on the deep western boundary current south of cape cod abstract using ctd oxygen data from eight cruises in the decade from 1994 2003 we have constructed a mean section of properties across the deep western boundary current dwbc south of cape cod near 70 w since all sections included direct velocity measurements our composite section enables us to portray the flow field as well as the mean water mass structure inshore of the gulf stream between the 2500 and 4000 m isobaths the flow is to the southwest along the bathymetry and is remarkably barotropic the equatorward flowing labrador sea water is shown to have high dissolved oxygen low salinity and low potential vorticity while the underlying overflow water is high in oxygen transport estimates for the cold limb of the thermohaline circulation give a range of 14 to 19 sv for the n atlantic deep water found on the section the greatest uncertainty is due to the presence of a warm core ring on one of the sections which apparently completely reversed the flow in the dwbc offshore of the dwbc some of the deep source waters are returned to the north in the deep gulf stream the section is compared to two other widely separated locations abaco and 55 w that have markedly different dwbc characteristics

interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

observed evolution of linear and nonlinear effects at the dahan downhole array taiwan analysis of the september 21 1999 m 7 3 chi chi earthquake sequence in this paper the site characteristics of the dahan downhole array are studied by analyzing the september 21 1999 m 7 3 chi chi earthquake sequence including the main shock and some aftershocks the four level array 0 m 50 m 100 m and 200 m is located to the north of hualien city in eastern taiwan polarization analysis is used to check the orientation errors of the seismometers at different levels of depth if the surface instrument is chosen as reference the angle between the major polarization axes of the surface and any downhole records is the orientation error that must be corrected for the downhole accelerographs the orientation errors at depths of 50 m 100 m and 200 m are 32 120 and 84 after the corrections the coherency between the surface and downhole records is substantially improved spectral ratio analysis shows that the predominant frequency of the chi chi main shock shifts to a lower frequency we also simulate ground motions at different depths by using the haskell method with a linear velocity structure model the record at surface is chosen as the input motion compared with the observed data ground acceleration can be well reproduced for the aftershocks weak motion events of the september 21 1999 m 7 3 chi chi earthquake however for the chi chi main shock the synthetic waveform cannot match well with the observation neither in amplitude nor in phase this indicates that large ground shaking probably induced the nonlinear site effect at that time and the model used cannot support it

on the average absolute transport of the deep western boundary currents east of abaco island the bahamas abstract data obtained from a line of absolute current profiler pegasus and ctd stations at 26 5 n east of abaco island in the northern bahamas are used to estimate the average transport and overall level of variability of the deep below 800 m southward flow associated with the so called deep western boundary current dwbc at this latitude from april 1985 to september 1987 a total of 11 sections were made along a line extending from near the island boundary to 85 km offshore as part of the subtropical atlantic climate studies stacs program in all but two cases when pegasus data were obtained over the total depth absolute velocities were determined from 0 to about 3000 db while ctd derived relative velocities were determined over the total depth using the combined datasets it is possible to reference the relative geostrophic velocity with pegasus data thereby obtaining an estimate of the absolute southward transport of the deep flow the average value of transport determined in this man

sh propagator matrix and qs estimates from borehole and surface recorded earthquake data summary a new method is proposed for the solving of elastic and anelastic parameters of a medium between two seismometers one in a borehole and one at the surface this allows us to retrieve equivalent quality factors of the materials and at the same time gives wave velocities as well as reflection and transmission coefficients in a layered structure the study of local site effects shows that the quality factor of shallow materials can be completely masked by resonance effects and near surface amplification our method is based on the sh propagator matrix in the time domain this separates the effects of resonance and amplification in the elastic structure from those of attenuation due either to scattering or physical attenuation and we find that the elastic problem is decoupled from the anelastic one after a synthetic test we applied the method to seismic records made with a borehole surface instrument operated in the southern rhine graben valley on the french german border due to high seismic noise at the surface we were only able to resolve a one layered mean 500 m deep structure between the borehole and surface instrument an apparent qs of 40 is found between 1 and 5 hz a frequency band where classical methods failed this is in close agreement with qs at frequencies larger than 15 hz obtained from the classical method based on the slope of spectral ratios

ambient noise and weak motion excitation of sediment resonances results from the tiber valley italy abstract we recorded ambient noise and weak earthquake ground motion on holocene sediments in the tiber valey north of rome in order to help predict the response of these sediments to strong ground motion ambient noise within the valley is high and sharply peaked near 2 5 hz similar peaks are observed along the tiber valley towards the south in and south of rome a small local earthquake simultaneously recorded within the valley and at a reference hard rock site is dominated by energy at 4 to 20 hz at the reference site but appears to excite low frequency resonances at the sediment site we also recorded noise at a small array of four stations within the valley and apply the spatial auto correlation method of aki 1957 1965 to obtain rayleigh wave dispersion between 1 and 5 hz the observed resonance and dispersion can be used to determine velocity structure in the near surface sediments between 1 and 5 hz we obtain phase velocities of approximately 400 to 500 m sec implying a shear wave velocity of 440 to 530 m sec the holocene sediment layer is constrained from geotechnical data to have a thickness of 40 m our velocity results thus predict a quarter wavelength resonance of 2 8 to 3 3 hz close to that observed in the ambient noise and weak motion data

biological and physical controls on the flux and characteristics of sinking particles on the northwest atlantic margin biogenic matter characteristics and radiocarbon contents of organic carbon oc were examined on sinking particle samples intercepted at three nominal depths of 1000 m 2000 m and 3000 m 50 m above the seafloor during a 3 year sediment trap program on the new england slope in the northwest atlantic we have sought to characterize the sources of sinking particles in the context of vertical export of biogenic particles from the overlying water column and lateral supply of resuspended sediment particles from adjacent margin sediments high aluminum al abundances and low oc radiocarbon contents indicated contributions from resuspended sediment which was greatest at 3000 m but also significant at shallower depths the benthic source i e laterally supplied resuspended sediment of opal appears negligible based on the absence of a correlation with al fluxes in comparison caco3 fluxes at 3000 m showed a positive correlation with al fluxes benthic sources accounted for 42 63 of the sinking particle flux based on radiocarbon mass balance and the relationship between al flux and caco3 flux episodic pulses of al at 3000 m were significantly correlated with the near bottom current at a nearby hydrographic mooring site implying the importance of current variability in lateral particle transport however al fluxes at 1000 m and 2000 m were coherent but differed from those at 3000 m implying more than one mode of lateral supply of particles in the water column

nonlinear ground motion amplification by sediments during the 1994 northridge earthquake it has been known since at least 1898 ref 1 that sediments can amplify earthquake ground motion relative to bedrock for the weak ground motion accompanying small earthquakes the amplification due to sediments is well understood in terms of linear elasticity hooke s law 2 but there has been a long standing debate regarding the amplification associated with the strong ground motion produced by large earthquakes the view of geotechnical engineers based largely on laboratory studies is that hooke s law breaks down at larger strains causing a reduced nonlinear amplification seismologists on the other hand have tended to remain sceptical of this nonlinear effect mainly because the relatively few strong motion observations seemed to be consistent with linear elasticity although some recent earthquake studies have demonstrated nonlinear behaviour under certain circumstances3 4 the significance of nonlinearity for the type of stiff soil sites found in the greater los angeles region remains unresolved5 here we report that ground motion amplification due to sediments for the main shock of the 1994 northridge earthquake was up to a factor of two less than the amplification observed for its aftershocks these observations imply significant nonlinearity in such amplification and bring into question the use of measurements of weak ground motion to predict the strong ground motion at sedimentary sites

circulation and deep water export at the western exit of the subpolar north atlantic the current system east of the grand banks was intensely observed by world ocean circulation experiment woce array acm 6 during 1993 95 with eight moorings reaching about 500 km out from the shelf edge and covering the water column from about 400 m depth to the bottom more recently a reduced array by the institut fur meerskunde ifm at kiel germany of four moorings was deployed during 1999 2001 focusing on the deep water flow near the western continental slope both sets of moored time series each about 22 months long are combined here for a mean current boundary section and both arrays are analyzed for the variability of currents and transports a mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper layer extrapolation and isopycnal subdivision of the mean transports into deep water classes the offshore part of the combined section is dominated by the deep reaching north atlantic current nac with currents still at 10 cm s 1 near the bottom and a total northward transport of about 140 sv sv 106 m3 s 1 with the details depending on the method of surface extrapolation used the mean flow along the western boundary was southward with the section mean north atlantic deep water outflow determined to be 12 sv below the 27 74 kg m 3 isopycnal however east of the deep western boundary current dwbc the deep nac carries a transport of 51 sv northward below 27 74 kg m 3 resulting in a large net northward flow in the western part of the basin from watermass signatures it is concluded that the deep nac is not a direct recirculation of dwbc water masses transport time series for the dwbc variability are derived for both arrays the variance is concentrated in the period range from 2 weeks to 2 months but there are also variations at interannual and longer periods with much of the dwbc variability being related to fluctuations and meandering of the nac a significant annual cycle is not recognizable in the combined current and transport time series of both arrays the moored array results are compared with other evidence on deep outflow and recirculation including recent models of different types and complexity

midlatitude equatorial dynamics of a grounded deep western boundary current part i midlatitude flow and the transition to the equatorial region abstracta comprehensive theoretical study of the nonlinear hemispheric scale midlatitude and cross equatorial steady state dynamics of a grounded deep western boundary current is given the domain considered is an idealized differentially rotating meridionally aligned basin with zonally varying parabolic bottom topography so that the model ocean shallows on both the western and eastern sides of the basin away from the equator the flow is governed by nonlinear planetary geostrophic dynamics on sloping topography in which the potential vorticity equation can be explicitly solved as the flow enters the equatorial region it speeds up and becomes increasingly nonlinear and passes through two distinguished inertial layers referred to as the intermediate and inner inertial equatorial boundary layers respectively the flow in the intermediate equatorial region is shown to accelerate and turn eastward forming a narrow equatorial jet the qualitative properties of the solution presented are consistent wi

large scale vertical and horizontal circulation in the north atlantic ocean abstract observations of large scale hydrography air sea forcing and regional circulation from numerous studies are combined by inverse methods to determine the basin scale circulation average diapycnal mixing and adjustments to air sea forcing of the north atlantic ocean dense overflows through the denmark strait and faroe bank channels are explicitly included and are associated with strong vertical and lateral circulation and mixing these processes in the far northern atlantic play a fundamental role in the meridional overturning circulation for the entire ocean accompanied by an upper cell of mode water and intermediate water circulation the two cells converge roughly at the mean depth of the midocean ridge crest the labrador sea water layer lies within this convergence south of the overflow region model derived mean diapycnal diffusivities are o 10 5 m2 s 1 or smaller at the base of the thermocline and diapycnal advection is driven primarily by air sea transformation on outcropping layers

how does the deep western boundary current cross the gulf stream abstract the manner in which the deep western boundary current dwbc crosses the gulf stream is investigated using data from a hydrographic survey conducted in 1990 absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level three density layers are considered in detail two mid depth layers which together make up the shallowest water mass component of the dwbc 500 1200 m and a deep layer consisting of the norwegian greenland overflow water 2500 3500 m the shallowest layer does not make it through the crossover and is completely entrained by the gulf stream however the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover in the intermediate layer which is denser than the gulf stream coming off the shelf part of the dwbc recirculates to the northeast while the onshoremost portion continues equatorward in the deep layer only a small amount of recirculation occurs the lateral

inversion of borehole weak motion records observed in istanbul turkey summary the estimation of shear wave velocity and attenuation in near surface geology is of primary importance in engineering seismology in fact such knowledge is essential for site response studies when preparing improved seismic hazard scenarios in this study we propose a linear inversion of the spectra of a deconvolved wavefield collected by a borehole array in istanbul turkey the spectra are calculated using as a reference the recordings collected by a sensor at the surface this allows us to minimize the effect of the deconvolution filter on the peaks amplitudes the feasibility of the proposed inversion scheme and the parametrization of the velocity and attenuation models were tested and assessed using synthetic data the real data inversion is carried out using observation from three weak motion events several starting models are used for each event changing the values of the quality factor qs whereas the starting vs profiles are fixed to what was obtained by the analysis of the deconvolved wavefield in the time domain our results showed that fairly well constrained vs models can be obtained while the variability of the qs profile both between different inversions of the same data set and between the inversions of different data sets might be large this indicates that qs variations observed in models derived by strong motion data inversions should be considered with care however the high agreement of the estimated soil profiles in the uppermost layer both when using different events and different starting models suggests that any variation in the s wave velocity that might be estimated when using strong motion data should be reliable

site amplification and attenuation of strong ground motion abstract at the mcgee creek california site three component strong motion accelerometers are located at depths of 166 m 35 m and 0 m the surface material is glacial moraine to a depth of 30 5 m overlying hornfels accelerations were recorded from two california earthquakes round valley m l 5 8 23 november 1984 18 08 utc and chalfant valley m l 6 4 21 july 1986 14 42 utc anti plane shear strains for the round valley and chalfant valley events are less than 1 8 10 5 and 1 2 10 5 respectively by separating out the sh components of acceleration we were able to determine the orientations of the downhole instruments peak sh accelerations at the surface were 5 7 times greater than those at 166 m in both earthquakes a constant phase velocity haskell thomson model was applied to generate synthetic sh seismograms at the surface using the accelerations recorded at 166 m in the frequency band 0 10 to 10 0 hz we compared the filtered synthetic records to the filtered surface data the onset of the sh pulse and the reflections from the interface at 30 5 m are clearly seen in both the synthetics and the recorded data the synthetic records closely match the data in both amplitude and phase the fit between the synthetic accelerograms and the data indicates that the seismic amplification at the surface is a result of the resonance of the surface layers and the contrast of the impedances shear stiffness of the near surface materials the impedance contrast of the materials does not account for all of the amplification of the peak accelerations at the surface the resonances of the layer system below 10 hz contribute the most to this amplification we find that a linear model predicts the soil response for frequencies up to 10 hz material damping is added to the soil layers of the model in the form of a quality factor q the damping moderates the strong resonance effects by allowing energy to radiate into the half space the quality factor q 10 is constant in this frequency range but does not accurately model the attenuation at frequencies above 10 hz

site amplification and attenuation via downhole array seismogram inversion a comparative study of the 2003 miyagi oki aftershock sequence weak motion geotechnical array recordings at 38 stations of the japanese strong motion network kik net from the 2003 m w 7 0 miyagi oki aftershock sequence are used here to quantify the amplification and attenuation effects of near surface formations to incident seismic motion initially a seismic waveform optimization algorithm is implemented for the evaluation of high resolution low strain velocity v s attenuation q s and density profiles at the sites of interest based on the inversion results v s versus q s correlations are developed and scattering versus intrinsic attenuation effects are accounted for in their physical interpretation surface to downhole traditional spectral ratios ssr cross spectral ratios c ssr and horizontal to vertical h v site response estimates are next evaluated and compared while their effectiveness is assessed as a function of the site conditions classified on the basis of the weighted average vs of the upper 30 m v s30 of the formations single and reference station site response estimates are successively compared to surface to rock outcrop amplification spectra and are evaluated by deconvolution of the downhole records based on the inversion results comparison of the observed ssr and estimated surface to rock outcrop amplification spectra illustrates the effects of destructive interference of downgoing waves at the downhole instrument level as a function of the site class site amplification factors are successively computed in reference to the national earthquake hazards reduction program nehrp b c boundary site conditions v s30 760 m sec and results are compared to published values developed on the basis of strong motion data and site response analyses finally weak motion ssr estimates are compared to the mainshock spectra and conclusions are drawn for the implications of soil nonlinearity in the near surface results presented in this article suggest that currently employed site classification criteria need to be reevaluated to ensure intraclass consistency in the assessment of amplification potentials and nonlinearity susceptibility of near surficial soil formations

variability in the deep western boundary current local versus remote forcing 1 horizontal velocity temperature and salinity measurements from the line w array for the period 2004 2008 show large changes in the water mass structure and circulation of the deep western boundary current dwbc fluctuations in the flow with periods from 10 to 60 days are bottom intensified signals most likely associated with topographic rossby waves trw a fraction 15 of the dwbc transport variability is caused by gulf stream rings and meanders these flow anomalies are surface intensified and fluctuate at frequencies lower than the trw interannual variability in the velocity field appears to be related to changes in the hydrographic properties the dominant mode of variability is characterized by an overall freshening cooling a potential vorticity pv increase in the deep labrador sea water dlsw and a pv decrease in the overflow water ow the variability in the flow associated with these property changes is not spatially homogeneous offshore water depths larger than 3500 m changes in the velocity are in phase with pv changes in the ow a decrease in the ow pv is accompanied by an increase in the southward negative transport conversely variations of the inshore flow are in phase with changes in the dlsw pv increasing pv and decreasing transport this trend true for most of the record reverses after the winter of 2007 2008 a sudden decrease of the dlsw pv is observed with a corresponding intensification of the flow in the inner dwbc as well as a northward shift in the gulf stream axis

the atakoy vertical array turkey insights into seismic wave propagation in the shallow most crustal layers by waveform deconvolution summary a vertical array of accelerometers was installed in atak oy western istanbul with the longterm aim of improving our understanding of in situ soil behaviour to assess the modelling and parametric uncertainties associated with the employed methodologies for strong motion site response analysis and for shallow geological investigations geotechnical and geophysical investigations were carried out to define the subsoil structure at the selected site data associated with 10 earthquakes 2 7 m 4 3 collected during the first months of operation of the array were used to image the upgoing and downgoing waves by deconvolution of waveforms recorded at different depths results have shown that the velocity of propagation of the imaged upgoing and downgoing waves in the borehole is consistent with that of s or p waves depending on the component of ground acceleration analysed but independent of the chosen signal window in particular an excellent agreement was found between the observed upgoing and downgoing wave traveltimes and the ones calculated using a model derived by seismic noise analysis of array data the presence of a smaller pulse on the waveforms obtained by deconvolution of the horizontal components suggests both internal s wave reflection and s to p mode conversion as well as a not normal incidence of the wavefield the presence of a pulse propagating with s wave velocity in the uppermost 25 m in the waveforms obtained by the deconvolution of the vertical components suggests p to s mode conversion these evidences imply that even when site amplification is mainly related to 1 d effects the standard practice in engineering seismology of deconvolving the surface recording down to the bedrock using an approximate s wave transfer function generally valid for vertical incidence of sh waves might lead to errors in the estimation of the input ground motion required in engineering calculations finally downgoing waves with significant amplitudes were found down to 70 m and even to 140 m depth this result provides a warning about the use of shallow borehole recordings as input for the numerical simulation of ground motion and for the derivation of ground motion prediction relationships

downhole receiver function a case study receiver function is defined as the spectral ratio of the radial component and the vertical component of the ground motion it is used to characterize converted waves we extend the use of the receiver function to downhole data using waves recorded in a borehole excited by an earthquake of magnitude 4 0 near san fran cisco california on 26 june 1994 the focal depth of the event was 6 6 km and the epicenter was located at a distance of 12 6 km from the borehole array six three component sensors were located at different depths in a borehole to extract a co herent response of the near surface from the incoherent earthquake waves we de convolve the waves recorded by the sensors at different depths with the waves recorded by the sensor on the surface deconvolution applied to the waves in the s time window recorded by the radial component result in an upgoing and adowngoing wave propagating with s wave velocity for the waves in the p time window re corded by the radial component deconvolution also gives an upgoing and a down going wave propagating with s wave velocity this interesting result suggests a p to s conversion at a depth below the deepest sensor to diagnose this we compute the receiverfunction forthe borehole recording of the earthquakewaves thereceiver function shows an upgoing wave with an arrival close to time t 0 for the deepest sensor the agreement of the upgoing wave in the receiver function with the travel time curve for the p to s converted wave calculated using the p and the s wave velocity profile supports the hypothesis of a pronounced p to s conversion we present a synthetic example to illustrate that the first arrival of the receiver function applied to borehole data gives the upward propagating p to s converted wave to corroborate the observation of the mode conversion we apply receiver function to a different earthquake data recorded by the same borehole array in 1998 the focal depth of the event was 6 9 km and the epicenter was located at a distance of 13 km from the borehole array the receiver function for these data also show an upgoing wave with a pulse close to time t 0 at the deepest sensor the moveout of the upgoing wave agrees with the travel time curve for the p to s converted wave hence supporting our observation of the mode conversion

on the path of the gulf stream and the atlantic meridional overturning circulation abstract the atlantic meridional overturning circulation amoc simulated in various ocean only and coupled atmosphere ocean numerical models often varies in time because of either forced or internal variability the path of the gulf stream gs is one diagnostic variable that seems to be sensitive to the amplitude of the amoc yet previous modeling studies show a diametrically opposed relationship between the two variables in this note this issue is revisited bringing together ocean observations and comparisons with the gfdl climate model version 2 1 cm2 1 both of which suggest a more southerly northerly gs path when the amoc is relatively strong weak also shown are some examples of possible diagnostics to compare various models and observations on the relationship between shifts in gs path and changes in amoc strength in future studies

propagation pathways of classical labrador sea water from its source region to 26 n 1 more than two decades of hydrography on the abaco line east of the bahamas at 26 n reveals decadal variability in the salinity of classical labrador sea water clsw despite the long distance from its source region in the north atlantic ocean hydrographic time series from the labrador sea and from the abaco line show a pronounced step like decrease in salinity between 1985 and 1995 in the labrador sea and between 1995 and 2010 at the abaco line suggesting a time lag between the two locations of approximately 9 years the amplitude of the anomaly at the abaco line is 50 of the amplitude in the labrador sea a similar time lag and reduction of amplitude is found in the high resolution ofes model in which salinity anomalies can be observed propagating through the deep western boundary current as well as through a broad interior pathway on its way south to the abaco line the clsw becomes 8 standard deviations saltier due to isopycnal mixing with mediterranean outflow water mow climatological data in the north atlantic suggests that the mixing ratio of mow to clsw at the abaco line is 1 4 and that no variability in mow is required to explain the observed variability at the abaco line the data studied here suggest that decadal clsw anomalies stay relatively coherent while getting advected despite the important role of interior pathways

tracking labrador sea water property signals along the deep western boundary current observations of the deep western boundary current dwbc at line w on the western north atlantic continental slope southeast of cape cod from 1995 to 2014 reveal water mass changes that are consistent with changes in source water properties upstream in the labrador sea this is most evident in the cold dense and deep class of labrador sea water dlsw that was created and progressively replenished and deepened by recurring winter convection during the severe winters of 1987 1994 the arrival of this record cold fresh and low potential vorticity anomaly at line w lags its formation in the labrador sea by 3 7 years complementary observations along the path of the dwbc provide further evidence that this anomaly is advected along the boundary and indicate that stirring between the boundary and the interior intensifies south of the flemish cap finally the consistency of the data with realistic advective and mixing time scales is assessed using the waugh and hall 2005 model framework the data are found to be best represented by a mean transit time of 5 years from the labrador sea to line w with a leading order role for both advection by the dwbc and mixing between the boundary flow and interior waters

transport of the north atlantic deep western boundary current about 39 n 70 w 2004 2008 abstract begun in spring 2004 a sustained measurement program line w is returning high resolution observations of the north atlantic s deep western boundary current dwbc southeast of new england the study focuses on the cold limb of the atlantic meridional overturning circulation near the boundary between the subpolar and subtropical gyres the field study consists of a 6 element continental slope spanning moored array on a line underlying an altimeter satellite ground track and periodic reoccupations of a full depth hydrographic section along the line extending from the continental shelf towards bermuda here data from the first 4 years of the array may 10 2004 april 9 2008 are analyzed along with 9 realizations of the section the array a mix of moored profiler and discrete fixed depth instrument moorings returned temperature salinity and horizontal velocity data with various temporal and depth resolutions after averaging to filter inertial tidal and other high frequency motions the combined moored data set was binned to the lowest common temporal resolution of 5 days the nominal burst sample interval of the moored profilers and interpolated to 2 dbar vertical resolution temperature salinity dissolved oxygen tracer chemical concentrations and direct velocity data were acquired on the hydrographic cruises the present work focuses on the 4 year mean and time varying meridional transport in 4 layers bounded by neutral density surfaces upper and classical labrador sea waters iceland scotland overflow waters and denmark strait overflow waters the 5 d 4 layer summed meridional transport estimates range between 3 5 and 79 9 sv with a record mean average transport of 25 1 sv and standard deviation of 12 5 sv bias adjustment to account for the finite width of the moored array increases the 4 layer mean transport estimate to 28 7 sv at time scales longer than about 1 month the variations in equatorward dwbc transport appear correlated with meridional position of the gulf stream north wall with stronger transport observed when the stream is displaced south

rupture processes of the 1999 august 17 izmit and november 12 duzce turkey earthquakes summary we derive the rupture history of the 1999 august 17 izmit mw 7 4 and 1999 november 12 duzce mw 7 1 earthquakes in turkey from teleseismic body waves using broad band data of the global seismograph network aftershock locations and mapped surface breaks the centroid solutions indicate strike slip mechanisms for both events the izmit earthquake was characterized by rupture propagating predominantly eastwards it consisted of a main rupture lasting about 25 s followed within 1 min by two more events of mw 6 9 and mw 7 0 with the teleseismic data we could not resolve the westward extent of rupture into the marmara sea however an upper bound of the seismic moment release west of the epicentre of the izmit event is estimated to be 1 9 1019 n m the duzce earthquake lasted about 14 s and was characterized by a bilateral mode of rupture in excellent agreement with mapped surface breaks and aftershock locations

seismic wave propagation in shallow layers at the gonaf tuzla site istanbul turkey using the first dataset available from the downhole geophysical observatory of the north anatolian fault we investigated near surface seismic wave propagation on the tuzla peninsula istanbul turkey we selected a dataset of 26 seismograms recorded at tuzla at sensor depths of 0 71 144 215 and 288 m to determine near surface velocities and attenuation structures the waveforms from all sensors were pairwise deconvolved and stacked this produced low noise empirical green s functions for each borehole depth interval from the green s functions we identified reflections from the free surface and a low velocity layer between 90 and 140 m depth the presence of a low velocity zone was also confirmed by a sonic log run in the borehole this structure plus high near surface p and s wave velocities of 3600 4100 and 1800 m s lead to complex interference effects between upgoing and downgoing waves as a result the determination of quality factors q with standard spectral ratio techniques was not possible instead we forward modeled the green s functions in the time domain to determine effective q values and to refine our velocity estimates the effective q p values for the depth intervals of 0 71 0 144 0 215 and 0 288 m were found to be 19 35 39 and 42 respectively for the s waves we obtained an effective q s of 20 in the depth interval of 0 288 m considering the assumptions made in our modeling approach it is evident that these effective quality factors are biased by impedance contrasts between our observation points our results show that even after correcting for a free surface factor of 2 the motion at the surface was found to be 1 7 times greater than that at 71 m depth our efforts also illustrate some of the difficulties of dealing with site effects in a strongly heterogeneous subsurface online material plots of resistivity and caliper logs and the spectra of all 26 events

spectral analysis of k net and kik net data in japan part ii on attenuation characteristics source spectra and site response of borehole and surface stations abstract in this study we apply a nonparametric spectral inversion scheme to a data set of accelerograms recorded by the k net and kik net networks in japan in order to derive attenuation characteristics source spectra and site response for this purpose we use a total of more than 67 000 s wave records from 2178 earthquakes m jma 2 7 8 obtained at 1555 stations at the earth s surface and more than 29 000 records from 1826 events recorded at 637 borehole stations at depths of 100 to 3000 m attenuation characteristics are investigated in five separate regions showing that crustal q depicts lower values in central compared to southern japan and a significant frequency dependence is observed in every region the source spectra follow the 2 model with higher stress drops for subcrustal earthquakes as compared with crustal ones while strong amplification effects dominate the site contributions for the surface sensors those for the borehole sensors are characterized by smaller variability nevertheless consistent with observations from deconvolution of borehole surface recording pairs downgoing wave effects are visible in the site contributions for many borehole stations finally the site amplification functions obtained at the surface are compared with surface to borehole s b and horizontal to vertical h v spectral ratios showing that the s b ratios generally provide better estimates of the horizontal amplification than the h v ratios due to amplification of the vertical component of ground motion

assessing the applicability of deconvolution of borehole records for determining near surface shear wave attenuationassessing the applicability of deconvolution of borehole records

the importance of distinguishing pseudoresonances and outcrop resonances in downhole array datathe importance of distinguishing pseudoresonances and outcrop resonances in downhole array data

evaluation and optimization of seismic networks and algorithms for earthquake early warning the case of istanbul turkey earthquake early warning eew systems should provide reliable warnings as quickly as possible with a minimum number of false and missed alarms using the example of the megacity istanbul and based on a set of simulated scenario earthquakes we present a novel approach for evaluating and optimizing seismic networks for eew in particular in regions with a scarce number of instrumentally recorded earthquakes we show that while the current station locations of the existing istanbul eew system are well chosen its performance can be enhanced by modifying the parameters governing the declaration of warnings furthermore unless using ocean bottom seismometers or modifying the current eew algorithm additional stations might not lead to any significant performance increase

combining genetic and linearized algorithms for a two step joint inversion of rayleigh wave dispersion and h v spectral ratio curves summary the joint inversion of rayleigh wave dispersion and h v curves from environmental noise measurements allows the retrieval of s wave velocity profiles for the shallow subsoil for this purpose genetic and linearized algorithm have been combined in a two step inversion procedure that allows the principal drawbacks typical of the application of each algorithm separately to be overcome in the first step a genetic algorithm procedure is used to constrain the subvolume of the parameter space where the absolute minimum of the misfit function is located in the second step a linearized inversion algorithm having as an initial guess the minimum misfit model deduced from the first step is applied to force the inversion towards the optimal solution to evaluate the feasibility and effectiveness of this approach seismic noise recordings at a test site in the po river valley north italy have been analysed here detailed geophysical and geological information is available along with earthquake recordings which allow a well constrained definition of both the local shear wave profile and transfer function comparisons between theoretical and experimental s wave velocity profiles and above all between the theoretical and experimental site response functions shows that this combination of inversion procedures can very efficiently to manage the extreme non linearity of the problem

subsonic near surface p velocity and low s velocity observations using propagator inversion detailed knowledge of near surface p and s wave velocities is important for processing and interpreting multicomponent land seismic data because 1 the entire wavefield passes through and is influenced by the near surface soil conditions 2 both source repeatability and receiver coupling also depend on these conditions and 3 nearsurface p and s wave velocities are required for wavefield decomposition and demultiple methods however it is often difficult to measure these velocities with conventional techniques because sensitivity to shallow wave velocities is low and because of the presence of sharp velocity contrasts or gradients close to the earth s free surface we demonstrate that these near surface p and s wave velocities can be obtained using a propagator inversion this approach requires data recorded by at least one multicomponent geophone at the surface and an additional multicomponent geophone at depth the propagator between them then contains all information on the medium parameters governing wave propagation between the geophones at the surface and at depth hence inverting the propagator gives local estimates for these parameters this technique has been applied to data acquired in zeist the netherlands the near surface sediments at this site are unconsolidated sands with a thin vegetation soil on top and the sediments considered are located above the groundwater table a buried geophone was positioned 1 05 m beneath receivers on the surface propagator inversion yielded low near surface velocities namely 270 15 m s for the compressional wave velocity which is well below the sound velocity in air and 150 9 m s for the shear velocity existing methods designed for imaging deeper structures cannot resolve these shallow material properties furthermore velocities usually increase rapidly with depth close to the earth s surface because of increasing confining pressure we suspect that for this reason subsonic near surface p wave velocities are not commonly observed

joint inversion of h v ratios and dispersion curves from seismic noise estimating the s wave velocity of bedrock 1 a joint inversion of phase velocity and h v ratio curves both obtained from seismic noise recordings permits the retrieval of the shear wave velocity structure of local sedimentary cover our inversion scheme uses a genetic algorithm and considers the influence of higher modes on the data sets encouraged by the results published previously on joint inversion parolai et al 2005 we went one step further we found using a synthetic data set that the impedance contrast at the sediment bedrock interface has a strong influence on the shape of the h v ratio curve which therefore allows the bedrock s wave velocity to be well constrained in the joint inversion procedure our observations were further confirmed using a real data set

determination of shallow shear wave velocity profiles in the cologne germany area using ambient vibrations summary we have used both single station and array methods to determine shallow shear velocity site profiles in the vicinity of the city of cologne germany from ambient vibration records based on fk analysis we assume that fundamental mode rayleigh waves dominate the analysed wavefield in the frequency range of 0 7 2 2 hz according to this view a close relation exists between h v spectral ratios and the ellipticity of the contributing rayleigh waves the inversion of the shape of h v spectral ratios then provides quantitative information concerning the local shear wave velocity structure however based on tests with synthetic data believed to represent a typical situation in the lower rhine embayment dispersion curves were found to provide stronger constraints on the absolute values of the velocity depth model than the ellipticities the shape of the ellipticities was found to be subject to a strong trade off between the layer thickness and the average layer velocity we have made use of this observation by combining the inversion schemes for dispersion curves and ellipticities such that the velocity depth dependence is essentially constrained by the dispersion curves while the layer thickness is constrained by the ellipticities in order to test this method in practice we have used array recordings of ambient vibrations from three sites where the subsurface geology is fairly well known and geotechnical information is at least partially available in order to keep the parameter space as simple as possible we attempted to fit only a single layer over a half space model however owing to earlier studies from the region we assume a power law depth dependence for sediment velocities for all three sites investigated the inversion resulted in models for which the shear wave velocity within the sediment layer both in absolute value at the surface and in depth dependence are found to be remarkably similar to the results obtained by budny from downhole measurements this is strong support for the interpretation of h v spectral ratios as rayleigh wave ellipticities for all three sites the predicted sh wave site amplification factors at the fundamental frequency are of the order of 5 6 with a slightly smaller value south of cologne

seismicity of yemen the arabian peninsula often appears totally aseismic on maps of world seismicity reflecting the apparent absence of earthquakes during the twentieth century and particularly since the improvement of the worldwide seismological network after the 1960s events in south west arabia have seldom attracted attention outside the area and this has hindered reporting of earthquakes in regional catalogues we show here that by contrast historical sources of local information describe a fairly continuous occurrence of medium magnitude damaging shocks over the past 12 centuries including a destructive event in 1941 that in many respects resembles the recent earthquake of december 1982 the common presumption that the arabian peninsula has a very low seismicity is incorrect

partitioning of dissolved iron and iron isotopes into soluble and colloidal phases along the ga03 geotraces north atlantic transect abstract the size partitioning of dissolved fe dfe 56 fe of 1 3 to 1 5 relative to dfe 56 fe 0 5 in the surface ocean where sfe and cfe cycle independently and identical sfe and dfe isotope ratios below 250 m where sfe and cfe are constantly partitioned and exchanged

determination of shallow s wave attenuation by down hole waveform deconvolution a case study in istanbul turkey summary the estimation of shear wave velocity and attenuation in near surface geology is of primary importance in engineering seismology in fact their knowledge is essential for site response studies when preparing improved seismic hazard scenarios in this study we propose two approaches for estimating the average shear wave quality factor qs by using recordings of a vertical array of accelerometers the methods are mainly based on the deconvolution of the wavefield recorded in a borehole with that recorded at the surface the first method requires the fourier transform of the deconvolved wavefield to be fitted with a theoretical transfer function valid for the vertical or nearly vertical in the case at hand up to 30 incidence angle propagation of s waves the second method is based on the spectral fitting of the fourier transform of only the acausal part of the deconvolved wavefield with a theoretical transfer function both methods can be applied without any prior knowledge of the subsoil structure since they are based on empirical data analysis and do not require a precise knowledge of the azimuthal orientation of the sensors in the boreholes which is seldom available first we describe the theoretical framework of the proposed methodologies for qs estimation which are based on the assumption that the structure in the borehole is weakly heterogeneous in the vertical direction i e no large impedance contrast exists between the borehole sensor and the surface second by using synthetic accelerograms we verify that in a realistic subsoil structure the assumption of vertical homogeneity can hold and we investigate the robustness and the suitability of the proposed methods finally only the method that was shown to provide the more stable results based on fitting the borehole to surface spectral ratio with a theoretical function is applied to earthquakes signals recorded by a vertical array of accelerometers installed in atak oy western istanbul results show that using borehole data provides a fair and robust estimate of an average qs of about 30 46 and 99 for the 0 50 0 70 0 140 m depth ranges respectively that can be used for numerical simulations of ground motion

gulf stream structure transport and recirculation near 68 w an analysis of the structure and transport of the gulf stream is undertaken using direct current meter observations from a 13 mooring array deployed near 68 w from june 1988 to august 1990 the analysis is based on a stream coordinate approach in which velocities are rotated into a local downstream coordinate frame and averaged according to their relative cross stream location within the current the picture so obtained represents the average synoptic structure of the gulf stream rather than the eulerian averaged structure in which the current is weakened and broadened by lateral meandering of the current and adjacent recirculations many familiar features of the gulf stream are reproduced in the analysis including an asymmetric velocity profile with larger shear on the cyclonic shoreward side of the current an offshore displacement of the velocity core with depth and a subsurface velocity maximum on the offshore side of the current westward recirculations are also seen on both sides of the gulf stream maximum downstream speeds at the axis of the gulf stream reach approximately 2 0 m s at the surface and 0 7 m s at 1000 m roughly twice the corresponding eulerian averaged values the analysis also reveals a deep extension of the gulf stream at 3500 m depth with a width of 130 km and average speeds of 3 4 cm s the transport of the gulf stream in the stream coordinate frame is 113 8 sv approximately 30 larger than the eulerian averaged transport of 88 sv on the basis of these results and other recent studies the downstream transport increase of the gulf stream and the inflow structure to the gulf stream are reconsidered it is concluded that approximately 30 sv or over half of the transport increase between cape hatteras and 68 w is fed by inflow from the northern side of the gulf stream and that this inflow is concentrated near cape hatteras and 68 w where the gulf stream flows steeply across isobaths converging from the north