conceptual thoughts on continuum damage mechanics for shallow ice shelves we consider a theory for shallow ice shelves that includes an isotropic damage variable a zeroth order shallow shelf approximation allows a simple yet consistent treatment of both ice dynamics and damage evolution we find that the damage variable like temperature has in general to vary with depth a purely two dimensional membrane theory can only be considered a rough approximation for isothermal ice shelves

a physically based calving model applied to marine outlet glaciers and implications for the glacier dynamics we present results from numerical ice flow models that include calving criteria based on penetration of surface and basal crevasses which in turn is a function of longitudinal strain rates near the glacier front the position of the calving front is defined as the point where either 1 surface crevasses reach the waterline model cdw or 2 surface and basal crevasses penetrate the full thickness of the glacier model cd for comparison with previous studies results are also presented for a height above buoyancy calving model qualitatively both models cdw and cd produce similar behaviour unlike previous models for calving the new calving criteria are applicable to both grounded termini and floating ice shelves and tongues the numerical ice flow model is applied to an idealized geometry characteristic of marine outlet glaciers results indicate that grounding line dynamics are less sensitive to basal topography than previously suggested stable grounding line positions can be obtained even on a reverse bed slope with or without floating termini the proposed calving criteria also allow calving losses to be linked to surface melt and therefore climate in contrast to previous studies in which calving rate or position of the terminus is linked to local water depth the new calving criterion is able to produce seasonal cycles of retreat and advance as observed for greenland marine outlet glaciers the contrasting dynamical behaviour and stability found for different calving models suggests that a realistic parameterization for the process of calving is crucial for any predictions of marine outlet glacier change

dynamics of izmit earthquake postseismic deformation and loading of the duzce earthquake hypocenter we have developed dynamic finite element models of izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip via either velocity strengthening frictional slip or linear viscous creep or by distributed linear viscoelastic relaxation of the lower crust we find that velocity strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time dependent global positioning system data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation our best frictional afterslip model fits the main features of postseismic slip inversions in particular high slip patches at and below the hypocenter and on the western karadere segment and limited afterslip west of the hersek delta burgmann et al 2002 the model requires a weakly velocity strengthening fault that is either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate dependence of friction a b than is indicated by laboratory experiments our best afterslip model suggests that the coulomb stress at the duzce hypocenter increased by 0 14 mpa 1 4 bars during the izmit earthquake assuming right lateral slip on a surface dipping 50 to the north and by another 0 1 mpa during the 87 days between the izmit and duzce earthquakes in the marmara sea region within about 160 km of the izmit earthquake rupture this model indicates that the coulomb stresses increased by 15 25 of the coseismic amount during the first 300 days after the earthquake three hundred days after the earthquake postseismic contributions to coulomb stressing rate on the maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate our estimates of postseismic coulomb stress are highly model dependent in the marmara region the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic coulomb stresses than the frictional afterslip model near field stress and fault zone rheology estimates are sensitive to the earth9s elastic structure when a layered elastic structure is incorporated in our model it yields a coulomb stress of 0 24 mpa at the duzce hypocenter significantly more than the 0 14 mpa estimated from the uniform elastic model because of the higher near field coseismic stresses the layered elastic model requires a higher value of velocity strengthening parameter a b a b times effective normal stress r to produce comparable postseismic slip a b is estimated at 0 4 and 0 2 mpa respectively for the layered and uniform elastic models these results highlight the importance of understanding the earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses

oceanic forcing of ice sheet retreat west antarctica and more ocean ice interactions have exerted primary control on the antarctic ice sheet and parts of the greenland ice sheet and will continue to do so in the near future especially through melting of ice shelves and calving cliffs retreat in response to increasing marine melting typically exhibits threshold behavior with little change for forcing below the threshold but a rapid possibly delayed shift to a reduced state once the threshold is exceeded for thwaites glacier west antarctica the threshold may already have been exceeded although rapid change may be delayed by centuries and the reduced state will likely involve loss of most of the west antarctic ice sheet causing 3 m of sea level rise because of shortcomings in physical understanding and available data uncertainty persists about this threshold and the subsequent rate of change although sea level histories and physical understanding allow the possibility that ice sheet response could be quite fast no strong constraints are yet available on

effect of orbital scale climate cycling and meltwater drainage on ice sheet grounding line migration 1 ice shelf response to climate cycling introduces hysteretic behavior in outlet glaciers with the evolution of meltwater drainage networks enhancing this effect the 2002 collapse of the larsen b ice shelf one of the floating extensions formed from glaciers flowing off the antarctic peninsula into the weddell sea followed the development of widespread poorly drained surface melt ponds during the atypical warmth of the 2001 2002 summer melt season yet for at least the past half century the floating termini of outlet glaciers in greenland did not collapse despite being exposed to summer mean temperatures and melt rates that far exceed those recorded in 2001 2002 on the antarctic peninsula we suggest that this paradox can be explained by the advection of well developed englacial drainage networks within greenland s extensive ablation zone the efficiency of this plumbing system limits meltwater ponding within crevasses and the subsequent ice shelf failure observed on the antarctic peninsula whether progressive or abrupt ice shelf removal leads to near synchronous glacier retreat conversely glaciation of fjords is delayed during ice shelf formation ultimately the relatively rapid loss and gradual establishment of ice shelf buttressing amid climate cycling leads to an asymmetric response in outlet glacier dynamics this asymmetry may have implications for the future of existing and recently calved ice shelves eustatic sea level and the hysteretic behavior of ice sheets

on the effects of thermally weakened ductile shear zones on postseismic deformation 1 we present three dimensional 3 d numerical models of postseismic deformation following repeated earthquakes on a vertical strike slip fault our models use linear maxwell burgers and temperature dependent power law rheology for the lower crust and upper mantle we consider effects of viscous shear zones that result from thermomechanical coupling and investigate potential kinematic similarities between viscoelastic models incorporating shear zones and elastic models incorporating rate strengthening friction on a deep aseismic fault root we find that the thermally activated shear zones have little effect on postseismic relaxation in particular the presence of shear zones does not change the polarity of vertical displacements in cases of rheologies that are able to generate robust postseismic transients stronger rheologies can give rise to an opposite polarity of vertical displacements but the amplitude of the predicted transient deformation is generally negligible we conclude that additional to thermomechanical coupling mechanisms of strain localization are required for a viscoelastic model to produce a vertical deformation pattern similar to that due to afterslip on a deep extension of a fault we also investigate the discriminating power of models incorporating burgers and power law rheology these rheologies were proposed to explain postseismic transients following large m7 earthquakes in the mojave desert eastern california numerical simulations indicate that it may be difficult to distinguish between these rheologies even with high quality geodetic observations for observation periods less than a decade longer observations however may potentially allow discrimination between the competing models as illustrated by the model comparisons with available gps and interferometric synthetic aperture radar data

earthquake ruptures with strongly rate weakening friction and off fault plasticity part 1 planar faults abstract observations demonstrate that faults are fractal surfaces with deviations from planarity at all scales we study dynamic rupture propagation on self similar faults having root mean square rms height fluctuations of order 10 3 to 10 2 times the profile length our 2d plane strain models feature strongly rate weakening fault friction and off fault drucker prager viscoplasticity the latter bounds otherwise unreasonably large stress concentrations in the vicinity of bends our choice of a cohesionless yield function prevents tensile stress states and thus fault opening a consequence of strongly rate weakening friction is the existence of a critical background stress level above which self sustaining rupture propagation in the form of self healing slip pulses first becomes possible around this level at which natural faults are expected to operate ruptures become extremely sensitive to fault roughness and exhibit substantial fluctuations in rupture velocity except for shallow inclinations of the maximum compressive stress to the fault less than about 20 the fluctuations are anticorrelated with the local fault slope these accelerations and decelerations of the rupture together with naturally emerging slip heterogeneity excite waves of all wavelengths and result in ground acceleration spectra that are flat at high frequency consistent with observed strong motion records

postseismic and interseismic displacements near a strike slip fault a two dimensional theory for general linear viscoelastic rheologies 1 we present an analytic solution for the deformation near an infinite strike slip fault in an elastic layer overlying a linear viscoelastic half space the theory is valid for any linear viscoelastic rheology and any earthquake sequence this is a generalization of the work of j c savage and colleagues which only holds for models with a uniform shear modulus maxwell viscoelastic lower half space and periodic rupture recurrence we demonstrate the theory for models of an elastic layer over a maxwell standard linear solid burgers and triviscous half space for each of these models we calculate example postseismic displacements interseismic displacements in a periodic earthquake sequence and interseismic displacements for a nonperiodic earthquake sequence our solution is for a simple geometry however the model presents an elegant tool to explore the evolution of displacements for relatively complex rheologies and rupture recurrence histories

contribution of viscoelastic flow in earthquake cycles within the lithosphere asthenosphere system faults slip relaxes lithospheric stress imposed by mantle flow and in turn transfers stress to the ductile regions the interplay of these systems governs the style of deformation at plate boundaries including the recurrence of seismic events however such deep processes remain challenging to incorporate in numerical simulations of earthquake cycles here we propose a model that couples fault slip and viscoelastic deformation to simulate fault dynamics in the lithosphere asthenosphere system our method resolves all phases of the earthquake cycle including dynamic rupture propagation afterslip slow slip events and the modulation of strain rate incurred in the ductile regions transient strain accelerations in the asthenosphere may follow both earthquakes and slow slip events shortly after the rupture depending on the rheology of the upper mantle and the magnitude of the event this study opens the door to greater insight into the variability of earthquake cycles by incorporating the dynamics of distributed deformation

under the hood of the earthquake machine toward predictive modeling of the seismic cycle advances in observational laboratory and modeling techniques open the way to the development of physical models of the seismic cycle with potentially predictive power to explore that possibility we developed an integrative and fully dynamic model of the parkfield segment of the san andreas fault the model succeeds in reproducing a realistic earthquake sequence of irregular moment magnitude m w 6 0 main shocks including events similar to the ones in 1966 and 2004 and provides an excellent match for the detailed interseismic coseismic and postseismic observations collected along this fault during the most recent earthquake cycle such calibrated physical models provide new ways to assess seismic hazards and forecast seismicity response to perturbations of natural or anthropogenic origins

a novel method for predicting fracture in floating ice basal crevasses may play an important precursory role in determining both the location and propagation of rifts and iceberg dimensions for example icebergs calved recently from thwaites glacier antarctica have the same width as surface undulations strengthening the connection between basal crevasses rifting and calving we explore a novel method for estimating the heights of basal crevasses formed at the grounding lines of ice shelves and ice streams we employ a thin elastic beam teb formulation and tensional yielding criterion to capture the physics of flexed ice at grounding lines observations of basal crevasse heights compare well with model predictions in the siple coast region of the ross ice shelf we find that the teb method is most accurate in areas of low strain rate we also test the method in other areas of antarctica to produce order of magnitude maps of grounding line basal crevasses and find general agreement with reported observations assuming basal crevasses develop in spatio temporal sequence and are advected downstream this method is computationally cheap and could be relatively easy to implement into damage oriented large scale ice models which aim at physically simulating calving and fracture processes

spectral element simulations of long term fault slip effect of low rigidity layers on earthquake cycle dynamics we develop a spectral element method for the simulation of long term histories of spontaneous seismic and aseismic slip on faults subjected to tectonic loading our approach reproduces all stages of earthquake cycles nucleation and propagation of earthquake rupture postseismic slip and interseismic creep we apply the developed methodology to study the effects of low rigidity layers on the dynamics of the earthquake cycle in 2 d we consider two cases small m 1 earthquakes on a fault surrounded by a damaged fault zone and large m 7 earthquakes on a vertical strike slip fault that cuts through shallow low rigidity layers our results indicate how the source properties of repeating earthquakes are affected by the presence of a damaged fault zone with low rigidity compared to faults in homogeneous media we find 1 reduction in the earthquake nucleation size 2 amplification of slip rates during dynamic rupture propagation 3 larger recurrence interval and 4 smaller amount of aseismic slip based on linear stability analysis we derive a theoretical estimate of the nucleation size as a function of the width and rigidity reduction of the fault zone layer which is in good agreement with simulated nucleation sizes we further examine the effects of vertically stratified layers e g sedimentary basins on the nature of shallow coseismic slip deficit our results suggest that low rigidity shallow layers alone do not lead to coseismic slip deficit while the low rigidity layers result in lower interseismic stress accumulation they also cause dynamic amplification of slip rates with the net effect on slip being nearly zero

potential antarctic ice sheet retreat driven by hydrofracturing and ice cliff failure abstract geological data indicate that global mean sea level has fluctuated on 103 to 106 yr time scales during the last 25 million years at times reaching 20 m or more above modern if correct this implies substantial variations in the size of the east antarctic ice sheet eais however most climate and ice sheet models have not been able to simulate significant eais retreat from continental size given that atmospheric co2 levels were relatively low throughout this period here we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model data conflict in response to atmospheric and ocean temperatures typical of past warm periods floating ice shelves may be drastically reduced or removed completely by increased oceanic melting and by hydrofracturing due to surface melt draining into crevasses ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing and may fail structurally if stresses exceed the ice yield strength producing rapid retreat incorporating these mechanisms in our ice sheet model accelerates the expected collapse of the west antarctic ice sheet to decadal time scales and also causes retreat into major east antarctic subglacial basins producing 17 m global sea level rise within a few thousand years the mechanisms are highly parameterized and should be tested by further process studies but if accurate they offer one explanation for past sea level high stands and suggest that antarctica may be more vulnerable to warm climates than in most previous studies

propagation of long fractures in the ronne ice shelf antarctica investigated using a numerical model of fracture propagation long rifts near the front of the ronne ice shelf antarctica are observed to begin as fractures along the lateral boundaries of outlet streams feeding the shelf these flaws eventually become the planes along which tabular icebergs calve the fractures propagate laterally as they advect through the shelf with orientations that can be explained by the glaciological stress field fracture length remains constrained over much of the advective path and locations of crack tip arrest are observed to coincide with structural boundaries such as suture zones between ice from adjacent outlet glaciers geomechanical principles and numerical models demonstrate that in the absence of these suture zones crack tips are unlikely to arrest in these locations we conclude that lateral inhomogeneity in the ice plays an important role in fracture mechanics through most of the ice shelf only near the shelf front are these local structural effects overcome such that the large rifts required for tabular iceberg production develop

continent wide estimates of antarctic strain rates from landsat 8 derived velocity grids strain rates are fundamental measures of ice flow and are used in a wide variety of glaciological applications including investigations of bed properties calculations of basal mass balance on ice shelves and constraints on ice rheological models however despite their extensive application strain rates are calculated using a variety of methods and length scales and the details are often not specified in this study we compare the results of nominal and logarithmic strain rate calculations based on a satellite derived velocity field of the antarctic ice sheet generated from landsat 8 satellite data our comparison highlights the differences between the two common approaches in the glaciological literature we evaluate the errors introduced by each approach and their impacts on the results we also demonstrate the importance of choosing and specifying a length scale over which strain rate calculations are made which can strongly influence other derived quantities such as basal mass balance on ice shelves finally we present strain rate data products calculated using an approximate viscous length scale with satellite observations of ice velocity for the antarctic continent

towards inferring earthquake patterns from geodetic observations of interseismic coupling ultimately seismotectonic studies seek to provide ways of assessing the timing magnitude and spatial extent of future earthquakes ample observations document the spatial variability in interseismic coupling defined as a degree of locking of a fault during the period of stress build up between seismic events fully or nearly locked fault patches are often surrounded by aseismically creeping areas however it is unclear how these observations could help assess future earthquakes here we simulate spontaneous seismic and aseismic fault slip with a fully dynamic numerical model our simulations establish the dependence of earthquake rupture patterns and interseismic coupling on spatial variations of fault friction we consider the long term evolution of slip on a model fault where two seismogenic locked segments are separated by an aseismically slipping patch where rupture is impeded we find that the probability for a large earthquake to break through the rupture impeding patch is correlated with the interseismic coupling averaged over this patch in addition the probability that an earthquake breaks through the rupture impeding patch and interseismic coupling are both related to fault friction properties through a single non dimensional parameter our study opens the possibility of predicting seismic rupture patterns that a fault system can produce on the basis of observations of its interseismic coupling and suggests that regions of low interseismic coupling may reveal permanent barriers to large earthquakes

seasonal and interannual variabilities in terminus position glacier velocity and surface elevation at helheim and kangerlussuaq glaciers from 2008 to 2016 the dynamic response of greenland tidewater glaciers to oceanic and atmospheric change has varied both spatially and temporally while some of this variability is likely related to regional climate signals glacier geometry also appears to be important in this study we investigated the environmental and geometric controls on the seasonal and interannual evolution of helheim and kangerlussuaq glaciers southeast greenland from 2008 to 2016 by combining year round satellite measurements of terminus position glacier velocity and surface elevation while helheim remained relatively stable with a lightly grounded terminus over this time period kangerlussuaq continued to lose mass as its grounding line retreated into deeper water by summer 2011 kangerlussuaq s grounding line had retreated into shallower water and the glacier had a 5 km long floating ice tongue we also observed seasonal variations in surface velocity and elevation at both glaciers at helheim seasonal speedups and dynamic thinning occurred in the late summer when the terminus was most retreated at kangerlussuaq we observed summer speedups due to surface melt induced basal lubrication and winter speedups due to ice shelf retreat we suggest that helheim and kangerlussuaq behaved differently on a seasonal timescale due to differences in the spatial extent of floating ice near their termini which affected iceberg calving behavior given that seasonal speedups and dynamic thinning can alter this spatial extent these variations may be important for understanding the long term evolution of these and other greenland tidewater glaciers

sea ice free conditions during the sturtian glaciation early cryogenian south australia in the neoproterozoic snowball earth hypothesis shutdown of the planet9s hydrological system has been attributed to a global ice cover during one or more extreme glaciations in the central flinders ranges south australia the yudnamutana subgroup of sturtian age includes diamictite sandstone and siltstone units of glaciomarine origin as much as 5000 m thick and is overlain by postglacial transgressive siltstone and shale of the tindelpina shale member tapley hill formation in the central flinders ranges the yudnamutana subgroup consists of 1 the pualco tillite gravity resedimented glacial deposits 2 the holowilena ironstone glacioturbidites 3 poorly stratified pebbly diamictite of the warcowie dolomite member lowermost wilyerpa formation gravity resedimented glacial deposits succeeded by 4 siltstones and sandstones with abundant hummocky cross stratification hcs storm deposits and 5 a lonestone bearing succession with cobble sized clasts in the upper wilyerpa formation ice rafted debris interpreted to record a glacial re advance in which hcs is absent because the action of oscillating waves is required to produce hcs on the seafloor its presence indicates an interval of significant meltback prior to glacial re advance given that the hcs occurs 2 km beneath the tindelpina shale member it signifies a major ice free interval during the sturtian glaciation

a unifying framework for iceberg calving models we propose a general framework for iceberg calving models that can be applied to any calving margin the framework is based on mass continuity the assumption that calving rate and terminus velocity are not independent and the simple idea that terminus thickness following a calving event is larger than terminus thickness at the event onset the theoretical near steady state analysis used to support and analyze the framework indicates that calving rate is governed to first order by ice thickness thickness gradient strain rate mass balance rate and backwards melting of the terminus the analysis furthermore provides a physical explanation for a previously derived empirical relationship for ice shelf calving alley and others 2008 in the calving framework the pre and post calving terminus thicknesses are given by two unknown but related functions the functions can vary independently of changes in glacier flow and geometry and can therefore account for variations in calving behavior due to external forcings and or self sustaining calving processes positive feedbacks although the calving framework does not constitute a complete calving model any thickness based calving criterion can easily be incorporated into the framework the framework should be viewed as a guide for future attempts to parameterize calving

the effect of buttressing on grounding line dynamics determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice sheet models here we investigate the role of lateral shear and ice shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets we derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non local ice shelf properties application of these results to a laterally confined version of the mismip 1a experiment shows that the boundary condition at the ice shelf front i e the calving law is a major control on the location and stability of the grounding line in the presence of buttressing allowing for both stable and unstable grounding line positions on downwards sloping beds these results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice shelf properties in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed consequently the marine ice sheet instability hypothesis may not apply to buttressed marine ice sheets

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

prevalence of viscoelastic relaxation after the 2011 tohoku oki earthquake seafloor global positioning system observations immediately after the great 2011 tohoku oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short term postseismic deformation rather than just afterslip on the fault as is commonly assumed this paper presents revised estimates of the afterslip following the 2011 tohoku oki earthquake based on seafloor gps observations made immediately after the earthquake tianhaozhe sun et al obtain unambiguous evidence for the dominant role of viscoelastic relaxation in short term postseismic deformation rather than elastic afterslip on the fault as is commonly assumed they conclude that models which erroneously assume an elastic earth substantially overestimate afterslip downdip of the rupture zone and underestimate afterslip updip after a large subduction earthquake crustal deformation continues to occur with a complex pattern of evolution1 this postseismic deformation is due primarily to viscoelastic relaxation of stresses induced by the earthquake rupture and continuing slip afterslip or relocking of different parts of the fault2 3 4 5 6 when postseismic geodetic observations are used to study earth s rheology and fault behaviour it is commonly assumed that short term a few years deformation near the rupture zone is caused mainly by afterslip and that viscoelasticity is important only for longer term deformation6 7 however it is difficult to test the validity of this assumption against conventional geodetic data here we show that new seafloor gps global positioning system observations immediately after the great tohoku oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short term postseismic deformation these data reveal fast landward motion of the trench area opposing the seaward motion of gps sites on land using numerical models of transient viscoelastic mantle rheology we demonstrate that the landward motion is a consequence of relaxation of stresses induced by the asymmetric rupture of the thrust earthquake a process previously unknown because of the lack of near field observations our findings indicate that previous models assuming an elastic earth will have substantially overestimated afterslip downdip of the rupture zone and underestimated afterslip updip of the rupture zone our knowledge of fault friction based on these estimates therefore needs to be revised

a numerical simulation of earthquake cycles along the nankai trough in southwest japan lateral variation in frictional property due to the slab geometry controls the nucleation position abstract the occurrence pattern of historical great earthquakes along the nankai trough indicates the existence of rupture segments that seem to correlate with 100 km scale lateral variation in the dip angle of the subducting philippine sea plate for instance the latest earthquakes the 1944 tonankai and the 1946 nankai earthquakes initiated their ruptures off kii peninsula where the dip angle of the slab is steeper than in adjacent areas to investigate how such heterogeneity in slab geometry affects the occurrence of earthquakes we conducted a three dimensional 3d numerical simulation of earthquake cycles for a large source area about 700 km long along the trough and 300 km wide in the dip direction based on a rate and state dependent friction law we used a simple flat dipping plane to model the plate boundary frictional parameters and effective normal stress were assumed to depend on the estimated depth of the real plate boundary based on seismic surveys and seismicity data the depth dependent frictional parameters were mapped on the flat model plane this made lateral variation in frictional property due to the variation in the dip angle of the slab the plate convergence rate in the model decreased eastward along the trough consistent with a recent global positioning system gps analysis in the simulation results preslip occurred and rupture started off kii peninsula both effects of a narrower locked area due to the higher dip angle and a higher plate convergence rate caused the highest stress accumulation rate to be in this area promoting slip nucleation our simple simulation suggests that lateral variation in the dip angle of the slab is one of the factors controlling the nucleation position of great interplate earthquakes

effect of a compliant fault zone on the inferred earthquake slip distribution 1 we present a new semi analytic method to evaluate the deformation due to a screw dislocation in arbitrarily heterogeneous and or anisotropic elastic half plane the method employs integral transformations to reduce the governing partial differential equations to the integral fredholm equation of the second kind dislocation sources as well as spatial perturbations in the elastic properties are modeled using equivalent body forces the solution to the fredholm equation is obtained in the fourier domain using a method of successive over relaxation and is mapped into the spatial domain using the inverse fast fourier transform we apply this method to investigate the effect of a soft damage zone around an earthquake fault on the co seismic displacement field and on the earthquake slip distribution inferred from inversions of geodetic data in the presence of a kilometer wide damage zone with a reduction of the effective shear modulus of a factor of 2 inversions that assume a laterally homogeneous model tend to underestimate the amount of slip in the middle of the seismogenic layer by as much as 20 this bias may accentuate the inferred maxima in the seismic moment release at depth between 3 6 km suggested by previous studies of large strike slip earthquakes

an observationally validated theory of viscous flow dynamics at the ice shelf calving front an analytical theory is developed for ice flow velocity in a boundary layer couplet at the calving front the theory has simple quantitative characteristics that relate ice front velocity to thickness strain rate and shelf width matching one set of empirically derived relationships alley and others 2008 and implying that these relationships predict ice velocity rather than calving rate the two boundary layers are where longitudinal and transverse flow fields change from the interior flow to patterns consistent with the calving front stress condition numerical simulations confirm the analytical theory the quantitative predictions of the theory have low sensitivity to unmeasured parameters and to shelf plan aspect ratio while its robustness arises from its dependence on the scale invariance of the governing equations the theory provides insights into calving the stability of ice shelf calving fronts the stability of the grounding line of laterally resisted ice streams and also suggests that the calving front is an instructive dynamical analogue to the grounding line

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

earthquake nucleation on rate and state faults aging and slip laws we compare 2 d quasi static earthquake nucleation on rate and state faults under both aging and slip versions of the state evolution law for both versions mature nucleation zones exhibit 2 primary regimes of growth well above and slightly above steady state corresponding respectively to larger and smaller fault weakening rates well above steady state aging law nucleation takes the form of accelerating slip on a patch of fixed length this length is proportional to b 1 and independent of a where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates the nucleation zone is guaranteed to remain well above steady state only for values of a b that are low by laboratory standards near steady state for both laws the nucleation zone expands the propagating front remains well above steady state giving rise to a simple expression for its effective fracture energy g c this fracture energy controls the propagation style for the aging law g c increases approximately as the square of the logarithm of the velocity jump this causes the nucleation zone to undergo quasi static crack like expansion to a size asymptotically proportional to b b a 2 for the slip law g c increases only as the logarithm of the velocity jump and crack like expansion is not an option instead the nucleation zone grows as an accelerating unidirectional slip pulse under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly b divided by the logarithm of the velocity jump where is the effective elastic shear modulus for this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 mpa

impact of tide topography interactions on basal melting of larsen c ice shelf antarctica 1 basal melting of ice shelves around antarctica contributes to formation of antarctic bottom water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers tides influence ice shelf basal melt rate wb by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf we use a three dimensional ocean model thermodynamically coupled to a nonevolving ice shelf to investigate the relationship between topography tides andwb for larsen c ice shelf lcis in the northwestern weddell sea antarctica using our best estimates of ice shelf thickness and seabed topography we find that the largest modeled lcis melt rates occur in the northeast where our model predicts strong diurnal tidal currents 0 4 m s 1 this distribution is significantly different from models with no tidal forcing which predict largest melt rates along the deep grounding lines we compare several model runs to explore melt rate sensitivity to geometry initial ocean potential temperature 0 thermodynamic parameterizations of heat and freshwater ice ocean exchange and tidal forcing the resulting range of lcis averagedwb is 0 11 0 44 m a 1 the spatial distribution of wb is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of wb is influenced by 0 these sensitivities in wbpredictions reinforce a need for high resolution maps of ice draft and sub ice shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models

the mechanisms behind jakobshavn isbrae s acceleration and mass loss a 3 d thermomechanical model study the mechanisms causing widespread flow acceleration of jakobshavn isbrae west greenland remain unclear despite an abundance of observations and modeling studies here we simulate the glacier s evolution from 1985 to 2016 using a three dimensional thermomechanical ice flow model the model captures the timing and 90 of the observed changes by forcing the calving front basal drag in the trough is low and lateral drag balances the ice stream s driving stress the calving front position is the dominant control on changes of jakobshavn isbrae since the ice viscosity in the shear margins instantaneously drops in response to the stress perturbation caused by calving front retreat which allows for widespread flow acceleration gradual shear margin warming contributes 5 to 10 to the total acceleration our simulations suggest that the glacier will contribute to eustatic sea level rise at a rate comparable to or higher than at present

ice velocity mapping of ross ice shelf antarctica by matching surface undulations measured by icesat laser altimetry abstract we present a novel method for estimating the surface horizontal velocity on ice shelves using laser altimetry data from the ice cloud and land elevation satellite icesat 2003 2009 the method matches undulations measured at crossover points between successive campaigns elevation measurements are first relocated into a time varying moving coordinate system using an initial velocity e g from velmap and then crossover height differences are minimized with an adjustment vector errors in geolocation of the icesat tracks result in some error in the adjustment vectors but these are small relative to the velocity adjustment for fast moving ice shelves we use the algorithm to estimate changes in the ice velocity of ross ice shelf between an earlier mapping from velmap and the icesat period the new velocity field is compared with velocities from in situ measurements and satellite radar interferometry the slowdown of 98 34 m yr 1 23 is observed in the ice shelf downstream of whillans ice stream and the deceleration rate is 3 1 1 1 m yr 2 during last three decades the method can be expanded to the simultaneous mapping of ice horizontal velocity ice thickness change and surface deformation for antarctic ice shelves as well as a more accurate mapping using future icesat 2 measurements

postseismic deformation following the 2010 m 7 2 el mayor cucapah earthquake observations kinematic inversions and dynamic models due to its location on a transtensional section of the pacific north american plate boundary the salton trough is a region featuring large strike slip earthquakes within a regime of shallow asthenosphere high heat flow and complex faulting and so postseismic deformation there may feature enhanced viscoelastic relaxation and afterslip that is particularly detectable at the surface the 2010 m 7 2 el mayor cucapah earthquake was the largest shock in the salton trough since 1892 and occurred close to the us mexico border and so the postseismic deformation recorded by the continuous gps network of southern california provides an opportunity to study the rheology of this region three year postseismic transients extracted from gps displacement time series show four key features 1 1 2 cm of cumulative uplift in the imperial valley and sim 1 cm of subsidence in the peninsular ranges 2 relatively large cumulative horizontal displacements 150 km from the rupture in the peninsular ranges 3 rapidly decaying horizontal displacement rates in the first few months after the earthquake in the imperial valley and 4 sustained horizontal velocities following the rapid early motions that were still visibly ongoing 3 years after the earthquake kinematic inversions show that the cumulative 3 year postseismic displacement field can be well fit by afterslip on and below the coseismic rupture though these solutions require afterslip with a total moment equivalent to at least a m 7 2 earthquake and higher slip magnitudes than those predicted by coseismic stress changes forward modeling shows that stress driven afterslip and viscoelastic relaxation in various configurations within the lithosphere can reproduce the early and later horizontal velocities in the imperial valley while newtonian viscoelastic relaxation in the asthenosphere can reproduce the uplift in the imperial valley and the subsidence and large westward displacements in the peninsular ranges we present two forward models of dynamically coupled deformation mechanisms that fit the postseismic transient well a model combining afterslip in the lower crust newtonian viscoelastic relaxation in a localized zone in the lower crust beneath areas of high heat flow and geothermal activity and newtonian viscoelastic relaxation in the asthenosphere and a second model that replaces the afterslip in the first model with viscoelastic relaxation with a stress dependent viscosity in the mantle the rheology of this high heat flow high strain rate region may incorporate elements of both these models and may well be more complex than either of them

what caused the low water phase of glacial lake agassiz abstract first order modeling suggests that a low water phase in late glacial lake agassiz can be explained through changes in the balance between evaporation precipitation and runoff rather than drainage the low water moorhead phase is often attributed to drainage through outlets opened by isostatic depression and retreat of the laurentide ice margin however new data indicate that the proposed outlets were ice covered during the moorhead phase instead the lake water levels dropped to the moorhead phase before the start of the younger dryas chronozone and remained there until 11 3 ka thus drainage seems to be an implausible explanation for younger dryas aged low water levels in lake agassiz an alternative explanation is that evaporation equaled or exceeded water inputs from the adjacent ice margin and the deglaciated parts of the drainage basin to evaluate whether this hypothesis is plausible we constructed a simple model that considers the paleo basin geometry hydrology and meltwater production from the adjacent ice margin modest hydrologic changes within the range of present day variability coupled with low meltwater production produce a closed basin shifts in the location of the polar jet driven by increased arctic albedo may explain our inferred hydrologic changes

a numerical study of open water formation in sea ice 1 a multicategory sea ice model that explicitly takes into account redistribution of undeformed ice into categories of ridged and rafted ice is introduced with the intention of investigating the simulation of the open water fraction within sea ice the performance of this model is compared with that of a reduced model that accounts for only two categories the latter type of sea ice model being most widely used in global climate models two idealized scenarios are considered polynya formation along a coastline under offshore winds and a low pressure system passing over an ice field furthermore the two models are compared under more real world forcing conditions as they exist in the weddell sea in simplified situations both models produce identical results upon introduction of more realistic ambient conditions however the multicategory model produces larger open water fractions than the two level model concomitantly and consistently with earlier findings the multicategory model produces thicker ice this is a result of the larger horizontal gradients of ice motion which in turn are due to the weaker ice strength that arises from a higher abundance of thinner ice this leads to more open water as well as increased dynamical growth of ice the open water fraction is of relevance in global ocean circulation models where the amount of open water influences the deep and bottom water formation rates such as in conjunction with antarctic coastal polynyas

implications of deformation following the 2002 denali alaska earthquake for postseismic relaxation processes and lithospheric rheology 1 during the first 2 years following the 2002 mw 7 9 denali alaska strike slip earthquake a large array of global positioning system gps receivers recorded rapid postseismic surface motions extending at least 300 km from the rupture and at rates of more than 100 mm yr in the near field here we use three dimensional 3 d viscoelastic finite element models to infer the mechanisms responsible for these postseismic observations we consider afterslip both from an inversion of gps displacements and from stress driven forward models poroelastic rebound and viscoelastic flow in the lower crust and upper mantle several conclusions can be drawn 1 no single mechanism can explain the postseismic observations 2 significant postseismic flow below a depth of 60 km is required to explain observed far field motions best explained by a weak upper mantle with a depth dependent effective viscosity that ranges from 1019 pa s at the moho 50 km depth to 3 4 1018 pa s at 100 km depth 3 shallow afterslip within the upper crust occurs adjacent to and beneath the regions of largest coseismic slip 4 there is a contribution from deformation in the middle and lower crust from either lower crustal flow or stress driven slip afterslip is preferred over broad viscoelastic flow owing to the existence of seismic velocity discontinuities across the fault at depth though our modeling does not favor either mechanism if the process is viscoelastic relaxation the viscosity is a factor of 3 greater than the inferred mantle viscosity 5 poroelastic rebound probably contributed to the observed postseismic deformation in the immediate vicinity of the denali totschunda junction these conclusions lead us to infer an alaskan mechanical lithosphere that is about 60 km thick overlying a weak asthenosphere and a denali fault that cuts through the entire lithosphere with shear accommodated by faulting in the top 20 km and time dependent aseismic shear below

frictional properties of olivine at high temperature with applications to the strength and dynamics of the oceanic lithosphere 1 faulting and brittle deformation of mantle rocks occurs in many tectonic settings such as oceanic transform faults oceanic detachment faults subduction zones and continental rifts however few data exist that directly explore the frictional properties of peridotite rocks improved constraints on the brittle deformation of peridotite is important for a more complete understanding of the rheological properties of the lithosphere furthermore our comparatively detailed understanding of plastic deformation in olivine allows us to explore the possible role of thermally activated intracrystalline deformation mechanisms in macroscopically brittle processes it has been hypothesized and some experimental data indicate that plastic yielding by dislocation glide low temperature plasticity determines the direct effect in the rate and state frictional constitutive formulation plastic flow may also have important implications for the blunting or necking at asperity contacts that influences the time and or displacement dependent friction evolution effect and frictional healing we present results from saw cut experiments on fine grained synthetic olivine fault gouge conducted in a gas medium deformation apparatus in the temperature range of 400 1000 c with 100 mpa confining pressure we conducted velocity stepping tests to explore the rate and temperature dependence of sliding stability we also conducted slide hold slide experiments to investigate the time and temperature dependence of fault zone restrengthening frictional healing the mechanical data and microstructural observations allow us to explore the role of thermally activated processes in frictional sliding the data indicate systematic temperature dependenceof rate and state variables that can be attributed to plastic yielding at grain to grain contacts we explore the implications of such temperature dependent behavior for controlling the base of the seismogenic zone in the oceanic lithosphere and we seek insight into possible mechanistic models for the interactions between fracture and flow that could lead to improved constraints on the strength of the lithosphere

a sensitivity study of annual area change for greenland ice sheet marine terminating outlet glaciers 1999 2013 the mass flux from marine terminating glacier outlets from the greenland ice sheet is a significant mass budget term yet glacier calving sensitivity to climate parameters is not well understood in this study the front area changes of 42 marine terminating greenland outlet glaciers were measured annually from 1999 to 2013 using optical satellite imagery thirty five glaciers retreated while seven remained stable the average front position change was 240 m a 1 and the total rate of front area change was 114 km 2 a 1 the yearly average area changes of all measured glaciers are compared with interannual variations in sea surface temperature sst sea ice concentration sic surface melt northern hemisphere nh surface air temperatures and the north atlantic oscillation nao index we find 1 northwestern glaciers area change have a significant correlation with surface melt and nh land temperature variations 2 southeastern glaciers have a significant correlation with local sst sic and nh temperatures and 3 southwestern glaciers area change correlate significantly with local sst and nao index we conclude that a climate sensitivity signal emerges considering a population of glaciers further we find a significant correlation between the date of minimum extent and glacier latitude area change data are available from http gac geus dk

an efficient numerical method for earthquake cycles in heterogeneous media alternating subbasin and surface rupturing events on faults crossing a sedimentary basin we present an efficient numerical method for earthquake cycle simulations that employs a finite difference discretization of the off fault material to accommodate spatially variable elastic properties the method is developed for the two dimensional antiplane shear problem of a vertical strike slip fault with rate and state friction we compare earthquake cycles in a homogeneous half space with those in which the upper portion of the fault cuts through a sedimentary basin in both cases we assume velocity weakening behavior over the full seismogenic depth even in the basin to isolate the influence of elastic heterogeneity in a homogeneous half space events rupturing the full seismogenic depth occur periodically event sequences are more complex in basin models with one or several subbasin events confined to the lower section of the fault followed by a much larger surface rupturing event that breaks through the basin this phenomenology emerges only for sufficiently compliant and deep basins predicted surface velocities are essentially identical before subbasin events and surface rupturing events suggesting that geodetic observations would not be useful in predicting the rupture mode the alternating sequence of subbasin and surface rupturing events would complicate interpretation of paleoseismic data our results also offer one potential explanation for the shallow slip deficit observed in many recent earthquakes namely that these events which lack appreciable surface slip are simply one style of rupture subsequent events on these faults might be larger with slip extending all the way to the surface the 1940 mw 7 0 and 1979 mw 6 5 imperial valley events might be considered as examples of these two rupture styles

widespread rifting and retreat of ice shelf margins in the eastern amundsen sea embayment between 1972 and 2011 the major outlet glaciers that drain the eastern sector of the amundsen sea embayment smith haynes thwaites and pine island are among the largest fastest flowing and fastest thinning glaciers in west antarctica their recent ice flow acceleration is linked to ocean induced ice shelf thinning but may also arise from additional losses of ice shelf buttressing that are not well understood here we present a comprehensive history of coastal change in the eastern amundsen sea embayment between 1972 and 2011 derived mostly from landsat imagery the termini of all four major outlet glaciers have retreated but retreat is most rapid along the ice shelf margins where progressive rifting has occurred this pattern of retreat coincides with the recent acceleration of grounded ice and contributed to loss of ice shelf buttressing the observed pattern of margin led gradual ice shelf disintegration appears to be common in accelerating ocean terminating outlet glaciers we hypothesize that this pattern is part of a positive feedback between glacier acceleration and rift growth that could drive further buttressing loss in the eastern amundsen sea embayment

a simple law for ice shelf calving a major problem for ice sheet models is that no physically based law for the calving process has been established comparison across a diverse set of ice shelves demonstrates that iceberg calving increases with the along flow spreading rate of a shelf this relation suggests that frictional buttressing loss which increases spreading also leads to shelf retreat a process known to accelerate ice sheet flow and contribute to sea level rise

links between ocean temperature and iceberg discharge during heinrich events the last glacial period was punctuated by several periods of massive iceberg discharge from the northern hemisphere ice sheets numerical simulations indicate that these discharge events are linked to an interplay between ice sheet accumulation marine ice shelf stability and periodically oscillating surface ocean temperature

evolution of basal crevasses links ice shelf stability to ocean forcing abstract iceberg calving is one of the primary mechanisms responsible for transferring ice from the antarctic ice shelves to the ocean but remains poorly understood previous theories of calving have sought to explain the calving process as a brittle phenomenon that occurs rapidly when surface or basal crevasses penetrate the entire ice thickness here we show that the strain rate weakening nature of ice permits initially narrow basal crevasses to seed an instability that gives rise to locally enhanced ductile deformation and thinning over length scales that are large compared to the ice thickness this ductile failure process called necking amplifies long wavelength features of bottom topography and allows basal crevasses to penetrate an increasing fraction of the ice thickness as they advect downstream application of the model to the four largest antarctic ice shelves shows that necking occurs downstream of pinning points and sharp protrusions in the ice shelf embayment where stress is highly concentrated however model predictions are sensitive to assumptions about basal melting and refreezing within crevasses suggesting that the combination of mechanical instability and ice ocean interaction on the scale of an individual crevasse may play a leading role in controlling ice shelf stability

a three dimensional calving model numerical experiments on johnsons glacier livingston island antarctica calving from tidewater glaciers and ice shelves accounts for around half the mass loss from both polar ice sheets yet the process is not well represented in prognostic models of ice dynamics benn and others proposed a calving criterion appropriate for both grounded and floating glacier tongues or ice shelves based on the penetration depth of transverse crevasses near the calving front computed using nye s formula the criterion is readily incorporated into glacier and ice sheet models but has not been fully validated with observations we apply a three dimensional extension of benn and others criterion incorporated into a full stokes model of glacier dynamics to estimate the current position of the calving front of johnsons glacier antarctica we find that two improvements to the original model are necessary to accurately reproduce the observed calving front 1 computation of the tensile deviatoric stress opening the crevasse using the full stress solution and 2 consideration of such a tensile stress as a function of depth our modelling results also suggest that johnsons glacier has a polythermal structure rather than the temperate structure suggested by earlier studies

on the coupled response to ice shelf basal melting ice shelf basal melting is tightly coupled to ice shelf morphology ice shelves in turn are coupled to grounded ice via their influence on compressive stress at the grounding line ice shelf buttressing here we examine this interaction using a local parameterization that relates the basal melt rate to the ice shelf thickness gradient this formulation permits a closed form solution for a steady state ice tongue time dependent numerical simulations reveal the spatial and temporal evolution of ice shelf ice stream systems in response to changes in ocean temperature and the influence of morphology dependent melting on grounding line retreat we find that a rapid 1 year re equilibration in upstream regions of ice shelves establishes a spatial pattern of basal melt rates relative to the grounding line that persists over centuries coupling melting to ice shelf shape generally but not always increases grounding line retreat rates relative to a uniform distribution with the same area average melt rate because upstream ice shelf thickness gradients and retreat rates increase nonlinearly with thermal forcing morphology dependent melting is more important to the response of weakly buttressed strongly forced ice streams grounded on beds that slope upwards towards the ocean e g those in the amundsen sea

bounds on the calving cliff height of marine terminating glaciers increased calving and rapid retreat of glaciers can contribute significantly to sea level rise but the processes controlling glacier retreat remain poorly understood we seek to improve our understanding of calving by investigating the stress field controlling tensile and shear failure using a 2d full stokes finite element model using idealized rectangular geometries we find that when rapidly sliding glaciers thin to near buoyancy full thickness tensile failure occurs similar to observations motivating height above buoyancy calving laws in contrast when glaciers are frozen to their beds basal crevasse penetration is suppressed and calving is minimal we also find shear stresses are largest when glaciers are thickest together the tensile and shear failure criteria map out a stable envelope in an ice thickness water depth diagram the upper and lower bounds on cliff height can be incorporated into numerical ice sheet models as boundary conditions thus bracketing the magnitude of calving rates in marine terminating glaciers

collapse of marine based outlet glaciers from the scandinavian ice sheet abstract we present a reconstruction of the timing and retreat rates of more than 2000 m thick younger dryas yd fjord glaciers in western norway using a detailed chronology of 10 be exposure ages from lateral moraines and 14 c dated end moraines a primary conclusion is that ice margins retreated up the 120 170 km long fjords at mean rates of 240 340 m yr 1 during the early holocene we further show that part of the south western sector of the scandinavian ice sheet collapsed in two distinct steps the first step occurred between 19 5 and 18 5 ka bp as break up of the norwegian channel ice stream which drained the ice sheet during the last glacial maximum lgm the second step was the rapid retreat up the fjords mentioned above dated to 11 6 11 1 ka bp during the intervening 7000 years no net retreat occurred despite oscillations of the ice margin this stepwise ice margin retreat strongly contrasts with the more monotonic decay of the ice sheet as a whole indicating that water depths set the pace for climate triggered ice margin retreat in this part of the ice sheet calving and melting of marine margins has dominated mass loss from modern ice sheets in recent decades however the mechanisms and long term 100 1000 yr rate of ice front retreat is less certain and empirical examples such as those given here may help in developing better numerical models

postseismic deformation following the 1999 chi chi earthquake taiwan implication for lower crust rheology on 1999 september 21 the mw 7 6 chi chi earthquake ruptured a segment of the chelungpu fault a frontal thrust fault of the western foothills of taiwan the stress perturbation induced by the rupture triggered a transient deformation across the island which was well recorded by a wide network of continuously operating gps stations the analysis of more than ten years of these data reveals a heterogeneous pattern of postseismic displacements with relaxation times varying by a factor of more than ten and large cumulative displacements at great distances in particular along the longitudinal valley in eastern taiwan where relaxation times are also longer we show that while afterslip is the dominant relaxation process in the epicentral area viscoelastic relaxation is needed to explain the pattern and time evolution of displacements at the larger scale we model the spatiotemporal behavior of the transient deformation as the result of afterslip on the decollement that extends downdip of the chelungpu thrust and viscoelastic flow in the lower crust and in the mid crust below the central range we construct a model of deformation driven by coseismic stress change where afterslip and viscoelastic flow are fully coupled the model is compatible with the shorter relaxation times observed in the near field which are due to continued fault slip and the longer characteristic relaxation times and the reversed polarity of vertical displacements observed east of the central range our preferred model shows a viscosity of 0 5 1 x 10 19 pa s at lower crustal depths and 5 x 10 17 pa s in the mid crust below the central range between 10 and 30 km depth the low viscosity zone at mid crustal depth below the central range coincides with a region of low seismicity where rapid advection of heat due to surface erosion coupled with underplating maintain high temperatures estimated to be between 300 c and 600 c from the modeling of thermo chronology and surface heat flow data

verification of model simulated mass balance flow fields and tabular calving events of the antarctic ice sheet against remotely sensed observations the antarctic ice sheet ais has the greatest potential for global sea level rise this study simulates ais ice creeping sliding tabular calving and estimates the total mass balances using a recently developed advanced ice dynamics model known as segment ice segment ice is written in a spherical earth coordinate system because the ais contains the south pole a projection transfer is performed to displace the pole outside of the simulation domain the ais also has complex ice water granular material bedrock configurations requiring sophisticated lateral and basal boundary conditions because of the prevalence of ice shelves a girder yield type calving scheme is activated the simulations of present surface ice flow velocities compare favorably with insar measurements for various ice water bedrock configurations the estimated ice mass loss rate during 2003 2009 agrees with grace measurements and provides more spatial details not represented by the latter the model estimated calving frequencies of the peripheral ice shelves from 1996 roughly when the 5 km digital elevation and thickness data for the shelves were collected to 2009 compare well with archived scatterometer images segment ice s unique non local systematic calving scheme is found to be relevant for tabular calving however the exact timing of calving and of iceberg sizes cannot be simulated accurately at present a projection of the future mass change of the ais is made with segment ice forced by atmospheric conditions from three different coupled general circulation models the entire ais is estimated to be losing mass steadily at a rate of 120 km3 a at present and this rate possibly may double by year 2100

modeling hydraulic fracture of glaciers using continuum damage mechanics the presence of water filled crevasses is known to increase the penetration depth of crevasses and this has been hypothesized to play an important role controlling iceberg calving rate here we develop a continuum damage based poro mechanics formulation that enables the simulation of water filled basal and surface crevasse propagation the formulation incorporates a scalar isotropic damage variable into a maxwell type viscoelastic constitutive model for glacial ice and the effect of the water pressure on fracture propagation using the concept of effective solid stress we illustrate the model by simulating quasi static hydrofracture in idealized rectangular slabs of ice in contact with the ocean our results indicate that water filled basal crevasses only propagate when the water pressure is sufficiently large and that the interaction between simultaneously propagating water filled surface and basal crevasses can have a mutually positive influence leading to deeper crevasse propagation which can critically affect glacial stability therefore this study supports the hypothesis that hydraulic fracture is a plausible mechanism for the accelerated breakdown of glaciers

iceberg calving during transition from grounded to floating ice columbia glacier alaska 1 the terminus of columbia glacier alaska unexpectedly became ungrounded in 2007 during its prolonged retreat visual observations showed that calving changed from a steady release of low volume bergs to episodic flow perpendicular rifting propagation and release of very large icebergs a style reminiscent of calving from ice shelves here we compare passive seismic and photographic observations through this transition to examine changes in calving mechanical changes accompany the visible changes in calving style post flotation generation of seismic energy during calving is substantially reduced we propose this is partly due to changes in source processes

the statistical physics of iceberg calving and the emergence of universal calving laws determining a calving law valid for all glaciological and environmental regimes has proven to be a difficult problem in glaciology for this reason most models of the calving process are semi empirical with little connection to the underlying fracture processes in this study i introduce methods rooted in statistical physics to show how calving laws valid for any glaciological domain can emerge naturally as a large spatial scale long temporal scale limit of an underlying continuous or discrete fracture process an important element of the method developed here is that iceberg calving is treated as a stochastic process and that the probability an iceberg will detach in a given interval of time can be described by a probability distribution function using limiting assumptions about the underlying probability distribution the theory is shown to be able to simulate a range of calving styles including the sporadic detachment of large tabular icebergs from ice tongues and ice shelves and the more steady detachment of smaller sized bergs from tidewater outlet glaciers the method developed has the potential to provide a physical basis to include iceberg calving into numerical ice sheet models that can be used to produce more realistic estimates of the glaciological contribution to sea level rise

the gravitationally consistent sea level fingerprint of future terrestrial ice loss 1 we solve the sea level equation to investigate the pattern of the gravitationally self consistent sea level variations fingerprints corresponding to modeled scenarios of future terrestrial ice melt these were obtained from separate ice dynamics and surface mass balance models for the greenland and antarctic ice sheets and by a regionalized mass balance model for glaciers and ice caps for our mid range scenario the ice melt component of total sea level change attains its largest amplitude in the equatorial oceans where we predict a cumulative sea level rise of 25 cm and rates of change close to 3 mm yr from ice melt alone by 2100 according to our modeling in low elevation densely populated coastal zones the gravitationally consistent sea level variations due to continental ice loss will range between 50 and 150 of the global mean this includes the effects of glacial isostatic adjustment which mostly contributes across the lateral forebulge regions in north america while the mid range ocean averaged elastic gravitational sea level variations compare with those associated with thermal expansion and ocean circulation their combination shows a complex regional pattern where the former component dominates in the equatorial pacific ocean and the latter in the arctic ocean

a mass flux perspective of the tidewater glacier cycle i explore the tidewater glacier cycle with a 1 d depth and width integrated flow model that includes a mass flux calving parameterization the parameterization is developed from mass continuity arguments and relates the calving rate to the terminus velocity and the terminus balance velocity the model demonstrates variable sensitivity to climate from an advanced stable configuration a small warming of the climate triggers a rapid retreat that causes large scale drawdown and is enhanced by positive glacier dynamic feedbacks eventually the terminus retreats out of deep water and the terminus velocity decreases resulting in reduced drawdown and the potential for restabilization terminus readvance can be initiated by cooling the climate terminus advance into deep water is difficult to sustain however due to negative feedbacks between glacier dynamics and surface mass balance despite uncertainty in the precise form of the parameterization the model provides a simple explanation of the tidewater glacier cycle and can be used to evaluate the response of tidewater glaciers to climate variability it also highlights the importance of improving parameterizations of calving rates and of incorporating sediment dynamics into tidewater glacier models

surface elevation changes at the front of the ross ice shelf implications for basal melting 1 rapid melting beneath the ross ice shelf ris occurs near the ice front likely in response to a secondary buoyant plume with tidal mixing and is sensitive to seasonal water temperatures in front of the ice shelf the front of the ris is examined using glas icesat laser altimetry data spatial and temporal changes in surface elevations are attributed to enhanced basal melting of the ice shelf near the ice front melt rates b increase exponentially as the front is approached from approximately zero at 40 km from the front to an average of 2 8 1 0 m a 1 within the front kilometer melt estimates within the front 60 km are best fit by the relationship b 2 0 exp x 11900 m a 1 where x denotes distance from the front frontal melt totals approximately 16 km3 a 1 in the front 40 km which accounts for between 10 and 40 of current published estimates of total melt beneath the ris spatial averaging along the front reveals a different pattern of melt in regions that have recently calved as compared to regions that have not calved frontal melt is modeled as a one dimensional buoyant plume with tidal currents included these model results imply that modest increases in sea surface temperatures will lead to considerable increases in melting beneath the ice shelf front

semiempirical and process based global sea level projections we review the two main approaches to estimating sea level rise over the coming century physically plausible models of reduced complexity that exploit statistical relationships between sea level and climate forcing and more complex physics based models of the separate elements of the sea level budget previously estimates of future sea level rise from semiempirical models were considerably larger than those from process based models however we show that the most recent estimates of sea level rise by 2100 using both methods have converged but largely through increased contributions and uncertainties in process based model estimates of ice sheets mass loss hence we focus in this paper on ice sheet flow as this has the largest potential to contribute to sea level rise progress has been made in ice dynamics ice stream flow grounding line migration and integration of ice sheet models with high resolution climate models calving physics remains an important and difficult modeling issue mountain glaciers numbering hundreds of thousands must be modeled by extensive statistical extrapolation from a much smaller calibration data set rugged topography creates problems in process based mass balance simulations forced by regional climate models with resolutions 10 100 times larger than the glaciers semiempirical models balance increasing numbers of parameters with the choice of noise model for the observations to avoid overfitting the highly autocorrelated sea level data all models face difficulty in separating out non climate driven sea level rise e g groundwater extraction and long term disequilibria in the present day cryosphere sea level system

calving event detection by observation of seiche effects on the greenland fjords with mass loss from the greenland ice sheet accelerating and spreading to higher latitudes the quantification of mass discharge in the form of icebergs has recently received much scientific attention here we make use of very low frequency 0 001 0 01 hz seismic data from three permanent broadband stations installed in the summers of 2009 10 in northwest greenland in order to monitor local calving activity at these frequencies calving seismograms are dominated by a tilt signal produced by local ground flexure in response to fjord seiching generated by major iceberg calving events a simple triggering algorithm is proposed to detect calving events from large calving fronts with potentially no user interaction our calving catalogue identifies spatial and temporal differences in calving activity between jakobshavn isbrae and glaciers in the uummannaq district similar to 200 km to the north the uummannaq glaciers show clear seasonal fluctuations in seiche based calving detections as well as seiche amplitudes in contrast the detections at jakobshavn isbrae show little seasonal variation which may be evidence for an ongoing transition to winter calving activity the results offer further evidence that seismometers can provide efficient and inexpensive monitoring of calving fronts

postseismic deformation due to the mw 6 0 2004 parkfield earthquake stress driven creep on a fault with spatially variable rate and state friction parameters 1 we investigate the coseismic and postseismic deformation due to the m w 6 0 2004 parkfield california earthquake we produce coseismic and postseismic slip models by inverting data from an array of 14 continuous gps stations from the scign network kinematic inversions of postseismic gps data over a time period of 3 years show that afterslip occurred in areas of low seismicity and low coseismic slip predominantly at a depth of 5 km inversions suggest that coseismic stress increases were relaxed by predominantly aseismic afterslip on a fault plane the kinetics of afterslip is consistent with a velocity strengthening friction generalized to include the case of infinitesimal velocities we performed simulations of stress driven creep using a numerical model that evaluates the time dependent deformation due to coseismic stress changes in a viscoelastoplastic half space starting with a coseismic slip distribution we compute the time dependent evolution of afterslip on a fault plane and the associated displacements at the gps stations data are best explained by a rate strengthening model with frictional parameter a b 7 x 10 3 at a high end of values observed in laboratory experiments we also find that the geodetic moment due to creep is a factor of 100 greater than the cumulative seismic moment of aftershocks the rate of aftershocks in the top 10 km of the seismogenic zone mirrors the kinetics of afterslip suggesting that postearthquake seismicity is governed by loading from the nearby aseismic creep the san andreas fault around parkfield is deduced to have large along strike variations in rate and state frictional properties velocity strengthening areas may be responsible for the separation of the coseismic slip in two distinct asperities and for the ongoing aseismic creep occurring between the velocity weakening patches after the 2004 rupture

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

future sea level rise from tidewater and ice shelf tributary glaciers of the antarctic peninsula abstract iceberg calving and increased ice discharge from ice shelf tributary glaciers contribute significant amounts to global sea level rise slr from the antarctic peninsula ap owing to ongoing ice dynamical changes collapse of buttressing ice shelves these contributions have accelerated in recent years as the ap is one of the fastest warming regions on earth further ice dynamical adjustment increased ice discharge is expected over the next two centuries in this paper the first regional slr projection of the ap from both iceberg calving and increased ice discharge from ice shelf tributary glaciers in response to ice shelf collapse is presented an ice sheet model forced by temperature output from 13 global climate models gcms in response to the high greenhouse gas emission scenario rcp8 5 projects ap contribution to slr of 28 16 to 32 16 mm by 2300 partitioned approximately equally between contributions from tidewater glaciers and ice shelf tributary glaciers in the rcp4 5 scenario sea level rise projections to 2300 are dominated by tidewater glaciers 8 18 mm in this cooler scenario 2 4 1 mm is added to global sea levels from ice shelf tributary drainage basins as fewer ice shelves are projected to collapse sea level projections from ice shelf tributary glaciers are dominated by drainage basins feeding george vi ice shelf accounting for 70 of simulated slr combined total ice dynamical slr projections to 2300 from the ap vary between 11 2 and 32 16 mm sea level equivalent sle depending on the emission scenario used these simulations suggest that omission of tidewater glaciers could lead to a substantial underestimation of the ice sheet s contribution to regional slr

assessment of ice flow dynamics in the zone close to the calving front of antarctic ice shelves this work was supported by a british antarctic survey studentship funded by the natural environment research council grant no ne k50094x 1

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

gps applications for geodynamics and earthquake studies geodetic measurements obtained with the global positioning system gps are increasingly more widely applied in geophysical studies in this paper we review the changes to the technology of gps geodesy over the last five years that are responsible for this increased applicability we survey geophysical investigations employing gps to measure coseismic postseismic and interseismic deformation plate motion and crustal deformation at plate boundaries volcano deformation and the deformation associated with glacial isostatic adjustment and its application to sea level studies we emphasize the use of gps determinations for the modeling of this wide variety of geophysical phenomena we also discuss the recent advent of permanent gps networks for regional geophysical studies as well as the possible future of gps surveying in light of the recent advances

avalanching glacier instabilities review on processes and early warning perspectives avalanching glacier instabilities are gravity driven rupture phenomena that might cause major disasters especially when they are at the origin of a chain of processes reliably forecasting such events combined with a timely evacuation of endangered inhabited areas often constitute the most efficient action recently considerable efforts in monitoring analyzing and modeling such phenomena have led to significant advances in destabilization process understanding improving early warning perspectives the purpose of this paper is to review the recent progress in this domain three different types of instabilities can be identified depending on the thermal properties of the ice bed interface if cold 1 the maturation of the rupture is associated with a typical time evolution of surface velocities and passive seismic activity a prediction of the final break off is possible using these precursory signs for the two other types water plays a key role in the development of the instability if the ice bed interface is partly temperate 2 the presence of meltwater may reduce the basal resistance which promotes the instability no clear and easily detectable precursory signs are known in this case and the only way to infer any potential instability is to monitor the temporal evolution of the thermal regime the last type of instability 3 concerns steep temperate glacier tongues switching for several days weeks during the melting season into a so called active phase followed in rare cases by a major break off event although the prediction of such events is still far from being achievable critical conditions promoting the final instability can be identified

spontaneous ice front retreat caused by disintegration of adjacent ice shelf in antarctica abstract antarctic ice discharge constitutes the largest uncertainty in future sea level projections floating ice shelves fringing most of antarctica exert retentive forces onto the ice flow while abrupt ice shelf retreat has been observed it is generally considered a localized phenomenon here we show that the disintegration of an ice shelf may induce the spontaneous retreat of its neighbor as an example we reproduce the spontaneous but gradual retreat of the larsen b ice front as observed after the disintegration of the adjacent larsen a ice shelf we show that the larsen a collapse yields a change in spreading rate in larsen b via their connecting ice channels and thereby causes a retreat of the ice front to its observed position of the year 2000 prior to its collapse this mechanism might be particularly relevant for the role of east antarctica and the antarctic peninsula in future sea level

glaciological advances made with interferometric synthetic aperture radar spaceborne interferometric synthetic aperture radar insar techniques for measuring ice flow velocity and topography have developed rapidly over the last decade and a half revolutionizing the study of ice dynamics spaceborne interferometry has contributed to major progress in many areas of glaciological study by providing the first comprehensive measurements of ice stream flow velocity over the major outlets of greenland and antarctica revealing that ice stream and outlet glacier flow can change rapidly months to years improving understanding of several ice sheet and ice shelf processes providing velocity for flux gate based mass balance assessment mapping flow of mountain glaciers and capturing the geomorphic traces of past ice flow we review the basic technique development the measurement characteristics and the extensive set of results yielded by these measurements

three dimensional boundary integral modeling of spontaneous earthquake sequences and aseismic slip 1 fault processes involve complex patterns of seismic events and aseismic slip this work develops a three dimensional 3 d methodology for simulating long term history of spontaneous seismic and aseismic slip on a vertical planar strike slip fault subjected to slow tectonic loading our approach reproduces all stages of earthquake cycles from accelerating slip before dynamic instability to rapid dynamic propagation of earthquake rupture to postseismic slip and to interseismic creep including aseismic transients we use the developed 3 d methodology to study interaction of fault slip with a small patch of higher normal stress over long term slip history for uniform initial prestress dynamic rupture is significantly affected by the stronger patch in the first simulated event but not in subsequent ones the change in behavior is due to redistribution of shear stress by prior slip which demonstrates that distributions of fault strength and stress are related and illustrates the importance of simulating long slip histories even in studies of dynamic rupture despite no long term effect on dynamic rupture the small patch of higher normal stress influences nucleation processes and hence long term slip patterns in the model comparison of the fully dynamic simulations and a widely used quasi dynamic approach shows that the quasi dynamic approach modifies long term slip patterns in addition to resulting in much smaller slip velocities and rupture speeds during dynamic events we show that the response of quasi dynamic formulations with reduced radiation damping terms can be scaled to match the results of the standard quasi dynamic formulation and hence cannot improve the comparison