electrical resistivity of ice from the antarctic peninsula antarctica l objectif etait de determiner le comportement electrique de la glace dans une zone ou existe une fonte estivale substantielle ainsi que des regimes thermiques contrastes des mesures electriques effectuees en trois sites le long d une ligne de courant sur le george vi ice shelf montrent que a la resistivite de la glace en profondeur est semblable a celle des autres shelfs antarctiques b la resistivite de la glace a la surface de la glace du shelf qui est soumise a la percolation et au regel de l eau de fusion est semblable a celle de la glace en profondeur et donc la glace a un caractere polaire

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

assessment and mitigation of coal bump risk during extraction of an island longwall panel abstract this study presents an integrated approach for field tests and numerical investigations to assess the risk of coal bumps this approach produces a stress relief technology using boreholes to mitigate risk during the extraction of an island longwall panel the field tests were conducted in an island longwall panel in the tangshan coal mine in the city of tangshan china in these tests roadway roof displacement and electromagnetic radiation emr of roadways in the panel were investigated to determine the zones of intensive roof deformation a numerical model flac 3d fast lagrangian analysis of continua in 3 dimensions was established to understand the results of the field tests and to map the zones in the panel with a high risk for coal bumps the results of the field tests and the numerical modeling show that the roof deformation starts to occur at more than 30 m ahead of the longwall face and the deformation starts to accelerate after a distance of 10 m in front of the longwall face large and rapid roof deformation is considered to be an important precursor of coal bump occurrence during the extraction of an island longwall panel based on these results a stress relief technology using boreholes was investigated through numerical methods the modeled results suggest that the abutment stress could be released by drilling boreholes in the zones prone to coal bumps the effectiveness of the stress release increased with the borehole length and decreased with the borehole spacing

impurity sources of f cl no3 and so4 2 in greenland and antarctic precipitation five polar impurity sources are proposed anthropogenic no3 and so42 in recent greenland but not antarctic snow marine aerosols the dominant cl source volcanic eruptions which produce detectable but variable peaks in so42 f and or cl concentrations biogenic sulfurous gases from soil and coastal waters the dominant source for so42 in nonvolcanic strata an unidentified source of background nitrate which is more active in the northern hemisphere and which displays a summer concentration maximum solar activity and supernova events do not modulate no3 concentrations in greenland snow

the east pacific rise and the thermal model although the thermal mode of lithosphere cooling can be used to predict the east pacific rise topography reasonably well it does not adequately predict the observed gravity when the zero temperature boundary condition is at the seafloor the misfit arises primarily because this model predicts thermally lowered densities in the crust at the rise axis which counteract the gravitational effect of the topography generating a theoretical gravity anomaly smaller than that observed it is shown using data at 12 n as an example that improved agreement with the thermal model is obtained if to first order the crust is treated as an isothermal constant thickness constant density layer with variable depth this result is consistent with seismic data and observations of convective cooling of the crust by seawater circulation it implies that convective cooling of the whole crust is very rapid even at the axis of fast spreading ridges the oceanic moho may represent the depth to which cracking and convective cooling extends

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

rock burst process of limestone and its acoustic emission characteristics under true triaxial unloading conditions abstract the rock failure process is associated with acoustic emission ae hence violent rock failure processes such as rock burst under different stress paths in laboratory can be studied using the ae monitoring technique in this paper single face dynamic unloading tests under true triaxial condition were carried out for paleozonic marine sedimentation limestone samples produced by blocks cored from 1140 m depth in jiahe coal mine of china frequency spectra analysis was conducted for the full wave ae data and figures showing the relationship of frequency amplitude of ae signals in rock burst stages were obtained the dynamical damage process and characteristics of limestone under the test condition are summarized it is found from the fast fourier transformation fft analysis of the ae signals that there exist two main frequency ranges i e lower 60 100 khz and higher 170 190 khz for the samples when the limestone samples are under relatively lower stress the ae signals are characterized by high frequency and low amplitude as the stress increases the ae signals tend to shift more towards a signature of high amplitude near the bursting failure of the rock samples it is observed that there are much higher amplitude and lower frequency events the ae accumulated energy release rapidly increases from the unloading state of the rock samples to its failure the dynamic unloading test facility provides a useful tool to gain an insight into the characteristics of rock bursts

dielectric behaviour of firn and ice from the antarctic peninsula antarctica les mesures dielectriques de 14 echantillons de glace et neve preleves en antarctique pour des temperatures de 2 a 70 c et des frequences de 10 hz a 100 khz ont ete comparees a celles de la glace prelevee au groenland

glaciological and oceanographic calculations of the mass balance and oxygen isotope ratio of a melting ice shelf on a etudie l alimentation du george vi ice shelf en relation avec l accumulation le velage et la fonte on releve la relation temperature taux d isotope de l oxygene ces donnees sont couplees a des donnees oceanologiques sur les eaux chaudes et son interaction avec le shelf

the role of acids in electrical conduction through ice 1 electrical conduction through meteoric polar ice is controlled by soluble impurities that originate mostly from sea salt biomass burning and volcanic eruptions the strongest conductivity response is to acids yet the mechanism causing this response has been unclear here we elucidate conduction mechanisms in ice using broadband dielectric spectroscopy of meteoric polar ice cores we find that conduction through polycrystalline polar ice is consistent with jaccard theory for migration of charged protonic point defects through single ice crystals except that bulk dc conduction is impeded by grain boundaries neither our observations nor modeling using archie s law support the hypothesis that grain boundary networks of unfrozen acids cause significant electrolytic conduction common electrical logs of ice cores by electrical conductivity measurement ecm or dielectric profiling dep and the attenuation of radio waves in ice sheets thus respond to protonic point defects only this response implies that joint interpretation of electrical and chemical logs can determine impurity partitioning between the lattice and grain boundaries or inclusions for example in the greenland ice core project grip ice core from central greenland on average more than half of the available lattice soluble impurities h cl nh4 create defects understanding this partitioning could help further resolve the nature of past changes in atmospheric chemistry

electrical resistivity sounding of the east antarctic ice sheet electrical resistivity soundings using a schlumberger array have been carried out at dome c east antarctica 74 39 s 124 10 e elevation 3400 m to sound the entire 3500 m depth of the ice sheet changes in density and temperature are largely separated in the ice at dome c so activation energies for both firn and ice could be determined we find an activation energy of 0 25 ev in both solid ice and firn between 15 c and 58 c a common value of the activation energy points to a single transport regime in which the charge carriers and conduction paths are the same in firn and ice to evaluate the variation of resistivity with density we have considered five dielectric mixture models that fit the available data on the high frequency dielectric constant of firn only looyenga s equation fits the field data for dc resistivity in the upper 900 m of the ice sheet where impurity concentrations are known from core samples we find no correlation between resistivities and the concentrations of salts or acids instead we find it likely that resistivities are correlated with the crystal size hence with the holocene wisconsin boundary in the ice column a pronounced increase in resistivity to a value comparable with that in temperate glacier ice occurs deep within the ice sheet we attribute this to a large increase in the size and irregularity of the ice crystals which destroys the continuity of the impurity shells surrounding the ice crystals that we believe supply the conduction paths shabtaie and bentley 1994a high resistivity does not imply removal of the impurities from the system moderate concentrations of impurities can be accommodated by locating them in disconnected domains

mechanism and risk assessment of overall instability induced rockbursts in deep island longwall panels abstract an increasing number of mines in china are being subjected to overall instability induced oii rockbursts in extracting deep remnant island longwall panels causing severe damage at far larger scale and intensity compared with common local rockbursts using the island longwall panel lw1305 at zhaolou coal mine as a case example this paper investigates the mechanism of the newly defined oii rockburst by means of stress and microseismic monitoring and then proposes a straightforward risk assessment method for oii rockbursts based on the actual bearing index in an attempt to provide guidelines for making engineering judgement the proposed risk assessment method was successfully applied to lw1305 and the assessment result is supported by the occurrence of an oii rockburst in lw1305 and further verified by numerical modeling and microseismic monitoring