Local coupling and conversion of surface waves due to Earth’s rotation. Part 1: theory

Snieder, Roel; Sens‐Schönfelder, Christoph

doi:10.1093/gji/ggaa587

Cited by 5 publications

(1 citation statement)

References 24 publications

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“…The Earth's Coriolis effect influences all seismic wave propagation, but it has the most noticeable effect on normal modes (Backus and Gilbert, 1961;Masters et al, 1983;Dahlen and Tromp, 1998) and surface waves (e.g., Park and Gilbert, 1986;Tromp, 1994;Snieder and Sens-Schönfelder, 2021). Body waves, particularly shear waves, can be modestly affected (Schoenberg and Censor, 1973;Snieder et al, 2016).…”

Section: Polarization Anomalies Caused By Earth's Coriolis Effectmentioning

confidence: 99%

On the measurement of Sdiff splitting caused by lowermost mantle anisotropy

Wolf

Long

Creasy

et al. 2022

Preprint

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Seismic anisotropy has been detected at many depths of the Earth, including its upper layers, the lowermost mantle, and the inner core. While upper mantle seismic anisotropy is relatively straightforward to resolve, lowermost mantle anisotropy has proven to be more complicated to measure. Due to their long, horizontal raypaths along the core-mantle boundary, S waves diffracted along the core-mantle boundary (Sdiff) are potentially strongly influenced by lowermost mantle anisotropy. Sdiff waves can be recorded over a large epicentral distance range and thus sample the lowermost mantle everywhere around the globe. Sdiff therefore represents a promising phase for studying lowermost mantle anisotropy; however, previous studies have pointed out some difficulties with the interpretation of differential SHdiff-SVdiff travel times in terms of seismic anisotropy. Here, we provide a new, comprehensive assessment of the usability of Sdiff waves to infer lowermost mantle anisotropy. Using both axisymmetric and fully 3D global wavefield simulations, we show that there are cases in which Sdiff can reliably detect and characterize deep mantle anisotropy when measuring traditional splitting parameters (as opposed to differential travel times). First, we analyze isotropic effects on Sdiff polarizations, including the influence of realistic velocity structure (such as 3D velocity heterogeneity and ultra-low velocity zones), the character of the lowermost mantle velocity gradient, mantle attenuation structure, and Earth's Coriolis force. Second, we evaluate effects of seismic anisotropy in both the upper and the lowermost mantle on SHdiff waves. In particular, we investigate how SHdiff waves are split by seismic anisotropy in the upper mantle near the source and how this anisotropic signature propagates to the receiver for a variety of lowermost mantle models. We demonstrate that, in particular and predictable cases, anisotropy leads to Sdiff splitting that can be clearly distinguished from other waveform effects. These results enable us to lay out a strategy for the analysis of Sdiff splitting due to anisotropy at the base of the mantle, which includes steps to help avoid potential pitfalls, with attention paid to the initial polarization of Sdiff and the influence of source-side anisotropy. We demonstrate our Sdiff splitting method using three earthquakes that occurred beneath the Celebes Sea, measured at many Transportable Array (TA) stations at a suitable epicentral distance. We resolve consistent and well-constrained Sdiff splitting parameters due to lowermost mantle anisotropy beneath the northeastern Pacific Ocean.

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Section: Polarization Anomalies Caused By Earth's Coriolis Effectmentioning

confidence: 99%

On the measurement of Sdiff splitting caused by lowermost mantle anisotropy

Wolf

Long

Creasy

et al. 2022

Preprint

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Local Coupling and Conversion of Surface Waves due to Earth’s Rotation. Part 2: Numerical Examples

Sens‐Schönfelder

Bozdag

Snieder

2020

Geophysical Journal International

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Summary Rotation of the Earth affects the propagation of seismic waves. The global coupling of spheroidal and toroidal modes by the Coriolis force over time is described by normal-mode theory. The local action of the Coriolis force on the propagation of surface waves can be described by coefficients for the coupling between propagating Rayleigh and Love waves as derived by (Landau & Lifshitz 1959). Using global wavefield simulations we show how the Coriolis force leads to coupling and conversion between both surface wave types depending on latitude, propagation direction, frequency, and local velocity structure. Surface wave coupling is most efficient for periods where the modes have similar phase velocities, a condition that is equivalent to the selection rules of the angular degree in the normal-mode framework, a phenomenon that we refer to as resonant coupling. In the time-domain, resonant coupling gradually converts energy from one wave type–Rayleigh waves or Love wave–into the other, which then propagates independently. Due to the lateral heterogeneity, the condition of equal phase velocity renders the rotational coupling location-dependent. East-west oriented ray path segments and segments at high latitudes (across the Poles) only weakly couple the fundamental mode Rayleigh and Love waves while coupling is strongest for propagation along the meridians across the equator. At 250 s period, where Love and Rayleigh waves have similar phase velocities, the net energy transfer from Rayleigh to Love wave reaches about 10% for one orbit.

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A spectral element approach to computing normal modes

Kemper

Driel

Munch

et al. 2021

Geophysical Journal International

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We introduce a new approach to the computation of gravito-elastic free oscillations or normal modes of spherically symmetric bodies based on a spectral element discretization of the radial ordinary differential equations (ODEs). Our method avoids numerical instabilities often encountered in the classical method of radial integration and root finding of the characteristic function. To this end, the code is built around a sparse matrix formulation of the eigenvalue problem taking advantage of state-of-the-art parallel iterative solvers. We apply the method to toroidal, spheroidal, and radial modes and we demonstrate its versatility in the presence of attenuation, fluid layers, and gravity (including the purely elastic case, the Cowling approximation, and full gravity). We demonstrate higher-order convergence and verify the software by computing seismograms and comparing these to existing numerical solutions. Finally, to emphasise the general applicability of our code, we show spectra and eigenfunctions of Earth, Mars, and Jupiter’s icy moon Europa and discuss the different types of modes that emerge.

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Local coupling and conversion of surface waves due to Earth’s rotation. Part 1: theory

Cited by 5 publications

References 24 publications

On the measurement of Sdiff splitting caused by lowermost mantle anisotropy

On the measurement of Sdiff splitting caused by lowermost mantle anisotropy

Local Coupling and Conversion of Surface Waves due to Earth’s Rotation. Part 2: Numerical Examples

A spectral element approach to computing normal modes

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