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  • 1
    Electronic Resource
    Electronic Resource
    Observably split multiplets—data analysis and interpretation in terms of large-scale aspherical structure (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 111 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We study aspherical structure of the deep Earth using normal mode splitting. Analysing 10 previously unresolved high-Q multiplets in the frequency band 5-10 mHz allows us to double the number of modal constraints on the aspherical structure of the core. We find that all the multiplets are anomalously split and exhibit predominantly parabolic splitting though some of the low-m singlets are not well-resolved. The parabolic splitting suggests that coupling to nearby modes is unimportant and that the structure sensed by these multiplets is dominantly axisymmetric and of harmonic degree 2. Four of the newly observed modes have less than 3 per cent of their potential energy in the inner core. Taken together with published observations of anomalously split multiplets, the following statements can be made. Anomalously split modes have typically 45 per cent of their potential energy below the core-mantle boundary (CMB) whereas modes with little energy in the core (less than 30 per cent) are not anomalously split. Some of the anomalously split modes have less than 3 per cent potential energy below the inner core boundary (ICB).These observations make it extremely difficult to invoke inner core structure of any kind as the source of anomalous splitting and have led to a search for alternative causes. Several studies have shown that the magnetic field is too small to cause anomalous splitting and a steady fluid flow in the outer core would cause splitting that is symmetrical in frequency and can therefore be ruled out. Anomalous structure of harmonic degree s= 2 in the outer core, with the density constrained to be in hydrostatic equilibrium, can explain a large amount of the signal but leads to an extremely unusual Vp/ρ relation. We therefore conclude that the cause of anomalous splitting is currently unknown.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb02112.x
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  • 2
    Electronic Resource
    Electronic Resource
    Inner Core Attenuation From Short-Period Pkp(Bc) Versus Pkp(Df) Waveforms (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 114 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Differential waveform analysis provides an excellent tool for studying the attenuation properties of the top of the inner core. We analyse 108 PKP(BC) versus PKP(DF) waveforms from Global Digital Seismograph Network (GDSN) vertical-component seismograms to constrain the frequency and depth dependency of Qα in this region. We use both frequency- and time-domain techniques. In the time-domain method, the BC phase is mapped onto the DF phase using an attenuation band operator. the mapping operator is parameterized by the upper and lower cut-off frequencies of the absorption band, the time shift required to align these two phases, and t*, the integrated effect of Qα−1 in the top of the inner core. In the frequency-domain analysis, multitaper spectral estimation is used to compute the complex spectrum of the two phases. the shape of the amplitude spectrum of the spectral ratio between these two phases gives an estimate of Qα.Similar results are obtained from frequency- and time-domain analysis but the Qα obtained from frequency-domain analysis is approximately 20 per cent greater than the value obtained from time-domain analysis. We prefer the frequency-domain results since they are not affected by the presence of noise at higher frequencies. Apparent Qα values exhibit considerable scatter with no clear frequency or depth dependence. We find that the average value of Qα in the top of the inner core is about 360 which is consistent with previous body wave studies but differs by a factor of two from values obtained from studies of the decay of free oscillations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb01461.x
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  • 3
    Electronic Resource
    Electronic Resource
    Spherically symmetric attenuation within the Earth from normal mode data (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 104 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Observations of the attenuation of free oscillations after large earthquakes are used to determine the spherically symmetric dissipative structure within the Earth. We model intrinsic attenuation of seismic energy between 0.3 and 10mHz with a frequency-independent Q model. The resolving power of our data indicates that simple models of Q can explain our observations and that we do not have enough information to constrain detailed absorption band models.The average shear Q is relatively well constrained for the mantle and is 250 ± 2 per cent. The shear Q in the inner core is mostly constrained by new observations of PKIKP equivalent modes and is 110 ± 25 per cent. This is lower than in some previous models, which have used possibly erroneous observations of core modes, though the new models still predict too little attenuation to match the observations of strong differential attenuation of the BC and DF branches of PKP at frequencies of about 1 Hz. Agreement between the mode and body-wave data sets should probably not be expected since it now seems likely that the PKIKP data sense an absorption band which is distinct from that sensed by the mode data.The distribution of bulk attenuation in the Earth remains ill-constrained though bulk attenuation is required somewhere to explain the attenuation of the radial modes. Successful models tend to put significant bulk Q in the upper mantle.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb05700.x
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  • 4
    Electronic Resource
    Electronic Resource
    Observation of low-order toroidal modes from the 1989 Macquarie Rise event (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 111 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: At frequencies below 1 mHz toroidal modes are more difficult to observe than spheroidal modes for two reasons. First, the comparatively rapid decay of the signal (low Q factor) of toroidal modes and the consequently short records which have to be analysed result in low resolution in frequency. Secondly, high noise on horizontal component seismometers due to their sensitivity to tilts induced by atmospheric disturbances makes it difficult to observe long-period motion. Here we present observations of the lowest order toroidal modes based largely on digital recordings of the 1989 Macquarie Rise event. We present what we believe to be the first unambiguous observation of the gravest toroidal mode 0T2. We estimate multiplet degenerate frequencies and Q for the fundamental modes up to 0T10 and the overtone 1T6. The frequencies of individual singlets are estimated for the modes 0T5-0T8, and for the three multiplets 0T8-0T10 we have sufficient signal to estimate also the degree 2 aspherical structure coefficients.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00572.x
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  • 5
    Electronic Resource
    Electronic Resource
    Upper mantle structure from long-period differential traveltimes and free oscillation data (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 109 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We have performed an inversion to model upper mantle structure using SS–S differential traveltime residuals in conjunction with previously reported structure coefficient estimates obtained from free oscillation frequency and attenuation measurements. The model we obtain is consistent with both the differential traveltime and structure coefficient data sets and provides nearly the same level of fit as independent inversions of these data sets.We find that a relatively simple model parametrization appears to be adequate for the data used. Upper mantle velocity perturbations and lateral variations in the radii of boundaries at 400 and 670km depth are parametrized with spherical harmonic expansions up to degree and order 8. The expansions are fixed at specific depth knots (separated by roughly 200km) spanning the range 20–670 km. Radial variations in velocity are obtained by linear interpolation between these depth knots. Smoothness constraints were applied as part of the inversion procedure, as well as corrections for the thickness and velocity variations of the crust. The smoothness constraints were adjusted to require a smoother structure in the bottom portion (400 to 670 km depth) of the model, where the structure coefficients have the greatest sensitivity. The upper part of the model is primarily constrained by the differential traveltime data.The resulting misfit of the final model shows that, on average, the individual structure coefficients and SS–S measurements are fit to within approximately 1.5 standard deviations. We suggest that some of the misfit is due to lower mantle structure which is not included in the modelling. The power as a function of harmonic degree for the top portion of the model (20 to 220 km depth) does not fall off significantly at the higher harmonic degrees, suggesting that the differential traveltime data may require models which include structure for degrees greater than 8. Boundary perturbations at depths of 400 and 670 km trade off strongly with the velocity variations. Allowing variations of the order of 10 km in the radii of the discontinuities at 400 and 670 km provides a slight improvement to the data fit.Comparisons of several depths of our upper mantle velocity model to the geoid show the strongest correlation at degree 2 for structure near 670 km depth. The distribution of slab structures is highly correlated to the degree 2 components of the model for depths of 200 to 670 km. Examination of the model in the vicinity of hotspots shows no consistent hotspot related signal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00098.x
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  • 6
    Electronic Resource
    Electronic Resource
    Tracking slabs in the lower mantle (1997)
    [s.l.] : Nature Publishing Group
    Nature 386 (1997), S. 558-558 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Sometimes it seems as if geoscientists as a group are exceptionally bad at solving problems. For example, for about 30 years we have been debating whether or not there is a significant barrier to mantle flow at a level close to the depth of the 660-km seismic discontinuity within the Earth. Our ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/386558a0
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  • 7
    Electronic Resource
    Electronic Resource
    Lower-mantle structure from ScS–S differential travel times (1991)
    [s.l.] : Nature Publishing Group
    Nature 352 (1991), S. 231-233 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] ScS-S differential travel times are ideal for investigating lower-mantle structure. They have almost no sensitivity to struc-ture in the upper mantle because, for large epicentral distances, their ray paths are nearly identical there (Fig. la). They are relatively insensitive to errors in source ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/352231a0
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  • 8
    Electronic Resource
    Electronic Resource
    Aspherical Earth structure from fundamental spheroidal-mode data (1982)
    [s.l.] : Nature Publishing Group
    Nature 298 (1982), S. 609-613 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Free-oscillation data reveal heterogeneity in the Earth's mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid. The heterogeneity can be modelled as localized in the transition zone ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/298609a0
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  • 9
    Electronic Resource
    Electronic Resource
    Limits on differential rotation of the inner core from an analysis of the Earth's free oscillations (1999)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 402 (1999), S. 66-69 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Differential rotation of the Earth's inner core has been inferred by several seismic ‘body-wave’ studies which indicate that the inner core is rotating at a rate between 0.2° and 3° per year faster than the Earth's crust and mantle. The wide range in inferred rotation ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/47011
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  • 10
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    Observation of low-order toroidal modes from the 1989 Macquarie Rise event (1992)
    Oxford University Press
    Details
    Publication Date: 1992-11-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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