eddy formation through the interaction between the leeuwin current leeuwin undercurrent and topography abstract the surface and subsurface circulation off south west australia was simulated using the regional ocean modelling system roms a primitive equation ocean model with terrain following s coordinates the major currents in this region the leeuwin current and leeuwin undercurrent were reproduced by specifying only temperature and salinity distributions from climatology the application of wind stress subsequent to the model reaching a quasi steady state resulted in the generation of the seasonal capes current a northward flowing current on the continental shelf associated with coastal upwelling the simulated currents compared well with the observed current patterns including the location and maximum current speeds shelf topography variations and bottom shear which generated vorticity influenced eddy formation eddies separated from the leeuwin current and the leeuwin undercurrent migrated westward and exited the model domain through merging dissipating or through an open boundary in the model simulations the undercurrent produced mostly cyclonic eddies due to strong negative vorticity where the current flowed against the continental slope the leeuwin current produced anticyclonic warm core eddies initiated from the formation of meanders which were strongest at the surface the eddy field was dominated by anticyclonic eddies at the surface and cyclonic eddies at 500 m the interaction between the leeuwin current and leeuwin undercurrent led to the formation of eddy pairs

the mean summertime circulation along australia s southern shelves a numerical study abstract for the first time a high resolution regional model is developed for the slope and shelf circulation within the great australian bight and for the gulfs region of south australia the results indicate the extent nature and dynamical interaction of a variety of circulation features that are most likely to be important for the region in particular the positive wind stress curl south of australia leads to an equatorward sverdrup transport in the deep ocean westward flinders current along the slope and upwelling of the 600 m deep permanent thermocline the wind stress curl also leads to a seaward topographic sverdrup transport within the wide sloping shelf of the bight and results in an anticyclonic gyre that is intensified off the eyre peninsula and reduced in magnitude by the joint effect of baroclinicity and topographic relief in the western half of the bight the seaward surface ekman and topographic transports are shown to converge with the onshore component of the flinders current lea

seismic reflections from the basal contacts of batholiths although seismic reflection data in batholithic terranes are scarce datasets available to us from three different regions show surprising similarities all three profiles one across precambrian rocks in texas and two across mesozoic batholiths in california and nevada show strong subhorizontal reflections at depths of 6 10 km these reflections are interpreted to originate from the base of the batholiths and indicate that these batholiths are tabular in shape the large amplitudes of the reflections require a marked contrast in acoustic impedance product of velocity and density between the batholith and the underlying rocks two of the profiles show reflective zones having appreciable thicknesses 0 5 2 5 km while the third profile shows a single continuous reflection the four most favored candidates for sub batholithic reflections are 1 underplated gabbro 2 cumulate layers 3 migmatites and 4 thrust faults cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance

a northern boundary current along australia s southern shelves the flinders current 1 south of australia the monthly mean wind stress curl is positive during both summer and winter and leads to ekman pumping and downwelling throughout the region sverdrup dynamics indicates that this downwelling should lead to a northward transport of around 5 10 sv 1 sv 10 m3 s 1 classical arguments for western boundary currents are adapted to show that this transport should be deflected into an upwelling favorable boundary current that flows from east to west along australia s southern shelves the flinders current support for this proposition is obtained from results of the ocean circulation and climate advanced modelling project occam sverdrup transports and limited observations for the region in addition the occam results show that the northward transport leads to upwelling at depths below 400 m and that the flinders current 1 intensifies from victoria to western australia transport 8 sv with speeds up to 15 cm s 1 2 can extend from the surface to depths of 800 m and 3 is found during both summer and winter during winter the winds are downwelling favorable and lead to a coastal current that flows from west to east and opposes the flinders current further support for the origin and nature of the flinders current is obtained from simple numerical experiments made using a rectangular domain and idealized representations of the summer and winter wind stress

model for the tectonic evolution of the mississippi embayment and its contemporary seismicity aeromagnetic and gravity surveys of the northern mississippi embayment have delineated a northeast trending basement depression about 70 km wide and more than 300 km long the feature the mississippi valley graben is remarkably linear over its known length has nearly parallel sides and has estimated depths relative to surrounding basement of more than 2 km both gravity and magnetic anomalies show an alignment of dense magnetic plutons along the boundaries of the graben estimated magnetization directions for the plutons indicate a mesozoic age the graben of probable earliest paleozoic age underlies the northwest side of the embayment and contains the area of major seismicity in particular much of the instrumentally located contemporary seismicity and the estimated locations of the large magnitude 1811 1812 earthquakes are in the central part of the graben we interpret the early paleozoic graben the mesozoic plutonism and the sedimentation cycles as evidence of stretching events seismicity is attributed to strain in and below the central graben area which would appear to be a focal point of deformation in the stretching model we suggest that the sequence of events can be explained by the action of a rotating horizontal stress field on a crustal flaw that persisted well back in pre cambrian time the postulated rotating stress field is presumably controlled by the system that drives the global plates

do east australian current anticyclonic eddies leave the tasman sea using satellite altimetry and high resolution model output we analyze the pathway of large long lived anticyclonic eddies that originate near the east australian current eac separation point we show that 25 30 of these eddies propagate southward around tasmania leave the tasman sea and decay in the great australian bight this pathway has not been previously documented owing to poor satellite sampling off eastern tasmania as eddies propagate southward they often stall for several months at near constant latitude along the pathway eddies become increasingly barotropic eddy intensity is primarily influenced by merging with other eddies and a gradual decay otherwise surface temperature anomaly associated with anticyclonic eddies changes as they propagate while surface salinity anomaly tends to remain relatively unchanged as they propagate

titanium vanadium and niobium mineralization and alkali metasomatism from the magnet cove complex arkansas the christy ti v nb deposit is hosted by the arkansas novaculite adjacent to the carbonatite alkaline magnet cove complex mineralized rock contains up to 8 wt percent ti 2 68 wt percent v and 0 124 wt percent nb minor amounts of li and mo are present and there is evidence for the preferential concentration of the middle to heavy rare earth elements ree in altered clay rich dikes mineralized rock consists of 1 thermally recrystallized novaculite that is partly replaced by actinolite sodic amphibole taeniolite klimg 2 si 4 o 10 f 2 pyrite and siderite 2 brookite bearing amphibole and taeniolite rich rock that contains siderite and pyrite 3 quartz brookite aggregates and 4 goethite rich rocks that form the oxidized zone of the deposit unaltered rock is scarce at the christy deposit comparison of mineralized lithologies from the christy deposit with those from two other areas of mineralization associated with the magnet cove complex the magnet cove titanium corporation deposit rutile deposit and the hardy walsh prospect provides mineralogical and chemical evidence for the source of the mineralizing fluids at the christy deposit massive clay rich kaolinite or smectite dikes are in contact with the mineralized zones of the christy deposit the presence of relict k feldspar and pyrite in the least altered dikes and certain characteristic trace elements e g ba rb suggest that the christy dikes are related to less pervasively altered rutile mineralized pyrite bearing carbonate feldspar dikes fromthe rutile deposit the feldspar carbonate dikes from the rutile deposit may be the products of intense alkali metasomatism of alkaline igneous silicate rocks that were emplaced prior to the intrusion of the magnet cove carbonatite the christy deposit formed through a series of complex processes the initial phase of mineralization is directly related to the infiltration of novaculite by alkali rich fluids that were probably derived from carbonatite magma introduction of the metasomatic fluids preceded emplacement of massive feldspar and pyrite bearing dikes that were subsequently altered to the clay rich masses titanium v nb and li were introduced by the alkali rich fluids at temperatures that were as indicated by fluid inclusion data willis et al 1991 as high as 600 degrees c during the initial stage of mineralization v was concentrated in aegirine and sodic amphibole li was concentrated in taeniolite minor amounts of ti were concentrated in aegirine and pyrite formed the replacement of novaculite by the aforementioned minerals yielded excess silica which precipitated as quartz in veins and as clear overgrowths on recrystallized inclusion rich partly replaced quartz niobium and v bearing brookite precipitated with the quartz minerals formed during the first stage reacted with a second fluid at temperatures of 100 degrees to 300 degrees c as indicated by fluid inclusion data willis et al 1991 and v was then concentrated in smectite and goethite the isotopic composition of siderite delta 18 o smow 23 8 25 0ppm delta 13 c pdb 7 1 to 8 9ppm suggests that the second fluid was a mixture of low temperature metasomatic fluid and ground water vanadium was further concentrated in clay minerals breit et al 1992 in goethite and in vug minerals as low temperature alteration proceeded brookite is not totally resistant to weathering and it is at least partly replaced by other oxides in the oxidized zone

eddy variability and energetics from direct current measurements in the antarctic circumpolar current south of australia two year time series measurements of current velocity and temperature in the subantarctic front saf south of australia from 1993 to 1995 provide estimates of eddy fluxes of heat and momentum across the antarctic circumpolar current acc and further insight into the variability of the acc the saf was found to be an energetic meandering jet with vertically coherent fluctuations varying on a timescale of 20 days with typical amplitude 30 cm s21 at 1150 dbar a daily varying coordinate frame that follows the direction of flow allowed mesoscale variability of the saf to be isolated from variability due to meandering of the front and proved very successful for examining eddy fluxes vertically averaged cross stream eddy heat flux was 11 3 kw m 22 poleward and significantly different from zero at 95 for fluctuations in the 2 90 day band zonally integrated this eddy heat flux 50 9 pw is more than large enough to balance heat lost south of the polar front and is as large as cross saf fluxes found in drake passage cross stream eddy momentum fluxes were small not significantly different from zero and of indeterminate sign but tending to decelerate the mean flow a relationship between vertical motion and meander phase identified in the gulf stream was found to hold for the acc eddy kinetic energy levels were similar to those in drake passage and southeast of new zealand eddy potential energy was up to an order of magnitude larger than at other acc sites most likely because meandering of the front is more common here baroclinic conversion was found to be the dominant mechanism by which eddies grow south of australia typical eddy growth time is estimated to be 30 days approximately twice as fast as in drake passage consistent with eddy energy growing rapidly downstream

latitudinal and frequency characteristics of the westward propagation of large scale oceanic variability abstract in the open ocean away from the equator westward propagation is a ubiquitous characteristic of oceanic variability the objectives of the study are to investigate the latitudinal dependence of the frequency of westward propagating variability two dimensional fourier analysis in frequency and zonal wavenumber is applied to time longitude records of sea surface height data obtained from the ocean topography experiment topex poseidon mission the focus of the study is placed on spatial scales larger than the mesoscale the frequency of westward propagation is generally lower than the critical frequency of the first mode baroclinic rossby waves as expected from the conventional theory of rossby waves in a midlatitude ocean however westward propagation with frequency of up to 2 times the critical frequency is also observed at most latitudes this supercritical propagation can be explained by the effects of the vertical shear of the mean flow at midlatitudes and by the effects of the equatorial

the 5500 km long boundary flow off western and southern australia 1 the path of the shelf edge flow off southwestern australia is documented using results from satellite altimetry and sea surface temperature sst and a climatogical in situ analysis during austral winter a continuous current is shown to extend from its origin at north west cape to the southern tip of tasmania a distance of 5500 km satellite sst observations and surface buoy tracks confirm the location and continuity of the current trajectory while the leeuwin current is forced by the strong alongshore pressure gradient associated with the meridional portion of the western australian coastal boundary our results suggest that the essentially zonal shelf edge flow along the southern australian coast arises from the setup of coastal sea level by onshore ekman flow driven by the winter westerly wind and possibly a further alongshore pressure gradient the timing of these two different forcing mechanisms means that the west coast pressure gradient delivers the leeuwin current to the south coast just as the winds reverse and are thus able to maintain the eastward passage of the current the shelf edge flow consists of two main water masses a low salinity warm water type of tropical origin associated with the leeuwin current and a high salinity warm water inflow formed on the western end of the great australian bight continental shelf a naming convention is proposed the leeuwin current representing flow from north west cape to the great australian bight gab the south australian current between the eastern gab and western bass strait and the zeehan current off western tasmania

a review of the shelf slope circulation along australia s southern shelves cape leeuwin to portland abstract a review is presented of the ocean circulation along australia s southern shelves and slope uniquely the long zonal shelf is subject to an equatorward sverdrup transport that gives rise to the flinders current a small sister to the world s major western boundary currents the flinders current is strongest near the 600 m isobath where the current speeds can reach 20 cm s and the bottom boundary layer is upwelling favourable it is larger in the west but likely intermittent in both space and time due to possibly opposing winds thermohaline circulation and mesoscale eddies the flinders current may be important to deep upwelling within the ubiquitous canyons of the region during winter the leeuwin current and local winds act to drive eastward currents that average up to 20 30 cm s the currents associated with the intense coastal trapped wave field 6 12 day band are of order 25 30 cm s and can peak at 80 90 cm s wintertime winds and cooling also lead to downwelling to depths of 200 m or more and the formation of dense coastal water within the great australian bight and the south australian sea within the great australian bight the thermohaline circulation associated with this dense water is unknown but may enhance the eastward shelf edge south australian current the dense salty water formed within spencer gulf is known to cascade as a gravity current to depths of 200 m off kangaroo island this dense water outflow and meanders in the shelf circulation also fix the locations of a sequence of quasi permanent mesoscale eddies between the eyre peninsula and portland during summer the average coastal winds reverse and surface heating leads to the formation of warm water in the western great australian bight and the south australian sea no significant exchange of shelf water and gulf water appears to occur due to the presence of a dense nutrient rich sub surface pool that is upwelled off kangaroo island the winds lead to weak average coastal currents increasing evidence suggests that el nino events 4 7 year period can have a major impact on the winter and summer circulation these events propagate from the pacific ocean and around the shelf slope wave guide of west australia and into the great australian bight during winter el nino events the average shelf currents may be largely shut down during summer the thermocline may be raised by up to 150 m the nature and role of tides and surface waves is also discussed along with uncertainties in the general circulation and future research

ocean circulation in the great australian bight in an eddy resolving ocean reanalysis the eddy field seasonal and interannual variability abstract the ocean circulation in the great australian bight gab is dominated by the surface intensified leeuwin current lc over the outer continental shelf and the flinders current farther offshore in the gab the lc flows eastwards and varies seasonally with strongest currents of up to 0 8 m s at 130 5 e bringing warm and salty waters as far east as western tasmania the flinders current flows westwards as an undercurrent at depths of around 500 m over the continental slope and at the surface offshore of the lc off the shelf the gab is rich in mesoscale eddies with surface intensified currents of up to 0 4 m s that are coherent over the full water depth getting weaker with depth within deep water eddies isotherms are displaced vertically by up to 200 m eddies in the eastern gab are most energetic in austral winter and spring coincident with the seasonal weakening of the lc the eastern most penetration of gab eddies occurs in austral winter when the lc starts to weaken we hypothesise that the horizontal shear associated with the lc generates eddies in the eastern gab the volume transport of the lc at the head of the gab is typically between 1 and 7 sv with a clear seasonal cycle and the interannual variability of lc transport is up to 2 sv

assimilation of surface velocity data into a primitive equation coastal ocean model summer of 1998 are 0 42 and 0 78 respectively demonstrating the effectiveness of the das our estimates of the error covariances are shown to be appropriate through a series of objective statistical tests analysis of the term balances of the model equations show that the dominant modeled dynamical balances are preserved by the das and that uncertainties in the spatial variability of the wind forcing are likely to be one source of model error by varying the relative magnitudes of the estimated forecast and observation error covariances the das is shown to be most effective when approximately 80 of the analysis is made up of the model solution index terms 4263 oceanography general ocean prediction 4219 oceanography general continental shelf processes 4255 oceanography general numerical modeling 3337 meteorology and atmospheric dynamics numerical modeling and data assimilation 4516 oceanography physical eastern boundary currents 4594 oceanography physical instruments and techniques keywords practical data assimilation numerical modeling upwelling highfrequency radar data

a generalization of cape into potential energy convertibility the concept of the potential energy convertibility pec is proposed as a generalization of convective available potential energy cape it is defined as a vertical integral of buoyancy weighted by a non dimensional normalized vertical momentum this is a measure of convertibility of potential energy into kinetic energy in the sense that the actual conversion rate is recovered when pec evaluated by the convective scale local buoyancy and vertical momentum as available from cloud resolving model crm simulations is multiplied by the normalization factor for the vertical momentum it reduces to cape when the standard parcel lifted buoyancy and a unit value for the normalized vertical momentum are used it is equivalent to arakawas schubert s cloud work function when the buoyancy and the vertical momentum profile for an entraining plume are used pec evaluated from locally defined buoyancy and vertical momentum in crm simulations correlates better with the convective precipitation than cape the evaluation of pec within a convective parametrization may be possible with an appropriate definition of the effective entrainment rate for example which is expected to improve cape based convective parametrizations copyright 2005 royal meteorological society

water mass analysis and alongshore variation in upwelling intensity in the eastern great australian bight 1 a study of climatological and conductivity temperature depth ctd data for 2004 is made to provide a conceptual model of upwelling for the eastern region of the great australian bight in particular the data and other studies provide strong evidence that shelf break upwelling is confined to the southwest kangaroo island region and does not occur farther to the west off the eyre peninsula rather the upwelled water is likely to remain in a kangaroo island pool until subsequent upwelling events draw the water to the shallower and surface coastal regions of the eastern bight in this manner the surface upwelling apparent off the bonney coast kangaroo island and the eastern great australian bight gab can appear to be simultaneous moreover it appears likely that the water within the kangaroo island pool remains nutrient rich support for this model comes from ctd sections collected in 2004 that show that the upwelled signal cool 26 kg m 3 diminishes in width and intensity with increasing distance from kangaroo island the pattern of fluorescence is similar to that for temperature in the upwelled plume and indicates that the kangaroo island pool remains nutrient rich relatively low oxygen concentrations may indicate a previous bloom the warmest water is found near the shelf break along with very low values of fluorescence and relatively higher levels of oxygen suggesting nutrient limited growth of phytoplankton these data also support the notion that the upwelled nutrient rich water is supplied from the kangaroo island pool and not by shelf break upwelling in the eastern gab anomalously salty and fresh water is identified as resulting from evaporation in coastal bays and groundwater aquifer discharge

estimates of m2 tidal energy dissipation from topex poseidon altimeter data most of the tidal energy dissipation in the ocean occurs in shallow seas as has long been recognized however recent work has suggested that a significant fraction of the dissipation perhaps 1 tw or more occurs in the deep ocean this paper builds further evidence for that conclusion more than 6 years of data from the topex poseidon satellite altimeter are used to map the tidal dissipation rate throughout the world ocean the dissipation rate is estimated as a balance between the rate of working by tidal forces and the energy flux divergence computed using currents derived by least squares fitting of the altimeter data and the shallow water equations such calculations require dynamical assumptions in particular about the nature of dissipation to assess sensitivity of dissipation estimates to input assumptions a large suite of tidal inversions based on a wide range of drag parameterizations and employing both real and synthetic altimeter data are compared these experiments and monte carlo error fields from a generalized inverse model are used to establish error uncertainties for the dissipation estimates owing to the tight constraints on tidal elevation fields provided by the altimeter area integrals of the energy balance are remarkably insensitive to required dynamical assumptions tidal energy dissipation is estimated for all major shallow seas excluding individual polar seas and compared with previous model and data based estimates dissipation in the open ocean is significantly enhanced around major bathymetric features in a manner consistent with simple theories for the generation of baroclinic tides

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

decay and expansion of the early aftershock activity following the 2011 mw9 0 tohoku earthquake the 2011 m w9 0 tohoku earthquake was followed by an abundant amount of seismicity providing a unique opportunity to analyze the triggering mechanism of great earthquakes although the tohoku earthquake occurred close to a dense seismic network many aftershocks that occurred in the first few hours after the mainshock are not recorded in the earthquake catalogs here we use a template waveform approach to recover as many as possible missing events in the first 12 hours following the tohoku mainshock our analysis is able to detect about 1 4 times more events than those listed in the high sensitivity seismograph hi net earthquake catalog combining our new dataset with earthquakes that occurred at latter times we are able to observe a continuous decay of the aftershock rate and along strike expansion of aftershock area we relate the latter observation to the occurrence of post seismic slip over the deep interface

on the leeuwin current system and its linkage to zonal flows in the south indian ocean as inferred from a gridded hydrography abstractthe leeuwin current system lcs along the coast of western australia consists of the poleward flowing leeuwin current lc the equatorward flowing leeuwin undercurrent luc and neighboring flows in the south indian ocean sio using geostrophic currents obtained from a highly resolved hydrographic climatology csiro atlas of regional seas cars this study describes the spatial structure and annual variability of the lc luc and sio zonal currents estimates their transports and identifies linkages among them in cars the lc is supplied partly by water from the tropics an annual mean of 0 3 sv 1 sv 106 m3 s 1 but mostly by shallow 200 m eastward flows in the sio 4 7 sv and it loses water by downwelling across the bottom of this layer 3 4 sv the downwelling is so strong that despite the large sio inflow the horizontal transport of the lc does not much increase to the south from 0 3 sv at 22 s to 1 5 sv at 34 s this lc transport is significantly smaller than previous

evolution movement and decay of warm core leeuwin current eddies abstract characteristics of eddies originating in the leeuwin current lc have been investigated using 10 day sea level anomaly maps of combined topex poseidon and ers altimeter data for the period 1995 2000 during these 6 years 37 long lived anticyclonic eddies were tracked from the west coast of australian to 90 e between three and nine eddies were spawned each year and their trajectories are described in detail most long lived eddies are generated in may june when the lc is most intense and at three preferred locations 20 21 5 s 24 25 s and 28 31 s the temporal evolution and latitudinal variations of the sea level anomalies and their radius and drift speed are discussed topography plays an important role in the evolution and structure of the eddies interactions with topography can induce splitting or merging of eddies which further affects the eddy decay

diapiric origin of the blytheville and pascola arches in the reelfoot rift east central united states relation to new madrid seismicity most of the earthquakes in the new madrid seismic zone correlate spatially with the blytheville arch and part of the pascola arch which are interpreted to be the same structure both arches may have formed by diapirism along the axis of the reeffoot rift seismic geophys ical and drill hole data indicate that the rocks in the arches are highly deformed and fractured and have gross lithologic properties that make them weaker than rocks adjacent to the arches the weaker rocks are inferred to fail seismically more readily than the stronger rocks adjacent to the arches

geologic structures related to new madrid earthquakes near memphis tennessee based on gravity and magnetic interpretations new inversions of gravity and magnetic data in the region north of memphis tennessee and south of latitude 36 define boundaries of regional structures and igneous complexes in the upper crust microseismicity patterns near interpreted boundaries suggest that igneous complexes influence the locations of microseismicity a weak seismicity cluster occurs near one intrusion covington pluton at the intersection of the southwest margin of the missouri batholith and the southeast margin of the reelfoot rift a narrow seismicity trend along the reelfoot rift axis becomes diffuse near a second intrusion osceola intrusive complex and changes direction to an area along the northwest flank of the intrusion the axial seismicity trend also contains a tight cluster of earthquakes located just outside the osceola intrusive complex the mechanical explanation of the two seismicity patterns is uncertain but the first cluster may be caused by stress concentration due to the high elastic stiffness and strength of the covington intrusion the spatially changing seismicity pattern near the osceola complex may be caused by the preceding factors plus interaction with faulting along the rift axis the axial seismicity strand itself is one of several connected and interacting active strands that may produce stress concentrations at strand ends and junctions the microseismicity clusters at the peripheries of the two intrusions lead us to conclude that these stress concentrations or stressed volumes may be locations of future moderate to large earthquakes near memphis

identification of a precambrian rift through missouri by digital image processing of geophysical and geological data digital free air and bouguer gravity anomaly images have been constructed from approximately 287 000 station readings for the region bordered by 25 49 n latitude and 80 110 w longitude the technique used to interpolate between station locations was based on a two dimensional spatial filter where the average of the anomaly values located within the filter area was computed the images contain as many as 256 contours values in byte variable so that subtle anomaly patterns can be identified and traced with much greater certainty than on most contour maps a newly discovered feature in the midcontinent is a gravity low that begins at a break in the midcontinent gravity high in se nebraska extends across missouri in a nw se direction and intersects the mississippi valley graben to form the pascola arch the anomaly varies from 120 to 160 km in width extends about 700 km and is best expressed in southern missouri where it has a bouguer amplitude of approximately 34 mgal the magnitude of the anomaly cannot be explained on the basis of a thickened section of paleozoic sedimentary rock the gravity data and the sparse seismic refraction data for the region are consistent with an increased crustal thickness beneath the gravity low some of the discrete positive magnetic anomalies in missouri are located along the borders of the gravity low digitally enhanced thermal infrared images from the heat capacity mapping mission show a distinct alignment of linear structures with the gravity feature the linear features in some cases correspond to mapped high angle normal faults to drape folds over relief within the precambrian basement and in some cases to extensions of mapped structures the gravity anomaly also cuts across the major precambrian boundary in se missouri marking the change from older sheared granites and metasedimentary rocks to younger granites and rhyolites given the cumulative evidence the gravity anomaly is probably the present expression of a failed arm of a rifting event perhaps one associated with the spreading that led to or preceded formation of the granite and rhyolite terrain of southern missouri

observational evidence for a southern hemisphere oceanic supergyre 1 a high resolution hydrographic atlas resolves the narrow boundary flow of the western pacific gyre and its pathway around tasmania southern australia and into the indian ocean basin this confirms recent model descriptions of a southern hemisphere supergyre a nested system of subtropical gyres the observations show that the gyre flow is squeezed into a narrow band between tasmania and the antarctic circumpolar current the residue of east australian current transport turns westward around tasmania and leaks into the indian ocean while there is also a limited form of gyre connection between the indian and atlantic systems south of africa with a mean outflow of agulhas current water into the atlantic basin these gyre circulations remain far more discrete the gyre coupling and the east australian current in particular provide the mechanism by which sub antarctic mode water and antarctic intermediate water are distributed between the ocean basins

global observations of large oceanic eddies 1 ten years of sea surface height ssh fields constructed from the merged topex poseidon t p and ers 1 2 altimeter datasets are analyzed to investigate mesoscale variability in the global ocean the higher resolution of the merged dataset reveals that more than 50 of the variability over much of the world ocean is accounted for by eddies with amplitudes of 5 25 cm and diameters of 100 200 km these eddies propagate nearly due west at approximately the phase speed of nondispersive baroclinic rossby waves with preferences for slight poleward and equatorward deflection of cyclonic and anticyclonic eddies respectively the vast majority of the eddies are found to be nonlinear

fault geometry and earthquakes in continental interiors abstract although large earthquakes in continental interiors are much less frequent than those along plate boundaries they have been responsible for a disproportionate amount of destruction i present a testable model for strain accumulation and seismicity in the upper crust of continental interiors intersecting faults provide a focus for strain build up resulting in local pockets of high strain accumulation rigid plates typically deform at strain rates 10 10 to 10 9 strain yr 1 but at intersecting fault zones strain accumulates at anomalously high rates of 10 8 to 10 7 strain yr 1 i e the strain rates are comparable with those at plate boundaries the resulting large earthquakes are associated with relatively short return times hundreds of years typically one of the intersecting faults appears to split and to offset the other main fault defined as a fault on which the larger offset occurs forming two parallel strands separated by a jog i also observe a rotation of one leg of the main fault with respect to the other the region of intersection is usually associated with intense fracturing increased microseismicity and uplift

pattern and velocity of propagation of the global ocean eddy variability 1 satellite altimeter data are used to study the characteristics of the horizontal propagation of eddy variability of the global oceans decade long time series of sea surface height is analyzed for finding the maximum cross correlation with neighboring time series within a window of space and time lags the space and time lags corresponding to the maximum correlation allow an estimate of the propagation velocity of the eddy variability that dominates the variance of sea surface height anomalies the method cannot distinguish the various forms of eddy variability isolated eddies and fronts the meandering of ocean currents or planetary waves however the results provide at a given location of the global oceans a uniquely determined propagation velocity that represents a time averaged description of the tendency of the movement of the local eddy variability the propagation velocity is highly inhomogeneous in space outside the equatorial zone the zonal propagation is intrinsically westward modified by ocean currents which could reverse the zonal propagation to eastward in regions like the gulf stream and the antarctic circumpolar current at midlatitudes and high latitudes the propagation pattern is highly affected by the path of ocean currents and the shape of bottom topography at tropical latitudes the meridional propagation is convergent toward the equator in the western basins and divergent away from the equator in the eastern basins comparison with the simulations of an eddy permitting ocean general circulation model shows overall agreement especially in the latitudinal variation of the zonal propagation velocity the result suggests that the model has captured the essence of the dynamics governing the propagation of ocean eddy variability

modeling studies of the leeuwin current off western and southern australia the leeuwin current strengthens considerably from february to may each year following the slackening of southerly coastal winds strong eddies develop a high resolution multilevel primitive equation ocean model is used to examine this eddy development in an idealized way by considering the development of flow from rest when temperatures are initially given the observed longshore gradients the system is allowed to geostrophically adjust in the absence of longshore winds and of any surface heat flux two types of experiments are conducted the first type uses the indian ocean climatological temperature gradient forcing case 1 and 2 while the second type repeats the first experiment with the added contribution of the north west shelf nws temperature profile cases 3 and 4 to investigate the additional effects of coastline irregularities cases 1 and 3 use an ideal coastline while cases 2 and 4 use an irregular realistic coastline in all cases maximum surface velocities occur at cape leeuwin where the leeuwin current changes direction and off southern australia maximum undercurrent velocities occur off western australia in case 1 cape leeuwin and the western australian coast are the preferred locations for the development of warm anticyclonic eddies which are generated due to a mixed instability mechanism in case 2 the warm anticyclonic eddies occur in the vicinity of coastal promontories and at cape leeuwin while advection of warm water is present along the entire coast in case 1 the irregular coastline geometry limits the extent of warm water in case 2 the added contribution from the nws water in cases 3 and 4 augments the onshore geostrophic inflow to produce a model leeuwin current and undercurrent that are more vigorous and unstable than in the previous cases in case 3 the nws water adds strong horizontal shear to the coastal equatorial region of the domain and vertical shear to the inshore current it also advects warmer water along the entire coast in case 4 the addition of both the nws water and the irregular coastline results in the establishment of a stronger surface current and undercurrent than in the previous cases however the irregular coastline limits the extent of the advection of the nws warmer water along the australian coast in all cases warm anticyclonic eddies develop at the coast cold cyclonic eddies form from the limbs of the established warm anticyclonic eddies with the result that two counterrotating cells are developed once the eddy pairs begin their westward propagation the leeuwin current intensifies as nonlinear effects result in a jet between the eddies and the coast these effects translate downstream to augment the current velocities at the coast which due to a mixed instability mechanism result in the development of new anticyclonic eddies at the coast the results from case 4 which has the most realistic features highlights the major characteristics of the leeuwin current and agrees well with available observations these results show that the model successfully captures the qualitative nature of the nonlinear eddying response of the leeuwin current

the influence of mixing on primary productivity a unique application of classical critical depth theory mixing and primary productivity was examined in upwelling influenced nearshore waters off south western eyre peninsula swep in the eastern great australian bight egab the economically and ecologically important shelf region off southern australia that forms part of the southern and indian oceans mixing stratification in the region was highly temporally variable with a unique upwelling circulation in summer autumn november april and downwelling through winter spring may september highest productivity was associated with upwelled stratified water up to 2958 mg c m

evidence of a large seasonal coastal upwelling system along the southern shelf of australia 1 we report observational existence of a large seasonal coastal upwelling system that establishes in austral summer december april along australian southern shelves wind driven upwelling events occur simultaneously in three upwelling centres spanning a distance of 800 km during each summer period there are 2 3 major upwelling events each lasting 1 week the simultaneous rapid response of sst to wind forcing in the upwelling centres which display vastly different shelf widths points to the existence of a larger scale process that carries cold water onto the shelf prior to the upwelling season exploration of a major upwelling event in march 1998 shows the evolution of peak surface chlorophyll a concentrations of 4 g l lagging the onset of upwelling by 1 week the associated exponential growth rate can be estimated at 0 4 d 1 another week later we found a distinct sub surface chlorophyll a maximum at a depth of 50 m centred along the upwelling front reasons for the formation of this maximum are not fully understood

tasman leakage in a fine resolution ocean model tasman leakage the westward flow of thermocline waters south of australia from the pacific to the indian ocean is one of the lesser studied of the inter ocean exchanges here some of the properties of the tasman leakage are inferred from lagrangian particles integrated using the three dimensional velocity fields of the 1 10 degree resolution ofes model the mean tasman leakage in this model is 4 2 sv with a standard deviation of 4 3 sv the heat flux associated with this leakage lies in the range 0 08 0 18 pw there is large variability in the tasman leakage on both sub weekly and inter annual scales but no trend over the 1983 1997 period despite the large weekly variability with peaks of more than 20 sv it appears that less than half of the tasman leakage is carried within eddies

aspects of the mean wintertime circulation along australia s southern shelves numerical studies abstract a high resolution numerical model is used to study the mean wintertime shelf slope circulation between the gulfs of south australia and the west coast of tasmania the mean downwelling favorable winds for the region result in a continuous eastward coastal current cc extending from cape leeuwin to the eastern coast of tasmania the magnitude of the cc is generally largest over the shelf break has speeds of more than 40 cm s 1 transport 2 4 sv sv 106 m3 s 1 and is generally in good agreement with observations the relative importance of the leeuwin current zonal winds and density gradients to the shelf circulation is established within bass strait the eastward circulation is in good agreement with observations and simple models show that about 30 of the transport arises from local winds and the remainder from the cc the total transport through the strait 0 95 sv is shown to be controlled by sea level on the east coast of tasmania a current of up to 10 cm s 1 is also found to flow

stressing of the new madrid seismic zone by a lower crust detachment fault a new mechanical model for the cause of the new madrid seismic zone in the central united states is analyzed the model contains a subhorizontal detachment fault which is assumed to be near the domed top surface of locally thickened anomalous lower crust rift pillow regional horizontal compression induces slip on the fault and the slip creates a stress concentration in the upper crust above the rift pillow dome in the coseismic stage of the model earthquake cycle where the three largest magnitude 7 8 earthquakes in 1811 1812 are represented by a single model mainshock on a vertical northeast trending fault the model mainshock has a moment equivalent to a magnitude 8 event during the interseismic stage corresponding to the present time slip on the detachment fault exerts a right lateral shear stress on the locked vertical fault whose failure produces the model mainshock the sense of shear is generally consistent with the overall sense of slip of 1811 1812 and later earthquakes predicted rates of horizontal strain at the ground surface are about 10 7 year 1 and are comparable to some observed rates the model implies that rift pillow geometry is a significant influence on the maximum possible earthquake magnitude