International audienc

[1] A total of 361 SKS and five local S wave splitting measurements obtained at global and regional seismic network stations in NE China and Mongolia are used to infer the characteristics of mantle fabrics beneath northeast Asia. Fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features. The dominant fast directions at the eastern part, beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone (MTZ), are about 100°from north, which are almost exactly the same as the motion direction of the Eurasian plate relative to the Pacific plate, and are independent of the direction of local geological features. The splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3% anisotropy. The shear wave splitting observations, complemented by the well-established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the Paleozoic era, can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected Pacific slab in the MTZ, or by back-arc asthenospheric flow in the mantle wedge above the slab.

Seismic anisotropy is obtained in the crust in northwestern capital area by shear‐wave splitting analysis, using the SAM technique. The seismic data was recorded at the Capital Area Seismic Network from Jan. 2002 to Dec. 2003. The results at 14 stations in all, every of which has at least 3 records available or more, are statistically discussed in this paper. The statistical results show that the average polarization of fast shearwaves is NE69.9° ±44.5° and the time delay of slow shear‐waves is 4.44±2.93 (ms/km). The average polarization of fast shear‐waves of NE69.9° ±44.5° suggests the direction of maximum horizontal principal compressive stress in this area. The most predominant polarization direction of fast shear‐wave suggests the tectonic implication of horizontal principal compressive stress at the direction NWW or nearly E‐to‐W, which exposes the Zhangjiakou‐Penglai depression fault zones with strike NWW. According to the polarization of fast shear‐wave, this study verifies that the predominant polarizations of fast shear‐wave at stations on active faults are consistent with fault strike. Possibly, both the Nankou‐Sunhe fault and Xiadian fault are two active faults while the Babaoshan fault is possibly a less active fault. The polarizations of fast shear‐wave in the North China Basin show the complexity, consistent with the complicated pattern of regional principal compressive stress controlled locally, induced by many faults crossing in the depression zone within the basin. This study also suggests that the quick change of time delays of slow shear‐waves is possibly related to the temperature change in deep crust.

Crustal anisotropy beneath 71 broadband seismic stations situated at the eastern Tibetan Plateau and the Sichuan Basin is investigated based on the sinusoidal moveout of P-to-S conversions from the Moho and an intracrustal discontinuity. Significant crustal anisotropy is pervasively detected beneath the study area with an average splitting time of 0.39±0.18 s. The resulting fast orientations are mostly parallel to the major shear zones in the Songpan-Ganzi Terrane, and can be explained by fluid-filled fractures, favoring the model of rigid block motion with deformations concentrated on the block boundaries. In the vicinity of the Xianshuihe-Xiaojiang Fault Zone in the southern Songpan-Ganzi Terrane, our results, when combined with previously revealed high crustal Poisson's ratio in the area, support the existence of mid/lower crustal flow. The Longmenshan Fault Zone and adjacent areas are dominated by strike-orthogonal fast orientations, which are consistent with alignments of cracks associated with compressional stress between the Plateau and the Sichuan Basin. The observations suggest that