propagation of rifting along the arabia somalia plate boundary into afar it is generally accepted that the aden ridge has propagated westward from 58 e to the western tip of the gulf of aden tadjoura at the edge of afar here we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the republic of djibouti we show that it veers northward forming a narrow zone of dense faulting along the northeastern edge of the afar depression the zone includes two volcanic rifts asal ghoubbet and manda inakir connected to one another and to the submarine part of the ridge by transfer zones both rifts are composite divided into two or three disconnected parallel nw se striking subrifts all of which appear to have propagated northwestward in asal ghoubbet as in manda inakir the subrifts appear to have formed in succession through north directed jumps from subrifts more farther south at present the northernmost subrifts manda and dirko koma of the manda inakir rift form the current tip of the northward propagating arabia somalia plate boundary in afar we account for most observations by a mechanical model similar to that previously inferred for the gulf of aden in which propagation is governed by the intensity and direction of the minimum horizontal principal stress 3 we interpret the northward propagation on land almost orthogonal to that in the gulf to be related to necking of the central afar lithosphere where it is thinnest such necking may be a consequence of differential magmatic thickening greater in the center of the afar depression where the ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression the model explains why the aden ridge foregoes its wsw propagation direction constant from 58 e to asal ghoubbet at a smaller scale individual rifts and subrifts keep opening perpendicular to the arabia somalia or danakil somalia motion vector and propagate northwestward concurrently such lithospheric cracks are forced to jump northward such that the plate boundary remains inside the regional n s necking zone changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns a small angle promoting long en echelon subrifts and a high angle smaller nested subrifts within subrifts the propagation mechanism is thus similar whether in oceanic or continental lithosphere the principal change being the overall propagation path here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the aden ridge finally because the same mechanism has led rifting along the red sea to propagate southward and jump to the western edge of afar the arabia somalia and arabia nubia plate boundaries tips have missed each other and keep overlapping further leading to strain transfer by large scale bookshelf faulting

empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

liquefaction and post liquefaction assessment of lightly cemented sands post liquefaction response of lightly cemented sands during an earthquake may change and become similar to uncemented sands due to bonding breakage in the current study the effect of degree of ce

the volcano seismic crisis in afar ethiopia starting september 2005 we describe the seismo volcanic crisis that occurred in northern afar in late 2005 which involved 15 earthquakes greater than m5 and a small explosive silicic eruption from a vent called da ure at 12 651 n 40 519 n close to dabbahu volcano the purpose is to pull together the different sources of information into a coherent preliminary interpretation of what happened the main geophysical data are the locations of the largest earthquakes and a radar interferogram of unusually high quality that reveals injection of a 60 km long dyke with surface deformation expressed as normal faulting subsidence occurred around the dabbahu volcanic edifice most of the dyke is likely to have been basaltic rather than silicic although the eruption was silicic the volume of the subsidence represents at most 25 of the magma injected into the dyke the silicic eruption was possibly triggered by interaction of basaltic magma with a shallow silicic reservoir at about the same time as the eruption and dyking episode some activity appears also to have taken place at the lava lake at erta ale volcano some 150 km to the north of the eruption site we evaluate the possibility that there may be some link by calculating stresses associated with opening of the fissure and looking at the activity of the lava lake as revealed by the thermal anomaly seen by weather satellites

plate dynamics near divergent boundaries geophysical implications of postrifting crustal deformation in ne iceland the bulk of a tectonic plate is thought to move continously at a rate consistent with the geologic average on the other hand movements are highly episodic at plate boundaries we study the plate dynamics that relate to these two different modes by modelling the displacements observed using the global positioning system in northeast iceland 1987 1990 these observations were made about 10 years after an episodic divergent movement between the north american and eurasian plates 1975 1981 the horizontal displacement field fits well a two dimensional model of postrupture stress relaxation assuming a thin elastic layer overlying a layer of newtonian viscosity this analysis indicates values of about 10 m2 s for the stress diffusivity and 0 3 2 1018 pa s for the newtonian viscosity of the lower layer however no significant correlation exists between the observed and modeled vertical displacements probably because of the relative inaccuracy of the vertical component observations assuming that contemporary plate motion is the sum of many displacements that have diffused from boundaries where episodic displacements occur periodically we simulate the spatial transition from episodic to continuous plate movements the plate boundary zone where movements are episodic or quasi episodic is of the order of 100 km wide depending on the stress diffusivity which may be fairly uniform throughout the world and the frequency of episodic movements

dual simulations of fluid flow and seismic wave propagation in a fractured network effects of pore pressure on seismic signature summary fluid flow in the earth s crust plays an important role in a number of geological processes in relatively tight rock formations such flow is usually controlled by open macrofractures with significant implications for ground water flow and hydrocarbon reservoir management the movement of fluids in the fractured media will result in changes in the pore pressure and consequently will cause changes to the effective stress traction and elastic properties the main purpose of this study is to numerically examine the effect of pore pressure changes on seismic wave propagation i e the effects of pore pressures on amplitude arrival time frequency content this is achieved by using dual simulations of fluid flow and seismic propagation in a common 2 d fracture network note that the dual simulations are performed separately as the coupled simulations of fluid flow and seismic wave propagations in such fracture network is not possible because the timescales of fluid flow and wave propagation are considerably different typically fluid flows in hours whereas wave propagation in seconds the flow simulation updates the pore pressure at consecutive time steps and thus the elastic properties of the rock for the seismic modelling in other words during each time step of the flow simulations we compute the elastic response corresponding to the pore pressure distribution the relationship between pore pressure and fractures is linked via an empirical relationship given by schoenberg and the elastic response of fractures is computed using the equivalent medium theory described by hudson and liu therefore we can evaluate the possibility of inferring the changes of fluid properties directly from seismic data our results indicate that p waves are not as sensitive to pore pressure changes as s and coda or scattered waves the increase in pore pressure causes a shift of the energy towards lower frequencies as shown from the spectrum as a result of scattering attenuation another important observation is that the fluid effects on the wavefield vary significantly with the source receiver direction that is the azimuth relative to the fracture orientation these results have significant implications for the characterization of naturally fractured reservoirs using seismic methods and may impact on experimental design to infer such attributes in a real reservoir situation particularly in acquiring time lapse seismic data

sensitivity of time lapse seismic to reservoir stress path the change in reservoir pore pressure due to the production of hydrocarbons leads to anisotropic changes in the stress field acting on the reservoir reservoir stress path is defined as the ratio of the change in effective horizontal stress to the change in effective vertical stress from the initial reservoir conditions and strongly influences the depletion induced compaction behaviour of the reservoir seismic velocities in sandstones vary with stress due to the presence of stress sensitive regions within the rock such as grain boundaries microcracks fractures etc since the response of any microcracks and grain boundaries to a change in stress depends on their orientation relative to the principal stress axes elastic wave velocities are sensitive to reservoir stress path the vertical p and s wave velocities the small offset p and sv wave normal moveout nmo velocities and the p wave amplitude versus offset avo are sensitive to different combinations of vertical and horizontal stress the relationships between these quantities and the change in stress can be calibrated using a repeat seismic sonic log checkshot or vertical seismic profile vsp at the location of a well at which the change in reservoir pressure has been measured alternatively the variation of velocity with azimuth and distance from the borehole obtained by dipole radial profiling can be used having calibrated these relationships the theory allows the reservoir stress path to be monitored using time lapse seismic by combining changes in the vertical p wave impedance changes in the p wave nmo and avo behaviour and changes in the s wave impedance

relating shear sonic anisotropy directions to stress in deviated wells we develop a model that relates the polarization direction of a dipole sonic fast shear wave propagating along the borehole hereafter called fsa to the relative deviatoric stress tensor for arbitrary well orientations we first define stress quantities or subsidiary principal stress relevant for sonic dipole shear characterized sufficiently far away into the formation to be unaffected by borehole stress concentration next we show analytically for wells oriented within principal stress planes and numerically for wells outside principal stress planes that the stress induced fsa coincides with the maximum normal stress direction orthogonal to the borehole maximum subsidiary principal stress using a nonlinear elastic model for isotropic unstressed background media and plane wave solutions this model is independent of stress sensitivity parameters this result is a consequence of first the direct relationship between the stressellipsoid factor and the shear stiffness difference ratio r 2

understanding of elastic anisotropy of shale under triaxial loading porosity deformation approach abstracttheoretical models for the description of stress dependencies for anisotropic rocks are important for building velocity models and the interpretation of seismic data with respect to stress lithology and pore content especially important is the accurate description of the change of anisotropy due to applied anisotropic stress by means of a quantitative model to aid the interpretation the porosity deformation approach pda originally introduced as the piezosensitivity theory describes stress induced changes of elastic properties without any knowledge about the geometry and orientation of the microcracks we have developed a formulation of the theory for a transverse isotropic rock which was loaded parallel to the bedding plane and we found that it can be described theoretically as an orthorhombic rock we also found out how properties that described the velocity change due to hydrostatic loading can be used to predict elastic anisotropy under additional axial loading the number of free para

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

transient deformation in the asal ghoubbet rift djibouti since the 1978 diking event is deformation controlled by magma supply rates the asal ghoubbet rift ag rift in djibouti lies in the subaerial continuation of the aden ridge system thereby constituting a unique location to study rifting processes and mechanisms involved in continental breakup and oceanic spreading continually upgraded and expanded geodetic technology has been used to record the 1978 asal rifting event and postdiking deformation in light of recent results obtained for the manda hararo dabbahu rifting event 2005 2010 we propose that the horizontal and vertical geodetic data can be modeled with a double source involving a dike like inflation component aligned along the rift axis and a spherical pressure source located at midsegment below the fieale caldera by revisiting the codiking data we propose that the reservoir below fieale could have fed at least partially the 1978 injection and the contemporaneous ardoukoba eruption and potentially induced local subsidence due to magma draining out of the central reservoir as an alternative to previously proposed viscoelastic relaxation models we reinterpret postdiking observations using a purely elastic rheology we determine the relative contribution of a midsegment reservoir inflation and a dike like opening component together with their respective time evolutions our results suggest that interactions between steadily accumulating tectonic strain and temporal variations in melt supply to the shallow magma plumbing system below the ag rift may entirely explain the geodetic observations and that viscoelastic deformation processes played a minor role in the 30 years following the 1978 rifting event

predicting stress induced velocity anisotropy in rocks a simple transformation using measured isotropic vp and vs versus hydrostatic pressure is presented for predicting stress induced seismic velocity anisotropy in rocks the compliant crack like portions of the pore space are characterized by generalized compressional and shear compliances that are estimated from the isotropic vp and vs the physical assumption that the compliant porosity is crack like means that the pressure dependence of the generalized compliances is governed primarily by normal tractions resolved across cracks and defects this allows the measured pressure dependence to be mapped from the hydrostatic stress state to any applied nonhydrostatic stress predicted p and s wave velocities agree reasonably well with uniaxial stress data for barre granite and massillon sandstone while it is mechanically similar to methods based on idealized ellipsoidal cracks the approach is relatively independent of any assumed crack geometry and is not limited to small crack densities

elastic piezosensitivity of porous and fractured rocks understanding the stress dependencies of seismic velocities is important for interpreting a variety of seismic data ranging from amplitude versus offset avo and velocity analysis to overpressure prediction and 4d seismic monitoring of reservoirs sometimes rather complex forms of these dependencies based on specific models of porous space geometry are used for example spherical contacts models duffy and mindlin 1957 merkel et al 2001 duffy merkel 2001 and crack contacts models gangi and carlson 1996 carcione and tinivella 2001 have been used in different studies gangi96 however usually the pore pressure velocity dependence along with the velocity dependence on the confining pressure are phenomenologically described by the following simple relationship zimmerman et al 1986 eberhart phillips et al 1989 freund 1992 jones 1992 prasad and manghnani 1997 khaksar et al 1999 carcione and tinivella 2001 kirstetter and macbeth 2001

an analytic model for the stress induced anisotropy of dry rocks one of the main causes of azimuthal anisotropy in sedimentary rocks is anisotropy of tectonic stresses in the earth s crust we have developed an analytic model for seismic anisotropy caused by the application of a small anisotropic stress we first considered an isotropic linearly elastic medium porous or nonporous permeated by a distribution of discontinuities with random isotropic orientation such as randomly oriented compliant grain contacts or cracks the geometry of individual discontinuities is not specified instead their behavior is defined by a ratio b of the normal to tangential excess compliances when this isotropic rock is subjected to a small compressive stress isotropic or anisotropic the number of cracks along a particular plane is reduced in proportion to the normal stress traction acting on that plane this effect is modeled using the sayers kachanov noninteractive approximation the model predicts that such anisotropic crack closure yields elliptical anisotropy regardless of

modeling elastic anisotropy resulting from the application of triaxial stress abstractelastic wave velocities in rocks vary with stress due to the presence of discontinuities and microcracks within the rock we analytically derived a model to account for stress dependency of seismic velocities when a rock is subjected to triaxial stresses we first considered a linearly isotropic elastic medium permeated by a distribution of cracks with random orientations the geometry of cracks is not specified instead their behavior is defined by a ratio b of their normal to tangential excess compliances when this isotropic rock is subjected to triaxial stresses cracks tend to close depending on their orientation with respect to the applied stresses for small stresses the model predicts ellipsoidal anisotropy and expresses the ratios of thomsen s parameters in the three orthogonal planes of symmetry as a function of the compliance and poisson s ratios it also establishes relationships between anisotropy parameters and stress ratios deriving the model for larger stresses shows that su

geodetic measurements of surface deformations during the present rifting episode in ne iceland iceland is part of the accreting plate boundary between the american and european plates since 1965 a number of repeated triangulation trilateration surveys spanning the neovolcanic zone in ne iceland were made in order to detect crustal movements all the measurements for a single observation epoch are separately adjusted by the method of least squares statistical tests are carried out to check the observations and adjustment models for outliers and model errors to detect significant movements in repeatedly measured geodetic networks with multivariate design a testing rule is described after unexpected horizontal contractions of the surface of approximately 0 5 m from 1965 to 1971 enormous deformations have been found since the beginning of a major rifting episode in december 1975 during the period from 1971 to 1980 a horizontal expansion of about 7 5 m and vertical displacements of 3 m were observed across the approximately 3 km wide active rifting zone of the krafla fissure swarm in adjacent zones compressional strains of up to 2 10 4 occurred rapidly decreasing to a value of 1 10 5 at a distance of 40 km from the rifting axis after 1980 the pattern of ground deformation has altered the east west shifts have decreased and are superposed by a radial component of motion around the krafla caldera west of the krafla fissure swarm a shear zone is found

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

first epoch geodetic gps measurements across the afar plate boundary zone in november 1991 a geodetic network was installed across the actively rifting plate boundary zone between africa and arabia using the microwave signals transmitted by the satellites of the global positioning system gps the relative positions of 37 stations are estimated with an average wrms scatter of 12 or 14 mm for lines shorter or longer than 100 km respectively assuming that the measurements can be repeated with the same precision in 1999 we expect to obtain horizontal uncertainties of 2 1 or 2 4 mm yr in the rate of change in the short and long lines respectively this level of precision will tightly constrain geological models for the spatial distribution of deformation around the rift in djibouti and for the far field plate motions surrounding the afar depression

static and dynamic pressure sensitivity anisotropy of a calcareous shale optimizing the productivity of nonconventional low permeability shale reservoirs requires detailed knowledge of the mechanical properties of such materials these rocks elastic anisotropy is acknowledged but usually ignored due to difficulties in obtaining such information here we study in detail the dynamic and static elastic properties of a suite of calcareous mudstones from the nonconventional duvernay reservoir of alberta canada the complete set of transversely isotropic elastic constants is obtained from strategically oriented ultrasonic transducers to confining pressures of 90 mpa wave speed anisotropies of up to 35 are observed at even the highest confining pressures furthermore the stress sensitivity of the wave speeds and hence moduli is itself highly dependent on direction with speeds taken perpendicular to the bedding plane being highly nonlinearly dependent on pressure whereas those along the bedding plane show unexpectedly nearly no pressure dependence these observations are in qualitative agreement with the preferentially oriented porosity and minerals seen in scanning electron microscope images these results may be significant to the interpretation of sonic logs and azimuthal amplitude versus offset for principal stress directions for the concentration of stress within such formations and for estimation of static engineering moduli from sonic log wave speeds

stress induced seismic azimuthal anisotropy sand shale content and depth trends offshore north west australia abstractseismic azimuthal anisotropy is apparent when p wave velocities vary with source receiver azimuth and downward propagating s waves split into two quasi s waves polarized in orthogonal directions not accounting for these effects can degrade seismic image quality and result in erroneous amplitude analysis and geologic interpretations there are currently no physical models available to describe how azimuthal anisotropy induced by differential horizontal stress varies with sand shale lithology and depth we develop a model that does so in unconsolidated sand shale sequences offshore north west australia our method naturally introduces two new concepts critical anisotropy and anisotropic depth limit critical anisotropy is the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth where the confining pressure and sediment compaction are minimal the anisotropic depth limit is the maximum depth where the stress induced azimuthal anisotropy is ex

frequency dependence of anisotropy in fluid saturated rocks part ii stress induced anisotropy case a major cause of attenuation in fluid saturated media is the local fluid flow or squirt flow induced by a passing wave between pores of different shapes and sizes several squirt flow models have been derived for isotropic media for anisotropic media however most of the existing squirt flow models only provide the low and high frequency limits of the saturated elastic properties we develop a new squirt flow model to account for the frequency dependence of elastic properties and thus gain some insight into velocity dispersion and attenuation in anisotropic media in a companion paper we focused on media containing aligned compliant pores embedded in an isotropic background matrix in this paper we investigate the case for which anisotropy results from the presence of cracks with an ellipsoidal distribution of orientations due to the application of anisotropic stress the low and high frequency limits of the predicted fluid saturated elastic properties are respectively consistent with gassmann theory and mukerji mavko squirt flow model in the most important case of liquid saturation analytical expressions are derived for elastic properties and thomsen anisotropy parameters the main observations drawn from this model are as follows crack closure perpendicular to the applied stress leads to an increase in seismic velocities as a function of stress in the direction of applied stress and a decrease in squirt flow induced dispersion and attenuation in this direction the anisotropy of squirt flow dispersion engenders a decrease in the degree of anisotropy with frequency the stress induced anisotropy remains elliptical even in saturated media for all frequency ranges

deformation across the asal ghoubbet rift djibouti uplift and crustal extension 1979 1986 geodetic networks of different sizes have been established since 1973 in the asal ghoubbet rift in south western afar in order to study crustal movements across an extensive plate boundary partial or entire remeasurements of these networks during the 1979 1986 interval provide detailed data about strain accumulation and vertical movements after a seismo volcanic rifting event that occured in november 1978 along this area similar to an oceanic accreting plate boundary the results of the geodetic monitoring indicate a continuous opening of the rift zone of about 6 cm yr since 1979 the deformation is principally distributed in the inner floor of the rift with a uniaxial extensional n40 e strain rate of about 15 strain yr the results of the 1979 and the 1984 85 levellings define the development of a large uplift across the rift zone amounting 15 cm during this 6 year interval this uplift is accompanied with a relative collapse of the inner floor zone of about 4 cm which is associated with creep on the active faults bounding the rift two stages of the rifting process are successively observed in djibouti an abrupt rifting event in 1978 and then a continuous accumulation stage which is controled by the plate motion and partially by magmatic inflation and active creep faulting

compressibility of porous rocks four compressibilities are defined for porous rock relating changes in the bulk and pore volumes to changes in the pore and confining pressures using a micromechanical theory based on classical linear elasticity three relations are found between these compressibilities two of these relations are verified experimentally for berea and bandera sandstone bounds are derived for these compressibilities involving only the porosity and the elastic moduli of the rock forming minerals for the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures this dependence is verified for berea bandera and boise sandstone while the strains cannot be expressed as functions of an effective stress regardless of how it is defined the elastic volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure

numerical modelling of quaternary deformation and post rifting displacement in the asal ghoubbet rift djibouti africa over the last three decades a host of information on rifting process relating to the geological and thermal structure long time scale deformation quaternary and holocene and rifting cycle displacement across the asal ghoubbet rift has been made available these data are interpreted with a two dimensional thermo mechanical model that incorporates rheological layering of the lithosphere dyke inflation and faulting active fault locations and geometry are mainly controlled by both thermal structure and magma intrusion into the crust the distributed slip throughout the inner rift is related to the closeness of magma chamber leading to additional stress into the upper thinned crust assuming a constant arabia somalia motion of 11 mm year the variation of subsidence rate between the last 100 and 9 ka is associated with a decrease of the average injection rate from 10 to 5 mm year these values about equal to the regional opening rate suggest that both volcanism and tectonic play an equivalent role in the rifting process our modelled sequence of events gives one possible explanation for both vertical and horizontal displacements observed since the 1978 seismovolcanic crisis although part of the post rifting deformation could be due to viscous relaxation the high opening rate in the first years after the event and the abrupt velocity change in 1984 1986 argue for a large dyke inflation of 12 cm year ending in 1985 the asymmetric and constant pattern of the gps velocity since 1991 suggests that present post rifting deformation is mainly controlled by fault creep and regional stretching this study demonstrates the internal consistency of the data set highlights the role of magmatism in the mechanics of crustal stretching and reveals a complex post rifting process including magma injection fault creep and regional stretching d 2005 elsevier b v all rights reserved

slip accumulation and lateral propagation of active normal faults in afar we investigate fault growth in afar where normal fault systems are known to be currently growing fast and most are propagating to the northwest using digital elevation models we have examined the cumulative slip distribution along 255 faults with lengths ranging from 0 3 to 60 km faults exhibiting the elliptical or bell shaped slip profiles predicted by simple linear elastic fracture mechanics or elastic plastic theories are rare most slip profiles are roughly linear for more than half of their length with overall slopes always 0 035 for the dominant population of nw striking faults and fault systems longer than 2 km the slip profiles are asymmetric with slip being maximum near the eastern ends of the profiles where it drops abruptly to zero whereas slip decreases roughly linearly and tapers in the direction of overall aden rift propagation at a more detailed level most faults appear to be composed of distinct shorter subfaults or segments whose slip profiles while different from one to the next combine to produce the roughly linear overall slip decrease along the entire fault on a larger scale faults cluster into kinematically coupled systems along which the slip on any scale individual fault or fault system complements that of its neighbors so that the total slip of the whole system is roughly linearly related to its length with an average slope again 0 035 we discuss the origin of these quasilinear asymmetric profiles in terms of initiation points where slip starts and barriers where fault propagation is arrested in the absence of a barrier slip apparently extends with a roughly linear profile tapered in the direction of fault propagation

fluid controlled faulting process in the asal rift djibouti from 8 yr of radar interferometry observations the deformation in the asal rift djibouti is characterized by magmatic inflation diking distributed extension fissure opening and normal faulting an 8 yr time line of surface displacement maps covering the rift constructed using radar interferometry data acquired by the canadian satellite radarsat between 1997 and 2005 reveals the aseismic behavior of faults and its relation with bursts of microseismicity the observed ground movements show the asymmetric subsidence of the inner floor of the rift with respect to the bordering shoulders accommodated by slip on three of the main active faults fault slip occurs both as steady creep and during sudden slip events accompanied by an increase in the seismicity rate around the slipping fault and the fieale volcanic center slip distribution along fault strike shows triangular sections a pattern not explained by simple elastic dislocation theory these observations suggest that the asal rift faults are in a critical failure state and respond instantly to small pressure changes in fluid filled fractures connected to the faults reducing the effective normal stress on their locked section at depth

a multidisciplinary study of the final episode of the manda hararo dyke sequence ethiopia and implications for trends in volcanism during the rifting cycle the sequence of dyke intrusions between 2005 and 2010 in the manda hararo rift segment ethiopia provided an opportunity to test conceptual models of continental rifting based on trends up to dyke 13 in the sequence it was anticipated that should magma supply con tinue dykes would shorten in length and eruptions would increase in size and decrease in distance from the segment centre as extensional stress was progressively released in this paper we revisit these predictions by presenting a comprehensive overview of the may 2010 dyke and fissure erup tion the 14th and last in the sequence from insar seismicity satellite thermal data ultraviolet so2 retrievals and multiple lidar surveys we find the dyke is longer than other eruptive dykes in the sequence propagating in two directions from the segment centre but otherwise fairly typical in terms of opening propagation speed and geodetic and seismic moment however though the eruption is located closer to the segment centre it is much smaller than pre vious events we interpret this as indicating that either the manda hararo rifting event was magma limited or that extensional stress varies north and south of the segment centre

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

approximate fluid substitution for vertical velocities in weakly anisotropic vti rocks despite the prevalence of elastic anisotropy in rocks most models used routinely for wave propagation imaging and rock physics assume an isotropic earth the main reason for using isotropic approximations is that we seldom measure enough parameters to characterize the stiffness tensor of a rock completely fluid substitution is an important example because of an incomplete knowledge of the stiffness tensor often we choose isotropic equations over their anisotropic form assuming weak anisotropy we derive an approximate form of the anisotropic fluid substitution equation for seismic waves propagating along the symmetry axis of a transversely isotropic medium the approximation takes the form of the usual isotropic calculation with a simple first order correction proportional to the thomsen anisotropic parameter thus requiring only three elastic constants because can be either greater than or less than zero in a vti medium we can explain why isotropic fluid substitution sometimes overpredicts

tectonics of the westernmost gulf of aden and the gulf of tadjoura from submersible observations the gulf of tadjoura separating the danakil and somalia plates1 represents the western extension of the sheba ridge between arabia and somalia in the gulf of aden fig 1 it has been assumed that a simple plate tectonic scheme could be applied to the area with ne sw transform faults offsetting roughly east west zones of accretion represented by the major bathymetrie troughs1 8 fig 1c more recently this boundary has been interpreted as a westward propagating rift with its present day tip on land in the region of the asal lake9 10 here we present data from dives with the submersible cyana which show that there have been two episodes of extensional faulting an older inactive phase parallel to the general ene wsw direction of the gulf and a younger fault set along discontinuous zones striking 140 there is no surface evidence for any transform fault zone in the area covered and the incipient spreading segments whose geometry implies extension on 050 do not define a plate boundary the faulting observed is at odds with that predicted by orthodox plate tectonics

the tectonic and geomagnetic significance of paleomagnetic observations from volcanic rocks from central afar africa deformation throughout afar over the past 2 myr has been characterized by widespread and intense crustal fragmentation that results from inhomogeneous extension across the region in eastern afar this situation has evolved to localized extension associated with the westward propagation of the gulf of aden gulf of tadjurah seafloor spreading system into the asal ghoubbet rift during the gradual process of rift propagation and localization crustal blocks in eastern afar sustained clockwise rotations of 11 to better understand the processes of rift propagation and localization and how they affect the rest of afar we have collected and analyzed over 400 oriented paleomagnetic samples from 67 lava flows from central and southern afar unlike eastern afar the mean paleomagnetic direction from central afar indicates that vertical axis rotations are statistically insignificant 3 6 4 4 though small clockwise rotations 8 are permitted thus propagation and localization in central afar have not had the same influence in causing crustal block rotations or perhaps more likely have not reached the same stage of evolution as seen in eastern afar in addition several of the lava flows record intriguing geomagnetic field behavior associated with polarity transitions excursions or large secular variation events interestingly the transitional or anomalous virtual geomagnetic poles vgps tend to cluster in two nearly antipodal regions one in the northern pacific ocean and the other in the southwest indian ocean one lava flow has recorded both of the antipodal transitional components with the two components residing in magnetic minerals with unblocking temperatures above and below 500 c respectively reheating and partial remagnetization by the overlying flow cannot explain either of the transitional directions because both differ significantly from that of the reversely magnetized overlying flow the high temperature component gives a vgp in the northern pacific whereas the lower temperature component gives a nearly antipodal vgp south of cape town south africa hence the configuration of the geomagnetic field appears to have jumped nearly instantaneously from a northern hemisphere transitional state to a southern hemisphere one during this normal to reverse polarity transition

sensitivity of elastic wave velocities to stress changes in sandstones the drilling of wells and production of hydrocarbons lead to changes in the stresses acting on the reservoir and surrounding rocks and these changes in stress may lead to problems such as wellbore instability sanding loss of permeability reservoir compaction casing deformation surface subsidence etc elastic wave velocities in sandstones are sensitive to changes in stress due to the presence of grain boundaries and microcracks within the rock this paper shows how changes in elastic wave velocities can be used to monitor the changes in stress that occur and a method for predicting the velocity changes due to a given change in stress is illustrated using the example of a borehole drilled into a sandstone reservoir

variation of liquefaction strength induced by monotonic and cyclic loading histories abstractonce loose sand has liquefied it becomes denser along with water drainage after liquefaction therefore liquefied sand is regarded as more robust against liquefaction such sand after liq

current vertical movement demonstrated by comparative levelling in brittany northwestern france abstract the current vertical movement of the earth s crust is demonstrated through comparative levelling the two methods presented are complementary first we constructed a profile along traverses in order to highlight localized features such as recurrent faulting or block tilting second we produced a map of relative current vertical movement rates and their error coefficients using a geostatistical kriging method these methods were applied to brittany northwestern france and revealed that 1 the ancient nw se rance and morlaix faults are presently reactivated and 2 the west of the region is tilted with uplift which is interpreted as a lithospheric deformation

contemporary holocene and quaternary deformation of the asal rift djibouti implications for the mechanics of slow spreading ridges because the frequency and character of rifting events along mid ocean ridges are largely unknown how the repetition of such events gives rise to rift structures is unexplored the asal rift in the afar depression of djibouti africa provides the world s best subaerial analogue for young slow spreading mid ocean ridges seismic geodetic and field observations of a seismovolcanic event in 1978 at asal yield estimates of the fault and dike locations geometry displacement and volume of basalt extruded in a rifting event a 6 9 kyr old lake shore highstand at asal has been warped downward by 70 m providing a holocene measure of the vertical deformation across the rift the rift topography furnishes an older datum which we infer to be 34 6 kyr old using the holocene deformation rate we find that faults throughout the rift valley are active holocene slip rates diminish beyond 4 km from the rift axis late quaternary rates decrease beyond 6 7 km the holocene slip rates are used to estimate repeat times by taking the displacement on the faults which slipped in 1978 as characteristic we find tectonic events on individual faults recur every 200 300 years half the rift faults slipped together in the 1978 event if this is typical then groups of faults are activated every 100 150 years we suggest that half the events take place in the rift axis accompanied by volcanic extrusion the remainder occur peripheral to the neovolcanic zone and involve fault slip only both events having a repeat time of 200 300 years given the 10 km width of the rift and its 16 mm yr 1 spreading rate the mean age of the material in the rift should be 350 kyr an order of magnitude older than the inferred age of the formation of the rift topography the subsidence rate of the rift axis during the past 35 kyr is 8 9 mm yr 1 with the rate of infilling by volcanic extrusion 1 mm yr 1 the resulting net subsidence rate about equal to the half spreading rate of the rift could not be sustained for 300 kyr without significant infilling by lavas thus both observations suggest that the long term vertical deformation in the rift has not been steady state instead we suggest that there is a rifting filling cycle at asal with the most recent filling episode ending 35 kyr

palaeoenvironments and palaeohydrology of a tropical closed lake lake asal djibouti since 10 000 yr b p in eastern africa republic of djibouti the hypersaline lake asal occupies an exceptional geological topographical and hydrological situation at 155 m below sea level and at 12 km from the sea it lies in the youngest and the deepest basin in a chain of closed depressions of central afar decreasing in elevation from west to east and represents the hydrogeological low of the whole system presently infiltration of marine water represents the major water input to the lake due to high evaporation conditions halite precipitates halocene water level fluctuations 315 m sedimentary conditions from fresh oligosaline waters to heavy brines and palaeohydrology are reconstructed field observations are supported by 85 14c dates on inorganic carbonates mollusc shells stromatolites and hydrothermal travertines stable isotope analyses on carbonates and sulphates mineralogy geochemistry and studies of biological remains discussion of the water balance for the major stages is based on reasonable assumptions about holocene conditions of precipitation and evaporation after an arid terminal pleistocene phase marked by coarse deposits due to exceptional floods the holocene cycle begins with complex strontium rich carbonate phases aragonite high and low mg calcites and dolomite these carbonates could reflect the first underground marine supply and waters concentrated through evaporation a high lake level ca 160 m a s l from ca 8600 to 6000 yr b p corresponds to the deposition of high mg calcite precipitated in a deep fresh water lake lake asal was draining the awash river system ethiopian plateau and escarpment through groundwater circulations the terminal basin of the system was probably the present day marine gulf of ghoubbat al kharab hydrologically separated from the open ocean by a sill at 4 m below sea level possibly due to fracture opening a dramatic regression in the lake level from 160 to 150 m occurred between ca 6000 and 5000 yr b p the inputs of continental water were consequently overwhelmed by marine infiltrations the carbonate phase exhibits clear resemblance with that of the transgressive episode from 5000 yr b p lake asal experienced minor water level fluctuations aragonite rich carbonates precipitated followed by calcium sulphates and halite stable isotopes of gypsum reflect its marine origin and various diagenetic processes the holocene evolution of asal which clearly differs from that of the neighbouring closed basins is due to both its specific geological and hydrological setting and to the regional climatic changes

1985 1999 gravity field variations across the asal rift insights on vertical movements and mass transfer repeated gravity measurements across the asal rift zone reveal that the central part of the northern rift might be subject to mass input in addition to vertical movements precise relative gravity measurements were made in 1985 1988 and 1999 temporal variations in the gravity field over the 11 and 14 yr periods reveal a probable relative gravity increase on the order of 30 gal 300 nm s2 in the central zone of the rift active zone since the ardoukoba rifting episode with respect to both sides of the rift the gravity data are compared with vertical deformation data collected by the french national geographical institute ign in 1984 and 2000 along 50 km of the across axis leveling profile the comparison shows that vertical movements alone cannot explain the observed gravity variations and that unlike in many other volcano studies our gravity and elevation changes cannot be modeled by a single process such as subsidence inflation or localized mass injection

effects of fracture intersections on seismic dispersion theoretical predictions versus numerical simulations abstract the detection and characterisation of domains of intersecting fractures are important goals in several disciplines of current interest including exploration and production of unconventional reservoirs nuclear waste storage co2 sequestration and groundwater hydrology among others the objective of this study is to propose a theoretical framework for quantifying the effects of fracture intersections on the frequency dependent elastic properties of fluid saturated porous and fractured rocks three characteristic frequency regimes for fluid pressure communication are identified in the low frequency limit fractures are in full pressure communication with the embedding porous matrix and with other fractures conversely in the high frequency limit fractures are hydraulically isolated from the matrix and from other fractures at intermediate frequencies fractures are hydraulically isolated from the matrix porosity but can be in hydraulic communication with each other depending on whether fracture sets are intersecting for each frequency regime the effective stiffness coefficients are derived using the linear slip theory and anisotropic gassmann equations explicit mathematical expressions for the two characteristic frequencies that separate the three frequency regimes are also determined theoretical predictions are then applied to two synthetic 2d samples each containing two orthogonal fracture sets one with and another without intersections the resulting stiffness coefficients thomsen style anisotropy parameters and the transition frequencies show good agreement with corresponding numerical simulations the theoretical results are applicable not only to 2d but also to 3d fracture systems and are amenable to being employed in inversion schemes designed to characterise fracture systems

correction to transient rift opening in response to multiple dike injections in the manda hararo rift afar ethiopia imaged by time dependent elastic inversion of interferometric synthetic aperture radar data 1 interferometric synthetic aperture radar insar data spanning the time intervals separating thirteen dike intrusions in the manda hararo dabbahu rift afar ethiopia from 2005 to 2009 show that transient deformation occurs in the inter diking period this deformation can be explained by the presence of seven inflating or deflating pressure sources by combining the data acquired on four different insar tracks through time dependent elastic models we are able to track these deformation modes with a time resolution smaller than 1 month sustained deflation of a deep magma reservoir at dabbahu in the 6 months following the main rifting event of 2005 and slow decelerating post eruptive re inflation of two shallow magma reservoirs below dabbahu and gabho volcanoes are monitored a deflation signal of deep origin on the neighboring rift system is also detected possibly caused by outflow of material from a preexisting reservoir into the deep plate boundary in contrast rapidly evolving deformation is observed at the center of the manda hararo rift segment transient deformation events are monitored in the weeks months following the diking events with pulses of localized rift opening after the dike intrusions followed by an exponential like decay of opening rate this signal may be associated with the replenishment of the central magma reservoir involved in feeding the 2005 2009 dikes alternatively the predominantly horizontal mode of deformation suggests an interaction between the response of the lithosphere to tectonic strain accumulation and the process of hydraulic connectivity within the central magma plumbing system

twenty five years of geodetic measurements along the tadjoura asal rift system djibouti east africa since most of tadjoura asal rift system sits on dry land in the afar depression near the triple junction between the arabia somalia and nubia plates it is an ideal natural laboratory for studying rifting processes we analyze these processes in light of a time series of geodetic measurements from 1978 through a network of about 30 gps sites covers the republic of djibouti additional points were also measured in yemen and ethiopia stations lying in the danakil block have almost the same velocity as arabian plate indicating that opening near the southern tip of the red sea is almost totally accommodated in the afar depression inside djibouti the asal ghoubbet rift system accommodates 16 1 mm yr of opening perpendicular to the rift axis and exhibits a pronounced asymmetry with essentially null deformation on its southwestern side and significant deformation on its northeastern side this rate slightly higher than the large scale arabia somalia motion 13 1 mm yr suggests transient variations associated with relaxation processes following the asal ghoubbet seismovolcanic sequence of 1978 inside the rift the deformation pattern exhibits a clear two dimensional pattern along the rift axis the rate decreases to the northwest suggesting propagation in the same direction perpendicular to the rift axis the focus of the opening is clearly shifted to the northeast relative to the topographic rift axis in the petit rift a rift in rift structure containing most of the active faults and the seismicity vertical motions measured by differential leveling show the same asymmetric pattern with a bulge of the northeastern shoulder although the inner floor of the rift is subsiding with respect to the shoulders all sites within the rift system show uplift at rates varying from 0 to 10 mm yr with respect to a far field reference outside the rift

strain accumulation across the asal ghoubbet rift djibouti east africa the asal ghoubbet rift in the republic of djibouti is a subaerial continuation of the gulf of aden ridge system many geological and tectonic arguments indicate that it represents one of the most typical spreading segments of the world rift system geodetic measurements regularly conducted from 1979 to 1983 across this area show an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm yr perpendicular to the normal faults that bound the rift periodic leveling surveys across major faults in the rift show vertical creep movements of about 5 to 10 mm yr although the observed deformations could be contamined by postseismic relaxation after a rifting episode in november 1978 the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley the deformation is uniformly distributed in time and in space inside the inner floor of the rift the necking of the lithosphere in the central part of the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process

post rifting stress relaxation at the divergent plate boundary in northeast iceland interaction of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the earth s surface in a diffusion like manner following tectonism at plate boundaries this process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary observations of such strain transients have been rare and have hitherto been confined to strike slip and underthrusting plate boundaries1 here we report the observation of a strain transient at the divergent spreading plate boundary in iceland a global positioning system survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion pulse shape and an amplitude three times greater than the 5 7 cm that would be expected from the average spreading rate2 a simple one dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1 1 a 0 3 m2s 1 combined with struc tural information from magnetotelluric measurements this implies a viscosity of 0 3 2 1019 pa s a value comparable to that derived for iceland from post glacial rebound23 but low compared with estimates for mantle viscosity obtained elsewhere3

nonlinear rock physics model for estimation of 3d subsurface stress in anisotropic formations theory and laboratory verification we develop a rock physics model based on nonlinear elasticity that describes the dependence of the effective stiffness tensor as a function of a 3d stress field in intrinsically anisotropic formations this model predicts the seismic velocity of both p and s waves in any direction for an arbitrary 3d stress state therefore the model overcomes the limitations of existing empirical velocity stress models that link p wave velocity in isotropic rocks to uniaxial or hydrostatic stress to validate this model we analyze ultrasonic velocity measurements on stressed anisotropic samples of shale and sandstone with only three nonlinear constants we are able to predict the stress dependence of all five elastic medium parameters comprising the transversely isotropic stiffness tensor we also show that the horizontal stress affects vertical s wave velocity with the same order of magnitude as vertical stress does we develop a weakanisotropy approximation that directly links commonly measured anisotropic thomsen parameters to the principal stresses each thomsen parameter is simply a sum of corresponding background intrinsic anisotropy and stressinduced contribution the stress induced part is controlled by the difference between horizontal and vertical stresses and coefficients depending on nonlinear constants thus isotropic rock stays isotropic under varying but hydrostatic load whereas transversely isotropic rock retains the same values of dimensionless thomsen parameters only unequal horizontal and vertical stresses alter anisotropy since thomsen parameters conveniently describe seismic signatures such as normal moveout velocities and amplitudevariation with offset gradients this approximation is suitable for designing new methods for the estimation of 3d subsurface stress from multicomponent seismic data

positioning results with doris on spot2 after first year of mission a new space system named doris has been developed by the centre national d etudes spatiales in france with the collaboration of the institut geographique national for precise orbit determination and positioning it is a doppler based system with a worldwide ground network of transmitting beacons and an onboard receiver the spot2 satellite launched in january 1990 carries doris as an experimental instrument in this study we present positioning results obtained from the analysis of the first 10 months of doris data through a semi dynamical approach i e the satellite orbit is improved simultaneously with the beacon positions external calibration with satellite laser ranging slr and very long baseline interferometry vlbi positions indicates that doris accuracy from a few months of data is presently 15 20 cm rms in absolute and relative baseline length 1000 km positioning for very short baselines 1000 km positioning results is in the range 10 20 cm rms for the shortest baselines 100 km the month to month repeatability is better than 5 cm in cartesian coordinates this repeatability is only 2 cm in the north and east components whereas it is 12 cm in the vertical component suggesting that tropospheric errors are important it should be possible to improve these results in the future by reducing the residual orbit error especially the atmospheric drag perturbation which was particularly troublesome in 1990 a year having very large solar activity and by improving modeling of the tropospheric correction

squirt flow in fully saturated rocks we estimate velocity frequency dispersion and attenuation in fully saturated rocks by employing the squirt flow mechanism of solid fluid interaction in this model pore fluid is squeezed from thin soft cracks into the surrounding large pores information about the compliance of these soft cracks at low confining pressures is extracted from high pressure velocity data the frequency dependence of squirt induced pressure in the soft cracks is linked with the porosity and permeability of the soft pore space and the characteristic squirt flow length these unknown parameters are combined into one expression that is assumed to be a fundamental rock property that does not depend on frequency the appropriate value of this expression for a given rock can be found by matching our theoretical predictions with the experimental measurements of attenuation or velocity the low frequency velocity limits as given by our model are identical to those predicted by gassmann s formula the high frequency limits may sign

effects of gravel content on liquefaction resistance and its assessment considering deformation characteristics in gravel mixed sand many reports describe overestimation of liquefaction resistance based on sounding data related to ground materials containing coarse particles such as gravel and cobbles better methods of liquefac

use of s wave anisotropy to quantify the onset of stress induced microfracturing abstractmicrofracturing and induced elastic anisotropy impart changes on body wave velocities with implications to seismic and wellbore testing methods and interpretation we have conducted simultaneous triaxial stress tests and ultrasonic wave propagation monitoring to quantify s wave anisotropy and microfracture development in berea sandstone and silurian dolomite the onset of stress induced microfracturing is detected at the beginning of appreciable s wave anisotropy called the s wave crossover swx the swx and subsequent increases in s wave anisotropy evidence microstructural damage development well before quasistatic indicators such as the volumetric strain point of positive dilatancy ppd and yield failure in all samples x ray microtomography confirmed fracture development and allowed for geometric assessment of fracture orientation stresses at the swx and ppd are compared with peak axial stress to understand linkages between damage and ultimate rock strength in berea sandstone the swx occ

geologic cause of seismic anisotropy a case study from offshore western australia velocity model building with a good understanding of anisotropy is one of the important elements in a prestack depth migration psdm project during a recently conducted psdm project we observed that 1 the application of negative delta epsilon is necessary to achieve reasonable depthing and gather flatness and 2 azimuthal variation in gather flatness exists in the interval where negative delta epsilon are necessary a geologic explanation of these interesting observations was necessary to justify the velocity model building result investigation of stress magnitude orientation and theoretical anisotropy of unconsolidated sandstone under nonhydrostatic stress revealed that these observations can be explained by stress induced anisotropy this concept may be applicable to other fields and the existence of azimuthal anisotropy and value of anisotropy parameters can be roughly estimated by the method described

liquefaction and flow failure during earthquakes two aspects of seismically induced liquefaction are discussed which are of vital engineering significance the triggering condition and the consequences of liquefaction the triggering condition is examined with respect to liquefaction analysis note being taken of the onset condition which is governed by cyclic strength consequences of liquefaction are discussed with respect to post seismic stability analysis in which the residual strength plays a major role procedures used for liquefaction analysis based on the results of in situ sounding tests are introduced and the applicability of this method for estimating associated ground settlements is discussed the evaluation of residual strength requires a better understanding of undrained sand behaviour results of extensive laboratory rests on japanese standard sand are examined and new index parameters are proposed to quantify undrained sand behaviour better the results of laboratory tests on silty sands are examined in the same way ah the results are

crustal structure and magmato tectonic processes in an active rift asal ghoubbet afar east africa 2 insights from the 23 year recording of seismicity since the last rifting event 1 the asal ghoubbet ag rift has sustained a major volcano tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys it is therefore an ideal place to study the transient magmato tectonic processes that operate after a rifting episode we examine the space time evolution of 2500 md 2 8 earthquakes recorded in the rift from 1979 to 2001 we focus on the relationships between this seismic activity and both the three dimensional structure of the rift and its postrifting behavior depicted from geodesy the results highlight the major role of the central magmatic system fieale shark bay on the structure seismic activity and overall behavior of the rift from 1978 to 1986 the rift opens at a fast rate yet mainly aseismically the opening is magmatically driven and accommodated since 1986 when the opening rate abruptly decreased the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system these pulses result from that magmatic zone undergoing alternating stretching and inflating episodes thus while the plate driven induced stresses have been rebuilding in the rift since 1987 the rift opening is still essentially accommodated in the axial magmatic zone the ag rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode that transient opening has essentially occurred aseismically and most tectonic faults remain relaxed and locked