Crustal seismic anisotropy in the west margin of the Ordos block : from the Yinchuan graben to the Haiyuan fault zone

Near field earthquake data from June 2010 to August 2017 recorded by seismic networks in the western edge of the Ordos block are collected, from which 137 waveforms of seismic events at 14 stations are chosen for shear-wave splitting analysis, yielding shear-wave splitting parameters, i. e. the polarizations of fast shear-waves (PFS) and the time delays of slow shear-waves. The results show that the PFS in the research area have a distinct zoning feature, largely consistent with the direction of the tectonic stress field or the fault strike. The PFS at stations in the western Ordos block, where are close to the block bordary, exhibits changes in NS, NNE and NE directions from north to south, which accords with the variation of the principal compressive stress in the northeastern Tibetan Plateau. The PFS is different in the eastern and western Yinchuan graben, the former is chiefly influenced by the NNE-directed extrusion from the Tibetan Plateau and the Huanghe-Linwu fault, and the latter is influenced by the principal compressive stress in NW direction between the Alax block and Ordos block and by internal stress distribution of the Alax block. The dominant direction of PFS in the intersection among the Ordos block, Alax block and Tibetan Plateau is in NE direction, consistent with the direction of the principal compressive stress of the northeastern margin of the Tibetan Plateau. The PFS dominant direction along the Haiyuan fault zone and its south is in WNW direction, which accords with the fault strike, suggesting the Haiyuan fault is active. Both tectonics and faults distribution are the main controlling factors to the PFS. The PFS of stations at the strike-slip faults are consistent with trends of these faults, which indicates that these stations are controlled by the faults, and the PFS of several other stations near the faults have the same direction with the tectonic stress, suggesting these stations are almst not affected by faults. The average delay time of slow shear-waves in the convergence zone of the Ordos block, Alax block and Tibetan Plateau are higher than other areas, suggesting that the degree of seismic anisotropy increases within this zone which is blocked by the Ordos block during the movement of the northeastern Tibetan Plateau towards northeast. Bounded by the Haiyuan fault zone to Liupanshan fault zone, the crustal seismic anisotropy on the both sides of the faults shows obvious differences, revealing the difference of stress and medium properties. It implies there may be risk of major earthquakes in adjacent areas, especially from the eastern end of the Haiyuan fault zone to the convergence zone between Liupanshan fault zone and the western Ordos block. In addition, the crust-upper mantle coupling in the research area is also preliminarily discussed in this paper.