on the application of simple rift basin models to the south polar region of enceladus 1 the south polar terrain spt of saturn s moon enceladus is a mysteriously active region that exhibits intriguing tectonic signatures and widespread fracturing the central region of the nearly circular spt is depressed into the surface by a few hundred meters and bounded by a ring of cliffs roughly 1 km high in this study we investigate whether this depression and surrounding mountainous uplift is consistent with the morphology of terrestrial rift basins and the possibility that the spt could have formed during a tectonic event analogous to those of such rift basins on earth using three mechanical models of basin formation we compare our predicted topography of the spt with observed topography of the region the first of three models we consider assumes crustal stretching by factor b and predicts a basin depth of roughly 600 m closely matching previously published estimates of the depth at the spt models of extension and compression assuming an elastic response in the ice crust predict best fit mountain uplift of roughly 1820 m and 1130 m respectively our preferred model suggests that the icy shell in the spt has been stretched but the extension is partially balanced by compression along the edges of the basin leading to the uplift of the mountains along the boundary thereby implying that the spt may have a tectonic origin analogous to that of a terrestrial basin

relating the occurrence of crevasses to surface strain rates the presence of crevasses on the surface of ice masses indicates that a fracture criterion has been met understanding how crevasses form will provide information about the stress and strain rate fields in the ice this study derives a relationship between measurements of strain rate and observations of crevassing on the surface of ice masses a literature search yielded 17 polar and alpine locations where strain rates had been measured and crevassing recorded by plotting strain rates converted to stresses using a creep law using axes representing the surface parallel principal stresses failure envelopes were derived by enclosing measurements where surface crevassing was absent the derived failure envelopes were found to conform well to theoretical ones predicted by the coulomb and the maximum octahedral shear stress von mises theories of failure the derived failure envelopes were scaled by the tensile strength which was found to vary from 90 to 320 kpa there was no systematic variation of tensile strength with either temperature at 10 m depth or the method used to locate the crevasses the observed variation in tensile strength could result from variations in ice properties e g crystal size impurity content or density or could be related to uncertainty in the constitutive relation creep flow and fracture share a very similar temperature dependence suggesting similar crystal scale processes are responsible for both the observed relationship will provide a supplementary tool with which to verify and test models of ice dynamics against remotely sensed imagery the study also indicates that a temperature rise of a few degrees throughout the ice column will not result directly in any increase in calving rates from the large antarctic ice shelves such as the filchner ronne or ross ice shelves

crustal thicknesses in se brazilian shield by receiver function analysis implications for isostatic compensation hypothesis are proposed to explain the data 1 a lower density by 30 40 kg m 3 i n the lithospheric mantle under the archean block of the sao francisco craton relative to the proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area relatively low density and high p wave velocity are compatible with a depleted low feo composition for the archean lithosphere 2 alternatively if the density contrasts between archean and proterozoic lithospheres are smaller than the values above then the crust beneath the paranabasin must be more dense than that of the craton higher crustal density and high poisson s ratio would be consistent with magmatic underplating in the lower crust beneath the paranabasin as inferred from other studies index terms 7205 seismology continental crust 1242 9360 information related to geographic region south america 7218 seismology lithosphere and upper mantle keywords crust upper mantle brazil parana basin san francisco craton

the dynamics of motion of the south american plate observations of the intraplate stress field as compiled and evaluated in the context of the world stress map project can be compared with stress modelling results to increase our understanding of the dynamics of motion of the lithospheric plates in this paper we investigate the driving and resistive plate tectonic forces acting on the south american plate in order to establish a detailed basis for future stress modelling our basic assumption is that the plate is in mechanical equilibrium that is the net torque of the forces acting on the plate vanishes the complexity of the forces exerted along the margins of the south american plate requires the dynamical analysis to be performed step wise incorporating the forces which are expected to be of major importance a first order model is defined the effect of additional plate boundary forces is subsequently examined in terms of superpositions onto the first order model with the first order model the relation between three resistive forces and shear stress on the base of the lithosphere is derived the ridge push force driving the motion of the south american plate can be quantified independently and is included in the calculations as a known quantity solutions are obtained using four different determinations of the euler vector of absolute motion of the south american plate which is taken to define the orientation of the basal shear stress the results demonstrate that the concept of a basal shear stress actively driving plate motion cannot be discarded on the basis of a dynamical analysis further with the assumption that basal drag is entirely concentrated below the continental pan of the lithosphere estimates of the maximum amount of continental basal shear resistance are derived evaluation of an alternative model for the south american plate excluding the northernmost pan of the andes demonstrates the significance of the resistive force exerted on the south american plate due to convergence with the caribbean plate it is to be expected that comparison of theoretical distributions of intraplate stress computed on the basis of the present dynamical analysis with observations of the actual state of stress will further constrain the range of feasible force models

melt under cutting and buoyancy driven calving from tidewater glaciers new insights from discrete element and continuum model simulations the simple calving laws currently used in ice sheet models do not adequately reflect the complexity and diversity of calving processes to be effective calving laws must be grounded in a sound understanding of how calving actually works here we develop a new strategy for formulating calving laws using a the helsinki discrete element model hidem to explicitly model fracture and calving processes and b the continuum model elmer ice to identify critical stress states associated with hidem calving events a range of observed calving processes emerges spontaneously from hidem in response to variations in ice front buoyancy and the size of subaqueous undercuts calving driven by buoyancy and melt under cutting is under predicted by existing calving laws but we show that the location and magnitude of hidem calving events can be predicted in elmer ice from characteristic stress patterns our results open the way to developing calving laws that properly reflect the diversity of calving processes and provide a framework for a unified theory of the calving process continuum

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

fracture of antarctic shelf ice 1 we have investigated the fracture of antarctic shelf ice core using two fracture mechanics test methods the chevron notched short rod specimen loaded in tension and the chevron notched round bar specimen loaded in three point bending these tests have been used to measure the fracture initiation toughness kinit at which crack growth starts on samples taken through the entire thickness of the ronne ice shelf from low density firn through consolidated meteoric ice to basal marine ice the fracture data are presented together with depth profiles of relevant physical and mechanical properties derived from the test specimens temperature density elastic modulus and grain size it is found that the trend in measured fracture toughness closely reflects changes in ice density and elastic modulus we augment the experimental study by presenting a fracture mechanics analysis of ice shelf surface and basal crevassing which directly incorporates our measurements for the examined ice shelf profiles basal crevasses are found to be inherently unstable unless an external restraining force is imposed which has important implications for overall ice shelf stability on the other hand surface crevassing is shown to be innately stable at depth our fracture mechanics model is used to predict local ice shelf back stresses in the vicinity of basal crevassing and is validated directly against field observations of crevasse penetration on the ronne ice shelf

tidal flexure at ice shelf margins hinge zones accommodate the differential tidal movement between floating ice shelves which move with the sea tide and grounded ice sheets which do not observations on rutford ice stream and ronne ice shelf antarctica using the kinematic method of global positioning system have yielded the first continuous tidal displacement profiles from an ice sheet hinge zone the form of these tidal displacement profiles indicates that the flexure can be fit to an elastic beam model using a parameter space optimization technique the model matches the observed tidal deflection profile to within 5 cm which is similar to the observational uncertainty the same model and parameter fitting technique is then successfully applied to various other published and unpublished tidal displacement data from doake ice rumples bach ice shelf and ekstrom ice shelf antarctica and jakobshavns glacier greenland i conclude that the elastic beam model with a single value of the elastic modulus 0 88 0 35 gpa adequately describes almost all the data and so can usefully be applied elsewhere earlier studies of tidal dissipation in ice shelf hinge zones based on a viscous or transient creep rheology showed that perhaps 30 of global tidal energy dissipation could be occurring in ice shelf hinge zones this study suggests however that an elastic rheology may be equally appropriate in which case this estimation of the tidal energy dissipation is excessively high

hydrological characteristics of the drainage system beneath a surging glacier a rare combination of natural circumstances permits assessment of current theories on water flow beneath glaciers outburst floods from the subglacial lake grimsvotn in iceland took place before during and after surging of skei ararjokull the glacier beneath which the outburst floods drain the observable drainage patterns associated with these floods show the different nature of the basal water conduit system of the glacier during surge and non surge phases during surge conditions basal water is dispersed slowly across the bed in a distributed drainage system but when the glacier is not surging water is transported rapidly through a system of tunnels

new insights into the subglacial and periglacial hydrology of vatnajokull iceland from a distributed physical model we apply a time dependent distributed glaciohydraulic model tovatna jo kull ice cap iceland aiming to determine the large scale subglacial drainage structure theimportance of basallyderivedmeltwater theinfluence ofapermeable glacierbedand vatnajo kull s discharge contribution to major rivers in iceland the model comprisestwo coupled layersthatrepresentthesubglacialhorizonperchedona subsurfaceaquifer inthe western sector and bedrock in the eastern sector to initialize and drive the simulations we use digital elevation models of the ice surface and bed the1999 2000 measured mass balance and an estimate of subglacial geothermal heat fluxes the modelled subglacial flow field differs substantially from that derived by hydraulic potential calculations and the corresponding distribution of basal effective pressure shows a strong correlation between low effective pressure and surge prone areas in northeastern and southern sectors of vatnajo kull simulations suggest that geothermally derived basal melt may account for upto 5 of the annualglacial discharge and buried aquifers may evacuate up to 30 ofsubglacialwater time dependenttestsyieldestimatesoftheglacialdischargecomponent in various outlet rivers and suggest a possible seasonal migration of subglacial hydraulic divides this study of present dayvatnajo kull hydrology formsthe starting point for inves tigationsof its future evolution

the sao vicente earthquake of 2008 april and seismicity in the continental shelf off se brazil further evidence for flexural stresses summary the continental margin and shelf of most stable intraplate regions tend to be relatively more seismically active than the continental interior in the southeast continental margin of brazil a seismiczoneextendsfromriograndedosultoespeismicactivityoccurring mainly along the continental slope and suggesting a close relationship with flexural stresses caused by the weight of the sediments in this region earthquakes with magnitudes larger than 5m b occurevery20 25yr onaverage thefocalmechanismsolutionsofpreviousearthquakes in this zone indicated reverse faulting on planes dipping approximately 45 with horizontal p axes the recent 5 2m b earthquake of 2008 april 23 occurred 125 km south of s ao vicente and was well recorded by many stations in se brazil as well as at teleseismic distances in north america and africa its focal depth was 17 km locating the hypocentre in the lower crust a well determined focal mechanism solution shows one vertical nodal plane and one subhorizontal nodal plane thep a ndt axes exhibit large dips which were confirmed by ar egional moment tensor inversion this unusual orientation of the fault mechanism can be attributed to a rotation of the principal stress directions in the lower crust caused by flexural effects due to the load of recent sedimentation

tensile strength of glacial ice deduced from observations of the 2015 eastern skafta cauldron collapse vatnajokull ice cap iceland

cauldron subsidence and subglacial floods ice cauldrons are depressions which form at the surface of ice sheets when an underlying subglacial lake empties notable examples of such cauldrons occur on the surface of the vatnajokull ice cap in iceland and in particular are formed when subglacial volcanic eruptions occur more generally cauldrons will form when a subglacial lake empties during a jokulhlaup the rate of subsidence of the ice surface is related to the rate at which the subglacial water empties from the lake we use a viscous version of classical beam theory applied to the ice sheet to determine the relation between the subsidence rate and flood discharge and we use the results to make inferences concerning ring fracture spacings in cauldrons the consequent effect on flood discharge dynamics and the likely nature of subsidence events in the antarctic ice sheet

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

causes of intraplate seismicity in central brazil from travel time seismic tomography abstract new results of travel time seismic tomography in central brazil provide evidence that the relatively high seismicity in this region is related to the thinner lithosphere at the limit between the amazonian and sao francisco paleocontinents the transition between these paleocontinents is marked by low velocity anomalies spatially well correlated with the high seismicity region which are interpreted as related to the lithospheric thinning and consequent rise of the asthenosphere which have increased the temperature in this region the low velocity anomalies suggest a weakness region favorable to the build up of stress the effective elastic thickness and the strain stress regime for the study area are in agreement with tomographic results a high velocity trend is observed beneath the parnaiba basin where low seismicity is observed indicating the presence of a cratonic core our results support the idea that the intraplate seismicity in central brazil is related to the thin lithosphere underlying parts of the tocantins province between the neighboring large cratonic blocks

a viscoelastic flowline model applied to tidal forcing of bindschadler ice stream west antarctica abstract the motion of bindschadler ice stream west antarctica is dominated by sliding over a nearly plastic bed according to analysis of kinematic gps data using a new viscoelastic flowline model inversions of time averaged velocity data with viscous ice flow models can be consistent with multiple sliding laws but propagation of velocity perturbations in a viscoelastic model can distinguish between sliding laws with different exponents we develop such a model and apply it to a time series of velocity for the tidally modulated flow of bindschadler ice stream formerly ice stream d observed velocity perturbations are found to be consistent with a flow law exponent m 8 which indicates basal motion with a relatively weak till bed lower exponents consistent with motion dominated by deformation within the ice over a hard or frozen bed are found to be unlikely this result suggests that bindschadler ice stream would respond rapidly and significantly to any future loss of buttressing from the ross ice shelf

glacier crevasses observations models and mass balance implications we review the findings of approximately 60 years of in situ and remote sensing studies of glacier crevasses as well as the three broad classes of numerical models now employed to simulate crevasse fracture the relatively new insight that mixed mode fracture in local stress equilibrium rather than downstream advection alone can introduce nontrivial curvature to crevasse geometry may merit the reinterpretation of some key historical observation studies in the past three decades there have been tremendous advances in the spatial resolution of satellite imagery as well as fully automated algorithms capable of tracking crevasse displacements between repeat images despite considerable advances in developing fully transient three dimensional ice flow models over the past two decades both the zero stress and linear elastic fracture mechanics crevasse models have remained fundamentally unchanged over this time in the past decade however multidimensional and transient formulations of the continuum damage mechanics approach to simulating ice fracture have emerged the combination of employing damage mechanics to represent slow upstream deterioration of ice strength and fracture mechanics to represent rapid failure at downstream termini holds promise for implementation in large scale ice sheet models finally given the broad interest in the sea level rise implications of recent and future cryospheric change we provide a synthesis of 10 mechanisms by which crevasses can influence glacier mass balance

breakup of the larsen b ice shelf triggered by chain reaction drainage of supraglacial lakes this research is supported by the u s national science foundation under grant geop ant 0944248 awarded to d r m and grants ant 0838811 and arc 0934534 awarded to o v s

stress observations and driving force models for the south american plate stress observations are compared to predictions of driving force models for the south american plate stress observations have the potential to give detailed information about the forces driving the plates in that sets of forces with the same net torque can produce quite different stress patterns for a two dimensional vertical section model of a plate there are three conditions for dynamical equilibrium the net forces and the net torque about any point must all be zero these conditions are used to estimate the magnitudes of forces as a function of age with ridge push and slab pull as active forces and plate drag and slab resistance as passive forces the results are consistent with earlier published models and indicate that the primary balance for an oceanic plate is between slab pull and the resisting drag forces we generalize the model for the possible forces driving the south american plate and compare with observed stress measurements and kinematics the south american plate has a small slab pull component amounting to about one fifth the ridge push force with the other components reasonably isolated geographically thus the plate balance is largely between ridge push and plate drag this gives an opportunity of resolving the effects of trench suction or collision at the subduction zones most remarkable is the nearly uniform e w orientation of the regional stress field shmax directions which is extensional in the high andes a second feature is the n s orientations in the amazon basin the two dimensional horizontal stress patterns are compared for successive models of combinations of driving forces models with ridge push slab pull at the scotia and caribbean arcs and trench suction near the west coast balanced by plate drag produce similar stress patterns to those observed the magnitude of a possible trench suction force is estimated using a model with an eddy between the nazca slab and the overlying south american plate

variations in the thermal output of the subglacial grimsvotn caldera iceland a 69 year record 1922 1991 of heat release has been obtained from the subglacial geothermal system of the grimsvotn caldera within the vatnajokull ice cap the data were derived from in situ measurements of the volume of meltwater accumulated in the caldera lake subtracting climatically induced melting the overall fluctuations in the heat flux are closely related to volcanic activity and are dominated by a main pulse of 11 600 mw caused by a major eruption in 1938 gradually declining to 1600 mw in 1976 1982 heat extracted from the roof of a magma chamber with the aid of hydrothermal convection may have given a basic contribution to the heat flux of 1500 to 2000 mw an upper bound the variable part of the heat flux from 0 to 10 000 mw was released from magma erupted at the base of the glacier and from shallow intrusions the total heat released over the period 1922 to 1991 was 8 1 1 6 1018j equivalent to the energy released by the solidification and cooling of 2 1 0 4 km of basaltic magma the contribution to the total heat flux was 45 max from a magma chamber 35 min from shallow intrusions and 20 from eruptions this implies that magma at the roof of a chamber solidified and cooled at the rate of 1 2 1 6 107 m a 1 or about 1 km over the last 69 years heat release at grimsvotn was probably more intense in the 19th century when volcanic eruptions were more frequent

tidal modulation of ice shelf buttressing stresses ocean tides influence the flow of marine terminating glaciers observations indicate that the large fortnightly variations in ice flow at rutford ice stream in west antarctica originate in the floating ice shelf we show that nonlinear variations in ice shelf buttressing driven by tides can produce such fortnightly variations in ice flow these nonlinearities in the tidal modulation of buttressing stresses can be caused by asymmetries in the contact stress from migration of the grounding line and bathymetric pinning points beneath the ice shelf using a simple viscoelastic model we demonstrate that a combination of buttressing and hydrostatic stress variations can explain a diverse range of tidal variations in ice shelf flow including the period phase and amplitude of flow variations observed at rutford and bindschadler ice streams

constraints on slow earthquake dynamics from a swarm in central italy several clustered slow earthquakes have been recorded by a geodetic interferometer in central italy the strain rise times of the events range from tens to thousands of seconds and the seismic moment scales with the square root of the rise time this scaling law contrasts with the conservative assumption of constant rupture velocity in fault modeling but is consistent with the occurrence of a slow rupture propagation analogous to heat diffusion in a slab

analysis of the south american intraplate stress field the first order south american intraplate stress field was modeled through a finite element analysis to evaluate the relative contribution of plate boundary forces and intraplate stress sources the finite element mesh consisted of 3100 nodes in a network of 5993 equal area triangular elements which provided a spatial resolution of about 1 at the equator an important aspect of our modeling is the inclusion of topographic forces due to the cooling oceanic lithosphere along the mid atlantic ridge e g ridge push the continental margins along the east coast of brazil and argentina and the elevated continental crust e g the andean cordillera predicted intraplate stresses for two representations of the western collisional boundary forces are evaluated pinned collisional boundaries and applied collisional boundary forces constraint for the modeling was provided by information about the orientation of the maximum horizontal compressive stress shmax provided by 217 stress indicators from the world stress map project as well as by shmax magnitude estimates and torque information from previous investigations our modeling results demonstrate that the first order features of the observed stress field can be explained with simple tectonic models which balance the torque acting on the plate either with a fixed western margin or drag forces applied along the base of the plate the predicted intraplate stress field is characterized by a nearly uniform e w shmax orientation throughout most regions of the plate with stress magnitudes generally less than 20 mpa averaged over a 100 km thick lithosphere significant perturbation of this regional stress field occurs in the western part of the plate in response to forces associated with the high topography of the andes although the magnitude of the collisional boundary forces acting along the western margin remains poorly constrained we estimate a plausible upper bound on the force per unit length acting along the peru chile trench to be about 2 5 1012 n m 1 while some of our models are consistent with a driving basal drag to balance the torques acting on the plate the magnitude of the drag torque is small compared to the contribution from other sources of stress such as the ridge push force

the importance of subduction for the modern stress field in the tyrrhenian area the tyrrhenian sea was formed by lithospheric extension in a context of continental convergence between africa and eurasia at present a compilation of strain and stress indicators in this region by rebai et al 1992 shows that an extensive regime is still predominant in the tyrrhenian sea while northern italy and southern sicily are undergoing compression this tectonic configuration is the result of two major contributions which act on a very heterogeneous lithosphere the africa eurasia collision on the one hand and the subduction along the calabrian arc on the other the latter is expected to create a trench suction force on the overriding tyrrhenian plate the objective of this paper is to investigate the relative importance of these contributions using a numerical model developed by p bird 1989 based on an improved version of the thin plate approach which models lithospheric deformation in an horizontal plane the results of this model demonstrate the necessity of considering the forces related to subduction to understand the modern stress and strain field an extrusion mechanism alone cannot explain present day observations the salient features of the current tectonic regime are reproduced by the model which however predicts more limited extension than expected further work will be needed to find a better tuning of the parameters and ascertain the quality of available data

spatiotemporal patterns in the energy release of great earthquakes for the past 80 years the energy released in great strike slip and thrust earthquakes has occurred in alternating cycles of 20 to 30 years this pattern suggests that a global transfer mechanism from poloidal to toroidal components of tectonic plate motions is operating on time scales of several decades the increase in seismic activity in california in recent years may be related to an acceleration of global strike slip moment release as regions of shear deformation mature after being reached by stresses that have propagated away from regions of great subduction decoupling earthquakes in the 1960s

ice ages and nuclear waste isolation abstract the greatest natural threats to the integrity of the geological barriers to nuclear wastes isolated in cavities mined at depths between 400 and 800 m are likely during rapid retreats of future ice sheets the next major glacial retreat is expected at ca 70 ka well within the lifetime of high grade nuclear waste but it is not yet clear how long man s greenhouse effect may delay it this contribution discusses the potential problems posed to european waste isolation sites during erosion by ice and over pressurizing of meltwater and gasses in a lithosphere flexed by major ice sheets these depend on the target rocks and the location of the site with respect to the ice streams and margins of future ice sheets of particular size no sites are planned under the centres of future ice sheets in europe where end glacial earthquakes can be expected to reactivate major faults nor where ice can be expected to deepen and lengthen fjords along the atlantic coast sites in the alps may be vulnerable to radical changes in the patterns of glacial troughs the stability and geohydrology of sites in coastal areas beyond future ice margins are threatened by river gorges when sea level falls ca 125 m or in enclosed basins like the mediterranean ever lower the greatest problems are likely in lowland regions exposed by the rapid retreat of thick ice fronts where large lakes on or under thick warm based ice are dammed by more distal cold based ice groundwater in subhorizontal fractures dilated by glacial unloading may reach over pressures capable of hydraulically lifting megablocks of bedrock with fracture permeability and or the ice damming them so that less permeable substrates are susceptible to incisions eroded to depths of ca 360 at locations controlled mainly by ice topography kinematics and history

a model of viscoelastic ice shelf flexure we develop a formal thin plate treatment of the viscoelastic flexure of floating ice shelves as an initial step in treating various problems relevant to ice shelf response to sudden changes of surface loads and applied bending moments e g draining supraglacial lakes iceberg calving surface and basal crevassing our analysis is based on the assumption that total deformation is the sum of elastic and viscous or power law creep deformations i e akin to a maxwell model of viscoelasticity having a spring and dashpot in series the treatment follows the assumptions of well known thin plate approximation but is presented in a manner familiar to glaciologists and with glen s flow law we present an analysis of the viscoelastic evolution of an ice shelf subject to a filling and draining supraglacial lake this demonstration is motivated by the proposition that flexure in response to the filling drainage of meltwater features on the larsen b ice shelf antarctica contributed to the fragmentation process that accompanied its collapse in 2002

experimental and theoretical fracture mechanics applied to antarctic ice fracture and surface crevassing recent disintegration of ice shelves on the antarctic peninsula has highlighted the need for a better understanding of ice shelf fracture processes generally in this paper we present a fracture criterion incorporating new experimental fracture data coupled with an ice shelf flow model to predict the spatial distribution of surface crevassing on the filchner ronne ice shelf we have developed experiments that have enabled us to quantify for the first time quasi stable crack growth in antarctic ice core specimens using a fracture initiation toughness kinit for which crack growth commences the tests cover a full range of near surface densities 560 871 kg m 3 10 9 75 7 m depth results indicate an apparently linear dependence of fracture toughness on porosity such that kinit 0 257 80 7 predicting a zero porosity toughness of ko 155 kpa m1 2 we have used this data to test the applicability to crevassing of a two dimensional fracture mechanics criterion for the propagation of a small sharp crack in a biaxial stress field the growth of an initial flaw into a larger crevasse which involves a purely tensile crack opening depends on the size of the flaw the magnitude of kinit and the nature of the applied stress field by incorporating the criterion into a stress map of the filchner ronne ice shelf derived from a depth integrated finite element model of the strain rate field we have been able to predict regions of potential crevassing these agree well with satellite imagery provided an initial flaw size is assumed in the range 5 50 cm

stress pulses in southern italy the crustal seismic activity in southern italy during the last three centuries has been analyzed in order to confirm or disprove the evidence of a northward migration of earthquake epicenters from calabria towards southern apennines and irpinia that we have found in a previous work the results suggest such a migration to be statistically significant at a velocity of about 13 km yr this in turns stands for the existence of stress pulses propagating northward along the apennine axis

ice collapse over trapped water bodies on enceladus and europa icy satellite surfaces are riddled with geological indicators that can inform what lies beneath here we investigate effects of trapped or perched subsurface liquid water on the surface above we use two models to investigate these effects first an analytical model to determine the flexural response of an ice shell to the existence of trapped water and second a discrete element model that simulates collapse of ice through granular interactions to determine possible topography that would result from this stress being placed on the ice we find that for europa s likely trapped water pockets below chaos terrains and a localized sea below enceladus south polar terrain these models can reproduce observed geology of both features thus we postulate that it is likely that europa s chaos terrains and enceladus south polar terrain formed due to a similar mechanism ice collapse above trapped water

intraplate seismicity in se brazil stress concentration in lithospheric thin spots summary intraplate seismicity has generally poor correlation with surface geological patterns except for major extensional features such as aborted continental rifts which may act as weak zones it is usually difficult to find simple geology based models to explain differences in seismic activity in stable continental regions seismicity in brazil is clearly not uniform and a few areas of higher activity have been identified however the seismic areas show almost no correlation with the main geological provinces which is typical of other intraplate settings a recent uppermantle tomography study in se and central brazil using approximately 8500 p wave and 2000 pkp wave arrivals recorded in 59 sites since 1992 has mapped p wave velocity anomalies from lithospheric depths down to 1300 km in this region higher seismic activity occurs preferentially in areas with low p wave velocities at 150 250 km depth the low p wave velocities are interpreted as shallower asthenosphere in such areas a hotter geotherm will reduce the strength of the lithospheric upper mantle causing most of the intraplate forces to be concentrated in the brittle upper crust the low velocity anomalies coincide with late cretaceous provinces of alkaline intrusions the proposed ponding of the trindade plume head beneath lithospheric thin spots is consistent with our tomography results suggesting that plume effects may have helped to preserve lithosphere asthenosphere topography although other factors are also important the present data show that stress concentrations resulting from lithosphere asthenosphere topography should play an important role in explaining the intraplate seismicity in the brazilian platform

evolution of basal crevasses links ice shelf stability to ocean forcing abstract iceberg calving is one of the primary mechanisms responsible for transferring ice from the antarctic ice shelves to the ocean but remains poorly understood previous theories of calving have sought to explain the calving process as a brittle phenomenon that occurs rapidly when surface or basal crevasses penetrate the entire ice thickness here we show that the strain rate weakening nature of ice permits initially narrow basal crevasses to seed an instability that gives rise to locally enhanced ductile deformation and thinning over length scales that are large compared to the ice thickness this ductile failure process called necking amplifies long wavelength features of bottom topography and allows basal crevasses to penetrate an increasing fraction of the ice thickness as they advect downstream application of the model to the four largest antarctic ice shelves shows that necking occurs downstream of pinning points and sharp protrusions in the ice shelf embayment where stress is highly concentrated however model predictions are sensitive to assumptions about basal melting and refreezing within crevasses suggesting that the combination of mechanical instability and ice ocean interaction on the scale of an individual crevasse may play a leading role in controlling ice shelf stability

dynamic damage model of crevasse opening and application to glacier calving 1 theory and applications of continuum damage mechanics for ice are discussed and on this basis an ice damage model valid at low stresses is proposed the model describes the damage itself the rheology of the damaged ice and the damage evolution a local damaging and healing law is considered and its parameterization is motivated the model parameters are inferred from published data of laboratory experiments the model is subsequently implemented in a finite element code and applied to the prediction of a calving process and to the destabilization of an ice chunk from a hanging glacier numerical results show good agreement with field measurements

bounds on the calving cliff height of marine terminating glaciers increased calving and rapid retreat of glaciers can contribute significantly to sea level rise but the processes controlling glacier retreat remain poorly understood we seek to improve our understanding of calving by investigating the stress field controlling tensile and shear failure using a 2d full stokes finite element model using idealized rectangular geometries we find that when rapidly sliding glaciers thin to near buoyancy full thickness tensile failure occurs similar to observations motivating height above buoyancy calving laws in contrast when glaciers are frozen to their beds basal crevasse penetration is suppressed and calving is minimal we also find shear stresses are largest when glaciers are thickest together the tensile and shear failure criteria map out a stable envelope in an ice thickness water depth diagram the upper and lower bounds on cliff height can be incorporated into numerical ice sheet models as boundary conditions thus bracketing the magnitude of calving rates in marine terminating glaciers

the 2012 2013 montes claros earthquake series in the sao francisco craton brazil new evidence for non uniform intraplate stresses in mid plate south america s u m m a r y on 2012 may 19 an mb 4 earthquake shook the town of montes claros brazil in the middle of the sao francisco craton because of the scarce seismicity in the area an event like this could provide valuable information to characterize the governing seismotectonics and stress field for the region here we present the results of more than 1 yr of local seismic monitoring after the main shock we found that the seismicity originated at approximately 1 km depth in an nnw oriented blind reverse fault dipping to the e the magnitude of the main shock was 4mb with aftershocks reaching up to 3 6mb focal mechanisms from first motion polarities and waveform moment tensor inversions indicate a reverse faulting in agreement with the orientation of the aftershock locations in addition we derived a new 1 d local velocity model using a simultaneous inversion of hypocentres and velocity layers the results indicate p wave velocities of 4 5 km s 1 for the upper layer of carbonate rocks and 5 23 and 5 69 km s 1 for the lower fractured and compact crystalline basement layers respectively higher vp vs ratios were obtained for the upper two layers compared to the lowermost layer possibly indicating presence of rock fracturing and percolated water the calculated stress drop for the main event is 0 33 mpa which is a relatively low value for an intraplate earthquake but still within the observed range the inversion of the main shock focal mechanism and previously published focal mechanisms suggests a compressional stress regime in the central part of the sao francisco craton which is different from the strike slip regime in the southern part although both have an ew oriented 1 on the other hand focal mechanisms of events located to the west of the craton indicate an nw se oriented 1 for central brazil this variability highlights the importance of local sources of stresses e g flexural stresses in mid plate south america unlike other mid plate areas of the world such as central and east north america where a more uniform stress field is observed

breaking the seal at grimsvotn iceland of several problems associa ted wit h theoretica l expla nation s of the jokul hla ups which emerge from the outlet gla cie r skeioarar jokull of the ice ca p vatna jokull in sou theast iceland the mechani sm of flood init iation is one that has hi therto defied expl anat ion we provide such an expl ana tion based on a car efu l analysis of the cla ssica l nye ro thlisbe rger model near the subglacial lake grimsvotn the hyd raulic potenti al gradie nt is towards the lake and there is theref ore a catchme nt boun dary under the ice whose location dep ends on the subglacial mel twater drai nage charac terist ics as the con dit ions for a flood approa ch we show that the wat er divi de migra tes towards the lake while at the sa me time the lake pre ssure in creases when the hydra ulic potential gradient to wards the lake is low and the refillin g rate is sl ow the se al will break whe n the cat chment boun dary reaches the lake while the la ke level is st ill below flotati on pressu re whereas if refillin g is rapid flo tation can be achie ved bef ore a flood is initia ted this theory can thus explain why the seal is normally broken when the lake le vel at grimsvotn is still some 60 m below flotation level in addit ion we are abl e to explain why thej okulhla up following the 1996 eruption did not occur until flo tation level was achie ved and we show how the cyeli cit y and magnitude ofjoku lhla ups can be explained wit hin this theory

landform and sediment imprints of fast glacier flow in the southwest laurentide ice sheet evidence for former fast glacier flow ice streaming in the southwest laurentide ice sheet is identified on the basis of regional glacial geomorphology and sedimentology highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream preliminary mapping from a digital elevation model of alberta identifies corridors of smoothed topography and corridor parallel streamlined landforms megaflutes to mega lineations that display high levels of spatial coherency ridges that lie transverse to the dominant streamlining patterns are interpreted as a series of minor recessional push moraines b thrust block moraines or composite ridges hill hole pairs constructed during readvances surges and c overridden moraines cupola hills apparently of thrust origin together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms localised cross cutting and or misalignment of flow sets indicates temporal separation and the overprinting of ice streams lobes the fast flow tracks are separated by areas of interlobate or inter stream terrain in which moraines have been constructed at the margins of neighbouring competing ice streams outlet glaciers this inter stream terrain was covered by more sluggish non streaming ice during full glacial conditions thin tills at the centres of the fast flow corridors in many places unconformably overlying stratified sediments suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams outlet glaciers is consistent with subglacial deformation theory in this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory sometimes with fast flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally the occurrence of minor push moraines and flutings and associated landforms because of their similarity to modern active temperate glacial landsystems are interpreted as indicative of ice lobe marginal oscillations possibly in response to seasonal climatic forcing in locations where meltwater was more effectively drained from the glacier bed further north the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice bed decoupling and drawdown initiated by the development of ice dammed lakes copyright 2008 john wiley sons ltd

modeling hydraulic fracture of glaciers using continuum damage mechanics the presence of water filled crevasses is known to increase the penetration depth of crevasses and this has been hypothesized to play an important role controlling iceberg calving rate here we develop a continuum damage based poro mechanics formulation that enables the simulation of water filled basal and surface crevasse propagation the formulation incorporates a scalar isotropic damage variable into a maxwell type viscoelastic constitutive model for glacial ice and the effect of the water pressure on fracture propagation using the concept of effective solid stress we illustrate the model by simulating quasi static hydrofracture in idealized rectangular slabs of ice in contact with the ocean our results indicate that water filled basal crevasses only propagate when the water pressure is sufficiently large and that the interaction between simultaneously propagating water filled surface and basal crevasses can have a mutually positive influence leading to deeper crevasse propagation which can critically affect glacial stability therefore this study supports the hypothesis that hydraulic fracture is a plausible mechanism for the accelerated breakdown of glaciers

stick slip motion of an antarctic ice stream the effects of viscoelasticity stick slip behavior is a distinguishing characteristic of the flow of whillans ice stream siple coast antarctica distinct from stick slip on northern hemisphere glaciers which is generally attributed to supraglacial melt the behavior is thought be controlled by basal processes and by tidally induced stress however the connection between stick slip behavior and flow of the ice stream on long time scales if any is not clear to address this question we develop a new ice flow model capable of reproducing stick slip cycles similar to ones observed on the whillans ice plain the model treats ice as a viscoelastic material and emulates the weakening and healing that are suggested to take place at the ice till interface the model results suggest the long term ice stream flow that controls ice discharge to surrounding oceans is somewhat insensitive to certain aspects of stick slip behavior such as velocity magnitude during the slip phase and factors that regulate it e g elastic modulus furthermore it is found that factors controlling purely viscous flow such as temperature influence stick slip contribution to long term flow in much the same way additionally we show that viscous ice deformation traditionally disregarded in analysis of stick slip behavior has a strong effect on the timing of slip events and therefore should not be ignored in efforts to deduce bed properties from stick slip observations

mechanisms of subglacial cavity filling in glacier de tete rousse french alps the deadliest outburst flood from an englacial cavity occurred on glacier de tete rousse in the mont blanc area french alps in 1892 a subglacial reservoir was discovered in the same glacier in 2010 and drained artificially in 2010 2011 and 2012 to protect the 3000 inhabitants downstream the mechanism leading to the spontaneous refilling of the cavity following these pumping operations has been analyzed for this purpose the subglacial water volume changes between 2010 and 2013 were reconstructed the size of the cavity following the pumping was found to have decreased from 53 500 m 3 in 2010 to 12 750 m 3 in 2013 creep and the partial collapse of the cavity roof explain a large part of the volume loss analysis of cavity filling showed a strong relationship between measured surface melting and the filling rate with a time delay of 4 6 hours a permanent input of 15 m 3 d 1 not depending on surface melt was also found the meltwater and rain from the surface is conveyed to bedrock through crevasses and probably through a permeable layer of rock debris at the glacier bed the drainage pathway permeability was estimated at 0 054 m s 1 from water discharge measurements and dye tracing experiments

seismic activity in monsuaba state of rio de janeiro brazil between 1988 december and 1989 february summary between 1988 december and 1989 february a sequence of microearthquakes occurred in monsuaba district of angra dos reis state of rio de janeiro brazil 24 km away from the angra dos reis nuclear power plant the activity was recorded initially by bma 35 km away from the epicentral area and later by a local network of five portable seismographs installed a few days after the activity began the hypocentral distribution and the composite fault plane solution suggest that the activity may have been related to a relatively small structure 2 1 5 km produced by a probable reverse fault 25 of azimuth and 35 of dip in the se direction representing a compressional stress regime with a nw se oriented p axis the temporal distribution of the monsuaba earthquakes shows a typical seismic sequence with foreshocks main shock mb 3 0 and aftershocks the frequency magnitude relations indicate a b value of 0 9 for the single frequency distribution and 1 0 for the cumulative one the main shock maximum intensity of vmm at monsuaba was felt up to a distance of 10 km

focal mechanisms of small earthquakes in the southeastern brazilian shield a test of stress models of the south american plate summary focal mechanisms of small earthquakes with magnitudes of about 3 in the se brazilian shield are calculated using s p amplitude ratios low attenuation q p from 400 to 800 in the shield upper crustal layers allowed sharp s arrivals to be recorded up to distances of 100 km besides p wave polarities sh wave first motions were also used to constrain the nodal plane orientations normal and reverse faulting mechanisms with strike slip components were found the inversion of four mechanisms to estimate the stress tensor indicated a strike slip stress regime with roughly e w orientated s 1 and n s s 3 both the orientations and the shape factor q 0 7 of the inverted stress are in excellent agreement with theoretical predictions for that part of brazil from the driving force model of coblentz richardson 1996 good agreement with the nature of the stress as well as its orientation was also found for the model of meijer 1995 both of these theoretical models include spreading stresses along the continent ocean lithospheric transition because the earthquakes are more than 300 km from the continental shelf they should not be avected by the local flexural forces caused by sediment load in the marginal basins the agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses

fracture propagation and stability of ice shelves governed by ice shelf heterogeneity tabular iceberg calving and ice shelf retreat occurs after full thickness fractures known as rifts propagate across an ice shelf a quickly evolving rift signals a threat to the stability of larsen c the antarctic peninsula s largest ice shelf here we reveal the influence of ice shelf heterogeneity on the growth of this rift with implications that challenge existing notions of ice shelf stability most of the rift extension has occurred in bursts after overcoming the resistance of suture zones that bind together neighboring glacier inflows we model the stresses in the ice shelf to determine potential rift trajectories calving perturbations to ice flow will likely reach the grounding line the stability of larsen c may hinge on a single suture zone that stabilizes numerous upstream rifts elevated fracture toughness of suture zones may be the most important property that allows ice shelves to modulate antarctica s contribution to sea level rise

average seismic source parameter relations for mid plate earthquakes mid plate earthquakes which are defined as having their epicenters at least 500 km from plate margins represent a small portion of the total global seismicity however because those occurring within continental interiors are characterized by anomalously large damage areas they are of particular interest empirical mb ms and m o data are used to develop scaling relations for mid plate earthquakes from them it is found that the corner period t o2 of their far field displacement spectra varies as the one fourth power of the seismic moment which indicates that average stress drop increases as seismic moment increases by use of savage s 1972 rectangular bilateral dislocation model the derived spectra of mid plate earthquakes yield estimates of fault rupture length and width average fault displacement and source rise time as a function of seismic moment these relations show that large mid plate earthquakes such as the 1886 south carolina and 1811 and 1812 new madrid events do not require large fault rupture length

the 1996 eruption at gjalp vatnajokull ice cap iceland efficiency of heat transfer ice deformation and subglacial water pressure the 13 day long gjlp eruption within the vatnajkull ice cap in october 1996 provided important data on ice volcano interaction in a thick temperate glacier the eruption produced 0 8 km 3 of mainly volcanic glass with a basaltic icelandite composition equivalent to 0 45 km 3 of magma ice thickness above the 6 km long volcanic fissure was initially 550 750 m the eruption was mainly subglacial forming a 150 500 m high ridge only 2 4 of the volcanic material was erupted subaerially monitoring of the formation of ice cauldrons above the vents provided data on ice melting heat flux and indirectly on eruption rate the heat flux was 5 6 10 5 wm 2 in the first 4 days this high heat flux can only be explained by fragmentation of magma into volcanic glass the pattern of ice melting during and after the eruption indicates that the efficiency of instantaneous heat exchange between magma and ice at the eruption site was 50 60 if this is characteristic for magma frag mentation in subglacial eruptions volcanic material and meltwater will in most cases take up more space than the ice melted in the eruption water accumulation would therefore cause buildup of basal water pressure and lead to rapid release of the meltwater continuous drainage of meltwater is therefore the most likely scenario in subglacial eruptions under temperate glaciers deforma tion and fracturing of ice played a significant role in the eruption and modified the subglacial water pressure it is found that water pressure at a vent under a subsiding cauldron is substantially less than it would be during static loading by the overlying ice since the load is partly compensated for by shear forces in the rapidly deforming ice in addition to intensive crevassing due to subsidence at gjlp a long and straight crevasse formed over the southernmost part of the volcanic fissure on the first day of the eruption it is suggested that the feeder dyke may have overshot the bedrock ice interface caused high deformation rates and fractured the ice up to the surface the crevasse later modified the flow of meltwater explaining surface flow of water past the highest part of the edifice the dominance of magma fragmentation in the gjlp eruption suggests that initial ice thickness greater than 600 700 m is required if effusive eruption of pillow lava is to be the main style of activity at least in similar eruptions of high initial magma discharge

a spectrum of jokulhlaup dynamics revealed by gps measurements of glacier surface motion gps campaigns on glaciers during jokulhlaups show how subglacial floods affect glacier motion and shed light on the dynamics of such floods three such campaigns have been carried out on southern and western vatnajokull southeast iceland over known jokulhlaup paths two slowly rising jokulhlaups from grimsvotn and two rapidly rising jokulhlaups from the western and eastern skafta cauldrons were captured in these campaigns with maximum discharge ranging from 240 to 3300 m 3 s 1 glacier surface movements measured in these campaigns are presented along with the corresponding discharge curves the measurements are interpreted as indicating 1 initiation of rapidly rising jokulhlaups with a propagating subglacial pressure wave 2 decreased glacier basal friction during jokulhlaups 3 subglacial accumulation of water in slowly rising jokulhlaups and 4 lifting of the glacier caused by subglacial water pressure exceeding overburden in both rapidly and slowly rising jokulhlaups the latter two observations are inconsistent with assumptions that are typically made in theoretical and numerical modelling of jokulhlaups both viscous and elastic deformation of the glacier as well as turbulent hydraulic fracture at the ice bedrock interface are important in the dynamics of the subglacial pressure wave at the front of rapidly rising jokulhlaups

an acoustic backscatter thermometer for remotely mapping seafloor water temperature a bottom mounted circularly scanning sonar operating at 40 khz has been used to map changes in water sound speed over a circular region 150 m in diameter if it is assumed that the salinity remains constant the change in sound speed can be converted to a change in temperature for the present system the spatial resolution is 7 5 m and the temperature resolution is 0 05 c the technique is based on comparison of successive sonar scans by means of a correlation algorithm the algorithm is illustrated using data from the sediment transport events on slopes and shelves stress experiment

sedimentary record of a late saalian jokulhlaup case study in siedlątkow outcrop central poland abstract the aim of the research conducted in an outcrop of late saalian sediments in siedl tkow central poland was to reconstruct extreme meltwater flows during a jokulhlaup on the basis of the geological record a combination of field and laboratory lithological methods was used which included lithofacies analysis analyses of grain size distribution shape and roundness of gravelly clasts examination of sediments made it possible to recognise palaeohydrological sedimentation and deformation processes during and immediately after the flood in particular the simultaneous operation of water flows in four paths in the immediate foreland of the ice front was reconstructed mechanisms of transport and sedimentation of deposits were determined characteristic features of the analysed jokulhlaup deposits include erosional and deformational contact with the substrate a high content of very coarse sediments poorly sorted with predominant massive clast supported gravels and boulders the presence of rip up clasts inverse grading in the lower part of the profile normal grading in the upper part and similarities in clast shape and roundness and the ratio of local rocks to northern origin rocks when compared to the surrounding saalian till the documented features allow for the following phases of jokulhlaup to be distinguished initial rising high stage and waning stage during the reconstructed event hyperconcentrated flow was dominant whereas fluvial flow and debris flow were of lesser importance the release of meltwater in the marginal part of the ice sheet came from the depression of the kolo basin to the north of the analysed sites complex deformation processes which occurred in the flood deposits and their substratum were also interpreted the analysed jokulhlaup event resembles some minor glacial floods in the ice contact zone of present day icelandic glaciers further research in marginal zones of pleistocene ice sheets should account for their occurrence as the analysed example indicates that jokulhlaups might have had considerable influence on the course of sedimentation processes in the marginal zones of ice lobes of pleistocene ice sheets as well as in the marginal valleys and sandars

stress orientations in brazilian sedimentary basins from breakout analysis implications for force models in the south american plate summary regional patterns of crustal stresses in brazil were studied with a detailed breakout analysis performed in 541 wells distributed throughout the country 481 from basins along the continental margin and 60 from intracratonic basins a total of about 591 km of cumulative well length digitally sampled at 0 15 m intervals was analysed most intervals 80 per cent are confined between 400 and 4000 m only wells that deviate less than 10d from the vertical were considered for a given well 1 the mean orientation and the standard deviation of each interval with breakouts were calculated 2 all intervals with mean breakout orientations less than 10d from the orientation of the hole deviation were discarded 3 all intervals with standard deviations less than 12d were selected 4 the selected breakouts were weighted by their interval length to calculate the mean orientation and standard deviation of the whole well and 5 the final results were classified according to the world stress map criteria thus 16 wells were classified as a a total of 7 2 km of breakouts 42 as b 7 1 km 78 as c 7 4 km 205 as d 3 3 km 184 as e 0 7 km and 16 were discarded in most basins the breakout orientations from different wells were usually consistent for qualities a b and c allowing a good estimate of the regional maximum horizontal stress shmax wells classified as quality d showed a large scatter the regional shmax determined from breakouts is generally in good agreement with the available nearby focal mechanisms the main trends of shmax are 1 nw se parallel to the coast along the equatorial marginal basins 2 e w in the alagoas basin 3 nne ssw parallel to the coast in the sergipe and reconcavo basins and 4 nw se in the middle amazon basin in the equatorial and eastern continental margins north of 15ds breakouts and focal mechanism measurements indicate that the shmax orientation is remarkably parallel to the coastline following a 90d bend of the coast in northeastern brazil because the theoretically predicted intraplate stresses in northern brazil trend about wnw to ene e g meijer 1995 coblentz richardson 1996 we interpret our observations as indicating that local sources of stress at the continental margins e g flexural stresses and lateral density contrasts dominate the plate wide stresses and may have been underestimated in theoretical stress models of the south american plate the new stress data 136 a c quality out of a population of 541 wells cover a region of mid plate south america where only 23 data points had been compiled previously in the world stress map database the more detailed observed patterns of the intraplate stress field should be helpful in better constraining future models of the forces driving the south american plate

the role of subduction on the horizontal motions in the tyrrhenian basin a numerical model the horizontal motions in the tyrrhenian basin and surrounding chains induced by subduction along the calabrian arc and northern apennines are analyzed by means of numerical models based on finite element techniques the driving mechanism in 2 dimensional vertical cross sections perpendicular to the trench is the slab pull due to the negative buoyancy of a stratified viscoelastic plate that models the ionic oceanic lithosphere or adriatic plate sinking in the upper mantle for the modern tectonic setting of the area we test the sensitivity of extension in the back arc and of roll back of the overriding plate to variations in the viscosity structure and in the geometry of subduction our results are compared with extensional velocities inferred from geology and slr vlbi data

three dimensional attenuation structure in and around the new madrid seismic zone abstract digital seismograms from the central mississippi valley seismic network have been used to study the three dimensional attenuation structure of the new madrid seismic zone and its surroundings attenuation time t values were calculated for each of 252 seismograms using a spectral decay technique assuming that their values are independent of frequency the observed t values were inverted using a stochastic least squares inversion scheme to determine a three dimensional image of the attenuation structure of the upper crust homogeneous and heterogeneous starting models of the upper crust produce similar images of that structure the resulting models include a substantial increase of attenuation within and close to the zones that contain the most active faults of the new madrid seismic zone this region was previously found al shukri and mitchell 1988 to be characterized by lower velocities than surrounding regions the reduced velocities and increased attenuation values in the active portion of the new madrid seismic zone are explained as being caused by fluid filled cracks in the upper crust the presence or absence of fluid filled cracks may govern whether or not faults in this region are active or inactive

termini of calving glaciers as self organized critical systems calving margins are highly sensitive to changes in climate and glacier terminus geometry numerical modelling suggests that calving glacier termini are self organized critical systems that are fluctuating between states of advance and retreat over the next century one of the largest contributions to sea level rise will come from ice sheets and glaciers calving ice into the ocean1 factors controlling the rapid and nonlinear variations in calving fluxes are poorly understood and therefore difficult to include in prognostic climate forced land ice models here we analyse globally distributed calving data sets from svalbard alaska usa greenland and antarctica in combination with simulations from a first principles particle based numerical calving model to investigate the size and inter event time of calving events we find that calving events triggered by the brittle fracture of glacier ice are governed by the same power law distributions as avalanches in the canonical abelian sandpile model2 this similarity suggests that calving termini behave as self organized critical systems that readily flip between states of sub critical advance and super critical retreat in response to changes in climate and geometric conditions observations of sudden ice shelf collapse and tidewater glacier retreat in response to gradual warming of their environment3 are consistent with a system fluctuating around its critical point in response to changing external forcing we propose that self organized criticality provides a yet unexplored framework for investigations into calving and projections of sea level rise

the regional intraplate stress field in south america a compilation of lithospheric stress directions for continental south america and the inferred major patterns of the regional intraplate stress field are presented stress orientations are based primarily on earthquake focal mechanisms and quaternary fault slip inversion published in the literature four new focal mechanisms based on short period p wave modeling and selected centroid moment tensor solutions published by u s geological survey consistent with p wave first motions at south american and other world wide standard seismograph network stations are also included in the data base the observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the south american plate in the andean plateau altitudes greater than 3000 m n s extensional stresses predominate e w compressional stresses are observed in the sub andean and platform regions extending up to about 1000 km east of the andes the maximum horizontal stress shmax is uniformly oriented in the e w direction throughout western south america averages of the shmax direction at grid points spaced 2 5 o were taken as representing the regional field the e w direction of this regional field is not affected by the change of strike of the andean chain nor by the contact of a flat subducted slab underneath the regional shmax direction is oriented 15 o more clockwise than the direction of the nazca plate convergence the difference between the regional shmax orientations and the absolute plate motion may be only about 6 o this may indicate that contact forces with the nazca plate may not be the only major contributor to the intraplate stresses in western south america the eastern limit of this andean stress province seems to coincide with aseismic regions in the upper amazon basin and in the paraml basin in the central amazonian region seismicity and stress data suggest a seismic province with n s compressional stresses the origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle amazon basin in northeastern brazil seismicity is characterized by upper crustal strike slip earthquakes bordering the potiguar marginal basin a model is proposed for this region in which the stress field is the result of a superposition of regional e w compressional stresses and local extensional stresses oriented perpendicular to the continental margin possibly related to density contrasts and sediment loading in the continental shelf

subglacial floods beneath ice sheets subglacial floods jokulhlaups are well documented as occurring beneath present day glaciers and ice caps in addition it is known that massive floods have occurred from ice dammed lakes proximal to the laurentide ice sheet during the last ice age and it has been suggested that at least one such flood below the waning ice sheet was responsible for a dramatic cooling event some 8000 years ago we propose that drainage of lakes from beneath ice sheets will generally occur in a time periodic fashion and that such floods can be of severe magnitude such hydraulic eruptions are likely to have caused severe climatic disturbances in the past and may well do so in the future

earthquake and flood induced turbidites in the saguenay fjord quebec a holocene paleoseismicity record abstract a 38 m long piston core from the deep basin of the saguenay fjord quebec recorded rapidly deposited layers rdl interpreted to represent major floods and earthquakes over the past 7200 years high resolution physical magnetic and sedimentological analyses revealed at least 14 rdl generally with a sandy base and a light gray color interbedded with hemipelagic sediments digital x radiography and grain size analyses at