精确测量的地幔体波走时及地幔非均匀性的强度(英文)
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摘要
用波形相关法精确地测定了在世界各地发生的87个6级以上地震的P波, PP波和Pdiff波的503个走时数据。记录这些地震波形的是新建于西太平洋地区的海洋半球地震观测网。我们利用这些高精度的走时数据研究了地幔体波的走时残差的范围及地幔非均匀性的强度。结果表明,P波、PP波和Pdiff波的走时残差最大分别为9 s ,11 s和15 s ,这为地幔层析成像反演中应该使用的体波走时残差数据的范围提供了重要信息。超出这一范围的走时残差数据不应该用于反演中,以免歪曲成像结果。我们发现,当震中距小于40°时,P波走时残差的范围为-6到+9 s。而对于40°到99°之间的震中距,P波走时残差的范围为-3到+5s。由于震中距越大,P波穿透地幔越深,我们这一结果提供了直接和确凿的证据,表明上地幔和地幔转换带中的横向非均匀性的强度要远胜于下地幔。我们精确测量的Pdiff波的走时数据表明,在地幔底部存在显著的低速异常,可能与地幔热柱或者超级地幔柱有关。我们使用了一个最新的三维全球层析成像模型来解释这些体波走时数据的空间变化。
We have measured 503 arrival times of P, PP, and Pdiff waves from seismograms of 87 global earthquakes with magnitudes≥M_w 6.0,as recorded by the newly installed Ocean Hemisphere Project Seismic Network located in the western Pacific region.These mantle body wave data are used to investigate the range of variations in the travel time residuals which indicate the degree of mantle heterogeneity.Our results show that variations of the residuals amount to 9 s for P data, 11 s for PP data, and 15 s for Pdiff data.These findings provide useful information on the maximum travel time residuals for the mantle body waves and on the criteria of using travel times for tomographic inversions.Thus outliers in a data set which exceed the plausible range of travel time residuals can be excluded to avoid biasing the inversion result.Our P-wave data show a large range of travel time residuals (-6 to +9 s) for epicentral distances shorter than 40 degrees and a smaller range of residuals (-3 to +5 s) for longer epicentral distances.Because P rays with a larger epicentral distance penetrate deeper in the mantle,our results provide sound and direct evidence for the inference that the upper mantle and transition zone have much stronger heterogeneities than the bulk of the lower mantle.Our Pdiff data show the existence of significant slow velocity anomalies in the D″ layer, which may be associated with the generation of plumes and superplumes.A new global tomographic model is employed to interpret the spatial variations of travel time residuals of the observed P, PP and Pdiff data.
We have measured 503 arrival times of P, PP, and Pdiff waves from seismograms of 87 global earthquakes with magnitudes≥M_w 6.0,as recorded by the newly installed Ocean Hemisphere Project Seismic Network located in the western Pacific region.These mantle body wave data are used to investigate the range of variations in the travel time residuals which indicate the degree of mantle heterogeneity.Our results show that variations of the residuals amount to 9 s for P data, 11 s for PP data, and 15 s for Pdiff data.These findings provide useful information on the maximum travel time residuals for the mantle body waves and on the criteria of using travel times for tomographic inversions.Thus outliers in a data set which exceed the plausible range of travel time residuals can be excluded to avoid biasing the inversion result.Our P-wave data show a large range of travel time residuals (-6 to +9 s) for epicentral distances shorter than 40 degrees and a smaller range of residuals (-3 to +5 s) for longer epicentral distances.Because P rays with a larger epicentral distance penetrate deeper in the mantle,our results provide sound and direct evidence for the inference that the upper mantle and transition zone have much stronger heterogeneities than the bulk of the lower mantle.Our Pdiff data show the existence of significant slow velocity anomalies in the D″ layer, which may be associated with the generation of plumes and superplumes.A new global tomographic model is employed to interpret the spatial variations of travel time residuals of the observed P, PP and Pdiff data.
引文
[ 1 ] WOODWARD R, MASTER G.Lower mantle structure from ScS-S differential travel ti mes[J] .Nature , 1991 , 352 : 231- 233 .
[ 2 ] GRAND S.Mantle shear structure beneath the Americas and surrounding oceans [J] .Journal of Geophysical Research, 1994 ,99 :11591-11621 .
[ 3 ] SU W, WOODWARD R,DZIEWONSKI A.Degree 12 model of shear velocity heterogeneity in the mantle[J] .Journal of Geophysical Research,1994 ,99 :6945-6980 .
[ 4 ] I NOUE H, FUKAO Y, TANABE K,et al .Whole mantle P wave travel ti me tomography[J] .Physics of the Earth and Planetary Interiors ,1990 ,59 :294-328 .
[ 5 ] ZHOU H.A high-resolution P wave model for the top 1 200 kmof the mantle[J] .Journal of Geophysical Research,1996 , 101 :27791-27810 .
[ 6 ] BIJ WAARD H, SPAKMAN W, ENGDAHL E.Closing the gap between regional and global travel ti me tomography[J] . Journal of Geophysical Research,1998 ,103 :30055-30078 .
[ 7 ] BOSCHI L, DZIEWONSKI A.High-andlow-resolutioni ma- ges of the Earth s mantle :i mplications of different approa- ches to tomographic modeling[J] .Journal of Geophysical Re- search,1999 ,104 :25567-25594 .
[ 8 ] ZHAO D.Seismic structure and origin of hotpots and mantle plumes[J] .Earth Planetary Science Letters ,2001 ,192 :251- 265 .
[ 9 ] ZHAO D.Global tomographic i mages of mantle plumes and subducting slabs :insight into deep Earth dynamics[J] .Phys- ics of the Earth and Planetary Interiors ,2004 ,146 :3-34 .
[10] ENGDAHL E, van der HILST R, BULAND R.Global teleseismic earthquake relocation with i mproved travel ti mes and procedures for depth determination[J] .Bulletin of Seis- mological Society of America ,1998 ,88 :722-743 .
[11] GRAND S, van der HILST R, WIDIYANTORO S.Global seismic tomography : a snapshot of convection in the Earth [J] .GSA Today ,1997 ,7 :1-7 .
[12] MORELLI A, DZIEWONSKI A.The harmonic expansion approach to the retrieval of deep Earth structure[ M]// NO- LET G.Seismic tomography . Dordrecht :Reidel Publishing Company ,1987 :251-274 .
[13] GRAND S.A possible station bias in travel ti me measure- ments reported to ISC[J] .Geophysical Research Letters , 1990 ,17 :17-20 .
[14] ZHAO D, HASEGAWA A, HORI UCHI S.Tomographic i maging of Pand S wave velocity structure beneath northeast- ern Japan[J] .Journal of Geophysical Research, 1992 , 97 : 19909-19928 .
[15] ZHAO D, XU Y, WIENS D,et al .Depth extent of the Lau back-arc spreading center and its relation to subduction processes[J] .Science ,1997 ,278 :254-257 .
[16] LAY T, WILLI AMS Q, GARNERO E.The core-mantle boundary layer and deep Earth dynamics[J] .Nature , 1998 , 392 :461-468 .
[17] YAMADA A, NAKANISHI I .Short-wavelengthlateral vari- ation of a D″P-wave reflector beneaththe southwestern Pacif- ic[J] .Geophysical Research Letters ,1998 ,25 :4545-4548 .
[18] GARNERO E.Heterogeneity of the lowermost mantle[J] . Annual Review of Earth and Planetary Sciences , 2000 , 28 : 509-537 .
[19] KANAZAWA T,SHI NOHARA M,SHIOBARA H,et al . Broad-band seismic network of the Ocean Hemisphere Project [J] .Earth Monthly ,2001 ,23 :18-26 .
[20] KENNETT B, ENGDAHL E.Travel ti mes for global earth- quake location and phase identification[J] .Geophysical Jour- nal International ,1991 ,105 :429-465 .
[21] DARRAGH R.Mapping of upper-mantle structure from dif- ferential PP-Ptravel ti me residuals[J] .Physics of the Earth and Planetary Interiors ,1985 ,41 :6-17 .
[22] WYSESSION M, OKAL E, BI NA C.The structure of the core-mantle boundary from diffracted waves[J] .Journal of Geophysical Research,1992 ,97 :8749-8764 .
[23] PAULSSEN H,STUTZMANN E.On PP-P differential trav- el ti me measurements [J] . Geophysical Research Letters , 1996 ,23 :1833-1836 .
[24] VALENZUELA R, WYSESSION M.Illuminatingthe base of the mantle with diffracted waves[ M]// GURNIS M, WY- SESSION ME, KNITTLE E,et al .The core-mantle bounda- ry . Washington DC: American Geophysical Union , 1998 : 57-71 .
[25] ZHAO D, HASEGAWA A, KANAMORI H.Deep structure of Japan subduction zone as derived fromlocal ,regional and teleseismic events [ J ] .Journal of Geophysical Research,1994 ,99 :22313-22329 .
[26] TAJI MA F, FUKAO Y, OBAYASHI M,et al .Evaluation of slab i mages in the northwestern Pacific[J] .Earth Planets Space ,1998 ,50 :953-964 .
[27] TURCOTTE D,SCHUBERT G.Geodynamics[ M] .[S.l .] : John Wiley and Sons Press ,1982 :1-450 .
[28] TATSUMI Y, MARUYAMA S, HONDA S.Mechanism of backarc opening in the Japan Sea :role of asthenospheric in- jection[J] .Tectonophysics ,1990 ,181 :299-306 .
[29] ZHAO D,LEI J , TANG R.Origin of the Changbai intraplate volcanismin Northeast China : evidence from seismic tomo- graphy[J] .Chinese Science Bulletin ,2004 ,49 :1401-1408 .
[30] LEI J , ZHAO D.P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia[J] .Tectono- physics ,2005 ,397 :281-295 .
[31] HUNG S, DAHLEN F, NOLET G.Frechet kernals for fi- nite-frequency travel ti mes , II .Examples [J] .Geophysical Journal International ,2000 ,141 :175-203 .
[32] MARUYAMA S.Plume tectonics[J] .Journal of Geological Society of Japan ,1994 ,100 :24-49 .
[33] MCNUTT M.Superswells[J] .Reviews of Geophysics ,1998 , 36 :211-244 .
[34] TANAKA S.Very lowshear wave velocity at the base of the mantle under the South Pacific Superswell[J] .Earth Planeta- ry Science Letters ,2002 ,203 :879-893 .
[35] TANI MOTO T.Predominance of large-scale heterogeneity and the shift of velocity anomalies between the upper and lower mantle[J] .Journal of Physics of Earth,1990 ,38 :493- 509 .
[36] FUKAO Y, WIDIYANTORO S, OBAYASHI M.Stagnant slabs in the upper and lower mantle transition region[J] .Re- views of Geophysics ,2001 ,39 :291-323 .
[37] GORBATOV A, KENNETT B.Joint bulk-sound and shear tomography for western Pacific subduction zone[J] .Earth Planetary Science Letters ,2003 ,210 :527-543 .
[38] ZHAO D, LEI J .Seismic ray path variations in a 3-D global velocity model[J] .Physics of the Earth and Planetary Interi- ors ,2004 ,141 :153-166 .
[ 2 ] GRAND S.Mantle shear structure beneath the Americas and surrounding oceans [J] .Journal of Geophysical Research, 1994 ,99 :11591-11621 .
[ 3 ] SU W, WOODWARD R,DZIEWONSKI A.Degree 12 model of shear velocity heterogeneity in the mantle[J] .Journal of Geophysical Research,1994 ,99 :6945-6980 .
[ 4 ] I NOUE H, FUKAO Y, TANABE K,et al .Whole mantle P wave travel ti me tomography[J] .Physics of the Earth and Planetary Interiors ,1990 ,59 :294-328 .
[ 5 ] ZHOU H.A high-resolution P wave model for the top 1 200 kmof the mantle[J] .Journal of Geophysical Research,1996 , 101 :27791-27810 .
[ 6 ] BIJ WAARD H, SPAKMAN W, ENGDAHL E.Closing the gap between regional and global travel ti me tomography[J] . Journal of Geophysical Research,1998 ,103 :30055-30078 .
[ 7 ] BOSCHI L, DZIEWONSKI A.High-andlow-resolutioni ma- ges of the Earth s mantle :i mplications of different approa- ches to tomographic modeling[J] .Journal of Geophysical Re- search,1999 ,104 :25567-25594 .
[ 8 ] ZHAO D.Seismic structure and origin of hotpots and mantle plumes[J] .Earth Planetary Science Letters ,2001 ,192 :251- 265 .
[ 9 ] ZHAO D.Global tomographic i mages of mantle plumes and subducting slabs :insight into deep Earth dynamics[J] .Phys- ics of the Earth and Planetary Interiors ,2004 ,146 :3-34 .
[10] ENGDAHL E, van der HILST R, BULAND R.Global teleseismic earthquake relocation with i mproved travel ti mes and procedures for depth determination[J] .Bulletin of Seis- mological Society of America ,1998 ,88 :722-743 .
[11] GRAND S, van der HILST R, WIDIYANTORO S.Global seismic tomography : a snapshot of convection in the Earth [J] .GSA Today ,1997 ,7 :1-7 .
[12] MORELLI A, DZIEWONSKI A.The harmonic expansion approach to the retrieval of deep Earth structure[ M]// NO- LET G.Seismic tomography . Dordrecht :Reidel Publishing Company ,1987 :251-274 .
[13] GRAND S.A possible station bias in travel ti me measure- ments reported to ISC[J] .Geophysical Research Letters , 1990 ,17 :17-20 .
[14] ZHAO D, HASEGAWA A, HORI UCHI S.Tomographic i maging of Pand S wave velocity structure beneath northeast- ern Japan[J] .Journal of Geophysical Research, 1992 , 97 : 19909-19928 .
[15] ZHAO D, XU Y, WIENS D,et al .Depth extent of the Lau back-arc spreading center and its relation to subduction processes[J] .Science ,1997 ,278 :254-257 .
[16] LAY T, WILLI AMS Q, GARNERO E.The core-mantle boundary layer and deep Earth dynamics[J] .Nature , 1998 , 392 :461-468 .
[17] YAMADA A, NAKANISHI I .Short-wavelengthlateral vari- ation of a D″P-wave reflector beneaththe southwestern Pacif- ic[J] .Geophysical Research Letters ,1998 ,25 :4545-4548 .
[18] GARNERO E.Heterogeneity of the lowermost mantle[J] . Annual Review of Earth and Planetary Sciences , 2000 , 28 : 509-537 .
[19] KANAZAWA T,SHI NOHARA M,SHIOBARA H,et al . Broad-band seismic network of the Ocean Hemisphere Project [J] .Earth Monthly ,2001 ,23 :18-26 .
[20] KENNETT B, ENGDAHL E.Travel ti mes for global earth- quake location and phase identification[J] .Geophysical Jour- nal International ,1991 ,105 :429-465 .
[21] DARRAGH R.Mapping of upper-mantle structure from dif- ferential PP-Ptravel ti me residuals[J] .Physics of the Earth and Planetary Interiors ,1985 ,41 :6-17 .
[22] WYSESSION M, OKAL E, BI NA C.The structure of the core-mantle boundary from diffracted waves[J] .Journal of Geophysical Research,1992 ,97 :8749-8764 .
[23] PAULSSEN H,STUTZMANN E.On PP-P differential trav- el ti me measurements [J] . Geophysical Research Letters , 1996 ,23 :1833-1836 .
[24] VALENZUELA R, WYSESSION M.Illuminatingthe base of the mantle with diffracted waves[ M]// GURNIS M, WY- SESSION ME, KNITTLE E,et al .The core-mantle bounda- ry . Washington DC: American Geophysical Union , 1998 : 57-71 .
[25] ZHAO D, HASEGAWA A, KANAMORI H.Deep structure of Japan subduction zone as derived fromlocal ,regional and teleseismic events [ J ] .Journal of Geophysical Research,1994 ,99 :22313-22329 .
[26] TAJI MA F, FUKAO Y, OBAYASHI M,et al .Evaluation of slab i mages in the northwestern Pacific[J] .Earth Planets Space ,1998 ,50 :953-964 .
[27] TURCOTTE D,SCHUBERT G.Geodynamics[ M] .[S.l .] : John Wiley and Sons Press ,1982 :1-450 .
[28] TATSUMI Y, MARUYAMA S, HONDA S.Mechanism of backarc opening in the Japan Sea :role of asthenospheric in- jection[J] .Tectonophysics ,1990 ,181 :299-306 .
[29] ZHAO D,LEI J , TANG R.Origin of the Changbai intraplate volcanismin Northeast China : evidence from seismic tomo- graphy[J] .Chinese Science Bulletin ,2004 ,49 :1401-1408 .
[30] LEI J , ZHAO D.P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia[J] .Tectono- physics ,2005 ,397 :281-295 .
[31] HUNG S, DAHLEN F, NOLET G.Frechet kernals for fi- nite-frequency travel ti mes , II .Examples [J] .Geophysical Journal International ,2000 ,141 :175-203 .
[32] MARUYAMA S.Plume tectonics[J] .Journal of Geological Society of Japan ,1994 ,100 :24-49 .
[33] MCNUTT M.Superswells[J] .Reviews of Geophysics ,1998 , 36 :211-244 .
[34] TANAKA S.Very lowshear wave velocity at the base of the mantle under the South Pacific Superswell[J] .Earth Planeta- ry Science Letters ,2002 ,203 :879-893 .
[35] TANI MOTO T.Predominance of large-scale heterogeneity and the shift of velocity anomalies between the upper and lower mantle[J] .Journal of Physics of Earth,1990 ,38 :493- 509 .
[36] FUKAO Y, WIDIYANTORO S, OBAYASHI M.Stagnant slabs in the upper and lower mantle transition region[J] .Re- views of Geophysics ,2001 ,39 :291-323 .
[37] GORBATOV A, KENNETT B.Joint bulk-sound and shear tomography for western Pacific subduction zone[J] .Earth Planetary Science Letters ,2003 ,210 :527-543 .
[38] ZHAO D, LEI J .Seismic ray path variations in a 3-D global velocity model[J] .Physics of the Earth and Planetary Interi- ors ,2004 ,141 :153-166 .
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