distributed subglacial discharge drives significant submarine melt at a greenland tidewater glacier this is the publisher s final pdf the article is copyrighted by american geophysical union and published by john wiley sons inc it can be found at http agupubs onlinelibrary wiley com agu journal 10 1002 28issn 291944 8007

global detection and location of seismic sources by using surface waves we develop an algorithm for the detection and location of seismic sources using intermediate period 35 150 sec surface waves recorded on a global array of stations continuous vertical seismic waveforms from the global network are collected and a 4 4 global grid of target locations is defined for each target location and each station a surface wave propagation operator is deconvolved from the seismogram to restore any source pulse present the envelope of the seismogram is calculated and cross correlated with a theoretical source pulse shape the resulting waveforms are stacked to improve signal to noise characteristics and the quality strength and timing of the potential detection are determined when a successful event detection is made a finer grid is applied to locate the event with greater precision we apply the algorithm systematically for the period 1993 2003 and catalog the events approximately 2000 events are detected and located for each year and 98 of shallow events in the harvard centroid moment tensor cmt catalog are detected and located by the new algorithm a comparison of 9482 events common to the two catalogs allows the detection strength to be calibrated against the cmt moment magnitude all detected events have estimated moment magnitudes m w 4 5 in each year approximately 100 events not listed in other global seismicity catalogs are detected and located many of these events lie along the ridge transform plate boundaries in the southern hemisphere and may be regular earthquakes that have gone undetected because of poor station coverage other events located in areas where global and regional networks provide good coverage are potentially anomalous and may have escaped detection as a result of their unusual source properties

ice front variation and tidewater behavior on helheim and kangerdlugssuaq glaciers greenland we used satellite images to examine the calving behavior of helheim and kangerdlugssuaq glaciers greenland from 2001 to 2006 a period in which they retreated and sped up these data show that many large iceberg calving episodes coincided with teleseismically detected glacial earthquakes suggesting that calving related processes are the source of the seismicity for each of several events for which we have observations the ice front calved back to a large pre existing rift these rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high which agrees well with earlier theoretical predictions in addition to the recent retreat in a period of higher temperatures analysis of several images shows that helheim retreated in the 20th century during a warmer period and then re advanced during a subsequent cooler period this apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water the cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in greenland can advance rapidly greenland s larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance

dynamics of glacier calving at the ungrounded margin of helheim glacier southeast greenland during summer 2013 we installed a network of 19 gps nodes at the ungrounded margin of helheim glacier in southeast greenland together with three cameras to study iceberg calving mechanisms the network collected data at rates up to every 7 s and was designed to be robust to loss of nodes as the glacier calved data collection covered 55 days and many nodes survived in locations right at the glacier front to the time of iceberg calving the observations included a number of significant calving events and as a consequence the glacier retreated 1 5 km the data provide real time high frequency observations in unprecedented proximity to the calving front the glacier calved by a process of buoyancy force induced crevassing in which the ice downglacier of flexion zones rotates upward because it is out of buoyant equilibrium calving then occurs back to the flexion zone this calving process provides a compelling and complete explanation for the data tracking of oblique camera images allows identification and characterisation of the flexion zones and their propagation downglacier interpretation of the gps data and camera data in combination allows us to place constraints on the height of the basal cavity that forms beneath the rotating ice downglacier of the flexion zone before calving the flexion zones are probably formed by the exploitation of basal crevasses and theoretical considerations suggest that their propagation is strongly enhanced when the glacier base is deeper than buoyant equilibrium thus this calving mechanism is likely to dominate whenever such geometry occurs and is of increasing importance in greenland

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

centroid single force inversion of seismic waves generated by landslides the centroid moment tensor cmt waveform inversion method of dziewonski et al 1981 is modified to analyze long period seismic waves generated by single force events such as the gigantic landslides associated with the 1980 eruption of mount st helens we refer to the method as centroid single force csf inversion as the result of the inversion we obtain the spatio temporal centroid and three components of a vector we call the csf vector which is the seismic point source representation of a landslide and whose direction is the opposite of the direction of sliding the sealer magnitude of the csf vector measures the overall size of the landslide and is given by mcsf md where m is the mass of the sliding object and d is the sliding distance we apply csf inversion to the long period surface waves generated by the 1980 mount st helens event the 1975 kalapana earthquake and the 1974 mantaro landslide using seismograms from the gdsn ida and hglp networks the csf solution for the st helens event is remarkably consistent with the actual geological observations and proves the efficacy of csf inversion the csf solution for the kalapana event does not fit the overall waveforms better than the cmt solution does however because the cmt solution does not explain the observed love wave radiation pattern it may be necessary to invoke a combination of both types of mechanism for this event although the geometry of the csf solution the direction of the force for the mantaro event is consistent with the actual landslide mcsf is about 5 times smaller than that expected from the mass and travel distance of the landslide estimated by geological observations this discrepancy may suggest either the relatively aseismic nature of this landslide or that the total volume of the slide was overestimated by analogy to single station cmt inversion single station csf inversion also appears to be stable and useful

ice melange dynamics and implications for terminus stability jakobshavn isbrae greenland 1 we used time lapse imagery seismic and audio recordings iceberg and glacier velocities ocean wave measurements and simple theoretical considerations to investigate the interactions between jakobshavn isbrae and its proglacial ice melange the melange behaves as a weak granular ice shelf whose rheology varies seasonally sea ice growth in winter stiffens the melange matrix by binding iceberg clasts together ultimately preventing the calving of full glacier thickness icebergs the dominant style of calving and enabling a several kilometer terminus advance each summer the melange weakens and the terminus retreats the melange remains strong enough however to be largely unaffected by ocean currents except during calving events and to influence the timing and sequence of calving events furthermore motion of the melange is highly episodic between calving events including the entire winter it is pushed down fjord by the advancing terminus at 40 m d 1 whereas during calving events it can move in excess of 50 103 m d 1 for more than 10 min by influencing the timing of calving events the melange contributes to the glacier s several kilometer seasonal advance and retreat the associated geometric changes of the terminus area affect glacier flow furthermore a force balance analysis shows that large scale calving is only possible from a terminus that is near floatation especially in the presence of a resistive ice melange the net annual retreat of the glacier is therefore limited by its proximity to floatation potentially providing a physical mechanism for a previously described near floatation criterion for calving

reverse glacier motion during iceberg calving and the cause of glacial earthquakes nearly half of greenland s mass loss occurs through iceberg calving but the physical mechanisms operating during calving are poorly known and in situ observations are sparse we show that calving at greenland s helheim glacier causes a minutes long reversal of the glacier s horizontal flow and a downward deflection of its terminus the reverse motion results from the horizontal force caused by iceberg capsize and acceleration away from the glacier front the downward motion results from a hydrodynamic pressure drop behind the capsizing berg which also causes an upward force on the solid earth these forces are the source of glacial earthquakes globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in greenland and antarctica

grounding line migration and high resolution calving dynamics of jakobshavn isbrae west greenland 1 we performed three field campaigns in 2004 2007 and 2010 at the southern margin of the jakobshavn isbrae west greenland in order to infer flow velocities and their changes from photogrammetric time lapse imagery with a temporal resolution of 20 min and a spatial spacing of about 30 m on the glacier surface area wide analysis of more than 3000 three dimensional trajectories at individual glacier positions allow for both the mapping of the grounding line and the detailed observation of flow variations during major calving events from 2004 to 2010 the grounding line of jakobshavn isbrae retreated 3 5 0 2 km considering previously published results the grounding line retreat amounts to 6 km since 1985 the glacier has an ephemeral floating tongue that can establish during the readvance of the glacier front and break apart after large calving events observations of a major calving event show that an acceleration of flow velocities coincides with the onset of the break up during which flow velocities of up to 70 m d can be reached moreover large vertical displacements of the glacier front in the order of 15 m and lowering of 8 m at positions 500 m beyond the calving front were observed 2 days before the calving event after the break up the glacier slowly adjusts to the new boundary conditions within the next 4 5 days flow velocity variations caused by calving were detected up to 1 km upstream only which indicates that individual calving events have no immediate effect on the large scale glacier dynamics

step wise changes in glacier flow speed coincide with calving and glacial earthquakes at helheim glacier greenland this is the publisher s version also available electronically from http onlinelibrary wiley com

possible mechanisms for glacial earthquakes the large glacial earthquakes reported on by ekstrom et al 2003 2006 and tsai and ekstrom 2007 have previously been evaluated in terms of their seismic characteristics in this paper we attempt to take constraints such as known glacial ice properties outlet glacier size calving style and meltwater variability to construct a self consistent physical model of the glacial earthquake process since many glaciological parameters are poorly constrained we parameterize a number of important processes and estimate a wide range of possible values for some properties the range of model outputs is thus fairly large but it is still difficult to match observational constraints under most conditions we find that only a small class of models is able to satisfy the major observational constraints these models are characterized by 1 lost basal resistance coupled to viscoelastic deformation with extensive internal crevassing or with low effective elastic modulus and possibly low effective viscosity or 2 by nonequilibrium calving such as having large icebergs capsize into the glacier front although observational constraints cannot definitively rule out any of the proposed classes of mechanisms the calving model has much stronger support fortunately the various models make different predictions regarding observables that can potentially be measured in the near future

simple scaling of catastrophic landslide dynamics catastrophic landslides involve the acceleration and deceleration of millions of tons of rock and debris in response to the forces of gravity and dissipation their unpredictability and frequent location in remote areas have made observations of their dynamics rare through real time detection and inverse modeling of teleseismic data we show that landslide dynamics are primarily determined by the length scale of the source mass when combined with geometric constraints from satellite imagery the seismically determined landslide force histories yield estimates of landslide duration momenta potential energy loss mass and runout trajectory measurements of these dynamical properties for 29 teleseismogenic landslides are consistent with a simple acceleration model in which height drop and rupture depth scale with the length of the failing slope

analysis of glacial earthquakes in 2003 ekstrom et al reported on the detection of a new class of earthquakes that occur in glaciated regions with the vast majority being in greenland the events have a characteristic radiation pattern and lack the high frequency content typical of tectonic earthquakes it was proposed that the events correspond to large and sudden sliding motion of glaciers here we present an analysis of all 184 such events detected in greenland between 1993 and 2005 fitting the teleseismic long period surface waves to a landslide model of the source we obtain improved locations timing force amplitudes and force directions after relocation the events cluster into seven regions all of which correspond to regions of very high ice flow and most of which are named outlet glaciers these regions are daugaard jensen glacier kangerdlugssuaq glacier helheim glacier the southeast greenland glaciers the northwest greenland glaciers rinks isbrae and jakobshavn isbrae event amplitudes range from 0 1 to 2 0 10 14 kg m force directions are consistent with sliding in the direction of glacial flow over a period of about 50 s each region has a different temporal distribution of events all glaciers are more productive in the summer but have their peak activity in different months over the study period kangerdlugssuaq has had a constant number of events each year whereas jakobshavn had most events in 1998 1999 and the number of events in helheim and the northwest greenland glaciers has increased substantially between 1993 and 2005 the size distribution of events in kangerdlugssuaq is peaked above the detection threshold suggesting that glacial earthquakes have a characteristic size

impact of hydrodynamics on seismic signals generated by iceberg collisions full glacier thickness icebergs are frequently observed to capsize as they calve into the ocean as they capsize they may collide with the glaciers termini previous studies have hypothesized that such collisions are the source of teleseismic glacial earthquakes we use laboratory scale experiments force balance modeling and theoretical arguments to show that 1 the contact forces during these collisions are strongly influenced by hydrodynamic forces and 2 the associated glacial earthquake magnitudes expressed as twice integrated force histories are related to the energy released by the capsizing icebergs plus a hydrodynamic term that is composed of drag forces and hydrodynamic pressure our experiments and first order modeling efforts suggest that due to hydrodynamic forces both contact force and glacial earthquake magnitudes may not be directly proportional to the energy released by the capsizing icebergs as might be expected most importantly however our results highlight the need to better understand the hydrodynamics of iceberg capsize prior to being able to accurately interpret seismic signals generated by iceberg collisions

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

analysis of low frequency seismic signals generated during a multiple iceberg calving event at jakobshavn isbrae greenland 1 we investigated seismic signals generated during a large scale multiple iceberg calving event that occurred at jakobshavn isbrae greenland on 21 august 2009 the event was recorded by a high rate time lapse camera and five broadband seismic stations located within a few hundred kilometers of the terminus during the event two full glacier thickness icebergs calved from the grounded or nearly grounded terminus and immediately capsized the second iceberg to calve was two to three times smaller than the first the individual calving and capsize events were well correlated with the radiation of low frequency seismic signals 0 1 hz dominated by love and rayleigh waves in agreement with regional records from previously published glacial earthquakes these low frequency seismic signals had maximum power and or signal to noise ratios in the 0 05 0 1 hz band similarly full waveform inversions indicate that these signals were also generated by horizontal single forces acting at the glacier terminus the signals therefore appear to be local manifestations of glacial earthquakes although the magnitudes of the signals twice time integrated force histories were considerably smaller than previously reported glacial earthquakes we thus speculate that such earthquakes may be a common if not pervasive feature of all full glacier thickness calving events from grounded termini finally a key result from our study is that waveform inversions performed on low frequency calving generated seismic signals may have only limited ability to quantitatively estimate mass losses from calving in particular the choice of source time function has little impact on the inversion but dramatically changes the earthquake magnitude accordingly in our analysis it is unclear whether the smaller or larger of the two calving icebergs generated a larger seismic signal

undercutting of marine terminating glaciers in west greenland marine terminating glaciers control most of greenland s ice discharge into the ocean but little is known about the geometry of their frontal regions here we use side looking multibeam echo sounding observations to reveal that their frontal ice cliffs are grounded deeper below sea level than previously measured and their ice faces are neither vertical nor smooth but often undercut by the ocean and rough deep glacier grounding enables contact with subsurface warm salty atlantic waters aw which melts ice at rates of meters per day we detect cavities undercutting the base of the calving faces at the sites of subglacial water sgw discharge predicted by a hydrological model the observed pattern of undercutting is consistent with numerical simulations of ice melt in which buoyant plumes of sgw transport warm aw to the ice faces glacier undercutting likely enhances iceberg calving impacting ice front stability and in turn the glacier mass balance

service life of a high density polyethylene geomembrane under simulated landfill conditions at 85 c abstractthe time to rupture of a 1 5 mm thick high density polyethylene geomembrane aged in a typical landfill composite liner configuration is investigated under a pressure of 250 kpa at 85 c the geomembrane was underlain with a geosynthetic clay liner and had a 560 g m2 needle pinched nonwoven geotextile protection layer separating it from 50 mm of drainage gravel containing leachate seventeen 0 6 m diameter tests were conducted in addition to 9 months required to deplete antioxidants stage i the tests indicated a lag period stage ii of 5 5 months and a time from the start of degradation to rupture stage iii of 20 months giving a total inferred time to rupture of 34 5 months 2 9 years there were up to 61 brittle ruptures per sample i e 2 million cracks per hectare the ruptures were predominately oriented in the machine direction and located 1 directly beneath a gravel contact 2 at the side of a gravel indentation or 3 between gravel indentations the ruptures between gravel

rapid retreat and acceleration of helheim glacier east greenland 1 a significant amount of the measured coastal thinning of the greenland ice sheet may be due to recent acceleration of outlet glaciers using remote sensing we measured two major periods of speedup on helheim glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km yr these speedups coincided with rapid retreats of the calving front totaling over 7 5 km the glacier also thinned by over 40 m from 2001 to 2003 retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area farther up glacier acceleration may be a delayed response to surface draw down and steepening of the glacier s main trunk if the 2005 speedup also produces strong thinning then much of the glacier s main trunk may un ground leading to further retreat citation howat i m i joughin s tulaczyk and s gogineni 2005 rapid retreat and acceleration of helheim glacier east greenland geophys res lett 32 l22502 doi 10 1029 2005gl024737

seasonal to decadal scale variations in the surface velocity of jakobshavn isbrae greenland observation and model based analysis 1 using new data we build upon the nearly two decade long record of observations from jakobshavn isbrae to investigate the processes driving its dynamic evolution while winter flow speed has not increased substantially over the last three winters there remains a strong seasonal variation in flow speed that coincides with a cycle of summer thinning and winter thickening we relate changes in glacier speed to geometry through variations in basal traction and horizontal stresses using ice flow models constrained by satellite and airborne observations these results suggest that the bed provides little flow resistance along the main trough within about 20 km of the terminus while the loss of buttressing from the retreat of grounded and floating ice likely contributed to the initial speedup other processes are of comparable significance at seasonal to decadal time scales from analysis of the models we hypothesize that thinning induced change in basal effective pressure is the dominant process influencing near terminus behavior while diffusive processes drive the upstream response the apparent need for the terminus to thin to near flotation before it can calve may limit the rate at which retreat occurs our analysis of the processes controlling the speed suggests little potential for further large acceleration thinning and elevated speeds may continue at rates similar to present however putting the glacier on course to retreat to the head of its deep trough in about a century at which point it likely would stabilize with a thinner terminus

framboidal pyrite evidence for persistent low oxygen levels in shallow marine facies of the nanpanjiang basin during the permian triassic transition abstract previous studies suggested that anoxia was a causal factor in the end permian mass extinction epme marked by abrupt enrichment of pyrite framboids in the post epme microbialites of the earliest triassic on shallow platforms and that this dysoxic anoxic phase followed a time of well oxygenated seafloors here we report persistent dysoxia throughout the latest permian and euxinia just before the epme based on a new redox history reconstruction study using framboidal pyrite size distribution as well as sulfur isotopic compositions of pyrites from the taiping section on the pingguo platform in the nanpanjiang basin in china further we show that the epme was followed here by rapid oxygenation not an anoxic incursion this revised redox history might be an unusual localized phenomenon when compared to other platforms or it could be solid sedimentary evidence for redox oscillations outside the bioclast enriched photic zone which broadens our understanding of terrestrial marine ecosystem interactions before and during the epme

seismic and hydroacoustic tremor generated by colliding icebergs 1 iceberg harmonic tremor iht emanating from tabular icebergs in the southern ocean and along calving margins of the antarctic ice sheet is a complex evolving signal at frequencies above approximately 0 5 hz iht has been observed as t phases on islands in the equatorial pacific as hydroacoustic signals in the indian ocean and by local and regional antarctic seismic networks to identify the iht source mechanism and to understand its relevance to iceberg calving evolution and breakup we deployed seismometers on a giant 25 by 50 km tabular iceberg called c16 in the ross sea antarctica during a uniquely accessible period austral summer 2003 2004 when it was aground against the northern shore of ross island during the deployment period c16 was in sporadic contact with another giant tabular iceberg b15a that was moving under the influence of local ocean currents this study reveals that the c16 associated iht was a manifestation of extended episodes of discrete repeating stick slip icequakes typically thousands of individual subevents per hour produced when the cliff like edges of the tabular icebergs underwent glancing strike slip collisions the iht signal that we observed is thus not a phenomenon associated with iceberg elastic or fluid resonance modes but is instead the consequence of long sequences of very regularly spaced and similar pulses of seismic radiation from these constituent stick slip subevents iht represents a newly identified glaciogenic source of seismicity that can be used to improve our understanding of iceberg dynamics and possibly of ice shelf disintegration processes

glacial rumblings from jakobshavn ice stream greenland the steep increase in greenland s glacial earthquake activity detected by the global seismographic network since the late 1990s suggests that a close inspection of these events might provide clues to the nature and origin of such seismic activity here we discuss the detection of large unexpected seismic events of extraordinarily long duration 10 40 min occurring about once every 2 days and localized in the ice stream that feeds the earth s fastest moving glacier jakobshavn isbrae from the east these glacial rumblings represent an ice mass wasting process that is greater and more frequent than glacial earthquakes have suggested probably triggered by calving the rumblings are all very similar regardless of duration and all end with a sharp earthquake like event in which the largest seismic amplitude is in the rumbling and that might signal the collapse of large ice masses upstream by calculating the total amount of seismic energy released as rumblings we estimate that the maximum seasonal amount of ice moved seismogenically down the ice stream is up to 12 km 3 or 30 of the average annual iceberg discharge in jakobshavn

glacial earthquakes in greenland and antarctica glacial earthquakes are a new class of seismic events first discovered as signals in long period seismograms recorded on the global seismographic network most of these events occur along the coasts of greenland where they are spatially related to large outlet glaciers glacial earthquakes show a strong seasonality with most events occurring during the late summer the rateofglacial earthquakeoccurrenceincreasedbetween2000and2005 with a stabilization of earthquake frequency at 2003 2004 levels in 2006 2008 recent observations establish a strong temporal correlation between the distinct seismic signals of glacial earthquakes and large ice loss events in which icebergs of cubic kilometer scale collapse against the calving face linking the seismogenic process to the force exerted by these icebergs on the glacier andtheunderlyingsolidearth asuddenchangeinglacierspeedresultsfrom these glacial earthquake calving events seasonal and interannual variations in glacier terminus position account for general characteristics of the temporal variation in earthquake occurrence glacial earthquakes in antarctica are less well studied but they exhibit several characteristics similar to glacial earthquakes in greenland