flow hydrography turbulent mixing and dissipation at fieberling guyot examined with a primitive equation model 1 the extensive set of measurements previously taken at fieberling guyot provides a backdrop for model results on flow hydrography turbulent mixing and derivative variables a z level primitive equation model driven at subinertial and superinertial tidal frequencies allows the nonlinear interaction of free and topographically trapped motions to jointly determine model outcomes model residual flows mean hydrographic features topographic amplification of oscillatory currents turbulent dissipation and vertical diffusivity kv profiles are all examined in the light of the observations azimuthal residual flows are toroidal above the summit with maximum speed 20 cm s 1 diurnal flow over the summit is intensified by a factor 20 downward vertical flow as a time mean w t occurs from 150 m above the summit apex to summit rim depth with radially inward flow above the apex and radial outward flow below upward w t rings the summit at larger r the existence of the observed cold dome is shown to depend on w t in the summit circulation cell vertical current shear is concentrated in a 150 m thick cap above the summit accounting for a domed distribution of and kv with maxima of 10 7 w kg 1 and 30 cm2 s 1 respectively profiles most like the observations of kv are obtained with a subgrid scale parameterization in which the turbulent prandtl number pr depends on the shear richardson number ri

topographically induced circulation patterns and mixing over condor seamount abstract analysis of mean and oscillatory circulation patterns over condor seamount situated near the central group of the azores islands was performed during 1 5 years of observations at least half of the time an anticyclonic cap was established over the summit the vortex was characterised by a strong asymmetry it was shifted to the south of the summit and strongly stretched along the gentle eastern and western slopes of the seamount three mechanisms are suggested to generate the anticyclonic cap impinging mean flow tidal rectification and vertical turbulent density fluxes our estimates suggest that tidal rectification alone can explain the formation of the observed anticyclonic cap but the character of temporal variation of its intensity gives evidence that impinging flow also plays an important role in the cap dynamics conditions for stability of the anticyclonic cap are found to be principally different depending on the impinging flow being unidirectional or dominated by synoptic oscillations for a unidirectional impinging flow in close accordance with the theory anticyclonic cap was readily formed over the top of the seamount the cap was stable except for the events of the cap shedding when the impinging flow accelerated above a critical level when the impinging flow varied its direction dominated by topographic rossby waves the anticyclonic cap was absent or its intensity got significantly reduced vertical turbulent density flux was of secondary importance for formation of the anticyclonic cap over the summit

observations of low frequency circulation and amplified subinertial tidal currents at cobb seamount we present acoustic doppler current profiler adcp survey results resolving three dimensional current structure in thermocline stratification atop cobb seamount 130 8 w 46 8 n mean flow includes clockwise circulation near the bottom and an east northeastward background current clockwise flow is not strong enough to isolate fluid from the background current as in a taylor cap unless this occurs in the deepest 50 m outside adcp coverage diurnal currents subinertial 0 69 are amplified within a few kilometers radially and about 100 m vertically of the seamount by up to 5 3 times ambient k1 tidal currents their diagnostics include clockwise propagation around the seamount with first azimuthal wavenumber clockwise rotation in time narrow current ellipses oriented nearly along bathymetric contours with positively correlated radial and azimuthal components counterclockwise turning with depth and downward propagation they are compared to the stratified seamount trapped wave of first azimuthal wavenumber and third highest subinertial resonant frequency 0 70 calculated numerically using measured bathymetric and stratification profiles inviscid free wave currents exhibit symmetric patterns not consistent with the measurements in a forced damped wave maintained by ambient tidal currents against parameterized friction of 2 day timescale as appropriate to dissipation timescales estimated from microstructure measurements symmetries are broken such that each observed current diagnostic is matched when the wave is linearized about a steady baroclinic clockwise current representing the measured mean flow its frequency and structure are very weakly modified

shear wave splitting and anisotropy from the aftershocks of the nahanni northwest territories earthquakes two large earthquakes ms 6 6 6 9 occurred in 1985 near the north nahanni river in the northwest territories canada the first on october 5 and the second on december 23 the earthquakes were followed by long aftershock sequences aftershocks between m 1 0 and 3 9 were recorded by five to eight three component stations during three field experiments in october 1985 and in january and september 1986 two distinct shear waves were generally observed in september 1986 the plane of polarization of the first arriving shear wave was well defined and averaged 156 8 measured clockwise from north the stress direction was inferred from the initial shear wave polarizations recorded at one station from a widespread aftershock distribution but was limited to no more than 10 km around it the delay between first and second arriving shear waves was up to 190 ms assuming that the two shear waves result from anisotropy and that the entire ray paths varying in length from 7 to 23 km contributed the maximum velocity anisotropy is 7 however the distribution of the anisotropy is not uniform the pressure axes determined from fault plane solutions for the two main shocks are nearly e w normal to the compressive direction found in the september 1986 field survey from shear wave splitting data four out of seven of the aftershocks for which focal mechanism orientations could be determined showed pressure axis orientations that agreed with the stress direction inferred from the initial shear wave polarizations from the field survey

azimuthal anomalies of short period p wave arrivals from nahanni aftershocks northwest territories canada and effects of surface topography abstract following two large earthquakes in the northwest territories of canada in 1985 and one in 1988 four field surveys were undertaken to study the extensive aftershock activity some of the seismographs employed recorded three component digital data which allowed the directions of approach of short period p wave arrivals to be analyzed at three stations observed azimuthal deviations reached as much as 40 from theoretically expected azimuths as computed from the inferred hypocenters for the events theoretical results are presented that support the hypothesis that the observed azimuthal deviations are caused principally by local topographic slopes in the vicinity of the recording instruments similar theoretical results indicate that effects of local surface topography on arrival azimuths of sh and sv waves are nearly an order of magnitude less than for p waves so that such effects will generally be unimportant in shear wave splitting studies

tides in the sea of okhotsk eight major tidal constituents in the sea of okhotsk have been investigated using a numerical solution of tidal equations on a 59 space grid the tides are dominated by the diurnal constituents diurnal tidal currents are enhanced in shelikhov bay and penzhinskaya guba at kashevarov bank in proximity to the kuril islands and at a few smaller locations the major energy sink for diurnal tides over 60 of the total energy is shelikhov bay and penzhinskaya guba the major portion of semidiurnal tide energy is dissipated in the northwestern region of the sea of okhotsk and in shelikhov bay and penzhinskaya guba nonlinear interactions of diurnal currents are investigated through k1 and o1 constituent behavior over kashevarov bank these interactions generate residual circulation of the order of 10 cm s 21 major oscillations at semidiurnal and fortnightly periods 13 66 days and higher harmonics of basic tidal periods the m2 tidal current caused by the nonlinear interaction of the diurnal constituents over kashevarov bank constitutes approximately a half of the total m2 tide current there the fortnightly current through nonlinear interactions also influences basic diurnal tidal currents by inducing fortnightly variations in the amplitude of these currents

the environment of hurricane debby 1982 part i winds abstract a three dimensional nested analysis of wind fields in the environment of hurricane debby 1982 has been completed the basic analysis tool uses a two dimensional least squares fitting algorithm combined with a derivative constraint that acts as a spatial low pass filter on the analyzed field gridded results of horizontally analyzed fields are combined into vertical cross sections and then analyzed to produce vertical continuity consequently a three dimensional analysis is obtained the database for the analysis comes primarily from omega dropwindsondes odws rawinsondes and satellite derived winds above 400 mb in the environment of hurricane debby near 0000 utc 16 september 1982 since these data come from many different sources and thus are not evenly distributed in the horizontal or vertical techniques have been developed to alleviate difficulties associated with inhomogeneous data the analyzed wind fields provide an independent evaluation of satellite derived winds at and below 400

observed deep anticyclonic cap over caiwei guyot the mean circulation pattern and its mechanism over caiwei guyot 1308 5600 m in the northwest pacific were studied utilizing three years of in situ data a deep anticyclonic cap was found to enclose the entire guyot from its bottom up to a depth of 728 m which is comprised of a stable but highly asymmetric anticyclonic circulation at the foot and a bottom trapped anticyclonic circulation over the summit on the slope the circulation is complex with a dominant anticyclonic circulation near the bottom and a weak cyclonic circulation at 2200 m the anticyclonic cap intensity over the summit is significantly modulated by the time varying impinging flow an intensified cold ring above the summit edge was observed at caiwei guyot which differs from the cold domes observed over traditional conic seamounts further analysis suggests that the impinging flow is primarily responsible for the cap formation and the m2 tide seamount interaction plays a secondary role the anticyclonic cap may play a role in the local geological distribution

surface wind fields of 1995 hurricanes erin opal luis marilyn and roxanne at landfall hurricanes erin opal luis marilyn and roxanne were the most destructive hurricanes of 1995 at landfall luis and marilyn contained maximum sustained winds marine exposure estimated at near 60 and 46 m s21 respectively the strongest landfalling storm of the 1995 season luis decreased in intensity from a category 4 to 3 on the saffir simpson scale shortly before the eyewall crossed the islands of antigua barbuda st kittsnevis st barthelemy st martin and anguilla hurricane marilyn strengthened as it approached the u s virgin islands with st thomas bearing the brunt of the north and south eyewall winds of 46 m s21 marine exposure and st croix being affected by the relatively weak western eyewall peak winds of 35 40 m s21 marine exposure for luis and marilyn only surface winds with marine exposures were analyzed because of unknown small scale interactions associated with complex island terrain with 500 1000 m elevations wind engineering studies suggest that wind acceleration over blunt ridges can increase or speed up winds by 20 80 topographic effects were evident in damage debris analyses and suggest that an operational method of assessing terrain induced wind gusts such as a scaled down mesoscale model is needed after landfall as a marginal hurricane over central florida hurricane erin regained strength over the gulf of mexico with a well defined radar reflectivity structure erin struck the florida panhandle near navarre beach with maximum sustained surface winds of 35 40 m s21 affecting the destin ft walton area hurricane opal made landfall in nearly the identical area as erin with maximum sustained surface winds of 40 45 m s21 having weakened from an intensity of nearly 60 m s21 only 10 h earlier opal was characterized by an asymmetric structure that was likely related to cold front interaction and an associated midlevel southwesterly jet roxanne struck cozumel mexico with sustained surface winds marine exposure of 46 s21 crossed the yucatan and meandered in the southwest gulf of mexico for several days while in the bay of campeche roxanne s large area of hurricane force winds disabled a vessel which lead to the drowning deaths of five oil industry workers high resolution wind records are critical to preserving an accurate extreme wind climatology required for assessment of realistic building code risks unfortunately power interruptions to automated surface observing stations on the u s virgin islands st croix st thomas and destin florida prevented complete wind records of the eyewall passages of marilyn and opal respectively

tidally induced turbulent mixing at irving seamount modeling and measurements 1 the relationship of tidal currents and turbulence over irving seamount is examined by model and measurement surface tides are partially scattered into internal tides and particularly over the seamount summit current strength and shear greatly increase current shear causes the model s turbulent mixing at irving modeled turbulent mixing as represented by vertical diffusivity is largest o 10 3 m2 s 1 in the bottom boundary layer and in a turbulent column rising to the sea surface directly above the seamount pockets of less intense mixing occur within the turbulent column but their locations are highly variable field measurements confirm a turbulent mixing column atop irving of comparable intensity increased model diffusivity average of 7 10 5 m2 s 1 also occurs to a distance of several hundred kilometers from the seamount flanks in a band vertically centered at 800 m depth

a three dimensional p wave velocity model for the charlevoix seismic zone quebec canada 1 a three dimensional p wave velocity model has been developed for the charlevoix seismic zone csz the csz is located along the st lawrence river 100 km northeast of quebec city canada and is one of the most active seismic zones in eastern north america five earthquakes with magnitudes equal to or exceeding 6 0 have occurred in the csz in historic time and around 200 earthquakes occur annually hypocenters are located in precambrian basement rocks basement rocks have been affected by numerous tectonic events including grenvillian collision iapetan rifting and meteor impact we performed a sequential tomographic inversion for p wave velocity structure based upon 3093 p wave arrivals from 489 earthquakes recorded by 12 stations high velocity is associated with the center of the impact crater the region of high velocity is surrounded by low velocities interpreted to be highly disrupted rocks an elongated high velocity region is present at midcrustal depths that trends parallel to the st lawrence river earthquakes avoid the high velocity body and separate into two bands one on either side of the feature larger earthquakes magnitude 4 have occurred along the northern edges of the high velocity region

25 day period large scale oscillations in the argentine basin revealed by the topex poseidon altimeter the measurement of the global sea surface height made by the topex poseidon satellite has provided the first synoptic view of large scale oceanic variability at intraseasonal scales from weeks to months areas of significant intraseasonal variability were found primarily in the tropics and the high latitude oceans the southern ocean in particular the focus of the paper is the finding of large scale oscillations at a period of 25 days in the argentine basin of the south atlantic ocean these oscillations exhibit a dipole pattern of counterclockwise rotational propagation centered at 458s 3178e over the zapiola rise the scale of the dipole is about 1000 km the peak to trough amplitude is on the order of 10 cm the amplitude of these oscillations has large seasonalto interannual variations these oscillations are shown to be associated with a free barotropic mode of the basin as a solution to a linearized barotropic vorticity equation closed f h contours provide a mechanism for the confinement of the waves to the topographic feature of the zapiola rise results from a numerical model simulation reproduced the patterns of the observed oscillations the resultant mass transport variability is on the order of 50 sv sv 106 m3 s21 deep current meters in the argentine basin reveal signals consistent with the altimetry observations

diurnal tides over kashevarov bank okhotsk sea diurnal tidal waves k1 and o1 were studied using a primitive equation ocean circulation model with fine vertical resolution the study focused on kashevarov bank a shallow submarine bank at the northern continental slope of the sea of okhotsk owing to resonance the currents over the bank are 20 30 times stronger than the off bank currents the large current amplitudes of the basic tidal waves k1 and o1 through nonlinear interactions generate residual currents and new oscillations in the semidiurnal fortnightly and other bands the model predicts a strong clockwise residual circulation with barotropic and baroclinic components the latter is about 50 of the total residual flow over the bank top to elucidate the role of various terms in maintenance of the residual motion the time averaged momentum and heat balance equations were analyzed the time averaged flow over the bank top is ageostrophic and the horizontal advection is dominant in the tidal rectification to demonstrate the influence of the tides on the thermohaline structure over kashevarov bank horizontally uniform and vertically stratified initial temperature and salinity distributions were considered strong tidal mixing results in an almost homogeneous thermohaline structure and well developed internal tides over the bank summit this fact demonstrates the role of tides in establishing the observed homogeneous stratification over kashevarov bank

shear wave splitting and anisotropy in the charlevoix seismic zone quebec in 1985 during the month of october 1985 a second experiment was undertaken in the charlevoix seismic zone to further test the hypothesis that shear wave splitting could be observed in a seismically active region the first experiment had been undertaken in 1984 and yielded only a very limited amount of data seismograms recorded by digital three component seismographs located very close to the epicentres of seven earthquakes showed shear wave splitting over 15 different paths the amount of wave variation varied from about 24 to 160 ms or from 0 4 to 8 7 of the wave velocity the largest value occurred over the shortest path of about 7 km for which essentially the whole path may be anisotropic leading to a crack density e of less than 0 09 for the rest of the data all with less than 3 shear wave velocity variation e varies from 0 005 to 0 03 if whole path anisotropy is assumed these values of e are not significantly different from those obtained in 1984 the average azimuth of the initial shear wave p

tidally driven vorticity diurnal shear and turbulence atop fieberling seamount abstract fine and microstructure profiles collected over fieberling seamount at 32 26 n in the eastern north pacific reveal a variety of intensified baroclinic motions driven by astronomical diurnal tides the forced response consists of three phenomena coexisting in a layer 200 m thick above the summit plain i an anticyclonic vortex cap of core relative vorticity 0 5f ii diurnal fluctuations of 15 cm s 1 amplitude and 200 m vertical wavelength and iii turbulence levels corresponding to an eddy diffusivity e 10 10 4 m2 s 1 the vortex cannot be explained by taylor proudman dynamics because of its 0 3fn2 negative potential vorticity anomaly the 0 3f fortnightly cycle in the vortex s strength suggests that it is at least partially maintained against dissipative erosion by tidal rectification the diurnal motions are slightly subinertial turning clockwise in time and counterclockwise with depth over the summit plain they also exhibit a fortnightly cycle in their amplitude pointing to

meiofauna assemblages of the condor seamount north east atlantic ocean and adjacent deep sea sediments seamounts are currently considered hotspots of biodiversity and biomass for macro and megabenthic taxa but knowledge of meiofauna is still limited studies have revealed the existence of highly diverse meiofauna assemblages however most data are mainly qualitative or focused only on specific groups thus preventing comparisons among seamounts and with other deep sea areas this study conducted on condor seamount azores north east atlantic ocean describes variation in abundance biomass community structure and biodiversity of benthic meiofauna from five sites located on the condor seamount and one site away from the seamount while the summit of the seamount hosted the highest alpha biodiversity the flanks and the bases showed a rich meiofauna assemblage in terms of abundance and biomass the observed marked differences in grain size composition of sediments reflected the oceanographic conditions impacting different sectors of the condor seamount and could play an important role in the spatial distribution of different meiofaunal taxa trophic conditions biochemical composition of organic matter explained 78 of the variability in the meiofauna biomass pattern while sediment grain influenced the vertical distribution of meiofauna and only partially explained meiofaunal taxa composition this study provides a further advancement in the knowledge of meiofaunal communities of seamounts only a deeper understanding of the whole benthic communities including meiofauna will allow to elaborate effective management and conservation tools for seamount ecosystems

numerical simulation of flow around a tall isolated seamount part ii resonant generation of trapped waves a sigma coordinate primitive equation ocean circulation model is used to explore the problem of the remnant generation of trapped waves about a tall circular isolated seamount by an incident oscillatory barotropic current the numerical solutions are used to extend prior studies into the fully nonlinear regime and in particular to quantify and interpret the occurrence of residual circulation specific attention is also devoted to the dependence of the resonance and rectification mechanisms on stratification forcing frequency and choice of subgrid scale viscous closure resonantly generated trapped waves of significant amplitude are found to occur broadly in parameter space a precise match between the frequency of the imposed incident current and the frequency of the trapped free wave is not necessary to produce substantial excitation of the trapped wave the maximum amplification factors produced in these numerical solutions o 100 times the strength of the incident current are consistent with previous studies in the presence of nonlinear advection strong residual currents are produced the time mean circulation about the seamount is dominated by a strong bottom intensified anticyclonic circulation closely trapped to the seamount maximum local time mean current amplitudes are found to be as large as 37 of the magnitude of the propagating waves in addition to the strong anticyclonic residual flow there is a weaker secondary circulation in the vertical radial plane characterized by downwelling over the top of the seamount at all depths maximum vertical downwelling rates of several tens of meters per day occur at the summit of the seamount the vertical mass flux implied by this systematic downwelling is balanced by a slow radial flux of mass directed outward along the flanks of the seamount time mean budgets for the radial and azimuthal components of momentum show that horizontal eddy fluxes of momentum are responsible for transporting net radial and azimuthal momentum from the far field to the upper flanks of the seamount there coriolis and pressure gradient forces provide the dominant balances in the radial direction however the coriolis force and viscous effects provide the primary balance for the azimuthal component

azimuthal variations in p wave travel times and shear wave splitting in the charlevoix seismic zone abstract a significant drop of up to 1 in the seismic p travel times recorded from controlled explosions occurred in the charlevoix seismic zone csz between 1979 and 1980 the travel time data exhibited variations not only with distance but also with azimuth these variations are interpreted in terms of extensive dilatancy anisotropy eda in 1984 and 1985 shear wave splitting was observed in the csz on micro earthquake seismograms under the assumption of eda the observations are interpreted in terms of a dominant crack orientation of about 36 and a crack density of around 0 05 to 0 02 the observations from the p and s waves are not incompatible with one another

observations of enhanced diapycnal mixing near the hawaiian ridge profiles of potential density obtained from ctd casts at two stations at different distances from the hawaiian ridge are examined for evidence of diapycnal turbulent mixing as indicated by density inversions and internalwave vertical strain results from independent casts are used to produce ensemble averaged vertical distributions for the number of inversions and the thorpe scale both parameters were found to be higher over the slope of the topography at 2500 m depth than in the deep ocean 110 km to the north thorpe scale based estimates of the rate of dissipation of turbulent kinetic energy and turbulent vertical diffusivity are elevated by an order of magnitude over the slope relative to deep ocean background levels the vertical distributions of these mixing parameters are nonuniform and exhibit signs of locally enhanced dissipation possibly due to internal tides generated at the ridge at the deep station turbulence is at background levels from the surface down to 2000 m below this a localized zone of enhanced mixing is observed within which the dissipation rate is o 1029 w kg21 and turbulent diffusivity is greater than o 1024 m2 s 21 perhaps due to an internal tide ray originating at the ridge the full depth topographical enhancement of mixing near the ridge also appears in the vertical strain field estimates of dissipation rate and turbulent diffusivity based on an internal wave wave interaction model give results similar to direct thorpe scale methods except in weakly stratified environments where both methods are subject to uncertainty near the topography the variation in mixing intensity observed between casts is sensitive to sporadic large mixing events which are triggered by internal waves associated with the spring tide the upper portion of the water column stronger stratification is more responsive to the tide than the deep regions

tidal and lower frequency currents above fieberling guyot an intensive yearlong deployment of current meter moorings was made over and around fieberling guyot 32 5 n 127 75 w in the north pacific the measurements are used to investigate tidal currents the mean flow and subinertial current fluctuations the k1 and o1 diurnal tides were greatly amplified over the seamount apparently through dynamics resembling nonlinear seamount trapped waves the diurnal tides interacted nonlinearly to induce constituents at the sum frequency m2 and difference frequency fortnightly above the summit diurnal amplification was strongest at about 450 m and it extended from the seamount center out over the flanks the s2 semidiurnal tide was not substantially amplified over the seamount the mean flow was primarily along isobaths and was strongest about 50 m above the seamount rim it was driven by tidal rectification so that mean along isobath velocity at a given location correlated well with the amplitude of the local fortnightly tide at that location accompanying the mean flow was a cold dome over the seamount that was presumably maintained by a balance of mean downward advection and eddy tidal radial heat transport lower frequency current fluctuations were enhanced near the seamount and were apparently driven by fluctuations in the ambient flow

seamounts in a restless ocean response of passive tracers to sub tidal flow variability 1 the action of sub tidal oceanic flow impinging on an isolated seamount and its consequences for biological tracer patterns was assessed using a three dimensional ocean circulation model passive tracers released above an isolated gaussian seamount were exposed to variations of flow magnitude frequency direction and seamount height the evolving patterns indicate significant qualitative differences steady and amplitude modulated unidirectional inflows generate advective downstream particle loss and patchiness non uniform inflow reduces advective particle loss and tracers are systematically redistributed and aggregated around the seamount summit tracer retention is mainly controlled by flow magnitude and summit depth variations of flow period were of lesser importance two important biological implications arise from this study first particle dynamics at isolated seamounts is strongly constrained by mesoscale variability of the sub tidal far field flow secondly the influence of seamounts on particle distribution is not restricted to local and small scale phenomena but can penetrate deep into the oceanic far field on a scale of tens to hundreds of kilometers

observations of amplified flows atop a large seamount current meter records from a mooring atop a large seamount in the eastern north pacific fieberling guyot are dominated by diurnal fluctuations at depths close to the seamount summit depth these oscillations are driven by diurnal tides to have amplitudes of 0 2 0 4 m s or more and exhibit characteristics of seamount trapped waves i e coastal trapped waves for a submerged periodic coast these waves rectify to produce anticyclonically directed low frequency currents in the vicinity of the seamount summit of magnitude comparable to the rms wave amplitude mean currents over several months are directed across isobaths at depths more than 200 m shallower than the seamount summit currents closer to the bottom have a component directed toward deeper water and are ageostrophic as a result of wave rectification internal wave band currents are more energetic with increasing proximity to the local bottom presumably as a result of wave reflection and focussing

a numerical simulation of flow at fieberling guyot a numerical sigma coordinate ocean circulation model is used to investigate tidally forced flow near fieberling guyot the aim is to reproduce the observed currents and to identify the dominant physical mechanisms that lead to the complex three dimensional flow fields at this tall and steep seamount in the northeast pacific our very high resolution simulation with 500 m horizontal and less than 20 m vertical grid spacing in the vicinity of the seamount summit was performed with the newest version of the terrain following sigma coordinate primitive equation model spem as in previous more idealized studies the seamount was placed in the center of a uniformly rotating periodic channel and forced by diurnal barotropic tides the exponential initial stratification the tidal forcing amplitude and its orientation were chosen based on observations from the fieberling area three major characteristics of the flow observed at fieberling guyot are reproduced both qualitatively and quantitatively diurnal currents reach about 20 cm s 1 a twentyfold amplification relative to the tidal flow away from the seamount the time mean anticyclonic flow speeds are close to the observed 10 cm s 1 maximum azimuthal velocity and the spatial structure of this vortex shows a maximum at about 6 km radius between 400 and 600 m depth clearly lifted off the bottom the time mean flows are found to be maintained by diurnal waves of a mixed type the net motion shows characteristics of both bottom intensified seamount trapped waves and vertically propagating vortex trapped waves while the former are mainly responsible for setting up a time mean anticyclonic flow along the upper flanks at the bottom the latter are needed to redistribute the momentum and time mean density thereby reproducing the observed structure with a flow maximum off the bottom an analysis of the energy momentum and density fluxes shows that rectification depends critically on the eddy fluxes that balance the time mean downwelling over the seamount summit and upper flanks the results of sensitivity and parameter studies are utilized to further interpret the role of individual physical mechanisms and the time mean dynamical balances

twelve years of geophysical research in the charlevoix seismic zone abstract various and extensive geophysical monitoring began in 1974 in the charlevoix seismic zone csz the historical record and a major earthquake in 1925 characterize this zone as the likely site of a future damaging earthquake i e m 5 5 or larger and the aim of the monitoring was to develop the capability of predicting such earthquakes the parameters studied included microseismicity seismic travel times electrical impedance vertical movement horizontal movement tilt gravity change and strain through water well level changes although no clear precursor was detected in the months before the largest seismic event that occurred during the observation period m 5 0 the multiparameter experiment provided new insights into the structure and the mechanics of this active region observations of several hundred well located microearthquakes have permitted the delineation of ancient rift faults that are now being activated by high horizontal compression the geographical extent of the zone is sharply defined by the microseismicity which occurs in an area of 35 km 80 km straddling the st lawrence river observation of p waves from accurately timed explosions and s waves from earthquakes support the hypothesis of extensive dilatancy anisotropy eda involving saturated parallel cracks in the crust the data have permitted an estimate of both crack density and the direction for the principal compressive stress in the region n 30 e changes in electrical parameters with periods of months to years were detected at five of the six magnetotelluric stations installed in the region changes in the polarization angle of the earth s electrical telluric field correlated well for seven years with calculated groundwater accumulation levels at one site electric impedance changes showed no clear correlation from station to station but changes at one station appear to correlate with a seismic p wave travel time change anomaly and with the disappearance of a negative gravity anomaly at the time of an unusual number of earthquakes in the range m 2 to 3 from mid 1976 to mid 1977 it is suggested that these correlated changes could be related to aseismic stress redistribution and fluid flow along one of the ne striking rift faults about two years before the m 5 0 earthquake in 1979 continuous monitoring of tilt surface and borehole and well water levels was carried out at a single site charlevoix geophysical observatory although there were unexplained near surface tilts of the order of the earth tide amplitude within two or three days before some earthquakes they were either not spatially coherent or were related to small unexplained temperature changes in the tiltmeter vault no intermediate term one week to one year precursors were seen against a noise level of 1 3 rad rms for near surface tilt observations and a noise level of 0 2 0 3 rad rms for borehole tilt measurements at a depth of 50 100 m the threshold of detectable tidal tilt admittance changes for the borehole tiltmeters was lowered to 2 but no significant changes were seen four coseismic changes in the levels of water in boreholes were observed all were drops in water level which were much larger than expected on the basis of elastic dislocation theory it is suggested that these could be explained either by the collapse of a dilatant zone around the respective hypocentres or by triggered aseisnuc movement on a nearby active fault borehole water level tides observed from 1980 to 1986 exhibit significant variations in amplitude and phase which are thought to be the result of time varying elastic properties in a nearby structure some constraints can now be placed on the long term deformation of the csz as a whole from tiltmeter levelling and horizontal surveys data from a levelling array observed from 1976 to 1986 puts a limit of 0 54 rad yr on the secular tilt rate at the observatory a further limit of 0 1 rad yr is provided by borehole tilt measurements over the period 1983 1986 a limit of 2 cm is placed on vertical movements northwest of the st lawrence river by levelling surveys in 1965 1978 1980 and 1982 no significant 90 confidence deviatoric strain has been detected across the csz from horizontal control surveys carried out in 1912 1919 1965 1978 and 1983 the maximum allowable long term strain rate across the csz from 1919 to 1983 is 0 16 strain yr

organic matter composition and macrofaunal diversity in sediments of the condor seamount azores ne atlantic in recent years increasing knowledge has been accumulated on seamounts ecology however their sedimentary environments and associated biological communities remain largely understudied in this study we investigated quantity and biochemical composition of organic matter and macrofaunal diversity in sediments of the condor seamount ne atlantic azores in order to test the effect of the seamount on organic matter distribution sediment samples were collected in 6 areas the summit the northern and southern flanks and bases and in an external far field site macrofauna abundance and diversity were investigated on the summit the southern flank and in the far field site the organic matter distribution reflected the complex hydrodynamic conditions occurring on the condor concentrations of organic matter compounds were generally lower on the whole seamount than in the far field site and on the seamount summit compared to flanks and bases a clear difference was also evident between the northern and southern slopes of the condor suggesting a role of the seamount in conditioning sedimentation processes and distribution of food resources for benthic consumers macrofauna assemblages changed significantly among the three sampling sites high abundance and dominance accompanied by low biodiversity characterized the macrofauna community on the condor summit while low dominance and high biodiversity were observed at the flank our results although limited to five samples on the seamount and two off the seamount do not necessarily support the paradigm that seamounts are more biodiverse than the surrounding seafloor however the abundance and biomass functional diversity and taxonomical distinctiveness of the macrofaunal assemblages from the condor seamount suggest that seamounts habitats may play a relevant role in adding to the regional biodiversity

the mont laurier quebec earthquake of 19 october 1990 and its seismotectonic environment on 19 october 1990 at 07 01 ut a magnitude mblg 5 0 earthquake occurred approximately 9 km southwest of mont laurier quebec canada latitude 46 47 n and longitude 75 59 w this earthquake is the largest to have occurred in the western quebec seismic zone since the 1944 magnitude 5 6 cornwall massena earthquake the mont laurier earthquake was widely felt up to distances of 500 km and caused some minor damage in the epicentral region intensity modified mercalli mm v during the week following the earthquake a field survey recorded 23 aftershocks magnitudes were all less than ml 1 5 analysis of the mainshock and its aftershocks indicates focal depths in the range 10 to 12 km of the two east west oriented nodal planes given by the predominantly thrust mechanism of the mainshock the steep northerly dipping one probably represents the fault plane as indicated by the trend of aftershock hypocenters the last 10 yr of recording have confirmed that most western quebec earthquakes including the mont laurier earthquake occur in a northwest southeast trending zone inside the central metasedimentary cm belt with most focal depths varying between 7 and 25 km although some northwest trending structural features are known correlating these with the epicentral trend is premature the mid crustal hypocentral depths of many earthquakes the east west trend of the fault plane of the mont laurier earthquake and variations in regional focal mechanisms all suggest reactivation of deep structural features which may not have a surface expression inside the central metasedimentary belt most earthquakes occur along the eastern side where the activity seems to correlate with the labelle shear zone and the morin anorthosite body which may act as a stress concentrator the western side of the activity does not end with the central metasedimentary belt a fact that implies that even though the factors controlling seismicity lie predominantly within the central metasedimentary belt the adjacent geologic domains are also seismogenic

upper ocean response to hurricane gilbert the evolving upper ocean response excited by the passage of hurricane gilbert september 14 19 1988 was investigated using current and temperature observations acquired from the deployment of 79 airborne expendable current profilers axcps and 51 airborne expendable bathythermographs from the national oceanic and atmospheric administration wp 3d aircraft in the western gulf of mexico the sea surface temperatures ssts mixed layer depths and bulk richardson numbers were objectively analyzed to examine the spatial variability of the upper ocean response to gilbert net decreases of the ssts of 3o 4oc were observed by the profilers as well as by the airborne infrared thermometer airt along the flight tracks and advanced very high resolution radiometer avhrr imagery the axcps indicated a marked cooling from 29oc to about 25 5oc on september 17 1988 which was about 1 2 inertial periods ip following storm passage this pool of cooler water 3 5 o was located further downstream in the hurricane wake by september 19 2 7 ip following the storm as a result of the near inertial currents in the mixed layer while there was a bias of about 0 6oc and 1 7oc between the in situ and avhrr derived ssts respectively both the avhrr images and the objectively analyzed fields indicated a rightward bias in the upper ocean cooling that extended from the storm track to about 4rmax where rmax the radius of maximum winds is equal to 50 km the larger sst offset of 1 7oc was due to the difference between the time of the avhrr image and the time of the aircraft experiment on september 19 the ssts derived from the avhrr images and the airt also indicated large gradients between the cold wake and the warm eddy in the central gulf of mexico the mixed layer deepened by about 30 35 m on the right side of the track during the storm and 1 2 ip later with little evidence of continued deepening afterward the mixed layer current vectors demonstrate that a strong near inertially rotating current was excited by the passage of gilbert with maxima of about 1 1 4 m s 1 the currents observed during and subsequent to 1 2 ip the storm diverged from the storm track whereas the mixed layer current vectors 2 7 ip after storm passage converged toward the track with relative maxima of 0 8 1 m s 1 this alternating pattern of convergence and divergence of the mixed layer current was associated with the upwelling and downwelling cycles of the baroclinic response considerable current shear existed between the mixed layer and the thermocline currents in the cool wake between the storm track and the 4rmax estimates of the bulk richardson numbers ranged between 0 2 and 1 0 during gilbert and at 1 2 ip which suggests that enhanced current shears were responsible for some of the mixed layer deepening