winter cascading of cold water in lake geneva 1 during the winters of 1998 and 1999 observations were made of the cascading of cold water from the nearshore shallow shelf zones and down the sloping sides of lake geneva cascading starts on the average 10 hours after the onset of surface cooling the draining cold water descends like a gravity current and the downslope speed of the head of these slugs of cold water u has a mean value of 5 2 cm s 1 with slugs persisting on the average for 8 hours when the monin obukov length scale at the water surface l is negative implying convection occurs and d over bar lg 1 where d is the mean shelf depth the nondimensionalized speed of the front of slugs u b 1 3 is found to be 1 3 0 4 where b is the surface buoyancy flux integrated over the time period from one slug to the next each slug is unsteady the head being followed by several fronts in which the temperature of the current decreases and its thickness increases these fronts travel faster than the mean flow by a factor of r 1 38 0 3 dynamical similarities are found with roll waves observed in turbulent open channel flows the circulation induced by the cascade is found to give a positive skewness to the time derivatives of near surface temperature in shallow waters in contrast with negative values close to the slope the volume of cold water carried by a slug increases with downslope distance as a consequence of turbulent entrainment and the contribution of convectively unstable plumes from the surface the average volume carried by the slug across the 21 m depth contour is about 1 9 times the volume of water in shallower water i e that on the shelf between shore and a depth of 21 m implying that cascading is an efficient means of flushing shelf water integrated around the lake the mean total volume flux amounts to 11 5 the average winter river inflow

flow adjustment and interior flow associated with a rectangular porous obstruction the flow at the leading edge and in the interior of a rectangular porous obstruction is described through experiments and scaling the porous obstruction consists of an emergent rectangular array of cylinders in shallow flow a configuration that mimics aquatic vegetation the main features of the flow depend upon the non dimensional canopy flow blockage which is a function of the obstruction width and porosity for the ranges of canopy flow blockage tested in this paper the fluid decelerates upstream of the obstruction over a length scale proportional to the array width for high flow blockage the interior adjustment length within the porous obstruction is set by the array width for low flow blockage the array s frontal area per unit volume sets the interior adjustment length downstream of the adjustment regions the interior velocity is governed by a balance between the lateral divergence of the turbulent stress and canopy drag or by a balance between the pressure gradient and canopy drag depending on the lateral penetration into the array of kelvin helmholtz kh vortices which is set by the non dimensional canopy flow blockage for a porous obstruction with two stream parallel edges the kh vortex streets along the two edges are in communication across the width of the array a phenomenon that results in cross array vortex organization which significantly enhances the vortex strength and creates significant lateral transport within the porous obstruction

applying the mixing layer analogy for flow resistance evaluation in gravel bed streams abstract previous studies show that the flow velocity profile is s shaped for low submergence flows in gravel bed streams as a consequence the power or logarithmic flow resistance equations cannot be theoretically deduced for a low submergence flow in this study the mixing layer analogy in an accurate and innovative way is applied to express the flow resistance in gravel bed streams laboratory experiments over 12 uniform gravel bed runs the relative submerged height ranging from 2 6 to 4 6 are used to determine the physical rationality and accuracy of the analogy that statistical flow turbulent properties are closer to typical values in the mixing layer than in the surface layer the key parameters of shear strength ls applied by an incomplete self similarity analysis for the dimensionless groups and difference coefficient cu defined as the ratio of velocity at the reference level to shear velocity deduced by the simplified darcy forchheimer brinkman equation are expressed and tested using experimental data the new approach is further underpinned by using 104 field measured reaches located in 40 different natural streams which adds to the more widely ranged field data with the relative submerged height ranging from 0 9 to 20 the analysis shows that the flow layer subdivisions theoretical approach based on the multivariable mixing layer analogy and optimized katul law characterized by the lowest root mean square error relative error and closest 1 nash sutcliffe index has potential in future research

depth averaged drag coefficient for modeling flow through suspended canopies aquatic suspended canopies are porous obstacles that extend down from the free surface but have a gap between the canopy and bed examples of suspended canopies include those formed by aquaculture structures or floating vegetation the major difference between suspended canopies and the more common submerged canopies which are located on the bottom boundary is the influence of the bottom boundary layer beneath the suspended canopy data from laboratory experiments are presented which explore aspects of the flow through and beneath suspended canopies constructed from rigid cylinders the experiments using both acoustic doppler and two dimensional 2d particle tracking velocimetry give details of the flow structure that may be divided vertically into a bottom boundary layer a canopy shear layer and an internal canopy layer the experimental data show that the penetration of the shear layer into the canopy is limited by the distance between the canopy and bottom boundary layer peaks in velocity spectra indicate an interaction between the bottom boundary and canopy shear layer an analytical model is also developed that can be used to calculate a drag coefficient that includes the effect of both canopy drag and bed friction this drag coefficient is suitable for use in 2d depth averaged hydrodynamic modeling the model also allows the average velocity within and beneath the canopy to be calculated and is used to investigate the effect of canopy density and thickness on both total drag and bottom friction

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

prognostic modeling studies of the keweenaw current in lake superior part i formation and evolution abstract the formation and evolution of the keweenaw current in lake superior were examined using a nonorthogonal coordinate primitive equation numerical model the model was initialized by the monthly averaged temperaturefield observed in june and september 1973 and run prognostically under different forcing conditions with and without winds as a rossby adjustment problem the model predicted the formation of a well defined coastal current jet within an inertial period of 16 4 h after the current field adjusted to the initial temperature field the magnitude and direction of this current jet varied with the cross shelf temperature gradient and wind velocity it tended to intensify during northeastward downwelling favorable winds and to lessen or even reverse during southwestward to northwestward upwelling favorable or southeastward downwelling favorable winds in a case with strong stratification and without external atmospheric forcings a well defined clockwise warm core eddy formed near the

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

flexural rigidity and shoot reconfiguration determine wake length behind saltmarsh vegetation patches vegetation patches play an important role in controlling sediment deposition in shallow aquatic environments such as coastal saltmarshes and fluvial systems however predicting deposition around vegetation patches is difficult due to the complexity of patch morphology and their dynamic interaction with the flow here we incorporate a biomechanical model parameterized using field data within a 3d computational fluid dynamics cfd model which allows prediction of individual shoot reconfiguration within patches due to flow forcing the model predicts velocity attenuation and bed shear stresses within the wake of the patch which agree spatially with accretion patterns measured in the field using terrestrial lidar the model is applied to sparse patches of suaeda maritima located in saltmarshes of coastal habitats to explore the role of i shoot distribution ii patch geometry iii shoot flexural rigidity and iv bulk flow velocity in determining the length of the predicted wake region we demonstrate that for suaeda maritima with intermediate rigidity the vertical shear layer over the vegetation controls the length of the predicted wake region consequently reconfiguration due to flexural rigidity strongly impacts on wake length confounding the relationship between patch height and wake length a simplified model for predicting wake length based on shoot reconfiguration is applied to the simulation data and shows good agreement the results demonstrate that the observed wake characteristics can be well explained by intraspecific variability in flexural rigidity thus demonstrating the importance of biomechanical traits in determining flow vegetation sediment interactions

improved parameterization of seagrass blade dynamics and wave attenuation based on numerical and laboratory experiments seagrass blade dynamics were explored through numerical and laboratory experiments in order to improve parameterization of wave attenuation by submerged aquatic vegetation in the presence of a background current in the numerical model a single blade was modeled as a series of rigid plates attached by torsion springs for the laboratory model strips of low density polyethylene were placed in a recirculating wave flume a new form of the keulegan carpenter number based on the horizontal excursion of the blade tip was found to be an excellent predictor of drag coefficient an algebraic model for predicting wave attenuation was developed based on the following observations during the portion of the wave period when the fluid velocities are highest the blade motion is almost completely arrested and the vast majority of the turbulence production occurs during this time turbulence production when the blade is pronated is accurately predicted by the maximum fluid velocity over the wave period the relative contribution to the total turbulence production over the wave period is determined by the relative strength of the waves and the current therefore using a simple algebraic fit the total depth integrated time averaged turbulence production can be accurately predicted by two flow parameters the maximum fluid velocity over the wave period and a non dimensional number that compares the wave and current velocities by fitting the algebraic model to data from a particular site it can be used to efficiently estimate wave attenuation and drag coefficient in seagrass exposed to waves with a background current

frictional strength and wear rate of carbonate faults during high velocity steady state sliding article history we ran an extensive series of shear experiments to test the effect of shear velocity and normal stress on wear rate and frictional strength the experiments were conducted on three types of carbonate samples with a rotary shear apparatus on solid ring shaped rock samples that slipped for displacements up to tens of meters at slip velocity of v 0 002 0 96 m s and normal stress n 0 25 6 9m pa the analysis reveals that during steady state stage the values of wear rate and frictional strength depend on both slip velocity and normal stress the wear rates at low slip velocity show linear relations to the normal stress archard s model however at high velocity v 0 5m s the wear rates are independent of the normal stress and may vanish at the highest velocity and normal stress of the present experiments the steady state friction coefficient correlates best with the experimental power density shear stress slip velocity we recognized three friction regimes high 0 8a t low power density low 0 3 at high power density and a transition regime of fast drop of friction coefficient as the power density increases from 0 03 to 0 3 mw m 2 runs of low power density high friction displayed fault surfaces covered with fine grained gouge whereas runs of high power density low friction displayed shiny smooth fault surfaces we interpret the observed intensity variations of wear rate and frictional strength as indicating a brittle to ductile transition associated with frictional heating

processes of deep water renewal in lake baikal deep water renewal in lake baikal siberia the world s deepest lake and largest lake by volume is relatively fast water age calculated from tritium and helium as well as from chlorofluorocarbons does not exceed 19 yr relative saturation of dissolved oxygen typically exceeds 80 the equation of state of baikal water was determined including the effect of dissolved ions and silicic acid based on nearly 600 ctd profiles taken between 1993 and 1995 two mechanisms of deep water mixing were identified 1 in spring cold and relatively saline water from the selenga the major inflow to the lake forms a density plume that reaches the bottom of the central basin during april and early may due to entrainment of lake water the plume transports about 125 km of water per year to the deepest part of the basin later in spring the river water forms the thermal bar observed along the eastern shore there are indications that parts of the selenga are also plunging to the deep part of the southern basin 2 at academician ridge separating the cold and saline water of the central basin from the warmer and slightly less saline water of the northern basin horizontal mixing results in a water mass that can sink on either side of the sill whereas in the central basin the water mass stays at intcrmediatc depth in the northern basin it sinks to the decpcst part more detailed data are needed to quantify this density flux no indication of a wind induced thermobaric instability was found

experimental study on vegetation flexibility as control parameter for wave damping and velocity structure abstract vegetation can contribute to coastal defence by damping incoming waves however prior studies have shown that attenuation varies greatly among plant species plant flexibility is a mechanical property that is commonly omitted but varies considerably between shrubs and grasses on salt marshes therefore we present an experimental study in a laboratory wave flume with artificial vegetation that differs in flexibility only we measured wave attenuation and water particle velocities around rigid and flexible salt marsh vegetation waves were measured using a series of gauges and particle image velocimetry piv was used to measure spatio temporal variations of water particle velocities in the x z plane around the vegetation our results show that flexible vegetation attenuates waves up to 70 less than rigid vegetation due to swaying of flexible plants furthermore we find that rigid vegetation modifies the velocity structure whereas flexible vegetation does not specifically a mean current in the direction of wave propagation develops around the canopy and the horizontal particle velocities are amplified directly above the canopy these results indicate that plant flexibility is a key parameter in the wave vegetation interaction that controls wave damping and velocity structure

geometrical and mechanical properties of four species of northern european brown macroalgae abstract aquatic vegetation interacts with the flow by posing an obstruction and in return experiences drag and is reconfigured by acting forces it is suggested that plant buoyancy and stiffness affect these bio physical interactions and hence should be considered when including vegetation in physical and numerical hydrodynamic models however data on these parameters is either lacking for many species or existing information is insufficient to model flexible aquatic vegetation correctly previous studies have focused on plant stems but did not take plant foliage into account the present study extends the existing knowledge base by providing data for four northern european brown macroalgae and also provides a comparison between the mechanical properties of stem and blade tissue for these species specimens of alaria esculenta laminaria digitata fucus serratus and fucus vesiculosus were collected from a small tidal inlet in norway other than f serratus the macroalgae were positively buoyant and all stems showed bending moduli in agreement with previous studies with respect to their order of magnitude only l digitata exhibited a significant difference between bending moduli for stems and blades and also between the tip and the base of the blades however there were differences in flexural rigidities product of bending modulus and second moment of cross sectional area of both stems and blades and the tip and the base of blades of all but f serratus this highlights the important influence of cross sectional area and cross sectional shape on stiffness

sediment transport and shear stress partitioning in a vegetated flow vegetation is a common feature in natural coastal and riverine water ways interacting with both the water flow and sediment transport however the physical processes governing these interactions are still poorly understood which makes it difficult to predict sediment transport and morphodynamics in a vegetated environment we performed a simple experiment to study how sediment transport responds to the presence of flexible single blade vegetation and how this response is influenced by the vegetation density we found that the skin friction and sediment transport are reduced in a plant patch and that this effect is larger for denser vegetation we then evaluated several methods to calculate the skin friction in a vegetated flow which is the key to sediment transport prediction among these the inversion of bed load transport formulas and the einstein and banks 1950 methods appeared to produce the most reasonable values of the skin friction finally we suggest using the parameter which is the ratio of the skin friction computed by these methods to the total bed shear stress to make more realistic sediment transport predictions in morphodynamic models

shear failure of a granite pin traversing a sawcut fault abstract fault heterogeneities such as bumps bends and stepovers are commonly observed on natural faults but are challenging to recreate under controlled laboratory conditions we study deformation and microseismicity of a 76 mm diameter westerly granite cylinder with a sawcut fault with known frictional properties an idealized asperity is added by emplacing a precision ground 21 mm diameter solid granite dowel that crosses the center of the fault at right angles this intact granite pin provides a strength contrast that resists fault slip upon loading to 80 mpa in a triaxial machine we first observed a m 4 slip event that ruptured the sawcut fault slipped 40 m but was halted by the granite pin with continued loading the pin failed in a swarm of thousands of m 6 to m 8 events known as acoustic emissions aes once the pin was fractured to a critical point it permitted complete rupture events m 3 on the sawcut fault stick slip instabilities subsequent slip events were preceded by clusters of foreshock like aes all located on the fault plane and the spatial extent of the foreshock clusters is consistent with our estimate of a critical nucleation dimension h we also identified an aseismic zone on the fault plane surrounding the fractured rock pin a post mortem analysis of the sample showed a thick gouge layer where the pin intersected the fault suggesting that dilatancy of this gouge propped open the fault and prevented microseismic events in its vicinity recorded microseismicity separates into three categories slip on the sawcut fault fracture of the intact rock pin and off fault seismicity associated with pin related rock joints we found that pin fracture events were exclusively implosive anticrack even though the shear process zone was overall dilatant this shows how aseismic effects can lead to unexpected seismic manifestations of certain faulting processes

deep water renewal in lake issyk kul driven by differential cooling during two field campaigns on lake issyk kul in march and august 2001 179 high resolution conductivitytemperature depth profiles were measured along with profiles of dissolved oxygen and light transmission on the basis of this extensive data set we investigated the large scale vertical advective processes responsible for deepwater renewal in lake issyk kul at some locations sampled in march variable peak structures in the tracer profiles were observed that indicate horizontally localized intrusions these intrusions have their origin in density plumes that propagate along the bottom of the channels in the eastern shelf region the cold dense water at the bottom of the channels is most likely generated by differential cooling of the water in the shallow shelf regions during winter during summer vertical advective processes appear to be limited to the upper 200 m where upwelling in the open water leads to a doming structure the august data support the results of earlier investigations that suggested that dynamic forcing by a basinwide gyre is responsible for the upwelling in the open water

observed source parameters for dynamic rupture with non uniform initial stress and relatively high fracture energy we have conducted dynamic rupture propagation experiments to establish the relations between insource stress drop fracture energy and the resulting particle velocity during slip of an unconfined 2 m long laboratory fault at normal stresses between 4 and 8 mpa to produce high fracture energy in the source we use a rough fault that has a large slip weakening distance an artifact of the high fracture energy is that the nucleation zone is large such that precursory slip reduces fault strength over a large fraction of the total fault length prior to dynamic rupture making the initial stress non uniform shear stress particle velocity fault slip and acceleration were recorded coseismically at multiple locations along strike and at small fault normal distances stress drop increases weakly with normal stress average slip rate depends linearly on the fault strength loss and on static stress drop both with a nonzero intercept a minimum fracture energy of 1 8 j m 2 and a linear slip weakening distance of 33 mm are inferred from the intercept the large slip weakening distance also affects the average slip rate which is reduced by in source energy dissipation from on fault fracture energy because of the low normal stress and small per event slip w86 mm no thermal weakening such as melting or pore fluid pressurization occurs in these experiments despite the relatively high fracture energy and the very low heat production energy partitioning during these laboratory earthquakes is very similar to typical earthquake source properties the product of fracture energy and fault area is larger than the radiated energy seismic efficiency is low at w2 the ratio of apparent stress to static stress drop is w27 consistent with measured overshoot the fracture efficiency is w33 the static and dynamic stress drops when extrapolated to crustal stresses are 2e7 3 mpa and in the range of typical earthquake stress drops as the relatively high fracture energy reduces the slip velocities in these experiments the extrapolated average particle velocities for crustal stresses are 0 18e0 6 m s that these experiments are consistent with typical earthquake source properties suggests albeit indirectly that thermal weakening mechanisms such as thermal pressurization and melting which lead to near complete stress drops dominate earthquake source properties only for exceptional events unless crustal stresses are low published by elsevier ltd

frictional melt and seismic slip 1 frictional melt is implied in a variety of processes such as seismic slip ice skating and meteorite combustion a steady state can be reached when melt is continuously produced and extruded from the sliding interface as shown recently in a number of laboratory rock friction experiments a thin low viscosity high temperature melt layer is formed resulting in low shear resistance a theoretical solution describing the coupling of shear heating thermal diffusion and extrusion is obtained without imposing a priori the melt thickness the steady state shear traction can be approximated at high slip rates by the theoretical form ss n1 4 a under a normal stress n slip rate v radius of contact area r a is a dimensional normalizing factor and w is a characteristic rate although the model offers a rather simplified view of a complex process the predictions are compatible with experimental observations in particular we consider laboratory simulations of seismic slip on earthquake faults a series of high velocity rotary shear experiments on rocks performed for n in the range 1 20 mpa and slip rates in the range 0 5 2 m s 1 is confronted to the theoretical model the behavior is reasonably well reproduced though the effect of radiation loss taking place in the experiment somewhat alters the data the scaling of friction with n r and v in the presence of melt suggests that extrapolation of laboratory measures to real earth is a highly nonlinear nontrivial exercise

the long precursory phase of most large interplate earthquakes many earthquakes are preceded by foreshocks however the mechanisms that generate foreshocks and the reason why they occur before some earthquakes and not others are unknown here we use seismic catalogues from the best instrumented areas of the north pacific to analyse the foreshock sequences preceding all earthquakes there between 1999 and 2011 of magnitude larger than 6 5 and at depths shallower than 50 km the data set comprises 31 earthquakes at plate boundaries and 31 in plate interiors we find that there is a remarkable contrast between the foreshock sequences of interplate compared with intraplate earthquakes most large earthquakes at plate interfaces in the north pacific were preceded by accelerating seismic activity in the months to days leading up to the mainshock in contrast foreshocks are much less frequent in intraplate settings we suggest that at plate boundaries the interface between the two plates begins to slowly slip before the interface ruptures in a large earthquake this relatively long precursory phase could help mitigate earthquake risk at plate boundaries

preslip and cascade processes initiating laboratory stick slip recent modeling studies have explored whether earthquakes begin with a large aseismic nucleation process or initiate dynamically from the rapid growth of a smaller instability in a cascade up process to explore such a case in the laboratory we study the initiation of dynamic rupture stick slip of a smooth saw cut fault in a 76 mm diameter cylindrical granite laboratory sample at 40 120 mpa confining pressure we use a high dynamic range recording system to directly compare the seismic waves radiated during the stick slip event to those radiated from tiny m 6 discrete seismic events commonly known as acoustic emissions aes that occur in the seconds prior to each large stick slip the seismic moments focal mechanisms locations and timing of the aes all contribute to our understanding of their mechanics and provide us with information about the stick slip nucleation process in a sequence of 10 stick slips the first few microseconds of the signals recorded from stick slip instabilities are nearly indistinguishable from those of premonitory aes in this sense it appears that each stick slip begins as an ae event that rapidly 20 s grows about 2 orders of magnitude in linear dimension and ruptures the entire 150 mm length of the simulated fault we also measure accelerating fault slip in the final seconds before stick slip we estimate that this slip is at least 98 aseismic and that it both weakens the fault and produces aes that will eventually cascade up to initiate the larger dynamic rupture

strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates 1 high velocity friction tests were conducted on solid and hollow cylinders of carrara calcite marble dolomite marble silicate bearing calcite marble and calcite gouge to investigate the strength of carbonate faults during seismic slip the experiments performed at normal stresses of 0 6 14 7 mpa slip rates of 0 03 1 60 m s and room temperature in a rotary shear friction testing machine yielded an extraordinarily low steady state friction coefficient 0 1 at slip rates of 1 1 1 2 m s the slip weakening distance of 4 28 m became shorter at higher normal stress or frictional work rate strong velocity weakening was observed not only in steady state but also in nonsteady state friction while the slip rate was changing thus slip deceleration was accompanied by fault strength recovery large rapid temperature rises in narrow shear localization zones less than a few micrometers induced carbonate decomposition such as the breakdown of calcite into aggregates of cao nanograins and co2 in carrara marble scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains mostly 1 m in diameter these microstructures and the measured high permeability 10 14 m2 of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication powder rheology at high slip rates is not yet well understood but the frictional behavior of nanograins appears to be strongly velocity dependent if decarbonation occurs during seismic slip in natural carbonate faults powder lubrication may make the faults slippery even under fluid drained conditions

lake baikal deepwater renewal mystery solved deepwater renewal by intrusions and turbulent diffusion in lake baikal is very effective despite the enormous depth of up to 1642 m and the permanently stable stratification below similar to 300 m depth temperature time series recorded at the bottom of a mooring installed since march 2000 in the south basin of the lake indicate recurrent freshwater intrusions with volumes of 50 to 100 km 3 about one order of magnitude larger than previously observed intrusions numerous mechanisms have been proposed to explain the advective deep water renewal here we present for the first time direct observations which prove that they are caused by coastal downwelling and subsequent thermobaric instability along the steep lake shores understanding these mechanisms is an important prerequisite for studying biogeochemical cycles for predicting the effects of climate change on this unique ecosystem and for evaluating the local climate history from the extraordinary sedimentary record of lake baikal

rock fails in shearing as a tuned critical system abstract shearing is the most widely exploited method to fracture and remove rock formations in many man made rock opening activities during shearing rock breaks into chips due to the periodic nucleation propagation and coalescence of numerous micro cracks the crack interactions along with the rock s complex physical properties lead to extreme challenges in reaching a consensus on the rock s failure behavior in shearing in this paper force drop events as a quantitative measure of the global rock breakage occurrence were recorded and analyzed a power law distribution with a tunable cut off between the frequency of the force drop events and their magnitudes is shown the obtained results for the first time reveal the rock s tuned critical behavior in shearing which is intrinsically similar to its failure behavior widely observed in earthquakes

growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting 1 to understand how frictional melting affects fault instability we performed a series of high velocity friction experiments on gabbro at slip rates of 0 85 1 49 m s 1 at normal stresses of 1 2 2 4 mpa and with displacements up to 124 m experiments have revealed two stages of slip weakening one following the initial slip and the other immediately after the second peak friction the first weakening is associated with flash heating and the second weakening is due to the formation and growth of a molten layer along a simulated fault the two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin continuous molten layer at the second peak friction the frictional coefficient decays exponentially from 0 8 1 1 to 0 6 during the second weakening the host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer melt viscosity increases notably soon after a molten layer forms however a fault weakens despite the increase in melt viscosity and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip weakening distance effect of frictional melting on fault motion can be predicted by solving a stefan problem dealing with moving host rock molten zone boundaries

the limited growth of vegetated shear layers 1 in contrast to free shear layers which grow continuously downstream shear layers generated by submerged vegetation grow only to a finite thickness because these shear layers are characterized by coherent vortex structures and rapid vertical mixing their thickness controls exchange between the vegetation and the overlying water experiments conducted in a laboratory flume show that the growth of these obstructed shear layers is arrested once the production of shear layer scale turbulent kinetic energy ske is balanced by dissipation of ske within the canopy this equilibrium condition along with a mixing length closure scheme was used in a one dimensional numerical model to predict the mean velocity profiles of the experimental shear layers the agreement between model and experiment is very good but field application of the model is limited by a lack of description of the drag coefficient in a submerged canopy

waving plants and turbulent eddies new large eddy simulations of flow over a flexible plant canopy by dupont et al j fluid mech 2010 this issue vol 652 pp 5 44 have produced apparently paradoxical results work over the last three decades had suggested that turbulent eddies could lock onto to the waving frequency of uniform cereal canopies their new simulations contradict this view although a resolution may lie in the essentially three dimensional nature of the instability process that generates the dominant eddies above plant canopies

single and double asperity failures in a large scale biaxial experiment stick slip experiments were performed in a direct shear apparatus using large inada granite blocks the dimensions of the pre existing fault surface were 100 cm length 10cm width by applying heterogeneous normal stress using three actuators by which forces could be independently controlled two asperities with different strengths were formed on a fault plane under a certain normal stress distribution single asperity failure and double asperity failure occurred alternately the single asperity failure refers to a stick slip event in which a weaker asperity ruptures alone without triggering the rupture of a stronger asperity the double asperity failure is an event in which a rupture of a weak asperity triggers the rupture of a strong asperity resulting in a failure of the whole fault for triggering of a rupture it is necessary that stress at the strong asperity is accumulated to a certain level just prior to the failure of the weak asperity

rupture preparation process controlled by surface roughness on meter scale laboratory fault abstract we investigate the effect of fault surface roughness on rupture preparation characteristics using meter scale metagabbro specimens we repeatedly conducted the experiments with the same pair of rock specimens to make the fault surface rough we obtained three experimental results under the same experimental conditions 6 7 mpa of normal stress and 0 01 mm s of loading rate but at different roughness conditions smooth moderately roughened and heavily roughened during each experiment we observed many stick slip events preceded by precursory slow slip we investigated when and where slow slip initiated by using the strain gauge data processed by the kalman filter algorithm the observed rupture preparation processes on the smooth fault i e the first experiment among the three showed high repeatability of the spatiotemporal distributions of slow slip initiation local stress measurements revealed that slow slip initiated around the region where the ratio of shear to normal stress was the highest as expected from finite element method fem modeling however the exact location of slow slip initiation was where became locally minimum probably due to the frictional heterogeneity in the experiment on the moderately roughened fault some irregular events were observed though the basic characteristics of other regular events were similar to those on the smooth fault local stress data revealed that the spatiotemporal characteristics of slow slip initiation and the resulting drop for irregular events were different from those for regular ones even under similar stress conditions on the heavily roughened fault the location of slow slip initiation was not consistent with anymore because of the highly heterogeneous static friction on the fault which also decreased the repeatability of spatiotemporal distributions of slow slip initiation these results suggest that fault surface roughness strongly controls the rupture preparation process and generally increases its complexity with the degree of roughness

hydrodynamics of flow through double layer rigid vegetation vegetation in riparian environment varies in size from grass to shrubs to trees a double layer approach with an array of short and tall acrylic dowels is employed here to simulate such an environment the focus of the present work is to study the response of the flow as it travels through a double layer arrangement for a variety of conditions for this purpose instantaneous measurements of velocity were collected using a one dimensional laser doppler velocimeter at multiple locations to create velocity and turbulence intensity profiles the results indicate that velocity is nearly constant within the short and tall dowel array and increases significantly at the top of the arrays the velocity profile behind a dowel is characterized by multiple inflection points and velocity spikes near the bed especially for locations directly behind a tall dowel where a bulge in the velocity at the top of the short dowel array is present followed by a significant drop in velocity the region immediately behind a tall dowel where the highest turbulence intensities are found is very unstable especially at the dowel boundaries where significant mass and momentum exchange occurs the better known case of submerged single layer flow is included for comparison

plant patch hydrodynamics in streams mean flow turbulence and drag forces 1 this paper investigates the effects of a finite size vegetation patch on flow turbulence variations in drag forces experienced by individual plants within the patch and flow drag interrelations the experiments were conducted in a 32 m long laboratory flume with a 2 8 m long artificial patch the plants were arranged to form a staggered pattern and three flow scenarios were tested velocities were recorded with an acoustic doppler velocimeter whereas specifically designed devices were used to simultaneously measure the drag forces acting on nine plants within the patch for all studied cases the results show zones of increased turbulent energy close to the leading edge and along the patch canopy top where turbulence shear production is enhanced zones of negative reynolds stresses u w are found inside the patch and they reflect the influence of plant morphology which affects the shape of the longitudinal velocity profile and associated turbulent fluxes modifications to the power spectral densities of velocity by the plants indicate the emergence of two plant induced mechanisms of energy production which are most likely related to the wake turbulence and shear layer turbulence drag fluctuations appear to be correlated with the velocity field with this correlation being especially profound at the highest studied flow rate the data suggest that the highlighted correlation stems from flow plant interactions at the top of the vegetation canopy likely due to the presence of large three dimensional coherent structures

evolution of fault interface rayleigh wave speed over simulated earthquake cycles in the lab observations interpretations and implications abstract to image fault zone elastic and mechanical properties is of great importance for the understanding of fault mechanics and earthquake physics in this study we show how a previously less known wave fault interface rayleigh wave firw can be used to probe the feedback between fault zone properties and earthquake ruptures we conducted meter scale rock friction experiments to simulate earthquake cycles in the lab three tests were performed under a fixed normal stress of 6 7 mpa negative for compression and a constant loading rate sequentially at 0 01 0 1 and 1 mm s we installed a strain gauge array near the fault to monitor the local dynamics focusing on mode ii rupture breaking the free edge of the fault and the breakout induced firw propagating backward along the fault by applying a waveform correlation method we systematically measured the firw speed over an 800 mm long interval in the central fault portion the results show that a relative reduction of firw speed by 1 4 can be detected near the end of the test loaded at 1 mm s such wave speed reduction can be explained by two mechanisms rupture induced brittle damage that softens fault zone rocks and some dissipative processes related to fault re rupturing viscous damping etc that take energy away from firw both mechanisms are also supported by other independent observations e g distribution of fault surface wear behavior of macroscopic friction implying that they may be coupled these observations and interpretations reveal an application potential of firw for constraining the elastic and mechanical properties of fault zones we suggest several directions for future research aimed at turning that application potential into reality

strain rate effect on fault slip and rupture evolution insight from meter scale rock friction experiments abstract we conduct meter scale rock friction experiments to study strain rate effect on fault slip and rupture evolution two rock samples made of indian metagabbro with a nominal contact dimension of 1 5 m long and 0 1 m wide are juxtaposed and loaded in a direct shear configuration to simulate the fault motion a series of experimental tests under constant loading rates ranging from 0 01 mm s to 1 mm s and under a fixed normal stress of 6 7 mpa are performed to simulate conditions with changing strain rates load cells and displacement transducers are utilized to examine the macroscopic fault behavior while high density arrays of strain gauges close to the fault are used to investigate the local fault behavior the observations show that the macroscopic peak strength strength drop and the rate of strength drop can increase with increasing loading rate at the local scale the observations reveal that slow loading rates favor generation of characteristic ruptures that always nucleate in the form of slow slip at about the same location in contrast fast loading rates can promote very abrupt rupture nucleation and along strike scatter of hypocenter locations at a given propagation distance rupture speed tends to increase with increasing loading rate we propose that a strain rate dependent fault fragmentation process can enhance the efficiency of fault healing during the stick period which together with healing time controls the recovery of fault strength in addition a strain rate dependent weakening mechanism can be activated during the slip period which together with strain energy selects the modes of fault slip and rupture propagation the results help to understand the spectrum of fault slip and rock deformation modes in nature and emphasize the role of heterogeneity in tuning fault behavior under different strain rates

plant stiffness and biomass as drivers for drag forces under extreme wave loading a flume study on mimics abstract moving water exerts drag forces on vegetation the susceptibility of vegetation to bending and breakage determines its flow resistance and chances of survival under hydrodynamic loading to evaluate the role of individual vegetation parameters in this water vegetation interaction we conducted drag force measurements under a wide range of wave loadings in a large wave flume artificial vegetation elements were used to manipulate stiffness frontal area in still water and material volume as a proxy for biomass the aim was to compare i identical volume but different still frontal area ii identical stiffness but different still frontal area and iii identical still frontal area but different volume comparison of mimic arrangements showed that stiffness and the dynamic frontal area i e frontal area resulting from bending which depends on stiffness and hydrodynamic forcing determine drag forces only at low orbital flow velocities did the still frontal area dominate the force velocity relationship and it is hypothesised that no mimic bending took place under these conditions mimic arrangements with identical stiffness but different overall material volume and still frontal area showed that forces do not increase linearly with increasing material volume and it is proposed that short distances between mimics cause their interaction and result in additional drag forces a model based on effective leaf length and characteristic plant width developed for unidirectional flow performed well for the force time series under both regular and irregular waves however its uncertainty increased with increasing interaction of neighbouring mimics

the age of water and ventilation timescales in a global ocean model abstract the age of water in the world ocean is studied using a passive age tracer introduced into a global ocean model additional information is derived from a transient dye tracer that tracks the time dependent spreading of surface waters into the model ocean interior of particular interest is the nature of ocean ventilation over the 10 100 yr timescale as well as the simulated age of deep and bottom water masses in the upper model levels young water is found to correspond with regions of convergence and downwelling in the surface ekman layer upwelling and convection are both shown to age the upper ocean by entraining older waters into the surface mixed layer in the deep model levels water age varies greatly between oceans with young water found in convectively active regions in the north atlantic and in the ross and weddell seas and old water found in the deep north pacific the oldest water mass mixture located at 2228 m depth in the western pacific ocean is dated at 1494 years made

modeling flow in coral communities with and without waves a synthesis of porous media and canopy flow approaches both canopy flow and porous media theories have been developed independent of one another to predict flow through submerged porous structures these approaches are very similar albeit with some key differences in how canopy resistance forces are parameterized canopy models provide a means of parameterizing the shear stresses that occur at the top of the canopy whereas porous media models can often provide a simpler and more tractable way of parameterizing turbulent form drag based on simple morphological metrics and empirical relationships already in the hydrology literature we developed a set of equations combining aspects of both models and applied this hybridized model to predict the flow structure within an experimental canopy formed by the branching coral porites compressa using model parameter values obtained from the literature results from the model predictions agreed well with direct measurements of flow speed and flow forces derived from particle image velocimetry under conditions of both unidirectional and wave driven oscillatory flow

laboratory measurements of flow and turbulence in discontinuous distributions of ligulate seagrass turbulent flow characteristics were investigated in laboratory flume studies of a ligulate plant canopy interrupted by a gap representing discontinuities observed in seagrass prairies the reliability of velocity measurements obtained using an acoustic doppler velocimeter within the canopy was shown using specifically designed experiments in relatively fast flow mean velocity 5 5 cm s 1 the mean flow profile was logarithmic above the canopy had an inflection point near its top and uniformly low values within it within the gap a recirculation cell formed reynolds stress maxima were approximately coincident with the mean flow inflection point quadrant analysis revealed an ejection dominated upper layer a sweep dominated region around the top of the canopy and within the gap and no dominant quadrant within the canopy in slower flow mean velocity 1 7 cm s 1 the plants were quasiemergent and the flow fields more uniform sweeps similarly dominated the region near the top of the canopy and within the gap in both flows autocorrelation of longitudinal velocity fluctuations showed a lagrangian time scale maximum at the downstream end of the gap

a vortex based model of velocity and shear stress in a partially vegetated shallow channel 1 this paper presents a method for predicting the distributions of velocity and shear stress in shallow channels with a boundary of emergent vegetation experiments in a laboratory channel with model vegetation show that the velocity profile exhibits a distinct two layer structure consisting of a rapidly varying shear layer across the vegetation interface and a more gradual boundary layer in the main channel in addition coherent vortices are observed which span both layers and are the dominant contributors to lateral momentum fluxes from these observations we propose a model for the vortex induced exchange and find expressions for the width of momentum penetration into the vegetation the velocity and shear stress at the vegetation edge and the width of the boundary layer in the main channel these variables along with a momentum balance in the main channel comprise a modeling framework which accurately reproduces the observed velocity and shear stress distributions the predictions for the velocity and shear stress can provide a basis for modeling flood conveyance overbank sediment transport and scalar residence time in the vegetated layer

nucleation of stick slip instability within a large scale experimental fault effects of stress heterogeneities due to loading and gouge layer compaction geodetic observations and large scale laboratory experiments show that seismic instability is preceded by slow slip within a finite nucleation zone in laboratory experiments rupture nucleation is studied mostly using bare rock interfaces whereas upper crustal faults are typically filled with gouge to investigate effects of gouge on rupture nucleation we performed a biaxial shearing experiment on a 350 mm long saw cut fault filled with gypsum gouge at room temperature and a minimum horizontal stress 2 0 3 5 mpa the gouge layer was sandwiched between polymethylmethacrylate pmma plates for reference also a fault without gouge was deformed strain gauges and digital image correlation were used to monitor the deformation field along the fault zone margins stick slip behavior occurred on both the gouge filled fault and the pmma fault nucleation of instability on the pmma fault persistently occurred from one location 2 3 to 3 4 along the fault adjacent to a slow slip zone at the fault end but nucleation on the gouge filled fault was more variable nucleating at the ends and or at approximately 2 3 along the fault with precursory slip occurring over a large fraction of the fault nucleation correlated to regions of high average fault stress ratio n which was more variable for the gouge filled fault due to small length scale variations in normal stress caused by heterogeneous gouge compaction rupture velocities and slip rates were lower for the gouge filled fault than for the bare pmma fault stick slip persisted when 2 was lowered and the nucleation zone length increased expanding from the center to the sample ends before transitioning into instability

extended nucleation of the 1999 mw 7 6 izmit earthquake laboratory and theoretical studies suggest that earthquakes are preceded by a phase of developing slip instability in which the fault slips slowly before accelerating to dynamic rupture we report here that one of the best recorded large earthquakes to date the 1999 moment magnitude mw 7 6 izmit turkey earthquake was preceded by a seismic signal of long duration that originated from the hypocenter the signal consisted of a succession of repetitive seismic bursts accelerating with time and increased low frequency seismic noise these observations show that the earthquake was preceded for 44 minutes by a phase of slow slip occurring at the base of the brittle crust this slip accelerated slowly initially and then rapidly accelerated in the 2 minutes preceding the earthquake

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

evolution of wear and friction along experimental faults we investigate the evolution of wear and friction along experimental faults composed of solid rock blocks this evolution is analyzed through shear experiments along five rock types and the experiments were conducted in a rotary apparatus at slip velocities of 0 002 0 97 m s slip distances from a few millimeters to tens of meters and normal stress of 0 25 6 9 mpa the wear and friction measurements and fault surface observations revealed three evolution phases a an initial stage slip distances 50 mm of wear by failure of isolated asperities associated with roughening of the fault surface b a running in stage of slip distances of 1 3 m with intense wear rate failure of many asperities and simultaneous reduction of the friction coefficient and wear rate and c a steady state stage that initiates when the fault surface is covered by a gouge layer and during which both wear rate and friction coefficient maintain quasi constant low levels while these evolution stages are clearly recognizable for experimental faults made from bare rock blocks our analysis suggests that natural faults bypass the first two stages and slip at gouge controlled steady state conditions

vegetated mixing layer around a finite size patch of submerged plants 1 theory and field experiments 1 dynamics of a vegetated mixing layer around a patch of submerged plants was examined theoretically and experimentally in a lowland river theoretical analysis explored analogy with hydrodynamic mixing layers and introduced equations that describe expansion of the layers and alterations in mean flow field experiments provided empirical data for validation of the theory and examination of the effects of population density on the mixing layer dynamics detailed measurements assessed the structure of turbulent flow plant morphology biomechanical properties bending and streamlining due to interaction with the flow the theory was compared with the field experiments and an agreement was concluded it was found that near the upstream edge of the patch the vegetated mixing layer expands similar to the canonical mixing layers although its statistics scale on the velocity differential characteristic for the stabilized part downstream mean momentum flux reduces in magnitude and reverses in direction while turbulence became dominant this was observed to stabilize the velocity differential and increase the mean velocity of the layer that result in its limited growth described by a logarithmic function the population density of vegetation was found to control the flow in dense vegetation the analogy with mixing layers resembled best while in sparse vegetation the flow behaved similar to the boundary layers this theory can be used for quantitative assessment of seasonal effects in natural vegetated rivers because the population density of vegetation varies during the vegetation season the companion paper focuses on the structural properties of turbulence in the vegetated shear layers

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

ventilation and dissolved oxygen cycle in lake superior insights from a numerical model ventilation and dissolved oxygen in lake superior are key factors that determine the fate of various natural and anthropogenic inputs to the lake we employ an idealized age tracer and biogeochemical tracers in a realistically configured numerical model of lake superior to characterize its ventilation and dissolved o2 cycle our results indicate that lake superior is preferentially ventilated over rough bathymetry and that spring overturning following a very cold winter does not completely ventilate the lake interior while this is unexpected for a dimictic lake no part of the lake remains isolated from the atmosphere for more than 300 days our results also show that lake superior s oxygen cycle is dominated by solubility changes as a result the expected relationship between biological consumption of dissolved o2 and ventilation age does not manifest

scale dependence of rock friction at high work rate in metre sized rock specimens rock friction starts to decrease at a much smaller work rate than in centimetre sized rock specimens thus demonstrating that rock friction is scale dependent frictional sliding at faults controls the rupture of earthquakes and much effort has gone into the study of the frictional properties of rocks in laboratory experiments this study shows that previously used scaling of results obtained in lab studies investigating the earth s crust didn t accurately take into account the effect of sample size futoshi yamashita et al used metre sized rock specimens not the centimetre sized samples commonly used and demonstrate that friction in the metre size range starts to decrease at a work rate that is an order of magnitude smaller than that of the smaller specimens the authors propose that stress concentrated areas exist in which more gouge materials are produced due to frictional slip resulting in further stress concentrations at these areas as such heterogeneity is common in nature the authors conclude that a natural fault could lose its strength faster than that expected from the properties estimated from small rock samples determination of the frictional properties of rocks is crucial for an understanding of earthquake mechanics because most earthquakes are caused by frictional sliding along faults prior studies using rotary shear apparatus1 2 3 4 5 6 7 8 9 10 11 12 13 revealed a marked decrease in frictional strength which can cause a large stress drop and strong shaking with increasing slip rate and increasing work rate the mechanical work rate per unit area equals the product of the shear stress and the slip rate however those important findings were obtained in experiments using rock specimens with dimensions of only several centimetres which are much smaller than the dimensions of a natural fault of the order of 1 000 metres here we use a large scale biaxial friction apparatus with metre sized rock specimens to investigate scale dependent rock friction the experiments show that rock friction in metre sized rock specimens starts to decrease at a work rate that is one order of magnitude smaller than that in centimetre sized rock specimens mechanical visual and material observations suggest that slip evolved stress heterogeneity on the fault accounts for the difference on the basis of these observations we propose that stress concentrated areas exist in which frictional slip produces more wear materials gouge than in areas outside resulting in further stress concentrations at these areas shear stress on the fault is primarily sustained by stress concentrated areas that undergo a high work rate so those areas should weaken rapidly and cause the macroscopic frictional strength to decrease abruptly to verify this idea we conducted numerical simulations assuming that local friction follows the frictional properties observed on centimetre sized rock specimens the simulations reproduced the macroscopic frictional properties observed on the metre sized rock specimens given that localized stress concentrations commonly occur naturally our results suggest that a natural fault may lose its strength faster than would be expected from the properties estimated from centimetre sized rock samples

deep ventilation of lake baikal waters due to spring thermal bars increase of density due to mixing cabbeling instability in a spring thermal bar in lake baikal results in the sinking of cold surface water t 3 2 3 4 c w 0 3 cm s 1 below the profile of tmd this density increase generates forced convection and subsequent movement of cold water along the underwater slope to maximum depths cooling of deep and near bottom waters caused by forced convection is compensated during other seasons of the year by eddy heat flux directed toward the bottom on a long term basis vertical temperature profiles in deep waters remain constant

melt welt mechanism of extreme weakening of gabbro at seismic slip rates a laboratory study of the frictional properties of the igneous rock gabbro at seismically relevant slip rates suggests that the initial weakening of a fault surface during earthquake rupture may be associated with hotspots and macroscopic streaks of melt which partially unload the rest of the slip interface

radiocarbon based circulation age of the world oceans 1 the distributions of deep ocean 14c data are often used to illustrate the rate of deep ocean circulation the associated conventional 14c ages show a difference of 1000 years between the north atlantic and the southern ocean and a difference of another 1000 years between the southern ocean and the north pacific these differences may be interpreted directly and mistakenly as the timescale of circulation the characterization of the deep ocean circulation being millennial is common using objectively gridded natural 14c data of key et al 2004 i recast 14c in terms of circulation by accounting for 1 long surface ocean reservoir 14c ages and 2 two sources of deep water formed in the north atlantic and around antarctica the new distribution of circulation 14c ages is more consistent with the deep ocean being characterized by a centennial timescale than a millennial timescale also the role of the southern sourced deep water is now made more obvious by accounting for the two important controls on oceanic 14c that are not well known outside the field of chemical oceanography the new map will be useful as an illustration of global deep ocean circulation to the wider scientific community and as a pedagogical tool to new students in earth sciences and oceanography

hydrodynamics of model posidonia oceanica patches in shallow water the hydrodynamics of simulated patches of the mediterranean seagrass posidonia oceanica were studied in laboratory flume experiments in which the height of the pronated canopy was always greater than half the total water depth the effects of variations in speed from 0 08 m s 21 to 0 24 m s 21 and patch configuration on the hydrodynamics were investigated significant speeds penetrated the patches to approximately half their height reducing speed did not change the flow patterns observed except to weaken and blur them flow encountering a single patch formed a turbulent wake at the height of the top of the canopy within this wake the vertical shear stress decreased monotonically downstream but the reynolds stress increased initially and then decayed when a second patch was positioned within the region where reynolds stress increased referred to as the 06 patch the wake center penetrated it causing average turbulent velocities with horizontal components 3 3 times higher and vertical components 4 2 times higher than in the upstream patch when this patch was positioned where the reynolds stress decayed referred to as the 14 patch the wake center rose above it nevertheless the turbulence in the 14 patch had horizontal components 12 higher and vertical components 22 higher on average than in the 06 patch because its upstream end was closer to the reynolds stress maximum thus the ratio of the patch separation to the length of wake in which the reynolds stress increased was identified as central to quantifying the turbulence within the downstream patch the increased turbulence is likely to be important in determining sedimentary and ecological patch characteristics by increasing retention of particulates in suspension and thus reducing depositional rates of for example larvae nutrients and dead organic matter

distribution of helium and tritium in lake baikal the 3 h 3 he age of a water mass is a measure of the time that has passed since the water mass was last in contact with the atmosphere between 1992 and 1995 a detailed study of 3 h 3 he ages was conducted in lake baikal the deepest and largest lake by volume on earth to investigate deep water renewal in its three major basins maximum 3 h 3 he ages are 14 17 years in the southern basin 16 18 years in the central basin and 10 11 years in the northern basin rates of renewal of deep water with surface water deduced from volume weighted mean 3 h 3 he ages below 250 m depth are about 10 yr 1 in the southern and central basins and 15 yr 1 in the northern basin in the southern basin the mean 3 h 3 he age below 250 m depth increased steadily from 9 6 years in 1992 to 11 years in 1995 indicating a slight diminution in deep water renewal during this time bottom water renewal by large scale advection was estimated from the mass balance of 3 he in the 200 m thick bottom layer of each basin bottom water renewal rates in the northern basin were found to be between 80 and 150 km 3 yr 1 and in the central basin between 10 and 20 km 3 yr 1 whereas in the southern basin they were practically zero correlating oxygen and dissolved helium 4 concentrations with 3 h 3 he age allowed us to determine the mean hypolimnetic oxygen depletion rate in the water column 4 5 mol l 1 yr 1 as well as mean helium fluxes from the lake bottom 2 8 x 10 11 atoms m 2 s 1 in the northern basin and 1 3 x 10 11 atoms m 2 s 1 in the central and southern basins the helium isotope ratio of the terrigenic helium component injected from the lake bottom determined from measurements of water from hydrothermal springs in the vicinity of the lake was found to be 2 2 x 10 7

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

flow and deposition in and around a finite patch of vegetation this laboratory study describes the flow and deposition observed in and around a finite patch of vegetation located at the wall of a channel two patch densities are considered with 2 and 10 solid volume fraction the velocity field measured in and around the patch by acoustic doppler velocimetry revealed three distinct zones first there is a diverging flow region at the leading edge of the patch where the flow in line with the patch decelerates and the bulk of the flow is diverted toward the open channel second there is a fully developed region within the vegetation where the velocity is uniform across the patch width and along the length of the patch third a shear layer forms at the interface between the patch and adjacent open channel the pattern of deposition in and around the vegetation was characterized by releasing 12 micron spherical glass particles and recording net deposition on a set of glass slides in the diverging region net deposition increases in the stream wise direction as the local velocity decreases in the fully developed region of the patch deposition decreases with longitudinal position as the concentration in the water column is depleted the deposition pattern is nearly uniform across the patch width consistent with the velocity field and suggesting that turbulent diffusive flux across the lateral edge of the patch is not a significant source of particles to the patch under the conditions studied here

scaling of the shear rupture process from nucleation to dynamic propagation implications of geometric irregularity of the rupturing surfaces a series of systematic high resolution laboratory experiments have been performed on the nucleation of propagating slip failure on preexisting faults having different surface roughnesses to demonstrate how the size scale and duration of shear rupture nucleation are affected by geometric irregularity of the rupturing surfaces on the basis of the experimental results it has been discussed theoretically how consistently scale dependent physical quantities inherent in shear rupture are scaled the experiments led to conclusive results that the nucleation process consists of two phases phase i an initial quasi static phase and phase ii a subsequent accelerating phase and that the nucleation process is greatly affected by geometric irregularities on the rupturing surfaces in phase i the rupture grows at a slow steady speed which is independent of the rupture growth length l in contrast during phase ii the rupture develops at accelerating speeds v which increase with an increase in l obeying a power law v vs l c n where vs is the shear wave velocity c is the characteristic length representing the geometric irregularity of the fault surfaces and and n are constants 8 87 10 29 and n 7 31 scale dependency of scale dependent physical quantities including the nucleation zone size and its duration is commonly ascribed to scale dependency of the slip dependent constitutive law parameter dc which is in turn governed by c it has been discussed that a unified comprehension can be provided for shear rupture of any size scale if the constitutive law for shear rupture is formulated as a slip dependent law

foreshocks during the nucleation of stick slip instability 1 we report on laboratory experiments which investigate interactions between aseismic slip stress changes and seismicity on a critically stressed fault during the nucleation of stick slip instability we monitor quasi static and dynamic changes in local shear stress and fault slip with arrays of gages deployed along a simulated strike slip fault 2 m long and 0 4 m deep in a saw cut sample of sierra white granite with 14 piezoelectric sensors we simultaneously monitor seismic signals produced during the nucleation phase and subsequent dynamic rupture we observe localized aseismic fault slip in an approximately meter sized zone in the center of the fault while the ends of the fault remain locked clusters of high frequency foreshocks mw 6 5 to 5 0 can occur in this slowly slipping zone 5 50 ms prior to the initiation of dynamic rupture their occurrence appears to be dependent on the rate at which local shear stress is applied to the fault the meter sized nucleation zone is generally consistent with theoretical estimates but source radii of the foreshocks 2 to 70 mm are 1 to 2 orders of magnitude smaller than the theoretical minimum length scale over which earthquake nucleation can occur we propose that frictional stability and the transition between seismic and aseismic slip are modulated by local stressing rate and that fault sections which would typically slip aseismically may radiate seismic waves if they are rapidly stressed fault behavior of this type may provide physical insight into the mechanics of foreshocks tremor repeating earthquake sequences and a minimum earthquake source dimension

shear induced instabilities of flows through submerged vegetation we consider the instabilities of flows through a submerged canopy and show how the full governing equations of the fluid structure interactions can be reduced to a compact framework that captures many key features of vegetative flow first by modelling the canopy as a collection of homogeneous elastic beams we predict the steady configuration of the plants in response to a unidirectional flow this treatment couples the beam equations in the canopy to the fluid momentum equations subsequently a linear stability analysis suggests new insights into the development of instabilities at the surface of the vegetative region in particular we show that shear at the top of the canopy is a dominant factor in determining the onset of instabilities known as monami based on numerical and asymptotic analysis of the quadratic eigenvalue problem the system is shown to be stable if the canopy is sufficiently sparse

granular nanoparticles lubricate faults during seismic slip nanoparticles are known to form in narrow slip zones in natural and experimental fault zones but their possible role in dynamic weakening of faults during seismic slip remains almost unexplored we conducted friction experiments on periclase mgo nanoparticles 50 nm in average size at rates as high as 1 3 m s 1 a typical speed of seismic slip the nanoparticles were used as the initial gouge to avoid complexities arising from comminution and fluid release the dynamic friction decreased with increasing slip rate and the steady state frictional coefficient reduced to as low as 0 1 at a slip rate of 1 3 m s 1 flash heating is not effective for nanoparticles and we propose that development of slickensides and dominant operation of nanoparticle rolling cause such a weakening nanoparticle lubrication appears to be as effective as melt lubrication and thermal pressurization and the formation of nanoparticles in slip zones may be an important fault lubrication process

mean and turbulent velocity fields near rigid and flexible plants and the implications for deposition 1 the transport of fine sediment and organic matter plays an important role in the nutrient dynamics of shallow aquatic systems and the fate of these particles is closely linked to vegetation we describe the mean and turbulent flow near circular patches of synthetic vegetation and examine how the spatial distribution of flow is connected to the spatial distribution of suspended sediment deposition patches of rigid emergent and flexible submerged vegetation were considered with two different stem densities for the rigid emergent vegetation flow adjustment was primarily two dimensional with flow deflected in the horizontal plane horizontal shear layers produced a von karman vortex street flow through the patch shifted the vortex street downstream resulting in a region directly downstream of the patch in which both the mean and turbulent velocities were diminished net deposition was enhanced within this region in contrast for the flexible submerged vegetation flow adjustment was three dimensional with shear layers formed in the vertical and horizontal planes because of strong vertical circulation turbulent kinetic energy was elevated directly downstream of the patch consistent with this deposition was not enhanced at any point in the wake this comparison suggests that morphological feedbacks differ between submerged and emergent vegetation further enhanced deposition occurred only in regions where both turbulent and mean velocities were reduced relative to the open channel reduced deposition indicating enhanced resuspension occurred in regions of high turbulence kinetic energy regardless of local mean velocity these observations highlight the importance of turbulence in controlling deposition

mixing layers and coherent structures in vegetated aquatic flows 1 to date flow through submerged aquatic vegetation has largely been viewed as perturbed boundary layer flow with vegetative drag treated as an extension of bed drag however recent studies of terrestrial canopies demonstrate that the flow structure within and just above an unconfined canopy more strongly resembles a mixing layer than a boundary layer this paper presents laboratory measurements obtained from a scaled seagrass model that demonstrate the applicability of the mixing layer analogy to aquatic systems specifically all vertical profiles of mean velocity contained an inflection point which makes the flow susceptible to kelvin helmholtz instability this instability leads to the generation of large coherent vortices within the mixing layer observed in the model at frequencies between 0 01 and 0 11 hz which dominate the vertical transport of momentum through the layer the downstream advection of these vortices is shown to cause the progressive coherent waving of aquatic vegetation known as the monami when the monami is present the turbulent vertical transport of momentum is enhanced with turbulent stresses penetrating an additional 30 of the plant height into the canopy

an experimental study of flow through rigid vegetation 1 better understanding of the role of vegetation in the transport of fluid and pollutants requires improved knowledge of the detailed flow structure within the vegetation instead of spatial averaging this study uses discrete measurements at multiple locations within the canopy to develop velocity and turbulence intensity profiles and observe the changes in the flow characteristics as water travels through a vegetation array simulated by rigid dowels velocity data was collected with a one dimensional laser doppler velocimeter under emergent and submerged flow conditions the effects of dowel arrangement density and roughness were also examined the results show that the velocity within the vegetation array is constant with depth and the velocity profile is logarithmic above it however the boundaries are marked by inflection points the strongest vortices and turbulence intensities can be found there especially in the region immediately downstream of a dowel these results support the idea that the flow in the region near the bed and at the top of the dowel array is very unstable leading to the formation of coherent structures and are areas of significant mass and momentum exchange

basal mechanics of ice streams insights from the stick slip motion of whillans ice stream west antarctica the downstream portion of whillans ice stream west antarctica moves primarily by stick slip motion the observation of stick slip motion suggests that the bed is governed by velocity weakening physics and that the basal physics is more unstable than suggested by laboratory studies the stick slip cycle of whilla s ice plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude to understand this variability we modeled the forces acting on the ice stream during the stick phase of the stick slip cycle the ocean tides introduce changes in the rate at which stress is applied to the ice plain increased loading rates promote earlier failure and vice versa results show that the bed of whillans ice stream strengthens over time healing during the quiescent intervals in the stick slip cycle with the bed weakening during slip events the time dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0 35 kpa during the course of a single day copyright 2009 by the american geophysical union

wind flow and sedimentation in artificial vegetation field and wind tunnel experiments abstract the flow dynamics within and immediately above an artificial plastic vegetation canopy are measured in the field and wind tunnel artificial plants created from wooden dowels and cable zip ties were arranged in a variety of arrays on pegboards where plant cover or lateral cover could be accurately controlled plant covers examined ranged from 12 to 60 wind velocity profiles and the bedforms incipient dunes that developed within and beyond the vegetation in a few hours were measured the results indicate that flow deceleration and drag increases within the canopy as plant density increases in general there is a marked reduction and inflection in velocity at the canopy top and a second major inflection in the velocity profiles occurs immediately below the base of the four arrays equal or 23 density tall vegetation allows flow to sweep under the canopy and local high speed maxima flows may develop leading to minimal sedimentation in those canopies but deposition downwind beyond the canopies short vegetation where the leaves fill the canopy close to the ground display significantly greater drag and rapid rates of sand deposition flow velocities above the canopy increase as plant cover or density increases due to flow compression and acceleration bedform development is strongly correlated with plant cover density or lateral cover and short plants such that shorter length higher bedforms develop in higher density and or shorter plants the relationship between plant cover density or lateral cover and within canopy velocity is non linear and a new tentative flow regime is proposed canopy flow which may occur between wake interference flow and skimming flow this study further confirms that plant density plays a defining role in determining bedform and dune morphology the results also apply to flow in semi arid grasslands

density driven exchange between the basins of lake lucerne switzerland traced with the 3h 3he method lake lucerne is divided by sills into four major and two minor basins during winter differential vertical mixing due to non uniform wind exposure and different total dissolved solid concentrations in the main tributaries of the lake cause considerable interbasin density gradients these gradients induce density driven currents across the sills that contribute significantly to the deep water exchange in the basins and gradually reduce the density gradients during summer over a period of 2 yr 1990 1992 the spatial and temporal evolution of water density and water age measured by the 3 h 3 he method was investigated variation in water age between the different basins and the occasional occurrence of age inversions older water overlying younger one can be explained in terms of the observed density distribution and the exchange flows the density gradients cause water age provides an integral measure of the magnitude of deep water renewal which in some basins can be roughly separated into contributions from density driven currents and wind induced vertical mixing by correlating water age with dissolved helium and oxygen concentrations radiogenic helium fluxes and average oxygen depletion rates were determined for the main basins of the lake the helium flux was between 1 and 2x10 10 atoms m 2 s 1 indicating dynamic equilibrium with helium production in the underlying crust oxygen depletion rates per unit sediment area were roughly equal in all basins between 220 and 290 g m 2 yr 1 whereas volumetric depletion rates varied from 1 8 g m 3 yr 1 in the deepest basin to 10 g m 3 yr 1 in the shallowest