the 1989 macquarie ridge earthquake seismic moment estimation from long period free oscillations we have analyzed the long period 1000 s part of the earth s free oscillation spectrum of the 1989 macquarie ridge earthquake this event stands out as the largest in the last decade and as the first great earthquake to have been recorded digitally on the new generation of very broadband seismic sensors around the globe its spectrum was modeled for five spectral peaks 0s4 0s6 0s9 1s8 and 5s3 taking into account the effects of lateral heterogeneity rotation and ellipticity which cause coupling within the spectral peaks self coupling and between different multiplets multiplet coupling the effects of self coupling were calculated following the formulation of woodhouse and girnius 1982 multiplet coupling only weakly affected the spectral peaks that were used a large discrepancy in phase and amplitude between the calculated spectrum and the data was observed which suggests that the seismic moment was 50 larger than that inferred from surface wave analysis satake and kanamori 1990 an error analysis was performed in order to detect inaccuracies that might have resulted in this discrepancy centroid time source finiteness correction and directivity effects are likely to cause a significant bias both in the moment and phase determination we saw no direct evidence that a precursory event could have been responsible for the additional energy release as was suggested by ihmle et al 1993

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

teleseismic detection of a slow precursor to the great 1989 macquarie ridge earthquake low frequency spectra for the 1989 macquarie ridge earthquake magnitude 8 2 show an amplitude increase and a phase delay decrease below 6 millihertz that require a short term slow precursor this earthquake can be modeled as a compound event in which a fast rupturing ordinary earthquake was initiated by an episode of slow smooth deformation that began more than 100 seconds before the main shock the moment released in the slow precursor was large about 3 x 10 20 newton meters equivalent to an event of magnitude 7 6 the data are consistent with the precursor being generated in a region of the oceanic upper mantle below the main rupture

channeling of plume flow beneath mid ocean ridges we investigate the interaction of mantle plumes with mid ocean ridges in 3 d numerical convection models using a finite volume multigrid technique with local grid refinement we are able to incorporate a much stronger temperature dependence of viscosity than was possible in previous models this allows lateral viscosity variations of more than six orders of magnitude which is realistic in a ridge situation in models of ridge centered plumes with constant flux we determine how the shape and the along axis width of the plume head depends on various model parameters the plume material is significantly channeled along the axis of a slow spreading ridge if the plume viscosity is in the order of 1017 pa s the plume material spreads radially below the ridge if the spreading rate is fast or the viscosity contrast between plume and ambient mantle is small in time dependent calculations we investigate the temporal evolution of short term variations of the plume flux at slow spreading ridges these perturbations propagate in the form of pulses along the ridge with a speed much larger than the spreading rate our results support the hypothesis that pulsations of the iceland plume are the cause for v shaped topography and gravity anomalies at the reykjanes ridge

influences on the carbonate hydrochemistry of mound spring environments lake eyre south region south australia abstract mound spring structures composed of tufa precipitated from near ambient temperature spring waters are rare consequently the hydrochemical evolution of the water and the processes that control carbonate deposition in these systems are not well understood this study analysed the water from 3 mound springs in an arid environment in the vicinity of lake eyre south south australia samples were tested for major ion chemistry and stable isotope ratios of water 2 h 1 h and 18 o 16 o most co 2 degasses from water i before emergence as it ascends in the fracture derived spring conduit system below the spring opening and ii after emergence in the turbulent upper tail environment between the spring vent pool and the tail delta however modern carbonate precipitation was generally restricted to tail delta environments spatially disconnected from the areas of strongest co 2 degassing in addition to co 2 degassing and carbonate precipitation evapotranspiration and heterotrophy were inferred to be processes controlling the chemical evolution of water flowing through the tail delta based on considerations of the geometry of the tail delta and evapotranspiration rates it was inferred that infiltration is an important process in determining the tail delta size and hence the morphology of the mound spring environment a reactive transport model was used to assess the importance of these processes for the evolution of carbonate hydrochemistry the model outcomes showed that water loss via evapotranspiration does not significantly affect tufa precipitation despite the springs being located within an arid environment heterotrophy was considered in the model in order to reproduce observed ph and carbonate concentrations highlighting the role of vegetation in controlling the water chemistry the modelling approach adopted here provides a generic framework for the analysis of calcareous spring deposits and places quantitative constraints on data based process conceptualisation

using environmental isotopes and dissolved methane concentrations to constrain hydrochemical processes and inter aquifer mixing in the galilee and eromanga basins great artesian basin australia summary groundwater recharge processes water rock interaction and the hydraulic connectivity between aquifers of the galilee and eromanga basins in central queensland australia were investigated using stable 2 h 18 o 13 c and 87 sr 86 sr and radiogenic 36 cl isotopes and dissolved methane concentrations complemented by major ion chemistry the central eromanga and the upper sequence of the galilee basins are both sub basins of the great artesian basin gab and the coal seams of the galilee basin are currently explored for their potential as commercial coal seam gas deposits in order to understand the potential influence of depressurisation of coal seams required to release the gas on adjacent aquifers a detailed understanding of recharge processes and groundwater hydraulics of these basins prior to any development is required each of the different isotope systems were used in this study to provide different information on specific processes for example the assessment of 13 c and 87 sr 86 sr ratios suggested that carbonate dissolution is one of the major processes controlling the water chemistry within some aquifers in addition the combined assessment of 2 h 18 o and major ion chemistry indicates that transpiration is the primary process controlling the solute concentration in the gab recharge area whereas evaporation appears to be less significant groundwaters in the galilee basin recharge area outside the limits of the gab are different to any groundwater within the gab units this difference is attributed to the dissolution of potassium bearing micas which are absent in the gab groundwater age estimates based on 36 cl cl ratios suggest that there is a steady increase along the flow paths and this lack of anomalous age estimates from the recharge areas to the deeper parts of the basin indicates that there is no evidence for regional inter aquifer mixing based in isotopes only however dissolved methane concentrations and groundwater chemistry near faults indicates the potential mix of gas and or water from the coal seams into the gab groundwaters suggesting that there may be local influence of faults as gas water conduits

radial mode observations from the 5 23 89 macquarie ridge earthquake the radial modes 1s0 5s0 axe clearly observable on several long period seismic records from the 5 23 89 macquarie ridge earthquake observations of 0s0 are weaker but stacking demonstrates significant excitation calculations suggest that for spherical earth models no simple double couple in the uppermost mantle can explain the excitation amplitudes of 1s0 5s0 relative to 0s0 calculations suggest that the anomalous amplitudes of 1s0 5s0 can be explained by coupling to other modes induced by plausible levels of deep earth lateral structure the coupling of 0s0 is much weaker and its observed amplitude suggests that roughly 4 of the total moment release inferred from surface wave studies deviated from pure strike slip geometry this can be modeled by 8 deviations from 0 90 in a simple double couple point source or by thrust subevents within the main shock

large scale adaptive mantle convection simulation summary a new generation parallel adaptive mesh mantle convection code rhea is described and benchmarked rhea targets large scale mantle convection simulations on parallel computers andthushasbeendevelopedwithastrongfocusoncomputationalefficiencyandparallelscalabilityofbothmeshhandlingandnumericalsolvers rheabuildsmantleconvectionsolversona collection ofparallel octree based adaptivefiniteelement librariesthatsupport new distributed data structures and parallel algorithms for dynamic coarsening refinement rebalancing and repartitioning of the mesh in this study we demonstrate scalability to 122 880 compute cores and verify correctness of the implementation we present the numerical approximation and convergence properties using 3 d benchmark problems and other tests for variable viscosity stokes flow and thermal convection

81kr in the great artesian basin australia a new method for dating very old groundwater abstract the measurement of cosmogenic 81kr t1 2 2 29 0 11 105 yr has been proposed for many years as a reliable tool for groundwater dating in the range from 105 to 106 yr in this paper we report on the first use of 81kr to determine the age of groundwater from four wells in the great artesian basin in australia as the concentration of 81kr in old groundwater is only a few hundred atoms per liter krypton was extracted from large 16 000 l groundwater samples and was analyzed for the isotopic abundance of 81kr by accelerator mass spectrometry ams with a cyclotron 81kr kr isotope ratios of 1 54 0 22 10 13 1 78 0 26 10 13 2 19 0 28 10 13 and 2 63 0 32 10 13 respectively were measured for these samples it is reasonable to assume that krypton dissolved in surface water in contact with the atmosphere has the known atmospheric 81kr kr ratio of 5 20 0 40 10 13 the observed reduction of isotope ratios in the groundwater samples can then be interpreted as being due to radioactive decay since recharge this results in respective groundwater ages of 4 02 0 51 105 yr 3 54 0 50 105 yr 2 87 0 38 105 yr and 2 25 0 42 105 yr the main emphasis of this paper lies on the description of the analytic procedure to extract a reliable 81kr signal from large groundwater samples although the uncertainties are still relatively large primarily due to counting statistics caused by the low cyclotron ams efficiency the new technique enabled for the first time a definite determination of residence times for old groundwater with 81kr it thus confirms the hope that this radionuclide may become a very valuable tool for groundwater dating

time domain observations of a slow precursor to the 1994 romanche transform earthquake low frequency spectral anomalies have indicated that some large earthquakes are preceded by extended episodes of smooth moment release but the reality of these slow precursors has been debated because they have not been directly observed in the time domain high gain seismograms from the 14 march 1994 romanche transform event moment magnitude m w 7 0 show a precursory ramp with a moment of 7 1018 newton meters beginning about 100 seconds before the arrival of the high frequency p waves this precursor was the initial phase of a slow component of slip that released nearly half of the total moment of the earthquake such behavior may be typical for large earthquakes on the oceanic ridge transform system

investigations into the applicability of adaptive finite element methods to two dimensional infinite prandtl number thermal and thermochemical convection an adaptive finite element procedure is presented for improving the quality of solutions to convection dominated problems in geodynamics the method adapts the mesh automatically around regions of high solution gradient yielding enhanced resolution of the associated flow features the approach requires the coupling of an automatic mesh generator a finite element flow solver and an error estimator in this study the procedure is implemented in conjunction with the well known geodynamical finite element code conman an unstructured quadrilateral mesh generator is utilized with mesh adaptation accomplished through regeneration this regeneration employs information provided by an interpolation based local error estimator obtained from the computed solution on an existing mesh the technique is validated by solving thermal and thermochemical problems with well established benchmark solutions in a purely thermal context results illustrate that the method is highly successful improving solution accuracy while increasing computational efficiency for thermochemical simulations the same conclusions can be drawn however results also demonstrate that the grid based methods employed for simulating the compositional field are not competitive with the other methods tracer particle and marker chain currently employed in this field even at the higher spatial resolutions allowed by the adaptive grid strategies

identifying recharge processes into a vast fossil aquifer based on dynamic groundwater 81kr age evolution abstract water in deep aquifers in arid regions is often considered to be fossil when modern recharge rates are negligible relative to the reservoir capacity over the past five decades the nubian sandstone aquifer nsa in the arid region of the sinai peninsula egypt and the negev desert israel has been considered to contain fossil water based on 14c dating which revealed 14c ages of about 30 kyr over most of the aquifer however this relatively homogeneous age distribution contradicts the expected increase in groundwater age in the direction of decreasing piezometric head along the flow trajectories here dating results with the longer lived 81kr radioisotope t1 2 229 11 kyr are presented highlighting a wide age range of 40 kyr to 630 kyr in the confined sections of the aquifer all with very low 14c activity

tracer applications of noble gas radionuclides in the geosciences noble gas radionuclides including 81kr t1 2 229 000 years 85kr t1 2 10 8 years and 39ar t1 2 269 years possess nearly ideal chemical and physical properties for studies of earth and environmental processes recent advances in atom trap trace analysis atta a laser based atom counting method have enabled routine measurements of the radiokrypton isotopes as well as the demonstration of the ability to measure 39ar in environmental samples here we provide an overview of the atta technique and a survey of recent progress made in several laboratories worldwide we review the application of noble gas radionuclides in the geosciences and discuss how atta can help advance these fields specifically determination of groundwater residence times using 81kr 85kr and 39ar dating old glacial ice using 81kr and an 39ar survey of the main water masses of the oceans to study circulation pathways and estimate mean residence times other scientific questions involving a deeper circulation of fluids in the earth s crust and mantle are also within the scope of future applications we conclude that the geoscience community would greatly benefit from an atta facility dedicated to this field with instrumentation for routine measurements as well as for research on further development of atta methods

a slow earthquake sequence on the san andreas fault earthquakes typically release stored strain energy on timescales of the order of seconds limited by the velocity of sound in rock over the past 20 years observations1 13 and laboratory experiments14 have indicated that rupture can also occur more slowly with durations up to hours such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones the detection of events with larger timescales requires near field deformation measurements in december 1992 two borehole strainmeters close to the san andreas fault in california recorded a slow strain event of about a week in duration and we show here that the strain changes were produced by a slow earthquake sequence equivalent magnitude 4 8 with complexity similar to that of regular earthquakes the largest earthquakes associated with these slow events were small local magnitude 3 7 and contributed negligible strain release the importance of slow earthquakes in the seismogenic process remains an open question but these observations extend the observed timescale for slow events by two orders of magnitude

radiometric 81kr dating identifies 120 000 year old ice at taylor glacier antarctica we present successful 81kr kr radiometric dating of ancient polar ice krypton was extracted from the air bubbles in four 350 kg polar ice samples from taylor glacier in the mcmurdo dry valleys antarctica and dated using atom trap trace analysis atta the 81kr radiometric ages agree with independent age estimates obtained from stratigraphic dating techniques with a mean absolute age offset of 6 2 5 ka our experimental methods and sampling strategy are validated by i 85kr and 39ar analyses that show the samples to be free of modern air contamination and ii air content measurements that show the ice did not experience gas loss we estimate the error in the 81kr ages due to past geomagnetic variability to be below 3 ka we show that ice from the previous interglacial period marine isotope stage 5e 130 115 ka before present can be found in abundance near the surface of taylor glacier our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available greatly enhancing their scientific value as archives of old ice and meteorites at present atta 81kr analysis requires a 40 80 kg ice sample as sample requirements continue to decrease 81kr dating of ice cores is a future possibility

a comparison of groundwater dating with 81kr 36cl and 4he in four wells of the great artesian basin australia the isotopic ratios 81 kr kr and 36 cl cl and the 4 he concentrations measured in groundwater from four artesian wells in the western part of the great artesian basin gab in australia are discussed based on radioactive decay along a water flow path the 81 kr kr ratios are directly converted to groundwater residence times results are in a range of 225 400 kyr with error bars in the order of 15 primarily due to counting statistics in the cyclotron accelerator mass spectrometer measurement additional uncertainties from subsurface production and or exchange with stagnant porewaters in the confining shales appear to be of the same order of magnitude these 81 kr ages are then used to calibrate the 36 cl and the 4 he dating methods based on elemental analyses of rock samples from the sandstone aquifer as well as from the confining bulldog shale the in situ flux of thermal neutrons and the corresponding 3 he 4 he and 36 cl cl ratios are calculated from a comparison of i the 3 he 4 he ratios measured in the groundwater samples with the calculated in situ ratios in rocks and ii the measured n 37 cl ratios with the 4 he concentrations measured in groundwater it is concluded that both helium and chloride are most likely added to the aquifer from sources in the stagnant porewaters of the confining shale by diffusion and or mixing based on this working hypothesis the 36 cl transport equation in groundwater is solved taking into account i radioactive decay ii subsurface production in the sandstone aquifer with an in situ 36 cl cl ratio of 6u10 315 and iii addition of chloride from a source in the confining shale with a 36 cl cl ratio of 13u10 315 lacking better information it is assumed that the chloride concentration increased linearly with time from an unknown initial value ci to its

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

radiokrypton dating finally takes off methods of age determination belong to the fundamental toolkit of modern earth and environmental sciences as well as archeology radiometric dating based on the well known radioactive decay of certain isotopes is the gold standard among the dating methods with radiocarbon 14c as the most famous example however many more radioisotopes are necessary to cover the wide range of dating applications among them 81kr or radiokrypton has long been recognized as a desirable tool especially for the dating of old groundwaters and ancient polar ice but this goal has remained elusive in pnas buizert et al 1 present to my knowledge the first successful 81kr dating of polar ice this breakthrough along with two recent applications of 81kr in groundwater 2 and thermal fluids 3 signals to me that the dream of radiokrypton dating has finally become reality

mantle circulation models with variational data assimilation inferring past mantle flow and structure from plate motion histories and seismic tomography summary mantle convection models require an initial condition some time in the past because this initial condition is unknown for earth we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent mesozoic and cenozoic geological history of our planet here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation we derive the generalized inverse of mantle convection and explore the initial condition problem in high resolution 3 d spherical mantle circulation models for a time period of 100 myr roughly comparable to half a mantle overturn we present a synthetic modelling experiment to demonstrate that mid cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling assuming present day mantle structure is well known even if an initial first guess assumption about the mid cretaceous mantle involved only a simple 1 d radial temperature profile we also demonstrate that convecting present day mantle structure back in time by reversing the time stepping of the energy equation is insufficient to model the mantle structure of the past the difficulty arises because such backward convection calculations ignore thermal diffusion effects and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle

one million year old groundwater in the sahara revealed by krypton 81 and chlorine 36 measurements of 81 kr kr in deep groundwater from the nubian aquifer egypt were performed by a new laser based atom counting method 81 kr ages range from 2 10 5 to 1 x 10 6 yr correlate with 36 cl cl ratios and are consistent with lateral flow of groundwater from a recharge area near the uweinat uplift in sw egypt low 2 h values of the 81 kr dated groundwater reveal a recurrent atlantic moisture source during pleistocene pluvial periods these results indicate that the 81 kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrolugic problems

fluidity a fully unstructured anisotropic adaptive mesh computational modeling framework for geodynamics we present a new computational modeling framework fluidity for application to a range of two and three dimensional geodynamic problems with the focus here on mantle convection the approach centers upon a finite element discretization on unstructured simplex meshes which represent complex geometries in a straightforward manner throughout a simulation the mesh is dynamically adapted to optimize the representation of evolving solution structures the adaptive algorithm makes use of anisotropic measures of solution complexity to vary resolution and allow long thin elements to align with features such as boundary layers the modeling framework presented differs from the majority of current mantle convection codes which are typically based upon fixed structured grids this necessitates a thorough and detailed validation which is a focus of this paper benchmark comparisons are undertaken with a range of two and three dimensional isoviscous and variable viscosity cases in addition model predictions are compared to experimental results such comparisons highlight not only the robustness and accuracy of fluidity but also the advantages of anisotropic adaptive unstructured meshes significantly reducing computational requirements when compared to a fixed mesh simulation

implementation and application of adaptive mesh refinement for thermochemical mantle convection studies numerical modeling of mantle convection is challenging owing to the multiscale nature of mantle dynamics high resolution is often required in localized regions with coarser resolution being sufficient elsewhere when investigating thermochemical mantle convection high resolution is required to resolve sharp and often discontinuous boundaries between distinct chemical components in this paper we present a 2 d finite element code with adaptive mesh refinement techniques for simulating compressible thermochemical mantle convection by comparing model predictions with a range of analytical and previously published benchmark solutions we demonstrate the accuracy of our code by refining and coarsening the mesh according to certain criteria and dynamically adjusting the number of particles in each element our code can simulate such problems efficiently dramatically reducing the computational requirements in terms of memory and cpu time when compared to a fixed uniform mesh simulation the resolving capabilities of the technique are further highlighted by examining plume induced entrainment in a thermochemical mantle convection simulation

krypton 81 dating of the deep continental intercalaire aquifer with implications for chlorine 36 dating abstract deep groundwater samples from the continental intercalaire ci aquifer in the northern tunisian sahara have been analyzed for noble gases 3he 4he ne and 81kr and for 14c to better constrain the groundwater residence time of this large transboundary aquifer its significant radiogenic 4he content and background level 14c both indicate water older than a few tens of thousands of years distinct helium concentrations and ne he ratios suggest different groundwater flow paths through the tozeur and kebili regions 81kr is applied for the first time on the ci aquifer providing more direct evidence for the presence of old groundwater with 81kr kr ratios at 63 15 of the atmospheric value corresponding to residence times of 150 630 ka these newly obtained 81kr ages enable a re evaluation of the previously reported data from the tozeur region in order to better constrain the input values of 36cl cl and cl the initial 36cl cl ratio is found to be 4 6 times larger than previously assumed and the initial cl concentration 6 15 times smaller the results of 81kr dating and the recalibrated 36cl dating define an eastward age progression across 500 km of flow path from the algerian to tunisian sections this study exemplifies the utility of 81kr not only as an independent and conservative chronometer but also as a supplemental measurement to better calibrate the parameters required for other age tracers including cosmogenic 36cl and radiogenic 4he

constraints on slow earthquake dynamics from a swarm in central italy several clustered slow earthquakes have been recorded by a geodetic interferometer in central italy the strain rise times of the events range from tens to thousands of seconds and the seismic moment scales with the square root of the rise time this scaling law contrasts with the conservative assumption of constant rupture velocity in fault modeling but is consistent with the occurrence of a slow rupture propagation analogous to heat diffusion in a slab

sources of chloride and implications for 36cl dating of old groundwater southwestern great artesian basin australia we investigate the relative importance of a number of processes that affect cl and 36cl distribution in the southwestern flow systems of the great artesian basin australia to provide more precise estimates of age and flow and recharge rates in situ production of 36cl secular variations in the rate of production of 36cl and its subsequent fallout as well as rates of diffusion of stable cl from adjacent aquitards mask the interpretation of rates of decay of 36cl cl along hydraulic gradients and can preclude estimating absolute groundwater ages however mean flow velocities calculated from absolute cl concentrations are 0 24 0 03 m yr and are a more reliable parameter than calculated ages because fewer assumptions and approximations are required recharge rates to the main cadna owie formation and algebuckina sandstone aquifer of the great artesian basin estimated from cl mass balance in the unconfined aquifers are 0 16 0 08 mm yr and were probably up to a factor of 3 higher 30 kyr ago as indicated by decreasing cl and 18o concentrations along the head gradient

the long wavelength geoid from three dimensional spherical models of thermal and thermochemical mantle convection the earth s long wavelength geoid anomalies have long been used to constrain the dynamics and viscosity structure of the mantle in an isochemical whole mantle convection model however there is strong evidence that the seismically observed large low shear velocity provinces llsvps in the lower mantle underneath the pacific and africa are chemically distinct and likely denser than the ambient mantle in this study we have formulated dynamically self consistent 3 d spherical mantle convection models to investigate how chemically distinct and dense piles above the core mantle boundary may influence the geoid our dynamic models with realistic mantle viscosity structure produce dominantly spherical harmonic degree 2 convection similar to that of the present day earth the models produce two broad geoid and topography highs over two major thermochemical piles in the lower mantle consistent with the positive geoid anomalies over the pacific and african llsvps our geoid analysis showed that the bottom layer with dense chemical piles contributes negatively to the total geoid while the layer immediately above the chemical piles contributes positively to the geoid canceling out the effect of the piles thus the bottom part of the mantle as a compensation layer has zero net contribution to the total geoid and the thickness of the compensation layer is 1000 km or 2 to 3 times as thick as the chemical piles our results help constrain and interpret the large scale thermochemical structure of the mantle using surface observations of the geoid and topography as well as seismic models of the mantle

teleseismic search for slow precursors to large earthquakes some large earthquakes display low frequency seismic anomalies that are best explained by episodes of slow smooth deformation immediately before their high frequency origin times analysis of the low frequency spectra of 107 shallow focus earthquakes revealed 20 events that had slow precursors 95 percent confidence level 19 were slow earthquakes associated with the ocean ridge transform system and 1 was a slow earthquake on an intracontinental transform fault in the east african rift system these anomalous earthquakes appear to be compound events each comprising one or more ordinary fast ruptures in the shallow seismogenic zone initiated by a precursory slow event in the adjacent or subjacent lithosphere

chlorine stable isotope studies of old groundwater southwestern great artesian basin australia abstract stable cl isotope ratios 37 cl 35 cl were measured in groundwater samples from the southwestern flow system of the great artesian basin australia to gain a better understanding of the cl sources and transport mechanisms 37 cl values range from 0 to 2 5 smoc and are inversely correlated with cl concentration along the inferred flow direction the cl isotopic compositions in conjunction with other geochemical parameters suggest that cl in groundwaters is not derived from salt dissolution mixing of the recharge water with saline groundwater cannot explain the relationship between 37 cl and cl concentration measured marine aerosols deposited via rainfall and subsequent evapotranspiration appear to be responsible for the cl concentrations observed in wells that are close to the recharge area and in groundwaters sampled along the southern transect 37 cl values measured in the leachate of the bulldog shale suggest that the aquitard is the subsurface source of cl for the majority of groundwater samples studied diffusion is likely the mechanism through which cl is transported from the pore water of the bulldog shale to the aquifer however a more detailed study of the aquitard rocks is required to verify this hypothesis

a review of the source behaviour and distribution of arsenic in natural waters the range of as concentrations found in natural waters is large ranging from less than 0 5 g l 1 to more than 5000 g l 1 typical concentrations in freshwater are less than 10 g l 1 and frequently less than 1 g l 1 rarely much higher concentrations are found particularly in groundwater in such areas more than 10 of wells may be affected defined as those exceeding 50 g l 1 and in the worst cases this figure may exceed 90 well known high as groundwater areas have been found in argentina chile mexico china and hungary and more recently in west bengal india bangladesh and vietnam the scale of the problem in terms of population exposed to high as concentrations is greatest in the bengal basin with more than 40 million people drinking water containing excessive as these large scale natural as groundwater problem areas tend to be found in two types of environment firstly inland or closed basins in arid or semi arid areas and secondly strongly reducing aquifers often derived from alluvium both environments tend to contain geologically young sediments and to be in flat low lying areas where groundwater flow is sluggish historically these are poorly flushed aquifers and any as released from the sediments following burial has been able to accumulate in the groundwater arsenic rich groundwaters are also found in geothermal areas and on a more localised scale in areas of mining activity and where oxidation of sulphide minerals has occurred the as content of the aquifer materials in major problem aquifers does not appear to be exceptionally high being normally in the range 1 20 mg kg 1 there appear to be two distinct triggers that can lead to the release of as on a large scale the first is the development of high ph 8 5 conditions in semi arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates this ph change leads either to the desorption of adsorbed as especially as v species and a range of other anion forming elements v b f mo se and u from mineral oxides especially fe oxides or it prevents them from being adsorbed the second trigger is the development of strongly reducing conditions at near neutral ph values leading to the desorption of as from mineral oxides and to the reductive dissolution of fe and mn oxides also leading to as release iron ii and as iii are relatively abundant in these groundwaters and so4 concentrations are small typically 1 mg l 1 or less large concentrations of phosphate bicarbonate silicate and possibly organic matter can enhance the desorption of as because of competition for adsorption sites a characteristic feature of high groundwater as areas is the large degree of spatial variability in as concentrations in the groundwaters this means that it may be difficult or impossible to predict reliably the likely concentration of as in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well arsenic affected aquifers are restricted to certain environments and appear to be the exception rather than the rule in most aquifers the majority of wells are likely to be unaffected even when for example they contain high concentrations of dissolved fe

chemical weathering processes in the great artesian basin evidence from lithium and silicon isotopes variations in lithium and silicon isotope ratios in groundwaters of the great artesian basin in australia and the causes of these variations have been explored the chemistries of li and si in groundwater are influenced by the dissolution of primary phases the formation of secondary minerals and the reaction of solid phases with dissolved constituents while isotopic variations are generated by uptake into clays which preferentially incorporate the light isotopes the lithium isotopic composition expressed as view the mathml source li7 of the groundwaters ranges from 9 to 16 16 and clearly reflects changes in aquifer conditions reaction transport modelling indicates that changes in li concentrations are principally controlled by the ratio of the weathering rate of primary minerals to the precipitation rate of secondary minerals whereas view the mathml source li7 is affected by the extent of isotope fractionation during secondary mineral formation which is dependent on mineralogy the patterns of groundwater si concentrations and view the mathml source si30 values versus flow distance suggest that si is at steady state in the aquifer the view the mathml source si30 value of most of the groundwater samples is close to 1 1 which is significantly lower than the view the mathml source si30 value of the reservoir rocks 0 0 since precipitation of clays preferentially removes the light si isotopes from solution the most plausible explanation for these low groundwater view the mathml source si30 values is addition of si by dissolution of isotopically light secondary minerals these data together with model calculations show that li isotopes are extremely sensitive to changes in the chemical and physical conditions in the aquifer whereas si is not importantly the model suggests that even in large aquifers with long fluid residence times where steady state would be expected to be reached the concentrations and isotopic fractionation of trace elements are not controlled by li adsorption the model developed here provides a basis for using li isotopes measured in groundwaters and surface waters to constrain weathering processes

using thermo mechanical models of subduction to constrain effective mantle viscosity mantle convection and plate dynamics transfer and deform solid material on scales of hundreds to thousands of km however viscoplastic deformation of rocks arises from motions of defects at sub crystal scale such as vacancies or dislocations in this study results from numerical experiments of dislocation dynamics in olivine for temperatures and stresses relevant for both lithospheric and asthenospheric mantle 800 1700 k and 50 500 mpa gouriet et al 2019 are used to derive three sigmoid parameterizations erf tanh algebraic which express stress evolution as a function of temperature and strain rate the three parameterizations fit well the results of dislocation dynamics models and may be easily incorporated into geodynamical models here they are used in an upper mantle thermo mechanical model of subduction in association with diffusion creep and pseudo brittle flow laws simulations using different dislocation creep parameterizations exhibit distinct dynamics from unrealistically fast sinking slabs in the erf case to very slowly sinking slabs in the algebraic case these differences could not have been predicted a priori from comparison with experimentally determined mechanical data since they principally arise from feedbacks between slab sinking velocity temperature drag and buoyancy which are controlled by the strain rate dependence of the effective asthenosphere viscosity comparison of model predictions to geophysical observations and to upper mantle viscosity inferred from glacial isostatic adjustment shows that the tanh parameterization best fits both crystal scale and earth scale constraints however the parameterization of diffusion creep is also important for subduction bulk dynamics since it sets the viscosity of slowly deforming domains in the convecting mantle within the range of uncertainties of experimental data and most importantly of the actual rheological parameters prevailing in the upper mantle e g grain size chemistry viscosity enabling realistic mantle properties and plate dynamics may be reproduced by several combinations of parameterizations for different deformation mechanisms deriving mantle rheology cannot therefore rely solely on the extrapolation of semi empirical flow laws the present study shows that thermo mechanical models of plate and mantle dynamics can be used to constrain the effective rheology of earth s mantle in the presence of multiple deformation mechanisms

evidence of bias in estimations of earthquake size magnitudes of earthquakes determined using traditional methods show systematic deviations dependent on tectonic setting from accurate estimates of earthquake size magnitudes are overestimated for most of the continental earthquakes underestimated for mid oceanic ridges and differences of varying sign are obtained for subduction zones this is important in evaluating the seismic risk or consideration of effects such as the seismic versus aseismic strain release as seismic moments determined from magnitudes can be wrong by as much as a factor of four

segmentation of the farallon slab abstract recent tomography images reveal a complex 3d mantle structure beneath western united states with feature morphology varying rapidly with depth by assimilating plate motion history paleo age of sea floor and paleo geography of plate boundaries in a 3 d numerical model we simulate the farallon juan de fuca subduction during the past 40 ma we find that the highly segmented upper mantle structure of western u s is a direct result of the farallon subduction we show that the tilted horseshoe shaped fast seismic anomaly beneath nevada and utah at 300 600 km depth range is in fact a segment of curled slab subducted since 15 ma and the shallower linear slab beneath the cascades is younger than 5 ma the distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench rollback since 20 ma the northward migrating jf pa na triple junction and the toroidal flow around slab edges the observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape depth and location of modeled subducted slab segments the inferred viscosity for the asthenosphere is 5 10 19 pa s and those for the transition zone and lower mantle are 1 5 10 21 pa s and 2 10 22 pa s respectively the slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth but of similar order of magnitude viscosity in the transition zone

slow earthquakes along oceanic fracture zones evidence for asthenospheric flow away from hotspots abstract the regime of strain release along transform faults of the mid oceanic ridge system is studied it is shown that earthquakes along certain fracture zones exhibit systematic discrepancies between values of their magnitudes measured at short and long periods implying a regime of slower strain release also observed in a pattern of complex body waveshapes these slow fracture zones do not correlate with simple geographic or kinematic properties but usually occur in the neighborhood of hotspot volcanism frequently also characterized by gravity and bathymetry anomalies we propose that regimes of slow strain release may be due to a partial lubrication of the fault along these fracture zones which may itself be due to asthenospheric flow from the nearby hotspots along the pattern of pipelines described by vogt and johnson and by morgan

frequency dependent relocation of the 1992 nicaragua slow earthquake an empirical green s function approach we present a two step empirical green s function analysis of the slow 1992 nicaragua earthquake mw 7 7 in the first step a narrow band cross correlation method recovers he relative source amplitude and phase delay spectra between the main shock and a suitable empirical green s function egf the analysis is performed at 2mhz intervals from 4 to 44mhz using first and second orbit fundamental mode rayleigh waves from the low frequency relative amplitudes we obtain the differences in mechanism and depth between the main shock and the egf and we present a method to correct for such differences in the second step we use the relative spectra to relocate the main shock relative to the egf position al a set of discrete frequencies in addition we invert the relative spectra for the relative source time functions using both a positivity and a smoothing constraint unlike standard egf deconvolution the spectral resolution achieved in our cross correlation method allows us to determine the conditions of validity of the egf approach from our analysis it appears that the centre of low frequency radiation 4 10 mhz during the slow 1 992 nicaragua earthquake is situated about 40 km trenchwards of the epicentre between 20 and 44 mhz centroids occupy intermediate positions between the low frequency points and the epicentre relative source time functions have an average duration of about 120 s they reveal a slow 1 km s1 smooth component of rupture with unilateral directivity towards the se along the trench axis a large amount of slip occurred at shallow depths at the interface between the subducted cocos plate and the caribbean plate in agreement with tsunami models of the event the low frequency radiation appears to originate from a zone devoid of teleseismic aftershocks this suggests that unusual frictional properties at the plate interface perhaps related to subducted sediments are responsible for the slow and smooth nature of the 1992 nicaragua earthquake

dynamics of a phase change at the base of the mantle consistent with seismological observations the phase change model for the origin of the d seismic discontinuity is tested by comparing the results of convection modeling with seismic observations we compute a number of global dynamic models that incorporate a phase change at the base of the mantle with different characteristics and transform the resulting temperature field and the distribution of phases to seismic velocities over 900 two dimensional synthetic waveforms are computed for each of the models from which s scs and scd phases are picked the distribution of the relative amplitudes and differential travel time residuals for these phases are statistically compared with the distribution of data from four well studied regions northern siberia alaska india and central america in a search for the characteristics of a phase transition that best match these seismic observations we find that the best fit among the models considered is obtained for phase transitions characterized by a clapeyron slope of 6 mpa k 1 and an elevation above the core mantle boundary of 150 km under adiabatic temperature or 127 gpa and 2650 k on a p t diagram dynamic models demonstrate that the value of clapeyron slope and the density difference between the phases can have significant influence on the dynamics of plumes but probably only a minor influence on the dynamics of subducted slabs we find that the thermal structure of subducted slabs can be important in giving rise to the seismic triplication the strongest scd arrivals in our models are observed in the area of subduction the folding of the slab at the base of the mantle leads to patterns in differential travel time distributions consistent with seismic observations and suggests that the largest heterogeneity occurs at the top of the d layer or just above it analysis of the spatial autocorrelation functions of the differential travel time residuals suggests that their characteristic peaks reflect the patterns of slab folding and may provide constraints on the rheology of slabs at the base of the mantle

a benchmark comparison for mantle convection codes summary we have carried out a comparison study for a set of benchmark problems which are relevant for convection in the earth s mantle the cases comprise steady isoviscous convection variable viscosity convection and time dependent convection with internal heating we compare nusselt numbers velocity temperature heat flow topography and geoid data among the applied codes are finite difference finite element and spectral methods in a synthesis we give best estimates of the true solutions and ranges of uncertainty we recommend these data for the validation of convection codes in the future

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

a reassessment of the rupture characteristics of oceanic transform earthquakes 1 we investigate the long period source spectra of oceanic transform earthquakes and find that previously proposed slow rupture components can be explained as artifacts generated by the modeling procedure we use low frequency 20 mhz rayleigh and love waves to calculate the amplitude spectra of five earthquakes on the romanche and chain transform faults in the equatorial mid atlantic ocean we find that errors and approximations in the centroid depth focal mechanism and earth structure at the source have significant effects on the shape of the source spectra if global catalog values and an average crustal model are assumed the spectra exhibit apparent anomalous energy at long periods which has previously been interpreted as a result of slow rupture we recalculate the source spectra using precise independently determined depths and moment tensors and a more realistic oceanic crustal structure in the source region the resulting source spectra are flat at long periods with no indication of anomalous long period energy our results imply that oceanic transform earthquakes do not commonly have detectable slow rupture components

latest development of radiokrypton dating a tool to find and study paleogroundwater abstract the study of paleogroundwater on a timescale of several thousands to a million years is interesting both in terms of understanding the aquifer system itself and water resource management it may also provide valuable information about the paleoclimate in their seminal work loosli and oeschger 1969 identified 81kr as an ideal age tracer for paleogroundwater up to one million years old since then radiokrypton dating has been a desirable tool recent developments in the atom trap trace analysis method have made radiokrypton dating available to the earth science community at large in this paper we will show the current status for radiokrypton dating with an emphasis on sample size reduction the latest advancements have allowed analysis with only 20 kg of water moreover we will review selected studies that have been carried out on some large aquifers with old groundwater around the world using this new tool

source time function complexity of the great 1989 macquarie ridge earthquake the rupture process of the great 1989 macquarie ridge mw 8 1 strike slip faulting earthquake is examined using an empirical green function deconvolution method which provides broadband source time functions from both body and surface waves recordings from a nearby much smaller mw 6 4 strike slip event provide accurate broadband green functions for pp s ss rayleigh and love waves spanning the period range 20 150 s body wave deconvolutions indicate that the principle short period 30 s moment release occurred in the first 25 s of rupture with no resolvable directivity only minor short period energy release occurred over the next 30 s surface wave deconvolutions for the usually neglected 20 150 s period range produce source time functions with slightly larger total moment than the body wave deconvolutions and no resolvable directivity from the onset to the peak of moment release in the first 20 s however azimuthally dependent broadening of the surface wave source functions which extend to more than 50 s duration indicates a component of northeastward unilateral rupture extension requiring about 30 50 km asymmetry in the overall rupture compatible with the northeastward asymmetry of the 1 day aftershock zone spectral inversions of long period love and rayleigh waves indicate a centroid time shift of approximately 25 3 s 50 7s duration trapezoid source in the period range of 150 280 s compatible with the deconvolved surface wave source time functions deconvolutions using both empirical and theoretical green functions are optimized to detect any long period precursory radiation for periods up to about 250 s but no convincing evidence for precursory radiation in the passband is found the long period surface wave phase data also do not require a precursor

mantle plume heads and the initiation of plate tectonic reorganizations we present results of numerical simulations which examine the plausibility of a reorganization of plate motions brought about by the interaction of a hot low viscosity plume head with the underside of a tectonic plate a numerical model of highly viscous fluid flow driven by thermal buoyancy is employed to examine the interaction of mantle plumes with surface plates in a two dimensional cartesian geometry plate like boundary conditions applied to the upper surface consist of a large active plate with fixed speed plus a smaller passive plate whose speed and direction of motion are permitted to react to tractions induced by the underlying mantle flow driven by the active plate the passive plate is also subject to applied end loads intended to simulate incipient subduction the mantle plume is initiated with a hot patch on the lower boundary and viscosity varies with temperature following an arrhenius rheology the calculations performed with fixed aspect ratio 4 1 and fixed rayleigh number ra 106 where ra refers to the strength of the starting plume were found to depend on three parameters the strength of the viscosity variation the size of the applied end load and the speed of the active plate these calculations indicate that plate motions can be sharply influenced by the lubricating effects of mantle plume heads even to the point of causing a reversal in the direction of plate motion

the 1992 nicaragua earthquake a slow tsunami earthquake associated with subducted sediments the 1992 nicaragua earthquake was a tsunami earthquake that is it generated tsunamis disproportionately large for its surface wave magnitude m s the moment magnitude m w determined from long period 250 s surface waves was 7 6 significantly larger than the 20 s m s of 7 this m s m w disparity is also characteristic of tsunami earthquakes the nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms we conclude that the nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the cocos and north american plates and because of the absence of sediments on the trench floor offshore of nicaragua the slip propagated up dip all the way to the ocean bottom exciting large tsunamis the occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction zone thrust earthquakes our results reinforce the idea that tsunami warning systems using long period 100 s waves are necessary to reduce the hazard from this type of earthquake

geomagnetic modulation of the 36cl flux in the grip ice core greenland geomagnetic field strength is expected to affect the production rate of cosmogenic isotopes such as beryllium 10 carbon 14 or chlorine 36 chlorine 36 data from the greenland ice core project grip ice core agree well with a production rate calculation based on a paleomagnetic reconstruction for the past 100 000 years over both long and short term variations a chlorine 36 peak at 38 000 years ago previously found in the beryllium 10 record from the vostok ice core can be explained by a period of low geomagnetic field intensity

are mantle plumes adiabatic abstract the issue concerning the state of adiabaticity of mantle plumes has been examined in a cartesian two dimensional box with an aspect ratio of six we have investigated in the quasi steady state regime high rayleigh number convection with both depth dependent viscosity and thermal expansivity for both the boussinesq and the extended boussinesq approximations we have also assessed the influence of various forms of thermal conductivity and internal heating we have generalized the classical bullen s parameter equation from one dimension to multidimensions for assessing the local state of adiabaticity inside plumes and in their surroundings we have extracted from the local geotherms and the local thermodynamic properties the corresponding bullen s parameter profiles and the two dimensional maps portraying the state of adiabaticity in the mantle histograms characterizing the frequency of adiabaticity are also employed for quantification purposes in general superadiabatic thermal gradients are found inside the thick plume limbs and sometimes along the central part of the plume the centers of plume heads are subadiabatic or nearly adiabatic but the edges of the plume heads are strongly subadiabatic alternating strips of subadiabaticity and adiabaticity are found in the downwellings the ambient mantle outside the plumes is generally adiabatic and is sometimes perforated with islands of marked deviations from adiabaticity

the 1994 java tsunami earthquake slip over a subducting seamount on june 2 1994 a large subduction thrust earthquake ms 7 2 produced a devastating tsunami on the island of java this earthquake had a number of unusual characteristics it was the first recorded large thrust earthquake on the java subduction zone all of the aftershock mechanisms exhibit normal faulting no mechanisms are similar to the main shock also the large tsunami and the relatively low energy radiated by the main shock have led to suggestions that this earthquake might have involved slow shallow rupture near the trench similar to the 1992 nicaragua earthquake we first relocate the main shock and the aftershocks we then invert long period surface waves and broadband body waves to determine the depth and spatial distribution of the main shock slip a dip of 12 hypocenter depth of 16 km and moment of 3 5 l020 n m mw 7 6 give the best fit to the combined seismic data and are consistent with the plate interface geometry the source spectrum obtained from both body and surface waves has a single corner frequency between 10 and 20 mhz implying a stress drop of 0 3 mpa the main energy release was preceded by a small subevent lasting 12 s the main slip occurred at 20 km depth downdip and to the nw of the hypocenter this area of slip is collocated with a prominent high in the bathymetry that has been identified as a subducting seamount we interpret the java earthquake as slip over this subducting seamount which is a locked patch in an otherwise decoupled subduction zone we find no evidence for slow shallow rupture no thrust aftershocks are expected if the entire locked zone slipped during the main shock but extension of the subducting plate behind the seamount would promote normal faulting as observed it seems probable that such a source model could also explain the size and timing of the observed tsunami

simulation of late cenozoic south american flat slab subduction using geodynamic models with data assimilation abstract the formation mechanisms of flat slabs in south america remain unclear to quantitatively evaluate the earlier proposed mechanisms we simulate the post 100 ma subduction history below south america using 4 d geodynamic models by progressively incorporating plate kinematics seafloor ages and key tectonic features including the buoyant oceanic crust continental cratons oceanic plateaus i e the inferred inca plateau subducting nazca ridge and juan fernandez ridge as well as deformable trench profiles according to recent geological reconstructions we find that in the absence of an overriding plate and subducting buoyancy features the seafloor age affects slab dip angle by controlling the slab s mechanical strength i e the resistance to bending and negative buoyancy integrated positive density anomaly that enhances bending our models show that slab strength dominates its buoyancy at age 30 ma and the opposite for younger ages the existence of a thick overriding plate reduces the slab dip by increasing dynamic suction and individual cratonic roots further lead to along trench variations of dip angle reduction while dynamic suction from the overriding plate generates a permanent reduction of the long wavelength slab dip angle it is the final addition of subducting oceanic plateau and aseismic ridges that produces the transient and localized flat slabs as observed these results suggest that all mechanisms except the buoyancy features affect the slab dip only at large spatial scales our best fit model with all the above tectonic features included provides a good match to both the upper mantle benioff zones and the temporal evolution of volcanic arcs since the mid miocene the imperfect match of the peruvian flat slab is likely associated with the uncertain 3 d configuration of the amazonian craton

interaction of subducted slabs with the mantle transition zone a regime diagram from 2 d thermo mechanical models with a mobile trench and an overriding plate transition zone slab deformation influences earth s thermal chemical and tectonic evolution however the mechanisms responsible for the wide range of imaged slab morphologies remain debated here we use 2 d thermo mechanical models with a mobile trench an overriding plate a temperature and stress dependent rheology and a 10 30 or 100 fold increase in lower mantle viscosity to investigate the effect of initial subducting and overriding plate ages on slab transition zone interaction four subduction styles emerge i a vertical folding mode with a quasi stationary trench near vertical subduction and buckling folding at depth vf ii slabs that induce mild trench retreat which are flattened horizontally deflected and stagnate at the upper lower mantle interface hd iii inclined slabs which result from rapid sinking and strong trench retreat isr iv a two stage mode displaying backward bent and subsequently inclined slabs with late trench retreat bir transitions from regime i to iii occur with increasing subducting plate age i e buoyancy and strength regime iv develops for old strong subducting and overriding plates we find that the interplay between trench motion and slab deformation at depth dictate the subduction style both being controlled by slab strength which is consistent with predictions from previous compositional subduction models however due to feedbacks between deformation sinking rate temperature and slab strength the subducting plate buoyancy overriding plate strength and upper lower mantle viscosity jump are also important controls in thermo mechanical subduction for intermediate upper lower mantle viscosity jumps 30 our regimes reproduce the diverse range of seismically imaged slab morphologies

further evidence for the compound nature of slow earthquakes the prince edward island earthquake of april 28 1997 seismograms of the april 28 1997 prince edward island transform fault earthquake mw 6 8 show clear evidence for the superposition of two types of rupture a normal earthquake and a slow component which primarily radiated energy at low frequencies we combined low frequency spectral data with p waveform data to invert for a source time function that satisfied data from 0 001 to 2 5 hz the results show that the earthquake started with a slow event which began 15 s before the fast mainshock and was part of a slow earthquake which lasted over 30 s the smoothness of the initial event is inferred from the low amplitude of high frequency energy at regional stations during the initial portion of the p wave arrival directivity analysis shows that the two components of the rupture occurred on different faults the slow event occurred on the main transform and the fast event occurred on a parallel fault 40 km to the east a similar compound sequence involving both slow and fast components and rupture on multiple faults occurred on the romanche transform in 1994 these and other results for slow earthquakes suggest that many large events on oceanic transform faults are compound events

global creepex distribution and its relation to earthquake source geometry and tectonic origin summary the ms mb ratio conventionally used for the discrimination of earthquakes and underground nuclear explosions has been studied for a pure earthquake catalogue in an attempt to classify earthquakes into two categories one related to extremely lowfrequency events of creep and the other to relatively high frequency explosions ms and mb measured at 20 s and 1 s respectively are considered as estimates of source spectra at two separate points thus allowing one to compare large samples of earthquakes from the spectral point of view creepex or the deviation of ms mfc points from the orthogonal regression of ms on mb is by definition a parameter independent of the size of seismic events it has been estimated from neic magnitudes and is studied in relation to parameters of central moment tensor solutions of the harvard group for this purpose the parameters of central moment tensors have been factorized to make classes of equivalence of events with the same dip and rake and arbitrary strike and size the resulting triangle representation of source mechanisms has been used to correct the global distribution of creepex for the type of source mechanism dip slip events have consistently lower creepex than strike slip ones however the global distribution of creepex either corrected or uncorrected for source mechanism has a clear tectonic pattern negative values in most of the subduction zones and positive ones in mid ocean ridges the dependence of creepex on focal depth is negligible according to empirical data for events shallower than 40 km limitation of the sample by considering only events in this depth range does not diminish considerably the amount of data and does not change the tectonic pattern of the creepex

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

using excess 4he to quantify variability in aquitard leakage summary fluid flux through aquitards controls the rate of recharge discharge cross formational fluid flow and contaminant transport in subsurface systems in this paper concentrations of 4 he are used to investigate the spatial distribution of vertical fluid flux through the regionally extensive great artesian basin aquitard system in northern south australia two vertical profiles of 4 he concentration in aquitard pore water augmented with regional sampling of aquifers above and below the aquitard were used to estimate fluid flux at multiple locations over a large spatial area 4 he concentrations in the shallow aquifer above the great artesian basin range from atmospheric equilibrium to 1000 times enriched over atmosphere fluid flux through the aquitard was estimated by fitting observed helium concentrations at each sampling site with a 1 d model of helium transport through the aquitard estimated fluid fluxes through the aquitard vary over three orders of magnitude across the study area in areas of competent aquitard fluid fluxes are less than 0 003 mm yr and mass transport of helium is dominated by molecular diffusion preferential discharge zones are clearly identifiable with fluid fluxes up to 3 mm yr our results show that fluid flux through a regionally extensive aquitard can be highly variable at large spatial scales and that 4 he concentrations in aquifers bounding the aquitard system provide a convenient and sensitive method for investigating aquitard flux at the regional scale

bias in the centroid moment tensor for central asian earthquakes evidence from regional surface wave data surface waves recorded on the chinese digital seismic network cdsn station urumqi wmq are analyzed for 26 centroid moment tensor cmt earthquakes located within an epicentral distance of 1300 km applying a simple grid search method for source parameter retrieval i obtain revised estimates of the focal depth fault strike and seismic moment m sub 0 from amplitude spectra of love and rayleigh waves and compare these estimates with the cmt best double couple solutions the comparisons show that residuals are unbiased for focal depth and fault strike where population spreads are plus minus 13 km and plus minus 17 degree respectively on the other hand m sub 0 estimates are found to be biased with cmt m sub 0 larger than regional estimates by an average of 0 27 plus minus 0 04 log units the results for focal depth and strike are consistent with previous global and regional studies comparing independent estimates with cmt results recent full waveform modeling studies for central asian earthquakes support the findings of bias in m sub 0 estimates i suggest that causes for m sub 0 bias may be related to great thicknesses of continental crust in hindu kush and tien shan regions and to data censoring practices at small magnitudes m sub 0 m sub b scaling relationships for central asianmore earthquakes show better agreement with western u s scaling when m sub 0 estimates determined in this study are used copyright 1998 american geophysical union less

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses