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

seismic moulin tremor through glacial moulins meltwater is routed from the glacier surface to its base moulins are a main feature feeding subglacial drainage systems and thus influencing basal motion and ice dynamics but their geometry remains poorly known here we show that analysis of the seismic wavefield generated by water falling into a moulin can help constrain its geometry we present modeling results of hour long seimic tremors emitted from a vertical moulin shaft observed with a seismometer array installed at the surface of the greenland ice sheet the tremor was triggered when the moulin water level exceeded a certain height which we associate with the threshold for the waterfall to hit directly the surface of the moulin water column the amplitude of the tremor signal changed over each tremor episode in close relation to the amount of inflowing water the tremor spectrum features multiple prominent peaks whose characteristic frequencies are distributed like the resonant modes of a semiopen organ pipe and were found to depend on the moulin water level consistent with a source composed of resonant tube waves water pressure waves coupled to elastic deformation of the moulin walls along the water filled moulin pipe analysis of surface particle motions lends further support to this interpretation the seismic wavefield was modeled as a superposition of sustained wave radiation by pressure sources on the side walls and at the bottom of the moulin the former was found to dominate the wave field at close distance and the latter at large distance to the moulin

experiments on saltation of sand in water a high speed video system was used to study saltation of sand in an open channel flow high rates of image acquisition provided the information needed to resolve details of the lagrangian properties of particle motion saltation of sand is described in terms of statistical properties of particle trajectories such as mean values and standard deviations of saltation length height and streamwise particle velocity results are compared with available empirical data saltation height seems to be independent of particle size when made dimensionless with the particle diameter however the dimensionless saltation length appears to increase as the particle size decreases particle collision with the bed is also analyzed observations of this process indicate that most of the interactions between saltating particles and the bed are of the collision rebound type which contradicts previous discussions on the subject the estimated value of the dynamic friction coefficient is about half that proposed by bagnold i

diurnal seismicity cycle linked to subsurface melting on an ice shelf seismograms acquired on the mcmurdo ice shelf antarctica during an austral summer melt season november 2016 january 2017 reveal a diurnal cycle of seismicity consisting of hundreds of thousands of small ice quakes limited to a 6 12 hour period during the evening in an area where there is substantial subsurface melting this cycle is explained by thermally induced bending and fracture of a frozen surface superimposed on a subsurface slush water layer that is supported by solar radiation penetration and absorption a simple one dimensional model of heat transfer driven by observed surface air temperature and shortwave absorption reproduces the presence and absence as daily weather dictated of the observed diurnal seismicity cycle seismic event statistics comparing event occurrence with amplitude suggest that the events are generated in a fractured medium featuring relatively low stresses as is consistent with a frozen surface superimposed on subsurface slush waveforms of the icequakes are consistent with hydroacoustic phases at frequency bf gt bf 75 bf hz and flexural gravity waves at frequency bf bf lt 25 bf hz our results suggest that seismic observation may prove useful in monitoring subsurface melting in a manner that complements other ground based methods as well as remote sensing

a note on the surface volume change of shallow earthquakes summary this note presents an exact analytical formula for determining the magnitude of coseismic surface volume change v of earthquake faults in a half space for a poisson solid the formula is remarkably simple v mzz 8 where mzz is one of the moment tensor elements of the source maximum v values derive from dip slip on faults plunging 45 for these events surface volume changes of 0 0001 and 4 3 km3 can be expected for magnitude 5 and 8 earthquakes respectively all of the coseismic surface volume change is recovered in the interseismic period through relaxation of the earth and rebound of the surface a useful rule of thumb for estimating the magnitude of vertical rebound in 45 dip slip events is hp s 24 where s is the coseismic slip on the fault

greenland ice sheet motion coupled with daily melting in late summer 1 we use ground based and satellite observations to detect large diurnal and longer period variations in the flow of the greenland ice sheet gris during late summer that are strongly coupled with changes in its surface hydrology the diurnal signals are associated with periodic changes in surface melting and the longer period signals are associated with the episodic drainage of supra glacial lakes ice velocity doubles around 2 hours after peak daily melting and returns approximately to wintertime levels around 12 hours afterwards demonstrating an intimate link between the surface and basal hydrology during late summer the ice sheet accelerates by 35 per positive degree day of melting the observed link between surface melting and enhanced flow is typical of alpine glaciers which may provide an appropriate analogue for the evolution of the gris in a warming climate

near surface environmentally forced changes in the ross ice shelf observed with ambient seismic noise author s chaput j aster r c mcgrath d baker m anthony r e gerstoft p bromirski p nyblade a stephen r a wiens d a das s b stevens l a

ground deformation and gravity changes accompanying the march 1981 eruption of mount etna abstract we present reults from simultaneous precise levelling and gravity surveys on mount etna covering the period august 1980 august 1981 the flank eruption of march 1981 erupted 18 35 10 5 m 3 of lava following it upward movements of more than 17 cm were observed close to the new fissure and a broad apparently independent uplift of 5 cm was observed 4 km to the west a zone of about 2 cm depression to the east of the fissure is insufficient to account for the volume of magma erupted gravity results show positive changes of up to 63 microgal and display good positive correlation with elevation changes both sets of measurements appear to be due to new intrusion of magma rather than subsurface magma drainage ground deformation close to the new fissure is well modelled by intrusion of a dyke in the zone 100 500 m below the surface striking along the fissure and of dip between 75 90 the gravity changes are modelled as due to a deeper intrusion of magma along the same line but some 1500 m below the surface the changes were not present immediately after the eruption but occurred during the ensuing 5 months it is proposed that this introduction of matter occurred by crack propagation along the fissure in the aftermath of the eruption towards the west of the fissure and some 4 km west of the summit ground deformation is modelled by intrusion of a dyke in the zone 300 1500 m below the surface and dipping at 80 85 again gravity changes appear to be due to magma intrusion at greater depth close to sea level in this case gravity changes are interpreted as due to magma density changes as a result of pressure increase in a larger scale fissure zone this same pressure increase may be forcing the new intrusion close to the surface and makes this part of the volcano a region of especially high risk

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

local gravity anomalies produced by dislocation sources rundle 1978 and walsh and rice 1979 have shown that the change in the vertical component of gravity is proportional to uplift for a spherical source of dilatation and for slip on an infinitely long dip slip fault in the first case no free air gravity anomaly is produced and in the second case no bouguer gravity anomaly gravity anomalies due to other dislocation sources in three dimensions are as follows for strike slip faulting the ratio of the gravity change to uplift depends upon position however the gravity change contours are roughly similar to those corresponding to a zero free air gravity anomaly nor is the ratio constant for dip slip faulting except for the two special cases of dip slip on a vertical fault and horizontal slip on a horizontal fault neither of which produce a bouguer anomaly the bouguer anomaly produced by an open horizontal crack is the same as would be produced had the material within the crack been mined out without deforming the solid if the horizontal crack were filled with material of density equal to the host rock a good approximation to sill formation no bouguer anomaly is produced for cracks of other inclinations the ratio of gravity change to uplift is not constant thus dilatancy in general does not correspond to the absence of a free air anomaly as might be suggested by the special case of a spherical source of dilatation for two dimensional models a cylindrical source of dilatation produces no free air gravity anomaly dip slip faulting produces no bouguer anomaly and open cracks produce a bouguer anomaly equal to that which would be produced had the material within the crack been mined out without deforming the solid a two dimensional crack filled with material of density equal to that of the host rock would produce no bouguer anomaly jachens et al 1983 have reported temporal changes in gravity elevation and area strain along the san andreas fault in southern california such that the bouguer anomaly apparently remains unchanged and the uplift to strain ratio is about 100 km several dislocation mechanisms are proposed that fulfill these constraints but these mechanisms appear to be rather contrived and are not regarded as satisfactory explanations

seasonal speedup of the greenland ice sheet linked to routing of surface water we use interferometric synthetic aperture radar observations recorded in a land terminating sector of western greenland to characterise the ice sheet surface hydrology and to quantify spatial variations in the seasonality of ice sheet flow our data reveal a non uniform pattern of late summer ice speedup that in places extends over 100 km inland we show that the degree of late summer speedup is positively correlated with modelled runoff within the 10 glacier catchments of our survey and that the pattern of late summer speedup follows that of water routed at the ice sheet surface in late summer ice within the largest catchment flows on average 48 faster than during winter whereas changes in smaller catchments are less pronounced our observations show that the routing of seasonal runoff at the ice sheet surface plays an important role in shaping the magnitude and extent of seasonal ice sheet speedup

post eruptive volcanic dome evolution as revealed by deformation and microgravity observations at usu volcano hokkaido japan abstract usu volcano hokkaido japan is a dacitic volcano known for its high production rate of lava domes and crypto domes it is thus a good target to study processes of volcanic dome evolution upheaval and or relaxation we carried out repeated gps and microgravity surveys on the three most recent domes of mt usu 1910 meiji shinzan 1943 1945 showa shinzan and 1977 1982 usu shinzan the repeat period was 1 to 2 months and extended from october 1996 to june 1997 we also compare new data with results from former studies more than 20 years after the start of usu shinzan dome growth there is still subsidence at a maximum rate of about 7 to 8 cm year the reasons for this subsidence are discussed repeated gravity surveys revealed an increase of gravity on the domes about 60 10 microgal year for usu shinzan about 15 microgal at showa shinzan and 10 to 20 microgal for meiji shinzan this gravity increase exceeds that expected due to subsidence we discuss and interpret the excess gravity change in terms of a density increase in the edifice caused by a combination of processes contraction of the edifice water level change devesiculisation cooling and magma intrusion quantification of these processes at usu volcano may help to understand the processes of evolution at domes on other volcanoes such as merapi indonesia unzen japan or montserrat west indies

links between acceleration melting and supraglacial lake drainage of the western greenland ice sheet 1 the impact of increasing summer melt on the dynamics and stability of the greenland ice sheet is not fully understood mounting evidence suggests seasonal evolution of subglacial drainage mitigates or counteracts the ability of surface runoff to increase basal sliding here we compare subdaily ice velocity and uplift derived from nine global positioning system stations in the upper ablation zone in west greenland to surface melt and supraglacial lake drainage during summer 2007 starting around day 173 we observe speedups of 6 41 above spring velocity lasting 40 days accompanied by sustained surface uplift at most stations followed by a late summer slowdown after initial speedup we see a spatially uniform velocity response across the ablation zone and strong diurnal velocity variations during periods of melting most lake drainages were undetectable in the velocity record and those that were detected only perturbed velocities for 1 day suggesting preexisting drainage systems could efficiently drain large volumes of water the dynamic response to melt forcing appears to 1 be driven by changes in subglacial storage of water that is delivered in diurnal and episodic pulses and 2 decrease over the course of the summer presumably as the subglacial drainage system evolves to greater efficiency the relationship between hydrology and ice dynamics observed is similar to that observed on mountain glaciers suggesting that seasonally large water pressures under the ice sheet largely compensate for the greater ice thickness considered here thus increases in summer melting may not guarantee faster seasonal ice flow

viscoelastic displacement and gravity changes due to point magmatic intrusions in a gravitational layered solid earth summary we present a method for the computation of time dependent geodetic and geophysical signatures deformation potential and gravity changes due to magmatic intrusions in a layered viscoelastic gravitational medium this work is an extension of a deformation model previously developed to compute effects due to volcanic loading in an elastic gravitational layered media the model assumes a planar earth geometry useful for near field problems and consists of welded elastic and viscoelastic layers overlying a viscoelastic half space every layer can either be considered elastic or viscoelastic the intrusion treated as a point source can be located at any depth in any of the layers or in the half space several examples of theoretical computations for different media are also presented we have found that in line with previous results obtained by other authors introducing viscoelastic properties in all or part of the medium can extend the effects displacements gravity changes etc considerably and therefore lower pressure increases are required to model given observed effects the viscoelastic effects seem to depend mainly on the rheological properties of the layer zone where the intrusion is located rather than on the rheology of the whole medium we apply our model to the 1982 1984 uplift episode at campi flegrei modelling simultaneously the observed vertical displacement and gravity changes the results clearly show that for a correct interpretation of observed effects it is necessary to include the gravitational field in the anelastic theoretical models this factor can change the value and pattern of time dependent deformation as well as the gravity changes explaining cases of displacement without noticeable gravity changes or vice versa cases with uplift and incremental gravity values and other cases the combination of displacement and gravity changes is found to be especially effective in constraining the possible characteristics of the magmatic intrusion as well as the rheology of the medium surrounding it

axi symmetric deformation of a thermo poro elastic half space inflation of a magma chamber summary the deformation observed at phlegraean fields italy during the recent 1982 1984 crisis is interpreted in terms of overpressure in a shallow magma chamber and migration of hot pressurized fluids elastic effects are studied first and previous results obtained from maruyama s strain nuclei are corrected poro elastic and thermo elastic effects are found to be as important as the elastic ones in a semi pervious medium the three terms are typically of the same magnitude results are applied to interpreting the bradyseismic crisis of 1982 84 at phlegraean fields pf near naples italy the concave shape of the first phase of the deformation history at pf can be reproduced by linearly increasing fluid pressure and or temperature at the boundary of a spherical cavity the convex shape of the second phase is interpreted in terms of a gradual return to normal conditions the late phase of slow subsidence is interpreted as the effect of diffusional dissipation of temperature and pressure excess the deconvolution of the real source time history at pf is largely non unique but in any case the importance of fluids in the deformation history is assessed furthermore the inclusion of fluid migration in the deformation model allows simple explanations of several geochemical and geophysical observations made during the bradyseismic crisis

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

seismologically observed spatiotemporal drainage activity at moulins hydrology is important for glacier dynamics but it is difficult to monitor the subsurface drainage systems of glaciers by direct observations since meltwater drainage generates seismic signals passive seismic analysis has the potential to be used to monitor these processes to study continuous seismic radiation from the drainage we analyze geophone data from six stations deployed at the kaskawulsh glacier in yukon canada during the summer of 2014 using ambient noise cross correlation techniques we locate the noise sources by back projecting the amplitude of the cross correlation to the glacier surface most of the ambient noise sequences are found in two clusters with each cluster located in the vicinity of a moulin identified at the surface stronger seismic radiation is observed during the day consistent with expected variability in melt rates we demonstrate that the sparse seismic network array with 2 km station separation has the ability to detect moulins within the array with a precision of 50 m we confirm that seismic activity is correlated with air temperature and thus melt on a diurnal timescale and precipitation correlates with the activity at longer timescales our results highlight the potential of passive seismic observations for monitoring water flow into subglacial channels through moulins with an affordable number of seismic stations but quantification of water flow rates still remains a challenge the cross correlation back projection technique described here can also potentially be applied to any localized source of ambient noise such as ocean noise tectonic tremor and volcanic tremor

continuous seismic threshold monitoring summary continuous threshold monitoring is a technique for using a seismic network to monitor a geographical area continuously in time the method provides at a given confidence level a continuous assessment of the upper magnitude limit of possible seismic events that might have occurred in the target area two approaches are presented in this paper site specific threshold monitoring by focusing a seismic network on a specific target site continuous threshold monitoring of that site is achieved we optimize the monitoring capability by tuning the frequency filters and array beams to known characteristics from previously recorded events at the site we define the threshold trace for the network as the continuous time trace of computed upper magnitude limits of seismic events in the target area at a 90 per cent confidence level as an example we have conducted a one week monitoring experiment of the northern novaya zemlya nuclear test site using the fennoscandian regional array network noress arcess finesa we find that the threshold trace is below mb 2 5 more than 99 per cent of the time 34 peaks exceed mb 2 5 all of these peaks correspond to interfering seismic events that have been independently located by a teleseismic or regional network during the entire one week time period the threshold trace exceeded mb 2 5 only for 43 min regional threshold monitoring this involves conducting site specific monitoring of a dense grid of geographical aiming points and requires the development of generic phase attenuation relationships for covering an extended geographical region using again the fennoscandian regional array network we illustrate the regional threshold monitoring approach by maps with colour contour displays we demonstrate that the network thresholds in fennoscandia and adjacent regions show strong regional dependence the thresholds are below mb 0 5 close to each array 300 km distance and range from mb 2 0 to 2 5 in parts of the norwegian sea and barents sea the thresholds also vary significantly under different background noise conditions and an increase of about 1 0 mb units is observed during a large teleseismic earthquake these regional threshold maps have advantages over standard network capability maps in being more accurate during time intervals when interfering seismic events occur they can also more easily reflect special conditions such as particularly favourable source station propagation paths and have the advantage of not being tied to specific event detection criteria the paper concludes that continuous threshold monitoring offers a valuable supplement to traditional seismic techniques used in nuclear test ban monitoring the method may also be useful for monitoring earthquake activity at low magnitudes for sites of special interest as well as for monitoring earthquake aftershock sequences

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

gravity fields and the interpretation of volcanic structures geological discrimination and temporal evolution abstract a review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes to a first approximation all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice positive anomalies with wavelengths less than 20 km and amplitudes up to ca 30 mgal characterise mainly basaltic volcanoes from various tectonic settings and are caused by a relatively dense intrusive complex magma body which contrasts with its surroundings either because the body is more mafic than average or more likely because near surface previously erupted materials are uncompacted negative anomalies with wavelengths up to 100 km and amplitudes up to ca 60 mgal occur over much larger volcanic calderas many of which have erupted highly silicic pyroclastic ash and pumice uncompacted silicic caldera infill with a possible contribution from low density magma bodies is responsible for the observed anomalies there is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction related basaltic andesite to mature high level silicic calderas repeated microgravity observations over active volcanoes enable magma movements variations in magma input and changes in magma density to be monitored closely dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events

seismic wave attenuation in the uppermost glacier ice of storglaciaren sweden we conducted seismic refraction surveys in the upper ablation area of storglaciaren a small valley glacier located in swedish lapland we estimated seismic wave attenuation using the spectral ratio method on the energy travelling in the uppermost ice with an average temperature of approximately 1 c attenuation values were derived between 100 and 300 hz using the p wave quality factor q p the inverse of the internal friction by assuming constant attenuation along the seismic line we obtained mean q p 6 1 we also observed that q p varies from 8 1 to 5 1 from the near offset to the far offset region of the line respectively since the wave propagates deeper at far offsets this variation is interpreted by considering the temperature profile of the study area far offset arrivals sampled warmer and thus more attenuative ice our estimates are considerably lower than those reported for field studies in polar ice 500 1700 at 28 c and 50 160 at 10 c and hence are supportive of laboratory experiments that show attenuation increases with rising ice temperature our results provide new in situ estimates of q p for glacier ice and demonstrate a valuable method for future investigations in both alpine and polar ice

surface radiation balance in the ablation zone of the west greenland ice sheet 1 we present 4 years of radiation observations and derived cloud properties from the ablation zone of the west greenland ice sheet 67 n data were collected using three automatic weather stations located at 6 38 and 88 km from the ice sheet margin at elevations of 490 1020 and 1520 m asl this part of greenland is characterized by a 150 km wide tundra a 100 km wide ablation zone and an average equilibrium line altitude of 1500 m asl at the lowest station snow is redistributed by the wind in crevasses and gullies leading to very little measured winter accumulation as a result glacier ice albedo 0 55 is at the surface throughout the melting season at 1020 m asl the winter snow cover typically disappears in mid to end june at 1520 m asl superimposed ice briefly reaches the surface at the end of the ablation season the combined effect of decreasing surface albedo and increasing cloud optical thickness as summer progresses causes absorbed shortwave radiation to peak in june at low elevations but progressively later at higher elevations incoming longwave radiation peaks in august in response to increased cloud optical thickness and heating of the atmosphere over the snow and ice free surroundings the ice sheet margin experiences continuous melting in june july and august this limits the emission of longwave radiation causing net longwave radiation to peak in august further enhancing melt the sum of these effects is that summer net radiation increases sharply toward the ice sheet margin to resolve this correctly requires high resolution climate models in the order of 10 km or better

changes in the stress regime and displacement field in the interior of mount etna deduced from surface observations abstract if measurements of all three components of displacement at the surface of a mountain are available the stress strain relations of the theory of linear elasticity can be used as a simple approximation to obtain insight into changes that take place on a relatively short time scale in its interior if such a simple elastic model is employed interpolated values derived from these data can be used as boundary conditions at the surface of the mountain and then integrated downwards this procedure yields values for the internal displacements that take place from year to year from which changes in the internal stress distributions can be calculated the location of regions in which the solutions obtained by this approach give inappropriate results indicate where the assumptions of linear elasticity do not hold and so provide further evidence in the task of determining internal structure measurements are available from 1987 to 1992 for all three components of displacement at a substantial number of points on the surface of mount etna in intervals of not more than a year and the method was applied to them to obtain evidence of regions in the interior of the volcano deserving further study

constraints on the lake volume required for hydro fracture through ice sheets 1 water filled cracks are an effective mechanism to drive hydro fractures through thick ice sheets crack geometry is therefore critical in assessing whether a supraglacial lake contains a sufficient volume of water to keep a crack water filled until it reaches the bed in this study we investigate fracture propagation using a linear elastic fracture mechanics model to calculate the dimensions of water filled cracks beneath supraglacial lakes we find that the cross sectional area of water filled cracks increases non linearly with ice sheet thickness using these results we place volumetric constraints on the amount of water necessary to drive cracks through 1 km of sub freezing ice for ice sheet regions under little tension lakes larger than 0 25 0 80 km in diameter contain sufficient water to rapidly drive hydro fractures through 1 1 5 km of subfreezing ice this represents 98 of the meltwater volume held in supraglacial lakes in the central western margin of the greenland ice sheet

nloadf a program for computing ocean tide loading the loading of the earth by the ocean tides produces several kinds of signals which can be measured by geodetic technique in order to compute these most accurately a combination of global and local models of the ocean tides may be needed the program nloadf convolves the green functions for loading with ocean tide models using a station centered grid with fixed dimensions making it easy to combine different ocean models without overlap in the convolution the program computes all the quantities of interest gravity displacement tilt and strain and includes the case where measurements are made beneath the surface of the ocean

mathematical modelling of the stress regime in mount etna using ground deformation measurements 1987 1992 abstract the stress strain relations of the theory of linear elasticity can be used as a simple approximation to the far more complicated relations that necessarily exist inside a volcano in order to obtain insight into changes that take place on a relatively short time scale or near the surface where stresses and temperature distributions are not so great as to suggest that other equations ought to be applied they are here used to study the stress distribution at the surface of mount etna when the edifice of the volcano is distorted by movements of magma or by straining in the structures below sufficient observational data exists to deduce the changes in the surface stress distribution resulting from the observed deformations this approach indicates that the regions of maximum stress most likely to lead to fissuring occur round the edge of the valle del bove

temporal variations in gravity at mt etna italy associated with the 1989 and 1991 eruptions results are presented from 11 microgravity surveys on mt etna between 1987 and 1993 a period including the major 1989 and 1991 1993 flank eruptions and subordinate 1990 activity measurements were made with lacoste and romberg d 62 and d 157 gravity meters along a network around the volcano between 1000 and 1900 m a s l and since 1992 a n s summit profile gravity changes of as much as 200 gal were observed at scales from the size of the summit region to that of the volcano none was associated with significant changes in ground elevation the data show an increase in gravity for 2 years before the 1989 eruption the increase is attributed to the accumulation of magma 0 25 1 7 109 m3 in an elongate zone oriented nnw sse between 2 5 and 6 km below sea level part of this magma was injected into the volcanic pile to supply the 1989 and 1990 eruptions it also probably fed the start of the 1991 1993 eruption since this event was not preceded by significant gravity changes a large gravity increase up to 140 gal detected across the volcano between june and september 1992 is consistent with the arrival in the accumulation zone of 0 32 2 2 109 m3 of new magma thus favoring continued flank effusion until 1993 a large gravity decrease 200 gal in the summit region marked the closing stages of the 1991 1993 event and is associated with magma drainage from the upper levels of etna s central feeding system

analysis of the time averaged properties of sediment motion in a local scour process 1 we examined the kinematics of sediment particles in the proximity of a vertical plate attached to the side wall of a pressurized duct the laboratory phenomenon investigated was archetypal of local scour near bridge abutments in covered rivers we conducted a clear water scour experiment where we measured the temporal evolution of the local erosion depth the latter was found to follow a logarithmic trend the concentration and velocity of the moving particles were measured on the basis of digital movies taken at various evolution stages of the process the spatial pattern of the sediment motion showed that the action of the principal vortex system becomes progressively more evident with the general lowering of the sediment transport intensity detected as the scour hole increases in time furthermore the phenomenology of local sediment transport was analyzed key findings of the research include 1 the time averaged values of the sediment transport rate are linked with those of concentration and velocity through relationships which are almost invariant regardless of the experimental time and of the location within the scour hole 2 intermittency and irregularity of the sediment fluxes increase as their mean intensity decreases 3 the relative weight of the temporal fluctuations of concentration and velocity in the time mean sediment transport activity is significant throughout the entire process potential implications of the present results for the modeling of local scour processes in pressurized flow conditions are discussed the considerations made are likely to have analogous applicability to local scour with a free surface since the large scale evolutions of the erosion depth in the two conditions may not be greatly different given the qualitative analogy of the present findings with earlier ones obtained for one dimensional bed load it is suggested that similar considerations may hold also for the small scale modeling of a wider class of sediment transport processes

gravel saltation 2 modeling a lagrangian model for the motion of a sediment particle in a water flow is proposed the particle motion equation is averaged over flow turbulence specialized to the case of coarse sediment particles saltating in water and coupled with a stochastic model for particle collision with the bed model predictions of statistics of gravel saltation generally agree well with experimental observations as an application of the saltation model bed load transport rates are estimated from modeled mean saltation streamwise velocity and a dynamic friction coefficient the results obtained compare favorably with typical bed load equations but they overestimate experimental measurements of gravel transport which would suggest that a bagnoldean formulation for bed load transport of coarse material by saltating particles may not be valid

caterpillar like ice motion in the ablation zone of the greenland ice sheet current understanding of ice dynamics predicts that increasing availability and variability of meltwater will have an impact on basal motion and therefore on the evolution and future behavior of the greenland ice sheet we present measurements of ice deformation subglacial water pressure and surface velocity that show periodic and episodic variations on several time scales seasonal multiday and diurnal these variations observed with gps and sensors at different depths throughout the ice column are not synchronous but show delayed responses of ice deformation with increasing depth and basal water pressure in antiphase with surface velocity with the help of a full stokes ice flow model these observations are explained as ice motion in a caterpillar like fashion caused by patches of different basal slipperiness horizontal stress transfer through the stiff central part of the ice body leads to spatially varying surface velocities and ice deformation patterns variation of this basal slipperiness induces characteristic patterns of ice deformation variability that explain the observed behavior ice flow in the ablation zone of the greenland ice sheet is therefore controlled by activation of basal patches by varying slipperiness in the course of a melt season leading to caterpillar like ice motion superposed on the classical shear deformation

supraglacial forcing of subglacial drainage in the ablation zone of the greenland ice sheet 1 we measure hydrological parameters in meltwater draining from an outlet glacier in west greenland to investigate seasonal changes in the structure and behaviour of the hydrological system of a large catchment in the greenland ice sheet gris our data reveal seasonal upglacier expansion and increase in hydraulic efficiency of the subglacial drainage system across a catchment 600 km2 to distances 50 km from the ice sheet margin this expansion occurs episodically in response to the drainage of surface meltwaters into a hitherto inefficient subglacial drainage system as new input locations become active progressively further upglacier this system is similar to alpine glaciers these observations provide the first synopsis of seasonal hydrological behaviour in the ablation zone of the gris

limits to future expansion of surface melt enhanced ice flow into the interior of western greenland moulins are important conduits for surface meltwater to reach the bed of the greenland ice sheet it has been proposed that in a warming climate newly formed moulins associated with the inland migration of supraglacial lakes could introduce surface melt to new regions of the bed introducing or enhancing sliding there by examining surface strain rates we found that the upper limit to where crevasses and therefore moulins are likely to form is 1600m this is also roughly the elevation above which lakes do not drain completely thus meltwater above this elevation will largely flow tens of kilometers through surface streams into existing moulins downstream furthermore results from a thermal ice sheet model indicate that the 1600m crevassing limit is well below the wet frozen basal transition 2000m together these data sets suggest that new supraglacial lakes will have a limited effect on the inland expansion of melt induced seasonal acceleration key points greenland ice sheet meltwater volumes are increasing notably at high elevations low strain rates limit the likelihood of moulin formation at high elevations high elevation meltwater will reach an already wet bed at lower elevations

seasonal variations in greenland ice sheet motion inland extent and behaviour at higher elevations abstract we present global positioning system observations that capture the full inland extent of ice motion variations in 2009 along a transect in the west greenland ice sheet margin in situ measurements of air temperature and surface ablation and satellite monitoring of ice surface albedo and supraglacial lake drainage are used to investigate hydrological controls on ice velocity changes we find a strong positive correlation between rates of annual ablation and changes in annual ice motion along the transect with sites nearest the ice sheet margin experiencing greater annual variations in ice motion 15 18 than those above 1000 m elevation 3 8 patterns in the timing and rate of meltwater delivery to the ice bed interface provide key controls on the magnitude of hydrologically forced velocity variations at each site in the lower ablation zone the overall contribution of variations in ice motion to annual flow rates is limited by evolution in the structure of the subglacial drainage system at sites in the upper ablation zone a shorter period of summer melting and delayed establishment of a hydraulic connection between the ice sheet surface and its bed limit the timeframe for velocity variations to occur our data suggest that land terminating sections of the greenland ice sheet will experience increased dynamic mass loss in a warmer climate as the behaviour that we observe in the lower ablation zone propagates further inland findings from this study provide a conceptual framework to understand the impact of hydrologically forced velocity variations on the future mass balance of land terminating sections of the greenland ice sheet

a decadal investigation of supraglacial lakes in west greenland using a fully automatic detection and tracking algorithm abstract the sudden drainage of supraglacial lakes has been previously observed to initiate surface to bed hydrologic connections which are capable of enhancing basal sliding in regions of the greenland ice sheet where ice thickness approaches 1 km in this study we develop a robust algorithm which automatically detects and tracks individual supraglacial lakes using visible satellite imagery to document the evolution of a population of west greenland supraglacial lakes over ten consecutive melt seasons validation tests indicate that the algorithm is highly accurate 99 0 of supraglacial lakes can be detected and tracked and 96 3 of reported lakes are true supraglacial lakes with accurate lake properties such as lake area and timing of formation and drainage investigation of the interannual evolution of supraglacial lakes in the context of annual melt intensity reveals that during more intense melt years supraglacial lakes drain more frequently and earlier in the melt season additionally the lake population extends to higher elevations during more intense melt years exposing an increased inland area of the ice sheet to sudden lake drainage events these observations suggest that increased surface meltwater production due to climate change will enhance the spatial extent and temporal frequency of lake drainage events it is unclear whether this will ultimately increase or decrease the basal sliding sensitivity of interior regions of the greenland ice sheet

shallow repeating seismic events under an alpine glacier at mount rainier washington usa we observed several swarms of repeating low frequency 1 5 hz seismic events during a 3 week period in may june 2010 near the summit of mount rainier washington usa that likely were a result of stick slip motion at the base of alpine glaciers the dominant set of repeating events multiplets featured 4000 individual events and did not exhibit daytime variations in recurrence interval or amplitude volcanoes and glaciers around the world are known to produce seismic signals with great variability in both frequency content and size the low frequency character and periodic recurrence of the mount rainier multiplets mimic long period seismicity often seen at volcanoes particularly during periods of unrest however their near surface location lack of common spectral peaks across the recording network rapid attenuation of amplitudes with distance and temporal correlation with weather systems all indicate that ice related source mechanisms are the most likely explanation we interpret the low frequency character of these multiplets to be the result of trapping of seismic energy under glacial ice as it propagates through the highly heterogeneous and attenuating volcanic material the mount rainier multiplet sequences underscore the difficulties in differentiating low frequency signals due to glacial processes from those caused by volcanic processes on glacier clad volcanoes

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

swarms of repeating stick slip icequakes triggered by snow loading at mount rainier volcano we have detected over 150 000 small m 3000 m on the glacier covered edifice and occur primarily in weeklong to monthlong swarms composed of simultaneous distinct families of events each family contains up to thousands of earthquakes repeating at regular intervals as often as every few minutes mixed polarity first motions a linear relationship between recurrence interval and event size and strong correlation between swarm activity and snowfall suggest the source is stick slip basal sliding of glaciers the sudden added weight of snow during winter storms triggers a temporary change from smooth aseismic sliding to seismic stick slip sliding in locations where basal conditions are favorable to frictional instability coda wave interferometry shows that source locations migrate over time at glacial speeds starting out fast and slowing down over time indicating a sudden increase in sliding velocity triggers the transition to stick slip sliding we propose a hypothesis that this increase is caused by the redistribution of basal fluids rather than direct loading because of a 1 2 day lag between snow loading and earthquake activity this behavior is specific to winter months because it requires the inefficient drainage of a distributed subglacial drainage system identification of the source of these frequent signals offers a view of basal glacier processes discriminates against alarming volcanic noises documents short term effects of weather on the cryosphere and has implications for repeating earthquakes in general

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

seismic activity and surface motion of a steep temperate glacier a study on triftgletscher switzerland the tongue of triftgletscher switzerland is particularly susceptible to major break off events due to its steep slope in order to detect precursors of such an event we monitored the local seismic activity and detected 2426 icequakes with sources located in an area ranging between 2050 and 2350ma s l events triggered by cracks and icefalls were recorded but no precise distinction between the two sources was possible using duration or frequency criteria clusters of seismic activity were located and confirmed by visual observations we performed a surface motion analysis and found that surface motion was driven significantly by runoff changes at a timescale of 2 3 days by means of a statistical analysis a power law behaviour of the released seismic energy distribution was discerned at certain times during the period investigated variations in power law exponent values indicated that low and high energy events predominantly occurred during phases of enhanced and reduced surface motion respectively substantial releases of seismic energy likely to signal the glacier recoupling were detected during phases of decreasing runoff clues to potential seismic precursors of break off events are discussed

on the relationship between fault permeability increases induced stress perturbation and the growth of aseismic slip during fluid injection author s cappa f guglielmi y nussbaum c birkholzer j abstract 2018 american geophysical union all rights reserved fluid injections into the deep subsurface can at times generate earthquakes but often they only produce aseismic deformations here we analyze the influence of fault hydromechanical properties on the growth of injection induced aseismic slip using hydromechanical modeling we show how permeability enhancement in addition to the background stress and frictional weakening has an important effect on the pressure diffusion and slip growth during injection we find that the more pronounced the fault permeability enhancement the stronger is the growth of the aseismic slip zone the effect of enhanced permeability is more pronounced when the fault is initially close to failure our results show that aseismic slip grows beyond the pressurized zone when the fault permeability increases while slip remains behind the pressurized zone when permeability does not vary from its initial preslip value thus fault permeability increases should be considered as complementary mechanism to current models of fluid induced aseismic slip

residual gravity changes and eruption magnitudes abstract research during the last four decades has demonstrated that systematic temporal gravity changes are associated with eruptive activity these gravity changes are caused by the free air effect of elevation changes induced by magma bodies and by mass redistributions where gravity change data are accompanied by concurrent elevation control observed gravity changes can be corrected for the free air effect giving residual gravity changes residual gravity changes reflect only mass redistributions at most volcanoes residual gravity changes can be accounted for by the addition of mass during pre eruption time and by the removal or addition of mass during post eruption time at explosive volcanoes however the data show mass must be removed from near surface magma systems during preeruptive time the only reasonable mechanism acounting for these negative pre eruption mass redistributions is displacing dense country rock or magma with gas bubbles in vesiculated magma application of gauss theorem to areal residual gravity changes and an assumed density for the displaced material give an estimate of the volume of exsolved gas contained within the magma body the kinetic energy released in explosive magmatic eruptions to a first approximation is caused by the explosive expansion of exsolved and dissolved gases contained within magma if the density and temperature of magma and if the amount pressure and composition of volatiles contained in magma bodies are known or assumed then the kinetic energy released by explosive gas decompression can be computed or estimated using simple thermodynamic relationships kinetic energy estimates for explosive magmatic eruptions computed from muzzle velocities and computed for isothermal and adiabatic gas decompression using assumed volatile contents pressures and temperatures give similar results minimum pre eruption exsolved volatile contents estimated from negative pre eruption mass deficits and assumed magma densities temperatures and pressures were used to compute predicted kinetic energies for five eruption gravity change sequences at pacaya mihara usu and mt st helens volcanoes although the data are too few to be conclusive they do indicate the possibility of forecasting eruption magnitudes at some volcanoes using precursory gravity changes

discussion of protecting vertical wall abutments with riprap mattresses by a h cardoso and c m s fael the authors are to be commended for their experimental program and for the attempt to provide guidelines for safe design of scour countermeasures by means of launching aprons this is a technical field in which the existing literature is still relatively scarce in this discussion some remarks will be given about the upstream width wu of the launching apron refer to fig 12 of the paper under discussion the authors did not report specific experimentation in this respect indeed the upstream width of the mattress has received relatively scarce attention in works on riprap countermeasures experimental evidence has shown that for short abutments as well the values of the time averaged shear stress distribution in the corner region upstream of the abutment face are lower than that in the incoming reach rajaratnam and nwachukwu 1983 molinas et al 1998 dey and barbhuiya 2005 if the phenomenon is modeled at threshold conditions as is typically done in clear water scour experiments no scour would be expected upstream of the abutment face in all the regions with stress lower than that in the upstream reach experimental evidence on sediment motion during the scour process at short and intermediate abutments radice et al 2006 2008 has shown that at the beginning of the erosion process the particle motion is most intense at the abutment nose where scour starts in contrast grain motion is almost negligible at the junction between the duct and abutment walls junction region here an appreciable rate of scour depth is observed only after the scour hole whose point of maximum depth is typically at the obstacle nose has reached a significant transverse slope in terms of design of scour countermeasures protecting the sediment bed around the abutment nose might be sufficient to prevent the scouring process an experimental test of local scour at a protected abutment was performed to check this issue a rectangular duct with a length of 5 8 m and a width of 0 40 m was used the recess section of the duct was filled with uniform plastic grains with a characteristic size of d 3 6 mm and a relative density of 1 43 water discharge was 18 5 l s corresponding to the threshold value for incipient particle motion in the undisturbed bed the abutment model was a vertical plate with a length of 10 cm and a thickness of 1 cm fig 1 the experimental installation has been described in detail by radice et al 2009 who performed an identical scour experiment without abutment protection in that study the possibility of comparing scour data taken under freesurface and covered flows was also demonstrated in the present experiment the sediment bed around the abutment nose was protected by means of a square plate as shown in fig 1 the plate dimension was 20 cm along both the flow and transverse directions plate thickness was 1 cm the plate was roughened gluing on its upper surface sediments identical to those of the loose bed thus the resulting thickness of the countermeasure was 1 4 mm the described protection device is actually a rough collar installed at the level of the original sediment bed however at scour initiation the collar is likely to behave similarly to a launching apron with an elevation identical to that of the protected sediment bed therefore the following observations on scour initiation should not be significantly affected by the different configuration of the countermeasure at the beginning of the process some sediment motion was evident in the junction region fig 2 in this region the timeaverage shear stress is expected to be smaller than the threshold value it is therefore reasonable to assume the motion events as due to the peak stress values during the motion events the grains assumed directions opposite to that of the incoming flow consistent with the structure of the principal vortex the sediment particles moved downstream of the abutment passing over the protection plate the grain motion in the junction region resulted in a weak but nonnegligible scour rate some particles coming from the upstream reach can be recognized this is consistent with the definition used here of incipient particle motion which corresponds to a weak sediment transport condition dimensionless sediment transport rate per unit width 6 10 5 after about 3 min of run time fig 3 the junction region scour depth exceeded the thickness of the protection plate and the sediments

subsidence above a planar reservoir 1 a tabular reservoir circular in plan is located at depth parallel to the surface compaction of the reservoir as fluid is withdrawn causes a change in the stress and displacement fields in the surrounding medium a solution to this problem is obtained by determining the boundary conditions at the interface between the reservoir and the half space using eshelby s 1957 cut and weld technique the displacement field in the reservoir and the surrounding medium is then calculated using the reciprocal theorem the displacement field is found to be the same as that calculated by geertsma 1973a 1973b and by segall 1992 segall et al 1994 using another technique

direct observations of evolving subglacial drainage beneath the greenland ice sheet simultaneous observations of moulins and boreholes in western greenland show that water delivery to the base of the ice sheet via moulins affects short term ice velocity fluctuations but not late season ice velocity decelerations increased meltwater delivery to the base of the greenland ice sheet will increase the ice sheet velocity accelerating its inevitable rush towards the ocean and subsequent sea level rise or will it debate on this topic is at the forefront of cryospheric research but has been hampered by the lack of simultaneous observations of hydraulic head in moulins vertical shafts that deliver water to the base of the ice sheet and boreholes which monitor basal water pressure lauren andrews and colleagues now provide these observations from a small region in western greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short term fluctuations in ice velocity late season decelerations in ice velocity however seem to be controlled by changes in unchannelized flow rather than any shifts in the moulin system seasonal acceleration of the greenland ice sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1 2 via crevasses and moulins vertical conduits connecting supraglacial water to the bed of the ice sheet as the melt season progresses the subglacial hydrologic system drains supraglacial meltwater more efficiently1 2 3 4 decreasing basal water pressure4 and moderating the ice velocity response to surface melting1 2 however limited direct observations of subglacial water pressure4 5 6 7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby 0 3 2 kilometres away boreholes indicating that moulins connect to an efficient channelized component of the subglacial hydrologic system which exerts the primary control on diurnal and multi day changes in ice velocity our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the paakitsoq region of western greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin connected channels to convey supraglacial melt instead these observations suggest that decreasing late season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt season dynamics affect ice velocity beyond the conclusion of the melt season8 9 10

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

glacial erosion status and outlook glacier erosion rates range across orders of magnitude and much of this variation cannot be attributed to basal sliding rates subglacial till acts as lubricating fault gouge or sawdust and must be removed for rapid subglacial bedrock erosion such erosion occurs especially where and when moulin fed streams access the bed and are unconstrained by supercooling or other processes streams also may directly erode bedrock likely with strong time evolution erosion is primarily by quarrying aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes debris bed friction significantly affects abrasion quarrying and general glacier flow frost heave drives cirque headwall erosion as winter cold air enters bergschrunds creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock and the glacier removes the resulting blocks recent subglacial bedrock erosion and sediment flux are in many cases much higher than long term averages over glacial cycles evolution of glacial valley form feeds back strongly on erosion and deposition most of this is poorly quantified with parts open to argument glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes motivating vigorous research

deformation induced gravity changes in volcanic regions summary we analyse a new model to estimate the change in gravity and in elevation caused by volcanic eruption or inflation in the new model a permeable porous zone surrounds a spherical magma chamber magma may enter or leave the magma chamber and independently the permeable zone this arrangement simulates the flow processes inferred from field studies and provides a degree of freedom not available in the classic model proposed by mogi and used by many others since then we find in agreement with field data and previous analyses of the mogi model that the change in edifice volume is smaller than the volume of magma which produces it the local change in gravity is found to be proportional to the local elevation change the constant of proportionality being positive or negative depending on whether magma flow is directed into or out of the chamber or infiltrated zone

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