thermochronometry reveals headward propagation of erosion in an alpine landscape glacial erosion of mountain ranges produces spectacular alpine landscapes and by linking climate with tectonics influences a broad array of geophysical phenomena although the resultant landforms are easily identified the timing and spatial pattern of topographic adjustment to pleistocene glaciations remain poorly known we investigated topographic evolution in the archetypal glacial landscape of fiordland new zealand using u th he thermochronometry we find that erosion during the past 2 million years removed the entire pre pleistocene landscape and fundamentally reshaped the topography erosion focused on steep valley segments and propagated from trunk valleys toward the heads of drainage basins a behavior expected if subglacial erosion rate depends on ice sliding velocity the fiordland landscape illustrates complex effects of climate on earth s surface morphology

hydraulics of subglacial supercooling theory and simulations for clear water flows 1 glaciohydraulic supercooling is a mechanism for accreting ice and sediment to the base of glaciers we extend existing models by reworking the spring hutter model for subglacial water flow in tubular conduits to allow for distributed water sheets our goal is to determine diagnostic features of supercooling relative to controlling variables results focus on along path water flow under time varying conditions with attention to ice accretion in and along subglacial overdeepenings we contrast simulations with constant recharge to diurnally varying recharge and expose behavior that cannot be inferred from simple models for example locations of simulated ice accretion differ from those found for steady state models even though total ice accretion remains comparable to field estimates downstream accretion influences upstream effective pressures that then modify the hydraulic gradient that drives water flow this modified gradient tends to inhibit additional accretion by increasing velocity and heat production via viscous dissipation during diurnal cycles accretion varies considerably in daytime viscous dissipation dominates the heat balance and ice melts in morning and evening when flow is rising or falling viscous dissipation is lower and accretion can proceed over nighttime the largest temperature depressions occur in the subglacial system but water flux is lowest and accretion rates are negligible we conclude by inferring that overdeepened glaciers with only clear water flows evolve toward stronger supercooling regimes rather than toward a dynamic equilibrium stabilizing feedbacks are unlikely to occur through glacier hydrology alone and other processes such as erosion sedimentation and sliding must play an important role

ground motion relations for eastern north america predictive relations are developed for ground motions from eastern north american earthquakes of 4 0 m 7 25 at distances of 10 r 500 km the predicted parameters are response spectra at frequencies of 0 5 to 20 hz and peak ground acceleration and velocity the predictions are derived from an empirically based stochastic ground motion model the relations differ from previous work in the improved empirical definition of input parameters and em pirical validation of results the relations are in demonstrable agreement with ground motions from earthquakes of m 4 to 5 there are insufficient data to adequately judge the relations at larger magnitudes although they are consistent with data from the saguenay m 5 8 and nahanni m 6 8 earthquakes the underlying model parameters are constrained by empirical data for events as large as m 6 8

erosion driven uplift in the gamburtsev subglacial mountains of east antarctica the relative roles of climate and tectonics in mountain building have been widely debated central to this debate is the process of flexural uplift in response to valley incision here we quantify this process in the gamburtsev subglacial mountains a paradoxical tectonic feature in cratonic east antarctica previous studies indicate that rifting and strike slip tectonics may have provided a key trigger for the initial uplift of the gamburtsevs but the contribution of more recent valley incision remains to be quantified inverse spectral free air admittance and bouguer coherence methods indicate that unusually for continents the coherence between free air gravity anomalies and bedrock topography is high 0 5 and that the elastic thickness of the lithosphere is anomalously low 15 km in contrast to previously reported values of up to 70 km the isostatic effects of two different styles of erosion are quantified dendritic fluvial incision overprinted by alpine style glacial erosion in the gamburtsevs and outlet glacier type selective linear erosion in the lambert rift part of the east antarctic rift system 3d flexural models indicate that valley incision has contributed ca 500 m of peak uplift in the gamburtsevs and up to 1 2 km in the lambert rift which is consistent with the present day elevation of oligocene miocene glaciomarine sediments overall we find that 17 25 of gamburtsev peak uplift can be explained by erosional unloading these relatively low values are typical of temperate mountain ranges suggesting that most of the valley incision in the gamburtsevs occurred prior to widespread glaciation at 34 ma the pre incision topography of the gamburtsevs lies at 2 2 5 km above sea level confirming that they were a key inception point for the development of the east antarctic ice sheet tectonic and or dynamic processes were therefore responsible for ca 80 of the elevation of the modern gamburtsev subglacial mountains

estimation of moment magnitude and stress parameter from shakemap ground motion parameters we describe an efficient method to determine moment magnitude and stress parameter in near real time in the immediate aftermath of a small to moderate earthquake m 3 6 from shakemap ground motion parameters 5 damped pseudospectral acceleration at 1 hz 0 33 hz 10 hz and or peak ground acceleration the methodology is based on relating shakemap parameters to source and attenuation parameters within the context of a generic stochastic point source model to provide an event specific ground motion prediction equation gmpe that will reliably predict amplitudes it is an approach that has been optimized for applications in regions with sparse networks an example is provided for southern ontario in brief we initially develop a simulation guided generic gmpe model based on the available database in the region we derive the applicable regional anelastic attenuation stress parameter and site amplification models then using the estimated regional models we show how to invert shakemaps parameters to estimate moment magnitude and stress parameter in near real time to define an event specific gmpe for an event that just happened the event specific gmpe can be used to provide robust calibrated shakemaps that are fully consistent with ground motion observations

glaciohydraulic supercooling in iceland we present evidence of glaciohydraulic supercooling under jokulhlaup and ablation dominated conditions from two temperate icelandic glaciers observations show that freezing of sediment laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes intraglacial frazil ice propagation under normal ablation dominated conditions can trap copious volumes of sediment which forms anomalously thick sections of debris rich ice glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling which may explain the concentration of englacial sediments deposited in heinrich layers in the north atlantic during the last glaciation

angles of friction and dilatancy of sand it is generally realized that the peakfriction angel depends not only on density but alsoon the stress path including differences betweenplane strain and triaxial testing conditions indeed plane strain and triaxial strain angles can differ bymore than 58 for a dense sand for a loose sand atthe critical density it is often suggested that similardifferences occur e g stroud 1971 lade 1984 however some authors have presented data thatsuggest a unique critical state angle e g rowe 1962 1971 bolton 1986 this technical note presents data on a uniquecritical state angle the implication is that thefailure criterion of a very loose sand is accuratelydescribed by the mohr coulomb condition whichgives the known six sided pyramid in principalstress space the test data on dense as well as loose hostunsand are also used to study the rate of dilation this topic was extensively treated by bolton 1986 and it is now generally accepted that thetriaxial rate of dilation coincides with the rate ofdilation found in plane strain tests followingroscoe 1970 bolton used an angle of dilatancyl

ground motion prediction model from local earthquakes of the mexico basin at the hill zone of mexico city abstract we developed ground motion prediction equations gmpes based on bayesian regressions to predict response spectra at sites located in the hill zone of mexico city for local earthquakes of the mexico basin the coefficients of the gmpes are built as functions of magnitude 2 3 m w 3 8 and hypocentral distance 8 r 60 km using 15 local earthquakes we compare our results with five gmpes an attenuation model obtained in the northern region of the subduction zone in the pacific coast of mexico from small earthquakes and four other existing gmpes for shallow crustal earthquakes two of the gmpes although not the most recent ones are still used to perform probabilistic seismic hazard analysis for the region under study

the evolution of the subglacial landscape of antarctica the aim is to investigate the evolution of the subglacial landscape of antarctica using an ice sheet and erosion model we identify different stages of continental glaciation and model the erosion processes associated with each stage the model links erosion to the basal thermal regime and indicates that much of the antarctic interior may have been subject to less than 200 m of erosion the depth of erosion reflects the presence or absence of warm based ice and the consistency of ice flow direction this information linked with knowledge about landscapes of glaciation in the northern hemisphere and some simple but robust assumptions about initial topography is used to generate a map of the subglacial antarctic landscape in which much of the lowland interior resembles the landscapes of areal scouring typical of the laurentian and scandinavian shields near the continental margins selective linear erosion has overdeepened pre existing river valleys by as much as 2 8 km high elevation plateaus adjacent to such drainage systems have survived largely unmodified under cold based ice high erosion rates result from steep thermal gradients in basal ice mountain regions such as the gamburtsev mountains uplands in dronning maud land and massifs in west antarctica are likely to bear features of local alpine glaciation such landscapes may have been protected under cold based ice for the last 34 myrs or possibly longer

ground motion prediction equation for small to moderate events at short hypocentral distances with application to induced seismicity hazards abstract the evaluation of seismic hazard from induced seismicity requires the development of ground motion prediction equations gmpes that are tuned to the key magnitude distance range for such applications i use events of m 3 6 at hypocentral distances less than 40 km drawn from the next generation attenuation west 2 nga west 2 database to develop a gmpe that accounts correctly for point source scaling in both magnitude and distance space for such events the developed gmpe is in demonstrable agreement with the nga west 2 database and with the predictions of a stochastic point source simulation model the database is sparse at close distances implying epistemic uncertainty of as much as a factor of 2 in ground motion amplitudes within 10 km of the hypocenter an important conclusion from this study is that the ground motion amplitudes for moderate induced events could be much larger near the epicenter than predicted by most of the nga west 2 gmpes the potential for large motions is a consequence of the shallow depth of induced events which places the earthquake fault only a short distance beneath the epicenter

freezing of ridges and water networks preserves the gamburtsev subglacial mountains for millions of years once an ice sheet grows beyond a critical thickness the basal thermal regime favors melting and development of subglacial water networks subglacial water is necessary for bedrock erosion but the exact mechanisms that lead to preservation of subglacial topography are unclear here we resolve the freezing mechanisms that lead to long term high altitude preservation across the gamburtsev subglacial mountains in east antarctica analyses of a comprehensive geophysical data set reveal a large scale water network along valley floors the ice sheet often drives subglacial water up steep topography where it freezes along high ridges beneath thinner ice statistical tests of hypsometry show the gamburtsevs resemble younger midlatitude mountains indicating exceptional preservation we conclude that the gamburtsevs have been shielded from erosion since the latest eocene 34 ma these freezing mechanisms likely account for the spatial and temporal patterns of erosion and preservation seen in other glaciated mountain ranges

average qlg qsn and observation of lg blockage in the continental margin of nova scotia the term lg blockage refers to the sudden disappearance of the lg phase along a particular propagation path which is commonly seen at continental oceanic transition zones in this paper we present observational evidence of lg blockage across the continental margin of nova scotia in eastern canada regional lg and sn spectra from 91 events with epicentral distances between 100 and 1200 km and magnitudes between 2 5 and 4 7 are inverted simultaneously for the source spectrum site amplification and average attenuation the vertical displacement spectra were estimated between 0 9 and 10 75 hz the assumptions include a fixed frequency independent geometric spreading rate for lg and a frequency dependent spreading model for the sn estimates for the apparent regional attenuations are qlg f 615 25 f0 35 0 04 and qsn f 404 23 f0 45 0 03 results from this study provide an accurate parameterization of observed amplitude spectra and are valuable for representing wave propagation in the region based on the observation of a strong trade off between sn and lg amplitudes which have different attenuation characteristics we conclude any attenuation study based on measuring amplitude of a package of several different phases without taking into consideration the propagation characteristics of individual waveforms at the region of study may bias the estimation of average regional q

regionally adjustable generic ground motion prediction equation based on equivalent point source simulations application to central and eastern north america abstract we develop a generic ground motion prediction equation gmpe that can be adjusted for use in any region by modifying a few key model parameters the basis of the gmpe is an equivalent point source simulation model whose parameters have been calibrated to empirical data in california in such a way as to determine the decoupled effects of basic source and attenuation parameters on ground motion amplitudes we formulate the generic gmpe as a function of magnitude distance stress parameter geometrical spreading rate and anelastic attenuation this provides a fully adjustable predictive model allowing users to calibrate its parameters using observed motions in the target region we also include an empirical calibration factor to account for residual effects that are different from and or missing in simulations compared to observed motions in the target region as an example we show how the generic gmpe can be adjusted for use in central and eastern north america cena and calibrated with the next generation attenuation east database we provide median predictions of ground motions in cena for average horizontal component peak ground motions and 5 damped pseudospectral acceleration periods up to t 10 s for magnitudes m 3 8 and distances up to 600 km

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice ii theory simple theory supports field observations lawson and others 1998 that subglacial water flow out of overdeepenings can cause accretion of layered debris bearing ice to the bases of glaciers the large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths if that flow ascends a sufficiently steep slope out of an overdeepening the water will supercool as the pressure melting point rises and basal ice accretion will occur diurnal occasional or annual fluctuations in water discharge will cause variations in accretion rate debris content of accreted ice or subsequent diagenesis producing layers under appropriate conditions net accretion of debris bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget

estimation of moment magnitude from ground motions at regional distances abstract we present a simple and robust approach to estimate moment magnitude m for events of m m can be estimated from 1 hz response spectral amplitudes for events in north america with an uncertainty standard deviation of residuals of m across regions and that it provides relationships that are consistent with the predictions of a stochastic point source model for typical regional attenuation models online material table of events for which the moment magnitude was estimated

estimates of te for continental regions using goce gravity satellite only gravity fields and surface gravity obtained from altimetric measurements now agree well at wavelengths greater than 180 km satellite gravity fields can therefore be used to estimate the elastic thickness tete in regions where surface observations are sparse they are used for this purpose in a number of continental regions of india africa and antarctica where the topography is sufficiently rough and also in regions of the usa china australia and siberia where there are surface measurements estimates of tete for antarctica depend on measurements of ice thickness which are now available for much of the continent values of tete are obtained using two methods from the admittance between the free air gravity and the topography and from the coherence between bouguer gravity anomalies and the topography the first but not the second gives values of tete that are everywhere less than the seismogenic thickness where there is sufficient topography estimates of tete from precambrian shields are all greater than 10 km and do not correlate with the lithospheric thickness they are probably are governed by variations in crustal heat generation rates values for regions strongly affected by phanerozoic tectonics are all less than 7 km and all such regions are underlain by thin lithosphere

tracking induced seismicity in the fort worth basin a summary of the 2008 2018 north texas earthquake study catalogtracking induced seismicity in the fwb a summary of the 2008 2018 north texas earthquake study catalog

early east antarctic ice sheet growth recorded in the landscape of the gamburtsev subglacial mountains the gamburtsev subglacial mountains are regarded as a key nucleation site for the antarctic ice sheet and they retain a unique long term record of pre glacial and early glacial landscape evolution here we use a range of morphometric analyses to constrain the nature of early glaciation and subsequent ice sheet evolution in the interior of east antarctica using a new digital elevation model of the gamburtsev subglacial mountains derived from an extensive airborne radar survey we find that an inherited fluvial landscape confirms the existence of the gamburtsev subglacial mountains prior to the onset of glaciation at the eocene oligocene climate boundary ca 34 ma features characteristic of glaciation at a range of scales are evident across the mountains high elevation alpine valley heads akin to cirques identified throughout the mountains are interpreted as evidence for early phases of glaciation in east antarctica the equilibrium line altitudes associated with these features combined with information from fossil plant assemblages suggest that they formed at or prior to 34 ma it cannot be ruled out that they may have been eroded by ephemeral ice between the late cretaceous and the eocene 100 34 ma hanging valleys overdeepenings truncated spurs and steep sided linear valley networks are indicative of a more widespread alpine glaciation in this region these features represent ice growth at or before 33 7 ma and provide a minimum estimate for the scale of the east antarctic ice sheet between ca 34 and 14 ma when dynamic fluctuations in ice extent are recorded at the coast of antarctica the implications are that the early east antarctic ice sheet grew rapidly and developed a cold based core that preserved the alpine landscape the patterns of landscape evolution identified provide the earliest evidence for the development of the east antarctic ice sheet and can be used to test coupled ice climate evolution models

positive feedbacks associated with erosion of glacial cirques and overdeepenings the principal points of water input to a glacier are the bergschrund in cirques and crevasse fields lower on the glacier crevasse fields commonly occur over convexities at the heads of overdeepenings in glacier beds the amplitude of subglacial water pressure fluctuations is large just down glacier from these points of water input erosion by quarrying is likely in such areas erosion is thus inferred to be localized on the headwalls of cirques and overdeepenings in the case of overdeepenings this leads to a positive feedback process in which a perturbation in the bed causes crevassing at the surface resulting in erosional forces that accentuate the perturbation when subglacial water flows up an adverse bed slope leading out of a cirque or overdeepening much of the viscous energy dissipated is used to warm the water to keep it at the pressure melting temperature as the ice thins and the pressure decreases in such situations subglacial conduits are maintained by high water pressures rather than by melting of conduit walls in the limit water pressures apparently become so high that water is forced out along the ice bed interface and the conduits collapse the products of erosion are then no longer flushed out and a protective till layer accumulates by limiting erosion on such adverse bed slopes this till layer controls the geometry of these overdeepened basins

glaciation as a destructive and constructive control on mountain building glacial erosion is thought to have an important role in controlling mountain height a common perception is that glaciers are powerful erosive agents inhibiting the ability of active mountains to rise significantly above the elevation of permanent glaciation through a process known as the glacial buzzsaw but now a study of erosion patterns and climate data from the late cenozoic in the glaciated patagonian andes suggests that glaciation can also have the reverse effect in effect making mountains higher at these polar latitudes glaciation acts as a protective layer preserving the uplifting mountains from erosion and allowing them to reach heights well above those predicted had a glacial buzzsaw been active it has been suggested that glacial erosion has an important role in controlling mountain height here spatial and temporal patterns of erosion in the glaciated patagonian andes have been assessed with the help of a low temperature thermochronologic data set the results show that between 38 s and 49 s accelerated erosion at the onset of widespread glaciation limits mountain height but at higher latitudes glaciation protects the landscape from erosion and leads to growth in mountain height and width theoretical analysis predicts that enhanced erosion related to late cenozoic global cooling can act as a first order influence on the internal dynamics of mountain building leading to a reduction in orogen width and height1 2 3 the strongest response is predicted in orogens dominated by highly efficient alpine glacial erosion producing a characteristic pattern of enhanced erosion on the windward flank of the orogen and maximum elevation controlled by glacier equilibrium line altitude3 4 where long term glacier mass gain equals mass loss however acquiring definitive field evidence of an active tectonic response to global climate cooling has been elusive5 here we present an extensive new low temperature thermochronologic data set from the patagonian andes a high latitude active orogen with a well documented late cenozoic tectonic climatic and glacial history data from 38 s to 49 s record a marked acceleration in erosion 7 to 5 myr ago coeval with the onset of major patagonian glaciation6 and retreat of deformation from the easternmost thrust front7 the highest rates and magnitudes of erosion are restricted to the glacial equilibrium line altitude on the windward western flank of the orogen as predicted in models of glaciated critical taper orogens where erosion rate is a function of ice sliding velocity3 8 in contrast towards higher latitudes 49 s to 56 s a transition to older bedrock cooling ages signifies much reduced late cenozoic erosion despite dominantly glacial conditions here since the latest miocene6 the increased height of the orogenic divide at these latitudes well above the equilibrium line altitude leads us to conclude that the southernmost patagonian andes represent the first recognized example of regional glacial protection of an active orogen from erosion leading to constructive growth in orogen height and width

the attenuation of fourier amplitudes for rock sites in eastern north america we develop an empirical model of the decay of fourier amplitudes for earthquakes of m 3 6 recorded on rock sites in eastern north america and discuss its implications for source parameters attenuation at distances from 10 to 500 km may be adequately described using a bilinear model with a geometric spreading of 1 r 1 3 to a transition distance of 50 km with a geometric spreading of 1 r 0 5 at greater distances for low frequencies and distances less than 50 km the effective geometric spreading given by the model is perturbed using a frequency and hypocentral depth dependent factor defined in such a way as to increase amplitudes at lower frequencies near the epicenter but leave the 1 km source amplitudes unchanged the associated anelastic attenuation is determined for each event with an average value being given by a regional quality factor of q 525f 0 45 this model provides a match on aver age between the known seismic moment of events and the inferred low frequency spectral amplitudes at r 1 km obtained by correcting for the attenuation model the inferred brune stress parameters from the high frequency source terms are about 600 bars 60 mpa on average for events of m 4 5

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

why the new madrid earthquakes are m 7 8 and the charleston earthquake is m 7 estimates of magnitudes of large historical earthquakes are an essential input to and can seriously affect seismic hazard estimates the earthquake intensity observations modified mercalli intensities mmi and assigned magnitudes m of the 1811 1812 new madrid events have been reinterpreted several times in the last decade and have been a source of controversy in making seismic hazard estimates in the central united states observations support the concept that the larger the earthquake the greater the maximum felt distance for the same crustal attenuation and local soil conditions magnitude should be the main influence on intensity values at large distances we apply this concept by comparing the mean mmi at distances of 600 1200 km for each of the four largest new madrid 1811 1812 earthquakes the 1886 charleston south carolina earthquake the 1929 m 7 2 grand banks earthquake and the 2001 m 7 6 bhuj india earthquake we fit the intensity observations using the form mmi a c dist 0 8 log dist to better define intensity attenuation in eastern north america ena the intensity attenuation in cratonic india differs from ena and is corrected to ena using both the above estimate and published intensity relations we evaluate source marine geophysical q and stress drop information as well as a 1929 milne shaw record at chicago to confirm that the 1929 grand banks earthquake occurred in ena crust our direct comparison of mean intensities beyond 600 km suggests m 7 5 7 3 7 7 and 6 9 for the three new madrid 1811 1812 mainshocks and the largest aftershock and m 7 0 for the 1886 charleston south carolina earthquake with an estimated uncertainty of 0 3 units at the 95 confidence level based on a monte carlo analysis our mean new madrid and charleston mainshock magnitudes are similar to those of bakun and hopper 2004 and are much higher than those of hough and page 2011 for new madrid online material tables of mean modified mercalli intensity for 800 1200 and 600 1000 km distance ranges and figures of least squares fit for all intensity measures used and for frequency dependent q in easternmost canada

frequency dependent lg wave attenuation in northern morocco abstract frequency dependent attenuation q 1 in the crust of northern morocco is estimated from lg wave spectral amplitude measurements every quarter octave in the frequency band 0 8 to 8 hz this study takes advantage of the improved broadband data coverage in the region provided by the deployment of the iberarray seismic network earthquake data consist of 71 crustal events with magnitudes 4 m b 5 5 recorded on 110 permanent and temporary seismic stations between january 2008 and december 2013 with hypocentral distances between 100 and 900 km 1274 high quality lg waveforms provide dense path coverage of northern morocco crossing a region with a complex structure and heterogeneous tectonic setting as a result of continuous interactions between the african and eurasian plates we use two different methods the coda normalization cn analysis that allows removal of the source and site effects from the lg spectra and the spectral amplitude decay sad method that simultaneously inverts for source site and path attenuation terms the cn and sad methods return similar results indicating that the lg q models are robust to differences in the methodologies larger errors and no significant frequency dependence are observed for frequencies lower than 1 5 hz for distances up to 400 km and the frequency band 1 5 hz 4 5 the model functions q f 529 22 23 f 1 5 0 23 0 06 and q f 457 7 7 f 1 5 0 44 0 02 are obtained using the cn and sad methods respectively a change in the frequency dependence is observed above 4 5 hz for both methods which may be related to the influence of the sn energy on the lg window the frequency dependent q 1 estimates represent an average attenuation beneath a broad region including the rif and tell mountains the moroccan and algerian mesetas the atlas mountains and the sahara platform structural domains and correlate well with areas of moderate seismicity where intermediate q values have been obtained

the effect of crustal structure on strong ground motion attenuation relations in eastern north america strong ground motion attenuation relations are usually described by smoothly decreasing functions of distance however consideration of wave propagation in the crust suggests that attenuation relations should be more complex such complexity may be present in strong ground motion data for eastern north american earthquakes which show amplitudes in the distance range of 60 to 150 km that lie above the trends at smaller and greater distances using a wavenumber integration method to compute green s functions and close in recordings of several earthquakes as empirical source functions we have generated synthetic seismograms that are in good agreement with regional and strong motion recordings of eastern north american earthquakes from these synthetic seismograms we have shown that the observed interval of relatively high amplitudes may be attributable to postcritically reflected s waves from the moho the presence and location of the interval of relatively high amplitudes is highly dependent on the crustal velocity structure and may therefore be expected to show regional variation however for any realistic structure model there will be a transition in the attenuation relation from an interval at shorter distances less than about 100 km that is dominated by direct waves to an interval at greater distances that is dominated by postcritically reflected waves the synthetic seismograms have response spectral velocities that match those of the recorded data and their m blg values are in good agreement with observed values

partially nonergodic empirical ground motion models for predicting horizontal and vertical pgv pga and 5 damped linear acceleration response spectra using data from the iranian plateau we present new ground motion prediction equations gmpes to estimate horizontal and vertical strong ground motion intensity measures gmims generated by shallow active crustal earthquakes occurring within the iranian plateau to this end a dataset containing 688 records from 152 earthquakes with moment magnitudes ranging from 4 7 to 7 4 and joyner boore distances up to 250 km has been used the effects of the local site condition are taken into account using the time averaged shear wave velocities in the upper 30 m v s 30 we decided not to include the style of faulting term in the final functional form because the total standard deviation is reduced 10 by removing this term from the functional form we used a nonlinear mixed effects regression to determine the coefficients of the functional form and to separate out the between event and between station standard deviations from the total standard deviation significant standard deviation of site to site variability demonstrates that the ergodic assumption is not able to account for the spatial variability of ground motions we introduced random effects coefficients to capture regional variations between different tectonic regions of the iranian plateau such as alborz and zagros in the regression analysis to investigate the effects of regionalization on gmpes the results showed that although the effects of regional variations for considered regions are negligible at close distances they are significant at longer distances the complexity and performance of the final functional form is justified by comparing akaike and bayesian information criteria values over many trial functional forms moreover the distribution of between event site to site and event station corrected residuals demonstrates that no trends are evident implying satisfactory performance of the proposed gmpes therefore the derived gmpes can be employed to predict gmims and to do seismic hazard assessments within the iranian plateau electronic supplement 1 lists of the events and stations considered in the final dataset and derived coefficients of the functional form 1 http www bssaonline org lookup suppl doi 10 1785 0120160205 dc1

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

cenozoic landscape evolution of an east antarctic oasis radok lake area northern prince charles mountains and its implications for the glacial and climatic history of antarctica abstract ice free areas antarctica reveal a multi million year history of landscape evolution but most attention up to now has focused on the transantarctic mountains the amery oasis in the northern prince charles mountains borders the lambert glacier amery ice shelf system that drains 1 million km2 of the east antarctic ice sheet and therefore provides a record of fluctuations of both local and regional ice since the ice sheet first formed in early oligocene time this glacial record has been deciphered by i geomorphological mapping from aerial photographs and on the ground ii documenting the relationship between thick well dated uplifted glaciomarine strata and the underlying palaeolandscape iii examining surficial sediment facies and iv surface exposure dating using 10be and 26al the se amery oasis records at least 10 million years of landscape evolution beginning with a pre late miocene phase of glacial erosion followed by deposition of glaciomarine strata of the battye glacier formation pagodroma group in late miocene time a wet based ice sheet next expanded over the se amery oasis following which deposition of the glaciomarine pliocene bardin bluffs formation pagodroma group took place both formations were uplifted by at least 500 and 200 m respectively their tops are characterised by geomorphological surfaces upon which intensive periglacial activity took place higher level bedrock areas were subjected to deep weathering and tor formation early pleistocene time was characterised by expansion of a cold based ice sheet across the whole area but it left little more than patches of sandy gravel and erratic blocks late pleistocene expansion of local ice the battye glacier saw deposition of moraine mound complexes on low ground around radok lake and ice dammed lake phenomena subglacial drainage of the lake escaped to the east exhuming the sediment filled gorges holocene landscape modification has been relatively superficial overall the landscape of the amery oasis evolved primarily under the influence of wet based probably polythermal glaciers in miocene and pliocene times whereas the quaternary period was characterised mainly by cold based glaciers that had comparatively little impact on the landscape

glacial effects limiting mountain height the height of mountain ranges reflects the balance between tectonic rock uplift crustal strength and surface denudation tectonic deformation and surface denudation are interdependent however and feedback mechanisms in particular the potential link to climate are subjects of intense debate spatial variations in fluvial denudation rate caused by precipitation gradients are known to provide first order controls on mountain range width crustal deformation rates and rock uplift moreover limits to crustal strength are thought to constrain the maximum elevation of large continental plateaus such as those in tibet and the central andes there are indications that the general height of mountain ranges is also directly influenced by the extent of glaciation through an efficient denudation mechanism known as the glacial buzzsaw here we use a global analysis of topography and show that variations in maximum mountain height correlate closely with climate controlled gradients in snowline altitude for many high mountain ranges across orogenic ages and tectonic styles with the aid of a numerical model we further demonstrate how a combination of erosional destruction of topography above the snowline by glacier sliding and commensurate isostatic landscape uplift caused by erosional unloading can explain observations of maximum mountain height by driving elevations towards an altitude window just below the snowline the model thereby self consistently produces the hypsometric signature of the glacial buzzsaw and suggests that differences in the height of mountain ranges mainly reflect variations in local climate rather than tectonic forces

origin of the adventure subglacial trench linked to cenozoic extension in the east antarctic craton abstract the antarctic plate occupies a unique geodynamic setting being mostly surrounded by divergent or transform margins major intracontinental basins and highlands characterize its bedrock buried under the 34 ma east antarctic ice sheet eais their formation atop of the cratonic lithosphere in the interior of east antarctica remains a major open question post mesozoic intraplate extensional tectonic activity has been proposed for their development and is supported by this work here we focus on the adventure subglacial trench ast whose origin is poorly constrained and controversial as currently available geophysical models suggest either extensional or compressional tectonic origin the ast is an over 250 km long 60 km wide subglacial trough elongated in the nnw sse direction adjacent to the westernmost flank of the wilkes subglacial basin and is parallel to regional scale alignments of magnetic and gravimetric anomalies geophysical campaigns allowed better definition of the ast physiography showing its typical half graben geometry the rounded morphology of the western flank of the ast was simulated through tectonic numerical modelling we consider the subglacial landscape to primarily reflect a preserved relict of the tectonic processes affecting the interior of east antarctica in the cenozoic due to the negligible erosion deposition capability of the eais the bedrock morphology was replicated through the activity of the listric adventure fault characterized by a basal detachment at the base of the crust 34 km and a vertical displacement of 2 5 km this length suggests its regional crustal importance the predicted displacement is interpreted either as a newly formed fault or as the partial reactivation of a weaker zone along a major precambrian crustal scale tectonic boundary the extensional regime in the ast is part of a more extensive 800 km long intraplate corridor characterized by nearly along strike extension in cenozoic times with a left lateral transpressional component this corridor may represent the effect of far field stresses induced by plate motions

extremely low long term erosion rates around the gamburtsev mountains in interior east antarctica the high elevation and rugged relief 3 km of the gamburtsev subglacial mountains gsm have long been considered enigmatic orogenesis normally occurs near plate boundaries not cratonic interiors and large scale tectonic activity last occurred in east antarctica during the pan african 480 600 ma we sampled detrital apatite from eocene sands in prydz bay at the terminus of the lambert graben which drained a large pre glacial basin including the northern gamburtsev mountains apatite fission track and u th he cooling ages constrain bedrock erosion rates throughout the catchment we double dated apatites to resolve individual cooling histories erosion was very slow averaging 0 01 0 02 km myr for 250 myr supporting the preservation of high elevation in interior east antarctica since at least the cessation of permian rifting long term topographic preservation lends credence to postulated high elevation mountain ice caps in east antarctica since at least the cretaceous and to the idea that cold based glaciation can preserve tectonically inactive topography

empirical attenuation of ground motion spectral amplitudes in southeastern canada and the northeastern united states a database of 1700 digital seismograms from 186 earthquakes of mag nitude mn 2 5 5 6 that occurred in southeastern canada and the northeastern united states from 1990 to 2003 was compiled maximum likelihood regression analysis of the database was performed to determine a model for the attenuation of fourier spectral amplitudes for the shear window for the vertical and horizontal component of motion for frequencies from 0 2 to 20 hz fourier amplitudes follow a hinged trilinear attenuation model fourier spectral amplitudes decay as r 1 3 where r is hypocentral distance within 70 km of the source there is a transition zone from 70 to 140 km as the direct waves are joined by strong postcritical reflections where the attenuation is described as r 0 2 spectral amplitudes actually increase with distance in this range for low frequencies beyond 140 km the attenuation is well described by r 0 5 corresponding to geometric spreading in two dimensions the associated model for the regional quality factor for frequencies greater than 1 hz can be ex pressed as q 893f 032 q can be better modeled over a wider frequency range 0 2 20 hz by a polynomial expression log q 3 052 0 393 log f 0 945 log f 2 0 327 log f 3 the polynomial expression accommodates the observation that q values are at a minimum about 1000 near 1 hz and rise at both lower and higher frequencies correction factors for the spectral amplitude model that describe the effects of focal depth on the amplitudes and their attenuation are developed using the subset of events with known focal depth the attenuation model is similar to that determined from an earlier study with more limited data atkinson and mereu 1992 but the enlarged database indicates more rapid near source amplitude decay and higher q the attenuation model is used to play back attenuation effects to determine the apparent source spectrum for each earthquake in the database and hence determine moment magnitude m and brune stress drop the events have moment magnitude in the range from 2 5 to 5 stress drop increases with moment magnitude for events of m 4 3 then appears to attain a relatively constant level in the range from 100 to 200 bars for the larger events as previously noted in atkinson 1993b the results of this study provide a useful framework for improving regional ground motion relations in eastern north america they further our understanding of attenuation in the region through analysis of an enlarged ground motion database in particular the inclusion of the three component broadband data gathered over the last decade allows extension of attenuation models to both horizontal and vertical components over a broad frequency range 0 2 20 hz

widespread persistent thickening of the east antarctic ice sheet by freezing from the base an international polar year aerogeophysical investigation of the high interior of east antarctica reveals widespread freeze on that drives substantial mass redistribution at the bottom of the ice sheet although the surface accumulation of snow remains the primary mechanism for ice sheet growth beneath dome a 24 of the base by area is frozen on ice in some places up to half of the ice thickness has been added from below these ice packages result from the conductive cooling of water ponded near the gamburtsev subglacial mountain ridges and the supercooling of water forced up steep valley walls persistent freeze on thickens the ice column alters basal ice rheology and fabric and upwarps the overlying ice sheet including the oldest atmospheric climate archive and drives flow behavior not captured in present models

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

attenuation of lg waves in the new madrid seismic zone of the central united states using the coda normalization method abstract unique properties of coda waves are employed to evaluate the frequency dependent quality factor of lg waves using the coda normalization method in the new madrid seismic zone of the central united states instrument and site responses are eliminated and source functions are isolated to construct the inversion problem for this purpose we used 121 seismograms from 37 events with moment magnitudes m ranging from 2 5 to 5 2 and hypocentral distances from 120 to 440 km recorded by 11 broadband stations a singular value decomposition svd algorithm is used to extract q values from the data while the geometric spreading exponent is assumed to be a constant inversion results are then fitted with a power law equation from 3 to 12 hz to derive the frequency dependent quality factor function the final results of the analysis are q v lg f 410 38 f 0 49 0 05 for the vertical component and q h lg f 390 26 f 0 56 0 04 for the horizontal component where the term after sign represents one standard error for stations within the mississippi embayment with an average sediment depth of 1 km around the memphis metropolitan area estimation of quality factor using the coda normalization method is not well constrained at low frequencies f

the shape of ground motion attenuation curves in southeastern canada the shape of the attenuation curve of the fourier spectrum of acceleration is established for southeastern canada from empirical data the dataset is comprised of 1200 digital vertical component seismograms from 100 earthquakes recorded on 30 stations of the eastern canada telemetered network the distance range of the observations is 10 to 1700 km focal depths are from 5 to 30 km results indicate that the attenuation curve has three distinct sections at distances less than 70 km corresponding to attenuation of the direct wave amplitudes decay slightly more rapidly than 1 r r 1 1 between 70 and 130 km where the direct wave is joined by sms spectral amplitudes are approximately constant beyond 130 km corresponding to the lg phase amplitudes decay at a rate that is consistent with r 0 5 and q 670 f 0 33 the findings support the hypothesis of burger et al 1987 that postcritical reflections from the moho discontinuity play an important role in determining the shape of the attenuation curve however the influence of sms is subtle allowing the shape to be approximated by simple functional forms for example the commonly assumed bilinear form r 1 r 0 5 with frequency dependent q fits well if the transition from direct wave to lg spreading is placed at approximately 60 km a linear form r 1 over all distances with constant q 2000 also provides a satisfactory fit overall we find little evidence for the dependence of attenuation on focal depth or tectonic province

formation of ice eddies in subglacial mountain valleys radar data from both greenland and antarctica show folds and other disruptions to the stratigraphy of the deep ice the mechanisms by which stratigraphy deforms are related to the interplay between ice flow and topography here we show that when ice flows across valleys or overdeepenings viscous overturnings called moffatt eddies can develop at the base of a subglacial valley the shear on the valley sidewalls is transferred through the ice forcing the ice to overturn to understand the formation of these eddies we numerically solve the non newtonian stokes equations with a glen s law rheology to determine the critical valley angle for the eddies to form when temperature is incorporated into the ice rheology the warmer basal ice is less viscous and eddies form in larger valley angles shallower slopes than in isothermal ice we also show that when ice flow is not perpendicular to the valley orientation complex 3d eddies transport ice down the valley we apply our simulations to the gamburtsev subglacial mountains and solve for the ice flow over radar determined topography these simulations show moffatt eddies on the order of 100 meters tall in the deep subglacial valleys

evaluation of ground motion models for usgs seismic hazard forecasts induced and tectonic earthquakes in the central and eastern united statesevaluation of gmms for usgs seismic hazard forecasts

evidence for early glaciation of southeastern beringia between the klondike plateau and yukon tanana highlands of yukon and alaska respectively current maps explain glaciated alpine locales and periglacial areas in terms of localized pliocene pleisto

modelled nucleation centres of the pleistocene ice sheets from an ice sheet model with subgrid topographic and glaciologic parameterizations abstract ice sheet inception at the onset of the last glacial epoch ca 115 kyr bp is not well understood to cause the northern hemisphere to descend into ice age conditions internal climate system feedbacks must have acted to amplify relatively low amplitude orbital cooling trends while a number of plausible internal mechanisms have been proposed for this internal amplification the dramatic continental ice sheet expansion that is recorded in the marine sediment record is an enduring mystery global sea level lowering of at least 20 m and as much as 75 m over 10 kyr appears typical of the early stages of glacial inception this rate of ice sheet growth has not been freely captured in either glaciologic or climatic models is it because we do not adequately understand the relevant internal feedbacks or fail to capture them in models is it simply related to technical model limitations e g topographic resolution this paper explores the premise that model resolution poses a critical limitation on simulations of ice sheet nucleation i present a series of ice sheet nucleation simulations in north america and eurasia based on an ice sheet model that includes parameterizations of km scale topographic detail snow ice accumulation and melt and glacier dynamics model climate is based on perturbations to modern observational climatology even with this topographic detail and with simulations long enough to allow ice to advect downslope and spread over the terrain it remains difficult to induce a large scale glacierization in both north america and eurasia severe climate perturbations are required cold temperatures need to be accompanied by substantial increases in precipitation in the north atlantic and polar regions in order to accrue continental ice at the rate demanded by sea level reconstructions the simulations presented here are too simple to separate climate perturbations from internal system feedbacks but with modern precipitation rates a combined climate cooling net positive feedback of more than 8 c is required to establish large scale ice sheets in

ground motions for induced earthquakes in oklahoma we examine ground motions for a sequence of earthquakes in oklahoma where assessment of hazard contributions from induced seismicity is of particular interest we aim to empirically calibrate a model driven equation that was derived for central and eastern north america cena so that it will match the observed ground motions of the 2011 prague oklahoma sequence we first show that ground motions in oklahoma decay at a rate similar to the average attenuation observed in the stable continental region of cena we then search for any needed adjustments to the cena source model to match ground motions from the induced seismicity sequence that occurred near prague in 2011 an interesting feature noted in the ground motion analysis is that the stress parameters for the prague mainshock events m 5 are notably higher than those for aftershocks moreover the stress parameter that characterizes the high frequency ground motion decays in both time and space relative to the three largest events the largest events in the prague sequence have similar source parameters to natural cena events of the same magnitude and focal depth but their aftershocks have weaker motions

frozen bed fennoscandian and laurentide ice sheets during the last glacial maximum the areal extents of the laurentide and fennoscandian ice sheets during the last glacial maximum about 20 000 years ago are well known1 but thickness estimates range widely from high domed2 to thin3 with large implications for our reconstruction of the climate system regarding for example northern hemisphere atmospheric circulation and global sea levels this uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials4 a knowledge of which would better constrain reconstructions of ice sheet thickness here we show that in the absence of direct data the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen and thawed bed conditions we argue that ribbed moraines were formed by brittle fracture of subglacial sediments induced by the excessive stress at the boundary between frozen and thawed bed conditions resulting from the across boundary difference in basal ice velocity maps of glacial landforms from aerial photographs of canada and scandinavia reveal a concentration of ribbed moraines around the ice sheet retreat centres of quebec keewatin newfoundland and west central fennoscandia together with the evidence from relict landscapes that mark glacial areas with frozen bed conditions the distribution of ribbed moraines on both continents suggest that a large area of the laurentide and fennoscandian ice sheets was frozen based and therefore high domed and stable during the last glacial maximum

position and variability of complex structures in the central east antarctic ice sheet although the flow of the east antarctic ice sheet is well constrained from surface measurements and altimetry our knowledge of the dynamic processes within the ice sheet remains limited recent high resolution radar data from the gamburtsev subglacial mountains in central east antarctica reveal a series of anomalous englacial reflectors in the lower half of the ice column that cannot be explained by conventional ice flow expanding on previous analyses we describe the geometrical and morphological features of 12 of these anomalous reflectors our description reveals a previously unacknowledged diversity in size geometry and internal structure of these reflectors we are able to identify four distinct morphological features 1 fingers 2 inclusions 3 sheets and 4 folds the fingers and inclusions probably form by shear instabilities at the boundary between the reflectors and the surrounding meteoric ice the sheets highlight that basal ice can be uplifted off of the bed and above surrounding meteoric ice and the folds may have formed in local regions of converging flow associated with subglacial topography the study provides key insights into the rheology stress and deformational regimes deep within the central east antarctic ice sheet

earthquake source spectra in eastern north america source spectra for earthquakes of 3 m 7 in eastern north america ena are determined from regional seismograph recordings strong motion records teleseismic data and modified mercalli intensity data the inclusion of mmi data is made possible by the finding that the felt area of an earthquake is an accurate measure of its high frequency spectral level at the source current ground motion relations for ena are based on a brune source model with a stress parameter of 100 bars this model is a poor fit to ena source spectra for events of m 4 at high frequencies 5 to 10 hz observed spectral amplitudes are more nearly matched by a brune stress parameter of 200 bars intermediate frequency amplitudes near 1 hz are matched by a stress parameter of approximately 50 bars an empirical two corner source model is proposed to describe ena source spectra for events of 4 m 7

earthquake ground motion prediction equations for eastern north america new earthquake ground motion relations for hard rock and soil sites in eastern north america ena including estimates of their aleatory uncertainty vari ability have been developed based on a stochastic finite fault model the model incorporates new information obtained from ena seismographic data gathered over the past 10 years including three component broadband data that provide new in formation on ena source and path effects our new prediction equations are similar to the previous ground motion prediction equations of atkinson and boore 1995 which were based on a stochastic point source model the main difference is that high frequency amplitudes f 5 hz are less than previously predicted by about a factor of 1 6 within 100 km because of a slightly lower average stress parameter 140 bars versus 180 bars and a steeper near source attenuation at frequencies less than 5 hz the predicted ground motions from the new equations are generally within 25 of those predicted by atkinson and boore 1995 the prediction equations agree well with available ena ground motion data as evidenced by near zero average residuals within a factor of 1 2 for all frequencies and the lack of any significant residual trends with distance however there is a tendency to positive residuals for moderate events at high frequencies in the distance range from 30 to 100 km by as much as a factor of 2 this indicates epistemic uncertainty in the prediction model the positive residuals for moderate events at 100 km could be eliminated by an increased stress parameter at the cost of producing negative residuals in other magnitude distance ranges adjustment factors to the equations are provided that may be used to model this effect

cenozoic extension along the reactivated aurora fault system in the east antarctic craton abstract the east antarctic craton is characterized by major intracontinental basins and highlands buried under the 34 ma east antarctic ice sheet their formation remains a major open question paleozoic to cenozoic intraplate extensional tectonic activity has been proposed for their development and in this work the latter hypothesis is supported here we focus on the aurora trench at within the aurora subglacial basin latitude 75 77 s longitude 117 118 e whose origin is still poorly constrained the at is an over 150 km long 25 km wide subglacial trough elongated in the nnw sse direction geophysical campaigns allowed better definition of the at physiography showing typical half graben geometry the rounded morphology of the western flank of the at was simulated through tectonic numerical modelling we consider the subglacial landscape to primarily reflect the locally preserved relict morphology of the tectonic processes affecting the interior of east antarctica in the cenozoic the bedrock morphology was replicated through the activity of the listric aurora trench fault characterized by a basal detachment at 34 km considered the base of the crust according to available geophysical interpretations and vertical displacements ranging between 700 and 300 m the predicted displacement is interpreted as the partial reactivation of a weaker zone along a major precambrian crustal scale tectonic boundary we propose that the aurora trench fault is the southern continuation of the 1000 km long aurora fault independently recognized by previous studies together they form the aurora fault system a long lived tectonic boundary with poly phased tectonic history within the eac that bounds the eastern side of the aurora subglacial basin the younger cenozoic reactivation of the investigated segment of the aurora fault system relates to the intraplate propagation of far field stresses associated to the plate scale kinematics in the southern ocean

near source geometrical spreading in the central virginia seismic zone determined from the aftershocks of the 2011 mineral virginia earthquake we used aftershocks of the 2011 mineral virginia earthquake to study geometrical spreading at hypocentral distances less than 60 km in the central virginia seismic zone sixty nine aftershocks occurring from 25 august 2011 through 24 december 2011 provided the data we used the coda normalization method to estimate the attenuation coefficient associated with geometrical spreading we filtered the time domain signals in several octave wide frequency bands and examined attenuation of peak s wave amplitude in the 1 0 30 0 hz frequency range amplitude was assumed to decrease as a function of hypocenter distance r according to r the coefficient of attenuation was examined for the three component s wave amplitudes with corrections for sh and sv radiation patterns we observed no systematic frequency dependence of the coefficient of attenuation for the radial and transverse components assuming infinite quality factor q derived as a weighted mean over the entire range of frequencies 1 30 hz are both 1 51 0 05 the weighted mean value of the attenuation coefficient on the vertical component over the same range of frequencies is 1 45 0 05 slightly less than for the horizontal components we corrected the data assuming three q models the estimated geometrical spreading coefficients are in the 1 30 1 46 range depending on the assumed q model and component which is only slightly less than the estimates of determined assuming infinite q the estimated attenuation coefficients differ significantly from the value of 1 0 expected for a whole space the results for the horizontal components are in agreement with previous full wavefield modeling however the observed vertical component attenuation is substantially less than that predicted by the synthetics the depths of the earthquakes are less than 8 km so these results may not be representative of geometrical spreading in parts of eastern north america where earthquakes occur at greater depths online material table of earthquake hypocenters and focal mechanisms

role of transient water pressure in quarrying a subglacial experiment using acoustic emissions 1 probably the most important mechanism of glacial erosion is quarrying the growth and coalescence of cracks in subglacial bedrock and dislodgement of resultant rock fragments although evidence indicates that erosion rates depend on sliding speed rates of crack growth in bedrock may be enhanced by changing stresses on the bed caused by fluctuating basal water pressure in zones of ice bed separation to study quarrying in real time a granite step 12 cm high with a crack in its stoss surface was installed at the bed of engabreen norway acoustic emission sensors monitored crack growth events in the step as ice slid over it vertical stresses water pressure and cavity height in the lee of the step were also measured water was pumped to the lee of the step several times over 8 days pumping initially caused opening of a leeward cavity which then closed after pumping was stopped and water pressure decreased during cavity closure acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically with repeated pump tests this crack grew with time until the step s lee surface was quarried our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded natural basal water pressure fluctuations should also concentrate stresses on rock steps increasing rates of crack growth stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size which may help explain the effect of sliding speed on erosion rates

history mass loss structure and dynamic behavior of the antarctic ice sheet antarctica contains most of earth s fresh water stored in two large ice sheets the more stable east antarctic ice sheet is larger and older rests on higher topography and hides entire mountain ranges and ancient lakes the less stable west antarctic ice sheet is smaller and younger and was formed on what was once a shallow sea recent observations made with several independent satellite measurements demonstrate that several regions of antarctica are losing mass flowing faster and retreating where ice is exposed to warm ocean waters the antarctic contribution to sea level rise has reached 8 millimeters since 1992 in the future if warming ocean waters and increased surface meltwater trigger faster ice flow sea level rise will accelerate

identification and control of subglacial water networks under dome a antarctica 1 subglacial water in continental antarctica forms by melting of basal ice due to geothermal or frictional heating subglacial networks transport the water from melting areas and can facilitate sliding by the ice sheet over its bed subglacial water flow is driven mainly by gradients in overburden pressure and bed elevation we identify small median 850 m water bodies within the gamburtsev subglacial mountains in east antarctica organized into long 20 103 km coherent drainage networks using a dense 5 km grid of airborne radar data the individual water bodies are smaller on average than the water bodies contained in existing inventories of antarctic subglacial water and most are smaller than the mean ice thickness of 2 5 km reflecting a focusing of basal water by rugged topography the water system in the gamburtsev subglacial mountains reoccupies a system of alpine overdeepenings created by valley glaciers in the early growth phase of the east antarctic ice sheet the networks follow valley floors either uphill or downhill depending on the gradient of the ice sheet surface in cases where the networks follow valley floors uphill they terminate in or near plumes of freeze on ice indicating source to sink transport within the basal hydrologic system because the ice surface determines drainage direction within the bed constrained network the system is bed routed but surface directed along flow variability in the structure of the freeze on plumes suggests variability in the networks on long 10s of ka timescales possibly indicating changes in the basal thermal state

quaternary relief generation by polythermal glacier ice the juxtaposition of wet based erosive ice in valleys and cold based non erosive ice atop felsenmeer covered interfluve plateaus has generated relief in the torngat mountains of northeastern canada measurements of in situ terrestrial cosmogenic nuclide tcn concentrations from 31 bedrock sites coupled with soils and geomorphology indicate that erosion of the valleys has been 2 m during a single glacial interglacial cycle however on summit plateaus the long term over several glacial interglacial cycles erosion rate is 1 4 m ma 1 tcn ratios reveal that the exposure plus ice cover history retained on some summit surfaces probably spans more than 800 ka despite complete ice cover as recently as 11 ka a thermodynamic ice sheet model with a basal water calculation is used to calculate the sliding distance normalized by the duration of ice cover for the region we formulate a general glacial erosion rule for the torngat mountains which correlates tcn derived erosion rates for terrain once partially covered by cold based ice with modelled average ice basal sliding velocities erosion rates vary linearly with average sliding velocity by a glacial erosion coefficient of 5 10 7 due to the significant distribution of cold based ice cover in this high latitude region our estimates of net regional glacial erosion and glacial erosion coefficient are orders of magnitude lower than a previously published value copyright 2005 john wiley sons ltd

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice i field evidence debris laden ice accretes to the base of matanuska glacier alaska u s a from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening frazil ice grows in the water column and forms aggregates while other ice grows on the glacier sole or on substrate materials sediment is trapped by this growing ice forming stratified debris laden basal ice growth rates of 0 1 m a 1 of debris rich basal ice are possible the large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings including heinrich events and the till sheets south of the laurentian great lakes

alternative hybrid empirical ground motion model for central and eastern north america using hybrid simulations and nga west2 models abstract an alternative hybrid empirical ground motion model for central and eastern north america cena is proposed the new ground motion model gmm is developed for the average horizontal components rotd50 of peak ground acceleration peak ground velocity and 5 damped pseudospectral accelerations at 0 01 10 s spectral periods hybrid empirical estimates are derived using the regional modification factors between two regions host and target along with empirical gmms from the host region the regional adjustment factors are ratios of the intensity measures from the generated synthetics in the host western north america wna and target cena regions in this study the recent updated empirical gmms developed by the pacific earthquake engineering research center for the next generation attenuation west2 nga west2 project bozorgnia et al 2014 are incorporated we used a broadband simulation technique proposed by the authors shahjouei and pezeshk 2015a to generate synthetics for both the wna and cena regions in which the high frequency and low frequency parts of synthetics are calculated through a stochastic finite fault method and kinematic source models along with the deterministic wave propagation respectively the updated seismological and geological parameters are deployed in simulations the new ground motion model is developed as part of the nga east research project considering multiple shaking scenarios that characterize the magnitude in the m 5 0 8 0 range the proposed gmm represents the level of ground shaking in the distance range of 2 1000 km and are developed for the reference rock site condition with v s 30 3 km s in cena the results are compared with some other existing models in the region in addition a comprehensive residual analysis is performed using the recorded earthquakes available in the nga east database

late cenozoic increase in accumulation rates of terrestrial sediment how might climate change have affected erosion rates abstract accumulation rates of terrestrial sediment have increased in the past few million years both on and adjacent to continents although not everywhere apparently erosion has increased in elevated terrain regardless of when last tectonically active or what the present day climate in many regions sediment coarsened abruptly in late pliocene time sparser data suggest increased sedimentation rates at 15 ma approximately when oxygen isotopes in benthic foraminifera imply high latitude cooling if climate change effected accelerated erosion understanding how it did so remains the challenge some obvious candidates such as lowered sea level leading to erosion of continental shelves or increased glaciation account for increased sedimentation in some but not all areas perhaps stable climates that varied slowly allowed geomorphic processes to maintain a state of equilibrium with little erosion until 3 4 ma when large oscillations in climate with periods of 20 000 40 000 years developed and de

basal roughness of the east antarctic ice sheet in relation to flow speed and basal thermal state basal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water to date basal motion represents one of the largest uncertainties in ice flow models it is that component of the total flow velocity that can change most rapidly and can therefore facilitate rapid variations in dynamic behaviour in this study we investigate the subglacial properties of the east antarctic ice sheet by statistically analysing the roughness of the bed topography inferred from radio echo sounding measurements we analyse two sets of roughness parameters one derived in the spatial and the other in the spectral domain with two roughness parameters each this enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet we further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet we find that one of the roughness parameters the hurst exponent derived in the spatial domain coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity

site effect evaluation using spectral ratios with only one station the spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity the purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station the technique originally proposed by nakamura to analyze rayleigh waves in the microtremor records is presented briefly and it is discussed why it may be applicable to study the intense s wave part in earthquake records results are presented for three different cities in mexico oaxaca oax acapulco gro and mexico city these cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city each time we compare the results of nakamura s technique with standard spectral ratios in all three cases the results are very encouraging we conclude that if site effects are caused by simple geology a first estimate of dominant period and local amplification level can be obtained using records of only one station

glacial hydrology and erosion patterns a mechanism for carving glacial valleys article i nfo article history most glacial erosion models assume that erosion rates are proportional to ice sliding velocity while recent studies have shown that water plays a major role in modulating sliding velocities the impact it might have on erosion rates is still unclear here we incorporate subglacial hydrology into a glacial erosion model that is based on a sliding rule our results explicitly highlight that adding subglacial hydrology has profound impacts on the temporal and spatial patterns of glacial erosion first it suggests that erosion might mainly occur during melting seasons when subglacial water pressure is large and effective pressure is low i e before the channels fully develop and reduce the water pressure at the ice bedrock interface which promotes sliding and erosion second the distribution of erosion is bimodal with a peak at the equilibrium line altitude and as we demonstrate enhanced erosion at much lower altitudes within the ablation area where water due to melting abounds this has important implications because it explains why glacial erosion can simultaneously

estimation of the coda wave attenuation and geometrical spreading in the new madrid seismic zone using the single backscattering method coda quality factor functions through coda window lengths of 20 30 40 50 and 60 s have been estimated for the new madrid seismic zone nmsz furthermore geometrical spreading functions for distances less than 60 km have been determined in this region at different center frequencies exploiting the coda normalization method a total of 284 triaxial seismograms with good signal to noise ratios snr 5 from broadband stations located in the nmsz were used the database consisted of records from 57 local earthquakes with moment magnitudes of 2 6 4 1 and hypocentral distances less than 200 km q factor values were evaluated at five frequency bands with central frequencies of 1 5 3 6 12 and 24 hz vertical components were utilized to estimate vertical coda q factor values horizontal coda q factor values were determined using the average amount of the q factor values estimated from two orthogonal horizontal components the coda q factor increases with increasing of the coda window length implying that with increasing the depth the coda q factor increases the intermediate values of the q factor and intermediate values of the frequency dependency indicate that the earth s crust and upper mantle beneath the entire nmsz is tectonically a moderate region with a moderate to relatively high degree of heterogeneities the geometrical spreading factors of s wave amplitudes are frequency dependent and determined to be 0 761 0 991 1 271 1 182 and 1 066 for center frequencies of 1 5 3 6 12 and 24 hz respectively at hypocentral distances of 10 60 km the geometrical spreading factors for lower frequencies are not recommended to be used due to the greater impact of the radiation pattern and directivity effect on low frequencies as well as the greater sensitivity of band pass filtered seismograms of small earthquakes to the noise in low frequencies

evaluation of functional forms for the attenuation of small to moderate earthquake response spectral amplitudes in north america abstract we studied several attenuation forms to evaluate their ability to describe the decay of response spectral amplitudes in eastern canada northeastern united states central united states pacific northwest british columbia northern california and southern california linear bilinear and trilinear regression forms with different combinations of geometric spreading coefficients were tested to assess their ability to describe spectral amplitude decay from 0 33 to 10 hz in the distance range from 10 to 400 km the residual trends and the shapes of the attenuation forms do not show significant differences in goodness of fit between these trial forms making it difficult to see a meaningful distinction between them furthermore statistical measures such as the akaike information criterion are not particularly helpful in drawing such distinctions a simple linear model as well as more complex forms can fit the data in almost every region however this linear model has steeper slope in the west 1 3 than in the east 1 and may not extend well over large distances the linear model effectively smoothes through complications in functional form such as the moho bounce or the transition to surface wave spreading at regional distances a trilinear model has the best statistical fit in most cases but the added complications in shape and parameterization do not appear warranted by the marginal improvement in residuals the bilinear form offers a good compromise between simplicity and the ability to model amplitude decay appropriately at both near and regional distances

determination of stress parameters for eight well recorded earthquakes in eastern north america we determined the stress parameter for the eight earthquakes stud ied by atkinson and boore 2006 using an updated dataset and a revised point source stochastic model that captures the effect of a finite fault we consider four geometrical spreading functions ranging from 1 r at all distances to two or three part functions the values are sensitive to the rate of geometrical spreading at close distances with 1 r 1 3 spreading implying much higher than models with 1 r spreading the important difference in ground motions of most engineering con cern however arises not from whether the geometrical spreading is 1 r 1 3 or 1 r at close distances but from whether a region of flat or increasing geometrical spreading at intermediate distances is present as long as is constrained by data that are largely at distances of 100 km 800 km the simple 1 r model fits the sparse data for the eight events as well as do more complex models determined from larger datasets where the larger datasets were used in our previous ground motion predic tion equations this suggests that uncertainty in attenuation rates is an important com ponent of epistemic uncertainty in ground motion modeling for the attenuation model used by atkinson and boore 2006 the average value of from the point source model ranges from 180 bars to 250 bars depending on whether or not the stress parameter from the 1988 saguenay earthquake is included in the average we also find that for a given earthquake is sensitive to its moment magnitude m with a change of 0 1 magnitude units producing a factor of 1 3 change in the derived

climatic limits on landscape development in the northwestern himalaya the interaction between tectonism and erosion produces rugged landscapes in actively deforming regions in the northwestern himalaya the form of the landscape was found to be largely independent of exhumation rates but regional trends in mean and modal elevations hypsometry frequency distribution of altitude and slope distributions were correlated with the extent of glaciation these observations imply that in mountain belts that intersect the snowline glacial and periglacial processes place an upper limit on altitude relief and the development of topography irrespective of the rate of tectonic processes operating

modeling geometrical spreading and the relative amplitudes of vertical and horizontal high frequency ground motions in eastern north america abstract horizontally layered velocity models were used with point source and finite fault sources to investigate geometrical spreading and the relative amplitudes of vertical and horizontal ground acceleration within 120 km of the source full wave field simulations were done for a range of focal depths and for strike slip and reverse focal mechanisms the attenuation of the geometric mean of randomly oriented horizontal component maximum acceleration amplitudes averaged over all azimuths significantly exceeds the theoretical geometrical spreading for far field body waves in a homogeneous whole space for hypocentral distances less than approximately 60 km the behavior of the vertical component is different from the horizontal vertical attenuation near the epicenter is greater and is more dependent on source mechanism and depth because of the rapid near source decay of the direct s wave reflections from the mid lower crust and moho control the maximum amplitude of the vertical component acceleration in the 60 120 km hypocenter distance range resulting in a flattening of the vertical amplitude distance relation near source vertical maximum amplitudes averaged over all source receiver azimuths tend to be less than the geometric mean horizontal amplitude for strike slip focal mechanisms but near the source for reverse faults the azimuthally averaged vertical component amplitude exceeds that of the geometric mean horizontal the modeling indicates that similar vertical and horizontal component geometrical spreading and approximately constant horizontal vertical amplitude ratios observed in connection with the lg phase at distances greater than approximately 100 km in eastern north america may not hold at smaller distances ground motion prediction models for the vertical component near the source may need to incorporate strong geometrical spreading and dependence on radiation pattern

peregrinations of the greenland ice sheet divide in the last glacial cycle implications for central greenland ice cores abstract the superb quality of the climate chronology archived in the summit greenland ice cores grip gisp2 testifies that the greenland ice sheet divide has been generally stable through the last glacial cycle the ice sheet has experienced a broad range of paleoclimate conditions ice sheet margin configurations and internal dynamical adjustments in glacial interglacial transitions however it is unlikely that the summit region escaped shifts in ice divide position geometry elevation and flow characteristics details of this dynamical history are important to several aspects of ice core studies the magnitudes of pure and simple shearing reconstruction of vertical ice velocity the explicit location of the ice divide and the divide residence time at different locations are all of interest in interpretation of climatic variables and physical properties of ice in the ice cores we apply a three dimensional thermomechanical ice sheet model to examine the evolution of these dynamical variables over the last 160 kyr in central greenland while a high elevation ice dome is present in the summit region throughout the simulation ice divide migrations of up to 150 km are predicted all points in the vicinity of the summit ice cores including the modern divide have been subject to flowline shifts and variable non zero shear deformation during the adjustment from glacial to holocene conditions from ca 10 ka to the present modelled divide peregrinations and strain rate history are consistent with the observed disturbance of deep ice in the grip and gisp2 ice cores which has muddled paleoclimate reconstructions for the last interglacial eemian period in greenland dynamical excursions are also evident north of the modern summit where the ngrip ice core is currently being drilled dahl jensen et al j glaciol 43 1997 300 306 however the prevailing flow direction and deformation regime at the ngrip site are much more stable than those at grip and gisp2 in the simulations combined with the greater depth of ice at this site this lends cautious optimism to the hope that eemian ice at ngrip may contain an intact record of eemian climate

ground motion amplification at rock sites across canada as determined from the horizontal to vertical component ratio we determine the frequency dependent amplification inherent in hard rock sites across canada under the assumption that it can be estimated from the ratio of the horizontal to vertical h v components of ground motion the use of h v as an estimate of amplification was originally proposed by nakamura 1989 for microtremors and then generalized to earthquake ground motions by lermo and chavez garcia 1993 the h v method was applied to a fourier spectra database compiled from 424 earthquakes of magnitude greater than or equal to 2 recorded on 32 three component stations of the canadian national seismograph network cnsn all sited on rock shear wave velocity 1500 m sec in each case we analyzed the s wave portion of the seismograms including the direct s and other strong s phases such as lg the average amplification h v for rock sites does not vary significantly across the country it is weakly frequency dependent increasing from a factor near unity at 0 5 hz to a maximum in the range from 1 2 to 1 6 at 10 hz this amplification is consistent with what would be expected from a gradual decrease in shear wave velocity as the seismic waves approach the surface due to factors such as near surface weathering the interpretation of the h v ratio as a measure of site response is consistent with the general geological conditions of the recording sites in particular the 5 hz h v ratio can be correlated with local geological conditions the central canada sites have the lowest estimated site amplification this is consistent with competent hard rock site conditions with high near surface shear wave velocities there are two stations that show anomalous h v ratios that suggest significant local site effects these are edm near edmonton which may be influenced by topographic amplification and a61 a station in the charlevoix seismic zone

regional spectral analysis of three moderate earthquakes in northeastern north america abstract we analyze fourier spectra obtained from the horizontal components of broadband and accelerogram data from the 1997 cap rouge the 2002 ausable forks and the 2005 riviere du loup earthquakes recorded by canadian and american stations sited on rock at hypocentral distances from 23 to 602 km we check the recorded spectra closely for anomalies that might result from site resonance or source effects we use beresnev and atkinson s 1997 near surface velocity structures and boore and joyner s 1997 quarter wave method to estimate site response at hard and soft rock sites we revise the street et al 1975 model for geometrical spreading adopting a crossover distance of r o 50 km instead of 100 km we obtain an average attenuation of q 410 25 f 0 50 0 03 for s l g surface waves with ray paths in the appalachian and southeastern grenville provinces we correct the recorded spectra for attenuation and site response to estimate source spectral shape and radiated energy for these three earthquakes and the 1988 m 5 8 saguenay earthquake the brune stress drops range from 130 to 419 bars and the apparent stresses range from 39 to 63 bars the corrected source spectral shapes of these earthquakes are somewhat variable for frequencies from 0 2 to 2 hz falling slightly below the fitted brune spectra

antarctic topography at the eocene oligocene boundary we present a reconstruction of the antarctic topography at the eocene oligocene ca 34 ma climate transition this provides a realistic key boundary condition for modeling the first big antarctic ice sheets at this time instead of using the present day bedrock topography which has changed significantly from millions of years of tectonism and erosion we reconstruct topography using a set of tools including ice sheet erosion models models of thermal subsidence and plate movement erosion estimates are constrained with offshore sediment volumes estimated from seismic stratigraphy maximum and minimum topographic reconstructions are presented as indicators of the range of uncertainty our results point to a significant upland area in the ross sea marie byrd land and weddell sea sectors in addition east antarctic coastal troughs are much shallower than today due to the restoration of material that has been selectively eroded by the evolving ice sheets parts of east antarctica have not changed since the e o boundary because they were protected under non erosive cold based ice the reconstructions provide a better defined boundary condition for modeling that seeks to understand interaction between the antarctic ice sheet and climate along with more robust estimates of past ice volumes under a range of orbital settings and greenhouse gas concentrations

a statistical model for ground motion produced by earthquakes at local and regional distances to adapt random process theory techniques for statistical estimation of peak ground motion to more realistic earth models we constrain the parameters of duration geometrical spreading and spectral shape by modeling the main ground motion as being the result of major contributions by the direct s wave and supercritically reflected s waves the results of our modeling are constrained to be consistent with values from full wave synthetics for the test models the combination of estimation theory and theoretical amplitude spectrum of the main ground motion within the ergodic window successfully predicts the mean peak vertical ground displacements velocities and accelerations of the 1982 miramichi earthquakes in new brunswick canada in addition upon considering the effects of source depth and crustal structure for the november 25 1988 saguenay earthquake m 5 8 in quebec canada the predicted mean peak horizontal ground accelerations match the observed data very well the effects of source depth and crustal structure on the peak ground motion are complicated for different source sizes and at different epicentral distance ranges

the gamburtsev mountains and the origin and early evolution of the antarctic ice sheet the manner in which the antarctic ice sheet first formed at a time of rapid global climate change about 34 million years ago has been the subject of considerable speculation several lines of evidence point to central antarctic mountain ranges as the site of the initial ice sheet but our knowledge of the present ice sheet topography is highly restricted in the very zone where ice sheet initiation is most likely along the subglacial gamburtsev mountain range at the centre of the present ice sheet now a detailed survey and analysis of the morphology of the gamburtsev mountains has been performed based on an ice penetrating radar study conducted over two seasons the data reveal a mountain landscape initially incised by rivers then over deepened by ice movement the topography beneath up to 3 000 metres of ice resembles an exploded version of classic alpine valley topography and seems to have developed more than 34 million years ago when mean summer temperatures were about 3 c the nature of initial glaciation on antarctica about 34 million years ago is a mystery results from an intensive radar survey now show that the gamburtsev mountains at dome a at the centre of the present ice sheet were initially incised by rivers and then subsequently overdeepened by ice movement this is suggestive of topographical development before 34 million years ago when mean summer temperatures were about 3 c ice sheet development in antarctica was a result of significant and rapid global climate change about 34 million years ago1 ice sheet and climate modelling suggest reductions in atmospheric carbon dioxide less than three times the pre industrial level of 280 parts per million by volume that in conjunction with the development of the antarctic circumpolar current led to cooling and glaciation paced by changes in earth s orbit2 based on the present subglacial topography numerical models point to ice sheet genesis on mountain massifs of antarctica including the gamburtsev mountains at dome a the centre of the present ice sheet2 3 our lack of knowledge of the present day topography of the gamburtsev mountains4 means however that the nature of early glaciation and subsequent development of a continental sized ice sheet are uncertain here we present radar information about the base of the ice at dome a revealing classic alpine topography with pre existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 c this landscape is likely to have developed during the initial phases of antarctic glaciation according to antarctic climate history estimated from offshore sediment records the gamburtsev mountains are probably older than 34 million years and were the main centre for ice sheet growth moreover the landscape has most probably been preserved beneath the present ice sheet for around 14 million years

entrainment at cold glacier beds here we present measurements of the gas content and isotopic composition of debris rich basal layers of a polar glacier meserve glacier antarctica which has a basal temperature of 17 c these measurements show that debris entrainment has occurred without alteration of the glacial ice and provide the most direct evidence to date that active entrainment occurs at the beds of cold glaciers without bulk freezing of water entrainment at subfreezing temperatures may have formed the u shaped trough containing meserve glacier in addition to possibly allowing some cold based glaciers to be important geomorphic agents entrainment at subfreezing temperatures provides a general mechanism for formation of the dirty basal layers of polar glaciers and ice sheets which are rheologically distinct and can limit the time span of ice core analyses furthermore accumulating evidence suggests that geomorphologists should abandon the assumption that cold based glaciers do not slide and abrade their beds

alpine glacial topography and the rate of rock column uplift a global perspective abstract the present study investigates the influence of alpine glacial erosion on the morphology and relief distribution of mountain regions associated with varying rock column uplift rates we take a global approach and analyse the surface area distribution of all mountain regions affected by glacial erosion between latitudes 60 s and 60 n most surface area in glaciated mountain ranges is concentrated below the modern snowline as predicted by the glacial buzzsaw hypothesis above the modern snowline the analysed mountain regions all exhibit an exponential decay of surface area with height on the basis of this decay the analysed mountain regions fall within three distinct groups primarily reflecting variations in average values of rock column uplift rates mountain ranges affected by rapid rock column uplift display high above snowline relief and large decay lengths whereas inactive orogens have little relief above the modern snowline we find a surprisingly simple relation between hypsometry above the local snowline and regional tectonic activity