discussion of inception of sediment transport on steep slopes by richard j s whitehouse richard l soulsby and jesper s damgaard the authors present in an interesting but not very original way a relationship for the critical shear stress on flow over a steep slope the resulting equation is given with 21 which they consider to be compatible with intuitive expectation with two independent data sets they show in fig 4 a reasonable good agreement the discusser was also concerned with this problem an equation of the form of 16 was presented by graf 1991 and compared with laboratory and field data also addressed in this research was the fact that flow on steep slopes is often but not always of small flow depth and over large grain sized material the dimensionless critical shear stress was then related to the relative roughness d50 h the authors might be interested in investigating this parameter in a recent review article by graf 1999 on the contribution of schoklitsch to the hydraulics of sediment transport it is shown that the authors 21 was presented already in 1914 this relation was obtained with a simple force balance consideration

experimental assessment of two theories for the effect of bedslope on the threshold of bedload transport abstract theoretical analyses for the effect of bed inclination on the threshold of sand transport over submarine slopes and bedforms have not yet been tested with laboratory and field data six hundred and seventy two laboratory experiments are described which determine the threshold velocity u cr for quartz sand on a range of slopes between positive and negative angles of repose the experimental results suggest a marked decrease in threshold velocities on increasingly steeper negative slopes downslope and a general increase for the positive upslope case the results are compared with and support earlier analyses although a marked divergence becomes apparent on slopes which are greater than about 14

insights on upper mantle melting rheology and anelastic behavior from seismic shear wave tomography in seismic tomography we map the wave speed structure inside the earth but we ultimately seek to interpret those images in terms of physical parameters this is challenging because many parameters can trade off with each other to produce a given wave speed the problem is compounded by the convention of mapping seismic structures as perturbations relative to a 1 d reference model rather than absolute wave speeds using a full waveform tomography model of europe as a case study we quantify the extent to which thermochemical and dynamic properties can be constrained using only s wave speed expressed in absolute values the wave speed distributions of this tomography model are compared with 4 million thermochemical models whose seismic properties are computed via thermodynamic modeling these models sample the full range of realistic mantle compositions including variable water and melt contents and mineral intrinsic anelasticity is taken into account intrinsic anelasticity causes waves to travel more slowly at higher temperatures leading to seismic attenuation but the sensitivity of the wave speed reduction to temperature is in turn controlled by the wave frequency global studies of surface waves indicate an anticorrelation between s wave speed and attenuation we therefore only retain thermochemical models satisfying this anticorrelation our study indicates that the frequency dependence of anelasticity depends on temperature or rheology with 0 1 being most appropriate in cold or lithospheric mantle and 0 3 in warmer regions i e the asthenosphere additionally the slowest regions require specific compositions and or a velocity weakening mechanism such as partial melting elastically accommodated grain boundary sliding or water

error bars for the global seismic q profile the radial attenuation profile of the earth is needed to account for dispersion effects when interpreting seismic velocities and can provide important constraints on composition to date most radial q models have been produced using traditional damped inversions of free oscillation and surface wave data because such inversions can severely underestimate the model uncertainties that are needed to guide mineralogical and dynamic interpretation and because the quality of data has continued to improve we revisit this seismic inverse problem using a model space search approach already proven effective with similar data we do indeed observe model uncertainties at least an order of magnitude greater than earlier estimates at the same time we find that q is determined well enough to confirm that the data favor several important features previously disputed because of questions of consistency these include shear attenuation that drops significantly in the lower third of the lower mantle and bulk attenuation that is negligible in the inner core but stronger in the outer core and lower mantle than suggested by most models d 2004 elsevier b v all rights reserved

role of a low viscosity zone in stabilizing plate tectonics implications for comparative terrestrial planetology 1 earth s near surface layer its lithosphere is broken into quasi rigid plates that form the upper thermal boundary layer for mantle convection since the discovery of plate tectonics it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone lvz over which the plates glide easily the lvz or asthenosphere concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the earth s gravity field here we show in both two dimensional 2 d cartesian and 3 d spherical earth models that combining a pronounced lvz and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection with 30 toroidal surface motion for the 3 d case recycling of water into the upper mantle at subduction zones is a plausible cause of earth s lvz whereas venus is dry and lacks both an lvz and plate tectonics

sand transport on steeply sloping plane and rippled beds experiments on sand transport have been carried out in the sloping sediment duct at hr wallingford the aim of the experiments was to investigate sediment transport mechanisms for sand of varying degree of grading on sloping beds the sloping sediment duct is a steady flow recirculating duct capable of generating mean flow speeds of up to 1 m s and tilting to 30 twenty two tests with two different sediments were conducted both sediments had a median grain size of about 0 23 mm but different standard deviations bed slopes up to 20 were used in the experiments the results show that bedforms have a significant effect on the transport rate since the bedforms in turn are affected significantly by the slope the relation between transport rate and slope is not a monotonic function maximum suspended transport rates were attained for downslope flows at angles of about 10 the transport rate for widely graded sediment was significantly larger than that for well sorted sediment for almost all flows and slopes

global models of surface wave attenuation 1 a large data set of fundamental mode rayleigh wave amplitudes is analyzed to derive global models of surface wave attenuation 1 q the data set consists of measurements of rayleigh wave amplitude anomalies in the period range 50 250 s for 347 earthquakes observed at 179 seismic stations the amplitude anomalies are considered to depend on four factors intrinsic attenuation along the ray path elastic focusing effects along the ray path a source factor accounting for uncertainties in the strength of excitation and a receiver factor accounting for uncertainties in the response at the station the amplitude data are inverted simultaneously for global maps of attenuation expanded in spherical harmonics up to degree 12 global maps of phase velocity expanded to degree 20 and source and receiver correction factors all four variable types are shown to be important in explaining the amplitude anomalies a data set of phase delay measurements provides additional constraints on velocity structure the maps of attenuation obtained by simultaneous inversion for elastic and anelastic models contain important features that are not robustly imaged when the effect of focusing on wave amplitude is ignored these include high attenuation along western north america and along the east pacific rise and other ridge systems and low attenuation associated with stable continental interiors the global attenuation maps exhibit a strong correlation with phase velocity maps corrected for the effect of the crust particularly for periods 200 s the correlation suggests that the variability in both q and velocity in the shallow upper mantle is primarily thermal in origin

attenuation tomography of the upper mantle we present qsadr17 a global shear wave attenuation model of the upper mantle synthetic tests confirm that large scale shear attenuation anomalies are resolved in the whole upper mantle with limited vertical smearing 50 km qsadr17 shows strong correlation with surface tectonics down to 200 km depth with low attenuation beneath continents and high attenuation beneath oceans the attenuation signal near 250 km depth is dominated by a high quality factor along subduction zones attenuating anomalies are found beneath mid ocean ridges down to 150 km and under most pacific hotspots from the lithosphere down to the transition zone the presence of broad attenuating anomalies at 150 km depth in the pacific ocean suggests that several thermal plumes pond in the asthenosphere evidence for compositional heterogeneities is found in the lithosphere at the base of cratons and in a number of active regions

an upper mantle seismic discontinuity beneath the galapagos archipelago and its implications for studies of the lithosphere asthenosphere boundary an upper mantle seismic discontinuity the gutenberg or g discontinuity at which shear wave velocity decreases with depth has been mapped from s to p conversions in radial receiver functions recorded across the galapagos archipelago the mean depth of the discontinuity is 91 8 km beneath the southeastern archipelago and 72 5 km beneath surrounding regions the discontinuity appears deeper beneath the portion of the nazca plate that we infer passed over the galapagos mantle plume than elsewhere in the region we equate the depth of the g discontinuity to the maximum depth extent of anhydrous melting which forms an overlying layer of dehydrated and depleted mantle we attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the galapagos spreading center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 115 30 c the g discontinuity lies within a high seismic velocity anomaly that we conclude forms by partial dehydration and a gradual but steady increase in seismic velocity with decreasing depth after upwelling mantle first encounters the solidus for volatile bearing mantle material at the depth of the solidus for anhydrous mantle material removal of remaining water creates a sharp decrease in velocity with depth this discontinuity may also mark a site of melt accumulation results from seismic imaging the compositions of galapagos lavas and rare earth element concentrations across the archipelago require that mantle upwelling and partial melting occur over a broad region within the dehydrated and depleted layer we conclude that the g discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere

sediment pick up functions experimental research has been performed to determine the sediment pick up by a uniform flow without initial sediment load a sediment lift was designed by which the sediment particles were moved upwards at a constant rate through a circular opening in the flume bottom five types of bed material sand have been used with d d5 d0 values in the range 130 1500 m the mean flow velocities were in the range 9 5 1 0 m s the experimental results have been used to derive an empirical pick up function the results have also been used to evaluate several existing pick up functions finally the proposed empirical pick up function has been applied to predict the bed load and suspended load transport in the latter application the proposed function has been used as a bed boundary condition in a mathematical model for suspended sediment transport

low seismic velocities below mid ocean ridges attenuation versus melt retention the first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low velocity zone between 40 and 100 km depth below the east pacific rise epr that has been attributed to substantial retained melt fractions of 0 3 2 such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry here we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions we start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure accounting for temperature pressure composition phase and melt dependent anharmonicity and temperature pressure frequency and hydration dependent anelasticity our models predict a double low velocity zone a shallow approximately triangular region due to dry melting and a low velocity channel between 60 and 150 km depth dominantly controlled by solid state high temperature seismic attenuation in a damp mantle with only a minor contribution of 0 1 melt we test how tomographic inversion influences the imaging of our modeled shear velocity features the epr experiment revealed a double low velocity zone but most tomographic studies would only resolve the deeper velocity minimum experimentally constrained anelasticity formulations produce vsas low as observed and can explain lateral variations in near ridge asthenospheric vs with 100 k temperature variations and or zero to high water content furthermore such qs formulations also reproduce low asthenospheric vs below older oceans and continents from basic lithospheric cooling models although these structures are compatible with global qs images they are more attenuating than permitted by epr data

zonal pressure gradient velocity and transport in the atlantic equatorial undercurrent from focal cruises july 1982 february 1984 the zonal pressure gradient zpg along the atlantic equator and the equatorial undercurrent euc transport are discussed for four cruises representative of each season a very clear sea surface slope reversal occurred in the eastern area during autumn as far west as 14 w an early onset of the equatorial thermocline rising was evident during spring 1983 an eastward equatorial surface jet clearly distinct from the euc was observed at 35 w and 29 w the greatest zpg but the lowest euc transport were observed during summer 1983 a nearly total absence of the zpg and a large surfacing of the euc as far as 10 w characterized the 1984 winter

melt in the mantle beneath the amagmatic zone southern nevada surface wave tomography of the western united states reveals the presence of an unusually slow shear wave velocity anomaly beneath a region in the basin and range province sometimes termed the amagmatic zone which is renowned for its lack of volcanic activity we pre sent a model of the three dimensional shear wave velocity structure beneath the southwestern united states constructed from the inversion of fundamental mode rayleigh wave dispersion the abundance of data from the deployment of the usarray a component of the earthscope project provides unprecedented resolution there is an excellent correlation between the location of slow shear wave velocity anomalies and recent younger than 1 ma volcanism this correlation and the unusually low values of absolute shear velocity beneath the magmatic gap lead us to conclude that there is melt present beneath this region previous studies have proposed that the preservation of cold continental lithosphere beneath the magmatic gap accounted for the lack of volcanism however this hypothesis is inconsistent with anomalously slow shear wave velocities in the 50 100 km depth range perhaps the prevalence of low angle normal faulting in this region has made it difficult for the melt to escape the mantle beneath an immediately adjacent region the southwestern nevada volcanic field where volcanism was abundant 10 15 m y ago but has since waned significantly higher shear velocities suggest that the melt has been extracted from the mantle

dimensionless erosion laws for cohesive sediment abstracta method of achieving a dimensionless collapse of erosion rate data for cohesive sediments is proposed and shown to work well for data collected in flume erosion tests on mixtures of sand and mud silt plus clay sized particles for a wide range of mud fraction the data collapse corresponds to a dimensional erosion law of the form e c m where e is erosion rate is shear stress c is the threshold shear stress for erosion to occur and m 7 4 this result contrasts with the commonly assumed linear erosion law e kd c where kd is a measure of how easily sediment is eroded the data collapse prompts a re examination of the way that results of the hole erosion test het and jet erosion test jet are customarily analyzed and also calls into question the meaningfulness not only of proposed empirical relationships between kd and c but also of the erodibility parameter kd itself fuller comparison of flume erosion data with hole erosion and jet erosion data will require revised analyses o

revisiting slope influence in turbulent bedload transport consequences for vertical flow structure and transport rate scaling gravity driven turbulent bedload transport has been extensively studied over the past century in regard to its importance for earth surface processes such as natural riverbed morphological evolution in the present contribution the influence of the longitudinal channel inclination angle on gravity driven turbulent bedload transport is studied in an idealised framework considering steady and uniform flow conditions from an analytical analysis based on the two phase continuous equations it is shown that i the classical slope correction of the critical shields number is based on an erroneous formulation of the buoyancy force ii the influence of the slope is not restricted to the critical shields number but affects the whole transport formula and iii pressure driven and gravity driven turbulent bedload transport are not equivalent from the slope influence standpoint analysing further the granular flow driving mechanisms the longitudinal slope is shown to not only influence the fluid bed shear stress and the resistance of the granular bed but also to affect the fluid flow inside the granular bed responsible for the transition from bedload transport to debris flow the relative influence of these coupled mechanisms allows us to understand the evolution of the vertical structure of the granular flow and to predict the transport rate scaling law as a function of a rescaled shields number the theoretical analysis is validated with coupled fluid discrete element simulations of idealised gravity driven turbulent bedload transport performed over a wide range of shields number values density ratios and channel inclination angles in particular all the data are shown to collapse onto a master curve when considering the sediment transport rate as a function of the proposed rescaled shields number

seafloor age dependence of rayleigh wave phase velocities in the indian ocean variations in the phase velocity of fundamental mode rayleigh waves across the indian ocean are determined using two inversion approaches first variations in phase velocity as a function of seafloor age are estimated using a pure path age dependent inversion method second a two dimensional parameterization is used to solve for phase velocity within 1 25 1 25 grid cells rayleigh wave travel time delays have been measured between periods of 38 and 200 s the number of measurements in the study area ranges between 4139 paths at a period of 200 s and 22 272 paths at a period of 40 s at periods 100 s the phase velocity variations are strongly controlled by seafloor age and shown to be consistent with temperature variations predicted by the half space cooling model for a mantle potential temperature of 1400 c the inferred thermal structure beneath the indian ocean is most similar to the structure of the pacific upper mantle where phase velocities can also be explained by a half space cooling model the thermal structure is not consistent with that of the atlantic upper mantle which is best fit by a plate cooling model and requires a thin plate removing age dependent phase velocity from the 2 d maps of the indian ocean highlights anomalously high velocities at the rodriguez triple junction and the australian antarctic discordance and anomalously low velocities immediately to the west of the central indian ridge

determination of intrinsic attenuation in the oceanic lithosphere asthenosphere system we recorded p and s waves traveling through the oceanic lithosphere asthenosphere system las using broadband ocean bottom seismometers in the northwest pacific and we quantitatively separated the intrinsic anelastic and extrinsic scattering attenuation effects on seismic wave propagation to directly infer the thermomechanical properties of the oceanic las the strong intrinsic attenuation in the asthenosphere obtained at higher frequency 3 hertz is comparable to that constrained at lower frequency 100 seconds by surface waves and suggests frequency independent anelasticity whereas the intrinsic attenuation in the lithosphere is frequency dependent this difference in frequency dependence indicates that the strong and broad peak dissipation recently observed in the laboratory exists only in the asthenosphere and provides new insight into what distinguishes the asthenosphere from the lithosphere

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

upper mantle seismic wave attenuation effects of realistic partial melt distribution frequency dependence of seismic velocity and attenuation resulting from viscoelastic relaxation of partially molten mantle is estimated we consider the contribution of the melt squirt mechanism through which pressure differences between disk shaped inclusions are equalized by melt passing through connecting tubes the pressure differences arise as a result of shear strain compressing disk shaped pores differently on the basis of disk orientation with respect to the applied shear the frequencies over which the transition from the unrelaxed to the relaxed states occurs are determined by representing the melt as a network of tubes connecting oblate ellipsoidal pores the pressure equalization process is modeled by a system of first order linear differential equations whose eigenvalues are the characteristic frequencies for melt squirt relaxation it is shown that in this framework the set of frequencies is invariant to the absolute scale of the system but is sensitive to melt bulk modulus and viscosity as well as distribution of melt inside pores and conduits use of realistic solid and melt physical properties and pore and conduit geometries demonstrates that it is the relaxed modulus that is most likely excited in the seismic band and that melt mobility has little effect on seismic attenuation some conceivable melt distributions however would result in detectable attenuation in the seismic band in all cases investigated attenuation increases with frequency indicating that melt squirt is not responsible for global upper mantle q observations

analysis of gravity and topography in the glimpse study region isostatic compensation and uplift of the sojourn and hotu matua ridge systems 1 the gravity lineations intraplate melting petrologic and seismic expedition glimpse experiment investigated the formation of a series of non hot spot intraplate volcanic ridges in the south pacific and their relationship to cross grain gravity lineaments detected by satellite altimetry using shipboard gravity measurements and a simple model of surface loading of a thin elastic plate we estimate effective elastic thicknesses ranging from 2 km beneath the sojourn ridge to a maximum of 10 km beneath the southern cross seamount these elastic thicknesses are lower than predicted for the 3 9 ma seafloor on which the volcanoes lie perhaps due to reheating and thinning of the plate during emplacement anomalously low apparent densities estimated for the matua and southern cross seamounts of 2050 and 2250 kg m 3 respectively probably are artifacts caused by the assumption of only surface loading ignoring the presence of subsurface loading in the form of underplated crust and or low density mantle using satellite free air gravity and shipboard bathymetry we calculate the age detrended residual mantle bouguer anomaly rmba the rmba corrects the free air anomaly for the direct effects of topography including the thickening of the crust beneath the seamounts and volcanic ridges due to surface loading of the volcanic edifices there are broad negative rmba anomalies along the sojourn and brown ridges and the hotu matua seamount chain that extend nearly to the east pacific rise these negative rmba anomalies connect to negative free air anomalies in the western part of the study area that have been recognized previously as the beginnings of the cross grain gravity lineaments subtracting the topographic effects of surface loading by the ridges and seamounts from the observed topography reveals that the ridges are built on broad bands of anomalously elevated seafloor this swell topography and the negative rmba anomalies contradict the predictions of lithospheric cracking models for the origin of gravity lineaments and associated volcanic ridges favoring models with a dynamic mantle component such as small scale convection or channelized asthenospheric return flow

structures of the oceanic lithosphere asthenosphere boundary mineral physics modeling and seismological signatures 1 we explore possible models for the seismological signature of the oceanic lithosphere asthenosphere boundary lab using the latest mineral physics observations the key features that need to be explained by any viable model include 1 a sharp 20 km width and a large 5 10 velocity drop 2 lab depth at 70 km in the old oceanic upper mantle and 3 an age dependent lab depth in the young oceanic upper mantle we examine the plausibility of both partial melt and sub solidus models because many of the lab observations in the old oceanic regions are located in areas where temperature is 1000 1200 k significant partial melting is difficult we examine a layered model and a melt accumulation model at the lab and show that both models are difficult to reconcile with seismological observations a sub solidus model assuming absorption band ab physical dispersion is inconsistent with the large velocity drop at the lab we explore a new sub solidus model originally proposed by karato 2012 that depends on grain boundary sliding in contrast to the previous model where only the ab behavior was assumed the new model predicts an age dependent lab structure including the age dependent lab depth and its sharpness strategies to test these models are presented

seismic attenuation near the east pacific rise and the origin of the low velocity zone abstract low shear wave velocities beneath mid ocean ridges and in the low velocity zone beneath oceanic plates commonly have been attributed to the presence of melt or dissolved water but several recent studies have challenged that interpretation the alternative is that the anelastic effects of increasing temperature may cause the observed drop in velocity along with a predicted increase in attenuation we report the first measurements of surface wave attenuation within regional arrays of seismometers on the seafloor near the east pacific rise there is much less attenuation than is predicted by models in which the velocity is controlled solely by the direct elastic and anelastic effects of changing temperature suggesting that melt and water concentration do play an important role there also is somewhat less attenuation than is found in global studies we speculate that scattering from unresolved velocity heterogeneities contribute to the apparent attenuation in global studies

slope correction for calculation of bedload sediment transport rates in steep channels abstractclassical theories developed for investigating bedload sediment transport are applicable largely for channel beds of a gentle slope examples are the well known shields diagram for the evaluation of the critical condition for initial sediment motion and the classical bedload transport formulas derived with deterministic or probabilistic approaches how to apply the existing bedload formulas for steep channels remains challenging in this study a simple procedure is proposed for correcting slope effects so that the existing bedload formulas can be extended for steep channels to verify the proposed correction factor two representative bedload formulas are modified and checked against published data of sediment transport rates that were significantly affected by steep slopes

morphodynamic modelling of rip channel growth the morphological evolution of a rip channel system due to normally incident waves has been investigated using two different approaches the first approach is a state of the art commercial 2dh coastal area model whereas the second approach is a linear stability analysis the results of the two different approaches have been compared and both models predict rip channel spacings and initial growth rates of similar magnitudes

upper mantle q structure beneath the east pacific rise a model of q as a function of depth has been determined for the upper mantle beneath the east pacific rise the model is appropriate for shear waves with periods from roughly 15 to 30 s the amplitudes of multibounce sh phases which propagate within the upper mantle were used to constrain the q structure the multibounce phase amplitudes were measured with respect to the s wave amplitude for each seismogram the data consist of s ss sss and ssss waves for distances from 30 to 80 the sources are earthquakes which occurred on transform faults associated with the east pacific rise and the seismograms were recorded by world wide standard seismograph network and canadian seismic network stations in western north america mantle beneath oceanic crust less than 15 ma and tectonically active continental crust is sampled since the s wave data primarily sample the deep mantle the resulting upper mantle q model depends on the q structure assumed for the lower mantle we assumed a constant q of 312 for the mantle beneath 650 km depth with this assumption the upper mantle phase amplitudes require a q of about 70 in the upper 150 km of the mantle from 150 to 400 km depth q increases to 180 the q in the transition zone from 400 to 650 km depth was found to be 180 significantly lower than the lower mantle q the resulting q model is consistent with several surface wave studies of mantle attenuation beneath the pacific however our results predict a qscs 210 which is significantly higher than many direct measurements of scs attenuation indicate furthermore the large range in measured qscs is inconsistent with lateral q variations being confined to the upper mantle if the region we have studied represents the low q extreme of upper mantle we conclude that large lateral variations in q exist in the lower mantle

crustal and uppermost mantle shear velocity structure adjacent to the juan de fuca ridge from ambient seismic noise 1 based on 6 months of obs data from the cascadia initiative experiment near the juan de fuca ridge we obtain rayleigh wave group and phase speed curves from 6 s to about 20 s period from ambient noise cross correlations among all station pairs we confirm the hypothesis that the dispersion data can be fit by a simple age dependent formula which we invert using a bayesian monte carlo formalism for an age dependent shear wave speed model of the crust and uppermost mantle between crustal ages of 0 5 and 3 5 ma igneous crustal structure is age invariant with a thickness of 7 km water depth varies in a prescribed way and sedimentary thickness and mantle shear wave speeds are found to increase systematically with crustal age the mantle model possesses a shallow low shear velocity zone lvz with a velocity minimum at about 20 km depth at 0 5 ma with lithosphere thickening monotonically with age minimum mantle shear velocities at young ages are lower than predicted from a half space conductively cooling model hscm and the lithosphere thickens with age faster than the hscm providing evidence for nonconductive cooling in the young lithosphere the shallow lvz is consistent with expectations for a largely dehydrated depleted harzburgite mantle with a small retained near ridge partial melt fraction probably less than 1 with melt extending to a lithospheric age of approximately 1 ma i e 30 km from the ridge

sea level rise will drive divergent sediment transport patterns on fore reefs and reef flats potentially causing erosion on atoll islands these data files and matlab scripts reproduce the model data and figures as published in bramante et al in prep modeling the impacts of a changing climate on cross shore sediment transport kwajalein atoll marshall islands

depth averaged two dimensional numerical modeling of unsteady flow and nonuniform sediment transport in open channels a depth averaged two dimensional 2d numerical model for unsteady flow and nonuniform sediment transport in open channels is established using the finite volume method on a nonstaggered curvilinear grid the 2d shallow water equations are solved by the isimple c n algorithms with the rhie and chow s momentum interpolation technique the proposed sediment transport model adopts a nonequilibrium approach for nonuniform total load sediment transport the bed load and suspended load are calculated separately or jointly according to sediment transport mode the sediment transport capacity is determined by four formulas which are capable of accounting for the hiding and exposure effects among different size classes an empirical formula is proposed to consider the effects of the gravity on the sediment transport capacity and the bed load movement direction in channels with steep slopes flow and sediment transport are simulated in a decoupled manner but the sediment module adopts a coupling procedure for the computations of sediment transport bed change and bed material sorting the model has been tested against several experimental and field cases showing good agreement between the simulated results and measured data

sediment transport on arbitrary slopes simplified model the paper presents a study on the influence of gravity on the incipient motion and the bed load transport of sediment the computation of critical bed shear stress is revisited considering the balance of forces hydrodynamic forces and submerged self weight acting on a solitary sediment particle lying on an arbitrary sloping bed modified effective bed shear stress and the corresponding critical bed shear stress which are defined to assess the incipient motion of sediment in the direction of resultant force are applied for the estimation of bed load transport rate in the direction of resultant force the sediment transport induced by the gravitational force which is oblique to the direction of the drag force induced by flow is incorporated into the bed load transport equation this modified model provides a reasonable prediction of the critical bed shear stress and the bed load transport rate the model is validated by experimental data it can be applied to steep slopes and can also avoid the problem of singularity that arises in numerically calculation of sediment transport rate additionally the vectorial transport rate obtained in the model calculation can be implemented in a numerical simulation of channel bed evolution

distribution of recent volcanism and the morphology of seamounts and ridges in the glimpse study area implications for the lithospheric cracking hypothesis for the origin of intraplate non hot spot volcanic chains 1 lithospheric cracking by remotely applied stresses or thermoelastic stresses has been suggested to be the mechanism responsible for the formation of intraplate volcanic ridges in the pacific that clearly do not form above fixed hot spots as part of the gravity lineations intraplate melting petrology and seismic expedition glimpse project designed to investigate the origin of these features we have mapped two volcanic chains that are actively forming to the west of the east pacific rise using multibeam echo sounding and side scan sonar side scan sonar reveals the distribution of rough seafloor corresponding to recent unsedimented lava flows in the hotu matua volcanic complex recent flows and volcanic edifices are distributed over a region 450 km long and up to 65 km wide with an apparent irregular age progression from older flows in the west to younger in the east the 550 km long southern cross seamount sojourn ridge brown ridge chain appears to have been recently active only at its eastern end near the east pacific rise a third region of recent flows is found 120 km north of southern cross seamount in seafloor approximately 9 myr old no indication of lithospheric extension in the form of faulting or graben formation paralleling the trend of the volcanic chains is found in the vicinity of recent flows or anywhere else in the study area thermoelastic cracking could be a factor in the formation of a few small very narrow volcanic ridges but most of the volcanic activity is broadly distributed in wide swaths with no indication of formation along narrow cracks the sojourn and brown chains appear to begin as distributed zones of small seamounts that later develop into segmented ridges perhaps under the influence of membrane stresses from self loading we suggest that the linear volcanic chains are created by moving melting anomalies in the asthenosphere and that lithospheric cracking plays at most a secondary role

effect of dynamic melting on acoustic velocities in a partially molten peridotite abstract the mechanical effect of materials with a zero shear modulus aggregated with solids with non zero shear moduli has been the primary motivation for why partially molten rocks should have lowered shear wave velocities we propose that elastic softening associated with the process of melting must also be considered in this view seismic waves propagating in regions of partial melting perturb the thermodynamic state and drive solid to liquid and liquid to solid thereby lowering the effective elastic moduli we present measurements of young s modulus for a mantle peridotite klb1 at lvz pressures and temperatures and seismic frequency that support this model the model predicts that the softening of elastic modulus is controlled by the pressure dependence of melt fraction f p and not the percentage of melt present in contrast to previous models the p wave velocity is decreased by nearly the same percentage as the s wave velocity

local high slope effects on sediment transport and fluvial bed form dynamics 1 we investigate the morphodynamic evolution of an arbitrarily tilted coarse mobile bed under the action of a free surface water flow the analysis is based on the experimental observations of the evolution of a mobile bed from its initial laterally tilted configuration to the final laterally flat state due to the presence of a lateral component of bed load transport measurements of the bed topography at different times allow one to reconstruct the flattening process the experimental observations are interpreted by means of a three dimensional numerical hydromorphodynamic model employing different relationships linear and nonlinear to evaluate bed load intensity and direction the analysis shows that the inclusion of nonlinear gravitational effects in the description of the bed load leads to a more satisfactory description of the bed evolution this finding opens important issues on how the inclusion of the nonlinear gravitational effects on the bed load can affect the morphodynamic evolution of complex bed topography occurring in natural rivers to investigate this point an application to the evolution of river bars is presented

transport of suspended sediment under the dam break flow on an inclined plane bed of arbitrary slope the problem of transport of suspended sediment after the break of a dam on an inclined bed is considered to that end we use the shallow water approximation for arbitrary constant slopes of the bottom taking into consideration the effect of friction the numerical technique and the frictional model are validated by comparison with available experimental data and asymptotic analytical solutions with special attention to the numerical solution near the wetting front the transport of suspended sediment down the inclined bed is obtained and discussed as a function of the slope of the bed for different values of the parameters characterizing the sediment and its transport properties for sufficiently large times we always find the formation of roll waves near the water front which affects the transport of sediments significantly these strong oscillations are accurately computed with the numerical method used the relative importance of the bed load to the suspended load sediment transport is also discussed as a function of the size of the sediment particles and the slope of the bed for different models on the initiation of sediment suspension from bed load we also check the dilute sediment approach and characterize the conditions for its failure finally the results of the present simplified model are intended to be used as tests of more complex numerical models

modeling the rollovers of sandy clinoforms from the gravity effect on wave agitated sand abstract clinoforms on coasts exposed to ocean waves form from sand exported from the shoreface during extreme wave conditions and deposited where wave action diminishes elsewhere it is shown that during upper 5 percentile wave conditions wave induced shear stresses exceed the sediment threshold of motion to below the clinoform rollovers experiments and theory suggest that where bed sediments are agitated by waves the effect of gravity should move particles down slope with a flux proportional to the slope combined with considerations of continuity this implies a diffusion of the sediment topography a property that would explain the smooth seabed morphology found at sandy rollovers where recorded with multibeam sonar for situations where this gravity effect dominates a simple analytical expression developed here shows how the rollover curvature should relate to wave properties and to the offshore component of sediment flux more sharply curved rollovers are expected where waves have short periods or where the sediment flux is large relative sediment fluxes were calculated using the model from rollover curvature and wave properties for sites from california southeast australia and atlantic and mediterranean iberia the relative magnitudes of the fluxes are roughly as would be expected from local physiography and coastal erosion rates for sandy clinoforms developed under mainly wave influences the model could be useful for exploring variations within clinoform datasets e g how varied convexity in an area of uniform wave properties reflects varied sediment flux and for interpreting how varied curvatures of rollovers within seismic stratigraphy reflect how wave climate and flux have varied in the past

shear wave attenuation and dispersion in melt bearing olivine polycrystals 2 microstructural interpretation and seismological implications 1 the torsional forced oscillation tests of melt bearing olivine aggregates reported by jackson et al 2004 consistently show a peak in attenuation that is absent from melt free aggregates tested under similar conditions and grain sizes characterization by sem shows that the melt resides in triple junction tubules and larger pockets as previously described tem imaging and eds analysis reveals that olivine olivine grain boundaries are characterized by a region 1 nm wide which is structurally and chemically distinct from olivine grain interiors from the possible mechanisms that can produce an anelastic attenuation peak melt squirt can be eliminated for our samples and experimental conditions we attribute the observed attenuation peak to elastically accommodated grain boundary sliding requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt while the nanometer scale grain boundary structure in the melt bearing aggregates is essentially the same as for melt free aggregates studied previously elastically accommodated sliding in the latter is apparently inhibited by tight three grain edge intersections the exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle

seismic velocity anisotropy and heterogeneity beneath the mantle electromagnetic and tomography experiment melt region of the east pacific rise from analysis of p and s body waves 1 we use teleseismic p and s delay times and shear wave splitting measurements to constrain isotropic and anisotropic heterogeneity in the mantle beneath the southern east pacific rise sepr the data comprise 462 p and s delay times and 18 shear wave splitting observations recorded during the mantle electromagnetic and tomography melt experiment we estimate the mantle melt content f and temperature t variation from the isotropic velocity variation our results indicate that the maximum variation in f beneath our array is between zero and 1 2 and maximum variation in t is between zero and 100 k we favor an explanation having partial contributions from both t and f we approximate the seismic anisotropy of the upper mantle with hexagonal symmetry consistent with the assumption of two dimensionality of mantle flow our new tomographic technique uses a nonlinear inversion of p and slow s polarization delay times to simultaneously solve for coupled vp and vs heterogeneity throughout the model and for the magnitude of anisotropy within discrete domains the domain dimensions and the dip of the anisotropy are fixed for each inversion but are varied in a grid search obtaining the misfit of the models to the body wave delay data and to split times of vertically propagating s waves the data misfit and the isotropic heterogeneity are sensitive to domain dimensions and dip of anisotropy in a region centered beneath the sepr the best average dip of the hexagonal symmetry axis is horizontal or dipping shallowly 30 west given the resolution of our data a subaxial region characterized by vertically aligned symmetry axes may exist but is limited to be 80 km deep we infer that the mantle flow beneath the sepr is consistent with shallow asthenospheric return flow from the direction of the south pacific superswell

sediment transport formula for steep channels a description is given and results presented of experiments to investigate the sediment capacity of stream flows on flume slopes of up to 20 a comparison of the results with values predicted by the meyer peter mueller equation shows that the latter equation seriously underestimates sediment transport capacity on slopes steeper than 3 mainly because of deficiencies in the form resistance factor a new equation is proposed giving the transport capacity for materials with grain sizes larger than 0 4 mm 0 016 in the equation is calibrated by both the original meyer peter data and new steep flume experiments and modified to incorporate the effect of nonuniform sediment grain size distributions with flow depths calculated by the manning strickler equation the new equation still gives excellent estimates of sediment capacity

coda wave seismic structure beneath the indian ocean region and its implications to seismotectonics and structural heterogeneity abstract the indian ocean region is one of the challenging areas for seismological research because of its complex seismotectonic settings and intricate structural heterogeneity prevails beneath the indian ocean region in order to address the problem we attempted to determine the attenuation structure for the indian ocean region using coda wave generation by single backscattering model in this study the central frequency of interest has been considered between 1 5 and 12 0 hz with lapse time window ltw of 20 30 40 50 and 60 s measured from the origin time of the earthquake we obtained the attenuation relation for different seismotectonic zones associated with varying estimate of attenuation parameters spreading zone qc spz 171f1 10 subduction zone qc sz 220f0 961 triple point junction qc tpj 156f1 15 prominent faults of indian ocean region qc pfioz 197f0 99 and volcanic and hotspot qc vz hs 209f0 97 we estimated average frequency dependent attenuation parameters of q0 that varies as 156 32 q0 220 33 with corresponding variability in frequency parameter as 0 96 0 05 1 15 0 09 at 40 s ltw for the entire indian ocean region suggesting that the entire indian ocean is seismo tectonically highly active and is attributed to the nature and extent of attenuation due to varying amount of structural heterogeneity in different tectonic zones characterized by the occurrence of earthquakes of varying strengths in the seismically active tectonic zones of the region we found that the triple junction rtj is associated with low qc and high attenuation because of deposition of a variety of sediments at the rtj which is in unison to the geological mapping of the region we observed that our estimated attenuation model is comparable that of the aleutian islands the adak seismic zone at low frequency 5 hz and to the philippine sea subduction zone petukhin at higher frequency range 5 hz which is very much correlative to seismo tectonic settings we infer that our assimilated attenuation model has potential to provide information on intricate seismotectonics and seismogenesis for computing earthquake source parameters and earthquake hazards that may be useful for assimilating detailed seismic velocity structure beneath the indian ocean region

high temperature elastic softening of orthopyroxene and seismic properties of the lithospheric upper mantle summary mineralogical models suggest that low velocity zones of the upper mantle may be accounted for by elasticity of dry rocks along geotherms away from the mid oceanic ridges mor while closer to mor anelasticity plays a significant role in reducing shear wave velocities vs and generating attenuation we investigate the potential influence of elastic softening precursor of high temperature phase transition in orthopyroxenes on the seismic properties of the upper mantle in situ brillouin and raman spectroscopy were used to evidence pre transitional behaviour at high temperature in natural san carlos orthopyroxene pre transitional behaviour induces a large softening of the acoustic and low frequency modes resulting in a large anharmonic decrease of sound velocities similar to that observed in the mgsio3 end member orthoenstatite the high temperature high pressure phase diagram of enstatite is revised to account for the new phase transition and a simple model is developed to evaluate elastic softening effects on the upper mantle seismic properties whose results depend much on the as yet poorly constrained pressure and compositional dependence of the phase transition boundary within the tested range of parameters opx softening is likely to affect the seismic properties of mantle rocks at depths shallower than 80 km in hot regions thus elastic softening of pyroxene is unlikely to affect the lvz or continental lithospheric mantle unless the transition temperature is drastically reduced by incorporation of aluminium in orthopyroxene it will contribute to a decrease of vs near mor hot spots and evolved continental rifts where it can explain part of the non linear high temperature decrease of vs in the lithospheric oceanic mantle it will affect the magnitude of anelastic effects and the nature of the likely mechanisms of attenuation required to match seismological observations

a depth averaged non cohesive sediment transport model with improved discretization of flux and source terms author s zhao j ozgen xian i liang d wang t hinkelmann r abstract 2019 elsevier b v this paper presents novel flux and source term treatments within a godunov type finite volume framework for predicting the depth averaged shallow water flow and sediment transport with enhanced the accuracy and stability the suspended load ratio is introduced to differentiate between the advection of the suspended load and the advection of water a modified harten lax and van leer riemann solver with the contact wave restored hllc is derived for the flux calculation based on the new wave pattern involving the suspended load ratio the source term calculation is enhanced by means of a novel splitting point implicit discretization the slope effect is introduced by modifying the critical shear stress with two treatments being discussed the numerical scheme is tested in five examples that comprise both fixed and movable beds the model predictions show good agreement with measurement except for cases where local three dimensional effects dominate

melt mapped inside earth s mantle an analysis of seismic data reveals the location and quantity of melted rock known as melt in earth s upper mantle the results show how these factors are correlated with the movement of the planet s tectonic plates a possible explanation for the low velocity zone in the mantle

seismic and electrical signatures of the lithosphere asthenosphere system of the normal oceanic mantle although plate tectonics started as a theory of the ocean basins nearly 50 years ago the mechanical details of how it works are still poorly known our understanding of these details has been hampered partly by our inability to characterize the physical nature of the lithosphere asthenosphere system las beneath the ocean we review the existing observational constraints on the seismic and electrical properties of the las particularly for normal oceanic regions away from mid oceanic ridges hot spots and subduction zones where plate tectonics is expected to present its simplest form whereas a growing volume of seismic data on land has provided remarkable advances in large scale pictures seafloor observations have been shedding new light on essential details by combing through these observational constraints researchers are unveiling the nature of the enigmatic las future directions for large scale seafloor observations are also discussed

deterministic coastal morphological and sediment transport modeling a review and discussion 1 modern coastal ocean modeling systems are now capable of numerically simulating a variety of coastal and estuarine problems and can thus provide useful information for managing coastal zones here we review state of the art eulerian implementations of bottom up sediment transport and morphological change in coastal ocean hydrodynamic models in order to investigate the fate of suspended sediment in coastal and estuarine waters as well as the evolution of sea or river beds sediment dynamics need to be represented at a scale relevant to the numerical discretized solution and significant effort is devoted to parameterize sediment processes we discuss boundary layer hydrodynamics and the computation of the bed shear stress we also focus on approaches used to represent near bed processes such as bed load transport and sediment erosion and deposition sediment diffusivities settling velocities and cohesive processes such as flocculation all have an impact on suspended sediment throughout the water column we then describe the implementation of process parameterizations in coastal hydrodynamic models explicitly reviewing five widely used systems the approaches implemented in these coastal models may present distinct strengths and shortcomings with regard to some important issues for coastal zones both numerical and physical while these detailed limitations need to be considered as part of model assessment more general issues also hinder present state of the art models in particular sediment transport is inherently highly empirical which is further compounded by issues arising from turbulence closure schemes we conclude by suggesting some possible directions toward improving sediment dynamics understanding and coastal scale predictive ability

insights into the lithospheric architecture of iberia and morocco from teleseismic body wave attenuation abstract the long and often complicated tectonic history of continental lithosphere results in lateral strength heterogeneities which in turn affect the style and localization of deformation in this study we produce a model for the attenuation structure of iberia and northern morocco using a waveform matching approach on p wave data from teleseismic deep focus earthquakes we find that attenuation is correlated with zones of intraplate deformation and seismicity but do not find a consistent relationship between attenuation and recent volcanism the main features of our model are low to moderate t in the undeformed tertiary basins of spain and high t in areas deformed by the alpine orogeny additionally low t is found in areas where the alboran slab is thought to be attached to the iberian and african lithosphere and high t where it has detached these features are robust with respect to inversion parameters and are consistent with independent data very mild backazimuthal dependence of the measurements and comparison with previous results suggest that the source of the attenuation is sub crustal in line with other recent studies the range of t we observe is much larger than can be expected from lithospheric thickness or temperature variations

surface wave tomography of the upper mantle beneath the reykjanes ridge with implications for ridge hot spot interaction 1 we modeled fundamental mode love and rayleigh waves to study the seismic properties of the upper mantle beneath the reykjanes ridge these waves were generated by regional earthquakes occurring in the north atlantic to the south of iceland and were recorded by stations located on iceland over 12 000 measurements of the phase group arrival time and amplitude of narrow pass filtered waveforms over the period range of 9 5 100 s were used to solve for mantle shear wave velocity structure and anisotropy in a vertical plane oriented normal to the ridge axis the velocity structure contains a broad and deep low velocity zone in the upper mantle beneath the ridge a joint analysis of the seismic structure with gravity data reveals that the low velocities are consistent with elevated temperatures 75 our study shows that plume material spreads broadly outward beneath the reykjanes ridge from iceland and is not confined to a narrow lithospheric channel at distances 200 km from the ridge shear wave anisotropy indicates a predominant horizontal alignment of the fast axes of anisotropic crystals mainly above 50 km depth which can be interpreted as past horizontal ridge perpendicular flow within 200 km of the ridge the anisotropy indicates a general vertical alignment of the fast axes or an alignment such that the fast axes point along the ridge the transition to this type of anisotropy coincides with the appearance of increased hot spot ridge interaction 20 myr ago indicating that plume flow has largely disrupted mantle flow beneath the ridge since that time

low velocity layers in the northwestern margin of the south china sea evidence from receiver functions of ocean bottom seismometer data abstract the receiver function method was applied to teleseismic recordings of an active source ocean bottom seismometer array in the northwestern margin of the south china sea forward modeling of the receiver functions was used to derive velocity interfaces beneath the stations two main groups of low velocity layers were imaged the first group in the reef area with a thickness of 7 10 km exhibits a seaward uptrend of the buried depth but poor continuity the second group with a thickness of 3 4 km is more continuous along the zhongsha trough the buried depth decreases asymmetrically from the flanks to the rift center a high shear wave velocity layer of 4 0 km s is imaged at the crust bottom beneath the zhongsha trough with thickness of 3 km the average vp vs in the cenozoic sediment is 2 0 3 0 and was affected by high fluid activity induced by intensive faults the vp vs of the crust varies between 1 77 and 1 88 suggesting an intermediate to basic composition under the continental extension together these results suggest a two stage evolution model of the continental crust in the first stage the crust deformed in a ductile manner and thus created considerable shearing structures with high anisotropy that caused the obvious decrease of the shear wave velocity in the second stage the shearing structures connected asymmetrically driven by detachment faults in the upper crust and upwelling of the lower crust the zhongsha trough then formed probably with magmatic underplating due to lithospheric extension or rising of hot mantle materials

bed load sediment transport on steep longitudinal slopes the results of new experiments on bed load transport on steep longitudinal slopes are presented the experiments were carried out in a large tilting duct with a fine sand median grain diameter of 208 micro m covering the range of slope angles beta of angle of repose for the sand phi sub s of 32 deg 0 56 rad measurements included bed shear stress flow discharge and the sediment pickup rate on slopes the results suggest that existing theories for bed load transport on slopes are not adequate for all bed slopes and the new data are used to formulate a semiempirical relation that satisfactorily predicts the transport rate on horizontal as well as mild and steep slopes the following are two important findings correcting the threshold term for slope in the meyer peter and muller formula adequately describes the transport rate on positive slopes elevation increasing in direction of flow and on small negative slopes while on steeper negative slopes an extra correction term is required although the value of beta phi sub s at which this term becomes necessary is a function of shear stress and the influence of slope vanishes at high shear stresses