ridge offsets normal faulting and gravity anomalies of slow spreading ridges we develop a model relating mid ocean ridge normal faulting and crustal structure by examining recently available high resolution gravity and multibeam bathymetric data of the mid atlantic ridge and other spreading centers results of the analysis reveal a consistent pattern of positive residual gravity anomalies along the crust paralleling all major atlantic offsets studied especially along the inside corner side of the offsets individual residual gravity anomaly spikes local peaks which have amplitudes of up to 20 mgal and typical across axis spacing of 10 30 km often coincide with individual major fault scarps suggesting crust thinned by normal faulting theoretical calculations indicate that the amplitude and spacing of the observed residual gravity spikes are consistent with the presence of successive ridge parallel low angle faults that originate episodically at inside corners of ridge offset intersections fault scarp heights and the amounts of crustal thinning as inferred from gravity anomalies are consistently larger at inside corners than at outside corners supporting a model in which tectonic extension near ridge offsets is asymmetric with low angle faults occurring preferentially at inside corners these results on spatial variations in seafloor morphology and gravity anomalies further support a three dimensional tectonic faulting model at oceanic spreading centers with three major characteristics 1 low angle faults form preferentially at inside corners where the mantle lithosphere is the strongest and the lithospheric plates are sufficiently decoupled across ridge axis offsets 2 low angle faults decrease in throw toward midpoints of long ridge segments where large low angle faults may not be sustained by a weak lithosphere and 3 the residual gravity peaks are statistically larger at transform than near nontransform offsets indicating that the length of a ridge offset and its tectonic style control the development of low angle faults

tectonic modification of axial crustal structure evidence from spectral analyses of residual gravity and bathymetry of the mid atlantic ridge flanks abstract bathymetry and gravity data of the northern atlantic suggest that oceanic crustal structure on the flanks of the mid atlantic ridge is strongly modified by tectonic extension at spreading ridge segments the seafloor along the zero age isochron is systematically more elevated and the crust thicker near the segment midpoint than at the ends suggesting focused magmatic accretion at segment centers in contrast seafloor older than 2 3 ma is usually the shallowest at inside corners of ridge offset intersections where positive residual gravity anomalies indicate tectonically thinned crust coherence between residual gravity and bathymetry for zero age crust is found at wavelength greater than 20 km which reflects the characteristic length of ridge segments and is consistent with an effective elastic plate thickness of 1 3 km no significant coherence however is observed on isochron profiles at ages of 2 5 ma and older at all wavelengths flow line profiles display coherence at wavelength greater than 5 km yielding an estimated effective elastic plate thickness of only 0 3 0 5 km synthetic fault models suggest that the flow line coherence may be controlled by the relatively close spacing of fault generated abyssal hills rather than reflecting the true elastic thickness of unfaulted lithosphere together these results indicate that the crustal structure of the zero age crust is modified strongly by tectonic extension along the rift valley walls of the slow spreading mid atlantic ridge

improved fourier terrain correction part ii fourier methods for potential fields have always been developed with the simplification that the calculation surface is a level plane the fourier approach can be extended to deal with an uneven observation surface i consider the case of terrain correction for gravity surveys in which the attraction of a variable thickness layer is calculated at points on its upper surface the main idea is to use a power series in topographic height that is then converted into a series of convolutions to avoid convergence problems a cylindrical zone around the observer must be removed from the fourier treatment and its contribution computed directly the resultant algorithm is very fast in an example based on a recent survey the new method is shown to be more than 300 times faster than a calculation based on summing contributions from a column of material under each topographic grid point

scotia sea tectonics from high resolution satellite gravity the release of altimetric data from the geosat geodetic mission by the us navy 1 is leading to a much improved understanding of tectonics in the southern ocean a region in which remoteness and adverse physical conditions have limited the acquisition of geophysical data by research ships the scotia sea is an outstanding example of back arc spreading which is revealed in some detail by free air gravity anomaly maps derived from the latest release of data acquired south of 30 s 2 sea surface height data for this region have been reduced to a grid of free air gravity anomalies and are illustrated here by means of colour shaded relief and contour maps the new data confirm the existence of a number of inactive spreading ridges within the scotia sea and surrounding small basins the amplitudes and wavelengths of gravity anomalies over these ridges conform in general to the expected relationship with spreading rate except in the central scotia sea where a proposed miocene slow spreading ridge appears to have left no clear signature the spreading ridge axis in the east scotia sea comprises seven or more segments separated by small mainly sinistral offsets and exhibits a median valley with depths of 200 1000 m that is reflected in free air lows of 10 40 mgal near both its northern and southern termini the gravity signature of the ridge becomes less distinct with a less pronounced axial low the northernmost segments of the ridge are displaced in a right lateral sense by a feature which appears to represent a southward migrating non transform offset whereas the process of spreading in drake passage and the east scotia sea was comparable to mid ocean ridges that in the central scotia sea may have been disorganized as observed in some western pacific back arc basins

phase relations for high frequency core mantle coupling and the earth s axial angular momentum budget abstract although it has long been thought that dynamical core mantle coupling is necessary for understanding decadal variations in the length of day lod the physics of the coupling mechanism are very poorly constrained there several hypotheses electromagnetic topographic gravitational viscous but it is notoriously difficult to distinguish between them on observational grounds there is both an expectation geophys j int 138 1999 679 and some evidence j geophys res 100 1995 8233 geophys res lett 24 1997a 1799 that there is significant core mantle coupling on much faster timescales down to subannual periods at such frequencies the core would no longer be the dominant driving force for lod but would instead interact with both the mantle and external reservoirs of angular momentum such as the atmosphere and ocean on these much faster timescales it is possible that some information that is obscured on decadal timescales becomes available in particular any characteristic timescales at very short periods may become observable here i examine the angular momentum budget of the earth on subannual to annual timescales for any additional implied constraints on the physics of angular momentum exchange between the core and the mantle i find that the discrepancy between atmospheric angular momentum and mantle angular momentum on timescales where the ocean is thought to be unimportant has a distinctive signature which is difficult to reproduce using simple models of core mantle coupling although an ad hoc frictional model does moderately well rather than indicating simplicity this may instead be symptomatic of great complexity in the physics of the core mantle boundary region

earth s variable rotation recent improvements in geodetic data and practical meteorology have advanced research on fluctuations in the earth9s rotation theinterpretation of these fluctuations is inextricably linked with studies of the dynamics of the earth moon system and dynamical processes in the liquid metallic core of the earth where the geomagnetic field originates other parts of the earth9s interior and the hydrosphere and atmosphere fluctuations in the length of the day occurring on decadal time scales have implications for the topography of the core mantle boundary and the electrical magnetic and other properties of the core and lower mantle investigations of more rapid fluctuations bear on meteorological studies of interannual seasonal and intraseasonal variations in the general circulation of the atmosphere and the response of the oceans to such variations

geodynamic estimates of the viscosity of the earth s inner core recent seismological studies1 2 have suggested that the inner core is rotating relative to the bulk of the earth a situation which according to numerical simulations3 may be sustained by convective flow in the liquid outer core on the other hand large gravitational forces due to the heterogeneous distribution of mass in the earth s mantle should be sufficient to keep the inner core aligned with the mantle4 here i show that the differential rotation of the inner core can be reconciled with these strong gravitational forces by allowing the shape of the inner core to adjust as it rotates so permitting an estimate of the effective viscosity of this innermost region of the earth the inferred rotation rate constrains the viscosity of the inner core to be less than 1016 pa s or greater than 1020 pa s as two different dynamical regimes are possible the viscosity estimates for these two regimes have very different implications for the origin of seismic anisotropy in the inner core

horizontal extent of anomalously thin crust near the vema fracture zone from the three dimensional analysis of gravity anomalies recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust with total crustal thickness less than half that of normal oceanic crust the horizontal extent of this anomalous crust is not well known to map the lateral distribution of thin crust in a major fracture zone we have constructed free air anomaly bathymetry and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the vema fracture zone with the mid allantic ridge we isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water crust interface the sediment layer and the crust mantle interface assuming constant crustal thickness and density these calculations are performed using a fully three dimensional model the remaining mantle bouguer anomaly is dominated by the effects of the cooling lithosphere variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three dimensional conduction of heat the attraction of this three dimensional density structure is subtracted from the mantle bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density the residual gravity anomaly map indicates that there is thin crust along much of the length of the vema fracture zone thin crust is concentrated primarily beneath the walls of the fracture zone rather than beneath the center of the fracture zone valley an interpretation confirmed by seismic refraction data the degree of thinning along the length of the fracture zone is variable ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km the excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust other major topographic features such as the median valley and the elevated transverse ridge along the south wall of the fracture zone are also not locally compensated by changes in crustal thickness

seismological evidence for differential rotation of the earth s inner core the travel times of seismic waves that traverse the earth s inner core show a small but systematic variation over the past three decades this variation is best explained by a rotation of the inner core that moves the symmetry axis of its known seismic anisotropy the inferred rotation rate is on the order of 1 per year faster than the daily rotation of the mantle and crust

a 3d gravimetric crustal model for the northeastern region of the iberian peninsula we combine different methodologies normally used separately in exploration geophysics to obtain the topographies of the moho discontinuity and the boundary between the upper and lower crusts by separating their effects in the bouguer anomaly map the sfm method was used to define the average depth of the layers causing the anomalies the resulting three dimensional model which reproduces the observed gravity anomaly was estimated by inverting the corresponding domains of the power spectrum a good correlation exists between the calculated depths and the upper and lower boundaries of the layered lower crust observed along the reflection seismic lines in the area

three dimensional gravity inversion model of the deep crustal structure of the central drake passage shackleton fracture zone and west scotia ridge antarctica 1 gravity and bathymetric data collected by the spanish r v hesperides over the shackleton fracture zone sfz and the west scotia ridge wsr were used to invert for the three dimensional 3 d structure of the deep crust data from the global gravity grid and global seafloor topography ggsft were also employed to enlarge the cruise area the merged data were analyzed to determine 3 d deep structure by numerical inversion water layer contribution to the gravity anomaly was eliminated taking into account the bathymetry spectral analysis of the reduced data yielded mean crust mantle interface cmi depths of 10 5 1 2 km inversion of the regional anomaly gave a 3 d detailed geometry of the cmi which generally agrees with the 2 d models established along profiles where gravity data and multichannel seismic lines are available the wsr shows an asymmetrical structure with a reverse fault located southeastward to the central valley this fault was developed probably as a consequence of the nw se compressive deformations which occurred following the spreading the 3 d view shows that the sw end of the spreading axis was affected more intensely by the compression the sfz represents an active sinistral transpressive fault zone of the scotia antarctica plate boundary and shows crustal thickening related to bathymetric highs the crustal thinning detected at the intersection with the inactive wsr suggests a complex interaction between these two structures

weak magnetic field changes over the pacific due to high conductance in lowermost mantle for the past few centuries the temporal variation in earth s magnetic field in the pacific region has been anomalously low the reason for this is tied to large scale flows in the liquid outer core near the core mantle boundary which are weaker under the pacific and feature a planetary scale gyre that is eccentric and broadly avoids this region however what regulates this type of flow morphology is unknown here we present results from a numerical model of the dynamics in earth s core that includes electromagnetic coupling with a non uniform conducting layer at the base of the mantle we show that when the conductance of this layer is higher under the pacific than elsewhere the larger electromagnetic drag force weakens the local core flows and deflects the flow of the planetary gyre away from the pacific the nature of the lowermost mantle conductance remains unclear but stratified core fluid trapped within topographic undulations of the core mantle boundary is a possible explanation high conductance in the lowermost mantle beneath the pacific deflects the planetary gyre which results in limited variation in the magnetic field in the region according to numerical modelling of earth s core dynamics

fluid flow near the surface of earth s outer core maps of the fluid flow at the core surface are important for a number of reasons foremost they may provide some insight into the workings of the geodynamo and may place useful constraints on geodynamo models from the flow the force balance at the top of the core can at least in part be deduced the flow can provide short term predictions of the secular variation the flow is important in understanding changes in the length of day and constraints on lateral temperature variations and topography at the core mantle boundary may be derived unlike the case of mantle convection only very small lateral variations in core density are required to drive the flow these density variations are too small by several orders of magnitude to be imaged seismically so instead we use the geomagnetic secular variation to infer the flow despite considerable recent progress in mapping the core flow substantial differences exist between maps produced by different researchers here we examine the possible underlying reasons for these differences paying particular attention to the inherent problems of nonuniqueness we focus on the aspects of the flow which do seem to be well determined and discuss their geophysical implications

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

inner core rotation rate from small scale heterogeneity and time varying travel times the time it takes seismic waves to propagate from south atlantic earthquakes through the inner core to station col in alaska has decreased systematically over the past 30 years travel times from three earthquakes in 1991 to an array of 37 seismometers in alaska suggest that lateral gradients in seismic wavespeeds are steep in this part of the inner core this combination of observations can be explained by postulating that the inner core is rotating 0 2 to 0 3 per year faster than the mantle

gravity fields of the southern ocean from geosat data in august 1990 the u s navy declassified all geodetic mission gm radar altimeter data acquired by the geosat satellite over oceanic regions south of 60 s we have used these gm data in conjunction with the unclassified lower resolution geosat exact repeat mission erm altimeter data to construct high resolution gravity fields on a 5 km grid covering the annular region of the southern ocean which lies between 60 s and 72 s and encircles antarctica during the gm a complete mapping of the marine geoid between 72 and 72 n was accomplished the gm produced more densely spaced ground tracks typically 2 or 3 km at 60 s than those of either the erm or seasat consequently we were able to use the gm data to map the marine gravity field at a higher resolution than was previously possible using satellite altimeter data this paper describes the techniques we used to derive these gravity fields and image them these techniques involve 1 computing along track sea surface height slopes 2 gridding of these ascending and descending slopes 3 converting the slopes to conventional deflections of the vertical 4 transforming the deflections to gravity anomalies in the frequency domain and 5 imaging the resulting images of the marine gravity field reveal much that is new about the seafloor and the tectonic fabric of the southern ocean a region which includes large expanses of seafloor that have never been surveyed by ships

planet within a planet rotation of the inner core of earth the time dependence of the orientation of earth9s inner core relative to the mantle was determined using a recently discovered 10 degree tilt in the axis of symmetry of the inner core9s seismic velocity anisotropy two methods of analyzing travel time variations for rays traversing the inner core on the basis of 29 years of data from the international seismological centre 1964 1992 reveal that the inner core appears to rotate about 3 degrees per year faster than the mantle an anomalous variation in inner core orientation from 1969 to 1973 coincides in time with a sudden change jerk in the geomagnetic field

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

inner core anisotropy in three dimensions the purpose of this paper is the investigation of cylindrical anisotropy in the inner core based on the travel time anomalies of the pkikp phase we use the arrival times reported in the international seismological centre bulletins for years 1964 1990 we select only earthquakes which have a good azimuthal coverage and a sufficiently large number of reporting stations the earthquakes are relocated using corrections for lateral heterogeneity computed for our most recent three dimensional mantle model we use a total of 313 422 observations of the df branch of pkp travel time anomalies within the epicentral distance ranges of 120 140 and 150 180 reported by 2335 stations for 26 377 earthquakes we process the data using an averaging procedure which we call cylindrical anisotropy stacking that enhances the effects of anisotropy but is expected to suppress those due to lateral heterogeneity and random errors the processed residuals show a remarkably consistent pattern this confirms the dominance of the cylindrical elastic anisotropy in the inner core with approximately a constant axis of symmetry this axis of symmetry is found to be tilted 10 5 1 from the earth s rotation axis in the direction 160 e 5 in the northern hemisphere in this new coordinate system we determine a four layer axisymmetric model of transverse anisotropy with each layer approximately 300 km thick the model shows that the anisotropy is strongest 3 within the innermost part of the core the travel time anomalies show significant 1 5 s longitudinal variations even when the tilt of the axis of symmetry is considered there is a substantial increase in the amplitude of the longitudinal variations which are dominated by the second and fourth harmonics for rays with bottoming depths exceeding 400 km below the inner core boundary the measurable tilt of the axis of symmetry and the presence of significant nonaxisymmetric signal may provide important clues with respect to the possible causes of anisotropy the increase in the anisotropy in the innermost part of the core departs from earlier inferences that anisotropy may be limited to the outermost 200 300 km of the earth s inner core

the springs of lake patzcuaro chemistry salt balance and implications for the water balance of the lake abstract lake patzcuaro the center of the ancient tarascan civilization located in the mexican altiplano west of the city of morelia has neither river input nor outflow the relatively constant lake salinity over the past centuries indicates the lake is in chemical steady state springs of the south shore constitute the primary visible input to the lake so influx and discharge must be via sub lacustrine ground water the authors report on the chemistry and stable isotope composition of the springs deeming them representative of ground water input the springs are dominated by ca mg and na whereas the lake is dominated by na combining these results with previously published precipitation rainfall measurements on the lake the authors calculate the chemical evolution from spring water to lake water and also calculate a salt balance of the ground water lake system comparing cl and 18o compositions in the springs and lake water indicates that 75 80 of the spring water is lost evaporatively during evolution toward lake composition during evaporation ca and mg are lost from the water by carbonate precipitation each liter of spring water discharging into the lake precipitates about 18 7 mg of caco3 salt balance calculations indicate that ground water input to the lake is 85 9 106 m3 a and ground water discharge from the lake is 23 0 106 m3 a thus the discharge is about 27 of the input with the rest balanced by evaporation a calculation of time to reach steady state ab initio indicates that the cl concentration of the present day lake would be reached in about 150 a

a comparison of geodetic and seismic estimates of inner core rotation seismological observations suggest the inner core has been rotating faster than the mantle during the past 30 50 years differential rotation can produce large gravitational torques on the mantle which if not balanced by other torques would be detectable as changes in the rotation rate of the mantle length of day or lod we compare seismological estimates of inner core rotation with estimates derived by inverting lod observations using models of gravitational coupling model predictions depend on several parameters but for the expected range of parameter values the seismological estimates of relative rotation are at least an order of magnitude larger than those predicted by the lod data furthermore the lod data predict oscillation in the angular alignment of the inner core and mantle during the past 30 50 years while the seismological data seem to require a component of steady rotation we show that steady rotation of the inner core implies a steady torque on the mantle this torque is not accelerating the rotation rate of the mantle so it must be opposed by another torque the required torque is consistent with a frictional electromagnetic stress on the base of the mantle due to a westward flow at the top of the core

tidal variations of earth rotation we have rederived the periodic variations of the earth s rotation due to the tidal deformation of the earth by the sun and moon and included all terms with amplitudes 0 002 milliseconds 1 mm this series applies to the mantle plus crust and oceans which rotate together for characteristic tidal periods the parameter which scales the rotational series is k c where k is that fraction of the love number which causes the tidal variation in the moment of inertia of the coupled mantle and oceans while c is the dimensionless polar moment of inertia of the coupled units if the whole earth minus oceans were coupled rotationally then k 0 30 and c 0 33 ocean tides increase k by 0 04 decoupling of the fluid core from the mantle decreases k by 0 06 while c cmantle 0 29 since neither pressure viscous nor hydromagnetic coupling locks the fluid to the mantle for periods less than 5 to 25 years from the analysis of lunar laser ranging data we find that k c at monthly and fortnightly frequencies equals 0 99 0 15 and 0 99 0 20 as compared with a theoretical value of 0 94 0 04 in addition we have estimated theoretically the effects of ocean tides on earth rotation nutation and polar motion using models based on the laplace tidal equations

inner core rotation a test at the worldwide scale abstract in order to determine the angular velocity of a possible inner core differential rotation with respect to the mantle we analyse travel times of waves sampling the inner core at the worldwide scale over a period of about 30 years for situations in which the inner core heterogeneities are dominant with respect to the mantle heterogeneities travel times of summary rays i e bunches of seismic rays with the same geometry must have smaller mean standard deviations when computed in the inner core reference frame than when computed in the mantle reference frame thus we build summary rays by applying a backward rotation of the inner core at angular velocity in order to sum residuals which sample the same inner core region at different times the mean standard deviation of the summary ray residuals computed for different values of should exhibit a minimum for the effective value this method has been applied to absolute pkp df residuals in considering only polar paths for which inner core heterogeneities are dominant with respect to mantle heterogeneities the data used come from a high quality file derived by engdahl et al engdahl e r van der hilst r buland r 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seismol soc am 88 722 743 from the catalogs of the international seismological centre isc for the period 1964 1995 the minimum standard deviation is obtained for the null rotation rate a similar analysis on the absolute pkp bc residuals which correspond to paths that do not sample the inner core shows that mantle heterogeneities are not at the origin of the observed minimum the same method has also been applied to a set of more than 1200 worldwide distributed pkp bc pkp df differential travel time residuals hand picked by ourselves or gathered by different authors for specific core studies here again the minimum standard deviation is obtained for the null rotation implying a synchronous rotation of the inner core with the mantle however the limited number of very old data and the geographic distribution of the paths prevent us from obtaining a resolution better than 1 year in particular a small super rotation with 1 year cannot be ruled out with the presently available data the problem of the level of the inner core heterogeneities compared to that of the mantle which is another possible limitation of the method is also discussed

what are the limitations of satellite altimetry radar altimeter measurements of the marine geoid collected during the seasat altimeter mission gave geophysicists hope of uncovering the gravity field over all the ocean basins however because of insufficient track density it has taken 16 years for the full potential of the satellite altimeter to be realized the high density coverage obtained by ers 1 during its geodetic mapping phase april 1994 march 1995 prompted the u s navy to declassify all of the geosat altimeter data in june 1995 the combination of these two high density data sets provided the first global view of all the ocean basins at a wavelength resolution of 20 30 km

the existence of an inner core super rotation questioned by teleseismic doublets abstract the first evidence for a differential rotation of the earth s inner core with respect to the mantle comes from the detection of a variation over 30 years of the differential travel time anomalies of seismic core phases song x richards p g 1996 seismological evidence for differential rotation of the earth s inner core nature 382 221 224 the comparison of teleseismic doublets provides a powerful means to measure such differential travel times and to test their temporal variation contrary to direct measurements of differential travel times this technique does not require to have an accurate location of the seismic events this method is first checked on examples of doublets for which no differential travel time is expected these tests allow us to point out the difficulties such as polarity reversals which may be encountered in the use of the doublet method it is then applied to deep tonga events recorded in france and to the south sandwich islands events recorded at station college col alaska which led song and richards to propose an inner core rotation for the first path we find no time variation in agreement with previous studies for the path from south sandwich island to col the doublet analysis shows that no significant temporal change in pkp travel time can be detected within the resolution of the method which is about 0 05 s the variation of the residuals observed by song and richards about 0 3 s over 30 years is in a large extent ascribable to hypocenter mislocations consequently present seismological observations do not detect the inner core differential rotation because if it exists it is smaller than about 0 2 year the present detection capacity of seismology

deep crustal structure of the area of intersection between the shackleton fracture zone and the west scotia ridge drake passage antarctica abstract the shackleton fracture zone which forms the boundary between the antarctic and scotia plates in the drake passage is characterized by a present day left lateral motion the west scotia ridge an extinct spreading centre formed the oceanic crust of the western scotia plate and intersects the shackleton fracture zone in a complex deformed area multichannel seismic gravity magnetic and multibeam swath bathymetry data were acquired during the antpac 97 98 cruise with the spanish vessel b o hesperides in the area of intersection of these two tectonic features the new data reveal its asymmetrical deep crustal structure which developed as a consequence of the overprinting of extensional and contractional deformation events the main seismic features of the crust of the scotia and antarctic plates are deep dipping braided reflectors which may be a consequence of an initial stretching deformation related to the shackleton fracture zone at present this fracture zone is characterized by thickened oceanic crust deformed by reverse and transcurrent faults and locally bounded by areas of crustal thinning the present morphology of the west scotia ridge has the characteristics of slow spreading centres with a central valley bounded by two elongated highs however its structure reveals that after spreading ended there was a nw se contractional deformation event with a thrust of about 40 km of slip that resulted in asymmetrical crustal thickening incipient subduction initiates subparallel to the strike of the spreading centre this tectonic event may be related to an episode of closure of south america and the antarctic peninsula probably of pliocene age we conclude that in oceanic domains areas with a weak crust fracture zones and spreading centres constitute the most sensitive regions for analysing the regional tectonic evolution since deformation events are better recorded there than in normal oceanic crust

new seismological constraints on differential rotation of the inner core from novaya zemlya events recorded at drv antarctica summary novaya zemlya nuclear test records at the seismic station drv antarctica are analysed in order to obtain further constraints on a possible diverential rotation of the inner core with respect to the mantle these data allow the sampling of the inner core along a nearly polar path in very stable conditions over more than two decades from 1966 to 1990 the pkp bc pkp df traveltime residuals which reflect the innercore anisotropy and or heterogeneities sampled along the path exhibit a great stability through time a computation of the residuals that are expected for various diverential rotation rates and the same rotation axis as the mantle has been performed using the worldwide residual catalogue of engdahl et al 1997 for summary rays that include the time as an additional parameter in data stacking comparison of data and predictions shows that an eastward diverential rotation with a rate as large as 3 yr 1 as suggested by some authors is not possible but an eastward rotation at 1 yr 1 or lower cannot be rejected