salt tectonics in salt bearing rift basins progradational loading vs extension abstract confined salt flow in narrow 3 km layer of evaporites results in a complex structural style consisting of vertical and closely spaced diapirs surrounded by thick 6 10 km and narrow minibasins with intricate internal patterns recording salt withdrawal these basins are usually explained as the result of thick skinned extension while other important factors such as sediment loading and subsalt basin configuration are not commonly addressed we use sandbox models 3d reconstruction of the models final stage and structural restoration to evaluate the impact of sediment loading thick skinned extension and different basin configurations on the suprasalt structural style and the spatial and temporal distribution of salt structures in these basins models simulating sediment loading display expulsion rollovers and younger salt structures in the progradation direction in contrast thick skinned extension models display stacked depocenters near basin boundary faults and opposite timing of diapir growth along strike changes in basin configuration and subsalt faults impact salt flow and salt structures as follows 1 faults perpendicularly facing the sediment progradation contribute to the formation of salt diapirs 2 oblique faults favour salt flow along strike 3 fault intersections cause salt inflation and salt diapirs and 4 basin confinement by graben narrowing or intrabasinal faults produces along strike differences in the timing of salt mobilization the sandbox models show close resemblance and are directly comparable to seismic sections through the nordkapp basin norwegian barents sea and other salt bearing rift basins with similar configuration

reconstructing thermal properties of firn at summit greenland from a temperature profile time series we have constrained the value for thermal diffusivity of near surface snow and firn at summit station greenland using a fourier type analysis applied to hourly temperature measurements collected from eight thermistors in a closed off air filled borehole between may 2004 and july 2008 an implicit finite difference method suggests that a bulk diffusivity of 25 3m2 a 1 is the most reasonable for representing macroscale heat transport in the top 30 m of firn and snow this value represents an average diffusivity and in a conduction only model generates temperature series whose phase shifts with depth most closely match those of the summit borehole data rms difference between measurements and model output is 6 days this bulk value derived numerically and corroborated analytically is useful over large tracts of the greenland ice sheet where density and microstructure are unknown

recent accumulation variability in northwest greenland from ground penetrating radar and shallow cores along the greenland inland traverse accumulation is a key parameter governing the mass balance of the greenland ice sheet several studies have documented the spatial variability of accumulation over wide spatial scales primarily using point data remote sensing or modeling direct measurements of spatially extensive detailed profiles of accumulation in greenland however are rare we used 400mhz ground penetrating radar along the 1009 km route of the greenland inland traverse from thule to summit during april and may of 2011 to image continuous internal reflecting horizons we dated these horizons using ice core chemistry at each end of the traverse using density profiles measured along the traverse we determined the depth to the horizons and the corresponding water equivalent accumulation rates the measured accumulation rates vary from 0 1mw e a in the interior to 0 7mw e a near the coast and correspond broadly with existing published model results though there are some excursions comparison of our recent accumulation rates with those collected along a similar route in the 1950s shows a 10 increase in accumulation rates over the past 52 years along most of the traverse route this implies that the increased water vapor capacity of warmer air is increasing accumulation in the interior of greenland

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

rapid techniques for determining annual accumulation applied at summit greenland we have determined accumulation histories by identifying annual layer horizons in records obtained by three independent methods 1 glaciochemical analysis on a core 2 density profiling in the borehole from which the core was taken using the neutron probe np technique and 3 borehole optical stratigraphy bos again in the same borehole we also used three different techniques for determining density to convert annual layer thickness to accumulation 1 gravimetric measurements on core samples 2 measurement of density using np and 3 a simple empirical model based on regional climatology the result is nine different accumulation time series three of which are completely independent the chemical analysis and np derived accumulation time series are correlated and the 70year means are in agreement the bos derived accumulation 70 year mean is slightly lower probably due to a combination of the empirical density model s underestimate of the density profile and the misidentification of sub annual events in the shallow part of the borehole as annual horizons

geometrical impact of the san andreas fault on stress and seismicity in california 1 most large earthquakes in northern and central california clustered along the main trace of the san andreas fault saf the north american pacific plate boundary however in southern california earthquakes were rather scattered here we suggest that such along strike variation of seismicity may largely reflect the geometrical impact of the saf using a dynamic finite element model that includes the first order geometric features of the saf we show that strain partitioning and crustal deformation in california are closely related to the geometry of the saf in particular the big bend is shown to reduce slip rate on southern saf and cause high shear stress and strain energy over a broad region in southern california and a belt of high strain energy in the eastern california shear zone

annual accumulation over recent centuries at four sites in central greenland 1 accurate accumulation records are fundamental to understanding mass balance of the greenland ice sheet ice cores can provide multicentury high resolution ground based point estimates of water accumulation in 2003 and 2004 four new ice cores were collected in the central western d4 and d5 and summit sandy and katie regions of greenland annual layer counting based on multiple glaciochemical parameters primarily hydrogen peroxide sea salt and dust proxies was used to date the ice cores resulting in annual dating the bottom depth ages for d4 d5 sandy and katie were estimated at 1738 1673 1844 and 1934 a d with mean annual accumulation rates of 41 4 35 2 22 4 and 22 4 cmweq yr 1 respectively these new records were in good agreement with previous accumulation maps and four previously published records from nearby multicentury ice cores spatial variability was quantified through an analysis of variance thereby allowing for more meaningful comparisons to atmospheric processes more regionally representative accumulation records were constructed for the central western and summit regions spanning recent centuries using a principle component analysis of both the new and previously reported ice core measurements these regional records exhibited 6 7 interannual variability 1 standard deviation of the mean and had lower spatial variability uncertainties than individual accumulation records correlations indicate that 20 of the variance in the central western region was explained by the north atlantic oscillation nao and suggests that an array of more northern ice cores may correlate more closely with nao

visual stratigraphic dating of the gisp2 ice core basis reproducibility and application annual layers are visible in the greenland ice sheet project 2 ice core from central greenland allowing rapid dating of the core changes in bubble and grain structure caused by near surface primarily summertime formation of hoar complexes provide the main visible annual marker in the holocene and changes in cloudiness of the ice correlated with dustiness mark wisconsinan annual cycles both markers are evident and have been intercalibrated in early holocene ice layer counts are reproducible between different workers and for one worker at different times with 1 error over century length times in the holocene reproducibility is typically 5 in wisconsinan ice age ice and decreases with increasing age and depth cumulative ages from visible stratigraphy are not significantly different from independent ages of prominent events for ice older than the historical record and younger than approximately 50 000 years visible observations are not greatly degraded by brittle ice or many other core quality problems allowing construction of long consistently sampled time series high accuracy requires careful study of the core by dedicated observers

coulomb stress accumulation along the san andreas fault system 1 stress accumulation rates along the primary segments of the san andreas fault system are computed using a three dimensional 3 d elastic half space model with realistic fault geometry the model is developed in the fourier domain by solving for the response of an elastic half space due to a point vector body force and analytically integrating the force from a locking depth to infinite depth this approach is then applied to the san andreas fault system using published slip rates along 18 major fault strands of the fault zone gps derived horizontal velocity measurements spanning the entire 1700 200 km region are then used to solve for apparent locking depth along each primary fault segment this simple model fits remarkably well 2 43 mm yr rms misfit although some discrepancies occur in the eastern california shear zone the model also predicts vertical uplift and subsidence rates that are in agreement with independent geologic and geodetic estimates in addition shear and normal stresses along the major fault strands are used to compute coulomb stress accumulation rate as a result we find earthquake recurrence intervals along the san andreas fault system to be inversely proportional to coulomb stress accumulation rate in agreement with typical coseismic stress drops of 1 10 mpa this 3 d deformation model can ultimately be extended to include both time dependent forcing and viscoelastic response

instruments and methods in situ measurements of firn compaction profiles using borehole optical stratigraphy we have developed a technique in which we use a borehole video camera and post processing software to make a record of the optical brightness as a function of depth in polar firn we call this method borehole optical stratigraphy to measure firn compaction we note the positions of optical features on the borehole wall detected by an initial baseline log we track the displacements of these features in subsequent logs the result provides a measurement of the relative vertical motion and thus compaction of the firn over the survey period we have successfully used this system at summit greenland to measure the depth distribution of firn column shortening experienced in a borehole over three 1 year periods the uppermost 30 m of the firn at summit is compacting as predicted by a simple steady state model implying that the firn density profile at summit is at or close to steady state over the past 70 years

revisiting sites of the south pole queen maud land traverses in east antarctica accumulation data from shallow firn cores 1 ground based accumulation measurements are scarce on the high east antarctic plateau but highly necessary for model validation and the interpretation of satellite data for the determination of antarctic mass balance here we present accumulation results obtained from four shallow firn cores drilled in the antarctic summer season 2007 2008 the cores were drilled along the first leg of the norwegian us ipy traverse through east antarctica visiting sites like plateau station and pole of relative inaccessibility that have been covered by the south pole queen maud land traverses spqmlt in the 1960s accumulation has been determined from volcanic chronology established from the conductivity records measured by dielectric profiling dep the tambora 1815 unknown 1809 double peak is clearly visible in the conductivity data and serves as a reliable time marker accumulation rates averaged over the period 1815 2007 are in the range of 16 to 32 kg m 2 a 1 somewhat lower than expected from the spqmlt data the spatial pattern is mainly influenced by elevation and continentality three of the firn cores show a decrease of more than 20 in accumulation for the time period 1815 2007 in relation to accumulation rates during the period 1641 1815 the spatial representativity of the firn cores is assessed by ground penetrating radar showing a rather smoothly layered pattern around the drill sites validation of the dep results is utilized by comparison with chemistry data proving the validity of the dep method for dating firn cores the results help understanding the status of the east antarctic ice sheet and will be important for e g future model derived estimates of the mass balance of antarctica

the effect of fluctuations in surface density accumulation and compaction on elevation change rates along the egig line central greenland repeated measurements of density profiles and surface elevation along a 515 km section of the greenland ice sheet have been used to determine elevation change rates and the error in determining mass balance from these rates which arises from short term fluctuations in mass input compaction and surface density over the 28 months from spring 2004 to summer 2006 the average error over 100 km sections of the traverse ranged from 0 006 to 0 100 m a 1 the lowest values comparable with the system accuracy of the cryosat radar altimeter 0 033 m a 1 were found below 3000 m the surface density required to translate the elevation change into mass change decreased from 0 40 g cm 3 at an elevation of 2348 m to 0 33 g cm 3 at an elevation of 3264 m from the density profiles the equivalent values for a time period of 10 years were found to be 0 48 and 0 38 g cm 3 respectively

physically based modeling of atmosphere to snow to firn transfer of h2o2 at south pole quantitative interpretation of ice core chemical records requires a detailed understanding of the transfer processes that relate atmospheric concentrations to those in the snow firn and ice a unique 2 year set of year round surface snow samples at south pole and snow pits with associated accumulation histories were used to test a physically based model for atmosphere to firn transfer of h2o2 the model which extends our previous transfer modeling at south pole into the snowpack is based on the advection dispersion equation and spherical diffusion within representative snow grains required physical characteristics of the snowpack such as snow temperature and ventilation were estimated independently using established physical models the surface snow samples and related model simulations show that there is a repeatable annual cycle in h2o2 in the surface snow at south pole it peaks in early spring and surface snow concentration is primarily determined by atmospheric concentration and temperature with some dependence on grain size the snow pits and associated model simulations point out the importance of accumulation timing and annual accumulation rate in understanding the deposition and preservation of h2o2 and 18o at south pole long term snowpack simulations suggest that the firn continues to lose h2o2 to the atmosphere for at least 10 12 years 3 m after burial at current south pole temperatures and accumulation rates

changes in greenland ice sheet elevation attributed primarily to snow accumulation variability the response of grounded ice sheets to a changing climate critically influences possible future changes in sea level recent satellite surveys over southern greenland show little overall elevation change at higher elevations but large spatial variability using satellite studies alone it is not possible to determine the geophysical processes responsible for the observed elevation changes and to decide if recent rates of change exceed the natural variability here we derive changes in ice sheet elevation in southern greenland for the years 1978 88 using a physically based model of firn densification and records of annual snow accumulation reconstructed from 12 ice cores at high elevation our patterns of accumulation driven elevation change agree closely with contemporaneous satellite measurements of ice sheet elevation change and we therefore attribute the changes observed in 1978 88 to variability in snow accumulation similar analyses of longer ice core records show that in this decade the greenland ice sheet exhibited typical variability at high elevations well within the long term natural variability our results indicate that a better understanding of ice sheet mass changes will require long term measurements of both surface elevation and snow accumulation

impurity influence on normal grain growth in the gisp2 ice core greenland intercept analysis of approximately bi yearly vertical thin sections from the upper part of the gisp2 ice core central greenland shows that grain size ranges increase with increasing age this demonstrates that something in the ice affects grain growth rates and that grain size cannot be used directly in paleothermometry as has been proposed correlation of grain growth rates to chemical and isotopic data indicates slower growth in ice with higher impurity concentrations and especially slow growth in forest fire layers containing abundant ammonium however the impurity grain growth relations are quite noisy little correlation is found between growth rate and isotopic composition of ice

dating the siple dome antarctica ice core by manual and computer interpretation of annual layering the holocene portion of the siple dome antarctica ice core was dated by interpreting the electrical visual and chemical properties of the core the data were interpreted manuallyandwith acomputeralgorithm the algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the gisp2 greenland ice sheet project 2 ice core 10 be stratigraphic ties to the dendrochronology 14 c recordandthedatedvolcanic stratigraphy thealgorithm interpretation ismorecon sistent andbetter quantifiedthanthe tedious and subjective manual interpretation

surface mass balance and stable oxygen isotope ratios from shallow firn cores on fimbulisen east antarctica the mass balance of antarctica is one of the crucial factors for determining sea level change in a warming climate the marginal zones of the continent namely the ice shelves are most sensitive to climate change during the 2009 10 austral summer an extensive glaciological field campaign was carried out on fimbulisen an ice shelf in east antarctica to investigate its recent surface mass balance shallow 10 18 m firn cores were drilled and accumulation rates and stable isotope ratios determined for firn core dating two different methods were compared 1 seasonal variations of stable oxygen isotope ratios d 18 o and 2 dielectric profiling including using the volcanic eruptions of pinatubo philippines 1991 and el chichon mexico 1982 as time markers the mean annual accumulation for the period 1992 2009 ranges from 298 to 349 mm w e a 1 the interannual variability at the drilling sites is 30 accumulation rates show a weak decreasing trend during the past 20 30 years which is statistically significant only for one of the cores stable isotope ratios were compared to the snowfall temperature of neumayer station neither the temperatures nor the d 18 o values show any trend for the investigated time period

annual accumulation at two sites in northwest greenland during recent centuries during summer 1995 150 m firn and ice cores were drilled to determine annual accumulation rates at two greenland sites 73 84 n 49 49 w nasa u site and 78 53 n 56 83 w humboldt glacier site annual layers were identified in the cores using multiple parameters 18 o and concentrations of dust h 2 o 2 nh 4 ca 2 and no 3 using all parameters together to define annual layers resulted in a 350 year record for the nasa u core with no dating uncertainty for the lower accumulation humboldt core the dating uncertainty is about 5 years over the 852 year period of record with no uncertainty over the past 200 years annual accumulation over the periods of record at the two sites averaged about 0 34 and 0 14 m water equivalent respectively a set of 20 m firn cores drilled near the main 150 m cores showed that interannual variability of accumulation exceeded spatial variability at nasa u the humboldt cores showed equal spatial and interannual variability the accumulation rates at both sites showed a low frequency variation of about 100 years and both sites showed 200 year cumulative fluctuations of about 2 m from mean accumulation rates compared to central greenland and to nasa u the humboldt core showed higher annual accumulation rates around 1760 1810 possibly indicating a changed circulation pattern for the more northern part of greenland in that period

tectonic stressing in california modeled from gps observations 1 what happens in the crust as a result of geodetically observed secular motions in this paper we find out by distorting a finite element model of california using gps derived displacements a complex model was constructed using spatially varying crustal thickness geothermal gradient topography and creeping faults gps velocity observations were interpolated and extrapolated across the model and boundary condition areas and the model was loaded according to 5 year displacements results map highest differential stressing rates in a 200 km wide band along the pacific north american plate boundary coinciding with regions of greatest seismic energy release away from the plate boundary gps derived crustal strain reduces modeled differential stress in some places suggesting that some crustal motions are related to topographic collapse calculated stressing rates can be resolved onto fault planes useful for addressing fault interactions and necessary for calculating earthquake advances or delays as an example i examine seismic quiescence on the garlock fault despite a calculated minimum 0 1 0 4 mpa static stress increase from the 1857 m 7 8 fort tejon earthquake results from finite element modeling show very low to negative secular coulomb stress growth on the garlock fault suggesting that the stress state may have been too low for large earthquake triggering thus the garlock fault may only be stressed by san andreas fault slip a loading pattern that could explain its erratic rupture history

inversion for the density depth profile of polar firn using a stepped frequency radar 1 translating satellite measurements of ice sheet volume change into sea level contribution requires knowledge of the profile of density as a function of depth within the ice sheet and how this profile changes over time this paper describes an interferometric method of inverting ground penetrating radar returns for the profile of firn density as a function of depth the method is an interferometric implementation of the common midpoint approach performed using a stepped frequency phase sensitive ground penetrating radar by recording the phase difference of returns with a range of antenna separations the different path lengths through the firn allow recovery of a smoothed representation of the density profile this density model is characterized by three parameters surface density and two decay lengths for porosity each operating over a different density range our results suggest that the stepped frequency radar used here can accurately recover differences in two way traveltime and produce useful estimates of the density profile in a test of the method performed at summit station in greenland the recovered density depth profile agreed with independent density measurements from an ice core and a neutron probe to within 6 root mean square error

observed and modeled greenland ice sheet snow accumulation 1958 2003 and links with regional climate forcing abstract annual and monthly snow accumulation for the greenland ice sheet was derived from ecmwf forecasts mainly 40 yr ecmwr re analysis era 40 and further meteorological modeling modeled accumulation was validated using 58 ice core accumulation datasets across the ice sheet and was found to be 95 of the observed accumulation on average with a mean correlation of 0 53 between modeled and observed many of the ice core datasets are new and are presented here for the first time central and northern interior parts of the ice sheet were found to be 10 30 too dry in era 40 in line with earlier ecmwf analysis although too much 50 locally snow accumulation was modeled for interior southern parts of greenland nevertheless 47 of 58 sites show significant correlation in temporal variability of modeled with observed accumulation the model also captures the absolute amount of snow accumulation at several sites most notably das1 and das2 in southeast greenland mean modeled accumulation over the

global positioning system constraints on fault slip rates in southern california and northern baja mexico we use global positioning system gps estimates of horizontal site velocity to constrain slip rates on faults comprising the pacific north america plate boundary in southern california and northern mexico we enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary we estimate a pacific north america relative plate motion rate of 49 3 mm yr one standard deviation consistent with nuvel 1a estimates we are able to resolve robust slip rate estimates for the southernmost san andreas san jacinto and elsinore faults 26 2 9 2 and 6 2 mm yr respectively and for the imperial and cerro prieto faults 35 2 and 42 1 mm yr respectively accounting for about 86 of the total plate motion the remaining 14 appears to be accommodated to the west of these fault systems probably via slip along the san clemente fault and or the san miguel vallecitos rose canyon and newport inglewood fault systems these results are highly consistent with paleoseismic estimates for slip rates implying that off fault strain accumulation within the deforming zone of the plate boundary is largely elastic we estimate that the seismically quiescent southernmost san andreas fault has incurred about 8 2 m of slip deficit over the last few hundred years presumably to be recovered during a future large earthquake

annual layers in polar firn detected by borehole optical stratigraphy 1 we have developed a system called borehole optical stratigraphy which can detect annual layers in boreholes in polar firn borehole optical stratigraphy consists of a downhole camera package and post processing routines which allow us to measure the intensity of light emitted from an on board source and returned from the borehole wall we manually identify the annual layers in this optical signal we used this system at siple dome antarctica in the 2001 2002 austral summer season our results agree well with counts of annual layers detected with two different methods in an ice core recovered 50 meters from our borehole

seasonal and interannual variations of firn densification and ice sheet surface elevation at the greenland summit seasonal and interannual variations in surface elevation at the greenland summit are modeled using a new temperature dependent formulation of firn densification and are compared with elevations from european remote sensing satellite ers 1 2 radar altimetry the rate constant and activation energy usually set as constants in the arrhenius type relation are strongly temperature dependent based on measurements of crystal growth rates a multiplicative factor in the densification rate accounts for differences between densification and grain growth rates and is chosen to match the modeled and measured density profiles from 0 to 40 m the stronger temperature dependence produces a significant seasonal cycle in the densification rate in the upper firn much of the densification and consequent surface lowering occur within 3 months in late spring early summer followed by a build up from accumulation modeled elevation changes using automatic weather station measurements of temperature and accumulation and modeled precipitation agree well with observations the respective seasonal amplitudes are 18 and 25 cm peak to peak with minima in mid summer the minimum elevation in 1995 is driven mainly by a temporary accumulation decrease and secondarily by warmer temperatures increased compaction driven by a summer warming trend decreases the modeled elevation 1992 99 by 20 cm but accumulation increases in latter years dominate the overall 4 2 cm a 1 trend

light propagation in firn application to borehole video borehole optical stratigraphy bos is a borehole video system and processing routine for investigating polar firn bos records brightness variations in the firn and is effective at identifying stratigraphic markers bos brightness logs have been used to count annual layers and measure vertical strain even though a specific cause of the brightness variations has not been determined here we combine two models of light transport to examine potential errors with bos and identify improvements which will allow the system to estimate optical grain size we use a monte carlo radiative transfer model to estimate the influence of firn microstructure variations on borehole reflectance we then use a ray tracing algorithm to model the multiple reflections within the borehole that cause measured brightness variations multiple reflections cause the brightness measured at a point on the borehole wall to not necessarily be equal to the local wall reflectance the ray tracing further shows that wall imperfections or variations in the camera position can produce brightness variations that are unrelated to changes in firn properties smooth walls and good stabilization of the camera help ensure that brightness variations result from variations in firn properties and thus are a measure of firn stratigraphy rather than artifacts

borehole optical stratigraphy and neutron scattering density measurements at summit greenland we have made side by side measurements in several boreholes at summit greenland using borehole optical stratigraphy bos and neutron scattering density logging techniques the bos logs show strong positive correlation at shallow depths with neutron scattering logs taken in the same borehole this supports the hypothesis that bos detects changes in density the positive correlation between returned brightness and density decreases with depth and finally becomes negative we conclude this inversion of correlation is related to changes in densification regime from grain boundary sliding to pressure sintering