DI31A-0245
Heterogeneity in Inner Core Anisotropy
A number of previous studies have investigated the existence of seismic anisotropy in the Earth's inner core. This anisotropic nature has been modelled using free oscillation data with widely varying results, which are formally inconsistent with each other. All of these models derived from normal mode data have been developed using a self coupling approximation which ignores the significance of coupling of different inner core modes through the anisotropic structure. This self coupling approximation of normal modes is not reliable for mantle structure (Deuss & Woodhouse 2001), here we show that this is also the case for the inner core. Body waves travelling through the inner core also show anisotropic behaviour; wave speed in the North-South direction is greater that in the equatorial plane. These differences can be readily seen in the differential travel times between waves which bottom in the outer core and those which penetrate into the inner core. Both PKPbc- PKPdf differential travel times and PKPab-PKPdf differential travel times display this anisotropy. Using these differential travel times, we find that there are lateral differences in the anisotropy of the inner core. Coupling of normal modes due to such lateral heterogeneity in the inner core should be observable in normal mode spectra. At present there are inconsistencies between the body wave and normal mode data which must be resolved. By comparing body wave and normal mode data with inner core anisotropy models we hope to find a picture of inner core anisotropy that is consistent with the seismic observations. Such information is a prerequisite for the integration of results from seismology and mineral physics, with the aim of establishing the details and causes of inner core anisotropy.
DI31A-0246
Geomagnetic Dipole Tilt Changes Induced by Core Flow
The tilt of the geomagnetic dipole decreased from about 11.7 degrees in 1960 to 10.5 degrees in 2005, following more than a century when it remained nearly constant. The recent poleward motion of the dipole axis is primarily due to a rapid decrease in the equatorial component of the dipole moment vector. Using maps of the equatorial dipole moment density and its secular change derived from core field models, we identify regions on the core- mantle boundary where the present-day tilt decrease is concentrated. Among the possible causes of equatorial dipole moment change on the core-mantle boundary, tangential magnetic diffusion is negligible on these time- scales, and although radial magnetic diffusion is potentially significant, the rapid changes in equatorial moment density indicate it is not the dominant mechanism. We show that magnetic flux transport can account for most of the observed equatorial dipole moment change. Frozen-flux core flow models derived from geomagnetic secular variation reveal a nearly- balanced pattern of sources and sinks for the equatorial dipole moment below the core-mantle boundary. The recent tilt decrease originates from two advective sinks, one beneath Africa where positive radial magnetic field is transported westward away from the equatorial dipole axis, the other beneath North America where negative radial magnetic field is transported northward away from the equatorial dipole axis. Each of these sinks is related to a prominent gyre that has evolved significantly over the past few decades, indicating the large-scale circulation in the outer core is variable on this time-scale.
DI31A-0247
Constraints on Earth's oldest magnetic field
The strength of the Earth's early geomagnetic field is of importance for understanding the evolution of the Earth's core, surface environment and atmosphere. Palaeomagnetic and palaeointensity data from rocks formed near the boundary of the Proterozoic and Archaean eons (~2.5 Ga), show many hallmarks of the more recent geomagnetic field: Reversals are recorded and available palaeointensity values are similar to those from younger rocks. Interestingly, paleosecular variation data indicate a dipole-dominated morphology, possible more dipolar than that seen in the 0-5 Ma geomagnetic field (Smirnov and Tarduno, GRL, 2004). This is consistent with some numerical geodynamo simulations with a smaller inner core. Here we discuss efforts to see through the ubiquitous low grade metamorphism that effects Archean rocks to obtain even older records of the magnetic field. Specifically, we use a CO2 laser heating approach and direct-current SQUID magnetometer measurements to obtain palaeodirections and intensities from single silicate crystals that host magnetite inclusions. We have found 3.2 Ga field strengths that are within 50% of the present-day value. This contrasts with some prior assertions that the mid-Archean field was some 10 times weaker than present-day (the prior studies were derived from rocks with secondary thermochemical remanent magnetizations rather than primary thermoremanent magnetizations). We will discuss our efforts to examine even older records potentially preserved in rocks of the Kaapvaal Craton of southern Africa, and the implications of these results for core evolution.
DI31A-0248
Normal mode constraints on shear and compressional wave velocity of the Earth's inner core
The solidity of the Earth's inner core was inferred in 1971 from normal mode observations. However, since then there has been little further proof that the inner core is solid and observations of PKJKP body wave arrivals are strongly dependend on the assumed inner core shear wave velocity. At the same time, many fundamental questions about the Earth's inner core remain unanswered. For example, it is unclear which high pressure and high temperature phases of iron, and its alloys, are stable in the inner core, and which light elements are present to explain the core's density. A key parameter required to answer these questions is the seismic inner core shear wave velocity. Free oscillation data are the most reliable source of seismic inner core shear wave velocity estimates; these data have been used here to determine the average compressional and shear wave velocity structure of the Earth's inner core and place tight constraints on their values. The new data reinforce the argument for a solid inner core, and the best data fit is obtained for average inner core velocities close to the PREM reference model: vs = 3.55 ± 0.05 km/s, vp = 11.15 ± 0.05 km/s. These values limit the time windows in which inner core shear waves, like PKJKP, may be observed. Inner core velocities for hcp and bcc iron at inner core conditions from ab initio molecular dynamics simulations and diamond anvil cell experiments are found to be incompatible with the seismological data. When light elements are taken into account, the misfit is worse than for a fluid inner core model. This discrepancy may be explained by the existence of fluid inclusions in the inner core, the effect of viscoelastic weakening, or polycrystalline behaviour.
DI31A-0249
Three-Dimensional Anisotropy and Texturing of Iron Crystals of Earth's Inner Core
Seismological studies have generally suggested that the Earth's inner core is anisotropic and the anisotropic structure changes significantly both laterally and with depth. Previous body-wave studies of the inner core have relied on ray tracing or waveform modeling using 1-D models. Here we present non-linear tomographic inversions of the inner core anisotropy using 3-D ray tracing, spline parameterization, and a large collection of PKP differential travel times. We adapt a pseudo-bending ray tracing (PBR) method in spherical coordinates for seismic rays that traverse the inner core. The 3-D anisotropic structure of the inner core is approximated to the first order as 3-D heterogeneous (but isotropic) structure for a given ray, making it possible to apply the PBR method. The inner core anisotropy model obtained has the following major features. (1) The model has strong hemispherical and depth variation. The isotropic velocity in the topmost inner core is greater in quasi-eastern hemisphere (QEH) (40 to 160oE) than in quasi-western hemisphere (QWH) (other longitudes). The anisotropy is weak in QEH to the depth of 600-700 km below the inner core boundary (ICB), while in QWH, the anisotropy increases at much shallower depth (about 100-200 km below the ICB) to about 3- 4%, then remains at about 2-4% throughout the rest of the inner core. (2) The anisotropy form changes abruptly (over a depth range of about 150 km) at the radius of about 600 km, slightly less than half of the inner core radius, forming a distinct inner inner core (IIC). The velocity in the IIC has maximums at equatorial and polar directions and minimum at an angle of about 40o from the equatorial plane. The velocity in the outer inner core (OIC), however, changes little for ray directions 0 to 40o from the equatorial plane. (3) Despite large variation of the anisotropy, the Voigt average velocity throughout the inner core is nearly uniform. The results suggest that the OIC is likely composed of iron crystals of a single phase with different degrees of preferred alignment along the spin axis of the Earth. The IIC may be composed of a different type of crystal alignment or a different iron phase. Our tests on model parameterization, mantle correction, and linear and non-linear inversions suggest the main features of our model are very robust. However, fine scale structures are likely to differ, particularly in the major transition zones, in the topmost QWH, between OIC and IIC, and between QEH and QWH in OIC. Searches for possible waveform complications from these boundaries need to be aware of the directional dependence and geographical variation to be successful.
DI31A-0250
Seismic Evidence for a Distinctly Anisotropic Innermost Inner Core
We observed clear underside reflections at the inner core boundary, PKIIKP, at two broadband seismic arrays in Venezuela and China from two deep focus earthquakes occurring in Indonesia and Argentina. Since PKIIKP traverses a very similar ray path with PKIKP, we anticipate that the differential travel time between PKIIKP and PKIKP is mostly sensitive to the seismic structure of the inner core. The uppermost 400 km of the inner core is also expected to produce no significant anomalies to the differential travel time along these two paths based on current inner-core models. The differential travel time is thus mainly subjected to heterogeneities and/or anisotropy in the center of the earth that affect the travel time of PKIKP. We found that the Indonesia-Venezuela path exhibits a ~1.8 s positive differential travel-time residual while the Argentina-China path shows no significant anomaly with respect to PREM. As the Indonesia-Venezuela and Argentina-China paths are in the directions of ~8° and 28° from the equatorial plane, respectively, our observation suggests that the slowest direction of wave propagation is no longer in the east-west direction for the innermost inner core. The Earth's center has a distinct seismic anisotropy relative to the rest part of the inner core.
DI31A-0251
Global Observations of Short Wavelength Topography on The Inner Core Boundary
Using high quality earthquake doublets from the South Sandwich Islands observed at the Yellowknife array, Cao et al. (2007) found evidence for significant anomalies in the amplitude of the post-critical reflected phase PKiKP, particularly in the frequency band 1-2 Hz, which they interpreted as evidence for short wavelength topography on the Inner Core Boundary (ICB). While the observations reported were limited to a small region of the ICB, an interesting question is whether such topography is globally distributed or presents differences between the eastern and western hemispheres, for which clear differential travel times and attenuation have been reported for these phases. Constraining the nature and lateral variations of the ICB and top of the inner core may help shed light on core dynamics and in particular the generation of the geodynamo. We have now scanned the IRIS database from 1972 to 2005, and assembled a high quality global dataset of waveforms in the distance range 134-144o, for which PKIKP and PKiKP are sufficiently separated to warrant the quantification of amplitude ratios between these two phases. The measured PKIKP/PKiKP amplitude ratio R in the frequency band 1-2 Hz was used to divide the data set into "normal" or "anomalous", for which values of R are larger than the maximum that can be explained by any realistic reference earth model. We found no systematic pattern in the distribution of normal and anomalous observations, both observed throughout the areas sampled by the available distribution of sources and receivers. In particular, there is no hemispherical pattern, indicating that the latter must originate at some depth below the ICB, rather than on the ICB itself.
DI31A-0252
Heat Transfer Scaling Regime Transitions in Rapidly Rotating Convection
Recent numerical studies suggest the possibility of a unifying heat transfer scaling law in rotating convection, for which Nu ~ Ra1.22. In a coupled experimental-numerical study of strongly supercritical rotating convective heat transfer, we show the emergence of scaling transitions beyond which the proposed law does not hold, and heat transfer scales more similarly to that found in non-rotating convection (i.e., Nu ~ Ra2/7). The transition is explored in convection ranging from non-rotating to rapidly rotating (E = 10- 6), with Rayleigh numbers ranging from 103 to 109, such that the degree of supercriticality, (Ra - Rac)/Rac, spans from 0 to 105. We hypothesize that the transition occurs when strongly supercritical convection isothermalizes the bulk interior of the fluid.
DI31A-0253
Improved Hard Lower Bounds on the Geodynamo Power Requirements
The Earth's thermal history is currently undergoing renewed scrutiny, the age of the inner core taking center stage of the debate. A key element of the controversy is estimates of the power required to drive the geodynamo or, equivalently, the power lost to Ohmic dissipation in the core. A recent prominent estimate of 1-2 TW (Roberts et al. 2003) presents substantial difficulty in forming a consistent Earth model, as the associated high heat flux leaking out of the core- mantle boundary would cool the core too fast, leading to an inner core far younger than expected. A current high- profile theory that sidesteps this difficulty suggests the existence of radiogenic heating in the core, reducing the solidification rate for a prescribed heat flux. Other lower estimates of 0.1-0.5 TW (Christensen & Tilgner, 2004) on the Ohmic dissipation do not require such a source of energy to explain an old inner core, of age comparable to that of the geomagnetic field. The current estimates of Ohmic dissipation are based on numerical geodynamo simulations, computations currently performed with parameter values far from those that are geophysically realistic. Despite the resulting magnetic and velocity fields being arguably at least Earth-like, the extrapolation of such models to the real Earth is nonetheless no more than suggestive. We present results concerning rigorous lower bounds on the Ohmic dissipation associated with the present day field. A lower bound exceeding any existing estimate would immediately rule that estimate as invalid; a sufficiently high value would lead to the inescapable inference of radiogenic heating in order to explain an old inner core. We build on previous work (Jackson and Livermore, 2007), where we considered the minimum Ohmic dissipation subject to various constraints: the field structure on the core-mantle boundary derived from observations of the present day geomagnetic field, and Earth-nutation models (Buffett et al. 2002) that predict rms field strengths on both the inner and outer core boundaries. With additional assumptions on the spectrum of the field, the lower bound was found to be approximately 0.1 TW, very close to the lower-end estimates derived from numerical simulations. However, such constraints effectively supply only boundary conditions on the field structure. We now add in a dynamical condition that the geomagnetic field must satisfy in the entire fluid core: Taylor's constraint. This supplies an infinite number of nonlocal constraints on the permissible structure of the field and must necessarily raise the previous lower bound. Although the improved lower bound will itself be of considerable interest, the associated optimizing field will provide the first example of a 3D magnetic field that is associated with the same dynamical regime as the core, namely, one that satisfies Taylor's constraint.
DI31A-0254
Preliminary Results From a Three-dimensional Numerical Simulation of Compositional Convection in the Earth's Fluid Core
Compositional buoyancy is believed to be important in driving core convection responsible for the geodynamo. As the Earth cools, heavy elements are segregated out of core fluid alloy and become part of the solid inner core (SIC), light elements and latent heat being simultaneously released from the inner core boundary (ICB). This release is probably non-uniform over the ICB, and occurs in discrete buoyant plumes or "jets". It has been argued by Loper and Moffatt that this discreteness has major implications for small- scale core dynamics. The aim of this project is to investigate this suggestion through numerical simulations. In the fluid outer core (FOC), the fluid presumably has very small compositional diffusivity κξ ~ O(10- 9)m2/s and viscosity ν ~ O(10-6)m2/s. If the light elements are ejected out of the ICB as a jet with velocity scale 0.001 m/s < Ujet < 0.1m/s and length scale (the width of the jet) 1 m < L < 103 m, the jet's Reynolds number would be between 103 and 1010 and its compositional Prandtl number would be O(103). For such large Reynolds and Prandtl numbers, severe numerical instability is unavoidable if the numerical resolution is not high enough. We report on a novel numerical approach to overcome this difficulty. Our mathematical model is three-dimensional compositional convection in a bounded fluid layer (representing the FOC) driven by a buoyant jet from the bottom boundary (representing the ICB). Some preliminary results from these calculations will be presented.
DI31A-0255
Dynamo Bifurcation in Geodynamo Models
We investigate the dynamo bifurcation --i.e. the onset of dynamo action-- in numerical models relevant to the Earth's core. As usual in such models, controlling parameters are very remote from relevant geophysical values, sometimes by factors in excess of a million. We show how the nature of the dynamo bifurcation happens to be very sensitive to the parameters regime. We report supercritical bifurcations, subcritical bifurcations and isola. We describe how this behavior depends on the relevant parameters. Finally, we show that two distinct dynamo branches with different energies can be described for a given set of parameters. While viscous effects are not negligeable in simulations, this could be related to the weak and strong field branches which have been proposed in the context of the Geodynamo.
DI31A-0256
Lateral Variations in the Scattering and Viscoelastic Properties of the Inner Core
Recent studies1 find unusual high-frequency coda following the PKiKP phase, reflected from the inner core boundary in the area of the Central Pacific and observed at teleseismic distances between 50 ° to 90 ° range. The majority of the observations of such codas were made at short-period teleseismic arrays in the USA, Eastern Asia and Australia. An array analysis of the coda wavetrains shows the apparent horizontal velocity of the coda to be consistent with scattering from the uppermost inner core. The codas have different durations and frequency content depending on the PKiKP location of the ray incident point on the inner core boundary, as well as the distance between source and receiver. We examine the frequency dependence of the signals as a function of the event depth, magnitude, distance, and the location of the PKiKP incidence point. The observations show that the low frequency limit of the signal changes from 2 Hz in the east to 0.5-1 Hz in the west, possibly correlated with east-west changes observed in attenuation and pulse broadening of PKIKP in the 130 ° -150 ° range sampling the same regions of the inner core along equatorial paths.. Radiative transport modeling of PKiKP coda shapes in the 1-4 Hz band is combined with pseudospectral modeling of PKIKP pulses in the 0.2 to 1 Hz band to assess and separate the effects of scattering by small-scale heterogeneity from the effects of viscoelasticity in the uppermost inner core. Observed lateral variations in elastic attenuation and scattering in the uppermost inner core may record lateral variations in the solidification process of the inner core and lateral variations in fluid velocities of the outer core near the inner core boundary. 1 Leyton, F., and K. Koper, J. Geophys. Res., 112, B05316, doi:10.1029/2006JB004369, 2007.
DI31A-0257
Effect of pressure on the melting behavior of the Fe-S system at moderate pressures: Insight into the state of Mercury's core
The effect of pressure on the liquidus curve of the iron-rich portion of the Fe-S system has been investigated using a multi-anvil apparatus. At 1 bar, the liquidus curve has two inflection points, indicating a positive deviation from the ideal liquidus curve. The curve gradually straightens out with increasing pressure. Our data show that at 10 GPa the curve is consistent with ideal solution behavior. However, the positive deviation returns at 14 GPa, leading to a significant drop in the liquidus temperature at relatively low sulfur conent. The observed pressure effect on the melting behavior of the Fe-S system allows us to establish a link between the current physical state and sulfur content of Mercury's core, and to investigate the history of solid iron precipitation in Mercury.
DI31A-0258
A thermo-chemical wind coupling the Earth''s inner core and deep mantle
The time-average dynamical processes occurring in the Earth''s fluid outer core are thought to depart from axial symmetry, as suggested by models of the paleomagnetic field at the core-mantle-boundary, by time-average core flows derived from the secular variation of the geomagnetic field, and by seismic maps of heterogeneity at the top of the solid inner core. Here we show that these three observations can be explained by the existence of a steady thermo-chemical wind connecting the Earth''s core-mantle and inner core boundaries. We use a numerical model of core dynamics and dynamo action driven by thermo- chemical convection. On top of the simulation shell, we impose a positive heat flow anomaly below China, consistent with deep-mantle seismic tomography, and its interpretation in terms of thermal versus chemical origins. This drives a columnar cyclone below Asia, locally concentrating the radial magnetic field into a persistent flux patch. The cyclone also permanently brings cold and chemically depleted material close to the inner-core-boundary, causing the inner core to crystallize at a faster pace below the Eastern hemisphere. Future progresses in inner-core seismology should allow to test a possible heterogeneity between the northern and southern hemispheres of the inner core, as predicted by our model. http://www.ipgp.jussieu.fr/~{}aubert
DI31A-0259
New estimate of the inner-outer core density ratio from previously unobserved steep- incidence inner core reflections
The density contrast at the inner-outer core boundary, which has significant geodynamical implications, has long been a subject of controversial findings. Seismological 1D models of the Earth indicate that this contrast is larger than it would be for just a phase transition. However, until recently, seismological studies using body waves and normal modes have given significantly different values for this ratio. When the ray theory is used, part of the problem lies in the fact that the convincing observations of PKiKP and PcP for epicentral distances less than 70 degrees are extremely rare. The relative amplitudes will be particularly sensitive to attenuation and radiation patterns, that are not indentical in the mantle. For short distances the mantle components are almost in common, and so the estimates of density conrasts should be most reliable. We report new observations of clear PKiKP arrivals at very short epicentral distances, sampling nearly the same path from the source to the reflection point and from the reflection point to the receiver . These observations are result of a systematic analysis of a global dataset of IRIS Passcal seismic broadband waveforms. We examine the propagations characteristics for the major discontinuities using ray theory and synthetic seismograms based on a reflectivity technique.
DI31A-0260
A New Approach to Determine the Inner-Core Rotation
We present a new approach to gain insight into the inner core rotation by separating the underlying structure from its time evolution. This is achieved by fitting existing seismic data with a smoothing spline analysis of variance model, which is implemented in the statistics package R. This model allows us to separate the structure from its time evolution without any a priori constraints, and also to estimate the error of the fit. Preliminary testing with synthetic datasets that feature both a non-linear structure and variable rotation and share existing earthquakes and stations geometry prove the success of this approach. The mantle and time-dependent inner-core structures are fitted simultaneously and can be separated. Applying this method to existing seismic data from S. Sandwich Islands earthquakes to Alaskan stations suggests an apparent change in the rotation rate of the inner-core within the last 20 years, implying a non-zero acceleration. We will include our newly acquired seismic data from the PASSCAL experiment ARCTIC in northern Alaska, which will increase our time-resolution as well as data coverage of the inner core structure. The results and possible implications on the inner-core rotation and its possible acceleration will be reported. http://www.geology.uiuc.edu/~xsong/arctic/arctic.html
DI31A-0261
Origin of the Low Rigidity of the Earth's Inner Core
The solid iron Earth's inner core has a low rigidity which manifests itself in the anomalously low velocities of shear waves as compared to those in iron alloys. Normally, when estimating elastic properties of a polycrystal one calculates an average over different orientations of a single crystal. This approach does not take into account the grain boundaries and defects likely to be abundant at high temperatures relevant for the inner core conditions. We show, by molecular dynamics simulations that if defects are considered, the calculated shear modulus and shear wave velocity decrease dramatically compared to the averaged single crystal values. Thus, the low shear wave velocity in the inner core receives its explanation (Science 316, 1603 (2007)).
DI31A-0262
Phase Relations of Iron and Iron-Nickel Alloys up to 3 Mbars
Iron is believed to be the major component of the Earth's core because it is the most abundant element that satisfies the observed seismic densities. Based on cosmochemical models and the studies of iron meteorites, it is generally accepted that the Earth's core also contains substantial amounts of nickel. Therefore, the high pressure behaviour of iron-nickel alloys is crucially important for interpreting and constraining geophysical and geochemical models of the Earth's core. The phase relation of iron at relatively low pressure has been well established. α-Fe with bcc structure at ambient condition transforms to γ-Fe at high temperature and to ε-Fe with hcp structure at above ~ 10 GPa. In contrast, the phase relation and the crystal structure at high pressure and temperature are still highly controversial. The phase relations of iron-nickel alloys were also studied in an externally-heated diamond-anvil cell (Huang et al. 1988, 1992) and in a laser-heated diamond-anvil cell (Lin et al. 2002, Mao et al. 2005, Dubrovinsky et al. 2007), but these experiments were limited to the pressure of 225 GPa. Applications of the previous results to the Earth's inner core conditions required significant extrapolations. In this study, we have investigated the phase relations of iron and a number of iron-nickel alloys in a wide range of pressures (>300 GPa), temperatures (>2000 K) and compositions (0-80 wt% Ni) using a laser-heated diamond-anvil cell with synchrotron x-ray diffraction. For iron, in-situ x-ray diffraction studies showed a wide range of stability of ε-Fe with an hcp structure up to 300 GPa and 2000 K and up to 343 GPa at room temperature. No evidence for the existence of phases other than ε-Fe, such as β-Fe with a dhcp structure (suggested by Dubrovinsky et al. 2000) or orthorhombic structure (suggested by Andrault et al. 1997), was observed. For iron-nickel alloys, high pressure and temperature experiments were conducted on Fe-18.4 wt% Ni, Fe-24.9 wt% Ni, Fe-35.7 wt% Ni, Fe-50.0 wt% Ni and Fe-80.0 wt% Ni up to 300 GPa. The experimental results indicate that the iron-nickel alloys strongly favour an fcc structure under multimegabar pressures. Our results can directly apply to the Earthfs inner core pressures and the phase relations of iron- nickel alloys may interpret seismically observed anisotropy and discontinuity in the Earth's inner core.
DI31A-0263
Experimental Compaction in a Growing Dendritic Zone
The Earth inner core is thought to be in a state of dynamical equilibrium between dendritic solidification and compaction of the resulting solid-liquid region (or 'mushy zone') (Sumita et al.,1996). One important question is how much liquid can be trapped in the inner core, or how efficient is compaction to squeeze out the liquid phase. While this can be estimated theoretically in the case of non-reacting liquid and solid phases, this problem is somewhat more complicated in the case of a crystallizing mushy zone, as it involves a continuous mass transfer between the two phases as the system evolves. Consequences on the evolution of the connectivity of the melt as solidification proceed are difficult to assess, making the dependence of the permeability on porosity difficult to predict theoretically, particularly when the liquid fraction becomes small. Other open questions includes how does compaction and convection compete in the mushy zone? What are the effects of compaction on the thickness of the convecting zone? on the interdendritic spacing? on the structure and dimensions of chimneys? We present here preliminary results of an experiment devoted to the study of compaction during the dendritic crystallization of a model material. In our experimental set-up, compaction is promoted by a high apparent gravity, which is imposed by putting the crystallizing sample in a standard lab centrifuge, where the centrifuge acceleration can reach a few thousand g. While solidification proceed, the sample is scanned in situ with ultrasounds, allowing us to follow the propagation of the solidification front and to investigate the variations of ultrasound velocity and attenuation in the liquid, mush and solid domains.
DI31A-0264
Simulations of Inner Core Coda Waves with a Multiple-Scattering Phonon Based Algorithm
Emergent, low-slowness coda waves following precritical PKiKP phases have now been observed at a geographically and geometrically diverse set of seismic arrays. Forward simulations using ray-theoretical single scattering theories have shown that it is difficult to match the observed properties with heterogeneity in the mantle, along the core-mantle boundary, or along the inner core boundary; only by placing heterogeneity within the volume of the inner core can the observations be well-matched. In this study, we present new simulations of PKiKP coda waves using a more complete, phonon-based approach that allows for multiple scattering between source and receiver. We experimented with a wide range of scattering models and again found that only by distributing heterogeneities within the volume of the inner core can the observations be fit. The thickness of the scattering layer must be at least 200-300~km and the data do not rule out the existence of heterogeneities throughout the inner core. There is a strong trade-off among the model parameters, and high-quality models can be found with correlation lengths of 1-16~km and velocity perturbations of 0.8%-10%. Perturbations in density are not required to fit the data. There is also a smaller trade-off with the value of Qp in the inner core. High Qp values require weaker models of inner core scattering to provide equivalent fits to the observations. Our preferred model has a correlation length of 1~km, an rms velocity perturbation of 0.8%, and an inner core Qp of 360.
DI31A-0265
The Earth's inner core deviation to plate movement
Under the premise of the Earth's spherical structure and mass distribution, the solid inner core cannot be stable in the center of the Earth. Thus, the deviation of the inner core is towards the sphere's barycenter and the liquid outer core must be brought about asymmetrical thermal convection. Based on the two suggestions, a concise and self-consistent global motion model can be built. The model consists of the following cycle: an asymmetrial thermal convection structure in the outer core led by the dislocation of the inner core¡úthe plume is in special upwell position because of differencial activation¡úthe formation and split of lithosphere¡úthe split plates drift and assemble in a new location, the mass of which causes the inner core to deviate towards the direction again¡úthe new asymmetry is formed. As this circulation continues, a definite and periodical motion emerges/forms. Its nonlinear features result in the Earth's motion with simple mechanisms but complex behavior. Ultraterrestrial events may disturb or even interrupt this movement but would not significantly affect the cycle forever.
DI31A-0266
Tetra-Frequency Spectrum Vector Method Of Resolving FCN Parameters From Tidal Data
The parameters of Earth free core nutation (FCN) are two fairly important geophysical parameters, which provide some valuable constraints on the Earth's deep construction. The theoretical estimations of FCN parameters were provided by some researchers such as Sasao et al. (1980), Wahr and Bergen (1986) and Mathews et al. (2002). Gwinn, Herring and Shapiro (1987) first obtained the observational values of FCN parameters by very long base interference (VLBI). Neuberg, Z¨¹rn and Hinderer (1987) began to retrieve FCN parameters from the observation of Earth tides and introduced the Stacking Method according to Marquardt algorithm. The later scholars basically followed the Stacking Method to check FCN parameters from tidal data. It is in fact a kind of non-linear inversion to resolve FCN parameters from tidal data, and there exists the phenomena of complex error transmission during the inversion process. Although Gauss distribution was satisfied by the observation errors of Earth tidal factors, the errors of resolved FCN parameters wouldn't follow the normal distribution. Not only the stacking method but also another resolution method (Florsch et al. 1994) could not provide the accurate description of resolution error of FCN parameters. We will introduce a new method called Tetra-Frequency Spectrum Vector Method (TFSVM) to resolve the FCN parameters from the gravity tidal data, which consists of two resolution steps: point resolution algorithm (PRA) and finite probability element method (FPEM). TFSVM can provide the accurate description of the error probability distribution of FCN parameters. After the precise analysis on the error probability distribution of FCN parameters resolved from Moxa station and Potsdam station, we found that the error distribution of FCN parameters is not satisfied with the normal distribution, and it might not be suitable for the classic error transmission law to study the resolution errors of FCN parameters. We believe that TFSVM would be able to promote the study of checking FCN parameters. Acknowledgements: This work is supported jointly by the National Natural Sciences Foundation of China (Grant No: 40404005) and the Excellent Prize of President Scholarship of CAS (Chinese Academy of Sciences). Thanks for the scientific invitation provided by Dr. Neumeyer and Prof. Rothacher in GFZ.