南极上地幔结构异常对长周期自由振荡的影响
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摘要
2012年发生的苏门答腊大地震激起了强烈的环形地球自由振荡,本文利用南极26个地震台站记录的此次大地震激发的0T10简正模,分析南极上地幔结构异常对长周期自由振荡的影响,我们将震后20h的自由振荡观测数据与利用PREM地球模型模拟的结果进行比较,结果表明:在震后较短时间内区域性(区域的范围远远小于简正模的波长)上地幔结构异常能显著影响长周期自由振荡质点的偏振,引起简正模强烈的偏振异常.以往研究主要关注地球自转和全球范围的上地幔结构异常对长周期自由振荡简正模的影响,局部范围的上地幔结构异常对长周期自由振荡的影响并未被重视.对南极地区0T10垂向偏振异常的进一步分析表明:南极大陆上地幔存在方位各向异性,上地幔各向异性主要分布在横贯南极山脉下方,深度范围约为70~660km.利用震后较短时间内的长周期地球自由振荡观测资料可以对局部区域上地幔的各向异性及其深度范围提供约束,可作为剪切波分裂和面波层析成像技术的补充.
This paper analyses the effect of upper mantle structure on0 T10 normal mode of free oscillation of Earth caused by 2012/04/11 MW8.6 Sumatra earthquake.We compare the observations with simulation calculated by the PREM,which shows a fact that is often ignored in previous studies of long period normal modes:lateral variation of upper mantle structure,whose wavelength is much smaller than the wavelengths of normal modes at low frequencies,can cause significant polarization anomalies for the normal modes,while usually only the effect of Earth′s rotation and global upper mantle lateral variation are taken into account.The further analysis of the0 T10 vertical polarization suggests that at South Polar the anisotropic area is mainly distributed beneath the Transantarctic Mountains with a depth range 70~660 km.Using the polarization of long period normal modes in a short time after the earthquake we can constrain the scope and depth ofthe lateral variation of local mantle structure.
This paper analyses the effect of upper mantle structure on0 T10 normal mode of free oscillation of Earth caused by 2012/04/11 MW8.6 Sumatra earthquake.We compare the observations with simulation calculated by the PREM,which shows a fact that is often ignored in previous studies of long period normal modes:lateral variation of upper mantle structure,whose wavelength is much smaller than the wavelengths of normal modes at low frequencies,can cause significant polarization anomalies for the normal modes,while usually only the effect of Earth′s rotation and global upper mantle lateral variation are taken into account.The further analysis of the0 T10 vertical polarization suggests that at South Polar the anisotropic area is mainly distributed beneath the Transantarctic Mountains with a depth range 70~660 km.Using the polarization of long period normal modes in a short time after the earthquake we can constrain the scope and depth ofthe lateral variation of local mantle structure.
引文
Backus G,Gilbert F.1961.The rotational splitting of the free oscillations of the Earth.Proceedings of the National Academy of Sciences of the United States of America,47(3):362-371.
Beghein C,Resovsky J,Van der Hilst R D.2008.The signal of mantle anisotropy in the coupling of normal modes.Geophys.J.Int.,175(3):1209-1234.
Dahlen F A.1968.The normal modes of a rotating,elliptical Earth.Geophys.J.Res.,16(4):329-367.
Dahlen F A,Tromp J.1998.Theoretical Global Seismology.New Jersey:Princeton University Press.
Dziewonski A M,Anderson D L.1981.Preliminary reference Earth model.Phys.Earth Planet.Inter.,25(4):297-356.
Geller R J,Stein S.1977.Split free oscillation amplitudes for the1960Chilean and 1964 Alaskan Earthquakes.Bulletin of the Seismological Society of America,67(3):651-660.
Hu X G,Liu L T,Kroner C,et al.2009.Observation of the seismic anisotropy effects on free oscillations below 4 mHz.J.Geophys.Res.,114:B07301,doi:10.1029/2008JB005713.
Lay T,Wallace T C.1995.Modern Global Seismology.San Diego:Academic Press.
Müller C.2001.Upper mantle seismic anisotropy beneath Antarctica and the Scotia Sea region.Geophys.J.Int.,147(1):105-122.
Masters G,Park J,Gilbert F.1983.Observations of coupled spheroidal and toroidal modes.J.Geophys.Res.,88(B12):10285-10298.
Oda H.2005.An attempt to estimate isotropic and anisotropic lateral structure of the Earth by spectral inversion incorporating mixed coupling.Geophys.J.Int.,160(2):667-682.
Park J.1986.Synthetic seismograms from coupled free oscillations:effects of lateral structure and rotation.J.Geophys.Res.,91(B6):6441-6464.
Resovsky J S,Ritzwoller M H.1998.New and refined constraints onthree-dimensional Earth structure from normal modes below 3mHz.J.Geophys.Res.,103(B1):783-810.
Shu P Y,Zhang B.1994.Crust-mantle structure under South Pole and Ross Sea Beach in Antarctica.Antarctic Research(in Chinese),6(3):15-22.
Sleep N H,Ebinger J,Kendall J M.2002.Deflection of mantle plume material by cratonic keels.∥The Early Earth:Physical,Chemical and Biological Development.Geological Society of London,Special Publication.
Smith M F,Masters G.1989.The effect of Coriolis coupling of free oscillation multiplets on the determination of aspherical Earth structure.Geophysical Research Letters,16(3):263-266.
Tsuboi S,Geller R J,Morris S P.1985.Partial derivatives of the eigenfrequences of a laterally heterogeneous Earth model.Geophysical Research Letters,12(12):817-820.
Xue X X,Hu X G,Hao X G,et al.2012.Constraining focal mechanism of the 2011Tohoku earthquake by gravity observations.Chinese J.Geophys.(in Chinese),55(9):3006-3015,dio:10.6038/j.issn.0001-5733.2012.09.019.
Xue Y,Cheng J,Liu J,et al.2014.Geodynamic mechanism of the2012Sumatra MW8.6earthquake and relationships with other great shocks in surrounding areas.Earth Science(Journal of China University of Geosciences)(in Chinese),39(4):481-491.
Zürn W,Laske G,Widmer-Schnidrig R,et al.2000.Observation of Coriolis coupled modes below 1 mHz.Geophys.J.Int.,143(1):113-118.
束沛镒,张钋.1994.南极点和罗斯海滨壳幔结构.南极研究,6(3):15-22.
薛秀秀,胡小刚,郝晓光等.2012.利用重力观测约束2011日本ToHoku大地震的震源机制.地球物理学报,55(9):3006-3015,doi:10.6038/j.issn.0001-5733.2012.09.019.
薛艳,程佳,刘杰等.2012.2012年苏门答腊MW8.6地震成因及与周边大震关系.地球科学(中国地质大学学报),39(4):481-491.
Beghein C,Resovsky J,Van der Hilst R D.2008.The signal of mantle anisotropy in the coupling of normal modes.Geophys.J.Int.,175(3):1209-1234.
Dahlen F A.1968.The normal modes of a rotating,elliptical Earth.Geophys.J.Res.,16(4):329-367.
Dahlen F A,Tromp J.1998.Theoretical Global Seismology.New Jersey:Princeton University Press.
Dziewonski A M,Anderson D L.1981.Preliminary reference Earth model.Phys.Earth Planet.Inter.,25(4):297-356.
Geller R J,Stein S.1977.Split free oscillation amplitudes for the1960Chilean and 1964 Alaskan Earthquakes.Bulletin of the Seismological Society of America,67(3):651-660.
Hu X G,Liu L T,Kroner C,et al.2009.Observation of the seismic anisotropy effects on free oscillations below 4 mHz.J.Geophys.Res.,114:B07301,doi:10.1029/2008JB005713.
Lay T,Wallace T C.1995.Modern Global Seismology.San Diego:Academic Press.
Müller C.2001.Upper mantle seismic anisotropy beneath Antarctica and the Scotia Sea region.Geophys.J.Int.,147(1):105-122.
Masters G,Park J,Gilbert F.1983.Observations of coupled spheroidal and toroidal modes.J.Geophys.Res.,88(B12):10285-10298.
Oda H.2005.An attempt to estimate isotropic and anisotropic lateral structure of the Earth by spectral inversion incorporating mixed coupling.Geophys.J.Int.,160(2):667-682.
Park J.1986.Synthetic seismograms from coupled free oscillations:effects of lateral structure and rotation.J.Geophys.Res.,91(B6):6441-6464.
Resovsky J S,Ritzwoller M H.1998.New and refined constraints onthree-dimensional Earth structure from normal modes below 3mHz.J.Geophys.Res.,103(B1):783-810.
Shu P Y,Zhang B.1994.Crust-mantle structure under South Pole and Ross Sea Beach in Antarctica.Antarctic Research(in Chinese),6(3):15-22.
Sleep N H,Ebinger J,Kendall J M.2002.Deflection of mantle plume material by cratonic keels.∥The Early Earth:Physical,Chemical and Biological Development.Geological Society of London,Special Publication.
Smith M F,Masters G.1989.The effect of Coriolis coupling of free oscillation multiplets on the determination of aspherical Earth structure.Geophysical Research Letters,16(3):263-266.
Tsuboi S,Geller R J,Morris S P.1985.Partial derivatives of the eigenfrequences of a laterally heterogeneous Earth model.Geophysical Research Letters,12(12):817-820.
Xue X X,Hu X G,Hao X G,et al.2012.Constraining focal mechanism of the 2011Tohoku earthquake by gravity observations.Chinese J.Geophys.(in Chinese),55(9):3006-3015,dio:10.6038/j.issn.0001-5733.2012.09.019.
Xue Y,Cheng J,Liu J,et al.2014.Geodynamic mechanism of the2012Sumatra MW8.6earthquake and relationships with other great shocks in surrounding areas.Earth Science(Journal of China University of Geosciences)(in Chinese),39(4):481-491.
Zürn W,Laske G,Widmer-Schnidrig R,et al.2000.Observation of Coriolis coupled modes below 1 mHz.Geophys.J.Int.,143(1):113-118.
束沛镒,张钋.1994.南极点和罗斯海滨壳幔结构.南极研究,6(3):15-22.
薛秀秀,胡小刚,郝晓光等.2012.利用重力观测约束2011日本ToHoku大地震的震源机制.地球物理学报,55(9):3006-3015,doi:10.6038/j.issn.0001-5733.2012.09.019.
薛艳,程佳,刘杰等.2012.2012年苏门答腊MW8.6地震成因及与周边大震关系.地球科学(中国地质大学学报),39(4):481-491.