seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

segment scale variations in the crustal structure of 150 300 kyr old fast spreading oceanic crust east pacific rise 8 15 n 10 5 n from wide angle seismic refraction profiles summary we have simultaneously inverted seismic refraction and wide angle moho reflection traveltimes for the 2 d crustal thickness and velocity structure of 150 300 kyr old crust along the east pacific rise epr between the siqueiros and clipperton fracture zones fzs our results show a strong correlation between ridge segmentation and upper and mid crustal seismic velocities with higher velocities near segment centres and lower velocities near segment ends low crustal velocities at the clipperton and siqueiros fzs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain a relict overlap basin left on the pacific plate by the 9 03 n overlapping spreading centre osc as it propagated southward is associated with a large 1 km s 1 negative upper and mid crustal velocity anomaly this anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration fracturing and shearing arising from rotation of the basin as it was formed the discordant zone left by this osc on the cocos plate is characterized by moderately low crustal velocities probably because of crustal fracturing as the osc propagated into older crust higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust and lower intensity tectonic alteration of the crust than near segment ends the mean crustal thickness along the epr between the siqueiros and clipperton fzs is 6 7 6 8 km the thickest crust is found beneath the lamont seamounts 9 km and in a southward pointing v shaped band located just north of the off axis trace of the 9 03 n osc 7 3 7 8 km the thinnest crust 6 km is found proximal to the clipperton and siqueiros fzs the crust associated with the off axis trace of the 9 03 n osc is not anomalously thin suggesting that magma supply beneath the osc is similar to that of the northern and southern segments we see a similar pattern of crustal thickness variation to that determined using multichannel reflection data including a gradual thickening of the crust from north to south along the northern ridge segment and the location of the thickest crust just north of the 9 03 n osc however the magnitude of the along axis crustal thickness variation we observe along the northern ridge segment between 9 50 n and 9 15 n 1 3 1 8 km excluding the lamont seamounts is significantly less than the 2 3 km of variation previously reported weakening the case for the existence of a low density mantle diapir at 9 50 n inferred from gravity data the band of thick crust located just north of the off axis trace of the 9 03 n osc suggests a close genetic link between this feature and the osc thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward propagating 9 03 n osc over the past 0 5 myr and not to along axis melt migration away from a mantle diapir as previously proposed

drilling deep into young oceanic crust hole 504b costa rica rift hole 504b is by far the deepest hole yet drilled into the oceanic crust in situ and it therefore provides the most complete ground truth now available to test our models of the structure and evolution of the upper oceanic crust cored in the eastern equatorial pacific ocean in 5 9 m y old crust that formed at the costa rica rift hole 504b now extends to a total depth of 1562 3 m below seafloor penetrating 274 5 m of sediments and 1287 8 m of basalts the site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust as a result the conductive heat flow approaches the value of about 200 mw m predicted by plate tectonic theory and the in situ temperature at the total depth of the hole is about 165 c the igneous section was continuously cored but recovery was poor averaging about 20 the recovered core indicates that this section includes about 575 m of extrusive lavas underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes the latter have been sampled in situ only in hole 504b the igneous section is composed predominantly of magnesium rich olivine tholeiites with marked depletions in incompatible trace elements nearly all of the basalts have been altered to some degree but the geochemistry of the freshest basalts is remarkably uniform throughout the hole successive stages of on axis and off axis alteration have produced three depth zones characterized by different assemblages of secondary minerals 1 the upper 310 m of extrusives characterized by oxidative seafloor weathering 2 the lower extrusive section characterized by smectite and pyrite and 3 the combined transition zone and sheeted dikes characterized by greenschist facies minerals a comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology alteration and porosity into three zones that are analogous to layers 2a 2b and 2c described by marine seismologists on the basis of characteristic seismic velocities many of the logs and experiments suggest the presence of a 100 to 200 m thick layer 2a comprising the uppermost rubbly pillow lavas which is the only significantly permeable interval in the entire cored section layer 2b apparently corresponds to the lower section of extrusive lavas in which original porosity is partially sealed as a result of alteration nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900 1000 m below seafloor within the transition between extrusives and sheeted dikes indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2b and 2c and confirming that layer 2c consists of intrusive sheeted dikes a vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends that between the sheeted dikes of seismic layer 2c and the gabbros of seismic layer 3 which has never been sampled in situ may be within reach of the next drilling expedition to hole 504b therefore despite recent drilling problems deep in the hole current plans now include revisiting hole 504b for further drilling and experiments when the ocean drilling program returns to the eastern pacific in 1991

palaeomagnetic and rock magnetic reliability criteria in ophiolitic rocks a case study from the palaeozoic ballantrae ophiolite scotland abstract palaeomagnetic studies of the ballantrae ophiolite southwest scotland are assessed in the light of palaeomagnetic and rock magnetic reliability criteria the remanence directions are palaeomagnetically reliable but have only limited significance for apparent polar wander path construction rock magnetic reliability criteria are found to be ambiguous when treated in isolation original grain size effects are detected in magnetic properties across three sections through pillow lavas we propose that a systematic study of spatial magnetic property variations within pillow lavas can provide an additional field test in the assessment of reliability thermomagnetic analyses and microscopic observations suggest three remanence carrying magnetic phases exist within the extrusive layer of the ophiolite these are almost pure magnetite hematite and titanomaghemite palaeomagnetic rock magnetic and geological observations are best satisfied by a pre obduction magnetisation age this study links the remagnetisation of an ultra oceanic conglomerate to local hydrothermal circulation rather than to a pervasive remagnetisation event as previously suggested

joining up land and marine seismic data case study from procida and ventotene islands tyrrhenian sea italy abstractthe seismic refraction survey routinely used to obtain information on subsurface geology has a limited use in the foreshore zone we have evaluated a nonstandard approach to acquire seismic data in the backshore foreshore area joining up land and marine seismic data the innovative part was the acquisition of a continuous seismic profile that allowed the recording of high resolution data in a transition zone tz to establish an integrated approach for improving the land marine seismic model in areas characterized by a complex near surface geology to achieve this objective two seismic tests were carried out along the beaches of the tyrrhenian islands of procida and ventotene we defined the p wave vp and s wave velocities vs with depth by integrating the tomography of the refracted first arrivals vp velocities versus depth with the inversion of the phase velocities dispersion curves extracted from the surface waves 1d vs velocities models working with both seismic methods allowed us to o

source of marine magnetic anomalies some results from dsdp leg 83 leg 83 of the deep sea drilling project represents the deepest penetration of undisturbed marine crust yet achieved providing samples from the lower part of the extrusive layer and the upper 300 m of the sheeted dike complex we present here measurements of the magnetic properties of rocks from leg 83 and address the question of their suitability as sources for marine magnetic anomalies we find that due to extensive hydrothermal alteration the extrusive part of the leg 83 section is insufficiently magnetized to be part of the magnetic source layer the sheeted dike complex on the other hand does have sufficient magnetization and contrary to recent models of marine magnetization based on ophiolite studies may be a contributor to magnetic anomalies the strong dependence of magnetic properties on the alteration history of the rocks however makes extension of these results to other regions problematical

greenland s pressure drag and the atlantic storm track abstract some effects of greenland on the northern hemisphere wintertime circulation are discussed inviscid pressure drag on greenland s slopes calculated from reanalysis data is related to circulation patterns greenland lies north of the core of the tropospheric westerly winds yet strong standing waves which extend well into the stratosphere produce a trough ridge system with jet stream lying close to greenland mean icelandic low in its wake and storm track that interacts strongly with its topography in the lower troposphere dynamic height anomalies associated with strongly easterly pressure drag on the atmosphere are quite localized in space and relatively short lived compared to upper levels yet they involve a hemispheric scale dislocation of the stratospheric polar vortex it is a two scale problem however the high pass time filtered part of the height field responsible for 73 of the pressure drag is quite different and expresses propagating cyclonic development in the atlantic storm

near surface velocity estimation using source domain full traveltime inversion and early arrival waveform inversion accurate estimation of near surface velocity is a key step for imaging deeper targets we present a new workflow to invert complex early arrivals in land seismic data for near surface velocities this workflow is composed of two methods source domain full traveltime inversion fti and early arrival waveform inversion ewi source domain fti automatically generates the background velocity that kinematically matches the reconstructed plane wave sources from early arrivals with true plane wave sources this method does not require picking first arrivals for inversion which is one of the most challenging and labor intensive steps in ray based first arrival traveltime tomography especially when the subsurface medium contains low velocity zones that cause shingled multi value arrivals moreover unlike conventional born based method source domain fti can determine if the initial velocity is slower or faster than the true one according to the gradient sign in addition the computational cost is reduced co

recent advances in fine particle magnetism implications for paleomagnetism it is becoming increasingly obvious in paleomagnetism that the older practice of using large numbers of samples to improve statistical measures of accuracy is often not adequate to reveal the signal of greatest geological importance blanket demagnetization treatments are now largely outdated except for the most ideally behaved samples a rapidly vanishing phenomenon in many locales variations in characteristic remanent directions between neighboring and supposedly contemporaneous but lithogically distinct rock units can no longer be simply treated as noise we now recognize that such differences may actually reflect rock magnetic effects rather than such factors as the short term behavior of the geomagnetic field or local variations in structure multiple overprints often the result of moderate reheating and or chemical alteration can result in an uncertain relationship between the timing of emplacement and the acquisition of remanence in the past rocks which presented such problems could often be avoided in favor of more straightforward units much of that geological cream has been skimmed however further progress in many areas particularly the determination of the apw path and reversal stratigraphy in the paleozoic will require a better understanding of magnetization processes in order to resolve the above mentioned problems and produce accurate and reliable results

satellite measurements reveal persistent small scale features in ocean winds four year averages of 25 kilometer resolution measurements of near surface wind speed and direction over the global ocean from the quikscat satellite radar scatterometer reveal the existence of surprisingly persistent small scale features in the dynamically and thermodynamically important curl and divergence of the wind stress air sea interaction over sea surface temperature fronts throughout the world ocean is evident in both the curl and divergence fields as are the influences of islands and coastal mountains ocean currents such as the gulf stream generate distinctive patterns in the curl field these previously unresolved features have important implications for oceanographic and air sea interaction research

paleomagnetic arguments for block rotations along the arakapas fault cyprus paleomagnetic data from the sheeted dike complex north of the arakapas fault reveal significant internal rotations the north south trending dikes have remanent magnetization directions close to the well known westerly paleomagnetic direction of the troodos massif whereas the east west trending dikes near the arakapas fault have northerly remanent magnetization directions the paleomagnetic data are in agreement with the apparent clockwise rotation of the dikes contrary to recent geological analyses which postulate that the pattern of dike trends is due to dike injection into a sigmoidal stress field along a sinistral transform fault the paleomagnetic results indicate clockwise rotations about vertical axes in response to a dextral shear in addition to the rotations around vertical axes small rotations around subhorizontal axes are inferred from the paleomagnetic data for tilted dikes thus these data reinforce the vertical intrusion hypothesis

winter atmospheric buoyancy forcing and oceanic response during strong wind events around southeastern greenland in the regional arctic system model rasm for 1990 2010 abstractstrong mesoscale tip jets and barrier winds that occur along the southeastern greenland coast have the potential to impact deep convection in the irminger sea the self organizing map som training algorithm was used to identify 12 wind patterns that represent the range of winter november march ndjfm wind regimes identified in the fully coupled regional arctic system model rasm during 1990 2010 for all wind patterns the ocean loses buoyancy primarily through the turbulent sensible and latent heat fluxes haline contributions to buoyancy change were found to be insignificant compared to the thermal contributions patterns with westerly winds at the cape farewell area had the largest buoyancy loss over the irminger and labrador seas due to large turbulent fluxes from strong winds and the advection of anomalously cold dry air over the warmer ocean similar to observations rasm simulated typical ocean mixed layer depths mld of approximately 400 m throughout the irminger basin with indi

a cause of rupture segmentation and synchronization in the nankai trough revealed by seismic imaging and numerical simulation 1 a giant earthquake occasionally occurs in a subduction zone owing to a simultaneous rupture in adjacent segments which have been previously ruptured by large earthquakes however it is still unknown if a giant earthquake coincidentally occurs or if there is a causal factor to control its generation in this study we show a cause and a growth process of a giant earthquake which may occur along southwestern japan on the basis of seismic images obtained from wide angle seismic data and a numerical simulation incorporating the structural images the wide angle seismic data were acquired along three trough parallel profiles crossing the rupture segmentation boundary between the 1944 tonankai moment magnitude mw 8 1 and the 1946 nankai mw 8 4 earthquakes the seismic imaging detected a high seismic velocity body forming a strongly coupled patch at the segmentation boundary the numerical simulation explained the historic rupture patterns and shows the occurrence of a giant earthquake along the entire nankai trough a distance of over 600 km long mw 8 7 the growth process revealed from the simulated slip history in and around the strongly coupled patch is 1 prior to the giant earthquake a small slow event or earthquake occurs near the segmentation boundary 2 this accelerates a very slow slip slower than the plate convergent rate at the strong patch which reduces a degree of coupling and 3 then a rupture easily propagates through the strong patch when the next earthquake is nucleated near the segmentation boundary consequently growing into a giant earthquake

magnetic properties and opaque mineralogy of rocks from selected seafloor hydrothermal sites at oceanic ridges magnetic properties natural remanent magnetization nrm susceptibility curie point temperature tc saturation isothermal remanent magnetization irms and koenigsberger ratio q and opaque mineralogy were determined for basalts diabases gabbros peridotites and serpentinites collected by dredging and submersible from the rift valley at five hydrothermal sites 15 n 17 n 23 n the snake pit hydrothermal field 26 n the tag hydrothermal field on the mid atlantic ridge and 42 n the sea cliff hydrothermal field on the northern gorda ridge the magnetomineralogy is interpreted in terms of deuteric hydrothermal metasomatic and ambient seawater alteration evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts but is pervasive in diabases gabbros and ultramafic rocks this lithologiec distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low temperature alteration in young surface basalts to more pervasive high temperature alteration in diabases gabbros and peridotites the studies reveal distinct correlations between magnetization intensity thermomagnetic behavior and magnetic mineralogy grain size style and intensity of alteration and rock type basalts have the highest nrm intensities and the lowest curie points 105 294 c basalts with nrm irms ratios less than 10 2 may have been remagnetized and have nrm intensities and q values much lower one third and one half respectively than basalts with nrm irms ratios of 10 2 or greater in the oceanic lithospheric suite examined remanent q 1 magnetization is present in basalts and induced magnetization q 1 occurs in metabasalts and the other rock types layer 2a basalts are the source of median valley magnetic anomalies the magnetic source may shift from the surface to deeper horizons with progressive seafloor aging and enhanced deuteric oxidation in layer 2b dikes and layer 3 gabbros and the formation of magnetite in serpentinized peridotites this shift is influenced by fluctuations in the curie isotherm related to the duration of active magma chambers and convective hydrothermal cells the magnetic and mineralogie properties of oceanic rocks determined show the effects of a wide range of alteration processes that are variously related to depth in oceanic lithosphere magmatic cooling history and hydrothermal circulation

impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of nicaragua impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of nicaragua

influence of the iceland mantle plume on oceanic crust generation in the north atlantic when a mantle plume with elevated temperature underlies an oceanic spreading centre it affects the generation of oceanic crust by creating thicker crust we map the variation in crustal thickness and seismic velocity along three long offset seismic profiles acquired over oceanic crust generated shortly after continental breakup in the north atlantic a 212 kmlong flowline from the faroes rifted continental margin across crust of 51 42 ma age where oceanic spreading developed close to the inferred centre of the iceland mantle plume a 256 km flowline extending from the hatton rifted continental margin across crust of 52 40 ma age about 800 km south of the presumed centre of the mantle plume and a 99 km strike line over oceanic crust formed at 43 ma in the iceland basin off the hatton continental margin the crustal velocity structure along each profile is constrained by multichannel seismic reflection data which is used primarily to map the sediments and by densely spaced ocean bottom seismometers which recorded wide angle reflections and refractions to offsets of more than 100 km over 56 000 crustal diving wave and moho wide angle reflection arrivals were used in joint crustal refraction and reflection tomographic inversions quantitative error analysis shows that the seismic velocity of the crust is mostly constrained to within 0 1 km s 1 and the depth of the moho to within 250 m we interpret the crustal thickness and velocity changes along the profiles as caused primarily by changes in the mantle temperature at the time of crustal formation if all the oceanic crustal thickness variations are ascribed to mantle temperature changes we infer that as mature seafloor spreading developed following continental breakup the mantle cooled by ca 75 c over a 10 myr period although it still remained hotter than the global average of normal oceanic crust the crust formed close to iceland is at all times thicker than that formed further away which we interpret as reflecting higher temperatures close to the centre of the thermal anomaly created by the mantle plume currently at the reykjanes ridge south of iceland we interpret thicker than normal oceanic crust as being caused by the presence of hotter mantle modulated by thickness variations of 1 5 2 0 km which are attributed to temporal variations in the mantle plume temperature of about 25 c on a 3 6 myr timescale a 1 5 km increase in thickness of oceanic crust generated between 48 and 45 ma on the faroes line is similar in magnitude and duration to those occurring on the present day reykjanes ridge which we suggest is due to a temperature pulse of 25 c gravity lineations in the northern north atlantic suggest that the oceanic crust has exhibited small thickness fluctuations of similar size throughout its history interpreted as due to small fluctuations in the temperature of the iceland mantle plume

seismic constraints of the formation process on the back arc basin in the southeastern japan sea to clarify the formation process of the back arc basin in the japan sea which is located next to the northwestern pacific a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern yamato basin to the coast of the northeastern japan island arc off awa shima island the crust beneath the northern yamato basin off awa shima island is approximately 16 km thick the upper and lower crusts are respectively about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient in the basin there are very few units with p wave velocity of 5 4 6 0 km s corresponding to the continental upper crust the crustal structure of the northern yamato basin has characteristics of thicker oceanic crust the high velocity lowermost crust in the northern yamato basin with 7 2 7 4 km s might show melt formed by a slightly high mantle temperature during the opening of the basin however the crust beneath the areas from the sado ridge to the coast which is approximately 25 26 km thick is slightly thinner than that of the continental crust and island arc crust the crustal structure beneath this area is inferred to be a rifted continental and or a rifted island arc crust

deep convection in the irminger sea forced by the greenland tip jet open ocean deep convection one of the processes by which deep waters of the world s oceans are formed is restricted to a small number of locations for example the mediterranean and labrador seas recently the southwest irminger sea has been suggested as an additional location for open ocean deep convection the deep water formed in the irminger sea has the characteristic temperature and salinity of the water mass that fills the mid depth north atlantic ocean which had been believed to be formed entirely in the labrador basin here we show that the most likely cause of the convection in the irminger sea is a low level atmospheric jet known as the greenland tip jet which forms periodically in the lee of cape farewell greenland and is associated with elevated heat flux and strong wind stress curl using a history of tip jet events derived from meteorological land station data and a regional oceanic numerical model we demonstrate that deep convection can occur in this region when the north atlantic oscillation index is high which is consistent with observations this mechanism of convection in the irminger sea differs significantly from those known to operate in the labrador and mediterranean seas

the influence of high frequency atmospheric forcing on the circulation and deep convection of the labrador sea abstractthe influence of high frequency atmospheric forcing on the circulation of the north atlantic ocean with emphasis on the deep convection of the labrador sea was investigated by comparing simulations of a coupled ocean ice model with hourly atmospheric data to simulations in which the high frequency phenomena were filtered from the air temperature and wind fields in the absence of high frequency atmospheric forcing the strength of the atlantic meridional overturning circulation and subpolar gyres was found to decrease by 25 in the labrador sea the eddy kinetic energy decreased by 75 and the average maximum mixed layer depth decreased by between 20 and 110 depending on the climatology in particular high frequency forcing was found to have a greater impact on mixed layer deepening in moderate to warm years whereas in relatively cold years the temperatures alone were enough to facilitate deep convection additional simulations in which either the wind or temperature was filtered revealed that

palaeomagnetic evidence for clockwise rotations related to dextral shear along the southern troodos transform fault cyprus abstract the southern troodos arakapas fault represents a late cretaceous oceanic fracture zone which was locally disrupted during late cretaceous eocene palaeorotation of the troodos microplate opinions are divided as to the sense of displacement along the transform fault and the exact timing of initiation of palaeorotation palaeomagnetic studies of turonian zeolite facies lavas and sediments exposed within the transform domain have revealed considerable variations in the declination of remanent magnetisation between sites along the arakapas fault belt the western margin of the limassol forest complex and the eastern flank of the troodos ophiolite at seven sites clockwise rotation of fault blocks has occurred about steeply inclined axes one fault block at the western end of the arakapas fault belt has experienced a net anticlockwise rotation while at six other sites only simple tilting about sub horizontal axes is indicated the overall clockwise sense of block rotation and initial dyke strikes calculated at three sites are consistent with right lateral slip along the transform cross cutting relationships revealed by the analysis of one site demonstrate that these rotations took place during crustal genesis and are not a product of post spreading disruption of the fracture zone sites located further south in the eastern limassol forest complex show no relative rotation with respect to the main troodos ophiolite to the north and have experienced only simple tilting however if our results are considered in conjunction with existing palaeomagnetic data from the area it appears that the entire area lay within a complicated zone of localised and predominantly clockwise block rotations produced by dextral slip along the transform some areas were rotated by over 100 about steeply inclined axes whereas others experienced only simple tilting the whole area was later subjected to a bulk 90 anticlockwise rotation along with the troodos microplate data obtained from the umbers and radiolarites perapedhi fm exposed on the eastern flank of the limassol forest block indicate that at least 30 and possibly up to 45 of this 90 rotation took place over a maximum of 15 ma

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

crustal structure of a rifted oceanic core complex and its conjugate side at the mar at 5 s implications for melt extraction during detachment faulting and core complex formation we present results of a seismic refraction experiment which determines the crustal and upper mantle structure of an oceanic core complex occ and its conjugate side located south of the 5 s ridge transform intersection at the mid atlantic ridge the core complex with a corrugated surface has been split by a change in location of active seafloor spreading resulting in two massifs on either side of the current spreading axis we applied a joint tomographic inversion of wide angle reflected and refracted phases for five intersecting seismic profiles the obtained velocity models are used to constrain the magmatic evolution of the core complex from the analysis of seismic layer 3 and crustal thickness an abrupt increase of crustal velocities at shallow depth coincides with the onset of the seafloor corrugations at the exposed footwall the observed velocity structure is consistent with the presence of gabbros directly beneath the corrugated fault surface the thickness of the high velocity body is constrained by pmp reflections to vary along and across axis between 3 and 5 km the thickest crust is associated with the central phase of detachment faulting at the higher elevated northern portion of the massif beneath the breakaway of the occ the crust is 2 5 km thick and reveals significantly lower velocities this implies that the fault initially exhumed low velocity material overlying the gabbro plutons in contrast crust formed at the conjugate side during occ formation is characterized by an up to 2 km thick seismic layer 2 overlying a 1 7 km thick seismic layer 3 obtained upper mantle velocities range from 7 3 to 7 9 km s 1 and seem to increase with distance from the median valley however velocities of 7 3 7 5 km s 1 beneath the older portions of the occ may derive from deep fluid circulation and related hydrothermal alteration which may likely be facilitated by the subsequent rifting our velocity models reveal a strongly asymmetric velocity structure across the ridge axis associated with the accretion of gabbros into the footwall of the detachment fault and upper crustal portions concentrated at the conjugate side our results do not support a substantial increase in the axial ridge s melt supply related to the final phase of detachment faulting hence the footwall rifting at 5 s may be a generic mechanism of detachment termination under very low melt conditions as predicted by recent numerical models of tucholke et al

advanced seismic imaging techniques characterize the alpine fault at whataroa new zealand the plate bounding alpine fault in new zealand is a 850 km long transpressive continental fault zone that is late in its earthquake cycle we have acquired and processed reflection seismic data to image the subsurface around the main drill site of the deep fault drilling project dfdp 2 the resulting velocity models and seismic images of the upper 5 km show complex subsurface structures around the alpine fault zone the most prominent feature is a strong reflector at depths of 1 5 2 2 km with an apparent dip of 48 to the southeast below the dfdp 2 borehole which we assume to be the main trace of the alpine fault above the main reflector parallel reflectors suggest the presence of a 600 m wide damage zone additionally subparallel reflectors are imaged that we interpret as secondary branches of the main fault zone conjugate faults imaged by the data show the complexity of the subsurface the derived p wave velocity model reveals a 300 600 m thick sedimentary layer with velocities of 2 3 km s above a schist basement with velocities of 4 5 5 5 km s a low velocity layer can be observed within the basement at 0 8 2 km depth a small scale low velocity anomaly appears at the top of the basement that can be correlated to the fault zone the results provide a reliable basis for a seismic characterization of the dfdp 2 drill site that can be used for further structural and geological investigations of the architecture of the alpine fault in this area

a model of ocean basin crustal magnetization appropriate for satellite elevation anomalies a model of ocean basin crustal magnetization measured at satellite altitudes is developed which will serve both as background to which anomalous magnetizations can be contrasted and as a beginning point for studies of tectonic modification of normal ocean crust the model is based on published data concerned with the petrology and magnetization of the ocean crust and consists of viscous magnetization and induced magnetization estimated for individual crustal layers thermal remanent magnetization and chemical remanent magnetization are excluded from the model because seafloor spreading anomalies are too short in wavelength to be resolved at satellite altitudes the exception to this generalization is found at the oceanic magnetic quiet zones where thermal remanent magnetization and chemical remanent magnetization must be considered along with viscous magnetization and induced magnetization

discrepancies between experimental observations in natural and synthetic samples abstract the evidence is reviewed by comparing the data obtained from synthetic samples with those derived from natural samples of magnetic minerals present in rocks it is shown that there are many issues which are still unresolved some suggestions are made concerning the future direction of rock magnetic research such as the need for intensified studies of natural samples the aim of this note is not to disclose anything new but rather to open up some lines of thought and draw attention to questions and ideas that may have been lurking in our minds from the very beginnings of paleomagnetism

atmospheric conditions associated with labrador sea deep convection new insights from a case study of the 2006 07 and 2007 08 winters abstractdeep convection in the labrador sea ls resumed in the winter of 2007 08 under a moderately positive north atlantic oscillation nao state this is in sharp contrast with the previous winter with weak convection despite a similar positive nao state this disparity is explored here by analyzing reanalysis data and forced ocean simulations it is found that the difference in deep convection is primarily due to differences in large scale atmospheric conditions that are not accounted for by the conventional nao definition specifically the 2007 08 winter was characterized by an atmospheric circulation anomaly centered in the western north atlantic rather than the eastern north atlantic that the conventional nao emphasizes this anomalous circulation was also accompanied by anomalously cold conditions over northern north america the controlling influence of these atmospheric conditions on ls deep convection in the 2008 winter is confirmed by sensitivity experiments where surface forcing and or in

spreading rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies summary the anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km myr i previous magnetization models of the oceanic lithosphere fail to explain this observation we propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization and the ratio of layer 3 over layer 2a saturation magnetizations varies linearly from about 0 4 at a spreading rate of 10kmmyr to about 0 at 55kmmyr i for spreading rates higher than 55 km myr layer 3 and uppermost mantle magnetizations are negligible recent advances concerning accretionary processes at mid ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation alteration of layer 2a and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate

comparison of wintertime mesoscale winds over the ocean around southeastern greenland in wrf and era interim strong mesoscale tip jets and barrier winds that occur over the ocean near southern greenland have the potential for impacting deep convection in the ocean the self organizing map som training algorithm was used to identify and classify the range of 10 m wind patterns present during ten winters 1997 2007 ndjfm in the ecmwf interim reanalysis era i and from a regional simulation using the weather research and forecasting wrf model at 50 km into a som the som is used to identify differences in the manifestation of westerly tip jets easterly tip jets and barrier flow the north atlantic oscillation nao index is well correlated with the type of tip jet present at cape farewell but the nao was not well correlated with the absence or presence of barrier flow wrf simulated patterns with strong barrier parallel flow more frequently than era i and wrf also had faster coastal winds than era i during all types of strong wind events the difference in coastal winds is likely related to model resolution and the resulting ability of each model to simulate strong mesoscale winds that are driven by greenland s steep terrain

tomo3d 3 d joint refraction and reflection traveltime tomography parallel code for active source seismic data synthetic test we present a new 3 d traveltime tomography code tomo3d for the modelling of active source seismic data that uses the arrival times of both refracted and reflected seismic phases to derive the velocity distribution and the geometry of reflecting boundaries in the subsurface this code is based on its popular 2 d version tomo2d from which it inherited the methods to solve the forward and inverse problems the traveltime calculations are done using a hybrid ray tracing technique combining the graph and bending methods the lsqr algorithm is used to perform the iterative regularized inversion to improve the initial velocity and depth models in order to cope with an increased computational demand due to the incorporation of the third dimension the forward problem solver which takes most of the run time 90 per cent in the test presented here has been parallelized with a combination of multi processing and message passing interface standards this parallelization distributes the ray tracing and traveltime calculations among available computational resources the code s performance is illustrated with a realistic synthetic example including a checkerboard anomaly and two reflectors which simulates the geometry of a subduction zone the code is designed to invert for a single reflector at a time a data driven layer stripping strategy is proposed for cases involving multiple reflectors and it is tested for the successive inversion of the two reflectors layers are bound by consecutive reflectors and an initial velocity model for each inversion step incorporates the results from previous steps this strategy poses simpler inversion problems at each step allowing the recovery of strong velocity discontinuities that would otherwise be smoothened

is the oceanic crust over 1 km necessary for the source of marine magnetic anomalies abstract a paleomagnetic vertical profile over 1 km through the oceanic crust is constructed by magnetic properties of 256 dsdp samples from hole 504b the costa rica rift the results of magnetic measurements of the relatively young oceanic crust indicate that the thickness of the marine magnetic source layer is a 0 6 km thick pillow lava layer of the uppermost oceanic crust

an overview of barrier winds off southeastern greenland during the greenland flow distortion experiment during the greenland flow distortion experiment barrier flow was observed by an instrumented aircraft on 1 2 5 and 6 march 2007 off southeastern greenland during this time period the barrier flow increased from a narrow jet 15 m s 1 to a jet filling almost the whole of the denmark strait with maximum wind speed exceeding 40 m s 1 dropsonde observations show that the barrier flow was capped by a sharp temperature inversion below mountain height below the inversion was a cold and dry jet with a larger northerly wind component than that of the flow above which was also warmer and more moist thus the observations indicate two air masses below mountain height a cold and dry barrier jet of northern origin and above this a warmer and moister air mass that was of cyclonic origin numerical simulations emphasize the non stationarity of the greenland barrier flow and its dependence on the synoptic situation in the greenland iceland region they show that the barrier jet originated north of the denmark strait and was drawn southward by a synoptic scale cyclone with the strength and location of the maximum winds highly dependent on the location of the cyclone relative to the orography of greenland experiments without greenland s orography suggest a 20 m s 1 enhancement of the low level peak wind speeds due to the presence of the barrier thus the greenland barrier flows are not classic geostrophically balanced barrier flows but have a significant ageostrophic component and are precisely controlled by synoptic scale systems copyright c 2009 royal meteorological society

inversion for salt flank geometry using transmitted p and s wave travel times accurate delineation of the salt flank is important for oil and gas exploration in areas with salt intrusion we describe the application of deformable layer tomography dlt to invert for the geometry of salt flank using travel times of transmitted p and s waves from surface sources to downhole receivers the dlt allows us to take advantage of a common situation that the salt velocity is known and generally invariant but we need to determine the variable geometry of salt flank we demonstrate our new method using a physical model experiment mimicking the setup of a walkaway vertical seismic profiling vsp survey we first use picked p wave arrivals to invert for the salt flank geometry the sinusoid shape of the salt flank is estimated fairly by the dlt due to the uneven p wave raypath coverage as an improvement we further incorporate the s wave arrivals in the dlt the picking of s wave arrivals is assisted with modeled s wave arrivals based on the p wave dlt model the dlt solution using both p and s wave arrivals delineates the salt flank geometry more accurately than that using p wave arrivals alone the salt flank delineation using dlt can complement with the conventional salt proximity and migration methods it could also serve as constraints or the initial model for the full waveform inversion of salt geometry

a case study of seismic zonation in municipal areas an earthquake of 7 4 magnitude in august 1999 caused major loss of life and severe damage within the municipality of izmit 170 km southeast of istanbul figure 1 a survey of the damaged buildings was made by the municipal authorities shortly after the earthquake the municipal government decided to conduct a pilot seismic zonation project to determine whether the cause of the damage was poor construction materials and methods or weak soil conditions in this project we investigated the soil conditions with two objectives in mind 1 to estimate the seismic model of the soil column at each district to determine the geotechnical earthquake engineering parameters and 2 to map active faults within the municipal area

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

analysis of neotectonic structures in the eastern precordillera of argentina in relation to seismic hazard by the application of integrated geophysical methods abstract the city of san juan in the central western region of argentina has been the target of very destructive superficial earthquakes some of which have not been associated to a clear structural source up to this date the city is constantly growing outside the valley where it is located towards the area of eastern precordillera which is currently having an increased socio cultural activity thus this study is focused on increasing the geological knowledge of the latter by studying the eastern flank of sierra chica de zonda eastern precordillera whose proved neotectonic activity represents a geohazard on the basis of the general geological setting the neotectonic structures in the study area are related to a major active synclinal folding located just under the western sector of the san juan city geophysical potential methods gravimetric and magnetometric surveys were used to recognize contacts by contrast of density and magnetic susceptibility in order to reduce the ambiguity of these methods the gravi magnetometric results were constrained by using seismic and electrical tomographies these contacts where geophysical properties abruptly change were interpreted as faults despite many of them not having a superficial expression the latter being of great importance to asses the seismic hazard of the study area

the impact of polar mesoscale storms on northeast atlantic ocean circulation every year thousands of mesoscale storms termed polar lows cross the climatically sensitive subpolar north atlantic ocean high resolution numerical simulations of the ocean circulation taking into account the effect of these storms on deep water formation suggest that polar lows significantly affect the global ocean circulation

nonlinear refraction traveltime tomography a few important issues for performing nonlinear refraction traveltime tomography have been identified they include the accuracy of the traveltime and raypath calculations for refraction the physical information in the refraction traveltime curves and the characteristics of the refraction traveltime errors consequently we develop a shortest path ray tracing method with an optimized node distribution that can calculate refraction traveltimes and raypaths accurately in any velocity model we find that structure ambiguity caused by short and long rays in the seismic refraction method may influence the inversion solution significantly therefore we pose a nonlinear inverse problem that explicitly minimizes the misfits of the average slownesses ratios of traveltimes to the corresponding ray lengths and the apparent slownesses derivatives of traveltimes with respect to distance as a result we enhance the resolution as well as the convergence speed to regularize our inverse problem we use the tikhon

arctic system reanalysis improvements in topographically forced winds near greenland southern greenland is home to a number of weather systems characterized by high speed low level winds that are the result of topographic flow distortion these systems include tip jets barrier winds and katabatic flows global atmospheric reanalyses have proven to be important tools in furthering our understanding of these systems and their role in the climate system however there is evidence that their mesoscale structure may be poorly resolved in these global products here output from the regional arctic system reanalysis asrv1 30 km and asrv2 15 km grid resolutions are compared to the global era interim reanalysis era i 80 km grid resolution focusing on their ability to represent winds in the vicinity of southern greenland comparisons are made to observations from surface and upper air stations as well as from research aircraft flights during the greenland flow distortion experiment gfdex the era i reanalysis has a tendency to underestimate high wind speeds and overestimate low wind speeds which is reduced in asrv1 and nearly eliminated in asrv2 in addition there is generally a systematic reduction in the root mean square error between the observed and the reanalysis wind speeds from era i to asrv1 to asrv2 the exception being low level marine winds where the correspondence is similar in all reanalyses case studies reveal that mesoscale spatial features of the wind field are better captured in asrv2 as compared to the era i or asrv1 these results confirm that a horizontal grid size on the order of 15 km is needed to characterize the impact that greenland s topography has on the regional wind field and climate however even at this resolution there are still features of the wind field that are under resolved

an image based effective medium modeling of near surface anomalies near surface modeling for statics corrections is an integral part of a land seismic data processing workflow the methods for near surface modeling can be categorized into five groups 1 uphole surveys 2 shallow seismic surveys 3 traveltime inversion 4 waveform inversion and 5 joint inversion of seismic and nonseismic data the first two are past methods the third usually referred to as traveltime tomography is at present the most widely accepted method and the last two methods are future methods with several practical aspects yet to be resolved

applications of engineering seismology in urban areas i present two case studies which describe applications of engineering seismology in urban areas high resolution shallow offshore 3d seismic survey for the bosphorus subsea road tunnel project and seismic geotechnical and earthquake engineering site characterization for a residential project in istanbul in case study 1 i conducted a 3d high resolution shallow seismic survey in 2011 to confirm the geometry of the future bosphorus subsea road tunnel traverse i processed the data to obtain 3d pstm and psdm volumes and interpreted these image volumes and correlated the interpretation results with the offshore borehole data with the recoverable signal band of 30 960 hz i attained a vertical resolution of less than 1 m

selective destructive demagnetization of breccias from dsdp leg 83 a microconglomerate test the magnetization of four breccia samples from the leg 83 section of dsdp hole 504b was analyzed by selective destructive demagnetization in order to study the origin and stability of hydrothermally altered basalts the nrm directions of the clasts for three of the four samples are randomly oriented and much more strongly magnetized than the bulk sample clasts which were individually demagnetized show two or more components of magnetization but neither are coincident with those of the bulk sample indicating that nrm was probably acquired prior to the consolidation of the breccia and suggesting that any overprint vrm or otherwise can be removed by af demagnetization to at most 50 oe reflected light microscopy and electron microprobe analysis of two samples show that the unexpectedly high nrm of the matrix regions is apparently the result of secondary magnetic phases precipitated from hydrothermal solutions

traveltime tomography of a dense wide angle profile across orphan basin abstractwe used first arrivals and moho reflections from the 500 km long orphan basin wide angle velocity experiment obwave profile with 3 5 km instrument spacing to construct a traveltime tomography section and to delineate the moho discontinuity across the orphan basin the orphan basin is a failed rift located offshore newfoundland canada showing thinned continental crust over an unusually wide region we observed 1 a zone of extreme crustal thinning 7 km thick crust with no evidence for mantle serpentinization 2 basement morphology exhibiting tilted blocks linked to the crustal thinning and 3 a thicker central crustal segment that is probably related to prerift structural inheritance comparison with the adjacent jeanne d arc basin to the southeast suggested the presence of a decoupling zone between the two basins accommodating the difference in extension rates there was a good correlation between the tomographic velocities and the reflection structure derived from a coincident seismic

impact of synoptic atmospheric forcing on the mean ocean circulation abstractthe impact of synoptic atmospheric forcing on the mean ocean circulation is investigated by comparing simulations of a global eddy permitting ocean sea ice model forced with and without synoptic atmospheric phenomena consistent with previous studies transient atmospheric motions such as weather systems are found to contribute significantly to the time mean wind stress and surface heat loss at mid and high latitudes owing to the nonlinear nature of air sea turbulent fluxes including synoptic atmospheric forcing in the model has led to a number of significant changes for example wind power input to the ocean increases by about 50 which subsequently leads to a similar percentage increase in global eddy kinetic energy the wind driven subtropical gyre circulations are strengthened by about 10 15 whereas even greater increases in gyre strength are found in the subpolar oceans deep convection in the northern north atlantic becomes significantly more vigorous which in turn leads to an increa

the derivation of an anisotropic velocity model from a combined surface and borehole seismic survey in crystalline environment at the cosc 1 borehole central sweden the scandinavian caledonides provide a well preserved example of a palaeozoic continent continent collision where surface geology in combination with geophysical data provides information about th

north atlantic interdecadal variability oceanic response to the north atlantic oscillation 1865 1997 abstract in contrast to the atmosphere knowledge about interdecadal variability of the north atlantic circulation is relatively restricted it is the objective of this study to contribute to understanding how the north atlantic circulation responds to a forcing by the north atlantic oscillation nao on interdecadal timescales for this purpose the authors analyze observed atmospheric and sea surface temperature sst data along with the response of an ocean general circulation model to a realistic monthly surface flux forcing that is solely associated with the nao for the period 1865 1997 in agreement with previous studies it is shown that the relationship between the local forcing by the nao and observed sst anomalies on interdecadal timescales points toward the importance of oceanic dynamics in generating sst anomalies a comparison between observed and modeled sst anomalies reveals that the model results can be used to assess interdecadal variability of the north atlantic circulation the observed

the oceanic response to mesoscale atmospheric forcing the response of the oceanic circulation to mesoscale atmospheric forcing is studied by comparing integrations of a global sea ice ocean model with high resolution european centre for medium range weather forecasts analysis data 0 4 to those with the same forcing coarse grained to a resolution typically employed in climate models and atmospheric reanalyses 1 8 it is shown that the representation of mesoscale features in atmospheric forcing fields leads to an increase in the strength of the wind driven gyres in the north atlantic and north pacific regions of about 5 10 of its mean value an increase of similar magnitude is found for the atlantic meridional overturning circulation from the results of this study it is argued that small scale atmospheric phenomena such as fronts mesoscale cyclones and topographic jets play an important role in driving the mean oceanic circulation

the source fault of the 1983 nihonkai chubu earthquake revealed by seismic imaging abstract from 2011 to 2012 we conducted marine seismic surveys using a multichannel seismic reflection mcs system and ocean bottom seismometers obss in and around the hypocentral region of the 1983 nihonkai chubu earthquake the survey covered the area from the japan coast of the japan sea to the yamato basin and the japan basin we investigated the relationship between the nihonkai chubu earthquake and the crustal structure obtained from seismic reflection imaging and the p wave velocity structure near the hypocentral region the focus in the present paper is on seismic data that were acquired using both the mcs system and obss based on the results the crustal structure of the japan sea around the hypocentral region of the 1983 nihonkai chubu earthquake is divided into three types island arc crust thick oceanic crust and oceanic crust the nihonkai chubu earthquake occurred near the boundary between the thick oceanic crust and the island arc crust there is a strong possibility that the east dipping reflector detected in the crust was the source fault of the nihonkai chubu earthquake in addition the location of anticlines formed by the east dipping reverse fault occurring in the boundary of the crustal structure associated with the source fault of the nihonkai chubu earthquake was confirmed in the range of latitude from 39 20 to 41 15 n although they were not found south of this segment there is also a concentrated area of reverse faults and folds approximately 100 km west of the source region of the nihonkai chubu earthquake this area is located near the boundary between the thick oceanic crust and the oceanic crust

unveiling seismic and density structure beneath the vrancea seismogenic zone romania an edited version of this paper was published by agu copyright 2009 american geophysical union

magnetic structure of the upper kilometer of the marine crust at deep sea drilling project hole 504b eastern pacific ocean deep sea drilling project hole 504b is located in 5 9 m y crust in the eastern pacific ocean about 200 km south of the costa rica rift at 1076 m subbasement it is the deepest penetration of marine crust yet achieved we present here magnetic data from this hole especially from the recently cored leg 83 section which constitutes the lowermost 500 m these data when combined with those of other studies yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available the basement rocks from hole 504b can be divided into three major magnetic units the upper units the top 500 m or so of basement are essentially similar to other shallow marine basement sections consisting of a mixture of various types of extrusive basalt low temperature altered titanomagnetite is the dominant magnetic carrier and the magnetic properties of these rocks are comparable to other shallow marine basalts below the upper units is a 200 m transition zone consisting of a mixture of extrusives and dikes high temperature hydrothermal alteration much of it greenschist facies grade has caused oxidation exsolution of primary titanomagnetite as well as its partial or total replacement by silicates the dominant magnetic carrier is an fe rich titanomagnetite magnetically indistinguishable from pure magnetite the combination of oxidation exsolution and silicate replacement results in a very low natural remanent magnetization nrm the remaining 300 m are in the upper portion of the sheeted dike complex primary titanomagnetite is exsolved but alteration is less intense than in the transition zone nrm values are substantially higher than those of the transition zone due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite which occurs as a silicate alteration product the nrm magnitude is sufficiently high that the sheeted dike complex in this location at least may make a significant if not substantial contribution to the magnetic anomaly overall the magnetic properties of this hole especially the lower 500 m are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling as hydrothermal temperatures do not appear to have exceeded 400 c the nrm in the lower two sections is apparently a chemical rather than a thermal remanence as such models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust a better understanding of the nature of postemplacement alteration in the marine crust is required especially its characteristics at high temperatures and its effects on magnetic properties

sea surface temperature anomalies planetary waves and air sea feedback in the middle latitudes the mechanisms that contribute to the generation and damping of large scale mid latitude sea surface temperature sst anomalies are discussed the sst anomalies reflect primarily the response of the upper ocean to the changes in air sea fluxes that are associated with daily weather fluctuations heat flux forcing is dominant in the lower middle latitudes while wind driven entrainment may be most effective in the high latitudes advection by anomalous ekman current is generally less important and ekman pumping is negligible the sst anomalies decay in part because of entrainment effects associated with mixed layer deepening and oceanic mixing and in part because of heat exchanges with the atmosphere the three approaches commonly used to model the evolution of sst anomalies are reviewed case studies based on monthly or seasonal anomaly maps of the large scale sst and atmospheric anomalies numerical simulations with one dimensional mixed layer models and stochastic forcing models we stress the similarities in the different approaches and discuss their main advantages and limitations the response of the atmosphere to mid latitude sst anomalies is considered first we discuss the poorly known relationship between sst anomalies and diabatic heating using a crude assumption for the air sea coupling we consider a two layer quasi geostrophic channel model and discuss the stationary wave response to sst anomaly forcing and the resulting air sea feedback it is found that the back interaction of the sst anomalies onto the atmosphere causes a weak sst anomaly damping at large scales and a strong one at small scales the air sea coupling should also act as an eastward propagator for the sst anomalies the response of more realistic linear wave models to prescribed diabatic heating is then reviewed and it is suggested that realistic mid latitude sst anomalies have a weak influence on the atmospheric circulation corresponding to changes in the geopotential height of 10 30 m at most this order of magnitude is consistent with the results of general circulation model experiments and with the limited climate predictability associated with mid latitude sst anomalies

crustal thickness variations along the southeast indian ridge 100 116 e from 2 d body wave tomography 1 axial morphology along the southeast indian ridge seir systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100 116 e west of the australian antarctic discordance aad basalt geochemistry has a consistent indian mid ocean ridge basalt morb type isotopic signature so changes in axial topography are attributed to variations in both mantle temperature and melt supply wide angle seismic refraction lines were shot to four ocean bottom hydrophones within seir segments p1 p2 s1 and t where each segment is characterized by a different morphology we constructed 2 d crustal velocity models by jointly inverting hand picked p wave refraction pg and moho reflection pmp traveltime data using a top down minimum structure methodology the results show a 1 5 km eastward decrease in crustal thickness across the study area with segment averages ranging from 6 1 km at p1 to 4 6 km at t melt generation models require a 30 c decrease in mantle temperature toward the aad to account for the crustal thickness trend significant changes in axial morphology accompany small scale variations in crustal thickness consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature melt supply and cooling from hydrothermal circulation layer 3 thins by 3 0 km as layer 2 thickens by 1 4 km between segments p1 and t reflecting the eastward decrease in melt supply and increase in melt lens depth the trade off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material

turning ray tomography turning ray tomography is useful for estimating near surface velocity structure in areas where conventional refraction statics techniques fail because of poor data or lack of smooth refractor velocity structure this paper explores the accuracy and inherent smoothing of turning ray tomography in its capacity to estimate absolute near surface velocity and the statics times derived from these velocities and the fidelity with which wavefields collapse to point diffractors when migrated through these estimated velocities the method comprises nonlinear iterations of forward ray tracing through triangular cells linear in slowness squared coupled with the lsqr linear inversion algorithm it is applied to two synthetic finite difference data sets of types that usually foil conventional refraction statics techniques these models represent a complex hard rock overthrust structure with a low velocity zone and pinchouts and a contemporaneous near shore marine trench filled with low velocity unconsolidated deposits exhibiting no seismically apparent internal structure in both cases velocities are estimated accurately to a depth of one fifth the maximum offset as are the associated statics times of equal importance the velocities are sufficiently accurate to correctly focus synthetic wavefields back to their initial point sources so migration datuming applications can also use these velocities the method is applied to a real data example from the timbalier trench in the gulf of mexico which exhibits the same essential features as the marine trench synthetic model the timbalier velocity inversion is geologically reasonable and yields long and short wavelength statics that improve the cmp gathers and stack and that correctly align reflections to known well markers turning ray tomography estimates near surface velocities accurately enough for the three purposes of lithology interpretation statics calculations and wavefield focusing for shallow migration and datuming