use of grace data to detect the present land uplift rate in fennoscandia after more than 13 years of grace monthly data the determined secular trend of gravity field variation can be used to study the regions of glacial isostatic adjustment gia here we focus on fenn

discrete element modeling of progressive failure in a wide coal roadway from water rich roofs abstract progressive failure of roadway roofs is a common failure mechanism in underground coal mines especially when water rich roofs are in close proximity to the roadway in this case study at a chinese coal mine the udec voronoi method was used to investigate the process of progressive roof failure in a wide coal roadway from water rich roofs in the numerical scheme the studied domain was partitioned into polygonal blocks bonded through contacts with pre defined dimensions the parameters of polygons and contacts in the voronoi program were calibrated to rock mass properties obtained through laboratory tests based upon laboratory tests and previous research findings a time dependent strength degradation process from water absorption was assumed and implemented in the numerical modeling next the progressive failure of a roadway from water rich roofs was analyzed in detail the numerical results agreed well with field measurements and observations this method was demonstrated to reproduce the real phenomenon of roof failure in all of its complexity the numerical results revealed that the progressive failure mechanism was characterized by an initial fracturing of the roof due to strength degradation which was followed by significant dilation as fractures grew the large span of the roadway further contributed to the roof failure process the results also clearly showed that shear cracks were predominant in the roof and played a major role in the behavior of the roadway roof additionally support and excavation schemes that affect roof stability were observed

pacific geoid anomalies revisited in light of thermochemical oscillating domes in the lower mantle abstract the geoid low over french polynesia has been previously interpreted as the signature of an upper mantle upwelling associated with a low viscosity zone located immediately below the lithosphere we propose here an alternative explanation using new high quality satellite gravity data and fluid mechanics constraints we thus discuss the origin of the high concentration of hotspots in the central pacific which is still debated satellite gravity data from the grace mission analyzed with wavelets allow us to better characterize the geoid low on french polynesia and the large scale positive geoid anomaly 600 km east of the line islands chain we show that the geoid anomalies are consistent with thermochemical oscillating domes and we bring constraints on their density contrast and their upper limit depth under this hypothesis the central pacific mantle would contain two thermochemical instabilities at different stages of their development a rising dome lighter extending approximately from the cmb up to the transition zone beneath the south pacific superswell toped by secondary plumes that generate short lived hot spot chains and a sinking dome denser in the lower mantle beneath the area located 600 km east of the line islands chain plate reconstructions further show that the latter could also have generated the southern line islands 35 m y ago and shatsky rise 140 m y ago when it was in its ascending phase while the south pacific dome probably created darwin rise 100 m y ago in a same type of event

moored wind temperature and current observations made during coastal ocean dynamics experiments 1 and 2 over the northern california continental shelf and upper slope moored observations of winds currents and temperature made over the northern california shelf and upper slope during the 1981 and 1982 upwelling seasons as part of the coastal ocean dynamics experiment code are described in both field experiments code 1 and code 2 the onset of the upwelling season is triggered by the strengthening of the north pacific high off northern california and signalled by the presence of water colder than 9 c near the bottom over the shelf vertical temperature stratification over the shelf is reduced during active upwelling local winds and subtidal currents over the shelf are strongly polarized in the alongshelf direction and the mean wind stress is directed equatorward response of alongshelf currents to wind forcing is similar at all depths over the shelf the response of the cross shelf current is more complex in the surface layer offshelf flow is significantly and positively correlated with the upwelling favorable alongshelf wind stress beneath this layer a weak return flow exists the thickness of the upper mixed layer varies greatly reaching 60 m during upwelling events thus direct wind forcing affects a substantial fraction of the water over the shelf low frequency wind stress current and temperature fields are adequately described by a few energetic empirical orthogonal functions with simple spatial structure the most energetic mode typically accounts for 75 or more of the variance of wind stress and temperature over the middle and inner shelf the most energetic current mode accounts for 90 or more of the variance is dominated by he alongshelf component and veers to the left with depth the low frequency alongshelf flow fluctuations are quasi barotropic with some vertical shear but no reversal with depth vertical shear in alongshelf current and the cross shelf density gradient obey the thermal wind relation the simple structure of the subdiurnal currents allow estimation of the alongshelf volume transport between the 60 and 130 m isobaths through three cross shelf transects in code 2 volume transport over the shelf is often nearly two dimensional but occasional large differences indicate significant mass exchange between the shelf and the adjacent upper slope correlation of low frequency currents and alongshelf wind stress shows the response of alongshelf current to be maximum near the surface and near midshelf where a 15 cm 1 alongshelf flow lags a 1 dyn cm 2 alongshelf wind stress by the order of 10 hours currents over the upper slope are not significantly correlated with local wind alongshelf transport over the shelf is highly correlated with local wind stress explaining 80 of the variance in the low frequency transport during code 2 volume transport is best correlated with wind stress approximately 30 km to the south winds in the code area during spring and summer are characterized by strong and persistent upwelling favorable winds interrupted by shorter periods of weak winds regression analysis indicates that in the absence of wind a poleward barotropic current jet exists over the shelf with a maximum amplitude over the inner shelf in the code area an upwelling favorable wind counters this jet over the shelf so that the alongshore flow is everywhere equator ward during strong wind events since the maximum response to wind forcing is found at midshelf and near surface the mean current over the upwelling season is strongly sheared in both the vertical and cross shelf directions mean flow over the outer shelf is surface intensified and is directed offshelf and equatorward while the deeper flow at midshelf and through most of the water column over the inner shelf is directed poleward

assessment of mineralogical and petrographic factors affecting petro physical properties strength and cracking processes of volcanic rocks abstract microtextural features influence the petrophysical and mechanical properties of rocks including the elastic properties and cracking processes under axial loads volcanic rocks with varying microtextural characteristics and compositions were evaluated to assess the effect of the main constituents mineralogical petrographic and micro structural features on the petrophysical properties elastic properties and strength furthermore detailed analyses were also used to understand the cracking processes under axial loads the analyses of the cracking processes were completed on thin sections obtained from the mechanically tested specimens the results from the quantitative mineralogical and petrographic studies and observations from thin sections revealed that the mineral mass fractions have an effect on the specific gravity and loss on ignition loi values on the other hand unconfined compressive strength ucs and elastic properties are mostly affected by petrographic variables e g mineral content the ucs values tend to decrease with a relative increase in the groundmass with respect to the phenocryst content and vice versa biotite is the only mineral that influences the ucs individually geometric features e g feret s diameter and perimeter of opaque minerals and biotite are determined to be important constituents influencing the young s modulus under axial loads crack propagation is strongly dependent on the proportional distribution of the phenocryst and groundmass it was observed that increasing groundmass content leads to predominantly axial cracks the cracks tend to bend or propagate as a boundary crack when they reach the boundary of an unaltered phenocryst thus axial and shear cracks co exist with increasing amounts of phenocrysts which hinders crack propagation occasionally cracks can penetrate the altered or opaque phenocrysts depending on the degrees of orientation with respect to applied load of the same minerals furthermore the synthesis of the measurements obtained from ucs tests and thin section observations reveals that an increase in the amount of phenocrysts which promote the formation of more shear cracks in addition to axial cracks causes an increase in the radial strain and poisson s ratio the other factors investigated had minor effects on physical and mechanical properties of studied rocks

simulation of heterogeneity creep damage and lifetime for loaded brittle rocks abstract a grain based heterogeneous numerical model originated from the discrete element method is developed and applied for lac du bonnet granite basing on voronoi cells the modelled microstructure considers different mineral components and grain size the model takes into account elastic grain and elasto plastic contact deformation inter and intra granular fracturing and lifetime prediction on basis of subcritical crack growth the procedure is successfully applied to simulate uniaxial compression tests brazilian tests and uniaxial creep tests the damage processes are studied in detail and mode i and mode ii fracture toughness were determined the proposed modelling allows the simulation of time dependent behaviour in terms of the damage process during primary secondary and tertiary creeps until the final failure characterized by macroscopic fracturing shear band and or macroscopic tensile fracture all simulations have shown reasonable agreement with macroscopic results obtained from lab tests on granite but have in addition delivered deeper insight into the microscopic damage process the proposed modelling approach is recommended for brittle rocks

quantifying the effects of scale and heterogeneity on the confined strength of micro defected rocks abstract a numerical study was performed to develop a framework for estimating the confined strength of rock blocks considering scale effects and in situ heterogeneity i e intensity of structural microdefects and degree of weathering grain boundary models using the voronoi tessellation scheme within udec have been used to simulate the results of small lab and large field scale compression unconfined and triaxial and indirect tensile brazilian tests on a series of progressively larger in size and degrading in quality numerical specimens accordingly relationships that link rock block strength with its volume and in situ condition were developed for the preliminary estimation of scaled mohr coulomb and hoek brown parameters the results from the scaling analysis generally suggest that cohesion decreases with both increasing scale and degrading sample condition in a manner similar to the scale condition dependant reduction of uniaxial compression strength ucs while the friction angle shows only minor variation with no apparent trend the measured peak confined strength values were also fitted to the generalized hoek brown criterion and a new block scale geological strength index parameter is introduced named micro gsi mgsi which was also linked to the scale condition dependant reduction of ucs by using the proposed linear and non linear approaches once the ucs reduction due to scaling effects is known the confined strength of rock blocks could be then defined and can be used to carry out preliminary rock engineering calculations and to run discontinuum numerical models in which rock blocks are simulated explicitly

the empirical age depth relation and depth anomalies in the pacific ocean basin this paper presents a new age depth relation and a comprehensive depth anomaly map for the pacific ocean age depth and sediment thickness were averaged for each 1 1 area bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep hot spots were treated as thermal anomalies and data within 800 km of them were eliminated from further analysis an age depth equation was determined by fitting a least squares line to the remaining data the resulting depth versus age distribution confirms the linear dependence of depth on age up to 80 ma all older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted this flattening of the age depth distribution requires that a separate function be fit to data older than 80 ma as a result data representing the deepest seafloor older than 80 ma were selected for curve fitting and the age depth distribution was approximated by an exponential function it is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age depth data from the pacific ocean a depth anomaly map prepared by comparing observed depth with the depth predicted by the new age depth formula shows several newly recognized features including swells over the louisville ridge in the south pacific and in the gulf of alaska seamounts

effect of heterogeneity of brittle rock on micromechanical extensile behavior during compression loading a grain based universal distinct element code model was developed to generate a deformable polygonal grain like structure to simulate the microstructure of brittle rock it takes into account grain scale heterogeneity including microgeometric heterogeneity grain scale elastic heterogeneity and microcontact heterogeneity the microgeometric heterogeneity can be used to match the grain size distribution of the rock the discrete element approach is able to simulate the microheterogeneity caused by elastic variation and contact stiffness anisotropy the modeling approach was evaluated using lac du bonnet granite and a aspo diorite the microheterogeneity played an important role in controlling both the micromechanical behavior and the macroscopic response when subjected to uniaxial compression loading the crack initiation stress was found to be controlled primarily by the microscale geometric heterogeneity while the microcontact heterogeneity controlled the strength characteristics the effect of heterogeneity on the distribution and evolution of tensile stresses and associated extension cracks was also examined

the effect of hot spots on the oceanic age depth relation hot spots located both midplate and on midocean ridges can bias the empirical age depth relation ocean floor bathymetry for seafloor greater than 80 m y b p in age has been previously used to determine if the oceanic lithosphere behaves as a cooling half space or as a cooling plate however large areas of the ocean basins do not fit a simple age depth relation anomalously shallow swells approximately 1000 km wide and 1 km high surround hot spots such as iceland the azores and the cape verde islands by sorting seafloor depth data by distance to the nearest hot spot track we show that the along isochron variation in depth in the north atlantic correlates with distance from a hot spot track due to the absence in the north atlantic of seafloor both old and far from hot spots we compare depth versus age at constant distance and do not observe flattening of the age depth relation in any distance range the age depth relation depth meters 2700 295 t m y 1 2 derived for seafloor 1200 to 1800 km from a hot spot track represents our best estimate of the age depth relation appropriate to seafloor unaffected by a hot spot track in the north atlantic the previously inferred flattening of the age depth relation appears to be caused by the inclusion of hot spot swells this implies that hot spots may be the primary means of heat transfer from the deeper mantle to the base of the lithosphere

identifying crack initiation and propagation thresholds in brittle rock recent work at the underground research laboratory of atomic energy of canada limited in pinawa manitoba has shown that high compressive stresses near the tunnel face significantly contribute to

revisiting grace antarctic ice mass trends and accelerations considering autocorrelation previous grace derived ice mass trends and accelerations have almost entirely been based on an assumption that the residuals to a regression model including also semi annual annual and tidal aliasing terms are not serially correlated we consider ice mass change time series for antarctica and show that significant autocorrelation is in fact present we examine power law and autoregressive models and compare them to those that assume white uncorrelated noise the data do not let us separate autoregressive and power law models but both indicate that white noise uncertainties need to be scaled up by a factor of up to 4 for accelerations and 6 for linear rates depending on length of observations and location for the whole of antarctica east antarctica and west antarctica the scale factors are 1 5 1 5 and 2 2 respectively for the trends and for the accelerations 1 5 1 5 and 2 1 substantially lower scale factors are required for offshore time series suggesting much of the time correlation is related to continental mass changes despite the higher uncertainties we find significant 2 sigma accelerations over much of west antarctica overall increasing mass loss and dronning maud land increasing mass gain as well as a marginally significant acceleration for the ice sheet as a whole increasing mass loss

influence of stress path on stress memory and stress fracturing in brittle rocks a two dimensional numerical model based on the distinct element method previously calibrated to the laboratory properties of undamaged and damaged lac du bonnet ldb granite was used to investigate the influence of stress path on the development of stress memory and stress fracturing in brittle rocks various cycles of loading and unloading similar to those imposed during kaiser effect tests were first applied to undamaged numerical specimens of ldb granite the results of kaiser effect test simulations were found to be consistent with those of published laboratory and numerical investigations further simulations were conducted to investigate the influence of stress path resulting from the excavation of a tunnel on the depth of stress fracturing around the excavation boundary for this purpose the stress paths at points on and near the tunnel wall obtained from a continuum finite element model were applied to the calibrated numerical specimen it was found that the amount of damage in the numeric

external validation of the grace ggm01c gravity field using gps and doris positioning results 1 we have used the new gravity field model ggm01c derived from grace data to reanalyze doris data from 1993 0 to 2003 2 using the gipsy oasis software and a free network approach we have estimated the position and velocity of each doris station in itrf2000 in order to test the accuracy of these results we have compared them to the positions and velocities of 43 collocated gps stations using local ties and covariance information doris results computed using the ggm01c gravity field instead of the egm96 gravity field show a significantly improved external agreement with gps position agreement of 12 26 mm was reduced to 10 13 mm and velocity agreement of 3 3 3 7 mm yr was reduced to 2 4 3 3 mm yr this can be interpreted as an external test of the accuracy of the new ggm01c gravity field

antarctic circumpolar current transport variability during 2003 05 from grace abstract gravity recovery and climate experiment grace gravity data spanning january 2003 november 2005 are used as proxies for ocean bottom pressure bp averaged over 1 month spherical gaussian caps 500 km in radius and along paths bracketing the antarctic circumpolar current s various fronts the grace bp signals are compared with those derived from the estimating the circulation and climate of the ocean ecco ocean modeling assimilation system and to a non boussinesq version of the regional ocean model system roms the discrepancy found between grace and the models is 1 7 cmh2o 1 cmh2o 1 hpa slightly lower than the 1 9 cmh2o estimated by the authors independently from propagation of grace errors the northern signals are weak and uncorrelated among basins the southern signals are strong with a common seasonality the seasonal cycle grace data observed in the pacific and indian ocean sectors of the acc are consistent with annual and semiannual amplitudes of 3 6 and 0 6 cmh2o 1 1 and 0

distribution characteristics and controlling factors of vertical joint spacing in sand mud interbedded strata abstract nearly 3 600 vertical joints were measured in sand mud interbedded rock masses from 30 outcrops in the yan an area china information such as joint spacing joint aperture and the thickness of sandstone formations was obtained results show that the vertical joints are well developed in the study areas and the fracture spacing tolayer thickness ratio s t can reach 0 79 the joint spacing has a strong positive linear relationship with the thickness of the sandstone layer and presents a lognormal distribution in each sandstone layer however the morphology of the lognormal distribution is related to the degree of joint development the higher the degree of joint development the smaller skewness of the joint spacing distribution in addition although the value of s t for saturate joint is generally 0 8 1 2 the vertical joints may reach saturation at greater s t by penetrating multiple layers of sandstone when the intervening soft layer is thin

nonlinear dynamic failure process of tunnel fault system in response to strong seismic event strong earthquakes and faults have significant effect on the stability capability of underground tunnel structures this study used a 3 dimensional discrete element model and the real records of ground motion in the wenchuan earthquake to investigate the dynamic response of tunnel fault system the typical tunnel fault system was composed of one planned railway tunnel and one seismically active fault the discrete numerical model was prudentially calibrated by means of the comparison between the field survey and numerical results of ground motion it was then used to examine the detailed quantitative information on the dynamic response characteristics of tunnel fault system including stress distribution strain vibration velocity and tunnel failure process the intensive tunnel fault interaction during seismic loading induces the dramatic stress redistribution and stress concentration in the intersection of tunnel and fault the tunnel fault system behavior is characterized by the complicated nonlinear dynamic failure process in response to a real strong seismic event it can be qualitatively divided into 5 main stages in terms of its stress strain and rupturing behaviors 1 strain localization 2 rupture initiation 3 rupture acceleration 4 spontaneous rupture growth and 5 stabilization this study provides the insight into the further stability estimation of underground tunnel structures under the combined effect of strong earthquakes and faults

new insights on intraplate volcanism in french polynesia from wavelet analysis of grace champ and sea surface data in this paper we discuss the origin of superswell volcanism on the basis of representation and analysis of recent gravity and magnetic satellite data with wavelets in spherical geometry we computed a refined gravity field in the south central pacific based on the grace satellite ggm02s global gravity field and the kms02 altimetric grid and a magnetic anomaly field based on champ data the magnetic anomalies are marked by the magnetic lineation of the seafloor spreading and by a strong anomaly in the tuamotu region which we interpret as evidence for crustal thickening we interpret our gravity field through a continuous wavelet analysis that allows to get a first idea of the internal density distribution we also compute the continuous wavelet analysis of the bathymetric contribution to discriminate between deep and superficial sources according to the gravity signature of the different chains as revealed by our analysis various processes are at the origin of the volcanism in french polynesia as evidence we show a large scale anomaly over the society islands that we interpret as the gravity signature of a deeply anchored mantle plume the gravity signature of the cook austral chain indicates a complex origin which may involve deep processes finally we discuss the particular location of the marquesas chain as suggesting that the origin of the volcanism may interfere with secondary convection rolls or may be controlled by lithospheric weakness due to the regional stress field or else related to the presence of the nearby tuamotu plateau

localization of geoid anomalies and the evolution of oceanic lithosphere a case study from the mendocino fracture zone 1 the thermal evolution of oceanic lithosphere is investigated by focusing on geoid offsets occurring across the mendocino fracture zone where plates with different ages are juxtaposed various processing techniques have been devoted to separate the geoid signal of the age contrast from regional and shorter wavelength components unrelated to the thermal structure of lithosphere nevertheless due to processing differences estimates of geoid offsets vary and no agreement on the thermal evolution of oceanic lithosphere has been found so far in this study we propose to use a continuous wavelet analysis to accurately characterize the components of the geoid at different spatial scales and to estimate a new geoid slope age relationship from localized signals we also apply the same wavelet transform on a set of synthetic geoid calculated with different assumptions on plate cooling the comparison of the observed geoid offsets with those predicted from cooling models indicates that our approach can successfully remove unwanted regional contributions and isolate the geoid signature due to lithospheric cooling our results suggest that contrary to previous studies geoid slopes measured at the mendocino fracture zone are compatible with both the half space cooling model and the plate model

geoid anomalies over the north east pacific fracture zones from satellite altimeter data summary seasat altimeter profiles have been used to recover geoid height anomalies over the north east pacific fracture zones geoid anomalies observed over fracture zones show a step like signal of 1 4m amplitude downward from the younger shallower side to the older deeper side of the fracture zone assuming that fracture zones are in isostatic equilibrium at the base of the lithosphere the geoid offset occurs because the portions of lithosphere on each side of the fracture have different thickness and density structures as predicted by thermal models of lithospheric cooling along the mendocino fracture zone whose age offset is of the order of 20 30myr the observed geoid anomalies present a clear change from east to west both in shape and in amplitude in the youngest part of mendocino around 130 w longitude there is a marked step in the geoid 3 4m amplitude while west of 140 w the older part of mendocino the geoid offset does not exceed 1 5 m the signal becoming more and more distorted the geoid offset age offset relation estimated at different locations along mendocino presents a progressive decrease from east to west e g from the young part to the old part of the fracture zone for the pioneer fracture zone the geoid offset is barely visible never exceeding 40 cm using the known bathymetry and crustal structure inferred from seismic refraction data at 128 30 w longitude as well as ages inferred from magnetic lineation data we have computed the theoretical geoid height anomaly at this location assuming local isostatic compensation at the base of the lithosphere we have deduced the subcrustal density structure from lithospheric cooling models taking account of the age thickness relationship comparison between computed and observed geoid anomaly at 128 30 w longitude is very satisfactory although a very slight overcompensation at depth needs to be assumed in order to give a better agreement over the western older part of mendocino observed geoid anomalies approach isostatic anomalies since subcrustal lateral density contrasts are smaller than in the young part of the fracture zone their isostatic shape and smaller amplitude imply smaller crustal discontinuities this suggests that the crustal thickness discontinuity across a fracture zone may decrease with age

a new method for assessing slope unloading zones based on unloading strain unloading zones are formed in shallow slopes after rock mass relaxation and crack extension generally slope unloading assessment depends on the traditional methods mainly related to the phenomena however unloading is a process of energy release based on the fracture mechanics and the statistical theory of rock mass structure a theoretical analysis was performed to establish the relationship between the accumulative opening displacement of unloading cracks and unloading strain and a new method was proposed to quantify the extent and damage degree of unloading zones according to unloading strain the new method was applied to assess the slope unloading zones at a hydropower station dam site in northwest china results showed that the accumulative opening displacement curve of unloading cracks regularly varied and could be easily divided into three parts the strong and weak unloading zones could be recognized from the slope of the first two parts while their extent was limited by the two inflexions on each curve by comparison with the traditional method the new method confirmed the reasonability in the slope unloading zone assessment however with an increase in the unloading degree the difference in the zonation results obtained from the two methods may increase

boundary flow on a lake slope during ekman layer arrest 1 the disruption of an arrested bottom ekman layer on a lake boundary slope is initiated when buoyancy enters the balance between the coriolis force and the pressure gradient this process is shown to behave in a lake in a qualitatively similar manner to the ocean a greater bottom slope and stronger stratification are found to dramatically shorten the shutdown timescale of the ekman flux compared to typical values on the continental shelf and slope convergence of the flow at the density front where the thermocline intersects the bottom leads to a detrainment of bottom boundary layer water into the interior of the lake and thus facilitates the transport of mass and heat independent of turbulence generation in the boundary layer by internal wave interactions internal waves complicate the flow structure the boundary water detrainment and gravitational instability due to ekman layer dynamics and the critical reflection and vertical overturns associated with the internal waves combine in effective boundary mixing

cooling of the oceanic lithosphere evidence from geoid anomalies across the udintsev and eltanin fracture zones abstract the often step like variation in the marine geoid across fracture zones is believed to reflect the change in age of the ocean floor and hence in temperature structure of the plate across the fracture zone the rate of change of the geoid with age has been estimated as a function of age from geoid offsets across fracture zones and used to constrain thermal models of lithospheric cooling results from three studies of the geoid offsets across the mendocino fracture zone are consistent with a thermal plate cooling model having a plate thickness between 100 and 125 km this is in contrast to results obtained from the combined measurements from fracture zones in the south pacific which are not consistent with either the plate model or boundary layer model of cooling to examine this inconsistency we have measured geoid offsets on seasat profiles across the udintsev and eltanin fracture zones in the south pacific and tested the robustness of the measurement technique we used a second order polynomial that included a step function centered on the fracture zone to estimate the regional geoid and the geoid offset the polynomials were fit to the geoid profiles using least squares minimization criteria data within a certain distance of the fracture zone were excluded from the fitting procedure in order to minimize the effects of horizontal cooling and lithospheric deformation near the fracture zone we find that the estimate of the geoid offset is very sensitive to the length of data excluded and that generally an exclusion width of 100 200 km is needed to avoid anomalies unrelated to the regional geoid offset near the fracture zone exclusion widths of less than 100 km generally lead to unpredictable errors in the geoid offset estimates using exclusion widths of between 150 and 200 km on most profiles we constructed geoid slope geoid offset divided by the age offset versus average age plots for each of the limbs of the two fracture zones similar trends were found on both the east and west limbs of the eltanin and the east limb of the udintsev the trends were inconsistent with a single thermal plate thickness and instead showed an initial decrease in slope between 0 and 35 m y followed by an abrupt increase followed again by a decrease in slope in contrast the west branch of the udintsev shows an increase in geoid slope values between 20 and 35 m y followed by a decrease the difference in observed trends across the western limb of the udintsev may be due to the effects of the louisville ridge north of the fracture zone on the regional geoid the similarity in trends on the remaining three limbs argues against the effects of isolated thermal or bathymetric anomalies and appears instead to reflect a general feature of the geoid slope versus average age relationship across fracture zones although the thermal plate cooling model is successful in predicting both seafloor depths and heat flow values out to ages of at least 80 m y b p it cannot explain the observed geoid slope values for these two fracture zones it is not clear at this point whether this is due to inadequacies in the cooling model or to peculiarities in fracture zone evolution possibly caused by small scale convection driven by the horizontal temperature gradients beneath fracture zones

an enhanced understanding of the basinal bowland shale in lancashire uk through microtextural and mineralogical observations abstract variability in the lower bowland shale microstructure is investigated here for the first time from the centimetre to the micrometre scale using optical and scanning electron microscopy om sem x ray diffraction xrd and total organic carbon content toc measurements a significant range of microtextures organic matter particles and fracture styles was observed in rocks of the lower bowland shale together with the underlying pendleside limestone and worston shale formations encountered the preese hall 1 borehole lancashire uk four micro texture types were identified unlaminated quartz rich mudstone interlaminated quartz and pyrite rich mudstone laminated quartz and pyrite rich mudstone and weakly interlaminated calcite rich mudstone organic matter particles are classified into four types depending on their size shape and location multi micrometre particles with and without macropores micrometre size particles in cement and between clay minerals multi micrometre layers and organic matter in large pores fractures are categorized into carbonate sealed fractures bitumen bearing fractures resin filled fractures and empty fractures we propose that during thermal maturation horizontal bitumen fractures were formed by overpressuring stress relaxation compaction and erosional offloading whereas vertical bitumen bearing resin filled and empty fractures may have been influenced by weak vertical joints generated during the previous period of veining for the majority of samples the high toc 2 wt low clay content 50 wt and the presence of a multi scale fracture network support the increasing interest in the bowland shale as a potentially exploitable oil and gas source the microtextural observations made in this study highlight preliminary evidence of fluid passage or circulation in the bowland shale sequence during burial

novel grain based model for simulation of brittle failure of alxa porphyritic granite abstract a grain based model gbm is generally employed by many researchers to simulate the microcracking process of crystalline brittle rock by the distinct element method in this study a novel grain based model ngbm in two dimensional particle flow code 5 0 pfc2d 5 0 is proposed to emulate the brittle failure of crystalline rock first the ngbm scheme is introduced in which the flat joint model is employed to model the contacts in mineral grains and the smooth joint model is assigned to the grain interface contacts then compressive tests are performed using alxa porphyritic granite and the strength values are fitted with the hoek brown failure criterion third a ngbm was generated based on the petrographic texture of realistic rock such as mineral constituents grain size and distribution the input microscale parameters are carefully calibrated according the experimental results finally this model is applied to study the microcracking process of alxa porphyritic granite and the influence of spatial distribution of minerals it is found that a relatively high ratio of uniaxial compressive strength to tensile strength of each mineral or synthetic samples can be reproduced by the ngbm moreover the simulated results show this model can capture the nonlinear characteristic of the hoek brown failure criterion after modeling numerous mineral grains were cut across which is consistent with the actual experimental observation the tensile crack is over the shear crack in the models while the number of intra grain shear crack increases dramatically with increasing confining pressure in addition the numerical results indicate that the effect of spatial distribution of minerals has limited influence on the evolution of microcracks but has great impact on microcrack distribution in summary the proposed ngbm in pfc2d can well reproduce the realistic failure process of granitic rocks under different loading conditions

coseismic and post seismic signatures of the sumatra 2004 december and 2005 march earthquakes in grace satellite gravity s u m m a r y the grace satellite mission has been measuring the earth s gravity field and its temporal variations since 2002 april although these variations are mainly due to mass transfer within the geofluid envelops they also result from mass displacements associated with phenomena including glacial isostatic adjustment and earthquakes however these last contributions are difficult to isolate because of the presence of noise and of geofluid signals and because of grace s coarse spatial resolution 400 km half wavelength in this paper we show that a wavelet analysis on the sphere helps to retrieve earthquake signatures from grace geoid products using a wavelet analysis of grace geoids products we show that the geoid variations caused by the 2004 december m w 9 2 and 2005 march m w 8 7 sumatra earthquakes can be detected at grace resolution the 2004 december earthquake produced a strong coseismic decrease of the gravity field in the andaman sea followed by relaxation in the area affected by both the andaman 2004 and the nias 2005 earthquakes we find two characteristic timescales for the relaxation with a fast variation occurring in the vicinity of the central andaman ridge we discuss our coseismic observations in terms of density changes of crustal and upper mantle rocks and of the vertical displacements in the andaman sea we interpret the post seismic signal in terms of the viscoelastic response of the earth s mantle the transient component of the relaxation may indicate the presence of hot viscous material beneath the active central andaman basin

grace detects coseismic and postseismic deformation from the sumatra andaman earthquake 1 we show that spherical harmonic sh solutions of the gravity recovery and climate experiment grace are now of sufficient quality to observe effects of co seismic and post seismic deformation due to the rupture from the mw 9 3 sumatra andaman earthquake on december 26 2004 and its companion nias earthquake mw 8 7 on march 28 2005 the improved ggm 03 sh level 2 solutions and improved filtering methods provide estimates with spatial resolution comparable to earlier estimates from range rate level 1 grace data the gravityfield disturbance extends over 1800 km along andaman and sunda subduction zones and changes with time following events gravity changes may be due to afterslip viscoelastic relaxation or other processes associated with dilatation satellite gravity measurements from grace provide a unique new measure of deformation and post seismic processes associated with major earthquakes especially in areas which are primarily oceanic citation chen j l c r wilson b d tapley and s grand 2007 grace detects coseismic and postseismic deformation from the sumatraandaman earthquake geophys res lett 34 l13302

geoid data and thermal structure of the oceanic lithosphere a long standing question is whether old oceanic lithosphere continues cooling as the boundary layer of a halfspace or approaches thermal equilibrium as modeled by a finite thickness plate although the latter is the most direct inference from seafloor depths and heat flow other explanations have been proposed we investigate this issue using published results for the derivative of the oceanic geoid with age estimated from geoid offsets across fracture zones such data have not been used extensively in analyses of the thermal evolution of the lithosphere primarily because they are inconsistent with two commonly used thermal models a halfspace or a 125 km thick plate recent studies however find that depth and heat flow data are better fit by a thinner 95 km plate model we thus compile published geoid slope results and find that these data though scattered can discriminate between the models geoid slope changes with age rather than being constant as predicted for a cooling halfspace this variation is greater than predicted for a thick plate and is better fit by a thin plate geoid data should thus be useful for improving thermal models of the lithosphere

temporal variations of the marine geoid the effects of temporal changes in the marine geoid on estimates of the ocean dynamic topography is being investigated influences from changes of land hydrology ice sheets post glacial rebound

the mantle wedge s transient 3 d flow regime and thermal structure arc volcanism volatile cycling mineralization and continental crust formation are likely regulated by the mantle wedge s flow regime and thermal structure wedge flow is often assumed to follow a regular corner flow pattern however studies that incorporate a hydrated rheology and thermal buoyancy predict internal small scale convection ssc here we systematically explore mantle wedge dynamics in 3 d simulations we find that longitudinal richter rolls of ssc with trench perpendicular axes commonly occur if wedge hydration reduces viscosities to 1 1019 pa s although transient transverse rolls with trench parallel axes can dominate at viscosities of 5 1018 1 1019 pa s rolls below the arc and back arc differ subarc rolls have similar trench parallel and trench perpendicular dimensions of 100 150 km and evolve on a 1 5 myr time scale subback arc instabilities on the other hand coalesce into elongated sheets usually with a preferential trench perpendicular alignment display a wavelength of 150 400 km and vary on a 5 10 myr time scale the modulating influence of subback arc ridges on the subarc system increases with stronger wedge hydration higher subduction velocity and thicker upper plates we find that trench parallel averages of wedge velocities and temperature are consistent with those predicted in 2 d models however lithospheric thinning through ssc is somewhat enhanced in 3 d thus expanding hydrous melting regions and shifting dehydration boundaries subarc richter rolls generate time dependent trench parallel temperature variations of up to 150 k which exceed the transient 50 100 k variations predicted in 2 d and may contribute to arc volcano spacing and the variable seismic velocity structures imaged beneath some arcs

thermal evolution of the lithosphere beneath fracture zones inferred from geoid anomalies a global survey of seasat derived geoid anomalies across fracture zones fz has been conducted to characterize the behaviour of the geoid slope with age of plate in response to lithospheric cooling we present new results for the geoid slope versus age along six fz of the atlantic ocean gibbs pico doldrums romanche ascension and agulhas falkland fz including results of a previous study on pacific fz mendocino murray clarion and unditsev fz we confirm that the geoid slope does not vary with age as conductive cooling models predict and show that its complex behaviour is a worldwide phenomenum two trends are clearly shown up by the data 1 a sharp initial decrease of the geoid slope at very young ages 2 a subsequent increase which typically starts at ages around 30 40 ma and culminates at 60 ma in a few cases the rise of the geoid slope occurs later beyond 80 ma according to the recent models developed by robinson parsons and driscoll these trends may result from small scale convection induced by the horizontal thermal gradients underneath the fz instabilities developing in a sublithospheric low viscosity layer whose properties vary with age

observations of high frequency internal waves in the coastal ocean dynamics region current meter data from the second coastal ocean dynamics experiment code ii for july 1982 are analyzed for internal waves in the 6 to 40 cycles per day cpd frequency band it is found that the wave field is anisotropic and that the current ellipses are oriented in approximately the cross isobath direction the squares of the ratio of the major to minor axes of the current ellipses the ellipticity are consistent with a continuum of internal waves propagating onshore but are not consistent with a single wave propagating onshore the reduction of internal wave energy across the shelf is consistent with propagation from the deep ocean or shelf break as is the correlation between vertical velocities and velocities parallel to the minor axis however there is evidence for the generation of additional internal wave energy on the shelf in the evolution of the current ellipses across the shelf and in the bluing of the internal wave spectra across the shelf internal wave energy levels are elevated by a factor of 1 5 to 5 above garrett and munk 1972 levels at the moorings on the 130 and 365 m isobaths the first vertical mode dominates at the 130 m isobath the only mooring for which the vertical modal analysis was done

subsidence of normal oceanic lithosphere apparent thermal expansivity and seafloor flattening abstract seafloor topography has been a key observational constraint on the thermal evolution of oceanic lithosphere which is the top boundary layer of convection in earth s mantle at least for the first 70 myr the age progression of seafloor depth is known to follow the prediction of half space cooling and the subsidence rate is commonly believed to be 350 m ma 1 2 here we show that based on a new statistical analysis of global bathymetry the average subsidence rate of normal oceanic lithosphere is likely to be 320 m ma 1 2 i e 10 lower than the conventional value we define the normal seafloor as regions uncorrelated with anomalous crust such as hotspots and oceanic plateaus but the lower subsidence rate appears to be a stable estimate not depending on how exactly we define the normal seafloor this low subsidence rate can still be explained by half space cooling with realistic mantle properties if the effective thermal expansivity of a viscoelastic mantle is taken into account in light of a revised model of half space cooling the normal seafloor unperturbed by the emplacement of anomalous crust exists for all ages and the so called seafloor flattening seems to be mostly caused by hotspots and oceanic plateaus

on modeling the thermal evolution of the oceanic upper mantle an assessment of the cooling plate model the plate cooling model for the thermal evolution of the oceanic upper mantle has been widely accepted to explain observed variations of depth to oceanic basement and conductive heat flow with the age of the seafloor several estimates of best fitting plate model parameters derived from depth heat flow and age data have been proposed but the viability of the plate model itself has not been rigorously evaluated we have used published mean depths and depths to basement at deep sea drilling project ocean drilling program dsdp odp drilling sites to test the plate cooling model based on two criteria first viable plate models must have coefficients that are consistent with the slope of the corresponding root t line because the half space or root t subsidence of young seafloor is implicit in the plate model i e the slope of the root t line can be calculated directly from the coefficients of the plate subsidence model second any viable physical model must fit the data with an acceptable degree of systematic misfit large systematic misfits indicate that the model cannot explain the observations fits of half space root t models to depth versus age data for young 80 ma seafloor indicate basal temperatures in the range 1300 to 1370 c based on the age at which the depths deviate from the root t line the minimum plate thickness that is compatible with the best fitting half space models for young seafloor is 120 km in contrast all best fitting plate models yield systematically higher temperatures 1450 to 1470 c and thinner plates 102 118 km the plate model can explain the depth to basement at dsdp odp drill sites with a satisfactory degree of systematic misfit but we find that there is no plate cooling model that can explain the variation of the mean depths derived from the dbdb5 database with age over the entire range of ages 0 to 165 ma models that best fit the entire data set have unacceptably large systematic misfits over the entire range of seafloor age whereas models that minimize the systematic misfit for young seafloor 0 to 81 ma fail for older seafloor the plate model clearly fails to explain the observations the cooling history of old seafloor is not simply an extension of the cooling history of young seafloor according to a simple plate cooling model previous studies have suggested as an alternative to the plate model that observed variations of basement depth with age are best explained by the combined effects of a normal half space cooling process and the dynamic and or thermal effects of hot spots or mantle plumes our results are entirely consistent with the half space cooling model and we find that the best reference model for normal subsidence is d t 2600 20 m 345 3 m m y t we also find that the heat flow predicted by this half space model is consistent with the most reliable average heat flow values from the pacific

flattening of the sea floor depth age curve as a response to asthenospheric flow the flattening of sea floor depths from the age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1 previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than 80 myr ref 2 thermal rejuvenation of lithosphere that passes over stationary hotspots3 5 or a whole mantle flow forced by two converging plates6 we suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere applying the model to the pacific plate we show that a solution for flow in an asthenosphere low viscosity channel which is consumed by plate accretion and the subduction of lithosphere at trenches and replenished by near ridge upwelling and near ridge hotspots generates the observed 1 km of sea floor flattening for asthenospheric viscosities of 2 1018 pa s and an absolute pacific plate motion of 100 mm yr 1 our model can also explain the asymmetric subsidence of the south american and african plates away from the mid atlantic ridge

experimental investigation of multi scale strain field heterogeneity in rocks abstract rocks have inherently heterogeneous micro structures the mechanical response of these heterogeneous micro structures primarily governs the overall macroscopic behavior of intact rocks as these features cause heterogeneity in the distribution of stress and strain within a rock volume these attributes have generally been studied through numerical tools but in this study the stress induced multi scale spatial heterogeneity in the strain field of three types of rocks was analyzed experimentally the full field non contact strain measurement approach of 2d digital image correlation dic was employed for real time evaluation of the progression of strain field heterogeneity across the surface of the rock specimens when subjected to uniaxial compression the results show that the strain field heterogeneity is a characteristic of the inherent heterogeneity of the rock specimens and that it amplifies with the magnitude of stress applied on the rock specimens the changes in the strain field heterogeneity with loading were consistent with established microcrack evolution processes in the rock specimens the statistical fluctuation in the strain field heterogeneity of the three rocks at varying gauge lengths gl was then analyzed for the determination of the representative volume element rve length scale at increasing load levels the evolution of the rve length scale with loading facilitates an objective way of determination of the stress at which the strain field heterogeneity increases and begins to influence the overall global response of the material the rve length scale variation with loading showed that at the rve length scale reaches the specimen scale at load levels well below the peak strength

mechanical degradation of emplacement drifts at yucca mountain a modeling case study part i nonlithophysal rock abstract this paper outlines rock mechanics investigations associated with mechanical degradation of planned emplacement drifts at yucca mountain which is the designated site for the proposed us high level nuclear waste repository the welded tuff emplacement horizon consists of two groups of rock with distinct engineering properties nonlithophysal and lithophysal units based on the relative proportion of lithophysal cavities the term lithophysal refers to hollow bubble like cavities in volcanic rock that are surrounded by a porous rim formed by fine grained alkali feldspar quartz and other minerals lithophysae are typically a few centimeters to a few decimeters in diameter part i of the paper concentrated on the degradation behavior of the generally hard strong and fractured nonlithophysal rock part ii concentrates on the host rock in the lithophysal units lithophysal rock is characterized by lithophysal cavities interconnected by fracturing fracture sets are not as clearly defined as in the nonlithophysal rock the rock mass porosity in the lithophysal units has been shown to be the primary physical factor that governs elastic and strength properties the degradation behavior of the tunnels in the lithophysal rock is controlled by the spalling of the surrounding rock mass a discontinuum material model has been developed to represent the drift scale rock mass properties in which slip and separation of contacting rock blocks can be estimated two dimensional discontinuum analyses were developed with the consideration of in situ thermal and seismic loads time dependent degradation was also considered in this study field and laboratory data and numerical analyses are well integrated to provide a solution for the unique problem of modeling drift degradation

geoid anomalies at fracture zones and thermal models for the oceanic lithosphere geoid anomalies derived from the geosat altimeter data set have been used to model the change in the geoid height across four pacific fracture zones after accounting for geoid signals from the flexural response to differential subsidence and thermal bending stresses we find that the estimates of geoid steps across the fracture zones are compatible with existing conductive models for the oceanic lithosphere if flexure is not accounted for the geoid steps will be underestimated by 30 40 departures from the predicted conductive trends may be explained by small scale convection and uncertainties in the crustal ages

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

the sustainability of water resources in high mountain asia in the context of recent and future glacier change high mountain asia contains the largest volume of glacier ice outside the polar regions and contain the headwaters of some of the largest rivers in central asia these glaciers are losing mass at a mean rate of between 0 18 and 0 5 m water equivalent per year while glaciers in the himalaya are generally shrinking those in the karakoram have experienced a slight mass gain both changes have occurred in response to rising air temperatures due to northern hemisphere climate change in the westerly influenced indus catchment glacier meltwater makes up a large proportion of the hydrological budget and loss of glacier mass will ultimately lead to a decrease in water supplies in the monsoon influenced ganges and brahmaputra catchments the contribution of glacial meltwater is relatively small compared to the indus and the decrease in annual water supplies will be less dramatic therefore enhanced glacier melt will increase river flows until the middle of the twenty first century but in the longer term into the latter part of this century river flows will decline as glaciers shrink declining meltwater supplies may be compensated by increases in precipitation but this could exacerbate the risk of flooding

the pole tide and its effect on grace time variable gravity measurements implications for estimates of surface mass variations gravity recovery and climate experiment grace provides monthly solutions for the earth s gravity field in the form of spherical harmonic coefficients these can be used to infer changes in mass at the earth s surface the pole tide the response of the earth and oceans to polar motion causes gravity signals dominated by harmonics of degree 2 order 1 if the pole tide is not removed from grace data it affects the coefficients of those harmonics c21 s21 and introduces errors when using those coefficients to determine surface mass variations the pole tide is partially removed by grace processing centers before solving for the gravity field but long period pole tide signals are not usually included in the grace pole tide correction and so those signals are still present in the grace coefficients we discuss this issue from the standpoint of somebody who uses the grace gravity fields to infer changes in surface mass we arrive at a recommendation for an optimal grace pole tide correction we describe how to modify the c21 s21 coefficients provided by the processing centers so that they conform with our recommendation we discuss the size of the pole tide contributions to c21 s21 compared to those of the direct load induced contributions as an example we show how an incompletely removed pole tide can impact grace results for the trend in ocean mass we consider the impact of mantle anelasticity on long period pole tide corrections and conclude that it is unlikely to affect those corrections by more than 20

the effect of a shallow low viscosity zone on the mantle flow the geoid anomalies and the geoid and depth age relationships at fracture zones summary using a 2 d finite element method we studied the flow that is driven by the horizontal temperature gradient at a fracture zone and calculated the resulting geoid and topography anomalies assuming a three layered viscosity structure for the upper mantle consisting of a conducting lid overlying a low viscosity channel which in turn overlies a unit viscosity layer extending to the base of the upper mantle we studied the effect of varying 1 the viscosity contrast between the fluid layers 2 the rayleigh number based on the viscosity of the bottom layer and 3 the thickness of the low viscosity channel the flow at first downwells immediately adjacent to the fracture zone on the older side its time scale and characteristic wavelength depend on the viscosity and depth of the top layer which is nearest the largest temperature gradient in the fluid after the initiation of the driven flow additional flows are generated away from the fracture zone through shear and thermal coupling and boundarylayer instabilities eventually however the flow extends throughout the fluid so that the time scales and the characteristic wavelengths of the flow depend on the thickness and viscosity of both layers when the rayleigh number that is based on the viscosity of the top layer and the depth of both fluid layers is less than lo6 the geoid anomalies are dominated by the convective signal when this rayleigh number exceeds lo6 the geoid anomalies retain a step across the fracture zone out to large ages we compared our results with geoid anomalies over the udintsev fracture zone and found that the predicted geoid anomalies are in good agreement with the observed anomalies when the viscosity of at least the top layer is greater than one order of magnitude less than post glacial rebound values 102 pas however we also compared the calculated topographic steps with those predicted by the average depth age relationships observed in the oceans and found that a viscosity contrast of at least one order of magnitude at a depth in the uppermost mantle is required to produce realistic topographic steps

thermomechanical evolution of oceanic fracture zones a fracture zone fz model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere as the lithosphere cools by conduction thermal and mechanical boundary layers develop and increase in thickness as age 1 2 surface expressions of this development such as topography and deflection of the vertical i e gravity field are most apparent along major fz s because of the sharp age contrast a simple model including the effects of lateral heat transport but with no elastic layer predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the fz ages observations however show that both the fz topography and the deflection of the vertical remain sharp as the fz evolves these two observations as well as the observed asymmetry in deflection of the vertical profiles across the udintsev romanche and mendocino fz s are explained by including a continuous elastic layer in the model the asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the fz modeling also shows that the evolution of the fz topography is extremely sensitive to the initial thermal structure near the ridge transform intersection model geoid steps and their development with age are used to access techniques for measuring geoid offsets across fz s reasonable step estimation techniques will underestimate the overall step amplitude by up to 50 this implies that abnormally thin thermal boundary layers derived from studies of geoid height versus age are not required by the data

the gravity recovery and climate experiment mission overview and early results 1 the grace mission is designed to track changes in the earth s gravity field for a period of five years launched in march 2002 the two grace satellites have collected nearly two years of data a span of data available during the commissioning phase was used to obtain initial gravity models the gravity models developed with this data are more than an order of magnitude better at the long and mid wavelengths than previous models the error estimates indicate a 2 cm accuracy uniformly over the land and ocean regions a consequence of the highly accurate global and homogenous nature of the grace data these early results are a strong affirmation of the grace mission concept

variability in spatial distribution of mineral phases in the lower bowland shale uk from the mm to μm scale quantitative characterization and modelling the microstructure of a highly laminated lower bowland shale sample is characterized at the micron to millimeter scale to investigate how such characterization can be utilized for microstructure based modelling of the shale s geomechanical behavior a mosaic of scanning electron microscope sem back scattered electron bse images was studied mineral and organic content and their anisotropy vary between laminae with a high variability in fracturing and multi micrometer aggregates of feldspars carbonates quartz and organics the different microstructural interface types and heterogeneities were located and quantified demonstrating the microstructural complexity of the bowland shale and defining possible pathways for fracture propagation a combination of counting box dispersion covariance and 2d mapping approaches were used to determine that the total surface of each lamina is 3 to 11 times larger than the scale of heterogeneities relative to mineral proportion and size the dispersion approach seems to be the preferential technique for determining the representative elementary area rea of phase area fraction for these highly heterogeneous large samples supported by 2d quantitative mapping of the same parameter representative microstructural models were developed using voronoi tessellation using these characteristic scales these models encapsulate the microstructural features required to simulate fluid flow through these porous bowland shales at the mesoscale

large scale ocean circulation from the grace ggm01 geoid 1 the grace gravity model 01 ggm01 computed from 111 days of grace k band ranging kbr data is differenced from a global mean sea surface mss computed from a decade of satellite altimetry to determine a mean dynamic ocean topography dot as a test of the ggm01 gravity model large scale zonal and meridional surface geostrophic currents are computed from the topography and are compared with those derived from a mean hydrographic surface reduction in residual rms between the two by 30 60 and increased correlation indicates that the ggm01 geoid represents a dramatic improvement over older geoid models which were developed from multiple satellite tracking data altimetry and surface gravity measurements for the first time all major current systems are clearly observed in the dot from space based measurements

ocean bathymetry and mantle convection 2 small scale flow numerical models demonstrate that a boundary layer instability of the lower lithosphere does not stop the lithosphere from subsiding and so it cannot explain the so called flattening of old seafloor the slower subsidence of the old seafloor is in any case better seen as a perturbation probably due to plumes furthermore if a boundary layer instability or any mode of upper mantle convection transported most of the heat emerging from old seafloor there would be pervasive large amplitude local bathymetrie gravity and geoid anomalies such anomalies have never been convincingly differentiated from the effects of the cooling lithosphere and hotspot tracks except for some low amplitude linear geoid anomalies near spreading centers the only modes of mantle convection with horizontal scales less than those of plates for which there is reasonably clear evidence are hotspot plumes and some possible near ridge boundary layer instabilities hotspots transport less than about 10 of the earth s heat flux and other small scale modes are probably less important

a joint analysis of gps displacement and grace geopotential data for simultaneous estimation of geocenter motion and gravitational field this work is funded by the university of newcastle to support nasa s grace and grace follow on projects as an international science team member to the missions and by australian research council dp170100224 as well as nasa support to j sauber as a science team member

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

new constraints on the origin of the hawaiian swell from wavelet analysis of the geoid to topography ratio abstract analyzing the formation mechanism of hotspot swells enhances our understanding of intraplate volcanism and the underlying geodynamical processes the two main hypotheses for the origin of the archetypal hawaiian swell are thermal lithospheric thinning and dynamic support by an ascending plume any successful model would have to be able to simultaneously explain the swell topography and the corresponding geoid anomaly in simple models of isostatic compensation the geoid to topography ratio gtr is linearly related to the depth of the compensating mass therefore it is often considered a fundamental parameter to assess swell support mechanisms previous estimates for the geoid to topography ratio gtr of the hawaiian swell however are biased towards low values by incomplete removal of the effects of volcanic loading and lithospheric flexure in order to resolve these issues we here apply a continuous wavelet transform which allows resolution of lateral variations of the gtr at various spatial scales in a series of synthetic tests the robustness of this approach and its power to identify the origin of hotspot swells are established with 8 m km on the youngest part of the chain the recovered gtr agrees well with the predictions for dynamic support therefore ruling out thermal rejuvenation as an important mechanism we also find that the depth of the compensating mass decays by 20 km over a distance of 500 km from hawaii to kauai and identify sublithospheric erosion by small scale convection in the ponded plume material as a viable mechanism to support this decay

time variations in ocean bottom pressure from a few hours to many years in situ data numerical models and grace satellite gravimetry in situ ocean bottom pressure obp obtained from 154 different locations irregularly scattered over the globe is carefully processed to isolate signals related to the ocean general circulation and large scale sea level changes comparison against a global numerical ocean model experiment indicates poor correspondence for periods below 24 hr possibly related to residual tidal signals and small timing errors in the atmospheric forcing applied to the ocean model correspondence increases rapidly for periods between 3 and 10 days where wind driven dynamics are already well understood and consequently well implemented into numerical models coherence decreases again for periods around 30 days and longer where processes not implemented into ocean general circulation models as barystatic sea level changes become more important correspondence between in situ data and satellite based obp as obtained from the gravity recovery and climate experiment grace german research centre for geosciences rl05a gravity fields critically depends on the postprocessing of level 2 stokes coefficients that also includes the selection of appropriate averaging regions for the grace based mass anomalies the assessment of other available grace level 2 products indicates even better fit of more recent solutions as itsg grace2016 and the center for space research and jet propulsion laboratory rl05 mascons in view of the strong high frequency component of obp however a higher temporal resolution of the oceanic grace products would be rather advantageous

modeling deformation and damage of rock salt using the discrete element method abstract rock salt is considered to be a suitable host rock formation for radioactive waste repositories however local stress changes caused by drift construction lead to the evolution of an excavation damaged zone edz such an edz can have a major impact on the safety of a radioactive waste repository because it can violate the integral sealing function due to the development of a permeable microcrack network the objective of this investigation is the development of a modeling strategy that can be used to simulate the damage behavior of rock salt at grain scale for this purpose discrete element modeling techniques were used in combination with polyhedral shaped elements voronoi polyhedra in order to duplicate the shape and arrangement of grains to calibrate the material parameters used in the constitutive models and to validate the relevant damage and fracture processes uniaxial compression tests were combined with acoustic emission measurements overall deformation behavior and crack evolution was reproduced by modeling and validated by lab experiments the new proposed modeling strategy based on dem and voronoi polyhedra shows new possibilities to simulate the mechanical behavior of rock salt in a realistic manner by explicitly considering the fracture processes at grain scale

experimental and numerical study on the effect of calcite on the mechanical behaviour of coal abstract the effects of the presence of calcite grains on the mechanical behaviour of a coal are investigated in this study coal specimens with and without calcite grains were obtained from the same coal seam and then subjected to confined compression with varying confining pressures mineral composition data of the coal containing calcite grains were obtained from petrographic analysis and subsequently used for constructing udec gbm model the model was calibrated to experimental results and then used to further the understanding of the influence of calcite grains on the mechanical behaviour of the coal it is found that the presence of calcite increases the young s modulus reduces the peak strength and the brittleness of the coal the numerical results suggest that the presence of calcite grains significantly increase the heterogeneities of the coal leading to differential and extensional straining which in turn causes internal tensile stresses and consequently tensile cracking the conformity of the numerical results with the experimental observations suggests the potential use of the udec gbm approach model for better understanding the mechanics of brittle rock failure at micro structure scale

space borne gravimetric satellite constellations and ocean tides aliasing effects summary ocean tides redistribute mass at high temporal frequencies satellite missions that aim to observe medium to low frequency mass variations need to take into account this rapidly varying mass signal correcting for the effects of ocean tides by means of imperfect models might hamper the observation of other temporal gravity field signals of interest this paper explores different methods for mitigating aliasing errors for the specific example of observing mass variations due to land hydrology including temporal filtering of time series of gravity solutions spatial smoothing and the use of satellite constellations for this purpose an earth system model esm was constructed which included state of the art time varying components for ocean atmosphere solid earth hydrology ice sheets and ocean tides using the esm we simulated the retrieval of the hydrologically driven gravity field changes using a number of different satellite constellations we find that 1 the ocean tide aliasing strongly depends on the satellite constellation the choice of orbital parameters and the length of the data span 2 the aliasing effect manifests itself differently for different geographical regions 3 the aliasing causes a peculiar striping pattern along the ground track of the satellite orbits 4 optimizing the choice of orbital parameters of a single grace type tandem can be more effective at reducing the aliasing of ocean tide model errors than flying more tandems finally we corroborate the experiences with grace data analysis that appropriate post processing techniques can significantly improve the quality of the retrieved gravity changes

microstructure in linear elasticity and scale effects a reconsideration of basic rock mechanics and rock fracture mechanics abstract an account on the role of higher order strain gradients in the mechanical behavior of elastic perfectly brittle materials such as rocks is given that is based on a special grade 2 elasticity theory with surface energy as this was originated by casal and mindlin and further elaborated by the authors the fundamental idea behind the theory is that the effect of the granular and polycrystalline nature of geomaterials i e their microstructural features on their macroscopic response may be modeled through the concept of volume cohesion forces as well as surface forces rather than through intractable statistical mechanics concepts of the boltzmann type it is shown that the important phenomena of the localization of deformation in macroscopically homogeneous rocks under uniform tractions and of dependence of rock behavior on the specimen s dimensions commonly known as size or scale effect can be interpreted by using this non local higher order theory these effects are demonstrated for the cases of the unidirectional tension test and for the small circular hole under uniform internal pressure commonly known as the inflation test the latter configuration can be taken as a first order approximation of the indentation test that is frequently used for the laboratory or in situ characterization of geomaterials in addition it is shown that the solution of the three basic crack deformation modes leads to cusping of the crack tips that is caused by the action of cohesive double forces behind and very close to the tips that tend to bring the two opposite crack lips in close contact and further it is demonstrated that the fracture toughness depends on the size of the crack and thus it is not a fundamental property of the material this latter outcome agrees with experimental results which indicate that materials with smaller cracks are more resistant to fracture than those with larger cracks

geoid height age relation from seasat altimeter profiles across the mendocino fracture zone twenty eight seasat altimeter profiles crossing the mendocino fracture zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age between ages of 15 and 135 m y an unbiased estimate of the overall geoid offset along each profile is determined from a least squares fit of the along track derivative of the geoid to the geoid slope predicted from a simple two layer gravitational edge effect model uncertainties based upon the statistical properties of each profile are also determined a geoid slope age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets the results are in agreement with geoid slope age relations determined from symmetrically spreading ridges sandwell and schubert 1980 however the fracture zone estimates have smaller uncertainties and show less scatter a comparison of these results with the geoid slope age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age 20 40 m y a plate cooling model with a thickness of 125 km is most compatible with the geoid slope age estimates although significant deviations occur these may indicate that the lithospheric thermal structure is not entirely age dependent

antarctic regional ice loss rates from grace abstract using recent improved time variable gravity solutions from the gravity recovery and climate experiment grace we estimate rates of antarctic ice mass change for the period january 2003 through september 2006 combined improvements in data and filtering techniques allow observation of ice loss in the northern antarctic peninsula ap and along the coast of the west and central amundsen sea embayment ase in west antarctica there is also evidence of ice loss along the coast near the stancomb wills sta and jutulstraumen jut glaciers in queen maud land apparent rates are adjusted for influences of limited spatial resolution filtering and estimated postglacial rebound pgr to obtain ice loss rates for the northern ap coastal ase and sta jut of 28 8 7 9 81 17 and 16 7 9 7 km3 yr respectively this is the first estimate for the northern ap from satellite gravity data the ase estimate 81 17 km3 yr is consistent with a previous value 77 14 km3 yr using an earlier grace data release these results indicate significant improvement in grace data quality increased spatial resolution and applicability of grace data to a wider class of problems than previously possible

a 2d dem based approach for modeling water induced degradation of carbonate rock abstract the mechanical characteristics of sedimentary rocks such as carbonates are highly sensitive to the influence of water in carbonate rich regions rock failures induced by water corrosion have been widely reported such as the collapse of cliffs and rockslides in this study a 2d model was built to investigate the time dependent strength degradation of carbonate rocks under the influence of water first degradation laws for two types of bonds were derived based on the different behaviors of depositional and diagenetic cements in water then via developing customized code using the fish language the degradation laws were coupled with udec to build the numerical procedure finally the ability of the proposed model to reproduce the macroscopic short and long term behaviors of carbonate rocks under the influence of water was validated by comparing the numerical results with the lab observations and measurements the results show that the presented procedure is capable of simulating the strength properties of carbonate rocks submerged in water for different time periods uncoupled tests as well as the lifetime time to failure of rocks in a chemo mechanically coupled condition

thermal cooling of the oceanic lithosphere new constraints from geoid height data abstract up to now tests of thermal models of the oceanic lithosphere as it cools and moves away from the ridge crest have been based mainly on topography and heat flow data however large areas of the ocean floor deviate from the normal subsidence due to thermal contraction and heat flow data are not very sensitive to the form of the model cooling of the lithosphere causes a short wavelength step in the geoid across fracture zones that can also be used to constrain thermal models we have analyzed geoid data at fracture zones from the seasat altimeter measurements in the entire pacific ocean and redetermined parameters of the cooling models we find that the data reveal two distinct regimes of cooling one for seafloor ages in the range 0 30 ma the other beyond 30 ma this does not appear to be correlated with particular fracture zones but rather it is representative of the whole area studied i e the entire south pacific and northeast pacific ocean these two trends may be interpreted in terms of two different asymptotic thermal thicknesses of the plate model the smaller thermal thickness 65 km found for ages from the results obtained in this study we conclude that the half space cooling model is unable to explain the data that beyond 30 ma a simple plate model gives a satisfactory fit to the data but in the younger plate portion ages

a systematic 2 d investigation into the mantle wedge s transient flow regime and thermal structure complexities arising from a hydrated rheology and thermal buoyancy 1 arc volcanism at subduction zones is likely regulated by the mantle wedge s flow regime and thermal structure and hence numerous studies have attempted to quantify the principal controls on mantle wedge conditions in this paper we build on these previous studies by undertaking a systematic 2 d numerical investigation into how a hydrated rheology and thermal buoyancy influence the wedge s flow regime and associated thermal structure we quantify the role of a range of plausible i water contents 0 5000 h 106si ii subduction velocities 2 10 cm yr and iii upper plate ages 50 120 myr finding that small scale convection ssc resulting from rayleigh taylor instabilities or drips off the base of the overriding lithosphere is a typical occurrence the morphology of ssc varies with viscosity and subduction parameters with drips at their most prominent when subduction velocities and wedge viscosities are low our results confirm that high subduction velocities and wedge viscosities promote a dominantly corner flow regime and strong upper plate erosion below the arc region by contrast we find that back arc upper plate erosion by ssc is largely controlled by wedge viscosity occurring when i viscosities are 5 1018 pa s and ii the length of the upper plate available for destabilization exceeds the characteristic wavelength of instabilities thus if hydrous weakening of wedge rheology extends at least 100 150 km from the trench our 2 d models predict an unstable flow regime resulting in temperature fluctuations of 50 100 k which are sufficient to influence melting and the stability of hydrous minerals

the effect of mineralogy and textural characteristics on the strength of crystalline igneous rocks using image based textural quantification abstract the attempts made to understand and quantify the relationship between rock texture and engineering properties have been accompanied by some ambiguities and uncertainties and the majority of the approaches though focusing on particular rock types have not been so successful for even the same rock types from other regions this study therefore was aimed to investigate the relationship between the texture and strength properties of a wide range of crystalline igneous rocks along with more aspects of textural analysis for this purpose fifteen fresh samples of crystalline igneous rocks belonging to the stone quarries of the nw iran were collected and subjected to compressive and tensile strength tests ucs and bts thin sections of the samples were used to prepare the full mosaic image layers in three different optical conditions about 18 000 mineral grains were manually traced on the processed images and the obtained vector data was used for the quantification of various textural parameters including size metrics shape descriptors microfabric indices and mineral content different phase considerations full phasic monophasic multiphasic and phase ratio were applied to derive the textural characteristics of the individual and multiple minerals and their proportional ratio finally simple linear regression analysis was performed to evaluate the relationship between the texture and strength properties the results indicated that the newly developed quartz to feldspar size ratio qfsr indicator was the only textural parameter that could be significantly correlated with both compressive and tensile strength in a wide range of rock types the results also highlighted the fact that the role of quartz and feldspar encountered with different rock failure mechanisms could be different in fact the tensile strength was obviously affected by the size and content of k feldspar while the compressive strength showed a more complicated mechanism and seemed to be controlled by the interference effects of plagioclase and quartz textural properties furthermore the study indicated that the influence of grain size on the rock strength was more important than the grain shape or mineral content

modeling failure of jointed rock slope with two main joint sets using a novel dem bond contact model abstract in order to investigate the failure processes and mechanisms of jointed rock slopes distinct element method dem was applied with a novel bond contact model the bond contact model is validated by a series of numerical direct shear tests on jointed models and the comparisons to laboratory test results dem jointed rock slope models were then generated and two arrangements of joint sets and two types of slope surface were chosen to test the capabilities of the bond contact model the slope models were destabilized progressively by the critical gravity approach and both the failure onset and the slope collapse process were analyzed microscopic information reveals that the initiation of slope failure can be divided into three phases and is induced from stress concentration within the rock bridges occurrence of tension cracks and their propagation and coalescence the slope surface properties reflected by different damping affect both post failure configuration and propagation distance of the rock slope the study demonstrates that the novel bond contact model implemented in the dem code is able to tackle the fundamental problems of jointed rock slope failure and helps to better understand the slope failure mechanisms from a macroscopic and microscopic level

acoustic travel time perturbations due to shallow water internal waves and internal tides in the barents sea polar front theory and experiment during august 1992 a combined acoustics physical oceanography experiment was performed to study both the acoustical properties and the ocean dynamics of the barents sea polar front in the region near bear island oceanographic observations from shipboard hydrography and moored sensors allowed the construction of the internal wave frequency spectrum for the area a rapidly sampled tomographic section from a 224 hz 16 hz bandwidth acoustic source to a 16 element vertical receiving array enabled the monitoring of travel time fluctuations over the internal wave frequency band to describe the measured acoustic fluctuations theoretical expressions have been developed for the travel time variances which are functions of the internal wave oceanographic field the local acoustic propagation characteristics and the acoustical system s properties both ray and mode theory expressions are generated as the experiment was performed in shallow water and both ray and mode arrivals were resolvable comparison of the

high frequency internal waves on a sloping shelf the behavior of an internal wave in a continuously stratified fluid over a sloping bottom is examined by finding approximate analytic solutions for the amplitude of waves in a coastal ocean with constant bottom slope linear bottom friction and barotropic mean flows these solutions are valid for frequencies higher than the frequency of critical reflection from the sloping bottom the solutions show that internal waves propagating toward the shore are refracted so that their crests become parallel to shore as they approach the coast and outward propagating waves are reflected back toward the coast from a caustic inviscid solutions predict that the amplitude of a wave goes to infinity at the coast but these infinite amplitudes are removed by even infinitesimal bottom friction these solutions for individual rays are then integrated for an ensemble of internal wave rays of random orientation that originate at the shelf break and propagate across the shelf it is found that for much of the shelf the shape of the current ellipse caused by these waves is nearly independent of the waves frequency the orientation of the current ellipse relative to isobaths is controlled by the redness of the internal wave spectrum at the shelf break and the strength of mean currents friction is more important on broader shelves and consequently on broad shelves the internal wave climate is likely to be dominated by any internal waves generated on the shelf not waves propagating in from the deep ocean

semidiurnal baroclinic tides on the central oregon inner shelf abstractsemidiurnal velocity and density oscillations are examined over the mid and inner continental shelf near heceta bank on the oregon coast measurements from two long term observation networks with sites on and off the submarine bank reveal that both baroclinic velocities and displacements are dominated by the first mode with larger velocities on the midshelf and northern parts of the bank midshelf sites have current ellipses that are near the theoretical value for single progressive internal tidal waves compared to more linearly polarized currents over the inner shelf baroclinic current variability is not correlated to the spring neap cycle and is uncorrelated between mooring locations an idealized model of two internal waves propagating from different directions reproduces some of the observed variability in semidiurnal ellipse parameters at times the phasing between moorings along a cross shelf transect are consistent with onshelf wave propagation a characteristic also present in the out

a new method to model the non linear crack closure behavior of rocks under uniaxial compression abstract the existence of micro cracks inside rocks influences the rock strength and deformation significantly and controls the presence of crack closure stage under compression however typical discrete element method dem fails to model the gradual crack closure behavior of rock due to insufficient consideration for pre existing micro cracks the stress strain curves simulated in previous dem are usually linear in initial stage until numerous cracks are induced under stress thus a new method implemented in discrete element method is proposed to model the crack closure behavior of rocks under uniaxial compression the pre existing micro cracks inside the rock sample are created and opened by introducing two notional contact surfaces and giving a negative reference gap these opened cracks are gradually closed under stress as the adjacent discs sustain sufficient relative movement re contact thus the gradual crack closure behavior of rock under compression is reproduced the crack distribution respect to the crack orientation is described with the crack tensor analysis to consider the effect of crack density and anisotropy three factors including crack intensity crack orientation and crack gap are discussed with parametric study to study their influence on the crack closure behavior the effects of pre existing cracks on the properties of rocks such as rock strength young s modulus crack closure stress levels are also analyzed and are in well agreement with observations by experiment a little modification is made to previous calibration process to consider the crack closure stage of rock finally two types of rocks including limestone and granite are calibrated and compared to the experiment results the simulated stress strain curves match the curves of experiment especially the initial crack closure segment perfectly which validates the proposed method

micro textural effects on crack initiation and crack propagation of andesitic rocks abstract the onset of dilation and uniaxial compressive strength ucs of rocks are known to be affected by mineralogical petrographic and microstructural features therefore micro textural variations are essential for understanding variation in strength and failure behaviour of rocks in this study volcanic rock samples with different micro textural characteristics from different locations in turkey were evaluated the mechanical tests revealed considerable variations in unconfined uniaxial compressive strength and the crack initiation ci threshold the ucs ranges between 108 289 mpa and ci ranges between 46 130 mpa detailed quantitative petrographic and mineralogical analyses were carried out on each tested specimen to understand these strength variations additionally microstructural studies were carried out on thin sections before and after mechanical perturbation to analyze 1 the effect of the main constituents of the rock specimens and mineral heterogeneity on crack initiation and propagation and 2 the orientation and distribution of micro and macro cracks formed during laterally unconfined compression the synthesis of mechanical mineralogical petrographic and microstructural data suggests that both the crack initiation threshold and peak strength are influenced by the ratio of the area of fine grain groundmass grains smaller than 200 m to amphibole and plagioclase phenocrysts in addition ci and ucs decrease with an increasing normalized area of biotite phenocryst

an analysis of geoid anomalies across the mendocino fracture zone implications for thermal models of the lithosphere geoid height increases in a steplike fashion across oceanic fracture zones owing to the juxtaposition of lithosphere with different ages and density structures at a ridge transform intersection the initial geoid offset depends on the age difference across the fault and is approximately equal to 0 15 m m y of age offset the amplitude and shape of this anomaly will change along the length of a fracture zone depending on how the lithosphere cools if the lithosphere thickens indefinitely boundary layer model then the geoid offset will remain constant although the width of the transition zone from one side of the fracture zone to the other will increase owing to both heat transfer across the fault and an increase in the average depth of compensation if however the lithosphere eventually approaches a constant thickness plate model the total geoid offset will decrease with time an analysis of 18 seasat altimetry profiles across the mendocino fracture zone offset 25 30 m y indicates geoid anomaly amplitudes systematically decrease from more than 4 m across the eastern younger part of the fracture zone to about 1 m across the western older part of the fault although the geoid offsets across the older portion of the mendocino fracture zone may have been underestimated by as much as 20 25 this large decrease in geoid offset strongly suggests that the lithosphere approaches a constant thickness along the older part of the fracture zone a plate thickness of 100 km fits the observed change in geoid offset along the mendocino but with uncertainties in the offset determination the age difference across the older part of the fault and the presence of areas of anomalous depths on both sides of the fracture zone this probably represents a minimum estimate of the total plate thickness

evolution of stress induced borehole breakout in inherently anisotropic rock insights from discrete element modeling the aim of this study is to better understand the mechanisms controlling the initiation propagation and ultimate pattern of borehole breakouts in shale formation when drilled parallel with and perpendicular to beddings a two dimensional discrete element model is constructed to explicitly represent the microstructure of inherently anisotropic rocks by inserting a series of individual smooth joints into an assembly of bonded rigid discs both isotropic and anisotropic hollow square shaped samples are generated to represent the wellbores drilled perpendicular to and parallel with beddings at reduced scale the isotropic model is validated by comparing the stress distribution around borehole wall and along x axis direction with analytical solutions effects of different factors including the particle size distribution borehole diameter far field stress anisotropy and rock anisotropy are systematically evaluated on the stress distribution and borehole breakout propagation simulation results reveal that wider particle size distribution results in the local stress perturbations which cause localization of cracks reduction of borehole diameter significantly alters the crack failure from tensile to shear and raises the critical pressure rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress far field stress anisotropy plays a dominant role in the shape of borehole breakout when drilled perpendicular to beddings while a secondary role when drilled parallel with beddings results from this study can provide fundamental insights on the underlying particle scale mechanisms for previous findings in laboratory and field on borehole stability in anisotropic rock

thermal cracking and the deep hydration of oceanic lithosphere a key to the generation of plate tectonics 1 the development of transient thermal stress in suboceanic mantle is investigated on the basis of two dimensional thermoviscoelastic models incorporating composite rheology appropriate for dry oceanic lithosphere thermal stress is shown to be sufficiently high to deeply fracture the coldest part of lithosphere e g to the depth of at least 30 km and possibly down to 50 km in 100 ma old lithosphere the release of thermal stress by tension cracking is limited to the vicinity of cracks and the cascade crack system is suggested to be required given the finite fracture strength of mantle materials possible physical and chemical consequences of deep thermal cracking are also discussed the rheological evolution of oceanic lithosphere is likely to be affected by thermal cracking and subsequent serpentinization which introduces the localized zones of weakness in the otherwise stiffest part of lithosphere this localized weakening may help to explain why plate tectonic convection not stagnant lid convection operates in earth s mantle

application of avhrr imagery and grace measurements for calculation of actual evapotranspiration over the quaternary aquifer lake chad basin and validation of groundwater models the paper presents an application of noaa avhrr and grace data for studying the regional hydrogeology of the quaternary aquifer in the lake chad basin the actual evapotranspiration et over the phreatic aquifer is computed using the s sebi algorithm and then compared to the et estimates derived from the grace land water solutions both the grace solutions and the s sebi results are similar in description of the actual et over the lake chad basin avhrr data helped to overcome non uniqueness in the modelling of the piezometry anomalies particularly in regional groundwater models the harr piezometric dome appeared to be an area of relatively high effective recharge while the chari baguirmi and the kadzell piezometric depressions and bahr el gahzal were probably areas of excess et over recharge

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

numerical simulation of progressive rock failure and associated seismicity in this paper a numerical approach to modeling progressive failure leading to collapse in rock and associated seismicity is reported in the first part a newly developed numerical code rfpaz rock failure process analysis is introduced the program allows modeling of the observed evolution of damage and associated seismic events due to progressive failure leading to collapse in brittle rock there are three features distinguishing the approach from conventional numerical methods such as fem 1 by introducing heterogeneity of rock parameters into the model rfpaz can simulate non linear behavior in rock using a linear method 2 by introducing elastic modulus reduction for failed elements rfpa2d can process discontinuum mechanics problems by a continuum mechanics method and 3 by recording the event rate of failed elements the seismicities associated with the progressive failure in rock can be simulated in the second part of the paper the applications of rfpa2d in simulating geological process and in solving mining design problems are illustrated numerical simulation of a fault initiation process indicates that some of the important phenomena such as coalescence of microfracture the nucleation and growth of crack clusters fault initiation and development elastic rebound dilatation uplift and seismic behavior etc can be simulated with this numerical code the simulation of progressive failure leading to collapse in underground openings demonstrates the capacity of this code in solving mining design problems