The lack of equipartitioning in global body wave coda

Sens‐Schönfelder, Christoph; Snieder, Roel; Stähler, Simon C.

doi:10.1002/2015gl065108

Cited by 49 publications

(32 citation statements)

References 44 publications

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“…In the late coda window, the main contribution to the correlation wavefield comes from waves that reverberate nearly vertically (as shown in a recent study by Sens-Schönfelder et al, 2015) so that surface waves are largely suppressed. All phases identified in the correlation wavefield correspond to differences between seismic arrivals with the same ray parameter and a subset of propagation legs in common.…”

Section: Discussion and Concluding Remarksmentioning

confidence: 88%

Earth's Correlation Wavefield: Late Coda Correlation

Phạm

Tkalčić

Sambridge

et al. 2018

Geophysical Research Letters

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Cross correlation of seismograms provides new information on the Earth both through the exploitation of ambient noise and specific components of earthquake records. Here we cross‐correlate recordings of large earthquakes on a planetary scale and identify a range of hitherto unobserved seismic phases in Earth's correlation wavefield. We show that both arrivals with the timing expected for the regular seismic wavefield and previously unexplained phases are produced by interference between seismic paths having the same ray parameter but with only a subset of propagation legs in common. This insight explains the origin and generation mechanism of the features of Earth's correlation wavefield and opens up new ways of addressing issues in global seismology. Strong similarity between observed and synthesized correlation wavefields indicates that the Earth's radial structure is remarkably well constrained in the intermediate period range.

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Section: Discussion and Concluding Remarksmentioning

confidence: 88%

Earth's Correlation Wavefield: Late Coda Correlation

Phạm

Tkalčić

Sambridge

et al. 2018

Geophysical Research Letters

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“…Array observations of the early coda suggest that it is mostly composed of a sum of scattered fundamental modes Rayleigh waves, whereas the late coda appears to be dominated by high − Q spheroidal higher-modes. Sens-Schönfelder et al (2015) used data from USArray to analyze the angular distribution of energy in the coda of the 2013 Okhotsk earthquake. They showed that multiple reverberations without significant effect of scattering persist at long lapse-time in the coda in certain period bands.…”

Section: Introductionmentioning

confidence: 99%

Breakdown of equipartition in diffuse fields caused by energy leakage

Margerin

2017

Eur. Phys. J. Spec. Top.

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Abstract. Equipartition is a central concept in the analysis of random wavefields which stipulates that in an infinite scattering medium all modes and propagation directions become equally probable at long lapse time in the coda. The objective of this work is to examine quantitatively how this conclusion is affected in an open waveguide geometry, with a particular emphasis on seismological applications. To carry our this task, the problem is recast as a spectral analysis of the radiative transfer equation. Using a discrete ordinate approach, the smallest eigenvalue and associated eigenfunction of the transfer equation, which control the asymptotic intensity distribution in the waveguide, are determined numerically with the aid of a shooting algorithm. The inverse of this eigenvalue may be interpreted as the leakage time of the diffuse waves out of the waveguide. The associated eigenfunction provides the depth and angular distribution of the specific intensity. The effect of boundary conditions and scattering anisotropy is investigated in a series of numerical experiments. Two propagation regimes are identified, depending on the ratio H * between the thickness of the waveguide and the transport mean path in the layer. The thick layer regime H * > 1 has been thoroughly studied in the literature in the framework of diffusion theory and is briefly considered. In the thin layer regime H * < 1, we find that both boundary conditions and scattering anisotropy leave a strong imprint on the leakage effect. A parametric study reveals that in the presence of a flat free surface, the leakage time is essentially controlled by the mean free time of the waves in the layer in the limit H * → 0. By contrast, when the free surface is rough, the travel time of ballistic waves propagating through the crust becomes the limiting factor. For fixed H * , the efficacy of leakage, as quantified by the inverse coda quality factor, increases with scattering anisotropy. For sufficiently thin layers H * ≈ 1/5, the energy flux is predominantly directed parallel to the surface and equipartition breaks down. Qualitatively, the anisotropy of the intensity field is found to increase with the inverse non-dimensional leakage time, with the scattering mean free time as time scale. Because it enhances leakage, a rough free surface may result in stronger anisotropy of the intensity field than a flat surface, for the same bulk scattering properties. Our work identifies leakage as a potential explanation for the large deviation from isotropy observed in the coda of body waves.

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“…Sens‐Schönfelder et al () conducted a thorough analysis of the equipartitioning properties of the coda waves by studying the deep Okhotsk event from 2013. They concluded that the earlier part of the coda of a teleseismic event (<5,000 s) is suitable for local body wave retrieval with mainly isotropic propagation.…”

Section: Discussionmentioning

confidence: 99%

Retrieval of Interstation Local Body Waves From Teleseismic Coda Correlations

Saygin

2019

JGR Solid Earth

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We retrieve the local P wave empirical Green's functions between the elements of five different regional arrays across the globe by cross‐correlating and bin stacking the teleseismic earthquake coda waves recorded at each array. The stack is made using the coda of P and S wave phases for events in the distance range from 40° to 50° from the center of the array. With a sequence of time windows along the coda the various body wave arrivals can be tracked, using record sections constructed by binning the stacked interstation correlograms in less than 1‐km distance increments. The correlation of the coda part of each principal seismic phase produces highly coherent interstation arrivals for different analysis windows. Such arrivals can be reproduced by just stacking 100 arrivals from a pool of more than a thousand events, showing the stability of the observed Green's functions. Modeling for the structure beneath the Warramunga array in the Northern Territory, Australia, demonstrates that these arrivals correspond to multiply reflected arrivals from layers at different depths. The recovery of high‐frequency interstation body waves from the teleseismic earthquake coda opens the prospect of conducting local high‐resolution seismic imaging with teleseismic energy.

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The lack of equipartitioning in global body wave coda

Cited by 49 publications

References 44 publications

Earth's Correlation Wavefield: Late Coda Correlation

Earth's Correlation Wavefield: Late Coda Correlation

Breakdown of equipartition in diffuse fields caused by energy leakage

Retrieval of Interstation Local Body Waves From Teleseismic Coda Correlations

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