greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

basal conditions and ice dynamics inferred from radar derived internal stratigraphy of the northeast greenland ice stream we analyze the internal stratigraphy in radio echo sounding data of the northeast greenland ice stream to infer past and present ice dynamics in the upper reaches of the ice stream we propose that shear margin steady state folds in internal reflecting horizons irhs form due to the influence of ice flow over spatially varying basal lubrication irhs are generally lower in the ice stream than outside likely because of greater basal melting in the ice stream from enhanced geothermal flux and heat of sliding strain rate modeling of irhs deposited during the holocene indicates no recent major changes in ice stream vigor or extent in this region downstream of our survey irhs are disrupted as the ice flows into a prominent overdeepening when combined with additional data from other studies these data suggest that upstream portions of the ice stream are controlled by variations in basal lubrication whereas downstream portions are confined by basal topography

water exchange between the continental shelf and the cavity beneath nioghalvfjerdsbrae 79 north glacier the mass loss at nioghalvfjerdsbrae is primarily due to rapid submarine melting ocean data obtained from beneath the nioghalvfjerdsbrae ice tongue show that melting is driven by the presence of warm 1 c atlantic intermediate water aiw a sill prevents aiw from entering the cavity from dijmphna sund requiring that it flow into the cavity via bathymetric channels to the south at a pinned ice front comparison of water properties from the cavity dijmphna sund and the continental shelf support this conclusion overturning circulation rates inferred from observed melt rates and cavity stratification suggest an exchange flow between the cavity and the continental shelf of 38msv sufficient to flush cavity waters in under 1 year these results place upper bounds on the timescales of external variability that can be transmitted to the glacier via the ice tongue cavity

crustal strain in the southern alps france 1948 1998 active tectonics in the western alps is revealed by a moderate level of seismic activity and geological evidences for quaternary deformation we present new geodetic determinations of the current strain rates in the southern part of the western alps that complement existing results in provence the jura and the subalpine and belledone massifs around grenoble we combined first and second order triangulation data collected in 1948 with global positioning system gps data collected in 1998 we estimate shear strain rates of 0 1 0 2 mrad yr 1 over distances on the order of 30 km significantly diverent from zero in most of the network we obtain nw se to n s compressive shear strain directions over most of the area in agreement with the seismological and geological data these geodetic estimates correspond to long term overall n s to nw se shortening of 2 4 mm yr 1 over the study area on the same order as the far field shortening measured by continuous gps on the grasse torino baseline 2 0 0 5 mm yr 1 that incorporates our geodetic network these results provide new constraints on interseismic strain in the western alps 2000 elsevier science b v all rights reserved

preliminary firn densification model with 38 site dataset firn densification modeling based on hot isostatic pressing with power law creep is investigated using depth density data from 38 sites that collectively have mean annual temperatures ranging from 216 to 256 k and accumulation rates ranging from 0 022 to 1 2 m w e a 1 we use an inverse technique to obtain free parameters in a simple physical model for different stages of time dependent firn densification our model works as well as or slightly better than previous models interpolating within the data range but extrapolating would require additional physics

fast retreat of zachariae isstrom northeast greenland after 8 years of decay of its ice shelf zachariae isstrom a major glacier of northeast greenland that holds a 0 5 meter sea level rise equivalent entered a phase of accelerated retreat in fall 2012 the acceleration rate of its ice velocity tripled melting of its residual ice shelf and thinning of its grounded portion doubled and calving is now occurring at its grounding line warmer air and ocean temperatures have caused the glacier to detach from a stabilizing sill and retreat rapidly along a downward sloping marine based bed its equal ice volume neighbor nioghalvfjerdsfjorden is also melting rapidly but retreating slowly along an upward sloping bed the destabilization of this marine based sector will increase sea level rise from the greenland ice sheet for decades to come

characteristics of ocean waters reaching greenland s glaciers interaction of greenland s marine terminating glaciers with the ocean has emerged as a key term in the ice sheet mass balance and a plausible trigger for their recent acceleration our knowledge of the dynamics however is limited by scarcity of ocean measurements at the glacier ocean boundary here data collected near six marine terminating glaciers 79 north kangerdlugssuaq helheim and petermann glaciers jakobshavn isbrae and the combined sermeq kujatdleq and akangnardleq are compared to investigate the water masses and the circulation at the ice ocean boundary polar water of arctic origin and atlantic water from the subtropical north atlantic are found near all the glaciers property analysis indicates melting by atlantic water aw found at the grounding line depth near all the glaciers and the influence of subglacial discharge at depth in summer aw temperatures near the glaciers range from 4 58c in the southeast to 0 168c in northwest greenland consistent with the distance from the subtropical north atlantic and cooling across the continental shelf a review of its offshore variability suggests that aw temperature changes in the fjords will be largest in southern and smallest in northwest greenland consistent with the regional distribution of the recent glacier acceleration

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

horizontal subduction and truncation of the cocos plate beneath central mexico 1 based on analysis of data from a trans mexico temporary broadband seismic network centered on mexico city we report that the subducting cocos plate beneath central mexico is horizontal and tectonically underplates the base of the crust for a distance of 250 km from the trench it is decoupled from the crust by a very thin low viscosity zone the plate plunges into the mantle near mexico city but is truncated at a depth of 500 km probably due to an e w propagating tear in the cocos slab unlike the shallow slab subduction in peru and chile there is active volcanism along the trans mexican volcanic belt tmvb that lies much further inland than regions to either side where subduction dip is not horizontal geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction

subducting slab ultra slow velocity layer coincident with silent earthquakes in southern mexico great earthquakes have repeatedly occurred on the plate interface in a few shallow dipping subduction zones where the subducting and overriding plates are strongly locked silent earthquakes or slow slip events were recently discovered at the down dip extension of the locked zone and interact with the earthquake cycle here we show that locally observed converted sp arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern mexico reveal that the ultra slow velocity layer usl varies spatially 3 to 5 kilometers with an s wave velocity of 2 0 to 2 7 kilometers per second most slow slip patches coincide with the presence of the usl and they are bounded by the absence of the usl the extent of the usl delineates the zone of transitional frictional behavior

dilatant till facilitates ice stream flow in northeast greenland abstract we present radio echo sounding res global positioning system gps and active source seismic data across the central portion of the northeast greenland ice stream negis negis widens downglacier from a small region of high geothermal flux near the ice divide our data reveal high porosity 40 water saturated till lubricating the ice stream ice accelerates and thins as it flows into negis producing marginal troughs in surface topography these troughs create steep gradients in the subglacial hydropotential that generate parallel sticky and slippery bands beneath the shear margins the low porosity sticky sediment bands limit ice entrainment across the margins and thus restrict further widening producing the long narrow and relatively stable ice stream however the observed relations among surface elevation basal water routing broad sedimentary drape and till dilatancy suggest that rapid shifts in ice dynamics are possible including rapid transmission of ocean forcing inland the source and routing of the subglacial till are unclear but our data help constrain hypotheses

influence of subglacial conditions on ice stream dynamics seismic and potential field data from pine island glacier west antarctica 1 we interpret seismic reflection and airborne potential field data acquired on pine island glacier west antarctica and find variations in the subglacial geology which correlate with variations in ice dynamics immediately beneath the glacier is a mixture of soft deforming sediments and harder non deforming sediments beneath this a sedimentary basin lies under part of the main glacier with another under one of its slower moving tributaries a tectonic boundary underlies the main trunk of the glacier separating these sedimentary basins to the north from crystalline rocks to the south which also include a thick rift related magmatic intrusion the boundary correlates with changes in the basal roughness ice flow speed and basal drag smoother bed faster flow and lower basal drag characterize the thicker sedimentary sequences to the north but there is no corresponding lateral change in the acoustic properties of the bed changes in the sub bed i e deeper than the ice bed interface lithology appear to account for the contrasting basal drag and ice velocity patterns over the glacier subglacial erosion could remove a thin layer of soft sediments to the south of the geological boundary leading to increased basal drag and reduced ice flow in the future we conclude that the subglacial geology plays a significant role in controlling the spatial pattern of present day ice flow and that the consequences of subglacial erosion may be reflected in temporal changes to the ice dynamics in the past and perhaps also in the near future

wave guide effects in subduction zones evidence from three dimensional modeling we have simulated seismic wave propagation in models of the subduction zone off the pacific coast of mexico using a three dimensional finite difference method the results show that a significant part of the energy generated by earthquakes in the subduction zone can be trapped inside a wave guide generated by the low velocity material of the accretionary prism the trapped waves in the simulations appear as long duration wave trains that follow the onset of the surface waves similar to the signature of broadband seismograms recorded by stations along the pacific coast of mexico for subduction earthquakes

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

subglacial controls on the flow of institute ice stream west antarctica the institute ice stream iis rests on a reverse sloping bed extending 150 km upstream into the 1 8 km deep robin subglacial basin placing it at the threshold of marine ice sheet instability understanding iis vulnerability has focused on the effect of grounding line melting which is forecast to increase significantly this century changes to ice flow dynamics are also important to iis stability yet little is known about them here we reveal that the trunk of the iis occurs downstream of the intersection of three discrete subglacial features a large active subglacial lake a newly discovered sharp transition to a zone of weak basal sediments and a major tectonic rift the border of iis trunk flow is confined by the sediment on one side and by a transition between basal melting and freezing at the border with the bungenstock ice rise by showing how basal sediment and water dictate present day flow of iis we reveal that ice sheet stability here is dependent on this unusual arrangement

tomography and thermal state of the cocos plate subduction beneath mexico city 1 the geometry and thermal state of the subducting cocos plate beneath mexico city has been enigmatic because of the absence of a deep wadati benioff zone we present a tomographic image of the slab based on inversion of 8869 teleseismic p wave travel times measured on a portable broadband seismic network the images combined with receiver function analysis show that the slab runs flat from the coast to near mexico city where it dives into the mantle just before the trans mexican volcanic belt with a dip of 75 it continues down to a depth of 500 km at a distance of 400 km from the trench where the tomography reveals that the dipping portion ends as well as standard block tomography we invert the travel time residuals for the parameters of a thermal slab model and find a slab thickness of 40 km that is consistent with the 15 ma age of the cocos plate the combination of a young hot plate and truncation at depth can explain the lack of deep seismicity due to high temperatures and lower negative buoyancy compared with an older thicker nontruncated plate

firn model intercomparison experiment firnmice evolution of cold dry snow and firn plays important roles in glaciology however the physical formulation of a densification law is still an active research topic we forced eight firn densification models and one seasonal snow model in six different experiments by imposing step changes in temperature and accumulation rate boundary conditions all of the boundary conditions were chosen to simulate firn densification in cold dry environments while the intended application of the participating models varies they are describing the same physical system and should in principle yield the same solutions the firn models all produce plausible depth density profiles but the model outputs in both steady state and transient modes differ for quantities that are of interest in ice core and altimetry research these differences demonstrate that firn densification models are incorrectly or incompletely representing physical processes we quantitatively characterize the differences among the results from the various models for example we find depth integrated porosity is unlikely to be inferred with confidence from a firn model to better than 2 m in steady state at a specific site with known accumulation rate and temperature firn model intercomparison experiment can provide a benchmark of results for future models provide a basis to quantify model uncertainties and guide future directions of firn densification modeling

scattering of teleseismic body waves by the lateral crustal heterogeneity at the pacific trench of mexico we examine the scattered field generated at the pacific trench of mexico upon the incidence of teleseismic body waves and the waveguide properties associated with the structure of the subduction zone in this study we analyze seismic records from a portable array perpendicular to the trench that show a distinctive phase with a phase velocity of graphic 1 following body wave phases this signal is better observed following phases with high incident angles such as ps sp and ss arriving from the ocean side of the trench and for events at teleseismic distances with magnitudes larger than m w 7 0 although weak scattered signals were detected for events down to m w 6 2 and for direct p and s phases we found that the lateral transition between the dipping slab and continental crust with an inclined boundary formed by the subducting slab at the trench generates a strong discontinuity that scatters seismic waves generating a secondary field with amplitude comparable to the incident wave this phase appears for incident body wave arrivals with characteristic wavelength comparable to the thickness of the continental crust analysis of the observed scattered field shows that the trapped waves are vertically polarized surface waves numerical modeling suggests that the impedance contrast at the trench and the geometry of the subducting slab form a waveguide like structure that efficiently generates and transmits trapped seismic waves along the continental margin 1 embed inline graphic 1 gif

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

ice stream related patterns of ice flow in the interior of northeast greenland ice flow in the interior of the ne quadrant of the greenland ice sheet is focused on the large ice stream draining the north side of the summit dome the rapid ice flow in the stream is apparent in the surface features in the stream and at the margins in the broad scale topography that drives ice flow and in satellite derived ice motion information the patterns of ice flow in the upper half of the stream are remarkable for their level of organization simple geometry and effects on local surface topography the stream begins less than 100 km from the ice divide as a current 15 km wide and then broadens symmetrically downstream by the addition of ice from the sides to a width of more than 60 km elevation data and visible band imagery show that the stream has marginal troughs tens of meters deep in its upper reach which are coincident with regions of high shear strain rate the topography of the margins and undulating surface of the stream is generated by the ice flow the surface undulations in the stream are fixed in location and shape over the 5 year period from 1994 to 1999 the enhanced flow presents a challenge for researchers trying to understand the history of ice discharge from a significant area in the interior of the ice sheet

high geothermal heat flow basal melt and the origin of rapid ice flow in central greenland age depth relations from internal layering reveal a large region of rapid basal melting in greenland melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet locally high melt rates indicate geothermal fluxes 15 to 30 times continental background the southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin

modeling of firn compaction for estimating ice sheet mass change from observed ice sheet elevation change changes in ice sheet surface elevation are caused by a combination of ice dynamic imbalance ablation temporal variations in accumulation rate firn compaction and underlying bedrock motion thus deriving the rate of ice sheet mass change from measured surface elevation change requires information on the rate of firn compaction and bedrock motion which do not involve changes in mass and requires an appropriate firn density to associate with elevation changes induced by recent accumulation rate variability we use a 25year record of surface temperature and a parameterization for accumulation change as a function of temperature to drive a firn compaction model we apply this formulation to icesat measurements of surface elevation change at three locations on the greenland ice sheet in order to separate the accumulation driven changes from the ice dynamic ablation driven changes and thus to derive the corresponding mass change our calculated densities for the accumulation driven changes range from 410 to 610kgm 3 which along with 900kgm 3 for the dynamic ablation driven changes gives average densities ranging from 680 to 790kgm 3 we show that using an average or effective density to convert elevation change to mass change is not valid where the accumulation and the dynamic elevation changes are of opposite sign

effects of trench zone scattering on receiver functions over a subduction zone a 3 d finite difference modelling study abstract as morozov morozov i b 2004 crustal scattering and some artefacts in receiver function images bull seismol soc am 94 4 1492 1499 suggested for a teleseismic array targeting subducting crust in a zone of active subduction scattering from the strong horizontal velocity heterogeneity beneath the trench zone itself produces subhorizontally propagating waves that should be observed as coherent dipping events in receiver functions rf due to similar rf delay times and moveouts these events could be difficult to distinguish from backscattered p and s wave modes to further verify this suggestion we performed a full waveform 3 d visco elastic finite difference modelling of teleseismic wave propagation within a simplified model of a subduction zone the synthetics show strong scattering from the area beneath the trench dominated by the mantle and crustal p waves propagating at 6 2 8 1 km s and slower these scattered waves occupy the same time and moveout intervals as the backscattered converted modes and also have similar amplitudes although their amplitude decay characters are different the uncertainty in the knowledge of the velocity and density structure of the subduction zone could make distinguishing between these modes difficult however under minimal assumptions recent observations of receiver function amplitudes decreasing away from the trench support the interpretation of sub trench zone scattering although still limited in its representation of crustal heterogeneity 3 d modelling suggests that scattering from near moho crustal structures plays a key role in the formation of teleseismic wavefields recognition of scattered noise in teleseismic records could help to constrain major crustal structures particularly those with strong horizontal velocity contrasts at near moho depths such as crustal sutures subduction fault zones and mountain roots matching of the observed arrivals with wavefield synthetics could help constrain the locations and parameters of such structures and also help substantiate the interpretations

a synthesis of the basal thermal state of the greenland ice sheet the basal thermal state of an ice sheet frozen or thawed is an important control upon its evolution dynamics and response to external forcings however this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes here we synthesize spatially extensive inferences of the basal thermal state of the greenland ice sheet to better constrain this state existing inferences include outputs from the eight thermomechanical ice flow models included in the searise effort new remote sensing inferences of the basal thermal state are derived from holocene radiostratigraphy modern surface velocity and modis imagery both thermomechanical modeling and remote inferences generally agree that the northeast greenland ice stream and large portions of the southwestern ice drainage systems are thawed at the bed whereas the bed beneath the central ice divides particularly their west facing slopes is frozen elsewhere there is poor agreement regarding the basal thermal state both models and remote inferences rarely represent the borehole observed basal thermal state accurately near northgrip and dye 3 this synthesis identifies a large portion of the greenland ice sheet about one third by area where additional observations would most improve knowledge of its overall basal thermal state

basal seismicity of the northeast greenland ice stream seismic studies of glaciers yield insights into spatio temporal processes within and beneath glaciers on scales relevant to flow and deformation of the ice these methods enable direct monitoring of the bed in ways that complement other geophysical techniques such as geodetic or ground penetrating radar observations in this work we report on the analysis of passive seismic data collected from the interior of the north east greenland ice stream the greenland ice sheet s largest outlet glacier we record thousands of basal earthquakes many of which repeat with nearly identical waveforms we also record many long duration glacial tremor episodes that migrate across the seismic network with slow velocities e g 4 12 m s 1 analysis of the basal earthquakes indicates a transition between times of individual event activity and times of tremor activity we suggest that both processes are produced by shear slip at localized asperities along the bed the transition between discrete and quasi continuous slipping modes may be driven by pore water pressure transients or heterogeneous strain accumulation in the ice due to strength contrasts of the underlying till

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

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

crustal scattering and some artifacts in receiver function images the existing 2d and 3d depth imaging approaches using multichannel teleseismic receiver functions rfs can result in significant artifacts caused by misinterpretation of signal generated noise resulting from broadside scattering within the crust a synthetic example using acquisition geometry of the 1993 cascadia experiment shows that scattering of the primary p wave arrivals from a sedimentary accretionary wedge could produce rf events close to those observed in real data quasi linear crustal structures with significant velocity contrasts near moho depths such as subduction zones should create particularly strong and coherent noise arrivals conventional interpretations of these arrivals based on assumptions of their origin in mode conversions could lead to images of spurious dipping structures of landward dips steeper than 10 that may be very difficult to distinguish from the true structures detailed analysis of prestack premigration rf data is required for correct identification of the signal generated noise and validation of the resulting depth images identification of scattered events in rf records could also help constrain major crustal structures such as crustal sutures and subduction fault zones and particularly those with strong moho expressions manuscript received 10 june 2003

s wave velocity structure below central mexico using high resolution surface wave tomography shear wave velocity of the crust below central mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense 500 km long linear seismic network vertical components of regional records from 90 well located earthquakes were used to compute rayleigh wave group velocity dispersion curves a tomographic inversion with high resolution in a zone close to the array obtained for periods between 5 and 50 s reveals significant differences relative to a reference model especially at larger periods 30 s a 2 d s wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions the results show large differences especially in the lower crust among back arc volcanic arc and fore arc regions they also show a well resolved low velocity zone just below the active part of the trans mexican volcanic belt tmvb suggesting the presence of a mantle wedge low densities in the back arc inferred from the low shear wave velocities can provide isostatic support for the tmvb

characteristics and small scale variability of gpr signals and their relation to snow accumulation in greenland s percolation zone we investigate snowpack properties at a site in west central greenland with ground penetrating radar gpr supplemented by stratigraphic records from snow pits and shallow firn cores gpr data were collected at a validation test site for cryosat t05 on the expglaciologiques internationales au groenland egig line over a 100 m 100 m grid and along 1 km sections at fre quencies of 500 and 800 mhz several internal reflection horizons irhs down to a depth of 10 m were tracked irhs are usually related to ice layer clusters in vertically bounded sequences that obtain their initial characteristics near the surface during the melt season warm conditions in the following melt season can change these characteristics by percolating meltwater in cold conditions smaller melt volumes at the surface can lead to faint irhs the absence of simple mechanisms for internal layer origin emphasizes the need for independent dating to reliably interpret remotely sensed radar data our gpr derived depth of the 2003 summer surface of 1 48 m measured in 2004 is confirmed by snow pit observations the distribution of irh depths on a 1 km scale reveals a gradient of increasing accumu lation to the northeast of about 5 cm w e km 1 we find that point measurements of accumulation in this area are representative only over several hundred metres with uncertainties of about 15 of the spatial mean

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

ice sheet velocity mapping a combined interferometric and speckle tracking approach the first and second radarsat antarctic mapping missions amm 1 and 2 have now acquired interferometric synthetic aperture radar sar overmuch of the ice sheet the radarsat 24 day repeat cycle is nearly ideal for measuring slow ice motion e g 1 but application of sar interferometry is limited in faster moving areas with a 1day repeat period ers 1 2 tandem sar data are much better matched to fast motion but are not always available fortunately several authors have demonstrated the ability to measure velocity in fast moving areas by tracking sar speckle from image to image which works well even in the absence of visible features while these estimates have intrinsically lower resolution and poorer accuracy than direct phase measurements they serve well in areas where there are no data suitable for conventional interferometry this paper describes algorithms i have developed for merging interferometric and speckle tracking data from multiple swaths to form a single seamless mosaic of velocity at each point in the mosaic all the available data are combined to produce estimates of the velocity and the associated error this technique is demonstrated using radarsat data collected over lambert glacier antarctica during amm 1 and 2

regional wave propagation from mexican subduction zone earthquakes the attenuation functions for interplate and inslab events the seismic waves from subduction zone earthquakes are significantly affected by the presence of 3d variation in crust and upper mantle structure around the source area these heterogeneous structures also profoundly modify the character of seismic waves as they propagate from the source area to regional distances this is illustrated by studying shallow interplate earthquakes along the mexican subduction zone and deeper inslab normal faulting earthquakes in the subducted cocos plate beneath mexican mainland the strong motion recordings of these earthquakes are used to evaluate the character of wave propagation along the path between the source region and mexico city we compare the wavefield from two large earthquakes of different source type during the shallow h 17 km interplate 1995 guerrero earthquake m w 7 3 the lg phase is the most prominent feature at regional distances of about 150 to a few hundred kilometers from the source the presence of a lateral velocity gradient in the crust caused by the subduction of the cocos plate enhances the lg wave amplitude which is then amplified further in the mexican volcanic belt by amplification in the low velocity volcanic rocks both effects lead to very large ground motions along the path from the coast to the mexican inland in the frequency band from 0 2 to 4 hz however for the deeper h 40 km inslab normal faulting 1999 oaxaca earthquake m w 7 5 the amplitude of the lg phase is too small to produce the abnormal wave propagation and the direct s wave and its multiple sms reflections between the free surface and moho show a simple attenuation with increasing distance we compare these observations with numerical simulations of seismic wave propagation using the fourier spectral method the results provide a key to the understanding of seismic wave field generated by shallow interplate and deeper inslab earthquakes in a realistic 3d heterogeneous structure

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

seasonal to decadal scale variations in the surface velocity of jakobshavn isbrae greenland observation and model based analysis 1 using new data we build upon the nearly two decade long record of observations from jakobshavn isbrae to investigate the processes driving its dynamic evolution while winter flow speed has not increased substantially over the last three winters there remains a strong seasonal variation in flow speed that coincides with a cycle of summer thinning and winter thickening we relate changes in glacier speed to geometry through variations in basal traction and horizontal stresses using ice flow models constrained by satellite and airborne observations these results suggest that the bed provides little flow resistance along the main trough within about 20 km of the terminus while the loss of buttressing from the retreat of grounded and floating ice likely contributed to the initial speedup other processes are of comparable significance at seasonal to decadal time scales from analysis of the models we hypothesize that thinning induced change in basal effective pressure is the dominant process influencing near terminus behavior while diffusive processes drive the upstream response the apparent need for the terminus to thin to near flotation before it can calve may limit the rate at which retreat occurs our analysis of the processes controlling the speed suggests little potential for further large acceleration thinning and elevated speeds may continue at rates similar to present however putting the glacier on course to retreat to the head of its deep trough in about a century at which point it likely would stabilize with a thinner terminus

advanced multifrequency radar instrumentation for polar research this paper presents a radar sensor package specifically developed for wide coverage sounding and imaging of polar ice sheets from a variety of aircraft our instruments address the need for a reliable remote sensing solution well suited for extensive surveys at low and high altitudes and capable of making measurements with fine spatial and temporal resolution the sensor package that we are presenting consists of four primary instruments and ancillary systems with all the associated antennas integrated into the aircraft to maintain aerodynamic performance the instruments operate simultaneously over different frequency bands within the 160 mhz 18 ghz range the sensor package has allowed us to sound the most challenging areas of the polar ice sheets ice sheet margins and outlet glaciers to map near surface internal layers with fine resolution and to detect the snow air and snow ice interfaces of snow cover over sea ice to generate estimates of snow thickness in this paper we provide a succinct description of each radar and associated antenna structures and present sample results to document their performance we also give a brief overview of our field measurement programs and demonstrate the unique capability of the sensor package to perform multifrequency coincidental measurements from a single airborne platform finally we illustrate the relevance of using multispectral radar data as a tool to characterize the entire ice column and to reveal important subglacial features

seismic channel waves in the accretionary prism of the middle america trench a visual examination of seismograms of earthquakes located near the middle america trench and recorded on vbb seismographs along the pacific coast of mexico when bandpass filtered between 6 and 30 s shows the presence of two types of waves the first is a short pulse and the second is a dispersed wavetrain the first type of wave is quickly attenuated while the second type propagates without significant attenuation we applied a frequency mime analysis to identify the nature of these waves a comparison of period group velocity diagrams for signals recorded on different stations shows that the first arrival is a rayleigh wave propagating in the continental plate while the second one propagates along the trench in the thickened sediment wedge of the accretionary prism we have also measured group velocities of rayleigh wave propagating in the cocos plate far from the coast at periods between 15 and 20 s the group velocities of the wave propagating along the trench are lower than the velocity of rayleigh waves propagating in adjacent continental and oceanic plates we conclude that the dispersed wavetrain observed near the coast is a wave trapped in a channel formed by low velocity sediments of the accretionary prism

heat flux variations beneath central greenland s ice due to anomalously thin lithosphere the thermal state of the earth s surface is usually influenced more by climate than by heating from the earth s interior numerical models show that in the oldest and thickest part of the greenland ice sheet geothermal heat flux through an anomalously thin lithosphere leads to strong regional variations in basal melting