Upper-mantle anisotropy in the southeastern margin of the Tibetan
Plateau revealed by fullwave SKS splitting intensity tomography
Abstract
The southeastern margin of the Tibetan Plateau has experienced complex
deformation since the Cenozoic, resulting in a high level of seismicity
and seismic hazard. Knowledge about the seismic anisotropy provides
important insight into the deformation mechanism and the regional
seismotectonics beneath this tectonically active region. In this study,
we conduct a fullwave multi-scale tomography to investigate the seismic
anisotropy in the southeastern margin of the Tibetan Plateau. Broadband
records from 470 teleseismic events at 111 permanent stations in the
region are used to obtain 5,216 high-quality SKS splitting intensity
measurements, which are then inverted in conjunction with 3D sensitivity
kernels to obtain the anisotropic model for the region with a
multi-scale resolution. Resolution tests show that our dataset recovers
anisotropy anomalies reasonably well on the scale of 1º x 1º
horizontally and ~100 km vertically. Our result suggests
that in the southeastern margin of the Tibetan Plateau the deformations
in the lithosphere and asthenosphere are decoupled. The anisotropy in
the lithosphere varies both laterally and vertically as a result of the
dynamic interactions of neighboring blocks as well as lithospheric
reactivation. The anisotropy in the asthenosphere largely follows the
direction of regional absolute plate motion, i.e. southeastward under
the Songpan-Ganzi Terrane and the Yangtze Craton and nearly east-west
south of 26ºN latitude. The SKS splitting observed at the surface can be
interpreted as the vertical integration of the contributions from
lithosphere and asthenosphere.