decadal scale variations in ice flow along whillans ice stream and its tributaries west antarctica this is the publisher s version also available electronically from http www ingentaconnect com

subglacial till a physical framework for its properties and processes the continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized the proposed framework is provisional in two senses the constitutive description of till will require improvement and not all potentially significant processes have been included till is treated as a three component mixture of water clasts coarse grained solids and fine grained matrix solids these components obey mass balance equations and their behavior is subject to constitutive relations that describe such processes as darcian water transport fine grained sediment transport consolidation shear deformation dilatancy and comminution five state variables uniquely determine the compressibility permeability shear strength and other physical properties numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

shear resistance and continuity of subglacial till hydrology rules the field observations of g s boulton stimulated widespread interest in deformable beds shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure during unsteady deformation pseudo viscous shear resistance can be caused by dilation of consolidated tills and resultant pore pressure decline this effect is probably uncommon however because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective pressure transients stick slip motion at whillans ice stream antarctica indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip recurrence intervals for rapid slip episodes there 6 18 hours indicate that till strength variations if driven by changes in pore pressure either related or unrelated to basal freezing are focused in the uppermost several centimeters of the bed ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate weakening during rapid slip with pore pressure decay causing strengthening between slip episodes by promoting shallow sluggish subglacial water flow and low effective pressure soft beds may help sustain themselves by slowing their own transport soft bed shear resistance kinematics and continuity are problems rooted in subglacial hydrology

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

in search of ice stream sticky spots the form drag of large bedrock bumps sticking into the base of an ice stream can produce effective sticky spots supporting large basal shear stress bedrock regions surrounded by lubricating till at the same topographic level can cause sticky spots but tend to collect lubricating water and thus are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps raised regions on the ice air surface also can cause moderate increases in the shear stress supported on the bed beneath surveys of large scale bedrock roughness strain grids across the margins of ice surface highs and possibly water pressure measurements in regions of thin or zero till would help identify and characterize sticky spots

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

dynamics of stick slip motion whillans ice stream antarctica article i nfo the stick slip motion and associated seismic emissions of whillans ice stream wis west antarctica are two of the many recent observations of unexpected ice sheet behavior that are challenging traditional models of rapid glacier motion here we find that the wis slip events repeatedly nucleate from a sticky spot located in the middle of the ice stream acting similar to an asperity in traditional models of earthquake physics this region shows less motion than surrounding areas during the inter slip periods thus concentrating stress and producing a pulse of seismic energy at the onset of slip the propagating rupture breaks through an additional asperity in the northern part of the ice stream producing another pulse of seismic energy 6 12 min after initiation both asperities are regions of higher hydraulic potential than surrounding regions suggesting they may have greater bed friction due to reduced water lubrication tidal pacing of the stress accumulation combined with fault healing controls the applied stress at failure with higher stress giving faster propagation of the rupture front and higher slip velocities these differences are reflected in the timing of the teleseismic arrivals our results highlight both the great sensitivity of large ice streams to small changes in external forcing and the importance of limited regions of the subglacial bed in controlling their motion as well as providing insights to the mechanics of repeating earthquakes

a laboratory study of particle ploughing and pore pressure feedback a velocity weakening mechanism for soft glacier beds if basal water discharge and pressure are sufficiently high a soft bedded glacier will slip over its bed by ploughing the process in which particles that span the ice bed interface are dragged across the bed surface results of laboratory experiments indicate that resistance to ploughing can decrease with increasing ploughing velocity velocity weakening during ploughing at various velocities 15 400 m a 1 till was compacted in front of idealized particles causing pore pressures there that were orders of magnitude higher than the ambient value this excess pore pressure locally weakened the till in shear thereby decreasing ploughing resistance by a factor of 3 0 6 6 with a six fold increase in ploughing velocity characteristic timescales of pore pressure diffusion and compaction down glacier from ploughing particles depend on till diffusivity ploughing velocity and sizes of ploughing particles these timescales accurately predict the ranges of these variables over which excess pore pressure and velocity weakening occurred existing ploughing models do not account for velocity weakening a new ploughing model with no adjustable parameters predicts ploughing resistance to no worse than 38 but requires that excess pore pressures be measured velocity weakening by this mechanism may affect fast glacier flow sediment transport by bed deformation and basal seismicity

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

modelling of short interval silent slip events in deeper subduction interfaces considering the frictional properties at the unstable stable transition regime summary recent high resolution observations of crustal movements have revealed silent slip events sses with propagation velocities of around 10 15 km d 1 and with intervals of 3 14 months along the deeper parts of the cascadia and nankai subduction zones this study develops 2 d and 3 d models of these short interval sses considering the frictional behaviour that was confirmed experimentally by shimamoto for the unstable stable transition regime to represent this frictional behaviour a small cut off velocity to an evolution effect is introduced in a rate and state dependent friction law when the cut off velocity to the evolution effect is significantly smaller than that to a direct effect steady state friction exhibits velocity weakening at low slip velocities and velocity strengthening at high slip velocities at the deeper cascadia and nankai subduction interfaces the pore pressure is inferred to be high because of the dehydration of materials in the descending plate under conditions where the pore fluid pressure is nearly equal to the lithostatic pressure and the critical weakening displacement is very small short interval sses with propagation velocities and slip velocities of 4 8 km d 1 and 2 4 10 7 m s 1 respectively can be reproduced the propagation velocity of short interval sses is in proportion to the slip velocity the results also show that during the nucleation process of large earthquakes the occurrence of short interval sses becomes irregular because of the accelerated slips that occur at the bottom of the seismogenic zone our results suggest that monitoring of short interval sses might be useful for forecasting the main earthquakes

dynamics of ice stream temporal variability modes scales and hysteresis 1 understanding the mechanisms governing temporal variability of ice stream flow remains one of the major barriers to developing accurate models of ice sheet dynamics and ice climate interactions here we analyze a simple model of ice stream hydrology coupled to ice flow dynamics and including drainage and basal cooling processes analytic and numerical results from this model indicate that there are two major modes of ice stream behavior steady streaming and binge purge variability the steady streaming mode arises from friction stabilized subglacial meltwater production which may also activate and interact with subglacial drainage the binge purge mode arises from a sufficiently cold environment sustaining successive cycles of thinning induced basal cooling and stagnation low prescribed temperature at the ice surface and weak geothermal heating typically lead to binge purge behavior while warm ice surface temperature and strong geothermal heating will tend to produce steady streaming behavior model results indicate that modern siple coast ice streams reside in the binge purge parameter regime near a subcritical hopf bifurcation to the steady streaming mode numerical experiments exhibit hysteresis in ice stream variability as the surface temperature is varied by several degrees our simple model simulates heinrich event like variability in a hypothetical hudson strait ice stream including dynamically determined purge time scale till freezing and basal cooling during the binge phase these findings are an improvement on studies of both modern and paleo ice stream variability and provide a framework for interpreting complex ice flow models

variable deceleration of whillans ice stream west antarctica the whillans ice stream ice plain wip has been slowing since at least 1963 prior constraints on this slowdown were consistent with a constant long term deceleration rate new observations of ice velocity from 11 continuous and 3 seasonal gps sites indicate the deceleration rate varies through time including on interannual time scales between 2009 and 2012 wip decelerated at a rate 6 1 to 10 9 2 m yr2 that was double the multidecadal average 3 0 to 5 6 2 m yr2 to identify the causes of slowdown we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher order inverse model all model results support the conclusion that the observed deceleration of wip is caused by an increase in basal resistance to motion at a rate of 10 to 40 pa yr subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on changes in subglacial hydrology or increases in the area of resistant basal substrate through differential erosion the observed variability in wip deceleration rate suggests that dynamics in subglacial hydrology plausibly driven by basal freeze on and or activity of subglacial lakes plays a key role in modulating basal resistance to ice motion in the region

friction of ice on ice 1 new measurements have been made of the friction coefficient of freshwater polycrystalline ice sliding slowly 5 10 8 to 1 10 3 m s 1 upon itself at temperatures from 98 to 263 k under low normal stresses 98 kpa sliding obeys coulomb s law the shear stress is directly proportional to the normal stress across the interface while cohesion offers little contribution to frictional resistance the coefficient of kinetic friction of smooth surfaces varies from k 0 15 to 0 76 and at elevated temperatures 223 k exhibits both velocity strengthening at lower velocities 10 5 to 10 4 m s 1 and velocity weakening at higher velocities strengthening and weakening are attributed to creep deformation of asperities and localized melting respectively at intermediate temperatures of 173 and 133 k the kinetic coefficient appears to not exhibit significant dependence upon velocity however at the low temperature of 98 k the coefficient of kinetic friction exhibits moderate velocity strengthening at both the lowest and the highest velocities but velocity independence over the range of intermediate velocities no effect was detected of either grain size or texture over the range of roughness 0 4 10 6 m ra 12 10 6 m a moderate effect was detected where k ra0 08 slide hold slide experiments revealed that the coefficient of static friction increases by an amount that scales logarithmically with holding time implications of the results are discussed in relation to shearing across tiger stripe faults within the icy crust of saturn s enceladus sliding of the arctic sea ice cover and brittle compressive failure of cold ice

deformation induced melting in the margins of the west antarctic ice streams flow of glacial ice in the west antarctic ice sheet localizes in narrow bands of fast flowing ice streams bordered by ridges of nearly stagnant ice but our understanding of the physical processes that generate this morphology is incomplete here we study the thermal and mechanical properties of ice stream margins where flow transitions from rapid to stagnant over a few kilometers our goal is to explore under which conditions the intense shear deformation in the margin may lead to deformation induced melting we propose a 2 d model that represents a cross section through the ice stream margin perpendicular to the downstream flow direction we limit temperature to the melting point to estimate melt rates based on latent heat using rheology parameters as constrained by laboratory data and observations we conclude that a zone of temperate ice is likely to form in active shear margins

seismic and geodetic evidence for grounding line control of whillans ice stream stick slip events the tidally modulated stick slip events of whillans ice stream in west antarctica produce seismic energy from three locations near the grounding line using ice velocity records obtained by combining time series from colocated broadband seismometers and gps receivers installed on the ice stream during the 2010 2011 and 2011 2012 austral summers along with far field seismic recordings of elastic waves we locate regions of high rupture velocity and stress drop these regions which are analogous to asperities in traditional seismic fault studies are areas of elevated friction at the base of the ice stream slip events consistently initiate at one of two locations near the center of the ice stream where events associated with the ross sea high tide originate or a grounding line spot where events associated with the ross sea low tide initiate as well as occasional high tide events following a skipped low tide event the grounding line site but not the central site produces rayleigh waves observable up to 1000 km away through fast expansion of the slip area grounding line initiation events also show strong directivity in the downstream direction indicating initial rupture propagation at 1 5 km s compared to an average of 0 150 km s for the entire slip event following slip initiation additional seismic energy is produced from two sources located near the grounding line first at the downstream end of subglacial lake engelhardt and second toward the farthest downstream extent of the ice stream this evidence suggests that the stronger higher friction material along the grounding line controls motion throughout the stick slip region

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

stick slip behavior of ice streams modeling investigations a puzzling phenomenon of ice stream flow is the stick slip motion displayed by whillans ice stream wis west antarctica in this study we test the hypothesis that the wis stick slip motion has features similar to those of other known stick slip systems and thus might be of the same origin to do so we adapt a simple mechanical model widely used in seismology to study classic stick slip behavior observed in tectonic faults in which the difference between static and dynamic friction allows for the generation and spatial propagation of abrupt slip events we show how spatial variability in friction properties as well as a periodic forcing intended to mimic the effect of tides can reproduce the observed duration and periodicity of stick slip motion in an ice stream an intriguing aspect of the association of wis with mechanical stick slip oscillators is that the onset of stick slip cycling from a condition of permanent slip appears to be associated with the reduction in overall speed of wis if this association is true then stick slip behavior of wis is a transitional phase of behavior associated with the ice stream s recent deceleration

fortnightly variations in the flow velocity of rutford ice stream west antarctica ice sheets like that of antarctica lose much of their ice via fast flowing ice streams hilmar gudmundsson has monitored the flow in one such stream the rutford ice stream in west antarctica over a period of two months the findings are a surprise surface flow velocities here fluctuate by as much as 20 over a period of two weeks perhaps under the influence of the spring neap fortnightly tidal cycle although the mechanism that couples these tides to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate monitoring of the flow of an ice sheet in antarctica finds that the surface velocity oscillates by as much as 20 per cent every two weeks the spring to neap tidal cycle seems to be the cause although the mechanism through which these tides couple to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate most of the ice lost from the antarctic ice sheet passes through a few fast flowing and highly dynamic ice streams1 quantifying temporal variations in flow in these ice streams and understanding their causes is a prerequisite for estimating the potential contribution of the antarctic ice sheet to global sea level change2 3 here i show that surface velocities on a major west antarctic ice stream rutford ice stream vary periodically by about 20 per cent every two weeks as a result of tidal forcing tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter term variations4 5 6 7 8 9 the existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long term change

motion of an antarctic glacier by repeated tidally modulated earthquakes subglacial seismicity reveals information about glacier behaviour analysis of repeated seismic events beneath an antarctic outlet glacier is consistent with sliding of debris laden ice over a bedrock asperity with an event frequency that is modulated by the ocean tides

younging directions in the dawros peridotite connemara the syn d2 dawros peridotite comprises a sequence of layered harzburgites associated with massive orthopyroxenites which pass upwards via a transition marked by the first appearance of chromitite layers into layered lherzolites microprobe analyses reveal that the ultramafic sequence exhibits a range in olivine composition from fo 91 9 to fo 82 3 and a range in orthopyroxene from en 91 9 to en 84 3 a transgressive metagabbro lens in the central outcrop is interpreted as a syn d2 intrusion into the ultramafic sequence and contains olivine fo 80 4 and a range in orthopyroxene from en 81 5 to en 76 0 mg fe ratios in coexisting olivines and pyroxenes are used with new structural data to elucidate younging directions in the layered parts of the peridotite the exposed sequence youngs consistently ne but is repeated by faulting and locally reversed by f4 folding the peridotite and its immediate hornfels became detached from the surrounding metasediments during d3 and now lie on the sw limb of a recumbent downward facing f3 antiform which plunges nw consideration of the structure in adjacent areas suggests that the dawros and currywongaun doughruagh intrusions lie within a single major f2 fold limb

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

ice shelf tidal flexure and subglacial pressure variations abstract we develop a model of an ice shelf ice stream system as a viscoelastic beam partially supported by an elastic foundation when bedrock near the grounding line acts as a fulcrum leverage from the ice shelf dropping at low tide can cause significant 1 cm uplift in the first few kilometers of grounded ice this uplift and the corresponding depression at high tide lead to basal pressure variations of sufficient magnitude to influence subglacial hydrology tidal flexure may thus affect basal lubrication sediment flow and till strength all of which are significant factors in ice stream dynamics and grounding line stability under certain circumstances our results suggest the possibility of seawater being drawn into the subglacial water system the presence of seawater beneath grounded ice would significantly change the radar reflectivity of the grounding zone and complicate the interpretation of grounded versus floating ice based on ice penetrating radar observations

slow slip events on the whillans ice plain antarctica described using rate and state friction as an ice stream sliding law the whillans ice plain wip antarctica experiences twice daily tidally modulated stick slip cycles slip events last about 30 minutes have sliding velocities as high as 0 5 mm s 15 km yr and have total slip 0 5 m slip events tend to occur during falling ocean tide just after high tide and just before low tide to reproduce these characteristics we use rate and state friction which is commonly used to simulate tectonic faulting as an ice stream sliding law this framework describes the evolving strength of the ice bed interface throughout stick slip cycles we present simulations that resolve the cross stream dimension using a depth integrated treatment of an elastic ice layer loaded by tides and steady ice inflow steady sliding with rate weakening friction is conditionally stable with steady sliding occurring for sufficiently narrow ice streams relative to a nucleation length stick slip cycles occur when the ice stream is wider than the nucleation length or equivalently when effective pressures exceed a critical value ice streams barely wider than the nucleation length experience slow slip events and our simulations suggest that the wip is in this slow slip regime slip events on the wip show a sense of propagation and we reproduce this behavior by introducing a rate strengthening region in the center of the otherwise rate weakening ice stream if pore pressures are raised above a critical value our simulations predict that the wip would exhibit quasi steady tidally modulated sliding as observed on other ice streams this study validates rate and state friction as a sliding law to describe ice stream sliding styles

elastic dynamics and tidal migration of grounding lines modify subglacial lubrication and melting 1 we combine the dynamics of ice bed and ocean in a new elastic model for the tidal timescale migration of grounding lines on deformable foundations previous interpretations of tidal flexure using models of elastic ice shelves with fixed grounding lines were found to be inconsistent suggesting an elasticity of ice that varies spatially and temporally and that is significantly smaller than measured experimentally we argue here that with our model a consistent purely elastic interpretation can be made combining this new approach with remote sensing measurements we show that the grounding line migrates several kilometers during a tidal cycle that we can infer the effective elastic properties of the bed and that the elastic pressure of the ice leads to a hydrological barrier near the grounding line that controls subglacial hydrology our findings imply that subglacial lubrication and melting induced by the ocean thermal forcing can increase substantially during high tide

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

effective friction law for small scale fault heterogeneity in 3d dynamic rupture 1 we address the problem of modeling dynamic rupture on multiscale heterogeneous faults in 3d under the assumption of slip weakening friction we numerically construct effective friction laws that integrate the effects of small scale heterogeneity during the rupture this homogenization process is based on the description of the initial phase of the rupture by the dominant unstable spectral mode its dynamics is influenced by the geometry of the fault the static friction heterogeneities and the friction law we first define a periodic small scale heterogeneous model introducing heterogeneity in the distribution of the static friction coefficient we then describe a method for constructing this effective friction law applying this new law homogeneously on the fault permits to reproduce the dynamic evolution of the heterogeneous fault furthermore we show that the effective friction law can be used to replace small scale heterogeneities in two scale heterogeneous models while preserving their effects we study three kinds of two scale models with growing complexity first periodic at both scales then periodic only at small scale and finally irregular at both scales this homogenization method can be adapted to the case where the heterogeneity is introduced in the initial stress rather than in the static friction value finally we show in a simple example that the effective friction law permits to reproduce the transition between subshear and supershear rupture propagation originally produced by heterogeneities on the fault

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event

transmission of basal variability to a glacier surface 1 transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium the three dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio that is the ratio of basal sliding to deformational velocity for any mean bedrock slope and above a minimum value of slip ratio the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3 8 times the mean ice thickness the transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio this explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas at slip ratios higher than about 100 the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope using linearized long wave theories at these slip ratios instead of the more accurate arbitrary wavelength theory gives estimates of decay times that are an order of magnitude too small and phase velocities several times too large the problem of the propagation and decay of small amplitude surface undulations on glaciers in three dimensions is solved small amplitude surface waves on glaciers are strongly diffusive and dispersive redistribution of mass on ice sheets and glaciers is a diffusion process and it is misleading albeit not mathematically incorrect to describe the reaction of glaciers to surface perturbations in terms of a wave propagation

similarity of organized patterns in driving and basal stresses of antarctic and greenland ice sheets beneath extensive areas of basal sliding the rate of ice transport from the interior of ice sheets to their margins and hence the rate with which it contributes to sea level is determined by the balance of driving stress basal resistance and ice internal deformation using recent high resolution observations of the antarctic and greenland ice sheets we compute driving stress and ice deformation velocities inferring basal traction by inverse techniques the results reveal broad scale organization in 5 20 km band like patterns in both the driving and basal shear stresses located in zones with substantial basal sliding both ice sheets experience basal sliding over areas substantially larger than previously recognized the likely cause of the spatial patterns is the development of a band like structure in the basal shear stress distribution that is the results of pattern forming instabilities related to subglacial water the similarity of patterns on the greenland and antarctic ice sheets suggests that the flow of ice sheets is controlled by the same fundamental processes operating at their base which control ice sheet sliding and are highly variable on relatively short spatial and temporal scales with poor predictability this has far reaching implications for understanding of the current and projection of the future ice sheets evolution

aseismic slip transients emerge spontaneously in three dimensional rate and state modeling of subduction earthquake sequences 1 to investigate the possible physical mechanisms of recently observed aseismic slip events in the cascadia japan and mexico subduction zones we apply a dieterich ruina rate and state friction law to a three dimensional model of a shallow subduction fault that is loaded by imposed steady plate slip rate far downdip along the thrust interface friction properties are temperature and hence depth dependent such that sliding is stable at depths below about 30 km the system is perturbed into a significantly nonuniform slip mode along strike by introducing small along strike variations in the constitutive parameters a and a b in addition to large heterogeneous earthquake slip at seismogenic depths and associated postseismic transients we found that slip events which have clearly aseismic slip rates emerge spontaneously around the downdip end of the seismogenic zone both transients which start well after a seismic event and those which are triggered by other transients are observed from the simulations the slip velocity depth range and sometimes along strike migration speed of simulated transients are similar to the observations from natural subduction zones unstable stable transitional friction properties near the downdip end of the seismogenic zone are suggested to be an ingredient allowing such transients simulated transients can weaken the locking intensity of the updip seismogenic zone while enhancing that of the transition zone spatial temporal correlation of aseismic transients and nearby seismicity in the guerrero mexico area suggests that transients may signal a period of increased probability for nucleating their high frequency counterparts as damaging subduction thrust events

slow earthquakes coincident with episodic tremors and slow slip events we report on the very low frequency earthquakes occurring in the transition zone of the subducting plate interface along the nankai subduction zone in southwest japan seismic waves generated by very low frequency earthquakes with seismic moment magnitudes of 3 1 to 3 5 predominantly show a long period of about 20 seconds the seismicity of very low frequency earthquakes accompanies and migrates with the activity of deep low frequency tremors and slow slip events the coincidence of these three phenomena improves the detection and characterization of slow earthquakes which are thought to increase the stress on updip megathrust earthquake rupture zones

stick slip motion of an antarctic ice stream the effects of viscoelasticity stick slip behavior is a distinguishing characteristic of the flow of whillans ice stream siple coast antarctica distinct from stick slip on northern hemisphere glaciers which is generally attributed to supraglacial melt the behavior is thought be controlled by basal processes and by tidally induced stress however the connection between stick slip behavior and flow of the ice stream on long time scales if any is not clear to address this question we develop a new ice flow model capable of reproducing stick slip cycles similar to ones observed on the whillans ice plain the model treats ice as a viscoelastic material and emulates the weakening and healing that are suggested to take place at the ice till interface the model results suggest the long term ice stream flow that controls ice discharge to surrounding oceans is somewhat insensitive to certain aspects of stick slip behavior such as velocity magnitude during the slip phase and factors that regulate it e g elastic modulus furthermore it is found that factors controlling purely viscous flow such as temperature influence stick slip contribution to long term flow in much the same way additionally we show that viscous ice deformation traditionally disregarded in analysis of stick slip behavior has a strong effect on the timing of slip events and therefore should not be ignored in efforts to deduce bed properties from stick slip observations

dynamic patterns of ice stream flow in a 3 d higher order ice sheet model with plastic bed and simplified hydrology 1 predicting ice sheet mass balance is challenging because of the complex flow of ice streams to address this issue we have coupled a three dimensional higher order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress the model was tested for its sensitivity to regional water availability first with an assumed undrained bed the ice stream oscillates between active and stagnant phases solely as a result of thermodynamic feedbacks occurring at the ice till interface however the velocity amplitude decreases over time as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing ice sheet mode second we assume that the till is able to assimilate water from a hypothetical regional hydrological system this leads to significantly different long term behavior as a continuously oscillating ice stream mode is maintained the extra water incorporated in the till leads to higher velocities triggering stronger thermodynamic feedbacks between the ice and till layer results also suggest that fast flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase similarly till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system underscoring the need to accurately describe the coupling between ice dynamics basal conditions and regional subglacial hydrology in ice sheet models

accelerated west antarctic ice mass loss continues to outpace east antarctic gains a b s t r a c t article history while multiple data sources have confirmed that antarctica is losing ice at an accelerating rate different measurement techniques estimate the details of its geographically highly variable mass balance with different levels of accuracy spatio temporal resolution and coverage some scope remains for methodological improvements using a single data type in this study we report our progress in increasing the accuracy and spatial resolution of time variable gravimetry from the gravity recovery and climate experiment grace we determine the geographic pattern of ice mass change in antarctica between january 2003 and june 2014 accounting for glacio isostatic adjustment gia using the ij05 r2 model expressing the unknown signal in a sparse slepian basis constructed by optimization to prevent leakage out of the regions of interest we use robust signal processing and statistical estimation methods applying those to the latest time series of monthly grace solutions we map antarctica s mass loss in space and time as well as can be recovered from satellite gravity alone ignoring gia model uncertainty over the period 2003 2014 west antarctica has been losing ice mass at a rate of 121 8g t yr and has experienced large acceleration of ice mass losses along the amundsen sea coast of 18 5g t yr 2 doubling the mass loss rate in the past six years the antarctic peninsula shows slightly accelerating ice mass loss with larger accelerated losses in the southern half of the peninsula ice mass gains due to snowfall in dronning maud land have continued to add about half the amount of west antarctica s loss back onto the continent over the last decade we estimate the overall mass losses from antarctica since january 2003 at 92 10 gt yr

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

the effects of entrained debris on the basal sliding stability of a glacier 1 new laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick slip motion of debris laden ice over rock with implications for glacier behavior friction between a rock substrate and clasts in ice generates heat which melts nearby ice to produce lubricating water an increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase low initial lubrication can result from cold or drained conditions whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads strengthening of the interface healing during stick intervals between slip events occurs primarily through meltwater refreezing when healing and unstable sliding are taken together the experiments reported here suggest that stick slip behavior is common from motion of debris laden glacier ice over bedrock

repeating earthquakes from rupture of an asperity under an antarctic outlet glacier the inner regions of the antarctic continent are generally regarded as nearly aseismic although microseismicity is known to occur beneath some outlet ice streams related to the interaction between the fast flowing ice and the bedrock here we show the occurrence of unusual earthquakes beneath an antarctic outlet glacier that share almost the same magnitude pointing to the repeated rupture of a single asperity these seismic events produce waveforms with very high similarity and uncommon spectrum and are tightly clustered in space but unlike other reported instances of repeating earthquakes on a patch of the san andreas fault they occur in frequent irregular swarms evidence locates these events at the rock ice interface under the glacier and shows the existence of stick slip motion on a smaller scale than the large slow slip events detected by global seismographs seismic behaviour of large glaciers can presumably be connected to surges in ice motion this study determines a little known environment for fracture dynamics studies while also contributing to the understanding of the coupling processes between fast flowing glaciers and bedrock that influence ice stream evolution and stability

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses

spontaneous and triggered aseismic deformation transients in a subduction fault model 1 aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault sliding experiments when that constitutive formulation is applied to model subduction earthquake sequences transients can arise spontaneously for certain effective stress variations with depth we show that if interstitial fluids are present and pore pressure is near lithostatic around and downdip from the frictional stability transition transients with recurrence intervals of 1 year are predicted on the basis of laboratory friction parameters and their temperature hence depth variations the recurrence interval decreases with and reaches 14 months when is 2 3 mpa dimensional analysis and numerical studies show that the fault response primarily depends on a parameter w h here the high pore pressure zone extends distance w updip from the stability transition and h is the stable patch size for steady sliding evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed transient sequences can also be triggered by a modest one time step like interseismic stress perturbation on the subduction fault due to nearby earthquakes or to pore pressure changes e g during episodes of metamorphic fluid release properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced its relative location along the subduction fault and its magnitude

laboratory study of the frictional rheology of sheared till 1 deformation of till produces power law creep for low strain at stresses high enough to cause permanent deformation but below the shear strength experiments were conducted on till a mixed size granular material from matanuska glacier alaska and the scioto ohio lobe of the laurentide ice sheet caesar till we deformed till in double direct shear under fixed shear velocity or shear stress creep normal stress ranged from 50 kpa to 5 mpa at shearing rates ranging from 1 to 300 m s for 1 cm thick samples creep was induced via small step perturbations in stress fabric development within till layers was investigated by varying shear strain prior to creep tests in velocity controlled experiments till deforms as a nearly coulomb plastic material with slight velocity strengthening corresponding to a stress exponent n 60 creep experiments conducted well below the shear strength indicate lower n values increasing as shear stress increases with increasing initial strain and inferred fabric development the creep strain rate decreases while n increases experiments at a normal stress of 1 mpa and no initial strain show n 6 8 increasing to n 17 5 at higher shear strains however strain rate was still decreasing and thus these values represent maximum estimates our data show that in the absence of dilatant hardening till exhibits rate sensitivity at strain of order 1 or less at low strains n likely depends on consolidation state permeability and dilation deformation is nearly rate insensitive coulomb plastic at shear stress near the shear strength or at high strain

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

basal mechanics of ice streams insights from the stick slip motion of whillans ice stream west antarctica the downstream portion of whillans ice stream west antarctica moves primarily by stick slip motion the observation of stick slip motion suggests that the bed is governed by velocity weakening physics and that the basal physics is more unstable than suggested by laboratory studies the stick slip cycle of whilla s ice plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude to understand this variability we modeled the forces acting on the ice stream during the stick phase of the stick slip cycle the ocean tides introduce changes in the rate at which stress is applied to the ice plain increased loading rates promote earlier failure and vice versa results show that the bed of whillans ice stream strengthens over time healing during the quiescent intervals in the stick slip cycle with the bed weakening during slip events the time dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0 35 kpa during the course of a single day copyright 2009 by the american geophysical union

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

sedimentary processes at the base of a west antarctic ice stream constraints from textural and compositional properties of subglacial debris samples of sediments from beneath ice stream b at camp upb west antarctica provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub ice stream environment piston coring in holes bored by hot water drilling yielded five 1 3 m long undisturbed subglacial sediment cores we analyzed granulometry composition and particle morphology in these cores the upb cores are composed of a clay rich unsorted diamicton containing rare marine diatoms sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments the presence of reworked diatoms and their state of preservation as well as the relative spatial homogeneity of this diamicton suggest that the upb cores sampled a several meter thick till layer and not in situ glacimarine sediments the till does incorporate material recycled from the subjacent poorly indurated tertiary glacimarine sediments of the ross sea sedimentary basin which extends beneath this part of the west antarctic ice sheet we propose that the lack of significant comminution in the upb till is ultimately due to its setting over these easily erodible clay rich source sediments the resulting fine grained till matrix inhibits glacial comminution because it facilitates buildup of high pore water pressures and hinders interparticle stress concentrations our observations are consistent with the conjecture that subglacial deformation of weak fine grained tills does not produce significant comminution of till debris elson 1988 based on our findings we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting poorly indurated fine grained sediments since such weak till layers create a permissive condition for ice streaming sub glacial geology may have an indirect but strong control over the location extent and basal mechanics of ice streams

on the physical mechanism of silent slip events along the deeper part of the seismogenic zone 1 to investigate the mechanism of recently observed silent slip events we simulated earthquake preparation processes using the dieterich ruina rate and state dependent friction law to ensure realistic modeling of the unstable stable transition we considered small cut off velocity to an evolution effect in the friction law for the transition zone when the cut off velocity to the evolution effect is significantly smaller than that of a direct effect steady state friction behaves as velocity weakening at low slip velocity and velocity strengthening at high slip velocity this frictional behavior was experimentally and theoretically confirmed for the unstable stable transition zone the results of our numerical simulations show that silent slips of which velocity is higher than the velocity of relative plate motion eventually propagates horizontally along the unstable stable transition over a period of several years silent slip events can be interpreted as being caused by the transitional behavior of the fault constitutive law