a model study of ocean circulation beneath filchner ronne ice shelf antarctica implications for bottom water formation 1 an isopycnic coordinate ocean circulation model has been applied to the southern weddell sea including the cavity beneath filchner ronne ice shelf with the aim of investigating the buoyancy forced circulation on the continental shelf buoyancy forcing is associated with both the annual growth and decay of sea ice and the interaction between ice shelf and ocean in the model a generally anti cyclonic circulation develops beneath the ice shelf so that new shelf waters entering the cavity in the west emerge colder and fresher in the east the outflow contributes to a dense current that spills off the continental shelf and descends the slope oceanographic observations from the region are consistent with this picture and highlight the overflow as a major source of weddell sea bottom water

altimeter derived variability of surface velocities in the california current system 2 seasonal circulation and eddy statistics abstract satellite surface height and temperature fields are used to examine the seasonal surface circulation in the california current system in spring and summer an equatorward jet develops next to the coast with an initial latitudinal structure that responds to the latitudinal distribution of equatorward alongshore winds this jet moves offshore from spring to fall and develops meanders and closed eddies that contribute eddy kinetic energy and water mass properties to the nearby deep ocean higher eke appears first in spring near the coast and moves offshore with the seasonal jet along altimeter tracks parallel to the coastline 200 and 400 km offshore the greatest eke is found in summer and fall respectively with wavelengths of approximately 300 km these are the wavelengths of the largest meanders and the distance between the largest eddies visible in altimeter snapshots of the jet at 600 800 km offshore the wavelengths are generally smaller 100 150 km similar to eddy sizes reported by others for the quiescent northeast pacific some energy with longer wavelengths arrives at the track 600 km offshore in winter and 800 km offshore in spring which we interpret as the most offshore extension of the eke generated by the jet and eddy system closer to the coast in the previous spring and summer a sequence of snapshots from october 1992 to october 1993 shows details of the evolution of the jet and eddy system over one complete annual cycle

time development of scour around a cylinder in simulated tidal currents a laboratory flume experiment was performed to investigate the time development of scour around a vertical cylinder acting as a scaled model of an offshore wind turbine monopile in tidal currents the tidal current was simulated by resolving each half cycle into three time steps between which flow velocity and depth were varied flow direction was reversed between half cycles which were otherwise identical between them the three time steps exhibited clear water transitional and live bed conditions the experiment was run over two full simulated tidal cycles the scour hole formed tended to a symmetrical shape after two half cycles and was both shallower and slower developing than the scour hole in a unidirectional current test carried out in the same flume this was due mainly to the variable rates of scour caused by the variable flow conditions within each half cycle and to a lesser extent to the infilling of the scour hole when the current direction reversed the lower scour depth recorded in tidal conditions implies that the amount of scour protection required may be less than previous studies suggest

far reaching effects of the hawaiian islands on the pacific ocean atmosphere system using satellite data we detected a wind wake trailing westward behind the hawaiian islands for 3000 kilometers a length many times greater than observed anywhere else on earth this wind wake drives an eastward ocean current that draws warm water from the asian coast 8000 kilometers away leaving marked changes in surface and subsurface ocean temperature standing in the path of the steady trade winds hawaii triggers an air sea interaction that provides the feedback to sustain the influence of these small islands over a long stretch of the pacific ocean

numerical simulation of scour and backfilling processes around a circular pile in waves abstract this study continues the investigation of flow and scour around a vertical pile reported by roulund et al 2005 flow and scour backfilling around a vertical pile exposed to waves are investigated by using a three dimensional numerical model based on incompressible reynolds averaged navier stokes equations the model incorporates 1 k turbulence closure 2 vortex shedding processes 3 sediment transport both bed and suspended load as well as 4 bed morphology the numerical simulations are carried out for a selected set of test conditions of the laboratory experiments of sumer et al 1997 2013a and the numerical results are compared with those of the latter experiments the simulations are carried out for two kinds of beds rigid bed and sediment bed the rigid bed simulations indicate that the vortex shedding for waves around the pile occurs in a one cell fashion with a uniform shedding frequency over the height of the cylinder unlike the case for steady current where a two cell structure prevails the rigid bed simulations further show that the horseshoe vortex flow also undergoes substantial changes in waves the amplification of the bed shear stress around the pile including the areas under the horseshoe vortex and the lee wake region is obtained for various values of the keulegan carpenter number the principal parameter governing the flow around the pile in waves the present model incorporated with the morphology component is applied to several scenarios of scour and backfilling around a pile exposed to waves in the backfilling simulations the initial scour hole is generated either by a steady current or by waves the present simulations indicate that the scour and backfilling in waves are solely governed by the lee wake flow in agreement with observations the numerical model has proven successful in predicting the backfilling of scour holes exposed to waves the results of the numerical tests indicate that the equilibrium depth of scour holes is the same for both the scour and the backfilling for a given keulegan carpenter number in full agreement with observations

experimental study on local scour at complex bridge pier under combined waves and current abstract this paper presents an experimental study on scour at complex bridge piers exposed to combined waves and current regular waves were produced with various strength and superimposed on co directional current with constant velocity the key parameters tested and discussed include keulegan carpenter k c number relative current strength u c w pile cap elevation and pier skewness the scour hole patterns and scour mechanism were found to depend on the flow type combination and compared with steady current the superposition of waves may restrain the development of scour hole due to migrating bed forms three types of scour temporal evolution i e ascend descend asymptotic and transferred type were identified and the scouring processes structure exposure cyclic sediment motion etc at the scour initiation and development stages were also discussed the dimensional time scale decreased for higher bed mobility with a power functional relationship while the non dimensional time scale peaked when the maximum near bed velocity components contributed by current and waves are similar the dependence of the equilibrium scour depths both original and normalized on the tested parameters was also analysed it was found that the normalized scour depth at complex piers increases with greater k c and u c w and tends to be much larger than that for single piers the increase of bed mobility did not lead to scour peaks at the clear water threshold as what is usually observed for current only scour the influence of pile cap elevation on the equilibrium scour depth was found to be much weaker than that under steady current furthermore new scour prediction method is proposed to incorporate extra terms taking the influence of pile cap elevation and pier skewness into account the prediction accuracy is improved significantly finally more suggestions regarding design considerations are also given

live bed local pier scour experiments local clear water and live bed scour tests were performed for a range of water depths and flow velocities with two different uniform cohesionless sediment diameters 0 27 and 0 84 mm and a circular pile with a diameter of 0 15 m the tests were performed in a tilting flume 1 5 m wide 1 2 m deep and 45 m long located in the hydraulics laboratory at the university of auckland in auckland new zealand these tests extend local scour data obtained in controlled experiments to velocity ratio v vc values as high as 6 this is near the velocity where the peak live bed scour occurs for the sediment and flow conditions scour depth predictions are made with four different local scour equations for the conditions of the tests and the results compared with the measured values

observations of divergence and upwelling around point loma california 1 historical records of near surface water temperatures in the southern californian bight often show a preferential cooling in the lee of headlands such as point dume palos verdes and point loma at times this cooler water is associated with an increase in chlorophyll a as is evident in satellite images of ocean color from the region here we combine hydrographic data from a 1 day cruise aboard the rv roger revelle a precursor to the 0304 california cooperative oceanic fisheries investigations calcofi cruise with high frequency hf radar codar measurements satellite images and long term thermistor records of near surface temperature to identify a small scale isolated upwelling in the lee of point loma 32 5 n associated with the more saline water downstream of the headland are higher nutrient concentrations an increase in chlorophyll a concentration and a bloom of chain forming diatoms indicative of a mature upwelling system it is suggested that this upwelling is not primarily due to local or remote wind forcing but rather to the divergence of the prevailing southerly flow as it passes the point loma headland time series of surface vorticity calculated from hf radar measurements of sea surface velocity show that as the flow separates from the headland relative vorticity increases offshore of the cape inshore the time series of divergence convergence shows a tendency toward divergence at the surface indicating a preferential upwelling which appears to raise the thermocline thus resulting in a flux of cold nutrient rich water to the surface in the presence of high nutrients and light photosynthetic organisms bloom in these upwelled waters as they are advected away from the headland and offshore by the prevailing surface currents

ocean response to wind sheltering in the southern california bight 1 a particularly warm surface oceanic wake persisted during the period 6 16 june 1999 in the south eastern part of santa catalina island in the southern california bight from satellite images regional atmospheric model results and hydrographic measurements around catalina island a strong correlation appears between observed sea surface temperature wind stress and surface stratification it is suggested that the warm island wake was probably due to lower turbulent mixing rates associated with wind sheltering effect we argue that the sheltering effect may also account for a portion of the relatively warm sea surface temperature characteristic of the whole bight opposing previous arguments that temperature evolution of the surface ocean in this region is primarily driven by lateral advective fluxes

circulation and multiple scale variability in the southern california bight abstract the oceanic circulation in the southern california bight scb is influenced by the large scale california current offshore tropical remote forcing through the coastal wave guide alongshore and local atmospheric forcing the region is characterized by local complexity in the topography and coastline all these factors engender variability in the circulation on interannual seasonal and intraseasonal time scales this study applies the regional oceanic modeling system roms to the scb circulation and its multiple scale variability the model is configured in three levels of nested grids with the parent grid covering the whole us west coast the first child grid covers a large southern domain and the third grid zooms in on the scb region the three horizontal grid resolutions are 20 km 6 7 km and 1 km respectively the external forcings are momentum heat and freshwater flux at the surface and adaptive nudging to gyre scale soda reanalysis fields at the boundaries the momentum flux is from a three hourly reanalysis mesoscale mm5 wind with a 6 km resolution for the finest grid in the scb the oceanic model starts in an equilibrium state from a multiple year cyclical climatology run and then it is integrated from years 1996 through 2003 in this paper the 8 year simulation at the 1 km resolution is analyzed and assessed against extensive observational data high frequency hf radar data current meters acoustic doppler current profilers adcp data hydrographic measurements tide gauges drifters altimeters and radiometers the simulation shows that the domain scale surface circulation in the scb is characterized by the southern california cyclonic gyre comprised of the offshore equatorward california current system and the onshore poleward southern california countercurrent the simulation also exhibits three subdomain scale persistent i e standing cyclonic eddies related to the local topography and wind forcing the santa barbara channel eddy the central scb eddy and the catalina clemente eddy comparisons with observational data reveal that roms reproduces a realistic mean state of the scb oceanic circulation as well as its interannual mainly as a local manifestation of an enso event seasonal and intraseasonal eddy scale variations we find high correlations of the wind curl with both the alongshore pressure gradient apg and the eddy kinetic energy level in their variations on time scales of seasons and longer the geostrophic currents are much stronger than the wind driven ekman flows at the surface the model exhibits intrinsic eddy variability with strong topographically related heterogeneity westward propagating rossby waves and poleward propagating coastally trapped waves albeit with smaller amplitude than observed due to missing high frequency variations in the southern boundary conditions

satellite observations of small coastal ocean eddies in the southern california bight this study describes the characteristics of extensive small scale coastal ocean eddies in the southern california bight these surface features were primarily detected by using ers 1 and ers 2 synthetic aperture radar sar satellite imagery from 1992 to 1998 the eddies predominantly cyclonic in their rotation appeared to result from several forcing mechanisms they were mainly observed within the santa barbara channel and the santa monica san pedro basin regions and appeared to be seasonal in their distribution observed eddy diameters were all less than 50 km with over 70 less than 10 km the sar data were complemented by sea surface temperature measurements derived from advanced very high resolution radiometer satellite imagery as well as in situ data from moorings and drifters that provided substantial verification of the small scale eddies these findings are significant in that the eddies were in general smaller in size and more abundant than previously reported additionally these results provide further evidence of the complex near surface circulation within the southern california bight with important implications for nutrient flux productivity plankton patchiness larval transport and recruitment and dispersal of pollutants

the transition zone of the canary current upwelling region abstract like all the major upwelling regions the canary current is characterised by intense mesoscale structure in the transition zone between the cool nutrient rich waters of the coastal upwelling regime and the warmer oligotrophic waters of the open ocean the canary island archipelago which straddles the transition introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and trade winds the combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability on the basis of historical data and a series of interdisciplinary field studies the principal features of the region are described these include a prominent upwelling filament originating near 28 n off the african coast cyclonic and anti cyclonic eddies downstream of the archipelago and warm wake regions protected from the trade winds by the high volcanic peaks of the islands the filament is shown to be a recurrent feature apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone its role in the transport and exchange of biogenic material including fish larvae is considered strong cyclonic eddies observed throughout the year drift slowly southwestward from gran canaria one sampled in late summer was characterised by large vertical isopycnal displacements apparent surface divergence and strong upwelling producing a fourfold increase in chlorophyll concentrations over background values such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen the lee region of gran canaria is shown to be a location of strong pycnocline deformation resulting from ekman pumping on the wind shear boundaries which may contribute to the eddy formation process

sea surface signatures of the island mass effect phenomena around madeira island northeast atlantic abstract this is an introductory work that describes the manifestation of the island mass effect phenomena in the atmosphere and at the sea surface for a region of the ocean depleted from oceanographic work the madeira archipelago 33 n 17 w the use of remote sensing tools becomes essential in recognizing some of the sea surface features that characterize the island mass effect phenomena avhrr coastal zone color scanner czcs and seawifs ancillary national center for environmental prediction ncep data together with highly developed processing tools from panorama processing and automatic navigation of real time images allowed the detection of atmospheric von karman vortex streets warm water surface wakes leeward eddies azorean front and localized upwelling results from the remote sensing analysis also helped to promote the reanalysis of historical in situ data for the manifestation of these island effects ipimar cruises carried out between 1979 and 1982 collected temperature salinity density and chlorophyll data which were reinterpolated at the sea surface to study the island mass effect phenomena for the first time results have shown the formation of a warm water wake south of madeira island expanding 400 km offshore the surface signature of the azorean front the so called subtropical front seems to take place within the madeira latitude 33 n north waters were consistently colder than the warm and salty southern waters avhrr data also revealed eddies and fronts being formed in the region leeward eddies were often observed in the flanks of the islands czcs data showed highly productive eddies in the west of madeira island localized cold water with high chlorophyll concentrations was also observed around the island s coast a particularly dynamic area was the underwater ridge that connected madeira and desertas islands future work should continue to sample these regions of interest with the simultaneous use of different satellite sensors vertical characterization of the different phenomena is needed

processes and evolution of scour around a monopile induced by tidal currents abstract despite offshore wind farms being mostly situated in tidally dominated waters only limited research on the effects of tidal flow on the scour process around offshore foundation structures has been carried out so far to further improve the prediction of scour around those structures systematic laboratory tests were conducted in a closed circuit flume on the processes and time development of scour around a monopile in tidal flow the tidal currents were adapted by bidirectionally reversing currents with continuously changing flow velocity therewith multidirectional flow aspects were simplified into flow components in tidal main direction tidal cycles and flow intensities were scaled with regard to field measurements at the fino 1 platform in the north sea the model tests were conducted at a length scale of 1 40 and scour depths were measured at multiple positions around the monopile by a camera system novel insights on the intrinsic progression of sediment displacement and time scale of the scour process were gained and in the following presented as a function of flow intensity for clear water and live bed conditions in addition baseline tests with unidirectional currents were conducted in which the constant flow velocity was either based on the maximum peak or the root mean square velocity of the tidal currents significantly slower scour progression and smaller scour depths in tidal currents were found compared to unidirectional currents based on the maximum peak velocity in contrast scour depths in tidal currents exceed those for unidirectional currents by up to 51 if the unidirectional currents are based on the root mean square value the comparison further implies that a flow velocity of around 15 20 larger than the root mean square flow velocity of the tide is needed to accurately estimate tidal current induced scour depth by unidirectional current thereby the results underline the importance of selecting suitable conditions for the design process of offshore structures for design purposes the evolution of tidally induced scour could be well predicted with a time discretised stepping approach

scour around coastal structures a summary of recent research abstract this paper summarizes the results of the european union marine science and technology eu mast iii project scour around coastal structures scarcost the summary is presented under three headings 1 introduction 2 flow and scour processes with the subheadings flow and scour processes around vertical cylinders flow and scour processes at detached breakwaters flow and scour processes at submerged breakwaters and the effect of turbulence on sediment transport and 3 sediment behaviour close to the structure with the subheadings field measurement and analysis of wave induced pore pressures and effective stresses around a bottom seated cylinder non linear soil modelling with respect to wave induced pore pressures and gradients wave induced pressures on the bottom for non linear coastal waves including also wave kinematics development of a numerical model linear soil modelling to calculate wave induced pore pressures the effect of liquefaction on sediment transport penetration of blocks in non consolidated fine soil and cyclic stiffness of loose sand the paper also includes a discussion of the role of scale effects in laboratory testing and the applicability of the results obtained in supporting engineering design

scour around a monopile induced by directionally spread irregular waves in combination with oblique currents abstract the progressive expansion of offshore wind energy towards greater water depths demands for an optimization of foundation structure designs to a wider range of load conditions in offshore waters wind driven wave irregularity and directionality become important aspects of realistic sea states to further improve the scour prediction in marine environment a novel experimental study was conducted to investigate the influence of directionally spread 3d irregular waves on the scouring process the tests were carried out in the 3d wave basin of the ludwig franzius institute leibniz university hannover germany a monopile structure was simulated by a transparent pile made of acrylic glass the study was set up to progress understanding and explore dissimilarities of scour development and patterns induced by directionally spread 3d and unidirectional 2d waves the model tests were complemented by tests with superimposed oblique currents despite identical total wave energy in terms of m 0 between the directionally spread and unidirectional wave spectra minor but distinct differences in scour depth and rate could be observed which inevitably can only be attributed to the presence and role of the wave spread for wave only conditions final scour depths s d induced by directionally spread waves were on average 33 smaller than those for unidirectional waves furthermore final scour depths decreased with increasing wave spreading and displayed a growing dependency on kc numbers with increasing wave spreading in combined wave and current conditions of up to u c w

time scale for local scour at bridge piers the temporal development of clear water local scour depth at cylindrical bridge piers in uniform sand beds is considered new data are presented and used to quantify the influence of flow duration on the depth of local scour an equilibrium time scale t is defined the data show that both t and the equilibrium scour depth dse are subject to similar influences of flow and sediment parameters as might be expected because they are inherently interdependent a method is given for determination of the time for development of dse for a given pier sediment and approach flow velocity and the concomitant estimation of the scour depth at any stage during development of the equilibrium scour hole the results show that the scour depth after 10 of the time to equilibrium is between about 50 and 80 of the equilibrium scour depth depending on the approach flow velocity of local scour d se is rapidly attained in live bed conditions but rather more slowly in clear water conditions fig 1 clear water scour occurs for mean flow velocities up to the threshold velocity for bed sediment entrainment i e v vc while live bed scour occurs for v vc the maximum equilibrium scour depth dse max occurs at v vc in armored cobble or cohesive sediment bed streams multiple flood events may be required before the maximum clear water scour is reached this may take many years the equilibrium scour depth in live bed con ditions fluctuates due to the effects of bed form migration the dashed lines in fig 1 represent the temporal average scour depth under live bed conditions the diagram also shows the time taken te for the equilibrium scour depth to develop the equilibrium time te is the focus of this paper it increases rapidly with flow velocity under clear water conditions but then decreases rapidly for live bed scour existing equations for depth of local scour at bridge piers give the equilibrium depth and are therefore conservative re garding temporal effects for the live bed conditions that typ ically pertain in floods equilibrium scour depths are appro priate however where clear water scour conditions exist the equilibrium depth of scour may be overly conservative peak flood flows may last only a number of hours or a few days in the field and short floods have insufficient time to generate equilibrium depths for example bridge piers situated on the floodplain may be wet for periods of less than one day during a flood typically clear water conditions pertain at such sites the actual scour may be only a small fraction of the equilib rium scour depth which could take weeks to fully develop johnson and mccuen 1991 developed an analytical model to simulate the temporal process of local scour at piers the model was applied to a hypothetical bridge pier using a gen erated sequence of flood flows over a 75 year period the in dividual storms being of 24 h duration at the end of the 75 year period the scour depth was still increasing

equilibrium scour depths around piles in noncohesive sediments under currents and waves abstract a universal formula for the estimation of equilibrium scour depth around a single cylindrical pile under the action of steady currents tidal and short waves is presented following a dimensional analysis the controlling parameters of scour was found to be the keulegan carpenter number and the ratio between the flow velocity and the critical velocity for the initiation of motion of the sediment particles u u c based on this finding a new formula for the prediction of equilibrium scouring depths was derived in this paper measurements of the scouring process and the action of waves and or currents from literature are used with kc and u u c ranging between 6 and 10 5 and 0 6 and 4 5 respectively for the parameterization and verification of the newly proposed formula the predicted scour depths and the investigated measurements show clearly the influence of the bed properties which have not been reported before in the literature predictions using the new approach have been compared with previous measurements and other formulations presented in the specialized literature the proposed empirical formula accounts for the effect of waves currents and sediment characteristics on the scouring process and shows better results than previous ones

numerical simulation of wave induced scour and backfilling processes beneath submarine pipelines abstract a fully coupled hydrodynamic morphodynamic numerical model is presented and utilized for the simulation of wave induced scour and backfilling processes beneath submarine pipelines the model is based on solutions to reynolds averaged navier stokes equations coupled with k turbulence closure with additional bed and suspended load descriptions forming the basis for sea bed morphology the morphological evolution is updated continuously rather than being based e g on period or other time averaging techniques simulations involving wave induced scour over the range of keulegan carpenter number 5 6 kc 30 demonstrate reasonable match with previous experiments both in terms of the equilibrium scour depth as well as the scour time scale wave induced backfilling processes are additionally studied by subjecting initial conditions taken from scour simulations with larger kc to new wave climates characterized by lower kc values the simulations considered demonstrate the ability of the model to predict backfilling toward expected equilibrium scour depths based on the new wave climate in line with experimental expectations the simulated backfilling process is characterized by two stages 1 an initial re distribution phase involving re organization of sediments in the immediate vicinity of the pipeline potentially followed by 2 a more lengthy backfilling evolution toward equilibrium scour depth the simulated backfilling time scales are of the same order of magnitude as in experiments though the multi stage process complicates a more systematic characterization the simulated sequences of scour and backfilling achieved within the present work are estimated to represent temporal durations of up to approximately 12 h at full practical scales

scour development around a jacket structure in combined waves and current conditions compared to monopile foundations abstract this paper presents the results of an experimental study on the scour development of a hydraulic transparent offshore foundation exposed to combined waves and current irregular waves propagating perpendicular to a current were simulated in a wave current basin the physical model tests were conducted in a length scale of 1 30 while measurements of the scour development over time were achieved by echo sounding devices placed at several locations at the upstream and downstream side of the jacket structure insights were gained on the scour development and time scale of the scouring process around a complex jacket structure for different wave current conditions the results were presented with respect to the keulegan carpenter kc number and the relative wave current velocity wave conditions were adjusted so that kc numbers between 6 7 and 23 4 could be tested in a systematic wave current test program with tests reaching from wave dominated conditions up to current dominated conditions measured scour depths were critically assessed by an extrapolation to expected equilibrium scour depths with respect to the current flow direction the experiments showed generally larger scour depths at the upstream side and lower scour depths on the downstream side for each pile of the jacket structure the development of global scour around the structure intensified with increasing relative wave current velocity as a result a practical formulation is proposed for the reliable prediction of local scour depths around a jacket foundation in combined wave current conditions finally dimensionless time scales and observed as well as predicted scour depths are compared to values for the scour development around monopiles

the geometry of aleutian subduction three dimensional seismic imaging we constrain the subduction geometry of the aleutian slab by the distribution of seismicity recorded at local seismic networks and by a teleseismic p wave residual sphere analysis of 11 earthquakes and one nuclear explosion the source depth and mechanism of each earthquake are determined by modeling teleseismically recorded long period p and sh waveforms we construct theoretical residual spheres by tracing rays through three dimensional thermal models of the subducting slab assuming a pressure independent thermal coefficient of velocity vp t rays are initiated at the estimated source depths and at epicenters whose relative locations are consistent with those obtained from the observed travel times after correction for slab effects for each event we determine the values of slab strike and dip that provide the best match to the observations average values of 67 confidence intervals determined by an f test are 5 in dip and 10 in strike strike varies along the arc by 60 for all but one of the modeled events the estimated orientation of the slab s aseismic extension agrees to within two standard deviations of that derived from seismicity distributions assuming a constant dip a slab penetration depth of 600 km fits the central aleutian events best this is at least 300 km below the deepest earthquakes slabs terminating above 400 km or below 1000 km provide significantly worse fits though there is a trade off between vp t and penetration depth the best fitting value of vp t using a 600 km deep slab is 0 5 0 1 ms 1 k 1 this is consistent with both ultrasonic laboratory measurements and values obtained for deeper earthquakes in several other subduction zones vp t estimated from shallow thrust events is 20 50 larger than that for intermediate focus events suggesting that a simple thermal model with constant vp t under predicts the velocity anomalies shallower than 100 km depth the fact that a 600 km deep slab provides better fits than a 1000 km deep slab implies that the downdip p rays exit the slab near a depth 600 km a slab bend to either a shallower or steeper dip would satisfy the p times hand picked pcp travel times are most consistent with the slab bending to a 20 steeper dip before plunging into the lower mantle in the western aleutians the relative plate motion becomes transform seismicity ceases at less than 100 km depth and volcanism is no longer active we image a slab like high seismic velocity zone apparently associated with the along arc lateral transport of cold subducted lithosphere from the east an earthquake whose location is known from local observations is analyzed in detail the velocity structure inferred using this location is nearly identical to that obtained using our scheme of annihilating location information from both theoretical and observed times a severe bias would result however from the commonly used procedure of analyzing travel times computed with respect to an incorrect event location such as that of the isc