deformation in rutford ice stream west antarctica measuring shear wave anisotropy from icequakes ice streams provide major drainage pathways for the antarctic ice sheet the stress distribution and style of flow in such ice streams produce elastic and rheological anisotropy which informs ice flow modelling as to how ice masses respond to external changes such as global warming here we analyse elastic anisotropy in rutford ice stream west antarctica using observations of shear wave splitting from three component icequake seismograms to characterize ice deformation via crystal preferred orientation over 110 high quality measurements are made on 41 events recorded at five stations deployed temporarily near the ice stream grounding line to the best of our knowledge this is the first well documented observation of shear wave splitting from antarctic icequakes the magnitude of the splitting ranges from 2 to 80 ms and suggests a maximum of 6 shear wave splitting the fast shear wave polarization direction is roughly perpendicular to ice flow direction we consider three mechanisms for ice anisotropy a cluster model vertical transversely isotropic vti model a girdle model horizontal transversely isotropic hti model and crack induced anisotropy hti model based on the data we can rule out a vti mechanism as the sole cause of anisotropy an hti component is needed which may be due to ice crystal a axis alignment in the direction of flow or the alignment of cracks or ice films in the plane perpendicular to the flow direction the results suggest a combination of mechanisms may be at play which represent vertical variations in the symmetry of ice crystal anisotropy in an ice stream as predicted by ice fabric models

on the role of the upper plate in great subduction zone earthquakes at subduction zones where convergence is not perpendicular to the trench i e oblique slip vectors of interplate thrust earthquakes are often systematically deflected away from the expected plate vector probably by deformation of the leading edge of the upper plate near the epicenters of great thrust earthquakes of mw 8 0 in this century that occurred at trenches systematic deflections of slip vectors for recent 1977 1992 5 interplate thrust earthquakes are significantly less than global averages statistical tests show that great earthquakes nucleate where deviations of slip vectors residuals are smaller and plate convergence is faster but not preferentially where the angle of obliquity is smaller than global averages the small slip vector residuals suggest that great earthquakes nucleate where forces in the forearc due to plate convergence are sustained elastically rather than anelastically and that shear stress on subduction thrust faults with great earthquakes is not necessarily higher than on faults without them shear stress magnitude is less important than rheology in determining the seismic behavior of subduction zones an observed decrease in the slip vector residuals with increasing rates of trench normal convergence is interpreted as evidence that the forearc is increasingly cooled and strengthened by faster subduction for the forearc to cool with increasing convergence rate the shear stress on the subduction thrust fault must be less than about 40 mpa lower temperatures on the fault surface and the possibility that thrust faults below undeforming forearcs are smoother than those below deforming forearcs both act to increase the frictional instability of the fault zone great thrust earthquakes at subduction zones seem to occur where convergence is fast and the forearc is cool and predominantly elastic the ability of the forearc of the upper plate to store elastic strain energy appears to be a major factor controlling where great subduction thrust earthquakes can and cannot occur

fabric and texture at siple dome antarctica preferred c axis orientations are present in the firn at siple dome west antarctica and recrystallization begins as shallow as 200 m depth in ice below 20 c based on digital analysis of c axis fabrics grain sizes and other characteristics of 52 vertical thin sections prepared in the field from the kilometer long siple dome ice core the shallowest section analyzed from 22 m shows clustering of c axes toward the vertical by 200 m depth girdle fabric and other features of recrystallized ice are evident in layers or regions separated by layers regions of typically finer grained ice lacking evidence of recrystallization ice from about 700 780m depth which was deposited during the last ice age is especially fine grained with strongly vertical c axes but deeper ice shows much larger crystals and strong evidence of recrystallization azimuthal asymmetry of some c axis fabrics trends in grain size and other indicators reveal additional information on processes and history of ice flow at siple dome

liquefaction resistance of soils from shear wave velocity a simplified procedure using shear wave velocity measurements for evaluating the liquefaction resistance of soils is presented the procedure was developed in cooperation with industry researchers and practitioners and evolved from workshops in 1996 and 1998 it follows the general format of the seed idriss simplified procedure based on standard penetration test blow count and was developed using case history data from 26 earthquakes and 70 measurement sites in soils ranging from fine sand to sandy gravel with cobbles to profiles including silty clay layers liquefaction resistance curves were established by applying a modified relationship between the shear wave velocity and cyclic stress ratio for the constant average cyclic shear strain suggested by r dobry these curves correctly predicted moderate to high liquefaction potential for 95 of the liquefaction case histories and are shown to be consistent with the standard penetration test based curves in sandy soils a case study is provided to illustrate application of the procedure additional data are needed particularly from denser soil deposits shaken by stronger ground motions to further validate the simplified procedure

characterization of norwegian marine clays with combined shear wave velocity and piezocone cone penetration test cptu data a database of research quality piezocone cone penetration test cptu and shear wave velocity vs information for norwegian marine clays has been assembled to study the small strain stiffness relationships for these materials and to examine the potential use of cptu and vs data in combination for the purposes of characterizing these soils data for sites where high quality block sampling was carried out have mostly been used improvements have been suggested to existing correlations between the small strain shear modulus gmax or vs and index properties for these soils recent research has shown that cptu corrected cone tip resistance qt and especially the pore pressure measured during cptus u2 and vs can be measured reliably and repeatably and are not operator or equipment dependant therefore a new soil classification chart involving the normalized cone resistance qt and normalized shear wave velocity vs1 or vs1 and u v0 where u is the pore water pressure and v0 is the in situ vertical

interpretation of in situ test results from the canlex sites one of the primary objectives of the canadian liquefaction experiment canlex project was to evaluate in situ testing techniques and existing interpretation methods as part of the overall goal to

antarctic ice sheet structures retrieved from p wave coda autocorrelation method and comparisons with two other single station passive seismic methods passive seismology is becoming increasingly popular for glacier ice sheet structure investigations in polar regions single station passive seismic methods including p wave receiver functions prfs horizontal to vertical spectral ratio hvsr and a recently proposed autocorrelation method have been used to retrieve glacier ice sheet structures despite their successful applications analysis regarding their detection abilities in different glaciological environments has not been reported in this study we compare ice thicknesses and v p v s ratios obtained from the three methods using data collected at gamseis and polenet anet seismic arrays in antarctica ice thickness estimates derived from the three methods are found to be consistent comparisons conducted under various model setups including those involving tiled layers and sedimentary layers show that the effectiveness of the autocorrelation method is not superior to the prf method for retrieving ice sheet structures the autocorrelation method however can complement other methods as it only requires a single component seismic record

the crystal fabric of ice from full waveform borehole sonic logging 1 in an ice sheet a preferred crystal orientation fabric affects deformation rates because ice crystals are strongly anisotropic shear along the basal plane is significantly easier than shear perpendicular to the basal plane the effect of fabric can be as important as temperature in defining deformation rates fabric is typically measured using analysis of thin sections under the microscope with co polarized light due to the time consuming and destructive nature of these measurements however it is difficult to capture the spatial variation in fabric necessary for evincing ice sheet flow patterns because an ice crystal is similarly elastically anisotropic the speed of elastic waves through ice can be used as a proxy for quantify anisotropy we use borehole sonic logging measurements and thin section data from dome c east antarctica to define the relations between apparent fabric and borehole measured elastic speeds compressionalvp and vertically polarized shear vsv these relations valid for single maximum fabrics allow in situ depth continuous fabric estimates of unimodal fabric strength from borehole sonic logging we describe the single maximum fabric usinga1 the largest eigenvalue of the second order orientation tensor for ice at 16 c anda1in the 0 7 1 range the relations arevp 248 a13 7 3755 m s 1 and vsv 210a17 3 1968 m s 1

crystal orientation fabrics within the antarctic ice sheet revealed by a multipolarization plane and dual frequency radar survey 1 to investigate the viscosity structure of ice sheets induced by crystal orientation fabric cof we carried out a multipolarization plane and dual frequency radar survey in east antarctica radar surveys were done along a 670 km long flow line from dome fuji toward the coast and two transverse lines of 300 km and 20 km length respectively the radar echoes were highly dependent on the polarization plane for ice depths between about 40 and 60 of the ice thickness in the lower reaches of the convergent ice flow sector approaching the outlet glacier when the polarization was perpendicular to the ice flow echoes were about 10 db stronger than when the polarization was parallel to the ice flow this feature was not clear in the upper part of this convergent flow sector farther inland where ice flow is divergent or parallel the radar echo varied by several decibels because of changes of the radar polarization and had maxima in two orientations dual frequency data showed that the cause of the reflections was changes in cof multipolarization data identified anisotropic reflectivities and birefringence as causes of the anisotropic radar echoes in the lower and upper reaches respectively with the aid of ice core based studies on cof we show that ice is composed of stacked layers of single pole and vertical girdle fabrics in the lower reaches in contrast we argue that changes of single pole clustering cause isotropic reflectivities in the upper reaches we also discuss on the development of cof along ice flow and its implication to ice sheet dynamics

sample disturbance effects in silt abstractthis paper presents results of testing good quality piston tube samples and parallel block samples from three silt sites in ireland and norway theoretical work and laboratory strain damage

textures and fabrics in the grip ice core a comprehensive study of textures and fabrics has been carried out on the greenland ice core project grip ice core crystal sizes and c axis orientations have been measured on thin sections with conventional techniques yielding new information on the growth rotation and recrystallization of ice crystals in the greenland ice sheet normal grain growth is found to persist to a depth of 700 m in the core where the onset of polygonization due to increasing strain prevents a further increase in grain size in the holocene ice smaller crystals are observed in the wisconsin ice larger crystals are found in the eemian ice and the crystal size is found to vary with climatic parameters in these periods this dependence which probably results from variable impurity content in the ice persists to a depth of 2930 m coarse grained ice probably resulting from rapid growth of crystals at comparatively high temperatures is found in the lowest 100 m of the core the data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet a significant strengthening is not observed at the holocene wisconsin transition the fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m the evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth stretched fabrics in coarse grained ice in the lowest 100 m could be due to tensional stresses this ice is unlikely to be undergoing any significant horizontal deformation at the present time

approximations to seismic ava responses validity and potential in glaciological applications amplitude variation with angle ava methods establish the seismic properties of material either side of a reflective interface and their use is growing in glaciology the ava response of an interface is defined by the complex knott zoeppritz k z equations numerous approximations to which we typically assume weak interface contrasts and isotropic propagation inconsistent with the strong contrasts at glacier beds and the vertically transverse isotropic vti fabrics were associated with englacial reflectivity we considered the validity of a suite of approximate k z equations for the exact p wave reflectivity rp of ice overlying bedrock sediment and water and englacial interfaces between isotropic and vti ice we found that the approximations of aki richards shuey and fatti match exact glacier bed reflectivity to within rp 0 05 smaller than the uncertainty in typical glaciological ava analyses but only for maximum incident angle i limited to 30 a stricter limit of i 20 o ffered comparable accuracy to a hydrocarbon benchmark case of shale overlying gas chargedsand the vti compliant rugerapproximation accurately described englacial reflectivity to within rp 0 01 and it can be modified to give a quadratic expression in sin 2 i suitable for curve matching operations having shown the circumstances under which ava approximations were valid for glaciological applications we have suggested that their interpretative advantages can be exploited in the future ava interpretations

sonic methods for measuring crystal orientation fabric in ice and results from the west antarctic ice sheet wais divide we describe methods for measuring crystal orientation fabric with sonic waves in an ice core borehole with special attention paid to vertical girdle fabrics that are prevalent at the wais divide the speed of vertically propagating compressional waves in ice is influenced by vertical clustering of the ice crystal c axes shear wave speeds particularly the speed separation between fast and slow shear polarizations are sensitive to azimuthal anisotropy sonic data from the wais divide complement thin section measurements of fabric thin sections show a steady transition to strong girdle fabrics in the upper 2000 m of ice followed by a transition to vertical pole fabrics below 2500 m depth compressional wave sonic data are inconclusive in the upper ice due to noise as well as the method s inherent insensitivity to girdle fabrics compared with available thin sections sonic data provide better resolution of the transition to pole fabrics below 2500 m notably including an abrupt increase in vertical clustering near 3000 m our compressional wave measurements resolve fabric changes occurring over depth ranges of a few meters that cannot be inferred from available thin sections but are sensitive only to zenithal anisotropy future logging tools should be designed to measure shear waves in addition to compressional waves especially for logging in regions where ice flow patterns favor the development of girdle fabrics

empirical correlations of shear wave velocity vs and penetration resistance spt n for different soils in an earthquake prone area erbaa turkey abstract the seismicity of the northern part of turkey is mainly controlled by the north anatolian fault zone nafz the nafz is one of the world s most active seismic zones and has produced destructive earthquakes and related hazards in the northern region of turkey several earthquakes and earthquake related hazards have occurred along different segments of this fault zone in the recent past the study area erbaa town is located along the eastern segment of north anatolian fault zone nafz and is one of the largest towns of tokat province in the middle black sea region of turkey the center of erbaa is located on the left embankment of the kelkit river after the disastrous 1942 ms 7 2 and 1943 ms 7 6 earthquakes the settlement was shifted southwards as a part of a seismic microzonation study of the erbaa area shear wave velocity vs values of the geological units exposed in this area were required for site response analyses the geological units in the study area consist mainly of alluvial and pliocene units these layers were evaluated on the basis of drilling in situ spt scptu and spt based uphole and laboratory testing in this study empirical correlations between shear wave velocity vs and standard penetration test blow counts spt n were considered in order to define shear wave velocity profiles for the study area the relationships between shear wave velocity standard penetration test spt blow counts and the soil properties were evaluated as functions of depth spt based uphole tests were performed to measure shear wave velocity during drilling operations in some of the borings the spt based vs values were computed with different empirical formulas and compared with the measured spt based uphole vs measurements the empirical correlations were found to require modification to provide the best correlation for this site the depth factor was considered during the development of new empirical equations therefore a site specific formula was proposed in order to obtain vs profiles for all layers in the study area

geotechnical site characterization in the greater memphis area using cone penetration tests abstract the determination of seismic ground hazards in memphis and shelby county tennessee is facilitated by the use of electronic cone penetration tests that can provide up to four independent readings with depth from a single sounding one series of soundings is being performed for site specific mapping to determine the presence and extent of potentially liquefiable sediments in situ soil resistance to liquefaction and initial soil stiffness for ground motion amplification studies another series of soundings is being conducted in conjunction with field paleoliquefaction mapping in the new madrid seismic zone to better define the intensity magnitude and geographic extent of ground failures caused by large past earthquake events as well as information about the source sands in this paper an overview is given on the types of multi channeled penetrometer data that are being collected including vertical profiles of cone tip stress qt sleeve friction fs penetration porewater pressure u1 or u2 downhole shear wave velocity vs and or electrical conductivity ke representative soundings are presented from select sites to illustrate repeatability data post processing methods and that derived downhole vs profiles are generally in good agreement with non invasive surface techniques at two memphis test sites cyclic stress based procedures for liquefaction are discussed with relation to data from a paleoliquefaction site in germantown tennessee and estimates of the minimum magnitude of the historic event are discussed while estimates of the earthquake magnitude are preliminary pending additional study on attenuation relationships and site response in the deep soils of the mississippi embayment it is inferred from evaluation of in situ test data using cyclic stress based techniques that the december 1811 new madrid earthquake was likely larger than a mw 7 5 event

mechanical behavior of silty soils of the venice lagoon as a function of their grading characteristics the main feature of the shallowest quaternary basin of the well renowned historic city of venice italy and its surrounding lagoon is the presence apparently without any regular trend in depth and site of a predominant silt fraction this is always combined with clay and or sand forming a chaotic and erratic interbedding of different sediments whose mineralogy is however variable in a relatively narrow range due to a unique geological origin and a common depositional environment after a brief description of the basic soil indexes of the venice lagoon soil the present study based on a comprehensive geotechnical laboratory investigation describes the range of variation of the most relevant time independent geotechnical properties moreover a new grain size index combining the geometrical characteristics of the particle distribution is introduced it is shown that the soil response at large and very small strains can be related to this grain size index which appears to be able to include the in

the distribution and timing of tephra deposition at siple dome antarctica possible climatic and rheologic implications approximately 300 volcanic ash and dust layers were observed in the siple dome antarctica ice core most of this tephra deposited between 700 and 800 m depth consisted primarily of glass shards with varying amounts of crystalline material and groundmass fragments the pattern of distribution of tephra fallout closely replicates that found in the byrd ice core indicative of contemporaneous deposition at both locations peak fallout occurred approximately 19 500 years ago based on methane tie points in the siple dome and greenland ice sheet project 2 gisp2 ice cores mount berlin was identified as a potential source of tephra although other volcanoes in west and east antarctica appear to have contributed ash and dust ice between 697 and 730 m in which fine grained tephra is concentrated has undergone enhanced thinning compared to ice with a similar concentration of tephra deposited contemporaneously between 1300 and 1540m at byrd it is speculated that this thinning has occurred in response to dynamic interaction between ice at siple dome and the two ice streams flanking it a dramatic change to a shear fabric appears to be directly related to the higher concentration of volcanic particles in the ice between 700 and 800 m

multichannel analysis of surface waves masw for offshore geotechnical investigations abstract multichannel analysis of surface wave masw surveys are commonly used onshore to produce shear wave velocity vs profiles this paper describes its use in offshore geotechnical investigations underwater masw umasw where the focus was on the upper 15 m of the sediment profile to aid the design of structures such as marine pipelines cable routes and shallow anchor systems umasw was shown to provide reliable data in irish offshore conditions in water depths of up to 50 m the receiver spacing and array length rather than the characteristics of the air gun seismic energy source were the primary control on signal penetration and resolution of individual sediment layers good resolution of the properties of the upper 5 m of sediment was found by reducing the receiver spacing to as low as 0 3 m a test site with a broad intertidal zone allowed coincident land marine and direct measurements to be compared and vs profiles obtained from umasw onshore masw and by seismic cone testing scptu were in good agreement vs profiles estimated from published correlations with cptu data in sandy sediments agreed well with the measured values inversion of data using different software packages showed only a minor variation in results

investigating englacial reflections with vibro and explosive seismic surveys at halvfarryggen ice dome antarctica explosive seismic reflection data from halvfarryggen a 910m thick local ice dome of the antarctic ice sheet show numerous laterally continuous reflections within the ice between 300 and 870m depth we compare the quality of data obtained with explosive sources with that obtained using a vibroseis source for detecting englacial reflections with a snowstreamer and investigate the origin of englacial reflections we find vibroseis in combination with a snowstreamer is ten times more productive than explosive seismics however englacial reflections are more clearly visible with explosives which have a broader bandwidth signature than the vibroseis which is band limited at the high frequency end to 100 hz only the strongest and deepest englacial reflection is detected with vibroseis we interpret the majority of englacial reflections to originate from changes in the crystal orientation fabric in closely spaced layers less than the vibro seismic tuning thickness of 13 5 m phase analysis of the lowermost englacial reflector 40m above the bed indicates a sharp increase in seismic wave speed we interpret this reflector as a transition to a vertical single maximum fabric our findings support current results from anisotropic ice flow models that crystal fabric is highly anisotropic at ice domes both laterally and vertically

physical conditions of fast glacier flow 2 variable extent of anisotropic ice and soft basal sediment from seismic reflection data acquired on store glacier west greenland outlet glaciers of the greenland ice sheet transport ice from the interior to the ocean and contribute directly to sea level rise because because discharge and ablation often exceed the accumulation to develop a better understanding of these fast flowing glaciers we investigate the basal conditions of store glacier a large outlet glacier flowing into uummannaq fjord in west greenland we use two crossing seismic profiles acquired near the centreline 30 km upstream of the calving front to interpret the physical nature of the ice and bed we identify one notably englacial and two notably subglacial seismic reflections on both profiles the englacial reflection represents a change in crystal orientation fabric interpreted to be the holocene wisconsin transition from amplitude versus angle ava analysis we infer that the deepest 80 m of ice of the parallel flow profile below this reflection is anisotropic with an enhancement of simple shear of 2 the ice is underlain by 45 m of unconsolidated sediments below which there is a strong reflection caused by the transition to consolidated sediments in the across flow profile subglacial properties vary over small scale and the polarity of the ice bed reflection switches from positive to negative we interpret these as patches of different basal slipperiness associated with variable amounts of water our results illustrate variability in basal properties and hence ice bed coupling at a spatial scale of 100 m highlighting the need for direct observations of the bed to improve the basal boundary conditions in ice dynamic models

georods the development of a four element geophone for improved seismic imaging of glaciers and ice sheets active seismic imaging of glaciers and ice sheets is important for constraining inputs to climate models such as englacial ice fabric and the nature of the basal interface however acquiring high quality seismic data is time consuming and resource intensive using traditional single element geophones requires ideal weather conditions e g light winds and excellent source coupling in addition deploying and retrieving these geophones is slow and cumbersome we have developed a four element georod that enhances signal levels by 20 30 db in a variety of conditions including blowing snow and poorly coupled source detonations the long slender design of these georods makes them easy to deploy and retrieve allowing researchers to acquire greater line kilometers of seismic data during field campaigns that are commonly time constrained

grain growth in polar ice i theory many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood here we develop the theory of grain growth in ice that is not deforming rapidly and in the succeeding paper we use this theory to explain observations from glacial ice in the absence of significant strain energy the driving force for grain growth arises from grain boundary curvature grain growth is slowed by the interaction of grain boundaries with extrinsic materials microparticles bubbles and dissolved impurities if the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material then the grain boundary velocity is determined by the velocity characteristic of the extrinsic material low velocity regime if the driving force is large enough to cause separation then boundaries migrate more rapidly than the extrinsic material high velocity regime but the net driving force is reduced through transient pinning by the extrinsic material polar ice is typically in the low velocity regime relative to dissolved impurities and the high velocity regime relative to microparticles and bubbles cross sectional area of grains is predicted to increase linearly with time under most but not all circumstances

analysis and interpretation of piezocone data on the silty soils of the venetian lagoon treporti test site this paper reports the main results of an extensive piezocone testing campaign performed over the last years at the treporti test site venice italy within a long lasting and comprehensive research project aimed at developing a reliable geotechnical model of the heterogeneous and highly stratified venetian lagoon basin the project included the construction of a full scale vertical walled cylindrical test bank together with a detailed monitoring system of surface settlements pore water pressures subsoil strains and stresses beneath the loaded area the analysis of the large amount of data thus available referring to different phases of the loading history confirms the great potential of piezocone as an in situ testing technique especially for stratigraphic profiling of the predominantly silty subsoil of the venetian lagoon but at the same time reveals some limitations of widely used empirical correlations for the relevant soil characterization with special reference to compressibility and conso

ice shelf basal melting at the grounding line measured from seismic observations analysis of seismic data provides a new technique to measure basal melting of an ice shelf close to the grounding line internal reflections have been observed within the ice on a seismic reflection profile at the grounding line of ronne ice shelf the changes in ice thickness above and below these reflections have been used in a steady state model to calculate the basal mass flux of the ice shelf soon after the ice begins to float strain rates within the ice calculated during the modeling agree with values from nearby survey data the calculated melt rates range between 0 and 7 m yr 1 with an estimated standard error of 2 4 m yr 1 in reasonable agreement with earlier estimates based on surface glaciological observations limitations in the method include the difficulties in determining reflector geometry from a single seismic section and also the limitations imposed by the assumption of steady state additional seismic data would greatly reduce these limitations

shear wave velocity as function of cone penetration test measurements in sand and silt mixtures abstract a database of research quality piezocone test cptu results and shear wave velocity v s measurements assembled in the context of an ambitious research project carried out at the treporti test site venice italy has been interpreted in order to study the seismic response of the predominantly silty sediments forming the venetian lagoon basin as well as to explore potential correlations for such intermediate soils the experience gained so far with the treporti database and other field data collected in the lagoon area has shown that the mechanical behavior of venetian soils cannot be easily interpreted using the existing and well established cptu based approaches hence the interest in examining whether or not the seismic response of these sediments expressed in terms of shear wave velocity follows the framework published for other soils the applicability to the available data of a number of existing correlations originally developed either for sands or fine sediments has been first investigated and updated relationships have been then suggested furthermore the small strain stiffness calculated from v s measurements has been compared with the field stiffness derived from the observed performance of a full scale test embankment built in the treporti site so as to verify common design assumptions the verification and calibration of existing approaches are likely to constitute a useful contribution to the practice of geotechnics not only in the venetian lagoon area but also in the silty deposits usually present in the surrounding alluvial plain and other similar depositional environments allowing rapid estimates of seismic soil classification as well as small strain stiffness properties for preliminary design or at sites where only cone penetration data are available

complex fabric development revealed by englacial seismic reflectivity jakobshavn isbrae greenland this is the published version copyright 2008 american geophysical union all rights reserved

fabrics in polar ice sheets development and prediction fabrics in polar ice sheets provide a record of deformational history and control the viscosity of ice during further deformation they affect geophysical sensing of ice sheets and provide an accessible analogue to fabric development during deformation of other geological and engineering materials a new synthesis of experimental and theoretical results shows that c axis fabrics are quantitatively related to cumulative strain and stress state in ice sheets for the full range of likely flow patterns basal shear divergent flow and parallel flow cause c axes to rotate toward the vertical axis whereas convergent flow causes c axes to rotate toward a vertical plane transverse to flow

bed conditions of pine island glacier west antarctica although 90 of antarctica s discharge occurs via its fast flowing ice streams our ability to project future ice sheet response has been limited by poor observational constraints on the ice bed conditions used in numerical models to determine basal slip we have helped address this observational deficit by acquiring and analyzing a series of seismic reflection profiles to determine basal conditions beneath the main trunk and tributaries of pine island glacier pig west antarctica seismic profiles indicate large scale sedimentary deposits combined with seismic reflection images measured acoustic impedance values indicate relatively uniform bed conditions directly beneath the main trunk and tributaries comprising a widespread reworked sediment layer with a dilated sediment lid of minimum thickness 1 5 0 4 m beneath a slow moving intertributary region a discrete low porosity sediment layer of 7 3 m thickness is imaged despite considerable basal topography seismic observations indicate that a till layer at the ice base is ubiquitous beneath pig which requires a highly mobile sediment body to maintain an abundant supply these results are compatible with existing ice sheet models used to invert for basal shear stress existing basal conditions upstream will not inhibit further rapid retreat of pig if the high friction region currently restraining flow directly upstream of the grounding line is breached however small changes in the pressure regime at the bed as a result of stress reorganization following retreat may result in a less readily deformable bed and conditions which are less likely to maintain high ice flow rates

englacial seismic reflectivity imaging crystal orientation fabric in west antarctica abrupt changes in crystal orientation fabric cof and therefore viscosity are observed near the base of the ice sheet throughout west antarctica we report on active source seismic observations from wais divide mid stream and downstream on thwaites glacier and the onset region of bindschadler ice stream these data reveal a prevalence of englacial seismic reflectivity in the bottom quarter of the ice sheet the observed seismic reflectivity is complex but largely bed conformable with long spatial wavelength features observed in the flow direction and short wavelength features observed across flow a correspondence of englacial structures with bed features is also observed we determine the origin of the reflectivity to be abrupt changes in the cof of ice based on the following 1 observations of englacial reflectivity are consistent with current knowledge of cof within ice sheets 2 englacial reflectivity caused by cof contrasts requires the simplest genesis especially at ice divides and 3 amplitude analysis shows that the observed englacial reflectivity can be explained by contrasts in seismic velocity due to cof changes we note that the downstream increase in the quantity and complexity of observations indicates that direct observations of cof at ice divides likely underestimate the role that fabric plays in ice sheet dynamics

physical properties crystalline textures and c axis fabrics of the siple dome antarctica ice core the quality of the ice core from siple dome west antarctica varied widely with significant fracturing below 400 m bubbly ice persisted to the ice rock interface at 1004 m and constituted the brittle zone the core has undergone minimal relaxation and has remained brittle and prone to fracturing more than 5 years after it was drilled this behavior is attributed to unrelieved stresses from kamb and bindschadler ice streams former ice streams c and d flanking the dome melt layers were identified sporadically throughout the core as were inclined layers tilted at angles that occasionally exceeded 10 structurally the ice was characterized by extensive recrystallization including grain size changes from 0 074 cm 2 at 59 m to 50 cm 2 at 992 m and major transitions in c axis fabrics unusual fabrics included vertical c axis clusters superimposed on vertical girdles that may reflect vertical compression acting in conjunction with horizontal tension the sudden appearance of a shear type fabric at 700 800 m appears closely linked to the occurrence of abundant tephra particles embedded in the ice the occurrence of dispersed sediment in the bottom 2 m is attributed to freeze on of basal meltwater

on the nature of the dirty ice at the bottom of the gisp2 ice core abstract we present data on the triple ar isotope composition in trapped gas from clean stratigraphically disturbed ice between 2800 and 3040 m depth in the gisp2 ice core and from basal dirty ice from 3040 to 3053 m depth we also present data for the abundance and isotopic composition of o2 and n2 and abundance of ar in the basal dirty ice the ar n2 ratio of dirty basal ice the heavy isotope enrichment reflecting gravitational fractionation and the total gas content all indicate that the gases in basal dirty ice originate from the assimilation of clean ice of the overlying glacier which comprises most of the ice in the dirty bottom layer o2 is partly to completely depleted in basal ice reflecting active metabolism the gravitationally corrected ratio of 40ar 38ar which decreases with age in the global atmosphere is compatible with an age of 100 250 ka for clean disturbed ice in basal ice 40ar is present in excess due to injection of radiogenic 40ar produced in the underlying continental crust the weak depth gradient of 40ar in the dirty basal ice and the distribution of dirt indicate mixing within the basal ice while various published lines of evidence indicate mixing within the overlying clean disturbed ice excess ch4 which reaches thousands of ppm in basal dirty ice at grip is virtually absent in overlying clean disturbed ice demonstrating that mixing of dirty basal ice into the overlying clean ice if it occurs at all is very slow order of magnitude estimates indicate that the mixing rate of clean ice into dirty ice is sufficient to maintain a steady thickness of dirty ice against thinning from the mean ice flow the dirty ice appears to consist of two or more basal components in addition to clean glacial ice a small amount of soil or permafrost plus preglacial snow lake or ground ice could explain the observations

prediction of long term settlement on soft clay using shear wave velocity and damping characteristics this paper presents a method of calculating long term settlement of a loaded pad on soft clay at klang selangor malaysia where the soil model is treated as an anelastic material of viscoelastic property initially an elastic shear modulus g value from shear wave velocity profiles of the seismic tests from spectral analysis of surface wave sasw and continuous surface wave csw tests was obtained a value of damping d at an equivalent elastic strain is then calculated from the hysteresis of the plate load tests corrected to equivalent strain using the damping strain formula the calculated elastic settlement and its equivalent damping are then used to calculate the long term settlement by applying the generalised viscoelastic formula comparisons to traditional methods of settlement predictions were made and the viscoelastic formula has shown better agreement to the observed settlement further modification of the settlement formula is introduced to improve the settlement accuracy to 10

nature of radio echo layering in the antarctic ice sheet detected by a two frequency experiment a two frequency radio echo sounding experiment was carried out at dome fuji the second highest dome in east antarctica and along a 1150 km long traverse line from the dome to the coast the goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions from the two frequency 60 mhz and 179 mhz radio echo responses at various sites we distinguished four zones each of the zones is characterized by a dominant cause of radio echo internal reflection as follows in the pd zone changes in dielectric permittivity are mainly due to density fluctuations in the pcof zone changes in dielectric permittivity are mainly due to changes in crystal orientation fabrics and in the ca zone changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions in each of these three zones the changes occur commonly along isochrones in addition a basal echo free zone the fourth zone was found to appear always below the pcof zone these four zones and their distribution suggested variations of the physical conditions within the ice sheet

physical and structural properties of the greenland ice sheet project 2 ice core a review substantial data sets have been collected on the relaxation characteristics density grain size c axis fabrics and ultrasonic velocities of the greenland ice sheet project 2 gisp2 core to its contact with bedrock at 3053 4 m changes in all these properties paralleled closely those found in cores from byrd station antarctica and dye 3 greenland density increased progressively with depth to a maximum of 0 921 mg m3 at about 1400 m at which depth the ice became bubble free below about 2000 m in situ densities began to decrease in response to increasing ice sheet temperatures since drilling much of the ice core has undergone significant volume expansion relaxation due to microcracking and the exsolving of enclathratized gases especially in the brittle ice zone between 650 and 1400 m grain size increased linearly to about 1000 m thereafter remaining fairly constant until the younger dryas event at 1678 m where a twofold to threefold decrease in grain size occurred these grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical including a small increase in c axis concentration across the younger dryas holocene boundary increased dust levels in the wisconsin ice have contributed to the maintenance of a fine grained texture which with its strong vertical c axis fabric persisted to nearly 3000 m however beginning at about 2800 m layers of coarse grained ice intermixed with the much finer grained matrix ice are observed below 3000 m the ice became very coarse grained this change attributed to annealing recrystallization at elevated temperatures in the ice sheet was accompanied by a dispersed or ring like redistribution of the c axes about the vertical ultrasonic measurements of vertical and horizontal p wave velocities made at 10 m intervals along the entire length of the gisp2 core fully confirmed the results of the crystallo optical observations a return to fine grained ice coincided with the first appearance of brown silty ice 13 m above bedrock bedrock material consisted of 48 cm of till including boulders and cobbles overlying gray biotite granite comprising the true bedrock there is evidence that disturbed structure in the gisp2 cores begins little more than 70 of the way through the ice sheet this disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10 of the ice sheet

subglacial seismic reflection strategies when source amplitude and medium attenuation are poorly known seismic reflection techniques are a powerful way to probe physical properties of subglacial strata inversion of seismic data for physical properties may be hampered however by lack of knowledge of the source amplitude as well as lack of knowledge of the compressional and shear attenuation in the ice new methods are described to measure the source signature that require no a priori knowledge of the ice attenuation profile another new method is described to obtain the angular dependence of the subglacial bed reflection coefficient that is relatively insensitive to knowledge of the ice attenuation finally a correction is provided to a long standing error in the literature regarding measurement of the bed normal incidence reflection coefficient

rapid development of anisotropic ice crystal alignment fabrics inferred from englacial radar polarimetry central west antarctica 1 anisotropy in englacial radar power was measured using 60 mhz and 179 mhz copolarized pulse modulated radar at 19 sites in central west antarctica the study region is a 100 300 km2area near the west antarctic ice sheet divide that separates ice flow toward the ross and amundsen embayments the frequency dependence of the returned power indicates that most of the radar data are affected by vertical variations in the crystal orientation fabric cof though the 60 mhz data are more affected by acidity contrasts in the top 1000 m significant polarimetric variations occur at most sites likely due to effects of the anisotropic cof patterns more anisotropic variations occur at sites with significant horizontal strain whereas more isotropic variations occur at sites where vertical compression dominates azimuthal shifts with depth of the principal axes of cof were found in shallow ice near the current flow divide and at greater depths over locations of rough bed the former indicates that the divide has differentially migrated resulting in a rotation of the principal cof axes nevertheless the regionally consistent radar signatures suggest that the first order ice properties in this area have remained constant and that no major changes in the strain configuration or ice topography have occurred for the past five to eight thousand years we conclude that shallow polarimetric features can be related to the current strain configurations and that englacial polarimetric features can help constrain current ice rheology and evolution of the ice topography

indication of active overthrust faulting along the holocene wisconsin transition in the marginal zone of jakobshavn isbrae 1 a zone of highly irregular ice motion has been detected within the ice sheet adjacent to jakobshavn isbrae a fast moving ice stream in west greenland two tilt sensors in different boreholes and frozen into the ice show angle variations of up to 0 1 and 0 3 within a 1200 s interval the amplitude of these flutter oscillations increases toward the transition from holocene to wisconsin ice at 682 m depth 150 m above bedrock all other tilt sensors above and below show low background noise a numerical model confirms that the stress configuration is exceptional in the neighborhood of the ice stream stress transfer from the ice stream to the margins causes an extra forcing of the basal ice at the drill site several possible explanations for the observed flutter oscillations are given the most likely being dynamic rupture events or stick slip motion along the holocene wisconsin transition if true this would correspond to an active overthrust fault with very high loading rates

field evaluation of deformation characteristics of a lacustrine clay deposit using seismic piezocone tests abstract the use of the seismic piezocone test scptu in geotechnical site investigation offers direct field measurement on stratigraphy and soil behaviour compared with some traditional investigation methods such as drilling sampling and field inspecting method or laboratory test procedures scptu can greatly accelerate the field work and hereby reduce corresponding operation cost this paper presents the methods review to predict the constrained modulus m and to measure small strain shear modulus gmax from scptu data the capacity of current interpretation methods to predict the constrained modulus m was evaluated for lixia river lagoonal soils several sites in lixia river area were selected for this study in each site scptu tests were performed and the soundings of cone tip resistance qc sleeve friction fs pore pressure u2 and shear wave velocity vs were recorded the predicted m values obtained from the scptu interpretation methods were compared with the reference values determined from the laboratory consolidation tests results of this study showed that the sanglerat method was able to predict m better than the other methods two new correlations were also developed to predict m from the corrected cone tip resistance qt and the net cone tip resistance qn small strain shear stiffness can be tentatively estimated in the absence of direct measurement of shear wave velocity the reliable result was obtained using an association between gmax qt and pore pressure parameter bq compared to field downhole wave velocity measurements scptu can lead to a more reliable measurement of maximum stiffness profile with depth the stress strain strength response of the clays was evaluated by the maximum shear stiffness and undrained shear strength

automated procedure for estimating vs30 utilizing p wave seismograms and its application to japan abstract we develop an automated procedure to estimate time averaged shear wave velocities in the upper 30 m of soil deposits vs30 rapidly and objectively based on initial p wave seismogram amplitudes in the radial and vertical directions we apply this procedure to 630 kiban kyoshin network stations in japan utilizing 9207 records it is clear that the estimates are in good agreement with real world measurements filtering records based on signal to noise ratio snr thresholds slightly improves vs30 estimation but a decrease in the number of records results in an inability to estimate vs30 at certain sites we propose bias correction models based on snrs and take off angles to facilitate vs30 estimation at numerous sites while maintaining suitable prediction performance we observe that the performance of the proposed method is similar to that of methods utilizing different types of proxies for various regions we successfully apply the proposed method to 989 kyoshin network stations utilizing 18 454 records and create a vs30 map for the entire area of japan

subglacial sediments as a control on the onset and location of two siple coast ice streams west antarctica laterally continuous subglacial sediments are a necessary component for ice streaming in the modern onset regions of the ice streams draining the siple coast of west antarctica on the basis of new seismic data combined with previous results we present geophysical results from seismic reflection and refraction experiments in the upper reaches of ice streams c and d that highlight continuous sedimentary basins within and upstream of the current onset regions of both ice streams with streaming ice overlying these sedimentary packages the subglacial environment changes from no sediment to discontinuous sediment to continuous sediment cover along a longitudinal profile from the ice sheet to tributary c1b along this same profile we observe a speedup of ice flow and then full development of the ice stream tributary ice stream d flows above a thick sedimentary package with an uppermost low seismic velocity zone indicative of soft till and the upglacier and lateral extensions of ice stream d are tightly constrained by the extent of continuous sediments the inland termination of these sediments suggests that future migration of high velocity low shear stress ice flow in these regions appears unlikely

anisotropy and crystalline fabric of whillans ice stream west antarctica inferred from multicomponent seismic data crystal orientation fabric cof describes the intrinsic anisotropic nature of ice and is an important parameter for modeling glacier flow we present the results of three component active source seismic observations from the whillans ice stream wis a fast flowing ice stream in west antarctica surface wave dispersion analysis ray tracing and traveltime inversion of compressional p and shear s waves reveal the presence of transversely isotropic ice with a vertical axis of symmetry vti beneath approximately 65 m of isotropic firn the ice stream is characterized by weak anisotropy involving an average ice thickness of approximately 780 m the analysis indicates that about 95 of the ice mass is anisotropic and the crystalline c axes span within an average broad cone angle of 73 10 with respect to the vertical axis moreover the mean temperature t below the firn estimated from seismic data is 15 5 c these data do not show evidence of englacial seismic reflectivity which indicates the lack of abrupt changes in the cof the presence of azimuthal anisotropy due to transversely compressive flow or fractures aligned along a preferential direction is also excluded we suggest that the observed vti ice structure is typical of large ice streams in regions where basal sliding and bed deformation dominate over internal glacial deformation

wet subglacial bedforms of the ne greenland ice stream shear margins we describe elongate wet subglacial bedforms in the shear margins of the ne greenland ice stream and place some constraints on their formation lateral shear margin moraines have been observed across the previously glaciated landscape but little is known about the ice flow conditions necessary to form these bedforms here we describe in situ sediment bedforms under the ne greenland ice stream shear margins that are observed in active source seismic and ground penetrating radar surveys we find bedforms in the shear margins that are 500 m wide 50 m tall and elongated nearly parallel to ice flow including what we believe to be the first subglacial observation of a shear margin moraine acoustic impedance analysis of the bedforms shows that they are composed of unconsolidated deformable water saturated till we use these geophysical observations to place constraints on the possible formation mechanism of these subglacial features

joint interpretation of explosive and vibroseismic surveys on cold firn for the investigation of ice properties two seismic surveys were carried out on the high altitude glacier saddle colle gnifetti monte rosa italy switzerland explosive and vibroseismic sources were tested to explore the best way to generate seismic waves to deduce shallow and intermediate properties 100 m of firn and ice the explosive source sissy excites strong surface and diving waves degrading data quality for processing no englacial reflections besides the noisy bed reflector are visible however the strong diving waves are analyzed to derive the density distribution of the firn pack yielding results similar to a nearby ice core the vibrator source elvis used in both p and sh wave modes produces detectable laterally coherent reflections within the firn and ice column we compare these with ice core and radar data the sh wave data are particularly useful in providing detailed high resolution information on firn and ice stratigraphy our analyses demonstrate the potential of seismic methods to determine physical properties of firn and ice particularly density and potentially also crystal orientation fabric

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

grain growth in polar ice ii application grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials microparticles bubbles and dissolved impurities are characterized fully dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice the high concentration of soluble impurities in wisconsinan ice from the dome c antarctica ice core and perhaps other ice cores probably causes the small grain sizes observed in that ice microparticles have little effect on grain growth in ordinary ice in ice layers that appear dirty owing to concentrations of volcanic tephra such as in the byrd station antarctica ice core or of morainal material micro particles reduce grain growth rates significantly the relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks so grain growth in firn is limited by boundary migration rather than by neck growth bubbles formed by pore close off at the firn ice transition are less mobile than grain boundaries causing bubble boundary separation whenever geometric constraints are satisfied however such separation reduces grain growth rates by only about 10 the observed linear increase of grain area with time is thus predicted by theory but the growth rate depends on soluble impurity concentrations as well as on temperature

creep of granular ice with and without dispersed particles the effects of silt sized particles average diameter of 50 m on the compressive creep of polycrystalline ice have been studied at stress levels from 0 1 to 1 45 mpa and temperatures of 12 c and 10 c dislocation densities during creep have been estimated using a dislocation based model of anelasticity the results indicate that at low concentrations up to 4wt particles increase the minimum creep rate power law behavior with an exponent of 3 was observed for both particle free ice and ice with 1 wt particles when the stress was 0 3 mpa in contrast linear behavior was observed when the stress was 0 3 mpa calculations show that the linear behavior is associated with a constant dislocation density and the power law behavior is associated with increasing dislocation densities with increasing stress

extensive storage of basal meltwater in the onset region of a major west antarctic ice stream a major meltwater body exists beneath a tributary of bindschadler ice stream west antarctica in a region where subglacial lakes have not been mapped but near where rapid vertical motion of the ice sheet surface has suggested shifting of a subglacial water body the water is trapped by a local reversal in ice air surface slope arising from ice flow over variable basal topography and from the positive feedback of basal lubrication from the trapped water strong variations in the water content of the sediments upglacier of the water body arise from a similar process these results are revealed by a novel application of the amplitude variation with offset avo seismic technique the existence of such water bodies and of the strong spatial variation in subglacial sediment properties is not captured in current models of subglacial hydrology lubrication of ice stream motion and sediment transport

use of piezocone to predict maximum stiffness of venetian soils a comprehensive geotechnical in situ and laboratory investigation into the soils forming the upper quaternary basin of the venetian lagoon has been carried out in addition to standard tests measurements of shear wave velocity were performed using crosshole seismic piezocone bender element system and resonant column tests despite the incompatible strain levels of the seismic and piezocone tests it is shown that piezocone tip resistance and excess pore pressure can be used for a preliminary estimate of the small strain shear modulus gmax for the venetian soils foreword flooding of the well known historic city of venice due to high tides is increasing in frequency as a consequence of both the natural and anthropic subsidence in the venice area and the eustatic sea level rise to prevent this construction of a mobile gate system located at the three lagoon inlets was pro posed at the end of the 1980s for the design of the gate foundations geotechnical inves tigations were carried out at the inlets and particularly in a limited area at the malamocco inlet herein called the ma lamocco test site mts several types of in situ tests in cluding crosshole tests cht and seismic piezocone tests scpt were performed on contiguous verticals ministero dei lavori pubblici 1994 the geotechnical laboratory inves tigation also included bender element bet and resonant col umn tests rct on high quality undisturbed samples obtained using a large diameter 220 mm sampler field and laboratory test results are considered here with the aim of relating g

flow law hypotheses for ice sheet modeling ice flow modeling requires a flow law relating strain rates to stresses in situ but a flow law cannot be measured directly in ice sheets microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain size and c axis fabric these microscopic processes can be inferred from observations of the physical properties and the flow law can then be estimated from the microscopic processes a review of available literature shows that this approach can be imperfectly successful interior regions of large ice sheets probably have depth varying flow law constants with the stress exponent n for power law creep less than 3 in upper regions and equal to 3 only in deep ice n probably equals 3 through most of the thickness of ice shelves and ice streams

basal conditions for pine island and thwaites glaciers west antarctica determined using satellite and airborne data we use models constrained by remotely sensed data from pine island and thwaites glaciers west antarctica to infer basal properties that are difficult to observe directly the results indicate strong basal melting in areas upstream of the grounding lines of both glaciers where the ice flow is fast and the basal shear stress is large farther inland we find that both glaciers have mixed bed conditions with extensive areas of both bedrock and weak till in particular there are weak areas along much of pine island glacier s main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line in agreement with earlier studies our forward ice stream model shows a strong sensitivity to small perturbations in the grounding line position these results also reveal a large sensitivity to the assumed bed sliding or deforming model with non linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear viscous till rheology finally comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress

relating bed character and subglacial morphology using seismic data from thwaites glacier west antarctica abstract seismic measurements on thwaites glacier show a spatially variable bed character with implications for ice sheet stability the west antarctic ice sheet is losing mass rapidly through outlet glaciers and ice streams in the amundsen sea embayment including thwaites glacier where limited observations and modeling suggest that ice flow rates depend on bed properties here we characterize bed properties of thwaites glacier based on amplitude analysis of reflection seismic data from a 40 km long profile collected in the approximate flow direction and two 10 km long profiles transverse to flow the upstream portion of the seismic profile reveals a 12 km long sedimentary basin with ice flow aligned bedforms capped by a continuous till layer that is likely soft and deforming porosity 0 4 0 45 with several locations where water has pooled at the bed downstream of the sedimentary basin the bed rises by 400 m over 25 km into subglacial highlands our seismic survey of these subglacial highlands reveals strong spatial variations in bed character across rugged topography 200 m amplitude at 2 to 5 km wavelength resembling crag and tails till on the stoss sides facing upglacier of topographic highs is more consolidated porosity 0 3 0 35 or lower whereas the lee sides facing downglacier and flat regions exhibit porosity similar to the till of the upstream sedimentary basin modeling studies could use the observed correlation between bed character and bed aspect and slope to extend our observations to other parts of thwaites glacier resulting in more realistic models of future grounding line retreat our findings highlight the need for more geophysical constraints on bed properties for important outlets in antarctica and greenland

grain scale processes folding and stratigraphic disturbance in the gisp2 ice core flow disturbances have been shown to alter stratigraphic order in the lower part of the ice sheet in central greenland vertical thin sections of the greenland ice sheet project 2 ice core show that in the lower 30 the expected c axis vertical fabric is interrupted by planes of grains stripes with c axes oriented approximately in the dip directions of the planes stripe parallel shear produces small scale folds the stripes can be explained qualitatively by a simple nucleation and growth model based on the strong anisotropy of ice crystal deformation nucleation probably is sensitive to spatially variable rates of polygonization producing spatially variable stripe densities stripes are modeled to affect the ice viscosity so variations in stripe density may contribute to viscosity contrasts that might produce larger deformational features and loss of stratigraphic order

recent thinning and migration of the western divide central west antarctica antarctica is currently thinning 0 08 m a 1 and migrating toward the ross sea at 10 m a 1 the asymmetric pattern of thickness change across the divide is not caused by changes in the accumulation gradient but rather by dynamical thinning that is stronger in the amundsen sea sector than in the ross sea sector available geological and glaciological data indicate that this pattern of thinning has persisted for at least two millennia with increased asymmetry likely over the past few centuries our data however are not sufficient to determine whether the present day migration of the western divide is a response to long term millennial forcing shorter term centennial forcing or both citation conway h and l a rasmussen 2009 recent thinning and migration of the western divide central west antarctica geophys res lett 36 l12502 doi 10 1029 2009gl038072

geotechnical reconnaissance and liquefaction analyses of a liquefaction site with silty fine sand in southern taiwan article i nfo geotechnical reconnaissance of a recurrent liquefaction site at a quaternary alluvial deposit in southern taiwan was conducted to establish a comprehensive case history for liquefaction on silty fine sand with high fines content the liquefaction occurred at a silty fine sand layer with d50 0 09 mm and fines content greater than 35 and was triggered by a mw 6 4 earthquake on march 4 2010 which induced maximum horizontal acceleration up to 0 189 g at the site in situ subsurface characterizations including standard penetration test cone penetration test and shear wave velocity measurement were performed as well as cyclic simple shear tests on undisturbed specimens retrieved by a modified hydraulic piston sampler comparisons of cyclic re sistance ratios crrs indicate that cpt sounding with standard penetration rate could overestimate the re sistance ratio and drainage conditions during penetration should be considered for high fines content soil in the liquefaction analysis additionally variations of crrs from different in situ tests indicate that correlations among in situ tests and crr could be soil specific and precautions should be taken when using these curves on silty fine sands

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

interpretation of topography and bed properties beneath thwaites glacier west antarctica using seismic reflection methods abstract thwaites glacier tg west antarctica is losing mass in response to oceanic forcing future evolution could lead to deglaciation of the marine basins of the west antarctic ice sheet depending on ongoing and future climate forcings but also on basal topography bathymetry basal properties and physical processes operating within the grounding zone hence it is important to know the distribution of bed types of tg s interior and grounding zone and to incorporate them accurately in models in order to improve estimates of retreat rates and stability here we estimate properties of the bed by determining its acoustic impedance from amplitude analysis of reflection seismic data we report on the results from two lines a longitudinal l line and a transverse n line on a central flowline of tg 100 km inland from the grounding zone the data show considerable spatial variability in bed forms and properties similar to results from a comparable survey farther inland notably we find the same pattern here of hard presumed bedrock material on the stoss side of bumps and soft presumed till on the lee side physical understanding indicates the basal flow law describing motion over different regions of tg s bed likely varies from nearly viscous over the bedrock regions to nearly plastic over till regions providing guidance for modeling

distribution of preferred ice crystal orientation determined from seismic anisotropy evidence from jakobshavn isbrae and the north greenland eemian ice drilling facility greenland abstractpreferred crystal orientation fabrics cofs within an ice sheet or glacier are typically found from ice cores we conducted experiments at the north greenland eemian ice drilling neem facility ice core location where cof data were available at jakobshavn isbrae west greenland to test if cof can be determined seismically we used observations of anisotropic seismic wave propagation on multioffset gathers and englacial imaging from a 2d reflection profile anisotropy analysis of the neem data yielded mean c axes distributed over a conical region of 30 to 32 from vertical no internal ice seismic reflectors were imaged direct cof measurements collected in the ice core agreed with the seismic observations at jakobshavn isbrae we used a multioffset gather and a 2d reflection profile but we lacked ice core data englacial reflectors allowed the determination of ice column interval properties anisotropy analysis found that the upper 1640 m of the ice column consisted of cold 10 c and mostly

seismic attenuation in glacial ice a proxy for englacial temperature 1 seismic attenuation or internal friction q 1 in glacial ice is highly sensitive to temperature particularly near the melting point here we detail a technique to estimate q and apply it to active source seismic data from jakobshavn isbrae greenland we compare our results to measured and modeled temperature profiles of the ice in the region we find an excellent match with differences between seismically estimated and modeled temperatures of less than 2 c mapping variations in seismic q through glacial ice thus is shown to allow detailed estimation of englacial temperature profiles which may be of special value in regions where in situ measurements are logistically difficult