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

plumes in a convecting mantle models and observations for individual hotspots the motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented a new list of 44 possible hotspots with associated tracks has been compiled for all of them calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow plume source depth has usually been assumed to be the top of d but an alternative source depth at the 670 km discontinuity has also been considered using models of relative plate motions and boundaries hotspot tracks on plates have been calculated and compared with age data ocean floor topography and distribution of volcanics on continents absolute plate motions have been redetermined under consideration of hotspot motion using a new least squares method for the hawaiian and yellowstone hotspots source locations and hotspot motion have been computed for a total of up to 23 different models the results show plume conduits being tilted with source regions at the d moving in the lowermost mantle flow generally toward large scale upwellings under southern africa and the south central pacific hotspot surface motion often represents the horizontal component of midmantle flow which is frequently opposite to plate motion toward ridges and away from subduction zones in particular almost all models tested predict southward motion of the hawaii and kerguelen hotspots and westward motion of the iceland hotspot for models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as or better than for the fixed hotspot model but sometimes fixed hotspots give the best fit in some cases where the track ends at a subduction zone e g for the bowie hotspot results can give indications about the otherwise unknown age of the hotspot in other cases especially for the tahiti hotspot results suggest an origin shallower than d and in yet other cases particularly east africa the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes

tectonic regionalization without a priori information a cluster analysis of upper mantle tomography abstract global mantle tomography can be improved through better use of data and application of more accurate wave propagation methods however few techniques have been developed for objective validation and exploration of the resulting tomographic models we show that cluster analysis can be used to validate and explore the salient features across such models we present a cluster analysis of a global upper mantle radially anisotropic model semum developed using full waveform tomography and the spectral element method applied to semum down to 350 km depth the cluster analysis reveals that absolute shear wave velocity vs depth profiles naturally group into families that correspond with known surface tectonics this allows us to construct a global tectonic regionalization based solely on tomography without the help of any a priori information we find that the profiles of stable platforms and shields consistently exhibit a mid lithospheric low velocity zone lvz between 80 and 130 km depth while the asthenosphere is found at depths greater than 250 km in both regions this global intra continental lithosphere low velocity zone agrees with recent receiver function studies and regional tomographic studies furthermore we identify an anomalous oceanic region characterized by slow shear wave speeds at depths below 150 km hotspots are found preferentially in the vicinity of this anomalous region in the pacific ocean where plate velocities are largest these regions have elongated shapes that align with absolute plate motion suggesting a relationship between the location of hotspots and small scale convection in the oceanic upper mantle

anisotropic shear wave velocity structure of the earth s mantle a global model 1 we combine a new large data set of surface wave phase anomalies long period waveforms and body wave travel times to construct a three dimensional model of the anisotropic shear wave velocity in the earth s mantle our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model simultaneous inversion for velocity and anisotropy as well as discontinuity topographies and implementation of nonlinear crustal corrections for waveforms a comparison of our new three dimensional model s362ani with two other models derived from comparable data sets but using different techniques reveals persistent features 1 strong 200 km thick high velocity anomalies beneath cratons likely representing the continental lithosphere underlain by weaker fast anomalies extending below 250 km which may represent continental roots 2 weak velocity heterogeneity between 250 and 400 km depths 3 fast anomalies extending horizontally up to 2000 3000 km in the mantle transition zone beneath subduction zones 4 lack of strong long wavelength heterogeneity below 650 km suggesting inhibiting character of the upper mantle lower mantle boundary and 5 slow velocity superplumes beneath the pacific and africa the shear wave radial anisotropy is strongest at 120 km depth in particular beneath the central pacific lateral anisotropic variations appreciably improve the fit to data that are predominantly sensitive to the uppermost and lowermost mantle but not to the waveforms that control the transition zone and midmantle depths tradeoffs between lateral variations in velocity and anisotropy are negligible in the uppermost mantle but noticeable at the bottom of the mantle

wavelet based directional analysis of the gravity field evidence for large scale undulations summary in the eighties the analysis of satellite altimetry data leads to the major discovery of gravity lineations in the oceans with wavelengths between 200 and 1400 km while the existence of the 200 km scale undulations is widely accepted undulations at scales larger than 400 km are still a matter of debate in this paper we revisit the topic of the large scale geoid undulations over the oceans in the light of the satellite gravity data provided by the grace mission considerably more precise than the altimetry data at wavelengths larger than 400 km first we develop a dedicated method of directional poisson wavelet analysis on the sphere with significance testing in order to detect and characterize directional structures in geophysical data on the sphere at different spatial scales this method is particularly well suited for potential field analysis we validate it on a series of synthetic tests and then apply it to analyze recent gravity models as well as a bathymetry data set independent from gravity our analysis confirms the existence of gravity undulations at large scale in the oceans with characteristic scales between 600 and 2000 km their direction correlates well with present day plate motion over the pacific ocean where they are particularly clear and associated with a conjugate direction at 1500 km scale a major finding is that the 2000 km scale geoid undulations dominate and had never been so clearly observed previously this is due to the great precision of grace data at those wavelengths given the large scale of these undulations they are most likely related to mantle processes taking into account observations and models from other geophysical information as seismological tomography convection and geochemical models and electrical conductivity in the mantle we conceive that all these inputs indicate a directional fabric of the mantle flows at depth reflecting how the history of subduction influences the organization of lower mantle upwellings

isotopic composition of waters from ethiopia and kenya insights into moisture sources for eastern africa 1 oxygen and deuterium isotopic values of meteoric waters from ethiopia are unusually high when compared to waters from other high elevation settings in africa and worldwide these high values are well documented however the climatic processes responsible for the isotopic anomalies in ethiopian waters have not been thoroughly investigated we use isotopic data from waters and remote data products to demonstrate how different moisture sources affect the distribution of stable isotopes in waters from eastern africa oxygen and deuterium stable isotopic data from 349 surface and near surface groundwaters indicate isotopic distinctions between waters in ethiopia and kenya and confirm the anomalous nature of ethiopian waters remote data products from the tropical rainfall measuring mission trmm and national centers for environmental prediction ncep reanalysis project show strong westerly and southwesterly components to low level winds during precipitation events in western and central ethiopia this is in contrast to the easterly and southeasterly winds that bring rainfall to kenya and southeastern ethiopia large regions of high equivalent potential temperatures e at low levels over the sudd and the congo basin demonstrate the potential for these areas as sources of moisture and convective instability the combination of wind direction data from ethiopia and e distribution in africa indicates that transpired moisture from the sudd and the congo basin is likely responsible for the high isotopic values of rainfall in ethiopia

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

observational hints for a plume fed suboceanic asthenosphere and its role in mantle convection an asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection in this study we discuss geochemical seismic and geoid depth evidence for lateral flow within this type of asthenosphere in particular we suggest that there are large scale layered horizontal flow structures that connect upward plume input beneath hotspots to near ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere lateral asthenosphere flow is also shaped by oceanic subduction zones with a partial return flow from trenches and by deep continental roots that are migrating barriers to asthenosphere flow this alternative paradigm offers relatively simple explanations for several puzzles about mantle convection for example the low mantle heat flow beneath continents it also offers an explanation for why mid ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large scale mantle flow while in contrast trenches appear to be strongly coupled to mantle thick regions of fast colder seismic velocity anomalies we also discuss several implications of this paradigm that should be testable in future studies such as the prediction of cogenetic off axis seamount volcanism that is created between an off axis hotspot and its neighboring ridge axis

waveform tomography reveals channeled flow at the base of the oceanic asthenosphere understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists using full waveform seismic tomography we imaged a pattern of horizontally elongated bands of low shear velocity most prominent between 200 and 350 kilometers depth which extends below the well developed low velocity zone these quasi periodic fingerlike structures of wavelength 2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers below 400 kilometers depth velocity structure is organized into fewer undulating but vertically coherent low velocity plumelike features which appear rooted in the lower mantle this suggests the presence of a dynamic interplay between plate driven flow in the low velocity zone and active influx of low rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer

inferring upper mantle structure by full waveform tomography with the spectral element method summary mapping the elastic and anelastic structure of the earth s mantle is crucial for understanding the temperature composition and dynamics of our planet in the past quarter century global tomography based on ray theory and first order perturbation methods has imaged long wavelength elastic velocity heterogeneities of the earth s mantle however the approximate techniques upon which global tomographers have traditionally relied become inadequate when dealing with crustal structure as well as short wavelength or large amplitude mantle heterogeneity the spectral element method on the other hand permits accurate calculation of wave propagation through highly heterogeneous structures and is computationally economical when coupled with a normal mode solution and applied to a restricted region of the earth such as the upper mantle sem importantly sem allows a dramatic improvement in accounting for the effects of crustal structure here we develop and apply a new hybrid method of tomography which allows us to leverage the accuracy of sem to model fundamental and higher mode long period 60 s waveforms we then present the first global model of upper mantle velocity and radial anisotropy developed using sem our model semum confirms that the long wavelength mantle structure imaged using approximate semi analytic techniques is robust and representative of the earth s true structure furthermore it reveals structures in the upper mantle that were not clearly seen in previous global tomographic models we show that semum favourably compares to and rivals the resolving power of continental scale studies this new hybrid approach to tomography can be applied to a larger and higher frequency data set in order to gain new insights into the structure of the lower mantle and more robustly map seismic structure at the regional and smaller scales

basin hydrogeological characterization using remote sensing hydrogeochemical and isotope methods the case of baro akobo eastern nile ethiopia hydrogeochemical and isotopic signatures of the waters of the baro akobo river basin show deviation from signatures in other ethiopian river basins in this study hydrogeochemical and isotope methods were employed to determine regional and local hydrogeology and characteristics of the basin optical thermal and radar remote sensing products were used to update geological and structural maps of the basin and determine sampling points using the judgment sampling method a total of 363 samples from wells springs rivers lakes swamps and rain were collected for this study and an additional 270 water quality data sets were added from previous studies these data were analyzed for their hydrogeochemical characteristics and isotope signatures analysis of the oxygen deuterium and tritium isotopes shows the groundwater of the basin is modern water among all basins in ethiopia the baro akobo basin shows the highest enrichment this indicates the proximity of the rainfall sources which presumably are the sudd and other wetlands in south sudan the hydrochemical properties of the waters show evapotranspiration is the dominant hydrologic process in the basin and explains the large amount of water that is lost in the lowland plain analysis of radon 222 shows no significant groundwater flux over the wetlands which are part of machar marshes this shows evaporation to be dominant hydrologic process in this zone results from all analyses help explain the limited holding capacity of the aquifers in the recharge zone and their vulnerability to anthropogenic impacts and climate variability there is a trend of decreasing surface flow and rainfall and increasing water soil erosion

intraplate volcanism with complex age distance patterns a case for small scale sublithospheric convection 1 many volcano chains in the pacific do not follow the most fundamental predictions of hot spot theory in terms of geographic age progressions one possible explanation for non hot spot intraplate volcanism is small scale sublithospheric convection ssc and we explore this concept using 3 d numerical models that simulate melting with rheology laws that account for the effects of dehydration ssc spontaneously self organizes beneath relatively mature oceanic lithosphere whenever this lithosphere is sufficiently young and thin ssc replaces the shallow layer of harzburgite which was formed by partial melting at the mid ocean ridge with fresh peridotite this mechanism enables magma generation without any preexisting thermochemical anomalies however the additional effect of melting induced dehydration to stiffen the harzburgite requires lower background viscosities to allow for vigorous ssc overturn of the compositional stratification and related magmatism the intrinsic stiffness of the dehydrated harzburgite furthermore restricts penetration of ssc into very shallow and cooler levels on the one hand such a restriction precludes high degrees of melting but on the other hand it slows asthenospheric cooling and thus prolongs the duration of melting to 25 ma volcanism over such an elongated melting anomaly continues for at least 10 20 ma and occurs on seafloor ages of 20 to 60 ma these seafloor ages increase with increasing mantle temperature due to the effect of forming a thicker harzburgite layer from more extensive mid ocean ridge melting the long durations of volcanism predicted reconcile observations of extended activity of individual seamounts and synchronous activity over great distances along some volcanic chains ssc thus gives an explanation for previously enigmatic volcano ages along the line islands and the gilbert and pukapuka ridges as well as along the individual subchains of the wakes marshalls and cook australs

mantle heterogeneity and off axis volcanism on young pacific lithosphere plate tectonics and mantle plumes explain most volcanism on earth but there are numerous actively forming linear volcanic chains in the middle of tectonic plates that are not explained by these theories using the multidisciplinary geophysical dataset of the melt and glimpse experiments we show that associated with 3 volcanic chains west of the east pacific rise there are low seismic velocities and densities in the asthenosphere that extend to the east pacific rise spreading center analogous to the hawaiian swell the low density anomalies produce swells beneath the volcanoes on young seafloor the associated gravity anomalies are part of a set of gravity lineaments that have been previously interpreted as being due to thermo elastic cracking of the lithosphere or small scale convection the correlation between the surface volcanism and subsurface density and velocity anomalies and their extension to the spreading center suggest that pre existing buoyant or fertile asthenospheric mantle heterogeneities are stretched in the direction of plate motion by shear between the plate and the underlying mantle these heterogeneities seed small scale convection producing upwelling and pressure release melting forming volcanic chains that extend nearly to the ridge axis

extension on the tibetan plateau recent normal faulting measured by insar and body wave seismology we use insar and body wave modelling to determine the faulting parameters for a series of five mw 5 9 7 1 normal faulting earthquakes that occurred during 2008 including the march 20 yutian earthquake mw 7 1 one of the largest normal faulting events to have occurred recently on the continents we also study three earlier normal faulting earthquakes that occurred in southern tibet between 1992 and 2005 coseismic deformation for each of these eight events is measured with ascending and descending interferograms from envisat ers and alos sar data elastic dislocation modelling of the line of sight insar displacements and body wave seismological modelling of p and sh waves are used to estimate fault parameters and are found to be in good agreement for all the events studied the use of insar to measure deformation allows a relatively precise determination of the fault location in addition to resolving the focal plane ambiguity only five of the eight events are associated with a clear surface topographic feature suggesting that an underestimation of the amount of extension would result from using the surface expressions of normal faulting alone the observations in all cases are consistent with slip on planar surfaces with dips in the range 40 50 that penetrates the uppermost crust to a depth of 10 15 km we find no evidence for active low angle dip less than 30 normal faulting the contribution of the normal faulting to overall extension estimated by summing seismic moments over earthquakes for the past 43 yr is 3 4 mm yr 1 or 15 20 per cent of the rates of extension measured across the plateau using gps 85 per cent of the moment release in normal faulting over the past 43 yr has occurred in regions whose surface height exceeds 5 km this observation adds weight to the suggestions that the widespread normal faulting on the plateau is the result of variations in the gravitational potential energy of the lithosphere

thickening of young pacific lithosphere from high resolution rayleigh wave tomography a test of the conductive cooling model direct seismic measurements of the thickening of oceanic lithosphere away from the spreading axis are rare due to the remoteness of mid ocean ridges on the east pacific rise at 17 s there were two long term broadband ocean bottom seismometer deployments the melt and glimpse experiments these arrays spanned young pacific plate seafloor ranging in age from 0 to 8 ma combining these two data sets we describe the increase in rayleigh wave phase velocities for 16 100 s period as function of distance from the ridge with two parameterizations an arbitrary function of seafloor age and a simple polynomial function of age on the pacific and nazca plates although resolution analysis shows that 10 to 15 independent pieces of information about the age variation of phase velocity on the pacific plate can be resolved at periods of 50 s the three parameter polynomial model fits the data almost as well indicating a simple pattern of the evolution of structure with age to compare our observations to the predictions for the conductive cooling of the lithosphere we convert a thermal half space cooling model to shear velocity using anharmonic and anelastic contributions the patterns in the velocities are not consistent with simple conductive cooling the conductive cooling model under predicts the changes in phase velocity with age observed in the 20 to 78 s period range we observe significant changes in shear velocity in the asthenosphere at depths greater than conductive cooling should extend the asthenospheric low velocity zone is asymmetric dipping to the west with the lowest velocities beneath the pacific plate west of the spreading center the conductive cooling model also over predicts the shear velocities in the lithosphere by 0 2 km s these anomalies indicate that more complicated mantle flow and significant mantle heterogeneities such as melt are required

chemical and isotopic δ18o δ2h δ13c 222rn multi tracing for groundwater conceptual model of carbonate aquifer gran sasso infn underground laboratory central italy summary a hydrochemical and isotope study was conducted on the drainage waters of an underground laboratory located inside the gran sasso massif central italy the study was expected to improve the conceptual model of groundwater circulation at the base of an over 1000 thick unsaturated zone in the gran sasso partitioned karst aquifer this lithostratigraphically and tectonically complex aquifer is typical of africa europe thrust and fold collision belt in the mediterranean area in this case investigations on water rock interactions during recharge in complex aquifers overlaid by a thick unsaturated zone have been made thanks to the strategic location of the gran sasso underground laboratories located in the core of a huge carbonate aquifer knowledge of the local basic hydrogeological setting was the starting point for a detailed hydrogeochemical and isotopic study which was carried out at the aquifer scale and at the fine scale in the underground laboratories the water rock interaction processes were investigated both spatially and in temporal sequences analysing recharge waters and groundwater in the underground laboratories by multitracing techniques including major ions and 18o 2h and 13c stable isotopes use of 222rn provides information on transit time in the aquifer processes proved to be typical of carbonate rocks with clear influence of vertical movement of water on chemical physical parameters through the unsaturated zone conversely in the saturated zone these processes proved to be dominantly affected by local geological structural conditions a conceptual model with dual flow velocity is proposed directly related to the local geological structural setting 222rn decay enables to calculate an effective velocity of around 10 m day for the fracture network through the sequence of less permeable dolomites and underlying limestone lag time between recharge and chemical changes in the saturated zone testifies to an effective velocity of about 35 m day for fast flow through recent and active extensional faults

using 222rn to examine groundwater surface discharge interaction in the rio grande de manati puerto rico 222rn was used in the karst drainage basin of the rio grande de manati in puerto rico to study groundwater surface flow relationships locations of groundwater influx along two sections of the rio grande de manati were identified the 222rn measurements were used together with stream discharge data in a mass balance equation to quantify the groundwater inputs the investigation established that both of the sections of the rio manati surveyed not only gained groundwater but lost surface flow it was calculated that the river gained about 1 2m3s 1 and lost 0 5m3s 1 to the aquifer between ciales and united states geological survey gauging station 5 between united states geological survey gauging stations 6 and 7 groundwater influx and stream flow loss occurred simultaneously with groundwater inputs equalling surface discharge losses of 4m3s 1 the study successfully demonstrated the innovative application of 222rn as a geochemical tracer in examining groundwater surface flow relationships in a karst system

full waveform tomography of the upper mantle in the south atlantic region imaging a westward fluxing shallow asthenosphere abstract a prominent feature of the south atlantic region is its strongly asymmetric residual bathymetry across the ocean basin it has been suggested that the residual bathymetry is dynamic in nature arising from the large slow velocity seismic anomaly located in the lower mantle beneath the african plate unfortunately the pattern of mantle heterogeneity particularly in the upper mantle is not well known owing to the sparsity of seismic stations and the existence of large aseismic regions on the african and south american plates here we present a new seismic tomographic study of the south atlantic upper mantle our model is based on a full seismic waveform inversion of 4000 high quality seismograms for isotropic 3 d seismic structure using a powerful adjoint methodology capable of extracting maximum information from each seismogram the theory requires simulation of seismic wave propagation in 3 d heterogeneous earth models computed with a spectral element method where the differences between observed and synthetic seismograms are quantified using phase misfits obtained through a time frequency transform the model images a continuous channel of pronounced slow seismic velocity in the shallow sublithospheric mantle between 150 and 300 km depth that branches in between the cratonic roots under the african and south american continents at greater depth below 300 350 km the slow anomalies are less pronounced and a change in the convective planform is indicated by isolated round shaped patches in an overall faster mantle it is possible that the depthwise change of the convective planform from vertical to horizontal advection of hot buoyant material in a low viscosity asthenosphere can reconcile the anomalous residual bathymetry in the region and we present a simple fluid dynamic model of pressure driven flow to assess the feasibility of this scenario

depth dependent viscosity and mantle stress amplification implications for the role of the asthenosphere in maintaining plate tectonics summary boundary layer theory is used to derive scaling relationships for plate stresses in a mantle convection system with a low viscosity asthenosphere the theory assumes a plate tectonic like mode of mantle convection withflow driven by an active upper boundary layer the theory predicts that the confinement of horizontal mantle flow within a low viscosity sublithospheric channel can lead to an increase in plate stress compared to the case lacking a channel even if the absolute viscosity of the sublithosphere mantle does not change between the two cases thetheoryfurtherpredictsincreasingshearstresswithdecreasinglow viscositychannelthickness if the thickness of tectonic plates is determined dominantly by a dehydrated chemical lithosphere then the plate normal stress is predicted to also increase with decreasing channel thickness we use 3 d spherical shell simulations of mantle convection with temperature depth and stress dependent rheology to test scaling trends the simulations and theoretical scalings demonstrate that a low viscosity layer asthenosphere can amplify convective stresses if the level of convective stress plays a role in maintaining and or reactivating plate boundaries this suggests that a relatively thin low viscosity layer may help to maintain plate tectonics the numerical simulations support this suggestion as they show that an increase in the thickness of a low viscosity channel can cause the system to transition from an active lid modeofconvectiontoastagnantlidstate collectively thesimulationsandtheoreticalscalings lead to the conclusion that the role of the asthenosphere in maintaining plate tectonics does not come principally from a basal lubrication effect associated with a low absolute asthenosphere viscosity but instead from a mantle flow channelization effect associated with a high viscosity contrast from the asthenosphere to the mantle below

chaotic topography mantle flow and mantle migration in the australian antarctic discordance oceanic crust formed over the past 30 million years at the australian antarctic discordance aad is characterized by chaotic sea floor topography reflecting a weak magma supply from an unusually cold underlying mantle during the past 3 4 million years however a source of increased magma supply coinciding with the known indian pacific mantle isotopic boundary has propagated into the eastern aad displacing the chaotic terrain and replacing it with normal sea floor pacific mantle reached the eastern boundary of the aad at least 7 million years ago but it was not until 3 4 million years ago that lavas derived from pacific mantle were first erupted within the aad this long hiatus combined with the ridge transform geometry across the aad boundary constrains the locus of mantle migration to a narrow relatively shallow region directly beneath the spreading axis of the southeast indian ridge

investigation of upper mantle structure under young regions of the southeast pacific using long period rayleigh waves abstract long period rayleigh wave group velocity dispersion curves are presented for paths across the east pacific ocean the records are from the ipg site at pamatai tahiti and the ida station at nana for the first time direct path observations of group velocities up to 300 s have been obtained this study shows that for young oceanic regions group velocities are low even at long periods the observations are interpreted in terms of an s wave velocity model by a generalized inversion scheme in the models for young ages the low velocity zone under the asthenosphere lid is well developed with a strong velocity gradient at the bottom of this zone followed at larger depths by a plateau representing a lower velocity zone and a marked gradient at 400 km