Influence of Mantle Structures on Measurements of Anisotropy in the Inner Core

2020 ◽  
Author(s):  
Sandra Beiers ◽  
Christine Thomas
Keyword(s):  
Seismic Anisotropy ◽  
Inner Core ◽  
Incidence Angle ◽  
Outer Core ◽  
Western Hemisphere ◽  
Travel Times ◽  
Array Seismology ◽  
Two Parameters ◽  
The Waves ◽  
Ray Paths

<p>The seismological exploration of the Earth’s inner core has revealed some structural complexities such as seismic anisotropy and hemispherical separation. Investigating the travel times of PKP waves from at least two different ray paths, a polar and an equatorial one, is one of the commonly used methods to probe the inner core’s anisotropy. Since the waves are traversing anomalous structures in the lowermost mantle before entering the core, these heterogeneities have to be taken into account when investigating anisotropy in the inner core.</p><p>In this study we use data from an equatorial path with events from Indonesia recorded in Morocco and a nearly polar one with earthquakes in New Zealand recorded in England. The two waves used in our study, PKPdf and PKPab, both propagate through mantle and outer core and PKPab additionally traverses the inner core. Within this work, we do not only analyse the travel times of the waves but rather investigate their deviations from the originally assumed path along with their incidence angle. This is done with the methods of array seismology, mainly its two parameters slowness and backazimuth.</p><p>The results of this study reveal opposite deviations of slowness and backazimuth of the polar in contrast to the equatorial path. While the polar waves travel shallower and closer to North, the equatorial waves propagate deeper and farther from North than predicted by ak135. Additionally we observe hemispherical differences between waves that sample the eastern and the ones that sample the western hemisphere for both ray paths, PKPdf and PKPab, which leads us to the assumption that the deviations are not caused by the inner core but are rather due to mantle structures.</p>

Revised velocities in the Earth's core

1973 ◽  
Vol 63 (3) ◽  
pp. 1073-1105 ◽  
Author(s):  
Anthony Qamar
Keyword(s):  
Transition Zone ◽  
Inner Core ◽  
Velocity Model ◽  
Outer Core ◽  
Travel Times ◽  
Zone Boundary ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Core Boundary ◽  
Velocity Jump

abstract Travel times and amplitudes of PKP and PKKP from three earthquakes and four underground nuclear explosions are presented. Observations of reflected core waves at nearly normal angles of incidence provide new constraints on the average velocities in the inner and outer core. Interpretation of these data suggests that several small but significant changes to Bolt's (1962) core velocity model (T2) are necessary. A revised velocity model KOR5 is given together with the derived travel times that are consistent with the 1968 tables for P. Model KOR5 possesses a velocity in the transition zone which is 112 per cent lower than that in model T2. In addition, KOR5 has a velocity jump at the transition zone boundary (r = 1782 km) of 0.013 km/sec and a jump at the inner core boundary (r = 1213 km) of 0.6 km/sec. These values are, respectively, 1/20 and 2/3 of the corresponding model T2 values.

scholarly journals Inner core anisotropy measured using new ultra-polar PKIKP paths

2020 ◽  
Vol 223 (2) ◽  
pp. 1230-1246
Author(s):  
Henry Brett ◽  
Arwen Deuss
Keyword(s):  
Seismic Anisotropy ◽  
Inner Core ◽  
Rotation Axis ◽  
Body Waves ◽  
P Wave ◽  
Mantle Structure ◽  
Data Set ◽  
Seismic Stations ◽  
Ray Path ◽  
Ray Paths

SUMMARY We measure the seismic anisotropy of the inner core using PKPbc-PKPdf and PKPab-PKPdf differential traveltimes, as a function of the angle ζ between the Earth’s rotation axis and the ray path in the inner core. Previous research relied heavily on body waves originating in the South Sandwich Islands (SSI) and travelling to seismic stations in Alaska to sample inner core velocities with low ζ (polar paths). These SSI polar paths are problematic because they have anomalous travel time anomalies, there are no ultra-polar SSI paths with ζ < 20° and they only cover a small part of the inner core. Here we improve constraints on inner core anisotropy using recently installed seismic stations at high latitudes, especially in the Antarctic, allowing us to measure ultra-polar paths with ζ ranging from 20°–5°. Our new data show that the SSI’s polar events are fast but still within the range of velocities measured from ray paths originating elsewhere. We further investigate the effect of mantle structure on our data set finding that the SSI data are particularly affected by fast velocities underneath the SSI originating from the subducted South Georgia slab, which is currently located just above the core mantle boundary. This fast velocity region results in mantle structure being misinterpreted as inner core structure and we correct for this using a P-wave tomographic model. We also analyse the effect of velocity changes on the ray paths within the inner core and find that faster velocities significantly change the ray path resulting in the ray travelling deeper into the inner core and spending more time in the inner core. To remove this effect, we propose a simple but effective method to correct each event-station pair for the velocity-dependent ray path changes in the inner core, producing a more reliable fractional traveltime measurement. Combining the new ultra-polar data with mantle and ray path corrections results in a more reliable inner core anisotropy measurement and an overall measured anisotropy of 1.9–2.3 per cent for the whole inner core. This is lower than previous body wave studies (3 per cent anisotropy) and in better agreement with the value of inner core anisotropy measured by normal modes (2 per cent anisotropy). We also identify regional variation of anisotropic structure in the top 500 km of the inner core, which appears to be more complex than simple hemispherical variations. These regional variations are independent of the SSI data and are still present when these data are excluded. We also find a potential innermost inner core with a radius of 690 km and stronger anisotropy.

Observing the signature of the magnetic field's behaviour in the radial variation of inner core anisotropy

2020 ◽  
Author(s):  
Janneke de Jong ◽  
Lennart de Groot ◽  
Arwen Deuss
Keyword(s):  
Magnetic Field ◽  
Heat Flux ◽  
Seismic Anisotropy ◽  
Inner Core ◽  
Body Wave ◽  
Outer Core ◽  
Core Structure ◽  
Reversal Rate ◽  
Seismic Structure ◽  
The Magnetic Field

<p>The release of latent heat and lighter materials during inner core solidification is the driving force of the liquid iron flow in the outer core which generates the Earth's magnetic field. It is well known that the behaviour of the magnetic field varies over long time scales. Two clearly identifiable regimes are recognized, (i) superchrons and (ii) periods of hyperactivity (Biggin et al. 2012). Superchrons are characterized by an exceptionally low reversal rate of the magnetic pole and are associated with a low core mantle boundary (CMB) heat flux. Hyperactive periods are defined by a high reversal rate and have a high CMB heat flux.</p><p>Here we investigate whether the occurrence of these two regimes is related to radial variations in inner core seismic structure. Using seismic body-wave observations of compressional PKIKP-waves (Irving & Deuss 2011, Waszek & Deuss 2011, Lythgoe et al. 2013)., we construct a model of inner core anisotropy by comparing the difference between travel times for polar and equatorial rays. Anisotropy is the directional dependence of wave velocity and is determined by the structure of iron crystals in the inner core, hence changes in seismic anisotropy are due to changes in inner core crystal texture. We invert for radial changes in anisotropy while allowing for lateral variations and find that a model of the inner core containing five layers best fits our data. The model contains an isotropic uppermost inner core and four deeper layers with varying degrees of anisotropy.</p><p>Texture differences of the inner core iron crystals have been linked to changes in the solidification process of the inner core (Bergman et al. 2005), i.e. the motor of outer core flow. Therefore, the observed anisotropy variation, caused by variations of inner core solidification, might be related to changes in the behaviour of the magnetic field. Using an inner core growth model (Buffett et al. 1996) we convert depth to time for a range of inner core nucleation ages between 3.0 and 0.5 Ga (Olsen 2016). We find a remarkable correlation between the solidification time of the seismically observed layers and the occurrence of the magnetic regimes for two inner core ages; one with a nucleation at 1.4 Ga and one at 0.6 Ga, corresponding to an average CMB heat flux of 7.6 TW and 16.7 TW respectively.</p><p>Although we currently cannot differentiate between these two inner core ages considering our results alone, they do show that a relation between inner core structure and the behaviour of the magnetic field is possible, and suggest that seismic observations of inner core structure might provide new and independent insights into the magnetic field and its history.</p>

scholarly journals Dynamic history of the inner core constrained by seismic anisotropy

2021 ◽  
Author(s):  
Daniel Frost ◽  
Marine Lasbleis ◽  
Brian Chandler ◽  
Barbara Romanowicz
Keyword(s):  
Seismic Anisotropy ◽  
Inner Core ◽  
Rotation Axis ◽  
Growth Models ◽  
Outer Core ◽  
Body Waves ◽  
Growth History ◽  
History Of ◽  
Earth’S Inner Core ◽  
Inner Core Growth

Abstract Progressive crystallisation of Earth's inner core over geological times drives convection in the outer core and the generation of the Earth’s magnetic field. Resolving the rate and pattern of inner core growth is thus crucial to understanding the evolution of the geodynamo. The growth history of Earth’s inner core is likely recorded in the distribution and strength of seismic anisotropy arising from deformation texturing constrained by boundary conditions at the inner-core solid-fluid boundary. Travel times of seismic body waves indicate that seismic anisotropy increases with depth. Here we find that the strongest anisotropy is offset from Earth's rotation axis. Using geodynamic growth models and mineral physics calculations, we simulate the development of inner core anisotropy in a self-consistent manner. We show for the first time that an inner core model composed of hexagonally close-packed iron-nickel alloy, deformed by a combination of preferential equatorial growth and slow translation can match the seismic observations without requiring the introduction of hemispheres with sharp boundaries. We find a model of the inner core growth history compatible with external constraints from outer core dynamics, supporting arguments for a relatively young inner core (~0.5-1.5 Ga) and a viscosity >1018 Pa-s.

The origin of precursors to core waves

1970 ◽  
Vol 60 (3) ◽  
pp. 939-952 ◽  
Author(s):  
Eystein Husebye ◽  
Raúl Madariaga
Keyword(s):  
Inner Core ◽  
Outer Core ◽  
P Wave ◽  
Long Tail ◽  
The Core ◽  
Short Period ◽  
Diffracted Waves ◽  
Multiple Paths ◽  
Core Boundary ◽  
The Waves

Abstract The origin of the precursors of the core waves in the range 105°-142° is studied. Between 105° and 125° a long tail is observed after the P wave diffracted by the core. In the range 130° ≦ Δ ≦ 142° we usually observe short-period onsets a few seconds before PKIKP; these are the waves called P(GH). Reflection at a discontinuity in the outer core, near the inner-core boundary, is shown to produce the P(GH) branch. Reflections in the outer core are rejected as a mechanism for the tail of the P diffracted wave. A theoretical study of diffraction of P by the core shows that higher modes of diffracted waves cannot explain the observations of the tail of P diffracted. We conclude, by elimination, that it is due to reflections or multiple paths in the upper mantle.

A velocity structure of the Earth's core

1971 ◽  
Vol 61 (2) ◽  
pp. 429-456 ◽  
Author(s):  
Goetz G. R. Buchbinder
Keyword(s):  
Transition Zone ◽  
Inner Core ◽  
Velocity Structure ◽  
Velocity Model ◽  
Outer Core ◽  
Travel Times ◽  
Amplitude Ratios ◽  
The Core ◽  
Short Period ◽  
Core Boundary

abstract Travel times and amplitudes of PKP, P2KP and higher multiple K phases are determined from a worldwide distribution of short-period seismograms. The sources are one explosion in Novaya-Zemlya and seven earthquakes, consisting of one intermediate focus event in the New Hebrides, and deep-focus events in Fiji, Java, Kermadec Islands, and Peru. The data are used to determine a new velocity model of the lowest mantle and the core. In the new velocity model 132, the velocity of the bottom of the mantle is 13.44 km/sec; the core mantle boundary is placed at 2892 ± 2 km. The velocity model of the core produces the PKP caustic B1 at 143° and the P2KP caustic B2 at −125°. A velocity discontinuity of 0.01 km/sec at a depth of 4550 km represents the top of the transition zone to account for the earliest forerunners of PKP. To account for the later forerunners a second discontinuity of 0.02 km/sec is placed at a depth of 4850 km. Since the forerunner data could not be resolved into branches, neither discontinuity is well defined. The top of the inner core boundary is placed at a depth of 5145 km with an uncertainty of at least 10 km and represents a discontinuity of 0.576 km/sec. Older core models have transition zone discontinuities an order of magnitude larger than those of model 132 with a discontinuity at the inner core boundary of about 1 km/sec. The smaller velocity discontinuities are a result of interpreting the amplitudes and travel times of PKP so that the turning points D and G are located at 120° and 140°, respectively, rather than at 110° and 125° as in previous interpretations. Amplitude ratios of PKP phases yield an inner core Q of about 400 and amplitude ratios of P3KP, P4KP and P5KP result in an outer core Q of about 4000.

scholarly journals Longitudinal waves through the Earth's Core*

1952 ◽  
Vol 42 (2) ◽  
pp. 119-134
Author(s):  
M. E. Denson
Keyword(s):  
Velocity Distribution ◽  
Travel Time ◽  
Focal Point ◽  
Epicentral Distance ◽  
Outer Core ◽  
Travel Times ◽  
Time Curve ◽  
Longitudinal Waves ◽  
The Core ◽  
The Waves

Abstract Amplitudes, periods, and travel times of the longitudinal P′ or PKP core waves have been investigated. Results indicate that the epicentral distance of the main focal point and the travel time of P′ phases vary with the periods of the waves. This variation would seem reasonably explained in terms of dispersion. The point of reversal in the travel-time curve of the waves through the outer core is believed to lie near 157°. Data suggest a discontinuity between 120° and 125° rather than 110°. Anomalies existing in energy, period, and travel-time relationships of the P′ phases indicate that current concepts of velocity distribution and of propagation paths within the core are in need of modification.

scholarly journals Osmotic stress and vesiculation as key mechanisms controlling bacterial sensitivity and resistance to TiO2 nanoparticles

2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Christophe Pagnout ◽  
Angelina Razafitianamaharavo ◽  
Bénédicte Sohm ◽  
Céline Caillet ◽  
Audrey Beaussart ◽  
...  
Keyword(s):  
Lipid Peroxidation ◽  
Osmotic Stress ◽  
Inner Core ◽  
Metal Oxide Nanoparticles ◽  
Membrane Vesicles ◽  
Outer Core ◽  
Membrane Composition ◽  
Cell Turgor ◽  
Bacterial Sensitivity ◽  
Membrane Components

AbstractToxicity mechanisms of metal oxide nanoparticles towards bacteria and underlying roles of membrane composition are still debated. Herein, the response of lipopolysaccharide-truncated Escherichia coli K12 mutants to TiO2 nanoparticles (TiO2NPs, exposure in dark) is addressed at the molecular, single cell, and population levels by transcriptomics, fluorescence assays, cell nanomechanics and electrohydrodynamics. We show that outer core-free lipopolysaccharides featuring intact inner core increase cell sensitivity to TiO2NPs. TiO2NPs operate as membrane strippers, which induce osmotic stress, inactivate cell osmoregulation and initiate lipid peroxidation, which ultimately leads to genesis of membrane vesicles. In itself, truncation of lipopolysaccharide inner core triggers membrane permeabilization/depolarization, lipid peroxidation and hypervesiculation. In turn, it favors the regulation of TiO2NP-mediated changes in cell Turgor stress and leads to efficient vesicle-facilitated release of damaged membrane components. Remarkably, vesicles further act as electrostatic baits for TiO2NPs, thereby mitigating TiO2NPs toxicity. Altogether, we highlight antagonistic lipopolysaccharide-dependent bacterial responses to nanoparticles and we show that the destabilized membrane can generate unexpected resistance phenotype.

On the reliability of PKIIKP phase identification at a single station

2021 ◽  
Author(s):  
Olga Usoltseva ◽  
Vladimir Ovtchinnikov
Keyword(s):  
Inner Core ◽  
Seismic Velocity ◽  
Seismic Station ◽  
Shear Velocity ◽  
Outer Core ◽  
Joint Analysis ◽  
Time Range ◽  
Model Parameters ◽  
Phase Identification ◽  
Single Station

<p><span>Study of the contact zone between the inner and outer core represents considerable interest for understanding of properties, structures and dynamic of the Earth's core. One of </span><span>the </span><span>sources of </span><span>the </span><span>data about the processes proceeding in the top part of the inner core is the seismic wave PKIIKP once reflected from an undersize inner core boundary. Amplitudes of these waves are sensitive to the shear velocity in the top part of the inner core and are small. Therefore their identification at a single seismic station is not reliable without application of additional methods of analysis. </span><span>Significant in this regard is the discussion about the source (in inner core or in mantle) of anomalous arrivals<!-- Это можно удалить --> detected at the TAM station in North Africa [1,2] in the time range of PKIIKP phase.</span></p><p><span>To estimate influence of model parameters (S and P seismic velocity) on the characteristics of PKIIKP wave (amplitude and travel time) we calculated sensitivity kernels for upper mantle and inner core for dominant period 1.2 s, azimuth step 0.2 degrees and radius step 20 km by using DSM Kernel Suite algorithm. It was revealed that PKIIKP amplitude is more sensitivities to mantle heterogeneities than to inner core ones. </span><span>For reducing the effects of the overlying structures we suppose to use </span>а <span>joint analysis PKIIKP and pPKIIKP waves. </span><span>With this approach, an incorrect i</span><span>dentification</span><span> of the PKIIKP wave is most likely excluded. </span><span>We<!-- Было бы хорошо привести пример --> demonstrate the effectiveness of the approach on the example of processing the seismogram of the 11.02.2015 earthquake re</span>с<span>o</span><span>rded at the GZH station in China at a distance of 179.4 degrees.</span></p><p><span>1. Wang W., Song X. Analyses of anomalous amplitudes of antipodal PKIIKP waves</span><span>,</span><span> E<!-- Удаляется вместе с текстом, выделенным выше Зеленым цветом. -->aPP. 2019. V. 3. P. 212-217. doi: 10.26464/epp2019023</span></p><p><span>2. Tsuboi S., Butler R. Inner core differential rotation inferred from antipodal seismic observations</span><span>,</span><span> PEPI</span><span>,</span><span> 2020. V.301. 106451. </span></p>

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