应用远震有限频率层析成像反演首都圈上地幔速度结构
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摘要
本文选用首都圈数字地震台网2003年9月～2005年12月记录的300多个远震事件的波形资料,采用分频带多道互相关方法得到三个不同频段的P波相对走时数据共18499个,计算了每个频段的走时灵敏度核,应用有限频率层析成像反演得到首都圈地区的上地幔三维P波速度结构模型.利用检测板估计了反演结果的分辨率,并与射线层析成像方法的结果进行了比较,说明了反演结果的可靠性.研究结果表明,各构造单元具有明显不同的速度结构特征,其差异可到150 km深:燕山隆起区表现高速;太行山隆起区整体以低速为主并存在小范围高速块体;华北盆地、渤海湾下浅层上地幔中存在大范围的强低速异常,其顶面在50～70 km,可视为软流圈顶面的埋深,这一结果说明华北盆地、渤海湾下岩石圈明显减薄;张家口蓬莱断裂带是上地幔浅部速度结构的变异带,也是岩石圈减薄的边界带,区内大部分强震都发生在该构造带上,由此看来该带上强震的发生不仅与地壳结构的不均匀性有关,还可能有较深的构造背景.
We selected waveform data from more than 300 teleseismic events recorded by the digital Capital Seismic Network during the period from September 2003 to December 2005, obtained 18499 P-wave relative travel-times by dividing these waveform data into three different frequency bands and multi-channel cross correlation measurement,calculated travel-time sensitivity kernels for each frequency band,applied the finite-frequency tomography to determine 3-D P-wave velocity structure model of the upper mantle under the Chinese capital region.We also analyzed the reliability of inversion results by using checkerboard resolution test and comparing present model with ray tomographic model.Our results show that there are distinct differences of deep velocity structure down to 150 km depth under three tectonic units.The Yanshan uplift exhibits the high velocity (high-V) features.Under the Taihangshan uplift,a broad low velocity (low-V) is visible,but there are also small high-V anomalies.A large scale prominent low-V anomaly is revealed in the shallow upper mantle under the North China basin and Bohai bay,and it extends up to the depth of 50～70 km which can be regarded as the top of asthenosphere.This result suggests lithospheric thinning in the North China basin and Bohai bay.Zhangjiakou-Penglai fault zone is a variation belt of velocity structure in the shallow upper mantle and a boundary of lithospheric thinning,most of large earthquakes has occurred in this belt.We consider that large earthquakes occurred in this belt are not only related to the crustal heterogeneity but also affected by the deeper velocity structure.
We selected waveform data from more than 300 teleseismic events recorded by the digital Capital Seismic Network during the period from September 2003 to December 2005, obtained 18499 P-wave relative travel-times by dividing these waveform data into three different frequency bands and multi-channel cross correlation measurement,calculated travel-time sensitivity kernels for each frequency band,applied the finite-frequency tomography to determine 3-D P-wave velocity structure model of the upper mantle under the Chinese capital region.We also analyzed the reliability of inversion results by using checkerboard resolution test and comparing present model with ray tomographic model.Our results show that there are distinct differences of deep velocity structure down to 150 km depth under three tectonic units.The Yanshan uplift exhibits the high velocity (high-V) features.Under the Taihangshan uplift,a broad low velocity (low-V) is visible,but there are also small high-V anomalies.A large scale prominent low-V anomaly is revealed in the shallow upper mantle under the North China basin and Bohai bay,and it extends up to the depth of 50～70 km which can be regarded as the top of asthenosphere.This result suggests lithospheric thinning in the North China basin and Bohai bay.Zhangjiakou-Penglai fault zone is a variation belt of velocity structure in the shallow upper mantle and a boundary of lithospheric thinning,most of large earthquakes has occurred in this belt.We consider that large earthquakes occurred in this belt are not only related to the crustal heterogeneity but also affected by the deeper velocity structure.
引文
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[15] Yang T,Grand S P,Wilson D,et al.Seismic structure beneath the Rivera subduction zone from finite-frequency seismic tomography.J.Geophys.Res.,2009,114,B01302,doi:10． 1029/2008JB005830
[16] Montelli R,Nolet G,Dahlen F A,et al.Finite-frequency tomography reveals a variety of plumes in the mantle.Science,2004,303:338～343
[17] Montelli R,Nolet G,Masters G,et al.Global P and PP traveltime tomography:rays versus waves.Geophys.J.Int.,2004,158:637～654
[18] Ren Y,Shen Y.Finite frequency tomography in southeastern Tibet:Evidence for the causal relationship between mantle lithosphere delamination and the north-south trending rifts.J.Geophys.Res.,2008,113,B10316,doi:10． 1029/2008JB005615
[19] Yang T,Shen Y,van der Lee S,et al.Upper mantle structure beneath the Azores hotspot from finite-frequency seismic tomography.Earth Planet.Sci.Lett.,2006,250:11～26
[20] Menke W.Geophysical Data Analysis:Discrete Inverse Theory.Academic Press,San Diego,Calif.,1989
[21] Paige C C,Saunders M A.LSQR:An algorithm for sparse linear equations and sparse least squares.ACM Trans.Math.Software,1982,8:43～71
[22] VanDecar J C,Crosson R S.Determination of teleseismic relative phase arrival times using multi-channel cross-correlation and least squares.Bull.Seismol.Soc.Am.,1990,80:150～169
[23] Kennett B L N,Engdahl E R.Traveltimes for global earthquake location and phase identification.Geophys.J.Int.,1991,105:429～465
[24] 国家地震局《中国岩石圈动力学地图集》编委会.中国岩石圈动力学地图集.北京:中国地图出版社,1989 Editorial Board for Lithospheric Dynamics Atlas of China,State Seismological Bureau.Lithspheric Dynamics Atlas of China(in Chinese).Beijing:China Cartographic Publishing House,1989
[25] 魏文博,叶高峰,金胜等.华北地区东部岩石圈导电性结构研究--减薄的华北岩石圈特点.地学前缘,2008,15(4) :204～216 Wei W B,Ye G F,Jin S,et al.Geoelectric structure of lithosphere beneath eastern North China:features of a thinned lithosphere from magnetotelluric soundings.Earth Science Frontiers(in Chinese),2008,15(4) :204～216
[26] Huang J,Zhao D.High-resolution mantle tomography of China and surrounding regions.J.Geophys.Res.,2006,111,B09305,doi:10． 1029/2005JB004066
[27] 王峻,刘启元,陈九辉等.首都圈地区的地壳厚度及泊松比.地球物理学报,2009,52(1) :57～66 Wang J,Liu Q Y,Chen J H,et al.The crustal thickness and Poisson's ratio beneath the Capital Circle Region.Chinese J.Geophys.(in Chinese),2009,52(1) :57～66
[28] 汪素云,许忠淮,裴顺平.华北地区上地幔顶部Pn波速度结构及其构造含义.中国科学(D辑),2003,33(增):91～98 Wang S Y,Xu Z H,Pei S P.Velocity structure of uppermost mantle beneath North China from Pn tomography and its implications.Science in China(Series D),2003,46(Suppl.):130～140
[29] Liang C,Song X,Huang J.Tomographic inversion of Pn travel times in China.J.Geophys.Res.,2004,109,B11304,doi:10． 1029/2003JB002789
[30] 国家地震局《深部物探成果》编写组.中国地壳上地幔地球物理探测成果.北京:地震出版社,1986 The Group of “Results of Deep Exploration”,State Seismological Bureau.Results of Geophysical Investigation in China Crust and Upper Mantle(in Chinese).Beijing:Seismological Press,1986
[2] 于湘伟,陈运泰,王培德.京津唐地区中上地壳三维P波速度结构.地震学报,2003,25(1) :1～14 Yu X W,Chen Y T,Wang P D.Three-dimensional P wave velocity structure in Beijing-Tianjin-Tangshan area.Acta Seismologica Sinica(in Chinese),2003,25(1) :1～14
[3] Huang J,Zhao D.Crustal heterogeneity and seismotectonics of the region around Beijing,China.Tectonophysics,2004,385:159～180
[4] 嘉世旭,齐诚,王夫运等.首都圈地壳网格化三维结构.地球物理学报,2005,48(6) :1316～1324 Jia S X,Qi C,Wang F Y,et al.Three-dimensional crustal gridded structure of the Capital area.Chinese J.Geophys.(in Chinese),2005,48(6) :1316～1324
[5] 齐诚,赵大鹏,陈顒等.首都圈地区地壳P波和S波三维速度结构及其与大地震的关系.地球物理学报,2006,49(3) :805～815 Qi C,Zhao D P,Chen Y,et al.3-D P and S wave velocity structures and their relationship to strong earthquakes in the Chinese capital region.Chinese J.Geophys.(in Chinese),2006,49(3) :805～815
[6] 金安蜀,刘福田,孙永智.北京地区地壳和上地幔的三维P波速度结构.地球物理学报,1980,23(2) :172～182 Jin A S,Liu F T,Sun Y Z.Three-dimensional P velocity structure of the crust and upper mantle under Beijing region.Chinese J.Geophys.(Acta Geophysica Sinica)(in Chinese),1980,23(2) :172～182
[7] 刘福田,曲克信,吴华等.华北地区的地震层面成像.地球物理学报,1986,29(5) :442～449 Liu F T,Qu K X,Wu H,et al.Seismic tomography of North China region.Chinese J.Geophys.(Acta Geophysica Sinica)(in Chinese),1986,29(5) :442～449
[8] Shedlock K,Roecker S.Elastic wave velocity structure of the crust and upper mantle beneath the North China basin.J.Geophys.Res.,1987,92(B9) :9327～9350
[9] 朱露培,曾融生,刘福田.京津唐张地区地壳上地幔三维P波速度结构.地球物理学报,1990,33(3) :267～277 Zhu L P,Zeng R S,Liu F T.Three-dimensional P-wave velocity structure under the Beijing network area.Chinese J.Geophys.(Acta Geophysica Sinica)(in Chinese),1990,33(3) :267～277
[10] Huang J,Zhao D.Seismic imaging of the crust and upper mantle under Beijing and surrounding regions.Phys.Earth Planet.Inter.,2009,173:330～348
[11] Ding Z,Zhou X,Wu Y,et al.Tomographic imaging of P wave velocity structure beneath the region around Beijing.Earthquake Science,2009,22(4) :403～408
[12] Dahlen F A,Hung S H,Nolet G.Frechet kernels for finite-frequency traveltimes-I.Theory.Geophys.J.Int.,2000,141:157～174
[13] Hung S H,Dahlen F A,Nolet G.Frechet kernels for finite-frequency traveltimes-Ⅱ.Examples.Geophys.J.Int.,2000,141:175～203
[14] Hung S H,Shen Y,Chiao L Y.Imaging seismic velocity structure beneath the Iceland hot spot:A finite frequency approach.J.Geophys.Res.,2004,109,B08305,doi:10． 1029/2003JB002889
[15] Yang T,Grand S P,Wilson D,et al.Seismic structure beneath the Rivera subduction zone from finite-frequency seismic tomography.J.Geophys.Res.,2009,114,B01302,doi:10． 1029/2008JB005830
[16] Montelli R,Nolet G,Dahlen F A,et al.Finite-frequency tomography reveals a variety of plumes in the mantle.Science,2004,303:338～343
[17] Montelli R,Nolet G,Masters G,et al.Global P and PP traveltime tomography:rays versus waves.Geophys.J.Int.,2004,158:637～654
[18] Ren Y,Shen Y.Finite frequency tomography in southeastern Tibet:Evidence for the causal relationship between mantle lithosphere delamination and the north-south trending rifts.J.Geophys.Res.,2008,113,B10316,doi:10． 1029/2008JB005615
[19] Yang T,Shen Y,van der Lee S,et al.Upper mantle structure beneath the Azores hotspot from finite-frequency seismic tomography.Earth Planet.Sci.Lett.,2006,250:11～26
[20] Menke W.Geophysical Data Analysis:Discrete Inverse Theory.Academic Press,San Diego,Calif.,1989
[21] Paige C C,Saunders M A.LSQR:An algorithm for sparse linear equations and sparse least squares.ACM Trans.Math.Software,1982,8:43～71
[22] VanDecar J C,Crosson R S.Determination of teleseismic relative phase arrival times using multi-channel cross-correlation and least squares.Bull.Seismol.Soc.Am.,1990,80:150～169
[23] Kennett B L N,Engdahl E R.Traveltimes for global earthquake location and phase identification.Geophys.J.Int.,1991,105:429～465
[24] 国家地震局《中国岩石圈动力学地图集》编委会.中国岩石圈动力学地图集.北京:中国地图出版社,1989 Editorial Board for Lithospheric Dynamics Atlas of China,State Seismological Bureau.Lithspheric Dynamics Atlas of China(in Chinese).Beijing:China Cartographic Publishing House,1989
[25] 魏文博,叶高峰,金胜等.华北地区东部岩石圈导电性结构研究--减薄的华北岩石圈特点.地学前缘,2008,15(4) :204～216 Wei W B,Ye G F,Jin S,et al.Geoelectric structure of lithosphere beneath eastern North China:features of a thinned lithosphere from magnetotelluric soundings.Earth Science Frontiers(in Chinese),2008,15(4) :204～216
[26] Huang J,Zhao D.High-resolution mantle tomography of China and surrounding regions.J.Geophys.Res.,2006,111,B09305,doi:10． 1029/2005JB004066
[27] 王峻,刘启元,陈九辉等.首都圈地区的地壳厚度及泊松比.地球物理学报,2009,52(1) :57～66 Wang J,Liu Q Y,Chen J H,et al.The crustal thickness and Poisson's ratio beneath the Capital Circle Region.Chinese J.Geophys.(in Chinese),2009,52(1) :57～66
[28] 汪素云,许忠淮,裴顺平.华北地区上地幔顶部Pn波速度结构及其构造含义.中国科学(D辑),2003,33(增):91～98 Wang S Y,Xu Z H,Pei S P.Velocity structure of uppermost mantle beneath North China from Pn tomography and its implications.Science in China(Series D),2003,46(Suppl.):130～140
[29] Liang C,Song X,Huang J.Tomographic inversion of Pn travel times in China.J.Geophys.Res.,2004,109,B11304,doi:10． 1029/2003JB002789
[30] 国家地震局《深部物探成果》编写组.中国地壳上地幔地球物理探测成果.北京:地震出版社,1986 The Group of “Results of Deep Exploration”,State Seismological Bureau.Results of Geophysical Investigation in China Crust and Upper Mantle(in Chinese).Beijing:Seismological Press,1986