Seismic anisotropy of upper mantle in eastern China

Abstract

Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks deployed in eastern China, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method, and then the image of upper mantle anisotropy in eastern China was acquired. In the study region, from south to north, the fast-wave polarization directions are basically EW in South China, gradually clockwise rotate to NWW-SEE in North China, then to NW-SE in Northeast China. The delay time falls into the interval [0.41 s, 1.52 s]. Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate. The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding, and then caused the upper mantle flow eastwards and southeastwards. The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction, implying that the crust-upper mantle deformation is possibly a vertically coherent deformation. eastern China, upper mantle anisotropy, SKS wave, fast-wave direction, lithosphere deformationSeismic anisotropy is an effective method for understanding the crust and upper mantle deformation. The anisotropy can be used to understand the intraplate deformation features and the deformation status of lithosphere mantle related to the plate tectonic movements. Recent researches show that the mantle anisotropy is a consequence of strain-induced lattice preferred orientation of olivine crystals [1,2] . There are several causes resulting in the mantle deformation. Among them, the plate motion is a direct cause. The size and direction of the mantle anisotropy strongly depend on the velocity of the plate motion. Based on the assumption of plate driving force, two kinematics models of crust-mantle deformation have been proposed recently, i.e., the vertically coherent deformation [3,4] and the simple asthenospheric flow [5] . The former predicts that the plates strongly have been coupled with the mantle and driven by the flow field generated from density heterogeneity in the mantle. The latter reflects that the plates are self-driven and decoupled from the mantle by mechanically weak asthenosphere, and the mantle deformation is attributed to the difference between the top and the bottom velocities in asthenosphere. In the various subjects of recent studies on geodynamics, the mantle anisotropy is one of the effective mea...

Crust-mantle coupling in North China: Preliminary analysis from seismic anisotropy

“…According to Table 2, the average fast shear-wave polarization of SKS through the upper mantle is obtained at NE 110.17° in this study. This differs from the NE 108.92° reported elsewhere [29,30] by only 1.25°. However, the average fast shearwave polarization of crustal direct shear-wave is NE 95.15°, 15.02° different with that of the fast SKS polarization obtained in this study.…”

“…In this region, SKS splitting results at BJT were obtained first by the Vinnik method [12]. Later, with more data, a study using the SC method obtained parameters of SKS splitting at BJT with results that are fully consistent with the Vinnik method [14] and also consistent with other independent studies [13,29,30]. The SC method has now been widely adopted globally to calculate SKS splitting.…”

“…The yellow lines are average polarizations of fast crustal shear-waves, the red lines are average polarizations of fast SKS phases in this study, the blue lines are average polarizations of fast SKS phases in[29,30].…”

“…There are two main crust-mantle deformation models, Sim- Polarization of fast SKS phases extracted from Chang et al [29,30] 108. ple Asthenospheric Flow (SAF) [48] and Vertical Coherent Deformation (VCD) [49], both of which are based on the hypothesis of driving-plate force. On the basis of SKS splitting information and surface GPS measurements, Wang et al [16] inferred that crust-mantle deformation within the Tibetan Plateau agrees with the strong coupling VCD crustmantle model.…”

“…Furthermore, we obtain average polarizations of fast shear-waves by station averaging in three different data sets (Table 2). In North China, Chang et al [29,30] measured SKS splitting from CASN stations and also from stations of the temporary North China Seismic Array, deployed by the Institute of Geophysics, China Earthquake Administration. Figure 1 displays the results of fast shear wave polarizations of SKS splitting at the 62 stations in this area; average results are listed in Table 2.…”