Anisotropy gradients from QL surface waves: Evidence for vertically coherent deformation in the Tibet region

- We have identified 50 events with quasi-Love (QL) waves observations in Tibet.
- We infer the locations of the scatterers from the delay times of the QL waves.
- The scatterers mostly lie near the boundary of the actively deforming region.
- Vertically coherent deformation in the Tibet region is suggested.

The India-Eurasia continental collision has not only caused the high uplift of the Tibetan Plateau, but also created a broad diffuse deformation zone in Central Asia. We relate the well-constrained extent of crustal deformation in the Tibet region with underlying mantle deformation by interpreting the quasi-Love (QL) surface wave scattering in the legacy data of Tibet. QL waves are waveform anomalies generated from cross-mode coupling of Earth's free oscillations, mainly through azimuthal anisotropy. Over 50 events with QL observations are identified using recordings in Tibet. The predominant frequency content of the QL waves is near 10 mHz, and reflects peak sensitivity of anisotropy at 150 km depth in the mantle assuming horizontally-oriented symmetry axes. By calculating the delay times between the QL waves and the main Love waves, we back-project the scatterers to cluster in areas like SE Tibet, Sayan Mountain and Iran. Noticeably the spatial distribution of these scatterers borders the crustal deformation pattern quite well, especially at the deformation limits of Central Asia. This linkage suggests a vertically coherent boundary condition through the crust and the upper mantle for the India-Eurasia continental collision in Central Asia.