potential flow models of suspension current air pressure we present analyse and discuss air pressure data from finite volume chute flows of dry fine snow in air these experiments have the correct similarity criteria to model powder snow avalanches and demonstrate the transition from a dense to a suspended flow we measured the dynamic air pressure at the base of the flow which features a marked negative pressure peak immediately behind the front this feature is also seen in observations of natural powder snow avalanches measured in russia japan and switzerland in direct numerical simulations of non boussinesq suspension flows and in ping pong ball avalanches this is evidence for large internal motions and suggests that there is a coherent vortex in the avalanche front this can result in impact pressures many times larger than those expected from the mean flow velocity we analyse the external air pressures using three models and show how the geometry and velocity of the flow can be found from this single air pressure measurement we also measured flow heights and speeds using image analysis and show that the speed is roughly independent of the slope angle and scales with the release size raised to the power 1 4 as predicted by similarity analysis for pseudo two dimensional 2 d flows

measured shear rates in large dry and wet snow avalanches we present estimates of internal shear rates of real scale avalanches that are based on velocity measurements optical velocity sensors installed on the instrument pylon at the swiss vallee de la sionne test site are used to measure flow velocities at different flow heights of three large dry and wet snow avalanches possible sources of error in the correlation analysis of the time lagged reflectivity signals measured by optical sensors are identified for real size avalanches these include spurious velocities due to noise and elongated peaks an appropriate choice of the correlation length is essential for obtaining good velocity estimates placing restrictions on the maximum possible accelerations in the flow improves the analysis of the measured data coherent signals are found only in the dense flowing cores we observe the evolution of shear rates at different depths between the front and tail of the flowing avalanche at the front large shear rates are found throughout the depth at the tail plug flows overriding highly sheared layers near the bottom of the flow are observed the measured velocities change strongly with height above the ground and fluctuations around the measured mean velocity can be identified we find that the dense flows are laminar undergoing a transition from supercritical to subcritical flow behaviour from the head to the tail furthermore we provide real scale experimental evidence that the mean shear rate and the magnitude of velocity fluctuations increase with the mean discharge

latest pleistocene ice rafting events on the scotian margin eastern canada and their relationship to heinrich events piston cores from the continental margin off nova scotia show up to four discrete intervals of brick red sandy mud which are up to 20 cm thick the ages of these intervals are bracketed by several radiocarbon dates and three fall in the range 12 5 14 1 ka radiocarbon years with 0 4 kyr reservoir correction the youngest dates from 10 4 ka placing it within the younger dryas the distribution of the beds and their petrographic character indicate a source in the gulf of saint lawrence the grain size of these beds suggests that they comprise a coarse component transported by ice rafting that diminishes distally and a fine component that represents suspension fallout from a surface plume and resulting nepheloid layers graded brick red beds in some cores were probably redeposited from turbidity currents the lowermost bed on the laurentian fan and east scotian rise is immediately overlain by a carbonate rich interval that can be identified all around the margin of the grand banks this interval is correlated with detrital carbonate bed dc 1 in the labrador sea and heinrich event h1 in the north atlantic the sequential occurrence of the two beds suggests that they may be a response to the same trigger probably sea level rise but that the gulf of saint lawrence source was more easily destabilized

observations of the dynamic structure of snow avalanches during two winters 1990 92 the dynamic structures of snow avalanches were studied in western norway artificially released wet snow avalanches ran down the avalanche chute and stopped in front of the retaining dam running velocity distributions were obtained not only by video tape recorder but also by various other recording instruments internal velocity was derived for the last avalanche by frequency analysis of impact pressure and optical sensor data the vertical velocity shear of the avalanche flow has been estimated to be in the range 1los

studies of coda using array and three component processing the application of standard array processing techniques to the study of coda presents difficulties due to the design criteria of these techniques typically the techniques are designed to analyze isolated short arrivals with definite phase velocity and azimuth and have been useful in the frequency range around 1 hz coda is long in time and may contain waves of different types phase velocities and azimuths nonetheless it has proved possible to use or adapt array methods to answer two questions what types of waves are present in coda and where are they scattered most work has been carried out on teleseismicp coda work on local coda has lagged due to lack of suitable data and the difficulties of dealing with high frequencies the time domain methods of beamforming and vespagram analysis have shown that there is coherent energy with a high phase velocity comparable top orpp in teleseismicp coda these methods can detect this coherent coda because it has a fairly definite phase velocity and the same or close to azimuth as firstp orpp this component must consist ofp waves and is either scattered near the source or reflected in the mantle path as apdpp or precursorpp reflection the fourier transform method of the frequency wavenumber spectrum has been adapted by integrating around circles of constant phase velocity constant total wavenumber to produce the wavenumber spectrum which shows power as a function of wavenumber or phase velocity for teleseismicp coda wavenumber spectra demonstrate that there is a diffuse coda of shear lg or surface waves scattered from teleseismicp near the receiver wavenumber spectra also suggest that the coherent coda is produced by near source scattering in the crust not mantle reflection since it is absent or weak for deep focus events crustal earthquakes have a very strong coherent component of teleseismic coda suggesting scattering from shear to teleseismicp near the source three component analysis of single station data has shown the presence of off azimuth arrivals and may lead to the identification of waves scattered from a single scatterer

velocity distribution in snow avalanches in order to investigate the detailed structure of snow avalanches we have made snow flow experiments at the miyanomori ski jump in sapporo and systematic observations in the shiai dani kurobe canyon in the winter of 1995 1996 a new device to measure static pressures was used to estimate velocities in the snow cloud that develops above the flowing layer of avalanches measurements during a large avalanche in the shiai dani which damaged and destroyed some instruments indicate velocities increased rapidly to more than 50 m s soon after the front velocities decreased gradually in the following 10 s velocities of the lower flowing layer were also calculated by differencing measurement of impact pressure both recordings in the snow cloud and in the flowing layer changed with a similar trend and suggest a close interaction between the two layers in addition the velocity showed a periodic change power spectrum analysis of the impact pressure and the static pressure depression showed a strong peak at a frequency between 4 and 6 hz which might imply the existence of either ordered structure or a series of surges in the flow

sedimentary facies of the nova scotian upper and middle continental slope offshore eastern canada a detailed survey of the upper and middle nova scotian continental slope at 42 50 n and 63 30 w indicates a complex morphology dominated by mass movements on various scales and an immature turbidity current channel the range of sediment facies is diverse including hemipelagic and turbidite muds turbidite sands and gravelly sandy muds of debris flow origin deformed units interpreted as slump deposits are also observed several facies associations related to discrete morphological environments are recognized thick turbidite sand units with minor intervening mud beds are characteristic of the high relief uppermost slope and channel margin thinner turbidite sands deformed slump beds and various mud facies are associated with small scale hummocky mid slope topography sand beds are more abundant in the depressions than on intervening hummocks indicating the preferred transport paths of small turbidity currents at the lower end of the main turbidity current channel frequent turbidite sand beds with relatively minor mud beds are deposited on a depositional lobe in areas unaffected by mass movements alternating bioturbated mud and sandy muds make up the core sequences a local model of sedimentation is proposed for this area and illustrates that simple models of continental slope sedimentation only apply to a limited range of settings

biogeochemical processes and geotechnical applications progress opportunities and challenges consideration of soil as a living ecosystem offers the potential for innovative and sustainable solutions to geotechnical problems this is a new paradigm for many in geotechnical engineering realising the potential of this paradigm requires a multidisciplinary approach that embraces biology and geochemistry to develop techniques for beneficial ground modification this paper assesses the progress opportunities and challenges in this emerging field biomediated geochemical processes which consist of a geochemical reaction regulated by subsurface microbiology currently being explored include mineral precipitation gas generation biofilm formation and biopolymer generation for each of these processes subsurface microbial processes are employed to create an environment conducive to the desired geochemical reactions among the minerals organic matter pore fluids and gases that constitute soil geotechnical applications currently being explored include cementation of sands to enhance bearing capacity and liquefaction resistance sequestration of carbon soil erosion control groundwater flow control and remediation of soil and groundwater impacted by metals and radionuclides challenges in biomediated ground modification include upscaling processes from the laboratory to the field in situ monitoring of reactions reaction products and properties developing integrated biogeochemical and geotechnical models management of treatment by products establishing the durability and longevity reversibility of the process and education of engineers and researchers

a comparison of powder snow avalanches at vallee de la sionne switzerland with plume theories powder snow avalanches are natural hazards which affect the way populations live in mountainous areas field measurements from avalanches remain one of the most significant and useful sources of information about their dynamics and behaviour in this paper we consider all the video data from the swiss vallee de la sionne test site from the years 2003 05 general scaling laws are sought for the avalanche front velocity based on plume theories avalanche froude numbers are found comparing three different length scales the cube root of the fracture volume the avalanche height and the depth of entrained snow cover we discuss the difficulties in defining the volume of a powder snow avalanche should we include just the head or also the turbulent wake that extends back to the starting zone this relates to whether we use a compact model for the avalanche such as the ksb model ancey 2006 turnbull and others 2006 or a plume model turner 1973 observations are made regarding the lateral spreading behaviour of the avalanches we show that the slow lateral spreading can be explained by large internal velocities and anisotropic turbulence generated by the large scale motion in the avalanche head

rg as a depth discriminant for earthquakes and explosions a case study in new england fundamental mode rayleigh waves with periods between about 0 4 and 2 5 sec rg are often observed on seismograms of explosions and very shallow focus earthquakes the rg phase is particularly prominent on seismograms of quarry blasts in this study rg waves generated by small earthquakes and explosions in new england are investigated to evaluate the extent to which rg waves can be used to estimate depths of events in the upper crust the data were recorded by the new england seismic network nesn operated by weston observatory the strongest rg signals recorded by the nesn are generally in the period range of 0 5 to 1 5 sec in that period range rg displacement is essentially confined to the upper 5 km of the crust with most of the rg wave energy in the upper 2 or 3 km sources deeper than about 4 km would not be expected to generate strong rg signals at these periods and so if rg can be clearly identified on a seismogram the source is most likely very shallow observed rg waves can therefore be used to discriminate very shallow focus events from deeper events provided that rg can be identified and distinguished from other phases a method for identifying rg waves at distances up to about 170 km is presented in this paper rg is identified and distinguished from other phases by measuring amplitudes at particular periods and arrival times using a narrow bandpass filter analysis rg p and rg lg ratios are estimated by forming the ratios of amplitudes in the appropriate group velocity frequency windows on vertical component seismograms the phase referred to as lg is actually a complicated wave train recorded on short period seismograms that travels with group velocities appropriate for s and lg waves an rg lg ratio appears to be a better measure of the presence of rg on a seismogram than an rg p ratio observed rg lg ratios are used to estimate depths of two earthquakes recorded by the nesn

a comparison of regional phases from underground nuclear explosions at east kazakh and nevada test sites abstract analysis of regional data from east kazakh ussr and nevada test site nts underground nuclear explosions suggests possible mechanisms for the generation of both low and high frequency lg spectral characteristics of pn and lg phases from 25 soviet nuclear shots recorded at the chinese digital seismic network cdsn station wmq are analyzed for their dependence on parameters such as m b generally related to shot depth and spatial location shagan versus degelen test sites the observed results are compared with those from the nevada test site nts where the near source conditions are better known the average amplitude ratio pn lg is found to be stable with m b but to vary strongly with frequency for both shagan and yucca flat explosions of similar yield the reduction in amplitude over the frequency range of about 1 to 5 hz is considerably larger for lg than for pn the low frequency lg appears to be generated by the scattering of explosion generated rg into s especially for the shagan river explosions for which good agreement between observation and theory is observed this mechanism also provides a possible explanation for the stability of lg as an estimator of explosive yields because the scattering should be expected to occur over a large region of the shallow crust another important result supported by theory is that the higher frequency lg is strongly dependent on the source medium velocity at higher frequencies 3 to 7 hz the amplitude ratios pn lg for explosions from shagan degelen pahute mesa and yucca flat test sites show significant differences that appear related to differences in their source medium velocities the observed large differences in the relative spectral content of pn and lg from east kazakh and nts explosions are mainly due to the vast differences in the crustal structure of the two test sites

evidence for wisconsinan glaciations in the verrill canyon area scotian slope a composite thickness of about 25 m of sediment has been cored from the verrill canyon on the scotian slope it is interpreted that the majority of this sequence was deposited in a glaciomarine environment during oxygen isotopic stage 2 and the top of stage 3 these sediments as seen in high resolution seismic reflection profiles are well stratified become thicker upslope are laterally variable in thickness and pass upslope into possible outer shelf tills three wedge shaped units of incoherent reflections interfinger with the parallel reflections and terminate in water depths greater than 700 m these wedge shaped units are interpreted as slumped diamict and outwash deposits the age of the uppermost wedge shaped unit is 26 000 21 000 yr based on extrapolation of radiocarbon dates this unit documents a late wisconsinan glacier readvance on the outer scotian shelf the underlying wedge shaped unit estimated to be 70 000 yrs old extends further west along the continental slope and may represent a more extensive early wisconsinan ice advance a third wedge shaped unit inferred to have formed during isotopic stage 6 is possibly a remnant of the first glaciation in the study area

dynamic pore pressure variations induce substrate erosion by pyroclastic flows field evidence shows that pyroclastic flows can entrain blocks from underlying substrates formed by earlier geological events yet counterintuitively they are less likely to erode unconsolidated layers of fine particles here we report laboratory experiments that reproduce these seemingly contradictory observations and also offer a means to infer pyroclastic flow velocity experiments demonstrate that the sliding head of a granular flow generates a dynamic upward pore pressure gradient at the flow substrate interface associated upward air flux is enough to fluidize a substrate of fines so that particles are not entrained individually but the substrate instead is subject to small shear instabilities in contrast coarse particles forming a non fluidized substrate are lifted at a critical upward force due to the pore pressure gradient according to their individual masses which provides a basis for a model to calculate the flow velocity application to the 18 may 1980 pyroclastic flow deposits at mount st helens washington state usa gives velocities of 9 13 m s 1 at 6 7 km from the vent on gentle slopes 4 6 in agreement with field observations at this volcano and at others

regional seismic recordings of the soviet nuclear explosion of the joint verification experiment during august and september 1988 the united states and the soviet union conducted a joint verification experiment in which each country was permitted to observe a nuclear explosion at the other country s test site using hydrodynamic means we have seismically recorded the soviet explosion at four stations at regional distances 750km and here report those observations the peak to peak particle motion in the p wave varied from 7 54 10 1 cm sec at 160 km to 1 02 10 2 cm sec at 740 km the average mb lg was 5 97 corresponding to a yield of 118 kt based on the nts mb lg yield relation of patton 1988 the average pn corner frequency of the soviet explosion was 4 hz with evidence of a second corner at about 0 7 hz both of these corners could be affected by spall near regional seismograms of the soviet jve explosion show a large arrival within the lg phase on the transverse component at all four sites this may correspond to tectonic release accompanying the explosion or to the effects of anisotropy on seismic wave propagation

proglacial sediment instability features on the scotian slope at 63 w abstract a small area of the scotian slope has been investigated in detail as a type example of a high latitude continental slope adjacent to a major ice outlet the geologic framework has been defined by airgun seismic profiles which show that late tertiary shales have irregular dips but overlie undisturbed strata these shales form a zone of diapirs at around 1200 m water depth which have ponded the prograding quaternary sediments upslope high resolution sparker profiles seamarc i mid range sidescan imagery and piston cores have been used to investigate the late quaternary sedimentary sequence a prominent gullied horizon about 40 m subbottom that is overlain by acoustically unstratified sediments sands is correlated with the occurrence of an early middle wisconsinan grounded icesheet at the top of the continental slope it is overlain by a thick sequence of proglacial muds and a thin veneer of holocene sediments much of the late wisconsinan sequence has been dissected by a series of closely spaced gullies both sidescan and subbottom profiler data show that slumping is common in the late wisconsinan sequence as a result of high sedimentation rates and local gully erosion piston cores provide evidence for debris flows and turbidity currents depositional sediment mounds in the diapir zone have accumulated sediment at three times the regional sedimentation rate suggesting that large amounts of fine sediment reached the slope in suspension and that much of the proglacial sediment by passed the continental slope

high frequency seismic observations in eastern kazakhstan ussr with emphasis on chemical explosion experiments two temporary three station seismic networks deploying surface and 100 m borehole high frequency seismometers of the order of 200 km from the kazakh test site in the ussr and the nevada test site in the united states are discussed with emphasis on chemical explosion experiments seismograms attained from the detonation of three buried explosions 10 t 20 t 10 t in eastern kazakhstan at distances between 156 and 637 km are examined in the frequency band of 1 80 hz observed signal to noise s n ratios were high reaching a maximum of 400 for pg waves and 200 for lg waves good signal to noise levels persisted to high frequencies s n 2 at about 50 hz for lg waves about 250 km from the source and at about 14 hz at 680 km distance for pg waves s n 2 at about 50 hz 270 km from the source shapes of displacement amplitude spectra were similar characterized by a broad maximum in signal to noise levels between 4 8 hz and a decay at higher frequencies e g above 10 hz of about f 3 5 f 4 1 for lg waves and f 3 1 f 4 5 for pg unconnected for distance magnitudes estimated from lg time domain amplitudes for the 10 t explosion are between 2 8 and 3 3 depending on the magnitude relation used spectral characteristics are used to put some constraint on lg q pg q is poorly constrained by the data a similar experiment in southern nevada showed much lower pg and lg signal to noise levels above 1 hz although kazakh and nevada absolute noise levels are comparable

variation of deposition depth with slope angle in snow avalanches measurements from vallee de la sionne 1 the snow surface height was precisely measured with a laser scanner before and after the passage of two dry mixed avalanches in vallee de la sionne during the winter of 2005 2006 the measurements were used to calculate the depth of the deposited snow along each entire avalanche path with a height resolution of 100 mm and a horizontal resolution of 500 mm these data are much more accurate than any previous measurements from large avalanches and show that the deposit depth is strongly negatively correlated with the slope angle that is on steep slopes the deposit is shallow and on gentle slopes the deposit is deep the time evolution of the snow depth showing the initial erosion and final deposition as the avalanche passed was also observed at one position using a frequency modulated continuous wave radar measurements at a nearby position gave flow speed profiles and showed that the avalanche tail consists of a steady state subcritical flow that lasts for about 100 s eventually the tail slowly decelerates as the depth slightly decreases and then it comes to rest we show that the dependency between the slope angle and the deposition depth can be explained by both a cohesive friction model and the pouliquen h stop model

on the nature of regional seismic phases i phase representations for pn pg sn lg summary an operator development of the seismic wavefield is used to generate descriptions of the propagation processes contributing to the main regional seismic phases pn pg sn lg these operator forms are valid for laterally heterogeneous crust and mantle models and include the major processes of interconversion between wavetypes these representations of the regional phases are used to examine the theoretical basis for discriminants between earthquakes and underground explosions based on the relative amplitudes of p and s phases the ratio of sn to pn amplitude looks promising as a high frequency discriminant however the ratio of lg to pn amplitudes is not as useful because of the complex nature of the propagation characteristics of lg

post glacial persistence of turbiditic activity within the rhone deep sea turbidite system gulf of lions western mediterranean linking the outer shelf and the basin sedimentary records emplacement of post glacial turbidites is commonly controlled by rapid changes in sea level or by seismicity on the continental rise of the gulf of lions western mediterranean an aseismic area we identified turbiditic beds deposited during the rising stage and highstand of sea level swath bathymetry sediment cores in situ cone penetrating tests cptu heavy mineral associations and radiocarbon dating determined the source composition distribution and age of the turbiditic beds turbidites are composed of homogeneous to positively graded silts to medium sand with quartz up to 90 shell debris and shelfal benthic faunas their distribution on the sea floor is very patchy and controlled by abundant inherited erosional bedforms their source is found in relict regressive sands at the outershelf their deposition occurred just after the onset of the post glacial sea level rise and the concomitant sediment starvation of the rhone deep sea turbiditic system until recently whilst canyons are fed with sand by strong seasonal hydro sedimentary dynamics on the outershelf the emplacement of post glacial turbidites is not controlled by sea level changes but probably by the periodic flushing of the canyons our study revealed that this low energy aseismic margin undergoes significant transport of sand down to the base of slope during the sea level rise and the holocene highstand

reply to comment by p a bartelt and o buser on role of pore pressure gradients in sustaining frontal particle entrainment in eruption currents the case of powder snow avalanches

relationship among submarine instabilities sea level variations and the presence of an ice sheet on the continental shelf an example from the verrill canyon area scotian shelf sea level low stands are associated with the growth of glacial ice sheets during maximum stages of glaciation ice sheets may extend to the outer continental shelf these glacial maxima normally correspond to sea level low stands consequently ice sheets may be supported directly by the seafloor inducing increased pore pressures in the sediment and increasing the overall bearing stresses on the margin these stresses trigger undrained failure in upper slope sediment and bearing capacity failure in the middle to lower slope after complete ice melting and dissipation of excess pore pressure the inter glacial sediments become overconsolidated shear resistance is higher and no gravity or earthquake induced failure occurs the gravity events can only occur in underconsolidated and normally consolidated glacial and postglacial deposits this explains the variation in volume and activity of mass wasting events during low and high sea level periods in areas where a glacial ice exists analyses of maximum ice loading events are presented for the scotian slope in contrast with traditional thought these analyses suggest that ice loading rather than high sedimentation rates is the dominant mechanism for slope instability on glaciated margins

a study of p waves from nevada test site explosions near source information from teleseismic observations spectral analyses of short period p arrivals on teleseismic recordings of nuclear explosions at yucca flats and pahute mesa regions of the nevada test site seem to provide explosion source medium information the spectral ratio p p coda on short period vertical component records at norsar shows significant correlation with explosion medium velocity for a given yield spectrum of the initial p waves from an explosion within a high velocity medium is richer in high frequencies as compared to that for p waves from an explosion within a low velocity medium the spectra of p codas on the other hand are considerably less sensitive to the near source velocity differences from one shot to another so that the spectral ratio p p coda is diagnostic of the shot medium velocity relative insensitivity of the p coda spectra and amplitudes to the overburden velocity is perhaps due to several factors including an effective source function controlled by average properties of rock within a fairly large nonlinear region around the shot point the study provides valuable insight into effects of the shot medium on the spectra and amplitudes of compressional arrivals and offers the exciting possibility of extracting near source information from teleseismic observations

in situ geotechnical characterization of sediments on the nova scotian slope eastern canadian continental margin abstract a seabed 2 m long cone penetrometer and coring system geotechnical module has been used at 17 stations in four transects on the scotian slope to characterise in situ shear strength and induced pore pressure on several different types of late pleistocene and early holocene failure study sites were selected using the sar high resolution deep towed acoustic system equipped with a digital 160 190 khz sidescan sonar and a 3 5 khz subbottom profiler several distinctive types of geotechnical signature were recognised from plots of cone resistance and induced pore pressure with depth in the sediment normally consolidated sediments show a progressive increase in cone resistance with depth to about 75 kpa at 2 m subbottom holocene surficial muds show spectacular apparent overconsolidation reaching a peak of 250 kpa at about 50 cm subbottom and then decreasing down to 1 5 m this overconsolidation is associated with zoophycos burrows late pleistocene sediments exhumed by bedding plane slides show strong true overconsolidation consistent with the original depth of burial inferred from high resolution seismic stratigraphy debris flows show only a slight shear stress gradient with depth 40 45 kpa over 0 5 1 m subbottom with under consolidation due to remoulding of sediment

late quaternary slumps and debris flows on the scotian slope the scotian slope just west of verrill canyon was surveyed using the sea marc i deep towed sidescan sonar system high resolution seismic reflection profiles and 25 piston cores the generally smooth continental slope has a gradient of 2 5 it is crossed by 2 small valleys that are 1 km wide and 100 m deep much of the sea bed shows evidence of surficial sliding that removed 10 20 m of sediment and the slide scars give the sea bed a steplike morphology from the 600 m isobath to 1 800 m there extend 2 zones of disturbed sediment these disturbed zones have the rough surface and transparent acoustic character previously regarded as characteristic of debris flows but sidescan sonar images and cores suggest that they are principally rotational slide deposits with true debris flows at their distal limits streamlined erosional depressions near the downslope edge of the debris flows cut both the flow and sediment farther down slope these may have been produced by a turbidity current associated with the debris flows sorted coarse sand in piston cores provides further evidence of current activity this widespread sediment failure on relatively low slopes was probably the result of a large earthquake that can be dated from cores as occurring between 5000 and 12000 yr b p

an updated radiocarbon based ice margin chronology for the last deglaciation of the north american ice sheet complex abstract the north american ice sheet complex naisc consisting of the laurentide cordilleran and innuitian ice sheets was the largest ice mass to repeatedly grow and decay in the northern hemisphere during the quaternary understanding its pattern of retreat following the last glacial maximum is critical for studying many facets of the late quaternary including ice sheet behaviour the evolution of holocene landscapes sea level atmospheric circulation and the peopling of the americas currently the most up to date and authoritative margin chronology for the entire ice sheet complex is featured in two publications geological survey of canada open file 1574 dyke et al 2003 quaternary glaciations extent and chronology part ii dyke 2004 these often cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka all ages in uncalibrated radiocarbon years using a combination of geomorphology stratigraphy and radiocarbon dating however by virtue of being over 15 years old the ice margin chronology requires updating to reflect new work and important revisions this paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies we also update the original radiocarbon dataset from the 2003 2004 papers with 1541 new ages to reflect work up to and including 2018 a major revision is made to the 18 ka ice margin where banks and eglinton islands once considered to be glacial refugia are now shown to be fully glaciated our updated 18 ka ice sheet increased in areal extent from 17 81 to 18 37 million km2 which is an increase of 3 1 in spatial coverage of the naisc at that time elsewhere we also summarize region by region significant changes to the deglaciation sequence this paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of dyke et al 2003 and dyke 2004 where new information is lacking this is a pragmatic solution to satisfy the needs of a quaternary research community that requires up to date knowledge of the pattern of ice margin recession of what was once the world s largest ice mass the 36 updated isochrones are available in pdf and shapefile format together with a spreadsheet of the expanded radiocarbon dataset n 5195 ages and estimates of uncertainty for each interval

analysis of slope failures in submarine canyon heads an example from the gulf of lions to improve understanding of evolution of submarine canyons a three dimensional slope stability model is applied to bourcart canyon in the western gulf of lions in the mediterranean sea the model builds on previous work by chen and others and it uses the upper bound theorem of plasticity to calculate the factor of safety of a kinematically admissible failing mass examples of three dimensional failure surfaces documented in the literature were used to test the model formulation model application to bourcart canyon employed the results of a detailed stratigraphic analyses based on data acquired by swath bathymetry sub bottom profiling high resolution seismic reflection surveys and piston coring the sediment layers were also characterized using in situ geotechnical measurements and laboratory tests the effects of three loading scenarios were analyzed 1 earthquake shaking 2 hemipelagic sedimentation and 3 axial incision these three mechanisms influenced the predicted volumes and shapes of slope failures along the flanks of bourcart canyon and comparison of these predictions with failure geometries inferred from seafloor morphology showed that mass failures could account for the observed morphology along the canyon walls as well as a mechanism of canyon widening

regional mb ms lg pg amplitude ratios and lg spectral ratios as criteria for distinguishing between earthquakes and explosions a theoretical study summary theoretical seismograms for point explosion and double couple sources in a uniform plane layered earth are used to evaluate the likely effectiveness of three criteria that have been proposed for distinguishing between earthquakes and explosions the criteria are regional ms ms lg pg amplitude ratios and lg spectral ratios for explosions in material with a high wave speed the near regional mb ms discriminant gives only marginal separation between the two types of source ranging from zero for strike slip earthquakes to on average 0 4 magnitude units for 45d dip slip mechanisms only for explosions in material with a low wave speed do the theoretical data show an overall separation lg pg amplitude ratios successfully separate the source types except for strike slip earthquakes where there is overlap with shallow explosions theoretical lg spectra for deep 1 5 km explosions are depleted in high frequencies when compared with those for earthquakes and shallow explosions discrimination between earthquakes and these deep explosions is therefore possible using the spectral ratio method an important mechanism underlying these lg based discriminants is the interaction of the spherical p wavefront from an explosion with the free surface which generates shear waves with amplitude depending both on frequency and source depth of burial

single station spectral discrimination using coda waves spectral discrimination methods applied at regional distances are based on the observation that there is a difference in the frequency content of the seismic waves radiated from the source region of earthquakes and explosions we have investigated a spectral discrimination method that exploits the scattered energy contained in the event codas of seismograms to measure the relative source excitation of earthquakes and explosions as a function of frequency key strengths of the coda method include the following 1 it is not affected by source radiation pattern because the scattering process averages the radiation over the focal sphere 2 path corrections are not required with the coda method because coda waves are scattered throughout a common crustal volume 3 it can be successfully applied using data from only a single station and 4 it can be successfully applied using data with clipped direct arrivals to implement the coda method we first estimate a type curve which represents the average shape of all the individual coda decay curves for a particular frequency band the shape of the type curve as well as the shape of all individual coda decay curves is controlled by the scattering and attenuation properties of the crustal volume sampled by the coda waves a common path effect the amplitude of the type curve is not defined and we arbitrarily adjust it to have a zero mean the amplitude difference between an individual event coda decay curve and the type curve provides a relative measure of the event s source size within the particular frequency band we define this amplitude difference as the relative source factor our approach exploits all of an event s usable coda for each spectral measurement thus providing a more stable measure of relative source size than could be obtained from using a predefined fixed length time window analysis of 27 nevada test site explosions and 15 southern great basin earthquakes 3 1 m 4 7 shows that the source factor difference between the 0 5 to 1 and 2 to 4 hz bands plotted against the 1 to 2 hz source factor is a stable event discriminant the method reliably distinguishes small explosions from small earthquakes 3 0 m 3 5 even if the explosions are overburied because the coda method requires only one seismic station we believe that it would be especially useful in identifying small clandestine underground nuclear tests that occur far from established networks

the effects of spall on regional seismograms the effects of spall on regional signals are investigated using synthetic seismograms to frequencies of 20 hz depending on the scaling relationships used it is found that spall may have an important effect on regional body waves and especially surface waves the frequency range in which the spall effect dominates is dependent upon the characteristic times of the spallation process but generally occurs in the frequency range of about 0 2 to 2 hz because of the peaked nature of the radiated spall spectra effects at frequencies outside the 0 2 to 2 hz frequency range will be dominated by the explosion source the frequency range and strength of the spall effect depend in a complicated manner upon the yield depth of burial near source structure and material strength uncertainties in the spall scaling relationships need to be better understood before the effects of spall on regional discrimination and yield estimation can be properly quantified copyright american geophysical union 1989

observations in support of rg scattering as a source for explosion s waves regional and local recordings of the 1997 kazakhstan depth of burial experiment abstract in august and september of 1997 three 25 ton chemical explosions were detonated at nominal depths of 550 300 and 50 m in boreholes at the former soviet test site at balapan kazakhstan one objective of this experiment was to evaluate the effect of differing source depth on the regional wave field analysis of regional seismic phases lead to the observation that regional p s wave amplitude ratios in the 1 to 5 hz band increase as a function of source depth however at frequencies greater than about 5 hz the relative amplitudes of p and s waves remain approximately constant for the differing depth shots similarly regional coda spectra are amplified in the 1 to 5 hz band for the shallow shots at local distances rg is the dominant seismic phase with peak amplitudes in the 0 7 to 5 hz range and rg is strongest for the shallower shots within the short distance spanned by the local stations rg is rapidly attenuated and the attenuation is accompanied by a significant shift in peak amplitude toward lower frequency at regional distances rg is below the noise level the coincident frequency band in which local rg rapidly loses energy and regional s phases are amplified points toward rg scattering as the dominant mechanism causing the discrepancies between p s amplitude ratios in this study these observations are particularly relevant to the understanding of physical processes affecting regional p s discriminants and may lead to improvements in discriminant methods

f k analysis of noress array and single station data to identify sources of near receiver and near source scattering abstract f k analyses of short period recordings of both u s and soviet underground nuclear explosions at the eskdalemuir eka scotland array indicate arrivals from a local source about 15 km due northwest of the array use of residual seismograms derived by subtracting the beamed record from each array channel confirmed the presence of the same scatterer the secondary seismic source appears to be short period surface waves due to the scattering of incident p waves at a nearby deep valley known as moffat water in agreement with key s 1967 earlier results similar analyses of the noress norway array data from u s and soviet nuclear shots also indicate secondary arrivals suggesting a local near surface scatterer about 25 30 km south west of the array in the region of lake mjosa thus short aperture array data can be useful in identifying and locating sources of near receiver scattering f k analyses of short period recordings of both explosion and earthquake sources at the high frequency noress array indicate secondary arrivals from a near receiver source about 25 30 km southwest of the array f k power difference plots obtained by subtracting after normalization the f k power versus slowness estimates for the initial p window from those for the later windows provided nearly identical results the secondary seismic source appears to be short period surface waves rg generated by the scattering of incident p waves in the region of lake mjosa 27 km sw of noress where there is about 1 km of relief from the bottom of the lake to the top of an adjacent hill

prediction of ignitive turbidity currents in scripps submarine canyon swift highly erosive continuous turbidity currents are known to be triggered in scripps submarine canyon during storms a relatively modest agitation due to edge waves can generate currents at least as swift as 1 9 m s 1 pantin 1979 and parker 1982 have described a mechanism by which a turbidity current can self accelerate by entraining sediment from the bed parker termed the mechanism ignition herein two models of steady spatially developing turbidity currents are developed and applied to scripps submarine canyon fairly specific predictions are obtained for the onset of ignitive acceleration and for the downstream development of the turbidity currents the results are in general agreement with available measurements in the same canyon it is speculated that ignitive turbidity currents constitute a major mechanism by which sand submarine canyons are eroded and by which sand is delivered to the abyssal plain

regional magnitude scaling transportability and ms mb discrimination at small magnitudes data sets of mb pn and mb lg measurements are presented for three continental regions in order to investigate scaling relationships with moment magnitude w and event discrimination at small magnitudes compilations of published measurements are provided for eastern north american and central asian earthquakes and new measurements are reported for earthquakes located in western united states statistical tests on w mb relationships show that the mb lg scale of nurru 1973 is transportable between tectonic regions and a single unified w mb lg relationship satisfies observations for m 6 5 in all regions a unified relationship is also developed for nuclear explosions detonated at the nevada test site and test sites of the former soviet union regional mb for explosions scale at higher rates than for earthquakes and of significance is the finding that mb pn for explosions scales at a higher rate than mb lg a model is proposed where differences in scaling rates are related to effects of spectral overshoot and near field rg scattering on the generation of pn and lg waves by explosions for earthquakes mb pn and mb lg scale similarly showing rates near 1 0 or 2 3 log1omo seismic moment

regional source discrimination of small events based on the use of lg wavetrain nuclear monitoring requires identification of small events observed at regional distances for which lg is generally the largest seismic phase recent research including ours has provided significantly better understanding of the generation of lg from explosions but the new concepts have not yet been fully exploited for improving discrimination between low magnitude explosions and earthquakes in different regions of the world our analysis of spectrograms which display spectral energy versus time shows that lg from explosions alone is generally followed by several distinct arrivals with progressively lower frequency contents probably due to the scattering of explosion generated rg into s at several discrete near source locations this promising new discriminant may offer a distinct advantage over other seismic methods of discrimination since it may not require the region of interest to be calibrated with the help of earlier known type seismic events moreover regional discrimination based on the lg wavetrain can be especially useful when signal to noise considerations for a small event preclude the use of other seismic phases

seismic yield determination of soviet underground nuclear explosions at the shagan river test site summary a study of p and lg based source size estimators has been conducted for 101 presumed underground nuclear explosions at the shagan river test site near semipalatinsk ussr a systematic variation in the relative patterns across the test site has been observed when comparing the p wave estimators world wide mb and ulm and the lg based observations rms lg determined from norsar and grafenberg array recordings these variations correlate well with geological features determined from satellite imagery and suggest in particular that the northeast and southwest portions of the test site divided by the chinrau fault are characterized by distinct geophysical properties a difference in m p mlg bias of as much as 0 15 magnitude units exists between these two subareas available data indicate that the mlg observations are very consistent with published yields and suggest that the magnitude yield relationship for p wave based estimators can be significantly improved by introducing regional bias corrections we have determined such corrections for three subareas of the test site and used the corrected mh and ym observations in combination with the lg data in a weighted averaging scheme to arrive at yield estimates for all events in the database for the explosion of 1988 september 14 we obtain yield estimates from the three estimators ranging from 106 to 118 kt which is consistent with the independently derived estimates of 115 122 kt quoted by sykes ekstrom 1989 a further assessment of the validity of our results would require access to independently measured yields of additional explosions

the stability of rms lg measurements and their potential for accurate estimation of the yields of soviet underground nuclear explosions data on underground nuclear explosions have recently become available from modern digital seismic stations installed within the soviet union and china observations of root mean square rms lg wave signals for soviet underground nuclear explosions at the shagan river test site in east kazakhstan show that the relative amplitudes of the rms signals at stations in norway the ussr and china are very similar for different explosions the standard deviation of the differences being only about 0 03 in logarithmic units i e magnitude units this is consistent with earlier observations comparing norsar and graefenberg array data and the observed scatter is significantly lower than has been reported for lg data from nevada test site explosions in view of the excellent correspondence found by nuttli 1986 and patton 1988 for lg versus yield at nevada this indicates that rms lg has a potential for yield estimation with very high accuracy at shagan river our study has shown that a selected stations in the ussr and china situated at regional distances provide a much improved signal to noise ratio of the lg phase for events at shagan river as compared to norsar array data b the scaling of rms lg amplitudes between different sized events recorded at the same single station site appears to be consistent with that of norsar indicating a remarkable degree of precision in single station measurements of lg signal c rms lg amplitude measurements for the best of these stations may be made at 1 5 to 2 0 magnitude units lower than at norsar or graefenberg allowing a much lower threshold for lg based yield determination and d the p wave detection capabilities of these single stations do not match those of the noress and arcess arrays thus teleseismic signals continue to be important for detection of small nuclear explosions our conclusion is that lg signals appear to provide an excellent basis for supplying estimates of the yields of nuclear explosions even down to below 1 kt when such signals are recorded at high quality digital in country seismic stations and when calibrated by access to independent nonseismic yield information for a few nuclear explosions at the test sites of interest in the context of monitoring a low yield threshold test ban treaty it will in addition be important to take into consideration various environmental conditions in the testing area such as the possible presence of cavities and to devise appropriate procedures for on site observations in this regard

a discrimination analysis of short period regional seismic data recorded at tonto forest observatory abstract a new seismic discriminant based on spectral differences of regional phases from earthquakes and explosions recorded at a single station has been tested and found to work remarkably well the test data consisted of a well constrained set of 30 nevada test site nts explosions and 21 earthquakes located within about 100 km of nts which were recorded on short period seismographs at the tonto forest observatory in central arizona at an epicentral distance averaging 530 km the events in the data set cover a magnitude range from 3 3 to 4 8 m b for which p n p g and l g phases have been analyzed we found that although l g phases from earthquakes are typically more prominent than for explosions with comparable p wave amplitude levels simple time domain l g p amplitude ratios do not result in a separation of the earthquake and explosion samples consistent enough to provide reliable discrimination however spectral analyses of the data over the frequency band from 0 5 to 5 0 hz revealed significant differences in the spectra of certain regional phases which proved to be a quite reliable discriminant in particular both the p g and l g spectra from earthquakes have been found to be richer in high frequency content than corresponding explosion spectra a discriminant measure defined as the ratio of average l g spectral amplitude level in the 0 5 to 1 0 hz passband to that in the 2 0 to 4 0 hz passband provides good separation of earthquake and explosion populations

air pressure data from the vallee de la sionne avalanches of 2004 1 despite their clear danger to humans snow avalanches are hard to document they occur in inaccessible and dangerous locations often at times of bad weather observation instruments frequently malfunction in the harsh conditions or are destroyed measurements of powder snow avalanches are particularly difficult as these occur less frequently and are usually very large to understand the air flow in front of and inside powder snow avalanches we have designed an air pressure sensor to survive the harsh conditions it consists of a differential pressure transducer with a high frequency response built into a specially designed housing unit we mounted this 10 m from the ground on a measurement mast in the vallee de la sionne avalanche test site in switzerland data from five powder snow avalanches over the winter of 2004 were recorded three of these were natural releases and two were artificially triggered with explosives we present an analysis of the sensor response and an interpretation of the signals in terms of simple flow fields we show how these data can be used to deduce information about the speed size and location of the avalanches using a dipole approximation our sensor has two major limitations the length of the internal tubing produces low frequency resonances and there is only one transducer so a complete flow model is needed to deduce the three velocity components and pressure we discuss these limitations and give a design for a new sensor to overcome them

measurement of frequency dependent lg attenuation in the great basin measurement of the spatial decay of spectral amplitudes of the higher mode seismic surface wave train lg indicates that in the great basin the apparent seismic quality factor q is a function of frequency observations along a 300 km profile recording of a nuclear explosion yield the function q 206 0 68 over the band 0 3 10 0 hz similar analysis using numerous recordings of earthquakes along paths which provide a good average sampling of the great basin crust gives q 214 15 0 54 0 09 for 0 3 5 0 hz if the crustal sampling by lg energy from nuclear explosions surface sources is primarily in the shallow crust as has been suggested then these results indicate a greater frequency dependence of apparent q there than at depth

chronology of late wisconsinan glaciation emerald basin scotian shelf eight new accelerator mass spectrometry ams radiocarbon dates from proglacial sediments in emerald basin indicate that the previously proposed chronology based on dates of total organic matter is much too old the new chronology indicates that grounded ice occupied emerald basin in the late wisconsinan

the intermittency regions of powder snow avalanches powder snow avalanches are typically composed of several regions characterized by different flow regimes these include a turbulent suspension cloud of fine particles a dense basal flow and an intermittency frontal region which is characterized by large fluctuations in impact pressure air pressure velocity and density but whose origin remains unknown in order to describe the physical processes governing the intermittency region we present data from four large powder snow avalanches measured at the vallee de la sionne test site in switzerland which show that the intermittency is caused by mesoscale coherent structures these structures have a length of 3 14 meters and a height of 10 m or more the structures can have velocities as much as 60 larger than the avalanche front speed and are characterized by an air particle mixture whose average density can be as high as 20 kg m 3 this average density increases the drag on large granules by a factor of up to 20 compared to pure air so that each structure can maintain denser snow clusters and single snow granules in suspension for several seconds the intermittency region has importance for the dynamics of an avalanche as it provides an efficient mechanism for moving snow from the dense layer to the powder cloud but also for risk assessment as it can cause large forces at large heights above the basal dense layer

pore fluid pressure and internal kinematics of gravitational laboratory air particle flows insights into the emplacement dynamics of pyroclastic flows 1 the emplacement dynamics of pyroclastic flows were investigated through noninvasive measurements of the pore fluid pressure in laboratory air particle flows generated from the release of fluidized and nonfluidized granular columns analyses of high speed videos allowed for correlation of the pressure signal with the flow structure the flows consisted of a sliding head that caused underpressure relative to the ambient followed by a body that generated overpressure and at the base of which a deposit aggraded for initially fluidized flows overpressure in the body derived from advection of the pore pressure generated in the initial column and decreased by diffusion during propagation relatively slow diffusion caused the pore pressure in the thinner flow to be larger than lithostatic at early stages furthermore partial auto fluidization revealed in initially nonfluidized flows also occurred and contributed to maintain high pore pressure whereas dilation or contraction of the air particle mixture with associated drag and or pore volume variation transiently led the pressure to decrease or increase respectively the combination of all these processes resulted in long lived high pore fluid pressure in the body of the flows during most of their emplacement in the case of the initially fluidized and slightly expanded 3 4 flows at least 70 100 of the weight of the particles was supported by pore pressure which is consistent with their inertial fluid like behavior dense pyroclastic flows on subhorizontal slopes are expected to propagate as inertial fluidized gas particle mixtures consisting of a sliding head possibly entraining basement derived clasts and of a gradually depositing body

frontal dynamics of powder snow avalanches 1 we analyze frontal dynamics of dilute powder snow avalanches sustained by rapid blow out behind the front such material injection arises as a weakly cohesive snow cover is fluidized by the very pore pressure gradient that the particle cloud induces within the snowpack we model cloud fluid mechanics as a potential flow consisting of a traveling source of denser fluid thrust into a uniform airflow stability analysis of a mass balance involving snow cover and powder cloud yields relations among scouring depth frontal height speed mixed mean density and impact pressure when the frontal region achieves a stable growth rate we compare predictions with field measurements show that powder clouds cannot reach steady frontal speed on a uniform snowpack with constant cloud width and derive a criterion for cloud ignition because static pressure is continuous across the mean air cloud interface and deviatoric stresses are negligible frontal acceleration is insensitive to local slope but instead arises from a deficit of flow induced suction in the wake we calculate how far a powder cloud travels until its frontal mixed mean density becomes stable and show how topographic spread can hasten its collapse

effects of flow regime and sensor geometry on snow avalanche impact pressure measurements impact pressures of snow avalanches have been measured at the vallee de la sionne experimental test site using two kinds of sensors with different sizes placed at different locations in the avalanche flow pressures measured for a fast dry snow avalanche and a slow wet snow avalanche are compared and discussed the pressures recorded in the dense flow of a dry snow avalanche agree well for the two types of sensors used showing negligible dependence on the measurement device on the other hand significantly different pressures are measured in the slow dense flow of a wet snow avalanche this is attributed to the slow drag and bulk flow of this type of avalanche leading to the formation and collapsing of force chain structures against the different surfaces of the sensors at a macroscopic scale limit state analysis can be used to explain such a mechanism by a shear failure occurring between freely flowing snow and a confined snow volume against the sensor according to a mohr coulomb failure criterion the proposed model explains how impact pressure can be up to 8 times higher than the hydrostatic snow pressure in wet cohesive slow avalanches and its dependence on sensor geometry

numerical analytical interfacing in two dimensions with applications to modeling nts seismograms a new method for interfacing numerical and integral techniques allows greater flexibility in seismic modeling specifically numerical calculations in laterally varying structure are interfaced with analytic methods that enable propagation to great distances such modeling is important for studying situations containing localized complex regions not easily handled by analytic means the calculations involved are entirely two dimensional but the use of an appropriate source in combination with a filter applied to the resulting seismograms produces synthetic seismograms which are point source responses in three dimensions the integral technique is called two dimensional kirchhoff because its form is similar to the classical three dimensional kirchhoff data from yucca flat at the nevada test site are modeled as a demonstration of the usefulness of the new method in this application both local and teleseismic records are modeled simultaneously from the same model with the same finite difference run this application indicates the importance of locally scattered rayleigh waves in the production of teleseismic body wave complexity and coda

a conceptual model of the deglaciation of atlantic canada abstract we present a conceptual model for the deglaciation of atlantic canada in which a role is played by ice streams the ice margin lay close to the edge of the continental shelf at the last glacial maximum ice streams occupying shelf troughs comprised 1 a major ice stream in the laurentian channel 2 secondary streams in the bay of fundy gulf of maine trinity trough and notre dame channel and 3 lesser ice streams elsewhere early retreat in the northeast notre dame bay and in the southwest bay of fundy gulf of maine was mainly by calving along embayments by 18 ka radiocarbon years a calving embayment had opened on emerald basin scotian shelf retreat along channels marooned ice on intervening banks a major calving episode beginning just before 14 ka removed ice from the gulf of st lawrence triggering re advances down secondary troughs isolation of a newfoundland ice cap produced radial drainage via fiords and more extensive flow into the st george s bay lowlands on the west coast by 13 ka most ice was on land and was ablating by melting rather than calving shelf ice caps persisted until 11 ka

micro scale geotechnical variability in continental slope and abyssal sediments influenced by the oxygen minimum zone in the arabian sea abstract we describe a new micro scale method of measuring the penetration resistance of the surficial layers of slope and abyssal plain sediments within and below the omz on the oman continental slope the micro scale load resistance penetrometer mlrp continuously measures the penetration resistance q of a cylindrical probe driven at constant velocity into cores of sediment recovered by sub coring box core samples measurements are recorded at 0 8 mm depth intervals into the sediment by an automated data acquisition system these provide continuous depth profiles of penetration resistance q and its variability qr in surficial sediments 0 100 mm the results represent the first published application of this technique to slope and abyssal plain sediments the q data divide the stations into shallow omz somz stations 391 854 m deep omz domz stations 987 1285 m and abyssal non omz anomz stations 3392 3396 m one station 688 m was anomalous penetration resistance q was positively related to shear strength and inversely related to water content but not to bioturbation measured as burrow numbers relaxation and pull out data were obtained from three stations three patterns of qr were observed consisting of well confined data data containing large negative and positive spikes and data showing a progressive down core increase in spikes four additional measures of q variability were defined which show statistically significant relationships with burrow numbers an increase in bioturbation estimated by burrow numbers is associated with a decrease in micro scale variability in q this is related to the activity of burrowing macrofauna our results are discussed in the context of biogeotechnical modifications of sedimentary environments

yield estimates of nevada test site explosions obtained from seismic lg waves a methodology is presented for determining the yield of underground nuclear explosions from lg wave amplitudes the methodology is applied to nevada test site nts explosions for which the data from short period vertical component analog seismographs at three stations are used to develop calibration curves for unsaturated material and water saturated rock source conditions the latter curves are found to provide reasonably accurate estimates of the yields of explosions in other areas of the united states and in the french sahara suggesting that they may be applicable to all continental areas if so they also can provide an estimate of the bias of mb p magnitudes between different continental sites for example the lg data from nts explosions indicate a 0 31 0 02 magnitude unit bias between nts and eastern north america similar to the approximately 0 33 unit bias found between western and eastern north america previously by use of earthquake data

impact pressures and flow regimes in dense snow avalanches observed at the vallee de la sionne test site 1 a fundamental problem in avalanche engineering is to determine the impact pressures exerted on structures this task is complicated because snow avalanches flow in a variety of regimes primarily depending on snow temperature and moisture content in this paper we address this problem by analyzing measured impact pressures flow velocities and flow depths of five vallee de la sionne avalanches the measurements are made on a 20 m high tubular pylon instrumented with high frequency pressure transducers and optoelectronic velocity sensors in the observed avalanches we find both subcritical and supercritical flow regimes typical froude numbers were smaller than 6 the subcritical regime fr 1 is characterized by a flow plug riding above a highly sheared basal layer the measured pressures are large and velocity independent in contradiction to calculation procedures pressure fluctuations increase with flow depth indicating a kinematic stick slip phenomena which is largest at the basal layer supercritical flow regimes 1 fr 6 are characterized by a sheared flow all over the avalanche depth in this regime the impact pressure is velocity dependent we derive relationships governing impact pressure as a function of the froude number and therefore flow regime encompassing all the observed avalanches

petrographic evidence from the eastern canadian margin of shelf crossing glaciations abstract it is difficult to evaluate when and where glaciers extended across the outer continental shelf off eastern canada the outer shelf and upper slope are commonly hard bottomed or have a great thickness of post glacial sediment in either case making direct sampling and dating of ice margin sediment impossible the timing and extent of glacier ice is important for understanding continental slope sedimentation and ice sheet dynamics we have therefore determined the lithologic components of the 1 4 mm granule fraction of over 100 well dated samples from 40 cores from the more distal parts of the deep water margin off nova scotia and newfoundland this fraction is demonstrably ice rafted in these cores and geographic variation in component results from input of icebergs at different calving margins the samples show that heinrich layers have a distinctive granule fraction that shows some areal variation as a result of dilution by sources other than hudson strait heinrich event 2 ca 21 ka is widespread as far southwest as the west scotian slope but by the last glacial maximum lgm numerous discrete sources of ice rafted granules are recognised implying that calving ice margins were present in several places off newfoundland and nova scotia including off banquereau a distinctive purplish lgm gravelly mud is recognised on the eastern newfoundland margin derived from a calving margin on the ne newfoundland shelf at the time of heinrich event 1 ca 14 ka there was a calving ice outlet somewhere on the grand banks and another in halibut channel retreat of ice from the gulf of st lawrence can be tracked by changing granule petrology from the lgm to 13 ka these data suggest that final ice retreat from many parts of the outer shelf was later than is shown in many recent ice margin reconstructions