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

circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

on the subsurface cfc maxima in the subtropical north pacific thermocline and their relation to mode waters and oxygen maxima the spatial structure of the subsurface chlorofluorocarbon cfc concentration maxima in the subtropical north pacific is examined using data from the pacific ocean sector of the world ocean circulation experiment woce hydrographic program collected in the period 1991 1994 comparison with potential vorticity pv sections reveals that the cfc maxima coincide with the pv minimum associated with subtropical mode water stmw in the western pacific as suggested by earlier observations warner et al 1996 in the eastern part of the central pacific the cfc maxima appear to be shallower than the central mode water cmw while they are deeper than the eastern subtropical mode water in the eastern pacific a subsurface oxygen maximum also coincides with stmw and the cfc maxima in the west is located between the cfc maxima and cmw in the central pacific and is absent in the east the collocation in the west is explained by the fact that stmw is the densest water to outcrop in the recirculation to the south of the kuroshio and the kuroshio extension while denser water is not directly ventilated in this region data from two post woce cruises and results from a steady state age model indicate that the cfc maxima are transient and are deepening such that the collocation of the cfc maxima and stmw does not persist in the late 1990s model experiments based on a smooth age profile demonstrate that the deepening of the cfc maxima is closely related to changes in the atmospheric cfc growth rates

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

shear at the base of the oceanic mixed layer generated by wind shear alignment observations are used to evaluate a simple theoretical model for the generation of near inertial shear spikes at the base of the open ocean mixed layer when the upper ocean displays a two layer structure the model predicts that large changes in shear squared can be produced by the alignment of the wind and shear vectors a climatology of stratification and shear variance in drake passage is presented which shows that these assumptions are most applicable to summer fall and spring but are not highly applicable to winter temperature salinity and velocity data from a high spatial resolution cruise in drake passage show that the model does not predict all large changes in shear variance the model is most effective at predicting changes in shear squared when it arises owing to near inertial wind driven currents without requiring a rotating resonant wind stress the model is also more effective where there is a uniform mixed layer above a strongly stratified transition layer rotary spectral and statistical analysis of an additional 242 drake passage transects from 1999 to 2011 confirmed the presence of this shear spiking mechanism particularly in summer spring and fall when stratification is stronger

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

warming and weakening of the abyssal flow through samoan passage abstractthe abyssal flow of water through the samoan passage accounts for the majority of the bottom water renewal in the north pacific thereby making it an important element of the meridional overturning circulation here the authors report recent measurements of the flow of dense waters of antarctic and north atlantic origin through the samoan passage a 15 month long moored time series of velocity and temperature of the abyssal flow was recorded between 2012 and 2013 this allows for an update of the only prior volume transport time series from the samoan passage from woce moored measurements between 1992 and 1994 while highly variable on multiple time scales the overall pattern of the abyssal flow through the samoan passage was remarkably steady the time mean northward volume transport of about 5 4 sv 1 sv 106 m3 s 1 in 2012 13 was reduced compared to 6 0 sv measured between 1992 and 1994 this volume transport reduction is significant within 68 confidence limits 0 4 sv but not at 95 conf

east australian current volume transports at 30 s estimates from the world ocean circulation experiment hydrographic sections pr11 p6 and the pcm3 current meter array between september 1991 and march 1994 the oceanic region off the east coast of australia at 30 s was the subject of an intense observational program part of the world ocean circulation experiment the objective was to improve our understanding of the dynamics of the east australian current eac and to measure its volume and energy flux the two main components were the pacific current meter array 3 pcm3 with six moorings spanning a total distance of 120 km and repeat high resolution hydrographic surveys the long term average frompcm3 shows the eac as a narrow swift and highly variable current centered at 40 km from the coast with strong shear in the upper 1000 m and mean southward velocities of 0 6 m s 1 in the core the measurements also revealed a thick countercurrent underneath the poleward flow the mean volume transport for the entire section covered by the array between the coast and 154 4 e was 22 1 4 6 sv toward the south with an rms variability of 30 sv combining the data from the pcm3 array with the hydrographic sections generated nine detailed snapshots of the absolute geostrophic current field the snapshots were used to evaluate the reliability of the pcm3 data for determining volume transports the rms difference between transports derived from direct current observations and from hydrographie data was 3 8 sv indicating that the pcm3 array can be used to describe the current variability despite the sparse spatial distribution of velocity measurements transport variability was dominated by periods between 90 and 140 days which are attributed to eddy shedding in the region occasional intense northward flow events were observed where total transports reached up to 50 sv northward on those occasions the southward boundary current changed from being surface intensified and mainly baroclinic to northward and more barotropic related to the northward migration past the array of the eac coastal separation point

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

from tabular to rhombohedral dolomite crystals in zechstein 2 dolostones from scharzfeld sw harz germany a case study with combined cl and ebsd investigations abstract tabular dolomite crystals found within dolomite rhombs have been investigated by cathodoluminescence cl microscopy and spectroscopy combined with electron backscatter diffraction ebsd for the first time the dolomites formed in the upper permian stassfurt carbonate ca2 at the southern margin of the german polish zechstein basin cathodoluminescence petrography of the dolostone succession revealed that the dolomites developed in four phases electron backscatter diffraction analysis reveals tabular crystal growth during the two first generations while the last two generations are characterized by rhombohedral crystal shapes the tabular dolomite cement crystals and their microcrystalline equivalents in matrix and components have a stoichiometric composition with good to very good lattice ordering manganese and iron contents of the tabular crystals are low and their carbon and oxygen isotope composition confirms an early diagenetic dolomite formation under marine evaporitic conditions from precursor carbonates of upper permian age cl spectroscopy reveals that the tabular dolomite generation 1 has a very high percentage of mn2 on the ca lattice position which results in a visually yellowish green cl emission although relatively increased mn2 contents at the ca lattice position appear to be rather common in evaporitic dolomites the combination of a tabular crystal shape and a preferred input of mn2 at the ca lattice position is a remarkable phenomenon as tabular dolomite crystals so far are exclusively reported from evaporitic diagenetic settings they could be the result of a high mg ca ratio which blocks c axis orientated growth of dolomite crystal the occurrence of well ordered dolomite of which the geochemical zoning can be studied in such detail is rare for the earliest synsedimentary stages of dolomite formation in marine environments because these early stages commonly consist of not or badly ordered ca dolomites a primary geochemical zoning of such dolomite usually gets lost during stabilisation and transformation to better ordering and stoichiometry

hydrography of the eastern south pacific ocean results from the sonne 102 cruise may june 1995 abstract full depth conductivity temperature depth hydrographic measurements were taken in may june 1995 along 28 15 35 15 and 43 15 s from the chilean coast to 86 88 w from the r v sonne sections of potential temperature salinity neutral density and oxygen are presented to illustrate the water column structure in this sparsely sampled region a more comprehensive database on all available hydrographic data from the region was compiled these data and available meteorological data are used to help in the interpretation of the sonne results a number of important features found in the sections are discussed including a shallow salinity minimum apparently ventilated from south of 37 s salt fingering as a possible mechanism to transport high salinity subtropical water into the shallow salinity minimum equatorial subsurface water subantarctic mode water and antarctic intermediate water geostrophic calculations of alongshore flow show the poleward peru chile undercurrent at each of the three sections with a poleward descending core and peak velocities of 10 cm s 1 as in earlier works a deep pacific outflow is found near the chilean coast part of a clockwise deep circulation in the chile basin

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

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

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

sources to the east greenland current and its contribution to the denmark strait overflow data from the east greenland current in 2002 are evaluated using optimum multiparameter analysis the current is followed from north of fram strait to the denmark strait sill and the contributions of different source waters in mass fractions are deduced from the results it can be concluded that at least in spring 2002 the east greenland current was the main source for the waters found at the denmark strait sill contributing to the overflow into the north atlantic the east greenland current carried water masses from different source regions in the arctic ocean the west spitsbergen current and the greenland sea the results agree well with the known circulation of the western nordic seas but also add knowledge both to the quantification and to the mixing processes showing the importance of the locally formed greenland sea arctic intermediate water for the east greenland current and the denmark strait

optimal estimation of hydrographic station data and derived fields abstract optimal estimation is applied to contouring and analysis of hydrographic sections measured fields such as temperature and salinity and derived fields such as geostrophic velocity are decomposed into large scale and small scale components first a heavily sampled basin scale field is estimated and subtracted from the data a small scale field barely resolved by the hydrography is then found from the residuals the power and utility of the technique are illustrated by means of examples using a short section from the straits of florida and a transatlantic section along 24 n

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

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