a few parts in 108 geodetic baseline repeatability in the gulf of california using the global positioning system global positioning system gps geodetic measurements of 350 650 km baselines across the pacific north america plate boundary in the gulf of california are presented the analysis employs a four station u s fiducial network and combined carrier phase and pseudorange data water vapor radiometer wvr data at the gulf sites are used to calibrate the gps signal for wet tropospheric path delays residual tropospheric delays are modeled as first order exponentially correlated stochastic processes the measurement precision for horizontal components is a few parts in 108 or better comparison of the gulf data with other geodetic techniques is not yet possible however the system accuracy appears to be about 1 part in 107 based on simultaneous gps solutions of a baseline in california collocated with vlbi the results are encouraging because a high level of precision with gps is demonstrated even with stations located outside a fiducial network and when wet tropospheric path delays are significant typically exceeding 20 cm at zenith

time averaged paleomagnetic field and secular variation predictions from dynamo solutions based on lower mantle seismic tomography abstract we compare three dynamo solutions incorporating laterally varying boundary heat flux with paleomagnetic models and data the boundary condition is defined by the d seismic shear wave velocity and the three solutions have boundary anomalies with different amplitudes the generated fields appear to divide into a stationary boundary locked part and a time varying part with persistent centres of activity both parts contribute to the time average a very long averaging time can be needed for nearly locked solutions but a rough time average that remains within the threshold set by the accuracy of paleomagnetic data is achieved in a few diffusion times the locked part dominates for larger amplitude boundary anomalies in previous work the locked field was shown to have strong similarities with the modern geomagnetic field previous dynamo solutions that were not locked to the boundary show similarities with our solutions with weak boundary forcing the axisymmetric time average has small g 2 0 and larger g 3 0 components and peaks in inclination anomaly in high latitudes associated with the locked field and low latitudes associated with the time average of the time varying fields the non axisymmetric time average displays a striking longitudinal variation in inclination anomaly with a large negative anomaly in the pacific region in agreement with observations none of the dominant geomagnetic coefficients are axisymmetric and g 2 0 negligible in all three models secular variation is concentrated in equatorial latitudes as in some recent paleomagnetic models the locked field agrees with the inclination difference found between hawai i and reunion in agreement with paleomagnetic averages the locked field agrees with the paleomagnetic time average rather better than the fields with less boundary variations we conclude that because the locked field agrees with the modern field as well as some aspects of the long term time average the geomagnetic field spends a considerable time in the present four lobe configuration it is not a coincidence that the present field resembles the time average longitudinal variations are likely to be at least as important as latitudinal variations in the paleomagnetic time average this presents a challenge for dynamo theory since a move to more geophysically realistic parameters would appear to destroy the locked solutions

paleomagnetic secular variation of the honolulu volcanic series 33 700 ka o ahu hawaii abstract thirty years after the precursory study j geophys res 13 1972 2129 19 dated sites with ages between 33 and 700 ka from the honolulu volcanic series hvs southeastern part of the island of o ahu in hawaii have been revisited a minimum of 16 samples per site have been studied using alternating field and thermal demagnetization both techniques yielded similar results coeval flows could be identified by relying on their k ar ages similar rock magnetic parameters and identical directions reliable paleomagnetic directions have been obtained for all sites two site mean directions which deviate significantly from the geocentric axial dipole gad correspond to periods of low dipole intensity and may thus reflect a larger proportion of non dipole components the average value of the site mean declinations is 358 6 while the mean inclination of 34 5 is 3 5 lower than the value of the geocentric dipole at the site the north geographic pole is largely contained within the uncertainties surrounding the position of the mean virtual geomagnetic poles vgp latitude the deviation of 2 3 in latitude is further reduced by 1 if we consider the existence of a standing small quadrupole component that would represent 5 of the dipole the angular dispersion of the virtual geomagnetic poles 12 is only 1 7 smaller than expected at the site latitude and thus not a firm indication of low secular variation thus there is no significant difference with the global time averaged field for the same period other recent results from the same island for an older time period display similar characteristics but with reverse and normal polarities that are not perfectly antipodal it is suggested that this could result from changes in the mean dipole field intensity during these time intervals this points out to the importance of scrutinizing the entire field vector in any analysis of paleosecular changes these results do not bring any evidence for a persistent field anomaly under hawaii

the effect of thermal core mantle interactions on the palaeomagnetic secular variation we compare a numerical dynamo model with a model of the palaeosecular variation and find that although there is good agreement in the meridional distribution of the secular variation the amplitude of secular variation in the dynamo model is too small increasing the rayleigh number does little to increase the amplitude of the secular variation while it increases the axial dipole component of the field to geophysically unreasonably large values however by introducing lateral variations in heat flux across the core mantle boundary into the dynamo model we are able to match the amplitude of the secular variation in addition we find that the dynamo model then exhibits persistently anomalous behaviour beneath the pacific region

long term geometry of the geomagnetic field for the last five million years an updated secular variation database lee 1983 assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years constable 1992 recently suggested that the virtual geomagnetic poles vgp derived from this data base have been preferentially located within two antipodal bands of longitude we have reanalyzed and updated the lee data base yielding a total of 3179 data from 86 distinct sites neither the total data set nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of vgps the shape of the common site longitude distribution is far more striking with a pronounced and robust minimum close to the common site longitude and secondary maxima about 120 o away from it the former first order feature is equivalent to the far sided effect discovered by wilson 1970 and corresponds to a persistent axial quadrupole amounting to about 5 of the axial dipole as previously found by several authors the latter second order feature could correspond to the vgp biasing effect described by egbert 1992 although its observed amplitude appears larger than predicted part of this effect an asymmetry in the maxima may also correspond to wilson s 1971 right handed effect given only a small axial quadrupolar component and the very uneven actual site distribution the expected longitude distribution of vgps can be calculated this is found to be in good agreement with the paleomagnetic observations i e rather flat with a moderate maximum near 120oe this study emphasizes problems related to the unsatisfactory distribution of sites the amplitude and significance of other terms beyond the dominant axial dipole and significant axial quadrupole contribution remain to be assessed

earth science time of reversal at intervals the polarity of the earth s magnetic field reverses the timescale for this reversal is unclear in fact it seems to depend on the latitude of the site from which the geological data are extracted

coupled dynamics of earth s geomagnetic westward drift and inner core super rotation this phd work focuses on the rotational dynamics of the coupled inner core outercore mantle system the conservation of the angular momentum our coupled earth model indeed involves two direct electromagnetic torques at the fluid core boundaries and a remote gravitational torque between the inner core and the mantle the rotational dynamics is described by four typical shears and studied in convective numerical simulations of the geodynamo which are able to reproduce the main characteristics of the geomagnetic field and its secular variation this secular variation is mainly embodied by the westward drift of magnetic flux patches at the cmb concentrated on the equator of the atlantic hemisphere and is well documented for the last four centuries we provide constrains on the inner core differential rotation by expressing its link to the geomagnetic westward drift this is performed through the formulation and the validation of dynamical electromagnetic torque models which are then introduced in the conservation of the angular momentum of the system in the long term state the global shear in the fluid outer core is distributed between the westward drift and the differential rotation of the inner core in proportions controlled by the state of couplings as a present day estimate of this shear is close to the observed westward drift we conclude there is no differential rotation of the inner core on time average in the time dependent state we observed that the strength of gravitational coupling is the dominant parameter this places limit on the decadal fluctuations of the inner core differential rotation which should not exceed a few hundredths of degree per year

oceanography when glacial giants roll over the energy released by capsizing icebergs can be equal to that of small earthquakes enough to create ocean waves of considerable magnitude should such glacial tsunamis be added to the list of future global warming hazards

global geomagnetic field models for the past 3000 years transient or permanent flux lobes psvmod1 0 is a compilation of globally distributed palaeodirectional data from archaeomagnetic artefacts lava flows and lake sediments at 24 sites evaluated at 100 year intervals from 1000 bc to ad 1800 we estimate uncertainty in these measures of declination and inclination by comparison with predictions from standard historical models in time intervals of overlap and use the 100 year samples and their associated uncertainties to construct a sequence of minimum structure global geomagnetic field models global predictions of radial magnetic field at the coremantle boundary cmb as well as inclination and declination anomalies at the earth9s surface provide an unprecedented view of geomagnetic secular variations over the past 3000 years and demonstrate a consistent evolution of the field with time resolution of the models is poorest in the southern hemisphere where only six of the 24 sites are located several with incomplete temporal coverage low flux regions seen in the historical field near the north pole are poorly resolved but the northern hemisphere flux lobes are clearly visible in the models these lobes are not fixed in position and intensity but they only rarely venture into the pacific hemisphere the pacific region is seen to have experienced significant secular variation a strong negative inclination anomaly in the region like that seen in 0 5 ma models persists from 1000 bc until ad 1000 and then gradually evolves into the smaller positive anomaly seen today on average between 1000 bc and ad 1800 the non axial dipole contribution to the radial magnetic field at the core mantle boundary is largest in the north central pacific and beneath central asia with clear non zonal contributions at the earth9s surface average inclination anomalies are large and negative in the central pacific and most positive slightly to the east of central africa inclination anomalies decrease with increasing latitude average declinations are smallest in equatorial regions again with strong longitudinal variations largest negative departures are centred over australia and eastern asia secular variation at the earth9s surface is quantified by standard deviation of inclination and declination about their average values and at the cmb by standard deviation in radial magnetic field all three show significant geographical variations but appear incompatible with the idea that secular variation in the pacific hemisphere is permanently attenuated by greatly enhanced conductivity in d 00 beneath the region

variation of seismic slip in the gulf of california and the possible effect on geodetic measurements of pacific north american plate motion the gulf of california is a unique tectonic transition zone which progresses from an oceanic ridge transform system at the mouth of the gulf to a predominantly continental transform system in southern california slip rates estimated from seismic moment data amount to about 17 to 30 of the accumulated tectonic slip in the gulf predicted by the nuvel 1 plate motion model even after accounting for the transition in crustal structure the seismic to tectonic slip ratios also appear to increase slightly from south to north this trend in the seismic moment release may reflect changes in the accommodation of pacific north american plate motion along the gulf which can be examined geodetically on the other hand geodetic measurements of the relative plate motion can be disturbed by short term effects associated with episodic seismicity to assess these effects the surface displacements due to typical transform events in the gulf are estimated using a simple dislocation model the results of this numerical calculation suggest that if a large transform event m0 1 5 1026 dyne cm were to occur within 100 to 200 km of a geodetic baseline for example one comprised in the geomex global positioning system network across the southern gulf the relative distance measurements could be affected by up to 15 mm this is marginally at the error level of a few millimeters plus 2 parts in 108 of baseline length for geomex measurements which thus are sensitive only to the far field displacement along the plate boundary over baselines of a few hundred kilometers in length

preliminary determination of pacfic north america relative motion in the southern gulf of calfornia using the global positioning system global positioning system gps data from experiments conducted in 1985 and 1989 in the southern gulf of california mexico allow a determination of relative motion between the pacific and north american plates the data indicate motion of cabo san lucas on the pacific plate relative to north america at a rate of 47 7 mrn yr and azimuth of 57 6 west of north 1 errors equivalent within uncertainties to the nuvel 1 global plate motion model

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

evaluation of the 1984 alaska global positioning system campaign with the bernese gps software during the summer of 1984 five ti 4100 global positioning system gps receivers were used to measure an eight station network in alaska and canada previously measured with mobile very long baseline interferometry vlbi systems with baselines ranging from 300 km to almost 2700 km the observations have been processed in the network mode with the bernese gps software using orbit improvement techniques the comparisons of the gps and vlbi solutions established through a seven parameter helmert transformation show an agreement of the order of 0 1 ppm for all solutions no significant scale factor between gps and vlbi coordinates could be detected these results demonstrate for the first time the power of orbit improvement in conjunction with network processing of gps phase observations

the myth of the pacific dipole window the angular dispersion of virtual geomagnetic poles vgp from lava flows is often cited as having been anomalously low in the pacific during the brunhes epoch because the dispersion from hawaiian lavas is said to be much lower than measured elsewhere this led to the concept of the pacific dipole window or pacific non dipole low because lavas tend to be erupted in bursts of activity many of the hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations geochronological controls as are available for hawaii have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field unfortunately in that analysis the angular dispersion of vgps was calculated about the mean vgp instead of about the spin axis and thus underestimated the dispersion we have therefore reanalyzed the relevant data using the following criteria 1 only those lavas with 95 10 have been considered 2 a latitude dependent cut off angle is used to eliminate those vectors that are not part of the normal secular variation 3 we have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector because suitable geochronological controls are rarely available application of this new method to the hawaiian data shows excellent agreement with the results from using geochronological controls where possible we have therefore included this method in our overall analysis as expected the resulting global data are compatible with a fisher distribution about the spin axis brunhes age data from hawaii n 96 independent measurements give a vgp angular dispersion about the spin axis of sf 12 4 and for the pacific region as a whole sf 12 5 n 190 between latitudes 15 and 30 north or south these values are the same as sf 12 4 n 160 calculated for the rest of the world in the same latitude range this clearly demonstrates that the hypothesis of the pacific dipole window may confidently be rejected

an introduction to the global positioning system and some geological applications receivers equipped to measure dual frequency carrier phase signals from satellites of the global positioning system gps have been capable under special conditions of determining relative horizontal positions among stations separated by one to a few hundred kilometers with a precision of one to several millimeters since the early 1980s the major obstacles to making this capability routine extending it to all parts of the globe and extending it to longer station separations have been equipment cost limitations in the gps satellite constellation arduous data analysis uncertainties in satellite orbits uncertainties in propagation delays associated with variable tropospheric water vapor and difficulties in resolving carrier phase cycle ambiguities recent improvements have occurred in all these areas the increasing ease and reduced cost of gps data acquisition and analysis are having a significant impact on studies of near fault crustal deformation and earthquake processes until recently the province of conventional terrestrial geodetic techniques the enhanced satellite constellation improved models and establishment of global tracking networks have extended several millimeters horizontal positioning capability to station separations of 1000 km or more in virtually all parts of the world this enables study of new classes of tectonic problems that previously were difficult to attack with any geodetic technique examples include a complete kinematic description of ongoing crustal deformation in broad complex continental plate boundary zones and measurement of relative plate motion at convergent boundaries where global models may be poorly constrained

assessment of global positioning system measurements for studies of crustal deformation the u s geological survey has been using the global positioning system gps to perform repeated surveys of networks of different sizes since january 1986 the intersite distances from these various networks are 223 km for a single line connecting the palos verdes and vandenberg sites in southern california 30 50 km for a network of three sites south of the san francisco bay near loma prieta 9 30 km for a network located near hebgen lake montana 7 12 km for a network of sites in the vicinity of parkfield california and 240 m for a line located at the goldstone antenna complex in the mojave desert the lengths quoted do not include the intersite distances for the fiducial sites included in the solutions for orbit determination which can be up to 8000 km we have attempted to assess the long term precision and accuracy of determinations of relative site position for a range of line lengths this assessment has consisted of several different studies in assessing precision we have generally investigated repeatability usually about a best fit linear temporal trend the smallest fractional repeatability was yielded by the measurements on the palos verdes to vandenberg line the root mean square rms residual about the best fit line for these determinations was 6 mm 0 03 parts per million or ppm for the north component 11 mm 0 05 ppm for the east component and 40 mm 0 18 ppm for the vertical component for these determinations gps data from 3 4 consecutive days were combined to produce a single estimate of the vector separation we also used the day to day scatter from these measurements to perform an analysis of variance and found that these multiday determinations can be significantly more precise than estimates determined from a single observing session the worst results were obtained for the loma prieta network where we obtained rms residuals of 11 12 mm 0 3 0 4 ppm for north 14 18 mm 0 3 0 6 ppm for east and 18 21 mm 0 4 0 7 ppm for vertical we cannot definitely explain these poor results but we have evidence to indicate that this poor scatter is due to the short observing session 4 5 hours compared to 6 5 hours for the palos verdes to vandenberg results in assessing accuracy we compared the gps results to those from independent techniques at parkfield we compared the gps estimates of fault parallel motion to those from creep meters and alignment arrays and found excellent agreement within 1 2 mm yr 1 not only for the slip but also for the variation of slip along the san andreas fault we compared the loma prieta and hebgen lake line length estimates to those obtained from a geodolite a high precision laser distance measurement device that the u s geological survey has used for 20 years for studies of crustal deformation if we model the geodolite minus gps line length differences l as l l where l is the line length then we find 0 6 0 5 mm and 0 1 0 1 mm km 1 the uncertainties quoted are one standard deviation from this comparison we conclude that at this level of significance there is no detectable difference between lengths determined from geodolite and gps data finally we compared the gps estimates of the palos verdes to vandenberg line length to those obtained from very long baseline interferometry vlbi the mean difference between the six gps estimates of line length and a best fit line through the vlbi estimates was 1 mm and the rms difference was 12 mm 0 05 ppm close to the standard deviation of the gps measurements for this predominantly east west line

small scale structure of the geodynamo inferred from oersted and magsat satellite data the geodynamo in the earth s liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the earth s surface1 2 here we use data from the currently operating danish oersted3 satellite and from the us magsat2 satellite that operated in 1979 80 to identify and interpret variations in the magnetic field over the past 20 years down to length scales previously inaccessible projected down to the surface of the earth s core we found these variations to be small below the pacific ocean and large at polar latitudes and in a region centred below southern africa the flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs and retrograde vortices with lows in the historical 400 year average magnetic field4 5 this pattern is analogous to those seen in a large class of numerical dynamo simulations6 except for its longitudinal asymmetry if this asymmetric state was reached often in the past it might account for several persistent patterns observed in the palaeomagnetic field7 8 9 10 we postulate that it might also be a state in which the geodynamo operates before reversing

accounting for magnetic diffusion in core flow inversions from geomagnetic secular variation summary we use numerical dynamos to investigate the possible role of magnetic diffusion at the top of the core we find that the contribution of radial magnetic diffusion to the secular variation is correlated with that of tangential magnetic diffusion for a wide range of control parameters the correlation between the two diffusive terms is interpreted in terms of the variation in the strength of poloidal flow along a columnar flow tube the amplitude ratio of the two diffusive terms is used to estimate the probable contribution of radial magnetic diffusion to the secular variation at earth like conditions we then apply a model where radial magnetic diffusion is proportional to tangential diffusion to core flow inversions of geomagnetic secular variation data we find that including magnetic diffusion does not change dramatically the global flow but some significant local variations appear in the non frozen flux core flow models termed diffusive the hemispherical dichotomy between the active atlantic and quiet pacific is weaker a cyclonic vortex below north america emerges and the vortex below asia is stronger our results have several important geophysical implications first our diffusive flow models contain some flow activity at low latitudes in the pacific suggesting a local balance between magnetic field advection and diffusion in that region second the cyclone below north america in our diffusive flows reconciles the difference between mantle driven thermal wind predictions and frozen flux core flow models and is consistent with the prominent intense magnetic flux patch below north america in geomagnetic field models finally we hypothesize that magnetic diffusion near the core surface plays a larger role in the geomagnetic secular variation than usually assumed

anisotropic paleosecular variation models implications for geomagnetic field observables abstract we present a family of statistical models for paleosecular variation psv of the geomagnetic field that are compatible with paleodirectional and paleointensity variations in lava flows sampling the last 5 ma and explore what paleomagnetic observables might be used to discriminate among the various family members we distinguish statistical models with axial anisotropy which provide a suitable description for an earth with homogeneous boundary conditions at the core mantle interface from those with more general anisotropy corresponding to geographically heterogeneous boundary conditions the models revise and extend earlier ones which are themselves descendants of cp88 devised by constable and parker constable c g parker r l 1988 statistics of the geomagnetic secular variation for the past 5 m y j geophys res 93 11569 11581 in cp88 secular variation is described by statistical variability of each gauss coefficient in a spherical harmonic description of the geomagnetic field with each coefficient treated as a normally distributed random variable the gauss coefficients of the non dipole part of the field exhibit isotropic variability and the variances are derived from the present field spatial power spectrum the dipole terms have a special status in cp88 with a non zero mean for the axial dipole and lower variance than predicted from the spatial power spectrum all non dipole terms have zero mean except the axial quadrupole cp88 is untenable for two reasons it fails to predict the observed geographic dependence of directional variability in the magnetic field and it grossly underpredicts the variance in paleointensity data the new models incorporate large variance in the axial dipole and in the non axial quadrupole gauss coefficients g21 and h21 the resulting variance in paleomagnetic observables depends only on latitude zonal models unless the variance in h21 is different from that in g21 non zonal models non zonal longitudinal variations in psv such as the flux lobes seen in the historical magnetic field are simulated using the non zonal models both the zonal and non zonal models fit summary statistics of the present dataset we investigate the influence of persistent non zonal influences in psv on various paleomagnetic observables it is shown that virtual geomagnetic pole vgp dispersion is rather insensitive to longitudinal variations in structure of psv and that inclination dispersion has the potential to be more informative given the right site distribution there is also the possibility of using paleointensity and geographic variations in the frequency of occurrence of excursional directions to identify appropriate psv models

secular variation in numerical geodynamo models with lateral variations of boundary heat flow we study magnetic field variations in numerical models of the geodynamo with convection driven by nonuniform heat flow imposed at the outer boundary we concentrate on cases with a boundary heat flow pattern derived from seismic anomalies in the lower mantle at a rayleigh number of about 100 times critical with respect to the onset of convection the magnetic field is dominated by the axial dipole component and has a similar spectral distribution as earth s historical magnetic field on the core mantle boundary cmb the time scales of variation of the low order gauss coefficients in the model agree within a factor of two with observed values we have determined the averaging time interval needed to delineate deviations from the axial dipole field caused by the boundary heterogeneity an average over 2000 years the archeomagnetic time scale is barely sufficient to reveal the long term nondipole field the model shows reduced scatter in virtual geomagnetic pole positions vgps in the central pacific consistent with the weak secular variation observed in the historical field longitudinal drift of magnetic field structures is episodic and differs between regions westward magnetic drift is most pronounced beneath the atlantic in our model although frozen flux advection by the large scale flow is generally insufficient to explain the magnetic drift rates there are some exceptions in particular equatorial flux spot pairs produced by expulsion of toroidal magnetic field are rapidly advected westward in localized equatorial jets which we interpret as thermal winds

the secular variation of earth s magnetic field models of the magnetic field at the core mantle boundary at selected epochs from 1715 0 to 1980 0 reveal novel features in the field at the core these suggest that the flow of core fluid is coupled to the mantle and that magnetic diffusion is significant

an assessment of long duration geodynamo simulations using new paleomagnetic modeling criteria qpm abstract long term temporal variations of the magnetic field timescales 10 myr characterized from paleomagnetic data have been hypothesized to reflect the evolution of earth s deep interior and couplings between the core and mantle by tying observed changes in the paleomagnetic record to mechanisms predicted from numerical geodynamo simulations we have a unique tool for assessing changes in the deep interior back in time however numerical simulations are not run in an earth like parameter regime and assessing how well they reproduce the geomagnetic field is difficult criteria have been proposed to determine the level of spatial and temporal agreement between simulations and observations spanning historical and holocene timescales but no such criteria exist for longer timescales here we present a new set of five criteria quality of paleomagnetic modeling criteria q pm that assess the degree of semblance between a simulated dynamo and the temporal and spatial variations of the long term 10 myr paleomagnetic field these criteria measure inclination anomaly virtual geomagnetic pole dispersion at the equator latitudinal variation in virtual geomagnetic pole dispersion normalized width of virtual dipole moment distribution and dipole field reversals we have assessed 46 geodynamo simulations using the q pm criteria the simulations have each been run for the equivalent of at least 300 kyr span reversing and non reversing regimes and include either homogeneous or heterogeneous heat flux boundary conditions we find that none of our simulations reproduce all salient aspects of the long term paleomagnetic field behavior for the past 10 myr nevertheless our simulations bracket earth values suggesting that an earth like simulation is feasible within the available computationally accessible parameter space this new set of criteria can inform future simulations that aim to reproduce all aspects of earth s long term magnetic field behavior

a strong lateral shear velocity gradient and anisotropy heterogeneity in the lowermost mantle beneath the southern pacific 1 velocity heterogeneity and seismic anisotropy in the d region beneath the southern pacific ocean is investigated using broadband shear waves from southwest pacific and south american earthquakes recorded by permanent and temporary stations in antarctica australia south america and on islands in the south pacific shear velocity perturbations vs are inferred from the differential travel times between hand picked horizontally polarized sh direct or diffracted s arrivals and vertically polarized sv sks arrivals derived patterns in vs roughly agree with global shear velocity models but reveal a stronger shear velocity lateral gradient north of approximately 52 s where vs transitions from approximately 0 5 to 1 0 relative to radially averaged reference models over less than 600 km along the base of d waveform analyses provide an even stronger constraint on the transitional region where there is a change in waveform behavior occurring over length scales less than 300 km differential travel times between skks and sks calculated by cross correlating sks with hilbert transformed skks support large vs amplitudes and help to constrain the location of the transitional region anisotropy in d ks is inferred from handpicked differential travel times between the sv and sh components of direct or diffracted s and display a slight spatial trend where the sh component precedes the sv in the north and east of the study area but arrives after sv in the center and southwest of it however there is high ks variability in the center of the study region there is no apparent correlation between vs and ks abrupt changes in the character of velocity and anisotropy in this region of d may be related to chemical heterogeneity at the boundary of the pacific superswell as well as small scale convection in the deep mantle

central and south america gps geodesy casa uno in january 1988 scientists from over 25 organizations in 13 countries and territories cooperated in the largest global positioning system gps campaign in the world to date table 1 43 gps receivers collected approximately 590 station days of data in american samoa australia canada colombia costa rica ecuador new zealand norway panama sweden united states west germany and venezuela the experiment was entitled casa uno an acronym for central and south america and uno is the spanish word for one designating first epoch measurements the casa uno experiment was the first civilian effort implementing a global gps satellite tracking network scientific goals of the project include measurement of strain in the northern andes measurement of subduction rates for the cocos and nazca plates beneath central and south america and measurement of relative motion between the caribbean plate and south america a second set of measurements are planned in 1991 casa dos and should provide preliminary estimates of crustal deformation and plate motion rates in the region the casa series of experiments are intended to be carried out over at least one decade

earthquakes and sea level space and terrestrial metrology on a changing planet the complacency with which man inhabits continent ocean margins or plate boundaries is based on the assumption that sea level rise and periodic seismicity are acceptable risks the forecast of future sea level or future seismicity is essential for informed assessment of these risks the historical data from which forecasts may be derived are in each case of limited antiquity and uneven spatial distribution developments in space geodesy and crustal deformation metrology in the past two decades offer the means to acquire a homogeneous global data set for monitoring relative vertical and horizontal motions of the earth s surface to within several millimeters these new tools are reviewed in terms of their application to forecasting trends in sea level rise and forecasting damaging earthquakes

thermal core mantle interactions maps of the magnetic field at the core mantle boundary for 1715 1980 reveal static features in the field and an absence of westward drift from much of the core mantle boundary the static features suggest that flow in the core is coupled to the mantle we examine different coupling mechanisms and predict lateral inhomogeneity at the core mantle boundary comparison with models of the lower mantle and core mantle boundary derived seismically and from the geoid is encouraging though their resolution is currently inadequate for a full comparison to be made with the much better resolved magnetic field

a revised estimate of pacific north america motion and implications for western north america plate boundary zone tectonics for the past 20 years much effort has been directed to determining the present day relative motion of the pacific and north american plates using two independent approaches one uses geologic observations and geodetic measurements along the san andreas fault and other faults in the plate boundary zone the other is based on plate motion models that incorporate spreading rates from marine magnetic anomalies transform azimuths and earthquake slip vectors geologic and geodetic studies find two principal elements of deformation slip along the san andreas of 34 mm yr directed n41 w and extension across the basin and range province of about 10 mm yr directed n56 w in contrast plate motion studies find 56 60 mm yr directed n35 w the discrepancy between these estimates a vector of about 15 mm yr oriented nearly due north is often attributed to a combination of slip along faults parallel to the san andreas and shortening normal to it here we revise the estimate of pacific north america motion by analyzing marine magnetic profiles from the gulf of california since 3 ma spreading has averaged 48 mm yr 10 mm yr slower than estimated before and consistent with the 49 mm yr spreading predicted by a new global plate motion model derived without any data along the pacific north america boundary the discrepancy with geodetic and geologic estimates is thus reduced to only 5 mm yr parallel to the san andreas 60 less than estimated before and 7 mm yr of shortening across the san andreas similar to prior estimates these results suggest that strike slip motion on faults west of the san andreas is less than thought before a conclusion consistent with geodetic seismological and other geologic observations

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity

implications of post perovskite transport properties for core mantle dynamics abstract recent evidence on perovskite to post perovskite phase change in the lowermost mantle suggests that post perovskite piles or lens should be present in the relatively cold downwelling areas while the roots of the hot upwelling plumes consist of perovskite post perovskite is often believed to be deformed predominantly by the dislocation creep and there are some hints that the activation parameters of the dislocation creep in post perovskite induce lower viscosity than that of perovskite at the same pressure and temperature conditions that can even result in a viscosity paradox in the lowermost mantle viscosity in cold downwellings transformed to post perovskite might be lower than the viscosity of warm perovskite plumes such a viscosity structure was indeed recently suggested by the geodynamical inversions of the geoid rheologically weak areas at the base of the subducted slabs may have important consequences for the slab deformation in the d we investigate the dynamics of the cold slabs transformed to post perovskite in the lowermost mantle by means of the simulations of thermal convection in a 2d cartesian model of the lower mantle with composite rheology including diffusion creep and dislocation creep different creep parameters are used for the perovskite lower mantle and for the post perovskite lens or layer respectively while perovskite is considered to be deformed via linear diffusion creep post perovskite is modeled either as a constant viscosity material with viscosity ranging from 1020 pa s to 1023 pa s or it is deformed purely by the dislocation creep the presence of the rheologically distinct post perovskite not only strongly influences the slab deformation above the core mantle boundary cmb but also results in different dynamic regimes of the cmb region characterised by e g the different length scales of the upwelling plumes depending on the creep parameters of post perovskite further the presence of very low viscosity lens or a layer above the cmb strongly enhances the cmb heat flux in the cold slab areas according to the amount of post perovskite present in the d and the rheology of post perovskite the heat flux across the cmb can vary considerably with time which may have important implications for the core mantle coupling and the character of the earth s geomagnetic field