基于全球小口径地震台阵的PcP和PKiKP震相研究
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摘要
由地球核幔边界(CMB)反射的PcP和由内外核边界(ICB)反射的PKiKP震相可以提供关于这两个界面物理性质最直接的信息,如密度和波速的变化,而这些信息对了解地球的核幔动力学过程具有重要的意义.结合PcP和PKiKP来研究CMB性质也比仅利用P和PcP更有优势,因为他们在上地幔的传播路径的相似性可以很好地消除上地幔结构对观测的影响.同时,短周期PcP和PKiKP对小尺度结构具有较高的分辨能力,可以其物性提供更好的约束.然而,由于CMB和ICB的P波反射系数较小,PcP和PKiKP信号与其他主要震相相比很弱,且容易受到其他信号干扰(如小震中距下PcP受P波尾波的影响),通常很难在单个地震记录上同时观测到它们.对于PKiKP,其振幅往往接近于噪声级别,提取该信号常常需要利用到台阵叠加技术.本文利用全球现有的小口径IMS台阵对这两个震相数据进行识取,并对两个震相的全球可观测性进行了比较和分析.在识别弱震相PKiKP的过程中使用相加权技术来增强信号.由于台间距仅有数千米,小口径台阵显著地增加了这两个震相观测的可靠性,同时由于加入多台数据,可以更好估计PKiKP/PcP振幅比等观测的不确定性.本文对前人关于CMB和ICB的研究工作以及该领域最新的研究进展进行了总结,并且对今后利用小口径台阵记录的PcP和PKiKP震相数据研究CMB和ICB结构进行了探讨(如CMB界面的起伏和其上方的超低速层,ICB随时间的变化等).
PcP and PKiKP that reflect from the Earth's core-mantle boundary and inner-outer core boundary respectively could provide most direct information about physical properties of the two interface in the Earth's deep interior,for example,density and velocity jumps across the boundary. Such information is critical for us to understand the core-mantle dynamics. Combining PcP and PKiKP to study CMB properties also has advantage over the combination of P and PcP,because ray paths of PcP and PKiKP are much more similar in the upper mantle than those of P and PcP,and effects of upper mantle heterogeneity can be well eliminated. However,due to the small reflection coefficients of PcP and PKiKP,they are usually weaker than other main seismic phases,and it's often difficult to observe both of them in a single seismograph. Especially for PKiKP,its amplitude is often close to noise level,and stacking techniques are usually required to identify the signal. During recent few decades,the CMB and ICB have been found not simple,and many seismic features related to them has been detected. For example,topography variation and strong heterogeneity near the boundary( mushy zone near the ICB and ultra-low velocity zone above the CMB). Short period PcP and PKiKP are very sensitive to those structure,and can provide good resolution for them. In this study,data of the two phases from global small aperture array are collected,and the global observability of the two phases are compared and analyzed. PhaseWeighted-Stacking( PWS) technique is used to identify the weak PKiKP signal. The small aperture array( inter-station distance of only few kilometers) significantly improves the reliability of the observation, and due to multi-station data of the array, the uncertainties of observation,for example,PKiKP/PcP amplitude ratio could be better estimated. After summarizing previous works,this study also discusses potential prospect of using this phases pair to study CMB and ICB structure,and proposes several applications( e. g.,ULVZ,CMB topography and temporary change of ICB).
PcP and PKiKP that reflect from the Earth's core-mantle boundary and inner-outer core boundary respectively could provide most direct information about physical properties of the two interface in the Earth's deep interior,for example,density and velocity jumps across the boundary. Such information is critical for us to understand the core-mantle dynamics. Combining PcP and PKiKP to study CMB properties also has advantage over the combination of P and PcP,because ray paths of PcP and PKiKP are much more similar in the upper mantle than those of P and PcP,and effects of upper mantle heterogeneity can be well eliminated. However,due to the small reflection coefficients of PcP and PKiKP,they are usually weaker than other main seismic phases,and it's often difficult to observe both of them in a single seismograph. Especially for PKiKP,its amplitude is often close to noise level,and stacking techniques are usually required to identify the signal. During recent few decades,the CMB and ICB have been found not simple,and many seismic features related to them has been detected. For example,topography variation and strong heterogeneity near the boundary( mushy zone near the ICB and ultra-low velocity zone above the CMB). Short period PcP and PKiKP are very sensitive to those structure,and can provide good resolution for them. In this study,data of the two phases from global small aperture array are collected,and the global observability of the two phases are compared and analyzed. PhaseWeighted-Stacking( PWS) technique is used to identify the weak PKiKP signal. The small aperture array( inter-station distance of only few kilometers) significantly improves the reliability of the observation, and due to multi-station data of the array, the uncertainties of observation,for example,PKiKP/PcP amplitude ratio could be better estimated. After summarizing previous works,this study also discusses potential prospect of using this phases pair to study CMB and ICB structure,and proposes several applications( e. g.,ULVZ,CMB topography and temporary change of ICB).
引文
Bolt B A,Qamar A.1970.Upper bound to the density jump at the boundary of the Earth's inner core[J].Nature,228(5267):148-50.
Colombi A,Nissen-Meyer T,Boschi L,et al.2014.Seismic waveform inversion for core-mantle boundary topography[J].Geophysical Journal International,198(1):55-71,doi:10.1093/gji/ggu112.
Deuss A,Woodhouse J H,Paulssen H,et al.2000.The observation of inner core shear waves[J].Geophysical Journal International,142(1):67-73,doi:10.1046/j.1365-246X.2000.00147.x.
Dziewonski A M,Anderson D L.1981.Preliminary reference Earth model[J].Physics of the Earth and Planetary Interiors,25(4):297-356,doi:10.1016/0031-9201(81)90046-7.
Engdahl E R,Flinn E A,Romney C F.1970.Seismic waves reflected from the Earth's inner core[J].Nature,228(5274),852-853,doi:10.1038/228852a0.
Garcia R,Souriau A.2000.Amplitude of the core-mantle boundary topography estimated by stochastic analysis of core phases[J].Physics of the Earth and Planetary Interiors,117(1-4):345-359,doi:10.1016/S0031-9201(99)00106-5.
Garnero E J,Mc Namara A K.2008.Structure and Dynamics of Earth's Lower Mantle[J].Science,320(5876):626-628,doi:10.1126/science.1148028.
Garnero E J,Revenaugh J,Williams Q,et al.1998.Ultralow Velocity Zone at the Core-Mantle Boundary[J].In The Core-Mantle Boundary Region,28,doi:10.1029/GD028p0319.
Gassner A,Thomas C,Krüger F,et al.2015.Probing the core-mantle boundary beneath Europe and Western Eurasia:A detailed study using Pc P[J].Physics of the Earth and Planetary Interiors,246,9-24,doi:10.1016/j.pepi.2015.06.007.
He Y M,Wen L X.2012.Geographic boundary of the“Pacific Anomaly”and its geometry and transitional structure in the north[J].Journal of Geophysical Research:Solid Earth,117(B9):1-16,doi:10.1029/2012JB009436.
He Y M,Wen L X,Zheng T Y.2006.Geographic boundary and shear wave velocity structure of the“Pacific anomaly”near the coremantle boundary beneath western Pacific[J].Earth and Planetary Science Letters,244(1-2):302-314,doi:10.1016/j.epsl.2006.02.007.
Hirose K,Takafuji N,Sata N,et al.2005.Phase transition and density of subducted MORB crust in the lower mantle[J].Earth and Planetary Science Letters,237(1-2):239-251,doi:10.1016/j.epsl.2005.06.035.
Idehara K,Yamada A,Zhao D P.2007.Seismological constraints on the ultralow velocity zones in the lowermost mantle from core-reflected waves[J].Physics of the Earth and Planetary Interiors,165(1-2):25-46,doi:10.1016/j.pepi.2007.07.005.
Jiang G,Zhao D P.2012.Observation of high-frequency PKi KP in Japan:Insight into fine structure of inner core boundary[J].Journal of Asian Earth Sciences,59,167-184,doi:10.1016/j.jseaes.2012.07.017.
Kampfmann W,Müller G.1989.Pc P amplitude calculations for a coremantle boundary with topography[J].Geophysical Research Letters,16(7):653-656,doi:10.1029/GL016i007p00653.
Kawakatsu H.2006.Sharp and seismically transparent inner core boundary region revealed by an entire network observation of nearvertical PKi KP[J].Earth,Planets and Space,58(7):855-863.
Koper K D,Dombrovskaya M.2005.Seismic properties of the inner core boundary from PKi KP/P amplitude ratios[J].Earth and Planetary Science Letters,237(3-4):680-694.
Koper K D,Pyle M L.2004.Observations of PKi KP/Pc P amplitude ratios and implications for Earth structure at the boundaries of the liquid core[J].Journal of Geophysical Research:Solid Earth,109(B3).
Koper K D,Pyle M L,Franks J M.2003.Constraints on aspherical core structure from PKi KP-PcP differential travel times[J].Journal of Geophysical Research:Solid Earth,108(B3).
Mc Namara A K,Garnero E J,Rost S.2010.Tracking deep mantle reservoirs with ultra-low velocity zones[J].Earth and Planetary Science Letters,299(1-2):1-9,doi:10.1016/j.epsl.2010.07.042.
Morelli A,Dziewonski A M.1987.Topography of the core-mantle boundary and lateral homogeneity of the liquid core[J].Nature,325(6106):678-683,doi:10.1038/325678a0.
Mori J,Helmberger D V.1995.Localized boundary layer below the midPacific velocity anomaly identified from a Pc P precursor[J].Journal of Geophysical Research:Solid Earth,100(B10):20359-20365,doi:10.1029/95JB02243.
Nataf H C.2000.Inner core takes another turn[J].Nature,405(6785):411-412.
Neuberg J,Wahr J.1991.Detailed investigation of a spot on the core mantle boundary using digital Pc P data[J].Physics of the Earth&Planetary Interiors,68(1-2):132-143,doi:10.1016/0031-9201(91)90013-8.
Niu F L,Chen Q F.2008.Seismic evidence for distinct anisotropy in the innermost inner core[J].Nature Geoscience,1(10):692-696,doi:10.1038/ngeo314.
Okada Y,Kasahara K,Hori S,et al.2014.Recent progress of seismic observation networks in Japan-Hi-net,F-net,K-NET and Ki K-net-[J].Earth,Planets and Space,56(8):xv-xxviii,doi:10.1186/BF03353076.
Poupinet G,Kennett B L N.2004.On the observation of high frequency PKi KP and its coda in Australia[J].Physics of the Earth and Planetary Interiors,146(3-4):497-511,doi:10.1016/j.pepi.2004.05.003.
Rost S,Earle P S.2010.Identifying regions of strong scattering at the core-mantle boundary from analysis of PKKP precursor energy[J].Earth and Planetary Science Letters,297(3-4):616-626,doi:10.1016/j.epsl.2010.07.014.
Rost S,Garnero E J,Thorne,M.S,et al.2010.On the absence of an ultralow-velocity zone in the North Pacific[J].Journal of Geophysical Research:Solid Earth,115(B4):doi:10.1029/2009JB006420.
Rost S,Garnero E J,Williams Q.2006.Fine-scale ultralow-velocity zone structure from high-frequency seismic array data[J].Journal of Geophysical Research:Solid Earth,111(B9):n/a-n/a,doi:10.1029/2005JB004088.
Rost S,Revenaugh J.2004.Small-scale changes of core-mantle boundary reflectivity studied using core reflected Pc P[J].Physics of the Earth and Planetary Interiors,145(1-4):19-36,doi:10.1016/j.pepi.2004.02.008.
Rost S,Thomas C.2002.Array seismology:methods and applications[J].Reviews of Geophysics,40(3):2-1-2-27.
Schimmel M,Paulssen H.1997.Noise reduction and detection of weak,coherent signals through phase-weighted stacks[J].Geophysical Journal International,130(2):497-505,doi:10.1111/j.1365-246X.1997.tb05664.x.
Shearer P,Masters G.1990.The density and shear velocity contrast at the inner core boundary[J].Geophysical Journal International,102(2):491-498.
Shen Z Y,Ai Y S,He Y M,et al.2013.A study on inner core boundary properties of East China using pre-critical Pc P-PKiKP data[J].Chinese Journal Of Geophysics(in chinese),56(10):3324-3333,doi:10.6038/cjg20131009.
Shen Z Y,Ai Y S,He Y M,et al.2016.Using Pre-critical PKi KP-PcPPhases to Constrain the Regional Structures of the Inner Core Boundary beneath East Asia[J].Physics of the Earth and Planetary Interiors,252,37-48,doi:10.1016/j.pepi.2016.01.001.
Song X D,Richards P G.1996.Seismological evidence for differential rotation of the Earth's inner core[J].Nature,382(6588):221-224.
Souriau A,Souriau M.1989.Ellipticity and density at the inner core boundary from subcritical PKi KP and Pc P data[J].Geophysical Journal International,98(1):39-54,doi:10.1111/j.1365-246X.1989.tb05512.x.
Stevenson D J.1987.Limits on lateral density and velocity variations in the Earth's outer core[J].Geophysical Journal International,88(1):311-319,doi:10.1111/j.1365-246X.1987.tb01383.x.
Sze E K M,van der Hilst R D.2003.Core mantle boundary topography from short period Pc P,PKP,and PKKP data[J].Physics of the Earth and Planetary Interiors,135(1):27-46,doi:10.1016/S0031-9201(02)00204-2.
Tanaka S.2010.Constraints on the core-mantle boundary topography from P4KP-PcP differential travel times[J].Journal of Geophysical Research:Solid Earth,115(B4):B04310,doi:10.1029/2009JB006563.
Tanaka S,Hamaguchi H.1997.Degree one heterogeneity and hemispherical variation of anisotropy in the inner core from PKP(BC)-PKP(DF)times[J].Journal of Geophysical Research:Solid Earth,102(B2):2925-2938,doi:10.1029/96JB03187.
Thorne M S,Garnero E J.2004.Inferences on ultralow-velocity zone structure from a global analysis of SPdKS waves[J].Journal of Geophysical Research:Solid Earth,109(B8):doi:10.1029/2004JB003010.
Tian D D,Wen L X.2017.Seismological evidence for a localized mushy zone at the Earth's inner core boundary[J].Nature Communications,8(1):165,doi:10.1038/s41467-017-00229-9.
Tkalcˇic'H,Young M,Bodin T,et al.2013.The shuffling rotation of the Earth's inner core revealed by earthquake doublets[J].Nature Geoscience,6(6):497-502,doi:10.1038/ngeo1813.
Vidale J E,Benz H M.1992.A sharp and flat section of the core-mantle boundary[J].Nature,359(6396):627-629,doi:10.1038/359627a0.
Waszek L,Deuss A.2015.Anomalously strong observations of PKi KP/Pc P amplitude ratios on a global scale[J].Journal of Geophysical Research:Solid Earth,120(7):5175-5190,doi:10.1002/2015JB012038.
Wen L X.2006.Localized Temporal Change of the Earth's Inner Core Boundary[J].Science,314(5801):967-970.
Wu W B,Ni S D,Shen Z C.2014.Constraining the short scale coremantle boundary topography beneath Kenai Peninsula(Alaska)with amplitudes of core-reflected Pc P wave[J].Physics of the Earth and Planetary Interiors,236,60-68,doi:10.1016/j.pepi.2014.09.001.
Zeng X,Ni S D.2013.Constraining shear wave velocity and density contrast at the inner core boundary with PKi KP/P amplitude ratio[J].Journal of Earth Science,24(5):716-724,doi:10.1007/s12583-013-0371-9.
李乐,周蕙兰,陈棋福.2004.地球内核的地震学研究进展[J].地球物理学进展,19(2):238-246.
申中寅,艾印双,何玉梅,等.2013.利用前临界Pc P-PKiKP资料研究中国东部内核边界性质[J].地球物理学报,56(10):3324-3333,doi:10.6038/cjg20131009.
Colombi A,Nissen-Meyer T,Boschi L,et al.2014.Seismic waveform inversion for core-mantle boundary topography[J].Geophysical Journal International,198(1):55-71,doi:10.1093/gji/ggu112.
Deuss A,Woodhouse J H,Paulssen H,et al.2000.The observation of inner core shear waves[J].Geophysical Journal International,142(1):67-73,doi:10.1046/j.1365-246X.2000.00147.x.
Dziewonski A M,Anderson D L.1981.Preliminary reference Earth model[J].Physics of the Earth and Planetary Interiors,25(4):297-356,doi:10.1016/0031-9201(81)90046-7.
Engdahl E R,Flinn E A,Romney C F.1970.Seismic waves reflected from the Earth's inner core[J].Nature,228(5274),852-853,doi:10.1038/228852a0.
Garcia R,Souriau A.2000.Amplitude of the core-mantle boundary topography estimated by stochastic analysis of core phases[J].Physics of the Earth and Planetary Interiors,117(1-4):345-359,doi:10.1016/S0031-9201(99)00106-5.
Garnero E J,Mc Namara A K.2008.Structure and Dynamics of Earth's Lower Mantle[J].Science,320(5876):626-628,doi:10.1126/science.1148028.
Garnero E J,Revenaugh J,Williams Q,et al.1998.Ultralow Velocity Zone at the Core-Mantle Boundary[J].In The Core-Mantle Boundary Region,28,doi:10.1029/GD028p0319.
Gassner A,Thomas C,Krüger F,et al.2015.Probing the core-mantle boundary beneath Europe and Western Eurasia:A detailed study using Pc P[J].Physics of the Earth and Planetary Interiors,246,9-24,doi:10.1016/j.pepi.2015.06.007.
He Y M,Wen L X.2012.Geographic boundary of the“Pacific Anomaly”and its geometry and transitional structure in the north[J].Journal of Geophysical Research:Solid Earth,117(B9):1-16,doi:10.1029/2012JB009436.
He Y M,Wen L X,Zheng T Y.2006.Geographic boundary and shear wave velocity structure of the“Pacific anomaly”near the coremantle boundary beneath western Pacific[J].Earth and Planetary Science Letters,244(1-2):302-314,doi:10.1016/j.epsl.2006.02.007.
Hirose K,Takafuji N,Sata N,et al.2005.Phase transition and density of subducted MORB crust in the lower mantle[J].Earth and Planetary Science Letters,237(1-2):239-251,doi:10.1016/j.epsl.2005.06.035.
Idehara K,Yamada A,Zhao D P.2007.Seismological constraints on the ultralow velocity zones in the lowermost mantle from core-reflected waves[J].Physics of the Earth and Planetary Interiors,165(1-2):25-46,doi:10.1016/j.pepi.2007.07.005.
Jiang G,Zhao D P.2012.Observation of high-frequency PKi KP in Japan:Insight into fine structure of inner core boundary[J].Journal of Asian Earth Sciences,59,167-184,doi:10.1016/j.jseaes.2012.07.017.
Kampfmann W,Müller G.1989.Pc P amplitude calculations for a coremantle boundary with topography[J].Geophysical Research Letters,16(7):653-656,doi:10.1029/GL016i007p00653.
Kawakatsu H.2006.Sharp and seismically transparent inner core boundary region revealed by an entire network observation of nearvertical PKi KP[J].Earth,Planets and Space,58(7):855-863.
Koper K D,Dombrovskaya M.2005.Seismic properties of the inner core boundary from PKi KP/P amplitude ratios[J].Earth and Planetary Science Letters,237(3-4):680-694.
Koper K D,Pyle M L.2004.Observations of PKi KP/Pc P amplitude ratios and implications for Earth structure at the boundaries of the liquid core[J].Journal of Geophysical Research:Solid Earth,109(B3).
Koper K D,Pyle M L,Franks J M.2003.Constraints on aspherical core structure from PKi KP-PcP differential travel times[J].Journal of Geophysical Research:Solid Earth,108(B3).
Mc Namara A K,Garnero E J,Rost S.2010.Tracking deep mantle reservoirs with ultra-low velocity zones[J].Earth and Planetary Science Letters,299(1-2):1-9,doi:10.1016/j.epsl.2010.07.042.
Morelli A,Dziewonski A M.1987.Topography of the core-mantle boundary and lateral homogeneity of the liquid core[J].Nature,325(6106):678-683,doi:10.1038/325678a0.
Mori J,Helmberger D V.1995.Localized boundary layer below the midPacific velocity anomaly identified from a Pc P precursor[J].Journal of Geophysical Research:Solid Earth,100(B10):20359-20365,doi:10.1029/95JB02243.
Nataf H C.2000.Inner core takes another turn[J].Nature,405(6785):411-412.
Neuberg J,Wahr J.1991.Detailed investigation of a spot on the core mantle boundary using digital Pc P data[J].Physics of the Earth&Planetary Interiors,68(1-2):132-143,doi:10.1016/0031-9201(91)90013-8.
Niu F L,Chen Q F.2008.Seismic evidence for distinct anisotropy in the innermost inner core[J].Nature Geoscience,1(10):692-696,doi:10.1038/ngeo314.
Okada Y,Kasahara K,Hori S,et al.2014.Recent progress of seismic observation networks in Japan-Hi-net,F-net,K-NET and Ki K-net-[J].Earth,Planets and Space,56(8):xv-xxviii,doi:10.1186/BF03353076.
Poupinet G,Kennett B L N.2004.On the observation of high frequency PKi KP and its coda in Australia[J].Physics of the Earth and Planetary Interiors,146(3-4):497-511,doi:10.1016/j.pepi.2004.05.003.
Rost S,Earle P S.2010.Identifying regions of strong scattering at the core-mantle boundary from analysis of PKKP precursor energy[J].Earth and Planetary Science Letters,297(3-4):616-626,doi:10.1016/j.epsl.2010.07.014.
Rost S,Garnero E J,Thorne,M.S,et al.2010.On the absence of an ultralow-velocity zone in the North Pacific[J].Journal of Geophysical Research:Solid Earth,115(B4):doi:10.1029/2009JB006420.
Rost S,Garnero E J,Williams Q.2006.Fine-scale ultralow-velocity zone structure from high-frequency seismic array data[J].Journal of Geophysical Research:Solid Earth,111(B9):n/a-n/a,doi:10.1029/2005JB004088.
Rost S,Revenaugh J.2004.Small-scale changes of core-mantle boundary reflectivity studied using core reflected Pc P[J].Physics of the Earth and Planetary Interiors,145(1-4):19-36,doi:10.1016/j.pepi.2004.02.008.
Rost S,Thomas C.2002.Array seismology:methods and applications[J].Reviews of Geophysics,40(3):2-1-2-27.
Schimmel M,Paulssen H.1997.Noise reduction and detection of weak,coherent signals through phase-weighted stacks[J].Geophysical Journal International,130(2):497-505,doi:10.1111/j.1365-246X.1997.tb05664.x.
Shearer P,Masters G.1990.The density and shear velocity contrast at the inner core boundary[J].Geophysical Journal International,102(2):491-498.
Shen Z Y,Ai Y S,He Y M,et al.2013.A study on inner core boundary properties of East China using pre-critical Pc P-PKiKP data[J].Chinese Journal Of Geophysics(in chinese),56(10):3324-3333,doi:10.6038/cjg20131009.
Shen Z Y,Ai Y S,He Y M,et al.2016.Using Pre-critical PKi KP-PcPPhases to Constrain the Regional Structures of the Inner Core Boundary beneath East Asia[J].Physics of the Earth and Planetary Interiors,252,37-48,doi:10.1016/j.pepi.2016.01.001.
Song X D,Richards P G.1996.Seismological evidence for differential rotation of the Earth's inner core[J].Nature,382(6588):221-224.
Souriau A,Souriau M.1989.Ellipticity and density at the inner core boundary from subcritical PKi KP and Pc P data[J].Geophysical Journal International,98(1):39-54,doi:10.1111/j.1365-246X.1989.tb05512.x.
Stevenson D J.1987.Limits on lateral density and velocity variations in the Earth's outer core[J].Geophysical Journal International,88(1):311-319,doi:10.1111/j.1365-246X.1987.tb01383.x.
Sze E K M,van der Hilst R D.2003.Core mantle boundary topography from short period Pc P,PKP,and PKKP data[J].Physics of the Earth and Planetary Interiors,135(1):27-46,doi:10.1016/S0031-9201(02)00204-2.
Tanaka S.2010.Constraints on the core-mantle boundary topography from P4KP-PcP differential travel times[J].Journal of Geophysical Research:Solid Earth,115(B4):B04310,doi:10.1029/2009JB006563.
Tanaka S,Hamaguchi H.1997.Degree one heterogeneity and hemispherical variation of anisotropy in the inner core from PKP(BC)-PKP(DF)times[J].Journal of Geophysical Research:Solid Earth,102(B2):2925-2938,doi:10.1029/96JB03187.
Thorne M S,Garnero E J.2004.Inferences on ultralow-velocity zone structure from a global analysis of SPdKS waves[J].Journal of Geophysical Research:Solid Earth,109(B8):doi:10.1029/2004JB003010.
Tian D D,Wen L X.2017.Seismological evidence for a localized mushy zone at the Earth's inner core boundary[J].Nature Communications,8(1):165,doi:10.1038/s41467-017-00229-9.
Tkalcˇic'H,Young M,Bodin T,et al.2013.The shuffling rotation of the Earth's inner core revealed by earthquake doublets[J].Nature Geoscience,6(6):497-502,doi:10.1038/ngeo1813.
Vidale J E,Benz H M.1992.A sharp and flat section of the core-mantle boundary[J].Nature,359(6396):627-629,doi:10.1038/359627a0.
Waszek L,Deuss A.2015.Anomalously strong observations of PKi KP/Pc P amplitude ratios on a global scale[J].Journal of Geophysical Research:Solid Earth,120(7):5175-5190,doi:10.1002/2015JB012038.
Wen L X.2006.Localized Temporal Change of the Earth's Inner Core Boundary[J].Science,314(5801):967-970.
Wu W B,Ni S D,Shen Z C.2014.Constraining the short scale coremantle boundary topography beneath Kenai Peninsula(Alaska)with amplitudes of core-reflected Pc P wave[J].Physics of the Earth and Planetary Interiors,236,60-68,doi:10.1016/j.pepi.2014.09.001.
Zeng X,Ni S D.2013.Constraining shear wave velocity and density contrast at the inner core boundary with PKi KP/P amplitude ratio[J].Journal of Earth Science,24(5):716-724,doi:10.1007/s12583-013-0371-9.
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