tide modulated ice flow variations drive seismicity near the calving front of bowdoin glacier greenland glacier microseismicity is a promising tool to study glacier dynamics however physical processes connecting seismic signals and ice dynamics are not clearly understood at present particularly the relationship between tide modulated seismicity and dynamics of calving glaciers remains elusive here we analyze records from an on ice seismometer placed 250 m from the calving front of bowdoin glacier greenland using high frequency glacier flow speed measurements we show that the microseismic activity is related to strain rate variations the seismic activity correlates with longitudinal stretching measured at the glacier surface both higher melt rates and falling tides accelerate glacier motion and increase longitudinal stretching long term microseismic monitoring could therefore provide insights on how a calving glacier s force balance and flow regime react to changes at the ice ocean interface

seismicity related to the glacier of cotopaxi volcano ecuador 1 significant seismic activity is generally recorded on volcanoes covered by an icecap this work was carried out in order to quantify the role of the glaciers in the generation of seismicity for cotopaxi volcano we compared the seismic activity registered on the glacier and on the rock near the snout of the north flank glacier we focused on the analysis of low frequency events 5 hz similar to volcanic lp events when recorded on rock base the particle motion analysis helps to estimate source locations which are distributed in crevasses areas high incident angles suggest a superficial origin these events are interpreted as icequakes for which we propose as source mechanism a fluid driven crack model triggered by ice cracking or hydraulic transients the low quality factor values estimated are consistent with the resonance of an ice crack filled with water this work shows that low frequency icequakes can be confusingly taken as volcanic lp events

microscale evidence of liquefaction and its potential triggers during soft bed deformation within subglacial traction tills published conceptual models argue that much of the forward motion of modern and ancient glaciers is accommodated by deformation of soft sediments within the underlying bed at a microscale this deformation results in the development of a range of ductile and brittle structures in water saturated sediments as they accommodate the stresses being applied by the overriding glacier detailed micromorphological studies of subglacial traction tills reveal that these polydeformed sediments may also contain evidence of having undergone repeated phases of liquefaction followed by solid state shear deformation this spatially and temporally restricted liquefaction of subglacial traction tills lowers the shear strength of the sediment and promotes the formation of transient mobile zones within the bed which accommodate the shear imposed by the overriding ice this process of soft bed sliding alternating with bed deformation facilitates glacier movement by way of stick slip events the various controls on the slip events have previously been identified as i the introduction of pressurised meltwater into the bed a process limited by the porosity and permeability of the till and ii pressurisation of porewater as a result of subglacial deformation to which we include iii episodic liquefaction of water saturated subglacial traction tills in response to glacier seismic activity icequakes which are increasingly being recognized as significant processes in modern glaciers and ice sheets as liquefaction operates only in materials already at very low values of effective stress its process form signatures are likely indicative of glacier sub marginal tills

controls on plate motion by oscillating tidal stress evidence from deep tremors in western japan deep tremors are considered to be shear slip on the plate interface and are often sensitive to tidal stress here we demonstrate that a small cluster of tremors is extremely well correlated with tide levels observed at nearby stations in western japan the correlation is interpreted as representing a nonlinear relationship between stress and slip which is similar to the rate dependent friction law an empirical relationship and observed tide records explain the temporal changes in tremor activity over 9 years the combination of the nonlinear fault rheology and oscillating tidal stress may control temporal changes in plate motion and earthquake occurrence remarkably the background seismicity is similar to the predicted tremor rate obtained from tidal observations over the past 50 years this mechanism may also explain the seasonality and decadal scale weak periodicity of large earthquakes and is likely to be helpful in probabilistic forecasting of future seismicity

pattern of cryospheric seismic events observed at ekstrom ice shelf antarctica mobility of glaciers such as rapid retreat or disintegration of large ice volumes produces a large variety of different seismic signals thus evaluating cryospheric seismic events e g changes of their occurrence in space and time allows to monitor glacier dynamics we analyze a one year data span recorded at the neumayer seismic network in antarctica events are automatically recognized using hidden markov models in this study we focused on a specifc event type occurring close to the grounding line of the ekstrom ice shelf observed waveform characteristics are consistent with an initial fracturing followed by the resonance of a water filled cavity resulting in a so called hybrid event the number of events detected strongly correlates with dominant tide periods we assume the cracking to be driven by existing glacier stresses through bending voids are then filled by sea water exciting the observed resonance in agreement with this model events occur almost exclusively during rising tides where cavities are opened at the bottom of the glacier i e at the sea ice interface

measured and simulated flow near a submerged spur dike to improve knowledge of the flow and scour processes associated with spur dikes three dimensional flow velocities were measured at 2 592 points using an acoustic doppler velocimeter over a fixed flat bed with a trapezoidal shaped submerged spur dike in a laboratory flume general velocity distribution and detailed near field flow structures were revealed by the measurements and numerical simulations performed using a free surface turbulent flow model with a k e closure scheme the three dimensional flow separation characterized in this study was found to yield forces on the bed that were significantly different from nonsubmerged vertical obstructions that have been measured in other studies values of bed shear stress derived from both measured and simulated values were similar but indicated that local scour would be initiated in one rather than in the two locations of initial local scour measured in previous experiments with a similar flow

tidal forcing of basal seismicity of ice stream c west antarctica observed far inland the seismicity rate beneath the downglacier 85 km of ice stream c west antarctica is modulated by the tide the tide beneath the ross ice shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude this tidal forcing travels up the ice stream as an attenuating wave at approximately 1 6m s 1 and is detectable 85km from the grounding line we successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity and thickness hb and calculate that the substrate has an apparent stiffness hb of o 108 pa s m 1 this finding suggests that the conditions of the till layer at the bed of ice stream c are similar to those of ice stream b and that the reason for the recent stagnation of ice stream c is other than loss of till we further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress

the search for seismic signatures of movement at the glacier bed in a polythermal valley glacier a passive seismology experiment was conducted across the main overdeepening of storglaciaren in the tarfala valley northern sweden to investigate the spatial and temporal distribution of basal microseismic waveforms in relation to known dynamics of this small polythermal sub arctic glacier the high ablation rate made it difficult to keep geophones buried and well coupled to the glacier during the experiment and reduced the number of days of good quality data collection the characterization of typical and atypical waveforms showed that the dominant waveforms were from near surface events such as crevassing waveforms resembling basal microseismic signals were very rare and seldom observed on more than two seismic stations simultaneously the analysis of waveforms amplitudes and particle motions suggested a near field origin for most events even though basal sliding is known to occur in the overdeepening no convincing examples of basal waveforms were detected suggesting basal microseismic signals are rare or difficult to detect beneath polythermal glaciers like storglaciaren we discuss the reasons for failing to locate basal signals consider the origin of common waveforms and make recommendations for setting up passive seismology experiments on glaciers with high ablation rates

calving flux estimation from tsunami waves abstract measuring glacier calving magnitude frequency and location in high temporal resolution is necessary to understand mass loss mechanisms of ocean terminating glaciers we utilized calving generated tsunami signals recorded with a pressure sensor for estimating the calving flux of bowdoin glacier in northwestern greenland we find a relationship between calving ice volume and wave amplitude this relationship was used to compute calving flux variation the calving flux showed large spatial and temporal fluctuations in july 2015 and in july 2016 with a mean flux of 2 3 0 15 105 m3 d 1 calving flux was greater during periods of fast ice flow high air temperature and at low falling tide indicating the importance of increased longitudinal strain due to glacier acceleration and or submarine melting at the calving front long term measurements with the method introduced here are promising for understanding the complex interplay of ice dynamics melting and calving at glacier fronts

localised thickening and grounding of an antarctic ice shelf from tidal triggering and sizing of cryoseismicity abstract we observe remarkably periodic patterns of seismicity rates and magnitudes at the fimbul ice shelf east antarctica correlating with the cycles of the ocean tide our analysis covers 19 years of continuous seismic recordings from antarctic broadband stations seismicity commences abruptly during austral summer 2011 at a location near the ocean front in a shallow water region dozens of highly repetitive events occur in semi diurnal cycles with magnitudes and rates fluctuating steadily with the tide in contrast to the common unpredictability of earthquake magnitudes the event magnitudes show deterministic trends within single cycles and strong correlations with spring tides and tide height the events occur quasi periodically and the highly constrained event sources migrate landwards during rising tide we show that a simple mechanical model can explain most of the observations our model assumes stick slip motion on a patch of grounded ice shelf which is forced by the variations of the ocean tide height and ice flow the well fitted observations give new insights into the general process of frictional triggering of earthquakes while providing independent evidence of variations in ice shelf thickness and grounding

tides modulate crevasse opening prior to a major calving event at bowdoin glacier northwest greenland retreat of calving glaciers worldwide has contributed substantially to sea level rise in recent decades mass loss by calving contributes significantly to the uncertainty of sea level rise projections at bowdoin glacier northwest greenland most calving occurs by a few large events resulting from kilometre scale fractures forming parallel to the calving front high resolution terrestrial radar interferometry data of such an event reveal that crevasse opening is fastest at low tide and accelerates during the final 36 h before calving using the ice flow model elmer ice we identify the crevasse water level as a key driver of modelled opening rates sea water level variations in the range of local tidal amplitude 1 m can reproduce observed opening rate fluctuations provided crevasse water level is at least 4 m above the low tide sea level the accelerated opening rates within the final 36 h before calving can be modelled by additional meltwater input into the crevasse enhanced ice cliff undercutting by submarine melt ice damage increase due to tidal cyclic fatigue crevasse deepening or a combination of these processes our results highlight the influence of surface meltwater and tides on crevasse opening leading to major calving events at grounded tidewater glaciers such as bowdoin

characterization of seasonal glacial seismicity from a single station on ice record at holtedahlfonna svalbard glacial seismicity provides important insights into glacier dynamic processes we study the temporal distribution of cryogenic seismic signals icequakes at holtedahlfonna svalbard between april

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

short lived ice speed up and plume water flow captured by a vtol uav give insights into subglacial hydrological system of bowdoin glacier abstract the subglacial hydrology of tidewater glaciers is a key but poorly understood component of the complex ice ocean system which affects sea level rise as it is extremely difficult to access the interior of a glacier our knowledge relies mostly on the observation of input variables such as air temperature and output variables such as the ice flow velocities reflecting the englacial water pressure and the dynamics of plumes reflecting the discharge of meltwater into the ocean in this study we use a cost effective vertical take off and landing vtol unmanned aerial vehicle uav to monitor the daily movements of bowdoin glacier north west greenland and the dynamics of its main plume using structure from motion photogrammetry and feature tracking techniques we obtained 22 high resolution ortho images and 19 velocity fields at the calving front for 12 days in july 2016 our results show a two day long speed up event up to 170 caused by an increase in buoyant subglacial forces with a strong spatial variability revealing that enhanced acceleration is an indication of shallow bedrock further we used the particle image velocimetry piv method to analyze water flow from successive uav images taken while flying over the main plume of the glacier we found that piv successfully captures the area of radially diverging flow of the plume and provides information on spatial and time variability as no other remote sensing technique can most interestingly the active part of the plume features pulsating water jets at the time scale of seconds and is 1 to 5 times smaller than its visual footprint defined by the iceberg free area combined with an ice flow model or a non steady plume model our approach has the potential to generate a novel set of input data to gather information about the depth of the bedrock the discharge of meltwater or the subglacial melting rate of tidewater glaciers

tidal and spatial variability of flow speed and seismicity near the grounding zone of beardmore glacier antarctica gps measurements of tidal modulation of ice flow and seismicity within the grounding zone of beardmore glacier show that tidally induced fluctuations of horizontal flow are largest near the grounding line and decrease downstream seismic activity is continuous but peaks occur on falling and rising tides beamforming methods reveal that most seismic events originate from two distinct locations one on the grid north side of the grounding zone and one on the grid south side the broad pattern of deformation generated as beardmore glacier merges with the ross ice shelf results in net extension along the grid north side of the grounding zone and net compression along the grid south side during falling tides seismic activity peaks on both sides because of increased vertical flexure across the grounding line during rising tides seismic activity in the region of extension on the grid north side is relatively low because the tidal influence on both horizontal strain rate and vertical flexure is small on the grid south side during rising tides however tidally induced horizontal strain rates promote increased seismicity in regions of long term compressional flow paths our study highlights how concurrent geodetic and seismic measurements provide insight into grounding zone mechanics and their influence on ice shelf buttressing

local scour associated with angled spur dikes a series of experiments were conducted in which the volume of the scour hole associated with model spur dikes was measured in a laboratory flume under clear water overtopping flows spur dike models were angled at 45 90 and 135 to the downstream channel sidewall with contraction ratios of 0 125 and 0 250 the main goals of the experiments were to evaluate the effect of the three angles on the volume of scour and potential aquatic habitat and on minimizing erosion adjacent to the streambanks the experiments showed that of the three angles tested the least erosion of the bed in the near bank region was associated with the spur dikes with 90 angles while the greatest volume of the scour hole was associated with the 135 spur dikes it was concluded that spur dikes with 135 angles showed the best potential for providing improved aquatic habitats while minimizing the potential for erosion of the channel bank

magma fragmentation by rapid decompression the processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology observations of explosive eruptions1 3 as well as theoretical analyses of the underlying processes4 have raised the question of whether the rapid decompression of highly viscous vesicular magma that results from the collapse of a lava dome or volcanic edifice can in itself produce explosive magma fragmentation and pyroclast formation here we report the results of a laboratory investigation of pyroclast formation following rapid decompression samples of magma from the 1980 eruption of mount st helens when rapidly depressurized from initial pressures of up to 12 mpa and at temperatures in the range 750 825 c fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions moreover we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events

subglacial discharge at tidewater glaciers revealed by seismic tremor subglacial discharge influences glacier basal motion and erodes and redeposits sediment at tidewater glacier termini discharge drives submarine terminus melting affects fjord circulation and is a central component of proglacial marine ecosystems however our present inability to track subglacial discharge and its variability significantly hinders our understanding of these processes here we report observations of hourly to seasonal variations in 1 5 10 hz seismic tremor that strongly correlate with subglacial discharge but not with basal motion weather or discrete icequakes our data demonstrate that vigorous discharge occurs from tidewater glaciers during summer in spite of fast basal motion that could limit the formation of subglacial conduits and then abates during winter furthermore tremor observations and a melt model demonstrate that drainage efficiency of tidewater glaciers evolves seasonally glaciohydraulic tremor provides a means by which to quantify subglacial discharge variations and offers a promising window into otherwise obscured glacierized environments

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

on the turbulent flow structure around an instream structure with realistic geometry we investigate the flow dynamics around a rock vane a widely used instream structure for stream restoration by conducting laboratory flume experiments and carrying out high resolution large eddy simulation les taking advantage of parallel computing the flume experiments are conducted under fixed and mobile bed conditions where the velocities and bed elevations are measured respectively the les is carried out for the fixed bed experiment by directly resolving the details of the rocks that constitute the vane and the individual roughness elements on the channel bed the les computed mean flow statistics show good agreement with the measurements and the analysis of the computed flow field reveals the existence of two counter rotating secondary flow cells downstream of the vane which originate from the plunging of the three dimensional streamlines onto a lower part of the sidewall downstream of the vane to further examine the role of the secondary flow cells under a mobile bed condition the les results are compared with the equilibrium bed elevation measured in the mobile bed experiment the mobile bed experiment reveals the existence of an oblique sand ridge downstream of the vane that is aligned with the line of flow convergence caused by the collision of the two secondary flow cells the results indicate that the two counter rotating cells downstream of the rock vane has a profound impact on the mean flow and bed shear stress as well as on the bed morphodynamics this article is protected by copyright all rights reserved

geometry of scour holes associated with 90 spur dikes the stable pools formed by spur dikes can be a significant factor for improving aquatic habitat in unstable degraded streams prediction of the volume and geometry of the scour hole associated with a spur dike yields information that is valuable for assessing the potential benefit to the aquatic habitat volumes of the scour hole in the vicinity of model spur dikes were measured in a laboratory flume under clear water overtopping flows spur dike length flow depth and shear velocity ratio were varied in the experiments and found to significantly influence the volume of the scour hole for overtopping flows the ratio of the flow depth to the spur dike height was found to be an important control on the geometry of the resulting scour hole at higher flow depth to spur dike height ratios the location of the maximum depth of scour changed from the upstream point of the dike toward the channel bank and caused a secondary scour zone to form downstream of the spur dike a preliminary technique is proposed to predict the volume of scour for spur dikes perpendicular to the bank

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

tidal influence on rutford ice stream west antarctica observations of surface flow and basal processes from closely spaced gps and passive seismic stations high resolution surface velocity measurements and passive seismic observations from rutford ice stream west antarctica 40 km upstream from the grounding line are presented these measurements indicate a complex relationship between the ocean tides and currents basal conditions and ice stream flow both the mean basal seismicity and the velocity of the ice stream are modulated by the tides seismic activity increases twice during each semi diurnal tidal cycle the tidal analysis shows the largest velocity variation is at the fortnightly period with smaller variations superimposed at diurnal and semi diurnal frequencies the general pattern of the observed velocity is two velocity peaks during each semi diurnal tidal cycle but sometimes three peaks are observed this pattern of two or three peaks is more regular during spring tides when the largest amplitude velocity variations are observed than during neap tides this is the first time that velocity and level of seismicity are shown to correlate and respond to tidal forcing as far as 40 km upstream from the grounding line of a large ice stream

the explosive eruption of soufriere hills volcano montserrat west indies 17 september 1996 on 17 september 1996 the soufriere hills volcano started a 9 hour period of dome collapse involving 11 7 106 m dre of lava after 2 5 hours of quiescence a sustained explosive eruption began estimated eruption parameters are plume height at least 11 3 km and a maximum of 15 km 180 m s for launch velocities of ballistic clasts peak explosion pressure of 27 5 mpa magma water contents of 2 5 5 magma discharge rates from 2300 to 4300 m s ejecta volume of 3 2 0 9 106 m dre ejecta consists of pumice 1160 kg m higher density vesiculated ejecta 1300 to 2000 kg m dense glassy clasts 2600 kg m breccias cut by tuffisite veins and hydrothermally altered lithics the ejecta are interpreted as a conduit assemblage with evacuation of the conduit down to depths of about 3 to 5 km the eruption was triggered by unloading of a gas pressurised conduit due to dome collapse

tidal and seasonal variations in calving flux observed with passive seismology the seismic signatures of calving events i e calving icequakes offer an opportunity to examine calving variability with greater precision than is available with other methods here using observations from yahtse glacier alaska we describe methods to detect locate and characterize calving icequakes we combine these icequake records with a coincident manually generated record of observed calving events to develop and validate a statistical model through which we can infer iceberg sizes from the properties of calving icequakes we find that the icequake duration is the single most significant predictor of an iceberg s size we then apply this model to 18 months of seismic recordings and find elevated iceberg calving flux during the summer and fall and a pronounced lull in calving during midwinter calving flux is sensitive to semidiurnal tidal stage large calving events are tens of percent more likely during falling and low tides than during rising and high tides consistent with a view that deeper water has a stabilizing influence on glacier termini multiple factors affect the occurrence of mechanical fractures that ultimately lead to iceberg calving at yahtse glacier seismology allows us to demonstrate that variations in the rate of submarine melt are a dominant control on iceberg calving rates at seasonal timescales on hourly to daily timescales tidal modulation of the normal stress against the glacier terminus reveals the nonlinear glacier response to changes in the near terminus stress field

investigating the dynamics of an alpine glacier using probabilistic icequake locations triftgletscher switzerland 1 in order to improve our understanding of the dynamics of potentially unstable steep glacier tongues we monitored during summer 2010 the micro seismicity of triftgletscher switzerland our system comprising 8 three component seismometers coupled with the ice surface was installed upstream of the glacier s tongue which is likely to evolve toward an unstable regime complementary surface motion and proglacial runoff measurements allowed the icequake activity to be interpreted in terms of glacier dynamics and hydraulics the strong contrast in seismic wave velocities due to the underlying bedrock was taken into account using a three dimensional 3 d velocity model implemented in a nonlinear probabilistic location procedure allowing to accurately define the hypocenter uncertainty we located 120 icequakes with a focal depth accuracy that allowed distinguishing between shallow events 87 events and near bedrock icequakes 33 events the first motions of most of the deep events argue against pure shear sources expected in case of stick slip motion and our suggested source mechanism is a superimposed tensile crack and shear dislocation the analysis of surface strain showed that near surface events represent the opening and transverse extension of surface crevasses in a confluent flow regime

tide modulated ice motion and seismicity of a floating glacier tongue in east antarctica we recorded the ice motion and icequakes on the floating part of langhovde glacier in east antarctica to better understand the dynamic behavior of ice shelves and floating tongues diurnal and semi diurnal variations in ice motion and seismicity were simultaneously observed at all four global navigation satellite system and three seismic stations over 2 weeks the short term along flow ice motion is explained by the elastic response of the glacier to ocean tide induced hydrostatic stress variations which decayed at a rate of 0 8 km 1 toward the grounding line we observed a large number of icequakes during mid rising and high tides that covered a broad frequency range and formed two major groups of events centered at 10 and 120 hz respectively the hourly occurrence rates were 500 events h 1 with the observed seismicity consistent with fracture due to floating tongue bending we also observed minor secondary peaks at high ice speeds which could reflect surface cracking due to stretching or basal friction our observation demonstrates that tidal modulation was the main factor to fracture the floating tongue of langhovde glacier

flow field at a vertical wall abutment experiments were conducted to measure the three dimensional turbulent flow field using the acoustic doppler velocimeter at a short vertical wall abutment ratio of abutment length to approach flow depth less than unity before and after the development of a scour hole under a clear water scour condition in the upstream the presentation of flow field through vectors at vertical sections shows a primary vortex associated with the downflow in the downstream the upward flow is comprised of with irregularities owing to the vortex shedding the flow separation near the bed and within the scour hole is evident from the turbulent kinetic energy distribution using reynolds stresses the bed shear stresses are calculated

intermediate depth icequakes and harmonic tremor in an alpine glacier glacier d argentiere france evidence for hydraulic fracturing we detected several thousand deep englacial icequakes on glacier d argentiere mont blanc massif between 30 march and 3 may 2012 these events have been classified in eight clusters inside each cluster the waveforms are similar for p waves and s waves although the time delay between the p waves and s waves vary by up to 0 03 s indicating an extended source area although these events were recorded by a single accelerometer they were roughly located using a polarization analysis the deepest events were located at a depth of 130 m 60 m above the ice bed interface the clusters are separated in space the largest cluster extends over about 100 m for this cluster the strike of the rupture plane is nearly parallel to the direction of the open crevasses and the dip angle is 56 deep icequakes occur in bursts of activity that last for a few hours and are separated by quiet periods many events occurred on 28 29 april 2012 during the warmest days when snow melting was likely important the distribution of events in time and space obeys a power law as also observed for earthquakes but with larger exponents the polarity of the p waves for all of the events is consistent with tensile faulting finally between 25 april and 3 may we observed a gliding harmonic tremor with a fun damental resonance frequency that varied between 30 hz and 38 hz with additional higher frequency harmonics during this time we also observed shallow hybrid events with high frequency onsets and a monochromatic coda these events might be produced by the propagation of fractures and the subsequent flow of water into the fracture the strongest resonance was observed just after a strong burst of deep icequakes and during an un usually warm period when the snow height decreased by 60 cm in one week the res onance frequency shows a succession of several sharp decreases and phases of progressive increases one of the strongest negative steps of the resonance frequency on 28 april coincides with a burst of deep icequakes these events appear to be associated with the propagation of fractures which can explain the decrease in the resonance frequency finally we observed an acceleration of glacier flow on 29 april and suggested that melt water had reached the ice bed interface these observations suggest that deep icequakes are due to hydraulic fracturing and that they can be used to track fluid flow inside glaciers

basal icequakes during changing subglacial water pressures beneath gornergletscher switzerland using dense networks of three component seismometers installed in direct contact with the ice the seismic activity of gornergletscher switzerland was investigated during the summers of 2004 and 2006 as subglacial water pressures varied drastically these pressure variations are due to the diurnal cycle of meltwater input as well as the subglacial drainage of gornersee a nearby marginal ice dammed lake up to several thousand seismic signals per day were recorded whereas most icequakes are due to surface crevasse openings about 200 events have been reliably located close to the glacier bed these basal events tend to occur in clusters and have signals with impulsive first arrivals at the same time basal water pressures and ice surface velocities were measured to capture the impact of the lake drainage on the subglacial hydrological system and the ice flow dynamics contrary to our expectations we did not observe an increase of basal icequake activity as the lake emptied thereby raising the subglacial water pressures close to the flotation level for several days in fact the basal icequakes were usually recorded during the morning hours when the basal water pressure was either low or decreasing during the high pressure period caused by the drainage of the lake no basal icequakes were observed furthermore gps measurements showed that the glacier surface was lowering during the basal seismic activity these observations lead us to conclude that such icequakes are connected to the diurnal variation in glacier sliding across the glacier bed

mean flow and turbulence around a laboratory spur dike the three dimensional turbulent flow field around a spur dike in a plane fixed bed laboratory open channel was studied experimentally using a microacoustic doppler velocimeter mean and turbulence characteristics in all three spatial directions were evaluated at upstream and downstream cross sections near the dike results showed that the primary flow separated in both lateral and vertical directions two counter rotating flow circulations consisting of the lateral and vertical velocity components originated at the dike section downstream of the dike the circulation in the flow separation zone is stronger than the one in the contracted primary flow zone the maximum bed shear stresses estimated using reynolds stresses is about three times as large as the mean bed shear stress of incoming flow

seismic detection and analysis of icequakes at columbia glacier alaska 1 contributions to sea level rise from rapidly retreating marine terminating glaciers are large and increasing strong increases in iceberg calving occur during retreat which allows mass transfer to the ocean at a much higher rate than possible through surface melt alone to study this process we deployed an 11 sensor passive seismic network at columbia glacier alaska during 2004 2005 we show that calving events generate narrow band seismic signals allowing frequency domain detections detection parameters were determined using direct observations of calving and validated using three statistical methods and hypocenter locations the 1 3 hz detections provide a good measure of the temporal distribution and size of calving events possible source mechanisms for the unique waveforms are discussed and we analyze potential forcings for the observed seismicity

scale effects in flume experiments on flow around a spur dike in flatbed channel this paper presents the findings from a series of flume experiments conducted to determine scale effects in small scale models of flow around a single spur dike wing dam groyne or abutment placed in a channel whose bed is fixed and flat the flow features of primary interest are flow thalweg alignment line of maximum streamwise velocity around a dike and area extent of the flow separation region wake immediately downstream of the dike those features are of practical concern in the deployment of dikes for various channel control purposes the scale effects of concern herein are those attributable to use small length scales together with a bed shear stress parameter e g shields parameter as the primary criterion for dynamic similitude small scale models especially micromodels often are used for investigating channel control issues also the shear stress criterion is commonly used for models of flow in loose bed channels whereas froude number commonly is the primary similitude criterion for models of fixed bed open channel flows the experiments show that use of a shear stress parameter as the primary criterion for dynamic similitude influences the flow thalweg and flow separation region at a dike it does so by distorting pressure gradients around the model dike and by affecting turbulence generated by the dike it also is shown that for a range of small models thalweg alignment and extent of separation region do not scale with model length scales these findings are important for interpreting results from small hydraulic models especially micromodels of flow in loose bed channels

tide induced microseismicity in the mertz glacier grounding area east antarctica the deployment of a seismic network along the adelie and george v coasts in east antarctica during the period 2009 2012 provides the opportunity to monitor cryoseismic activity and to obtain new insights on the relationship between tidal cycles and coastal glacier dynamics here we focus on records from a seismometer located on a rocky outcrop in the vicinity of the grounding line of the 35 km broad mertz glacier a major outflow of this region we detect numerous icequakes 50 000 events within 10 months and up to 100 events h and demonstrate their clear tidal modulation we suggest that they result from ice friction and fracturing around the rocky peak and from the glacier flexure in response to the falling and rising tides at its grounding area we propose that such icequake monitoring could be used as a climate proxy since grounding lines are subject to migrate with sea level changes

an investigation into the forces that drive ice shelf rift propagation on the amery ice shelf east antarctica for three field seasons 2002 03 2004 05 2005 06 we have deployed a network of gps receivers and seismometers around the tip of a propagating rift on the amery ice shelf east antarctica during these campaigns we detected seven bursts of episodic rift propagation to determine whether these rift propagation events were triggered by short term environmental forcings we analyzed simultaneous ancillary data such as wind speeds tidal amplitudes and sea ice fraction a proxy variable for ocean swell we find that none of these environmental forcings separately or together correlated with rift propagation this apparent insensitivity of ice shelf rift propagation to short term environ mental forcings leads us to suggest that the rifting process is primarily driven by the internal glaciological stress our hypothesis is supported by order of magnitude calculations that the glaciological stress is the dominant term in the force balance however our calculations also indicate that as the ice shelf thins or the rift system matures and iceberg detachment becomes imminent short term stresses due to winds and ocean swell may become more important

coherent structure dynamics in turbulent flows past in stream structures some insights gained via numerical simulation large scale coherent vortical structures in natural streams and rivers dominate flow and transport processes and impact the stability of stream banks the diversity and abundance of organisms and the quality of running waters in aquatic ecosystems thus understanding and being able to model the dynamics of energetic coherent structures in such flows at ecologically relevant scales are crucial prerequisites for developing a science based ecosystem restoration framework we review recent progress toward the development of coherent structure resolving csr computational fluid dynamics techniques based on hybrid urans les modeling strategies for simulating turbulent flows in open channels with hydraulic structures csr simulations of the turbulent horseshoe vortex thsv past bed mounted piers explained the physical mechanism leading to the experimentally documented bimodal velocity fluctuations of the vortex and underscored the importance of the reynolds number as a key parameter governing the thsv dynamics simulations of high reynolds number flows past surface piercing groynelike structures in open channels revealed the complexity of the recirculating region at the upstream face of the groyne underscored the interaction of the flow in this region with the energetic shear layer shed from the point of separation at the upstream side wall and demonstrated the importance of flow depth in the vorticity dynamics of such flows the paper also identifies areas for future work and modeling challenges that need to be addressed for the computational tools to be able to accurately predict flow and transport processes in real life aquatic environments

sustained seismic tremors and icequakes detected in the ablation zone of the greenland ice sheet during summer 2011 seismic activity in the ablation zone of the western greenland ice sheet gris was monitored using a network of three component seismometers the seismic record includes a large variety of icequakes and seismic tremors that demonstrate a clear correlation with subglacial water flow we verified the existence of well known shallow icequakes related to surface crevasse formation deep icequakes located at 100 160 m depth and narrow banded short term seismic tremors tens of seconds in duration in addition we present previously unreported long term tremors lasting several hours using attenuation of the measured tremor amplitude we locate the epicentre of this long term tremor to a large moulin within our study area between 3 and 11 hz our continuous seismic record is dominated by this moulin tremor and shows strong correlation with the water level of the generating moulin we argue that monitoring of icequake and glacial tremor sources bears high potential for investigating glacier hydraulics and dynamics and is thus an ideal supplement to traditional glaciological measurements

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

calving seismicity from iceberg sea surface interactions 1 iceberg calving is known to release substantial seismic energy but little is known about the specific mechanisms that produce calving icequakes at yahtse glacier a tidewater glacier on the gulf of alaska we draw upon a local network of seismometers and focus on 80 hours of concurrent direct observation of the terminus to show that calving is the dominant source of seismicity to elucidate seismogenic mechanisms we synchronized video and seismograms to reveal that the majority of seismic energy is produced during iceberg interactions with the sea surface icequake peak amplitudes coincide with the emergence of high velocity jets of water and ice from the fjord after the complete submergence of falling icebergs below sea level these icequakes have dominant frequencies between 1 and 3 hz detachment of an iceberg from the terminus produces comparatively weak seismic waves at frequencies between 5 and 20 hz our observations allow us to suggest that the most powerful sources of calving icequakes at yahtse glacier include iceberg sea surface impact deceleration under the influence of drag and buoyancy and cavitation numerical simulations of seismogenesis during iceberg sea surface interactions support our observational evidence our new understanding of iceberg sea surface interactions allows us to reattribute the sources of calving seismicity identified in earlier studies and offer guidance for the future use of seismology in monitoring iceberg calving

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

flow and turbulence structure around a spur dike in a channel with a large scour hole 1 the flow and turbulence structure around a vertical wall spur dike that extends over the whole depth of a straight channel are investigated using detached eddy simulation des the channel reynolds number in des and the corresponding experiment is red 2 4 105 which is typical of applications in small rivers and outside the range where well resolved large eddy simulation les can be conducted the bathymetry at equilibrium scour conditions is obtained from a loose bed experiment this paper discusses the main mechanisms which drive the growth of the scour hole upstream and downstream from the spur dike during the later stages of the scour process and how these mechanisms change between the initial flat bed conditions and later stages of the scour process scale effects are investigated by comparing simulation results at red 2 4 105 with those from simulations performed at a much lower reynolds number red 18 000 results show that while the structure of the horseshoe vortex hv system changes with respect to the case in which the bed is flat the main necklace vortex of the hv system is still subject to large scale aperiodic oscillations similar to the ones observed in flows past in stream bluff body obstacles mounted on a flat surface present results show that the amplification of the horizontal vorticity within the lower part of the separated shear layer ssl and the associated formation of streaks of high bed shear stress below the region where the ssl eddies are convected in the near bed region is a general feature of high reynolds number flow past a vertical wall spur dike placed in a loose bed channel at all stages of the scour process

glacier dynamics near the calving front of bowdoin glacier northwestern greenland to better understand recent rapid recession of marine terminating glaciers in greenland we performed satellite and field observations near the calving front of bowdoin glacier a 3 km wide outlet glacier in northwestern greenland satellite data revealed a clear transition to a rapidly retreating phase in 2008 from a relatively stable glacier condition that lasted for 20 years ice radar measurements showed that the glacier front is grounded but very close to the floating condition these results in combination with the results of ocean depth soundings suggest bed geometry in front of the glacier is the primary control on the rate and pattern of recent rapid retreat presumably glacier thinning due to atmospheric and or ocean warming triggered the initial retreat in situ measurements showed complex short term ice speed variations which were correlated with air temperature precipitation and ocean tides ice speed quickly responded to temperature rise and a heavy rain event indicating rapid drainage of surface water to the bed semi diurnal speed peaks coincided with low tides suggesting the major role of the hydrostatic pressure acting on the calving face in the force balance these observations demonstrate that the dynamics of bowdoin glacier are sensitive to small perturbations occurring near the calving front