morphology and environment of cold water coral carbonate mounds on the nw european margin cold water coral carbonate mounds owing their presence mainly to the framework building coral lophelia pertusa and the activity of associated organisms are common along the european margin with their spatial distribution allowing them to be divided into a number of mound provinces variation in mound attributes are explored via a series of case studies on mound provinces that have been the most intensely investigated belgica hovland pelagia logachev and norwegian mounds morphological variation between mound provinces is discussed under the premise that mound morphology is an expression of the environmental conditions under which mounds are initiated and grow cold water coral carbonate mounds can be divided into those exhibiting inherited morphologies where mound morphology reflects the morphology of the colonised features and developed morphology where the mounds assume their own gross morphology mainly reflecting dominant hydrodynamic controls finer scale surface morphological features mainly reflecting biological growth forms are also discussed

alignment of near surface inclusions and appropriate crack geometries for geothermal hot dry rock experiments ubiquitous splitting of seismic shear waves indicates that most rocks in the upper half of the crust are pervaded by stress aligned fluid filled inclusions called eda cracks these inclusions are expected to be aligned perpendicular to the minimum compressional stress by stress relationships similar to those aligning industrial hydraulic fractures at depths where the overburden stress is sufficiently large typically below a few hundred metres this minimum stress is usually horizontal so that the eda cracks and hydraulic fractures are typically aligned vertically striking parallel or subparallel to the direction of maximum compression this is confirmed by the polarizations of the split shear waves along raypaths at depth in the crust at the free surface however the vertical stress is zero or approximately zero and cracks and hydraulic fractures at shallow depths in intact rock tend to be horizontal thus the directions of minimum stress and the orientations of hydraulic fractures are likely to swing through 90 near the surface of the earth since the behaviour of cracks and stress is often crucial to drilling operations the rotation of the crack and stress geometry near surface has important implications particularly for optimizing hydrocarbon production and geothermal reservoir management consequently evidence gained from experiments for example in hot dry rock geothermal heat extraction in inappropriate crack geometries at shallow depths may not be valid when applied to other crack and stress geometries at depth in hot rock

a seismic refraction study of the crustal structure associated with the adamawa plateau and garoua rift cameroon west africa summary recordings of quarry blasts along a 300 km profile across the adamawa plateau at the eastern end of the cameroon volcanic line permitted the determination of the crustal structure of the region to the north of the plateau in the area of the garoua rift an eastern arm of benue trough the crust is about 23 km thick and is underlain by upper mantle with a p wave velocity of 7 8 kms 1 the southern part of the plateau by comparison is associated with a more normal crustal thickness of about 33 km and underlain by upper mantle with a velocity of 8 0 kms 1 between these two areas the seismological experiment was not able to delineate how the crust changes however if the topography and gravity field reflect crustal thickness then there is a rapid change in crustal thickness at the northern edge of the adamawa plateau the study thus provides the first seismic evidence for major crustal thinning beneath the garoua arm of the west african rift system as well as providing evidence for an alternative model for the formation of the cameroon volcanic line

stable sr isotope sr ca mg ca li ca and mg li ratios in the scleractinian cold water coral lophelia pertusa the aragonitic skeletons of scleractinian cold water corals can serve as valuable archives in paleoceanographic studies the potential of 88 86sr sr ca mg ca li ca and mg li ratios of the cold water coral lophelia pertusa to record intermediate water mass properties has been investigated here we used samples from several locations along the european continental margin spanning a large temperature range from 6 to 14 c stable strontium isotope measurements were carried out with the recently developed double spike tims technique and our results differ from those obtained with less precise methods in contrast to the strong positive relationship with temperature of previous studies our results suggest that 88 86sr measured in scleractinian cold water corals is not controlled by seawater temperature but reflects the sr isotopic composition of seawater with an offset of 88 86sr 0 196 as found in previous studies the elemental ratios sr ca li ca and mg li measured in corals are significantly related to water temperature and do not correlate with salinity moreover sr ca ratios in l pertusa display the expected inverse correlation with temperature however the variance in the sr ca data severely limits the accuracy of paleotemperature estimates the li ca and mg ca ratios reveal other influences besides temperature such as ph and or growth or calcification rate however corresponding mg li ratios in l pertusa are more tightly related to temperature as they remove these secondary effects in particular the mg li ratio in l pertusa may serve as a new promising paleotemperature proxy for intermediate water masses our dataset represents the most extensive geochemical examination of l pertusa to date revealing a temperature sensitivity of 0 015 mol mmol c for mg li however using this temperature dependence and the precision of 5 3 2sd only temperature variations larger than 1 5 c can be resolved with 95 confidence

circulation phenomena and frontal dynamics of the norwegian coastal current after a short general description of the norwegian coastal current some recent investigations concerning its variability are reviewed the variability caused by meteorological effects and fresh water fluxes is shown to be the dominant feature the coastal current not only responds to local wind forcing but is also a signal channel of sea level variations in the north sea as a whole the variability at times manifests itself as mesoscale waves and eddies the subsurface frontal region is characterized by interleaving of coastal and atlantic water and strong mixing lateral and longitudinal frontal motions are dealt with in relation to plumes of brackish water from the fjords and outbreaks of brackish and or warm water masses from the skagerrak the inflow of atlantic water through the norwegian trench and the formation of deep and bottom water in skagerrak are discussed

giant carbonate mud mounds in the southern rockall trough seismic profiles sidescan sonar data bottom photography and sampling proved for the first time that there are giant carbonate mounds bioherms in the rockall trough the pelagia mounds on the northern porcupine bank are relatively isolated whereas most of the logachev mounds on the southeast rockall bank form a field of closely spaced contiguous mounds they are all found on the upper slope at depths between about 500 and 1200 m and as a result of this study about 500 have been mapped they have a variety of shapes and the larger ones are very steep sided up to 350 m high and 2 km wide at the base sediment samples show that the mounds consist of pale coloured muds mainly aragonite usually with live and or dead cold water corals at the seafloor and with buried dead corals coral thickets appear to be detected as a brush like signature on high resolution profiles and as a speckled pattern on high resolution sidescan sonar records shelly sands are found on the seabed between the mounds currents in the mound areas are strong enough to transport sands along slope as medium sized sand waves and to prevent pelagic deposition thus providing surfaces for initial coral settlement by the winnowing of fines to leave glacial dropstones and exposed rocks these observations show that the poleward directed upper slope current of the eastern north atlantic extends further south than hitherto known and that there is a southwesterly directed current on rockall bank there are waves on the logachev mounds with a wavelength of about 20 30 m that are thought to be moulded in carbonate muds by across slope directed internal tidal currents and or cascading currents mounds develop above erosional surfaces seen on seismic data the huge amount of carbonate mud seems to be produced by the rapid growth and breakdown of cold water corals living in a very favourable environment intermediate nepheloid layers formed upstream by the strong currents may provide the food source for the deep water suspension feeders conditions for giant mound growth require an ideal balance between current speed and sedimentation rate among other factors no evidence for the suggested link to methane seepage has been found so far

vp vs ratio along the voring margin ne atlantic derived from obs data implications on lithology and stress field a total of 13 regional ocean bottom seismograph obs profiles with an accumulated length of 2207 km acquired on the voring margin ne atlantic have been travel time modelled with regards to s waves the vp vs ratios are found to decrease with depth through the tertiary layers which is attributed to increased compaction and consolidation of the rocks the vp vs ratio in the intra campanian to mid campanian layer 1 75 1 8 in the central voring basin is significantly lower than for the layers above and beneath suggesting higher sand shale ratio this layer was confirmed by drilling to represent a layer of sandstone this mid cretaceous anomaly is also present in the northern voring basin as well as on the southern lofoten margin further north the vp vs ratio in the extrusive rocks on the voring plateau is estimated to be 1 85 conformable with mafic basaltic rocks landward of the continent ocean transition cot the vp vs ratio in the layer beneath the volcanics is estimated to be 1 67 1 75 these low values suggest that this layer represents sedimentary rocks and that the sand shale ratio might be relatively high here the vp vs ratio in the crystalline basement is estimated to be 1 67 1 75 in the basin and on the landward part of the voring plateau indicating the presence of granitic granodioritic continental crust in the lower crust the vp vs ratio in the basin decreases uniformly from southwest to northeast from 1 85 1 9 to 1 68 1 73 suggesting a gradual change from mafic gabbroic to felsic granodioritic lower crust significant 3 5 azimuthal s wave anisotropy is observed for several sedimentary layers as well as in the lower crust all these observations can be explained by invoking the presence of liquid filled microcracks aligned vertically along the direction of the present day maximum compressive stress nw se

a simple model of fjord and coastal circulation interaction abstract the dynamical interaction of a narrow fjord with a wind driven coastal regime is investigated using a linear two layer numerical model the coriolis acceleration is important in the coastal regime but assumed to he unimportant in the fjord dynamical because v 0 for a wide variety of wind conditions bottom topography and model parameters the wind forced coastal circulation with its geostrophic alongshore currants has a strong effect on the circulation within the fjord these geostrophic currents control the free surface and pycnocline displacement at the fjord mouth thereby strongly affecting fjord circulation this mechanism is an alternative to the classical idea of hydraulic control at the mouth by sills or constrictions model simulators also show that the free surface slope is a baroclinic effect and that alongshore and across shore winds affect the fjord differently alongshore winds produce flooding while up and down fjord winds simply sat up the surface we find that offshore win

carbonate mounds from paradox to world heritage the recent marine carbonate world comprises two major compartments 1 the surface domain of the photozoan carbonates confined in space by water depth and by the penetration of light and 2 a deep domain where heterozoan mound builder guilds directly forage on fluxes of nutrients which primarily percolate from the photic zone and or are generated by in situ benthic processes locally giant cold water coral mounds tower up to heights of 150 to 250 m above the sea floor in general between 500 and 1300 m water depth and within sharply delineated provinces some 15 years of research on these giant mound provinces conveys a picture of their distribution in space and possibly sheds light on controls acting in concert globally there is no counterpart for the prolific north atlantic mound basin namb a chemical control is seen by an overlay of the mound provinces on a map of the aragonite saturation horizon ash an external physical control is inferred fromthe position of themound provinces girdling a vigorous north atlantic subtropical gyre systemand clustering close to the roof of the intermediate to deep water masses of a dynamically stratified ocean on the eastern boundary of the namb nutrient fluxes are enhanced by mixing processes driven either by internal waves between galicia and the shetlands or by the vast and heterogeneous eastern boundary upwelling system along the iberian african margins down to 10 n early diagenesis by carbonate dissolution and re precipitation driven by convecting or advecting internal fluids can contribute to stabilize such constructions facilitating an exuberant vertical accretion it is speculated that in thenorth atlantic ocean the deep water carbonate factory outclasses in size the shallow water coral reefs giant mound formation is a recurrent play of life since the dawn of the metazoans nama group upper neoproterozoic however with actors and plots varying from act to act remarkably literature reports only three occurrences of deep water mounds in the phanerozoic the modern ocean possibly the danian and the carboniferous some striking parallelisms in the development of the atlantic and the paleo tethys oceans combined with the developing insights in the controls on deep watermounds in the present ocean invite for a comparative study this has the potential to eventually shed light on the full circulation pattern of the paleo tethys ocean surface and deep comparative studies will build upon 1 modeling of ocean circulation constrained by the record of deep water carbonate systems and supported by advances in tracer and proxy tools and 2 field studies on representative and accessible continental locations the mound route that develops in morocco under the auspices of ioc unesco will provide tomulti disciplinary teams with marine and continental experience opportunities for confronting observations from the modern ocean and on key records of past oceanic basins it has the potential to eventually qualify for a unesco recognition as world heritage

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

geochemical and physical constraints for the occurrence of living cold water corals cold water coral communities cover a wide range of possible habitats in terms of latitude ocean basins and depth with ongoing studies continually expanding occurrences in various regions of the global ocean a range of factors determines the formation of cold water coral reefs such as physical hydrochemical and biological e g food supply factors recently more and more modeling studies have emerged using a variety of mathematical approaches have emerged including environmental niche factor analysis enfa and predictive habitat suitability models however only few studies have attempted to characterize the underlying suite of hydro biogeochemical and physical constraints of cold water coral reefs and to differentiate between pristine reef growth vs sites with reduced or no coral occurrences this study concentrates on new data and a compilation of existing data sets on the physical and chemical properties in the ne atlantic and the mediterranean it explores the influence of ambient bottom waters and its characteristics on living cold water reefs and mounds formed by lophelia pertusa several questions are addressed 1 what are the physical and geochemical boundary conditions of living cold water corals 2 do these geochemical parameters correlate with proposed physical prerequisites 3 is there a general difference in the signature of living and dead coral sites

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

shear wave polarizations near the north anatolian fault i evidence for anisotropy induced shear wave splitting summary the turkish dilatancy projects tdp1 in 1979 and tdp2 in 1980 recorded small earthquakes near the north anatolian fault with closely spaced networks of three component seismometers in order to investigate the possibility of diagnosing dilatancy from its effects of shear wave propagation this paper examines the polarizations of shear wavetrains recorded in the shear wave window immediately above the earthquake foci abrupt changes in the orientation and or ellipticity of the shear wave polarizations are almost always observed during the first few cycles following the initial shear wave arrival on each seismogram the horizontal projections of the polarizations of the first shear wave arrivals at any given station show nearly parallel alignments with approximately the same orientations at each of the recording sites with one exception it is difficult to explain this uniform alignment over a wide area in terms of scattering at the irregular surface topography or by earthquake focal mechanisms we demonstrate that the shear wave splitting is likely to be the result of anisotropy in the region above the earthquake foci which could produce polarizations displaying the observed alignments the temporal change of the azimuth of alignment observed at one locality between 1979 and 1980 may be due to the release of a local stress anomaly by a very near earthquake

gas hydrate crystals may help build reefs during a recent cruise in the porcupine basin off southwest ireland we discovered two extensive and hitherto largely unsuspected deep water reef provinces including a giant cluster of hundreds of buried mounds the ring shapes of many reefs suggest that they are caused by an axial fluid expulsion at the sea bed a transient flow well confined in space and time we are exploring various hypotheses but a stimulating avenue for research is opened by a glacially controlled growth pulse and subsequent decay of a shallow layer of gas hydrates as a methane buffer and probably indirectly as a ground for overlying biological communities

near fault anisotropy following the hector mine earthquake 1 we present anisotropy measurements from shear wave splitting along the hector mine rupture zone six major arrays were deployed in four locations in the year following the m7 1 hector mine earthquake the dense station coverage wide distribution of the arrays and repeated deployments show a clear predominant fast direction and spatial variation of splitting along the fault but no resolvable temporal variations we determined splitting parameters using an automated cross correlation method discarding fast directions with initial source polarizations near crack parallel or perpendicular directions only two of the four array locations give reliable measurements of anisotropy at depth fast directions and delay times are constant across the 1 km wide array length however some spatial variation of splitting is observed along fault strike delay times decrease from north to south with greater splitting in areas of higher slip a change in splitting parameters along fault strike likely reveals the orientation of cracking in the near fault region during a major quake average fast directions are between fault parallel and the regional maximum compressive stress direction we do not see temporal evolution in anisotropy however measured splitting suggests a heterogeneous stress field partially created during rupture that persists over at least a 1 year timescale

controlled sources for shear wave surveys in mines the ability to analyse shear wave anisotropy in a mine environment is greatly aided by using multiple source orientations of a reproducible impulsive shear wave source the analysis of what is probably the first controlled source shear wave experiment in a mine environment demonstrates clearly that shear wave polarizations and time delays between split shear wave arrivals are reliably measured because of the use of multiple source orientations rather than a single shear wave source reliability is further aided by modelling the shear wave source radiation pattern which allows for the unequivocal discrimination between seismic raypaths where shear wave splitting did and did not occur the analysis also demonstrates the great importance of high reproducibility of the seismic source for the use of shear waves in time lapse surveys to monitor changes in a rockmass

effects of the free surface on shear wavetrains summary the behaviour of shear waves is of great importance in identifying and investigating seismic anisotropy in the earth however shear wavetrains recorded at the earth s surface do not always reflect the motion at depth introducing practical problems of interpretation shear wavetrains incident on the surface of an isotropic half space at angles less than critical about 35 are broadly preserved but at greater angles substantial distortions can occur for stations situated close to the source as in local earthquake studies the local sp phase a radially polarized precursor to s may occur the behaviour at the surface of an anisotropic half space is further complicated by the divergence of phase and energy propagation vectors all of these complications suggest that detailed seismogram modelling is essential to any study of shear wave propagation in the earth and in particular to investigations of anisotropy induced shear wave splitting

biomarker hydrogen isotope composition δd as proxy for holocene hydroclimatic change and seismic activity in sw peloponnese greece we present a 6000 year long record tracing hydroclimate changes in sw greece based on hydrogen isotope composition of aquatic plant derived n c 23 alkanes d c23 in a sediment core from the messe

inversion of arrival times in a region of dilatancy anisotropy shear wave splitting has been identified in many three component seismograms from two separate field experiments on a section of the north anatolian fault in north west turkey these observations are consistent with shear wave propagation through a zone of extensive dilitancy anisotropy a preliminary attempt has been made to confirm this interpretation by simultaneously inverting suites of arrival times for hypocentral locations and for parameters describing an anisotropic halfspace although the inversion procedure is not globally convergent it is possible to recognize the true solution by systematically varying the initial conditions applied to selected data sets the inversion defines several anisotropic models that fit the data significantly better than a simple isotropic model and display the anisotropy required by the shear wave splitting however most of these anisotropic models are not superior when they are used to individually locate events in a much larger data set however for each experiment there is a single model that produces clearly superior locations for the larger data sets than those of other anisotropic or simple isotropic models both models display similar velocity variations which are characteristic of propagation through distributions of biplanar cracks displaying orthorhombic symmetry the principal axes of the two models are oriented in similar directions and are within 20 of the principal axis of regional stress derived from fault plane solutions the solutions indicate low velocities close to the tensional axis as would be expected in extensive dilatancy anisotropy

the sardinian cold water coral province in the context of the mediterranean coral ecosystems abstract a new cold water coral cwc province has been identified in the mediterranean sea in the capo spartivento canyon system offshore the southern coast of sardinia the sardinia cold water coral province is characterized in the nora canyon by a spectacular coral growth dominated by the branching scleractinian madrepora oculata at a depth of 380 460 m the general biohermal frame is strengthened by the common occurrence of the solitary scleractinian desmophyllum dianthus and the occasional presence of lophelia pertusa as documented by remotely operated vehicle survey this area is a hotspot of megafaunal diversity hosting among other also live specimens of the deep oyster neopycnodonte zibrowii the new coral province is located between the central mediterranean cwc provinces bari canyon santa maria di leuca south malta and the western and northern ones melilla catalan provencal ligurian canyons as for all the best developed cwc situations in the present mediterranean sea the new sardinian province is clearly influenced by levantine intermediate water which appears to be a main driver for cwc distribution and viability in this basin

the basis for earthquake prediction summary recent advances in understanding the behaviour of shear waves propagating in the crust make the routine prediction of earthquakes seem practicable accumulating evidence suggests that most of the earth s crust is pervaded by distributions of fluid filled cracks and microcracks that are aligned by the contemporary stress field so that the cracked rockmass is effectively anisotropic to seismic waves this causes shear waves to split and shear wave splitting is observed whenever shear waves propagating along suitable raypaths in the crust are recorded by three component instruments these distributions of cracks are known as extensive dilatancy anisotropy or eda many characteristics of the crack and stress geometry can be monitored by analyzing shear waves propagating through the cracked rockmass observations of temporal variations of the behaviour of shear wave splitting in seismic gaps confirm these hypotheses and suggest that stress changes before earthquakes may be monitored by analyzing shear waves in particular monitoring earthquake preparation zones with three component shear wave vertical seismic profiles could lead to techniques for the routine prediction of earthquakes

estimating the internal structure of reservoirs with shear wave vsps in the past 10 years vertical seismic profiles vsps have played an increasingly important role in identifying and delineating in situ layers such vsps have usually recorded p wave data three component shear wave trains carry three or four times more information about the source and structure of the rocks along the raypath than the equivalent p wave train so in principle shear wave vsps should be a particularly valuable technique for examining in situ layers

analysis of shear waves with respect to splitting near the san andreas fault summary shear wave recordings acquired by vertical seismic profiling in the cajon pass deep scientific drillhole california have been investigated with respect to shear wave splitting using various methods the methods used should enable detection of possible anisotropy induced ai shear wave splitting consistent with a system with hexagonal symmetry and a horizontally oriented axis of symmetry careful investigation of the direct shear waves leads to the conclusion that a1 shear wave splitting consistent with this simple model cannot be observed with the methods applied the lack of such a1 shear wave splitting is probably partly caused by the low difference between the maximum and minimum principal stress in the area which is probably too modest to vertically align cracks and microcracks to any significant degree the propagation and polarization of the shear waves in this area is thus probably more controlled by factors related to the complex geology rather than to the current stress field the lack of detected a1 shear wave splitting does not however imply that the medium is isotropic the polarization diagrams show clear evidence of shear wave splitting which most likely indicates the presence of an anisotropy of higher order

shear wave splitting and anisotropy in the charlevoix seismic zone quebec during the month of august 1984 an experiment was undertaken in the charlevoix seismic zone to test the hypothesis that shear wave splitting could be observed in a seismically active region seismograms recorded by digital 3 component seismographs located very close to the epicentres of three earthquakes showed shear wave splitting the amount of splitting was estimated to be 1 6 to 2 of the shear wave velocity from particle motion diagrams the average azimuth of the direction of polarization was 30 these observations can be explained by the presence of vertical cracks striking 30 east of north with a minimum crack density of e 0 03

swas a shear wave analysis system for semi automatic measurement of shear wave splitting above small earthquakes the complexity of shear wave arrivals above small earthquakes makes the polarisations and time delays of shear wave splitting above small earthquakes difficult to measure we report a semi automatic shear wave analysis system swas that appears to combine the benefits of both visual and automatic techniques initially swas automatically estimates shear wave polarisations and picks shear wave arrivals by an expert system which provides sufficiently accurate initial measurements for visual adjustment swas then allows easy comparison and adjustment of picks between screen images of original seismograms seismograms rotated into anisotropic polarisations and polarisation diagrams hodograms with immediate plotting in various standard or non standard configurations this speeds up visual measurements by well over an order of magnitude and typically allows records of almost all small earthquakes m 1 0 to be reliably measured for shear wave splitting polarisations and time delays swas was developed and tested for data from the sil seismic network in iceland

suggestions for a consistent terminology for seismic anisotropy seismic anisotropy is an unfamiliar concept to many geophysicists and the use of misleading and ambiguous terminology has made it more difficult to understand i suggest here a consistent terminology in which simple expressions have specific meanings similar to their colloquial meanings it is hoped that use of such language will help to make the increasing number of papers reporting seismic anisotropy more readily comprehensible to the non specialist this not a manual of anisotropy and it is not intended for theoreticians it is a list of terms which may make anisotropy a little easier to understand for those more familiar with wave propagation in isotropic solids

shear wave polarizations near the north anatolian fault iii observations of temporal changes summary almost all shear waves from local earthquakes recorded on closely spaced three component seismometer networks deployed near the north anatolian fault turkey in two experiments in 1979 and 1980 display shear wave splitting the observations are consistent with the presence of eda extensive dilatancy anisotropy distributions of fluid filled cracks and microcracks aligned by the regional stress field temporal changes in the stress field which may occur before an earthquake may modify the geometry and possibly the orientation of the eda microcracks and lead to corresponding changes in the behaviour of the split shear waves a third experiment was undertaken in 1984 to investigate eda further and to search for possible temporal variations of the polarization of the leading split shear wave and the time delay between split shear waves observations indicate that the polarization alignments which are parallel to the strike of the parallel vertical eda cracks are unaltered between 1979 and 1984 implying that the direction of the regional stress field has not changed significantly temporal changes in the stress field are more likely to cause changes in the crack density and or aspect ratio which would result in a corresponding change in time delay between the split shear waves we examine observations of time delay in relation to their propagation path with respect to the crack geometry since it is then possible to separate the effects of changes in crack density and changes in aspect ratio with this procedure a small temporal variation of time delays is found between 1979 and 1984 consistent with a decrease in crack density and consequently a relaxation of stress in this time period no evidence was found for any observable variation of time delay over a six month observation span in 1984 we suggest that analysis of repeated shear wave vsps offers a technique for monitoring stress changes before earthquakes

resonant response models for the valley of mexico ii the trapping of horizontal p waves summary the strong ground motion recorded during the 1989 april 25 earthquake ms 6 9 shows many of the previously observed features in the soft lacustrine clays over which downtown mexico city is built a dominant response frequency around 0 4 hz enormous time prolongation of the seismic signal with characteristic beating phenomena and conspicuous differential amplification following the two step process suggested in part i seligman et al 1989 we add two new theoretical aspects the generation of horizontally propagating sp waves in the layered structure beneath mexico city for incoming sv waves beyond critical incidence and the subsequent lateral trapping of these sp waves in the lacustrine clays the time evolution of the lateral resonant pattern produced by these waves is calculated for the realistic geometry of the lake beds synthetic accelerograms and fourier spectra at different sites within the clay region are thus obtained our results can explain the monochromaticity and the long duration of the strong motion observed systematically in the soft clays of the valley of mexico

analysis of records of local earthquakes the turkish dilatancy projects tdp1 and tdp2 summary this is the introduction to a series of papers analysing records from three component seismometers immediately above a swarm of small earthquakes near the north anatolian fault in turkey the networks were set up to investigate the possibility of estimating dilatancy induced adisotropy by monitoring shear wave splitting in shear waves from local earthquakes as suggested by the hypothesis of extensive dilatancy anisotropy eda these networks were among the first closely spaced digital three component seismometers to be deployed in the shear wave window immediately over a seismic source zone where the polarizations of incident shear waves can be recognized from surface recordings consequently the seismograms display several new phenomena that have not been previously observed 1 the polarizations of almost all shear waves arriving within the shear wave window showed abrupt changes in direction characteristic of shear wave splitting in synthetic seismograms modelling wave propagation through anisotropic rocks 2 the polarizations of the faster split shear waves are in patterns characteristic of propagation through distributions of cracks aligned by the principal axes of stress but not characteristic sf shear waves propagating through isotropic rock from a double couple source oriented by the same stress axes 3 many characteristics of the observed polarization patterns are similar to synthetic polarization diagrams for wave propagation through vertical parallel cracks 4 the orientation of the anisotropy derived from the shear wave polarizations is consistent with the common axes of stress of the fault plane mechanisms 5 the orientation of the shear wave anisotropy is also similar to the orientation of the anisotropy derived independently from inverting p and s wave arrival times and 6 there is some indication of a temporal change in polarization in one locality such as might be expected if a change in a local stress anomaly altered the crack geometry

isotope hydrology of the aghios nikolaos area of crete greece abstract deuterium and oxygen 18 data have been used to identify the origin of recharge to a coastal complex of springs and also aquifers developed for supplying water to aghios nikolaos crete tritium data have been used to estimate the age of groundwater at different sampling locations the results of this study indicate that the almyros aghios nikolaos springs are recharged mainly from the part of the gavrovo tripolitza limestones forming the mountains of selena katharo tsivi and lassithiotika while the part of the same limestones forming the mountainous area of mavro dassos provides recharge to the northern part of the aghios nikolaos plateau the total surfaces of these areas their corresponding potential annual recharges as well as the flow velocity of the water recharging the springs of almyros aghios nikolaos during the last few kilometers of its way are estimated

influence of stratification and topography upon internal wave spectra in the region of sills 1 a non hydrostatic model in vertical cross section form is used to examine the role of sill width and buoyancy upon the energy cascade into the internal tidal band and higher frequencies in sill regions power spectra analysis of computed time series show that as sill width is reduced the energy transfer from the baroclinic tide to its higher harmonics is reduced however there is an increase in energy at higher frequencies corresponding to lee wave periods in addition internal mixing on the lee side of the sill increases the addition of small scale topography on the sill slope produces a similar spectral change in the internal wave distribution and mixing for a narrow sill the addition of a surface buoyant layer with a higher buoyancy frequency significantly reduces the energy cascade to higher frequencies with a corresponding reduction in mixing the associated spectra have characteristics resembling wide sill topography

shear wave splitting in cross hole surveys modeling shear wave splitting diagnostic of some form of effective seismic anisotropy is observed along almost all near vertical raypaths through the crust the splitting is caused by propagation through distributions of stress aligned vertical parallel fluid filled cracks microcracks and preferentially oriented pore space that exist in most crustal rocks shear waves have severe interactions with the free surface and may be seriously disturbed by the surface and by near surface layers in principle cross hole surveys chss should be free of much of the near surface interference and could be used for investigating shear waves at higher frequencies and greater resolution along shorter raypaths than is possible with reflection surveys and vsps synthetic seismograms are examined to estimate the effects of vertical cracks on the behavior of shear waves in chs experiments the azimuth of the chs section relative to the strike of the cracks is crucial to the amount of information about seismic anisotropy that can

on the influence of cold water coral mound size on flow hydrodynamics and vice versa using a combination of in situ observations and idealistic 2 d nonhydrostatic numericalsimulations the relation between cold water coral cwc mound size and hydrodynamics is explored forthe rockall bank area in the north atlantic ocean it is shown that currents generated by topographicallytrapped tidal waves in this area cause large isopycnal depressions resulting from an internal hydrauliccontrol above cwc mounds the oxygen concentration distribution is used as a tracer to visualize the flowbehavior and the turbulent mixing above the mounds by comparing two cwc mounds of different sizesand located close to each other it is shown that the resulting mixing is highly dependent on the size of themound the effects of the hydraulic control for mixing nutrient availability and ecosystem functioning arealso discussed

observations of shear wave polarizations from rockbursts in a south african gold field an analysis of acceleration and velocity recordings summary rockbursts in the deep gold mines of south africa have seismic magnitudes ranging from ml less than zero to ml more than 5 they occur in very confined volumes surrounding the working face of the mining excavations an examination of three component acceleration and velocity seismograms shows that the polarizations of shear waves recorded within the shear wave window above an active mining area have a nearly uniform alignment the polarization alignment and the measured time delays are consistent with shear waves propagating through the effective anisotropy of parallel vertical microcracks throughout the rockrnass polarizations measured from velocity transducers were compared with polarizations measured from strongmotion acceleration recordings to show the amount of scatter in the data we conclude that the dry fractures caused by the high stresses during normal mining processes have negligible effect at the wavelengths at which shear waves are recorded at the surface the anisotropy observed at the surface appears to be due to microcracks aligned by the regional stress regime rather than local disturbances to the stress regime due to mining operations

shear wave polarizations in the peter the first range indicating crack induced anisotropy in a thrust fault regime summary three component seismograms of small local earthquakes recorded in the peter the first range of mountains near garm tadzhikistan ssr display shear wave splitting similar to that previously observed near the north anatolian fault in turkey the peter the first range is in a region of compressional tectonics whereas the north anatolian fault is a comparatively simple strike slip fault detailed analysis of the turkish records suggests that the splitting is diagnostic of crack induced anisotropy caused by vertical microcracks aligned parallel to the direction of maximum compression preliminary examination of paper records from garm shows that most shear waves arriving within the shear wave window display shear wave splitting and that the polarizations of leading shear waves are consistently aligned in a ne sw direction the area is complicated and the tectonics are not well understood but the ne sw direction is approximately perpendicular to the compressional axis in many of the fault plane mechanisms of the earthquakes these earthquakes are usually at depths between 5 and 12 km although there are some deeper events nearby parallel shear wave polarizations such as those observed are expected to indicate the strike of nearly vertical parallel microcracks which would be aligned parallel to the direction of maximum compression thus the shear wave polarizations in the peter the first range indicate that the directions of principal stress are reversed in the rock above the earthquake foci where thrust faulting is taking place

aggradation and carbonate accumulation of holocene norwegian cold water coral reefs cold water coral ecosystems present common carbonate factories along the atlantic continental margins where they can form large reef structures there is increasing knowledge on their ecology molecular genetics environmental controls and threats available however information on their carbo nate production and accumulation is still very limited even though this information is essential for their evaluation as carbonate sinks the aim of this study is to provide high resolution reef aggradation and carbonate accumulation rates for norwegian cold water coral reefs from various settings sunds inner shelf and shelf margin furthermore it introduces a new approach for the evaluation of the cold water coral preservation within cold water coral deposits by computed tomography analysis this approach allows the differentiation of various kinds of cold water coral deposits by their macrofossil clast size and orientation signature the obtained results suggest that preservation of cold water coral frameworks in living position is favoured by high reef aggradation rates while preservation of coral rubble prevails by moderate aggradation rates a high degree of macrofossil fragmentation indicates condensed intervals or unconformities the observed aggradation rates with up to 1500 cm kyr 1 exhibit the highest rates from cold water coral reefs so far reef aggradation within the studied cores was restricted to the early and late holocene available datings of norwegian cold water corals support this age pattern for other fjords while on the shelf cold water coral ages are reported additionally from the early middle holocene the obtained mean carbonate accumulation rates of up to 103 g cm 2 kyr 1 exceed previous estimates of cold water coral reefs by a factor of two to three and by almost one order of magnitude to adjacent sedimentary environments shelf slope and deep sea only fjord basins locally exhibit carbonate accumulation rates in the range of the cold water coral reefs furthermore cold water coral reef carbonate accumulation rates are in the range of tropical reef carbonate accumulation rates these results clearly suggest the importance of cold water coral reefs as local maybe regional to global carbonate sinks

tectonics of the marmara sea region of turkey new evidence from micro earthquake fault plane solutions summary composite and single event fault plane solutions for microearthquakes in the izmit bay area of the marmara sea indicate right lateral strike slip motion and tension on this extension of the north anatolian fault this interpretation is consistent with teleseismically determined fault plane solutions obtained for larger earthquakes on the marmara sea seismic lineation consideration of the microplate geometry of north western turkey inferred from seismicity as well as earthquake mechanisms suggests that the region comprises two seismotectonic units with differing styles of deformation the anatolid structures of south and central western anatolia are undergoing major extension whereas the pontid structures of the marmara sea region are being sheared resulting in a mixed regime of both strike slip and extensional faulting

seismic anisotropy in the fore arc region of the hikurangi subduction zone new zealand abstract shear waves from local earthquakes recorded on an array of nine broad band seismograph stations have been used to investigate the anisotropic structure within the crust of the overlying australian plate and within and below the subducting slab underlying the southern north island new zealand earthquakes near the plate interface were used to image the crustal anisotropy whereas deeper down dip subduction zone events were used to sample the subducting slab and the mantle below apart from a small amount of variable near surface anisotropy top few kilometres of the crust the measured fast shear wave directions of both the crust of the overlying plate 51 18 and the mantle 41 15 are similar and are nearly parallel to the strike of the subduction zone and the dominant geological features of the region sks measurements which mainly sample the mantle beneath the subducting plate give a value of 28 11 and measurements made elsewhere in the new zealand region using sks and scs2 produce similar results the measurements of crustal anisotropy are similar to those found further south on the wellington peninsula and confirm the presence of near surface anisotropy the maximum delay times for crustal earthquakes indicate a pervasive shear wave velocity anisotropy of 4 in the crust of the overlying plate and the increase in the delay times with depth for the subduction zone earthquakes translates to a shear wave velocity anisotropy of 1 4 in the mantle below the subducting slab these results when combined with the sks measurements suggest trench parallel flow in the mantle below the subducted slab and a coherence of the deformation processes in the crust of the overlying plate and the mantle beneath the subducting slab

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

late weichselian deglaciation and early holocene development of a cold water coral reef along the lopphavet shelf northern norway recorded by benthic foraminifera and ostracoda cold water coral cwc settlement in northern norway is strongly related to the outlet glaciers of the fennoscandian ice sheet and dating of known cwc structures show clearly post glacial ages two gravity cores pos391 559 2 277 cm long and pos391 559 3 282 cm long were recovered on a cwc reef in the area of lopphavet northern norway detailed investigations on lithology sediment structures and composition micropaleontology foraminifera and ostracoda and ams 14c dating on the epibenthic foraminifera discanomalina coronata were performed on the two cores phosphorus analyses were performed only on core pos391 559 3 results indicate that the whole core pos391 559 2 is representative of a cwc reef environment the base of the core is dated at 10 600 120 cal yr bp thus representing one of the oldest ages of a norwegian coral reef core pos391 559 3 documents the passage from a proximal glacier environment characterized by fine silty sediments with intercalation of several dropstone layers to a cwc ecosystem the transition from the glacial to the interglacial stage is dated as old as 10 725 205 cal yr bp whereas the base of the core is dated to an age of 15 300 550 cal yr bp diversity of benthic foraminifera is higher within the cwc especially in the intervals containing coral framework five clusters are identified based on the bray curtis similarity term analyses and the interpretation of data shows that they are related to different ecological settings e g fluctuations of the sea ice cover influence of the warmer and more saline atlantic water masses transitional to a fully interglacial environment well oxygenated nutrient rich and high current setting being conducive to cwc ostracod assemblages show that these crustaceans may be also used to characterize sedimentary facies on cwc reefs

environmental constraints on holocene cold water coral reef growth off norway insights from a multiproxy approach high latitude cold water coral cwc reefs are particularly susceptible due to enhanced co2 uptake in these regions using precisely dated u th cwcs lophelia pertusa retrieved during research cruise pos 391 lopphavet 70 6 n oslofjord 59 n we applied boron isotopes 11b ba ca li mg and u ca ratios to reconstruct the environmental boundary conditions of cwc reef growth the sedimentary record from these cwc reefs reveals a lack of corals between 6 4 and 4 8 ka the question remains if this phenomenon is related to changes in the carbonate system or other causes the initial postglacial setting had elevated ba ca ratios indicative of meltwater fluxes showing a decreasing trend towards cessation at 6 4 ka with a oscillation pattern similar to continental glacier fluctuations downcore u ca ratios reveal an increasing trend which is outside the range of modern u ca variability in l pertusa suggesting changes of seawater ph near 6 4 ka the reconstructed bwt at lopphavet reveals a striking similarity to barent sea surface and sub sea surface temperature records we infer that meltwater pulses weakened the north atlantic current system resulting in southward advances of cold and co2 rich arctic waters a corresponding shift in the 11b record from 25 0 to 27 0 probably implies enhanced ph up regulation of the cwcs due to the higher pco2 concentrations of ambient seawater which hastened mid holocene cwc reef decline on the norwegian margin

perspectives on seismic and rockburst research in the south african gold mining industry 1983 1987 this paper summarizes seismic and rockburst research activities related to south african deep level gold mines over the period 1983 to 1987 it covers continued research in directions that were considered in the seismicity in mines symposium in 1982 as well as in several new areas of research five broad areas are identified 1 seismic data acquisition and processing improved seismic systems are being developed velocity models related to known stratigraphy are being used to provide more accurate estimates of seismic locations 2 source mechanisms and near source effects on seismic wave transmission this work provides fundamental insights into seismicity and rock behaviour and is being applied in rockburst prediction research 3 mine layouts excess shear stress is being investigated as a design parameter by analyzing mining configurations and resultant seismicity in addition better understanding of the behaviour of highly stressed remnants and pillars is also being obtained from seismic studies 4 strong ground motion studies evaluation of the performance of support elements including recently developed backfill materials requires better knowledge of ground motion around underground excavations during seismic events and rockbursts 5 rockburst prediction and control rockburst prediction research continues with some reported success in addition the feasibility of actively triggering fault slip or conditioning the rock ahead of the stope face to ameliorate the rockburst hazard is currently being investigated

erosion isostatic rebound models for uplift an application to south eastern australia summary a mountain s history includes two distinctive phases one of active tectonism and construction followed by one of erosion and passive isostatic rebound in the first phase uplift is driven by tectonic mechanisms while in the second phase base levels of the terrain are regionally uplifted it is this latter phase that is modelled here the starting model is a mountain range initially in isostatic equilibrium on a viscoelastic plate defined by the effective flexural rigidity d and relaxation time 7 the rate of erosion at any time t is assumed to be proportional to the elevation at that time with an erosional time constant 7 for a given present day topography the uplift erosion gravity and stress can be computed through time as functions of d 7 7 and the time to at which the erosional rebound mechanism became the dominant landscaping process the model has been applied to the highlands of south eastern australia which we assume to be an erosional residue of the palaeozoic lachlan fold belt observations of rivers cutting through cainozoic basalts and other geomorphological indicators of uplift can then be interpreted in terms of the isostatic rebound and there is no need to invoke active tectonic uplift mechanisms the model parameters that fit the observations are t 150 3 50myr dt 2 5 x ioz4nmmyr and to 180 200myr the predicted late palaeozoic fold belt topography is about 75 per cent greater than the present day values topographic depressions are predicted to have occurred along the margins of the formerly elevated areas and the model is relevant to understanding the evolution of basins flanking the highlands within the highland area erosion generally exceeds uplift but in some regions the two are approximately equal in keeping with recent geomorphological observations the flexural stresses associated with the erosion initially increase with time and maximum values in south eastern australia are predicted to occur in late cainozoic time with the stress state near the surface being mainly tensional

hydrodynamics and cold water coral facies distribution related to recent sedimentary processes at galway mound west of ireland abstract cold water coral carbonate mound development is the result of complex and interactive hydrographical biological and geological processes that can result in morphostructures several hundred meters high the case study presented here investigates one of these large mounds galway mound in the eastern porcupine seabight to build an understanding of mound forming processes and the driving factors for the first time bottom current data have been recorded at six locations over a mound thus allowing an interpretation of the local flow field to be made in addition to the overall flow pattern in the porcupine seabight the recorded data display distinct diurnal tides comparison of the local flow field coral facies distributions current induced seabed features and grain size distributions over the galway mound highlights a correlation between the abundance of living corals with areas of enhanced bottom currents however the interplay of contour currents tidal currents and the local topography further influences the coral facies and results in a distinct asymmetry in the coral facies distribution at galway mound by baffling sediment the corals also affect sedimentation on the mound

large deep water coral banks in the porcupine basin southwest of ireland the porcupine basin southwest of ireland was one of the earliest sites from where the deep water corals lophelia sp and madrepora sp were recovered these deep water corals have since been found all along the atlantic margins of europe in water depths ranging from 50 to more than 2000 m recent geophysical studies have demonstrated the mound building potential of deep water corals available data indicate that three major provinces of coral bank occurrences can be identified in the porcupine basin 1 high relief surface mounds which have a dimension of 1 by 5 km and a height up to 200 m hovland mounds flanked to the north by 2 a swarm of buried mounds somewhat smaller up to 90 m and with more irregular shapes than those recognised in area 1 magellan mounds and 3 outcropping or buried conical mounds single or in elongated clusters up to 150 m high occurring on the southeastern slope of the basin belgica mounds as far as can be inferred from shallow cores the surface lithology predominantly consists of an upper layer rich in foraminiferal sand and terrigenous silty clay with intercalations of biogenic rubble the banks host a remarkable number of colonies of living and dead lophelia pertusa and madrepora oculata the living and dead assemblages are underlain by a significant layer of coral debris in a muddy matrix deep water coral debris together with a living association of the same species covers the surface of the belgica and hovland mounds which may suggest that these corals have played a significant role in the development of the mound structures the capacity for mound formation by scleractinian corals in the aphotic zone has been known for some time examples are found at different locations along the shelves and the continental margins of the north atlantic the role of the corals in these deep water build ups is still a point of debate though the genesis and initial control of mound settings in this basin might be related to hydrocarbon seeps it appears that the major development of the porcupine coral banks in recent geological times has most likely been controlled by oceanic circulation and dynamics in water masses and nutrient supply

generation of inhomogeneous p waves in a layered medium abstract the generation of inhomogeneous horizontally propagating p waves in a stratified elastic medium were analyzed for incident sv waves beyond the critical angle using realistic values of s and p wave velocities and considering both clay and alluvial sedimentary layers overlying a bedrock half space we obtained enormous amplifications for the horizontal displacements at the free surface these amplifications are of the order of several thousands for the fundamental mode of the system which systematically dominates all other frequencies

isotope hydrology study of areas in eastern macedonia and thrace northern greece abstract sampling of springs for which the mean elevation of recharge could be estimated provided an estimate of 0 44 100 m 1 for the groundwater altitude effect on the 18o value in eastern macedonia and 0 21 in thrace a slight difference in both the slope and the intercept of the lines expressing the regression of elevation against 18o values in the continental part of thrace and the island of thassos is also observed as case studies the isotopic composition of surface water and groundwater was used to investigate the mechanism of recharge of groundwater in the areas of lekani mountains and thassos island as a result the contribution of water leaking from the river nestos to the recharge of the springs of paradissos near to the river bed kastro filippon and aghios athanassios at a distance of over 25 km from the river bed as well as of the aquifers tapped by boreholes in the plain between kastro filippon and aghios athanassios was indicated the estimate is 54 12 for the contribution of the river water to the spring yield in the case of aghios athanassios and 43 10 in the case of the springs of paradissos as another result the hydraulic connection between the marbles of profitis ilias and the marbles and dolomites of potamia series in the case of thassos island was confirmed finally the extent of the carbonate rocks and the amount of precipitation effective in recharge to the main springs as well as to the aquifers tapped by boreholes in the different plains of the areas under investigation were estimated

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

shear wave polarization anisotropy observed in a rift zone in japan abstract we have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in central kyushu japan we have recognized two different directions in the polarization of the faster shear waves about e w directions at three stations located in the rift zone and about n s at two stations possibly located outside and close to the edge of the rift zone the former case e w is consistent with a hypothesis that shear wave splitting is caused by vertical cracks or fractures striking in the e w direction which have formed under the present tectonic stress inside a rift zone extending in the n s direction with the maximum horizontal compressive stress striking in the direction about e w and the minimum compressive stress vertical in contrast the latter case n s is inconsistent with the above hypothesis this inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone alternatively it may result from a complicated stress regime within the crust close to the edge of the rift zone arrival time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0 12 s while those from the upper mantle events are less than about 0 25 s the relatively small arrival time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust

shear wave splitting observed above small earthquakes in a geothermal area of japan summary the phenomenon of shear wave splitting has been observed at two seismic stations with three component seismographs above shallow microcarthquakes which occurred at depths of 1 2 km in the takinoue geothermal area of northern honshu an almost uniform alignment of the polarizations of the faster split shear wave in the direction of ene wsw may result from the presence of non randomly distributed liquid filled microcracks two models of crack distribution reproduce the observed polarization pattern in the first model vertical microcracks are aligned in the ene wsw direction while the second supposes a distribution of microcracks striking in the ene wsw direction but dipping randomly the ene wsw strike of the cracks in both models is consistent with the direction of the ene wsw maximum horizontal compressive stress inferred from the analysis of earthquake mechanisms in the study area it differs slightly from the strike of vertical fractures wnw ese obtained by hydraulic fracturing experiments at sites about 10 km west of the study area the tectonic stress regime obtained from an analysis of focal mechanisms of microearthquakes occurring in this area seems to be consistent with the latter model of the crack distribution from the shear wave analysis at least at depths less than about 700 m an analysis of the delay times between the faster and the slower split shear waves has revealed a region with anomalously large delay times normalized to ray path lengths a 3 d p wave velocity inversion indicates that this region is a low p wave velocity zone the low p wave velocities and high shear wave birefringence is consistent with a relatively dense distribution of liquid filled microcracks

benthic dynamics at the carbonate mound regions of the porcupine sea bight continental margin a brief review is given of some dynamical processes that influence the benthic dynamics within the carbonate mound provinces located at the porcupine bank sea bight margin ne atlantic the depth range of the mounds in this region 600 1 000 m marks the upper boundary of the mediterranean outflow water above which eastern north atlantic water dominates both water masses are carried northwards by the eastern boundary slope current in the benthic boundary layer both the action of internal waves and other tidal period baroclinic waves may enhance the bottom currents and add to both the residual and maximum flow strength both residual and maximum bottom currents vary at different mound locations with stronger currents found at belgica se porcupine sea bight mound and pelagia nw porcupine bank mound regions whilst weakest currents are found at the hovland and magellan mounds at the northern sea bight margin the differences may be attributed to the presence of internal waves pelagia or bottom intensified diurnal waves belgica these different dynamical regimes are likely to have implications for the distribution patterns of live coral at the different locations

analysis of exceptionally large tremors in two gold mining districts of south africa an investigation of ground motion recorded using broad band wide dynamic range digital seismographs of large mine tremors from two south african mining districts with different geologic settings reveals some essential differences in both seismic source and ground motion parameters in the klerksdorp district where the strata are offset by major throughgoing normal faults the largest tremors with magnitudes ranging as high as 5 2 tend to be associated with slip on these pre existing faults moreover the seismic source and ground motion parameters are quite similar to those of natural crustal earthquakes in the carletonville district by contrast where substantial faults do not exist the large magnitude tremors appear to result from the failure of relatively intact rock and cause seismic stress drops and ground motion parameters higher than normally observed for natural shocks additionally there appears to be an upper magnitude limit of about 4 in the carletonville district detailed analyses of an exceptionally large event recorded locally from each of these districts serve to highlight these contrasts

paleoseawater density reconstruction and its implication for cold water coral carbonate mounds in the northeast atlantic through time carbonate buildups and mounds are impressive biogenic structures throughout earth history in the recent ne atlantic cold water coral cwc reefs form giant carbonate mounds of up to 300 m of elevation the expansion of these coral carbonate mounds is paced by climatic changes during the past 2 7 myr environmental control on their development is directly linked to controls on its main constructors the reef building cwcs seawater density has been identified as one of the main controlling parameter of cwc growth in the ne atlantic one possibility is the formation of a pycnocline above the carbonate mounds which is increasing the hydrodynamic regime supporting elevated food supply and possibly facilitating the distribution of coral larvae the potential to reconstruct past seawater densities from stable oxygen isotopes of benthic foraminifera has been further developed a regional equation gives reliable results for three different settings peak interglacials e g holocene peak glacials e g last glacial maximum and intermediate setting between the two extremes seawater densities are reconstructed for two different ne atlantic cwc carbonate mounds in the porcupine seabight indicating that the development of carbonate mounds is predominantly found at a seawater density range between 27 3 and 27 7 kg m 3 notation comparable to recent conditions we interpret the reconstructed density range as a pycnocline serving as boundary layer on which currents develop carrying nutrition and possibly coral larvae the close correlation of cwc reef growth with reconstructed seawater densities through the pleistocene highlights the importance of pycnoclines and intermediate water mass dynamics

preboreal onset of cold water coral growth beyond the arctic circle revealed by coupled radiocarbon and u series dating and neodymium isotopes cold water coral mounds of lophelia pertusa are widespread across the scandinavian shelf which was completely ice covered during the last glacial maximum between 22 to 18 ka bp rapid deglacial meltdown of the fennoscandian inland ice and the retreat of its ice streams freed most of the shelf of ice by 15 ka bp however cold water coral growth commenced only after the pleistocene holocene transition at 11 65 ka bp when modern like climatic patterns and oceanographic conditions were established a tight climatic coupling has been constrained with u series ages coupled 14c ages provide local reservoir ages from various gravity cores in a fjord setting in stjernsund at 70 n and on the open shelf in traenadjupet at 66 n reinvestigation of earlier 14c coral chronologies suggests that coral ecosystems widely established themselves across the entire 3000 km long scandinavian shelf prior to 10 ka bp the earliest occurrence of madrepora oculata at 2 4 ka bp suggests a late holocene colonization of the norwegian shelf which is linked to a prominent mound growth hiatus in traenadjupet 64 n mound growth rates near the northern biogeographic boundary of l pertusa with up to 614 cm ka 1 during certain growth periods are much higher than the previously reported fastest rates of 220 cm ka 1 from the irish margin contemporaneous rapid fjordbasin sedimentation is slower with 63 cm ka 1 matrix 14c ages overlap with coral 14c ages from the same horizon this indicates rapid framework construction and efficient trapping of background sediment hiatuses are frequent in on mound sediments and only short periods of coral growth are recorded coupled 14c and end values indicate a persistent holocene inflow of the north atlantic current in stjernsund but also deglacial meltwater mixing during the early holocene prior to 9 5 ka bp reservoir ages are overall close to the surface marine reservoir age but r is highly localized

high resolution facies zonation within a cold water coral mound the case of the piddington mound porcupine seabight ne atlantic abstract framework forming cold water corals cwc s such as lophelia pertusa and madrepora oculata generate positive topographic features on the seabed called cwc mounds in the north east atlantic cwc mounds have been studied in detail and reveal heterogeneous spatial on mound organisation of coral patches many of these studies are limited by a paucity of remotely sensed and video imagery at an appropriate resolution and coverage this study is the first attempt to video mosaic an entire cwc mound the piddington mound of the moira mounds porcupine seabight irish margin the mosaic is divided into 18 980 0 25 m 2 cells with a manual classification applied to each within a geographic information system gis geospatial analysis shows that cell distribution is not random but clustered significantly across the mound surface these clusters of cells make up a ring like facies pattern a model for the processes that lead to this facies pattern is suggested based on contemporary environmental controls parallels to shallow water reef atolls are also drawn which subsequently has implications for interpreting fossil coral outcrops

surface seismic and electrical methods to detect fluids related to faulting in the absence of drilling surface based geophysical methods are necessary to observe fault zones and fault zone physical properties at seismogenic depths these in situ physical properties can then be used to infer the presence and distribution of fluids along faults although such observations are by nature indirect and become less exact with greater depth multiple observations of a range of such geophysical properties as compressional and shear seismic velocity vp and vs vp v5 ratio related to poisson s ratio resistivity and attenuation in and adjacent to fault zones offer the greatest hope of making inferences of the fault zone geometry fluids in the fault zone and fluid reservoirs in the surrounding crust for simple geometries fault zone guided waves can provide information on fault zone width and velocities for faults of the order of 200 m wide to address the question of whether a narrow fault zone can be imaged well enough at depths of seismic rupture to infer the presence of anomalously high fluid rock ratios we present synthetic seismic tomography and magnetotelluric examples for an ideal case of a narrow fault zone with a simple geometry large changes in material properties and numerous earthquakes within the fault zone a synthetic 0 5 km wide fault zone with 20 velocity reduction is well imaged using local earthquake tomography when sequential velocity inversions are done the true fault width is found even to 9 km depth although the calculated amplitude of the velocity reduction is lower than the actual amplitude vp vs is as well determined as vp magnetotelluric imaging of a synthetic fault zone shows that a conductive fault zone can be well imaged within the upper 10 km further a narrow 1 km very low resistivity 3 ohm m fault core can be imaged within a broad 5 km low resistivity 10 ohm m fault zone illustrating that regions of a fault containing large quantities of interconnected fluids within a broader conductive fault zone should be detectable thus variations in fluid content and fluid pressure can be inferred from electrical and seismic methods but there will always be uncertainty in these inferences due to the trade off with other factors such as intrinsic variations in porosity mineralogy and pore geometry the best approach is combined modeling of varied seismic and electrical data

the scattering of shear waves in the crust the two major sources of scattering for shear waves in the crust interactions with the topography at the surface and the effective anisotropy of aligned cracks throughout the rockmass introduce first order changes to the shear wave particle motion at the surface shear waves are scattered by the topography within a wavelength or two of the recording site so that unless the effective incidence angle is less than the critical angle sin 1vs vp the recorded waveforms may bear little relationship to the waveforms of the incident wave within the rockmass shear waves are scattered by extensive dilatancy anisotropy eda the distribution of stress aligned fluid filled cracks microcracks and preferentially oriented pore space pervading most rocks in the crust analysis of this shear wave splitting yields new information about the internal structure of thein situ rockmass which is not otherwise available

extensive dilatancy anisotropy relative information in vsps and reflection surveys1 yardley g and crampin s 1990 extensive dilatancy anisotropy relative information in vsps and reflection surveys geophysical prospecting 39 337 355 shear waves have complicated interactions with the free surface particularly in the presence of low velocity surface layers topographic irregularities and the expected near surface crack and stress anomalies consequently it has been suggested that shear waves should be recorded subsurface in vertical seismic profiles vsps in order to extract accurate information about the in situ crack and stress geometry contained in shear wave splitting this paper compares the information in synthetic shear waves in reflection gathers and vsps in order to assess the relative merits of the two techniques for investigating shear wave splitting synthetic seismograms demonstrate that in the presence of even very simple surface layers shearwaves recorded in reflection surveys at the surface have polarizations which may not indicate crack and stress geometry at depth in contrast shear waves recorded in vsps are relatively unaffected by surface layers and near surface stress and crack anomalies and the behaviour of shear wave splitting is dominated by the structure of the rock mass in the vicinity of subsurface geophones matrix rotations of multicomponen t multisource shear wave reflection data to extract the information contained in the split shear waves are found to be directly meaningful only in situations where crack orientations do not change with depth

a four year study of shear wave splitting in iceland 1 background and preliminary analysis this is part 1 of a report of a four year study of seismic shear wave splitting in iceland designed to seek temporal variations before earthquakes shearwave splitting is observed routinely in iceland whenever shear waves arrive within the shear wave window of seismic stations and whenever adequate data are available temporal and spatial variations in shear wave splitting are observed before both earthquakes and volcanic eruptions shear wave splitting is caused principally by the stress aligned fluid saturated microcracks and pore throats in almost all in situ rocks fluid saturated microcracks are the most compliant elements of the rock mass and changes in splitting can be directly interpreted and modelled as the effects of changing stress on the microcrack geometry in the rock mass often at considerable distances from the immediate earthquake source zone such changes were found and are reported in paper 2 this paper presents the background preliminary observations and analysis of shear wave splitting in iceland

crust and upper mantle seismic anisotropy variations from the coast to inland in central and southern mexico subduction zones are among the most dynamic tectonic environments on earth deformation mechanisms of various scales produce networks of oriented structures and faulting systems that result in a highly anisotropic medium for seismic wave propagation in this study we combine shear wave splitting inferred from receiver functions and the results from a previous sks wave study to quantify and constrain the vertically averaged shear wave splitting at different depths along the 100 station mesoamerican subduction experiment array this produces a transect that runs perpendicular to the trench across the flat slab portion of the subduction zone below central and southern mexico strong anisotropy in the continental crust is found below the trans mexican volcanic belt tmvb and above the source region of slow slip events we interpret this as the result of fluid melt ascent the upper oceanic crust and the overlying low velocity zone exhibit highly complex anisotropy while the oceanic lower crust is relatively homogeneous regions of strong oceanic crust anisotropy correlate with previously found low v p v s regions indicating that the relatively high v s is an anisotropic effect upper mantle anisotropy in the southern part of the array is in trench perpendicular direction consistent with the alignment of type a olivine and with entrained subslab flow the fast polarization direction of mantle anisotropy changes to n s in the north likely reflecting mantle wedge corner flow perpendicular to the tmvb

evidence for shallow and pervasive seismic anisotropy in the wellington region new zealand the shear waves of local earthquakes were recorded during a 5 month deployment of seven three component digital seismographs on the wellington peninsula new zealand the seismographs were spaced an average of less than 5 km apart and over 300 local earthquakes were recorded with phase arrivals within the shear wave window a significant number 37 of the earthquakes recorded showed clear evidence of shear wave splitting identifiable fast and slow shear wave arrivals with similar pulse shapes consistent polarization directions at particular stations were also observed even when poor signal to noise ratio or scattering meant that no slow shear wave arrival could be identified however there were large station to station differences in the polarization directions correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double couple focal mechanism the cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy large station to station differences in the polarization alignments ranging from 61 24 to 137 18 suggest that most anisotropy is confined to the top 2 3 km of the crust however there is evidence from one station for a small amount of pervasive anisotropy if such a trend existed on the other stations it could not be identified because of the large scatter in the data points the measured delay times between split shear waves vary from 0 02 to 0 22 s with a mean value of 0 1 0 06 s this translates to a near surface shear wave velocity anisotropy of about 10 with up to 2 pervasive anisotropy possible throughout the crust this data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important there is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment

isotope hydrology study of the kalamos attikis and assopos riverplain areas in greece variations in the isotopic composition of water were used to define the mechanism of recharge of the assopos riverplain and the kalamos attikis coastal brackish karst springs the plain is mainly recharged by local rain water while the karst springs are fed by the parnis mountain system the mean recharge altitude being estimated in the order of 870 m the apparent velocity and the underground pathway of the water feeding the springs were deduced and the most convenient sites for the drilling of production wells for recovery of the spring water before salinization were sited as a result 73 000 m3 of water are now pumped per day and used as additional potable water for the athens area comprizing about one sixth of the mean daily consumption

crustal anisotropy from local observations of shear wave splitting in west bohemia czech republic abstract the results of a systematic study of shear wave splitting observed in the three component digital seismograms of about 400 local microearthquakes that occurred during the 1985 1986 west bohemian earthquake swarm are presented two shear phases polarized nearly perpendicularly in horizontal projection were observed at all five stations of the local seismic network for most of the events the polarization of the faster shear wave is aligned in the wnw ese direction regardless of the mechanism and hypocenter location of the individual events and coincides with the direction of the maximum horizontal compressive tectonic stress in the region these effects are interpreted in terms of effective anisotropy of the upper crust in the region the maximum delay time between the split shear waves is 0 15 sec and corresponds to 6 anisotropy directional variation of the delay time was observed at the nearest station epicentral distance 5 km this variation was compared with functions predicted by theoretical models of cracked media to resolve which model is the most appropriate for crustal anisotropy surprisingly no agreement could be found for hudson9s model of dry or water filled parallel cracks that is widely used by many authors a successful fit was obtained for schoenberg douma9s model of a medium with parallel fractures the fracture normal pointed n31 e and the optimum fracture parameters were e n 0 4 and e t 0 02 the theoretically calculated fractures are aligned parallel to the tectonic stress direction and do not correspond to any real tectonic lines in the region

water mass characteristics and sill dynamics in a subpolar cold water coral reef setting at stjernsund northern norway the stjernsund located in subpolar setting at 70 5 n off northern norway hosts a thriving cold water coral reef community on a morainic sill dives with manned submersible jago identified the different reef zones and sedimentary facies on top and on the slopes of the sill hydrographic investigations indicate different water mass distribution east and west of the sill winter mode water and norwegian coastal water variability depends on the runoff and freshwater discharge into the fjord atlantic water dynamics are almost entirely tidally driven high resolution ctd time series covering a full tidal cycle demonstrate mixing processes occurring east of the sill additionally the different bathymetric distribution of living corals on the western and eastern slope of the sill portrays the dependence on these tidal dynamics the living corals thrive just below the isopycnal of 27 5 kg m 3 which marks the boundary between norwegian coastal water and atlantic water

seismic anisotropy the state of the art ii summary the theory causes observations and possible applications of seismic anisotropy in the earth have developed considerably since the previous state of the art paper was published in 1977 the behaviour of waves in layered anisotropic media is now much better understood and the evidence for seismic anisotropy indicates that anisotropy is likely to be present throughout much of the crust and upper mantle the top few hundred kilometres of the mantle appears to be anisotropic with the orientations aligned by the present or palaeo stress field the upper part of the crust is frequently anisotropic probably due to cracks differentially aligned by the non lithostatic stresses the possibility of being able to monitor crack geometry by seismic techniques opens a wide range of applications in currently important activities

shear waves from three component ocean bottom seismographs off lofoten norway indicative of anisotropy in the lower crust summary data from four three component ocean bottom seismographs obs situated on a 145 km long profile on the rost high on the continental shelf off lofoten northern norway have been modelled using 2 d seismic ray tracing and a p and s wave velocity model along the profile is presented the s wave model has been obtained from a study of the high quality horizontal component data a vp vs ratio of 1 95 2 15 is found in the sedimentary layers on the rost high these high values are consistent with shaly sediments which are identified in dredged bedrock samples south of the rost high a considerably lower vp vs ratio ca 1 6 is estimated for the sediments which can be attributed in part to a higher degree of compaction in the deepest sediments and in part to a more sandy lithology in the upper and middle crystalline crust a vp vs ratio of 1 75 is obtained strong s reflections and p to s conversions at the moho are observed on all four instruments and the modelling of these arrivals indicates that the s wave velocity in the lower crust varies with angle of incidence from 3 5 km s 1 vp vs 1 95 at vertical incidence to 4 0 km s 1 vp vs 1 7 at 60 70d this angle of incidence dependent s wave anisotropy the order of 10 per cent is consistent with some horizontal or subhorizontal layering in the lower crust

crustal composition of the more margin and compilation of a conjugate atlantic margin transect highlights the basement at the mid norwegian more margin is dominantly felsic in composition a lower crustal body is interpreted as a mixture of continental blocks and eclogite the thickness of the outer lower crustal body is twice as thick on the east greenland margin the thinning during this first phase of post caledonian extension was highest for proto norway abstract the inner part of the volcanic passive more margin mid norway expresses an unusual abrupt thinning from high onshore topography with a thick crust to an offshore basin with thin crystalline crust previous p wave modeling of wide angle seismic data revealed the presence of a high velocity 7 7 8 0 km s body in the lower crust in this transitional region these velocities are too high to be readily interpreted as early cenozoic intrusions a model often invoked to explain lower crustal high velocity bodies in the region we present a vp vs model derived from the modeling of wide angle seismic data acquired by use of ocean bottom seismograph horizontal components the modeling suggests dominantly felsic composition of the crust an average vp vs value for the lower crustal body is modeled at 1 77 which is compatible with a mixture of continental blocks and caledonian eclogites the results are compiled with earlier results into a transect extending from onshore norway to onshore greenland back stripping of the transect to early cenozoic indicates asymmetric conjugate magmatism related to the continental break up further back stripping to the time when most of the caledonian mountain range had collapsed indicates that the thinning during the first phase of extension was about 25 higher for proto norway than proto greenland

anisotropy in the shallow crust observed around the san andreas fault before and after the 2004 m 6 0 parkfield earthquake local seismic arrays were deployed at two locations along the san an dreas fault saf near parkfield california before and after the 2004 m 6 0 parkfield earthquake using local earthquakes we determine the anisotropic field within 1 2 km of the main trace of the saf at the two array locations separated by 12 km the initial array near the safod site was deployed for six weeks in october and november 2003 and the second array located near the town of parkfield was de ployed for 3 months following the 28 september 2004 m 6 0 parkfield earthquake we find the fast shear wave polarization direction nearly fault parallel n40w for stations on the main fault trace and within 100 m to the southwest of the saf at both array locations these fault parallel measurements span the 100 to 150 m wide zone of pervasive cracking and damage interpreted from fault zone trapped waves associated with the main fault core li et al 2004 2006 outside of this zone the fast orientations are scattered with some preference for orientations near n10e roughly parallel to the regional maximum horizontal compressive stress direction rh in addition fast directions are preferentially oriented parallel to a northern branch of the saf recorded on stations in the 2004 parkfield deployment the measured anisotropy is likely due to a combination of stress aligned micro cracks away from the fault and shear fabric within the highly evolved fault core the majority of our measurements are taken outside of the main fault core and we es timate the density of microcracks from the measured delay times apparent crack densities are approximately 3 with large scatter the data suggest weak depth dependence to the measured delay times for source depths between 2 and 7 km below 7 km source depth the delay times do not correlate with depth suggesting higher confining pressure is forcing the microcracks to close no coseismic variation in the anisotropic parameters is observed suggesting little to no influence on measured splitting due to the 2004 m 6 0 parkfield earthquake however the premainshock and postmainshock data presented here are from arrays separated by 12 km limiting our sensitivity to small temporal changes in anisotropy

shear wave splitting above small earthquakes in the kinki district of japan abstract we have identified shear wave splitting diagnostic of the effective anisotropy induced by aligned microcracks in the wavetrains of micro earthquakes at four stations of the abuyama network in the kinki district of japan we find that the directions of polarization of the faster split shear waves are nearly parallel for all azimuths of arrival and for all angles of incidence less than the critical angle at three of the four stations these directions of polarization are consistent with the axis of maximum compression obtained from earthquake fault plane mechanisms and also agree with the directions of the general trends of geological structures which represent the orientations of the cleavage or lamination these results suggest that crack induced anisotropy is present in the brittle upper crust beneath japan as has been found elsewhere but we could not distinguish whether this reflects the distributions of cracks induced by the present stress field or results from the general trends of surface geology although the delay times between faster and slower shear waves are difficult to estimate reliably because of their high sensitivity to internal interfaces the delay times can be interpreted as the result of a distribution of parallel vertical cracks with a crack density of about 0 04 the consistency or lack of consistency of the directions of the shear wave polarizations at the four stations demonstrates the effects of surface topography and near surface layering on the shear wave polarizations

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

shear wave polarizations near the north anatolian fault ii interpretation in terms of crack induced anisotropy summary the polarizations of shear waves recorded by networks of digital thxee component seismometers immediately above small earthquakes near the north anatolian fault in turkey display shear wave splitting on almost all shear wave seismograms recorded within the shear wave window this splitting is incompatible with source radiation patterns propagating through simple isotropic structures but is compatible with effective anisotropy of the internal structure of the rock along the ray paths this paper interprets the phenomena in terms of widespread crack induced anisotropy distributions of stress induced cracks model many features of the observations and synthetic polarization diagrams calculated for propagation through simulated cracked rock are similar to the observed patterns this evidence for widespread crack induced anisotropy lends strong support to the hypothesis of extensive dilatancy anisotropy eda suggested by laboratory experiments in subcritical crack growth the crucial evidence confirming some form of eda would be observations of temporal changes in shear wave splitting as the stress field alters the crack density and crack geometry there is some weak evidence for such temporal changes at one site but further analysis of suitable digital three component seismometer networks in seismic areas is required to confirm eda

extensive dilatancy anisotropy beneath mount hood oregon and the effect of aspect ratio on seismic velocities through aligned cracks the azimuthal variations of p wave velocity measured in u s geological survey usgs seismic refraction experiments around mount hood oregon are modeled by propagation through the effective anisotropy of parallel vertical cracks the three refractors at 1 3 7 and 8 5 km depth show different behavior the shallow refractor at 1 km displays p wave velocity variations typical of the parallel water filled vertical microcracks found throughout much of the upper continental crust the arrivals from the refractor at 3 7 km display velocity variations consistent with either dry cracks or more probably with cracks filled with supercritical water as one interpretation of the heat flow around mount hood suggests that temperatures at 3 7 km would be above the critical temperature for water at in situ pressures the velocity variations at the deepest refractor at 8 5 km depth are poorly constrained but again suggest liquid filled cracks the possibility of high temperatures suggests that at this depth the cracks could be thin skins of melt along grain boundaries the vertical cracks in all three refractors strike in the northwest to southeast quadrants which is consistent with the compressional quadrants in the mechanisms of local earthquakes shear wave splitting reported elsewhere indicates that there are stress aligned water filled microcracks extensive dilatancy anisotropy or eda throughout at least the brittle upper 10 or 20 km of the continental crust the high heat flow in the high cascades may confine the range of crack behavior usually spread over the whole thickness of the crust to the top 10 km beneath mount hood thus the p wave velocity variations beneath mount hood may indicate that eda cracks occur throughout the whole of the continental crust the large number of shot to recorder travel times measured by the usgs allows extensive dilatancy anisotropy to be recognized from the analysis of p wave velocity variations for the first time