scholarly journals Seismological expression of the iron spin crossover in ferropericlase in the Earth’s lower mantle

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Grace E. Shephard ◽  
Christine Houser ◽  
John W. Hernlund ◽  
Juan J. Valencia-Cardona ◽  
Reidar G. Trønnes ◽  
...  
Keyword(s):  
Lower Mantle ◽  
Spin Crossover ◽  
Shear Velocity ◽  
Oceanic Lithosphere ◽  
P Wave ◽  
Depth Range ◽  
S Wave ◽  
Seismic Profiles ◽  
P Waves ◽  
Wave Tomography

AbstractThe two most abundant minerals in the Earth’s lower mantle are bridgmanite and ferropericlase. The bulk modulus of ferropericlase (Fp) softens as iron d-electrons transition from a high-spin to low-spin state, affecting the seismic compressional velocity but not the shear velocity. Here, we identify a seismological expression of the iron spin crossover in fast regions associated with cold Fp-rich subducted oceanic lithosphere: the relative abundance of fast velocities in P- and S-wave tomography models diverges in the ~1,400-2,000 km depth range. This is consistent with a reduced temperature sensitivity of P-waves throughout the iron spin crossover. A similar signal is also found in seismically slow regions below ~1,800 km, consistent with broadening and deepening of the crossover at higher temperatures. The corresponding inflection in P-wave velocity is not yet observed in 1-D seismic profiles, suggesting that the lower mantle is composed of non-uniformly distributed thermochemical heterogeneities which dampen the global signature of the Fp spin crossover.

scholarly journals Comparing ray-theoretical and finite-frequency teleseismic traveltimes: implications for constraining the ratio of S-wave to P-wave velocity variations in the lower mantle

2020 ◽  
Vol 224 (3) ◽  
pp. 1540-1552
Author(s):  
Carlos A M Chaves ◽  
Jeroen Ritsema ◽  
Paula Koelemeijer
Keyword(s):  
North America ◽  
Wave Velocity ◽  
Lower Mantle ◽  
P Wave ◽  
Finite Frequency ◽  
S Wave ◽  
P Waves ◽  
Long Period ◽  
P Wave Velocity ◽  
The Pacific

SUMMARY A number of seismological studies have indicated that the ratio R of S-wave and P-wave velocity perturbations increases to 3–4 in the lower mantle with the highest values in the large low-velocity provinces (LLVPs) beneath Africa and the central Pacific. Traveltime constraints on R are based primarily on ray-theoretical modelling of delay times of P waves (ΔTP) and S waves (ΔTS), even for measurements derived from long-period waveforms and core-diffracted waves for which ray theory (RT) is deemed inaccurate. Along with a published set of traveltime delays, we compare predicted values of ΔTP, ΔTS, and the ΔTS/ΔTP ratio for RT and finite-frequency (FF) theory to determine the resolvability of R in the lower mantle. We determine the FF predictions of ΔTP and ΔTS using cross-correlation methods applied to spectral-element method waveforms, analogous to the analysis of recorded waveforms, and by integration using FF sensitivity kernels. Our calculations indicate that RT and FF predict a similar variation of the ΔTS/ΔTP ratio when R increases linearly with depth in the mantle. However, variations of R in relatively thin layers (< 400 km) are poorly resolved using long-period data (T > 20 s). This is because FF predicts that ΔTP and ΔTS vary smoothly with epicentral distance even when vertical P-wave and S-wave gradients change abruptly. Our waveform simulations also show that the estimate of R for the Pacific LLVP is strongly affected by velocity structure shallower in the mantle. If R increases with depth in the mantle, which appears to be a robust inference, the acceleration of P waves in the lithosphere beneath eastern North America and the high-velocity Farallon anomaly negates the P-wave deceleration in the LLVP. This results in a ΔTP of about 0, whereas ΔTS is positive. Consequently, the recorded high ΔTS/ΔTP for events in the southwest Pacific and stations in North America may be misinterpreted as an anomalously high R for the Pacific LLVP.

The effects of seismic anisotropy on  S-wave travel time-tomography: the problem of apparent anomalies and possible solutions

2021 ◽  
Author(s):  
Francesco Rappisi ◽  
Brandon Paul Vanderbeek ◽  
Manuele Faccenda
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Shear Velocity ◽  
P Wave ◽  
Travel Times ◽  
S Wave ◽  
Velocity Models ◽  
Travel Time Tomography ◽  
Wave Travel ◽  
Wave Tomography

<p>Teleseismic travel-time tomography remains one of the most popular methods for obtaining images of Earth's upper mantle. While teleseismic shear phases, most notably SKS, are commonly used to infer the anisotropic properties of the upper mantle, anisotropic structure is often ignored in the construction of body wave shear velocity models. Numerous researchers have demonstrated that neglecting anisotropy in P-wave tomography can introduce significant imaging artefacts that could lead to spurious interpretations. Less attention has been given to the effect of anisotropy on S-wave tomography partly because, unlike P-waves, there is not a ray-based methodology for modelling S-wave travel-times through anisotropic media. Here we evaluate the effect that the isotropic approximation has on tomographic images of the subsurface when shear waves are affected by realistic mantle anisotropy patterns. We use SPECFEM to model the teleseismic shear wavefield through a geodynamic model of subduction that includes elastic anisotropy predicted from micromechanical models of polymineralic aggregates advected through the simulated flow field. We explore how the chosen coordinates system in which S-wave arrival times are measured (e.g., radial versus transverse) affects the imaging results. In all cases, the isotropic imaging assumption leads to numerous artefacts in the recovered velocity models that could result in misguided inferences regarding mantle dynamics. We find that when S-wave travel-times are measured in the direction of polarisation, the apparent anisotropic shear velocity can be approximated using sinusoidal functions of period pi and two-pi. This observation allows us to use ray-based methods to predict S-wave travel-times through anisotropic models. We show that this parameterisation can be used to invert S-wave travel-times for the orientation and strength of anisotropy in a manner similar to anisotropic P-wave travel-time tomography. In doing so, the magnitude of imaging artefacts in the shear velocity models is greatly reduced.</p>

scholarly journals Bridgmanite-Predominant Lower Mantle Evidenced from Velocity and Density Profiles across the 660-km Discontinuity

2021 ◽  
Author(s):  
Suyu Fu ◽  
Yanyao Zhang ◽  
Takuo Okuchi ◽  
Jung-Fu Lin
Keyword(s):  
Lower Mantle ◽  
P Wave ◽  
S Wave ◽  
Seismic Profiles ◽  
Density Profiles ◽  
Deep Earth ◽  
Casio3 Perovskite ◽  
Diamond Anvil ◽  
Heat Capacity Measurements ◽  
Best Fit

Abstract Earth’s mantle composition is essential to our understanding of its physics and dynamics. Here we report single-crystal elasticity (Cij) of (Al,Fe)-bearing bridgmanite, Mg0.88Fe0.1Al0.14Si0.90O3 with Fe3+/∑Fe=~0.65, up to ~82 GPa measured in diamond anvil cells. Together with heat capacity measurements on bridgmanite and ferropericlase, we develop a fully internally-consistent thermoelastic model to simultaneously evaluate lower-mantle mineralogy and geotherm via comparisons of P-wave, S-wave velocities, and density (VP, VS, and ρ) with one-dimensional seismic profiles. Our best-fit model demonstrates the lower mantle consists of ~89 vol% (Al,Fe)-bearing bridgmanite, ~4 vol% ferropericlase, and ~7 vol% CaSiO3 perovskite. A chemically layered mantle with pyrolitic upper mantle and bridgmanite-predominant lower mantle would display ~3.2(±1.5)%, ~5.2(±1.5)%, and ~5.0(±1.0)% jumps in VP, VS, and ρ, respectively, across the 660-km discontinuity, which are well consistent with seismic reflection observations. The lower mantle could have become bridgmanite-predominant via accumulations of ancient silica-rich materials, which helps explain current deep-Earth seismic and geochemical signatures.

scholarly journals P-wave tomography beneath Greenland and surrounding regions-II. Lower mantle

2020 ◽  
Author(s):  
Genti Toyokuni ◽  
Takaya Matsuno ◽  
Dapeng Zhao
Keyword(s):  
Lower Mantle ◽  
P Wave ◽  
Wave Tomography

Numerical simulation of azimuthal acoustic logging in a borehole penetrating a rock formation boundary

Geophysics ◽  
2016 ◽  
Vol 81 (3) ◽  
pp. D283-D291 ◽  
Author(s):  
Peng Liu ◽  
Wenxiao Qiao ◽  
Xiaohua Che ◽  
Xiaodong Ju ◽  
Junqiang Lu ◽  
...  
Keyword(s):  
Domain Model ◽  
P Wave ◽  
S Wave ◽  
Angle Variation ◽  
P Waves ◽  
Acoustic Logging ◽  
S Waves ◽  
Rock Formation ◽  
Logging Tool ◽  
Difference Time

We have developed a new 3D acoustic logging tool (3DAC). To examine the azimuthal resolution of 3DAC, we have evaluated a 3D finite-difference time-domain model to simulate a case in which the borehole penetrated a rock formation boundary when the tool worked at the azimuthal-transmitting-azimuthal-receiving mode. The results indicated that there were two types of P-waves with different slowness in waveforms: the P-wave of the harder rock (P1) and the P-wave of the softer rock (P2). The P1-wave can be observed in each azimuthal receiver, but the P2-wave appears only in the azimuthal receivers toward the softer rock. When these two types of rock are both fast formations, two types of S-waves also exist, and they have better azimuthal sensitivity compared with P-waves. The S-wave of the harder rock (S1) appears only in receivers toward the harder rock, and the S-wave of the softer rock (S2) appears only in receivers toward the softer rock. A model was simulated in which the boundary between shale and sand penetrated the borehole but not the borehole axis. The P-wave of shale and the S-wave of sand are azimuthally sensitive to the azimuth angle variation of two formations. In addition, waveforms obtained from 3DAC working at the monopole-transmitting-azimuthal-receiving mode indicate that the corresponding P-waves and S-waves are azimuthally sensitive, too. Finally, we have developed a field example of 3DAC to support our simulation results: The azimuthal variation of the P-wave slowness was observed and can thus be used to reflect the azimuthal heterogeneity of formations.

Elastic Parameters from Compressional and Shear Wave Tomographic Survey: A Case Study from Kuala Lumpur, Malaysia

2017 ◽  
Vol 22 (4) ◽  
pp. 427-434
Author(s):  
Julius K. von Ketelhodt ◽  
Thomas Fechner ◽  
Musa S. D. Manzi ◽  
Raymond J. Durrheim
Keyword(s):  
Wave Train ◽  
Kuala Lumpur ◽  
S Wave ◽  
Elastic Parameters ◽  
High Quality ◽  
P Waves ◽  
Receiver Array ◽  
Wave Tomography ◽  
Karstic Limestone

An integrated P- and S-wave cross-borehole tomographic survey was performed in the city center of Kuala Lumpur, Malaysia, with the aim of exploring a karstic limestone area near an area that previously encountered cavities. Horizontally polarized shear waves were generated with two opposing, perpendicular strike directions and recorded with a multi-level, three-component receiver array. This allowed a high quality picking of the traveltimes, whereby the wave train reverses at the time of the S-wave arrival. In addition, high quality sparker generated P-waves were recorded. The P- and S-wave traveltimes were used to invert for two co-located tomograms. These tomograms enabled a better interpretation capability than a P- or S-wave tomogram on its own. The tomograms enabled the calculation of the elastic parameters, i.e., P- to S-wave velocity (Vp/Vs) ratio, Poisson's ratio, bulk modulus, Young's modulus and the shear modulus, on a 2D surface between the boreholes. This further aided the interpretation, as areas with limited traveltime accuracy and thus, an increase in tomographic error, could be easily identified, and the extent of a large cavity could be estimated. The interpretation of the tomograms was constrained by two additional boreholes, which provided more confidence on the delineation and location of cavities at depths. The survey shows the benefit of co-locating P- and S-wave tomography surveys.

The Australian Plate and underlying mantle from waveform tomography with massive datasets

2021 ◽  
Author(s):  
Janneke de Laat ◽  
Sergei Lebedev ◽  
Bruna Chagas de Melo ◽  
Nicolas Celli ◽  
Raffaele Bonadio
Keyword(s):  
Structural Information ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Depth Range ◽  
S Wave ◽  
Crust And Upper Mantle ◽  
Southern Boundary ◽  
Velocity Anomaly ◽  
Australian Plate ◽  
Australian Continent

<p>We present a new S-wave velocity tomographic model of the Australian Plate, Aus21.  It is constrained by waveforms of 0.9 million seismograms with both the corresponding sources and stations located within the half-hemisphere centred at the Australian continent. Waveform inversion extracts structural information from surface, S- and multiple S-waves on the seismograms in the form of a set of linear equations. These equations are then combined in a large linear system and inverted jointly to obtain a tomographic model of S- and P-wave speeds and S-wave azimuthal anisotropy of the crust and upper mantle. The model has been validated by resolution tests and, for particular locations in Australia with notable differences with previous models, by independent inter-station measurements of surface-wave phase velocities, which we performed using available array data. </p><p> </p><p>Aus21 offers new insights into the structure and evolution of the Australian Plate and its boundaries. The Australian cratonic lithosphere occupies nearly all of the western and central Australia but shows substantial lateral heterogeneity. It extends up to the northern edge of the plate, where it is colliding with island arcs, without subducting. The rugged eastern boundary of the cratonic lithosphere provides a lithospheric definition of the Tasman Line. The thin, warm lithosphere below the eastern part of the continent, east of the Tasman Line, underlies the Cenozoic volcanism locations in the area. The lithosphere is also thin and warm below much of the Tasman Sea, underlying the Lord Howe hotspot and the submerged part of western Zealandia. A low velocity anomaly that may indicate the single source of the Lord Howe and Tasmanid hotspots is observed in the transition zone offshore the Australian continent, possibly also sourcing the East Australia hotspot. Another potential hotspot source is identified below the Kermadec Trench, causing an apparent slab gap in the overlying slab and possibly related to the Samoa Hotspot to the north. Below a portion of the South East Indian Ridge (the southern boundary of the Australian Plate) a pronounced high velocity anomaly is present in the 200-400 km depth range just east of the Australian-Antarctic Discordance (AAD), probably linked to the evolution of this chaotic ridge system.</p>

On the Green function of the Lord–Shulman thermoelasticity equations

2019 ◽  
Vol 220 (1) ◽  
pp. 393-403 ◽  
Author(s):  
Zhi-Wei Wang ◽  
Li-Yun Fu ◽  
Jia Wei ◽  
Wanting Hou ◽  
Jing Ba ◽  
...  
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Parabolic Type ◽  
Second Order ◽  
P Wave ◽  
Thermal Mode ◽  
S Wave ◽  
Order Tensor ◽  
P Waves ◽  
Thermal Conductivities

SUMMARY Thermoelasticity extends the classical elastic theory by coupling the fields of particle displacement and temperature. The classical theory of thermoelasticity, based on a parabolic-type heat-conduction equation, is characteristic of an unphysical behaviour of thermoelastic waves with discontinuities and infinite velocities as a function of frequency. A better physical system of equations incorporates a relaxation term into the heat equation; the equations predict three propagation modes, namely, a fast P wave (E wave), a slow thermal P wave (T wave), and a shear wave (S wave). We formulate a second-order tensor Green's function based on the Fourier transform of the thermodynamic equations. It is the displacement–temperature solution to a point (elastic or heat) source. The snapshots, obtained with the derived second-order tensor Green's function, show that the elastic and thermal P modes are dispersive and lossy, which is confirmed by a plane-wave analysis. These modes have similar characteristics of the fast and slow P waves of poroelasticity. Particularly, the thermal mode is diffusive at low thermal conductivities and becomes wave-like for high thermal conductivities.

scholarly journals POLENET/LAPNET teleseismic <i>P</i> wave travel time tomography model of the upper mantle beneath northern Fennoscandia

Solid Earth ◽  
2016 ◽  
Vol 7 (2) ◽  
pp. 425-439 ◽  
Author(s):  
Hanna Silvennoinen ◽  
Elena Kozlovskaya ◽  
Eduard Kissling
Keyword(s):  
Upper Mantle ◽  
Fennoscandian Shield ◽  
P Wave ◽  
Significant Feature ◽  
Depth Range ◽  
Low Velocity ◽  
Lateral Velocity ◽  
International Polar Year ◽  
Northern Fennoscandia ◽  
Wave Tomography

Abstract. The POLENET/LAPNET (Polar Earth Observing Network) broadband seismic network was deployed in northern Fennoscandia (Finland, Sweden, Norway, and Russia) during the third International Polar Year 2007–2009. The array consisted of roughly 60 seismic stations. In our study, we estimate the 3-D architecture of the upper mantle beneath the northern Fennoscandian Shield using high-resolution teleseismic P wave tomography. The P wave tomography method can complement previous studies in the area by efficiently mapping lateral velocity variations in the mantle. For this purpose 111 clearly recorded teleseismic events were selected and the data from the stations hand-picked and analysed. Our study reveals a highly heterogeneous lithospheric mantle beneath the northern Fennoscandian Shield though without any large high P wave velocity area that may indicate the presence of thick depleted lithospheric “keel”. The most significant feature seen in the velocity model is a large elongated negative velocity anomaly (up to −3.5 %) in depth range 100–150 km in the central part of our study area that can be followed down to a depth of 200 km in some local areas. This low-velocity area separates three high-velocity regions corresponding to the cratonic units forming the area.

scholarly journals Phased array compaction cell for measurement of the transversely isotropic elastic properties of compacting sediments

Geophysics ◽  
2011 ◽  
Vol 76 (3) ◽  
pp. WA113-WA123 ◽  
Author(s):  
Kurt T. Nihei ◽  
Seiji Nakagawa ◽  
Frederic Reverdy ◽  
Larry R. Myer ◽  
Luca Duranti ◽  
...  
Keyword(s):  
Elastic Properties ◽  
Phased Array ◽  
Transversely Isotropic ◽  
P Wave ◽  
S Wave ◽  
P Waves ◽  
Array Measurements ◽  
Experimental Apparatus ◽  
Marine Sediment Sample ◽  
Plane P Waves

Sediments undergoing compaction typically exhibit transversely isotropic (TI) elastic properties. We present a new experimental apparatus, the phased array compaction cell, for measuring the TI elastic properties of clay-rich sediments during compaction. This apparatus uses matched sets of P- and S-wave ultrasonic transducers located along the sides of the sample and an ultrasonic P-wave phased array source, together with a miniature P-wave receiver on the top and bottom ends of the sample. The phased array measurements are used to form plane P-waves that provide estimates of the phase velocities over a range of angles. From these measurements, the five TI elastic constants can be recovered as the sediment is compacted, without the need for sample unloading, recoring, or reorienting. This paper provides descriptions of the apparatus, the data processing, and an application demonstrating recovery of the evolving TI properties of a compacting marine sediment sample.

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