effects of lateral viscosity variations on postglacial rebound implications for recent sea level trends global sea level changes have been occurring since the last pleistocene deglaciation recently the contribution of current postglacial rebound to local sea level variations has been removed by means of a radially stratified earth model in order to assess the amount of sea level rise due to recent climatic changes the authors have studied the effects of lateral variations in the viscosity structure on the postglacial uplift rates along the continental margins finite element calculations in cylindrical geometry show that the spread in the cumulative vertical displacement may be affected by 20 in the presence of lateral rheological contrasts but the vertical rates of deformation can attain differences of up to 50 for the different models horizontal deformation rates along continental margins are much more affected by the lateral variations in viscosity and can be used in the future as constraints on mantle rheology

linear filtering of vlf dip angle measurements the suggested interpretation technique is based on discrete linear filtering of vlf data the output of the described filtering results is expressed in terms of an equivalent current density at a specified depth that would cause the measured magnetic field the most practical six point filter gives an accuracy of 8 the filter is an extension of the commonly used fraser filter to process vlf dip angle data filtering the same data set for various depths gives an idea about the change of current density with depth areas with high current density correspond to good conductors the conductor dip can also be determined the use of the technique is illustrated on theoretical and field examples in all cases a good correlation with original models and other types of geophysical measurements was obtained as shown in the last example the filtering technique is also applicable in interpretation of other electromagnetic methods

fossil strain from the 1811 1812 new madrid earthquakes observations of postseismic deformation generally suggest that the effects of postseismic viscoelastic relaxation may persist for many decades after an earthquake in particular strain effects from the three great earthquakes that occurred in the new madrid seismic zone in 1811 1812 may influence present day measurements of ground deformation in this active seismic region of the central united states forward calculations using an earth rheology that may be appropriate for a continental intraplate region and a simple fault model for the 1811 1812 sequence suggest that postseismic relaxation may be an important factor in driving the unexpectedly high rate of shear deformation recently observed in the southern new madrid seismic zone

computation of apparent resistivity profiles from vlf em data using linear filtering a simple filter is developed which transforms vlf em real magnetic field transfer functions into apparent resistivities it is based on the relationship between the horizontal derivative of the surface electric field and the vertical magnetic field at the surface of a two dimensional earth model the performance of this simple autoregressive filter is tested for modelled and real survey data the technique yields profiles of apparent resistivity very similar both in magnitude and in wavelength to those which would have been obtained using vlf em resistivity measurements or d c resistivity profiling this low pass filter has the advantage of reducing high wavenumber noise in the data therefore only the major features of the vlf em profile are displayed

baroclinic instability in the venus atmosphere abstract a three dimensional spherical primitive equation eigenvalue model is used to investigate the baroclinic stability properties of the wind and temperature fields in the venus atmosphere as measured by pioneer venus it is found that baroclinic instability occurs in the region of the middle cloud deck the most unstable modes have growth times less than eight days and are vertically confined to the region near the middle cloud layer the most unstable baroclinic mode at zonal wavenumber 2 has characteristics similar to those observed for the high latitude rotating dipole thermal feature certain planetary scale baroclinic modes can penetrate to relatively high altitudes under the right circumstances and may therefore explain some of the wave features observed between 60 and 90 km for example thermal oscillations with periods between four and seven days occurring at middle latitudes have characteristics which appear to be consistent with computed properties of planetary scale baroclinic modes

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

lithospheric extension near lake mead nevada a model for ductile flow in the lower crust small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions in this study we examine the boundary between the unextended colorado plateau and a strongly extended domain in the basin and range province in the lake mead nevada region bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required finite element modeling of newtonian flow and power law creep shows that flow over the length scale of the eastern basin and range 500 km or more corresponding to upper crustal extension by a factor of 1 4 3 over 10 m y requires effective viscosities less than 10 18 10 20 pa s for ductile channels 10 25 km thick flow over shorter length scales 150 km may be accommodated with effective viscosities as high as 10 21 pa s modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory derived power law creep rates the longer scale flow may require elevated crustal temperatures more than 700 c depending on the composition and material properties assumed under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep this work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions and thereby explaining the relative uniformity in gravity and topography between such regions

the recent crustal stress field in central europe trajectories and finite element modeling the recent crustal stress field of central europe and especially of the adjoining areas to the east is presented in terms of the directions of maximum horizontal stress shmax the analysis is based on fault plane solutions in situ stress measurements geologic fault slip determinations and repeated precise geodetic triangulations a bending of the direction of shmax from the well known nw se direction in the western part of the study area to directions of ne sw to e w in the eastern part is shown first results on the recent crustal stress field of the study area were presented by grunthal and stromeyer 1986 who substantiated this tendency of bending in the central and eastern parts of the study area by few observations only therefore one aim of this paper was to compile observations on the areas with few data points generally these additional data confirm the previously established pattern in some areas especially in the pannonian basin the stress features became more complicated compared with those solely based on a few data points the second part of the paper presents steady state elastic finite element model calculations to provide some possible explanations of the observed stress orientations as a result of plate driving forces simulation of the north atlantic seafloor spreading and the northward directed motion of the african plate by appropriate boundary loads produces a pattern of shmax directions for the western part of the eurasian plate which is compatible with the broad scale observed stress directions subregional anomalies such as the fanlike stress pattern perpendicular to the arc of the western alps or the radial directions around the pannonian basin can be explained only when additional stress producing features are introduced overmodulating the regional field rigorous introduction of physically constrained model parameters for all these features was not feasible to date therefore the preliminary empirical model calculations presented in this paper are attempts to discuss which features have the most influence on the stress field they are in a regional scale the north atlantic seafloor spreading the northward motion of the african and the arabian plate and obviously subregionally increased stiffness of the apulian promontary as well as of the bohemian massif

images from galileo of the venus cloud deck images of venus taken at 418 violet and 986 near infrared nir nanometers show that the morphology and motions of large scale features change with depth in the cloud deck poleward meridional velocities seen in both spectral regions are much reduced in the nir in the south polar region the markings in the two wavelength bands are strongly anticorrelated the images follow the changing state of the upper cloud layer downwind of the subsolar point and the zonal flow field shows a longitudinal periodicity that may be coupled to the formation of large scale planetary waves no optical lightning was detected

temporal variability of ultraviolet cloud features in the venus stratosphere abstract statistics on the temporal variability of uv cloud features on venus during 66 days of nominal mission imaging by the pioneer venus orbiter cloud photopolarimeter reveal at least five types of systematic variability on large scales 1 a low latitude global scale wave of period 3 94 0 1 days corresponding to longitudinal motion of the dark equatorial band and propagating westward relative to the mean flow 2 a midlatitude wave of period 5 20 0 2 days corresponding to wavenumber 1 oscillations of the latitude of the bright polar bands and propagating eastward relative to the mean flow 3 2 to 3 week fluctuations in the slope of longitudinal cloud brightness power spectra at intermediate wavenumbers manifested by variations in the intensity of large bow shaped features 4 2 month variations in polar region brightness consistent with polar brightening episodes observed from earth and 5 a monotonic decrease in the disk integrated brightness of venus during the nominal mission which may be either a true time variation or a solar locked longitudinal dependence of brightness small scale features appear to correlate with large scale albedo patterns specifically cellular features exist primarily where large scale dark material is present while the orientation of streak features with respect to latitude circles oscillates with the same 4 day period as the large scale features at low latitudes the wide range of time scales present in the data suggests the complexity of venus stratospheric dynamics extended observations over many years may be becessary to define the general circulation

deglacial land emergence and lateral upper mantle heterogeneity in the svalbard archipelago i first results for simple load models summary the glacial isostatic adjustment following the removal of the pleistocene ice sheets furnishes basic information on the thickness of the earth s lithosphere and the viscosity of the subjacent mantle paper i is concerned with an elementary interpretation of the post glacial land emergence observed at a restricted set of six locations in the svalbard archipelago the interpretation is based on a three layer incompressible maxwell viscoelastic earth model which is forced by a time dependent load distribution simulating the deglaciation history of the svalbard archipelago and the barents sea in view of inconclusive geomorphological evidence suggesting either total or partial glaciation of the area at glacial maximum two simple load models barents t1 and barents p1 are considered in the first instance a comparison between the calculated and observed values of land emergence suggests increases in lithosphere thickness and asthenosphere viscosity with increasing distance of the location from the continental margin the trends are qualitatively insensitive to the load model load thickness or load cross section used for load model barents t1 with parabolic cross section an initial load thickness of 1 50 km and a relaxed fitting criterion the lateral variations are only weak if a more restrictive fitting criterion is employed the variations become more prominent and suggest increasing values of lithosphere thickness from below 80 km to above 140 km and of asthenosphere viscosity from below 3 1018 pa s to above 2 1019 pa s over a distance of about 300 km in paper ii kaufmann wolf 1995 a high resolution load model and an extended set of land emergence observations are used to study further the possibility of resolving lateral heterogeneity in the upper mantle below the svalbard archipelago

the fully developed superrotation simulated by a general circulation model of a venus like atmosphere abstract formation and maintenance of the fully developed superrotation in the venus atmosphere are investigated by using a center for climate system research national institute for environmental study ccsr nies venus like atmospheric general circulation model under the condition that zonally uniform solar heating is used the meridional circulation is dominated by a single cell in a whole atmosphere and the superrotation with velocities faster than 100 m s 1 is formed near 60 km altitude the meridional circulation effectively pumps up angular momentum from the lower to the middle atmosphere then the angular momentum is transported by poleward flows of the meridional circulation and a part of the transported momentum is returned back to the low latitudinal regions by waves as a result the simulated superrotation is formed by the gierasch mechanism equatorward angular momentum flux required in the gierasch mechanism is caused by not only barotropic waves but also various waves rossby mixed ross

effect of temporal and spatial variations of the primary signal on vlf total field surveys most of the airborne and ground vlf instruments presently used measure the total field response in addition to field ratios results of surveys using these instruments are adversely affected by spatial and temporal variations in the vlf primary field until now the nature of such variations has not been studied from the point of view of geophysical surveying practice spatial variations are analyzed using radio propagation models the most important result is the identification of primary field minima where surveys would be unreliable their dependence on the transmitter location is rather complex and modeling should be carried out before specifying vlf stations for a survey area spatial and temporal variations have been studied using field monitoring of the transmitted signal the results of field experiments indicate that the nature of the received vlf fields changes significantly even over moderate distances 20 30 km and that data cannot be reliably corrected over larger distances this observatio

modelling of stresses in the fennoscandian lithosphere induced by pleistocene glaciations the seismicity observed in fennoscandia is usually explained by tectonic stresses the question of the contribution of stresses induced by glacial isostatic adjustment to the seismicity has not finally been settled yet in order to quantify the isostatic stress field generated by the fennoscandian glacial load we consider layered maxwell viscoelastic half spaces loaded by axisymmetric discs in particular we calculate the maximum shear stress distribution induced by the load model and the associated stress regimes in the lithosphere emphasis is placed on a discussion of the sensitivity of the calculated isostatic stresses to variations in load cross section lithosphere thickness and asthenosphere viscosity we also examine the effects produced by the viscous relaxation of the lithosphere whose viscosity is parameterized according to the stratification expected for temperature activated diffusion creep our results show that the isostatic shear stresses typically reach 2 mpa near the surface at the present time but that their distribution may be modified strongly by the details of the earth and load models investigated the viscous relaxation of the lithosphere becomes particularly important if more than one glaciation is considered since the last ice age in fennoscandia consisted of several glaciations the model of a perfectly elastic lithosphere conventionally employed in models of glacial isostatic adjustment therefore may not be justified when calculating the isostatic stresses

lithospheric thickness inferred from australian post glacial sea level change the influence of a ductile crustal zone 1 we investigate the influence of a crustal low viscosity zone clvz on numerical predictions of post glacial late holocene sea level variations in the australian region specifically we compare predictions from a model with a 15 km thick clvz within an otherwise elastic plate to results from a suite of models in which the thickness of a purely elastic crust lithosphere is varied we conclude that previous inferences of lithospheric thickness based on differential highstands from the region represent approximately the thickness of the elastic layer above the clvz and thus if a clvz exists these inferences may be significantly underestimating the total depth extent of the crust lithosphere system

oscillation of venus upper atmosphere 1 little is known about the variability of venus upper atmosphere we report discovery of a large 30 50 9 day period density oscillation derived from radar tracking of the magellan spacecraft from 15 sept 92 24 may 93 the densities correspond to 164 184 km altitude 11 n latitude and cover all local times the wave is presumed to propagate upward from lower atmosphere regions and occurs mainly between dusk and midnight suggesting that local time differences in forcing and or mean wind filtering are affecting accessibility to the upper atmosphere possible sources for the wave are discussed including wave wave interactions many questions remain concerning the origin and nature of the 9 day oscillation including its persistence instruments on venus express now orbiting venus have the opportunity to further elucidate the 9 day wave and understand its role in the dynamics of venus upper atmosphere

geophysical surveys of burial sites a case study of the oaro urupa a maori family burial site was surveyed using shallow electromagnetic em magnetometer gradiometer and ground penetrating radar gpr techniques the locations of some older graves are no longer known partly through the loss of markers and partly through the loss of the oral record there are at least 150 such burial sites throughout new zealand and similar problems occur elsewhere in the world the knowledge that maori graves are traditionally set out facing east made survey design easier background trends are removed and possible anomalous features are identified despite the complicating presence of metal fencing the em in phase response the vertical gradient of the total magnetic field and the gpr response were particularly useful in combination gpr surveys were carried out using two frequencies 450 and 200 mhz the higher frequency radar surveys were limited in extent because these data had a limited depth of penetration and were prone to signal ringing in the surficial clay the lower

lower crustal flow in an extensional setting constraints from the halloran hills region eastern mojave desert california postextensional uplift and tilting of quaternary lake sediments in the halloran hills eastern mojave desert california suggest that regional scale flow within the lower crust is occurring in response to tectonic loads created during upper crustal extension in late miocene time the concept that significant flow within the lower crust may occur for millions of years after the cessation of extension indicates that topographic gradients can be modified by regional scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting an analytic solution developed in this paper combines viscous flow in the lower crust flexural bending within the midcrust and isostasy into an invertible linear differential equation that describes the topographic response to upper crustal extension assuming that there was no topographic relief across the halloran hills region prior to extension inversion of modern topographic data indicates that crustal thinning above the halloran hills detachment increases westward with a listric geometry and that little crustal thinning has occurred east of the western flank of clark mountain in good agreement with the known geometry of the fault and the location of its breakaway zone incorporation of quaternary tilting data indicates that the modern viscosity beneath the halloran hills assuming a 10 km lower crustal channel is a maximum of 1019 pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 ma this corresponds to a temperature increase of at least 75 to 100 c at the moho in our opinion the most likely source of this temperature increase is a regional scale thermal event within the underlying mantle and diffusion of heat upward into the lower crust if correct these results have important implications for the way in which crustal extension is linked to mantle heating within the basin and range province

zonal winds in the middle atmosphere of venus from pioneer venus radio occultation data abstract pioneer venus radio occultation data for four seasons between december 1978 and october 1981 are used to construct meridional cross sections of temperature and zonal wind velocity for the middle atmosphere of venus in the altitude range of 40 80 km and in the latitude range of 1 5 85 wind speeds are derived assuming cyclostrophic balance the wind field contains as many as three jets first a prominent one above the clouds situated between 50 and 55 latitude with a maximum speed of 140 m s 1 a second with a speed of 95 m s 1 centered at 70 and at 60 km altitude and a possible third jet at 15 and 65 km altitude with a speed of 100 m s 1 the notable midlatitude jet is associated with the cold polar collar wind speeds above 70 km generally decrease with height because of the warm pole at these levels however within the jet the speed is 100 m s 1 at 83 km the jets may be barotropically unstable and inertial instability may occur above the clouds on the equatorial side of the main

magnetic susceptibility and vlf r investigations for determining geothermal blowout contaminated area a case study from alasehir manisa turkey western turkey is one of the most spectacular regions of widespread active continental extension in the world the most prominent structures of this region are e w trending gediz and buyuk menderes grabens geothermal activity around city of manisa in gediz graben has been investigated by many researchers and many geothermal boreholes were drilled in order to produce electricity in the middle of may 2012 a geothermal blowout occurred 150 m away from the deep drilling depth 1 100 m in alkan village of alasehir resort in city of manisa after that four big blowouts took place at the same area and thermal water had been diverted to alkan stream for almost 4 months the boron b and fe levels of thermal water are 87 and 11 2 ppm where the background b and fe values of topsoil vary between 0 2 1 5 ppm and 3 08 5 63 ppm respectively the objective of this study was to utilize joint magnetic susceptibility and vlf r investigations to determine possibly contaminated area with b related to high fe level on the topsoil and thermal water in the subterranean layers it is observed that magnetic susceptibility values vary between 500 and 790 cgs 10 6 in the vicinity of blowouts and 150 340 cgs 10 6 at a distance of 50 m around where the background average value is 20 cgs 10 6 by using the data collected at 346 stations besides the vlf r studies were carried out along 18 profiles at three different frequencies laterally constrained two layer inversion was applied to each station in addition to the inversion of all profiles for each frequency all lines were stacked 2 d resistivity maps were obtained and the contaminated area was determined approximately as 35 000 m2 the results show not only satisfactory agreement between magnetic susceptibility and vlf r data but also provide fast and valued interpretation of geochemical and geophysical properties of the geothermal contaminated area

geoid heights due to a crustal low viscosity zone in glacial isostatic adjustment modeling a sensitivity analysis for goce 1 seismic observations and the occurrence of post seismic deformation in several continental regions indicate the presence of one or more layers with relative low 1018 1019 pas viscosity in the earth s crust this study simulates the effect of thickness viscosity and depth of such a low viscosity zone lvz on geoid heights that are the result of glacial isostatic adjustment to pleistocene ice loads it is found that an lvz introduces variations in geoid heights up to 1 meter with spactial scales larger than a few hundred kilometers underneath and just outside former glaciated areas the response to changes in the properties of the lvz is shown to be wavelength dependent the lvz induced geoid signal is within the accuracy of the upcoming goce satellite mission which means that in principle it should be possible to detect the presence of a supposed lvz with goce data

zonal winds near venus cloud top level a model study of the interaction between the zonal mean circulation and the semidiurnal tide abstract a primitive equation wave mean flow interaction model designed by j r holton and used originally to study earth s middle atmosphere has been adapted to venus in order to clarify our understanding of the interaction between the semidiurnal tide and the thermally driven mean meridional circulation near the cloud top level with or without the tide the model produces midlatitude jets whose structure is insensitive to vertical shear of the background angular velocity above and below the cloud top level but it is sensitive to background angular velocity at the cloud top level when this background angular velocity is close to that of venus the latitudes and speeds of these jets are similar to the latitudes and speeds of jets at the venus cloud top level as inferred from observed temperatures and the cyclostrophic balance condition in agreement with the hypothesis of fels and lindzen the model tide accelerates the equatorial zonal wind near the cloud top level and decelerates it at higher levels the tidal vertical wavelength maximum amplitude dissipative decay length and zonal flow accelerations are sensitive functions of background angular velocity in agreement with elementary gravity wave theory in the equatorial cloud top region tidal acceleration is comparable in magnitude to the decelerative effects of vertical advection and the model s rayleigh friction damping for sufficiently rapid initial zonal flow near the cloud top level the area weighted global mean cloud top level zonal wind increases with time over a 50 day model run as a result of tidal acceleration agreement between the model tide and the observed tide or the tide determined in the more detailed calculations of pechmann and ingersoll is best when the background angular velocity at the jet level is about 30 larger than that observed

fast computation of a viscoelastic deformable earth model for ice sheet simulations the model used by lingle and clark 1985 to approximate the deformation of the earth under a single ice stream is adapted to the purposes of continent scale ice sheet simulation the model combines a layered elastic spherical earth farrell 1972 with a viscous half space overlain by an elastic plate lithosphere cathles 1975 for the half space model we identify a new mathematical formulation essentially a time dependent partial differential equation which generalizes and improves upon the standard elastic plate lithosphere with relaxing asthenosphere model widely used in ice sheet simulation the new formulation allows a significantly faster numerical strategy a spectral collocation method based directly on the fast fourier transform we verify this method by comparing to an integral formula for a disk load we also demonstrate that the magnitudes of numerical errors made in approximating coupled ice flow earth deformation systems are significantly smaller than pairwise differences between several earth models our implementation of the lingle and clark 1985 model offers important features of spherical layered self gravitating viscoelastic earth models without the computational expense

a bulk cloud parameterization in a venus general circulation model a condensing cloud parameterization is included in a super rotating venus general circulation model a parameterization including condensation evaporation and sedimentation of mono modal sulfuric acid cloud particles is described saturation vapor pressure of sulfuric acid vapor is used to determine cloud formation through instantaneous condensation and destruction through evaporation while pressure dependent viscosity of a carbon dioxide atmosphere is used to determine sedimentation rates assuming particles fall at their terminal stokes velocity modifications are described to account for the large range of the reynolds number seen in the venus atmosphere two gcm experiments initialized with 10 ppm equivalent of sulfuric acid are integrated for 30 earth years and the results are discussed with reference to y shaped cloud structures observed on venus the gcm is able to produce an analog of the y shaped cloud structure through dynamical processes alone with contributions from the mean westward wind the equatorial kelvin wave and the mid latitude polar mixed rossby gravity waves the cloud top height in the gcm decreases from equator to pole and latitudinal gradients of cloud top height are comparable to those observed by pioneer venus and venus express and those produced in more complex microphysical models of the sulfur cycle on venus differences between the modeled cloud structures and observations are described and dynamical explanations are suggested for the most prominent differences

implications of a ductile crustal layer for the deformation caused by the fennoscandian ice sheet summary previous studies of glacial isostatic adjustment have usually considered earth models with a purely elastic lithosphere a possibly significant deviation from this assumption is the presence of a ductile crustal layer embedded in an otherwise elastic lithosphere such a layer has been suggested for various continental regions on the basis of seismic and tectonic evidence the present study investigates the implications of a ductile crustal layer for the interpretation of glacial isostatic adjustment using a layered incompressible maxwell viscoelastic earth model and a simplified representation of the fennoscandian glaciation the relaxation time and amplitude spectra show that besides the conventional buoyancy mode m0 also present in earth models with a purely elastic lithosphere the ductile layer supports a slowly decaying mode mc which is capable of modifying the deformation markedly thus measures of the absolute deformation such as the stress clearly reflect the presence of the ductile layer in contrast to this are measures of the relative deformation such as the vertical displacement rate and the rate of gravity change to a good approximation these rates are inversely proportional to the relaxation time of the more quickly decaying mode m0 and are therefore less affected by the ductile layer taken together the present study suggests that provided a ductile crustal layer exists its presence should be accounted for in interpretations of the glacial isostatic adjustment following the fennoscandian or smaller glaciations

response of normal faults to glacial interglacial fluctuations of ice and water masses on earth s surface 1 changes in surface loads during glacial interglacial cycles and the associated flexure and rebound of the lithosphere alter the stress field in actively extending regions and may lead to slip rate variations on normal faults in particular loading may cause a period of seismic quiescence that is followed by temporal clustering of earthquakes during and after unloading here we present a suite of finite element experiments to evaluate how the magnitude distribution and temporal evolution of the load as well as rheological parameters of the lithosphere and asthenosphere influence the response of a normal fault the results show that the duration of the seismically quiet period during loading and the intensity of the slip rate increase during unloading are primarily controlled by the magnitude of the load the time lag between the changes in loading and the reaction of the fault is mainly determined by the viscosity of the asthenosphere parameters that play only a minor role for the fault s response include the rate of load removal fault strength and the thickness of the lithosphere our results imply that earthquake recurrence models derived from holocene slip histories may not be representative for the long term evolution of seismogenic faults

geophysical imaging of municipal solid waste contaminant pathways surface electrical and electromagnetic surveys were conducted on top of a solid waste facility in unguwan dosa kaduna state northwest nigeria the aim of the geophysical survey was to detect vertical and subvertical fractures that may provide pathways for groundwater and contaminant transport results from the 2d electrical resistivity imaging showed vertical and subvertical contacts overlain by 6 10 m thick overburden quantitative interpretation of the vlf em data correlates well with the results of the 2d resistivity imaging delineating the vertical and subvertical contacts as good and weak conductors fractures zones with resistivity values of 40 220 and 300 420 m respectively azimuthal schlumberger ves measurements yield apparent anisotropy values ranging from 1 01 to 1 47 for electrode spacings of 1 45 m with the highest value recorded at spacing of 2 m however azimuthal variations at large spacings 30 45 m showed no fracture anisotropy due possibly to the array s low sensitivity to anisotropy at these spacings the result of the study showed that pollutants in the leachate can reach and contaminate the groundwater therefore urgency for leachate treatment at this site is recommended to prevent contamination of groundwater

glacio seismotectonics ice sheets crustal deformation and seismicity abstract the last decade has witnessed a significant growth in our understanding of the past and continuing effects of ice sheets and glaciers on contemporary crustal deformation and seismicity this growth has been driven largely by the emergence of postglacial rebound models pgm constrained by new field observations that incorporate increasingly realistic rheological mechanical and glacial parameters in this paper we highlight some of these recent field based investigations and new pgms and examine their implications for understanding crustal deformation and seismicity during glaciation and following deglaciation the emerging glacial rebound models outlined in the paper support the view that both tectonic stresses and glacial rebound stresses are needed to explain the distribution and style of contemporary earthquake activity in former glaciated shields of eastern canada and fennoscandia however many of these models neglect important parameters such as topography lateral variations in lithospheric strength and tectonic strain built up during glaciation in glaciated mountainous terrains glacial erosion may directly modulate tectonic deformation by resetting the orogenic topography and thereby providing an additional compensatory uplift mechanism such effects are likely to be important both in tectonically active orogens and in the mountainous regions of glaciated shields in general the short term response to ice fluctuations is similar to the earth s response to fluctuations in water reservoirs with the subsequent increase or decrease in seismicity which depends on the pre existing stress state when the ice fluctuations occur on the spatial scale and magnitude of the late pleistocene glaciation and deglaciation however the viscoelastic response of the earth especially the mantle causes significant changes in crustal deformation and earthquake activity that is spatially extensive and temporally complex the regions of greatest ice thickness and the regions marginal to the late pleistocene ice sheets indicate the most dramatic evidence of earthquake faulting the mantle response to these large ice mass fluctuations and the change in mass between the oceans and land caused and continues to cause measureable crustal deformation at hundreds of kilometres from the ice margins for both tectonically active and cratonic regions palaeoseismic investigations of late pleistocene and holocene faults are an important tool in evaluating earthquake hazard the predicted response of the earth over this time however is very dependent on the model assumed for most regions far more carefully designed field observations are needed to constrain the existing rheological and ice models

slip rate variations on faults during glacial loading and post glacial unloading implications for the viscosity structure of the lithosphere abstract many active faults in extensional and contractional tectonic settings experienced a slip rate increase after the last glacial period when the volume of nearby glaciers and lakes decreased this post glacial slip rate increase is caused by transient stresses that are generated by the unloading induced rebound and superimposed on the tectonic background stress field as the latter is different for normal and thrust faults the response to loading and unloading should depend on the fault type here we use finite element models including a fault in rheologically layered lithosphere to explore the conditions under which both normal and thrust faults experience a post glacial slip rate increase the results show that a post glacial slip rate increase occurs on normal faults if the lower crust is stronger than the lithospheric mantle whereas thrust faults accelerate if the lower crust is weaker than the lithospheric mantle these findings imply that the response of faults to mass fluctuations on the earth9s surface may provide constraints on the rheological stratification of the lithosphere we use our results to make predictions on the viscosity structure of the basin and range province and northern scandinavia where palaeoseismological data document a pronounced increase in seismicity as a result of post glacial unloading and rebound

maintenance of strong rotational winds in venus middle atmosphere by thermal tides the cloud level atmosphere of venus takes little more than 4 days to complete one rotation whereas the solid planet below has a 243 day period computer simulations of the circulation of the venus middle atmosphere between 40 and 85 kilometers as driven by solar radiation absorbed in the clouds reproduce i the observed cloud level rotation rate ii strong vertical shears above and below the cloud tops and iii midlatitude jets and strong poleward flow on the day side simulated circulations converge to yield nearly the same zonal winds when initialized with both stronger or weaker rotation rates these results support the hypothesis that the observed cloud top rotation rate is maintained by statistical balance between fluxes of momentum by thermal tides and momentum advection by mean meridional circulation

a two level concept of plate tectonics application to geodynamics abstract a two level plate tectonics concept is developed on the basis of data on lithosphere rheological stratification this approach differentiates between crust and subcrust plate ensembles separated by a lower crust viscoplastic asthenolayer similarly to classical plate tectonics three types of boundaries are distinguished in the lower layer which do not always coincide with crust plate boundaries especially for continents applications of this concept to geodynamics are considered and a corresponding quantitative analysis for several important processes is carried out a quantitative model of mountain formation and collision plateau origins is proposed also a geodynamic model of the evolution of passive margins taking into account a lower crust viscous flow is considered and its geological consequences are discussed a mechanism of rifting taking into consideration rheological lithospheric layering and its vertical movements caused by extension is developed both a qualitative scheme and quantitative analysis of the slow evolution of intracraton structures of shield basin type taking into account erosion and sedimentation processes are worked through also historical aspects of plate tectonics are discussed from the point of view of the proposed concept

fennoscandian earthquakes whole crustal rupturing related to postglacial rebound local and regional earthquake locations provide seismic evidence that large shield earthquakes have occurred in northern fennoscandia these paleoearthquakes with fault lengths of up to 160 kilometers and average displacements of up to 15 meters were triggered by nonisostatic compressive stresses caused by the removal of the ice at the end of the last deglaciation the fennoscandian faults were probably formed by single events that ruptured through most of the crust the largest event moment magnitude mw 8 2 was larger than other known stable continent earthquakes outside failed rifts or extended crust

can deglaciation trigger earthquakes in n america a spherical self gravitating viscoelastic earth model is used to calculate the spatial and temporal evolution of the glacially induced lithospheric stress and fault stability in north america the predicted onset time of a pulse of earthquake activities the mode of failure and the magnitude of instability are investigated for three sites with different distance from the former ice margin it is found that glacial unloading is able to trigger paleo earthquakes within the ice margin near charlevoix 47 5 n 70 1 w and in wabash valley 38 5 n 87 w outside the ice margin however rebound stress decays away from the former ice margin thus glacial unloading is unlikely to have triggered the large m8 earthquakes in new madrid 36 6 n 89 5 w

mining induced crustal deformation in northwest germany modelling the rheological structure of the lithosphere abstract large scale coal mining in germany causes deloading of the crust by the removal of weight the crust responds by regional uplift while locally in the mining areas proper the surface subsides by sagging above extracted coal seams and removed groundwater in this study only the regional uplift is addressed the deloading consists of the total annual mass of coal and ground water removal since the 19th century for all significant mines the uplift data are from repeated levellings between about 1950 and 1980 reduced to mean annual rates of elevation change these observations are interpreted as the visco elastic response of the crust and uppermost mantle but alone they cannot discriminate layering from a homogeneous viscous half space or an elastic layer above such a half space if the seismically determined layered structure of upper and lower crust lithosphere mantle and asthenosphere is assumed to constrain the rheological structure a plausible model can be found by inversion only the lower crustal and asthenospheric effective maxwell viscosities are subjected to inversion a model consistent with the observations has the following properties 15 km of elastic upper crust 15 20 km thick lower crust of surprisingly low viscosity of 6 10 17 0 3 pas 30 50 km visco elastic mantle lithosphere with an assumed average viscosity of about 10 22 pas a visco elastic asthenosphere assumed bottomless with 5 10 19 0 6 pas the low viscosity values are consistent with active rifting and above average heat flow in the region the very low effective viscosity of the lower crust in this model it could be even lower if the thickness is smaller may be explained by elevated stresses quartz rheology and water content as suggested by xenoliths from the lower crust below the eifel region as a by product the tectonic subsidence in the lower rhine embayment filled with tertiary and quaternary sediments can be better defined by taking the anthropogenic effect into account

interpretation of vlf em in phase data using current density pseudosections1 the interpretation of vlf em surveys in terms of buried conductors can be assisted by the application of a linear filter to the observed in phase component of the vertical magnetic field one such scheme is examined critically by using the calculated response from a variety of synthetic models to compute theoretical current density pseudosections the results confirm that this filter technique provides a useful complementary tool for studying the third i e depth dimension for single steeply dipping plates 45 diagnostic information may be derived concerning the depth size lateral location and direction of dip since the current density maxima seems always to occur within the conductor or at least within one data interval however there are some limitations which do not appear to be widely recognized for single plates the angle of dip cannot be resolved by the current density transformation pattern distortions can occur where targets are in close proximity where the cross sectional form of the conductor is complex or where the dip is shallow 45 in these latter cases the current density maxima may not occur within the conductive structure and therefore cannot be used to infer depth of burial and or conductor shape with the same degree of confidence

dependence of horizontal stress magnitude on load dimension in glacial rebound models summary it has been proposed that the deglaciation of the northern hemisphere triggered large earthquakes within intraplate environments and in this paper we examine this hypothesis by evaluating quantitatively the stress state in the lithosphere produced by time dependent surface loads a series of models demonstrate the dependence of horizontal incremental stress in an elastic plate overlying an inviscid or viscoelastic mantle on the lateral extent of a load applied at the surface the horizontal stress is largest when the dominant wavelength that is twice the diameter of the load is close to eight times the elastic thickness of the plate when the mantle is inviscid and for the particular viscosity model employed in this paper close to 12 times the elastic thickness for a viscoelastic mantle at wavelengths close to the critical wavelength the horizontal incremental stress may be up to six times as large as the vertical incremental stress for appropriate earth model parameters amplification of horizontal stress is close to maximum for ice loads with a radius of 280 km comparable to the dimensions of the former ice sheet over great britain this amplification may be sufficiently large that loading by small ice sheets can lead to failure on marginally stable faults in contrast to the behaviour for large ice sheets the models also predict greater fault instability for fennoscandia than for the larger laurentide ice sheet consistent with the observation of large postglacial faults in northern sweden the model is used to predict the stability of faults and style of faulting due to rebound stresses in the absence of a background tectonic stress field since the last glacial maximum 18 000 years ago in northern europe within the formerly glaciated region thrust faulting is predicted to occur at the end of deglaciation and normal faulting is predicted to occur in peripheral regions for the entire period since the last glacial maximum

mapping groundwater contamination using dc resistivity and vlf geophysical methods a case study geophysical methods can be helpful in mapping areas of contaminated soil and groundwater electrical resistivity and very low frequency electromagnetic induction vlf surveys were carried out at a site of shallow hydrocarbon contamination in utah county utah previously installed monitoring wells facilitated analysis of water chemistry to enhance interpretation of the geophysical data the electrical resistivity and vlf data correlate well and vertical cross sections and contour maps generated from these data helped map the contaminant plume which was delineated as an area of high interpreted resistivities

formation and maintenance of the 4 day circulation in the venus middle atmosphere abstract the venus 4 day circulation with wind speeds 60 times greater than the surface rotation is located at the top of the cloud layer 65 70 km in the middle atmosphere of the planet this dramatic phenomenon is related to the combination of two processes the meridional circulation induced by solar heating in the cloud layer and its associated poleward momentum transport and the additional momentum from a 4 day wave originating in the equatorial lower atmosphere numerical simulations are conducted for two cases of 4 day wave forcing at the lower boundary of the middle atmosphere if no wave is forced no significant zonal mean wind appears in the equatorial middle atmosphere while a midlatitudinal jet develops due to centrifugal force when an appropriate range of the 4 day wave forcing is supplied a large zonal mean flow can be realized from the equator to midlatitudes planetary rossby waves having periods of 5 6 days are likely to be generated in baroclinically unstable regions an ultravio

shallow upper mantle rheology and secular ice sheet fluctuations on time scales from decades to centuries continental cryospheric forcing in response to climate change constitutes a major source of isostatic disequilibrium that may influence future regional sea level variations current vertical displacements and gravity field variations are often estimated neglecting rheological effects and thus assuming a fully elastic response of the earth in this study we adopt a more general point of view aiming at describing ongoing surface movements resulting from recent glacial instabilities also taking into account the effects associated with shallow upper mantle and crustal rheologies our computations are based on the post widder laplace inversion formula which permits the straightforward computation of load deformation coefficients for steady state and transient rheologies up to very high harmonic degrees using a surface load with a simple geometry and time history we compare the classical elastic solutions to those obtained considering the rheological response of the shallow upper mantle while at the center of the ice sheet rheology only magnifies the elastic response the pattern and time history of vertical displacement at the ice sheet margins show a greater complexity mainly due to the development of lateral forebulges whose shape and amplitude are particularly sensitive to the rheology of the shallow upper mantle in this region assuming an elastic rheology is generally appropriate on a century time scale but significant deviations from a purely elastic response both sign and amplitude are observed at longer time scales or when a low viscosity zone with maxwell rheology is taken into account

induced stresses and fault potential in eastern canada due to a realistic load a preliminary analysis summary in order to understand the causal relation between postglacial rebound and earthquakes a simple disc load model is used to 1 calculate stresses induced in the lithosphere and mantle by glacial loading melting and postglacial rebound and 2 evaluate the effect of glacial loading rebound on the failure potential for earthquakes in the upper crust the dependence of the failure potential and the actual mode of failure on the coefficient of friction the ambient tectonic stress magnitude direction the stress due to the overlying rocks and lithospheric thickness are investigated prominent features of this paper are the inclusion of 1 a viscoelastic mantle and thus the migration of stress and 2 the ambient tectonic stress and overburden stress contributions in the calculation of the total stress field it is assumed that throughout the earth there are optimally oriented pre existing virtual faults that are initially close to but not at failure thus a time dependent quantity called dfsm related to the coulomb mohr failure criterion can be defined such that a negative value of dfsm would advocate faulting or earthquake activities whereas a positive value of dfsm would promote stability the results indicate that under all combinations of tectonic stress magnitude and overburden stress crustal loading promotes fault stability directly underneath the load upon the removal of the load thrust faulting is predicted within the ice margin if the horizontal stress s induced by the overburden is greater than or equal to the vertical component s of the overburden stress 121 where sjs under this condition theory predicts that faulting or earthquake activity should have reached a maximum immediately after deglaciation if the horizontal stress induced by the overburden is less than the vertical component of the overburden stress 1 l then theory predicts fault stability within the ice margin the theory predicts fault instability both north and south of the ice margin the mode of failure however is completely determined by the value of i the trade off between the tectonic stress magnitude and the overburden stress parameter 1 is also investigated it is shown that a larger tectonic stress magnitude can be used to compensate a smaller value of 1 the results of this analysis show that variations in the coefficient of friction lithospheric thickness and a ductile zone below the upper crust do not significantly affect the above conclusions

shallow karst exploration using mt vlf and dc resistivity methods1 a geophysical test was carried out over a well located and fairly embedded karstic conduit the mt vlf method was selected because of its high resolution and its ability to provide a resistivity parameter sensitive to water and clay this method was used together with dc resistivity methods which allow vertical adjustment of the vlf data and show consistency between the investigation and target depths after correcting the deformations due to the polarization of the primary field the mt vlf data show clearly in the central part of the site that the conduit does not coincide with an anomaly axis but coincides with the boundary between a conductive area and a resistive area 2d modelling confirms that direct detection of the conduit is not feasible and that the conduit is located close to a conductive zone corresponding to a completely clay filled fractured zone this situation was observed on the whole site and the conduit seemed systematically to thread its way between the conductive zones to join the outlet the distribution of the conductive fractured zones and the direction of the hydraulic gradient were two important elements in predicting the location of the conduits a 3d approach would increase the probability of finding the conduits in such a case

viscous gravitational relaxation summary this paper is concerned with a detailed examination of the response of maxwell models of the planet to surface mass loads particular attention is devoted to an examination of the factors which determine the isostatic response since the understanding of this response is crucial in a number of different geodynamic problems one particular example which we discuss in detail is concerned with the prediction of free air gravity anomalies produced by large scale deglaciation events using the methods developed here we are able to provide the first direct assessment of the importance of initial isostatic disequilibrium on the observed relative sea level variations and free air gravity anomalies forced by the melting of the laurentide ice sheet we are therefore able to estimate the extent to which such initial disequilibrium might influence the inference of mantle viscosity from isostatic adjustment data our calculations establish that free air gravity data although they are sensitive to the degree of initial disequilibrium provide an extremely high quality constraint upon the viscosity of the lower mantle

application of the vlf em method with eemd to the study of a mud volcano in southern taiwan abstract a three year geophysical investigation was carried out in the wu shan ding mud volcano nature reserve in southwest taiwan this study utilized the very low frequency electromagnetic vlf em method with a recent data processing technique ensemble empirical mode decomposition eemd to investigate the subsurface structure of the mud volcanic site through the single profile modeling and the surface 2d contour analysis with the help of this new data processing technique the signal of vlf em data was enhanced to an extent that a regularized inversion with finite element forward modeling scheme can be applied to map the subsurface resistivity image of a single profile with satisfactory accuracy this study successfully predicted the evolving of a gryphon and proposed a model showing that the mechanism of fissure eruptions is more appropriate than a point source model for the shallow structure of the wu shan ding mud volcano the results also indicate that the chi shan fault is still active and that it traverses the study area with a nearly vertical up thrown angle

suppression of earthquakes by large continental ice sheets the interior regions of antarctica and greenland are aseismic no earthquake larger than body wave magnitude 4 5 5 0 is known for either except along coastal zones or continental shelves here i advance an explanation for this lack of seismic activity in terms of pressure effects produced by the continental ice sheets that mantle both continents

asthenospheric viscosity and stress diffusion a mechanism to explain correlated earthquakes and surface deformations in ne japan summary a significant correlation is found in both space and time between the intraplate land and interplate sea thrust zone only earthquakes in tohoku ne japan that has persisted since the times of reliably reported events in ad 1600 the correlation peaks at a land lead of about 36 yr with an average correlation distance of 200km with the implication of an average strain migration rate of 5 6 km yr 1 the correlation is highly significant 99 per cent both from formal statistics and from tests of random shuffles of the data additional analysis of the data as a point process confirms the results of the correlation analysis the sharpness of the correlation peak when compared to the individual times of occurrence of the land and sea events suggests a trigger mechanism to explain the correlation the general model ofsubduction rupture rebound is extended to include additional features the buckling of the land plate from the force of the subducting slab and the viscoelastic coupling of the plate to the underlying asthenosphere a buckle produces a high stress region in the continental plate where earthquakes are more prone to occur thus producing the spatial correlation in the data this may also explain the preferred location on land for the smaller modern day seismic events in ne japan the viscoelastic coupling controls the interaction between the land and sea events resulting in the temporal correlation in the data because of viscosity the model equations are diffuse like with strain pulses as solutions thus from the inferred strain migration rate it is possible to estimate asthenospheric viscosity 7 1018pa s using this model a large land shock generates a strain pulse that affects the locked fault at the thrust zone several decades later as the continental plate tends to pull away from the subducting slab the frictional force arising from the overburden pressure is reduced thus unlocking the fault and triggering a sea earthquake the viscoelastic model is also used to explain surface deformations measured by triangulation surveys in japan in 1904 and 1964 horizontal displacements which we believe are surface manifestations of the strain pulse from the large 1896 riku u land shock m 7 5 in ne japan are fit well by the model and provide a viscosity estimate 13 1018 pa s

a very low frequency electromagnetic study of the geo environmental hazardous areas in taiwan this study utilized the very low frequency electromagnetic vlf em technique a passive electromagnetic prospecting method working in the very low frequency range 15 30 khz to investigate the geo environmental problems of shallow low conductivity sedimentary layers in taiwan field examples successfully demonstrate the advantages of using this method in locating non mineralized shallow fault zones the zero crossings of in phase and quadrature measurements in 2 d contour maps clearly locate the position of subsurface anomalous source bodies further analysis of the measured vlf single profile peaks reveals that this method is useful in determining subsurface structures and conductivity the advantages of nondestructive noninvasive and low consumption of power make this method extremely friendly to the environment the authors anticipate this method will have more profound impacts on the interactions between prospecting technology and the earth

ionospheric induced very low frequency electric field wavetilt changes very low frequency vlf electric field wavetilt measurements were made continuously over a period of four months at a fixed location daytime and nighttime values of the wavetilt were found to be relatively constant during sunrise and sunset the wavetilt decreased to 20 percent of the daytime value making wavetilt measurements meaningless for two three hour intervals during a 24 hour period a mechanism involving modal interference is postulated this phenomenon may be a problem in other passive electromagnetic em prospecting systems including audio frequency magnetic and audiomagnetotelluric

mantle insulation beneath the west african craton during the precambrian cambrian transition at the time of the precambrian cambrian transition the west african craton underwent widespread magmatism hydrothermal activity and thermal rejuvenation this tectonothermal event gave rise to an anorogenic ring of fire along the rim of this craton following the pan african brasiliano belt that was reactivated by extension and transtension the thermal phenomena were due to the progressive peripheral release of mantle heat that had built up beneath this craton because of strong insulating conditions the west african craton at the precambrian cambrian transition can thus be envisioned in terms of a gigantic pressure cooker with a thick blanketing lithospheric lid these insulation processes triggered an unusually hot mantle that was channeled by edge driven convection toward the peri west african craton extensional corridors and released through magmatic pressure relief valves massive ice melting and outgassing of volcanic co2 gave rise to a planet scale sea level rise a greenhouse effect and the end of the icehouse snowball earth these processes played an important role in the phanerozoic explosion of life on earth

the influence of a ductile crustal zone on glacial isostatic adjustment geodetic observables along the u s east coast we consider the influence of a 15 km thick ductile crustal layer of viscosity 1018 pa s embedded in an elastic 80 km lithosphere on predictions of on going glacial isostatic adjustment henceforth gia along the u s east coast crustal velocities are perturbed by 0 5 mm yr or more by the inclusion of the ductile layer furthermore perturbations in predictions of sea level rates along the u s east coast due to the ductile layer have a geographic pattern that is similar to residual trends evident in previous gia corrections of tide gauge records along this coast

ground based observation of the cyclic nature and temporal variability of planetary scale uv features at the venus cloud top level abstract a planetary scale bright and dark uv feature known as the y feature rotates around venus with a period of 4 5 days and has been long time interpreted as planetary waves when assuming this its rotation period and spatial structure might help to understand the propagation of the planetary scale waves and find out their role in the acceleration deceleration of the zonal wind speed which is essential for understanding the super rotation of the planet the rotation period of the uv feature varied over the course of observation by the pioneer venus orbiter however in previous explorations of venus such as pioneer venus and venus express the spacecraft were operated in nearly fixed inertial space as a result the periodicity variations on sub yearly timescales one venusian year is 224 earth days were obscured by the limitations of continuous dayside observations we newly conducted six periods of ground based venus imaging observations at 365 nm from mid august 2013 to the end of june 2014 each observation period spanned over half or one month enabling long term monitoring of venus atmosphere above the equator region distributions of the relative brightness were obtained from the equatorial eq to mid latitudinal regions in both hemispheres and from the cyclical variations of these distributions we deduced the rotation periods of the uv features of the cloud tops brightness the relative brightness exhibited periods of 5 2 and 3 5 days above 90 of significance the relative intensities of these two significant components also seemed subject to temporal variations although the 3 5 day component considered persists throughout the observation periods its dominance over the longer period varied in a cyclic fashion the prevailing first significant mode seems to change from 5 2 day waves to 3 5 day waves in about nine months which is clearly inconsistent with the venusian year clear periodic perturbations indicating stability of the planetary scale uv feature were observed only in the presence of single longer or shorter periodic waves during the transition periods of dominant wave changing the amplitude of the relative brightness was largely changed this can be explained by the deformation of the y shaped uv feature as observed by pioneer venus in 1979

effect of viscosity structure on fault potential and stress orientations in eastern canada summary previous investigations of the causal relationship between postglacial rebound and earthquakes in eastern canada have focused on the mode of failure and the observed timing of the pulse of earthquake faulting activity following deglaciation in this study the observational database has been extended to include observed orientations of the contemporary stress field and the rotation of stress since deglacial times it is shown that many of these observations can be explained by a realistic ice history and a viscoelastic earth with a uniform 1021 pa s mantle the effects of viscosity structure on the above predictions are also examined it is shown that since most of the above observations are found within the ice margin they are not very sensitive to lithospheric thickness also the inclusion of a 25 or 50 km ductile layer within the lithosphere will not decouple the seismogenic upper crust high viscosity 1022 pa s in the lower mantle is rejected by the stress orientation and rotation observations a low viscosity 6 times 1020pa s upper mantle with 1 6 times 1021 pa s in the upper part of the lower mantle and 3 times 1021 pa s in the lower part of the lower mantle below 1200 km depth has been found to give predictions that are in general agreement with the observations

vlf ground surveys a powerful tool for the study of shallow two dimensional structures the suitability of vlf ground surveys in the investigation of shallow two dimensional structures is analyzed for such structures the polar formalism is derived necessary in practice since the transmitters are generally not in the structural strike or profile a simple vertical dike is considered to demonstrate the striking anisotropy which can be expected over such a structure in particular the high apparent resistivity along the direction of a well conducting dike and the low resistivity across it the theory is then confronted with the practical example of an asymmetrical vertical dike resulting from a strike slip fault modelling of the survey results is very successful and yields good confirmation of the polar behaviour vlf ground surveys thus provide a quick and powerful tool for the study of geological accidents within about 100 m of the surface

cloud tracked winds from pioneer venus ocpp images abstract analysis of ultraviolet image sequences obtained from the pioneer venus orbiter cloud photopolarimeter and covering five 80 day periods from 1979 1985 provides the first climatological description of the cloud top circulation on venus the average zonal winds can be characterized as a 5 day retrograde rotation of the whole cloud level atmosphere with weak jets at middle to high latitudes both the midlatitude and equatorial zonal winds vary by about 5 8 m s 1 over time spans of 1 6 years the average meridional circulation is poleward in both hemispheres up to at least 60 latitude consistent with the presence of a thermally direct hadley circulation associated with the clouds the strength of the hadley circulation also varies with time four wave modes are clearly identified a diurnal solar tide a semi diurnal solar tide a 4 day equatorial wave and a 5 day midlatitude wave the semidiurnal tide appears to have an amplitude of about 5 m s 1 and to be approximately constant with time

a forward approach to the problem of nonlinear viscoelasticity and the thickness of the mechanical lithosphere the physical significance of the mechanical lithosphere is clarified using a nonlinear maxwell flexure model this model reconciles high temperature creep data for rock with the observed long term strength of the mechanical lithosphere an approximate correspondence is established between the nonisothermal maxwell model and an elastic flexure model to define an effective lithosphere thickness he the model calculations are essentially forward since he does not depend in a significant way upon the nominal or prescribed value of beam thickness depending rather upon such variables as temperature and creep activation energy the effective thickness of the maxwell model though a function of time is stable enough over appropriate time scales to explain the long term integrated strength of the lithosphere the strength index is defined as the ratio of axial stress to the axial stress that would be present in a purely elastic fiber suffering the same strain the base of the effective lithosphere is marked by a critical isopleth of strength c 0 3 this critical level though not its position is independent of time since loading the strength index also maps out zones of predominant rheology the narrowness of the transition zone between the elastic lithosphere and the viscous asthenosphere explains the success of the elastic plate in modeling many examples of lithosphere flexure the elastic plate approximation may apply best where the lithosphere is relatively young and hot isolated weak zones in the continental crust may or may not weaken the lithosphere as a whole depending upon the distribution of these zones and the manner in which they deform in general such weakening is likely to be most effective when the base of the crust occurs at midlithospheric depths