a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

accurate modelling and inversion of electrical resistivity data in the presence of metallic infrastructure with known location and dimension electrical resistivity tomography ert has been widely used in environmental applications to study processes associated with subsurface contaminants and contaminant remediation anthropogenic alterations in subsurface electrical conductivity associated with contamination often originate from highly industrialized areas with significant amounts of buried metallic infrastructure the deleterious influence of such infrastructure on imaging results generally limits the utility of ert where it might otherwise prove useful for subsurface investigation and monitoring in this manuscript we present a method of accurately modeling the effects of buried conductive infrastructure within the forward modeling algorithm thereby removing them from the inversion results the method is implemented in parallel using immersed interface boundary conditions whereby the global solution is reconstructed from a series of well conditioned partial solutions forward modeling accuracy is demonstrated by comparison with analytic solutions synthetic imaging examples are used to investigate imaging capabilities within a subsurface containing electrically conductive buried tanks transfer piping and well casing using both well casings and vertical electrode arrays as current sources and potential measurement electrodes results show that although accurate infrastructure modeling removes the dominating influence of buried metallic features the presence of metallic infrastructure degrades imaging resolution compared to standard ert imaging however accurate imaging resultsmore may be obtained if electrodes are appropriately located less

detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

sediment properties along gradients of siltation in south east asia marine sediment properties were quantified along se asian siltation gradients and across different habitat types to assess the extent of terrestrial sediment influence and derive indicators of of terrestrial sediment input three sites in the andaman sea ranong phuket and trang thailand one site in the gulf of thailand pak phanang thailand and three sites in the south china sea bolinao at the luzon island puerto galera at the mindoro island and el nido at the palawan island all in the philippines were studied the following main habitat types were covered mangroves seagrass beds coral reefs mud flats river mouths and deep channels particularly in bolinao and el nido distinct fronts in siltation were identified at about 7 km from the source mud 250 m total and inorganic carbon as well as calcium interstitial ammonia and phosphate varied with the fine sand fraction along the second axis two way analysis of variance showed that water depth distance from silt source site and habitat type all contributed to the variance but site explained most linear regressions showed positive correlations between silt and water content organic matter total nitrogen total phosphorus and iron but negative correlations with calcium suggesting iron and calcium as markers for terrigeneous and marine origin respectively the composition of the sediment particulate matter groups the habitat types in two clusters 1 silty types as river mouths shallow mud bottoms deep channels and mangrove stands with high contents of organic matter and nutrients versus 2 non silty types as seagrass beds and coral reefs with low contents of nutrients and organic matter median settling velocities of the silt loads varied from 0 6 m d 1 to 27 m d 1 experimentally determined susceptibility to resuspension identified a critical water content of 50 2002 elsevier science ltd

2d optimized electrode arrays for borehole resistivity tomography and co 2 sequestration modelling modern optimization approaches for electrode configurations can significantly improve the resolution of 2 5d resistivity imaging surveys this study presents a brief review of the 2 5d optimization approach particularly for borehole borehole surveys with applications for mapping virtual co2 plumes sequestrated in deep saline reservoir formations the applied algorithm searches for arrays that maximize the spatial resolution of the survey among the comprehensive dataset of best possible spatial resolution i e least temporal resolution a main goal of this study is to increase the temporal resolution of ert borehole borehole surveys by selecting optimized electrode configurations in order to minimise the required data acquisition time while sustaining a high spatial resolution the optimized dataset starts with a base set and is iteratively increased based on the model resolution matrix r until the required number of data points is achieved among four different optimization methods the compare r cr method of the best resolution is applied to directly calculate r for each new array added to the optimized dataset small optimized datasets generated by this technique are only 0 95 i e almost the same resolution with increasing the size of the optimized dataset during its generation the algorithm progressively enhances r r values in the central interwell region of low sensitivities and low resolution far higher than in the near borehole region of high sensitivities also the inverted tomogram reliability increases by increasing the optimized data size briefly the optimized arrays improve the resolution in the interwell region which is commonly low in borehole borehole ert studies the inverted output model is evaluated quantitatively using the model difference relative to the input model the results reflect the common smearing effects and artefacts of varying degrees that overpredict volumes underpredict magnitudes and blur boundaries of the target anomalies this input model is a synthetic resistivity model that was used to generate synthetic forward solution data used during the inversion applications on synthetic co2 models show that the mapping resolution for optimized datasets is better than that for other highly resolving arrays of the same number of data points problems of smeared boundaries and thin layers are less visible in the optimized array than in the other highly resolving arrays

the depth distribution of mantle anisotropy beneath the tonga subduction zone abstract shear wave splitting recorded by a temporary deployment of broadband seismometers located above the tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the tonga back arc region splitting parameters obtained for local s and teleseismic sks phases were modeled using a ray based method that predicts shear wave splitting on individual phase paths and determines the best fitting values of anisotropic strength orientation and depth extent splitting in teleseismic sks phases is identical to that in s phases from local earthquakes that occur at the base of the transition zone demonstrating that there is no significant splitting due to anisotropy in the lower mantle splitting times for local s phases do not vary significantly with depth and for models in which the strength of anisotropy is laterally uniform they rule out anisotropy in the transition zone finally the observed splitting indicates that anisotropy of more than 0 8 exists in the upper mantle with a fast symmetry axis at 60 w roughly parallel to the absolute plate motion vector of the subducting pacific lithosphere

archaeological investigation by electrical resistivity tomography a preliminary study b this paper describes a method in which vertical resistivity sections are generated tomographically from measurements on a linear array of equally spaced electrodes inserted at the ground surface the array is multiplexed to a resistivity meter which gathers one set of all possible independent apparent resistivity measurements and the geophysical section is then reconstructed by backprojecting these weighted data along equipotentials into the subsurface the technique has been evaluated numerically and in field trials over shallow archaeological structures at fountains abbey

melt extraction from partially molten regions beneath mid ocean ridges abstract a complete set of two dimensional equations for conservation of mass momentum and energy of a two phase system of melt in a deformable matrix is used to derive analytically a model for the vertical distribution of the melt fraction also a set of asymptotic relations for parameters in the velocity field of the matrix and the melt in the neighbourhood of the symmetry axis of sea floor spreading is obtained these relations have been used to derive solutions appropriate to the melt extraction from the partially molten regions beneath mid ocean ridges and the conditions for their existence the results show the convergence of the melt streamlines towards the spreading center it is caused by 1 a horizontal gradient of the piezometric pressure associated with the deformations of the matrix and 2 an upward increase of the porosity due to melting for accepted values of the relevant parameters a focusing effect of melt towards spreading centers exists thereby providing an explanation for the mid ocean paradox of a narrow region of ridge axis volcanism overlying a broad melt production zone

alternatives to core mantle boundary topography abstract nuclear explosions at the french test sites in the south pacific have been recorded at the regional seismic networks in the western united states a total of approximately 600 good quality short period seismograms were selected from an initial set of 900 the dense receiver spacing particularly in california allows the division of the networks into numerous subarrays and the data from each subarray can be analysed using conventional array techniques such as beam forming and frequency wave number analysis the seismic rays provide dense sampling of a small area of the core mantle boundary cmb beneath the eastern pacific from 5 to 13 n and 133 to 126 w pcp has a shallow incidence angle on the cmb at an epicentral distance of 65 and d is sampled over a large swathe from 2 s to 24 n and from 134 to 123 w beam traces have been formed for the p and pcp phases for each of the subarrays using slowness and azimuth values derived from array methods good quality beams have been selected for further analysis by applying a matched filter to the beamed traces an accurate set of pcp p differential travel times has been determined and compared with iasp91 the residuals show a smooth trend over the area sampled for the given epicentral distances p and pcp have similar paths through the crust and upper mantle so we make the assumption that near source and near receiver structures have negligible effect on the differential times we also assume that the midmantle is relatively homogenous many previous studies have interpreted pcp p residuals in terms of cmb topography by mapping the residuals directly to topography a range in elevation of 40 km over a lateral extent of about 700 km results alternatively we attempted to interpret the residuals in terms of velocity heterogeneity in d a range of d models have been tested including lateral velocity variation in a d layer with constant thickness the pcp p residuals may be explained by allowing the p wave velocity to vary laterally by a factor of 2 over the region sampled with most of the region being marginally fast this suggests that d heterogeneity has large spectral power at relatively short wavelengths in this scenario previous tomographic inversions for cmb topography will have large errors since in these inversions corrections for d structure are based on low order spherical harmonic models

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

seismic evidence of partial melt within a possibly ubiquitous low velocity layer at the base of the mantle three source regions show evidence of a low velocity layer that is less than 15 kilometers thick on top of the core mantle boundary and require about a 3 1 ratio of shear to compressional velocity reduction which is consistent with partial melt layer thickness is correlated with travel time residuals of the seismic phases that are most sensitive to the lowermost mantle velocity these observations suggest that the layer is thinned beneath downwellings but is present everywhere along the core mantle boundary low viscosity accompanying partial melt can localize the upwelling of warmed mantle making the low velocity layer a plausible source of mantle plumes

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

lower mantle structure from scs s differential travel times knowledge of the spectrum of structure in the lower mantle is crucial to our understanding of the dynamical evolution of the earth and in principle can be inferred from the analysis of global seismic data sets the long wavelength compressional velocity structure has usually been constrained through tomographic inver sion of the isc catalogue of p wave travel times1 3 whereas shear velocity has been inferred from waveform modelling of long period shear waves4 6 the models that have been produced are similar in only the largest scale features7 and explain little of the variance of the raw data used in their construction thus there is a legitimate concern that the current generation of global scale models give only a crude approximation to the largest scale structure and that there may be significant aliasing of short wavelength heterogeneity here we use scs s differential travel times to demonstrate that the three dimensional structure of the lower mantle is indeed dominated by continental scale features particularly convincing is the fact that the features are apparent in the raw data and are not the product of a complicated modelling procedure

simulation of electrical conduction in geomaterials by spice electrical responses of the subsurface can be used to identify geologic strata locate anomalies detect and delineate contamination among many other applications all these applications depend on the spatial variations of electrical properties in the subsurface and the resulting flow pattern of electric current due to the heterogeneity of the subsurface and complex boundary conditions three dimensional electric current flow problems are not easy to analyze in particular when the response is frequency and or time dependent in this paper a method of electric circuit analogy is proposed to simulate the electrical responses of geomaterials using the circuit simulator spice the technique will allow simulation of more complex electrical conduction behavior of geomaterials without much extra effort the excellent agreement between simulated results and analytical solutions developed for surface geophysical techniques establishes the viability of the method limitations of the approach and potential solutions to relax these limitations and other potential applications of the technique in geosciences are also discussed

dynamical geochemistry of the hawaiian plume this paper presents a simple dynamical model for melting and trace element distribution in the hawaiian mantle plume i model the plume as a partially molten stagnation point flow against the oceanic lithosphere and obtain solutions for the temperature melt migration rate and trace element concentration within it trace element concentrations in the melt exceed simple batch melting predictions by up to 70 the magnitude of this effect depends strongly on the solid melt partition coefficientk trace elements with differentk therefore experience a dynamical fractionation within the plume and incompatible trace element ratios such asla ce always exceed the batch melting predictions i suggest a simple model for plume lithosphere interaction in which melts from these two sources mix in proportions determined by thermodynamic constraints the model can explain the composition of basalts from haleakala if the degree of melting of the lithospheref1 decreases with time from roughly 10 for tholeiites to 2 for alkalic basalts these values are considerably higher than previous estimates off1 1 and imply correspondingly smaller and more realistic values 10 km for the thickness of the melted part of the lithosphere partial melting of additional depleted sources such as the asthenosphere is therefore not required by the haleakala data estimates off1 are highly sensitive to the values chosen for the partition coefficients however and should therefore be interpreted with caution

the generation and composition of partial melts in the earth s mantle abstract a set of equations is presented which combines the constraints of fluid dynamics and multicomponent phase equilibrium to provide a unified description of partial melting in the earth s mantle the equations are applied to a one dimensional model for pressure release melting of a simplified mantle material which contains only two chemical components exhibiting either a complete solid solution or b a binary eutectic in both cases melting occurs over a range of depths the unmelted crystalline residue matrix is modeled as a saturated porous medium through which the melt can migrate because of its differential buoyancy since melt interacts continuously with the matrix during ascent melting occurs by equilibrium rather than fractional fusion this equilibrium fusion is not the same as batch fusion however since material elements are quickly dispersed by migration of melt relative to the matrix to a first approximation the temperature profiles adiabats in the partially molten zone are independent of melt migration the slope of the adiabats varies in inverse proportion to the number of degrees of freedom which characterizes the melting melting of a complete solid solution occurs along a wet adiabat whose slope is controlled by absorption of latent heat melting of a eutectic system occurs along a steeper univariant adiabat until one solid phase is exhausted and subsequently along a wet adiabat the velocity of melt migration can exceed the mantle upwelling velocity by an order of magnitude or more the volume fraction of melt present is always less than the fraction of the material which has melted and is unlikely to exceed a few percent for a wide range of initial conditions melting of a eutectic system produces erupted melts having constant major element composition and widely varying trace element composition this result may provide a partial explanation for the characteristic major and lil element patterns observed in morb liquid compositional paths during pressure release melting cannot be determined from isobaric sections of phase diagrams and can only be calculated by solving the energy equation subject to the appropriate depth dependent phase equilibrium constraints

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

3d geoelectric tomography and archaeological applications following a previous paper in which the principles of a 3d ground surface tomographic processing of self potential data were established we extend the method to active source geoelectric surveying the main purpose of the new tomographic approach is to obtain a physical image reconstruction of the induced electric charges distributed over buried resistivity discontinuities the information is produced in a probabilistic sense as the mathematical formulation underlying the method treats only the intrinsic physical nature of the generated electric field underground and the method of its ground surface detection independently of the geometry of the unknown structures in practice a 3d tomography is realized by cross correlating a set of distributed electric field ground surface data with a scanning function representing a unit positive point charge located anywhere in the lower half space the resolution of the method is tested on the synthetic response of a 3d structural simulation of an archaeological target consisting of an infinitely resistive prismatic body immersed in a half space including surface inhomogeneities and layering finally the field response of a 3d structure consisting of a hypogeal dromos chamber tomb inside the sabine necropolis at colle del forno close to rome is presented and discussed

lateral variations near the core mantle boundary differential travel times between s and sks ts sks and skks and sks tskks sks along with amplitude behavior have been used to study lateral variations at the core mantle boundary the data were recorded by the world wide seismographic station network wwssn and the canadian seismograph network csn from source regions in south america and the fiji tonga area a core mantle boundary region traversed by sks and s beneath the mid pacific from the fiji tonga area to north american stations shows the most anomalous behavior in ts sks sks and s are anomalously separated by up to 8 seconds in addition to ts sks anomalies sks and skks are anomalously separated by 2 to 3 seconds a model fitting ts sks for this anomalous region requires one or more of the following 1 up to 5 increase in the top 300 km of outer core velocity 2 a large scale high vs region 2 increase in the lower mantle ne of tonga 3 lateral variations in d shear wave velocities of up to 3 however 1 above predicts a decrease in tskks sks contradicting the observations this suggests that 2 and 3 are more likely explanations in modeling this data set

modelling seismic reflections from d using the kirchhoff method abstract the observation of seismic reflections from the top of d is restricted to the epicentral distance range near the critical angle where the amplitude is large and a phase distortion occurs this phase advance in the reflected wavelet is examined and implications for commonly used seismic processing techniques such as phase picking stacking and deconvolution are made kirchhoff synthetics are generated to study several models of d reflectors such as steeply inclined interfaces and laterally varying velocity contrasts observations of strong amplitude variations in seismic reflections the bounce points of which lie well within a fresnel zone are successfully explained in a search for reflections from d investigators have exclusively used models of the d layer parallel to the core mantle boundary we speculate about a steep slope as it may be produced by deposited slab material and model the influence of such features on travel time and waveform

global shear velocity heterogeneities in the d layer inversion from sd sks differential travel times a global map of shear velocity in the d layer results from the inversion of 340 differential travel times of diffracted s sh minus sks sv sd sks from long period records of global seismic networks the two phase design reduces contamination from upper mantle heterogeneities and errors in location and origin time of the events additional corrections are made for 1 azimuthal anisotropy at stations where shear wave splitting parameters are available and for 2 travel time perturbations due to lower mantle asphericity although both effects are minor compared with the observed residuals with respect to the preliminary reference earth model prem dziewonski and anderson 1981 the corrected residuals ranging from 16 to 18 s are attributed to anomalies in d sampled by both phases taking these residuals as data and assuming a constant 250 km thick d layer we invert for a lateral velocity variation model of d using spherical harmonics in parameterizing d velocities a high degree expansion l 14 avoids aliasing but only the reliably determined low degree components li l produce optimal models the optimal model li 6 achieves a variance reduction of 56 and mean error 0 019 km s and the velocity is insensitive to the thickness of the d layer assumed a baseline shift of 0 022 km s from prem is resolved the magnitude of variation 0 2 km s or 3 is nearly twice as large as that of tanimoto 1990 and su et al 1994 while comparable to that of liu and dziewonski 1994 which includes s sks data and li and romanowicz 1996 degree 2 variation dominates the d with positive anomalies beneath the asian australian and american continents and southernmost pacific and negative anomalies beneath the pacific western africa atlantic and the southern indian ocean this pattern is similar to that found in previous tomograpic models significant discrepancies in nondegree 2 features are discussed in terms of the constraints from sd the fast and slow regions of d correlate with the inferred paleoslab positions and the major hotspot distribution respectively both at the 99 confidence level implications of these correlations for mantle dynamics are suggested the global model agrees reasonably well with the velocities determined regionally from the sd slowness measurement and the scs s residuals in other studies the latest radial models of d constrained from s triplications at distances of 75 90 provide a completely independent benchmark models for eurasia alaska and the caribbean predict sd travel times corresponding to 0 03 0 08 km s anomalies consistent with our lateral variation model in respective regions these vertical structures are also consistent with this study in scs times indicating that they agree in a vertically averaged sense major inconsistency exists for the india model syll young and lay 1987b which predicts sd and scs arrivals much slower than those expected from the 0 1 km s fast anomaly we found suggesting either the presence of strong lateral heterogeneities or the need for further mapping in this area

integrating pedological and geophysical methods to enhance the informative value of an archaeological prospection the example of a roman villa rustica near regensburg germany abstract geophysical prospection techniques are widely used to visualize the buried past various methods such as magnetometry electric resistance mapping and electric resistivity tomography and ground penetrating radar yield different results the use of all three techniques in combination with aerial photography interpretation and pedological mapping is highly effective but it requires a multi layer approach this paper presents such a multi layer approach carried out at a site with buried remnants of a roman villa rustica in southern germany the integration of the various results into a geographical information system leads to a geocodation of all outcomes and a final archaeological interpretation several buildings in different states of preservation different kinds of ground floors both with and without hypocausts perimeter walls and kilns could all be detected the soil mapping results helped in the geophysical interpretation by outsourcing soil erosion and accumulation areas it is shown that none of the employed methods could have supplied all the compiled information on their own and the strengths and weaknesses of each method is discussed in order to point out the implications for archaeologists

co2 plume modeling in deep saline reservoirs by 2d ert in boreholes the need to manage co2 emissions for mitigating the greenhouse effect has led to worldwide research to reduce atmospheric co2 techniques of carbon capture and storage ccs must 1 be effective and cost competitive 2 provide stable long term storage and 3 be environmentally benign potential terrestrial media for co2 storage include depleted oil and gas reservoirs coal seams that cannot be mined and deep saline water reservoirs capped by impermeable rock to prevent upward leakage

magma ascent by porous flow porous flow of buoyant liquid through partially molten rock is regarded as the initial transport process leading to magma segregation in the mantle recent work has identified the importance of matrix deformation and compaction in this process we present finite difference calculations based on a generalized form of darcy s law that includes matrix deformation in two dimensional cartesian and cylindrical coordinates we emphasize the existence of solitary wave solutions called magmons these waves are regions of locally high porosity that ascend through regions of low uniform porosity they differ from diapirs where the liquid and matrix ascend together the one dimensional waves that were previously reported are found to be unstable in two dimensions breaking down to form two dimensional waves of circular cross section both the development of the instability and the form of the two dimensional waves are supported by theoretical analysis numerical experiments in which two dimensional waves collide show that the extent to which they are conserved after collision depends on the lateral offset of the waves the conservation can be very good in other cases larger waves appear to grow at the expense of smaller waves although the quantification of the relevant permeability and rheological parameters remains uncertain geophysically plausible estimates suggest wavelengths of kilometers and velocities of centimeters per year in our preliminary assessment of the relevance of these results to igneous processes we find that magmons are unlikely to be important in regions of broad upwelling such as beneath spreading centers here the transport of liquid adjusts to match the supply by melting and compaction processes are not important if liquid is supplied from below to a stable partially molten region of the asthenosphere it is expected to ascend in magmons this may occur beneath oceanic volcanic centers or in subcontinental mantle it is possible that magma ascent in magmons leads to spatial and temporal episodicity of volcanic activity if higher level processes do not obscure this influence the waveform of a magmon ascends faster than the liquid within the magmon so new liquid is taken in from above while original liquid is lost from below consequently magmons can mobilize small degrees of partial melt and deliver it rapidly to the surface

plume formation in the d double prime layer and the roughness of the core mantle boundary numerical simulations of the formation of thermal plumes in the d layer at the base of the earth s mantle show that plumes are initiated by coalescence of small scale convective instabilities within the low viscosity region immediately above the core mantle boundary cmb these instabilities support topographic roughness on the cmb having horizontal scales of 20 50 km and provide a source for scattered p waves seen as precursors to the phases pkikp and pkkp the calculated structure of fully developed plumes emerging from d consists of 5 50 cm yr 1 flow confined to 50 100 km thick vertical conduits with strongly temperature dependent viscosity plumes exhibit time dependent behaviour including upward propagating solitary conduit waves which may contribute to episodicity in hotspot volcanism

anisotropy beneath 9 stations of the geoscope broadband network as deduced from shear wave splitting polarization of sks scs and s waves has been analyzed on digital 3 components seismograms recorded at 9 stations of the recently installed geoscope broadband network splitting was observed for about 15 records and is interpreted as being due to seismic anisotropy beneath the stations from every record it is possible to retrieve the fast direction at the station and the time difference between the fast and slow polarization directions some 30 observations of quasi linear s pulses also help constrain the direction of fast velocity synthetic seismograms were built to test the validity of the method used to retrieve the parameters of anisotropy typical time differences are about 1 sec corresponding for example to a 80 km thick zone of perfectly oriented pyrolite the fast directions obtained at 3 continental stations in america westford santa cruz heredia could be related to plate tectonic processes on the other hand the fast directions found in 3 oceanic islands kipapa papete la reunion differ strongly from the oceanic plate velocity direction there

small scale lateral shear velocity and anisotropy heterogeneity near the core mantle boundary beneath the central pacific imaged using broadband scs waves core reflected scs waves from 49 large m 5 1 deep tonga fiji events recorded in western north america are used to study a localized region of the core mantle boundary cmb under the central pacific ocean a total of 248 observations from the berkeley digital seismic network bdsn caltech united states geological survey usgs terrascope and the incorporated research institutions for seismology iris broadband arrays span epicentral distances of 73 85 scs reflection points sample a cmb patch southeast of the hawaiian islands at latitude 4 to 16 n and longitude 156 to 144 w near the proposed source location of the hawaiian plume highly variable scs travel times amplitudes waveforms and shearwave splitting indicate that the lowermost mantle in this region is heterogeneous both laterally and radially scsh sh differential travel times are on average 4 s larger than predicted by the preliminary reference earth model prem this is due to delayed scs arrivals and is largely accounted for by a model with a strong velocity decrease in d the scsh sh residuals also show a spatial trend not accounted for by a radial model indicating a lateral decrease in lower mantle shear velocity to the northeast this lateral velocity gradient appears to cause focusing of scs energy scsh sh amplitude ratios are larger than predicted by prem or by a model with a strong negative gradient with depth in d which enhances scs scs splitting corrected for lithospheric anisotropy beneath the receivers indicates spatial variations in d anisotropy with a systematic change in the orientation of the fast polarization direction from transverse to the ray path fast scsh to parallel to the ray path fast scsv along a northeast traverse these spatial trends suggest lateral gradients in the boundary layer shear flow on scale lengths of a few hundred kilometers which may be related to dynamical flow near the near the root of the large hawaiian plume

a stagnation point flow model for melt extraction from a mantle plume we present a two dimensional dynamical model for melt extraction from a mantle plume mantle material rises at constant velocity wo until it reaches a depth at which melting begins the flow in the melt zone above this depth has the form of a stagnation point flow melt migrates through the unmelted matrix in response to the total nonhydrostatic pressure gradient which is the sum of contributions from the deformation of the matrix and the buoyancy of the melt melt extraction is modeled as a melt flux through the upper surface of the melt zone our main conclusions are as follows 1 the nonhydrostatic pressure gradient associated with the deformation of the matrix is small relative to the buoyancy of the melt everywhere within the melt zone and the melt migration within the plume obeys darcy s law the rate of melt extraction from a mantle plume is therefore controlled by the buoyancy and viscosity of the melt itself and not by the viscosity of the matrix 2 melt extraction from a plume occurs over an area comparable to that of the plume itself and about 60 80 of the melt generated within the plume is extracted both the geometry and rate of extraction predicted by the model are consistent with observations in the hawaiian islands 3 layers of segregated melt magma chambers are likely to form at the the base of the lithosphere under intraplate volcanoes

seismic anisotropy in the d layer we present observations of diffracted sv svd for a path between the fiji tonga islands and the eastern coast of north america at distances greater than 110 observed features of s diffracted suggest that coupling between svd and shd can be ruled out as a first order effect for this path arrivals of shd are late relative to iasp91 travel times by about 10 s and those of svd are late relative to shd by 3 s for most records the slope of the log svd shd spectral ratio is around 3hz 1 in the range 0 06 0 15 hz a transversely isotropic low velocity layer in the lowermost mantle with a thickness of 200 300 km may account for most of the observed properties of svd

the dynamical and thermal structure of deep mantle plumes abstract if the interpretation of the d layer at the base of the mantle as a thermal boundary layer with a temperature increment in the order of 800 k is correct then the formation of deep mantle plumes to vent material from it appears inevitable we demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long lived chimneys that are 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material the speed of flow up the axis of the plume is correspondingly fast 1 6 m y 1 at the base and 4 8 m y 1 at 670 km depth thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there this convergence which contributes only a small increment to the flux of material up the plume also serves to throttle the flow in the chimney the global plume mass flux necessary to transport 1 6 10 12 w of core heat upward through the mantle is 1 8 10 6 kg s 1 at its base plume material is probably still significantly below its solidus or eutectic temperature but substantial partial melting is very likely as it rises we speculate that a small fraction of this fluid component eventually emerges at the surface in hot spots with the fate of the remainder being unknown the behaviour and properties of d and of plumes are closely coupled not only are plumes a necessary consequence of a thermal boundary layer but their existence is impossible without that layer

scale lengths of shear velocity heterogeneity at the base of the mantle from s wave differential travel times resolving the seismic velocity heterogeneity in the lowermost mantle is essential for understanding the chemical and dynamical processes operating in the boundary layer between the core and the mantle several regions of the lowermost mantle appear to have abrupt increases in shear velocity several hundred kilometers above the core mantle boundary at the top of the d sup prime prime layer this structure is revealed by seismic wave energy refracted by the velocity increase resulting in an extra arrival scd in teleseismic s wavetrains at distances from 65 to 95 degree anomalies in differential travel times of the extra arrival relative to direct s which turns at shallower depths in the midmantle or scs which reflects from the core mantle boundary below the velocity increase provide probes of the velocity heterogeneity in the lowermost mantle with better vertical resolution than provided by conventional scs s measurements we explore the spatial patterns in scd s scs scd and scs s differential time residuals for paths through the lower mantle beneath alaska eurasia and india all being regions with coherent scd phases to place constraints on the strength and scale lengths of shear velocity heterogeneity and or variable topography of the lower mantle discontinuity in these regions the observed patterns are poorlymore predicted by existing global tomographic models significant small scale heterogeneity with lateral length scales of 200 endash 500 km or less exists even within regions that display a relatively uniform d sup prime prime discontinuity structure over scale lengths of 1500 endash 2000 km the strongest travel time variations are associated with structure above the d sup prime prime region in contrast to common assumptions rapid lateral fluctuations in scs scd differential times suggest that the anomalies accumulate within a relatively thin zone less than the overall thickness of d sup prime prime abstract truncated less

numerical and experimental mapping of small root zones using optimized surface and borehole resistivity tomography an exact mapping of root zones is essential to understand plant growth root biomass and soil functions important for environmental and climatic management and protection numerical and experimental techniques of the electrical resistivity tomography were applied in 2d and 3d to resolve small root zones in the centimeter range numerically we studied two scenarios of conductive and resistive root zones as a function of 1 eight different quadripole electrode configurations standard nonstandard and optimized 2 four different survey designs with electrode arrays at the soil surface and in boreholes and 3 eight different inversion constraints the best resolved output tomogram was evaluated semiquantitatively using the criteria of visual similarity to the input model least data set rms error and iteration number and quantitatively by the model difference relative to the input model the results showed that the surface borehole configurations have the best resolution for the whole root zone the single surface and borehole surveys resolve only the respective upper and middle lower root parts the results reflect the potential of the optimization approach to generate small data sets of far higher resolution than the standard sets based on these results we used the surface borehole survey around a young hibiscus planted in a sandy soil in a laboratory experiment the surface borehole surveys using small optimized configurations result in an optimum spatiotemporal resolution for simultaneous applications for 3d mapping of targets root zones and water and soil heterogeneities and 4d monitoring of their processes

seismic anisotropy in the lowermost mantle beneath the pacific onset time differences of up to 3 s are observed between transverse ssh and longitudinal ssv components of broadband s waves at distances of 85 to 120 for paths traversing the lowermost mantle d beneath the pacific after correction for upper mantle anisotropy ssh usually arrives earlier than ssv with the splitting increasing with distance from 100 to 120 the data yield two possible models of anisotropy 1 anisotropy may vary laterally with transverse isotropy existing in higher than average shear velocity regions beneath the northeastern pacific or 2 anisotropy may vary with depth with transverse isotropy concentrated in a thin 100 km thermal boundary layer at the base of d a few recordings at distances less than 105 show that ssv arrives earlier than ssh indicating that general anisotropy likely exists in shallower regions of d

short period p wave constraints on d reflectivity we present evidence of a p wave reflector in d beneath the central and eastern equatorial pacific obtained from nearly 7000 short period seismic recordings of earthquakes in the tonga fiji and south america subduction complexes both regions overlap previous observations of an s wave triplication at similar depths double array waveform stacking places the reflector 190 20 km above the core mantle boundary and allows for a velocity transition zone no thicker than 50 km the data admit a wide range of compressional v p and shear wave velocities v s the 2 3 increase in v s suggested by earlier long period s wave studies however necessitates only a 0 5 0 6 increase in v p the data stacks represent spatial averages over large areas of the lower mantle such that reflector intermittence and topography decrease stack amplitude and lower the apparent v p jump our results constrain these processes to act over length scales of 10 100 km

effluent diffusion and sediment dispersion at microtidal river mouths predicted using mathematical models plume characteristics associated with fluvial discharge into a microtidal environment are predicted using mathematical models such predictions are compared with field measurements from the river louros western greece combined frictionally dominated and buoyant plume expansion models show encouraging agreement i e to within 1 with temperature salinity and velocity observations the inclusion of particle settling velocities into the analysis enables surficial sea bed sediment size distribution to be determined the predicted patterns show close correspondence with those collected from the subaqueous delta of a small river in greece the approach described may form the more general basis for the prediction of riverine sediment deposition on inner continental shelves

investigation of a tertiary maar structure using three dimensional resistivity imaging this paper presents the application of the electrical resistivity tomography ert method to the investigation of the tertiary maar structure of baruth germany known from previous gravimetric surveys ert was applied to support the optimum location for a palaeoclimatological drill hole special modifications of data acquisition signal processing and inversion are introduced to adapt the method of ert to the special requirements for the 3 d investigation of structures with horizontal extensions of 1 km or more more than 5000 dipole dipole combinations were recorded at three concentric circular electrode arrangements using stand alone transient data acquisition systems reftek we present a fast approximate imaging technique based on the simultaneous iterative reconstruction technique sirt as the complete calculation of the inverse frechet matrix is avoided the algorithm is especially suitable for large data and model spaces where complete inversion is beyond the limits of available computing hardware the single step method is applicable to arbitrary irregular electrode layouts synthetic tests show that the imaging procedure reconstructs the main features of the subsurface a low resistivity body could be interpreted as limnic sediments filling the interior of the tertiary maar crater considering the horizontal resistivity gradient estimates for the lateral and depth extents of the structure were made an optimum position for a palaeoclimatological borehole was found and was in good agreement with the gravimetric minimum

how human activities affect the fine sediment distribution in the dutch coastal zone seabed abstract the fine sediment distribution in the seabed is an important indicator for the ecological functioning of shallow coastal seas in this paper we investigate the processes and conditions that determine the fine sediment distribution in the dutch coastal zone surficial seabed while also assessing the response of the system to human interventions an extensive sediment dataset collected in the dutch coastal zone from 2006 to 2014 is presented these data are used to map the distribution of fines in the seabed of the dcz at unique spatiotemporal scales for the entire dutch coastal zone the distribution of fines generally agrees well with previous studies the recent extension of the port of rotterdam the maasvlakte 2 reclamation was found to locally change the distribution of fines in the sand mining pit and directly south of the reclamation fines percentages in the seabed increased by more than 10 we developed a conceptual framework to analyse the distribution of fines and how it is affected by human interventions three components are distinguished within this framework 1 sources of fines 2 transport pathways and 3 accumulation potential these components are determined both qualitatively and quantitatively based on high resolution bathymetric and hydrodynamic model data the distinction between the three components makes it possible to unravel the contributions of different human interventions to the changes in the fines distribution in the case of maasvlakte 2 the local increase of fines percentage in the seabed could thus be attributed to a temporary additional source of fines and enhanced accumulation potential the high spatiotemporal resolution of the new sediment dataset proved crucial to enable development and testing of the framework to evaluate the impact of large engineering works on the spatial distribution of fines

sediment accumulation and organic material flux in a managed mangrove ecosystem estimates of land ocean atmosphere exchange in peninsular malaysia rates of sediment accumulation and organic matter decomposition in mangrove forests of different age were examined at the matang mangrove forest reserve in peninsular malaysia these data were used with previous findings to construct a first order mass balance of carbon to determine whether the ecosystem is net heterotrophic or autotrophic and to estimate land ocean atmosphere exchange measurements of various carbon and nitrogen processes in sediments suggest that organic matter is rapidly and efficiently mineralized to a depth of 1 m rates of total carbon oxidation in sediments tcox ranged from 77 to 118 mmol c m 2 day 1 with no significant differences with forest age sulfate reduction range 19 53 mmol s m 2 day 1 appeared to be the dominant decomposition pathway accounting for an average of 51 75 of sediment tcox aerobic respiration range 5 20 mmol c m 2 day 1 accounted for 5 20 of mean tcox methanogenesis was not measurable denitrification range 0 4 11 0 mmol n2 m 2 day 1 was rapid but very variable and may have accounted for 25 of tcox at the old growth forest rates of sediment mass accumulation range 2 2 11 4 kg m 2 year 1 were rapid compared with previous measurements in mangroves efficiency of carbon burial range 23 29 mmol c m 2 day 1 in sediments increased from 16 to 27 from the youngest to the oldest forest the ratio of sediment tcox to mangrove net primary production decreased from 28 to 7 with increasing forest age the efficiency of n mineralization range 67 81 and burial range 10 29 in sediments showed the opposite pattern tree n demand required from 8 to 22 mmol n m 2 day 1 surface n2 fixation was measurable only in two of six trials but was rapid range 2 3 3 0 mmol n2 m 2 day 1 a mass balance of carbon for the entire reserve suggests that the ecosystem is currently net autotrophic with a net ecosystem production of 21 109 mol c year 1 and an ecosystem p r of 1 4 despite considerable uncertainty forest production and respiration clearly dominates carbon flow accounting for 95 of total inputs and 79 of total outputs respectively river inputs phytoplankton primary production and trash fish inputs to fish cage aquaculture in the waterways were comparatively minor export of mangrove detritus equated to 25 of net canopy production only 2 of total carbon input to the ecosystem was buried in mangrove sediments clear felling and thinning of forests accounted for 1 of total outputs 73 of total carbon inputs to the ecosystem were respired our budget indicates that the major exchange of carbon in this managed ecosystem is between forest and atmosphere

a combined seismic and geoelectrical monitoring approach for co2 storage using a synthetic field site this paper presents an approach to assess the methods of 2d elastic seismic full waveform inversion fwi and electrical resistivity tomography ert for monitoring storage of co2 in deep saline formations as at a real site the subsurface is not fully known this approach uses a synthetic field site densities and saturations are obtained from a numerical simulation of the injection process and are introduced into geophysical forward models which simulate the geophysical data acquisition from the surface fwi and boreholes ert these synthetic geophysical data are then evaluated with respect to changes in co2 saturation and are compared to the fully known co2 saturation of the numerical process model to verify the geophysical monitoring methods results show that both methods are capable of detecting a thin co2 phase body in about 2 200 m depth for a synthetic site in the north german basin inverted co2 saturations are in good agreement however both methods cannot resolve the co2 phase edges of less than 4 m saturated thickness the maximum error of estimated co2 saturation is 10 for the fwi method and 15 for the ert method if accurate baseline models are available the fwi method sensitivity on the baseline model is tested by a sensitivity analysis demonstrating high sensitivity on bulk density but low sensitivity on fluid densities and porosity ert results are considerably improved using structural information from the fwi as constraint

the influence of waves and seagrass communities on suspended particulates in an estuarine embayment resuspension of bottom sediments by waves corresponding changes in suspended particulate material spm concentrations in the overlying water column and transport pathways of spm were investigated in a shallow estuarine embayment colonized by seagrass communities in chesapeake bay in shallow 2 m unvegetated regions significant resuspension of bottom sediments was evident when northerly winds exceeded 25 km h 1 increasing spm concentrations up to 10 fold high concentrations of spm generated by resuspension dissipated rapidly within 24 h after winds became calm patterns of spm along the embayment s depth gradient suggest that part of this resuspended material was transported offshore into deeper reaches of the estuary in areas of the embayment colonized by seagrasses wave energy was attenuated by the vegetation suppressing resuspension and enhancing deposition as a result spm concentrations were significantly lower inside seagrass beds than in adjacent unvegetated areas during periods of high winds when wave induced resuspension occurred in unvegetated areas spm concentrations remained unchanged inside the bed at normal water levels however when water levels were elevated by spring tides or storm surges plants were less effective at attenuating wave energy and spm concentrations increased inside the seagrass bed due to resuspension and advective processes calculations based on the results of this study indicate that sedimentation rates are substantially higher in seagrass communities than in unvegetated areas

lateral variations in lowermost mantle shear wave anisotropy beneath the north pacific and alaska s waves recorded by long period world wide standardized seismograph network and broadband stations in north america for deep northwest pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north pacific and alaska systematic delays of up to 4 s are observed between longitudinal components sv and transverse components sh of motion for core reflected scs waves as well as for core grazing and diffracted s waves the absence of significant splitting for s waves that have turning points more than a few hundred kilometers above the core mantle boundary indicates that anisotropy is localized within the d region the lowermost portion of the mantle sv sh differential arrival times for both scs and sdiff along with path length estimates assuming a 250 km thick d region indicate spatial variations in the strength of shear wave anisotropy the strongest anisotropy 1 1 5 is found in the eastern part of the study area with systematic reduction in magnitude toward the west a transverse isotropy model can explain the data with the velocity structure for horizontally polarized waves vsh having a 2 3 discontinuous shear velocity increase at the top of d as proposed in earlier studies of the region and a similar structure for s wave particle motion in the direction normal to the core mantle boundary vsv but with the velocity jump at the top of d and the velocity within d being reduced from that for vsh by 0 5 1 5 km s large uncertainties exist for velocity gradients above and below the velocity jump but the requirement of a reduced vsv relative to vsh in d is clear synthetic waveforms calculated by using separate isotropic structures for sh and sv match the observations well and constrain the basic anisotropic structure because the shear waves all traverse the region with near grazing geometries the study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the d layer topography of the d layer is not well resolved because of trade off with volumetric heterogeneity

modelling d structure beneath central america with broadband seismic data deep south american events recorded at distances between 70 and 96 are used to constrain the lower mantle structure beneath central america employing data from the terrascope bdsn iris and cnsn canadian national seismic network broadband arrays we obtain a 1 d shear model with a 200 km thick d layer with a 3 velocity jump at the top and a negative gradient within the layer from broadband waveform modelling of sh polarized s scs and scd the shear structure above d is similar to the preliminary reference earth model prem dziewonski and anderson 1981 preliminary reference earth model physics of earth planetary interiors 25 297 356 in terms of dt d and s sks differential times sv synthetics for this model assuming a prem core produce a satisfactory fit to the data except at the larger ranges to the north where the waveforms display rapid variation this feature is indicative of lateral variation of the type suggested by grand 1994 where the discontinuity becomes less well defined synthetics computed for a simplified 2 d cross section through this structure produce the scd phase and simple s at larger distances while scd is easily observed in the shear wave data there is little evidence of its counterpart in the p data by testing several models with different p velocity jumps at the same depth as the d jump in shear structure we conclude that the velocity jump must be less than 1 in fact prem provides an excellent fit both in timing and waveform shapes and therefore we conclude that lower mantle structure beneath central america shows clear evidence for a discontinuity in s velocity but not in p

inhomogeneities near the core mantle boundary evidenced from scattered waves a review statistical properties of small scale inhomogeneities wavelengths between 20 and 70 km near the core mantle boundary are inferred from scattered core waves observations of scattered core waves at large seismic arrays and worldwide networks indicate that the inhomogeneities have a global nature with similar characteristics however there may exist a few regions having markedly stronger or weaker strengths scattering by volumetric inhomogeneities of about 1 inp wave velocity in the lower mantle or by about 300 m of topographic relief of the core mantle boundary can explain the observations at present it is not possible to rule out either of these two alternatives or a combination of both

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt

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