a glassy lowermost outer core summary new theories for the viscosity of metallic melts at core pressures and temperatures together with observations of translational modes of oscillation of earth s solid inner core suggest a rapid increase in the dynamic viscosity near the bottom of the liquid outer core if the viscosity of the lowermost outer core f region is sufficiently high it may be in a glassy state characterized by a frequency dependent shear modulus and increased viscoselastic attenuation in testing this hypothesis the amplitudes of high frequency pkikp waves are found to be consistent with an upper bound to shear velocity in the lowermost outer core of 0 5 km s 1 at 1 hz the fit of a maxwell rheology to the frequency dependent shear modulus constrained by seismic observations at both low and high frequency favours a model of the f region as a 400 km thick chemical boundary layer this layer has both a higher density and higher viscosity than the bulk of the outer core with a peak viscosity on the order of 10 9 pa s or higher near the inner core boundary if lateral variations in the f region are confirmed to correlate with lateral variations observed in the structure of the uppermost inner core they may be used to map differences in the solidification process of the inner core and flow in the lowermost outer core

localized temporal change of the earth s inner core boundary compressional waves of an earthquake doublet two events occurring in the south sandwich islands on 1 december 1993 and 6 september 2003 recorded at three seismic stations in russia and kyrgyzstan and reflected off earth s inner core boundary arrived at least from 39 to 70 milliseconds earlier in the 2003 event than in the 1993 event such changes indicate that earth s inner core radius enlarged locally beneath middle africa by 0 98 to 1 75 kilometers between the times of these two events changes of the inner core radius may be explained by either a differential motion of the inner core assuming that irregularities are present at the inner core boundary and fixed to the inner core or a rapid growth of the inner core by this amount

inner core anisotropy and heterogeneity level an analysis of core phase travel times is performed in order to estimate the relative contributions of heterogeneities and anisotropy and their distribution inside the inner core a stochastic analysis performed on the equatorial paths which are poorly sensitive to anisotropy indicates that the inner core exhibits a heterogeneity level smaller than 0 3 at all depths on scale lengths larger than 200 km this result extended to all path orientations suggests a predominant effect of the anisotropy on travel times anomalies a simultaneous inversion of absolute and differential core phase travel times for any path orientation favours a simple inner core model in which a large homogeneous structure with 3 uniform anisotropy is asymmetrically surrounded by a homogeneous isotropic layer with a thickness of 100 200 km on the western hemisphere increasing to 400 km over a large part of the eastern hemisphere

seismic evidence for an inner core transition zone seismic waves that traverse earth s inner core along north south paths produce unusually broad pulse shapes at long periods compared with waves along east west paths and reflections from below the inner core boundary at short periods the observations provide compelling evidence for a seismic velocity discontinuity along north south paths about 200 kilometers below the inner core boundary separating an isotropic upper inner core from an anisotropic lower inner core the triplication associated with such a structure might be responsible for reported waveform complexity of short period inner core arrivals along north south paths and if the depth of the boundary is laterally variable their large travel time variation

seismic velocity and attenuation structures in the top 400 km of the earth s inner core along equatorial paths 1 we study seismic velocity and attenuation structures in the top 400 km of the earth s inner core based on modeling of differential traveltimes amplitude ratios and waveforms of the pkikp pkikp phases observed at the epicentral distance range of 120 141 and the pkpbc pkikp phases observed at the distance range of 146 160 along equatorial paths our data are selected from the seismograms recorded in the global seismographic network from 1990 to 2001 and many regional seismic networks the observed pkikp pkikp and pkpbc pkikp phases exhibit distinctive east west hemispheric patterns 1 at the distance ranges of 131 141 and 146 151 pkikp phases arrive about 0 3 s earlier than the theoretical arrivals based on the preliminary reference earth model prem for the pkikp phases sampling the eastern hemisphere 40 e 180 e of the inner core and about 0 4 s later for those sampling the western hemisphere 180 w 40 e at the distance range of 151 160 pkikp phases arrive about 0 7 s earlier than the predicted arrivals based on prem for those sampling the eastern hemisphere and about 0 1 s later for those sampling the western hemisphere 2 amplitude ratios of the pkikp pkikp phases at the distance range of 131 141 and of the pkikp pkpbc phases at the distance range of 146 151 are in general smaller for the pkikp phases sampling the eastern hemisphere than for those sampling the western hemisphere at distances greater than 151 the pkikp pkpbc amplitude ratios become indistinguishable for the two hemispheres these observations can be best explained by two different types of seismic velocity and attenuation models along equatorial paths one for each hemisphere in the top 400 km of the inner core for the eastern hemisphere the velocity structure has a velocity increase of 0 748 km s across the inner core boundary icb a small velocity gradient of 0 0042 km s 100 km in the top 235 km followed by a steeper velocity gradient of 0 1 km s 100 km extending from 235 km to 375 km and a velocity gradient of 0 01 km s 100 km in the deeper portion of the inner core the attenuation structure has an average q value of 300 in the top 300 km and an average q value of 600 in the deeper portion of the inner core for the western hemisphere the velocity structure has a velocity increase of 0 645 km s across the icb and a velocity gradient of 0 049 km s 100 km in the top 375 km the attenuation structure has an average q value of 600 in the top 375 km of the inner core our results suggest that the inner core hemispheric variations in velocity extend deeper than 375 km below the icb and the top 235 km of the inner core in the eastern hemisphere is anomalous compared to the rest of the inner core in having a small velocity gradient high velocity and high attenuation

the inner core hemispheric boundary near 180 w abstract the inner core ic east west hemispheric dichotomy is widely recognized but the reported position of the hemispheric boundary varies among studies due to uneven sampling coverage and the data analyzed this study investigates the sharpness of the western hemispheric boundary whb near 180 w by analyzing differential time residuals of pkikp pkpdf and pkp bc df for pkpdf phases that sample 155 e 130 w in various azimuthal directions using pkikp pkpdf observations the whb is located at 175 e 180 w in the southern hemisphere based mainly on the lateral isotropy anisotropy transition however based on the lateral isotropic velocity contrast and this isotropy anisotropy transition between the two hemispheres its location is 170 160 w in the northern hemisphere these findings indicate that the whb is sharp and exhibits a latitudinal dependence with a 10 20 kink as well as 1 75 anisotropy in the uppermost ic across the 180 155 w range of the western hemisphere as suggested by pkp bc df the whb could remain at 160 w at depth the isotropic velocity contrast near the whb 160 w between the eastern and western hemispheres is lower than previous estimates using pkpdf phases sampling the bulk part of each hemisphere

ab initio calculations on the free energy and high p t elasticity of face centred cubic iron ab initio finite temperature molecular dynamics simulations have been used to calculate the free energy and elasticity of face centred cubic fcc iron at a state point representative of the earth s inner core whilst the free energy of this phase is found to be higher than that of hexagonal close packed hcp iron the difference is only 14 mev atom it is possible that this difference might be overcome by the presence of light elements as previous calculations at zero kelvin have shown that the addition of elements such as silicon stabilise fec fe with respect to hcp fe by at least 40 mev atom the calculated elastic constants at core pressures and temperatures of pure fec fe and of alloys of fe with sulphur and nickel fe3s and fe3ni derived from the fee structure lead to average shear wave velocities that are considerably higher than those inferred from seismology however these mineralogical and seismological results could be reconciled by the presence of partial melt in the inner core the calculated p wave anisotropy of fec fe is comparable with the seismological values but only if there is a high degree of crystal alignment although the necessity for alignment can be reduced if a layered model for the inner core is invoked the results presented in this paper therefore suggest that fcc fe cannot be ruled out as a candidate for the dominant phase of the earth s inner core c 2008 elsevier b v all rights reserved

the search for evidence of large prehistoric earthquakes along the atlantic seaboard the spacial distribution of seismically induced liquefaction features discovered along the atlantic seaboard suggest that during the last 2000 to 5000 years large earthquakes body wave magnitude m b 5 8 0 4 in this region may have been restricted exclusively to south carolina paleoliquefaction evidence for six large prehistoric earthquakes was discovered there at least five of these past events originated in the charleston south carolina area the locale of a magnitude 7 event in 1886 during the past two millennia large events may have occurred about every 500 to 600 years

equatorial anisotropy in the inner part of earth s inner core from autocorrelation of earthquake coda the speed of seismic waves passing through the earth s inner core varies with direction analysis of earthquake seismic data suggests that this directional dependence differs between innermost and outer inner core

thermochemical convection in earth s inner core summary the dynamics of earth s inner core depends critically on whether it is stably stratified or unstably stratified we propose here a general analysis of the thermal evolution of the inner core whether the geotherm in the inner core is superadiabatic or not depends on the inner core solidification rate on the thermal diffusivity of iron at inner core conditions and on the ratio of the clapeyron slope to the adiabatic gradient in the inner core the temperature field within the inner core can be destabilizing and could drive convection if the growth of the inner core is fast enough the effect of radiogenic heating is probably small and perhaps surprisingly can even stabilize the inner core against convection the uncertainties are such that it is not possible at present to conclude about the likelihood of thermal convection in the inner core but recent estimates of the core mantle boundary cmb heat flux and inner core conductivity favour convection thermal convection is more likely early in the inner core history a consequence of the secular decrease in cooling rate of the core in addition solidification induced partitioning of the light elements may induce a stable density stratification within the inner core we develop a numerical model of thermochemical convection in a growing inner core which couples the evolution and dynamics of the inner core with the thermal and compositional evolution of the outer core melting and crystallization associated with deformation of the inner core boundary icb would be of importance for the style of convection if the viscosity is large but we focus here on the case of low viscosity for which phase change associated with dynamic topography at the icb is expected to play a secondary role in this regime convection is typical of high rayleigh number internally heated convection with cold plumes falling from the icb several possible scenarios can lead to a layered inner core either because of cessation of thermal convection due to the decrease in cooling rate of the core or because of a compositional stratification which can confine convection in the deep inner core or stabilize the whole inner core for each of these scenarios it is possible to find plausible sets of parameters inner core age viscosity magnitude of the compositional stratification for which the radius at which convection stops corresponds to the radius of the seismically inferred innermost inner core

test of the innermost inner core models using broadband pkikp travel time residuals 1 two different innermost inner core imic models have recently been proposed each with different radii and different orientation and strength of anisotropy in order to test these imic models we have systematically assembled a high quality dataset of broadband pkikp data in the epicentral distance range of 150 to 180 our new dataset consists of 1 100 individually measured absolute pkikp arrival times also pkikp bottoming points and ray paths in the inner core are well distributed in the east west and north south hemispheres we invert this dataset for two layer models of anisotropy in the inner core and compare the fits obtained with those predicted from the existing imic models our results show that if there is an imic its radius is most likely around 500 km rather than the 300 km originally proposed and is in better agreement with the layering in the inner core based on pkikp waveform modeling

rivera ocean seismic experiment rose overview the rivera ocean seismic experiment rose was designed as a combined sea and land seismic program to utilize both explosive and earthquakes to study a number of features of the structure and evolution of a mid ocean ridge a major oceanic fracture zone and the transition region between ocean and continent the primary region selected for the experiment included the rivera fracture zone the crest and eastern flank of the east pacific north of the rivera and adjacent areas of baja california and mainland mexico the experiment included 1 study of the east pacific rise south of the orozco fracture zone primarily using ocean bottom recording and explosive sources 2 a seismicity program at the orozco and 3 a land based program of recording natural events along the coastal region of mexico a considerable amount of useful data was obtained in each of the three subprograms in the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle structure and dynamics of the east pacific rise seismicity of the orozco fracture zone and partitioning of energy transmission between the ocean volume and the crust lithosphere on land the fortuitousmore occurrence of the petatlan m7 6 earthquake of march 14 1979 permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event which provide new insight into the filling of a major seismic gap in the region this overview describes the scientific rationale and the design of the experiments along with some general results less

seismic anisotropy of the earth s inner core resulting from flow induced by maxwell stresses seismological observations indicate that the inner core of the earth is elastically anisotropic1 anisotropic structures are likely to be formed by dynamic processes and therefore such observations have the potential to provide constraints on flow in the inner core and on the geodynamo itself but in addition to the difficulties in estimating the relevant physical properties of iron under inner core conditions2 3 4 even the macroscopic processes responsible for generating seismic anisotropy in this region have yet to be determined5 6 7 8 9 as a result the geodynamic significance of seismic anisotropy in the inner core has remained unknown here i propose based on geodynamic and mineral physics considerations that flow induced by the stress due to the magnetic field the maxwell stress near the inner core boundary produces an axisymmetric fabric responsible for the observed seismic anisotropy the resultant seismic anisotropy reflects the geometry of the magnetic field near the inner core boundary and therefore seismological observations might provide constraints on the geodynamo this flow also causes non uniform release of energy at the inner core boundary associated with solidification and melting which may affect the pattern of convection in the outer core

anisotropy in the deep earth abstract seismic anisotropy has been found in many regions of the earth s interior its presence in the earth s crust has been known since the 19th century and is due in part to the alignment of anisotropic crystals in rocks and in part to patterns in the distribution of fractures and pores in the upper mantle seismic anisotropy was discovered 50 years ago and can be attributed for the most part to the alignment of intrinsically anisotropic olivine crystals during large scale deformation associated with convection there is some indication for anisotropy in the transition zone particularly in the vicinity of subducted slabs here we focus on the deep earth the lower mantle and core where anisotropy is not yet mapped in detail nor is there consensus on its origin most of the lower mantle appears largely isotropic except in the last 200 300 km in the d region where evidence for seismic anisotropy has been accumulating since the late 1980s mostly from shear wave splitting measurements recently a picture has been emerging where strong anisotropy is associated with high shear velocities at the edges of the large low shear velocity provinces llsvps in the central pacific and under africa these observations are consistent with being due to the presence of highly anisotropic mgsio 3 post perovskite crystals aligned during the deformation of slabs impinging on the core mantle boundary and upwelling flow within the llsvps we also discuss mineral physics aspects such as ultrahigh pressure deformation experiments first principles calculations to obtain information about elastic properties and derivation of dislocation activity based on bonding characteristics polycrystal plasticity simulations can predict anisotropy but models are still highly idealized and neglect the complex microstructure of polyphase aggregates with strong and weak components a promising direction for future progress in understanding the origin of seismic anisotropy in the deep mantle and its relation to global mantle circulation is to link macroscopic information from seismology and microscopic information mineral physics through geodynamics modeling anisotropy in the inner core was proposed 30 years ago to explain faster p wave propagation along the direction of the earth s axis of rotation as well as anomalous splitting of core sensitive free oscillations there is still uncertainty about the origin of this anisotropy in particular it is difficult to explain its strength based on known elastic properties of iron as it would require almost perfect alignment of iron crystals indeed the strongly anomalous p travel times observed on paths from the south sandwich islands to alaska may or may not be due to inner core anisotropy and will need to be explained before consensus can be reached on the strength of anisotropy in the inner core and its origin

rapid intraplate strain accumulation in the new madrid seismic zone remeasurement of a triangulation network in the southern part of the new madrid seismic zone with the global positioning system has revealed rapid crustal strain accumulation since the 1950s this area experienced three large moment magnitudes 8 earthquakes in 1811 to 1812 the orientation and sense of shear is consistent with right lateral strike slip motion along a northeast trending fault zone as indicated by current seismicity detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time

onset and orientation of convection in the inner core summary thermal convection is an effective mechanism for producing seismic anisotropy in many regions of the earth however it is not known whether this mechanism is relevant for the anisotropy observed in the inner core here we establish the requirements for convection in the inner core by combining a simple model for inner core growth with a solution for conductive cooling thermal convection is likely during the early stages of inner core growth but becomes less probable as the volume of the inner core increases the transition to a non convecting state is driven by changes in both the thermal and compositional stratification a decrease in the vigour of convection near the transition to a non convecting state isolates the pattern of flow with the lowest critical rayleigh number rac we apply a perturbation method based on rayleigh s principle to calculate rac in a hydrostatically flattened inner core we find that the final pattern of flow is characterized by a degree l 1 mode in which the axis of upwelling aligns with the rotation axis this pattern of flow has the symmetry required to explain the distribution of anisotropy at the largest scales the extent of crystal alignment depends on the level of stress during this final convective state one possible interpretation of the observed anisotropy is that the final convective state establishes the large scale crystal alignment without completely erasing a more complicated texture that developed when convection was more vigorous

microearthquakes on the east pacific rise at 21 n and the rivera fracture zone during march and april of 1974 three ocean bottom seismometer obs capsules were deployed in arrays on the east pacific rise epr at 21 n for 8 days on the rivera fraction zone rfz for 9 days and at the epr rfz intersection for 12 days seismicity on the epr is characterized by extremely small events none large enough to trigger all of the instruments simultaneously a weak 24 hour periodicity in obs event trigger rate is observed this could be related to earth strain tides or a modulation in the trigger sensitivity due to a varying noise level on the rivera fracture zone an en echelon pattern of earthquakes with a maximum depth of 10 km was observed seismic activity at the junction of the epr and rfz becomes more diffuse and terminates abruptly at the epr the junction array was slightly offset from the active zone of seismicity and the depth of only one event is reliably determined at 5 4 2 km the s wave spectra of microearthquakes at the junction and on the rfz are computed and seismic moments and corner frequencies determined stress drops are between 0 1 and 10 bars and show no systematic difference at the two deployment sites larger earthquakes observed at teleseismic distances do show markedly lower stress drops when they originate near to spreading centers but these differences are not observed at the microearthquake level in this study it is suggested that this is due to a natural tendency for small earthquakes to have small stress drops and a lack of large earthquakes during the rfz deployment

space geodetic evidence for rapid strain rates in the new madrid seismic zone of central usa a series of large earthquakes centred on the town of new madrid in the mississippi valley in the winter of 1811 12 still reverberates in the world of seismology the controversy relates to their origin over 2 000 km from the nearest plate boundary and their relevance to earthquake hazard today the population of new madrid in the early nineteenth century was tiny a similar event today would have much more serious consequences new evidence based on gps data points to the existence of rapid strain rates in the area comparable in magnitude to those across active plate boundaries and consistent with known active faults in the region in the winter of 1811 1812 near the town of new madrid in the central united states and more than 2 000 km from the nearest plate boundary three earthquakes within three months shook the entire eastern half of the country and liquefied the ground over distances far greater than any historic earthquake in north america1 2 the origin and modern significance of these earthquakes however is highly contentious3 geological evidence demonstrates that liquefaction due to strong ground shaking similar in scale to that generated by the new madrid earthquakes has occurred at least three and possibly four times in the past 2 000 years refs 4 6 consistent with recurrence statistics derived from regional seismicity7 here we show direct evidence for rapid strain rates in the area determined from a continuously operated global positioning system gps network rates of strain are of the order of 10 7 per year comparable in magnitude to those across active plate boundaries and are consistent with known active faults within the region these results have significant implications for the definition of seismic hazard and for processes that drive intraplate seismicity

waveform search for the innermost inner core abstract waveforms of the pkikp seismic phase are inconsistent with the effects of a sharp boundary at 300 km radius of an inner most inner core seismograms synthesized for the distance range 150 to 180 in models having a discontinuity at 300 km radius in the inner core exhibit focused diffractions around the innermost sphere at antipodal range that are inconsistent with observed pkikp waveforms a few examples however can be found of pkikp complexity near 165 consistent with a triplication created by a 475 km radius discontinuity a rapid change in the magnitude and variance of seismic attenuation of pkikp whose rays turn between 400 and 600 km radius is also observed consistent with an innermost sphere of low regionally uniform seismic attenuation models predicting waveforms in agreement with observed waveforms have either a transition in elastic and anelastic properties spread over a depth interval greater than 100 km or an innermost sphere of differing elastic properties that exceeds 450 km radius the weak but suggestive evidence for an innermost inner core may simply represent a transition in fabric spread over 100 km or more centered near 500 km radius this change may signify the end stage of solidification flow and re crystallization resulting in the highest ordering and largest grain sizes of intrinsically anisotropic crystals

geophysics tectonics in the earth s core the complex three dimensional structure of the earth s solid inner core reveals how it has grown through time numerical simulations of the solidification process suggest that part of this structure has resulted from recent tectonic activity

geodetic measurement of horizontal deformation in the northern san francisco bay region california analysis of geodolite observations in the coast ranges of northern california indicates that relative plate motion is spread over a wide zone little or no deformation occurs to the southwest of the san andreas fault itself to the northeast of the san andreas fault a region transected by the parallel rodgers creek and maacama faults 25 6 mm yr of right lateral slip is distributed over about 60 km there is a peak shear strain of 0 6 0 1 rad yr near the san andreas fault elsewhere in the deforming zone shear strain rates are 0 3 0 04 rad yr at the northeastern boundary of the study area the strain rate is less than 0 2 rad yr except for the high strain rate near the san andreas fault there is no evidence of any localization of deformation to the surface traces of the faults nor is there any evidence of creep or aseismic slip on any of the faults in the area the data indicate that the relative motion between the pacific and north american plate occurs primarily to the northeast of the san andreas fault the observations are inconsistent with models that include slip at depth on only the san andreas fault the data can be fit equally well by models with slip at depth on all three of the major faults or with motion distributed across the entire zone from the san andreas fault to the napa valley fault preliminary analysis of observations to the farallon islands about 40 km southwest of the san andreas fault suggests that little or no additional relative motion is accommodated on that side of the san andreas fault in particular it is unlikely that the 20 mm yr difference between the plate motion rate about 55 mm yr and the san andreas fault system rate about 35 mm yr can be explained by deformation between the san andreas fault and the farallon islands

seismological evidence for differential rotation of the earth s inner core the travel times of seismic waves that traverse the earth s inner core show a small but systematic variation over the past three decades this variation is best explained by a rotation of the inner core that moves the symmetry axis of its known seismic anisotropy the inferred rotation rate is on the order of 1 per year faster than the daily rotation of the mantle and crust

state of stress and intraplate earthquakes in the united states recently compiled data on the state of stress have been used to define stress provinces in the conterminous united states in which the orientation and relative magnitude of the horizontal principal stresses are fairly uniform the observed pattems of stress constrain mechanisms for generating intraplate lithospheric stresses coupled with new information on geologic structure and tectonism in seismically active areas of the midcontinent and east these data help to define some characteristics common to these areas and to identify key questions regarding why certain faults seem to be seismically active

inner core transition zone and anomalous pkp df waveforms from polar paths 1 short period pkp df waves which travel through the inner core from polar paths nearly ns were observed to have small amplitudes and complex waveforms but the cause has been a mystery here we examine both short period and broadband pkp record sections from a single earthquake in south sandwich islands ssi the data from polar paths show clear evidence for an inner core triplication our waveform modeling suggests a 250 km thick isotropic upper inner core uic overlying an anisotropic lower inner core lic with about 8 anisotropy the synthetics match the observed waveforms and amplitudes well with a normal value of inner core attenuation we propose that such a uic lic transition zone structure may be the cause of anomalous df waveforms along polar paths the inferred level of anisotropy in the lic 2 3 times previous estimates has important implications for understanding physical properties of the inner core and sources of inner core anisotropy

depth extent of hemispherical inner core from pkp df and pkp cdiff for equatorial paths abstract differential travel times and amplitude ratios of pkp df and pkp cdiff partly including pkp bc for equatorial paths at epicentral distances from 150 to 160 are examined to discuss the radial structure of the uppermost 500 km of the inner core by using seismograms with a predominant frequency of approximately 0 5 hz 649 measurements are successfully collected including 295 and 354 readings for the eastern and western hemispheres respectively forward modeling with the reflectivity method suggests that the best fit velocity structures in the eastern and western hemispheres are 0 5 faster and 0 9 slower respectively than an average structure at the inner core boundary icb and that the velocity perturbations become linearly closer to the average with depth finally coinciding with the average at 400 km and 250 km below the icb respectively if ak135 is adopted for the structure above the icb in the eastern hemisphere the q values in the uppermost 250 km of the inner core and that in the range from 300 to 450 km below the icb are approximately 180 and 1000 respectively in contrast the q value in the western hemisphere is almost constant at approximately 300 400 in the uppermost 450 km of the inner core

depth dependence of anisotropy of earth s inner core both body wave pkp travel times creager 1992 song and helmberger 1993a mcsweeney and creager 1993 shearer 1994 and fits to the splitting of core modes tromp 1993 show general agreement that the top 300 km of inner core is very anisotropic the anisotropy displays axial symmetry around the earth s spin axis with the polar direction 3 faster than the equatorial direction one key problem now is the depth dependence of the inner core anisotropy here we extend our polar path studies to include both long period and short period modeling for the pkp phases at ranges 120 to 173 arrivals from the top of the inner core pkikp and reflections from the inner core boundary pkikp can be observed distinctly in short period records at ranges 130 to 140 and as waveform distortions in the long period records at ranges 130 to 146 these waveforms provide a new set of data for examining the topmost 150 km of the inner core which is not well sampled by the previous body wave travel times record sections of waves traversing the inner core nearly parallel to the earth s spin axis polar paths from three events two beneath the south sandwich islands and one along the macquarie ridge recorded at world wide standardized seismograph network canadian network and long range seismic measurements stations are analyzed our results suggest that the top 150 km of the inner core is only weakly anisotropic less than 1 with strong evidence indicating that the top 60 km is not anisotropic at all

the innermost inner core of the earth evidence for a change in anisotropic behavior at the radius of about 300 km since the discovery of the inner core in 1936 no additional spherical subshell of the earth has been observed based on an extensive seismic data set we propose the existence of an innermost inner core with a radius of 300 km that exhibits a distinct transverse isotropy relative to the bulk inner core specifically within the innermost inner core the slowest direction of wave propagation is 45 from the east west direction in contrast the direction of the slowest wave propagation in the overlying inner core is east west the distinct anisotropy at the center of the earth may represent fossil evidence of a unique early history of inner core evolution

ab initio calculations of the elasticity of iron and iron alloys at inner core conditions evidence for a partially molten inner core abstract ab initio finite temperature molecular dynamics simulations have been used to calculate the elasticity of hcp fe bcc fe fes and fesi at core conditions the calculated compressional wave velocities are in excellent agreement with the most recent experimental data however the calculated shear wave velocities of all phases studied are significantly higher than those inferred from seismology this discrepancy can only be explained if the inner core is partially molten containing more than 8 liquid

dynap software for estimating crustal deformation from geodetic data abstract the acronym dynap dynamic adjustment program identifies a software package for estimating crustal dynamic parameters from a collection of geodetic data dynap accepts various data types including horizontal directions and angles distances azimuths zenith distances and intersite vectors such as those obtainable with the global positioning system gps or with very long baseline interferometry vlbi dynap incorporates the simultaneous reduction technique whereby crustal dynamic parameters are estimated simultaneously along with station positional coordinates and various nuisance parameters all via the least squares process the estimated crustal dynamics parameters come in groups of six or fewer where each group corresponds to a prespecified geographic region and period of time the six parameters convert mathematically to yield the three components of the horizontal strain rate tensor the horizontal rotation rate and the two components of the vertical tilt rate vector dynap s applicability is illustrated with two examples the first involves over 60 years of triangulation trilateration data in the cascadia subduction zone of northwestern california here a maximum shearing rate was found of 0 161 0 074 radians yr with the corresponding axis of maximum contraction oriented n25 e 14 the second involves a 1980 triangulation trilateration survey and a 1986 gps survey both spanning the imperial fault of southern california here a maximum dextral shearing rate was found of 1 073 0 171 radians yr in the direction of n31 w 5

tomographic inversion for three dimensional anisotropy of earth s inner core abstract seismological studies generally suggest 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 one dimensional 1d models here we present non linear tomographic inversions of the inner core anisotropy using three dimensional 3d 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 pkp df phase the method iteratively perturbs each discontinuity point and continuous segment of the ray through 3d earth structure so that its travel time is minimum the 3d anisotropic structure of the inner core is approximated to the first order as 3d heterogeneous but isotropic structure for a given ray the data are corrected using a scaled mantle tomographic model 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 40 160 e 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 40 from the equatorial plane the velocity in the outer inner core oic however changes little for ray directions 0 40 from the equatorial plane 3 despite large variation of the anisotropy the isotropic velocity voigt average throughout the inner core is nearly uniform the results suggest that the oic is likely composed of the same type of iron crystals with uniform chemistry but the iic may be composed of a different type of crystal alignment a different iron phase or a different chemical composition our tests on model parameterization mantle correction and linear and non linear inversion suggest the main features of our model are very robust however fine scale structures are likely to differ particularly in the major transition zones e g in the topmost qwh isotropy to anisotropy between oic and iic change in the form of anisotropy and between qeh and qwh in oic difference in anisotropy strength searches for possible waveform complications from these boundaries need to be aware of the directional dependence and geographical variation to be successful

inner core rotation rate from small scale heterogeneity and time varying travel times the time it takes seismic waves to propagate from south atlantic earthquakes through the inner core to station col in alaska has decreased systematically over the past 30 years travel times from three earthquakes in 1991 to an array of 37 seismometers in alaska suggest that lateral gradients in seismic wavespeeds are steep in this part of the inner core this combination of observations can be explained by postulating that the inner core is rotating 0 2 to 0 3 per year faster than the mantle

structure and dynamics of earth s inner core abstract the seismological exploration of the earth s inner core has revealed unexpected and puzzling structural complexities its elastic anisotropy is now well established and has been shown to vary spatially there is a well defined hemispherical dichotomy in anisotropy and attenuation and significant variations in the radial direction with weaker anisotropy in the uppermost regions and possibly different properties in an innermost inner core of radius 300 600 km perhaps even more puzzling is the observation of a stably stratified layer at the base of the outer core which has recently been proposed to result from melting of inner core material if confirmed the presence of this layer has major implications for our understanding of convection in the outer core inner core differential rotation with respect to the mantle remains controversial we will review here recent progress in understanding the dynamics of the inner core a number of different models have been proposed for the development of the inner core s structure and formation of the stable layer at the base of the outer core and particular care will be taken to clarify under which conditions the proposed mechanisms can be active

joint inversion for inner core rotation inner core anisotropy and mantle heterogeneity observational evidence for differential rotation of the inner core has recently been reported from time dependent travel times of the seismic inner core phase pkp df these analyses are hindered by potential biases from the mantle heterogeneity and systematic earthquake location errors which have raised doubts about the reality of the inferred differential rotation here we report time dependent pkp df pkp bc differential travel times of a total of 611 measurements obtained from 92 earthquakes in the south sandwich islands ssi over the past nearly half a century recorded at the college alaska station col and alaska seismic network asn the events are relocated using the joint hypercenter determination jhd technique dewey 1971 travel time residuals at col from ssi earthquakes over a period of 45 years and at three additional stations from ssi earthquakes over periods of about 22 years show steady increases with time the temporal trends are demonstrated to be statistically very significant with dense samples of the pathway at different time periods we use a joint inversion technique that allows us to separate the time dependent inner core structure from the time independent mantle biases surprisingly the inner core rotation seems resolvable even from digital data observed at asn in the 1990s spanning over 8 years alone in such a joint inversion the results provide strong support for a differential inner core rotation the inferred rotation rate ranges from 0 3 to 1 1 per year faster than the mantle when we take into account the uncertainty in the tilt of the anisotropy axis of the patch of the inner core that was sampled a westward inner core rotation can be ruled out

seismological and mineralogical constraints on the inner core fabric 1 under the assumption of lattice preferred orientation of iron hcp crystals the seismic anisotropy of the inner core is related to single crystal elastic properties by a fabric model this model is able to explain a new seismological constraint the constant behaviour of the ratio k of two anisotropy coefficients despite radial and lateral variations of the anisotropy level in the inner core the relevance of the theoretical calculations of iron single crystal elastic properties at inner core conditions are tested by a comparison between seismic estimates and computed values of this ratio k taking into account the error bar of the seismic estimate iron hcp structures presenting fast and slow c axis could fit the data equally well

structuring the inner core earth s rocky mantle and solid inner core are separated by the 2 300 kilometre deep layer of molten iron that constitutes the outer core yet the sluggish pattern of mantle convection creates structure in the inner core

the inner inner core of the earth texturing of iron crystals from three dimensional seismic anisotropy abstract we present a three dimensional model of the seismic anisotropy and texturing of iron crystals in the inner core the form of the anisotropy changes suddenly at slightly less than half of the inner core radius the outer part is composed of iron crystals of a single phase with different degrees of preferred alignment along the spin axis of the earth the inner part may be composed of a different phase of crystalline iron or have a different pattern of alignment

constraints on inner core anisotropy from pkp df travel times pkp df travel times reported by the international seismological centre isc are analyzed to study the depth dependence of inner core anisotropy plots of pkp df travel time residuals versus range for different subsets of the data clearly show large anomalies at ranges beyond about 149 for those paths with inner core ray angles within 30 of the rotation axis on average the times are about 3 to 5 s earlier for these paths than for rays at other angles consistent with recent waveform measurements of anomalous pkp df times by creager 1992 and song and helmberger 1993 and indicative of inner core anisotropy in which the p velocity is about 3 faster for n s paths than e w paths the size of these apparent anomalies in the isc data are much larger than those inferred from several previous analyses of isc pkp df data which indicated 0 5 to 2 s anomalies at these ranges the earlier studies applied binning and averaging schemes to the data using windows that were overly tight thus excluding some of the most anomalous data points and biasing the results toward smaller anomalies to avoid this problem i apply a method for computing summary ray residuals that finds the maxima in smoothed histograms of the residuals and does not rely on a fixed interval to window the data points standard errors on each summary ray residual are estimated using a bootstrap technique that randomly resamples the data plots of the summary ray residuals for several range intervals reveal a small number of independent paths nearly parallel to the rotation axis for which the arrivals are 3 to 5 s early although the data are sparse for paths within 30 of the rotation axis anisotropy does not appear to diminish with depth within the inner core and may cause a much larger anomaly near 180 than the 2 s value obtained in several prior studies of isc data the uniform anisotropy models of creager 1992 and song and helmberger 1993 fit the bulk of the data reasonably well however the observed residuals at ranges between 132 and 140 are smaller than those predicted by most anisotropy models suggesting that the anisotropy may diminish within the outermost 50 km of the inner core

microearthquakes in the black smoker hydrothermal field east pacific rise at 21 n in july and august 1980 an array of five ocean bottom seismographs was deployed within 3 km of the 350 c hydrothermal vents at the rivera submersible experiment rise site at 21 n on the east pacific rise two of these instruments were placed within 600 m of the vents using a transponder navigation network the array detected four basic types of events the first type consisted of local very small microearthquakes locations obtained for 11 of these events place three within 1 km of the vents with the others elsewhere along the rise crest they appear to originate either from movement on the faults in the area or from the hydrothermal system beneath this area a study of the s p times of this type indicates a maximum hypocentral depth of 2 3 km implying a similar limit to the depth of hydrothermal circulation and brittle fracturing in the vicinity of the vents the second type of event found consisted of emergent earthquakes that have many of the characteristics of volcanic harmonic tremor the frequency of these events falls in the 1 5 hz range and are similar in appearance to those seen at mount st helens prior to and during its may 1980 eruption they may be either hydrothermal or volcanic in origin the third type of event produced a very monochromatic high frequency seismogram with the energy concentrated at 20 hz these events also appear to have a local origin the particle motion of these earthquakes shows almost pure stonely wave propagation and they appear to originate from a regularly oscillating dilatational source the nature of the source of event types two and three is clearly not the usual sort of motion on a fault and is still not understood the fourth type of earthquake recordings were regional events apparently from the nearby rivera and tamayo fracture zones the low frequency acoustic noise in the water was 16 64 times higher within 300 m of the vents than it was 2 km away this implies that the black smoker vents may be generating a large acoustical disturbance as the 350 c water pours into the ocean the observations are among the first of seismicity on moderate to fast spreading ridges and many of the events appear to be related to the hydrothermal activity found there

earth science a deep foundry melting and solidification of iron alloys in earth s core may explain structural complexity in the solid inner core and alter the way we think about the dynamics of the deep interior see letter p 361 david gubbins and colleagues present geodynamo simulations showing that variations in heat flow at the core mantle boundary are transferred to the inner core boundary they find that the variations can be large enough to cause heat to flow into the inner core and if this were to occur in the earth to cause localized melting such melting would release heavy liquid that could form a variable composition layer providing a simple explanation for seismic velocity anomalies seen immediately above the inner core boundary localized melting and freezing could also provide a mechanism to create seismic anomalies in the inner core itself

formation of a solid inner core during the accretion of earth the formation of an inner core during the accretion of earth is investigated using self gravitating and compressible earth models formed by accreting a total of 25 or 50 moon to mars size planetary embryos the impact of an embryo heats the proto earth s interior differentially more below the impact site than elsewhere the rotating core dynamically overturns and stratifies shortly after each impact creating a spherically symmetric and radially increasing temperature distribution relative to an adiabatic profile merging of an embryo to the proto earth increases the lithostatic pressure that results in compressional temperature increase while further enhances the melting temperature of the core causing solidification a total of 36 thermal evolution models of the growing proto earth s core are calculated to investigate effects of major physical parameters no solidification is considered in the first 21 models where modified two body escape velocities are used as the impact velocities of the embryos at the end of accretion temperatures in the upper part of the core are significantly different among these models whereas temperatures in the deeper parts are similar the core solidification considered in the remaining 15 models where impact velocities higher than the modified two body escape velocities are adopted drastically changes the temperature distribution in the deeper parts of the core all of the models produce partially solidified stiff inner cores 1000 2100 km in radius at the end of accretion where the solid fraction is larger than 50 the innermost of the stiff inner cores are completely solidified to radii 250 1500 km

tectonic history of the earth s inner core preserved in its seismic structure seismic data show that the earth s inner core is structurally complex numerical simulations suggest that whereas the deeper structure may be inherited from past episodes of inner core growth the origin of the shallow structure is due to ongoing deformation

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

inner core differential motion confirmed by earthquake waveform doublets we analyzed 18 high quality waveform doublets with time separations of up to 35 years in the south sandwich islands region for which the seismic signals have traversed the inner core as pkp df the doublets show a consistent temporal change of travel times at up to 58 stations in and near alaska and they show a dissimilarity of pkp df coda using waveform doublets avoids artifacts of earthquake mislocations and contamination from small scale heterogeneities our results confirm that earth s inner core is rotating faster than the mantle and crust at about 0 3 degrees to 0 5 degrees per year

evidence for lower crustal ductile strain localization in southern new york historic triangulation data have been analysed to determine whether intraplate seismicity is associated with ongoing ductile deformation in the lower crust the model we have attempted to test is basically analogous to strain accumulation and release along plate boundary strike slip faults like the san andreas fault in california that is beneath an elastic seismogenic upper crust 20 km thick strain is preferentially localized within ductile shear zones in the lower crust due to broad scale plate driving forces the localized lower crustal ductile strain causes stress and strain to accumulate elastically in the brittle crust which is eventually released in crustal earthquakes at greater depths this localized shear deformation probably develops into pervasive ductile flow numerous geodetic measurements along the san andreas fault confirm that earthquakes in the brittle upper crust are produced by the release of elastic strain that results from ongoing ductile shear or slip in the lower crust1 2 we have found evidence of high rates of crustal deformation in southern new york which suggest that localized ductile shear is occurring in the lower crust

convection and anisotropy of the inner core estimation of the rayleigh number of the inner core suggests that this region is convecting the flow pattern is likely to produce crystallographic preferred orientation of the elastically anisotropic hexagonal closest packed phase of iron such that compressional wave velocities would be greater in the axial relative to the equatorial direction by roughly 1 percent this result is in agreement with seismological evidence that the inner core is elastically anisotropic

crystal structure prediction for iron as inner core material in heavy terrestrial planets abstract the relative stability of different crystal structures for pure fe under applied pressure is calculated from quantum mechanics using the highly accurate apw lo method in the pressure range of 0 100 tpa we corroborate the prediction that iron adopts subsequently the bcc hcp fcc hcp and bcc structures in contrast to previous studies we identify a family of stacking fault structures that are competing with the ground state structure at all pressures implications for the properties of the inner core of the earth and heavy terrestrial exoplanets are discussed

fine scale heterogeneity in the earth s inner core the seismological properties of the earth s inner core have become of particular interest as we understand more about its composition and thermal state1 2 observations of anisotropy and velocity heterogeneity in the inner core are beginning to reveal how it has grown and whether it convects3 4 the attenuation of seismic waves in the inner core is strong and studies of seismic body waves5 6 have found that this high attenuation is consistent with either scattering or intrinsic attenuation5 the outermost portion of the inner core has been inferred to possess layering and to be less anisotropic than at greater depths7 8 9 10 here we present observations of seismic waves scattered in the inner core which follow the expected arrival time of the body wave reflection from the inner core boundary the amplitude of these scattered waves can be explained by stiffness variations of 1 2 with a scale length of 2 kilometres across the outermost 300 km of the inner core these variations might be caused by variations in composition by pods of partial melt in a mostly solid matrix or by variations in the orientation or strength of seismic anisotropy

seismic structures in the earth s inner core below southeastern asia documenting seismic heterogeneities in the earth s inner core ic is important in terms of getting an insight into its history and dynamics a valuable means for studying properties and spatial structure of such heterogeneities is provided by measurements of body waves refracted in the vicinity of the inner core boundary icb here we investigate eastern hemisphere of the solid core by means of pkpbc pkpdf differential travel times that sample depths from 140 to 360 km below its boundary we study 292 polar and 133 equatorial residuals measured over the traces that probe roughly the same volume of the ic in both planes equatorial residuals show slight spatial variations in the sampled ic volume mostly below the level of 0 5 whereas polar residuals are up to three times as big direction dependent and can exhibit higher local variations the measurements reveal fast changes in seismic velocity within a restricted volume of the ic we interpret the observations in terms of anisotropy and check against several anisotropy models few of which have been found capable of fitting the residuals scatter we particularly quantify the model where a dipping discontinuity separates fully isotropic roof of the ic from its anisotropic body whereas the depth of isotropy anisotropy transition increases in southeast direction from 190 km below southeastern asia off the coast of china to 350 km beneath australia another acceptable model cast in terms of localized anisotropic heterogeneities is valid if 33 largest polar measurements over the rays sampling a small volume below southeastern asia and the rest of polar data are treated separately this model envisages almost isotropic eastern hemisphere of the ic at least down to the depth of 360 km below the icb and constrains the anisotropic volume only to the ranges of north latitudes from 18 to 23 east longitudes from 125 to 135 and depths exceeding 170 km the anisotropy strength in either model is about 2 further effective pursuit of the models presents challenges in terms of resolution and coverage and basically requires a significant dataset extension

hemispherical variations in seismic velocity at the top of the earth s inner core knowledge of the seismic velocity structure at the top of the earth s inner core is important for deciphering the physical processes responsible for inner core growth1 2 3 previous global seismic studies4 5 6 7 8 9 have focused on structures found 100 km or deeper within the inner core with results for the uppermost 100 km available for only isolated regions10 11 12 here we present constraints on seismic velocity variations just beneath the inner core boundary determined from the difference in travel time between waves reflected at the inner core boundary and those transmitted through the inner core we found that these travel time residuals observed on both global seismograph stations and several regional seismic networks are systematically larger by about 0 8 s for waves that sample the eastern hemisphere of the inner core 40 e to 180 e compared to those that sample the western hemisphere 180 w to 40 e these residuals show no correlation with the angle at which the waves traverse the inner core this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0 8 higher than that of the western hemisphere

an earthquake sequence studied with ocean bottom seismographs summary a tripartite ocean bottom seismograph array at the junction of the east pacific rise and rivera fracture zone recorded an eathquake sequence consisting of three main shocks mb 4 3 4 3 and 4 8 and numerous aftershocks from the fracture zone in the distance range 35 50 km delineation of the rupture zones by aftershocks indicates that the first two main shocks took place on overlapping fault areas while the third occurred over a fault area separated from the first by several kilometres both rupture zones were about 4 km long surface wave spectra indicate a shallow about 3 km below the sea floor source as does obs array phase velocity data the seismic moments obtained from teleseismic surface wave data of 1 3 2 1 and 2 8 1023 dyn cm with the fault areas as delineated by aftershocks imply a stress drop of about 8 bars for the main shocks aftershock sequences of each of the main shocks are similar with a b value of about 0 65 teleseismic p travel times are similar to those from near surface sources in nevada

geoarchaeological evidence of strong prehistoric earthquakes in the new madrid missouri seismic zone sand blows and fissures that cover 10 500 km sup 2 in northeastern arkansas and southeastern missouri attest to the severity of the 1811 1812 earthquake series in the new madrid seismic zone however except for one occurence near new madrid missouri the region has been devoid of any evidence of other major shocks for at least 1 3 ka prior to 1811 and possibly for 9 ka stratigraphic relations and radiocarbon dating at a recently excavated archaeological site near east prairie missouri have revealed liquifaction phenomena attributable to a shock dated to within about 100 yr prior to a d 539 and a probable second one dated between about a d 539 and 991

constraints on inner core anisotropy using array observations of p p author s frost da romanowicz b abstract 2017 american geophysical union all rights reserved recent studies of pkpdf travel times suggest strong anisotropy 4 or more in the quasi western inner core hemisphere however the availability of paths sampling at low angles to the earth s rotation axis the fast axis is limited to augment this sampling we collected a travel time data set for the phase p p df pkppkpdf for which at least one inner core leg is quasi polar at two high latitude seismic arrays we find that the inferred anisotropy is weak on the order of 0 5 to 1 5 confirming previous results based on a much smaller p p data set while previous models of inner core anisotropy required very strong alignment of anisotropic iron grains our results are more easily explained by current dynamic models of inner core growth we observe large travel time anomalies when one leg of p p df is along the south sandwich to alaska path consistent with pkpdf observations and warranting further investigation

enhanced intraplate seismicity along continental margins some causes and consequences abstract in australia much of the seismic activity is restricted to zones up to several hundred kilometres wide inboard of continental margins at high angle to the trend in maximum horizontal compression shmax intriguingly along the margin side of one such zone a near optimally oriented crustal scale structure the darling fault has no recorded historical seismicity to explain these enigmatic features we explore how the compositional and thermal structuring of continental passive margins may lead to variations in mechanical response to imposed forcing using forward modelling we show how lateral heat flow across such margins can produce increases in moho temperatures by several 10s of degrees relative to the continental interior extending up to 200 km inboard of the ocean continent transition provided the step in thermal lithosphere thickness into the continent is greater than 30 km for smaller lithospheric steps moho temperatures beneath continental margins will be cooler than beneath interiors our mechanical models show that such thermal effects can elevate strain rates along the margin by several factors relative to continental interiors when subject to far field forcing in the shallow seismogenic realm the strain rate maximum is concentrated at around 100 150 km inboard of the ocean to continent lithospheric step providing a plausible explanation for the pattern of seismicity observed in some of australia s marginal seismic zones this interpretation requires that strength changes associated with moho temperature variations of several 10s of degrees are sufficient to override the reactivation potential of near optimally oriented crustal penetrative structures with thermal processes providing an important switch controlling reactivation potential of existing structures it also provides an insight into the mechanism of formation of continental ribbons during rifting of thermally mature margins

earthʼs inner core innermost inner core or hemispherical variations the structure of earth s deep inner core has important implications for core evolution since it is thought to be related to the early stages of core formation previous studies have suggested that there exists an innermost inner core with distinct anisotropy relative to the rest of the inner core using an extensive new data set of handpicked absolute travel time observations of the inner core phase pkikp we find that the data are best explained by variations in anisotropy between two hemispheres and do not require an innermost inner core we demonstrate that observations of an innermost inner core are an artifact from averaging over lateral anisotropy variations more significantly we show that hemispherical variations in anisotropy previously only imaged in the upper inner core continue to its centre the eastern region has 0 5 1 5 anisotropy whereas the western region has 3 5 8 8 anisotropy increasing with depth with a slow direction at 57 61 to the earth s rotation axis at all depths such anisotropy is consistent with models of aligned hcp or bcc iron aggregates