中天山地区的Pn波速度结构与各向异性
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摘要
利用宽频带流动地震台阵GHENGIS和吉尔吉斯地震台网KNET记录的地震波走时数据,反演了中天山地区的Pn波速度结构和各向异性.结果表明,中天山上地幔顶部平均速度偏低,具有构造活动地区的特点和明显的横向非均匀性;中天山南部地幔上涌区的Pn波速度非常低,表明存在较高的热流活动.Pn波速度的变化与地震分布有着密切的对应关系:地震大都发生在中天山北部Pn波高速区上方,而南部的Pn波低速区上方几乎没有地震.这一现象说明地幔上涌引起高温极大地降低了岩石层地幔的强度,并以热传导的方式进入地壳使其失去地震破裂强度而发生韧性变形.中天山北部和南部的各向异性也存在一定的差异,南部各向异性的快波方向为近南北方向,与SKS波的各向异性特征基本一致,反映了地幔物质的迁移方向;北部各向异性的快波方向呈向南凸出的旋转趋势,估计与哈萨克地台南缘楚河盆地地壳块体向天山挤入造成应力场的改变和岩石层变形有关.
Pn wave velocity structure and anisotropy in the central Tien Shan are investigated by using seismic arrival data from the temporary broadband network (GHENGIS) and the Kyrgyzstan seismic network (KNET). The results reveal lower average Pn velocities and strong heterogeneity at the uppermost mantle beneath the central Tien Shan, which are often found in tectonically active zones. Particularly, very low Pn velocities are found in the south-central Tien Shan, which is believed to be underlain by an unusual mantle with higher heat flows. There is a clear correlation between seismic activity and Pn velocity variation. Most earthquakes are concentrated in the north-central Tien Shan with high Pn velocities, while almost no earthquakes located in the south-central Tien Shan with very low Pn velocities. Our result suggests that the very low Pn velocity beneath the south-central Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has been conducted up into the crust, inducing a ductile rheology that prevents significant seismic activity. Moreover, strong anisotropy is observed in the north- and south-central Tien Shan. Fast Pn velocity in the south-central Tien Shan is nearly in N-S directions, which is consistent with the SKS wave anisotropy concerning mantle flow. However, fast Pn velocity directions in the north-central Tien Shan show a southward rotation variation. We estimate that it is probably related to the inserting of the crust of the Chu basin from the southern Kazakhstan platform to the central Tien Shan, which caused a change of stress fields and lithosphere deformation.
Pn wave velocity structure and anisotropy in the central Tien Shan are investigated by using seismic arrival data from the temporary broadband network (GHENGIS) and the Kyrgyzstan seismic network (KNET). The results reveal lower average Pn velocities and strong heterogeneity at the uppermost mantle beneath the central Tien Shan, which are often found in tectonically active zones. Particularly, very low Pn velocities are found in the south-central Tien Shan, which is believed to be underlain by an unusual mantle with higher heat flows. There is a clear correlation between seismic activity and Pn velocity variation. Most earthquakes are concentrated in the north-central Tien Shan with high Pn velocities, while almost no earthquakes located in the south-central Tien Shan with very low Pn velocities. Our result suggests that the very low Pn velocity beneath the south-central Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has been conducted up into the crust, inducing a ductile rheology that prevents significant seismic activity. Moreover, strong anisotropy is observed in the north- and south-central Tien Shan. Fast Pn velocity in the south-central Tien Shan is nearly in N-S directions, which is consistent with the SKS wave anisotropy concerning mantle flow. However, fast Pn velocity directions in the north-central Tien Shan show a southward rotation variation. We estimate that it is probably related to the inserting of the crust of the Chu basin from the southern Kazakhstan platform to the central Tien Shan, which caused a change of stress fields and lithosphere deformation.
引文
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[24]黄金莉,宋晓东,汪素云.川滇地区上地幔顶部Pn速度细结构.中国科学(D辑),2003,33(增刊):144～150Huang J L,Song XD,Wang S Y.Pnvelocitystructure of uppermost mantle beneath Sichuan-Yunnan region and continent dynamicsimplication.Science in China(Series D),2003,33(Suppl.):144～150
[25]Liang C,Song X.Alowvelocity belt beneath northern and eastern Tibetan Plateau fromPn tomography.Geophys.Res.Lett.,2006,33,L22306,doi:10.1029/2006GL027926
[26]Bielinski R A,Park S K,Rybin A,et al.Lithospheric heterogeneity inthe Kyrgyz Tien Shanimaged by magnetotelluric studies.Geophys.Res.Lett.,2003,30(15):1806,doi:10.1029/2003GL017455
[27]Abdrakhmatov K Y,Aldazhanov S A,Hager B H.Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates.Nature,1996,384:450～453
[28]Thompson S C,Weldon RJ,Rubin C M,et al.Late Quaternaryslip rates across the central Tien Shan,Kyrgyz Republic,Central Asia.J.Geophys.Res.,2002,107,2203,doi:10.1029/2001JB000596
[2]Roecker S W,Sabitova T M,Vinnik L P,et al.Three-dimensional elastic wave velocity structure of the western and central Tien Shen.J.Geophys.Res.,1993.98(B9):15779～15795
[3]胥颐,刘福田,刘建华等.中国大陆西部造山带及其毗邻盆地的地震层析成像.中国科学(D辑),2000,30(2):113～122Xu Y,Liu F T,Liu J H,et al.Seismic tomography beneath the orogenic belts and adjacent basins of northwestern China.Science in China(Series D),2001,44(5):468～480
[4]Makeyeva LI,Vinnik L P,Roecker S W.Shear-wave splitting and small-scale convectioninthe continent upper mantle.Nature,1992,358(9):144～147
[5]Wolfe CJ,Vernon III F L.Shear-wave splitting at central Tien Shan:evidence for rapid variation of anisotropic patterns.Geophys.Res.Lett.,1998,25(8):1217～1220
[6]Kosarev GL,Petersen NV,Vinnik L P,et al.Receiver functions for the Tien Shan analog broadband network:Contrasts in the evolution of structures acrossthe Talasso-Ferganafault.J.Geophys.Res.,1993,98(B3):4437～4448
[7]Bump HA,Sheehan A F.Crustal thickness variations across the northern Tien Shan from telesiesmic receiver functions.Geophys.Res.Lett.,1998,25(7):1055～1058
[8]Oreshin S,Vinnik L,Peregoudov D,et al.Lithosphere and asthenosphere of the Tien Shan imaged by S receiver functions.Geophys.Res.Lett,2002,29(8):1191～1194
[9]Vinnik L P,Reigber C,Aleshin I M,et al.Receiver function tomography of the central Tien Shan.Earth Planet.Sci.Lett.,2004,225:131～146
[10]Ghose S,Hamburger W.Source parameters of moderate-sized earthquakes in the Tien Shan,central Asia from regional moment tensor inversion.Geophys.Res.Lett.,1998,25(16):3181～3184
[11]Ghose S,Hamburger M W,Virieux J.Three-dimensional velocitystructure and earthquake location beneath the northern Tian Shan of Kyrgyzstan,central Asia.J.Geophys.Res.,1998,103(B2):2725～2784
[12]Mellors RJ,Vernon F L,Pavlis G L,et al.TheMs=7.3Suusamyr,Kyrgyzstan,Earthquake:1.Constrains on fault geometry and source parameters based on aftershocks and body-wave modeling.Bull.Seism.Soc.Am.,1997,87(1):11～22
[13]胥颐,Roecker S W,魏若平等.天山中部的地震定位和地壳活动性分析.地球物理学报,2005,48(6):1308～1315Xu Y,Roecker S W,Wei RP,et al.Analysis of seismic activityin the crust from earthquake relocation in the Central Tian Shan.Chinese J.Geophys.(in Chinese),2005,48(6):1308～1315
[14]Hearn T M.Anisotropic Pntomographyinthe western United States.J.Geophys.Res.,1996,101(B4):8403～8414
[15]汪素云,Hearn T M,许忠淮等.中国大陆上地幔顶部Pn速度结构.中国科学(D辑),2001,31(6):449～454Wang S Y,Hearn T M,Xu Z H,et al.Velocity structure of uppermost mantle beneath China continent from Pn tomography.Sciencein China(Series D)(in Chinese),2001,31(6):449～454
[16]Hearn T M,Wang S,Ni J F,et al.Uppermost mantle velocities beneath China and surrounding regions.J.Geophys.Res.,2004,109(B11301),doi:10.1029/2003JB002874
[17]Liang C,Song X D,Huang J L.Tomographic inversion of Pn travel times in China.J.Geophys.Res.,2004,109(B11304),doi:10.1029/2003JB002789
[18]Lees J M,Crosson R S.Tomographic inversion for three-dimensional velocity structure at Mount St.Helens using earthquake data.J.Geophys.Res.,1989,94:5716～5728
[19]Paige C C,Saunders M A.LSQR:An algorithmfor sparse linear equations andleast squares problems.ACMTrans.Math.Software,1982,8(1):43～71
[20]Song L P,Koch M,Koch K,et al.Isotropic and anisotropic Pn velocity inversion of regional earthquake traveltimes underneath Germany.Geophys.J.Int.,2001,146:795～800
[21]刘启元,陈九辉,李顺成等.新疆伽师强震群区三维地壳上地幔S波速度结构及其地震成因的探讨.地球物理学报,2000,43(3):356～365Liu QY,Chen J H,Li S C,et al.Passive seismic experiment in Xingjiang Jiashi strongearthquake regionand discussiononits seismic genesis.Chinese J.Geophys.(in Chinese),2000,43(3):356～365
[22]Laske G,Masters G,Reif C.Crust2.0:a newglobal crustal model at2×2degrees.http://mahi.ucsd.edu/Gabi/rem.dir/crust2.html
[23]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(增刊):91～98Wang S Y,Xu Z H,Pei S P.Velocitystructure of uppermost mantle beneath North China from Pn tomography and its implications.Science in China(Series.D),2003,33(Suppl.):91～98
[24]黄金莉,宋晓东,汪素云.川滇地区上地幔顶部Pn速度细结构.中国科学(D辑),2003,33(增刊):144～150Huang J L,Song XD,Wang S Y.Pnvelocitystructure of uppermost mantle beneath Sichuan-Yunnan region and continent dynamicsimplication.Science in China(Series D),2003,33(Suppl.):144～150
[25]Liang C,Song X.Alowvelocity belt beneath northern and eastern Tibetan Plateau fromPn tomography.Geophys.Res.Lett.,2006,33,L22306,doi:10.1029/2006GL027926
[26]Bielinski R A,Park S K,Rybin A,et al.Lithospheric heterogeneity inthe Kyrgyz Tien Shanimaged by magnetotelluric studies.Geophys.Res.Lett.,2003,30(15):1806,doi:10.1029/2003GL017455
[27]Abdrakhmatov K Y,Aldazhanov S A,Hager B H.Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates.Nature,1996,384:450～453
[28]Thompson S C,Weldon RJ,Rubin C M,et al.Late Quaternaryslip rates across the central Tien Shan,Kyrgyz Republic,Central Asia.J.Geophys.Res.,2002,107,2203,doi:10.1029/2001JB000596
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