青藏高原中部Quasi-Love波的识别及其转换点揭示的东西向方位各向异性变化
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摘要
目前人们利用4种基本的地震波现象研究地震各向异性,如横波双折射、面波散射、与传播方向有关的走时异常和PS转换波震相.本文利用面波散射产生的Quasi-Love(QL)波研究青藏高原上地幔顶部的各向异性结构特征.首先利用中国地震台网昌都(CAD)台记录的地震波形资料识别出产生QL波的路径,并利用合成地震记录和垂直偏振极性分析证实所观测到的为QL波,而不是高阶振型的Rayleigh波或其他体波震相;然后由Rayleigh波、Love波和QL波的群速度估算了各向异性结构横向变化的转换点;不同周期时,转换点的位置不同.这种频率依赖性还需要进一步的模拟研究.Love波向Rayleigh波耦合(产生QL波)的转换点位置揭示了青藏高原面波方位各向异性变化特征,并以南北向构造带的东西分段性、上地幔流引起的地球内力诱导岩石形变解释了青藏高原各向异性的东西向差异性.
Seismic anisotropy can be studied by four basic seismic wave behaviors, e. g, directiondependent travel time anomalies, shear-wave birefringence, surface-wave scattering, and direction-dependent conversion of compressional waves to shear waves. In this paper, Quasi-Love (QL) waves, generated by surface wave scattering, are utilized to determine the variation of uppermost mantle anisotropy of Tibet plateau. Firstly, QL waves are identified from the seismograms of the selected paths recorded by the CAD station. The observed QL waves are confirmed by distinguishing QL waves from higher mode Rayleigh waves or other body waves, which are absent in synthetic seismograms. Furthermore, the location of azimuthal anisotropy gradient is estimated from the group velocities of Rayleigh wave, Love wave and QL wave. Finally, the characteristic of azimuthal anisotropy of upper mantle in Tibet plateau is summarized. The reason of the existence of azimuthal anisotropy gradient can be explained by east-west variation of tectonics. Also the other reason is the force from the upper mantle flow which has led to voluminous rock deformation.
Seismic anisotropy can be studied by four basic seismic wave behaviors, e. g, directiondependent travel time anomalies, shear-wave birefringence, surface-wave scattering, and direction-dependent conversion of compressional waves to shear waves. In this paper, Quasi-Love (QL) waves, generated by surface wave scattering, are utilized to determine the variation of uppermost mantle anisotropy of Tibet plateau. Firstly, QL waves are identified from the seismograms of the selected paths recorded by the CAD station. The observed QL waves are confirmed by distinguishing QL waves from higher mode Rayleigh waves or other body waves, which are absent in synthetic seismograms. Furthermore, the location of azimuthal anisotropy gradient is estimated from the group velocities of Rayleigh wave, Love wave and QL wave. Finally, the characteristic of azimuthal anisotropy of upper mantle in Tibet plateau is summarized. The reason of the existence of azimuthal anisotropy gradient can be explained by east-west variation of tectonics. Also the other reason is the force from the upper mantle flow which has led to voluminous rock deformation.
引文
[1] Woodhouse J H, Dahlen F A. The effect of a general aspherical perturbation on the free oscillations of the Earth.Geophys. J. R. astr. Soc. , 1978, 53:335-354
[2] Park J. Synthetic seismograms from coupled free oscillations:effects of lateral structure and rotation. J. Geophys. Res. ,1986, 91: 6441-6464
[3] 张素芳,张智.均匀介质背景中三维异常体的面波波场响应及其动力学特征分析.地球物理学报,2008,51(4) :1180-1187 Zhang S F, Zhang Z. Surface wavefield and dynamic analysis for three-dimensional abnormal structures with homogenous background velocity model. Chinese J. Geophys. (in ChinevSe),2008,51(4) :1180-1187
[4] Levin V, Park J, Margheriti L, et al. Structure and texture of the upper mantle beneath Northern Apennines: evidence from quasi-Love waves. Geophysical Research Abstracts, 2006, 8, 1607-7962/gra/EGU06-A-05413
[5] Crampin S. Distinctive particle motion of surface waves as a diagnostic of anisotropic layering. Geophys. J. R. astr. Soc.,1975,40:177-186
[6] Park J, Yu Y. Anisotropy and coupled free oscillations: simplified models and surface wave observations. Geophys.J. Int. ,1992,110:401-420
[7] Yu Y, Park J. Upper mantle anisotropy and coupled-mode long-period surface waves. Geophys. J. Int., 1993,114:473-489
[8] Oda H, Onishi S. The effect of regional variation of lattice preferred orientation on surface waveforms. Geophys. J.Int. , 2001,144:247-258
[9] Yu Y, Park J. Hunting for azimuthal anisotropy beneath the Pacific Ocean region. Journal of Geophysical Research , 1994,99:15399-15421
[10] Kobayashi R. Polarization anomalies of Love waves observed in and around Japan. Earth Planets Space, 2002,54:357-365
[11] Yu Y, Park J, Wu Francis. Mantle anisotropy beneath the Tibetan Plateau: evidence from long-period surface waves.Physics of Earth and Planetary Interiors, 1995,87:231-246
[12] Levin V, Park I, Lucente F P, et al. End of subduction in northern Apennines confirmed by observations of quasi-Love waves from the great 2004 Sumatra-Andaman earthquake.Geophysical Research Letters, 2007, 34, doi:10． 1029/2006GL028860
[13] Zhang Z J, Li Y K, Wang G J, et al. East-west crustal structure and "down-bowing" Moho under the northern Tibet revealed by wide-angle seismic profile. Science in China (Series D) , 2002,45(6) :550-558
[14] Zhang Z J, Teng J W, Li Y K, et al. Crustal structure of seismic velocity in southern Tibet and east-westward escape of the crustal material. Science in China (SeriesD) , 2004,47 (6) :500-506
[15] Zhang Z J, Klemperer S. West-east variation in crustal thickness in northern Lhasa block, central Tibet, from deep seismic sounding data. J. Geophys. Res. , 2005, 110,B09403, doi:10． 1029/2004JB003139
[16] 吴庆举,曾融生,赵文津.喜马拉雅-青藏高原的上地幔倾斜构造与陆-陆碰撞过程.中国科学,2004,34(10) :910-925 Wu Q J, Zeng R S, Zhao W J. Oblique tectonic and continental-continental collision in the upper mantle of Himalaya-Tibet plateau. Science in China ( Series D) ( in Chinese) ,2004,34(10) :910-925
[17] 吴庆举,曾融生.用宽频带接收函数研究青藏高原的地壳结构.地球物理学报,1998,41(5) :669-679 Wu Q J, Zeng R S. The crustal structure of Qinghai-Xizang Plateau inferred from broadband teleseismic waveform.Chinese J. Geophys. (in Chinese) , 1998,41(5) :669-679
[18] Huang Z X, Peng Y, Luo Y, et al. Azimuthal anisotropy of Rayleigh waves in East Asia. Geophysical Research Letters, 2004,31,doi:10． 1029/2004GL020399
[19] Tanimoto T. The azimuthal dependence of surface wave polarization in a slightly anisotropic medium. Geophys. J.Int. ,2004,156:73-78
[20] Kobayashi R, Nakanishi I. Location of Love-to-Rayleigh conversion due to lateral heterogeneity or azimuthal anisotropy in the upper mantle. Geophysical Research Letters, 1998,25(7) :1067-1070
[21] Herrmann R B, Ammon C J. Computer programs in seismology-surface waves, receiver functions and crustal structure. 2002, http://www. eas. slu. edu/People/RBHerrmann/index. html
[22] McNamara D E, Owens T J, Silver P G, et al. Shear wave anisotropy beneath the Tibetan Plateau. Journal of Geophysical Research , 1994,99:13655-13665
[23] Liu K, Zhang Z J, Hu J F. Frequency band-dependence of S-wave splitting in China mainland and its implications. Science inChina (Series D), 2001,44(7) : 659-665
[24] 姜枚,许志琴,Hirn A等.青藏高原及其邻区地震各向异性和上地幔特征.地球学报,2001,22(2) :111-116 Jiang M, Xu Z Q, Hirn A, et al. Teleseismic anisotropy and corresponding features of the upper mantle in Tibet Plateau and its neighboring areas. Acta Geoscientia Sinica ( in Chinese), 2001,22(2) ,111-116
[25] Pei S P, Zhao J, Sun Y, et al. Upper mantle seismic velocities and anisotropy in China determined through Pn and Sn tomography. J. Geophys. Res. , 2007,112,doi:10． 1029/ 2006JB004409
[26] 吕庆田,姜枚,马开义等.由震源机制和地震波各向异性探讨青藏高原岩石圈变形.地质论评,1997,43(4) :337-346 L( ) Q T, Jiang M, Ma K Y, et al. The deformation characteres of Qinghai-Xizang lithosphere: implication from earthquake mechanism and seismic anisotropy. Geological Review (in Chinese), 1997 ,43(4) :337-346
[27] 杨晓松,金振民,马瑾等.青藏高原北部异常SKS分裂成因的初步探讨--被熔体强化的岩石圈各向异性.地球物理学报,2002,45(6) :821-831 Yang X S, Jin Z M, Ma J, et al. Genesis of SKS splitting in the north-central Qinghai-Xizang Plateau: melt alignment enhanced lithosphere anisotropy. Chinese J . Geophys. (in Chinese), 2002 ,45(6) :821-831
[28] Zhang P Z, Shen Z K, Wang M, et al. Continuous deformation of the Tibetan Plateau from global positioning system data. Geology,2004,32(9) :809-812
[29] Dricker I G, Roecker S W. Lateral heterogeneity in the upper mantle beneath the Tibetan plateau and its surroundings from SS-S travel time residuals. J. Geophys. Res. , 2002, 107,2305,doi:10． 1029/2001JB000797
[30] 张进,马宗晋.西藏高原西、中、东的分段性及其意义.地质学报,2004,78(2) :218-228 Zhang J, Ma Z J. East-west segmentation of the Tibetan Plateau and its implication. Acta Geologica Sinica ( in Chinese), 2004 ,78(2) :218-228
[2] Park J. Synthetic seismograms from coupled free oscillations:effects of lateral structure and rotation. J. Geophys. Res. ,1986, 91: 6441-6464
[3] 张素芳,张智.均匀介质背景中三维异常体的面波波场响应及其动力学特征分析.地球物理学报,2008,51(4) :1180-1187 Zhang S F, Zhang Z. Surface wavefield and dynamic analysis for three-dimensional abnormal structures with homogenous background velocity model. Chinese J. Geophys. (in ChinevSe),2008,51(4) :1180-1187
[4] Levin V, Park J, Margheriti L, et al. Structure and texture of the upper mantle beneath Northern Apennines: evidence from quasi-Love waves. Geophysical Research Abstracts, 2006, 8, 1607-7962/gra/EGU06-A-05413
[5] Crampin S. Distinctive particle motion of surface waves as a diagnostic of anisotropic layering. Geophys. J. R. astr. Soc.,1975,40:177-186
[6] Park J, Yu Y. Anisotropy and coupled free oscillations: simplified models and surface wave observations. Geophys.J. Int. ,1992,110:401-420
[7] Yu Y, Park J. Upper mantle anisotropy and coupled-mode long-period surface waves. Geophys. J. Int., 1993,114:473-489
[8] Oda H, Onishi S. The effect of regional variation of lattice preferred orientation on surface waveforms. Geophys. J.Int. , 2001,144:247-258
[9] Yu Y, Park J. Hunting for azimuthal anisotropy beneath the Pacific Ocean region. Journal of Geophysical Research , 1994,99:15399-15421
[10] Kobayashi R. Polarization anomalies of Love waves observed in and around Japan. Earth Planets Space, 2002,54:357-365
[11] Yu Y, Park J, Wu Francis. Mantle anisotropy beneath the Tibetan Plateau: evidence from long-period surface waves.Physics of Earth and Planetary Interiors, 1995,87:231-246
[12] Levin V, Park I, Lucente F P, et al. End of subduction in northern Apennines confirmed by observations of quasi-Love waves from the great 2004 Sumatra-Andaman earthquake.Geophysical Research Letters, 2007, 34, doi:10． 1029/2006GL028860
[13] Zhang Z J, Li Y K, Wang G J, et al. East-west crustal structure and "down-bowing" Moho under the northern Tibet revealed by wide-angle seismic profile. Science in China (Series D) , 2002,45(6) :550-558
[14] Zhang Z J, Teng J W, Li Y K, et al. Crustal structure of seismic velocity in southern Tibet and east-westward escape of the crustal material. Science in China (SeriesD) , 2004,47 (6) :500-506
[15] Zhang Z J, Klemperer S. West-east variation in crustal thickness in northern Lhasa block, central Tibet, from deep seismic sounding data. J. Geophys. Res. , 2005, 110,B09403, doi:10． 1029/2004JB003139
[16] 吴庆举,曾融生,赵文津.喜马拉雅-青藏高原的上地幔倾斜构造与陆-陆碰撞过程.中国科学,2004,34(10) :910-925 Wu Q J, Zeng R S, Zhao W J. Oblique tectonic and continental-continental collision in the upper mantle of Himalaya-Tibet plateau. Science in China ( Series D) ( in Chinese) ,2004,34(10) :910-925
[17] 吴庆举,曾融生.用宽频带接收函数研究青藏高原的地壳结构.地球物理学报,1998,41(5) :669-679 Wu Q J, Zeng R S. The crustal structure of Qinghai-Xizang Plateau inferred from broadband teleseismic waveform.Chinese J. Geophys. (in Chinese) , 1998,41(5) :669-679
[18] Huang Z X, Peng Y, Luo Y, et al. Azimuthal anisotropy of Rayleigh waves in East Asia. Geophysical Research Letters, 2004,31,doi:10． 1029/2004GL020399
[19] Tanimoto T. The azimuthal dependence of surface wave polarization in a slightly anisotropic medium. Geophys. J.Int. ,2004,156:73-78
[20] Kobayashi R, Nakanishi I. Location of Love-to-Rayleigh conversion due to lateral heterogeneity or azimuthal anisotropy in the upper mantle. Geophysical Research Letters, 1998,25(7) :1067-1070
[21] Herrmann R B, Ammon C J. Computer programs in seismology-surface waves, receiver functions and crustal structure. 2002, http://www. eas. slu. edu/People/RBHerrmann/index. html
[22] McNamara D E, Owens T J, Silver P G, et al. Shear wave anisotropy beneath the Tibetan Plateau. Journal of Geophysical Research , 1994,99:13655-13665
[23] Liu K, Zhang Z J, Hu J F. Frequency band-dependence of S-wave splitting in China mainland and its implications. Science inChina (Series D), 2001,44(7) : 659-665
[24] 姜枚,许志琴,Hirn A等.青藏高原及其邻区地震各向异性和上地幔特征.地球学报,2001,22(2) :111-116 Jiang M, Xu Z Q, Hirn A, et al. Teleseismic anisotropy and corresponding features of the upper mantle in Tibet Plateau and its neighboring areas. Acta Geoscientia Sinica ( in Chinese), 2001,22(2) ,111-116
[25] Pei S P, Zhao J, Sun Y, et al. Upper mantle seismic velocities and anisotropy in China determined through Pn and Sn tomography. J. Geophys. Res. , 2007,112,doi:10． 1029/ 2006JB004409
[26] 吕庆田,姜枚,马开义等.由震源机制和地震波各向异性探讨青藏高原岩石圈变形.地质论评,1997,43(4) :337-346 L( ) Q T, Jiang M, Ma K Y, et al. The deformation characteres of Qinghai-Xizang lithosphere: implication from earthquake mechanism and seismic anisotropy. Geological Review (in Chinese), 1997 ,43(4) :337-346
[27] 杨晓松,金振民,马瑾等.青藏高原北部异常SKS分裂成因的初步探讨--被熔体强化的岩石圈各向异性.地球物理学报,2002,45(6) :821-831 Yang X S, Jin Z M, Ma J, et al. Genesis of SKS splitting in the north-central Qinghai-Xizang Plateau: melt alignment enhanced lithosphere anisotropy. Chinese J . Geophys. (in Chinese), 2002 ,45(6) :821-831
[28] Zhang P Z, Shen Z K, Wang M, et al. Continuous deformation of the Tibetan Plateau from global positioning system data. Geology,2004,32(9) :809-812
[29] Dricker I G, Roecker S W. Lateral heterogeneity in the upper mantle beneath the Tibetan plateau and its surroundings from SS-S travel time residuals. J. Geophys. Res. , 2002, 107,2305,doi:10． 1029/2001JB000797
[30] 张进,马宗晋.西藏高原西、中、东的分段性及其意义.地质学报,2004,78(2) :218-228 Zhang J, Ma Z J. East-west segmentation of the Tibetan Plateau and its implication. Acta Geologica Sinica ( in Chinese), 2004 ,78(2) :218-228
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