石岛地震台远震记录反演研究
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摘要
利用石岛地震台的远震体波记录,采用旋转相关函数法和接收函数法分别反演了台站下方介质的各向异性特征和速度结构.(1)对震中距25°~35°且记录良好的5次地震的ScS震相,采用旋转相关函数法反演了岩石圈的剪切波分裂参数.对深源地震的反演结果表明,石岛地震台快波偏振方向为N94°E,这意味着西沙附近处于近东西向微偏南的拉张或地壳下方的地幔流方向为近东西微偏南,西沙地区地壳是过渡性的,其底部的驱动力主要来自与欧亚板块运动一致的物质流.快慢波时间延迟为1.3 s,估算各向异性层厚度为100 km左右.(2)对震中距20°~60°的9次远震P波波形三分向记录,采用接收函数法反演了地壳和上地幔的S波速度结构.反演结果表明,石岛地震台下方地壳分为3层:约5 km以上有一速度梯度带,S波速度从1.5 km/s逐渐增加到3.5 km/s,其间有若干小的分层;在5~16 km的平均速度为3.8 km/s左右,其间有若干小的分层;在16.0~26.5 km的速度为3.6 km/s左右,这是一个明显的低速层;莫霍面埋深为26.5 km,莫霍面以下平均速度为4.7 km/s,也有若干小的分层,尤其是在莫霍面之下有一个明显的低速层.根据转换波到时分析和速度剖面左右摆动现象,认为反演结果中的小分层可能是不真实的,但在16.0~26.5 km的低速层的真实程度还是较高的,表明下地壳具有一定的塑性.
Far-field seismic waves recorded at the Shidao Seismographic Station,located in the South China Sea,are used to inverse earth anisotropy and shear wave velocity vertical distribution under the station respectively.(1)The approach of rotated correction function is applied to five satisfactory ScS wave recordings at distance of 25°～35° to inverse shear wave splitting parameters of lithosphere.The inversion result of the deepest earthquake shows that the polarization of fast shear wave is N94°E and the extensional stress or the moving direction of the mantle mass in the Xisha region is nearly EW.It suggests that the crust in this region is a transitional one and driving force beneath comes from the mass flow whose flowing direction is consistent with the moving of Eurasian plate in the South China Sea.The time delay between fast and slow shear waves is 1.3 s,from which the anisotropy effective thickness is estimated about 100 km.(2)The method of receiver function is used for nine high quality P wave recordings with three components at distance of 20°～60° to inverse vertical distribution of shear wave velocity beneath the station.The result indicates that the crust can be divided into three layers: above 5 km where there is a gradient zone including several small layers,in which shear wave velocity increases from 1.5 to 3.5 km/s gradually;between 5 and 16 km the velocity is about 3.8 km/s in average including several small layers;between 16.0 and 26.5 km the velocity is about 3.6 km/s which is an obvious low velocity layer.The buried depth of Moho discontinuity is 26.5 km and beneath the Moho the velocity is about 4.7 km/s in average,including several small layers,especially an obvious low velocity layer just beneath the Moho.The analysis of the arrival time of transformed waves and the velocity swinging variation around initial model suggest the smaller layers maybe are not reliable,but the velocity layer between 16.0 and 26.5 km maybe is real one implying the plasticity of the lower crust.
Far-field seismic waves recorded at the Shidao Seismographic Station,located in the South China Sea,are used to inverse earth anisotropy and shear wave velocity vertical distribution under the station respectively.(1)The approach of rotated correction function is applied to five satisfactory ScS wave recordings at distance of 25°～35° to inverse shear wave splitting parameters of lithosphere.The inversion result of the deepest earthquake shows that the polarization of fast shear wave is N94°E and the extensional stress or the moving direction of the mantle mass in the Xisha region is nearly EW.It suggests that the crust in this region is a transitional one and driving force beneath comes from the mass flow whose flowing direction is consistent with the moving of Eurasian plate in the South China Sea.The time delay between fast and slow shear waves is 1.3 s,from which the anisotropy effective thickness is estimated about 100 km.(2)The method of receiver function is used for nine high quality P wave recordings with three components at distance of 20°～60° to inverse vertical distribution of shear wave velocity beneath the station.The result indicates that the crust can be divided into three layers: above 5 km where there is a gradient zone including several small layers,in which shear wave velocity increases from 1.5 to 3.5 km/s gradually;between 5 and 16 km the velocity is about 3.8 km/s in average including several small layers;between 16.0 and 26.5 km the velocity is about 3.6 km/s which is an obvious low velocity layer.The buried depth of Moho discontinuity is 26.5 km and beneath the Moho the velocity is about 4.7 km/s in average,including several small layers,especially an obvious low velocity layer just beneath the Moho.The analysis of the arrival time of transformed waves and the velocity swinging variation around initial model suggest the smaller layers maybe are not reliable,but the velocity layer between 16.0 and 26.5 km maybe is real one implying the plasticity of the lower crust.
引文
[1]胥颐,刘建华,傅竹武,等.西沙群岛和哀牢山-红河地区的宽频带地震观测[A].李家彪,高抒.中国边缘海形成演化系列研究:第2卷.中国边缘海岩石层结构与动力过程[M].北京:海洋出版社,2003.216—223.
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[5]YAN Pin,ZHOU Di,LIU Zhao-shu.A crust structure profile across the northern continental margin of the South China Sea[J].Tec-tonophysics,2001,338:1—21.
[6]QIU Xue-lin,YE San-yu,WU Shi-min,et al.Crustal structure across the Xisha Trough,northwestern South China Sea[J].Tectono-physics,2001,341:179—193.
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[14]阮爱国,王椿镛,李清河,等.上地幔各向异性分析中地壳的影响[J].地球物理学报,2004,47(3):441—448.
[15]阮爱国,王椿镛.上地幔各向异性反演方法[J].西北地震学报.2002,24(2):104—112.
[16]BOWMAN J R,ANDO M.Shear-wave splitting in the upper mantle wedge above the Tonga subduction zone[J].Geophys J R AstronSoc,1987,88:25—41.
[17]SILVER P G,CHAN W W.Shear wave splitting and subcontinental mantle deformation[J].J Geophys Res,1991,96:16 429—16 454.
[18]马宗晋,陈鑫连,叶叔华,等.中国大陆现今地壳运动的GPS研究[J].科学通报,2001,46(13):1 118—1 120.
[19]臧绍先,吴忠良,李爱兵.南海、中印半岛及其邻近地区的地震活动及应力状态[J].地震学报,1991,13(2):129—138.
[20]CHRISTENSEN N I.The magnitude,symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonics[J].Geophys J R Astron Soc,1984,76:89—112.
[21]邹最红,陈晓非.利用SV分量接收函数反演地壳横波速度结构[J].地震学报,2003,25(1):15—23.
[22]吴庆举,曾融生.用宽频带远震接收函数研究青藏高原的地壳结构[J].地球物理学报,1998,41(5):669—679.
[23]KENNETT B L N,KERRY N J.Seismic waves in a stratified half space[J].Geophys J R Astron Soc,1979,57:557—583.
[24]KENNETT B L N.Seismic waves in a stratified half space:II.Theoretical seismograms[J].Geophys J R Astron Soc,1980,61:1—10.
[25]SHAW P R,ORCUTT J A.Waveform inversion of seismic refraction data and application to young Pacific crust[J].Geophys J R As-tron Soc,1985,82:375—415.
[2]陈雪,林进峰,许时耕.南沙海槽南缘逆掩推复构造地区的动力学分析[J].海洋学报,2002,24(1):73—85.
[3]陈雪,林进峰.南海中央海盆岩石圈厚度和地壳年代的初步分析[J].海洋学报,1997,19(2):71—84.
[4]刘建华.南海中部地震反射波特征及其地质解释[J].海洋学报,2000,22(6):73—80.
[5]YAN Pin,ZHOU Di,LIU Zhao-shu.A crust structure profile across the northern continental margin of the South China Sea[J].Tec-tonophysics,2001,338:1—21.
[6]QIU Xue-lin,YE San-yu,WU Shi-min,et al.Crustal structure across the Xisha Trough,northwestern South China Sea[J].Tectono-physics,2001,341:179—193.
[7]HESS H H.Seismic anisotropy of the uppermost mantle under ocean[J].Nature,1964,203:629—631.
[8]NICOLAS A,CHRISTENSEN N I.Formation of anisotropy in upper mantle peridotites—A review in composition,structure and dynam-ics of the lithosphere[A].FUCHES K,FROIDEVAUX C.Asthenosphere System:Vol.16[M].Washington D C:Am Geophys Un,1987.111—123.
[9]KEITH C M,CRAMPIN S.Seismic body waves in anisotropic media:synthetic seismograms[J].Geophys J R Astron Soc,1977,49:225—243.
[10]STUART CRAMPIN.A review of the effects of anisotropic layering on the propagation of seismic waves[J].Geophys J R Astron Soc,1977,49:9—27.
[11]LANGSTON C A.The effect of planar dipping structure on source and receiver responses for constant ray parameter[J].Bull SeismolSoc Am,1977,67:1 029—1 050.
[12]BURDICK L J,HELMBERGER D V.Time functions appropriate for deep earthquakes[J].Bull Seismol Soc Am,1974,64:1 419—1 428.
[13]BURDICK L J,LANGSTON C A.Modelling crustal structure through the use of converted phases in teleseismic body waves[J].BullSeismol Soc Am,1977,67:677—691.
[14]阮爱国,王椿镛,李清河,等.上地幔各向异性分析中地壳的影响[J].地球物理学报,2004,47(3):441—448.
[15]阮爱国,王椿镛.上地幔各向异性反演方法[J].西北地震学报.2002,24(2):104—112.
[16]BOWMAN J R,ANDO M.Shear-wave splitting in the upper mantle wedge above the Tonga subduction zone[J].Geophys J R AstronSoc,1987,88:25—41.
[17]SILVER P G,CHAN W W.Shear wave splitting and subcontinental mantle deformation[J].J Geophys Res,1991,96:16 429—16 454.
[18]马宗晋,陈鑫连,叶叔华,等.中国大陆现今地壳运动的GPS研究[J].科学通报,2001,46(13):1 118—1 120.
[19]臧绍先,吴忠良,李爱兵.南海、中印半岛及其邻近地区的地震活动及应力状态[J].地震学报,1991,13(2):129—138.
[20]CHRISTENSEN N I.The magnitude,symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonics[J].Geophys J R Astron Soc,1984,76:89—112.
[21]邹最红,陈晓非.利用SV分量接收函数反演地壳横波速度结构[J].地震学报,2003,25(1):15—23.
[22]吴庆举,曾融生.用宽频带远震接收函数研究青藏高原的地壳结构[J].地球物理学报,1998,41(5):669—679.
[23]KENNETT B L N,KERRY N J.Seismic waves in a stratified half space[J].Geophys J R Astron Soc,1979,57:557—583.
[24]KENNETT B L N.Seismic waves in a stratified half space:II.Theoretical seismograms[J].Geophys J R Astron Soc,1980,61:1—10.
[25]SHAW P R,ORCUTT J A.Waveform inversion of seismic refraction data and application to young Pacific crust[J].Geophys J R As-tron Soc,1985,82:375—415.
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