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Seismic anisotropy in the upper mantle beneath East Asia has been studied extensively using shear wave (SKS) splitting measurements, which have provided important information on mantle dynamics in this region. However, SKS measurements have poor vertical resolution, and so their interpretations are usually not unique. In this work we use a large number of traveltime data from 34,036 local earthquakes recorded by 1563 seismic stations to determine the first model of 3-D P wave azimuthal anisotropy in the lithosphere beneath China. Our results show that the fast velocity directions (FVDs) are generally correlated with the surface geologic features, such as the strikes of the orogens, active faults, and tectonic boundaries. The FVDs in the upper crust are normal to the maximal horizontal stress (σ H ) in regions with extensive compression such as the Tibetan Plateau, whereas they are subparallel to σ H in strike-slip shear zones such as the western and eastern Himalayan syntax. The comparison of the FVDs of P wave anisotropy with SKS splitting measurements indicates that beneath the Tibetan Plateau the seismic anisotropy in the lithosphere contributes significantly to the SKS splitting observations. In contrast, in east China the P wave FVDs in the lithosphere are different from the SKS splitting measurements, suggesting that the SKS splitting is mainly caused by the anisotropy in the deeper mantle such as the asthenosphere and the mantle transition zone under east China. These novel results provide important new information on the lithospheric deformation and mantle dynamics in East Asia.

The subduction of the Indian Plate beneath SE Tibet and its related volcanism in Tengchong are important geologic processes that accompany the evolution of the Tibetan Plateau. However, it is still not clear whether the subduction and volcanism are confined to the upper mantle or if they extend deep into the mantle transition zone (MTZ). Here, we imaged MTZ structures by using receiver function methods with the waveforms recorded by more than 300 temporary stations in SE Tibet. The results show significant depressions of both the 410-km and 660-km discontinuities and a thickened MTZ (260-280 km) beneath SE Tibet. The depression of the 660-km discontinuity (by 10-30 km) and the thickened MTZ correlate well with high P-wave velocity anomalies in the MTZ, indicating the presence of a subducted Indian slab within the MTZ. Significant depression of the 410-km discontinuity (by 10-20 km) beneath the Tengchong volcano indicates that the volcano originates from the MTZ and is closely related to the subducted Indian slab. Our results confirm the deep subduction of the Indian plate and the deep origin of the Tengchong volcano.However, it remains unknown whether a slab gap exists and contributes to the Tengchong volcano.

We applied the gCAP algorithm to determine 239 focal mechanism solutions 3:0 M W 6:0 ð Þwith records of dense ChinArray stations deployed in Yunnan, and then inverted 686 focal mechanisms (including 447 previous results) for the regional crustal stress field with a damped linear inversion. The results indicate dominantly strike-slip environment in Yunnan as both the maximum (r 1 ) and minimum (r 3 ) principal stress axes are sub-horizontal. We further calculated the horizontal stress orientations (i.e., maximum and minimum horizontal compressive stress axes: S H and S h , respectively) accordingly and found an abrupt change near *26°N. To the north, S H aligns NW-SE to nearly E-W while S h aligns nearly N-S. In contrast, to the south, both S H and S h rotate laterally and show dominantly fan-shaped patterns. The minimum horizontal stress (i.e., maximum strain axis) S h rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in west Yunnan, and further to NE-SW in the South China block in the east. The crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions. Therefore, the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.