Seismic anisotropy and mantle flow beneath northeast China inferred from regional seismic networks

Northeast China is situated partly on the Sino-Korean craton and partly on a portion of the Inner Mongolia fold belt. Characterized by widespread Cenozoic intraplate volcanism and deformation caused by multiepisode extension, northeast China is an ideal place to study the underlying physical processes of rifting and active magmatism. We measured SKS wave splitting parameters from several regional networks of broadband stations recently deployed in northeast China. We employed a multievent signal-to-noise ratio (SNR) weighted method and obtained measurements of splitting parameters at 108 stations. These stations cover a variety of tectonic terranes, including the Songliao basin, the Jiamusi massif, the Inner Mongolia belt and the Yanshanian orogenic belt. Overall, the observed SKS splitting times are of low amplitude (similar to 0.8 s), indicating that the underlying mantle experienced relatively weak deformation in this region. The observed anisotropy varies consistently from place to place and exhibits an asymmetric pattern across the Songliao basin. At the southeast edge of the Songliao basin, the fast axis aligns nicely along the NW-SE direction, whereas the axis rotates slightly to NNW toward north beneath the Jiamusi massif. At the west edge of the basin where the north-south gravity lineament (NSGL) is located, we found significant scatter in the fast-axis direction. At the Yanshanian orogenic belt in the southwest corner of the basin, the fast axis aligns along the EW direction. The amplitude of the splitting time appears to be positively correlated with lithospheric thickness, suggesting that lithospheric deformation is the likely cause of the observed seismic anisotropy.