larsen ice shelf has progressively thinned the retreat and collapse of antarctic peninsula ice shelves in tandem with a regional atmospheric warming has fueled speculation as to how these events may be related satellite radar altimeter measurements show that between 1992 and 2001 the larsen ice shelf lowered by up to 0 27 0 11 meters per year the lowering is explained by increased summer meltwater and the loss of basal ice through melting enhanced ocean driven melting may provide a simple link between regional climate warming and the successive disintegration of sections of the larsen ice shelf

measurement and modeling of ablation of the bottom of supraglacial lakes in western greenland 1 we report measurements of ablation rates of the bottom of two supraglacial lakes and of temperatures at different depths collected during the summers of 2010 and 2011 in west greenland to our knowledge this is the first time that such data sets are reported and discussed in the literature the measured ablation rates at the bottom of the two lakes are of the order of 6 cm day versus a rate of 2 5 3 cm day in the case of bare ice of surrounding areas though our measurements suggest the presence of a vertical temperature gradient it is not possible to draw final conclusions as the measured gradient is smaller than the accuracy of our temperature sensors in situ measurements are compared with the results of a thermodynamic model forced with the outputs of a regional climate model in general the model is able to satisfactorily reproduce the measured quantities with rmse of the order of 3 4 cm for the ablation and 1 5 c in the case of water temperature our results confirm that the ablation at the bottom of supraglacial lakes plays an important role on the overall lake volume with the ablation in the case of ice covered by a lake being 110 135 of that over bare ice at nearby locations beside ice sheet hydrological implications melting at the bottom of a supraglacial lake might affect estimates of lake volume from spaceborne visible and near infrared measurements

supraglacial lakes on the larsen b ice shelf antarctica and at paakitsoq west greenland a comparative study we acknowledge the support of the u s national science foundation under grant ant 0944248

catastrophic ice shelf break up by an ice shelf fragment capsize mechanism two disintegration events leading to the loss of larsen a and b ice shelves antarctic peninsula in 1995 and 2002 respectively proceeded with extreme rapidity order of several days and reduced an extensive seemingly integrated ice shelf to a jumble of small fragments these events strongly correlate with warming regional climate and accumulation of surface meltwater supporting a hypothesis that meltwater induced propagation of pre existing surface crevasses may have initiated ice shelf fragmentation we address here an additional subsequent mechanism that may sustain and accelerate the ice shelf break up once it begins the proposed mechanism involves the coherent capsize of narrow less than thickness ice shelf fragments by rolling 90 in a direction toward or away from the ice front fragment capsize liberates gravitational potential energy forces open ice shelf rifts and contributes to further fragmentation of the surrounding ice shelf

breakup of the larsen b ice shelf triggered by chain reaction drainage of supraglacial lakes this research is supported by the u s national science foundation under grant geop ant 0944248 awarded to d r m and grants ant 0838811 and arc 0934534 awarded to o v s

laboratory investigations of iceberg capsize dynamics energy dissipation and tsunamigenesis 1 we present laboratory experiments designed to quantify the stability and energy budget of buoyancy driven iceberg capsize box shaped icebergs were constructed out of low density plastic hydrostatically placed in an acrylic water tank containing freshwater of uniform density and allowed or forced if necessary to capsize the maximum kinetic energy translational plus rotational of the icebergs was 15 of the total energy released during capsize and radiated surface wave energy was 1 of the total energy released the remaining energy was directly transferred into the water via hydrodynamic coupling viscous drag and turbulence the dependence of iceberg capsize instability on iceberg aspect ratio implied by the tank experiments was found to closely agree with analytical predictions based on a simple hydrostatic treatment of iceberg capsize this analytical treatment along with the high reynolds numbers for the experiments and considerably higher values for capsizing icebergs in nature indicates that turbulence is an important mechanism of energy dissipation during iceberg capsize and can contribute a potentially important source of mixing in the stratified ocean proximal to marine ice margins

a computational investigation of iceberg capsize as a driver of explosive ice shelf disintegration potential energy released from the capsize of ice shelf fragments icebergs is the immediate driver of the brief explosive phase of ice shelf disintegration along the antarctic peninsula e g the larsen a larsen b and wilkins ice shelves the majority of this energy powers the rapidly expanding plume of ice shelf fragments that expands outward into the open ocean a smaller fraction of this energy goes into surface gravity waves and other dynamic interactions between ice and water that can sustain the continued fragmentation and break up of the original ice shelf as an initial approach to the investigation of ice shelf fragment capsize in ice shelf collapse we develop a simple conceptual model involving ideal rectangular icebergs initially in unstable or metastable orientations which are assembled into a tightly packed mass that subsequently disassembles via massed capsize computations based on this conceptual model display phenomenological similarity to aspects of real ice shelf collapse a promising result of the conceptual model presented here is a description of how iceberg aspect ratio and its statistical variance the two parameters related to ice shelf fracture patterns influence the enabling conditions to be satisfied by slow acting processes e g environmentally driven melting that facilitate ice shelf disintegration

ice shelf disintegration by plate bending and hydro fracture satellite observations and model results of the 2008 wilkins ice shelf break ups abstract satellite remote sensing observations of three break up events in 2008 for the wilkins ice shelf 28 february to 6 march 27 may to 31 may and 28 june to mid july provide unprecedented detail of ice shelf calving during rapid break up the observations reveal that the wilkins break ups occur through a distinctive type of shelf calving which we term disintegration as well as more typical rifting and calving here we focus on the disintegration process which is characterized by repeated rapid fracturing that creates narrow ice edge parallel blocks with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass we use these data to develop and test a model of floating ice plate disintegration in which ice plate bending stresses at the ice front arising from buoyancy forces can lead to runaway calving when free mobile water is available high resolution satellite images and laser altimetry of the first break up event provide details of fracture spacings ice thicknesses and plate bending profiles that agree well with our model predictions we suggest that surface or near surface meltwater is the main pre condition for disintegration and that hydro fracture is the main mechanism brine layers from near waterline brine infiltration can support a similar process but this is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing a combination of brine enhanced fracturing and changing internal net extensional stresses was the likely mechanism behind the latter two wilkins events

the flexural dynamics of melting ice shelves a conspicuous precursor of catastrophic ice shelf break up along the antarctic peninsula reported widely in the literature is the gradual increase in surface melting and consequent proliferation of supraglacial lakes and dolines here we present analytical and numerical solutions for the flexure stresses within an ice shelf covered by lakes and dolines both isolated and arrayed we conclude that surface water promotes ice shelf instability in two ways 1 by water assisted crevasse penetration as previously noted and 2 by the inducement of strong tensile flexure stresses exceeding background spreading stress by 10 100 times in response to surface water mass loads and hydrostatic rebound occurring when meltwater lakes drain

source array analysis of coda waves near the 1989 loma prieta california mainshock implications for the mechanism of coseismic velocity changes coda waves are often considered to be generated by backscattering of primary waves from randomly distributed heterogeneities in the crust however modeling and experimental work indicates that much of the coda may be due to scattering near the receiver knowing where the coda is generated is important for several reasons coda waves have been used to characterize site amplification scattering and attenuation throughout the crust and velocity changes associated with large earthquakes however a lack of knowledge of where coda waves are generated can lead to ambiguous interpretations in this study we analyze 26 s of coda waves recorded at up to 78 stations using slowness stacking on two source arrays and find that at nearly all stations regardless of distance from the source nearly all the arrivals are within one hemisphere most are strongly clustered near the direct arrival and the clusters are in general symmetrically disposed about the array axis fault plane this finding suggests that the coda consists primarily of waves scattered near the stations rather than waves scattered throughout the crust this result holds even at stations very close to the sources where the amount of coda examined is about 6 times the direct s travel time and it suggests that much of the coseismic velocity decrease associated with the 1989 loma prieta earthquake ellsworth et al 1992 occurred in the shallow crust near the stations one possible mechanism for inducing such a change is a decrease in the mean stress from the mainshock slip however on the basis of static stress modeling we find that this mechanism would produce a velocity increase in the region where velocity was observed to decrease we conclude that the velocity decrease was more likely caused by crack opening or crack coalescence due to strong shaking from the mainshock

calving and rifting on the mcmurdo ice shelf antarctica on 2 march 2016 several small en echelon tabular icebergs calved from the seaward front of the mcmurdo ice shelf and a previously inactive rift widened and propagated by 3 km 25 of its previous length setting the stage for the future calving of a 14 km2 iceberg within 24 h of these events all remaining land fast sea ice that had been stabilizing the ice shelf broke up the events were witnessed by time lapse cameras at nearby scott base and put into context using nearby seismic and automatic weather station data satellite imagery and subsequent ground observation although the exact trigger of calving and rifting cannot be identified definitively seismic records reveal superimposed sets of both long period 10 s sea swell propagating into mcmurdo sound from storm sources beyond antarctica and high energy locally sourced short period 10 s sea swell in the 4 days before the fast ice break up and associated ice shelf calving and rifting this suggests that sea swell should be studied further as a proximal cause of ice shelf calving and rifting if proven it suggests that ice shelf stability is tele connected with far field storm conditions at lower latitudes adding a global dimension to the physics of ice shelf break up

a model of viscoelastic ice shelf flexure we develop a formal thin plate treatment of the viscoelastic flexure of floating ice shelves as an initial step in treating various problems relevant to ice shelf response to sudden changes of surface loads and applied bending moments e g draining supraglacial lakes iceberg calving surface and basal crevassing our analysis is based on the assumption that total deformation is the sum of elastic and viscous or power law creep deformations i e akin to a maxwell model of viscoelasticity having a spring and dashpot in series the treatment follows the assumptions of well known thin plate approximation but is presented in a manner familiar to glaciologists and with glen s flow law we present an analysis of the viscoelastic evolution of an ice shelf subject to a filling and draining supraglacial lake this demonstration is motivated by the proposition that flexure in response to the filling drainage of meltwater features on the larsen b ice shelf antarctica contributed to the fragmentation process that accompanied its collapse in 2002

a structural glaciological analysis of the 2002 larsen b ice shelf collapse this study provides a detailed structural glaciological analysis of changes in surface structures on the larsen b ice shelf on the antarctic peninsula prior to its collapse in february march 2002 mapped features include the ice shelf front rifts crevasses longitudinal linear surface structures and meltwater features we define domains on the ice shelf related to glacier source areas and demonstrate that prior to collapse the central larsen b ice shelf consisted of four sutured flow units fed by crane jorum punchbowl and hektoria green evans glaciers between these flow units were suture zones of thinner ice where the feeder glaciers merged prior to collapse large open rift systems were present offshore of foyn point and cape disappointment these rifts became more pronounced in the years preceding break up and ice blocks in the rifts rotated because of the strong lateral shear in this zone velocity mapping of the suture zones indicates that the major rifts were not present more than about 20 years ago we suggest that the ice shelf was preconditioned to collapse by partial rupturing of the sutures between flow units this we believe was the result of ice shelf front retreat during 1998 2000 reducing the lateral resistive stress on the upstream parts of the shelf and glacier flow units ice shelf thinning and pre shelf break up glacier acceleration

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

tsunami and infragravity waves impacting antarctic ice shelves the responses of the ross ice shelf ris to the september 16 2015 8 3 mw chilean earthquake tsunami 75 s period and to oceanic infragravity ig waves 50 300 s period were recorded by a broadband seismic array deployed on the ris from november 2014 to november 2016 here we show that tsunami and ig generated signals within the ris propagate at gravity wave speeds 70 m s as water ice coupled flexural gravity waves ig band signals show measureable attenuation away from the shelf front the response of the ris to chilean tsunami arrivals is compared with modeled tsunami forcing to assess ice shelf flexural gravity wave excitation by very long period vlp 300 s gravity waves displacements across the ris are affected by gravity wave incident direction bathymetry under and north of the shelf and water layer and ice shelf thicknesses horizontal displacements are typically about 10 times larger than vertical displacements producing dynamical extensional motions that may facilitate expansion of existing fractures vlp excitation is continuously observed throughout the year with horizontal displacements highest during the austral winter with amplitudes exceeding 20 cm because vlp flexural gravity waves exhibit no discernable attenuation this energy must propagate to the grounding zone both ig and vlp band flexural gravity waves excite mechanical perturbations of the ris that likely promote tabular iceberg calving consequently affecting ice shelf evolution understanding these ocean excited mechanical interactions is important to determine their effect on ice shelf stability to reduce uncertainty in the magnitude and rate of global sea level rise

iceberg capsize tsunamigenesis calving from the floating termini of outlet glaciers and ice shelves is just the beginning of an interesting chain of events that can subsequently have important impacts on human life and property immediately after calving many icebergs capsize roll over by 90 due to the instability of their initial geometry as icebergs melt and respond to the cumulative effects of ocean swell they can also reorient their mass distribution by further capsize and fragmentation these processes release gravitational potential energy and can produce impulsive large amplitude surface gravity waves known as tsunamis a term derived from the japanese language iceberg capsize tsunamis in greenland fjords can be of sufficient amplitude to threaten human life and cause destruction of property in settlements iceberg capsize tsunamis may also have a role in determining why some ice shelves along the antarctic peninsula disintegrate explosively in response to general environmental warming to quantify iceberg tsunami hazards we investigate iceberg capsize energetics and develop a rule relating tsunami height to iceberg thickness this rule suggests that the open water tsunami height located far from the iceberg and from shorelines where the height can be amplified has an upper limit of 0 01h where h is the initial vertical dimension of the iceberg

fracture propagation and stability of ice shelves governed by ice shelf heterogeneity tabular iceberg calving and ice shelf retreat occurs after full thickness fractures known as rifts propagate across an ice shelf a quickly evolving rift signals a threat to the stability of larsen c the antarctic peninsula s largest ice shelf here we reveal the influence of ice shelf heterogeneity on the growth of this rift with implications that challenge existing notions of ice shelf stability most of the rift extension has occurred in bursts after overcoming the resistance of suture zones that bind together neighboring glacier inflows we model the stresses in the ice shelf to determine potential rift trajectories calving perturbations to ice flow will likely reach the grounding line the stability of larsen c may hinge on a single suture zone that stabilizes numerous upstream rifts elevated fracture toughness of suture zones may be the most important property that allows ice shelves to modulate antarctica s contribution to sea level rise

damage accelerates ice shelf instability and mass loss in amundsen sea embayment pine island glacier and thwaites glacier in the amundsen sea embayment are among the fastest changing outlet glaciers in west antarctica with large consequences for global sea level yet assessing how much and how fast both glaciers will weaken if these changes continue remains a major uncertainty as many of the processes that control their ice shelf weakening and grounding line retreat are not well understood here we combine multisource satellite imagery with modeling to uncover the rapid development of damage areas in the shear zones of pine island and thwaites ice shelves these damage areas consist of highly crevassed areas and open fractures and are first signs that the shear zones of both ice shelves have structurally weakened over the past decade idealized model results reveal moreover that the damage initiates a feedback process where initial ice shelf weakening triggers the development of damage in their shear zones which results in further speedup shearing and weakening hence promoting additional damage development this damage feedback potentially preconditions these ice shelves for disintegration and enhances grounding line retreat the results of this study suggest that damage feedback processes are key to future ice shelf stability grounding line retreat and sea level contributions from antarctica moreover they underline the need for incorporating these feedback processes which are currently not accounted for in most ice sheet models to improve sea level rise projections

ice dolines on larsen ice shelf antarctica abstract ice dolines on the larsen ice shelf antarctica are observed to beelongated depressions a few hundred meters across and up to 19m deep one meterresolution imagery is used to quantify these dimensions elevation profiles across fivedolines are derivedby photoclinometry landsat and radar imagery is also usedto showthatdolinescanforminasinglemeltseasonandpersistforyears dolinesoccurinclustersandindirectproximitytosurfacemeltwaterlakes fieldobservationssuggestdolinesformby collapse into a subsurface cavity a direct hydraulic connection with the underlyingocean is believed necessary to drain water that would otherwise collect in dolines aformation hypothesis is discussed consistent with these observations and with energy andhydrostatic imbalance considerations introduction doline isageologictermdescribingaroundedhollowthatcanrange in width from 1 to 100m and in depth from ten tohundredsof meters agi 1976 mellor 1960 wasthefirsttosuggest the term icedoline be applied to depression featuresseen on ice their size hundreds of meters across is theprincipal characteristic distinguishing them from

seasonal evolution of supraglacial lakes on a floating ice tongue petermann glacier greenland supraglacial lakes are known to trigger antarctic ice shelf instability and break up however to date no study has focused on lakes on greenland s floating termini here we apply lake boundary area and depth algorithms to landsat 8 imagery to analyse the inter and intraseasonal evolution of supraglacial lakes across petermann glacier s 81 n floating tongue from 2014 to 2016 while also comparing these lakes to those on the grounded ice lakes start to fill in june and quickly peak in total number volume and area in late june early july in response to increases in air temperatures however through july and august total lake number volume and area all decline despite sustained high temperatures these observations may be explained by the transportation of meltwater into the ocean by a river and by lake drainage events on the floating tongue further as mean lake depth remains relatively constant during this time we suggest that a large proportion of the lakes that drain do so completely likely by rapid hydrofracture the mean areas of lakes on the tongue are only 20 of those on the grounded ice and exhibit lower variability in maximum and mean depth differences likely attributable to the contrasting formation processes of lakes in each environment

evolution and relationships between volcanism and tectonics in the central eastern part of the oligocene borovitsa caldera eastern rhodopes bulgaria abstract the nested borovitsa caldera emplaced during the collision related paleogene volcanism in the eastern rhodopes the pre caldera succession consists in priabonian to early oligocene sediments and lavas absarokites shoshonites latites the caldera filling corresponds to an acid volcanism early oligocene in age the tectono magmatic evolution of the caldera can be divided into six main stages 1 ignimbritic units more than 1 5 km thick with a trachydacitic to trachytic composition deposited the k ar method yields an age of 34 33 5 ma the volcanic products are either strongly or not welded in the western and eastern parts of the caldera respectively 2 an initial murga caldera 7 10 km in diameter collapsed this event was accompanied by the intrusion of a circular body consisting of lenses bearing rocks of trachyrhyodacitic to rhyolitic composition within the border faults 3 the emission of pyroclastic rocks continued and a large sub volcanic body 33 ma of trachydacitic to trachyrhyolitic composition intruded in the western part of the circular body 4 the borovitsa caldera 15 km 34 km collapsed rhyolitic and trachydacitic dykes dated at 32 5 ma intruded along its border faults 5 high si trachyrhyolitic perlitic domes intruded in the eastern part of the borovitsa caldera at 30 32 ma and the dushka caldera collapsed within the borovitsa structure 6 dykes of various compositions from shoshonite to rhyolite and trachydacitic to rhyolitic sub volcanic stocks finally intruded within the caldera and along its rims at 27 5 29 5 ma observations on radar and optical satellite imagery allowed both a new mapping of the structural pattern in the borovitsa caldera and the understanding of the relationships between faulting and volcanism in this area horse tail features accommodating the right lateral throw component at the termination of nw se and n s right lateral strike slip faults are superimposed upon the murga caldera and the eastern part of the borovitsa caldera respectively wnw ese to nw se extension along the horse tail patterns was responsible for the ascent of pressurized magma through the crust via vertical open fractures leading to volcanic activity and subsequent emplacement of domes and crypto domes during the fifth magmatic stage this extension was followed by a compression with various orientations which may be interpreted as a switch of the tensile stress to a compressive one at the onset of subsidence of the dushka caldera the last tectonic event in the area is characterized by a n s extension that led to the reactivation of part of the previous structures the formation of nw se to e w normal faults and the emplacement of dykes and stocks during the sixth magmatic stage

fracture field for large scale ice dynamics recent observations and modeling studies emphasize the crucial role of fracture mechanics for the stability of ice shelves and thereby the evolution of ice sheets here we introduce a macroscopic fracture density field into a prognostic continuum ice flow model and compute its evolution incorporating the initiation and growth of fractures as well as their advection with two dimensional ice flow to a first approximation fracture growth is assumed to depend on the spreading rate only while fracture initiation is defined in terms of principal stresses the inferred fracture density fields compare well with observed elongate surface structures since crevasses and other deep reaching fracture structures have been shown to influence the overall ice shelf dynamics we propose the fracture density field introduced here be used as a measure for ice softening and decoupling of the ice flow in fracture weakened zones this may yield more accurate and realistic velocity patterns in prognostic simulations additionally the memory of past fracture events links the calving front to the upstream dynamics thus the fracture density field proposed here may be employed in fracture based calving parameterizations the aim of this study is to introduce the field and investigate which of the observed surface structures can be reproduced by the simplest physically motivated fracture source terms

alternative ice shelf equilibria determined by ocean environment 1 dynamic and thermodynamic regimes of ice shelves experiencing weak 1 m year 1 to strong 10 m year 1 basal melting in cold bottom temperature close to the in situ freezing point and warm oceans bottom temperature more than half of a degree warmer than the in situ freezing point are investigated using a 1 d coupled ice ocean model complemented with a newly derived analytic expression for the steady state temperature distribution in ice shelves this expression suggests the existence of a basal thermal boundary layer with thickness inversely proportional to the basal melt rate model simulations show that ice shelves afloat in warm ocean waters have significantly colder internal ice temperatures than those that float in cold waters our results indicate that in steady states the mass balance of ice shelves experiencing strong and weak melting is controlled by different processes in ice shelves with strong melting it is a balance between ice advection and basal melting and in ice shelves with weak melting it is a balance between ice advection and deformation sensitivity simulations show that ice shelves in cold and warm oceans respond differently to increase of the ocean heat content ice shelves in cold waters are more sensitive to warming of the ocean bottom waters while ice shelves in warm waters are more sensitive to shallowing of the depth of the thermocline

observations of buried lake drainage on the antarctic ice sheet between 1992 and 2017 the antarctic ice sheet ais lost ice equivalent to 7 6 3 9 mm of sea level rise ais mass loss is mitigated by ice shelves that provide a buttress by regulating ice flow from tributary glaciers however ice shelf stability is threatened by meltwater ponding which may initiate or reactivate preexisting fractures currently poorly understood processes here through ground penetrating radar gpr analysis over a buried lake in the grounding zone of an east antarctic ice shelf we present the first field observations of a lake drainage event in antarctica via vertical fractures concurrent with the lake drainage event we observe a decrease in surface elevation and an increase in sentinel 1 backscatter finally we suggest that fractures that are initiated or reactivated by lake drainage events in a grounding zone will propagate with ice flow onto the ice shelf itself where they may have implications for its stability

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis