No abstract

Global stacks of receiver functions clearly exhibit the upper mantle stratification. Besides the most prominent seismic discontinuities, such as the Moho and the 410 and 660 km discontinuities, a negative discontinuity is detected at a depth of~600 km, indicating a low-velocity layer at the base of the mantle transition zone. The slant-slack technique helps to identify the primary conversions from the multiple reverberations. Presence of the negative 600 km discontinuity underneath both continent and ocean island stations, where the crustal thickness significantly differs, also precludes the possible cause of crustal reverberations. We conclude that the negative 600 km discontinuity could be a global feature, possibly resulted from accumulation of ancient subducted oceanic crust. The X-discontinuity at 300 km depth is also observed in our global stacks, which can be explained by the coesite-stishovite phase transformation.

No abstract

No abstract

Detailed seismic structure in the crust beneath the northeastern margin of Tibetan Plateau was revealed by receiver functions of a regional permanent seismic network. At most stations, negative P-to-S converted phases can be detected in the radial receiver functions, prior to the Moho phases, indicating low velocities in the midlower crust. Prominent azimuthal variations in the transverse receiver functions with polarity reversal suggest azimuthal anisotropy in the crust. We used time variations of the P-to-S converted phases at the Moho in the radial receiver functions and the azimuth-weighted stacking of transverse receiver functions to determine the fast direction and magnitude of anisotropy. The low-velocity midlower crust with the coherent azimuthal anisotropy in the northeastern margin of Tibetan Plateau is consistent with the lower crustal channel flow model.