A new 3-D mantle density model from recent normal-mode measurements

2021 ◽  
Author(s):  
Rûna van Tent ◽  
Arwen Deuss ◽  
Andreas Fichtner ◽  
Lars Gebraad ◽  
Simon Schneider ◽  
...  
Keyword(s):  
Normal Mode ◽  
Lower Mantle ◽  
Velocity Structure ◽  
Normal Modes ◽  
Shear Velocity ◽  
Boundary Region ◽  
Splitting Function ◽  
The Pacific ◽  
Compositional Variations ◽  
Body Tides

<p>Constraints on the 3-D density structure of Earth’s mantle provide important insights into the nature of seismically observed features, such as the Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle under Africa and the Pacific. The only seismic data directly sensitive to density variations throughout the entire mantle are normal modes: whole Earth oscillations that are induced by large earthquakes (M<sub>w</sub> > 7.5). However, their sensitivity to density is weak compared to the sensitivity to velocity and different studies have presented conflicting density models of the lower mantle. For example, Ishii & Tromp (1999) and Trampert et al. (2004) have found that the LLSVPs have a larger density than the surrounding mantle, while Koelemeijer et al. (2017) used additional Stoneley-mode observations, which are particularly sensitive to the core-mantle boundary region, to show that the LLSVPs have a lower density. Recently, Lau et al. (2017) have used tidal tomography to show that Earth's body tides prefer dense LLSVPs.</p><p>A large number of new normal-mode splitting function measurements has become available since the last density models of the entire mantle were published. Here, we show the models from our inversion of these recent data and compare our results to previous studies. We find areas of high as well as low density at the base of the LLSVPs and we find that inside the LLSVPs density varies on a smaller scale than velocity, indicating the presence of compositionally distinct material. In fact, we find low correlations between the density and velocity structure throughout the entire mantle, revealing that compositional variations are required at all depths inside the mantle.</p>

Global observations of 3D mantle attenuation using normal modes 

2021 ◽  
Author(s):  
Sujania Talavera-Soza ◽  
Arwen Deuss
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Velocity Structure ◽  
3D Structure ◽  
Normal Modes ◽  
Seismic Attenuation ◽  
Body Waves ◽  
Splitting Function ◽  
Thermal Origin

<div> <div> <div> <p>Seismic tomographic models based solely on wave velocities are unable to distinguish between a temperature or compositional origin for Earth’s 3D structure variations, such as the Large Low Shear Velocity Provinces (LLSVPs) beneath the lower mantle of Africa and the Pacific. Seismic attenuation or damping is able able to provide additional information that may help to unravel the origin of the LLSVPs, which is fundamental to understand mantle convection evolution. For example, a thermal origin for the LLSVPs will point to them being short-lived anomalies, whereas a compositional origin will point to them being long-lived, forming mantle 'anchors' and influencing the pattern of mantle convection for a large part of Earth’s history. Seismic attenuation is able to make that distinction, because it is directly sensitive to temperature variations. So far, global 3D attenuation models have only been available for the upper mantle, with only two regional body waves studies exploring the lower mantle (Lawrence and Wysession, 2006; Hwang and Ritsema, 2011).<br>Here, we use normal mode data to measure elastic splitting functions (dependent on velocity and density) and anelastic splitting functions (dependent on attenuation). The advantage of normal modes is that they allow us to include focussing and scattering due to the velocity structure without the need for approximations, because we measure the elastic splitting function jointly with the anelastic splitting function. In our measurements for upper mantle sensi- tive modes, we find anti-correlation between the elastic and anelastic splitting functions, suggesting a thermal origin for low velocity spreading ridges, and agreeing with previous studies. On the other hand, for lower mantle sensitive modes, we find correlation, suggesting the averagely attenuating LLSVPs are surrounded by strongly attenuating regions potentially due to the presence of post-perovskite.</p> </div> </div> </div>

scholarly journals A new catalogue of toroidal-mode overtone splitting function measurements

2020 ◽  
Author(s):  
Simon Schneider ◽  
Arwen Deuss
Keyword(s):  
Normal Mode ◽  
Large Scale ◽  
Velocity Structure ◽  
Normal Modes ◽  
Splitting Function ◽  
Data Set ◽  
Toroidal Mode ◽  
Function Coefficient ◽  
Fiji Island ◽  
Shear Wave Velocity Structure

Abstract Spectra of whole Earth oscillations or normal modes provide important constraints on Earth’s large scale structure. The most convenient way to include normal mode constraints in global tomographic models is by using splitting functions or structure coefficients, which describe how the frequency of a specific mode varies regionally. Splitting functions constrain 3D variations in velocity, density structure and boundary topography. They may also constrain anisotropy, especially when combining information from spheroidal modes, which are mainly sensitive to P-SV structure, with toroidal modes, mainly sensitive to SH structure. Spheroidal modes have been measured extensively, but toroidal modes have proven to be much more difficult and as a result only a limited number of toroidal modes have been measured so far. Here we expand the splitting function studies by Resovsky and Ritzwoller (1998) and Deuss et al. (2013), by focusing specifically on toroidal mode overtone observations. We present splitting function measurements for 19 self-coupled toroidal modes of which 13 modes have not been measured before. They are derived from radial and transverse horizontal component normal mode spectra up to 5 mHz for 91 events with MW ≥ 7.4 from the years 1983-2018. Our data include the Tohoku event of 2011 (9.1MW), the Okhotsk event of 2013 (8.3MW) and the Fiji Island event from 2018 (8.2MW). Our measurements provide new constraints on upper and lower mantle shear wave velocity structure and in combination with existing spheroidal mode measurements can be used in future inversions for anisotropic mantle structure. Our new splitting function coefficient data set will be available online.

Viscoelasticity of the lower mantle from forward modeling of normal modes and solid Earth tides

2020 ◽  
Author(s):  
Ulrich Faul ◽  
Harriet Lau
Keyword(s):  
Absorption Band ◽  
Solid Earth ◽  
Lower Mantle ◽  
Normal Modes ◽  
Earth Tides ◽  
Solid Earth Tides ◽  
Anelastic Behavior ◽  
Body Tides ◽  
Anelastic Deformation ◽  
Intrinsic Dissipation

<p>Grain scale diffusive processes are involved in the rheology at convective timescales, but also at the transient timescales of seismic wave propagation, solid Earth tides and post-glacial rebound. Seismic and geodetic data can therefore potentially provide constraints on lower mantle properties such as grain size that are unconstrained otherwise. Current models of the transient viscosity of the lower mantle infer an absorption band of finite width in frequency. Seismic models predict a low frequency end to the absorption band at timescales corresponding to the longest normal modes of about an hour. By contrast, geodetic models infer the onset of an absorption band at these frequencies to cover anelastic deformation at timescales up to 18.6 years. A difficulty in extracting frequency dependence from mode and tide data is its convolution with depth dependence.</p><p>To circumvent this problem we select a distinct set of seismic normal modes and solid Earth body tides that have similar depth sensitivity in the lower mantle. These processes collectively span a period range from 7 minutes to 18.6 years. This allows the examination of frequency dependent energy dissipation over the lower mantle across 6 orders of magnitude. To forward model the transient creep response of the lower mantle we use a laboratory-based model of intrinsic dissipation that we adapt to the lower mantle mineralogy. This extended Burgers model represents an empirical fit to data principally from olivine, but also MgO and other compounds. The underlying microphysical model envisions a sequence of processes that begin with a broad plateau in dissipation at the highest frequencies after the onset of anelastic behavior, followed by a broad absorption band spanning many decades in frequency. The absorption band transitions seamlessly into viscous behavior. Since dissipation both for the absorption band and for (Newtonian) viscous behavior is due to diffusion along grain boundaries there can be no gap between the end of the absorption band and onset of viscous behavior.</p><p>Modeling of the planetary response to small strain excitation necessitates consideration of inertia and self gravitation. The phase lag due to solid Earth body tides therefore does not correspond directly to the intrinsic dissipation measured in the laboratory as material property. We have developed a self consistent theory that combines the planetary response with time-dependent anelastic deformation of rocks. Results from a broad range of forward models show that at lower mantle pressures periods of modes fall onto the broad plateau in dissipation at the onset of anelastic behavior. This explains the apparent frequency independence or even negative frequency dependence observed for some normal modes. At longer timescales, solid Earth tides fall on the frequency-dependent absorption band. This reconciles seemingly contradictory results published by seismic and tidal studies. Observations at even longer timescales are needed to constrain the transition from absorption band to viscous behavior.</p>

Improving Stoneley-mode constraints on the structures near the core-mantle boundary

2020 ◽  
Author(s):  
Harry Matchette-Downes ◽  
Robert D. van der Hilst ◽  
Jingchen Ye ◽  
Jia Shi ◽  
Jiayuan Han ◽  
...  
Keyword(s):  
Material Properties ◽  
Three Dimensional ◽  
Normal Modes ◽  
Shear Velocity ◽  
Boundary Region ◽  
Earth Model ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Special Cases

<p>Although observations of seismic normal modes provide constraints on the structure of the entire Earth, the core-mantle boundary region remains poorly understood. Stoneley modes should offer better constraints, because they are confined near to the fluid-solid interface, but this property also makes them difficult to detect. In this study, we use recently-developed finite-element approach to show that Stoneley modes can be excited and detected, but only in certain special cases. We first investigate the physical explanation for these cases. Next, we describe how they could be detected in seismic data, and the sensitivity of these data to the material properties. We illustrate this sensitivity by calculating the modes of a three-dimensional Earth model containing a large low-shear-velocity province (LLSVP). Finally, we present some preliminary observations. We hope that this new understanding will lead to new constraints on the material properties and morphology of the core-mantle boundary region. In turn, this information, especially the constraints on density, should help to answer questions about the Earth, for example in mantle convection (are LLSVPs thermally or chemically buoyant? Primordial or slab graveyards? Passive or active?) and core convection (does the outermost core have a stable stratification?).</p>

scholarly journals The signal of outermost-core stratification in body-wave and normal-mode data

2020 ◽  
Vol 223 (2) ◽  
pp. 1338-1354
Author(s):  
Rûna van Tent ◽  
Arwen Deuss ◽  
Satoshi Kaneshima ◽  
Christine Thomas
Keyword(s):  
Normal Mode ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Body Wave ◽  
Normal Modes ◽  
Outer Core ◽  
Core Structure ◽  
Seismic Velocities ◽  
Data Types ◽  
Additional Information

Summary Seismological models of the outer core’s radial velocity structure show that the outermost core is slower than PREM. For models derived from body-wave data these low velocities are confined to the top of the outer core, while normal-mode data prefer a velocity gradient that deviates from PREM throughout the entire outer core. These different models have led to conflicting interpretations regarding the presence of stratification at the top of the outer core. While body-wave based models have been shown to require a compositionally stratified outermost core, the velocity and density profiles obtained from normal-mode data correspond to a homogeneous outer core. In addition, the observed low velocities in the outermost core are difficult to reconcile with compositional models of stratification, as the required enrichment in light elements would generally increase seismic velocities. Here, we investigate how well-suited both seismic body-wave and normal-mode data are to constrain the velocity and density structure of the outer core. To this end, we model and compare the effects of outer-core structure and D″ structure on the differential traveltimes of body-wave phases SmKS and on the centre frequencies of normal modes. We find that a trade-off between outer-core structure and D″ structure exists for both data types, but neither data can be readily explained by reasonable D″ velocities and densities. Low outermost-core velocities are therefore still required by seismological data. Using additional information from the centre frequencies of Stoneley modes—normal modes that are particularly sensitive to variations in velocity and density at the top of the outer core—we confirm that normal-mode data indeed require low velocities with respect to PREM in the outermost core, similar to a recent normal-mode model, and an overall higher outer-core density. The presence of buoyant stratification in the outermost core is therefore not immediately supported by the centre frequencies of Stoneley modes. Stratification with high seismic velocity, as one would expect from most straightforward stratification-forming processes, is directly contradicted by our results.

FrosPy: A Modular Python Toolbox for Normal Mode Seismology

2022 ◽  
Author(s):  
Simon Schneider ◽  
Sujania Talavera-Soza ◽  
Lisanne Jagt ◽  
Arwen Deuss
Keyword(s):  
Normal Mode ◽  
Free Oscillation ◽  
Large Time ◽  
Reference Model ◽  
Normal Modes ◽  
Earth Model ◽  
Splitting Function ◽  
Quality Factor Q ◽  
Earth’S Free Oscillation ◽  
Q Values

Abstract We present free oscillations Python (FrosPy), a modular Python toolbox for normal mode seismology, incorporating several Python core classes that can easily be used and be included in larger Python programs. FrosPy is freely available and open source online. It provides tools to facilitate pre- and postprocessing of seismic normal mode spectra, including editing large time series and plotting spectra in the frequency domain. It also contains a comprehensive database of center frequencies and quality factor (Q) values based on 1D reference model preliminary reference Earth model for all normal modes up to 10 mHz and a collection of published measurements of center frequencies, Q values, and splitting function (or structure) coefficients. FrosPy provides the tools to visualize and convert different formats of splitting function coefficients and plot these as maps. By giving the means of using and comparing normal mode spectra and splitting function measurements, FrosPy also aims to encourage seismologists and geophysicists to learn about normal mode seismology and the study of the Earth’s free oscillation spectra and to incorporate them into their own research or use them for educational purposes.

The upper mantle beneath the Aleutian Island Arc from pure-path Rayleigh-wave dispersion data

1972 ◽  
Vol 62 (6) ◽  
pp. 1439-1453 ◽  
Author(s):  
K. H. Jacob ◽  
K. Hamada
Keyword(s):  
Velocity Structure ◽  
Shear Velocity ◽  
Wave Dispersion ◽  
Island Arc ◽  
Aleutian Island ◽  
Low Velocity ◽  
The Pacific ◽  
Average Shear ◽  
Tectonically Active ◽  
Group Velocities

abstract Group velocities of Rayleigh waves recorded at a long-period seismograph station on Amchitka Island were obtained for mixed tectonic paths across the Pacific and the Aleutian Island Arc Ridge. The mixed-path group velocities for periods between 20 and 60 sec were then separated into pure-oceanic and pureridge path group velocities. The group velocities for the pure paths along the Aleutian ridge are on the average 0.36 km/sec lower than those for the purely oceanic paths across the Pacific. Inversion of the pure-ridge group velocities yields almost continental shear velocities in the crust, a very gradual crust-mantle transition at depths between 20 and 40 km, a thin lithospheric lid of uppermost mantle material between 30 and 70 km with relatively low maximum shear velocities approaching 4.4 km/sec, and a very pronounced low-velocity zone at depths below 70 km with an average shear velocity of 4.1 km/sec. The computed shear velocity structure beneath the Aleutian Ridge is compared to models for other tectonically active and stable regions.

Sign in / Sign up

Export Citation Format

Share Document