fracture propagation to the base of the greenland ice sheet during supraglacial lake drainage surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change the process whereby such a pathway is created through thick cold ice has not however been previously observed we describe the rapid 2 hours drainage of a large supraglacial lake down 980 meters through to the bed of the greenland ice sheet initiated by water driven fracture propagation evolving into moulin flow drainage coincided with increased seismicity transient acceleration ice sheet uplift and horizontal displacement subsidence and deceleration occurred over the subsequent 24 hours the short lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially the integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup

vibrational modes of hydraulic fractures inference of fracture geometry from resonant frequencies and attenuation oscillatory seismic signals arising from resonant vibrations of hydraulic fractures are observed in many geologic systems including volcanoes glaciers and ice sheets and hydrocarbon and geothermal reservoirs to better quantify the physical dimensions of fluid filled cracks and properties of the fluids within them we study wave motion along a thin hydraulic fracture waveguide we present a linearized analysis valid at wavelengths greater than the fracture aperture that accounts for quasi static elastic deformation of the fracture walls as well as fluid viscosity inertia and compressibility in the long wavelength limit anomalously dispersed guided waves known as crack or krauklis waves propagate with restoring force from fracture wall elasticity at shorter wavelengths the waves become sound waves within the fluid channel wave attenuation in our model is due to fluid viscosity rather than seismic radiation from crack tips or fracture wall roughness we characterize viscous damping at both low frequencies where the flow is always fully developed and at high frequencies where the flow has a nearly constant velocity profile away from viscous boundary layers near the fracture walls most observable seismic signals from resonating fractures likely arise in the boundary layer crack wave limit where fluid solid coupling is pronounced and attenuation is minimal we present a method to estimate the aperture and length of a resonating hydraulic fracture using both the seismically observed quality factor and characteristic frequency finally we develop scaling relations between seismic moment and characteristic frequency that might be useful when interpreting the statistics of hydraulic fracture events

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

depth determination for shallow teleseismic earthquakes methods and results the depths of moderate sized shallow earthquakes can be routinely determined to an accuracy of several kilometers using teleseismic data alone making it possible to determine the vertical positions of earthquakes within the lithosphere this capability provides a valuable tool for plate tectonic studies since earthquake depths can be used to constrain lithospheric thermal and mechanical structure at plate boundaries and in intraplate regions two basic techniques are used for depth analyses surface wave spectra can be used to determine focal depths as mode excitation reflects the behavior of the eigenfunctions with depth the primary limitation on such studies stems from lateral heterogeneity of velocity structure the waveforms of body waves are also diagnostic of focal depth which controls the time separation between the direct arrival and near source surface reflections these studies can be limited by a partial trade off between depth and source time function duration the reliability of the body wave modeling approach now in general use is shown by excellent agreement between depths determined by different investigators using different algorithms the depth determination is robust to typical uncertainties in focal mechanism and near source structure the trade off with source time function duration generally does not produce major difficulties for small events ms 6 5 for larger events the trade off can be resolved using a approach in which for a given depth the time function yielding the best fit to a suite of seismograms is found by deconvolution deconvolution at a series of depths yields a misfit function whose minimum indicates the focal depth tests with synthetic data show that the deconvolution process yields reliable depth estimates even given uncertainties in focal mechanism and near source structure it successfully handles multiple sources moderate sized finite sources and horizontally propagating ruptures good results are obtained for vertically propagating ruptures until the fault area becomes so large that waveforms differ significantly from the point source approximation a number of important tectonic results have emerged from studies using these depth determination methods the maximum depth of oceanic intraplate seismicity increases with lithospheric age and is approximately bounded by the 750 c isotherm this depth roughly equals the flexural thickness of the lithosphere but is much less than the depth to the low velocity zone indicated by surface wave dispersion the maximum depth of continental intraplate seismicity is also temperature limited but by a lower temperature these observations are consistent with the rapid weakening of the appropriate rocks with depth predicted by standard temperature dependent rheologies the maximum depth of oceanic transform seismicity corresponds to an isotherm of about 400 c this temperature is lower than would be predicted from the intraplate results and may indicate that transforms are either hotter or weaker than expected the depths of ridge crest earthquakes large enough for teleseismic analysis are extremely shallow 0 8 km the depth distribution of seismicity in continental normal faulting regions provides constraints on the nature of the extension process at subduction zones precise depth determinations can resolve the upper and lower flexural regions in the bending downgoing plate precisely locate the plate interface and map the vertical extent and position of faulting in large earthquakes

basal crevasses and associated surface crevassing on the larsen c ice shelf antarctica and their role in ice shelf instability we identify a series of basal crevasses along a 31 km transect across the northern sector of the larsen c ice shelf antarctica using in situ ground penetrating radar the basal crevasses propagate from a region of multiple shallow basal fractures to form widely spaced 0 5 2 0 km but deeply incised 70 134 m features surface troughs observed in visible imagery exist above the basal crevasses as the ice vertically shears to reach hydrostatic equilibrium while widespread surface crevassing occurs along the crests and on the flanks of the undulations primarily aligned with the topography we suggest based on the location of the surface crevasses and the along flow evolution of the basal crevasses that the former are induced by a bending stress created by gradients in hydrostatic forces using a linear elastic fracture mechanics model we investigate the sensitivity of basal crevasse propagation to observed trends of ice shelf thinning and acceleration basal crevasses are large scale structural weaknesses that can both control meltwater ponding and induce surface crevassing together these features may represent an important mechanism in both past and future ice shelf disintegration events on the antarctic peninsula

glacier crevasses observations models and mass balance implications we review the findings of approximately 60 years of in situ and remote sensing studies of glacier crevasses as well as the three broad classes of numerical models now employed to simulate crevasse fracture the relatively new insight that mixed mode fracture in local stress equilibrium rather than downstream advection alone can introduce nontrivial curvature to crevasse geometry may merit the reinterpretation of some key historical observation studies in the past three decades there have been tremendous advances in the spatial resolution of satellite imagery as well as fully automated algorithms capable of tracking crevasse displacements between repeat images despite considerable advances in developing fully transient three dimensional ice flow models over the past two decades both the zero stress and linear elastic fracture mechanics crevasse models have remained fundamentally unchanged over this time in the past decade however multidimensional and transient formulations of the continuum damage mechanics approach to simulating ice fracture have emerged the combination of employing damage mechanics to represent slow upstream deterioration of ice strength and fracture mechanics to represent rapid failure at downstream termini holds promise for implementation in large scale ice sheet models finally given the broad interest in the sea level rise implications of recent and future cryospheric change we provide a synthesis of 10 mechanisms by which crevasses can influence glacier mass balance

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

basal icequakes recorded beneath an alpine glacier glacier d argentiere mont blanc france evidence for stick slip motion while basal icequakes associated with glacier motion have been detected under antarctica for several decades there remains very little evidence of stick slip motion for alpine glaciers here we analyzed 2357 basal icequakes that were recorded at glacier d argenti ere mont blanc massif between february and november of 2012 and that are likely to be associated with basal sliding these events have been classified into 18 multiplets based on their waveforms the strong similarity of the waveforms within each multiplet suggests an isolated repeating source despite this similarity the peak amplitude within each multiplet varies gradually in time by up to a factor of 18 the distribution of these events in time is relatively complex for long time scales we observe progressive variations in the amplitudes of events within each multiplet for intermediate time scales hours the events occur regularly in time with typical return times of several minutes up to several hours for short time scales from 0 01 to 100 s the largest multiplet shows clus tering in time with a power law distribution of the interevent times the location of these events and their focal mechanisms are not well constrained because most of these events were detected by a single seismometer nevertheless the locations can be estimated with an accuracy of a few tens of meters using a polarization analysis the estimated average depth of the basal events is 179 m which is in good agreement with the estimated glacier thickness the relative changes in distance between the source and the sensor can be measured accurately by correlating separately the p wave and s wave parts of the seismograms of each event with the template waveforms which are obtained by averaging the signals within each multiplet we observed small variations in the times between the p wave and the s wave of up to 0 6 ms over 50 days these variations cannot be explained by displacement of the sensor with respect to the glacier but might be due to small changes in the seismic wave velocities with time finally we found using numerical simulations that the observed signals are better explained by a horizontal shear fault with slip parallel to the glacier flow than by a tensile fault these results suggest that the basal events are associated with stick slip motion of the glacier over rough bedrock the rupture length and the slip are difficult to estimate nonetheless the rupture length is likely to be of the order of meters and the total seismic slip accumulated over one day might be as large as the glacier motion during the most active bursts

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

the robustness of seismic moment and magnitudes estimated using spectral analysis we present an assessment of how microseismic moment magnitude mw e stimates vary with the method and parameters used to calculate seismic moment this is an important topic for operators and regulators who require good magnitude estimates when monitoring induced seismicity it is therefore imperative that these parties know and understand what errors exist in given magnitude values something that is poorly reported this study concentrates on spectral analysis techniques and compares mw computed in the time and frequency domains using recordings of mw 1 5e vents at cotton valley east texas the maximum discrepancy between mw estimated using the different methods is 0 6 units a significant variation by adjusting parameters in the mw calculation we find that the radiation pattern correction term can have the most significant effect on m w generally up to 0 8 units following this investigation we make a series of recommendations for estimating microseismic m w using spectral methods noise should be estimated and removed from recordings and an attenuation correction should be applied the spectral level can be measured by spectral fitting or taken from the low frequency level significant factors in obtaining reliable microseismic mw estimates include using at least four receivers recording at 1000 hz and making radiation pattern corrections based on focal mechanism solutions not average values

joint inversion for earthquake depths using local waveforms and amplitude spectra of rayleigh waves reliable earthquake depth is fundamental to many seismological problems in this paper we present a method to jointly invert for centroid depths with local distance 5 seismic waveforms and regional distance of 5 15 rayleigh wave amplitude spectra on sparse networks we use earthquake focal mechanisms and magnitudes retrieved with the cut and paste cap method to compute synthetic amplitude spectra of fundamental mode rayleigh wave for a range of depths then we grid search to find the optimal depth that minimizes the joint misfit of amplitude spectra and local waveforms as case studies we apply this method to the 2008 wells nevada mw6 0 earthquake and a mw5 6 outer rise earthquake to the east of japan trench in 2013 uncertainties estimated with a bootstrap re sampling approach show that this joint inversion approach constrains centroid depths well which are also verified by independent teleseismic depth phase data

behavior and origin of helium neon argon and nitrogen from active faults compositional variations of gas species occluded in fracture zones type i within active faults are essentially the same as those in bubble gases type ii separating from mineral springs along faults the variation in type ii is hydrogeochemically interpreted as a result of dissolving and degassing of atmospheric air in groundwater at various temperatures the comparison of these variations in both types of gases suggests that ne ar and n2 in the type i gases are of atmospheric origin and their fluctuation is strongly controlled by the circulation of groundwater which recharges and discharges atmospheric air at various temperatures further hydrogeochemical evaluation shows that a part of he observed in fault zones is of radiogenic origin but its proportion is very small in active faults in these zones juvenile he emission related to the tectonic stresses causing earthquake may be substantially modified by the compositional fluctuation of gases controlled by the water circulation within the fault system accordingly monitoring of rare gases except for h2 from historically active faults may be less effective for earthquake prediction in contrast gas bubbles of high he content issuing from mineral springs which usually occur along prehistorically active faults are less contaminated by air and may provide some general precursory information concerning regional stresses at depth prior to an earthquake

observation of surface seismic activity changes of an alpine glacier during a glacier dammed lake outburst during the 2004 outburst flood of gornersee switzerland we observed surface microseismic activity so called icequakes near the glacier dammed lake on gornergletscher using surface waves we located near surface tensile fracturing events by inverting time delays using a quasi newton scheme we located 8520 near surface events with uncertainties of less than 10 m seismic activity increased during the lake drainage and the icequake epicenters migrated downglacier furthermore we identified events whose fault plane orientations are nearly perpendicular to the local pattern of surface crevassing these observations can be explained by the rotation of principal strains at the glacier surface deduced from local ice flow measurement which occurred during the flotation of the ice dam at the onset of the lake drainage in general we suggest that our technique is a useful tool to locate large data sets of microseismic events and can be used to monitor the temporal evolution of fracture processes and their dependence on glacier flow and glacier hydrology

crevasse patterns and the strain rate tensor a high resolution comparison values of the strain rate tensor represented at a 20 m length scale are found to explain the pattern and orientation of crevasses in a 0 13 km 2 reach of worthington glacier alaska u s a the flow field of the reach is constructed from surveyed displacements of 110 markers spaced 20 30 m apart a velocity gradient method is then used to calculate values of the principal strain rate axes at the nodes of a 20 m 20 m orthogonal grid crevasses in the study reach are of two types splaying and transverse and are everywhere normal to the trajectories of greatest most tensile principal strain rate splaying crevasses exist where the longitudinal strain rate x is 0 and transverse crevasses are present under longitudinally extending flow i e x 0 the orientation of crevasses changes in the down glacier direction but the calculated rotation by the flow field does not account for this change in orientation observations suggest that individual crevasses represent local values of the regional flow field and are transient on the time scale of 1 2 years they are not persistent features that are translated and rotated by flow crevasse patterns are thus found to be a useful tool for mapping the strain rate tensor in this reach of a temperate valley glacier

breaking the ice identifying hydraulically forced crevassing hydraulically forced crevassing is thought to reduce the stability of ice shelves and ice sheets affecting structural integrity and providing pathways for surface meltwater to the bed it can caus

on the disintegration of ice shelves the role of fracture dans une plateforme glaciaire contrairement au cas des glaciers les crevasses de surface peuvent atteindre le niveau de mer et les crevasses de fond remonter jusqu a ce niveau les crevasses jouent alors un role important dans la desintegration de la plateforme la fracturation est examinee en fonction des contraintes generales et locales on applique l etude a la stabilite de plateformes de l antarctique ouest

pressure dependence of viscosity of rhyolitic melts abstract viscosity of silicate melts is a critical property for understanding volcanic and igneous processes in the earth we investigate the pressure effect on the viscosity of rhyolitic melts using two methods indirect viscosity inference from hydrous species reaction in melts using a piston cylinder at pressures up to 2 8 gpa and direct viscosity measurement by parallel plate creep viscometer in an internally heated pressure vessel at pressures up to 0 4 gpa comparison of viscosities of a rhyolitic melt with 0 8 wt water at 0 4 gpa shows that both methods give consistent results in the indirect method viscosities of hydrous rhyolitic melts were inferred based on the kinetics of hydrous species reaction in the melt upon cooling i e the equivalence of rheologically defined glass transition temperature and chemically defined apparent equilibrium temperature the cooling experiments were carried out in a piston cylinder apparatus using hydrous rhyolitic samples with 0 8 4 wt water cooling rates of the kinetic experiments varied from 0 1 k s to 100 k s hence the range of viscosity inferred from this method covers 3 orders of magnitude the data from this method show that viscosity increases with increasing pressure from 1 gpa to 3 gpa for hydrous rhyolitic melts with water content 0 8 wt in the high viscosity range we also measured viscosity of rhyolitic melt with 0 13 wt water using the parallel plate viscometer at pressures 0 2 and 0 4 gpa in an internally heated pressure vessel the data show that viscosity of rhyolitic melt with 0 13 wt water decreases with increasing pressure combining our new data with literature data we develop a viscosity model of rhyolitic melts as a function of temperature pressure and water content

seismicity on the western greenland ice sheet surface fracture in the vicinity of active moulins we analyzed geophone and gps measurements collected within the ablation zone of the western greenland ice sheet during a 35 day period of the 2011 melt season to study changes in ice deformation before during and after a supraglacial lake drainage event during rapid lake drainage ice flow speeds increased to 400 of winter values and icequake activity peaked at times 7 days after drainage this seismicity developed variability over both diurnal and longer periods 10 days while coincident ice speeds fell to 150 of winter values and showed nightly peaks in spatial variability approximately 95 of all detected seismicity in the lake basin and its immediate vicinity was triggered by fracture propagation within near surface ice 330m deep that generated rayleigh waves icequakes occurring before and during drainage frequently were collocated with the down flow west end of the primary hydrofracture through which the lake drained but shifted farther west and outside the lake basin after the drainage we interpret these results to reveal vertical hydrofracture opening and local uplift during the drainage followed by enhanced seismicity and ice flow on the downstream side of the lake basin this region collocates with interferometric synthetic aperture radar measured speedup in previous years and could reflect the migration path of themeltwater supplied to the bed by the lake the diurnal seismic signal can be associated with nightly reductions in surface melt input that increase effective basal pressure and traction thereby promoting elevated strain in the surficial ice

seismic multiplet response triggered by melt at blood falls taylor glacier antarctica 1 meltwater input often triggers a seismic response from glaciers and ice sheets it is difficult however to measure melt production on glaciers directly while subglacial water storage is not directly observable therefore we document temporal changes in seismicity from a dry based polar glacier taylor glacier antarctica during a melt season using a synthesis of seismic observation and melt modeling we record icequakes using a dense six receiver network of three component geophones and compare this with melt input generated from a calibrated surface energy balance model in the absence of modeled surface melt we find that seismicity is well described by a diurnal signal composed of microseismic events in lake and glacial ice during melt events the diurnal signal is suppressed and seismicity is instead characterized by large glacial icequakes we perform network based correlation and clustering analyses of seismic record sections and determine that 18 of melt season icequakes are repetitive multiplets the epicentral locations for these multiplets suggest that they are triggered by meltwater produced near a brine seep known as blood falls our observations of the correspondingp wave first motions are consistent with volumetric source mechanisms we suggest that surface melt enables a persistent pathway through this cold ice to an englacial fracture system that is responsible for brine release episodes from the blood falls seep the scalar moments for these events suggest that the volumetric increase at the source region can be explained by melt input

the role of hydrologically driven ice fracture in drainage system evolution on an arctic glacier 1 observations from ellesmere island suggest that the connection between surface and subglacial drainage on a predominantly cold glacier is made abruptly by hydrologically driven propagation of fractures from the surface to the bed where ice is 150 m thick water ponded to a depth of 6 9 m within a supraglacial stream system before establishing a permanent bed connection multiple premonitory drainage events preceded the final drainage of ponded water implying that fracturing is necessary but insufficient to establish a permanent link between surface and subglacial drainage refreezing of water that penetrates the first fractures to form may reseal the connection while flow resistance within the subglacial system may delay the onset of continuous through flow a large volume of ponded water is required to enlarge fractures sufficiently by melting to maintain continuous drainage while feedbacks between subglacial hydrology and ice dynamics may assist in maintaining the connection and initiating subglacial outflow

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