s velocity variations beneath north america 1 we investigate s velocity variation in the upper mantle beneath north american to better understand the effects of data heterogeneity model parameterization and regularization to this end we analyzed and fit regional s and rayleigh wave trains generated by earthquakes around north america that occurred between the years 2000 through 2006 including waveforms from the transportable array stations of earthscope s usarray these new data were combined with constraints used for the 3 d s velocity model na04 in order to create a new model na07 another model was created from a suite of good fit models to provide a useful guide for model velocities and uncertainties by estimating ranges of probable velocity variations throughout the upper mantle we find that the southern and eastern edges of the north american craton appear to be defined by paleozoic orogens rather than proterozoic ones on average the archean portion of the craton is 200 km thick while the paleozoic part averages 175 km thick with an 80 m s lower s velocity the horizontal gradients in velocity are over 1 0 100 km at the western margin of the craton 0 5 100 km in the south and 1 100 km at the eastern margin

dipoles of the south east madagascar current from an analysis using almost 20 years of satellite altimetry data and in situ observations we have shown thatat the southern tip of madagascar the semc breaks up in a series of nearly symmetric dipolar vortex pairs of approximately zero net relative vorticity they appear at a frequency of about 4 6 per year most vortex pairs break up shortly after their formation and interact with previously formed cyclones and anticyclones the velocity shear across the semc is so large that the dynamics of the evolution of the dipole is dominantly nonlinear a surprising consequence is that there is no retroflection of the semc nor does it continue as an unstable westward jet instead the lobes develop symmetrically and the dynamics appear more similar to that of modon formation flierl et al 1983 the eke south of madagascar was shown to correlate with the strength of the semc prior to separation such a relationship could not be found by palastanga et al 2006 probably because they used a more upstream satellite transect to determine the surface velocity of the semc within the semc there is a short decorrelation length scale also observed in the gulf stream sturges and hong 2001 and the velocity at our transect does not significantly correlate with the velocity at locations further than 450 km upstream the very early retroflection of the ac in 2001 was an exceptional one only one more appeared in the 18 year period in 2008 in both cases it needed a concerted action of cyclonic meanders of the ac and the arc to shortcut the retroflection loop with an active role of the madagascar dipoles in generating the agulhas cyclones interannual variability in the separation region appeared related with the enso cycle but not significantly with the iod significant correlations were found between the nino3 4 index and both the strength of the semc and the eke southwest of madagascar positive and negative lags are between 250 and 500 days preceding a positive negative enso phase stronger weaker velocities in the semc were observed following a positive negative phase weaker stronger velocities in the semc were observed we are presently investigating the possible explanations of these relations and their impact

imaging the lithosphere beneath ne tibet teleseismic p and s body wave tomography incorporating surface wave starting models the northeastern margin of the tibetan plateau which includes the qiangtang and songpan ganzi terranes as well as the kunlun shan and the qaidam basin continues to deform in response to the ongoing india eurasia collision to test competing hypotheses concerning the mechanisms for this deformation we assembled a high quality data set of approximately 14 000 p and 4000 s wave arrival times from earthquakes at teleseismic distances from the international deep profiling of tibet and the himalaya phase iv broad band seismometer deployments we analyse these arrival times to determine tomographic images of p and s wave velocities in the upper mantle beneath this part of the plateau to account for the effects of major heterogeneity in crustal and uppermost mantle wave velocities in tibet we use recent surface wave models to construct a starting model for our teleseismic body wave inversion we compare the results from our model with those from simpler starting models and find that while the reduction in residuals and results for deep structure are similar between models the results for shallow structure are different checkerboard tests indicate that features of 125 km length scale are reliably imaged throughout the study region using synthetic tests we show that the best recovery is below 300 km and that broad variations in shallow structure can also be recovered we also find that significant smearing can occur especially at the edges of the model we observe a shallow dipping seismically fast structure at depths of 140 240 km which dies out gradually between 33 n and 35 n based on the lateral continuity of this structure from the surface waves we interpret it as indian lithosphere alternatively the entire area could be thickened by pure shear or the northern part could be an underthrust lhasa terrane lithospheric slab with only the southern part from india we see a deep fast wave velocity anomaly below 300 km that is consistent with receiver function observations of a thickened transition zone and could be a fragment of oceanic lithosphere in ne tibet it appears to be disconnected from faster wave velocities above i e it is not downwelling or subducting here our models corroborate results of previous work which imaged a relatively slow wave velocity region below the kunlun shan and northern songpan ganzi terrane which is difficult to reconcile with the hypothesis of southward directed continental subduction at the northern margin wave velocities in the shallow mantle beneath the qaidam basin are faster than normal and more so in the east than the west

unraveling the geometry of the farallon plate synthesis of three dimensional imaging results from usarray abstract we compare 12 recent three dimensional 3d seismic imaging results that made extensive use of data from the earthscope transportable array ta our goal is to sort out what can be said about the geometry of the farallon plate our main approach is 3d visualization using a kinematic plate motion model as a framework comparison of results from all 12 image volumes indicates that the results are most consistent with a single coherent farallon slab overridden by north american the farallon can be tracked from the trench in the pacific northwest to its remnants in the lower mantle under eastern north america from the trench the lithosphere has a low dip to the volcanic arc immediately east of the arc the slab steepens sharply before undergoing a decrease in dip above the 410 km discontinuity the gently dipping section varies along strike under washington the deflection is minor but to the south the slab flattens to become nearly horizontal beneath southern idaho there is a strong agreement that the high velocity anomaly associated with the slab vanishes under eastern oregon scattered wave imaging results however suggest the top of the anomaly is continuous these can be reconciled if one assumes the wavespeed anomaly has been neutralized by processes linked to the yellowstone system we find that all results are consistent with a 4d kinematic model of the mendocino slab window under nevada and utah in the eastern us the larger scale models all show a lower mantle anomaly related to the older history of farallon subduction the link between the lower mantle and new results in the u s cordillera lies under the high plains where the required usarray coverage is not yet complete image volumes in a unified format are supplied in an electronic supplement

seismic tomography of the southern california crust based on spectral element and adjoint methods we iteratively improve a 3 d tomographic model of the southern california crust using numerical simulations of seismic wave propagation based on a spectral element method sem in combination with an adjoint method the initial 3 d model is provided by the southern california earthquake center the data set comprises three component seismic waveforms i e both body and surface waves filtered over the period range 2 30 s from 143 local earthquakes recorded by a network of 203 stations time windows for measurements are automatically selected by the flexwin algorithm the misfit function in the tomographic inversion is based on frequency dependent multitaper traveltime differences the gradient of the misfit function and related finite frequency sensitivity kernels for each earthquake are computed using an adjoint technique the kernels are combined using a source subspace projection method to compute a model update at each iteration of a gradient based minimization algorithm the inversion involved 16 iterations which required 6800 wavefield simulations the new crustal model m 16 is described in terms of independent shear v s and bulk sound v b wave speed variations it exhibits strong heterogeneity including local changes of 30 per cent with respect to the initial 3 d model the model reveals several features that relate to geological observations such as sedimentary basins exhumed batholiths and contrasting lithologies across faults the quality of the new model is validated by quantifying waveform misfits of full length seismograms from 91 earthquakes that were not used in the tomographic inversion the new model provides more accurate synthetic seismograms that will benefit seismic hazard assessment

q tomography of the upper mantle using three component long period waveforms summary we present a degree 8 3 d q model qrlw8 of the upper mantle derived from three component surface waveform data in the period range 60 400 s the inversion procedure involves two steps in the first step 3 d whole mantle velocity models are derived separately for elastic sh transverse component and sv vertical and longitudinal component velocity models using both surface and body waveforms and the non linear asymptotic coupling theory nact approach in the second step the surface waveforms thus aligned in phase are inverted to obtain a 3 d q model in the depth range 80 670 km various stability tests are performed to assess the quality of the resulting q model and in particular to assess possible contamination from focusing effects we find that the 3 d patterns obtained are stable but the amplitude of the lateral variations in q is not well constrained because large damping is necessary to extract the weak q signal from data the model obtained agrees with previous results in that a strong correlation of q with tectonics is observed in the first 250 km of the upper mantle with high attenuation under oceanic regions and low attenuation under continental shields it is gradually replaced by a simpler pattern at larger depth at the depths below 400 km the q distribution is generally dominated by two strong minima one under the southern pacific and one under africa yielding a strong degree 2 pattern most hotspots are located above regions of low q at this depth ridges are shallow features in both velocity and q models

short period surface wave phase velocities across the conterminous united states abstract surface wave phase velocity maps for the full footprint of the usarray transportable array ta across the conterminous united states are developed and tested three component long period continuous seismograms recorded on more than 1800 seismometers most of which were deployed for 18 months or longer are processed using a noise cross correlation technique to derive inter station love and rayleigh dispersion curves at periods between 5 and 40 s the phase velocity measurements are quality controlled using an automated algorithm and then used in inversions for love and rayleigh phase velocity models at discrete periods on a 0 25 by 0 25 pixel grid the robustness of the results is examined using comparisons of maps derived from subsets of the data a winter summer division of the cross correlation data results in small model differences indicating relatively minor sensitivity of the results to seasonal variations in the distribution of noise sources division of the dispersion data based on inter station azimuth does not result in geographically coherent model differences suggesting that azimuthal anisotropy at the regional scale is weak compared with variations in isotropic velocities and does not substantially influence the results for isotropic velocities the phase velocity maps and dispersion measurements are documented and made available as data products of the 10 year long usarray ta deployment

circulation stratification and seamounts in the southwest indian ocean abstract circulation in the vicinity of six seamounts along the southwest indian ridge was studied as part of a multidisciplinary survey in november 2009 examination of altimetric data shows that several of the seamounts lie in the area of slow mean westward flow between the southern tip of madagascar 25 s and the agulhas return current arc flowing eastward between 37 s and 40 s the mean westward drift of mesoscale features was 4 1 0 9 cm s 1 integrated between madagascar and 37 s this westward drift can account for 50 sv 1 sv 106 m3 s 1 which added to 25 sv of southward flow past madagascar is sufficient to account for the total agulhas current transport of 70 21 sv the transport of the arc was also measured at two longitudes down to 2000 m combined with earlier crossings of the arc in 1986 and 1995 the full depth transport of the arc is estimated at 71 85 sv at longitudes 40 50 e indicating that the agulhas current then arc transport continues unreduced as far as 50 e before beginning to recirculate in the southwest indian ocean subtropical gyre the primary control on the circulation near each seamount was its position relative to any mesoscale eddy at the time of the survey melville lay on the flank of a cyclonic eddy that had broken off the arc and was propagating west before remerging with the next meander of the arc nearby sapmer on the other hand was in the centre of an anticyclonic eddy resulting in very weak stratification over the seamount at the time of the survey middle of what lies most often on the northern flank of the arc in strong currents but was at the time of the survey near the edge of the same eddy as sapmer coral in the subtropical front south of the arc was in waters much colder fresher denser and more oxygenated than all the other seamounts walter was close to the path of eddies propagating southwest from east of madagascar while atlantis the furthest east and north seamount experienced the weakest eddy currents

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

partitioned waveform inversion and two dimensional structure under the network of autonomously recording seismographs nonlinear optimization methods are very effective in finding models that predict optimal waveform fits to observed seismograms provided the number of model parameters is small to reduce the size of the large scale two or three dimensional seismic modelling problem path integrals over seismic velocities may be defined as model parameters instead of the seismic velocities themselves this partitions the large scale optimization problem into a number of independent small scale problems and reduces the computational effort by several orders of magnitude the path integral parameters are transformed such that their correlation matrix becomes diagonal a linear inversion performs the mapping from the space of path integral parameters into the model space of seismic velocities and makes backus gilbert resolution analysis possible the method has been applied to body wave and surface wave data from the broadband portable network of autonomously recording seismographs nars in western europe using a surface wave ray approximation to generate synthetics by mode summation waveforms were fitted for frequencies as high as 50 mhz variations in s velocity as large as 6 1 have been established with a vertical resolution of 100 150 km and a horizontal resolution as small as 400 km in parts of the model although a low velocity layer is present under the southern part of the nars array such layer is very weak or absent north of the paris basin horizontal velocity gradients are well resolved down to 400 km depth

simultaneous inversion for 3 d variations in shear and bulk velocity in the mantle until recently most of the seismic tomographic modeling has been addressing the question of lateral heterogeneity either in p or s wave velocities the s wave velocity variations are larger and hence provide stronger signal on long period waveforms the direct p travel times being the first arrivals on the other hand are most frequently reported in the international seismological centre isc bulletins in mineral physics experiments the variation in bulk velocity is more often measured to better understand the differences between vp and vp patterns and better link the results from mineral physics to those of seismic tomography we formulate the inverse problem in terms of relative perturbations in the shear velocity vs 12 and bulk sound velocity v k 12 we use a large data set which consists of waveforms waveform derived travel times and travel times from the isc bulletins the earthquakes are relocated using corrections for lateral heterogeneity the events which cannot be reliably determined are discarded the model is defined as spherical harmonics to degree 12 horizontally and as chebyshev polynomials to order 13 radially for both shear and bulk sound velocity the inversion is performed under smoothness constraint the resolution tests and bootstrapping analysis indicate that the model is well recovered particularly at long wavelength the results indicate a much larger variability of shear than bulk sound velocity the model explains observations well the most intriguing result obtained in this study is that the variations in shear velocity and bulk sound velocity are negatively correlated in the lowermost mantle the explanation is not very clear from the mineral physics point of view it is not unlikely that this could be explained in terms of thermal variation even though we are unwilling to rule out the possibility of large wavelength compositional variations

lithosphere asthenosphere interaction beneath the western united states from the joint inversion of body wave traveltimes and surface wave phase velocities summary the relation between the complex geological history of the western margin of the north american plate and the processes in the mantle is still not fully documented and understood several pre usarray local seismic studies showed how the characteristics of key geological features such as the colorado plateau and the yellowstone snake river plains are linked to their deep mantle structure recent body wave models based on the deployment of the high density large aperture usarray have provided far more details on the mantle structure while surface wave tomography ballistic waves and noise correlations informs us on the shallow structure here we combine constraints from these two data sets to image and study the link between the geology of the western united states the shallow structure of the earth and the convective processes in mantle our multiphase dna10 s model provides new constraints on the extent of the archean lithosphere imaged as a large deeply rooted fast body that encompasses the stable great plains and a large portion of the northern and central rocky mountains widespread slow anomalies are found in the lower crust and upper mantle suggesting that low density rocks isostatically sustain part of the high topography of the western united states the yellowstone anomaly is imaged as a large slow body rising from the lower mantle intruding the overlying lithosphere and controlling locally the seismicity and the topography the large e w extent of the usarray used in this study allows imaging the slab graveyard a sequence of farallon fragments aligned with the currently subducting juan de fuca slab north of the mendocino triple junction the lithospheric root of the colorado plateau has apparently been weakened and partly removed through dripping the distribution of the slower regions around the colorado plateau and other rigid blocks follows closely the trend of cenozoic volcanic fields and ancient lithospheric sutures suggesting that the later exert a control on the locus of magmato tectonic activity today the dna velocity models are available for download and slicing at http dna berkeley edu

a 3 d shear velocity model of the crust and uppermost mantle beneath the united states from ambient seismic noise summary in an earlier study bensen et al measured surface wave dispersion curves from ambient noise using 203 stations across north america which resulted in rayleigh and love wave dispersion maps from 8 70 s period and 8 20 s period respectively we invert these maps in a two step procedure to determine a 3 d shear wave velocity model v s of the crust and uppermost mantle beneath much of the contiguous us the two steps are a linearized inversion for a best fitting model beneath each grid node followed by a monte carlo inversion to estimate model uncertainties in general a simple model parametrization is sufficient to achieve acceptable data fit but a rayleigh love discrepancy at periods from 10 to 20 s is observed in which simple isotropic models systematically misfit rayleigh and love waves in some regions crustal features observed in the model include sedimentary basins such as the anadarko green river williston basins as well as california s great valley and the mississippi embayment the east west velocity dichotomy between the stable eastern us and the tectonically deformed western us is shown to be abrupt in the crust and uppermost mantle but is not coincident in these regions crustal high velocity material tends to lap over the high velocities of the uppermost mantle the rayleigh love discrepancy between 10 and 20 s period is crustal in origin and is observed in a number of regions particularly in extensional provinces such as the basin and range it can be resolved by introducing radial anisotropy in the lower or middle crust with v sh v sh by about 1 per cent

tomographic inversion of p and pp data for aspherical mantle structure below the northwest pacific region summary to investigate the morphology of subducted slab in the mantle below northwest pacific island arcs we inverted traveltime residuals for aspherical variations in p wave propagation velocity relative to the radially symmetric iasp91 reference model the tomographic method used is based on a step wise linearization of the inversion problem first we relocated isc international seismological centre hypocentres with re identified p and pp phase data using the iasp91 traveltime tables the variance of p residuals relative to iasp91 traveltimes was 17 per cent less than the variance of p data reported by the isc relative to the jeffreys bullen j b traveltime tables second we performed a linearized lsqr inversion for earth structure and source relocation with the p and pp residuals obtained from the first step using iasp91 as the reference model for seismic velocities the incorporation of the depth phase pp in the tomographic inversions has two major advantages 1 the pp data provide constraints on focal depth and thus reduce the trade off between source relocation and structure and 2 the pp ray paths improve the sampling of earth structure in the shallow mantle and transition zone we used more than 2 times 106 and about 1 times 105p and pp wave traveltime residuals respectively from about 40 000 earthquakes with epicentres in the study region that were recorded at one or more of the 2300 globally distributed seismological stations considered in this study we assessed the spatial resolution in the tomographic images with checker board type sensitivity tests these tests reveal high resolution of upper mantle and transition zone structure particularly below the central part of our study region structure with wavelengths of the order of 100 km is resolved below japan whereas structure with wavelengths of the order of 300 km is well resolved below the kuril izu bonin and ryukyu arcs small scale structure is poorly resolved in depth below the northern part of the kuril kamchatka arc and below the izu bonin and mariana arcs this limits the interpretation of slab structure and mantle flow from tomographic images alone with this limitation in mind we conclude from the tomographic images that subducted slab deflects in the mantle transition zone below the geographical area encompassed by the kuril basin the japan sea and the northern part of the philippine sea this is in good agreement with the results of other recently published tomographic studies the occurrence of earthquakes several hundred kilometres off the inclined wadati benioff seismic zones and inferences about 660 km discontinuity topography in contrast slab like structures of high p wave velocity are imaged in the lower mantle below the deepest earthquakes of the northern kuril kamchatka and mariana seismic zones this is indicative of local slab penetration of the lower mantle from tomographic images we cannot discern between compositionally or thermally induced variations in seismic velocity however with regard to the nature of the boundary between upper and lower mantle our observations argue against either compositional mantle layering with large contrasts in intrinsic density or phase changes with steep clapeyron slopes

high resolution crustal model of se tibet from joint inversion of seismic p wave travel times and bouguer gravity anomalies and its implication for the crustal channel flow abstract we present a high resolution 3 d crustal model of southeastern tibetan plateau se tibet by joint inversion of local earthquake p wave travel times and bouguer gravity anomalies the joint inversion improves the resolution compared with travel time only tomography and more clearly depicts the spatial distribution of low velocity zones in the crust our joint inversion model finds no evidence for crustal channel flow in this part of se tibet however our velocity model shows three distinct low velocity zones in the mid crust which are generally associated with the major fault systems that control the deformation of se tibet in the cenozoic the thick mafic ultramafic underplating beneath the emeishan large igneous province effectively blocks the crustal flow from tibetan plateau and the crustal low velocity zone beneath the luzhijiang fault and xiaojiang fault is more likely due to localized asthenospheric upwelling at the southern end of nujiang fault the joint inversion model reveals the magmatic system beneath the tengchong volcano that facilitates melt migration from uppermost mantle to the magma chambers in the upper to mid crust

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

lithospheric expression of geological units in central and eastern north america from full waveform tomography abstract the earthscope ta deployment has provided dense array coverage throughout the continental us and with it the opportunity for high resolution 3d seismic velocity imaging of both lithosphere and asthenosphere in the continent building upon our previous long period waveform tomographic modeling in north america we present a higher resolution 3d isotropic and radially anisotropic shear wave velocity model of the north american lithospheric mantle constructed tomographically using the spectral element method for wavefield computations and waveform data down to 40 s period the new model exhibits pronounced spatial correlation between lateral variations in seismic velocity and anisotropy and major tectonic units as defined from surface geology in the center of the continent the north american craton exhibits uniformly thick lithosphere down to 200 250 km while major tectonic sutures of proterozoic age visible in the surface geology extend down to 100 150 km as relatively narrow zones of distinct radial anisotropy with vsv vsh notably the upper mantle low velocity zone is present everywhere under the craton between 200 and 300 km depth east of the continental rift margin the lithosphere is broken up into a series of large somewhat thinner 150 km high velocity blocks which extend laterally 200 300 km offshore into the atlantic ocean between the craton and these deep rooted blocks we find a prominent narrow band of low velocities that roughly follows the southern and eastern laurentia rift margin and extends into new england we suggest that the lithosphere along this band of low velocities may be thinned due to the combined effects of repeated rifting processes and northward extension of the hotspot related bermuda low velocity channel across the new england region we propose that the deep rooted high velocity blocks east of the laurentia margin represent the proterozoic gondwanian terranes of pan african affinity which were captured during the rodinia formation but left behind after the opening of the atlantic ocean our results suggest that recurring episodes of tectonic events that are well exposed at the surface also leave persistent scars in the continental lithosphere mantle marked by isotropic and radially anisotropic velocity anomalies that reach as deep as 100 150 km

relationship between observed upper mantle structures and recent tectonic activity across the southeastern united states the lithospheric structure of the southeastern united states is a product of earlier episodes of continental collision and breakup the region is located in the interior of the north american plate away from active plate margins however there is ongoing tectonism in the region with multiple zones of seismicity uplifting arches and cenozoic intraplate volcanism the mechanisms controlling this activity and the state of stress remain enigmatic two important factors are plate strength and preexisting inherited structures here we present new tomographic images of the upper mantle beneath the southeastern united states revealing large scale structural variations in the upper mantle examples include the relatively thick lithospheric mantle of stable north america that abruptly thins beneath the paleozoic appalachian orogeny and the slow upper mantle of the proterozoic reelfoot rift our results also indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering this provides a viable explanation for seismicity uplifting and young intraplate volcanism we postulate that not only tectonic inheritance but also continuing lithospheric foundering may control the ongoing activity of the region long after it became a passive margin based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere we propose that piecemeal delamination has occurred beneath the region throughout the cenozoic removing a significant amount of reworked deformed mantle lithosphere ongoing lithospheric foundering beneath the eastern margin of stable north america explains significant variations in thickness of lithospheric mantle across the former grenville deformation front

a simple approach to the joint inversion of seismic body and surface waves applied to the southwest u s 1 body and surface wave tomography have complementary strengths when applied to regional scale studies of the upper mantle we present a straight forward technique for their joint inversion which hinges on treating surface waves as horizontally propagating rays with deep sensitivity kernels this formulation allows surface wave phase or group measurements to be integrated directly into existing body wave tomography inversions with modest effort we apply the joint inversion to a synthetic case and to data from the ristra project in the southwest u s the data variance reductions demonstrate that the joint inversion produces a better fit to the combined dataset not merely a compromise for large arrays this method offers an improvement over augmenting body wave tomography with a one dimensional model the joint inversion combines the absolute velocity of a surface wave model with the high resolution afforded by body waves both qualities that are required to understand regional scale mantle phenomena index terms 7218 seismology lithosphere and upper mantle 7255 seismology surface waves and free oscillations 7294 seismology instruments and techniques 8109 tectonophysics continental tectonics extensional 0905 8180 tectonophysics tomography citation west m w gao and s grand 2004 a simple approach to the joint inversion of seismic body and surface waves applied to the southwest u s geophys res lett 31 l15615

heterogeneity and anisotropy of the lithosphere of se tibet from surface wave array tomography 1 understanding the geotectonic evolution of the southeastern tibetan plateau requires knowledge about the structure of the lithosphere using data from 77 broadband stations in sw china we invert rayleigh wave phase velocity dispersion curves from ambient noise interferometry t 1 0 40 s and teleseismic surface waves t 2 0 150 s for 3 d heterogeneity and azimuthal anisotropy in the lithosphere to 150 km depth our surface wave array tomography reveals 1 deep crustal zones of anomalously low shear wave speed and 2 substantial variations with depth of the pattern of azimuthal anisotropy upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern himalayan syntaxis with fast directions generally parallel to the main strike slip faults the mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south the tomographically inferred 3 d variation in azimuthal anisotropy helps constrain the source region of shear wave splitting south of 26 n off the high plateau most of the observed splitting can be accounted for by upper mantle anisotropy but for stations on the plateau proper with thick crust crustal anisotropy cannot be ignored on long wavelengths the pattern of azimuthal anisotropy in the crust differs from that in the mantle this is easiest explained if deformation varies with depth the deep crustal zones of low shear wave speed and presumably mechanical strength may represent loci of ductile deformation but their lateral variation suggests that in se tibet localized crustal channel flow and motion along the major strike slip faults are both important

vorticity and vertical circulation at an ocean front abstract density and velocity data with 4 km horizontal resolution from a survey of a front during fasinex frontal air sea interaction experiment are combined to describe the structure of the top 300 m of the ocean the geostrophic velocity field is derived and is used to examine the relative importance of stratification relative vorticity and twisting terms in ertel s potential vorticity q tenfold isopycnic changes in q are found across a horizontal scale of only 10 km these changes are confined to isopycnals that outcrop from the seasonal thermocline into the mixed layer the ageostrophic velocity field is quantified by solution of the omega equation and vertical velocities of up to 40 m day 1 are found small 40 km surface trapped top 200 m eddies are found to play a crucial role in the transport and effective diffusion of properties across the thermocline out of the mixed layer

an introduction to the physical oceanography of six seamounts in the southwest indian ocean exploratory surveys of six seamounts in the southwest indian ocean provide a description of physical processes induced by seamounts along the southwest indian ridge mean currents 15 25 cm s 1 in the vicinity of each seamount were dominated by mesoscale eddies the dominant seamount driven process was the generation of internal tides by the barotropic tide interacting with the seamount crests this led to enhanced shear in the vicinity of the crests resulting in mixing where stratification was weak for example in the core of an anticyclonic mesoscale eddy or where there had been a winter mixed layer tidally driven up and downwelling was observed at the seabed with associated variability in bottom temperature of up to 3 c over a tidal cycle vertical displacement of isopycnals by internal tidal waves reached 200 m peak to trough fluorescence in the surface eutrophic layer could thus extend down to the seamount crest on each tidal cycle apparently spatial variations in short conductivity temperature depth sections across each seamount were probably aliased temporal variations from the strong tidal signal evidence for taylor caps or other potential trapped circulations at the seamount crest was weak most likely because currents associated with mesoscale eddies were too strong to allow their formation

constraining p wave velocity variations in the upper mantle beneath southeast asia we have produced a p wave model of the upper mantle beneath southeast se asia from reprocessed short period international seismological centre isc p and pp data short period p data of the annual bulletin of chinese earthquakes abce and long period pp p data we used 3d sensitivity kernels to combine the datasets and mantle structure was parameterized with an irregular grid in the best sampled region our data resolve structure on scale lengths less than 150 km the smearing of crustal anomalies to larger depths is reduced by a crustal correction using an a priori 3d model our tomographic inversions reveal high velocity roots beneath the archean ordos plateau the sichuan basin and other continental blocks in se asia beneath the himalayan block we detect high seismic velocities which we associate with subduction of indian lithospheric mantle this structure is visible above the 410 km discontinuity and may not connect to the remnant of the neo tethys oceanic slab in the lower mantle our images suggest that only the southwestern part of the tibetan plateau is underlain by indian lithosphere and thus that the upper mantle beneath northeastern tibet is primarily of asian origin our imaging also reveals a large scale high velocity structure in the transition zone beneath the yangtze craton which could have been produced in multiple subduction episodes the low p wave velocities beneath the hainan island are most prominent in the upper mantle and transition zone they may represent counter flow from the surrounding subduction zones and may not be unrelated to processes beneath eastern tibet 2005 elsevier b v all rights reserved

tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases pkp pdiff the mapping of variations in p wave speed in the deep mantle is restricted by the uneven sampling of p waves in particular beneath the southern hemisphere to enhance data coverage we augmented the 1 6 million summary rays of p pp and pwp that we used in previous studies with differential travel times of diffracted and refracted core phases for the core refracted differential travel time residuals pkpab pkpdf and pkpab pkpbc we used 1383 cross correlated digital waveforms as well as 27 000 routinely processed bulletin data we used the waveform data to define quality criteria for the selection and reduction of the bulletin pkp data for pkpdf pdiff we only considered 543 records derived from waveform cross correlation no pcp data were used in this study we used optical ray theory to calculate the ray paths associated with the p pp pwp and pkp data which are measured at 1 hz however to account for the large fresnel zones of the low frequency 50 mhz pkpdf pdiff data we estimated the three dimensional shape of the frechet sensitivity kernels from kernels calculated by normal mode summation the use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high frequency data to constrain small scale structure the differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary cmb however images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle here we do not make a priori assumptions about the mantle source of anomalous time differentials from test inversions we conclude that both upper and lower mantle structures that are poorly resolved by p data can be mapped into the core along pkp paths but that the effect of outer core structures if any on the mantle model is small compared to the inversion of the p pp and pwp alone the inclusion of the pkpab pkpdf and pkpab pkpbc and pkpdf pdiff data improves the resolution of structure beneath 2200 km depth in particular the joint inversion puts better constraints on the long wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the cmb that is in agreement with but still smaller than inferences from shear wave studies resolution tests indicate that in some regions the enhanced definition of structure is significant but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

a moore s cellular automaton model to get probabilistic seismic hazard maps for different magnitude releases a case study for greece abstract cellular automata are simple mathematical idealizations of natural systems and they supply useful models for many investigations in natural science examples include sandpile models forest fire models and slider block models used in seismology in the present paper they have been used for establishing temporal relations between the energy releases of the seismic events that occurred in neighboring parts of the crust the catalogue is divided into time intervals and the region is divided into cells which are declared active or inactive by means of a threshold energy release criterion thus a pattern of active and inactive cells which evolves over time is determined a stochastic cellular automaton is constructed starting with these patterns in order to simulate their spatio temporal evolution by supposing a moore s neighborhood interaction between the cells the best model is chosen by maximizing the mutual information between the past and the future states finally a probabilistic seismic hazard map is given for the different energy releases considered the method has been applied to the greece catalogue from 1900 to 1999 the probabilistic seismic hazard maps for energies corresponding to m 4 and m 5 are close to the real seismicity after the data in that area and they correspond to a background seismicity in the whole area this background seismicity seems to cover the whole area in periods of around 25 50 years the optimum cell size is in agreement with other studies for m 6 the optimum area increases according to the threshold of clear spatial resolution and the active cells are not so clustered the results are coherent with other hazard studies in the zone and with the seismicity recorded after the data set as well as provide an interaction model which points out the large scale nature of the earthquake occurrence

multimode rayleigh wave inversion for heterogeneity and azimuthal anisotropy of the australian upper mantle summary we present an azimuthally anisotropic 3 d shear wave speed model of the australian upper mantle obtained from the dispersion of fundamental and higher modes of rayleigh waves we compare two tomographic techniques to map path average earth models into a 3 d model for heterogeneity and azimuthal anisotropy method i uses a rectangular surface cell parametrization and depth basis functions that represent independently constrained estimates of radial earth structure it performs an iterative inversion with norm damping and gradient regularization method ii uses a direct inversion of individual depth layers constrained by bayesian assumptions about the model covariance we recall that bayesian inversions and discrete regularization approaches are theoretically equivalent and with a synthetic example we show that they can give similar results the model we present here uses the discrete regularized inversion of independent path constraints of method i on an equal area grid with the exception of westernmost australia we can retrieve structure on length scales of about 250 km laterally and 50 km in the radial direction to within 0 8 per cent for the velocity 20 per cent for the anisotropic magnitude and 20 for its direction on length scales of 1000 km and longer down to about 200 km there is a good correlation between velocity heterogeneity and geologic age at shorter length scales and at depths below 200 km however this relationship breaks down the observed magnitude and direction of maximum anisotropy do not in general appear to be correlated to surface geology the pattern of anisotropy appears to be rather complex in the upper 150 km whereas a smoother pattern of fast axes is obtained at larger depth if some of the deeper directions of anisotropy are aligned with the approximately n s direction of absolute plate motion this correspondence is not everywhere obvious despite the fast 7 cm yr 1 northward motion of the australian plate more research is needed to interpret our observations in terms of continental deformation predictions of sks splitting times and directions an integrated measure of anisotropy are poorly matched by observations of shear wave birefringence

high resolution teleseismic tomography of upper mantle structure using an a priori three dimensional crustal model summary the effect of an a priori known 3 d crustal model in teleseismic tomography of upper mantle structure is investigated we developed a 3 d crustal p wave velocity model for the greater alpine region encompassing the central and western alps and the northern apennines to estimate the crustal contribution to teleseismic traveltimes the model is constructed by comparative use of published information from active and passive seismic surveys the model components are chosen to represent the present large scale alpine crustal structure and for their significant effect on the propagation of seismic wavefields they are first order structures such as the crust mantle boundary sedimentary basins and the high velocity ivrea body teleseismic traveltime residuals are calculated for a realistic distribution of azimuths and distances by coupling a finite difference technique to the iasp91 traveltime tables residuals are produced for a synthetic upper mantle model featuring two slab structures and the 3 d crustal model on top of it the crustal model produces traveltime residuals in the range between 0 7 and 1 5 s that vary strongly as a function of backazimuth and epicentral distance we find that the non linear inversion of the synthetic residuals without correcting for the 3 d crustal structure erroneously maps the crustal anomalies into the upper mantle correction of the residuals for crustal structure before inversion properly recovers the synthetic slab structures placed in the upper mantle we conclude that with the increasing amount of high quality seismic traveltime data correction for near surface structure is essential for increasing resolution in tomographic images of upper mantle structure

descent of the ancient farallon slab drives localized mantle flow below the new madrid seismic zone 1 the great earthquake sequence that occurred in the central mississippi river valley during the winter of 1811 1812 is unprecedented in the historical record of seismicity within stable continental plate interiors we show using viscous flow models based on high resolution seismic tomography that the descent of the ancient farallon slab into the deep mantle beneath central north america induces a highly localized flow directly below the new madrid seismic zone nmsz this localization arises because of structural variability in the farallon slab and the low viscosity of the sub lithospheric upper mantle and it represents a heretofore unrecognized and possibly significant driving mechanism for the enigmatic intraplate seismicity

global upper mantle tomography with the automated multimode inversion of surface and s wave forms summary we apply the automated multimode inversion of surface and s wave forms to a large global data set verify the accuracy of the method and assumptions behind it and compute an s vvelocity model of the upper mantle crust 660 km the model is constrained with 51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations structure of the mantle and crust is constrained by waveform information both from the fundamentalmode rayleigh waves periods from 20 to 400 s and from s and multiple s waves higher modes in order to enhance the validity of the path average approximation we implement the automated inversion of surface and s wave forms with a three dimensional 3 d reference model linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3 d reference in this study composed of a 3 d model of the crust and a one dimensional 1 d global average depth profile in the mantle below waveform information is related to shear and compressional velocity structure within approximate waveform sensitivity areas we use two global triangular grids of knots with approximately equal interknot spacing within each a finely spaced grid for integration over sensitivity areas and a rougher spaced one for the model parametrization for the tomographic inversion we use lsqr with horizontal and vertical smoothing and norm damping we invert for isotropic variations in s and p wave velocities but also allow for s wave azimuthal anisotropy in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity the lateral resolution of the resulting isotropic upper mantle images is a few hundred kilometres varying with data sampling we validate the imaging technique with a spectral element resolution test inverting a published global synthetic data set computed with the spectral element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately this test confirms both the accuracy of the implementation of the method and the validity of the jwkb and path average approximations as applied in it reviewing the tomographic model we observe that low s v velocity anomalies beneath mid ocean ridges and backarc basins extend down to 100 km depth only shallower than according to some previous tomographic models this presents a close match to published estimates of primary melt production depth ranges there in the seismic lithosphere beneath cratons unambiguous high velocity anomalies extend to 200 km pronounced low velocity zones beneath cratonic lithosphere are rare where present south america tanzania they are neighboured by volcanic areas near cratonic boundaries the images of these low velocity zones may indicate hot material possibly of mantle plume origin trapped or spreading beneath the thick cratonic lithosphere

observation of a mesoscale eddy dipole on the northern madagascar ridge consequences for the circulation and hydrography in the vicinity of a seamount abstract based on satellite and in situ data the dynamic characteristics and vertical structure of a surface intensified mesoscale eddy dipole recently expelled from the south east madagascar current semc is described for the first time the dipole was surveyed 250 nautical miles south of madagascar between 14 and 23 november 2016 during west east and south north transects carried out over the northern madagascar ridge the dipole consisted of two counter rotating vortices of similar size 100 km and intensity 0 7 f and an intense southwestward jet 150 cm s 1 in the frontal region between the two eddies the cyclonic eddy was lying on the western side of the anticyclonic eddy with azimuthal velocities reaching 100 cm s 1 at the surface and decreasing slowly with depth 40 cm s 1 at 600 m this dipole was defined as a highly non linear ro 0 7 isolated eddy type structure c 11 cm s 1 and u c 0 7 capable of trapping and advecting water masses over large distances the enhanced concentration of chlorophyll a found in the cyclone relative to the anticyclone could be tracked back to the spin up phase of the two eddies and attributed to eddy pumping the eddy cores were located above the pycnocline 1026 4 kg m 3 within the upper 600 m and consisted of varieties of subtropical underwater stuw found within the semc the stuw found in the anticyclone was more saline and oxygenated than in the cyclone highlighting mixing with the inshore shelf waters from the southeastern coastal upwelling cell off madagascar observations suggest that the dipole interacted strongly with the chaotic bathymetry of the region characterized by a group of five seamounts lying between 240 m and 1200 m the bathymetry blocked its westward advection trapping it in the vicinity of one shallow seamount for more than 4 weeks so enhancing the role of the eddy induced velocities in stirring the surrounding water masses squeezed between the southern madagascan shelf and the northern flank of the anticyclone two filament like dynamic features with very different water mass properties could be observed on the south north transect i one filament highly concentrated in chlorophyll a demonstrating the capacity of the eddy to export shelf water offshore ii intrusions of southern type stuw generally found south of the south indian counter current sicc recirculating on the external flanks of the anticyclone although the observed circulation and hydrography were largely constrained by the presence of the mesoscale eddy dipole unmistakable fine scale dynamics were also observed in the vicinity of the mad ridge seamount superimposed onto the mesoscale eddy flow

the northern gulf anomaly p and s wave travel time delays illuminate a strong thermal feature beneath the northern gulf of mexico abstract seismic measurements are used in a detailed investigation of a region of extremely low asthenospheric seismic velocities along the us gulf coast first imaged in continental scale geotomography which we term the northern gulf anomaly nga differential p and s wave arrival times from teleseisms at a variety of back azimuths observed on earthscope transportable array stations near the us gulf coast demonstrate that asthenospheric seismic velocities are 8 10 lower than the neighboring craton and define the spatial extent and character of the anomaly travel time anomalies are calculated relative to the ak135 earth model and corrected to account for the effect of the kilometers thick sedimentary cover in the region the nga is most intense at the southernmost coast of louisiana and east texas with an eastern edge at 89 w and smoothly tapers away in a triangular wedge that extends inland as far as 300 km it has sharper edges and a smaller areal extent by 50 than previously published geotomography has indicated both the magnitude and ratio of delays indicate that the nga has a thermal origin which may represent past or present day small scale convective upwelling near the southeastern edge of the north american continent the nga suggests that large scale but poorly understood asthenospheric processes are at work beneath the us gulf coast notwithstanding this region s reputation as an aseismic passively subsiding continental margin

new images of the earth s upper mantle from measurements of surface wave phase velocity anomalies 1 the sensitivity of a surface wave to the lateral structure of the earth s mantle depends on the local structure and thickness of the crust particularly at short periods the strong lateral heterogeneity of the crust has a significant nonlinear effect on the surface wave sensitivity functions thus far this effect has not been quantified and tomographic studies of the upper mantle based on surface wave observations have not taken it into account on the basis of a jeffreys wentzel kramers brillouin jwkb description of surface wave propagation we find and discuss the lateral variations of the sensitivity of love and rayleigh waves to mantle seismic anomalies associated with a realistic model of the crust mooney et al 1998 we then use the resulting laterally varying sensitivity kernels to determine a new tomographic image of horizontally and vertically polarized shear velocity in the upper mantle from love and rayleigh wave phase velocity measurements periods from 35 to 300 s in terms of large scale patterns of high and low velocity our model is similar to previously published ones confirming among other features the anomalous anisotropy underlying the central pacific ekstrom and dziewonski 1998 we find however that the lateral dependence of the surface wave sensitivity kernels has a significant effect on the absolute amplitude of the anomalies which should not be neglected as we attempt to obtain higher resolution images of the upper mantle