on geoid heights and flexure of the lithosphere at seamounts the sea surface height has now been mapped to an accuracy of better than 1 m by using radar altimeters on board orbiting satellites the major influence on the mean sea surface height is the marine geoid which is an equipotential surface we have carried out preliminary studies of how oceanic volcanoes which rise above the ocean floor as isolated seamounts and oceanic islands or linear ridges contribute to the marine geoid simple one and two dimensional models have been constructed in which it is assumed that the oceanic lithosphere responds to volcanic loads as a thin elastic plate overlying a weak fluid substratum previous studies based on gravity and bathymetry data and uplift subsidence patterns show that the effective flexural rigidity of oceanic lithosphere and the equivalent elastic thickness te increase with the age of the lithosphere at the time of loading the models predict that isolated seamounts emplaced on relatively young lithosphere on or near a mid ocean ridge crest will be associated with relatively low amplitude geoid anomalies about 0 4 0 5 m km of height while seamounts formed on relatively old lithosphere on ridge flanks will be associated with much higher amplitude anomalies 1 4 1 5 m km studies of the seasat altimetric geoid prepared by nasa s jet propulsion laboratory support these model predictions geoid amplitudes are relatively low over the mid pacific mountains and line islands which formed on or near a mid ocean ridge crest and relatively high over the magellan seamounts and wake guyots which formed off ridge direct modeling of the altimetric geoid over these features is complicated however by the wide spacing of the satellite tracks which can exceed 100 km and poor bathymetric control beneath individual satellite tracks in regions where multibeam bathymetric surveys are available models can be constructed that fit the altimetric geoid to better than 1 m studies of geoid anomalies over the emperor seamount chain for example suggest that these seamounts formed on 20 30 ma old oceanic lithosphere while anomalies over the louisville ridge and the hawaiian ridge suggest that these seamounts formed on lithosphere older than about 80 ma this indicates that geoid anomalies over bathymetrie features depend not only on the crustal structure prior to loading and the overall shape and density of the features but also on their tectonic setting we examine the implications of these results to studies which attempt 1 to separate temporal variations of the sea surface due to oceanographic effects 2 to predict seamounts directly from satellite derived gravity and geoid data and 3 to isolate the geoid effects of deep processes occurring beneath the lithosphere such as those due to mantle convection

a model for the plate tectonic evolution of the east central pacific based on seasat investigations abstract a complete seasat dataset is used to investigate the small scale bathymetry of a 15 by 15 area of the east central pacific northeast of pitcairn island which was involved in the miocene ridge jump and reorientation two fracture zones fz1 and fz2 are mapped along 450 km at an azimuth of n95 e while the signature of the austral fracture zone disappears east of 135 w several seamounts are discovered or confirmed along the oeno henderson ducie lineament and the degree of isostatic compensation of a number of islands at the eastern end of the tuamotu group is studied islands of the northern branch have practically no signatures in the geoid while the southern ones must have formed on lithosphere at least 10 ma old on the basis of these constraints we present a model of the evolution of spreading since 40 ma b p in which the deactivation of the austral fracture zone and the transfer of its offset to fz1 in the late oligocene is interpreted as due to rift propagation triggered by a northern tuamotu hotspot we propose that the oeno ducie chain is the surface expression of a southern tuamotu hotspot deviated 15 in azimuth by the presence of a young and fresh fracture zone thus our model predicts that hotspots and ridge systems can have a moderate influence on each other s kinematics and surface expressions which does not however extend to permanent trapping

the seasat altimeter data and its accuracy assessment the seasat satellite launched on june 27 1978 carried a radar altimeter designed to measure the altitude of the satellite above the ocean surface the surface wave height and the ocean surface backscatter coefficient from which wind speed can be inferred postlaunch engineering assessment indicated that the seasat altimeter met the performance specification for 10 cm precision noise in the altitude measurements however to determine the accuracy of this measurement as well as the accuracy of the wave height and wind speed measurements a significant calibration validation and model development effort was required this summary describes the instrument atmospheric and geophysical effects which influence the radar altimeter measurement accuracies and the attendant correction models adopted for the altimeter geophysical data record also summarized are the activities of the seasat altimeter precision orbit determination experiment team directed towards the validation and improvement of these models as well as the investigations required to assess the accuracy of the altimeter measurement and the computed satellite altitude ephemeris finally an accuracy assessment is made for the various altimeter measurement corrections which are included on the altimeter geophysical data record

an analysis of the altimetric geoid in various wavebands in the central pacific ocean constraints on the origin of intraplate features abstract seasat profiles were filtered in three different wavebands and interpolated to elaborate maps of the very short 25 110 km and short 80 320 km and 280 820 km wavelengths of the geoid anomalies for the south pacific for the very short wavelengths we mapped the geoid roughness i e the variations of amplitude of the anomalies two lines following present day plate motions and separated by 800 1000 km were identified the first is formed by the tuamotu archipelago the pitcairn crough alignment and the easter sala y gomez chain the second comprises the cook austral islands a seamount cluster mapped close to the east pacific rise south of 30 s and the chile rise alignments with a n150 e direction and corresponding to traces of old thermal activity were mapped on lithosphere older than 40 m y these features are also spaced of 800 1000 km the map for the 80 320 km waveband shows a pattern of 200 km wavelength undulations several trends are superimposed but the n95 e direction is dominant in the area we associate these geoid undulations to lithospheric phenomena such as fracturations and or small magmatic ridges the 280 820 km waveband reveals linear anomalies roughly following the direction of present day absolute plate motions the linear anomalies have a 400 500 km wavelength and are nearly continuous between the pacific and the nazca plates the topography is located on the highs but the anomalies generally precede the active ends of the volcanic chains this result favours the hot line hypothesis previously suggested by several authors we associate these bands to thermal heterogeneities probably due to a convective regime since the main topographic features shall be emplaced over the ascending limbs of two neighbouring cells the 800 1000 km spacing of the topography imposes a minimum size of 400 500 km for the cells so the convective cells cannot be limited to a thin layer beneath the lithosphere but will affect part of the upper asthenosphere the existence of hot lines aligned parallel to plate motions can explain the intermittent magmatic activity observed for the volcanic lineaments of the south pacific volcanic activity can be the result of the interaction between the hot lines and lithospheric zones of weakness and or of instabilities in the convective cells the regular spacing observed for topography formed before and after 40 m y suggests that the size of the cells is rather constant through time

three dimensional deformation analysis global vector spherical harmonic and local finite element representation abstract deformation elements which have global support such as tidal induced displacements are represented in terms of vector spherical harmonics in contrast local strain and local rotation are represented in terms of irregularly shaped three dimensional finite elements these geodetic finite elements reflect the structure of a geodetic network which is arbitrarily shaped various continuity classes are discussed numerical examples are presented

compensation of swells and plateaus in the north pacific no direct evidence for mantle convection at intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation a low geoid topography ratio 8 m km in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection after subtracting a reference geoid from the observed geoid previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000 4000 km since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid topography ratios 10 20 m km they were believed to reflect the planform of mantle convection we show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10 since the geoid spectrum is red the residual geoid is dominated by degree 11 when the harmonics of the reference geoid are rolled off gradually the regular pattern of geoid highs and lows disappears in the northeast pacific the new residual geoid reflects the lithosphere age offsets across the major fracture zones in the northwest pacific the residual geoid corresponds to isostatically compensated swells and plateaus we have calculated the geoid topography ratio for 10 swells and plateaus and have found a range of compensation depths the highest geoidaopography ratio of 5 5 m km occurs on the flanks of the hawaiian swell intermediate ratios occur in four areas including the midway swell these intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of airy compensated seamounts low geoid topography ratios occur over the remaining five areas e g emperor seamounts reflecting the absence of a thermal swell our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid we see only indirect evidence of thermal plumes reheating the lower lithosphere

on the strain analysis of tectonic movements using fault crossing geodetic surveys abstract a generalized approach to the analysis of deformation surveys has been developed by the authors and utilized in the analysis of tectonic movements the approach is applicable to any type of repeated geometrical measurements geodetic and non geodetic surveys any type of deformations including rigid body displacements and strain and geometrical configuration of the observation network the approach is based on the least squares fitting of selected deformation models to the displacement field obtained from repeated observations of deformations the approach consists of three basic processes 1 1 preliminary identification of the deformation models 2 2 estimation of the deformation parameters using a generalized mathematical model for the least squares fitting 3 3 diagnostic checking of the deformation models and the final selection of the best model based on global statistical tests and on calculated significance levels of the deformation parameters a numerical example is given using survey data from four epochs of observations of a small geodetic network which was established across an active fault in the peruvian andes

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

the determination of displacement fields from geodetic data along a strike slip fault observations of angles or distances between stations of a geodetic network are commonly used to infer information about the movement of the surface of the earth the absence of any observations external to the network leads to an ambiguous displacement field existing techniques for eliminating this ambiguity are all unsatisfactory in some respect the best technique an inner coordinate solution is not appropriate for networks located in a strike slip fault environment the inner coordinate solution zeros the rotation of all stations about their center of mass along a strike slip fault like the san andreas however motion normal to the fault is less likely than motion parallel to the fault the solution presented here an outer coordinate solution finds the rotation of the network that minimizes the components of displacement normal to the fault since motion along a strike slip fault is generally expected to be parallel to the fault the displacements obtained with the outer coordinate solution are more reasonable than those obtained with other techniques examination of a trilateration network near san francisco bay demonstrates the large effect that the choice of adjustment technique can have on the inferred relative motion of the two sides of the fault the inner coordinate solution gave a rate of about 1 mm yr whereas the preferred outer coordinate solution rate was 36 mm yr

a comparison of surface fitting algorithms for geophysical data many treatments in geophysics require regular grids of data since the data are generally recorded irregularly e g gravity measurements along roads or even on tracks e g satellite measurements it is necessary to grid the observed data we present the result of a comparison of various surface fitting algorithms carried out in order to check their reliability two different types of sampling have been verified i clouds of points and ii points along tracks five algorithms have been extensively tested 1 polynomial fit 2 algorithm using a combination of spline laplace 3 krigeage 4 least squares fitting method and 5 finite element method the suitability of each method for different sets of data and the limitations in terms of amplitude and gradient are discussed

grim gravity model improvement using lageos grim3 l1 improved modeling of the earth s geopotential is one of the major tasks of satellite geodesy the availability of a new set of precise lageos satellite laser ranging slr data as well as seasat acquired observations enabled us to compute an improved solution in the grim model series the new geopotential grim3 l1 recombined the satellite perturbation normals of the former grim3 solution with new normal equation subsystems derived from an additional 16 months of lageos slr data oceanic 1 1 mean free air anomalies recovered from seasat altimetry an updated set of terrestrial 1 1 mean free air anomalies and several condition equations for zonal and resonant harmonics the combined normal equations were solved for a complete set of spherical harmonics up to degree and order 36 and the coordinates of 109 tracking stations comparisons with altimetry derived quantities terrestrial gravity anomalies from satellite to satellite tracking longitude accelerations of 24 hour satellites and satellite orbit fits show a substantial improvement over previous grim solutions and indicate that the lageos data have contributed considerably to the reliable estimation of the long wavelength portion of the gravity field

are earthquakes predictable summary the answer to the above question depends on the definition of earthquake prediction we discuss several definitions and possible classifications of earthquake prediction methods we also consider various measures of prediction efficiency review several recent examples of earthquake prediction and describe the methods that can be used to verify prediction schemes we conclude that an empirical search for earthquake precursors that forecast the size of an impending earthquake has been fruitless despite considerable effort in several countries no statistically rigorous validation of proposed precursory phenomena is available therefore reported cases of precursors can be explained by random noise or by chance coincidence we present evidence that earthquakes are non linear chaotic scale invariant phenomena the most probable consequence of earthquake self similarity is a lack of earthquake predictability as popularly defined that is a forecast of a specific individual earthquake many small earthquakes occur throughout any seismic zone demonstrating that the critical conditions for earthquake nucleation are satisfied almost everywhere apparently any small shock can grow into a large event thus it is likely that an earthquake has no preparatory stage this sceptical view of current earthquake prediction efforts should not be interpreted as a statement that any further attempts to mitigate the destructive effects of earthquakes are futile the seismic moment conservation principle when combined with geodetic deformation data offers a new way to evaluate the seismic hazard not only for tectonic plate boundaries but also for areas of low seismicity that is the interiors of continents earthquake clustering with a power law temporal decay omori s law can be used to estimate the rate of future earthquake ocurrence realtime seismology can facilitate relief efforts after large earthquakes and eventually provide an immediate warning of severe shaking a few seconds or tens of seconds before the shaking starts

invariant geodynamical information in geometric geodetic measurements summary repeated geodetic measurements have been used to extract geodynamical quantities such as displacements velocities of movement and crustal strains historical geodetic networks especially those established before the space geodetic era were and still are very important in providing a unique insight into the local or regional historical deformation state of the earth for the geodetic network without a tie to an external reference frame free network adjustment methods have been widely applied to derive geodynamical quantities currently it is commonly accepted that absolute displacements cannot be uniquely determined from triangulation trilateration measurements but relative displacements can be found uniquely if the geodetic network is geometrically overdetermined see e g segall matthews 1988 strain tensors were derived using the coordinate method and were reported to be uniquely determined we have carried out a theoretical analysis of invariant geodynamical information in geometric geodetic observations and concluded 1 that relative displacements are not invariant quantities and thus cannot be uniquely determined from the geodetic network without a tie to an external reference frame and 2 the components of the strain tensors are not all invariant and thus cannot individually be determined uniquely from the network however certain combinations of strain components are indeed invariant and can be uniquely determined from geometric geodetic measurements the theory of invariant information is then applied to the analysis of the tokai first order triangulation trilateration network spanning an interval of more than 100 yr the results show that the normal and principal strains are significantly affected by the unknown scaling biases and orientation differences thus any attempt at geophysical interpretation of these quantities must be exercised with great care if the scaling bias and the orientation difference are small the shear strain is shown to be practically invariant the invariant analysis of the tokai network has also shown that a belt near shizuoka has been significantly deformed in the past 100 yr this has not been seen in any previous analysis of displacements and or strains

statistical aspects of parkfield earthquake sequence and parkfield prediction experiment abstract this note discusses three interconnected statistical problems concerning the parkfield sequence of moderate earthquakes and the parkfield prediction experiment a is it possible that the quasi periodic parkfield sequence of characteristic earthquakes is no uncommon specific phenomenon the research hypothesis but can be explained by a preferential selection from available earthquake catalogs to this end we formulate the null hypothesis earthquakes occur according to the poisson process in time and their size follows the gutenberg richter relation we test whether the null hypothesis can be rejected as an explanation for the parkfield sequence b if the null hypothesis cannot be refuted what is the probability of magnitude m 6 earthquake occurrence in the parkfield region c the direct goal of the parkfield experiment is the registration of precursory phenomena prior to a m6 earthquake however in the absence of the characteristic earthquake can the experiment resolve which of the two competing hypotheses is true in a reasonable time statistical analysis is hindered by an insufficiently rigorous definition of the research model and inadequate or ambiguous data however we show that the null hypothesis cannot be decisively rejected the quasi periodic pattern of intermediate size earthquakes in the parkfield area is a statistical event likely to occur by chance if it has been preferentially selected from available earthquake catalogs the observed magnitude frequency curves for small and intermediate earthquakes in the parkfield area agree with the theoretical distribution computed on the basis of a modified gutenberg richter law gamma distribution using deformation rates for the san andreas fault we show that the size distribution of the parkfield characteristic earthquakes can also be attributed to selection bias according to the null hypothesis the yearly probability of a m 6 earthquake originating in the parkfield area is less than 1 signifying that several more decades of observation may be needed before the expected event occurs by its design the parkfield experiment cannot be expected to yield statistically significant conclusions on the validity of the research hypothesis for many decades

the fixed hotspot hypothesis and origin of the easter sala y gomez nazca trace reconstructions of the pacific and nazca plates suggest that both the nazca and tuamotu ridges originated from a melting anomaly which existed beneath the pacific farallon ridge during the time interval between anomalies 19 and 11 as would be predicted from the fixed hotspot hypothesis the hotspot hypothesis interpreted according to morphology of the easter sala y gomez nazca trace would further suggest that the hotspot was located entirely beneath the nazca plate during the time interval between anomalies 11 and 2 at the end of this interval the eastern boundary of the easter plate formed centered above the hotspot according to this scheme the present location of the hotspot is about 200 km west of easter island however the easter model is incompatible with the hotspot model for the hawiian emperior chain if the hotspots are fixed relative to one another as the age of the point on the tuamotu ridge which corresponds with the hawaiian emperor bend 53 to 55 m y is significantly greater than the age of the bend 43 m y the fixed hostpot hypothesis can be satisfied by the two traces only if one or a combination of the following conditions is met the kinematic age of the hawaiian emperor bend is substantially greater than the isotopically dated age the magnetic anomaly time scale is in significant error and or magnetic anomaly identifications in the vicinity of the tuamotu ridge are erroneous it might appear that other mechanisms for the origin of hotspot traces are preferable to the hotspot hypothesis the most promising of these involves propagating fractures resulting from intraplate stress this hypothesis has the advantage of accommodating anomalies in some hotspot traces such as persistence of igneous activity long after passage of the trace over the hotspot

earthquake prediction a critical review summary earthquake prediction research has been conducted for over 100 years with no obvious successes claims of breakthroughs have failed to withstand scrutiny extensive searches have failed to find reliable precursors theoretical work suggests that faulting is a non linear process which is highly sensitive to unmeasurably fine details of the state of the earth in a large volume not just in the immediate vicinity of the hypocentre any small earthquake thus has some probability of cascading into a large event reliable issuing of alarms of imminent large earthquakes appears to be effectively impossible key word earthquake prediction

estimability analysis of geodetic astrometric and geodynamical quantities in very long baseline interferometry summary within the framework of newtonian kinematics vlbi observations are analysed with respect to estimability of geodetic and astrometric quantities an earth model of either rigid or deformable type is designed instrumental clock offsets and clock drifts are included observational patterns are studied in all detail reviewed in seven tables appendix a is an

a comparison of computation methods for crustal strains from geodetic measurements abstract repeated geodetic measurements are widely used for the determination of crustal strain rates in this paper two computational methods for strain analysis are reviewed the conventional analysis using observation differences and an approach using the inner coordinates of a geodetic network the mathematical development of both methods is presented the case of two dimensional homogeneous strain is considered for observations taken at two different epochs from computer simulation studies the attainable accuracies in estimating the strain rates are compared for each computational method taking the precision of the geodetic observations into account results point to the fact that the two methods are equivalent under certain conditions but there are significant advantages gained in using the inner coordinate approach the main advantage is that non repeated observations can be utilized in the inner coordinate adjustment which contributes additional information about the strain components

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

the elastic thickness of the lithosphere in the pacific ocean abstract in this study we present determinations of the effective elastic thickness t e of the oceanic lithosphere along pacific chains or archipelagoes t e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes and constrained by geoid heights provided by seasat in the south central pacific estimates of 14 km for the marquesas and 6 km or less for the pitcairn mururoa gloucester chain are in good agreement with a previous work in this region cook austral and society chains around the line islands chain seasat data reveal that the bathymetry is poorly known preventing fine analysis meanwhile t e looks globally very low 6 km except for three volcanoes but these results may be unreliable the easter chain features low t e values 6 km with no noticeable variation along the chain higher values are found for a samoan island manuae 24 km and along the hawaiian emperor seamounts chain from 32 km at the eastern end of the chain to 21 5 km for the hawaiian volcanoes and from 25 5 to 15 km for the emperor seamounts the large number of t e estimates obtained in this study points out a noticeable difference between north and south pacific results those from the north pacific agree with the general trend increase with the square root of age plate at loading time while those from the south central pacific are much lower according to their plate age these low t e results from the south pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model therefore these results then point out a regional difference in oceanic lithosphere

a directional analysis of the small wavelength geoid in the pacific ocean the geoid over the pacific ocean has been filtered in order to enhance wavelengths of the order of 200 km as noticed by previous authors lineations elongated in the direction of present plate motion are observed in the east central pacific elongated features as clearly visible with a larger amplitude and a totally different direction are also present south of the hawaiian chain and west of baja california the dominant directions visible on the filtered geoid map have been confirmed by spectral analyses near the californian coast the ne sw lineations seem to be related to extensional tectonics south of hawaii the nw se directions correspond to the various directions of the pacific plate prior to 40 ma magmas implaced in the lithosphere over fixed convective plumes may be the source of these geoid lineations

seasat altimetry and the south atlantic geoid 2 short wavelength undulations this paper is a discussion of the short wavelength 25 100 km undulations of the geoid recovered from seasat altimeter data in the south atlantic these undulations are known to be highly correlated to seafloor topography and they considerably improve our knowledge about global bathymetry of the south atlantic which remains poorly surveyed the orientation of fracture zones fz with respect to the seasat profiles ensures an efficient recovery of the fz pattern in the younger 65 ma parts of the south atlantic between 20 n and 50 s we used 22 well marked fzs regularly distributed between 12 n and 47 s to compute eulerian poles describing the finite relative movement between africa and south america from the present back to 35 ma fz trends south of the equator are well accounted for with a single rotation and standard statistical tests do not indicate a need for distinct and successive rotations however these tests indicate an improvement of the fit when distinct poles are adjusted to separate subsets of our 22 fzs either fzs north and south of the equator or fzs north and south of tristan da cunha this points out the possibility of the existence of either corridors in which slight relative movements occur or diffuse elastic deformation over the entire south atlantic although this is really a second order effect we also present and discuss a map of the roughness of the geoid for the 25 to 110 km wave band strong disagreements between the roughness map and the gebco maps exist in regions of sparse ship track coverage for example the rio grande rise has an associated roughness feature which appears elongated and more symmetric with respect to the walvis ridge than what is displayed on the gebco maps the roughness map also clearly documents a long wavelength 400 km segmentation of the mid atlantic ridge between the equator and 35 s

vertical crustal movement in the boso peninsula south kanto japan as deduced from the adjustment of a geodetic network with signals abstract in the adjustment of a leveling network that is frequently resurveyed we introduce corrections not only to the approximated heights of the stations but also to their rates of vertical movement which are linear over time as an unknown thus we can apply least squares collocation to the problem and have some cases corresponding to the rank deficiency of the design matrix in the observation equation we have applied this method to the study of the vertical crustal deformation in the kanto district particularly to the boso peninsula honshu japan rates of vertical crustal movement and their covariance matrices are deduced for two periods for the kanto district 1 1965 1978 and 2 1978 1989 and for three periods for the boso peninsula 1 1965 1972 2 1973 1976 and 3 1976 1988 the deduced rates range from 10 mm yr to 10 mm yr and their standard deviations amount to 11 mm yr for the kanto district for the boso peninsula they range from 22 mm yr to 6 mm yr and amount to 5 mm yr this peninsula is very close to the convergent plate boundary between the philippine sea plate and south kanto and the results of this analysis reveal the typical fluctuating behavior of the crustal movements associated with the subduction process during the interseismic period the author interprets this episodic crustal movement as being due to intermittent creep dislocation along the deeper part of the interface between the kanto district and the philippine sea plate

possible perturbations to hotspot traces and implications for the origin and structure of the line islands perturbations to hotspot signals recorded on the surface of the earth will produce hotspot traces with characteristics more complex than those of simple time progressive volcanic chains some of these perturbations are a consequence of normal plate tectonics some are observed interactions of hotspot magmatism with lithospheric structure and processes and others are hypothetical the line islands volcanic chain has a complex structure and some characteristics that cannot be reconciled with those expected of a simple time progressive hotspot trace two models for the development of the line islands are presented model 1 assumes that the line islands were formed between 130 and 67 ma and model 2 assumes an age span of 90 to about 15 ma and incorporates the tuamotu islands as an extension of the line islands in model 1 most of the complex structure and subsidence of the chain can be explained in terms of perturbations to the hotspot signal model 2 gives a good geometric fit to the volcanic chain but fails to explain the anomalous subsidence of the line islands

shallow velocity structure along the hirapur mandla profile using traveltime inversion of wide angle seismic data and its tectonic implications summary in order to investigate the velocity structure and hence shed light on the related tectonics across the narmada son lineament traveltimes of wide angle seismic data along the 240 km long hirapur mandla profile in central india have been inverted a blocky laterally heterogeneous three layer velocity model down to a depth of 10 km has been derived the first layer shows a maximum thickness of the upper vindhyans 4 5 km s1 of about 1 35 km and rests on top of normal crystalline basement represented by the 5 9 km s1 velocity layer the anomalous feature of the study is the absence of normal granitic basement in the great vindhyan graben where lower vindhyan sediments 5 3 km s1 were deposited during the precambrian on high velocity 6 3 km s1 metamorphic rock the block beneath the narmada son lineament represents a horst feature in which high velocity 6 5 km s1 lower crustal material has risen to a depth of less than 2 km south of the lineament the deccan traps were deposited on normal basement during the upper cretaceous period and attained a maximum thickness of about 800 m

geodetic strain measurements in washington two new geodetic measurements of strain accumulation in the state of washington for the interval 1972 1979 are reported near seattle the average principal strain rates are 0 07 0 03 strain yr n 19 w and 0 13 0 02 strain yr n71 e and near richland south central washington the average principal strain rates are 0 02 0 01 strain yr n36 w and 0 04 0 01 strain yr n54 e extension is taken as positive and the uncertainties quoted are standard deviations a measurement of shear strain accumulation dilatation not determined in the epoch 1914 1966 along the north coast of vancouver island by the geodetic survey of canada indicates a marginally significant accumulation of right lateral shear 0 06 0 03 rad yr across the plate boundary n40 w strike although there are significant differences in detail these strain measurements are roughly consistent with a crude dislocation model that represents subduction of the juan de fuca plate the observed accumulation of strain implies that large shallow thrust earthquakes should be expected off the coast of washington and british columbia however this conclusion is not easily reconciled with either observations of elevation change along the washington coast or the focal mechanism solutions for shallow earthquakes in washington

159 ma kjakebeinet lamproites dronning maud land antarctica and their implications for gondwana breakup processes lamproite dykes that cross cut middle jurassic karoo related flood basalts in vestfjella dronning maud land antarctica have been dated at 158 7 1 6 ma by 40ar 39ar incremental heating of phlogopite compared to most lamproites these dykes are unusually low in sio2 40 09 40 18 wt and high in cao 12 35 16 24 wt and p2o5 3 03 3 81 wt they have broad affinities to spatially and temporally related group ii kimberlites of southern africa but the geochemically closest correlatives are cao and p2o5 rich diopside madupitic lamproites from leucite hills usa the kjakebeinet lamproites are notably high in incompatible elements show negative initial end values 6 0 6 7 and positive initial esr values 14 17 with tdm model ages of c 1 1 ga and probably record melting of metasomatized subcontinental lithospheric mantle beneath western dronning maud land their emplacement records the youngest recognized magmatic event in the region coincides with the proposed second and final stage of gondwana breakup between africa and antarctica and may be associated with deep seated coast parallel strike slip faulting in dronning maud land survival of old easily fusible lithospheric mantle material 20 ma after the eruption of voluminous flood basalt magmas suggests that the latter were emplaced within narrow zones of lithospheric thinning and that the source of lamproites remained largely intact between these zones

crustal deformation across and beyond the los angeles basin from geodetic measurements we combine 6 years of global positioning system gps data with 20 years of trilateration data and a century of triangulation taped distance and astronomic azimuth measurements to derive 66 interseismic station velocities in the greater los angeles region we interpolate the velocities to construct a regional strain rate map beyond the los angeles basin our results generally agree with the model proposed by the working group on california earthquake probabilities in 1995 important regional findings of this study are as follows 1 there is a significant n s convergence and e w extension about 0 22 and 0 17 0 05 strain yr respectively for the two components along the southern frontal fault system of the san gabriel mountains 2 the crustal deformation around the big bend of the san andreas fault saf cannot be explained solely by wrench style motion across the saf remaining motion could be part of a nw se extension which is the response to nfsw compression in the central transverse ranges region alternatively it could be caused by left lateral faulting on an oblique blind thrust beneath the san gabriel mountains 3 low strain rates axe found along the elsinore fault and newport inglewood 4 north south compression decreases from the raymond hill fault westward to the santa monica fault there is little east west extension along this fault system

finite element analysis of strain patterns from geodetic observations across a plate margin abstract repeated trilateration measurements of local or regional networks are frequently the basis of geodetic strain analyses as a rule the strain field is considered homogeneous for the whole area under investigation or at least for parts of it however the treatment of strain as homogeneous over a large area means averaging strains and is often not desired to get an insight into the local variety of strain patterns the area under investigation can be dissected into finite elements i e triangles the individual analyses of these elements lead to meaningful results which can serve as a basis for refined models of interpretation emphasis is put on a strict analysis considering all correlations rigorously this concerns 1 1 point displacements and their variance covariance matrix as derived from the original geodetic observations through network adjustments 2 2 distortions of observables i e distances and angles as derived from the point displacements and their variance covariance matrix 3 3 global regional and local strain parameters and their variance covariance matrix as derived from point displacements or from distorted observables the method of strain analysis using finite elements is illustrated by a network spanning the san andreas and calaveras faults near hollister california

comparison of geos 3 and seasat altimeter resolution capabilities a comparison is made between geos 3 and seasat altimeter data with regard to noise power spectral density psd derived geoid psd and short wavelength resolution at wavelengths greater than 450 km the noise psds for geos 3 and seasat are essentially the same and are attributed to mesoscale oceanographic effects at shorter wavelengths the noise spectra diverge because of the higher geos 3 instrument noise the average along track geoid psds are nearly identical between wavelengths of 4500 and 72 km the spectral coherence between repeat tracks shows that the short wavelength resolution of seasat data is 32 km and for geos 3 it is 60 km

tectonics and evolution of the juan fernandez microplate at the pacific nazca antarctic triple junction in this study magnetic and bathymetric data from an r v endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the juan fernandez microplate at the pacific nazca antarctic triple junction magnetic lineations along the east ridge of the microplate fan to the south indicating that the nazca juan fernandez pole is near the northern end of this feature relative motions of the juan fernandez pacific nazca antarctic four plate system are determined using relative plate motion inversions the resulting tectonic model predicts that the northern boundary of the juan fernandez microplate is a zone of compression and that the west ridge and southwestern boundary are spreading obliquely the southeastern boundary is predicted to be obliquely divergent but present data fail to constrain its geometry we also present a schematic evolutionary model which relates the formation of the juan fernandez microplate to differential spreading rates at the triple junction and discuss related complexities the future evolution of the fourplate system depends on whether one or both ridges remain active

horizontal crustal movements in the kanto tokai district japan as deduced from geodetic data abstract the kanto and tokai district japan is located very close to the northern convergent boundary of the philippine sea plate and has been attacked by great earthquakes several times during the historic past this district has been resurveyed several times since the first observation the authors compiled all the geodetic data and deduced the horizontal crustal movements in the new net adjustment for geodetic network the six stations tsukubasan teruishizan dodairayama kokushidake kaikomagadake and enasan are assumed not to have moved during the observational period the horizontal displacement vectors for four periods are deduced also error ellipses of displacement vectors are estimated the earth s crustal strains are calculated from obtained vectors it is concluded that the kanto district is compressed from ocean to inland during the interseismic period and reversed completely by faulting during the 1923 kanto earthquake the secular rate of north northwest displacement of the triangulation stations close to the sagami trough is estimated to be of the order of 3 cm year the tokai district has been compressed from ocean to inland during the past 90 years except for the western part where the 1944 tonankai earthquake occurred the tectonic strain field in the kanto tokai district is expressed by a line of contraction the observed compression and extension cycle is due to north northwest drifting of the philippine sea plate and reverse faulting along its convergent plate boundary the results of geodetic observations and unified net adjustments considering their possible errors completely revealed the earthquake cycle

fast rift propagation along the east pacific rise near easter island during a 1983 seabeam cruise we surveyed the easter nazca spreading center which forms the eastern boundary of the easter microplate we have analyzed all available bathymetry and magnetic anomaly data to determine the present fine scale tectonic configuration and recent tectonic evolution of this spreading center the narrowing of the axial brunhes anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm yr assuming an average spreading rate of 120 mm yr there are indications of a short episode of faster propagation 350 mm yr prior to 0 4 ma these are the fastest rates of rift propagation yet documented the tip of the propagating rift is at about 25 s 112 25 w 180 km south of the previously hypothesized position the recent spreading direction is n85 w a large well defined sheared zone to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the easter pacific spreading center 360 km to the west is being replaced by the fast propagator