical cracks are orientated in an approximate E-W direction. This interpretation is consistent with geological information, fault-plane solutions, lineations mapped from satellite observations, and reported heat flow values, but the possibility that these variations are caused by lateral changes of velocity in the crust of western Turkey cannot be completely ruled out at the present time.