slow dynamics and strength recovery in unconsolidated granular earth materials a mechanistic theory rock materials often display long time relaxation commonly termed aging or slow dynamics after the cessation of acoustic perturbations in this paper we focus on unconsolidated rock materials and propose to explain such nonlinear relaxation through the shear transformation zone stz theory of granular media adapted for small stresses and strains the theory attributes the observed relaxation to the slow irreversible change of positions of constituent grains and posits that the aging process can be described in three stages fast recovery before some characteristic time associated with the subset of local plastic events or grain rearrangements with a short time scale log linear recovery of the elastic modulus at intermediate times and gradual turnover to equilibrium steady state behavior at long times we demonstrate good agreement with experiments on aging in granular materials such as simulated fault gouge after an external disturbance these results may provide insights into observed modulus recovery after strong shaking in the near surface region of earthquake zones

how do bedform patterns arise new views on the role of bedform interactions within a set of boundary conditions one explanation for bedform patterns is self organization in which the pattern emerges because of interactions among the bedforms themselves models remote images field studies and lab experiments have identified bedform interactions that involve whole bedforms only bedform defects or that are remote interactions between bedforms it is proposed that bedform interactions form a spectrum from constructive to regenerative in pattern development constructive interactions including merging lateral linking cannibalization and remote transfer of sediment push the system toward fewer larger more widely spaced bedforms regenerative interactions including bedform splitting defect creation and calving push the system back toward a more initial state other interactions including off center collision defect migration and bedform and defect repulsion cause pattern change but may not be strongly constructive or regenerative although bedform interactions are ubiquitous to any field of bedforms their dynamics flow field modification and impact upon measurable pattern parameters are yet poorly understood most bedform interactions span bedform types and fluids supporting the hypothesis that pattern emerges from dynamics at the bedform level in a hierarchy that includes lower levels of bedform flow and grain fluid interactions bedform interactions alone however cannot account for the rich diversity of bedform patterns in nature it is proposed that field diversity arises because of boundary conditions which are the environmental variables within which a field evolves conceptually boundary conditions modify the shape of the attractor toward which a field evolves possibly by altering the type and frequency of bedform interactions boundary conditions are broadly similar within system types but are unique for each bedform field so that no two are ever exactly alike although aeolian and fluvial systems share some types of boundary conditions flow depth is a unique boundary condition in shallow fluvial systems copyright 2009 john wiley sons ltd

coagulation and transport of sediments in the gironde estuary the distribution of suspended particle size and concentration were measured along the gironde estuary france from the river seaward to the ocean the suspended particle size and volume concentration were measured using in situ holography and onboard optical techniques utilizing special procedures in order to avoid floc breakage sediments discharged by the rivers coagulate upon encountering the very low salinities 0 2 of the upper estuary confirmed with laboratory experiments and are then transported and deposited by currents in the remainder of the estuary this coagulation coupled with estuarine circulation produces a turbidity maximum which is offset between the surface and bottom waters the floc size maximum is oceanward of the turbidity maximum and is likewise offset along the estuary by about 30 km the estuary can be subdivided into the following zones 1 coagulation 2 hydrodynamic landward of the null point and 3 hydrodynamic seaward of the null point initial coagulation appears to be completed in coagulation zone 1 and particles are transported and settled with very little floc breakage and recoagulation in zones 2 and 3 only the floc settling velocities coupled with estuarine circulation control the concentration and size distributions of flocs in the water column and eventually control the deposition of sediments

nonlinear dynamical triggering of slow slip on simulated earthquake faults with implications to earth 1 among the most fascinating recent discoveries in seismology are the phenomena of dynamically triggered fault slip including earthquakes tremor slow and silent slip during which little seismic energy is radiated and low frequency earthquakes dynamic triggering refers to the initiation of fault slip by a transient deformation perturbation most often in the form of passing seismic waves determining the frictional constitutive laws and the physical mechanism s governing triggered faulting is extremely challenging because slip nucleation depths for tectonic faults cannot be probed directly of the spectrum of slip behaviors triggered slow slip is particularly difficult to characterize due to the absence of significant seismic radiation implying mechanical conditions different from triggered earthquakes slow slip is often accompanied by nonvolcanic tremor in close spatial and temporal proximity the causal relationship between them has implications for the properties and physics governing the fault slip behavior we are characterizing the physical controls of triggered slow slip via laboratory experiments using sheared granular media to simulate fault gouge granular rock and glass beads are sheared under constant normal stress while subjected to transient stress perturbation by acoustic waves here we describe experiments with glass beads showing that slow and silent slip can be dynamically triggered on laboratory faults by ultrasonic waves the laboratory triggering may take place during stable sliding constant friction and slip velocity and or early in the slip cycle during unstable sliding stick slip experimental evidence indicates that the nonlinear dynamical response of the gouge material is responsible for the triggered slow slip

flood ebb tidal asymmetry in a shallow sandy estuary and the impact on net sand transport flood ebb tidal dominance plays a pivotal role in estuarine sediment transport the dyfi estuary mid wales uk is used as a case study together with an idealized trumpet shaped estuary morphology to illustrate key processes giving rise to flood ebb dominance observations made in the main entrance channel to the dyfi indicate that the system is presently ebb dominant with regard to both the relatively long duration of the ebb tide and also the faster ebb velocities predictions made across the mouth of the estuary using the telemac modelling system suggest further that the net sand transport presently occurs out of the estuary in contrast due to the varying distribution of channels and flats within the estuary the tidal flow in the upper estuary is flood asymmetric and here the net transport is up estuary flood versus ebb dominance is shown to be attributable to the relative extent of the channels and sandflats in relation to the mean water level an attempt is made to develop a simple metric to determine present day transport pathways within an estuary in response to its cross channel flow asymmetries

the effects of channels and shoals on exchange between the chesapeake bay and the adjacent ocean the role of bathymetric changes in determining the transport of water and salt in the lower chesapeake bay lcb was investigated using high resolution acoustic doppler current profiler adcp and conductivity temperature depth profiles a cross channel transect was repeated eight times during neap tides on october 6 7 1993 which illustrated the lateral structure of the longitudinal and transverse flow fields and the intratidal variations in the flow structure across the lcb amplitude and phase of the m2 tidal component as well as the mean flow velocity were calculated using least squares fitting at every point of a uniform grid obtained from the adcp data the results differ from the classical two layer pattern of estuarine circulation modified by coriolis effects but are consistent with recent hydrographic observations semidiurnal flow was highest over the navigational channels and lateral gradients were strongest in regions of sharp bathymetric changes the phase lag of the semidiurnal flow also showed lateral and vertical gradients that represented advances at the bottom with respect to the surface and over the shoals in relation to the channels the section of the water column measured indicated a mean outflow of 0 7 104 m3 s and a mean inflow of 1 3 104 m3 s the apparent gain of water by the estuary during the period of observation can be explained by meteorologically forced net barotropic inflow the depth averaged mean longitudinal flow consisted of inflow in the navigational channels and outflow over the shoals the mean transverse flow showed near surface convergence over the channels we propose a possible explanation for the observed flow and density structure as follows the barotropic and baroclinic forcing interact with the bathymetry to extend the inflow from the bottom to the surface thereby inducing a transverse circulation that yields near surface convergence over the channels

a note on the stress dilatancy relation for simulated fault gouge theoretical constraints on the stress dilation relation for a deforming coulomb material requirev ifc 0 andv sin 1 m m always wherev is the dilation angle is the friction angle c is cohesion m is the maximum shear stress and m is the mean effective stress recent laboratory measurements of friction and dilatancy of simulated fault gouge show that small amplitude shear load cycling causes compaction and consolidation comparison of the data with theory indicates that such load cycling produces 1 increased coefficient of friction or friction angle 2 increased cohesion and 3 increased dilatancy rate or dilation angle under certain conditions of load cycling without significant plastic shear strain accumulation p 0 005 we find thatv exceeds both and in contrast to theory sin 1 m m this result is interpreted in terms of enhanced cohesion and overconsolidation which lead to residual stresses within the gouge an analogy is drawn between these special loading conditions and those extant on natural faults in particular our results imply that jostling and minor stress variations associated with microearthquakes may produce strengthening of fault gouge and changes in the fault zone s stress dilatancy relation hence compaction associated with microseismicity may lead to subsequent dilation of fault gouge even for faults with large displacement rates and large net offsets e g san andreas in regions where such dilation persists over sufficient displacements on the order of the critical slip distance for seismic faulting it may tend to inhibit unstable slip

geometry and migration characteristics of bedforms under waves and currents part 1 sandwave morphodynamics abstract in this paper the geometric and migration characteristics of sandwaves generated under the combined effect of waves and currents are described description and analysis of the flow structure along the centerline of developed sandwaves are also presented the velocity distribution along the sandwave is used to explain in part the size and shape characteristics of the ripples superimposed upon the sandwaves as discussed in the companion paper catano lopera y and garcia m h in press geometry and migration characteristics of bedforms under waves and currents part 2 ripples superimposed upon sandwaves coastal engineering doi 10 1016 j coastaleng 2006 03 008 variation in roughness height along the considered bed feature is also discussed for a particular experiment velocity measurements within the fluid were recorded with an acoustic doppler velocimeter adv an acoustic level sensor was employed to determine surface wave characteristics such as amplitude and period 3d mapping of the bottom was done via a 32 composite element array of sub aquatic acoustic sensors seatek multiple transducer arrays bottom records were statistically analyzed to obtain height length migration rates of ripples and sandwaves and vertical growth rate of sandwaves experiments were conducted for the mobility factor and the reynolds wave number within the ranges 10

nonlinear dynamics granular media and dynamic earthquake triggering the landers earthquake of 1992 triggered a series of earthquakes in other parts of california focusing attention on the mechanism of remote triggering laboratory experiments on granular material under pressure suggest that triggering is a result of seismic waves impinging on a fault and inducing elastic nonlinear behaviour of the fault core accompanied by instantaneous weakening and failure the resulting softening to weakening model fits in well with field observations of real earthquakes the 1992 magnitude 7 3 landers earthquake triggered an exceptional number of additional earthquakes within california and as far north as yellowstone and montana1 2 3 since this observation other large earthquakes have been shown to induce dynamic triggering at remote distances for example after the 1999 magnitude 7 1 hector mine1 and the 2002 magnitude 7 9 denali4 earthquakes and in the near field as aftershocks5 the physical origin of dynamic triggering however remains one of the least understood aspects of earthquake nucleation1 2 3 4 5 the dynamic strain amplitudes from a large earthquake are exceedingly small once the waves have propagated more than several fault radii for example a strain wave amplitude of 10 6 and wavelength 1 m corresponds to a displacement amplitude of about 10 7 m here we show that the dynamic elastic nonlinear behaviour of fault gouge perturbed by a seismic wave may trigger earthquakes even with such small strains we base our hypothesis on recent laboratory dynamic experiments conducted in granular media a fault gouge surrogate6 7 from these we infer that if the fault is weak8 9 10 seismic waves cause the fault core modulus to decrease abruptly and weaken further if the fault is already near failure this process could therefore induce fault slip

auto acoustic compaction in steady shear flows experimental evidence for suppression of shear dilatancy by internal acoustic vibration journal of geophysical research vol 117 b09314 doi 10 1029 2011jb008897 2012 auto acoustic compaction in steady shear flows experimental evidence for suppression of shear dilatancy by internal acoustic vibration nicholas j van der elst 1 emily e brodsky 1 pierre yves le bas 2 and paul a johnson 2 received 22 september 2011 revised 9 august 2012 accepted 20 august 2012 published 28 september 2012 1 granular shear flows are intrinsic to many geophysical processes ranging from landslides and debris flows to earthquake rupture on gouge filled faults the rheology of a granular flow depends strongly on the boundary conditions and shear rate earthquake rupture involves a transition from quasi static to rapid shear rates understanding the processes controlling the transitional rheology is potentially crucial for understanding the rupture process and the coseismic strength of faults here we explore the transition experimentally using a commercial torsional rheometer we measure the thickness of a steady shear flow at velocities between 10 a3 and 10 2 cm s at very low normal stress 7 kpa and observe that thickness is reduced at intermediate velocities 0 1 10 cm s for angular particles but not for smooth glass beads the maximum reduction in thickness is on the order of 10 of the active shear zone thickness and scales with the amplitude of shear generated acoustic vibration by examining the response to externally applied vibration we show that the thinning reflects a feedback between internally generated acoustic vibration and granular rheology we link this phenomenon to acoustic compaction of a dilated granular medium and formulate an empirical model for the steady state thickness of a shear zone in which shear induced dilatation is balanced by a newly identified mechanism we call auto acoustic compaction this mechanism is activated when the acoustic pressure is on the order of the confining pressure and results in a velocity weakening granular flow regime at shear rates four orders of magnitude below those previously associated with the transition out of quasi static granular flow although the micromechanics of granular deformation may change with greater normal stress auto acoustic compaction should influence the rheology of angular fault gouge at higher stresses as long as the gouge has nonzero porosity during shear citation van der elst n j e e brodsky p y le bas and p a johnson 2012 auto acoustic compaction in steady shear flows experimental evidence for suppression of shear dilatancy by internal acoustic vibration j geophys res 117 b09314 doi 10 1029 2011jb008897 1 introduction 2 frictional sliding processes in geophysics often involve granular shear flows at the sliding interface this is true for landslides and debris flows as well as for earthquake rup tures within granulated damage zones or gouge filled faults the frictional strength in these contexts is controlled by the rheology of the granular flow which has a strong dependence department of earth and planetary science univ of california santa cruz california usa geophysics group los alamos national laboratory los alamos new mexico usa corresponding author nicholas j van der elst department of earth and planetary science 1156 high st univ of california santa cruz ca 95060 usa nvandere ucsc edu 2012 american geophysical union all rights reserved 0148 0227 12 2011jb008897 on shear rate and boundary conditions campbell 2006 clement 1999 iverson 1997 savage 1984 3 for different shear rates confining stresses and pack ing densities the description of a granular flow can range from solid like to gas like jaeger et al 1996 albeit with complicated second order behavior in each regime the appropriate description for a particular flow is typically determined by the dimensionless inertial number which compares the magnitude of the grain inertial stresses to the confining stress bocquet et al 2001 campbell 2006 clement 1999 iverson 1997 lu et al 2007 savage 1984 i rd g 2 p where r is density d is grain diameter g is the strain rate and p is the confining normal pressure the shear rate profile in boundary driven flows is commonly observed to decay b09314 1 of 18

high resolution swath sonar investigation of sand ridge dune and ribbon morphology in the offshore environment of the new jersey margin abstract sand ridges on the northeast us atlantic shelf form in the near shore environment most likely in response to storm driven flows as the holocene transgression has continued the ridges have been transferred to an offshore hydrodynamic regime where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly here we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller scale features in an effort to place constraints on bedform development in these deeper waters a recent high resolution swath sonar survey mapped a portion of the new jersey shelf from 20 m water depth to the shelf break 120 m imaging both sand ridges and smaller scaled dunes and ribbons in far greater detail than has been previously possible using a robust statistical analysis we find that the gross morphology of ridges height width length does not change with depth beyond 20 m water depth and changes in ridge orientation generally mirror changes in regional contour orientation hence ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime however on the inner shelf 20 m water depth to the mid shelf shore we do find evidence in the ridge shape which has an asymmetry opposite to that seen near shore and in the complex backscatter response that some important modifications to ridges are taking place at these water depths in contrast on the mid shelf from the mid shelf shore to the franklin shore ridges tend to have higher backscatter at the crests implying that these are largely winnowed relict features lineated smaller scale 100 500 m wavelength

classification of seafloor geology using multibeam sonar data from the scotian shelf abstract we describe a method for classifying multibeam sonar data and illustrate the method using data collected with a simrad em1000 sonar on the scotian shelf canada the method involves comparing various attributes of the bathymetry and backscatter with typical examples of specified seafloor types and computing their statistical degree of similarity sediment ponds are identified as areas of low echo amplitude and low topographic gradient and curvature ridges and troughs are identified by fitting a paraboloid to patches of the bathymetry once the sediments ridges and troughs are located we are able to use the database to extract orientations and other characteristics of these features for example the orientation of the topography can be computed from the paraboloid surface and rose diagrams of the ridge and trough orientations reflect the fabric of up turned cambro ordovician sedimentary beds outcropping in this area the unconsolidated sediments ponded within small basins have low topographic gradients with consistent tilts towards the southeast mean 133 which we interpret as due to offshore transport of holocene sediments possibly in response to storms

lateral variability of subtidal flow at the mid reaches of a macrotidal estuary transverse variations of tidal and subtidal flow were investigated in a macrotidal and convergent estuary this was accomplished by combining data analysis of current velocities and water density with numerical modeling at the mid reaches of the gironde estuary france nonlinear mechanisms responsible for overtide generation and hence subtidal flows were found to vary across the estuary and from neap to spring tides subtidal flows were driven by a combination of internal asymmetry tidal advective accelerations nonlinear effects of water level variations quadratic friction and river discharge the quarter diurnal overtide band d4 in the flow was generated by internal asymmetry and tidal advective accelerations during neap tide the ratio of quarter diurnal to squared semidiurnal bands d4 d22 was largest 0 3 in sections of the channel showing subtidal outflow river discharge increased from neap to spring tides causing a subsequent increase of seaward subtidal currents during spring tide d4 was generated by tidal advective accelerations and quadratic friction combined with river discharge rather than by internal asymmetry the sixth diurnal overtide d6 in the flow was comparable to d4 for both neap and spring tides largest d6 d23 ratios were found in the shallowest cross channel locations during both neap and spring tides

geometry and migration characteristics of bedforms under waves and currents part 2 ripples superimposed on sandwaves this paper presents results of a series of detailed measurements of geometric and migrating characteristics of ripples superimposed upon sandwaves under the action of combined waves and currents velocity measurements within the fluid surface wave characteristics and 3d mapping of the bottom were recorded with an acoustic doppler velocimeter adv an acoustic water level sensor and a 32 composite element array of sub aquatic acoustic sensors respectively bottom records were statistically analyzed to obtain height length and migration rates of ripples experiments examined ripple heights and wavelengths for the mobility factors as defined in eq 4 and the reynolds wave number within the ranges 10 88 and 16 103 rew 5 105 respectively measured values were compared with laboratory and field data together with semi empirical and analytical formulae from the literature good correlation was obtained when plotting measured ripple length and length in dimensionless form as a function of the reynolds wave number rew under a given hydraulic condition it was observed that ripples with different geometric characteristics may coexist at different locations over the sandwave ripple steepness is presented as a function of the shields parameter although characterized with rather large scatter finally average ripple migration speed is presented as a function of the shields parameter and the mobility number

dynamical weakening of faults by acoustic fluidization when a fault slips seismically some of the energy released may excite strong short wavelength vibrations near the fault core such vibrations can temporarily reduce the normal stress on the fault allowing it to slip at lower shear stresses than predicted by laboratory coefficients of friction this phenomenon of acoustic fluidization provides an alternative to theories that invoke pressurized fluids as an explanation for why some faults appear to be so weak

pattern analysis of dune field parameters as a basic form of pattern analysis the parameters of dune spacing defect density crest orientation and crest length are measured from remote images and treated statistically for dunes at white sands in new mexico the algodones in california the agneitir in mauritania and the namib in namibia statistical populations are identified from frequency plots of dune spacing and crest length field scale calculations of defect density and rose diagrams of crest orientation single populations characterize simple dune fields white sands whereas multiple populations characterize compound complex dunes algodones namib and complex dune fields agneitir as time increases dune fields show an increase in dune spacing and crest length a decrease in defect density more tightly clustered crest orientation and a reduction in the variance associated with measurements of these parameters the results are consistent with models of dune fields as self organizing complex systems in which a characteristic pattern emerges as a function of constructional time because pattern evolution is a function of time it may be possible to use pattern analysis to augment current methods of age determination statistically defined populations can be used in geomorphic backstripping to unstack generations of simple patterns that give rise to complex patterns and to reconstruct each generation in terms of construction time and palaeo wind regime copyright 2006 john wiley sons ltd

a note on tidal asymmetry in the severn estuary a one dimensional non linear numerical model is used to investigate the tidally averaged frictional stress and set up of water level due to tidal asymmetry in the severn estuary these quantities depend on the overtides in the region a linearized model of the overtides is applied to calculations of the m4 currents in order to delineate the mechanisms responsible for their generation the relative importance of individual non linear mechanisms to the tidally averaged stress and set up is determined these mechanisms are interactions between tidal flow and changes in depth or breadth over a cross section frictional interaction between the tidal flow and stokes drift interaction between the tidal fluctuations in water depth and frictional retardation and non linear advection

a laboratory scale study on the role of mechanical vibrations in pore pressure generation in pyroclastic materials implications for pyroclastic flows the results of laboratory scale experiments with ignimbrite material are reported and discussed the ignimbrite material contained in a rotating drum is subjected to mechanical vibrations simulating particle agitation at the base of a pyroclastic flow propagating on an irregular substrate it is observed that vibrations lead to a severe decrease of internal friction evidenced by a drastic drop of the avalanche angle of the material in the rotating drum we propose that acoustic streaming a well known phenomenon in engineering applications operates in the ignimbrite material vibrations cause the development of rotational air flow around the particles and in consequence the enhancement of hydrodynamic viscous stresses promotes pore fluid pressure which leads to dynamical weakening through fluidization a fluid mechanical theoretical model accounts for the decrease of the friction coefficient of the ignimbrite material under a wide range of experimental conditions it can be reasonably inferred that sustained high gas pore pressure resulting from mechanical agitation could be present in pyroclastic flows and might contribute to their high mobility

late pleistocene and holocene dune activity and wind regimes in the western sahara desert of mauritania the western sahara desert in mauritania is dominated by extensive sand seas consisting largely of linear dunes analyses of landsat images geomorphic and stratigraphic studies and optically stimulated luminescence dating of dunes in the azefal agneitir and akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25 15 ka centered around the last glacial maximum 10 13 ka spanning the younger dryas event and after 5 ka the wind regimes that occurred during each of these periods were significantly different leading to the formation of dunes on three distinct superimposed trends northeast north northeast and north and the development of the sand seas as composite geomorphic features

automatic processing of high rate high density multibeam echosounder data 1 multibeam echosounders mbes are currently the best way to determine the bathymetry of large regions of the seabed with high accuracy they are becoming the standard instrument for hydrographic surveying and are also used in geological studies mineral exploration and scientific investigation of the earth s crustal deformations and life cycle the significantly increased data density provided by an mbes has significant advantages in accurately delineating the morphology of the seabed but comes with the attendant disadvantage of having to handle and process a much greater volume of data current data processing approaches typically involve computer aided human inspection of all data with time consuming and subjective assessment of all data points as data rates increase with each new generation of instrument and required turn around times decrease manual approaches become unwieldy and automatic methods of processing essential we propose a new method for automatically processing mbes data that attempts to address concerns of efficiency objectivity robustness and accuracy the method attributes each sounding with an estimate of vertical and horizontal error and then uses a model of information propagation to transfer information about the depth from each sounding to its local neighborhood embedded in the survey area are estimation nodes that aim to determine the true depth at an absolutely defined location along with its associated uncertainty as soon as soundings are made available the nodes independently assimilate propagated information to form depth hypotheses which are then tracked and updated on line as more data is gathered consequently we can extract at any time a current best estimate for all nodes plus co located uncertainties and other metrics the method can assimilate data from multiple surveys multiple instruments or repeated passes of the same instrument in real time as data is being gathered the data assimilation scheme is sufficiently robust to deal with typical survey echosounder errors robustness is improved by pre conditioning the data and allowing the depth model to be incrementally defined a model monitoring scheme ensures that inconsistent data are maintained as separate but internally consistent depth hypotheses a disambiguation of these competing hypotheses is only carried out when required by the user the algorithm has a low memory footprint runs faster than data can currently be gathered and is suitable for real time use we call this algorithm cube combined uncertainty and bathymetry estimator we illustrate cube on two data sets gathered in shallow water with different instruments and for different purposes we show that the algorithm is robust to even gross failure modes and reliably processes the vast majority of the data in both cases we confirm that the estimates made by cube are statistically similar to those generated by hand

aeolian and fluvial evidence of changing climate and wind patterns during the past 100 ka in the western simpson desert australia abstract sediments near finke in central australia provide evidence of late quaternary evolution and the interaction of aeolian and fluvial systems in response to changing climate in the western simpson desert thermoluminescence tl dating is used to develop a chronology of aeolian and alluvial activity and to identify differences in sand provenance quaternary alluviation in the finke valley at this location occurred at or prior to c 90 ka and with no surviving evidence of subsequent activity until the holocene in contrast to rivers in the eastern part of the lake eyre basin substantial alluviation took place here in the early to mid holocene probably due to reactivation of the northern monsoon and its partial penetration into central australia the regional dunefield near finke consists of linear dunes largely reworked and aligned during the last glacial 30 12 ka as part of the great anticlockwise whorl of dunes in central australia the oldest dated source bordering dunes from the finke river are bright red in colour were deposited at c 100 ka and are now buried beneath paler source bordering dunes the latter consist of two exposed units a lower one of unknown orientation that dates at 17 9 ka and an upper unit aligned to the northwest and parallel to the prevailing winds that dates at 5 0 ka the tl signature and sediment texture of the oldest source bordering dunes show them to have probably been derived from weathered alluvial red beds of similar or greater age nearby and to have contributed abundant sand to the regional dunefield immediately north of the river in contrast sand from the younger paler source bordering dunes appears to be from a different source and to have only recently extended into the regional field the finke region would seem to be in a pivotal position for the study of palaeowind patterns that have created and modified australia s whorl of continental dunes the 30 18 ka regional linear dunes near finke are oriented almost due north their cross sectional asymmetry steeper eastern slopes suggests a response to southwesterly or westerly sand transporting winds between 18 and 10 ka these winds appear to have shifted to their present southeast orientation during the past 5 ka from dune ages alignments and asymmetry it is proposed here that there was a northward shift in the wind pattern at finke by about 100 150 km 1 1 5 of latitude during the last glacial but that through this period the regional dune pattern has remained remarkably stable

dynamic friction in sheared fault gouge implications of acoustic vibration on triggering and slow slip friction and deformation in granular fault gouge are among various dynamic interactions associated with seismic phenomena that have important implications for slip mechanisms on earthquake faults to this end we propose a mechanistic model of granular fault gouge subject to acoustic vibrations and shear deformation the grain scale dynamics is described by the shear transformation zone theory of granular flow which accounts for irreversible plastic deformation in terms of flow defects whose density is governed by an effective temperature our model accounts for stick slip instabilities observed at seismic slip rates in addition as the vibration intensity increases we observe an increase in the temporal advancement of large slip events followed by a plateau and gradual decrease furthermore slip becomes progressively slower upon increasing the vibration intensity the results shed important light on the physical mechanisms of earthquake triggering and slow slip and provide essential elements for the multiscale modeling of earthquake ruptures in particular the results suggest that slow slip may be triggered by tremors

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

investigating parabolic and nebkha dune formation using a cellular automaton modelling approach vegetation plays an important role in shaping the morphology of aeolian dune landscapes in coastal and semi arid environments where ecogeomorphic interactions are complex and not well quantified we present a discrete ecogeomorphic aeolian landscape model decal capable of simulating realistic looking vegetated dune forms permitting exploration of relationships between ecological and morphological processes at different temporal and spatial scales the cellular automaton algorithm applies three simple rules that lead to self organization of complex dune environments including nebkhas with distinctive deposition tails that form in association with mesquite type shrubs and hairpin long walled parabolic dunes with trailing ridges that evolve from blowouts in association with vegetation succession changing the conditions of simulations produces differing landscapes that conform qualitatively to observations of real world dunes the model mimics the response of the morphology to changes in sediment supply vegetation distribution density and growth characteristics as well as initial disturbances the introduction of vegetation into the model links spatial and temporal scales previously dimensionless in bare sand cellular automata grid resolutions coarser than the representative size of the modelled vegetation elements yield similar morphologies but when cell size is reduced to much smaller dimensions the resultant landscape evolution is dramatically different the model furthermore demonstrates that the relative response characteristics of the multiple vegetation types and their mutual feedback with geomorphological processes impart a significant influence on landscape equilibria suggesting that vegetation induces a characteristic length scale in aeolian environments this simple vegetated dune model illustrates the power and versatility of a cellular automaton approach for exploring the effects of interactions between ecology and geomorphology in complex earth surface systems

multi scale variability of beach profiles at duck a wavelet analysis beach profiles have been observed to change over a range of spatial and temporal scales however techniques for quantifying this variability have not been fully established in this paper a wavelet technique is introduced as a method to study the multi scale variability of beach profiles the beach profile data comprising a 22 year time series surveyed at the us army corps of civil engineers field research facility frf at duck are analysed using the adapted maximal overlap discrete wavelet transform amodwt the analysis successfully identifies strong local features in the variability of beach profiles in time and space separately that cannot be isolated by traditional statistical methods the analysis of spatial wavelet variances provides a new means of investigating the depth of closure analysis of variances by temporal scales shows that the combined effects of several temporal scales with one or two dominant scales can be seen at particular points across profiles whilst the dominant temporal scales are different at different portions of the profiles the method allows for the extremely nonstationary behaviour of beach profile to be analysed into separate frequency bands that can facilitate the interpretation of morphological changes in terms of physical processes

simple models for the shape and symmetry of tidal sand waves 3 dynamically stable asymmetrical equilibrium forms without flow separation abstract the complex interaction between the local steepness of a sand bed and the tidally induced bedload transport over it is theoretically such that dynamically stable sand waves of asymmetrical cross section can exist beneath a tidal current in which one stream dominant exceeds in peak speed the other subordinate calculations based on a mathematical model embodying these effects show that as with naturally occurring sand waves the degree of sand wave morphological asymmetry increases with growing hydraulic asymmetry it being supposed as a compromise and for simplicity that the streams are of equal duration and simple harmonic in variation but of different peak speeds the calculated sand waves are translatory and their steeper side faces in the direction of the dominant tidal stream and the net bedload transport as would appear to be the case for naturally occurring forms however the precise form of the increase of morphological asymmetry with hydraulic asymmetry together with the bounds on the existence field for sand waves is influenced by the magnitude of the bed material entrainment threshold relative to the peak speeds of the tidal streams the main limitations on the model are its severely simplified treatment of flow and bedload transport over sand waves and the lack of empirical data by which to test the proposed bed steepness effects

the use of new elevation data srtm aster for the detection and morphometric quantification of pleistocene megadunes draa in the eastern sahara and the southern namib abstract new freely available digital elevation data derived from the shuttle radar topography mission srtm 3 and the advanced spaceborne thermal emission and reflection radiometer aster allow for the first time the use of area wide high resolution elevation models they are useful for various investigations especially in regions that are devoid of detailed topographic maps in this contribution the geomorphological sedimentological geochronological and archaeological results of the cologne collaborative research centre acacia are presented showing that the southern great sand sea of egypt eastern sahara is composed of a complex dune pattern of pleistocene megadunes draa and holocene dunes a conceptual model is presented suggesting draa formed by a strong unimodal trade wind regime that acted most probably as helical roll vortices the draa differ conspicuously in their dimensions and in their evolution from the smaller longitudinal dunes which arise from the recent bimodal wind regime first results of comparative investigations in the southern namib erg namibia yielded similar conditions and a synchronous landscape development based on these results the second part of the paper evaluates the elevation data for both regions with respect to their availability for the determination of draa and dunes and the morphometric quantification of the draa due to their different resolution srtm 3 90 m aster 30 m and their different sensor technology srtm 3 radar aster stereo data both models have assets and drawbacks but can complement each other with the calculation of geomorphometric parameters elevation slope the aster digital elevation model allows the location of the draa of the great sand sea of egypt the quantification of their spatial distribution size volume and consequently the holocene dune movement in conclusion the resolution of the aster elevation model is applicable for quantitative analyses of well defined morphological elements with altitudinal variations 40 50 m the volume estimation presented provides a tool for a realistic assessment of the sand mass balance of sand seas this is not possible with the srtm 3 model and generally impossible with both models if the altitudinal variation is

aeolian dune field self organization implications for the formation of simple versus complex dune field patterns abstract the interpretation of aeolian dune field patterns as self organizing complex systems is a new paradigm in which pattern evolution may be addressed computer simulations supported by field and experimental data indicate that a given wind regime produces a simple dune field pattern dune type and crest orientation are determined by wind regime and pattern ordering occurs through dune dune interactions over time because dunes reorient only at their crest terminations with a change in wind regime the rate of formation of a new pattern of small dunes is typically faster than the rate of reorientation of the existing pattern resulting in the superposition of simple patterns to give rise to complex patterns complex patterns are distinct from spatial changes in a simple pattern and from the type of superposition that characterizes compound complex dunes complex patterns necessarily indicate a rate of pattern formation that is rapid compared to the rate of sediment accumulation on the depositional surface

tidal bore dynamics in funnel shaped estuaries the formation and dynamics of tidal bores in funnel shaped estuaries is investigated from both a global tidal wave scaling analysis and new quantitative field observations we show that tidal bore occurrence in convergent estuaries can be estimated from a dimensionless scaling parameter characterizing the relative intensity of nonlinear friction versus local inertia in the momentum equation a detailed analysis of tidal bore formation and secondary wave structure is presented from a unique long term database observations of more than 200 tides acquired during four campaigns in the two main french tidal bore estuaries the seine and gironde garonne estuaries we describe the effect of freshwater discharge on the global tidal wave transformation at the estuarine scale and on local tidal bore occurrence in the upper estuary our field data suggest that the tidal bore intensity is mainly governed by the dimensionless tidal range which characterizes the local tidal wave nonlinearity we also show that the secondary wavefield associated with tidal bore propagating in natural estuaries differs significantly from those associated to undular bores in rectangular channels in particular we observe an abrupt decrease of the whelp steepness when the froude number goes below 1 1 this secondary field transition can explain why tidal bore occurrence in worldwide estuaries is certainly underestimated

a prescriptive model of stratification induced by freshwater runoff abstract stratification induced by freshwater buoyancy input constitutes a key determinant of the environment both in estuaries and adjacent regions of freshwater influence rofis predicting the development and breakdown of estuarine stratification is inherently more difficult than the prediction of thermal stratification because the buoyancy input is localized at the lateral boundaries rather than being uniformly distributed over the surface the primary processes responsible for controlling water column stability have been identified in a previous paper simpson et al 1990 estuaries 12 125 132 as i the estuarine circulation driven by horizontal density gradients ii stirring by tidal and wind stresses and iii straining of the density field by tidal shear these processes are incorporated in a prescriptive model of the time evolution of stratification which utilizes analytical solutions for the tidal and density current profiles specifies the mixing in terms of energy conditions previously used in the heating stirring models and assumes a horizontal density gradient which is invariant with depth the behaviour of this simple model is described and compared with recent time series observations of the variability of stratification in a tidally energetic rofi which indicate the presence of strong semi diurnal and semi monthly cycles in water column stability the model reproduces the main qualitative features of the observations but is restricted as a predictive tool by the constant density gradient assumption and the omission of the interaction between the tidal and density driven flows the inclusion of this latter interaction which occurs via the frictional stresses and the control of eddy viscosity by stratification and the turbulent energy intensity requires a dynamically active model which specifies the feedback processes in terms of an appropriate closure scheme

a study of non linear tidal propagation in shallow inlet estuarine systems part i observations the offshore tide becomes strongly distorted as it propagates into shallow estuarine systems observations of sea surface elevation and horizontal currents over periods ranging from three days to one year at nine stations within nauset inlet estuary document the non linear interaction of the off shore equilibrium tidal constituents despite strong frictional attenuation within the estuary the overtides and compound tides of m2 s2 and n2 in particular reach significant amplitude resulting in strong tidal distortion high frequency forced constituents in sea surface are phase locked consistently leading the forcing tides by 60 70 resulting in a persistent distortion where falling tide is longer than rising tide forced constituents in currents are more nearly in phase with equilibrium constituents producing flood currents which are shorter but more intense than ebb currents a compound fortnightly tide msf modulates the mean water level such that lowest tides occur during neap phase instead of spring phase this fortnightly tide can be contaminated by storm surge changing the phase characteristics of this constituent implications of the overtides compound tides and lower frequency tides on near bed suspended and dissolved material transport are profound

laboratory observation of acoustic fluidization in granular fault gouge and implications for dynamic weakening of earthquake faults 1 several lines of evidence including remote triggering of earthquakes and modulation of seismic tremor by earth tides suggest that faults weaken when subject to shaking and dynamic stresses associated with the passage of seismic waves however the origin of such dynamic weakening is poorly understood here we explore the role of acoustic resonance for dynamic fault weakening using laboratory measurements experiments were conducted using a split hopkinson pressure bar assembly with dynamic stressing via impact loading samples were composed of crushed rock particles from mine tailings with a particle size distribution similar to that found in a natural fault gouge we used pulse shaper techniques and carefully evaluated dynamic stresses recorded at the front and rear of the sample to ensure that dynamic force balance was satisfied our experiments document acoustic induced fluidization and dramatic dynamic weakening frictional strength and elastic modulus of a simulated fault gouge are reduced by a factor of 5 10 via acoustic fluidization we find a threshold acoustic pressure for fluidization that varies systematically with gouge zone properties our observations could help explain dynamic fault weakening and triggering of earthquake fault slip by dynamic stressing

eolian dunes computer simulations and attractor interpretation a simple computer simulation algorithm for the transport of sand by wind produces forms resembling barchan crescentic ridge linear and star natural dune classes sand is moved as slabs composed of many grains that are picked up at random transported in a specified direction and deposited 1 with a probability that depends on the local presence or absence of sand or 2 in shadow zones in the lee of dunes the simulated dune fields are interpreted as complex systems with sand dune classes being dynamical attractors of these systems the evolution of dunes once formed becomes decoupled from the details of eolian sand transport

a method for semi automated objective quantification of linear bedforms from multi scale digital elevation models the increasing availability of large detailed digital representations of the earth s surface demands the application of objective and quantitative analyses given recent advances in the understanding of the mechanisms of formation of linear bedform features from a range of environments objective measurement of their wavelength orientation crest and trough positions height and asymmetry is highly desirable these parameters are also of use when determining observation based parameters for use in many applications such as numerical modelling surface classification and sediment transport pathway analysis here we 1 adapt and extend extant techniques to provide a suite of semi automatic tools which calculate crest orientation wavelength height asymmetry direction and asymmetry ratios of bedforms and then 2 undertake sensitivity tests on synthetic data increasingly complex seabeds and a very large scale 39 000 km2 aeolian dune system the automated results arecompared with traditional manually derived measurements at each stage this new approach successfully analyses different types of topographic data from aeolian and marine environments from a range of sources with tens of millions of data points being processed in a semi automated and objective manner within minutes rather than hours or days the results from these analyses show there is significant variability in all measurable parameters in what might otherwise be considered uniform bedform fields for example the dunes of the rub al khali on the arabian peninsula are shown to exhibit deviations in dimensions from global trends morphological and dune asymmetry analysis of the rub al khali suggests parts of the sand sea may be adjusting to a changed wind regime from that during their formation 100 to 10 ka bp

flow effects on the morphology and dynamics of aeolian and subaqueous barchan dunes 1 barchans are aeolian sand dunes that form where unidirectional winds blow on firm ground with limited sand supply although they have been analyzed for decades the difficulty of conducting experiments in the field due to the large lengthscale and timescale involved is a considerable obstacle to understand their dynamics however crescentic patterns also appear underwater on a much smaller scale allowing easier study it is shown here that down scaled solitary barchans can be created in the laboratory using the periodic and asymmetric motion of a plate underwater to simulate the effect of a unidirectional flow those subaqueous barchans compare very well to their aeolian relatives their speeds scale as the cube of the speed of the flow times the inverse of their typical dimension finally the exploration of new topics such as the influence of the directionality of the wind or that of binary collisions illustrate that barchans can eventually adapt to any external perturbations by emitting one or several small barchans

deformation at the frictional viscous transition evidence for cycles of fluid assisted embrittlement and ductile deformation in the granitoid crust abstract mid crustal deformation is classically characterized by the transition from ductile to brittle deformation defining the frictional to viscous transition fvt here we investigate an exhumed continental mid crustal basement section in order to envisage the relationship between ductile and brittle deformation at the fvt our detailed study from km to micro scale shows that under greenschist metamorphic conditions deformation is accommodated by a dense network of highly localized ductile shear zones in the investigated case it is not quartz which defines the overall ductile deformation behavior but the viscous granular deformation in shear zones with an ultrafine grained polymineralic matrix consisting of quartz feldspar sheet silicates and epidote during viscous granular flow mass transfer processes under the presence of fluids promote a chemo mechanical mixing resulting in grain size reduction and reaction softening coeval with this ductile deformation fluid assisted embrittlement occurs as indicated by biotite coated fractures cataclasites and injection of non cohesive polymineralic gouge material into secondary fractures inside the host rock the embrittlement during predominant ductile deformation occurs in cycles i e prolonged periods of slow viscous granular flow are interrupted by rapid brittle deformation we interpret this fluid assisted cyclic embrittlement evidenced by injection of the fluidized material into off fault fractures as an alternative equivalent to pseudotachylites and as a microstructural indicator for paleo seismic activity with exhumation and associated cooling localized deformation persists in the ultrafine grained polymineralic shear zones but progressively transitions to cataclastic flow and finally to pressure dependent frictional flow always showing cycles of slow interseismic flow and fast seismic injection events overall in the granitic crust of the aar massif brittle and ductile deformation coexist up to deformation temperatures of minimum 450 c indicating that the fvt has to be placed in a rather wide range from 8 km up to 18 20 km in the granitoid crust

aeolian dune field pattern boundary conditions aeolian dune field patterns reflect the complex external environment within which the pattern evolves these external environmental controls are the boundary conditions on aeolian dune field pattern formation the influences of boundary conditions such as wind regime and sediment supply are well known however boundary conditions represent a relatively unexplored area of dune field pattern formation source area geometry and areal limits are two newly recognized boundary conditions measurements of crest spacing and crest length from satellite images of dune field patterns with point and line source area geometries show an increase in spacing and crest length over distance whereas spacing and crest length in plane sourced patterns emerge equally across the dune field the impact of the size and shape of a dune field on crest spacing is tested using a previously established analytical model model results indicate that the area of a dune field limits the maximum spacing that can occur within a given area and that dune fields that are five times longer in the direction of dune migration than in the crest parallel direction can achieve the greatest spacing within a given area empirical measurements of spacing defect density and dune field area from ten different dune fields ranging over four orders of magnitude in size show that spacing increases and defect density decreases as dune field area increases because the formation of all dune fields involves a wind regime and sediment supply there must be interplay of boundary conditions within the same dune field at different stages of pattern development one boundary condition may dominate over others and with changing climatic eustatic or tectonic parameters boundary conditions may change to modify an existing pattern recognizing boundary condition controls on aeolian dune field pattern formation provides a framework for recognizing the signature of the external environment in which a pattern developed and can be used for reconstructing past dune constructional events and climatic change