a high resolution p wave model for the top 1200 km of the mantle this work attempts to map aspherical velocities in the top 1200 km of the mantle that are illuminated by the international seismological centre p wave data the processing includes summary ray sorting and hypocentral redetermination to cope with the extreme variation of ray coverage a multicell inversion is used to simultaneously constrain anomalies in overlapping cells the multicell solutions are summed after inversion to form the final model named p1200 at cell size of 1 1 and about 50 km in thickness the lateral resolution of the model is about 1 2 in major subduction zones and about 5 15 in most places major features in the model withstood tests on signal coherency and different reference models the long wavelength parts of the p1200 model correlate well with previous global models especially the s12 model of shear wave among the large features in the transition zone and below there are two fast bands one from canada to south america and another from siberia through east asia and the philippine sea to western australia there are also slow patches under the central pacific and from india to eastern africa though most subduction zones have slab like high velocity anomalies they are sandwiched by broad slow velocities at shallow depths some of the slabs appear to penetrate the 660 km discontinuity and stagnant slabs are also seen in the transition zone plume like slow anomalies exist below some prominent hot spots like hawaii iceland yellowstone and french polynesia

volcanic eruption velocities measured with a micro radar a novel technique to quantitatively assess volcanic eruption dynamics is presented we report on measurements of volcanic eruption velocities with a modified micro radar doppler anemometer on stromboli volcano italy the mean vertical eruption velocity recorded during a four day period in late 1996 was around 10 m s which is somewhat on the small side for strombolian eruptions however the activity of stromboli volcano during this period was very weak explaining the relatively small velocities the technique used is much better in resolution than older techniques based on static photos and allows a continuous monitoring of the volcanic activity in addition to the velocity information recorded by the doppler spectrum it also contains information on the density of the ejected material and the size distribution of the particles in the volcanic jet

strombolian explosions 1 a large bubble breaking at the surface of a lava column as a source of sound strombolian activity consists of a series of explosions caused by the breaking of large overpressurized bubbles at the surface of the magma column acoustic pressure has been measured for 36 explosions at stromboli we propose that sound is generated by the vibration of the bubble before it bursts oscillations are driven by an initial overpressure inside the bubble assumed to be initially at rest just below the magma air interface inertia effects cause the bubble to overshoot its equilibrium radius then the bubble becomes underpressurized and contracts because of gas compressibility these oscillations are only slightly damped by viscous effects in the magma layer above the bubble the bubble cannot complete more than one cycle of vibration because of instabilities developing on the magma layer that lead to its breaking near the minimum radius assuming a simple geometry we model this vibration and constrain the radius and length of the bubble and the initial overpressure by fitting a synthetic waveform to the measured acoustic pressure the fit between synthetic and observed waveforms is very good both for frequency 60 rad s 1 and amplitude the initial bubble radius is 1 m and the length varies between several and a few tens of meters from the initial overpressure approximately 105 pa we calculate the maximum radial velocity of ejecta 30 m s 1 the generally good agreement between data and predictions of our model leads us to suggest that acoustic measurements are a powerful tool for the understanding of eruption dynamics

environmental isotope study of seawater intrusion in the coastal aquifer syria seawater intrusion into the shallow aquifer in the syrian coast north of latakia damsarkho ras ibn hani and south of tartous al hamidieh ein zarka was studied using hydrochemical and isotopic techniques the electrical conductivity ec distribution map north of latakia revealed that mixing in this area is the consequence of a frontal intrusion of seawater within the fresh groundwater aquifer which in turn results from intensive pumping since the 1960s which has lowered the water table inland below sea level in ein zarka south of tartous in contrast the ec distribution revealed that seawater intrusion is due to local up coning as a result of intensive pumping the deuterium and oxygen 18 relationship is that of a mixing line with a slope of 5 55 indicating an intrusion between freshwater and seawater in addition the relationship between oxygen 18 and chloride reveals that the mixing has a dominant role compared to evaporation process the mixing ratios are estimated to be between 6 and 10 north of latakia while they do not exceed 3 south of tartous a tritium model was applied to compute the mean transit time which is estimated to be around 10 years on average to reach the equilibrium that existed originally between the fresh groundwater and seawater provided that severe pumping is completely halted and the aquifer is naturally recharged by rainfall and deep percolation of irrigation water thereby allowing the restoration of the hydraulic gradient

radial models of mantle viscosity results from a genetic algorithm summary the long wavelength component of the geoid has been used to constrain radial variations in mantle viscosity using 3 d velocity anomaly models determined from seismic tomography as the buoyancy forces driving viscous flow using a genetic algorithm ga the robustness of mantle viscosity models obtained by calculusbased optimization methods is investigated gas are a relatively new class of optimization techniques which are generating a growing interest in the geophysics community these methods can be particularly useful for solving highly non linear optimization problems unlike traditional techniques gas do not require derivative information to form a jacobian matrix or sensitivity kernels models are constructed from pieces of successful models following a simple set of rules these simple rules are driven by stochastic rather than deterministic means because of the efficiency of the ga for the mantle viscosity problem it is possible to explore a greater range of potential solutions than with a traditional optimization method it is found that while both calculus based and ga optimization methods find viscosity models with a low viscosity transition zone the ga finds another class of models with a high viscosity in the transition zone that also satisfies the geoid data the geoid alone is unable to resolve low viscosity transition zone models from high viscosity transitionzone models the effect of the scaling between seismic velocity and density on the preferred viscosity model is also explored while a velocity to density scaling with a pronounced increase in the transition zone provides the best fit to the geoid the preferred vertical viscosity profile is insensitive to the velocity to density scaling

can long wavelength dynamical signatures be compatible with layered mantle convection analyses of the long wavelength geoid with seismic tomographic models have been providing for a long time important estimates of mantle viscosity these estimates have nearly been derived under the assumption of whole mantle flow it has been commonly held that a fully impermeable boundary at 660 km depth is incompatible with the long wavelength gravity signal on the other hand models with whole mantle circulation which can explain a large portion of the geoid signal usually produce excessive amplitudes of the dynamical topography especially for long wavelengths using recent tomographic models together with genetic algorithm we have successfully demonstrated that the layered convection model can also produce a reasonable fit to the geoid which is comparable in quality with that obtained for the whole mantle model the layered model can simultaneously yield realistic amplitudes of the dynamical topographies of the surface and the 660 km discontinuity

tides for a convective earth in this paper we give values of the tidal gravimetric factor as well as of the love numbers for the tidal surface displacement and for the tidal mass redistribution potential that are consistent with the presently adopted definitions we present analytical expressions for these quantities and compute numerical values for two rotating nonspherical earth models in the first model the earth is everywhere in hydrostatic equilibrium and the inner core and mantle are both elastic in the second model the earth is ellipsoidal with an inelastic mantle and with a nonhydrostatic initial state for which the effects of mantle convection and its associated boundary deformations are considered this latter model is constrained to reproduce the observed free core nutation period and global earth dynamical flattening

geoid anomalies and the structure of continental and oceanic lithospheres geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere on the nature of upper mantle seismic anomalies and on the thermal evolution of the oceanic lithosphere for the shallow lithospheric density anomalies considered here the geoid anomaly is shown to follow approximately the moment law derived from isostasy by a procedure of least squares inversion the geoid over old cratons is found to be equal to that over relatively young 50 myr oceanic lithosphere the oceanic geoid as a function of age follows the expected linear trend with flattening at old ages these results are independent of the degree at which the spherical harmonics are truncated if the tectosphere consisted only of a thick thermal boundary layer marked geoid lows with an amplitude larger than 20 m should be present over cratons in disagreement with the present observation the predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere this analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir the density of these cold roots is similar to that of mantle material elsewhere because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small scale convection

the gas content and buoyancy of strombolian ash plumes plinian plumes erupt with a bulk density greater than that of air and depend upon air entrainment during their gas thrust phase to become buoyant if entrainment is insufficient the column collapses into a potentially deadly pyroclastic flow this study shows that strombolian ash plumes can be erupted in an initially buoyant state due to their extremely high initial gas content and in such cases are thus impervious to column collapse the high gas content is a consequence of decoupled gas rise in the conduit in which particles are ultimately incidental the relations between conduit gas flow eruption style and plume density are explored here for strombolian scenarios and contrasted with conventional wisdom derived from plinian eruptions considering the inherent relation between gas content and initial plume density together with detailed measurements of plume velocities can help unravel ambiguities surrounding conduit processes eruption styles and hazards at poorly understood volcanoes analysis of plume dynamics at santiaguito volcano guatemala adds further support for a model involving decoupled gas rise in the conduit

the role played by a low viscosity zone under a 660 km discontinuity in regional mantle layering abstract our previous work from regional geoid inversion over oceans has shown that there exists a low viscosity zone lvz under the 660 km boundary nonlinear inversion has been applied for fitting to the intermediate wavelength oceanic geoid without invoking a priori any impermeable boundary in this study we have conducted a systematic search of the dependence of the depth of this regional boundary from 660 to 1000 km we found that the use of this boundary at the depth of the lvz exerts little influence on the inversion the lvz can act regionally as an impermeable boundary and enhances very fast lateral flow at the uppermost lower mantle on the other hand impermeable boundary alone is not caused by a lvz as shown also by analysis of the 2d flow patterns driven by internal density anomalies inferred from tomography from the intermediate wavelength geoid we can not resolve the existence of layering when the lvz exists beneath the 660 km boundary however a lvz is needed to fit the geoid this lvz can have profound influence on the geodynamical behavior of mantle circulation and the dispersal of geochemical anomalies and the generation of seismic anisotropy

radial profiles of temperature and viscosity in the earth s mantle inferred from the geoid and lateral seismic structure abstract in the framework of dynamical modelling of the geoid we have estimated basic features of the radial profile of temperature in the mantle the applied parameterization of the geotherm directly characterizes thermal boundary layers and values of the thermal gradient in the upper and lower mantle in the inverse modelling scheme these parameters are related to the observables geoid and seismic structure of the mantle through the viscosity profile which is parameterized as an exponential function of pressure and temperature we have tested 104 model geotherms for each of them we have found proper rheological parameters by fitting the geoid with the aid of a genetic algorithm the geotherms which best fit the geoid show a significant increase of temperature 600 800 c close to the 660 km discontinuity the value of the thermal gradient in the mid mantle is found to be sub adiabatic both a narrow thermal core mantle boundary layer and a broad region with a superadiabatic regime can produce a satisfactory fit of the geoid the corresponding viscosity profiles show similarities to previously presented models in particular in the viscosity maximum occurring in the deep lower mantle the best fitting model predicts the values of activation volume v and energy e which are in a good agreement with the data from mineral physics except for v in the lower mantle which is found somewhat lower than the estimate based on melting temperature analysis an interesting feature of the viscosity profiles is a local decrease of viscosity somewhere between 500 and 1000 km depth which results from the steep increase of temperature in the vicinity of the 660 km discontinuity

subducted slabs and lateral viscosity variations effects on the long wavelength geoid summary the characteristic broad local maxima exhibited by the long wavelength geoid over subduction zones are investigated with a numerical model of mantle flow in a spherical axisymmetric geometry a synthetic model of buoyancy driven subduction is used to test the effects on the geoid caused by the depth of penetration of the lithosphere into the mantle by the viscosity stratification and by lateral viscosity variations lvv in the lithosphere upper and lower mantle the presence of anomalous slab density in the lower mantle guarantees geoid amplitudes comparable with the observations favouring the picture of slabs that penetrate the transition zone and sink into the deep mantle the viscosity of the lower mantle controls the long wavelength geoid to the first order ensuring a clear positive signal when it is at least 30 times greater than the upper mantle viscosity the presence of lvv in the lithosphere in the form of weak plate margins helps to increase the contribution of the surface topography causing a pronounced reduction of the geoid localized lvv associated with the cold slab play a secondary role if they are in the upper mantle on the other hand highly viscous slabs in the lower mantle exert a large influence on the geoid they cause its amplitude to increase dramatically way beyond the values typically observed over subduction zones long wavelength flow becomes less vigorous as the slab viscosity increases deformation in the upper mantle becomes more localized and power is transferred to short wavelengths causing the long wavelength surface topography to diminish and the total geoid to increase slabs may be then weakened in the lower mantle or retain their high viscosity while other mechanisms act to lower the geoid it is shown that a phase change from perovskite to post perovskite above the core mantle boundary can cause the geoid to reduce significantly thereby helping to reconcile models and observations

new perspectives on mantle dynamics from high resolution seismic tomographic model p1200 recently a high resolution tomographic model the p1200 based on p wave travel times was developed which allowed for detailed imaging of the top 1200 km of the mantle this model was used in diverse ways to study mantle viscosity structure and geodynamical processes in the spatial domain there are lateral variations in the transition zone suggesting interaction between the lower mantle plumes and the region from 600 km to 1000 km some examples shown here include the continental region underneath manchuria ukraine and south africa where horizontal structures lie above or below the 660 km discontinuity the blockage of upwelling is observed under central africa and the interaction between the upwelling and the transition zone under the slow icelandic region appears to be complex an expansion of the aspherical seismic velocities has been taken out to spherical harmonics of degree 60 for degrees exceeding around 10 the spectra at various depths decay with a power law like dependence on the degree with the logarithmic slopes in the asymptotic portion of the spectra containing values between 2 and 2 6 these spectral results may suggest the time dependent nature of mantle convection details of the viscosity structure in the top 1200 km of the mantle have been inferred both from global and regional geoid data and from the high resolution tomographic model we have considered only the intermediate degrees l 12 25 in the nonlinear inversion with a genetic algorithm approach several families of acceptable viscosity profiles are found for both oceanic and global data the families of solutions for the two data sets have different characteristics most of the solutions asociated with the global geoid data show the presence of asthenosphere below the lithosphere in other families a low viscosity zone between 400 and 600 km depth is found to lie atop a viscosity jump other families evidence a viscosity decrease across the 660 km discontinuity solutions from oceanic geoid show basically two low viscosity zones one lying right below the lithosphere the other right under 660 km depth all of these results bespeak clearly the plausible existence of strong vertical viscosity stratification in the top 1000 km of the mantle the presence of the second asthenosphere may have important dynamical ramifications on issues pertaining to layered mantle convection numerical modelling of mantle convection with two phase transitions and a realistic temperature and pressure dependent viscosity demonstrates that a low viscosity region under the endothermic phase transition can indeed be generated self consistently in time dependent situations involving a partially layered configuration in an axisymmetric spherical shell model

a global geoid model with imposed plate velocities and partial layering most inversions of the long wavelength geoid in conjunction with the seismic tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation moreover modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid we have tested whether a good prediction of the geoid can also be achieved by including two constraints a semipermeable behavior of the 660 km discontinuity and surface plate velocities equal to the observed ones the mass transfer between upper and lower mantle has been changed by imposing a surface density anomaly at a depth of 660 km which is proportional to the mass anomaly needed to achieve perfectly layered circulation on the top of the mantle we assume a stiff lithosphere that moves with a velocity corresponding to the observed plate motion the viscosity only varies with depth considering a simple three layer viscosity structure and changing the permeability of the 660 km interface we have obtained a satisfactory variance reduction of the geoid data 75 for degrees 2 12 the best fit to the geoid is obtained if the mass transfer across the 660 km boundary is reduced to one third in comparison with the purely whole mantle model the best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle the amplitudes of dynamic topography predicted by our model are remarkably small 100 m thus fully compatible with the observation

modeling the dynamic component of the geoid and topography of venus 1 we analyze the venusian geoid and topography to determine the relative importance of isostatic elastic and dynamic compensation mechanisms over different degree ranges the geoid power spectrum plotted on a log log scale shows a significant change in its slope at about degree 40 suggesting a transition from a predominantly dynamic compensation mechanism at lower degrees to an isostatic and or elastic mechanism at higher degrees we focus on the dynamic compensation in the lower degree interval we assume that 1 the flow is whole mantle in style 2 the long wavelength geoid and topography are of purely dynamic origin and 3 the density structure of venus mantle can be approximated by a model in which the mass anomaly distribution does not vary with depth solving the inverse problem for viscosity within the framework of internal loading theory we determine the families of viscosity models that are consistent with the observed geoid and topography between degrees 2 and 40 we find that a good fit to the data can be obtained not only for an isoviscous mantle without a pronounced lithosphere as suggested in some previous studies but also for models with a high viscosity lithosphere and a gradual increase in viscosity with depth in the mantle the overall viscosity increase across the mantle found for the latter group of models is only partially resolved but profiles with a 100 km thick lithosphere and a viscosity increasing with depth by a factor of 10 80 hence similar to viscosity profiles expected in the earth s mantle are among the best fitting models

spatial characterization of the seawater upconing process in a coastal mediterranean aquifer plana de castellon spain evolution and controls in this contribution we describe the formation and evolution of the upconing process in a mediterranean coastal aquifer the study area has experienced severe salinization over the last 40 years because of intensive exploitation of groundwater we used historical and current records of piezometric levels and chloride concentrations to trace the development of the salinization of the aquifer we defined the 3d shape of the saline wedge from the spatial distribution of chloride concentrations and vertical well logs of electrical conductivity using monitoring network data upconing first appeared in the early 90s and has continued until the present day in this study we examined the intensity of the upconing process dry periods and the associated increases in pumping caused the advance of seawater intrusion the sharp reduction in groundwater withdrawals over the last 10 years has caused the saline wedge to move backwards although the ongoing pumping and the climate conditions mean that this retreat is quite slow

crust 5 1 based inference of the earth s dynamic surface topography geodynamic implications summary we employ the recently published crust 5 1 model of global crustal structure mooney et al 1998 to estimate the topographic contributions of isostatically compensated crustal loads and in turn infer a long wavelength field for earth s dynamic surface topography our inference of dynamic surface topography is characterized by peak to peak variations of the order of 4 km we argue that the square root of age variation of the oceanic bathymetry has a dynamic origin remarkably continental regions are the site of deep approximately 1 5 km dynamic surface topography depressions the power spectrum of the field is dominated by spherical harmonic degrees l 1 4 and 5 suggesting the importance of continent ocean differences rather than deep lower mantle heterogeneity to its origin we model the crust 5 1 based inference of dynamic surface topography within the context of seismic tomography based internal loading theories we compute the depth dependences of dynamic surface topography response functions for various viscosity profiles that we consider in our analyses our best descriptions of the crust 5 1 based inference achieve total variance reductions of the order of 70 per cent in the spherical harmonic degree range l 1 8 these are obtained for wholemantle circulation models that preclude lower mantle heterogeneity from maintaining significant vertical stresses on the outer surface these models are characterized by a significant increase in viscosity across the depth of the 660 km seismic discontinuity by a factor of at least 50 relative to the average viscosity of the upper mantle we also consider results of a layered circulation model but in this case we are unable to reconcile constraints provided by large scale superswell topography we propose that the antipodal pacific and african superswells are dynamically maintained by positively buoyant lower mantle superplumes imparting on the outer surface a large scale low amplitude predominantly degree 2 pattern of vertical stresses finally we note that the crust 5 1based inference of dynamic surface topography provides useful geodynamic constraints on the nature of deep subcontinental structure optimal descriptions of the data require deep subcontinental keels composed of anomalously dense material

topography of the ocean floor thermal evolution of the lithosphere and interaction of deep mantle heterogeneities with the lithosphere topography associated with large wave length geoid is derived by two methods first the bathymetry corrected for sediment loading is decomposed using a least squares method as a function of age plus terms proportional to the geoid the topography at old ages is flatter predicted by the law at degrees 2 and 3 very little topography is associated with the geoid the transfer function is not even certain in sign but is never smaller than 0 6 more topography seems associated with higher degree harmonics 6 and 7 second the function of age alone best fitting the topography is determined the correlation coefficients of the residual topography with the geoid are then computed a slight correlation appears for degree 6 but not for degrees 2 and 3 these results suggest a mechanical decoupling between the lower mantle mass heterogeneities and the lithosphere

hydrochemical characteristics and salinization processes of groundwater in the shallow aquifer of eastern laizhou bay china groundwater salinization has become a crucial environmental problem worldwide and is considered the most widespread form of groundwater contamination in the coastal zone in this study a hydrochemical investigation was conducted in the eastern coastal shallow aquifer of laizhou bay to identify the hydrochemical characteristics and the salinity of groundwater using ionic ratios deficit or excess of each ions saturation indices and factor analysis the results indicate that groundwater in the study area showed wide ranges and high standard deviations for most of hydrochemical parameters and can be classified into two hydrochemical facies ca2 mg2 cl facies and na cl facies the ionic ratio deficit or excess of each ions and si were applied to evaluate hydrochemical processes the results obtained indicate that the salinization processes in the coastal zones were inverse cation exchange dissolution of calcite and dolomite and intensive agricultural practices factor analysis shows that three factors were determined factor 1 tds ec cl mg2 na k ca2 and so42 factor 2 hco3 and ph factor 3 no3 and ph representing the signature of seawater intrusion in the coastal zone weathering of water soil rock interaction and nitrate contamination respectively copyright c 2011 john wiley sons ltd

permeability changes during remediation of an aquifer affected by sea water intrusion a laboratory column study summary during flushing of a seawater affected aquifer with freshwater fine particles can be mobilized and can subsequently clog up pores thereby causing a reduction of permeability here we used packed columns 25 cm length to experimentally investigate the changes in permeability during such a remediation scenario we hypothesized that a gradual and abrupt change in salinity as well as the pressure boundary conditions along the column will affect the particle release and permeability changes we collected sediments from a coastal aquifer in china in which seawater had intruded the sediments were packed into laboratory columns the columns were oriented horizontally and instrumented with a series of piezometers to measure hydraulic heads along the column several sequences of displacements of seawater electrical conductivity ec 48 5 ms cm with freshwater ec 0 78 ms cm were passed through the columns permeability was determined from measured flow rates and hydraulic heads experiments were conducted both with constant head as well as with constant flux conditions column outflow was analyzed for suspended particles and electrical conductivity the results indicate that the permeability was reduced by up to 70 when seawater was displaced with freshwater under constant head conditions permeability reductions were more pronounced than under constant flux the critical salinity i e electrical conductivity associated with the critical salt concentration for particle release in the sediments was 7 ms cm when salinity was changed abruptly fine particles were eluted from the column however when the salinity change was gradual no fines were eluted in agreement with previous literature data we observed a more pronounced reduction of permeability under an abrupt than under a gradual salinity change permeability reduction was not uniform along the column with generally decreasing permeability with increasing distance from the column inlet

on the possibility of a folded crustal layer stored in the hydrous mantle transition zone abstract previous numerical studies of mantle convection focusing on subduction dynamics have indicated that viscosity rheological heterogeneity in the subducting plates and the surrounding mantle may have a primary effect on the behavior of subducting plates meanwhile the existence of a low viscosity layer lvl beneath the mantle transition zone mtz has been suggested by several geodynamic studies we investigated the effects of a rheologically weaker crustal layer in a hydrous wet mtz with an underlying lvl on the behavior of subducting plates and determined the trace of crustal material using a numerical simulation model of subduction dynamics in three dimensional regional spherical geometry the combined effect of a rheologically weaker crustal layer and an lvl beneath the mtz produces characteristic transient behavior of the crustal layer whose trace shows a folded loop like feature in the wet mtz along with a folded subducting slab in the lvl this phenomenon is caused by mechanical interaction of the crustal layer with an upward positive bending force due to the main body of the subducting slab folding in the lvl and an enhanced downward negative buoyancy force due to an increase in density from the garnet to the post garnet phase of crustal material the transiently folded crustal layer behind the root of the stagnant slab is also observed this result is consistent with seismological evidence suggesting a piece of subducted oceanic crust in the uppermost lower mantle beneath the subducting slab in the mariana trench over half of the subducted crust is temporarily stored in the mtz however this stored crustal material begins to penetrate into the lower mantle within 10 myr a relatively short timeframe relative to the overturn time of mantle convection eventually most of the crustal material is entrained into the lower mantle but a non negligible portion remains stored in the mtz for 50 myr after the beginning of the accumulation of subducted crust in the mtz

locally layered convection inferred from dynamic models of the earth s mantle the structure of convection in the mantle is still the subject of considerable debate the now standard modelling of the convective flow as driven in a viscous mantle by density anomalies derived from seismic tomography has successfully explained the longest wavelength degree 2 to 8 geoid anomalies and provided important information concerning the viscosity structure of the mantle1 8 with this approach however the predicted response of surface topography to convective stresses the dynamic topography has a typical magnitude of several kilometres which does not conform with observations9 11 a possible source of this discrepancy lies in the severe underestimation by tomography of density anomalies due to deflections of the boundary between the upper and lower mantle at 660 km depth here we model the mantle flow implied by seismically derived density heterogeneities using an empirical method to account for the 660 km boundary topography the predicted dynamic surface topography thus obtained is significantly reduced to values that conform with the observations in addition the 660 km boundary topography appears to have a strong influence on the computed mantle circulation inducing local layering of the convective flow

subducting slabs stagnant in the mantle transition zone the p wave velocity structure beneath the western pacific is found from the international seismological center first arrival data special attention was paid to the deep structure beneath the wadati benioff zone we discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks this way of discretization minimizes a problem with tomographic studies of regional scale difficulty in making corrections for the effects outside the region of interest our solution is iterative with the alternate step of the relocation of earthquakes using the whole mantle model of inoue et al 1990 as a starting model a first order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model the essential result depends little on whether the reference spherical model is smooth or discontinuous near 400 and 670 km depths we examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere the resolution is depth dependent but in general good enough to see the slab configuration beneath the southern kurile japan izu bonin arcs and the java arc it is relatively poor beneath the northern kurile and mariana arcs the seismic image of subducting slab beneath the southern kurile to bonin arcs bends to subhorizontal near the leading edge of the wadati benioff zone and extends continentward over a distance of more than 1000 km the subhorizontal portion of the slab connects a high velocity blob to the bottom that reaches a depth of at least 800 km across the 670 km discontinuity under the japan arc although the image of the java slab directly penetrates the 670 km discontinuity it then bends to a shallow dip with a considerable spread reaching a depth of about 1200 km these results suggest that descending slabs of lithosphere in the western pacific tend to be stagnant in the transition zone under a subtle control of the 670 km discontinuity although stagnant slab materials eventually descend into the lower mantle they no longer maintain their original configuration below the 670 km discontinuity

layered mantle convection a model for geoid and topography the long wavelength geoid and topography are dynamic effects of a convecting mantle the long wavelength geoid of the earth is controlled by density variations in the mantle and has been explained by circulation models involving whole mantle flow however the relationship of long wavelength topography to mantle circulation has been a puzzling problem in geodynamics we show that the dynamic topography is mainly due to density variations in the upper mantle even after the effects of lithospheric cooling and crustal thickness variation are taken into account layered mantle convection with a shallow origin for surface dynamic topography is consistent with the spectrum small amplitude and pattern of the topography layered mantle convection with a barrier about 250 km deeper than the 670 km phase boundary provides a self consistent geodynamic model for the amplitude and pattern of both the long wavelength geoid and surface topography

understanding the origin of salinization of the plio quaternary eastern coastal aquifer of cap bon tunisia using geochemical and isotope investigations as in many other semi arid regions the plio quaternary aquifer of the eastern coast of cap bon peninsula ne tunisia shows a parallel increase in overexploitation and mineralization of groundwater resources and so the water quality is deteriorating different methods using geochemistry ions na cl ca2 mg2 br and stable isotopes 18o 2h are compared with the hydrodynamic information for identifying the main processes involved in the increase of salinization along the coast intrusion of seawater resulting from groundwater overexploitation is identified but is not the only cause of qualitative degradation the development of irrigation that induces soil leaching and transfer of fertilizers to groundwater over the whole aquifer extent is another major reason for the increase in salinization a total of 48 groundwater wells were sampled to obtain additional information on the hydrochemical characteristics of the groundwater defined in previous studies

mantle heterogeneities geoid and plate motion a monte carlo inversion seismic tomography in both the upper and the lower mantle as well as subducting oceanic slabs defined by seismicity has been translated into density heterogeneities to generate models of mantle circulation these models can predict both the surface velocities and the geoid which can be compared with plate tectonics and gravity data a given model is specified by 6 parameters related to the viscosities of 3 mantle layers and the absolute amplitudes of density variations in the upper and lower mantle as well as in the slabs the values of these parameters are chosen at random within an acceptable range each model is submitted to an appropriate test comparing observations with predictions the results of the most successful models selected by this monte carlo inversion are displayed they yield preferred mantle viscosity structures exhibiting large variations at depth with a physical interface between upper and lower mantle i e with the possibility for the circulation to penetrate the 650 km discontinuity two classes of viscosity profiles stand out the first one implies a regular increase of the viscosity in the sublithospheric mantle with reasonable values for the density parameters the second one is unexpected in the sense that it predicts a very stiff bottom for the upper mantle it also requires vanishingly small amplitudes for the upper mantle density heterogeneities defined by tomography which would thus have to be of lithological rather than thermal origin with a chemical interface at 650 km the outcome is very similar the same two classes of viscosity structures do yield a satisfactory geoid prediction however only the class of models with a stiff layer at midmantle depths predicts acceptable surface velocities altogether the best models out of some 60 000 which have been tested only explain one third of the geoid and two thirds of the surface divergence for spherical harmonic degrees 1 to 6 nevertheless the main features of these two observed patterns are present in the computed maps and 4 out of 6 correlation coefficients lie close to the 90 confidence level this is true for the geoid as well as for the surface divergence of the displacement velocity however as the internal viscosity structure has been assumed to have spherical symmetry the rotational component of the surface velocities cannot be predicted

dynamical consequences of mid mantle viscosity stratification on mantle flows with an endothermic phase transition recent geophysical evidences from seismology and geoid inversions have pointed out the existence of some layering between 900 and 1000 km depth and the possibilities of a low viscosity zone between 660 and 1000 km depth using a finite difference and spectral code we have conducted 3 d calculations of thermal convection with the endothermic phase transition at 670 km depth and a family of viscosity profiles with emphasis on the presence of two low viscosity zones one in the upper mantle between 100 and 250 km depth and the second one between 670 and 1000 km depth below 1000 km the viscosity increases with depth the surface rayleigh number used was in the range of 10 7 the dominating effect is that due to the second low viscosity zone below the phase change layered convection is induced by the presence of this second asthenosphere which causes horizontal flow to develop in there alternatively a viscosity increase at 670 km depth would shift convection to the single layer regime the potential sharp variations of the viscosity structure between 670 and 1000 km depth can greatly influence the global flow dynamics of the mantle

seismic anisotropy in the deep mantle boundary layers and the geometry of mantle convection an attempt is made to explore the geodynamical significance of seismic anisotropy in the deep mantle on the basis of mineral physics the mineral physics observations used include the effects of deformation mechanisms on lattice and shape preferred orientation the effects of pressure on elastic anisotropy and the nature of lattice preferred orientation in deep mantle minerals in dislocation creep regime many of these issues are still poorly constrained but a review of recent results shows that it is possible to interpret deep mantle seismic anisotropy in a unified fashion based on the solid state processes without invoking partial melting the key notions are i the likely regional variation in the magnitude of anisotropy as deformation mechanisms change from dislocation to diffusion creep or superplasticity associated with a change in the stress level and or grain size in the convecting mantle with a high rayleigh number and ii the change in elastic anisotropy with pressure in major mantle minerals particularly in mg fe o the results provide the following constraints on the style of mantle convection i the sh sv anisotropy in the bottom transition zone and the sv sh anisotropy in the top lower mantle can be attributed to anisotropy structures lattice preferred orientation and or laminated structures caused by the horizontal flow in this depth range suggesting the presence of a mid mantle boundary layer due to partially layered convection ii the observed no significant seismic anisotropy in the deep mantle near subduction zones implies that deformation associated with subducting slabs is due mostly to diffusion creep or superplasticity and therefore slabs are weak in the deep mantle and hence easily deformed when encountered with resistance forces and iii the sh sv anisotropy in the cold thick portions of the d layer is likely to be due to horizontally aligned shape preferred orientation in perovskite plus magnesiowustite aggregates formed by strong horizontal shear motion in the recent past

groundwater salinization processes in shallow coastal aquifer of djeffara plain of medenine southeastern tunisia urban and industrial development and the expansion of irrigated agriculture have led to a drastic increase in the exploitation of groundwater resources the over exploitation of coastal aquifers has caused a seawater intrusion and has seriously degraded groundwater quality the shallow coastal aquifer of the djeffara plain southeastern tunisia constitutes an example of water resource suffering an intensive and uncontrolled pumping for irrigation intensive exploitation of the aquifer and climate aridity caused a decrease in piezometric level and an increase in salinity according to the hydrochemical data cl so4 2 no3 hco3 br ca2 mg2 na k and the stable isotope composition oxygen 18 and deuterium content groundwater salinization in the investigated system is caused by three main processes i salts dissolution especially in the central part of jerba and around medenine plain ii evaporation process and iii seawater intrusion which caused the increase in salinity in the peninsula of el jorf in jerba and in the north of ben gardane

sound velocities in olivine at earth mantle pressures the independent elastic constants of an upper mantle mineral san carlos olivine mg1 8fe0 2 sio4 were measured from 0 to 12 5 gigapascals evidence is offered in support of the proposition that the explicit temperature dependence of the bulk modulus is small over the range of temperatures and pressures thought to prevail above the 400 kilometer discontinuity and thus the data can be extrapolated to estimate the properties of olivine under mantle conditions at a depth of 400 kilometers in the absence of high temperature data at high pressures estimates are made of the properties of olivine under mantle conditions to a depth of 400 kilometers in contrast with low pressure laboratory data the predicted covariance of shear and compressional velocities as a function of temperature nearly matches the seismically estimated value for the lower mantle

identification of a holocene aquifer lagoon system using hydrogeochemical data abstract the hydrogeochemical characteristics of the cabo de gata coastal aquifer southeastern spain were studied in an attempt to explain the anomalous salinity of its groundwater this detritic aquifer is characterised by the presence of waters with highly contrasting salinities in some cases the salinity exceeds that of seawater multivariate analysis of water samples indicates two groups of water g1 and g2 group g1 is represented in the upper part of the aquifer where the proportion of seawater varies between 10 and 60 whilst g2 waters taken from the lower part of the aquifer contain 60 70 seawater in addition hydrogeochemical modelling was applied which reveals that the waters have been subject to evaporation between 25 and 35 there was a good agreement between the modelled results and the observed water chemistry this evaporation would have occurred during the holocene in a coastal lagoon environment the resulting brines would have infiltrated into the aquifer and due to their greater density sunk towards the impermeable base the characteristics of this water enabled us to reconstruct the interactions that must have occurred between the coastal aquifer and the lagoon and to identify the environmental conditions that prevailed in the study area during the middle holocene

effusive to explosive transition during the 2003 eruption of stromboli volcano the persistent explosive activity of stromboli volcano italy ceased in december 2002 and correlated with the onset of a seven month long effusive eruption on the volcano flank from new vents that opened just below the summit craters we intensively monitored this effusive event collecting and interpreting in real time an extensive multiparametric geophysical data set the resulting data synergy allowed detailed insights into the conduit dynamics that drove the eruption and the transition back to the typical strombolian activity we present a direct link between gas flux magma volume flux and seismicity supporting a gas driven model whereby the balance between gas flux and gas overpressure determines whether the system will support effusive or explosive activity this insight enabled us to monitor the migration of the magma column up the conduit and to explain the onset of explosive activity

the role of slabs and oceanic plate geometry in the net rotation of the lithosphere trench motions and slab return flow 1 absolute plate motion models with respect to a deep mantle reference frame e g hot spots typically contain some net rotation nr of the lithosphere global mantle flow models for the present day plate setting reproduce similarly oriented nrs but with amplitudes significantly smaller than those found in some high nr pacific hot spot reference frames it is therefore important to understand the mechanisms of nr excitation which we attempt here with two dimensional cylindrical models of an idealized pacific domain we study the influence of slab properties oceanic ridge position continental keels and a weak asthenospheric layer on nr and trench migration fast slab return flow develops in models with stiff slabs and moderate slab dips rapid nrs comparable to the high nr pacific hot spot reference frames are primarily induced by asymmetric slab dips in particular a shallow slab beneath south america and a steep slab in the western pacific a scaling relationship links the amplitude of nr to plate size slab dip angle and slab viscosity asymmetric ridge positions also promote nr through asymmetric plate sizes continental keels have less impact in contrast to what has been found in earlier global studies several models yield unidirectional pacific trench motions such as slab advance in the western pacific and simultaneously slab retreat in the eastern pacific our model provides a physical explanation for nr generation in the present day pacific setting and hints at mechanisms for the temporal evolution of the basin