controls on turbulent mixing in a strongly stratified and sheared tidal river plume abstractconsiderable effort has been made to parameterize turbulent kinetic energy tke dissipation rate e and mixing in buoyant plumes and stratified shear flows here a parameterization based on kunze et al is examined which estimates e as the amount of energy contained in an unstable shear layer ri ric that must be dissipated to increase the richardson number ri n2 s2 to a critical value ric within a turbulent decay time scale observations from the tidal columbia river plume are used to quantitatively assess the relevant parameters controlling e over a range of tidal and river discharge forcings observed e is found to be characterized by kunze et al s form within a factor of 2 while exhibiting slightly decreased skill near ri ric observed dissipation rates are compared to estimates from a constant interfacial drag formulation that neglects the direct effects of stratification this is found to be appropriate in energetic regimes when the bulk averaged richardson number rib is less than

redistribution of dynamic stress during coseismic ruptures evidence for fault interaction and earthquake triggering we investigate the spatiotemporal evolution of dynamic stress outside a rupturing extended fault the dynamic stress variations caused by a coseismic rupture in a half space are computed by using the discrete wavenumber and reflectivity methods after a transient phase the stress time history evolves to the final static stress value we compare the static stress changes resulting from this model with those computed from a static dislocation model we have applied this method to study the interactions between the first two normal faults which ruptured during the 1980 ms 6 9 irpinia earthquake these two subevents are separated in time by nearly 20 s while the third and last subevent occurred 40 s after the rupture onset we compute the dynamic stress changes caused by the rupture of the first subevent our modeling results show that the dynamic stress peak on the second subevent fault plane is reached between 7 s and 8 s after the rupture initiation on the main fault on the average the static stress level on the second subevent 20 s fault plane is reached nearly after 14 s the dynamic rupture did not jump from a rupturing segment to the adjacent one immediately but the triggering of the 20 s subevent is delayed by roughly 10 s with respect to the instant of occurrence of the dynamic stress peak induced by the 0 s event the dynamic stress pulse propagates along the strike direction of the second subevent fault plane at an average velocity of nearly 3 4 km s the delayed triggering of the second subevent can be interpreted in terms of the frictional properties of the faults in particular rate and state dependent frictional law can explain a delayed instability after a sudden change in stress using the estimated values of the subevent triggering delay and the shear stress change we attempt to constrain the parameter a on the 20 s fault the values here inferred agree well with those resulting from previous studies

shoreline response to autogenic processes of sediment storage and release in the fluvial system 1 experimentally determined shoreline migration rates show high frequency autogenic variability superimposed on low frequency allogenic shoreline responses induced by eustatic base level change this variability persists even when the shoreline migration is averaged laterally indicating time variation in total sediment discharge reaching the shoreline the magnitude of autogenic variability in shoreline migration rate changes by roughly a factor of 3 depending on the shoreline migration direction it is strongest during transgression when the shoreline is on average migrating against the mean sediment flow and weakest during regression when the shoreline is migrating with the mean sediment flow we propose that this time variation is due to overall sediment storage and release in the fluvial system we use a one dimensional geometric model to model the autogenic signals observed in the experiment the model uses small periodic changes in the fluvial slope to represent the effects of storage and release of supplied sediment due to intrinsic variation in the transport efficiency of the fluvial system the periodic pulses of sediment discharge to the shoreline required in the model to explain the variations in shoreline migration can exceed by an order of magnitude the mean allogenic sediment discharge the slope variability required in the model to explain the autogenic shoreline signals is 1 4 of the mean slope this is well within the range of observed variability in depositional fluvial slopes we propose that at least part of this observed variability is real that is the long profile slope of a depositional river system may have an intrinsic elasticity of a few percent of its mean value even under steady forcing autogenic slope variation of this magnitude could readily produce parasequence scale deposits in the stratigraphic record

reconstructing relative flooding intensities responsible for hurricane induced deposits from laguna playa grande vieques puerto rico extreme coastal flooding primarily during hurricane strikes has deposited sand rich layers in laguna playa grande a back barrier lagoon located on the island of vieques puerto rico silici clastic grain size distributions within these overwash deposits fine landward away from the barrier and toward the mainland a simple advective settling model can explain this pattern of lateral sorting and is used to constrain the relative magnitude of past flooding events a deposit associated with the a d 1928 san felipe hurricane is used as a modern analogue to test the technique which produces reasonable estimates for wave heights that exceed the barrier during the event a 5000 yr reconstruction of local flooding intensity is developed that provides a measure of the competence for each overwash event to transport coarser grained sediment a fixed distance into the lagoon this reconstruction indicates that although the laguna playa grande record exhibits large scale changes in hurricane frequency on centennial to millennial time scales the magnitude of these events has stayed relatively constant over the last 5000 yr no evidence exists for an anomalously large hurricane or tsunami event with a competence for sediment transport greater than historical hurricane events

dynamic faulting studied by a finite difference method we have developed a finite difference method that is especially adapted to the study of dynamic shear cracks we studied a number of simple earthquake source models in two and three dimensions with special emphasis on the modeling of the stress field we compared our numerical results for semi infinite and self similar shear cracks with the few exact solutions that are available in the literature we then studied spontaneous rupture propagation with the help of a maximum stress criterion from dimensional arguments and a few simple examples we showed that the maximum stress criterion depended on the physical dimensions of the fault for a given maximum stress intensity the finer the numerical mesh the higher the maximum stress that had to be adopted a study of in plane cracks showed that at high rupture velocities the numerical results did not resolve the stress concentration due to the rupture front from the stress peak associated with the shear wave propagating in front of the crack we suggest that this is the reason why transonic rupture velocities are found in the numerical solutions of in plane faulting when the rupture resistance is rather low finally we studied the spontaneous propagation of an initially circular rupture two distinct modes of nucleation of the rupture were studied in the first a plane circular shear crack was formed instantaneously in a uniformly prestressed medium after a while once stress concentrations had developed around the crack edge the rupture started to grow in the second type of nucleation a preexisting circular crack became unstable at time t 0 and started to grow the latter model appeared to us as a more realistic simulation of earthquake triggering in this case the initial stress was nonuniform and was the static field of the preexisting fault

numerical simulation of three dimensional faulting processes with heterogeneous rate and state dependent friction abstract miyatake t 1992 numerical simulation of three dimensional faulting processes with heterogeneous rate and state dependent friction in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 223 232 numerical simulations of three dimensional faulting processes with heterogeneous rate and state dependent friction were carried out by using a multi mass spring model the rate and state dependent friction law proposed by dieterich and later developed by ruina has much capability to simulate a wide variety of source processes e g precursory slip post seismic slip co seismic slip creep events and earthquake cycles since the original equation is a stiff ordinary equation which has rapidly decaying solutions and is difficult to solve we modified the equation so that numerical computation are made stable this rate and approximate state dependent friction law is applied to a single degree of freedom elastic system to compare its dynamical behaviours with those of the original friction law and found that our friction law was a good approximation of the original formula then we apply this modified friction law to a multi mass spring model to simulate a whole earthquake source process in this paper we studied mainly the effect of heterogeneity of frictional parameters three types of barriers i e a strong barrier a weak barrier and a relaxation barrier can be simulated by certain frictional parameters for each case large mainshocks occurred repeatedly in the case of a weak barrier the barrier region slipped with some delay time during the mainshock i e the occurrence of a multiple shock in the case of a strong barrier the barrier region was left unslipped during the mainshock a small part of this barrier region slipped sequentially as small aftershocks and eventually the entire barrier region slipped as the largest aftershock a fault with a relaxation barrier was also left unslipped during the mainshock similarly to a strong barrier case but a small part of the barrier slipped as creep events

frictional constraints on crustal faulting we consider how variations in fault frictional properties affect the phenomenology of earthquake faulting in particular we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes we model these variations using a rate and state dependent friction law where we differentiate velocity weakening behavior into two fields the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening similarly we differentiate velocity strengthening behavior into two fields the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening the strong seismic field comprises the seismic slip concentrations or asperities the two intermediate fields weak seismic and compliant have frictional velocity dependences that are close to velocity neutral these fields modulate both the tectonic loading and the dynamic rupture process during the interseismic period the weak seismic and compliant regions slip aseismically while the strong seismic regions remain locked evolving into stress concentrations that fail only in main shocks the weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically this mixed frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance the model also provides a mechanism for rupture arrest dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas producing broad areas of accelerated afterslip aftershocks occur on both the weak seismic and compliant areas around a fault but most of the stress is diffused through aseismic slip rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed we test this frictional model by comparing the seismicity and the coseismic slip for the 1966 parkfield 1979 coyote lake and 1984 morgan hill earthquakes the interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip these regions are interpreted as either weak seismic or compliant depending on whether or not they manifest interevent seismicity

mixing by shear instability at high reynolds number 1 shear instability is the dominant mechanism for converting fluid motion to mixing in the stratified ocean and atmosphere the transition to turbulence has been well characterized in laboratory settings and numerical simulations at moderate reynolds number it involves rolling up i e overturning of the density structure within the cores of the instabilities in contrast measurements in an energetic estuarine shear zone reveal that the mixing induced by shear instability at high reynolds number does not primarily occur by overturning in the cores rather it results from secondary shear instabilities within the zones of intensified shear separating the cores this regime is not likely to be observed in the relatively low reynolds number flows of the laboratory or in direct numerical simulations but it is likely a common occurrence in the ocean and atmosphere

laboratory investigation of the impact of lateral spreading on buoyancy flux in a river plume abstractthe relationship between lateral spreading and mixing in stratified gravity currents is investigated by comparing laterally confined and unconfined currents in a series of laboratory experiments the vertical turbulent buoyancy flux is determined using a control volume approach with velocity and density fields derived from combined particle image velocimetry piv and planar laser induced fluorescence plif lateral spreading is determined in the unconfined experiments based on plan view imaging using the optical thickness method otm the authors find that lateral spreading dramatically modifies the plume structure the spreading plume layer consists of approximately linear density and velocity profiles that extend to the surface whereas the channelized plume profiles are uniform near the surface lateral spreading decreases the average plume density relative to laterally confined currents with similar inflow conditions however the local turbulent buoyancy flux in the spreading experiments i

two dimensional time dependent hurricane overwash and erosion modeling at santa rosa island abstract a 2dh numerical model which is capable of computing nearshore circulation and morphodynamics including dune erosion breaching and overwash is used to simulate overwash caused by hurricane ivan 2004 on a barrier island the model is forced using parametric wave and surge time series based on field data and large scale numerical model results the model predicted beach face and dune erosion reasonably well as well as the development of washover fans furthermore the model demonstrated considerable quantitative skill upwards of 66 of variance explained maximum bias 0 21 m in hindcasting the post storm shape and elevation of the subaerial barrier island when a sheet flow sediment transport limiter was applied the prediction skill ranged between 0 66 and 0 77 in a series of sensitivity tests in which several hydraulic forcing parameters were varied the sensitivity studies showed that the variations in the incident wave height and wave period affected the entire simulated island morphology while variations in the surge level gradient between the ocean and back barrier bay affected the amount of deposition on the back barrier and in the back barrier bay the model sensitivity to the sheet flow sediment transport limiter which served as a proxy for unknown factors controlling the resistance to erosion was significantly greater than the sensitivity to the hydraulic forcing parameters if no limiter was applied the simulated morphological response of the barrier island was an order of magnitude greater than the measured morphological response

strong motion prediction using mathematical modeling techniques abstract the parkfield california earthquake of 1966 produced the first recording of ground motion in the immediate vicinity of an earthquake fault the simplicity of displacement pulse observed at station 2 aroused great interest among seismologists working in the area of earthquake source mechanism since then numerous theoretical works on simulating strong motion have been carried out starting with the use of a simple kinematic model assuming a uniform slip over a fault plane the assumption of uniform slip had to be modified to explain the strong motion observed during the parkfield earthquake of 1966 the borrego mountain earthquake of 1968 the san fernando earthquake of 1971 the imperial valley earthquake of 1979 the el asnam earthquake of 1980 and others through these studies the simulation technique has been advanced to include a more realistic medium in the beginning most methods used green9s function for an unbounded homogeneous medium then the free surface effect the effect of a sedimentary layer as well as laterally heterogeneous basin structure have been included in the simulation the above simulation studies were however restricted to relatively low frequency waves represented in a displacement or velocity seismogram in order to predict an acceleration seismogram dominated by high frequency waves hybrid models have been proposed in which gross features of rupture propagation are specified deterministically but the details of the process are described by a stochastic model specified by a small number of parameters the application of these models to several california earthquakes revealed encouraging results that some of the key parameters of the model are stable among earthquakes mathematical modeling techniques will play an important role in the prediction of strong motion for a great earthquake in california as well as for major earthquakes outside california based on the records of moderate earthquakes already acquired in california

a backbarrier overwash record of intense storms from brigantine new jersey abstract analysis of aerial photographs and historic charts indicates that the barrier beach at brigantine nj has migrated landward 300 to 400 m since 1869 primarily as a result of overwash during hurricanes and winter storms a series of vibracores from the backbarrier salt marsh reveals a millennial scale stratigraphic record of overwash deposition carbon 14 c 14 and cesium 137 cs 137 radioisotopic methods were used to date overwash deposits washovers the ages of recent washovers are consistent with deposition during intense storms in 1938 1944 1950 and 1962 an additional overwash deposit recovered in five of the sediment cores was likely deposited by an intense hurricane strike in 1821 or possibly in 1788 two prehistoric overwash fans were likely deposited by intense storms striking the new jersey coast in the 7th to 14th centuries and 6th to 7th centuries a d the landward barrier migration indicates that the older overwash sediments were likely transported a considerably greater distance than the more recent overwash fans the greater distance of transport may indicate that the prehistoric storms that deposited overwash fans across the study site were more intense than the most intense storm to strike this coast in the historic period the hurricane of 1821 the spatially variable occurrence of overwash deposition at this site points to a need for multisite stratigraphic surveys of extensive stretches of the coast in order to develop reliable records of past intense storm frequency from backbarrier environments

buoyant gravity currents released from tropical instability waves abstracttwo extremely sharp fronts with changes in sea surface temperature 0 4 c over lateral distances of 1 m were observed in the equatorial pacific at 0 140 w and at 0 75 n 110 w in both c

numerical modelling of space and time variations of seismic activity before major earthquakes summary the spatial and temporal variations of seismic activity before large earthquakes including precursory swarm earthquakes clustering pre seismic quiescence in the epicentral area doughnut patterns and foreshocks are numerically modelled by quasi static and dynamical rupture processes on three dimensional heterogeneous faults with various types of non uniform strength distributions the main physical parameters that could yield these precursory changes of seismicity patterns are the form of distributions of static and sliding frictional strengths and their spatial variations over the fault and also the rate of increase of tectonic stress for weakly heterogeneous faults a large scale main shock rupturing the entire fault surface occurs without any precursory indications if the fault has moderately to heavily non uniform strengths on a small scale gradually increasing activity tends to occur over a long period prior to a main event with no swarms or quiescence for extremely heterogeneous faults including a number of small to moderate size asperities with medium to high strengths a typical sequence appears with swarms quiescence foreshocks and a main shock event the temporal behaviour of these events seems to depend primarily on the form of distribution of the strengths if the fault includes a large size asperity with medium to high strengths in its central part a doughnut pattern of seismicity repeatedly appears with high activity and quiescence in the surrounding and central asperity regions if a belt like barrier region with extremely high strengths lies in the central part of the fault dividing the entire region into two sections large scale shocks take place successively in the adjacent unruptured regions at slightly different times and a main shock event rupturing the entire fault occurs after a long period of quiescence these results from numerical simulations could provide possible explanations of observed seismicity patterns prior to major earthquakes

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

rollover drowning and discontinuous retreat distinct modes of barrier response to sea level rise arising from a simple morphodynamic model we construct a simple morphodynamic model to investigate the long term dynamic evolution of a coastal barrier system experiencing sea level rise using a simplified barrier geometry the model includes a dynamic shoreface profile that can be out of equilibrium and explicitly treats barrier sediment overwash as a flux with barrier behavior primarily controlled by the maximum potential overwash flux and the rate of shoreface response the modeled barrier system demonstrates four primary behaviors height drowning width drowning constant landward retreat and a periodic retreat height drowning occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep pace with sea level rise on the other hand width drowning occurs when the shoreface response rate is insufficient to maintain the barrier geometry during overwash driven landward migration during periodic barrier retreat the barrier experiences oscillating periods of rapid overwash followed by periods of relative stability as the shoreface resteepens this periodic retreat which occurs even with a constant sea level rise rate arises when overwash rates and shoreface response rates are large and of similar magnitude we explore the occurrence of these behaviors across a wide range of parameter values and find that in addition to the maximum overwash flux and the shoreface response rate barrier response can be particularly sensitive to the sea level rise rate and back barrier lagoon slope overall our findings contrast with previous research which has primarily associated complex barrier behavior with changes in external forcing such as sea level rise rate sediment supply or back barrier geometry

frictional behavior of granite at low and high sliding velocities to study how the velocity dependence of steady state friction of granite varies as a function of sliding rate we slid initially bare surfaces of westerly granite at rates of from 0 001 m s 32 mm yr to over 3 mm s accumulating 376 mm of total displacement the normal stress was 50 mpa we find that the velocity dependence measured by a b changes from negative velocity weakening at 32 m s modeling of experimental data using a simple constitutive law shows that the magnitude of a the direct dependence of friction on velocity remains nearly constant at all rates up to at least 100 m s the increase in a b with velocity is due to a reduction in the magnitude of b our results are consistent with the view that a measures the deformation rate sensitivity of the fault zone material and that b measures the amount of change of some fault zone property

nonlinear internal waves and plumes generated in response to sea loch outflow auv and time lapse photography observations the tidally modulated outflow of brackish water from a sea loch forms a thin surface layer that propagates into the coastal ocean as a buoyant gravity current transporting nutrients and sediments as well as fresh water heat and momentum the fresh intrusion both propagates into and generates a strongly stratified environment which supports trains of nonlinear internal waves nliws nliws are shown to propagate ahead of this buoyancy input in response to propagation of the outflow water into the stratified environment generated by the previous release as well as in the opposing direction after the reflection from steep bathymetry oblique aerial photographs were taken and photogrammetric rectification led to the identification of the buoyant intrusion and the subsequent generation of nliws an autonomous underwater vehicle auv was deployed on repeated reciprocal transects in order to make simultaneous ctd adcp and microstructure shear measurements of the evolution of these phenomena in conjunction with conventional mooring measurements auv based temperature and salinity signals of nliws of depression were observed together with increased turbulent kinetic energy dissipation rates of over two orders of magnitude within and in the wake of the nliws repeated measurements allow a unique opportunity to investigate the horizontal structure of these phenomena simple metric scaling demonstrates that these these processes are likely to be feature of many fjordic systems located on the west coast of scotland but may also play a key role in the assimilation of the outflow from many tidally dominated fjordic systems throughout the world

gravity current dynamics and entrainment a process study based on observations in the arkona basin abstract a 19 h time series of dissipation stratification and horizontal velocities has been obtained for a dense gravity current flowing into the arkona basin in the western baltic sea the observations are compared with one dimensional quasi steady theory in which the gravity component in the flow direction is balanced by bottom friction while that in the cross flow direction is balanced by the coriolis force the observations deviate from the theory in that the bottom shear stress is more than 2 times as large as that required to balance the gravity several reasons for this discrepancy are discussed a 1d turbulence model is also compared with the observations profiles of velocity stratification and dissipation rates generally show similar variations with depth as the observations although the observed dissipation rates are somewhat larger than the modeled and the modeled transverse velocities are much larger than the observed subsequently the model is used to investigate the variation of t

a new parameterization for entrainment in overflows abstract dense overflows entrain surrounding waters at specific locations for example sills and constrictions but also along the descent over the continental slope the amount of entrainment dictates the final properties of these overflows and thus is of fundamental importance to the understanding of the formation of deep water masses even when resolving the overflows coarse resolution global circulation and climate models cannot resolve the entrainment processes that are often parameterized a new empirical parameterization is suggested obtained using an oceanic and laboratory dataset which includes two novel aspects first the parameterization depends on both the froude number fr and reynolds number of the flow second it takes into account subcritical fr 1 entrainment a weak but nonzero entrainment can change the final density and consequently the depth and location of important water masses in the open ocean this is especially true when the dense current follows a long path over

the role of turbulence stress divergence in decelerating a river plume 1 turbulence controls the composition of river plumes through mixing and alters the plume s trajectory by diffusing its momentum while believed to play a crucial role in decelerating river source waters the turbulence stress in a near field river plume has not previously been observationally quantified in this study finely resolved density velocity and turbulence observations are combined with a control volume technique to describe the momentum balance in the columbia river s near field plume during 10 tidal cycles that encompass both large and small river flow turbulence stress varies by 2 3 orders of magnitude both within a given ebb and between ebbs with different tidal or river forcing its magnitude scales with the strength of the instantaneous ebb outflow i e high stresses occur during peak flow of strong ebbs during these periods the momentum equation is represented by a balance between stress divergence and plume deceleration as the flow relaxes the stress divergence weakens and other terms pressure gradient and coriolis may become appreciable and influence plume deceleration while the momentum balance could not be closed during these weaker flow periods during strong tidal pulses the time scale for decay based on observed stress is significantly less than a tidal half period indicating that stress divergence plays a fundamental role in the initial deceleration of the plume

seismicity simulation with a rate and state dependent friction law the dynamic motions and stabilities of a single degree of freedom elastic system controlled by different friction laws are compared the system consists of a sliding block connected to an elastic spring driven at a constant velocity the friction laws are a laboratory inferred friction law called the rate and state dependent friction law proposed by dieterich and ruina and a simple friction law described by dynamic and static frictions we further extend the solution to a one dimensional mass spring model which is an analog of a fault controlled by the rate and state dependent friction law this model predicts non uniform slip and stress drop along the rupture length of a heterogeneous fault this result is very different from some earlier modelings based on the simple friction law and a slip weakening friction law in those earlier modelings the stress and slip functions become smoother with time along the length of the fault rupture owing to the interactions between fault segments during slip because of this smoothing process the number of small events will decrease with time and the universilly observed stationary magnitude frequency relation cannot be explained the interaction between a fault segment and its neighboring segments can be measured when the post slip stress on this segment is compared with the stress on an identical segment represented by a block in this modeling without neighboring segments if the post slip stress of the former is much higher than that of the latter strong interaction exists if the two are close only weak interaction exists the interaction is determined by the relative motion between fault segments and the time duration of interaction our new modeling with the rate and state dependent friction law appears to show no such smoothing effect and provides a physical mechanism for the roughening process in the difference between the fault strength and stress that is necessary to explain the observed stationary magnitude frequency relation the noninstantaneous healing predicted by the rate and state dependent friction law may be repsonsible for the recurring nonuniform slip and stress drop and may be explained by the reduction of interaction among fault segments due to the low frictional strength during the fault stopping the very low friction during slip stopping allows much longer times than does the higher friction due to instantaneous healing for the fault segments to adjust their motions from an upper limit slip velocity to almost rest according to newton s second law a process with fixed masses and constant velocity changes involves low forces and weak interactions if it is accomplished in a long time period and vice versa our modeling also indicates that the existence of strong patches with higher effective stress on a fault is needed for the occurrence of major events the creeping section of a fault such as the one along the san andreas fault in central california on the other hand can be simulated with the rate and state dependent friction law by certain model parameters which however must not include strong patches in this case small earthquakes and aseismic creep relieve the accumulating strain without any large events

bedform and stratification characteristics of some modern small scale washover sand bodies washover sand bodies commonly develop along microtidal coastlines in beach barrier island or spit settings wave runup usually in conjunction with an abnormally high water level may overtop the most landward berm of the beach and the foredune crest if one exists to produce overwash and subsequent runoff across the more landward subaerial surface two main elements of the resulting deposit are the washover fan and runoff channel newly formed small scale washover deposits were examined along the outer banks north carolina near pt mugu california and at presque isle lake erie pennsylvania the fans were formed in response to unidirectional landward transport and the runoff channels in response to unidirectional flow usually in a landward direction but sometimes in shore parallel then seaward direction where overwash carried across the fan surface and entered a pond or lagoon a small scale delta microdelta developed in this case the washover fan consisted of two subfacies the wetted but subaerial part of the fan and the subaqueous washover delta flow associated with the development of the fan and runoff channel produced distinctive sets of bedforms and internal stratification high velocity discontinuous surges moving across the fan surface resulted in the development of a plane bed and subhorizontal to low angle landward dipping planar stratification which comprised the major part of the fan similarly rhomboid forms were produced by high velocity sheet flow across the fan surface where flow carried into a standing body of water delta type foreset strata developed for this case the lateral structural sequence was subhorizontal planar stratification merging landward into landward dipping delta tabular foreset strata in the runoff setting where flow became channelized and continuous both upper flow and lower flow regime currents were typical upper flow regime bedforms included antidunes standing waves and plane beds the most commonly observed lower flow regime bedforms included microdelta like bars low amplitude bars linguoid ripples and sinuous crested current ripple trains the sets of sedimentary structures comprising modern washover sand bodies provide criteria for the identification of similar deposits in ancient sediments and for more specific interpretation of the environment

observations of shear and vertical stability from a neutrally buoyant float measurements of 1 to 5 m shear and strain from a neutrally buoyant float are used to examine the statistics and causes of shear instability in a 9 day record unstable conditions defined as vz n 2 where vz vz are due to higher than average shear rather than lower than average buoyancy frequency and there are no instances of unstable buoyancy frequency or overturns on the 0 5 m spacing of the thermistors shear is dominated by upward propagating near inertial motions unstable events were more frequent when a near inertial wave packet occupied the water parcel tracked by the float groups of unstable events occur roughly every 5 7 hours at scales 2 5 m and much less often at larger scales events typically last 10 min one buoyancy period or longer these time scales appear to be controlled by vertical advection of 1 m fine structure past the float sensors so do not represent the intrinsic time scales of instability turbulent and mixing quantities are estimated from these fine scale measurements giving a dissipation rate of e 2 3 16 10 10 w kg and an eddy diffusivity of k 3 10 6 m2 s in agreement with direct microstructure estimates therefore the float adequately resolves those quantities needed to estimate turbulent dissipation and could be used to monitor turbulent mixing in the ocean

model for calculating bed load transport of sediment a bed load transport model based on the mechanics of sediment moving by saltation yields predicted values of bed load flux as a function of boundary shear stress grain diameter and density the parameters required to calculate bed load transport particle velocity bed load sediment concentration and the height of the bed load layer can all be determined from our model which computes sequences of trajectories of individual saltating grains as well as the concentration of moving grains that the flow can support the latter is related to the momentum the accelerating grains extract from the flow predicted curves of bed load transport vs boundary shear stress agree well with transport data collected by gilbert meyer peter iet al n and the waterways experimental station measured shear stress was corrected for pressure drag when bed forms were present comparison of predicted bed load transport with common bed load equations reveals considerable similarity among the relationships the best agreement with the data is produced by relationships in which the transport rate vanishes as the shear stress approaches the critical shields number as in our model and yalin s equation

700 yr sedimentary record of intense hurricane landfalls in southern new england five intense category 3 or greater hurricanes occurring in 1635 1638 1815 1869 and 1938 have made landfall on the new england coast since european settlement historical records indicate that four of these hurricanes 1635 1638 1815 and 1938 and hurricane carol a strong category 2 storm in 1954 produced significant storm surges 3 m in southern rhode island storm surges of this magnitude can overtop barrier islands removing sediments from the beach and nearshore environment and depositing overwash fans across back barrier marshes lakes and lagoons in a regime of rising sea level accumulation of marsh lake or lagoon sediments on top of overwash deposits will preserve a record of overwash deposition we examined the record of overwash deposition at succotash salt marsh in east matunuck rhode island and tested the correlation with historical records of intense storms aerial photographs taken after hurricanes in 1954 and 1938 show overwash fans deposited at the site analysis of 14 sediment cores from the back barrier marsh confirmed the presence of these fans and revealed that 4 additional large scale overwash fans were deposited within the marsh sediments the four overwash fans deposited since the early seventeenth century at succotash marsh matches the historical record of significant hurricane induced storm surge these fans were most likely deposited by hurricanes in 1954 1938 1815 and either 1638 or 1635 radiocarbon dating of two prehistoric overwash fans indicated that these were deposited between a d 1295 1407 and 1404 1446 and probably represent intense hurricane strikes in the past 700 yr at least 7 intense hurricanes struck the southern rhode island coast and produced a storm surge that overtopped the barrier at succotash marsh

impacts of hurricane ike on the beaches of the bolivar peninsula tx usa abstract hurricane ike caused substantial beach erosion along the coast of the bolivar peninsula tx much of the erosion was caused by the offshore ebb flow of the ca 5 m storm surge that formed spatially discrete scour features using aerial photography and repeat lidar data we identify five types of scour features and describe the alongshore distribution in four flow environments type 1 scours are relatively small and compact features associated mainly with flow off a wide vegetated grasses shrubs trees surface across a wide beach type 2 scours are large and branching forms associated mainly with flow that was channeled by streets or gaps between structures type 3 scours are large and blocky features associated with flow off a marsh surface across a highway which removed almost all beach sands from the surface type 4 scours are elongated shore perpendicular channels associated with the same flow characteristics as type 3 scours type 5 scours are elongated shore perpendicular features sometimes branching associated with flow through gaps in a destroyed shore protection structure repeat imagery indicates that many of the features persisted for at least seven months recent aerial photography indicates that aspects of some features remained evident more than three years after ike s landfall

erosional and depositional characteristics of regional overwash deposits caused by multiple hurricanes regional scale washover deposits along the florida gulf and atlantic coasts induced by multiple hurricanes in 2004 and 2005 were studied through coring trenching ground penetrating radar imaging aerial photography and prestorm and poststorm beach profile surveys erosional and depositional characteristics in different barrier island sub environments including dune field interior wetland and back barrier bay were examined over the eroded dune fields the washover deposits are characterized by an extensive horizontal basal erosional surface truncating the old dune deposits and horizontal to slightly landward dipping stratification over the marshes in the barrier island interior the washover deposits are characterized by steep tabular bedding with no erosion at the bottom overwash into the back barrier bay produced the thickest deposits characterized by steep prograding sigmoidal bedding no significant erosional feature was observed at the bottom washover deposits within the dense interior mangrove swamp demonstrate both normal and reversed graded bedding the washover deposits caused by hurricanes frances 2004 and jeanne 2004 along the southern florida atlantic coast barrier islands are substantially different from those along the northern florida barrier islands caused by ivan 2004 and dennis 2005 in terms of regional extension erosional features and sedimentary structures these differences are controlled by different overall barrier island morphology vegetation type and density and sediment properties the homogeneity of sediment along the northern florida coast makes distinguishing between washover deposits from ivan and dennis difficult in contrast along the atlantic coast barrier islands the two overwash events as demonstrated by two phases of graded bedding of the bimodal sediments are easily distinguishable

speculations on the geometry of the initiation and termination processes of earthquake rupture and its relation to morphology and geological structure earthquake initiation and termination processes are commonly described in terms of barriers and asperities barriers fall into two classes geometric barriers are associated with places where the orientation of a failure surface changes and relaxation barriers where stress is low because aseismic creep processes outpace tectonic loading geometric barriers fall into conservative and nonconservative subgroups according to whether finite fault motion can proceed without the creation of new structures or whether it demands the creation of new faulting or void space the multiple faulting or fragmentation associated with some nonconservative barriers can disrupt fault planes and form asperities by means of selected examples it is shown that a description in terms of these barriers can help one to visualise the processes of earthquake rupture and its relation to the geological environment

turbulence from breaking surface waves at a river mouth abstractobservations of surface waves currents and turbulence at the columbia river mouth are used to investigate the source and vertical structure of turbulence in the surface boundary layer tu

stratigraphy of washover deposits in florida implications for recognition in the stratigraphic record abstract twelve washover deposits were cored on the west central gulf coast of florida to provide data to permit development of a model to help identify washover facies in the stratigraphic record typical modern washover stratigraphy displays landward dipping plane beds comprised of well sorted sand with distinct laminae of shells and heavy minerals five subfacies are delineated which show variations in composition texture and bioturbation throughout the washover facies these subfacies represent differences in flow conditions during overwash position relative to sea level and variable degrees of reworking after deposition three shell assemblages aid in identification of washover deposits backbarrier sediments composed of shoreface open water species or mixed shoreface backbarrier species may potentially be washover in origin sediments with purely backbarrier quiet water shell species are likely to have been deposited independently of washover activity examination of washover deposits of differing ages reveals that preservation of washover stratigraphy is not exclusively a function of time reworking of small scale stratification can occur in as short as a decade however this same stratification was found to be preserved in deposits several hundred years old destruction of original washover signatures is related to the position of the deposits relative to sea level and the rate and depth of burial even after the destruction of small scale stratigraphic features washover deposits may still be identified as such due to their texture composition and shell assemblages key features in recognizing the facies after bioturbation and reworking are 1 the presence of clean sand in otherwise muddy backbarrier sediments 2 the landward thinning of the facies and 3 the presence of shoreface shells or mixed shoreface backbarrier shells on landward portions of the barrier island system if reworking is severe and or there are limited subsurface data distinguishing washovers from genetically similar deposits e g flood tidal deltas and spillover deposits in the stratigraphic record is difficult and when considered out of stratigraphic context may not be recognizable

tracking hurricane generated storm surge with washover fan stratigraphy we use the stratigraphy preserved in a washover fan to reconstruct the timing or emplacement and environmental conditions along the matagorda peninsula texas during hurricane ike in 2008 washover fan stratigraphy preserves a topset foreset break tfb that rises 0 42 m in elevation as the fan built landward we constrain overwash flow depths to 0 1 0 32 m through deposit sedimentology and tie the rising trajectory of the tfb to rising storm surge water levels measured in the back barrier bay 0 03 m h 1 as the hurricane approached the coast this relation allows us to estimate that the fan took 0 52 0 90 days to build and was finished building before the storm surge peaked this is 15 25 of the 3 5 days of hurricane induced storm surge near the site we show how washover stratigraphy can be used to constrain the timing and amount of sediment redistribution on a coast associated with a hurricane information that is necessary to test and or calibrate existing numerical models that predict shoreline change during hurricanes

3d eight order elastic finite difference modeling of refraction and strong motion data from the coyote lake region california abstract 3d elastic modeling of vertical component explosive source refraction data is performed to constrain the shallow part of a velocity model followed by 3d modeling of three component strong motion data from a spatially and temporally extended earthquake source in that model modeling is by eighth order finite differencing on a staggered grid which automatically generates all body and surface waves including multiples and conversions application is to the 1981 usgs refraction survey and the 6 august 1979 coyote lake event in central california 3d modeling was able to reproduce the main observed features in both the refraction and earthquake data for the refraction data the synthetics explain local variations in first arrival travel times reverberations in the near surface sediments and surface waves for the strong motion data the initial arrival times and amplitudes are determined by the position and orientation of the initial rupture the shapes of the trailing edges of the pulses are modified by the actual source distribution changes of source position of 1 to 2 km are easily resolvable through changes in the position of nodal planes discontinuities in material properties e g layering produce a coda of multiples and converted waves lateral variations in velocities especially in the upper few kilometers change the energy distribution among the trace components by 3d refraction and so shift the apparent source directivity and position as well as contributing to the coda remaining differences between the observed and synthetic data are attributed to local seismometer coupling effects very local site structures and phenomena such as anelasticity and anisotropy which are not explicitly modeled

entrainment and turbulence in saline underflow in lake ogawara under certain tidal conditions a saline underflow originating in the pacific ocean moves into lake ogawara japan the underflow consists of a uniform saline bottom layer that is slightly warmer than the ambient and an interfacial shear layer in which the velocity and density are decreased within the experimental area the underflow is confined to a channel approximately 1 km wide and is essentially two dimensional the underflow had a bulk richardson number defined in terms of the mean properties between 1 and 2 the rate of entrainment into the bottom layer was calculated using two distinct methods the first method used the change in the maximum salinity of the underflow measured at two stations along the path of the underflow to infer the amount of ambient water entrained the second method made direct measurements of vertical mass fluxes with a profiler the agreement between the two methods was excellent the measured entrainment coefficients were consistent with the derived entrainment law the turbulent structure of the flow was mapped for a 3 h quasi steady period of the flow turbulence is predominantly generated on the bottom boundary and is transported vertically to the density interface where it leads to mixing

tide induced variation of the dynamics of a salt wedge estuary abstract the time variations of the salt wedge intrusion in the fraser estuary british columbia were monitored during a variety of tidal and run off conditions using instruments and sampling methods that provided high resolution of the velocity and water properties in space and time the salt wedge was found to vary considerably in position and vertical structure through the tidal cycle due to the interaction of the tidal flow with the density driven motion of the salt wedge during the flood the salt wedge progressed up the estuary as a gravity current while during the ebb the salinity structure was eroded by shear instability the difference in character of the flow between flood and ebb is attributed to the transition between a subcritical and supercritical internal froude number during flood tide the internal state is subcritical and the estuarine shear flow is maintained with a small amount of vertical mixing while in the supercritical ebb flow vertical shears become so large that the pycnoc

hydraulics and mixing in a laterally divergent channel of a highly stratified estuary author posting american geophysical union 2017 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 122 2017 4743 4760 doi 10 1002 2016jc012455

structure and dynamics of the columbia river tidal plume front 1 time dependent buoyant plumes form at the outflow of tidally dominated estuaries when estuary discharge velocity exceeds plume internal wave speed c a sharp front forms at the plume s leading edge that expands from the time dependent source using observations of the columbia river tidal plume from multiple tidal cycles we characterize time evolving plume structure and quantify front speed uf plume internal wave speed c front curvature and ultimate extent we identify three distinct stages of propagation 1 initially the plume is strongly influenced by shallow bathymetry near the river mouth 2 as the front advances offshore the plume detaches from the bottom and expands as a freely propagating gravity current with relatively constant uf c and frontal froude number f uf c ambient currents explain intracycle variability in uf and winds alter front shape variability in ambient stratification associated with previous cycles plume remnants leads to complex fronts and internal waves 3 finally the plume decelerates adjusts toward geostrophy and may radiate additional internal waves using a simple kinematic model we suggest that constant frontal propagation speed uf 0 9 0 1 m s during stage 2 is primarily controlled by linearly increasing volume flux from the columbia river mouth as this discharge rate subsides the plume expands as a fixed volume with decreasing front speed stage 3 the plume s final extent is controlled by the rossby radius which scales with a length based on the total volume discharged this provides an integral description of plume front evolution based on the time dependent estuary discharge

seasonality of deep cycle turbulence in the eastern equatorial pacific abstractthe seasonal cycles of the various oceanic and atmospheric factors influencing the deep cycle of turbulence in the eastern pacific cold tongue are explored moored observations at 140 w have shown seasonal variability in the stratification velocity shear and turbulence above the pacific equatorial undercurrent euc in boreal spring the thermocline and euc shoal and turbulence decreases marginal instability clustering of the local gradient richardson number around the critical value of 1 4 evident throughout the rest of the year has not been detected during spring while the daily averaged turbulent energy dissipation in the euc is weakest during the spring it is not clear whether the diurnal fluctuations that define the deep cycle cease large eddy simulations are performed using climatological initial and boundary conditions representative of january april july and october deep cycle turbulence is evident in all cases the mechanism remains the same and the maximum turbulence leve