Crustal shear-wave velocity structure beneath the western Tibetan plateau revealed by receiver function inversions

The collision of Indian and Eurasian plates is the most significant geological event on the Earth since Cenozoic era. How the subduction of the Indian plates occurs under the Tibetan plateau is one of the topics receiving much attention. Whether the boundaries between inner micro-blocks of the Tibetan plateau cut through the crust is another focused issue, which is of great significance for the deformation mechanism of the Tibetan plateau. To help address these issues, we performed a passive-source seismic survey profiling through the western Tibetan plateau. The receiver function method was used to study the crustal shear-wave velocity structure along the survey line. Firstly, we selected the similar receiver functions for each station by discarding the weird waves, then stacked the rest of results to obtain waves of high signal-to-noise ratio. Secondly, the inversion was operated with the Neighborhood algorithm. The Neighborhood algorithm made use of the geometrical constructs known as Voronoi cells to derive the search in the parameter space. It is known that this algorithm produces a self adaptive search behavior. The inversion results of all stations are shown as a 2D image. As we expected, the shear-wave velocity structure along the survey line coincides with the migration of common conversion point (CCP) of the receiver functions. The earthquake of M-s>4. 0 mainly happened beneath the Karakorum fault zone and the Longmu Co fault zone, which are featured by low shear-wave velocity in the upper-middle crust at. both ends of the profile. The subduction of Indian plate under the Tibetan plateau probably reaches the Bangong-Nujiang suture based on the comprehensive analysis in this study and the previous work on the western Tibetan plateau. The Karakorum fault zone, Bangong-Nujiang suture and the Altyn-Tagh fault zone cut through the whole crust leading to the Moho offset. The absence of low shear-wave velocity in the upper-middle crust between the Karakorum fault zone and the Longmu Co fault zone probably indicates the lack of the decoupling layer, which supports the assumption that western Tibet has been experienced crustal shortening and thickening.