Helmberger D, Lay T, Ni S, Gurnis M
Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
Proc Natl Acad Sci U S A. 2005 Nov 29;102(48):17257-63. doi: 10.1073/pnas.0502504102. Epub 2005 Oct 10.
Seismologists have known for many years that the lowermost mantle of the Earth is complex. Models based on observed seismic phases sampling this region include relatively sharp horizontal discontinuities with strong zones of anisotropy, nearly vertical contrasts in structure, and small pockets of ultralow velocity zones (ULVZs). This diversity of structures is beginning to be understood in terms of geodynamics and mineral physics, with dense partial melts causing the ULVZs and a postperovskite solid-solid phase transition producing regional layering, with the possibility of large-scale variations in chemistry. This strong heterogeneity has significant implications on heat transport out of core, the evolution of the magnetic field, and magnetic field polarity reversals.
地震学家多年来一直知道地球最下层地幔的情况很复杂。基于对该区域进行观测的地震波相位所建立的模型显示，这里存在相对明显的水平间断面以及强烈的各向异性区域、结构上近乎垂直的差异，还有小范围的超低速带（ULVZ）。从地球动力学和矿物物理学的角度来看，人们开始逐渐理解这种结构的多样性，即高密度的部分熔融导致了超低速带的形成，而后钙钛矿的固-固相变产生了区域分层现象，同时化学组成也可能存在大规模的变化。这种强烈的非均质性对从地核向外的热传输、磁场的演化以及磁场极性反转都有着重大影响。
