Upper-Mantle Discontinuities
Abrupt increases in density and seismic velocity occur near 410- and 660-km depth, caused by mineral phase changes in olivine. These discontinuities can be observed using reflected and converted seismic waves, most easily at long periods using stacking methods. Precursors to SS have proven particularly useful in mapping global variations in 410- and 660-km topography, showing that large depressions in the 660-km discontinuity are found in subduction zones (Shearer, 1991; Flanagan and Shearer, 1994). Other discontinuities, such as the Moho and LAB (lithosphere-asthenosphere boundary) can also be mapped using these methods.
Relevant papers with contributions from my group include:
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Liu, T., and P.M. Shearer, Complicated lithospheric structure beneath the contiguous US revealed by teleseismic S reflections, J. Geophys. Res., doi: 10.1029/2020JB021624, 2021.
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Wei, S. S., P. M. Shearer, C. Lithgow-Bertelloni, L. Stixrude, and D. Tian, Oceanic plateau of the Hawaiian mantle plume head subducted to the uppermost lower mantle, Science, 370, 983-987, doi: 10.1126/science.abd0312, 2020.
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Tian, D., M. Lv, S. S. Wei, S. M. Dorman, and P. M. Shearer, Global variations of Earth's 520- and 560-km discontinuities, Earth Planet. Sci. Lett., 552, doi: 10.1016/j.epsl.2020.116600, 2020.
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Imaging upper-mantle structure under USArray using long-period reflection seismology, J. Geophys. Res., 124, doi: 10.1029/2019JB017326, 2019.
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Wei, S. S., and P. M. Shearer, A sporadic low-velocity layer atop the 410 km discontinuity beneath the Pacific Ocean, J. Geophys. Res., 122, 5144-5159, doi: 10.1002/2017JB014100, 2017.
(PDF)
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Rychert, C. A., and P. M. Shearer, Imaging the lithosphere-asthenosphere boundary beneath the Pacific
using SS waveform modeling, J. Geophys. Res., 116, doi: 10.1029/2010JB008070, 2011.
(PDF)
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Rychert, C. A. and P. M. Shearer, Resolving crustal thickness using SS waveform stacks,
Geophys. J. Int., 180, 1128-1137, doi:10.1111/j.1365-246X.2009.04497.x, 2010.
(PDF)
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Rychert, C. A., P. M. Shearer and K. M. Fischer, Scattered wave imaging of the
lithosphere-asthenosphere boundary, Lithos, doi: 1016/j.lithos.2009.12.006, 2010.
(PDF)
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Rychert, C. A., and P. M. Shearer, A global view of the lithosphere-asthenosphere
boundary, Science, 324, doi: 10.1126/science.1169754, 2009.
(PDF)
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Houser, C., G. Masters, M. Flanagan, and P. Shearer, Determination and analysis of
long-wavelength transition zone structure using SS precursors, Geophys. J. Int., 174, 178-194,
doi: 10.1111/j.1365-246X.2008.03719.x, 2008.
(PDF)
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Lawrence, J. F., and P. M. Shearer, Imaging mantle transition zone thickness with SdS-SS
finite-frequency sensitivity kernels, Geophys. J. Int., 174, 143-158,
doi: 10,1111/j.1365-246X.2007.03673.x, 2008.
(PDF)
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Lawrence, J. F., and P. M. Shearer, A global study of transition zone thickness using receiver
functions, J. Geophys. Res., 111, B06307, doi:10.1029/2005JB003973, 2006.
(PDF)
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Shearer, P.M., Upper mantle seismic discontinuities, in Earth's Deep Interior: Mineral Physics and Tomography from the Atomic to the Global Scale, AGU Geophysical Monograph 117, 115-131, 2000. (PDF)
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Sheehan, A.F., P.M. Shearer, H.J. Gilbert and K.G. Dueker, Seismic migration processing of P-SV converted phases for mantle discontinuity structure beneath the Snake River Plain, western United States, J. Geophys. Res., 105, 19,055-19,065, 2000. (PDF)
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Flanagan, M.P. and P.M. Shearer, A map of topography on the 410-km discontinuity from PP precursors, Geophys. Res. Lett., 26, 549-552, 1999. (PDF)
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Shearer, P.M., M.P. Flanagan and M.A.H. Hedlin, Experiments in migration processing of SS precursor data to image upper mantle discontinuity structure, J. Geophys. Res., 104, 7229-7242, 1999. (PDF)
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Shearer, P.M. and M.P. Flanagan, Seismic velocity and density jumps across the 410- and 660-kilometer discontinuities, Science, 285, 1545-1548, 1999. (PDF)
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Flanagan, M.P. and P.M. Shearer, Global mapping of topography on transition
zone velocity discontinuities by stacking SS precursors, J. Geophys. Res.,
103, 2673-2692, 1998. (PDF)
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Flanagan, M.P. and P.M. Shearer, Topography on the 410-km seismic velocity
discontinuity near subduction zones from stacking of sS, sP, and pP
precursors, J. Geophys. Res., 103, 21,165-21,182, 1998. (PDF)
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Shearer, P.M., Transition zone velocity gradients and the 520-km
discontinuity, J. Geophys. Res., 101, 3053-3066, 1996. (PDF)
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Shearer, P.M., Seismic studies of the upper mantle and transition zone,
Reviews of Geophysics, Supplement, U.S. National Report to IUGG
1991-1994, 321-324, 1995.
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Shearer, P.M., Seismic discontinuities in Earth's upper mantle,
McGraw Hill Yearbook of Science and Technology, McGraw Hill, New York,
79-81, 1995.
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Shearer, P.M., Global mapping of upper mantle reflectors from long-period
SS precursors, Geophys. J. Int., 115, 878-904, 1993.
(PDF)
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Shearer, P.M., Imaging Earth's upper mantle, EOS Trans. AGU (article),
74, 602, 1993 (this article also appeared in AGU's Geophysics News
1993, p 7-8).
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Shearer, P.M. and T.G. Masters, Global mapping of topography on the
660 km discontinuity, Nature, 355, 791-796, 1992. (PDF)
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Shearer, P.M., Constraints on upper-mantle discontinuities from observations
of long-period reflected and converted phases,
J. Geophys. Res., 96, 18,147-18,182, 1991. (PDF)
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Shearer, P.M., Seismic imaging of upper-mantle structure with new evidence
for a 520-km discontinuity, Nature, 344, 121-126, 1990. (PDF)