低速区

低速区（LVZ）是上地幔接近岩石圈与软流圈边界处，地震波横波速度显著偏低、电导率显著偏高的区域。深度从80km至300km。其中部分区域的地震波纵波缺失。^[2]^[2]^[3]^[4]

另外一个地震波速显著降低的区域在核幔边界的不超过50km的薄层，称为超低速区（英语：Ultra low velocity zone）（ULVZ）。^[3]

低速区可以用部分熔融（约1%）解释。^[2]水可以降低熔点，扮演了重要角色。^[4]^[5]

地震波的横波在低速区下降了3–6%，比纵波波速变化更显著。^[7]雷氏不连续面是低速区的下界。

只需要0.05–0.1 %的水就可导致1%的熔融，产生低速区。地盾下缺乏低速区可解释为地热梯度太小，阻止了部分熔融。^[8]

识别

低速区的存在最早是由宾诺·古登堡 (Beno Gutenberg) 在1959年观察到地震波到达速度比预期慢时提出的。^[9] 他指出，在距震中 1° 到 15° 之间，纵向到达的振幅呈指数下降，之后突然大幅增加。低速区的存在使地震能量散焦，随后是高速梯度使其集中，这为这些观察结果提供了解释。^[10]

参见

参考文献

[1]
GR Helffrich & BJ Wood. The Earth's Mantle (PDF). Nature (Macmillan Magazines). 2002, 412 (2 August): 501; Figure 1. doi:10.1038/35087500.^{[失效链接]}
[2]
L Stixrude & C Lithgow-Bertolloni. Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone (PDF). Journal of Geophysical Research. 2005, 110: B03204 [2018-12-16]. Bibcode:2005JGRB..11003204S. doi:10.1029/2004JB002965. （原始内容 (PDF)存档于2011-08-30）.
[3]
EJ Garnero, MS Thorne, A McNamara & S Rost. Chapter 6: Fine-scale ultra-low-velocity zone layering at the core-mantle boundary and superplumes. David A Yuen; Shigenori Maruyama (编). Superplumes: beyond plate tectonics. Springer. 2007: 139. ISBN 1-4020-5749-0.
[4]
Philip Kearey; Keith A. Klepeis; Frederick J. Vine. Global tectonics 3rd. Wiley-Blackwell. 2009: 32. ISBN 1-4051-0777-4.
[5]
It is hypothesized that the absence of plate tectonics on the planet Venus is due to the absence of water in its crust and upper mantle. Cooling occurs largely through mantle plumes. See Gillian R. Foulger. Plates, plumes, and paradigms; Volume 388 of Special papers. Geological Society of America. 2005: 857. ISBN 0-8137-2388-4.
[6]
Figure patterned after Don L Anderson. New theory of the earth 2nd. Cambridge University Press. 2007: 102, Figure 8.6. ISBN 0-521-84959-4.; Original figure attributed to Grand & Helmberger (1984)
[7]
Brown, G.C.; Mussett A.E. The inaccessible earth. Taylor & Francis. 1981: 235 [2010-02-20]. ISBN 978-0-04-550028-4.
[8]
Condie, K.C. Plate tectonics and crustal evolution. Butterworth-Heinemann. 1997: 282 [2010-02-20]. ISBN 978-0-7506-3386-4.
[9]
Gutenberg, B. Physics of the Earth's Interior. New York: Academic Press. 1959: 240. ISBN 0-12-310650-8.
[10]
Anderson, D.L. 3. The Crust and Upper Mantle. Theory of the Earth (PDF). Boston: Blackwell Scientific Publications. 1989 [2010-02-20]. ISBN 0-521-84959-4. （原始内容 (PDF)存档于2010-06-23）.

[Helffrich-1] [1]
GR Helffrich & BJ Wood. The Earth's Mantle (PDF). Nature (Macmillan Magazines). 2002, 412 (2 August): 501; Figure 1. doi:10.1038/35087500.^{[失效链接]}

[Stixrude-2] [2]
L Stixrude & C Lithgow-Bertolloni. Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone (PDF). Journal of Geophysical Research. 2005, 110: B03204 [2018-12-16]. Bibcode:2005JGRB..11003204S. doi:10.1029/2004JB002965. （原始内容 (PDF)存档于2011-08-30）.

[Rost-3] [3]
EJ Garnero, MS Thorne, A McNamara & S Rost. Chapter 6: Fine-scale ultra-low-velocity zone layering at the core-mantle boundary and superplumes. David A Yuen; Shigenori Maruyama (编). Superplumes: beyond plate tectonics. Springer. 2007: 139. ISBN 1-4020-5749-0.

[Vine-4] [4]
Philip Kearey; Keith A. Klepeis; Frederick J. Vine. Global tectonics 3rd. Wiley-Blackwell. 2009: 32. ISBN 1-4051-0777-4.

[Venus-5] [5]
It is hypothesized that the absence of plate tectonics on the planet Venus is due to the absence of water in its crust and upper mantle. Cooling occurs largely through mantle plumes. See Gillian R. Foulger. Plates, plumes, and paradigms; Volume 388 of Special papers. Geological Society of America. 2005: 857. ISBN 0-8137-2388-4.

[Anderson1-6] [6]
Figure patterned after Don L Anderson. New theory of the earth 2nd. Cambridge University Press. 2007: 102, Figure 8.6. ISBN 0-521-84959-4.; Original figure attributed to Grand & Helmberger (1984)

[Brown-7] [7]
Brown, G.C.; Mussett A.E. The inaccessible earth. Taylor & Francis. 1981: 235 [2010-02-20]. ISBN 978-0-04-550028-4.

[Condie-8] [8]
Condie, K.C. Plate tectonics and crustal evolution. Butterworth-Heinemann. 1997: 282 [2010-02-20]. ISBN 978-0-7506-3386-4.

[Bruno-9] [9]
Gutenberg, B. Physics of the Earth's Interior. New York: Academic Press. 1959: 240. ISBN 0-12-310650-8.

[Don-10] [10]
Anderson, D.L. 3. The Crust and Upper Mantle. Theory of the Earth (PDF). Boston: Blackwell Scientific Publications. 1989 [2010-02-20]. ISBN 0-521-84959-4. （原始内容 (PDF)存档于2010-06-23）.

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