three dimensional structure of the african superplume from waveform modelling summary previous 2 d modelling of seismic waveforms and traveltimes has revealed a large scale ridge like velocity anomaly beneath africa which is usually referred to as the african superplume the structure starts from the southern indian ocean and extends northwestwards into the atlantic ocean with its base on the core mantle boundary the structure has relatively sharp lateral boundaries with the shear velocity inside 3 per cent lower than the ambient mantle while the compressional velocity is almost normal the 3 d structure is best illustrated by seismic waveforms recorded by the south africa array generated by earthquakes in the western pacific ocean the diffracted s phase travels along the axis of the structure for over 60 with the northern and southernmost paths sampling its edges the s waveforms are simple but delayed by up to 20 s for paths sampling the middle of the structure whereas they display two arrivals for paths along the boundaries these complex waveforms can be explained by 3 d multipathing due to rapid lateral variations in shear wave velocity along the edges of the structure these sharp features are confirmed by modelling broadband records associated with the proposed ridge structure with two independent methods the spectral element method and a ray based 3 d code dwkm

seismic wavefield calculation for laterally heterogeneous whole earth models using the pseudospectral method summary a method for simulating seismic wave propagation in a laterally heterogeneous whole earth model is presented by solving the elastodynamic equations in 2 d cylindrical coordinates using the pseudospectral method psm the psm is an attractive timedomain technique that uses the fast fourier transform for an accurate diierentiation of eld variables in the equations since no dispersion error arises in fourier diierentiation even when using a large grid spacing computer memoryand time are reduced by several orders of magnitude compared to traditional nite diierence methods in order to examine body wave phases with current computing resources a slice through the sphere is approximated with a 2 d cylindrical model an irregular grid spacing is used in the vertical coordinate to improve the treatment of the various structural boundaries appearing in the earth model by matching the heterogeneity in the model synthetic seismograms obtained by the psm calculation are compared with those calculated from an exact simulation method for a spherically homogeneous 1 d earth model and achieve good agreement the psm method is illustrated by constructing the seismic p sv wave eld for strongly heterogeneous earth models including a shield structure near the free surface and velocity perturbations just above the core mantle boundary the visualization of the evolution of seismic p and s waves in time and space is tracked using a sequence of snapshots and synthetic seismograms these displays allow direct insight into the nature of the complex seismic wave behaviour in the earth s interior

sks and spdks sensitivity to two dimensional ultralow velocity zones 1 seismic wave propagation through two dimensional core mantle boundary cmb ultralow velocity zones ulvzs is modeled using a global pseudospectral algorithm seismograms are synthesized for several types of ulvz models to investigate the effect of these structures on sks and spdks phases one dimensional models and two dimensional models with different quasi 1 d velocity structures on the source and receiver sides of the cmb provide a baseline for comparison with other 2 d models models with finite length ulvzs are used to test the sensitivity of the spdks travel time and waveform to different portions of the p diffracted wave path this test shows that spdks waves are only sensitive to ulvzs with lengths 100 km our results give three tools for identifying and characterizing 2 d ulvz structures first dual spdks pulses indicate exposure to at least two separate cmb velocity structures either different source and receiver side cmb velocities or different adjacent velocity regions for which pdiff inception occurs outside of and propagates into a ulvz second high amplitude sks precursors indicate a very strong i e thick and or large velocity perturbations ulvz hence the absence of sks precursors in most previous ulvz studies indicates that very strong sharp ulvzs are not common third mean spdks delays relative to prem constrain the minimum ulvz strength and length combinations required to produce a given travel time delay

adjoint tomography of the southern california crust using an inversion strategy based on adjoint methods we developed a three dimensional seismological model of the southern california crust the resulting model involved 16 tomographic iterations which required 6800 wavefield simulations and a total of 0 8 million central processing unit hours the new crustal model reveals strong heterogeneity including local changes of 30 with respect to the initial three dimensional model provided by the southern california earthquake center the model illuminates shallow features such as sedimentary basins and compositional contrasts across faults it also reveals crustal features at depth that aid in the tectonic reconstruction of southern california such as subduction captured oceanic crustal fragments the new model enables more realistic and accurate assessments of seismic hazard

a doubling of the post perovskite phase boundary and structure of the earth s lowermost mantle a new model of the deep earth allows geophysicists to measure variations in earth s temperature near the boundary between the rocky mantle and core using a recently discovered phase change in the lowest part of the earth s mantle where atoms rearrange themselves in crystals under extreme pressure and temperature the model also explains odd features of this part of the mantle such as patchy regions that reflect seismic waves generated by large earthquakes and inferences of dense partial melting combined with observations the results imply very large variations in temperature providing further evidence for circulation of material throughout the earth s mantle the thermal structure of the earth s lowermost mantle the d layer spanning depths of 2 600 2 900 kilometres1 is key to understanding the dynamical state and history of our planet earth s temperature profile the geotherm is mostly constrained by phase transitions such as freezing at the inner core boundary or changes in crystal structure within the solid mantle that are detected as discontinuities in seismic wave speed and for which the pressure and temperature conditions can be constrained by experiment and theory a recently discovered phase transition at pressures of the d layer2 3 4 is ideally situated to reveal the thermal structure of the lowermost mantle where no phase transitions were previously known to exist here we show that a pair of seismic discontinuities observed in some regions of d can be explained by the same phase transition as the result of a double crossing of the phase boundary by the geotherm at two different depths this simple model can also explain why a seismic discontinuity is not observed in some other regions and provides new constraints for the magnitude of temperature variations within d

structural features and shear velocity structure of the pacific anomaly 1 we constrain structural features and shear velocity structure of a low velocity anomaly in the lower mantle beneath the pacific we term it the pacific anomaly on the basis of forward travel time and waveform modeling of the observed direct s sdiff scs sks and skks phases sampling a great arc across the anomaly from eastern eurasia to southern south america after correction for the effects of earthquake mislocation and the seismic heterogeneities outside the pacific anomaly seismic observations suggest that the pacific anomaly along the great arc consists of at least two separated portions with a 740 km wide gap between them the western portion of the anomaly is about 1050 km wide extends at least 740 km above the core mantle boundary cmb and exhibits a trapezoidal shape with lateral dimensions increasing slightly with depth the velocity structure of the western portion varies from 3 0 at the top 740 km above the cmb to 3 5 at 100 km above the cmb and an average shear velocity reduction of 5 in the bottom 100 km of the mantle the eastern portion of the anomaly reaches at least 340 km above the cmb beneath the mid pacific with an 1800 km wide base and has a uniform velocity reduction of 3 waveform modeling further suggests a very low velocity layer with a shear velocity reduction of 10 located at the edge of the western portion of the anomaly combining the latest results from others we present a general picture of structural and velocity structures of the pacific anomaly the structural and velocity features suggest that the anomaly represents a cluster of metastable thermo chemical piles

seismic imaging of the laterally varying d region beneath the cocos plate summary we use an axisymmetric spherical earth finite difference algorithm to model sh wave propagation through cross sections of laterally varying lower mantle models beneath the cocos plate derived from recent data analyses synthetic seismograms with dominant periods as short as 4 s are computed for several models 1 a d reflector 264 km above the core mantle boundary with laterally varying s wave velocity increases of 0 9 2 6 per cent based on localized structures from a 1 d double array stacking method 2 an undulating d reflector with large topography and uniform velocity increase obtained using a 3 d migration method and 3 cross sections through the 3 d mantle s wave velocity tomography model txbw we apply double array stacking to assess model predictions of data of the models explored the s wave tomography model txbw displays the best overall agreement with data the undulating reflector produces a double scd arrival that may be useful in future studies for distinguishing between d volumetric heterogeneity and d discontinuity topography synthetics for the laterally varying models show waveform variability not observed in 1 d model predictions it is challenging to predict 3 d structure based on localized 1 d models when lateral structural variations are on the order of a few wavelengths of the energy used particularly for the grazing geometry of our data iterative approaches of computing synthetic seismograms and adjusting model characteristics by considering path integral effects are necessary to accurately model fine scale d structure

estimate of the rigidity of eclogite in the lower mantle from waveform modeling of broadband s to p wave conversions broadband usarray recordings of the 21 july 2007 western brazil earthquake mw 6 0 depth 633 km include high amplitude signals about 40 s 75 s and 100 s after the p wave arrival they are consistent with s wave to p wave conversions in the mantle beneath northwestern south america the signal at 100 s denoted as s1750p has the highest amplitude and is formed at 1 750 km depth based on slant stacking and semblance analysis waveform modeling using axisymmetric finite difference synthetics indicates that s1750p is generated by a 10 km thick heterogeneity presumably a fragment of subducted mid ocean ridge basalt in the lower mantle the negative polarity of s1750p is a robust observation and constrains the shear velocity anomaly vs of the heterogeneity to be negative the amplitude of s1750p indicates that vs is in the range from 1 6 to 12 4 the large uncertainty in vs is due to the large variability in the recorded s1750p amplitude and simplifications in the modeling of s1750p waveforms the lower end of our estimate for vs is consistent with ab initio calculations by tsuchiya 2011 who estimated that vs of eclogite at lower mantle pressure is between 0 and 2 due to shear softening from the poststishovite phase transition

evaluation of 1 d and 3 d seismic models of the pacific lower mantle with s sks and skks traveltimes and amplitudes 1 in this study we analyzed the seismic phases s sks and skks from 31 deep focus earthquakes in the tonga fiji region recorded in north america between epicentral distances of 85 and 120 the differential traveltimes and amplitude ratios for these phases reveal clear epicentral distance trends not predicted by standard one dimensional 1 d reference earth models the increase of the s sks amplitude ratio up to a factor of 10 is accompanied by an increase of the s sks differential traveltime of up to 10 s skks sks differential traveltimes of 2 3 s and skks sks amplitude ratios of a factor of 2 4 across the epicentral range have maxima near 107 we examined these observations using full 1 d and 3 d waveforms for three 1 d seismic velocity profiles for the central pacific region and for the tomographic model s40rts including modifications different regularization parameters great circle path azimuthal variation strength of s wave velocity perturbations s wave velocity gradients in the lower mantle and ultra low velocity zones to explain these data we constructed a hybrid model that combines both features of s40rts and short wavelength features from the 1 d profiles the large scale seismic structure is represented by s40rts embedded within s40rts are a 20 km thick ultra low velocity zone at the core mantle boundary near the source side and a 200 km thick negative velocity gradient zone near the receiver side of the paths our analysis demonstrates that the s wave velocity structure of the pacific large low shear velocity province cannot be interpreted solely by global tomographic or regional modeling approaches in exclusion of each other

evidence for a deep asthenosphere beneath north america from western united states sks splits the uniform southwesterly trend of fast traveling split sks waves that traverse the upper mantle beneath the yellowstone swell provides good evidence for both southwesterly motion of north america over a relatively stable deep earth interior and young strain accommodation within the yellowstone swell mantle via dislocation creep of olivine these results contrast with the many sks splits recorded across the western united states the splitting behavior of which often is very complex at individual sites and the orientations of which as a set cannot be attributed simply to plate motion or to small scale convection beneath north america lithosphere these data suggest that away from yellowstone 1 mantle fabric is complex and 2 north america motion is accommodated by strain at depths greater than those typically associated with asthenosphere

deep mantle seismic modeling and imaging detailed seismic modeling and imaging of earth s deep interior is providing key information about lower mantle structures and processes including heat flow across the core mantle boundary the configuration of mantle upwellings and downwellings phase equilibria and transport properties of deep mantle materials and mechanisms of core mantle coupling multichannel seismic wave analysis methods that provide the highest resolution deep mantle structural information include network waveform modeling and stacking array processing and 3d migrations of p and s wave seismograms these methods detect and identify weak signals from structures that cannot be resolved by global seismic tomography some methods are adapted from oil exploration seismology but all are constrained by the source and receiver distributions long travel paths and strong attenuation experienced by seismic waves that penetrate to the deep mantle large and small scale structures with velocity variations ranging from a fraction of a percent to tens of percent have been detected and are guiding geophysicists to new perspectives of thermochemical mantle convection and evolution

localized double array stacking analysis of pcp d and ulvz structure beneath the cocos plate mexico central pacific and north pacific abstract a large high quality p wave data set comprising short period and broadband signals sampling four separate regions in the lowermost mantle beneath the cocos plate mexico the central pacific and the north pacific is analyzed using regional one dimensional double array stacking and modelling with reflectivity synthetics a data screening criterion retains only events with stable pcp energy in the final data stacks used for modelling and interpretation this significantly improves the signal stacks relative to including unscreened observations allows confident alignment on the pcp arrival and allows tight bounds to be placed on p wave velocity structure above the core mantle boundary cmb the pcp reflections under the cocos plate are well modelled without any ultra low velocity zone from 5 to 20 n at latitudes from 15 to 20 n we find evidence for two p wave velocity discontinuities in the d region the first is 182 km above the cmb with a ln vp of 1 5 near the same depth as a weaker discontinuity

extent of the low velocity region in the lowermost mantle beneath the western pacific detected by the vietnamese broadband seismograph array 1 we present evidence showing the extent of the low velocity region in the lowermost mantle beneath the western pacific we analyzed s ss scs s and sscs ss travel times observed by the vietnamese broadband seismograph array deployed as part of the ocean hemisphere project the abrupt changes in scs s and sscs ss travel times suggest that the western geographical boundary of the low velocity region is located around 140 e and is sharp more than 4 velocity contrast within 200 km the dependency of s and ss travel time anomalies of epicentral distances suggests that the strong low velocity region is confined to within 400 km from the cmb core mantle boundary the existence of lateral heterogeneities with a 100 km scale inside the low velocity region is also suggested by the abrupt changes in s and scs waveforms

very low shear wave velocity at the base of the mantle under the south pacific superswell abstract s wave velocity heterogeneity at the base of the mantle beneath the south pacific superswell has been examined using skks sks and s sdiff sks differential travel times the residuals of skks sks and s sdiff sks with respect to the preliminary reference earth model prem are compared to the residuals predicted using recently published three dimensional s wave velocity models we have obtained a three dimensional model that explains the s sdiff sks residuals well and applied this model to the correction of the skks sks residuals the corrected skks sks residuals beneath the south pacific superswell remain large and positive forward modeling reveals that these residuals can be accounted for by a 5 7 low velocity region with a lateral extent of about 500 km in the lowermost 200 km of the mantle although the magnitude of the velocity reduction and the thickness of the anomalous region trade off against one another and cannot be uniquely determined the existence of a very low s wave velocity region at the base of the mantle is a robust feature this feature lies close to the ultra low velocity zone ulvz identified by p velocity data beneath the southwestern pacific and is located just below a 2 3 low s velocity body in the lower mantle at depths of 2300 2700 km beneath the south pacific superswell

modeling the ratios of skks and sks amplitudes with ultra low velocity zones at the core mantle boundary 1 between 105 115 degrees the sks waveform is complicated by the formation of spdks a wave that has segments of p diffraction along the core mantle boundary while previous studies have primarily focused on the move out of spdks from sks we analyze the concomitant reduction of the sks amplitude long period skks sks amplitude ratios present a coherent global pattern skks sks is relatively large in north and south american recordings of deep tonga fiji earthquakes but prem like in european recordings of earthquakes in south america and north american recordings of earthquakes in indonesia modeling of skks sks indicate that ultra low velocity zones ulvzs layers at the base of the mantle with a thickness of about 10 20 km and a shear velocity reduction between 20 30 are required to explain high skks sks ratios and the early move out of spdks

coexisting shear and compressional wave seismic velocity discontinuities beneath the central pacific stacking of broadband shear waves s and short period compressional waves p provides compelling evidence for the existence of a localized discontinuous increase in both s and p wave velocity 230 km above the core mantle boundary cmb beneath the central pacific modelling indicates a 1 7 increase in s velocity v s and a 0 75 increase in p velocity vp similar features have been detected intermittently on a global basis but seldom with s and p structure being resolved in the same location previous evidence for anisotropy near the cmb beneath the central pacific suggests that the discontinuity may be related to anisotropic fabrics induced by flow in the thermal boundary layer above the core

seismic anisotropy in the core mantle transition zone split s waves observed at hockley texas from events in the tonga fiji region of the southwest pacific show predominantly vertically polarized shear wave sv energy arriving earlier than horizontally polarized sh energy for rays propagating horizontally through d after corrections are made for the effects of upper mantle anisotropy beneath hockley a time lag of 1 5 to 2 0 s remains for the furthest events 93 9 100 6 while the time lags of the nearer observations 90 5 92 9 nearly disappear at closer distances the s waves from these same events do not penetrate as deeply into the lower mantle and are not split these observations suggest that a patch of d beneath the central pacific is anisotropic while the mantle immediately above the patch is isotropic the thickness of the anisotropic zone appears to be of the order of 100 200 km observations of shear wave splitting have previously been made for paths that traverse d under the caribbean and under alaska sh leads sv the reverse of the hockley observations but in these areas the fact that sv leads sh in the hkt data shown here suggests a different sort of anisotropy under the central pacific from that under alaska and the caribbean the case of sh travelling faster than sv is consistent with transverse isotropy with a vertical axis of symmetry vti and does not require variations with azimuth the case of sv leading sh is consistent with transverse isotropy with a horizontal axis of symmetry hti an azimuthally anisotropic medium and with a vti medium formed by a hexagonal crystal given that mg fe sio3 perovskite appears unlikely to form anisotropic fabrics on a large scale the presence of anisotropy may point to chemical heterogeneity in the lowermost mantle possibly due to mantle core interactions

joint seismic geodynamic and mineral physical constraints on three dimensional mantle heterogeneity implications for the relative importance of thermal versus compositional heterogeneity summary the joint interpretation of seismic and geodynamic data requires mineral physical parameters linking seismic velocity to density perturbations in the earth s mantle the most common approach is to link velocity and density through relative scaling or conversion factors r s dln dlnv s however the range of possible r s values remains large even when only considering thermal effects we directly test the validity of several proposed depthdependent conversion profiles developed from mineral physics studies for thermally varying properties of mantle materials the tests are conducted by simultaneously inverting shear wave traveltime data and a diverse suite of convection related constraints interpreted with viscous flow response functions of the mantle these geodynamic constraints are represented by surface spherical harmonic basis functions up to harmonic degree 16 and they consist of the global free air gravity field tectonic plate divergences dynamic surface topography and the excess ellipticity of the core mantle boundary cmb the tests yield an optimum 1 d thermal r s profile that is generally compatible with all considered data with the exception of the dynamic surface topography that is most sensitive to the shallow upper mantle this result is not surprising given that cratonic roots are known to be compositionally distinct from the surrounding ambient mantle moreover it is expected that the temperature dependence of the thermal r s in the upper mantle is significant due to the temperature dependence of seismic attenuation or q therefore a simple 1 d density velocity relationship is insufficient to address this problem we obtained independent estimates of the first order correction factors to the selected r s profile within the cratonic roots and in the ambient thermal upper mantle these correction factors defined as r s lnv s are highly negative within the cratons signifying considerable compositional buoyancy this result confirms that the negative buoyancy associated with the low temperatures in the cratons is significantly counteracted by the composition induced positive buoyancy resulting in near neutral buoyancy of the cratonic roots within the ambient upper mantle the correction factors are found to be positive which is consistent with the expected behaviour of the r s relationship in thermally varying upper mantle material we obtain a significantly greater reconciliation of the global gravity anomalies and dynamic surface topography when applying these laterally varying corrections compared to a simple 1 d r s relationship inversion for a fully 3 d r s relationship reveals secondary effects including additional compositional variation within the cratonic roots and the deep mantle superplume structures we estimate the relative magnitude of the thermal and compositional non thermal contributions to mantle density anomalies and conclude that thermal effects dominate shear wave and density heterogeneity throughout the non cratonic mantle we also demonstrate the potential pitfalls of scaling a purely seismically derived model to obtain density rather than performing a true joint inversion to obtain velocity and density simultaneously

right lateral oblique slip movements followed by post ottawan 1050 1020 ma orogenic collapse along the carthage colton shear zone data from the dana hill metagabbro body adirondack mountains new york

constructing synthetics from deep earth tomographic models recent studies of deep mantle structure indicate strong heterogeneity to conduct high resolution waveform modelling of these structures we have developed a new method to construct 2 d synthetics directly from block style tomographic models unlike the wkbj approximation which utilizes rays overshooting and undershooting receivers our method wkm approximation uses rays that arrive at the receiver first the ray paths from the 1 d layered reference model are used to localize each ray segment where the anomalous velocities are applied by overlay as in tomography next new p i t i p i ray parameter ti traveltime are computed to satisfy snell s law along with their numerical derivative p t which is used to construct a synthetic seismogram similar to the wkbj method as a demonstration of the usefulness of this method we generated wkm synthetics for the d region of high velocities beneath central america based on grand s tomography model reasonable fits to broad band data are obtained by condensing his distributed anomalies into his lowermost mantle layer such a 2 d model predicts synthetics containing a laterally varying s cd triplication similar to observations

ultralow velocity zone geometries resolved by multidimensional waveform modelling ultra low velocity zones ulvzs are thin patches of material with strongly reduced seismic wave speeds situated on top of the core mantle boundary cmb a common phase used to detect ulvzs is spdks skpds an sks wave with a short diffracted p leg along the cmb most previous efforts have examined ulvz properties using 1d waveform modeling approaches we present waveform modeling results using the 2 5d finite difference algorithm psvaxi allowing us better insight into ulvz structure and location we characterize ulvz waveforms based on ulvz elastic properties shape and position along the spdks raypath in particular we vary the ulvz location e g source or receiver side ulvz topographical profiles e g boxcar trapezoidal or gaussian and ulvz lateral scale along great circle path 2 5o 5o 10o we observe several waveform effects absent in 1d ulvz models and show evidence for waveform effects allowing the differentiation between source and receiver side ulvzs early inception of the spdks skpds phase is difficult to detect for receiver side ulvzs with maximum shifts in skpds initiation of 3o in epicentral distance whereas source side ulvzs produce maximum shifts of spdks initiation of 5o allowing clear separation of source versus receiver side structure we present a case study using data from up to 300 broadband stations in turkey recorded between 2005 and 2010 we observe a previously undetected ulvz in the southern atlantic ocean region centered near 45o s 12 5ow with a lateral scale of 3 vp reduction of 10 vs reduction of 30 and density increase of 10 relative to prem

monomeric versus mixed monomeric polymeric models for aqueous aluminium species constraints from low temperature natural waters in equilibrium with gibbsite under temperate and tropical climate abstract spring waters from fougeres armorican massif margeride and mont lozere central massif france groundwaters flood waters and recession waters from sinnamary french guyana were collected in small watersheds on granitic or metamorphic rocks leaching experiments were also conducted on soils from fougeres waters were analyzed for major elements and al in order to check the control on dissolved al by gibbsite kaolinite imogolite and other possible secondary minerals and of different models concerning the speciation of al in aqueous solution the results show that 1 the single monomeric model hitherto widely used leads to large supersaturations with respect to gibbsite kaolinite and even in some cases to amorphous al hydroxide and to a great scattering of data 2 the polymeric monomeric model leads to values of iap ionic activity product in agreement with control by gibbsite 3 total dissolved al decreases at constant ph and temperature and gets closer to the solubility curve of gibbsite in deeper springs in fougeres and in recession waters in guyana as contrary to shallower springs in fougeres and flood waters in guyana this allows us to propose a new reaction scheme for al precipitation as ph increases to 5 in a first step a polymeric species forms and in a second step gibbsite crystallization from monomeric species proceeds coupled with depolymerization this picture is consistent with classical data if one considers no more polymerization to occur by an edge to edge mechanism but by a face to face mechanism the polymeric form thus appears to constitute a pool buffering al activity iap of gibbsite is well correlated with temperature but uncertainty on the solubility product of gibbsite is amplified by the polymer monomer relationship which limits the use of the relation obtained as a low temperature geothermometer

an analysis of large scale variations in small scale mantle heterogeneity using global seismographic network recordings of precursors to pkp high frequency precursors to the core phase pkp are caused by scattering off heterogeneities in the lowermost mantle and d regions and they provide a unique window into the small scale structure of the deep earth we study lower mantle scattering by analyzing 412 high quality pkp precursor records at ranges between 120 and 137 5 as obtained from the global seismic networks during the last 10 years to examine regional variations in scattering strength we compare individual records with the globally averaged pkp precursor stack of hedlin et al 1997 we identify strong differences in apparent scattering strength among specific source receiver paths inversion of these data for scattering source regions is complicated by ambiguity between source and receiver side scattering and the sparse and uneven data coverage synthetic tests however suggest that inversions with applied smoothness constraints can resolve large scale differences in scattering strength over significant parts of the lower mantle we use a conjugate gradient method based on an approximation to rayleigh born scattering theory to image differences in the average strength of scattering within the lowermost 1000 km of the mantle our results indicate particularly strong scattering beneath central africa parts of north america and just north of india whereas weaker scattering is seen beneath south and central america eastern europe and indonesia some regions of strong scattering correlate roughly with large scale anomalies revealed by seismic tomography including the african plume and the tethys trench these correlations are tentative rather than definitive because bootstrap resampling tests show that many details in our model are not reliably resolved and the network data alone do not permit complete resolution of the source receiver ambiguity in all areas further progress in this area will require integration of available network recordings with data collected by regional networks and arrays and consideration of the phase velocity of the precursors as well as their temporal variations

a strong lateral shear velocity gradient and anisotropy heterogeneity in the lowermost mantle beneath the southern pacific 1 velocity heterogeneity and seismic anisotropy in the d region beneath the southern pacific ocean is investigated using broadband shear waves from southwest pacific and south american earthquakes recorded by permanent and temporary stations in antarctica australia south america and on islands in the south pacific shear velocity perturbations vs are inferred from the differential travel times between hand picked horizontally polarized sh direct or diffracted s arrivals and vertically polarized sv sks arrivals derived patterns in vs roughly agree with global shear velocity models but reveal a stronger shear velocity lateral gradient north of approximately 52 s where vs transitions from approximately 0 5 to 1 0 relative to radially averaged reference models over less than 600 km along the base of d waveform analyses provide an even stronger constraint on the transitional region where there is a change in waveform behavior occurring over length scales less than 300 km differential travel times between skks and sks calculated by cross correlating sks with hilbert transformed skks support large vs amplitudes and help to constrain the location of the transitional region anisotropy in d ks is inferred from handpicked differential travel times between the sv and sh components of direct or diffracted s and display a slight spatial trend where the sh component precedes the sv in the north and east of the study area but arrives after sv in the center and southwest of it however there is high ks variability in the center of the study region there is no apparent correlation between vs and ks abrupt changes in the character of velocity and anisotropy in this region of d may be related to chemical heterogeneity at the boundary of the pacific superswell as well as small scale convection in the deep mantle

some dynamical consequences of partial melting in earth s deep mantle we use regional scale numerical models of mantle convection to investigate the simple hypothesis that seismically anomalous thin patches of earth s lowermost mantle termed ultralow velocity zones or ulvz are derived from partial melting of ordinary mantle the models span the lower 500 km of earth s mantle employ both temperature and melting related contributions to buoyancy and include a cold randomly moving downwelling introduced from above to maintain a thermal boundary layer partial melting of ordinary mantle introduces a ubiquitous partially molten layer above an isothermal core mantle boundary as a consequence of its isothermal and isobaric conditions although it naturally develops variations in thickness greater than two orders of magnitude with the thickest portions occurring at the base of upwelling plumes and a thin layer elsewhere we find that only a dense partially molten mixture produces partial melt distributions that are compatible with seismic observations of ulvz however if such a melt percolates downward a dense basal liquid layer accumulates above the core mantle boundary the apparent requirement of a volumetrically dense and non percolating melt phase in the lowermost mantle presents serious problems for the hypothesis that ulvz arise from melting of ordinary mantle and suggests that such features likely form as a consequence of more complex processes furthermore these considerations suggest that the solidus of ordinary mantle is a reasonable upper bound on the present day temperature of the cmb 2007 published by elsevier b v

anomalously low amplitude of s waves produced by the 3d structures in the lower mantle abstract direct s and sdiff phases with anomalously low amplitudes are recorded for the earthquakes in papua new guinea by seismographs in northern america according to the prediction by a standard 1d model the amplitudes are the lowest at stations in southern california at a distance and azimuth of around 95 and 55 respectively from the earthquake the amplitude anomaly is more prominent at frequencies higher than 0 03 hz we checked and ruled out the possibility of the anomalies appearing because of the errors in the focal mechanism used in the reference synthetic waveform calculations the observed anomaly distribution changes drastically with a relatively small shift in the location of the earthquake the observations indicate that the amplitude reduction is likely due to the 3d shear velocity vs structure which deflects the wave energy away from the original ray paths moreover some previous studies suggested that some of the s and sdiff phases in our dataset are followed by a prominent postcursor and show a large travel time delay which was explained by placing a large ultra low velocity zone ulvz located on the core mantle boundary southwest of hawaii in this study we evaluated the extent of amplitude anomalies that can be explained by the lower mantle structures in the existing models including the previously proposed ulvz in addition we modified and tested some models and searched for the possible causes of low amplitudes full 3d synthetic waveforms were calculated and compared with the observations our results show that while the existing models explain the trends of the observed amplitude anomalies the size of such anomalies remain under predicted especially at large distances adding a low velocity zone which is spatially larger and has less vs reduction than ulvz on the southwest side of ulvz contributes to explain the low amplitudes observed at distances larger than 100 from the earthquake the newly proposed low velocity zone mostly overlaps with the northern part of the pacific large low shear velocity province llsvp revealed in tomographic models although the very low amplitudes observed at a distance of about 95 remain unexplained our results indicate that the boundary of the pacific llsvp is sharp and the amplitude of s waves at these large distances is lowered by strong vertical and or lateral deflection at the boundary toward the interior of the low velocity province

tomographic filtering of geodynamic models implications for model interpretation and large scale mantle structure 1 the resolution operator r is a critical accompaniment to tomographic models of the mantle r facilitates the comparison between conceptual three dimensional velocity models and tomographic models because it can filter these theoretical models to the spatial resolution of the tomographic model we compute r for the tomographic model s20rts ritsema et al 1999 2004 and two companion models that are based on the same data but derived with different norm damping values the three models explain within measurement uncertainty s sks and s skks travel times equally well to demonstrate how artifacts distort tomographic images and complicate model interpretation we apply r to 1 a thermochemical and 2 an isochemical model of convection in the mantle that feature different patterns of shear velocity heterogeneity in the deep mantle if we assume that shear velocity heterogeneity is caused by temperature variations only r suppresses short wavelength structures removes strong velocity gradients and introduces artificial stretching and tilting of velocity anomalies temperature anomalies in the thermochemical model resemble the spatial extent of low seismic velocity anomalies and the shear velocity spectrum in the d region better than the isochemical model however the thermochemical model overpredicts the amplitude of shear velocity variation and places the african and pacific anomalies imperfectly we suspect that inaccurate velocity scaling laws and uncertain initial conditions control these mismatches extensive hypothesis testing is required to identify successful models

a strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the pacific a new approach for constraining the seismic shear velocity structure above the core mantle boundary is introduced whereby sh sks differential travel times amplitude ratios of sv sks and sdiff waveshapes are simultaneously modeled this procedure is applied to the lower mantle beneath the central pacific using da ta from numerous deep focus southwest pacific earthquakes recorded in north america we analyze 90 broadband and 248 digitized analog recordings for this source receiver geometry sh sks times are highly variable and up to 10 s larger than standard reference model predictions indicating the presence of laterally varying low shear velocities in the study area the travel times however do not constrain the depth extent or velocity gradient of the low velocity region sv sks amplitude ratios and sh waveforms are sensitive to the radial shear velocity profile and when analyzed simultaneously with sh sks times rnveal up to 3 shear velocity reductions restricted to the lowermost 190 50 km of the mantle our preferred model for the central eastern pacific region ml has a strong negative gradient with 0 5 reduction in velocity relative to the preliminary reference earth model prem at 2700 km depth and 3 reduction at 2891 km depth and slight velocity reductions from 2000 to 2700 km depth 0 0 5 lower than prem significant small scale 100 500 km shear velocity heterogeneity 0 5 1 is required to explain scatter in the differential times and amplitude ratios