subaru telescope observations of deep impact the impact cratering process on a comet is controversial but holds the key for interpreting observations of the deep impact collision with comet 9p tempel 1 mid infrared data from the cooled mid infrared camera and spectrometer comics of the subaru telescope indicate that the large scale dust plume ejected by the impact contained a large mass 106 kilograms of dust and formed two wings approximately 45 from the symmetric center both consistent with gravity as the primary control on the impact and its immediate aftermath the dust distribution in the inner part of the plume however is inconsistent with a pure gravity control and implies that evaporation and expansion of volatiles accelerated dust

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

satellite radar interferometry 1993 1999 suggests deep accumulation of magma near the crust mantle boundary at the krafla volcanic system iceland 1 deep magma accumulation near the crust mantle boundary 21 km depth at the krafla volcanic system is suggested from insar observations a best fit model derived from four interferograms covering 1993 1999 comprises an opening dike representing plate spreading and post rifting deformation and two mogi sources a mogi source deflating at a rate of 0 3 106 m3 yr coincides with the shallow krafla magma chamber while a deeper inflating mogi source further north at 21 km depth inflates at a rate of 26 106 m3 yr the inflating source is at or near the crust mantle boundary as identified by seismic studies and is interpreted as accumulating magma

tectonics and magma dynamics coupling in a dyke swarm of iceland abstract to study the dynamics of dyke and fissure swarms in iceland we analyse the distribution of the dykes across the tertiary alftafjordur dyke swarm and that of open fissures and faults across the active krafla fissure swarm within these narrow 5 to 10 km wide and long length width ratio 10 swarms deformation is accommodated both by tectonics and magmatism the dyke swarm built during the westward regional tilting of the host rock lava pile the populations of dykes suggest that a characteristic thickness at about 3 m is involved during dyking dykes show a quasi periodic distribution across the swarm and form clusters spaced by about 2 5 km the analysis of the fissure distribution in the krafla volcanic system reveals a cluster spacing of about half of that of the dyke clusters we discuss the potential origins of these features as a result of magma dynamics host rock properties and tectonics clustering possibly reflects the intrinsic mechanical response to stretching of the upper levels of the brittle crust we propose a three dimensional model of dyke swarms in which magma likely migrates laterally within the upper crust from igneous centres where high magmatic supply rates locally perturb the regional stress field

viscosity structure beneath northeast iceland the dynamics of crustal rifting in iceland depend strongly on the lower crustal rheology which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume while magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle since viscosity is also sensitive to the degree of partial melt present viscosity estimates for these regions could shed light on the factors responsible for these observations in this study we utilize horizontal and vertical displacement vectors determined in gps campaigns in northeast iceland since 1986 these are modeled in terms of steady state tectonic loading plus postseismic postdiking relaxation following the 1975 1984 krafla rifting episode as first proposed by foulger and others with the elastic part of the model fixed by external constraints these data have a high sensitivity to the viscosity structure beneath iceland lower crust and upper mantle viscosities of about 3 1019 pa s and 3 1018 pa s respectively yield the closest agreement with the data our lower crustal viscosity estimate is consistent with the low attenuation and low subsolidus temperature for the lower crust inferred in recent studies inversions for fissure opening during the krafla rifting episode yield about 7 m of opening centered on the krafla rift as is observed allowing for contemporaneous deep rifting on vertical faults along the askja segment partially accounts for the observed increase in separation across the rift during 1987 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the askja rift during 1987 1990 recent deep normal faulting beneath the askja rift and further south might explain all of these remaining features

post glacial rebound of iceland during the holocene abstract the geodynamic context of iceland provides the rare opportunity to analyse the combined effects of a ridge and a hotspot on deformation processes from a digital elevation model field analysis and a compilation of previous work we propose a synthesis of vertical motions of iceland during the holocene with a focus on the post glacial rebound we determined two ancient marine limits one above and one below the present sea level estimated at 10 ka 300 years and 8150 350 years bp respectively we calculated an uplift phase of 40 170 m with a rate of 2 1 9 2 cm a 1 between 10 ka 300 years and 8150 350 years bp corresponding to the post glacial rebound of iceland following the weichselian glaciation spatial variations of the rebound are mainly related to the local glacial dynamics ice load and deglaciation history rather than the geodynamic context however the relaxation time deduced from uplift data is 4167 years in west iceland and 2000 years in ssw iceland we estimated viscosity from relaxation time ranging from 2 1 10 19 pa s to 3 2 10 19 pa s the significant difference in the relaxation time is due to local variation of the lithospheric thickness as a result of rifting rather than because of a variation of the viscosity

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

the measurements of soil parameters relevant to tunnelling in clays a comprehensive laboratory program was carried out on specimens trimmed from 152 mm diameter piston samples of a soft silty clay and a varved clay at a tunnel site in thunder bay results of conventional triaxial tests as well as special tests for the determination of anisotropic elastic parameters simple shear tests and stress path tests are presented results indicate that the unloading moduli are about twice the loading moduli the poisson s ratios in unloading are about three times those in loading but the independent shear modulus is relatively unaffected while the modulus is sensitive to mode of consolidation drainage and direction of stress path the stress states at failure of all the different types of tests fall close to a single envelope for compression and extension the choice of soil parameters for the analysis of deformation in tunnelling in soft clays is discussed key words clay tunnelling stress path deformation modulus shear modulus anisotropic elastic parameters failure env

holocene vertical deformation outside the active rift zone of north iceland in iceland the main deformation activity occurs not only in the active rift zone where volcanism and seismicity coexist in fissure swarms but also in the transfer zones north and south where the main part of the seismic activity is located many studies described extension and vertical displacement associated with rifting and magmatism inside this rift zone they have classically assumed that no deformation occurs outside the rift zone however few studies have already been conducted outside the rift zone this work focuses on an area west of the rift zone in the akureyri area which displays an anomalous topography we illustrate an ene tilting of 0 23 since approximately 17 000 years ago using ancient lacustrine shorelines mapped in detail with a differential gps survey and analysis of spot images this degree of tilting corresponds to a vertical displacement of 160 m i e a deformation rate of 1 cm year which is close to values measured in the active zone this long wavelength deformation is compatible both with i the topographic anomaly of the akureyri area and with ii the very diffuse microseismicity that affects northern iceland the origin of this tilting is discussed and the different hypotheses glacial loading response rift uplift transform zone effect thermal doming local magmatic intrusion are reviewed our results structural data and the topography are more consistent with a local magmatic intrusion inducing local doming this work is the first attempt at quantifying long term deformation outside the rift zone in iceland

experimental ejection angles for oblique impacts implications for the subsurface flow field a simple analytical solution for subsurface particle motions during impact cratering is useful for tracking the evolution of the transient crater shape at late times a specific example of such an analytical solution is maxwells z model which is based on a point source assumption here the parameters for this model are constrained using measured ejection angles from both vertical and oblique experimental impacts at the nasa ames vertical gun range data from experiments reveal that impacts at angles as high as 45 to the targets surface generate subsurface flow fields that are significantly different from those created by vertical impacts the initial momentum of the projectile induces a subsurface momentum driven flow field that evolves in three dimensions of space and in time to an excavation flow field during both vertical and oblique impacts a single stationary pointsource model specifically maxwells z model however is found inadequate to explain this detailed evolution of the subsurface flow field during oblique impacts because 45 is the most likely impact angle on planetary surfaces a new analytical model based on a migrating point source could prove quite useful such a model must address the effects of the subsurface flow field evolution on crater excavation ejecta deposition and transient crater morphometry

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

fast and partitioned postglacial rebound of southwestern iceland located both on the mid atlantic ridge and above a mantle plume iceland is subject to horizontal and vertical motions many studies described these deformations in terms of rifting episodes that have combined both extensional tectonics and magmatism however few studies have described the glacio isostatic response induced by the retreat of the weichselian ice cap the melting of this ice cap induced a postglacial rebound for the whole of iceland that may be controlled by the geodynamic setting and the rheological layering of the lithosphere this study is devoted to 1 understanding the holocene rebound on the southwestern coast and 2 estimating the asthenosphere viscosity and depth beneath iceland two stages of holocene evolution were determined by means of gps profiles morphological observations and data compilation the first stage corresponds to a vertical uplift of 67 5 to 157 5 m it started at 10 000 years bp and ended at 8500 years bp implying uplift rates between 4 5 and 10 5 cm a it was a quick isostatic response to the fast ice retreat the second stage had vertical motion of tens of meters with a probable tectonic origin and started at 8500 years bp the uplift rate is 1 to 2 orders of magnitude slower than the one during the first stage uplift partitioning during the first stage was controlled by the thermal state of the lithosphere the highest geothermal flux inducing the maximum uplift rates the relaxation time for uplift provides a viscosity estimate of 5 4 5 8 1019 pa s for the asthenosphere this value is similar to those determined for glacial areas in different continental contexts however the flexural wavelength indicates a shallower asthenosphere than that occurring in continental domains therefore this study highlights a coupling between the thermal structure of the icelandic asthenosphere and the glacial rebound

radar interferometry and its application to changes in the earth s surface geophysical applications of radar interferometry to measure changes in the earth s surface have exploded in the early 1990s this new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times the resulting interferogram is a contour map of the change in distance between the ground and the radar instrument these maps provide an unsurpassed spatial sampling density 100 pixels km 2 a competitive precision 1 cm and a useful observation cadence 1 pass month 1 they record movements in the crust perturbations in the atmosphere dielectric modifications in the soil and relief in the topography they are also sensitive to technical effects such as relative variations in the radar s trajectory or variations in its frequency standard we describe how all these phenomena contribute to an interferogram then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments including the four satellites currently in orbit ers 1 ers 2 jers 1 and radarsat the next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement respecting the limits of the technique assessing its uncertainty recognizing artifacts and discriminating different types of signal we then review the geophysical applications published to date most of which study deformation related to earthquakes volcanoes and glaciers using ers 1 data we also show examples of monitoring natural hazards and environmental alterations related to landslides subsidence and agriculture in addition we consider subtler geophysical signals such as postseismic relaxation tidal loading of coastal areas and interseismic strain accumulation we conclude with our perspectives on the future of radar interferometry the objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it

large heterogeneities in comet 67p as revealed by active pits from sinkhole collapse pits have been observed on many cometary nuclei mapped by spacecraft1 2 3 4 it has been argued that cometary pits are a signature of endogenic activity rather than impact craters such as those on planetary and asteroid surfaces impact experiments5 6 and models7 8 cannot reproduce the shapes of most of the observed cometary pits and the predicted collision rates imply that few of the pits are related to impacts8 9 alternative mechanisms like explosive activity10 have been suggested but the driving process remains unknown here we report that pits on comet 67p churyumov gerasimenko are active and probably created by a sinkhole process possibly accompanied by outbursts we argue that after formation pits expand slowly in diameter owing to sublimation driven retreat of the walls therefore pits characterize how eroded the surface is a fresh cometary surface will have a ragged structure with many pits while an evolved surface will look smoother the size and spatial distribution of pits imply that large heterogeneities exist in the physical structural or compositional properties of the first few hundred metres below the current nucleus surface

deep impact excavating comet tempel 1 deep impact collided with comet tempel 1 excavating a crater controlled by gravity the comet s outer layer is composed of 1 to 100 micrometer fine particles with negligible strength 1000 kelvins a large increase in organic material occurred during and after the event with smaller changes in carbon dioxide relative to water on approach the spacecraft observed frequent natural outbursts a mean radius of 3 0 0 1 kilometers smooth and rough terrain scarps and impact craters a thermal map indicates a surface in equilibrium with sunlight

dynamics of volcanic systems in iceland example of tectonism and volcanism at juxtaposed hot spot and mid ocean ridge systems volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate pull as the plates are pulled apart associated with the mid ocean ridges and the magma dynamics of the iceland mantle plume most systems are 40 150 km long 5 20 km wide and develop a central volcano they supply magma to all eruptions in iceland data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment as a result the geometry of the plume is better constrained and the crust previously considered thin 10 km is now modeled as thick 20 40 km depending on the location of the volcanic systems their activity either decreases or increases faulting in the two main seismic zones from this we can infer that emplacement of the feeder dike to the largest historical eruption in iceland that of laki in 1783 increased shear stress in the south iceland seismic zone and almost certainly triggered the largest m 7 1 in 1784 historical earthquake in ic

widening of the krafla fissure swarm during the 1975 1981 volcano tectonic episode frequent distance measurements across the krafla fissure swarm north iceland recorded the extension accompanying the sequence of rifting events which started in december 1975 and lasted for 6 years an 80 to 90 km long section of the fissure swarm extended during this sequence of rifting and volcanic events maximum widening of about 8 m occurred 10 to 12 km north of leirhnjukur which is located above the center of the krafla magma reservoir from that location the amount of widening decreased north wards and is estimated to exceed 2 m where the seismicity indicated the northern termination of the present rifting off the north coast about 70 km north of leirhnjukur the amount of widening also decreased southwards and approached zero at 15 to 20 km south of leirhnjukur the ground deformation associated with these rifting events can be summarised as a narrow strip 1 to 2 km wide along the fissure swarm is heavily fractured with numerous open cracks parallel to the fissure swarm this fractured strip has subsided 2 to 3 m relative to its flanks the flanks of the fractured zone have been uplifted relative to regions farther away the uplift is not well constrained but tilt observations at several locations indicate about 1 m uplift the flanks of the rift zone have contracted perpendicular to the fissure swarm the maximum contractional strain exceeds 300 mm per km the amount of areal expansion windening of the fissure swarm times the length of the fractured zone associated with these rifting events is estimated to be about 0 30 km2 for individual events the area of expansion has been roungly proportional to the volume of subsidence above the krafla magma reservoir if the width of a new dike is equal widening of surface fissures the ratio of the subsidence volume to the area of expansion for the best observed events indicated a height of a new dike system as 2 4 to 2 8 km this ratio is significantly less for events of large lava production but even during these events the majority of magma leaving the krafla reservoir was apparently emplaced in subsurface fissures

installation effects of a bored pile wall in overconsolidated clay in situ embedded retaining walls are used to form basements and cuttings in urban areas throughout the world particularly where space is restricted it is generally accepted that the process of installing such a wall may influence the stress state of the soil and hence the post construction displacements wall bending moments and prop loads however wall installation is a complex three dimensional problem and its effects are not well understood this paper describes a study carried out on a section of contiguous bored pile retaining wall in an overconsolidated clay in kent england using instrumentation to monitor total horizontal stresses and pore water pressures before during and after wall installation reductions in horizontal stress were recorded during wall installation bringing the ratio of horizontal to vertical effective stress k from an initial value of about 1 to about 0 8 following wall installation there was no further change in horizontal stress over an additional period of 10 month

crustal deformation from 1992 to 1995 at the mid atlantic ridge southwest iceland mapped by satellite radar interferometry satellite radar interferometry observations of the reykjanes peninsula oblique rift in southwest iceland show that the reykjanes central volcano subsided at an average rate of up to 13 millimeters per year from 1992 to 1995 in response to use of its geothermal field interferograms spanning up to 3 12 years also include signatures of plate spreading and indicate that the plate boundary is locked at a depth of about 5 kilometers below that depth the plate movements are accommodated by continuous ductile deformation which is not fully balanced by inflow of magma from depth causing subsidence of the plate boundary of about 6 5 millimeters per year

a method of estimating surface settlement above tunnels constructed in soft ground a technique suitable for the analysis of lined tunnels constructed in soft soil is described this technique permits consideration of the soil lining interaction lining weight and plastic failure within the soil particular attention is given to the simulation of loss of ground including the annular void created by the difference between the tunnelling machine and lining diameters the analysis is used in a parametric study to identify potentially significant factors affecting the prediction of surface settlement due to tunnel construction consideration is given to the effects of elastic modulus the distance between the tunnel and the underlying rigid base stratum plastic failure initial stresses ground loss including the annular void and the injection of clay grout into the tailpiece void the results of this analysis indicate that elastic anisotropy ground loss and clay grout injection are all important factors that are not usually considered in analyses in particular a parameter called th

engineering characteristics of british over consolidated clays and mudrocks i tertiary deposits abstract there is much information in the literature relating to the classification strength deformability permeability and other geotechnical properties displayed by british overconsolidated clays and mudrocks the opportunity has been taken in this paper to collate published values of the more important geotechnical parameters for a number of tertiary formations and to present ranges and averages in a convenient form the data are intended to serve as a reference of typical values of parameters but it must be recognised that the results were derived from tests on material from particular locations and will not necessarily be typical of formations as a whole meaningful interpretation is possible only when there is complete awareness as to why values may vary since the degree of weathering is particularly important values for both weathered and unweathered material are presented where possible however discussion of the data indicates that mineralogical composition geological loading present burial depth and method of testing are also significant causes of variation some of the implications regarding the choice of design parameters are discussed of particular relevance is the degree and orientation of fissuring present in the material

surface deformation at the krafla volcano north iceland 1982 1992 extensive measurements of ground deformation at the krafla volcano iceland have been made since the beginning in 1975 of a series of eruptions and intrusions into the fissure system that extends north and south of the volcano i concentrate on measurements before and after the eruption of september 1984 the last event of this series when the largest volume of lava was erupted the patterns of ground deformation associated with the 1984 eruption determined by precision levelling electronic distance measurements and lake level observations were similar to earlier intrusions and eruptions in that the surface of the volcano subsided and the fissure system widened as magma moved laterally from a shallow central reservoir into the fissure system the shallow magma reservoir of krafla continued to expand for about five years after the eruption but a slow subsidence of the central area began in 1989 besides the presence of an inflating and deflating shallow magma reservoir at a depth of 2 5 km beneath the krafla caldera another inflating magma reservoir may exist at much greater depth below krafla the accumulation of compressive strain by numerous rift intrusions and eruptions since 1975 along the flanks of the north south krafla fissure swarm is being released slowly and will probably be reflected in the results of deformation measurements near krafla for the next several decades the total horizontal extension of the krafla rift system in 1975 1984 was about 9 m equal to about 500 years of constant plate divergence the extension is twice the accumulated divergence since previous rifting events and eruptions in 1724 1729

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

sources of crustal deformation associated with the krafla iceland eruption of september 1984 a decade long plate boundary rifting episode in northern iceland ended with the september 1984 fissure eruption of krafla volcano we apply a nonlinear inversion method to geodetic data collected before and after the eruption to infer the location geometry and strengths of deformation sources associated with the eruption the net outflow of magma from a 3 km deep magma chamber beneath the krafla caldera was 30 120 106 m a similar volume of magma 50 70 106 m was emplaced in a 1 meter wide 9 km long dike extending from the surface to 7 km depth furthermore at least 110 106 m of magma erupted accordingly a surplus of magma must have been expelled from a second reservoir the location of which although uncertain is likely to lie at depths greater than 5 km beneath krafla volcano it would be difficult to detect this deeper source because of the narrow aperture of the geodetic networks

discrimination of geophysical phenomena in satellite radar interferograms various geophysical phenomena are recorded in the interference patterns formed by differencing two synthetic aperture radar sar images the fringes generated by the topographic relief can be removed using a digital elevation model dem the remaining fringes map the change in satellite to ground range which occurred between the acquisition times of the two images by comparing different pairs of images spanning different intervals of time it is possible to discriminate between geophysical signal and interferometric artifact here we apply this pair wise logic to the area around the 1992 landers california earthquake using sar images acquired by the ers 1 satellite the range varies with time as a constant step impulsive or sloping function examples of each type include a dem error an aftershock two atmospheric perturbations and postseismic afterslip respectively their signatures are identified and separated using the pair wise logic

hypervelocity impact fragmentation of basalt and shale projectiles abstract results are presented for the fragmentation of projectiles in laboratory experiments 1 5 mm cubes and spheres of basalt and shale were impacted onto water at normal incidence and speeds from 0 39 to 6 13 km s 1 corresponding to peak shock pressures 0 7 32 gpa projectile fragments were collected and measured over 100 000 fragments in some impacts at sizes down to 10 m power laws were fitted to the cumulative fragment size distributions and the evolution of the exponent vs impact speed and peak shock pressure found the gradient of each of these power laws increased with increasing impact speed peak shock pressure the percentage of the projectiles recovered in the impacts was found and used to estimate projectile remnant survival in different solar system impact scenarios at the mean impact speed appropriate to that scenario for pluto the moon and in the asteroid belt approximately 55 40 and 15 respectively of an impactor could survive and be recovered at an impact site finally the catastrophic disruption energy densities of basalt and shale were measured and found to be 24 104 j kg 1 and 9 104 j kg 1 respectively a factor of 2 5 difference these corresponded to peak shock pressures of 1 to 1 5 gpa basalt and 0 8 gpa shale this is for near normal incidence impacts where tensile strength is dominant for shallow angle impacts we suggest shear effects dominate resulting in lower critical energy densities and peak shock pressures we also determine a method to ascertain information about fragment sizes in solar system impact events using a known size of impactor the results are used to predict projectile fragments sizes for the veneneia and rheasilvia crater forming impacts on vesta and similar impacts on ceres

atmospheric effects on ejecta emplacement and crater formation on venus from magellan the venus cratering record provides a unique environment for assessing the effects of both gravity and an atmosphere on impact crater formation this contribution uses surface signatures of energy partitioning as a framework for testing extrapolations from laboratory experiments and other planetary settings seven general conclusions can be drawn first the dense lower atmosphere of venus takes on the role of a low density target for bodies smaller than about 4 km in diameter air blasts created by cratering in the atmosphere create distinctive surface signatures that allow the derivation of an independent assessment of impactor energy at the limit of break up second dynamic pressures during entry of larger bodies will exceed their strength limit but may not prevent penetration of the atmosphere due to aerodynamic reshaping that minimizes the drag coefficient such a process may account for the formation of unusually small craters 1 3 km third the dense atmosphere of venus preserves signatures of early time cratering processes on the surface that are typically lost on atmosphere free surfaces such signatures not only provide another estimate of impactor energy but also include a distinctive record of the impactor i e comet versus asteroid in distinctive run out flows created before the crater has finished formation strong winds and turbulence associated with the atmospheric disturbance at later times create wind streaks behind topographic barriers fourth ballistic ejection of excavated debris occurs from craters on venus just as it does on planets without an atmosphere thereby underscoring the fundamental mechanical transfer of energy from impactor to target fifth ejecta emplacement is nonballistic due to the large dynamic forces acting on the advancing curtain and its constituent ejecta the outward moving ejecta curtain induces strong response winds that entrain ejecta and drive a ground hugging debris flow outward without returning to the cavity flow separation creates an overriding run out ejecta flow further sustained by atmospheric turbulence and identified as radar dark lobate lobes sixth radar dark parabolas are proposed to be late time fallout deposits created as the downrange fireball evolves aloft perhaps analogous to terrestrial tektite strewn fields and seventh the response of crater formation to the atmosphere is conversely expressed by a reduction in cratering efficiency as revealed by the unusually large central peak complexes and the unexpected diameter depth relations of craters hence surface ages may be significantly underestimated

estimation of in situ stresses using anisotropic elasticity and suction measurements sampling and specimen preparation produce changes in mean effective stresses and pore water pressures even with perfect sampling the paper takes an existing simplified three parameter cross anisotropic elastic model and uses it to model these changes the required ratio of cross anisotropic parameters j 3g can be obtained from standard ciu triaxial tests if measurements are also made of suctions in unloaded specimens in the laboratory then a combination of j 3g the measured suction and the effective overburden pressure permits an estimation of the horizontal effective pressure and the k0 at rest coefficient this can be helpful in numerical modelling that needs to start from in situ conditions and in planning pressure levels for reconsolidation of clay specimens in the laboratory tests were done on belfast upper boulder clay from a depth of 28 m values of horizontal in situ effective stress estimated from these measurements compare favorably with conventional estimates of the at rest coef

smart 1 end of life shallow regolith impact simulations the smart 1 end of life impact with the lunar surface was simulated with impacts in a two stage light gas gun onto inclined basalt targets with a shallow surface layer of sand this simulated the probable impact site where a loose regolith will have overlaid a well consolidated basaltic layer of rock the impact angles used were at 5 and 10 from the horizontal the impact speed was 2 km s 1 and the projectiles were 2 03 mm diameter aluminum spheres the sand depth was between approximately 0 8 and 1 8 times the projectile diameter implying a loose lunar surface regolith of similar dimensions to the smart 1 spacecraft a crater in the basement rock itself was only observed in the impact at 10 incidence and where the depth of loose surface material was less than the projectile diameter in which case the basement rock also contained a small pit like crater in all cases the projectile ricocheted away from the impact site at a shallow angle this implies that at the smart 1 impact site the crater will have a complicated structure with exposed basement rock and some excavated rock displaced nearby and the main spacecraft body itself will not be present at the main crater

the strength of london clay in relation to the design of embedded retaining walls in the design of embedded retaining walls in stiff clay the lateral earth pressures acting on the wall in limiting equilibrium are derived from the shear strength parameters of the clay these parameters are usually determined by triaxial compression tests at stress levels greater than those in the field consequently they may not realistically represent the actual strength of the soil adjacent to the wall which experiences decreasing effective stress as excavation proceeds in front of the wall the effect of stress level and stress path on the strength parameters has been investigated in a series of undrained triaxial tests carried out at comparatively low stress levels in compression and extension on undisturbed samples of london clay the samples were obtained from a site where soil stresses have been monitored adjacent to a propped embedded retaining wall author trrl

long period interferograms reveal 1992 1998 steady rate of deformation at krafla volcano north iceland we formed interferograms of ers sar scenes covering the area of krafla n iceland with time span values of up to six years 1992 1998 our data reveals a steady deformation rate at krafla and within its fissure swarm with values reaching 2 1 cm y in the ground to satellite direction at the volcano the area affected by deformation extends 20 km both north and south of the volcano the best fit dislocation model consists of sills to the north and south of the volcano and a magma chamber located below the volcano all of them undergoing contraction

compaction as the origin of the unusual craters on the asteroid mathilde the asteroid mathilde has suffered at least five giant impacts previous studies suggest that mathilde s giant craters should be surrounded by blankets of ejecta that are kilometres deep1 2 yet the craters show no evidence of filling by material excavated during later nearby impacts1 3 computer simulations of impacts have been used to suggest that the absence of ejecta arises because the impact energy is deposited in a small volume due to mathilde s unusually high porosity4 which produces ejecta velocities so high that nearly all of the material escapes mathilde s gravitational field5 here we report laboratory measurements of high velocity impacts into porous material which support an alternative explanation3 the crater is formed mainly by compaction not excavation the small amount of ejecta lofted in our experiments have velocities sufficiently low that nearly all of the material is redeposited within the crater bowl the crater itself results from material being compressed rather than ejected this type of cratering implies that highly porous asteroids are minor contributors of meteorites because essentially no material escapes the asteroids

contrasting meteoritic signatures within the clearwater east and clearwater west impact structures the view from osmium isotopes abstract osmium isotopes provide a powerful tool for identifying meteoritic signatures in impactites we apply the osmium isotope method to impact melt and country rocks from the clearwater east and clearwater west craters located in quebec canada impact melts from clearwater east have 187 os 188 os ratios of 0 1281 0 1285 these values indicate a significant meteoritic component which exceeds that in impact melts at all terrestrial craters studied to date except morokweng such findings align with earlier results from chromium isotopes and platinum group elements in contrast impact melts from clearwater west have 187 os 188 os ratios between 6 604 and 59 12 these highly radiogenic ratios are indistinguishable from the 187 os 188 os ratios in country rocks hence osmium isotopes provide no evidence for a meteoritic component in impact melts at clearwater west the clearwater craters formed in almost identical targets therefore target effects cannot readily explain the stark difference between the two clearwater craters if melt sheet heterogeneity is similar at the two craters the probability that melts at clearwater west host an undetected chondritic component is 1 may best explain the unusually strong meteoritic signature although the signature or its nondetection at each crater may be related to asymmetric preservation of the impactor component the results presented here provide further evidence that clearwater east and clearwater west were temporally separate impact events

the deep impact crater on 9p tempel 1 from stardust next abstract the stardust next sdn mission returned to comet 9p tempel 1 and viewed the site of the deep impact di collision just over one comet year later comparisons between pre impact images from the its camera on the di probe and sdn images reveal a 50 m diameter crater surrounded by a low rim about 180 m in diameter the removal of a small mound uprange but offset from the trajectory from the impact site can be related to changes in the evolution of ejecta a narrow 6 gap in the ejecta curtain downrange indicates that a ridge extending from the impact facing scarp downrange interrupted the final stages of cratering in one small region together these observations indicate that the di excavation crater diameter was about 200 m 20 m a value consistent with the ejected mass derived from earth and space based observations with the assumption that this mass represents only 10 20 of the total ejected mass as a result the di crater visible today is consistent with either a larger transient crater which collapsed or a central crater of a nested crater resembling an inverted sombrero the latter alternative would be expected from a layered target a loose particulate surface about 1 2 m deep over a slightly more competent substrate

magmatic fissures and their systems in iceland a tectonomagmatic model 1 volcanic systems in iceland comprise a central volcano linked to magmatic fissures whose subsurface architecture remains enigmatic the surface dimensions and trends of active fissures show their spatial relation with fault systems structure of the fremri namur and hengill volcanic systems was analyzed the linear vent arrays marking recent fissures are tightly bound by vertical segmented faults while normal faults accommodating extension are common on the flanks of the fissure swarm evidence of normal frictional slip on the fissure bounded faults suggests that failure occurred in response to subsidence with subsequent dilation this is supported by inward tilting of strata adjacent to faults compatible with a downsag phase we propose that magmatic fissures have vertical feeders with lateral offshoots extending along the rift zone their inflation deflation during an eruptive cycle causes subsidence such magmatically generated faults can be subsequently modified by tectonic extension

return to comet tempel 1 overview of stardust next results on february 14 2011 stardust next sn flew by comet tempel 1 the target of the deep impact di mission in 2005 obtaining dust measurements and high resolution images of areas surrounding the 2005 impact site and extending image coverage to almost two thirds of the nucleus surface the nucleus has an average radius of 2 83 0 1 km and a uniform geometric albedo of about 6 at visible wavelengths local elevation differences on the nucleus reach up to 830 m at the time of encounter the spin rate was 213 per day period 40 6 h and the comet was producing some 130 kg of dust per second some 30 of the nucleus is covered by smooth flow like deposits and related materials restricted to gravitational lows this distribution is consistent with the view that the smooth areas represent material erupted from the subsurface and date from a time after the nucleus achieved its current shape it is possible that some of these eruptions occurred after 1609 when the comet s perihelion distance decreased from 3 5 au to the current 1 5 au much of the surface displays evidence of layering some related to the smooth flows and some possibly dating back to the accretion of the nucleus pitted terrain covers approximately half the nucleus surface the pits range up to 850 m in diameter due to their large number they are unlikely to be impact scars rather they probably result from volatile outbursts and sublimational erosion the di impact site shows a subdued depression some 50 m in diameter implying surface properties similar to those of dry loose snow it is possible that the 50 m depression is all that remains of an initially larger crater in the region of overlapping di and sn coverage most of the surface remained unchanged between 2005 and 2011 in albedo photometric properties and morphology significant changes took place only along the edges of a prominent smooth flow estimated to be 10 15 m thick the margins of which receded in places by up to 50 m coma and jet activity were lower in 2011 than in 2005 most of the jets observed during the sn flyby can be traced back to an apparently eroding terraced scarp the dust instruments detected bursts of impacts consistent with a process by which larger aggregates of material emitted from the nucleus subsequently fragment into smaller particles within the coma

integration of micro gravity and geodetic data to constrain shallow system mass changes at krafla volcano n iceland new and previously published micro gravity data are combined with insar data precise levelling and gps measurements to produce a model for the processes operating at krafla volcano 20 years after its most recent eruption the data have been divided into two periods from 1990 to 1995 and from 1996 to 2003 and show that the rate of deflation at krafla is decaying exponentially the net micro gravity change at the centre of the caldera is shown using the measured free air gradient to be 85 gal for the first and 100 gal for the second period after consideration of the effects of water extraction by the geothermal power station within the caldera the net gravity decreases are 73 17 gal for the first and 65 17 gal for the second period these decreases are interpreted in terms of magma drainage following a mogi point source model we calculate the mass decrease to be 2 1010 kg year reflecting a drainage rate of 0 23 m3 s similar to the 0 13 m3 s drainage rate previously found at askja volcano n iceland based on the evidence for deeper magma reservoirs and the similarity between the two volcanic systems we suggest a pressure link between askja and krafla at deeper levels at the lower crust or the crust mantle boundary after the krafla fires co rifting pressure decrease of a deep source at krafla stimulated the subsequent inflow of magma eventually affecting conditions along the plate boundary in n iceland as far away as askja we anticipate that the pressure of the deeper reservoir at krafla will reach a critical value and eventually magma will rise from there to the shallow magma chamber possibly initiating a new rifting episode we have demonstrated that by examining micro gravity and geodetic data our knowledge of active volcanic systems can be significantly improved

multiple volcano deformation sources in a post rifting period 1989 2005 behaviour of krafla iceland constrained by levelling tilt and gps observations abstract the krafla rifting episode which occurred in north iceland in 1975 1984 was followed by inflation of a shallow magma chamber until 1989 at that time gradual subsidence began above the magma chamber and has continued to the present at a declining rate pressure decrease in a shallow magma chamber is not the only source of deformation at krafla as other deformation processes are driven by exploitation of two geothermal fields together with plate spreading in addition deep seated magma accumulation appears to take place with its centre 10 km north of the krafla caldera the relative strength of these sources has varied with time new results from a levelling survey and gps measurements in 2005 allow an updated view on the deformation field deformation rates spanning 2000 2005 are the lowest recorded in the 30 year history of geodetic studies at the volcano the inferred rate of 2000 2005 subsidence related to processes in the shallow magma chamber is less than 0 3 cm yr whereas it was 5 cm yr in 1989 1992 currently the highest rate of subsidence takes place in the leirbotnar area within the krafla caldera and appears to be a result of geothermal exploitation

pore water pressure and total horizontal stress response to epbm tunnelling in london clay the ground response in terms of surface and subsurface displacements to twin bore crossrail tunnel construction beneath a research monitoring site in hyde park london using earth pressure balan

postrifting anelastic deformation around the spreading plate boundary north iceland 1 modeling of the 1987 1992 deformation field using a viscoelastic earth structure a third global positioning system gps survey of a regional network surrounding the krafla volcanic system north iceland was conducted in 1992 following a major crustal spreading episode which began in this system in 1975 differencing the 1992 results with those from 1987 and 1990 reveals a regional deformation field with a maximum rift normal expansion rate of 4 5 cm year near the rift decreasing to 3 cm year at large distances the time averaged spreading rate in north iceland 1 8 cm year cannot account for this deformation the vertical deformation field reveals regional uplift throughout the network area at its maximum closest to the rift and decreasing with distance three different models are applied to study the postdike injection ground deformation 1 stress redistribution in an elastic layer over a viscoelastic half space 2 stress redistribution in an elastic viscous layered medium and 3 continued opening at depth on the dike plane in an elastic half space using model 1 the effects of historical episodes in the region are subtracted from the observed displacement fields and the remaining motion is modeled as relaxation following the recent krafla rifting episode the best fit model involves a half space viscosity of 1 1 x 10 18 pa s a relaxation time of 1 7 years and an elastic layer thickness for northeast iceland of 10 km the vertical field indicates that the krafla dike complex rifted the entire elastic layer using model 2 the motion 1987 1990 and 1990 1992 can be simulated adequately given the survey errors but the 1987 1992 deformation is poorly fitted suggesting that a more realistic geophysical model is required using model 3 a range of dikes will fit the deformation field