a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

antarctic ice sheet melting in the southeast pacific the first oceanographic measurements across a deep channel beneath the calving front of pine island glacier reveal a sub ice circulation driven by basal melting of 10 12 m yr 1 a salt box model described here gives a melt rate similar to that of ice balance and numerical models 5 50 times higher than averages for the george vi and ross ice shelves melting is fueled by relatively warm circumpolar deep water that floods the deep floor of the amundsen and bellingshausen sea continental shelves reaching the deep draft of this floating glacier a revised melt rate for ice shelves in the southeast pacific sector raises circumpolar ice shelf melting to 756 gt yr 1 given prior estimates of surface accumulation and iceberg calving this suggests that the antarctic ice sheet is currently losing mass to the ocean

mass balance of east antarctic glaciers and ice shelves from satellite data thevelocityandmassdischarge of ninemajoreast antarcticglaciersnot draining into the ross or filchner ronne ice shelves is investigated using interferometric syntheticapertureradar insar datafromtheeuropeanremote sensingsatellite1and2 ers 1 2 andradarsat 1 theglaciersare david ninnis mertz totten scott denman lambert shiraseandstancomb wills insarisusedtolocatetheirgroundinglinewithpre cision ice velocity is measured with either insar or a speckle tracking technique ice thickness is deduced from prior determined ice shelf elevation assuming hydrostatic equi librium mass fluxes are calculated both at the grounding line and at a flux gate located downstream the grounding line flux is compared to a mass input calculated from snow accumulation to deduce the glacier mass balance the calculation is repeated at the flux gate downstream of the grounding line to estimate the average bottom melt rate of the ice shelf under steady state conditions the main results are 1 grounding lines are found several tens of km upstream of prior identified positions not because of a recent ice sheet retreatbutbecause of the inadequacy of prior determined grounding line positions 2 no gross imbalance between outflow and inflow is detected on the nine glaciersbeing investi gated with anuncertainty of 10 20 prior determined largely positive mass imbalances wereduetoanincorrectlocalizationofthegroundingline 3 highratesofbottommelting 24 7 micea 1 are inferred neargrounding zones where ice reaches the deepestdraft a few glaciers exhibit lower bottom melt rates 4 7 micea 1 bottom melting however appearsto be a major source of mass loss on antarctic ice shelves

borehole imagery of meteoric and marine ice layers in the amery ice shelf east antarctica a real time video camera probe was deployed in a hot water drilled borehole through the amery ice shelf east antarctica where a total ice thickness of 480 m included at least 200 m of basal marine ice down looking and side looking digital video footage showed a striking transition from white bubbly meteoric ice above to dark marine ice below but the transition was neither microscopically sharp nor flat indicating the uneven nature at centimetre scale of the ice shelf base upstream where the marine ice first started to accrete marine ice features were imaged including platelet structures cell inclusions entrained particles and the interface with sea water at the base the cells are assumed to be entrained sea water and were present throughout the lower 100 150m of the marine ice column becoming larger and more prevalent as the lower surface was approached until near the base they became channels large enough that the camera field of view could not contain them platelets in the marine ice at depth appeared to be as large as 1 2 cm in diameter particles were visible in the borehole meltwater probably marine and mineral particles liberated by the drill but their distribution varied with depth

the 5 september 2012 nicoya costa rica mw 7 6 earthquake rupture process from joint inversion of high rate gps strong motion and teleseismic p wave data and its relationship to adjacent plate boundary interface properties 1 on 5 september 2012 a large thrust earthquake mw 7 6 ruptured a densely instrumented seismic gap on the shallow dipping plate boundary beneath the nicoya peninsula costa rica ground motion recordings directly above the rupture zone provide a unique opportunity to study the detailed source process of a large shallow megathrust earthquake using very nearby land observations hypocenter relocation using local seismic network data indicates that the event initiated with small emergent seismic waves from a hypocenter 10 km offshore 13 km deep on the megathrust a joint finite fault inversion using high rate gps strong motion ground velocity recordings gps static offsets and teleseismic p waves reveals that the primary slip zone slip 1 m is located beneath the peninsula the rupture propagated downdip from the hypocenter with a rupture velocity of 3 0 km s the primary slip zone extends 70 km along strike and 30 km along dip with an average slip of 2 m the associated static stress drop is 3 mpa the seismic moment is 3 5 1020 nm giving mw 7 6 the coseismic large slip patch directly overlaps an onshore interseismic locked region indicated by geodetic observations and extends downdip to the intersection with the upper plate moho at deeper depths below the upper plate moho seismic tremor and low frequency earthquakes have been observed most tremor locates in adjacent areas of the megathrust that have little coseismic slip a region of prior slow slip deformation to the southeast also has no significant coseismic slip or aftershocks an offshore locked patch indicated by geodetic observations does not appear to have experienced coseismic slip and aftershocks do not overlap this region allowing the potential for a comparable size rupture offshore in the future

reassessment of net surface mass balance in antarctica abstract recent in situ measurements of surface mass balance and improved calculation techniques are used to produce an updated assessment of net surface mass balance over antarctica a new elevation model of antarctica derived from ers 1 satellite altimetry supplemented with conventional data was used to delineate the ice flow drainage basins across antarctica the areas of these basins were calculated using the recent digital descriptions of coastlines and grounding lines the delineation of drainage basins was achieved using an automatic procedure which gave similar results to earlier hand drawn catchment basins more than 1800 published and unpublished in situ measurements of net surface mass balance from antarctica were collated and then interpolated a net surface mass balance map was derived from passive microwave satellite data being employed as a forcing field to control the interpolation of the sparse in situ observations basinwide integrals of net surface mass balance were calculated using t

fast recession of a west antarctic glacier satellite radar interferometry observations of pine island glacier west antarctica reveal that the glacier hinge line position retreated 1 2 0 3 kilometers per year between 1992 and 1996 which in turn implies that the ice thinned by 3 5 0 9 meters per year the fast recession of pine island glacier predicted to be a possible trigger for the disintegration of the west antarctic ice sheet is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters

the response of ice shelf basal melting to variations in ocean temperature abstract a three dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean warming scenarios the model predicts that the total ice shelf basal melt increases quadratically as the ocean offshore of the ice front warms this occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf both of which are found to increase linearly with ocean warming the behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature this study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results particularly the claim that a single melt rate sensitivity to warming is universally valid the hypothesized

warm ocean is eroding west antarctic ice sheet 1 satellite radar measurements show that ice shelves in pine island bay have thinned by up to 5 5 m yr 1 over the past decade the pattern of shelf thinning mirrors that of their grounded tributaries the pine island thwaites and smith glaciers and ocean currents on average 0 5 c warmer than freezing appear to be the source the synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini and the drawdown of grounded ice shows that antarctica is more sensitive to changing climates than was previously considered

redefinition of the amery ice shelf east antarctica grounding zone 1 new evidence is presented which shows that the amery ice shelf east antarctica extends 240 km upstream of the previously reported position we combine a digital elevation model of the amery ice shelf created from ers 1 satellite radar altimetry with measured ice thicknesses and a simple density model in a hydrostatic buoyancy calculation to map the extent of the floating ice this reveals that the ice is floating as far south as 73 2 s the result is confirmed by static gps measurements collected during three consecutive field campaigns on the amery ice shelf where the vertical component of the gps shows a clear tidal signal at 72 98 s other evidence for the grounding zone position comes from an analysis of satellite imagery mass flux calculations and ice radar data the southward extension of the grounding line substantially alters the shape and dimensions of the ocean cavity beneath the ice shelf which has implications for modeling studies of sub ice shelf processes such as basal melting and freezing ocean circulation and tides the new grounding line position will also improve geophysical studies where the computation of ocean tidal loading corrections is important for postglacial rebound estimates and correction of satellite altimetry measurements within the region

properties of a marine ice layer under the amery ice shelf east antarctica the amery ice shelf east antarctica undergoes high basal melt rates near the southern limit of its grounding line where 80 of the ice melts within 240 km of becoming afloat a considerable portion of this later refreezes downstream as marine ice this produces a marine ice layer up to 200 m thick in the northwest sector of the ice shelf concentrated in a pair of longitudinal bands that extend some 200 km all the way to the calving front we drilled through the eastern marine ice band at two locations 70 km apart on the same flowline we determine an average accretion rate of marine ice of 1 1 0 2 m a 1 at a reference density of 920 kg m 3 between borehole sites and infer a similar average rate of 1 3 0 2 m a 1 upstream the deeper marine ice was permeable enough that a hydraulic connection was made whilst the drill was still 70 100 m above the ice shelf base below this marine close off depth borehole video imagery showed permeable ice with water filled cavities and individual ice platelets fused together while the upper marine ice was impermeable with small brine cell inclusions we infer that the uppermost portion of the permeable ice becomes impermeable with the passage of time and as more marine ice is accreted on the base of the shelf we estimate an average closure rate of 0 3 m a 1 between the borehole sites upstream the average closure rate is faster at 0 9 m a 1 we estimate an average porosity of the total marine ice layer of 14 20 such that the deeper ice must have even higher values high permeability implies that sea water can move relatively freely through the material and we propose that where such marine ice exists this renders deep parts of the ice shelf particularly vulnerable to changes in ocean properties

radarsat 1 antarctic mapping project change detection and surface velocity campaign theradarsat 1antarcticmappingproject ramp isacollaboration betweennasaandthecanadianspaceagencytomapantarcticausingsyntheticaperture radar sar the first antarctic mapping mission amm 1 wassuccessfullycompleted in october 1997 data from the acquisition phase of the 1997 campaign have been used to achievetheprimarygoalofproducingthefirsthigh resolutionsarimagemapofantarctica the modified antarctic mapping mission mamm occurredduring thefallof 2000 the acquisition strategy concentrated on collecting highest resolution radarsat 1 data of antarctica s fast glaciers for change detection and feature retracking estimates of surface velocity additionally extensive data were acquired for interferometric analysis over the entire viewable region which extends north of 80 1 s latitude this paper summarizes the goalsand strategybehind mamm it goes onto discuss ice sheet marginchanges observed on several ice shelvesaroundthecontinent margin changesare documentedbycomparing amm 1and mamm datawith earlier datasets including european remote sensing satel lite 1sar imagery landsat imagery the antarctic digital database version1 coastline and declassified intelligence satellite photography analysis reveals a complex pattern of ice margin advance and retreat without indicating any systematic behavior in ice sheet extentabouttheice sheetperimeter

velocity and strain rates derived from insar analysis over the amery ice shelf east antarctica we use displacements derived from matching complex synthetic aperture radar data using maximum coherence to generate a dense network of velocity estimates over the amery ice shelf from these velocities we generate the horizontal strain rate components and resolve them with respect to the local flow direction we present the spatial distributions of velocity and transverse shear strain rate and use them to investigate features of the flow regime for the shelf from the southern end of the shelf velocity decreases from a high of about 800ma 1 to around 300 ma 1 and then increases to a maximum of about 1350ma 1 at the centre of the front strain rates vary systematically across and along the shelf the pattern of the transverse shear strain rate clearly identifies the shear margins where values exceed 0 1 a 1 in the southern section of the shelf the pattern also shows longitudinal bands of enhanced shear strain rate containing ice with a strong preferred crystal fabric that was advected from shear margins upstream in the northern section of the shelf significant values of longitudinal and traverse stresses lead to enhanced shear deformation through their effect on the octahedral shear stress term

impact of antarctic ice shelf basal melting on sea ice and deep ocean properties 1 an approximation of antarctica s rocky and icy coastline normally forms the southern boundary in global climate models such a configuration neglects extensive ice shelf areas where ocean ice interaction initiates a net freshwater flux to the circumpolar continental shelf equal to 75 of the annual mean net precipitation in coastal seas the results of a numerical model for the southern ocean using two contrasting configurations with and without caverns beneath major antarctic ice shelves are compared they show that the freshwater flux due to deep basal melting significantly stabilizes the shelf water column in front of an ice shelf as well as downstream due to advection by the coastal current if the freshwater from the caverns is absent sea ice is thinner shelf waters are warmer and saltier and the southern ocean deep basins are flushed by denser waters

mass budget of the grounded ice in the lambert glacier amery ice shelf system we used remote sensing and in situ measurements of surface accumulation rate ice surface velocity thickness and elevation to evaluate the mass budgets of grounded ice flow regimes that form the lambert glacier amery ice shelf system three distinct drainage regimes are considered the western and eastern margins of the ice shelf and the southern grounding line at the major outlet glacier confluence which can be identified with drainage zones 9 11 and 10 respectively of giovinetto and zwally 2000 our findings show the entire grounded portion of the basin is approximately in balance with a mass budget of 4 2 9 8gta 1 drainages 9 10 and 11 are within balance to the level of our measurement uncertainty with mass budgets of 2 5 2 8 gta 1 2 6 7 8 gta 1 and 0 9 2 3 gta 1 respectively the region upstream of the australian lambert glacier basin lgb traverse has a net mass budget of 4 4 6 3 gt a 1 while the downstream region has 8 9 9 9 gt a 1 these results indicate that glacier drainages 9 10 and 11 upstream and downstream of the australian lgb traverse are in balance to within our measurement error

tsunami early warning within five minutes tsunamis are most destructive at near to regional distances arriving within 20 30 min after a causative earthquake effective early warning at these distances requires notification within 15 min or less the size and impact of a tsunami also depend on sea floor displacement which is related to the length l width w mean slip d and depth z of the earthquake rupture currently the primary seismic discriminant for tsunami potential is the centroid moment tensor magnitude m w cmt representing the product lwd and estimated via an indirect inversion procedure however the obtained m w cmt and the implied lwd value vary with rupture depth earth model and other factors and are only available 20 30 min or more after an earthquake the use of more direct discriminants for tsunami potential could avoid these problems and aid in effective early warning especially for near to regional distances previously we presented a direct procedure for rapid assessment of earthquake tsunami potential using two simple measurements on p wave seismograms the predominant period on velocity records t d and the likelihood t 50 ex that the high frequency apparent rupture duration t 0 exceeds 50 55 s we have shown that t d and t 0 are related to the critical rupture parameters l w d and z and that either of the period duration products t d t 0 or t d t 50 ex gives more information on tsunami impact and size than m w cmt m wp and other currently used discriminants these results imply that tsunami potential is not directly related to the product lwd from the seismic faulting model as is assumed with the use of the m w cmt discriminant instead information on rupture length l and depth z as provided by t d t 0 or t d t 50 ex can constrain well the tsunami potential of an earthquake we introduce here special treatment of the signal around the s arrival at close stations a modified real time m wpd rt magnitude and other procedures to enable early estimation of event parameters and tsunami discriminants we show that with real time data currently available in most regions of tsunami hazard event locations m b and m wp magnitudes and the direct period duration discriminant t d t 50 ex can be determined within 5 min after an earthquake occurs and t 0 t d t 0 and m wpd rt within approximately 10 min this processing is implemented and running continuously in real time within the early est earthquake monitor at ingv rome http early est rm ingv it we also show that the difference m b log10 t d t 0 forms a rapid discriminant for slow tsunami earthquakes the rapid availability of these measurements can aid in faster and more reliable tsunami early warning for near to regional distances

a 4 decade record of elevation change of the amery ice shelf east antarctica 1 we report on long term surface elevation changes of the central amery ice shelf ais by comparing elevation records spanning 4 decades 1968 2007 we use elevation records acquired with the following methods optical leveling 1968 1969 ers radar altimetry 1992 2003 gps 1995 2006 and ice cloud and land elevation satellite icesat laser altimetry 2003 2007 we compute multidecadal elevation trend dh dt values at crossovers between the leveling route and each of the gps and icesat tracks as well as shorter period dh dt at ers ers gps gps and icesat icesat crossovers at gps leveling crossovers the mean long term dh dt is 0 003 m a 1 and at icesat leveling crossovers the mean dh dt is 0 013 m a 1 neither trend is significantly different from zero the data do however exhibit variable trends near zero change between 1991 and mid 1996 then thickening to 2003 followed by thinning 2003 2007 with 5 year dh dt averages exceeding 0 1 m a 1 the changes in dh dt pattern in mid 1996 and again in 2003 occur with unexpected speed the ice shelf exhibits different dh dt patterns than does the surrounding grounded ice suggesting that surface mass balance variations or longer term variations in firn densification processes are unlikely to be major causes we conclude that these observed multiyear elevation changes must be due to currently unexplained or presently poorly quantified phenomena involving surface or basal processes and or ice dynamics with the multidecadal stability of the ais established the short term fluctuations that we observe suggests that for other ice shelves observed strong dh dt signals over short time periods do not necessarily indicate ice shelf instability

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

emplacement mechanisms of contrasting debris avalanches at volcan mombacho nicaragua provided by structural and facies analysis we study the lithology structure and emplacement of two debris avalanche deposits dads with contrasting origins and materials from the quaternary holocene mombacho volcano nicaragua a clear comparison is possible because both dads were emplaced onto similar nearly flat 3 slope topography with no apparent barrier to transport this lack of confinement allows us to study in nature the perfect case scenario of a freely spreading avalanche in addition there is good evidence that no substratum was incorporated in the events during flow so facies changes are related only to internal dynamics mombacho shows evidence of at least three large flank collapses producing the two well preserved debris avalanches of this study one on its northern flank las isletas directed northeast and the other on its southern flank el crater directed south other south eastern features indicate that the debris avalanche corresponding to the third collapse la danta occurred before las isletas and el crater events the materials involved in each event were similar except in their alteration state and in the amount of substrata initially included in the collapse while el crater avalanche shows no signs of substratum involvement and has characteristics of a hydrothermal weakening related collapse the las isletas avalanche involves significant substratum and was generated by gravity spreading related failure the latter avalanche may have interacted with lake nicaragua during transport in which case its run out could have been modified through a detailed morphological and structural description of the mombacho avalanches we provide two contrasting examples of non eruptive volcanic flank collapse we show that remarkably even with two distinct collapse mechanisms the debris avalanches developed the same gross stratigraphy of a coarse layer above a fine layer this fine layer provided a low friction basal slide layer whereas dad layering and the run outs are roughly similar the distribution of structures is different and related to lithology las isletas has clear proximal faults replaced distally by inter hummock depressions where basal unit zones are exhumed whereas el crater has faults throughout but the basal layer is hidden in the distal zone hummocky forms depend on material type with steep hummocks being formed of coherent lava units and low hummocks by matrix rich units in both avalanches extensional structures predominate the upper layers exclusively underwent longitudinal and lateral extension this is consistent with evidence of only small amounts of block to block interactions during bulk horizontal spreading the base of the moving mass accommodated transport by large amounts of simple shear we suggest that contractional structures and inter block collisions seen in many other avalanches are artifacts related to topographic confinement

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

ocean interactions with the base of amery ice shelf antarctica using a two dimensional ocean thermohaline circulation model we varied the cavity shape beneath amery ice shelf in an attempt to reproduce the 150 m thick marine ice layer observed at the gi ice core site most simulations caused melting rates which decrease the ice thickness by as much as 400 m between grounding line and g1 but produce only minor accumulation at the ice core site and closer to the ice front changes in the seafloor and ice topographies revealed a high sensitivity of the basal mass balance to water column thickness near the grounding line to submarine sills and to discontinuities in ice thickness model results showed temperature salinity gradients similar to observations from beneath other ice shelves where ice is melting into seawater modeled outflow characteristics at the ice front are in general agreement with oceanographic data from prydz bay a freshwater flux across the grounding line derived from melting beneath the grounded ice sheet would have to be anomalously large to produce the basal marine ice layer and account for the ice shelf water outflow we concur with morgan s inference that the g 1 core may have been taken in a basal crevasse filled with marine ice this ice is formed from water cooled by ocean ice shelf interactions along the interior ice shelf base

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

ocean circulation and ice ocean interaction beneath the amery ice shelf antarctica simulations of the ocean dynamics in the cavity under the amery ice shelf antarctica were carried out using a three dimensional numerical ocean model two different boundary conditions were used to describe the open ocean barotropic exchange at the ice front the simulations show that the circulation in the ocean cavity is predominantly barotropic and is generally steered by the cavity topography the circulation is driven by the density gradient in the cavity which is strongly influenced by the heat and salt fluxes from melting and freezing processes at the ice ocean interface and by the horizontal exchange of heat and salt across the open ocean boundary at the ice front the interaction at the ice ocean interface allows the basal component of the mass loss of the amery ice shelf to be estimated in the two simulations the computed losses were 5 8 gt yr 1 and 18 0 gt yr 1 values consistent with observations the bulk of the melting occurred near the southern grounding line of the ice shelf although substantial melting also occurred in areas where heat transport by horizontal circulation was large accretion was restricted to areas where water from upstream melting became supercooled as it ascended the ice shelf base

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

a geophysical study of the hatton bank volcanic margin a summary of the results from a combined seismic gravity and magnetic experiment abstract this paper summarizes the results of a geophysical study of the hatton bank volcanic margin in the north atlantic normal incidence and synthetic aperture profiles saps show seaward dipping reflectors interpreted as subaerial basalt flows on the continental slope and in the adjacent ocean basin velocity depth profiles derived from expanding spread profiles esps shot along strike show elevated velocities in the lower crust on the continental slope the high velocities are interpreted as evidence for igneous underplating or intrusion gravity modelling shows high densities in the lower crust supporting a model of igneous accretion an ocean bottom seismometer obs and multichannel refraction line shot perpendicularly to strike through the midpoints of the esps further constrained the velocity structure across the margin the velocity model was refined as each dataset was processed clearly showing that multiple datasets were required in order to constrain the velocity structure tightly in particular esps were found to produce misleading results in places where the assumption that the sub surface structure is one dimensional inherent in the esp technique was not valid the final velocity model is used to infer the thickness of igneous material accreted across the hatton margin which is estimated to increase westwards from zero at the top of the bank to 18 km at the bottom of the continental slope and then thin rapidly to 11 km in the adjacent iceland basin as the oceanic crust thins its seismic velocity decreases suggesting a decrease in percentage partial melting these observations strongly support a model in which volcanic margins are produced by elevated asthenospheric temperatures

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

basal melting and freezing under the amery ice shelf east antarctica the basal melting and freezing rates under the amery ice shelf east antarctica are evaluated and their spatial distributions mapped ice velocity surface elevation and accumulation rate datasets are employed in the analysis along with a column averaged ice density model our analysis shows that the total area of basal melting is 34 700 km 2 with a total annual melt of 62 5 9 3 gt and an average melting rate of 1 8 0 3 m a 1 basal freezing mainly occurs in the northwestern part of the ice shelf over a total area of 26 100 km 2 and with a maximum freezing rate of 2 4 0 4 m a 1 the total marine ice that accretes to the ice shelf base is estimated to be 16 2 2 4 gt a 1 using a redefined grounding line and geometry of the amery ice shelf we estimate the net melt over the ice shelf base is about 46 4 6 9 gt a 1 which is higher than previous modeling and oceanographic estimates net basal melting accounts for about half of the total ice shelf mass loss with the rest being from iceberg discharge our basal melting and freezing distribution map provides a scientific basis for quantitative analysis of ice ocean interaction at the ice shelf ocean interface

volcanogenic tsunamis in lakes examples from nicaragua and general implications this paper emphasizes the fact that tsunamis can occur in continental lakes and focuses on tsunami triggering by processes related to volcanic eruptions and instability of volcanic edifices the two large lakes of nicaragua lake managua and lake nicaragua host a section of the central american volcanic arc including several active volcanoes one case of a tsunami in lake managua triggered by an explosive volcanic eruption is documented in the geologic record however a number of events occurred in the past at both lakes which were probably tsunamigenic these include massive intrusion of pyroclastic flows from apoyo volcano as well as of flank collapse avalanches from mombacho volcano into lake nicaragua maar forming phreatomagmatic eruptions which repeatedly occurred in lake managua are highly explosive phenomena able to create hugh water waves as was observed elsewhere the shallow water depth of the nicaraguan lakes is discussed as the major limiting factor of tsunami amplitude and propagation speed the very low profile shores facilitate substantial in land flooding even of relatively small waves implications for conceiving a possible warning system are also discussed

evidence for rapid retreat and mass loss of thwaites glacier west antarctica thwaites glacier the second largest ice stream in west antarctica drains an area of 166 500 2000 km2 which accumulates 55 5 gt a 1 or 60 6 km3 ice a 1 into the amundsen sea unrestrained by an ice shelf using interferometric synthetic aperture radar insar data collected by the european remote sensing satellites ers 1 and 2 in 1996 an output flux of 71 7 gt a 1 or 77 8 km3 ice a 1 is estimated at the grounding line where ice thickness is deduced from hydrostatic equilibrium a similar flux 70 7 gt a 1 or 76 8 km3 ice a 1 is obtained at a gate located 20 km upstream where ice thickness was measured in 1978 by ice sounding radar total accumulation in between the two gates is 1 6 gt a 1 or 1 8 km3 ice a 1 ice discharge therefore exceeds mass accumulation by 30 15 and thwaites glacier must be thinning and retreating at present the insar data show that the glacier floating ice tongue exerts no back pressure on the inland ice calves into tabular icebergs along a significant fraction of its grounding line and has a grounding line thickness which exceeds a prior calculated limit for stability glacier thinning is confirmed at the coast by the detection of a 1 4 0 2 km retreat of its grounding line between 1992 and 1996 with insar which implies 3 2 0 6 m ice a 1 thinning at the glacier center and less near the sides these results complement the decimeter scale annual surface lowering observed with satellite radar altimetry several hundred km inland of the grounding line the magnitude of ice thinning estimated at the coast however rules out temporal changes in accumulation as the explanation for surface lowering ice thinning must be due to changes in ice flow

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

digital elevation models for the lambert glacier amery ice shelf system east antarctica from ers 1 satellite radar altimetry the lambert glacier amery ice shelf system is a major component of the east antarctic ice sheet this paper presents two digital elevation models dems that have been generated for the lambert amery system from validated european remote sensing satellite ers 1 radar altimeter waveform data the first dem covers the amery ice shelf only and was produced using kriging on a 1 km grid the second is a coarser 5 km dem of the entire lambert amcry system generated via simple averaging procedures the dems provide unprecedented surface elevation information for the lambert amery system and allow new insight into the glaciology of the region

observations and modeling of the august 27 2012 earthquake and tsunami affecting el salvador and nicaragua on 27 august 2012 04 37 utc 26 august 10 37 p m local time a magnitude mw 7 3 earthquake occurred off the coast of el salvador and generated surprisingly large local tsunami following the event local and international tsunami teams surveyed the tsunami effects in el salvador and northern nicaragua the tsunami reached a maximum height of 6 m with inundation of up to 340 m inland along a 25 km section of coastline in eastern el salvador less severe inundation was reported in northern nicaragua in the far field the tsunami was recorded by a dart buoy and tide gauges in several locations of the eastern pacific ocean but did not cause any damage the field measurements and recordings are compared to numerical modeling results using initial conditions of tsunami generation based on finite fault earthquake and tsunami inversions and a uniform slip model

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

distribution of marine ice beneath the amery ice shelf we present a map of the marine ice accreted to the base of the amery ice shelf ais east antarctica this map is obtained by converting a digital elevation model dem of the ais generated from satellite radar altimeter data to an ice thickness map assuming hydrostatic equilibrium and subtracting from that a second ice thickness map derived from airborne radio echo sounding res measurements the res signal does not penetrate the marine ice so the measurement is only to the meteoric marine ice boundary and therefore the difference between the two maps is the marine ice thickness the marine ice is up to 190 m thick and accounts for about 9 of the shelf volume it is concentrated in the northwest of the shelf a result of the clockwise ocean circulation in the cavity below

the cavity under the amery ice shelf east antarctica ocean circulation under ice shelves and associated rates of melting and freezing are strongly influenced by the shape of the sub ice shelf cavity we have refined an existing method and used additional in situ measurements to estimate the cavity shape under the amery ice shelf east antarctica a finite element hydrodynamic ocean tide model was used to simulate the major tidal constituents for a range of different sub amery ice shelf cavity water column thicknesses the data are adjusted in the largely unsurveyed southern region of the ice shelf cavity by comparing the complex error between simulated tides and in situ tides derived from gps observations we show a significant improvement in the simulated tides with a combined complex error of 1 8 cm in comparison with past studies which show a complex error of 5 3 cm our bathymetry incorporates ice draft data at the grounding line and seismic surveys which have provided a considerable amount of new data this technique has particular application when the water column beneath ice shelves is inaccessible and in situ gps data are available

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

melting and freezing beneath filchner ronne ice shelf antarctica 1 we use remote sensing data sets to evaluate the spatial distribution of melt beneath the filchner ronne ice shelf fris the net melt rate of 83 4 24 8 gtons yr is 2 5 5 times lower than previous glaciological estimates but is similar to existing oceanographic estimates the spatial distribution however differs significantly from standard conceptual and numerical models in which most melt occurs along the grounding lines our results suggest most grounding line melt is refrozen while the dominant ice shelf water isw source is melting near the ice shelf front probably associated with tidal action this suggests that changes in ice shelf extent can impact isw production rates in the weddell sea

mechanism of the 1992 nicaragua tsunami earthquake the 1992 nicaragua earthquake generated larger tsunamis than expected from its surface wave magnitude ms 7 2 and is known as a tsunami earthquake seismological studies showed that the duration was very long for its size about 100 s other studies have shown that the seismic moment estimated from tsunamis is an order of magnitude larger than that from seismic waves even after the long duration is accounted for numerical computations of tsunamis from various fault models are made to reconcile this discrepancy comparison of calculated waveforms with tide gauge records shows that the fault width is 40 km much narrower than the aftershock area and extends only into the upper 10 km of the ocean bottom slip amount on the fault is estimated to be 3 m from amplitude comparisons the fault length is estimated to be 250 km slightly longer than the aftershock area from comparison of the tsunami height distribution the rigidity around the shallow fault may be smaller than that of a standard underthrust fault and the seismic moment is estimated as 3 1020 nm consistent with the seismic observations a slow rupture on the shallow fault presumably in the subducted sediments is responsible to the unusually large tsunami excitation