Hemispherical transition of seismic attenuation at the top of the earth's inner core

Cao, A.; Romanowicz, Barbara

doi:10.1016/j.epsl.2004.09.032

Cited by 114 publications

(141 citation statements)

References 39 publications

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“…For the 2003 event, the waveforms of PKiKP are simply reversed in polarity with respect to those of PKIKP, as theoretically predicted for postcritical reflections (Fig. 3D) (7). For the 1993 event, the amplitudes of PKiKP are much reduced (by a factor of 3.0).…”

Section: Data Results and Discussionmentioning

confidence: 54%

Short wavelength topography on the inner-core boundary

Cao

Masson

Romanowicz

2007

Proc. Natl. Acad. Sci. U.S.A.

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Constraining the topography of the inner-core boundary is important for studies of core-mantle coupling and the generation of the geodynamo. We present evidence for significant temporal variability in the amplitude of the inner core reflected phase PKiKP for an exceptionally high-quality earthquake doublet, observed postcritically at the short-period Yellowknife seismic array (YK), which occurred in the South Sandwich Islands within a 10-year interval (1993/2003). This observation, complemented by data from several other doublets, indicates the presence of topography at the innercore boundary, with a horizontal wavelength on the order of 10 km. Such topography could be sustained by small-scale convection at the top of the inner core and is compatible with a rate of super rotation of the inner core of Ϸ0.1-0.15°per year. In the absence of inner-core rotation, decadal scale temporal changes in the innercore boundary topography would provide an upper bound on the viscosity at the top of the inner core.PKiKP ͉ super rotation ͉ doublet

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Section: Data Results and Discussionmentioning

confidence: 54%

Short wavelength topography on the inner-core boundary

Cao

Masson

Romanowicz

2007

Proc. Natl. Acad. Sci. U.S.A.

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“…Thus, the shallower hemispherical layers of the IC can be adjusted by a cubic polycrystalline aggregate using the same LPO percentage, though they bear a clear difference in their travel-time arrivals. A commonly accepted idea (15,61) is that the western inner core is characterized by slower PKIKP arrivals and higher attenuation factors (Q α ), whereas the eastern hemisphere has rather faster P-wave propagation and low Q α (Fig. 1).…”

Section: Resultsmentioning

confidence: 99%

Hemispherical anisotropic patterns of the Earth’s inner core

Mattesini

Belonoshko

Buforn

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

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It has been shown that the Earth's inner core has an axisymmetric anisotropic structure with seismic waves traveling ∼3% faster along polar paths than along equatorial directions. Hemispherical anisotropic patterns of the solid Earth's core are rather complex, and the commonly used hexagonal-close-packed iron phase might be insufficient to account for seismological observations. We show that the data we collected are in good agreement with the presence of two anisotropically specular east and west core hemispheres. The detected travel-time anomalies can only be disclosed by a lattice-preferred orientation of a body-centered-cubic iron aggregate, having a fraction of their [111] crystal axes parallel to the Earth's rotation axis. This is compelling evidence for the presence of a body-centered-cubic Fe phase at the top of the Earth's inner core.elastic anisotropy | Fe-bcc/-hcp | PKiKP/PKIKP waves | molecular dynamics T he Earth's inner core (IC) is a small spherical body (with a radius of 1,200 km) located at the center of our home planet. Since its discovery in the late 1936 by Inge Lehmann (1), it has long been the most out of reach and enigmatic place of the Earth. Through comparison of the equation of state to seismic data and the abundance of iron, it has been established that the solid IC mainly consists of iron (2-4). The growth of the IC from the freezing of the molten iron alloy at the inner-core boundary (ICB) (5) drives the convection in the outer core and provides the energy source for the geodynamo (6). From the analysis of the normal modes of the Earth's free oscillations and the body wave data, it has been found that compressional P waves travel about 3% faster along the Earth's spin axis than in the equatorial plane (7-10). The growing evidence of an elastic anisotropic IC has continued to generate interesting understandings of the deepest part of our planet, though not all the scientific efforts have converged to the same scenario. Surprisingly, the picture has become more and more confusing as further observations have been collected (7). Explaining the anisotropic IC behavior within a unified and well-accepted geophysical model has become an increasingly complicated issue. Nowadays, we know that the Earth's inner core has a rather complex three-dimensional structure, where texture (11-13) and degree of seismic anisotropy (14-20) change considerably with depth. The most recent view of the solid core corresponds to an uppermost isotropic layer characterized by faster P waves in the eastern hemisphere than in the western one (15,21,22). Although the existence of an outermost isotropic layer has somewhat been questioned (23, 24), more consensus has been reached about the presence of a deeper and more anisotropic region (13,16,25).Different mechanisms have been proposed to explain the IC anisotropy (26, 27), though most of them have firmly relied on the lattice-preferred orientation (LPO) of both the hexagonalclose-packed (hcp) (12,(28)(29)(30)(31) and the body-centered-cubic (bcc) iron crystals (32...

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“…Studies have shown evidence of anisotropy, oriented with the fast direction aligned to Earth's rotation axis (Poupinet et al, 1983;Morelli et al, 1986;Woodhouse et al, 1986;Creager, 1992;Vinnik et al, 1994;Irving and Deuss, 2011). A distinct east-west asymmetry has also been observed in the inner core, whereby the west hemisphere has lower velocity, weaker attenuation and stronger anisotropy than the east (Tanaka and Hamaguchi, 1997;Niu and Wen, 2001;Cao and Romanowicz, 2004;Oreshin and Vinnik, 2004;Deuss et al, 2010;. The difference in isotropic velocity between the hemispheres has been found to be between 0.5% (Sun and Song, 2008) and 1.5% (Garcia, 2002), with most studies finding a value of 0.8%-1% (Tanaka and Hamaguchi, 1997;Niu and Wen, 2001;Cao and Romanowicz, 2004).…”

Section: Introductionmentioning

confidence: 98%

Measuring the seismic velocity in the top 15 km of Earth’s inner core

Godwin

Waszek

Deuss

2018

Physics of the Earth and Planetary Interiors

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A B S T R A C TWe present seismic observations of the uppermost layer of the inner core. This was formed most recently, thus its seismic features are related to current solidification processes. Previous studies have only constrained the eastwest hemispherical seismic velocity structure in the Earth's inner core at depths greater than 15 km below the inner core boundary. The properties of shallower structure have not yet been determined, because the seismic waves PKIKP and PKiKP used for differential travel time analysis arrive close together and start to interfere. Here, we present a method to make differential travel time measurements for waves that turn in the top 15 km of the inner core, and measure the corresponding seismic velocity anomalies. We achieve this by generating synthetic seismograms to model the overlapping signals of the inner core phase PKIKP and the inner core boundary phase PKiKP. We then use a waveform comparison to attribute different parts of the signal to each phase. By measuring the same parts of the signal in both observed and synthetic data, we are able to calculate differential travel time residuals. We apply our method to data with ray paths which traverse the Pacific hemisphere boundary. We generate a velocity model for this region, finding lower velocity for deeper, more easterly ray paths. Forward modelling suggests that this region contains either a high velocity upper layer, or variation in the location of the hemisphere boundary with depth and/or latitude. Our study presents the first direct seismic observation of the uppermost 15 km of the inner core, opening new possibilities for further investigating the inner core boundary region.

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Hemispherical transition of seismic attenuation at the top of the earth's inner core

Cited by 114 publications

References 39 publications

Short wavelength topography on the inner-core boundary

Short wavelength topography on the inner-core boundary

Hemispherical anisotropic patterns of the Earth’s inner core

Measuring the seismic velocity in the top 15 km of Earth’s inner core

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Hemispherical transition of seismic attenuation at the top of the earth's inner core

Cited by 114 publications

References 39 publications

Short wavelength topography on the inner-core boundary

Short wavelength topography on the inner-core boundary

Hemispherical anisotropic patterns of the Earth’s inner core

Measuring the seismic velocity in the top 15 km of Earth’s inner core

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Product

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Measuring the seismic velocity in the top 15 km of Earth’s inner core