中国东北上地幔各向异性研究
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摘要
利用中国数字地震台网记录的地震波形,观测其穿过地球外核和地幔的SKS波分裂特征,确定了快波S波的振动方向和快慢波时间延迟。这种经过地幔传播的SKS震相的横波分裂主要来源于上地幔的各向异性。可以用应变引起的上地幔中矿物质的结晶优势排列来解释。结果表明,对于所分析的10个观测台站均发现了明显的横波分裂现象。时间延迟在0.4～1.8 s。在东部地区,快波振动方向与中国东北西太平洋俯冲带的板块相对运动方向基本一致,说明俯冲的太平洋板块地幔流对各向异性有重要意义。在西部地区,快波振动方向和时间延迟都具有较大差异,说明其总体形变的起因是复杂的,各向异性受到地幔流和历史构造运动的双重影响。
By using the seismic waveforms recorded by the China Digital Seismic Network,the SKS wave splitting is observed,which travels through the outer core and mantle of the Earth,and the fast polarization direction and delay time are determined.The shear-wave splitting of mantle SKS phase is mainly attributed to the upper mantle anisotropy.The observed anisotropy can be explained by crystallographic mineral alignment achieved through dislocation motion.The analysis indicates that there are clear shear-wave splittings beneath all ten seismic stations.The delay time is between 0.4 s and 1.8 s.For the eastern area,the fast polarization direction is consistent with the relative plate motion direction of the Western Pacific subduction zone,showing that the mantle flow within the subducting Pacific plate has a major effect on the observed anisotropy.In contract, for the western area,there are wide variations for both the fast polarization direction and delay time. This observation suggests that the origin of the dislocation motion is complex.The anisotropy is controlled by both the current mantle flow and the tectonic motion in the history.
By using the seismic waveforms recorded by the China Digital Seismic Network,the SKS wave splitting is observed,which travels through the outer core and mantle of the Earth,and the fast polarization direction and delay time are determined.The shear-wave splitting of mantle SKS phase is mainly attributed to the upper mantle anisotropy.The observed anisotropy can be explained by crystallographic mineral alignment achieved through dislocation motion.The analysis indicates that there are clear shear-wave splittings beneath all ten seismic stations.The delay time is between 0.4 s and 1.8 s.For the eastern area,the fast polarization direction is consistent with the relative plate motion direction of the Western Pacific subduction zone,showing that the mantle flow within the subducting Pacific plate has a major effect on the observed anisotropy.In contract, for the western area,there are wide variations for both the fast polarization direction and delay time. This observation suggests that the origin of the dislocation motion is complex.The anisotropy is controlled by both the current mantle flow and the tectonic motion in the history.
引文
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[2]SILVER P G,CHAN W W.Shear wave splitting and subcontinental mantle deformation[J].J Geophys Res,1991,96: 16429 - 16454.
[3]IIDAKA T,NIU F.Mantle and crustal anisotropy in the eastern China region inferred from waveform splitting of SKS and PpSms[J].Earth Planets Space,2001,53:159-168.
[4]罗艳,黄忠贤,彭艳菊,等.中国大陆及邻区SKS波分裂研究[J].地球物理学报,2004,47(5):812-821.
[5]LIU K H,GAO S S,YAO Y,et al.Shear wave splitting and mantle flow associated with the deflected Pacific slab beneath northeast Asia[J].J Geophys Res,2008,113:B01305.
[6]BAI L,KAWAKATSU H,MORITA Y.Two anisotropic layers in central orogenic belt of North China Craton[J].Tectonophysics, 2010,494:138-148.
[7]HESS H H.Seismic anisotropy of the uppermost mantle under oceans[J].Nature,1964,203:629-631.
[8]NATAF H C,NAKANISHI I,ANDERSON D L.Anisotropy and shear-velocity heterogeneities in the upper mantle[J].Geophys Res Lett,1984,11(2):109-112.
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