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an automatic procedure which gave similar results to earlier hand drawn catchment basins more than 1800 published and unpublished in situ measurements of net surface mass balance from antarctica were collated and then interpolated a net surface mass balance map was derived from passive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

iceberg trajectory modeling and meltwater injection in the southern ocean this is the first large scale modeling study of iceberg trajectories and melt rates in the southern ocean an iceberg model was seeded with climatological iceberg calving rates based on a calculation of the net surface accumulation from each snow catchment area on the antarctic continent in most areas modeled trajectories show good agreement with observed patterns of iceberg motion though discrepencies in the weddell sea have highlighted problems in the ocean general circulation model output used to force the iceberg model the coriolis force is found to be important in keeping bergs entrained in the coastal current around antarctica and topographic features are important in causing bergs to depart from the coastal regions the modeled geographic distribution of iceberg meltwater joining the ocean has been calculated and is found in many near coastal regions to be comparable in magnitude to the excess of precipitation over evaporation p e

crustal and upper mantle responses to lithospheric segmentation in the northern apennines lithospheric tear faults are expected to develop in response to along strike variations in the rates of slab rollback however the exact geometry of such structures and their crustal and upper mantle expressions are still debated we present an analysis of seismic structural and morphological features that possibly represent the expression of lithospheric segmentation in the northern apennines geophysical observations show evidence for the existence of a discontinuity in the lithospheric structure beneath the northern apennines characterized by a change in the spatial distribution of intermediate depth seismicity along strike variations in the pattern of crustal seismicity and a bend in the moho topography the near surface expression of this discontinuity is associated with an abrupt change in the morphology and exhumation history of the northern apennines in the proximity of the livorno sillaro lineament we interpret these features as evidence for incipient tearing of the lithospheric slab beneath the northern apennines marking the boundary between domains that underwent contrasting styles of lithospheric deformation which are either associated with different rates of slab rollback or a transition from underplating to retreat we suggest that similar types of structures may play a crucial role in the evolution of convergent plate boundaries allowing segmentation of orogenic belts and facilitating the development of orogenic curvatures ultimately further tearing along such structures could potentially lead to the occurrence of tear related magmatism and the formation of slab windows

sea ice and iceberg dynamic interaction 1 a model of iceberg motion has been implemented in the los alamos sea ice model cice individual bergs are tracked under the influence of winds currents sea surface tilt coriolis and sea ice forcing in turn sea ice is affected by the presence of icebergs primarily as obstacles that cause the sea ice to ridge on the upstream side or create open water on the downstream side of the bergs open water formed near icebergs due to sea ice ridging and blocking of sea ice advection increases level and ridged ice downstream of the bergs through increased frazil ice formation resulting anomalies in sea ice area and thickness compared with a simulation without icebergs are transported with the sea ice flow expanding over time although local changes in the sea ice distribution may be important for smaller scale studies these anomalies are small compared with the total volume of sea ice and their effect on climate scale variables appears to be insignificant

transoceanic wave propagation links iceberg calving margins of antarctica with storms in tropics and northern hemisphere 1 we deployed seismometers on the ross ice shelf and on various icebergs adrift in the ross sea including b15a a large 100 km by 30 km fragment of b15 which calved from the ross ice shelf in march 2000 the data reveal that the dominant energy of these floating ice masses is in the 0 01 to 0 1 hz band and is associated with sea swell generated in the tropical and extra tropical pacific ocean in one example a strong storm in the gulf of alaska on 21 october 2005 approximately 13 500 km from the ross sea generated swell that arrived at b15a immediately prior to and during its break up off cape adare on 27 october 2005 if sea swell influences iceberg calving and break up a teleconnection exists between the antarctic ice sheet mass balance and weather systems worldwide

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

the fingerprints of flexure in slab seismicity earthquake moment tensors in eastern pacific epac slabs typically show downdip tensional dt axes whereas in the western pacific wpac they typically show downdip compressional dc axes or have mixed orientations indicative of unbending prevailing conceptual models emphasize uniform stress deformation modes that is bulk slab stretching or shortening as the dominant control on intermediate depth seismic expression in contrast we propose that a diversity of seismic expression including dt and dc dominated regions is consistent with expectations of flexural strain accumulation based on systemic differences in slab geometry our analysis reveals two largely unrecognized features of epac intraslab seismicity first earthquake clusters consistent with slab unbending are present in epac slabs albeit at much shallower depths than typical of wpac slabs second intermediate depth epac dt seismicity is strongly localized to the upper half of zones undergoing curvature increase such as flat slab segments our study highlights how the seismic expression of slab flexure is impacted by the relative contribution of brittle and ductile deformation the strongly asymmetric temperature structure that is preserved in sinking slabs means that seismicity disproportionately records the deformation regime in the colder part of the slab above the neutral plane of bending the expression of in plane stress may be discernible in terms of a systematic modifying effect on the seismic expression of flexure

tracking large tabular icebergs using the seawinds ku band microwave scatterometer abstract knowledge of iceberg locations is important for safety reasons as well as for understanding many geophysical and biological processes originally designed to measure wind speed and direction over the ocean seawinds is a microwave scatterometer that operates at 13 4 ghz ku band on the quikscat satellite radar measurements from seawinds are collected and processed on a daily basis using resolution enhancement techniques to produce daily radar images because icebergs scatter microwave energy more than sea ice and sea water icebergs are detected as high backscatter targets surrounded by lower backscatter regions in daily seawinds images as a result iceberg positions are determined in real time and a time series of iceberg positions is maintained in an antarctic iceberg database by brigham young university s microwave earth remote sensing mers laboratory since seawinds operates independent of both solar illumination and cloud cover and has a large daily spatial coverage this paper demonstrates that seawinds is an excellent platform to detect and track large tabular icebergs these icebergs are generally larger than 5 km and are typically characterized as a rough ice plateau above the surrounding sea water or sea ice the number of icebergs tracked in the mers antarctic iceberg database is found to be generally greater than the number of icebergs tracked by the national ice center the movement patterns of all icebergs detected by seawinds are also analyzed and 90 of icebergs are found to travel a counter clockwise path around antarctica and accumulate in the weddell and scotia seas iceberg detection and tracking is demonstrated via multiple case studies that highlight icebergs c 19a and a 22a using the mers database and through real time operational support of the 2005 2008 and 2009 nsf antarctic cruises iceberg positions are validated by using collocated high resolution satellite imagery and by navigating the nsf ships to physically intercept several large tabular icebergs in the weddell and scotia seas

observations of unusual fast ice conditions in the southwest ross sea antarctica preliminary analysis of iceberg and storminess effects massive tabular icebergs 1000 km 2 surface area 1011 kg mass arrived in the southwest ross sea in early 2001 where they remained relatively immobile for the next 4 years during the period of their presence extensive landfast sea ice fast ice waxed and waned along the victoria land coast with maximum coverage exceeding typical coverage prior to the arrival of the icebergs by a factor of 5 the purposes of this study are to determine 1 whether the extensive iceberg blockade extending from ross island to drygalski ice tongue was indeed as intuition suggests responsible for the unusual fast ice conditions and 2 how storm frequency intensity and seasonal timing may have mitigated the effects of the icebergs our simple analysis of glaciological and atmospheric conditions observed during 2001 05 suggests that iceberg movement alone is not sufficient to explain fast ice variability and in fact it is the detailed interplay between storms and iceberg location that determine this variability

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

calving of large tabular icebergs from ice shelf rift systems 1 we used interferometric synthetic aperture radar to study the detachment process that allowed two large icebergs to calve from the ross ice shelf antarctica time series of rift geometries indicate that rift widths increased steadily whereas rift lengths increased episodically through several discrete rift tip propagation events we also conducted modeling experiments constrained by the observed rift geometry both the observations and model suggest that rift opening and thus tabular iceberg calving are largely driven by glaciological stresses stress introduced by the effect of gravity on the ice shelf rather than by stress introduced by the ocean and atmosphere e g tides and storms this style of rift propagation is expected to determine the steady background calving rate of ice shelves and thus differs significantly from styles that led to the recent disintegration of ice shelves in response to climate warming e g the larsen b ice shelf on the antarctic peninsula

tabular iceberg collisions within the coastal regime during 2000 07 five giant icebergs b15a b15j b15k c16 and c25 adrift in the south western ross sea antarctica were instrumented with global positioning system gps receivers and other instruments to monitor their behavior in the near coastal environment the measurements show that collision processes can strongly influence iceberg behavior and delay their progress in drifting to the open ocean collisions appear to have been a dominant control on the movement of b15a the largest of the icebergs during the 4 year period it gyrated within the limited confines of ross island the fixed ross ice shelf and grounded c16 iceberg interactions in the near coastal regime are largely driven by ocean tidal effects which determine the magnitude of forces generated during collision and break up events estimates of forces derived from the observed drift trajectories during the iceberg collision induced calving of iceberg c19 from the ross ice shelf during the iceberg induced break off of the tip of the drygalski ice tongue and the break up of b15a provide a crude estimate of the stress scale involved in iceberg calving considering the total area the vertical face of new rifts created in the calving or break up process and not accounting for local stress amplification near rift tips this estimated stress scale is 10 4 pa

chilean flat slab subduction controlled by overriding plate thickness and trench rollback how flat slab geometries are generated has been long debated it has been suggested that trenchward motion of thick cratons in some areas of south america and cenozoic north america progressively closed the asthenospheric wedge and induced flat subduction here we develop time dependent numerical experiments to explore how trenchward motion of thick cratons may result in flat subduction we find that as the craton approaches the trench and the wedge closes two opposite phenomena control slab geometry the suction between ocean and continent increases favoring slab flattening while the mantle confined within the closing wedge dynamically pushes the slab backward and steepens it when the slab retreats as in the peru and chile flat slabs the wedge closure rate and dynamic push are small and suction forces generate in some cases flat subduction we model the past 30 m y of subduction in the chilean flat slab area and demonstrate that trenchward motion of thick lithosphere 200 300 km currently 700 800 km away from the peru chile trench reproduces a slab geometry that fits the stress pattern seismicity distribution and temporal and spatial evolution of deformation and volcanism in the region we also suggest that varying trench kinematics may explain some differing slab geometries along south america when the trench is stationary or advances the mantle flow within the closing wedge strongly pushes the slab backward and steepens it possibly explaining the absence of flat subduction in the bolivian orocline

tectonic segmentation of the north andean margin impact of the carnegie ridge collision the north andean convergent margin is a region of intense crustal deformation with six great subduction earthquakes mw 7 8 this century the regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the andes segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction this segmentation is related to heterogeneity on the subducting nazca plate in particular the influence of the carnegie ridge collision is investigated four distinct seismotectonic regions can be distinguished region 1 from 6on to 2 5on with steep ese dipping subduction and a narrow volcanic arc region 2 from 2 5on to 1os showing an intermediate depth seismic gap and a broad volcanic arc region 3 from 1os to 2os with steep ne dipping subduction and a narrow volcanic arc region 4 south of 2os with flat subduction and no modern volcanic arc the carnegie ridge has been colliding with the margin since at least 2 ma based on examination of the basement uplift signal along trench parallel transects the subducted prolongation of carnegie ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed broad volcanic arc these findings conflict with previous tectonic models suggesting that the carnegie ridge entered the trench at 1 ma furthermore the anomalous geochemical adakitic signature of the volcanoes in the broad ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant shallowly subducting carnegie ridge from segments of steep subduction in regions 1 and 3 it is further suggested that carnegie ridge supports a local flat slab segment similar to that observed in peru the impact of the carnegie ridge collision on the upper plate causes transpressional deformation extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the san andreas fault system the pattern of instrumental and historical seismicity indicates 1 great earthquakes on the northern and southern flanks of the colliding ridge 2 a slight reduction in observed seismicity at the trench ridge intersection 3 increased stress far into the continent and 4 a nne displacement of the n andes block to be further effects of the collision 1999 elsevier science b v all rights reserved

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

a trapdoor mechanism for slab tearing and melt generation in the northern andes segments of flat subduction are recognized in subduction zones but little is known about the behavior of slabs along the edges of flat slab segments well constrained earthquake data from the northern edge of the peru flat slab segment reveal that the subducting nazca slab is more contorted than previously assumed with a prominent tear developed along a lateral ramp that bounds the peruvian flat slab beneath southern ecuador the slab geometry and associated tear are spatially linked to sumaco volcano similar to 100 km east of the arc front sumaco mafic lavas are ultrapotassic enriched in incompatible elements and show little evidence for crustal assimilation these geochemical signatures are consistent with low degree 80 km depth we propose that focused melting at the slab edge was enabled by the combination of tearing and the generation of poloidal mantle flow in the proximity of the tear a possible explanation for this process is the opening of a trapdoor style tear which induced a poloidal mantle flow by the gravitational foundering of the relatively old and dense edge of the flat slab segment

modeling giant iceberg drift under the influence of sea ice in the weddell sea antarctica the drift trajectory of giant iceberg c 7 traversing the weddell sea antarctica for 2 years was successfully simulated application of the classical driving forces like wind and ocean currents resulted in a significant discrepancy between modeled and observed iceberg velocities in the western weddell sea the most realistic drift pattern in space and time was achieved by adding a sea ice force which represents the ability of a dense sea ice cover 90 to lock in icebergs and collect the momentum of the wind over an area much larger than the area of the iceberg proper this process was parameterized using a sea ice strength p which depends on sea ice concentration and thickness both having highest values in winter and in the western weddell sea which is covered with multi year sea ice as a consequence of the sensitivity to sea ice the timing of the iceberg drift becomes important revealing the region off brunt ice shelf eastern weddell sea as a location where bergs either continue westward with the coastal current or follow a southern branch onto the shallow continental shelf

antarctic ice shelf calving triggered by the honshu japan earthquake and tsunami march 2011 we use european space agency envisat data to present the first observational evidence that a northern hemisphere tsunami triggered antarctic ice shelf calving more than 13 000 km away the honshu tsunami of 11 march 2011 traversed the pacific ocean in 46 years this event further illustrates the growing evidence of ocean wave impact on antarctic calving and emphasizes the teleconnection between the antarctic ice sheet and events as far away as the northern hemisphere

contribution of giant icebergs to the southern ocean freshwater flux 1 in the period 1979 2003 the mass of giant icebergs icebergs larger than 18 5 km in length calving from antarctica averaged 1089 300 gt yr 1 of ice under half the snow accumulation over the continent given by the intergovernmental panel on climate change 2246 86 gt yr 1 here we combine a database of iceberg tracks from the national ice center and a model of iceberg thermodynamics in order to estimate the amount and distribution of meltwater attributable to giant icebergs by comparing with published modeled meltwater distribution for smaller bergs we show that giant icebergs have a different melting pattern an estimated 35 of giant icebergs mass is exported north of 63 s versus 3 for smaller bergs although giant bergs spend more of the earlier part of their history nearer to the coast we combine both estimates to produce the first iceberg meltwater map that takes into account giant icebergs the average meltwater input is shown to exceed precipitation minus evaporation p e in certain areas and is a nonnegligible term in the balance of freshwater fluxes in the southern ocean the calving of giant icebergs is however episodic this might have implications for their impact on the freshwater budget of the ocean it is estimated that over the period 1987 2003 the meltwater flux in the weddell and ross seas has varied by at least 15 000 m3 s 1 over a month because of the potential sensitivity of the production of deep waters to abrupt changes in the freshwater budget variations in iceberg melt rates of this magnitude might be climatologically significant

flow of the ross ice shelf antarctica is modulated by the ocean tide the ice streams feeding the ross ice shelf antarctica have large tidally modulated sinusoidal and stick slip flow but the interaction with the ice shelf is poorly understood we show that the flow of the ross ice shelf front up to 650 km from the ice streams exhibits smooth sinusoidal motions corresponding to tidal modulation these observations suggest a possible linking of the ice shelf with the ice streams to form a unified system that responds to small perturbations in stresses associated with ocean tides if this is the case the presence of the sinusoidal motion but the absence of stick slip motion suggests there is damping of very high frequency signals the dissimilar signatures of the motions observed in the ice streams and at the front of the ice shelf present challenges to model development aimed at understanding the dynamics of coupled ice stream ice shelf flow and the movement of ice across grounding lines

subduction controls on the compositions of lavas from the ecuadorian andes three volcanoes of the ecuadorian andes atacazo antisana and sumaco lie in a transect perpendicular to the trench and the main trend of the andean arc each of the volcanoes lies on crust of substantially different age composition and thickness few compositional or isotopic features correspond in a straightforward way to the type of the crust through which the magmas have passed isotopic data limit assimilation to 15 at each of the volcanoes instead a systematic relationship exists between the compositions of the lavas and the depth to the benioff zone suggesting that subduction imparts the principal control on the compositions of the magmas atacazo s lavas have low concentrations of the incompatible trace elements and very large lil hfs ratios sumaco s lavas are strongly enriched in the incompatible trace elements and have small lil hfs ratios antisana s lavas are intermediate in almost every respect these features are consistent with devolatilization of the subducted slab controlling the extent of partial melting of a depleted mantle source a mixing and melting model suggests the volcanic front magmas are made by large extent of partial melting 15 and include a large slab input 1 1 added to the depleted mantle the magmas of the middle belt of volcanoes are made by smaller extent of partial melting 3 induced by moderate amounts of slab derived fluid 0 06 the back arc magmas result from small degrees of melting 2 and small slab input

numerical simulations of the ross sea tides tidal currents below the floating ross ice shelf are reconstructed by using a numerical tidal model they are predominantly diurnal achieve maximum strength in regions near where the ice shelf runs aground and are significantly enhanced by topographic rossby wave propagation along the ice front a comparison with observations of the vertical motion of the ice shelf surface indicates that the model reproduces the diurnal tidal characteristics within 20 similar agreement for the relatively weak semi diurnal tides was not obtained and this calls attention to possible errors of the open boundary forcing obtained from global ocean tidal simulations and to possible errors in mapping zones of ice shelf grounding air sea contact below the ice shelf is eliminated by the thick ice cover the dominant sub ice shelf circulation may thus be tidally induced a preliminary assessment of sub ice shelf conditions based on the numerical tidal simulations suggests that 1 strong barotropic circulation is driven along the ice front and 2 tidal fronts may form in the sub ice shelf cavity where the water column is thin and where the buoyancy input is weak

seismic observations of glaciogenic ocean waves micro tsunamis on icebergs and ice shelves seismometers deployed over a 3 year period on icebergs in the ross sea and on the ross ice shelf antarctica reveal that impulsive sources of ocean surface waves are frequent e g 200 events per year in the ross sea in the ice shelf and iceberg covered environment of coastal antarctica the 368 events recorded by our field deployment suggest that these impulsive events are generated by glaciological mechanisms such as 1 small scale calving and edge wasting of icebergs and ice shelf fronts 2 edge on edge closing and opening associated with iceberg collisions and 3 possibly the impulsive opening of void space associated with ice shelf rifting and basal crevasse formation the observations described here provide a background of glaciogenic ocean wave phenomena relevant to the ross sea and suggest that these phenomena may be exploited in the future using more purposefully designed observation schemes to understand iceberg calving and ice shelf disintegration processes

rupture history of the 1997 cariaco venezuela earthquake from teleseismic p waves a two step finite fault waveform inversion scheme is applied to the broadband teleseismic p waves recorded for the strike slip cariaco venezuela earthquake of 9 july 1997 to recover the distribution of mainshock slip the earthquake is first analyzed using a long narrow fault with a maximum rise time of 20 sec this line source analysis indicates that slip propagated to the west with a constant rupture velocity and a relatively short rise time the results are then used to constrain a second inversion of the p waveforms using a 60 km by 20 km two dimensional fault the rupture shows a zone of large slip 1 3 m peak near the hypocenter and a second broader source extending updip and to the west at depths shallower than 5 km the second source has a peak slip of 2 1 meters and accounts for most of the moment of 1 1 1026 dyne cm 6 6 mww estimated from the p waves the inferred rupture pattern is consistent with macroseismic effects observed in the epicentral area

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

a new view for the geodynamics of ecuador implication in seismogenic source definition and seismic hazard assessment a new view of ecuador s complex geodynamics has been developed in the course of modeling seismic source zones for probabilistic seismic hazard analysis this study focuses on two aspects of the plates interaction at a continental scale a age related differences in rheology between farallon and nazca plates marked by the grijalva rifted margin and its inland projection as they subduct underneath central ecuador and b the rapidly changing convergence obliquity resulting from the convex shape of the south american northwestern continental margin both conditions satisfactorily explain several characteristics of the observed seismicity and of the interseismic coupling intermediate depth seismicity reveals a severe flexure in the farallon slab as it dips and contorts at depth originating the el puyo seismic cluster the two slabs position and geometry below continental ecuador also correlate with surface expressions observable in the local and regional geology and tectonics the interseismic coupling is weak and shallow south of the grijalva rifted margin and increases northward with a heterogeneous pattern locally associated to the carnegie ridge subduction high convergence obliquity is responsible for the north andean block northeastward movement along localized fault systems the cosanga and pallatanga fault segments of the north andean block south american boundary concentrate most of the seismic moment release in continental ecuador other inner block faults located along the western border of the inter andean depression also show a high rate of moderate size earthquake production finally a total of 19 seismic source zones were modeled in accordance with the proposed geodynamic and neotectonic scheme

quantifying flow regimes in a greenland glacial fjord using iceberg drifters funded by the national science foundation grant numbers oce 1130008 arc 0909274 and the university of oregon

abnormal seismological and magmatic processes controlled by the tearing south american flat slabs abstract the influence of flat slab subduction on the formation of intra slab earthquakes volcanic activities and mantle seismic velocity anomalies remains unclear we attempt to better understand these processes by simulating the two flat slabs in peru and chile using data orientated geodynamic models our results successfully reproduce the observed flat slabs as mainly due to two subducting aseismic ridges in contrast to the traditional view of flat slab subduction we find that these slabs are internally torn as is due to the 3d nature of the subducting buoyancy features this broken slab configuration confirmed by regional tomography naturally explains the abnormal distribution of and stress regimes associated with the intermediate depth earthquakes we further show that the slab tearing process could also better explain the formation of adakitic and ore forming magmatism the evolution of the magmatic arc and the enigmatic mantle seismic structures beneath these regions we propose that slab tearing may represent a common result of buoyancy feature subduction and that the resulting mantle processes could affect the long term geodynamic evolution of continents

seismic and hydroacoustic tremor generated by colliding icebergs 1 iceberg harmonic tremor iht emanating from tabular icebergs in the southern ocean and along calving margins of the antarctic ice sheet is a complex evolving signal at frequencies above approximately 0 5 hz iht has been observed as t phases on islands in the equatorial pacific as hydroacoustic signals in the indian ocean and by local and regional antarctic seismic networks to identify the iht source mechanism and to understand its relevance to iceberg calving evolution and breakup we deployed seismometers on a giant 25 by 50 km tabular iceberg called c16 in the ross sea antarctica during a uniquely accessible period austral summer 2003 2004 when it was aground against the northern shore of ross island during the deployment period c16 was in sporadic contact with another giant tabular iceberg b15a that was moving under the influence of local ocean currents this study reveals that the c16 associated iht was a manifestation of extended episodes of discrete repeating stick slip icequakes typically thousands of individual subevents per hour produced when the cliff like edges of the tabular icebergs underwent glancing strike slip collisions the iht signal that we observed is thus not a phenomenon associated with iceberg elastic or fluid resonance modes but is instead the consequence of long sequences of very regularly spaced and similar pulses of seismic radiation from these constituent stick slip subevents iht represents a newly identified glaciogenic source of seismicity that can be used to improve our understanding of iceberg dynamics and possibly of ice shelf disintegration processes

antarctic iceberg distribution and dissolution from ship based observations the australian antarctic program s iceberg dataset from ship based observations including information from the austral summer seasons 1984 85 to 1999 2000 is examined and used to extend earlier studies using snapshots of the iceberg population to provide an idea of the iceberg life cycle the distribution of icebergs between 60 and 1508 e is discussed in terms of calving regions and ocean currents temporal changes are also examined the discussion leads us to the point where we can define an area bounded to the north by the maximum sea ice limit and to the south by the antarctic divergence in which icebergs are confined as they drift eastward this allows estimation of total dissolution in terms of iceberg numbers and volume within 108 longitudinal sectors and with knowledge of drift speeds iceberg movement rates and freshwater input across the sector iceberg dissolution rates are found to be 0 03 0 05 m d 1 and the total mass contribution of fresh water to the ocean as the icebergs traverse our 308 of longitude study sector is 32 gt this amounts to a contribution equivalent to precipitation of 15 5 cm a 1 accounting for 2 of the total iceberg discharge from the antarctic ice sheet

instability of the antarctic ross sea embayment as climate warms collapse of the antarctic ice sheet since the last glacial maximum 18 000 years ago is most pronounced in the ross sea embayment which is partly ice free during antarctic summers thereby breaching the o ring of ice shelves and sea ice surrounding antarctica that stabilizes the ice sheet the o ring may have vanished during early holocene 5000 to 3000 b c roman 1 to 400 a d and medieval 900 to 1300 a d warm periods and reappeared during the little ice age 1300 to 1900 a d we postulate further collapse in the embayment during the post 1900 warming may be forestalled because east antarctic outlet glaciers nail the ross ice shelf to the transantarctic mountains so it can resist the push from west antarctic ice streams our hypothesis is examined for byrd glacier and a static ice shelf using three modeling experiments having plastic viscous and viscoplastic solutions as more data and improved modeling became available observed crevasse patterns were not reproduced a new research study is needed to model a dynamic ross ice shelf with all its feeder ice streams outlet glaciers and ice calving dynamics in three dimensions over time to fully test our hypothesis the required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf calving must then successively pull the outlet glacier nails so collapse of the marine west antarctic ice sheet proceeds to completion

mass balance of polar ice sheets recent advances in the determination of the mass balance of polar ice sheets show that the greenland ice sheet is losing mass by near coastal thinning and that the west antarctic ice sheet with thickening in the west and thinning in the north is probably thinning overall the mass imbalance of the east antarctic ice sheet is likely to be small but even its sign cannot yet be determined large sectors of ice in southeast greenland the amundsen sea embayment of west antarctica and the antarctic peninsula are changing quite rapidly as a result of processes not yet understood

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

slab2 a comprehensive subduction zone geometry model subduction zones are home to the most seismically active faults on the planet the shallow megathrust interfaces of subduction zones host earth s largest earthquakes and are likely the only faults capable of magnitude 9 ruptures despite these facts our knowledge of subduction zone geometry which likely plays a key role in determining the spatial extent and ultimately the size of subduction zone earthquakes is incomplete we calculated the three dimensional geometries of all seismically active global subduction zones the resulting model called slab2 provides a uniform geometrical analysis of all currently subducting slabs