地球内核及其边界的结构特征和动力学过程

Seismic structure and dynamic process of the Earth′s inner core and its boundary

现代地震学展现出了一个复杂的地球内核内部和表面结构.地球内核内部结构的主要特征表现为其地震波速度和衰减呈现各向异性,且各种结构(速度、衰减和各向异性)均呈现东西半球差异,而内核表面的新发现则包括其局部区域存在起伏的地形和固液并存的糊状层.地球内核压缩波速度和衰减均呈现以地球旋转轴为轴的柱对称各向异性,沿地球旋转轴方向传播的压缩波比沿赤道方向传播的压缩波传播更快且衰减更强烈.同时,内核各向异性结构随深度而变化:内核顶部约100~400km接近各向同性,而在内核最深处300~600km内则可能存在一个具有不同各向异性特征的内内核.地球内核的东西半球差异表现在多方面:在内核顶部~100km厚度内,东半球的各向同性速度比西半球快约0.8%,东半球具有较强的衰减(Q=250),而西半球则具有较弱的衰减(Q=600);西半球的顶部各向同性层厚度约为100km,而东半球顶部各向同性层厚度则约为400km;在各向同性层底下,西半球具有较强的各向异性(~4%),而东半球则具有较弱的各向异性(~0.7%).地球内核边界在菲律宾海、黄海、西太平洋以及中美洲下方存在1~14km高的地形起伏,在鄂霍次克海西南部下方存在4~8km厚的糊状层.地球内核的这些新发现引发了对许多可能的新物理机制的探讨,也促使我们重新评估我们对外核成分、外核热化学对流、内核凝固过程和地球磁场驱动力的认识.这些结果表明内核凝固过程和地球磁场的热和化学驱动力远比传统观念认为的横向均匀分布复杂得多.内核西半球可能不断凝固并释放潜热和轻元素,而东半球则可能不断熔化并吸收潜热和轻元素,外核对流的驱动力在东西半球可能截然不同,甚至呈现相反方向.这些凝固与熔化交替过程也发生在局部地形起伏区域.在糊状层区域,地球内核凝固释放潜热和化学能,而在大部分无糊状地区,内核凝固只释放潜热.

Modern seismology has revealed complex structures of the Earth＇s inner core and its boundary. The interior of the Earth＇s inner core exhibits both velocity and attenuation anisotropy, and hemispherical variations of velocity, attenuation and anisotropy, while the surface of the inner core possesses topography and mushy zone in some localized regions. The inner core velocity and attenuation anisotropy is cylindrical with the axis of high velocity and high attenuation parallel with the Earthrs rotation axis, and depth dependent with the topmost 100- 400 km close to be isotropic and the innermost 300-600 km possibly possessing a distinct anisotropy. The hemispherical variations in the inner core are manifested in many aspects, the isotropic velocity of the topmost -100 km of the eastern hemisphere is 0.8% faster than that of the western hemisphere, the attenuation in the top -100 km of the eastern hemisphere （Q=250） is stronger than that of the western hemisphere （Q=600）, the top isotropic layer is thinner in the western hemisphere （-100 km） than in the eastern hemisphere （-400 km）, and the inner core anisotropy in the western hemisphere （44%） is stronger than in the eastern hemisphere （-0.7 %）. In the boundary, the inner core exhibits topographic variations with height changes of 1-14 km beneath the Philippine Sea, the Yellow Sea, the western Pacific and Central America, and a mushy zone with a thickness of 4-8 km beneath the southwest Okhotsk Sea. These new findings have motivated a series of new physical mechanisms in the scientific community and demand for reevaluation of outer core composition, thermal-compositional convection in the outer core, inner core solidification process and driving force of Earthrs geodynamo. These results suggest that the inner core solidification process and the driving force of Earthrs geodynamo are not laterally homogeneous, as it has long been held in the traditional views. Solidification may occur in the western hemisphere releasing latent heat and light elements, while melting may occur in the eastern hemisphere absorbing latent heat and light elements. Therefore, the driving forces for the outer core convection in these two hemispheres may be different and may even be in opposite signs. Such alternative solidification and melting should also occur in the localized topographic regions. Moreover, the solidification in the region of a mushy zone would release both thermal and chemical energy, while that in other regions of a sharp inner core boundary only releases thermal energy.