Frequency-dependent Pms splitting measurements across the Longmenshan thrust belt in the eastern Tibetan Plateau

Abstract

The mechanism of the crustal deformation at different depths under the eastern Tibetan Plateau is still unresolved. In this study, we conduct shear-wave splitting measurements using two-frequency bands (0.08–0.6 Hz and 0.6–1.0 Hz) Moho-converted Pms receiver-function analyses of teleseismic events recorded at 29 temporary seismic stations densely and linearly deployed across the Longmenshan thrust belt. Our results show that the average delay time between the fast and slow shear waves is ~0.37 s at the high-frequency band, which is smaller than ~0.49 s at the low-frequency band. The fast polarization directions are consistent beneath the Songpan-Ganzi Block between the high- and low-frequency bands, but they are much more heterogeneous at the high-frequency band than those at the low-frequency band beneath the Longmenshan thrust belt and the western Sichuan Basin. The dominated fast polarization direction at the high-frequency band, consistent with the strike of active faults on the surface, possibly reflects the upper crustal deformation, whereas at the low-frequency band it may reflect the lower crustal deformation. All these results suggest that the deformation between the upper and lower crust beneath the Songpan-Ganzi Block could be coupled, but it might be decoupled beneath the Longmenshan thrust belt and the western Sichuan Basin.