Depth localized shear wave splitting from SKS and P receiver functions: Method and results

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Vinnik, Lev; Aleshin , Igor; Kiselev, Segey; Kosarev, Greigoriy; Makeyeva, Larissa
Holding Date 23 September 2008

Shear wave splitting in the SKS seismic phase provides a unique possibility to judge on deformations at depths inaccessible for direct observations. This technique combines high lateral resolution with a nearly missing resolution for depth, which undermines scientific significance of the results. We describe a technique which provides high resolution for both dimensions, which is achieved by joint inversion of P receiver functions and SKS waveforms.
The technique involves azimuthal filtering of the horizontal components of the receiver functions and provides a criterion to discriminate between effects of azimuthal anisotropy and lateral heterogeneity of the isotropic medium. A search for the optimum models is conducted with a technique similar to simulated annealing. Testing with synthetics demonstrates that this a approach is robust. We present in detail the data for the central Tien Shan. The results for 10 seismograph stations reveal a pronounced change in the patterns of azimuthal anisotropy at a depth around 100 km.
In the mantle lithosphere (at depths less than 100 km), anisotropy is relatively weak, and fast wave polarization direction varies laterally in a broad range. Anisotropy in this layer can be a combined effect of present day thrusting and of deformations of the geologic past. In the lower layer (asthenosphere) the average azimuth of fast wave polarization is close to the trend of the belt, whereas magnitude of S wave anisotropy is stable and large (between 5 and 6%). This anisotropy is a likely result of recent uniaxial shortening at right angle to the trend of the belt. Different patterns of anisotropy in the lithosphere and asthenosphere are found in a few other regions.