the closing of tethys and the tectonics of the himalaya recent geological and geophysical data from southern tibet allow refinement of models for the closing of southern neo tethys and formation of the himalaya shelf sediments of the indian passive continental margin which pass northward into deep sea tethyan sediments of the indus tsangpo suture zone were deposited in the late cretaceous an andean type margin with a 2 500 km long trans himalayan kohistan ladakh gangdese granitoid batholith formed parallel to the southern margin of the lhasa block together with extensive andesites rhyolites and ignimbrites lingzizong formation the southern part of the lhasa block was uplifted deformed and eroded between the cenomanian and the eocene in the western himalaya the kohistan island arc became accreted to the northern plate at this time the northern part of the lhasa block was affected by jurassic metamorphism and plutonism associated with the mid jurassic closure of the bangong nujiang suture zone to the north the timing of collision between the two continental plates ca 50 40 ma marking the closing of tethys is shown by 1 the change from marine flysch like to continental molasse like sedimentation in the indus tsangpo suture zone 2 the end of gangdese i type granitoid injection 3 eocene s type anatectic granites and migmatites in the lhasa block and 4 the start of compressional tectonics in the tibetan tethys and indus tsangpo suture zone south facing folds south directed thrusts after the eocene closure of tethys deformation spread southward across the tibetan tethys zone to the high himalaya deep crustal thrusting barrovian metamorphism migmatization and generation of oligocene miocene leucogranites were accompanied by south verging recumbent nappes inverting metamorphic isograds and by south directed intracontinental shear zones associated with the main central thrust continued convergence in the late tertiary resulted in large scale north directed backthrusting along the indus tsangpo suture zone more than 500 km shortening is recorded in the foreland thrust zones of the indian plate south of the suture and 150 km shortening is recorded across the indian shelf zanskar range and the indus suture in ladakh there was also large scale shortening of the karakoram and tibetan microplates north of the suture as much as 1 000 km shortening occurred in tibet the more recent deformation however involved the spreading of this thickened crust and the lateral motion of the tibetan block along major approximately east west trending strike slip fault zones

stratigraphic and textural variations in the chromite composition of the ophiolitic sakhakot qila complex pakistan the deposits and their host rocks compare well with podiform ores hosted by alpine type peridotites of ophiolites unlike most alpine type complexes predominant stratigraphic variations in the chemical composition of chromite exist the chromite deposits fall in two major zones the first consisting of three subzones regular compositional correlations between the accessory chromites of the host rocks and the associated segregated chromites are drawn indicating relatively later crystallization of the former relict cumulate chromitite textures cover the podiform range and are examined in the light of compositional data the inequigranular textural varieties probably developed by crystallization of successive crops from magmatic fractionation with their coarser grained massive chromite components always being slightly higher in mg mg fe super 2 than the associated finer grained chromite components the mg fe super 2 exchange between chromite and silicates seems to start during the cumulus stage and continues during the postcumulus stage but is reduced to a very small intensity during the subsolidus stage until it ends at about 700 degrees c modified journal abstract

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

salinity driven thermocline transients in a wind and thermohaline forced isopycnic coordinate model of the north atlantic abstract an isopycnic coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation the model performance is illustrated by a 30 year spinup run with coarse horizontal resolution 2 mesh in a domain with north atlantic topography extending from 10 to 60 n latitude the vertical structure encompasses 10 isopycnic layers in steps of 0 2 units capped by a thermodynamically active mixed layer from an initially isohaline state with isopycnals prescribed by zonally averaged climatology the model is forced by seasonally varying wind stress radiative and freshwater fluxes and by a thermal relaxation process at the surface after a mechanical spinup time of about 15 years a quasi stationary pattern of mean circulation and annual variability ensues characterized by pronounced subtropical mode water formation

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

palaeoseismology from microfabric and geochemical analysis of lacustrine sediments windermere uk lake sediments commonly contain detrital layers that record events such as floods or earthquakes but these may be disturbed or partially destroyed by bioturbation here we use a novel combination of techniques to relate microscopic sediment fabric features to processes at the lake basin scale x radiography and micro x ray fluorescence analysis of cores are complemented by back scattered electron imagery and energy dispersive x ray microanalysis of resin embedded sediment together the microfabric and geochemical methods enable the identification of clay layer mass transport deposits despite bioturbational mixing of the original end members two cores with robust radionuclide chronologies contain correlative clay layers dated to 1979 1974 1982 and 1979 1980 1973 1986 respectively these clay layers probably represent the distal turbidite generated by a major mass flow deposit identified from multi beam swath bathymetry and sediment grab sampling a probable trigger for the mass flow and associated turbidity current is the 4 7 ml 1979 carlisle earthquake the lake basin slope was probably preconditioned for failure by increased sedimentary biogenic gas production and sediment in wash as a result of anthropogenic activities coupled with sediment disruption and dredging this study highlights the effectiveness of microstratigraphic techniques in the recognition and characterization of event layers in sediments where bioturbative disruption has occurred

analysis of a rapid sea ice retreat event in the bellingshausen sea the winter advance of the sea ice edge in the bellingshausen sea is frequently interrupted by periods of rapid retreat lasting a few days the frequency and duration of such events strongly controls the location of the late winter sea ice edge in this sector of the antarctic we examine the dynamics and thermodynamics of a retreat event that occurred in may 2001 using data from a drifting buoy array together with diagnostics from a kinematic thermodynamic ice growth model and a high resolution 11 km regional coupled ocean ice model during the retreat event the ice edge retreated by 250 km over 13 days in response to strong and persistent northerly winds associated with a quasi stationary low pressure system ice motion in the outer part of the pack was convergent and correlated strongly with local wind forcing by contrast in the region closer to the coast ice motion was less well correlated with wind forcing model diagnostics indicate that ice thickening resulting from convergence in the outer pack was largely balanced by basal melting in the outer pack ice was in a state close to free drift while closer to the coast internal ice stresses became significant the ocean ice model simulated the characteristics of the retreat event realistically giving us confidence in the ability of such models to reproduce ice conditions in this sector

role of the atmospheric and oceanic circulation in the tropical pacific sst changes a coupled climate model is used to explore the response of the tropical sea surface temperature sst to positive sst anomalies in the global extratropics the main model results here are consistent with previous numerical studies in response to prescribed sst anomalies in the extratropics the tropical ssts rise rapidly and reach a quasi equilibrium state within several years and the tropical subsurface temperatures show a slow response the annual mean hadley cell as well as the surface trades are weakened the weakened trades reduce the poleward ekman transports in the tropical ocean and furthermore lead to anomalous positive convergences of heat transport which is the main mechanism for maintaining the tropical pacific sst warming the process of an extratropical influence on the tropics is related to both the atmospheric and oceanic circulations the intertropical convergence zone itcz moves southward and eastward in the pacific corresponding to a reduction of the hadley circulation and walker circulation at the same time convective precipitation anomalies are formed on the boundary of the climatological itcz while the climatological mean convections centered in the southeast asia region are suppressed the largely delayed response of the tropical subsurface temperature cannot be explained only by the strength change of the subtropical cells stcs but can be traced back to the slow changing of subsurface temperature in the extratropics in the extratropical oceans warming and freshening reduce the surface water density and the outcropping lines of certain isopycnal layers are moved poleward this poleward movement of outcropping lines can weaken the positive temperature anomalies or even lead to negative anomalies on given isopycnal layers displayed on time dependent isopycnal layers positive subsurface temperature anomalies are present only in the region after subduction and are subsequently replaced by negative temperature anomalies in the deep tropics regions the noticeable features of the density compensation of temperature and salinity indicate that diapycnal processes play an important role in the equatorward transport of the temperature and salinity anomalies from the midlatitude

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

composition of modern deep sea sands from arc related basins summary petrographic and chemical examination of modern deep sea sands shows a clear distinction between active and passive plate tectonic settings among active settings sands from forearc basins of island arcs and basins from strike slip continental margins can be distinguished but those from other arc related settings overlap considerably in composition in particular sands from continental margin subduction zones and those from the backarc of island arcs appear to be indistinguishable chemically arc related sands are very close in composition to ancient greywacke sandstones for instance sodium almost always exceeds potassium thus it is not necessary for sodium to be added to sands diagenetically to make greywackes as has been suggested the influence of tectonics on the composition of sandstones has long been a central feature of sandstone petrology the geosynclinal model of global tectonics stimulated much thought on this subject probably the best known ideas are those of krynine e g 1942 subsequently such research declined but the plate model for global tectonics has produced a new wave of interest e g crook 1974 schwab 1975 potter 1978 dickinson suczek 1979 much of this work has relied on data from ancient sediments where diagenetic changes and difficulty in de ciphering the tectonic setting limit the useful ness of the conclusions much more work is needed on recent sediments to establish a base line against which ancient sediments can be compared we have previously reported valloni maynard 1981 the results of our work on the petrography of a set of recent deep sea sands from piston cores in this paper we shall com pare this petrographic information with bulk chemistry and attempt to relate these to tecto nics in arc related settings

dynamical and observational constraints on tropical pacific sea surface temperatures at the last glacial maximum asynchronously coupled atmosphere and ocean general circulation model simulations are used to examine the consequences of changes in the west east sea surface temperature sst gradient across the equatorial pacific at the last glacial maximum lgm simulations forced by the climap sst for the lgm where the west east sst gradient across the pacific is reduced compared to present produce a reduction in the strength of the trade winds and a decrease in the west east slope of the equatorial thermocline that is incompatible with thermocline depths newly inferred from foraminiferal assemblages stronger than present trade winds and a more realistic simulation of the thermocline slope are produced when eastern pacific ssts are 2 c cooler than western pacific ssts our study highlights the importance of spatial heterogeneity in tropical ssts in determining key features of the glacial climate

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

eddy formation in the denmark strait overflow on the basis of observations along the continental slope of east greenland a primitive equation open boundary model is used to study the formation of eddies in the denmark strait overflow the eddies are produced by vortex tube stretching initiated by cross slope components of the velocity field a sequence of cyclones and anticyclones is formed the cyclones are located along the deeper part of the slope and the anticyclones are along the shallower slope the eddies are almost barotropic with horizontal and vertical velocities of 30 cm s 1 and 0 1 cm s 1 respectively the cyclones migrate along the isobaths with a slight downhill component and intensify on their path until they become influenced by the flat deep basin their translation speed is typically 20 30 cm s 1 and the local change in relative vorticity is nearly entirely determined by vortex tube stretching and vorticity advection the overflow water flows mainly between the cyclonic and anticyclonic eddy chains the overflow is strongly accelerated by the eddies the existence of anticyclonic eddies along the shallower part of the east greenland continental slope predicted by the numerical model has been confirmed by observations

geochemical characterization of proterozoic upper crustal metamorphic terrain of southern malakand agency pakistan abstract proterozoic crystalline schists that have extensive outcrops in the southern malakand agency and adjacent regions were sampled from localities west of dargai town to include lithologically different samples their major and trace element chemistry and mineralogy indicate a sedimentary origin protolith variations and a passive continental margin tectonic setting the ree contents indicate considerable mixing of various source lithologies during sedimentation the samples have high tree and lree hree compared with the post archaean upper crustal values the chemical data indicate their derivation from a typical granodioritic upper crust with a large component of k rich granitic rocks

inception of the northern european ice sheet due to contrasting ocean and insolation forcing about 115 000 yr ago the last interglacial reached its terminus and nucleation of new ice sheet growth was initiated evidence from the northernmost nordic seas indicate that the inception of the last glacial was related to an intensification of the atlantic meridional overturning circulation amoc in its northern limb the enhanced amoc combined with minimum northern hemisphere insolation introduced a strong sea land thermal gradient that together with a strong wintertime latitudinal insolation gradient increased the storminess and moisture transport to the high northern european latitudes at a time when the northern hemisphere summer insolation approached its minimum

role of greenland freshwater anomaly in the recent freshening of the subpolar north atlantic the cumulative greenland freshwater flux anomaly has exceeded 5 000 km3 since the 1990s the volume of this surplus freshwater is expected to cause substantial freshening in the north atlantic analysis of hydrographic observations in the subpolar seas reveals freshening signals in the 2010s the sources of this freshening are yet to be determined in this study the relationship between the surplus greenland freshwater flux and this freshening is tested by analyzing the propagation of the greenland freshwater anomaly and its impact on salinity in the subpolar north atlantic based on observational data and numerical experiments with and without the greenland runoff a passive tracer is continuously released during the simulations at freshwater sources along the coast of greenland to track the greenland freshwater anomaly tracer budget analysis shows that 44 of the volume of the greenland freshwater anomaly is retained in the subpolar north atlantic by the end of the simulation this volume is sufficient to cause strong freshening in the subpolar seas if it stays in the upper 50 100 m however in the model the anomaly is mixed down to several hundred meters of the water column resulting in smaller magnitudes of freshening compared to the observations therefore the simulations suggest that the accelerated greenland melting would not be sufficient to cause the observed freshening in the subpolar seas and other sources of freshwater have contributed to the freshening impacts on salinity in the subpolar seas of the freshwater transport through fram strait and precipitation are discussed

simulated variability of the circulation in the north atlantic from 1953 to 2003 abstract the variability of the circulation in the north atlantic and its link with atmospheric variability are investigated in a realistic hindcast simulation from 1953 to 2003 the interannual to decadal variability of the subpolar gyre circulation and the meridional overturning circulation moc is mostly influenced by the north atlantic oscillation nao both circulations intensified from the early 1970s to the mid 1990s and then decreased the monthly variability of both circulations reflects the fast barotropic adjustment to nao related ekman pumping anomalies while the interannual to decadal variability is due to the baroclinic adjustment to ekman pumping buoyancy forcing and dense water formation consistent with previous studies an original characteristic of the oceanic response to nao is presented that relates to the spatial patterns of buoyancy and wind forcing over the north atlantic anomalous ekman pumping associated with a positive nao phase first induces a decrease of the southern sub

a model study of upper ocean sea ice interactions abstract a thermodynamic sea ice model including leads has been coupled to a one dimensional oceanic mixed layer model in order to investigate the upper ocean sea ice interactions the oceanic heat flux at the base of the ice layer fb is predicted not imposed assuming a thermodynamic equilibrium between the bottom of the ice and the water just below for testing of the model the annual cycle of the arctic sea ice and upper ocean is simulated along longitude 169 5 w the model produces a realistic evolution of the sea ice thickness and extent and of the upper mean salinity and temperature profiles the major deficiencies in the simulation are linked to the absence of ocean and ice dynamics the modeled value of fb is far from being constant below thin ice it can be larger than 5 w m 2 due to the important fraction of the solar irradiance that is transmitted through the ice absorbed in the mixed layer and then returned to the ice below the thicker perennial sea ice fb takes values between 0 and

isopycnal eofs in the azores current region a statistical tool fordynamical analysis and data assimilation abstract an empirical orthogonal function analysis of the vertical space time variability of the thermohaline structure of the ocean in summer and fall 1993 in the azores current region has been performed the analysis used data from ctd casts of the semaphore intensive experiment referred to as a seasonally varying climatology the relative merits of depth coordinate and isopycnal coordinate representations have been examined in regard to the quality of identification of physical processes and to the effectiveness of the extrapolation from a sea level anomaly in an attempt to improve the treatment of the vertical dimension in oceanic assimilation problems the isopycnal eofs consistently proved more efficient in capturing the vertical structure of both dominant processes in the area of investigation the variability linked to the mediterranean water and the coherent physical system made up of the azores front and the azores current the isopycnal analysis also proved more robust when meddies mediterra

soil response on the fraser delta to the mw 5 1 duvall washington earthquake strong motion recordings of the 3 may 1996 mw 5 1 duvall washington earthquake made in southwestern british columbia provide the best constraints to date on soil response to seismic shaking on the fraser delta just south of vancouver a total of nine strong motion instruments in southern b c triggered during this earthquake seven of which are on or near the fraser delta these recordings were made at epicentral distances of 140 to 180 km a preliminary analysis of these data clearly shows that for low frequencies 3 hz seismic spectral amplification by a factor of 2 to 7 occurs on the fraser delta relative to nearby bedrock or thin firm soil sites however at higher frequencies moderate attenuation is observed at the fraser delta sites the largest peak ground accelerations recorded generally around 2 to 3 hz were not on the thickest delta sediments but nearer the edge of the delta peak amplification is more pronounced in the horizontal component of ground motion than in the vertical the average sh z ratio for peak values at the fraser delta sites is 4 5 0 9 compared to 1 6 0 6 for the bedrock and firm soil sites in addition to the strong motion records we also utilize five three component broadband recordings of this earthquake made in southern b c epicentral distances of 140 to 520 km and 12 short period vertical component recordings epicentral distances of 130 to 380 km to document attenuation of peak acceleration with distance for this earthquake the observed values are slightly lower than those predicted by the ground motion relations proposed for the year 2000 national building code of canada

subdivision of precambrian time recommendations and suggestions by the subcommission on precambrian stratigraphy abstract the subcommission on precambrian stratigraphy consisting at the time of its last formal meeting in 1982 of 12 titular members and 30 correspondents has been actively working toward an acceptable subdivision of precambrian time for more than a decade comprehensive time rock charts have been prepared for major precambrian shields and this composite record has been analysed for patterns and events common to several or to all a geochronologic subdivision scheme has evolved that reflects these patterns of major events in earth history while still recognizing that those events are typically varied in intensity and timing from one region to another time unit boundaries have been selected so as to enclose or delimit principal cycles of sedimentation orogeny and magmatism but the boundaries are defined in years without specific reference to any bodies of rock several recommendations developed sequentially at formal meetings held at intervals of 2 4 years and announced in geologic journals with calls for comment and criticism are proposed 1 1 the recommendation to establish two major time units of eon rank proterozoic and archaean archean separated by a time boundary at 2500 ma is reaffirmed 2 2 the provisional recommendation to subdivide the proterozoic eon into three units of era rank provisionally labelled proterozoic i ii and iii with time boundaries at 2500 1600 900 and about 570 ma respectively is confirmed and 3 3 further subdivision of the proterozoic into eight time units of period rank as yet unnamed with time boundaries at 2500 2100 1800 1600 1400 1200 900 700 and about 570 ma respectively is provisionally recommended the nature of the subcommission s deliberations and the guidelines developed are described herein comment and criticism both on the procedure followed and on the recommendations themselves are invited

constraints on the frequency magnitude relation and maximum magnitudes in the uk from observed seismicity and glacio isostatic recovery rates summary earthquake populations have recently been shown to have many similarities with critical point phenomena with fractal scaling of source sizes energy or seismic moment corresponding to the observed gutenberg richter g r frequency magnitude law holding at low magnitudes at high magnitudes the form of the distribution depends on the seismic moment release rate mand the maximum magnitude m max the g r law requires a sharp truncation at an absolute maximum magnitude for finite m i n contrast the gamma distribution has an exponential tail which allows a soft or credible maximum to be determined by negligible contribution to the total seismic moment release here we apply both distributions to seismic hazard in the mainland uk and its immediate continental shelf constrained by a mixture of instrumental historical and neotectonic data tectonic moment release rates for the seismogenic part of the lithosphere are calculated from a flexural plate model for glacio isostatic recovery constrained by vertical deformation rates from tide gauge and geomorphological data earthquake focal mechanisms in the uk show near vertical strike slip faulting with implied directions of maximum compressive stress approximately in the nnw sse direction consistent with the tectonic model maximum magnitudes are found to be in the range 6 3 7 5 for the g r law or 7 0 8 2 m l for the gamma distribution which compare with a maximum observed in the time period of interest of 6 1 m l the upper bounds are conservative estimates based on 100 per cent seismic release of the observed vertical neotectonic deformation glacio isostatic recovery is predominantly an elastic rather than a seismic process so the true value of m max is likely to be nearer the lower end of the quoted range

the exceptional earthquakes in kaliningrad district russia on september 21 2004 abstract the earthquakes of magnitudes m w 5 0 and 5 2 in the kaliningrad enclave of russia on september 21 2004 were unexpected in a low seismicity area the earthquakes caused moderate damage in the kaliningrad enclave and smaller damage in northern poland and in southern and western lithuania the largest earthquake was the strongest ever recorded instrumentally in the region and it was felt at distances up to 800 km in directions towards the west and south the perceptibility area is abruptly cut off by the tornquist teisseyre zone the south west margin of the east european craton the earthquakes are instrumentally located at depths 16 20 km under the central northern part of the sambia peninsula in the kaliningrad enclave for these events it is noted that the macroseismic calculations of 10 19 km depths are in reasonable agreement the source mechanism of the largest earthquake was determined to be a right lateral strike slip on a wnw ese near vertical fault of orientation almost parallel to the tornquist teisseyre zone and to the north coast of the sambia peninsula based on available stress information it is interpreted that the underlying cause of the earthquakes is the absolute plate motion historical information is scanty it is searched in an attempt to evaluate past seismic activity in the region and to evaluate vulnerable weakness zones in the geological structures

vertical mixing and cabbeling in layered models abstract a consistent scheme for vertical mixing in layered numerical models is derived in this paper the fact that the vertical coordinate density depends on the properties being transported namely salinity and potential temperature renders the inclusion of vertical mixing in layered models a subtle problem the approach the authors have taken is based upon the entrainment into a layer being proportional to the turbulent activity in that layer across each interface there are then two entrainment velocities one upward velocity that is the entrainment of fluid into the layer above the interface and one downward velocity being the entrainment velocity into the layer below the interface this double entrainment accounts for both the diffusive and the advective consequences of turbulent mixing the proposed scheme works without approximation for a nonlinear equation of state and can readily handle the production of density caused by cabbeling several examples are given

a numerical scheme for modeling tidal wetting and drying 1 this paper introduces a numerical scheme developed to model the wetting and drying of coastal areas by the tide suitable for an oceanic circulation model using finite differences it is based on existing numerical schemes developed for isopycnic models for which the layer thickness can vanish when they interact with the bottom topography some original features are added and the scheme is optimized to accurately reproduce analytical solutions of an idealized problem the main parameters of this numerical scheme are 1 a critical height which depends on the local bottom slope and the grid step and 2 the rossby number this original numerical scheme is applied to a realistic barotropic configuration of the normand breton gulf where tidal amplitudes are large and where the wet dry front location varies greatly

on the geochemical evolution of sedimentary rocks the average major element compositions of clastic sedimentary rock assemblages i e carbonates evaporites and iron formations not considered for various times in earth history archean early proterozoic late proterozoic phanerozoic are reported based largely on the compilations of f l schwab uncertainties in the averages are considerable there is little evidence of secular change in the average composition of sedimentary rocks during post archean times such data do not support the idea of a gradual change in composition of the upper continental crust from mafic to felsic composition trace element patterns in sedimentary rocks particularly ree and th distributions support the idea of constancy of upper crustal composition during the post archean the average composition of archean clastic sedimentary rocks differs significantly from the post archean average being enriched in na mg and ca and depleted in si and k such differences are consistent with the archean upper crust having a more mafic composition than post archean upper crust the trace element data further support this idea it is noted that the average composition of post archean clastic sedimentary rocks predicts to within a factor of 1 2 the composition of the exposed continental crust for the elements si al fe k mg and ti the abundances of ca and na are depleted in the sedimentary rocks due to the fact that the compositions of carbonates evaporites and seawater were not considered the average composition of archean clastic sedimentary rocks does not compare favourably with an independent estimate of archean upper crust composition being enriched in si k and depleted in ca fe mg and ti such a comparison indicates that the archean upper crust may have had a more felsic major element composition than previously thought

seismogenesis and state of stress in the uk abstract in this paper i present a compilation of focal mechanisms for earthquakes with magnitudes greater than 3 0 ml in the british isles that can be used to help constrain our understanding of seismicity and it s driving forces in the british isles the fault plane solutions consist of both previously published mechanisms for significant british earthquakes and new solutions calculated from regional and local data for more recent and smaller earth quakes that were previously unpublished focal mechanisms for earthquakes in the uk are dominantly strike slip with northwest southeast compression and northeast southwest tension or reverse with northwest southeast com pression in many cases there is also an oblique component to the slip p and t axes from individual solutions are relatively well constrained in azimuth though less so in dip with p axes orientation for most events clustering between north and north west indicating sub horizontal compression however some spatial variation in p and t axes orientation is also apparent with near north northeast compression and east west extension in north west scotland changing to northwest southeast compression in england and wales i estimate a best fitting stress tensor under the assumption of uniform stress using two different inversion methods for both the entire focal mechanism data set and two different subsets of the data the results from the two different datasets suggest that there is a significant difference in the stress state between northwest scotland and england and wales calculated 1 directions for england and wales are northwest southeast consistent both with existing stress data and expected stresses from first order plate motions by contrast the inversion results for northwest scotland show near east west extension with possible 1 and 2 directions lying in a north south band and that the magnitudes of 1 and 2 are similar the relative magnitude of the principal stresses r determined for england and wales suggests that the intermediate stress 2 is close to the average value of 1 and 3 the clear difference in the stress inversion results between northwest scotland and england and wales suggests that the principal stress directions expected from first order plate motions have been modified in scotland by local stress conditions due to glacio isostatic adjustment

on anomalous sea surface temperatures in the nordic seas abstract gridded sea surface temperature sst sea level pressure 10 m wind field and ice concentration data from the winters 1982 98 are used in a study of the large scale variability of the ssts in the nordic seas mean fields are extracted and areas of maximum variability identified a complex principal component analysis is applied to identify coherent structure of variability in the sst field the leading mode accounting for 39 of the variance reveals a band of high correlations at increasing phase lags along the west coast of norway across the greenland sea and south through the denmark strait the sst data contain two cycles of low frequency variability during the 17 winters studied having approximately 5 yr and 12 yr periods for the eastern parts of the nordic seas propagation speeds agree with other transport estimates this indicates that sst anomalies are representing upper ocean heat anomalies advected by the mean flow from north of scotland toward the barents sea this is not true

a possible mechanism for the strong weakening of the north atlantic subpolar gyre in the mid 1990s 1 the extent and strength of the north atlantic subpolar gyre spg changed rapidly in the mid 1990s going from large and strong in 1995 to substantially weakened in the following years the abrupt change in the intensity of the spg is commonly linked to the reversal of the north atlantic oscillation nao index changing from strong positive to negative values in the winter 1995 96 in this study we investigate the impact of the initial spg state on the subsequent behavior of the spg by means of an ocean general circulation model driven by ncep ncar reanalysis fields our sensitivity integrations suggest that the weakening of the spg cannot be explained by the change in the atmospheric forcing alone rather for the time period around 1995 the spg was about to weaken irrespective of the actual atmospheric forcing due to the ocean state governed by the persistently strong positive nao during the preceding seven years 1989 1995 our analysis indicates that it was this preconditioning of the ocean in combination with the sudden drop in the nao in 1995 96 that lead to the strong and rapid weakening of the spg in the second half of the 1990s this hypothesis explains the diverging evolution of the strength of the spg and the atmospheric forcing winter nao after 1995 as has been suggested recently

precambrian deformed granites of possible basement in the himalayas abstract granitic rocks are abundant in the axial zone the core of the himalayas the lesser himalayan unfossiliferous metasedimentary sequence lying to the south of the axial zone also contains concordant sheets of granitic rocks which were earlier described either as intrusive or metamorphosed volcaniclastic sediments these granites have yielded precambrian radiometric ages the lesser himalayan deformed granites have been demarcated and we suggest that they represent thrust wedges of the precambrian older than 1000 ma basement rocks of the himalayas the deformed granite sheets of the eastern himalayas show the development of various types of mylonite the chemical characters of these granites vary within reasonable limits geochemistry and sr isotope ratios suggest their derivation by melting from crustal material and sediments probably greywacke to variable degrees from geochemical evidence it is suggested that the central crystalline rocks of the axial zone may contain reworked precambrian basement components the studied granites provide evidence of a precambrian metamorphic and anatectic event in the himalayas

the dudley earthquake of 2002 a moderate sized earthquake in the uk the 4 7 ml dudley earthquake on 22 september 2002 at 23 53 utc was widely felt throughout england and wales and was the largest earthquake to occur onshore in the united kingdom uk since the magnitude 5 1 ml bishop s castle earthquake in 1990 the earthquake hypocentre determined from inversion of observed p and s wave travel time data suggests a source depth of 14 km and this depth estimate is supported by forward modelling of observed waveforms focal mechanisms obtained from both first motion polarities of local observations and moment tensor inversion of regional observations show left lateral strike slip faulting along a near vertical near north south striking fault plane whose orientation is in good agreement with the surface expression of the observed faults in the region two aftershocks were recorded within the location error ellipsoid of the mainshock comparison of the waveform signals revealed that the mainshock and aftershocks were nearly co located and possibly had the same source mechanism the observed peak ground acceleration is found to be less than that predicted using empirical relations which have been considered applicable in the uk seismic moment m0 and stress drop were measured from on scale records where lg arrivals were clear and then used to give better estimates of the peak ground accelerations using a stochastic approach

erosion of a surface vortex by a seamount abstract numerical experiments are carried out on the f plane using a shallow water isopycnal model to analyze the behavior of a surface intensified anticyclonic vortex when it encounters an isolated seamount the advection by the vortex of deep fluid parcels across the isobaths is known to generate deep anticyclonic and cyclonic circulations above and near the bathymetry respectively these circulations are shown to exert a strong shear on the upper layers which causes an erosion of the initial vortex by filamentation the erosion often results in a subdivision of the eddy while the eroded original structure forms a dipole with the deep cyclone and is advected away the filaments torn off from the original core aggregate into a new eddy above the seamount splitting in more than two structures is sometimes observed the erosion process is quantified by the bulk volume integral of the eddy potential vorticity anomaly a sensitivity study to different parameters of the configuration distance between

moment magnitude determination for local and regional earthquakes based on source spectra we investigated the use of an automated routine to determine moment magnitudes from the displacement spectra of local and regional earthquakes two algorithms a genetic algorithm and a converging grid search were developed and tested with earthquake data from mexico norway and deception island antarctica it was found that compared with manual analysis the algorithms give reliable automatic moment magnitude m w estimates in the range 1 m 8 the converging grid search appeared to be more cost effective than the genetic algorithm m w at local and regional distances seems superior to amplitude based magnitudes that saturate for large earthquakes the application of the automated algorithm in near real time may help to obtain a nonsaturated magnitude estimate in the case of a large earthquake immediately after the earthquake has occurred also the method can be useful for processing large amounts of data manuscript received 27 july 2001

long term coordinated changes in the convective activity of the north atlantic the north atlantic is a peculiarly convective ocean the convective renewal of intermediate and deep waters in the labrador sea and greenland iceland sea both contribute significantly to the production and export of north atlantic deep water thus helping to drive the global thermohaline circulation while the formation and spreading of 18 degree water at shallow to intermediate depths off the us eastern seaboard is a major element in the circulation and hydrographic character of the west atlantic for as long as time series of adequate precision have been available to us it has been apparent that the intensity of convection at each of these sites and the hydrographic character of their products have been subject to major interannual change as shown by aagaard 1968 clarke et al 1990 and meincke et al 1992 for the greenland sea in the ows bravo record from the labrador sea eg lazier 1980 et seq and at the panulirus hydrostation s site in the northern sargasso off bermuda eg jenkins 1982 talley and raymer 1982 this paper reviews the recent history of these changes showing that the major convective centres of the greenland and labrador seas are currently at opposite convective extrema in our postwar record with vertical exchange at the former site limited to 1000 m or so but with labrador sea convection reaching deeper than previously observed to over 2300 m as a result greenland sea deep water has become progressively warmer and more saline since the early 70 s due to increased horizontal exchange with the arctic ocean through fram strait while the labrador sea water has become progressively colder and fresher over the same period through increased vertical exchange most recently convection has become deep enough there to reach into the more saline nadw which underlies it so that cooler but now saltier and denser lsw has resulted

decreasing overflow from the nordic seas into the atlantic ocean through the faroe bank channel since 1950 the overflow of cold dense water from the nordic seas across the greenland scotland ridge and into the atlantic ocean is the main source for the deep water of the north atlantic ocean this flow also helps drive the inflow of warm saline surface water into the nordic seas the faroe bank channel is the deepest path across the ridge and the deep flow through this channel accounts for about one third of the total overflow previous work has demonstrated that the overflow has become warmer and less saline over time here we show using direct measurements and historical hydrographic data that the volume flux of the faroe bank channel overflow has also decreased estimating the volume flux conservatively we find a decrease by at least 20 per cent relative to 1950 if this reduction in deep flow from the nordic seas is not compensated by increased flow from other sources it implies a weakened global thermohaline circulation and reduced inflow of atlantic water to the nordic seas

geochemistry of the early proterozoic metasedimentary rocks of the alligator rivers region northern territory australia abstract the early proterozoic metasedimentary sequence of the alligator rivers region a part of the pine creek geosyncline in the northern territory australia overlies an archaean granitoid basement early proterozoic sedimentary sequences in general record important changes in the composition of the upper continental crust about the archaean proterozoic boundary however the geochemistry of only a few of these sequences has been documented the geochemistry of the early proterozoic succession in the alligator rivers region is reported here and the results are interpreted in terms of differences between the stratigraphic units their provenance particularly in relation to crustal evolution and their subsequent metamorphism and weathering clastic metasedimentary rocks throughout the alligator rivers region have a remarkably uniform major and trace element geochemistry the kakadu group and upper member of the cahill formation are relatively more enriched in sio 2 and correspondingly more depleted in al 2 o 3 than the rest of the sequence reflecting the greater dominance of metapsammitic assemblages the lower member of the cahill formation which hosts the major u deposits of the alligator rivers region and the metasedimentary sequence in general exhibit no significant enrichment in u above normal background values rare earth element ree concentrations in the metasedimentary units within the alligator rivers region are uniform though in detail there are some important differences within and between formations the composition of the early proterozoic clastic metasediments in the alligator rivers region is consistent with the composition of similar material of the same age from other areas and supports current ideas on crustal evolution the alligator rivers metasediments are enriched in si and k and depleted in mg ca and na relative to the archaean average for clastic sedimentary rocks and their ree geochemistry resembles typical post archaean sedimentary rocks having a light ree enriched pattern and a distinct eu eu depletion compared to typical archaean sediments however the ree data indicate that two compositionally distinct sources are involved in the provenance of the kakadu group and possibly the lower member of the cahill formation where two types of ree patterns can be distinguished on their hree concentration and eu eu anomaly

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

a local magnitude scale ml for the united kingdom we have developed a new local magnitude scale ml for the united kingdom uk to replace the hutton and boore 1987 scale developed for southern california which has been used in the uk until now the new uk scale is developed from 1482 observations of 85 earthquakes on 50 stations located across the british isles and ireland most of the observations are from epicentral distances of less than 600 km and only few from greater distances up to 900 km the distance range of the scale is therefore 0 600 km the amplitude observations were used to invert for the parameters defining distance dependence in the ml scale and station corrections synthetic tests showed that the inversion was robust the new ml scale for the uk is given by ml loga 0 95logr 0 00183r 1 76 in which a is horizontal component ground displacement amplitude in nanometers the amplitudes are measured on traces that are filtered to simulate the wood anderson seismograph r is the hypocentral distance in km the uk scale is intermediate between scales determined for california and those of other intraplate areas such as norway or the northeastern united states the absolute station corrections found are all less than 0 5 the scale derived for the uk helps to reduce the overall variance of the mean magnitude estimates by 30 much of this improvement is due to the use of station corrections applying the uk scale to the database of recorded earthquakes results in a reduction of magnitude for earthquakes above ml 2 and a slight increase in magnitude for earthquakes below ml 2 the biggest change to the ml computation is likely to be for small earthquakes with few amplitude readings where the use of station corrections makes a significant difference

relation between the wind stress curl in the north atlantic and the atlantic inflow to the nordic seas 1 in this study an isopycnic coordinate ocean model has been used to investigate the relationships between the north atlantic wind stress curl wsc and the inflow of atlantic water to the nordic seas for the period 1995 2001 there is a maximum in the correlation between the zonally averaged wsc at 55 n and the inflow with a 15 month time lag capturing a relation already found in observational data in the model this relation is linked to the mixing along the western flank of the rockall bank 56 n 15 w for the period 1995 2001 the atmospheric forcing in the northeastern north atlantic is relatively weak and the depth of the mixed layer is shallower than the sill depths of the greenland scotland ridge gsr slowly moving baroclinic disturbances caused by anomalies in the wind forcing will then be transmitted into the nordic seas where they are recorded as anomalous volume transports in the norwegian atlantic current in contrast for the pentad prior to this period the atmospheric forcing is much more intense and generates mixing well below sill depths of the gsr for all winters baroclinic disturbances forced by variations in the atmospheric forcing will then tend to follow f h contours that do not enter the nordic seas and the 15 month lagged relations between the wind and the volume transports will vanish recent observational data support this view

the sensitivity of the atlantic meridional overturning circulation to enhanced freshwater discharge along the entire eastern and western coast of greenland the possible responses of the atlantic meridional overturning circulation amoc to increased freshwater discharge along the greenland coast has become an issue of growing concern given the increasing rate of the greenland ice sheet gris melting a recent model study suggested a weakened amoc of about 13 30 when a freshwater anomaly of 0 1 sv 1 sv 106 m3 s 1 was released along the entire greenland coast during the late twentieth century 1965 2000 in this study we use a fully coupled climate model to examine the sensitivity of amoc to a similar amount of freshwater forcing but released separately along the eastern the western and the entire greenland coast our results show that in all three cases there is a general weakening of the amoc mainly due to a reduced formation of labrador sea water moreover when additional freshwater is released along the eastern coast of greenland the amoc weakens more compared to the other two cases the different degree of convective mixing reduction in the irminger sea is the main reason for the spread in amoc responses in the three freshwater hosing experiments compared to the other two experiments the eastern coast experiment shows a relative warming in the labrador sea and the generation of a negative greenland tip jet like wind pattern anomaly these anomalies lead to a weaker convective mixing in the southern irminger sea and result subsequently in less formation of the simulated upper labrador sea water ulsw in the eastern coast experiment this study therefore highlights a potential important role for ulsw formation in determining the sensitivity of the amoc in response to large gris melting

computing marine ice thickness at an ice shelf base the freezing of sea water to the base of an ice shelf can give rise to large patches of accumulated ice a phenomenon known as marine ice in this study a numerical method is presented for calculating the thickness of the marine ice layer using an ice shelf ocean model the present day modeling paradigm of ice shelf ocean interaction usually involves the fixed specification of the ice shelf geometry while the ocean circulation in the cavity beneath the ice shelf evolves freely this approach relies on several assumptions such as steady state ice shelf thickness and ice shelf flow fields in order to make reasonable quantitative estimates of the thermodynamic exchange processes occurring at the ice shelf base this paper discusses the impact of these and other assumptions on the estimation of the thickness of the marine ice layer model simulation results are presented for an idealized ice shelf ocean configuration as a demonstration of the feasibility of the numerical method a sensitivity analysis is given so as to quantify the relative uncertainty in the marine ice thickness that arises from uncertainties in the model input parameters these being principally the ice shelf flow field the basal accumulation rate and the ice shelf thickness field

an inflated subpolar gyre blows life toward the northeastern atlantic abstract deep convection in the labrador and irminger seas inflates the cold and low saline subpolar gyre which is a rich nutrient and zooplankton source for the surrounding warmer waters of subtropical origin the zooplankton abundances on the south iceland shelf show characteristic sub decadal variability which closely reflect the oceanic abundances of the ecologically most important zooplankton species calanus finmarchicus much higher abundances of this species are observed during years when the winter mixed layer depths mld in the labrador irminger sea and over the reykjanes ridge are deep furthermore a tight relationship is identified between on shelf zooplankton abundances and lateral shifts of the biologically productive subarctic front southwest of iceland thus we suggest that northeastward expansion of the subpolar gyre results in biologically productive periods in the waters southwest of iceland both oceanic and on the shelf in addition to local atmospheric forcing we find that the mld and frontal position are also impacted by remote heat losses and convection in the labrador sea through northward advection of unstable mode waters the sub decadal oceanic and on shelf biological production peaks are possibly predictable by half a year local winter convection to subsequent summer production and the advective time lag from the labrador sea might induce an even longer predictability horizon up to 1 5 years

the seismic reflection moho beneath the united kingdom and adjacent areas abstract onshore and offshore deep seismic reflection data have been integrated to produce two way travel time and depth maps of the seismic reflection moho beneath the uk moho travel times are mostly in the range 9 5 11 5 s showing only weak correlation with specific upper crustal structural features but decreasing generally northwestward towards the atlantic margin moho depths show a much stronger correlation with upper crustal structure than do travel times there is pronounced crustal thinning moho depths 32 km are found generally beneath basement massifs particularly onshore short wavelength topographic features on the moho are quite rare and are generally restricted to ancient zones of continental collision and or subduction some of these ancient collision and or subduction zones are spatially associated with a highly reflective moho this is particularly evident where low angle shear zones in the lower crust or uppermost mantle converge with or intersect the moho it is suggested that development of a moho detachment surface can tectonically smooth the moho and thereby markedly increase its reflectivity a tendency in certain areas for the moho to remain at roughly constant two way travel time though not constant depth beneath both sedimentary basins and intervening basement massifs is interpreted as indicating that structural features on the scale of several tens of kilometres are in regional isostatic equilibrium conversely travel time perturbations beneath smaller features 10 km across indicate that these are at least partially uncompensated the uk crust is therefore weak but not sufficiently weak to attain local airy isostasy basement massifs west of scotland show systematically smaller moho two way travel times than similar structures farther south within the constraints of regional isostasy this can be interpreted in two main ways greater rigidity and strength of lewisian rocks within the west scottish massifs manifest as higher seismic velocity relative to density could be a cause of the observed travel time differences alternatively low density material beneath the crust could buoy up the western scottish massifs if this buoyancy were the consequence of igneous underplating then any underplated material would have to lie beneath the seismic reflection moho

uk crustal structure close to the iapetus suture a receiver function perspective summary teleseismic receiver functions have been calculated for data from 10 short period three component seismic recording stations across northern britain to investigate variations in crustal and upper mantle velocity structure the stations straddle the iapetus suture zone the inferred boundary between two of the continents fused together during the caledonian orogeny the receiver function data shows that there is considerable azimuthal variation in both crustal and upper mantle structure beneath several stations the data are projected on to a 2 d profile showing laterally continuous moho ps conversions at delay times between 3 and 4 s synthetic receiver functions generated using the velocity model from a previous deep seismic reflection refraction survey show ps and pppms phases comparable to the observed data 1 d forward modelling of the data gives crustal thicknesses of 30 km there is a significant velocity depth trade off in the receiver function method and the crustal thicknesses have been constrained by a priori velocity information investigation of the data from station gim close to the iapetus suture shows a sub moho phase which is found only on data from northerly backazimuths phase modelling is consistent with the presence of a gradational low velocity zone with a minimum vp of 6 5 km s 1 at a depth of 43 km this feature has similar characteristics to the wide angle seismic reflection and refraction velocity model of the w reflector in northern scotland the large scale heterogeneity at gim is attributed to structures associated with the iapetus suture however modelling has been performed using 1 d approximations while the phase signature could result from complex 3 d morphology we therefore conclude only that the results provide evidence of significant lateral variation in subcrustal structure

source parameters for the 28 april 2007 mw 4 0 earthquake in folkestone united kingdom a moderate size earthquake m l 4 3 m w 4 0 occurred in southeastern england on 28 april 2007 the earthquake was of some significance as it caused damage in the town of folkestone and produced the largest peak horizontal ground acceleration pga 0 1 g measured in the united kingdom to date it was followed by 12 aftershocks between m l 0 8 and 1 7 the earthquake was the first of this size recorded by a significant number of newly installed broadband stations in the united kingdom the hypocenter of the event was at a depth of about 5 km beneath folkestone with an error ellipse indicating horizontal errors in a location of about 5 km the depth was well constrained using a number of techniques of which local travel time inversion and teleseismic depth phase modelling are most reliable a stress drop of 28 6 bars and a source radius of 0 5 km were determined from the analysis of displacement source spectra we derived a near surface attenuation factor 0 02 from the aftershock data that were used in the spectral analysis of the mainshock applying the horizontal to vertical h v spectral ratio technique to microtremor data recorded at a station 2 km from the epicenter revealed site amplification at frequencies of 0 4 and 3 9 hz this amplification is likely to have contributed to the mainshock pga of 0 1 g measured at the same site similar site conditions may have been responsible for the damage in parts of folkestone the moment tensor computed from regional broadband data showed a strike slip mechanism with a normal component and either right lateral movement on a west southwest east northeast striking or left lateral movement on a north northwest south southeast striking nodal plane the north northwest south southeast striking nodal plane matches the trend of the main faults affecting the kent coalfield and also possibly the variscan front it is thus possible that the causative fault was associated with the variscan front a major structural boundary at the northern limit of late carboniferous folding and thrusting

mesoscale variability in denmark strait the pv outflow hypothesis abstract the outflow through denmark strait shows remarkable mesoscale variability characterized by the continuous formation of intense mesoscale cyclones just south of the sill these cyclones have a diameter of about 30 km and clear signatures at the sea surface and in currents measured near the bottom they have a remnant of arctic intermediate water aiw in their core the authors hypothesis is that these cyclones are formed by stretching of the high potential vorticity pv water column that outflows through denmark strait the light upper layer of the outflow the east greenland current remains on the surface in the irminger sea while the dense overflow water descends the east greenland continental slope the midlevel waters mostly aiw could thus be stretched by more than 100 which would induce very strong cyclonic relative vorticity the main test of this new hypothesis is by way of numerical experiments carried out with an isopycnal coordinate ocean model configured to have a marginal sea

the variability of the atlantic meridional overturning circulation the north atlantic oscillation and the el nino southern oscillation in the bergen climate model abstract the link between the interannual to interdecadal variability of the atlantic meridional overturning circulation amoc and the atmospheric forcing is investigated using 200 yr of a control simulation of the bergen climate model where the mean circulation cell is rather realistic as is also the location of deep convection in the northern north atlantic the amoc variability has a slightly red frequency spectrum and is primarily forced by the atmosphere the maximum value of the amoc is mostly sensitive to the deep convection in the irminger sea which it lags by about 5 yr the latter is mostly forced by a succession of atmospheric patterns that induce anomalous northerly winds over the area the impact of the north atlantic oscillation on deep convection in the labrador and greenland seas is represented realistically but its influence on the amoc is limited to the interannual time scale and is primarily associated with wind forcing the tropical pacific shows a strong variability in the model

performance of a second order moments advection scheme in an ocean general circulation model 1 the reliability of ocean general circulation models ogcms strongly depends on the quality of their tracer advection schemes for the sake of simplicity and computing time tracer advection schemes most commonly used in large scale ogcms tend to be low order schemes which suffer from spurious numerical diffusion and dispersion that result in distorted solutions the application of high order schemes would reduce numerical errors but at a considerable cost in terms of computing time an alternative to the use of high order methods is the implementation of algorithms that take into account the sub grid distribution of tracers one such method is the second order moments som scheme of prather 1986 which is more accurate than a fourth order scheme but at the time consumption of a second order algorithm this article presents results from coarse resolution global ocean simulations with very low explicit diapycnal mixing in which active and passive tracers were advected with the som method we compare the performance of the method with that of more traditional schemes namely the fct flux corrected transport and quicker quadratic upstream interpolation for convective kinematics schemes in general the use of the som method significantly improves tracer distributions and transports compared to fct and quicker thus leading to a better representation of ocean currents notably boundary currents and frontal systems while model simulations employing the fct and quicker schemes recreate a global overturning circulation with strong upwelling occurring in low latitudes the som simulations admit a circulation pattern closer to that known as the reconfigured conveyor belt toggweiler and samuels 1993 in which the bulk of the global ocean upwelling occurs in the southern ocean

on the role of the azores current in the ventilation of the north atlantic ocean three high resolution ocean circulation models of the north atlantic differing chiefly in their description of the vertical coordinate are used to examine the ventilation of the north atlantic subtropical gyre all the models produce mode waters of realistic densities in the sargasso sea and the european basin but have azores currents of differing strengths which are categorised as strong of realistic transport intermediate and weak these differences have a critical impact upon the ventilation of the gyre most importantly the strong azores current forms an effective barrier to the southward ventilation of eastern north atlantic water from the northern european basin initially driving it southwestwards into the central gyre before turning it back eastwards again in a general cyclonic circulation north of the current the intermediate and weak azores currents instead allow the southward ventilation of this water mass near the european and african coasts the situation in nature appears to be intermediate between these two cases with the azores current acting as a partial block the study also raises the possibility of the azores current forming an advective connection between the sargasso sea mode waters in the western basin and modes of similar densities found in the eastern basin on the southern side of the current although there are high levels of variability in the extent of these lighter modes in the eastern basin in nature this postulate is supported by a number of observational studies in addition the present study also provides some support for the local production of madeira mode water in the eastern basin associated with retroflecting current branches on the southern side of the azores current overall the azores current is therefore likely to have a critical impact upon the ventilation of the subtropical gyre over a large area rather than just locally affecting the potential vorticity and density structure of the upper ocean between subtropical latitudes and the northern european basin the study also contributes to an ongoing community effort to assess the realism of our current generation of ocean models

temporal and spatial variability of the sea surface salinity in the nordic seas 1 in this paper the temporal and spatial variability of the sea surface salinity sss in the nordic seas is investigated the data include a russian hydrographical database for the nordic seas and daily to weekly observations of salinity at ocean weather station mike owsm located at 66 n 2 e in the norwegian sea in addition output from a medium resolution version of the miami isopycnic coordinate ocean model micom forced with daily national center for environmental prediction national center for atmospheric research ncep ncar reanalysis data is used to complement the analysis of the temporal and spatial fields constructed from the observational data sets the nordic seas show a strong seasonal variability in the vertical density stratification and the mixed layer ml depth with a weak stratification and a several hundred meters deep ml during winter and a well defined shallow ml confined to the upper few tens of meters during summer the seasonal variability strongly influences the strength of the high frequency variability and to what extent subsurface anomalies are isolated from the surface high frequency variability has been investigated in terms of standard deviation of daily sss calculated for the different months of the year from observations at owsm typical winter values range from 0 03 to 0 04 psu and summer values range from 0 06 to 0 07 psu results from the model simulation show that highest variability is found in frontal areas and in areas with strong stratification and lowest variability in the less stratified areas in the central norwegian sea and south of iceland investigation of the interannual variability over the last 50 years shows a marked freshening of the atlantic water in the norwegian and greenland seas moreover the strength of the southern sector of the polar front as defined by the 34 8 35 0 psu isohalines along the western boundary of the inflowing atlantic water undergoes significant interannual variability with gradient stretching reaching up to 300 km in comparison the variability in the strength of the eastern front and northern sector of the polar front seemingly controlled by the shelf break off norway and the ridge between the norwegian and the greenland seas typically undergoes stretching only between 60 and 80 km the investigation also demonstrates that the low frequency variability in the upper ocean density field in the greenland sea a key factor for the deep water convection is governed by the variability in the sea surface field since the early 1960s there has been a negative trend in the salinity probably contributing to the observed decrease in the deep water production in that period