texture of the uppermost inner core from forward and back scattered seismic waves body waves interacting with the boundary of the solid inner core at narrow and wide angles of incidence provide independent constraints on a heterogeneous texture that may originate from the process of solidification the equatorial quasi eastern hemisphere of the uppermost 50 100 km of the inner core is characterized by a higher isotropic p wave velocity lower q s inferred from pkikp and simpler pkikp pulses compared to adjacent regions in the western hemisphere and polar latitudes compared to this region the adjacent western primarily pacific equatorial region is characterized by higher q s and a higher level of coda excitation following pkikp lateral variations in both inner core q inferred from transmitted pkikp and inner core heterogeneity inferred from the coda of reflected pkikp can be modeled by lateral variations in a solidification fabric in an actively crystallizing eastern equatorial region characterized by upwelling flow in the outer core fabrics that explain strong attenuation and the absence of attenuation and velocity anisotropy in short range 120 140 pkikp and weak pkikp codas have an anisotropy of scale lengths with longer scale lengths in the vertical direction perpendicular to the inner core boundary in less actively solidifying regions in the equatorial western hemisphere longer scale lengths tend to be more parallel to the inner core boundary consistent with outer core flow tangent to the inner core boundary or viscous shearing and recrystallization in the horizontal direction away from more actively crystallizing regions in the eastern hemisphere this texture is less effective in attenuating pkikp by forward scattering lateral variation in the equatorial western hemisphere between vertical versus horizontal oriented plate like textures may explain lateral variations from weak to strongly back scattered pkikp coda and from strong to weak velocity and attenuation anisotropy in short range pkikp 2007 elsevier b v all rights reserved

uppermost inner core seismic structure new insights from body waveform inversion abstract differential travel times and waveforms of pkikp and pkikp phases in the 129 141 distance range deconvolved for the effects of source time functions and average mantle attenuation operators are inverted for velocity and attenuation in the uppermost 80 km of the inner core and the velocity gradient and attenuation in the lowermost 200 km of the outer core f region results confirm degree one velocity structure in the inner core whose fast and slow regions appear to be separated by transitions with a velocity contrast of about 1 although attenuation in the inner core also exhibits degree one pattern a high attenuation region q p 1 0 002 0 0067 extends to the central pacific which has been previously considered a part of the western hemisphere characterized by slower velocity and lower attenuation the rest of the inner core exhibits a positive correlation between velocity and attenuation as observed previously an observed mantle like relationship between velocity and attenuation in the central pacific can be interpreted as originating from higher than average homologous temperature consistent with a model of inner core solidification driven by outer core convection coupled to core mantle boundary thermal heterogeneity alternative mechanisms including differences in grain size and impurity concentration and lateral differences in solidification rate might be required to explain the structure of the inner core the f region velocity and attenuation are best explained by the ak135 f velocity model and q p 1 0 0 001 respectively

distinct layering in the hemispherical seismic velocity structure of earth s upper inner core the existence of hemispherical variation in the earth s inner core is well documented but consensus has not yet been reached on its detailed structure the uppermost layers are a region of particular importance as they are directly linked to the growth processes and post solidification mechanisms of the inner core here we use a large pkikp pkikp differential travel time residual data set to derive a model for the upper inner core providing new constraints on its isotropic and anisotropic velocity and the amount of scattering we find that the eastern and western hemisphere are separated by sharp boundaries this is incompatible with the recently proposed inner core translation model but might be explained by differences in outer core convection and inner core solidification rates the eastern hemisphere displays weak anisotropy of 0 5 1 0 the western hemisphere on the other hand is characterized by the presence of an isotropic upper layer with a thickness of 57 5 km with anisotropy of 2 8 appearing at deeper depths the boundary between the isotropic layer and the deeper anisotropy appears sharp we also detect for the first time a high velocity layer at the top of the eastern hemisphere with a thickness of 30 km which we interpret as being due to an increased amount of light elements there appears to be no relationship between the layered structure in the two hemispheres with abrupt changes in velocity with depth in one hemisphere without any significant change at the same depth in the other hemisphere our results indicate that there is a difference in composition and mineral structure between the hemispheres resulting in differing responses to external processes

imaging the inner core under africa and europe abstract the inner core under africa is thought to be a region where the nature of inner core texture changes from the strongly anisotropic western part of the inner core to the weakly anisotropic or isotropic eastern part of the inner core additionally observations of a difference in isotropic velocity between the two hemispheres have been made a very large new dataset of simultaneous pkpdf and pkpbc observations on which differential travel times have been measured is used to examine the upper 360 km of the inner core under europe africa and the surrounding oceans inversion of the differential travel time data for laterally varying inner core anisotropy reveals that inner core anisotropy is stronger under central africa and the atlantic ocean than under the western indian ocean no hemispherical pattern is present in voigt isotropic velocities indicating that the variation in anisotropy is due to differing degrees of crystal alignment in the inner core not material differences when anisotropy is permitted to change with depth the upper east most part of the study region shows weaker anisotropy than the central and western regions when depth dependence in the inner core is neglected the hemisphere boundary is better represented as a line at 40 e than one at 10 e however it is apparent that the variation of anisotropy as a function of depth means that one line of longitude cannot truly separate the more and less anisotropic regions of the inner core the anisotropy observed in the part of the inner core under africa which lies in the western hemisphere is much weaker than that under central america showing that the western hemisphere is not uniformly anisotropic as the region of low anisotropy spans a significant depth extent it is likely that heterogeneous heat fluxes in the core which may cause variations in inner core anisotropy have persisted for several hundred million years

aspherical structural heterogeneity within the uppermost inner core insights into the hemispherical boundaries and core formation abstract lateral heterogeneities at the top of the inner core are investigated using earthquakes that occurred in indonesia and southeast asia and were recorded in the southeastern caribbean using seismic observations of attenuation and seismic velocity we were able to constrain the characteristics of the boundary between the inner and outer core to further investigate the dynamics and evolution of the earth s core our seismic observations from core phases confirm that the outermost inner core is asymmetrically heterogeneous and we are able to further constrain the morphology and physical properties of this layer comparison of data from earthquakes with ray paths traversing from east to the west versus those with ray paths from west to east allow us to map the aspherical heterogeneity of the boundary layer and specifically image the boundary between the proposed quasi eastern and western hemispheres of the inner core the variation of differential travel times between pkp df and pkp bc attenuation in terms of q factor and latitudinal changes for both of these observations can be attributed to localized heterogeneity at the quasi hemispherical boundaries of the inner core we constrain the change in the thickness of outermost core boundary layer from 100 to 250 km within a distance of a few 10s of kilometers at 45 e 2 for the western boundary with an overall p wave velocity decrease in the western hemisphere of 0 5 and increase of 0 5 in the eastern hemisphere we constrain the eastern boundary at latitudes greater than 45 n to 173 e 4 with an overall p wave velocity decrease in the western hemisphere of 1 0 in the uppermost 200 km of the inner core the eastern boundary at equatorial latitudes is constrained to a region

normal mode coupling due to hemispherical anisotropic structure in earth s inner core summary we present illustrative calculations of the effect of hemispherical variation in inner core anisotropy on earth s normal modes of oscillation body wave studies show that the anisotropy in the inner core is not simple cylindrical anisotropy which is often portrayed in models derived from normal mode data but varies with longitude hemispherical or odd degree structure has to be studied by cross coupling normal modes as the self coupling technique is sensitive only to even degree earth structure a completely general definition of inner core anisotropy would require a prohibitive number of degrees of freedom however we show that any existing cylindrical anisotropy model can be confined to only one part of the inner core using our new theory we find that hemispherical anisotropy causes significant changes in the frequency and quality factor of several inner core sensitive normal modes the effect of hemispherical inner core anisotropy can also be seen in synthetic seismograms radial pkikp and pkjkp modes all respond to the presence of hemispherical variation in inner core anisotropy if the variations in inner core anisotropy seen in body wave data are part of a gross large scale pattern then this structure should also affect normal mode data on an observable scale

anomalously strong observations of pkikp pcp amplitude ratios on a global scale the inner core boundary marks the phase transition between the solid inner core and the fluid outer core as the site of inner core solidification the boundary provides insight into the processes generating the seismic structures of the inner core in particular it may hold the key to understanding the previously observed hemispherical asymmetry in inner core seismic velocity anisotropy and attenuation here we use a large pkikp pcp amplitude ratio and travel time residual data set to investigate velocity and density contrast properties near the inner core boundary although hemispherical structure at the boundary has been proposed by previous inner core studies we find no evidence for hemispheres in the amplitude ratios or travel time residuals in addition we find that the amplitude ratios are much larger than can be explained by variations in density contrast at the inner core boundary or core mantle boundary this indicates that pkikp is primarily observed when it is anomalously large due to focusing along its raypath using data in which pkikp is not detected above the noise level we calculate an upper estimate for the inner core boundary icb density contrast of 1 2 kg m 3 the travel time residuals display large regional variations which differ on long and short length scales these regions may be explained by large scale velocity variations in the f layer just above the inner core boundary and or small scale topography of varying magnitude on the icb which also causes the large amplitudes such differences could arise from localized freezing and melting of the inner core

seismic velocity and attenuation structures in the top of the earth s inner core 1 we collect a global data set of pkikp and pkikp phases recorded by the global seismic network and many regional seismic arrays to study seismic structure in the top of the earth s inner core the pkikp and pkikp observations show different characteristics between those sampling the eastern hemisphere 40 e 180 e of the inner core and those sampling the western hemisphere 180 w 40 e pkikp phases 1 arrive about 0 4 s earlier than the theoretical arrivals based on preliminary reference earth model prem for those sampling the eastern hemisphere of the inner core and about 0 3 s later for those sampling the western hemisphere 131 141 2 bifurcate at smaller epicentral distances for those sampling the eastern hemisphere compared to those sampling the western hemisphere and 3 have smaller amplitudes for those sampling the eastern hemisphere waveform modeling of these observations suggests two different types of models for the two hemispheres of the top of the inner core with a model in the eastern hemisphere having a p velocity increase of 0 765 km s across the inner core boundary a small radial velocity gradient of 0 000055 km s km and an average q value of 250 and a model in the western hemisphere with a p velocity increase of 0 633 km s across the inner core boundary a radial velocity gradient of 0 000533 km s km and an average q value of 600 the hemispherical difference of seismic structures may be explained by different geometric inclusions of melt and or different alignments of iron crystals with anisotropic properties in both velocity and attenuation we speculate that this large scale pattern of seismic heterogeneities may be caused by a large scale heat flow anomaly at the bottom of the outer core and or different vigorousness of convection in the top of the inner core between the two hemispheres

normal mode splitting due to inner core attenuation anisotropy the earth s inner core displays transverse velocity anisotropy with cylindrical symmetry which causes the anomalous zonal splitting of inner core sensitive normal modes in this paper we extend existing theory for calculating normal mode splitting from models of cylindrical velocity anisotropy to include models of anelastic attenuation anisotropy furthermore we derive the equations that can be used to rewrite the attenuation anisotropy parameters natural to normal mode considerations in terms of body wave attenuation anisotropy for compressional and shear waves

the mw 5 4 reggio emilia 1996 earthquake active compressional tectonics in the po plain italy summary we have analysed the seismic sequence that occurred in october 1996 near the town of reggio emilia on the southern edge of the po plain the onset of the sequence was marked by a 5 4 moment magnitude main shock located at 15 km depth the main shock focal mechanism is a reverse solution with a strike slip component and the scalar moment is 1 46 1017 n m we used broad band digital recordings from a borehole station located at about 70 km from the epicentre for a spectral analysis in order to estimate attenuation and source parameters for the main shock in addition the empirical green s function method has been applied to evaluate the source time function in terms of both moment rate and stress rate we infer an asperity like rupture process for the main shock as suggested by the short duration of the stress release with respect to the overall duration of the moment rate function this analysis also allows us to estimate the average dynamic stress drop of the main shock 600 bar we analysed the digital recordings of the temporary local seismic network deployed after the main shock and of a permanent local network maintained by the italian petroleum agency agip during 15 days of field experiments we recorded more than 800 aftershocks which delineate a 9 km long ne elongated distribution confined between 12 and 15 km depth suggesting that the basement is involved in the deformation processes 102 focal mechanism of aftershocks have been computed from p wave polarities showing mainly pure reverse solutions we calculate the principal stress axes from a selected population of earthquakes providing a constraint on the stress regime of this part of the po plain the focal mechanisms are consistent with a n s subhorizontal 1 all the seismological data we have analysed confirm that this region is undergoing active compressional tectonics as already inferred from recent earthquakes geomorphological data and other stress indicators moreover the elongation of the reggio emilia aftershock sequence is consistent with the regional direction of the thrust fronts cropping out in the area suggesting that they are still active

depth dependent attenuation in the uppermost inner core from the taiwan short period seismic array pkp data inner core attenuation properties from two spots were examined in detail using pkikp and pkp bc phases with a distance range of between 147 and 154 as recorded by both the short period regional seismic array in taiwan and the broadband stations of the german regional seismic network the ray paths from the six south american events to taiwan sampled the inner core region beneath the eastern part of the northern pacific ocean while those from fiji to europe reflected the inner core boundary under the western part of the northern pacific ocean results of this study show the depth dependent attenuation of qp beneath the eastern part of the northern pacific ocean from 241 21 8 at depths 270 km with a weighted mean of 305 25 4 within the frequency band 0 5 2 0hz the average qp under the western part of the northern pacific ocean is 160 33 5 0 5 1 5hz which is consistent with that previously reported it is found that qp under the western part of the northern pacific is clearly lower than that under the eastern part this discrepancy might be related to regional variations in the uppermost inner 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

the sedimentology and dynamics of crater affiliated wind streaks in western arabia terra mars and patagonia argentina abstract wind streaks comprise recent aeolian deposits that have been extensively documented on venus earth and mars martian wind streaks are among the most abundant surface features on the planet and commonly extend from the downwind margins of impact craters previous studies of wind streaks emerging from crater interior deposits suggested that the mode of emplacement was primarily related to the deposition of silt sized particles as these settled from plumes we have performed geologic investigations of two wind streaks clusters one situated in western arabia terra a region in the northern hemisphere of mars and another in an analogous terrestrial site located in southern patagonia argentina where occurrences of wind streaks emanate from playas within maar craters in both these regions we have identified bedforms in sedimentary deposits on crater floors along wind facing interior crater margins and along wind streaks these observations indicate that these deposits contain sand sized particles and that sediment migration has occurred via saltation from crater interior deposits to wind streaks in arabia terra and in patagonia wind streaks initiate from crater floors that contain lithic and evaporitic sedimentary deposits suggesting that the composition of wind streak source materials has played an important role in development spatial and topographic analyses suggest that regional clustering of wind streaks in the studied regions directly correlates to the areal density of craters with interior deposits the degree of proximity of these deposits and the craters rim to floor depths in addition some but not all wind streaks within the studied clusters have propagated at comparable yearly earth years rates extensive saltation is inferred to have been involved in its propagation based on the studied terrestrial wind streak that shows ripples and dunes on its surface and the martian counterpart changes orientation toward the downslope direction where it extends into an impact crater

anisotropy of earth s inner core intrinsic attenuation from seismic normal mode models the earth s inner core the slowly growing sphere of solid iron alloy at the centre of our planet is known to exhibit seismic anisotropy both normal mode and body wave studies have established that when the global average is taken compressional waves propagate faster in the north south direction than in the equatorial plane recent body wave studies also indicate that this fast direction may be more attenuating and interpret this anisotropic attenuation in terms of anisotropic scattering due to inner core texturing here we use the earth s normal modes to study the attenuation anisotropy of both compressional and shear waves in the inner core as normal modes have wavelengths several orders of magnitude longer than estimates of inner core grain size any attenuation anisotropy quantified using normal modes must reflect the anisotropy of intrinsic viscoelastic attenuation of the crystalline inner core alloy by inverting zonal anelastic and elastic normal mode splitting function coefficients of twenty inner core sensitive modes we construct models of inner core intrinsic attenuation and velocity anisotropy we find that for compressional waves the north south direction is both fast and more strongly attenuating the existence of intrinsic inner core attenuation anisotropy can be interpreted in terms of anisotropic zener relaxation in the metallic alloy comprising the inner core such anisotropic zener relaxation has only been observed in the presence of solute atoms and is thus entirely consistent with the presence of a few atomic per cent of light elements in the earth s inner core

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

using pkikp coda to determine inner core structure 2 determination of qc 1 despite the similar and rather uniform composition of two components of the core the solid inner part presents a much larger attenuation compared to the liquid outer part more over several authors have reported evidence of scattered seismic energy coming from the inner core ic this implies that the ic has lateral variations in structure or composition with a scale length from one to tens of kilometers these seismic scatterers could be caused by changes in crystal grain size variations in the orientation of anisotropy or the presence of partial melt and or impurities this work pursues the characterization of inner core scattering ics using a global data set of high frequency pkikp waveforms recorded at short period small aperture seismic arrays we apply a methodology used in the study of heterogeneities in the crust and upper mantle that consists of curve fitting of the observed scattered energy with a standard model along with a stripping technique to isolate the pkikp coda from the rest e g p pcp and pp codas from the observed pkikp codas we found an average qc 500 for the inner core reflecting a scattering attenuation at least comparable to the intrinsic attenuation we found a geographical dependence in the observations of pkikp coda with most of them coming from the pacific ocean and asia and relatively few observations coming from the atlantic ocean the simplest explanation is that the ics has a quasi hemispherical variation similar to other properties such as p wave velocity jump at the inner core boundary attenuation in the upper ic strength of anisotropy and or thickness of an overlaying isotropic layer this phenomenon is probably related to the solidification texturing of iron crystals showing hemispherical differences in the growth of the inner core

q in the inner core short period vertical seismograms from ten earthquakes and one nuclear explosion in the distance range of 140 170 degrees are examined for a study of the anelastic properties of the inner core the depth of the earthquake foci varies between 116 and 625 km the phases of interest are pkp bc branch and pkikp with bottoming depths in the upper few hundred kilometers of the inner core the data consist of high quality digital data at sampling rates 20 40 samples per second selected after an extensive search of the 1985 1986 gdsn event tapes for short period p waves with a high signal to noise ratio the results are based primarily upon observations between distances of 147 and 154

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

reconstruction of late pleistocene climate in the valsequillo basin central mexico through isotopic analysis of terrestrial and freshwater snails we aim to reconstruct the climatic and environmental conditions in the valsequillo basin during the deposition of the valsequillo gravels between c 40 000 and 8000 years ago when large mega fauna and potentially humans occupied the basin fossil freshwater fossaria sp and sphaeriidae family and terrestrial polygyra couloni holospira sp and cerionidae family snail shells from sections within the barranca caulapan were collected for oxygen and carbon stable isotope analysis oxygen and carbon isotopes in terrestrial and freshwater snail shells relate to local climatic parameters and environmental conditions prevailing during the lifetime of the snail whole shell isotope analysis showed that c 35 000 years ago climate in the valsequillo basin was similar to the present day between c 35 000 and 20 000 bp conditions became increasingly dry after which conditions became wetter again although this record is truncated intra shell isotopic analyses show that the amount of precipitation varied seasonally during the late pleistocene if people did reach this part of the americas in the late pleistocene they would have experienced changing long term and seasonal climatic conditions and would have had to adapt their life strategies accordingly

regionally heterogeneous uppermost inner core observed with hi net array studies of velocity structure in the earth s inner core and its volumetric variations illuminate our understanding of inner core dynamics and composition here we use an extensive number of seismograms recorded by the hi net array to construct complete empirical curves of pkpbc pkpdf differential traveltimes the nature of these curves implies that significant variations in traveltimes are accumulated during the passage of pkpdf waves through the uppermost inner core and rules out outer core structure effects uniform cylindrical anisotropy of a plausible strength in the uppermost inner core can also be ruled out as a cause of the observed traveltime variations because the range of sampled ray angles is too narrow the configuration and strength of inhomogeneities from a recent tomographic model of the lowermost mantle cannot account for the observed traveltime variations therefore we infer that either variations of p wave isotropic velocity on the scale of about hundred km and less are present in the uppermost inner core or the material may be organized in distinctive anisotropic domains and both of these features may be superimposed on long wavelength hemispherical structure if the former holds true the absolute magnitude of required p wave velocity perturbations from referent values is 0 60 0 10 in the quasi eastern and 1 55 0 15 in the quasi western hemisphere 0 85 0 05 and 1 10 0 10 respectively with the lowermost mantle correction the existence of these variations is a plausible physical outcome given that vigorous compositional convection in the outer core and variations in heat exchange across the inner core boundary may control the process of solidification

anisotropy in inner core attenuation a new type of data to constrain the nature of the solid core during the last decade the presence of velocity anisotropy inside the inner core with a fast axis parallel to the earth s rotation axis has been well established the quantitative analysis of the amplitudes of waves which sample a particular region of the inner core under africa but exhibit various orientations allows us to document and characterize the presence of anisotropy in attenuation the analysis is based on a comparison of both amplitudes and travel times of the pkp df wave which samples the inner core and the pkp bc wave which has nearly the same path but bottoms inside the liquid core the data reveal that the direction of strong attenuation correlates with that of fast velocity when referred to the same epicentral distance and focal depth the pkp df pkp bc amplitude ratio are about five times lower for paths tilted by 25 with respect to the earth rotation axis than for nearly equatorial paths a clear negative correlation is observed between travel time and amplitude residuals when the angle to earth rotation axis varies this first quantitative analysis combined with experimental results on oriented crystals and mushy media may bring important constraints on the mechanism responsible for anisotropy in the inner core

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

hemispherical transition of seismic attenuation at the top of the earth s inner core in contrast to the liquid outer core the earth s inner core is mostly solid and its composition is more pure iron based on dynamic arguments related to the freezing process of the inner core and the observation of much lower p wave quality factor in the inner core q 10 000 it has been suggested that a mushy layer with liquid inclusions may exist at the top of the inner core on the other hand seismic measurements indicate that q increases towards the center of the inner core we here present estimates of q in the depth range 32 110 km beneath the inner core boundary icb based on the measurement of pkikp pkikp amplitude ratios after a narrow band pass filtering 0 7 2 0 hz our measurements indicate that there are pronounced hemispherical differences in the values of q 335 and 160 in the western 180 w to 40 e and eastern 40 e to 180 e hemispheres respectively and in the depth of transition from decreasing to increasing q 32 km beneath the icb in the eastern hemisphere and 85 km in the western hemisphere below 85 km the hemispherical pattern disappears we also confirm the existence of a correlated hemispherical pattern in p velocity down to 85 km the p velocity and q variations are compatible with an interpretation in terms of small hemispherical variations of temperature at the top of the inner core and their influence on the morphology of porosity and connectivity of liquid inclusions in the mushy zone the disappearance of the differences in q beneath 85 km provide constraints on the likely depth extent of the mushy zone

frequency dependent seismic attenuation in the inner core 1 a viscoelastic interpretation 1 using reference source time functions broadband velocity waveforms of pkikp in the distance range 150 180 are inverted for a viscoelastic model of inner core attenuation a mean q at 1 hz of 307 90 is determined from 345 available pkikp ray paths both global and regional results find a depth dependent attenuation in the deep inner core with anisotropic attenuation resolved in regional results attenuation is much stronger in the upper 300 km of the inner core the preferred model of viscoelastic attenuation is frequency dependent with very weak velocity dispersion our data do not resolve any hemispherical differences of attenuation in the deep inner core

lateral variations in p velocity and attenuation in the d layer from diffracted p waves abstract diffracted p waves recorded at two stations along the same great circle are analysed to specify the lateral variations in velocity and velocity gradient in the d layer at the base of the mantle data come from long period and broadband geoscope and gdsn stations the variations in both apparent ray parameter and amplitude decay are studied as a function of the dominant period of the p wave regional variations in the ray parameter indicate a 2 5 variation for the p velocity in d with high velocities beneath the north pacific central america and northern asia and low velocities beneath the south pacific south atlantic and northern indonesia the absence of correlation between these velocity variations and the lateral variations in p attenuation favour a chemical origin rather than a thermal origin for these heterogeneities on the other hand lateral variations in the velocity gradient are suggested from the frequency dependence of both ray parameter and p amplitude decay

possible heterogeneity of the earth s core deduced from pkikp travel times the core of the earth is usually described by spherically symmetrical velocity models the core is made of two main spherical layers the fluid outer core with a radius close to 3 480 km and a p velocity increasing with depth from 8 km s 1 to 10 3 km s 1 and the solid inner core with a radius of 1 220 km and a p velocity close to 11 km s 1 refs 1 2 station residuals of the seismic core phase pkikp have been computed for 400 seismological observatories worldwide using 5 yr of the international seismological centre isc bulletins pkikp travel times can be corrected for upper mantle propagation by subtracting p delays thus pkikp p residuals are a measurement of the average vertical travel times in the lower mantle and in the core of the earth beneath seismic stations a spherical harmonic development of pkikp p delays up to degree 4 explains 58 of the variance in the data pkikp p exhibit a latitudinal dependence polar stations tend to be faster than equatorial stations we show here that this pattern may reflect a departure from spherical symmetry in the p velocity distribution in the vicinity of the inner core boundary of the earth

heterogeneity and anisotropy of seismic attenuation in the inner core 1 we evaluate seismic attenuation in the inner core from the spectral ratio between the seismic phase pkp df and either pkp ab or pkp bc at the epicentral distances from 150 to 170 the global data set is divided into two groups according to the position of the turning point of pkp df either in the eastern from 50 e to 120 w or the western hemisphere in the eastern hemisphere the parameter t shows strong dependence on the epicentral distance and no dependence on the angle between the earth s spin axis and pkp df ray at the turning point in the western hemisphere t is weakly dependent on the distance and varies between approximately 0 7 s and 0 1 s for the angles between 0 and 90 respectively our results allow us to reconcile seemingly incompatible results of many previous studies the anisotropy of attenuation is correlated with the anisotropy of p velocity

hemispherical structure in inner core velocity anisotropy 1 we investigate the hemispherical pattern in inner core velocity anisotropy using a new and independent high quality data set of pkpbc pkpdf and pkpab pkpdf body wave observations our data show no evidence for a tilted anisotropy axis with respect to the earth s rotation axis and is significantly better fit by a model with 4 4 anisotropy in the western hemisphere and only 1 0 in the eastern hemisphere than by a model of uniform anisotropy we carry out variance minimization and find the boundaries between the hemispheres at lines of constant longitude at 14 e and 151 w variance minimization enables us to extract the imprint of hemispherical structure from all of the data and not just polar paths resulting in boundaries which are nearly 30 from the results of previous studies the high quality of the data allows us to provide robust evidence that the isotropic velocity in the eastern hemisphere is 0 2 lower than in the western hemisphere in the top 660 km of the inner core our data set also suggests an increase in inner core anisotropy in the eastern hemisphere from 1 to around 6 at depths deeper than 660 km indicating that the hemispherical pattern in anisotropy may disappear at greater depths the presence of hemispherical structure rules out mechanisms for creating anisotropy which are unable to sustain longitudinal variations in the inner core furthermore steady inner core superrotation of the order of 0 1 year would eradicate the hemispherical differences though inner core oscillation would still be permissible

short period pkp phases and the anelastic mechanism of the inner core abstract short period seismograms are synthesized for pkp phases in anelastic earth models the synthetics were constructed using a synthetic technique valid at grazing incidence a source time function appropriate for deep focus earthquakes and an instrument response for either a short period wwssn or sro seismograph the agreement between predicted and observed amplitudes and spectral ratios requires neither a low q zone at 0 2 2 hz nor a low or negative p velocity gradient at the bottom of the outer core thin low q zones beneath the inner core boundary fit spectral ratio data that sample the upper 200 km of the inner core but fail to fit data that sample the lower inner core only a model having q 1 0 003 0 004 at 0 2 2 hz nearly constant with depth in the inner core satisfies all of the spectral ratio and amplitude data the assumption of a bulk viscosity of 10 10 3 pa s for the liquid phase of a partially molten inner core combined with the observation of low shear attenuation in the inner core at frequencies less than 0 005 hz limit the physical parameters associated with two possible attenuation mechanisms 1 fluid flow and viscous relaxation due to ellipsoidally shaped inclusions of melt and 2 the solid liquid phase transformation induced by the stress change during the passage of a seismic wave both mechanisms require an order of 0 1 partial melt to reproduce the observed q 1 in the outer core the time constant of the mechanism of phase transformation is predicted to be 10 4 10 6 s confirmation of small shear attenuation in the inner core in the frequency band of seismic body waves would favor the mechanism of phase transformation

relationship between anisotropy of p and s wave velocities and anisotropy of attenuation in serpentinite and amphibolite velocities and q values of p and s waves as functions of pressure and temperature at 100 and 600 mpa are presented for a serpentinite and an amphibolite both rocks exhibit a strong lattice preferred orientation lpo of the major mineral phases antigorite and hornblende respectively velocities and q values increase with pressure the rate of increase is different in the three orthogonal directions normal and parallel to foliation and lineation and closely related to progressive closure of microcracks increasing temperature decreases velocities and q values only slightly as long as thermal cracking is prevented by the applied confining pressure substantial anisotropy of velocities and q in p and s waves is observed in both rocks but is found to be different in origin anisotropy of p and s wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the lpo of major minerals increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate the residual anisotropy of p and s wave velocities shear wave splitting at high confining pressure is mainly a result of preferred mineral orientation by contrast anisotropy of q is very low at low confining pressure and markedly enhanced as pressure is increased at high confining pressure substantial anisotropy of q in p waves is apparent but reversed from that of p wave velocities qp is highest in the direction normal to the foliation plane whereas vp and vs is lowest in this direction the generation of a pronounced anisotropy of qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation the increase of q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain the reverse is true for the x and y directions serpentinite and x direction amphibolite parallel to the foliation plane

reconciling the hemispherical structure of earth s inner core with its super rotation earth s solid inner core grows through solidification of material from the fluid outer core onto its surface at rates of about 1 mm per year1 freezing in core properties over time and generating an age depth relation for the inner core a hemispherical structure of the inner core is well documented an isotropic eastern hemisphere with fast seismic velocities contrasts with a slower anisotropic western hemisphere2 3 4 independently the inner core is reported to super rotate at rates of up to 1 per year5 6 7 considering the slow growth steady rotation rates of this magnitude would erase frozen in regional variation and cannot coexist with hemispherical structure here we exploit the age depth relation using the largest available pkikp pkikp seismic travel time data set to confirm hemispherical structure in the uppermost inner core and to constrain the locations of the hemisphere boundaries we find consistent eastward displacement of these boundaries with depth from which we infer extremely slow steady inner core super rotation of 0 1 1 per million years our estimate of long term super rotation reconciles inner core rotation with hemispherical structure two properties previously thought incompatible it is in excellent agreement with geodynamo simulations8 9 while not excluding the possibility that the much larger rotation rates inferred earlier5 6 7 correspond to fluctuations in inner core rotation on shorter timescales10

a local crossing path study of attenuation and anisotropy of the inner core 1 we report results from studying a region under the north pacific using 46 ray paths along east west and north south directions to study the nature of inner core anisotropy and find an anisotropy signal in differential pkpbc pkpdf at about the resolution of the data with north south paths faster than east west but no dependence of attenuation on wavespeed or depth in this part of the inner core inner core q is 130 52 255 between 140 340 km depth the observations also provide constraints on the degree of homogeneous meridional anisotropy possibly present in the core to between 0 1 and 0 6 velocity differences in the fast and slow directions significantly smaller than the 2 4 axi symmetric anisotropy in the inner core the small meridional component to anisotropy argues against significant contributions to anisotropy from low order inner core convection or stresses imposed on the inner core by the outer core field

constraints on upper inner core structure from waveform inversion of core phases summary the determination of the inner core boundary structure is key to understanding the mechanisms of inner core growth iron solidification and anisotropy formation in the inner core a waveform inversion of a worldwide data set of pkikp and pkikp core phases is performed to investigate the velocity and the attenuation structure of this region the data set is chosen to sample the top 50 km of the inner core two inversion methods combining time and frequency domain observations are presented synthetic tests are performed in order to evaluate the resolution and the confidence intervals of the model parameters the attenuation of p waves in the uppermost inner core is poorly resolved but appears to be high q 100 the uppermost inner core interface is divided into two regions a region extending from 180 w to 60 e and characterized by negative p wave velocity perturbations dv v 0 per cent and a region extending from 60 e to 180 e and characterized by positive p wave velocity perturbations dv v 1 per cent the results of the inversion do not favour the presence of transverse isotropy aligned along the spin axis of the earth in the uppermost inner core for these two regions however these regions are correlated with those where large scale variations of the inner core anisotropy have been determined previously in the deeper inner core 100 km beneath the inner core boundary