continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

surface electric variations induced by deep hydraulic stimulation an example from the soultz hdr site 1 we carried out monitoring of surface electric potential variations at the soultz hot dry rock site france during a stimulation experiment where 23 000 m3 of water were injected in granite at 5 km depth we have observed a long period 9 10 days potential variation with a maximum amplitude of 5 mv thanks to the conductive well casing the start of this anomalous potential coincides with that of the water injection and its magnitude increases as more water is injected before returning toward its original value after shut in we interpret this anomalous potential as an electrokinetic effect which is caused by the injection of water circulating in deep fractures we modeled the pressure distribution during the stimulation and obtained the surface sp using electrokinetic coupling relationships we found that an in situ coupling coefficient of 1 mv mpa at the low end of laboratory values gives a good fit to the data

forecasting change of the magnetic field using core surface flows and ensemble kalman filtering 1 accurate forecasting of the change of the earth s internal magnetic field over short intervals of time e g less than five years has many applications for government academic and commercial users forecasting can be achieved by making a number of reasonable assumptions about how the main field interacts with the flow in the liquid outer core in particular the magnetic field can be considered to be entrained in the large scale flow along the core mantle boundary surface over short time periods giving rise to measurable change at the earth s surface the observed change or secular variation at or above the surface of the earth can thus be inverted to produce flow models these can be used to propagate fluid parcels threaded by the field forwards in time to forecast the non linear change of the magnetic field in addition to prediction of field change by flow models it would be advantageous to include observations of the field from satellite measurements or ground based observatories we therefore present a method using ensemble kalman filtering enkf to produce an optimal assimilation between magnetic field change as forecast from core flow models and direct observations of the field we show by assuming a steady flow and assimilating field observations annually it is possible to produce a forecast over five years with less than 30nt root mean square difference from the true field within an assumed error budget the enkf method also allows sensitivity analysis of the field models to noise and uncertainty within the physical representation

steady surficial core motions an alternate method a new method for deriving steady surficial core motions from geomagnetic field models is presented and applied the original method determines the steady velocity field at the top of a frozen flux core which best fits in both the spatial and temporal linear least squares sense a model of the large scale geomagnetic secular variation sv at the base of a source free mantle voorhies 1986 the new method is based upon the same physical assumptions but fits sv at earth s surface instead of the core mantle boundary cmb the newly derived flow differs somewhat from prior solutions and shows but slight westward drift however other key global properties are virtually unchanged over a 15 year interval the new method provides a better fit to the time varying geomagnetic field at earth s surface than does the old

the kapuskasing uplift a geological and geophysical synthesis over the past decade the kapuskasing uplift has been the subject of intense geological and geophysical investigation as lithoprobe s window on the deep crustal structure of the archean superior province enigmatic since its recognition as a positive gravity anomaly in 1950 the structure has been variably interpreted as a suture rift transcurrent shear zone or intracratonic thrust diverse studies including geochronology geothermobarometry and various geophysical probes provide a comprehensive three dimensional image of archean 2 75 2 50 ga crustal evolution and proterozoic 2 5 1 1 ga cooling and uplift the data favour an interpretation of the structure as an intracratonic uplift related to hudsonian collision eastward across the southern kapuskasing uplift erosion levels increase from 10 km in the michipicoten greenstone belt through the wawa gneiss domain 10 20 km into granulites 20 30 km of the kapuskasing structural zone juxtaposed against the low grade swayze greenstone belt al

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

can downwelling at the top of the earth s core be detected in the geomagnetic secular variation abstract it has been argued based on recent seismic and mineral physics studies that the top of earth s liquid outer core is stably stratified here i analyze persistent geomagnetic secular variation features on the core mantle boundary to examine whether a kinematic signature of core fluid upwelling downwelling can be detected i focus on regions of intense high latitude geomagnetic flux patches that may be maintained by fluid downwelling in order to identify persistent patterns the radial field and its secular variation are stacked in the flux patch moving reference frame these stacked images are compared with forward solutions to the radial induction equation based on idealized field flow models clear advective secular variation signature below north america indicates that these intense flux patches may exhibit significant mobility stretching signature in the form of persistent positive secular variation correlated with the intense flux patch below the southern indian ocean may be considered as regional scale geomagnetic evidence for whole core convection although pure toroidal flow cannot be outruled

structural and rheological implications of lower crustal earthquakes below northern switzerland abstract a detailed study of the recent seismicity in the northern alpine foreland of switzerland reveals that earthquakes with magnitudes between 0 9 and 4 2 occur not only in the upper part of the crust but contrary to observations in most other intracontinental settings seismicity extends down to depths of about 30 km in the lower crust observed p wave velocities in the focal region of even the deepest earthquakes are less than 6 3 km s 1 the average value of poisson s ratio for the entire focal depth range 6 30 km lies between 0 23 and 0 24 focal mechanisms are mostly a combination of strike slip and normal faults with consistent orientations of p and t axes but without any systematic dependence on focal depth from geological observations regional strain rates are of the order of 10 13 s 1 two dimensional temperature modelling indicates that temperatures in the lower crustal seismogenic zone are well above 450 c under the assumption of a dry environment the observed p wave velocities and poisson s ratios imply either a granitic or quartz rich gneissic composition or possibly a gabbroic quartz free granulite the deformation would be governed either by the rheology of quartz or of plagioclase the onset of ductility observed in laboratory studies lies around 300 c for quartz and around 450 c for plagioclase thus conditions in the lower crust beneath northern switzerland appear to be incompatible with the occurrence of either brittle failure or plastic instabilities as is implied by the existence of earthquakes alternatively lower crustal earthquakes could be a manifestation of the presence of fluids in the lower crust fluids under near lithostatic pressure either evolved in situ from metamorphic reactions or from infiltration from the mantle would tend to decrease the effective stress on pre existing faults and thus allow brittle friction failure to occur at depths where under dry conditions or low fluid pressures only ductile deformation would be expected

seismic velocities and composition of the canadian crust abstract to investigate variations in crustal structure and composition we analyze approximately 80 000 teleseismic p wave seismograms recorded at 343 broadband stations distributed about the canadian landmass these data are binned by horizontal slowness and simultaneously deconvolved into receiver functions we apply stacking methods to retrieve estimates of the bulk crustal velocity ratio vp vs and thickness h from this receiver function dataset under the assumption of locally 1d structure these crustal parameters are analyzed together with a compilation of velocity and thickness estimates from previous active source experiments and global crustal thickness models to investigate competing models of crustal composition and evolution crustal thicknesses vary between 25 km and 48 km with an area weighted average of 37 km for all stations vp vs estimates range from 1 65 to 1 95 and display a systematic increasing trend from nw to se across the churchill superior and grenville provinces analysis of intra crustal conversions indicates little evidence for a well defined widespread conrad discontinuity although the lower crust appears to be more transparent to teleseismic converted waves than the upper crust this observation runs contrary to that of p reflectivity in many active source studies and may be reconciled through a dominant scale length of heterogeneity that is less than 800 m the vp vs observations and vp constraints from active source studies are compared with laboratory measurements for common crustal rocktypes and found to be consistent with bulk compositions falling between granite gneiss and diorite previously reported systematic variations in crustal thickness and vp vs with age in a global data set are not supported by our data set

waning buoyancy in the crustal roots of old mountains when mountains form through the collision of lithospheric plates uplift of the earth s surface is accompanied by thickening of the crust and the buoyancy of these deep crustal roots relative to the surrounding mantle is thought to contribute to the support of mountain topography once active tectonism ceases continuing erosion will progressively wear away surface relief here i provide new constraints on how crustal roots respond to erosional unloading over very long timescales in old collisional mountain belts ratios of surface relief to the thickness of the underlying crustal root are observed to be smaller than in young mountains on the basis of gravity data this trend is best explained by a decrease in the buoyancy of the crustal root with greater age since the most recent mountain building episode which is consistent with metamorphic reactions1 2 produced by long term cooling an approximate balance between mountain and root mass anomalies suggests that the continental lithosphere remains weak enough to permit exhumation of crustal roots in response to surface erosion for hundreds of millions of years the amount of such uplift however appears to be significantly reduced by progressive loss of root buoyancy

a reduced stochastic model of core surface dynamics based on geodynamo simulations we make use of recent geodynamo simulations to propose a reduced stochastic model of the dynamics at the surface of earth s core on decadal and longer periods this model replicates the most energetic eigen directions of the geodynamo computation towards shorter timescales it proposes a compensation for weaknesses of these simulations this model furthermore accounts for the signature in the geomagnetic secular variation of errors of representativeness associated with unresolved processes we incorporate the reduced stochastic model into a geomagnetic data assimilation algorithm an augmented state ensemble kalman filter and apply it to re analyze magnetic field changes over the period 1880 2015 errors of representativeness appear to be responsible for an important fraction of the observed changes in the secular variation as it is the case in the dynamo simulation recovered core surface motions are primarily symmetric with respect to the equator we observe the persistence of the eccentric westward gyre over the whole studied era and vortices that partly follow isocontours of the radial magnetic field at the core surface our flow models provide a good fit to decadal changes in the length of day and predict its interannual variations over 1940 2005 the largest core flow acceleration patterns are found in an equatorial belt below 10 in latitude and are associated with non axisymmetric features no systematic longitudinal drift of acceleration patterns is found even over the past decades where satellite data are available the acceleration of the high latitude westward jet in the pacific hemisphere is during the satellite era a factor 5 smaller than previously reported and its structure shows some evidence for equatorial asymmetry the era 1 of continuous satellite records provides enhanced contrast on the rapid core flow variations the proposed assimilation algorithm offers the prospect of evaluating earth likeness of geodynamo simulations

waveform joint inversion of seismograms and electrograms for moment tensor characterization of fracking events electromagnetic signals have been observed in association with fracking experiments in the laboratory and in the field we have developed a seismoelectric forward modeling approach to produce synthetic seismograms and electrograms generated by fracking events using the finite element method with perfect matched layer boundary conditions the poroelastodynamic equations are solved in the frequency domain using a formulation based on the solid phase displacement and the pore pressure these results are used to compute the electrical field disturbances of electrokinetic nature three types of electrical signals are generated type i disturbance is associated with the seismic source itself type ii disturbance corresponds to seismoelectric conversions and type iii corresponds to coseismic signals this model is applied to simulate the seismic and electrical signals corresponding to the occurrence of a fracking event in a two layers system we perform a stochastic joint inversion of the seismograms and electrograms using the adaptive metropolis algorithm ama to obtain the posterior probability density functions of the parameters characterizing the seismic source assuming that the velocity model is perfectly known the joint waveform inversion is performed on synthetic noise free data and the ama algorithm is successful in retrieving the true values of the unknown parameters the proposed approach is then tested on the same synthetic data after being contaminated with 15 random noise with respect to the maximum amplitude of the signals the model parameters are better determined for the joint inversion of seismic and electrical data by comparison with the inversion of the seismic time series alone we also propose a deterministic tomographic algorithm that is successful in locating the in situ source current density distribution for types i and ii anomalies from the electrical data alone

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

limits on lateral pressure gradients in the outer core from geodetic observations abstract lateral variations in pressure associated with flow at the core mantle boundary will produce deformations of the mantle it is shown here that geostrophically plausible pressure gradients are sufficient to produce measurable changes in the length of day polar motion j2 sea level tilt gravity and strain of these polar motion is the most discriminating and limits intra annual variations in the lateral pressure field at planetary wavelength to less than 104 pa satellite observations of changes in j2 are potentially even more useful however at present the time span of observations is short and the separation of a residual which might be due to a change in the distribution of pressure at the core mantle boundary from other causes such as post glacial rebound is ambiguous it is suggested that not more than 10 of the decade variations in the length of day may be caused by pressure fluctuations at the core mantle boundary

tangentially geostrophic flow at the core mantle boundary compatible with the observed geomagnetic secular variation the large scale component of the flow abstract we present a method for determining the large scale component of a tangentially geostrophic flow beneath the core mantle boundary compatible with magnetic secular variation observations we use a tangentially geostrophic basis to ensure the geostrophy of the motion the fit of the secular variation sv generated by the motion to the observed sv in fact sv models is adequate taking into account the existing error level as in any horizontal geostrophic motion the flow is expressed as a sum of two independent tangentially geostrophic flows a zonal component which is toroidal and a non zonal component which is directly linked with the motions deeper in core the flow derived for the recent epoch 1970 1985 presents interesting symmetry properties the non zonal velocities are the same at two antipodal points while the zonal velocities are the same at two points symmetrical about the equator the non zonal component of the flow is more vigorous than the zonal one the consoidal ingredient though weaker than the toroidal ingredient is essential and indicates strong vertical motion at depth in low latitude areas the sv is actually compatible with a geostrophic motion at the core mantle boundary and appears to be mainly due to the action of the non zonal component of the flow

testing recent geomagnetic field models via magnetic flux conservation at the core mantle boundary abstract the 28 candidate main field models submitted for the international geomagnetic reference field model revision 1985 reveal that a dramatic increase in the rate of decline of the earth s total pole strength began around 1960 nevertheless all but one field model conserves the absolute magnetic flux linking the core mantle boundary cmb fairly well during the 40 year time span 1945 1985 this supports the frozen flux approximation on global length and decade time scales geostrophic fluid motion at the top of a frozen flux core conserves the total magnetic flux linking either geographic hemisphere of the cmb this constraint is poorly satisfied by the candidate secular variation sv models yet other sv models truncated at higher degree and models based upon temporal interpolation rather than extrapolation satisfy this constraint very well these results lead us to make several recommendations for the 1985 and future igrf revisions

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

geomagnetic evidence for fluid upwelling at the core mantle boundary summary previous studies both geomagnetic and seismic have been unable to show conclusively whether or not there is fluid upwelling at the core mantle boundary here a new method is developed in which an attempt is made to invert geomagnetic secular variation data measured at the earth s surface for a frozen flux purely toroidal core mantle boundary cmb velocity field under the assumption that the mantle is electrically insulating and flux is frozen in at the cmb these data have previously been inverted for the core mantle boundary radial secular variation from which the appropriate fit between model and data is known two different main field models were used to assess the effect of uncertainty in its radial component at the cmb the conclusions were the same in both cases frozen flux purely toroidal motions provide a poor fit a statistical test allows very firm rejection of the hypothesis that the residuals are not significantly larger whereas there is no statistical difference between the residuals of inversions for radial secular variation and frozen flux velocity fields at the cmb if upwelling and down welling is included the inherent non uniqueness in the velocity field obtained is not of concern since only their statistical properties are utilized and no physical significance is attached to the flows obtained

teleseismic studies of the canadian landmass lithoprobe and its legacy national research council of canada nrc research press http www nrc cnrc gc ca notices e html copyright permission to reproduce materials on this web site were produced and or compiled by the national research council for the purpose of providing canadians with direct access to information about the programs and services offered by the government of canada the material on this site is covered by the provisions of the copyright act by canadian laws policies regulations and international agreements such provisions serve to identify the information source and in specific instances to prohibit reproduction of materials without written permission

mixing and compositional stratification produced by natural convection 1 experiments and their application to earth s core and mantle an extensive series of laboratory experiments is used to quantify the circumstances under which fluids can be mixed by natural convection at high flux rayleigh number a compositionally buoyant fluid was injected at a fixed rate into an overlying layer of ambient fluid from a planar horizontally uniform source the nature of the resulting compositional convection was found to depend on two key dimensionless parameters a reynolds number re and the ratio u of the ambient fluid viscosity to the input fluid viscosity increasing the reynolds number corresponded to increasing the vigor of the convection while the viscosity ratio was found to determine the spacing between plumes and whether buoyant fluid rose as sheets u 1 from measurements of the final density profile in the fluid after the experiments we quantified the extent to which buoyant liquid was mixed in terms of a thermodynamic mixing efficiency e the mixing efficiency was found to be high e 0 9 when either the reynolds number was large re 100 or the viscosity ratio was small u 200 the amount of mixing was related to whether ascending plumes generated a large scale circulation in the ambient fluid when our results are applied to the differentiation of the earth s core we suggest that the convection resulting from the release of buoyant residual liquid into the liquid outer core due to crystallization at the boundary between the inner and the outer core will probably lead to nearly complete mixing in the dynamically very different context of the mantle mantle plumes are predicted to ascend through the mantle and pond beneath the lithosphere whereas convection driven by the subduction of oceanic lithosphere is expected to produce moderate to extensive mixing of the mantle when the competing plate and plume modes of mantle convection are considered together we find that owing to a larger driving buoyancy flux the plate scale flow will destroy any stratification at the top of the mantle produced by mantle plumes applying our results to the stagnant lid style of thermal convection predicted to occur in the mantles of the moon mercury mars venus and pre archean earth we expect the respective flows to produce minor thermal stratification at the respective core mantle boundaries in part 2 of this study jellinek and kerr this issue we apply our results to the differentiation of magma chambers and komatiite lava flows

the steady part of the secular variation of the earth s magnetic field the secular variation of the earth s magnetic field results from the effects of magnetic induction in the fluid outer core and from the effects of magnetic diffusion in the core and the mantle adequate observations to map the magnetic field at the core mantle boundary extend back over three centuries providing a model of the secular variation at the core mantle boundary here we consider how best to analyze this time dependent part of the field we propose that the first step should be to seek the steady core flow that best fits the field maps isolating what we call the steady secular variation the part of the secular variation explained by steady core flow to calculate steady core flow over long time periods we introduce an adaptation of our earlier method of calculating the flow in order to achieve greater numerical stability we perform this procedure for the periods 1840 1990 and 1690 1840 and find that well over 90 of the variance of the time depedent field can be explained by simple steady core flow the core flows obtained for the two intervals are broadly similar to each other and to flows determined over much shorter recent intervals although we can explain a large part of the signal with these flows none of them provides an adequate explanation of the secular variation in particular the fit to the original observations from which the field models were derived is inadequate we examine the residual secular variation the part of the signal that remains we argue that it is dominated by the effects of nonsteady flow and magnetic diffusion we do not find any compelling evidence for torsional oscillations as the dominant ingredient of this nonsteady part of the secular variation

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

estimation of crustal stochastic parameters from seismic exploration data stochastic models for the crystalline crust produce synthetic seismograms that compare well with recorded data for a variety of crustal ages and tectonic environments in this paper we explore the parameter space describing such stochastic models as a basis for formulating the inverse problem that is we wish to estimate the parameters which define a stochastic model from the recorded backscattered wave field we base the estimation on approximate relations between the primary reflectivity series which is the ideal wave field response of a medium and various seismic gathers a two dimensional lateral correlation method is used to investigate the sensitivity of synthetic wave fields to horizontal characteristic length a derived empirical relationship relates the scale length to the half width of the correlation coefficient a horizontal wavenumber time domain spectral analysis successfully estimates the horizontal characteristic length ax and the fractal dimension d of the stochastic medium from which the wave field was backscattered we employ a monte carlo based optimization scheme to fit the observed spectral values to a one dimensional von karman function and simultaneously estimate values for ax and the hurst number the method yields consistent results when tested on synthetic data application of this technique to data sorted into different domains shows that zero offset sections yield the best estimate we also present results of our estimates using this method on a program for array seismic studies of the continental lithosphere passcal dynamite source shot gather and a vibroseis source consortium for continental reflection profiling cocorp data set recorded in the basin and range province

seismic waves converted from velocity gradient anomalies in the earth s upper mantle summary modelling of elastic wave propagation in 1 d structures is frequently performed using reflectivity techniques in which the earth s velocity profile is approximated by stacks of homogeneous layers the complete reflection transmission r t response of a zone with arbitrary 1 d depth variation including both gradients and discontinuities in material properties can however be calculated using invariant embedding techniques results from earlier studies are here extended to derive exact expressions for r t matrices in arbitrary 1 d anisotropic media using a form of born approxi mation valid for thin scatterers and which does not assume small perturbations in material properties the r t matrices are solutions to a system of non linear ordinary differential equations of ricatti type and may be manipulated using standard r t matrix algebra in an equivalent description the wavefield within the heterogeneous zone is considered in terms of depth dependent contributions from up and downgoing waves propagating within the embedding reference medium this leads to efficient calculation of the internal wavefield using r t matrices of the heterogeneous stratification and portions thereof at minor additional expense mode conversion of teleseismic p and s phases from velocity gradients is examined by way of examples and comparison with three component data from broad band stations of the yellowknife seismic array the frequency dependence of such wave interactions depends on the differences in vertical slowness between incident and scattered modes it is shown that significant energy is converted from transition zones with extent l p 2 a broader interval than will generally produce intramode reflections a layer structure identified from ps conversions near 75 km depth below the slave craton is shown to be compatible with a 10 km thick gradient zone in which anisotropy increases from ambient levels to vp 5 per cent vs 2 5 per cent at a discontinuous upper boundary this characterization supports a previous interpretation as the upper strata of a former oceanic plate juxtaposed against overriding lithosphere during an ancient episode of shallow subduction

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

derivation and use of core surface flows for forecasting secular variation improving forecasts of the temporal and spatial changes of the earth s main magnetic field over periods of less than 5 years has important scientific and economic benefits various methods for forecasting the rate of change or secular variation have been tried over the past few decades ranging from the extrapolation of trends in ground observatory measurements to computational geodynamo modeling with data assimilation from historical magnetic field models we examine the utility of an intermediate approach using temporally varying core surface flow models derived from relatively short periods of magnetic field data to produce by advection secular variation estimates valid for the earth s surface we describe a new method to compute a core flow changing linearly with time from magnetic secular variation and acceleration data we invert a combination of data from the champ satellite mission and ground observatories over the period 2001 0 to 2010 0 for a series of such models we assess their ability to forecast magnetic field changes by comparing them to chaos 4 a state of the art model using data from 1997 to 2014 5 we show that the magnetic field predictions tend to have a lower root mean square difference from chaos 4 than the international geomagnetic reference field or world magnetic map series of secular variation models

electric potential source localization reveals a borehole leak during hydraulic fracturing abstracta laboratory experiment was performed to see if passively recorded electric signals can be inverted to retrieve the position of fluid leakages along a well during an attempt to hydraulically fracture a porous block in the laboratory the cubic block was instrumented with 32 nonpolarizing sintered ag agcl electrodes during the test several events were detected corresponding to fluid leakoff along the seal of the well each event showed a quick burst in the electric field followed by an exponential type relaxation of the potential distribution over time the occurrence of these electric events was always correlated with a burst in the acoustic emissions and a change in the fluid pressure these self potential data were inverted in two steps 1 using a deterministic least square algorithm with focusing to retrieve the position of the source current density in the block for a given snapshot in the electric potential distribution and 2 using a genetic algorithm to refine the position of the sou

ambient seismic noise tomography of canada and adjacent regions part i crustal structures this paper presents the first continental scale study of the crust and upper mantle shear velocity v s structure of canada and adjacent regions using ambient noise tomography continuous waveform data recorded between 2003 and 2009 with 788 broadband seismograph stations in canada and adjacent regions were used in the analysis the higher primary frequency band of the ambient noise provides better resolution of crustal structures than previous tomographic models based on earthquake waveforms prominent low velocity anomalies are observed at shallow depths 20 km beneath the gulf of st lawrence in east canada the sedimentary basins of west canada and the cordillera in contrast the canadian shield exhibits high crustal velocities we characterize the crust mantle transition in terms of not only its depth and velocity but also its sharpness defined by its thickness and the amount of velocity increase considerable variations in the physical properties of the crust mantle transition are observed across canada positive correlations between the crustal thickness moho velocity and the thickness of the transition are evident throughout most of the craton except near hudson bay where the uppermost mantle v s is relatively low prominent vertical v s gradients are observed in the midcrust beneath the cordillera and beneath most of the canadian shield the midcrust velocity contrast beneath the cordillera may correspond to a detachment zone associated with high temperatures immediately beneath whereas the large midcrust velocity gradient beneath the canadian shield probably represents an ancient rheological boundary between the upper and lower crust

self potential observations during hydraulic fracturing 1 the self potential sp response during hydraulic fracturing of intact sierra granite was investigated in the laboratory excellent correlation of pressure drop and sp suggests that the sp response is created primarily by electrokinetic coupling for low pressures the variation of sp with pressure drop is linear indicating a constant coupling coefficient cc of 200 mv mpa however for pressure drops 2 mpa the magnitude of the cc increases by 80 in an exponential trend this increasing cc is related to increasing permeability at high pore pressures caused by dilatancy of microcracks and is explained by a decrease in the hydraulic tortuosity resistivity measurements reveal a decrease of 2 prior to hydraulic fracturing and a decrease of similar to 35 after fracturing an asymmetric spatial sp response created by injectate diffusion into dilatant zones is observed prior to hydraulic fracturing and in most cases this sp variation revealed the impending crack geometry seconds before failure at rupture injectate rushes into the new fracture area where the zeta potential is different than in the rock porosity and an anomalous sp spike is observed after fracturing the spatial sp distribution reveals the direction of fracture propagation finally during tensile cracking in a point load device with no water flow a sp spike is observed that is caused by contact electrification however the time constant of this event is much less than that for transients observed during hydraulic fracturing suggesting that sp created solely from material fracture does not contribute to the sp response during hydraulic fracturing

the axial dipole strength and flow in the outer core from 1590 to 1833 although extensive measurements of the geomagnetic field were made no direct measurements of its intensity are available field models for this period either make use of indirect palaeomagnetic intensity determinations or assume a similar change in dipole strength to that seen presently here we examine the impact of different choices of field model strength on models of the core surface flow derived from the geomagnetic secular variation we find that for a reasonable range of possible changes in the field strength the variability that this unknown imposes on the flow structure is small compared with other choices in the modelling strategy such as the degree to which the geomagnetic models are fit the smoothness of the flows and a priori information included to reduce non uniqueness in the flow calculation the flows predict variations in core angular momentum of similar magnitude to those seen in recent times these variations are larger than seen in models from geodetic data but do not seem inconsistent with the geodetic data themselves we therefore suggest that it is reasonable to study core flows back at least until 1650 thereby doubling the time interval for which flows can be considered to constrain the behaviour of the geodynamo

stable regions in the earth s liquid core summary slow cooling of the whole earth can be responsible for the convection in the core that is required to generate the magnetic field previous studies have assumed the cooling rate to be high enough for the whole core to convect here we study the effects of a low rate of cooling by assuming the temperature at the base of the mantle to remain constant with an initially entirely molten adiabatic core we argue that in such a situation convection would stop at the top of the core and calculate the consequent thermal evolution a stable density stratified layer grows downwards from the core mantle boundary reaching a thickness of 100 1000 km in a few thousands of millions of years there is some geomagnetic evidence to support belief in the existence of such a stable layer

metastability mechanical strength and the support of mountain belts exhumed high pressure rocks from the caledonian root zone in norway provide analogues for processes occurring today under southern tibet allowing large scale geophysical observations from gravity and earthquakes to be linked with the mechanical properties of metastable rocks under high mountains metastability is essential for the survival of thick mountain roots and hence of high mountains and is in turn controlled by water if water is absent dry granulite formed from earlier melting episodes is both stable and strong and likely to survive in the eclogite stability field of deep root zones for hundreds of million years but if hydrous fluid is introduced the transformation of granulite to eclogite is relatively rapid and accompanied by a dramatic loss of strength in norway this transformation was initiated by water infiltration along fractures formed by earthquakes the same process as marked by deep earthquakes may be occurring today beneath southern tibet

geomagnetic field analysis v determining steady core surface flows directly from geomagnetic observations summary geomagnetic secular variation is a result of inductive effects of fluid motion at the top of the earth s liquid core which can be mapped using the frozen flux approximation and inverting mathematical models of the secular variation nonuniqueness may be removed by assuming the flow to be stationary the use of an intermediate mathematical model of secular variation makes comparison of the derived models with observation unsatisfactory and here we devise a new method inverting the geomagnetic measurements directly to obtain steady core motion the method is applied to observatory annual mean data for the 30yr period 1960 1990 to produce six separate models of steady flow three that are stationary over 10yr intervals 1960 1970 1970 1980 and 1980 1990 two 20 yr models 1960 1980 and 1970 1990 and one 30yr model 1960 1990 the method is not restricted to continuous time series or linear x y z data the 10 yr models fit the weighted data with misfits of 1 0 1 3 the 20 yr models fit less well and the 30yr model has a misfit of 1 7 when constrained to have the same norm as the 10yr models all models appear to fit the central decade with a gross misfit close to unity suggesting that changes in core motion may be associated with the two geomagnetic jerks that occurred around 1969 and 1978 furthermore close inspection of the fits at individual observatories shows that steady core motions with this norm cannot produce the secular accelerations required at a large number of high quality observatories steady flows can predict magnetic fields that follow the trend at most observatories but cannot follow the detailed time variations of the past three

how the crust meets the mantle lithoprobe perspectives on the mohorovicic discontinuity and crust mantle transition application of regional geophysical and geological methods throughout two decades of canada s lithoprobe project provides new opportunities to analyze the mohorovicic discontinuity moho and crust mantle transition the transect format employed during lithoprobe in which 10 specified regions of canada were targeted for approximately a decade each between 1984 and 2003 permitted teams of scientists to focus on geological geophysical and tectonic issues for each transect as a primary objective was to enhance knowledge of the structure of the crust and lithosphere an obvious target in each transect was the nature and origin of the moho and crust mantle transition accordingly the combined results provide new perspectives on the moho and the relationship of the moho to the crust mantle transition perhaps the most important result is that the continental geophysical moho is a deceptively simple feature it has a variety of signatures at different scales that preclude a single universally applicable interpretation in methods that provide large scale information such as regional seismic studies it is a relatively abrupt refraction velocity contrast that often displays a dramatic downward decrease in seismic reflectivity however its origin in a geological or tectonic sense is perhaps best determined by careful analyses of structural details near the geophysical moho which are complex and varied in some areas within canada it appears that the geophysical moho may be old and perhaps remains from the time the crust formed in other areas it appears to be a relatively young feature that was superimposed onto older crustal fabrics

simple models of fluid flow at the core surface derived from geomagnetic field models summary we examine the problem of determining the fluid flow at the core surface given a model of the temporal variation of the magnetic field seeking to fit the field over a finite time interval the flow is assumed steady over the time interval and we adopt the frozen flux approximation we produce a range of solutions of varying complexity and subject to different constraints estimate the errors on the solutions and assess their consistency with the underlying assumptions we find both direct evidence for ageostrophic flow in the form of flow across the geographical equator and indirect evidence in the form of poorer fits to the field when the flow is contained to be geostrophic on account of this evidence we do not adopt the geostrophic approximation however we find that unconstrained flows are poorly determined and consider the various alternatives for producing better determined solutions we choose to restrict our attention to determining the large scale toroidal part of the flow and find a simple pattern of flow which is nearly symmetric about the equator from a sequence of maps of the flow over 5 year time intervals back to 1915 we are still able to identify many of the basic aspects of this simple pattern but changes in the flow pattern are discernible although our models of the flow explain a large proportion of the signal typically 95 per cent of the variance the fits which we obtain are not as good as might be expected failure of the underlying assumptions together with our decision to map only the large scale toroidal part of the flow explains the poorer than expected fit

minimal crustal magnetizations from satellite data abstract the minimum amplitude crustal magnetization needed to explain magsat vector anomaly data is found using harmonic splines as basis functions shure et al 1982 phys earth planet inter 28 215 229 and choosing a subset of the original basis parker and shure 1982 geophys res lett 9 812 815 to represent magnetization we suggest the optimum model of this type is obtained by associating representers only with north and vertically downwards data points given the larger uncertainties associated with east component magsat data following parker et al 1987 rev geophys 25 17 40 we find a model minimizing the r m s volume integral of magnetization over the magnetized lithosphere the numerical effort involved in producing a regional or continental scale model with given horizontal spatial detail with our method is less than that for an equivalent source dipole model when the magnetization is not assumed induced approximately equal to that for an induced magnetization equivalent dipole model and considerably less than that for a spherical harmonic apparent susceptibility model the method is demonstrated by producing a solution for two regions the contiguous usa for comparison with previously published models and africa and its continental margins magnetization models fit the data slightly worse than those of the downward continued anomaly field at the earth s surface comparison of our component of magnetization for the usa in the direction of the core field with the induced magnetization equivalent dipole model of mayhew 1984 earth planet sci lett 71 290 296 is very good as expected for magnetization either induced by or frozen remanently in an approximately dipole field either parallel or antiparallel to the current rotation axis the east component of magnetization is much smaller than the north and vertically downwards components we make no assumptions about the relative contributions of remanent and induced magnetization and the resulting models have a substantial remanent component however our models suffer from the severe non uniqueness common to all magnetization models whereby magnetization in a uniform susceptibility and thickness shell concentric with the earth s centre owing to any internal magnetic field that is then switched off generates no external magnetic field

the moho of north america a brief review focused on recent studies abstract recent national programs in canada lithoprobe and the u s earthscope are providing vast quantities of data and many new scientific insights concerning crustal and lithospheric mantle structure and evolution more modest but significant results are also being produced in and about mexico this paper primarily focuses on results published since 2005 and presents an updated moho map of north america this overview of crustal structure does reveal a number of interesting observations for example two buried distinct crustal blocks have been discovered in the past 20 years one was caught in a precambrian suture zone in canada and the other is buried under the gulf coast region and is still of unknown origin the precambrian assembly of the continent has been documented by the mapping of numerous suture zones and shallow portions of subducted slabs have been preserved in several cases not surprisingly the internal structure of these suture zones is complex and highly variable isostatic adjustment has been shown to be a complex process involving the lithospheric mantle not just crustal structure variations specifically the correlation of crustal thickness with elevation is only approximate at best several crustal blocks that have been stable over very long periods of time possess unusually thick crust whose lower portion is usually fast vp 7 0 km s rifting has been shown to affect the crust in a variety of ways ranging from the massive magmatic modification that produced the mid continent rift system to the broad region of extension in the basin and range province crustal structure along the active modern western margin of north america is highly complex as would be expected as a result of transitions between subduction and transform faulting and variations in volcanism and intraplate magmatism we clearly still have a lot to learn about the structure and evolution of this continent

steady flows at the top of earth s core derived from geomagnetic field models select models of the main geomagnetic field and its secular variation are extrapolated to the base of an insulating mantle and used to estimate the adjacent fluid motion of a perfectly conducting outer core the assumption of steady motion provides formally unique solutions and is tested along with that of no upwelling the hypothesis of no upwelling is found to be substantially worse than that of steady motion although the actual motion is not thought to be steady the large scale secular variation at the top of the core can be adequately described by a large scale combined toroidal poloidal circulation which is steady for intervals of at least a decade or two the derived flows include a bulk westward drift but are complicated by superimposed jets gyres and surface divergence indicative of vigorous vertical motion at depth the circulation pattern and key global properties including rms speed upwelling and westward drift are found to be fairly insensitive to variations in modeling parameters

steady core flow in an azimuthally drifting reference frame summary flows at the top of the earth s core generating the observed geomagnetic secular variation sv can be deduced in the frozen flux approximation with various nonuniqueness reducing assumptions such as tangential geostrophy and steadiness steady flows are attractive because they require only a small number of parameters to explain the gross features of the sv however they are unable to reproduce the fine detail contained in the sv and cannot be used to explain observed decadal changes in the length of day here we find flows steady in a local reference frame within the core but the frame is allowed to rotate relative to the mantle previous investigations have studied steady drift with respect to the mantle introducing just a single extra parameter into the calculation here we also allow the drift to vary with time we then minimize a linear combination of the fit to time varying coefficients expressing the sv a measure of the complexity of the flow and the drift acceleration the resulting non linear inverse problem is solved in a two stage iterative process for a given drift we solve for the best fitting steady flow and then adjust the drift to improve the fit we seek solutions for the intervals 1900 1980 and 1840 1990 over both epochs allowing the reference frame of a steady flow to drift gives a strikingly improved fit for flows with a relatively high misfit the frame drift is westwards at a rate similar to the observed westward drift rate of the geomagnetic field at the earth s surface approximately 0 2u yr x1 requiring a tighter fit and hence a more complex flow gives rise to two solutions one with a westward frame drift with respect to the mantle the other eastward and the relative drift rates gradually increase to a maximum of 0 9u yr x1 as the misfit decreases flows in the mantle reference frame are similar to those deduced previously by any of the nonuniqueness reducing assumptions however in the drifting frame the flows are almost completely dominated by the drift between the two reference frames although the time dependence of the drift is weak and results in only a small additional misfit reduction over a uniform drift by assuming that the variation in drift reflects variation in solid body rotation of the whole core it can explain decadal length of day changes almost as well as and prior to 1900 perhaps better than fully time dependent tangentially geostrophic flows with vastly more free parameters we examine the significance of these models in terms of large scale wave motion in the core

the evolution of the core surface flow over the last seven thousands years 1 we analyze the core surface flow over the last seven thousands years derived from cals7k 2 a continuous model of the geomagnetic field and its secular variation for the period 5000 bc to 1950 ad this model relies on indirect observations of the geomagnetic field through the sampling of natural archives lake sediments lavas and archaeological artifacts an extensive comparison between gufm1 a historical field model spanning the interval 1590 to 1990 and cals7k 2 is carried out while both are similar in their descriptions of the large scale core field their agreement compares poorly in terms of secular variation the differences are primarily caused by the different data quality used to build these models such a comparison is essential for assessing the reliability of the results of flow inversion we seek instantaneous core flow solutions that are purely toroidal and tangentially geostrophic for which we can reasonably neglect the effect of magnetic diffusion our flow solutions suggest that the core surface flow undergoes different regimes of zonal flow direction with epochs of mainly westward flow alternating with epochs of mainly eastward flow the episodes of eastward and westward flow are robust as they are obtained for purely toroidal and tangentially geostrophic flow the changes in the large scale zonal flow direction seem to recur with a typical period of between 540 and 1050 years variations in thermal and magnetic winds occurring on timescales comparable with the convective overturn of the liquid outer core could account for the observed zonal flow variations

hydraulic stimulation of geothermal reservoirs fluid flow electric potential and microseismicity relationships summary we have observed surface electric potential variations of several mv during a reservoir stimulation experiment run in 2000 at the soultz sous forets france hot dry rock site that we have interpreted as deep electrokinetic processes using data from another experiment run in 2003 at the same site we show here that the electrically conducting steel casing makes possible the measurement of surface sp anomalies even 5 km above the injection zone but obliterates any information about the geometry of the electrokinetic sources we also observed a slow temporal sp decay after the end of the that we interpret as large fluid flow persisting long after shut in at the edge of the stimulated area this persisting deep fluid flow explains why large induced microseismic events are recorded in this area long after the end of the stimulation experiment as fluid overpressures diffuse far away from the injection area this interpretation is consistent with the diffusive pattern of seismicity analysis of shapiro et al

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

multi genetic origin of the continental moho insights from lithoprobe this paper reviews four hypotheses for the origin of the continental crust mantle boundary and discusses seismic parameters with which these hypotheses might be tested the relict moho hypothesis posits that the oceanic moho is preserved during continental assembly the magmatic underplating hypothesis posits formation of a new moho by episodic emplacement of sill like intrusive bodies the metamorphic or metasomatic front hypothesis posits that the moho is overprinted by a phase transformation and the regional decollement hypothesis posits that the moho behaves as a structural detachment these hypotheses are not mutually exclusive and examples from the canadian lithoprobe program suggest that all four may be applicable in different regions of north america comparison of seismic images from a fossil subduction zone with modern subduction at cascadia suggests that serpentinization of the forearc mantle a previously unrecognized mechanism for overprinting and erasing the reflection moho may have occurred in the proterozoic

geophysical characterization of the northwest geysers geothermal field california abstract the clear lake volcanic field in northern california is the youngest and northern most part of a long lived volcanic system that has produced recent 10 ka eruptions adjacent to the clear lake volcanic field is the world s largest energy producing geothermal field the geysers the hottest part of the geyser s geothermal field is in the northwest where temperatures reach 400 c at 3 km depth low permeability high thermal gradients and low steam saturation prescribed development of an enhanced geothermal system egs in the northwest geysers to increase energy producing capacity though the northwest geysers is known to be the hottest part of the field geophysical methods have failed to adequately image any inferred heat source this project aims to image the heat source of the northwest geysers using newly collected gravity and magnetotelluric mt measurements gravity data were jointly modeled with existing magnetic data along a two dimensional profile aligned with an existing geologic cross section the key feature of the potential field model is a low density low susceptibility body at 5 km depth bmsl under the egs mt data were modeled in three dimensions to characterize subsurface resistivity structure where the upper 3 km of the resistivity model agrees well with existing data lithologic and steam saturation are estimated from modeled resistivity values using existing geophysical data below 3 km depth bmsl the resistivity model images a possible young intrusion under the egs a possible zone of partial melt

mapping the fluid flow and shear near the core surface using the radial and horizontal components of the magnetic field summary we examine the problem of determining the fluid flow and the shear the radial derivative of the flow at the core surface given a model of the temporal variation of the magnetic field whereas most previous work has focused on determining only the flow which requires only the use of the radial component of the magnetic field here in addition we determine the shear for which we must use the horizontal component of the magnetic field estimates of the jump in the value of the horizontal magnetic field bh across the boundary layer between the top of the free stream and the base of the mantle are small and suggest that to a high level of accuracy the mantle valuks of bh can be used at the top of the core except in the special case of an insulating mantle only the horizontal poloidal field is known at the core mantle boundary and supplies one extra equation for the determination of velocity and shear we show how the matrix elements relating the coefficients of the spectral expansion of the flow and shear are related to the geomagnetic secular variation coefficients in closed form we examine the uniqueness of the resulting inverse problem and show that one part of the non uniqueness from which the shear suffers is particularly easy to describe it takes the same form as that which affects the flow namely a toroidal ambiguity in the field u b however certain uniqueness theorems can be derived we extend the steady motions theorem of voorhies backus 1985 and the geostrophic motions theorem of hills 1979 and backus lemouel 1986 to the determination of the flow and shear and derive closely analogous results uniqueness in the steady case depends on the value of the same discriminant as the velocity and in the geostrophic case the shear can be determined uniquely in the same areas as can the velocity i e outside certain ambiguous patches for the geostrophic regime the lateral density or temperature variations at the top of the core can be found in a self consistent manner we apply our method to the temporal evolution of the field over the period 1960 1980 and produce solutions for each of the assumptions of unconstrained steady motions geostrophic motions and purely toroidal motions we find that the form of the flow changes very little from solutions based only on the radial induction equation and that the shear is weak and aligned with the flow with a sense such that the strength of the flow decreases with depth with a length scale for linear decay of half the core radius this suggests that the flow near the core surface is indicative of whole core flow rather than a flow confined to a layer near the core surface

variability modes in core flows inverted from geomagnetic field models the flow of liquid metal inside the earth s core produces the geomagnetic field and its time variations understanding the variability of those deep currents is crucial to improve the forecast of geomagnetic field variations which affect human spacial and aeronautic activities moreover it may provide relevant information on the core dynamics the main goal of this study is to extract and characterize the leading variability modes of core flows over centennial periods and to assess their statistical robustness to this end we use flows that we invert from two geomagnetic field models gufm1 and cov obs and apply principal component analysis and singular value decomposition of coupled fields the quasi geostrophic qg flows inverted from both geomagnetic field models show similar features however cov obs has a less energetic mean and larger time variability the statistical significance of flow components is tested from analyses performed on subareas of the whole domain bootstrapping methods are also used to extract significant flow features required by both gufm1 and cov obs three main empirical circulation modes emerge simultaneously constrained by both geomagnetic field models and expected to be robust against the particular a priori used to build them large scale qg dynamics mode 1 exhibits three large vortices at medium high latitudes with opposite circulation under the atlantic and the pacific hemispheres mode 2 interestingly accounts for most of the variations of the earth s core angular momentum in this mode the regions close to the tangent cylinder and to the equator are correlated and oscillate with a period between 80 and 90 years each of these two modes is energetic enough to alter the mean flow sometimes reinforcing the eccentric gyre and other times breaking it up into smaller circulations the three main circulation modes added together to the mean flow account for about 70 of the flows variability 90 of the root mean square total velocities and 95 of the secular variation induced by the total flows direct physical interpretation of the computed modes is not straightforward nonethe less similarities found between the two first modes and time spatial features identified in different studies of core dynamics suggest that our approach can help to pinpoint the relevant physical processes inside the core on centennial timescales

magnetic boundary layers in numerical dynamos with heterogeneous outer boundary heat flux abstract it has been proposed that magnetic flux expulsion due to outer core fluid upwellings may affect the geomagnetic secular variation on the core mantle boundary bloxham 1986 in this process intense horizontal field lines are concentrated below the outer boundary introducing small radial length scales and consequently strong radial diffusion here we explore such magnetic boundary layers in numerical dynamo simulations with heterogeneous outer boundary heat flux inferred from a tomographic model of lower mantle seismic shear waves velocity anomalies our scheme associates magnetic boundary layers to peak horizontal magnetic fields at the top of the shell in our models mean magnetic boundary layer thickness ranges 200 400 km and decreases with increasing magnetic reynolds number extrapolation or interpolation to earth s core conditions based on total core flow amplitude or its poloidal part gives mean magnetic boundary layer thickness of 220 and 260 330 km respectively we find magnetic boundary layers associated with the azimuthal field at the equatorial region whereas magnetic boundary layers associated with the meridional field are found at mid latitudes negative outer boundary heat flux anomalies yield preferred locations of expulsion of azimuthal field below africa and the pacific while positive outer boundary heat flux anomalies yield preferred locations of expulsion of meridional field below the americas and east asia furthermore we find a tendency of the azimuthal field to low latitudes of the northern hemisphere our results suggest that the local diffusion time is on the order of several kyr and the local magnetic reynolds number is on the order of 10 both much smaller than classical estimates

time varying fluid flow at the top of earth s core derived from definitive geomagnetic reference field models the effects of a time varying fluid velocity have been included in the estimation of frozen flux core surface flow this has been done to improve the quality performance and accuracy of kinematic analyses of the geomagnetic secular variation the derived flows consist of a reference velocity plus a steady acceleration and thus evolve linearly with time the method features simultaneous solution for the initial radial geomagnetic field component at the core mantle boundary and the subadjacent time varying fluid motion the trade off between time averaged spatial complexity of the derived flows and misfit to the weighted definitive geomagnetic reference field models for 1945 1980 is explored at fixed flow complexity simultaneous solution for both initial geomagnetic field and steadily accelerating flow gives substantial reductions in the square weighted residuals per degree of freedom when compared with previous solutions for either initial field and steady flow or steady flow alone the apparent timescale for flow change varies nonmonotonically with flow complexity between about 23 and 41 years the single maximum value is associated with unit negative slope of the log log trade off curve and is used to help identify a preferred solution this solution has an rms speed of but 7 50 km yr l and an rms acceleration of 0 183 km yr 2 or 1 84x10 13 m s 2 the latter indicates extremely weak net forces upon the liquid outer core

variability of the topographic core mantle torque calculated from core surface flow models abstract with the prospect of studying the relevance of the topographic core mantle coupling to the variations of the earth s rotation and also its applicability to constraining the core surface flow we investigate the variability of the topographic torque estimated by using core surface flow models accompanied by a uncertainty due to the non uniqueness problem in the flow inversion and b variance originating in that of geomagnetic secular variation models employed in the inversion various flow models and their variances are estimated by inverting prescribed geomagnetic models at the epoch 1980 the subsequent topographic torque is then calculated by using a core mantle boundary topography model obtained by seismic tomography the calculated axial and equatorial torques are found subject to the variability of order 1 0 19 and 1 0 20 nm respectively on which b is more effective than a the variability of the torque is attributed even to a and b of the large scale flows degrees 2 and 3 yet it still seems unlikely for the decadal polar motion with the observed amplitude to be excited exclusively by the equatorial topographic torque associated with any of reasonable core surface flow models it is also confirmed that with the topography model adopted here the axial topographic torque on a rigid annulus in the core coaxial with the earth s rotation axis associated with any of reasonable flow models is larger by two orders of magnitude than the plausible inertial torque on such cylinders this implies that any core surface flow model consistent with the topographic coupling does not exist unless the topography model is appropriately modified nevertheless the topographic coupling might provide not only a weak constraint for explaining the decadal lod variations but also the possibility to probe the core surface flow and the core dynamics

the deep crust of the southern rhine graben reflectivity and seismicity as images of dynamic processes abstract the rhine graben part of the european cenozoic rift system deserves special attention because of its location in the foreland of the alpine orogen the phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the variscan orogeny late orogenic crustal re equilibration to the interference of rifting and alpine orogeny in tertiary times the rhine graben is one of the most detailedly studied continental grabens prospecting for hydrocarbons in its sedimentary fill and intensive pre site studies for the continental deep drilling program ktb on its eastern crystalline flank black forest provided a comprehensive set of geoscience data seismic investigations ranging from a deep seismic network of near vertical reflection to concurrent wide angle refraction experiments were accompanied by seismological and geological surveys therefore a direct observational comparison between the two depth definitions of crustal subdivisions elastic vs strength rheological was possible in large parts of western europe deep seismic sounding dss investigation distinguishes between a crystalline upper crust of comparatively low p wave velocities 5 9 6 0 km s with little and discrete reflectivity and a strongly reflective lower crust with higher p wave velocities about 6 4 6 8 km s both are separated by the conrad discontinuity in contrast strength and rheological considerations define a brittle upper crust and a ductile lower crust the boundary between these two realms is usually referred to as the brittle ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres the present brittle ductile transition in the rhine graben region as defined by the deepest hypocentres does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies this suggests that the laminated lower crust developed during the crustal re equilibration phase at the end of the variscan orogeny the second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the variscan lower crust a remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern rhine graben and is possibly induced by the young rifting process we suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the alpine orogen this allows us to propose the distinction between the influence of compression extension continental subduction and intra crustal detachment during graben formation the dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small scale collisional escape tectonics associated with an intra continental transform fault at the southern end of the rhine graben the rhine sao ne transform zone

the scattering of seismic waves through a crust and upper mantle with random lateral and vertical inhomogeneities abstract the propagation of seismic waves through earth models with slightly random lateral and vertical inhomogeneities superimposed on one composed of layers with vertical velocity gradients was investigated the maximum deviation of velocity from a mean value at a given depth and a correlation distance derived from a two dimensional smoothing filter were two parameters used to vary the amplitude and size of the velocity anomalies the resulting models show short discontinuous reflectors scattered about at various depths throughout the model and are thus in agreement with many deep seismic reflection experiments on the other hand numerical experiments using ray tracing techniques showed that the effect of the lateral and vertical velocity anomalies is to scatter the energy and break up the continuous travel time lines from vertical gradient models into travel time segments with different slopes similar to those observed in many long range seismic refraction experiments and to those resulting from layering effects in the media many of the numerical experiments which modelled the random crust produced a pg segment and a p segment with an apparent conrad discontinuity at a depth of 10 20 km this apparent depth being related to the correlation distance when a seismic wave propagates through a heterogeneous earth the amount of its energy which is converted into scattered energy will be a function of the inhomogeneous characteristics of the medium through which it has passed if a ray passes through a homogeneous earth the energy arriving at an array station should be relatively coherent whereas if the ray encounters lateral and vertical inhomogeneities its energy will be incoherent and much more complex a series of coherency measurements done on array recordings of earthquakes at various distances showed that large lateral and vertical variations in complexity exist for different ray paths through the earth with the region below the 650 km discontinuity in the mantle tending to be much simpler than the region just below the lithosphere

an 1800 km cross section of the lithosphere through the northwestern north american plate lessons from 4 0 billion years of earth s history the lithoprobe slave northern cordillera lithospheric evolution snorcle study across northwestern north america in combination with related crustal studies has been synthesized into an 1800 km long cross section of the lithosphere that is constrained by high resolution geophysical data seismic reflection refraction electromagnetic potential fields and detailed bedrock geology the cross section offers one of the longest continuous profiles of the continental lithosphere anywhere in the world that is constrained by combined geophysical measurements and electromagnetic properties and exposed bedrock geological relationships the primary conclusion of the study is that during all major orogenic episodes recorded from archean to present in that part of earth s lithosphere the crust and perhaps much of the mantle was reorganized and redistributed rather than being differentiated from the mantle at the time of orogenesis the observed subsurface geometries of relict subduction zones accretion

reservoir monitoring by a 4 d electrical technique an advanced geoelectrical technique for imaging potential fractures has been developed by engineering geophysics laboratory of kyushu university the method fluid flow tomography fft has been applied to monitor fluid flow behaviors during massive water steam injection operations through a borehole fluid flow behaviors in potential fractures could be continuously traced and visualized as a function of time by the fft system potential fractures can be evaluated by 3 d inversion of self potential sp anomalies based on a potential theory similar to the hypocenter of the earthquakes the fft method has been applied to determine the location of fractures in a geothermal area in japan

convection in the earth s core driven by lateral variations in the core mantle boundary heat flux moving core fluid maintains an isothermal core mantle boundary cmb so lateral variations in the cmb heat flow result from mantle convection such variations will drive thermal winds even if the top of the core is stably stratified these flows may contribute to the magnetic secular variation and are investigated here using a simple non magnetic numerical model of the core the results depend on the equatorial symmetry of the boundary heat flux variation large scale equatorially symmetric es heat flux variations at the outer surface of a rapidly rotating spherical shell drive deeply penetrating flows that are strongly suppressed in stratified fluid smaller scale es heat flux variations drive flows less dominated by rotation and so less inhibited by stratification equatorially anti symmetric flux variations drive flows an order of magnitude less energetic than those driven by es patterns but due to the nature of the coriolis force are less suppressed by stratification the response of the rotating core fluid to a general cmb heat flow pattern will then depend strongly on the subadiabatic temperature profile imposing a lateral heat flux variation linearly related to a model of seismic tomography in the lowermost mantle drives flow in a density stratified fluid that reproduces some features found in flows inverted from geomagnetic data