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Constraints on the anisotropic contributions to velocity discontinuities at ∼60 km depth beneath the Pacific

Constraints on the anisotropic contributions to velocity discontinuities at ∼60 km depth beneath the Pacific
Constraints on the anisotropic contributions to velocity discontinuities at ∼60 km depth beneath the Pacific
Strong, sharp, negative seismic discontinuities, velocity decreases with depth, are observed beneath the Pacific seafloor at ∼60 km depth. It has been suggested that these are caused by an increase in radial anisotropy with depth, which occurs in global surface wave models. Here we test this hypothesis in two ways. We evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests. We do this by fitting an example surface wave data set near the East Pacific Rise. We also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in S-to-P and P-to-S receiver functions and SS precursors using synthetic seismograms. We test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering. The result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0–10 Ma seafloor, which would also have a radial anisotropy signature. An increase in radial anisotropy with depth at 60 km depth is not well-resolved in surface wave models, and could be artificially observed. Shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops (<6%) in SS precursor imaging, but not receiver functions. The effect is diminished if strong anisotropy also exists at 0–60 km depth as suggested by surface waves. Overall, an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations.
1525-2027
2855-2871
Rychert, Catherine A.
70cf1e3a-58ea-455a-918a-1d570c5e53c5
Harmon, Nicholas
10d11a16-b8b0-4132-9354-652e72d8e830
Rychert, Catherine A.
70cf1e3a-58ea-455a-918a-1d570c5e53c5
Harmon, Nicholas
10d11a16-b8b0-4132-9354-652e72d8e830

Rychert, Catherine A. and Harmon, Nicholas (2017) Constraints on the anisotropic contributions to velocity discontinuities at ∼60 km depth beneath the Pacific. Geochemistry, Geophysics, Geosystems, 18 (8), 2855-2871. (doi:10.1002/2017GC006850).

Record type: Article

Abstract

Strong, sharp, negative seismic discontinuities, velocity decreases with depth, are observed beneath the Pacific seafloor at ∼60 km depth. It has been suggested that these are caused by an increase in radial anisotropy with depth, which occurs in global surface wave models. Here we test this hypothesis in two ways. We evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests. We do this by fitting an example surface wave data set near the East Pacific Rise. We also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in S-to-P and P-to-S receiver functions and SS precursors using synthetic seismograms. We test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering. The result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0–10 Ma seafloor, which would also have a radial anisotropy signature. An increase in radial anisotropy with depth at 60 km depth is not well-resolved in surface wave models, and could be artificially observed. Shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops (<6%) in SS precursor imaging, but not receiver functions. The effect is diminished if strong anisotropy also exists at 0–60 km depth as suggested by surface waves. Overall, an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations.

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Rychert_et_al-2017-Geochemistry,_Geophysics,_Geosystems - Version of Record
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Accepted/In Press date: 21 July 2017
e-pub ahead of print date: 4 August 2017
Published date: August 2017

Identifiers

Local EPrints ID: 414972
URI: http://eprints.soton.ac.uk/id/eprint/414972
ISSN: 1525-2027
PURE UUID: a10e7ed6-a1bc-4a97-a150-ff1ed11948e4
ORCID for Nicholas Harmon: ORCID iD orcid.org/0000-0002-0731-768X

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Date deposited: 19 Oct 2017 16:30
Last modified: 07 Oct 2020 01:56

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