Deep Earth Structure: Q of the Earth from Crust to Core

Romanowicz, Barbara; Mitchell, Brian J.

doi:10.1016/b978-0-444-53802-4.00021-x

Cited by 68 publications

(66 citation statements)

References 327 publications

(308 reference statements)

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“…The extrapolation of the preferred model for the viscoelasticity of polycrystalline MgO to conditions prevailing in the uppermost lower mantle (23 GPa, and 1500°C on the mantle adiabat, and a grain size of 10 mm (Figure e)), suggests values of Q −1 increasing with increasing period across the seismic band from ~10 −3 at 3 s to ~10 −2 at 3000 s period. That these values are broadly comparable with the seismologically inferred dissipation of ~0.003 for the lower mantle [e.g., Romanowicz and Mitchell , ] suggests that the ferropericlase phase might be a significant contributor to seismic wave attenuation and related dispersion in the Earth's lower mantle.…”

Section: Discussionsupporting

confidence: 54%

“…High‐temperature viscoelastic relaxation of the rocks and minerals of the Earth's deep interior is responsible for the attenuation and associated dispersion of seismic shear waves. Such departures from perfectly elastic behavior are most pronounced in the asthenosphere of the upper mantle and in the inner core, in each of which regions maximum dissipation ( Q −1 ) values of order 10 −2 are inferred, whereas lower values near 0.003 are found for the lower mantle [e.g., Romanowicz and Mitchell , ]. In the largely crystalline materials of the Earth's mantle, such relaxation is likely to involve both the stress‐induced migration of dislocations within individual mineral grains and also sliding on grain and interphase boundaries.…”

Section: Introductionmentioning

confidence: 99%

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Grain size‐sensitive viscoelastic relaxation and seismic properties of polycrystalline MgO

et al. 2016

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Torsional forced‐oscillation experiments on a suite of synthetic MgO polycrystals, of high‐purity and average grain sizes of 1–100 µm, reveal strongly viscoelastic behavior at temperatures of 800–1300°C and periods between 1 and 1000 s. The measured shear modulus and associated strain energy dissipation both display monotonic variations with oscillation period, temperature, and grain size. The data for the specimens of intermediate grain size have been fitted to a generalized Burgers creep function model that is also broadly consistent with the results for the most coarse‐grained specimen. The mild grain size sensitivity for the relaxation time τL, defining the lower end of the anelastic absorption band, is consistent with the onset of elastically accommodated grain boundary sliding. The upper end of the anelastic absorption band, evident in the highest‐temperature data for one specimen only, is associated with the Maxwell relaxation time τM marking the transition toward viscous behavior, conventionally ascribed a stronger grain size sensitivity. Similarly pronounced viscoelastic behavior was observed in complementary torsional microcreep tests, which confirm that the nonelastic strains are mainly recoverable, i.e., anelastic. With an estimated activation volume for the viscoelastic relaxation, the experimentally constrained Burgers model has been extrapolated to the conditions of pressure and temperature prevailing in the Earth's uppermost lower mantle. For a plausible grain size of 10 mm, the predicted dissipation Q−1 ranges from ~10−3 to ~10−2 for periods of 3–3000 s. Broad consistency with seismological observations suggests that the lower mantle ferropericlase phase might account for much of its observed attenuation.

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Section: Discussionsupporting

confidence: 54%

Section: Introductionmentioning

confidence: 99%

Grain size‐sensitive viscoelastic relaxation and seismic properties of polycrystalline MgO

et al. 2016

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“…12 and 15) are relatively large. Studies based on earthquake data suggest values of α on the order of 10 −6 m −1 , but quickly growing, with decreasing period, within the period range of interest to us [Mitchell , 1995;Romanowicz and Mitchell , 2015]. Surface waves at those periods are perhaps best sampled by ambient-noise cross correlations; Prieto et al…”

Section: 23)mentioning

confidence: 98%

On seismic ambient noise cross-correlation and surface-wave attenuation

Boschi

Magrini

Cammarano

et al. 2019

Geophysical Journal International

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SUMMARY We derive a theoretical relationship between the cross correlation of ambient Rayleigh waves (seismic ambient noise) and the attenuation parameter α associated with Rayleigh-wave propagation. In particular, we derive a mathematical expression for the multiplicative factor relating normalized cross correlation to the Rayleigh-wave Green’s function. Based on this expression, we formulate an inverse problem to determine α from cross correlations of recorded ambient signal. We conduct a preliminary application of our algorithm to a relatively small instrument array, conveniently deployed on an island. In our setup, the mentioned multiplicative factor has values of about 2.5–3, which, if neglected, could result in a significant underestimate of α. We find that our inferred values of α are reasonable, in comparison with independently obtained estimates found in the literature. Allowing α to vary with respect to frequency results in a reduction of misfit between observed and predicted cross correlations.

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“…The common features in these studies include low Q l values in the uppermost mantle ($80-200 km), intermediate Q l values in the transition zone (200-650 km), and highest Q l values in the lower mantle. While several discrepancies persist in attenuation tomography [Romanowicz and Mitchell, 2015], all models show a somewhat abrupt jump to high Q l in the lower mantle that is significant beyond the 2r uncertainties on either side of the 650 km discontinuity as reported by Resovsky et al [2005]. Moulik [2016] evaluated the robustness of this feature by modulating the jump in Q l through regularization and found that it is required to fit recent normal-mode observations.…”

Section: Translating Physical Properties To Seismic Structure: Modelimentioning

confidence: 94%

The importance of grain size to mantle dynamics and seismological observations

Dannberg

Eilon

Faul

et al. 2017

Geochem Geophys Geosyst

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Grain size plays a key role in controlling the mechanical properties of the Earth's mantle, affecting both long‐time‐scale flow patterns and anelasticity on the time scales of seismic wave propagation. However, dynamic models of Earth's convecting mantle usually implement flow laws with constant grain size, stress‐independent viscosity, and a limited treatment of changes in mineral assemblage. We study grain size evolution, its interplay with stress and strain rate in the convecting mantle, and its influence on seismic velocities and attenuation. Our geodynamic models include the simultaneous and competing effects of dynamic recrystallization resulting from dislocation creep, grain growth in multiphase assemblages, and recrystallization at phase transitions. They show that grain size evolution drastically affects the dynamics of mantle convection and the rheology of the mantle, leading to lateral viscosity variations of 6 orders of magnitude due to grain size alone, and controlling the shape of upwellings and downwellings. Using laboratory‐derived scaling relationships, we convert model output to seismologically observable parameters (velocity and attenuation) facilitating comparison to Earth structure. Reproducing the fundamental features of the Earth's attenuation profile requires reduced activation volume and relaxed shear moduli in the lower mantle compared to the upper mantle, in agreement with geodynamic constraints. Faster lower mantle grain growth yields best fit to seismic observations, consistent with our reexamination of high‐pressure grain growth parameters. We also show that ignoring grain size in interpretations of seismic anomalies may underestimate the Earth's true temperature variations.

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Deep Earth Structure: Q of the Earth from Crust to Core

Cited by 68 publications

References 327 publications

Grain size‐sensitive viscoelastic relaxation and seismic properties of polycrystalline MgO

Grain size‐sensitive viscoelastic relaxation and seismic properties of polycrystalline MgO

On seismic ambient noise cross-correlation and surface-wave attenuation

The importance of grain size to mantle dynamics and seismological observations

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