利用中国地震台网(CSN)记录到的PKP波研究中太平洋下地幔底部的小尺度P波速度变化
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摘要
PKP震相包含了下地幔底部P波速度结构的重要信息.中国地震台网(CSN)台站记录到的南美洲地震的PKP波的射线,对中太平洋下的D″层有很好的采样.本研究采用这些PKP波的AB和DF两个分支的走时差,研究了中太平洋下地幔底部P波速度的小尺度变化.AB-DF的走时差减小了上地幔横向不均匀性的影响,而对下地幔底部P波速度的横向变化十分敏感.与此同时,AB-DF的走时差也减小了地震定位误差的影响,消除地震发震时间测定误差的影响.本研究的结果表明,在中太平洋的地幔底部存在着大范围的AB-DF走时正残差,也即低速异常区,这可能是太平洋下超大地幔热柱的源处.观测到的P波速度异常的空间分布总体上与Grand通过层析成像得到的CMB的S波的速度异常相一致,并在变化的幅度上很好地相关,P波速度的扰动值(在D″层大约为2%)是Grand速度模型中的S波异常的36%.这一结果有助于太平洋下超大地幔热柱的结构和性质的进一步研究.
Seismic PKP arrivals are sensitive to P velocity structure in the lowermost mantle.Earthquakes in South America recorded at China Seismographic Network(CSN) provide excellent samples of the lowermost mantle underneath the Central Pacific.Here we use differential travels between the AB and DF branches of PKP waves to constrain the small-scale variation of P waves beneath the Central Pacific.The use of differential AB-DF times reduces the biases of errors of earthquake locations and origin time and heterogeneity of the upper mantle but is sensitive to the structure in the lowermost mantle.Our results show large positive AB-DF differential time residuals across large areas of the lowermost mantle beneath the central Pacific,which may indicate the source region of the "Pacific Super Plume".The observed PKP residuals consist with the predictions for Grand's tomographic S model.Our inferred P velocity variation(which is about 2% in the bottommost 200 km D″ layer) is about 36% of the S velocity variation.However,we observe significant variations in travel-time residuals both inside the Central Pacific Plume and near its boundaries.The results have implications for understanding the structure and nature of the Pacific Super Plume.
Seismic PKP arrivals are sensitive to P velocity structure in the lowermost mantle.Earthquakes in South America recorded at China Seismographic Network(CSN) provide excellent samples of the lowermost mantle underneath the Central Pacific.Here we use differential travels between the AB and DF branches of PKP waves to constrain the small-scale variation of P waves beneath the Central Pacific.The use of differential AB-DF times reduces the biases of errors of earthquake locations and origin time and heterogeneity of the upper mantle but is sensitive to the structure in the lowermost mantle.Our results show large positive AB-DF differential time residuals across large areas of the lowermost mantle beneath the central Pacific,which may indicate the source region of the "Pacific Super Plume".The observed PKP residuals consist with the predictions for Grand's tomographic S model.Our inferred P velocity variation(which is about 2% in the bottommost 200 km D″ layer) is about 36% of the S velocity variation.However,we observe significant variations in travel-time residuals both inside the Central Pacific Plume and near its boundaries.The results have implications for understanding the structure and nature of the Pacific Super Plume.
引文
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[15]Lay T,Helmberger D V.A lower mantle S wave triplication and theshear velocity structure of D″.Geophys.J.R.Astron.Soc.,1983,75:799-837
[16]Garnero E J,Grand S P,Helmberger D V.Low P-wave velocity atthe base of the mantle.Geophysics Research Letters,1993,20(17):1843-1846
[17]Wen L X.Seismic evidence for a rapidly varying compositionalanomaly at the base of the Earth’s mantle beneath the Indian Ocean.Earth and Planetary Science Letters,2001,194(1-2):83-95
[18]Ni S D,Tan E,Gurnis M,Helmberger D.Sharp sides to the Africansuperplume.Science,2002,296(5574):1850-1852
[19]Song X D.Anisotropy of the Earth’s inner core.Rev.Geophys.,1997,35:297-313
[20]Song X D,Helmberger D V.Effect of velocity structure in D″on PKPphase.Geophys.Res.Lett.,1993,20:285-288
[21]Song X D,Helmberger D V.PKP differential travel times:Implications from three-dimensional lower mantle structure.Geophys.Res.Lett.,1997,24:1863-1866
[22]Sun X L,Song X D.PKP travel times at near antipodal distances:implications for inner core anisotropy and lowermost mantle structure.Earth and Planetary Science Letters,2002,199:429-445
[23]Dziewonski A M,Anderson D L.Preliminary reference earth model.Physics of the Earth and Planetary Interiors,1981,25(4):297-356
[24]Wessel P,Smith W H F.New improved version of Generic MappingTools released.EOS Trans.Amer.Geophys.U.,1998,79(47):579
[2]Wysession M E,Lay T,Revenaugh J,et al.The D″discontinuityand implications.In:Gurnis M,Wysession M E,Knittle E eds.TheCore-Mantle Boundary.AGU,Washington,D.C.1998
[3]Garnero E J.Heterogeneity of lowermost mantle.Ann.Rev.EarthPlanet.Sci.,2000,28:509-537
[4]Garnero E J.A new paradigm for Earth’s core-mantle boundary.Science,2004,304(5672):834-836
[5]Liu X F,Dziewonski A M.Global analysis of shear wave velocityanomalies in the lower-most mantle.In:Gurnis M,Wysession M E,Knittle E eds.The Core-Mantle Boundary.AGU,Washington,D.C.1998
[6]Masters G,Johnson S,Laske G,Bolton H.A shear-velocity modelof the mantle.Philos.Trans.R.Soc.Lond.A.,1996,354:1385-1411
[7]Grand S P,Van der Hilst R D,Widiyantoro S.Global seismictomography:a snapshot of convection in the Earth.GSA Today,1997,7:1-7
[8]Van der Hilst R D,Karason H.Compositional heterogeneity in thebottom 1000 kilometers of Earth’s mantle:toward a hybrid convectionmodel.Science,1999,283:1885-1888
[9]Ritsema J,Van Heijst H J,Woodhouse J H.Complex shear wavevelocity structure imaged beneath Africa and Iceland.Science,1999,286(5446):1925-1928
[10]Kuo B Y,Garnero E J,Lay T.Tomographic inversion of S-SKStimes for shear velocity heterogeneity in D”:Degree 12 and hybridmodels.J.Geophys.Res.,2000,105:28139-28157
[11]Bijwaad H,Spakman W.Non-linear global P-wave tomography byiterated linearized inversion.Geophysics Journal International,2000,141(1):71-82
[12]Romanowicz B.Global mantle tomography:Progress status in the past10 years.Ann.Rev.Earth Planet.Sci.,2003,31:303-328
[13]Zhao D P.Global tomographic images of mantle plumes andsubducting slabs:insight into deep Earth dynamics.Physics of theEarth and Planetary Interiors,2004,146(1-2):3-34
[14]Dziewonski A M.Mapping the lower mantle—Determination of lateralheterogeneity in P velocity up to degree and order 6.J.Geophys.Res.,1984,89:5929-5952
[15]Lay T,Helmberger D V.A lower mantle S wave triplication and theshear velocity structure of D″.Geophys.J.R.Astron.Soc.,1983,75:799-837
[16]Garnero E J,Grand S P,Helmberger D V.Low P-wave velocity atthe base of the mantle.Geophysics Research Letters,1993,20(17):1843-1846
[17]Wen L X.Seismic evidence for a rapidly varying compositionalanomaly at the base of the Earth’s mantle beneath the Indian Ocean.Earth and Planetary Science Letters,2001,194(1-2):83-95
[18]Ni S D,Tan E,Gurnis M,Helmberger D.Sharp sides to the Africansuperplume.Science,2002,296(5574):1850-1852
[19]Song X D.Anisotropy of the Earth’s inner core.Rev.Geophys.,1997,35:297-313
[20]Song X D,Helmberger D V.Effect of velocity structure in D″on PKPphase.Geophys.Res.Lett.,1993,20:285-288
[21]Song X D,Helmberger D V.PKP differential travel times:Implications from three-dimensional lower mantle structure.Geophys.Res.Lett.,1997,24:1863-1866
[22]Sun X L,Song X D.PKP travel times at near antipodal distances:implications for inner core anisotropy and lowermost mantle structure.Earth and Planetary Science Letters,2002,199:429-445
[23]Dziewonski A M,Anderson D L.Preliminary reference earth model.Physics of the Earth and Planetary Interiors,1981,25(4):297-356
[24]Wessel P,Smith W H F.New improved version of Generic MappingTools released.EOS Trans.Amer.Geophys.U.,1998,79(47):579
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