synthesis of realistic oceanic pn wave trains the synthesis of realistic oceanic pn phases was accomplished using wave number integration wave trains with durations of the order of 100 s following the initial compressional wave arrival were generated for a laterally homogeneous vertically inhomogeneous oceanic lithosphere model the excitation of these long wave trains is primarily a result of near receiver water and sediment reverberations and does not involve scattering a late arriving wave train with a group velocity of 1 6 km s having an emergent onset and substantial duration characteristic of oceanic abyssal t phases was also generated even with phase velocities restricted to be greater than 5 5 km s the synthetic pn wave train was compared to data collected in the southwest pacific during the ngendei seismic experiment many of the spectral characteristics associated with sediment and water reverberation observed in the synthetics were also observed in the data first it was found that the amplitude falloff rate of the pn wave train is a strongly increasing function of frequency in direct contrast to results predicted for forward scattered coda this leads to a predominance of low frequencies late in the wave train which is easily interpreted in terms of the constructive interference of different modes of reverberation second the presence of spectral peaks at frequencies associated with both sediment and water reverberations was observed the predicted frequencies were computed using values of the water depth sediment thickness and sediment velocity obtained in an independent study strong alignment of the spectral peaks with the predicted frequencies illustrates the reverberating nature of the pn wave train the presence of substantial power in excess of 15 hz can be attributed to the confinement of pn to the oceanic lithosphere the oceanic pn phase is thus characterized as a refraction from the lower lithosphere with multiple reverberations in the oceanic water column and sediment layer comprising the subsequent coda complicated oceanic lithosphere models are not needed to explain the gross characteristics of pn propagation

the confluence zone of the intense katabatic winds at terra nova bay antarctica as derived from airborne sastrugi surveys and mesoscale numerical modeling the surface wind field inland of the intense coastal kataba c wind regime at terra nova bay antarctica has been studied both observationally and numerically airborne surveys of windinduced features on the snow surface have been used to construct the time averaged winter surface airflow pattern the surface motion field has also been simulated by a mesoscale primitive equation model using terrain slopes with a horizontal resolution of 32 km both methods of analys s demonstrate that the intense katabatic airstream at terra nova bay is forced by converging air currents in the continental interior the broadscale confluence zone becomes organized into two regions within about 180 km of the coast the primary route for katabatic mass transport into the terra nova bay area is reeves glacier valley but an important secondary source is provided by airflow down david glacier the former is generated by the coriolis induced concentration of mass transport on the left side looking downwind of the broadscale confluence zone as well as by the near coastal mountain deflection of airflow into the valley head the confluence zone feeding into david glacier valley stretches over 100 km into the interior and is forced by the broadscale terrain configuration of the ice sheet source areas for the two airstreams differ with reeves glacier valley being fed by cold air formed well into the interior whereas the airflow down david glacier is sustained by radiative cooling over lower parts of the ice sheet which are much closer to the ross sea evaluation of the observational and modeling results reveals both the strengths and weaknesses of each approach airborne surveys of sastrugi orientations are a highly successful method for establishing the detailed pattern of surface airflow however a systematic examination of sastrugi dimensions suggests that such work could also be carried out using spot type satellite observations which is a more cost effective approach than aircraft surveying the double jet structure of katabatic airflow into the terra nova bay area is well represented by the primitive equation modeling but much finer grids are needed to resolve the near coastal drainage details

anomalous mantle structure at the proterozoic paleozoic boundary in northeastern us using teleseismic p wave tomography we detect very large 5 variation in the compressional velocity of the upper mantle beneath the proterozoic paleozoic boundary in the northeastern us in this region the upper mantle is heterogeneous on the 100 km scale to a depth of 300 km below this depth the mantle appears to be laterally homogenous at least on the scale of 200 250 km given the tectonic history of the region the large amplitude short wavelength heterogeneities can not be caused by temperature variations but may be due to chemical heterogeneity or variations in the amount and orientation of flow induced seismic anisotropy

deep water facies processes and models a review and classification scheme for modern and ancient sediments abstract a review of previous work on modern and ancient deep water facies processes and models is presented with a new classification scheme involving 40 distinct facies related to 15 conceptually distinct facies groups these facies are fixed points in a spectrum of facies generated in a process continuum from resedimentation processes through semi permanent bottom currents to pelagic settling in essence the scheme is descriptive of the sedimentary attributes of sediments although it is designed to aid interpretation of possible sediment transport deposition processes the classification scheme is three tier with facies classes groups and constituent facies and is hierarchical to allow flexibility in its use there are seven facies classes with classes a e defined largely on the basis of grain size differences class f on the basis of internal organization and class g on composition the facies classes are class a gravels muddy gravels gravelly muds and pebbly sands with 5 gravel grade class b sands with 80 sand grade and 80 mud 40 silt and 0 20 sand class e muds and clays with 95 mud grade for the purpose of large scale mapping or reconnaissance fieldwork either the level of facies classes or groups may be appropriate whereas for more detailed sedimentology the more detailed facies level will be necessary

accumulation over the greenland ice sheet from historical and recent records water accumulation defined as precipitation minus evaporation was estimated over all of greenland as part of a program to understand changes in ice sheet mass and elevation over 360 historical and recent point accumulation estimates on the greenland ice sheet were evaluated and 276 estimates that were judged to be high quality were used to develop the accumulation map the data set includes 99 points developed as part of four investigations of the past 5 15 years these are judged to have the greatest accuracy using kriging the average accumulation over greenland is estimated to be 30 g cm 2 yr 1 for the interior part of the ice sheet above 1800 m elevation where most of the data were acquired the average accumulation is also estimated to be 30 g cm 2 yr 1 there are still many areas on the ice sheet including northwest southeast and southern greenland where accumulation is highly uncertain exceeding the mean ice sheet uncertainty at a point of 20 25 in these regions further sampling will be required to reduce uncertainty in both regional and ice sheet wide accumulation

mantle discontinuities and temperature under the north american continental keel a ubiquitous feature of upper mantle seismic velocity models has been the presence of high velocity keels beneath stable continental interiors1 2 3 4 5 uncertainty remains however regarding the maximum depth to which continental keels extend the degree to which they have cooled the mantle that surrounds them and their role in mantle flow here we investigate thermal anomalies across the eastern margin of the north american continental keel by imaging the seismic discontinuities at depths of 410 and 660 km with compressional to shear converted waves recorded by a 1 500 km long seismometer deployment in the eastern united states the thickness of the transition zone the region nominally between depths of 410 and 660 km and the depth to the 410 km discontinuity indicate that cold keel material and sub keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than 150 k a 20 km depression of the 660 km discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone2 and may be associated with the remnants of the subducted farallon plate1 2 4

greenland snow accumulation estimates from satellite radar scatterometer data data collected by the c band ers 2 wind scatterometer escat the ku band adeos 1 nasa scatterometer nscat and the ku band seawinds on quikscat qscat satellite instruments are used to illustrate spatiotemporal variability in snow accumulation on the greenland ice sheet microwave radar backscatter images of greenland are derived using the scatterometer image reconstruction sir method at 3 day intervals over the periods 1991 1998 and 1996 1997 for escat and nscat respectively the backscatter coefficient normalized to 40 incidence a and gradient in backscatter b in the range 20 60 are compared with historical snow accumulation data and recent measurements made in the program for arctic regional climate assessment parca shallow snow pits empirical relationships derived from these comparisons reveal different exponential relationships between c and ku band a values and dry snow zone mean annual accumulation q frequency difference images between overlapping scatterometer images suggest that c band data are more sensitive to snow layering and buried inhomogeneities whereas ku band data are more sensitive to volume scattering from recently accumulated snow direct comparisons between nscat b values and in situ q measurements show a linear relationship between ln q and b with a negative rank correlation of r 0 8 the root mean square residual in fitting regression line equation ln q 3 08 17 83b to the data is 0 05 m snow water equivalent this simple ku band empirical relationship is exploited to investigate decadal changes in dry snow zone accumulation between seasat 1978 and nscat 1996 additional comparisons between nscat and recent qscat 1999 data reveal significant upslope shifts in the dry snow line along the southwestern flank of the ice sheet recent acceleration in the increase in intensity of scattering is observed in the percolation zone suggesting increased melting between 2000 and 3000 m elevation in the southern half of the ice sheet

azimuth variation in microwave scatterometer and radiometer data over antarctica while designed for ocean observation scatterometer and radiometer data have proven very useful in a variety of cryosphere studies over large regions of antarctica ice sheet and bedrock topography and the snow deposition drift and erosional environment combine to produce roughness on various scales roughness ranges from broad basin scale ice sheet topography at spl sim 100 km wavelengths to large spatially coherent dune fields at spl sim 10 km wavelength to erosional features on the meter scale known as sastrugi these roughness scales influence the microwave backscattering and emission properties of the surface combining to introduce azimuth angle dependencies in the satellite observation data in this paper the authors explore the use of nasa scatterometer nscat data european remote sensing ers advanced microwave instrument ami scatterometer mode data and special sensor microwave imager ssm i data to study surface roughness effects in antarctica all three sensors provide strong evidence of azimuth modulation which is correlated with the surface slope environment and results in a katabatic wind flow regime due to its broad azimuth coverage nscat data appears to be the best suited for azimuth angle observations a simple empirical model for the azimuth variation in the radar backscatter is developed and an algorithm for computing the parameters of the model from nscat data at a fine scale is presented results indicate relationships exist between the azimuthal variation of the data and the orientation of the surface slope and small scale roughness relative to the sensor look direction

thinning of the ice sheet in northwest greenland over the past forty years thermal expansion of the oceans as well as melting of glaciers ice sheets and ice caps have been the main contributors to global sea level rise over the past century the greatest uncertainty in predicting future sea level changes lies with our estimates of the mass balance of the ice sheets in greenland and antarctica1 satellite measurements have been used to determine changes in these ice sheets on short timescales demonstrating that surface elevation changes on timescales of decades or less result mainly from variations in snow accumulation2 here we present direct measurements of the changes in surface elevation between 1954 and 1995 on a traverse across the north greenland ice sheet measurements over a time interval of this length should reflect changes in ice flow the important quantity for predicting changes in sea level relatively unperturbed by short term fluctuations in snow accumulation we find only small changes in the eastern part of the transect except for some thickening of the north ice stream on the west side however the thinning rates of the ice sheet are significantly higher and thinning extends to higher elevations than had been anticipated from previous studies3

mantle helium in ground waters of eastern north america time and space constraints on sources mantle helium in continental environments is generally considered to be the result of active volcanism and or active extension the latest episodes of volcanism in northeastern north america are the track of the new england hotspot 95 190 ma and the closure of the iapetus sea before 300 ma thus the identification of mantle helium in young ground waters of central new england is counter to the conventional wisdom on the basis of evaluation of helium evolution in emplaced magmas we postulate an aged mantle source for the excess helium component in ground waters of central new england that is either 1 a local near surface emplaced gas rich magma that has retained significant volatiles e g in fluid inclusions or 2 a deeply emplaced gas rich magma with high initial 2 he 4 he 10 5 and helium transport with dispersion through the crust over time this gas rich initial condition may support the concept of a volatile enriched mantle wedge and thus explain the increased buoyancy flux of the new england hotspot as it traversed eastern north america as has been suggested by others

sks splitting beneath transportable array stations in eastern north america and the signature of past lithospheric deformation seismic anisotropy in the upper mantle beneath continental interiors is generally complicated with contributions from both the lithosphere and the asthenosphere previous studies of sks splitting beneath the eastern united states have yielded evidence for complex and laterally variable anisotropy but until the recent arrival of the usarray transportable array ta the station coverage has been sparse here we present sks splitting measurements at ta stations in eastern north america and compare the measured fast directions with indicators such as absolute plate motion surface geology and magnetic lineations we find few correlations between fast directions and absolute plate motion except in the northeastern u s and southern canada where some stations exhibit variations in apparent splitting with backazimuth that would suggest multiple layers of anisotropy a region of the southeastern u s is dominated by null sks arrivals over a range of backazimuths consistent with previous work we document a pattern of fast directions parallel to the appalachian mountain chain suggesting a contribution from lithospheric deformation associated with appalachian orogenesis overall our measurements suggest that upper mantle anisotropy beneath the eastern united states is complex with likely contributions from both asthenospheric and lithospheric anisotropy in many regions

isotopic geochemistry of the saratoga springs implications for the origin of solutes and source of carbon dioxide we report the results of an isotopic study designed to determine the source of solutes and carbon dioxide in the famed saratoga springs new york mineral waters these waters have thousands of milligrams per liter total dissolved solid concentrations and are highly charged with carbon dioxide gas the spring waters are cold 12 c and there is no local deep seated thermal anomaly they emerge through thick shale caprock along the surface expression of normal faults the 13c 5 8 to 0 8 vienna peedee belemnite of the dissolved inorganic carbon and elevated 3he 4he ratios suggest that the source of the co2 is the mantle or an ancient deep crystallized igneous melt the stable isotopic content of the spring waters defines a mixing line between modern local meteoric waters 70 and a component with heavier d but similar 18o values this trend and that of 87sr 86sr of dissolved strontium versus 1 sr are consistent with the hypothesis that canadian shield type brines contribute salinity to the springs these brines plausibly migrate from the adirondack mountains to the topographically low mcgregor fault system in the hudson river lowlands where the saratoga springs discharge

enigmatic deep water depositional mechanisms upper part of the oquirrh group utah abstract textures body fossil assemblages trace fossil assemblages and lack of sedimentary structures formed by tractive currents indicate that upper pennsylvanian and wolfcampian sandstones and limestones of the oquirrh group in northwestern utah were deposited in deep water however the abundant thick bedded to massive sandstones are not classical turbidites the depositional mechanisms of five deep water lithofacies have been inferred 1 massive and thickly bedded sands were transported by turbidity currents and deposited by grain flows 2 rippled sands were deposited from turbidity currents 3 bioclast packstones similar to walker s 1978 pebbly sandstones formed from sediments emplaced by turbidity currents and grain flows 4 non channelized conglomerates were deposited by dense turbid flows gram flows and debris flows and 5 thinly bedded siltstones are of hemipelagic origin the lateral continuity of individual beds is generally not known but it is well documented that 100 m thick sets of beds are continuous over several kilometers of distance and that units dominated by a single lithofacies can be mapped from 10 to over 10 kilometers nevertheless the lateral depositional geometry of the upper part of the oquirrh group remains enigmatic indications of an integrated submarine fan distribution system are vague at best

turbidite facies fluid escape structures and mechanisms of emplacement of the oligo miocene aljibe flysch gibraltar arc betics southern spain abstract the aljibe flysch the largest tectono sedimentary unit in the gibraltar arc provides a record of oligo miocene sedimentation in the betic orogen southern spain the aljibe flysch can be subdivided into two sub units the beneiza flysch late oligocene and the aljibe arenites early miocene which have very different depositional histories and mechanisms of emplacement the beneiza flysch is characterised by laterally continuous and thin bedded lithic arenites siltstones and mudstones which contain complete and partial bouma sequences these are interpreted to have been deposited in a sheet turbidite system which may have formed part of the north african flysch trough by contrast the aljibe arenites comprise of thick bedded super mature quartz arenites which are dominated by fluid escape structures these sandstones are similar to facies group b1 b2 disorganised sands of pickering et al 1989 which are interpreted to be the product of deposition from high density turbidity currents fluid escape structures have been classified according to the scheme of lowe 1975 and their vertical arrangement indicates that a large volume of sediment was deposited extremely rapidly during frequent remobilisation events consideration of the origin of the fluid escape structures and their vertical arrangement has identified problems in their interpretation and identified key areas for future research the boundary between the beneiza flysch and aljibe arenites is interpreted to be a significant stratigraphic boundary across which there is a major change in sediment source and mechanism of sediment emplacement several models can be put forward for the generation of this stratigraphic break and these serve to set the agenda for future research into the emplacement of other similar flysch sequences and the origin of the gibraltar arc flysch

observation and characterization of radar backscatter over greenland characterization of the microwave signature of the greenland snow surface enables delineation of the different snow facies and is a tool for tracking the effects of climate change a new empirical observation model is introduced that uses a limited number of parameters to characterize the snow surface based on the dependence of radar backscatter on incidence angle azimuth angle spatial gradient and temporal rate of change the individual model parameters are discussed in depth with examples using data from the nasa scatterometer nscat and from the c band european remote sensing ers satellite advanced microwave instrument in scatterometer mode the contribution of each model term to the overall accuracy of the model is evaluated the relative contributions of the modeled dependencies vary by region two studies illustrating applications of the model are included first interannual changes over the greenland ice sheet are investigated using nine years of ers data surface changes are observed as anomalies in the spl sigma spl deg model parameters second intraannual variations of the surface are investigated these changes are observed in the average backscatter normalized to a given observation geometry the results indicate that the model can be used to obtain a more complete understanding of multiyear change and to obtain low variance high temporal resolution observations of intraannual changes the model may be applied for increased accuracy in scatterometer synthetic aperture radar sar and wide angle sar studies

the seismic 8 discontinuity and partial melting in continental mantle strong scattered reflections beyond 8 degrees 8 offset are characteristic features of all high resolution seismic sections from the continents the reflections identify a low velocity zone below approximately 100 kilometers depth beneath generally stratified mantle this zone may be caused by partial melting globally initiated at equal depth in the continental mantle solid state is again attained at the lehmann discontinuity in cold stable areas whereas the zone extends to near the 400 kilometer discontinuity in hot tectonically active areas thus the depth to the lehmann discontinuity may be an indicator of the thermal state of the continental mantle

layered structure in the upper mantle across north america from joint inversion of long and short period seismic data abstract we estimate crustal and uppermost mantle shear velocity structure beneath 30 stations in north america by jointly inverting the high frequency scattered wavefield observed in the p wave coda together with long period surface wave phase and group dispersion data several features distinguish our approach from previous such joint inversions 1 we apply a cross convolution method rather than more standard deconvolution approaches used in receiver function studies and consider both love and rayleigh wave dispersion allowing us to infer profiles of radial anisotropy 2 we generate probabilistic 1d radially anisotropic depth profiles across the whole uppermost mantle down to 350 km depth 3 the inverse problem is cast in a trans dimensional bayesian formalism where the number of isotropic and anisotropic layers is treated as unknown allowing us to obtain models described with the least number of parameters results show that the tectonically active region west of the rocky mountain front is marked by a lithospheric asthenosphere boundary and a lehmann discontinuity occurring at relatively shallow depths 60 150 km and 100 200 km respectively whereas further east in the stable craton these discontinuities are deeper 170 200 km and 200 250 km respectively in addition in the stable part of the continent at least two mid lithospheric discontinuities are present at intermediate depths suggesting the existence of strong lithospheric layering and a mechanism for lithospheric thickening by underplating of additional layers as cratonic age increases the moho across the continent as well as mid crustal discontinuities in the craton are also imaged in agreement with independent studies

snow facies over ice sheets derived from envisat active and passive observations this paper aims to separate different snow regions over the terrestrial ice sheets based on their measured microwave signatures it takes advantage of coregistered data from passive and active sensors on the environmental satellite envisat to directly derive a snow facies indicator in a point by point basis this paper represents the first attempt of this kind in exploiting nadir viewing and dual frequency data from both altimeter and radiometer sensors the approach is based on a clustering method such representation of the data by means of fewer clusters necessarily loses fine details but achieves simplification in geographical representation and eases the description of the condition of the ice sheets in 2004 our approach broadens the description of the snow pack by taking into account characteristics such as surface roughness grain size stratification and snowmelt effects whereas the latter has often solely been considered in most previous work such partition of the ice sheets might help to better understand relationships between microwave signatures and snow morphology it could also be useful for estimating elevation uncertainty in altimeter data which in turn is essential to correctly interpret the significance of the rates of elevation change in a changing climate and to convert elevation change to snow mass change

greenland ice sheet surface properties and ice dynamics from ers 1 sar imagery c band synthetic aperture radar sar imagery from the european space agency s ers 1 satellite reveals the basic zonation of the surface of the greenland ice sheet the zones have backscatter signatures related to the structure of the snowpack which varies with the balance of accumulation and melt at various elevations the boundaries of zones can be accurately located with the use of this high resolution imagery the images also reveal a large flow feature in northeast greenland that is similar to ice streams in antarctica and may play a major role in the discharge of ice from the ice sheet

a sharp lithosphere asthenosphere boundary imaged beneath eastern north america although a rigid lithosphere about 100 km of crust and uppermost mantle moving over a weaker asthenosphere a few hundred kilometres of more deformable rock is a widely accepted aspect of plate tectonics the properties of the boundary between them are poorly understood when compressional waves p waves from an earthquake encounter an interface at an angle some of the energy is converted to shear waves or s waves these converted waves can be used to construct an image of the interface based on converted waves recorded in eastern north america the lithosphere asthenosphere boundary has been resolved as a sharp gradient that cannot be explained by thermal gradients alone indicating that the boundary is controlled by melt or high water content in the asthenosphere plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere yet the nature of the lithosphere asthenosphere boundary remains poorly understood the gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers for example if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures mantle flow models1 2 indicate that the velocity gradient at its base would occur over tens of kilometres in contrast if the asthenosphere is weak owing to volatile enrichment3 4 5 6 or the presence of partial melt7 the lithosphere asthenosphere boundary could occur over a much smaller depth range here we use converted seismic phases in eastern north america to image a very sharp seismic velocity gradient at the base of the lithosphere a 3 11 per cent drop in shear wave velocity over a depth range of 11 km or less at 90 110 km depth such a strong sharp boundary cannot be reconciled with a purely thermal gradient but could be explained by an asthenosphere that contains a few per cent partial melt7 or that is enriched in volatiles relative to the lithosphere3 4 5 6

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

an edge detection technique to estimate melt duration season and melt extent on the greenland ice sheet using passive microwave data the melt extent duration and melt season on the greenland ice sheet were estimated using an edge detection technique on passive microwave data from the ssm i and smmr instruments 18 19v ghz channel for the period 1979 to 1997 the annual brightness temperature tb time series at a pixel location that experiences summer melt has a steep rise and drop in tb with the onset and end of melt respectively a derivative of gaussian edge detector is used to detect edges corresponding to the onset and end of melt the time lapsed between the first upward and last downward edge on the annual tb time series gives an estimate of the melt season and the time lapsed between successive upward and downward edges gives the duration of melt while the maximum melt extent increased by 18 the total duration of melt increased by 3 7 and the total melt season decreased by 3 8 during the period 1979 1997

the northern appalachian anomaly a modern asthenospheric upwelling the northern appalachian anomaly naa is an intense laterally localized 400km diameter low velocity anomaly centered in the asthenosphere beneath southern new england its maximum shear velocity contrast at 200km depth is about 10 and its compressional to shear velocity perturbation ratio is about unity values compatible with it being a modern thermal anomaly although centered close to the track of the great meteor hot spot it is not elongated parallel to it and does not crosscut the cratonic margin in contrast to previous explanations we argue that the naa s spatial association with the hot spot track is coincidental and that it is caused by small scale upwelling associated with an eddy in the asthenospheric flow field at the continental margin that the naa is just one of several low velocity features along the eastern margin of north america suggests that this process may be globally ubiquitous

radar measurements of melt zones on the greenland ice sheet surface based microwave radar measurements were performed at a location on the western flank of the greenland ice sheet here firn metamorphosis is dominated by seasonal melt which leads to marked contrasts in the vertical structure of winter and summer firn this snow regime is also one of the brightest radar targets on earth with an average backscatter coefficient of 0 db at 5 3 ghz and an incidence angle of 25 by combining detailed observations of firn physical properties with ranging radar measurements we find that the glaciological mechanism associated with this strong electromagnetic response is summer ice lens formation within the previous winter s snow pack this observation has important implications for monitoring and understanding changes in ice sheet volume using space borne microwave sensors

wintertime surface winds over the greenland ice sheet abstract clear sky wintertime surface winds over the greenland ice sheet are simulated with a three dimensional mesoscale numerical model it is shown that the simulated winds blow from the broad gently sloped interior to the steep coastal margins this general wind pattern is similar to that found over antarctica due to the same governing dynamics the longwave radiational cooling of the sloping ice terrain is the key driving force of this cold airflow in some coastal areas the downslope winds converge into large fjords such as kangerlussuaq and sermilik this is consistent with the frequent presence in these areas of warm signatures on cloud free thermal infrared satellite images that are generated by katabatic winds the shape of the greenland ice sheet plays an important role in directing the flow of the surface winds the study demonstrates that the surface wind pattern is only moderately affected by climatological flow around and over the ice sheet the mass redistribution associated with the kat

on reconciling ground based with spaceborne normalized radar cross section measurements this study examines differences in the normalized radar cross section derived from ground based versus spaceborne radar data a simple homogeneous half space model indicates that agreement between the two improves as 1 the distance from the scatterer is increased and or 2 the extinction coefficient increases

comparison of microwave backscatter anisotropy parameterizations of the antarctic ice sheet using ascat the c band eumetsat advanced scatterometer ascat was launched in 2007 and provides backscatter measurements with excellent azimuth and incidence angle diversity in polar regions c band backscatter measurements can be used to retrieve near surface snow firn properties such as grain size and accumulation rate however the antarctic ice sheet ais exhibits strong microwave backscatter anisotropy as a function of both incidence and azimuth angles and this anisotropy must be well understood before such physical parameter retrieval can be accurately performed this paper presents a detailed comparison of several different parameterizations of both the azimuth and the incidence anisotropy parameterizations used here result in residuals similar to the radiometric performance of ascat indicating accurate characterization of the anisotropy the analysis of these parameterizations provides an insight into the physical mechanisms taking place near the surface of the ais 30 day composite maps of several parameters are presented and discussed in the context of near surface glaciology this anisotropy characterization will form the basis of a technique for extraction of physical parameters of the ais

seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of kalahari craton 1 based on joint consideration of s receiver functions and surface wave anisotropy we present evidence for the existence of a thick and layered lithosphere beneath the kalahari craton our results show that frozen in anisotropy and compositional changes can generate sharp mid lithospheric discontinuities mld at depths of 85 and 150 200 km respectively we found that a 50 km thick anisotropic layer containing 3 s wave anisotropy and with a fast velocity axis different from that in the layer beneath can account for the first mld at about 85 km depth significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere as inferred from chemical tomography and those of our second mld led us to characterize it as a compositional boundary most likely due to the modification of the cratonic mantle lithosphere by magma infiltration the deepening of this boundary from 150 to 200 km is spatially correlated with the surficial expression of the thabazimbi murchison lineament tml implying that the tml isolates the lithosphere of the limpopo terrane from that of the ancient kaapvaal terrane the largest velocity contrast 3 6 4 7 is observed at a boundary located at depths of 260 280 km beneath the archean domains and the older proterozoic belt this boundary most likely represents the lithosphere asthenosphere boundary which shallows to about 200 km beneath the younger proterozoic belt thus the kalahari lithosphere may have survived multiple episodes of intense magmatism and collisional rifting during the billions of years of its history which left their imprint in its internal layering

shear velocity structure and azimuthal anisotropy beneath eastern north america from rayleigh wave inversion 1 we have obtained shear velocity structure beneath the northeastern united states and southeastern canada using rayleigh wave phases and amplitudes thin crust 36 42 km is observed along the atlantic coast and in the eastern appalachian orogen and thick crust 42 46 km is imaged in the western appalachians and in the western new york portion of the grenville province the variation of crustal thickness correlates well with the observed bouguer gravity anomalies in the upper mantle the high velocity continental keel of cratonic north america is present in the western part of the study area while a broad low velocity region is imaged in new england from the hudson river valley to the white mountains this low velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel in addition to lateral variations in velocity we estimate the azimuthal dependence of phase velocity strong and relatively uniform shear wave splitting is observed in the study region but at periods of 100 s or less the average azimuthal anisotropy of rayleigh waves is less than 1 and is not significantly different from zero at any individual period this small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere much of the source of the shear wave splitting must lie deeper than 200 km

thin lithosphere beneath the central appalachian mountains constraints from seismic attenuation beneath the magic array abstract the passive margin of the eastern coast of the united states is known to be geologically active with recently rejuvenated topography intraplate seismicity and volcanism of eocene age this study uses seismic data from the mid atlantic geophysical integrative collaboration magic experiment to constrain lateral variations in the attenuation of teleseismic p waves beneath the central appalachian mountains to shed light on the structure and dynamics of the upper mantle at this active passive margin we use a monte carlo approach to estimate variations in attenuation along with both data and model uncertainties the quality factor of the upper mantle dramatically decreases over a distance of less than 50 km on the western side of the central appalachian mountains where a low velocity anomaly has been previously inferred extrinsic factors such as scattering or focusing are rejected as explanations for the observations on the basis of finite difference waveform modeling experiments the peak in attenuation beneath the crest of the appalachian mountains requires that near to super solidus conditions occur in the upper mantle and is co located with volcanism of eocene age our preferred interpretation is that the attenuation reflects the removal of the mantle lithosphere via delamination beneath the mountains followed by ongoing small scale convection

the lithosphere asthenosphere boundary and cratonic lithospheric layering beneath australia from sp wave imaging sp and ps scattered wave receiver functions were calculated for nineteen stations across australia and the island of tasmania in order to image the lithosphere asthenosphere boundary and layering within the lithosphere within phanerozoic eastern australia and the eastern margin of the south australia craton prominent sp phases from a negative velocity contrast were found at depths of 61 11 km to 131 9 km consistent with the lithosphere asthenosphere boundary depth range from surface wave tomography these phases imply significant velocity drops over depth ranges of 30 40 km or less and thus cannot be explained by a lithosphere asthenosphere boundary that is controlled by temperature alone rather they imply that the asthenosphere is hydrated with respect to a drier depleted lithosphere or contains a small amount of partial melt the shallowest sp phases have the largest amplitudes and occur in regions with the most recent voluminous volcanism strengthening the link to partial melt at the base of the lithosphere in contrast no significant negative sp phases were found at the base of the thick cratonic lithosphere at the stations in central and western australia implying that the cratonic lithosphere asthenosphere velocity gradient is distributed over more than 50 70 km in depth this gradient may be purely thermal in origin although gradational changes in composition or melt content cannot be ruled out a negative sp phase was observed at depths of 69 8 km to 85 14 km at stations in central and western australia indicating the presence of a drop in velocity internal to the lithosphere this interface within the lithosphere may be a relic of cratonic mantle formation or the result of alteration by melt and metasomatism

variations in snowpack melt on the greenland ice sheet based on passive microwave measurements a simple microwave emission model is used to simulate 37 ghz brightness temperatures associated with snowpack melt conditions for locations across the greenland ice sheet the simulated values are utilized as threshold values and compared to daily gridded smmr and ssm i passive microwave data in order to reveal regions experiencing melt the spatial extent of the area classified as melting is examined on a daily monthly and seasonal may august basis for 1979 91 the typical seasonal cycle of melt coverage shows melt beginning in late april a rapid increase in the melting area from mid may to mid july a rapid decrease in melt extent from late july through mid august and cessation of melt in late september seasonal averages of the daily melt extents demonstrate an apparent increase in melt coverage over the 13 year period of approximately 3 8 annually significant at the 95 confidence interval this increase is dominated by statistically significant positive trends in melt coverage during july and august in the west and southwest of the ice sheet we find that a linear correlation between microwave derived melt extent and a surface measure of ablation rate is significant in june and july but not august so caution must be exercised in using the microwave derived melt extents in august nevertheless knowledge of the variability of snowpack melt on the greenland ice sheet as derived from microwave data should prove useful in detecting climate change in the arctic and examining the impact of climate change on the ice sheet

analysis of synthetic aperture radar data collected over the southwestern greenland ice sheet analyses of the first aircraft multi frequency polarimetric synthetic aperture radar sar data acquired over the southwestern greenland ice sheet are presented data were collected on 31 august 1989 by the jet propulsion laboratory sar using the nasa dc 8 aircraft along with curvilinear patterns associated with large scale morphologic features such as crevasses lakes and streams frequency and polarization dependencies are observed in the p l and c band image products

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

recent changes in the microwave scattering properties of the antarctic ice sheet time series satellite microwave data are used to monitor and quantify changes in the scattering properties of the antarctic ice sheet daily ers scatterometer escat and special sensor microwave imager ssm i image data acquired since 1992 are analyzed to understand the seasonal and interannual changes over the ice sheet for regions of the ice sheet where azimuthal modulation is negligible seasonal cycles are observed in both the escat amplitude spl sim 0 5 db and ssm i data amplitude spl sim 10 k these cycles are attributed to seasonal changes in surface temperature interannual variability in the time series signatures appears to be associated with accumulation there is also evidence to suggest that shifts in the wind direction can alter the backscatter through azimuthal modulation over the period 1992 97 large trends are observed in the escat 1 k yr sup 1 signatures over several regions in antarctica these changes typically occur over ice shelves and at the margins of the ice sheet where previous melt events have occurred and where accumulation is relatively high 300 mm yr sup 1 it is likely the large changes result from the successive burial of an efficient scattering layer formed by refreezing after a melt event prior to 1992 there is also evidence to suggest that similarly large changes can be observed in the interior of the ice sheet due to the burial of depth hear layers in order to monitor long term change in the properties of the antarctic ice sheet it is necessary to remove the seasonal cycle from time series microwave data such anomaly data can then be used to understand the link between escat and ssm i with accumulation and wind shifts

a contrast in anisotropy across mid lithospheric discontinuities beneath the central united states a relic of craton formation the mid lithospheric discontinuity mld is a seemingly sharp decrease in seismic velocity at depths internal to the lithosphere and appears to be a pervasive feature beneath continental interiors its presence within cratons which have remained relatively stable since formation suggests that the mld may result from processes associated with continent formation we use p to s wave receiver functions to interrogate seismic anisotropy across the mld within the ca 1 35 1 55 ga granite rhyolite province of the central united states our analysis reveals strong evidence for sharp changes in the orientation of anisotropy across multiple mlds with an approximately north to northwest fast orientation of anisotropy in the upper lithosphere the consistency of this signature over a large region suggests that the observed anisotropy is a relic of north american craton formation in addition the presence of several distinct anisotropic layers within the cratonic lithosphere supports models for craton formation via stacked subducted slabs or a series of underthrusting events

conductivity structure of the lithosphere asthenosphere boundary beneath the eastern north american margin tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere asthenosphere boundary lab yet this boundary remains enigmatic regarding its properties and geometry the processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies here we employ marine magnetotelluric mt data acquired along a 135 km long profile offshore martha s vineyard new england usa to image the electrical conductivity structure beneath the new england continental margin for the first time we invert the data using two different mt 2 d inversion algorithms and present a series of models that are obtained using three different parameterizations fully unconstrained unconstrained with an imposed lab discontinuity and a priori constrained lithosphere resistivity this suite of models infers variability in the depth of the lab with an average depth of 115 km at the eastern north america passive margin models robustly detect a 350 m lithospheric anomalous conductivity zone lacz that extends vertically through the entire lithosphere our preferred conductivity model is consistent with regional p to s receiver function data shear wave velocity gravity anomalies and prominent geological features we propose that the lacz is indicative of paleo lithospheric thinning either resulting from kimberlite intrusions associated with rifting and the new england great meteor hotspot track or from shear driven localized deformation related to rifting

monteregian hotspot track a long lived mantle plume the monteregian hills the younger white mountains and the new england seamounts constitute a well defined hotspot track the evolution of the plume through time is obtained from the marine track the buoyancy flux of the plume when it made the new england seamounts was about 2 mg s 1 which would make it the seventh or eighth most active plume today the hotspot declined in activity after it made nashville seamount the buoyancy flux when it made corner seamounts and today is less than 0 5 mg s 1 the changes in the flux of the marine hotspot limit how far the marine flux can be extrapolated to the west onto the continental track and what can be learned in general about vulnerability of the continental lithosphere to hotspots the lack of an obvious track west of montreal may be due either to failure of the plume to penetrate the canadian shield to the lack of recognizable intrusions or to strengthening of the plume when it approached montreal

no regional anisotropic domains in the northeastern u s appalachians a fabric frozen within the continental lithosphere should lead to regional anisotropic domains with close correlation between tectonics seismic anisotropy and shear wave travel time delays alternatively seismic anisotropy could be maintained by active deformation in the sub lithospheric mantle this scenario would predict broad spatial consistency in shear wave splitting and weak correlation between the splitting surface tectonics and travel time delays a coherent pattern of shear wave splitting is observed in the northeastern united states covering large parts of new york and new england spanning the appalachian orogen and areas underlain by the grenvillian basement the direction of fast shear wave propagation as measured from core refracted sks skks and pks phases varies systematically with event back azimuth at all sites examined and is fit well by two layers of anisotropy with subhorizontal symmetry axes relative travel time delays of shear waves that sample the region vary over 100 km length scale and correlate with surface geology tomographic images suggest lateral shear velocity variations 3 including a slow anomaly beneath the grenvillian basement exposed in the adirondack mountains there is little correlation between this horizontally rough seismic velocity and the horizontally smooth anisotropic model consistent with shear wave splitting we therefore conclude that in the northeastern us the concept of regional anisotropic domains i e distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation does not apply

seismic velocity structure of the crust and shallow mantle of the central and eastern united states by seismic surface wave imaging seismic surface waves from the transportable array of earthscope s usarray are used to estimate phase velocity structure of 18 to 125 s rayleigh waves then inverted to obtain three dimensional crust and upper mantle structure of the central and eastern united states ceus down to 200 km the obtained lithosphere structure confirms previously imaged ceus features e g the low seismic velocity signature of the cambrian reelfoot rift and the very low velocity at 150 km depth below an eocene volcanic center in northwestern virginia new features include high velocity mantle stretching from the archean superior craton well into the proterozoic terranes and deep low velocity zones in central texas associated with the late cretaceous travis and uvalde volcanic fields and beneath the south georgia rift which contains jurassic basalts hot spot tracks may be associated with several imaged low velocity zones particularly those close to the former rifted laurentia margin

thin lithosphere beneath the central appalachian mountains a combined seismic and magnetotelluric study abstract a joint analysis of magnetotelluric and sp receiver function data collected along a profile across the central appalachians highlights variations in regional lithospheric structure while the interpretation of each data set by itself is non unique we identify three distinct features that are consistent with both the resistivity model and the receiver function image 1 thin lithosphere beneath the appalachian mountains 2 somewhat thicker lithosphere to the east of the mountains beneath the coastal plain and 3 a lithosphere asthenosphere boundary that deepens to the west of the mountains in some regions the correspondence between seismic velocity discontinuities and resistivity mark the base of the lithosphere while in other locations we see seismic discontinuities that are contained within the lithosphere at the western end of our profile a transition from highly resistive lithosphere to more conductive mantle represents the transition across the grenville front the thickness of lithosphere beneath the grenville terrain is 140 km lithosphere at the eastern end of the profile has a thickness that is not well constrained by our coverage but is at least 110 km thick this lithosphere can be associated with a broader region of high resistivity material seen to extend further south directly beneath the appalachian mountains lithospheric thickness is inferred to be as thin as 80 km based on observations of elevated mantle conductivities and a westward dipping seismic converter electrical conductivities in the uppermost asthenospheric mantle are sufficiently high 0 1 s m to require the presence of a small volume of partial melt the location of these elevated conductivities is close offset 50 km to the west to eocene volcanic outcrops in and around harrisonburg va our observations speak to mechanisms of intraplate volcanism where there is no divergent or convergent plate motion to trigger mantle upwelling or obvious fluid release either of which can facilitate melting instead we suggest that small scale mantle convection related either to pre existing lithospheric thickness variations or to lithospheric loss through delamination coupled with relative plate motion with respect to the underlying asthenosphere can trigger small amounts of melting this melt migrates upslope along the base of the lithosphere potentially thermally eroding the lithosphere resulting in further thinning

snowmelt on the greenland ice sheet as derived from passive microwave satellite data abstract the melt extent of the snow on the greenland ice sheet is of considerable importance to the ice sheet s mass and energy balance as well as arctic and global climates by comparing passive microwave satellite data to field observations variations in melt extent have been detected by establishing melt thresholds in the cross polarized gradient ratio xpgr the xpgr defined as the normalized difference between the 19 ghz horizontal channel and the 37 ghz vertical channel of the special sensor microwave imager ssm i exploits the different effects of snow wetness on different frequencies and polarizations and establishes a distinct melt signal using this xpgr melt signal seasonal and interannual variations in snowmelt extent of the ice sheet are studied the melt is found to be most extensive on the western side of the ice sheet and peaks in late july moreover there is a notable increasing trend in melt area between the years 1979 and 1991 of 4 4 per year which came to an abrupt halt in 1

toward a robust retrieval of snow accumulation over the antarctic ice sheet using satellite radar 1 the sensitivity of radar measurements to snow accumulation rate is determined by the firn volume characteristics of the ice sheets here we present a new approach for calculating the volume backscattering of dry firn which is combined with recently developed empirical parameterizations of firn grain size and density as functions of depth surface temperature and accumulation rate to this end dense medium radiative transfer theory is applied to calculate the volume scattering and absorption coefficients the coefficients for the density transition between snow and dense firn are evaluated using polynomial interpolation for testing the method we used measured accumulation rates envisat asar c band wide swath mode images and quikscat ku band backscattering data from dronning maud land antarctica the comparison between measured and simulated backscattering coefficients shows that no tuning parameter is necessary to obtain the correct absolute level of scattering intensity the robustness of accumulation rate retrievals depends on the consideration of technical and environmental factors due to the presence of sastrugi on the ice sheet surface the measured intensities are sensitive to the radar look direction wind compaction of snow and depth hoar formation change the depth dependent snow density and grain size profiles theoretical simulations revealed that the backscattering coefficient at c band is more sensitive to changes of accumulation rates than at ku band penetration depths can vary significantly dependent on radar frequency and firn characteristics this has to be taken into account when comparing accumulation rates from different locations

relating microwave backscatter azimuth modulation to surface properties of the greenland ice sheet azimuth dependence of a normalized radar cross section over the greenland ice sheet is modeled with a simple surface scattering model the model assumes that azimuth anisotropy in surface roughness at scales of 3 300 m is the primary mechanism driving the modulation to evaluate the contribution of azimuth anisotropy in surface roughness to the radar backscatter the model is compared to models based on isotropic surface roughness the models are inverted to estimate snow surface properties using measurements from the c band european remote sensing satellite advanced microwave instrument in scatterometer mode results indicate that the largest mesoscale rms surface slopes are found in the lower portions of the dry snow zone estimates of the preferential direction in surface roughness are highly correlated with katabatic wind fields over greenland which is consistent with wind formed sastrugi as the dominant mechanism causing azimuth modulation of the maximum improvement of the azimuth modulation surface model compared to its isotropic counterparts occurs in the lower regions of the dry snow zone where the azimuth variability of is the largest in regions with azimuth modulation over 1 db the mean root mean square error estimate of the azimuth dependent surface scattering model is 0 46 db compared with 0 70 db for similar models using isotropic roughness

spectral characteristics of high frequency pn sn phases in the western pacific pn and sn phases from 25 selected earthquakes recorded since july of 1979 on ocean bottom hydrophones near wake island are used to complement and extend prior investigations of high frequency pn sn spectra in the western pacific at a distance of about 18 sup 0 approx 2000 km frequencies for pn and sn are as high as 30 and 35 hz respectively at a distance of about 30 sup 0 approx 3300 km as high as 15 and 20 hz respectively pn phases lose their high frequency energy more rapidly than sn phases do yet pn wavetrains are much longer than sn wavetrains pn wavetrains of longer duration more energy and higher frequencies are found for travel paths primarily in the northwestern pacific basin than for travel paths across the transition zone from the shallow ontong java plateau to the deep northwestern pacific basin sn phases are extremely weak or absent for travel paths crossing this transition zone from the shallower ontong java plateau to the deeper northwestern pacific basin whereas sn phases are well recorded for travel paths crossing the transition zone in the opposite direction although normal mantle refracted p phases are well recorded beyond about 21 sup 0 approx 2300 km available data indicatemore that detectable normal mantle refracted p phases may not exist at distances from about 17 sup 0 to 21 sup 0 less

windsat passive microwave polarimetric signatures of the greenland ice sheet windsat has systematically collected the first global fully polarimetric passive microwave data over both land and ocean as the first spaceborne polarimetric microwave radiometer it was designed to measure ocean surface wind speed and direction by including the third and fourth stokes parameters which are mostly related to the asymmetric structures of the ocean surface roughness although designed for wind vector retrieval windsat data are also collected over land and ice and this new data has revealed for the first time significant land signals in the third and fourth stokes parameter channels particularly over greenland and the antarctic ice sheets the third and fourth stokes parameters show well defined large azimuth modulations that appear to be correlated with geophysical variations particularly snow structure melting and metamorphism and have distinct seasonal variation the polarimetric signatures are relatively weak in the summer and are strongest around spring this corresponds well with the formation and erosion of the sastrugi in the dry snow zone and snowmelt in the soaked zone in this paper we present the full polarimetric signatures obtained from windsat over greenland and use a simple empirical observation model to quantify the azimuthal variations of the signatures in space and time

spatial and temporal behavior of microwave backscatter directional modulation over the saharan ergs radar backscatter sigmadeg from ergs is modulated with view direction incidence thetas and azimuth phi angles where the modulation characteristics reflect the surface geometry sigmadeg also varies spatially and reflects the spatial inhomogeneity of the sand surface we use sigmadeg measurements at different thetas and phi angles from the nasa european remote sensing satellite and seawinds scatterometers to understand the relationship between wind and erg bedforms a model incorporating the sigmadeg phi modulation and spatial inhomogeneity is proposed surface slope variations are related to the sigmadeg spatial inhomogeneity we compare the backscatter model results with numerically predicted wind direction data provided by the european centre for medium range weather forecasts ecmwf over the erg surfaces we use the maxima of the phi modulation at thetas 33deg to infer the orientation of the dominant slip sides on the sand surface these orientations are consistent with the ecmwf wind directions spatially and temporally

azimuthal anisotropy of scatterometer measurements over land studies of the earth s land surface involving scatterometers are becoming an increasingly important application field of microwave remote sensing similarly to scatterometer observations of ocean waves the backscattering coefficient sigma0 response of land surfaces depends on both the incidence and azimuth angle under which the observations are made in order to retrieve geophysical parameters from scatterometer data it is necessary to account for azimuthal modulation effects of the backscattered signal in the present study this paper localizes the regions affected by a strong azimuthal signal dependence when observed with the european remote sensing satellite scatterometer and the seawinds scatterometer on quikscat qscat the possible physical reasons for the azimuthal effects relating the very detailed qscat azimuthal response to the spatial orientation of special topographic features and land cover within the sensor footprint were then discussed different methods for normalizing the backscattering coefficient with respect of observation azimuth angle were also proposed and evaluated first the mean local incidence angle of the sensor footprint using the shuttle radar topography mission digital elevation model dem were modeled and concluded that the resolution of the dem is too coarse to characterize most of the observed azimuthal effects a more effective way of normalizing the backscatter with respect to azimuth is then found to be by using historical backscatter observations to statistically determine the expected backscatter at each observation azimuth and incidence angle as well as time of the year the efficiency of this method is limited to the availability of past measurements for each location on the earth

deep mantle contributions to the surface dynamics of the north american continent abstract the regional and continental scale dynamics of north america and vicinity are explored using a high resolution model of mantle flow the model is constrained by simultaneously inverting global seismic and mantle convection data sets and it includes an explicit treatment of the positive chemical buoyancy of the continental tectosphere moreover it adopts a depth dependent mantle viscosity structure which reconciles both glacial isostatic adjustment gia and convection data the flow model successfully reproduces plate velocities and observations of surface gravity and topography including the continent scale quasi linear depression after corrections for gia and crustal heterogeneity extending from northern alaska to venezuela the predictions also match lithospheric flow and stress fields inferred from local and regional measurements of seismic anisotropy and surface deformation we demonstrate that these signals are largely driven by viscous flow coupled to density anomalies in the deep portions of the upper mantle and within the lower mantle where the latter may be associated with the descent of the ancient kula farallon plate system and an active mantle upwelling below the pacific margin of the north american plate more importantly the flow calculations elucidate how these large scale heterogeneities give rise to regional scale flow and stress patterns below the southwestern u s and below the central u s

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

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

anomalous seaward dip of the lithosphere asthenosphere boundary beneath northeastern usa detected using differential array measurements of rayleigh waves summary rayleigh wave phase velocities and azimuth anomalies in the period range of 30 100 s are measured for a set of four triangular arrays of broad band seismometers in coastal northeastern usa this is a region in which a strong upper mantle slow shear velocity anomaly a new england anomaly crosses the continental margin earthquakes from a wide range of directions are used to detect the variation of parameters with azimuth of propagation no lateral heterogeneity in phase velocity is detected at these periods between stations at the centre and the edge of the anomaly however large 10 20 per cent azimuthal variations occur and have a cos 1 dependence which is indicative of a dipping structure in the upper mantle corresponding azimuth variations with a magnitude of 5 are also detected this behaviour is consistent with a southeasterly n150 e dip of the lithosphere asthenosphere boundary beneath new england this dip is associated with the shoaling of the new england anomaly beneath the adirondack mountains west of the array it is opposite to the dip associated with lithospheric thickening toward the interior of the craton

an accumulation map for the greenland dry snow facies derived from spaceborne radar 1 an important component of ice sheet mass balance calculations is the snow accumulation rate traditionally determined across the ice sheet from interpolated ice core data we derive an accumulation rate map within the greenland dry snow facies using c band radar backscatter and the mean annual temperature values for the backscatter were obtained from the ers 1 synthetic aperture radar mosaic acquired during september november 1992 an empirical equation based on elevation and latitude is used to determine the mean annual temperature we examine the influence of accumulation rate and mean annual temperature on c band 5 3 ghz radar backscatter using a forward model which incorporates snow metamorphosis and radar backscatter components our model is run over a range of accumulation and temperature conditions consistent with the greenland dry snow facies on the basis of our modeled results a look up table is generated which uniquely maps the measured radar backscatter and mean annual temperature to accumulation rate our results compare favorably 15 4 average difference with 124 in situ accumulation rate measurements falling within our study area we attribute some of the differences to 1 unreliable in situ data points which were based on a single year s accumulation and 2 a systematic underestimate of accumulation rate by our forward model in the low accumulation regions associated with the northern portion of our map with questionable data points and those above 77 n removed the average differences were less than 10 3 as determined from 73 ice cores

observations of the antarctic ice sheet with the seasat scatterometer relation to katabatic wind intensity and direction the seasat a satellite scatterometer radar initially designed to measure ocean wind intensity and direction also provided observations on the antarctic ice sheet the signal of the back scatter coefficient decreases strongly from 10 to 20 db when the incidence angle of the observations increases from 0 to 65 an additional 5 db signal is found which is correlated with the direction and intensity of katabatic winds independent of the incidence angle and polarization of the signal by using simplified models of the volume scattering within the snowpack which is mostly sensitive to snow grain size and surface scattering from the air snow interface which depends on roughness it is evident that the signal of the scatterometer could result from the effects of snow dunes at low incidence angle and of micro roughness and volume back scatter at incidence angles greater than 25 the instrument therefore provides a means of measuring the direction and intensity of katabatic winds

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

upper mantle stratification by p and s receiver functions summary seismic stratification of the upper mantle is investigated by applying two complementary techniques to the records of the graefenberg array in southern germany the anisotropic p receiver function technique kosarev et al 1984 vinnik montagner 1996 is modified by using summary seismic events instead of individual events and different weighting functions instead of the same function for the harmonic angular analysis of the sv and t components of the pds phases the summary events provide better separation of the second azimuthal harmonic than the individual events the parameters of the second harmonics of sv and t thus evaluated should be similar if they reflect the effects of azimuthal anisotropy this can be used as a criterion to identify the anisotropy to detect the sdp phases and their azimuthal variations caused by azimuthal anisotropy we have developed a stacking technique which can be termed the s receiver function technique it includes axis rotation to separate interfering p and s arrivals determination of the principal m component of the s wave motion deconvolution of the p components of many recordings by their respective m components and stacking of the deconvolved p components with weights depending on the level of noise and the angle between the m direction and the backazimuth of the event both techniques yield consistent results for the graefenberg array as indicated by the p receiver functions the upper layer of the mantle between the moho and 80 km depth is anisotropic with dvs vs around 0 03 and the fast direction close to 20 clockwise from north the fast direction of anisotropy below this layer is around 110 the boundary between the upper and the lower anisotropic layers is manifested by the detectable pds and sdp converted phases shear wave splitting in sks is strongly dominated by azimuthal anisotropy in the lower layer asthenosphere

measurements of coda buildup and decay rates of western pacific p po and so phases and their relevance to lithospheric scattering we use empirical functions to characterize the shape of band passed p po and so coda envelopes of 93 earthquakes the data sets consist of earthquake data from the western and northwestern pacific recorded by arrays of several stations and include both hydrophone and geophone recordings on analog and digital instruments each earthquake was band passed into six octave wide frequency bands between 0 8 and 50 hz the empirical function a poisson function pn t a n 1 tn e t n has parameters a a measure of energy falloff rate and nc time of the maximum the study showed that these parameters varied in systematic ways po so area a ratios were found to be considerably higher for hydrophone data in the 20 35 range than for geophone and hydrophone data in the 8 20 range the difference was significantly larger than between vertical and hydrophone data in the same range in some instances a strong frequency dependence or reverse dispersion of the time of maximum amplitude n was observed with the lower 1 2 hz frequencies arriving up to 25s later than the higher 10 20 hz frequencies p po and so average falloff rates a vary systematically with frequency where the lowest frequencies most commonly have the minimum falloff rates and the middle frequencies have the maximum falloff rates the variation in falloff between earthquakes is large with the trends only becoming apparent upon averaging many measurements a strong increase in falloff rates at about 3 hz may be associated with a decrease in the amount of scatterers with scale lengths of about one half the wavelength of a 3 hz compressional wave which is about 1 3 km falloff rates were seen to decrease with epicentral range reflecting an increased scattering with length of propagation path the scattering that produces the coda is thus not concentrated in the source region but distributed along the propagation path falloff rates were seen to decrease with increasing age of crust the difference may be due to the different average age of the lithosphere in these two geographical areas earthquakes at a depth of 100 300 km have systematically lower po falloff values than shallower events this effect is clearly seen in trench earthquakes and indicates that significant scattering occurs within or near the deeper buried part of the subducting plate the so po falloff ratio is a factor of 2 lower for the hydrophone data than for the geophone measurements this difference is most likely due to conversion of shear waves at or below the seafloor if we assume that the coda decay is controlled by intrinsic attenuation then measurements of coda decay can be converted into estimates of the quality factor q at 10 hz our falloff rates give qpo 1250 which is a reasonable number for a compressional wave quality factor in the uppermost mantle and qso qpo 2 the low snear wave q is in disagreement with inferences made by other observers however the fact that the falloff rates are fairly constant in the frequency range 5 30 hz but vary with propagation distance suggests that attenuation cannot be the only phenomenon determining the decay of the coda other factors such as reverberations in the water column may play an important part in the coda formation as well and may bias these estimates q

discrimination of glacier facies using multi temporal sar data glacier facies from the greenland ice sheet and the wrangell st elias mountains alaska are analyzed using multi temporal synthetic aperture radar sar data from the european space agency ers 1 satellite distinct zones and facies are visible in multi temporal sar data including the dry snow facies the combined percolation and wet snow facies the ice facies transient melt areas and moraine in greenland and south central alaska very similar multi temporal signatures are evident for the same facies although these facies are found at lower altitude in west greenland where the equilibrium line appears to be found at sea level at 71 30 n during the year analyzed 1992 93 probably because of the cooling effect of the eruption of mount pinatubo in greenland both the percolation and dry snow facies are excellent distributed targets for sensor calibration with backscatter coefficients stable to within 0 2 db however the percolation facies near the top of mount wrangell are more complex and less easily delineated than in greenland and at high altitude the glacier facies have a multi temporal signature which depends sensitively on slope orientation