Upper‐Mantle Anisotropy and Dynamics Beneath Northeast Asia: Insight From SKS and Local S Splitting Analysis

Abstract

Tectonics in Northeast Asia are characterized as the deep subduction of the Pacific Plate and the induced volcanism in Northeast China. Seismic anisotropy due to deformations is essential for understanding the mantle dynamics. However, the routine SKS splitting captures the accumulated seismic anisotropy from core‐mantle boundary to the surface, so the interpretation of the SKS splitting measurements is nonunique. Here, benefiting from abundant deep‐focus earthquakes in the subducting Pacific Plate, we measured shear wave splitting using both the SKS and local S waves recorded at Station MDJ to discriminate the anisotropy at different depths under Northeast China. We obtained 61 SKS and 22 local S wave splitting parameters. The SKS splitting measurements show apparent azimuthal variations that can be modeled by two‐layer anisotropy. The fast polarization directions (FPDs) of anisotropy in the upper and lower layers are NE‐SW and NW‐SE, which may be explained by the lithospheric deformation accompanying the Mesozoic fault activity and the asthenospheric deformation due to motion of the Eurasian Plate, respectively. The FPDs of the local S wave splitting measurements are parallel to the direction of the Pacific Plate subduction; the variation of the delay times is related to the take‐off angles. The observations can be explained by the inclined symmetry axis of the aligned metastable olivine in the subducting Pacific Plate.

Anisotropic Tomography and Dynamics of the Big Mantle Wedge

“…Previous SWS studies (Figure S8 in the Supporting Information ) show nearly NW‐SE FVDs under the Songliao basin (J. Li & Niu, 2010; Lu et al., 2020), and nearly E‐W FVDs under the North China basin (Y. Yang et al., 2018; T. Zheng et al., 2019; T. Zhu & Ma, 2021). Near the TLF, the FVDs are normal to the strike of the Tanlu fault (K. Liu et al., 2008; Z. Huang et al., 2011), which represent an integration of the NE‐SW anisotropy in the lithosphere and NW‐SE anisotropy in the asthenosphere (Bi et al., 2020). Beneath the North China basin, the FVDs change slightly at depths of 100–300 km (Figures 2c and S8 in the Supporting Information ), which may reflect the contribution of fossil anisotropy in the crust and/or lithosphere.…”

“…Huang et al, 2011),. which represent an integration of the NE-SW anisotropy in the lithosphere and NW-SE anisotropy in the asthenosphere(Bi et al, 2020). Beneath the North China basin, the FVDs change slightly at depths of 100-300 km (Figures2c and S8in the Supporting Information S1), which may reflect the contribution of fossil anisotropy in the crust and/or lithosphere.…”

Distinct Lithospheric Structure in the Xing'an‐Mongolian Orogenic Belt

“…The arc‐shaped LAB and the variations of Moho depth under the Changbaishan range are likely caused by the upwelling asthenosphere beneath the Changbaishan volcano related to the present Pacific subduction. The local NE‐SW FPDs beneath the TLFZ may reflect the lithospheric deformation accompanying the Dunhua‐Mishan fault zone (the eastern branch of the TLFZ) since the Mesozoic (Bi et al., 2020; C. Liu et al., 2018). The crustal FPD from the anisotropic Rayleigh wave tomographic results is also uniformly oriented in NE‐SW around the TLFZ, parallel to its strike trend (Fan et al., 2020).…”

Panoptic View of Mantle Flow Beneath Trans‐Continental Northeast Asia: Distinct Variation Detected From ∼2,000 km Shear Wave Splitting Profile

“…We argue that the upwelling of hot material from the deep mantle resulted in the change of the horizontal flow direction of the asthenosphere (Figure 4b) and thus induced the vertical seismic anisotropy that is undetectable for the steeply propagating SKS waves and would be ultimately featured as weak or nonsplittings (Li et al., 2021). Anisotropy investigation using over 20 years of seismic records showed that in this subduction region, a kind of convection flow associated with the BMW as deep as the MTZ might also contribute to the observed seismic anisotropy (Bi et al., 2020; G. R. Guo et al., 2021).…”

“…Due to the limited back-azimuth distribution, it is hard to investigate the detailed variation of the fast polarization direction with event azimuth, which could provide clues on multi-layered anisotropy (Cherie et al, 2016;Li & Chen, 2006;Makeyeva et al, 1992). Although some complexity beneath a GSN station MDJ (44.617°N, 129.591°E) has been revealed using more than 20 years of waveform data (Bi et al, 2020;Guo et al, 2021), most previous measurements of shear wave splitting in NE China did not find apparent azimuthal variation of the splitting parameters at most of the permanent stations (Gao & Sun, 2021;Li & Niu, 2010;Lu et al, 2019). Therefore, it is reasonable to obtain a salient anisotropy feature beneath this dense and long profile, which could be used to diagnose the systematic variation of the anisotropy pattern in the mantle.…”