超低速区研究进展与展望
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摘要
作为固态地幔和液态外核的过渡区,D″层是地球内部最重要的边界层之一,也是物理、化学现象最丰富的区域之一.而D″层的底部零落散布着厚5~60 km,横向尺度200~300 km,剪切波(Vs)速异常可达-30%,压缩波(Vp)异常达-10%和密度异常可达+10%的异常体,称为超低速区(ultra low velocity zone,ULVZ).研究超低速区的地震学手段主要是分析体波(PcP,ScP,ScS,SKPdS,PKKP,PKP)波形的复杂性.本文评述了超低速区的地震学研究的历史及现状,并介绍超低速区的可能成因及其地球动力学作用,最后展望超低速区研究的未来.
As the transition zone between the solid mantle and the liquid outer core,D″ is one of the most important boundary layers in the earth,featuring complicated physical and chemical processes.And at the bottom of D″,there are small patches about 5~60 km high and 200~300 km across,with Vs reduced up to 30%,Vp up to 10% but with density increase up to 10%,as compared to typical anomaly of a few percent in other parts of the earth.These extreme anomalies are named ultra low velocity zones(ULVZ),and are best modeled with seismic body waves such as PcP,ScP,SKPdS,ScS,PKP and PKKPs.The history and state of the art researches on ULVZs,their probable origin and influences on geodynamics were reviewed.Finally,a discussion was given about what can be expected of studies on ULVZs in the near future.
As the transition zone between the solid mantle and the liquid outer core,D″ is one of the most important boundary layers in the earth,featuring complicated physical and chemical processes.And at the bottom of D″,there are small patches about 5~60 km high and 200~300 km across,with Vs reduced up to 30%,Vp up to 10% but with density increase up to 10%,as compared to typical anomaly of a few percent in other parts of the earth.These extreme anomalies are named ultra low velocity zones(ULVZ),and are best modeled with seismic body waves such as PcP,ScP,SKPdS,ScS,PKP and PKKPs.The history and state of the art researches on ULVZs,their probable origin and influences on geodynamics were reviewed.Finally,a discussion was given about what can be expected of studies on ULVZs in the near future.
引文
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[3]Ritsema J,Heijst H J,Woodhouse J H.Complexshear wave velocity structure i maged beneath Africaand Iceland[J].Science,1999,286:1 925-1 928.
[4]Van der Hilst R D,Widiyantoro S,Engdahl E R.Evidence for deep mantle circulation from globaltomography[J].Nature,1997,386:578-584.
[5]Kuo B Y,Garnero EJ,Lay T.Tomographic inversionof S-SKS ti mes for shear velocity heterogeneity in D″:Degree 12 and hybrid models[J].Journal ofGeophysical Research-Solid Earth,2000,105(B12):28 139-28 157.
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[7]Ni S D,Ding X M,Hel mberger D V,et al.Low-velocity structure beneath Africa from forwardmodeling[J].Earth and Planet Sci Lett,1999,170:497-507.
[8]Ni S D,Hel mberger D V.Probing an ultra-lowvelocity zone at the core mantle boundary with P and Swaves[J].Geophysical Research Letters,2001,28(12):2 345-2 348.
[9]Ni S D,Hel mberger D V.Seismological constraints onthe African superplume,could be the oldest structurein the lower mantle[J].EPSL,2003,206:119-131.
[10]Hedlin M A H,Shearer P M,Earle P S.Seismicevidence for small-scale heterogeneity throughout theEarth s mantle[J].Nature,1997,387(6629):145-150.
[11]Niu F L,Wen L X.Strong seismic scatterers near thecore-mantle boundary west of Mexico[J].Geophys ResLett,2001,28:3 557-3 560.
[12]Ni S D,Hel mberger D V,Tromp J.Three-di mensional structure of he African superplume fromwaveform modelling[J].Geophysical JournalInternational,2005,161(2):283-294.
[13]Lay T,Garnero E J,William Q.Partial melting in athermo-chemical boundary layer at the base of themantle[J].Physics of the Earth and PlanetaryInteriors,2004,146(3-4):441-467.
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[15]Buffett B A.Esti mates of heat flowin the deep mantlebased on the power requirements for the geodynamo[J].Geophys Res Lett,2002,29(12).doi:10.1029/2001GL014649.
[16]Kellogg L H,Hager B H,Van der Hilst R D.Compositional stratification in the deep mantle[J].Science,1999,283:1 881-1 884.
[17]Grand S P,Van der Hilst R D,Widiyantoro S.Globalseismic tomography:A snapshot of convection in theEarth[J].GSA Today,1997,7(4):1-7.
[18]Sidorin Igor,Gurnis M,Hel mberger D V.Dynamicsof a phase change at the base of the mantle consistentwith seismological observations[J].J Geophys Res,1999,104(B7):15 005-15 023.
[19]Ni S D,Ding X M,Hel mberger D V.Constructingsynthetics from deep earth tomographic models[J].Geophysical Journal International,2000,140(1):71-82.
[20]Wysession M E,Lay T,Revenaugh J,et al.The D″Discontinuity and its I mplications[M]//The Core-Mantle Boundary Region.Washington:AGU Press,1998:273-297.
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[24]Shi m S H,Duffy T S,Jeanloz R,et al.Stability andcrystal structure of MgSi O3perovskite to the core-mantle boundary[J].Geophys Res Lett,2004,31.L10603.doi:10.1029/2004GL019639.
[25]Iitaka T,Hirose K,Kawamura K,et al.The elasticityof the MgSi O3post-perovskite phase in the Earth slowermost mantle[J].Nature,2004,430:442-445.
[26]Lay T,Heinz D,Ishii M,et al.Multi-displinaryi mpact of the deep mantle post-perovskite phasetransition[J].EOS,2005,86(1):1-5.
[27]Garnero E J,Maupin V,Lay T,et al.Variableazi muthal anisotropy in Earth s lowermost mantle[J].Science,2004,306(5694):259-261.
[28]Garnero EJ,Hel mberger D V.Avery slowbasal layerunderlying large-scale low-velocity anomalies in thelower mantle beneath the Pacific:evidence from corephases[J].Phys Earth&Planet Inter,1995,91:161-176.
[29]Garnero E J,Hel mberger D V.Seismic detection of athin laterally varying boundary layer at the base of themantle beneath the central-Pacific[J].Geophys ResLett,1996,23:977-980.
[30]Wen L X,Hel mberger D V.Atwo-di mensional P-SVhybrid method and its application to modeling localizedstructures near the core-mantle boundary[J].Journalof Geophysical Research-Solid Earth,1998,103(B8):特约评述17 901-17 918.
[31]Hel mberger D V,Ni S D,Wen L X,et al.Seismicevidence for ultra lowvelocity zones beneath Africa andeastern Atlantic[J].Journal of Geophysical Research,2000,105:23 865-23 878.
[32]Garnero E J,MS Thorne,McNamara A,et al.Fine-scale ultra-lowvelocity zone layering at the core-mantleboundary and superplumes[M]//SuperplumeMonograph.Springer,2007(in press).
[33]Thorne M S,Garnero E J,Grand S.Geographiccorrelation between hot spots and deep mantle lateralshear-wave velocity gradients[J].Phys Earth PlanetInter,2004,146:47-63.
[34]Wen L X,Hel mberger D V.Ultra-low velocity zonesnear the core-mantle boundary from broadband PKPprecursors[J].Science,1998,279(5357):1 701-1 703.
[35]Avants M,Lay T,Garnero E J.Determining shearvelocity structure of the ULVZ beneath the centralPacific using stacked ScS data[J].Geophys Res Lett,2006,33.L07314.doi:10.1029/2005GL024989.
[36]Luo S N,Ni S D,Hel mberger D V.Evidence for asharp lateral variation of velocity at the core-mantleboundary from multipathed PKPab[J].Earth andPlanetary Science Letters,2001,189(3-4):155-164.
[37]Mori J,Hel mberger D V.Localized boundary layerbelow the mid-Pacific velocity anomaly from a PcPprecursor[J].J Geophys Res,1995,100:20 359-20 365.
[38]Garnero E J,Vidale J.ScP:a probe of ultralow-velocity zones at the base of the mantle[J].GeophysRes Lett,1999,26:377-380.
[39]Rost S,Revenaugh J.Small-scale ultra-low velocityzone structure i maged by ScP[J].J Geophys Res,2003,108(B1).2 056.doi:10.1029/2001JB001627.
[40]Rost S,Garnero E J,Williams Q,et al.Seismicconstraints on a possible plume root at the core-mantleboundary.Nature,2005,435:666-669.
[41]McSweeney T J,Creager K C,R T Merrill.Depthextent of inner-core seismic anisotropy andi mplicationsfor geomagnetism.Phys Earth Planet Inter,1997,101:131-156.
[42]Rost S,Garnero EJ.Detection of an ultra-lowvelocityzone at the CMB using diffracted PKKPab waves[J].JGeophys Res,2006,111.B07309,doi:10.1029/2005JB003850.
[43]Williams Q,Garnero E J.Seismic evidence for partialmelt at the base of Earth s mantle[J].Science,1996,273:1 528-1 530.
[44]Buffett B A,Garnero EJ,Jeanloz R.Sedi ments at thetop of Earth s core[J].Science,2000,290:1 338-1 342.
[45]Dobson D P,Brodholt J P.Subducted banded ironformations as a source of ultralow-velocity zones at thecore-mantle boundary[J].Nature,2005,434:371-374.
[46]Mao W L,Mao H-k,Sturhahn W,et al.Iron-richpost-perovskite and the origin of ultralow-velocityzones[J].Science,2006,312:564-565.
[47]Wen L X.Seismic evidence for a rapidly varyingcompositional anomaly at the base of the Earth s mantlebeneath the Indian Ocean[J].Earth Planet Sci Lett,2001,194:83-95.
[48]McNamara A K,Zhong S J.Thermochemicalstructures within a spherical mantle:Superplumes orpiles[J].J Geophys Res,2004,109.B07402.doi:10.1029/2003JB002847.
[49]Wang Y,Wen L X.Mapping the geometry andgeographic distribution of a very low velocity provinceat the base of the Earth s mantle[J].J Geophys Res,2004,109.B10305.doi:10.1029/2003JB002674.
[50]Ni S D,Hel mberger D V,Tromp J.Three-di mensional structure of the African superplume fromwaveform modelling[J].Geophys J Int,2005,161:283-294;doi:10.1111/j.1365-246X.2005.02508.x.
[51]Hel mberger D V,Ni S D.Approxi mate 3D body-wavesynthetics for tomographic models[J].Bulletin of theSeismological Society of America,2005,95(1):212-224.
[52]Komatitsch D,Ritsema J,Tromp J.The spectral-element method,beowulf computing,and globalseismology[J].Science,2002,298:1 737-1 742.
[53]Hung S H,Dahlen FA,Nolet G.Wavefront healing:a banana-doughnut perspective[J].Geophysical JournalInternational,2000,146(2):289-312.
[54]Zhao L,Jordan T H,Chapman C H.Three-di mensional Frechet differential kernels for seismicdelay ti mes[J].Geophysical Journal International,2000,141(3):558-576.
[55]Liu Q Y,Tromp J.Finite-frequency kernels basedupon adjoint methods[J].Bull SeismSoc Amer,2006,96:2 383-2 397.
[56]An Q,Zheng Lianqing,Fu Rongshan,et al.Solid-liquid transitions of sodium chloride at high pressures[J].J Chem Phys,2006,125,(154510):1-10.
[2]Grand S P.Mantle shear structure beneath theAmerican and surrounding oceans[J].J Geophys Res,特1994,99:11 591-11 621.
[3]Ritsema J,Heijst H J,Woodhouse J H.Complexshear wave velocity structure i maged beneath Africaand Iceland[J].Science,1999,286:1 925-1 928.
[4]Van der Hilst R D,Widiyantoro S,Engdahl E R.Evidence for deep mantle circulation from globaltomography[J].Nature,1997,386:578-584.
[5]Kuo B Y,Garnero EJ,Lay T.Tomographic inversionof S-SKS ti mes for shear velocity heterogeneity in D″:Degree 12 and hybrid models[J].Journal ofGeophysical Research-Solid Earth,2000,105(B12):28 139-28 157.
[6]Lay T,Hel mberger D V.A lower mantle S wavetriplication and the shear velocity structure of D″[J].Geophys J R Astron Soc,1983,75:799-837.
[7]Ni S D,Ding X M,Hel mberger D V,et al.Low-velocity structure beneath Africa from forwardmodeling[J].Earth and Planet Sci Lett,1999,170:497-507.
[8]Ni S D,Hel mberger D V.Probing an ultra-lowvelocity zone at the core mantle boundary with P and Swaves[J].Geophysical Research Letters,2001,28(12):2 345-2 348.
[9]Ni S D,Hel mberger D V.Seismological constraints onthe African superplume,could be the oldest structurein the lower mantle[J].EPSL,2003,206:119-131.
[10]Hedlin M A H,Shearer P M,Earle P S.Seismicevidence for small-scale heterogeneity throughout theEarth s mantle[J].Nature,1997,387(6629):145-150.
[11]Niu F L,Wen L X.Strong seismic scatterers near thecore-mantle boundary west of Mexico[J].Geophys ResLett,2001,28:3 557-3 560.
[12]Ni S D,Hel mberger D V,Tromp J.Three-di mensional structure of he African superplume fromwaveform modelling[J].Geophysical JournalInternational,2005,161(2):283-294.
[13]Lay T,Garnero E J,William Q.Partial melting in athermo-chemical boundary layer at the base of themantle[J].Physics of the Earth and PlanetaryInteriors,2004,146(3-4):441-467.
[14]Ni S D,Tan E,Gurnis M,et al.Sharp sides to theAfrican superplume[J].Science,2002,296(5574):1 850-1 852.
[15]Buffett B A.Esti mates of heat flowin the deep mantlebased on the power requirements for the geodynamo[J].Geophys Res Lett,2002,29(12).doi:10.1029/2001GL014649.
[16]Kellogg L H,Hager B H,Van der Hilst R D.Compositional stratification in the deep mantle[J].Science,1999,283:1 881-1 884.
[17]Grand S P,Van der Hilst R D,Widiyantoro S.Globalseismic tomography:A snapshot of convection in theEarth[J].GSA Today,1997,7(4):1-7.
[18]Sidorin Igor,Gurnis M,Hel mberger D V.Dynamicsof a phase change at the base of the mantle consistentwith seismological observations[J].J Geophys Res,1999,104(B7):15 005-15 023.
[19]Ni S D,Ding X M,Hel mberger D V.Constructingsynthetics from deep earth tomographic models[J].Geophysical Journal International,2000,140(1):71-82.
[20]Wysession M E,Lay T,Revenaugh J,et al.The D″Discontinuity and its I mplications[M]//The Core-Mantle Boundary Region.Washington:AGU Press,1998:273-297.
[21]Murakami M.Phase transition of lower mantle mineralandits geophysical i mplications[D].Tokyo Institute ofTechnology,2004.
[22]Murakami M,Hirose K,Kawamura K,et al.Post-perovskite phase transition in MgSi O3[J].Science,2004,304:855-858.
[23]Oganov A R,Ono S.Theoretical and experi mentalevidence for a post-perovskite phase of MgSi O3inEarth s D″layer[J].Nature,2004,430:445-448.
[24]Shi m S H,Duffy T S,Jeanloz R,et al.Stability andcrystal structure of MgSi O3perovskite to the core-mantle boundary[J].Geophys Res Lett,2004,31.L10603.doi:10.1029/2004GL019639.
[25]Iitaka T,Hirose K,Kawamura K,et al.The elasticityof the MgSi O3post-perovskite phase in the Earth slowermost mantle[J].Nature,2004,430:442-445.
[26]Lay T,Heinz D,Ishii M,et al.Multi-displinaryi mpact of the deep mantle post-perovskite phasetransition[J].EOS,2005,86(1):1-5.
[27]Garnero E J,Maupin V,Lay T,et al.Variableazi muthal anisotropy in Earth s lowermost mantle[J].Science,2004,306(5694):259-261.
[28]Garnero EJ,Hel mberger D V.Avery slowbasal layerunderlying large-scale low-velocity anomalies in thelower mantle beneath the Pacific:evidence from corephases[J].Phys Earth&Planet Inter,1995,91:161-176.
[29]Garnero E J,Hel mberger D V.Seismic detection of athin laterally varying boundary layer at the base of themantle beneath the central-Pacific[J].Geophys ResLett,1996,23:977-980.
[30]Wen L X,Hel mberger D V.Atwo-di mensional P-SVhybrid method and its application to modeling localizedstructures near the core-mantle boundary[J].Journalof Geophysical Research-Solid Earth,1998,103(B8):特约评述17 901-17 918.
[31]Hel mberger D V,Ni S D,Wen L X,et al.Seismicevidence for ultra lowvelocity zones beneath Africa andeastern Atlantic[J].Journal of Geophysical Research,2000,105:23 865-23 878.
[32]Garnero E J,MS Thorne,McNamara A,et al.Fine-scale ultra-lowvelocity zone layering at the core-mantleboundary and superplumes[M]//SuperplumeMonograph.Springer,2007(in press).
[33]Thorne M S,Garnero E J,Grand S.Geographiccorrelation between hot spots and deep mantle lateralshear-wave velocity gradients[J].Phys Earth PlanetInter,2004,146:47-63.
[34]Wen L X,Hel mberger D V.Ultra-low velocity zonesnear the core-mantle boundary from broadband PKPprecursors[J].Science,1998,279(5357):1 701-1 703.
[35]Avants M,Lay T,Garnero E J.Determining shearvelocity structure of the ULVZ beneath the centralPacific using stacked ScS data[J].Geophys Res Lett,2006,33.L07314.doi:10.1029/2005GL024989.
[36]Luo S N,Ni S D,Hel mberger D V.Evidence for asharp lateral variation of velocity at the core-mantleboundary from multipathed PKPab[J].Earth andPlanetary Science Letters,2001,189(3-4):155-164.
[37]Mori J,Hel mberger D V.Localized boundary layerbelow the mid-Pacific velocity anomaly from a PcPprecursor[J].J Geophys Res,1995,100:20 359-20 365.
[38]Garnero E J,Vidale J.ScP:a probe of ultralow-velocity zones at the base of the mantle[J].GeophysRes Lett,1999,26:377-380.
[39]Rost S,Revenaugh J.Small-scale ultra-low velocityzone structure i maged by ScP[J].J Geophys Res,2003,108(B1).2 056.doi:10.1029/2001JB001627.
[40]Rost S,Garnero E J,Williams Q,et al.Seismicconstraints on a possible plume root at the core-mantleboundary.Nature,2005,435:666-669.
[41]McSweeney T J,Creager K C,R T Merrill.Depthextent of inner-core seismic anisotropy andi mplicationsfor geomagnetism.Phys Earth Planet Inter,1997,101:131-156.
[42]Rost S,Garnero EJ.Detection of an ultra-lowvelocityzone at the CMB using diffracted PKKPab waves[J].JGeophys Res,2006,111.B07309,doi:10.1029/2005JB003850.
[43]Williams Q,Garnero E J.Seismic evidence for partialmelt at the base of Earth s mantle[J].Science,1996,273:1 528-1 530.
[44]Buffett B A,Garnero EJ,Jeanloz R.Sedi ments at thetop of Earth s core[J].Science,2000,290:1 338-1 342.
[45]Dobson D P,Brodholt J P.Subducted banded ironformations as a source of ultralow-velocity zones at thecore-mantle boundary[J].Nature,2005,434:371-374.
[46]Mao W L,Mao H-k,Sturhahn W,et al.Iron-richpost-perovskite and the origin of ultralow-velocityzones[J].Science,2006,312:564-565.
[47]Wen L X.Seismic evidence for a rapidly varyingcompositional anomaly at the base of the Earth s mantlebeneath the Indian Ocean[J].Earth Planet Sci Lett,2001,194:83-95.
[48]McNamara A K,Zhong S J.Thermochemicalstructures within a spherical mantle:Superplumes orpiles[J].J Geophys Res,2004,109.B07402.doi:10.1029/2003JB002847.
[49]Wang Y,Wen L X.Mapping the geometry andgeographic distribution of a very low velocity provinceat the base of the Earth s mantle[J].J Geophys Res,2004,109.B10305.doi:10.1029/2003JB002674.
[50]Ni S D,Hel mberger D V,Tromp J.Three-di mensional structure of the African superplume fromwaveform modelling[J].Geophys J Int,2005,161:283-294;doi:10.1111/j.1365-246X.2005.02508.x.
[51]Hel mberger D V,Ni S D.Approxi mate 3D body-wavesynthetics for tomographic models[J].Bulletin of theSeismological Society of America,2005,95(1):212-224.
[52]Komatitsch D,Ritsema J,Tromp J.The spectral-element method,beowulf computing,and globalseismology[J].Science,2002,298:1 737-1 742.
[53]Hung S H,Dahlen FA,Nolet G.Wavefront healing:a banana-doughnut perspective[J].Geophysical JournalInternational,2000,146(2):289-312.
[54]Zhao L,Jordan T H,Chapman C H.Three-di mensional Frechet differential kernels for seismicdelay ti mes[J].Geophysical Journal International,2000,141(3):558-576.
[55]Liu Q Y,Tromp J.Finite-frequency kernels basedupon adjoint methods[J].Bull SeismSoc Amer,2006,96:2 383-2 397.
[56]An Q,Zheng Lianqing,Fu Rongshan,et al.Solid-liquid transitions of sodium chloride at high pressures[J].J Chem Phys,2006,125,(154510):1-10.
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