iceland scotland overflow water transport variability through the charlie gibbs fracture zone and the impact of the north atlantic current the charlie gibbs fracture zone cgfz a deep and wide gap in the mid atlantic ridge near 52 n is a gateway between the eastern and western subpolar regions for the atlantic meridional overturning circulation amoc in 2010 2012 an eight mooring array of current meters and temperature salinity sensors was installed across the cgfz between 500 m and the sea floor to measure the mean transport of westward flowing iceland scotland overflow water isow and investigate the impact of the eastward flowing north atlantic current nac on isow transport variability the 22 month record mean isow transport through the cgfz 1 7 0 5 sv 95 confidence interval is 30 lower than the previously published estimate based on 13 months of current only measurements 2 4 sv 1 2 sv saunders 1994 the latter mean estimate may have been biased high due to the lack of continuous salinity measurements although the two estimates are not statistically different due to strong mesoscale variability in both data sets empirical orthogonal function analysis and maps of satellite derived absolute dynamic topography show that weak westward isow transport events and eastward reversals are caused by northward meanders of the nac with its deep reaching eastward velocities these results add to growing evidence that a significant fraction of isow exits the iceland basin by routes other than the cgfz

variability of the overflow water transport in the western subpolar north atlantic 1950 97 abstract one of the major topics in current field research is the question of whether or to what extent the north atlantic ocean is subject to changes in water mass transports and how they are related to atmospheric phenomena like the north atlantic oscillation nao bottle and ctd data from the 1950s to 1990s are presented to reconstruct spatially and temporally the baroclinic contribution to the deep water transports in the western subpolar north atlantic the focus is on the two densest components of north atlantic deep water the gibbs fracture zone water gfzw and the denmark strait overflow water dsow direct velocity measurements in the considered time period are sparse for this reason it was decided to calculate the geostrophic velocity relative to 1400 dbar this level is located in the weakly stratified labrador sea water the combined baroclinic volume transport of gfzw and dsow during the early 1990s was about 5 sv sv 106 m3 s 1 in the irminger sea and 7 8 sv in the labrador sea nea

observations of the antarctic circumpolar current and deep boundary current in the southwest atlantic fourteen month velocity and temperature records from an array of 14 moorings north and west of the falkland plateau and supporting hydrographic and tracer data reveal a narrow boundary current that carries dense antarctic waters the current flows west along the northern flank of the falkland plateau with mean speeds of more than 10 cm s 1 at 5000 m and more than 30 cm s 1 at 2500 m the westward flow extends from the bottom to at least 1000 m but the upper portion of the current is a branch of the antarctic circumpolar current acc following the only deepwater route between the scotia sea and the argentine basin waters colder than 0 2 c are too cold to be associated with the acc at drake passage and must ultimately derive from the weddell sea as part of the deep thermohaline circulation the westward transport of water colder than 0 2 c is 8 2 106 m3 s 1 in the mean the bottom boundary current is similar to that predicted by the stommel arons model but considerable variability is introduced by the meandering of the overlying acc chlorofluorocarbon data suggest that new antarctic water from the georgia basin enters the argentine basin via the deep boundary current which passes beneath the acc some new water is also advected east after being entrained in the acc most of the water in the deep boundary current is recirculated water that has been in residence in the argentine basin for some time water colder than 0 2 c is relatively new to the basin and comprises about 2 5 106 m3 s 1 of the westward flow of the boundary current

a model of isotope fractionation in reacting geochemical systems the authors present a numerical technique that predicts how the stable isotopes sup 2 h sup 13 c sup 18 o and sup 34 s fractionate among solvent aqueous species minerals and gases over the course of a geochemical reaction process this model is based on mass balance techniques similar to those already presented in the literature but differs from previous techniques in that it allows minerals to be segregated form isotopic exchange instead of remaining in isotopic equilibrium such an approach allows us to simulate the fractionation of isotopes between rock and fluid resulting solely from mineral dissolution and precipitation the technique was tested by modeling isotopic fractionation during several reaction processes including 1 dolomitization of limestone by a migrating pore fluid 2 diagenetic alteration of the permian lyons sandstone in the denver basin and 3 hydrothermal alteration of the okanagan batholith in southern british columbia the results of calculations in which minerals are segregated from isotopic exchange compare well to isotopic trends observed in nature but differ markedly from calculations that assume isotopic equilibrium 54 refs 4 figs 3 tabs

five decades of triggered earthquakes in koyna warna region western india a review abstract we review reservoir triggered seismicity rts in the koyna warna region of india since the impoundment of the koyna reservoir in 1962 until the year 2015 we include seismicity that occurred farther south in response to the impoundment of warna reservoir in 1993 about 35 km to the south of koyna reservoir the on going earthquake activity for more than five decades in a small seismic volume of about 30 20 10 km3 has characterized the koyna warna region as globally unique and significant to date 22 earthquakes of m 5 about 200 earthquakes of m 4 and several thousand smaller earthquakes have been recorded by a dense network of stations in the region various seismological geophysical and hydrological studies have been carried out to understand the phenomenon of triggered seismicity including source mechanism fault geometry crustal structure earthquake processes causal relationship with reservoir water level changes and anomalous water level fluctuations in cased bore wells the present review takes stock of all such studies undertaken so far to summarise the understanding on various geo scientific issues and milestone achievements with regard to mechanism of triggered seismicity and source processes fault plane solutions commensurate with geometry of faulting source parameters seismogenic depth crustal structure and the role of reservoir water level two stage increase in seismic energy release coinciding with peaks of annual filling and draining cycle of reservoir with one month delay vis a vis spurt of moderate earthquakes m 5 due to i rapid rise in reservoir level 12 m week ii reservoir water level exceeding the previous maxima and iii duration for which high reservoir level is retained are characteristic features numerical models simulating diffusion of pore fluid pressure fronts during the filling stage of reservoir suggest stress perturbations of the order of 0 75 2 25 bar at hypocentral depth triggering earthquakes on critically stressed pre existing faults in the year 2005 a 13 station digital seismometers network became functional that characterized the distribution of epicentres in the area with increased accuracy and suggested four major seismic zones with well defined clusters scientific drilling carried out recently provide several new information regarding the subsurface geology and structure in the koyna region such as i thickness of deccan traps 933 in the koyna area and 1185 m in the warna area ii presence of granite gneiss basement directly underlying the deccan traps iii absence of infratrappean sediments and iv temperature not exceeding 150 c at hypocentral depth it is planned to establish a deep borehole observatory close to the seismic source zone for direct measurements of physical and mechanical properties of rocks pore fluid pressure hydrology thermal condition and other parameters of an intra plate active fault zone in the near field of earthquakes before during and after their occurrence results from such experiment and long term in situ monitoring of critical parameters would enhance our understanding on the mechanism of triggered earthquakes and the role of reservoir water level fluctuations

periodic energy release in the new madrid seismic zone the annual release of strain energy by earthquakes in the new madrid seismic zone as represented by the strain factor is calculated and cumulated when the main annual energy release in the form of a linear regression is subtracted from the data the residual curve suggests that the energy release rate is cyclical with a period of 30 to 35 yr for earthquakes of magnitude 5 5 or less furthermore the current rate is well below normal and may be expected to significantly increase in the near future although the increase may be in the form of several small earthquakes the historical pattern suggests that at least one earthquake of body wave magnitude 5 0 to 5 2 may be anticipated

north atlantic climate and deep ocean flow speed changes during the last 230 years variations in the near bottom flow speed of iceland scotland overflow water isow are documented in a 230 year long deep sea sediment record from the eastern flank of reykjanes ridge in the subpolar north atlantic at sub decadal time scales for recent decades the isow palaeocurrent reconstructions show similarities with observational hydrographic data furthermore recent isow flow changes fall mostly within the range of its variability of the past 230 years the record also reveals a hitherto unrecognized coupling of deep flow speeds in the subpolar north atlantic with the north atlantic oscillation nao index with more less vigorous isow flow during negative positive phases of the nao our results suggest that the changes in isow vigor are largely controlled by the transport and characteristics of labrador sea water rather than variations in the overflow itself with implications for the meridional overturning of the atlantic ocean and climate

a stratigraphic case history using three dimensional seismic data in the gulf of thailand a three dimensional marine seismic survey was conducted in the gulf of thailand to aid in the development of a gas field indicated by three wildcat wells the results and interpretation reported previously demonstrated improved fault resolution and better structural definition five successful appraisal wells have now been drilled and these show that most of the sands have limited extent widespread character changes in the seismic data also support stratigraphic variations in many of the sands several new methods of 3d stratigraphic interpretation have been developed while investigating the depositional history of this area anomalous seismic amplitudes tied to sands penetrated by wells and mapped from seiscroptm horizontal sections in time and depth have indicated the distribution of bars and channels horizon seiscrop sections each sliced through a single bed have been used to delineate these depositional features directly g logtm sections displaying seismic logs derived by rigorous wave equation inversion confirm the existence of these features sands greater than 10 m thick have proved mappable

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

the search for hydrothermal sources on the mid atlantic ridge abstract in a search for hydrothermal vents in the median valley of the mid atlantic ridge twenty seven lowerings of a telemetering thermistor probe were made between 43 and 47 n no hot spots were detected below 3 000 m valleys were found filled with water of more or less uniform potential temperature indicating weak but efficient mixing the mechanisms for the renewal of the mixed water and the control exercised by the sill on them is reviewed sill locations and depths are identified for seven valley segments in the study area sill depths range from 2 100 to 2 850 m within the valleys the temperature at a depth of 3 000 m ranges from 3 56 to 3 17 c decreasing as the sill depth increases a strategy is proposed for exploring a series of such valleys in a systematic search for the presence of hydrothermal vents employing chemical sampling of mn ch4 or 3he sampling concentrated near the upper boundary of the mixed region and near the sill of a valley are novel features of the proposed method

flow through discovery gap abstract a narrow gap discovery gap in the east azores fracture zone at 37 n in the eastern atlantic provides a channel for the exchange of bottom water between the madeira and iberian abyssal basins a detailed survey defines its length 150 km width 10 50 km depth of sills near 4800 m and sediment thickness 0 500 m year long measurements of deep flow are made from six moorings and ten current meters mean flows are about 5 cm s 1 these data are supplemented by 10 days of float tracking near 4700 m and numerous density profiles within and around the gap a persistent southwest northeast flow of dense water is found in discovery gap and the flux of water colder than a potential temperature of 2 05 c is measured to be 0 21 04 106 m3 s 1 this discharge spreads over an area of about 1011 m2 beyond the gap exit where it is warmed both by geothermal heating and by mixing with overlying water an estimate of 1 5 to 4 cm2 s 1 is derived for the diapycnal diffusivity similar to values dete

cold outflow from the faroe bank channel abstract the exchange of water between the norwegian sea and the north atlantic around the faroe islands was investigated in 1987 and 1988 combining ctd data with shipborne acoustic doppler current profiler adcp measurements tests were devised to assess the accuracy of the adcp measurements and the results are described it is found that estimating a reference velocity for geostrophic measurements is limited by the presence of tides of order 10 cm s 1 in the outflow of cold water from the faroe bank channel which reaches speeds of nearly 1 m s 1 the error is acceptably small but strong time dependent components are revealed the discharge of cold water is traced 75 km beyond the sill of this channel and changes in potential temperature salinity and potential vorticity on an isopycnal surface are attributed to intense mixing an effective diffusivity near 100 cm2 s 1 is derived and k h instability demonstrated as a plausible source of the turbulent energy six current meter moorings were deployed from

observed and modeled pathways of the iceland scotland overflow water in the eastern north atlantic abstract the spreading of iceland scotland overflow water isow in the eastern north atlantic has largely been studied in an eulerian frame using numerical models or with observations limited to a few locations no study to date has provided a comprehensive description of the isow spreading pathways from both eulerian and lagrangian perspectives in this paper we use a combination of previously unreported current meter data hydrographic data rafos float data and a high resolution 1 12 numerical ocean model to study the spreading pathways of isow from both of these perspectives we identify three isow transport cores in the central iceland basin 59 n with the major core along the eastern boundary of the reykjanes ridge rr and the other two in the basin interior based on trajectories of observed and or numerical floats seeded along 59 n we also describe the isow spreading pathways and quantify their relative importance within 10 years 7 11 of isow from 59 n escapes into the irminger sea via gaps in the rr north of the charlie gibbs fracture zone cgfz the water that moves through these gaps principally originates from the shallower isow layer along the rr eastern boundary 10 13 travels further southward until the cgfz where it crosses westward into the western subpolar gyre 18 21 of isow spreads southward along the eastern flank of the mid atlantic ridge into the western european basin web most of the remaining water stays in the iceland basin over the 10 year period a model based investigation provides a first look at the temporal variability of these isow pathways we find that the fraction of southward water exported into the web is anti correlated with the export through the cgfz a result assumed to reflect these pathways interactions with the north atlantic current in magnitude and or position shift

interaction of the north atlantic current with the deep charlie gibbs fracture zone throughflow the charlie gibbs fracture zone cgfz a passage of 3600 m sill depth through the mid atlantic ridge near 52 n is a known gateway for the passage of deep waters from the northeast atlantic into the western basin during a shipboard survey of august 1997 deep current profiling yielded eastward deep flow through the passage while geostrophy calculated against an intermediate reference level resulted in westward relative deep transport the reason was an unusual and deep reaching northward excursion of the north atlantic current nac inspection of historical data showed that such interference of the nac with the cgfz regime occured occasionally in the past relocation of surface circulation patterns by decadal ocean climate anomalies may thus be of significance also for the deep circulation

seismic hydraulic fracture migration originated by successive deep magma pulses the 2011 2013 seismic series associated to the volcanic activity of el hierro island in this manuscript we present a new interpretation of the seismic series that accompanied eruptive activity off the coast of el hierro canary islands during 2011 2013 we estimated temporal variations of the gutenberg richter b value throughout the period of analysis and performed high precision relocations of the preeruptive and syneruptive seismicity using a realistic 3 d velocity model our results suggest that eruptive activity and the accompanying seismicity were caused by repeated injections of magma from the mantle into the lower crust these magma pulses occurred within a small and well defined volume resulting in the emplacement of fresh magma along the crust mantle boundary underneath el hierro we analyzed the distribution of earthquake hypocenters in time and space in order to assess seismic diffusivity in the lower crust our results suggest that very high earthquake rates underneath el hierro represent the response of a stable lower crust to stress perturbations with pulsatory character linked to the injection of magma from the mantle magma input from depth caused large stress perturbations to propagate into the lower crust generating energetic seismic swarms the absence of any preferential alignment in the spatial pattern of seismicity reinforces our hypothesis that stress perturbation and related seismicity had diffusive character we conclude that the temporal and spatial evolution of seismicity was neither tracking the path of magma migration nor it defines the boundaries of magma storage volumes such as a midcrustal sill our conceptual model considers pulsatory magma injection from the upper mantle and its propagation along the moho we suggest within this framework that the spatial and temporal distributions of earthquake hypocenters reflect hydraulic fracturing processes associated with stress propagation due to magma movement

interpretative lessons from three dimensional modeling three dimensional 3 d seismic modeling has been accomplished by describing geologic surfaces with triangular plates and then computing the seismic response by kirchhoff wave theory the resulting time sections illustrate many interesting 3 d phenomena which are useful in interpreting geologic structures three dimensional resolution studies relate the concept of fresnel zone reflection to seismic resolution if high resolution is desired both horizontally and vertically then not only is a dense field survey required but also a detailed amplitude study the dense seismic coverage is required to map the focal line of concave boundary edges which are difficult to delineate with conventional seismic data additional studies on complex models such as grabens and 3 d permeability traps associate interpretational pitfalls to a wandering specular reflection path that is side swipe in each geologic model maximum resolution is obtained on a principal plane line dip line if a seismic dip line is not

trawled megafaunal invertebrate assemblages from bathyal depth of the mid atlantic ridge 48 54 n we investigated the effects of contrasting surface primary production on the benthic invertebrate megafauna at four sites on the mid atlantic ridge the sites designated nw ne sw and se were located to the west and east of the ridge axis and to the north and south of the charlie gibbs fracture zone benthic megafauna were sampled in 2007 and 2009 with a semi balloon otter trawl at a target depth of 2 500 m the total biomass and density of major taxonomic groups did not differ significantly between sites despite those to the north being characterised by greater surface productivity than those to the south however the density and biomass of individual taxonomic groups as well as diversity and body size all showed significant differences between sites diversity was highest at the se and lowest at the ne site most species were larger to the north community composition was significantly different between all sites with the greatest number of unique species found at the se and noticeably fewer unique species at the northern sites there was no clear correlation between the surface productivity and community structure suggesting complex ecological controls on the communities it is speculated that in addition to the energy supply drivers such as strong currents and sediment characteristics play an important role in shaping the communities at the different sites to what extent the ridge acts as a dispersal barrier for benthic invertebrate fauna remains unclear however high numbers of species unique to the southern site suggest a limited dispersal between the northern and southern areas

field appraisal with three dimensional seismic surveys offshore trinidad a consortium comprising texaco trinidad inc trinidad and tobago oil company ltd and trinidad tesoro petroleum company ltd commenced exploration in the south east coast consortium block offshore trinidad in 1973 after four years of intensive exploration a gas condensate discovery was announced in early 1977 on the pelican prospect later that year in anticipation of the possible future need to site drilling production platforms a three dimensional 3 d seismic survey was recorded over the prospect this survey resulted in improvements in seismic record quality multiple attenuation and fault resolution a coordinated geologic geophysical interpretation based on the 3 d seismic survey a reevaluation of log correlations and the use of seismic logs differed significantly from earlier interpretations because of this it is anticipated that development of the field will need to be initiated in a different fault block from that previously envisioned a second 3 d survey contiguous to the pelican su

lagrangian observations of the middepth and deep velocity fields of the northeastern atlantic ocean the circulation of the northeastern atlantic ocean at intermediate depths is characterized by watermass transformation processes that involve iceland scotland overflow water isow from the northeast labrador sea water lsw from the west and mediterranean water from the south field observations were carried out with 89 eddy resolving floats rafos and marvor types the data coverage achieved is remarkably high and enables a comprehensive study of the eastern basins between iceland and the azores the trajectories show typical pathways of the water masses involved and the role that the complex bottom topography plays in defining them the isow paths tend to lean against the slopes of the reykjanes ridge and rockall plateau westward escapes through multiple gaps in the ridge are possible superimposed on a sustained southward flow in the eastern basin along the mid atlantic ridge lsw pathways leading to the eastern basins are subject to high variability in flow direction and eddy activity in addition to a selection of characteristic trajectories maps of the horizontal distributions of lagrangian eddy kinetic energy and integral time scales are presented these reveal distinct areas of intensified mixing in the iceland basin as well as the sharp contrast between the subpolar and subtropical dynamics a self contained eddy detection scheme is applied to obtain statistics on individual eddy properties and their abundance it is suggested that much of the intensified mixing can be related to cyclonic activity particularly in the subpolar region

structural interpretation from horizontal seismic sections the interpreter of a 3 d survey must utilize a data volume horizontal slices through a data volume called seiscrop sections have unique properties and structural interpretation from them is fast convenient and effective an event on a seiscrop section displays local strike a property which permits direct contouring of a structural surface without any timing and posting the width of an event on a seiscrop section is a composition of the frequency of the data and the structural dip event terminations indicate faults or other discontinuities when they are transverse to structural strike faults parallel to structural strike are much less evident on a single seiscrop section but become apparent with the relative movement of events from section to section in practical mapping the interpreter normally contours one fault block before proceeding to the next with the correlation between them being established from the vertical sections with dual polarity variable area displays the interpreter can perc

analysis of seismic data using horizontal cross sections three dimensional data provide the interpreter with a new dimension for interpretation horizontal cross sections this article suggests formats for the display of the data and techniques for deriving contour maps of horizons strike directions dip angles thicknesses of formations and fault blocks

dynamics of north atlantic deep water masses during the holocene high resolution flow speed reconstructions of two core sites located on gardar drift in the northeast atlantic basin and orphan knoll in the northwest atlantic basin reveal a long term decrease in flow speed of northeast atlantic deep water neadw after 6 500 years benthic foraminiferal oxygen isotopes of sites currently bathed in neadw show a 0 2 depletion after 6 500 years shortly after the start of the development of a carbon isotope gradient between neadw and norwegian sea deep water we consider these changes in near bottom flow vigor and benthic foraminiferal isotope records to mark a significant reorganization of the holocene deep ocean circulation and attribute the changes to a weakening of neadw flow during the mid to late holocene that allowed the shoaling of lower deep water and deeper eastward advection of labrador sea water into the northeast atlantic basin

direct velocity measurements in the samoan passage an array of six current meter moorings was deployed in the samoan passage 10 s 170 w from september 1992 to february 1994 with the goal of determining the northward abyssal transport the 17 month mean transport beneath 4000 m was 6 0 sv 106 m3 s 1 the time series were low passed at a period of 100 hours to remove near inertial and tidal variability and the resulting transport had a standard deviation of 1 5 sv a minimum of 1 1 sv and a maximum of 10 7 sv the dominant low frequency variability in transport and area averaged temperature was at a period of 30 days where the temperature led the transport as would be expected if the heat balance were advective the temporal and spatial variability suggested that the mean transport is known to within a 1 standard deviation error of 0 5 sv the along passage flow was intensified to the west and toward the bottom as might be expected in a rotating geostrophic system three pathways through the sill region were identified on the basis of bathymetry and current direction the flow was strongly controlled by topography as indicated by the flow s orientation and directional steadiness the 30 day variability in transport might be described as a resonance of the northward abyssal flow

isow spreading and mixing as revealed by deep argo floats launched in the charlie gibbs fracture zone to improve our understanding of deep circulation we deployed five deep argo floats 0 4 000 m in the charlie gibbs fracture zone cgfz which channels the flow of iceland scotland overflow water isow a dense water mass of the north atlantic ocean the floats were programed to drift at 2 750 dbar in the isow layer the floats mainly moved westward in the cgfz although some of them followed different routes for few cycles depending on northward intrusions of the north atlantic current over the cgfz one float revealed a direct route for isow from cgfz to the deep western boundary current at flemish cap in the cgfz oxygen data acquired by the floats revealed that the isow layer characterized by salinity higher than 34 94 and density greater than 27 8 kg m was mainly composed of the highly oxygenated isow and the less oxygenated north east atlantic deep water neadw a complex water mass from the east atlantic in the isow layer the relative contribution of isow was generally larger in the northern valley than in the southern valley of cgfz northward intrusions of the north atlantic current above the cgfz increased the relative contribution of neadw in the northern valley and favors mixing between isow and neadw the isow neadw signal flowing westward from the cgfz toward the deep western boundary current was progressively diluted by labrador sea water and denmark strait overflow water oxygen measurements from deep argo floats are essential for a better understanding and characterization of the mixing and spreading of deep water masses

analysis of fluid pressure propagation in heterogeneous rocks implications for hydrologically induced earthquakes mathematical models are developed to study the propagation of excess pore pressure in heterogeneous and fractured rocks for a homogeneous rock the time needed for a pore pressure front to migrate downward is directly proportional to the square of the depth and inversely proportional to the permeability of the rock variational studies show that pore pressure propagation is highly influenced by the permeability heterogeneity of the crust the results argue for the importance of accounting for geologic complexity when using mathematical models to estimate the extent of downward transmission of increases in hydraulic heads the models presented in the paper are used to estimate a range of possible hypocentral depths of periodic seismicity observed near mt ogden on the alaska british columbia border the time lag between these earthquakes and hydrologic loading is on the order of days or weeks indicating a quick response of seismicity to the increased surface water input from rainfall or glacial discharge if such a causal relationship exists our models estimate that the hypocentral depths of these earthquakes could be on the order of several kilometers if a high degree of vertical interconnectivity of fractures exists

recirculating components to the deep boundary current of the northern north atlantic abstract the meridional overturning system of the north atlantic is conventionally thought of as transporting warm water 4 c to high latitudes and covd water to low latitudes the northern cold water sources for this system are dense overflow from the nordic seas and less dense water from the labrador sea lsw the overflow components are carried westward by a deep northern boundary current dnbc that is shown here actually to begin southeast of the overflow regions the dnbc lies along the southern side of a system of islands and submarine ridges that divides the nordic seas from the subpolar basins the overflows act to increase the transport of the dnbc and this transport is further increased through the entrainment of warm waters once inside the labrador sea the dnbc is joined by lsw before continuing south to low latitudes as a cold deep western boundary current dwbc however published transport estimates are larger than can be explained by warm water entrainment alone thus indicating entrainment also of recirculating cold components the subject of this paper their sources are shown to be the lsw and cold abyssal waters originating from the southern hemisphere the lsw is entrained downward into the denser part of the dnbc and laterally into the upper part of the dwbc whereas the cold abyssal waters are supplied by an eastern intensified northward flow in the eastern atlantic that serves as the initial source of the dnbc and by a similar northward flow in the western atlantic the meridional overturning system described includes recirculations and poleward transport of cold water in addition tothe components described by the conventional system the cold abyssal waters in both the eastern and western basins have relatively low levels of oxygen and high concentrations of silicate reflecting their southern origin but their influence in the northwestern atlantic is somewhat obscurred by strong recirculating cyclonic gyres in the newfoundland and labrador basins this influence is detected by a deep silicate maximum extending poleward from the mid latitude western atlantic through the two gyres with eastern concentration and recirculating back diluted toward the south in the dwbc within the respective basins such a maximum also extends northward through the irminger basin into the dnbc but there is an ambiguity as to whether this is a direct extension of the signal from the south in the western basin or if it comes from the mid latitude eastern basin by way of westward flow through the charlie gibbs fracture zone it is perhaps consequence of both estimates of the volume transports of the various recirculating cold components indicate that they supply about as much water to the deep boundary currents as do the combined cold water sources in the north

the flux of overflow water through the charlie gibbs fracture zone we report the results of a set of measurements made within the charlie gibbs fracture zone in 1988 1989 the principal data sources are year long current meter records 16 in number and 23 conductivity temperature depth stations the aim was to determine the flux of overflow water passing westward out of the iceland basin in the eastern north atlantic within the fracture zone overflow water is defined either as having salinity greater than 34 94 at the present time or more generally as having a potential density 0 27 8 kg m3 in 1988 1989 the core salinity had a value equal to that reported in 1981 and 1983 but was fresher by about 0 02 than was found in 1964 the flux was estimated twice per day and was found to fluctuate strongly with energy spread in a broad band of periods ranging from 10 to 400 days no annual signal could be detected the mean value was 2 4 0 5 sv less than all previous estimates based on hydrography alone it is suggested that hydrography may systematically overestimate transport because diffusion is neglected flux estimates from current meter arrays are rarely used in initializing numerical ocean models consequently measurements like those reported here are an independent and important means for assessing the performance of such models at the deepest levels water enters the two transform valleys of the fracture zone from the west in the northern valley the flow is periodically flushed out by energetic bursts of overflow water a critical value of an inverse froude number is shown to define the flushing threshold

the water masses of the central north atlantic in 1983 84 abstract hydrographic surveys carried out in 1983 84 along both sides of the mid atlantic ridge between 24 and 53 n provide a detailed description of the well known north atlantic water masses with particular emphasis on their meridional distribution and zonal dissymetry in the upper layers the dense horizontal sampling resolves the several narrow gulf stream extensions into the ocean interior giving the image in the central water density range of a mosaic of mode waters separated by fronts at intermediate depths a vertical shear in the distributions of the mediterranean water and labrador sea water stands out both water masses having their lower part displaced southwards relative to their upper parts bottom waters containing nearly 20 percent of pure antarctic bottom water are observed at 50 n in the eastern basin in contrast with the western basin where proportions greater than 10 percent were found only south of 36 n along our section this water mass analysis also gives indications that stron

the overflows across the ninetyeast ridge abstract water deeper than say 3800 m in the central indian basin comes from the west australian basin across deep saddles on the ninetyeast ridge near lats 5 s 10 s and 28 s the woce hydrographic program in the indian ocean 1994 1996 provided opportunities for more detailed measurements of water properties in these overflows than any made earlier with zero velocity surfaces inferred from contrasts in oxygen and silicic acid concentrations and from distributions of specific volume anomaly geostrophic estimates of these overflow rates are respectively 1 0 1 1 and 0 1 10 6 m 3 s 1 westward these constructions also imply strong zonal flows of mid depth water between 2000 db and the zero velocity surfaces through the passages 2 2 and 6 7 10 6 m 3 s 1 westward at lats 5 s and 10 s respectively and 1 8 10 6 m 3 s 1 eastward at lat 28 s as in the southward flow through denmark strait density stratification explains the splitting of westward currents at 5 s and 10 s into mid depth and deep flows as they pass over their saddles but not why such vertically extended flows should exist in the first place the cause of opposed eastward flow at 28 s is less certain but might be related to the particular geometry of the west australian basin

diurnal variations in flow over a succession of ridges and valleys observations of flow in a nearly two dimensional valley located within a succession of similar ridges and valleys are presented measurements of the mean flow have been made at all times of the day and are found to be well characterized by a suitably defined froude number the unstable and neutral daytime flow shows a flow separation whilst when the froude number has a value of about two usually around evening and dawn transitions the flow is attached and the valley well ventilated for smaller values of froude number the flow speed in the valley reduces and a characteristic drainage flow occurs in addition to mean flow measurements and soundings of temperature structure measurements were also made of turbulence energy and stress this allows an estimate of the effective value of the surface roughness zo and an indication of the stability dependence of the momentum transfer to be made

areal seismic methods for determining the extent of acoustic discontinuities the collection of areal seismic reflection data is becoming fairly routine it is now generally realized that the solution of three dimensional 3 d structural problems is only possible when the target has been adequately sampled and the data have been correctly migrated to produce an accurate image of the subsurface however many of our exploration prospects are associated with lithological changes or stratigraphic features rather than structural features we show how areal seismic techniques can provide an added dimension in determining the extent of acoustic discontinuities in areas where the strata are generally flat

the active part of charlie gibbs fracture zone a study using sonar and other geophysical techniques a short survey with gloria side scan sonar and other geophysical instruments has revealed new information about charlie gibbs fracture zone between 29 and 36 w the traces of two transform faults have been clearly delineated they fit small circles about the pole of rotation with an rms error of only about 1 km but they do not always follow the deepest parts of the transform valleys the transforms are joined by a short spreading center at 31 45 w the median transverse ridge appears to have been produced by normal seafloor spreading at this center and bears identifiable vine matthews magnetic anomalies a transverse ridge along the eastern inactive part of the northern transform may be an intrusive feature considerable thicknesses of sediment appear to have been deposited in the northern transform valley from norwegian sea overflow water passing through the fracture zone but transverse ridges have prevented the sediment reaching the southern valley

surface effects of triggered fault slip on reykjanes peninsula sw iceland three large earthquakes mw 4 5 were triggered within 5 min 85 km west of a mw 6 5 earthquake in the south iceland seismic zone sisz we report on surface effects of these triggered earthquakes which include fresh rupture widespread rockfall disrupted rockslides and block slides field data confirm that the earthquakes occurred along n striking right lateral strike slip faults field data also support the conclusion from modeling of insar data that deformation from the second triggered event was more significant than for the other two a major hydrological effect was the draining of water through an open fissure on a lake bed lowering the lake level by greater than 4 m field relationships suggest that a component of aseismic slip could have been facilitated by water draining into the fault zone d 2003 elsevier science b v all rights reserved