chronology of late wisconsinan glaciation in middle north america abstract we propose a chronology of late wisconsinan glacial fluctuations in middle north america from alberta to wisconsin based on radiocarbon dates derived solely from wood previous chronologies of the southwestern margin of the north american continental ice sheet have depended to a considerable degree on radiocarbon dates from fine grained organic sediment this material is commonly contaminated with older carbon resulting in chronologic confusion by using only dates from wood much of the confusion disappears however because of the scarcity of wood dates only four of the sixteen identified fluctuations are accurately dated an advance into iowa about 14 000 to 13 500 bp an advance into south dakota and iowa about 12 300 bp an advance into the lake michigan basin about 11 700 bp and an advance into the lake superior basin about 9900 bp in addition the beginning of late wisconsinan glaciation before 20 000 bp is fairly well documented none of the fluctuations in the western part of the region are accurately dated

origin of a washboard moraine of the des moines lobe inferred from sediment properties abstract geometric characteristics of the washboard moraines of the des moines lobe dml of the laurentide ice sheet agree with their proposed origin as crevasse squeeze ridges but study of their sediments is required to help further test this hypothesis a 70 m long 3 5 m high section through a moraine ridge in central iowa revealed till with irregular isolated lenses of silt sand and gravel that dip to varying extents upglacier the texture and density of the till are like those of the basal till of the dml studied elsewhere in iowa and preconsolidation pressures determined from tests on till and silt of the ridge indicate that it developed subglacially rather than at the glacier margin preconsolidation pressures additionally demonstrate that pore water pressures in the bed supported most of the glacier s weight which would have contributed to till mobility fabrics based on the anisotropy of magnetic susceptibility of 3125 intact till specimens collected at 125 locations in the section indicate two end member states of strain that varied with location in the ridge and caused sediment mounding simple shear that was directed downglacier along shear planes inclined upglacier together with pure shear where an overlying crevasse allowed the sediment bed to extend upward and laterally meltwater that likely flowed along the crevasse deposited sorted sediments that were incorporated in till deformed and rotated this positive test of the crevasse squeeze hypothesis indicates that the dml was in longitudinal extension near its margin reinforcing previous arguments that the lobe surged the predominance of fabrics caused by simple shear demonstrates that crevasse filling was underway before the surge had fully halted this study should prompt caution in using similar transverse ridges such as those geophysically imaged in some submarine glacier forefields as indicators of retreat rates

laboratory earthquakes the sub rayleigh to supershear rupture transition we report on the experimental observation of spontaneously nucleated supershear rupture and on the visualization of sub rayleigh to supershear rupture transitions in frictionally held interfaces the laboratory experiments mimic natural earthquakes the results suggest that under certain conditions supershear rupture propagation can be facilitated during large earthquake events

self healing slip pulses in dynamic rupture models due to velocity dependent strength abstract seismological observations of short slip duration on faults short rise time on seismograms during earthquakes are not consistent with conventional crack models of dynamic rupture and fault slip in these models the leading edge of rupture stops only when a strong region is encountered and slip at an interior point ceases only when waves from the stopped edge of slip propagate back to that point in contrast some seismological evidence suggests that the duration of slip is too short for waves to propagate from the nearest edge of the ruptured surface perhaps even if the distance used is an asperity size instead of the entire rupture dimension what controls slip duration if not dimensions of the fault or of asperities in this study dynamic earthquake rupture and slip are represented by a propagating shear crack for all propagating shear cracks slip velocity is highest near the rupture front and at a small distance behind the rupture front the slip velocity decreases as pointed out by heaton 1990 if the crack obeys a negative slip rate dependent strength relation the lower slip velocity behind the rupture front will lead to strengthening that further reduces the velocity and under certain circumstances healing of slip can occur the boundary element method of hamano 1974 is used in a program adapted from andrews 1985 for numerical simulations of mode ii rupture with two different velocity dependent strength functions for the first function after a slip weakening displacement the crack follows an exponential velocity weakening relation the characteristic velocity v 0 of the exponential determines the magnitude of the velocity dependence at dynamic velocities the velocity dependence at high velocity is essentially zero when v 0 is small and the resulting slip velocity distribution is similar to slip weakening if v 0 is larger rupture propagation initially resembles slip weakening but spontaneous healing occurs behind the rupture front the rise time and rupture propagation velocity depend on the choice of constitutive parameters the second strength function is a natural log velocity dependent form similar to constitutive laws that fit experimental rock friction data at lower velocities slip pulses also arise with this function for a reasonable choice of constitutive parameters slip pulses with this function do not propagate at speeds greater than the raleighwave velocity the calculated slip pulses are similar in many aspects to seismic observations of short rise time in all cases of self healing slip pulses the residual stress increases with distance behind the trailing edge of the pulse so that the final stress drop is much less than the dynamic stress drop in agreement with the model of brune 1976 and some recent seismological observations of rupture

southern laurentide ice lobes were created by ice streams des moines lobe in minnesota usa abstract regional mapping in southern minnesota has illuminated a suite of landforms developed by the des moines lobe that delimit the position of the lobe at its maximum and at lesser readvances the ice lobe repeatedly advanced discharged its subglacial water and subsequently stagnated recent glaciological research on antarctic ice streams has led some glacial geologists to postulate that ice streams drained parts of the marine based areas of the laurentide ice sheet i postulate that such ice streams may develop in land based areas of an ice sheet as well and that the des moines lobe 200 km wide and 900 km long was an outlet glacier of an ice stream it appears to have been able to advance beyond the laurentide ice sheet as long as adequate water pressure was maintained however the outer part of the lobe stagnated because subglacial water that facilitated the flow was able to drain away through tunnel valleys stagnation of the lobe is not equivalent to stoppage of the ice stream because ice repeatedly advanced into and onto the stagnant margins stacking ice and debris similar landforms are also seen in other lobes of the upper midwestern united states

three dimensional dynamic simulation of the 1992 landers earthquake the 1992 landers california earthquake magnitude 7 3 was modeled as the propagation of a spontaneous rupture controlled by a realistic prestress distribution with the use of a three dimensional finite difference method the dynamic rupture reproduces the general slip pattern used to compute the initial stress distribution and generates near fault ground motions at the surface similar to observations the simulated rupture propagates on the fault along a complex path with highly variable speed and rise time changing the magnitude and pattern of the stress significantly the method provides the framework to estimate earthquake rupture parameters from recorded seismic and geodetic data

fracture roughness and gouge distribution of a granite shear band 1 localization of deformation during fracture mechanical tests leads to the development of shear bands we performed triaxial tests using sidobre granite at four different confining pressures from 20 to 80 mpa we compared two sets of tests one set was stopped immediately after the formation of the shear band a second one included additional shear deformation from the analysis of thin sections of these laboratory samples we characterize the typical microstructures in the shear band mode i and ii cracks riedel cracks cataclastic flow statistical properties of rupture surface roughness and gouge grain size reveals scaling invariance using a mechanical profiler the fracture roughness is measured along parallel profiles and shown to be correctly described over up to 3 orders of magnitude by self affine geometry with a roughness exponent close to 0 80 this property is very similar to tensile crack even if local processes are different the influence of the slip is observed fracture surfaces are rougher along the slip direction 0 74 than perpendicular to it 0 80 the confining pressure is shown to have a weak effect on the fracture roughness it smoothes the surface slight increase of the roughness exponent gouge particles extracted from the shear band present a power law distribution with an exponent ranging from 1 44 to 1 91 this exponent appears to increase with the shearing displacement and the confining pressure when a significant shear of the band is combined with a high confining pressure i e impeded dilation of the band the hallmark of fragmentation is observed for the particle distribution and related to a smoothing of the band boundary

new empirical polyaxial criterion for rock strength abstract a new criterion is presented for prediction of intact rock and rock mass failure under polyaxial state of stresses a comprehensive database of the results of uniaxial triaxial and polyaxial tests on intact rock was utilized for evaluation of the new criterion and comparison of its accuracy with the most accurate and frequently used criteria analysis of 195 individual data groups of brittle failure in the form of 3 1 for twelve different rock types showed that the proposed criterion lead to determination coefficients higher than 0 99 in most cases it also gave the lower values of root mean squared errors relative to mohr coulomb and hoek brown criteria in fitting the normalized strength data for each rock type the criteria were used to fit a typical series of triaxial strength data including brittle and ductile behavior it showed that the new criterion can maintain its accuracy over a wider range of stresses in the absence of rock mass strength data applicability of the new criterion for rock mass was verified by fitting it to typical hoek brown failure envelopes regression analysis of the polyaxial strength data in the form of 3 2 1 for six rock types showed that the new criterion predict the strength more accurately than the modified wiebols cook and you criteria in all cases

simulation of heterogeneity creep damage and lifetime for loaded brittle rocks abstract a grain based heterogeneous numerical model originated from the discrete element method is developed and applied for lac du bonnet granite basing on voronoi cells the modelled microstructure considers different mineral components and grain size the model takes into account elastic grain and elasto plastic contact deformation inter and intra granular fracturing and lifetime prediction on basis of subcritical crack growth the procedure is successfully applied to simulate uniaxial compression tests brazilian tests and uniaxial creep tests the damage processes are studied in detail and mode i and mode ii fracture toughness were determined the proposed modelling allows the simulation of time dependent behaviour in terms of the damage process during primary secondary and tertiary creeps until the final failure characterized by macroscopic fracturing shear band and or macroscopic tensile fracture all simulations have shown reasonable agreement with macroscopic results obtained from lab tests on granite but have in addition delivered deeper insight into the microscopic damage process the proposed modelling approach is recommended for brittle rocks

origin of washboard moraines of the des moines lobe spatial analyses with lidar data abstract the des moines lobe dml the largest lobe along the southern margin of the laurentide ice sheet deposited low relief 1 5 m transverse moraine ridges herein called washboard moraines that are most prominent in central iowa their origin as either subglacial or ice marginal features bears on the dynamics of the lobe geomorphic reconstructions of it and interpretations of similar ridges elsewhere data from airborne laser swath mapping provide the first digital elevation models with sufficient spatial resolution to study in detail the geometric attributes of these topographically subtle features spectral analysis of profiles perpendicular to ridge crests indicates that most of them are spaced with statistically significant periodicities with dominant wavelengths of 70 150 m normalizing and stacking these profiles indicate that on average they display no systematic asymmetry locally washboard moraines are intersected by discontinuous longitudinal ridge segments trends of the crests of moraines are scalloped with cusps that point upglacier and are coincident with outwash trains which were overridden by the lobe our interpretation of these data is that the moraines formed subglacially as crevasse squeeze ridges overridden outwash trains likely supported low basal water pressures and resulted in anomalously slow basal slip causing upglacier deflection of crevasses that extended to the bed this crevasse squeeze interpretation reinforces evidence that the dml surged to its maximum position bering glacier as a temperate surge type glacier that deposited crevasse squeeze ridges similar to the washboard moraines of the dml may be a good analog for the lobe

rogen ribbed moraine identification and possible origin abstract it is emphasized that rogen moraine is a morphological term applied to a special kind of transverse moraine characteristically these ridges show transitions to drumlins it is inferred that this morphology is created in at least two steps including remoulding of pre existing landforms

imaging surface contacts power law contact distributions and contact stresses in quartz calcite glass and acrylic plastic a procedure has been developed to obtain microscope images of regions of contact between roughened surfaces of transparent materials while the surfaces are subjected to static loads or undergoing frictional slip static loading experiments with quartz calcite soda lime glass and acrylic plastic at normal stresses to 30 mpa yield power law distributions of contact areas from the smallest contacts that can be resolved 3 5 m2 up to a limiting size that correlates with the grain size of the abrasive grit used to roughen the surfaces in each material increasing normal stress results in a roughly linear increase of the real area of contact mechanisms of contact area increase are by growth of existing contacts coalescence of contacts and appearance of new contacts mean contacts stresses are consistent with the indentation strength of each material contact size distributions are insensitive to normal stress indicating that the increase of contact area is approximately self similar the contact images and contact distributions are modeled using simulations of surfaces with random fractal topographies the contact process for model fractal surfaces is represented by the simple expedient of removing material at regions where surface irregularities overlap synthetic contact images created by this approach reproduce observed characteristics of the contacts and demonstrate that the exponent in the power law distributions depends on the scaling exponent used to generate the surface topography

triaxial strength and deformability of intact and increasingly jointed granite samples abstract triaxial strength tests were performed on artificially jointed standard size granite specimens with two sub vertical and three sub horizontal 2 3 joints a fully servo controlled press was used to load the samples and attain residual strength these tests add to an existing test database for intact and artificially jointed 1 2 joints samples of the same rock the main parameters from each test were used to analyse tendencies in particular changes in elasticity and peak and residual strength in line with increased jointing peak strength and apparent young s modulus decreased from the intact to the jointed specimens reflecting a tendency analogous to that deriving from the use of classification systems to characterize rock masses mohr coulomb and hoek brown failure criteria were fit to peak and residual strength tests results in the case of the hoek brown criterion it seems that an equivalent of the geological strength index gsi which reflects structure at the specimen scale could be of use in estimating the decrease in strength of increasingly jointed samples residual strength envelopes were practically the same for the intact and jointed specimens our results for granite would suggest that the so called rock mechanics scale effect at large i e the relevant change in material properties from rock samples at lab scale to that of the rock mass including the occurrence of natural discontinuities at the engineering work scale is more due to structure number orientation spacing and features of occurring joints than to sample scale effect i e the variation in properties when testing different size from a few mm to some decimetres unjointed intact rock samples at the laboratory

flow mechanism of the des moines lobe of the laurentide ice sheet rapid flow of the des moines lobe of the laurentide ice sheet may have been related to its unlithified substrate new reconstructions of the lobe based on moraine elevations sediment subsidence during moraine deposition and flow direction indicators indicate that the lobe may have been 3 times thicker than in previous reconstructions nevertheless implied basal shear stresses are 15 kpa so internal ice deformation was not significant instead the lobe likely moved by a combination of sliding plowing of particles through the bed surface and bed shear consolidation tests on basal till yield preconsolidation stresses of 125 300 kpa so effective normal stresses on the bed were small a model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed resulting in too small a shear traction on the bed to shear it at depth consistent with this prediction measurements of orientations of clasts in basal till yield a weak fabric implying pervasive bed shear strain less than 2 although some stronger fabrics have been reported by others we infer tentatively that movement was principally at the bed surface by plowing

magnetic fabrics of the douglas till of the superior lobe exploring bed deformation kinematics abstract the superior lobe of the laurentide ice sheet during its last advance into northwestern wisconsin may have moved and transported sediment primarily through deep widespread shear of its bed to high strains 100 to study the kinematics of this deformation we measured the anisotropy of magnetic susceptibility of 3750 intact samples of its basal till the douglas member of the miller creek formation collected from eight profiles at 0 2 m depth increments a benchmark for interpretations was provided by results of ring shear experiments conducted on the douglas till which provided fabric characteristics for the case of horizontal simple shear orientations of maximum susceptibility k1 were controlled by preferred orientations of silt sized magnetite grains and were similar to sand grain orientations measured in thin sections strengths of fabrics formed by orientations of k1 indicate that most of the till sampled was deformed but to variable strains deformation averaged over the 20 km spanned by the study approximated horizontal simple shear however large differences in k1 fabric azimuths 45 over lateral distances of meters to tens of meters indicate highly heterogeneous bed deformation perhaps focused in anastomosing zones with associated divergent and convergent till shear this interpretation is supported by orientations of principal susceptibilities that imply in many cases either that shear planes were steeply dipping particularly transverse to the shearing direction or that there were major components of pure shear variations in k1 fabric azimuth with depth indicate that most of the till thickness did not shear simultaneously rather till accumulated at the bed as shear direction changed in response to temporally shifting zones of shallow deformation

strength of jointed rock masses jointed rock masses comprise interlocking angular par ticles or blocks of hard brittle material separated by discontinuity surfaces which may or may not be coated with weaker materials the strength of such rock masses depends on the strength of the intact pieces and on their freedom of movement which in turn depends on the number orientation spacing and shear strength of the discontinuities a complete understanding of this problem presents formidable theoretical and experi mental problems and hence simplifying assumptions are required in order to provide a reasonable basis for estimating the strength of jointed rock masses for engineering design purposes this paper summarizes some of the basic information upon which such simplifying assumptions can be made a simple empirical failure criterion is presented and its application in engineering design is illustrated by means of a number of practical examples des masses jointives de rochers comprennent des particules angulaires enchevetrees ou des

influence of the rupture initiation on the intersonic transition crack like versus pulse like modes we numerically investigate the supershear transition of inplane rupture under slip weakening friction and tectonic loading using a non smooth spectral element method nucleation of the rupture in the vicinity of an initial stress concentration is consistently solved together with the dynamic propagation nucleation is shown to influence the transition to the supershear propagation and depending upon the initiation process and the level of the stress out of the nucleating asperity crack like or pulse like solutions can be selected in the intersonic regime pulses are triggered by dynamic unloading of the traction ahead of the rayleigh wave and tend to become metastable the asymptotic numerical solution is always a crack propagating close to p wave speed the rupture front and the arrest velocities are numerically characterized we argue that the nucleation phase introduces a trade off with the state of the stress in determining the distance at which supershear appears

stress strain time numerical modelling of a deep seated gravitational slope deformation preliminary results abstract an analysis of the effects of creep on the development of a deep seated gravitational slope deformation dsgsd carried out by numerical modelling is described the slope rock mass evolution has been simulated applying different creep rheological models the studied dsgsd is located in the central italian alps val s giacomo it is characterised by three main scarps different sets of tensile trenches and counterscarps mapped during a detailed geomorphological survey the mechanical behaviour of the rock masses in terms of elasto plastic parameters has been defined on the base of laboratory and in situ tests in order to simulate this instability process a stress strain time numerical modelling has been performed by a finite difference numerical code fast lagrangian analysis of continua flac the effects of different constitutive model laws have been considered the visco elastic maxwell model the visco elastic burger model and the elasto visco plastic burger model the modelling has been performed simulating the melting of the ancient glacier starting from its maximum load because of the difficulty to determine rock mass creep parameters for each of the rheological models a sensitivity analysis has been performed varying the physical mechanical properties the visco elasto plastic model allows to underline the formation of shear surface showing a depth and a shape in good agreement with the morphologic features of the slope the only action of unloading due to glacier melting seems to be insufficient to generate deep critical surfaces which can be related with the formation of the dsgsd

subglacial drainage eskers and deforming beds beneath the laurentide and eurasian ice sheets glaciological theory predicts that the subglacial drainage network at the base of gently sloping ice sheets resting on deforming sediment should consist of many wide shallow probably braided canals distributed along the ice sediment interface rather than an arborescent network of relatively few large tunnels as would develop over a rigid substrate a corollary prediction examined here is that eskers which form in large subglacial tunnels should be rare where subglacial bed deformation occurred but they may be relatively common where the bed was rigid bed deformation would be most likely where subglacial till was relatively continuous fine grained and of low permeability that is in regions where till is derived primarily from underlying sedimentary bedrock but unlikely where discontinuous coarse grained high permeability till was derived from underlying crystalline bedrock the observed distribution of eskers in areas covered by the laurentide and eurasian british scandinavian and barents sea ice sheets during the last glaciation shows that most eskers occur over crystalline bedrock overlain by discontinuous high permeability till but are rare or absent over sedimentary bedrock overlain by fine grained low permeability till thus matching reasonably well our prediction glaciological theory and geologic evidence indicate that esker systems on a subcontinental scale are time transgressive sedimentological evidence for a canal drainage system appears to be present in fine grained tills where eskers are largely absent

complexity of seismicity due to highly rate dependent friction we study a simple antiplane fault of finite length embedded in a homogeneous isotropic elastic solid in order to understand the origin of seismic source heterogeneity in the presence of nonlinear rate dependent friction all the mechanical properties of the medium and friction are assumed to be homogeneous starting from a heterogeneous initial stress distribution we apply a slowly increasing uniform stress load far from the fault and we simulate the seismicity for more than 20 000 events in some cases the style of seismicity produced by this model is determined by a control parameter which measures the degree of rate dependence of friction for classical friction models with rate independent friction no complexity appears and seismicity is perfectly periodic for weakly rate dependent friction seismicity becomes slightly nonperiodic but most events are still characteristic earthquakes when friction is highly rate dependent seismicity is completely irregular and ruptures of all sizes occur inside the fault highly rate dependent friction destabilizes the healing process producing premature healing of slip and partial stress drop premature healing causes rupture to take the form of narrow propagating slip episodes the so called heaton s 1990 pulses partial stress drop produces large variations in the state of stress which in turn produce earthquakes of different sizes we make the conjecture that all models in which static stress drop is only a fraction of the dynamic stress drop produce stress heterogeneity

p y criterion for rock mass drilled shafts socketed in rock mass have been used frequently as a foundation system to support both vertical and lateral loads traditionally the lateral interaction between the drilled shaft and the surrounding rock medium has been characterized by means of nonlinear p y curves however there is a lack of well verified p y criterion for rock mass in this paper a hyperbolic p y criterion is developed based on both theoretical derivations and numerical finite element parametric analysis results the methods for determining pertinent rock parameters needed for constructing the proposed p y curves are presented in the paper two full scale lateral load tests on large diameter fully instrumented drilled shafts socketed in rock conducted by the writers together with additional four load test results reported by gabr et al were used to validate the applicability of the proposed hyperbolic p y curves for rock mass the comparisons between the computed shaft responses both deflections and bending mome

the effects of blast damage zone thickness on rock slope stability the selection of blast damage zone t of the hoek brown hb criterion is significant in open pit slope design and stability analysis traditional slope numerical analysis adopts a single value of blast damage factor d to the entire rock mass which results in underestimation of slope stability in this research the parallel layer model plm which rock mass is divided into a number of layers parallel to slope surface with a decreasing value of d applied to each layer in slope modeling is used in the limit equilibrium method to investigate the effect of blast damage zone t on the stability of rock slopes based on extensive parametric studies a blast damage zone weighting factor ft is proposed to quantify the influence of t on the evaluation of the factor of safety fos of given slopes results show that the selection of t in the slope model plays an important role in terms of calculation of fos especially when the ratio of t to slope height h is less than 1 0 finally based on ft and the existing stability charts a stability model is proposed for estimating the fos of slopes with various slope geometries and rock mass properties the reliability of the proposed stability model is tested against numerical solutions the results show that fos estimated from the proposed stability model exhibits only 5 6 average discrepancy from numerical solutions using 1254 sets of data the proposed stability model is simple and straightforward and can be used for the preliminary rock slope stability analysis considering blast damage zone effects

preconsolidation of tills and intertill clays by glacial loading in southern saskatchewan canada preconsolidation of pre battleford tills and intertill clays resulted in a narrow range of effective pressures 1800 200 kpa at 10 sites along a 1300 km transect across southern saskatchewan these data suggest an almost invariant loading system that is independent of total ice thickness which ranged from 1150 to 2000 m the preconsolidation process apparently took place in a temperate glacier environment the loading system is characterized as a variable thickness of ice related to distance from the glacier terminus a steady hydraulic grade line sustained a potentiometric surface within the subglacial sediments at an elevation consistently 69 76 of the total ice thickness this mechanism maintains small fluctuations in effective stress

origin of the active drumlin field at mulajokull iceland new insights from till shear and consolidation patterns abstract stratigraphic and morphologic data previously collected from the forefield of mulajokull iceland suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them we have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier s forefield patterns of till strain indicated by the anisotropy of magnetic susceptibility ams of 2300 intact till samples indicate that till was deposited during shear deformation with shearing azimuths and planes that conform to the drumlin morphology thus till deposition occurred as drumlins grew in agreement with lidar data indicating that the degree of aggradation of the glacier forefield is largest in areas subjected to the most surges previously described unconformities on the drumlin flanks however indicate that drumlin relief at mulajokull has resulted in part from erosion given that the last surge deposited a till layer both on and between drumlins a reasonable hypothesis is that erosion between drumlins occurred during normal quiescent flow of the glacier between surges densities of till samples analyzed in conjunction with laboratory consolidation tests indicate that effective stresses on the bed during such periods were on the order of 100 kpa larger between drumlins than within them an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins the observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation moreover the lack of ams fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till flux gradients in a deforming bed

dynamic faulting under rate dependent friction we discuss the effects of rate dependent friction on the propagation of seismic rupture on active faults several physicists using burridge and knopoff s box and spring model of faulting have proposed that fault complexity may arise from the spontaneous development of a self similar stress distribution on the fault plane if this model proves to be correct it has important consequences for the origin of the complexity of seismic sources in order to test these ideas on a more realistic earthquake model we developed a new boundary integral equation method for studying rupture propagation along an antiplane fault in the presence of nonlinear rate dependent friction we study rupture dynamics of models with single and twin asperities in our models asperities are places on the fault with a higher value of prestress othewise all fault parameters are homogeneous we show that for models with such asperities a slip velocity weakening friction leads to the propagation of supersonic healing phases and to the spontaneous arrest of fracture if the prestress outside the asperities is low enough for models with asperities we can also observe narrow slip velocity pulses qualitatively similar to the so called heaton pulses observed in some earthquake accelerograms we also observe a complex distribution of stress after the rupture that depends on details of the initial distribution of asperities and on the details of the friction law

the eyjabakkajokull glacial landsystem iceland geomorphic impact of multiple surges abstract a new glacial geomorphological map of the eyjabakkajokull forefield in iceland is presented the map covers c 60 km2 and is based on high resolution aerial photographs recorded in august 2008 as well as field checking landforms are manually registered in a geographical information system arcgis based on inspection of orthorectified imagery and digital elevation models of the area we mapped subglacially streamlined landforms such as flutes and drumlins on the till plain supraglacial landforms such as ice cored moraine pitted outwash and concertina eskers and ice marginal landforms such as the large multi crested 1890 surge end moraine and smaller single crested end moraines the glaciofluvial landforms are represented by outwash plains minor outwash fans and sinuous eskers extramarginal sediments were also registered and consist mainly of old sediments in wetlands or locally weathered bedrock eyjabakkajokull has behaved as a surge type glacier for 2200 years hence the mapped landforms originate from multiple surges landforms such as large glaciotectonic end moraines hummocky moraine long flutes crevasse fill ridges and concertina eskers are characteristic for surge type glaciers the surging glacier landsystem of eyjabakkajokull serves as a modern analog to the landsystems of terrestrial paleo ice streams

incorporation of non associative flow rules into rock slope stability analysis abstract the circular shear failure mechanism in rock slopes occurs in large scale heavily jointed or weak rock masses most rock slope stability analysis problems do not consider the effect of flow rules i e the dilatancy angle the failure criteria commonly applied are the linear mohr coulomb or the non linear hoek and brown in general all slope stability methods originate from the assumption of the associative flow rule calculating equivalent values of the cohesive strength and friction angle in this study the non linear hoek and brown failure criteria are applied in the parametric form which permits the incorporation both the associative and non associative flow rules a theoretical approach of the limit equilibrium method is performed for circular slip surfaces under the assumption that there are only self weight forces acting on the failure mass of the slope taking the critical plane failure mode as the foundation both the analysis and results are performed in dimensionless form the study emphasizes the importance of the value of dilatancy angle and non conservative character of the hypothesis of the associative flow law i e the overestimation of the value of the factor of safety specific examples are analysed and compared to the finite difference method and a real case is presented

bayesian estimation of the normal and shear stiffness for rock sockets in weak sedimentary rocks abstract the design of socketed foundations in weak sedimentary rocks is often governed by settlement the shear stiffness of the rock socket sidewalls and the normal stiffness of the rock socket base are the main parameters that affect the settlement of rock sockets at service load conditions and are needed for settlement calculation in load transfer method or using the theory of elasticity in this paper in situ load test data for socketed foundations in weak rocks are collected the shear stiffness of the rock socket sidewalls and the normal stiffness for the rock socket base are respectively estimated based on the measured side shear stress and shear displacement and base contact pressure and normal displacement relationships the parameters affecting shear and normal stiffnesses are discussed a bayesian approach is used to develop probabilistic models for the rock socket shear and normal stiffnesses which are functions of both rock properties and the rock socket size

the petrographic and geomechanical properties of some sandstones from the newspaper member of the natal group near durban south africa abstract sandstones are by far the most important rock type in the natal group which is mainly a terrestrial succession formed during ordovician silurian times the most notable member of the group is the newspaper member from which sample material was obtained most of the material sampled was fine to medium grained arkosic sandstone they are well cemented with generally low porosity accordingly sandstones of the newspaper member tend to be very strong and of low deformability few of the petrological properties of these sandstones have a notable influence on the mechanical properties however increasing clay content tends to lower the unconfined compressive strength whilst increasing quartz content tends to enhance it on the other hand many of the mechanical properties are interrelated significant or highly significant relationships occur amongst the strength parameters and between hardness and strength young s modulus also has significant relationships with strength and hardness another property which has a significant influence on strength and deformability is porosity

fabric signature of subglacial till deformation breidamerkurjokull iceland the foreland of breidamerkurjokull iceland is the only locality where tills known to have undergone subglacial deformation are exposed till on the foreland has a two tiered structure consisting of a dilatant upper horizon c 0 5 m thick and a compact lower till these horizons correspond to the ductile deforming a horizon and the brittle ductile b horizon observed below the glacier by g s boulton and co workers the relationship between known strain history and a variety of macrofabric elements is examined for these two genetic facies of deformation till the upper horizon exhibits variable a axis fabrics and abundant evidence for clast re alignment reflecting ductile flow and rapid clast response to transient strains in contrast the lower horizon has consistently well organized a axis fabrics with a narrow range of dip values recording clast rotation into parallel with strain axes during brittle or brittle ductile shear the data indicate that till strain history imparts identifiable macrofabric signatures providing important analogues to guide the interpretation of pleistocene tills

excavation induced zonal disintegration of the surrounding rock around a deep circular tunnel considering unloading effect abstract a new non euclidean dynamic model is proposed to investigate the zonal disintegration mechanism of isotropic rock masses around a deep circular tunnel subjected to dynamic unloading in which the defect parameter r is introduced to describe effects of micro defects on the deformation and failure of deep rock masses on the basis of the deformation incompatibility condition the non euclidean dynamic equation is established then laplace transformation and inverse laplace transformation theory are applied to solve the non euclidean dynamic equations the stress fields are obtained from the non euclidean dynamic equations and the boundary conditions the number and size of fractured and non fractured zones are determined using the hoek brown criterion numerical computation is carried out it is found from numerical results that the number and size of fractured and non fractured zones significantly depend on unloading rate in situ stress and dynamic mechanical parameters of deep rock masses

pulse like and crack like ruptures in experiments mimicking crustal earthquakes theoretical studies have shown that the issue of rupture modes has important implications for fault constitutive laws stress conditions on faults energy partition and heat generation during earthquakes scaling laws and spatiotemporal complexity of fault slip early theoretical models treated earthquakes as crack like ruptures but seismic inversions indicate that earthquake ruptures may propagate in a self healing pulse like mode a number of explanations for the existence of slip pulses have been proposed and continue to be vigorously debated this study presents experimental observations of spontaneous pulse like ruptures in a homogeneous linear elastic setting that mimics crustal earthquakes reveals how different rupture modes are selected based on the level of fault prestress demonstrates that both rupture modes can transition to supershear speeds and advocates based on comparison with theoretical studies the importance of velocity weakening friction for earthquake dynamics

clast fabric development in a shearing granular material implications for subglacial till and fault gouge elongate clasts in subglacial till and in fault gouge align during shearing but the relation between clast fabric strength and cumulative shear strain for such materials is effectively unknown this relation was explored in experiments with a large ring shear device in which a till and a viscous putty that contained isolated clasts were sheared to high strains as expected rotation of clasts in the putty is closely approximated by the theory of g b jeffery who derived the orbits of rigid ellipsoids in a slowly shearing fluid clast rotation in the till however is strikingly different rather than orbiting through the shear plane as predicted by jeffery most clasts rotate into the shear plane and remain there resulting in strong fabrics regardless of the aspect ratios and initial orientations of clasts this divergent behavior is likely due to slip of the till matrix along the surfaces of clasts which is a natural expectation in a granular material but violates the no slip condition of jeffery9s model these results do not support the widespread belief that subglacial till deformation results in weak clast fabrics thus many tills with weak fabrics thought to have been sheared subglacially to high strains like many basal tills of the laurentide ice sheet may have been sheared only slightly with little effect on either ice sheet dynamics or sediment transport in addition these results indicate that in simple shear the rotation of clasts in till and in fault gouge is best analyzed with the model of a march who treated inclusions as passive markers

nature and origin of corrugated ground moraine of the des moines lobe story county iowa abstract corrugated ground moraine in story county iowa consists of low relief 1 2 m ridges which most commonly consist of late wisconsinan till the average spacing between ridges is about 105 m and they generally parallel the configuration of the bemis moraine the terminal moraine of the des moines glacial lobe till fabric pebble orientations and anisotropy of magnetic susceptibility facies variations and landform fabric analyzed at three sites in typical corrugated ground moraine landscape near ames in story county iowa indicate that at sites 1 and 2 the ridges consist of till probably deposited by lodgement at site 3 two ridges consist of till and massive to cross bedded sand till at site 3 is also probably lodgement till the nature of the till and related glaciofluvial sediments at each site suggests that the corrugation ridges formed in basal cracks or crevasses in the des moines lobe the cracks are postulated to have formed during extending flow of the des moines lobe as it advanced toward its terminal position retreat of the des moines lobe was rapid enough to preclude significant accumulations of supraglacial sediment upon corrugated ground moraine

a survey of the engineering properties of some anhydrite and gypsum from the north and midlands of england abstract the permian and triassic systems in the north and midlands of england contain notable beds of anhydrite and gypsum these rocks contain small but varying amounts of muddy material which often has some influence on their geomechanical properties anhydrite is a strong to very strong rock in terms of both its unconfined compressive strength and point load index whereas gypsum is of medium strength according to its unconfined compressive strength because of the low porosity of these rocks porosity does not have a significant influence on their strength however the strength and hardness of these rocks are strongly correlated in terms of the slake durability test anhydrite has a very high durability and gypsum a high to very high durability with the stronger rocks yielding higher values of durability most anhydrite and gypsum exhibit plastic elastic plastic deformation subsequent plastic deformation occurring at an earlier stage during loading of gypsum than of anhydrite as far as deformability is concerned that of anhydrite is very low while that of gypsum varies from low to high both rock types experience hysteresis on cyclic loading that of gypsum being more notable than that of anhydrite and the amount of hysteresis undergone increases with successive cycles of loading and unloading anhydrite and gypsum both undergo creep when subjected to constant loading gypsum being the more prone to creep under constant load

submarine glacial landforms and rates of ice stream collapse the rate of deglacial ice sheet retreat across polar continental shelves and possible ice stream collapse and sea level rise has been much debated high resolution imagery of seafloor morphology is available for many polar shelves and fjords the rapidity of ice retreat is inferred from diagnostic assemblages of submarine landforms produced at ice stream sedimentary beds these landforms exposed by ice retreat across high latitude shelves demonstrate that deglaciation occurs in three main ways rapidly by flotation and breakup episodically by still stands and or grounding events punctuating rapid retreat or by slower retreat of grounded ice submarine landform assemblages imply through the presence of grounding zone wedges overprinting mega scale glacial lineations on many polar shelves that ice stream retreat is more often episodic than catastrophic these observations provide a robust test of the ability of numerical models to predict the varied response of ice sheet basins to environmental changes

contributions to geotechnique 1948 2008 engineering geology rock mechanics and rock engineering throughout its history geotechnique has regularly published papers on the fundamentals of engineering geology the core concepts of rock mechanics and the practice of rock engineering in totality these papers provide a concise history of the development of these areas of geotechnics here a review of many of these papers is presented in terms of the specific themes of engineering geology rock slope instability behaviour of intact rock and discontinuities in rock field measurements in rock mechanics discontinuous rock masses numerical modelling and rock engineering papers covering a wide range of issues within these themes are outlined together with an indication of how they have influenced later developments the review clearly demonstrates how geotechnique has been at the forefront of rock mechanics rock engineering and engineering geology for the whole of its 60 year existence

evaluation of peak side resistance for rock socketed shafts in weak sedimentary rock from an extensive database of published field load tests a limit state approach this paper presents analyses of the measured peak side resistance of rock sockets constructed in weak claystone shale limestone siltstone and sandstone the peak side resistance is obtained fro

stability of highly fractured infinite rock slopes with nonlinear failure criteria and nonassociated flow laws the effect of the dilatancy phenomenon on rock masses is not usually considered when calculating slope stability a theoretical analysis has been conducted on the stability of infinite rock slopes

marginal formation of de geer moraines and their implications to the dynamics of grounding line recession de geer moraine ridges occur in abundance in the coastal zone of northern sweden preferentially in areas with proglacial water depths in excess of 150 m at deglaciation from detailed sedimentological and structural investigations in machine dug trenches across de geer ridges it is concluded that the moraines formed due to subglacial sediment advection to the ice margin during temporary halts in grounding line retreat forming gradually thickening sediment wedges the proximal part of the moraines were built up in submarginal position as stacked sequences of deforming bed diamictons intercalated with glaciofluvial canal infill sediments whereas the distal parts were built up from the grounding line by prograding sediment gravity flow deposits distally interfingering with glaciolacustrine sediments the rapid grounding line retreat ca 400 m yr 1 was driven by rapid calving in turn enhanced by fast iceflow and marginal thinning of ice due to deforming bed conditions the spatial distribution of the moraine ridges indicates stepwise retreat of the grounding line it is suggested that this is due to slab and flake calving of the ice cliff above the waterline forming a gradually widening subaqueous ice ledge which eventually breaks off to a new grounding line followed by regained sediment delivery and ridge build up less

rupture modes in laboratory earthquakes effect of fault prestress and nucleation conditions seismic inversions show that earthquake risetimes may be much shorter than the overall rupture duration indicating that earthquakes may propagate as self healing pulse like ruptures several mechanisms for producing pulse like ruptures have been proposed including velocity weakening friction interaction of dynamic rupture with fault geometry and local heterogeneity and effect of bimaterial contrast we present experimental results on rupture mode selection in laboratory earthquakes occurring on frictional interfaces which were prestressed both in compression and in shear our experiments demonstrate that pulse like ruptures can exist in the absence of a bimaterial effect or of local heterogeneities we find a systematic variation from crack like to pulse like rupture modes with both 1 decreasing nondimensional shear prestress and 2 decreasing absolute levels of shear and normal prestress for the same value of nondimensional shear prestress both pulse like and crack like ruptures can propagate with either sub rayleigh or supershear rupture speeds our experimental results are consistent with theories of ruptures on velocity weakening interfaces implying that velocity weakening friction plays an important role in governing the dynamic behavior of earthquake ruptures we show that there is no measurable fault normal stress decrease on the fault plane due to the nucleation procedure employed in experiments and hence this is not a factor in the rupture mode selection we find that pulse like ruptures correspond to the levels of nondimensional shear prestress significantly lower than the static friction coefficient suggesting that faults hosting pulse like ruptures may operate at low levels of prestress compared to their static strength

deformation of the batestown till of the lake michigan lobe laurentide ice sheet deep pervasive shear deformation of the bed to high strains 100 may have been primarily responsible for flow and sediment transport of the lake michigan lobe of the laurentide ice sheet to test this hypothesis we sampled at 0 2 m increments a basal till from one advance of the lobe batestown till along vertical profiles and measured fabrics due to both anisotropy of magnetic susceptibility and sand grain preferred orientation unlike past fabric studies interpretations were guided by results of laboratory experiments in which this till was deformed in simple shear to high strains fabric strengths indicate that more than half of the till sampled has a 5 probability of having been sheared to moderate strains 7 30 secular changes in fabric azimuth over the thickness of the till probably due to changing ice flow direction as the lobe receded indicate that the bed accreted with time and that the depth of deformation of the bed did not exceed a few decimeters orientations of principal magnetic susceptibilities show that the state of strain was commonly complex deviating from bed parallel simple shear deformation is inferred to have been focused in shallow temporally variable patches during till deposition from ice

the experimental basis for interpreting particle and magnetic fabrics of sheared till particle fabrics of basal tills may allow testing of the bed deformation model of glacier flow which requires high bed shear strains 100 field studies however have not yielded a systematic relationship between shear strain magnitude and fabric development to isolate this relationship four basal tills and viscous putty were sheared in a ring shear device to strains as high as 714 fabric was characterized within a zone of shear deformation using the long axis orientations of fine gravel and sand particles and the anisotropy of magnetic susceptibility ams of small 5 8 cm3 intact samples results indicate that till particles rotate toward the plane of shearing with long axis orientations that become tightly clustered in the direction of shear 0 78 s1 0 94 for three dimensional data these strong steady state fabrics are attained at shear strains of 7 30 with no evidence of fabric weakening with further strain regardless of the specific till or particle size fraction under consideration these results do not support the jeffery model of particle rotation which correctly describes particle rotation in the viscous putty but not in the tills owing to fluid mechanical assumptions of the model that are violated in till the sensitivity of fabric development to shear strain magnitude indicates that for most till units where shear strain magnitude is poorly known attributing fabric variations to spatial differences in other variables such as till thickness or water content will be inherently speculative attributing fabric characteristics to particular basal till facies is uncertain because shear strain magnitude is unlikely to be closely correlated to till facies weak or spatially variable fabrics in the absence of post depositional disturbance or major deviations from unidirectional simple shear indicate that till has not been pervasively sheared to the high strains required by the bed deformation model strong flow parallel fabrics are a necessary but insufficient criterion for confirming the model copyright 2008 john wiley sons ltd

seafloor slopes at mid ocean ridges from submersible observations and implications for interpreting geology from seafloor topography abstract observations from 145 submersible dives are used to create a database of mid ocean ridge scarp topography and lithology seafloor lithologies are classified into extrusives basaltic talus dykes gabbros and serpentinites and the dive locations are broadly classified according to whether they are close to transform valleys and as a function of ridge spreading rate the database is used to determine whether there is any difference in the maximum slope for each rock type which might relate to differences in rock jointing cohesion or friction properties there is a common perception that lower crustal rocks form steeper slopes than shallower crustal rocks but until now there has been little evidence to support or refute this idea from our analysis we find a tendency for gabbro and dykes to form steeper slopes than serpentinite the 90th percentile of each lithology slope distribution used as a measure of limiting slope is 43 39 and 32 for gabbro dykes and serpentinite respectively lithologic control on slope is weak however compared to overall slope variability in these mid ocean ridge settings so seafloor relief is likely to be a poor guide to underlying geology we speculate on the structure of eroding fault scarps outline the implications for attempts to infer active faults from talus ramp activity and discuss more generally factors affecting the geomorphology of mid ocean ridge slopes

slip pulse rupture behavior on a 2 m granite fault we describe observations of dynamic rupture events that spontaneously arise on meter scale laboratory earthquake experiments while low frequency slip of the granite sample occurs in a relatively uniform and crack like manner instruments capable of detecting high frequency motions show that some parts of the fault slip abruptly velocity 100 mm s 1 acceleration 20 km s 2 while the majority of the fault slips more slowly abruptly slipping regions propagate along the fault at nearly the shear wave speed we propose that the dramatic reduction in frictional strength implied by this pulse like rupture behavior has a common mechanism to the weakening reported in high velocity friction experiments performed on rotary machines the slip pulses can also be identified as migrating sources of high frequency seismic waves as observations from large earthquakes show similar propagating high frequency sources the pulses described here may have relevance to the mechanics of larger earthquakes

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

relative scale and the strength and deformability of rock masses abstract the strength and deformation of rocks depend strongly on the degree of fracturing which can be assessed in the field and related systematically to these properties appropriate mohr envelopes obtained from the rock mass rating rmr classification system and the hoek brown criterion for outcrops and other large scale exposures of fractured rocks show that rock mass cohesive strength tensile strength and unconfined compressive strength can be reduced by as much as a factor often relative to values for the unfractured material the rock mass deformation modulus is also reduced relative to young s modulus a cook book example illustrates the use of rmr in field applications the smaller values of rock mass strength and deformability imply that there is a particular scale of observation whose identification is critical to applying laboratory measurements and associated failure criteria to geologic structures

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

self healing pulse like shear ruptures in the laboratory models predict that dynamic shear ruptures during earthquake faulting occur as either sliding cracks where a large section of the interface slides behind a fast moving rupture front or self healing slip pulses where the fault relocks shortly behind the rupture front we report experimental visualizations of crack like pulse like and mixed rupture modes propagating along frictionally held incoherent interfaces separating identical solids and we describe the conditions under which those modes develop a combination of simultaneously performed measurements via dynamic photoelasticity and laser interferometry reveals the rupture mode type the exact point of rupture initiation the sliding velocity history and the rupture propagation speed

short slip duration in dynamic rupture in the presence of heterogeneous fault properties recent studies of strong motion data consistently show that the risetime duration of slip at particular locations on the fault is significantly shorter than the overall rupture duration the physical explanation for this observation and its implications have become central issues in earthquake source studies two classes of mechanisms have been proposed to explain short risetimes one explanation is that velocity weakening frictional behavior on the fault surface causes the fault to self heal this possibility is suggested by rate dependent friction observed in laboratory experiments and by some two dimensional dynamic numerical simulations of earthquake rupture it has recently been demonstrated however that the velocity dependence of friction observed in the laboratory is too weak to cause faults to self heal an alternative explanation for short risetimes is that spatially heterogeneous fault strength e g barriers limit the slip duration in this paper we investigate this second explanation for short risetimes by constructing a three dimensional dynamic rupture model for the 1984 morgan hill california earthquake mw 6 2 using a kinematic model previously obtained from waveform inversion of strong motion data we assume velocity independent friction and a critical stress fracture criterion and derive a dynamic model specified by the spatial distribution of dynamic stress drop and strength excess that reproduces the slip and rupture time of the kinematic model the slip velocity time functions calculated from this dynamic model are then used in a subsequent inversion to fit the strong motion data by alternating between dynamic and kinematic modeling we obtain a dynamic model that provides an acceptable fit to the recorded waveforms in this dynamic model the risetime is short over most of the fault which is attributable entirely to the short scale length slip stress drop heterogeneity required by the strong motion data a self healing mechanism such as strongly velocity dependent friction is not required to explain the short risetimes observed in this earthquake

dynamic stress changes during earthquake rupture we assess two competing dynamic interpretations that have been pro posed for the short slip durations characteristic of kinematic earthquake models de rived by inversion of earthquake waveform and geodetic data the first interpretation would require a fault constitutive relationship in which rapid dynamic restrengthen ing of the fault surface occurs after passage of the rupture front a hypothesized mechanical behavior that has been referred to as self healing the second inter pretation would require sufficient spatial heterogeneity of stress drop to permit rapid equilibration of elastic stresses with the residual dynamic friction level a condition we refer to as geometrical constraint these interpretations imply contrasting pre dictions for the time dependence of the fault plane shear stresses we compare these predictions with dynamic shear stress changes for the 1992 landers m 7 3 1994 northridge m 6 7 and 1995 kobe m 6 9 earthquakes stress changes are com puted from kinematic slip models of these earthquakes using a finite difference method for each event static stress drop is highly variable spatially with high stress drop patches embedded in a background of low and largely negative stress drop the time histories of stress change show predominantly monotonic stress change after passage of the rupture front settling to a residual level without significant evidence for dynamic restrengthening the stress change at the rupture front is usu ally gradual rather than abrupt probably reflecting the limited resolution inherent in the underlying kinematic inversions on the basis of this analysis as well as recent similar results obtained independently for the kobe and morgan hill earthquakes we conclude that at the present time the self healing hypothesis is unnecessary to explain earthquake kinematics

fabric shape and the interpretation of sedimentary fabric data abstract the concept of fabric shape provides a powerful means of visualizing and analyzing data on sedimentary fabric the shape of a fabric is defined in terms of the ratios between eigenvalues s1 s2 and s3 derived using the orientation tensor method a continuum of all possible orthorhombic fabric shapes can be clearly represented on equilateral ternary diagrams originally developed for the presentation of particle shape data two indices are of particular value in scaling these diagrams fabric isotropy i s3 s1 and fabric elongation e 1 s2 s1 together these indices uniquely define the shape of any orthorhombic fabric and provide a rational and quantitative basis for reconstructing depositional and de ormational processes equilateral ternary diagrams provide a more versatile and powerful basis for analyzing fabric data than other eigenvalue diagrams now in use use of the diagrams in reconstructing processes of sediment transport and deposition is illustrated using fabric data from subglacial tills and a variety of slope deposits

magnetic fabric of sheared till a strain indicator for evaluating the bed deformation model of glacier flow wet based portions of ice sheets may move primarily by shearing their till beds resulting in high sediment fluxes and the development of subglacial landforms this model of glacier movement which requires high bed shear strains can be tested using till microstructural characteristics that evolve during till deformation here we examine the development of magnetic fabric using a ring shear device to deform two wisconsin age basal tills to shear strains as high as 70 hysteresis experiments and the dependence of magnetic susceptibility of these tills on temperature demonstrate that anisotropy of magnetic susceptibility ams develops during shear due to the rotation of primarily magnetite particles that are silt sized or smaller at moderate shear strains 6 25 principal axes of maximum magnetic susceptibility develop a strong fabric s 1 eignevalues of 0 83 0 96 without further strengthening at higher strains during deformation directions of maximum susceptibility cluster strongly in the direction of shear and plunge up glacier consistent with the behavior of pebbles and sand particles studied in earlier experiments in contrast the magnitude of ams does not vary systematically with strain and is small relative to its variability among samples this is because most magnetite grains are contained as inclusions in larger particles and hence do not align during shear although processes other than pervasive bed deformation may result in strong flow parallel fabrics ams fabrics provide a rapid and objective means of identifying basal tills that have not been sheared sufficiently to be compatible with the bed deformation model

retreat signature of a polar ice stream sub glacial geomorphic features and sediments from the ross sea antarctica abstract three research expeditions to the ross sea antarctica resulted in collection of a dataset of more than 270 km of side scan and chirp sonar data more than 330 km of swath bathymetry and 3 5 khz data and 24 cores within a glacially carved trough the former ice stream flow path is divided into six zones covering a distance of approximately 370 km distinguished by unique stratigraphic signatures and geomorphic features an erosional surface with thin patchy lodgement till characterizes zone 1 this region is interpreted as having experienced relatively high basal shear stress conditions zones 2 3 and 4 are characterized by an erosional surface and thin time transgressive subglacial and grounding line proximal deposits that include back stepping moraines flutes transverse moraines and corrugation moraines these zones represent the transition between erosional and depositional regimes under the expanded lgm ice sheet material eroded from the inner shelf was transported toward the outer shelf possibly as a thin deforming till layer the two outer zones are depositional and include maximum grounding line zone 5 and pro glacial deposits that were overridden subsequently by the ice sheet zone 6 surface features include mega scale glacial lineations corrugation moraines and iceberg furrows ice in these zones is interpreted as having experienced relatively lower basal shear stress an extensional regime and faster flow this advance may have destabilized the ice sheet initiating local draw down and production of icebergs that furrowed the sea floor corrugation moraines are thought to represent annual retreat moraines constraining the retreat rate of the ice sheet across the continental shelf to a consistent 40 to 100 m a 1

earthquake nucleation size evidence of loading rate dependence in laboratory faults recent gps observations of major earthquakes such as the 2014 chile megathrust show a slow pre slip phase releasing a significant portion of the total moment ruiz et al 2014 despite advances from theoretical stability analysis rubin ampuero 2005 ruina 1983 and modeling kaneko carpenter nielsen 2017 it is not fully understood what controls the prevalence and the amount of slip in the nucleation process here we present laboratory observations of slow slip preceding dynamic rupture where we observe a dependence of nucleation size and position on the loading rate laboratory equivalent of tectonic loading rate the setup is composed of two polycarbonate plates under direct shear with a 30 centimeters long slip interface the results of our laboratory experiments are in agreement with the pre slip model outlined by ellsworth and beroza 1995 and observed in laboratory experiments latour schubnel nielsen madariaga vinciguerra 2013 nielsen taddeucci vinciguerra 2010 ohnaka kuwahara 1990 which show a slow slip followed by an acceleration up to dynamic rupture velocity however further complexity arises from the effect of 1 rate of shear loading and 2 inhomogeneities on the fault surface in particular we show that when the loading rate is increased from 10 2 mpa s 1 to 6 mpa s 1 the nucleation length can shrink by a factor of three and the rupture nucleates consistently on higher shear stress areas the nucleation lengths measured fall within the range of the theoretical limits lb and l derived by rubin and ampuero 2005 for rate and state friction laws

end bearing response of open ended pipe piles embedded in rock abstract prediction of bearing capacity of steel pipe piles in rock masses is an important consideration in civil engineering especially as such prediction influences the safety of the supported superstructures as well as the pile integrity in pile driving operations provided that the rock mass is described as a linear elastic and perfectly plastic material obeying the hoek brown failure criterion a finite element analysis is performed to investigate the embedment depth effect on the annular base bearing capacity and the failure mechanism of typical open ended pipe piles in sedimentary rock masses the pipe pile has smooth walls and rough toe surface annular toe resistance of pipe piles can serve as an estimate of the rock mass resistance to driving in a fully coring mode which is usually expected for large diameter open ended pipe piles pipe pile results are also extended to circular piles and embedded strip foundations socketed in rock masses the analysis is shown to highlight the influence of the annular geometry of the pipe pile causing an unsymmetrical failure mechanism with respect to pipe wall center as well as an inclination of rock mass reaction which if sufficiently large may lead to pile convergence and damage during pile driving operations the failure mechanism legitimates the plug tendency to rise up in the pipe and explains the plug formation the study demonstrates that in most practical applications the bearing capacity of pipe piles approaches a limiting value which is less than or at most equal to the end bearing capacity of an embedded strip foundation of width equal to the pipe wall thickness a comparison has been made with experimental data it is shown that the results are relatively in good agreement with test data in terms of rock mass resistance and the mechanism of rock plug formation

a risk assessment proposal for underground cavities in hard soils soft rocks abstract underground calcarenite quarries in marsala sicily have been involved in a number of collapses that have seriously damaged numerous buildings the stability conditions were therefore examined in order to assess risk conditions within the historical centre of the town and the surrounding areas which are subject to urban expansion starting with an extensive collection of historical information the research was carried out through surveys of the cavities systematic sampling of material petrographic analysis and geotechnical testing the results of laboratory tests and in situ investigations provided a geotechnical characterization of both the intact material and the rock mass correlating the deformability yielding and strength parameters of the intact rock to the structural features it was possible to define the range of variability of the shear strength parameters of the rock mass the geotechnical parameters obtained were then introduced in the three dimensional finite element codes and the stability and safety conditions of underground quarries were evaluated once the geometrical conditions of the single cavity in its own location relative to the surrounding environment were defined by means of a new parameter k the safety factor was defined according to the intact rock and the rock mass strength and deformation modulus using the proposed abacuses solutions provide risk charts that allow the quantification of the hazard due to the presence of cavities adjacent in the subsoil the applicability of the abacus was investigated through an analysis of a real existing geometry in which collapse is not yet occurred and through a back analysis process on already collapsed underground cavities in three sites risk charts can be considered as a useful starting tool to be used in order to optimize the planning of specific surveys

the influence of the lithospheric thickness on bilateral slip summary a 2 d model of slip formation and dynamic propagation in an elastic lithospheric plate coupled to the asthenosphere and having a fault plane is investigated using a finite element method slip is nucleated at a weak portion of the fault plane and propagates bilaterally at the edges slip is assumed to develop according to a critical shear stress criterion combined with a slip weakening law except for the weak portion the slip resistance is constant along the fault plane the influence of resistive stresses from the asthenosphere at the bottom of the plate is taken into account the results indicate that bilateral slip lengths are limited to about the lithospheric plate thickness or somewhat larger such lengths will only be reached for sufficiently high tectonic loading after maximum slip length has been reached the slipping region is split into two regions of unidirectional slip moving in opposite directions the results thus illustrate one mechanism which limits the maximum amount of slip during an earthquake even in the absence of strong barriers

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

experimental investigation of strong ground motion due to thrust fault earthquakes thrust fault earthquakes are studied in a laboratory earthquake setup previously used to investigate analog strike slip seismic events dynamic mode ii ruptures are generated along preexisting faults in an analog material homalite h 100 and their interaction with the free surface is studied for both sub rayleigh and supershear rupture speeds high speed digital photography and laser velocimeter diagnostics are used synergistically to identify and study the ground velocity signatures caused by the various features of the generated ruptures the obtained surface normal motions of both sub rayleigh and supershear ruptures show substantial asymmetry between the hanging and footwall with the hanging wall experiencing much larger velocity amplitudes the main features of the surface velocity traces at various stations can be explained by the calculated arrivals of various waves and fronts mach cones pand s waves and sub rayleigh features in both the sub rayleigh and supershear cases the arrival of the rupture tip generates a prominent rayleigh wave traveling along the simulated earth s surface supershear events feature larger amplitudes of ground shaking profiles all signatures in the surface motion records attenuate and broaden with increasing distance from the fault trace the signatures corresponding to the arrival of the mach fronts attenuate with distance at a slower rate than those from sub rayleigh ruptures the arrival of the updip supershear rupture at the free surface creates a downdip propagating slip feature with its own mach cone these additional mach fronts further amplify ground shaking on the hanging and footwalls

pore pressure changes and strength mobilization of soft rocks in consolidated undrained cyclic loading triaxial tests changes in the pore pressure during cu consolidated undrained cyclic loading triaxial tests on fully saturated specimens of four miocene soft rocks i e ohya tuff yokohama siltstone kobe mudstone and sandstone are analyzed as a result three modes of failure i e brittle brittle ductile and ductile can be recognized and other characteristic stress levels e g crack initiation stress plastic flow gliding stress dilation stress etc identified the mobilization of the strength of these rocks in terms of cohesion and internal friction angle is then discussed a procedure for the estimation of the rock cohesion strength is first introduced on the basis of the modified griffith s failure criterion the determination of the mobilized friction angle at any state of stress during the tests follows

limiting envelopes of a dry porous limestone under true triaxial stress states abstract the present study investigates the influences of the confining pressure and the intermediate principal stress on the mechanical behaviour of a porous limestone true triaxial tests are carried out on a broad range of confining stresses covering both brittle and ductile regimes the experiments included the special cases of triaxial compression extension and isotropic compression as well as tests at constant levels of the intermediate principal stress the resulting stress strain curves and yield stresses are then discussed three dimensional yield envelopes are subsequently built in meridian and octahedral planes the shape of the octahedral sections changes with the mean stress from triangular to hexagonal and then quasi circular at even higher mean stresses the evolution continues towards a triangle with apexes oriented in the direction of triaxial extension stress states various existing yield criteria are fitted and compared to the experimental data finally a three dimensional yield surface is proposed combining the previous observations

spatiotemporal properties of sub rayleigh and supershear ruptures inferred from full field dynamic imaging of laboratory experiments many earthquakes propagate at sub rayleigh speeds earthquakes propagating at supershear speeds though less common are by far more destructive hence it is important to quantify the motion characteristics associated with both types of earthquake ruptures here we report on the spatiotemporal properties of dynamic ruptures measured in our laboratory experiments using the dynamic digital image correlation technique earthquakes are mimicked by the frictional rupture propagating along the interface of two homalite plates digital images of the propagating ruptures are captured by an ultrahigh speed camera and processed with digital image correlation in order to produce sequences of evolving displacement and velocity maps our measurements reveal the full field structure of the velocity components bridge the gap between previous spatially sparse velocimeter measurements available only at two to three locations and enable us to quantify the attenuation patterns away from the interface

estimation of rock mass properties in excavation damage zones of rock slopes based on the hoek brown criterion and acoustic testing abstract when slopes are excavated in rock masses with the blasting method blasting and stress relaxation will produce an excavation damage zone edz in the remaining rock mass this study presents an estimation of the rock mass properties in the edz based on the generalized hoek brown failure criterion and the varying disturbance factor in this estimation the disturbance factor is quantified by using the acoustic p wave velocities tested in undamaged and damaged rock masses instead of a single constant value chosen from the descriptive guidelines statistical analysis of the acoustic test data shows that with an increase in depth behind the slope face the disturbance factor declines from 1 0 to 0 almost in a linear manner for blasting excavation of slopes in weaker rock masses the zone in the immediate vicinity of the slope face is fully damaged and thus over this range the disturbance factor remains the constant value of 1 0 as the disturbance factor linearly decreases within the edz the deformation modulus uniaxial compressive strength and equivalent cohesive strength of the damaged rock mass linearly increase to the undamaged values by assigning the varying parameters to the rock mass in the edz a numerical calculation based on the flac program is conducted to assess the slope stability it is found that using the varying rock mass parameters within the edz will yield completely different failure surfaces and factors of safety from that uses the constant parameters therefore it is significant to develop such an approach to quickly estimate the varying rock mass properties in the edz particularly when in situ tests are not available

strain patterns in glacitectonically thrusted sediments and conditions during thrusting abstract glacitectonically thrusted blocks of unlithified sediment are commonly inferred to have been frozen during thrusting with implications for paleoclimate and glacier dynamics we inferred strain patterns within thrusted blocks of glaciolacustrine mud in the rubjerg knude and mons klint glacitectonic complexes of coastal denmark to try to assess whether the mud was frozen during thrusting fabrics based on the anisotropy of magnetic susceptibility ams of the muds are interpreted with results of ring shear experiments and models of ams fabric development during layer parallel shortening lps in intact distal stratigraphy fabrics indicate moderate lps fabrics where thrusts have disrupted more proximal stratigraphy indicate rotation of mud sheets during thrusting with insufficient penetrative strain to reset the lps fabric the exception is within decimeters of faults at rubjerg knude where shear overprinted the lps fabric although these data indicate that mud sheets were mostly rigid during thrusting this is not necessarily evidence of permafrost critical taper theory indicates that even a gentle slope of the proglacial surface away from the glacier margin would have allowed slip without internal deformation of thrust sheets requiring only minimal pore water pressures

an assessment of surge induced crevassing and the formation of crevasse squeeze ridges 1 evidence for extensive crevassing is preserved on the deglaciated forelands of many surging glaciers as crevasse squeeze ridges csrs at some point these crevasses make direct connection with the bed in order to become sediment filled and full depth connections have been inferred from turbid water up wellings in crevasses and the formation of concertina eskers the dynamics of seven surging glaciers are assessed using a linear elastic fracture mechanics approach to determine the likely directions of fracture and controlling parameters for mode i crevasses extensional surface strain rates are insufficient to promote top down full depth penetration for small crevasse spacing 5 m surface strain rates are sufficient for top down crevassing to depths of 4 12 m explaining the extensive surface crevassing associated with glacier surging as has been shown in other settings top down full depth crevassing is only possible when water is added and approaches 97 of the crevasse depth the provision of sufficient meltwater to facilitate this is problematic due to the extensive surface crevassing unless water can move along connected crevasses to a dominant water capturing crevasse for ice thicknesses greater than 200 m basal water pressures in excess of 80 90 of flotation are required for full depth bottom up crevassing field evidence suggests that this is the default for surging glaciers and that on occasion water pressures may even become artesian csrs found across many surging glacier forelands and ice margins most likely result from the infilling of basal crevasses driven for the most part bottom up by high basal water pressures

unstable behavior of the laurentide ice sheet over deforming sediment and its implications for climate change geologic records of fluctuations of the laurentide ice sheet margin following the most recent glacial maximum ca 20 000 14c yr b p identify fundamental differences in ice sheet behavior depending on subglacial bed conditions rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet deforming sediment dynamics in contrast largely uninterrupted ice margin retreat with no evidence of significant readvance occurred over rigid bed areas indicating stable behavior unstable ice sheet behavior was most pronounced from 15 000 until 10 000 14c yr b p by which time most of the ice margin had retreated onto a rigid bed unstable ice sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater as well as meltwater routing to the north atlantic thus affecting thermohaline circulation the abrupt climate oscillations in the north atlantic region that ended at 10 000 14c yr b p may thus have their origin in the inherently unstable behavior of the laurentide ice sheet overriding deforming sediment

terrestrial ice streams a view from the lobe abstract the glacial landforms of minnesota are interpreted as the products of the lobate extensions of ice streams that issued from various ice sheds within the laurentide ice sheet low relief till plains trough shaped lowlands boulder pavements and streamlined forms make up the subglacial landsystem in minnesota that is interpreted as having been formed by streaming ice extremely uniform tills are created subglacially in a way that remains somewhat mysterious at the ice margins thrust moraines and hummocky stagnation topography are more common than single crested simple moraines if the ice lobes had repeated advances subglacial drainage features are obscure up ice but are present down ice in the form of tunnel valleys eskers spooner hills and associated ice marginal fans ice streaming may occur when basal shear stress is lowered as a result of high subglacial water pressure subglacial conditions that allow the retention of water will allow an ice lobe to extend far beyond the ice sheet as long as the ice shed also supports the advance by supplying adequate ice even with adequate ice flux however the advance of an ice lobe may be terminated at least temporarily if the subglacial water is drained through tunnel valleys or perhaps a permeable substrate thrust moraines and ice stagnation topography will result from sudden drainage although climate change is ultimately responsible for the accumulation of ice in the laurentide ice sheet the asynchronous advances and retreats of the ice lobes in the mid continent are strongly overprinted by the internal dynamics of individual ice streams as well as the interaction of ice sheds which obscure the climate signal

power law singularity as a possible catastrophe warning observed in rock experiments the question which type of signals can be determinately related to catastrophic rupture in heterogeneous brittle media still remains open here we report a specific precursor of catastrophic rupture i e a power law singularity of responses based on rock experiments our experimental observations show that the singularity with power exponent b where b 0 51p0 10 meanps d appears ahead of catastrophic rupture in some rocks and the singularity does not appear at all for gradual failure it is indicated that the power law singularity can emerge well only close to catastrophic rupture and thus it could serve as a specific warning for catastrophic rupture to address the potential forewarning of imminent catastrophic rupture a fitting process based on the data before catastrophic rupture was developed to determine the two unknowns related to the occurrence catastrophe point uf and the power exponent bu it is demonstrated that the power law singularity appears only in the vicinity of catastrophe and that the power law singularity does not occur for gradual failure

pattern recognition of lidar data and sediment anisotropy advocate a polygenetic subglacial mass flow origin for the kemijarvi hummocky moraine field in northern finland abstract object based image analysis and supervised machine learning on a digital elevation model derived from airborne laser scanning data were conducted to assess the aggregate raw material potential over the kemijarvi hummocky moraine field in finnish lapland geomorphometric analysis revealed surface stoniness to be diagnostic of moraine hummocks it was utilized to detect true moraine hummocks from among the potential hummocks delineated using object based image analysis with logistic regression of curvature features derived from the digital elevation model area under the receiver operating characteristics curve auc 0 77 based on the electrical sedimentary anisotropy data the kemijarvi moraine is polygenetic such that the ridges are diagonally and erosionally fluted from sector 280 290 on their surfaces while the ridge cores are deposited from the nw from sector 310 340 the cores are composed of infill gravels and crudely stratified diamict sequences we consider the core of the kemijarvi hummocky moraine field to represent pre existing early weichselian subglacial ridges rather than being generated through monogenetic late weichselian compressional shearing and stacking frozen bed extension or bed ribbing instability mechanisms as presented for ribbed moraines that are transverse to ice flow recently evidence has been presented of multiple slip events earthquakes with moment magnitudes mw 6 7 7 0 up ice from kemijarvi we therefore propose that the genesis of the hummocks may be associated with past early weichselian fault instability within glacio isostatic adjustment s which was able to generate subglacial water bodies possibly lakes and saturated materials that were forced towards the se based on this new geomorphological evidence from mapping using a digital elevation model derived from airborne laser scanning data and azimuthal conductivity surveys combined with the reinterpretation of previously presented geophysics and sedimentological observations we suggest the kemijarvi hummocky moraine field to be reconsidered as the kemijarvi subglacial mass flow field which holds significant potential for construction aggregate materials

elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rate and state dependent friction we present an efficient and rigorous numerical procedure for calculating the elastodynamic response of a fault subjected to slow tectonic loading processes of long duration within which there are episodes of rapid earthquake failure this is done for a general class of rate and state dependent friction laws with positive direct velocity effect the algorithm allows us to treat accurately within a single computational procedure loading intervals of thousands of years and to calculate for each earthquake episode initially aseismic accelerating slip prior to dynamic rupture the rupture propagation itself rapid post seismic deformation which follows and also ongoing creep slippage throughout the loading period in velocity strengthening fault regions the methodology is presented using the two dimensional 2 d antiplane spectral formulation and can be readily extended to the 2 d in plane and 3 d spectral formulations and with certain modifications to the space time boundary integral formulations as well as to their discretized development using finite difference or finite element methods the methodology can be used to address a number of important issues such as fault operation under low overall stress interaction of dynamic rupture propagation with pore pressure development patterns of rupture propagation in events nucleated naturally as a part of a sequence the earthquake nucleation process earthquake sequences on faults with heterogeneous frictional properties and or normal stress and others the procedure is illustrated for a 2 d crustal strike slip fault model with depth variable properties for lower values of the state evolution distance of the friction law small events appear the nucleation phases of the small and large events are very similar suggesting that the size of an event is determined by the conditions on the fault segments the event is propagating into rather than by the nucleation process itself we demonstrate the importance of incorporating slow tectonic loading with elastodynamics by evaluating two simplified approaches one with the slow tectonic loading but no wave effects and the other with all dynamic effects included but much higher loading rate

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

surface form of the southern laurentide ice sheet and its implications to ice sheet dynamics reconstructions of the ice surface morphology of several lobes of the southern laurentide ice sheet reinforce previous arguments that this sector of the ice sheet was thin and low sloping driving stresses estimated from the geometry of the reconstructed ice surfaces are 0 7 4 3 kpa for the 14 ka des moines lobe 0 9 1 2 kpa for the 14 ka james lobe 0 9 1 7 kpa for the 18 20 ka lake michigan lobe 1 8 2 9 kpa for the 15 18 ka chippewa sublobe and 17 22 kpa for the 15 18 ka green bay lobe previous estimates of rates of ice margin advance 450 2 000 m yr indicate moderate to fast ice velocities for the ice lobes reconstructed driving stresses and velocity estimates of the des moines james and lake michigan lobes are analogous to the distal ends ice plains of low sloping 0 4 x 10 3 but fast moving 500 m yr west antarctic ice streams whose dynamics have been attributed to sliding and or subglacial sediment deformation by pervasive shear these reconstructions support recent models of the laurentide ice sheet which include movement by sliding or by subglacial sediment deformation along its southern western and northwestern sectors evidence for either mechanism should be represented in the sedimentologic and geomorphic records thin ice in these regions indicates that the laurentide ice sheet contained less ice volume and represented less of an orographic obstacle to atmospheric circulation than has been considered in models of the ice sheet on a rigid bed with steep profiles

three dimensional finite difference simulation of fault dynamics rectangular faults with fixed rupture velocity we analyze three dimensional finite difference solutions for a simple shear crack model of faulting to determine the effects of fault length and width on the earthquake slip function the fault model is dynamic with only rupture velocity fault dimensions and dynamic stress drop prescribed the numerical solutions are accurate for frequencies up to 5 hz and are combined with asymptotic results for shear cracks in order to characterize the slip function at higher frequencies near the hypocenter the slip velocity exhibits a square root singularity whose intensity increases with hypocentral distance at distances greater than the fault width w growth of the velocity intensity ceases and the slip function becomes nearly invariant with distance along the fault length closed form expressions are developed for the dependence of static slip s slip rise time tr and slip velocity intensity v on fault geometry along the center line of a long narrow fault at hypocentral distances exceeding w these expressions reduce to s w tr 0 5 w vr and v w 2 vr where is the dynamic stress drop the shear modulus and vr the rupture velocity the numerical results imply that uniform dislocation kinematic earthquake models in which slip is represented by a ramp time function will underpredict high frequency ground motion relative to low frequency ground motion a further implication of the numerical solutions is that the nature of inelastic processes at the advancing edge of a long fault will depend on fault width but will be independent of rupture length

the structural geology of a surge type glacier abstract the range of deformation structures found in surge type variegated glacier alaska is similar to that found in many thin skinned thrust belts and includes foliations folds dip slip and strike slip faults thrust faults and fractures the development of structural relationships and the overall structural assemblage can be related to the deformation histories of quiescent and surge phases of motion the surge phase dominates the formation of brittle structures because of the large stresses and strain rates associated with it the relationship between downglacier verging overturned folds with hinge lines transverse to flow and a family of transverse arcuate thrust faults is characteristic of structural development in the lower part of the glacier during the surge longitudinal foliation develops mainly from sedimentary stratification in the upper part of the glacier in a marginal transpressive deformation regime during quiescent phase flow from broad accumulation basins the structural relationships in this surge type glacier may aid in the interpretation of structures in other gravity driven tectonic settings