natural magnetic disturbance fields not precursors preceding the loma prieta earthquake 1 available records of the magnetic indices dst and ap together with standard observatory recordings of 1 min field levels were examined for the period preceding the earthquake of october 1989 centered near loma prieta california the magnetic records showed that the fraser smith et al 1990 report claiming the existence of a 100 s ultralow frequency geomagnetic field precursor signal at corralitos california foretelling a nearby earthquake is not valid my study shows that the stanford ulf signal was not local but rather widespread throughout the western united states and therefore expected to be due to a coincidental geomagnetic solar terrestrial disturbance field

frequency spectral properties of an ulf electromagnetic signal around the 21 july 1995 m 5 7 yong deng china earthquake abstract the frequency temporal analysis of the electromagnetic field recorded from 30 may 1995 to 24 august 1995 at a site about a hundred kilometer north of the epicenter of the 21 july 1995 yong deng earthquake china has shown a clear systematic variation on an electric dipole in the ultra low frequency range from 0 01 to 0 0005 hz a three fold decrease of the frequency of the maximal energy concentration was observed on 30 may 1995 from 0 005 to 0 0010 hz few hours at the time of the earthquake this decrease was nearly monotonic with superimposed small scale step like fluctuations and continued after the yong deng earthquake for about two weeks when the minimum of 0 001 hz was observed just before its abrupt disappearance the ulf signal is observed on one of the four electric dipoles the instrumentation and the data acquisition system do not generate this signal the signal could be generated by an electrokinetic effect depending on stress field changes in the local environment of one or two probes the results need further understanding and interpretation

shallow water turbulence modeling and horizontal large eddy computation of river flow by introducing the concept of sds subdepth scale turbulence to model three dimensional 3d turbulence with length scales less than the water depth and treating it explicitly with a proper separate modeling an sds 2dh model has been developed to simulate the evolution of horizontal large scale eddies in shallow water applying this model to river flows with transverse shear due to vegetation drag the horizontal large scale hls eddies were found to dominate horizontal momentum mixing the bottom friction and vegetation drag acting as sinks of vorticity play the key roles in the development of the horizontal large scale eddies and in reynolds stress generation the sds 2dh model can directly describe effects of flow geometry such as vegetation layer width on the large scale eddy development and hence predicts turbulence mixing better than the k e model

estuary enhanced upwelling of marine nutrients fuels coastal productivity in the u s pacific northwest 2014 american geophysical union all rights reserved the pacific northwest pnw shelf is the most biologically productive region in the california current system a coupled physical biogeochemical model is used to investigate the influence of freshwater inputs on the productivity of pnw shelf waters using realistic hindcasts and model experiments that omit outflow from the columbia river and strait of juan de fuca outlet for the salish sea estuary outflow from the strait represents a critical source of nitrogen to the pnw shelf accounting for almost half of the primary productivity on the vancouver island shelf a third of productivity on the washington shelf and a fifth of productivity on the oregon shelf during the upwelling season the columbia river has regional effects on the redistribution of phytoplankton but does not affect pnw productivity as strongly as does the salish sea a regional nutrient budget shows that nitrogen exiting the strait is almost entirely 98 of ocean origin upwelled into the strait at depth mixed into surface waters by tidal mixing and returned to the coastal ocean from the standpoint of nitrogen availability in the coastal euphotic zone the estuarine circulation driven by freshwater inputs to the salish sea is more important than the supply of terrigenous nitrogen by rivers nitrogen rich surface waters exiting the strait follow two primary pathways to the northwest in the vancouver island coastal current and southward toward the washington and oregon shelves nitrogen flux from the juan de fuca strait and eddy region to these shelves is comparable to flux from local wind driven upwelling

magnetic and electric fields associated with changes in high pore pressure in fault zones application to the loma prieta ulf emissions we determined the electric and magnetic fields generated during failure of faults containing sealed compartments with pore pressures ranging from hydrostatic to lithostatic levels exhumed fault studies and strain measurement data limit the possible size of these compartments to less than 1 km in extent rupture of seals between compartments produces rapid pore pressure changes and fluid flow and may create fractures that propagate away from the high pressure compartment along the fault face nonuniform fluid flow results from pressure decrease in the fracture from crack generated dilatancy partial blockage by silica deposition and clearing as pressure increases a direct consequence of this unsteady fluid flow may be associated transient magnetic signals caused by electrokinetic piezomagnetic and magnetohydrodynamic effects models of these processes for fault geometries with 1 km high pressure compartments show that electrokinetic effects are several orders of magnitude larger than the other mechanisms the electrokinetic signals produced by this unsteady flow are comparable in magnitude and frequency to the magnetic signals observed prior to the ml 7 1 loma prieta earthquake of october 18 1989 provided fracture lengths are less than 200 m

mapping turbidity patterns in the po river prodelta using multi temporal landsat 8 imagery abstract thirty meters resolution turbidity maps derived from landsat 8 l8 images were used to investigate spatial and temporal variations of suspended matter patterns and distribution in the area of po river prodelta italy in the period from april 2013 to october 2015 the main focus of the work was the study of small and sub mesoscale structures linking them to the main forcings that control the fate of suspended sediments in the northern adriatic sea a number of hydrologic and meteorological events of different extent and duration was captured by l8 data quantifying how river discharge and meteo marine conditions modulate the distribution of turbidity on and off shore at sub mesoscale peculiar patterns and smaller structures as multiple plumes and sand bars were identified thanks to the unprecedented spatial and radiometric resolution of l8 sensor the use of these satellite derived products provides interesting information particularly on turbidity distribution among the different delta distributaries in specific fluvial regimes that fills the knowledge gap of traditional studies based only on in situ data a novel approach using satellite data within model implementation is then suggested

detection of ultra low frequency emissions connected with the spitak earthquake and its aftershock activity based on geomagnetic pulsations data at dusheti and vardzia observatories abstract ulf electromagnetic emissions from the spitak armenia earthquake site have been detected observations were carried out at the dusheti and vardzia georgia observatories at distances 120 200 km from the epicentre in a frequency range 0 005 1 hz it is shown that the emission appears several hours before the main shock and some of the powerful aftershocks and is not connected with geomagnetic pulsations of magnetospheric sources

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

the relative importance of head flux and prior information in hydraulic tomography analysis using cross correlation analysis we demonstrate that flux measurements at observation locations during hydraulic tomography ht surveys carry nonredundant information about heterogeneity that are complementary to head measurements at the same locations we then hypothesize that a joint interpretation of head and flux data even when the same observation network as head has been used can enhance the resolution of ht estimates subsequently we use numerical experiments to test this hypothesis and investigate the impact of flux conditioning and prior information such as correlation lengths and initial mean models i e uniform mean or distributed means on the ht estimates of a nonstationary layered medium we find that the addition of flux conditioning to ht analysis improves the estimates in all of the prior models tested while prior information on geologic structures could be useful its influence on the estimates reduces as more nonredundant data i e flux are used in the ht analysis lastly recommendations for conducting ht surveys and analysis are presented

hydrologic effects on water level changes associated with episodic fault creep near parkfield california as part of the parkfield california earthquake prediction experiment water level is monitored in a well 460 m from the main trace of the san andreas fault on middle mountain in the preparation zone of the anticipated parkfield earthquake the well configuration allows water level to be monitored in two fluid reservoirs at depths of 85 and 250 m below land surface during 1987 water level changes were recorded during 12 of the 18 episodes of accelerated fault creep detected by a creep meter spanning the fault trace 750 m northwest of the well the creep related water level changes in the shallow reservoir have durations of less than 1 day whereas in the deeper reservoir the changes persist for as long as 2 months these data suggest that the transient nature of the water level changes in the shallow interval is due to vertical flow to the water table and is not evidence that creep events propagate past the well phase leads of earth tidal constituents in the water level data from the shallow interval relative to the same constituents in the local volume strain tide support the interpretation of significant flow to the water table at periods of 1 day or less the form of the water level changes in the deep interval is affected by horizontal flow to the well bore this effect can be removed from the water level records using a theoretical response curve constrained by the phases of earth tidal constituents in the deep interval relative to the local volume strain tide for the events where the signal in the shallow interval has been large enough to measure the sizes of the simultaneous water level changes in the two reservoirs are consistent with the same amounts of volume strain occurring at both depths

sediment eddy diffusivity in meandering turbulent jets implications for levee formation at river mouths 1 depositional patterns characteristic of river mouths are controlled by the sediment laden turbulent jet exiting from rivers here we show that jet instability developing at river mouths with small width to depth ratio can affect these patterns numerical simulations indicate that sediment eddy diffusivity which is a measure of the spreading of sediments out of the jet core depends on the interplay between sediment settling and large coherent flow structures which are associated with unstable jets our results suggest that optimal conditions for eddy diffusivity are met when the time scale of sediment settling is equal to the characteristic eddy time scale in this condition sediments are deposited at the farthest distance from the jet centerline thus promoting the formation of lateral deposits settling time scales much greater or much smaller than the eddy time scale is associated with low eddy diffusivity which promotes the formation of a central deposit such eddy mediated sediment transport is more evident in the zone of established flow than in the zone of flow establishment since the former is characterized by fully developed vortices a simplified model is proposed to estimate the settling and eddy time scales and to predict the optimal condition for levee formation

improved predictions of saturated and unsaturated zone drawdowns in a heterogeneous unconfined aquifer via transient hydraulic tomography laboratory sandbox experiments summary interpretation of pumping tests in unconfined aquifers has largely been based on analytical solutions that disregard aquifer heterogeneity in this study we investigate whether the prediction of drawdown responses in a heterogeneous unconfined aquifer and the unsaturated zone above it with a variably saturated groundwater flow model can be improved by including information on hydraulic conductivity k and specific storage s s from transient hydraulic tomography tht we also investigate whether these predictions are affected by the use of unsaturated flow parameters estimated through laboratory hanging column experiments or calibration of in situ drainage curves to investigate these issues we designed and conducted laboratory sandbox experiments to characterize the saturated and unsaturated properties of a heterogeneous unconfined aquifer specifically we conducted pumping tests under fully saturated conditions and interpreted the drawdown responses by treating the medium to be either homogeneous or heterogeneous we then conducted another pumping test and allowed the water table to drop similar to a pumping test in an unconfined aquifer simulations conducted using a variably saturated flow model revealed 1 homogeneous parameters in the saturated and unsaturated zones have a difficult time predicting the responses of the heterogeneous unconfined aquifer 2 heterogeneous saturated hydraulic parameter distributions obtained via tht yielded significantly improved drawdown predictions in the saturated zone of the unconfined aquifer and 3 considering heterogeneity of unsaturated zone parameters produced a minor improvement in predictions in the unsaturated zone but not the saturated zone these results seem to support the finding by mao et al 2011 that spatial variability in the unsaturated zone plays a minor role in the formation of the s shape drawdown time curve observed during pumping in an unconfined aquifer

flow patterns and morphology of a prograding river delta the transition of flow between laterally confined channels and the unchannelized delta front controls the morphodynamic evolution of river deltas but has rarely been measured at the field scale we quantify flow patterns and bathymetry that define the evolution of the subaqueous delta front on the wax lake delta a rapidly prograding delta in coastal louisiana a significant portion of flow 59 departs the channel network over lateral channel margins as opposed to the downstream channel tips bathymetric surveys and remotely sensed estimates of flow direction allow spatial changes in flow velocity to be quantified and patterns of erosion and deposition to be estimated shallowing along channel margins produces spatial acceleration and erosion lateral spreading deceleration and deposition occur within three to eight channel widths outside of the channel margins in interdistributary bays the shape of each flow path is constrained by nourishment boundaries that separate the outflows from neighboring channels deposit elevation decreases with a basinward slope of 2 4 10 4 with distance from a channel margin along any flow path regardless of the channel or location that flow departed the network bathymetric depressions called interdistributary troughs form along nourishment boundaries where flow paths are the longest and deposit elevation is correspondingly low we conclude that the deposit morphology exerts a strong control on bathymetric evolution and that interaction between neighboring channels and even neighboring deltas can influence delta front morphology

validation of hydraulic tomography in an unconfined aquifer a controlled sandbox study in this study we demonstrate the effectiveness of hydraulic tomography ht that considers variably saturated flow processes in mapping the heterogeneity of both the saturated and unsaturated zones in a laboratory unconfined aquifer the successive linear estimator sle developed by mao et al 2013c for interpreting ht in unconfined aquifers is utilized to obtain tomograms of hydraulic conductivity k specific storage ss and the unsaturated zone parameters pore size parameter and saturated water content s for the gardner russo s model the estimated tomograms are first evaluated by visually comparing them with stratigraphy visible in the sandbox results reveal that the ht analysis is able to accurately capture the location and extent of heterogeneity including high and low k layers within the saturated and unsaturated zones as well as reasonable distribution patterns of and s for the gardner russo s model we then validate the estimated tomograms through predictions of drawdown responses of pumping tests not used during the inverse modeling effort the strong agreement between simulated and observed drawdown curves obtained by pressure transducers and tensiometers demonstrates the robust performance of ht that considers variably saturated flow processes in unconfined aquifers and the unsaturated zone above it in addition compared to the case using the homogeneous assumption ht results as expected yield significantly better predictions of drawdowns in both the saturated and unsaturated zones this comparison further substantiates the unbiased and minimal variance of ht analysis with the sle algorithm

ballooning of river plume bulge and its stabilization by tidal currents when freshwater debouches into an adjacent ocean an anticyclonic eddy bulge is formed in front of the river mouth it is well known that a bulge growing offshore ballooning hardly reaches a steady state in the absence of either ambient currents or wind forcing this study provides a physical interpretation for the ballooning of river plume bulges by conducting numerical experiments in which a river plume is induced by a coastal freshwater source part of the freshwater released to the model ocean undergoes inertial instability near inertial oscillations are predominant when disturbances are not forced in ambient waters of the river plume these isotropic disturbances are amplified by inertial instability so that unstabilized freshwater can move in arbitrary directions thus unstabilized freshwater does not need to move toward the coastal boundary current on the right hand side of the river mouth freshwater unstabilized for a long time can stay in the bulge for a long time unstabilized freshwater accumulates gradually in the bulge and so ballooning occurs when the direction of disturbances is prescribed in ambient waters unstabilized freshwater is forced to move in the same direction thereby the motion of unstabilized freshwater is restricted in the alongshore direction when background disturbances are induced by alongshore tidal currents it is therefore concluded that tidal currents play a role in stabilizing the offshore growth of river plume bulges in coastal and shelf waters

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

dynamic characteristics of the yellow river mouth the yellow river delta is dynamic because the river carries a heavy sediment load and serious siltation occurs in the area massive sediment transport brings fertile soil to the river mouth leading to its expansion by 20 25 km2 per year the dramatic reduction in runoff and sediment in the delta due to increasing water demand has extended the period of flow cut off the river changes its delta channel about once every ten years the channel shifts affect the upstream flood stages since the river shifted to the present delta channel in 1976 the shape of the delta has changed remarkably sediment laden flow has been building channels by depositing sediment in low velocity areas and has extended the river mouth into the sea for more than 40 km the velocity of sea currents is enhanced by the growth of the river mouth spit at a rate of 1 m s 1 per 10 km extension which results in high capacity of sea currents carrying sediment away from the mouth the new river mouth area was silted up by 14 m and the old river mouth area was eroded by 5 m siltation takes place in the low flow season and scouring occurs in the flood season scouring occurs during the rising limb of a flood and siltation occurs during the receding limb of a flood the roughness of the river changes largely following the increase in discharge the minimum manning roughness occurs at non siltation and non erosion discharge tidal influence in the river is restricted to a section extending over only a few kilometres unlike in many other coastal areas the river water with its high sediment concentration develops into density current and flows underneath the seawater during flood this mechanism contributes to sediment transportation in the delta copyright 2000 john wiley sons ltd

a geostatistical inverse method for variably saturated flow in the vadose zone a geostatistical inverse technique utilizing both primary and secondary information is developed to estimate conditional means of unsaturated hydraulic conductivity parameters saturated hydraulic conductivity and pore size distribution parameters in the vadose zone measurements of saturated hydraulic conductivity and pore size distribution parameters are considered as the primary information while measurements of steady state flow processes soil water pressure head and degree of saturation are regarded as the secondary information this inverse approach relies on the classical linear predictor cokriging theory and takes the advantage of the spatial cross correlation between the soil water pressure head and each of the following degree of saturation saturated hydraulic conductivity and a pore size distribution parameter using an approximate perturbation solution for steady variably saturated flow under general boundary conditions the cross covariances between the primary and secondary information are derived the approximate solution is formulated on the basis of a first order taylor series expansion of a discretized finite element equation the sensitivity matrix in the solution is evaluated by an adjoint state sensitivity approach for flow in heterogeneous media under variably saturated conditions through several numerical examples the inverse model demonstrates its ability to improve the estimates of the spatial distribution of saturated hydraulic conductivity and pore size distribution parameters using the secondary information

postseismic rebound in fault step overs caused by pore fluid flow near field strain induced by large crustal earthquakes results in changes in pore fluid pressure that dissipate with time and produce surface deformation synthetic aperture radar sar interferometry revealed several centimeters of postseismic uplift in pull apart structures and subsidence in a compressive jog along the landers california 1992 earthquake surface rupture with a relaxation time of 270 45 days such a postseismic rebound may be explained by the transition of the poisson s ratio of the deformed volumes of rock from undrained to drained conditions as pore fluid flow allows pore pressure to return to hydrostatic equilibrium

formation and maintenance of single thread tie channels entering floodplain lakes observations from three diverse river systems 1 tie channels connect rivers to floodplain lakes on many lowland rivers and thereby play a central role in floodplain sedimentology and ecology yet they are generally unrecognized and little studied here we report the results of field studies focused on tie channel origin and morphodynamics in the following three contrasting systems the middle fly river papua new guinea the lower mississippi river and birch creek in alaska across these river systems tie channels vary by an order of magnitude in size but exhibit the same characteristic morphology and appear to develop and evolve by a similar set of processes in all three systems the channels are characterized by a narrow leveed single thread morphology with maximum width approximately one tenth the width of the mainstem river the channels typically have a v shaped cross section unlike most fluvial channels these channels develop as lakes become isolated from the river by sedimentation narrowing of the connection between river and lake causes a sediment laden jet to develop levees develop along the margins of the jet leading to channel emergence and eventual levee aggradation to the height of the mainstem levees bidirectional flow in these channels is common outflows from the lake scour sediment and prevent channel blockage we propose that channel geometry and size are then controlled by a dynamic balance between channel narrowing by suspended sediment deposition and incision and widening by mass failure of banks during outflows tie channels are laterally stable and may convey flow for hundreds to a few thousand of years

hydrodynamic and suspended sediment transport controls on river mouth morphology 1 river mouths building into standing bodies of water have strikingly varied growth habits this presents a compelling pattern formation problem that is also of great practical relevance for subsurface prediction and managing coastal wetlands here we present a generalized 2 5 dimensional potential vorticity pv theory that explains sedimentation patterns of a sediment laden stationary jet by coupling an understanding of vorticity with suspended sediment concentration fields we explore the physical meaning of this new sediment pv definition and its impact on outflow depositional patterns by analyzing data from a shallow wall bounded plane jet experiment and by discussing new theoretical insights a key result is that lateral advection and diffusion of suspended sediment are directly proportional to jet vorticity a feature that reveals the mechanistic process that forms elongated channels by focused levee deposition the new pv theory constitutes a more generalized mathematical framework that expands the rouse theory for the equilibrium of suspended sediment

growth of river mouth bars in sheltered bays in the presence of frontal waves 1 one of the key processes for the formation of deltas and their fluvial networks is the deposition of mouth bars in front of prograding distributaries waves influence mouth bar growth but it is not clear how and to what extent toward this end we conduct a modeling study on deltas forming in sheltered bays where waves are locally generated and both longshore currents and surf zone are absent we focus on the simplified case of a homopycnal river plume subject to frontal wave attack and we begin by isolating the effects of waves on jet spreading an analytical model for the hydrodynamic interaction between incoming waves and a turbulent expanding jet is developed and tested with the numerical model delft3d coupled to the wave model swan both the analytical model and delft3d predict that incoming surface gravity waves increase the spreading of the jet and the interaction between wave and current boundary layers causes an increase in bottom friction to investigate how waves influence mouth bar morphodynamics a set of numerical simulations is run with delft3d swan utilizing a geometry and wave characteristics typical of sheltered bays our numerical results show that in the presence of waves mouth bars form up to 35 closer to the river mouth and 40 faster when compared to cases without waves the distance from the river mouth to the stagnated mouth bar decreases with increasing wave height and wave period the timescale of bar formation is inversely proportional to wave height and directly proportional to wave period our modeling study suggests that wave influence on mouth bar growth is complex small waves like the ones modeled here promote mouth bar formation via increased jet spreading and faster formation time which in turn should create deltas with more distributary channels on the other hand large waves suppress mouth bar formation as seen in other studies leading to fewer distributary channels

characterization of aquifer heterogeneity using transient hydraulic tomography 1 hydraulic tomography is a cost effective technique for characterizing the heterogeneity of hydraulic parameters in the subsurface during hydraulic tomography surveys a large number of hydraulic heads i e aquifer responses are collected from a series of pumping or injection tests in an aquifer these responses are then used to interpret the spatial distribution of hydraulic parameters of the aquifer using inverse modeling in this study we developed an efficient sequential successive linear estimator ssle for interpreting data from transient hydraulic tomography to estimate three dimensional hydraulic conductivity and specific storage fields of aquifers we first explored this estimator for transient hydraulic tomography in a hypothetical one dimensional aquifer results show that during a pumping test transient heads are highly correlated with specific storage at early time but with hydraulic conductivity at late time therefore reliable estimates of both hydraulic conductivity and specific storage must exploit the head data at both early and late times our study also shows that the transient heads are highly correlated over time implying only infrequent head measurements are needed during the estimation applying this sampling strategy to a well posed problem we show that our ssle can produce accurate estimates of both hydraulic conductivity and specific storage fields the benefit of hydraulic tomography for ill posed problems is then demonstrated finally to affirm the robustness of our ssle approach we apply the ssle approach to a hypothetical three dimensional heterogeneous aquifer

semidiurnal switching of stratification in the region of freshwater influence of the rhine observations in the rhine region of freshwater influence rofi system in the north sea show evidence of large semidiurnal oscillations in stability superimposed on a mean stratification occurring throughout the stratified region at times of reduced mixing the amplitude of this semidiurnal variation is of the same order as the mean stability and frequently results in conditions being mixed or nearly mixed once per tide it is deduced that this semidiurnal variation results primarily from cross shore tidal straining which interacts with the density gradient to induce stratification this conceptual picture of the contributing processes has been tested in a one dimensional point model forced by the observed slopes and the local density gradients the model exhibits the same qualitative behavior as the observations produces oscillations in stratification of the amplitude observed and confirms the critical role of cross shore tidal straining the large cross shore shear nder stratified conditions is identified with the changes in ellipse configuration which are observed between mixed and stratified conditions the occurrence of semidiurnal variations in stability in the rhine rofi is thus inferred to be a consequence of the development of mean stability whenever the horizontal density gradients relax in conditions of low stirring

coastal mixing in multiple mouth deltas a case study in the po delta italy abstract satellite imagery provides evidence of complex mixing dynamics in the coastal zone in front of multiple mouth deltas one peculiar feature identified in front of the po delta italy consists in warmer water bulges present in some periods in the coastal zone between the river mouths such features are evident during both high and low river discharge through an integrated approach based on the analysis of satellite imagery in situ field data and a high resolution oceanographic model representing the whole river delta sea system we investigated the relative contribution of the different forcing in controlling coastal mixing of riverine waters the results evidence that the occurrence of these warmer saltier water bulges is due to upwelling induced by the combined action of tides and wind regimes aligned along coastline winds from the land and along the coast drive the upwelling through the well known mechanism described by ekman the presence of river discharge enhance the water column stratification creating the conditions in which tidal action follows the tidal straining theory both processes are identified in modelling results the occurrence of these localized coastal waters with peculiar thermohaline characteristics detectable on satellite imagery of the area can be relevant in the definition of the freshwater areas of influence and the mechanisms of riverine water mixing in the near coastal zone this can shed some light eventually on characterizing the sediment dynamics as well as the thermohaline properties of waters in the area and also to identify eventual impacts on the local ecosystems and fishery

relating river plume structure to vertical mixing abstract the structure of a river plume is related to the vertical mixing using an isohaline based coordinate system salinity coordinates offer the advantage of translating with the plume as it moves or expanding as the plume grows this coordinate system is used to compare the relative importance of different dynamical processes acting within the plume and to describe the effect each process has on the structure of the plume vertical mixing due to inertial shear in the outflow of a narrow estuary and wind mixing are examined using a numerical model of a wind forced river plume vertical mixing and the corresponding entrainment of background waters is greatest near the estuary mouth where inertial shear mixing is large this region is defined as the near field with the more saline far field plume beyond wind mixing increases the mixing throughout the plume but has the greatest effect on plume structure at salinity ranges just beyond the near field wind mixing is weaker at high salinity classes tha

co seismic geoelectric potential changes observed in japan co seismic signals of geoelectric potential have been observed for several earthquakes eqs in japan it has been argued that if there are precursory changes stronger signals should be at the time of eq our results prove that co seismic signals do exist but they start with the arrival of seismic waves and not at the origin time of eqs their amplitude does not scale with the dipole length both facts indicate that the changes are local effects of passing seismic waves and not due to electric signals emitted from the seismic source observed changes contain two components oscillatory and offset decay ones the former is synchronized with seismic waves whereas the latter with a rise time of a few seconds and a decay time of 0 1 1 hour is suspected to be related to eq induced subsurface hydrological change in both cases the origin of electric potential variations may be electrokinetic

a temporal sampling strategy for hydraulic tomography analysis 1 this paper investigates optimal sampling times of drawdowns for the analysis of hydraulic tomography ht survey the investigation was carried out by analyzing the spatial and temporal evolution of cross correlations between the head responses at an observation well and transmissivity t and storage coefficient s properties during a pumping test in homogeneous and heterogeneous aquifers the analysis shows that the cross correlation between the head and s values is limited to the region between the observation and the pumping well in the aquifers it reaches the highest value near the early time tm and decays to zero afterwards the time tm is approximately equal to the time t0 at which the extrapolated drawdown from the first straight line portion of an observed drawdown log time plot becomes zero at early times the high cross correlation between the head and t is confined to the region between the observation and the pumping well this region then evolves into two humps one on each side of the circular region encompassing the observation well and the pumping well the size of the two humps expands and their values reach the maximum as flow reach steady state as a consequence we hypothesize that pairs of head data at t0 and those at either the steady state or a late time during an ht survey could yield the best estimates of the heterogeneous t and s fields results from numerical experiments have verified this hypothesis and demonstrated that this sampling strategy is generally applicable even when the boundary condition is unknown we therefore recommend in principle that 1 carrying out pumping tests of ht surveys for sufficiently long period of time such that drawdown reaches the entire area of interest and 2 using a constant head or zero drawdown for all boundaries during the inverse modeling analysis

detection of karst conduit patterns via hydraulic tomography a synthetic inverse modeling study abstract karst aquifers are geologic media that consist of enlarged solution conduits the geometry of such conduits is very difficult to delineate and its hydraulic parameters such as hydraulic conductivity k and specific storage ss are difficult to estimate hydraulic tomography ht shown to successfully map the hydraulic heterogeneity of porous and fractured rock aquifers may also be effective in karst aquifers in this study we utilize two synthetic karst aquifers with the branchwork and network conduit patterns bcp and ncp to explore the ability of ht to map conduit patterns and their hydraulic parameters prior to the ht analysis forward models are built to study groundwater flow through synthetic karst aquifers to generate data and to gain insights into flow patterns forward modeling results reveal that 1 flow rates at the outlet of the bcp model is higher than the ncp model even when the same water injection rates are applied at the wells 2 the location of the injection well has a significant impact on the flow rate at the outlet and 3 flow lines mainly follow the conduit pattern these results suggest that the accurate mapping of karst networks is critical next the ht analysis of these data is conducted using a set of six models bcp and ncp to map the heterogeneity of different conduit systems under steady state and transient flow conditions inverse modeling results reveal that the estimated k maps become more reliable as the number of injection tests and the corresponding monitoring data are increased in the ht analysis moreover the results of the bcp model is closer to the original conduit pattern in comparison to the ncp model a comparative study of three inverse models with different number of observation wells and assuming the availability of geological data has shown that a more reliable pattern of the conduits can be mapped by adding geological information even when a small number of observation wells are available overall our results confirm the feasibility of ht in detecting major conduits as well as being able to differentiate between the bcp and ncp network types

high resolution electromagnetic imaging of the san andreas fault in central california although there is increasing evidence that fluids may play a significant role in the earthquake rupture process direct observation of fluids in active fault zones remains difficult since the presence of an electrically conducting fluid such as saline pore water strongly influences the overall conductivity of crustal rocks electrical and electromagnetic methods offer great potential for overcoming this difficulty here we present and compare results from high resolution magnetotelluric mt profiles across two segments of the san andreas fault saf which exhibit very different patterns of seismicity parkfield which has regular small earthquakes and creep events and in the carrizo plain where the fault is seismically quiescent and apparently locked in both surveys electric fields were sampled continuously with 100 m long dipoles laid end to end across the fault from 100 to 0 1 hz the data from both profiles are consistent with a two dimensional 2 d fault parallel resistivity model when both transverse electric and magnetic te and tm mode data are included in the interpretation narrow approximately 300 endash 600 m wide zones of low resistivity extending to depths of 2 endash 4 km in the core of the fault are required at both locations however at parkfield the conductance conductivity thickness product of the anomalous region ismore an order of magnitude larger than at carrizo plain suggesting much higher concentrations of fluids for the more seismically active parkfield segment we also image structural differences between the two segments at carrizo plain resistive presumably crystalline rocks are present on both sides of the fault at depths below 3 endash 4 km in particular we clearly image resistive basement extending approximately 10 km or more east of the saf beneath the elkhorn hills and temblor range at parkfield the situation is quite different with a resistive block of salinian granite west of the fault and an electrically conductive presumably fluid rich franciscan complex to the east it is possible that these structural differences control the difference in mechanical behavior of the fault either directly by affecting fault strength or indirectly by controlling fluid supply copyright 1999 american geophysical union less

alongshore sediment bypassing as a control on river mouth morphodynamics river mouths shoreline locations where fluvial and coastal sediments are partitioned via erosion trapping and redistribution are responsible for the ultimate sedimentary architecture of deltas and because of their dynamic nature also pose great management and engineering challenges to investigate the interaction between fluvial and littoral processes at wave dominated river mouths we modeled their morphologic evolution using the coupled hydrodynamic and morphodynamic model delft3d swan model experiments replicate alongshore migration of river mouths river mouth spit development and eventual spit breaching suggesting that these are emergent phenomena that can develop even under constant fluvial and wave conditions furthermore we find that sediment bypassing of a river mouth develops though feedbacks between waves and river mouth morphology resulting in either continuous bypassing pathways or episodic bar bypassing pathways model results demonstrate that waves refracting into the river mouth bar create a zone of low alongshore sediment transport updrift of the river mouth which reduces sediment bypassing sediment bypassing in turn controls the river mouth migration rate and the size of the river mouth spit as a result an intermediate amount of river discharge maximizes river mouth migration the fraction of alongshore sediment bypassing can be predicted from the balance between the jet and the wave momentum flux quantitative comparisons show a match between our modeled predictions of river mouth bypassing and migration rates observed in natural settings

simulation of field injection experiments in heterogeneous unsaturated media using cokriging and artificial neural network 1 simulations of moisture flow in heterogeneous soils are often hampered by lack of measurements of soil hydraulic parameters making it necessary to rely on other sources of information in this paper we develop a methodology to integrate data that can be easily obtained for example initial moisture content i bulk density and soil texture with data on soil hydraulic properties via cokriging and artificial neural network ann based pedotransfer functions the method is applied to generate heterogeneous soil hydraulic parameters at a field injection site in southeastern washington state stratigraphy at the site consists of imperfectly stratified layers with irregular layer boundaries cokriging is first used to generate three dimensional heterogeneous fields of bulk density and soil texture using an extensive data set of field measured i which carry signature about site heterogeneity and stratigraphy soil texture and bulk density are subsequently input into an ann based site specific pedotransfer function to generate three dimensional heterogeneous soil hydraulic parameter fields the stratigraphy at the site is well represented by the estimated pedotransfer variables and soil hydraulic parameters the parameter estimates are then used to simulate a field injection experiment at the site a relatively good agreement is obtained between the simulated and observed moisture contents the spatial distribution pattern of observed moisture content as well as the southeastward moisture movement is captured well in the simulations in contrast to earlier work using an effective parameter approach yeh et al 2005 we are able to reproduce the observed splitting of the moisture plume in a coarse sand unit that is sandwiched between two fine textured units the simple method of combining cokriging and ann for site characterization provides unbiased prediction of the observed moisture plume and is flexible so that additional measurements of various types can be included as they become available

electromagnetic precursors to earthquakes in the ulf band a review of observations and mechanisms despite over 2 decades of international and national monitoring of electrical signals with the hope of detecting precursors to earthquakes the scientific community is no closer to understanding why precursors are observed only in some cases laboratory measurements have demonstrated conclusively that self potentials develop owing to fluid flow and that both resistivity and magnetization change when rocks are stressed however field experiments have had much less success many purported observations of low frequency electrical precursors are much larger than expectations based on laboratory results in some cases no precursors occurred prior to earthquakes or precursory signals were reported with no corresponding coseismic signals nonetheless the field experiments are in approximate agreement with laboratory measurements maximum resistivity changes of a few percent have been observed prior to some earthquakes in china but the mechanism causing those changes is still unknown anomalous electric and magnetic fields associated with fluid flow prior to earthquakes may have been observed finally piezomagnetic signals associated with stress release in earthquakes have been documented in measurements of magnetic fields

the october 17 1989 loma prieta california earthquake and its aftershocks geometry of the sequence from high resolution locations hypocenters of the loma prieta sequence form a dipping zone that rises from the mainshock hypocenter and is parallel to the mainshock nodal plane most aftershocks cluster around the perimeter of the zone surrounding a relatively aseismic center which approximates the region of mainshock rupture at its southeastern end the dipping aftershock zone warps into a vertical surface that corresponds to the san andreas fault in the central and northwestern parts of the zone at depths above 10 km the aftershocks define numerous disjoint fault structures the large component of reverse slip observed in this event agrees with a simple model for slip on a dipping plane within a compressional fault bend we do not believe that the loma prieta earthquake occurred on the sargent fault however we are unable to conclude whether it ruptured the principal plate boundary fault or a less frequently active fault

turbulent characteristics of a shallow wall bounded plane jet experimental implications for river mouth hydrodynamics jets arising from rivers streams and tidal flows entering still waters differ from most experimental studies of jets both in aspect ratio and in the presence of a solid bottom boundary and an upper free surface despite these differences the applicability of experimental jet studies to these systems remains largely untested by either field or realistically scaled experimental studies here we present experimental results for a wall bounded plane jet scaled to jets created by flow discharging into floodplain lakes a characteristic feature of both our prototype and experimental jets is the presence of large scale meandering turbulent structures that span the width of the jets in our experimental jets we observe self similarity in the distribution of mean streamwise velocities by a distance of six channel widths downstream of the jet outlet after a distance of nine channel widths the velocity decay and the spreading rates largely agree with prior experimental results for plane jets the magnitudes and distributions of the cross stream velocity and lateral shear stresses approach self preserving conditions in the upper half of the flow but decrease in magnitude and deviate from self preserving distributions with proximity to the bed the presence of the meandering structure has little influence on the mean structure of the jet but dominates the jet turbulence a comparison of turbulence analysed at time scales both greater than and less than the period of the meandering structure indicates that these structures increase turbulence intensities by 3 5 times and produce lateral shear stresses and momentum diffusivities that are one and two orders of magnitude greater respectively than turbulence generated by bed friction alone

results of ultra low frequency magnetic field measurements during the guam earthquake of 8 august 1993 we report the results of measurements of ultra low frequency magnetic noise during a large earthquake ms 7 1 at guam of 8 august 1993 depth 60 km the ulf observing system is located in the guam island about 65 km from the epicenter several distinct features of this analysis are summarized 1 we have proposed rather sophisticated statistical analyses monthly mean standard deviation in order to estimate the wave intensity and polarization i e ratio z h 2 a comparison between the ulf wave activity and kp is useful in distinguishing between the space geomagnetic pulsations and non space emissions 3 then the use of the ratio z h is found to be of essential importance in discrimating the emissions presumably of seismic origin from space plasma waves 4 the statistical analysis of the temporal evolution of this ratio has yielded that it shows a broad maximum only about one month before the earthquake and this suggests that the emissions during this period are very likely to be magnetic precursors 5 the temporal variation of z component is similar to that for the loma prieta earthquake such that it shows a broad maximum ten days two weeks before the earthquake and another increase a few days before the earthquake 6 the emissions presumably associated with the earthquake are of noise like nature and their main frequency is 0 02 0 05 hz with maximum intensity 0 1 nt

low frequency magnetic field measurements near the epicenter of the ms 7 1 loma prieta earthquake the authors report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the m sub s 7 1 loma prieta earthquake of 17 october 1989 their measurements cover 25 narrow frequency bands in the more than six decade frequency range 0 01 hz 32 khz with a time resolution varying from a half hour in the ulf range 0 01 10 hz to one second in the elf vlf range 10 hz 32 khz the ulf system is located near corralitos about 7 km from the epicenter the elf vlf system is located on the stanford campus about 52 km from the epicenter analysis of the elf vlf system is located on the stanford campus about 52 km from the epicenter analysis of the elf vlf data has revealed no precursor activity that they can identify at this time however the ulf data have some distinctive and anomalous features first a narrow band signal appeared in the range 0 05 0 2 hz around september 12 and persisted until the appearance of the second anomalous feature which consisted of a substantial increase in the noise background starting on 5 october and covering almost the entire frequency range of the ulf system third there was an anomalousmore dip in the noise background in the range 0 2 5 hz starting one day ahead of the earthquake finally and perhaps most compelling there was an increase to an exceptionally high level of activity in the range 0 01 0 5 hz starting approximately three hours before the earthquake there do not appear to have been any magnetic field fluctuations originating in the upper atmosphere that can account for this increase further while the systems are sensitive to motion seismic measurements indicate that there were no significant shocks preceding the quake thus the various anomalous features in the data and in particular the large amplitude increase in activity starting three hours before the quake may have been magnetic precursors less

a systematic compilation of earthquake precursors article a survey of published scientific literature was undertaken to identify and catalog observed earthquake pre cursors the earthquake precursors selected for analysis included electric and magnetic fields gas emissions groundwater level changes temperature changes surface deformations and seismicity for each of these precursors the published scientific literature was searched to document the statistics of each reported earth quake precursor spatial extent time duration amplitude signal noise ratio to analyze dependence of the observable for each precursor on earthquake magnitude and to explore proposed physical models to explain eachearthquakeprecursor somegeneralcharacteristics wereobservedfortheseprecursoryphenomena first the largest amplitude precursory anomalies tend to occur before the largest magnitude earthquakes also the number of precursory anomalies tends to increase the closer in time to the occurrence of the earthquake finally theprecursoryanomaliestendtooccurclosetotheeventualepicenterof theearthquake ingeneral the physical models indicate that all of the precursory phenomena are related to deformation that occurs near the fault prior to the main earthquake while the models provide plausible physical explanations for the pre cursors there are many free parameters in the models that are poorly resolved

mechanics of river mouth bar formation implications for the morphodynamics of delta distributary networks 1 in this paper we use observational data and numerical modeling to present a new explanation for the formation of river dominated delta networks observational data from deltas throughout the world show that distributary channel widths depths and lengths decrease nonlinearly with successive bifurcations trends in width and depth are an outcome of hydraulic geometry scaling the trend in channel length is a consequence of delta growth analyses of serial maps show that the positions of bifurcations are the fossilized locations of river mouth bars also called middle ground and distributary mouth bar in front of old delta channel mouths therefore the trend in channel length can be explained through the mechanics of river mouth bar formation and evolution we use delft3d a coupled hydrodynamic and morphodynamic model to simulate the process of river mouth bar formation within an expanding turbulent jet in front of distributary channel mouths our results describe in detail the formation and evolution of a river mouth bar system and demonstrate that the distance to the river mouth bar is proportional to jet momentum flux and inversely proportional to grain size therefore channel length decreases down delta because with each successive bifurcation the jet momentum flux decreases these results can be used to predict the future evolution of river dominated deltas and can be used to aid in hydrocarbon exploration of these depositional environments

morphodynamics of subaqueous levee formation insights into river mouth morphologies arising from experiments 1 levee formation by deposition from sediment laden flows debouching into open waters has been conceptually and analytically linked to jet hydrodynamics since the middle of the 20th century despite this long standing association between jets and levees the morphodynamics controlling subaqueous levee development remain largely unquantified and poorly understood here we report the results of physical experiments on subaqueous levee development modeled on floodplain tie channel processes in the laboratory we created subaqueous levees from a sediment laden jet entering a basin of still water levee formation occurred where the local shear velocity of the jet declined below the critical shear velocity for entrainment of sediment into suspension the rate of levee deposition depended on the rate of lateral sediment transfer to the jet margins and the settling velocity of suspended sediment in our friction dominated flows lateral sediment flux to the levees was primarily due to dispersive lateral transport driven by turbulence high rates of sediment transfer from the jet core to the margins led to levee growth that outpaced deposition along the flow centerline under hypopycnal conditions we failed to produce levees we observed distinct differences in levee morphology due to changes in the size and density of suspended sediment under identical hydrodynamic conditions our experimental results and review of published data on hypopycnal river mouths suggest that suspended sediment characteristics relative magnitude of bed friction and channel outlet aspect ratios may have a more significant influence on river mouth morphology than the buoyancy of discharging waters as envisioned in existing conceptual models

results of ulf magnetic field measurements near the epicenters of the spitak ms 6 9 and loma prieta ms 7 1 earthquakes comparative analysis the characteristics of the ulf magnetic field emissions measured at two magnetic observatories in the republic of georgia prior to and after the ms 6 9 earthquake that occurred near spitak armenia on december 7 1988 are compared with the apparently similar emissions associated with the ms 7 1 earthquake that occurred near loma prieta california on october 17 1989 the main features of the spitak measurements according to observations made at the dusheti station 128 km to the spitak epicenter as compared with the loma prieta measurements which were made at corralitos california 7 km to the loma prieta epicenter are the following 1 the intensity of ulf background activity started growing 3 to 5 days before the spitak earthquake whereas the corresponding increase in activity began 12 days before the loma prieta earthquake 2 a substantial ulf emission burst was recorded at dusheti starting 4 hours prior to the main shock a similar large burst of ulf activity commenced 3 hours before the loma prieta event and continued until the occurrence of the main shock 3 ulf activity remained high for about two weeks after the spitak earthquake and for several months after the loma prieta earthquake 4 ulf noise bursts were observed 1 to 6 hours before powerful aftershocks at spitak during the period of enhanced activity but there was no conclusive link between the ulf noise at corralitos and the after shocks a major difference in the ulf activity preceding the two earthquakes is a difference in amplitude 0 2 nt at spitak and 5 nt at loma prieta but this is easily explained as being caused by the different distances of the observation stations from the epicenters

controls on river delta formation insights from numerical modelling abstract primary hydraulic and sedimentary controls on river deltas were previously postulated in classification schemes in the field of sedimentary geology based on many field observations however detailed mechanistic models were restricted to the river dominated delta class analogous to the concept of morphological conditioning here we show that antecedent stratigraphy controls morphometrics of prograding river dominated delta distributary networks under steady sea level further we use coupled hydrodynamic morphodynamic stratigraphic detailed numerical modelling to assess the influence of wind generated waves and tides on clastic river delta formation our synthetic simulations show that deltas forming under mere riverine forcing prograde via sequences of mouth bar induced flow bifurcation and upstream channel shifting windwave action suppresses sequestration of fine sediments on the developing delta plain entailing relatively smooth shorelines perturbed by a limited number of distributary channels in contrast tide influenced river deltas are found to prograde mainly via lengthening of initially formed relatively stable distributaries as well as being characterized by cyclicity in deposits interbedding of sands and silts these results provide a framework for physics based river delta modelling under various environmental conditions particularly our findings suggest that relatively low energetic basin conditions can already significantly impact morphological and stratal patterns of prograding river deltas

comparative hydrodynamics of 10 mediterranean lagoons by means of numerical modeling a comparison study between 10 mediterranean lagoons has been carried out by means of the 3 d numerical model shyfem the investigated basins are the venice and marano grado lagoons in the northern adriatic sea the lesina and varano lagoons in the southern adriatic sea the taranto basin in the ionian sea the cabras lagoon in sardinia the ganzirri and faro lagoons in sicily the mar menor in spain and the nador lagoon in morocco this study has been focused on hydrodynamics in terms of exchange rates transport time scale and mixing water exchange depends mainly on the inlet shape and tidal range but also on the wind regimes in the case of multi inlet lagoons water renewal time which is mostly determined by the exchange rate is a powerful concept that allows lagoons to be characterized with a time scale in the case of the studied lagoons the renewal time ranged from few days in the marano grado lagoon up to 1 year in the case of the mar menor the analysis of the renewal time frequency distribution allows identifying subbasins the numerical study proved to be a useful tool for the intercomparison and classification of the lagoons these environments range from a leaky type to a choked type of lagoons and give a representative picture of the lagoons situated around the mediterranean basin mixing efficiency turns out to be a function of the morphological complexity but also of the forcings acting on the system

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

delta progradation driven by an advancing sediment source coupled theory and experiment describing the evolution of elongated deltas 1 most of the models of alluvial fan and delta morphodynamics to date have used one of two assumptions concerning the upstream boundary condition 1 a fixed sediment feed point at the vertex or 2 a feed point corresponding to a bedrock alluvial transition that e g migrates upstream under conditions of constant base level here however we present both experimental and numerical results pertaining to a new configuration i e one in which the sediment feed point migrates downstream the research is motivated by the operation of the tailings pond of a mine in which a delta forms the mine operators must frequently move the outfalls of their slurry pipelines downstream to avoid their burial under sediment as the delta aggrades this downstream migration results in a characteristic delta configuration under the right conditions as the tailings delta progrades downstream it does not have time to fill all the available space in the lateral direction thus leaving unfilled open water on either side the result is the formation of an elongated delta here we use both experiments and numerical modeling to characterize the control of the feed point migration rate on the lateral extent of the prograding deltas we use these results to obtain a partial explanation of natural elongated deltas in the case of such deltas the downstream progradation of natural levees provides an analog to the downstream migration of the outfall of a slurry pipeline

ultra low frequency electromagnetic measurements associated with the 1998 mw 5 1 san juan bautista california earthquake and implications for mechanisms of electromagnetic earthquake precursors abstract ultra low frequency ulf 0 01 10 hz magnetic field anomalies prior to m 6 0 earthquakes have been reported in various regions of the world here we test whether smaller magnitude earthquakes have associated ulf anomalies by using continuous measurements made 9 km above and 2 km ne of the epicenter of the m w 5 1 8 12 98 san juan bautista ca earthquake half hour spectral averages of the magnetic field data in nine different ulf frequency bands show no long term days to weeks magnetic field anomaly prior to the earthquake in addition magnetic and electric field polarization shows no anomalous behavior clearly associated with seismic activity an 0 02 nt increase in activity for 2 h prior to the earthquake is close to the background noise levels precluding identification of a precursor we present scaling calculations to show if precursory ulf anomalies are related to the size of the earthquake the observed 0 02 nt increase is the approximately expected magnetic anomaly prior to the san juan bautista sbj earthquake based on the anomaly observed prior to the 1989 loma prieta lp earthquake our reexamination of previously proposed dilatant conductive and piezomagnetic mechanisms for ulf precursors shows that these effects are consistent with the observations prior to the sjb earthquake based on the observations i e lack of a strong precursor in contrast previously proposed electrokinetic and magnetohydrodynamic mhd mechanisms do not appear to be appropriate for this particular earthquake this analysis further supports the hypothesis that precursory ulf signals are related to the size of the earthquake

analysis of ultralow frequency electromagnetic field measurements associated with the 1999 m 7 1 hector mine california earthquake sequence we installed two electromagnetic em monitoring systems in the im mediate aftermath of the 16 october 1999 hector mine earthquake to search for possible continuing ultralow frequency ulf em activity due to the mainshock as well as for any precursory or coseismic em signals that might be associated with large aftershocks we installed the first portable monitoring system 2 5 days after the m 7 1 hector mine earthquake at a location 16 km southeast of the epicenter and 2 km east of the surface rupture a second system was installed on 29 october 1999 10 km northwest of the epicenter and within 100 m of the surface rupture our continuous measurements of multiple component magnetic field electric field and ground motion span the low frequencies appropriate for recording possible em sig nals generated at seismogenic depths and were carried out during 3 months following the mainshock continuous magnetic field measurements at observatory em stations operating in california are used as remote reference sites to remove global atmo spheric signals which helps isolate local terrestrial sources of interest our analysis of preseismic ulf em variations the coseismic response the 2 month long magnetic field power spectra and electric field polarization shows no anomalous behavior clearly associated with seismic activity

comparison of ultra low frequency electromagnetic signals with aftershock activity during the 1989 loma prieta earthquake sequence abstract ultra low frequency 0 01 to 10 0 hz magnetic field fluctuations near the epicenter of the 1989 loma prieta earthquake rose sharply immediately before the earthquake following indications of increased disturbance during the previous 12 days the magnetic activity remained much higher than the pre earthquake background level for 6 weeks following the mainshock these observations suggest a causal relationship between the earthquake failure process and the magnetic signals a search for similar precursory electromagnetic signals associated with aftershocks of this earthquake yields negative results specifically no correlation appears to exist between the amplitude of the electromagnetic activity and the frequency or magnitude of aftershocks following the mainshock either a threshold earthquake magnitude larger in this case than m l 5 5 may be necessary to generate precursory electromagnetic signals or the continued generation of magnetic signals related to the mainshock may have masked signals generated by the larger aftershocks

observation and three dimensional simulation of chloride plumes in a sandy aquifer under forced gradient conditions two well forced gradient tracer experiments over a distance of 5 m were carried out in a coastal sandy aquifer at georgetown south carolina the evolution of three dimensional chloride plumes during two tracer experiments was observed a three dimensional finite element model for flow and transport was used with extensive hydraulic conductivity data obtained from slug tests to assess our ability to predict solute transport in the aquifer results showed that our predictive ability is limited to the bulk behavior of the plumes which is mainly controlled by some significant heterogeneities in addition hydraulic conductivity values estimated by the hvorslev method for analysis of slug tests best represented the hydraulic conductivity distribution of the sandy aquifer finally the results of a long term experiment illustrate the importance of temporal variability in boundary conditions in the prediction of solute transport in aquifers

cross correlation analysis and information content of observed heads during pumping in unconfined aquifers 1 using a first order cross correlation analysis this paper investigates the relationship between observed heads and hydraulic properties in the saturated and vadose zones at different times and locations of three dimensional unconfined aquifers during pumping tests cross correlation analysis is a weighted sensitivity analysis casted into a stochastic framework it determines the relative impact of each parameter with respect to others in time and space on the observed heads according to uncertainty or spatial variability of each parameter it reveals the information content in measured drawdowns about heterogeneity during a pumping test in an unconfined aquifer which is critical for aquifer parameter estimation based on a synthetic numerical example our cross correlation analysis reveals that heads in the saturated zone at late times carry the greatest nonsymmetrically weighted information content about the hydraulic conductivity ks distribution within the cone of depression on the other hand heads in the saturated zone at early times contain the most information about the specific storage ss heterogeneity in a narrow region between the observation and pumping locations during intermediate and late times heads in the saturated zone largely reflect the effects of saturated water content s and pore size parameter in the thin unsaturated region near the water table above the pumping and observation locations at last heads in the vadose zone at late times carry the greatest information about s and around the observation point

close temporal correspondence between geomagnetic anomalies and earthquakes during the 2002 2003 eruption of etna volcano 1 the early stages of the 2002 2003 lateral eruption at mount etna were accompanied by slow changes over some hours and some rapid step offsets in the local magnetic field at five monitoring locations the total magnetic field intensity has been measured using continuously operating overhauser magnetometers at a sampling rate of 10 s the very unique aspect of these observations is the close temporal correspondence between magnetic field offsets and earthquakes that occurred in the upper northern flank of the volcano on 27 october 2002 prior to a primary eruption rapid coseismic changes of the magnetic field were clearly identified for three of the most energetic earthquakes which were concentrated along the northeast rift at a depth of about 1 km below sea level coseismic magnetic signals with amplitudes from 0 5 to 2 5 nt have been detected for three of the largest seismic events located roughly midway between the magnetic stations we quantitatively examine possible geophysical mechanisms which could cause the magnetic anomalies the comparison between magnetic data seismicity and surface phenomena implies that piezomagnetic effects are the primary physical mechanism responsible for the observed magnetic anomalies although the detailed cause of the rapid high stress change required is not clear the modeling of the observed coseismic magnetic changes in terms of piezomagnetic mechanism provides further evidence of the complex interaction between volcanic and tectonic processes during dike propagation along the northeast rift

the effects of rotation and river discharge on net mixing in small mouth kelvin number plumes abstractsmall mouth kelvin number plumes or plumes with a source width smaller than the deformation radius are characterized by near field plume regions of rapid lateral expansion and strong vertical mixing net plume mixing or the dilution of a plume by ocean water between the estuary mouth and the far field plume is examined using idealized numerical experiments with the regional ocean modeling system roms the density anomaly of plume water entering the far field is determined from isohaline analysis of the modeled salinity field the experiments indicate that when estuarine discharge increases net plume mixing decreases in a rotating environment but increases in a nonrotating environment scaling analysis supports that this opposite trend in behavior is related to rotation turning the plume limiting the lateral expansion and suppressing shear mixing the results of this study explain different trends in net plume mixing reported in previous studies and compare favorably to observations of the

the dynamics of a river bay delta system an analysis has been made to aid our understanding of various phenomena associated with turbulent plane jets that issue from river outlets and discharge into quiescent bays an integrated form of the hydrodynamic equations of flow continuity and momentum balance coupled with advective diffusion mass transport equations has been formulated into a two dimensional spatial domain closed form analytical solutions are obtained with the aid of similarity functions for the velocity and sediment concentration profiles a numerical procedure for estimating the areal and volume deposition of the delta sediments is presented the atchafalaya river delta a newly developing delta in the south central louisiana coast is used to illustrate analytical techniques the results of analytical predictions are compared with the results predicted by other statistical and generic approaches an agreement is found a sensitivity analysis is performed to assess the relative importance of various parameters in the river bay delta system

a potential vorticity theory for the formation of elongate channels in river deltas and lakes 1 rivers empty into oceans and lakes as turbulent sediment laden jets which can be characterized by a gaussian horizontal velocity profile that spreads and decays downstream because of shearing and lateral mixing at the jet margins recent experiments demonstrate that this velocity field controls river mouth sedimentation patterns in nature diffuse jets are associated with mouth bar deposition forming bifurcating distributary networks while focused jets are associated with levee deposition and the growth of elongate channels that do not bifurcate river outflows from elongate channels are similar in structure to cold filaments observed in ocean currents where high potential vorticity helps to preserve coherent structure over large distances motivated by these observations we propose a hydrodynamic theory that seeks to predict the conditions under which elongate channels form our approach models jet velocity patterns using the flow vorticity both shearing and lateral spreading are directly related to the vertical component of vorticity we introduce a new kind of potential vorticity that incorporates sediment concentration and thus allows study of jet sedimentation patterns the potential vorticity equation reduces the number of fluid momentum equations to one without losing generality this results in a compact analytical solution capable of describing the streamwise evolution of the potential vorticity of a sediment laden jet from initial conditions at the river mouth our theory predicts that high potential vorticity is a necessary condition for focused levee deposition and the creation of elongate channels comparison to numerical laboratory and field studies indicates that potential vorticity is a primary control on channel morphology our results may be useful for designing river delta restoration schemes such as the proposed mississippi delta diversion

recent movement on the garlock fault as suggested by water level fluctuations in a well in fremont valley california water levels have been continuously recorded since march 1978 in a well in fremont valley where several strands of the adjacent garlock fault zone have exhibited both left lateral displacement and components of normal displacement differences in water levels indicate that a fault segment lies between the observation well and a nearby irrigation well during the 4 year recording period six sharp fluctuations or spikes were noted these fluctuations occurring over 2 to 4 day periods have amplitudes of 15 30 cm they appear to be the result of creep events on a nearby fault two types of creep events are plausible 1 normal slip on an en echelon trace of the garlock fault less than 300 m south of the well with the north side up relative to fremont valley or 2 left lateral slip on the same fault because of the nature of the fluctuations we favor the latter interpretation dislocation models utilizing exponential arc tangent and skewed cosine functions were used to analyze the water level fluctuations associated pressure distribution and fault displacements the results suggest that creep on the fault ranges from several millimeters to a centimeter for individual events estimates of cumulative creep for the period 1978 1982 range from 20 to 50 mm depending on the particular model employed

late holocene sea level changes and paleoclimate recorded in lake lungue southern mozambique abstract we report on late holocene paleoenvironmental and sea level changes in southern mozambique based on analysis of diatom stratigraphy mineral magnetic susceptibility saturation isothermal remnant magnetization sirm and organic carbon content in a sediment core from lake lungue located ca 30 km north of xai xai city in the limpopo river floodplain eleven radiocarbon dates performed on terrestrial shells allowed establishment of an age depth model high content of brackish marine taxa especially diploneis suborbicularis and navicula yarrensis suggests that the lake lungue basin was part of the indian ocean coastal zone between ca 740 and 910 ce suggesting higher relative sea level during this phase a similar diatom distribution in older parts of the sequence of unknown age indicates that the site was connected to the indian ocean also at some stage prior to 740 ce between ca 910 and 1130 ce the basin was under a combined effect of sporadic marine water influx and limpopo river actions a freshwater lake was established sometime between 1130 and 1360 ce when the basin became less affected by marine influences due to lower sea levels instead the lake system was more influenced by limpopo river meandering dynamics and flooding events and eventually isolated into a more or less independent lake ecosystem where diatom diversity most likely responded to lake levels fluctuations driven by shifts in relative humidity and rainfall amounts within the floodplain and also from upstream sources during this phase high abundance of freshwater planktonic species i e aulacoseira granulata and a ambigua indicates high lake levels and wetter conditions dated to ca 1360 1560 ce from 1560 ce until present the lake has been subject to drier conditions and higher evaporation as indicated by lower lake levels reported by a decline in freshwater planktonic taxa and increase in brackish taxa e g amphora robusta

the effect of wind waves on the development of river mouth bars 1 river deltas are among the most important environments on earth housing productive ecosystems and a large fraction of the human population the key process for delta development is the deposition of mouth bars in front of delta distributaries predicting mouth bar formation on marine coastlines is complex because of the interactions between waves tides water and sediment discharge here we study the effects of waves on mouth bar growth with the coupled sediment transport and wave model delft3d swan our results show that wave characteristics height period and direction play an important role in the formation of mouth bars in our numerical experiments waves affect bar development in three ways by modifying the direction of the river jet by increasing bottom shear stresses at the river mouth and by changing bottom friction and hence increasing jet spreading we further show that high waves with long period prevent the formation of mouth bars in particular wave angles between 45 and 60 are the least favorable to bar formation likely producing a deflected river mouth

po river plume on the adriatic continental shelf dispersion and sedimentation of dissolved and suspended matter during different river discharge rates abstract suspended matter and dissolved nutrients were studied in the prodelta of the po river northern adriatic sea immediately after one of the greatest floods of the last century which occurred in october 2000 and in four successive seasonal conditions hydrology total suspended matter tsm particulate organic carbon poc and particulate nitrogen pn content organic carbon stable isotopes 13 c poc particle grain size spectra phytoplankton and dissolved inorganic nutrients i e nitrate nitrite ammonium phosphate and silicate were measured in different discharge conditions to describe processes occurring in the po river delta the main effect of high river discharge was a large volume of low salinity water at the surface that extended over the entire study area the concentration of most sampled variables decreased along a dilution gradient from the po river distributaries mouths to the offshore for tsm at the surface riverine input and dilution were the main factors affecting its distribution at the bottom however increased tsm in the nearshore was due to rapid sinking of suspended matter at the river mouth while resuspension and advection dominated the transport of near bottom sediments offshore at 15 20 m depth in the offshore where resuspension was active an increase in ammonium and decrease in oxygen supported the presence of regeneration processes occurring at the bottom during the october 2000 flood event high nutrient concentrations promoted new production triggering an algal bloom in the southern region of the study area the origin of particulate organic carbon in the river delta was then estimated from a mixing model based on 13 c poc values and carbon nitrogen c n molar ratios and as the contribution of three distinct sources of organic carbon were determined i e marine riverine and terrestrial the riverine end member accounted for 50 of the surface poc and generally decreased towards the offshore the terrestrial carbon contribution was generally the least abundant 20 28 at the surface and at depth respectively and exhibited peak values 30 40 during the october 2000 flood last the marine contribution was the most abundant at the bottom 40 and generally increased offshore during the october 2000 flood the fluvial contribution of particulate organic carbon that is the terrestrial and riverine fractions represented approximately 80 of the total particulate organic carbon in the po river delta

the adriatic sea general circulation part ii baroclinic circulation structure abstract in the second part of the paper dedicated to the adriatic sea general circulation the horizontal structure of the hydrographic parameters and dissolved oxygen fields is described on a seasonal timescale maps of temperature and salinity climatological fields reveal the enhanced seasonal variability of the adriatic sea which at the surface is associated with the major dilution effects of river runoff the density and derived dynamic height fields show for the first time the baroclinic geostrophic structure of the general circulation winter is dominated by compensation effects between temperature and salinity fronts along the western coastline the resulting baroclinic circulation is weak and suggests the presence of barotropic current components not accessible by the dataset spring and summer seasons have the smallest spatial scales in the temperature and salinity fields and stronger subbasin scale gyres and current systems which have been classified in a schematic representation of the circu

lagrangian motion and fluid exchange in a barotropic meandering jet kinematic models predict that a coherent structure such as a jet or an eddy in an unsteady flow can exchange fluid with its surroundings the authors consider the significance of this effect for a fully nonlinear dynamically consistent barotropic model of a meandering jet the calculated volume transport associated with this fluid exchange is comparable to that of fluid crossing the gulf stream through the detachment of rings although the model is barotropic and idealized in other ways the transport calculations suggest that this exchange mechanism may be important in lateral transport or potential vorticity budget analyses for the gulf stream and other oceanic jets the numerically simulated meandering jet is obtained by allowing a small amplitude unstable meander to grow until a saturated state occurs the resulting flow is characterized by finite amplitude meanders propagating with nearly constant speed and the results clearly illustrate the stretching and stirring of fluid particles along the edges of the recirculation regions south of the meander crests and north of the troughs the fluid exchange and resulting transport across boundaries separating regions of predominantly prograde retrograde and recirculating motion is quantified using a dynamical systems analysis the geometrical structures that result from the analysis are shown to be closely correlated with regions of the flow that are susceptible to high potential vorticity dissipation moreover in a related study this analysis has been used to effectively predict the entrainment and detrainment of particles to and from the jet