Texture of the uppermost inner core from forward- and back-scattered seismic waves

Cormier, Vernon F.

doi:10.1016/j.epsl.2007.04.003

Cited by 56 publications

(58 citation statements)

References 44 publications

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“…Overall the residuals are smaller than those at COL-average 0.5 s at BOSA and LBTB compared to 3 s at COL. The Aleutian Islands residuals are also more scattered, spanning a range of 1.2 s compared to 0.5 s at COL; this is in agreement with the hemispherical differences and a more scattering eastern inner-core hemisphere (Cormier 2007). As in evident in Fig.…”

Section: Aleutian Islands Eventssupporting

confidence: 86%

“…Studying regions of the inner core that exhibit linear variations in spatial velocity structure with longitude is particularly well suited for testing the rotation hypothesis, since this rotation is most likely to occur about the Earth's rotation axis (Song 2000a , 1997-1999Silver et al (2001)] has provided data. For Kuril Islands events, the experiment BL [Brazilian Lithosphere Seismic Project;2001-2007 has enabled us to span the inner core. For each experiment, PKPbcPKPdf residuals for events derived from the same earthquake lists as those used in probing the temporal variations have been analysed at a number of stations.…”

Section: Figurementioning

confidence: 99%

“…For South Sandwich Islands events, data from the PASSCAL experiment ARCTIC ['Structure and Rotation of the Inner Core', 2004-2007Lindner et al (2010)] have been used. For Aleutian Islands events, the PASSCAL experiment SA ['Anatomy of an Archean Craton', South Africa…”

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confidence: 99%

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Global seismic body-wave observations of temporal variations in the Earth's inner core, and implications for its differential rotation

Mäkinen¹,

Deuss²

2011

Geophysical Journal International

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S U M M A R YDifferential rotation of the Earth's inner core has been predicted in some geodynamo models, and seismic studies over the past 15 yr have resolved rotation rates up to 1 • yr −1 . Most previous seismic body-wave studies have focussed on South Sandwich Islands events recorded at station COL in Alaska. Here, we present a globally extended study into temporal variations in the inner core over some 25 yr, using PKPbc-PKPdf traveltime residuals. To test for differential rotation of the inner core, displacement of inner-core heterogeneities over time is sought. We introduce a new method of space-flattening to remove the effect of spatial variations on the time variations; this allows for the use of both polar, semi-equatorial and equatorial geometries. First, we reanalyse polar paths from South Sandwich Islands events to stations COL and INK in North America. These stations yield a differential rotation of the inner core at a rate of 0.12-0.38 • yr −1 in an eastward direction, in agreement with previous studies. However, station DAWY, which has a very similar path through the inner core as COL, yields at best a westward differential rotation of the inner core. Thus DAWY results are incompatible with the COL/INK inferred rotation. Secondly, earthquakes in the Aleutian Islands region, observed at BOSA and LBTB in southern Africa, exhibit temporal variations that are incompatible with the South Sandwich Islands-COL/INK inferred rotation rate. Thirdly, Kuril Islands events, recorded in South America at station BDF, yield inconclusive results. Finally, our final piece of evidence for the irreconcilability of differential inner-core rotation with global data comes from using earthquakes in the Vanuatu region, recorded at BCAO/BGCA in Central Africa, an equatorial geometry. These residuals resolve a westward inner-core rotation at a rate of 0.14 • yr −1 , over the same time period that South Sandwich Islands events indicate an eastward rotation. As any rigid-body rotation should yield the same direction and rate independent of where the inner core is sampled, our results allow us to reject previously reported inner-core differential rotation rates of up to 0.1-0.5 • yr −1 . Instead, our results suggest that structure in either the inner or the outer core is varying with time, over relatively short timescales and in ways that cannot be explained by, and do not support, a differentially rotating inner core.

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Section: Aleutian Islands Eventssupporting

confidence: 86%

Section: Figurementioning

confidence: 99%

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Global seismic body-wave observations of temporal variations in the Earth's inner core, and implications for its differential rotation

Mäkinen¹,

Deuss²

2011

Geophysical Journal International

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“…On the other hand, there is strong evidence from the analysis of P-waves traversing the inner core for scattering in the inner core indicating heterogeneities of up to 1.2 % in velocity and scale lengths of 2 km in the outermost 300 km (Cormier 2007;Koper et al 2004;Leyton and Koper 2007;Poupinet and Kennett 2004;Rost and Garnero 2004;. These heterogeneities are likely due to inclusion of melt in the solid core, compositional changes, or changes in the anisotropy of the core material .…”

Section: Other Probes (P Pcp Pp)mentioning

confidence: 90%

Seismic Detections of Small-Scale Heterogeneities in the Deep Earth

Rost

Earle

Shearer

et al. 2015

The Earth's Heterogeneous Mantle

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We report the detection of coherent scattered energy related to the phase PKPPKP (P′P′) in the data of medium aperture arrays. The scattered energy (P′•P′) is weak and requires array processing techniques to extract the signal from the noise. The arrival time window of P′•P′ is mostly free from other interfering body wave energy and can be detected over a large distance range. P′•P′ has been detected in the data of large aperture arrays previously, but the detection in the data of smaller arrays shows its potential for the study of the small-scale structure of the Earth. Here, we show that P′•P′ can detect scattering off small-scale heterogeneities throughout the Earth's mantle from crust to core making this one of the most versatile scattering probes available. We compare the results of P′•P′ to a related scattering probe (PK•KP). The detected energy is in agreement with stronger scattering, i.e., more heterogeneous structure, in the upper mantle and in an approximately 800-km-thick layer above the core-mantle boundary. Lateral variations in heterogeneity structure can also be detected through differences in scattered energy amplitude. We use an application of the F-statistic in the array processing allowing us a precise measurement of the incidence angles (slowness and backazimuth) of the scattered energy. The directivity information of the array data allows an accurate location of the scattering origin. The combination of high-resolution array processing and the scattering of P′•P′

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“…This result indicates that the strong scattering region is located beneath the Pacific Ocean and Asia, which covers parts of the eastern and western hemispheres. On the basis of scattering properties and Q structure, Cormier (2007) inferred textural differences between the eastern and western hemispheres near the surface of the inner core, including vertically oriented structures in the eastern hemisphere.…”

Section: Introductionmentioning

confidence: 99%

Complex inner core boundary from frequency characteristics of the reflection coefficients of PKiKP waves observed by Hi-net

Tanaka

Tkalčić

2015

Prog. in Earth and Planet. Sci.

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Frequency-dependent reflection coefficients of P waves at the inner core boundary (ICB) are estimated from the spectral ratios of PKiKP and PcP waves observed by the high-sensitivity seismograph network (Hi-net) in Japan. The corresponding PKiKP reflection locations at the ICB are distributed beneath the western Pacific. At frequencies where noise levels are sufficiently low, spectra of reflection coefficients show four distinct sets of characteristics: a flat spectrum, a spectrum with a significant spectral hole at approximately 1 or 3 Hz, a spectrum with a strong peak at approximately 2 or 3 Hz, and a spectrum containing both a sharp peak and a significant hole. The variety in observed spectra suggests complex lateral variations in ICB properties. To explain the measured differences in frequency characteristics of ICB reflection coefficients, we conduct 2D finite difference simulations of seismic wavefields near the ICB. The models tested in our simulations include a liquid layer and a solid layer above the ICB, as well as sinusoidal and spike-shaped ICB topography with varying heights and scale lengths. We find that the existence of a layer above the ICB can be excluded as a possible explanation for the observed spectra. Furthermore, we find that an ICB topographic model with wavelengths and heights of several kilometers is too extreme to explain our measurements. However, restricting the ICB topography to wavelengths and heights of 1.0-1.5 km can explain the observed frequency-related phenomena. The existence of laterally varying topography may be a sign of lateral variations in inner core solidification.

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Texture of the uppermost inner core from forward- and back-scattered seismic waves

Cited by 56 publications

References 44 publications

Global seismic body-wave observations of temporal variations in the Earth's inner core, and implications for its differential rotation

Global seismic body-wave observations of temporal variations in the Earth's inner core, and implications for its differential rotation

Seismic Detections of Small-Scale Heterogeneities in the Deep Earth

Complex inner core boundary from frequency characteristics of the reflection coefficients of PKiKP waves observed by Hi-net

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